JP4526489B2 - Wavelength conversion module, wavelength conversion light source, and wavelength conversion device - Google Patents

Description

  The present invention relates to a wavelength conversion module, a wavelength conversion light source, and a wavelength conversion device, and more particularly, a wavelength conversion module that converts the wavelength of input light using sum frequency generation and difference frequency generation, and outputs the wavelength. The present invention relates to a conversion light source and a wavelength conversion device.

  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 element that converts a signal wavelength into an arbitrary signal wavelength. Conventionally, as a wavelength conversion element for converting the wavelength of light, an element using a semiconductor optical amplifier, an element using four-wave mixing, second harmonic generation, a kind of second-order nonlinear optical effect, sum frequency generation, difference frequency A wavelength conversion element using generation is known (for example, see Patent Document 1). Wavelength conversion using the second-order nonlinear optical effect can convert the wavelength of an existing laser light source into various other wavelengths, so that it can be applied not only to the communication field described above but also to various industrial fields. Aiming at research.

  Among the second-order nonlinear optical effects, second-harmonic generation can obtain light having a wavelength half that of the excitation light by entering one type of excitation light into a nonlinear optical medium having the second-order nonlinear optical effect. . In the second harmonic generation, since the wavelength is uniquely determined by the wavelength of the laser used for the excitation light, the obtained wavelength range is limited. On the other hand, sum frequency generation or difference frequency generation is performed by combining two types of excitation light (hereinafter, signal light and excitation light for distinction) and entering the nonlinear optical medium, and the optical frequency of the signal light and the excitation light. The light having a wavelength corresponding to the sum or difference of the optical frequencies is generated. Therefore, light of various wavelengths can be generated by combining the wavelengths of the signal light and the excitation light.

  When sum frequency generation or difference frequency generation is used, signal light and excitation light output from two laser light sources having different wavelengths are combined and incident on a nonlinear optical medium. In order to obtain high conversion efficiency even when the incident power is relatively small, it is desirable that a waveguide be formed in the nonlinear optical medium. When wavelength conversion is performed using a nonlinear optical medium in which a waveguide is formed, for example, a spatial beam of laser light output from two laser light sources is combined by an optical component such as a dichroic mirror, and is converted by a lens or the like. Condensed and coupled to the waveguide. However, when an optical system comprising such optical components is used, the optical adjustment of the apparatus becomes complicated and lacks practicality. Therefore, a wavelength conversion module that makes incident light incident through an optical fiber is configured. The wavelength conversion module has a function of adjusting the temperature of the nonlinear optical medium in order to obtain phase matching necessary for wavelength conversion.

  On the other hand, when a semiconductor laser is used as the pumping light, various semiconductor lasers developed for optical communications in the 0.85 μm, 1.3 μm, 1.55 μm band, or 0.94 to 1.06 μm band can be used. Can generate light of various wavelengths. Many of these laser light sources are configured as an optical module to which an optical fiber called a pigtail is attached, so that they can be easily handled as a light source. Therefore, the pigtails from the two laser light sources are connected to an optical multiplexer composed of fiber components such as optical fiber couplers, and signal light and excitation light are incident on the waveguide of the nonlinear optical medium from one optical fiber. A method for performing wavelength conversion is known.

JP 2003-140214 A

  At this time, two signal lights having different wavelengths and pumping light are incident on the optical waveguide from one optical fiber. However, since the mode diameter in the optical fiber differs depending on the wavelength, there is a problem that it is difficult to achieve both high coupling efficiency in the wavelength of the signal light and the wavelength of the excitation light.

  The optical fiber connecting the optical multiplexer and the wavelength conversion module needs to have a waveguide mode that does not change even when the optical fiber is bent from the viewpoint of mounting. That is, it is necessary to satisfy single mode propagation at the wavelength of the signal light and the wavelength of the excitation light. For example, when light with wavelengths of 0.98 μm and 1.55 μm is incident, in order to realize single mode propagation at both wavelengths, a single mode in which the cutoff wavelength of the higher order mode is shorter than 0.98 μm. It is necessary to use fiber. For example, when a single mode optical fiber for 0.98 μm is used, single mode propagation is guaranteed at a relatively long wavelength of 1.55 μm. However, the longer the wavelength, the larger the mode diameter of the optical fiber. For example, the mode diameter of the wavelength 0.95 μm is 6.6 μm, and the mode diameter is 1.55 μm, which is about 12 μm.

  For example, even if the mode diameter is converted using two lenses, high coupling efficiency cannot be obtained unless the ratio of the mode diameter of the optical fiber is equal to the ratio of the mode diameter of the waveguide. There is also a problem that it is extremely difficult to obtain such a combination of an optical fiber and a waveguide. Even when the end face of the optical fiber and the end face of the waveguide are directly connected to obtain a coupling, it is difficult to achieve both high coupling efficiencies at two different wavelengths.

  The present invention has been made in view of such a problem, and an object thereof is to efficiently couple signal light having two different wavelengths and excitation light into a waveguide of a nonlinear optical medium. The object is to provide a wavelength conversion module, a wavelength conversion light source, and a wavelength conversion device.

  In order to achieve the above object, according to the present invention, the invention described in claim 1 inputs the excitation light having the wavelength λ1 and the signal light having the wavelength λ2 having different wavelengths, and generates the converted light having the wavelength λ3. A non-linear optical crystal in which a waveguide structure including a lower clad having a refractive index n1, a core having a refractive index n2, and an upper clad having a refractive index n3 is formed. At any wavelength λ of the light and the converted light,

Where d is the thickness of the core, w is the width of the core, and nc is the larger value of n1 or n3, and the waveguide is optically coupled And an input photonic crystal fiber that couples the signal light and the excitation light at the input end of the waveguide with the same mode diameter .

  The invention according to claim 2 is the wavelength conversion module according to claim 1, further comprising an output photonic crystal fiber that is optically coupled to the waveguide and emits the converted light from the waveguide. It is characterized by that.

  According to a third aspect of the present invention, the input photonic crystal fiber according to the first or second aspect is optically coupled to the waveguide through a lens, and a mode diameter of the input photonic crystal fiber is determined. The beam diameter converted by the lens is made equal to the mode diameter of the waveguide.

  According to a fourth aspect of the present invention, the input photonic crystal fiber according to the first or second aspect is optically coupled by directly facing the waveguide, and the input photonic crystal fiber is The mode diameter is equal to the mode diameter of the waveguide.

  According to a fifth aspect of the present invention, the photonic crystal fiber according to any one of the first to fourth aspects is a polarization maintaining type.

The invention according to claim 6 is the wavelength conversion module according to any one of claims 1 to 5, wherein the lower clad is made of LiTaO ₃ and the core is doped with Zn whose polarization is periodically inverted. The upper clad is made of LiNbO ₃ and has a ridge-type waveguide structure made of air.

