Wavelength conversion device

Abstract

A wavelength conversion device according to an embodiment of the present disclosure comprises: a wavelength conversion element (417); a first optical system (410) configured to cause fundamental-wave light and light of two wavelengths in a wavelength band of the fundamental-wave light to be incident on the wavelength conversion element; second optical systems (411, 412, 413) configured to separate converted light of the fundamental-wave light, the light of the two wavelengths, and remaining fundamental-wave light from the wavelength conversion element; photodetectors (421, 422) configured to detect a light intensity for each of the separated light of the two wavelengths; and a controller (420) configured to control an operating temperature of the wavelength conversion element on the basis of the difference between the light intensities respectively detected for the separated light of the two wavelengths.

Description

Wavelength conversion device

[0001] The present disclosure relates to a wavelength conversion device, and more particularly to a wavelength conversion device including a nonlinear optical element.

[0002] Conventionally, wavelength conversion devices using nonlinear optical elements are known. In a wavelength conversion device using a second-order nonlinear optical element, when signal light and pump light having a wavelength approximately half that of the signal light are input into the second-order nonlinear optical element, wavelength-converted signal light (also referred to as wavelength-converted light in the present specification) can be obtained through an optical parametric amplification (OPA) process (see, for example, Patent Document 1). For example, a wavelength conversion device using a second-order nonlinear optical element can perform wavelength conversion on 1.55 μm band light while maintaining the light as light without converting it into an electrical signal, by using pump light of high optical power in the 0.78 μm band.

[0003] FIG. 1 shows a schematic configuration of a conventional wavelength conversion device. The wavelength conversion device 100 shown in FIG. 1 includes a metal housing, and inside the metal housing, the wavelength conversion device includes a wavelength conversion element 117, lenses 109-2 and 111-2 disposed on the input and output sides of the wavelength conversion element 117, and dichroic mirrors 113 and 114 as well as 115 and 116.

[0004] The wavelength conversion element 117 is a nonlinear medium such as a second-order nonlinear optical element, and a waveguide 118 is formed therein. The nonlinear medium is, for example, Periodically Poled Lithium Niobate (PPLN).

[0005] The metal housing 110 is provided with sealing windows 119 and 121. Signal light 101 emitted from the 1.55 μm optical fiber 106 is converted into parallel light by lens 109-1 and incident into the metal housing through the input-side sealing window 119. Excitation light 102 emitted from the 0.78 μm optical fiber 105 is converted into parallel light by lens 120 and incident into the metal housing through the input-side sealing window 121. Wavelength-converted light 104, which is the wavelength-converted signal light, is emitted out of the metal housing through the output-side sealing window 119. The wavelength-converted light 104 is focused by lens 111-1 and incident into the 1.55 μm optical fiber 107. The remaining excitation light 103 is emitted out of the metal housing through the output-side sealing window 121. The remaining excitation light 103 is focused by lens 112 and incident into the 0.78 μm optical fiber 108.

[0006] Within the metal housing 110, the signal light 101 is reflected by the dichroic mirror 114 and the dichroic mirror 113, and together with the excitation light 102 that has passed through the dichroic mirror 113, is focused by the lens 109-2 and incident on the waveguide 118 of the wavelength conversion element 117.

[0007] In the wavelength conversion element 117, the signal light 101 is amplified by the OPA process, and wavelength-converted light of the signal light 101 is generated. Such wavelength-converted light is produced when high-power excitation light is input to the wavelength conversion element. Conventionally, a second harmonic generator has been used to generate such high-power excitation light. The second harmonic generator amplifies the fundamental wave light (also referred to as fundamental wave light in this specification), which serves as the seed for the second harmonic, with a high-power optical amplifier such as an erbium-doped fiber amplifier (EDFA), and utilizes the second harmonic generation (SHG) process, which is a second-order nonlinear optical effect in a nonlinear medium. If light in the wavelength band of the signal light described above is used as the fundamental wave light, high-power excitation light (also referred to as SHG light in this specification) with a wavelength of approximately half the wavelength of the fundamental wave light can be obtained.

