Abstract:
A method for tuning a semiconductor laser including a plurality of wavelength selection portions, each of which has a periodic wavelength characteristic, including: controlling a value of a refractive index controlling means of the wavelength selection portions to achieve a desired output wavelength of the laser; and shifting the value when the value is equal to or excess of a predetermined value to a basal value side until achieving the desired output wavelength, the basal value being a value without applying refractive index variation by the refractive index controlling means, the predetermined value being a value for shifting one period of the periodic wavelength characteristic.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of and claims priority to International Patent Application No. PCT/JP2009/069957 filed on Nov. 26, 2009, which claims priority to Japanese Patent Application No. 2008-303939 filed on Nov. 28, 2008, subject matter of these patent documents is incorporated by reference herein in its entirety. 
     BACKGROUND 
     (i) Technical Field 
     A certain aspect of the embodiments discussed herein is related to a method for tuning a semiconductor laser. 
     (ii) Related Art 
     A wavelength tunable laser disclosed in Japanese Patent Application Publication No. 2007-48988 has a SG-DBR (Sampled Grating Distributed Bragg Reflector) region having a plurality of wavelength selection portions. A heater controls a temperature of each wavelength selection portion. Thus, wavelength selection characteristics are controlled with refraction index changing. 
     In the wavelength tunable laser, a reflection spectrum wavelength selected through overlapping of wavelength characteristics of each wavelength selection portion of the SG-DBR region and a gain spectrum wavelength of a SG-DFB (Sampled Grating Distributed Feedback) region are made to correspond to each other, and an oscillation wavelength is fixed to a predetermined wavelength. 
     SUMMARY 
     It is necessary to tune a parameter value of the wavelength selection portion of the wavelength tunable laser with respect to each wavelength channel in advance using the tunable laser. After tuning the parameter value of the wavelength selection portion of the wavelength tunable laser, the parameter value is stored to a memory (EP-ROM etc.). In a field where the wavelength tunable laser is operating, a controller of the wavelength tunable laser reads the parameter value with respect to a target wavelength of the wavelength tunable laser from the memory and supplies the value to target electrodes of the laser. On the other hand, it is necessary to control a predetermined relation between each wavelength selection portion in order to select only a predetermined single wavelength. However, it is confirmed that a very large parameter value is needed when the field of the wavelength tunable laser is operating by a parameter value which is acquired by tuning the above relation is keeping. 
     According to an aspect of the present invention, there is provided a method for tuning a semiconductor laser including a plurality of wavelength selection portions, each of which has a periodic wavelength characteristic, including: controlling a value of a refractive index controlling means of the wavelength selection portions to achieve a desired output wavelength of the laser; and shifting the value when the value is equal to or excess of a predetermined value to a basal value side until achieving the desired output wavelength, the basal value being a value without applying refractive index variation by the refractive index controlling means, the predetermined value being a value for shifting one period of the periodic wavelength characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic view of a laser device in accordance with a first embodiment; 
         FIG. 2A  and  FIG. 2B  illustrate a reflection spectrum of a segment before heating by a heater; 
         FIG. 3A  and  FIG. 3B  illustrate a reflection spectrum of a CSG-DBR region and a SG-DFB region; 
         FIG. 4A  to  FIG. 4D  illustrate a principle of a control of the semiconductor laser in accordance with the first embodiment; 
         FIG. 5  illustrates an example of a flowchart executed by a controller; 
         FIG. 6A  to  FIG. 6E  illustrate a relation between a temperature of a segment and the reflection spectrum of the CSG-DBR region; 
         FIG. 7  illustrates an actual temperature control of the heater; and 
         FIG. 8  illustrates a schematic view of a semiconductor laser in accordance with a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A description will be given of a best mode for carrying the present invention. 
     First Embodiment 
       FIG. 1  illustrates a schematic view of a laser device  100  in accordance with a first embodiment. As illustrated in  FIG. 1 , the laser device  100  has a semiconductor laser  10 , a temperature control device  20  and a controller  30 . The semiconductor laser  10  is provided on the temperature control device  20 . A description will be given of details of each component. 
