Patent Publication Number: US-7589888-B2

Title: Optical amplifier having a constant gain profile

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/389,486, filed Mar. 14, 2003, now U.S. Pat. No. 7,042,634 which claims priority to Japanese Patent Application Nos. 2002-070974 filed Mar. 14, 2002 and 2002-301155 filed Oct. 15, 2002, the contents of which are incorporated herein by specific reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to optical amplifiers, and more particularly to a technique for controlling the gain profile of an optical amplifier equipped with an amplification medium containing rare-earth ions. 
     2. The Relevant Technology 
     Wavelength division multiplexing (WDM) communications technology is one of the most effective means for meeting the sharply increasing demand for optical communication. Since WDM increases the communications capacity by increasing signal channels, the available bandwidth for signals must be expanded. 
     The current WDM communications use rare-earth ion doped optical fiber amplifiers in many cases. One of the bands used for signal amplification is the C band (1530-1570 nm) where the Erbium doped optical fiber amplifier (EDFA) can work effectively. 
     Meanwhile, the S band (1460-1530 nm) is drawing attention as the next-generation band that has a transmission loss and a low dispersion as low as that in the C band. The Thulium doped optical fiber amplifier (TDFA), of which amplification band lies in the S-band, is under intensive research. 
     S-Band TDFA has attained a power conversion efficiency (40%) as high as that accomplished by EDFA for use in the L band, and the experiment of its WDM transmission was successful (for example, see OFC2001 PD-1). Furthermore, C-band EDFA is used together with L-band EDFA for band combination to widen the band for signal amplification. Indeed, there is a report that this combination attained a broadband data transmission of 10.9 Tbit/s (for example, see OFC2001 PD-24). 
     In WDM communications using plural amplifiers equipped with rare-earth doped optical fibers as amplification media, such a problem is posed that the spectrum of output light from the amplifiers changes and occasionally deforms significantly. 
     This is because the power of input signal light to the optical amplifier fluctuates with time-varying transmission loss and with changes in the number of signal channels, thereby the gain spectrum of the optical amplifier changes, and such changes are accumulated to result in signal deformations. 
     Since deformations of the gain spectrum in the amplifier become a cause of limiting the transmission distance, the gain spectrum must be held constant. For this purpose, it is effective to control the gain profile of each amplifier to be constant (hereinafter, may be called the gain profile constant control (GPCC)). 
     As an example of such a control method for silica-based EDFA, there is a method that monitors a gain of a signal light in a channel and controls the power of pump light so that the gain in a channel becomes constant. This is an almost established control technique for maintaining the gain profile constant in silica-based EDFA. The gain profile in fluoride PDFA can be held constant by a similar control method. The amplification medium (optical fiber) of such optical amplifiers operates according to the amplification mechanism in which levels related with the amplification are substantially two. This is a mechanism rarely seen that allows to handle the amplification process only related with the upper level related with the amplification (hereinafter which also refers the amplification upper level) and the lower level involved in the amplification (hereinafter which also refers the amplification lower level) or ground level. 
     The above mechanism will be described with reference to  FIG. 1 .  FIG. 1A  is a schematic diagram illustrating the excitation energy levels in silica-based EDFA, and  FIG. 1B  is a schematic diagram illustrating the excitation levels in the Praseodymium doped optical fiber amplifier (PDFA). 
     In the silica-based EDFA, the amplification lower level agrees with the ground level, as shown in  FIG. 1A . Thus the energy levels related with amplification are the only two levels—the amplification upper level ( 4 I 13/2 ) and the amplification lower level ( 4 I 15/2 ). By monitoring the gain of a signal right in a channel and adjusting an intensity of pump light source so that the gain in the channel becomes constant, the gain profile can be held constant. 
     Referring now to  FIG. 1B , PDFA has three energy levels related with amplification—amplification upper level, amplification lower level and ground level. In PDFA, however, since the lifetime of fluorescence at the amplification lower level is very short, compared with that at the amplification upper level, the lower level can be neglected. Then it becomes possible to maintain the gain profile constant by the same technique employed in the silica-based EDFA. 
     When the energy levels related with amplification are substantially two, the gain profile can be relatively easily maintained constant. 
     In general, however, there are few cases where the energy levels related with amplification can be regarded as only two in optical amplifier using rare-earth ion doped medium. The above method of monitoring the gain at one signal wavelength and maintaining the gain profile unchanged by adjusting the intensity of the pump light based on changes in the gain of the signal is not applicable to common optical amplifiers. In other words, the gain profile cannot be maintained constant against fluctuation of input signal levels and other conditions (for example, changes in temperature) only by adjusting the intensity of pump light to hold the gain at one single wavelength constant. 
     Thus, in principle, the gains at two different signal wavelengths must be monitored, and the gain control has been complex. 
     As mentioned above, an optical amplifier used an amplification medium doped with the part of rare-earth ion (an optical fiber or an optical waveguide) can be controlled a gain profile constantly by monitoring a gain at one signal wavelength and controlling an intensity of a pump light. However, an optical amplifier used other rare-earth ion doped amplification medium cannot be controlled a gain profile constantly. 
     Apart from the above mentioned the gain profile constant control method that the gain at one signal wavelength is monitored and an intensity of a pump light is adjusted based on change thereof, a method for maintaining a gain profile constantly by using oscillator or control light is provided. 
     As an example of using oscillator, in E. Desurvire et al., “Gain control in erbium-doped fiber amplifiers by all-optical feedback loop”, IEEE, Electronics Letters. Vol. 27, No. 7, pp. 560-561, 28 Mar. 1991, the technique is described that the construction of feedback loop is introduced, a laser oscillation is caused at one wavelength of ASE, the gain at the wavelength is clumped, and thus gain spectrum is controlled. In addition of the literature, the techniques of clumping a gain includes EP 0 497 491, Japanese Patent Application Laid-open No. 9-509012 (1997), and Japanese Patent Application Laid-open No. 11-145533 (1999). 
     As an example of using control light, E. Desurvire et al. (IEEE, Photonics technology letters, vol. May 1991 entitled Dynamic gain compensation in saturated erbium-doped fiber amplifiers) discloses that ASE level at one wavelength is monitored and a power of control light is controlled so that the ASE level becomes constant. 
     Japanese Patent Application Laid-open No. 2000-261079 discloses the optical amplifier and the controlling method that a power of control light which is inputted from back-side of a rare-earth ion doped optical fiber and outputted from the optical fiber is monitored and a control light source is controlled so that the power of control light becomes constant. 
     Japanese Patent Application Laid-open No. 8-304856 (1996) discloses that a pump power inputted to a amplification medium and a pump power outputted from the amplification medium are monitored and a power of a control light is controlled based on a ratio between both pump powers. 
     Further, it is also disclosed that a total input power of a control light and signal light are monitored and a power of the control light is controlled so that the total input power becomes constant, and a total output power of a control light and signal light are monitored and a power of the control light is controlled so that the total output power becomes constant. 
     The methods of using oscillator or control light as described above can control the gain spectrum of silica-based EDFA wherein the levels greatly related to amplification (i.e., it refers the level that should be considered for determining the state of the gain spectrum) are only two, but they cannot control TDFA and other rare-earth ion doped optical amplifiers. 
     Thus it has been desired that an optical amplifier and a control method which can be applicable to the optical amplifier use the common rare-earth doped amplification medium and control the gain profile constant by an easy method. 
     SUMMARY OF THE INVENTION 
     This invention has been made to solve above problems, and one of its objects is to provide such an optical amplifier equipped with a common rare-earth ion doped amplification medium that can easily hold the gain profile constant. 
     Another object of the invention is to provide a control method for holding the gain profile constant using an optical amplifier equipped with a common rare-earth ion doped amplification medium. 
     One aspect of the invention relates to such an optical amplifier equipped with a common rare-earth ion doped amplification medium that can easily hold the gain profile constant. The optical amplifier of the invention particularly relates to an optical amplifier using an amplification medium where one or more common rare-earth elements are doped. 
     According to a first embodiment, the optical amplifier of the invention comprises a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, an optical resonator that causes laser oscillation at one or more wavelength of amplified spontaneous emission light produced in the amplification medium, a monitoring means that monitors a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light inputted to the amplification medium and a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light outputted from the amplification medium, and a control unit that controls the excitation means based on a value from monitoring means. 
     According to a second embodiment, the optical amplifier of the invention comprises a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, a plurality of optical resonators that cause laser oscillation at a plurality of wavelengths of amplified spontaneous emission light produced in the amplification medium. 
     According to a third embodiment, the optical amplifier of the invention comprises a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, one or more control light source that can control population inversion state of the amplification medium by inputting a control light at a wavelength in the amplification band (that has same meaning as the gain band) of the amplification medium into the amplification medium and changing a power of the inputted control light, a monitor means that monitors powers of lights inputted into and outputted from the amplification medium, and a control unit where a gain is calculated from lights inputted into or outputted from the amplification medium from values from the monitoring means and controls the control light source so that the calculated gain is matched with a prescribed value or an externally decided value. 
     A second aspect of the invention is a control method for maintaining the gain profile of the optical amplifier constant. 
     The control method of the invention is a control method for holding constant the gain profile of the optical amplifier. 
     The first embodiment of the control method relates to a gain profile constant control method of an optical amplifier comprising a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, an optical resonator that causes laser oscillation, a monitoring means, and a control unit that controls the excitation means based on a value from monitoring means, the method comprising the steps of causing laser oscillation at one or more wavelength of amplified spontaneous emission light produced in the amplification medium; and monitoring a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light inputted to the amplification medium and a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light by the monitoring means, and controlling an intensity of the pump light source by controlling the excitation means by the control unit based on the value from the monitoring means. 
     The second embodiment of the control method relates to a gain profile constant control method of an optical amplifier comprising a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, a plurality of optical resonator, the method comprising causing laser oscillation at a plurality of wavelengths of amplified spontaneous emission light produced in the amplification medium. 
     The third embodiment of the control method relates to a gain profile constant control method of an optical amplifier comprising a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, one or more control light source that can control population inversion state of the amplification medium by changing a power of the inputted control light, a monitor means, and control unit, the method comprising inputting the control light with a wavelength in the amplification band of the amplification medium into the amplification medium, and monitoring powers of the light inputted into and outputted from the amplification medium, and calculating a gain of the light inputted into or outputted from the amplification medium and controlling the control light source so that the calculated gain is matched with a prescribed value or an externally decided value. 
     The first optical amplifier of the invention has at least an optical resonator that produces laser oscillation therein and controls the gain of an arbitrary light signal, or has at least an optical resonator that produces laser oscillation therein and controls the sum of powers of laser oscillation and signal light to be constant. The second optical amplifier can hold the gain spectrum constant with two or more optical resonators installed therein. 
     If the optical amplifier of the invention is applied to a WDM transmission system, the gain spectrum of signal light can be controlled despite factors causing changes in the gain spectrum due to input level fluctuation along with changes in the number of signal channels and temperature change. 
     The optical amplifier of the invention can hold the output constant, compensating time-varying changes in transmission loss, and can prevent hole burning in the oscillation wavelength. 
     According to the third embodiment of the invention, the optical amplifier using a rare-earth ion doped amplification medium has at least a control light source therein and controls the gain of control light or signal light, or has at least a control light source therein and controls the gains of control light and signal light, or otherwise has at least a control light source therein and controls the gains of two light signals, in order to hold the gain spectrum constant. 
     The control method of the invention can be applied to a wide range of optical amplifier using common rare-earth ion doped amplification medium. 
     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. 
         FIGS. 1A and 1B  are schematic diagrams illustrating the energy levels greatly related with amplification of Er ions and Pr ions in Er doped fiber and Pr doped fiber. 
         FIGS. 2A and 2B  are diagrams illustrating the energy levels of Er ions and Tm ions. 
         FIG. 3  is a diagram illustrating the result of adjustment of pump light intensity for TDFA under a gain held constant at wavelength λ 1 . 
         FIG. 4  is a diagram illustrating the gain spectra obtained from adjustment of intensity of the control light source so that the total input power of control light and signal light is always constant. 
         FIG. 5  is a diagram illustrating the gain spectra obtained from adjustment of intensity of the control light source so that the total output power of control light and signal light is always constant. 
         FIG. 6  is a diagram illustrating a first embodiment of the optical amplifier of the invention. 
         FIG. 7  is a diagram illustrating a second embodiment of the optical amplifier of the invention. 
         FIG. 8  is a diagram illustrating a third embodiment of the optical amplifier of the invention. 
         FIG. 9A  is a diagram illustrating Example 1 of the optical amplifier of the invention;  FIG. 9B  is a diagram illustrating the positions of optical splitters used in the optical resonator of the example shown in  FIG. 9A ; and  FIG. 9C  is a diagram where the maximum deviations of gain profile are plotted against varied input signal levels. 
         FIG. 10A  is a diagram illustrating Example 2 of the optical amplifier of the invention;  FIG. 10B  is a diagram illustrating the insertion positions of optical splitters in example 2 (of a double-path configuration); and  FIG. 10C  is a diagram where the maximum deviations of gain profile are plotted against input signal levels varied from −25 to 10 dBm/ch. 
         FIG. 11A  is a diagram illustrating Example 3 of the optical amplifier of the invention; and  FIG. 11B  is a diagram illustrating the insertion positions of optical splitters used in example 3. 
         FIG. 12  is a diagram illustrating Example 4 of the optical amplifier of the invention. 
         FIG. 13  is a diagram illustrating Example 5 of the optical amplifier of the invention. 
         FIG. 14  is a diagram illustrating Example 6 of the optical amplifier of the invention. 
         FIG. 15  is a diagram illustrating Example 7 of the optical amplifier of the invention. 
         FIG. 16  is a diagram illustrating Example 8 of the optical amplifier of the invention. 
         FIG. 17  is a diagram illustrating Example 9 of the optical amplifier of the invention. 
         FIG. 18  is a diagram illustrating Example 10 of the optical amplifier of the invention. 
         FIG. 19  is a diagram illustrating Example 11 of the optical amplifier of the invention. 
         FIG. 20A  is a diagram illustrating Example 12 of the optical fiber amplifier of the invention; and  FIG. 20B  is a diagram where the maximum deviations of gain profile are plotted against varied input signal levels. 
         FIG. 21A  is a diagram illustrating Example 13 of the optical fiber amplifier of the invention; and  FIG. 21B  is a diagram where the maximum deviations of gain profile are plotted against varied input signal levels. 
         FIG. 22A  is a diagram illustrating Example 14-1 of the optical fiber amplifier of the invention;  FIG. 22B  is a diagram where the sum Gs of gains of two monitored signal lights and their gain difference ΔGs were plotted when the input signal light power was fixed at −15 dBm/ch, the pump light power was fixed at 400 mW and the power of control light was varied;  FIG. 22C  is a diagram where the sum Gs of gains of two monitored signal lights and their gain difference ΔGs were plotted when the input signal light power was fixed at −15 dBm/ch, the control light power was fixed at −15 dBm and the pump light power was varied;  FIG. 22D  is a diagram demonstrating the results where the deviations of gain spectra at individual input conditions (−25, −20, −15 and −10 dBm/ch) were plotted based on the gain spectra when the gain is controlled and when −10 dBm/ch is inputted; and  FIG. 22E  is a diagram illustrating the configuration of Examples 14-2 to 14-4. 
         FIG. 23  is a diagram illustrating Example 15 of the optical fiber amplifier of the invention. 
         FIG. 24A  is a diagram illustrating Example 16 of the optical fiber amplifier of the invention; and  FIG. 24B  is a diagram illustrating the outputs of control light observed before and after correction in Example 16. 
         FIG. 25A  is a diagram illustrating Example 17 of the optical fiber amplifier of the invention; and  FIG. 25B  is a diagram illustrating the outputs of control light observed before and after correction in Example 17. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first aspect of the invention relates to an optical amplifier that can maintain the gain profile constant in such an optical amplifier using a rare-earth doped amplification medium. 
     A second aspect of the invention relates to a control method for maintaining the gain profile constant in the optical amplifier using a rare-earth doped amplification medium. 
     The present invention has the optical amplifiers and control methods of the first embodiment to the third embodiment as described above. The principles of the first embodiment and the second embodiment, and then the principle of the third embodiment will be described in below. 
     First described are an optical amplifier using an amplification medium where a rare-earth ion of which energy levels relevant to amplification are only two—the amplification upper level and the amplification lower level—is doped, and a control method for holding the gain profile constant. Such an amplifier may include silica-based Erbium doped optical fiber (EDF) as the amplification medium. 
     The optical amplifier using silica-based EDF as the amplification medium is a relatively simple system where the energy levels relevant to amplification are just two levels-the amplification upper level ( 4 I 13/2 ) and the amplification lower level ( 4 I 15/2 ). 
     Such an amplifier using, for example, silica-based EDF as the amplification medium has at least a silica-based EDF, an excitation means for exciting the silica-based EDF, a monitor means for monitoring input signal light or the like and a control unit for controlling the excitation means based on the information provided by the monitor means. 
       FIG. 2A  is a diagram illustrating the energy levels of Er 3+  ions in the optical amplifier using silica-based EDF as the amplification medium. This type optical amplifier carries out amplification by exciting ions from the amplification lower level ( 4 I 15/2 ) to the amplification upper level ( 4 I 13/2 ). An example of the control methods for maintaining the gain profile constant in silica-based EDFA is to monitor the gain of signal light in one channel (hereinafter, may be described “ch”) and to adjust the power of pump light so that the gain of the channel does not change. 
     Now the principle of this control method will be described below. 
     Gain(λ, x) at wavelength λ that is the gain per unit length at a position, x, in the longitudinal direction in an Er doped optical fiber amplifier is expressed by the following equation (for example, see E. Desurvire, “Erbium-Doped Fiber Amplifiers” A. Wiley-Interscience Publication, Chapter 1, 1994).
 
