JP2007194366A - Semiconductor laser module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor laser module which can attain a narrowed spectral line width. <P>SOLUTION: The semiconductor laser module which is a combined semiconductor laser diode 84 and light reflector 82 includes a lens 85 for converting the light emitted from the semiconductor laser diode 84 to parallel light, and a lens 86 for optically coupling the parallel light to a fiber. The light reflector 82 is arranged on the optical axis between the lens 85 and the lens 86. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor laser module, and more particularly to a semiconductor laser module in which a semiconductor laser diode and a light reflector are combined into a module.

  In recent years, in order to increase the communication capacity of an optical communication system, a wavelength division multiplexing (WDM) communication system that multiplexes and transmits a plurality of lights having different wavelengths has been actively introduced. In such a WDM communication system, in order to effectively use a limited number of wavelengths, there is a demand for practical use of a wavelength conversion element that converts a signal wavelength into an arbitrary signal wavelength. Conventionally, as a wavelength conversion element for converting the wavelength of light, an element using a semiconductor optical amplifier, an element using four-wave mixing, second harmonic generation by quasi-phase matching, which is a kind of second-order nonlinear optical effect, and sum frequency Wavelength conversion elements using generation and difference frequency generation are known (for example, see Patent Document 1).

FIG. 1 shows a configuration of a wavelength conversion device using a conventional quasi phase matching type wavelength conversion element. The wavelength conversion device combines and outputs the semiconductor laser 11 that outputs the signal light A having the wavelength λ _A , the semiconductor laser 12 that outputs the excitation light B having the wavelength λ _B , and the signal light A and the excitation light B. The optical coupler 14 includes a wavelength conversion element 13 that inputs the combined signal light A and pumping light B and outputs converted light C having a wavelength λ _C. Drive circuits 11a and 12a and temperature control circuits 11b and 12b are connected to the semiconductor lasers 11 and 12, respectively. Further, a fiber grating 15 is inserted between the semiconductor laser 11 and the optical coupler 14.

Since the intensity of the converted light C is proportional to the product of the intensity of the signal light A and the excitation light B, only the wavelength can be converted from the signal light A to the converted light C if the excitation light is kept constant. . For example, when λ _A = 1.55 μm and λ _B = 0.98 μm, λ _C = 0.60 μm is obtained as the sum frequency. When λ _A = 1.31 μm and λ _B = 1.06 μm, λ _C = 0.59 μm is obtained as the sum frequency. Further, when λ _A = 1.55 μm and λ _B = 0.77 μm, λ _C = 1.53 μm is obtained as the difference frequency. Therefore, in order to obtain a specific wavelength, it is necessary to strictly control the wavelengths of the signal light A and the excitation light B.

  FIG. 2 shows a phase matching curve when wavelength conversion is performed in order to obtain green light of 0.53 μm by second harmonic generation. Since the phase matching band of the wavelength conversion element is very narrow, a semiconductor laser that oscillates in a single mode is desirable in order to stably output converted light.

  The wavelengths of 1.55 μm and 1.31 μm are long wavelength bands used in optical communication, and a laser diode that oscillates at a single wavelength, such as a DFB laser diode, can be used as a semiconductor laser. On the other hand, the wavelength in the short wavelength band of 0.98 μm, 1.06 μm, and 0.77 μm is very difficult to produce a DFB laser diode and the demand is small. Yes. Therefore, a fiber grating that partially reflects only a specific wavelength is connected to the output of the semiconductor laser, and a part of the output light is reinjected into the semiconductor laser to control the oscillation wavelength to oscillate at the grating wavelength. ing.

JP 2003-140214 A A. Ferrari, et al., “Subkilohertz Fluctuations and Mode Hopping in High-Power Grating-Stabilized 980-nm Pumps,” IEEE J. of Lightwave Tech., Vol.20, pp.515-518, 2002/3 W. Streifer, et al., “Effect of external reflectors on longitudinal modes of distributed feedback lasers,” IEEE J. of Quantum Electronics, vol. 10, pp. 154-161, 1975/4

  When a semiconductor laser diode (hereinafter referred to as “LD”) and a fiber grating (hereinafter referred to as “FBG”) are combined, the FBG is often used at a location about 1 m or more away from the LD (see, for example, Non-Patent Document 1). ). As an example, when an FBG with a reflection band of 60 pm is fusion spliced away from an LD of 976 nm band by 1.2 m, a reflection peak appears repeatedly at 0.27 pm (corresponding to a resonance interval of 1.2 m) in the reflection band. It will be.

