Patent Publication Number: US-2006002436-A1

Title: Wavelength tunable laser and method of controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-195879, filed on Jul. 1, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a wavelength tunable laser of which an oscillation wavelength is tunable and a method of controlling the same.  
      2. Description of the Related Art  
      Along with dramatic increase in demands for communication in recent years, development of multi-wavelength communication systems (wavelength division multiplexing (WDM) systems), which realize high-capacity transmission by a single optical fiber by way of multiplexing plural single beams of different wavelength, shows progress. For such a wavelength division multiplexing system, a wavelength tunable laser capable of selecting a desired wavelength from a wide range of wavelengths is strongly expected in building the systems.  
      FIGS.  11  and FIGS.  12  are schematic diagrams showing basic structures of a wavelength tunable laser using a conventional wavelength tunable filter, and  FIG. 11A  and  FIG. 12A  show wavelength tunable lasers using transmission-type wavelength tunable filters, and  FIG. 11B  and  FIG. 12B  show wavelength tunable lasers using reflection-type tunable wavelength filters, respectively.  
      The transmission-type wavelength tunable laser shown in  FIG. 11A  in which a resonator  111  is composed of a pair of reflectors  101 ,  102  arranged to face each other, includes, in the resonator  111 , a semiconductor optical amplifier: SOA  103  radiating a laser beams with a gain for a wide range of wavelengths, a transmission-type wavelength tunable filter  104  allowing an oscillation wavelength to be tunable and capable of selecting a desired wavelength from a wide range of wavelengths, and a phase controller  105  controlling the phase of the laser beam resonating in the resonator  111 .  
      On the other hand, a reflection-type wavelength tunable laser shown in  FIG. 11B  in which a resonator  112  is composed of a reflector  101  and a reflection-type wavelength tunable filter  106  arranged to face the reflector  101 , includes, in the resonator  111 , a SOA  103  and a phase controller  105 .  
      In these wavelength tunable lasers, in order to achieve a laser oscillation at a desired wavelength, the following controls are required.  
      A first control is, as shown in  FIG. 13A , a control such that a transmissive peak wavelength or a reflective peak wavelength of a tunable filter (hereinafter, it is simply designated as the peak wavelength) represented by a dotted line BL in the drawing is adjusted in the direction of, for example, an arrow A so that the peak wavelength is allowed to be a target wavelength λ 1  represented by a solid line SL in the drawing. A second control is, as shown in  FIG. 13B , a control such that a longitudinal mode position of the resonators  111 ,  112  represented by a dotted line BL in the drawing is adjusted in the direction of, for example, an arrow B so that the longitudinal mode position practically coincides with the wavelength λ 1  represented by a solid line SL by means of the phase controller  105 .  
      The transmission-type wavelength tunable laser shown in  FIG. 12A  is constituted with an etalon  107  as being an optical element having a cyclic transmissive wavelength added to the structure of the wavelength tunable laser in  FIG. 11A . The reflection-type wavelength tunable laser shown in  FIG. 12B  is similarly constituted with an etalon  107  added to the structure of the wavelength tunable laser in  FIG. 11B . The etalons  107  are arranged respectively, for example, between the phase controller  105  and the reflector  102  inside the resonator  111 , or, for example, between the phase controller  105  and the wavelength tunable filter  106  inside the resonator  112 .As shown in  FIG. 14A , a semiconductor laser without filter has a possibility of oscillating at all wavelengths coinciding with the longitudinal modes of the resonator. Whereas, the above-described wavelength tunable laser can oscillate only at wavelengths in the longitudinal modes positioned in the vicinity of the cyclic transmissive wavelength of the etalon, as shown in  FIG. 14B . In this case, as shown in  FIG. 14C , the oscillation at the arbitrary transmissive wavelength of the etalon is possible by choosing one of the cyclic transmissive wavelengths of the etalon by the wavelength tunable filter.  
      In these wavelength tunable lasers, in order to achieve a laser oscillation at a desired wavelength, the following controls are required.  