  According to a seventh aspect of the present invention, there is provided the wavelength conversion module according to any one of the first to sixth aspects, a multiplexer connected to the input photonic crystal fiber, and one input of the multiplexer. And a light source that outputs the signal light and a light source that is connected to the other input of the multiplexer and outputs the excitation light.

  The invention according to claim 8 is the wavelength conversion module according to any one of claims 1 to 6, a multiplexer connected to the input photonic crystal fiber, and one input of the multiplexer. And a light source that outputs the excitation light. The wavelength conversion module converts the signal light input to the other input end of the multiplexer to a wavelength of 400 nm to 1000 nm by sum frequency generation. It is characterized by outputting light.

  As described above, according to the present invention, the wavelength of the signal light and the wavelength of the excitation light are set by setting the normalized frequency V> 4 in the wavelengths of the signal light, the excitation light, and the converted light involved in the wavelength conversion. The mode diameter of the waveguide can be made substantially equal, and it becomes possible to efficiently couple to the waveguide of the nonlinear optical medium.

  In addition, according to the present invention, the waveguide of the nonlinear optical medium is more efficiently introduced by inputting the signal light and the excitation light through the photonic crystal fiber capable of almost equal mode diameters at the wavelengths of the excitation light and the converted light. Can be combined.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. As described above, it is necessary to suppress a difference in mode diameter between the optical fiber coupled to the wavelength conversion element and the waveguide of the nonlinear optical medium. In recent years, optical fibers called photonic crystal fibers or holey fibers have been developed (see, for example, Non-Patent Documents 1 and 2). A photonic crystal fiber has a waveguide structure formed by providing a large number of holes in the cross section, and various characteristics can be controlled by changing the arrangement of the holes. For example, zero dispersion can be achieved at wavelengths shorter than 1.3 μm, which cannot be achieved with ordinary optical fibers, a single mode fiber with a very small mode diameter can be realized to increase the nonlinear effect, or the nonlinear effect can be reduced. In order to transmit a large amount of power, a single mode fiber having a large core diameter can be realized.

  The photonic crystal fiber described in Non-Patent Document 1 can obtain single-mode propagation even when the wavelength changes greatly. In addition, the photonic crystal fiber described in Non-Patent Document 2 has a smaller mode wavelength dependency than a normal optical fiber. Using such a photonic crystal fiber, it is possible to propagate the above-mentioned light with a wavelength of 0.98 μm and 1.55 μm in a single mode, and to make the mode diameters at both wavelengths substantially equal.

In the present embodiment, as an example, a wavelength conversion element using LiNbO ₃ which is a typical second-order nonlinear optical crystal is considered. Known methods for forming a waveguide in a nonlinear optical medium made of LiNbO ₃ include a proton exchange method and a method using Ti diffusion. However, since these methods use a change in refractive index due to element diffusion, the refractive index of the core portion of the fabricated waveguide is a graded index type in which the boundary with the refractive index of the cladding changes slowly. Become. When two light beams with different wavelengths are incident on a graded index waveguide, the light confinement effect is weak. Therefore, long-wavelength light is affected by the refractive index of the cladding, and the mode is shorter than that of short-wavelength light. The diameter increases.

On the other hand, according to the direct bonding method described in Non-Patent Document 3, it is possible to form a so-called step index type waveguide in which the refractive index at the boundary between the core and the cladding changes sharply. For example, LiTaO ₃ having a lower refractive index than LiNbO ₃ is used as a clad substrate, and a LiNbO ₃ substrate is directly bonded onto this substrate. A LiNbO ₃ substrate is polished to a thickness necessary for the formation of the waveguide to form a waveguide layer, and processed into a ridge shape by dry etching or dicing to form a core. This embodiment is characterized in that the wavelength dependence of the mode diameter in such a step index type waveguide is reduced.

  FIG. 1 shows a ridge-type waveguide according to the first embodiment of the present invention. In order to simplify the discussion, it is assumed that the width and thickness of the core 12 of the waveguide are the same T, the refractive index of the lower cladding 11 is n1, and the refractive index of the core 12 is n2. The upper cladding 13 around the core 12 is covered with air having a refractive index of 1. In order to generalize and discuss the dependence on the wavelength λ, the normalized frequency V of the waveguide is defined as follows.

Increasing the wavelength λ incident on the nonlinear optical medium corresponds to decreasing the normalized frequency V. Further, the normalized refractive index B when the equivalent refractive index of the waveguide mode is set to neq is defined as follows.

  Here, when the equivalent refractive index neq becomes equal to the cladding refractive index n1, the normalized refractive index B becomes zero. This indicates that light propagating through the core leaks out to the clad, and indicates that there is no mode in which light is guided through the waveguide.

  FIG. 2 shows changes in the normalized refractive index B of the fundamental mode (0th order mode) and the primary mode when the normalized frequency V is changed. According to FIG. 2, the V value needs to be about 6 or less in order to obtain a single mode waveguide having no higher-order mode, and the V value needs to be about 4 or more in order to obtain a fundamental mode waveguide. FIG. 3 shows how the value obtained by normalizing the full width of the core by the thickness T of the core where the longitudinal electric field of the waveguide is 1 / e (e is a natural logarithm) of the maximum value with respect to the normalized frequency V. Shows how it changes.

  Next, in order to investigate the wavelength dependence of the mode diameter, the electric field distribution of the fundamental mode when the normalized frequency V was changed was calculated. FIG. 4 shows the vertical electric field distribution of the waveguide when the normalized frequency V is changed in the fundamental mode, and FIG. 5 shows the horizontal electric field distribution.

  According to FIGS. 3 to 5, in the ridge-type waveguide, the optical confinement effect in the lateral direction of the waveguide is large due to the steep refractive index difference between the core 12 and the upper cladding 13 which is air. Therefore, even if the normalized frequency V changes, that is, the wavelength changes, the transverse mode diameter does not change. On the other hand, in the longitudinal direction of the waveguide, the mode diameter slightly changes with the change of the normalized frequency V. However, when the normalized frequency V at which the fundamental mode can exist is 4 or more, it can be seen that the change in the mode diameter is very small.

  In this way, in the ridge-type waveguide, the wavelength dependence of the mode diameter can be reduced, so standardization is possible so that a propagation mode can exist in the wavelength of signal light, excitation light, and converted light involved in wavelength conversion. The frequency V can be set. By setting V> 4 at these wavelengths, the mode diameter of the waveguide at the wavelength of the signal light and the wavelength of the excitation light can be made substantially equal.

