JP5180914B2 - 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.

  7A and 7B show temperature changes of the active layer and the thermistor (the solid line is the active layer temperature and the broken line is the thermistor temperature) when the drive current supplied to the LD is changed. FIG. 7A is a graph when temperature control by TEC is not performed (without TEC control), and FIG. 7B is a graph when temperature control using TEC is performed (during TEC control). . FIG. 7C shows a state where a relatively small drive current is supplied to the LD (solid line) and a relatively large drive 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. 7A, 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. 7B, 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 temperature of the active layer increases, so that the wavelength of the emitted laser light (light) is shifted to the long wavelength side (FIG. 7C). 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 a heating element that generates heat by energization is provided on a surface of the temperature detection element other than the surface in contact with the holding base, and the semiconductor gain The driving current supplied to the chip is also supplied to the heating element. 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 a drive current is supplied to 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 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.

  When a drive current is supplied to the semiconductor gain chip to emit light, the drive current (or part thereof) is also supplied to the heating element. The thermistor is heated (heated) by the heat generated by the heating element.

  When a drive current is supplied to the semiconductor gain chip, the heat of the active layer of the semiconductor gain chip is transferred to the temperature detection element via the holding base, and the heating element provided in contact with the temperature detection element Since the temperature detection element is heated by generating heat by the drive current, the temperature divergence between the semiconductor gain chip and the temperature detection element can be reduced. When the thermoelectric cooler is controlled so that the temperature measured by the temperature detecting element becomes a predetermined temperature, the active layer is also maintained at a substantially predetermined temperature. The oscillation wavelength can be stabilized.

  The driving current may be supplied to the semiconductor gain chip via the heating element in series, or the driving current may be branched to supply one to the semiconductor gain chip and the other to the heating element. May be. In any case, when a driving current is supplied to the semiconductor gain chip, the heating element generates heat and the temperature detecting element is heated, so that the temperature between the semiconductor gain chip (active layer) and the thermistor is increased. The divergence becomes smaller.

  As a heating element provided in the temperature detection element, in one embodiment, a thin film resistor provided in a state insulated from the temperature detection element is used. When the driving current is supplied to the thin film resistor, the thin film resistor is heated and transmitted to the temperature detecting element.

  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 partially expanded perspective view which shows the other example of 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 provided 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.

  The thermistor 13 is also provided with an upper surface electrode 36 in the range of half of its upper surface and a lower surface electrode 37 on the entire lower surface. 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 a change in electrical resistance of the thermistor 13, a current is also passed through the thermistor 13 via the upper surface electrode 36 and the lower surface electrode 37.

An insulating film (for example, SiO ₂ film) 38 is provided in the remaining half of the upper surface of the thermistor 13 (the area where the upper surface electrode 36 is not formed), and a thin film resistor (for example, Au (gold)) is provided on the insulating film 38. 39) is provided.

  The thin film resistor 39 is formed in a frame shape on the upper surface of the insulating film 38, and two wire bonding pads are formed on both ends thereof so as to be separated from each other. Two wires 42 and 43 are bonded to the two wire bonding pads of the thin film resistor 39, respectively. One wire 42 is connected to one of the lead terminals 26 (anode electrode terminal) via an electrode pad 33 provided on the submount 21 and a wire 41 bonded to the electrode pad 33. The other wire 43 is connected to the upper surface electrode 31 of the semiconductor gain chip 11. Thus, the lead terminal (anode electrode terminal) 26 and the upper surface electrode 31 of the semiconductor gain chip 11 are electrically connected through the wire 41, the electrode pad 33, the wire 42, the thin film resistor 39, and the wire 43.

  The lower surface electrode 32 of the semiconductor gain chip 11 is in electrical contact with an electrode pad 34 provided on the submount 21. The electrode pad 34 has a size that protrudes from the mounting range of the semiconductor gain chip 11, and one end of a wire 44 whose other end is connected to one of the lead terminals 26 (cathode electrode terminal) at the protruding portion. , Bonded. The lead terminal (cathode electrode terminal) 26 and the lower surface electrode 32 of the semiconductor gain chip 11 are electrically connected via the wire 44 and the electrode pad 34.

  For the thermistor 13, one of the upper electrode 36 and the lead terminal 26 is connected by a wire 45, and the electrode pad 35 on the submount 21 in contact with the lower electrode 37 and one of the lead terminals 26 is connected by a wire 46. ing. A current is passed through the thermistor 13 through the wires 45 and 46.