     The semiconductor laser  10  has a structure in which a CSG-DBR (Chirped Sampled Grating Distributed Bragg Reflector) region  11 , a SG-DFB region  12  and a SOA (Semiconductor Optical Amplifier) region  13  are coupled in this order. 
     The CSG-DBR region  11  has an optical waveguide having gratings at a given interval. The optical waveguide of the CSG-DBR region  11  has a plurality of segments in which a diffraction grating region having a grating is coupled to a space region. In the CSG-DBR region  11 , the segments of the CSG-DBR region  11  have a different optical length. 
     The optical waveguide of the CSG-DBR region  11  is made of semiconductor crystal of which absorption edge wavelength is shorter than a laser oscillation wavelength. In the embodiment, the CSG-DBR region  11  has three segments (a segment CSG 1  to a segment CSG 3 ). The CSG-DBR region  11  has heaters according to each segment. In the embodiment, three heaters  14   a  to  14   c  are provided on the CSG-DBR region  11  according to the segments CSG 1  to the segment CSG 3 . 
     The SG-DFB region  12  has an optical waveguide having gratings at a given interval. The optical waveguide of the SG-DFB region  12  has a plurality of segments in which a diffraction grating region having a grating is coupled to a space region. In the SG-DFB region  12 , each segment has substantially the same optical length. The optical waveguide of the SG-DFB region  12  is made of semiconductor crystal having a gain with respect to a laser oscillation at an objective wavelength. An electrode  15  is provided on the SG-DFB region  12 . 
     The SOA region  13  has an optical waveguide made of semiconductor crystal for amplifying or absorbing a light with a current control. An electrode  16  is provided on the SOA region  13 . The optical waveguides of the CSG-DBR region  11 , the SG-DFB region  12  and the SOA region  13  are optically coupled to each other. 
     The temperature control device  20  has a pertier element and so on, and controls a temperature of the semiconductor laser  10 . The controller  30  has a control portion having a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory) and so on, and has a power supply. The ROM of the controller  30  stores control information of the semiconductor laser  10 , a control program and so on. 
     Next, a description will be given of an operation of the laser device  100 . The controller  30  provides a given current to the electrode  15 . Thus, the optical waveguide of the SG-DFB region  12  generates a light. The generated light propagates in the optical waveguides of the CSG-DBR region  11  and the SG-DFB region  12 , is reflected and amplified repeatedly, and the light propagates in the optical waveguides of the SOA region  13 , is amplified, and is emitted outside. The controller  30  provides a given current to the electrode  16 . Thus, an output of the semiconductor laser  10  is kept constant. 
     Next, the controller  30  controls the temperature of the segments CSG 1  to CSG 3  by controlling the heaters  14   a  to  14   c . Thereby, equivalent refraction index of the segments CSG 1  to CSG 3  is changed. In this case, the reflection characteristics of the segments CSG 1  to CSG 3  are changed. Thus, the oscillation wavelength of the semiconductor laser  10  may be changed. With the control, the laser device  100  makes the semiconductor laser  10  oscillate at a desirable wavelength. 
       FIG. 2A  illustrates a schematic view of reflection spectrums of the segments CSG 1  to CSG 3  before heating by the heaters  14   a  to  14   c .  FIG. 2B  illustrates overlapped reflection spectrums of the segments CSG 1  to CSG 3 . As illustrated in  FIG. 2A , the reflection spectrums of the segments CSG 1  to CSG 3  have a different periodic peak, because the segments CSG 1  to CSG 3  have a different optical length. Therefore, as illustrated in  FIG. 2B , the reflection spectrums do not overlap at a given wavelength, and overlap at another wavelength. 
