Wavelength tunable laser diode

Abstract: A wavelength tunable laser diode comprises a temperature variable heater (8) separated from an active layer (1) by a distance less than the thickness of a compound semiconductor substrate (3). Because the heater (8) is located very close to the active layer (1), the response time of temperature change is improved. That in turn widens the tunable range of the laser diode.

Description: The present invention relates to a wavelength tunable laser diode and, more particularly, to a wavelength tunable laser diode especially suitable for use with coherent optical communication systems. Wavelength-division-multiplexing communication involves having a light beam of multiple wavelengths carry signals so as to transmit data of multiple channels over a single optical fiber cable. Thus an increased transmission capacity is made available with this method. In this case, the receiver side requires installing a laser diode for a local oscillator. The laser diode allows the receiver to tune in through interference to signals of a specific wavelength out of the received wavelength-division-multiplexed light. Generally, a wavelength tunable laser diode is used as the laser diode for the local oscillator. The wavelength tunable laser diode permits selection of signals of any specific wavelength out of a plurality of wavelengths in the received light. This laser diode must meet three requirements: (1) the tunable range must be wide; (2) the spectral line width must be narrow enough to permit utilization of interference; and (3) the optical output power must be high so as to improve the S/N ratio for the signal. A first example of the prior art wavelength tunable laser diode is one described in "Electronics Letters," Vol. 25, No. 15 (1989), pp. 990-991. With a grating arrangement inside, this laser diode is a distributed feedback laser diode having a device length of 1.2 mm. The large device length allows the electrode arrangement to be divided into a plurality of electrodes. Varying the injecting current ratio between the multiple electrodes provides a laser beam having a tunable range of 2.2 nm, an optical output power of 20 mW or more and a spectral line width of 1 MHz or less. A second example of the prior art wavelength tunable laser diode is one described in "Electronics Letters," Vol. 26, No. 1 (1990), pp. 46-47. This is a distributed feedback laser diode with a waveguide inserted therein, the waveguide varying the refractive index. The laser diode of this structure provides a tunable range of 7 nm while its optical output is as low as 4 mW or less. A third example of the prior art wavelength tunable laser diode is one that utilizes one known laser diode characteristic: that the wavelength typically varies at a rate of 0.1 nm/°C. The laser diode of the above characteristic is attached to a Peltier device used for temperature control. As the temperature of the laser diode is varied, the wavelength emitted there from is controlled accordingly. With the first prior art example comprising multiple electrodes, the reflecting band width of 3 nm for the internal grating limits the tunable range to about 3 nm. Given that constraint, it is difficult to widen the tunable range to 4 nm or more. With the second prior art example incorporating the refractive-index-tuning waveguide, the carrier doped to vary the refractive index worsens the optical output power through absorption. Given that kind of laser diode, it is impossible to obtain an optical output of 10 mW or higher. With the third prior art example that uses the Peltier device for laser diode temperature control, the response time of temperature change is prolonged. That is, wavelengths are switched in units of as long as seconds. This is because the Peltier device is attached to the back of the substrate detached from an active layer of the laser diode, with the result that the temperature of the whole laser diode can only be controlled from the back of the substrate. As described, the conventional wavelength tunable laser diodes have yet to fulfill in practical terms the requirements of output, tunable range and response time at the same time. There has been an urgent need to develop a laser diode that would meet all such requirements. Applied Physics Letters, vol. 47, No. 3, August 1985, pages 222-224, discloses a thermoelectrically controlled laser diode where the laser active portion is formed in a mesa structure and a Peltier cooler is disposed on the substrate surface laterally of the protruding mesa portion. SUMMARY OF THE INVENTION: It is therefore an object of the present invention to overcome the above-described drawbacks and disadvantages and to provide a wavelength tunable laser diode which provides a high optical output power, a wide tunable range and a high wavelength switching speed. It is desirable to provide a practical wavelength tunable laser diode which provides an optical output power of 10 mW or higher, a spectral line width of 10 MHz or less, a tunable range of 4 nm or more, and a response time on the order of 10⁻³ seconds (1 ms) or less. These objects are met by a wavelength tunable laser diode as defined in claim 1. According to an embodiment of the invention, there is provided a wavelength tunable laser diode comprising at least: a semiconductor substrate; a first semiconductor layer formed on the semiconductor substrate and constituting an active layer whose band gap energy is less than that of the semiconductor substrate; a second semiconductor layer formed on the semiconductor substrate and having the polarity type opposite to that of the semiconductor substrate; and electrodes formed above the second semiconductor layer and under the first semiconductor layer, the electrodes being used for carrier injection; wherein a resistance film is deposited as a temperature variable heater on the electrode above the second semiconductor