Abstract:
A wavelength tunable laser device includes: a pair of reflection mirrors; a semiconductor element disposed between the pair of reflection mirrors, the semiconductor element integrating a region for providing an optical gain, a region having a wavelength tunable filter function and a phase control region; and an optical filter disposed between the semiconductor element and one of the pair of reflection mirrors, the optical filter having periodical transmission wavelengths. A wavelength tunable laser device is provided which is easy to be controlled and can be made compact.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and claims priority of Japanese Patent Application No. 2004-118194 filed on Apr. 13, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     A) Field of the Invention  
         [0003]     The present invention relates to a laser device and more particularly to a wavelength tunable laser device.  
         [0004]     B) Description of the Related Art  
         [0005]     In order to cope with increasing data traffic, a wavelength division multiplexing (WDM) optical communication system has been developed and is in practical use, which transmits optical signals of a plurality of wavelengths via a single optical fiber at a time. In a conventional WDM optical communication system, sophisticated processes such as optical add drop multiplexer (OADM), wavelength routing and optical packet transmission have been studied in order to realize a large capacity flexible system by positively utilizing the wavelength information of an optical signal. In order to realize such processes, a wavelength tunable laser of a single device having a wide wavelength tunable range and a high output power has been desired as a light source.  
         [0006]     Japanese Patent Laid-open Publication No. 2003-283024 proposes a novel structure of a wavelength tunable laser of a single device and a high output having a wide wavelength tunable range.  
         [0007]      FIG. 5A  shows the structure proposed in Japanese Patent Laid-open Publication No. 2003-283024. A gain medium  54  having an optical gain in a wide wavelength range, a band-pass filter  55  and an etalon filter  56  are disposed along an optical axis  57  between a reflection mirror  51  and a semi-transparent mirror  52 . The reflection mirror  51  and semi-transparent mirror  52  define a cavity  50 . The gain medium  54  is structured, for example, by a semiconductor optical amplifier (SOA). The band-pass filter  55  is structured, for example, by an acousto-optical tunable filter (AOTF). A frequency controller  58  controls the pass band of the band-pass filter  55 . The etalon filter  56  is structured by a Fabry-Perot etalon filter. The etalon filter is designed and disposed so that it has a periodical sharp transmission (wavelength) spectrum, for example, satisfying the specifications of an International Telecommunications Union (ITU) grid.  
         [0008]     Optical gain extending over a wide frequency range are generated by the gain medium  54 , and only narrow band energies are filtered by the band-pass filter  55 , and further filtered by the etalon filter having transmission wavelengths of the periodical sharp transmission characteristics. Resonance is formed by the cavity. An SOA-AOTF laser using such SOA and AOTF generates a high output laser beam in a single mode.  
         [0009]     The specific structure and measured performances of such an SOA-AOTF laser was pronounced by K. Takabayashi et al. in Proc. of ECOC 2003, vol. 4, 890.  
         [0010]      FIG. 5B  shows the structure of an SOA-AOTF laser. The layout of this structure has the reversed right and left of the structure shown in  FIG. 5A . A semiconductor optical amplifier SOA is used as the gain medium  54 , whose one facet constitutes the semi-transparent mirror  52 . The band-pass filter  55  and Fabry-Perot etalon  56  are disposed between SOA and the reflection mirror  51 .  
         [0011]     The band-pass filter  55  is constituted of two AOTF having a symmetrical structure to eliminate a Doppler shift. A lens  59  is disposed between the band-pass filter  55  and Fabry-Perot etalon  56  to improve an optical coupling efficiency. The cavity  50  defined between the semi-transparent mirror  52  and reflection mirror  51  has a length of 50 mm. About 90 nm of a wavelength tunable width Δλ is obtained.  
         [0012]     A combination of these three devices can be considered promising: a device having an optical gain over a wide wavelength range; a wavelength filter having a sharp (periodical, fixed or finely adjusted) wavelength selectivity; and a wavelength filter having a wide wavelength tunable range with a coarse wavelength selectivity.  
