JP5190027B2 - Semiconductor laser module and Raman amplifier having the same - Google Patents

Description

  The present invention relates to a semiconductor laser module and a Raman amplifier including the semiconductor laser module.

  A semiconductor laser or a semiconductor gain chip (hereinafter referred to as LD) includes an active layer, and generates laser light (light) by supplying current to the active layer. When current is supplied to the active layer, the temperature of the active layer rises. Since the temperature variation of the active layer affects the wavelength of the laser beam (light) emitted from the LD, a thermoelectric cooler (TEC) for cooling the LD is generally provided inside the module in which the LD is stored. LD is mounted on the TEC. A thermistor for measuring temperature is also mounted on the TEC. The TEC is controlled so that the temperature measured by the thermistor becomes a predetermined temperature (for example, 25 ° C.).

  The TEC is provided in the module for the purpose of maintaining the active layer temperature of the LD at a predetermined temperature. As described above, the TEC maintains the thermistor temperature, not the active layer temperature, at a predetermined temperature. Operate. That is, the thermistor temperature can be maintained at a predetermined temperature by temperature control (cooling) by TEC, but this does not necessarily lead to the constant active layer temperature of the LD.

  6A and 6B show temperature changes of the active layer and the thermistor when the drive current supplied to the LD is changed (the solid line indicates the active layer temperature and the broken line indicates the thermistor temperature). FIG. 6A is a graph when temperature control by TEC is not performed (without TEC control), and FIG. 6B is a graph when temperature control using TEC is performed (during TEC control). . FIG. 6C shows a state where a relatively small driving current is supplied to the LD (solid line) and a relatively large driving current is supplied to the LD (one-dot chain line) in a state where temperature control using TEC is performed. It is a graph which shows the relationship between the wavelength of the laser beam (light) radiate | emitted from LD, and output (power).

  As shown in FIG. 6A, when temperature control by TEC is not performed, the active layer temperature and the thermistor temperature both increase as the drive current increases. In particular, the active layer temperature rises rapidly. As the drive current increases, the difference between the active layer temperature and the thermistor temperature increases.

  Referring to FIG. 6B, when temperature control using TEC is performed, as described above, the thermistor temperature is maintained at a predetermined temperature. However, as the drive current increases, the active layer temperature rises and the deviation from the thermistor temperature increases. When the drive current is increased, the active layer temperature rises, so that the wavelength of the emitted laser light (light) is shifted to the longer wavelength side (FIG. 6C). In an external cavity semiconductor laser module that uses a diffraction grating (FBG) formed in an optical fiber as an external reflector, the reflection wavelength of the FBG and the optical gain wavelength of the semiconductor gain chip are particularly large. If it deviates, it will be out of the pull-in range and oscillation by FBG cannot be obtained.

  By disposing the thermistor as close to the LD as possible, the difference between the thermistor temperature and the active layer temperature is improved to some extent, and the wavelength shift of the laser beam is improved to some extent (for example, Patent Document 1). However, it does not bring about fundamental improvement.

JP 2002-141599 A

  An object of the present invention is to enable the active layer temperature to be kept substantially constant.

  In the semiconductor laser module according to the present invention, an optical waveguide having an active layer is formed on a semiconductor substrate, and light generated by injecting a current into the active layer is guided through the optical waveguide. A semiconductor gain chip that emits the light from a front end surface formed at the end of the optical fiber, and an optical fiber that is optically coupled to the front end surface and has a diffraction grating that reflects light of a predetermined wavelength inside, An external resonator type semiconductor laser in which a resonator is configured by reciprocating the light between a rear end face formed at the other end of the optical waveguide and the diffraction grating, and a temperature change is output as a change in resistance value. A semiconductor laser module including a temperature detection element and a thermoelectric cooler controlled so that a resistance value output from the temperature detection element becomes a predetermined value, the semiconductor gain chip and the temperature detection element Is disposed on a holding base mounted on the thermoelectric cooler, and the semiconductor gain chip and the temperature detecting element have a thermal connection state using the holding base as a heat path and pass through the holding base. In addition to the heat path, at least one other heat path for connecting the semiconductor gain chip and the temperature detecting element is formed. The thermoelectric cooler is controlled by a control circuit (cooler control circuit) provided outside the semiconductor laser module. Based on data (resistance value) representing the measured temperature from the temperature detection element, the cooler control circuit controls the thermoelectric cooler so that the temperature detection element maintains a predetermined temperature.

