Patent Publication Number: US-2017353001-A1

Title: Tunable laser

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-109903, filed on Jun. 1, 2016, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a tunable laser and a small-size wavelength locker in a tunable laser used as a light source for optical communications. 
     BACKGROUND 
     In recent years, mainly a tunable laser has been used as a light source of an optical communication system using wavelength multiplexing. In the tunable laser, a wavelength locker for precisely controlling the oscillation wavelength of the tunable laser is used. 
       FIG. 13  is a conceptual configuration diagram of a related-art wavelength locker. As illustrated in  FIG. 13 , the wavelength locker includes a beam splitter  202  that splits part of an output light beam of a tunable laser  201  and a beam splitter  203  that causes the split light beam to be further split into two light beams. Furthermore, the wavelength locker includes a photodiode  206  for monitoring the light intensity of one of the light beams split by the beam splitter  203  and a photodiode  205  that monitors the transmitted light intensity after passing through a periodic filter, typically an etalon  204 , regarding the other of the light beams split by the beam splitter  203 . 
     The ratio of the monitored values of an output S PD1  of the optical detector  206  and an output S PD2  of the optical detector  205  (S PD1 /S PD2 ) represents the transmittance of the etalon  204  at the wavelength of the output light of the tunable laser  201 . Therefore, it becomes possible to cause the oscillation wavelength of the tunable laser  201  to match a desired wavelength by obtaining the transmittance of the etalon  204  at the desired wavelength in advance and carrying out feedback control to cause S PD1 /S PD2  to correspond with the transmittance of the etalon  204  at the desired wavelength. 
     In the related-art wavelength multiplexing communication system, the wavelength of the tunable laser is used while being fixed to a wavelength grid with substantially equal interval defined in advance, for example, a grid with a 50-GHz interval defined in the international telecommunication union telecommunication standardization sector (ITU-T). In this case, as illustrated in  FIG. 14 , the period (free spectrum range (FSR)) of the etalon used for the wavelength locker is set to 50 GHz and the peak wavelength positions of the transmission spectrum of the etalon are set in such a manner that the ITU-T grid wavelengths correspond with vicinities of intermediate points between the peak and bottom of the transmission spectrum of the etalon. This may enhance the efficiency of change in the transmittance (=S PD1 /S PD2 ) of the etalon with respect to wavelength change and may cause the oscillation wavelength of the tunable laser to precisely match the grid wavelength. 
     Conversely, if the grid wavelengths correspond with the peaks or bottoms of the transmission spectrum of the etalon, the change in S PD1 /S PD2  with respect to the wavelength becomes small. Thus, it is preferable to avoid the corresponding of the grid wavelengths with the bottoms or peaks. 
     As described above, in the wavelength locker, it is preferable to shift the grid wavelengths from the peak or bottom wavelengths of the etalon inside the wavelength locker by causing the FSR of the etalon to precisely match the grid interval and precisely adjusting the peak wavelength positions of the transmission spectrum of the etalon. This matching and adjustment may be implemented by precisely adjusting the thickness of the etalon, the angle of incidence of laser light to the etalon, and the temperature of the etalon. However, there is a problem that the adjustment takes high cost regarding each parameter. 
     Moreover, studies are being made on introduction of a flexible grid system based on the supposition that the grid interval is arbitrarily changed in the future. In this system, as illustrated in  FIG. 15 , the minimum grid interval is 6.25 GHz and it is conceivable that the grid interval is shorter than the FSR of the etalon. Thus, it becomes difficult to completely avoid the corresponding of the grid wavelengths with the peaks or bottoms of the etalon even when various kinds of adjustment of the etalon like the above-described ones are carried out. 
     Therefore, as a technique for avoiding the corresponding with the peak wavelength or bottom wavelength of the etalon with any wavelength, a wavelength locker using two etalons has been proposed.  FIG. 16  is a conceptual configuration diagram of a related-art improved wavelength locker. The wavelength locker is obtained by adding a beam splitter  207 , an etalon  208 , and a photodiode  209  to the configuration illustrated in  FIG. 13 . 