      A first control is, as shown in  FIG. 15A , a control such that a peak wavelength of a tunable filter represented by a dotted line BL in the drawing is adjusted in the direction of, for example, an arrow A so that the peak wavelength is allowed to be a transmissive wavelength of the etalon  107  λ 2  represented by a solid line SL in the drawing. A second control is, as shown in  FIG. 15B , a control such that the longitudinal mode position of the resonators  111 ,  112  represented by a dotted line BL in the drawing is adjusted in the direction of, for example, an arrow B so that the longitudinal mode position practically coincides with the wavelength λ 2  represented by a-solid line SL in the drawing by means of the phase controller  105 .  
      In the wavelength tunable laser shown in  FIG. 11A  and  FIG. 11B , when the second control, namely, the control such that the longitudinal mode position is adjusted to the peak wavelength of the wavelength tunable filters  104 ,  106  is performed, a control method of, for example, feeding back an optical output power so as to be maximum by monitoring the output power can be considered. This utilizes the fact that a resonator loss in the wavelength tunable laser becomes minimum and the optical output power becomes maximum when oscillating at the peak wavelength of the wavelength tunable filters  104 ,  106 .  
      In the wavelength tunable laser of  FIGS. 12 , the control method of feeding back so that the optical output power becomes maximum can be considered also in the case that the first control, namely the control such that the transmissive wavelength or the reflective wavelength of the wavelength tunable filters  104 ,  106  is adjusted to a desired transmissive wavelength of the etalon  107  is performed. In this case, the point where the optical output power becomes maximum is to be retrieved by moving the peak wavelength of the wavelength tunable filter. From the relative standpoint, this is similar to the case that the longitudinal mode position is moved with the wavelength of the wavelength filters  104 ,  106  fixed, in the second control of the wavelength tunable laser in  FIGS. 11 .  
      In general, it has been considered that a filter having a symmetric spectrum shape with respect to the peak wavelength as the wavelength tunable filter is desirable to perform a stable wavelength control in the wavelength tunable laser as described above. It is because a state of variation in the laser characteristic becomes same in the case that the oscillation wavelength shifts to the long wavelength side from the filter peak wavelength, as well as in the case that it shifts to the short wavelength side, as a result, a simple control can be performed.  
      [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-261086  
      [Non-Patent Document 1] Kotaki, Y.;Ishikawa, H.;,IEEE Journal of Quantum Electronics volume:25, Issue: 6 Jun. 1989 Pages:1340-1345  
      However, in the case that the wavelength tunable filter having a symmetric spectrum shape with respect to the peak wavelength is used in the wavelength tunable laser of  FIGS. 11 , the state of variation in the laser characteristic, for example optical output power, is different between in the case that the oscillation wavelength shifts to the long wavelength side from the filter peak wavelength and in the case that it shifts to the short wavelength side. As shown in  FIG. 16  indicating the relation of the longitudinal mode position and the optical output power, the variation of the optical output power is asymmetric against peak wavelength in which optical output power becomes maximum, and the peak wavelength approaches extremely the wavelength where the optical output power varies discontinuously at the short wavelength side. The discontinuity represents the unstable condition such that the oscillation wavelength hops to an adjacent longitudinal mode and the noise characteristic deteriorates. Therefore, the control should be performed, avoiding the discontinuity, however, the control avoiding the discontinuity is exceedingly difficult because a tolerance of control is narrow due to the approach of the discontinuity to the optical output power peak wavelength.  
      Similarly, in the wavelength tunable laser of  FIGS. 12 , when the first control, namely, the control in which the transmissive wavelength or the reflective wavelength of the wavelength tunable filters  104 ,  106  is adjusted to a desired transmissive wavelength of the etalon  107  is performed, there also exists unstable points where the optical output power are discoutinuous near the optical output power peak wavelength (in this case, the points where a wavelength hopping occurs between the transmissive wavelength of the etalon  107 ), therefore a stable control is difficult to be performed.  
     SUMMARY OF THE INVENTION  
      This invention has been made in view of the above-described problems, and an object thereof is to provide a wavelength tunable laser and a method of controlling the same, capable of radiating a stable laser beam at a desired oscillation wavelength with a good noise characteristic.  
