Optical signal processing device

Abstract

This wavelength conversion device (500) is provided with: a wavelength conversion element (512) provided with a waveguide array (511); a first optical system (533) for causing the signal light and the excitation light to enter a waveguide (511a) in the waveguide array (511) and causing the monitoring light to enter a waveguide (511b) adjacent to the waveguide (511a); a second optical system (534) for demultiplexing the emitted light from the waveguides (511a, 511b) into signal light, excitation light, and monitoring light; a polarizer (525) having a transmission axis orthogonal to the input polarization plane angle of the monitoring light; a wave detector (526) for receiving the monitoring light; and a measurement unit (532) for measuring the power of the monitoring light on the basis of the output of the wave detector (526).

Description

Optical signal processing device

[0001] The present invention relates to an optical signal processing device such as a wavelength conversion device or an optical amplification device.

[0002] Wavelength conversion technology has been put to practical use in various fields, such as optical signal wavelength conversion in optical communication systems, optical processing, medicine, and bioengineering. The wavelength range of light that can be converted by such wavelength conversion technology ranges from the ultraviolet to the visible, infrared, and even the terahertz range, which cannot be directly output by semiconductor lasers. Furthermore, even in wavelength ranges that can be directly output by semiconductor lasers, wavelength conversion technology can be used as an alternative when sufficient output cannot be obtained.

[0003] In optical communication systems, wavelength conversion techniques based on second-order nonlinear optical effects are widely used, such as wavelength conversion devices that use wavelength conversion operation by difference frequency generation (described later) and optical amplification devices that use optical amplification operation by the parametric effect. In such wavelength conversion techniques based on second-order nonlinear optical effects, lithium niobate (LiNbO), which is a second-order nonlinear material and has a large nonlinear constant, is used as a wavelength conversion element. ３ In particular, waveguide-type LN, which has a structure in which the polarization of the crystal is periodically inverted (periodically polarized structure), is widely used as a light source due to its high wavelength conversion efficiency.

[0004] Generally, in wavelength conversion based on the second-order nonlinear optical effect, the wavelength λ １ of light and wavelength λ ２ By utilizing the second-order nonlinear optical effect that occurs between light of wavelength λ ３ Among them, the wavelength conversion expressed by the formula (1) is called sum frequency generation (SFG).

[0005]

[0006] Also, in formula (1), λ １ = λ ２When , the formula (2) holds true. Wavelength conversion that satisfies the formula (2) is particularly called second harmonic generation (SHG).

[0007]

[0008] On the other hand, wavelength conversion that satisfies formula (3) is called difference frequency generation (DFG).

[0009]

[0010] In the DFG that satisfies equation (3), the wavelength λ １ The light of wavelength λ is used as excitation light. ２ The light of wavelength λ ３ The light is called idler light.

[0011] Furthermore, as another application example of DFG, a second-order nonlinear optical material is placed inside a resonator, and the wavelength λ １ Only the light of wavelength λ is input, and the wavelength λ satisfies equation (3). ２ of light and wavelength λ ３ It is also possible to generate light of this order. An oscillator that operates in this way is called an optical parametric oscillator.

[0012] Wavelength converters in optical communication systems typically perform wavelength conversion based on the principle of the second-order nonlinear optical effect described above. In addition, in recent years, improvements in the wavelength conversion efficiency (the ratio of the intensity of wavelength-converted light to the intensity of incident light) of wavelength converters have made it possible to realize optical amplification operations utilizing the second-order nonlinear optical effect with a configuration similar to that of a wavelength converter. Because such optical amplification devices utilize phase-sensitive operation to amplify input light without degrading the signal-to-noise ratio, they are expected to replace erbium-doped fiber amplifiers as optical amplifiers for long-distance transmission.

[0013] Generally, two optical amplification operations are known for phase-sensitive amplifiers. One optical amplification operation utilizes degenerate parametric amplification, in which signal light and pump light with a wavelength half that of the signal light are input to a second-order nonlinear optical material, and the signal light is amplified (see, for example, Non-Patent Document 1). The other optical amplification operation utilizes non-degenerate parametric amplification, in which a pair of signal light and idler light is input to a second-order nonlinear optical material, and pump light with a wavelength equal to the sum frequency of the signal light and idler light is input, and the signal light and idler light are amplified (see, for example, Non-Patent Document 2). The pair of signal light and idler light is generated by the DFG mechanism described above.

[0014] In the field of communications, such as optical communications systems, when wavelength conversion technology and optical amplification technology using second-order nonlinear optical effects are used, DFG and parametric amplification are mainly used. When DFG and parametric amplification are used, the signal light and idler light exist in the 1.55 μm communication wavelength band, so the pump light is light in the 0.78 μm band.

[0015] Fig. 5 is a diagram showing the basic configuration of a conventional optical amplifier 100 using degenerate parametric amplification. While Fig. 5 illustrates an example of the optical amplifier 100 in which amplified signal light is output, a wavelength converter using DFG can also be realized with a similar configuration. In a wavelength converter using DFG, idler light is output.

[0016] 5, optical amplifier 100 includes second-order nonlinear optical element 101, dichroic mirror 102 disposed on the input side of second-order nonlinear optical element 101 and configured to combine signal light 104 and pump light 105 and guide the combined light to second-order nonlinear optical element 101, and dichroic mirror 103 disposed on the output side of second-order nonlinear optical element 101 and configured to demultiplex output light from second-order nonlinear optical element 101 into amplified signal light 106 and pump light 107. For example, assuming that signal light 104 is in the optical fiber communication band, as described above, signal light 104 will be in the 1.55 μm band. Pump light 105 will be in the 0.78 μm band, which is half the wavelength of the signal light.

