JP5421230B2 - Wavelength conversion device and wavelength conversion device - Google Patents

Description

  The present invention relates to a wavelength conversion device and a wavelength conversion device, and more specifically, a wavelength conversion device in which optical multiplexers / demultiplexers using multi-mode interference (MMI) are integrated, and a wavelength conversion device using the same About.

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 and electro-optical devices is underway. Various materials have been researched and developed as nonlinear optical media and electro-optical media used in such elements. An oxide-based compound substrate such as lithium niobate (LiNbO ₃ , hereinafter referred to as LN) is known as a promising material because it has a very high second-order nonlinear optical constant / electro-optic constant. As an example of an optical device using non-linearity with high LN, a wavelength conversion element using difference frequency generation (DFG) by pseudo phase matching is known.

  In recent years, in order to increase the communication capacity of an optical communication system, a wavelength division multiplexing (WDM) communication system that multiplexes and transmits a plurality of lights having different wavelengths has been actively introduced. In such a WDM communication system, in order to effectively use a limited number of wavelengths, there is a demand for practical use of a wavelength conversion device that converts a signal wavelength into an arbitrary signal wavelength.

  Conventionally, as a wavelength conversion element for converting the wavelength of light, a device using a semiconductor optical amplifier, a device using four-wave mixing, and the like are known. However, these wavelength conversion elements have not been able to satisfy conditions such as high efficiency, high speed, wide band, low noise, and polarization independence required in an optical communication system.

FIG. 1 shows a configuration of a conventional quasi phase matching type wavelength conversion element using LN. The signal light A having a relatively small light intensity and the excitation light B having a relatively large light intensity are combined by a multiplexer 1 and are incident on an optical waveguide 2 of a nonlinear optical medium having a polarization inversion structure. In the optical waveguide 2, the signal light A is converted into converted light C having a different wavelength by difference frequency wave generation (DFG) due to a nonlinear optical effect. The converted light C is emitted from the optical waveguide 2 together with the excitation light B. The emitted converted light C and excitation light B are separated by the duplexer 3. If the wavelengths of the signal light A and the excitation light B are λ ₁ and λ ₃ , respectively, the wavelength λ ₂ of the converted light C is
1 / λ ₂ = 1 / λ ₃ −1 / λ ₁
Satisfied. The wavelength λ ₂ of the converted light C is a wavelength obtained by turning the wavelength λ ₁ of the signal light A around the wavelength λ ₃ twice that of the pumping light B. For example, when the wavelength λ ₃ of the excitation light B is 0.78 μm, the signal light A having a wavelength λ ₁ = 1.54 μm is converted into converted light C that is a difference frequency light having a wavelength λ ₂ = 1.58 μm. be able to.

  The conversion band for the signal light A and the converted light C is as wide as ± 30 nm or more with respect to the wavelength of the excitation light. For example, a WDM signal bundled in a wavelength band C band used for wavelength division multiplexing (WDM) optical communication is L It is possible to perform batch conversion of wavelength groups such as to the band or from the L band to the C band.

  Conventionally, a wavelength conversion element using quasi-phase matching has been manufactured by forming a periodic polarization reversal structure on a nonlinear optical crystal substrate such as LN and then manufacturing a proton exchange waveguide. On the other hand, in order to improve the optical confinement in the optical waveguide and realize highly efficient wavelength conversion using the bulk or near-bulk nonlinear effect, a wavelength conversion element with a ridge-type optical waveguide structure is proposed Has been. When producing a ridge-type optical waveguide, first, a periodically poled structure is produced on an Mg-added LN substrate, and then adhered to an LN substrate prepared separately using an adhesive. Next, after reducing the thickness of the Mg-added LN substrate by surface grinding, a ridge-type waveguide is produced by precision grinding using a dicing saw (see, for example, Non-Patent Document 1).

  If a 0.78 μm band light source is used for the excitation light in this method, a stable, high wavelength accuracy and high output light source is not widely used at 0.78 μm, and it is difficult to prepare easily. Since the wavelengths of the light and the pumping light are different by about half, the optimum size of the optical waveguide is different, and there is a problem that it is necessary to suppress excitation other than the desired mode when the light enters the waveguide. For this reason, it is preferable to use a stable and highly reliable 1.5 μm band light source as a light source of excitation light. Moreover, high output light can be easily obtained by using an optical fiber amplifier or the like. As wavelength conversion using LN using pseudo phase matching, a technique called cascade excitation of second harmonic generation (SHG) and difference frequency generation (DFG) (hereinafter, SHG-DFG cascade excitation method) has been widely used. (For example, refer nonpatent literature 2).

If the cascade excitation method is used, the light having a wavelength twice as large as the excitation wavelength necessary for wavelength conversion in the 1.5 μm band by the difference frequency generation as the wavelength conversion excitation light using LN using pseudo phase matching Can be used. In the example of the wavelength conversion described with reference to FIG. 1, by using the pumping light of the wavelength lambda ₃ '= 1.56 .mu.m in place of the wavelength lambda ₃ = 0.78 .mu.m, a SHG-DFG cascade excitation method. In FIG. 1, when the wavelength λ ₃ ′ of the excitation light B is 1.56 μm, the second harmonic (wavelength: 0.78 μm) of the excitation light is generated inside the nonlinear optical medium. According to the cascade excitation, the converted light C can be further obtained by the difference frequency generation (DFG) between the second harmonic (SHG) generated inside the nonlinear optical medium and the signal light A.

  On the other hand, in the wavelength conversion by SHG-DFG cascade excitation, the quality of the converted light is likely to deteriorate, and a 1.5 μm band light source is used for the excitation light, so it is difficult to separate the excitation light with the near wavelength from the signal light / converted light. Therefore, it is necessary to provide a certain band between the pumping light wavelength called the guard band and the signal light / converted light. However, by securing this band, the wavelength conversion band that can be used is narrowed, and the number of wavelengths that can be collectively converted is reduced. If a guard band is provided, conversion to a wavelength close to the signal light becomes impossible. Furthermore, when an optical fiber amplifier is used to obtain high excitation light, the quality of signal light / converted light deteriorates due to an increase in ASE noise. In cascade excitation, crosstalk light due to sum frequency generation (SFG) between excitation light and signal light increases, and signal quality deteriorates. Therefore, for high-quality wavelength conversion, wavelength conversion by excitation of 0.78 μm band light is desired. In this 0.78 μm band light, signal light / excitation light is not converted. It is necessary to easily perform wavelength conversion without exciting a mode other than the desired mode with low loss of input.

  However, from the viewpoint of difficulty in obtaining a light source in the 0.78 μm band and the necessity of suppressing other modes, a difference from the second harmonic light (SHG) process due to the nonlinear effect using a light source in the 1.5 μm band as the excitation light is used. There has been a demand for a method (independent multistage excitation) in which the frequency generation (DFG) process is performed separately rather than in cascade.

  Furthermore, since cascade excitation generates difference frequency simultaneously while generating second harmonic light internally, compared with the optical power of the same second harmonic light, cascade excitation is more effective in cascade excitation and independent multistage excitation methods. Only one quarter is inefficient. Therefore, if the independent multistage excitation method is used, there is an advantage that four times the efficiency can be obtained as compared with the cascade excitation.

  In addition to the SHG-DFG cascade excitation method, a sum frequency generation (SFG) and a difference frequency can be used as a wavelength conversion method using LN using pseudo phase matching to convert a signal wavelength to an arbitrary signal wavelength. A technique called cascade excitation of generation (DFG) (hereinafter referred to as SFG-DFG cascade excitation method) has been proposed.

FIG. 2 shows the configuration of a conventional SFG-DFG cascade excitation method using a quasi phase matching wavelength conversion element using LN. The signal light A having the wavelength λ ₁ , the two pumping lights 1B (wavelength λ ₂ ) and the pumping light 2B (wavelength λ ₄ ) having two different wavelengths are combined by the multiplexer 1 and are polarized. Is incident on the optical waveguide 2 of the nonlinear optical medium. In the optical waveguide 2, the signal light A is converted into converted light C (wavelength λ ₃ ) having a different wavelength by generating a sum frequency wave due to the nonlinear optical effect. The signal light A (wavelength λ ₁ ), the pumping light 1 B (wavelength λ ₂ ), and the converted light C have a relationship of 1 / λ ₃ = 1 / λ ₂ + 1 / λ ₁ . Further, converted light D can be obtained by generating a difference frequency between the sum frequency light (converted light C) generated inside the nonlinear optical medium and the excitation light 2B (wavelength λ ₄ ). The converted light D (wavelength λ ₅ ), converted light C (wavelength λ ₃ ), and pumping light 2B (wavelength λ ₄ ) have a relationship of 1 / λ ₅ = 1 / λ ₃ -1 / λ ₄ . The converted light D is emitted from the optical waveguide 2 together with the converted light C, the excitation light 1B, the excitation light 2B, and the signal light A.