      A wavelength tunable laser of the present invention includes a resonator, an optical amplifier provided inside the resonator, radiating a laser beam, a wavelength tunable filter provided inside the resonator or as one part of the resonator, allowing an oscillation wavelength to be tunable, and a phase controller controlling a phase of the laser beam resonating inside the resonator, in which the wavelength tunable filter has an asymmetric filter characteristic and is designed so that a loss given to a long wavelength side with respect to a peak wavelength of the filter is larger than a loss given to a short wavelength side.  
      A method for controlling a wavelength tunable laser in the present invention is performed using the wavelength tunable laser including a resonator, an optical amplifier provided inside the resonator, radiating a laser beam, a wavelength tunable filter provided inside the resonator or as one part of the resonator, allowing an oscillation wavelength to be tunable, and a phase controller controlling a phase of the laser beam resonating inside the resonator, in which the wavelength tunable filter has an asymmetric filter characteristic and is designed so that a loss given to a long wavelength side with respect to the peak wavelength of the filter is larger than a loss given to a short wavelength side, so that the oscillation wavelength of the laser beam from the optical amplifier is allowed to coincide with the peak wavelength of the filter in the wavelength tunable filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A and 1B  are schematic diagrams showing basic structures of a wavelength tunable laser of the present invention;  
       FIGS. 2A and 2B  are schematic diagrams showing basic structures of the wavelength tunable lasers of the present invention;  
       FIG. 3  is a characteristic chart explaining problems of a conventional wavelength tunable laser;  
       FIG. 4  is a characteristic chart showing a result of an experiment in which the optimum range of filter loss asymmetricity in the wavelength tunable laser in the present invention is examined;  
       FIGS. 5A  to  5 D are characteristic charts showing relations between longitudinal mode positions and optical output powers when using a conventional wavelength tunable filter having a symmetric filter characteristic with respect to a peak wavelength;  
       FIGS. 6A and 6B  are characteristic charts showing the relations between wavelengths and the transmittance or the reflectance in a wavelength tunable filter of the present invention, based on the comparison with the conventional wavelength tunable filter;  
       FIGS. 7A  to  7 D are characteristic charts showing relation between the longitudinal mode positions and the optical output powers when using the wavelength tunable filter in the present invention having an asymmetric filter characteristic with respect to the peak wavelength;  
       FIGS. 8A and 8B  are schematic diagrams showing a principal structure of a transmission-type wavelength tunable laser according to a first embodiment;  
       FIG. 9  is an explanatory chart showing an example realizing an asymmetric filter characteristic by an AOTF according to the first embodiment;  
       FIGS. 10A and 10B  are schematic diagrams showing a principal structure of a reflection-type wavelength tunable laser according to a second embodiment;  
       FIGS. 11A and 11B  are schematic diagrams showing basic structures of the conventional wavelength tunable lasers;  
       FIGS. 12A and 12B  are schematic diagrams showing basic structures of the conventional wavelength tunable lasers;  
       FIGS. 13A and 13B  are characteristic charts showing an oscillation control of the wavelength tunable laser in  FIGS. 11 ;  
       FIGS. 14A  to  14 C are characteristic charts showing an oscillation control of the wavelength tunable laser in  FIGS. 12 ;  
       FIGS. 15A and 15B  are characteristic charts showing an oscillation control of the wavelength tunable laser in  FIGS. 12 ; and  
       FIG. 16  is a characteristic chart showing a relation between longitudinal mode positions and optical output powers to explain problems when using the conventional wavelength tunable filter having the symmetric filter characteristic with respect to the peak wavelength. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     BASIC GIEST OF PRESENT INVENTION  
      FIGS.  1  and FIGS.  2  are schematic diagrams showing basic structures of a wavelength tunable laser of the present invention.  FIG. 1A  and  FIG. 2A  show transmission-type wavelength tunable lasers and  FIG. 1B  and  FIG. 2B  show reflection-type wavelength tunable lasers respectively.  
