JP2004134811A - Semiconductor stimulated solid-state laser and optical device utilizing it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor stimulated solid-state laser which can efficiently output a stable infrared laser beam by improving the stimulation light absorption efficiency, and an optical device utilizing it. <P>SOLUTION: This semiconductor stimulated solid-state laser is equipped with a solid-state laser medium, a semiconductor laser for stimulating the solid-state laser medium, and a pair of opposite mirrors for resonating the light stimulated in the solid-state laser medium. The distance between the pair of mirrors is determined so that the relationship between the gain width GW of the solid-state laser medium and the frequency spacing FS in the resonator mode is 0.1≤FS/GW<1. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a semiconductor laser pumped solid-state laser that uses laser light emitted from a semiconductor laser as excitation light, and an optical device using the same.

Conventionally, there has been known a semiconductor laser-excited solid-state laser in which a solid-state laser crystal is excited in the axial direction by laser light emitted from a semiconductor laser, and an infrared laser light is oscillated by a resonator formed in the axial direction.

Such a semiconductor laser-pumped solid-state laser has the advantages of being ultra-small, long-lived, capable of single longitudinal mode oscillation, and capable of high-frequency modulation.

As this semiconductor laser, one having an oscillation wavelength of 808 to 810 nm, which is the absorption wavelength range of a solid laser crystal, and having an output of 100 mW or more, is used. Yttrium aluminum garnet) and the like are used.

In a semiconductor laser pumped solid-state laser, the output of the semiconductor laser must be efficiently absorbed by the solid-state laser crystal. At present, the oscillation wavelength of a semiconductor laser that provides the highest output is in the 800 nm band, but since Nd ³⁺ (neodymium) absorption is in the vicinity of this, an oxide crystal or the like to which Nd is added is widely used as a solid-state laser crystal. ing.

By the way, in a semiconductor laser-pumped solid-state laser, in order to oscillate in a single longitudinal mode and obtain low-noise laser light with a very stable oscillation frequency, it is required to increase the frequency interval in the resonator mode. This requires the solid-state laser crystal to be thin.

When the solid-state laser crystal is thinned, the amount of laser light emitted from the semiconductor laser that passes through the solid-state laser crystal increases, while the amount of light that is absorbed by the solid-state laser crystal and excites the solid-state laser crystal decreases. There is a problem that an infrared laser cannot be efficiently obtained with respect to the laser light emitted from the laser.

The present invention has been made in order to solve the above-described problems, and it is an object of the present invention to improve a semiconductor laser-excited solid-state laser capable of improving absorption efficiency of excitation light and efficiently obtaining a stable infrared laser output. It is an object of the present invention to provide an optical device.

The invention according to claim 1 includes a solid-state laser medium, a semiconductor laser for exciting the solid-state laser medium, and a pair of mirrors facing each other for resonating light excited by the solid-state laser medium. A semiconductor laser-excited solid-state laser, wherein a relationship between a gain width GW of the solid-state laser medium and a frequency interval FS of a resonator mode is within a range of 0.1 ≦ FS / GW <1; interval is selected, the transverse mode is a _{TEM 01} following mode, the solid-state laser medium consists of YVO ₄ crystal Nd is added, the thickness of the solid-state laser medium is 0.4mm or more, below 3.75mm The pair of mirrors is configured by a reflection film directly coated on two opposing surfaces of the solid-state laser medium, and laser light is emitted from the semiconductor laser of the solid-state laser medium. One surface of the solid-state laser medium is coated with a mirror that is non-reflective for the laser light and highly reflective for oscillation light having a wavelength of 1340 nm, and the other surface of the solid-state laser medium facing the one surface has a wavelength of 1340 nm. A mirror that reflects the oscillating light and transmits a part of the oscillating light.

According to a second aspect of the present invention, there is provided the semiconductor laser-excited solid-state laser according to the first aspect, wherein the solid-state laser medium, the semiconductor laser, and a pair of mirrors are mounted on the same heat conductive support, A temperature control device for controlling the temperature of the porous support.

According to a third aspect of the present invention, there is provided the semiconductor laser-excited solid-state laser according to the first aspect, wherein the solid-state laser medium, the semiconductor laser, and the pair of mirrors have a window on an emission surface of the light excited by the solid-state laser medium. And a wave plate or a polarizer is provided in a window of the case.

