Abstract:
In order to form a wavelength tunable laser capable of tuning a wave over a wide range by simple control means, a thin film heater is mounted either over an upper electrode of a ridge waveguide semiconductor laser having ridge waveguides on a semiconductor substrate or over the semiconductor substrate and on both sides of the ridge waveguide with a gap of a few μm. By controlling a current passed to the thin film heater, the oscillation wavelength of the semiconductor laser is tuned. In the case where the thin film heater is mounted over an upper electrode of a ridge waveguide, a nonconductor is formed on both sides of the ridge conductor to more efficiently enable heat from the heater to reach an active layer of the ridge waveguide more efficiently.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a wavelength tunable laser and an optical device using the wavelength tunable laser. More particularly, the invention relates to a semiconductor laser capable of tuning a lasing wavelength over a wide range, an optical modulator using the semiconductor laser, and a wavelength-division multiplexing transmission system employing, as a light source, a semiconductor laser used for a wavelength-division multiplexing optical system for multiplexing a plurality of different signal light and transmitting the multiplexed signal. 
     One of the important techniques in a wavelength-division multiplexing optical system is management of a wavelength of a light source of each of a plurality of channels. In the present optical communication systems, in order to maintain the wavelength of the light source at a predetermined value, a wavelength monitor and means for stabilizing the wavelength of the light source by feedback are provided for each channel and a spare light source prepared for a failure is provided for each of all of the channels. The number of related electronic devices therefore increases according to the number of channels. It is also necessary to control each semiconductor laser so that its lasing wavelength is within a predetermined narrow wavelength band. It is consequently difficult to improve the manufacturing yield. Such issues interfere with the attempt to achieve miniaturization and reduction in cost of an optical transmission system and are significant issues in the case of further narrowing the interval between waves of channels or the case of increasing the number of channels. 
     On the other hand, there is an idea such that the lasing wavelengths necessary for a plurality of channels are covered by a single backup light source by using a lasing wavelength tunable semiconductor laser. In this case, a wavelength tunable semiconductor laser capable of easily and successively sweeping the lasing wavelength is necessary, but has not been realized until now. 
     In particular, in an optical multiplexing transmission of a long distance, it is necessary to realize the system in a form that an optical modulator is monolithically integrated by which chirping can be reduced. In a monolithic integrated light source in which an optical modulator is incorporated, by adjusting the temperature of the whole light modulator, the wavelength of a channel can be adjusted. Presently, however, the operating temperature range of the monolithic integrated optical modulator is as narrow as ±5 degrees centigrade. The width of the wavelength which can be swept in practice is therefore only about 0.5 nm. 
     FIG. 9 shows the configuration in cross section of a wavelength tunable semiconductor laser capable of easily and successively sweeping the wavelength, in which a heater electrode is attached to a conventional buried heterostructure semiconductor laser. (For example, a technique described in IEEE Photonics Technology Letters, Vol. 4, p. 321, 1992 can be mentioned as a wavelength-division multiplexing system light source of this kind). According to the technique, a heater layer is formed over an upper electrode of a buried heterostructure semiconductor laser via an insulating film to control the temperature of an active layer. As shown by arrows with thick lines, since the heat generated by the heater layer escapes into not only the active layer but also a buried layer, the active layer cannot be efficiently heated. It is therefore a problem that the wavelength tuning efficiency, that is, the wavelength fluctuation range per unit power in the wavelength tunable semiconductor laser is as low as 3.2 nm/W. 
     SUMMARY OF THE INVENTION 
     It is therefore a main object of the invention to realize a wavelength tunable laser capable of tuning a wavelength over a wide range by simple means. 
     It is another object of the invention to realize a wavelength-division multiplexing transmission system which achieves the object and is suitable for a long distance transmission by using the wavelength tunable laser. 
     In order to achieve the objects, a wavelength tunable laser according to the invention is formed by mounting a thin film heater layer over and/or on a side of an upper electrode of a ridge waveguide semiconductor laser on a semiconductor substrate. The ridge waveguide semiconductor laser is obtained by forming a waveguide constructing a semiconductor laser in a ridge shape on a semiconductor substrate including a light emitting layer. The cross section of the ridge can have a shape of rectangle, trapezoid, or the like. An inverse trapezoid (inverse mesa) shape in which the side in contact with the semiconductor substrate is narrower than the upper side is preferable. 
