Abstract:
Arrayed DBR (Distributed Bragg Reflector) laser shows a problem that spectrum purity is deteriorated when a current is flowed in a semiconductor optical amplifier for attaining a sufficient optical output. In addition, the arrayed waveguide grating laser shows a problem that the spectrum purity is deteriorated by leakage of light. An output end of each of the laser channels is provided with a gate (a core) that can be controlled through bias application. The gate has a function for amplifying light when the laser channels are operated and for absorbing light when the laser channels are not operated.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese Application Serial No. 334676/2004, filed on Nov. 18, 2004, the content of which is hereby incorporated by reference into this application.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to semiconductor laser apparatuses, and more particularly, to an arrayed Distributed Bragg Reflector (DBR)-semiconductor laser apparatus that realizes a tunable semiconductor laser capable of setting any optional wavelength.  
         [0004]     2. Description of the Related Art  
         [0005]     As tunable lasers capable of optionally setting a wavelength of the entire C-band of wavelength band used in a wavelength division multiplexer (WDM: Wavelength Division Multiplexer) system, DBR lasers having a special diffraction grating such as SG (Sampled Grating), SSG (Super Structure Grating) or the like have been developed. In order to perform laser oscillation at a desired wavelength, DBR lasers needed current control for optical phase adjustment in addition to current control for wavelength adjustment. The control system became complex and additionally it was hard to ensure long-term wavelength durability. In order to overcome the aforesaid problems, a short cavity DBR laser not requiring any optical phase adjustment has been recently developed (refer to “Selected Topics in Quantum Electronics”, IEEE Journal, Vol. 9, September/October of 2003, p. 1132-1137, for example). Since the variable range of wavelength per channel in this short cavity DBR laser is limited to 10 nm or less, it is necessary to array a plurality of DBR lasers for covering the entire C-band. As a form for realizing the foregoing arrangement, it has been known in the art to provide a semiconductor laser for combining DBR laser arrays through a multi-mode interferometer (MMI: Multi-Mode Interferometer) and amplifying it through a semiconductor optical amplifier (SOA: Semiconductor Optical Amplifier).  FIG. 1  shows an example of a laser chip having the DBR laser arrays, MMI and SOA integrated in monolithic form. The laser chip is made such that gain electrodes  101 ,  102 ,  103  and  104 , DBR electrodes  105 ,  106 ,  107  and  108  and SOA electrode  114  are formed on the surface of an InP substrate  100 . A waveguide structure is such that a DBR laser channel ch 1   115 , a DBR laser channel ch 2   116 , a DBR laser channel ch 3   117 , and a DBR laser channel ch 4   118  are arranged in parallel to one another, to each of which corresponding one of optical waveguides  109 ,  110 ,  111  and  112  is connected on the optical output side. The optical waveguides  109 ,  110 ,  111  and  112  are connected to an MMI multiplexer  113 , which is further connected to an SOA waveguide below an SOA electrode  114 .  FIG. 2A  is a plan view of the chip shown in  FIG. 1 . This laser chip includes a rear DBR region  138 , a gain region  139 , a front DBR region  140 , an S-shaped waveguide region  141 , an MMI region  142  and an SOA region  143 , which are integrated therein. A front end surface  144  and a rear end surface  145  are coated with a low reflection film.  FIG. 2B  is an ABCDE-sectional view of the chip shown in  FIG. 2A . In this case, the ABCDE-section is defined as sections of an optical path ranging from a semiconductor laser shown at the upper-most part in  FIG. 2A  to a semiconductor optical amplifier through the optical waveguides, a multiplexer for multiplexing the optical waveguides. The DBR laser part is made such that a core layer  132  in the gain region, a refractive index control core layer  134  in the rear DBR region and a refractive index control core layer  135  in the front DBR region are connected to one another. Refractive grating supplying layers  136  are disposed on the refractive index control core layer  134  in the rear DBR region and the refractive index control core layer  135  in the front DBR region. The refractive index control core layer  135  in the front DBR region is connected to a core layer  137  of the low loss optical waveguide, which forms the S-shaped waveguide region  141  and the MMI region  142 . Further, the core layer  137  is connected to the core layer  133  in the SOA region.  
