Polarization diversity wavelength conversion device

Abstract

A polarization diversity wavelength conversion device (10) according to one embodiment of the present disclosure is provided with: a first and second wavelength converter (102, 103) arranged between a polarization beam splitter (121) and a polarization beam combiner (122); a first excitation light source (100) for supplying excitation light to the first wavelength converter (102); a first temperature regulator (104) for regulating the temperature of the first wavelength converter (102); a second excitation light source (101) for supplying excitation light to the second wavelength converter (103); a second temperature regulator (105) for regulating the temperature of the second wavelength converter (103); optical intensity detectors (108cx, 108cy) for outputting the intensity of light of a first wavelength and the intensity of light of a second wavelength related to wavelength conversion in the polarization diversity wavelength conversion device (10); and a controller (111). The controller (111) is connected to the first and second excitation light sources (100, 101), and is connected to the first and second temperature regulators (104, 105).

Description

Polarization diversity wavelength conversion device 【0001】 This disclosure relates to a polarization diversity wavelength conversion device, and more particularly to a polarization diversity wavelength conversion device using optical parametric amplification. 【0002】 Conventionally, wavelength band converters using optical parametric amplification (OPA) with periodically polarized lithium niobate (PPLN), which converts multiple wavelength optical signals simultaneously while maintaining their optical properties (hereinafter also referred to as OPA-PPLN), are known (see Non-Patent Literature 1). OPA-PPLN is expected to be a technology that efficiently realizes functions such as signal wavelength band selection and wavelength collision avoidance in optical communication systems such as All Photonics Networks (APNs) without optical-electrical-optical (OEO) conversion. 【0003】 Conventionally, configurations and methods for temperature control to maintain an appropriate gain spectrum have been proposed for the stabilization control of OPA-PPLNs. Conventional methods focus on two wavelengths λL and λH and control the temperature so that the gain difference between the two wavelengths becomes a desired value (for example, so that the gains are equalized), thereby stably maintaining an appropriate signal gain across the entire wavelength band used. (See Patent Document 1) Conventional methods for stabilizing OPA-PPLNs are control methods for OPA-PPLNs corresponding to a single polarization. 【0004】 On the other hand, there is a need for polarization-independent wavelength converters that can be applied to communication systems, namely polarization diversity wavelength converters that handle both X-polarization and Y-polarization. 【0005】 Japanese Patent Publication No. 2020-76834 【0006】T. Umeki, O. Tadanaga, and M. Asobe, 'Highly Efficient Wavelength Converter Using Direct-Bonded PPZnLN Ridge Waveguide,'2010 IEEE Journal of Quantum Electronics, Vol. 46, No. 8, pp. 1206-1213 【0007】 This disclosure provides an embodiment of a polarization diversity wavelength converter. The polarization diversity wavelength converter according to this embodiment comprises a polarization separator and a polarization combiner, a first wavelength converter disposed between the polarization separator and the polarization combiner, a first excitation light source that supplies excitation light to the first wavelength converter, a first temperature controller that adjusts the temperature of the first wavelength converter, a second wavelength converter disposed between the polarization separator and the polarization combiner, a second excitation light source that supplies excitation light to the second wavelength converter, a second temperature controller that adjusts the temperature of the second wavelength converter, an optical intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength related to wavelength conversion in the polarization diversity wavelength converter, and a controller, the controller being connected to the first and second excitation light sources and to the first and second temperature controllers. 【0008】 As described above, according to one embodiment of the present disclosure, it is possible to provide a polarization-independent wavelength conversion device. 