JP2015225127A - Wavelength conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an output-stabilized wavelength conversion device that shows no decrease in an output even when the device is subjected to accurate wavelength control and that can achieve a stable high output in a wavelength conversion device for converting wavelengths by use of a nonlinear optical effect.SOLUTION: An output unit of a wavelength conversion element module separates second harmonic waves from an output of fundamental wave light; and an optical circulator 111 is disposed in a route of the separated fundamental wave light. The light passing through the optical circulator 111 is guided to an optical phase modulator 112 and subjected to phase modulation for achieving frequency modulation by phase modulation. The fundamental wave light subjected to the phase modulation and frequency modulation is returned by the optical circulator 111 through an input/output port 108 into the module. Second harmonic waves converted from the phase-modulated fundamental wave light are extracted from the output port 109 and detected by a photodetector 113. A feedback circuit 114 controls a temperature regulator 115 to increase an intensity of the second harmonic waves to the maximum based on an electric signal generated by photo-electric conversion in the photodetector 113.

Description

  The present invention relates to a wavelength converter using a linear optical effect used in an optical communication system or an optical measurement system.

  Many for the generation and modulation of coherent light in the ultraviolet, visible, infrared, and terahertz range for applications such as optical signal wavelength conversion, optical modulation, optical measurement, optical processing, medicine, and biotechnology in optical communications Development of non-linear optical devices is underway.

Various materials have been researched and developed as nonlinear optical media used in such elements, and oxide-based compound substrates such as lithium niobate (LiNbO ₃ ) have very high second-order nonlinear optical constants as promising materials. Are known. As an example of an optical device using the high nonlinear optical characteristics of such a material, a wavelength conversion element using second harmonic generation, difference frequency generation, and sum frequency generation by pseudo phase matching is known.

  For example, strong absorption lines corresponding to various vibrations of environmental gases and the like exist in the mid-infrared wavelength region of 2 to 5 μm, and therefore, development of a small mid-infrared light source is desired. Such a light source in the mid-infrared region for sensing needs to have a narrow line width, and it is promising to generate a difference frequency using a technically mature excitation light source near 1 μm and signal light in the communication wavelength band. It is considered.

  In addition, since there is a wavelength range that is difficult to realize with a semiconductor laser in the visible light wavelength range near 0.5 μm, green light is generated by second harmonic generation or sum frequency generation using an excitation light source near 1 μm. A wavelength conversion technique capable of generating visible light such as light is promising.

  In recent years, since the bulk characteristics of crystals can be used as they are, ridge-type optical waveguides having characteristics such as high light damage resistance, long-term reliability, and easy device design have been researched and developed using materials such as lithium niobate. A ridge-type optical waveguide can be formed by thinning one substrate of the optical element formed by joining two substrates and then performing ridge processing.

  The optical waveguide made of lithium niobate and the like thus processed can be used by directly installing the processed element (chip) when used in the laboratory, but it can be used in an apparatus or commercially available. In the case of shipping as a product, for protection and convenience of the product, there is a case in which a group of related elements necessary for use is packaged as a functional part called a module in a single package container. It is common.

  FIG. 7A shows a configuration of a wavelength conversion element module including a typical nonlinear optical element. FIG. 7A shows an example of a fiber pigtail type module that can be coupled using an optical fiber at each of the input port 204 and the output port 205 in order to easily connect the input side and output side optical fibers. It is. This wavelength conversion element module includes an optical waveguide element 201, a temperature adjustment element such as a Peltier element for controlling the temperature of the optical waveguide element, an optical lens 203 used for optical coupling on the input / output side, an input / output optical fiber 211, and a temperature adjustment element. And the like, and a package container 202 for housing these elements.

  In FIG. 7A, the fundamental wave light used for conversion is input from the light source 210 via the input port 204 from the input end of the optical waveguide device 201, and the converted light and the conversion light output from the output end of the optical waveguide device 201 are converted. The fundamental wave light that has not been output is output via the output port 205. The electrode terminal 206 and the package container 202 are usually made of a metal material having excellent electrical conductivity and thermal conductivity. The temperature adjustment element that controls the temperature of the element is disposed, for example, below the optical waveguide element 201 and is connected to the electrode terminal 206 of the package by electric wiring.

