Patent Publication Number: US-2013235891-A1

Title: Wavelength monitor, wavelength lockable laser diode and method for locking emission wavelength of laser diode

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wavelength monitor, a wavelength lockable laser diode (hereafter dentoed as LD) implementing with the wavelength locker, and a method for locking the emission wavelength of a tunable LD. 
     2. Related Background Art 
     Kimoto et al. disclosed a semiconductor laser module including a tunable distributed feedback (hereafter denoted as DFB) LD and a wavelength locker in a single package (Furukawa Technical Report 112, July, 2003, pp. 1 to 4). The wavelength locker disclosed therein had two photodiodes (hereafter denoted as PD), one of which detected a portion of back facet light directly from the DFB-LD, while the other of which detected another portion of the back facet light through an etalon filter. The emission wavelength of the DFB-LD may be tuned through these two detections. 
     Recent wavelength division multiplexing (hereafter denoted as WDM) system has ruled a span between nearest two grid wavelengths as 50 GHz within the wavelength region of 192 to 197 THz, which corresponds to the 1550 nm band. In such a system, an optical signal source is required to control the emission wavelength further precisely and stably. The emission wavelength of an LD often fluctuates due to operating temperature and/or a long-term degradation of device performance. The wavelength locker for such an LD is inevitable in the WDM system. 
     Conventional wavelength lockers have been implemented with an optical component having a periodic transmission spectrum against wavelengths. An etalon filter is one of typical components showin such periodic transmission spectrum. The period between the transmission maxima of the etalon filter matches with the span of the grid wavelengths of the WDM system. 
     However, the wavelength locker described above leaves a subject that the locking performance may be available only within a narrow wavelength range. That is, when the emission wavelength of the LD shifts more than one period of the transmission spectrum, the wavelength locker tunes the emission wavelength next to the target wavelength, which is a fatal subject when such a wavelength locker is going to be applied to the recent WDM system. 
     Moreover, recent optical apparatus further requests to make the housing or package thereof as compact as possible. When the wavelength locker is realized by discrete components of the DFB-LD, the etalon filter, and PDs; these devices are arranged independently and coupled with, for instance, a condenser lens to obtain a satisfactory coupling condition, which inevitably enlarges the size of the housing/package. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention relates to a wavelength monitor monolithically integrated with a tunable LD. The wavelength monitor and the tunable LD may form a wavelength lockable LD. The wavelength monitor according to the present invention includes a first optical filter and a second optical filter, each of which may transmit light generated by the tunable LD and have the transmission spectrum periodically varying in a wavelength range attributed to the WDM system. A feature of the wavelength monitor of the present invention is that two transmission spectra have a combination in respective transmittances which is specific to the grid wavelength of the WDM system but different from combinations of respective transmittance at other grid wavelengths. 
     Another aspect of the present invention relates to a wavelength lockable LD that includes a tunable LD, a wavelength monitor monolithically integrated with the tunable LD, and a controller. The tunable LD may emit light with an emission wavelength substantially coincident with a target grid wavelength of the WDM system. The wavelength monitor may include first and second optical filters, each having the transmission spectrum periodically varying in the wavelength range of the WDM system. A feature of the present wavelength lockable LD is that two transmission spectra have a combination of transmittances thereof which is specific to the target grid wavelength but different from combinations in the transmittances at other grid wavelengths. The controller may tune the emission wavelength of the tunable LD such that two optical filters show a combination in respective transmittances equal to the combination at the grid wavelength. 
     Still another aspect of the present invention relates to a method to tune the emission wavelength of the wavelength lockable LD to a target wavelength. The wavelength lockable LD monolithically integrates a tunable LD with a wavelength monitor. The wavelength monitor includes first and second optical filters each having a periodic transmission spectrum whose transmittances at the target wavelength constitutes a combination which is specific to the target wavelength but different from combinations of transmittances at other wavelengths. The method may include steps of: (a) guiding light generated by the tunable LD to the first and second optical filters; (b) detecting respective outputs of the first and second optical filters; (c) tuning the emission wavelength of the tunable LD based on the output of the first optical filter so as to set the output of the first optical filter equal to a first preset transmittance for the transmission spectrum of the first optical filter; and (d) verifying the emission wavelength by comparing the output of the second optical filter with a second preset transmittance for the transmission spectrum of the second optical filter. A feature of the present method is that the first preset transmittance and the second preset transmittance constitute the specific combination of the transmittances at the target grid wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
         FIG. 1  is a plan view showing a wavelength lockable LD according to the first embodiment of the present invention; 
         FIG. 2  magnifies the wavelength monitor shown in  FIG. 1 ; 
         FIG. 3  shows a block diagram of a control circuit to tune the emission wavelength of the wavelength lockable LD shown in  FIG. 1 ; 
         FIG. 4  shows a flow chart to tune the emission wavelength to a target wavelength; 
         FIGS. 5A to 5C  show transmission spectra of the ring filters implemented within the wavelength monitor shown in  FIG. 2 ; 
         FIG. 6  is a plan view showing another wavelength lockable LD according to the second embodiment of the invention; 
         FIG. 7  is a plan view showing still another wavelength lockable LD according to the third embodiment of the invention, where the wavelength lockable LD of the present embodiment implements with Mach-Zender filters in the wavelength monitor; 
         FIG. 8  magnifies the wavelength monitor implemented within the wavelength lockable LD shown in  FIG. 7 ; 
         FIGS. 9A to 9C  show transmission spectra of the Mach-Zender filters implemented within the wavelength monitor shown in  FIG. 8 ; 
         FIG. 10  is a plan view showing still another wavelength lockable LD according to the fourth embodiment of the invention; 
         FIG. 11  shows a cross section of another tunable LD able to be integratable with the wavelength monitor of the present invention; 
         FIG. 12  is a plan view showing still another wavelength lockable LD according to the fifth embodiment of the invention, where the wavelength lockable LD integrates the tunable LD shown in  FIG. 11 ; 
         FIG. 13  shows a block diagram of the circuit to tune the emission wavelength of the wavelength lockable LD shown in  FIG. 12 ; and 
         FIG. 14  shows a cross section of still another tunable LD integratable with the wavelength monitor according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Next, some preferred embodiments according to the present invention will be described in detail. In the description of the drawings, the same numerals or symbols will refer to the same elements if possible without overlapping explanations. Aspect ratio in respective drawings does not always reflect the practical dimensions, and sometimes modified by the explanation sake. 