  When using sum frequency generation, the wavelength of the converted light is shorter than the wavelength of the signal light and the wavelength of the pumping light, so that V> 4 in the light having the longer wavelength of the signal light or the pumping light. Set to. When the difference frequency generation is used, the wavelength of the converted light is longer than the wavelength of the signal light and the wavelength of the excitation light, so that V> 4 is set in the wavelength of the converted light. In this way, all the light can be guided to the waveguide of the nonlinear optical medium, and the difference between the waveguide modes of the signal light and the excitation light can be suppressed small.

  By combining a ridge-type waveguide with a small mode diameter wavelength dependency and a photonic crystal fiber in this way, signal light and pump light with different wavelengths are incident, and signal light and pump light are incident with high efficiency. can do. When the mode diameters are approximately equal, the photonic crystal fiber can be coupled to the waveguide by so-called butt joint, in which the optical axes are aligned and directly opposed to each other. Also, when the mode diameters of the photonic crystal fiber and the waveguide are different, optically coupled through a lens or the like, the beam diameter obtained by converting the mode diameter of the photonic crystal fiber by the lens, and the mode diameter of the waveguide Can be combined with high efficiency.

  In addition, depending on the combination of the waveguide parameter and the wavelength, V> 6 at the wavelength of the signal light or the excitation light, which may be a multimode waveguide. Even in such a case, high coupling efficiency to the fundamental mode can be obtained by coupling the photonic crystal fiber and the waveguide with substantially equal mode diameters. Since the waveguide of the nonlinear optical medium is not used by being freely bent, if the fundamental mode is excited at the input end, the coupling to the higher-order mode does not occur even if the waveguide is a multimode.

  FIG. 6 shows an embedded waveguide according to the second embodiment of the present invention. The waveguide core 22 has the same width and thickness T, the lower cladding 21 has a refractive index n1, and the core 22 has a refractive index n2. The upper clad 23 around the core 22 is covered with the same refractive index n1 as that of the lower clad 21. As the normalized frequency V and the normalized refractive index B, the above-described formulas 1 and 2 are used.

  FIG. 7 shows changes in the normalized refractive index B of the fundamental mode (0th order mode) and the primary mode when the normalized frequency V is changed. According to FIG. 7, the V value needs to be about 4.2 or less in order to obtain a single mode waveguide having no higher-order mode, and the V value needs to be about 2 or more in order to obtain a fundamental mode waveguide. . FIG. 8 shows how the value obtained by normalizing the entire width of the core with the thickness T of the core where the longitudinal electric field of the waveguide is 1 / e (e is a natural logarithm) of the maximum value with respect to the normalized frequency V. Shows how it changes.

  Next, in order to investigate the wavelength dependence of the mode diameter, the electric field distribution of the fundamental mode when the normalized frequency V was changed was calculated. FIG. 9 shows the vertical electric field distribution of the waveguide when the normalized frequency V is changed in the fundamental mode. In the case of the buried waveguide, the vertical and horizontal refractive index distributions are almost equal in the case of the buried waveguide, so that the vertical and horizontal electric field distributions are also almost equal.

  8 and 9, it can be seen that, in the buried waveguide, the mode diameter changes greatly depending on the change in the normalized frequency V at which the fundamental mode can exist. However, it can be seen from FIG. 8 that when the normalized frequency V is 4 or more, the change in mode diameter can be suppressed to a small value. Under these conditions, the wavelength dependence of the mode diameter can be reduced even in the buried waveguide, and the mode diameter can be made substantially equal for the wavelengths of the signal light, the excitation light, and the converted light involved in wavelength conversion. it can.

  When using sum frequency generation, the wavelength of the converted light is shorter than the wavelength of the signal light and the wavelength of the pumping light, so that V> 4 in the light having the longer wavelength of the signal light or the pumping light. Set to. When using the difference frequency generation, the wavelength of the converted light is longer than the wavelength of the signal light and the wavelength of the excitation light. Therefore, the wavelength of the converted light is set to satisfy V> 2, and the signal light or It sets so that it may become V> 4 in light with a long wavelength among excitation light. In this way, all the light can be guided to the waveguide of the nonlinear optical medium, and the difference between the waveguide modes of the signal light and the excitation light can be suppressed small.

  In this way, if a buried waveguide with a small mode diameter wavelength dependency and a photonic crystal fiber are combined and signal light and pump light of different wavelengths are incident, signal light and pump light are incident with high efficiency. can do. The coupling between the photonic crystal fiber and the waveguide can be carried out by a so-called butt joint in which the optical axes coincide with each other when the mode diameters are approximately equal. Further, when the mode diameters of the photonic crystal fiber and the waveguide are different, they can be coupled with high efficiency by converting the mode diameter via a lens or the like.

  In the case of the embedded waveguide, if V> 4 at the wavelength of the signal light or the excitation light, a multimode waveguide is formed as shown in FIG. Even in such a case, high coupling efficiency to the fundamental mode can be obtained by coupling the photonic crystal fiber and the waveguide with substantially equal mode diameters.

  FIG. 10 shows an embedded waveguide according to the third embodiment of the present invention. The waveguide core 32 has the same width and thickness T, the lower cladding 31 has a refractive index n1, and the core 32 has a refractive index n2. The upper clad 33 around the core 32 is covered with a refractive index n3. In order to generalize and discuss the dependence on the wavelength λ, the normalized frequency V of the waveguide is defined as follows.

  Here, nc is the larger value of n1 or n3. The ridge-type waveguide of the first embodiment is an extreme example of an asymmetric waveguide as in the third embodiment because the refractive index of the upper cladding is 1. If the refractive index of the upper cladding is an intermediate value between the refractive index of the lower cladding and the air refractive index of 1, the dependence of the equivalent refractive index and mode diameter on the normalized frequency is It shows an intermediate property between the waveguide and the buried waveguide.

  When the refractive index difference between the upper clad 33 and the core 32 is large, the same properties as the ridge-type waveguide are exhibited. When the refractive index of the upper cladding 33 is larger than the refractive index of the lower cladding 31, there is no mode in which light is guided when the equivalent refractive index of the guided mode is the same as the refractive index of the upper cladding. Since the optical roles of the upper clad and the lower clad are substantially the same, in the case of an asymmetric waveguide, if the normalized frequency V of the waveguide is defined as shown in Equation 3, It can be handled in the same way as an embedded waveguide.

  As an example, a buried waveguide in which n2> n1> n3 and n2-n1 has an equivalent refractive index 2.3 times that of n2-n3 will be described. FIG. 11 shows changes in the normalized refractive index B of the fundamental mode (0th order mode) and the primary mode when the normalized frequency V is changed. According to FIG. 11, in the case of an asymmetric waveguide, an intermediate property between the ridge-type waveguide shown in FIG. 2 and the buried-type waveguide shown in FIG. 7 is shown, and it is guided by the refractive index of the cladding. The shape of the wave mode changes.