【0009】This figure shows the schematic configuration of a polarization diversity wavelength converter according to one embodiment of the present disclosure. This figure shows the schematic configuration of an optical intensity detector in a polarization diversity wavelength converter according to various embodiments of the present disclosure. This figure illustrates the monitor light used to control a polarization diversity wavelength converter according to one embodiment of the present disclosure. This figure shows an example of the intensity of parametric fluorescence output from a wavelength band converter (OPA-PPLN) using optical parametric amplification with periodically polarized lithium niobate. This figure shows an example of the temperature dependence of the intensity of parametric fluorescence output from an OPA-PPLN. This figure shows an example of the excitation light intensity dependence of the intensity of parametric fluorescence output from an OPA-PPLN. This figure shows the schematic configuration of a modified form of the polarization diversity wavelength converter according to one embodiment of the present disclosure. This figure shows the schematic configuration of another modified form of the polarization diversity wavelength converter according to one embodiment of the present disclosure. This figure illustrates a method for controlling a polarization diversity wavelength converter according to one embodiment of the present disclosure. This figure shows the schematic configuration of a polarization diversity wavelength converter according to another embodiment of the present disclosure. 【0010】 Embodiments of the present disclosure will be described in detail below with reference to the drawings. Identical or similar reference numerals indicate identical or similar elements, and repeated descriptions may be omitted. Material names and numerical values ​​in the following description are illustrative, and embodiments of the present disclosure may be carried out using other materials and numerical values ​​without departing from the spirit of the present disclosure. 【0011】 Polarization diversity wavelength converters according to the various embodiments described below are configured to separate an input signal light into X-polarized and Y-polarized signals, then wavelength-convert each of them using an OPA-PPLN, and then combine them again. The polarization diversity wavelength converters of this disclosure are configured to control the gain of the OPA-PPLN corresponding to the X-polarization and the gain of the OPA-PPLN corresponding to the Y-polarization to be equal. Polarization diversity wavelength converters are disclosed that are capable of handling signal light in which the polarization state varies randomly and independently over time for each wavelength. 【0012】(First Embodiment) A polarization diversity wavelength converter according to a first embodiment of the present disclosure will be described with reference to Figures 1 and 2. The polarization diversity wavelength converter 10 includes a polarization beam splitter (PBS) 121 that separates the X-polarization and Y-polarization of the signal light, a polarization beam combiner (PBC) 122 that combines the X-polarization and Y-polarization of the signal light, and wavelength band converters (OPA-PPLN) 102 and OPA-PPLN 103 positioned between the PBS 121 and the PBC 122. The OPA-PPLN 102 is positioned to receive the X-polarization of the signal light, and the OPA-PPLN 103 is positioned to receive the Y-polarization of the signal light. The signal light is a wavelength division multiplexing signal (WDM) in which different wavelengths are multiplexed. 【0013】 The polarization diversity wavelength conversion device 10 includes an excitation light source 100, a temperature regulator (TEC) 104, an excitation light source 101, and a temperature regulator (TEC) 105. 【0014】 The excitation light source 100 is a light source that emits excitation light (hereinafter also referred to as X-excitation light) that is combined with the X-polarization of the signal light and incident on the OPA-PPLN 102. The excitation light source 100 is configured to change the intensity of the X-excitation light according to instructions from the controller 111. Similarly, the excitation light source 101 is a light source that emits excitation light (hereinafter also referred to as Y-excitation light) that is combined with the Y-polarization of the signal light and incident on the OPA-PPLN 103. The excitation light source 101 is configured to change the intensity of the Y-excitation light according to instructions from the controller 111. 【0015】 TEC 104 is configured to adjust the operating temperature of OPA-PPLN 102 according to instructions from controller 111. Similarly, TEC 105 is configured to adjust the operating temperature of OPA-PPLN 103 according to instructions from controller 111. 【0016】 OPA-PPLN 102 and OPA-PPLN 103 each include a dichroic mirror type multiplexer 142, a PPLN waveguide 143, and a dichroic mirror type demultiplexer 145. The nonlinear optical medium constituting the PPLN waveguide 143 is, for example, LiNbO ３ , LiTaO３ , LiNb(x)Ta(1-x)O ３ (0 ≤ x ≤ 1), or these may contain at least one additive selected from the group consisting of Mg, Zn, Sc, and In. 