Second harmonic generation device

Abstract

A second harmonic generation device according to the present disclosure branches off, by a dichroic mirror inside an optical module, a part of second harmonic light (SHG light) generated by a second-order nonlinear optical element such as a PPLN element, and supplies monitoring light or a monitoring voltage to the outside of the module. In an output optical circuit or an input optical circuit of the PPLN element, two dichroic mirrors are combined to extract SHG light in a collimated light state, thereby avoiding output level limitation of the SHG light and damage to a connector connection part. In the input optical circuit, return light of the SHG light or return light of a fundamental wave light can also be utilized as monitoring light. Provided is a second harmonic generation device that suppresses a reduction in wavelength conversion efficiency and damage to a connector connection part.

Description

Second Harmonic Generation Apparatus

[0001] The present invention relates to a wavelength conversion technique utilizing a second-order nonlinear optical effect.

[0002] Wavelength conversion techniques have been put into practical use in various fields such as optical communication systems, optical processing, medical treatment, and biotechnology. In optical communication systems, wavelength conversion techniques based on second-order nonlinear optical effects are widely used, including wavelength conversion devices and optical amplification devices utilizing optical amplification operation based on the parametric effect. As a material for second-order nonlinear optical elements, lithium niobate (LiNbO₃, hereinafter referred to as LN) having a large nonlinear constant is often used. In particular, periodically polarization-reversed LN is called PPLN (Periodically Poled Lithium Niobate), and can obtain an overwhelmingly higher nonlinear optical effect than the original LN crystal. The nonlinear optical effect increases as the overlapping density of light that causes nonlinear interaction becomes higher. By forming a ridge waveguide in a PPLN element and adopting a waveguide structure that confines light in a small cross-sectional area and guides light over a long distance, more efficient wavelength conversion can be achieved.

[0003] In the second-order nonlinear optical effect, light with wavelengths λ₁ and λ₂ is input to generate new light with wavelength λ₃. Wavelength conversion satisfying the following formula is called Sum Frequency Generation (SFG).

[0004] 1 / λ₃ = 1 / λ₁ + 1 / λ₂ Formula (1) When λ₁=λ₂ in Formula (1), further transformation is performed, and wavelength conversion satisfying the following formula is called Second Harmonic Generation (SHG).

[0005] λ₃ = λ₁ / 2 Formula (2) Wavelength conversion satisfying the following formula is called Difference Frequency Generation (DFG).

[0006] 1 / λ3 = 1 / λ1 - 1 / λ2 Equation (3) By combining the above wavelength conversion mechanisms, wavelength conversion systems and optical amplification systems can be constructed. There is a growing demand for pigtail-type optical modules that include fibers with fused optical connectors, with optical components mounted together with PPLN elements inside a metal housing.

[0007] International Publication No. 2024 / 247066

[0008] Optical modules using PPLN elements, or systems combining these optical modules, require higher precision and stability in wavelength conversion operation. PPLN elements have problems with refractive index changes due to thermo-optic effects caused by temperature inhomogeneity and temporal fluctuations, EO effects due to electric field generation caused by pyroelectricity due to temperature changes, and optical damage. This change in refractive index causes a change in the birefringence state of the second-order nonlinear optical element, requiring monitoring of the birefringence state (Patent Document 1). In addition, in order to understand the stability of the wavelength-converted light, it is also necessary to monitor the level of the second-harmonic light (SHG light) used as excitation light. In pigtail-type optical modules, the SHG light monitoring mechanism had problems such as limiting the output level of the SHG light or damaging the connector connection.

[0009] One aspect of the present invention is a second harmonic generator comprising a nonlinear optical element in a metal housing, the second harmonic generator comprising: an input optical circuit that receives fundamental light and optically couples it to one end of the waveguide of the nonlinear optical element; and an output optical circuit that receives second harmonic light (SHG light) generated by the nonlinear optical element, the output optical circuit comprising: a first lens that collimates the SHG light from the other end of the waveguide of the nonlinear optical element; a first dichroic mirror that reflects the collimated SHG light at a predetermined rate and totally reflects the residual component of the fundamental light; a second lens that focuses the collimated SHG light transmitted through the first dichroic mirror onto the core of an output fiber; and a second dichroic mirror that reflects the SHG light reflected by the first dichroic mirror, wherein the second harmonic generator is configured such that the reflected light from the second dichroic mirror can be used as monitoring light for the SHG light.