     First Embodiment 
     A wavelength lockable LD according to the first embodiment of the present invention will be described first.  FIG. 1  is a plan view showing the wavelength lockable LD  1 A according to the first embodiment of the invention. 
     The wavelength lockable LD  1 A includes a tunable LD  3 , a wavelength monitor  5 , a semiconductor optical amplifier (hereafter denoted as SOA)  7 , and an optical modulator  9 . 
     First, the wavelength monitor  5  will be described.  FIG. 2  magnifies a primary portion of the wavelength monitor  5 . The wavelength monitor  5  includes the base PD  60 , the first PD  61 , the second PD  62 , and some waveguides  59  which includes three waveguides,  50 - 52 . The base waveguide  50  has two waveguides,  50   a  and  50   b , putting the base PD  60  therebetween. One of the base waveguides  50   a , which is the front waveguide, optically couples with the active waveguide  311  in the tunable LD  3 ; while, the other waveguide  50   b , the rear waveguide, optically couples with the optical coupler  69 . The optical coupler  69  is a type of the multi-mode interference (MMI) coupler that divides an optical beam incoming from the rear waveguide  50   b  into two beams each propagating in the first and second waveguides,  51  and  52 . 
     The first waveguide  51  includes a linear waveguide  51 L extending horizontally in  FIG. 2  and a ring filter  51 F that comprises a pair of liner waveguides  51 R 1  and a pair of semicircular waveguides  51 R 2 , where these waveguides,  51 R 1  and  51 R 2 , form an oval and one of the liner waveguides  51 R 1  couples with the liner waveguide  51 L. The first waveguide  51  has a periodic transmission spectrum, details of which depend on the optical path length of the first ring filter  51 F. That is, adjusting the refractive index of the ring filter  51 F and the dimensions thereof, the period of the periodic transmission spectrum of the first waveguide  51  may be controlled. 
     The second waveguide  52  also includes another ring filter  52 F comprising a pair of linear waveguides  52 R 1  and a pair of semicircular waveguides  52 R 2 , where those waveguides form another oval. One of the linear waveguides  52 R 1  couples with the linear waveguide  52 L that propagates one of optical beams divided by the MMI coupler  69 . The period of the periodic transmission spectrum of the second waveguide  52  may be adjusted by setting the optical path length of the second ring filter  52 F. In the present embodiment shown in  FIG. 2 , the optical path length of the second ring filter  52 F is slightly different from that of the first ring filter  51 F. Details of the optical path lengths and differences of the periodic transmittance of respective ring filters,  51 F and  52 F, will be described later. 
     The first PD  61  receives light transmitting through the first waveguide  51 , while, the second PD  62  receives light transmitting through the second waveguide  52 . 
     The waveguides  59  in the wavelength monitor  5  propagates a portion of the light generated in the tunable LD  3  to respective PDs,  60  to  62 , and detected thereby. Specifically, the front waveguide  50   a  first transmits a portion of the light generated in the tunable LD  3  to the base PD  60 . The PD may detect a portion of thus transmitted light and pass a rest portion of the light. The rear waveguide  50   b  may carry this rest portion of the light to the MMI coupler  69 . The MMI coupler  69  may divide this rest portion of the light into two parts, one of which propagates on the first waveguide  51 , modulated by the first ring filter  51 F, and detected by the first PD  61 ; while, the other part propagates on the second waveguide  52 , modulated by the second ring filter  52 F, and detected by the second PD  62 . 
     Next, details of the optical modulator  9  will be described. The optical modulator  9  according to the present invention includes a pair of waveguides  91  extending in parallel to each other between two MMI couplers,  81  and  82 , with the arrangement of 2×2. That is, two modulation waveguides  91  are divided by the MMI coupler  81  which optically couples with the active waveguide  311  through the SOA  7 , while, combined by the other MMI coupler  82  which optically couples with the output port  1 P of the wavelength lockable LD  1 A. The later MMI coupler  82  also couples with a surplus waveguide  98 , where it is terminated in a side of the tunable LD  3 . The light, generated in the tunable LD  3  and amplified by the SOA  7 , is divided into two parts by the first MMI coupler  81 . The divided beams each propagates in respective modulation waveguides  91  and merges in the second MMI coupler  82  to output from the port  1 P. During the propagation in respective modulation waveguides  91 , each beam senses the electric field different from the other, which may cause a phase difference between them; the light combined by the second MMI coupler  82  may be modulated. Specifically, applying an electrical modulation signal between two electrodes  95  in the modulation waveguides  91 , the light propagating in one of the modulation waveguides  91  advances or delays the phase thereof by π with respect to the light propagating in the other modulation waveguide  91 ; accordingly, the merged light substantially vanishes when the phase difference between two beams is π but receives no effect when the phase of respective beams coincides to the other. Thus, the light emitted from the tunable LD  3  may be modulated by the modulation signal applied to the electrodes  95 . The electrodes  96  which are also arranged in the modulation waveguides  91  may preset the phase difference between two beams. 
     Next, a method to control the wavelength lockable LD  1 A, specifically, a method to tune the emission wavelength of the tunable LD  3 , according to an embodiment of the present invention, will be described. The emission wavelength of the wavelength lockable LD  1 A may be tuned so as to coincide with one of grid wavelengths of the WDM system. In the explanation below, this one of grid wavelengths is called as the target wavelength. 
       FIG. 3  is a block diagram of the control system for the wavelength lockable LD  1 A, where the control system primarily includes a micro control unit (MCU)  121 . The MCU  121 , by receiving tree outputs each coming from the base PD  60 , and the first and second PDs,  61  and  62 , may control the tunable LD  3 , exactly, the gain region  31  thereof, the thermo-electric controller (hereafter denoted as TEC)  2 , the SOA  7  and the modulator  9  so as to tune the emission wavelength of the tunable LD  3  coincident with the target wavelength. The MCU  121  may output control signals, V 31 , V 7 , V 9  and V 2 , to respective elements. 