     One of optical devices according to the invention is an integrated optical device in which the wavelength tunable laser and an external optical modulator are integrated on a semiconductor substrate of the wavelength tunable laser. 
     Further, another optical device according to the invention constructs a wavelength-division multiplexing transmission system for multiplexing light signal of a plurality of channels of different wavelengths and transmitting the light signal through a light transmission line. One or more wavelength tunable lasers are used as spare light source(s) of the plurality of light sources of the plurality of channels. When one of the light sources of the channels becomes faulty or the like and has to be replaced, the spare light source is allowed to operate and its wavelength is made coincide with the wavelength of the light source of the channel to be replaced by using the wave tuning function of the wavelength tunable laser. 
     The wavelength tunable laser of the invention enables the heat generated by the thin film heater to be efficiently applied to the light emitting part of the semiconductor laser. A monolithic integrated device is formed by combining the wavelength tunable laser with an optical modulator to thereby provide each of many optical devices such as a wavelength-division multiplexing transmission system with the effective means. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent in view of the following description of the preferred embodiments in conjunction with accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view showing the structure of a monolithic integrated optical device as an embodiment of a wavelength tunable laser according to the invention. 
     FIG. 1B shows an enlarged portion of FIG.  1 A. 
     FIG. 2 shows the result of measurement of the wavelength tuning characteristic of the wavelength tunable laser according to an embodiment of the present invention. 
     FIG. 3A is a perspective view showing the structure of a monolithic integrated optical device constructing an optical modulator as an embodiment of an optical device using the wavelength tunable laser according to the invention. 
     FIG. 3B shows an enlarged portion of FIG.  3 A. 
     FIG. 4A is a perspective view showing the structure of a monolithic integrated optical device constructing an optical modulator as an embodiment of an optical device using the wavelength tunable laser according to the invention. 
     FIG. 4B shows an enlarged portion of FIG.  4 A. 
     FIG. 5 is a system configuration diagram showing the configuration of a wavelength-division multiplexing optical system to which a wavelength tunable laser according to the invention is applied. 
     FIG. 6 is a block diagram showing the configuration of the main part of FIG.  5 . 
     FIG. 7 is a perspective view for explaining an embodiment of a spare light source corresponding to a spare light source  507  in FIG. 6 or the like. 
     FIG. 8 is a block diagram showing another embodiment of a wavelength-division multiplexing transmission system. 
     FIG. 9 is a cross section of a wavelength tunable laser which is a conventionally known buried semiconductor laser. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 1A is a perspective view showing the configuration of a monolithic integrated optical device as an embodiment of a wavelength tunable laser according to the invention. The cross sectional configuration of the main part of the embodiment is shown in an enlarged diagram FIG.  1 B. 
     In the wavelength tunable laser, a buffer layer  102  and an active layer  103  are formed on a semiconductor substrate  101  and a ridge waveguide which includes a cladding layer  106  and a cap layer  107  and has an inverse mesa shape is formed. On the active layer  103 , polyimide as a nonconductor  115  is formed via SiO 2  on both sides of the ridge waveguide. Further, an upper electrode  108  of a semiconductor laser is formed on the ridge waveguide and a thin film heater  110  is mounted over the upper electrode  108  via an insulating film  109 . 
     An embodiment of a method of fabricating the wavelength tunable laser will be described hereinbelow. On an n type ( 100 ) InP semiconductor substrate  101 , 1.0 μm of an n type InP buffer layer  102 , the active layer  103 , and 0.02 μm of a first p type InP cladding layer  104  are sequentially deposited by metalorganic vapor-phase epitaxy. The active layer  103  has 0.05 μm of an n type InGaAsP lower guide layer (composition wavelength of 1.10 μm), a multiple quantum well layer of 5 cycles (a well layer made of InGaAsP having a thickness of 6.0 nm and compressive strain of 1% (composition wavelength of 1.70 μm) and a barrier layer made of InGaAsP having a thickness of 10 nm (composition wavelength of 1.15 μm)) and 0.05 μm of an upper guide layer made of InGaAsP (composition wavelength of 1.15 μm). The wavelength of light emitted from the multiple quantum well active layer  103  is about 1.56 μm. 