Gain(λ, x )=(σ e (Δ) N 2( x )−σ a (λ) N 1( x ))  (eq. 1)
 
 N 1 +N 2 =N total  (eq. 2)
         σe: Cross-section area of induced emission   σa: Cross-section area of induced absorption   N2(x): Number of ions at the amplification upper level ( 4 I 13/2 )   N1(x): Number of ions at the amplification lower level ( 4 I 15/2 )   Ntotal: Number of all ions       

     From (eq. 1) and (eq. 2), the following is provided:
 
Gain(λ, x )=(σ e (λ) N 2( x )−(σ a (λ)( N total− N 2( x ))  (eq. 3)
 
     As indicated by (eq. 3), the parameter is only N2. 
     Since the cross-section area of induced emission and the cross-section area of induced absorption are determined by the physical properties of ions doped in the amplification medium, N2 must be controlled unchanged, namely the numbers of ions at the amplification upper level and amplification lower level must be kept unchanged, in order to hold the gain spectrum constant. 
     In WDM signal transmission, the power of input signal light coming in the optical amplifier changes with transmission loss and the number of channels. Such changes lead to changes in the rate of induced emission for Er 3+  ions at  4 I 13/2  in the case of silica-based EDFA. As a result, N2 changes and consequently the gain spectrum changes, as indicated by (eq. 3). 
     Thus, in the case of an amplifier using silica-based EDF as the amplification medium where the energy levels related to amplification are only two, the gain spectrum can be maintained constant as indicated by (eq. 3) regardless of the input power level in the optical amplifier, if one signal light wavelength is monitored and the pump light power is controlled so that the gain at this signal wavelength becomes constant. 
     The follows are the specific steps of the control method of holding the gain constant: branching signal light at the input and output sides of the optical fiber, measuring the power of signal light at each side, calculating the gain of signal light from this measurement, and adjusting the power of pump light so that the discrepancy between the calculated gain and the prescribed value becomes zero. This is the basic operation of the control method for maintaining the gain profile constant in the optical amplifier using silica-based EDF as the fiber for amplifier. 
     Meanwhile, there are few cases where amplification mechanism can be described by ions only at the amplification upper level and amplification lower level. For instance, when the host glass of the Er-doped fiber is replaced by fluoride glass, an excitation of Er ion to another energy level higher than the amplification upper level and lifetime at the higher level cannot be negligible, even if using the same induced emission as above. In such a case, even if the gain at a wavelength (for example, a wavelength in the input signal light) is maintained at a constant level, the gain profile cannot be fixed. This is because the number of ions at the upper level, lower level and other level must be considered when describing the amplification mechanism. For example, in TDFA, the total ion number is expressed by the following equation (eq. 2′), and thus it becomes insufficient to consider the one parameter (N 2 ) alone as in (eq. 3). Namely, in TDFA, the ion number of other level must be considered.
 
 N 1 +N 2 +N 3 =N total  (eq. 2′)
 