According to a fourth aspect of the present invention, there are provided a first and a second laser light sources that emit laser lights having polarization planes orthogonal to each other, and a polarization beam splitter that combines the laser lights from the first and the second laser light sources. An optical device comprising: a polarizing beam splitter that combines laser light combined from a polarizing beam splitter; and a coupling optical system that collects the laser light combined from the polarizing beam splitter and guides the combined laser light to an end surface of an optical fiber. The first and second laser light sources include a solid-state laser medium, a semiconductor laser for exciting the solid-state laser medium, and a pair of opposed mirrors for resonating light excited by the solid-state laser medium. Wherein the relationship between the gain width GW of the solid-state laser medium and the frequency interval FS of the resonator mode is 0.1 ≦ FS / GW <1. That the within the interval selected between a pair of mirrors, the transverse mode is a TEM ₀₁ following mode, the solid-state laser medium consists of YVO ₄ crystal Nd is added, the thickness of the solid laser medium 0.4 mm or more and 3.75 mm or less, and the pair of mirrors are constituted by reflection films directly coated on two opposing surfaces of the solid-state laser medium, and a laser beam is emitted from the semiconductor laser of the solid-state laser medium. Is coated with a mirror that is non-reflective for the laser light and highly reflective for the oscillating light having a wavelength of 1340 nm, and the other surface facing the one surface of the solid-state laser medium has a wavelength A first laser light source that is coated with a mirror that reflects 1340 nm oscillating light and transmits a part of the oscillating light, before entering the coupling optical system; The distance between the beam waist of the laser beam from the laser beam and the polarization beam splitter is equal to the distance between the beam waist of the laser beam from the second laser light source before entering the coupling optical system and the polarization beam splitter. It is characterized by having done.

According to the first aspect of the present invention, an interval between a pair of mirrors is selected in a range where the relationship between the gain width GW of the solid-state laser medium and the frequency interval FS of the resonator mode is 0.1 ≦ FS / GW <1. Laser-excited solid-state having a low-noise frequency below 2 GHz, which is a practical frequency range for communication, and having a thickness of a solid-state laser medium that can be easily manufactured without lowering the pumping efficiency. A laser can be obtained.

Further, since the pair of mirrors is constituted by the reflection films directly coated on the two opposing surfaces of the solid-state laser medium, a semiconductor laser-excited solid-state laser that can be easily manufactured can be obtained.

Further, the solid-state laser medium, it is possible Nd is to obtain a high laser-diode pumped solid-state laser of excitation efficiency since it is configured such that the addition has been YVO ₄ crystal.

According to a second aspect of the present invention, there is provided a temperature control device for mounting the solid-state laser medium, the semiconductor laser, and the pair of mirrors on the same thermally conductive support, and controlling the temperature of the thermally conductive support. With the configuration, the temperature control performance can be improved, and the oscillation output of the semiconductor laser pumped solid-state laser can be improved.

According to a third aspect of the present invention, the solid-state laser medium, the semiconductor laser, and the pair of mirrors are built in a case having a window on an emission surface of light excited by the solid-state laser medium, and a wave plate or Since the polarizer is provided, the number of components can be reduced, and as a result, optical aberrations can be reduced.

According to a fourth aspect of the present invention, a polarization beam splitter combines laser beams having mutually orthogonal polarization planes from the first and second laser light sources, collects the combined laser beams, and collects the end face of the optical fiber. And a distance between the beam waist of the laser light from the first laser light source before entering the coupling optical system and the polarizing beam splitter, and a distance before entering the coupling optical system. Since the distance between the beam waist of the laser light from the second laser light source and the distance between the polarization beam splitters is made equal, it is possible to obtain an optical device having good laser light coupling efficiency.

According to the present invention, it is possible to provide a semiconductor laser-excited solid-state laser capable of improving the absorption efficiency of excitation light and obtaining an efficient and stable infrared laser output, and an optical device using the same.

FIG. 1 shows a semiconductor laser pumped solid-state laser according to an embodiment of the present invention.

