Abstract:
This invention provides an improved optical amplifier, which comprises a rare earth doped optical fiber for amplifying the optical signal, at least one pump laser for providing a pump signal to the rare earth doped optical fiber, a wavelength division multiplexer for extracting a backward amplified spontaneous emission (ASE) signal in the C-band wavelength (1525-1565 nm) differ from the L-band wavelength (1565-1610 nm) containing the optical signal to be transmitted, a gain controller for controlling a power level of the pump signal based on a received power of the backward ASE signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to optical amplifiers, and more specifically relates to rare earth doped fiber amplifiers for amplifying optical signals by providing a pump signal. 
     2. Description of the Related Art 
     In general, rare earth doped fiber amplifiers such as erbium doped fiber amplifiers have been used for optical communication systems. In erbium doped fiber amplifiers, it is well known that an amplified optical signal can be obtained by providing to the erbium doped fiber a pump signal having a shorter wavelength than that of the optical signal to be transmitted. Furthermore, the pump power of the pump signal can be adjusted automatically by monitoring an amplified spontaneous emission (ASE) signal. However, when monitoring the forward propagating ASE signal, conventional gain control methods require a complicated extracting means, such as Fiber Bragg Gratings (FBG) or Array Waveguide Gratings (AWG) to extract the forward ASE signal from the amplified optical signal. On the other hand, when monitoring the backward propagating ASE signal, since a backward ASE signal with enough power can not be obtained, complicated control techniques are needed to control the pump power based on such weak backward ASE powers. 
     As a result, conventional erbium doped fiber amplifiers each provided with a gain controller for performing the conventional gain control methods mentioned above would become larger and more complicate as the number of channels in a density multiplexed input signal increase. It is thus desired to establish a simple gain controller for controlling the gain of rare earth doped fiber amplifiers. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved optical amplifier with a simple gain controller. 
     It is another object of the present invention to provide an improved rare earth doped fiber amplifier with a gain controller for controlling the gain of a pump signal and a monitor coupler for monitoring an amplified spontaneous emission (ASE) signal. 
     In order to achieve the above object of the present invention, an optical amplifier for amplifying an optical signal, wherein the amplifier comprises: 
     an input port for inputting the optical signal to be amplified, 
     a rare earth doped optical fiber, coupled to said input port, for amplifying the optical signal, 
     a first pump laser, coupled to said rare earth doped optical fiber, for providing a first pump signal to said rare earth doped optical fiber in the direction of the optical signal, 
     a wavelength division multiplexer, coupled between said input port and said rare earth doped optical fiber, for extracting a backward traveling amplified spontaneous emission signal in a wavelength band different from a wavelength band containing the optical signal, 
     a gain controller for controlling a power level of the first pump signal based on a received power of the backward traveling amplified spontaneous emission signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred form of the present invention is illustrated in the accompanying drawings in which: 
     FIG. 1 is a simplified block diagram of an optical amplifier in accordance with the first embodiment of the invention; 
     FIG. 2 is a graphical representation showing the simulated gain spectra for three input optical signals with different total input powers +6, 0, −6 dBm without feedback control operation; 
     FIG. 3 is a graphical representation showing the change in the forward and backward traveling ASE powers without feedback control operation; 
     FIG. 4 is a graphical representation showing the simulated gain variation with feedback control operation in accordance with the first embodiment of the invention; 
     FIG. 5 is a graphical representation showing the simulated gain spectra for three input optical signals with different total input powers +6, 0, −6 dBm with feedback control operation; 
     FIG. 6 is a simplified block diagram of a two stage optical amplifier in accordance with the second embodiment of the invention; 
     FIG. 7 is a graphical representation showing the simulated gain variation with feedback control operation in accordance with the second embodiment of the invention; 
     FIG. 8 is a graphical representation showing the change in the backward traveling ASE power with feedback control operation in accordance with the second embodiment of the invention; 
     FIG. 9 is a simplified block diagram of a two stage optical amplifier in accordance with the third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A. First embodiment of the invention 
     The first embodiment of the present invention will be described below with reference to a preferred embodiment in conjunction with the accompanying drawings. 
     FIG. 1 shows a simplified block diagram of an optical amplifier  10 , which is provided with an input port  11   a , an output port  11 B, a single erbium doped fiber  11 , a forward pump laser  12 , a backward pump laser  13 , optical isolators  14  and  15 , wavelength division multiplexers (WDM)  16  and  17 , a gain controller  18  and a monitoring WDM  19 . 
     Optical signals having a L-band wavelength (1565-1610 nm) are supplied to the input port  11   a  of the single erbium doped fiber  11 , which is doped with erbium and further co-doped with aluminum. For example, a single erbium doped fiber of 28 m is used so as to satisfy the requirement that the amplification level of the optical signals in the L-band wavelength may be approximately 16 dB. The fiber thus shows the characteristic that absorption loss around 1550 nm is 8 dB/m. 