  FIG. 3 shows an output spectrum of a light source in which a conventional semiconductor laser diode and a fiber grating are combined. When evaluated with an optical spectrum analyzer with a resolution of 10 pm, it is measured as a single spectrum with a half-value width of about 20 pm. However, in reality, the half-value width of about 20 pm is in a multimode state having about 70 peaks. FIG. 4 shows the results measured with an electric spectrum analyzer. It has a peak at a position that is a multiple of about 85 MHz corresponding to 0.27 pm. The upper side of FIG. 4 is a result of measurement by peak hold, and the lower side is a result of measurement by averaging. Since both agree with each other, it can be seen that the oscillation is in a stable multimode state.

  A light source that outputs mid-infrared light using a wavelength conversion element is also used in an apparatus that analyzes an absorption spectrum of environmental gas. For example, in the case of directly observing linear absorption of about the Doppler width, a semiconductor laser that outputs pumping light used as a light source needs to realize a single spectral line width of a half width of 0.8 pm (250 MHz) or less. .

  In order to narrow the spectral width of a semiconductor laser singly, a method has been considered in which a reflection band is narrowed and an LD and an FBG are connected close to each other. FIG. 5 shows a configuration of a conventional semiconductor laser module in which an LD and an FBG are brought close to each other. An FBG 52 is formed in the vicinity of the tip of the fiber 51, and a tip portion 53 that is coupled to the LD 54 is processed into a tip shape. Since the FBG 52 is preferably fixed so that the optical axis is in a straight line, a metal ferrule is attached and soldered so as to include the FBG 52 from the vicinity of the tip of the fiber 51. According to this configuration, it is possible to realize characteristics with a half-value width of 2 pm or less. However, there is a problem in that the component cost is high because the front end portion 53 serving as a lens and the FBG 52 are integrated. Further, there is a problem that the tip portion 53 and the LD 54 need to be close to about several μm, the characteristics during monitoring change depending on the distance, and the mounting process is complicated because the manufacturing tolerance is narrow.

  FIG. 6 shows a configuration of a conventional semiconductor laser module using lens coupling. An FBG 62 is formed near the tip of the fiber 61 and is coupled to the LD 64 using the lenses 65 and 66. Since the lenses 65 and 66 are used for coupling, the manufacturing tolerance can be increased, and the mounting process is facilitated. However, since the lenses 65 and 66 and the FBG 62 need to be fixed in the metal ferrule, the length of the FBG 62 is limited to the length of the metal ferrule. In this method, since the length of the FBG 62 can be increased only to about 6 mm, the reflection band can be narrowed only to about 70 pm.

  Increasing the element length is effective for increasing the output from the LD. Therefore, if the element length of the LD in the 976 nm band is 2 mm, peaks at intervals of about 60 pm occur on the wavelength spectrum due to resonance between the end faces of the LD. Since this peak interval is slightly narrower than the reflection band of the FBG, a state in which two modes are selected in the reflection band can occur. Therefore, a semiconductor laser module having a long element length using lens coupling has a problem that it cannot operate stably in a single mode.

  The present invention has been made in view of such problems, and an object thereof is to provide a semiconductor laser module capable of obtaining a narrow spectral line width.

  In order to achieve the above object, the present invention provides a semiconductor laser module in which a semiconductor laser diode and an optical reflector are combined, and the emitted light from the semiconductor laser diode is transmitted through a fiber. The optical reflector is disposed on an optical axis between the lens and the fiber.

  According to a second aspect of the present invention, there is provided a semiconductor laser module in which a semiconductor laser diode and a light reflector are combined, a first lens that converts light emitted from the semiconductor laser diode into parallel light, and the parallel light as a fiber. And a second lens optically coupled to the optical reflector, wherein the light reflector is disposed on an optical axis between the first lens and the second lens.

  According to a third aspect of the present invention, in the semiconductor laser module in which the semiconductor laser diode and the light reflector are combined, a first lens that optically couples the emitted light from the emission end face of the semiconductor laser diode to a fiber; A second lens that converts the emitted light from the end face opposite to the emitting end face into parallel light, wherein the light reflector reflects the parallel light.

According to a fourth aspect of the present invention, in the optical reflector according to the _first , _second , or third aspect, two materials having refractive indexes of n ₁ and n ₂ are respectively 1 / (4n ₁ ) times the set wavelength. And 1 / (4n ₂ ) times as thick as a bulk-shaped element alternately stacked.

  According to a fifth aspect of the present invention, the light reflector according to the first, second, or third aspect is a Fabry-Perot resonator.

