JP2011064895A - Wavelength conversion device and wavelength conversion apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact and inexpensive wavelength conversion device which is easily mounted, suppresses power loss of an input light source, and achieves a high power input. <P>SOLUTION: The device includes an optical multiplexing part in which, by the multi-mode propagation of incident light with wavelength λ<SB>1</SB>and λ<SB>2</SB>or of incident light with wavelength λ<SB>1</SB>and λ<SB>3</SB>, the light with the first wavelength (λ<SB>1</SB>) and the light with the second wavelength (λ<SB>2</SB>or λ<SB>3</SB>) are coupled through mode interference inside and in which an output end face is installed in the point where the light with the first wavelength and with the second wavelength converge. From a non-linear optical medium coupled in the convergent point of the output end face, there are outputted a conversion light with the wavelength λ<SB>3</SB>for the incident light with the wavelength λ<SB>1</SB>and λ<SB>2</SB>and a conversion light with the wavelength λ<SB>2</SB>for the incident light with the wavelength λ<SB>1</SB>and λ<SB>3</SB>. The optical multiplexing part and the non-linear optical medium are integrated on the same substrate in the device. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical multiplexer, a wavelength conversion device, and a wavelength conversion apparatus, and more specifically, an optical multiplexer using multi-mode interference (MMI) and a wavelength conversion device in which the optical multiplexer is integrated. The present invention relates to a wavelength conversion device using a wavelength conversion device.

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 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 another wavelength by generation of a difference frequency wave 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 around the wavelength that is twice the wavelength of the excitation light λ ₃ . For example, when the wavelength λ ₁ of the excitation light B is 0.78 μm, the signal light A having the wavelength λ ₁ = 1.54 μm is converted into converted light C that is a difference frequency light having the 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.

  In the conventional method for manufacturing a wavelength conversion element using such quasi-phase matching, a wavelength conversion element is manufactured by manufacturing a proton exchange waveguide after forming a periodically poled structure on a nonlinear optical crystal substrate such as LN. Was making. 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.

  A method of manufacturing a ridge type optical waveguide will be described. First, after a periodically poled structure is fabricated on an Mg-added LN substrate, it is bonded to a separately prepared LN substrate using an adhesive. After the substrate thickness of the Mg-added LN substrate is reduced by surface grinding, a ridge-type waveguide is manufactured by precision grinding using a dicing saw (see, for example, Non-Patent Document 1).

As the pumping light for wavelength conversion using LN using such quasi-phase matching, light having a wavelength twice as large as the pumping wavelength necessary for wavelength conversion in the 1.5 μm band by difference frequency generation is used. There were many things. In the example of the wavelength conversion described with reference to FIG. 1, so that the pumping light of the wavelength λ ₃ '= 1.56μm is used instead of the wavelength lambda ₃ = 0.78 .mu.m. This is a method called cascade excitation. When the wavelength λ ₃ ′ of the excitation light B is 1.56 μm in FIG. 1, the second harmonic (wavelength: 0.78 μm) of the excitation light is generated inside the nonlinear optical medium. To do. By generating a difference frequency between the second harmonic generated in the nonlinear optical medium and the signal light A, the converted light C can be further obtained.

In the method using the 0.78 μm band for the excitation light, a stable light with a wavelength of 0.78 μm, high wavelength accuracy, and a high-output light source are not widely used, and it is difficult to prepare easily. Further, since the wavelengths of the signal light and the excitation light are different by half, the optimum size of the optical waveguide is different. As a result, when light is incident on the waveguide, it is difficult to suppress excitation other than the desired mode. On the other hand, in the cascade excitation method, a stable and highly reliable light source of 1.5 μm band that is widely used can be used as a light source of excitation light. Furthermore, since high output light can be easily obtained by using an optical fiber amplifier or the like, it has been widely used. (For example, see Non-Patent Document 2)
However, wavelength conversion by cascade excitation has a problem that the quality of the converted light tends to deteriorate. Since the cascade excitation method uses a 1.5 μm band light source for the excitation light, it becomes difficult to separate the excitation light and the signal light / converted light having a wavelength close to each other. It was necessary to provide a certain band between the two. By securing this band, there is a problem that the wavelength conversion band that can be used is narrowed, and the number of wavelengths that can be collectively converted is reduced.

Further, when the guard band is provided, there is a problem that conversion to a wavelength close to the signal light becomes impossible. Furthermore, when an optical fiber amplifier is used to obtain high excitation light, there is a problem that the quality of signal light / converted light deteriorates due to an increase in ASE noise. Further, in the cascade excitation, there is a problem that the crosstalk light due to the sum frequency generation between the excitation light and the signal light is increased, and the signal quality is deteriorated. (For example, see Non-Patent Document 3)
Therefore, wavelength conversion by excitation of 0.78 μm band light is desired. However, this wavelength conversion method requires a technique in which the input of signal light / excitation light is low loss and can be easily performed without exciting modes other than the desired mode.