1 is a semiconductor laser for excitation having a maximum output of 1 W and an oscillation wavelength of 808 nm, 2 is a condensing lens for condensing a laser beam for excitation emitted from the semiconductor laser for excitation, and 3 is YVO ₄ (added with Nd). It is a solid-state laser medium (solid-state laser crystal) composed of a crystal of yttrium vanadium oxide) or a YAG crystal to which Nd is added.

(4) A mirror R1 composed of a dichroic mirror that is non-reflective to the excitation laser light (808 nm) and totally reflects oscillation light (1340 nm) is coated on the incident surface of the excitation laser light of the solid-state laser medium 3. The output surface of the solid-state laser medium 3 for oscillating light is coated with a mirror R2 that transmits several percent of the oscillating light (1340 nm) and reflects the rest. A resonator is constituted by the pair of mirrors R1 and R2.

FIG. 2 shows the relationship between the frequency, the gain width of the solid-state laser medium, and the resonator mode.

In FIG. 2, f20 indicates the relationship between the gain and the frequency of the solid-state laser medium, GW indicates the gain width of the solid-state laser medium, and f24 to f28 indicate the resonator modes. The frequency interval FS between adjacent resonator modes f24 to f28 is given by FS = c / 2nl. Here, c is the speed of light, n is the refractive index of the solid-state laser medium, and l is the resonator length.

When the frequency interval FS in the resonator mode is larger than the gain width GW of the solid-state laser medium, oscillation occurs in a single longitudinal mode. In the present invention, however, the gain width GW of the solid-state laser medium and the frequency interval FS in the resonator mode are used. The distance l between the pair of mirrors (resonator length, that is, the thickness of the solid-state laser medium) l is selected so that the relationship with satisfies 0.1 ≦ FS / GW <1. In this case, the number of vertical modes is 2 to 11 as described later. By setting the resonator length l in this manner, the level of laser noise can be minimized in practical use, and infrared laser light can be efficiently obtained with respect to laser light emitted from a semiconductor laser. it can.

Hereinafter, the reason why the relationship between the gain width GW of the solid-state laser medium and the frequency interval FS of the resonator mode is set as described above will be described. Nd: YVO _4, which is a solid-state laser medium capable of obtaining oscillation light having a wavelength of 1340 nm, has a higher absorptance of excitation laser light emitted from a semiconductor laser than other crystals (for example, about 3 times the absorptivity of YAG). Times). However, in order to have a single longitudinal mode, the thickness of the solid-state laser medium needs to be about 0.2 mm. In this case, processing and handling of the crystal become difficult, and the doping amount of Nd is reduced. The excitation laser light from the semiconductor laser absorbed as 1 atm% is only about 30%, and the oscillation efficiency of the infrared laser is poor.

The absorptance of the excitation laser light is related to the Nd doping amount and the thickness of the solid-state laser medium. To obtain an absorptivity of 99%, the product of the Nd doping amount and the thickness of the solid-state laser medium is 3 atm%. Mm or more, it is necessary to be about 1.2 atm% mm to obtain an absorptivity of 85%.

FIG. 3 is a graph showing the relationship between the absorptance of the excitation laser light, the doping amount of Nd, the thickness of the solid-state laser medium, and the FS / GW according to the present invention. For example, when the doping amount of Nd is 3 atm%, when the thickness (crystal thickness) of the solid laser medium is 0.4 mm, the absorptivity of the excitation laser is 85% or more, and the thickness of the solid laser medium is When it is 1.0 mm, the absorption rate of the excitation laser is 99% or more. When the thickness of the solid-state laser medium is 0.4 mm, FS / GW is 1.0, and when the thickness of the solid-state laser medium is 3.75 mm, FS / GW is 1/10.

When the doping amount of Nd is constant, the thickness of the solid-state laser medium for setting the absorption rate of the excitation laser light to 99% is set to the thickness of the solid-state laser medium for setting the absorption rate of the excitation laser light to 85%. It is about 2.5 times the length.

FIG. 4 is a diagram showing the relationship between the output of the excitation laser light and the infrared laser output when the absorptance of the excitation laser light is changed. The measured values are indicated by solid lines, and the theoretical values are indicated by broken lines.