  FIGS. 8A and 8B are schematic diagrams of wavelength conversion element modules that generate second harmonics as converted light. FIG. 8A shows a system in which a fiber is not used on the output side and output to space. Since only the second harmonic is used, an appropriate wavelength filter 220 is disposed to cut the unconverted fundamental wave light output from the output side. In FIG. 8B, a fiber is also used on the output side. In this case, a wavelength separation filter 208 such as a dichroic mirror that reflects only a specific wavelength is disposed inside the package to separate the fundamental light in a direction different from the second harmonic. In FIG. 8B, the fundamental wave light is output to the output port 209 and the second harmonic is output to the output port 205.

  The configuration as described above is the basic configuration of the wavelength conversion module, but the configuration changes according to the required specifications. In particular, when the output light intensity is made constant or precisely controlled, the output of the converted light cannot be controlled with the configuration of FIGS. 8A and 8B alone. In such a case, a mechanism for monitoring the output and performing feedback control is necessary.

  FIGS. 9A and 9B are explanatory diagrams of a configuration in which the output of the second harmonic is stabilized by precise wavelength control in the conventional wavelength converter. In this configuration, a part of the second harmonic of the output light is branched by the optical branching means 212 such as an optical coupler and received by the photodetector 213, and the temperature controller 215 is controlled by the control unit 214 based on the result. The temperature of the optical waveguide device 201 is adjusted by control. In a wavelength conversion element such as lithium niobate, the refractive index of the material is changed by changing the element temperature, and the output wavelength can be changed to a wavelength showing the maximum conversion efficiency.

  By such a method, the output light intensity can be precisely controlled by adjusting the waveguide element temperature so as to maximize the detected optical signal at the start of use of the optical waveguide element 201 or at regular intervals. It is.

  FIG. 9B shows a module configuration in which when the light source 210 is a variable wavelength light source, the control unit 214 also controls the light source 210 to control the wavelength of the fundamental wave light.

  On the other hand, in addition to controlling the temperature of the optical waveguide device 201, a method of matching the output wavelength of the light source with the wavelength exhibiting the maximum conversion efficiency is also known for the purpose of making the output wavelength and the output light intensity constant. (See Patent Document 1 and Patent Document 2).

Japanese Patent No. 3329446 Japanese Patent No. 3526282 JP 2006-19603 A

Amnon Yariv, co-translated by Kunio Tada and Takeshi Kamiya, “Basics of Optoelectronics”, Maruzen Co., Ltd., PP. 247-248

  However, in the module configuration that performs the feedback control as described above, it is necessary to branch a part of the converted light that is output in order to monitor the output, and there is a problem that the available output becomes weak accordingly. Usually, an output of about 1/20 is monitored using an optical coupler having a branching ratio of about 1:20. However, if the branching ratio is significantly lowered, detection of monitor light becomes difficult.

  When the branching ratio is kept lower than usual, it is possible to detect monitor light by inserting an optical amplifier or using a highly sensitive detector, but the problem is that the module is expensive. There is. If the input power of the fundamental wave light can be increased sufficiently by inserting an optical amplifier into the fundamental light source, etc., some branching may be compensated by adjusting the amplifier output, but there are also wavelength bands where inexpensive optical amplifiers are not available. .

  Even when the output is split and monitored, control such as modulation of the fundamental light source is necessary to maximize the output, but such control increases the line width. However, there is a problem that high-performance wavelength conversion with a narrow line width cannot be realized.

  The present invention has been made in view of such problems, and the object of the present invention is to reduce output even when precise wavelength control is performed in a wavelength conversion device that performs wavelength conversion using a nonlinear optical effect. It is an object of the present invention to provide an output stabilization wavelength converter capable of providing a stable and high output.