  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.

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.H.Chou et al., “Optical Signal processing and Switching with Second-Order Nonlinearities in Waveguides,” IEICE Trans. Electorn., E83-C, 2000, p.869-874 J. Yamawaku et al. “Low-Crosstalk 103 Channel × 10 Gb / s (1.03 Tb / s) Wavelength Conversion With a Quasi-Phase-Matched LiNbO3 Waveguide,” IEEE J. Select. Topics Quantum Electron., Vol.12, No.4, 2006, p.521-528

  However, it is difficult to realize a laser with a wavelength of 0.5 to 0.6 μm such as green, yellow-green, and orange using a semiconductor, and a laser light source device that combines a highly efficient nonlinear optical medium and a semiconductor laser has been developed. Yes.

  FIG. 2 shows the configuration of a conventional green light source. The laser light source includes two excitation lasers 20 and 21, an LN nonlinear optical crystal 24 whose polarization is periodically inverted, a multiplexer 23 that combines the laser beams output from the excitation lasers 20 and 21, and multiplexing. And a filter 25 that separates the laser light of the excitation laser that has passed through the LN nonlinear optical crystal 24 and the sum frequency light generated by the LN nonlinear optical crystal 24.

The wavelength lambda ₁ of the excitation laser 20, the wavelength lambda ₂ of the excitation laser 21,
1 / λ ₁ + 1 / λ ₂ = 1 / λ ₃
Is a combination that satisfies For example, by setting the wavelength λ ₁ = 980 nm of the excitation laser 20 and the wavelength λ ₂ = 1300 nm of the excitation laser 21, converted light having a wavelength λ ₃ = 559 nm can be obtained. A highly efficient and highly stable wavelength conversion light source can be realized by combining a semiconductor laser and a wavelength conversion element.

  Normally, an optical fiber type directional coupler is used for the multiplexer 23. However, the method of combining input light using a conventional multiplexer and inputting the input light to the wavelength conversion element has a problem that it is difficult to reduce the size of the wavelength conversion device (wavelength conversion light source). The multiplexer has a size of about several centimeters as a single unit, and it is necessary to secure a region for handling the input / output fiber of the optical fiber type directional coupler, which is a major obstacle to miniaturization.

  Next, in the case of combining light having significantly different wavelengths, there is a problem that the loss of at least one wavelength of light is increased in the output fiber of the multiplexer. This is because the optimum core size of the optical fiber differs depending on the wavelength. For example, consider a case where green light of 560 nm is extracted by wavelength conversion using the sum frequency generation of two lights of 1300 nm and 980 nm. The core diameter of the 1300 nm single mode optical fiber is about 9 μm, whereas the core diameter of the 980 nm single mode optical fiber is different from about 6 μm. A single mode optical fiber of each wavelength can be used as the input fiber of the multiplexer, but an optical fiber with a core diameter intermediate between the core diameters of either one or both fibers can be used as the output fiber after multiplexing. Will be used. As a result, when either one of the single mode optical fibers is selected, the loss during propagation of the other light increases. When an optical fiber having an intermediate core diameter is selected, propagation loss increases for both wavelengths of light.

  As described above, a single mode optical fiber cannot be used for an output fiber of a normal multiplexer, that is, an input fiber to a nonlinear optical crystal for light of at least one wavelength. For this reason, there has been a problem that a desired mode in wavelength conversion cannot be excited and wavelength conversion efficiency is lowered.

  The wavelength conversion element is designed so that conversion is performed in a specific mode (usually a fundamental mode). Light that has propagated through an optical fiber that is not a single mode may undergo mode conversion depending on the handling of the optical fiber, and may propagate through the optical fiber in another mode. When light of a specific mode cannot be incident on the wavelength conversion element, the wavelength conversion efficiency is significantly reduced. Also, even for light propagating in a single mode, the efficiency other than the desired mode is excited depending on the incident condition to the waveguide.

  In order to suppress the excitation of other modes in order to suppress the decrease in conversion efficiency, first, increase the curvature of the output side fiber, and then adjust the alignment while monitoring not only the excitation light but also the converted light at the time of input. There was a problem that complicated adjustments such as to do would be necessary. Here, the example of input light of 1300 nm and 980 nm has been described. However, in the above-described wavelength conversion for communication, the input light has a larger wavelength difference between 1560 nm and 780 nm, and the difference in diameter of the single mode optical fiber. The problem becomes significant.

  Wavelength converters (wavelength-converted light sources) are widely used in medical, bio, and environmental measurement, and there is a great need for industrial applications, and there are great expectations for miniaturization. In the case of using a conventional multiplexer so far, there is a problem that it is difficult to reduce the cost because the cost of the multiplexer and the cost of mounting are required. In addition to measurement applications, applications to devices that require high power output such as laser displays are also being considered. However, since a normal directional coupler type multiplexer uses optical fiber fusion, there is a problem that it is difficult to input a watt-class high power.