     In this case, the ratio S PD1 /S PD3  of the output S PD1  of the optical detector  206  and an output S PD3  of the optical detector  209  is the monitored value of the transmittance of the etalon  204 , and the ratio S PD2 /S PD3  of the output S PD2  of the optical detector  205  and the output S PD3  of the optical detector  209  is the monitored value of the transmittance of the etalon  208 . In this case, as illustrated in  FIG. 17 , the FSRs of the two etalons  204  and  208  are identical to each other and are both 50 GHz, for example. In addition, the peak wavelengths of the transmission spectra of the etalons  204  and  208  are adjusted to be shifted from each other by ¼ of the FSR, i.e. 12.5 GHz. 
     As above, due to the use of the two etalons  204  and  208 , the peak wavelength or bottom wavelength of one etalon  204  is not the peak wavelength or bottom wavelength in the other etalon  208 . Therefore, by selecting which of the monitored values of the etalons  204  and  208  is to be used according to the target wavelength, it becomes possible to keep each wavelength from overlapping with the peak wavelengths or bottom wavelengths of the two etalons  204  and  208  simultaneously. 
     However, in the related-art method, because the FSRs of the etalon  204  and the etalon  208  are made to precisely correspond with each other and the peak wavelengths of the etalons  204  and  208  are precisely shifted from each other by ¼ of the FSR, the thickness, the angle of incidence, the temperature, and so forth of the two etalons  204  and  208  are precisely adjusted. Therefore, there is a problem that the cost taken for the adjustment increases even compared with the related-art configuration using one etalon, illustrated in  FIG. 13 . 
     Moreover, there is a problem that the size of the wavelength locker becomes larger due to the configuration using the two etalons. As described above, with the configurations of the related-art wavelength lockers, it is difficult to implement a wavelength locker capable of stable wavelength control with respect to an arbitrary wavelength with a small size and at low cost. 
     The followings are reference documents. 
     [Document 1] Japanese Laid-open Patent Publication No. 2015-060961, and 
     [Document 2] Seok Hwan Jeong and Ken Morito,“Compact and wideband optical 90° hybrid based on a one-way tapered MMI coupler”, 2011 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, 6-11 Mar. 2011. 
     SUMMARY 
     According to an aspect of the embodiments, a tunable laser includes a semiconductor optical amplifier, a waveguide wavelength-tunable filter that forms the tunable laser with the semiconductor optical amplifier, an optical splitting mechanism set on a coupling optical waveguide that couples the wavelength-tunable filter and the semiconductor optical amplifier, a first optical splitter of a waveguide type that splits at least part of a light beam split by the optical splitting mechanism into two light beams, a first optical waveguide coupled to one output end of the first optical splitter, a second optical waveguide that is coupled to another output end of the first optical splitter and includes a delay waveguide, a 90° hybrid waveguide that includes two input ports to which an output light beam from the first optical waveguide and an output light beam from the second optical waveguide are input and four output ports that output four output light beams; a first output waveguide and a second output waveguide coupled to two output ports that output at least light beams whose phases are shifted from each other by 90° among the four output ports; a first optical detector that receives an output light beam of the first output waveguide; and a second optical detector that receives an output light beam of the second output waveguide. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual configuration diagram of a tunable laser of an embodiment of the present disclosure; 
         FIG. 2  is an explanatory diagram of a transmission characteristic of a wavelength locker of a tunable laser of the embodiment of the present disclosure; 
         FIG. 3  is a conceptual configuration diagram of a tunable laser of embodiment example 1 of the present disclosure; 
         FIG. 4  is a sectional view of a major part of a wavelength-tunable filter used for a tunable laser of embodiment example 1 of the present disclosure; 
         FIG. 5  is a schematic sectional view of a semiconductor optical amplifier (SOA) used for a tunable laser of embodiment example 1 of the present disclosure; 
         FIG. 6  is an explanatory diagram of a monitored value of an output power of a tunable laser of embodiment example 1 of the present disclosure; 