[0008] Figure 2 shows a schematic configuration of a conventional second harmonic generator. The second harmonic generator 200 in Figure 2 has a metal housing 210, and inside the metal housing are a wavelength conversion element 217 and lenses 212 and 213 arranged on the input and output sides of the wavelength conversion element 217.

[0009] The wavelength conversion element 217 is a second-order nonlinear optical element such as a PPLN, and a waveguide 218 is formed therein.

[0010] The metal housing 210 is provided with sealing windows 219 and 220. Fundamental wave light emitted from the 1.55 μm optical fiber 201 enters the metal housing through the input-side sealing window 219, converted into parallel light by lens 211, and then focused by lens 212 before entering the waveguide 218. The SHG light (i.e., high optical power excitation light) and amplified spontaneous emission (ASE) light generated during the SHG process emitted from the waveguide 218 are converted into parallel light by lens 213, and then separated into ASE light and SHG light by optical filter 205, with only the SHG light exiting the metal housing through the output-side sealing window 220. The SHG light is focused by lens 214 and enters the 0.78 μm optical fiber 202.

[0011] Furthermore, the second harmonic generator 200 includes a high-power optical amplifier 230 and a bandpass filter 231 outside the metal housing 210.

[0012] The optical amplifier 230 is a high-power optical amplifier such as an EDFA and a semiconductor optical amplifier (SOA) for amplifying light in the 1.55 μm band.

[0013] The bandpass filter 231 is a bandpass filter that separates the ASE light and the fundamental wave light generated by the optical amplifier 230 and extracts the fundamental wave light. The bandpass filter 321 is provided to suppress the degradation of the signal-to-noise ratio (SNR) of the SHG light generated by the wavelength conversion element 217.

[0014] The efficiency of SHG light generation in the SHG process at the wavelength conversion element 217 is temperature-dependent. In second-order nonlinear optical elements such as PPLNs, the phase-matching wavelength fluctuates sensitively with temperature. Therefore, the second harmonic generator 200 is stabilized by adjusting the operating temperature of the wavelength conversion element 217 using a temperature detector such as a thermistor and a temperature controller such as a Peltier element and heater.

[0015] The stabilization of the second harmonic generator will be explained with reference to Figures 3A, 3B, and 3C. Figure 3A shows a schematic configuration of the second harmonic generator 300. High-power fundamental wave light (pump) emitted from the laser light source (LD) 320 and amplified by the EDFA 330 is incident on a wavelength conversion element 317, which is a second-order nonlinear optical element such as a PPLN. An SHG process occurs in the wavelength conversion element 317. The SHG light (SH-pump) emitted from the wavelength conversion element 317 is partially branched by the branching coupler 302 and its optical power is detected by the photodetector 303.

[0016] The controller 304 controls the thermoelectric controller (TEC) 301 based on the optical power of the SHG light detected by the photodetector 303.

[0017] Figure 3B shows the phase matching characteristics of the SHG. As shown in Figure 3B, when the temperature changes, the wavelength that is phase-matched with the nonlinear optical element changes (the phase matching curve shifts in the wavelength axis direction when the temperature changes). Therefore, the controller 304 controls the TEC 301 to maintain the optical power of the SHG light detected by the photodetector 303 at its maximum value. Specifically, it searches for the optimal operating temperature (temperature T in the example of Figure 3B) by making minute fluctuations in the operating temperature. The controller 304 also provides feedback to the EDFA 330 to maintain the optical power of the SHG light. (See, for example, Patent Document 2)

[0018] As shown in Figure 3C, when the SHG process occurs in the wavelength conversion element 317 of the second harmonic generator 300, the inverse OPA process also occurs simultaneously. Therefore, the wavelength conversion element 317 emits SHG light (also referred to as SH-pupm in this specification) generated in the SHG process, as well as the fundamental light remaining after the SHG process and ASE light generated in the OPA process. As can be seen from the spectrum of light generated in the OPA process shown in Figure 3C, the shape of the spectrum changes depending on the operating temperature of the nonlinear optical element. By controlling the operating temperature of the wavelength conversion element 317 so that the difference in light intensity between two specific wavelengths becomes a desired value (for example, so that they are equivalent), the spectral shape can be stabilized (i.e., the generation of the second harmonic can be stabilized). (See, for example, Patent Document 3)