[0010] Another aspect of the present invention is a second harmonic generator comprising a metal housing containing a nonlinear optical element, the input optical circuit receiving fundamental light and optically coupling it to one end of the waveguide of the nonlinear optical element, the input optical circuit including a first lens for collimating the fundamental light, a first dichroic mirror that fully transmits the collimated fundamental light and totally reflects the return light of the second harmonic light (SHG light) generated in the nonlinear optical element, a second lens that focuses the fundamental light transmitted through the first dichroic mirror to one end of the waveguide of the nonlinear optical element, and a second dichroic mirror that reflects the return light of the SHG light reflected by the first dichroic mirror, and the output optical circuit that receives the SHG light generated in the nonlinear optical element, wherein the return light of the SHG light from the second dichroic mirror is configured to be usable as monitoring light.

[0011] A further aspect of the present invention is a second harmonic generator comprising a metal housing containing a nonlinear optical element, the input optical circuit receiving fundamental light and optically coupling it to one end of the waveguide of the nonlinear optical element, the input optical circuit including a first lens for collimating the fundamental light, a first dichroic mirror that fully transmits the collimated fundamental light and totally reflects the return light of the fundamental light from the nonlinear optical element, a second lens that focuses the fundamental light that has passed through the first dichroic mirror to one end of the waveguide of the nonlinear optical element, and a second dichroic mirror that reflects the return light of the fundamental light reflected by the first dichroic mirror, and an output optical circuit that receives second harmonic light (SHG light) generated by the nonlinear optical element, wherein the return light of the fundamental light from the second dichroic mirror is configured to be usable as monitoring light.

[0012] The present invention provides a second harmonic generator that suppresses the reduction in wavelength conversion efficiency and damage to the connector connection.

[0013] This diagram illustrates the configuration of a wavelength conversion system including an SHG device, its operation, the SHG device and the principle of SHG light generation, a more specific configuration of a conventional SHG device, a diagram illustrating the SHG light monitoring mechanism in a conventional SHG device, the configuration of an SHG device with a monitoring output function according to Embodiment 1, the filter characteristics of the SHG device according to Embodiment 1, the configuration of an SHG device according to Modification 1 of Embodiment 1, the configuration of an SHG device according to Modification 2 of Embodiment 1, the configuration of an SHG device according to Modification 3 of Embodiment 1, the configuration of an SHG device according to Modification 4 of Embodiment 1, the configuration of an SHG device with a monitoring output function according to Embodiment 2, the configuration of an SHG device with a monitoring output function according to Embodiment 3, the filter characteristics when monitoring SHG light in Embodiment 2, and the filter characteristics when monitoring fundamental light in Embodiment 2.

[0014] The second harmonic generator of this disclosure splits a portion of the second harmonic light (SHG light) generated by a second-order nonlinear optical element such as a PPLN element using a mirror inside the module, and supplies monitoring light or monitoring voltage outside the module. By combining two dichroic mirrors on the output or input side of the PPLN element and extracting the SHG light in a collimated state, it is possible to avoid limiting the output level of the SHG light and damaging the connector connection. Below, we will first describe the conventional SHG light monitoring configuration and its problems. Then, we will describe the second harmonic generator having the monitoring mechanism of this disclosure. Hereinafter, for simplicity, the second harmonic generator will be referred to as the SHG device.

[0015] Figure 1 is a conceptual diagram illustrating the configuration and operation of a wavelength conversion system including an SHG device. Figure 1(a) is a configuration diagram of the wavelength conversion system 10, which includes an SHG device 300 that generates SHG light, which is excitation light 22, and a wavelength conversion device 200 that generates wavelength-converted light 23 from signal light 20 and excitation light 22 by DFG operation. Figure 1(b) is a diagram illustrating the wavelength conversion operation in the wavelength conversion system 10 on the wavelength axis. The SHG device 300 receives fundamental wave light 21 as input, and the SHG mechanism generates excitation light 22, which is SHG light. Furthermore, in the wavelength conversion device 200, wavelength-converted light 23 is generated by the DFG process of the excitation light 22 and signal light 20.

[0016] Fibers 31 and 34 for the 1.55 μm band are used for the fundamental light 21, signal light 20, and wavelength-converted light 23, for example, and fiber 33 for the 0.78 μm band is used for the excitation light 22. Both the SHG device 300 and the wavelength converter 200 are composed of pigtail-type optical modules, and the excitation light 22 passes through the optical connector connection section 153. In the following description, unless otherwise specified, the term "optical module" refers to the SHG device.

[0017] Figure 2 is a diagram illustrating the SHG device and the principle of SHG light generation. Figure 2(a) shows the principle of SHG light generation 22 in an SHG device 300 using a PPLN element 102. When a sufficient level of fundamental wave light 21 is input to one end of the PPLN element 102, SHG light 22-1 is output from the other end of the PPLN element 102 due to the nonlinear optical effect that occurs in the waveguide. At the same time, a residual component 21-1 of the residual fundamental wave light is also output, so an optical filter 110 is required to remove the residual component 21-1.