  In recent years, an optical access visualization technique using wavelength conversion in an optical communication network has been proposed. This technology converts the light in the communication band into visible light that can be seen by humans, thereby improving maintenance and operation work efficiency in optical access networks such as opening tests, service type confirmation, and failure cause isolation, thereby reducing costs. It is expected. However, the configuration of the conventional wavelength conversion element described above has a problem that it is difficult to reduce the cost of the device.

  The object of the present invention is a compact, low-cost wavelength conversion that does not require complicated adjustments such as a power monitor for converted light, is easy to mount, suppresses power loss of the input light source, and allows high power input. It is to provide a device and a wavelength converter. In particular, high-quality wavelength conversion is possible in the conversion of light in the communication wavelength band.

In order to achieve such an object, according to the present invention, among wavelengths having a relationship of 1 / λ ₃ = 1 / λ ₂ + 1 / λ ₁ , _two wavelengths λ ₁ and λ ₂ or wavelengths λ ₁ and λ ₃ are used. A wavelength conversion device that inputs two incident lights to a nonlinear optical medium and outputs converted light of wavelength λ ₃ or λ ₂ respectively, and incident light of wavelengths λ ₁ and λ ₂ or incident light of wavelengths λ ₁ and λ ₃ Is propagated in multimode, the light of the _first wavelength (λ ₁ ) and the light of the second wavelength (λ ₂ or λ ₃ ) are coupled by the mode interference inside, and the light of the first wavelength And an optical combining part provided with an output end face at a point where the second wavelength converges, and incident light having wavelengths λ ₁ and λ ₂ from a nonlinear optical medium coupled to the converging point on the output end face to the converted light having a wavelength lambda ₃ are converted light having a wavelength lambda ₂ to the incident light of wavelength lambda ₁ and lambda ₃ is outputted, the said optical multiplexing unit nonlinear Characterized in that the optical medium is integrated on the same substrate.

The wavelength conversion device according to the present invention includes the wavelength conversion device, a first laser that inputs incident light having the wavelength λ ₁ from an input end face of the optical multiplexing unit, and incident light having the wavelength λ ₁ or λ _3. A second laser input from the input end face of the optical multiplexing unit and a filter for separating the converted light having the wavelength λ ₃ or λ ₂ output from the nonlinear optical medium are provided.

  As described above, according to the present invention, since the multiplexing / demultiplexing function and the mode filter function necessary for wavelength conversion can be integrated on the same wavelength conversion device, mounting is easy, optical loss is low, and high power input is achieved. A highly durable and compact wavelength conversion device can be provided at a low price.

It is the schematic which shows the structure of the wavelength conversion element of the quasi phase matching type | mold using the conventional LN. It is the schematic which shows the structure of the conventional green light source. It is a figure which shows the structure of the wavelength conversion device which integrated the optical multiplexer concerning the 1st Embodiment of this invention. It is a figure of the simulation result which shows the coupling | bonding of the light in the mode interference waveguide shown in FIG. It is a figure which shows the process of producing the wavelength conversion waveguide shown in FIG. It is a figure which shows the dimension of the wavelength conversion device concerning one Example of this invention. It is sectional drawing which shows the input waveguide of the wavelength conversion device concerning one Example of this invention. It is a perspective view which shows the preparation methods of the wavelength conversion device concerning one Example of this invention. It is a spectrum figure which shows the 1st experimental result of the wavelength conversion device concerning one Example of this invention. It is a spectrum figure which shows the 2nd experimental result of the wavelength conversion device concerning one Example of this invention. It is the schematic which shows the structure of the green light source concerning the 2nd Embodiment of this invention. It is a figure of the simulation result which shows the coupling | bonding of the light in the mode interference waveguide shown in FIG. It is the schematic which shows the structure of the blue and green light source concerning the 3rd Embodiment of this invention. It is a figure of the simulation result which shows the coupling | bonding of the light in the mode interference waveguide shown in FIG. It is the schematic which shows the structure of the in-line type | mold wavelength converter concerning the 4th Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 3 shows a configuration of a wavelength conversion device in which the optical multiplexer according to the first embodiment of the present invention is integrated. A wavelength conversion device in which an optical multiplexer is integrated includes a first input waveguide 31, a second input waveguide 32, a mode interference waveguide 33 that functions as an optical multiplexer, and a wavelength conversion waveguide on a substrate 30. 34. The input waveguides 31 and 32 are coupled to the input side of the mode interference waveguide 33 at a position shifted from the center line 35 indicating the optical axis of the mode interference waveguide 33. The first input waveguide 31 guides signal light having a wavelength of 1.56 μm, and the second input waveguide 32 guides excitation light having a wavelength of 0.78 μm.