【0017】 In OPA-PPLN 102, the dichroic mirror multiplexer 142 is configured to combine the X-polarized signal light from PBS 121 with the X-excitation light from the excitation light source 100. The combined X-polarized and X-excitation light is then incident on the PPLN waveguide 143. Similarly, in OPA-PPLN 103, the dichroic mirror multiplexer 142 is configured to combine the Y-polarized signal light from PBS 121 with the Y-excitation light from the excitation light source 101. The combined Y-polarized and Y-excitation light is then incident on the PPLN waveguide 143. 【0018】 In OPA-PPLN 102, the PPLN waveguide 143 is configured to generate X-polarized conversion light of the signal light by difference frequency generation (DFG). The X-polarized conversion light of the signal light is emitted from the PPLN waveguide 143 together with the amplified X-polarized signal light and excitation light. Similarly, in OPA-PPLN 103, the PPLN waveguide 143 is configured to generate Y-polarized conversion light of the signal light by difference frequency generation (DFG). The Y-polarized conversion light of the signal light is emitted from the PPLN waveguide 143 together with the amplified Y-polarized signal light and excitation light. 【0019】 In OPA-PPLN 102, the dichroic mirror demultiplexer 145 is configured to separate the excitation light from the light coming from the PPLN waveguide 143. The separated light (the converted X-polarized signal light and the amplified X-polarized signal light) is output from OPA-PPLN 102. Similarly, in OPA-PPLN 103, the dichroic mirror demultiplexer 145 is configured to separate the excitation light from the light coming from the PPLN waveguide 143. The separated light (the converted Y-polarized signal light and the amplified Y-polarized signal light) is output from OPA-PPLN 103. 【0020】The polarization diversity wavelength converter 10 includes an optical branching coupler 106a that branches a portion of the signal light incident on the PBS 121 (branching approximately 1% to 10% of the light intensity, and so on), and an optical intensity detector 108a that detects the light intensity of the branched signal. The polarization diversity wavelength converter 10 also includes an optical branching coupler 106bx that branches a portion of the light intensity of the X-polarization of the signal light from the PBS 121, and an optical intensity detector 108bx that detects the light intensity of the X-polarization of the branched signal. Similarly, the polarization diversity wavelength converter 10 includes an optical branching coupler 106by that branches a portion of the Y-polarization of the signal light from the PBS 121, and an optical intensity detector 108by that detects the light intensity of the Y-polarization of the branched signal. Furthermore, the polarization diversity wavelength converter 10 includes an optical branching coupler 106cx that branches off the converted X-polarized signal light from the OPA-PPLN 102 and a portion of the X-polarized signal light, and an optical intensity detector 108cx that detects the light intensity of the converted X-polarized signal light and the X-polarized signal light of the branched signal light. Similarly, the polarization diversity wavelength converter 10 includes an optical branching coupler 106cy that branches off the converted Y-polarized signal light from the OPA-PPLN 103 and a portion of the Y-polarized signal light of the amplified signal light, and an optical intensity detector 108cy that detects the light intensity of the converted Y-polarized signal light and the Y-polarized signal light of the branched signal light. Furthermore, the polarization diversity wavelength converter 10 includes an optical branching coupler 106d that branches off a portion of the light from the PBC 122 (converted signal light obtained by combining the converted signal light with the X polarization of the signal light and the converted signal light with the Y polarization of the signal light, and amplified signal light obtained by combining the amplified X polarization of the signal light and the amplified Y polarization of the signal light), and an optical intensity detector 108d that detects the light intensity of the branched converted signal light and the signal light. The light intensity detected by the optical intensity detectors 108a, 108bx, 108by, 108cx, 108cy, and 108d is supplied to the controller 111. 【0021】 In this embodiment, the optical branching coupler 106a and the light intensity detector 108a, as well as the optical branching coupler 106d and the light intensity detector 108d, are optional components that do not participate in controlling the gain of the OPA-PPLN 102 and OPA-PPLN 103. 