         [0013]     Japanese Patent Laid-open Publication No. HEI-6-29628 proposes an optical coupler and a semiconductor laser made of a gain section and a wavelength selection coupler integrated together. Lower and upper waveguides are formed in a semiconductor structure, and a diffraction grating is formed above the lower and upper waveguides to provide selective coupling between the lower and upper waveguides. The expected filter band width is described as 2.5 nm.  
         [0014]     IEEE Photonics Technology Letters, vol. 7, no. 7, (1995) 697-699 propose a grating assisted codirectional coupler laser with super structure grating reflector (GCSR) laser having the above-described three functions integrated together. A gain section, a coupler section, a phase control section and a reflection section are formed on an InP substrate and electrodes are formed independently for each section. The coupler section has upper and lower waveguides and a diffraction grating formed above the upper and lower waveguides, similar to the wavelength selection coupler of Japanese Patent Laid-open Publication No. HEI-6-29628, and realizes the wavelength tunable filter function by utilizing the phenomenon that light in a specific wavelength range determined by an equivalent refractive index difference between two waveguides controllable by current injection is selectively moved between two waveguides. The reflection section is structured by a superstructure grating reflector and has the periodical sharp wavelength filer characteristics. The transmission peak wavelength of this filter can also be finely adjusted by current injection. A single longitudinal mode oscillation can be realized and a wide wavelength tunable range Δλ of 100 nm can be obtained.  
       SUMMARY OF THE INVENTION  
       [0015]     An object of this invention is to provide a wavelength tunable laser device easy to be controlled and excellent in a wavelength selectivity.  
         [0016]     Another object of this invention is to provide a wavelength tunable laser device easy to be controlled and capable of a high optical output.  
         [0017]     According to one aspect of the present invention, there is provided a wavelength tunable laser device comprising: a pair of reflection mirrors; a semiconductor device disposed between the pair of reflection mirrors, the semiconductor device integrating a region for providing an optical gain, a region having a wavelength tunable filter function and a phase control region; and an optical filter disposed between the semiconductor device and one of the pair of reflection mirrors, the optical filter having periodical transmission wavelengths.  
         [0018]     The semiconductor device having the gain region, wavelength tunable region and phase control region can be controlled easily by three electrodes. A combination of the optical filter having the periodical sharp transmission wavelength characteristics and the semiconductor device can realize oscillation at a single wavelength.  
         [0019]     A wavelength tunable laser device can be provided which has a high output, a broad tunable wavelength range and an easy control. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a longitudinal cross sectional view showing briefly the structure of a wavelength tunable laser device according to an embodiment of the invention.  
         [0021]      FIG. 2  is a graph showing an oscillation mode of the wavelength tunable laser device shown in  FIG. 1 .  
         [0022]      FIGS. 3A  to  3 E are cross sectional views illustrating an example of main manufacture processes for the wavelength tunable laser device.  
         [0023]      FIG. 4  is a perspective view briefly showing the structure of an optical communication system.  
         [0024]      FIGS. 5A and 5B  are a block diagram and a cross sectional view showing the structure of a wavelength tunable laser device having an SOA-AOTF structure according to prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]      FIG. 1  is a cross sectional view schematically showing the structure of a wavelength tunable semiconductor laser device according to an embodiment of the present invention. A semiconductor device  30 , a Fabry-Perot etalon  31  and a reflection mirror  32  are disposed along an optical axis. If necessary, a lens  34  is mounted. The left side end face of the semiconductor device  30  is a cleavage plane or a low reflection (partial reflection) plane  27  low-reflection coated with a dielectric multi-layer film. The right end face of the semiconductor device is a non-reflection plane  26  non-reflection coated with a dielectric multi-layer film. The low reflection plane  27  and mirror  32  define a cavity. The semiconductor device  30  has a gain region G, a phase control region PH, a directional coupler region DC and an absorption region AB. Although the etalon filter is made of a solid state etalon filter, it may be made of an air gap etalon filter.  