         [0006]      FIG. 3  shows an example of the wavelength characteristic of the laser chip in  FIG. 1 . The DBR lasers of  4  channels, ch 1 , ch 2 , ch 3  and ch 4 , covers wavelength regions different from one another so as to cover the entire C-band. In order to provide a wavelength of 1540 nm, for example, a current is inputted to the gain layer and SOA layer of ch 2  and then a current of 100 mA is inputted to the DBR layer. At this time, no current is allowed to flow in the gain layer and DBR layer of the DBR laser with other channels.  
         [0007]     However, when a current is allowed to flow to provide a sufficient optical output, a problem arises of degrading spectrum purity.  FIG. 4  shows a spectrum obtained by operating the laser chip in  FIG. 1 . The laser chip is operated such that the DBR laser channel ch 4   118  is selected, a current of 20 mA is inputted to the gain electrode  102 , 5 mA to the DBR electrode  106  and 200 mA to the SOA electrode  114 . In addition to a main signal  301  of 1560 nm, both background light  302  with a narrow wavelength range and background light  303  with a wide wavelength range are generated, thus, degrading spectrum purity. As a result, an intensity ratio between the main signal and the background light (here, referred to as an SNR: Signal Noise Ratio) is 35 dB, which does not satisfy 40 dB requisite for optical communications in general.  
         [0008]     The background light  302  with a narrow wavelength range is probably due to the following: Spontaneous emission light occurring upon application of a current to the core layer  133  in the SOA region passes through the MMI region  142  and the S-shaped waveguide region  141 . Then, it reaches the refractive index control core layer  135  in the front DBR region in each of the DBR laser channels. Each of the front DBR refractive index control layers  135  of the DBR laser channels reflects spontaneous emission light. The reflected light is returned back to the core layer  133  in the SOA region, amplified there and then the amplified light is output from the end surface  144 . In addition, the background light  303  with a wide wavelength range is probably due to the following: Intensity of signal light inputted to the core layer  133  in the SOA region is weak because signal light generated by the DBR laser  118  is lost at the MMI. This leads to a large amount of current not used for amplifying the inputted light. This surplus current generates the background light  303  with a wide wavelength range.  
       SUMMARY OF THE INVENTION  
       [0009]     The foregoing discussion makes it clear that it is necessary to suppress the reflection at the front DBR region  140  so as to minimize the background light  302  with a narrow wavelength range. In addition, it is necessary to improve intensity of signal light generated by the DBR laser so as to minimize the background light  303  with a wide wavelength range. To meet the necessities, as shown in  FIG. 5 , the core layer  405  in the gate region (not referred sometimes to as a gate region specifically, but as a core layer in the claims and the present specification) and gate electrodes  401 ,  402 ,  403  and  404  are added at the emitting end surfaces of the DBR laser channels to the prior art configuration. This gate increases an optical output by applying current thereto to make the gate serving as a semiconductor optical amplifier when the laser channel is operated. When the DBR laser channel is not operated, the gate provides the following function by applying no current to the gate: The light is absorbed at the gate, the spontaneous emission light from the core layer  133  in the SOA region is reflected at the refractive index control core layer  135  in the front DBR region so as to prevent the light from being returned back to the core layer  133  in the SOA region. As a result, a laser operation with improved spectrum purity is accomplished.  