  When current is injected into the active layer included in the semiconductor gain chip, light is generated in the active layer. Light emitted from the front end face of the semiconductor gain chip is incident on an optical fiber having a diffraction grating (FBG) partially formed through a lens. Laser oscillation is caused by repeated light reflection between the rear end face of the semiconductor gain chip and the diffraction grating. The obtained laser light is guided through the optical fiber.

  Since the rear end face of the semiconductor gain chip functions as a light reflector, it is necessary to reflect light with high reflectivity. Further, in order to obtain the external resonance mode by FBG more stably, the front end face of the semiconductor gain chip needs to have a low light reflectance. In one embodiment, an antireflection film is provided on the front end surface of the semiconductor gain chip, and a reflection film is provided on the rear end surface. For example, as the reflective film provided on the rear end face, a film having a light reflectance of 90% or more is used.

  Since the semiconductor gain chip and the temperature detection element are disposed on the holding table, heat from the semiconductor gain chip is conducted to the temperature detection element via the holding table. That is, the semiconductor gain chip and the temperature detection element have a thermal connection state in which the holding table is a heat path. The semiconductor laser module according to the present invention has another heat path for connecting the semiconductor gain chip and the temperature detecting element by heat conduction in addition to the heat path by the holding table. Even through this other heat path, the heat from the semiconductor gain chip is conducted to the temperature detection element, and the temperature detection element is heated (heated).

  According to the present invention, in addition to the heat from the semiconductor gain chip being conducted to the temperature detection element via the holding table, the heat is conducted to the temperature detection element via another heat path different from the holding table. Therefore, the difference between the temperature of the active layer of the semiconductor gain chip and the temperature of the temperature detection element is reduced. Therefore, when the thermoelectric cooler is controlled so that the temperature measured by the thermistor becomes a predetermined temperature, the active layer is also maintained at a substantially predetermined temperature. The oscillation wavelength can be stabilized.

  In one embodiment, the additional heat path is formed by a metal wire. In this case, at least one of the semiconductor gain chip and the thermistor is provided with an insulating film that prevents energization between the semiconductor gain chip and the temperature detection element via the metal wire. In order to generate light in the semiconductor gain chip and in order to measure the resistance value in the temperature detection element, a current is supplied to each of the temperature detection element. Therefore, a metal wire (another thermal wire) connecting the semiconductor gain chip and the temperature detection element. This is because the path) should not electrically short-circuit the semiconductor gain chip and the temperature detection element. By providing an insulating film on at least one of the semiconductor gain chip and the thermistor, energization (short circuit) between the semiconductor gain chip and the thermistor is prevented.

  When another heat path is formed by a metal wire, a larger amount of heat can be transferred by using a metal wire having a large diameter. Instead of or in addition to using a metal wire having a large diameter, the semiconductor gain chip and the temperature detection element may be connected by a plurality of metal wires.

  Since the semiconductor laser module according to the present invention can emit laser light that has been stably FBG oscillated over a wide current range, it is suitable for use as an excitation light source in a Raman amplifier. The present invention also provides the above-described semiconductor laser module, and a Raman amplifier including an optical fiber that causes the laser light from the semiconductor laser module to be incident as excitation light and cause stimulated Raman amplification.

(A) is a plan view showing the internal structure of the semiconductor laser module, and (B) is a side sectional view showing the internal structure of the semiconductor laser module. It is a partially expanded perspective view which shows the wiring of a semiconductor gain chip and a thermistor. It is a block diagram which shows the electric constitution of the laser unit containing a semiconductor laser module. (A) is a graph showing the relationship between the drive current when the temperature control by TEC is not performed and the temperature change of the active layer and the thermistor, and (B) is the drive current when the temperature control by TEC is performed and the active layer and The graph which shows the relationship of the temperature change of a thermistor, (C) respectively shows the graph which shows the relationship between the wavelength of a laser beam, and an output when performing temperature control by TEC. 1 shows a block diagram of a Raman amplifier. The prior art is shown. (A) is a graph showing the relationship between the drive current and the temperature change of the active layer and the thermistor when temperature control by TEC is not performed, and (B) is temperature control by TEC. (C) shows a graph showing the relationship between the wavelength of the laser beam and the output when temperature control is performed by TEC.