[0017] As shown in FIG. 5 , the optical amplifier 100 has multiple different optical inputs and outputs. The optical components, such as the second-order nonlinear optical element 101, that make up the optical amplifier 100 are bulk-type components. For this reason, the optical amplifier 100 has often been configured with a spatial optical system including multiple bulk-type components. However, in recent years, with the advancement of development of optical fiber communication devices, many optical waveguide-type and optical fiber-type optical components have become commercially available as optical components for communication wavelength bands. Accordingly, wavelength converters and optical amplifiers have increasingly adopted structures using pigtail-type modules, with a focus on optical coupling with optical fibers. By adopting a structure using a pigtail-type module, wavelength converters and optical amplifiers can be made smaller and require no optical alignment compared to those using conventional bulk-type optical components. For this reason, structures using pigtail-type modules have been attracting attention in wavelength converters and optical amplifiers in recent years, and there has been a growing demand for the development of low-cost module manufacturing methods and manufacturing apparatuses.

[0018] 6 is a diagram showing a conceptual configuration of a conventional wavelength conversion device 200 using a pigtail type module. As shown in Fig. 6, the wavelength conversion device 200 includes a wavelength conversion element 201 having a waveguide 201a made of a second-order nonlinear optical material and a clad 201b arranged under the waveguide 201a, a lens 202a arranged on the input side of the wavelength conversion element 201 for collecting signal light 206 and pumping light 207 and guiding them to the waveguide 201a, a lens 202b arranged on the output side of the wavelength conversion element 201 for converting the output light into parallel light, a dichroic mirror 203a for reflecting the input signal light 206, and a lens 202b for converting the output light into parallel light. a dichroic mirror 203b that multiplexes the incident signal light 206 and the excitation light 207; a dichroic mirror 203c that splits the output light output from the wavelength conversion element 201 into wavelength-converted signal light 208 and excitation light 209; a dichroic mirror 203d that reflects the signal light 208 split by the dichroic mirror 203c and leads it to the outside; and a metal housing 204 that encloses the wavelength conversion element 201, lenses 202a, 202b, and dichroic mirrors 203a, 203b, 203c, and 203d inside.

[0019] The waveguide 201a of the wavelength conversion element 201 is made of, for example, LN or LiTaO having a periodically poled structure. ３ , or LiNb ｘ Ta （１－ｘ） O ３ (0≦x≦1) or any of these materials to which at least one selected from the group consisting of Mg, Zn, Sc, and In is added as an additive.

[0020] 6 illustrates an example in which signal light 206 is input from optical fiber 205a and pumping light 207 is input from optical fiber 205b. Signal light 206 and pumping light 207 are converted into parallel light by lenses 202c and 202d, respectively, and guided into metal housing 204. In the example of FIG. 6, as in the example of FIG. 5, an optical communication system is assumed, in which the wavelength band of signal light 206 is 1.55 μm and the wavelength band of pumping light 207 is 0.78 μm, which is half the wavelength of the signal light. Therefore, optical fiber 205a is a 1.55 μm-band optical fiber, and optical fiber 205b is a 0.78 μm-band optical fiber.

[0021] The wavelength-converted signal light 208 is focused by lens 202e, then optically coupled to optical fiber 205c, and then guided to the outside. The pumping light 209 is focused by lens 202f, then optically coupled to optical fiber 205d, and then guided to the outside. The optical fiber 205c is a 1.55 μm band optical fiber, and the optical fiber 205d is a 0.78 μm band optical fiber. However, it is only necessary to demultiplex the 0.78 μm band light from the output light of the wavelength conversion element 201, and the demultiplexed pumping light 209 does not necessarily have to be guided to optical fiber 205d.

[0022] Fig. 7 is a diagram showing the transmittance and reflectance of the dichroic mirrors 203a to 203d with respect to the wavelength of incident light. In Fig. 7, 210 represents transmittance, and 211 represents reflectance. As shown in Fig. 7, the dichroic mirrors 203a to 203d have high transmittance and low reflectance for light in the 0.78 μm band, which is the wavelength band of the pump light. On the other hand, they have low transmittance and high reflectance for light in the 1.55 μm band, which is the wavelength band of the signal light. The characteristics of the dichroic mirrors 203a to 203d enable the wavelength conversion device 200 to multiplex or demultiplex the signal light and the pump light.

[0023] The wavelength conversion device 200 having the above-described configuration has a structure that allows it to be optically coupled to an optical fiber, and therefore is a smaller and simpler wavelength conversion device than conventional wavelength conversion elements that use bulk-type optical components.

[0024] However, in the wavelength conversion device 200, the refractive index of the wavelength conversion element 201 may change due to a thermo-optic effect caused by temperature non-uniformity or temporal fluctuations in the wavelength conversion element 201, an electro-optic (EO) effect caused by a photoelastic phenomenon caused by stress changes in the wavelength conversion element 201, an EO effect caused by an electric field caused by a pyroelectric effect due to temperature adjustment for phase matching, or optical damage. The change in the refractive index of the wavelength conversion element 201 causes a change in the birefringence state of the wavelength conversion element 201. This change poses a problem of a decrease in the polarization extinction ratio, amplification gain, or conversion efficiency of the wavelength conversion device 200.

[0025] Furthermore, second-order nonlinear optical materials such as LN exhibit polarization dependence. Therefore, in order to amplify a polarization multiplexed signal, a polarization diversity mechanism that amplifies the two polarization components independently is required. However, there is a problem in that it is difficult to amplify a polarization multiplexed signal due to changes in the birefringence state of the wavelength conversion element 201.