         [0010]     Then, referring to  FIGS. 6A and 6B , a more practical configuration and controlling method will be described. A band-gap value at the core layer  133  in the SOA region is defined as E SOA , an energy value of the signal light from the DBR laser channel ch 1  ( 115 ) as E sig1 , and a band-gap value of the core layer  405  in the first gate region optically connected to the DBR laser channel ch 1  ( 115 ) as E 1 . When the DBR laser channel ch 1  ( 115 ) is not operated (no signal light is outputted), light from the SOA is absorbed at the core layer in the first gate region with a relation of E SOA&gt;E   1 . In addition, when the DBR laser channel ch 1  ( 115 ) is operated, a relation of E SIG1&gt;E   1  is established, and then signal light from the DBR laser channel ch 1  ( 115 ) is amplified by inputting a current to the core layer  405  in the first gate region through the gate electrode. Further, an energy value of the signal light from the DBR laser channel ch 2  ( 116 ) is defined as E sig2 , and a band-gap value at the core layer  405  in the second gate region optically connected to the DBR laser channel ch 2  ( 116 ) is defined as E 2 . When the DBR laser channel ch 2  ( 116 ) is not operated (no signal light is outputted), light from the DBR is absorbed at the core layer in the second gate region under a relation of E SOA&gt;E   2 . When the DBR laser channel ch 2  ( 116 ) is operated, a relation of E SIG2&gt;E   2  is established, and signal light from the DBR laser channel ch 2  ( 116 ) is amplified by inputting an current to the core layer  405  in the second gate region through the gate electrode.  
         [0011]     Then, a controlling method will be described below. When the DBR laser channel ch 1  ( 115 ) is operated, the first gate region is operated to amplify the signal light and the second gate region is operated to absorb light from the SOA. To the contrary, when the DBR laser channel ch 2  ( 116 ) is operated, the second gate region is operated to amplify the signal light, the first gate region is operated to absorb light from the SOA. The aforesaid configuration and control accomplishes the laser operation with improved spectrum purity.  
         [0012]     There has been described up to now of a case where when the DBR laser channel ch 1  ( 115 ) is operated, the first gate region amplifies the signal light from the DBR laser channel ch 1  ( 115 ) and when the DBR laser channel ch 2  ( 116 ) is operated, the second gate region amplifies the signal light from the DBR laser channel ch 2  ( 116 ). However, it is not necessarily required for the gate region to amplify the signal light, but the gate region is needed only to allow signal light to pass therethrough without any loss. A practical configuration and controlling method of a system different from those of the foregoing will be described below.  
         [0013]     When the DBR laser channel ch 1  ( 115 ) is not operated (no signal light is outputted), light from the SOA is absorbed at the core layer in the first gate region with a relation of E SOA&gt;E   1 . When the DBR laser channel ch 1  ( 115 ) is operated (signal light is outputted), a relation of E SIG1 &lt;E 1  is established and the signal light from the DBR laser channel ch 1  ( 115 ) is allowed to pass. An amplifying action at the core layer is not substantially carried out. Further, when the DBR laser channel ch 2  ( 116 ) is not operated, a relation of E SOA &gt;E 2  is established, and light from the SOA is absorbed at the core layer in the second gate region. When the DBR laser channel ch 2  ( 116 ) is operated, a relation of E SIG2 &lt;E 2  is established, and the signal light from the DBR laser channel ch 2  ( 116 ) is allowed to pass. Also in this case, no amplifying action is substantially found at the core layer. A controlling method is the same as that found when the aforesaid gate region performs the amplifying operation. Such a configuration and controlling method described above accomplish a laser operation with improved spectrum purity.  
         [0014]     Both the aforesaid cases have a problem in that as a gate electrode is increased in number, the number of control electrodes is increased, which complicates control. Since current is inevitably allowed to flow the gain electrode and gate electrode of the operating DBR channel simultaneously, the gain electrode and the gate electrode can be used in common, whereby the present number of electrodes can be maintained.  
         [0015]     In addition, also when the gate layer is not especially installed, it is possible to improve spectrum purity by minus-biasing the front DBR region  140  when the DBR laser channel is not operated, thereby absorbing the spontaneous emission light from the core layer  133  in the SOA region into the DBR layer to prevent its reflection.  