     Next, an algorithm to tune the emission wavelength of the wavelength lockable LD  1 A according to the present embodiment will be described.  FIG. 4  is a flow chart of the algorithm to tune the emission wavelength. The method according to the present embodiment includes steps of: 
     (S 1 ) preparing a tunable LD  3  and measuring initial conditions for the tuning;
 
(S 3 ) guiding light generated in the tunable LD  3  to respective PDs,  60  to  62 , and detecting it by respective PDs in the wavelength monitor  5 ;
 
(S 5 ) tuning the emission wavelength based on the outputs of the base and the first PDs,  60  and  61 ;
 
(S 7 ) verifying the emission wavelength to be coincide with the target wavelength based on the base and second PDs,  60  and  62 ; and
 
(S 9 ) when the emission wavelength is different from the target wavelength, tuning the emission wavelength based on the outputs of the base and the second PDs,  60  and  61 , again.
 
     (S 1  Pre-Process) 
     The step S 1  prepares a wavelength lockable LD  1 A and initial operating conditions of the wavelength lockable LD  1 A. Specifically, step S 1  first sets the wavelength lockable LD  1 A in a preset temperature by controlling the TEC  2 ; then monitors the emission wavelength of the LD  3  by an external wavelength detector under a condition where the gain region  31  of the tunable LD  3  is biased by the signal V 31  through the electrode  315  thereof. Next, shifting the temperature of the lockable LD  1 A by controlling the TEC  2  such that the emission wavelength of the LD  3  coincides with one of grid wavelength of WDM communication system, which will be called as the target wavelength, parameters listed below are recorded when the emission wavelength just coincides with the target wavelength. Parameters to be recorded are the transmittance of the first ring filter  51 F, that of the second ring filter  52 F, the temperature of the TEC  2 , the bias V 31  for the gain region  31 , and the bias V 7  for the SOA  7 . Two transmittances may be calculated from the outputs of three PDs,  60  to  61 . The initial conditions described above are measured for respective grid wavelengths of the WDM system, and saved in, for instance, a look-up-table prepared in the MCU  121 . 
     (S 3  Guiding Light) 
     Step S 3  selects one of grid wavelengths, which is the target wavelength, and sets parameters of the temperature of the TEC  2 , the bias V 31  and the bias V 7  in respective devices to generate light by the tunable LD  3 . In this process, the bias V 7  applied to the SOA  7  and the bias V 9  applied to the modulator  9  are preferably set such that the light emitted from the output port  1 P substantially vanishes. 
     A portion of light generated by the tunable LD  3  enters respective PDs,  60  to  62 , through the waveguides  59  and detected thereby. The first PD  61  may detect light filtered by the first ring filter  51 F, while, the second PD  62  may detect light filtered by the second ring filter  52 F. 
     (S 5  First Tuning) 
     Step S 5  tunes the emission wavelength based on the output of the first and second PDs,  61  and  62 . The transmission spectra of the first and second ring filters,  51 F and  52 F, will be explained in advance to describe steps S 5 . 
       FIGS. 5A to 5C  show the transmission spectra of two ring filters,  51 F and  52 F, where the horizontal axis shows the wavelength and the vertical axis corresponds to the transmittance. Bold lines in these figures are the transmission spectrum for the first ring filter  51 F, while, thin lines are those of the second ring filter  52 F. The grid wavelengths of the WDM system, which are denoted by the mark “G” in the figures, are arranged with a constant span of 50 GHz (about 0.4 nm) 
     As explicitly shown in the figures, especially, in  FIGS. 5B and 5C , the period of the transmission spectrum of the first ring filter  51 F is different from the period of the transmission spectrum of the second ring filter  52 F. In the present embodiment, the latter period for the second ring filter  52 F is slightly greater than that of the former period for the first ring filter  51 F. 
     Moreover, the period of the transmission spectrum for the first ring filter  51 F is substantially equal to the span of the grid wavelengths of the WDM system. Accordingly, the transmittance of the first ring filter  51 F at the grid wavelengths, which is denoted by filled circles, becomes substantially constant to be about 0.817. 
     In a wavelength region where the tunable LD  3  may emit light, which is 1520 to 1570 nm in the present embodiment, the first ring filter  51 F and the second ring filter  52 F satisfy the following conditions: the transmission spectrum of the first ring filter  51 F shows N cycles, where N is an integer equal to or greater than 2, and the transmission spectrum of the second ring filter  52 F shows less than N cycles but equal to or greater than N−0.5 cycles, preferably, the second ring filter  52 F shows less than N−0.1 cycles but equal to or greater than N−0.5 cycles. In the present embodiment, N is 125, that is, the transmission spectrum of the first ring filter  51 F shows 125 cycles in the wavelength range of 1520-1570 nm, while, the second ring filter  52 F shows about 124.5 cycles in the same wavelength range, which is just less than that of the first ring filter  51 F. 
     Moreover, as shown in  FIGS. 5A to 5C , the transmittance of the second ring filter  52   f  at the grid wavelengths G, which are illustrated by open squares, monotonically varies from the shortest grid wavelength to the longest grid wavelength, where the transmittance monotonically decreases in the present embodiment, which may give a combination of transmittances which is specific to the target wavelength bur different from combinations in other grid wavelengths. 
     The transmission spectrum of the first ring filter  51 F described above is available in an arrangement thereof where a length of the linear waveguide  51 R 1  is 496 μm, a diameter of the semi-circular waveguide  51 R 1  is 50 μm, and a material thereof has refractive index of 3.83005 at the wavelength of 1520 nm. While, the periodic transmission spectrum for the second ring filter  52 F is available in an arrangement where a length of the linear waveguide  52 R 1  is 495 μm, a diameter of the semi-circular waveguide is 50 μm and a material thereof has the refractive index same with that of the first ring filter  51 F. 
     The first tuning process S 5  tunes the emission wavelength of the tunable LD  3  based on the output of the base PD  60  and that of the first PD  61 . For instance, one case will be explained where the emission wavelength of the tunable LD  3  is put close to or substantially equal to one of grid wavelengths, which is marked by “GP 1 ” in  FIG. 5B . Referring to the lock-up table above described, the MCU  121  adjusts the temperature of the TEC  2  by sending the signal V 2  to the TEC  2  such that the transmittance of the first ring filter  51 F obtained by a ratio of the output of the first PD  61  against the output of the base PD  60  becomes equal to a value at the grid wavelength GP 1 , which is about 0.817 in  FIG. 5B . 