[0018] Figure 2(b) schematically shows the characteristics of the optical filter on the output side of the SHG device. It is required to have a transmittance of 100% at a wavelength of 775 nm for SHG light and a transmittance of 0% at a wavelength of 1550 nm for the fundamental wave light.

[0019] Figure 3 shows a more specific configuration of a conventional SHG device. In the SHG device 300 in Figure 3, the paths and elements of each light source are schematically depicted, and the specific configuration of components and holding mechanisms are omitted or simplified. The fundamental wave light is amplified to a level sufficient to produce a nonlinear optical effect by the optical amplifier 150, and then input to the SHG device 300 via the bandpass filter (BPF) 151.

[0020] The SHG device 300 is entirely housed inside a metal casing 101. The fundamental wave light 21 is guided into the metal casing 101 by a fiber, passes through an optical window 120, and is optically coupled to the PPLN element 102 from outside the device. A ridge-type optical waveguide 103 is formed in the PPLN element 102. On the input side of the PPLN element 102, the incident fundamental wave light is collimated by a lens 104, and then focused again by a lens 105 to the end face of the optical waveguide of the PPLN element 102, thereby optically coupling the fundamental wave light. On the output side of the PPLN element 102, similar to the input side, the SHG light emitted from the optical waveguide 103 is collimated by a lens 106, and then focused again by a lens 107 to the core of the fiber 33, thereby optically coupling the SHG light 22.

[0021] In an SHG device with the configuration described above, it is necessary to monitor the level of excitation light, i.e., SHG light, output from the SHG device, in order to understand the operating status of the system shown in Figure 1 and to detect and analyze problems.

[0022] Figure 4 is a diagram illustrating the monitoring mechanism for SHG light in a conventional SHG device. The wavelength conversion system 10-1 in Figure 4 is generally the same configuration as the wavelength conversion system 10 shown in Figure 1, but it further includes an optical splitter 152 for extracting monitoring light 24 and a detector 130. The optical splitter 152 supplies 90% of its power to the wavelength conversion device 200 and leads 10% of its power to the detector 130. The monitoring voltage 25 is obtained as the output of the detector 130.

[0023] The monitoring mechanism for SHG light in the SHG apparatus shown in Figure 4 had the following problem: The optical splitter 152 operates in a relatively short wavelength band of excitation light wavelength 775 nm, resulting in high losses in the optical splitter. For example, in the case of a fused optical splitter, with a branching ratio of 9:1, the overall insertion loss is about 1.5 dB.

[0024] Furthermore, in order to obtain sufficient wavelength conversion efficiency in the wavelength converter 200, a sufficiently high level of excitation light must be supplied from the SHG device 300 to the wavelength converter 200. However, in a system configuration including two pigtail-type optical modules, two connector connections 153-1 and 153-2 are required on both sides of the optical splitter 152. At these connector connections, high levels of excitation light can damage the fiber end faces. Since such damage causes permanent performance degradation of the system, it was necessary to limit the level of excitation light to a level that would not cause damage. For example, at the connector connection of a single-mode fiber with a core diameter of about 5 μm, the maximum power required to avoid damage with excitation light of wavelength 775 nm is about 1 W. The losses that occur in the optical splitter mentioned above also contributed to suppressing the level of excitation light supplied to the wavelength converter 200. The decrease in the excitation light level led to a decrease in the wavelength conversion efficiency of the wavelength converter 200.

[0025] The configuration and operation of the SHG apparatus of this disclosure will be described below, along with various modifications. In each embodiment, the wavelengths of the signal light, wavelength-converted light, and fundamental wave light are generally in the 1550 nm band, and the wavelength of the excitation light, the SHG light, is in the 775 nm band, but the invention is not limited to these examples. An example using a PPLN element as the second-order nonlinear optical element will be described, but the material of the second-order nonlinear optical element is not limited to this. For example, LN, LiTaO3, or LiNb having a periodic polarization reversal structure. (x) Ta (1-x)O3 (0 ≤ x ≤ 1) may be any of these, or may contain at least one selected from the group Mg, Zn, Sc, and In as an additive. As mentioned above, in the following description, the second harmonic generator will be abbreviated as the SHG device. [Embodiment 1] Figure 5 is a diagram showing the configuration of the SHG device having a monitoring output function according to Embodiment 1. The SHG device 100-1 in Figure 5 is a pigtail type optical module, in which a PPLN element 102 and optical components are mounted inside a box-shaped metal housing 101. A ridge-type optical waveguide 103 is formed in the PPLN element 102. Parts of the metal housing protrude from both sides of the PPLN element, and have port portions that include space. An external fiber is fixed to the port portion, and light is input or output inside and outside the optical module. The input side of the metal-cased PPLN element is equipped with an input optical circuit including two lenses 104 and 105, which optically couples the core of the 1.55 μm band fiber that guides the fundamental wave light 21 with the optical waveguide 103 of the PPLN element 102. The configuration of the SHG apparatus 100-1 shown in Figure 5 above, including the configuration of the optical amplifier 150 and BPF 151 connected to the input port side, is the same as that of the conventional SHG apparatus 300. The output optical circuit, which includes lenses 106 and 107 that optically couple the core of the optical waveguide 103 of the PPLN element 102 with the core of the 0.78 nm band fiber 33-1 for SHG light, is also the same as that of the conventional SHG apparatus 300.

