Abstract:
An optical transceiver that installs an optical modulator with the Mach-Zehnder type and made of primarily semiconductor materials, and an Erbium Doped Fiber Amplifier (fiber amplifier) is disclosed. The fiber amplifier and the MZ modulator, in addition to a wavelength tunable laser diode, an intelligent coherent receiver, and a polarization maintaining splitter, are installed within a compact case following the standard of CFP2. The fiber amplifier provides a wavelength tunable filter that passes light amplified by the fiber amplifier but eliminates amplified stimulated emission in regions out of the wavelength band of the light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 62/281,517, filed on Jan. 21, 2016, and U.S. Provisional Application Ser. No 62/310,282, filed on Mar. 18, 2016; the contents of which are relied upon and incorporated herein by reference in its/their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates to an optical transceiver, in particular, the application relates to an optical transceiver that implements a fiber amplifier and is primarily used in the coherent wavelength division multiplexing (WDM) system. 
       BACKGROUND 
       [0003]    Abrupt increase of information to be transmitted on an optical communication system has requested an optical transceiver operable in further faster speed exceeding 10 Gbps and sometimes reaching 100 Gbps. In addition to the increase of the operation speed, some optical transceivers implement a function of the coherent modulation where an optical signal is modulated in phase of light. Such a transceiver or the optical communication system to modulate the phase of the light are called as the coherent optical transceiver and the coherent optical system. 
         [0004]    In a coherent optical transceiver, an optical modulator types of, what is called, the Mach-Zehnder (MZ) modulator, is inevitable to modulate a phase of light. The MZ modulator is conventionally made of dielectric material, typically a lithium niobate (LiNbO 3 ), because of a large coupling efficiency between electrical properties and optical properties thereof. However, the MZ modulator made of the dielectric material has large dimensions to show an enough interaction, which makes hard to be installed within an optical transceiver with a limited outer dimensions. 
         [0005]    Another type of the MZ modulator primarily made of semiconductor material has been developed. Because of larger refractive index of semiconductor materials compared with those of dielectric materials, the MZ modulator made of semiconductor materials has smaller dimensions so as to be installed within a small sized optical transceiver. However, as a compensation of the smaller dimensions, the MZ modulator of semiconductor materials inevitably or inherently shows a greater optical loss. Accordingly, a means to amplify an optical signal output from the MZ modulator, or entering the MZ modulator, that is, an optical amplifier type of erbium doped fiber amplifier (fiber amplifier), is necessary to be installed within the optical transceiver. 
       SUMMARY 
       [0006]    One aspect of the present application relates to a method of controlling an optical transceiver that implements a wavelength tunable optical source and a fiber amplifier. The fiber amplifier provides an erbium doped fiber (EDF), an amplified stimulated emission (ASE) filter, a variable optical attenuator (VOA), a first monitor photodiode (mPD), and a second mPD. The first mPD senses an output of the VOA, while, the second mPD senses an output of the ASE filter. The ASE filter has a tunable wavelength passing band but filters out amplified stimulated emissions out of the tunable wavelength passing band. The method of the present application comprises steps of: (a) tuning the tunable wavelength passing band of the ASE filter to a preset wavelength of the wavelength tunable optical source by sensing the output of the ASE filter with the second mPD; (b) adjusting output power of the fiber amplifier to a preset power by sensing the output of the VOA with the first mPD; and (3) iterating the steps of tuning the tunable wavelength passing band of the ASE filter and adjusting the output power of the fiber amplifier so that the fiber amplifier emits light with the preset power at the preset wavelength. 
         [0007]    The second aspect of the present application relates to an optical transceiver that communicates with a host system. The optical transceiver of the preset application includes a wavelength tunable optical source and a fiber amplifier, where the wavelength tunable optical source emits light with a preset wavelength that set by a command provided from the host system. The fiber amplifier includes an EDF, a pumping source, an ASE filter and a VOA. The pumping source provides a pump beam to the EDF by which the EDF amplifies the light provided from the wavelength tunable optical source. The ASE filter, which is implemented downstream of the EDF, has a tunable wavelength passing band that passes the light amplified by the EDF but filters out amplified stimulated emissions generated by the EDF and having wavelengths out of the tunable wavelength passing band. The VOA, which is implemented downstream of the ASE filter, sets output power of the fiber amplifier in preset power by attenuating the amplified light received from the EDF and passing the ASE filter. 