     An optical amplifier using a rare-earth doped optical fiber in which three energy levels relate to amplification greatly included the amplifier using Thulium doped optical fiber (TDF). This optical amplifier has four level transition, but, among them, the energy levels related to amplification greatly is three as shown in  FIG. 2B . Namely, Thulium doped optical fiber amplifier (TDFA) has the amplification upper level, amplification lower level and ground level, which are greatly involved in amplification. Since all these levels must be considered, the number of ions at those three levels cannot be held constant only by monitoring the gain at a single wavelength to make it unchanged like the case of EDFA. 
       FIG. 3  is a diagram illustrating the characteristics of the optical amplifier observed during amplification of WDM signals when the power of pump light in TDFA is adjusted to make the gain constant that is calculated from the signal light power of wavelength λ 1  coming in the optical amplifier in TDFA and the signal light power of wavelength λ 1  coming out from TDFA. 
     Designated a, b and c in  FIG. 3  are the characteristics of the optical amplifier observed when the total input signal power is low (a), medium (b) and high (c). In this measurement, the concentration of Tm ions doped in TDF was 6000 ppm, and the excitation wavelength was 1400 nm. Excitation in Tm-doped fluoride fiber for amplification is a bi-directional excitation that excites the fiber from forward and backward sides. The power ratio between the forward pump power and backward pump power was kept constant. As shown in  FIG. 3 , even if the gain at a single wavelength is kept constant, the gain profile changes as the power of input signal light changes. 
     Such a phenomenon also causes in the case of using gain clump employing an oscillator. Namely, the gain at the oscillation wavelength is held constant, but the gains at other wavelengths are not settled in a unique value, and thus a gain profile is not held constant. 
     The following methods have been proposed as a control method for controlling the amplifier having such three levels which relate to amplification greatly. These control methods are used in the TDFA optical amplifier, which include two methods. One control method is used in TDFA of bi-directional excitation. This method monitors the gains of two kinds of signal light. Based on the monitor result, the power ratio between the forward and backward excitation is controlled to adjust the gain band, and the overall gain is controlled by adjusting the power of total pump light. In TDFA, the gain spectra shift with wavelength if the powder ratio between the forward and backward excitation changes. 
     As second control method for TDFA, there is a method that controls the intensity of pump light sources at two wavelengths (1.4 μm and 1.56 μm) and adjusts the gain spectrum (see Won Jae Lee et al., “Gain excursion &amp; tilt compensation algorithm for TDFA using 1.4 mm/1.5 mm dual wavelength control”, OFC2002, ThZ3.). 
     In the optical amplifiers using a rare-earth ion doped amplification medium, except for those which the levels related to amplification are substantially two levels, the gain profile cannot be kept constant against fluctuation in input signal levels and other changes in conditions (for example, temperature changes), only by adjusting the intensity of pump light to maintain the gain of a single wavelength constant. In principle, such systems need a complex control that monitors the gains of signal light of two wavelengths. 
     The optical amplifier of the invention can use an amplification medium where a rare-earth ion of which amplification upper, lower and other levels are involved in amplification is doped. The case where other levels are third level only is described below, but the present invention is not limited to the case, and includes the case where more levels are related to amplification. 
     The optical amplifier of the present invention includes those in which the third level is related to energy transition via induced emission, and the lower level or the third level is the ground level. Meanwhile, the control method of the invention holds the gain profile constant in a simple manner by controlling the number of ions at those three energy levels using the following (1)-(4) techniques. 
     (1) First Technique 
     In this technique, laser oscillation is performed in the light path including the optical amplifier using energy transition between two of the above three energy levels involved in induced emission. At the same time, an arbitrary input signal light in the wavelength-multiplexed signal light or an input signal light of the same amplification band separately introduced into the amplifier is monitored to hold its gain constant. 
     (2) Second Technique 
     In this technique, laser oscillation is performed in the light path including the optical amplifier using energy transition between two of the above three energy levels involved in induced emission. At the same time, the power of input light entered to the optical amplifier and the power of oscillated laser is monitored, gain profile is controlled constantly based on the value obtained from these powers. 
     (3) Third Technique 
     In this technique, laser oscillation is performed in the light path including the optical amplifier using energy transition between two of the above three energy levels involved in induced emission. At the same time, the power of light outputted from the optical amplifier and the power of oscillated laser is monitored, gain profile is controlled constantly based on the value obtained from these powers. 
     (4) Fourth Technique 
     This is a technique that does not monitor signal light. In this technique, laser oscillation is performed by two transitions between different pairs of the above three energy levels. In other words, a first laser oscillation is performed in the light path including the optical amplifier via a first transition between any pair of energy levels involved in induced emission in the amplification medium. At the same time, a second laser oscillation is performed in the light path including the optical amplifier via a second transition between another pair of energy levels involved in induced emission in the amplification medium. 
     According to the above first to third techniques (first embodiment), since laser oscillation is performed in a light path including the optical amplifier, the numbers of ions at the two energy levels involved in laser oscillation have a fixed relation. Under such a condition, if the intensity of pump light is further controlled to keep the gain of an arbitrary signal light or power of input light or power of output light constant, the numbers of ions at three levels including the rest level can be uniquely determined. As a result, the gain profile can be fixed. 
     In the fourth technique (second embodiment), since all the three energy levels are related with laser oscillation, the numbers of ions at three energy levels hold a fixed relation. If it is assumed that the number of rare-earth ions doped in the amplification medium is constant, the numbers of ions in the three energy levels are uniquely determined. As a result, the gain profile can also be determined uniquely. 
     A plurality of the laser oscillation can be performed in the amplifier in the present invention. 
     The feature of the above first to third techniques is to hold the gain profile constant by performing laser oscillation in the optical amplifier and at the same time monitoring the gain of an arbitrary signal light, or by performing laser oscillation in the optical amplifier and monitoring the intensities of signal light and oscillated laser. The above first to third techniques can use the following three signals as specific monitor signals; however, these are just examples and others may be used as well. 
     (I) An electric signal proportional to the gain of at least one signal light, calculated from the ratio between an electric signal converted from the power of light branched from signal light coming into the amplification medium and an electric signal converted from the power of light branched from signal light coming out from the amplification medium (hereinafter, called the first monitoring method). The input signal light may be a signal light for WDM transmission or a separate signal light introduced into the optical amplifier as a monitor signal. 
     (II) An electric signal which is the sum of or the linear combination of an electric signal converted from the power of light branched from the total input signal light and an electric signal (proportional to the power of oscillated laser) converted from the power of light branched from the oscillated laser at the input side of the amplification medium (hereinafter, called the second monitoring method). 
     (III) An electric signal which is the sum or the linear combination of an electric signal converted from the power of light branched from the total output signal light and an electric signal (proportional to the power of oscillated laser) converted from the power of light branched from the oscillated laser at the output side of the amplification medium (hereinafter, called the third monitoring method). 
     In the technique conducting gain control using no monitor signal (the above fourth technique), two oscillations are performed by two optical resonators in the optical amplifier including the amplification medium. In this technique, the wavelengths of the two lasers emitted from the two optical resonators are selected from, for example, the following three wavelengths. In the present invention, the different two wavelengths can be used. 
     (A) The wavelength of the laser is included within the amplified spontaneous emission (ASE) spectrum of induced emission from the amplification upper level to the lower level (hereinafter, called the first optical resonator). 
     (B) The wavelength of the laser is included within the amplified spontaneous emission light (ASE) spectrum of induced emission from the amplification upper level to the ground level (hereinafter, called the second optical resonator). 
     (C) The wavelength of the laser is included in the amplified spontaneous emission light (ASE) spectrum of induced emission from the amplification lower level to the ground level (hereinafter, called the third optical resonator). 
     According to the first embodiment of the invention, a first optical amplifier that operates on the above principle is provided. The optical amplifier of the invention comprises an amplification medium, an excitation means for exciting the amplification medium, an optical resonator that causes laser oscillation at least one wavelength of amplified spontaneous emission light produced in the amplification medium, a monitoring means that monitors a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light inputted to the amplification medium and a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light outputted from the amplification medium, and a control unit that controls the excitation means based on a value obtained from monitoring means. 
     According to the first embodiment of the invention, a first control method that operates on the above principle is provided. The first control method includes for example a gain profile constant control method using the first optical amplifier, the method comprising the steps of causing laser oscillation at one or more wavelength of amplified spontaneous emission light produced in the amplification medium; and monitoring a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light inputted to the amplification medium and a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light outputted from the amplification medium by the monitoring means, and controlling an intensity of the pump light source by controlling the excitation means by the control unit based on the value obtained from the monitoring means. 
     According to the second embodiment of the invention, a second optical amplifier that operates on the above principle is provided. The optical amplifier of the invention comprises an amplification medium, an excitation means for exciting the amplification medium, a plurality of optical resonators that causes laser oscillation at a plurality of wavelengths of amplified spontaneous emission light produced in the amplification medium. 
     To provide such optical resonators having the above oscillation wavelengths, the optical resonator needs an optical band-pass filter for selecting a wavelength for the resonator or a wavelength selector comparable to the optical band-pass filter. Preferably, it should further comprise a means for controlling loss during one round of travel in the optical resonator at the given laser wavelength, for example, variable or invariable optical attenuator. The devices for controlling loss and wavelength can be incorporated as embedded functions in the optical splitter employed in constructing an optical resonator (for example, means for splitting or combining laser light). 
     According to the second embodiment of the invention, a second control method that operates on the above principle is provided. The control method includes a gain profile constant control method using the second optical amplifier, the method comprising causing laser oscillation at a plurality of wavelengths of amplified spontaneous emission light produced in the amplification medium. 
     In the first and second embodiments, it is preferable that the monitor means comprises optical band-pass filter. The first and second embodiments include the first to third techniques, the first to third monitoring methods, and the first to third optical resonator as described above (1) to (3), (I) to (III) and (A) to (C), respectively. 
     Now, the principle of the gain profile constant control in the third embodiment of the invention will be described. The feature of the third embodiment of the control method for holding the gain profile constant is that the population inversion in the amplification medium is held constant by adjusting the intensity of control light in accordance with the level of input signal. In other words, the basic concept of the third embodiment is a compensation of input signal level fluctuation with the control light of which wavelength lies in the amplification range. According to the third embodiment, the gain profile is maintained constant by adjusting the control light so that the gain of a wavelength in the amplification range becomes constant for various rare-earth ion doped optical amplifiers. The feature of third embodiment is to monitor a gain in a wavelength (or wavelength region) of a light in order to adjust the control light source. Adjusting the control light source by monitoring the control light or at least one signal light is because the gain spectrum deforms in the method which controls the power of the control light by monitoring a total output or a total input power of the control light and signal light and the gain profile cannot keep constant. Referring  FIG. 4  and  FIG. 5 , this will be described from the case where a control light is controlled by monitoring a total output or input power of control light and signal light. 
       FIG. 4  illustrates that a gain profile cannot keep constant only by maintaining the total input power constant.  FIG. 4  is a diagram illustrating the gain profile observed during amplification of WDM signals when the intensity of a control light in TDFA, which is equipped with a control light source outputting the control light with a wavelength within the amplification band other than the wavelength of signal light at input side, is adjusted so that the sum of input power of control light and the power of total input signal light (hereinafter, also called total input power) is kept constant toward the sift of the input of the total signal light power into the optical amplifier. 
     Designated a, b and c in  FIG. 4  are the gain spectra of the optical amplifier observed when the total input signal power is low (a), medium (b) and high (c). In this measurement, the concentration of Tm ions doped in TDF was 4000 ppm, and the excitation wavelength was 1410 nm. Excitation in Tm-doped fluoride fiber for amplification is a bi-directional excitation that excites the fiber from forward and backward sides. The total power of pump lights was kept constant. As shown in  FIG. 4 , even if the total input power into the optical amplifier is kept constant, the gain profile changes as the power of input signal light changes. 
       FIG. 5  illustrates that a gain profile cannot keep constant only by maintaining the total output power constant.  FIG. 5  is a diagram illustrating the gain profile observed during amplification of WDM signals when the intensity of a control light in TDFA, which is equipped with a control light source outputting the control light with a wavelength within the amplification band other than the wavelength of signal light at input side, is adjusted so that the sum of output power of control light and the power of total output signal light (hereinafter, also called total output power) is kept constant toward the sift of the input of the total signal light power into the optical amplifier. 
     Designated a, b and c in  FIG. 5  are the gain spectra of the optical amplifier observed when the total input signal power is low (a), medium (b) and high (c). In this measurement, the concentration of Tm ions doped in TDF was 5000 ppm, and the excitation wavelength was 1420 nm. Excitation in. Tm-doped fluoride fiber for amplification is a bi-directional excitation that excites the fiber from forward and backward sides. The total power of pump lights was kept constant. As shown in  FIG. 5 , even if the total output power into the optical amplifier is kept constant, the gain profile changes as the power of input signal light changes. 
     As described above, even if the total input power or the total output power is kept constant, the gain spectrum cannot be held constant. 
     The optical amplifier of the third embodiment of the invention comprises an amplification medium, an excitation means for exciting the amplification medium, one or more control light source that can control population inversion state of the amplification medium by inputting the control light with a wavelength in the amplification band of the amplification medium into the amplification medium and changing a power of the inputted control light, a monitor means that monitors powers of lights inputted into and outputted from the amplification medium, and a control unit where a gain is calculated from lights inputted into and outputted from the amplification medium from values obtained from the monitoring means and controls the control light source so that the calculated gain is matched with a prescribed value or an externally decided value. 
     The optical amplifier of the third embodiment has a means for controlling the intensity of control light emitted from the control light source. This means includes controlling the current in the control light source and controlling the degree of attenuation with a variable attenuator installed in the output side of the control light source. 
     The third embodiment of the invention includes the optical amplifier which comprises an amplification medium, an excitation means for exciting the amplification medium, one or more control light source that can control population inversion state of the amplification medium by inputting the control light with a wavelength in the amplification band of the amplification medium into the amplification medium and changing a power of the inputted control light, a monitor means that monitors powers of lights at a plurality of wavelengths inputted into and outputted from the amplification medium, and a control unit where gains are calculated from lights at the plurality of wavelengths inputted into or outputted from the amplification medium from values obtained from the monitoring means, then calculates the difference and the sum of the plurality of gains, and controls the control light source so that the difference calculated from the plurality of gains is matched with a prescribed value or an externally decided value and controls the excitation means so that the sum calculated from the plurality of gains is matched with a prescribed value or an externally decided value. 
     In the present invention, the monitor means may include those which monitors the control light, at least one channel of signal light, or the light with the prescribed wavelength within the amplification band of the optical amplifier. 
     In the third embodiment, preferably, according to the value obtained from the monitoring means, a correction means corrects the value from the monitoring means, or the gain, the difference of gains or the sum of gains calculated from the control unit. 
     Further, it is preferable that the third embodiment includes an optical band-pass filter. 
     Furthermore, the present invention includes a third control method according to the principle of the third embodiment as described above. 
     One example of the gain profile constant control method of an optical amplifier of the third embodiment is the method for the optical amplifier that comprises a rare-earth doped optical fiber or rare-earth doped optical waveguide serving as an amplification medium where rare-earth ions have been doped in its core and/or clad, an excitation means for exciting the amplification medium, one or more control light source that can control population inversion state of the amplification medium by changing the power of an inputted control light, a monitor means, and control unit. The method comprises steps of inputting the control light with a wavelength in the amplification band of the amplification medium into the amplification medium; monitoring powers of the light inputted into and outputted from the amplification medium; and calculating a gain of the light inputted into or outputted from the amplification medium and controlling the control light source so that the calculated gain is matched with a prescribed value or an externally decided value. 
     In the present invention, an optical fiber in the optical amplifier of the present invention may be doped with one or more rare earth ion. 
     Now the optical amplifier and gain profile constant control method of the present invention will be described further specifically with reference to the accompanying drawings. These are, however, just examples, and the invention is not limited to such specific examples. 
     The first embodiment of the invention relates to an optical amplifier including an optical resonator forming a loop between the output side and input side of the optical amplifier.  FIG. 6  shows the schematic structure of the optical amplifier of the first embodiment, while  FIG. 7  shows that of the second embodiment. 
     The optical amplifier  400  ( FIG. 6 ) of the first embodiment is an optical amplifier equipped with an optical resonator  402 , a monitoring means  404  and control unit  420 . The optical resonator  402  is equipped with an amplification medium  410 , an excitation means  412 , optical splitters  414 ,  416  and a wavelength selector  418 . The monitoring means  404  monitors a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light inputted to the amplification medium and a power of at least one light selected from a power of at least one light with at least one prescribed wavelength band selected from the light. The control unit  420  controls the excitation means  412  based on the value from the monitoring means. In the present embodiment, the monitoring means includes the optical splitter  426 ,  428  for inputting the light to be monitored into the control unit. 
     The amplification medium  410  can be a rare-earth doped optical fiber where a rare-earth element has been doped in the core and/or clad. Particularly, the present invention can use the Thulium doped optical fiber, Erbium doped optical fiber and Holmium doped optical fiber, for example. Specifically, the amplification medium may be a rare-earth ion doped optical fiber of which host glass is silica glass, Bismuth-based glass, fluoride-based ZBLAN glass of little non-radiation transition, In—Pb glass or Tellurite glass, for example. Rare-earth ion used in the present invention is not limited, but it is preferable to use Thulium ion (Tm 3+ ), Holmium ion (Ho 3+ ) or Erbium ion (Er 3+ ). The rare-earth ion can be used alone or in combination. 
     The excitation means  412  includes pump light sources  422 ,  424  and couplers  436 ,  438  that combine light from the pump light sources. These light sources are those commonly used for exciting rare-earth doped fibers. For example, they are semiconductor laser diode (LD) and InGaAs distorted quantum well LD, for example. Specifically, they are, for example, solid lasers such as Nd-YLF laser, Nd-YAG laser, Ti sapphire laser, semiconductor lasers or fiber lasers. The optical amplifier shown in  FIG. 6  is excited from its front and back sides. However, the present invention is not limited to this example but may be excited from either front or back side only. 
     The optical splitters are fusion spliced fibers (split type, wavelength division multiplexing type), dielectric multi-layered films, or circulators combined with fiber gratings, which are devices that sample part of the input signal and/or amplified spontaneous emission light (ASE) by reflection. 
     The wavelength selector used in the oscillation system includes optical filters such as dielectric multi-layered films and fiber gratings. 
     In the invention, the optical resonator may include a variable optical attenuator or the like. In the optical resonator of the invention, oscillation takes place when the gain of the amplification medium agrees with the loss during one round of travel in the oscillation system. Using a variable optical attenuator or the like, it becomes possible to control the relationship between the gain of the amplification medium and the loss during one round of travel in the oscillation system. 
     As couplers, common couplers such as pump light/signal light optical couplers may be used. 
     In the monitoring means  404 , the monitored signals (hereinafter referred monitor signal) (first signal and second signal in  FIG. 6 ) branched off by the optical splitters  426 ,  428  come in the monitoring unit  419 . The control unit performs a calculation based on the information from the monitoring unit. The monitoring means may include an optoelectronic transducer that converts the monitor signals divided at the optical splitters  426 ,  428  to electric signal. The optoelectronic transducer converts a monitor signal to an electric signal. In the present invention, the monitor signal or electric signal is monitored. 
     The control unit  420  includes, for example, a differential signal generating circuit, a gain signal calculation circuit (division circuit), and combination thereof; and adder circuit and linear coupler circuit. In the present invention, the monitoring means and the control unit are those which perform the following controls. 
     (i) The monitoring means monitors an inputted power and an outputted power of at least one signal light; and the control unit calculates a signal gain based on the value obtained from the monitoring means, and controls the excitation means so that the signal gain agree agrees with a prescribed value or an externally decided value. 
     (ii) The monitoring means monitors a power of a laser oscillation light at the input side of the amplification medium in the resonator and a power of signal light inputted to the amplification medium; and the control unit calculates a sum of the laser oscillation light power and the signal light power obtained from the monitoring means, or the value obtained from a linear combination of the laser oscillation light power and the signal light power, and controls the excitation means so that the value agrees with a prescribed value or an externally decided value. 
     (iii) The monitoring means takes out simultaneously part of the laser oscillation light in the optical resonator at signal input side of the amplification medium and part of the input signal entering the optical amplifier, and monitors the total power thereof, and control unit controls the excitation means so that the value from the monitoring means agrees with a prescribed value or an externally decided value. 
     (iv) The monitoring means monitors a power of a laser oscillation light at signal output side of the amplification medium in the resonator and a power of signal light outputted from the amplification medium; and the control unit calculates a sum of the laser oscillation light power and the signal light power obtained from the monitoring means, or the value obtained from a linear combination of the laser oscillation light power and the signal light power, and controls the excitation means so that the value agrees with a prescribed value or an externally decided value. 
     (v) The monitoring means takes out simultaneously part of the laser oscillation light in the optical resonator at signal output side of the amplification medium and part of the signal light outputted from the optical amplifier, and monitors the total power thereof; and control unit controls the excitation means so that the value from the monitoring means agrees with a prescribed value or an externally decided value. 
     Preferably in the first embodiment, isolators  430 ,  432  are installed, as shown in  FIG. 6 . 
     The configuration of components in the optical resonator and control unit are not limited to that shown in  FIG. 6  but may be varied. 
     The control method for holding the gain profile constant in the first embodiment takes out part of the input signal entering the optical amplifier and/or the amplified spontaneous emission light (ASE) in the amplification medium of the optical amplifier, and oscillates it. The monitored signals (hereinafter referred monitor signal) (first signal and second signal in  FIG. 6 ) comes in the control unit  420 . The control unit performs a calculation based on the information from the monitoring unit. In the present invention, the monitor signal or electric signal is monitored. 
     In the present invention, the monitoring means and the control unit perform the controls as described above (i) to (v). 
     Now the control method using the optical amplifier of the invention (operation of the optical amplifier of the invention) will be described specifically below. The following is the description of the single-path configuration; however, the present invention can use the double-path configuration and bi-directional amplification. 
     The signal light comes in from the left side in  FIG. 6 , and part of this light is branched off by an optical splitter  426 . The branched signal light is then entered to the monitoring unit  419  as the first signal. The first signal is preferably an electric signal. Thus, in the present embodiment, the monitor signal light is preferably converted into an electric signal by an optoelectronic transducer and the like that converts the branched signal light into an electric signal. The signal light that has passed the optical splitter  426  now goes through the optical splitter  414 , isolator  432  and coupler  436 , and enters the amplification medium  410 . 
     Pump light is supplied to the amplification medium  410  from its front and back sides via couplers  436 ,  438 . Amplified signal light goes out of the system after passing an isolator  430  and optical splitters  416 ,  428 . The optical splitter  428  split splits part of the signal light to monitor part of the output signal light. The light branched here enters the monitoring unit  419  as the second signal. This second signal is also preferably an electric signal. Thus, in this embodiment, the branched signal light is preferably converted into electric signal by an optoelectronic transducer and the like that converts the branched signal light into an electric signal. 
     A monitored light is WDM light of a number of wavelengths of at least one channel, or light of a wavelength in the amplification band that is entered to the optical amplifier separately. In the present invention, the signal light is preferably the signal that is monitored by the monitoring means as described above (i) to (v). 
     The first and second signals enter the control unit  420  via the monitoring unit  419 , and values such as the gains of those input signals is calculated, and current values of the pump light sources are calculated so that the values agree with a prescribed value. The pump light sources  422 ,  424  are controlled so that the intensities of the pump light sources become the current value. The control unit performs operations and controls as described in above (i) to (v). 
     The signal light and ASE light coming out from the isolator  430  are sampled by the optical splitter  416 , and the sampled light is returned to the amplification medium  410  via the wavelength selector  418  and the optical splitter  414 . In the present embodiment, the optical resonator is a loop formed by at least an amplification medium  410 , an excitation means  412 , optical splitters  414 ,  416  and a wavelength selector  418 . 
     The oscillation wavelength of this optical resonator is determined by the employed optical splitters  414 ,  416  that have wavelength-dependence for splitting or a function of the wavelength selector  418  or both. The optical resonator oscillates at the point where the loss in one round of travel in the resonator becomes equal to the gain of the amplification medium  410  at the oscillation wavelength. If a stable oscillation is continuously provided, the gain of the amplification medium  410  does not change at the oscillation wavelength selected by the wavelength selector and the like, even when pump light of a power higher than necessary for such oscillation is injected from the pump light source. 
     As described above, laser oscillation is induced by an optical resonator in the optical amplifier, and light intensity of the pump light source is controlled so that the gain profile becomes constant with monitoring by the monitoring means described above (i) to (v). 
     The second embodiment of the invention is an optical amplifier equipped with two or more resonators and control method for holding their gain profiles constant. 
       FIG. 7  is the schematic diagram illustrating the configuration of the optical amplifier of the second embodiment.  FIG. 7  shows the optical amplifier equipped with two oscillators. The following explanation is based on the construction. The optical amplifier  500  of the present embodiment includes a first optical resonator  502  and second optical resonator  504 . The first optical resonator  502  has an amplification medium  510 , an excitation means  512  for exciting this medium, optical splitters  514 ,  516  and a wavelength selector  518 . The second optical resonator  504  has an amplification medium  510 , an excitation means  512  for exciting this medium, optical splitters  520 ,  522  and a wavelength selector  524 . 
     The excitation means  512  includes pump light sources  526 ,  528  that excite the amplification medium, and couplers  530 ,  532  that combine light beams from the pump light sources. Excitation by the optical amplifier shown in  FIG. 7  is bi-directional, but either forward or backward excitation may be employed in the invention. 
     Referring now to  FIG. 7 , the present embodiment may have an oscillation decision circuit  534  for maintaining oscillation in the two resonators. The oscillation decision circuit forms an oscillation decision means along with optical splitters  536 ,  538  that sample part of oscillation light from each resonator. For example, the optical splitters  536 ,  538  sample signal lights, and they are converted into electric signals by an optoelectronic transducer or the like. The electric signals enter the oscillation decision circuit  534 . 
     This embodiment may also employ a variable optical attenuator and the like in the optical resonator. As mentioned above, the optical resonator of the invention oscillates at the point where the gain of the amplification medium agrees with the loss in one round of travel in the oscillation system. By introducing a variable attenuator and the like, it becomes possible to control the relationship between the gain of the amplification medium and the loss in the one round of travel in the oscillation system. 
     As shown in  FIG. 7 , the second embodiment preferably has isolators  540 ,  542 . 
     The constituting elements such as optical components and oscillation decision circuit used in the optical resonators may take various configurations other than that shown in  FIG. 7  in the embodiment. 
     The components used in the first embodiment may be used also in this embodiment. 
     Now the control method (operation of the optical amplifier of the invention) using the optical amplifier of the present embodiment will be specifically described below. The following description adopts a single-path configuration, but the present invention may employ the double-path configuration or a configuration using a bi-directional optical amplifier. 
     Signal light enters the amplification medium  510  via optical splitters  514 ,  520 , an isolator  540 , and a coupler  530 . The signal light from the amplification medium  510  leaves the system via the coupler  532 , isolator  542  and optical splitters  522 ,  516 . 
     Preferably, the present embodiment has an oscillation decision circuit for maintaining oscillation in the first and second resonators. Light in the first resonator is split off by the optical splitter  538  and then entered to the oscillation decision circuit  534  as a monitor signal. In turn, light in the second resonator is split off by the optical splitter  536  and then entered to the oscillation decision circuit  534  as a monitor signal. The light serving as a monitor signal is preferably converted into an electric signal by an optoelectronic transducer or the like. Based on the two monitor signals, the intensity of pump light emitted from each of the pump light sources  526 ,  528  are adjusted so that the two resonators may oscillate. 
     The third embodiment of the invention relates to an optical amplifier configured to use control light for holding the gain profile of the optical amplifier constant. 
     Control light is introduced in the optical amplifier, and the gain profile of the optical amplifier is controlled to be constant with reference to this control light. The feature of the third embodiment is to monitor a gain at a wavelength (or a wavelength region) of a light in order to adjust a control light source. 