  FIG. 1A shows the internal structure of the semiconductor laser module by a plan view, and FIG. 1B shows the internal structure of the semiconductor laser module by a side sectional view. In FIGS. 1A and 1B, illustration of wires (conductive wires) is omitted.

  Referring to FIGS. 1A and 1B, a semiconductor laser module 1 includes a rectangular parallelepiped package (housing) 2 having a hollow inside, and a semiconductor gain chip 11 that emits light inside the package 2. , A lens 12 that collects light emitted from the semiconductor gain chip 11, a thermistor 13 that measures temperature (outputs a temperature change as a change in resistance value), and receives light emitted from the rear end face of the semiconductor gain chip 11. A photodiode 14 for monitoring the output of the semiconductor gain chip 11 is stored. The semiconductor gain chip 11 and the thermistor 13 are placed on a submount (holding base) 21, the lens 12 is held by a lens holder 22, and the photodiode 14 is fixed to the wall surface of the PD submount 23. The submount 21, the lens holder 22, and the PD submount 23 are all fixed on the substrate 24.

  Although not shown in detail, the semiconductor gain chip 11 is configured by laminating a plurality of semiconductor layers having different compositions including active layers and types and amounts of impurities on a semiconductor substrate. The active layer is formed in a stripe shape in the light guiding direction. Then, light generated in the active layer is confined in a plurality of semiconductor layers formed above and below the active layer, and an optical waveguide is constituted by a plurality of layers including the active layer.

  A thermoelectric cooler (for example, Peltier element) (hereinafter referred to as TEC) 15 is fixed to the bottom surface inside the package. A substrate 24 on which the semiconductor gain chip 11, the lens 12, the thermistor 13, and the photodiode 14 are mounted is fixed on the TEC 15.

  A plurality of lead terminals 26 protrude outward from the left and right wall surfaces of the package 2. Inside the package 2, the end of the lead terminal 26 and the semiconductor gain chip 11, the thermistor 13, the photodiode 14 and the TEC 15 are electrically connected by wires.

  A cylindrical ferrule 3 having an optical fiber 4 disposed at the center is fixed to the front wall surface of the package 2.

  When a driving current is supplied to the semiconductor gain chip 11 in the forward direction, light is generated in the active layer. The light generated in the active layer is emitted from the front end face (exit end face) of the semiconductor gain chip 11 and enters the lens 12. The light collected by the lens 12 enters the optical fiber 4.

  In the optical fiber 4, a diffraction grating (FBG) 4a (hereinafter referred to as FBG 4a) is formed. Of the light emitted from the front end face of the semiconductor gain chip 11, light having the Bragg wavelength of the FBG 4a and a wavelength in the vicinity thereof is reflected by the FBG 4a. The light reflected by the FBG 4a returns to the semiconductor gain chip 11 again through the optical fiber 4 and the lens 12, is amplified by receiving the optical gain in the active layer, and is reflected on the rear end face of the semiconductor gain chip 11. Laser reflection is caused by repeated light reflection between the rear end face of the semiconductor gain chip 11 and the FBG 4 a, and the laser light enters the optical fiber 4. Thus, the external resonator type semiconductor laser is composed of the semiconductor gain chip 11 that generates light and reflects the light at the rear end face, and the FBG 4a that functions as an external reflector. Incidentally, since it is necessary to return the light reflected by the FBG 4 a to the semiconductor gain chip 11, no isolator is provided in the package 2. The semiconductor gain chip 11 may be provided with an antireflection film on the front end surface and a reflection film on the rear end surface. As the reflective film provided on the rear end face, for example, a film having a light reflectance of 90% is used.

  FIG. 2 shows an enlarged perspective view of the semiconductor gain chip 11 and the thermistor 13 mounted on the submount 21.