  However, in wavelength conversion by SFG-DFG cascade excitation, the quality of the converted light tends to deteriorate. When an optical fiber amplifier is used to obtain pump light, the quality of signal light deteriorates due to generation of the sum frequency of ASE light. Further, since there is a limit to the wavelength that the excitation light 2B can take, the wavelength conversion band is limited. For example, when the excitation light 2B is set to the wavelength of the signal light or the double wavelength of the light obtained by sum frequency wave generation, the second harmonic generation of the second excitation light 2B or the second excitation light 2B Since the CW light is clogged into the converted light C on which the signal is superimposed by generating the sum frequency with the one excitation light 1B, the signal receiving sensitivity is significantly lowered. For this reason, the wavelength which the 2nd excitation light 2B can take is restrict | limited.

  Therefore, even in the SFG-DFG configuration, a method of separately performing the sum frequency generation (SFG) process and the difference frequency generation (DFG) process by the non-linear effect instead of the cascade has been desired.

  On the other hand, in addition to wavelength converters in the communication wavelength band, laser light sources in the visible range or mid-infrared range that have not been realized with semiconductor lasers have been put into practical use by using quasi-phase matching type wavelength conversion elements.

  Currently, gas lasers such as He—Ne laser and Ar laser, solid-state lasers such as Nd: YAG laser, dye lasers and semiconductor lasers are known as lasers in practical use. In recent years, small, lightweight, and inexpensive semiconductor lasers have become widespread mainly in the visible and near-infrared wavelength bands. In particular, in the field of optical communication, 1.3 μm band and 1.5 μm band semiconductor lasers for signal light sources and 0.98 μm band and 1.48 μm band semiconductor lasers for pumping fiber amplifiers are widespread. Further, CD (0.78 μm band), DVD (0.65 μm band), and Blu-ray (0.4 μm band) semiconductor lasers are also widely used as light sources for pickup of optical recording medium readers.

  However, since there are wavelength bands that are difficult to realize with semiconductors, laser light source devices that combine highly efficient nonlinear optical media and semiconductor lasers with widely used wavelength bands have been developed. For example, it is difficult to realize a laser with a wavelength of 0.5 to 0.6 μm such as green, yellow green, and orange with a semiconductor, and a laser light source using second harmonic generation or sum frequency generation by a highly efficient nonlinear optical medium Has been put to practical use. Furthermore, the third harmonic generation can be realized by the SHG-SFG cascade excitation method using the second-order nonlinear optical medium and combining the second harmonic generation (SHG) and the sum frequency generation (SFG).

  However, with the conventional SHG-SFG configuration, it is difficult to obtain a high power output. In the conventional SHG-SFG configuration, first, SH light (frequency 2ω) is obtained by generating the second harmonic of the input pumping light (frequency ω), and then pumping light (frequency ω) and SH light (frequency 2ω) are obtained. The third harmonic generation light (TH light: frequency 3ω) is obtained by generating the sum frequency. In this case, the power of the excitation light is attenuated by the second harmonic generation at the first stage. The output of the third harmonic generation light is proportional to the multiplication of the power of the SH light required for sum frequency generation and the excitation light, so that the efficiency of the second harmonic generation process in the first stage is increased to obtain high power SH light. However, since the power of the excitation light is attenuated accordingly, it is difficult to increase the output of the net third harmonic generation light.

  Therefore, there has been a demand for a technique for performing the second harmonic generation (SHG) process and the sum frequency generation (SFG) process by the nonlinear effect separately instead of the cascade.

Tatsuo Kawaguchi et al. “Waveguide SHG Device Using LiNbO3 Epitaxial Growth and Ultraprecision Processing Technology”, Laser Research, Vol. 28, No. 9, September 2000, p. 601-603 M.M. H. Chou et al. , “Optical Signal Processing and Switching with Second-Order Nonlinearities in Waveguides,” IEICE Trans. Electricn. , E83-C, 2000, p. 869-874 J. et al. Yamawaku et al. “Low-Crosstalk 103 Channel × 10 Gb / s (1.03 Tb / s) Wavelength Conversion With a Quasi-Phase-Matched LiNbO 3 Waveguide,” IEEE J. Select. Topics Quantum Electron. , Vol. 12, no. 4, 2006, p. 521-528

  As explained above, for example, in the SHG-DFG cascade excitation method, in order to perform high-quality wavelength conversion, wavelength conversion by excitation of 0.78 μm band light is desired. However, stable wavelength accuracy is high at 0.78 μm. However, high-power light sources are not widely used, and it is difficult to prepare them easily. In addition, since the wavelengths of the signal light and the pumping light are different by half, the optimal size of the optical waveguide is different, which makes it difficult to suppress excitation other than the desired mode when light enters the waveguide. was there. Therefore, as a pumping light source, a widely used stable and reliable 1.5 μm band light source is used, firstly 0.78 μm band light is generated, and the generated 0.78 μm band light is used for wavelength. A technique for performing conversion, that is, a technique capable of performing SHG-DFG in multiple stages has been desired.

  FIG. 3 shows a configuration example of a wavelength conversion method using 0.78 μm band light generation by 1.5 μm band optical excitation using a conventional SHG-DFG multi-stage method. The pump light A in the 1.5 μm band is input to the first wavelength conversion device (nonlinear optical medium) 3A, and light in the 0.78 μm band (hereinafter referred to as SH light B) is generated by second harmonic generation (SHG). The dichroic mirror M1 (or prism filter, etc.) outputs the 1.5 μm band excitation light A that is output from the first wavelength conversion device 3A and the 0.78 μm band SH light B obtained by wavelength conversion. To separate. The separated 0.78 μm-band SH light B and signal light C are combined by a multiplexer M2 and input to the second wavelength conversion device 3B, and difference frequency generation in the second wavelength conversion device 3B ( The converted light D can be obtained by DFG).

  However, in this SHG-DFG multistage method, the second harmonic generation (SHG) and the difference frequency generation (DFG) need to be performed using separate elements 3A and 3B, and thus the characteristics of the elements are not completely matched. Therefore, it is necessary to adjust the center wavelength and the like by individually adjusting the temperature and adjusting each temperature. In addition, since the generated 0.78 μm band light is separated and input, there is a problem that the optical power loss is large and the wavelength conversion efficiency is lowered. Furthermore, when inputting light in the 0.78 μm band, it is necessary to suppress excitation other than the desired mode, and complex adjustments such as alignment while monitoring not only the excitation light but also the converted light at the time of input are necessary. turn into. Moreover, since it is necessary to produce two wavelength conversion devices to produce one wavelength conversion device, the cost increases.

  Therefore, a technique is desired in which multistage conversion of second harmonic generation (SHG) and difference frequency generation (DFG) is performed using the same waveguide. FIG. 4 shows an SHG-DFG multi-stage configuration using a conventional folding method. First, the 1.5 μm band excitation light A is input to the wavelength conversion waveguide, and 0.78 μm band light (hereinafter referred to as SH light B) is generated by second harmonic generation (SHG). One end face of the wavelength conversion waveguide is coated with a reflection film (HR coating) for the SH light B in the 0.78 μm band, and an antireflection film (AR coating) for the excitation light A in the 1.5 μm band. Has been. The unconverted 1.5 μm band excitation light A and the 0.78 μm band SH light B obtained by wavelength conversion are separated by the HR / AR optical film 41, and only the 0.78 μm band SH light B has a wavelength. Folded back to the conversion waveguide. The signal light C is inputted from the end face with the HR / AR coating, and the converted light D can be obtained by the difference frequency generation (DFG) with the 0.78 μm band light.

However, in the conventional folding method described above, the excitation light A in the 1.5 μm band and the SH light B in the 0.78 μm band are separated only by the coating film. There is a problem that the return light of the light A cannot be sufficiently suppressed. Specifically, it is technically difficult for the coating film to suppress reflected light, and the amount of reflected light suppression is usually about 20 to 30 dB. This means that when 1 W of 1.5 μm band excitation light is input, 1.5 μm band light of 1 to several tens of mW returns to the wavelength conversion waveguide. There arises a problem of conversion and a problem of signal quality degradation due to crosstalk light due to generation of a sum frequency between pump light and signal light. In addition, ASE light generated when an optical fiber amplifier is used for pumping light in the 1.5 μm band also returns to the wavelength conversion waveguide, thereby degrading the quality of signal light and converted light due to ASE noise. There was also a problem. In addition, since ASE light is generated over a wide wavelength band, the antireflection film needs to cover a wide wavelength band, and there is a problem from the viewpoint of cost and reliability, for example, a multilayer film is required.
Accordingly, there has been a demand for an SHG-DFG multi-stage method that can fold only the SH light in the 0.78 μm band into the same waveguide and multiplex the signal light with the reflection in the 1.5 μm band completely suppressed. .