[0019] Japanese Patent No. 7417160, International Publication WO2009 / 081577, Japanese Patent No. 7087928

[0020] However, the method of slightly varying the operating temperature of the wavelength conversion element as described above has the problem of slow control speed, making it impossible to track rapid fluctuations in the operating temperature of the nonlinear optical element. Furthermore, the configuration shown in Figure 3A requires a 0.78 μm band tap monitor such as the branch coupler 302 and the photodetector 303, which causes loss of 0.78 μm band excitation light (i.e., SHG light). In addition, the fiber guiding the 0.78 μm band light has a small core diameter of about 5 μm, and there is also the problem of large losses due to fiber connection.

[0021] This disclosure has been made in view of the above problems, and its purpose is to provide a wavelength conversion element that can stabilize the generation of second harmonics in the wavelength conversion element without branching off a portion of the generated harmonic light.

[0022] To achieve this objective, a wavelength conversion element according to one embodiment of the present disclosure comprises: a wavelength conversion element; a first optical system configured to incident fundamental light and light of two wavelengths in the wavelength band of the fundamental light onto the wavelength conversion element; a second optical system configured to separate the converted light of the fundamental light from the wavelength conversion element, the light of the two wavelengths, and the remaining fundamental light; a photodetector configured to detect the light intensity of each of the two separated wavelengths of light; and a controller configured to control the operating temperature of the wavelength conversion element based on the difference in light intensity detected for each of the two separated wavelengths of light.

[0023] As described above, according to one embodiment of the wavelength conversion device of this disclosure, it is possible to stabilize the generation of second harmonics in the wavelength conversion element based on the light intensity of light with wavelengths in the wavelength band of the fundamental light, without branching a portion of the generated harmonic light.

[0024] This figure shows a schematic configuration of a wavelength conversion device using a conventional nonlinear optical element. This figure shows a schematic configuration of a conventional second harmonic generator. This figure illustrates the stabilization of a conventional second harmonic generator. This figure shows the phase matching characteristics of second harmonic generation (SHG). This figure illustrates the SHG / OPA cascade process. This figure shows a schematic configuration of a wavelength conversion device according to one embodiment of the present disclosure. This figure shows the spectrum of ASE light in the wavelength conversion element of the wavelength conversion device according to this embodiment. This is a graph plotting the difference between two wavelengths contained in the ASE light in the wavelength conversion element of the wavelength conversion device according to this embodiment. This figure shows a schematic configuration of a wavelength conversion device according to another embodiment of the present disclosure. This figure shows a schematic configuration of a wavelength conversion device according to yet another embodiment of the present disclosure.

[0025] Embodiments of this disclosure will be described in detail below with reference to the drawings. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. Material names and numerical values ​​in the following description are illustrative, and embodiments of this disclosure may use other materials and numerical values ​​without departing from the spirit of the invention.

[0026] (Embodiment 1) Figure 4 shows a schematic configuration of a wavelength conversion device according to one embodiment of the present invention. In the following description, the fundamental wave light is defined as light in the 1.55 μm wavelength band, and the wavelength λ0 of the fundamental wave light is defined as 1.530 μm. The wavelengths λ1 and λ2 of the fundamental wave light wavelength band are defined as 1.532 μm and 1.540 μm. The light with wavelength λ' generated when light with wavelength λ is incident on the wavelength conversion element is called the converted light. For example, the excitation light (SHG light) in the 0.78 μm band generated when fundamental wave light with wavelength λ0 (1.530 μm) is incident on the wavelength conversion element is called the converted light of the fundamental wave.

[0027] The wavelength conversion device 400 in Figure 4 has a metal housing 401, and inside the metal housing are a wavelength conversion element 417, a multibandpass filter 410 and lens 406 arranged on the input side of the wavelength conversion element 417, and a lens 407, a short-pass filter 411, and a wavelength filter 412 arranged on the output side of the wavelength conversion element 417.