      A transmission-type wavelength tunable laser shown in  FIG. 1A  in which a resonator  11  is composed of a pair of reflectors  1 ,  2  arranged to face each other includes, in the resonator  11 , a semiconductor optical amplifier (SOA)  3  radiating a laser beam with a gain for a wide range of wavelengths, a transmission-type wavelength tunable filter  4  allowing an oscillation wavelength to be tunable and capable of selecting a desired wavelength from a wide range of wavelengths, and a phase controller  5  controlling a phase of the laser beam resonating in the resonator  11 .  
      On the other hand, a reflection-type wavelength tunable laser shown in  FIG. 1B  in which a resonator  12  is composed of the reflector  1  and a reflection-type wavelength tunable filter  6  arranged to face the reflector  1  includes, in the resonator  12 , the SOA  3  and the phase controller  5 .  
      As shown in  FIG. 2A , a structure such that an etalon  7  which is an optical element having a cyclic transmissive wavelength is added to the transmission-type wavelength tunable laser shown in  FIG. 1 , or a structure such that the etalon  7  is similarly added to the reflection-type wavelength tunable laser shown in  FIG. 1B  are also suitable.  
      In the present invention, filter characteristics of the wavelength tunable filters  4 ,  6  are asymmetric, in this case asymmetric between a short wavelength side and a long wavelength side with a peak wavelength as the center, and are designed so that losses given to the long wavelength side are larger than losses given to the short wavelength side. Specifically, the loss in a wavelength which is apart from the peak wavelength of the wavelength tunable filter to the long wavelength side by a half value of an oscillatable mode interval is from 0.5 dB to 10 dB larger than the loss in a wavelength which is apart to the short wavelength side by a half value of the occillatable longitudinal mode.  
      The operational principles of the wavelength tunable filters  4 ,  6  are now described as follows.  
      It is considered that the asymmetricity in a relation between an optical output power and a phase in the conventional wavelength tunable laser shown in  FIGS. 11 , or in a relation between the optical output power and the peak wavelength of the wavelength tunable filter when a symmetric wavelength tunable filter is used in the conventional wavelength tunable laser shown in FIGS.  12  is caused by so-called asymmetric gain saturation in the SOA. This is the phenomenon such that a gain in the long wavelength side of the oscillation wavelength is increased and a gain in the short wavelength side is decreased in the SOA when the laser oscillation is generated as shown in  FIG. 3 .  
      A difference between the loss in the wavelength which is apart from the peak wavelength to the long wavelength side by the a half value of an oscillatable longitudinal mode interval and the loss in the wavelength which is apart to the short wavelength side by the a half value varies in accordance with a structure of an active layer used for the SOA or a wavelength difference from the oscillation wavelength (mode interval) and the like. Suppose that the mode interval is in a range from 0.01 nm to 5 nm using the active layer of MQW, for example, it is proved from our experiment that the difference between the gain in the wavelength which is apart from the oscillation wavelength to the long wavelength side by the a half value of an oscillatable longitudinal mode interval and the loss in the wavelength which is apart to the short wavelength side by the a half value is approximately 0.5 dB to 10 dB. The experimental result thereof is shown in  FIG. 4 . Here, a left side of the oscillation wavelength shown by a dotted line in the drawing is the short wavelength side, and a right side thereof is the long wavelength side, and it is found out that the difference between these sides is approximately at a maximum of 10 dB and at a minimum of 0.5 dB.  
      FIGS.  5  are characteristic charts showing relations between longitudinal mode positions and optical output powers when the conventional wavelength tunable filter having a symmetric filter characteristic with respect to the peak wavelength is used.  
       FIG. 5A  is a characteristic chart showing a relation between the longitudinal mode positions and the optical output powers,  FIG. 5B  is a characteristic chart showing a relation between a wavelength and a gain shown in numeral (1) of  FIG. 5A , in the vicinity of a point where the optical output power varies discontinuously and a mode hopping occurs (discontinuity) in the shorter wavelength than peak wavelength of the filter,  FIG. 5C  is a characteristic chart showing a relation between a wavelength and a gain when the longitudinal mode shown in numeral (2) of  FIG. 5A , in the vicinity of a point where the longitudinal mode position coincides with the peak wavelength of the filter , and  FIG. 5D  is a characteristic chart showing a relation between a wavelength and a gain shown in numeral (3) of  FIG. 5A , at the vicinity of the discontinuity in the longer wavelength then peak wavelength of the filter.  