         FIG. 7  is an explanatory diagram of monitored values of a wavelength locker of a tunable laser of embodiment example 1 of the present disclosure; 
         FIG. 8  is a schematic plan view of a 90° hybrid waveguide in a tunable laser of embodiment example 2 of the present disclosure; 
         FIG. 9  is a conceptual configuration diagram of a tunable laser of embodiment example 3 of the present disclosure; 
         FIG. 10  is a conceptual configuration diagram of a tunable laser of embodiment example 4 of the present disclosure; 
         FIG. 11  is a conceptual configuration diagram of a tunable laser of embodiment example 5 of the present disclosure; 
         FIG. 12  is a conceptual configuration diagram of an optical module of embodiment example 6 of the present disclosure; 
         FIG. 13  is a conceptual configuration diagram of a related-art wavelength locker; 
         FIG. 14  is an explanatory diagram of a relationship between a monitored signal of a wavelength locker in a related-art wavelength locker and grid wavelengths; 
         FIG. 15  is an explanatory diagram of a relationship between a monitored signal of a wavelength locker in a related-art wavelength locker and grid wavelengths in a flexible grid system; 
         FIG. 16  is a conceptual configuration diagram of a related-art improved wavelength locker; and 
         FIG. 17  is an explanatory diagram of a relationship between monitored signals of a wavelength locker in a related-art improved wavelength locker and grid wavelengths. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A tunable laser of an embodiment of the present disclosure will be described with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is a conceptual configuration diagram of a tunable laser of the embodiment of the present disclosure. The tunable laser of the present disclosure includes a semiconductor optical amplifier  20 , a waveguide wavelength-tunable filter  11  that forms the tunable laser with the semiconductor optical amplifier  20 , and a wavelength locker  30 . An optical splitting mechanism  13  that splits part of light in a laser resonator including the wavelength-tunable filter  11  and the semiconductor optical amplifier  20  is provided and at least part of the light split by the optical splitting mechanism  13  is guided to the wavelength locker  30 . It is to be noted that numeral  12  denotes an optical waveguide that couples the wavelength-tunable filter  11  and the semiconductor optical amplifier  20 . 
     The wavelength locker  30  includes a first optical splitter  31  of a waveguide type, a first optical waveguide  32  coupled to one output end of the first optical splitter  31 , a second optical waveguide  33  that is coupled to the other output end of the first optical splitter  31  and includes a delay waveguide  34 , and a 90° hybrid waveguide  35  including two input ports and four output ports. The wavelength locker  30  includes a first output waveguide  36   1  and a second output waveguide  36   2  coupled to two output ports that output at least light beams whose phases are shifted from each other by 90° among the four output ports of the 90° hybrid waveguide  35 . The first output waveguide  36   1  and the second output waveguide  36   2  are coupled to a first optical detector  37   1  and a second optical detector  37   2 , respectively. 
     In this case, it is desirable to at least monolithically integrate the wavelength-tunable filter  11 , the optical splitting mechanism  13 , the first optical splitter  31 , the first optical waveguide  32 , the second optical waveguide  33  including the delay waveguide  34 , the 90° hybrid waveguide  35 , the first output waveguide  36   1 , and the second output waveguide  36   2 . 
     For example, the wavelength-tunable filter  11  may be a vernier-type wavelength-tunable filter including three straight-line optical waveguides that are juxtaposed, two ring resonators disposed one by one among the three optical waveguides, and a loop mirror provided at an end part of the optical waveguide remotest from the semiconductor optical amplifier  20  among the three optical waveguides. Alternatively, a vernier-type wavelength-tunable filter including a sampled grating distributed Bragg reflector may be used. The sampled grating distributed Bragg reflector includes two distributed Bragg reflectors whose periods are different from each other. Effects of the present disclosure are similarly achieved with any waveguide wavelength-tunable filter. 
     The 90° hybrid waveguide  35  may be a 4×4 multimode interference waveguide or may be a multimode interference waveguide with a two-stage configuration obtained by coupling four 2×2 multimode interference waveguides. 