         [0026]     The semiconductor device  30  has an InGaAsP lower waveguide layer  12 , an n-type InP spacer layer  13  and an upper waveguide layer formed on an n-type InP substrate in this order from the bottom. The lower waveguide layer  12  has a bandgap wavelength longer than the bandgap of InP and shorter than an object oscillation wavelength (having a higher refractive index than that of InP). The upper waveguide layer has a bandgap wavelength longer than that of InP (having a higher refractive index than InP).  
         [0027]     The upper waveguide layer is constituted of: a multiple quantum well active layer  14  capable of generating light having a wavelength in the objective 1.5 micron band, formed on the left side; a multiple quantum well absorbing layer  15  capable of absorbing light generated by the multiple quantum well active layer  14 , formed on the right side; and waveguide layers  16  and  17  capable of transmitting light generated by the multiple quantum well active layer  14 , formed between the multiple quantum well active layer  14  and multiple quantum well absorbing layer  15 . The upper and lower waveguide layers  17  and  12  have different reflectivities (refractive index difference Δn). The multiple quantum well absorbing layer  15  can be made of the same lamination structure as that of the multiple quantum well active layer  14 . The multiple quantum well structure has preferably an optical confinement structure (semiconductor confinement hetrojunction: SCH) by adding a low reflective layer above and under a multiple quantum well.  
         [0028]     A p-type InP upper spacer layer  18  is formed on the upper waveguide layer, and a diffraction grating  19  having a period of 15 μm is formed on a partial upper surface of the upper spacer layer  18 . A p-type InP buffer layer  20  covers the diffraction grating. On the p-type buffer layer  20 , electrodes  22 ,  23  and  24  are formed at positions corresponding to the upper waveguide layers  14 ,  16  and  17 . An electrode  21  is formed on the bottom of the substrate  11 .  
         [0029]     A light amplifying function of the semiconductor device can be realized by the composite structure of the electrode  22 , buffer layer  20 , spacer layer  18 , multiple quantum well active layer  14 , spacer layer  13 , lower waveguide layer  12 , substrate  11  and electrode  21 , respectively in the gain region G. Light can be generated in the multiple quantum well by flowing a forward current from the electrode  22  toward the electrode  21 .  
         [0030]     A phase control function can be realized by the composite structure of the electrode  23 , buffer layer  20 , spacer layer  18 , upper waveguide layer  16 , spacer layer  13 , lower waveguide layer  12 , substrate  11  and electrode  21 , respectively in the phase control region PH. The upper waveguide layer  16  in the phase control region has a composition transparent to light generated from the multiple quantum well, and the refractive index can be controlled by flowing a forward current from the electrode  23  toward the electrode  21 .  
         [0031]     A diffraction grating loading directional coupler can be realized by the composite structure of the electrode  24 , buffer layer  20 , diffraction grating  19 , spacer layer  18 , upper waveguide layer  17 , spacer layer  13 , lower waveguide layer  12 , substrate  11  and electrode  21 , respectively in the directional coupler region DC. Light having a specific wavelength moves between the upper and lower waveguide layers, depending upon the period of the diffraction grating and the equivalent refractive index difference An between those of the upper and lower waveguide layers.  
         [0032]     The composite structure of the buffer layer  20 , spacer layer  18 , multiple quantum well absorbing layer  15 , spacer layer  13 , lower waveguide layer  12  and substrate  11 , respectively in the absorbing region AB, has no upper electrode. The multiple quantum well absorbing layer  15  of the upper waveguide layer absorbs light generated from the multiple quantum well active layer  14  and transmitted through the upper waveguide layers  16  and  17 . The lower waveguide layer  12  transmits light moved from the upper waveguide layer  17 .  