  Similar to the SHG-DFG cascade excitation method, the SFG-DFG cascade excitation method and the SHG-SFG cascade excitation method are similar to the above because it is difficult to completely separate each frequency conversion process due to the nonlinear effect. There was a problem.

In order to solve the above-mentioned problems, a first aspect of the present invention includes a nonlinear optical medium, an optical multiplexer / demultiplexer, first and second output waveguides, and the first and second output waveguides. The light is reflected at the first wavelength band that is a specific wavelength band, and is in the second wavelength band that is different from the first wavelength band. An optical film that suppresses reflection of light, and the nonlinear optical medium generates light of the first wavelength band from light of the second wavelength band input from one end , The first wavelength band light and the second wavelength band light are output from the other end, and the optical multiplexer / demultiplexer outputs the first wavelength band light output from the nonlinear optical medium to the first wavelength band. Output to the output waveguide, output light of the second wavelength band to the second output waveguide, and the first output waveguide has the first wavelength The output end is vertically treated so as to reflect the light at the optical film, and the light of the first wavelength band reflected at the optical film is input to the optical multiplexer / demultiplexer, In the output waveguide 2, the output end is obliquely end-treated so that reflection of the light in the second wavelength band on the optical film is suppressed, and the output subjected to the oblique end-face treatment Signal light is input to the optical multiplexer / demultiplexer from an end, and the optical multiplexer / demultiplexer inputs the light of the first wavelength band and the signal light to the other end of the nonlinear optical medium, It is a wavelength conversion device.

According to the second aspect of the present invention, the nonlinear optical that outputs any one of the second harmonic light, the difference frequency light, and the sum frequency light by SHG, DFG, and SFG in the nonlinear optical effect with respect to the input light. and a medium, is connected before Symbol nonlinear optical medium, a demultiplexer for outputting the separated using a mode interference to different wavelengths of light output from the nonlinear optical medium, before Symbol demultiplexer a first output waveguide connected to one of two outputs, and a second output waveguide connected to the other of the two outputs of the previous SL demultiplexer, before Symbol two output provided at the output end of the waveguide, to suppress the reflection of the first wavelength band light, and an optical film that reflects light in the second wavelength band, before SL output terminal of the first output waveguide An end face is formed so that a portion is inclined with respect to the first output waveguide, and the second output waveguide is formed. By configuring the end face so that the output end is perpendicular to the second output waveguide, the return light of the first wavelength band is prevented from being input again to the nonlinear optical medium. , entered when signal light is inputted from the output end of the first output waveguide, the nonlinear optical medium with light of a second wavelength band signal light is reflected by the second output waveguide And a non-linear optical change independent of the non-linear optical change and outputting the non-linear optical change.

  Invention of Claim 3 is the wavelength conversion device of Claim 1 or 2, Comprising: The optical path length in the propagation direction of the said optical multiplexing part is the light of the said 1st wavelength or the 1st wavelength in the said output end surface. It is characterized in that at least one of the two wavelengths of light is set to an extreme value.

  A fourth aspect of the present invention is the wavelength conversion device according to any one of the first to third aspects, wherein the width of the optical multiplexing portion is 5 μm or more and 100 μm or less.

The invention according to claim 5 is the wavelength conversion device according to any one of claims 1 to 4, wherein the nonlinear optical medium is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _{(1- x)} O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or at least one selected from the group consisting of Mg, Zn, Sc, and In is contained as an additive.

  The invention according to claim 6 is the wavelength conversion device according to claim 5, wherein the nonlinear optical medium is a crystal film grown by a liquid phase epitaxial method.

  The invention according to claim 7 is the wavelength conversion device according to claim 5 or 6, wherein the nonlinear optical medium has a first substrate having a nonlinear optical effect and a refractive index compared to the first substrate. It is a thin film substrate manufactured by pasting together a small second substrate.

  The invention according to claim 8 is the wavelength conversion device according to claim 7, wherein the first substrate has a structure in which a nonlinear constant is periodically inverted.

  The invention according to claim 9 is the wavelength conversion device according to claim 7 or 8, wherein the first substrate and the second substrate are directly bonded together by diffusion bonding by heat treatment. Features.

  A tenth aspect of the present invention is characterized in that a plurality of the wavelength conversion devices according to any of the first to ninth aspects are integrated on the same substrate.

  The invention according to claim 11 is the wavelength conversion device according to any one of claims 1 to 10, a first laser light source unit that inputs the incident light to an input end face of the wavelength conversion device, and the wavelength conversion. A wavelength conversion apparatus comprising: a second laser light source unit that inputs the incident light to an output end face of the device; and a filter that separates the converted light output from the wavelength conversion device.

  As described above, according to the present invention, there are provided a wavelength conversion device capable of performing non-linear optical change of any one of SHG, DFG, and SFG on input light in multiple stages and a wavelength conversion apparatus using the same. can do.

It is a figure which shows the structure of the wavelength conversion element of the quasi phase matching type | mold using the conventional LN. It is a figure which shows the structure of the SFG-DFG cascade excitation method by the quasi phase matching type | mold wavelength conversion element using the conventional LN. It is a figure which shows the structural example of the wavelength conversion using the conventional SHG-DFG multistage method. It is a figure which shows the SHG-DFG multistage structure by the conventional folding method. It is a figure which shows the structure of the wavelength conversion device which integrated the optical multiplexer of this invention. It is a figure of the simulation result which shows the coupling of the light in a mode interference waveguide. It is a figure which shows the process of producing a wavelength conversion waveguide. It is a figure which shows the dimension example of the wavelength conversion device of this invention. It is sectional drawing which shows the structural example of the wavelength conversion waveguide of this invention. It is a perspective view which shows the preparation methods of the wavelength conversion device of this invention. It is a figure which shows the end surface processing process of the wavelength conversion device of this invention. It is a figure which shows the structure of the wavelength converter concerning the 1st Example of this invention. It is a figure which shows the structure of the wavelength converter concerning the 2nd Example of this invention. It is a figure which shows the structure of the wavelength converter concerning 3rd Example of this invention. It is a figure which shows the structure of the wavelength converter concerning the 4th Example of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
SHG-DFG Multistage Independent Configuration The wavelength converter of this embodiment performs SHG (second harmonic generation) -DFG (difference frequency generation) independently in multiple stages.

  First, the configuration of a wavelength conversion device that can be used in the wavelength conversion device of the present embodiment will be described with reference to FIGS. This wavelength conversion device will be described below assuming that light having a wavelength of 1.5 μm band and light having a wavelength of 0.78 μm band are used.

  FIG. 5 shows a part of the configuration of the wavelength conversion device in which the optical multiplexer / demultiplexer according to the first embodiment of the present invention is integrated. The wavelength conversion device in which the optical multiplexer / demultiplexer is integrated includes a wavelength conversion waveguide 34 on the substrate 30, a mode interference waveguide 33 connected to the output end of the wavelength conversion waveguide, and functioning as an optical multiplexer / demultiplexer, and a mode. A first output waveguide 31 and a second output waveguide 32 connected to the output of the interference waveguide 33 are provided.

  The wavelength conversion waveguide 34 is made of a nonlinear optical medium, causes nonlinear optical change of any one of SHG, DFG, and SFG to light input from one end and outputs it to the other end.

  The mode interference waveguide 33 has a multiplexing / demultiplexing function, and has a wavelength at a position (backward direction on the paper surface) shifted from the center line 35 in the width direction of the mode interference waveguide 33 (from the front side to the back side in FIG. 5). By providing the conversion waveguide 34 and the output waveguides 31 and 32, the first input / output waveguide 31 guides light having a wavelength of 1.5 μm, and the second input / output waveguide 32 has a wavelength of 0. It is configured to guide light in the 78 μm band.

  Here, the multiplexing / demultiplexing function of the mode interference waveguide will be described with reference to FIG. FIG. 6 is a simulation result showing light coupling / separation in the mode interference waveguide. A state in which the signal light having a wavelength of 1.56 μm and the excitation light having a wavelength of 0.78 μm are combined is shown by a simulation by BPM (Beam Propagation Method).

  The mode interference waveguide 36 of FIG. 6 is a slab type waveguide, and two input waveguides 37 and 38 are provided on the input side thereof. These input waveguides 37 and 38 are provided at positions shifted from each other by a distance Δ with respect to the center line in the width direction of the mode interference waveguide 36. This interval Δ is set so that 2 × Δ is one third of Wm. For example, as shown in FIG. 6A, the width Wm of the mode interference waveguide 36 = 30 μm, the axial deviation amount Δ = 5 μm of the input waveguide 37, the axial deviation amount Δ = 5 μm of the output waveguide 38, and the refractive index of the cladding. = 1.0, the refractive index of the core = about 2.1.