  FIG. 4 is a simulation result showing light coupling in the mode interference waveguide shown in FIG. 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). Here, as shown in FIG. 4A, the width Wm of the mode interference waveguide 33 = 30 μm, the axial deviation amount Δ1 = 5 μm of the input waveguide, the output waveguide axial deviation amount Δ2 = 5 μm, the refractive index of the cladding = 1.0, the refractive index of the core = about 2.1.

  FIG. 4B is a diagram illustrating the behavior of excitation light having a wavelength of 0.78 μm. The two excitation lights incident from the second input waveguide 32 connected at a position shifted from the center of the mode interference waveguide 33 (Δ1) are developed into a plurality of modes inherent to the mode interference waveguide 33. The multimode propagation in the mode interference waveguide 33 occurs. At this time, an extreme value (convergence point) is obtained after propagating through an optical path length having a light amount of a wavelength of 0.78 μm due to mode interference caused by a difference in propagation constant of each mode.

  FIG. 4C is a diagram illustrating the behavior of signal light having a wavelength of 1.56 μm. The signal light incident from the first input waveguide 31 connected at a position shifted from the center of the mode interference waveguide 33 (Δ1) is developed into a plurality of modes unique to the mode interference waveguide 33, and the mode Multimode propagation is performed in the interference waveguide 33. At this time, an extreme value (convergence point) is obtained after propagating through an optical path length having a light quantity of wavelength 1.56 μm due to mode interference caused by a difference in propagation constant of each mode.

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, follow the following approximate equation.

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 is affected by the reciprocal for each wavelength, when the wavelength is different by half as in this embodiment, 1.56 μm beats twice while 0.78 μm beats once. By providing an output end face near the point where both converge, and coupling the output waveguide 34, it is possible to couple 0.78 μm light and 1.56 μm light.

  In the simulation result shown in FIG. 4, the output waveguide axis deviation amount Δ2 is the same as the input waveguide axis deviation amount Δ1. 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 deviation Δ2) depends on the position of the input waveguide (input waveguide deviation Δ1). 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. Therefore, the installation position of the output waveguide (axis deviation amount Δ2) 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, light having wavelengths of 0.78 μm and 1.56 μm can be easily combined with a simple configuration using only a waveguide.

FIG. 5 shows a process of manufacturing the wavelength conversion waveguide shown in FIG. In the first embodiment, the first substrate 11 that is a nonlinear optical medium is a Z-cut Zn-added LN substrate made of a crystal film grown by a liquid phase epitaxial method. 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.

  After the surfaces of the prepared first and second substrates 11 and 12 are made hydrophilic by normal acid cleaning or alkali cleaning, these two substrates are superposed in a clean atmosphere in which microparticles do not exist as much as possible. 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. 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, thereby producing a ridge type optical waveguide (see FIG. 7 described later).

  FIG. 6 shows the dimensions of the wavelength conversion device according to one embodiment of the present invention. The waveguide width of the input waveguide 61 for signal light having a wavelength of 1.56 μm and the input waveguide 62 for excitation light having a wavelength of 0.78 μm is 5 μm. The interval between the input waveguides is 127 μm so as to meet the half pitch of the optical fiber array. The input waveguide 61 for signal light having a wavelength of 1.56 μm is a straight waveguide, and the input waveguide 62 for excitation light having a wavelength of 0.78 μm is a light having a wavelength of 1.56 μm with a gentle curve having a curvature of 5 mm. Asymptotically approach the input waveguide 61 for use. The mode interference waveguide 63 is coupled with the input waveguide interval of 10 μm. 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. An output waveguide of a mode interference waveguide is connected on the same optical axis as that of the input waveguide 61 for signal light having a wavelength of 1.56 μm. The width of the wavelength conversion waveguide 64 which is an output waveguide is 5 μm. The wavelength conversion waveguide 64 is previously provided with a polarization inversion structure, and its length is 45 mm.

  FIG. 7 shows an input waveguide of the wavelength conversion device. A ridge type optical waveguide 14 having a height of 5 μm and a waveguide width of about 5 μm is formed on the first substrate 11 of the thin film substrate 13. As shown in the drawing, since the etching selectivity between the mask and the film is not large in the dry etching process, the ridge type optical waveguide 14 has a mesa shape.

FIG. 8 shows a method for manufacturing a wavelength conversion device. FIG. 8A 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. 5 and having a periodically poled structure. The thin film substrate 13 is bonded to the 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. 8B).

  For each of these wavelength conversion devices, the thin film substrate 13 is cut into a strip shape, and the end face 14a of the input optical waveguides 15 and 16 and the end face 14b of the wavelength conversion waveguide 18 are optically polished to produce a wavelength conversion device having a length of 51 mm. (FIG. 8C).

  In order to evaluate the characteristics of the multiplexer integrated in the wavelength conversion device, the branching ratio is 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 that is an output waveguide to the multiplexer. 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 can be manufactured.

  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 078 μm are used.