      As shown in  FIG. 5A , when a wavelength tunable filter having a symmetric filter characteristic with respect to the peak wavelength is used, the discontinuity i.e., the point where the gain of the oscillation mode becomes equivalent to the gain of the adjacent longitudinal mode shifts to the long wavelength side from the point where the central wavelength between the two adjacent longitudinal modes coincides with the peak wavelength of the wavelength tunable filter. As a result, it is considered that intervals between the peak position (optical output power peak wavelength) where the optical output power becomes maximum and the discontinuities are in different states in the right and the left, and a tolerance at the short wavelength side of which the interval is narrower becomes narrow. Namely, the distance between the peak wavelength in the characteristic of the filter itself and the wavelength where the mode hop occurs becomes small especially in the case of  FIG. 5B  as compared with in the case of  FIG. 5D , therefore the laser oscillation becomes unstable when the oscillation wavelength is coincide with the peak wavelength of the filter.  
      In order to expand the interval between the optical output power peak wavelength and the discontinuity at the maximum, the point where the optical output power becomes maximum may be in an almost mid-point of the adjacent two discontinuities. And to realize this, in a state that the peak wavelength of the wavelength tunable filter coincides with the central wavelength between the two adjacent longitudinal modes, effective gains including asymmetric gains of the two longitudinal modes may be equivalent.  
      FIGS.  6  are characteristic charts showing relations between wavelengths and a transmittance or a reflectance in the wavelength tunable filter of the present invention, based on a comparison with the conventional wavelength tunable filter.  
      Since the conventional wavelength tunable filter has a symmetric filter characteristic with respect to the peak wavelength, a loss given to the long wavelength side and a loss given to the short wavelength side are approximately equal value as shown in  FIG. 6A . Whereas, in the wavelength tunable filter of the present invention, a transmission spectrum or a reflection spectrum is asymmetric as shown in  FIG. 6B , and a loss in a wavelength apart from the peak wavelength to the long wavelength side by a half value of the longitudinal mode interval is 0.5 dB to 10 dB larger than a loss in a wavelength apart from the peak wavelength to the short wavelength side by a half value of the longitudinal mode. Thereby, the effect by the asymmetric gain saturation is denied.  
      FIGS.  7  are characteristic charts showing relations between longitudinal mode positions and optical output powers when the wavelength tunable filter of the present invention having an asymmetric filter characteristic with respect to the peak wavelength is used.  
       FIG. 7A  is a characteristic chart showing the relation between longitudinal mode positions and optical outputs,  FIG. 7B  is a characteristic chart showing a relation between a wavelength and a gain in the vicinity of a discontinuity at the short wavelength side shown in a code (1) of  FIG. 7A ,  FIG. 7C  is a characteristic chart showing a relation between a wavelength and a gain when the longitudinal mode shown in numeral (2) of  FIG. 7A  approximately coincides with the peak wavelength, and  FIG. 7D  is a characteristic chart showing a relation between a wavelength and a gain in the vicinity of a discontinuity at the long wavelength side shown in numeral (3) of  FIG. 7A .  
      When the wavelength tunable filter having the asymmetric filter characteristic with respect to the peak wavelength, a point where a mode hopping occurs, i.e., a point where gains between the oscillation mode and the adjacent longitudinal mode are equivalent approximately agrees with a point where the central wavelength between the two adjacent longitudinal modes coincides with the peak wavelength of the wavelength tunable filter. As a result, as shown in  FIG. 7A , the interval between the peak wavelength of the optical output power and the discontinuities are approximately equivalent in the right and the left, namely, a distance between the peak wavelength in a characteristic of a filter itself and an oscillation mode in the case of  FIG. 7B  is approximately equivalent to a distance in the case of  FIG. 7D . Therefore, respective discontinuities of the right and the left can be maximally held off from the peak wavelength of the optical output. Due to the technique controlling the wavelength tunable filter so that the optical output power becomes maximum , this means the oscillation wavelength coincides the peak wavelength of the wavelength tunable filter , the highly stable control can be achieved.  