     Now the third embodiment is described with reference to  FIG. 8 . 
     The optical amplifier  600  of the third embodiment has monitoring means  601 ,  603 , an amp unit  602  and a control system  604 . The monitoring means  601  is constructed by an optical detector included in the control light source  610 . The monitoring means  603  is constructed by an optical splitter  634  and an optical detector  635  such as an optoelectronic transducer. The monitoring means  601  shown in  FIG. 8  is an example where the optical detector is included in the control light source  610 , but the present invention is not limited thereto, the optical detector may be provided separately. In such a case, an optical splitter (splitter) is provided to the optical amplifier, and part of signal light input may be split. The signal from the optical detector is inputted to the control system. The optical detector includes an optoelectronic transducer or the like, for example. The amp unit  602  comprises a control light source  610 , amplification medium  612 , pump light sources  614 ,  616 , coupler  618  combining light from the control light source, and couplers  620 ,  622  that combine light from the pump light sources. Referring to  FIG. 8 , the amp unit preferably has isolators  624 ,  626 . 
     The control light source  610  may preferably contain a photodetector. In the third embodiment, the control light source is not limited to such a case, and it does not need to contain the photodetector therein. In the case where the control light source  610  contains the photodetector, it can transmit a first signal  628  for monitoring. In the third embodiment, one or more control light source may be present in the optical amplifier. 
     The control system  604  has a control unit  630  that controls the control light source and a calculation unit  632 . To this calculating unit, a second signal  636  is sent from the optical splitter  634  that samples part of the output light from the amp unit  602 . The calculation unit  632  calculates the gain of control light from the first signal provided by the control light source  610  and the second signal provided by the optical splitter  634 , and then calculates a current for the control light source  610  so that the gain becomes equal to a prescribed value. The calculation result is sent to the control unit  630 . The control unit  630  controls the intensity of control light in the control light source  610  in accordance with the current value sent from the calculation unit  632 . The control system  604  is also referred as control unit in the specification. In such a case, the control unit includes control unit  630  and the calculation unit  632  having the above functions. 
     The constituting elements comprising the optical resonators of the embodiment may take various configurations other than that shown in  FIG. 8 . For example, the optical amplifier shown in  FIG. 8  takes a configuration of bi-directional excitation, but the present invention may employ either forward or backward excitation. 
     The components used in the first embodiment can be used also in this embodiment. 
     Specifically, the amplification medium may be a rare-earth ion doped optical fiber of which host glass is silica glass, Bismuth-based glass, fluoride-based ZBLAN glass of little non-radiation transition, In—Pb glass or Tellurite glass, for example. Rare-earth ion is not limited, but it is preferable to use Thulium ion (Tm 3+ ), Holmium ion (Ho 3+ ) or Erbium ion (Er 3+ ). The couplers and splitters may be fission spliced fibers (split type, wavelength division multiplexing type), dielectric multi-layered films, or circulators combined with fiber gratings. The control light source that emits control light and the pump light sources that emit pump light are, for example, solid lasers such as Nd-YLF laser, Nd-YAG laser, Ti sapphire laser, semiconductor lasers or fiber lasers. 
     Now the control method (operation of the optical amplifier of the invention) using the optical amplifier of the present embodiment will be specifically described below. The following explanation is a case where the optical detector in the monitoring means  601  is included in the control light source, and a gain of a control light is monitored. 
     Signal light is combined by the coupler  618  with control light provided by the control light source  610 . Next, the combined signal light passing through the isolator  624  is combined by the coupler  620  with pump light sent from the pump light source  614 . The signal light combined with pump light enters the amplification medium  612 . The amplification medium is excited by the pump light sources  614 ,  616  from both sides. Signal light coming out from the amplification medium passes the coupler  622  and the isolator  626 , and split by the splitter  634 . Part of the signal light that is not split at the splitter  634  is outputted as output signal light. 
     The control light injected from the control light source to the optical fiber is detected by a photodetector inside the control light source  610 , and the power of the control light is sent to the calculation unit  632  as the first signal. The control light coming out from the amplification medium is split by the splitter, and the power of the control light is sent to the calculation unit  632  as the second signal  636 . The powers of control light at the input and output sides are preferably converted into electric signals before entering the calculation unit  632 . 
     The calculation unit  632  calculates the gain of control light based on the received detected value, calculates a current for the control light source  610  so that the gain becomes equal to the prescribed value, and this value is sent to the control unit  630 . The control unit  630  adjusts the light intensity of the control light source  610  so that the current becomes equal to the value sent from the calculation unit  632 . 
     Because the wavelength of control light falls in the amplification band, the gain profile can be held constant by adjusting the control light so that the gain of control light becomes constant. 
     In the present embodiment, the signal inputted to the calculation unit is not limited to the signal based on the control light. The signal includes one which is based on at least one channel signal light, or which is based on a light within a prescribed wavelength band in an amplification band. In the control method of the present embodiment, correction is made to correct the value from the monitoring means, or the gain, the difference of gains and the sum of gains that are calculated in the control unit in accordance with the value obtained from the monitoring means by correction means. 
     In the present embodiment, preferably, the monitoring means comprises an optical band-pass filter, and a monitor signal is passed through the filter. 
     In the optical amplifiers of the above embodiments, the direction the signal light travels in the amplification medium was not specified. However, the optical amplifier may take (a)-(c) configurations, and the signal light may travel in the direction shown below. 
     (a) An optical isolator is connected to either of the input or output side of the amplification medium or both of them. Signal light travels only one direction in the amplification medium (called the single-path in this specification). 
     (b) An optical circulator is connected to either side of the amplifier, and the other side is connected to a device having a mirror function that reflects at least single light. Signal light travels in the amplification medium via the optical circulator, and then the signal light is amplified. Next, the single light is reflected by the mirror and travels in the reverse direction in the amplification medium for amplification. Amplified signal light goes out via the circulator (called the double-path in the specification). Considering the above first to third monitoring methods where an optical splitter is installed for sampling a monitor signal from signal light, it can be installed in each of the signal output port that is disposed via an optical circulator with looking from the optical amplifier side. The optical splitting means may be installed between the optical circulator and the amplification medium. In this case, the directivity of the optical splitting means can be used to distinguish the input side monitoring signal from the output side monitoring signal. 
     (c) An optical circulator is connected to both sides of the amplification medium. Two signal lights travel in different directions in the amplification medium simultaneously or in separate time periods for amplification (hereinafter, called the bi-directional optical amplifier). In this case, considering the above first to third monitor methods, an optical splitting means is installed to sample a monitor signal from the signal light, and its position may be input and output side of the amplification medium with reference to either of the signal lights traveling in opposite directions. Specifically, the splitting means may be disposed outside of the circulator or between the circulator and the amplification medium. 
     EXAMPLES 
     Now the embodiments of the invention will be described with reference to the accompanying drawings. 
     Example 1 
     The present example is an example of a first embodiment of the invention, and particularly relates to an optical amplifier and control method according to above (1). 
       FIG. 9A  is a diagram illustrating the optical amplifier of the invention. Referring now to  FIG. 9A , the optical amplifier of the invention includes an optical resonator, monitoring means, and a control unit. The optical resonator includes an amplification medium  1306 , pump light sources  1314 ,  1317  for exiting the amplification medium, a wavelength selector  1305 , a variable optical attenuator  1307 , optical splitters for branching signal light to the wavelength selector and variable attenuator, and an optical splitter  1302  for combining signal light coming from the wavelength selector and variable attenuator. The monitoring means has an optical splitter  1301  for sampling the first monitored signal light, splitter  1311  for sampling the second monitored signal, control unit  1316 , optical band pass filters  1312 ,  1320  for permitting only wavelengths common in the first and second monitored signal lights, and optoelectronic transducing circuits  1313 ,  1319  for converting these monitored signal lights into electric signals. The control unit has pump light driver circuits  1315 ,  1318  for controlling the pump light sources. The present example is an example of bi-directional excitation, but the invention is not limited to this example and may use either forward or backward excitation. 
     The employed amplification medium  1306  was glass where Tm was doped to host glass, ZBLAN glass, which was one of fluoride-based glasses. The concentration of doped Tm ions was 6000 ppm. The Tm-dope ZBLAN fluoride-based fiber was 5 m. The pump light sources  1314 ,  1317  were semiconductor lasers of which oscillation wavelengths were 1400-1430 nm. The optical splitters  1301 ,  1311  in the monitor signal light sampling areas were mirrors made of dielectric multi-layered films capable of sampling 1% of incident signal light by reflection. 
     The light signals were 16 wavelengths in the 1480-1510 nm range at intervals of 2 nm. The optical band-pass filters  1312 ,  1320  permitting light of which central wavelength was 1480 nm of a 0.8 nm half-width. The optoelectronic transducing circuits  1313 ,  1319  were InGaAs-PIN-PDs. The optical splitters  1302 ,  1310  comprising the optical resonator had a loss of 0.2 dB or less in 1480-1510 nm, and a splitting ratio of 95% in 1513-1516 nm. 
     The wavelength selector  1305  was a dielectric multi-layered film permitting light of which central wavelength was 1513 nm of a 0.8 nm half-width with a loss of 0.5 dB. The variable optical attenuator  1307  was controlled so that the loss in 1513-1516 nm during one round of travel in the optical resonator was 18 dB. The variable attenuator was a device used in common optical amplifiers. 
     The monitoring means  1316  included a division circuit for calculating the gain of signal light based on the ratio between the first monitor signal (the first electric signal in this example) and second monitor signal (the second electric signal in this example). More specifically, it included a differential signal generating circuit that generated differential signal and a gain signal calculating circuit (division circuit). 
     Now the operation of the optical amplifier of the invention will be specifically described with reference to the configuration shown in  FIG. 9A . Signal light comes in from the left side in  FIG. 9A , and part of the signal light is sampled by branching at the optical splitter  1301 . The sampled monitored signal light enters the optoelectronic transducing circuit  1313  via an optical band-pass filter  1312  and becomes the first electric signal. The signal light that has passed the optical splitter  1301  comes in the amplification medium  1306  via an optical splitter  1302 , isolator  1303  and pump light/signal light coupler  1304 . 
     Pump light may be injected from the back side via the pump light/signal light coupler  1308 . Amplified signal light passes the isolator  1309  and optical splitters  1310 ,  1311  before leaving the system. The optical splitter  1311  monitors part of the output signal light, which becomes the second electric signal entering the optoelectronic transducing circuit  1319  via an optical band-pass filter  1320 . 
     The signal light is WDM light of at least one wavelength channel. The two optical band-pass filters  1312 ,  1320  permit the same wavelength which agrees with the wavelength of a specific one channel in the WDM signal light. 
     The optical splitter  1310  samples part of the signal light and ASE light emitted from the isolator  1309 , and the sampled light returns to the amplification medium  1306  via the variable optical attenuator  1307 , wavelength selector  1305  and optical splitter  1302 . These components form the optical resonator. 
     The oscillation wavelength of the optical resonator can be decided by splitters  1302 ,  1310  which have wavelength-dependency, or the function of the wavelength selector  1305 , or both. At this wavelength, the optical resonator oscillates at the point where the loss during one round of travel in the resonator becomes equal to the gain of the amplification medium  1306 . When a stable oscillation is possible, the gain of the amplification medium  1306  does not change at the oscillation wavelength even if pump light of an intensity higher than that needed for oscillation is provided. 
     The above first and second electric signals are converted by the division circuit  1316  into gain signals. Based on the gain signals, the pump light driver circuits  1315 ,  1318  are adjusted so that the light intensity of the pump light source agrees with a prescribed value. Pump light is injected in the amplification medium via the pump light/signal light couplers  1304 ,  1308 . 
       FIG. 9B  is a diagram illustrating the positions of splitters comprising the optical resonator. The same components have the same numerals in  FIGS. 9A and 9B . The optical splitters  1302 ,  1310  comprising the optical resonator may be installed in various positions. The optical resonator is a loop comprising an optical splitter  1302 , amplification medium  1306 , optical splitter  1310 , variable optical attenuator  1307  and wavelength selector  1305 . The optical splitters  1302 ,  1301  and pump light/signal light coupler  1304  may be inserted in A or B sandwiching the optical isolator  1303 . 
     Particularly when the loss in the optical isolator  1303  for the excitation wavelength is high, the pump light/signal light coupler  1304  is preferably inserted in B. These components are inserted in A or B in light of losses in splitters and coupler at the excitation wavelength, but may be inserted in positions other than those shown in  FIG. 9A . 
     Likewise, the optical splitters  1311 ,  1310  and pump light/signal light coupler  1317  may be inserted in C or D. In general, since the optical isolator  1309  has an excellent capability of blocking light traveling in the reverse direction over a wide range of wavelengths, the pump light/signal light coupler  1317  should be inserted in C in most cases. The optical splitters  1311 ,  1310  and pump light/signal light coupler  1317  are inserted in C or D in light of losses in splitters and coupler at the excitation wavelength, but may be inserted in positions other than those shown in  FIG. 9A . 
       FIG. 9C  is a graph where the maximum values in deviations in gain profile are plotted against varied input signal levels. With reference to the gain spectrum at −25 dBm/ch, the maximum deviations of gain spectra observed when the input signal levels set at −25, −20, −15 and −10 dBm/ch were inputted, were plotted in this graph. 
     The intensity of pump light was set at a value approximately 1.3 times the threshold value needed for oscillation of the optical resonator. The gains were estimated by scanning a −35 dBm weak probe light. The dotted line in  FIG. 9C  shows the result obtained with no gain control using monitored signal lights. The solid line in  FIG. 9C  shows the values obtained when the control is performed so that the gain at 1480 nm was held constant by the control circuit that monitored the 1480 nm signal light.  FIG. 9C  demonstrates that the proposed control succeeded in holding the gain profile constant. 
     Example 2 
     This example is an example of the optical amplifier described in above (1). 
       FIG. 10A  is a diagram illustrating a second example of the optical amplifier of the invention. This example is an example of the double-path type amplifier, and in particular an example of the reflection type optical amplifier. 
     The optical resonator of the example has the same configuration as that of the first example. The present example has a circulator  1503  at the input side of the optical resonator and a mirror  1508  at the output side of the optical resonator. Referring now to  FIG. 10A , the monitoring means of the optical resonator of the example has the same configuration as that of the first example except that the optical splitter  1502  sampling the second signal light is installed on the output side of the circulator  1503  sampling the amplified output signal. Although the present example shows an example of the bi-directional excitation, the invention may use the forward or backward excitation. 
     Referring to  FIG. 10A , the amplification medium  1526  used an In-based fluoride fiber where 2000 ppm Tm ions were doped. This fiber was 20 m long. Its pump light source was an InGaAs distorted quantum well LD of which oscillation wavelength was 1030 nm. The optical amplifier of the present example includes a mechanism for sampling a first electric signal and a mechanism for sampling a second electric signal in order to split off monitored signal light. The mechanism for sampling a first electric signal includes an optical splitter  1501 , an optical band-pass filter  1517  and an optoelectronic transducing circuit  1519 . The mechanism sampling a second electric signal includes an optical splitter  1502 , an optical band-pass filter  1518  and an optoelectronic transducing circuit  1520 . 
     The optical splitters  1501 ,  1502  are devices each using a dielectric multi-layered film mirror for splitting off 1% of input signal light by reflection. 
     Light signals were 16 wavelengths from 1460 to 1490 nm at intervals of 2 nm. The optical band-pass filters  1517 ,  1518  passed light of which central wavelength was 1460 nm of a 0.8 nm half-width. The optoelectronic transducing circuits  1519 ,  1520  were InGaAs-PIN-PDs. The optical splitters  1504 ,  1507  which are included in the optical resonator had a passing loss of 0.2 dB or less and a splitting ratio of 95% in 1493-1496 nm. 
     The wavelength selector  1510  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1493 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  1511  was adjusted so that the loss in 1493-1496 nm during one round of travel in the optical amplifier became 20 dB. 
     Now the operation of the optical amplifier of the invention will be specifically described below with reference to the configuration shown in  FIG. 10A . Signal light that has passed the optical splitter  1501  enters the amplification medium  1526  via the circulator  1503 , then reflected by the mirror  1508 , passes the amplification medium  1526  again, and goes out to the optical splitter  1502  via the circulator  1503 . 
     The optical splitters  1501 ,  1502  split off part of signal light as monitored signal light. Only necessary light signals are selected by screening the sampled monitored signal light in the optical band-pass filters  1517 ,  1518  that screen the same band, and the taken out signals are entered to the optoelectronic transducing circuits  1519 ,  1520  to be converted into electric signals. The first and second electric signals are taken out as gain signals by the division circuit  1516 . Based on this gain signal, the pump light driver circuits  1514 ,  1515  are controlled to adjust the intensity of pump light in the pump light sources  1512 ,  1513  so that the gain agrees with the prescribed value. Pump light is injected to the amplification medium via the pump light/signal light couplers  1505 ,  1506 . 
     The optical resonator is a loop comprising optical splitters  1504 ,  1507 , a wavelength selector  1510 , a variable optical attenuator  1511  and an amplification medium  1526 . Otherwise, it is possible to add an isolator in the optical path extending from the optical splitter  1504  to the optical splitter  1507  via the wavelength selector  1510  and the variable optical attenuator  1511  to fix the travel direction of oscillated light in the optical resonator. Regardless of the presence/absence of this isolator, the gain in the amplification medium at the wavelength of laser oscillation is fixed by the loss during one round of travel in the optical amplifier, and the gain of monitored signal light at the selected wavelength is also fixed by an electric feedback system using the division circuit  1516 . Then the gain profile of the amplifier is uniquely held constant. 
       FIG. 10B  is a diagram illustrating the positions where the optical splitters are installed with a double-path configuration. The optical splitters included in a monitoring means, optical resonator and excitation means may be installed in various positions other than those indicated in  FIG. 10A . In  FIG. 10B , when the components are the same as those in  FIG. 10A , the same numerals as those in  FIG. 10A  are used. 
     Now the relative positions of the monitor circuit for input signal light (combination of the optical splitter  1501 , optical band-pass filter  1517  and optoelectronic transducing circuit  1519 ), the monitor circuit for output signal light (combination of the optical splitter  1502 , optical band-pass filter  1518  and optoelectronic transducing circuit  1520 ), the circuit for pump light (combination of the pump light/signal light coupler  1505 , pump light source  1512 , pump light/signal light coupler  1506  and pump light source  1513 ) and the optical resonator (combination of the optical splitters  1504 ,  1507 , variable optical attenuator  1511 , and wavelength selector  1510 ) are explained below. 
     If the optical splitters  1501 ,  1502  are directional couplers, the optical splitter  1501  may be installed in A or B; and optical splitter  1502  may be installed in D or B. Meanwhile, the optical splitters  1504  and  1507  may be installed in A and D, B and C, A and C, or D and C, respectively. The pump light/signal light coupler  1506  is preferably positioned in C. The pump light/signal light coupler  1505  is preferably positioned in B, although it may be positioned also in A or D, if its forward or backward loss in the circulator  1503  at the oscillation wavelength is sufficiently low. These devices may be installed anywhere within the A-D range if their optical performance is well taken into account. 
       FIG. 10C  is a graph where the maximum values in deviations in gain profile are plotted against varied input signal levels. With reference to the gain spectrum at −25 dBm/ch, the maximum deviations of gain spectra observed when the input signal level was varied from −25 to −10 dBm/ch are plotted in this graph. The dotted line in  FIG. 10C  shows the result obtained with no gain control at the wavelengths of monitored signal lights, while the solid line shows the result obtained when the gain control was carried out.  FIG. 10C  demonstrates that the proposed control succeeded in holding the gain profile constant. 
     Example 3 
     This example is an example of the optical amplifier described in above (1). 
       FIG. 11A  is a diagram illustrating a third example of the optical amplifier of the invention. This example is an example of the bi-direction amplifier. The optical resonator, monitoring means and control unit of this example have the same structures as those of the first example. Referring now to  FIG. 11A , the present example has circulators  1702 ,  1707  at both sides of the optical resonator, and the first and second light signals come in the resonator from the opposite directions. Although the present example shows an example of the bi-directional excitation, the invention may use the forward or backward excitation. 
     The amplification medium  1726  used a Tellurite-based fiber where 2000 ppm of Tm ions were doped. This fiber was 20 m long. The pump light source  1715  was a LD of which oscillation wavelength was 1400 nm, while the pump light source  1717  used combined light emitted by LDs that oscillated at 1400 nm and 1600 nm, respectively. 
     The present example monitors a state of a gain profile by using monitored signal lights, and holds the gain profile constant by controlling the pump light sources with the pump light driver circuits  1716 ,  1718 . The two LDs that were connected to the pump light driver circuit  1718  and oscillated at 1400 nm and 1600 nm respectively were controlled by feedback to drive these pump light sources. Sampling of monitored signal lights was carried out by optical splitters  1701 ,  1708 , which were fusion spliced type fiber couplers. These optical splitters are the devices that take out 1% of input signal light by reflection. 
     Light signals were 16 wavelengths from 1480 to 1510 nm at intervals of 2 nm. 
     The optical band-pass filters  1711 ,  1713  passed light of which central wavelength was 1510 nm of a 0.8 nm half-width. The optoelectronic transducing circuits  1712 ,  1714  were InGaAs-PIN-PDs. The optical splitters  1703 ,  1706  included in the optical resonator had a passing loss of 0.2 dB or less in 1480-1510 nm and a splitting ratio of 95% in 1474-1477 nm wavelengths. 
     The wavelength selector  1709  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1476 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  1710  was adjusted so that the loss in 1474-1477 nm during one round of travel in the optical amplifier became 20 dB. 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference observed against varied input levels appeared at 1510 nm. Its deviation was 7 dB with no gain control, while 0.3 dB with gain control. 
     Now the operation of the optical amplifier of the invention will be specifically described below with reference to the configuration shown in  FIG. 11A . Signal light that has passed the optical splitter  1701  enters the amplification medium  1726  via the circulator  1702 . That is, signal light enters again the amplification medium  1726  for amplification via the optical splitter  1703  and pump light/control light coupler  1704 . Backward-excited light is combined by the pump light/signal light coupler  1705 . 
     The amplified signal light passes the optical splitter  1706  and circulator  1707 , and then the light is split off by the optical splitter  1708  for sampling. Part of input signal light is converted into electric signals by the optical splitter  1701 , optical band-pass filter  1711  and optoelectronic transducing circuit  1712 . Meanwhile, part of output signal light is converted into electric signals by the optical splitter  1708 , optical band-pass filter  1713  and optoelectronic transducing circuit  1714 . 
     The individual electric signals are converted into gain signals by the division circuit  1720 . Based on this gain signal, the pump light driver circuits  1716 ,  1718  control the intensity of pump light in the pump light sources  1715 ,  1717  so that the gain agrees with the prescribed value. The gain at the wavelength of monitored signal light in the input signal light is controlled by electric feedback. 
     Meanwhile, the optical resonator is a loop comprising optical splitters  1703 ,  1706 , a wavelength selector  1709 , a variable optical attenuator  1710  and an amplification medium  1726 . 
     Otherwise, it is possible to add an isolator in the light path extending from the optical splitter  1703  to the optical splitter  1706  via the wavelength selector  1709  and the variable optical attenuator  1710  to fix the travel direction of oscillated light in the optical resonator. Regardless of the presence/absence of this isolator, the gain in the amplification medium at the wavelength of laser oscillation is fixed by the loss during one round of travel in the optical amplifier, and the gain of monitored signal light at the selected wavelength is also fixed by an electric feedback system using the division circuit  1720 . Then the gain profile of the amplifier is uniquely held constant. 
       FIG. 11B  is a diagram illustrating the positions where the optical splitters included in the monitoring means, optical resonator and excitation means are installed in a configuration shown in  FIG. 11A . The optical splitters included in the monitoring means and optical resonator may be installed various positions other than those indicated in  FIG. 11A . The same components have the same numerals in  FIGS. 11A and 11B . 
     Now the relative positions of the monitor circuit for input signal light (combination of the optical splitter  1701 , optical band-pass filter  1711  and optoelectronic transducing circuit  1712 ), the monitor circuit for output signal light (combination of the optical splitter  1708 , optical band-pass filter  1713  and optoelectronic transducing circuit  1714 ), the circuit of pump light (combination of the pump light/signal light coupler  1704 , pump light source  1715 , pump light/signal light coupler  1705  and pump light source  1717 ) and the optical resonator (combination of the optical splitters  1703 ,  1706 , variable optical attenuator  1710  and wavelength selector  1709 ) are explained below. 
     The optical splitter  1701  may be installed in A or B; and optical splitter  1708  may be installed in C or D. Meanwhile, the optical splitters  1703  and  1706  may be installed in A and D, B and C, A and C, or B and C, respectively. The pump light/signal light coupler  1704  may be positioned in A or B, but preferably in B. The pump light/signal light coupler  1705  is preferably placed in C, although it may be positioned in C or E. These devices may be installed anywhere within the A-E range if their optical performance is well taken into account. 
     Example 4 
     The present example is an example of the optical amplifier described in above (2). 
       FIG. 12  illustrates a fourth example of the optical amplifier of the invention, which has a single-path configuration. The optical amplifier of the invention includes an optical resonator of the same configuration as that employed in the first example, and a monitoring means that monitors the sum of the entire power of input signals and the power of oscillated light in the resonator at an input side of the amplification medium and control unit that controls the pump light sources. The monitoring means comprises an optical splitter  1902  for sampling signal light and oscillation light, an optoelectronic transducing circuit  1910  for converting light power into electric signals, and a monitoring unit (for example, a differential signal generating circuit  1915  that sends a signal to the pump light driver circuits  1912 ,  1914  based on the electric signals. The control unit includes pump light driver circuits  1912 ,  1914  that control the pump light sources  1911 ,  1913 . 
     The employed amplification medium  1916  was a Zr-based fluoride fiber where 2000 ppm of Er ions were doped. This fiber was 7 m long. 
     The pump light sources  1911 ,  1913  employed multi-polarized pumping units where LDs oscillating at 980 nm were installed so that polarized waves crossed at right angles. The present example monitors a state of a gain profile by using monitor signal lights, and holds the gain profile constant by controlling the pump light sources with the pump light driver circuits  1912 ,  1914 . In the present example, all the LDs (pump light sources) are controlled by feedback; and the intensity of pump light from the pump light/signal light coupler  1905  became 1.4 times that of pump light from the pump light/signal light coupler  1904 . 
     The monitor signal light sampling unit comprises an optical splitter  1902  and an optoelectronic transducing circuit  1910 . The optical splitter  1902  is a bulk type coupler using a dielectric multi-layered film as a filter. Its splitting ratio was 1% in the 900-1000 nm and 2500-3000 nm ranges. Light signals were 16 wavelengths from 2600-2630 nm at intervals of 2 nm. The optoelectronic transducing circuit  1910  was a PIN-PD. 
     The optical splitters  1901 ,  1907  included in the optical resonator had a passing loss of 0.2 dB or less in 2600-2630 nm and a splitting ratio of 95% in 2610-2650 nm. The wavelength selector  1908  was a device using a dielectric multi-layered film that passed light of a central wavelength of 2630 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  1909  was adjusted so that the loss in 2610-2650 nm during one round of travel in the optical resonator became 20 dB. 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference observed against varied input levels was 5 dB with no gain control, while 0.2 dB with gain control. 
     Now the operation of the optical amplifier of the invention will be specifically described below with reference to the configuration shown in  FIG. 12 . This example can be applied to optical amplifiers of the single-path configuration, double-path configuration and bi-directional configuration. 
     The optical amplifier of the present example uses an optical resonator and the second monitoring method at a time. Particularly in this example, the second monitoring method is featured by conducting sampling of the total signal light and sampling of oscillated laser light at the input side of the amplification medium in the resonator, with a single optical splitter. The power of the sampled total signal light and the power of oscillated laser are entered together to a single optoelectronic transducer to be converted into an electric signal. 
     The pump light driver circuit is controlled so that the difference between this electric signal and a prescribed level becomes zero. The structure of the pump light driver circuit is the same as that in the above example. The employed optical components are the same as those described in the above example. 
     Signal light that has passed the optical splitters  1901 ,  1902 , isolator  1903  and pump light/signal light coupler  1904  enters the amplification medium. The amplified signal light passes the pump light/signal light coupler  1905 , isolator  1906  and optical splitter  1907  before leaving the system. The optical resonator is a loop comprising optical splitters  1901 ,  1907 , a wavelength selector  1908 , a variable optical attenuator  1909  and an amplification medium  1916 . 
     The optical splitter  1902  has a function of taking out the oscillation light and the total input signal light in the optical resonator at a prescribed ratio, and the taken out monitor signal light is converted into an electric signal by the optoelectronic transducing circuit  1910 . This electric signal has a signal level proportional to the intensity of light coming in the amplification medium  1916 , and this level is compared with a prescribed level. The compared signal level is provided as a differential signal by a differential signal generating circuit  1915  that generates the differential signal as output, and the pump light driver circuit and pump light source are controlled by feedback based on this differential signal. 
     This control makes the light coming in the amplification medium  1916  agree with a prescribed value or an externally decided value. The splitters can be installed in various patterns. Denoted  1912 ,  1914  are pump light driver circuits. 
     Example 5 
       FIG. 13  illustrates a fifth example of the optical amplifier of the invention, which has a single-path configuration. 
     Referring to  FIG. 13 , the present example has the same configuration as that for example 4 except that it comprises an optical splitter  2002  for sampling the first monitored signal light at the input side of the resonator, a monitored signal light sampling unit including an optoelectronic transducing circuit  2012  for transducing the sampled monitored signal light into an electric signal and an isolator  2011  between the variable optical attenuator  2010  and the optical splitter  2007 . 
     The employed amplification medium  2026  was a Zr-based fluoride fiber where 6000 ppm of Tm ions were doped. This fiber was 7 m long. The pump light sources  2014 ,  2016  were LDs that oscillate at 1400 nm. 
     The present example monitors a state of a gain profile by using monitor signal lights, and holds the gain profile constant by controlling the pump light sources with the pump light driver circuits  2015 ,  2017 . In the present example, all the LDs that oscillate at 1400 nm and are connected to the pump light driver circuits  2015 ,  2017  are subject to a feedback control. Assuming that the pump light intensity at the pump light/signal light coupler  2005  is P1 and that at the pump light/signal light coupler  2006  is P2, P1 and P2 are controlled so as to meet the following relations:
 