         [0016]     Additionally, if the DBR laser, gate region, optical waveguide, multiplexer and SOA are integrated as a monolithic, the apparatus more preferably becomes small in size.  
         [0017]     In general, since the substrate is set to a ground potential, and the first and second gate regions are each provided with an electrode, associated components can be controlled.  
         [0018]     In addition, the respective core layers of the first and second gate regions may each be a multiple quantum wells layer or a bulk layer.  
         [0019]     The DBR laser has been described so far by way of example. Examples of more commonly used laser apparatuses include one which has a laser with a plurality of channels, an optical multiplexer for multiplexing the output beams from the plural channel laser, and an optical waveguide for connecting the output end of the plural channel laser with the optical multiplexer. In this laser apparatus, it is possible to attain a similar effect of improving a spectral purity by mounting a gate in the waveguides to be connected.  
         [0020]     Although there has been described up to now the case of two lasers, if the number of lasers were plural, the number of lasers is not limited. Four lasers may be applicable as indicated in the preferred embodiment of the present invention. When one laser outputs signal light, the other lasers are controlled not to output any signal light. When one laser outputs a signal light, the core layer connected to the laser is merely controlled to transmit the signal light therethrough or to amplify the signal light. At this time, an individual core layer connected to the other laser (one or a plurality of lasers) is controlled to absorb light from the semiconductor amplifier.  
         [0021]     The semiconductor laser apparatus of the preferred embodiment of the present invention enables a tunable laser that has high spectral purity and is operated with high-output to be realized by an easy process. The preferred embodiment of the present invention remarkably improves element performance and yields. In addition, an optical communication system to which the semiconductor laser apparatus is applied is easily realized to have a lower price, a large capacity and a long distance operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a perspective of a laser chip having a DBR laser array, MMI and SOA integrated as a monolithic;  
         [0023]      FIG. 2A  is a plan view of the laser chip shown in  FIG. 1 ;  
         [0024]      FIG. 2B  is a cross-sectional view of the laser chip shown in  FIG. 1 ;  
         [0025]      FIG. 3  shows an example of the wavelength characteristics of the laser chip shown in  FIG. 1 ;  
         [0026]      FIG. 4  shows an example of a spectrum of the laser chip shown in  FIG. 1 ;  
         [0027]      FIG. 5  is a perspective view of a laser chip based on the present invention;  
         [0028]      FIG. 6A  is a plan view of the laser chip shown in  FIG. 5 ;  
         [0029]      FIG. 6B  is a cross-sectional view of the laser chip shown in  FIG. 5 ;  
         [0030]      FIG. 7  is a block diagram for showing the configuration of a semiconductor laser apparatus in accordance with a preferred embodiment of the present invention;  
         [0031]      FIG. 8  is a plan view of the laser chip in accordance with the embodiment of the present invention;  
         [0032]      FIG. 9  is a block diagram for showing a configuration of the semiconductor laser apparatus based on the present invention;  
         [0033]      FIG. 10  is a block diagram for showing a configuration of the semiconductor laser apparatus based on the present invention; and  
         [0034]      FIG. 11  is a configuration view for showing the semiconductor laser in accordance with the preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     The preferred embodiments of the present invention will be described in detail below.  