  An upper surface electrode 31 and a lower surface electrode 32 are formed on the upper surface and the lower surface of the semiconductor gain chip 11, respectively. By applying a forward drive current between the upper surface electrode 31 and the lower surface electrode 32, light is emitted from the front end surface and the rear end surface of the semiconductor gain chip 11 as described above.

  Three electrode pads 33, 34, and 35 are provided on the submount 21.

  The electrode pad 33 is used to electrically connect the upper surface electrode 31 of the semiconductor gain chip 11 and one of the lead terminals 26 (see FIG. 1A). One end of two metal (for example, gold, copper, etc.) wires 41 and 42 is bonded to the electrode pad 33. The other end of one wire 41 is bonded to the lead terminal 26, and the other end of the other wire 42 is bonded to the upper surface electrode 31 of the semiconductor gain chip 11. The upper electrode 31 of the semiconductor gain chip 11 and the lead terminal 26 are electrically connected via the electrode pad 33 and the two wires 41 and 42.

  The electrode pad 34 electrically connects one of the lower surface electrode 32 and the lead terminal 26 of the semiconductor gain chip 11 (which is of course different from the lead terminal connected to the upper surface electrode 31 of the semiconductor gain chip 11). Used to connect. The electrode pad 34 is formed in the mounting range of the semiconductor gain chip 11 on the submount 21, and a part thereof is formed so as to protrude from the mounting range of the semiconductor gain chip 11. One end of the wire 43 is bonded to the position of the electrode pad 34 that protrudes from the mounting range of the semiconductor gain chip 11. The other end of the wire 43 is bonded to the lead terminal 26. The lower surface electrode 32 of the semiconductor gain chip 11 and the lead terminal 26 are electrically connected via the electrode pad 34 and the wire 43.

  The thermistor 13 measures the temperature using the fact that the electrical resistance of a metal oxide (manganese, cobalt, nickel, etc.), semiconductor (silicon), etc. changes according to the temperature. In order to measure the electrical resistance change of the thermistor 13, a current is also passed through the thermistor 13. An upper surface electrode 36 is formed on a part of the upper surface of the thermistor 13, and a lower surface electrode 37 is formed on the entire lower surface of the thermistor 13. The electrode pad 35 is formed in the mounting range of the thermistor 13 on the submount 21, and a part of the electrode pad 35 extends beyond the mounting range of the thermistor 13. The electrode pad 35 and the lower electrode 37 of the thermistor 13 are energized. One end of each of the wires 44 and 45 is bonded to the electrode pad 35 energized with the upper surface electrode 36 of the thermistor 13 and the lower surface electrode 37 of the thermistor 13, and the other end is bonded to the two lead terminals 26, respectively. . A current is passed through the thermistor 13 via the wires 44 and 45.

  In addition to the wires 41 to 45 for supplying current to the semiconductor gain chip 11 and the thermistor 13, there are metal (for example, gold, copper, etc.) wires 46 that connect the semiconductor gain chip 11 and the thermistor 13. One end of the wire 46 is bonded to the upper surface electrode 31 of the semiconductor gain chip 11 and the other end is bonded to the upper surface of the thermistor 13.

Unlike the wires 41 to 45, the wire (thermal conductor) 46 is not for supplying current to the semiconductor gain chip 11 and the thermistor 13, but to obtain a thermal connection state between the semiconductor gain chip 11 and the thermistor 13. belongs to. An insulating film (for example, SiO ₂ film) 38 is provided on a part of the upper surface of the thermistor 13, and a pad 39 is provided on the insulating film 38. The other end of the wire 46 is bonded to the pad 39 on the insulating film 38. Since the insulating film 38 exists, no current flows between the semiconductor gain chip 11 and the thermistor 13 through the wire 46. The wire 46 that is not used for energization but is used for heat conduction is hereinafter referred to as a “heat path wire 46”. Of course, the insulating film 38 may be provided not on the thermistor 13 side but on the semiconductor gain chip 11 side. Heat generated in the semiconductor gain chip 11 (temperature rise) is transmitted to the thermistor 13 by the heat path wire 46, and a thermal connection state between the semiconductor gain chip 11 and the thermistor 13 is obtained. Details of the operation and effect of introducing the heat path wire 46 will be described later.