When the resonator is formed by increasing the parallelism and flatness of both surfaces of the solid-state laser medium and forming a pair of mirrors R1 and R2 on both surfaces of the solid-state laser medium as described above, the solid-state laser medium of the laser light for excitation is used. Due to the internal profile and the relationship between the laser gain and the loss in the solid-state laser medium, the thinner the solid-state laser medium, the higher the efficiency of the infrared laser output at 1340 nm may be and the more efficient.

条件 The above condition of 0.1 ≦ FS / GW is determined from the condition of low noise below 2 GHz which is a practical frequency range for communication. What may possibly occur around 2 GHz due to laser noise is beat noise generated from the frequency interval FS in the resonator mode. Further, when the transverse mode is a low-order but multi-mode, noise due to the longitudinal and transverse resonance mode is generated, and the frequency band of this noise may be around 1/10 of beat noise.

Therefore, the frequency band of the beat noise needs to be 20 GHz or more, and therefore, the frequency interval FS in the resonator mode needs to be 20 GHz or more. Here, the gain width GW of the Nd: YVO ₄ crystal in which the doping amount of Nd is 1 atm% is experimentally found to be about 200 GHz.

From the above, it is necessary to satisfy 0.1 ≦ FS / GW in order to achieve low noise below 2 GHz. When 0.1 = FS / GW, the number of vertical modes is 11 at the maximum.

Commonly available Nd: YVO ₄ doped amount of the crystal of Nd is 0.5～3Atm%, the absorption rate of the excitation laser beam of 808nm as doping amount of Nd is small becomes lower. When the doping amount of Nd is small, the absorptance of the laser light for excitation is increased by increasing the thickness of the crystal, but as a result, the cavity length l is widened and the frequency interval FS of the cavity mode is narrowed.

From the relationship between the absorption rate of the excitation laser light and the product of the Nd doping amount and the thickness of the solid-state laser medium, in order to set the absorption rate of the excitation laser light to 99%, for example, the Nd doping amount is set. In the case of 1 atm%, the thickness l of the solid-state laser medium needs to be at least 3 mm or more. Since the cavity length derived from the condition of 0.1 ≦ FS / GW is 3.75 mm or less, the absorptance of the excitation laser beam can be 99% even when the Nd doping amount is 1 atm%. . Further, even when the doping amount of Nd is set to 0.5 atm%, the absorptance of the excitation laser beam can be set to 95%.

On the other hand, the condition of FS / GW <1 is determined from the condition that when the doping amount of Nd is 3%, the absorptance of the excitation laser beam is 85% or more, and the thickness of the solid-state laser medium can be reduced as much as possible.

(4) In the case where the output of the semiconductor laser for excitation is 1 W, if the absorption rate of the laser light for excitation of the solid laser medium is 85% or more, an infrared laser output of 230 mW or more can be obtained. In this case, the rated output of the laser device can be set to 200 mW, and for example, the performance is sufficient for communication.

From the relationship between the absorptance of the excitation laser light and the product of the Nd doping amount and the thickness of the solid-state laser medium, for example, when the Nd doping amount is 3 atm%, the thickness l of the solid-state laser medium is 0.4 mm. In this case, the absorptance of the excitation laser light becomes 85% or more. With such a thickness, processing and handling of the solid-state laser medium is not difficult. Frequency interval FS of the resonator mode in this case is about 190GHz, Nd: YVO ₄ substantially coincides with the gain width GW of the crystal, FS / GW <1 is obtained.

As described above, when the interval l between the pair of mirrors is selected so that the relationship between the gain width GW of the solid-state laser medium and the frequency interval FS of the resonator mode is 0.1 ≦ FS / GW <1. As a result, the absorptance of the excitation laser beam becomes 85% or more, and the infrared laser output (1340 nm) can be obtained with an efficiency of 23% or more. At this time, the transverse mode is a low-order mode of TEM ₀₁ or lower.

で は Note that, in the practical frequency range of 2 GHz or less, the noise level is −120 dB / Hz or less (the curve indicated by “a” in FIG. 5 shows a region of 30 MHz or less). The curve shown by b in FIG. 5 shows the background noise level in a region of 30 MHz or less.