       Embodiment 1  
       [0036]      FIGS. 5, 6A  and  6 B illustrate an example of a semiconductor laser with a 1550-nm-band according to the prevent invention. This semiconductor laser has a capability to change an oscillation wavelength by inputting a current to a refractive index control layer and selecting a DBR laser channel.  FIG. 5  is a perspective view,  FIG. 6A  is a top plan view, and  FIG. 6B  is an FGHIJ-sectional view of the chip shown in  FIG. 6A . The FGHIJ-section is defined as sections of an optical path ranging from a semiconductor laser shown at the upper-most part of  FIG. 6A  to a semiconductor optical amplifier through gate areas, optical waveguides, a multiplexer for multiplexing the optical waveguides. A channel DBR laser array, an MMI multiplexer and an SOA are integrated as a monolithic  
         [0037]     A method for manufacturing the laser apparatus will be described below. A core layer  132  in a gain region, a core layer  133  in an SOA region and a core layer  405  in a gate region are grown on an n-type ( 100 ) InP semiconductor substrate  131  by an organic metal vapor phase epitaxy. The core layers are made of a strain InGaAsP material, and composed of multiple quantum wells with 10 periods. A light-emitting wavelength of the multiple quantum wells is about 1550 nm. Later, the core layer  132  becomes a core layer in the gain region of the DBR laser, the core layer  405  becomes a core layer in the gate region and the core layer  133  becomes a core layer in the SOA region. Subsequently, a refractive index control core layer  134  in the InGaAsP (a composition wavelength of 1.40 μm) rear DBR region with a thickness of 0.4 μm, a refractive index control core layer  135  in the front DBR region and an InGaAsP (a composition wavelength of 1.3 μm) diffraction grating supply layers  136  with a thickness of 50 nm are formed in sequence by an organic metal vapor phase epitaxy using the well-known selective etching and a direct coupling technology for different waveguides. Then, a uniform diffraction grating is printed on the diffraction grating supply layers  136  by the well-known process. Diffraction grating periods are set to 236.6, 238.1, 239.7 and 241.2 nm so as to obtain different oscillation wavelengths at the DBR laser channels ch 1  ( 115 ), ch 2  ( 116 ), ch 3  ( 117 ) and ch 4  ( 118 ), respectively. In addition, the periods of the laser channels in the rear DBR region are made the same as those in the front DBR region. Subsequently, while the refractive index control core layer  134  in the rear DBR region, the core layer  132  in the gain region, the refractive index control layer  135  in the front DBR region, the core layer  405  in the gate region and the core layer  133  in the SOA region are protected using the well-known selective etching process and the direct coupling technology for different waveguides, a core layer  137  in the InGaAsP (a composition wavelength of 1.3 μm) low loss optical waveguide is formed in sequence by an organic metal vapor phase epitaxy. Then, a p-type InP clad layer with a thickness of 1.5 μm and a high density p-type InGaAs cap layer with a thickness of 0.2 μm are formed in sequence by an organic metal vapor phase epitaxy.  
         [0038]     Then, the waveguide is formed by the well-known selective dry etching technology using an insulation stripe shaped mask. Subsequently it is selectively implanted with Fe-doped InP by the organic metal vapor phase epitaxy. A mesa width is 1.3 μm. A rear DBR region  138 , a gain region  139 , a front DBR region  140 , a gate region  406 , an S-shaped waveguide region  141 , an MMI region  142  and an SOA region  143  are formed to have lengths of 300, 35, 120, 100, 500, 200 and 600 μm, respectively. A separation region of 5 μm is disposed between each region. The entire laser chip has a length of 2,000 μm. Thereafter, electrodes are provided on the front surface of the chip so as to supply current to the regions as shown in  FIG. 6A . A common electrode is provided on the rear surface of the chip. After the chip is cut into a laser chip length of 2,000 μm, the front end surface  144  and rear end surface  145  of the laser chip are each formed with a low reflection film with a reflectance of 0.01%.  
         [0039]     The distribution reflection type laser manufactured exhibited single mode oscillation with a 1550-nm-band at each of the four channels. In addition, this laser provided an output sufficient for an optical communication use with a chip light output of about 30 mW by use of a gain current of 20 mA, gate current of 50 mA and SOA layer current of 200 mA. As shown in  FIG. 3 , the laser provides as wide as a wavelength range of 1530 to 1570 nm by changing the DBR current and selecting the operating laser channel. An SNR at this time is 40 dB or more, which a sufficient value for the optical communication use.  