       FIG. 3A  illustrates a reflection spectrum of the CSG-DBR region  11 . The reflection spectrum of the CSG-DBR region  11  is obtained by overlapping the reflection spectrums of the segments CSG 1  to CSG 3 . As illustrated in  FIG. 3A , reflection intensity differs at each peak wavelength. Thus, an envelope curve having a bell shape is formed. When wavelength range is enlarged, an envelope curve in which a plurality of bells are arrayed is formed. Thus, the CSG-DBR region  11  has wavelength dependency with respect to the reflection intensity. 
       FIG. 3B  illustrates a reflection spectrum of the SG-DFB region  12 . The reflection intensity is approximately constant at each peak wavelength, because the optical length of each segment of the SG-DFB region  12  is substantially the same and each temperature of the segments is kept constant by the temperature control device  20 . 
     When a wavelength of the reflection spectrum of the CSG-DBR region  11  having relatively large reflection intensity corresponds to any wavelength of the reflection spectrum of the SG-DFB region  12 , the semiconductor laser  10  laser-oscillates at the corresponding wavelength. It is therefore possible to select a laser oscillation wavelength by changing a relation between wavelength and reflection intensity of the CSG-DBR region  11 . 
     Next, a description will be given of a principle of a control of the semiconductor laser in accordance with the embodiment, with reference to  FIG. 4A  to  FIG. 4D . It is assumed that the number of the segments of the CSG-DBR region subjected to the temperature control is two for simplifying the drawings. As illustrated in  FIG. 4A , the semiconductor laser laser-oscillates at a given temperature. In this case, the reflection spectrum of whole of the CSG-DBR region is determined with the overlapping of the reflection spectrums of the segments of the CSG-DBR region. Each temperature of the segments before electrical power supply to the heaters is hereinafter referred to as a basal value. In  FIG. 4A , the basal value is zero degrees C. The basal value is meaning a heat value of the heaters. In the case where the basal value is zero degrees C., the CSG-DBR region has an environmental temperature. 
     Next, as illustrated in  FIG. 4B , power provision to the heaters makes temperature gradient in the segments. In this case, the temperature gradient is made so that the semiconductor laser laser-oscillates at a wavelength near the desirable wavelength. Each temperature of the segments in this case is hereinafter referred to as an initial value. In  FIG. 4B , the initial temperature value of one segment is 15 degrees C., and the initial temperature value of the other segment is zero degrees C. 
     Then, as illustrated in  FIG. 4C , the temperature gradient is substantially kept, and the temperature of the both segments is increased so that the oscillation wavelength of the semiconductor laser reaches to the desirable wavelength. In the example of  FIG. 4C , the initial temperature value of one segment is 35 degrees C., and the initial temperature value of the other segment is 20 degrees C. 
     A description will be given of a case where the temperature of each segment is increased, and the oscillation wavelength shifts toward long wavelength. In this case, the oscillation wavelength shifts toward long wavelength with temperature increase of each segment, and the oscillation wavelength jumps to another predetermined value of shorter wavelength side when the temperature reaches a predetermined value. The oscillation wavelength shifts toward long wavelength with further temperature increase of each segment. In this way, the oscillation wavelength repeats in a single direction within a given wavelength range in a given periodic temperature. In the example of  FIG. 4C , the oscillation wavelength repeats in a single direction in the periodic temperature of 30 degrees C. 
     Much power provision to the heaters is needed for keeping of high temperature of each segment. And so, in the embodiment, it is determined whether the temperature of each segment is controlled to be higher than the basal value by one period or more. If the temperature of each segment is higher than the basal value by one period or more, the temperature of the segment is shifted to the basal value side by periodic. In the example of  FIG. 4D , the temperature of each segment is increased to 70 degrees C., and after that, the temperature of each segment is decreased by more than one period and is decreased to 5 degrees C. 
     In this case, the power amount provided to the heaters may be reduced without changing of the oscillation wavelength. Thus, the power consumption of the laser device  100  may be reduced. The degradation of the semiconductor laser is reduced and the reliability of the semiconductor laser is improved because the temperature of the semiconductor laser is reduced. 