In the above-described embodiment, an example in which an Nd: YVO ₄ crystal is used as the solid-state laser medium has been described. However, the same operation and effect can be obtained by using an Nd: YAG crystal instead.

However, Nd: a YAG crystal, absorption rate of the excitation laser beam is Nd: Since YVO ₄ a 1/3 degree of crystallinity should be about 3 times the thickness of the crystal.

FIGS. 6A and 6B show an example of an optical device using the above-described semiconductor laser pumped solid-state laser as a laser light source. FIG. 6A is a cross-sectional view as viewed from the side of the laser light source 20a, and FIG. 6B is a plan view showing the configuration of the entire optical system.

The laser light sources 20a and 20b are, as described above, an excitation semiconductor laser 1 having an oscillation wavelength of 808 nm, a condenser lens 2 for condensing an excitation laser beam emitted from the excitation semiconductor laser, and a YVO ₄ crystal to which Nd is added. Or a solid-state laser medium 3 made of a YAG crystal doped with Nd, a pair of mirrors R1 and R2 facing each other for resonating light excited by the solid-state laser medium 3, polarizers 4 and 6, and a Faraday element 5. And outputs an infrared laser beam of 1340 nm.

Laser beams having polarization planes orthogonal to each other emitted from the laser light sources 20a and 20b are guided to the polarization beam splitter 10 via the optical isolator 7 and combined, and are combined by the condenser lens 8 (one coupling optical system) into an optical fiber connector. It is coupled to the optical fiber 9 via 12.

As described above, when the laser beams having the polarization planes orthogonal to each other are combined and coupled to the optical fiber 9, the power coupled to the optical fiber 9 is increased, and light of a predetermined polarization plane is extracted at the exit of the optical fiber 9. A constant power can be obtained without depending on the angle of the polarization plane.

When the two laser light sources 20a and 20b are the same, the distance between the beam waist before entering the condenser lens 8 of each laser beam and the polarization beam splitter 10 is made equal (L1 = L2), so that the optical fiber It is possible to improve the efficiency of coupling to

The semiconductor laser 1, the condenser lens 2, the solid-state laser medium 3, the pair of mirrors R1, R2, and the optical isolator 7 are made of a material having a window on an emission surface of light excited by the solid-state laser medium and having heat conductivity. The polarizer 6 of the optical isolator 7 is built in the case 11 and is used as a window (optical window) of the case.

As described above, the modularization of the laser light sources 20a and 20b facilitates replacement work, and the number of components can be reduced by one by using an optical thin film having birefringence such as a polarizer as a window of a case. As a result, aberration can be reduced. When a wave plate is used inside the case, the wave plate may be used as a window of the case.

The laser light sources 20a and 20b, the polarizing beam splitter 10, the condenser lens 8 and the optical fiber connector 12 are mounted on the same heat conductive support 13. The heat conductive support 13 is fixed on a Peltier device 15 joined to a heat sink 14.

A semiconductor laser 1, a condenser lens 2, a solid-state laser medium 3, a pair of mirrors R1 and R2, an optical isolator 7, a polarizing beam splitter 10, a condenser lens 8, and an optical fiber connector 12 are disposed near the semiconductor laser 1. In response to a temperature detection signal from a temperature sensor not shown, a temperature control circuit (not shown) drives and controls the Peltier element 15 to maintain a predetermined temperature.

The semiconductor laser 1, the condenser lens 2, the solid-state laser medium 3, the pair of mirrors R1, R2, the optical isolator 7, the polarization beam splitter 10, the condenser lens 8, and the optical fiber connector 12 are housed and sealed in a housing 16. ing. Further, a moisture absorbing agent 17 is provided in the housing 16. Thereby, the internal space in the housing 16 is kept at a low humidity, and dew condensation can be prevented.