[0026] As an existing countermeasure technology for the above-mentioned problems, there is a method of checking the birefringence state of the second-order nonlinear optical element (wavelength conversion element 201). However, this method poses another problem in that the operation of the wavelength conversion device 200 must be stopped every time the birefringence state of the second-order nonlinear optical element is checked. Therefore, there is a demand for a technology that can monitor the birefringence state of the second-order nonlinear optical element without stopping the operation of the device.

[0027] T. Umeki, O. Tadanaga, A. Takada, and M. Asobe, “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides”, Optics Express, Vol. 19, No. 7, pp. 6326-6332, 2011 T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto, and H. Takenouchi, “First demonstration of “high-order QAM signal amplification in PPLN-based phase sensitive amplifier”, Optics Express, Vol.22, No.3, pp.2473-2482, 2014

[0028] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an optical signal processing device that can simultaneously monitor the birefringence state of a second-order nonlinear optical element installed inside the device while the device is in operation.

[0029] an optical signal processing device according to the present invention, which comprises: a second-order nonlinear optical element having a waveguide array made of a second-order nonlinear optical material; a first optical system configured to input signal light and pump light into a first waveguide in the waveguide array, and to input monitor light that is linearly polarized and has the same wavelength band as the signal light or the pump light into a second waveguide in the waveguide array adjacent to the first waveguide; a second optical system configured to demultiplex the light emitted from the first and second waveguides into the signal light, the pump light, and the monitor light; a polarizer configured to have a transmission axis orthogonal to the input polarization plane angle of the monitor light demultiplexed by the second optical system; a detector configured to receive the monitor light demultiplexed by the second optical system and passed through the polarizer; and a measurement unit configured to measure the power of the monitor light based on an output from the detector.

[0030] According to the present invention, the waveguides in the second-order nonlinear optical element are arrayed, linearly polarized monitor light is input to the second waveguide adjacent to the first waveguide for wavelength conversion, and the power of the monitor light output from the second-order nonlinear optical element is measured. This makes it possible to monitor the birefringence state of the second-order nonlinear optical element during operation based on the results of the power measurement.

[0031] Fig. 1 is a diagram showing an example of the relationship between the rate of change of birefringence of a wavelength conversion element and the power of light output from a polarizer. Fig. 2 is a diagram showing the configuration of a wavelength conversion device according to a first embodiment of the present invention. Fig. 3 is a diagram showing an example of power measurement results by a measurement unit of the wavelength conversion device according to the first embodiment of the present invention. Fig. 4 is a diagram showing the configuration of a wavelength conversion device according to a second embodiment of the present invention. Fig. 5 is a diagram showing the configuration of a conventional optical amplifier. Fig. 6 is a diagram showing the configuration of a conventional wavelength conversion device. Fig. 7 is a diagram showing the transmittance and reflectance of a dichroic mirror relative to the wavelength of incident light in a conventional wavelength conversion device.

[0032] [Principles of the Invention] Various embodiments of the present invention will be described in detail below with reference to the drawings. The numerical values ​​described in the embodiments are for illustrative purposes only and are not intended to limit the technical scope of the present invention. Furthermore, the following description is an example, and some configurations may be omitted, modified, or added without departing from the spirit of the embodiments of the present invention. Furthermore, in the following embodiments, a wavelength conversion device will be described as an example, but the same device configuration can also be used as an optical amplification device.

[0033] The wavelength conversion device of the present invention arrays the waveguides within the wavelength conversion element, and inputs linearly polarized monitor light with an arbitrary input polarization angle into a waveguide adjacent to the wavelength conversion waveguide. The monitor light output from the wavelength conversion element is then passed through a polarizer with a transmission axis perpendicular to the input polarization angle, after which the power of the monitor light is measured. The wavelength conversion device of the present invention can monitor the birefringence state of the wavelength conversion element during operation based on the results of the power measurement. Retardation is used to monitor the birefringence state of the wavelength conversion element based on the power of the monitor light. The principle is explained below.

[0034] The second-order nonlinear optical material used in the wavelength conversion element is an anisotropic material. Therefore, birefringence occurs in the light propagating inside. Assume a crossed Nicol optical system in which linearly polarized light with an input polarization angle θ is input to the second-order nonlinear optical material, and the light polarized by the second-order nonlinear optical material passes through a polarizer with a transmission axis perpendicular to the input polarization angle θ. Here, the input polarization angle θ is set to 45°. If the retardation amount is Γ, the power I of the light output from the polarizer is expressed by equation (4).

[0035]

[0036] The retardation amount Γ is given by equation (5).

[0037]

[0038] In equation (5), λ is the wavelength of light incident on the wavelength conversion element, L is the length of the wavelength conversion element, n ｏ is the refractive index of the wavelength conversion element for ordinary light, n ｅ is the refractive index of the wavelength conversion element for extraordinary rays.

[0039] By utilizing the principles explained in equations (4) and (5), the birefringence state of the wavelength conversion element can be monitored from the measurement results of the power of the light output from the polarizer.

[0040] 1 is a diagram showing an example of the relationship between the rate of change of the birefringence of a wavelength conversion element and the power of light output from a polarizer. In the example of FIG. 1, the birefringence amount |n ｏ -n ｅ | is 0.08, the length L of the wavelength conversion element is 50 mm, the wavelength λ of light incident on the wavelength conversion element is 0.85 μm, and the input polarization plane angle θ is 45°, the rate of change in the birefringence of the wavelength conversion element is ±5×10 －５ The power of the output light for the range of

[0041] By using the above-described crossed Nicol optical system, it is possible to obtain the relationship between the rate of change of birefringence and the power of output light as shown in Fig. 1. The wavelength conversion device of the present invention is configured so that the birefringence state of the wavelength conversion element can be monitored during operation based on the relationship between the change of birefringence and the power of output light.