     As the optical splitting mechanism  13 , any of a directional coupler, a multimode interferometer, and a Y-branch waveguide may be used. Alternatively, the optical splitting mechanism  13  may be formed of a partial reflection mechanism in which a loop mirror is used for partial reflection and an optical waveguide that propagates a light beam that is not reflected by the partial reflection mechanism. Furthermore, the first optical splitter  31  may be any of a directional coupler, a multimode interferometer, and a Y-branch waveguide. 
     For size reduction, it is desirable to form at least the waveguide wavelength-tunable filter  11 , the first optical waveguide  32 , the second optical waveguide  33 , the delay waveguide  34 , the first output waveguide  36   1 , and the second output waveguide  36   2  by silicon wire waveguides by using a Si waveguide substrate having a silicon on insulator (SOI) structure as a substrate  10 . In this case, it is also possible to mount the semiconductor optical amplifier  20  in a recess part made in the substrate  10 . 
     Moreover, for size reduction, as the first optical detector  37   1  and the second optical detector  37   2 , photodiodes that include a Ge layer and are monolithically integrated on silicon wire waveguides serving as the first output waveguide  36   1  and the second output waveguide  36   2 , respectively, may be used. 
     Alternatively, a compound semiconductor waveguide may be used as the waveguide wavelength-tunable filter  11 . In this case, the wavelength-tunable filter  11  may be monolithically integrated with the semiconductor optical amplifier  20 . Therefore, size reduction of the whole device is possible and an assembly for establishing optical coupling from the tunable laser to the wavelength locker  30  becomes unnecessary. Moreover, the wavelength-tunable filter  11  or the wavelength locker  30  may be formed of a quartz waveguide. 
     Moreover, a second optical splitter of a waveguide type that splits the light beam split by the optical splitting mechanism  13  into two light beams may be further provided at the previous stage of the first optical splitter  31 . Furthermore, a third optical detector that receives a light beam other than the light beam split to the first optical splitter  31  may be provided and a power monitoring mechanism may be added. 
     In this case, a first monitoring mechanism that takes the ratio of monitored values of the first optical detector  37   1  and the third optical detector and a second monitoring mechanism that takes the ratio of monitored values of the second optical detector  37   2  and the third optical detector are provided. To control the wavelength, it is desirable to provide a wavelength control mechanism that controls the oscillation wavelength of the tunable laser in such a manner that the ratio of the monitored value of the first monitoring mechanism and the monitored value of the second monitoring mechanism becomes a prescribed value. As the wavelength control mechanism in this case, a mechanism that causes a current to flow to a heater provided on the waveguide that forms the waveguide wavelength-tunable filter  11  may be used. 
     Alternatively, the power monitoring mechanism may be a mechanism that adds an output light beam from one output port of the 90° hybrid waveguide  35  and an output light beam from the output port at which the phase is shifted from the output light beam from the one output port by 180° among the four output ports of the 90° hybrid waveguide  35 . Alternatively, a power monitoring mechanism that monitors part of an output light beam from the semiconductor optical amplifier  20  may be employed. 
       FIG. 2  is an explanatory diagram of a transmission characteristic of a wavelength locker of a tunable laser of the embodiment of the present disclosure. In the waveguide obtained by combining the delay waveguide  34  and the 90° hybrid waveguide  35 , transmission spectra having a sine wave shape with a period according to the delay amount of the delay waveguide  34  are obtained with respect to the wavelength at the four output ports of the 90° hybrid waveguide  35 . The spectra whose period is the same among the four output ports and whose transmission peak wavelengths are shifted from each other by every ¼ of the period among the four output ports are obtained. 
     The reason why the period is the same among the four output ports is because the same delay waveguide  34  is used. Furthermore, the relationship in which the peak positions are shifted from each other by every ¼ period among the four output ports is a characteristic ensured because the phases at the respective output ports of the 90° hybrid waveguide  35  are shifted from each other by every n/2. Therefore, adjustment to cause the FSRs to correspond with each other, which is carried out in the related-art case using two etalons, illustrated in  FIG. 16 , and adjustment to shift the peak wavelengths from each other by every ¼ period are unnecessary, which may reduce the adjustment cost. 