  In order to solve the above-described problems, the present invention provides a nonlinear optical element that generates converted light having a different wavelength from one or more fundamental wave lights, and an input port that inputs the fundamental wave light to the first end face of the nonlinear optical element. A wavelength conversion device comprising: a first output port that emits first converted light from a second end face of the nonlinear optical element; and a temperature adjusting unit that adjusts a temperature of the nonlinear optical element, A first wavelength separation filter disposed between a second end face and the output port for separating the first converted light and the fundamental light; and the first wavelength separation filter for the first wavelength separation filter. An input / output port that emits the fundamental light separated from the converted light, and a phase modulation that applies phase modulation to the fundamental light emitted from the input / output port and enters the phase-modulated light into the input / output port Means and said A second wavelength separation filter that is disposed between the end face of 1 and the input port and separates the second converted light generated from the phase-modulated fundamental light and the phase-modulated fundamental light; A second output port for emitting the second converted light separated from the phase-modulated fundamental light by the second wavelength separation filter; and the second conversion emitted from the second output port. A photodetector for detecting light; and a feedback circuit for controlling the phase modulation amount in the phase modulation unit and the temperature adjustment unit based on a change in signal intensity detected by the photodetector. .

  According to a second aspect of the present invention, in the wavelength converter according to the first aspect, the feedback circuit further controls a fundamental wave light source.

  According to a third aspect of the present invention, in the wavelength conversion device according to the first or second aspect, the nonlinear optical element generates a second harmonic as the converted light.

  According to a fourth aspect of the present invention, in the wavelength conversion device according to the first or second aspect, the nonlinear optical element generates a sum frequency or a difference frequency as the converted light, and the phase modulation means includes optical multiplexing / demultiplexing. A circuit further comprising: phase modulating the fundamental wave light of a predetermined wavelength separated by the optical multiplexing / demultiplexing circuit among the fundamental wave light output from the input / output port; The fundamental wave light including wave light is incident on the input / output port.

  According to a fifth aspect of the present invention, in the wavelength converter according to the first, second, or fourth aspect, the phase modulation means includes an optical circulator and an optical phase modulator, and is emitted from the input / output port. The fundamental light is incident on the optical phase modulator via the optical circulator, and the fundamental light phase-modulated by the optical phase modulator is incident on the input / output port via the optical circulator. Features.

  According to a sixth aspect of the present invention, in the wavelength converter according to the first, second, or fourth aspect, the phase modulation means includes an optical multiplexing / demultiplexing circuit and an optical phase modulator, and the input / output port The emitted fundamental wave light is incident on the optical phase modulator via the optical multiplexing / demultiplexing circuit, and the fundamental wave light phase-modulated by the optical phase modulator is input / output port via the optical multiplexing / demultiplexing circuit. It is characterized by being incident on.

  According to a seventh aspect of the present invention, in the wavelength converter according to the first, second, or fourth aspect, the phase modulation means is a reflection type optical phase modulator.

  The invention according to claim 8 is the wavelength converter according to any one of claims 1 to 7, wherein the first and second wavelength separation filters are dichroic mirrors using a dielectric film. And

  The invention according to claim 9 is the wavelength conversion device according to any one of claims 1 to 8, wherein the nonlinear optical element is made of a second-order nonlinear optical material that is periodically poled. .

  According to a tenth aspect of the present invention, in the wavelength converter according to the ninth aspect, the nonlinear optical element is a ridge type optical waveguide.

  According to the present invention, in a wavelength conversion device that performs wavelength conversion using a nonlinear optical effect, stable high output is possible without reducing output for precise control. Further, as a particularly excellent effect, it is possible to perform wavelength conversion with a narrow line width while stabilizing the output for a long time.

(A), (b) is the schematic of the wavelength converter which stabilized the output which concerns on Embodiment 1 of this invention. It is a figure explaining the frequency component of the light which performed the phase modulation. (A) is the figure explaining the relationship between the conversion light intensity | strength and wavelength in a wavelength converter, (b) is the figure explaining the relationship between the modulation | alteration component intensity | strength corresponded to the differential coefficient of conversion light intensity | strength, and a wavelength. . It is the schematic of the wavelength converter which stabilized the output which concerns on Embodiment 2 of this invention. It is the schematic of the wavelength converter which stabilized the output which concerns on Embodiment 3 of this invention. It is the schematic of the wavelength converter which stabilized the output which concerns on Embodiment 4 of this invention. (A) is a figure which shows the structure of the wavelength conversion element module provided with the typical nonlinear optical element. (A), (b) is a schematic diagram of the wavelength conversion element module provided with the nonlinear optical element which generates a 2nd harmonic as converted light. (A), (b) is explanatory drawing of the structure which stabilizes the output of a 2nd harmonic by wavelength precise control in the conventional wavelength converter.