     The process of the first tuning S 5  is thus carried out. However, the transmittance of the first ring filter  51 F shows the periodic characteristic whose period is substantially equal to the span of the grid wavelengths G as described above; accordingly, the MCU  121  may occasionally tune the emission wavelength to another grid wavelength GP 2  shown in  FIG. 5C  when the emission wavelength fluctuates more than one period of the transmission spectrum of the first ring filter  51 F, which is 0.4 nm in the present embodiment. In other words, the first ring filter  51 F may provide a plurality of equivalent wavelengths, and the MCU  121  may operate to set the emission wavelength in one of the equivalent wavelengths, which is sometimes different from the target wavelength GP 1 . 
     (S 7  Verification) 
     The verifying process S 7  may be carried out after the first tuning S 5 . As described above, even if the first tuning S 5  sets the emission wavelength in another grid wavelength different from the target wavelength, the verification process S 7  may detect the fact that the emission wavelength is not the target wavelength by the outputs of the second PD  62  and the base PD  60 . 
     Specifically, the transmittance of the second ring filter  52 F at the grid wavelengths G are different from each other as shown in  FIGS. 5A to 5C  in the wavelength range. Accordingly, the transmittance of the second ring filter  52 F obtained by the second PD  62  and that of the base PD  60  when the emission wavelength is set in another grid wavelength GP 2  is shifted from the transmittance thereof when the emission wavelength is just set in the target wavelength GP 1 . For instance, the transmittance of the second ring filter  52 F, when the emission wavelength is improperly set in the other wavelength GP 2 , becomes about 0.220, while, the transmittance thereof when the emission wavelength is properly set to the target wavelength GP 1  is about 0.595. 
     Accordingly, the MCU  121  may verify, based on the transmittance of the second ring filter  52 F, whether the emission wavelength is properly set to the target wavelength GP 1  among the grid wavelengths G. When the MCU  121  decides that the emission wavelength is properly controlled to the target wavelength GP 1 , the process to control the emission wavelength of the tunable LD  3  is completed. However, the MCU  121  decides that the emission wavelength is improperly controlled; the MCU  121  may proceed to the second tuning S 9 . 
     (S 9  Second Tuning) 
     The second tuning S 9  controls the emission wavelength first by the output of the second PD  62  and that of the base PD  60 ; then by the output of the first PD  61  and that of the base PD  60 . Specifically, the MCU  121  first controls the TEC  2  such that the transmittance of the second ring filter  52 F obtained by the ratio of the output of the second PD  62  against that of the base PD  60  puts close to the value of the transmittance thereof at the target wavelength GP 1 , which is about 0.595 in the present embodiment. After the adjustment above, the emission wavelength is fallen within a wavelength range of one period of the transmission spectrum in which the target wavelength GP 1  belongs. 
     Then, the MCU  121  precisely controls the TEC  2  such that the transmittance of the first ring filter  51 F calculated by the output of the first PD  61  and that of the base PD  60  becomes the value at the target wavelength GP 1 , which is 0.817 in the present embodiment. Because the emission wavelength of the tunable LD  3  is set in the wavelength close to the target wavelength GP 1  in advance to the precise tuning, the MCU  121  may properly tune the emission wavelength of the tunable LD  3  in the target wavelength GP 1 . 
     After the tuning of the emission wavelength, the MCU  121  may further adjust the emission magnitude of the tunable LD  3  so as to be equal to a preset magnitude by control the bias V 7  applied to the SOA  7 . Furthermore, the MCU  121  may adjust the bias V 9  applied to the modulator  9  so as to get a properly modulated light from the modulator  9 . Thus, the light having the target wavelength which is precisely adjusted, the preset magnitude, and the properly modulated may be obtained from the output port  1 P. 
     Second Embodiment 
     Next, the second embodiment of the wavelength lockable LD according to the present invention will be described in detail.  FIG. 6  is a plan view of the wavelength lockable LD  1 B according to the second embodiment of the invention. The wavelength lockable LD  1 B has a different arrangement from those of the first wavelength lockable LD  1 A in viewpoints of the SOA-less tunable LD  3  and a different wavelength monitor  5 B. 
     The wavelength monitor  5 B of the present embodiment exists in downstream of the modulator  9 ; that is, the wavelength monitor  5 B couples in the first waveguide  51  thereof with the end of the active waveguide  311  of the tunable LD  3  through the surplus waveguide  98 , the modulation waveguide  91 , and the MMI coupler  81  of the modulator  9 . The other end of the active waveguide  311  is terminated in the end facet of the wavelength lockable LD  1 B. 
     The wavelength monitor  5 B of the present embodiment also includes the base PD  160 , the first PD  161 , and the second PD  162 . The light entering the waveguide  150  divided by the MMI coupler  82  is partially absorbed by the base PD  160  but a most part of the light passes the base PD  160  and divided into two beams, one of which enters the first ring filter  151 F, while, the other of which enters the second ring filter  152 F. Both light beams entering respective waveguides,  151  and  152 , are terminated after passing the first PD  161  and the second PD  162 . 
     The wavelength monitor  5 B may tune the emission wavelength of the tunable LD  3  even the emission wavelength thereof remarkably shift from the target wavelength by the mechanism same as those of the wavelength monitor  5 A according to the first embodiment. 
     Third Embodiment 
       FIG. 7  is a plan view of a wavelength lockable LD  1 C according to the third embodiment of the present invention. The wavelength lockable LD  1 C includes the tunable LD  3 , the SOA  7 , the modulate  9 , and another type of wavelength monitor  5 C. 
     The wavelength monitor  5 C according to the present embodiment will be described in detail.  FIG. 8  is a plan view of the wavelength monitor  5 C. The wavelength monitor  5 C includes three PDs, namely, the base PD  260 , the first PD  261 , and the second PD  262 , and the waveguides  259  that comprises the primary waveguide  250 , and the first and second waveguides  251  and  252 , each divided from the primary waveguide  250 . The primary waveguide  250  includes the front waveguide  250   b  arranged in upstream of the base PD  260  and the rear waveguide  250   a  disposed in downstream of the base PD  260 . 
     The front waveguide  250   b  optically couples with the active waveguide  311  of the tunable LD  3  as illustrated in  FIG. 7 . The base PD  250 , which is put between the front waveguide  250   b  and the rear waveguide  250   a , may receive raw light of the tunable LD  3 , where the raw light means that the light directly comes from the tunable LD  3  without passing any optical filters. 