     A grating  105  of uniform cycles of 241 nm is formed on the whole face of the substrate by holographic photolithography and wet etching. The depth of the grating is about 50 nm. Subsequently, 1.7 μm of the second p type InP cladding layer  106  and 0.2 μm of a highly doped p type InGaAs cap layer  107  are formed by metalorganic vapor-phase epitaxy. 
     Subsequently, a process is performed to obtain an inverse-mesa ridge waveguide type laser structure having the width of about 2.0 μm and, after that, the upper electrode  108  is formed. The upper electrode  108  for laser driving is patterned and the silicon oxide film  109  having a thickness of 200 nm is formed on the entire face. Further, the platinum thin film heater  110  having a width of about 10 μm and a thickness of 300 nm is formed only over the ridge waveguide by electron beam evaporation, photolithography, and ion million. Au heater electrode pads  114  for connection are connected to both ends of the platinum thin film heater  110 . 
     Finally, after opening a window in the upper electrode  108  for laser driving, a lower electrode  111  is formed. The resultant is cut by a cleavage process into devices each having a length of d=400 μm, a low reflecting film  112  of about 1% of reflectance is formed on the front end face of the device and a high reflecting film  113  of about 90% of reflectance is formed on the rear end face by a known method. 
     A distributed feedback semiconductor laser device in a 1.55 μm band fabricated by the above fabricating method is mounted on a heat sink (not shown) using a silicon carbide material and the upper electrode  108  for laser driving and the heater electrode are wired. 
     FIG. 2 shows the result of measurement of the wavelength tuning characteristic of the wavelength tunable laser according to the embodiment. In the graph, the lateral axis denotes power consumption (mW) of the heater  110  and the vertical axis denotes a change value (nm) of the wavelength. In the graph, black dots indicate the embodiment and blank dots show the conventional technique shown in FIG.  9 . The measurement is carried out under the condition that the heat sink temperature is set at 20 degrees centigrade and the heater current passing through the heater  110  is changed in a range from 0 to 100 mA. As understood from the measurement result, by changing the heater current within the range from 0 to 100 mA, the wavelength tuning range of 5 nm or larger is obtained. In this case, the wavelength tuning efficiency of about 10 nm/W that is about 10 times as high as the conventional one is obtained. Since the temperature coefficient of the lasing wavelength of the laser is 0.11 nm/deg., the temperature of the laser active layer is heated to 20 to 57 degrees centigrade. In this case, the laser driving current necessary for a constant output of 10 mW changes from 50 mA to 70 mA. An increase is therefore suppressed to only 20 mA. 
     On the other hand, the longitudinal mode of the distributed feedback laser during sweeping of the wavelength by the current is stable, so that complete continuous wavelength sweeping is realized without mode hopping since the reflectance of the laser cavity uniformly changes by the heating and, in principle, there is no change in the longitudinal mode. 
     In the embodiment, as mentioned above, since the characteristic fluctuation at the time of high temperature is smaller as compared with the conventional buried hetero structure semiconductor laser shown in FIG. 9, the ridge waveguide structure semiconductor laser has an advantage such that the laser characteristic deterioration at the time of heating of the heater is a little. Since heat generated by the heater  110  does not easily escape to the polyimide portion  104 , the active layer can be efficiently heated via the ridge section. Consequently, the wavelength tuning operation can be realized with a smaller amount of power consumption. 
     The wavelength tunable laser on which the heater having the multiple quantum well active layer made of InGaAsP is mounted has been described in the embodiment. When the laser has an active layer made of another material such as InGaAlAs or GaInNAs having an excellent characteristic at high temperature, the high temperature characteristic of the active layer is more excellent than that of the InGaAsP material laser of the embodiment. Consequently, the wavelength sweep can be realized over a wider range. 
     In the structure of the embodiment, by passing the current to the heater electrode, the laser active layer is heated to change the reflectance. Thus, the lasing wavelength of the distributed feedback laser can be changed over a wide range. 