  Hereinafter, embodiments of the present invention will be described in detail.

(Embodiment 1)
Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1A and 1B are schematic views of a wavelength conversion device that stabilizes an output according to Embodiment 1 of the present invention.

  The nonlinear optical element 101 that performs wavelength conversion mounted on the fiber pigtail type wavelength conversion element module has, for example, a waveguide substrate made of Zn-doped lithium niobate having a domain-inverted structure and a base substrate made of Mg-doped lithium niobate directly. A ridge-type optical waveguide formed by processing a waveguide substrate bonded by a bonding method can be obtained. When one or more fundamental wave lights (input lights) are incident on the nonlinear optical element 101, wavelength converted light is output. As the nonlinear process, second harmonic generation, sum frequency generation, difference frequency generation, and the like are possible. In the present embodiment, the case where second harmonic generation is performed from the point that the effect can be described most simply will be described. However, output stabilization is possible even in other nonlinear processes.

  As described above, the nonlinear optical element 101 is disposed in the metal package container 102 having high thermal and electrical conductivity via a temperature adjusting element such as a Peltier element. Electrodes are connected to the package container 102 for temperature control and other uses, and electrical exchange with the outside is possible. In the present embodiment, four electrodes 105 connected to a temperature adjustment element used for temperature adjustment are shown, but electrodes for applications other than temperature adjustment may be provided.

  The nonlinear optical element 101, the input port 103, the output ports 104 and 109, and the input / output port 108 are lens-coupled via an optical lens, and are coupled to the optical fiber by a pigtail. The output unit uses a dichroic mirror 107 as a wavelength separation filter to separate the output of the fundamental wave light that has not been converted from the second harmonic wave. In the figure, the fundamental wave light is output to the input / output port 108 and the second harmonic wave is output. Is output to the output port 104. The length of the nonlinear optical element 101 is, for example, 50 mm, and the total length of the package container 102 is, for example, 80 mm.

  The configuration so far described is the same as that described with reference to FIGS. 7 and 8, but the characteristic configuration of the present invention will be described below. An optical circulator 111 is arranged in the fundamental wave path separated on the output side. The light that has passed through the optical circulator 111 is introduced into the optical phase modulator 112 and subjected to phase modulation at a modulation speed of, for example, several MHz to several hundred kHz for the purpose of frequency modulation by phase modulation. The amount of modulation in the optical phase modulator 112 is controlled by the feedback circuit 114.

  The fundamental wave light subjected to frequency modulation is returned into the module by the optical circulator 111 via the input / output port 108 and is incident on the nonlinear optical element 101 again by the dichroic mirror 107. Since the nonlinear optical element 101 that performs wavelength conversion has no directionality, even when fundamental light is incident from the output side, the wavelength-converted light can be extracted from the original input side corresponding to the end of the waveguide. Therefore, a dichroic mirror 106 that reflects the second harmonic wave and transmits the fundamental wave light is disposed on the input side, and the second harmonic wave converted from the phase-modulated fundamental wave light is extracted from the output port 109, and the photodetector 113. Detect with. The electrical signal photoelectrically converted by the photodetector 113 is input to the feedback circuit 114. The feedback circuit 114 extracts only the component corresponding to the frequency modulation, and the temperature so that the intensity of the output second harmonic is maximized. The regulator 115 is controlled. The temperature adjuster 115 controls the temperature of the nonlinear optical element 101 by controlling the voltage applied to the electrode 105 connected to the temperature adjusting element. If the fundamental light source 110 is a wavelength tunable light source as shown in FIG. 1B, the wavelength of the light source 110 can also be controlled by the feedback circuit 114.