     It is to be noted that a supposition will be made about the case in which two individual periodic wavelength filters include waveguide filters, for example, the case in which the wavelength filters include two ring resonator waveguides, similarly to the case of using the etalons of the related-art example. In this case, similarly to the case of the etalons of the related-art example, adjustment of the FSRs and peak positions of the two wavelength filters is carried out and it is difficult to automatically obtain the relationship in which the peak positions are shifted by the ¼ period as in the present disclosure. Therefore, adjustment of the peak wavelength positions is carried out and it is difficult to realize the reduction in the cost taken for the adjustment of the peak positions, which is an issue of the related art. 
     Embodiment Example 1 
     Next, a tunable laser of embodiment example 1 of the present disclosure will be described with reference to  FIG. 3  to  FIG. 7 .  FIG. 3  is a conceptual configuration diagram of a tunable laser of embodiment example 1 of the present disclosure. The major part of the tunable laser is formed of a Si waveguide substrate  40  and an SOA  80  including a multi-quantum well (MQW) active layer serving as a gain waveguide. In the Si waveguide substrate  40 , a wavelength-tunable filter  50  and a wavelength locker  70  are provided. It is to be noted that the SOA  80  is mounted in a recess part made in the Si waveguide substrate  40 . 
     The wavelength-tunable filter  50  includes three straight-line optical waveguides  51 ,  53 , and  55  based on Si wire waveguides, a loop mirror  56  as a total reflection mirror, and two ring resonators  52  and  54  different in the radius of curvature for obtaining the Vernier effect of selecting the wavelength. The optical waveguide  51  coupled to the SOA  80  is provided with a directional coupler  61  as an optical splitting mechanism and the directional coupler  61  guides split light to a directional coupler  63  through an optical waveguide  62 . 
     Furthermore, the two ring resonators  52  and  54  are provided with heaters  57  and  58  in order to change the refractive index and shift the resonance wavelength of the ring resonator to carry out wavelength tuning. A phase adjustment heater  59  is provided immediately before the loop mirror  56  of the optical waveguide  55  and these heaters are coupled to a drive electronic circuit separately disposed in the module through the element surface. 
       FIG. 4  is a sectional view of a major part of a wavelength-tunable filter used for a tunable laser of embodiment example 1 of the present disclosure and is illustrated as a sectional view of the optical waveguide  55  here. The Si wire waveguide is formed by utilizing an SOI substrate and is formed by etching a single-crystal Si layer provided over a single-crystal Si substrate  41  with the intermediary of a BOX layer  42  that doubles as a lower clad layer. The Si wire waveguide is formed of a Si core layer whose sectional shape has a width of 500 nm and a thickness of 250 nm and has a shape surrounded by a SiO 2  upper clad layer  43 . Furthermore, the heaters such as the phase adjustment heater  59  are formed by patterning Ti deposited on the SiO 2  upper clad layer  43  and are covered by a SiO 2  protective film  60 . 
     The laser resonator is formed between a cleavage end surface of the SOA  80  and the loop mirror  56  of the wavelength-tunable filter  50 . The ring resonators  52  and  54  have periods of resonance wavelength (FSRs) minutely different from each other, for example, the FSR of one of the two ring resonators  52  and  54  is 5 nm and the other is 5.5 nm. The ring resonators  52  and  54  form a vernier-type wavelength-tunable filter that selects one wavelength based on the overlapping of the resonance wavelengths of the two ring resonators. A tunable laser that carries out laser oscillation at an arbitrary wavelength may be implemented by arbitrarily setting the wavelength at which the resonance wavelengths of the two ring resonators  52  and  54  overlap and making a combination with the SOA  80 . 
