JP2011128368A - Wavelength conversion light source - Google Patents

Abstract

In a wavelength conversion light source that generates a mid-infrared light using a waveguide type nonlinear optical crystal, it is intended to reduce power consumption by suppressing excitation light intensity.
A nonlinear optical crystal is provided with a waveguide structure. First and second high reflectivity films 303 and 304 having high reflectivity with respect to the wavelength λ ₂ are applied to both end faces of the waveguide 302. Incident light from an excitation light source 305 that emits excitation light having a wavelength λ ₁ is supplied from a fiber 306 and is coupled to the waveguide 302 through the first and second lenses 307 and 308 through the high reflectivity film 303. Yes. On the other hand, the incident light from the seed light source 309 that emits the seed light of the signal light having the wavelength λ ₂ is supplied from the fiber 310 and passes through the third lens 311 and the mirror 312 to the waveguide 302 through the high reflectivity film 304. Are combined. The mirror 312 has a property of reflecting the seed light and transmitting the mid-infrared light.
[Selection] Figure 3

Description

  The present invention relates to a wavelength conversion light source, and more particularly to a wavelength conversion light source including a wavelength conversion element that generates mid-infrared light used for environmental gas measurement and the like.

From the viewpoints of environmental protection and safety and health, greenhouse gases such as CH ₄ , CO ₂ , CO, N ₂ O, environmental gases such as NO _x , SO _x , ammonia, water absorption peaks, and many organic gases It is strongly desired to establish a trace analysis technique for residual agricultural chemicals. As one method for measuring the gas concentration, a method is known in which laser light is applied to a gas to be measured and its absorption characteristics are observed. Since each gas has its own absorption line, the gas concentration can be measured by scanning a laser beam having a wavelength near the absorption line and observing the absorption spectrum.

  Many of the environmental gases have fundamental vibrations or low-order harmonic absorption lines in the mid-infrared light region having a wavelength of 2 μm or more. Accordingly, there is an increasing demand for a mid-infrared light source capable of continuous oscillation at room temperature in the mid-infrared region having a wavelength of 2 μm or more. As such a light source, a number of light sources that output laser light in the mid-infrared region, which is the absorption line wavelength of gas, have been announced by generating a difference frequency by a quasi phase matching wavelength conversion element (for example, Non-Patent Document 1). reference). This light source can be easily put to practical use because a technically stable semiconductor laser having a wavelength of 2 μm or less can be used as a laser for inputting excitation light and signal light to the wavelength conversion element.

As a method of obtaining mid-infrared light by wavelength conversion, there is optical parametric oscillation in addition to difference frequency generation (see, for example, Non-Patent Document 2). A block diagram of optical parametric oscillation is shown in FIG. Since bulk type nonlinear optical crystals have been used in optical parametric oscillation, bulk nonlinear optical crystals will be used for explanation. The nonlinear optical crystal 11 is disposed so as to be sandwiched between the concave mirrors 12 and 13. When excitation light having a wavelength λ ₁ is input along the optical axis 14, light having wavelengths λ ₂ and λ ₃ satisfying 1 / λ ₁ = 1 / λ ₂ + 1 / λ ₃ due to a nonlinear optical effect is generated in the nonlinear optical crystal 11. appear. The mirrors 12 and 13 are highly reflective at least with respect to the light with the wavelength λ _{2, and} the light with the wavelength λ ₂ reciprocates between the mirrors 12 and 13 along the optical axis 15. The actual light follows a locus in which the light is narrowed down in the nonlinear optical crystal 11 like a light shape 16 indicated by a dotted line. In the nonlinear optical crystal 11, light having wavelengths λ ₂ and λ ₃ is generated. This acts as a so-called gain medium, and is sandwiched between mirrors 12 and 13 to form an optical oscillator. When the gain in the nonlinear optical crystal 11 exceeds the loss in the mirrors 12 and 13, oscillation light having a wavelength λ ₂ including oscillation is obtained along the optical axis 17. In this case, light having a strong wavelength λ ₃ is simultaneously obtained by being induced by light having a strong wavelength λ ₂ .

