JP2007316339A - Wavelength conversion laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To output coherent light utilizing a nonlinear optical effect with a simple construction. <P>SOLUTION: A wavelength conversion laser is provided with a laser (51) generating light having a wavelength λ1, a first mirror (54) transmitting light having the wavelength λ1 and reflecting light having wavelength λ2 longer than the wavelength λ1, a second mirror (55) which transmits laser light having the wavelength λ1 and reflects light having the wavelength λ2 and whose reflectance of the wavelength λ2 is lower than that of the first mirror, a gain medium (52) disposed on an optical axis between the first and the second mirrors, absorbing a portion of light having the wavelength λ1 and having gain in the wavelength λ2 and a nonlinear optical crystal (53) in which light having the wavelengths λ1 and λ2 and emitted from the gain medium via the second mirror is made incident and from which conversion light having a third wavelength λ3 is emitted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength conversion laser, and more particularly to a wavelength conversion laser that outputs a coherent light having a desired wavelength by difference frequency generation or sum frequency generation using a nonlinear optical medium.

  In recent years, environmental problems have been greatly highlighted and measurement of environmental gases has become important. Many of the environmental gases have a fundamental vibration or an absorption line of lower harmonics in the mid-infrared region of a wavelength of 2 μm to 5 μm. Accordingly, there is an increasing demand for mid-infrared light sources that generate high-power coherent light in the mid-infrared region. As such a light source, a Pb salt laser that outputs laser light having a wavelength of 3 μm or more, a quantum cascade laser that outputs laser light having a wavelength of 5 μm or more, and the like are known. However, since it is a low-temperature operation and oscillates only by pulse driving, it is not practical.

  On the other hand, a fluorescence microscope is known in which cell tissue is stained with a fluorescent label to analyze cell functions. The fluorescence microscope uses a light source having a wavelength of 500 to 600 nm, which is an absorption wavelength band of red fluorescent protein. However, a small and compact light source, particularly a semiconductor laser, has not been put into practical use in this wavelength band.

  In recent years, a wavelength conversion laser (for example, Non-Patent Document 1) that generates laser light in the mid-infrared region using difference frequency generation using a second-order nonlinear optical effect, or 500 to 600 nm using sum frequency generation. There has been a report of a wavelength conversion laser (for example, Non-Patent Document 2) that generates laser light. Here, the difference frequency generation means that laser light having a wavelength λ1 and a wavelength λ2 is incident on a nonlinear optical crystal to obtain converted light having a wavelength λ3 that satisfies the relationship 1 / λ1-1 / λ2 = 1 / λ3. The sum-frequency generation means that laser beams having wavelengths λ1 and λ2 are incident on the nonlinear optical crystal to obtain converted light having a wavelength λ3 that satisfies the relationship 1 / λ1 + 1 / λ2 = 1 / λ3.

O. Tadanaga, et al., “Widely tunable 2-micron-band difference-frequency generation in direct-bonded quasi-phase-matched LiNbO3 ridge waveguide”, CLEO / QELS CML5 2005 M. Asobe, et al., “Laser diode pumped 560-590 nm light source using sum-frequency generation in direct-bonded quasi-phase-matched LiNbO3 waveguide”, CLEO / QELS CML3 2005 A. Keating, et al., “High-temperature optically pumpued 1.55 μm VCSEL operating at 6 Gb / s”, IEEE Photon Tech. Lett. Vol.12, No.2, pp.116-118 (2000)

  FIG. 1 shows a configuration of a conventional wavelength conversion laser. As shown in FIG. 1A, the wavelength conversion laser is incident on a laser 11 that outputs laser light having a wavelength λ1, a laser 12 that outputs laser light having a wavelength λ2, and laser light having wavelengths λ1 and λ2. Then, it is comprised from the nonlinear optical crystal 13 which radiate | emits the laser beam of wavelength (lambda) 3.

  As shown in FIG. 1B, the means for combining the two laser beams is to convert the output beams from the lasers 11 and 12 into parallel beams using the lenses 21 and 22, and combine the two beams using the combiner 24. Wave and enter the nonlinear optical crystal 13 through the lens 23. Further, as shown in FIG. 1C, output light from the lasers 11 and 12 is incident on the nonlinear optical crystal 13 using an optical fiber and a multiplexer 31. Such a wavelength conversion laser requires two laser light sources and multiplexing means, and has a problem that the number of parts is large.

  The present invention has been made in view of such problems, and an object thereof is to provide a wavelength conversion laser that outputs coherent light using a nonlinear optical effect with a simple configuration.