         [0008]    The third aspect of the present application also relates to an optical transceiver that communicates with a host system. The optical transceiver includes a wavelength tunable optical source and a fiber amplifier. The wavelength tunable optical source emits light with a preset wavelength set by a command provided from the host system. The fiber amplifier includes an EDF, a pumping source, an ASE filter, an mPD, and a controller. The pumping source provides a pump beam to the EDF by which the EDF amplifies the light provided from the wavelength tunable optical source. The ASE filter, which is implemented downstream of the EDF, has a tunable wavelength passing band for passing the light amplified by the EDF but filters out amplified stimulated emissions having wavelengths out of the tunable wavelength passing band. The optical transceiver of the present aspect has a feature that the mPD, the pumping source, and the controller form a feedback loop that adjusts the output of the fiber amplifier to preset power, and the mPD, the ASE filter, and the controller form another feedback loop that tunes the tunable wavelength passing band of the ASE filter to the preset wavelength of the wavelength tunable optical source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
           [0010]      FIG. 1  shows an outer appearance of an optical transceiver according to an embodiment of the present invention; 
           [0011]      FIG. 2A  shows a block diagram of optical components implemented in the optical transceiver shown in  FIG. 1 , and  FIG. 2B  shows a block diagram of a fiber amplifier in the optical transceiver; 
           [0012]      FIG. 3  shows optical spectra of optical signals output from the fiber amplifier; 
           [0013]      FIG. 4  shows an optical spectrum of the optical signal output from the fiber amplifier and a method of aligning the center of the wavelength passing band of the ASE filter implemented in the fiber amplifier with the peak wavelength of the optical signal; 
           [0014]      FIG. 5  shows a block diagram of a fiber amplifier according to the first embodiment of the present application; 
           [0015]      FIG. 6  is a flow chart for controlling the fiber amplifier shown in  FIG. 5  in an initializing routine; 
           [0016]      FIG. 7  is a flow chart for controlling the fiber amplifier shown in  FIG. 5  in an ordinary routine; 
           [0017]      FIG. 8  shows a block diagram of a fiber amplifier according to the second embodiment of the present application; 
           [0018]      FIG. 9A  is a flow chart for controlling the fiber amplifier shown in  FIG. 8  in the initializing routine, and  FIG. 9B  is a flow chart for controlling the fiber amplifier shown in  FIG. 8  in the ordinary routine; 
           [0019]      FIG. 10  is another flow chart for controlling the fiber amplifier modified from the algorithm shown in  FIG. 6 ; 
           [0020]      FIG. 11  shows a block diagram of a fiber amplifier according to a modification of the second embodiment shown in  FIG. 8 ; 
           [0021]      FIG. 12  shows a block diagram of a fiber amplifier according to the third embodiment of the present application; and 
           [0022]      FIG. 13  shows a typical behaviors of the output power of the fiber amplifier of the modified arrangement of the second embodiment shown in  FIG. 11  and the output of the monitor photodiode during the initializing routine for setting the center of the passing band of the ASE filter and the output power of the fiber amplifier. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Next, some examples of an optical transceiver according to the present application will be described in detail as referring to accompanied drawings. In the description of the drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without duplicating explanations. 
         [0024]      FIG. 1  shows an outer appearance of an optical transceiver of the present application, where the optical transceiver  1  of the present application follows the standard of the centum form factor pluggable  2  (CFP 2 ), which is one of multi-source agreements (MSA) defining electrical/optical specifications and physical dimensions of an optical transceiver widely used in the field of the optical communication system. The optical transceiver  1  has a housing  10  whose dimensions follow the CFP 2  standard; that is, the housing  10  has dimensions of 91.5 mm in length, 41.5 mm in width, and 12.4 mm in height, respectively. As shown in  FIG. 1 , the housing  10  of the present embodiment comprises a top housing  12  or a top cover, a frame  14 , and a bottom housing  16  or a bottom cover. The top housing  12  and the bottom housing  16  sandwich the frame  14 , which forms an inner space for enclosing optical and electrical components therein. The front wall  14   a  of the frame  14  provides an optical receptacle  18  of a type of LC (Lucent Connector) receptacle. The optical receptacle  18  provides two optical ports, one of which is for the optical transmission and the other is for the optical reception. Accordingly, the optical transceiver  1  may be operable in the full-duplex optical communication. Moreover, as described below, the optical transceiver  1  may be operable for the modulation system using the DP-QPSK (Dual Polarization Quadrature Phase Shift Keying) algorithm, where an optical signal entering the optical transceiver  1  has four degrees of the multiplicity, namely, two multiplicities in the phase of the optical signal and two multiplicities in the polarization. 
         [0025]    Respective sides of the front wall  14   a  provide a mechanism including a bail  18   a  and the slider  18   b  that is slidable rearward and forward working with the rotation of the bail  18   a.  Thus, the optical transceiver  1  may be plugged with or released from the host system. Although not explicitly illustrated in  FIG. 1 , the optical transceiver  1  provides an electrical plug in a rear end thereof. The electrical plug is to be mated with an electrical connector provided in the host system, which establishes the electrical communication with respect to the host system. In the present specification, the term “front” and/or “forward” is assumed to be a direction where the optical receptacle  18  is provided. On the other hand, the term “rear” and/or “back” corresponds to an opposite direction where the electrical plug is provided. But those notations could not affect the scope of the present invention. 