Although the case where the mirrors 12 and 13 are mirrors constituting a resonator with respect to light of wavelength λ ₂ has been described here, the mirrors 12 and 13 are mirrors that constitute a resonator with respect to light of wavelength λ _3. It does not matter if it is.

Such optical parametric oscillation becomes a standing wave in the optical axis 15, and can oscillate at all wavelengths within the gain wavelength region of the nonlinear optical crystal 11 exceeding the loss in the mirrors 12 and 13. There is a possibility of oscillation. Therefore, it is well known that stable single mode oscillation is possible when laser light having a fixed wavelength λ ₂ in the gain region is incident on the nonlinear optical crystal 11 as seed light in addition to the excitation light having wavelength λ ₁ .

D. G. Lancaster et al., Optics Lett., Vol. 24, No. 23, 1744 (1999). L. E. Myers et al., J. Opt. Soc. Am. B., Vol. 12, No. 11, 2102 (1995).

  In the optical parametric oscillation using the bulk type nonlinear optical crystal as described above, the nonlinear optical effect is small, and the excitation light intensity required for oscillation is as very large as several W and the power consumption is large. It is considered best to use a waveguide-type nonlinear optical crystal with high light density and high conversion efficiency.

FIG. 2 shows a configuration diagram when a waveguide-type nonlinear optical crystal is used. In this configuration diagram, a case where input light is supplied from an optical fiber will be described. The structure of the waveguide 22 is applied to the nonlinear optical crystal 21. High reflectivity films 23 and 24 are provided on both end faces of the waveguide 22. Incident light is supplied from the fiber 25 and is coupled to the waveguide 22 via lenses 26 and 27. For such a structure, the two waves of the wavelength lambda ₁ of the excitation light and the wavelength lambda ₂ of the seed beam when entering the fiber 25, since the two waves is wavelength different for chromatic aberration of the lenses 26, 27 There is a problem that the optimum coupling efficiency cannot be obtained at the same time for the two lights.

  The present invention has been made in view of such problems, and an object thereof is to suppress the excitation light intensity in a wavelength conversion light source that generates a mid-infrared light using a waveguide type nonlinear optical crystal. The purpose is to reduce power consumption.

In order to achieve such an object, according to a first aspect of the present invention, there is provided a wavelength-converted light source that generates mid-infrared light, an excitation light source that emits excitation light having a _first wavelength λ 1, A seed light source that emits seed light having a _second wavelength λ ₂ different from the wavelength λ ₁ and a nonlinear optical crystal waveguide are provided, and the reflectance of the seed light is 70 at both end faces of the nonlinear optical crystal waveguide. % Of the reflection film is installed, and the excitation light emitted from the excitation light source is incident on the nonlinear optical crystal waveguide from one end of the nonlinear optical waveguide via a lens, and the seed light emitted from the seed light source Light enters the nonlinear optical crystal waveguide from the other end of the nonlinear optical waveguide via a lens and a mirror, and the mid-infrared light is generated by optical parametric oscillation in the nonlinear optical crystal waveguide. / Characterized in that it satisfies _{1 -1 / λ 2 = 1 /} λ 3 is the third mid-infrared light of wavelength lambda _3.

  According to a second aspect of the present invention, in the first aspect, the mirror has a characteristic of reflecting the seed light and transmitting the mid-infrared light.

  According to a third aspect of the present invention, in the first aspect, the mirror has a characteristic of transmitting the seed light and reflecting the mid-infrared light.

  According to a fourth aspect of the present invention, in the second or third aspect, a polarization inversion structure is applied to a core layer of the nonlinear optical crystal waveguide.

  According to a fifth aspect of the present invention, in the fourth aspect, the material of the core layer is lithium niobate.

  According to a sixth aspect of the present invention, in the second or third aspect, the light from the excitation light source and the seed light source is coupled to the nonlinear optical crystal waveguide through a fiber. To do.

According to the present invention, by entering the first wavelength lambda ₁ of the excitation light and the first wavelength lambda ₁ and the second wavelength different from lambda ₂ of the seed beam from another direction of the nonlinear optical crystal waveguide end, Light of the first wavelength λ ₁ and the second wavelength λ ₂ can be optimally incident on the nonlinear optical crystal waveguide. This leads to an improvement in wavelength conversion efficiency by replacing a conventional bulk crystal with a waveguide element, and a reduction in coupling loss. Therefore, the wavelength conversion light source with low power consumption can be achieved by suppressing the output of the excitation light source.