  FIG. 12 shows how the value obtained by normalizing the full width of the core by the thickness T of the core where the longitudinal electric field of the waveguide is 1 / e (e is a natural logarithm) of the maximum value with respect to the normalized frequency V. Shows how it changes. Since the behavior is intermediate between the ridge waveguide shown in FIG. 3 and the buried waveguide shown in FIG. 8, when the normalized frequency V is 4 or more, the change in mode diameter can be suppressed to a small value. Under these conditions, the wavelength dependence of the mode diameter can be reduced even in an asymmetric buried type waveguide, and the mode diameters are almost equal for the wavelengths of signal light, excitation light, and converted light involved in wavelength conversion. can do.

  When sum frequency generation and difference frequency generation are used, V> 4 is set for light having a long wavelength among signal light and excitation light. In this way, all the light can be guided to the waveguide of the nonlinear optical medium, and the difference between the waveguide modes of the signal light and the excitation light can be suppressed small. Furthermore, if a buried waveguide having a small wavelength dependence of the mode diameter and a photonic crystal fiber are combined, and signal light and pump light having different wavelengths are incident, signal light and pump light are incident with high efficiency. be able to. The coupling between the photonic crystal fiber and the waveguide can be carried out by a so-called butt joint in which the optical axes coincide with each other when the mode diameters are approximately equal. Further, when the mode diameters of the photonic crystal fiber and the waveguide are different, they can be coupled with high efficiency by converting the mode diameter via a lens or the like.

  In the case of an asymmetric buried waveguide, depending on the combination of wavelengths, if V> 4 at the wavelength of the signal light or the excitation light, a multimode waveguide is obtained. Even in such a case, high coupling efficiency to the fundamental mode can be obtained by coupling the photonic crystal fiber and the waveguide with substantially equal mode diameters.

  In the above-mentioned teacher form, the thickness of the core and the width of the core are set to the same T. However, even if the ratio of the thickness to the width is other than 1, the tendency of the mode diameter to the normalized frequency V is It does n’t change much. Therefore, if the thickness of the core is d and the width of the core is w,

It is sufficient to satisfy at least one of these, preferably both.

  As described above, according to the present embodiment, the wavelength dependency of the mode diameter of the optical fiber used for incidence of the signal light and the excitation light and the optical waveguide for performing wavelength conversion can be suppressed to be small. Therefore, signal light and excitation light having different wavelengths can be efficiently incident on the optical waveguide, and an efficient wavelength conversion device can be realized. Also, two laser light sources having different wavelengths, a multiplexer that multiplexes two laser lights, a photonic crystal fiber for making the laser light incident on the wavelength conversion element, and a wavelength conversion element having an optical waveguide structure By using the sum frequency generation or the difference frequency generation, it is possible to configure a wavelength conversion light source that generates a short wavelength light source such as the visible region, or a long wavelength light source such as the near infrared or the mid infrared. . Further, it is desirable that the optical fiber and the photonic crystal fiber used when configuring the wavelength conversion light source be of a polarization preserving type in order to stably perform wavelength conversion.

The optical waveguide that performs wavelength conversion may be any element that can perform wavelength conversion using the second-order nonlinear optical effect by phase matching or pseudo-phase matching. For example, the materials used for the core include LiNbO ₃ , LiTaO ₃ , KNbO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1), Li _x K _1-x Ta _y Nb _1-y O ₃ , There are oxide ferroelectrics such as KTiOPO ₄ or those obtained by adding elements such as Mg, Zn, Sc, and In to these. In addition, when using quasi phase matching, a polarization inversion structure in which the polarization is periodically inverted can be used.

  The material used for the clad may be any material having a refractive index smaller than that of the core material. However, from the viewpoint of forming the optical waveguide, a material having a thermal expansion coefficient close to that of the core material is desirable. The method for forming the waveguide is not particularly limited. For example, a wafer bonding method can be used for forming the lower cladding, and a sputtering method, a vapor deposition method, or the like can be used for forming the upper cladding. .

  Further, according to the present embodiment, when V> 4 is set in the wavelengths of the signal light, the excitation light, and the converted light, the mode diameters of the three wavelengths substantially coincide with each other in input / output. Therefore, the converted light can be extracted by coupling the photonic crystal fiber also to the output side of the wavelength conversion element.

  According to this embodiment, when wavelength conversion (up-conversion) to a short wavelength is performed using sum frequency generation of relatively low power signal light and relatively high power excitation light, loss of input light is achieved. Can be kept small. Therefore, the signal light can be efficiently converted into short wavelength light. This property is not limited to simply increasing the output light power, and the following effects can be obtained.

  For example, when detecting light in the communication wavelength band of 1.3 μm to 1.55 μm, a photodetector made of InGaAs is used. A photodetector made of InGaAs has a lower quantum efficiency than a photodetector made of Si used at a wavelength of 1 μm or less. Therefore, there is a problem that photon counting for detecting very weak light and high detection sensitivity cannot be used for the required quantum cryptography communication. While suppressing the loss of weak signal light in the 1.3 μm to 1.55 μm band, the wavelength conversion according to the present embodiment is applied to perform wavelength conversion (up-conversion) to the wavelength band of a photodetector made of Si by sum frequency generation. . In such an application example, even if the power of the pumping light is increased, converted light exceeding the number of photons of the input signal light is not generated. Therefore, it is possible to effectively utilize the property of suppressing the loss of signal light. it can.

FIG. 13 shows the configuration of the wavelength conversion module according to the first embodiment of the present invention. The wavelength conversion module 101 includes a wavelength conversion element 111, a lens 112 that optically couples the polarization maintaining photonic crystal fiber 102 and the wavelength conversion element 111, and a dichroic mirror 113 that separates and outputs only converted light. including. The wavelength conversion element 111 includes a nonlinear optical medium 121 made of LiNbO ₃ on which a waveguide 122 is formed, a lens 123 that is optically coupled to the polarization maintaining photonic crystal fiber 102, and light output from the waveguide 122. And a lens 124 for collimating. Example 1 is a wavelength conversion module that outputs 600 nm converted light by sum frequency generation from signal light having a wavelength of 1.55 μm and excitation light having a wavelength of 0.98 μm.

  The wavelength conversion element 111 is mounted on one carrier, and the temperature of the entire carrier is controlled by a Peltier element and a thermistor. The exit end face of the photonic crystal fiber 102 and the entrance end face of the waveguide 122 have antireflection coatings at wavelengths of 1550 nm and 980 nm, and the exit end face of the waveguide 122 has antireflection coating at a wavelength of 600 nm. ing.