         [0040]     Then, a controlling method will be described below.  FIG. 7  shows a block diagram illustrating a configuration of a semiconductor laser apparatus according to the present invention. A tunable laser chip  502  is mounted in a semiconductor laser apparatus  501 . DBR layer electrodes  503 - 506 , gain layer electrodes  507 - 510 , gate layer electrodes  511 - 514  and an SOA electrode  515  are formed on the tunable laser chip  502 . In addition, this apparatus includes: a DBR power supply change-over switch  516  for selecting a DBR laser channel to input a wavelength adjustment current; a DBR power supply  517 ; a gain current change-over switch  518  for selecting a channel to input a gain current; a gain power supply  519 ; gate power supplies  522 - 525 ; an external communications port  526 ; an internal memory  527 ; and an interface  528 .  
         [0041]     A wavelength and optical output setting signal from the outside is inputted to the external communication port  526  and transferred to the internal memory  527  having a look-up table. The internal memory  527  sets the operating DBR laser channel, a DBR current and a gain current in response to the wavelength and optical output setting signal. In response to the setting of the internal memory, the DBR power supply  517  and the gain power supply  519  each generate a current, and the DBR power supply change-over switch  516  and the gain power. supply change-over switch  518  each select an electrode. In addition, the internal memory determines an SOA current in response to an optical output set signal and then the SOA power supply  520  generates a current. The gate power supplies  522 ,  523 ,  524  and  525  are operated such that a current is inputted from the gain power supply  519  to the operating DBR laser channel, and the remaining three channels not operated are short circuited or minus-biased to absorb the spontaneous emission light from the SOA.  
         [0042]     It becomes possible to install the semiconductor laser apparatus of the present invention in a small-sized module because the controlling method of this configuration is simplified.  
         [0043]     Although the foregoing description relates to the DBR laser array having four channels, the number of channels may be 2 or more, e.g., 6, 8 or 10. Additionally, as a typical preferred embodiment of the present invention, the buried laser structure using materials of InGaAsP has been described. The present invention can be similarly applied to all the semiconductor laser materials such as InGaAlAs, GaInNAs, InGaAs, InGaAlP and the like. In addition, the present invention can be applied similarly not only to the buried type laser apparatus, but also to the apparatus using the so-called ridge waveguide structure or a buried ridge structure. Additionally, although the wavelength band is a 1550-nm-band, it may also be a 1300-nm-band, which is frequently used for communications. Further, although the DBR laser array has been described, a distributed feedback (DFB: Distributed Feedback) laser array may be applied.  
       Embodiment 2  
       [0044]      FIG. 8  illustrates an example of a semiconductor laser manufactured with a band of 1550 nm in which a gain electrode and a gate electrode are used in common for each DBR laser channel. The laser in  FIG. 8  is different from that in  FIG. 7  in that a gain electrode and a gate electrode are used in common for each DBR laser channel, that is, gain and gate common electrodes  601 ,  602 ,  603  and  604  are formed. In the first embodiment, the number of the gate electrodes is increased, but a current is allowed to flow the gain electrode and gate electrode of a DBR channel that is inevitably operated, which makes it possible to use the gate electrode in common with the gain electrode. At this time, the current number of the electrodes can therefore be maintained.  FIG. 9  illustrates a block diagram for showing a configuration of the semiconductor laser apparatus in reference to  FIG. 8 . As the gain electrode and the gate electrode are used in common, the gain power supply and the gate power supply are made in common. In other words, gate and gate common power electrodes  701 ,  702 ,  703  and  704  are installed and concurrently gain and gate common power supplies  705 ,  706 ,  707  and  708  are installed.  