It is a block diagram of the conventional optical parametric oscillation. It is the conventional block diagram at the time of using a waveguide type nonlinear optical crystal. It is a figure which shows the structure of the wavelength conversion light source concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the wavelength conversion light source concerning the 2nd Embodiment of this invention. 1 is a diagram illustrating a configuration of a wavelength conversion light source according to Example 1. FIG. 6 is a diagram illustrating a configuration of a wavelength conversion light source according to Example 2. FIG. FIG. 6 is a diagram illustrating a configuration of a wavelength conversion light source according to a third example. It is a figure which shows the structure of the wavelength conversion light source concerning Example 4. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the first and second embodiments will be described, followed by examples.

(First embodiment)
FIG. 3 shows the configuration of the wavelength conversion light source according to the first embodiment of the present invention. The nonlinear optical crystal 301 is provided with a structure of a waveguide 302 (hereinafter also referred to as “nonlinear optical crystal waveguide”). Both end faces of the waveguide 302 have a first high-reflectivity film 303 having a reflectance of 90% or more with respect to the wavelength λ ₂ (corresponding to the “second wavelength”) and a reflectance of 70% or more. A second high reflectivity film 304 is applied. Incident light from an excitation light source 305 that emits excitation light having a wavelength λ ₁ (corresponding to “first wavelength”) is supplied from a fiber 306, and has high reflectivity via first and second lenses 307 and 308. Coupled to waveguide 302 through membrane 303. On the other hand, the incident light from the seed light source 309 that emits the seed light of the signal light having the wavelength λ ₂ is supplied from the fiber 310 and passes through the third lens 311 and the mirror 312 to the waveguide 302 through the high reflectivity film 304. Are combined. The mirror 312 has a property of reflecting the seed light and transmitting the mid-infrared light.

The incident seed light having the wavelength λ ₂ is reflected by the first and second high reflectivity films 303 and 304 on both end faces of the waveguide 302 and reciprocates in the waveguide 302. The pumping light of wavelength λ _{1 in} the waveguide 302 traveling in the direction from the first high reflectance film 303 to the second high reflectance film 304 amplifies the seed light of wavelength λ ₂ traveling in the same direction. receive. In the seed light having the wavelength λ ₂ , laser oscillation occurs when the amplification exceeds the sum of the loss in the first and second high reflectivity films 303 and 304 and the waveguide loss due to the waveguide in the waveguide 302. Accompanying the laser oscillation of this signal light, the mid-infrared light having a high intensity wavelength λ ₃ (corresponding to the “third wavelength”) satisfying 1 / λ ₁ −1 / λ ₂ = 1 / λ ₃ can be extracted. it can. At this time, the mid-infrared light having the wavelength λ ₃ passes through the mirror 312 and is extracted through the lens 313.

In the first embodiment described above, the excitation light having the wavelength λ ₁ can be optimally coupled to the waveguide 302 by the installation positions of the first and second lenses 307 and 308 with respect to the fiber 306 and the waveguide 302. On the other hand, the light from the seed light source 309 is coupled to the waveguide 302 by the independent third lens 311, and only the coupling between the light from the seed light source 309 and the waveguide 302 is taken into consideration. Can be installed.

  In the above configuration, the light from the excitation light source and the seed light source is coupled to the waveguide through the fiber, but the output from the semiconductor LD chip is coupled to the waveguide through the lens without using the fiber. May be.

  In addition, periodic polarization inversion may be applied to the nonlinear optical crystal. Normally, phase matching cannot be achieved between the three lights of excitation light, seed light, and mid-infrared light, resulting in low generation efficiency of mid-infrared light. There is an advantage that the generation efficiency of the mid-infrared light in the nonlinear optical waveguide is increased.