  In order to achieve the above object, according to the present invention, a wavelength conversion laser according to claim 1 transmits a light having a wavelength λ1 and a light having the wavelength λ1, and transmits the light having the wavelength λ1. A first mirror (54) that reflects light having a longer wavelength λ2, and transmits the laser light having wavelength λ1, reflects the light having wavelength λ2, and the reflectance of the wavelength λ2 is that of the first mirror; A second mirror (55) having a reflectance lower than that of the first mirror and the second mirror is disposed on the optical axis between the first mirror and the second mirror to absorb a part of the light having the wavelength λ1, and the wavelength λ2 A gain medium (52) having a gain and a nonlinear optical crystal that enters the light of wavelength λ1 and wavelength λ2 emitted from the gain medium via the second mirror and emits converted light of wavelength λ3 (53).

  The invention according to claim 2 is the wavelength conversion laser according to claim 1, wherein the first mirror, the gain medium, and the second mirror are sequentially stacked on a semiconductor substrate. To do.

  According to a third aspect of the present invention, in the wavelength conversion laser according to the first aspect, the laser includes a third mirror, an active layer, and a fourth mirror sequentially stacked on a semiconductor substrate, and the fourth mirror. Further, the first mirror, the gain medium, and the second mirror are sequentially stacked.

  According to a fourth aspect of the present invention, the nonlinear optical crystal according to the first, second, or third aspect has a periodic domain-inverted structure. The nonlinear optical crystal may have a waveguide structure.

The invention according to claim 6 is the nonlinear optical crystal according to any one of claims 1 to 5, wherein the nonlinear optical crystal is LiNbO ₃ , LiTaO ₃ , KNbO ₃ , K _1-y Li _y Ta _1-x Nb _x O ₃ ( 0 <x <1, 0 ≦ y <1), or KTiOPO ₄ . The nonlinear optical crystal can also contain at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive.

  As described above, according to the present invention, the first mirror (54), the gain medium (52), and the second mirror (55) function as a light-excited surface-emitting laser, and light having a wavelength λ1. And the light of wavelength λ2 are incident on the nonlinear optical crystal (53), and the converted light of wavelength λ3 is output by the difference frequency generation or the sum frequency generation, so the wavelength conversion laser using the nonlinear optical effect with a simple configuration Can be configured.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 2 shows the configuration of a wavelength conversion laser according to an embodiment of the present invention. The wavelength conversion laser includes a laser 51 that outputs laser light having a wavelength λ1 and a nonlinear optical crystal 53. On the optical axis on which the laser light from the laser 51 is incident on the nonlinear optical crystal 53, a mirror 54 and a gain medium are provided. 52 and the mirror 54 are arranged in order.

  The laser light having the wavelength λ1 is transmitted through the mirror 54, part of it is absorbed by the gain medium 52, the rest is transmitted through the gain medium 52, passes through the mirror 55, and is incident on the nonlinear optical crystal 53. In the gain medium 52, carriers are generated by the absorption of the laser light having the wavelength λ 1, and light excitation type laser oscillation occurs according to the oscillation mode formed by the mirror 54 and the mirror 55. The laser beam having the oscillation wavelength λ2 is also incident on the nonlinear optical crystal 53 at the same time. The nonlinear optical crystal 53 outputs converted light of wavelength λ3, which is mid-infrared light or visible light, by difference frequency generation or sum frequency generation by two laser beams of wavelength λ1 and wavelength λ2.

FIG. 3 shows the configuration of the wavelength conversion laser according to Example 1 of the present invention. The wavelength conversion laser includes a laser 101 that outputs laser light having a wavelength λ1 and a nonlinear optical crystal 103, and a lens 104 and a gain medium are placed on an optical axis on which the laser light from the laser 101 is incident on the nonlinear optical crystal 103. The semiconductor element 102 including the lens 105 and the lens 105 are sequentially arranged. The laser 101 outputs laser light having a wavelength λ1 = 980 nm. The nonlinear optical crystal 103 is made of lithium niobate (LiNbO ₃ : hereinafter referred to as LN).

  The semiconductor element 102 includes a DBR mirror 122 made of a GaAs / AlGaAs multilayer film, a GaInAsN active layer 123 as a gain medium, and a DBR mirror 124 made of a GaAs / AlGaAs multilayer film by crystal growth on a GaAs semiconductor substrate 121. Are sequentially stacked. The DBR mirror 122 and the DBR mirror 124 transmit 980 nm light and reflect 1.27 μm light. The reflectance of 1.27 μm light is set so that the DBR mirror 122 is higher than the DBR mirror 124.