 P 2 (mW)=400 t  
 
 P 1 (mW)=100 t+ 300
 
     where t is a real number between 0 and 1. 
     The optical splitter  2002  in the monitored signal light sampling unit and the optical splitter  2004  in the monitor signal light sampling unit are fission spliced type fiber couplers. These are fission spliced type couplers that have an almost constant splitting ratio of 1% in 1470-1530 nm. The optical splitter  2002  splits only light traveling from the left to the right in  FIG. 13 , while the optical splitter  2004  splits only light traveling from the right to the left in  FIG. 13 . Light signals were 16 wavelengths from 1480 to 1510 nm at intervals of 2 nm. 
     The optoelectronic transducing circuits  2012 ,  2013  were InGaAs-PIN-PDs. The optical splitters  2003 ,  2007  included in the optical resonator had a passing loss of 0.2 dB or less in 1480-1510 nm and a splitting ratio of 95% in 1513-1517 nm. 
     The wavelength selector  2009  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1515 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  2010  was adjusted so that the loss at 1515 nm during one round of travel in the optical resonator became 17 dB. 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference observed at 1515 nm against varied input levels was 6 dB with no gain control, while 0.3 dB with gain control. Denoted  2001 ,  2008  and  2011  in the figure are optical isolators. 
     Now the operation of the optical amplifier of the invention will be specifically described below with reference to the configuration shown in  FIG. 13 . The present example uses the above second monitoring method, and the optical splitter that samples the total signal light is a separate device from the optical splitter that samples laser oscillation light at the input side of the amplification medium in the resonator. By these splitters, the different light beams are introduced into separate optoelectronic transducers and then converted into independent electric signals corresponding to individual light powers. The obtained electric signals are converted by a certain electric means into an electric signal that is a linear combination of the two signals or their sum. This signal is taken out, and then the pump light driver circuits are adjusted so that the difference between this electric signal and a prescribed level becomes zero. The operation of the pump light driver circuit is the same as that in the above example. The optical components are the same as those described in the above example. 
     Signal light that has passed the optical splitters  2002 ,  2003 , isolator  2001  and pump light/signal light coupler  2005  enters the amplification medium  2026 . The amplified signal light passes the pump light/signal light coupler  2006 , optical splitter  2007  and isolator  2008  before leaving the system. 
     The optical resonator is a loop comprising optical splitters  2003 ,  2007 , a wavelength selector  2009 , a variable optical attenuator  2010 , an isolator  2011  and an amplification medium  2026 . The optical splitters  2002 ,  2004  both work as directional couplers. This optical splitter  2004  takes out only laser oscillation light traveling in the direction opposite to the signal light in the optical resonator. 
     The monitored signal light and the monitor signal light corresponding to input signal light and the monitor signal light corresponding to laser oscillation light, which are sampled by the optical splitters  2002 ,  2004 , are converted into electric signals by the optoelectronic transducing circuits  2012 ,  2013 . Those signals are summed up in the adder signal generating circuit  2018 , and this sum is compared with the externally decided level to provide their differential signal. 
     The pump light driver circuits  2015 ,  2017  and pump light sources  2014 ,  2016  are controlled by feedback so that the differential signal becomes zero. The optical splitters can be installed with flexibility as to location. 
     Example 6 
     This example is an example of the optical amplifier described in above (3). 
       FIG. 14  illustrates a sixth example of the optical amplifier of the invention, which has a single-path configuration. 
     The present example is the same as Embodiment 4 except that it has replaced the monitoring means and the control unit, which monitors the sum of the power of total input signal light and the power of oscillation light in the resonator at an input side of the amplification medium and controls the pump light sources based on this sum, by a monitoring means and a control unit that monitors the sum of the power of total output signal light and the power of oscillation light in the resonator at an output side of the amplification medium and controls the pump light sources based on this sum. This control unit is inserted between the isolator  2105  and the optical splitter  2107 , as shown in  FIG. 14 . 
     The employed amplification medium  2116  was a Zr-based fluoride fiber where 6000 ppm of Tm ions were doped. This fiber was 7 m long. The pump light sources  2110 ,  2112  were LDs that oscillate at 1400 nm. 
     The present example monitors a state of a gain profile by using monitor signal lights, and holds the gain profile constant by controlling the pump light sources with the pump light driver circuits  2111 ,  2113 . In the present example, all the LDs that oscillate at 1400 nm and are connected to the pump light driver circuits  2111 ,  2113  are subject to a feedback control. Assuming that the light intensity of pump light from the pump light/signal light coupler  2103  is P1 and that from the pump light/signal light coupler  2104  is P2, P1 and P2 are controlled so as to meet the following relations:
 
 P 1 (mW)=2 ×P 2 (mW).
 