     The rear waveguide  250   a  couples with the coupler  269  with the arrangement of 1×2 multi-mode coupler that divides light passing through the base PD  260  into two beams, one of which enters the first waveguide  251 , while, the other of which enters the second waveguide  252 . 
     One feature of the wavelength monitor  5 C according to the present embodiment is that the first waveguide  251  has an arrangement of the Mach-Zender filter  251 F. Specifically, the first waveguide  251  is divided into two waveguides,  251 R 1  and  251 R 2 , then these two waveguides,  251 R 1  and  251 R 2 , merge again to constitute the Mach-Zender filter  251 F. But the optical path length of two waveguides from the branch to the merger is different from the other. 
     The transmission spectrum of the Mach-Zender filter  251 F shows the periodic behavior with respect to the wavelength of the light passing therethrough. A difference of the physical dimensions of two waveguides from the branch to the merger and the refractive index of material constituting two branches may influence the period of the periodic transmission spectrum. 
     The second Mach-Zender filter  252 F has the mechanism same with those of the first Mach-Zender filter  251 F above described. However, the difference of the optical path length between two waveguides from the branch to the merger is different from the other Mach-Zender filter. Specifically, the period of the transmission spectrum of the first Mach-Zender filter  251 F is different from the period of the transmission spectrum of the second Mach-Zender filter  252 F. 
     As illustrated in  FIG. 8 , the first Mach-Zender filter  251 F is put between the base PD  260  and the first PD  261 ; that is, the light output from the base PD  260  and transmitted through the first Mach-Zender filter  251 F enters the first PD  261 . While, the second Mach-Zender filter  252 F is put between the base PD  260  and the second PD  262 . The light coming from the base PD  260  and passing through the second Mach-Zender filter  252 F enters the second PD  262 . 
     The transmission spectra of the Mach-Zender filters,  251 F and  252 F, and an algorithm to decide the current emission wavelength of the tunable LD  3  will be described in detail. 
       FIGS. 9A to 9C  show transmission spectra of respective Mach-Zender filters,  251 F and  252 F, in a wavelength range from 1525 to 1570 nm which is attributed to the WDM system.  FIGS. 9A to 9C  also denote the grid wavelengths G defined in the WDM system, where a span between the nearest grids is set to be 50 GHz, or about 0.4 nm. 
     The transmission spectra,  251 T and  252 T, of the Mach-Zender filters,  251 F and  252 F, are different from those of the ring filters,  51 F and  52 F, shown in  FIGS. 5A to 5C , that is, although the transmission spectra,  251 T and  252 T, show a periodic behavior but the shape thereof is sinusoidal compared with those of the ring filter,  51 F and  52 F. Similar to two ring filters,  51 F and  52 F, the period of the transmission spectra,  251 F and  252 F, are different from the other. 
     The period of the first Mach-Zender filter  51 F is preferably different from the span of the grid wavelengths. Moreover, the period of the second Mach-Zender filter  52 F is preferably different from the span of the grid wavelengths and also from the period of the first Mach-Zender filter  51 F. In  FIGS. 9A to 9C , filled circles each corresponds to the transmittance of the first Mach-Zender filter  251 F at respective grid wavelengths, while, filled squares each corresponds to the transmittance of the second Mach-Zender filter at respective grid wavelengths. 
     The peak wavelengths of the transmission spectrum of the first Mach-Zender filter, and those of the second Mach-Zender filter depend of the refractive index of materials of two waveguides,  251 R 1  and  251 R 2 , and the difference of the optical path length between two waveguides,  251 R 1  and  251 R 2 . The same situation may be applicable to the second Mach-Zender filter  252 F. 
     The MCU  121  first selects one of two Mach-Zender filters,  251 F and  252 F, based on a condition that which slopes in the transmission spectrum becomes abrupt at the target wavelength. Then, the MCU  121  receives the output from the PD that couples with the selected Mach-Zender filter,  251 F or  252 F. 
     In an example, assuming the target wavelength is given by GP 1  in  FIG. 98 , the transmission spectrum of the first Mach-Zender filter  251 T shows a greater slope, or the rate of change in the transmission spectrum for the second Mach-Zender filter  251 F becomes larger compared with that of the other transmission spectrum. Then, the MCU  121  selects the first Mach-Zender filter  251 F and the first PD  261  for the wavelength tuning. 
     In another example, when the target wavelength is given by GP 2 , the rate of change in the first transmission spectrum  251 T for the first Mach-Zender filter  251  becomes substantially zero, while, that of the second Mach-Zender filter  252  becomes substantially maximum. Then the MCU  121  may select the second filter  252 F and second PD  62  for tuning the emission wavelength of the tunable LD  3 . 
     Then, the MCU  121  may adjust the temperature of the tunable LD  3  such that the output of the first PD  261  with respect to the output of the base PD  260  becomes substantially equal to the transmittance of the first Mach-Zender filter  251 F, where it seems to be about 0.266 in the present case. 
     Similarly, when the emission wavelength of the LD  3  is selected to be the other grid wavelength GP 2  in  FIG. 9B , the MCU  121  may control the temperature of the tunable LD  3  such that the transmittance of the second Mach-Zender filter  252 F, which may be obtained from the output of the second PD  262  with respect to the output of the base PD  260 , becomes close to a value of 0.305. 
     After roughly setting the emission wavelength of the tunable LD  3 , the bias V 7  applied to the SOA  7  is set so as to increase the optical absorption by the SOA  7  not to output the light from the output port  1 P. Moreover, when the optical modulator  9  controls the output thereof, the MCU  121  sets the bias V 7  to make the SOA  7  active and the bias V 9  to the modulator  9  to modulate light from the SOA  7 . Thus, the wavelength lockable LD  1 C may output the modulated light from the output port  1 P. 
     The wavelength lockable LD  1 C may monitor the wavelength of the light generated in the tunable LD  3  by the base PD  260 , the first and second PDs,  261  and  262 . The first PD  261  may monitor the light passing through the first Mach-Zender filter  251 F, while, the second PD  262  may monitor the light passing through the second Mach-Zender filter  252 F. The transmission spectra of two Mach-Zender filters,  251 F and  252 F, are different from each other; accordingly, the wavelength monitor  5 C may monitor the wavelength in a wide range. Still further, the transmission spectra of two Mach-Zender filters,  251 F and  252 F, may be set in a combination which is specific to the grid wavelengths within the wavelength range. Therefore, even the Mach-Zender filter shows a periodic transmission spectrum, which means that a plurality of equivalent wavelengths gives the substantially same transmittance, only one wavelength specific to the combination of two transmittance may be selected. 