  The active layer 123 has a band gap designed to have an oscillation wavelength at a wavelength λ2 = 1.27 μm, has an absorptivity at 980 nm, and emits light of 1.27 μm. Here, the active layer 123 is set to a thickness that does not absorb all light of 980 nm. That is, the 980 nm light incident on the semiconductor element 102 is transmitted through the DBR mirror 122, a part is absorbed by the active layer 123, the rest is transmitted through the active layer 123, and is transmitted through the DBR mirror 124 to be nonlinear optical crystal. 103 is incident.

  The active layer 123 that absorbs light of 980 nm emits light of 1.27 μm by photoexcitation, and is oscillated by a resonator composed of the DBR mirror 122 and the DBR mirror 124, and emits laser light having a wavelength λ2 = 1.27 μm. Emits light. As described above, the semiconductor element 102 functions as an optically pumped surface emitting laser. The 980 nm light and the 1.27 μm laser light transmitted through the active layer 123 are incident on the nonlinear optical crystal 103 through the lens 105.

When the light output from the output side of the nonlinear optical crystal 103 is observed through a filter made of Ge, converted light having a wavelength λ3 = 4.3 μm and a light intensity of 3 nW can be observed. Mid-infrared light having a wavelength of 4.3 μm is effective for detecting CO ₂ gas.

FIG. 4 shows the configuration of a wavelength conversion laser according to Example 2 of the present invention. The difference from the configuration of the first embodiment shown in FIG. 3 is that the polarization of the nonlinear optical crystal 131 made of LN has a structure that is periodically inverted. The inversion period is 26.7 μm. The composition of the active layer 123 of the semiconductor element 102 and the reflection characteristics of the DBR mirrors 122 and 124 are controlled to output light having a wavelength λ2 = 1.30 μm. When the light output from the output side of the nonlinear optical crystal 131 is observed through a filter made of Ge, converted light having a wavelength λ3 = 4.0 μm and a light intensity of 160 nW can be observed. Mid-infrared light having a wavelength of 4.0 μm is effective for detecting SO ₂ gas.

  Next, the inversion period of the nonlinear optical crystal 131 is set to 7.83 μm. When the light output from the output side of the nonlinear optical crystal 131 is observed, converted light having a wavelength λ3 = 0.56 μm and a light intensity of 2.2 μW can be observed. Visible light having a wavelength of 0.56 μm is effective as an excitation light source for a dye used in a fluorescence microscope.

  FIG. 5 shows the configuration of a wavelength conversion laser according to Example 3 of the present invention. The difference from the configuration of the first embodiment shown in FIG. 3 is that the waveguide 133 is formed with a structure in which the polarization of the nonlinear optical crystal 132 made of LN is periodically inverted. The inversion period is 25.0 μm. When the light output from the emission side of the nonlinear optical crystal 132 is observed through a filter made of Ge, converted light having a wavelength λ3 = 4.3 μm and a light intensity of 5 μW can be observed.

FIG. 6 shows a method for manufacturing a nonlinear optical crystal having a periodic domain-inverted structure and having a waveguide formed therein. First, a LiNbO ₃ substrate 141 and a LiTaO ₃ substrate 142 on which a periodically poled structure is prepared in advance are prepared. Each of the substrates 141 and 142 is a 3-inch wafer whose both surfaces are optically polished, and has a thickness of 500 μm. After making the surfaces of the substrates 141 and 142 hydrophilic by normal acid cleaning or alkali cleaning, the two substrates are superposed in a clean atmosphere. The superposed substrates are put in an electric furnace and bonded by heat treatment at 500 ° C. for 3 hours (first step). The bonded substrates are void-free, and cracks do not occur even when returned to room temperature.

  Next, the thickness of the substrate 41 is reduced to 14 μm using a grinding device such as a grinder and a polishing device. After polishing the substrate 141, polishing is performed to obtain a mirror-polished surface (second step). Next, the polished thin film substrate is set on a dicing saw, and a ridge waveguide having a core width of 18 μm is manufactured by precision processing using a diamond blade having a particle diameter of 4 microns or less (third step).

  FIG. 7 shows the configuration of a wavelength conversion laser according to Example 4 of the present invention. The difference from the configuration of the first embodiment shown in FIG. 3 is that the semiconductor substrate 151 of the semiconductor element 150 is subjected to lens processing. The surface of the semiconductor substrate 151 facing the laser 101 was processed into a convex shape to give the role of the lens 103 shown in FIG. When the light output from the output side of the nonlinear optical crystal 103 is observed through a filter made of Ge, converted light having a wavelength λ3 = 4.3 μm and a light intensity of 2 nW can be observed.