         [0026]      FIG. 2A  schematically illustrates an optical coupling system in the optical transceiver  1  shown in  FIG. 1 . The optical transceiver  1  installs, in the inner space, a wavelength tunable optical source  20 , an optical modulator  40 , an optical receiver  50 , and a fiber amplifier  60 . The wavelength tunable optical source  20  may be a wavelength tunable laser diode (t-LD) that emits continuous wave (CW) light as a local beam, where the CW light whose wavelength is tunable depending on biases supplied thereto. The local beam is split by a polarization maintaining splitter (PMS)  30  into two beams, one of which is provided to the optical modulator  40  as the CW beam to be modulated; while, the other is provided to the optical receiver  50  as a local beam to be interfered with an optical signal entering the optical transceiver  1 . The PMS  30  is optically coupled with the t-LD  20  by a polarization maintaining fibers (PMF),  20   a  and  30   a,  as interposing the first polarization maintaining connector (PMC)  70   a.  The PMS  30  is coupled with the optical modulator  40  by the PMFs,  30   b  and  40   a,  as interposing the second PMC  70   b;  also coupled with the optical receiver  50  by the PMFs,  30   c  and  50   a,  as interposing the third PMC  70   c.  Thus, the polarization of the light output from the t-LD  20  may be maintained to the optical modulator  40  and the optical receiver  50 . 
         [0027]    The optical receiver  50  is coupled with the PMS  30  through the PMFs,  30   c  and  50   a,  as interposing the third PMC  70   c.  The optical receiver  50  receives the incoming optical signal, which contains a plurality of extractable optical signals depending on the phases and the polarizations thereof, through a single mode fiber (SMF)  50   b  from the input port IN of the optical transceiver  1 . 
         [0028]    The optical modulator  40  is disposed between two PMCs,  70   b  and  70   d.  The former PMC  70   b  couples with the optical modulator  40  through the PMF  40   a,  while, the latter PMC  70   d  couples with the optical modulator  40  by an SMF  40   b.  In  FIG. 2A , bold lines denote the SMFs, while, slim lines denote the PMFs. Because the output of the optical modulator  40  is extracted through the SMF  60   b,  the connector  70   d  is unnecessary to be a type of a PMC. However, the optical transceiver  1  of the embodiment uses the PMC  70   d  to couple SMFs,  40   b  and  60   a,  because of the simplicity. The output of the optical modulator  40 , as described above, is provided to the fiber amplifier  60  through the SMF  60   a.    
         [0029]      FIG. 2B  schematically shows a functional block diagram of a fiber amplifier  60  in the optical transceiver  1 . The fiber amplifier  60  receives an optical signal from the optical modulator  40  through the SMFs,  40   b  and  60   a,  and an optical isolator  63 A that prevents light from returning the optical modulator  40 . The optical signal output from the optical modulator  40  is merged with a pump beam, which is generated by a pumping source  62  of a type of an LD through the wavelength selective coupler (WSC)  63 . The optical signal merged with the pump beam enters the erbium doped fiber (EDF)  61  to be optically amplified thereby. The amplified optical signal is provided to the gain flattening filter (GFF)  64 . The fiber amplifier  60  may further provides another optical isolator  63 B downstream of the EDF  61  that prevents an amplified optical beam from returning the EDF  61  as stray light, which becomes optical noises for the EDF  61 . Because the EDF  61  has an optical gain largely depending on wavelengths of the optical signal; the GFF  64  equalizes the gain spectrum of the EDF  61 . The amplified and equalized optical signal is provided to a variable optical attenuator (VOA)  65 . The VOA  65  attenuates the amplified optical signal to an adequate power level by adjustable attenuation. The monitor photodiode (mPD)  66  with an optical tap is put downstream of the VOA  65 . The mPD  66  senses power of the optical beam output from the VOA  65  and keeps the power thereof in a target level defined by the specification of the MSA by controlling the VOA  65 . The optical signal thus amplified, equalized, and variably attenuated to the target level is output from the output port OUT through the SMF  60   b.  In the optical transceiver  1  with the fiber amplifier  60  in the output thereof, the amplified output power of the optical signal may be maintained by a feedback loop of the mPD  66 , a controller  100 , and the VOA  65  or the pumping source  62 , which is often called as an automatic power control (APC). Also, the pumping source  62  accompanies with another mPD  62   a  that senses the optical power output from the pumping source  62 . That is, another APC loop that includes the mPD  62   a,  the controller  100 , and the pumping source  62  may maintain the optical power of the pumping source  62  in a preset power. 
         [0030]    One typical application of the optical transceiver  1  shown in  FIGS. 1, 2A, and 2B  is the coherent optical system, where an optical transceiver receiving an optical signal and outputting another optical signal having a wavelength same with that of the received optical signal. The received optical signal and the transmitted optical signal may have one specific wavelength but multiplexing two or more phases, two or more signal amplitudes, and two polarizations. However, the former two techniques, namely, the multiplicity by the phase and the multiplicity by the amplitude, are currently restricted to the dual multiplicity because of the operating speed of the electrical circuits. Even recent advanced electrical circuits could not follow the speed of light, or the frequency of light. Accordingly, other alternatives that enhance the multiplicity of the transmitted signal have been proposed. That is, optical transceivers each generating an optical signal whose wavelength is specific thereto and different from each other are passively coupled in the optical outputs therefrom by, for instance, optical couplers. The optical signals, which are passively coupled, may be transmitted on a single fiber as the wavelength multiplexed signal. An optical transceiver implemented in such a system receives the WDM signal; and, without de-multiplexing the WDM signal, extracts and recovers data contained in one of the optical signals whose wavelength matches with the wavelength of the transmitted signal that the optical transceiver just transmits. Because an optical transceiver applicable to the coherent system implements an optical phase comparator, which is often called as an optical hybrid, and recovers data contained in an optical signal whose wavelength matches with the wavelength which the t-LD  20  in the optical transceiver  1  just outputs. 