Details of the waveguide 122 of the nonlinear optical medium 121 will be described. The nonlinear optical medium 121 is produced by a direct bonding method described in Non-Patent Document 3. FIG. 14 shows a cross-sectional view of the nonlinear optical medium 121. The lower cladding 131 is a Z-cut substrate made of LiTaO ₃ , and the waveguide core 132 is made of a Z-cut substrate made of LiNbO ₃ doped with Zn. The core 132 has a ridge type optical waveguide formed by dicing. The core 132 has a thickness of 8 μm and a width of 8 μm. LiNbO ₃ to which Zn is added has a domain-inverted structure with a period of 9.95 μm. The length of the waveguide is 20 mm, and the normalized SFG conversion efficiency is 2900% / W.

Here, because it uses the nonlinear coefficient d ₃₃ of LiNbO _3, all light incident is TM mode, the converted light is also emitted in the TM mode. The refractive index in the TM mode, that is, the refractive index of the core 132 at the extraordinary light polarization in LiNbO ₃ is about 2.1. The refractive index difference between the core 132 and the lower cladding 131 is 0.6% at a wavelength of 1550 nm, 0.8% at 980 nm, and 1.0% at 600 nm. The upper cladding 133 is air having a refractive index of 1.

In Expression 3, the normalized frequencies V at three wavelengths related to wavelength conversion are V = 7.7 at a wavelength of 1550 nm, V = 13.9 at a wavelength of 980 nm, and V = 26 at a wavelength of 600 nm. FIG. 15 shows the vertical electric field distribution of the waveguide at each wavelength, and FIG. 16 shows the horizontal electric field distribution. It can be seen that the ridge-type waveguide structure of Example 1 has substantially the same mode diameter at each wavelength when V> 4 at all wavelengths. The total width at which the light intensity becomes 1 / e ^{2 of the} peak is 7.0 μm in the vertical direction and 6.4 μm in the horizontal direction at a wavelength of 1550 nm, 6.5 μm in the vertical direction at a wavelength of 980 nm, 6.4 μm in the horizontal direction, and vertical at 600 nm. The direction is 6.0 μm and the lateral direction is 6.4 μm.

  FIG. 17 shows a change in coupling efficiency with respect to the incident beam diameter when light having wavelengths of 1550 nm and 980 nm is incident on the waveguide 122 by a Gaussian beam. By setting the incident mode diameter to about 6 μm at wavelengths of 1550 nm and 980 nm, both wavelengths can be efficiently incident. The mode diameter of the polarization maintaining photonic crystal fiber 102 used in Example 1 is 7 μm at wavelengths of 1550 nm and 980 nm. Therefore, the diameter of the beam is converted using the lenses 112 and 123 to convert it to 6 μm at which the coupling efficiency to the waveguide 122 is maximized. As a result, the coupling efficiency between the polarization maintaining photonic crystal fiber 102 and the waveguide 122 shows a high coupling efficiency of 97% at a wavelength of 1550 nm and 97% at a wavelength of 980 nm. The conversion efficiency of the entire wavelength conversion module 101 viewed from the input side of the polarization maintaining photonic crystal fiber 102 is 2200% / W. By making excitation light with a wavelength of 980 nm 100 mW, signal light with a wavelength of 1550 nm 1 mW, and incident on the wavelength conversion module 101, 2.2 mW of converted light with a wavelength of 600 nm can be obtained. This corresponds to conversion of incident light having a wavelength of 1550 nm into light having a wavelength of 600 nm with an efficiency of 88%.

  For comparison, the wavelength conversion module shown in FIG. 13 is configured using a normal polarization maintaining fiber for 980 nm instead of the polarization maintaining photonic crystal fiber 102. The polarization maintaining fiber for 980 nm has a mode diameter of 12 μm at a wavelength of 1550 nm and a mode diameter of 6.6 μm at a wavelength of 980 nm. Referring to FIG. 17, when the mode diameter at the wavelength of 1550 nm is 12 μm, the coupling efficiency is reduced to about 63%. When up-converting to the wavelength band of a photodetector made of Si, if the coupling efficiency of the signal light with a wavelength of 1.55 μm is low, the power of the signal light does not increase even if the power of the excitation light with a wavelength of 980 nm is increased. The light receiving sensitivity will be lowered.

  According to the first embodiment, signal light having a wavelength of 1550 nm can be efficiently converted into converted light having a wavelength of 600 nm by suppressing a decrease in coupling efficiency of signal light.

FIG. 18 shows a configuration of a wavelength conversion module according to the second embodiment of the present invention. The outputs of the light source having a wavelength of 1550 nm and the light source having a wavelength of 980 nm are combined by a fiber coupler 205 made of a polarization maintaining fiber for 980 nm and output to the polarization maintaining fiber 204 for 980 nm. Further, the signal is input to the wavelength conversion module 201 through the polarization maintaining photonic crystal fiber 202 fused and connected to the polarization maintaining fiber 204 for 980 nm. The wavelength conversion module 201 includes a wavelength conversion element 211, a lens 212 that optically couples the polarization maintaining photonic crystal fiber 202 and the wavelength conversion element 211, and a dichroic mirror 213 that separates and outputs only the converted light. And a lens 214 that optically couples the polarization maintaining photonic crystal fiber 203 and the wavelength conversion element 211. The wavelength conversion element 211 includes a nonlinear optical medium 221 made of LiNbO ₃ in which a waveguide 222 is formed, a lens 223 optically coupled to the polarization maintaining photonic crystal fiber 202, and a polarization maintaining photonic crystal fiber. 203 and a lens 224 that optically couples the wavelength conversion element 211 and the wavelength conversion element 211. The second embodiment is a wavelength conversion module that outputs 600 nm converted light by generating a sum frequency from signal light having a wavelength of 1.55 μm and excitation light having a wavelength of 0.98 μm.

The wavelength conversion element 211 is mounted on one carrier, and the temperature of the entire carrier is controlled by a Peltier element and a thermistor. The exit end face of the photonic crystal fiber 202 and the entrance end face of the waveguide 222 have antireflection coatings at wavelengths of 1550 nm and 980 nm, and the exit end face of the waveguide 222 has antireflection coating at a wavelength of 600 nm. ing. The nonlinear optical medium 221 is made of LiNbO ₃ and has a domain-inverted structure. The manufacturing method and the structure of the waveguide are the same as those in the first embodiment. The polarization maintaining photonic crystal fiber is the same as that of the first embodiment.