  When the excess optical loss due to the mode interference waveguide 63 functioning as a multiplexer was evaluated, the light was combined with a very small loss of 0.5 dB for 1.56 μm light and 1.0 dB for 078 μm light. A wavelength variable light source was used as the light source in the 1.56 μm band, and the wavelength dependence of the optical excess loss by the multiplexer was measured. Compared with 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.

  A wavelength conversion experiment was performed using difference frequency generation using two waves combined by a multiplexer. An 8-wave C-band signal light group arranged at 100 GHz intervals is input to the first input waveguide 61. Excitation light having a wavelength of 777.7 nm is input to the second input waveguide 62. The signal light group and the excitation light are multiplexed by the mode interference waveguide 63 functioning as a multiplexer, and input to the wavelength conversion waveguide 64, and a wavelength converted light group to the L band is obtained by the difference frequency generation. .

  Since the mode interference waveguide 63 performs optical multiplexing by inter-mode interference, when the input light from the input waveguides 61 and 62 propagates in a mode other than the predetermined mode, the loss of the mode interference waveguide 63 increases. To do. This is because the mode interference waveguide 63 has a function of multiplexing and plays a role of a mode filter. Therefore, according to the present embodiment, if light is input to the input waveguide so that the transmitted light of the excitation light and the signal light is maximized without performing special adjustment such as monitoring the power of the converted light. Good. Thereby, the optimal incident condition to the wavelength conversion device is obtained.

  FIG. 9 shows a first experimental result of the wavelength conversion device. It is an output speckle of signal light and converted light when 130 mW excitation light is input. It can be seen from the parametric gain that the converted light has a gain with respect to the input signal light. 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 having an SNR of 40 dB or more with little influence of ASE noise.

  FIG. 10 shows a second experimental result of the wavelength conversion device. This is an output spectrum when the wavelength of the signal light is changed from 1490 nm to 1555.2 nm. It can be seen that wavelength conversion with gain over 30 nm is possible. In addition, unlike the case of cascade excitation, it is not necessary to use strong excitation light in the 1.56 μm band. Therefore, even if the difference between signal light and converted light is close wavelength conversion of 50 GHz, high SNR can be obtained. be able to.

(Second Embodiment)
FIG. 11 shows a configuration of a green light source according to the second embodiment of the present invention. The wavelength conversion device in which the optical multiplexer is integrated includes a first input waveguide 111, a second input waveguide 112, a mode interference waveguide 113, and a wavelength conversion waveguide 114. The input waveguides 111 and 112 are coupled to the input side of the mode interference waveguide 113. The first input waveguide 111 guides signal light having a wavelength of 1.3 μm, and the second input waveguide 112 guides excitation light having a wavelength of 0.98 μm.

  FIG. 12 is a simulation result showing light coupling in the mode interference waveguide shown in FIG. A state in which excitation light having a wavelength of 0.98 μm and signal light having a wavelength of 1.3 μm are coupled is shown by a simulation by BPM. Here, as shown in FIG. 12A, the width Wm of the mode interference waveguide 113 = 11 μm, the axial shift amount Δ1 = 3.5 μm of the input waveguide, the output waveguide axial shift amount Δ2 = 3.5 μm, the cladding The refractive index of the core is 1.0 and the refractive index of the core is about 2.1.

  FIG. 12B is a diagram illustrating the behavior of excitation light having a wavelength of 0.98 μm. The excitation light incident from the second input waveguide 112 connected at a position shifted from the center of the mode interference waveguide 113 (Δ1) is developed into a plurality of modes unique to the mode interference waveguide 113, and the mode Multimode propagation in the interference waveguide 113. At this time, an extreme value (convergence point) is obtained after propagating an optical path length having a light amount of a wavelength of 0.98 μm due to mode interference caused by a difference in propagation constant of each mode.

  FIG. 12C is a diagram illustrating the behavior of signal light having a wavelength of 1.3 μm. The signal light incident from the first input waveguide 111 connected at a position shifted from the center of the mode interference waveguide 113 by the axis (Δ1) is developed into a plurality of modes unique to the mode interference waveguide 113, and the mode Multimode propagation in the interference waveguide 113. At this time, an extreme value (convergence point) is obtained after propagating through an optical path length having a light quantity of wavelength of 1.3 μm due to mode interference caused by a difference in propagation constant of each mode.

  Since the beat length influences each wavelength by a reciprocal number, when the ratio of the excitation light to the signal light is 3: 4 as in the present embodiment, the 0.98 μm excitation light strikes the beat three times. 1.3 μm signal light beats 4 times. By providing an output end face in the vicinity of the point where both converge and coupling the output waveguide 114, it is possible to couple 0.98 μm excitation light and 1.3 μm signal light.