  FIG. 8 shows the configuration of a wavelength conversion laser according to Example 5 of the present invention. The fifth embodiment uses a multi-wavelength laser 160 in which the laser of the wavelength conversion laser shown in the first embodiment and a semiconductor element are manufactured on one semiconductor substrate. The multi-wavelength laser 160 includes a DBR mirror 162 made of a GaAs / AlGaAs multilayer, an active layer 163 made of a GaAs / GaInAs quantum well, and a GaAs / AlGaAs multilayer by crystal growth on a GaAs semiconductor substrate 161. A DBR mirror 164 is sequentially stacked to constitute a surface emitting laser. This surface emitting laser outputs a laser beam having a wavelength λ1 = 980 nm from the DBR mirror 164 by current injection.

  Further, a DBR mirror 165 composed of a GaAs / AlGaAs multilayer film, a GaInAsN active layer 166, and a DBR mirror 167 composed of a GaAs / AlGaAs multilayer film are sequentially stacked on the DBR mirror 164 by crystal growth, and a gain medium is formed. It is composed. The DBR mirror 165 and the DBR mirror 167 transmit light of 980 nm and reflect light of 1.30 μm. The reflectance of 1.30 μm light is set so that the DBR mirror 165 is higher than the DBR mirror 167.

  The active layer 166 has a band gap designed to have an oscillation wavelength at a wavelength 2 = 1.30 μm, has an absorptivity at 980 nm, and emits light of 1.30 μm. Here, the active layer 166 is set to a thickness that does not absorb all light of 980 nm. That is, the 980 nm light incident on the gain medium is transmitted through the DBR mirror 165, a part is absorbed by the active layer 166, and the rest is transmitted through the active layer 166, and is transmitted through the DBR mirror 167. Is incident on.

  The active layer 166 that has absorbed the light of 980 nm emits light of 1.30 μm by photoexcitation, becomes an oscillation state by a resonator composed of the DBR mirror 165 and the DBR mirror 167, and emits laser light having a wavelength λ2 = 1.30 μm Emits light. Thus, the gain medium functions as an optically pumped surface emitting laser. The 980 nm light and the 1.30 μm laser light transmitted through the active layer 166 are incident on the nonlinear optical crystal 103 through the lens 105.

  When the light output from the output side of the nonlinear optical crystal 103 is observed through a filter made of Ge, converted light having a wavelength λ3 = 4.0 μm and a light intensity of 12 nW can be observed.

(Mounting method of wavelength conversion laser)
FIG. 9 shows a mounting method of the wavelength conversion laser according to the fifth embodiment. The multi-wavelength laser 160 is mounted on the metal member 200 of the TO-38 standard system, for example. Further, the multi-wavelength laser 160 is sealed by a CAN package 201 having a window in the emission direction of the laser light from the multi-wavelength laser 160. A lens holder 202 having a lens 105 fixed on the optical axis of the laser beam is connected to the surface of the CAN package 201 having a window. The lens 105 may be incorporated in the window portion of the CAN package 201.

  The lens holder 202 is connected to a holder 203 on which the nonlinear optical crystal 103 is fixed on the optical axis of the laser beam. Furthermore, a filter or the like fixed to the holder may be disposed on the optical axis of the light output from the nonlinear optical crystal 103. In this way, the wavelength conversion laser is improved by sealing the multi-wavelength laser with the CAN package, and the wavelength conversion laser is assembled by fixing the lens and the nonlinear optical crystal through the holder. Alignment can be facilitated. Further, the wavelength conversion laser can be reduced in size by joining the CAN package and the holder.

(Other examples)
In Examples 1 to 5, a laser having a wavelength λ1 = 980 nm and a gain medium that emits 1.3 μm band light are used. For example, a laser having a wavelength λ1 = 850 nm may be used.

  In Examples 1 to 5, the DBR mirror is composed of a GaAs / AlGaAs multilayer film, but a dielectric multilayer film may be formed by vapor deposition or the like. The active layer is made of GaInAsN, but may be made of a III-V group compound semiconductor using elements such as Al as the group III and Sb, P as the group V. Further, although the semiconductor device emits light of the wavelength λ2 = 1.3 μm band, it may emit light of wavelengths 1.2 μm and 1.55 μm longer than the laser wavelength of 980 nm. Furthermore, the active layer may be a quantum well or a quantum box.