  FIG. 6B is a diagram illustrating the behavior when excitation light having a wavelength of 0.78 μm is input from the input waveguide 37 of the mode interference waveguide 36. The excitation light incident from the input waveguide 37 connected to the position shifted from the center of the mode interference waveguide 36 by the axis (Δ) is developed into a plurality of modes inherent to the mode interference waveguide 36, and the mode interference waveguide Multi-mode propagation in 36. At this time, due to mode interference that occurs because the propagation constant of each mode is different, after propagating an optical path length with a light amount of 0.78 μm, the axis is shifted from the center of the mode interference waveguide 36 by Δ opposite to the input side. An extreme value (convergence point) is taken at the position.

  FIG. 6C is a diagram illustrating the behavior when signal light having a wavelength of 1.56 μm is input from the input waveguide 38 of the mode interference waveguide 36. The signal light incident from the input waveguide 38 connected to the position shifted from the center of the mode interference waveguide 36 (Δ) is developed into a plurality of modes unique to the mode interference waveguide 36, and the mode interference waveguide Multi-mode propagation in 36. At this time, due to mode interference caused by the propagation constant of each mode being different, after propagating an optical path length having a light amount of 1.56 μm, the axis was shifted by Δ from the center of the mode interference waveguide 36 opposite to the input side. After the first extreme value (convergence point) is taken at the position, the second extreme value (convergence point) is taken at a position shifted by Δ on the opposite side.

The optical path length from the convergence point to the next convergence point is referred to as beat length, when the length and L _[pi, according to formula 1 of substantially less.

Here, W _e is the width of the effective mode interference waveguide sensed by light, _ng is the effective refractive index, and λ ₀ is the wavelength of the input light. Since the beat length affects each wavelength by a reciprocal number, when the wavelength is different to the half of the other wavelength as in the present embodiment, the light of one wavelength strikes the beat once while the other Wavelength light hits the beat twice. Specifically, 1.56 μm beats twice, while 0.78 μm beats once.

  In addition, the position where the beat is first hit in the width direction of the mode interference waveguide is a position where the input axis shift position Δ is shifted by Δ to the opposite side across the center line. After that, beats are beaten at positions that are alternately shifted by Δ across the center line.

  As described above, in the mode interference waveguide, a beat is hit at the convergence point at a position determined in the width direction for each wavelength. Therefore, the length of the mode interference waveguide 36 shown in FIG. By setting the length to be the least common multiple of the beat length of the band and providing an output near the point where both converge (the position shifted by the Δ axis in the width direction), the light of 0.78 μm and the light of 1.56 μm are obtained. Combined output is possible. If this characteristic is used in reverse, 0.78 μm light and 1.56 μm light can be demultiplexed into two optical waveguides. That is, if a waveguide for inputting light of two wavelength bands is provided on the output side of FIG. 6A and the input waveguides 37 and 38 are used as output waveguides, the light of the two wavelength bands is converted into two light guides. It can be demultiplexed into the waveguide and output.

  In the simulation result shown in FIG. 6, the axial shift amount of the output-side waveguide is the same as the axial shift amount of the input-side waveguide. This is because the two input waveguides are installed at positions at which the width Wm = 30 μm of the mode interference waveguide 33 is equally divided, and the convergence point converges on the extension of the input waveguide. In general, the position of the convergence point (output waveguide axis shift amount) depends on the position of the input waveguide (shift amount of the input waveguide). In addition, the number of convergence points is not limited to one, and when an output waveguide is provided at a position having a plurality of convergence points, the amount of axial deviation of the output waveguide to be installed differs depending on which convergence point is used. Accordingly, the installation position (axis shift amount) of the output waveguide is determined in accordance with the desired convergence position in consideration of conditions such as the wavelength, the position of the input waveguide, the number of multiplexed / demultiplexed waves, and the like. In this way, the mode interference waveguide 33 can easily transmit light having a wavelength of 1.56 μm (first wavelength band) and light having a wavelength of 0.78 μm (second wavelength band) with a simple configuration including only the waveguide. Can be combined / demultiplexed.

  Further, in the wavelength conversion device of the present invention, the two output waveguides provided on the output side of the mode interference waveguide 33 are subjected to different end face treatments, so that the first wavelength band (1. The return light in the 5 μm band) is prevented from being input again to the nonlinear optical medium.

  This end face processing will be described with reference to FIG. In the wavelength conversion device of this embodiment, the two output waveguides 112 and 113 connected to the mode interference waveguide are formed in different shapes. Specifically, the output waveguide 113 for introducing light in the 0.78 μm band is formed in a straight line, and the output waveguide 112 for introducing light in the 1.5 μm band is formed to have a curved portion. Yes. One end face 111 common to the output end portions of the two output waveguides is determined, and the output waveguides 112 and 113 are cut along the end face 111 to perform end face processing. The position of the end face 111 is such that the output waveguide 112 that introduces light in the 1.5 μm band is inclined with respect to the end face, and the output waveguide 113 that introduces light in the 0.78 μm band is perpendicular to the end face 111. The end face is processed by adjusting the shape of the output end at the position. As a result, the end face 111 of the output waveguide for introducing the 1.5 μm band light can be processed into a shape having an angle of 6 °.

  After the end face processing of the output ends of the output waveguides 112 and 113 is performed, the end face 111 is antireflective (AR) for light in the 1.5 μm band and reflected for light in the 0.78 μm band ( HR) was deposited using an ion-assisted sputtering apparatus. When the characteristics of the antireflection (AR) film were evaluated for light in the 1.5 μm band, the reflectance was 0.5%.

  Further, the end face 114 on the input side of the wavelength conversion waveguide 115 is also processed so that the end of the wavelength conversion waveguide 115 has an angle of 6 °, so that the light in the 1.5 μm band and the 0.78 μm band An optical laminated film 117 was formed so as to be antireflective (AR) with respect to light.

Next, a method for manufacturing a wavelength conversion device will be described. First, based on FIG. 7, the process of producing the wavelength conversion waveguide shown in FIG. 3 will be described. In the first embodiment, the first substrate 11 that is a nonlinear optical medium is a Z-cut Zn-added LN substrate. First, as shown in FIG. 7A, two substrates 11 and 12 are prepared. The first substrate 11 has a periodically poled structure so that the phase matching condition is satisfied in the 1.5 μm band in advance. A Z-cut LiTaO ₃ substrate is used as the second substrate 12. As the nonlinear optical medium, in addition to LN, KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn, Sc A material containing at least one selected from the group consisting of In as an additive can be used.

  The first substrate 11 and the second substrate 12 have substantially the same thermal expansion coefficient. Further, the refractive index of the second substrate 12 is smaller than the refractive index of the first substrate 11. Each of the first and second substrates 11 and 12 is a 3-inch wafer having both surfaces optically polished. The thickness of the first substrate 11 is 300 μm, and the thickness of the second substrate 12 is 500 μm.

  Next, after the surfaces of the prepared first and second substrates 11 and 12 are made hydrophilic by ordinary acid cleaning or alkali cleaning, these two substrates are superposed in a clean atmosphere in which microparticles do not exist as much as possible. (FIG. 7 (b)). Then, the superposed first and second substrates 11 and 12 are put in an electric furnace and subjected to diffusion bonding by heat treatment at 400 ° C. for 3 hours. The bonded substrate is free from microparticles and the like on the bonding surface, is void free, and does not crack when returned to room temperature.

  Next, using a polishing apparatus in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the first substrate 11 of the bonded substrates reaches 20 μm (FIG. 7C). A polishing surface having a mirror surface can be obtained by performing a polishing process after the polishing process. When the parallelism of the substrate (difference between the maximum height and the minimum height) was measured using an optical parallelism measuring instrument, submicron parallelism was obtained almost entirely except for the periphery of a 3-inch wafer. A thin film substrate 13 suitable for manufacturing a conversion element can be manufactured. Since this thin film substrate 13 was prepared by directly bonding the first substrate 11 and the second substrate 12 by diffusion bonding by heat treatment without using an adhesive, the film has a uniform composition and film over the entire area of the 3-inch wafer. Have a thickness.

  Thereafter, a wavelength conversion waveguide is manufactured by using a dry etching process as an optical waveguide manufacturing means. Of the thin film substrate 13, a waveguide pattern is formed on the surface of the first substrate 11 by a normal photolithography process. Thereafter, the substrate is set in a dry etching apparatus, and the surface of the first substrate 11 of the thin film substrate 13 is etched using Ar gas as an etching gas to produce a ridge type optical waveguide (see FIG. 10 described later).