In Example 2, since the fiber coupler 205 made of the polarization maintaining fiber for 980 nm is used, the mode diameter is different from that of the polarization maintaining photonic crystal fiber 203. However, since the coupling between the fibers performs fusion splicing, the mode diameter between the two fibers in the connection portion changes continuously. Therefore, although the mode diameters between the fibers are different, the connection loss at the connection part can be kept relatively small. The loss due to the fusion splicing between the polarization maintaining fiber 204 for 980 nm and the polarization maintaining photonic crystal fiber 203 with the output of the fiber coupler 205 in Example 2 is 0.6 dB at a wavelength of 1550 nm and 0.3 dB at a wavelength of 980 nm. It is.
The nonlinear optical medium 221 is made of LiNbO ₃ and has a domain-inverted structure. The manufacturing method and the structure of the waveguide are the same as those in the first embodiment. The polarization maintaining photonic crystal fiber is the same as that of the first embodiment.

  In the second embodiment, the structure of the waveguide 222 of the nonlinear optical medium 221 and the polarization maintaining photonic crystal fiber 202 are the same as those in the first embodiment. As shown in FIGS. 15 and 16, the mode diameter of the converted light having a wavelength of 600 nm in the waveguide is substantially equal to the wavelengths of 1550 nm and 980 nm. Since the mode diameter of the polarization maintaining photonic crystal fiber at a wavelength of 600 nm is almost the same as the wavelengths of 1550 nm and 980 nm, the same lens and polarization maintaining photonic crystal fiber are used on the output side as well. Thus, the converted light can be taken out.

  According to the second embodiment, the coupling efficiency between the waveguide 222 and the polarization maintaining photonic crystal fiber 203 can be as high as 91%. In addition, since the converted light is taken out by an optical fiber, the output can be easily handled, and can be realized with the same components as those on the incident side. Therefore, there is an advantage that the components can be manufactured without increasing the types of components.

  In the second embodiment, the conversion efficiency of the entire wavelength conversion module viewed from the input side of the polarization maintaining photonic crystal fiber 202 is 2000% / W. By making the excitation light with a wavelength of 980 nm 100 mW, the signal light with a wavelength of 1550 nm 1 mW, and entering the wavelength conversion module 201, 20 mW of converted light with a wavelength of 600 nm can be obtained. This corresponds to conversion of incident light having a wavelength of 1550 nm into light having a wavelength of 600 nm with an efficiency of 80%.

FIG. 19 shows a wavelength conversion module according to Example 3 of the present invention. Outputs of the light source 308 having a wavelength of 1307 nm and the light source 309 having a wavelength of 980 nm are combined by a multiplexer 305 and input to the wavelength conversion module 301 via the polarization maintaining photonic crystal fiber 302. The wavelength conversion module 301 includes a wavelength conversion element 311, a lens 312 that optically couples the polarization maintaining photonic crystal fiber 302 and the wavelength conversion element 311, and a dichroic mirror 313 that separates and outputs only converted light. including. The wavelength conversion element 311 includes a nonlinear optical medium 321 made of LiNbO ₃ in which a waveguide 322 is formed, a lens 323 that is optically coupled to the polarization maintaining photonic crystal fiber 302, and light output from the waveguide 322. And a lens 324 for collimating. Example 3 is a wavelength conversion module that outputs converted light of 560 nm by sum frequency generation from signal light of wavelength 1307 nm and excitation light of 980 nm.

  The light source 308 having a wavelength of 1307 nm uses a DFB-LD, and the light source 309 having a wavelength of 980 nm uses a Fabry-Perot type LD (FP-LD) whose wavelength is stabilized using a fiber grating 306. The output of the light source 308 is output to the 1.3 μm band polarization maintaining fiber 307, and the output of the light source 309 is output to the 0.98 μm polarization maintaining fiber 306. The multiplexer 305 includes a lens and a dichroic mirror. The multiplexer 305 makes the beam diameters of the light input from the 1.3 μm-band polarization-maintaining fiber 307 and the 0.98 μm-polarization-maintaining fiber 306 approximately equal to each other using lenses having different focal lengths. After being collimated in this way, it is multiplexed by a dichroic mirror. The combined light is condensed and output to the polarization maintaining photonic crystal fiber 302 by the condensing lens.

The wavelength conversion element 311 is mounted on one carrier, and the temperature of the entire carrier is controlled by a Peltier element and a thermistor. The exit end face of the photonic crystal fiber 302 and the entrance end face of the waveguide 322 are provided with antireflection coats at wavelengths of 1307 nm and 980 nm, and the exit end face of the waveguide 322 is provided with antireflection coat at a wavelength of 560 nm. ing. The nonlinear optical medium 321 is made of LiNbO ₃ and has a domain-inverted structure. The polarization maintaining photonic crystal fiber is the same as in the first and second embodiments.

Details of the waveguide 322 of the nonlinear optical medium 321 will be described. The nonlinear optical medium 321 is manufactured by the direct bonding method described in Non-Patent Document 3. FIG. 20 shows a cross-sectional view of the nonlinear optical medium 321. The lower cladding 331 is a Z-cut substrate made of LiTaO ₃ , and the waveguide core 332 is made of a Z-cut substrate made of LiNbO ₃ doped with Zn. The core 332 has a ridge type optical waveguide formed by dry etching. An upper clad 333 made of Ta ₂ O _{5 is} deposited by sputtering so as to cover the core 332 and the lower clad 331, thereby forming a buried waveguide. The core has a thickness of 7 μm and a width of 7 μm. In LiNbO ₃ to which Zn is added, polarization inversion is formed at a period of 7.85 μm. The length of the waveguide is 20 mm, and the normalized SFG conversion efficiency is 3900% / W.

Here, because it uses the nonlinear coefficient d ₃₃ of LiNbO _3, all light incident is TM mode, the converted light is also emitted in the TM mode. The refractive index in the TM mode, that is, the refractive index of the core 132 at the extraordinary light polarization in LiNbO ₃ is about 2.1. The refractive index difference between the core 332 and the lower clad 331 is 0.7% at a wavelength of 1307 nm, 0.8% at 980 nm, and 1.1% at 560 nm. The refractive index of the upper cladding 333 is slightly smaller than that of the lower cladding 331, and the refractive index difference between the core 332 and the upper cladding 333 is 1.7% at a wavelength of 1307 nm, 1.8% at 980 nm, and 1.9% at 560 nm. is there.

In Expression 3, the normalized frequencies V at three wavelengths related to wavelength conversion are V = 8.4 at a wavelength of 1307 nm, V = 11.9 at a wavelength of 980 nm, and V = 26.4 at a wavelength of 560 nm. FIG. 21 shows the vertical electric field distribution of the waveguide at each wavelength, and FIG. 22 shows the horizontal electric field distribution. It can be seen that the embedded waveguide structure of Example 3 has very close mode diameters at each wavelength when V> 4 at all wavelengths. The total width at which the light intensity becomes 1 / e ^{2 of the} peak is 6.3 μm in the vertical direction and 6.1 μm in the horizontal direction at a wavelength of 1307 nm, and 6.1 μm in the vertical direction and 6.1 μm in the horizontal direction at a wavelength of 980 nm.