     Embodiment 2 
     FIG. 3A is a perspective view showing the configuration of a monolithic integrated optical device as another embodiment of the wavelength tunable laser according to the invention. The cross sectional configuration of the main part of the embodiment is shown in an enlarged diagram FIG.  3 B. The different point from the embodiment shown in FIGS. 1A and 1B is the position of the heater electrode. In the second embodiment, a heater electrode  210  is formed over an active layer  203  via a silicon oxide film  209  on a side of the ridge waveguide. The ridge waveguide and the heater electrode  210  are provided so as to have a predetermined gap and a nonconductor is removed. The other structure is substantially the same as that of FIGS. 1A and 1B. Specifically, 1.0 μm of an n type InP buffer layer  202 , an active layer  203 , and a first p type InP cladding layer  204  are sequentially formed on an n type ( 100 ) InP semiconductor substrate  201 . 
     The device has a grating  205  formed on the entire face of the substrate, a second p type InP cladding layer  206 , a highly doped p type InGaAs cap layer  207 , an inverse mesa ridge guide laser structure, an upper electrode  208 , and a silicon oxide film  209 . Au heater electrode pads  214  for wiring are connected to both ends of the platinum thin film heater  210 . A low reflecting film  212  of reflectance of about 1% is formed on the front end face of the integrated device and a high reflecting film  213  of reflectance of about 90% is formed on the rear end face. The embodiment has also the inverse mesa ridge waveguide shown in FIGS. 1A and 1B and can realize the wavelength tuning characteristic in a manner similar to the first embodiment. 
     Embodiment 3 
     FIG. 4A is a perspective view showing the configuration of a monolithic integrated optical device constructing an optical modulator as an embodiment of an optical device using the wavelength tunable laser according to the invention. The cross sectional configuration of the main part of the embodiment is shown in an enlarged diagram FIG.  4 B. 
     In the embodiment, a wavelength tunable laser of the operating principle similar to that of the embodiment 1 and an electro-absorption optical modulator are monolithic integrated. On an n type ( 100 ) InP semiconductor substrate  301 , an n type InP buffer layer  302 , an active layer  303 , and a first p type InP cladding layer  104  are sequentially deposited. The active layer  303  comprises 0.05 μm of an n type InGaAsP lower guide layer (composition wavelength of 1.10 μm), a multiple quantum well layer of 5 cycles (a well layer made of InGaAsP having a thickness of 6.0 nm and compressive strain of 1% (composition wavelength of 1.70 μm) and a barrier layer made of InGaAsP having a thickness of 10 nm (composition wavelength of 1.15 μm), and 0.05 μm of an upper guide layer made of InGaAsP (composition wavelength of 1.15 μm). A grating  305  of uniform cycles of 241 nm is formed on the whole face of the substrate. Subsequently, a second p type InP cladding layer  306  and a highly doped p type InGaAs cap layer  307  are formed. Further, a process is performed to obtain an inverse mesa ridge waveguide laser structure having a width of about 2.0 μm and, after that, an upper electrode  308  is formed. On the upper electrode  308 , a silicon oxide film  309  is formed on the entire face. A platinum thin film heater  310  is formed only over the ridge waveguide. Au heater electrode pads  314  for connection are connected to both ends of the platinum thin film heater  310 . After opening a window in the upper electrode  308  for laser driving, a lower electrode  311  is formed. A low reflecting film  312  of about 1% of reflectance is formed on the front end face of the device and a high reflecting film  313  of about 90% of reflectance is formed on the rear end face. An enlarged diagram shows a cross section of the semiconductor layer of the main part taken along line X-X′. 
     In the embodiment, an electro-absorption optical modulator is formed on the InP semiconductor substrate  301 . The interval of 150 μm or more is provided between the wavelength tunable laser and the electro-absorption optical modulator. It is designed so that heat applied into the laser at the time of tuning the wavelength does not reach the optical modulator. In a manner similar to the first embodiment, the basic lateral structure is a known inverse mesa ridge waveguide laser in which polyimide is embedded such that a nonconductor  315  such as polyimide is formed on both sides of the ridge waveguide. According to the third embodiment, when the oscillation wavelength is 1550 to 1554 nm and the current of heating is changed from 0 to 100 mA, 4 nm of the wavelength tuning width is obtained. When the wavelength is tuned, a stable long distance transmission characteristic is obtained at 10 gigabits per second within the wave sweeping range of 4 nm since the change in the chirping characteristic of the electro-absorption optical modulator is slight in the wavelength range of about 4 nm. 