  When combining after hitting multiple beats, the length of the multiplexer becomes long. In order to solve this, the beat length increases as the square of the width of the mode interference waveguide as in (Equation 1), so the input waveguides 111 and 112 are arranged at both ends of the mode interference waveguide 113, The width of the mode interference waveguide 113 is reduced and the beat length is shortened. From the calculation result, the length of the mode interference waveguide 113 in which the excitation light having a wavelength of 0.98 μm and the signal light having a wavelength of 1.3 μm are combined is estimated to be 3.3 mm. In this way, light having a wavelength of 0.98 μm and 1.3 μm can be easily combined with a simple configuration of only a waveguide.

  Similarly to the first embodiment, a thin film substrate is manufactured using the manufacturing process shown in FIG. 5, and a wavelength conversion waveguide in which a multiplexer is integrated is manufactured by dry etching. As shown in FIG. 8, these optical waveguides were cut into strips, and both end faces of the optical waveguides were optically polished to produce wavelength conversion waveguides. The produced wavelength conversion waveguide is mounted, and the green light source shown in FIG. 11 is produced. The wavelength conversion apparatus includes two excitation lasers 117 and 118, a wavelength conversion device, and a fiber array 119 that inputs laser light of the excitation laser to the wavelength conversion device. The fiber array 119 is fixed with a UV curable adhesive after alignment. The wavelength conversion apparatus also includes a filter 115 that separates the laser beams of the excitation lasers 117 and 118 that have passed through the wavelength conversion device and the sum frequency light generated by the wavelength conversion device.

The wavelength lambda ₁ of the excitation laser 117, the wavelength lambda ₂ of the excitation laser 118,
1 / λ ₁ + 1 / λ ₂ = 1 / λ ₃
Is a combination that satisfies When the wavelength λ ₁ = 980 nm of the pump laser 117 and the wavelength λ ₂ = 1300 nm of the pump laser 118, green converted light having the wavelength λ ₃ = 559 nm can be obtained.

At this time, for example, even if the wavelength conversion waveguide is designed to be a single mode at λ ₂ = 1300 nm, it becomes a multimode at λ ₁ = 980 nm. For this reason, in the prior art, it is difficult to excite only the fundamental mode necessary for wavelength conversion, and much time is required for alignment of incident light, and cost reduction is difficult. In this embodiment, since the multiplexer using mode interference functions as a mode filter, the fiber array 119 is aligned so that the transmission power of the signal light and the excitation light is maximized, and the transmission power of the excitation light is maximized. It is possible to excite the fundamental mode necessary for wavelength conversion. As a result, good wavelength conversion efficiency can be obtained, so that the assembly of the apparatus can be simplified. In addition, the effect of this embodiment eliminates the need for a fiber-type directional coupler that has been used in a conventional wavelength conversion laser light source, and can achieve downsizing of the wavelength conversion laser light source.

(Third embodiment)
FIG. 13 shows a configuration of a blue / green light source according to the third embodiment of the present invention. The wavelength conversion device in which the optical multiplexer is integrated includes a first input waveguide 131, a second input waveguide 132, a mode interference waveguide 133, and wavelength conversion waveguides 134A and 134B. The input waveguides 131 and 132 are coupled to the input side of the mode interference waveguide 133 at a position shifted from the central axis of the mode interference waveguide 133. The first input waveguide 131 guides signal light having a wavelength of 1.3 μm, and the second input waveguide 132 guides excitation light having a wavelength of 0.98 μm.

  FIG. 14 is a simulation result showing light coupling in the mode interference waveguide shown in FIG. The state of branching / coupling of 0.98 μm signal light and 1.3 μm excitation light was shown by BPM simulation. Here, as shown in FIG. 14A, the width Wm of the mode interference waveguide 133 = 30 μm, the input waveguide axis shift amount Δ1 = 5 μm, the output waveguide axis shift amount Δ2 = 5 μm, and the clad refractive index = 1. 0.0, the refractive index of the core = about 2.1.

  FIG. 14B is a diagram illustrating the behavior of excitation light having a wavelength of 0.98 μm. The excitation light incident from the second input waveguide 132 connected at a position shifted from the center of the mode interference waveguide 133 by the axis (Δ1) is developed into a plurality of modes unique to the mode interference waveguide 133, and the mode Multimode propagation occurs in the interference waveguide 133. At this time, an extreme value (convergence point) is obtained after propagating an optical path length having a light amount of a wavelength of 0.98 μm due to mode interference caused by a difference in propagation constant of each mode.

  FIG. 14C is a diagram showing the behavior of signal light having a wavelength of 1.3 μm. The signal light incident from the first input waveguide 131 connected at a position shifted from the center of the mode interference waveguide 133 by the axis (Δ1) is developed into a plurality of modes unique to the mode interference waveguide 133, and the mode Multimode propagation occurs in the interference waveguide 133. At this time, an extreme value (convergence point) is obtained after propagating through an optical path length having a light quantity of wavelength of 1.3 μm due to mode interference caused by different propagation constants of the modes.