     Thus, the wavelength lockable LD  1 C may tune the emission wavelength thereof precisely in the preset grid wavelength by the algorithm below. 
     That is, one of two Mach-Zender filters,  251 F and  252 F, is first selected under a condition that the slope of the transmittance thereof against the wavelength is larger in the requested grid wavelength, such as GP 1  and GP 2 , which means that one of the PDs,  261  and  262 , is selected to monitor the wavelength. Next, the emission wavelength of the tunable LD  3  is controlled such that the transmittance of the selected Mach-Zender filter,  251 F or  252 F, calculated by the output of the selected PD,  261  or  262 , and the base PD  260  becomes equal to the designed transmittance of the selected Mach-Zender filter,  251 F or  252 F, by controlling the temperature of the tunable LD  3 . In this process, because the selected Mach-Zender filter,  251 F or  252 F, shows a greater rate of the change in the transmittance compared with that of the unselected Mach-Zender filter,  251 F or  252 F, the precise monitoring of the emission wavelength may be performed, which means that the precise tuning of the emission wavelength may be realized. 
     The wavelength lockable LD  1 C of the present embodiment couples the optical output port  1 P with the end of the active waveguide  311  opposite to the end with which the wavelength monitor  5 C couples. This arrangement enables to tune the emission wavelength of the tunable LD  3  in the desired grid wavelength under a condition where the output port  1 P emits no light. 
     Moreover, the wavelength monitor  5 C of the present embodiment may have two Mach-Zender filter,  251 F and  252 F, showing respective transmittances in a combination thereof specific to the grid wavelength in the preset wavelength range. Accordingly, even the Mach-Zender filter shows the periodic transmission spectrum, which inevitably has a plurality of equivalent wavelengths each showing the same transmittance, the wavelength monitor  5 C of the present embodiment may determine the specific wavelength in the wavelength range. Thus, even the tunable LD  3  shifts the emission wavelength more than one period of the periodic transmission spectrum of the Mach-Zender filter,  251 F or  252 F, the wavelength monitor  5 C may securely detect the shift and the wavelength lockable LD  1 C may recover the target emission wavelength. 
     Fourth Embodiment 
     Next, another wavelength lockable LD  1 D according to the fourth embodiment of the present invention will be described.  FIG. 10  is a plan view of the wavelength lockable LD  1 D of the present embodiment, where the wavelength lockable LD  1 D has substantially same arrangement with those of the previous device  1 C shown in  FIG. 7  except for the arrangement of the wavelength monitor  5 D. 
     The wavelength monitor  5 D of the present embodiment optically couples with the tunable LD  3  in the front end of the active waveguide  311  not in the rear end thereof as those of the previous wavelength monitor  5 C. Specifically, the waveguide  350  of the wavelength monitor  5 D optically couples with the active waveguide  311  through the MMI coupler  82 , the waveguide  91 , another MMI coupler  81 , where they are in the optical modulator  9 , and SOA  7 . The rear end of the active waveguide  311  terminates at the facet perpendicular to the facet,  1 E 1  or  1 E 2 . 
     The wavelength monitor  5 D of the present embodiment has the waveguides,  351  and  352 , extending along the direction of the active waveguide  311 , which is perpendicular to the direction along which the waveguide of the optical modulator  9  extends. Two waveguides,  351  and  352 , optically couple with respective Mach-Zender filters,  351 F and  352 F, and the PDs,  361  and  362 . 
     The wavelength lockable LD  1 D, similar to the previous LD  1 C, may keep the emission wavelength thereof in the target gird wavelength by the mechanism same as those of the previous LD  1 C, even when the LD  1 D shifts the emission wavelength thereof broadly by some reasons. Moreover, because the active waveguide  311  and the waveguides,  351  and  352 , in the wavelength monitor  5 D extend along the direction perpendicular to the direction along which the waveguide  91  of the optical modulator extends, the total length of the wavelength lockable LD  1 D may be decreased, which may facilitate the assembly of the LD  1 D compared with those of the previous embodiment, where the plane size thereof is an extended rectangle. 
     The wavelength monitor of the present invention may have various alternatives. For instance, although the wavelength monitors,  5 A to  5 D, has two waveguides,  50   a  and  50   b , putting the base PD  60  therebetween, the wavelength monitor  5 A may have another arrangement where the waveguide  50  is divided into two waveguides, one of which couples with the base PD  60 , while, the other of which is further divided into two waveguides each coupled with the first and second PDs,  61  and  62 . 
     The wavelength monitor  5 C in the third embodiment couples with the rear end of the active waveguide, that is, optically couples with the end opposite to the one coupling with the SOA  7 , the wavelength monitor  9 , and the output port  1 P. However, the wavelength monitor  5 C may couple with the front end of the active waveguide  311  as those of the fourth embodiment  5 D through the optical modulator  9  and the SOA  7 . On the other hand, the wavelength monitor  5 D of the force embodiment may optically couple with the rear end of the active waveguide  311 . 
     Fifth Embodiment 
     Finally, a detail of the tunable LD  3  will be further described.  FIG. 11  shows a cross section of the tunable LD  3 A along the optical axis thereof according to the present invention, while,  FIG. 12  is a plan view of a wavelength lockable LD  1 E implemented with the tunable LD  3 A. 
     The tunable LD  3 A of the present embodiment includes the gain region  531 , the phase adjusting region  533 , the first and second sampled grating distributed Bragg reflector (hereafter denoted as SG-DBR) regions,  535  and  537 . The gain region  531  has the same arrangement with those  311  in the former embodiments. These four regions,  531  to  537 , are arranged on the common semiconductor substrate  511 , and along the optical axis of the tunable LD  3 A. 
     The gain region  531  has the active waveguide  531   a , the upper cladding layer  512  above the active waveguide  531   a , a contact layer  531   c  above the upper cladding layer  512 , and the anode  531   d  above the contact layer  531   d . The active waveguide  531   a  may be made of material with a longer bandgap wavelength, equivalently, a smaller bandgap energy, and extends along the optical axis with a length of, for instance, 500 μm. The active waveguide  311  may include a lower separate confinement hetero-structure layer (hereafter denoted as SCH layer), an upper SCH layer, and an active layer put between these two SCH layers. The SCH layers may be made of GaInAsP with a bandgap wavelength of 1.25 μm and a thickness of 50 nm. 