  By setting the incident mode diameter to about 6.1 μm at wavelengths of 1307 nm and 980 nm, both wavelengths can be efficiently incident. The mode diameter of the polarization maintaining photonic crystal fiber 302 used in Example 3 is 7 μm at wavelengths of 13070 nm and 980 nm. Therefore, the lens diameters 312 and 323 are used to convert the beam diameter so that the coupling efficiency to the waveguide 322 is maximized to 6.1 μm. As a result, the coupling efficiency between the polarization maintaining photonic crystal fiber 302 and the waveguide 322 exhibits a high coupling efficiency of 98% at a wavelength of 1307 nm and 98% at 980 nm. The conversion efficiency of the entire wavelength conversion module 301 viewed from the input side of the polarization maintaining photonic crystal fiber 302 is 3000% / W. By entering the wavelength conversion module 301 with 30 mW of excitation light with a wavelength of 980 nm and 30 mW of signal light with a wavelength of 1307 nm, 25 mW of converted light with a wavelength of 560 nm can be obtained.

  According to the third embodiment, since the coupling loss between the pumping light and the signal light can be suppressed to be small, the power of the pumping light necessary for obtaining the desired converted light output can be reduced. As a result, the power consumption of the light source can be reduced, and effects such as a longer laser life can be obtained. In the third embodiment, an asymmetric buried waveguide in which the upper clad and the lower clad have different refractive indexes is used. However, a symmetrical buried waveguide in which the upper clad and the lower clad have the same refractive index may be used. Similar effects can be obtained.

FIG. 23 shows a wavelength conversion module according to Example 4 of the present invention. The outputs of the 1.3 μm band light source 408 and the 1.02 μm band light source 409 are combined by a multiplexer 405 and input to the wavelength conversion module 401 via the polarization maintaining photonic crystal fiber 402. The wavelength conversion module 401 includes a wavelength conversion element 411 and a dichroic mirror 413 that separates and outputs only converted light. The wavelength conversion element 411 includes a nonlinear optical medium 421 made of LiNbO ₃ in which a waveguide 422 is formed, and a lens 424 that collimates the light output from the waveguide 422. Example 4 is a wavelength conversion module that outputs converted light in the 4500 nm band from the 1.3 μm band signal light and the 1.02 μm band excitation light by generating a difference frequency.

  The 1.3 μm band light source 408 uses a DFB-LD, and the 1.02 μm band light source 409 uses a Fabry-Perot type LD (FP-LD) whose wavelength is stabilized by using a fiber grating 406. The output of the light source 408 is output to the 1.3 μm band polarization maintaining fiber 407, and the output of the light source 409 is output to the 0.98 μm polarization maintaining fiber 406. The multiplexer 405 includes a lens and a dichroic mirror. The multiplexer 405 makes the beam diameters of the light input from the 1.3 μm band polarization-maintaining fiber 407 and the 0.98 μm polarization-maintaining fiber 406 approximately equal to each other using lenses having different focal lengths. After being collimated in this way, it is multiplexed by a dichroic mirror. The combined light is condensed and output to the polarization maintaining photonic crystal fiber 402 by the condensing lens.

The wavelength conversion element 411 is mounted on one carrier, and the temperature of the entire carrier is controlled by a Peltier element and a thermistor. The polarization maintaining photonic crystal fiber 402 and the optical waveguide 422 of the nonlinear optical medium 421 are directly coupled by a butt joint. The exit end face of the photonic crystal fiber 402 and the entrance end face of the waveguide 422 are provided with antireflection coatings at wavelengths of 1.3 μm and 0.98 μm, and the exit end face of the waveguide 422 is antireflection at a wavelength of 4500 nm. There is a coat. The nonlinear optical medium 321 is made of LiNbO ₃ and has a domain-inverted structure. The polarization maintaining photonic crystal fiber is the same as in the first to third embodiments.

Details of the waveguide 422 of the nonlinear optical medium 421 will be described. The nonlinear optical medium 421 is manufactured by the direct bonding method described in Non-Patent Document 3. The lower clad is a Z-cut substrate made of LiTaO ₃ , and the waveguide core is made of a Z-cut substrate made of LiNbO ₃ doped with Zn. The core has a ridge type optical waveguide formed by dicing. The upper cladding is air having a refractive index of 1. The core has a thickness of 14 μm and a width of 17 μm. In LiNbO ₃ to which Zn is added, polarization inversion is formed at a period of 25.15 μm. The length of the waveguide is 60 mm, and the normalized SFG conversion efficiency is 125% / W.

Here, because it uses the nonlinear coefficient d ₃₃ of LiNbO _3, all light incident is TM mode, the converted light is also emitted in the TM mode. The refractive index in the TM mode, that is, the refractive index of the core 132 at the extraordinary light polarization in LiNbO ₃ is about 2.1. The refractive index difference between the core and the lower cladding is 0.66% at a wavelength of 1325 nm, 0.72% at 1024 nm, and 0.96% at 4500 nm.

In Expression 3, the normalized frequencies V at the three wavelengths related to wavelength conversion are V = 16.5 at a wavelength of 1325 nm, V = 22.2 at a wavelength of 1024 nm, and V = 5.5 at a wavelength of 4500 nm. Using the width of the core, V = 22.4 at a wavelength of 1325 nm, V = 30.1 at a wavelength of 1024 nm, and V = 7.46 at a wavelength of 4500 nm. FIG. 24 shows the vertical electric field distribution of the waveguide at each wavelength, and FIG. 25 shows the horizontal electric field distribution. In the ridge-type waveguide structure of Example 4, when V> 4 at all wavelengths, the V-values of the signal light wavelength and the excitation light wavelength are particularly large, so that the mode diameter at each wavelength is very large. You can see that it is close to. The total width at which the light intensity becomes 1 / e ^{2 of the} peak is 12.5 μm in the vertical direction and 13.1 μm in the horizontal direction at a wavelength of 1325 nm, 12.4 μm in the vertical direction and 13.5 μm in the horizontal direction at a wavelength of 1024 nm.

  By setting the incident mode diameter to about 12 μm at wavelengths of 1325 nm and 1024 nm, both wavelengths can be efficiently incident. Since the mode diameter of the polarization-maintaining photonic crystal fiber 402 used in Example 4 is 12 μm at wavelengths of 1325 nm and 1024 nm, the polarization-maintaining photonic crystal fiber 402 and the waveguide 422 are directly coupled. Then enter. As a result, the coupling efficiency between the polarization-maintaining photonic crystal fiber 401 and the waveguide 422 exhibits a high coupling efficiency of 98% at a wavelength of 1325 nm and 97% at a wavelength of 1024 nm. The conversion efficiency of the entire wavelength conversion module 401 viewed from the polarization maintaining photonic crystal fiber 402 is 100% / W. By making the excitation light with a wavelength of 1024 nm 100 mW and the signal light with a wavelength of 1325 nm incident on the wavelength conversion module 401, converted light of 5500 m with a wavelength of 4500 nm can be obtained.