         [0031]    Such an optical transceiver implementing an fiber amplifier and applied in the coherent WDM system is necessary to reduce optical noises not only the transmitted wavelength but whole wavelengths defined in the WDM system.  FIG. 3  explains a reason why the noise reduction becomes a key factor in the coherent WDM system. 
         [0032]    As described above, the fiber amplifier  60  implemented in the output stage of the optical transceiver keeps the optical output thereof in the target power by the APC loop of the mPD  66 , the controller  100 , and the pumping source  62  or the VOA  65 . However, the optical amplification by an EDF inevitably accompanies, what is called, the amplified stimulated emission (ASE).  FIG. 3  shows various output spectra of the fiber amplifier  60  in the optical transceiver  1 . In  FIG. 3 , floor noises or terrace noises around −60 dB in a wavelength range from 1520 nm to 1575 nm are called as the ASE. Moreover, the magnitude of the ASE depends on the wavelength. Specifically, the EDF  61  has the wavelength dependent optical gain; that is, the gain of the EDF  61  reduces in relatively longer wavelengths. Accordingly, even when the fiber amplifier  60  receives an optical signal with preset amplitude, the 
         [0033]    EDF  61  is necessary to be provided with the pumping power dependent on the wavelength of the input optical signal. In particular, when the input optical signal has a relatively longer wavelength, the pumping power becomes greater in order to maintain the optical output power in the preset level, which means that the ASE increases in particular, the ASE in shorter wavelengths becomes larger compared with those in longer wavelengths because of the gain characteristic of the EDF  61 . 
         [0034]    When the optical signal output from one optical transceiver like the present embodiment is wavelength multiplexed with those output from other optical transceivers to realize the coherent WDM system, the respective ASEs are inevitably superposed to each other. Specifically, when the system implements four optical transceivers each generating output signals with wavelengths like those shown in  FIG. 3  and implementing respective fiber amplifiers like the present embodiment accompanying with the ASEs of around −60 dB, total ASEs after passively superposing these four optical signals increases to −54 dB even the ASEs show wavelength independent magnitude. When the ASE becomes larger in shorter wavelengths as shown in  FIG. 3 , the total magnitude of the ASE further degrades. 
         [0035]    The optical transceiver that receives such passively superposed optical signals is necessary to extract one optical signal from the degraded input optical signal, which not only reduces the accuracy of the recovery but also sometimes makes unable to recover. Thus, the optical transceiver like the present embodiment that implements the fiber amplifier in the output stage thereof is inevitable to reduce the ASE. However, the wavelength of the output optical signal of the optical transceiver  1  is optional, namely, depending on the wavelength of the input optical signal, a configuration where the EDF  61  accompanies with a simply band-passing filter (BPF) downstream thereof could not solve the subject described above. A BPF, whose wavelength passing band is necessary to be variable as indicated by a broken line in  FIG. 3 , namely, an optionally selectable wavelength passing band becomes necessary. The peak wavelength of the BPF is necessary to be varied or scanned to align with the wavelength of the output optical signal, which is shown in  FIG. 4 . 
       FIRST EMBODIMENT 
       [0036]      FIG. 5  schematically illustrates a functional block diagram of a fiber amplifier  60 A implemented within an optical transceiver that is applicable to the coherent WDM system. The fiber amplifier  60 A shown in  FIG. 5  provides a wavelength tunable filter, which will be denoted as ASE filter  69 , downstream of the GFF  64  and a second mPD  66 B for sensing the output of the ASE Filter  69 . The second mPD  66 B, the controller  100 , and the ASE Filter  69  constitute a feedback loop that aligns the wavelength passing band of the ASE Filter  69  with the peak wavelength of the optical signal. That is, scanning the wavelength passing band of the ASE Filter  69  as sensing the output power of the ASE Filter  69  by the second mPD  66 B, the wavelength passing band of the ASE Filter  69  may be determined at a wavelength where the output of the second mPD  66 B becomes a maximum. The other feedback loop of the first mPD  66 A, the controller  100 , and the VOA  68  maintains the output power of the optical signal output from the fiber amplifier  60 A in the target power. Also, the third feedback loop of the PD  62   a,  the controller  100 , and the pumping source  62  maintain the pumping power for the EDF  61 . However, the third APC loop may be removed because the output power of the pumping source  62  may be controlled based on the output of the first and the second mPDs,  66 A and  66 B. In the fiber amplifier  60 A shown in  FIG. 5 , because the ASE filter  69  is set downstream of the EDF  61 , the noises due to the amplified stimulated emission whose wavelengths are set outside of the passing band of the ASE filter  69  may be effectively eliminated. 