  FIG. 8 shows the dimensions of a wavelength conversion device according to an embodiment of the present invention. The wavelength conversion waveguide 64 is previously provided with a polarization inversion structure, and its length is 45 mm. A mode interference waveguide 63 is coupled to one end of the wavelength conversion waveguide 64. The width 63 of the mode interference waveguide is 30 μm, and the optical path length of the mode interference waveguide 63 is 3.5 mm. It is coupled to the mode interference waveguide 63 where the input / output waveguide spacing is 5 μm. The waveguide widths of the input / output waveguide 61 for excitation light and signal light in the wavelength 1.5 μm band and the input / output waveguide 62 for excitation light in the wavelength 0.78 μm are 5 μm. The input / output waveguide 62 for excitation light having a wavelength of 0.78 μm is a straight waveguide, and the input / output waveguide 61 for excitation light and signal light having a wavelength of 1.5 μm has a gentle curve with a curvature of 3 mm. It is away from the input / output waveguide 62 for excitation light having a wavelength of 0.78 μm. The input / output waveguide 62 for excitation light having a wavelength of 0.78 μm has an end face so as to intersect perpendicularly to the end face. On the other hand, the pumping light and signal light input / output waveguides 61 having a wavelength of 1.5 μm are installed so as to obliquely intersect the end face, and the angle is 6 °. The end face on the side having the two input / output waveguides 61 and 62 is coated with an optical film for reflecting light having a wavelength of 0.78 μm and preventing reflection of light having a wavelength of 1.5 μm. The actual end face structure and manufacturing method will be described later with reference to FIG.

  FIG. 9 shows an input waveguide of the wavelength conversion device. Since the ridge type optical waveguide 11 having a height of 5 μm and a waveguide width of about 5 μm, particularly the characteristics of the multimode interference waveguide, the waveguide width has a great influence as can be seen from Equation 1, the thickness of the first substrate 11 It is desirable to etch deeper and completely remove the first substrate material on both sides of the ridge waveguide. However, in this case, since the bonding surface with the second substrate 12 becomes extremely thin, a sufficient bonding strength to withstand it is required. The direct bonding method in this embodiment does not cause peeling even in a structure in which the first substrate 11 and the second substrate 12 are bonded only by the surface 13 directly under the waveguide, and sufficient bonding strength is obtained. As a result, a structure in which both sides of the ridge waveguide are completely dropped to the second substrate 12 as shown in FIG.

FIG. 10 shows a method for manufacturing a wavelength conversion device. FIG. 10A shows a first substrate 11 (Z-cut Zn-added LN substrate) and a second substrate 12 (Z-cut LiTaO ₃ ) formed by the method shown in FIG. 7 and having a periodically poled structure. The thin film substrate 13 is bonded to the substrate substrate. As shown in the figure, the first substrate 11 is formed with a portion where the periodic domain-inverted structure is formed and a portion where it is not formed.

  Patterns of the input waveguides 15 and 16, the mode interference waveguide 17 and the wavelength conversion waveguide 18 are formed on the surface of the first substrate 11 by a normal photolithography process. The input waveguides 15 and 16 and the mode interference waveguide 17 are formed in a portion where the periodic polarization inversion structure is not formed, and the wavelength conversion waveguide 18 is manufactured in a portion where the periodic polarization inversion structure is formed, and 3 inches. A plurality of wafers are produced in parallel with the thin film substrate 13 which is a wafer. Thereafter, the substrate is set in a dry etching apparatus, and the surface of the first substrate 11 of the thin film substrate 13 is etched using Ar gas as an etching gas, thereby producing a plurality of wavelength conversion devices (FIG. 10B).

  The thin film substrate 13 is cut into a strip shape for each of these wavelength conversion devices, and the wavelength conversion device having a length of 51 mm is cut out by optically polishing the end surfaces 14a of the input optical waveguides 15 and 16 and the end surface 14b of the wavelength conversion waveguide 18. Thus, the wavelength conversion device before the end face processing can be obtained (FIG. 10C). When the device shown in FIG. 10C is subjected to end face processing as described in FIG. 11, a wavelength conversion device used in the present invention can be obtained.

  Here, in order to evaluate the characteristics of the mode interference waveguide integrated in the wavelength conversion device, the branching ratio was measured. Hereinafter, a description will be given with reference to FIG. The branching ratio is a ratio of demultiplexing of light output to the input waveguide ports 61 and 62 when light is input from the wavelength conversion waveguide 64 to the mode interference waveguide. The smaller the branching ratio value, the better the characteristics of the multiplexer (demultiplexer). The light of 1.56 μm is input from the wavelength conversion waveguide 64, and the ratio of the optical power output from the second input waveguide 62 to the sum of the optical power output to the input waveguides 61 and 62 is branched. Define ratio. At this time, the value of the branching ratio was sufficiently small as 2%.

  Similarly, 0.78 μm excitation light is input from the wavelength conversion waveguide 64, and the optical power output from the first input waveguide 61 with respect to the sum of the optical power output to the input waveguides 61 and 62. Is defined as the branching ratio. At this time, the value of the branching ratio is sufficiently small as 2%, and it can be confirmed that a good multiplexer / demultiplexer can be produced.

  Subsequently, using an optical fiber array in which optical fiber core wires are arranged at intervals of 127 μm, 1.56 μm signal light is excited in the first input waveguide 61 and 0.78 μm is excited in the second input waveguide 62. Incident light. The two optical fiber core wires used in the optical fiber array have different mode diameters, and optical fiber core wires that are single mode at 1.56 μm and 0.78 μm are used.

  When the excess optical loss due to the mode interference waveguide 63 functioning as a multiplexer / demultiplexer was evaluated, the light was multiplexed with a very small loss of 0.5 dB for 1.56 μm light and 1.0 dB for 0.78 μm light. It was. The wavelength dependence of the excess optical loss due to the mode interference waveguide 63 was measured by using a wavelength variable light source as a light source of 1.56 μm band. Compared to the peak output light amount, the wavelength range in which the additional excess loss is within 1 dB is as wide as about 40 nm.

  Next, in order to obtain characteristics as a wavelength conversion device, signal light in the 1.56 μm band was input from the first input waveguide 61, and the efficiency of wavelength conversion from the second harmonic generation was evaluated. The normalized conversion efficiency was as high as 1300% / W at a wavelength of 1555.4 nm.

  Further, it is preferable that a drain waveguide 65 is provided in the mode interference waveguide 63 of the wavelength conversion device. The 1.5 μm band light that has been introduced and reflected by the wavelength conversion waveguide 64 is emitted from the drain waveguide 65, so that unwanted return light of 1.5 μm band does not enter the wavelength conversion waveguide 64. It is like that. The drain waveguide 65 is shifted from the center by the same shift amount as the output waveguide 62 in which the 0.78 μm band is conducted so that the 1.5 μm band light introduced from the output waveguide 62 to the mode interference waveguide 63 can be output. The axis is misaligned.

  Next, a wavelength conversion apparatus using the wavelength conversion device will be described. The wavelength converter according to the present embodiment realizes an SHG-DFG multistage method in which second harmonic generation (SHG) and difference frequency generation (DFG) are performed in multiple stages. Specifically, after generating 0.78 μm band light by SHG process using 1.5 μm band excitation light, it is generated by further inputting 1.5 μm band signal light having a wavelength different from the excitation light. It is a device that obtains light of a desired wavelength by causing light and a DFG process in the .78 μm band.

  As shown in FIG. 12, the wavelength conversion apparatus of this embodiment is provided with a semiconductor laser Y1, an erbium-doped optical fiber amplifier (EDFA) Y2, and an optical circulator Y3 on the input side of the created wavelength conversion device 10, thereby converting the wavelength. A plurality of signal light generating means (not shown), an optical coupler Y11, and an optical circulator Y12 are provided on the output side of the device 10.

  Using the wavelength converter shown in FIG. 12, a wavelength conversion experiment using the SHG-DFG multistage method in which second harmonic generation (SHG) and difference frequency generation (DFG) are performed independently in multiple stages was attempted.

  As pumping light, 1.56 μm light emitted from the external cavity type semiconductor laser Y1 is amplified by an erbium-doped optical fiber amplifier (EDFA) Y2, passed through the optical circulator Y3, and then incident on the wavelength conversion waveguide Y4. More incident. The second harmonic generation in the wavelength conversion waveguide Y4 generates light having a wavelength of 0.78 μm, which is half of 1.56 μm. The 1.56 μm light and the 0.78 μm light are transmitted through the wavelength conversion waveguide Y4 and then incident on the mode interference waveguide Y5. The 1.56 μm light and the 0.78 μm light are separated by the interference in the mode interference waveguide Y5, the 0.78 μm light is introduced into the output waveguide Y6, and the 1.56 μm light is introduced into the output waveguide Y7. be introduced.

  The 0.78 μm light introduced into the output waveguide Y6 is reflected by the reflection film Y8 formed on the output side end face, passes back through the output waveguide Y6 and the mode interference waveguide Y5, and enters the wavelength conversion waveguide Y4. The Since the reflectance of the reflection film is as high as 99%, the loss of light due to folding is dominated by the loss when reciprocating in the mode interference waveguide Y5, but the excess optical loss due to the mode interference waveguide Y5 is very high. Since the loss is small, the light can be folded back and forth with a very small loss of 2.0 dB.