       FIG. 5  is a schematic sectional view of an SOA used for a tunable laser of embodiment example 1 of the present disclosure. Over an n-type InP substrate  81 , an n-type InP clad layer  82 , an MQW active layer  83 , a p-type InP clad layer  84 , and a p-type InGaAs contact layer  85  are sequentially deposited. Subsequently, part of the layers from the p-type InGaAs contact layer  85  to the n-type InP clad layer  82  is etched in a stripe manner to form a mesa structure and this stripe-manner mesa structure is buried by a Fe-doped InP buried layer  86 . An n-side electrode  89  is formed on the back surface of the n-type InP substrate  81  and a p-side electrode  88  is provided on the p-type InGaAs contact layer  85  through a stripe-manner opening made in an SiO 2  film  87 . As the MQW active layer  83 , GaInAsP well layers whose thickness of six layers is 5.1 nm and GaInAsP barrier layers whose thickness of seven layers is 10 nm are alternately stacked and formed, for example. 
     The end surface on the side coupled to the optical waveguide  51  is supplied with an anti-reflection coating. At the other end surface, a cleavage surface or a reflective film having certain reflectance is formed. The end surface of the side on which the cleavage surface or the reflective film having certain reflectance is formed functions as a one-side reflective mirror that forms a resonator of a laser with the loop mirror  56 . 
     It is to be noted that, in  FIG. 5 , the stripe-manner mesa structure is formed into a straight line shape. However, the stripe-manner mesa structure may be formed of an inclined waveguide having an angle of 7° with respect to the normal to the end surface, a bent waveguide, and a straight-line waveguide from the side of receiving light of the optical waveguide  51 , and undesired reflection may be reduced. At this time, the end part side of the optical waveguide  51  is also inclined in conformity to the inclined waveguide so that the angle of departure may match the angle of the inclined waveguide. 
     Referring to  FIG. 3  again, one light beam split by the directional coupler  63  is guided to a photodiode  66  via an optical waveguide  64 . The other light beam split by the directional coupler  63  is guided to the wavelength locker  70  via an optical waveguide  65 . 
     The wavelength locker  70  includes a directional coupler  71 , an optical waveguide  72 , an optical waveguide  73  including a delay waveguide  74  in which the delay amount is approximately 1.4 mm, and a 90° hybrid waveguide  75  including a 4×4 multimode interference (MMI) waveguide that couples the optical waveguides  72  and  73  to first and third input ports and includes four output ports. Output waveguides  76   1  to  76   4  are coupled to the respective output ports of the 90° hybrid waveguide  75  and two output waveguides  76   1  and  76   2  that output light beams whose phases are shifted from each other by 90° are guided to photodiodes  77   1  and  77   2 , respectively. It is to be noted that, instead of the directional couplers  61 ,  63 , and  71 , 1×2 MMI waveguides or Y-branch waveguides may be used. 
       FIG. 6  is an explanatory diagram of a monitored value of an output power of a tunable laser of embodiment example 1 of the present disclosure. The photodiode  66  is used as a simple power monitor for directly monitoring part of light split from the inside of the resonator. As illustrated in  FIG. 6 , the output power is almost steady with respect to the wavelength as long as there is no fluctuation due to temperature change or the like. 
       FIG. 7  is an explanatory diagram of monitored values of a wavelength locker of a tunable laser of embodiment example 1 of the present disclosure. Because the photodiodes  77   1  and  77   2  receive light that has passed through the delay waveguide  74  and the 90° hybrid waveguide  75 , transmission characteristics that are periodic with respect to the wavelength are obtained. The period depends on the delay amount of the delay waveguide  74  and is approximately 0.4 nm (=50 GHz). At the first and second output ports of the 90° hybrid waveguide  75 , the light beams incident from the optical waveguides  72  and  73  are coupled with the phases shifted from each other by n/2. This provides the relationship in which the transmission peak wavelengths are shifted from each other by ¼ of the period as illustrated in  FIG. 7 . This makes it possible to realize the relationship in which the periods with respect to the wavelength are the same and the peak wavelengths are shifted by ¼ of the period as two wavelength locker outputs, only by fabrication of Si waveguides without fine adjustment. 
     It is to be noted that, in  FIG. 7 , the monitored values of the two photodiodes  77   1  and  77   2  are divided by the monitored value of the photodiode  66 , which serves as a simple optical output monitor. For example, S PD1 /S PD3  and S PD2 /S PD3  are calculated. This enables conversion into the transmittance of the wavelength locker waveguide similarly to the related-art wavelength locker and makes it possible to control the wavelength without being affected by overall increase and decrease in the intensity of light split into the wavelength locker  70  due to increase and decrease in the laser output power. 