[0026] The difference from the conventional SHG device 300 is that the output optical circuit of the PPLN element 102 includes two dichroic mirrors 111 and 112 that branch off a portion of the SHG light output from the PPLN element 102. In the conventional SHG device, the SHG light was branched at an optical splitter outside the optical module to obtain monitoring light. In contrast, in the SHG device of this disclosure, a portion of the SHG light is branched inside the optical module in a collimated state. Based on the branched SHG light, monitoring light or monitoring voltage of the SHG light can be output to the outside of the optical module.

[0027] The SHG device 100-1 includes a first dichroic mirror 111 that branches a portion of the collimated SHG light between lenses 106 and 107, and a second dichroic mirror 112 that enables the branched SHG light to be output externally. The second dichroic mirror 112 focuses the branched SHG light with lens 108 and optically couples it to the core of the monitoring light fiber 33-2. The filter characteristics of the first dichroic mirror 111 and the second dichroic mirror 112 will be described later.

[0028] In the SHG device 300, the SHG light collimated between lenses 106 and 107 is split at a predetermined ratio (branching ratio). As a result, the power density of the collimated SHG light is orders of magnitude smaller than when using a conventional fiber-based optical splitter. The power density ratio of the SHG light can be reduced to about 1 / 10000 compared to the conventional technology. Therefore, the limitation on the excitation light level that was a problem in the conventional technology is largely eliminated. In the wavelength conversion system shown in Figure 4, an optical splitter was inserted between two pigtail-type optical modules, requiring at least two connector connections. With the configuration of the SHG device 100-1 of this disclosure shown in Figure 5, the number of connector connections can be reduced to at least one. Furthermore, if an implementation method is adopted in which the two optical modules are directly connected by a fiber, the connector connection can be eliminated in the path of the excitation light.

[0029] Figure 6 shows the dichroic mirror characteristics of the SHG apparatus of Embodiment 1. Figure 6(a) shows the transmission and reflection characteristics of the first dichroic mirror. Focusing on the excitation light, i.e., SHG light wavelength of 775 nm and the fundamental wave light wavelength of 1550 nm, the transmittance is shown by a solid line and the reflectance by a dotted line. The first dichroic mirror transmits a predetermined percentage (for example, 90%) of the SHG light from the PPLN element 102 toward the lens 107. The remaining 10% of the SHG light is reflected toward the second dichroic mirror 115. The residual fundamental wave from the PPLN element 102 is totally reflected toward the second dichroic mirror 115.

[0030] Figure 6(a) shows the transmission and reflection characteristics of the second dichroic mirror 115. For SHG light with a wavelength of 775 nm, total internal reflection is directed toward lens 108. The two dichroic mirrors 114 and 115 allow 10% of the SHG light to be output to fiber 33-2 as monitoring light 24. The residual fundamental light is transmitted directly through the second dichroic mirror 115. The two dichroic mirrors with the filter characteristics shown in Figure 5 allow the SHG light, which is the excitation light, to be split at a predetermined ratio (branching ratio) and supplied outside the optical module as monitoring light 24. As will be described later, it can also be input to a detector inside the optical module. The predetermined ratio of the transmittance mentioned above can be set to any value by adjusting the number of layers of the dielectric multilayer film of the dichroic mirror.

[0031] Accordingly, the SHG apparatus of this disclosure is a second harmonic generator comprising a nonlinear optical element 102 in a metal housing, an input optical circuit that receives fundamental wave light 21 and optically couples it to one end of the optical waveguide 103 of the nonlinear optical element, and an output optical circuit that receives second harmonic light (SHG light) generated by the nonlinear optical element, a first lens 106 that collimates the SHG light from the other end of the waveguide of the nonlinear optical element, and reflects the collimated SHG light at a predetermined ratio, and the fundamental wave The system can be implemented with an output optical circuit including a first dichroic mirror 111 that totally reflects the residual component of light, a second lens 107 that focuses the collimated SHG light transmitted through the first dichroic mirror onto the core of an output fiber, and a second dichroic mirror 112 that reflects the SHG light reflected by the first dichroic mirror, wherein the reflected light from the second dichroic mirror can be used as a monitoring light 24 for the SHG light.