  The invention according to claim 6 is characterized in that the optical reflector according to claim 4 or 5 is set to have a total length of 6 mm or less and a reflection bandwidth of 70 pm or less.

  As described above, according to the present invention, a narrow spectral line width can be obtained by using a light reflector having a high refractive index instead of the fiber grating.

  Further, according to the present invention, by using the semiconductor laser module as a light source excitation laser that outputs mid-infrared light using a wavelength conversion element, it is possible to stably output converted light. Therefore, the present invention can also be applied to an apparatus for analyzing an absorption spectrum of environmental gas.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
When a grating structure is considered as an optical reflector, a reflection band can be calculated (for example, see Non-Patent Document 2). FIG. 7 shows the relationship between the reflection bandwidth Δλ _SB and the refractive index fluctuation value Δn of the concavo-convex portion in the grating at a wavelength of 976 nm. The vertical axis represents the reflection bandwidth Δλ _SB in units of pm, and is a value obtained by multiplying the length L (unit: mm) of the light reflector by the average value n of the refractive indexes of the two materials of the light reflector. is there. The horizontal axis represents a value obtained by multiplying the refractive index fluctuation value Δn by L.

Here, if Δλ _SB = 70 pm and L = 6 mm and n = 1.45 are substituted, the value on the vertical axis becomes 610 pm * mm. At this time, the value on the horizontal axis is 4 × 10 ⁻⁵ and Δn = 6.7 × 10 ⁻⁶ . That is, when the refractive index fluctuation value and the length of the optical reflector are determined, the product of the refractive index of the fiber core and the reflection bandwidth is fixed. For this reason, if the refractive index variation values are the same, the reflection bandwidth can be narrowed even with the same length of light reflector by increasing n.

  Therefore, by using an optical reflector having a high refractive index instead of the fiber grating, it is possible to realize feedback of light with high reflection in a narrow band. Specifically, a bulk light reflector is disposed between the lenses and fixed inside the package of the semiconductor laser module. Since coupling is performed using a lens, a large manufacturing tolerance can be obtained and a conventional mounting process can be used, so that an inexpensive semiconductor laser module can be produced.

  FIG. 8 shows a configuration of the semiconductor laser module according to the first embodiment. The LD 84 and the end face of the fiber 81 are coupled using a lens 85 that converts light emitted from the LD 84 into parallel light and a lens 86 that optically couples the parallel light to the fiber 81. Regarding the end face reflectance of the LD 84, the reflectance on the side close to the lens 85 is 0.1%, and the opposite side is 90%. The light reflector 82 is disposed on the optical axis between the lenses 85 and 86 and is fixed inside the package of the semiconductor laser module.

The light reflector 82 is composed of a material Al _x Ga _1-x As (i = 1) and a material Al _y Ga _1-y As (i = 2), and each has a setting wavelength of 976 nm of 1 / (4n _i ). The layers are repeatedly laminated at a thickness corresponding to twice (i represents the number of 1 or 2 of the material). The composition ratio x and y are adjusted so that Δn = 3 × 10 ⁻⁶ , assuming that the selected wavelength bandwidth of the light reflector 82 is 30 pm, n = 3.8, and the total length of the light reflector 82 is 5.0 mm. The reflectance of the light reflector 82 is 20%. The laminated material is cut into 2 mm squares, fixed to a jig, and fixed inside the package of the semiconductor laser module.

  FIG. 9 shows an output spectrum of the semiconductor laser module according to the first example. The result measured using etalon (FSR = 8 GHz) as a spectrum measuring apparatus is shown. FIG. 10 shows an enlarged view of the output spectrum. The spectral line width is a half-width of 4.5 pm (165 MHz) or less, which indicates that it is a narrow band.

  FIG. 11 shows the configuration of the semiconductor laser module according to the second embodiment. A lens 115 that optically couples light emitted from the LD 114 to the end face of the fiber 111 is provided. Regarding the end face reflectance of the LD 114, the reflectance on the side close to the lens 115 is 0.1%, and the opposite side is 90%. The light reflector 112 is disposed on the optical axis between the lens 115 and the end face of the fiber 111.

  The light reflector 112 is the same as that of the first embodiment, and the reflectance is 20%. A narrow-band spectral line width can be obtained as in the first embodiment.