     In embodiment example 1 of the present disclosure, by using the wavelength locker mechanism formed of Si waveguides, it becomes possible to implement two monitors of the wavelength locker having the same period with respect to the wavelength and having peak wavelengths shifted by the ¼ period without carrying out precise adjustment. Therefore, it becomes possible to implement, at low cost, the wavelength locker mechanism for properly selecting the two monitors according to the target wavelength and keeping the target wavelength from corresponding with the peak or bottom of the monitor output. 
     Furthermore, the wavelength locker mechanism of the present disclosure is monolithically integrated with a waveguide wavelength-tunable filter and thus it is also possible to reduce the size compared with the related-art configurations using an etalon or the like. It is to be noted that, in embodiment example 1, the position at which light from the laser resonator is split to the wavelength locker  70  is set near the coupling part with the SOA  80  and light in the direction from the SOA  80  toward the ring resonator  52  is split. However, the position of the splitting does not have to be this position. However, if light is split at this position and with this direction, a more desirable configuration is obtained because there is an advantage that the light may be split from the part at which the light intensity is the highest in the resonator due to optical amplification in the SOA  80  and thus the light may be efficiently supplied to the wavelength locker  70 . 
     Embodiment Example 2 
     Next, a tunable laser of embodiment example 2 of the present disclosure will be described with reference to  FIG. 8 . The tunable laser of embodiment example 2 is obtained by replacing the 90° hybrid waveguide  75  in the tunable laser of embodiment example 1 of the present disclosure illustrated in  FIG. 3  by a different 90° hybrid waveguide  90 . Therefore, only the structure of the 90° hybrid waveguide  90  will be described here. 
       FIG. 8  is a schematic plan view of a 90° hybrid waveguide in a tunable laser of embodiment example 2 of the present disclosure. The 90° hybrid waveguide  90  is obtained by arranging four 2×2 MMI waveguides  91   1  to  91   4  into a two-stage configuration with the intermediary of a 90° phase shifter  92 , and four outputs ch 1  to ch 4  represented in  FIG. 2  are obtained from four output ports  93   1  to  93   4  of two 2×2 MMI waveguides  91   3  and  91   4  at the latter stage. 
     Embodiment Example 3 
     Next, a tunable laser of embodiment example 3 of the present disclosure will be described with reference to  FIG. 9 . The tunable laser of embodiment example 3 is obtained by replacing the photodiodes  66 ,  77   1 , and  77   2  in the tunable laser of embodiment example 1 illustrated in  FIG. 3  by Ge photodiodes  67 ,  78   1 , and  78   2  that are monolithically integrated.  FIG. 9  is a conceptual configuration diagram of a tunable laser of embodiment example 3 of the present disclosure and the basic configuration is similar to the above-described embodiment example 1. 
     In the embodiment example 3, the width of the single crystal silicon layer on the output end side of the optical waveguide  64  and the output waveguides  76   1  and  76   2  formed of Si wire waveguides is extended and a Ge layer is epitaxially grown thereon to form the p-i-n-type Ge photodiodes  67 ,  78   1 , and  78   2 . 
     In embodiment example 3 of the present disclosure, because the photodiodes are also formed on Si waveguides, it becomes possible to further reduce the size of the tunable laser including the wavelength locker. It is to be noted that, also in the embodiment example 3, the 90° hybrid waveguide  90  illustrated in  FIG. 8  may be used. 
     Embodiment Example 4 
     Next, a tunable laser of embodiment example 4 of the present disclosure will be described with reference to  FIG. 10 . The tunable laser of embodiment example 4 is obtained by replacing the wavelength-tunable filter  50  in the tunable laser of embodiment example 1 illustrated in  FIG. 3  by a Y-branch sampled grating distributed Bragg reflector (SG-DBR).  FIG. 10  is a conceptual configuration diagram of a tunable laser of embodiment example 4 of the present disclosure and the basic configuration is similar to the above-described embodiment example 1. 