[0028] Furthermore, the wavelength conversion device 400 has a high-power optical amplifier 430 and a lens 405 on the input side outside the metal housing 401, and lenses 408 and 409, a wavelength separator 413, and photodetectors 421 and 422 on the output side outside the metal housing 401. The optical amplifier 430 is a high-power optical amplifier such as an EDFA and a semiconductor optical amplifier (SOA) for amplifying light in the 1.55 μm band.

[0029] Furthermore, the wavelength conversion device 400 includes a controller 420 and a TEC 421.

[0030] The wavelength conversion device 400 in Figure 4 differs from the second harmonic generator 200 in Figure 2 in that it does not have a bandpass filter to separate the ASE light in the 1.55 μm wavelength band generated by the optical amplifier 430 from the fundamental wave light and extract the fundamental wave light. The wavelength conversion device 400 in Figure 4 stabilizes the generation of the second harmonic in the wavelength conversion element 417 by utilizing the ASE light in the 1.55 μm wavelength band generated by the optical amplifier 430.

[0031] The wavelength conversion element 417 is a second-order nonlinear optical element such as a PPLN, and a waveguide (not shown) is formed therein.

[0032] The multibandpass filter 410 is an element configured to transmit light in the 1.55 μm band at wavelengths λ0 = 1.530 μm, as well as λ1 = 1.532 μm and λ2 = 1.540 μm.

[0033] The short-pass filter 411 is an element configured to transmit light in the 0.78 μm band (i.e., SH-pump) and reflect light in the 1.55 μm band.

[0034] The wavelength filter 412 is an element configured to transmit wavelength λ0 in the 1.55 μm band and reflect light of wavelengths λ1 and λ2, and is also called, for example, a wavelength division multiplexing (WDM) filter.

[0035] The wavelength separator 413 is an element configured to separate light of wavelengths λ1 and λ2, and for example, has one or more dielectric multilayer mirrors.

[0036] The photodetectors 421 and 422 are elements configured to detect the light intensity of wavelengths λ1 and λ2, respectively.

[0037] The controller 420 is an element having a processor configured to control the TEC 421 and the optical amplifier 430 based on the light intensity of wavelengths λ1 and λ2 detected by the photodetectors 421 and 422.

[0038] The metal housing 401 is provided with sealing windows 402, 403, and 404. Fundamental wave light (pump) and ASE light emitted from the 1.55 μm band optical fiber 414 are converted into parallel light by the lens 405 and incident into the metal housing through the input-side sealing window 402. Subsequently, the fundamental wave light with wavelength λ0, as well as ASE light with wavelength λ1 and ASE light with wavelength λ2, which have passed through the multibandpass filter 410, are focused by the lens 406 and incident into the waveguide of the wavelength conversion element 417.

[0039] In the SHG process in the wavelength conversion element 417, an SH-pump is generated and emitted from the wavelength conversion element 417. The SH-pump is light in the 0.78 μm band, and its wavelength is approximately half the wavelength λ0 of the fundamental light. In addition, in the OPA process in the wavelength conversion element 417, ASE light with wavelengths λ1 and λ2 is amplified, and simultaneously, converted light (wavelengths λ1' and λ2') of ASE light with wavelengths λ1 and λ2 is generated, and further ASE light is generated and emitted from the wavelength conversion element 417. Furthermore, any remaining fundamental light from the SHG and OPA processes in the wavelength conversion element 417 is also emitted from the wavelength conversion element 417 as is.

[0040] The light emitted from the wavelength conversion element 417 is converted into parallel light by the lens 407, then the SH-pump is separated by the short-pass filter 411, and then the remaining fundamental wave light (pump, λ0) is separated by the wavelength filter 412.

[0041] The SH-pump, separated by the short-pass filter 411, exits the metal housing through the sealing window 43, is then focused by the lens 408, and incident on the 0.78 μm optical fiber 415.

[0042] The amplified ASE light (wavelengths λ1 and λ2), the converted light (wavelengths λ1' and λ2'), and the ASE light generated during the OPA process are emitted from the sealing window 404 to the outside of the metal housing, then focused by the lens 409 and incident on the 1.55 μm band optical fiber 416.