     The optical isolator  14  is connected to the input terminal  11   a  and prohibits the optical signals from back propagating in an opposite direction to the incident optical signals. Similarly, the isolator  15  is connected to the output terminal  11   b  and prohibits the optical signals from back propagating in an opposite direction to the incident optical signals. 
     The forward pump laser  12  comprises, for example, a laser diode operable to change the output power thereof in proportion to a bias current and is coupled between the input port  11   a  and the single erbium doped fiber  11 . The forward pump laser  12  then provides to the single erbium doped fiber  11 , via the WDM  16  in a co-propagating direction with the optical signal, a forward pump signal of 1480 nm whose energy is higher than that of the optical signals in the L-band wavelength. 
     The backward pump laser  13  comprises, for example, a laser diode of a constant output power, and is located between the output port  11   b  and the single erbium doped fiber  11 . The backward pump laser  13  then provides to the single erbium doped fiber  11 , via the WDM  17  in a counter-propagating direction of the optical signals, a backward pump signal of 1480 nm whose energy is higher than that of the optical signal in the L-band wavelength. 
     When providing the forward and backward pump signals from the forward and backward pump lasers  12  and  13 , the single erbium doped fiber  11  is population inverted. As a result of the stimulated emission based on the population inversion, the optical signals supplied to the input port  11   a  can be amplified. The amplified optical signals are thus output to a communication path (not shown) via the output port  11   b.    
     To control the amplification factor of the optical signals in the L-band wavelength, the forward pump laser  12  changes the pump power of the forward pump signal based on the bias current. The bias current is controlled by the gain controller  18  such that the received backward ASE signal power extracted by the monitoring WDM  19  is constant. On the other hand, the backward pump laser  13  outputs a backward pump signal at a constant power. 
     The monitoring WDM  19  is coupled between the WDM  16  and the single erbium doped fiber  11  and extracts the backward traveling ASE signal at a C-band wavelength (1530 nm), traveling in the opposite direction to the optical signals, as a monitor signal. The backward traveling ASE signal extracted by the monitoring WDM  19  is supplied to the gain controller  18 , which comprises a photo-diode  20  and a control element  21 . The photo-diode  20  performs a photoelectric conversion of the backward traveling ASE signal in order to obtain an electrical signal corresponding to the received power of the backward traveling ASE signal. The control element  21  controls the bias current of the forward pump laser  12  so that the received power of the backward traveling ASE signal is fixed at a predetermined power level. 
     FIG. 2 show simulated gain spectra for three total input powers of −6, 0 and +6 dBm with no feedback control operation and assuming eight input channels. The vertical axis indicates gain (dB) and the horizontal axis indicates an input wavelength (nm). The simulation results have been obtained on condition that the forward traveling ASE signal and the backward traveling ASE signal are adjusted to obtain a gain flattened spectra (approximately 16 dB), when the total input power of the input optical signals comprising eight multiplexed input channel is 0 dBm. In FIG. 2, the curve  22  indicates the characteristic gain spectra for an input optical signal with total input power of 0 dBm (that is, 10 mW). Curve  23  indicates the characteristic gain spectra for an input optical signal with total input power of −6 dBm. Curve  24  indicates the gain characteristic for an input optical signal with total input power of +6 dBm. As shown in FIG. 2, the gain of the optical amplifier  10  generally changes in response to the total input power of each input optical signal if the pump power of the forward pump laser  12  is maintained at a constant level. 
     FIG. 3 shows changes in the forward and backward ASE power when the total input power is changed (−6, 0, +6 dBm). In FIG. 3, the curves  25 ,  26  and  27  indicate the ASE power of the backward traveling ASE signal for the three input signals with total input powers of −6, 0 and +6 dBm respectively. Similarly, the curves  28 ,  29  and  30  indicate the ASE power of the forward traveling ASE signal for the three input signals with total input powers of −6, 0 and +6 dBm respectively. As to the backward traveling ASE signal, the ASE power of the backward traveling ASE signal can be detected at relatively high power in the C-band (1525-1565 nm), in contrast to the backward travelling ASE signal in the L-band (1565-1610 nm), regardless of the input power of the input optical signals. In addition, the ASE powers of the backward traveling ASE signal are peaked around 1530 nm. On the other hand, as to the forward traveling ASE signals, the ASE powers of the forward traveling ASE signals can be also detected at relatively high power in the L-band (1565-1610 nm) overlapping with that of the input optical signal, but can not be detected at relatively high power in the C-band (1525-1565 nm) not overlapping with that of input optical signals. Consequently, the backward traveling ASE signal in the C-band is fit to use as a monitor signal in consideration of both the wavelength band and the ASE power. 