         [0037]    Various types of the ASE filter  69 , namely, the wavelength tunable filter, attributed to a narrow passing band, have been known in the field. For instance, a Mach-Zander interferometer, which has two arm waveguides with respective lengths different from each other, may show the function of the tunable filter. Adjusting a bias supplied to one of the arm waveguides may tune the passing band thereof. Because the lengths of the arm waveguides are different, optical signals with wavelengths outside of the passing band disappear at the output of the MZ interferometer. A diffraction grating is also known as the type of the wavelength tunable filter having a narrow passing band. Adjusting an incident angle of the optical signal to the diffraction grating, the passing band thereof varies. A physical mask or the like adequately set downstream of the diffraction grating may effectively eliminate optical signals caused by the second or higher order diffraction. An etalon filter is kwon as still another type of a wavelength tunable filter. An etalon filter inherently shows a periodic transmission spectrum whose maximum transmittance may be varied depending on a temperature thereof. Accordingly, adjusting the temperature, the maximum transmittance may be aligned with the emission wavelength of the t-LD  20 . 
         [0038]      FIG. 6  shows a flow chart for controlling the ASE filter  69  and the VOA  68  in an initializing routine. Step S 11  practically provides an optical signal having an optical wavelength to the fiber amplifier  60 A, where the optical signal is output from the t-LD  20  in the optical transceiver  1 . Step S 12  sets the tunable wavelength passing band of the ASE Filter  69  in a preset wavelength. As described later, the preset wavelength is preferable to be set in a shortest wavelength or a longest wavelength in a wavelength range used in the coherent WDM system, because the center wavelength will be swept in subsequent steps. Accordingly, the shortest or the longest wavelength for the preset wavelength may simplify this sweep. Next, step S 13  sets the pumping power of the pumping source  62  in a preset power, which is optional and may be selected such that the EDF  61  shows a practical optical gain but does not show excess ASE noises. Then, the sweep of the tunable wavelength passing band of the ASE Filter  69  begins as sensing the output power of the ASE Filter  69  at steps S 14  to S 16 . The sweep iterates until the sensed power begin to decrease. When the sensed power decreases at step S 16 , then, the tunable wavelength passing band of the ASE filter  69  may be determined at the wavelength set in previous one cycle at step S 17 . 
         [0039]    Next, the process adjusts the output power of the fiber amplifier  60 A in the target power. After setting the tunable wavelength passing band of the ASE filter  69  at step S 17 , the process compares the output power sensed by the mPD  66 A with the target power. When the sensed power is less than the target power, which corresponds to “No” at step S 18 , the controller  100  adjusts the attenuation of the VOA  68  until the output power of the fiber amplifier  60 A becomes the target power. The output power is still less than the target power even when the attenuation of the VOA  68  is set to be 0 dB, the controller  100  increases the pump power by an amount and restarts the initializing control from step S 14 , because the variation of the pump power affects the tunable wavelength passing band of the ASE Filter  69 ; but the sweep of the tunable wavelength passing band may be started from once determined wavelength. 
         [0040]      FIG. 7  shows a flow chart for controlling the output power of the fiber amplifier  60 A concurrently with the tunable wavelength passing band of the ASE Filter  69  during a stable state of the fiber amplifier  60 A. 
         [0041]    The fiber amplifier  60 A first senses the output power by the mPD  66 A at step S 21 . When the output power deviates from the target power, the controller sets new attenuation in the VOA  68  at steps, S 22  and S 23 , which may be derived from a difference between the sensed power and the target power, and by referring to a look-up-table that co-relates the difference of the power to the attenuation. Subsequently, the controller  100  adjusts the tunable wavelength passing band of the ASE Filter  69  when the control mode is in the mode for adjusting the center wavelength. When the operation of the controller  100  is in the mode for correcting the output power, the control iterates the sense of the output power at step S 21 . 
         [0042]    In the mode for adjusting the center wavelength, the controller  100  temporarily acquires the output power p 1  of the ASE Filter  69  of the center wavelength currently set therein at step S 25 , shifts the center wavelength by δw at step S 26 , and senses the output power p 2  of the ASE Filter  69  again at step S 27 . Comparing the former output power p 1  with the latter output power p 2  at step S 28 , the control procedure backs to the start when the latter output power p 2  exceeds the former output power p 1 ; that is, the current tunable wavelength passing band of the ASE Filter  69  becomes closer to the peak wavelength of the optical signal. On the other hand, when the latter output power p 2  becomes less than the former output power p 1 , which means that the current tunable wavelength passing band of the ASE Filter  69  becomes apart from the peak wavelength of the optical signal, the controller  100  shifts the center wavelength by −2.3 w at step S 29 ; that is, the controller  100  shifts the center wavelength by δw toward the direction opposite to the former direction, and the procedure of the control resumes step S 21 . Thus, the procedure shown in  FIG. 7  concurrently matches the center wavelength with the peak wavelength of the optical signal and the output power in the target power. In the procedure thus described, two modes of the adjustment of the center wavelength and the correction of the output power are iterated; that is, the controller  100  stably or continuously iterates only the mode for correcting the output power; and intermittently performs the mode for adjusting the center wavelength. 