  FIG. 3 is a block diagram showing an electrical configuration of a laser unit including the semiconductor laser module 1 described above. The laser unit is constituted by the semiconductor laser module 1, the drive power supply 51 provided outside the semiconductor laser module 1, an automatic output control circuit (APC: Auto Power Control circuit) 52, and the TEC controller 53.

  A drive current is supplied from the drive power supply 51 to the semiconductor gain chip 11. A drive current from the drive power supply 51 is controlled based on a control signal supplied from the APC circuit 52.

  The APC circuit 52 controls the drive power supply 51 so that the output current from the photodiode 14 becomes a predetermined value. The photodiode 14 is disposed behind the semiconductor gain chip 11, receives light emitted from the rear end face, and outputs a current corresponding thereto. Since the output of light emitted from the rear end face of the semiconductor gain chip 11 is proportional to the output of light emitted from the front end face, the drive power supply 51 is controlled so that the output current from the photodiode 14 becomes a predetermined value. Thus, the light output (power) emitted from the front end face of the semiconductor gain chip 11 is maintained at a predetermined value.

  Based on the setting of the APC circuit 52, the drive current supplied to the semiconductor gain chip 11 is determined, and the power of light emitted from the semiconductor gain chip 11 (power of laser light after laser oscillation) is determined. Depending on the required output characteristics, light of various powers can be emitted from the semiconductor laser module 1.

  When the drive current from the drive power supply 51 is supplied to the semiconductor gain chip 11, the active layer of the semiconductor gain chip 11 generates heat. The semiconductor gain chip 11 is cooled by a TEC 15 that operates according to a control signal supplied from the TEC controller 53.

  The TEC controller 53 controls the TEC 15 so that the temperature measured by the thermistor 13 becomes a predetermined temperature (for example, 25 ° C.). That is, a current is supplied to the thermistor 13 from the drive power supply 51, and a resistance value that changes with a temperature change is detected by the TEC controller 53 connected to the thermistor 13. Based on the resistance value of the thermistor 13, the TEC controller 53 controls the TEC 15 so that the resistance value is maintained at a predetermined value. Thereby, the measurement temperature of the thermistor 13 is maintained at a predetermined temperature (for example, 25 ° C.).

  As described above, when the drive current is supplied, the active layer of the semiconductor gain chip 11 generates heat. The submount 21 is made of a material having thermal conductivity such as aluminum nitride or copper tungsten, and the temperature of the thermistor 13 is also increased by the heat transmitted from the semiconductor gain chip 11 via the submount 21. In this case, the TEC controller 53 controls the TEC 15 so as to lower the temperature of the raised thermistor 13.

  Here, the semiconductor gain chip 11 and the thermistor 13 are connected by a heat path wire 46. Therefore, when the driving current is supplied to the semiconductor gain chip 11 and the active layer of the semiconductor gain chip 11 generates heat, the thermistor 13 is heated by the heat transmitted through the heat path wire 46. That is, the thermistor 13 is heated by heat conduction from the semiconductor gain chip 11 via the submount 21 and also by heat conduction via the heat path wire 46.

  If the heat generated in the semiconductor gain chip 11 is transmitted to the thermistor 13 only by heat conduction through the submount 21, the temperature of the thermistor 13 does not match the temperature of the active layer of the semiconductor gain chip 11. In particular, when the drive current supplied to the semiconductor gain chip 11 is large, a large divergence occurs between the temperature of the thermistor 13 and the temperature of the active layer of the semiconductor gain chip 11. As a result, the thermistor 13 is maintained at a predetermined temperature, but the active layer rises without being maintained at the predetermined temperature. The oscillation wavelength of the laser light changes to the long wavelength side (see FIGS. 6A to 6C).

  However, as described above, the thermistor 13 transmits not only the heat conduction via the submount 21, but also the heat from the heat path wire 46 that directly connects the semiconductor gain chip 11 and the thermistor 13. As the temperature of the active layer of the semiconductor gain chip 11 increases, the amount of heat transferred to the thermistor 13 via the heat path wire 46 increases. Therefore, the temperature measured by the thermistor 13 reflects the temperature rise of the active layer of the semiconductor gain chip 11.