  In Examples 1 to 5, the active layer is produced by growing a DBR mirror and an active layer in this order on a GaAs substrate. For example, after growing a DBR mirror on a GaAs substrate, a separately grown InGaAsP active layer or the like may be bonded by a wafer fusion method or the like.

In Examples 1 to 5, LN was used as the nonlinear optical crystal, but LiNbO ₃ , LiTaO ₃ , KNbO ₃ , K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 ≦ y <1), it is possible to apply any of KTiOPO _4. Moreover, these crystals may contain at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive.

  In the first to third embodiments, the semiconductor element 102 may be disposed in the opposite direction with respect to the traveling direction of the laser light, and the light from the laser 101 may be incident from the DBR 124 side. In this case, the DBR 124 is set to have a higher light reflectance than the DBR 122.

  Instead of the waveguide fabrication method shown in Embodiment 3, a metal diffusion method such as Ti or a proton exchange method can be applied.

  In Examples 4 and 5, a periodic polarization inversion structure may be introduced into the nonlinear optical crystal. Furthermore, a waveguide structure may be introduced.

  In the fifth embodiment, the DBR mirror 164 and the DBR mirror 165 are individually manufactured. However, an integrated DBR mirror that reflects 980 nm light and 1.3 μm band light may be used. At this time, the reflectance is set to be lower than the DBR mirror 162 for 980 nm light and higher than the DBR mirror 166 for 1.3 μm band light.

It is a figure which shows the structure of the conventional wavelength conversion laser. It is a figure which shows the structure of the wavelength conversion laser concerning one Embodiment of this invention. It is a figure which shows the structure of the wavelength conversion laser concerning Example 1 of this invention. It is a figure which shows the structure of the wavelength conversion laser concerning Example 2 of this invention. It is a figure which shows the structure of the wavelength conversion laser concerning Example 3 of this invention. It is a figure which shows the manufacturing method of the nonlinear optical crystal which has a periodic polarization inversion structure and in which the waveguide was formed. It is a figure which shows the structure of the wavelength conversion laser concerning Example 4 of this invention. It is a figure which shows the structure of the wavelength conversion laser concerning Example 5 of this invention. It is a figure which shows the mounting method of the wavelength conversion laser concerning Example 5. FIG.

Explanation of symbols

11, 12, 51, 101 Laser 13, 53, 103, 131, 132 Nonlinear optical crystal 21, 22, 23, 104, 105 Lens 24, 31 Multiplexer 25, 54, 55 Mirror 52, 102, 150 Semiconductor element 121 , 151, 161 Semiconductor substrate 122, 124, 152, 154, 162, 164, 165, 167 DBR mirror 123, 153, 163, 166 Active layer 133 Waveguide 141 LiNbO ₃ substrate 142 LiTaO ₃ substrate 160 Multi-wavelength laser 200 Metal member 201 CAN package 202 Lens holder 203 Holder

Claims (7)

A laser that generates light of wavelength λ1,
A first mirror that transmits light of wavelength λ1 and reflects light of wavelength λ2 longer than wavelength λ1,
A second mirror that transmits the laser light of the wavelength λ1, reflects the light of the wavelength λ2, and has a reflectance of the wavelength λ2 lower than that of the first mirror;
A gain medium that is disposed on the optical axis between the first mirror and the second mirror, absorbs part of the light of the wavelength λ1, and has a gain at the wavelength λ2.
A non-linear optical crystal that receives the light of the wavelength λ1 and the wavelength λ2 emitted from the gain medium through the second mirror and emits the converted light of the wavelength λ3. laser.   2. The wavelength conversion laser according to claim 1, wherein the first mirror, the gain medium, and the second mirror are sequentially stacked on a semiconductor substrate. In the laser, a third mirror, an active layer, and a fourth mirror are sequentially stacked on a semiconductor substrate,
2. The wavelength conversion laser according to claim 1, wherein the first mirror, the gain medium, and the second mirror are sequentially stacked on the fourth mirror.   The wavelength conversion laser according to claim 1, wherein the nonlinear optical crystal has a periodic polarization inversion structure.   The wavelength conversion laser according to claim 4, wherein the nonlinear optical crystal has a waveguide structure. The nonlinear optical crystal is any one of LiNbO ₃ , LiTaO ₃ , KNbO ₃ , K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 ≦ y <1), or KTiOPO _4. The wavelength conversion laser according to any one of claims 1 to 5, wherein The wavelength conversion laser according to claim 6, wherein the nonlinear optical crystal contains at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive.

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