  On the other hand, the 1.56 μm light introduced into the output waveguide Y7 is output from the wavelength conversion device 10, branched by the optical circulator Y12, and output. At this time, reflection of light of 1.56 μm is suppressed by a synergistic effect of the antireflection film Y8 formed on the output side end face of the output waveguide Y7 and the end face processing of 6 ° cut. Although there is a component in which the light reflected without being completely suppressed is reflected by the output waveguide Y7 and the mode interference waveguide Y5 and enters the wavelength conversion waveguide Y4, the antireflection film Y8 and the oblique end face Due to the synergistic effect of processing, the amount of reflected return light is very small, -50 dB with respect to the amount of incident light. Further, since the antireflection effect by the oblique end face processing has little wavelength dependency, an antireflection effect of −45 dB or more was obtained over a wide band of 100 nm or more.

  Further, since the branching ratio of the mode interference waveguide Y5 is 2%, the amount of light of about 2% of the 1.56 μm light incident on the mode interference waveguide Y5 after passing through the wavelength conversion waveguide Y4 is about 2%. Leakage light is introduced into the output waveguide Y6 instead of the output waveguide Y8 that should be originally introduced. Since the end face of the output waveguide Y6 is vertical, there is return light that could not be suppressed by the antireflection film Y8 alone. The return light of this leaked light is transmitted again through the mode interference waveguide Y5 and guided to the drain. Return light can be prevented by flowing into the waveguide Y9. That is, most of the return light of the leaked light passes through the mode interference waveguide Y7 and is then introduced into the drain waveguide Y9 and discharged out of the device. As a result, the demultiplexing effect (about −17 dB) by the first incidence on the mode interference waveguide Y5, the reflection suppression effect (about −23 dB) by the antireflection film Y8, and the drain after being reflected back to the mode interference waveguide Y5. Due to the demultiplexing effect (about −17 dB) by the waveguide Y9, the reflected return light amount can be suppressed to a very small amount of −57 dB with respect to the incident light amount.

  Further, an 8-wave C-band signal light group arranged at 100 GHz intervals with a wavelength of 1.54 μm as the center is generated, combined by the optical coupler Y11, passed through the optical circulator Y12, and then output from the output end face side It entered the waveguide Y7. The signal light group input to the output waveguide Y7 passes through the mode interference waveguide Y5 and then enters the wavelength conversion waveguide Y4. At this time, 0.78 μm light reflected by the optical film Y8 on the end face of the output waveguide Y6 and again passing through the output waveguide Y6 and the mode waveguide Y5 is also introduced into the wavelength conversion waveguide Y4. When these lights are incident, 8 waves arranged at intervals of 100 GHz with a wavelength of 1.58 μm as the center are generated by the difference frequency generation (DFG) between 0.78 μm light and the signal light group in the wavelength conversion waveguide Y4. L-band wavelength conversion signal light group is generated. The generated wavelength conversion signal group is output from the input side of the wavelength conversion device 10 and is output after passing through the optical circulator Y3.

  As described above, according to the wavelength conversion device of the wavelength conversion device of the present embodiment, the amount of return light in the 1.56 μm band used in the second harmonic generation process can be suppressed to a very small amount. Can be a process independent of the second harmonic generation process.

  Further, since the mode interference waveguide Y5 performs optical multiplexing by inter-mode interference, when the input light propagates in a mode other than the predetermined mode, the loss of the mode interference waveguide Y5 increases. This is because the mode interference waveguide Y5 has a function of multiplexing and plays a role of a mode filter. Therefore, according to the present embodiment, light may be input so as to maximize the transmitted light of the excitation light and the signal light without performing special adjustment such as monitoring the power of the converted light. Thereby, the optimal incident condition to the wavelength conversion device is obtained.

  When 1 W pumping light was input, the converted light was converted with gain to the input signal light by the parametric gain. In addition to the high conversion efficiency of the wavelength conversion device, the entire wavelength conversion device is a direct-junction ridge waveguide, which is good without causing optical damage such as a photorefractive effect for high-power inputs. This is due to the fact that excellent wavelength conversion characteristics are obtained. In addition, unlike cascade excitation, it is not necessary to use strong excitation light in the 1.56 μm band, so that it is possible to obtain high-quality converted light with little influence of ASE noise and an SNR of 40 dB or more. Furthermore, the wavelength of the signal light was brought close to 1.56 μm, and an attempt was made to perform wavelength conversion in the vicinity. Unlike cascade excitation, strong excitation light in the 1.56 μm band is suppressed, so that even if the difference between signal light and converted light is close wavelength conversion of 50 GHz, high SNR can be obtained. did it.

(Second Embodiment)
SFG-DFG Multistage Independent Configuration FIG. 13 shows the configuration of a wavelength converter according to the second embodiment of the present invention. The wavelength converter according to the present embodiment realizes an SFG-DFG multistage method in which sum frequency generation (SFG) and difference frequency generation (DFG) are independently performed in multiple stages. The same wavelength conversion device as that in the first embodiment can be used.

The wavelength conversion apparatus of the present embodiment includes an excitation light source W1, an erbium-doped optical fiber amplifier (EDFA) W2, an optical coupler W3, and an optical circulator W4 on the input side of the optical conversion device 10, and An optical circulator W14, an erbium-doped optical fiber amplifier (EDFA) W13, and a control light source W12 are provided on the output side.
Using the wavelength converter shown in FIG. 13, a wavelength conversion experiment by the SFG-DFG multistage method in which sum frequency generation (SFG) and difference frequency generation (DFG) are performed in multiple stages was attempted. The 1.57 μm pumping light output from the pumping light source W1 and amplified by the erbium-doped optical fiber amplifier (EDFA) W2 and the 1.55 μm signal light generated by the signal light source (not shown) are combined by the optical coupler W3, and then the light After passing through the circulator W4, the light enters from the incident side of the wavelength conversion waveguide W5. The signal light of 1.55 μm is intensity-modulated at 40 Gb / s. By generating the sum frequency in the wavelength conversion waveguide W5, light having a wavelength of 0.78 μm having a frequency of the sum of 1.55 μm and 1.57 μm is generated. At this time, a modulation signal of signal light is superimposed on 0.78 μm light.

  The 1.55 μm signal light, the 1.57 μm excitation light, and the 0.78 μm light pass through the wavelength conversion waveguide W5 and then enter the mode interference waveguide W6. The light is split by the interference in the mode interference waveguide W6, 0.78 μm light is introduced into the output waveguide W7, 1.55 μm signal light and 1.57 μm excitation light are introduced into the output waveguide W8.

  The light of 0.78 μm introduced into the output waveguide W7 is reflected by the reflection film formed on the output side end face, passes back through the output waveguide W7 and the mode interference waveguide W6, and enters the wavelength conversion waveguide W5. . Since the reflectance of the reflection film is as high as 99%, the loss of light due to folding is dominated by the loss when reciprocating the mode interference waveguide W6, but the excess optical loss due to the mode interference waveguide W6 is very high. Since the loss is small, the light can be folded back and forth with a very small loss of 2.0 dB.

  The reflection of the 1.55 μm signal light and the 1.57 μm excitation light introduced into the output waveguide W8 is suppressed by the synergistic effect of the antireflection film W9 formed on the output side end face and the end face processing of 6 ° cut. The light that has been reflected without suppressing the reflection is folded back through the output waveguide W8 and the mode interference waveguide W6 and is incident on the wavelength conversion waveguide W5, but due to the synergistic effect of the antireflection film and the oblique end surface processing. The amount of reflected return light was as very small as −50 dB with respect to the amount of incident light. Further, since the antireflection effect by the oblique end face processing has little wavelength dependency, an antireflection effect of −45 dB or more was obtained over a wide band of 100 nm or more.

  Further, since the branching ratio of the mode interference waveguide W6 is 2%, the light quantity of about 2% of the 1.55 μm band light introduced into the mode interference waveguide W6 after passing through the wavelength conversion waveguide W5. In this case, leakage light is introduced into the output waveguide W7, not the output waveguide W8 to be originally introduced. Since the end face of the output waveguide W7 is vertical, there may be return light that could not be suppressed by the antireflection film W9 alone. The return light of this leaked light is transmitted again through the mode interference waveguide W6. The return light can be prevented by flowing into the drain waveguide W10. That is, most of the return light of the leaked light passes through the mode interference waveguide Y7 and is then introduced into the drain waveguide Y9 and discharged out of the device. As a result, the demultiplexing effect (about −17 dB) by the first incidence on the mode interference waveguide Y5, the reflection suppression effect (about −23 dB) by the antireflection film Y8, and the drain after being reflected back to the mode interference waveguide Y5. Due to the demultiplexing effect (about −17 dB) by the waveguide Y9, the reflected return light amount can be suppressed to a very small amount of −57 dB with respect to the incident light amount.