         [0043]    The reason why the procedure sequentially carries the control of the output power and the adjustment of the center wavelength is that the tunable wavelength passing band of the ASE Filter  69  or the wavelength at which the output of the mPD  66 B becomes maximum that deviates from the wavelength once set thereat depending on variation of the temperature and so on. Accordingly, the procedures carry out the correction of the output power after the tunable wavelength passing band of the ASE Filter  69  is shifted at steps, S 26  and S 29 , regardless of the state of the ASE Filter  69  whether the tunable wavelength passing band thereof matches with the peak wavelength of the optical signal. 
         [0044]      FIG. 10  shows another flow chart for controlling the output power of the fiber amplifier  60 A at the tuned wavelength, modified from the algorithm shown in  FIG. 6 . In a practical use of the optical transceiver  1  in the WDM system, the aforementioned algorithm for tuning the output wavelength thereof outputs an optical signal with substantial power during the tuning of the ASE filter  69 . That is, the flow chart shown in  FIG. 6  first activates the pump source  62  then the ASE filter  69  is tuned in the tunable wavelength passing band thereof, which probably leaks the pump beam in the output port OUT of the fiber amplifier  60 A even when the tunable wavelength passing band of the ASE filter  69  mismatches with the wavelength of the optical signal coining from the optical modulator  40 , which should be avoided from a view point of increasing optical noises for other optical transceivers whose communication wavelengths match with the passing band of the ASE filter  69 . 
         [0045]    The modified flow chart shown in  FIG. 10  may prevent the fiber amplifier  60 A from leaking the optical signal during the tuning of the ASE filter  60 , which is often called as the dark tuning. Specifically, when the fiber amplifier  60 A receives a command to change the wavelength of the optical source  20  at step S 31 , namely, the wavelength of the optical signal output from the optical modulator  40 , first sets the pumping power completely off and the attenuation of the VOA  68  in maximum to prevent the optical signal from being output from the fiber amplifier  60 A, at step S 32 . Then, the algorithm sets the tunable wavelength passing band of the ASE filter  69  in a preset wavelength corresponding to the changed wavelength, or, in the shortest or the longest wavelength in a wavelength range used in the coherent WDM system, at step S 33 . Subsequently, the pumping power is set in a minimum power by which the tuning of the tunable wavelength passing band of the ASE filter  69  may be carried out, at step S 34 . Because the VOA  68  is set in the maximum attenuation, the pumping beam is unable to be output from the fiber amplifier  60 A even the tunable wavelength passing band of the ASE filter  69  matches with the wavelength of the optical source  20 . 
         [0046]    Then, the algorithm begins the sweep of the tunable wavelength passing band of the ASE filter  69  as sensing the optical power output from the ASE filter  69  by the mPD  66 B. When the sensed power is out of a preset range, the sweep of the tunable wavelength passing band of the ASE filter  69  continues. On the other hand, the sensed power of the mPD  66 B becomes within in the present range, the coarse tuning of the tunable wavelength passing band of the ASE filter  69  at steps S 35  and S 36  is completed, and the algorithm advances the next step S 37 . 
         [0047]    At step S 37 , the pumping power is set in a preset power, which is ordinary greater than the pumping power set in the coarse tuning but the fiber amplifier  60 A still outputs substantially no optical signal because the VOA  68  in the attenuation thereof is set in maximum. The algorithm begins the fine tuning of the tunable wavelength passing band of the ASE filter  69  by the feedback loop of the mPD  66 B, the controller  100 , and the ASE filter  69  such that the sensed power by the mPD  66 B becomes a maximum, at steps S 38  and S 39 . Finally, the algorithm shown in  FIG. 10  sets the output power of the fiber amplifier  60 A by the APC loop of the mPD  66 A, the controller  100 , and the VOA  68 . Details of the fine tuning above described are substantially same with the flow chart shown in  FIG. 7 . 
       SECOND EMBODIMENT 
       [0048]      FIG. 8  schematically shows a functional block diagram of another fiber amplifier  60 B according to the second embodiment of the present invention. The fiber amplifier  60 B shown in  FIG. 8  removes the VOA  68  provided in the former fiber amplifier  60 A. Instead, the fiber amplifier  60 B adjusts the output power thereof by varying the pumping power of the pumping source  62 . That is, the mPD  66 A the controller  100 , and the pumping source  62  constitute the first feedback loop for maintaining the output power in the target power, which is the APC loop; and the other feedback loop of the mPD  66 , the controller  100 , and the ASE filter  69  constitute the second one for adjusting the tunable wavelength passing band of the ASE filter  69 . Because the fiber amplifier  60 B removes the VOA  68 , the fiber amplifier  60 B may be formed in compact. As  FIG. 1  shows the outer appearance of the optical transceiver  1  implementing the fiber amplifier therein, the optical transceiver  1  has an extremely restricted inner space. Accordingly, removal of electrical and/or optical components is strictly preferable. The fiber amplifier  60 B shown in  FIG. 8  without the VOA  68  is installed within the optical transceiver. 