     The active layer in the active waveguide  531   a  may have the multiple quantum well (hereafter denoted as MQW) structure including a plurality of well layers made of GaInAsP with a thickness of 5 nm and a plurality of barrier layers made of also GaInAsP with a thickness of 10 nm but having a composition different from the composition of the well layer. The well layers and the barrier layers are alternately stacked to each other. The peak wavelength of the active layer measured by the photoluminescence spectrum is 1550 nm. The total thickness of the active waveguide  531   a  is, for example, about 0.2 μm. 
     The upper cladding layer  512  may be made of p-type semiconductor material, for instance p-type InP, with a thickness of about 1500 nm. The upper cladding layer  512  is common in all regions of the gain region  531 , the phase adjusting region  533 , the first SG-DBR region  535  and the second SG-DBR region  537 . 
     The contact layer  531   c  may be made of p-type semiconductor material such as p-type InGaAs with a thickness of about 200 nm. The anode electrode  531   d  may be made of eutectic metal such as AuZn to make an ohmic contact to the contact layer  531   c . The anode electrode  531   d  and the cathode electrode  515  in the back surface of the substrate  511  may inject carriers into the active waveguide  531   a  which may induce the recombination of electrons and holes in the active waveguide  531   a  and photons are generated therein. The photon may be converted into the laser light by propagating within the active waveguide  531   a.    
     The phase adjusting region  533 , which locates outside of the gain region  531 , exactly between the gain region  531  and the first SG-DBR region  535 . The phase adjusting region  533  includes a portion of the waveguide  535   a , the contact layer  533   c  and the anode electrode  533   d . The waveguide  535   a  may be made of material having a bandgap wavelength shorter than that of the active waveguide  531   a , namely, the bandgap energy of the waveguide in the phase adjusting region  533  is wider than that of the active waveguide  531   a , which means that the waveguide  535   a  is substantially transparent for the light generated in the gain region  531 . The phase adjusting region has a length of about 200 μm along the optical axis, and may be made of un-doped InGaAsP with the photoluminescence peak at 1350 nm and a thickness of about 0.35 μm. 
     The contact layer  533   c  and the anode electrode  533   d  of the phase adjusting region  533  may be made of materials same with those of the gain region,  531   c  and  531   d , and may have a thickness same with those of the gain region, respectively. The phase adjusting region  533  has a function to adjust phase of the light propagating in the waveguide thereof. Specifically, injecting carriers into the waveguide  535   a  from the anode  533   d  and the cathode electrode  515 , the refractive index of the waveguide  535   a  may vary, which also varies the phase propagating therein. Thus, the side mode suppression ratio of the light emitted from the tunable LD  3  may be enhanced. 
     The first SG-DBR region  535  is in an outside of the phase adjusting region  533 ; while, the second SG-DBR region  537  is in an outside of the gain region  531 . Two SG-DBR regions,  535  and  537 , each has a length of bout 600 μm along the optical axis thereof. 
     The first SG-DBR region  535  includes a portion of the waveguide  535   a , the upper cladding layer  512 , the contact layer  535   c  and the anode electrode  535   d , where each of layers is stacked on the substrate  511  in this order; while, the second SG-DBR region  537  includes the waveguide  537   a , the upper cladding Lauer  512 , the contact layer  537   c , and the anode electrode  537   d , where each of layers is also stacked on the substrate  511  in this order. 
     Portions of the upper cladding layer  512  not converted with the contact layer,  531   c  to  537   c , and the anode electrodes,  531   d  to  537   d , are covered with an insulating film made of, for instance, silicon die-oxide (SiO 2 ), which securely isolate respective contact layers,  531   c  to  537   c , and respective anode electrodes,  531   d  to  537   d.    
     The contact layers,  355   c  and  357   c , and the anode electrodes,  355   d  and  357   d , of the first and second SG-DBR regions,  355  and  357 , respectively, may be made of material same with those of the gain region  531 . 
     Furthermore, the first and second SG-DBR regions,  535  and  537 , includes a sampled grating (SG) in an interface between the waveguide,  535   a  or  537   a , and the upper cladding layer  512 , which comprises of a plurality of grating regions and a plurality of space regions alternately arranged to the others along the optical axis. The SG shows a periodic reflectance spectrum. 
     Two SG-DBR regions,  535  and  537 , may tune the emission wavelength of the tunable LD  3 A. Specifically, because two SG-DBR regions,  535  and  537 , forms the optical cavity for the photons generated in the gain region  531 , and the periodic reflectance spectrum of the SG-DBR region,  535  and  537 , depends on the injected carriers; the emission wavelength of the tunable LD  3 A may be tuned by adjusting the bias conditions, namely, injected carriers of two SG-DBR regions,  535  and  537 . Ordinarily, the period of the periodic reflectance spectrum of two SG-DBR regions,  535  and  537 , are set to be slightly different from the others. The laser oscillation may occur at the wavelength where respective reflectance peaks of two SG-DBR regions,  535  and  537 , coincide with others, or the emission wavelength may be tuned by adjusting the injected carriers so as to emission wavelength becomes equal to the target wavelength. 
       FIG. 13  shows a block diagram to control the emission wavelength of the wavelength lockable LD  1 E of the present embodiment, which is modified from those shown in  FIG. 3 . Although previous embodiments concentrate of the tunable LD whose emission wavelength may be tuned by varying the temperature thereof. While, the LD  3 A of the present embodiment may tune the emission wavelength by adjusting the bias conditions; accordingly, the MCU first sets the temperature of the tunable LD  3 A by setting the bias V 2  to the TEC in a preset value and maintains the temperature during the operation. 
     The MCU  121  then sets the biases, V 31  to V 37 , in preset conditions in order to set the emission wavelength roughly coincide with one of the grid wavelength, which is called as the target wavelength. The bias conditions, V 31  to V 37 , are measured in advance to the practical operation of the wavelength lockable LD  1 E and stored in a memory as a look-up-table. At this step, the emission wavelength becomes nearly coincident with the target wavelength but not exactly coincident therewith. The MCU then precisely tunes the emission wavelength by monitoring the output of the first PD 1  and comparing the transmittance of the first ring filter calculated from the monitored output of the first PD 1  with the designed transmittance. The MCU tunes the biases, V 33  to V 37 , such that the transmittance of the ring filter  51 F measured from the output of the first PD 1  becomes coincident with the designed value. 