  Furthermore, the control light having a wavelength of 1.58 μm generated by the control light source W12 was allowed to enter the output waveguide W8 from the output end face side after passing through the erbium-doped optical fiber amplifier (EDFA) W13 and the optical circulator W14. The control light passes through the mode interference waveguide W6 and then enters the wavelength conversion waveguide W5. At this time, 0.78 μm light reflected by the optical film W9 on the end face of the output waveguide W7 and again passing through the output waveguide W7 and the mode waveguide W6 is also introduced into the wavelength conversion waveguide W5. When these lights are incident, converted light having a wavelength of 1.54 μm can be obtained by generating a difference frequency from 0.78 μm light in the wavelength conversion waveguide W5. At this time, the modulation signal was again superimposed on the converted light from the 0.78 μm light, and as a result, the signal light having a wavelength of 1.55 μm was converted into a converted light having a wavelength of 1.54 μm. ASE light and 1.55 μm signal light and 1.57 μm excitation light return light are sufficiently suppressed, so SNR degradation due to ASE noise, and sum frequency generation of control light and signal light / excitation light Therefore, high-quality wavelength conversion can be realized.

  Thus, according to the wavelength conversion device of the wavelength conversion device of the present embodiment, the amount of return light in the 1.57 μm band and 1.55 μm band used in the sum frequency generation process can be suppressed to a very small amount. The frequency generation process can be a process independent of the sum frequency generation process.

  Moreover, by changing the wavelength of the control light from 1.62 μm to 1.50 μm, the wavelength of the converted light could be changed from 1.50 μm to 1.62 μm over a wide band of 120 nm or more. In the conventional SFG-DFG cascade excitation method, since the sum frequency generation (SFG) process and the difference frequency generation (DFG) process occur simultaneously, the wavelength of the excitation light, the signal light, and the sum frequency generation light is the same or close to each other. Since it was difficult to use for control light, the variable range of converted light was limited. However, in this method, since the sum frequency generation (SFG) process and the difference frequency generation (DFG) process can be controlled independently, it is possible to bring the control light to an arbitrary wavelength. A variable range was obtained.

(Third embodiment)
SHG-DFG Multistage Independent Array Configuration FIG. 14 shows the configuration of a wavelength converter according to the third embodiment of the present invention. The wavelength conversion apparatus of this embodiment arranges a plurality of wavelength conversion devices having the same configuration as the wavelength conversion device of the first embodiment in parallel, and each wavelength conversion device uses second harmonic generation (SHG) and difference frequency generation (DFG). The SHG-DFG multi-stage method is performed by an array. In the present embodiment, the wavelength conversion device 20 may be configured by arranging a plurality of the same configurations as those in the first embodiment in parallel.

  For example, as shown in FIG. 14, the wavelength conversion device 10 according to the first embodiment is manufactured by arranging four wavelength conversion waveguides Q14, Q24, Q34, and Q44 in parallel so that the wavelength conversion waveguides Q14, Q24, Q34, and Q44 have an interval of 127 μm. be able to. In the first embodiment, the periodic polarization reversal structure has a polarization reversal period set so as to achieve phase matching with the second harmonic generation of 1.56 μm. However, in the wavelength conversion device 20 of the present embodiment, the periodic polarization reversal structure is parallel. The periodical polarization inversion periods on the four wavelength conversion waveguides arranged in the above are set so that phase matching can be obtained for the second harmonic generation of 1.560 μm, 1.561 μm, 1.562 μm, and 1.563 μm, respectively.

  The wavelength conversion apparatus of this embodiment corresponds to the input of the wavelength conversion device 20, and includes excitation light sources Q11, Q21, Q31, Q41, erbium-doped optical fiber amplifiers (EDFA) Q12, Q22, Q32, Q42, and an optical circulator. Three devices Q13, Q23, Q33, and Q43 are arranged, and two signal light sources (not shown) and two optical circulators Q101, Q102, Q103, and Q104 corresponding to the output of the wavelength conversion device 20 are arranged. Devices were arranged and provided.

  Using the wavelength converter shown in FIG. 14, a wavelength conversion experiment by the SHG-DFG multistage method in which second harmonic generation (SHG) and difference frequency generation (DFG) are performed in independent multistages was attempted.

  As the pumping light, erbium-doped optical fiber amplifiers emit light having wavelengths of 1.560 μm, 1.561 μm, 1.562 μm, and 1.563 μm emitted from the four external resonator type semiconductor lasers Q11, Q21, Q31, and Q41, respectively. (EDFA) Amplified by Q2, passes through optical circulators Q13, Q23, Q33, and Q43, and then enters from the incident side of wavelength conversion waveguides Q14, Q24, Q34, and Q44. In the wavelength conversion waveguides Q14, Q24, Q34, and Q44, a second harmonic generation (SHG) process is caused to generate 0.78 μm light, which is output to the mode conversion waveguide Q15 connected thereto. The

  On the other hand, signal light having a wavelength of 1.55 μm was incident on the first wavelength conversion waveguide Q14 phase-matched to a wavelength of 1.560 μm from the output waveguide Q17 on the output end face side. The 1.55 μm signal light is phase-modulated with 40 Gb / s DQPSK. The signal light passes through the mode interference waveguide Q15 and then enters the wavelength conversion waveguide Q14. At this time, since 0.78 μm light generated and turned back in the second harmonic generation (SHG) process is also incident on the wavelength conversion waveguide Q14, signal light having a wavelength of 1.55 μm in the wavelength conversion waveguide Q14 Wavelength converted signal light having a wavelength of 1.57 μm is generated by difference frequency generation (DFG) with 0.78 μm light. The generated wavelength conversion signal light having a wavelength of 1.57 μm is output from the wavelength conversion device 20 and is branched and output after passing through the optical circulator Q13.

  Unlike the first embodiment, the wavelength converter of this embodiment uses an optical switch to switch the signal light having the same wavelength of 1.55 μm and select the output waveguides Q17, Q27, Q37, and Q47 to be input. Can do. The signal light with a wavelength of 1.55 μm of this wavelength conversion device was switched and entered into the second wavelength conversion waveguide Q24 that was phase-matched to the wavelength of 1.561 μm. At this time, the wavelength converted signal light having a wavelength of 1.572 μm is generated by the difference frequency generation between the 0.78 μm light generated and folded in the wavelength conversion waveguide Q24 and the signal light having a wavelength of 1.55 μm as in the case of Q14. It is generated and output after passing through the optical circulator Q23. Similarly, when the wavelength conversion waveguide to which signal light is input is switched to Q34 and Q44 by the optical switch, the wavelength converted signal light having wavelengths of 1.574 μm and 1.576 μm can be taken out by the optical circulators Q33 and Q43, respectively.

  As described above, according to the wavelength conversion device of the present embodiment, in addition to the effects of the first embodiment, the wavelength conversion device that can switch the wavelength conversion destination and can perform high-quality wavelength conversion is provided. It can be realized at a low price by integrating into one element.

(Fourth embodiment)
SHG-SFG Multistage Independent Configuration FIG. 15 shows the configuration of a wavelength conversion device according to the fourth embodiment of the present invention. The wavelength conversion apparatus of this embodiment is a SHG-SFG multistage in which second harmonic generation (SHG) and sum frequency generation (SFG) are independently performed in multiple stages using a wavelength conversion device having the same configuration as the wavelength conversion device of the first embodiment. It is to realize the law. The wavelength conversion device having the same configuration as that of the first embodiment can be used. However, in the first embodiment, the periodic polarization inversion structure sets the polarization inversion period so that phase matching can be achieved for the second harmonic generation of 1.56 μm. Both the domain-inverted periodic structure QPM1 capable of phase matching for 56 μm second harmonic generation and the domain-inverted periodic structure QPM2 capable of phase matching for sum frequency generation of wavelengths 1.56 μm and 0.78 μm were produced in series. Yes.

  Using the wavelength converter shown in FIG. 15, a wavelength conversion experiment by the SHG-SFG multistage method in which second harmonic generation (SHG) and sum frequency generation (SFG) are performed in multiple stages was attempted.

  As pumping light, 1.56 μm light emitted from the external cavity type semiconductor laser K1 is amplified by an erbium-doped optical fiber amplifier (EDFA) K2, passed through an optical circulator K3, and then converted into wavelength by a dichroic mirror K4. Incident from the incident side of the waveguide K5. The second harmonic generation (SHG) in the wavelength conversion waveguide K5 generates light having a wavelength of 0.78 μm, which is half of 1.56 μm. The 1.56 μm light and the 0.78 μm light are incident on the mode interference waveguide K6 from the wavelength conversion waveguide K5. Due to the interference in the mode interference waveguide K6, the 1.56 μm light and the 0.78 μm light are separated and introduced into separate output waveguides K8 and K9, respectively.