(Second Embodiment)
FIG. 4 shows a configuration of a wavelength conversion light source according to the second embodiment of the present invention. The nonlinear optical crystal 401 has a structure of a waveguide 402 (hereinafter also referred to as “nonlinear optical crystal waveguide”). The first high reflectivity film 403 having a reflectivity of 90% or more with respect to the wavelength λ ₂ (corresponding to the “second wavelength”) and the first high reflectivity film having a reflectivity of 70% or more on both end faces of the waveguide 402 Two high reflectivity films 404 are provided. Incident light from an excitation light source 405 that emits excitation light having a wavelength λ ₁ (corresponding to a “first wavelength”) is supplied from a fiber 406 and passed through first and second lenses 407, 408. It is coupled to the waveguide 402 through the reflectivity film 403. On the other hand, incident light from a seed light source 409 that emits seed light of signal light having a wavelength λ ₂ is supplied from the fiber 410 and passes through the third lens 411, the mirror 412, and the fourth lens 413. Coupled to waveguide 402 through high reflectivity film 404. The mirror 412 has characteristics of transmitting signal light and reflecting mid-infrared light.

The incident seed light having the wavelength λ ₂ is reflected by the first and second high reflectivity films 403 and 404 on both end faces of the waveguide 402 and reciprocates in the waveguide 402. The pumping light of wavelength λ _{1 in} the waveguide 402 traveling in the direction from the first high reflectance film 403 to the second high reflectance film 404 amplifies the seed light of wavelength λ ₂ traveling in the same direction. receive. In the seed light of wavelength λ ₂ , laser oscillation occurs when the amplification exceeds the sum of the loss in the first and second high reflectivity films 403 and 404 and the waveguide loss due to the waveguide of the waveguide 402. Accompanying the laser oscillation of the seed light, mid-infrared light having a high intensity wavelength λ ₃ (corresponding to the “third wavelength”) satisfying 1 / λ ₁ -1 / λ ₂ = 1 / λ ₃ can be extracted. it can. At this time, the mid-infrared light having the wavelength λ ₃ is reflected by the mirror 413 and extracted.

In the second embodiment described above, the excitation light having the wavelength λ ₁ can be optimally coupled to the waveguide 402 according to the installation positions of the first and second lenses 407 and 408 with respect to the fiber 406 and the waveguide 402. On the other hand, light from the light source 409 is coupled to the waveguide 402 by independent third and fourth lenses 411 and 413, and only the coupling between the light from the seed light source 409 and the waveguide 402 is considered. The positions of the fourth lenses 411 and 413 can be set.

  In the above configuration, the light from the excitation light source and the seed light source is coupled to the waveguide through the fiber, but the output from the semiconductor LD chip is coupled to the waveguide through the lens without using the fiber. May be.

  In addition, periodic polarization inversion may be applied to the nonlinear optical crystal. Similar to the first embodiment, there is an advantage that the generation efficiency of mid-infrared light in the nonlinear optical waveguide is increased.

FIG. 5 shows a configuration of the wavelength conversion light source according to the first embodiment. This wavelength conversion light source is an example of the first embodiment. A nonlinear optical crystal 501 having a waveguide 502 and composed of LiNbO ₃ subjected to periodic polarization inversion was prepared. The polarization inversion period was 28.4 μm and the waveguide length was 5 cm. First and second high reflectivity films 503 and 504 having a high reflectivity with respect to a wavelength of 1550 nm were applied to both end faces of the waveguide of the nonlinear optical crystal 501. The reflectivity at 1550 nm of the first high reflectivity film 503 was 99%, and the reflectivity at 1550 nm of the second high reflectivity film 504 was 70%.

  An excitation light source 505 having a wavelength of 1064 nm has a fiber 506 as an output port. The output from the excitation light source 505 was coupled to the waveguide 502 through the first and second lenses 507 and 508 from the side where the high reflectance film 503 was applied. At this time, the optical coupling between the fiber 506 and the waveguide 502 only needs to consider 1064 nm, and the focal length of the first and second lenses 507 and 508 does not need to consider chromatic aberration, and the fiber 506 with respect to light having a wavelength of 1064 nm. The first and second lenses 507 and 508 having a focal length ratio can be selected so as to compensate the ratio between the mode diameter inside the waveguide and the mode diameter inside the waveguide 502. In addition, since the installation position of the first and second lenses 507 and 508 can be determined only at 1064 nm without considering the difference in refraction angle depending on the wavelength, the installation location is 1064 nm more efficiently than in the case of considering the seed light. Can combine light.