[0042] The birefringence state of a wavelength conversion element can be monitored by arraying the waveguides within the wavelength conversion element and irradiating monitoring light into a waveguide adjacent to the wavelength conversion waveguide. This is because, although distributions in the refractive index and waveguide size occur within the wavelength conversion element, the characteristics of two adjacent waveguides spaced apart by a distance of several tens to several hundreds of microns can be considered to be nearly identical. Therefore, in this invention, the waveguides within the wavelength conversion element are arrayed, and information on the change in birefringence of the wavelength conversion element is obtained from the waveguide adjacent to the wavelength conversion waveguide.

[0043] When injecting monitor light into a waveguide, it is necessary to consider the matching of the mode field diameter (MFD) of the optical fiber with that of the waveguide. To achieve this matching, a pseudo-confocal optical system configuration is often used, in which collimated light is formed between two lenses, as shown in Figure 6. In this configuration, if the focal length on the input side is f1 and the focal length on the output side is f2, the beam diameter magnification is f2 / f1, so the required lens combination must be selected.

[0044] In a configuration using a pseudo-confocal optical system, the optical coupling is optimized by the combination of lens focal lengths. However, once the combination of focal lengths is determined, the exit position relative to the entrance position is also determined as f2 / f1 times, so the relationship between the entrance position and the exit position must be taken into consideration. When two light beams, including a monitor light, are input from an optical fiber to two adjacent waveguides, the pitch interval determined by the image magnification is changed. This requires changing the interval between the incident light beams on the optical fiber side, changing the interval between the waveguides, or changing both the interval between the incident light beams and the interval between the waveguides.

[0045] The inventors have conducted extensive research into waveguide spacing matching associated with MFD matching. For example, for single-mode fibers, the MFD and pitch of optical fibers are standardized as 10 μm for MFD and 250 μm or 0.127 μm for fiber arrangement pitch between two fibers. Therefore, it is desirable to accommodate pitch conversion on the wavelength conversion element side. Setting the waveguide spacing in a wavelength conversion element can be easily achieved by photolithography, machining, or the like in the fabrication process for forming the waveguides.

[0046] As described above, by arraying the waveguides in the wavelength conversion element and directing monitoring light into the waveguide adjacent to the wavelength conversion waveguide, it is possible to monitor the birefringence state of the wavelength conversion element simultaneously with wavelength conversion.

[0047] 2 is a diagram showing the configuration of a wavelength converter 500 according to a first embodiment of the present invention. The wavelength converter 500 includes a 1.55 μm band polarization-maintaining optical fiber 501 for inputting a signal light 601, a 0.78 μm band polarization-maintaining optical fiber 502 for inputting a pumping light 602 and a supervisory light 603, an optical connector 505 having a ferrule 503 and a lens 504 for fixing the optical fiber 501, an optical connector 508 having a multi-fiber ferrule 506 and a lens 507 for fixing the optical fiber 502, and a dichroic mirror 509 for reflecting the signal light 601. 509, a dichroic mirror 510 (wavelength multiplexer) that multiplexes the signal light 601, the pumping light 602, and the monitoring light 603, a wavelength conversion element 512 (second-order nonlinear optical element) that has a waveguide array 511 made of a second-order nonlinear optical material, a lens 513 that couples the light emitted from the dichroic mirror 510 to two adjacent waveguides in the waveguide array 511, a lens 514 that converts the light emitted from the wavelength conversion element 512 into parallel light, and a lens 515 that converts the light emitted from the lens 514 into the signal light 604 and the pumping light. a dichroic mirror 515 (wavelength demultiplexer) that demultiplexes the signal light 604 into the light 605 and the monitoring light 606; a dichroic mirror 516 that reflects the signal light 604; a 1.55 μm band polarization-maintaining optical fiber 517 that guides the signal light 604 to the outside; a 0.78 μm band polarization-maintaining optical fiber 518 that guides the pumping light 605 and the monitoring light 606 to the outside; an optical connector 521 that has a ferrule 519 and a lens 520 for fixing the optical fiber 517; and a multi-core optical fiber 521 for fixing the optical fiber 518. The optical connector 524 includes a ferrule 522 and a lens 523, a polarizer 525 having a transmission axis perpendicular to the input polarization plane angle of the monitor light 606, a detector 526 that receives the monitor light 606 output from the polarizer 525, a metal housing 527 that houses dichroic mirrors 509, 510, 515, and 516, a wavelength conversion element 512, and lenses 513 and 514, and a measuring unit 532 that measures the power of the monitor light 606 based on the output of the detector 526.

[0048] The dichroic mirrors 509 and 510 and the lens 513 constitute a first optical system 533 , and the dichroic mirrors 515 and 516 and the lens 514 constitute a second optical system 534 .

[0049] The end of the optical fiber 501 on the wavelength conversion device 500 side is fixed by a ferrule 503 of an optical connector 505. Signal light 601 is guided by the optical fiber 501, converted into parallel light by a lens 504, and passes through an optical window 528 provided in a metal housing 527. The signal light 601 that enters the metal housing 527 is reflected by a dichroic mirror 509.

[0050] The end of the ribbon core of the polarization-maintaining optical fiber 502, which is a multi-core optical fiber, on the wavelength converter 500 side is fixed by a multi-core ferrule 506 of an optical connector 508. The pumping light 602 and the monitor light 603 are each guided by different cores of the optical fiber 502, converted into parallel light by a lens 507, and pass through an optical window 529 provided in a metal housing 527. The dichroic mirror 510 reflects the signal light 601 from the dichroic mirror 509 and transmits the pumping light 602 and monitor light 603 that have entered the metal housing 527, thereby multiplexing the signal light 601, pumping light 602, and monitor light 603. At this time, the optical fibers 501, 502, the optical connectors 505, 508, and the dichroic mirrors 509, 510 are arranged so that the optical axis of the signal light 601 reflected by the dichroic mirror 510 coincides with the optical axis of the excitation light 602 transmitted through the dichroic mirror 510.