         [0049]    Because the VOA  68  is removed from the fiber amplifier  60 B, the correction of the output power is done by the modified procedures shown in  FIGS. 6 and 7 .  FIG. 9A and 9B  show portions of the procedures for controlling the pumping source  62 . In the initializing routine shown in  FIG. 9A , the pumping power is once set in the preset amount for determining the tunable wavelength passing band of the ASE Filter  69  at steps S 12  and S 13 . After aligning the tunable wavelength passing band of the ASE Filter  69  with the peak wavelength of the optical signal, the pumping power set in the pumping source  62  is adjusted such that the output power of the fiber amplifier  60 B becomes the target power. That is, comparing the current output power of the fiber amplifier  60 B with the target power at step S 18 , the controller  100  calculates revised pumping power next supplied to the pumping source  62  at step S 19   a  when the current output power is different from the target power. Then, the controller  100  sets the revised pumping power in the pumping source  62  then carries out the procedure of step S 18  for comparing two powers again. The steps, S 18  to S 19   b,  are iterated until the two powers become substantially equal to each other. 
         [0050]    On the other hand, in an ordinary operation of the fiber amplifier  60 B, the controller  100  first senses the current output power through the mPD  66 A at step S 21 . Then, the controller  100  calculates a revised pumping power next supplied to the pumping source  62  from a difference between the sensed output power and the target power at step S 22   a.  The controller  100  may access a table that co-relates the difference of the two powers with the pumping power, and sets the revised pumping power in the pumping source  62 . Other procedures under the ordinary operation are same with those shown in  FIG. 7 . Thus, the arrangement shown in  FIG. 8  may show the function same with that realized in the arrangement shown in  FIG. 5  even the arrangement shown in  FIG. 8  removes the VOA  63 . 
         [0051]    Moreover, the arrangement of  FIG. 8  may further remove the PD  62   a  that senses the output of the pumping source  62 . The pumping source  62  in the fiber amplifier  60 B is ordinarily controlled based on the output of the fiber amplifier  60 B not the PD  62   a;  accordingly, the fiber amplifier  60 B may omit the PD  62   a.  In a case where the control through the mPD  66 A takes a time because the control includes the adjustment of the tunable wavelength passing band of the ASE Filter  69 , the PD  62   a  may be effective to realize a local APC loop for the pumping source  62 . 
       MODIFICATION OF SECOND EMBODIMENT 
       [0052]      FIG. 11  schematically illustrates an arrangement of a fiber amplifier  60 C which is modified from the arrangement of the fiber amplifier  60 B of the second embodiment. The fiber amplifier  60 C shown in  FIG. 11  provides the VOA  68  downstream of the ASE filter  69  for enabling the tuning. The tuning of the fiber amplifier  60 C shown in  FIG. 11  becomes complex compared with that carried out in the fiber amplifier  60 A shown in  FIG. 5  because the fiber amplifier  60 C of the present embodiment omits the mPD  66 B that may sense the optical output directly from the ASE filter  69 . The tuning of the center wavelength of the ASE filter  69 , as described above, may be carried out by the feedback loop of the mPD  66 A, the controller  100 , and the ASE filter  69 . 
         [0053]    Accordingly, the tuning of the fiber amplifier  60 C when the optical transceiver  1  changes the wavelength of the optical source  20  is carried out by setting the pumping power in the minimum power during the procedures of the tuning. Referring to the flow chart shown in  FIG. 10 , the pumping power in the coarse tuning is set in the minimum power at which the tuning of the tunable wavelength passing band of the ASE filter  69  is enabled. That is, the minimum pumping power is the minimum power only enabling the tuning of the center wavelength. However, the fiber amplifier  60 A of the first embodiment increases the pumping power to the preset power, which is greater than the minimum power, during the fine tuning because the fiber amplifier  60 A provides the VOA  68  the ASE filter  69  and this VOA  68  effectively and enough attenuates the output of the ASE filter  69 . 
         [0054]    The fiber amplifier  60 C of the present embodiment, because of no mPD  66 B, keeps the pumping power in the minimum power during the fine tuning. After the fine tuning, the output power of the fiber amplifier  60 C is controlled by the APC loop of the mPD  66 A, the controller  100 , and the pumping source  62 . It is preferable that the fine tuning of the center wavelength is carried out after the APC control because the APC control possibly varies the center wavelength of the optical signal just output from the EDF  61 . 
         [0055]    The algorithm to tune the center wavelength in the fiber amplifier  60 C will be further described. First, turning off the pumping source  62  and setting the VOA  68  in the maximum attenuation, the ASE filer  69  is driven by preset conditions. For instance, the ASE filer  69  of a type of etalon filter, heaters implemented therein are provided with predetermined power corresponding to the wavelength to be output from the fiber amplifier  60 C. Concurrently with the initialization of the ASE filter  69 , the t-LD  20  is conditioned so as to emit light with this wavelength. 