     Specific Embodiments To Which Present Invention Is Applied  
      Hereinafter, based on the contents of above-described basic gist, specific embodiments to which the present invention is applied will be explained in detail with reference to drawings.  
     First Embodiment  
      In this embodiment, a specific example of a wavelength tunable laser including a transmission-type wavelength tunable filter having an asymmetric filter characteristic.  
      FIGS.  8  are schematic diagrams showing a principal structure of a transmission-type wavelength tunable laser according to the first embodiment.  
      As shown in  FIG. 8A , the transmission-type wavelength tunable laser includes a semiconductor optical amplifier (SOA)  21  radiating a laser beam, an acousto-optic wavelength tunable filter (AOTF)  22  as being a transmission-type wavelength tunable filter having an asymmetric filter characteristic, a lens  23  condensing the laser beam, an etalon  24  as being an optical element having a cyclic transmissive wavelength, and a reflector  25 .  
      The SOA  21  has an end surface  21   a  which is a cleavage surface functioning as a reflector, and a resonator  31  is formed between the end surface  21   a  and the reflector  25 . As the SOA  21 , for example, one of the SOAs using a bulk-structured waveguide as an active layer, the one using a MQW structured waveguide, or the one using a quantum-dot structure can be applied. Because an asymmetric gain saturation is generated in all the SOAs having respective structures described above. A Waveguide for controlling a phase is integrated in the SOA  21 , therefore a longitudinal mode in the resonator  31  can be moved by injecting an electric current. As the etalon  24 , for example, the one of which free spectrum range is 100 GHz is used.  
      The AOTF  22  is a waveguide-type filter as shown in  FIG. 8B , and three input ports  22   a,    22   b,  and  22   c  are provided in an input side and three output ports  22   d,    22   e,  and  22   f  are provided in an output side respectively. Polarization beam splitters (PBS)  32  are arranged to an input end, an output end and a central part respectively. These input ports  22   a,    22   b,  and  22   c  and these output ports  22   d,    22   e,  and  22   f  and the PBSs  32  form two waveguides  33 , 34 . In the AOTF  22 , in order to set off the Doppler shift generated thereinside, the waveguides have a two-stage configuration.  
      On the basis of the input ports  22   a,    22   b  and  22   c,  a comb electrode  35  in which electrode material is engaged in a shape like teeth of a comb and to which RF is applied, and a SAW guide  36  propagating a surface acoustic wave (SAW) generated from the comb electrode  35  are provided in a forward region and a backward region of the waveguide respectively.  
      In the AOTF  22 , when a light is incident from the input port  22   a,  only a light having a specific wavelength which is decided by a RF frequency is radiated from the output port  22   e.  Therefore, the change of the RF frequency applied to the comb electrode  35  enables a wavelength tuning operation.  
      In the AOTF  22 , an asymmetric filter characteristic is achieved as follows.  
       FIG. 9  is an explanatory chart showing an example realizing the asymmetric filter characteristic using the AOTF according to the present embodiment.  
      In the AOTF  22 , in the region where the SAW guide  36  is provided at the waveguide  34 , a distribution is given in a width of the waveguide  34  as shown in  FIG. 9 . In this case, the AOTF  22  is designed so that the width of the waveguide  34  is changed in a range of approximately 0.2 μm at the maximum. Accordingly, a difference of losses between a long-wavelength side and a short-wavelength side in a wavelength shifted by 50 GHz from a transmission peak wavelength, which corresponds to a half of the free spectrum range of the etalon  24  can be 1 dB. The difference of losses can be set to a desired value by changing the distribution of the width of the waveguide  34 . This enables the control such that an oscillation wavelength of the laser is allowed to coincide with the peak wavelength of the filter in the AOTF  22  easily and accurately.  
      As described above, according to the present embodiment, a wavelength tunable laser whereby a stable laser radiation can be achieved at a desired oscillation wavelength, having a good noise characteristic can be actualized.  