1 is a diagram illustrating a semiconductor laser-pumped solid-state laser according to an embodiment of the present invention. FIG. 6 is a diagram illustrating a relationship between a frequency of a semiconductor laser pumped solid-state laser, a gain width of a solid-state laser medium, and a resonator mode. FIG. 3 is a diagram showing the interrelationship between the absorptance of excitation laser light, the doping amount of Nd, the thickness of a solid-state laser medium, and FS / GW according to the present invention. FIG. 4 is a diagram illustrating a relationship between the output of the excitation laser light and the infrared laser output when the absorption rate of the excitation laser light according to the present invention is changed. FIG. 3 is a diagram illustrating noise characteristics of the excitation laser light according to the present invention. FIG. 2 is a diagram illustrating an example of an optical device using a semiconductor laser-excited solid-state laser according to the present invention as a laser light source.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Condensing lens 3 Solid-state laser medium 4 Polarizer 5 Faraday element 6 Polarizer 7 Optical isolator 8 Condensing lens 9 Optical fiber 10 Polarization beam splitter 11 Case 12 Optical fiber connector 13 Thermal conductive support 14 Heat sink 15 Peltier element 16 Case 17 Hygroscopic agent 20a Laser light source 20b Laser light source R1 mirror R2 mirror

Claims (4)

A solid-state laser medium, a semiconductor laser for exciting the solid-state laser medium, and a semiconductor laser-excited solid-state laser including a pair of opposed mirrors for resonating light excited by the solid-state laser medium,
The relationship between the gain width GW of the solid-state laser medium and the frequency interval FS of the resonator mode is as follows:
0.1 ≦ FS / GW <1
The distance between the pair of mirrors is selected within a range of
The transverse mode is a mode below TEM ₀₁ ,
The solid-state laser medium is made of a YVO ₄ crystal doped with Nd,
A thickness of the solid-state laser medium is 0.4 mm or more and 3.75 mm or less;
The pair of mirrors are configured by reflecting films directly coated on two opposing surfaces of the solid-state laser medium,
One surface of the solid-state laser medium on which laser light is incident from the semiconductor laser is coated with a mirror that is non-reflective for the laser light and highly reflective for oscillating light having a wavelength of 1340 nm. A semiconductor laser-excited solid-state laser, characterized in that a mirror that reflects oscillation light having a wavelength of 1340 nm and transmits a part thereof is coated on the other surface facing the surface. 2. The temperature control device according to claim 1, wherein the solid-state laser medium, the semiconductor laser and the pair of mirrors are mounted on the same heat-conductive support, and a temperature controller for controlling the temperature of the heat-conductive support is provided. Semiconductor laser pumped solid state laser. The solid-state laser medium, the semiconductor laser, and the pair of mirrors are built in a case having a window on an emission surface of light excited by the solid-state laser medium, and include a wave plate or a polarizer in the window of the case. The semiconductor laser pumped solid-state laser according to claim 1, wherein: First and second laser light sources that emit laser light having polarization planes orthogonal to each other, a polarization beam splitter that synthesizes laser light from the first and second laser light sources, and a laser that is synthesized from the polarization beam splitter An optical device including a polarization beam splitter that synthesizes light and a coupling optical system that condenses the laser light synthesized from the polarization beam splitter and guides the laser light to an end surface of the optical fiber,
The first and second laser light sources include a solid-state laser medium, a semiconductor laser for exciting the solid-state laser medium, and a pair of mirrors facing each other for resonating light excited by the solid-state laser medium. A semiconductor laser pumped solid state laser comprising:
The relationship between the gain width GW of the solid-state laser medium and the frequency interval FS of the resonator mode is as follows:
0.1 ≦ FS / GW <1
The distance between the pair of mirrors is selected within a range of
The transverse mode is a mode below TEM ₀₁ ,
The solid-state laser medium is made of a YVO ₄ crystal doped with Nd,
A thickness of the solid-state laser medium is 0.4 mm or more and 3.75 mm or less;
The pair of mirrors are configured by reflecting films directly coated on two opposing surfaces of the solid-state laser medium,
One surface of the solid-state laser medium on which laser light is incident from the semiconductor laser is coated with a mirror that is non-reflective for the laser light and highly reflective for oscillating light having a wavelength of 1340 nm. The other surface opposite to the surface is coated with a mirror that reflects oscillating light having a wavelength of 1340 nm and transmits part of the light.
The distance between the beam waist of the laser light from the first laser light source before entering the coupling optical system and the polarization beam splitter, and the laser from the second laser light source before entering the coupling optical system An optical device wherein a beam waist of light and a distance between the polarizing beam splitters are equalized.

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