         [0045]     An external wavelength and optical output setting signal is inputted to an external communication port  526  and transferred to an internal memory  527  having a look-up table. The internal memory sets a DBR laser channel to be operated, a DBR current, a gain current and a gate current in response to the wavelength-setting signal. The DBR power supply  517  generates a current according to the settings of the internal memory, and the DBR power supply change-over switch  516  selects an electrode. In addition, the internal memory determines the SOA current in response to the optical output-setting signal, and the SOA power supply  520  generates a current. A current is supplied from the gain and gate power supply to the gain and gate layers in the DBR laser channel to be operated, and the gain and gate layers in the remaining three channels not to be operated are short circuited or minus-biased.  
         [0046]     The DBR laser manufactured oscillated in a single mode with a 1550-nm-band at each of the four channels. The laser obtained the sufficient output, an optical output of about 30 mW, for optical communication use by use of the sum of a gain current and gate current of 70 mA and the SOA current of 200 mA. As shown in  FIG. 3 , the laser provides as wide as a wavelength range of 1530 to 1570 nm by changing the DBR current and selecting the operating laser channel. An SNR at this time is 40 dB or more, which a sufficient value for the optical communication use.  
       Embodiment 3  
       [0047]      FIG. 10  is a block diagram for illustrating a configuration of a semiconductor laser apparatus that improves an SNR by modifying the controlling method by use of the prior art laser chip with a 1550-nm-band (having no gate layer). In this configuration, the DBR electrodes of three channels not operated are minus-biased and the bandgap of the DBR layer is enlarged in wavelength. Thus, spontaneous emission light from a core layer  133  of an SOA region is absorbed, thereby improving spectral purity. In this configuration, the DBR power supplies  801 ,  802 ,  803  and  804  are installed.  
         [0048]     An external wavelength and optical output setting signal is inputted to an external communication port  506  and transferred to an internal memory  527  having a look-up table. The internal memory sets a gain current in response to the wavelength-setting signal. The internal memory determines an SOA current in accordance with the optical output setting signal so that an SOA power supply  520  generates a current. The DBR power supply of an operating channel generates a predetermined current in response to a setting of the internal memory. The DBR power supplies for the other three channels not operated generate a minus-bias (e.g., −10 V).  
         [0049]     The DBR laser manufactured oscillated in a single mode with a 1550-nm-band at each of the four channels. The laser obtained the sufficient output, a optical output of about 30 mW, for optical communication use by use of the sum of a gain current and gate current of 70 mA and the SOA current of 200 mA. As shown in  FIG. 3 , the laser provides as wide as a wavelength range of 1530 to 1570 nm by changing the DBR current and selecting the operating laser channel. An SNR at this time is 40 dB or more, which a sufficient value for the optical communication use.  
       Embodiment 4  
       [0050]     This embodiment corresponds to a case where the first embodiment is generalized so that a gate structure is useful in improving spectral purity. A semiconductor laser apparatus is configured to include a laser with a plurality of channels, an optical multiplexer for multiplexing the output beams from the plural channel laser, and an optical waveguide for connecting the output end of the plural channel laser with the optical multiplexer.  
         [0051]     A tunable laser with sixteen tunable arrayed waveguide gratings (AWG: Arrayed Waveguide Grating) is shown as an example of the aforesaid structure.  FIG. 11  illustrates a configuration of a laser where a gate electrode is added to the tunable AWG laser. A tunable AWG laser  901  includes a multiplexer  902 , gate electrodes  903 ,  904 ,  905  and  906 , front gain electrodes  907 ,  908 ,  909  and  910 , a  4 × 4  AWG  911  and rear gain electrodes  912 ,  913 ,  914  and  915 . Its operating principle will be described below. For example, when a current is inputted to the front gain electrode  907  and the rear gain electrode  912 , laser oscillation occurs through the 4×4 AWG  911 . In this case, the AWG has wavelength selectivity, whereas the front gain electrode  907  and the rear gain electrode  912  cause only light with a certain specified wavelength to pass therethrough. Thus, a single mode oscillation occurs at a specific wavelength. Part of the oscillation light is taken out through the optical coupler and outputted through the gate electrode  906  and the multiplexer  902 . At this time, there occurs a problem in that spontaneous emission light from the front gain electrode  907  and the rear gain electrode  915  leaks into the other channels of the AWG, outputted through the other channels, deteriorating the SNR of the signal light. To prevent this problem, the gate electrodes  904 ,  905  and  906  not related to the oscillation are short circuited or minus-biased, thus, preventing the spontaneous emission light from being outputted.  