     The MCU  121  then verifies the emission wavelength based on the output of the second PD 2 , that is, when the transmittance of the second ring filter  52 F also has the periodic spectrum, however, the period thereof is different from the others. Then, transmittances of the second ring filter at the grid wavelengths show a monotonic behavior; accordingly, the MCU  121  may verify whether the emission wavelength is in the target wavelength or not, even the transmittance of the first ring filter obtained from the output of the first PD 1  is in accordance with the designed transmittance. 
     When the emission wavelength is tuned in a wavelength different from the target wavelength, the MCU adjusts the biases, V 33  to V 37 , so as to make the transmittance of the second ring filter  52 F coincident with the designed transmittance. Thus, the emission wavelength of the tunable LD  3 A may be precisely tuned in the target wavelength. After tuning the emission wavelength, the MCU  121  sets other biases, V 7  and V 9 , in respective conditions. The bias V 7  applied to the SOA  7  may adjust the magnitude of the output from the wavelength lockable LD  1 E, while, the condition V 9  to the optical modulator  9  may drive ideally. 
     Sixth Embodiment 
     The present invention does not restrict the tunable LD in an arrangement shown in  FIG. 11 .  FIG. 14  is a cross section of still another tunable LD  3 B, which includes a modified gain region  631  with a structure of the sampled grating distributed feedback (SG-DFB) structure, and a chirped SG-DBR regions  635 . Specifically, The SG-DFB region  631  includes, on the semiconductor substrate  611 , the lower cladding layer  631   e , the active layer  631   a , the upper cladding layer  631   b , the contact layer  631   c  and the anode electrode  631   d . The SG-DFB region  631  further includes a plurality of grating regions  631 A and a plurality of space regions  631 B alternately arranged to each other along the optical axis. One grating region  631 A and one space region  631 B continuous to the grating region  631 A constitutes one segment; and the SG-DGB region includes a plurality of segments. The optical grating in the grating regions  631 A may be made of material different from that of the lower cladding layer  631   e  and buried within the lower cladding layer  631   e . When the lower cladding layer is made of InP, the grating optical grating may be made of, for instance, Ga 0.22 In 0.78 As 0.47 P 0.53 . 
     On the other hand, the CSG-DBR region  635  includes, on the semiconductor substrate  611 , the lower cladding layer  631   e , the waveguide layer  635   a , the upper cladding layer  631   b , the insulating film  635   c , and a plurality of heaters,  635 Ah to  635 Ch. The waveguide layer  635   a  may be made of Ga 0.22 In 0.78 As 0.4 P 0.53 , which has a shorter bandgap wavelength compared with that of the active layer  631   a  in the gain region  631 . That is, the waveguide layer  635   a  in the CSG-DBR region  635  is substantially transparent to the light generated in the gain region  631 . Each of heaters,  635 Ah to  635 Ch, has electrodes,  635 Ae to  635 Ce. The lower cladding layer  631   e  and the upper cladding layer  631   b  of the gain region  631  extend in the CSG-DBR region  635 , namely, these two layers,  631   e  and  631   b , are common in the gain region  631  and the CSG-DBR region  635 . The CSG-DBR region  635  includes three blocks,  635 A to  635 C. Each of blocks has a plurality of segments comprised of a grating region  631 A and the space region  631 B as those in the gain region  631 . A feature of the CSG-DBR region  635  is that at least one block,  635 A to  635 C, has an optical length of the space region different from the optical length of the other blocks, and each of blocks,  635 A to  635 C, accompanies with a monolithic heater,  635 Ah to  635 Ch, in the top of the device. 
     While, as described above; each segment in the gain region  631  has the same optical length. Accordingly, the SG-DFB region  631  may have a plurality of gain peaks with a constant pitch, while, the CSG-DBR region  635  includes three units,  635 A to  635   c , at least one of the units has a specific optical length in the space region  631   b , and the optical length may be varied by the temperature controllable by respective heaters,  635 Ah to  635 Ch. Therefore, the laser oscillation may occur in a strict condition where one of the discrete gain peaks in the SG-DFB region  631  coincides with one of the reflectance peaks in the CSG-DBR region. Moreover, because the reflectance peaks in the CSG-DBR region  635  may be controlled by adjusting the temperature of respective blocks,  635 A to  635 C, one of segments dominates the wavelength tuning, which may not only tune the emission wavelength precisely but widen the tuning range of the emission wavelength. 
     The tunable LD  3 B of the present embodiment may further include an absorption region  639  in a side opposite to the gain region  631  with respect to the CSG-DBR region  635 . That is, the absorption region  639  and the gain region  631  put the CSG-DBR region  635  therebetween. The absorption region  639  includes, on the substrate  611 , the lower cladding layer  631   e , the absorption layer  639   a , the upper cladding layer  631   c , the contact layer  639   c  and the electrode  639   d . The absorption layer  639   a  may be made of material that can absorb the light emitted in the gain region  631 . The absorption layer  639   a  preferably has a bandgap wavelength longer than the emission wavelength of the LD  3 B. Further preferably, the bandgap wavelength of the absorption layer is longer than a longest wavelength at which the tunable LD  3 B may oscillate. Specifically, the absorption layer  639   a  may have the MQW structure constituted by well layers made of Ga 0.47 In 0.53 As with a thickness of 5 nm and barrier layers made of Ga 0.28 In 0.72 As 0.61 P 0.39  with a thickness of 10 nm. The absorption layer  639   a  may be made of a bulk of Ga 0.46 In 0.54 As 0.98 P 0.02 . The embodiment shown in  FIG. 3B  has the absorption layer  639   a  made of the same material with those of the active layer  631   a  of the gain region  631 . The absorption layer may have the function to absorb light generated in the gain region  631  but leaked through the CSG-DBR region  635 . When the leaked light increases, the light reflected at the face of the absorption region  639  backs in the gain region  641 , which becomes an optical noise and degrades the emission characteristic of the tunable LD  3 B. Varying the bias condition applied to the electrode  639   d , the absorption characteristic of the region  639  may be widely changed. When the tunable LD  3 B integrates the absorption region  639 , the wavelength monitor according to the present invention may optically couple with the front side of the gain region  631 . 
     While particular embodiments 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. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.