     A description will be given of the above-mentioned control of the laser device  100  with reference to  FIG. 5  and  FIG. 6A  to  FIG. 6E .  FIG. 5  illustrates a flowchart executed by the controller  30 .  FIG. 6A  to  FIG. 6E  illustrate a relation between the temperature and the reflection spectrum of the segments CSG 1  to CSG 3 . 
     As illustrated in  FIG. 5 , the controller  30  provides a predetermined current to the electrodes  15  and  16 , and provides electrical power to the heaters  14   a  to  14   c  so that the temperature of the segments CSG 1  to CSG 3  reaches the initial value (Step S 1 ). In this case, the controller  30  controls the electrical power provided to the heaters  14   a  to  14   c  so that the temperature difference between the segments CSG 1  to CSG 3  is set according to the ratio of the optical lengths of the segments CSG 1  to CSG 3 . Thus, as illustrated in  FIG. 6B , the reflection spectrum of the CSG-DBR region  11  is controlled to be a bell shape. In the example of  FIG. 6B , the wavelength of the reflection spectrum having the largest reflection intensity is λ 1 . 
     Next, the controller  30  increases the temperature of the segments CSG 1  to CSG 3  (Step S 2 ). In this case, the controller  30  keeps the temperature difference between the segments CSG 1  to CSG 3  substantially constant and increases the temperature of the segments CSG 1  to CSG 3 , as illustrated in  FIG. 6C . Thus, the peak wavelength may be shifted without the changing of the wavelength range of the bell-shaped wavelength characteristics, as illustrated in  FIG. 6D . In the example of  FIG. 6D , the peak wavelength is shifted from λ 1  to λn. This allows the laser oscillation of the semiconductor laser  10  at the desirable wavelength. 
     Then, the controller  30  determines whether the temperature of the segments CSG 1  to CSG 3  is higher than the basal value by one periodic or more (Step S 3 ). If it is not determined that the temperature of the segments CSG 1  to CSG 3  is higher than the basal value by one periodic or more in Step S 3 , the controller  30  executes Step S 2  again. 
     If it is determined that the temperature of the segments CSG 1  to CSG 3  is higher than the basal value by one periodic or more in Step S 3 , the controller  30  reduces the temperature of the segment by one periodic or more as illustrated in  FIG. 6E  (Step S 4 ). After that, the controller  30  executes Step S 2  again. 
     With the flowchart of  FIG. 5 , the electrical power provision amount to the heaters  14   a  to  14   c  may be reduced without the changing of the oscillation wavelength. Thus, the power consumption of the laser device  100  may be reduced. And, the degradation of the semiconductor laser  10  may be restrained and the reliability of the semiconductor laser  10  may be improved, because the temperature of the semiconductor laser  10  is reduced. 
     It is preferable that the temperature difference error of the segments CSG 1  to CSG 3  is within −0.5 degrees C. to 0.5 degrees C., when the temperature difference between the segments CSG 1  to CSG 3  is increased in Step S 2 . 
       FIG. 7  illustrates an actual temperature control of the heaters  14   a  to  14   c . A dotted line of  FIG. 7  indicates a case where the temperature of the segments CSG 1  to CSG 3  was increased. A solid line of  FIG. 7  indicates a case where the temperature of the segments CSG 1  and CSG 2  was reduced by one periodic. Therefore, the temperature of the segment CSG 3  was the same in both of the cases. In the case of the dotted line of  FIG. 7 , the temperature of the segments CGS 2  was reduced after the temperature of the segment CSG 1  was reduced. 
     As illustrated in  FIG. 7 , total amount of the electrical power of the heaters was reduced by reducing the temperature of the segment CSG 1 , and was further reduced by reducing the temperature of the segment CSG 2 . On the other hand, the oscillation wavelength was hardly changed. Thus, the oscillation wavelength may be kept even if the temperature of the segment is reduced by per periodic. 