[0032] The SHG device 100-1 shown in Figure 5 can be modified in various ways to accommodate different monitoring functions and to stabilize the monitoring mechanism. [Modification 1 of Embodiment 1] Figure 7 shows the configuration of the SHG device of Modification 1 of Embodiment 1. In the SHG device 100-1 of Figure 5, the monitoring light was configured to be taken out of the optical module as monitoring information for the SHG light, which is the excitation light. The SHG device 100-2 of Modification 1 in Figure 7 is configured to take out the monitoring information as an electrical signal. Therefore, the basic configuration for branching the excitation light having two dichroic mirrors is the same as that of the SHG device in Figure 5, and only the differences will be explained.

[0033] The SHG light from the PPLN element is reflected at a predetermined rate by the first dichroic mirror 111 and then totally reflected by the second dichroic mirror 112. In the configuration shown in Figure 7, a detector 130 that converts the SHG light totally reflected by the second dichroic mirror into an electrical signal is provided on the inside of the metal housing. The detector 130 generates an electrical signal corresponding to the power level of the received SHG light and can output the electrical signal as a monitoring voltage 25 at the voltage output port on the outside of the metal housing 101. Since the electrical signal from the optical module can be used as monitoring information for the SHG light, all elements of the monitoring mechanism of the SHG device 300 shown in Figure 4, including the two connector connection parts 153-1 and 153-2, can be replaced by the SHG device 100-1. [Modification 2 of Embodiment 1] Figure 8 shows the configuration of the SHG device of Modification 2 of Embodiment 1. The modified SHG device 100-3 in Figure 8 has a configuration in which a filter 113 for removing ASE noise is incorporated into the optical module, compared to the SHG device 100-1 in Figure 5.

[0034] In the SHG device 100-1 shown in Figure 5, a BPF 151 was provided upstream of the optical module to reduce ASE noise. Inserting the BPF required two connector connections between the optical amplifier 150 and the optical module. In the SHG device 100-3, the function of the BPF 151 is realized by a filter 113 placed between lenses 104 and 105 of the input optical circuit.

[0035] In the SHG apparatus 100-1, the externally positioned BPF 151 is an optical fiber-based optical component and experiences loss even at the fundamental wave wavelength of 1.55 μm. As in the SHG apparatus 100-3 of this modified example 2, the loss to the fundamental wave light can be reduced by filtering the large beam diameter fundamental wave light that has been collimated by the lens 104. Specifically, when the external BPF is configured as a fiber pigtail type, the loss at a wavelength of 1.55 μm of fundamental wave light is approximately 3 dB. On the other hand, when the filter 113 is configured as a dielectric multilayer film, the loss at a length of 1.55 μm is approximately 0.2 dB.

[0036] It goes without saying that the configuration of this modified example, which includes the ASE filter in the input optical circuit, can also be applied to the SHG devices in Figures 5 and 7. [Modified Example 3 of Embodiment 1] Figure 9 shows the configuration of the SHG device of Modified Example 3 of Embodiment 1. The SHG device 100-4 of Modified Example 1 in Figure 9 has a configuration for processing the residual component of the fundamental wave light that appears on the output side of the PPLN element, compared to the SHG device 100-1 in Figure 5. Therefore, only the differences from the SHG device 100-1 in Figure 5 will be explained.

[0037] The input fundamental wave light 21 is converted to SHG light in the SHG process in the PPLN element 102 and consumed, and is significantly attenuated at the output terminal of the PPLN element. The remaining fundamental wave component is totally reflected by the first dichroic mirror 111, as shown in the filter characteristics in Figure 6, and reaches the second dichroic mirror 112. Since the second dichroic mirror 112 has 100% transmittance at the wavelength of the fundamental wave, the residual component of the fundamental wave strikes the inner surface of the metal housing and is absorbed and reflected. A portion of the residual component of the fundamental wave returns in the reverse direction and propagates in the reverse direction through the two dichroic mirrors to the PPLN element 102. Such reflected fundamental wave light can adversely affect the operation of the SHG device.

[0038] The SHG apparatus 100-4 is equipped with a light absorber 131 on the inner surface of the metal housing near the second dichroic mirror 112. The light absorber 131 absorbs the residual component 21-1 of the fundamental wave light transmitted through the second dichroic mirror 112, preventing the reflected light from reaching the PPLN element 102. The configuration of the SHG apparatus in Modified Example 3 makes it possible to suppress the destabilization of the SHG apparatus due to the reflected light of the fundamental wave light. Examples of the light absorber 131 include dyes and organic compounds.