         [0052]     The tunable arrayed waveguide grating laser oscillated in a single mode at each of the sixteen wavelengths with a 1550-nm-band. An SNR at this time was 40 dB or more, which is a sufficient value for optical communication use.  
         [0053]     A description of reference numerals used in the drawings of the present application is as follows:  
         [0000]      100 : InP substrate  
         [0000]      101 ,  102 ,  103 ,  104 : gain electrode  
         [0000]      105 ,  106 ,  107 ,  108 : DBR electrode  
         [0000]      109 ,  110 ,  111 ,  112 : optical waveguide  
         [0000]      113 : MMI multiplexer  
         [0000]      114 : SOA electrode  
         [0000]      115 : DBR laser channel ch 1   
         [0000]      116 : DBR laser channel ch 2   
         [0000]      117 : DBR laser channel ch 3   
         [0000]      118 : DBR laser channel ch 4   
         [0000]      131 : n-type ( 100 ) InP semiconductor substrate  
         [0000]      132 : core layer in the gain region  
         [0000]      133 : core layer in the SOA region  
         [0000]      134 : refractive index control core layer in the rear DBR region  
         [0000]      135 : refractive index control core layer in the front DBR  
         [0000]     region  
         [0000]      136 : diffractive grating supplying layer  
         [0000]      137 : core layer in the low loss optical waveguide  
         [0000]      138 : rear DBR region  
         [0000]      139 : gain region  
         [0000]      140 : front DBR region  
         [0000]      141 : S-shaped waveguide region  
         [0000]      142 : MMI region  
         [0000]      143 : SOA region  
         [0000]      144 : front end surface  
         [0000]      145 : rear end surface  
         [0000]      301 : major signal  
         [0000]      302 : background light of narrow wavelength range  
         [0000]      303 : background light of wide wavelength range  
         [0000]      401 ,  402 ,  403 ,  404 : gate electrode  
         [0000]      405 : core layer in the gate region  
         [0000]      406 : gate region  
         [0000]      501 : semiconductor laser apparatus  
         [0000]      502 : tunable laser chip  
         [0000]      503 - 506 : DBR electrode  
         [0000]      507 - 510 : gain electrode  
         [0000]      511 - 514 : gate electrode  
         [0000]      515 : SOA electrode  
         [0000]      516 : DBR power supply change-over switch  
         [0000]      517 : DBR power supply  
         [0000]      518 : gain power supply change-over switch  
         [0000]      519 : gain power supply  
         [0000]      522 - 525 : gate power supply  
         [0000]      526 : external communications port  
         [0000]      527 : internal memory  
         [0000]      528 : interface unit  
         [0000]      601 ,  602 ,  603 ,  604 : gain and gate common electrode  
         [0000]      701 ,  702 ,  703 ,  704 : gain and gate common electrode  
         [0000]      705 ,  706 ,  707 ,  708 : gain and gate common power supply  
         [0000]      801 ,  802 ,  803 ,  804 : DBR power supply  
         [0000]      901 : tunable wavelength AWG laser  
         [0000]      902 : multiplexer  
         [0000]      903 ,  904 ,  905 ,  906 : gate electrode  
         [0000]      907 ,  908 ,  909 ,  910 : front gain electrode  
         [0000]      911 :  4 × 4  AWG  
         [0000]      912 ,  913 ,  914 ,  915 : rear gain electrode  
         [0000]      916 : optical path in AWG