[0051] The lens 513 causes the signal light 601 and the excitation light 602, which are outgoing lights from the dichroic mirror 510, to enter a wavelength conversion waveguide 511a in the waveguide array 511 formed in the wavelength conversion element 512. The lens 513 also causes the monitor light 603, which is outgoing lights from the dichroic mirror 510, to enter a waveguide 511b adjacent to the waveguide 511a in the waveguide array 511.

[0052] The focal lengths of the lenses 513 and 514 are set so that the MFDs of the 0.78 μm band polarization-maintaining optical fibers 502 and 518 match the MFDs of the waveguides 511 a and 511 b in the 0.78 μm band. The waveguide array 511 is formed by photolithography so that the spacing between the waveguides is equal to the spacing between the optical fiber cores in the multi-core ferrules 506 and 522 multiplied by the image magnification (f2 / f1, the ratio of the focal length f1 of the lens 513 to the focal length f2 of the lens 514).

[0053] The monitor light 603 is light in the same wavelength band as the pump light 602. For example, assuming an optical communication system in which the signal light 601 is in the 1.55 μm band and the pump light 602 is in the 0.78 μm band, the wavelength band of the monitor light 603 may be the 0.85 μm band, which has little absorption loss and for which commercial semiconductor light sources are relatively easy to obtain. In addition, the monitor light 603 is linearly polarized light with an arbitrary input polarization angle (45° in this embodiment), and is input together with the pump light 602.

[0054] 2, when the pump light 602 and the monitor light 603 propagate through the ribbon of the optical fiber 502 and are guided into the metal housing 527, the optical fiber 502 is a 0.78 μm band polarization-maintaining optical fiber. The monitor light 603 is input into the metal housing 527 at a polarization angle of 45° with respect to the slow axes of the optical fibers 502 and 518.

[0055] The wavelength conversion element 512 outputs wavelength-converted signal light 604, pump light 605, and monitor light 606. The signal light 604 is converted into parallel light by the lens 514, reflected by the dichroic mirror 515, and further reflected by the dichroic mirror 516, and passes through an optical window 530 provided in the metal housing 527. The signal light 604 that has passed through the optical window 530 is condensed by the lens 520 and enters the 1.55 μm-band polarization-maintaining optical fiber 517. The end of the optical fiber 517 on the wavelength conversion device 500 side is fixed by a ferrule 519 of the optical connector 521.

[0056] The pumping light 605 and the monitor light 606 are converted into parallel light by the lens 514, transmitted through the dichroic mirror 515, and passed through an optical window 531 provided in the metal housing 527. The pumping light 605 and the monitor light 606 that have passed through the optical window 531 are condensed by the lens 523 and respectively enter different core fibers of the 0.78 μm band polarization-maintaining optical fiber 518. The end of the ribbon core fiber of the optical fiber 518 on the wavelength converter 500 side is fixed by a multi-core ferrule 522 of the optical connector 524.

[0057] The monitor light 606 is guided by the optical fiber 518 and enters the polarizer 525, which has a transmission axis orthogonal to the input polarization angle of the monitor light 606 (the transmission axis is at −45° with respect to the slow axes of the optical fibers 502 and 518). The detector 526 receives the light output from the polarizer 525 and converts it into an electrical signal. The measuring unit 532 measures the power of the monitor light 606 based on the output of the detector 526.

[0058] The waveguide array 511 of the wavelength conversion element 512 is made of, for example, LN or LiTaO having a periodically poled structure, similar to the wavelength conversion device 200. ３ , or LiNb ｘ Ta （１－ｘ） O ３ (0≦x≦1) or any of these materials to which at least one selected from the group consisting of Mg, Zn, Sc, and In is added as an additive.

[0059] The characteristics of the dichroic mirrors 509, 510, 515, and 516 are the same as those of the wavelength conversion device 200. The dichroic mirrors 509, 510, 515, and 516 are configured to have high transmittance and low reflectance for light in the respective wavelength bands of the pump light and the supervisory light (0.78 μm band and 0.85 μm band), and low transmittance and high reflectance for light in the 1.55 μm band, which is the wavelength band of the signal light. The characteristics of the dichroic mirrors 509, 510, 515, and 516 enable multiplexing and demultiplexing of the signal light, pump light, and supervisory light. The transmittance and reflectance of the dichroic mirrors 509, 510, 515, and 516 can be set by designing the dielectric multilayer film.

[0060] According to the wavelength conversion device 500 of this embodiment, it is possible to monitor the birefringence state of the wavelength conversion element 512 based on the light power measured by the measuring unit 532 while the wavelength conversion device 500 is in operation.

[0061] 3 is a diagram showing an example of a power measurement result obtained by the measurement unit 532 of the wavelength conversion device 500. As shown in FIG. 3, the power of the monitor light in an arbitrary time range fluctuates with time. When such a power measurement result is obtained, it can be seen that the birefringence state of the wavelength conversion element 512 fluctuates with time from the known relationship between the power of the monitor light and the rate of change of the birefringence of the wavelength conversion element 512.

[0062] A detailed investigation into the factors behind the time dependency of the power of the monitoring light revealed that there is a correlation with changes in the environmental temperature in which the wavelength conversion device 500 is installed. It was also found that taking measures to suppress environmental temperature changes can suppress fluctuations in the power of the monitoring light. In this way, the wavelength conversion device 500 of this embodiment can monitor the change over time in the birefringence state of the wavelength conversion element 512 based on the power of the monitoring light. Depending on the monitoring results, it is possible to estimate the cause of the change in the birefringence state and implement measures to suppress the change.