         [0056]    After the t-LD  20  stabilizes the emission thereof, in particular, the wavelength thereof, and the ASE filter  69  is also stably operated so as to pass the wavelength of the t-LD  20 , the pumping source  62  is practically activated by providing a preset bias, which may be determined based on the output power of the fiber amplifier  60 C of +5 dBm at the attenuation in the VOA  68  to be 20 dB. Here, the attenuation of the VOA  68  has a value equal to or greater than zero, and a larger value of the attenuation causes a smaller value of power of light output from the VOA  68 . Namely, the VOA  68  with the attenuation of 20 dB, for example, will cause output light with power of −20 dBm from input light with power of 0 dBm. Setting the loop gain of the feedback loop of the VOA  68 , mPD  66 A, and the controller  100  in a maximum, the ASE filter  69  in the pass band thereof may be finely adjusted by sweeping the pass band and sensing the output of the fiber amplifier  60 C by the mPD  66 A. 
         [0057]    After the ASE filter  69  is finely adjusted in the pass band thereof to the wavelength corresponding to the emission wavelength of the t-LD  20 , the closed loop gain of the feedback loop is set in the ordinary gain, namely, decreasing the closed loop gain, and the output power of the fiber amplifier  60 C may be adjusted in the predetermined value by the closed loop of the VOA  68 , the mPD  66 A, and the controller  100 . Thus, the optical output of the fiber amplifier  60 C may be adjusted in both of the wavelength and the power thereof. 
         [0058]      FIG. 13  shows the optical output of the fiber amplifier  60 C in the right axis and the output of the mPD  66 A in the left axis during the initialization of the fiber amplifier  60 C. During the coarse tuning of the ASE filter  69 , the output power of the VOA  68  is kept around −20 dBm under the maximum closed loop gain for setting the wavelength tunable passing band of the ASE filter  69 . After the coarse turning, the closed loop gain of the APC loop formed by the mPD  66 A, the controller  100 , and the VOA  68  is once set in a practical gain that appears a dip in the beginning of the APC loop. Finally, under the practical closed loop gain, the output power of the fiber amplifier  60 C is adjusted in a preset power, which is 0 dBm in the embodiment shown in  FIG. 13 . 
       THIRD EMBODIMENT 
       [0059]      FIG. 12  schematically illustrates still another arrangement of a fiber amplifier  60 D according to the third embodiment of the present invention. The fiber amplifier  60 D of the present embodiment, compared with those aforementioned embodiment, provides the mPD  66 B set just downstream of the ASE filter  69  but omits the other mPD  66 A set in downstream, or in the output of the VOA  68 . Because the fiber amplifier  60 D of the present embodiment provides the mPD  66 B, the dark tuning of the tunable wavelength passing band of the ASE filter  69  may be carried out by the procedures shown in the flow chart of  FIG. 10 , but, the APC loop for setting the output power of the fiber amplifier  60 D in the target power becomes hard because the fiber amplifier  60 D has no mPD in the output thereof. 
         [0060]    The fiber amplifier  60 D, instead of the APC loop using an mPD, carries out the feedforward control for adjusting the output power. That is, the mPD  66 B set in the output of the ASE filter  69  may sense the output power of the ASE filter  69 , which is just the input optical power of the VOA  68 . Accordingly, comparing the sensed output power of the ASE filter  68 , the input power of the VOA  68 , with the target power, the controller  100  may set the attenuation of the VOA  68  by referring to a table stored in the controller  100  or else, where the table co-relates the difference between the input and the output of the VOA  68  to the attenuation to be set in the VOA  68 . Thus, the fiber amplifier  60 D may keep the output power thereof in the target power. 
         [0061]    While particular examples of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. For instance, the fiber amplifier  60 B shown in  FIG. 8  may further provide attenuators with fixed attenuation between the EDF  61  and the GFF  64 , between the GFF  64  and the ASE Filter  69 , and in downstream of the mPD  66 A, in addition to the GFF  64  and the ASE Filter  69 . In order to enhance the OSNR (optical signal to noise ratio) of the optical signal output from the fiber amplifier  60 B; a relatively larger current is preferably provided to the pumping source  62 . An EDF is known to increase the OSNR as increasing the pumping current. In such a case, when the optical input signal output from the optical modulator  40  has substantial intensity, the output power of the EDF  61 , that of the GFF  64 , and that of the ASE Filter  69  sometimes exceed an allowable range. The attenuators with the fixed attenuation may substantially reduce the optical power in respective stages. The APC control by the mPD  66 A, the controller  100 , and the pumping source  62  may perform the fine adjustment of the output power of the fiber amplifier  60 B. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention. 
         [0062]    In the dark tuning, the VOA  68  is first set in the condition where substantially no power is output therefrom, then, the tuning of the ASE filter  69  is carried out so as to the output of the mPD.