  Here, the converted light extracted from the output port 104 is defined as the first converted light, and the converted light converted from the phase-modulated fundamental light returned to the module via the input / output port 108 is converted into the second converted light. This is converted light.

  The fundamental wave light returned from the input / output port 108 to the nonlinear optical element 101 enters the input port 103 without being reflected by the dichroic mirror 106. An ordinary light source is equipped with an optical isolator that prevents return light, but when using a light source that does not come with an optical isolator, an optical isolator is provided in front of the light source to prevent this return light from entering the light source. May be. In this embodiment, a dichroic mirror is used as the wavelength separation filter. However, a wavelength separation filter using a multimode interferometer (MMI) or a directional coupler may be used.

  Next, the operating principle of this apparatus will be described.

  The fundamental light on the output side that is not normally used is reused, and phase modulation is performed by the optical phase modulator 112. As shown in FIG. 2, the phase-modulated light is accompanied by light waves having different frequencies called sidebands at frequencies around the reference frequency, which is the frequency of the fundamental light (see Non-Patent Document 1). Therefore, the frequency is also modulated by this phase modulation. The frequency modulation can be adjusted by the modulation intensity.

  The light subjected to the phase modulation and frequency modulation is returned again into the module by the optical circulator 111, and wavelength conversion is performed by the optical waveguide which is the nonlinear optical element 101 to generate the second harmonic. The second harmonic is extracted from the output port 109 by using the dichroic mirror 106 and detected by the photodetector 113. The electrical signal representing the intensity of the detected second harmonic output from the photodetector 113 is processed in the feedback circuit 114, and only the signal component corresponding to the frequency modulation by the phase modulation in the optical phase modulator 112 is extracted.

FIG. 3A shows the relationship between the converted light intensity and the wavelength in the wavelength converter. In the wavelength conversion by the nonlinear optical effect, efficient conversion is performed only when the phase relationship between the fundamental wave light and the converted light satisfies an appropriate relationship, that is, only at a wavelength that satisfies the phase matching condition. In the second harmonic generation by the second-order nonlinear optical effect, the converted light intensity, that is, the second harmonic intensity I is I は sinc ² (ΔkL / 2) (1)
Δk = k _SH -2k _F = n _SH / λ _SH -2n _F / λ _F (2)
It is represented by k is the wave number of light, L is the element length, λ is the wavelength, n is the refractive index, the subscript SH is the second harmonic, and F is the fundamental wave.

  Since Δk also changes as the wavelength changes, the second harmonic intensity I has a characteristic as shown in FIG. Modulating a wavelength (frequency) and extracting a component corresponding to the modulation is equivalent to obtaining a differential coefficient at the wavelength (frequency). Therefore, if a component corresponding to modulation is extracted by a feedback circuit, FIG. FIG. 3B is a waveform obtained by differentiating a).

  FIG. 3B shows the relationship between the modulation component intensity corresponding to the derivative of the converted light intensity and the wavelength. At a wavelength different from the maximum conversion efficiency, the derivative takes a non-zero value. Therefore, it is always the maximum by applying feedback to the element temperature or the element temperature and the light source wavelength so that the component corresponding to the modulation becomes zero. Wavelength conversion with high efficiency can be realized. Since Δk changes when the refractive index changes due to a change in element temperature, the waveforms in FIGS. 3A and 3B shift to the left and right, and the second harmonic intensity I can be adjusted. it can.

  However, since there is a point where the derivative coefficient becomes zero outside the maximum and minimum values of the differential value, the temperature is greatly changed at the start of operation to grasp the positional relationship between the maximum and minimum values of the differential. It is necessary to be able to maintain a zero value between the two points.

  The advantages of the configuration of the present invention are as follows.

  The first point is to reuse the fundamental light on the output side that is not normally used, so there is no need to branch to monitor the second harmonic for output, and the second harmonic is not wasted efficiently. It can be used. As described above, since it is usually necessary to branch about 1/20, the output is improved by about 5% by using the configuration of the present invention. This effect is very significant especially in wavelength conversion in a wavelength band where it is difficult to use an optical amplifier.