[0063] 4 is a diagram showing the configuration of a wavelength converter 700 according to a second embodiment of the present invention. The wavelength converter 700 includes a 1.55 μm band polarization-maintaining optical fiber 701 for inputting a signal light 801 and a supervisory light 802, a 0.78 μm band polarization-maintaining optical fiber 702 for inputting a pumping light 803, an optical connector 705 having a multi-core ferrule 703 and a lens 704 for fixing the optical fiber 701, an optical connector 708 having a ferrule 706 and a lens 707 for fixing the optical fiber 702, and a dichroic mirror 709 for reflecting the signal light 801 and the supervisory light 802. a dichroic mirror 710 (wavelength multiplexer) that multiplexes the signal light 801, the monitoring light 802, and the pumping light 803; a wavelength conversion element 712 (second-order nonlinear optical element) that has a waveguide array 711 made of a second-order nonlinear optical material; a lens 713 that couples the light emitted from the dichroic mirror 710 to two adjacent waveguides in the waveguide array 711; a lens 714 that converts the light emitted from the wavelength conversion element 712 into parallel light; a dichroic mirror 715 (wavelength demultiplexer) that demultiplexes into monitoring light 805 and pumping light 806; a dichroic mirror 716 that reflects signal light 804 and monitoring light 805; a 1.55 μm band polarization-maintaining optical fiber 717 that guides signal light 804 and monitoring light 805 to the outside; a 0.78 μm band polarization-maintaining optical fiber 718 that guides pumping light 806 to the outside; an optical connector 721 that has a multi-core ferrule 719 and a lens 720 for fixing the optical fiber 717; the optical connector 724 having a ferrule 722 and a lens 723 for receiving the monitoring light 805; a polarizer 725 having a transmission axis perpendicular to the input polarization plane angle of the monitoring light 802 and 805; a detector 726 that receives the monitoring light 805 output from the polarizer 725; a metal housing 727 that houses dichroic mirrors 709, 710, 715, and 716, a wavelength conversion element 712, and lenses 713 and 714; and a measuring unit 732 that measures the power of the monitoring light 805 based on the output of the detector 726.

[0064] The dichroic mirrors 709 and 710 and the lens 713 constitute a first optical system 733 , and the dichroic mirrors 715 and 716 and the lens 714 constitute a second optical system 734 .

[0065] The end of the ribbon core of polarization-maintaining optical fiber 701, which is a multi-core optical fiber, on the wavelength converter 700 side is fixed by a multi-core ferrule 703 of an optical connector 705. Signal light 801 and monitor light 802 are each guided by different cores of the optical fiber 701, converted into parallel light by a lens 704, and pass through an optical window 728 provided in a metal housing 727. The signal light 801 and monitor light 802 that enter the metal housing 727 are reflected by a dichroic mirror 709.

[0066] The end of the optical fiber 702 on the wavelength conversion device 700 side is fixed by a ferrule 706 of an optical connector 708. Pumping light 803 is guided by the optical fiber 702, converted into parallel light by a lens 707, and passes through an optical window 729 provided in a metal housing 727. The dichroic mirror 710 reflects the signal light 801 and the monitor light 802 from the dichroic mirror 709, and transmits the pumping light 803 that has entered the metal housing 727, thereby multiplexing the signal light 801, the monitor light 802, and the pumping light 803. At this time, the optical fibers 701 and 702, the optical connectors 705 and 708, and the dichroic mirrors 709 and 710 are arranged so that the optical axis of the signal light 801 reflected by the dichroic mirror 710 coincides with the optical axis of the pumping light 803 that has transmitted through the dichroic mirror 710.

[0067] The lens 713 causes the signal light 801 and the excitation light 803, which are among the light emitted from the dichroic mirror 710, to enter a wavelength conversion waveguide 711a in the waveguide array 711 formed in the wavelength conversion element 712. The lens 713 also causes the monitor light 802, which is among the light emitted from the dichroic mirror 710, to enter a waveguide 711b adjacent to the waveguide 711a in the waveguide array 711.

[0068] The focal lengths of the lenses 713 and 714 are set so that the MFDs of the 1.55 μm band polarization-maintaining optical fibers 701 and 717 match the MFDs of the waveguides 711 a and 711 b in the 1.55 μm band. The waveguide array 711 is formed by photolithography so that the spacing between the waveguides is equal to the spacing between the optical fiber cores in the multi-core ferrules 703 and 719 multiplied by the image magnification (f2 / f1, the ratio of the focal length f1 of the lens 713 to the focal length f2 of the lens 714).

[0069] The monitor light 802 is light in the same wavelength band as the signal light 801. In addition, the monitor light 802 is linearly polarized light with an arbitrary input polarization angle (45° in this embodiment) and is input together with the signal light 801. For example, as shown in Figure 4, when the signal light 801 and the monitor light 802 propagate through the ribbon of the optical fiber 701 and are guided into the metal housing 727, the optical fiber 701 is a 1.55 μm-band polarization-maintaining optical fiber. The monitor light 802 is input into the metal housing 727 at a polarization angle of 45° with respect to the slow axes of the optical fibers 701 and 717.

[0070] The wavelength conversion element 712 outputs wavelength-converted signal light 804, monitor light 805, and pump light 806. The signal light 804 and monitor light 805 are converted into parallel light by a lens 714, reflected by a dichroic mirror 715, and further reflected by a dichroic mirror 716, and pass through an optical window 730 provided in a metal housing 727. The signal light 804 and monitor light 805 that have passed through the optical window 730 are focused by a lens 720 and each enters a different core of a 1.55 μm-band polarization-maintaining optical fiber 717. The end of the ribbon core of the optical fiber 717 on the wavelength conversion device 700 side is fixed by a multi-core ferrule 719 of an optical connector 721.