[0043] The ASE light (wavelengths λ1 and λ2), converted light (wavelengths λ1' and λ2'), and ASE light generated during the OPA process, propagating through the 1.55 μm optical fiber 416, are separated by the wavelength separator 413 into ASE light of wavelength λ1, ASE light of wavelength λ2, and other light. The ASE light of wavelengths λ1 and λ2 are incident on photodetectors 421 and 422, respectively, via the 1.55 μm optical fiber 418. The other light is incident on the 1.55 μm optical fiber 419.

[0044] The controller 420 controls the TEC 421 based on the detected light intensities of wavelengths λ1 and λ2. Specifically, the controller 420 controls the TEC 421 such that the difference between the light intensities of wavelengths λ1 and λ2 becomes a desired value (e.g., 0).

[0045] FIG. 5A shows spectra of ASE light when the wavelength conversion element 417 of the wavelength conversion device 400 according to the present embodiment is operated at different temperatures. The operating temperature is changed from the optimal operating temperature of 50°C in steps of 0.1°C. The central wavelength of the spectrum is the wavelength λ0 (=1.530 µm) of the fundamental wave light. The spectrum near the central wavelength at the optimal operating temperature of 50°C is flat, but when the temperature is changed, the spectrum near the central wavelength becomes non-flat.

[0046] FIG. 5B shows the difference between the light intensity at wavelength λ1 (=1.532 µm) and the light intensity at wavelength λ2 (=1.540 µm) in the spectrum shown in FIG. 5A. When the temperature is the optimal operating temperature of 50°C, the difference in light intensity is 0, and the difference in light intensity increases as the temperature deviates further from the optimal operating temperature. Therefore, the controller 420 can stabilize the second harmonic generation in the wavelength conversion element 417 by controlling the TEC 421 so that the difference between the light intensities of wavelengths λ1 and λ2 becomes 0, and controlling the optical amplifier 430 so that the light intensity of wavelength λ1 or λ2 becomes constant.

[0047] As described above, according to the wavelength conversion device 400 of the present embodiment, by controlling the operating temperature of the wavelength conversion element 417 based on the light intensities of light at wavelengths λ1 and λ2 in the 1.55 µm band, which is the pump wavelength band, it is possible to stabilize the second harmonic generation in the wavelength conversion element 417. Furthermore, according to the wavelength conversion device 400 of the present embodiment, since a part of the generated SH-pump is not branched, excessive loss does not occur.

[0048] It is also conceivable to use ASE light generated in the OPA process without using the ASE light generated in the optical amplifier 430. In the wavelength conversion device 400 according to the present embodiment, the ASE light of wavelength λ1 and the ASE light of wavelength λ2 that have transmitted through the multi-band pass filter 410 are amplified in the OPA process, so that the SN ratio is increased. Therefore, the influence of noise on the operation of the controller is reduced.

[0049] In the present embodiment, as light having a wavelength within the wavelength band of the fundamental wave light, ASE light of wavelengths λ1 and λ2 in the 1.55 μm wavelength band generated by the optical amplifier 430 is used. However, light having a wavelength within the wavelength band of the fundamental wave light can be made incident on the wavelength conversion element 417 by any method.

[0050] (Embodiment 2) FIG. 6 shows a schematic configuration of a wavelength conversion device according to another embodiment of the present invention. Compared with the wavelength conversion device 400 of FIG. 4, the wavelength conversion device 600 shown in FIG. 6 differs in that a sealing window 602 is added to the metal housing 401, a wavelength filter 611 is added inside the metal housing 401, the multi-band pass filter between the sealing window 402 and the lens 406 is replaced with a wavelength filter 610, and a lens 605 is added outside the metal housing 401. The wavelength conversion device 600 stabilizes the second harmonic generation in the wavelength conversion element 417 by using light of wavelengths λ1 and λ2 in the 1.55 μm wavelength band of the fundamental wave light incident from the sealing window 602. The light of wavelengths λ1 and λ2 is light emitted from a light source (not shown) different from that of the fundamental wave light, and is not light emitted from the optical amplifier 430.

[0051] The wavelength filters 610 and 611 are elements configured to transmit light with wavelength λ0=1.530 μm in the 1.55 μm band and reflect light with λ1=1.532 μm and λ2=1.540 μm.