[0039] It goes without saying that the configuration for processing the unwanted residual components of fundamental light in this modified example can be directly applied to any of the SHG devices in Figures 5, 7, and 8. [Modified Example 4 of Embodiment 1] Figure 10 shows the configuration of the SHG device of Modified Example 4 of Embodiment 1. The SHG device 100-5 of Modified Example 4 in Figure 10 provides another configuration for processing the residual components of fundamental light appearing on the output side of the PPLN element, similar to the SHG device 100-4 in Figure 9. In the SHG device 100-4, the residual components of fundamental light were absorbed inside the optical module, but in the SHG device 100-5 of this modified example, the residual components are released outside the optical module.

[0040] The SHG apparatus 100-5 is equipped with a sealing window 123 in the metal housing near the second dichroic mirror 112 to allow residual components of the fundamental wave light to escape to the outside. The sealing window 123 is configured to optically transmit the wavelength components of the fundamental wave light as they are. The residual components 21-1 of the fundamental wave light that pass through the second dichroic mirror 112 are allowed to escape to the outside of the optical module, and the reflected light does not reach the PPLN element 102. Similar to the SHG apparatus of Modified Example 3, it is possible to suppress instability of the SHG apparatus due to reflected light of the fundamental wave light.

[0041] It goes without saying that the configuration for processing residual components of unnecessary fundamental wave light in the present modification can be directly applied to any of the SHG devices shown in FIGS. 5, 7 and 8. [Embodiment 2] In the SHG device of Embodiment 1 described above, two dichroic mirrors are provided in an output optical circuit on the output side of a PPLN element, and monitoring light or monitoring voltage for SHG light serving as excitation light is output to realize a monitoring function. Monitoring of excitation light can also be performed using SHG light or fundamental wave light obtained on the input side of the PPLN element.

[0042] FIG. 11 is a diagram showing the configuration of an SHG device having a monitoring output function according to Embodiment 2. The SHG device 100-6 in FIG. 11 is a pigtail-type optical module, in which a PPLN element 102 and optical components are mounted inside a box-shaped metal housing 101. A ridge-type optical waveguide 103 is formed on the PPLN element 102. Portions for fixing external fibers are provided on both sides of the PPLN element, and light is input or output between the inside and outside of the optical module. An input optical circuit including two lenses 104 and 105 is provided on the input side of the PPLN element in the metal housing, and optically couples the core of a 1.55 μm band fiber that guides fundamental wave light 21 and the core of the optical waveguide 103 of the PPLN element 102. The output side of the PPLN element also has the same configuration as that of the SHG device 300 according to the prior art.

[0043] The difference from the prior art SHG device 300 is that, as the input optical circuit of the PPLN element 102, dichroic mirrors 114 and 115 for branching return light of fundamental wave light or return light of SHG light are provided between the lens 104 and the lens 105. Further, similarly to Modification 2 of Embodiment 1, a filter 113 for reducing ASE noise is also provided.

[0044] In the SHG device 100-6 of the present embodiment, return light of SHG light generated by the PPLN element 102, or fundamental wave light reflected by the PPLN element 102 is used as monitoring light for SHG light. SHG light or fundamental wave light propagating from the PPLN element 102 to the input side also indirectly reflects the SHG process occurring in the SHG device 300, and can be used as monitoring light for SHG light.

[0045] The SHG light or fundamental wave light propagating through the PPLN element 102 to the input side is totally reflected by the first dichroic mirror 114, and further reflected by the second dichroic mirror 115. The reflected SHG light or fundamental wave light is condensed by the lens 109 onto the core of the fiber for the monitoring light 24, and is optically coupled. A fiber of a corresponding wavelength band is used depending on whether SHG light or fundamental wave light is used as the monitoring light 24. Specific examples of the filter characteristics of the first dichroic mirror 114 and the second dichroic mirror 115 will be described later with reference to FIGS. 13 and 14. It should be noted that although the same first dichroic mirror as in Embodiment 1 is used in terms of terminology, the first dichroic mirror 114 in Embodiments 2 and 3 is different from the first dichroic mirror 111 in Embodiment 1. The same applies to the terms "second dichroic mirror" 112 and 115.

[0046] According to the SHG device 100-6 having the monitoring output function of Embodiment 2, since monitoring light from the input port side of the pigtail-type optical module is used, an optical splitter is not required on the output port side. With the SHG device 100-6 of Embodiment 2, the number of connector connection portions on the output port side of the optical module can be reduced to at least one. Furthermore, if a mounting method in which two optical modules are directly connected by a fiber is adopted, the connector connection portion can be eliminated in the pump light output path.