  Since the beat length is different for each wavelength, a convergence point that converges to one point and a convergence point that converges to two points appear by selecting an appropriate length of the mode interference waveguide as in this embodiment. Converging at two points means that the signal light or the excitation light is branched into two. Thus, the excitation light having a wavelength of 0.98 μm can be branched into two, and the branched light having a wavelength of 0.98 μm and the signal light having a wavelength of 1.3 μm can be combined. According to the calculation result, the length of the mode interference waveguide 133 for splitting the excitation light having a wavelength of 0.98 μm into two branches and combining one of the 0.98 μm light and the 1.3 μm signal light is 4.0 mm. It is estimated. In this way, light having wavelengths of 0.98 μm and 1.3 μm can be easily branched / coupled with a simple configuration of only a waveguide.

  Similarly to the first embodiment, a thin film substrate is manufactured using the manufacturing process shown in FIG. 5, and a wavelength conversion waveguide in which a multiplexer is integrated is manufactured by dry etching. As shown in FIG. 8, these optical waveguides were cut into strips, and both end faces of the optical waveguides were optically polished to produce wavelength conversion waveguides. The produced wavelength conversion element is mounted to produce a blue / green light source shown in FIG. The wavelength conversion apparatus includes two excitation lasers 137 and 138, a wavelength conversion device, and a fiber array 139 that inputs laser light of the excitation laser to the wavelength conversion device. The fiber array 139 is fixed with a UV curable adhesive after alignment. The wavelength conversion apparatus also includes a filter 135 that separates the laser light of the excitation lasers 137 and 138 transmitted through the wavelength conversion element device and the sum frequency light generated by the wavelength conversion device.

The wavelength conversion device has two wavelength conversion waveguides 134A and 134B. In the wavelength conversion waveguide 134A, the wavelength lambda ₁ of the excitation laser 137, the wavelength lambda ₂ of the excitation laser 138,
1 / λ ₁ + 1 / λ ₂ = 1 / λ ₃
The conversion light that satisfies the above is obtained. When the wavelength λ ₁ = 980 nm of the pump laser 137 and the wavelength λ ₂ = 1300 nm of the pump laser 138, green converted light having a wavelength λ ₃ = 559 nm can be obtained.

In the wavelength conversion waveguide 134B, light having a wavelength λ ₁ of the branched excitation laser 138 is incident,
2 / λ ₁ = 1 / λ ₃
The conversion light that satisfies the above is obtained. When the wavelength of the excitation laser 137 is λ ₁ = 980 nm, blue converted light having the wavelength λ ₃ = 490 nm can be obtained.

  Since a satisfactory wavelength conversion efficiency can be obtained by maximizing the transmission power of the excitation light and the signal light, it is possible to use a commercially available alignment device. In addition, the effect of this embodiment eliminates the need for a fiber-type directional coupler that has been used in the conventional wavelength conversion laser light source, and the mode interference waveguide 133 also has a function of multiplexing and demultiplexing. A wavelength conversion laser light source having a wavelength output can be reduced in size.

(Fourth embodiment)
FIG. 15 shows the configuration of an inline-type wavelength converter according to the fourth embodiment of the present invention. The wavelength conversion device in which the optical demultiplexer is integrated includes a wavelength conversion waveguide 154, a mode interference waveguide 153, a first output waveguide 151, and a second output waveguide 152. The output waveguides 151 and 152 are coupled to the output side of the mode interference waveguide 153 at a position shifted from the central axis of the mode interference waveguide 153. The first output waveguide 151 guides light having a wavelength of 0.65 μm, and the second output waveguide 152 guides light having a wavelength of 1.3 μm.

  In the wavelength conversion waveguide 154, light having a wavelength of 0.65 μm is obtained by second harmonic generation of the input light having a wavelength of 1.3 μm. The input light having a wavelength of 1.3 μm and the light having a wavelength of 0.65 μm obtained by wavelength conversion are output from the wavelength conversion waveguide 154. The light output from the wavelength conversion waveguide 154 is input to the mode interference waveguide 153, separated into the respective wavelengths, and output to the output waveguides 151 and 152, respectively.

  In this way, light having wavelengths of 0.65 μm and 1.3 μm can be easily demultiplexed with a simple configuration using only a waveguide. The wavelength conversion device in which the optical demultiplexer according to the fourth embodiment is integrated is provided at a lower price than a conventional wavelength conversion device that requires a fiber type directional coupler and a demultiplexing filter. The In addition, since the mode interference waveguide functions not only as a demultiplexing function but also as a mode filter, the fiber alignment can be performed only with the power monitor of the excitation light, and the maximum efficiency of the wavelength conversion waveguide can be obtained. I can suppress it.