     Embodiment 4 
     FIGS. 5 and 6 are a system configuration diagram and a diagram showing the configuration of the main section of a wavelength-division multiplexing transmission system using the wavelength tunable laser according to the invention. 
     Light signals of a plurality of channels whose wavelengths are multiplexed, which are generated by a wavelength-division multiplexing transmission system  501  according to the invention are amplified by a fiber amplifier  502  and the amplified signals are transmitted through an optical fiber  503  for transmission and demodulated by an optical receiver  504  via an optical amplifier on the receiving side. As necessary, one or a plurality of optical amplifiers  502  for relay are provided in some midpoints in the optical fiber  503 . 
     The wavelength-division multiplexing transmission system  501  is formed as a monolithic integrated device having optical devices of a plurality of light signal sources  505  of different wavelengths of a plurality of channels ch. 1  to ch. 32 , a spare light source  507 , a Mach-Zehnder type optical modulator  508  for optical modulating an output of the spare light source  507 , and an optical multiplexer  506  for multiplexing output light of the light signal sources  505  and the modulator  508 . The wavelength set in the light signal source  505  is 1534.25 nm to 1558.98 nm and the wavelength interval is set to 100 GHz. A single spare light source  507  covers the entire wavelength range from 1534.25 nm to 1558.98 nm. 
     The light output of the spare light source  507  is led to the Mach-Zehnder type optical modulator  508  which is a single waveguide optical modulator and is made of lithium niobate, and subjected to high-speed optical modulation. Since the modulation characteristic hardly fluctuates according to the operation wavelength in the Mach-Zehnder type optical modulator  508 , laser beams of different wavelengths from the spare light source  507  are modulated with the same chirping characteristic. According to the embodiment, when a fault occurs in any of the main light sources  505  of  32  channels, by setting the wavelength of the spare light source  507  to the wavelength of the faulty light source, the function of the wavelength-division multiplexing transmission system is recovered at high speed. All of the channels can be backed up by the single spare light source  507 , the single Mach-Zehnder type optical modulator  508 , and a single driver. Consequently, as compared with the conventional configuration in which spare parts are prepared for each of the channels, the miniaturization of the system and the cost reduction are greatly improved. 
     Embodiment 5 
     FIG. 7 is a perspective view for explaining an embodiment of a spare light source corresponding to the spare light source  507  in FIG. 6 or the like. In the embodiment, eight distributed feedback semiconductor lasers  701  are monolithic integrated on a semiconductor integrated substrate  705 . Output light of the semiconductor lasers  701  are converged to an outgoing waveguide  704  by a known optical multiplexer  702  integrated on the same substrate  705 . A semiconductor light amplifier  703  is connected to the outgoing waveguide  704  to compensate a multiplexing loss. The oscillation wavelength of each of the eight distributed feedback semiconductor lasers  701  is set to a range from 1530 to 1562 nm and the ranges are set at intervals of 4 nm by controlling a grating cycle and a gain peak wavelength of each of the lasers  701  in accordance with a known method. The configuration of the semiconductor laser  701  is according to the embodiment shown in FIG.  1 . When the carrier temperature of the semiconductor laser  701  was set to 20 degrees centigrade and the current of heating was changed in a range from 0 to 100 mA, the wavelength tuning width of 4 nm was realized. 
     Embodiment 6 
     FIG. 8 is a block diagram showing another embodiment of the wavelength-division multiplexing transmission system. In the embodiment shown in FIG. 6, only one spare light source  507  is provided. In the sixth embodiment, a plurality of channels ch. 1  to ch.  33  are divided into eight groups  805  each having four channels of close wavelengths and spare light sources  801  to  808  are provided for the eight groups, respectively. The other configuration and operation are similar to those of FIG.  6 . 
     The wavelength tunable laser according to the invention can realize the wide wavelength tuning range and the wave tuning efficiency a few times as high as that of the known conventional buried hetero structure semiconductor laser by the simple configuration of using the ridge structure semiconductor laser. By monolithic integrating the wavelength tunable laser and the optical modulator and assembling the integrated device to a communication system, a high-reliability high-quality wavelength-division multiplexing transmission system can be realized. Further, a very reliable optical transmitter capable of continuously tuning the wavelength of a transmission signal can be easily realized at low manufacturing cost.