【0022】The controller 111 is configured to change the intensity of the X-polarized excitation light emitted from the X-polarized excitation light source 100 and the Y-polarized excitation light emitted from the Y-polarized excitation light source 101 based on the supplied light intensity, and is configured to adjust the operating temperature of the TEC 104 of the OPA-PPLN 102 and the TEC 105 of the OPA-PPLN 103. 【0023】 Referring to Figure 2, the configuration of the light intensity detector 108 will be described. It includes an optical branching coupler 200 that branches the light from the optical branching coupler 106, a wavelength filter 201L into which one of the branched lights is incident, a wavelength filter 201H into which the other branched light is incident, a light intensity detector 202L that detects the light intensity from the wavelength filter 201L, and a light intensity detector 202H that detects the light intensity from the wavelength filter 201H. 【0024】 Wavelength filters 201L and 201H are wavelength filters configured to transmit light of predetermined wavelengths (hereinafter also referred to as monitor light). Wavelength filter 201H is configured to transmit monitor light of higher wavelengths, and wavelength filter 201L is configured to transmit monitor light of lower wavelengths. 【0025】 The light intensity detectors 202L and 202H are time-resolved light intensity detectors that can time-resolve the intensity of incident light and detect the moment-by-moment changes in light intensity. The light intensity detectors 108a, 108bx, 108cx, 108by, 108cy, and 108d are configured such that the integration time Ti of the measured values ​​is set to be equal to each other, the output timing of the detected values ​​is synchronized with each other, and the synchronization accuracy (time difference in output timing) is set to be smaller than the integration time Ti. 【0026】 (Control of the spectral shape of the wavelength band converter) Here, with reference to Figures 3 to 5, the control of the spectral shape of the wavelength band converter (OPA-PPLN) will be explained. Figure 3 shows the relationship between the frequencies of the excitation light, signal light, and converted light in the OPA-PPLN. Figure 3 shows the wavelength λ of the fundamental wave. ０ (frequency ω ０ ) at 1573 nm, excitation light wavelength λ ｐ (Frequency 2ω０ shows the wavelength conversion band of the PPLN waveguide when ) is 786.5 nm. The wavelength λs (frequency ω ｓ of the input signal light (WDM signal light) is 1536 to 1567 nm, then 2ω ０ −ω ｓ generates converted light (converted light of the WDM signal) with a wavelength λc of 1579 to 1611. The wavelength (frequency) of the predetermined monitor light described above is the wavelength (frequency) above the upper limit value of the wavelength range (band) of the input signal light (WDM signal light) and below the wavelength λ ０ of the fundamental light, and the wavelength (frequency) below the lower limit value of the wavelength range (band) of the input signal light (WDM signal light) and near the lower limit value. Alternatively, the wavelength (wavelength) of the predetermined monitor light is the wavelength above the wavelength λ ０ of the fundamental light and below the lower limit value of the wavelength range (band) of the converted light (converted light of the WDM signal), and the wavelength (frequency) near the upper limit value above the upper limit value of the wavelength range (band) of the converted light (converted light of the WDM signal). In another example, the wavelength (frequency) of the predetermined monitor light can be two wavelengths (for example, the shortest wavelength and the longest wavelength among the multiplexed multiple wavelengths) within the wavelength range (band) of the input signal light (WDM signal light). 【0027】 Fig. 4 shows an example of the intensity (gain) of the parametric fluorescence output from the OPA-PPLN. As shown in Fig. 4, the shape of the gain spectrum changes according to the operating temperature of the OPA-PPLN. By controlling the operating temperature of each of the OPA-PPLN corresponding to the X polarization and the OPA-PPLN corresponding to the Y polarization, the shape of the gain spectrum becomes optimal. Fig. 4 also shows, as an example, the wavelengths of two monitor lights (λL and λH). λL is a wavelength of 1533 nm near the lower limit value of the wavelength λs (1536 to 1567 nm) of the input signal light (WDM signal light), and λH is a wavelength of 1571 nm near the upper limit value. 【0028】Figure 5 shows an example of the temperature dependence of the intensity (gain) of parametric fluorescence output from an OPA-PPLN. The intensity (gain) on the two monitor lights (λL = 1533 nm and λH = 1571 nm) shown in Figure 4, and the difference between these gains are shown. As shown in Figure 5, if two appropriate wavelengths (λL and λH) are selected, the gain difference between these wavelengths changes almost linearly with respect to the operating temperature of the OPA-PPLN, and corresponds one-to-one with the operating temperature of the OPA-PPLN. Therefore, by controlling the operating temperature of the OPA-PPLN so that the gain difference between these two wavelengths becomes a desired value (for example, so that they are equalized), it is possible to control the shape of the gain spectrum (constant control) in a stable manner (unaffected by the discrepancy between the set temperature and the actual operating temperature of the OPA-PPLN due to changes in ambient temperature, for example). 