  The 0.78 μm light introduced into the output waveguide K7 is reflected by the reflective film K9 formed on the output side end face, passes back through the output waveguide K7 and the mode interference waveguide K6, and enters the wavelength conversion waveguide K5 again. Is done. Since the reflectivity of the reflection film K9 is as high as 99%, the loss of light due to folding is dominated by the loss when reciprocating through the mode interference waveguide K6, but the excess optical loss due to the mode interference waveguide K6 is very large. Since the loss is very small, the light can be folded back and forth with a very small loss of 2.0 dB. Further, the 1.56 μm light leaked into the output waveguide K7 and reflected by the reflective film K9 is output from the drain waveguide K10.

  The reflection of 1.56 μm light introduced into the output waveguide K8 is suppressed by the synergistic effect of the antireflection film K9 formed on the output side end face and the end face processing of 6 ° cut. Although there is a component in which the light reflected without being suppressed is reflected by the output waveguide K8 and the mode interference waveguide Y6 and enters the wavelength conversion waveguide K5, the antireflection film and the oblique end surface processing are present. Because of this synergistic effect, the amount of reflected return light was as small as -50 dB with respect to the amount of incident light. Further, since the antireflection effect by the oblique end face processing has little wavelength dependence, an antireflection effect of −45 dB or more was obtained over a wide band of 100 nm or more, and the reflection of ASE light could be sufficiently suppressed.

  Further, second excitation light having a wavelength of 1.56 μm was incident from the output end face side of the output waveguide K8. The second excitation light passes through the mode interference waveguide K6 and then enters the wavelength conversion waveguide K5. Wavelength converted signal light having a wavelength of 0.520 μm is generated by sum frequency generation (SFG) with 0.78 μm light in the wavelength conversion waveguide K5, and is output after passing through the dichroic mirror K4. In the configuration according to the present embodiment, the second excitation light can be amplified and input by another EDFAK 12 independent of the first excitation light, so that the power attenuation when the second harmonic of the first excitation light is generated is reduced. It was possible to compensate, and high power third harmonic generation was possible.

  As described above, according to the wavelength conversion device of the wavelength conversion device of the present embodiment, the amount of return light in the 1.56 μm band used in the second harmonic generation process can be suppressed to a very small amount. Can be a process independent of the second harmonic generation process.

  In the example shown in FIG. 15, the first excitation light and the second excitation light are output from the same light source. However, the second excitation light source may be prepared separately from the first excitation light source. Good. In that case, if the characteristics of the domain-inverted periodic structure that can be phase-matched to the second harmonic generation arranged in series and the domain-inverted periodic structure that can be phase-matched to the sum frequency generation do not match, the light source of the second excitation light By changing the wavelength, the mismatch of characteristics can be compensated and wavelength conversion can be performed with high efficiency.

  In addition, if the inversion period of the polarization inversion periodic structure capable of phase matching for sum frequency generation is appropriately adjusted, light having a wavelength different from that of the first excitation light can be used as the wavelength of the second excitation light. it can. For example, in the fourth embodiment, a domain-inverted periodic structure capable of phase matching for second harmonic generation with a wavelength of 1.56 μm, and domain inversion capable of phase matching for sum frequency generation with a wavelength of 1.57 μm and a wavelength of 0.78 μm. If both periodic structures are manufactured in series, 0.521 μm light can be finally generated. Thus, since each frequency conversion process by a non-linear effect can be caused independently, conversion by a combination of arbitrary wavelengths is easily possible. Also, by making the wavelengths of the first excitation light and the second excitation light intentionally different, inverse frequency conversion, etc., due to the inverse process of each frequency conversion due to nonlinear effects such as second harmonic generation and sum frequency generation This is a very effective effect when performing watt-class high power output.

  As described above, according to the present invention, the reflection function necessary for folding, the high suppression function of unnecessary return light, the multiplexing function of signal light and pumping light, and the mode filter function are combined in the same wavelength conversion device. Since it can be integrated on top, non-linear optical changes of SHG, DFG, and SFG can be performed in multiple stages independently with respect to input light, so mounting is easy, optical loss is low, and resistance to high power input is strong Thus, a device capable of high-quality wavelength conversion can be provided in a small size and at a low price.

10, 20: Wavelength conversion device 30: Substrate 34, 115, Y4, W5, Q14, Q24, Q34, Q44, K5: Wavelength conversion waveguide 33, Y5, W6, Q19, Q29, Q39, Q49, Q49, K6: Mode interference Waveguide 31, 32, 112, 113, Y6, Y7, W7, W8, Q16, Q17, Q26, Q27, Q36, Q37, Q46, Q47, K7, K8: Output waveguide

Claims (11)

A first wavelength which is provided at a nonlinear optical medium, an optical multiplexer / demultiplexer, first and second output waveguides, and output ends of the first and second output waveguides and has a specific wavelength band An optical film that reflects light with respect to light in a band and suppresses reflection of light with respect to light in a second wavelength band that is a wavelength band different from the first wavelength band ;
The nonlinear optical medium generates light of the first wavelength band from light of the second wavelength band input from one end, and outputs the light of the first wavelength band and the light of the second wavelength band. Output from the end,
The optical multiplexer / demultiplexer outputs the light in the first wavelength band output from the nonlinear optical medium to the first output waveguide, and outputs the light in the second wavelength band to the second output. Output to the waveguide,
In the first output waveguide, the output end is vertically end-treated so that light in the first wavelength band is reflected by the optical film, and the first wavelength reflected by the optical film is reflected. The band light is input to the optical multiplexer / demultiplexer,
In the second output waveguide, the output end is obliquely end-treated so that reflection of the light in the second wavelength band on the optical film is suppressed, and the oblique end-face treatment is performed. The signal light is input to the optical multiplexer / demultiplexer from the output end,
The optical multiplexer / demultiplexer inputs the light in the first wavelength band and the signal light to the other end of the nonlinear optical medium . A nonlinear optical medium that outputs any one of the second harmonic light, the difference frequency light, and the sum frequency light by SHG, DFG, and SFG in the nonlinear optical effect with respect to the input light;
A multiplexer / demultiplexer that is connected to the nonlinear optical medium and separates and outputs light of different wavelengths output from the nonlinear optical medium using mode interference;
A first output waveguide connected to one of the two outputs of the multiplexer / demultiplexer;
A second output waveguide connected to the other of the two outputs of the multiplexer / demultiplexer;
An optical film that is provided at an output end of the two output waveguides, suppresses reflection of light in the first wavelength band, and reflects light in the second wavelength band;
The end face is configured so that the output end of the first output waveguide is inclined with respect to the first output waveguide, and the output end of the second output waveguide is the second output guide. By configuring the end face to be perpendicular to the waveguide, it is possible to prevent the return light of the first wavelength band from being input again to the nonlinear optical medium, and to output the first output waveguide. when the signal light from the end portion is inputted, is inputted together with the light in the second wavelength band signal light is reflected by the second output waveguide in said nonlinear optical medium, independent of the said non-linear optical change A wavelength conversion device characterized by causing a nonlinear optical change to output.   The optical path length in the propagation direction of the optical multiplexing unit is set such that at least one of the light of the first wavelength and the light of the second wavelength has an extreme value at the output end face. The wavelength conversion device according to claim 1, wherein the device is a wavelength conversion device.   4. The wavelength conversion device according to claim 1, wherein a width of the optical multiplexing unit is 5 μm or more and 100 μm or less. 5. The nonlinear optical medium may be LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _(x) Ta _(1-x) O ₃ (0 ≦ x ≦ 1), KTiOPO ₄ , or Mg, Zn, Sc, and In The wavelength conversion device according to claim 1, comprising at least one selected from the group consisting of additives.   6. The wavelength conversion device according to claim 5, wherein the nonlinear optical medium is a crystal film grown by a liquid phase epitaxial method.   The non-linear optical medium is a thin film substrate manufactured by bonding a first substrate having a non-linear optical effect and a second substrate having a refractive index smaller than that of the first substrate. Item 7. The wavelength conversion device according to Item 5 or 6.   The wavelength conversion device according to claim 7, wherein the first substrate has a structure in which a nonlinear constant is periodically inverted.   The wavelength conversion device according to claim 7 or 8, wherein the first substrate and the second substrate are directly bonded to each other by diffusion bonding by heat treatment.   A wavelength conversion device, wherein a plurality of wavelength conversion devices according to claim 1 are integrated on the same substrate. The wavelength conversion device according to any one of claims 1 to 10,
A first laser light source unit that inputs the incident light to an input end face of the wavelength conversion device;
A wavelength conversion apparatus comprising: a second laser light source unit that inputs the incident light to an output end face of the wavelength conversion device; and a filter that separates the converted light output from the wavelength conversion device.