[0071] The excitation light 806 is converted into parallel light by the lens 714, transmitted through the dichroic mirror 715, and passes through an optical window 731 provided in the metal housing 727. The excitation light 806 that has passed through the optical window 731 is condensed by the lens 723 and enters the 0.78 μm band polarization-maintaining optical fiber 718. The end of the optical fiber 718 on the wavelength conversion device 700 side is fixed by a ferrule 722 of an optical connector 724.

[0072] The monitor light 805 is guided by the optical fiber 717 and enters the polarizer 725, which has a transmission axis orthogonal to the input polarization angle of the monitor light 805 (the transmission axis is at −45° with respect to the slow axes of the optical fibers 701 and 717). The detector 726 receives the light output from the polarizer 725 and converts it into an electrical signal. The measuring unit 732 measures the power of the monitor light 805 based on the output of the detector 726.

[0073] The characteristics of the dichroic mirrors 709, 710, 715, and 716 are the same as those of the wavelength conversion device 200. The dichroic mirrors 709, 710, 715, and 716 are configured to have high transmittance and low reflectance for light in the wavelength band of the pumping light, and low transmittance and high reflectance for light in the wavelength bands of the signal light and supervisory light. The characteristics of the dichroic mirrors 709, 710, 715, and 716 enable the multiplexing and demultiplexing of the signal light, pumping light, and supervisory light.

[0074] As a result of power measurement by the measuring unit 732 of the wavelength conversion device 700 in this embodiment, the time change in the power of the monitor light was obtained as shown in Fig. 3. In this embodiment, as in the first embodiment, it is possible to monitor the time change in the birefringence state of the wavelength conversion element 712 based on the power of the monitor light. Depending on the monitoring result, it is possible to estimate the cause of the change in the birefringence state and take measures to suppress the change.

[0075] In the first and second embodiments, a wavelength converter was used as an example of an optical signal processing device, but as mentioned above, an optical amplifier can also be realized with a configuration similar to that of a wavelength converter. Furthermore, in the present embodiments, a wavelength converter using a pigtail-type module was used as an example, but the present invention can also be applied to a wavelength converter or optical amplifier configured with a free-space optical system using bulk-type optical components.

[0076] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes.

[0077] (Supplementary Note 1) An optical signal processing device of the present invention comprises a second-order nonlinear optical element having a waveguide array made of a second-order nonlinear optical material; a first optical system configured to input signal light and pump light into a first waveguide in the waveguide array, and to input monitor light that is linearly polarized and has the same wavelength band as the signal light or the pump light into a second waveguide in the waveguide array adjacent to the first waveguide; a second optical system configured to demultiplex the light emitted from the first and second waveguides into the signal light, the pump light, and the monitor light; a polarizer configured to have a transmission axis orthogonal to the input polarization plane angle of the monitor light demultiplexed by the second optical system; a detector configured to receive the monitor light demultiplexed by the second optical system and passed through the polarizer; and a measurement unit configured to measure the power of the monitor light based on an output of the detector.

[0078] (Supplementary Note 2) In the optical signal processing device described in Supplementary Note 1, the first optical system includes a wavelength multiplexer configured to multiplex the signal light, the pump light, and the monitor light, and a first lens configured to input the signal light and the pump light out of the light emitted from the wavelength multiplexer into the first waveguide and to input the monitor light out of the light emitted from the wavelength multiplexer into the second waveguide, and the second optical system includes a second lens configured to convert the light emitted from the first and second waveguides into parallel light, and a wavelength demultiplexer configured to demultiplex the light emitted from the second lens into the signal light, the pump light, and the monitor light.

[0079] (Supplementary Note 3) The optical signal processing device described in Supplementary Note 2 further comprises a first polarization-maintaining optical fiber configured to guide the signal light, or a set of the signal light and the monitor light, to the first optical system; a second polarization-maintaining optical fiber configured to guide the set of the pumping light and the monitor light, or the pumping light, to the first optical system; a third polarization-maintaining optical fiber configured to guide the signal light demultiplexed by the second optical system, or the set of the signal light and the monitor light demultiplexed by the second optical system; and a fourth polarization-maintaining optical fiber configured to guide the set of the pumping light and the monitor light demultiplexed by the second optical system, or the monitor light demultiplexed by the second optical system.

[0080] (Supplementary Note 4) In the optical signal processing device described in Supplementary Note 3, the focal lengths of the first and second lenses are set so that input and output mode field diameters of the signal light, the pumping light, and the monitoring light match the mode field diameters of the first and second waveguides, and when the first and third polarization-maintaining optical fibers guide the signal light and the second and fourth polarization-maintaining optical fibers are multi-core optical fibers that guide the pumping light and the monitoring light using two different core fibers, the interval between the first waveguide and the second waveguide is the interval obtained by multiplying the arrangement interval of the two core fibers by the ratio of the focal lengths of the first and second lenses.

[0081] (Supplementary Note 5) In the optical signal processing device described in Supplementary Note 3, the focal lengths of the first and second lenses are set so that input and output mode field diameters of the signal light, the pumping light, and the monitoring light match the mode field diameters of the first and second waveguides, the first and third polarization-maintaining optical fibers are multi-core optical fibers that guide the signal light and the monitoring light using two different core fibers, and the second and fourth polarization-maintaining optical fibers guide the pumping light, the interval between the first waveguide and the second waveguide is the interval obtained by multiplying the arrangement interval of the two core fibers by the ratio of the focal lengths of the first and second lenses.