[0047] The use of the detector 130 in the SHG device 100-2 shown in FIG. 7 can also be applied to the SHG device 100-6 in FIG. 11. That is, the detector 130 that receives SHG light or fundamental wave light can be arranged on the inner surface of the metal housing at the output destination of the second dichroic mirror 115 in FIG. 11, and an electrical signal can be output. [Embodiment 3] It is also possible to provide both the configuration of the two dichroic mirrors 111 and 112 of the output optical circuit in the SHG device 100-1 of the above-described Embodiment 1, and the configuration of the two dichroic mirrors 114 and 115 of the input optical circuit in the SHG device 100-1 of Embodiment 2.

[0048] Figure 12 shows the configuration of an SHG device having a monitoring output function according to Embodiment 3. The SHG device 100-7 in Figure 12 is a pigtail-type optical module, with a PPLN element 102 and optical components mounted inside a box-shaped metal housing 101. The basic configuration of the optical module in Embodiment 3 is the same as that of the SHG devices in Embodiments 1 and 2. The SHG device 100-7 in Figure 12 can be obtained by replacing the output optical circuit in the SHG device 100-6 in Figure 11 with a configuration that includes the two dichroic mirrors 111 and 112 shown in Figure 5.

[0049] Therefore, the SHG device 100-7 can utilize two types of monitoring light from the input port and output port sides of the optical module. One is a first monitoring light 24-1 based on the return light from the PPLN element 12 at the input port side of the optical module. The other is a second monitoring light 24-2 based on the branched light of the SHG light from the PPLN element 102 at the output port side of the optical module. By using two types of monitoring light, it becomes easier to detect and analyze malfunctions in the operation of the SHG device. For example, possible causes of a decrease in HG light output include attenuation of excitation light inside the PPLN element, and the attenuation of SHG light within the element even though sufficient SHG light output is generated. This can be used as a clue for isolating and analyzing such problems in the SHG device. Furthermore, it can also be used to detect and analyze malfunctions in the overall operation of the wavelength conversion system shown in Figure 1.

[0050] Figure 13 shows the filter characteristics when monitoring the reflected light of SHG light. It shows the transmission and reflection characteristics of two dichroic mirrors when using the reflected light of SHG light as the monitoring light 24 in Embodiment 2 and the monitoring light 24-1 in Embodiment 3. Focusing on the excitation light, i.e., the wavelength of SHG light of 775 nm and the wavelength of the fundamental light of 1550 nm, the transmittance is shown by a solid line and the reflectance by a dotted line. Figure 13(a) shows the characteristics of the first dichroic mirror 114, and (b) shows the characteristics of the second dichroic mirror 115.

[0051] As shown in Figure 13(a), the first dichroic mirror 114 totally reflects the reflected light of the SHG light propagating in the reverse direction toward the input port side of the PPLN element 102. On the other hand, it totally transmits the input fundamental wave light 21 toward the PPPLN element.

[0052] As shown in Figure 13(b), the second dichroic mirror 115 further totally reflects the SHG light that has been totally reflected by the first dichroic mirror 114. As shown in Figure 12, the SHG light totally reflected by the second dichroic mirror 115 is focused by the lens 109 onto the core of a 775 nm band fiber and output as monitoring light 24.

[0053] Accordingly, the present invention can be implemented as an input optical circuit that receives fundamental light 21 and optically couples it to one end of the optical waveguide 103 of the nonlinear optical element, comprising: a first lens 104 for collimating the fundamental light; a first dichroic mirror 114 that fully transmits the collimated fundamental light and fully reflects the return light of the second harmonic light (SHG light) generated in the nonlinear optical element; a second lens 105 that focuses the fundamental light transmitted through the first dichroic mirror to one end of the waveguide of the nonlinear optical element; and a second dichroic mirror 115 that reflects the return light of the SHG light reflected by the first dichroic mirror; and an output optical circuit that receives the SHG light generated in the nonlinear optical element, configured so that the return light of the SHG light from the second dichroic mirror can be used as monitoring light 24.

[0054] Figure 14 shows the filter characteristics when monitoring the reflected light of the fundamental wave. In Embodiments 2 and 3, the transmission and reflection characteristics of two dichroic mirrors are shown when the reflected light of the fundamental wave is used as the monitoring light 24 and 24-1. Focusing on the excitation light, i.e., SHG light wavelength of 775 nm and the fundamental wave wavelength of 1550 nm, the transmittance is shown by a solid line and the reflectance by a dotted line. Figure 14(a) shows the characteristics of the first dichroic mirror 114, and (b) shows the characteristics of the second dichroic mirror 115.

[0055] As shown in Figure 14(a), the first dichroic mirror 114 transmits the input fundamental wave light 21 in the direction of the PPPLN element at a predetermined rate (for example, a transmittance of 90%). On the other hand, it reflects the return light of the fundamental wave light propagating from the PPLN element 102 toward the input port toward the second dichroic mirror 115 at a predetermined rate (for example, 10%).