[0052] The fundamental wave light (pump) and ASE light emitted from the 1.55 μm band optical fiber 414 are converted into parallel light by the lens 405 and incident into the metal housing through the sealing window 402 on the input side, and then only the fundamental wave light with wavelength λ0 is transmitted through the wavelength filter 610. In addition, light with wavelengths λ1 and λ2 emitted from the 1.55 μm band optical fiber 614 are converted into parallel light by the lens 605 and incident into the metal housing through the sealing window 602 on the input side, and then reflected by the wavelength filter 611 and further reflected by the wavelength filter 610.

[0053] The fundamental wave light of wavelength λ0 that has passed through the wavelength filter 610, as well as the light of wavelengths λ1 and λ2 that have been reflected by the wavelength filter 610, are focused by the lens 406 and incident on the waveguide of the wavelength conversion element 417.

[0054] In the SHG process in the wavelength conversion element 417, an SH-pump is generated and emitted from the wavelength conversion element 417. The SH-pump is light in the 0.78 μm band, and its wavelength is approximately half the wavelength λ0 of the fundamental light. In addition, in the OPA process in the wavelength conversion element 417, light with wavelengths λ1 and λ2 is amplified, and at the same time, converted light (wavelengths λ1' and λ2') of wavelengths λ1 and λ2 is generated, and further ASE light is generated and emitted from the wavelength conversion element 417. Furthermore, any remaining fundamental light from the SHG process and OPA process in the wavelength conversion element 417 is also emitted from the wavelength conversion element 417 as is.

[0055] The light emitted from the wavelength conversion element 417 is converted into parallel light by the lens 407, then the SH-pump is separated by the short-pass filter 411, and then the remaining fundamental wave light (pump, λ0) is separated by the wavelength filter 412.

[0056] The SH-pump, separated by the short-pass filter 411, exits the metal housing through the sealing window 43, is then focused by the lens 408, and incident on the 0.78 μm optical fiber 415.

[0057] The amplified light (wavelengths λ1 and λ2), the converted light (wavelengths λ1' and λ2'), and the ASE light generated during the OPA process are emitted from the sealing window 404 to the outside of the metal housing, then focused by the lens 409 and incident on the 1.55 μm band optical fiber 416.

[0058] The light propagating through the 1.55 μm optical fiber 416 (wavelengths λ1 and λ2), the converted light (wavelengths λ1' and λ2'), and the ASE light generated during the OPA process are separated by the wavelength separator 413 into light of wavelength λ1, light of wavelength λ2, and other light. Light of wavelengths λ1 and λ2 are incident on photodetectors 421 and 422, respectively, via the 1.55 μm optical fiber 418. The other light is incident on the 1.55 μm optical fiber 419.

[0059] The controller 420 controls the TEC 421 based on the detected light intensities of wavelengths λ1 and λ2. Specifically, the controller 420 controls the TEC 421 so that the difference in light intensities of wavelengths λ1 and λ2 becomes a desired value (e.g., 0), and controls the optical amplifier 430 so that the light intensity of wavelength λ1 or λ2 becomes constant.

[0060] As described above, the wavelength conversion device 600 according to this embodiment, similar to the wavelength conversion device 400, makes it possible to stabilize the generation of second harmonics in the wavelength conversion element 417 by controlling the operating temperature of the wavelength conversion element 417 based on the light intensity of the light with wavelengths λ1 and λ2 in the 1.55 μm band, which is the wavelength band of the pump. Furthermore, the wavelength conversion device 400 according to this embodiment does not branch off a portion of the generated SH-pump, so no excessive losses occur.

[0061] (Embodiment 3) Figure 7 shows a schematic configuration of a wavelength conversion device according to yet another embodiment of the present invention. The wavelength conversion device 700 shown in Figure 7 differs from the wavelength conversion device 400 in Figure 4 in that a sealing window 702 is added to the metal housing 401, a short-pass filter 711 is added inside the metal housing 401, the multiband-pass filter between the sealing window 402 and the lens 406 is replaced with a multiband-reflection filter 710, the lens 407 is replaced with a lens 707 that has a coating that reflects a part of the SH-pump, and a lens 705 and a photodetector 721 are added outside the metal housing 401. The wavelength conversion device 700 uses ASE light with wavelengths λ1 and λ2 in the 1.55 μm wavelength band of the fundamental wave to stabilize the generation of second harmonics in the wavelength conversion element 417, and controls the optical amplifier 430 based on the light intensity of the reflected light of the generated SH-pump.