  The thin film resistor 39 provided on the upper surface of the thermistor 13 exists on a path for supplying current to the upper surface electrode 31 of the semiconductor gain chip 11 and is connected in series with the semiconductor gain chip 11. Therefore, when a current is passed between the upper surface electrode 31 and the lower surface electrode 32 of the semiconductor gain chip 11 in order to emit light from the semiconductor gain chip 11 (to obtain a laser beam), the current is a thin film resistor on the upper surface of the thermistor 13. 39 is also energized. When the current is applied, the thin film resistor 39 generates heat. Details of the operation and effect of using the thin film resistor 39 will be described later.

  FIG. 3 shows the semiconductor gain chip 11 and the thermistor 13 mounted on the submount 21, which is different from that shown in FIG. 2 in that the thin film resistor 39 and the semiconductor gain chip 11 are connected in parallel. Is different.

  When a current flows between the wire 41 and the wire 44 (voltage is applied), the electrode pad 33, the wire 47, the upper surface electrode 31 of the semiconductor gain chip 11, the semiconductor layer of the semiconductor gain chip 11, and the semiconductor gain chip 11 Branching into a loop consisting of the lower electrode 32 and electrode pad 34, and a loop consisting of the electrode pad 33, wire 42, thin film resistor 39, wire 43 and electrode pad 34 (lower electrode 32 of the semiconductor gain chip 11). The same voltage is applied to the chip 11 and the thin film resistor 39. Also in this case, the thin film resistor 39 generates heat.

  In order to change the branching ratio of the current to the semiconductor gain chip 11 and the thin film resistor 39, the shape of the electrode pad 33 or the wires 42 and 47 are changed so as to obtain a desired branching ratio. That's fine.

  FIG. 4 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 semiconductor gain chip 11 and the thermistor 13 are disposed on a submount 21 made of a material having thermal conductivity such as aluminum nitride and copper tungsten and have a thermal connection state. The heat generated in the layer is transmitted to the thermistor 13 via the submount 21, and the temperature of the thermistor 13 also rises. In this case, the TEC controller 53 controls the TEC 15 so as to lower the temperature of the raised thermistor 13.

  Here, as described above, when the drive current (drive voltage) is supplied to the semiconductor gain chip 11, the drive current is also supplied to the thin film resistor 39 provided on the thermistor 13, and the thin film resistor 39 generates heat. . The thermistor 13 is heated by heat conduction from the semiconductor gain chip 11 via the submount 21 and also heated by heat generated by the thin film resistor 39.

  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. 7A to 7C).

  However, as described above, the thermistor 13 is heated not only by heat conduction via the submount 21, but also by heat from the thin film resistor 39 provided on the upper surface of the thermistor 13. When the current (voltage) supplied to the semiconductor gain chip 11 is increased, the heat generation amount of the active layer is increased and the heating amount of the thin film resistor 39 is also increased. As a result, the temperature gradient inside the thermistor is relaxed, and a temperature closer to the active layer temperature is detected.

  FIGS. 5A to 5C correspond to FIGS. 7A to 7C and show graphs in the case where a thin film resistor 39 is provided on the upper surface of the thermistor 13.

  Referring to FIG. 5A, by connecting thin film resistor 39 in series with semiconductor gain chip 11 (FIG. 2) or in parallel (FIG. 3), the difference between the active layer temperature and the thermistor temperature is reduced. be able to. Referring to FIG. 5B, as a result of the difference between the active layer temperature and the thermistor temperature being reduced, if the thermistor 13 is maintained at a predetermined temperature by the TEC controller 53 and TEC15, the active layer temperature is also brought to a predetermined temperature. Maintained at close temperature. Even if the drive current is increased, the wavelength of the emitted light hardly shifts (FIG. 5C). 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.

  FIG. 6 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
15 Thermoelectric cooler (TEC)
21 Submount
38 Insulating film
39 Thin film resistors
60 Raman amplifier
62 Optical fiber for amplification

Claims (5)

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,
A heating element that generates heat when energized is provided on the surface of the temperature detection element other than the surface in contact with the holding table,
The drive current supplied to the semiconductor gain chip is also supplied to the heating element,
Semiconductor laser module. The drive current is supplied to the semiconductor gain chip via the heating element in series.
The semiconductor laser module according to claim 1. The drive current is branched, one is supplied to the semiconductor gain chip, and the other is supplied to the heating element.
The semiconductor laser module according to claim 1. The heating element is a thin film resistor provided in a state insulated from the temperature detection element.
The semiconductor laser module according to any one of claims 1 to 3. A semiconductor laser module according to any one of claims 1 to 4, 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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