[0082] (Supplementary Note 6) The optical signal processing device described in Supplementary Note 1 further includes a housing configured to house the second-order nonlinear optical element and the first and second optical systems, and introduces the signal light, the excitation light, and the monitoring light into the housing through an optical window on an entrance side installed on a wall surface of the housing, and emits the signal light, the excitation light, and the monitoring light separated by the second optical system to the outside of the housing through an optical window on an exit side installed on a wall surface of the housing.

[0083] (Supplementary Note 7) In the optical signal processing device according to Supplementary Note 1, the first and second waveguides are made of a second-order nonlinear optical material having a periodically poled structure.

[0084] (Supplementary Note 8) The optical signal processing device according to Supplementary Note 1 is a wavelength conversion device or an optical amplification device.

[0085] The present invention can be applied to wavelength converters or optical amplifiers. The wavelength converters or optical amplifiers of the present invention are expected to be applied to optical communication systems and the like as devices with lower loss than conventional devices.

[0086] 500, 700... Wavelength converter, 501, 517, 701, 717... 1.55 μm band polarization-maintaining optical fiber, 502, 518, 702, 718... 0.78 μm band polarization-maintaining optical fiber, 503, 506, 519, 522, 703, 706, 719, 722... Ferrule, 504, 507, 513, 514, 520, 523, 704, 707, 713, 714, 720, 723... Lens, 505, 508, 521, 524, 705, 708, 721, 724 ...optical connector, 509, 510, 515, 516, 709, 710, 715, 716...dichroic mirror, 511, 711...waveguide array, 511a, 511b, 711a, 711b...waveguide, 512, 712...wavelength conversion element, 525, 725...polarizer, 526, 726...detector, 527, 727...metal housing, 528-531, 728-731...optical window, 532, 732...measuring unit, 533, 733...first optical system, 534, 734...second optical system.

Claims

1. A second-order nonlinear optical element comprising a waveguide array made of a second-order nonlinear optical material, a first optical system configured to cause signal light and pump light to enter a first waveguide in the waveguide array and to cause monitoring light, which is linearly polarized and has the same wavelength band as the signal light or the pump light, to enter a second waveguide adjacent to the first waveguide in the waveguide array, a second optical system configured to demultiplex the output light of the first and second waveguides into the signal light, the pump light, and the monitoring light, a polarizer configured to have a transmission axis orthogonal to the input polarization plane angle of the monitoring light demultiplexed by the second optical system, a detector configured to receive the monitoring light demultiplexed by the second optical system and passing through the polarizer, and a measurement unit configured to measure the power of the monitoring light based on the output of the detector. A optical signal processing apparatus characterized by comprising the above components.

2. In the optical signal processing apparatus according to claim 1, the first optical system includes a wavelength multiplexer configured to multiplex the signal light, the pump light, and the monitoring light, and a first lens configured to cause the signal light and the pump light among the output light of the wavelength multiplexer to enter the first waveguide and to cause the monitoring light among the output light of the wavelength multiplexer to enter the second waveguide. The second optical system includes a second lens configured to convert the output light of the first and second waveguides into parallel light, and a wavelength demultiplexer configured to demultiplex the output light of the second lens into the signal light, the pump light, and the monitoring light. A optical signal processing apparatus characterized by comprising the above components.

3. In the optical signal processing apparatus according to claim 2, a first polarization-maintaining optical fiber configured to guide the signal light, or a set of the signal light and the monitoring light, to the first optical system; a second polarization-maintaining optical fiber configured to guide the set of the excitation light and the monitoring light, or the excitation light, to the first optical system; a third polarization-maintaining optical fiber configured to guide the signal light demultiplexed by the second optical system, or the set of the signal light and the monitoring light demultiplexed by the second optical system; and a fourth polarization-maintaining optical fiber configured to guide the set of the excitation light and the monitoring light demultiplexed by the second optical system, or the monitoring light demultiplexed by the second optical system. The optical signal processing apparatus is characterized by further comprising these components.

4. In the optical signal processing apparatus according to claim 3, the focal lengths of the first and second lenses are set such that the input / output mode field diameters of the signal light, the excitation light, and the monitoring light, and the mode field diameters of the first and second waveguides are matched. When the first and third polarization-maintaining optical fibers are multi-core optical fibers that guide the signal light, and the second and fourth polarization-maintaining optical fibers are multi-core optical fibers that guide the excitation light and the monitoring light by two different cores, the distance between the first waveguide and the second waveguide is the distance obtained by multiplying the arrangement interval of the two cores by the ratio of the focal lengths of the first and second lenses. The optical signal processing apparatus is characterized by this feature.

5. In the optical signal processing apparatus according to claim 3, the focal lengths of the first and second lenses are set such that the input / output mode field diameters of the signal light, the excitation light, and the monitoring light, and the mode field diameters of the first and second waveguides are matched. When the first and third polarization-maintaining optical fibers are multi-core optical fibers that guide the signal light and the monitoring light by two different cores, and the second and fourth polarization-maintaining optical fibers are multi-core optical fibers that guide the excitation light, the distance between the first waveguide and the second waveguide is the distance obtained by multiplying the arrangement interval of the two cores by the ratio of the focal lengths of the first and second lenses. The optical signal processing apparatus is characterized by this feature.

6. The optical signal processing apparatus according to claim 1, further comprising a housing configured to accommodate the second-order nonlinear optical element and the first and second optical systems, introducing the signal light, the excitation light, and the monitoring light into the housing through an incident-side optical window installed on a wall surface of the housing, and emitting the signal light, the excitation light, and the monitoring light separated by the second optical system out of the housing through an emission-side optical window installed on the wall surface of the housing.

7. The optical signal processing apparatus according to claim 1, wherein the first and second waveguides are made of a second-order nonlinear optical material having a periodically poled inversion structure.

8. The optical signal processing apparatus according to claim 1, which is a wavelength conversion device or an optical amplification device.

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