  FIG. 12 shows a configuration of the semiconductor laser module according to the third embodiment. The LD 124 and the end face of the fiber 121 are coupled using a lens 125 that converts the light emitted from the exit end face of the LD 124 into parallel light and a lens 126 that optically couples the parallel light to the fiber 121. Regarding the end face reflectance of the LD 124, the reflectance on the side close to the lens 125 is 0.1%, and the opposite side is 0.1%. The light reflector 122 is disposed through the lens 127 so as to be optically coupled to the end surface opposite to the emission end surface of the LD 124.

As in the first embodiment, the light reflector 122 has a selected wavelength band of 30 pm, n = 3.8, the total length of the light reflector 122 is 5.0 mm, and a composition ratio x such that Δn = 3 × 10 ⁻⁶ . Adjust y. The reflectance of the light reflector 122 is 90%. The laminated material is cut into 2 mm squares, fixed to a jig, and fixed inside the package of the semiconductor laser module. As in the first embodiment, a narrow band spectral line width can be obtained.

  The configuration of the semiconductor laser module according to the fourth embodiment is the same as the configuration of the semiconductor laser module according to the first embodiment shown in FIG. The light reflector 82 uses a GaAs substrate having a thickness of 1.5 mm, and the opposite incident surface and exit surface are polished. In this case, an etalon is configured using both end faces with a reflectance of 34% as mirrors. Thereby, since a reflection band of 64 pm is obtained, a narrow-band spectral line width can be obtained as in the first embodiment.

  In this embodiment, a bulk-like element in which a grating is formed is used as the light reflector, but a Fabry-Perot resonator may be used. Further, a chirp structure may be applied in order to improve reflection characteristics. The material of the bulk element is not limited to a semiconductor, and a dielectric material can be combined.

It is a figure which shows the structure of the wavelength converter using the conventional quasi phase matching type | mold wavelength conversion element. It is a figure which shows the phase matching curve in the case of performing wavelength conversion by 2nd harmonic generation. It is a figure which shows the output spectrum of the light source which combined the conventional laser diode and fiber grating. It is a figure which shows the result of having measured the output spectrum with the electric spectrum analyzer. It is a figure which shows the structure of the conventional semiconductor laser module. It is a figure which shows the structure of the semiconductor laser module using the conventional lens coupling | bonding. It is a figure which shows the relationship between a stop band width and the refractive index fluctuation value in a grating. 1 is a diagram illustrating a configuration of a semiconductor laser module according to Example 1. FIG. It is a figure which shows the output spectrum of the semiconductor laser module concerning Example 1. FIG. It is a figure which shows the result of having expanded the output spectrum. FIG. 6 is a diagram illustrating a configuration of a semiconductor laser module according to Example 2. FIG. 6 is a diagram illustrating a configuration of a semiconductor laser module according to Example 3.

Explanation of symbols

11, 12, 54, 64, 84, 114, 124 Semiconductor laser (LD)
11a, 12a Drive circuit 11b, 12b Temperature control circuit 13 Wavelength conversion element 14 Optical coupler 15, 52, 62 Fiber grating (FBG)
51, 61, 81, 111, 121 Fiber 53 Tip portion 65, 66, 85, 86, 115, 125-127 Lens 82, 112, 122 Light reflector

Claims (6)

In a semiconductor laser module combining a semiconductor laser diode and a light reflector,
A lens for optically coupling the light emitted from the semiconductor laser diode to a fiber;
The semiconductor laser module according to claim 1, wherein the optical reflector is disposed on an optical axis between the lens and the fiber. In a semiconductor laser module combining a semiconductor laser diode and a light reflector,
A first lens for converting light emitted from the semiconductor laser diode into parallel light;
A second lens for optically coupling the parallel light to a fiber;
The semiconductor laser module according to claim 1, wherein the light reflector is disposed on an optical axis between the first lens and the second lens. In a semiconductor laser module combining a semiconductor laser diode and a light reflector,
A first lens for optically coupling light emitted from an emission end face of the semiconductor laser diode to a fiber;
A second lens that converts the emitted light from the end face opposite to the emitting end face into parallel light;
The semiconductor laser module, wherein the light reflector reflects the parallel light. The optical reflector has a bulk shape in which two materials having refractive indexes of n ₁ and n ₂ are alternately stacked at a thickness of 1 / (4n ₁ ) times and 1 / (4n ₂ ) times of a set wavelength. 4. The semiconductor laser module according to claim 1, 2, or 3, wherein   4. The semiconductor laser module according to claim 1, wherein the light reflector is a Fabry-Perot resonator. 6. The semiconductor laser module according to claim 4, wherein the optical reflector has a total length set to 6 mm or less and a reflection bandwidth set to 70 pm or less.

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