[0062] The short-pass filter 711 is an element configured to transmit light in the 0.78 μm band (i.e., SH-pump) and reflect light in the 1.55 μm band.

[0063] The multiband reflection filter 710 is an element configured to reflect light in the 1.55 μm band at wavelengths λ0 = 1.530 μm, as well as λ1 = 1.532 μm and λ2 = 1.540 μm.

[0064] The coated lens 707 is a lens in which a coating that reflects a portion of the SH-pump is applied to the surface facing the emission surface of the wavelength conversion element 417.

[0065] The fundamental wave light (pump) and ASE light emitted from the 1.55 μm band optical fiber 414 enter the metal housing through the sealing window 402 on the input side, converted into parallel light by the lens 405, and then reflected by the multiband reflection filter 710 and the short-pass filter 711. The fundamental wave light with wavelength λ0, as well as the ASE light with wavelength λ1 and ASE light with wavelength λ2, reflected by the short-pass filter 711, are focused by the lens 406 and incident into the waveguide of the wavelength conversion element 417.

[0066] In the SHG process in the wavelength conversion element 417, an SH-pump is generated and emitted from the wavelength conversion element 417. The SH-pump is light in the 0.78 μm band, and its wavelength is approximately half the wavelength λ0 of the fundamental light. In addition, in the OPA process in the wavelength conversion element 417, ASE light with wavelengths λ1 and λ2 is amplified, and simultaneously, converted light (wavelengths λ1' and λ2') with wavelengths λ1 and λ2 is generated, and further ASE light is generated and emitted from the wavelength conversion element 417. Furthermore, any remaining fundamental light from the SHG and OPA processes in the wavelength conversion element 417 is also emitted from the wavelength conversion element 417 as is.

[0067] Light emitted from the wavelength conversion element 417 is reflected by a lens 707 coated to reflect a portion of the SH-pump and re-entered into the wavelength conversion element 417. The remainder is converted into parallel light by the coated lens 707, after which the SH-pump is separated by the short-pass filter 411, and then the remaining fundamental wave light (pump, λ0) is separated by the wavelength filter 412.

[0068] Similar to the wavelength conversion device 400 in Figure 4, the light intensity of ASE light at wavelengths λ1 and λ2 is detected by photodetectors 421 and 422.

[0069] The controller 420 controls the TEC 421 based on the detected light intensities of wavelengths λ1 and λ2.

[0070] A portion of the SH-pump that is re-incident to the wavelength conversion element 417 passes through the short-pass filter 711 and exits the metal housing 401 through the sealing window 702, where it is focused by the lens 705 and incident on the 0.78 μm band optical fiber, after which the light intensity is detected by the photodetector 721.

[0071] Based on the detected light intensity of the SH-pump, the controller 420 controls the optical amplifier 430 so that the light intensity of the SH-pump output from the wavelength conversion element 417 becomes a desired value.

[0072] As described above, according to the wavelength conversion device 600 of this embodiment, similar to the wavelength conversion device 400, it is possible to stabilize the generation of second harmonics in the wavelength conversion element 417 by controlling the operating temperature of the wavelength conversion element 417 based on the light intensity of the light with wavelengths λ1 and λ2 in the 1.55 μm band, which is the wavelength band of the pump.

[0073] In this embodiment, the lens positioned on the output side of the wavelength conversion element 417 is coated to reflect a portion of the SH-pump. However, the output surface of the wavelength conversion element 417 or the sealing window 403 may be coated to reflect a portion of the SH-pump, thereby causing a portion of the SH-pump to be re-incident to the wavelength conversion element 417.

[0074] According to the wavelength conversion device of the embodiment of this disclosure, the generation of second harmonics in the wavelength conversion element can be stabilized.