  The second point is that control for maximizing the output is always possible without affecting the light source of the fundamental wave light. In the conventional method, it is necessary to perform such output control only once before entering the steady operation, or to perform control by adding modulation to the light source of the fundamental light. When the control for adjusting to the phase matching wavelength is performed only once before the steady operation, the output wavelength and the phase matching wavelength are gradually shifted due to continuous use for a long time, and the output is lowered. On the other hand, if the control for applying phase modulation to the light source of the fundamental wave light is performed, the wavelength of the fundamental wave light inevitably fluctuates as shown in FIG. Becomes difficult. In the configuration of the present invention, in order to apply phase modulation to the fundamental wave light different from the fundamental wave light that generates the second harmonic extracted as the external output, control for always maximizing the output is performed without affecting the line width. It can be carried out.

  The third point is to input the fundamental wave light again to the optical waveguide element and monitor the second harmonic. A method of fixing the output using the fundamental wave light on the output side is described in Patent Document 3, for example. However, when the fundamental wave light power on the output side is large, as occurs in the wavelength band where the conversion efficiency is low, the attenuation amount of the fundamental wave light is reduced. Based on this attenuation amount, the wavelength at which the maximum conversion occurs is determined. It is difficult to fix the output. For example, assuming a conversion efficiency of 1000% / W with an input of 5 mW fundamental wave light, the second harmonic output is 0.25 mW. When viewing the fundamental wave light itself, it is necessary to fix the wavelength based on the attenuation of about 5%, and depending on the noise state, it may be difficult to detect the attenuation, that is, the dent. In that respect, in the configuration of the present invention, since the second harmonic output value having a peak is monitored instead of the attenuation amount of the fundamental light, monitoring is easy even when the conversion efficiency is low, and the output is maximized. The wavelength can be easily adjusted. In this example, the second harmonic generated by the second wavelength conversion using the fundamental wave light that appears on the output side is about 0.23 mW. This is approximately the same value as the second harmonic output of the output, and is a light intensity that can be easily monitored.

  In a wavelength converter that inputs light having a wavelength of 1.56 μm as fundamental light and outputs 0.78 μm as second harmonic, for example, when 10 mW second harmonic is generated by inputting 20 mW fundamental light, In the prior art, about 5% was split as monitor light, and an output of about 9.5 mW was taken out after branching.

  In contrast, the present invention can extract all 10 mW as an output. On the other hand, the fundamental wave light that was not converted through the nonlinear optical element was 10 mW. However, when this was returned to the nonlinear optical element having a conversion efficiency of, for example, 2500% / W, the converted fundamental light was re-converted for monitoring. The second harmonic output was about 2 mW including input / output loss. This is a value of about 20% of the output 10 mW and can be sufficiently monitored. In the present invention, since the fundamental wave light source is not subjected to phase modulation, it can be configured with a very narrow line width and at a low cost.

(Embodiment 2)
FIG. 4 is a schematic diagram of a wavelength conversion device with stabilized output according to Embodiment 2 of the present invention. In this embodiment, the configuration in which the optical circulator 111 is used when the fundamental wave light on the output side is modulated and returned to the module in the first embodiment is replaced with an optical coupler 121 that is one of optical multiplexing / demultiplexing circuits. It was realized.

  This optical coupler 121 is called a 3 dB coupler and divides the input into two equal parts and branches right and left. The branched light propagates through the optical fiber clockwise and counterclockwise and is subjected to phase modulation, and then is multiplexed again by the optical coupler 121 and returns to the module. It is necessary to install the optical phase modulator 112 at an intermediate point of the optical path so that there is no phase difference between clockwise and counterclockwise light when combined, but if this is satisfied, the same function as in the first embodiment Can be given.

  Also in the present embodiment, all 10 mW could be extracted as output in the same conversion as in the first embodiment. On the other hand, the fundamental wave light that has not been converted is 10 mW, but if this is returned to a nonlinear optical element having a conversion efficiency of, for example, 2500% / W, the second harmonic output reconverted for monitoring becomes the input / output loss. Including, it was about 2 mW. This is a value of about 20% of the output 10 mW and can be sufficiently monitored. Further, in the present invention, since the fundamental wave light source is not modulated, it can be configured with a very narrow line width and at a low cost.