  FIGS. 4A to 4C correspond to FIGS. 6A to 6C, and show graphs when the semiconductor gain chip 11 and the thermistor 13 are directly connected by the heat path wire 46. FIG. ing.

  Referring to FIG. 4A, the gap between the active layer temperature and the thermistor temperature can be reduced by connecting the semiconductor gain chip 11 and the thermistor 13 using the heat path wire 46. Referring to FIG. 4B, as a result of the difference between the active layer temperature and the thermistor temperature being reduced, when the thermistor 13 is maintained at a predetermined temperature by the TEC controller 53 and TEC 15, the active layer temperature is also brought to a substantially predetermined temperature. Maintained at close temperature. Even when the drive current is increased, the wavelength shift of the emitted light hardly occurs (FIG. 4C). Even if the semiconductor laser module 1 is used as a light source for obtaining laser light having various powers (even if the drive current is varied), laser light having a stable wavelength can be obtained.

  The heat path wire 46 is made of a material having good thermal conductivity such as Au (gold), Al (aluminum), Cu (copper). Further, in order to shorten the time required to reach the thermal equilibrium state, a wire having a large diameter may be used as the heat path wire 46, or the semiconductor gain chip 11 and the thermistor 13 may be connected by a plurality of heat path wires 46.

  FIG. 5 shows a block diagram of a Raman amplifier using the above-described semiconductor laser module 1 (laser unit) as a pumping light source.

  In the Raman amplifier 60, the laser light emitted from the semiconductor laser module 1 is input to the amplification optical fiber 62 through the coupler 61 as excitation light. Stimulated Raman scattering occurs in the amplification optical fiber 62, and a gain is generated on the longer wavelength side by about 100 nm from the wavelength of the laser light (excitation light wavelength). When the signal light enters the amplification optical fiber 62, the signal light is amplified by the gain generated in the amplification optical fiber 62 (Raman amplification). Since the semiconductor laser module 1 can emit laser light with relatively high power, it can transmit signal light over a long distance. Further, since the semiconductor laser module 1 stably oscillates without causing a wavelength shift even when the driving current is increased, the Raman gain can be stably obtained.

DESCRIPTION OF SYMBOLS 1 Semiconductor laser module 4 Optical fiber 4a Diffraction grating (FBG)
11 Semiconductor gain chip
13 Thermistor
14 photodiode
15 Thermoelectric cooler (TEC)
21 Submount
38 Insulating film
46 Heat path wire
60 Raman amplifier
62 Optical fiber for amplification

Claims (4)

An optical waveguide having an active layer is formed on a semiconductor substrate, and light generated by injecting a current into the active layer is guided through the optical waveguide and is formed at one end of the optical waveguide. A semiconductor gain chip from which the light is emitted, and an optical fiber that is optically coupled to the front end face and has a diffraction grating that reflects light of a predetermined wavelength inside, at the other end of the optical waveguide An external resonator type semiconductor laser in which a resonator is formed by reciprocating the light between the formed rear end face and the diffraction grating;
A temperature detection element that outputs a temperature change as a change in resistance value;
A semiconductor laser module including a thermoelectric cooler controlled so that a resistance value output from the temperature detection element becomes a predetermined value;
The semiconductor gain chip and the temperature detection element are disposed on a holding base mounted on the thermoelectric cooler,
The semiconductor gain chip and the temperature detection element have a thermal connection state with the holding table as a heat path, and connect the semiconductor gain chip and the temperature detection element in addition to a heat path passing through the holding table. At least one other heat path is formed,
Semiconductor laser module. Said another heat path is formed by a metal wire,
At least one of the semiconductor gain chip and the temperature detection element is provided with an insulating film that prevents energization between the semiconductor gain chip and the temperature detection element via the metal wire.
The semiconductor laser module according to claim 1. The semiconductor gain chip and the temperature detection element are connected by a plurality of metal wires,
The semiconductor laser module according to claim 2. The semiconductor laser module according to any one of claims 1 to 3, and an optical fiber in which laser light from the semiconductor laser module is incident as excitation light and causes stimulated Raman amplification,
Raman amplifier equipped with.

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