【0029】 (Control of Wavelength Band Converter Gain) Figure 6 shows an example of the dependence of the intensity (gain) of parametric fluorescence output from an OPA-PPLN on the excitation light intensity. As shown in Figure 6, the intensity (gain) on two example monitor lights (λL = 1533 nm and λH = 1571 nm) changes according to the intensity of the excitation light input to the OPA-PPLN. In this embodiment, the magnitude of the gain of the OPA-PPLN corresponding to X polarization and the OPA-PPLN corresponding to Y polarization are controlled by controlling the intensity of the X-polarized excitation light and the Y-polarized excitation light, respectively. 【0030】 Next, the control of the operation of the controller 111 in the polarization diversity wavelength conversion device 10 of this embodiment will be described. In this embodiment, for the X polarization and Y polarization of the input signal light (WDM signal), two wavelengths (for example, the shortest wavelength and the longest wavelength) from among the multiplexed wavelengths are used as monitor light (λL, λH). The controller 111 receives in real time the intensities Pinx(λL,t) and Pinx(λH,t) detected from the light intensity detector 108bx, the intensities Poutx(λL,t) and Poutx(λH,t) detected from the light intensity detector 108cx, the intensities Piny(λL,t) and Piny(λH,t) detected from the light intensity detector 108by, and the intensities Pouty(λL,t) and Pouty(λH,t) detected from the light intensity detector 108cy. 【0031】The controller 111 calculates the X-polarization gain Gx(λL,t) = Poutx(λL,t) / Pinx(λL,t) and the X-polarization gain Gx(λH,t) = Poutx(λH,t) / Pinx(λH,t) for the signal light in the OPA-PPLN 102. Similarly, the controller 111 calculates the Y-polarization gain Gy(λL,t) = Pouty(λL,t) / Piny(λL,t) and the Y-polarization gain Gy(λH,t) = Pouty(λH,t) / Piny(λH,t) for the signal light in the OPA-PPLN 103. 【0032】 The controller 111 controls the excitation light sources 100 and 101 and the temperature controllers 104 and 105 so that Gx(λL)=Gx(λH)=Gy(λL)=Gy(λH)=Gt in the OPA-PPLN 102 and OPA-PPLN 103 of the polarization diversity wavelength converter 10. 【0033】 Specifically, the controller 111 constantly adjusts the intensity of the X-polarization light output from the excitation light source 100 so that the X-polarization gain Gx(λH) becomes the target gain value Gt. The period for adjusting the intensity of the excitation light can be, for example, 100 Hz. In addition, the controller 111 periodically adjusts the operating temperature of the OPA-PPLN 102 by controlling the temperature controller 104 so as to equalize the X-polarization gains Gx(λL) and Gx(λH) (so as to make the shape of the gain spectrum constant). The period for adjusting the temperature can be, for example, 0.1 Hz. 【0034】 Similarly, the controller 111 constantly adjusts the intensity of the Y excitation light output from the excitation light source 101 so that the Y polarization gain Gy(λH) becomes the target gain value Gt (equal to the X polarization gain Gx(λH)). The period for adjusting the intensity of the excitation light can be, for example, 100 Hz. In addition, the controller 111 periodically adjusts the operating temperature of the OPA-PPLN 103 by controlling the temperature controller 105 so that the Y polarization gains Gy(λL) and Gy(λH) are equal (to make the shape of the gain spectrum constant). The period for adjusting the temperature can be, for example, 0.1 Hz. 【0035】As described above, the polarization diversity wavelength conversion device 10 can handle signal light whose polarization state randomly varies with time independently for each wavelength. 【0036】 In addition, the controller 111 may control the excitation light sources 100 and 101 and the temperature regulators 104 and 105 so that the gain differences between Gx(λL) and Gx(λH) and between Gy(λL) and Gy(λH) in the OPA-PPLN 102 and OPA-PPLN 103 of the polarization diversity wavelength conversion device 10 are within a predetermined range ΔG (Gx(λL)=Gx(λH)+ΔG and Gy(λL)=Gy(λH)+ΔG), and Gx(λH)=Gy(λH)=Gt. 【0037】 In the above description, an example is shown in which the monitor lights (λL, λH) use two wavelengths (for example, the shortest wavelength and the longest wavelength) out of the multiple wavelengths multiplexed in the input signal light (WDM signal). However, two wavelengths (for example, the shortest wavelength λH’ and the longest wavelength λL’) out of the multiple wavelengths in the converted light of the WDM signal may be used. 