         [0033]     Light generated in the multiple quantum well active layer  14  in the gain region G moves to the lower waveguide layer  12  via the phase control region PH and diffraction grating loading directional coupler DC, and is subjected to gentle wavelength selection. Light transmitted through the lower waveguide layer and output from the non-reflection plane  26  at the right side end face of the semiconductor device  30  is given a sharp wavelength selectivity by transmitting through the Fabry-Perot etalon  31 . Light reflected at the mirror  32  and returned to the Fabry-Perot etalon  31  and to the semiconductor device  30  moves in the semiconductor device  30  along a path reversing the path described above, and is returned at the low reflection plane  27  at the left side end face to repetitively reciprocate in the cavity. With these operations, oscillation occurs in the single longitudinal mode.  
         [0034]      FIG. 2  is a graph showing the wavelength selectivity of the oscillation mode. The abscissa represents a wavelength and the ordinate represents a light intensity or transmissivity in a simplified shape. The longitudinal mode s has a wavelength interval defined by a cavity length. In order to broaden the interval of longitudinal modes s, it is preferable to shorten the cavity length.  
         [0035]     The structure shown in  FIG. 1  using the semiconductor device  30  integrating the gain device and the wavelength filter with a wide wavelength tunable range, can shorten the cavity length more than the structure (SOA-AOTF) using a semiconductor optical amplifier (SOA) and an acousto-optical tunable filter (AOTF) having a wide wavelength tunable range. The structure shown in  FIG. 1  can be implemented easily, and the coupling loss in the resonator is small so that a high output is possible.  
         [0036]     Wavelength selectivities f 1  of the Fabry-Perot etalon  31  are sharp and distributed periodically on the wavelength axis. The transmission wavelength of the Fabry-Perot etalon is preferably made in conformity with the grid stipulated by ITU (e.g., wavelengths at an interval of 100 GHz or 50 GHz at the center wavelength of 194.1 THz). With this arrangement, the laser oscillation wavelength is always on the predetermined wavelength grid, without electric control.  
         [0037]     A wavelength selectivity f 2  of the diffraction loading directional coupler is coarse in a relatively broad wavelength range. A combination of the etalon and diffraction grating directional coupler in a short cavity length can provide oscillation at the single longitudinal mode wavelength.  
         [0038]     In the structure shown in  FIG. 1 , the normal to the Fabry-Perot etalon is not made coincident with the optical axis, but is inclined several degrees. This inclination can prevent light reflected at the Fabry-Perot etalon from returning to the optical waveguide path of the semiconductor device. A wavelength tunable laser device having a cavity length of about 1 cm can be formed by setting the length of the semiconductor device to about 2 mm and the length, on the optical axis, of the structure of the lens, etalon and mirror, to about 8 mm.  
         [0039]     Description will be made on a manufacture method for the semiconductor device shown in  FIG. 1  including a 1.55 μm band semiconductor laser.  
         [0040]     As shown in  FIG. 3A , on an n-type InP base substrate  11   a,  an n-type InP buffer layer  11   b  is grown to a thickness of 500 nm to form an n-type InP substrate  11 . Crystal growth can be performed, for example, by metal organic chemical vapor deposition (MOCVD) method. As shown in  FIG. 1 , the gain region G, phase control region PH, directional coupler (grating coupler) region DC and absorbing region AB are reserved on the substrate from the left to right in this order. On the n-type InP substrate  11 , an n-type InGaAsP lower waveguide layer  12  is grown and an n-type InP lower spacer layer  13  having a thickness of 900 nm is grown on the lower waveguide layer. The lower waveguide layer has a bandgap of 1.1 μm and a thickness of 100 nm.  
         [0041]     On the n-type InP lower spacer layer  13 , an non-doped InGaAsP lower optical confinement layer  14   a  is grown to a thickness of 180 nm. On the lower optical confinement layer  14   a,  a multiple quantum well layer  14   x  having a thickness of 103 nm is formed. The multiple quantum well layer realizes an oscillation frequency of 1.55 μm and is a lamination of six layers of quantum well layers and corresponding barrier layers alternately stacked. On the multiple quantum well layer  14   x,  another non-doped optical confinement layer  14   b  having a thickness of 180 nm is formed, to thereby obtain a multiple quantum well active layer  14  having a total thickness of 460 nm. The optical confinement layers  14   a  and  14   b  have a refractive index lower than that of the multiple quantum well layer  14   x  and provide a function of confining light in the multiple quantum well layer in the longitudinal direction. On the multiple quantum well structure, a p-type InP first upper spacer layer  18   a  is grown to a thickness of 60 nm.  