      In the present embodiment, the AOTF  22  is used as a wavelength tunable filter having an asymmetric filter characteristic. The AOTF  22  is designed so that the width of the waveguide  34  is changeable with respect to an optical axis direction, therefore the transmissive spectrum is allowed to be asymmetric without difficulty, as a result, a reliable and stable laser radiation can be achieved.  
      In the present embodiment, the case that the transmission-type wavelength tunable laser having the asymmetric filter characteristic is achieved by using the AOTF  22  has been described, however, the invention is not limited to this embodiment. For example, by using a simple structured reflection-type AOTF, a reflection-type tunable laser having an asymmetric filter characteristic can also be achieved. In addtion, a wavelength tunable laser may be constituted not using the etalon  24 .  
     Second Embodiment  
      In this embodiment, a specific example of a wavelength tunable laser including a reflection-type wavelength tunable filter having an asymmetric filter characteristic will be described.  
      FIGS.  10  are schematic diagrams showing a principal structure of a reflection-type wavelength tunable laser according the second embodiment.  
      The reflection-type wavelength tunable laser is a so-called 3-electrode DBR (distribution Bragg reflection-type mirror) laser in which a filter characteristic of a DBR unit is asymmetric. The DBR unit can change a reflection wavelength thereof by injecting a electric current.  
      The wavelength tunable laser is constituted by including an active layer unit  41 , a phase control unit  42 , and the DBR unit  43  as shown in  FIG. 10A . An electrode  41   a  is pattern-formed on an upper surface of the active layer unit  41 , an electrode  42   a  is pattern-formed on an upper surface of the phase control unit  42  and an electrode  43   a  is pattern-formed on an upper surface of the DBR unit  43  respectively. The active layer unit  41  corresponds to a semiconductor optical amplifier (SOA) radiating a laser beam, and the DBR unit  43  corresponds to a reflection-type wavelength tunable filter. An element end surface  41   a  corresponding to an end portion of the SOA is a cleavage surface functioning as a reflector. A resonator  51  is formed between the element end surface  41   a  and the DBR unit  43 . An active layer  41   b  is formed in the active layer unit  41  and a diffraction grating  43   b  is formed in the DBR unit  43  respectively.  
      By injecting the electric current to the DBR unit  43 , a reflection peak wavelength can be changed. In addition, by injecting the electric current to the phase control unit  42 , the position of a resonator longitudinal mode can be changed.  
      In the wavelength tunable laser of this embodiment, a difference from a conventional 3-electrode DBR laser is a shape of a reflection spectrum at the DBR unit  43 . This can be achieved by changing a cycle of the diffraction grating  43   b  to an optical axis direction. In the conventional 3-electrode DBR laser, the cycle Λ of the diffraction grating  43   b  is constant, for example, 240 nm when the laser oscillates in 1.55 μm zone. Whereas, in this embodiment, when z denotes the position in the optical axis direction at the DBR unit  43  as shown in  FIG. 10B , a distribution is given to the cycle of the diffraction grating as follows, for example. 
 
Λ=240 nm+Λoffset+ f ( z ): z 1 &lt;z&lt;z 2, Λoffset+ f ( z )&gt;0 
 
Λ=240 nm :z&lt;z1, z2&lt;z 
 
      In this case, by designing f(z), Λoffset, a desired asymmetry can be obtained. Accordingly, a control such that an oscillation wavelength of the laser beam from the active layer unit  41  is allowed to coincide with a peak wavelength of a filter in the DBR unit  43  easily and accurately can be achieved.  
      As described above, according to the present embodiment, a wavelength tunable laser whereby a stable radiation of the laser beam at a desired oscillation wavelength can be possible, having a good noise characteristic can be realized.  
      In the present embodiment, a stable radiation of the laser beam with a simple structure can be possible by applying a reflection-type wavelength tunable laser, in this case, a 3-electrode DBR laser.  
      In the present embodiment, the case in which one DBR unit  43  is provided as a wavelength tunable filter is illustrated, however, this invention is not limited to this embodiment. For example, even if two or more DBR units are combined to be used as the wavelength tunable filter, it is suitable that the filter characteristic combining these characteristics of these DBR units may be designed to be asymmetric.  
      The present embodiments are to be considered in all respects as illustrative and no restrictive, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.