【0038】 (Second Embodiment) A polarization diversity wavelength conversion device according to the second embodiment of the present disclosure will be described with reference to FIG. 7. The polarization diversity wavelength conversion device 10 in FIG. 1 uses two of the input signal lights (WDM signals) as monitor lights. The polarization diversity wavelength conversion device 20 of the present embodiment uses two monitor lights having wavelengths (λL and λH) outside the wavelength band of the input signal light (WDM signal) as shown in FIG. 3. 【0039】The polarization diversity wavelength converter 20 of this embodiment has the configuration of the polarization diversity wavelength converter 10 of Figure 1, with the addition of a monitor light source 130, a polarization scrambler 131, and an optical multiplexer coupler 132. Furthermore, as shown in Figure 7, the polarization diversity wavelength converter 20 can omit (or may not omit) the optical branching coupler 106bx that branches off a portion of the X-polarization light intensity of the signal light from the PBS 121, the optical intensity detector 108bx that detects the X-polarization light intensity of the branched signal, the optical branching coupler 106by that branches off a portion of the Y-polarization light of the signal light from the PBS 121, and the optical intensity detector 108by that detects the Y-polarization light intensity of the branched signal. 【0040】 The monitor light source 130 is configured to generate two monitor lights (λL and λH). 【0041】 The polarization scrambler 131 is configured to randomly change the polarization state of the two monitor lights from the monitor light source 130. The polarized scrambled monitor light in the polarization scrambler 131 is converted into light in which the intensity ratio of the mutually orthogonal first polarization direction (X polarization) and second polarization direction (Y polarization) is randomly changed after passing through the PBS 121. As a result of the random change in the intensity ratio of X polarization and Y polarization, the time-averaged value of the light intensity separated in the PBS 121 is the same (1:1) for both X polarization and Y polarization. Furthermore, the polarization scrambler 131 is configured such that the polarization scrambling time constant Tsc is smaller than the integration time Ti of the measurements in the light intensity detectors 108a, 108cx, and 108cy. This allows these intensity detectors to obtain the average value of the intensity of the monitor light detected over an integration time Ti that is longer than the polarization scrambling time constant Tsc. Alternatively, instead of the monitor light source 130 and polarization scrambler 131, an ASE (Amplified Spontaneous Emission) light source that generates unpolarized light with a wide spectral width and a wavelength filter that extracts only two monitor wavelengths (λL and λH) from the unpolarized light as monitor light may be used. 【0042】In the polarization diversity wavelength conversion device 10 of FIG. 1, the optical branching coupler 106a and the optical intensity detector 108a, which are arbitrary components, are configured to branch a part of the signal light and the monitor light, filter two monitor lights (λL and λH), and detect the optical intensity. 【0043】 Similar to the polarization diversity wavelength conversion device 10 of FIG. 1, the optical branching coupler 106cx and the optical intensity detector 108cx are configured to branch a part of the X polarization of the signal light and the X polarization of the monitor light, filter two monitor lights (λL and λH), and detect the optical intensity. The optical branching coupler 106cy and the optical intensity detector 108cy are configured to branch a part of the Y polarization of the signal light and the Y polarization of the monitor light, filter two monitor lights (λL and λH), and detect the optical intensity. 【0044】 The operation control of the controller 111 in the polarization diversity wavelength conversion device 20 of the present embodiment will be described. The controller 111 receives the intensities Pin(λL,t) and Pin(λH,t) from the optical intensity detector 108a, the intensities Poutx(λL,t) and Poutx(λH,t) from the optical intensity detector 108cx, and the intensities Pouty(λL,t) and Pouty(λH,t) from the optical intensity detector 108cy in real time, respectively. 