     As shown in FIGS. 2 and 3, the ASE power of the backward traveling ASE signals and the gain changes over the whole wavelength band (1520-1600 nm) in response to a change of the total input power of the input optical signals. However, FIG. 4 shows that the gains spectra of the three input optical signal levels can be obtained at an identical level, without regard to the change in the total input power and the component of wavelength, by adjusting the pump power of the forward pump laser  12 , such that the power of the backward traveling C-band ASE signal is constant. In FIG. 4, a black circle mark indicates the relation between the gain and the wavelength for the input optical signal with total input power of 0 dBm. A cross mark indicates the relation between the gain and the wavelength for the input optical signal with total input power of −6 dBm. An “I” shaped mark indicates the relation between the gain and the wavelength for an input optical signal with total input power of +6 dBm. As a result, based on the simulated result shown in FIG. 4, it can be understood that an unchanged gain level of the L-band erbium doped fiber amplifier can be obtained without regard to the changes in the total input powers of the input optical signals. 
     FIG. 5 shows the changes in the forward and backward ASE powers when the pump power of the forward pump laser  12  is controlled such that the ASE power of the backward traveling C-band ASE signal is constant. As indicated by the curve  32 , the three ASE spectra characteristics of the backward traveling ASE signals corresponding to total input powers of −6, 0 and +6 dBm coincide with each other since the ASE power characteristics do not vary in response to the changes in the input powers and the wavelengths. Similarly, the three ASE spectra characteristics of the forward traveling ASE signals corresponding to the input powers of −6, 0 and +6 dBm coincide with each other as indicated by a curve  33 . 
     As mentioned above, since the backward traveling ASE signal in the C-band wavelength (1525-1565 nm) is monitored, the monitoring WDM  19  can detect the backward traveling ASE signal at relatively high power level, without complicated extracting means such as Fiber Bragg Gratings (FBG) or Array Waveguide Gratings (AWG). Further the gain controller  18  can easily adjust the pump power of the forward pump laser  12  so that the monitored backward traveling ASE signal in the C-band can be maintained at a predetermined power level. 
     B. Second embodiment of the invention 
     FIG. 6 shows a schematic block diagram of a two stage optical amplifier  110 , where an optical amplifier  10  shown in FIG.  1  and an optical amplifier  30  are cascaded together. In general, a two stage optical amplifier is used to reduce optical noise figure without reducing the gain thereof. The first stage of the optical amplifier  110  (the optical amplifier  30 ) acts as a pre-amplifier for the second stage (the optical amplifier  10 ). The optical amplifier  110  then comprises the optical amplifier  10  and the optical amplifier  30 . The structure and operation of the optical amplifier  10  are the same as the first embodiment of the invention. The optical amplifier  30  comprises an input port  11   a , a single erbium doped fiber  31 , a forward pump laser  32 , an optical isolator  34 , a wavelength division multiplexer (WDM)  36 , a gain controller  38  and a monitoring WDM  39 . 
     Referring to the two stage optical amplifier  110  in detail. Optical signals in the L-band wavelength (1565-1610 nm) are supplied to the input port  11   a  of the single erbium doped fiber  31  which is doped with erbium and further co-doped with aluminum. For example, for the single erbium doped fiber  31 , a single erbium doped fiber of 7.8 m is used so as to satisfy the requirement that the first EDF  31  is highly inverted and the noise figure of the signal erbium doped fiber  31  is low. The fiber thus shows the characteristic that the absorption loss around 1550 nm is 2.7 dB/m. 
     The optical isolator  34  is connected to the input terminal  11   a  and prohibits the optical signals to be transmitted for propagating in the opposite direction of the input optical signals. 
     The forward pump laser  32  is provided with, for example, a laser diode operable to change the pump power thereof in proportion to a bias current. The forward pump laser  32  is located between the input port  11   a  and the single erbium doped fiber  31 . The forward pump laser  32  then provides, to the single erbium doped fiber  11  via the WDM  16 , a pump signal of 980 nm whose energy is higher than that of the input optical signals in the L-band wavelength (1565-1610 nm). 
     When providing the forward pump signal from the forward pump laser  32 , the single erbium doped fiber  31  is population inverted. As a result of the stimulated emission based on the population inversion, the optical signals supplied to the input port  11   a  can be amplified. The amplified optical signals are thus output to the optical amplifier  10 . 
     Referring to the feedback control operation of the optical amplifier  30 . To control an amplification factor of the optical signals to be amplified, the forward pump laser  32  is provided with a laser diode that is operable to change pump power thereof in response to a bias current provided by the controller  38 . The gain controller  38  then controls the bias current based on an ASE power of a backward traveling ASE signal in the C-band wavelength (1525-1565 nm). 