layer with an insulation film interposed between the film and the electrode. This constitution provides very favorable results. According to the invention, the temperature variable heater is located near the active layer. This means that the temperature around the active layer is varied at a high rate. In particular, the heater may be deposited on the upper-surface electrode opposite to the semiconductor substrate facing the active layer. In that case, there exists no thick semiconductor substrate having a high thermal capacity between the heater and the active layer. This structure makes the response time of temperature change around the active layer shorter than ever before as the temperature of the heater is varied. When the temperature around the active layer changes, the refractive index around the active layer varies correspondingly. Where the oscillation wavelength of the laser diode is 1.55 µm, the wavelength thereof varies at a rate of 0.1 nm/°C. Thus if the temperature around an active layer 1 in Fig. 1 is changed by 40°C, a tunable oscillation wavelength range of approximately 4 nm is made available. The resistance film used as the heater is located on the device surface very close to the active layer, the distance between the heater and the active layer being as small as several µm, compared with about 200 µm, i.e., the typical thickness of the prior art semiconductor substrate. Thus controlling the heater temperature with the wavelength tunable laser diode of the invention makes the response time of temperature change near the active layer significantly shorter than conventional temperature-controlled wavelength tunable laser diodes. Fig. 2a graphically shows how the response time varies when temperature is varied from 20°C to 60°C on a 200 µm thick InP substrate. A curve 10 plots response time measurements taken when the heater is located at the back of the substrate, and another curve 11 plots response time measurements in effect when the heater is provided over the entire substrate surface. When temperature is controlled from the back of the substrate, the heat dissipates throughout the substrate and thus a time delay of about 0.03 ms is experienced, as indicated by the curve 10. When the heater is provided over the entire surface of the substrate whose back is kept to a constant 20°C, no time delay occurs; the response is immediate. If the resistance film deposited as the heater over the substrate surface is made substantially the same in width as the active layer, the response time is further shortened. Because the active layer is formed near the surface where the resistance film is deposited as the heater, making the heater width the same as that of the active layer allows the temperature of the active layer 1 to be raised with very little heat dissipation. This setup makes the width of the heater less than if the entire surface of the substrate is heated. Since the change in temperature is dependent on the power density per unit area, the power density is raised when the heater width is reduced. The raised power density leads to higher energy efficiency, which in turn improves the response time of temperature change significantly. Referring to Fig. 2b, a curve 13 plots response time measurements taken when the heater is provided over the entire width of an InP substrate 400 µm wide; another curve 12 plots response time measurements taken when a heater 10 µm wide is provided on the surface of the InP substrate 400 µm wide. In either case, the heater is deposited on the surface of the InP substrate. The graphic representation of Fig. 2B shows that when the heater is made smaller in width than the substrate, the power density is boosted. In turn, the raised power density makes the response time higher by about one order of magnitude than when the heater is provided over the entire substrate surface. Where a resistance film 10 µm wide is used, the injection power is reduced by one sixth. The injection power is not strictly proportionate to the ratio of the resistance film width of 10 µm to the diode width of 400 µm because the heat dissipates crosswise within the substrate. As described, depositing the heater on the surface of the diode surface makes the response time of temperature change around the active layer shorter than the conventional setup of the heater being located at the back of the substrate. In addition, reducing the heater width enhances the energy efficiency for temperature control and further promotes temperature change around the active layer. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a schematic view of a wavelength tunable laser diode embodying the invention; Fig. 2a is a graphic representation of temperature rise versus response time measurements taken in two cases: one in which the semiconductor substrate is heated on the surface thereof and the other from the back thereof; and Fig. 2b is a graphic representation of temperature rise versus response time measurements taken in two cases: one in which the entire surface of the substrate is heated by a resistance film, and the other in which part of the substrate surface is heated by the resistance film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: Fig. 1 is a schematic view showing a wavelength tunable laser diode practiced as one preferred embodiment of the invention. Referring to Fig. 1, an active layer 1 and a p-type InP layer 2 are formed on an n-type InP substrate 3 using the known metal-organic vapor phase deposition epitaxial growth technique. The active layer 1 has a multiple quantum well structure of InGaAs/InGaAsP. After this, the active layer 1 is subjected to etching so that an active layer strip about 1 µm wide is left and the rest removed. An Fe-doped high resistance layer 10 is formed where the active layer 1 was removed so that a carrier is injected only into the remaining active layer 1. Above the