(Embodiment 3)
FIG. 5 shows a schematic diagram of a wavelength conversion device with stabilized output according to Embodiment 3 of the present invention. In this embodiment, the configuration in which the optical circulator 111 is used when the fundamental wave light on the output side is modulated and returned to the fiber pigtail type wavelength conversion element module in the first embodiment is used as the reflection type optical phase modulator 131. This is realized by using

  The reflection type optical phase modulator 131 is constituted by a waveguide type phase modulator whose end face is mirror-finished, and is a modulator capable of performing phase modulation on both the forward path and the return path of the waveguide. The configuration of the present embodiment can also have the same function as that of the first embodiment.

  Also in the present embodiment, all 10 mW could be extracted as output in the same conversion as in the first embodiment. On the other hand, the fundamental wave light that was not converted was 10 mW, but if this was returned to an optical waveguide having a conversion efficiency of, for example, 2500% / W, the second harmonic output that was reconverted for monitoring would have an input / output loss. 2mW including that. This is a value of about 20% of the output of 10 mW, and can be sufficiently monitored. Further, in the present invention, since the fundamental wave light source is not modulated, it can be configured with a very narrow line width and at a low cost.

(Embodiment 4)
The present invention can be applied not only to second harmonic generation, but also to sum frequency generation and difference frequency generation as long as the wavelength of the converted light and the wavelength of the fundamental light are different enough to be separated by the wavelength separation filter. The following is a configuration adapted for sum frequency generation and difference frequency generation.

  FIG. 6 shows a schematic diagram of a wavelength conversion device with stabilized output according to Embodiment 4 of the present invention. This embodiment is a form adapted to a sum frequency generation or difference frequency generation apparatus in the case where there are two types of fundamental wave light, and includes a light source 141-1 for fundamental light 1 and a light source 141-2 for fundamental light. . The fundamental wave light 1 and the fundamental wave light 2 are combined by the optical coupler 142-1 and are incident on the fiber pigtail type wavelength conversion element module. Similar to the first to third embodiments, the combined fundamental wave light is transmitted through the nonlinear optical element 101, separated from the sum frequency or difference frequency by the dichroic mirror 107, and emitted from the input / output port 108.

  Even if there are two types of fundamental light, only one fundamental wave is required to perform phase modulation. Therefore, in this embodiment, for example, the fundamental light 1 is configured to be phase-modulated. An optical coupler 142-2 having high separation capability is connected to the input / output port 108 and separates the fundamental wave light 1 and the fundamental wave light 2 into two optical paths.

  An optical circulator 111-1 and an optical phase modulator 112 are arranged in the optical path of the fundamental wave light 1, phase-modulated, and then returned from the input / output port 108 to the fiber pigtail type wavelength conversion element module. On the other hand, only the optical circulator 111-2 is arranged in the optical path of the fundamental wave light 2, and is returned to the fiber pigtail type wavelength conversion element module without being subjected to phase modulation.

  The sum frequency or difference frequency generated again by the fundamental wave light 1 and the fundamental wave light 2 subjected to frequency modulation incident on the nonlinear optical element 101 is reflected by the dichroic mirror 106 and emitted from the output port 109. Detected.

  The feedback circuit 114 controls the optical phase modulator 112 to extract only components corresponding to frequency modulation of sum frequency generation or difference frequency generation, and maximizes sum frequency generation or difference frequency generation based on the extracted components. The temperature controller 115 and the light sources 141-1 and 141-2 are controlled.

  In this embodiment, light having a wavelength of 1.064 μm is used as fundamental wave light 1, light having a wavelength of 1.55 μm using an optical amplifier is input as fundamental wave light 2, and light having a wavelength of 3.4 μm, which is a difference frequency light. In the present invention, for example, when a difference frequency light of about 3.2 mW is generated by inputting a fundamental wave light 1 of 40 mW and a fundamental wave light 2 of 200 mW, the difference frequency light 3 output in the present invention is All 2mW could be removed. Further, since the fundamental wave light source is not modulated, it can be configured with a very narrow line width and at a low cost.