[0056] As shown in Figure 14(b), the second dichroic mirror 115 further totally reflects the return light of the fundamental wave reflected by the first dichroic mirror 114. As shown in Figure 12, the return light of the fundamental wave totally reflected by the second dichroic mirror 115 is focused by the lens 109 onto the core of the 1550 nm band fiber and output as monitoring light 24.

[0057] Accordingly, the present invention can also be implemented as a second harmonic generator comprising a metal housing containing a nonlinear optical element 102, an input optical circuit that receives fundamental wave light 21 and optically couples it to one end of the waveguide of the nonlinear optical element, the input optical circuit including a first lens 104 for collimating the fundamental wave light, a first dichroic mirror 114 that fully transmits the collimated fundamental wave light and fully reflects the reflected light of the fundamental wave light from the nonlinear optical element, a second lens 105 that focuses the fundamental wave light that has passed through the first dichroic mirror to one end of the waveguide of the nonlinear optical element, and a second dichroic mirror 115 that reflects the reflected light of the fundamental wave light reflected by the first dichroic mirror, and an output optical circuit that receives second harmonic light (SHG light) generated by the nonlinear optical element, wherein the reflected light of the fundamental wave light from the second dichroic mirror can be used as monitoring light 24.

[0058] As described in detail above, the second harmonic generator of the present invention can suppress the decrease in wavelength conversion efficiency and damage to the connector connection in the wavelength conversion system.

[0059] This invention can be used in optical signal processing and optical communication systems.

Claims

1. A second harmonic generator comprising a metal housing equipped with a nonlinear optical element, the second harmonic generator comprising: an input optical circuit that receives fundamental light and optically couples it to one end of the waveguide of the nonlinear optical element; and an output optical circuit that receives second harmonic light (SHG light) generated by the nonlinear optical element, the output optical circuit comprising: a first lens that collimates the SHG light from the other end of the waveguide of the nonlinear optical element; a first dichroic mirror that reflects the collimated SHG light at a predetermined rate and totally reflects the residual component of the fundamental light; a second lens that focuses the collimated SHG light transmitted through the first dichroic mirror onto the core of an output fiber; and an output optical circuit including a second dichroic mirror that reflects the SHG light reflected by the first dichroic mirror, wherein the reflected light from the second dichroic mirror is configured to be usable as monitoring light for the SHG light.

2. The second harmonic generator according to claim 1, further comprising a third lens that focuses the SHG light reflected by the second dichroic mirror onto the core of an output fiber for monitoring light and outputs the monitoring light.

3. The second harmonic generator according to claim 1, further comprising a photodetector inside the metal housing that receives the SHG light reflected by the second dichroic mirror and outputs an electrical signal.

4. The second harmonic generator according to claim 1, wherein the input optical circuit includes a fourth lens for collimating the fundamental wave light, a fifth lens for focusing the collimated fundamental wave light to one end of the waveguide of the nonlinear optical element, and an optical filter between the fourth lens and the fifth lens for removing spontaneous emission noise.

5. The second harmonic generator according to claim 1, further comprising a light absorber for the residual component of the fundamental wave light transmitted through the second dichroic mirror.

6. The second harmonic generator according to claim 1, further comprising an optical window for extracting the residual components of the fundamental wave light transmitted through the second dichroic mirror to the outside of the metal housing.

7. A second harmonic generator comprising a metal housing equipped with a nonlinear optical element, the input optical circuit receiving fundamental light and optically coupling it to one end of the waveguide of the nonlinear optical element, comprising: a first lens for collimating the fundamental light; a first dichroic mirror that fully transmits the collimated fundamental light and totally reflects the reflected light of the second harmonic light (SHG light) generated in the nonlinear optical element; a second lens that focuses the fundamental light transmitted through the first dichroic mirror to one end of the waveguide of the nonlinear optical element; and an output optical circuit that receives the SHG light generated in the nonlinear optical element, wherein the input optical circuit is configured so that the reflected light of the SHG light from the second dichroic mirror can be used as monitoring light.

8. A second harmonic generator comprising a metal housing equipped with a nonlinear optical element, the input optical circuit receiving fundamental light and optically coupling it to one end of the waveguide of the nonlinear optical element, the input optical circuit including a first lens for collimating the fundamental light, a first dichroic mirror that fully transmits the collimated fundamental light and totally reflects the return light of the fundamental light from the nonlinear optical element, a second lens that focuses the fundamental light that has passed through the first dichroic mirror to one end of the waveguide of the nonlinear optical element, and a second dichroic mirror that reflects the return light of the fundamental light reflected by the first dichroic mirror, and an output optical circuit that receives second harmonic light (SHG light) generated by the nonlinear optical element, wherein the return light of the fundamental light from the second dichroic mirror is configured to be usable as monitoring light.

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