     To monitor the backward traveling ASE signal, the monitoring WDM  39  is coupled between the WDM  36  and the single erbium doped fiber  31  and extracts the backward traveling ASE signal in the C-band wavelength (1530 nm) traveling in opposite direction to the input optical signals. The backward traveling ASE signal is supplied to the controller  38  which comprises a photo-diode  20  and a control element  21 . The photo-diode  20  performs a photoelectric conversion of the backward traveling ASE signal in order to obtain an electrical signal corresponding to the received power of the backward traveling ASE signal. The control element  21  controls the bias current of the forward pump laser  12  so that the received power of the backward traveling ASE signal becomes a predetermined power level. 
     Although the input optical signal amplified by the single erbium doped fiber  31  is supplied to the optical fiber  10 , the explanation of the optical amplifier  10  is omitted here since it is the same as the first embodiment. 
     The amplitude characteristic of the two stage optical amplifier  110  comprising the optical amplifiers  10  and  30  is almost the same as that of the first embodiment as shown in FIGS. 7 and 8. FIG. 7 shows simulated gain spectra for three total input powers of −6, 0 and +6 dBm with no feedback control operation and assuming eight input channels. In FIG. 7, curve  22  indicates the gain variation of the input optical signal with total input power of 0 dBm (that is, 10 mW). Curve  23  indicates the gain variation of the input optical signal when the total input power is −6 dBm. Curve  24  indicates the gain variation of the input optical signal when the total input power is +6 dBm. Consequently, it can be seen that the gain of the two stage optical amplifier  110  changes with the component of wavelengths when the feedback control operation is not performed and the total input power of the input optical signals change as in the first embodiment. In addition, a curve  31  overlapping with the curve  22  indicates the gain variations of the above-mentioned three input optical signal levels, with different total input powers, when the forward pump power of the forward pump lasers  12  and  32  are adjusted such that the ASE power of the backward traveling ASE signals is constant. Accordingly, it can be seen that constant gain can be obtained by controlling the ASE powers of the backward traveling ASE signals without regard to the wavelengths of the input optical signals and the change in the total input power of the input optical signal. 
     In FIG. 8, curves  25 ,  26  and  27  indicate the ASE power variation of the backward traveling ASE signals corresponding to the input optical signals with different input powers of +6, 0 and −6 dBm. Furthermore, a curve  32  almost overlapping to the curve  25  indicates an ASE power variation of a backward traveling ASE signal when the pump power of the forward pump lasers  12  and the forward pump laser  32  are controlled such that the backward ASE powers of the backward traveling C-band ASE signal is constant. Accordingly, the two stage optical amplifier  110  can amplify the input optical signals at a constant gain without changing the gain response of the amplifier with respect to the wavelength, by the feedback gain control operation for maintaining the ASE power of the backward traveling ASE signals at the predetermined power level. In addition, the optical noise figure (NF total ) is given by the following equation. 
     
       
         NF total =NF 1 +(1/G 1 ) NF 2    (1)  
       
     
     Here, NF 1  is an optical noise figure of the optical amplifier  30 , NF 2  is an optical noise figure of the optical amplifier  10 , and G 1  is the gain of the optical amplifier  30 . 
     For example, assuming that the optical amplifier  30  having a noise figure NF of 4 dB and a gain of 10 dB is used, and the optical amplifier  10  having a noise figure NF 2  of 8 dB is used. The two-stage optical amplifier  110  comprising the optical amplifiers  10  and  30  can realize the low noise figure of 5 dB. 
     C. Third embodiment of the invention 
     In the first and second embodiments, the monitoring WDM  19  or  39  has been used as an extracting means to extract the backward traveling ASE signals from the optical fiber  11  or  31 . However, as shown in FIG. 9, it is possible to alter the monitoring WDM  19  to the combination of a circulator  19   a  and a filter  19   b . The circulator  19   a  leads to the controller  18  optical components (including the backward traveling ASE signal) that propagate in the opposite direction of the input optical signals. The filter  19   b  then passes the desired wavelength band including the backward traveling ASE signal and rejects the undesired wavelength band including the input optical signals. 
     Although the input optical signals are in the L-band wavelength (1570-1600 nm) and the monitor signal is in the C-band wavelength (1530-1560 nm), the combination of two types of wavelengths can be changed suitably. 
     The backward pump laser  13  is not always required to the optical amplifiers  10 ,  110  and  120 . 
     Referring to the feedback control operation of the optical amplifier  110 , since the gain characteristics of a two stage optical amplifier  110  are determined principally by the second optical amplifier  10 , pump power control of the first stage amplifier  30  is not always required. In this instance, the forward pump laser  32  then provides a constant pump signal to the single erbium doped fiber, at a predetermined constant power. 
     The 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 the invention, therefore, as defined in the appended claims is intended to cover all such changes and modifications as fall within the true spirit of the invention.