  For comparison, the wavelength conversion module shown in FIG. 23 is configured using a normal polarization maintaining fiber for 980 nm instead of the polarization maintaining photonic crystal fiber 402. The polarization maintaining fiber for 980 nm has a mode diameter of 10 μm at a wavelength of 1325 nm and a mode diameter of 6.8 μm at a wavelength of 980 nm. The coupling efficiency at a wavelength of 1325 nm is reduced to 68%. According to the fourth embodiment, it is possible to efficiently convert signal light having a wavelength of 1325 nm into converted light having a wavelength of 4500 nm by suppressing a decrease in the coupling efficiency between the signal light and the excitation light.

It is sectional drawing which shows the ridge type | mold waveguide concerning the 1st Embodiment of this invention. It is a figure which shows the change of the standardized refractive index B of a fundamental mode (0th-order mode) when changing the normalized frequency V and a primary mode. It is a figure which shows the value which normalized the full width of the core in which the electric field of the vertical direction of a waveguide becomes 1 / e of the maximum value with the thickness T of the core. It is a figure which shows the electric field distribution of the vertical direction of a waveguide when the normalization frequency V is changed. It is a figure which shows the electric field distribution of the horizontal direction of a waveguide when the normalization frequency V is changed. It is sectional drawing which shows the buried type waveguide concerning the 2nd Embodiment of this invention. It is a figure which shows the change of the standardized refractive index B of a fundamental mode (0th-order mode) when changing the normalized frequency V and a primary mode. It is a figure which shows the value which normalized the full width of the core in which the electric field of the vertical direction of a waveguide becomes 1 / e of the maximum value with the thickness T of the core. It is a figure which shows the electric field distribution of the vertical direction of a waveguide when the normalization frequency V is changed. It is sectional drawing which shows the embedded type waveguide concerning the 3rd Embodiment of this invention. It is a figure which shows the change of the standardized refractive index B of a fundamental mode (0th-order mode) when changing the normalized frequency V and a primary mode. It is a figure which shows the value which normalized the full width of the core in which the electric field of the vertical direction of a waveguide becomes 1 / e of the maximum value with the thickness T of the core. It is a figure which shows the wavelength conversion module concerning Example 1 of this invention. FIG. 3 is a cross-sectional view illustrating a nonlinear optical medium of Example 1. It is a figure which shows the electric field distribution of the longitudinal direction of the waveguide in each wavelength. It is a figure which shows the electric field distribution of the horizontal direction of the waveguide in each wavelength. It is a figure which shows the change of the coupling efficiency with respect to an incident beam diameter when the light of a Gaussian beam injects into a waveguide. It is a figure which shows the wavelength conversion module concerning Example 2 of this invention. It is a figure which shows the wavelength conversion module concerning Example 3 of this invention. 6 is a cross-sectional view showing a nonlinear optical medium of Example 3. FIG. It is a figure which shows the electric field distribution of the longitudinal direction of the waveguide in each wavelength. It is a figure which shows the electric field distribution of the horizontal direction of the waveguide in each wavelength. It is a figure which shows the wavelength conversion module concerning Example 4 of this invention. It is a figure which shows the electric field distribution of the longitudinal direction of the waveguide in each wavelength. It is a figure which shows the electric field distribution of the horizontal direction of the waveguide in each wavelength.

Explanation of symbols

11, 21, 31, 131, 331 Lower clad 12, 22, 32, 132, 332 Core 13, 23, 33, 133, 333 Upper clad 101, 201, 301, 401 Wavelength conversion modules 102, 202, 203, 302, 402 Polarization-maintaining photonic crystal fiber 111, 211, 311, 411 Wavelength conversion element 112, 123, 124, 212, 214, 223, 224, 312, 323, 324, 424 Lens 113, 213, 313, 413 Dichroic mirror 121, 221, 321, 421 Nonlinear optical medium 122, 222, 322, 422 Waveguide 204, 304, 404 980 nm polarization maintaining fiber 205 fiber coupler 305, 405 multiplexer 306, 406 fiber grating 307, 40 7 1.3 μm band polarization maintaining fiber 308, 309, 408, 409

Claims (8)

In a wavelength conversion module that inputs excitation light having a wavelength λ1 and signal light having a wavelength λ2 having different wavelengths, and generates converted light having a wavelength λ3,
A nonlinear optical crystal in which a waveguide structure comprising a lower clad having a refractive index n1, a core having a refractive index n2, and an upper clad having a refractive index n3 is formed, and includes the signal light, the excitation light, and the converted light. At any wavelength λ

Where d is the thickness of the core, w is the width of the core, and nc is the larger value of n1 or n3,
A wavelength conversion module comprising: an input photonic crystal fiber optically coupled to the waveguide and configured to input the signal light and the excitation light at an input end of the waveguide with the same mode diameter .   The wavelength conversion module according to claim 1, further comprising an output photonic crystal fiber optically coupled to the waveguide and emitting the converted light from the waveguide.   The input photonic crystal fiber is optically coupled to the waveguide through a lens, and a beam diameter obtained by converting the mode diameter of the input photonic crystal fiber by the lens, and a mode diameter of the waveguide. The wavelength conversion module according to claim 1, wherein the wavelength conversion modules are equal.   The input photonic crystal fiber is optically coupled by directly facing the waveguide, and the mode diameter of the input photonic crystal fiber is made equal to the mode diameter of the waveguide. The wavelength conversion module according to claim 1 or 2.   5. The wavelength conversion module according to claim 1, wherein the photonic crystal fiber is a polarization maintaining type. The lower clad is made of LiTaO ₃ , the core is made of LiNbO ₃ doped with Zn whose polarization is periodically inverted, and the upper clad has a ridge-type waveguide structure made of air. The wavelength conversion module according to claim 1. The wavelength conversion module according to any one of claims 1 to 6,
A multiplexer connected to the input photonic crystal fiber;
A light source connected to one input of the multiplexer and outputting the signal light;
A wavelength-converted light source comprising: a light source connected to the other input of the multiplexer and outputting the excitation light. The wavelength conversion module according to any one of claims 1 to 6,
A multiplexer connected to the input photonic crystal fiber;
A light source connected to one input end of the multiplexer and outputting the excitation light;
The wavelength conversion module, wherein the wavelength conversion module outputs converted light having a wavelength of 400 nm to 1000 nm by generating a sum frequency of the signal light input to the other input terminal of the multiplexer.

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