     The optical splitter  2106  employed in monitor signal light sampling area is a fusion spliced type fiber coupler. This is a fusion spliced type coupler that has an almost constant splitting ratio of 1% in 1470-1530 nm. Light signals were 16 wavelengths from 1480 to 1510 nm at intervals of 2 nm. The optoelectronic transducing circuit  2114  was an InGaAs-PIN-PD. 
     The optical splitters  2101 ,  2107  comprising the optical resonator had a passing loss of 0.2 dB or less in 1480-1510 nm and a splitting ratio of 95% in 1513-1517 nm. The wavelength selector  2108  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1515 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  2109  was adjusted so that the loss at 1515 nm during one round of travel in the optical resonator became 17 dB. 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference observed at 1515 nm against varied input levels was 6 dB with no gain control, while 0.3 dB with gain control. 
     Now the operation of the optical amplifier of the invention will be specifically described below with reference to the configuration shown in  FIG. 14 . The present example uses an optical resonator along with the above third monitoring method. 
     Signal light that has passed the optical splitter  2101 , isolator  2102  and pump light/signal light coupler  2103  enters the amplification medium. The amplified signal light passes the pump light/signal light coupler  2104 , isolator  2105  and optical splitters  2106 ,  2107  before leaving the system. 
     The optical resonator is a loop comprising optical splitters  2101 ,  2107 , a wavelength selector  2108 , a variable optical attenuator  2109 , and an amplification medium  2116 . The optical splitter  2106  is a splitting device that does not work as a directional coupler but splits off the total signal light and the oscillation laser light at a given ratio as monitor elements. The monitor signal light split by the optical splitter  2106  is converted into an electric signal corresponding to the power of the monitor signal light by the optoelectronic transducing circuit  2114 . This signal is compared with an externally decided level by the differential signal generating circuit  2115 , and the difference between them is provided as output. 
     The pump light driver circuits  2111 ,  2113  and pump light sources  2110 ,  2112  are controlled by feedback so that the differential signal becomes zero. The optical splitters can be installed with flexibility as to location. Optical splitters may be prepared for each of the laser oscillation light element and amplified signal light element. 
     Example 7 
     This example is an example of the optical amplifier described in above (4). 
       FIG. 15  illustrates a seventh example of the optical amplifier of the invention, which has a single-path configuration. 
     The present example holds the gain profile of the optical amplifier using two optical resonators. Since the two resonators share the same amplification medium, the content of rare-earth ions in the amplification medium is fixed. Thus, even when two resonators are controlled to hold the gain profile constant, the requirement that the total number of rare-earth ions in the resonators be constant is fulfilled. 
     The employed amplification medium  2226  was a Zr-based fluoride fiber where 6000 ppm of Tm ions were doped. This fiber was 7 m long. The pump light sources  2217 ,  2219  were LDs that oscillate at 1400 nm. In the present control, all the LDs that oscillate at 1400 nm and are connected to the pump light driver circuits  2218 ,  2220  are subject to a feedback control. 
     The optical splitters  2201 ,  2208  comprising the first optical resonator had a passing loss of 0.2 dB or less in 1480-1510 nm and a splitting ratio of 95% in 1513-1517 nm. The wavelength selector  2209  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1515 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  2212  was adjusted so that the loss at 1515 nm during one round of travel in the optical resonator became 17 dB. 
     The optical splitters  2202 ,  2207  comprising the second optical resonator had a passing loss of 0.2 dB or less in 1480-1510 nm and a splitting ratio of 95% in 1600-1800 nm. The wavelength selector  2210  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1700 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  2213  was adjusted so that the loss at 1700 nm during one round of travel in the optical resonator became 16 dB. 
     This example has a control part that makes the two resonators oscillate at all times, and this control part has a monitor signal light sampling unit. The monitor signal light sampling unit includes optical splitters  2215 ,  2211  and optoelectronic transducing circuits  2214 ,  2216 . The optical splitters  2215 ,  2211  are fusion spliced type fiber couplers, and have an almost constant splitting ratio of 1% in 1470-1750 nm. The optoelectronic transducing circuits  2216 ,  2214  were InGaAs-PIN-PDs. Light signals were 16 wavelengths from 1480 to 1510 nm at intervals of 2 nm. 
     The feedback control of the pump light sources is carried out by an oscillation decision circuit  2221  so that the first and second resonators maintain oscillation all the times based on the electric signal from the optoelectronic transducing circuits  2216 ,  2214 . 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference observed against varied input levels was 6 dB with no gain control, while 0.3 dB with gain control. 
     Now the operation of the optical amplifier of the invention will be specifically described below with reference to the configuration shown in  FIG. 15 . This is an example which uses two optical resonators that are a first optical resonator and a second optical resonator. In this example, the two optical resonators must be controlled to keep oscillation together in the optical amplifier. For this goal, the optical amplifier includes a function of monitoring each laser oscillation condition (function composed of an optical splitter  2211 , optoelectronic transducing circuit  2216 , oscillation decision circuit  2221 , optical splitter  2215 , and optoelectronic transducing circuit  2214 , oscillation decision circuit  2221  and others). This function monitors the status of laser oscillation and controls the pump light sources to cause laser oscillation in both resonators all the times. 
     As shown in  FIG. 15 , the optical amplifier may include variable attenuators  2212 ,  2213  and the like that can change the loss during one round of travel in the optical resonator. 
     Signal light that has passed the optical splitters  2201 ,  2202 , isolator  2203  and pump light/signal light coupler  2204  enters the amplification medium  2226 . The amplified signal light from the amplification medium  2226  passes the pump light/signal light coupler  2205 , isolator  2206  and optical splitters  2207 ,  2208  before leaving the system. 
     The optical resonators are a first optical resonator having an optical splitter  2201 , a wavelength selector  2209 , a variable optical attenuator  2212 , and optical splitters  2215 ,  2208 , and a second optical resonator having optical splitters  2202 ,  2211 , a wavelength selector  2210 , a variable optical attenuator  2213 , and an optical splitter  2207 . 
     The optical splitter  2215  splits off light in the first resonator and provides it to the optoelectronic transducing circuit  2214 . The obtained electric signal is sent to the oscillation decision circuit  2221 . The optical splitter  2211  splits off light in the second resonator and provides it to the optoelectronic transducing circuit  2216 . The obtained electric signal is sent to the oscillation decision circuit  2221 . Based on these electric signals, the pump light driver circuits  2218 ,  2220  control the intensity of pump light provided by the pump light sources  2217 ,  2219  so that both resonators may oscillate. The optical splitters can be installed with flexibility as to location. 
     Example 8 
       FIG. 16  illustrates an eighth example of the optical amplifier of the invention, which is the case where two single paths are serially connected. Now the first optical amplifier is described. The first optical amplifier  2301  is the same one as that shown in  FIG. 9A . Thus the configuration and operation of the optical amplifier is the same as that described in Example 1. 
     The second optical amplifier  2302  is the same one as that shown in  FIG. 12  (Example 4). Thus  FIG. 12  is used to describe; individual components of this amplifier. 
     The employed amplification medium  1916  was a Zr-based fluoride fiber where 6000 ppm of Tm ions were doped. This fiber was 7 m long. The pump light sources  1911 ,  1913  were multi-polarized pumping units where LDs oscillating at 1400 nm were installed so that polarized waves crossed at right angles. The present example using monitor signal lights applied feedback control to all the LDs so that the intensity of pump light from the pump light/signal light coupler  1905  became twice that from the coupler  1905 . 
     The optical splitter  1902  in the monitor signal light sampling part was a bulk type coupler using a dielectric multi-layered film as a filter. Its splitting ratio was 1% in 1460-1530 nm. Light signals were 16 wavelengths from 1480 to 1510 nm at intervals of 2 nm. 
     The optoelectronic transducing circuit  1910  was a PIN-PD. The optical splitters  1901 ,  1907  included in the optical resonator had a passing loss of 0.2 dB or less in 1480-1510 nm and a splitting ratio of 95% in 1470-1477 nm. The wavelength selector  1908  was a device using a dielectric multi-layered film that passed light of a central wavelength of 1475 nm of a 0.8 nm half-width with a passing loss of 0.5 dB. The variable attenuator  1909  was adjusted so that the loss at 1475 nm during one round of travel in the optical resonator became 15 dB. 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference observed against varied input levels was 10 dB with no gain control, while 0.3 dB with gain control. 
     It is possible to arbitrarily select optical amplifiers described in Examples 2, 3, 5, 6 and 7 and serially connect them. Serially connected optical amplifiers are expected to show a high operating efficiency. 
     Example 9 
       FIG. 17  illustrates a ninth example of the optical amplifier of the invention, which is the case where two single paths are serially connected. The optical amplifier of the present example is a device where a gain equalizer  2402  and a variable optical attenuator  2403  are inserted in the junction of the two-stage type optical amplifier described in Example 8. 
     The gain equalizer  2402  uses a long-interval fiber grating, while the variable optical attenuator  2403  has a flatness of loss of 0.1 dB or less in 1480-1510 nm. 
     By employing the gain equalizer  2402  and the variable optical attenuator  2403 , the present example succeeded in lowering the gain flatness to 2%, compared with 30% obtained for 1480-1510 nm light in Example 8. 
     The gain flatness is defined as follows:
 
(Gain flatness)={(Maximum gain in the amplification band)−(Minimum gain in the amplification band)}/(Minimum gain in the amplification band)
 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference against varied input levels was 10 dB with no gain control, while 0.3 dB with gain control. 
     The first and second optical amplifiers  2401  and  2404  can be selected from those described in Examples 1-7. The gain equalizer  2402  is inserted to flatten the gain profile, while the variable optical attenuator  2403  adjusts the intensity of input signal light provided to the second amplifier. 
     Example 10 
       FIG. 18  illustrates a tenth example of the optical amplifier of the invention, which is the case where two single paths are serially connected. The optical amplifier of the present example is a device where a gain equalizer  2502  and a variable optical attenuator  2503  are inserted in the junction of the two-stage type optical amplifier described in Example 8. The optical amplifier of the present example further comprises an optical splitter  2504 , optical band-pass filter  2506 , optoelectronic transducing circuit  2507  and differential signal generating circuit  2508 . The optical splitter  2404  is a bulk-type device of a splitting ratio of 1% in 1480-1510 nm. The optical band-pass filter  1506  is a device using a dielectric multi-layered film that passes light of a central wavelength of 1480 nm of a 0.8 nm half-width with a passing loss of 1 dB. 
     The gain equalizer  2502  uses a long-interval fiber grating, while the variable optical attenuator  2503  has a flatness of loss of 0.1 dB or less in 1480-1510 nm, being capable of controlling the degree of attenuation based on external signals. 
     In the present example, we monitored the 1480 nm signal light with the above control unit, and applied a feedback control to the variable optical attenuator  2503  via the differential signal generating circuit  2508  in the control unit so as to keep the signal level constant. By such feedback control, the light signals in individual channels show constant levels in the second amplifier  2505 . 
     The present example succeeded in lowering the gain flatness to 2%, compared with 30% obtained for 1480-1510 nm light in Example 8. 
     The gain flatness is defined as follows:
 
(Gain flatness)={(Maximum gain in the amplification band)−(Minimum gain in the amplification band)}/(Minimum gain in the amplification band)
 