  The seed light source 509 is a semiconductor laser module having an oscillation wavelength of 1550 nm. The seed light source 509 has a fiber 510 as an output port. A third lens 511 and a mirror 512 are installed so that the output from the seed light source 509 is optimally coupled to the waveguide 502 from the side where the high reflectance film 504 is applied. The mirror 512 used had a characteristic of reflecting a wavelength of 1550 nm and transmitting 3 to 4 μm of mid-infrared light. A third lens 513 was placed through the mirror 512 on the optical axis from the end of the waveguide 502 on the side on which the second high reflectance film 504 was applied, and the mid-infrared light output was measured.

  The mid-infrared light output started to be observed when the output intensity of the excitation light source 505 was around 100 mW. A mid-infrared light output of 6 mW was observed when the output intensity of the excitation light source 505 was 1 W.

FIG. 6 shows a configuration of a wavelength conversion light source according to the second embodiment. This wavelength conversion light source is an example of the second embodiment. A nonlinear optical crystal 601 having a waveguide 602 and composed of LiNbO ₃ subjected to periodic polarization inversion was prepared. The polarization inversion period was 28.4 μm and the waveguide length was 5 cm. First and second high reflectivity films 603 and 604 having high reflectivity with respect to a wavelength of 1550 nm were applied to both end faces of the waveguide of the nonlinear optical crystal 601. The reflectivity at 1550 nm of the first high reflectivity film 603 was 99%, and the reflectivity at 1550 nm of the second high reflectivity film 604 was 80%.

  An excitation light source 605 having a wavelength of 1064 nm has a fiber 606 as an output port. First and second lenses 607 and 608 are installed so that the output from the excitation light source 605 is optimally coupled to the waveguide 602 from the side where the first high reflectance film 603 is applied.

  The seed light source 609 is a semiconductor laser module having an oscillation wavelength of 1550 nm. The seed light source 609 has a fiber 610 as an output port. A third lens 611, a mirror 612, and a fourth lens 613 are installed so that the output from the seed light source 609 is optimally coupled to the waveguide 602 from the side where the second high-reflectivity film 604 is applied. did. A mirror 612 having a characteristic of transmitting a wavelength of 1550 nm and reflecting mid-infrared light of 3 to 4 μm was used. The mid-infrared light output reflected by the mirror 612 was measured.

  The mid-infrared light output started to be observed when the output intensity of the excitation light source 505 was around 90 mW. A mid-infrared light output of 4 mW was observed when the output intensity of the excitation light source 505 was 1 W.

FIG. 7 shows a configuration of a wavelength conversion light source according to the third embodiment. This wavelength conversion light source is an example of the first embodiment. A nonlinear optical crystal 701 having a waveguide 702 and made of LiNbO ₃ subjected to periodic polarization inversion was prepared. The polarization inversion period was 28.4 μm and the waveguide length was 5 cm. First and second high reflectivity films 703 and 704 having high reflectivity with respect to a wavelength of 1550 nm were applied to both end faces of the waveguide of the nonlinear optical crystal 701. The reflectivity at 1550 nm of the first high reflectivity film 703 was 99%, and the reflectivity at 1550 nm of the second high reflectivity film 704 was 90%.

  A distributed feedback type semiconductor laser chip was used as the excitation light source 705 having a wavelength of 1064 nm. First and second lenses 706 and 707 are installed so that the output from the excitation light source 705 is optimally coupled to the waveguide 702 from the side where the high reflectance film 703 is applied.

  As the seed light source 708, a distributed feedback semiconductor laser chip having an oscillation wavelength of 1550 nm was used. In addition, a third lens 709 and a mirror 710 are installed so that the output from the seed light source 708 is optimally coupled to the waveguide 702 from the side where the high reflectance film 704 is applied. The mirror 710 used has a characteristic of reflecting a wavelength of 1550 nm and transmitting 3 to 4 μm of mid-infrared light. A third lens 711 was placed through the mirror 710 on the optical axis from the end of the waveguide 702 on the side where the second high reflectance film 704 was applied, and the mid-infrared light output was measured.