     In the embodiment example 4, as a wavelength-tunable filter, a Y-branch SG-DBR  100  formed of a branch waveguide including two distributed Bragg reflectors whose periods are different from each other is used. Also in this configuration, the directional coupler  61  is provided close to the SOA  80  on an optical waveguide  101  that couples the Y-branch SG-DBR  100  and the SOA  80 . 
     Similar effects to embodiment example 1 may be expected also in the configuration using the Y-branch SG-DBR as in embodiment example 4 of the present disclosure. It is to be noted that, also in the embodiment example 4, the 90° hybrid waveguide  90  illustrated in  FIG. 8  may be used and the Ge photodiodes  67 ,  78   1 , and  78   2  illustrated in  FIG. 9  may be used. 
     Embodiment Example 5 
     Next, a tunable laser of embodiment example 5 of the present disclosure will be described with reference to  FIG. 11 . The tunable laser of embodiment example 5 is obtained by replacing the loop mirror  56  in the tunable laser of embodiment example 1 illustrated in  FIG. 3  by a partial reflection loop mirror.  FIG. 11  is a conceptual configuration diagram of a tunable laser of embodiment example 5 of the present disclosure and the basic configuration is similar to the above-described embodiment example 1. 
     In the embodiment example 5, a partial reflection loop mirror  102  is used as a loop mirror that forms the wavelength-tunable filter and the placement of the optical waveguides  51 ,  53 , and  55  and the ring resonators  52  and  54  are inverted. Furthermore, the partial reflection loop mirror  102  is provided with an optical waveguide  103 . Here, light that is not reflected by the partial reflection loop mirror  102  and propagates into the optical waveguide  103  is guided to the directional coupler  63 . 
     In embodiment example 5 of the present disclosure, because the wavelength-tunable filter is formed by using the partial reflection loop mirror  102 , one directional coupler ( 61 ) becomes unnecessary. It is to be noted that, also in the embodiment example 5, the 90° hybrid waveguide  90  illustrated in  FIG. 8  may be used and the Ge photodiodes  67 ,  78   1 , and  78   2  illustrated in  FIG. 9  may be used. 
     Embodiment Example 6 
     Next, an optical module of embodiment example 6 of the present disclosure will be described with reference to  FIG. 12 . The optical module of embodiment example 6 is obtained by providing the tunable laser of embodiment example 1 illustrated in  FIG. 3  with a monitoring mechanism and a wavelength control mechanism.  FIG. 12  is a conceptual configuration diagram of an optical module of embodiment example 6 of the present disclosure and the basic configuration is similar to the above-described embodiment example 1. 
     In the optical module of the embodiment example 6, by a monitoring mechanism  110 , the ratio of the monitored values of the photodiode  66  and the photodiode  77   1  (S PD1 /S PD3 ) and the ratio of the monitored values of the photodiode  66  and the photodiode  77   2  (S PD2 /S PD3 ) are calculated. Based on these monitored values, by a wavelength control mechanism  120 , the values of currents to the heaters  57  and  58  on the ring resonators  52  and  54  configuring the wavelength-tunable filter  50  and the phase adjustment heater  59  are controlled to control the resonance wavelengths of the ring resonators  52  and  54 . 
     Conversion into the transmittance of the wavelength locker is enabled by taking the ratios of the monitored values in this manner, and laser oscillation with a desired wavelength is enabled by controlling the oscillation wavelength in such a manner that these transmittances become prescribed steady values. It is to be noted that, which monitored value ratio of S PD1 /S PD3  and S PD2 /S PD3  is to be employed is selected at each wavelength grid as the target wavelength. In this case, the wavelength dependence of the monitored value ratios of S PD2 /S PD3  and S PD2 /S PD3  is obtained in advance and, based on the result, the monitored value ratio with which the target wavelength does not correspond with the peak or bottom wavelength is selected. Due to this, with any wavelength, wavelength control is allowed in the state in which the target wavelength does not correspond with the peak or bottom of the monitored value ratio. Thus, stable wavelength control is allowed with an arbitrary wavelength. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.