active layer 1 is formed an SiO₂ film 5 having a window above the remaining active layer strip. Above the film 5 is a p-type electrode 6 made principally of gold (Au) and into which a carrier is injected for laser oscillation. The manufacturing stages so far are the same as those of conventional laser diode manufacture. To implement a wavelength tunable laser having a single wavelength and a narrow spectral line width, there is provided a distributed feedback laser diode structure under the active layer 1 in the optical axis direction (in the direction of depth in Fig. 1). This distributed feed-back laser diode structure has a grating 0.24 µm in pitch. Fig. 1, on the right-hand side of the p-type electrode 6 are formed electrodes 9-1 and 9-2 in an insular fashion. Thereafter, an SiO₂ layer 7 about 20 µm wide is deposited on the active layer 1. On top of the layer 7 is formed a resistance film 8 made of a Pt film, which is 0.7 µm thick and 10 µm wide. The two electrodes 9-1 and 9-2 are connected to respective ends of the resistance film 8. Finally, an n-type electrode 4 is deposited over the entire back of the substrate 3, the electrode 4 being made primarily of gold (Au). After producing a specimen 200 µm thick and 400 µm wide in the above-described manner, the inventors attached the back of the specimen to a heat radiation block that could be controlled in temperature using a Peltier device, and tested the specimen for characteristic evaluation. During the test, the block was kept to a constant 20°C. When a carrier of 250 mA was injected into the p-type electrode 6, a laser beam 1.540 µm in wavelength, 1 MHz in spectral line width and 20 mW in optical output power was obtained. When a current of 400 mA was injected into the resistance film 8 via the electrodes 9-1 and 9-2, the temperature rose by 20°C 10 µs later, with the wavelength shifted by 2 nm to reach 1.542 µm. After a period of 100 µs, the temperature rose by 40°C and the wavelength moved by 4 nm to the long wave side, reaching 1.544 µm. Within one millisecond after this, feedback control by the receiver produced a laser beam of stable oscillation at 1.544 µm. At this point, the optical output power was 10 mW or higher and the spectral line width 10 MHz or less. Although one specific embodiment of the invention has been described, the embodiment is not limitative of the invention and may be modified in various ways. As described and according to the invention, the temperature variable heater is provided close to the active layer of the laser diode. This structure widens the tunable range of the laser diode and improves the thermal response characteristic thereof. Illustratively, the embodiment depicted above provides a tunable range of 4 nm within one microsecond. With conventional temperature-controlled laser diodes, the Peltier device is attached to the electrode at the back of the semiconductor substrate not directly but through a heat sink metal block. This has further prolonged the response time of temperature change. The time delay involved entails an overshoot phenomenon. This means that it takes nearly one second to acquire a laser beam of sufficiently stable oscillation. By contrast, the present invention not only improves the response time of temperature change but also stabilizes quickly the temperature near the active layer of the device through the improved response. Since a resistance film is used as the heater, reducing the film width boosts the injection energy density. This further enhances the response time and energy efficiency, which permits easer control. For example, the above-described embodiment is a wavelength tunable laser diode that provides a laser beam 1.54 µm in wavelength, 1 MHz in spectral line width and 20 mW in optical output power at a response time of one millisecond or less over a tunable range of 4 nm.

Claims: A wavelength tunable laser diode comprising a compound semiconductor substrate (3) of a first polarity type, an active layer (1) formed on the surface of the substrate (3) and being made of a first compound semiconductor layer with a band gap energy level lower than that of the substrate (3), a second compound semiconductor layer (2) formed on the active layer (1) and having a second polarity type opposite to said first polarity type, a first electrode (6) electrically connected to the surface of the second semiconductor layer (2), and temperature controlling means (8) separated from the active layer (1) by a distance smaller than the thickness of the substrate (3), characterised in that the temperature controlling means is a heater constituted by a resistance film (8) disposed on the second semiconductor layer (2), with said first electrode (6) and an insulating film (7) being interposed between the resistance film (8) and the second semiconductor layer (2), and a pair of electrodes (9-1, 9-2) electrically connected to the resistance film (8). The laser diode of claim 1, wherein the width of the resistance film (8) is smaller than that of the substrate (3). The laser diode of claim 1 or 2, wherein the resistance film (8) is made of platinum. The laser diode of any one of claims 1 to 3, wherein said pair of electrodes (9-1, 9-2) is disposed on the surface of the substrate (3), with an insulating film (5) being interposed between the electrodes (9-1, 9-2) and the substrate (3). The laser diode of any one of claims 1 to 4, wherein the substrate (3) is constituted by an InP layer. The laser diode of any one of claims 1 to 5, wherein the active layer (1) has a multiple quantum well structure of GaAs/InGaAsP. The laser diode of any one of claims 1 to 6, wherein the second semiconductor layer (2) is constituted by an InP layer.

Inventors: CHINONE NAOKI, OKA AKIHIKO, SAITO KATUTOSHI, SAKANO SHINJI, CHINONE, NAOKI, OKA, AKIHIKO, SAITO, KATUTOSHI, SAKANO, SHINJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01S3
IPC codes: H01S5