     The input signal level was changed between −25 and −10 dBm/ch, and the gain profile at −25 dBm/ch was determined as the reference gain profile. The maximum deviation from this reference against varied input levels was 10 dB with no gain control, while 0.3 dB with gain control. The levels of individual WDM signals were kept at a constant value of 3 dBm at the signal output side of the optical amplifier of the Example. 
     As described in Example 10, the optical amplifier of the invention can include a control unit that controls the variable optical attenuator. The degree of attenuation by the variable attenuator in the optical amplifier can be controlled by such a control unit that has the function of electric control based on sampled monitor signal lights. 
     The first and second optical amplifiers  2501  and  2505  can be selected from those described in Examples 1-7. The gain equalizer  2502  is inserted to flatten the gain profile. The optical splitter  2504  splits off part of signal light. The optical band-pass filter  2506  takes out signals in an arbitrary channel, and the optoelectronic transducing circuit  2507  converts such signals into electric signals. From the electric signal and a prescribed value set in the differential signal generating circuit  2508 , a control signal is generated and this control signal is used to control the variable optical attenuator. 
     The degree of attention in the variable optical attenuator is controlled by feedback so that the gap between the outputs from the first optical amplifier  2501  and the second optical amplifier  2505  becomes smaller, and preferably zero. 
     In this example, the input levels in individual channels of WDM signals become equal to each other in the second optical amplifier. 
     Example 11 
       FIG. 19  illustrates an eleventh example of the optical amplifier of the invention, which is the case where more than one single path is serially connected. This example uses two optical amplifiers  2618 ,  2619 . This example is an example of dividing the input signal provided to the optical amplifier into two bands, amplifying the divided signals with individual amplifiers, and combining the amplified signals again. In the figure, denoted  2602  is a first optical amplifier,  2603  a gain equalizer,  2604  a variable optical attenuator,  2605  an optical splitter,  2606  a second optical amplifier,  2607  an optical band-pass filter,  2608  and  2615  optoelectronic transducing circuits, and  2609  a differential signal generating circuit. 
     The optical amplifier  2618  is the same as that used in Example 10. Thus the configuration and operation of the optical amplifier  2618  is the same as that described in Example 10. Now the optical amplifier  2619  is described below. 
     A third optical amplifier  2610  in the optical amplifier  2619  is the optical amplifier used in Example 1. The optical splitter  2613  is a device for wavelengths between 1460-1490 nm and a bulk-type device of a splitting ratio of 1%. The optical band-pass filter  2614  is a device using a dielectric multi-layered film that passed light of a central wavelength of 1460 nm of a 0.8 nm half-width with a passing loss of 1 dB. 
     The gain equalizer  2611  uses a long-interval fiber grating, while the variable optical attenuator  2612  has a flatness of loss of 0.1 dB or less in 1460-1490 nm, being capable of controlling the degree of attenuation based on external signals. 
     The control using the optical amplifier of the present example monitors the 1460 nm signal light and applies a feedback control to the variable optical attenuator  2612  via the differential signal generating circuit  2616  so that the monitored signal becomes constant. Then the levels of the individual light signals coming out from the optical amplifier  2619  become constant. 
     The signal band splitter  2601  and the signal band coupler  2617  combine the 1460-1476 nm band and the 1480-1510 nm band. The employed light signals are 25 WDM waves in total consisting of 9 waves from 1460 to 1476 nm at intervals of 2 nm and 16 waves from 1480 to 1510 nm at intervals of 2 nm. 
     When the input signal level was changed between −25 and −10 dBm/ch, the level of output light from the signal coupler  2617  fluctuated 10 dB or more under no control of the invention, while the level fell 5.0-5.2 dBm when the control of the invention was employed. 
     Example 12 
     This example is an example using the optical amplifier described in third embodiment. 
       FIG. 20A  illustrates a diagram illustrating the present example of the optical amplifier of the invention. Referring now to  FIG. 20A , this optical amplifier has an amplification medium, for example, an optical fiber  3304  of which core is doped with rare-earth ions, isolators  3302 ,  3305 ,  3308 , a coupler  3301  for coupling control light and signal light, a coupler  3303  for coupling signal/control light from the isolator  3302  with pump light supplied by a pump light source  3309  via an isolator  3308 , a splitter  3306  for splitting off output light taken out from the fiber  3304  via an isolator  3305 , a filter  3310  for extracting control light from an output of the splitter  3306 , a control light source  3307  having a photodetector therein, a pump light source  3309  for generating pump light, a photodetector  3311  for detecting control light from the filter  3310 , a calculation unit  3313  that receives the signal from the control light source  3307  and the signal from the photodetector  3311 , and a control unit  3312  for controlling the control light source  3307  based on the output from the calculation unit  3313 . 
     The host glass of the rare-earth doped optical fiber can be silica-based glass, bismuth-based glass, fluoride-based ZBLAN glass that is less likely to cause non-radiation transition, In—Pb glass and Tellurite glass. 
     The coupler  3301  and the splitter  3306  may be fusion spliced fibers (splitter type, wavelength division multiplexing type), dielectric multi-layered film, or a circulator coupled with a fiber grating. As the coupler  3303 , a fusion spliced fiber, a dielectric multi-layered film, or a circulator coupled with a fiber grating can be used. The control light source  3307  that emits control light and the pump light source  3309  that emits pump light can be solid lasers such as Nd-YLF laser, Nd-YAG laser, Ti-sapphire laser, semiconductor lasers or fiber lasers. The filter  3310  can be a dielectric multi-layered film, a fiber grating or the like. 
     In the present example, the light signals were 16 waves from 1480 to 1510 nm at intervals of 2 nm, the optical fiber  3304  for the amplification was a 5 m fiber of which host glass was ZBLAN fluoride glass with core where 6000 ppm of Thulium ions were doped, the wavelength of control light was 1520 nm, the wavelength of pump light was 1400 nm, the central wavelength of isolators  3302 ,  3305  was 1480 nm, the coupler  3301  and the splitter  3306  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1512 nm or shorter bands and the 1518 nm or longer band, the coupler  3303  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1430 nm or shorter band and the 1440 nm or longer band, and the filter  3310  passed light of a central wavelength of 1520 nm of a 0.8 nm half-width. The power of pump light was fixed at 300 mW. 
     As shown in  FIG. 20A , the signal light was combined with the control light from the control light source  3307  by the coupler  3301 , passing the isolator  3302 , combined with pump light from the isolator  3308  by the coupler  3303 , passing the optical fiber  3304  and the isolator  3305 , and split off signal light as output at the splitter  3306 . 
     The power of the input control light provided to the optical fiber  3304  was detected by a photodetector installed in the control light source  3307 . The power of output control light from the optical fiber  3304  was split by the splitter  3306 , passing the filter  3310 , detected by the photodetector  3311 , and then sent to the calculation unit  3313  in the form of an electric signal. 
     The calculation unit  3313  calculated the gain of control light from the received detection value, calculated the current for the control light source  3307  that was equal to the prescribed value, and the value was sent to the control unit  3312 . 
     The control unit  3312  controlled the light intensity in the control light source  3307  so that its current became equal to that sent from the calculation unit  3313 . 
     Since the wavelength of control light lies in the amplification band, the gain profile can be held constant by adjusting the intensity of control light so that the gain of the control light does not change. 
       FIG. 20B  is a graph where the maximum values in deviations in gain profile were plotted against varied input signal levels in Example 12. With reference to the gain spectrum at −25 dBm/ch, the maximum deviation of each gain spectrum observed when the input signal levels set at −25, −20, −15 to −10 dBm/ch were inputted, were plotted in this graph. The gains were measured by scanning a −35 dBm weak probe light. The dotted line in  FIG. 20B  shows the result provided by a conventional method adopting no gain control using monitored signal lights. The solid line in  FIG. 20B  shows the values obtained when the gain at 1520 nm was held constant by the monitoring system using 1520 nm control light.  FIG. 20B  demonstrates that the proposed control succeeded in holding the gain profile constant. 
     Example 13 
       FIG. 21A  is a diagram illustrating the 13th example of the optical amplifier of the invention. The same components have the same numerals in  FIGS. 21A and 20A , and their description is not repeated here. Referring now to  FIG. 21A , in addition to the components shown in  FIG. 20A , this optical amplifier has a splitter  3501  that splits off signal light sent to/from the amplifier, a filter  3502  for extracting signal light from the splitter  3501 , a control light source  3504 , and a photodetector  3503  that detects the power of signal light sent from the filter  3502 . In the present example, the splitter  3306  splits output signal light, filter  3310  extracts signal light and photodetector  3311  detects the signal light power. 
     The splitter  3501  may be a fusion spliced fiber (splitter type, wavelength division multiplexing type), dielectric multi-layered film, or a circulator coupled with a fiber grating. The control light source  3504  that emits control light can be solid lasers such as Nd-YLF laser, Nd-YAG laser, Ti-sapphire laser, semiconductor lasers or fiber lasers. The filter  3502  can be a dielectric multi-layered film, a fiber grating or the like. 
     In the present example, the signal waves were 16 waves of 1470 to 1500 nm at intervals of 2 nm, the optical fiber  3304  was a 10 m fiber of which host glass was ZBLAN fluoride glass with core where 4000 ppm of Thulium ions were doped, the wavelength of control light was 1460 nm, the wavelength of pump light was 1400 nm, the central wavelength of isolators  3302 ,  3305  was 1480 nm, the coupler  3501  and the splitter  3306  were fiber couplers of a splitting ratio of 95:5, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1462 nm or shorter band and the 1468 nm or longer band, the coupler  3501  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1430 nm or shorter band and the 1440 nm or longer band, and the filters  3502 ,  3310  passed light of a central wavelength of 1510 nm of a 0.8 nm half-width. The power of pump light was fixed at 300 mW. One channel of wavelength division multiplexed light to be monitored was 1510 nm input light. 
     As shown in  FIG. 21A , the signal light was split by the splitter  3501 , the signal light split by the splitter  3501  was combined with control light from the control light source  3504  by the coupler  3301 , passing the isolator  3302 , combined with pump light from the isolator  3308  by a WDM coupler  3303 , passing the optical fiber  3304  and the isolator  3305 , and split off to provide signal light as output at the splitter  3306 . 
     The power of the 1510 nm input signal light was split by the splitter  3501 , passing the filter  3502 , detected by the photodetector  3503 , while the 1510 nm output signal light was split by the splitter  3306 , passing the filter  3310 , detected by the photodetector  3311 , and the detected value was sent to the calculation unit  3313  in the form of an electric signal. 
     The calculation unit  3313  calculated the gain of the 1510 nm signal light based on the received detection value, and the current in the control light source  3504  was calculated so as to agree with a prescribed value, and the calculated current value was sent to the control unit  3312 . The control unit  3312  controlled the light intensity of the control light source  3504  so that its current became equal to the current sent from the calculation unit  3313 . In this example, the light intensity of the control light source  3504  was controlled with reference to the detected signal light power of one channel at 1510 nm. However, two or more channels can be used for such control. Further, such control can be implemented by utilizing the entire signal light of the prescribed wavelength range for monitored signal light. For example, the signal light power in each channel in 1470-1500 nm is summed up, and the gain calculated from this sum can be used to control the light intensity in the control light source  3504  as described above. 
       FIG. 21B  is a graph where the maximum values in deviations in gain profile were plotted against varied input signal levels. With reference to the gain spectrum at −25 dBm/ch, the maximum deviation of each gain spectrum observed when the input signal levels set at −25, −20, −15 and −10 dBm/ch were inputted, were plotted in this graph. The gains were measured by scanning a −35 dBm weak probe light. The dotted line in  FIG. 21B  shows the result provided by a conventional method adopting no gain control using monitored signal lights. The solid line in  FIG. 21B  shows the values obtained when the gain at 1510 nm was held constant by the monitoring system utilizing 1510 nm signal light. This figure demonstrates that the proposed control succeeded in holding the gain profile constant. 
     Example 14 
     Example 14-1 
       FIG. 22A  is a diagram illustrating a 14 th  example of the optical amplifier of the invention. The same components have the same numerals in  FIGS. 21A and 22A , and their description is not repeated here. Referring now to  FIG. 22A , in addition to the components shown in  FIG. 21A , this optical amplifier has a splitter  3702  that splits output light from the splitter  3306 , a filter  3703  for extracting control light from the splitter  3702 , a photodetector  3704  that detects the power of control light extracted by the filter, a splitter  3701  splits pump light from the isolator  3308  to couplers  3303 ,  3705 , and a coupler  3705  that combines signal light from the optical fiber  3304  and pump light from the splitter  3701 . The control light source  3307  has a photodetector therein that detects the power of input control light source. 
     The coupler  3705  may be a fusion spliced fiber dielectric multi-layered film, or circulator coupled with a fiber grating. As the filter  3703 , a dielectric multi-layered film or a fiber grating can be used. 
     In the present example, the signal light waves were 16 waves of 1460 to 1490 nm at intervals of 2 nm, the optical fiber  3304  was a 20 m fiber of which host glass was ZBLAN fluoride glass with core where 2000 ppm of Thulium ions were doped, the wavelength of control light of the control light source  3307  was 1450 nm, the wavelength of pump light was 1390 nm, the central wavelength of isolators  3302 ,  3305  was 1480 nm, the splitters  3501 ,  3306  were fiber couplers of a splitting ratio of 95:5, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1452 nm or shorter band and the 1458 nm or longer band, the splitters  3701 ,  3702  were fiber couplers of which splitting ratio was 3 dB, the couplers  3303 ,  3705  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1430 nm or shorter band and the 1440 nm or longer band, the filters  3502 ,  3310  passed light of a central wavelength of 1490 nm of a 0.8 nm half-width, and the filter  3703  passed light of a central wavelength of 1450 nm of a 0.8 nm half-width. The monitored signal lights were 1450 nm control light and 1490 nm signal light. 
     Now suppose that the gains of two monitored signal lights are expressed by G(1450 nm) and G(1490 nm), and define the sum of gains Gs and the gain difference □G as follows:
 
 Gs=G (1450 nm)+ G (1490 nm)  (1)
 
Δ G=G (1450 nm)− G (1490 nm)  (2)
 
     If Gs and ΔG are determined uniquely, then G(1450 nm) and G(1490 nm) are determined. Thus G(1450 nm) and G(1490 nm) can be controlled by adjusting Gs and ΔG. 
       FIG. 22B  is a graph that plotted the gain sum Gs and gain difference ΔG of two monitored signal lights observed when the control light power was varied, with the input signal light power and the pump light power being fixed at −15 dBm/ch and 400 nW, respectively, in this example. Gs is shown with a solid line, while ΔG is shown with a broken line. Gs and ΔG decreased monotonously as the control light power increased. 
       FIG. 22C  is a graph that plotted the gain sum Gs and gain difference ΔG of two monitored signal lights observed when the control light power was varied, with the input signal light power and the control light power being fixed at −15 dBm/ch and −15 dBm, respectively, in the this example. Like  FIG. 22B , Gs is shown with a solid line, while ΔG is shown with a broken line. Gs increased monotonously as the control light power increased, while ΔG did not change almost at all. 
     The results shown in  FIGS. 22B and 22C  indicate that the gain spectrum can be controlled by the following control algorithm. If the monitored signal light gain has changed, the control light power, which does not change almost at all even when the pump light power is varied, is controlled so that ΔG becomes a desired value, while fixing the pump light power. Next, the control light power is fixed, and the pump light power is adjusted to get a desired Gs. The gain spectrum can thereby be controlled. 
     As shown in  FIG. 22A , the signal light is partially split by the splitter  3501 , combined with control light from the control light source  3307  by the coupler  3301 , passing the isolator  3302 , combined with pump light from the splitter  3701  by the coupler  3303 , passing the optical fiber  3304  and then the coupler  3705  that combines pump light from the splitter  3701  with signal light, further passing the isolator  3305 , and partially split by the splitter  3306  to be outputted as output signal light. 
     From the signal light partially split by the splitter  3501 , the filter  3502  extracts only 1490 nm signal light, and then the photodetector  3503  detects this signal light. The photodetector in the control light source  3307  detects the power of control light provided to the amplifier. The output light split off by the splitter  3306  is further split by the splitter  3702 . From one of the split light beams, only 1490 nm signal light is extracted by the filter  3310 , and the extracted light is detected by the photodetector  3311 . Meanwhile, from the other split light beam, only 1450 nm control light is extracted by the filter  3703  and detected by the photodetector  3704 . These detected light signals are converted into electric signals, and then sent to the calculation unit  3313 . 
     The calculation unit  3313  calculates the gains of signal light and control light based on the received detection values to calculate Gs and ΔG, then calculates the currents for the control light source and the pump light source so that Gs and ΔG agree with prescribed values. These calculated current values are sent to the control unit  3312 . 
     The control unit  3312  adjusts the control light source  3307  and pump light source  3309  so as to attain the current values sent from the calculation unit  3313 . 
       FIG. 22D  is a graph where gain spectrum deviations from the reference gain spectrum at −10 dBm/ch observed when the input signal levels (−25, −20, −15 and −10 dBm/ch) were varied under the gain control of the invention are plotted. The deviations were 0.3 dB or less for all the input conditions, and this demonstrates the effectiveness of the present invention. 
     Example 14-2 
     The configuration of the present example is the same as that described in Example 14-1 referred to  FIG. 22A , except for performing numerical settings in the calculation unit. Thus the components and units other than the calculation unit are the same as those employed in Example 14-1. The following description refers to  FIG. 22E . 
     In the present example, the signal light waves were 16 waves of 1490 to 1520 nm at intervals of 2 nm, the optical fiber  3304  was a 4 m fiber of which host glass was ZBLAN fluoride glass with core where 8000 ppm of Thulium ions were doped, the wavelength of control light of the control light source  3307  was 1470 nm, the wavelength of pump light was 1400 nm, the central wavelength of isolators  3302 ,  3305 ,  3308  was 1470 nm, the splitters  3501 ,  3306  were fiber couplers of a splitting ratio of 95:5, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1472 nm or shorter band and the 1478 nm or longer band, the splitters  3701 ,  3702  were fiber couplers of which splitting ratio was 3 dB, the couplers  3303 ,  3705  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1430 nm or shorter band and the 1440 nm or longer band, the filters  3502 ,  3310  passed waves of 1490-1520 nm, and the filter  3703  passed light of central wavelength of 1470 nm of a 0.8 nm half-width. The monitored signal lights were control light at 1470 nm and all the light signals of prescribed 1490-1520 nm. 
     Now suppose that the gains of two monitored signal lights are expressed by G(1470 nm) and G(1490-1520 nm), and define the sum of gains Gs and the gain difference ΔG as follows:
 
 Gs=G (1470 nm)+ G (1490-1520 nm)  (1)
 
Δ G=G (1470 nm)− G (1490-1520 nm)  (2)
 
     If Gs and ΔG are determined uniquely, then G(1470 nm) and G(1490-1520 nm) are determined. Thus G(1470 nm) and G(1490-1520 nm) can be controlled by adjusting Gs and ΔG. 
     Because Gs and ΔG of this example behave like those of Example 14-1 against changes in the pump light power and control light power, the gain spectrum can be controlled by the same control algorithm. As is the case with Example 14-1, the input signal light enters the splitter  3501 , being split by the splitter  3306 , and one of the split beams comes out as signal light. 
     From the signal light split by the splitter  3501 , the filter  3502  extracts only the prescribed 1490-1520 nm signal light, the photodetector  3503  detects this light. The photodetector in the control light source detects the power of control light to be provided to the optical fiber. The signal light and control light split by the splitter  3306  is further split by the splitter  3702 . From one of the split light beams, only 1490-1520 nm signal light is extracted by the filter  3310 , and the extracted light is detected by the photodetector  3311 . Meanwhile, from the other split light beam, only 1470 nm control light is extracted by the filter  3703  and detected by the photodetector  3704 . These detected light signals are converted into electric signals, and then sent to the calculation unit  3313 . Much like Example 14-1, the control unit  3312  controls light intensity in the control light source  3307  and pump light source  3309 . 
     Gain spectrum deviations from the reference gain spectrum observed when the input signal level was varied from −25 to −10 dBm/ch under the gain control of the invention were 0.3 dB or less, and this demonstrates the effectiveness of the present invention. 
     Example 14-3 
     The configuration of the present example is the same as that described in Example 14-1 referred to  FIG. 22A , and the employed components and units are the same as those used in Example 14-1. The optical fiber is, however, a Ho-doped fiber, and this is a difference from Example 14-1. Now the following description refers to  FIG. 22A . 
     In the present example, the signal light waves were 16 waves of 2890 to 2920 nm at intervals of 2 nm, the optical fiber  3304  was a 20 m fiber of which host glass was ZBLAN fluoride glass with core where 1000 ppm of Holmium ions (Ho 3+ ) were doped, the wavelength of control light of the control light source  3307  was 2880 nm, the wavelength of pump light was 1200 nm, the central wavelength of isolators  3302 ,  3305  was 2900 nm, the splitters  3501 ,  3306  were fiber couplers of a splitting ratio of 95:5, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 2882 nm or shorter band and the 2888 nm or longer band, the splitters  3701 ,  3702  were fiber couplers of which splitting ratio was 3 dB, the couplers  3303 ,  3705  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1300 nm or shorter band and the 1310 nm or longer band, the filters  3502 ,  3310  passed waves of 2890-2920 nm, and the filter  3703  passed light of central wavelength of 2880 nm of a 0.8 nm half-width. The monitored signal lights were 2880 nm control light and all the light signals of prescribed 2890-2920 nm. 
     Now suppose that the gains of two monitored signal lights are expressed by G(2880 nm) and G(2890-2920 nm), and define the sum of gains Gs and the gain difference ΔG as follows. As is the case with Example 14-2, gain sum Gs and gain difference ΔG are calculated, the currents in the control light source and pump light source are calculated so that Gs and ΔG become equal to those externally provided, and these current values are sent to the control unit  3312 . The control unit  3312  controls the control light source  3307  and pump light source  3309  so that their currents agree with the received values.
 