  In the configurations of the second and third embodiments, an optical coupler having a high separation capability is connected to the input / output port 108 to separate the fundamental wave light 1 that undergoes phase conversion and the fundamental wave light 2 that does not undergo phase conversion, and the separation capability is By adding a loop circuit in which only the optical coupler and the optical circulator are arranged in the optical path of the fundamental light 2 of the high optical coupler, a configuration suitable for sum frequency generation and difference frequency generation can be achieved as in the fourth embodiment.

101, 201 Nonlinear optical element 102, 202 Package container 103, 204 Input port 104, 109, 205, 209 Output port 105, 206 Electrode 106, 107, 208 Dichroic mirror 108 Input / output port 110, 141, 210 Light source 111 Optical circulator 112 Optical phase modulator 113, 213 Optical detector 114, 214 Feedback circuit 115, 215 Temperature controller 121, 142, 212 Optical coupler 131 Reflective optical phase modulator 203 Optical lens 211 Optical fiber 220 Wavelength filter

Claims (10)

A nonlinear optical element that generates converted light having different wavelengths from one or more fundamental wave lights; an input port for entering fundamental light into a first end face of the nonlinear optical element; and a first end face from the second end face of the nonlinear optical element. A wavelength converter comprising: a first output port that emits the converted light; and a temperature adjusting unit that adjusts the temperature of the nonlinear optical element,
A first wavelength separation filter disposed between the second end face and the output port and separating the first converted light and the fundamental light;
An input / output port that emits the fundamental light separated from the first converted light by the first wavelength separation filter;
Phase modulation means for performing phase modulation on the fundamental wave light emitted from the input / output port and entering the fundamental wave light phase-modulated into the input / output port;
A second wavelength separation filter disposed between the first end face and the input port, for separating the second converted light generated from the phase-modulated fundamental light and the phase-modulated fundamental light When,
A second output port for emitting the second converted light separated from the phase-modulated fundamental light by the second wavelength separation filter;
A photodetector for detecting the second converted light emitted from the second output port;
A feedback circuit that controls the phase modulation amount in the phase modulation means and the temperature adjustment means based on a change in signal intensity detected by the photodetector;
A wavelength conversion device comprising: The wavelength converter according to claim 1, wherein the feedback circuit further controls a fundamental light source.   The wavelength conversion device according to claim 1, wherein the nonlinear optical element generates a second harmonic as the converted light. The nonlinear optical element generates a sum frequency or a difference frequency as the converted light,
The phase modulation means further includes an optical multiplexing / demultiplexing circuit, and performs phase modulation on the fundamental wave light of a predetermined wavelength separated by the optical multiplexing / demultiplexing circuit among the fundamental wave light output from the input / output port. 3. The wavelength conversion device according to claim 1, wherein the fundamental wave light including the modulated fundamental wave light having the predetermined wavelength is incident on the input / output port. The phase modulation means includes
An optical circulator,
An optical phase modulator;
The fundamental wave light emitted from the input / output port is incident on the optical phase modulator via the optical circulator, and the fundamental wave light phase-modulated by the optical phase modulator is transmitted via the optical circulator. The wavelength conversion device according to claim 1, wherein the wavelength conversion device is incident on the input / output port. The phase modulation means includes
An optical multiplexing / demultiplexing circuit;
An optical phase modulator;
The fundamental light emitted from the input / output port is incident on the optical phase modulator via the optical multiplexing / demultiplexing circuit, and the fundamental light phase-modulated in the optical phase modulator is the optical multiplexing / demultiplexing The wavelength converter according to claim 1, wherein the wavelength converter is incident on the input / output port via a circuit.   5. The wavelength conversion device according to claim 1, wherein the phase modulation means is a reflective optical phase modulator.   8. The wavelength conversion device according to claim 1, wherein the first and second wavelength separation filters are dichroic mirrors using a dielectric film.   9. The wavelength conversion device according to claim 1, wherein the nonlinear optical element is made of a second-order nonlinear optical material whose polarization is periodically inverted.   The wavelength conversion device according to claim 9, wherein the nonlinear optical element is a ridge type optical waveguide.

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