     The CSG-DBR is used as a distributed reflector in the embodiment. However, the structure in not limited to the embodiment. A SG-DBR region including segments having substantially the same optical length may be used. In this case, a reflection spectrum of the SG-DBR may be formed to be the bell shape by making the temperature gradient of the segments. Therefore, the electrical power provision amount to the heaters may be reduced without the changing of the oscillation wavelength by reducing the temperature of the segment toward the basal value side by per periodic when the temperature of the segment is higher than the basal value by one period or more. 
     The temperature of the segment is used as the parameter for controlling the refraction index of the segment in the embodiment. However, the structure in not limited to the embodiment. For example, the temperature of the heaters or the electrical power provision amount to the heaters may be used as the parameter for controlling the refraction index of the segment. The refraction index of the segment may be controlled by providing the current to the segment, and the current value may be used as the parameter for controlling the refraction index. 
     Second Embodiment 
       FIG. 8  illustrates a schematic view of a semiconductor laser  10   a  in accordance with the second embodiment. The semiconductor laser  10   a  is a laser of ring resonator type. As illustrated in  FIG. 8 , the semiconductor laser  10   a  has ring resonators  61 ,  62  and  63  optically coupled to each other, and a SOA region  64  optically coupled to the ring resonators  61 ,  62  and  63 . The ring resonator  61 , the ring resonator  62  and the ring resonator  63  are optically coupled in this order from the SOA region  64  side. An AR (Anti Reflection) film  66  is formed on one edge face on the side of the ring resonator  61 . A HR (High Reflection) film  67  is formed on the other edge face on the side of the ring resonator  63 . 
     The ring resonator  61  is a resonator having a period peak in the wavelength characteristics, and acts as a filter having a peak of reflection spectrum periodically at a given wavelength interval. The ring resonator  61  has the same wavelength characteristics as the SG-DFB region  12  of the semiconductor laser  10  of the first embodiment, and determines a wavelength at which the semiconductor laser  10   a  can oscillate. 
     The ring resonators  62  and  63  are a resonator having a period peak in the wavelength characteristics, and acts as a filter having a peak in the reflection spectrum periodically at a given interval. Both of the ring resonators  62  and  63  have a different diameter from the ring resonator  61 . The periodic peak of the reflection spectrum appears only in a given wavelength range because the ring resonators  62  and  63  are provided. Therefore, the ring resonators  62  and  63  have the same wavelength characteristics as the CSG-DBR region  11  of the semiconductor laser  10   a  of the first embodiment. 
     The ring resonators  62  and  63  have a heater on a ring and under the ring. Each heater controls the refraction index of the ring resonators  62  and  63 . Therefore, the oscillation wavelength of the semiconductor laser  10   a  may be controlled by controlling the temperature of each heater. 
     In the semiconductor laser  10   a , a vernier effect occurs with the overlapping between the peak of the reflection spectrum of the ring resonator  61  and that of the ring resonators  62  and  63 , and a wavelength at which an oscillation is established is selected. The SOA region  64  is a semiconductor optical amplifier allowing a gain in the resonators. 
     In the semiconductor laser  10   a , the wavelength characteristics illustrated in  FIG. 4  appears. Therefore, when the temperature of the ring resonators  62  and  63  is higher than the basal value by one periodic or more, the temperature of the ring resonator is reduced toward the basal value side by periodic. In this case, the electrical power provision amount to the heaters may be reduced without changing of the oscillation wavelength. Thus, the power consumption may be reduced. 
     The temperature of the ring resonator is used as the parameter for controlling the refraction index of the ring resonator in the embodiment. However, the structure is not limited to the embodiment. For example, the temperature of the heaters or the electrical power provision amount to the heaters may be used as the parameter for controlling the refraction index of the resonator. The refraction index of the resonator may be controlled with the current to the resonator, and the current value may be used as the parameter for controlling the refraction index of the resonator. 
     The present invention is not limited to the specifically described embodiments and variations but other embodiments and variations may be made without departing from the scope of the claimed invention.