 Gs=G (2880 nm)+ G (2890-2920 nm)  (1)
 
Δ G=G (2880 nm)− G (2890-2920 nm)  (2)
 
     Gain spectrum deviations from the reference gain spectrum observed when the input signal level was varied from −25 to −10 dBm/ch under the gain control of the invention were 0.3 dB or less, and, this demonstrates the effectiveness of the present invention. 
     Example 14-4 
     The configuration of the present example is the same as that described in Example 14-1 referred to  FIG. 22A , and the employed components and units are the same as those used in Example 14-1. The amplifier filter is, however, an Er-doped fiber and this is a difference from Example 14-1. Now the following description refers to  FIG. 22A . 
     In the present example, the signal light waves were 16 waves of 1690 to 1720 nm at intervals of 2 nm, the optical fiber  3304  was a 15 m fiber of which host glass was ZBLAN fluoride glass with core where 2000 ppm of Erbium ions (Er 3+ ) were doped, the wavelength of control light of the control light source  3307  was 1680 nm, the wavelength of pump light was 800 nm, the central wavelength of isolators  3302 ,  3305  was 1700 nm, the splitters  3501 ,  3306  were fiber couplers of a splitting ratio of 95:5, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1682 nm or shorter band and the 1688 nm or longer band, the splitters  3701 ,  3702  were fiber couplers of which splitting ratio was 3 dB, the couplers  3303 ,  3705  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1300 nm or shorter band and the 1310 nm or longer band, the filters  3502 ,  3310  passed waves of 1690-1720 nm, and the filter  3703  passed light of central wavelength of 1680 nm of a 0.8 nm half-width. The monitored signal lights were 1680 nm control light and the entire light signals of prescribed 1690-1720 nm. 
     Now suppose that the gains of two monitored signal lights are expressed by G(1680 nm) and G(1690-1720 nm), and define the sum of gains Gs and the gain difference ΔG as follows. As is the case with Example 14-2, gain sum Gs and gain difference ΔG are defined as follows, the currents in the control light source and pump light source are calculated so that Gs and ΔG become equal to those externally provided, and these values are sent to the control unit  3312 . The control unit  3312  controls the control light source  3307  and pump light source  3309  so that their currents agree with the received values.
 
 Gs=G (1680 nm)+ G (1690-1720 nm)  (1)
 
Δ G=G (1680 nm)− G (1690-1720 nm)  (2)
 
     Gain spectrum deviations from the reference gain spectrum observed when the input signal level was varied from −25 to −10 dBm/ch under the gain control of the invention were 0.3 dB or less, and this demonstrates the effectiveness of the present invention. 
     In Example 14, the doped ions were Tm 3+ , Ho 3+  and Er 3+ . However, the present invention is not limited by those examples. Instead, the invention can be applied to any rare-earth doped optical amplifiers where the sum of ions in the amplification upper level and the amplification lower level is not constant. 
     Example 15 
       FIG. 23  is a diagram illustrating a 15th example of the optical amplifier of the invention. The same components have the same numerals in  FIGS. 23 and 22A , and their description is not repeated here. Referring now to  FIG. 23 , in addition to the components shown in  FIG. 22A , this optical amplifier has a splitter  4101  that splits signal light, a filter  4102  for extracting signal light from the splitter  4101 , a photodetector  4103  that detects the power of signal light from the filter  4102 . Also in this example, the filter  3703  extracts signal light from the splitter  3702 , and the photodectector  3704  detects the power of extracted signal light. The power of pump light was fixed at 400 mW. 
     The splitter  4101  may be a fusion spliced fiber (splitter type, wavelength division multiplexing type), dielectric multi-layered film, or circulator coupled with a fiber grating. As the filter  4102 , a dielectric multi-layered film or a fiber grating can be used. 
     In the present example, the signal light waves were 16 waves of 1480 to 1510 nm at intervals of 2 nm, the optical fiber  3304  was a 5 m fiber of which host glass was ZBLAN fluoride glass with core where 6000 ppm of Thulium ions were doped, the wavelength of control light of the control light source  3504  was 1470 nm, the wavelength of pump light was 1410 nm, the central wavelength of isolators  3302 ,  3305 ,  3308  was 1480 nm, the splitters  3501 ,  3306  were fiber couplers of a splitting ratio of 95:5, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1472 nm or shorter band and the 1478 nm or longer band, the splitters  3701 ,  3702 ,  4101  were fiber couplers of which splitting ratio was 3 dB, the couplers  3303 ,  3705  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1430 nm or shorter band and the 1440 nm or longer band, the filters  3502 ,  3310  passed light of a central wavelength of 1480 nm of a 0.8 nm half-width, and the filters  4102 ,  3703  passed light of a central wavelength of 1510 nm of a 0.8 nm half-width. The monitored signal lights were 1480 nm signal light and 1510 nm signal light. 
     Now suppose that the gains of two monitored signal lights are expressed by G(1480 nm) and G(1510 nm), and define the sum of gains Gs and the gain difference ΔG as follows:
 
 Gs=G (1480 nm)+ G (1510 nm)  (1)
 
Δ G=G (1480 nm)− G (1510 nm)  (2)
 
     Gs and ΔG are determined uniquely, then G(1480 nm) and G(1510 nm) are determined. Thus G(1480 nm) and G(1510 nm) can be controlled by adjusting Gs and ΔG. 
     Because Gs and ΔG of this example behave like those of Example 14-1 against changes in the pump light power and control light power, the gain spectrum can be controlled by the same control algorithm. As is the case with Example 14-1, Gs and ΔG are calculated, and the light intensities in the control light source  3504  and pump light source  3309  can be controlled. 
     In this example, two channels (1480 nm and 1510 nm) of the WDM light signals were used to calculate gains and Gs as well as ΔG to control the light intensities of the control light source  3504  and the pump light source  3309 . However, more channels can be used for this control. For example, instead of ΔG, the difference in gain between arbitrary two channels can be used. Like the case using Gs and ΔG, it is possible to define parameters so that the gains in individual channels can be controlled for adjusting light intensity. 
     Gain spectrum deviations from the reference gain spectrum observed when the input signal level was varied from −25 to −10 dBm/ch under the gain control of the invention were 0.3 dB or less, and this demonstrates the effectiveness of the present invention. 
     Example 16 
       FIG. 24A  is a diagram illustrating a 16th example of the optical amplifier of the invention. The same components have the same numerals in  FIGS. 24A and 22A , and their description is not repeated here. Referring now to  FIG. 24A , in addition to the components shown in  FIG. 22A , this optical amplifier has a calculation unit  4313  incorporating a correction circuit  4301  that corrects the signal light power detected by the photodetector  3503  and a calculation circuit  4302  that conducts calculation based on the corrected signal light power and sends the calculation result to the control unit  3312 . Like Example 14-2, the employed components and units were the same as those used in Example 14-1. 
     In the present example, the signal light waves were 16 waves of 1480 to 1510 nm at intervals of 2 nm, the optical fiber  3304  was a 5 m fiber of which host glass was ZBLAN fluoride glass with core where 6000 ppm of Thulium ions were doped, the wavelength of control light of the control light source  3307  was 1470 nm, the wavelength of pump light was 1390 nm, the central wavelength of isolators  3302 ,  3305  was 1480 nm, the coupler  3301  was a dielectric multi-layered film type wavelength division multiplexing coupler that split/combined the 1472 nm or shorter band and the 1478 nm or longer band, the splitters  3701 ,  3702  were fiber couplers of which splitting ratio was 3 dB, the couplers  3303 ,  3705  were dielectric multi-layered film type wavelength division multiplexing couplers that split/combined the 1430 nm or shorter band and the 1440 nm or longer band, the splitters  3501 ,  3306  were fiber couplers of a splitting ratio of 95:5, the filters  3502 ,  3310  passed 1480-1510 nm waves, and the filter  3703  passed light of a central wavelength of 1470 nm of a 0.8 nm half-width. The monitored signal lights were 1470 nm control light and all the signal light of prescribed 1480-1510 nm. 
     Now suppose that the gains of two monitored signal lights are G(1470 nm) and G(1480-1510 nm). However, the total signal light power detected by the photodetector  3503  includes light other than signal light (for example, amplified spontaneous emission light) that may have a non-negligible intensity. This phenomenon becomes pronounced in multiple-stage relays. Then the correction circuit  4301  generates a correction value calculated by a function corresponding to input signal light power, and the value is added to or multiplied by G(1480-1510 nm) that was calculated by the calculation unit for correction of G(1480-1510 nm). The corrected G(1480-1510 nm) is now expressed by G′(1480-1510 nm). Gain sum Gs and gain difference ΔG are defined as follows:
 
 Gs=G (1470 nm)+ G′ (1480-1510 nm)  (1)
 
Δ G=G (1470 nm)− G′ (1480-1510 nm)  (2)
 
     If Gs and ΔG are determined uniquely, then G′(1470 nm) and G′(1480-1510 nm) are determined. Thus G(1470 nm) and G′(1480-1510 nm) can be controlled by adjusting Gs and ΔG. 
     Because Gs and ΔG of this example behave like those of Example 14-1 against changes in the pump light power and control light power, the gain spectrum can be controlled by the same control algorithm. As is the case with Example 14-1, the input signal light is split off by the splitters  3501 ,  3306 , and the split light goes out as signal light. 
     From the signal light split by the splitter  3501 , the filter  3502  extracts the prescribed 1480-1510 nm signal light, and the photodetector  3503  detects this light. The photodetector  3307  installed in the control light source detects the power of control light. The signal light and control light split by the splitter  3306  is further split by the splitter  3702 . From one of the split light beams, 1470 nm control beam is extracted by a filter  3703  and detected by a photodetector  3704 . From the other split light beam, the entire 1480-1510 nm signal light is extracted by the filter  3310  and then detected by a photodetector  3311 . These detected light signals are converted into electric signals, and sent to the calculation circuit in the calculation unit  3313 . The value detected by the photodetector  3503  is sent to a correction circuit  4301 . 
     The value detected by the photodetector  3503  is now corrected by a correction circuit  4301  in the calculation unit  4313  as follows:
 
Pre= Pc ×(1.0029−0.2530 ×Pc− 3.0981 ×Pc×Pc ),  Pc≧ 0.005  (3)
 
     Pre: Control light power after correction (mW) 
     Pc: Control light power before correction (mW) 
     The above corrections are made to correct the power of control light detected by the photodetector  3503  by adding weights in accordance with the value of the power of control light.  FIG. 24B  shows the relationship between the power of control light before and after correction. The lateral axis represents the power of control light before correction, while the vertical axis represents the difference (ΔP) between the powers before and after correction. As the power of control light before correction increases, ΔP increases sharply. This correction is an example, and various correction functions may be defined as long as they meet the operation requirements of the amplifier during control. Although in this example only correction of the output from the photodetector  3503  has been described, it is possible to correct the output from the photodetector  3704  or both outputs from the photodetectors  3503  and  3704 . 
     The corrected power of control light after correction is sent to the calculation circuit  4302 . 
     The calculation circuit  4302  calculates these gains of signal light and control light from received detection value and correction value and then calculates Gs and ΔG. Currents for the control light source and pump light source are calculated so that prescribed Gs and ΔG are provided, and those calculated current values are sent to the control unit  3312 . 
     The control unit  3312  adjusts the control light source  3307  and pump light source  3309  so that they have currents sent from the calculation circuit  4302 . 
     Gain spectrum deviations from the reference gain spectrum observed when the input signal level was varied from −25 to −10 dBm/ch under the gain control of the invention were 0.3 dB or less, and this demonstrates the effectiveness of the present invention. 
     Example 17 
       FIG. 25A  is a diagram illustrating a 17 th  example of the optical amplifier of the invention. The same components have the same numerals in  FIGS. 25A and 24A , and their description is not repeated here. Referring now to  FIG. 25A , in addition to the components shown in  FIG. 24A , this optical amplifier has circulators  4502 ,  4504  coupled with fiber gratings  4501 ,  4505  that combine/split control light and signal light, and a coupler  4503  that combines pump light from the isolator  3308  with signal light and control light from the circulator  4502 . 
     Instead of circulators  4502 ,  4504  coupled with fiber gratings  4501 ,  4505 , a fusion spliced fiber (splitter type, wavelength division multiplexing type) or dielectric multi-layered film can be employed. As the coupler  4503 , a fusion spliced fiber, dielectric multi-layered film or circulator coupled with a fiber grating can be used. 
     In the present example, the signal light waves were 16 waves of 1478, 1480, 1482, 1484, 1486, 1488, 1490, 1492, 1496, 1498, 1500, 1502, 1504, 1506, 1508 and 1510 nm. The optical fiber  3304  was a 7 m fiber of which host glass was ZBLAN fluoride glass with core where 6000 ppm of Thulium ions were doped, the wavelength of control light of the control light source was 1494 nm, the wavelength of pump light was 1400 nm, the central wavelength of isolator  3308  was 1400 nm, the circulators  4502 ,  4504  coupled with fiber gratings  4501 ,  4505  combined/split the 1494 nm wave (band width 0.8 nm), and the coupler  4503  was a dielectric multi-layered film type wavelength division multiplexing coupler that combined the 1430 nm or shorter band and the 1440 nm or longer band. The power of pump light was fixed at 300 mW. 
     As shown in  FIG. 25A , the signal light is combined with control light by the coupler  4501 , passing the circulator  4502  coupled with the fiber grating  4501 , further combined with pump light by the coupler  4503 , passing the optical fiber  3304 , and partially split by the circulator  4504  coupled with the fiber grating  4505  to be outputted as an output signal light. 
     The power of control light entered to the optical fiber  3304  is detected by a photodetector installed in the control light source  3307 . The power of control light provided by the optical fiber is divided by the circulator  4504  coupled with the fiber grating  4505 , detected by the photodetector  4313 , and sent to the calculation unit  3313  in the form of an electric signal. 
     The detection value detected by the photodetector in the control light source  3307  is now corrected by a correction circuit  4301  in the calculation unit  4313  as follows in the same manner as described in Example 16.  FIG. 25B  shows the relationship between the powers of control light before and after correction. 
     The detection value detected by the photodetector in the control light source  3307  is now corrected by a correction circuit  4301  in the calculation unit  3313  as follows in the same manner as described in Example 16.  FIG. 25B  shows the relationship between the powers of control light before and after correction.
 
Pre= Pc ×(1.0106−0.73559 ×Pc ), Pc≧0.005
 
     Pre: Control light power after correction (mW) 
     Pc: Control light power before correction (mW) 
     This function for correction is an example, and various correction functions may be defined as long as they meet the operation requirements of the amplifier during control. Although in this example only correction of the output from the photodetector in the control light source  3307  has been described, it is possible to correct the output from the photodetector  3311  or from both outputs of the photodetector  3311  and control light source  3307 . 
     The power of control light after correction is sent to the calculation circuit  4302 , and the light intensity of the control light source  3307  is adjusted in accordance with this correction. 
     Gain spectrum deviations from the reference gain spectrum observed when the input signal level was varied from −25 to −10 dBm/ch under the gain control of the invention were 0.3 dB or less, and this demonstrates the effectiveness of the present invention. 
     The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.