1, 23 multiplexer 2 optical waveguide 3 demultiplexer 11 first substrate 12 second substrate 13 thin film substrate 14 ridge type optical waveguides 15, 31, 61, 111, 131 first input waveguides 16, 32, 62, 112, 132 Second input waveguide 17, 33, 63, 113, 133, 153 Mode interference waveguide 18, 34, 64, 114, 134 Wavelength conversion waveguide 20, 21, 117, 118, 137, 138 Excitation lasers 22a, 22b Lens 24 LN nonlinear optical crystals 25, 115, 135 Filter 30 Substrate 35 Center lines 119, 139, 159 Fiber array 151 First output waveguide 152 Second output waveguide

Claims (11)

Among the wavelengths having a relationship of 1 / λ ₃ = 1 / λ ₂ + 1 / λ ₁ , two incident lights having wavelengths λ ₁ and λ ₂ or wavelengths λ ₁ and λ ₃ are input to the nonlinear optical medium, and the wavelength λ ₃ Or a wavelength conversion device that outputs converted light of λ ₂ respectively,
The incident light of the wavelengths λ ₁ and λ ₂ or the incident light of the wavelengths λ ₁ and λ ₃ propagates in multimode, so that the first wavelength (λ ₁ ) and the second wavelength (λ ₂ or λ ₃ ), and an optical multiplexing unit having an output end face provided at a point where the light of the first wavelength and the second wavelength converge.
From the nonlinear optical medium that is coupled to a point where the convergence of the output end face converted light having a wavelength lambda ₃ to the incident light of wavelength lambda ₁ and lambda ₂ is the wavelength lambda ₂ with respect to incident light of wavelength lambda ₁ and lambda ₃ The wavelength conversion device is characterized in that the converted light is output and the optical multiplexing unit and the nonlinear optical medium are integrated on the same substrate. Among the wavelengths having a relationship of 1 / λ ₃ = 1 / λ ₂ + 1 / λ ₁ , one incident light of wavelength λ ₁ = λ ₂ is input to the nonlinear optical medium, and converted light of wavelength λ ₃ is output, A wavelength conversion device that inputs two incident lights having wavelengths λ ₁ and λ ₂ or wavelengths λ ₁ and λ ₃ to a nonlinear optical medium and outputs converted light having a wavelength λ ₃ or λ ₂ , respectively.
The incident light of the wavelengths λ ₁ and λ ₂ or the incident light of the wavelengths λ ₁ and λ ₃ propagates in multimode, so that the light of the first wavelength (λ ₁ ) is branched by the mode interference inside and branched. A first convergence point where the light of one first wavelength (λ ₁ ) and the light of the second wavelength (λ ₂ or λ ₃ ) are combined and the branched light of the first wavelength converges. And an optical multiplexing part provided with an output end face at a second convergence point at which the branched first light and the second wavelength converge,
The converted light having the wavelength λ ₃ is output from the first nonlinear optical medium coupled to the first convergence point, and the wavelengths λ ₁ and λ are output from the second nonlinear optical medium coupled to the second convergence point. converted light having a wavelength lambda ₃ to the _second incident light, converted light having a wavelength lambda ₂ to the incident light of wavelength lambda ₁ and lambda ₃ is outputted,
The wavelength conversion device, wherein the optical multiplexing unit and the first and second nonlinear optical media are integrated on the same substrate.   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. Among the wavelengths having a relationship of 1 / λ ₃ = 1 / λ ₂ + 1 / λ ₁ , a wavelength at which one incident light having the wavelength λ ₁ = λ ₂ is input to the nonlinear optical medium and converted light having the wavelength λ ₃ is output. A conversion device,
The output end of the nonlinear optical medium is connected to the input end face, and light having the _first wavelength (λ ₁ = λ ₂ ) and light having the second wavelength (λ ₃ ) propagate in multimode, thereby An optical multiplexing part provided with an output end face at a point where the light of the wavelength and the second wavelength converge,
The light having the first wavelength and the second wavelength are output from the output waveguide coupled to the converging point on the output end face, and the optical multiplexing unit and the nonlinear optical medium are on the same substrate. A wavelength conversion device that is integrated.   5. 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. 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 6. The wavelength conversion device according to claim 1, further comprising at least one selected from the group consisting of additives.   The wavelength conversion device according to claim 6, 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 8. The wavelength conversion device according to Item 6 or 7.   The wavelength conversion device according to claim 8, wherein the first substrate has a structure in which a nonlinear constant is periodically inverted.   The wavelength conversion device according to claim 8 or 9, wherein the first substrate and the second substrate are directly bonded to each other by diffusion bonding by heat treatment. The wavelength conversion device according to any one of claims 1 to 10,
A first laser that inputs incident light having the wavelength λ ₁ from an input end face of the optical multiplexing unit;
A second laser that inputs incident light of the wavelength λ ₁ or λ ₃ from an input end face of the optical multiplexing unit;
And a filter that separates the converted light having the wavelength λ ₃ or λ ₂ output from the nonlinear optical medium.

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