[0075] 100, 400, 600, 700 Wavelength conversion device 101 Signal light 102 Excitation light 103 Residual excitation light 104 Wavelength converted light, which is the signal light with converted wavelength 105, 108, 202, 415 0.78 μm band optical fiber 106, 107, 201, 414, 416, 418, 419, 614, 714 1.55 μm band optical fiber 110, 210, 401 Metal housing 109, 111, 112, 120, 211, 212, 213, 214, 405, 406, 407, 408, 409, 605, 705 Lens 113, 114, 115, 116 Dichroic mirror 117, 217, 317, 417 Wavelength conversion element 118, 218 Waveguides 119, 121, 219, 220, 402, 403, 404, 602, 702 Sealing window 200 Second harmonic generator 205 Optical filter 230, 430 Optical amplifier 231 Bandpass filter 300 Second harmonic generator 301, 423 Thermoelectric controller (TEC) 302 Branch coupler 303, 421, 422, 721 Photodetector (PD) 304, 420 Controller 320 Laser light source 330 EDFA 410 Multibandpass filter 411, 711 Shortpass filter 412, 610, 611 Wavelength filter 413 Wavelength separator 707 Coated lens 710 Multiband reflective filter

Claims

1. A wavelength conversion device comprising: a wavelength conversion element; a first optical system configured to incident fundamental light and light of two wavelengths in the wavelength band of the fundamental light onto the wavelength conversion element; a second optical system configured to separate the converted light of the fundamental light from the wavelength conversion element, the light of the two wavelengths, and the remaining fundamental light; a photodetector configured to detect the light intensity of each of the two separated wavelengths of light; and a controller configured to control the operating temperature of the wavelength conversion element based on the difference in the detected light intensity of each of the two separated wavelengths of light.

2. The wavelength conversion device according to claim 1, wherein the fundamental wave light incident on the wavelength conversion element is light amplified by an optical amplifier, and the controller is further configured to control the optical amplifier such that the detected light intensity for either of the two separated wavelengths of light becomes constant.

3. The wavelength conversion apparatus according to claim 1, wherein the second optical system includes: a short-pass filter that transmits the converted light of the fundamental wave and reflects the light of the two wavelengths and the remaining fundamental wave; a wavelength filter configured to transmit the remaining fundamental wave reflected by the short-pass filter and reflect the light of the two wavelengths; and a wavelength separator configured to separate the light of the two wavelengths reflected by the wavelength filter.

4. The wavelength conversion device according to claim 1, wherein the fundamental wave light incident on the wavelength conversion element is light amplified by an optical amplifier, and the first optical system includes a multibandpass filter that transmits light of two wavelengths in the wavelength band of the fundamental wave light, among the fundamental wave light and the naturally radiated amplified light (ASE light) generated by the optical amplifier.

5. The wavelength conversion device according to claim 3, wherein the fundamental wave light incident on the wavelength conversion element is light amplified by an optical amplifier, the two wavelengths of light in the wavelength band of the fundamental wave light are not light emitted from the optical amplifier, and the first optical system includes two wavelength filters configured to transmit the fundamental wave light and reflect the two wavelengths of light in the wavelength band of the fundamental wave light.

6. The wavelength conversion device according to claim 1, wherein the fundamental light incident on the wavelength conversion element is light amplified by an optical amplifier, the first optical system includes a multiband reflective filter configured to reflect light of two wavelengths in the wavelength band of the fundamental light from among the fundamental light and the naturally radiated amplified light (ASE light) generated by the optical amplifier, the wavelength conversion device further comprises a photodetector configured to detect the light intensity of a portion of the converted light of the fundamental light reflected at the exit end of the wavelength conversion element or in the second optical system, and the controller is further configured to control the optical amplifier based on the detected light intensity of a portion of the converted light of the fundamental light.

7. The wavelength conversion device according to claim 1, further comprising a thermoelectric controller, the controller being configured to control the thermoelectric controller based on the difference in the detected light intensity for each of the two separated wavelengths of light.

8. The wavelength conversion device according to claim 1, wherein the wavelength conversion element is a second-order nonlinear optical element, the fundamental wave light is light in the 1.55 μm wavelength band, and the converted light of the fundamental wave light is light in the 0.78 μm band.

Images