  The mid-infrared light output started to be observed when the output intensity of the excitation light source 705 was around 60 mW. A mid-infrared light output of 4 mW was observed when the output intensity of the excitation light source 705 was 800 mW.

FIG. 8 shows a configuration of a wavelength conversion light source according to the fourth embodiment. This wavelength conversion light source is an example of the second embodiment. A nonlinear optical crystal 801 having a waveguide 802 and made of LiNbO ₃ subjected to periodic polarization inversion was prepared. The polarization inversion period was 28.4 μm and the waveguide length was 5 cm. First and second high reflectivity films 803 and 804 having high reflectivity with respect to the wavelength 1550 were applied to both end faces of the waveguide of the nonlinear optical crystal 801. The reflectivity at 1550 nm of the first high reflectivity film 803 was 99%, and the reflectivity at 1550 nm of the second high reflectivity film 804 was 70%.

  A distributed feedback semiconductor laser chip was used as the excitation light source 805 having a wavelength of 1064 nm. First and second lenses 806 and 807 are installed so that the output from the excitation light source 805 is optimally coupled to the waveguide 802 from the side where the high reflectance film 803 is applied.

  As the seed light source 808, a distributed feedback semiconductor laser chip having an oscillation wavelength of 1550 nm was used. In addition, a third lens 809, a mirror 810, and a fourth lens 811 are installed so that the output from the seed light source 808 is optimally coupled to the waveguide 802 from the side where the high reflectance film 804 is provided. . A mirror 810 having a characteristic of transmitting a wavelength of 1550 nm and reflecting mid-infrared light of 3 to 4 μm was used. The mid-infrared light output reflected by the mirror 810 was measured.

  The mid-infrared light output began to be observed when the output intensity of the excitation light source 805 was around 120 mW. A mid-infrared light output of 12 mW was observed when the output intensity of the excitation light source 805 was 1 W.

302, 402, 502, 602, 702, 802 Nonlinear optical crystal waveguides 303, 304, 403, 404, 503, 504, 603, 604, 703, 704, 803, 804 High reflectivity films 306, 310, 406, 410 , 506, 510, 606, 610 Fiber 307, 308, 311, 313, 407, 408, 411, 413, 507, 508, 511, 513, 607, 608, 611, 613, 706, 707, 709, 711, 806 , 807, 809, 811 Lens 305, 405, 505, 605, 705, 805 Excitation light source 309, 409, 509, 609, 708, 808 Seed light source 301, 401, 501, 601, 701, 801 Nonlinear optical crystal

Claims (6)

In the wavelength conversion light source that generates mid-infrared light,
An excitation light source that emits excitation light of a _first wavelength λ ₁ ;
A seed light source that emits seed light having a _second wavelength λ ₂ different from the _first wavelength λ ₁ ;
A non-linear optical crystal waveguide,
Reflective films having a reflectance of 70% or more with respect to the seed light are disposed on both end faces of the nonlinear optical crystal waveguide,
The excitation light emitted from the excitation light source is incident on the nonlinear optical crystal waveguide through a lens from one end of the nonlinear optical waveguide,
The seed light emitted from the seed light source is incident on the nonlinear optical crystal waveguide from the other end of the nonlinear optical waveguide via a lens and a mirror,
The mid-infrared light is mid-infrared light of the third wavelength λ ₃ that is generated by optical parametric oscillation in the nonlinear optical crystal waveguide and satisfies 1 / λ ₁ −1 / λ ₂ = 1 / λ _3. A wavelength conversion light source characterized by that.   The wavelength conversion light source according to claim 1, wherein the mirror has a characteristic of reflecting the seed light and transmitting the mid-infrared light.   The wavelength conversion light source according to claim 1, wherein the mirror has a characteristic of transmitting the seed light and reflecting the mid-infrared light.   4. The wavelength conversion light source according to claim 2, wherein a polarization inversion structure is applied to a core layer of the nonlinear optical crystal waveguide.   The wavelength conversion light source according to claim 4, wherein a material of the core layer is lithium niobate.   4. The wavelength conversion light source according to claim 2, wherein light from the excitation light source and the seed light source is coupled to the nonlinear optical crystal waveguide through a fiber.

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