         [0042]     As shown in  FIG. 3B , on the first upper spacer layer  18   a  in the gain region and absorbing region, a hard mask layer HM 1  of SiO 2  or the like is formed and the first upper spacer layer  18   a  and multiple quantum well structure  14  in the exposed phase control region and grating coupler region are etched. The multiple quantum well structure is left in the gain region and absorbing region.  
         [0043]     In the etched and removed region, an InGaAsP upper waveguide layer  16  ( 17 ) having a band edge wavelength λ=1.4 μm and a thickness of 380 nm and a p-type InP second upper spacer layer  18   b  having a thickness of 140 nm are grown through butt joint. The hard mask layer HM 1  is thereafter removed. The InP first upper spacer layer  18   a  and InP second upper spacer layer  18   b  form an InP layer continuous in a lateral direction.  
         [0044]     As shown in  FIG. 3C , on the InP layers  18   a  and  18   b,  a p-type InP third spacer layer  18   c  having a thickness of about 50 nm is grown. The InP layers  18   a,    18   b  and  18   c  form an InP upper spacer layer  18 . On the InP upper spacer layer  18 , an InGaAsP diffraction grating layer  19   a  is grown which has a band edge wavelength λ=1.38 μm and a thickness of 70 nm. On the diffraction grating layer  19   a,  a hard mask HM 2  of SiO 2  or the like is formed, which has a diffraction grating pattern at a period of 15 μm. By using the hard mask HM 2 , an unnecessary region of the diffraction grating layer  19   a  is etched and removed. The hard mask HM 2  is thereafter removed.  
         [0045]     As shown in  FIG. 3D , a p-type InP buffer layer  20  is grown to a thickness of about 300 nm as measured on the diffraction grating  19 , burying the diffraction grating  19 . The structure formed is a basic structure of the semiconductor device.  
         [0046]     As shown in  FIG. 3E , on the semiconductor lamination structure, a hard mask of a stripe pattern is formed which is made of SiO 2  and has a mesa width of about 1.5 μm. By using this hard mask, a mesa stripe structure is formed by dry etching.  FIG. 3E  is a cross sectional view taken along a direction perpendicular to the direction in  FIG. 3D . After the mesa etching, a p-type InP burying layer  35  and an n-type InP burying layer  36  are grown to pn-burying the mesa stripe structure (a portion of a p-type InP layer  38  to be formed thereafter may be grown). After the hard mask is removed, a p-type InP layer  38  and an InGaAsP contact layer (λ=1.1 μm)  39  are formed. Electrodes (electrodes  21  to  24  shown in  FIG. 1 ) are formed on the bottom of the substrate and on the upper surface of the contact layer, by well-known methods. Dielectric multi-layer films of low reflection coating and non-reflection coating are formed on opposite end faces to complete the semiconductor device shown in  FIG. 1 .  
         [0047]     The substrate of the semiconductor device is not limited only to the InP substrate. Other group III-V compound semiconductor substrates such as a GaAs substrate may also be used for forming a laser device having a different wavelength band such as a 1.3 μm band.  
         [0048]      FIG. 4  is a schematic diagram showing an optical communication system using the wavelength tunable laser device described above. In an optical network  40 , a plurality of optical add drop multiplexers (OADMs)  41 ,  42 ,  43  and  44  are interconnected by an optical fiber  48 . The optical fiber  48  transmits optical signals having wavelengths λ 1 , λ 2 , . . . , λn. The optical add drop multiplexer  44  has a roll of picking up light having a wavelength λ y  via a wavelength selection filter  45  and transmitting light having a wavelength λ x  generated by a waveform tunable laser  46  to the optical fiber  48 . The wavelength tunable laser device  46  is realized by the structure shown in  FIG. 1 .  
         [0049]     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.