【0045】The controller 111 calculates the X-polarization gain Gx(λL,t) = Poutx(λL,t) / Pinx(λL,t) and the X-polarization gain Gx(λH,t) = Poutx(λH,t) / Pinx(λH,t) of the signal light in the OPA-PPLN 102. In this embodiment, since the polarization scrambler 131 makes the time average of the separation ratio between X-polarization and Y-polarization one-to-one, Pinx(λL,t) = (1 / 2)Pin(λL,t) and Pinx(λH,t) = (1 / 2)Pin(λH,t). Therefore, the intensities Pin(λL,t) and Pin(λH,t) from the light intensity detector 108a can be used to calculate the X-polarization gains Gx(λL,t) and Gx(λH,t). Furthermore, the controller 111 calculates the Y polarization gain Gy(λL,t) = Pouty(λL,t) / Piny(λL,t) and Y polarization gain Gy(λH,t) = Pouty(λH,t) / Piny(λH,t) of the signal light in the OPA-PPLN 103. Similar to the X polarization, Piny(λL,t) = (1 / 2)Pin(λL,t) and Piny(λH,t) = (1 / 2)Pin(λH,t) also apply to the Y polarization, so the controller 111 can use the intensities Pin(λL,t) and Pin(λH,t) from the light intensity detector 108a to calculate the Y polarization gain Gy(λL,t) and Y polarization gain Gy(λH,t). 【0046】 Similar to the first embodiment, the controller 111 controls the excitation light sources 100 and 101 in the OPA-PPLN 102 and OPA-PPLN 103 of the polarization diversity wavelength converter 10 so that Gx(λL)=Gx(λH)=Gy(λL)=Gy(λH)=Gt. The controller 111 also controls the temperature controllers 104 and 105 to stabilize the shape of the gain spectrum. 【0047】 As explained above, the polarization diversity wavelength converter 20 can handle signal light in which the polarization state fluctuates independently and randomly over time for each wavelength. Furthermore, the polarization diversity wavelength converter 20 can be configured with fewer components compared to the polarization diversity wavelength converter 10. 【0048】In addition, similar to the first embodiment, the controller 111 may control the excitation light sources 100 and 101 and the temperature controllers 104 and 105 so that the gain difference between Gx(λL) and Gx(λH) and the gain difference between Gy(λL) and Gy(λH) in the polarization diversity wavelength converter 10's OPA-PPLN 102 and OPA-PPLN 103 are within a predetermined range ΔG (Gx(λL)=Gx(λH)+ΔG and Gy(λL)=Gy(λH)+ΔG), and Gx(λH)=Gy(λH)=Gt. 【0049】 (Third Embodiment) A polarization diversity wavelength converter according to a third embodiment of the present disclosure will be described with reference to Figure 8. The polarization diversity wavelength converter 30 of this embodiment utilizes two monitor lights having wavelengths (λL and λH) other than the wavelength band of the signal light (WDM signal), similar to the configuration of the polarization diversity wavelength converter 20 in Figure 7. In this embodiment, instead of the monitor light source 130 and the polarization scrambler 131, a PDL measuring device capable of outputting light of the wavelengths of the two monitor lights and measuring the PDL (Polarization Dependent Loss) of each light may be used. In that case, the light output from the PDL measuring device is input to the optical multiplexer coupler 132, and the light branched by the optical branching coupler 106d is returned to the PDL measuring device. The optical branching coupler 106a and the optical intensity detector 108a serve as input intensity monitors to compensate for the loss of monitor light generated in the optical multiplexer coupler 132. 【0050】 The polarization diversity wavelength converter 30 of this embodiment can omit (or may not omit) the following components from the polarization diversity wavelength converter 20 configuration shown in Figure 7: an optical branching coupler 106cx that branches off a portion of the X-polarization of the signal light from OPA-PPLN 102 and the X-polarization of the monitor light, an optical intensity detector 108cx that detects the light intensity of the branched X-polarization of the monitor light, an optical branching coupler 106cy that branches off a portion of the Y-polarization of the signal light from OPA-PPLN 103 and the Y-polarization of the monitor light, and an optical intensity detector 108cy that detects the light intensity of the branched Y-polarization of the monitor light. 【0051】In the polarization diversity wavelength converters 10 and 20 shown in Figures 1 and 7, the optical branching coupler 106d and the optical intensity detector 108d, which were previously optional components, are configured to branch a portion of the signal light and monitor light from the polarized beam combiner (PBC) 122, filter two monitor beams (λL and λH), and detect the optical intensity. 【0052】 In this embodiment, the controller 111 uses the light intensity values ​​from the light intensity detector 108a and the light intensity values ​​from the light intensity detector 108d to detect the total gain G(λ,t) of the gain in the OPA-PPLN 102 and the gain in the OPA-PPLN 103. 【0053】 The polarization scrambler 131 is configured to randomly change the polarization state of the two monitor lights from the monitor light source 130 so that the time average of the separation ratio of the X-polarized and Y-polarized waves separated in the PBS 121 is 1:1. The actual separation ratio of the X-polarized and Y-polarized waves will fluctuate over time. The controller 111 acquires the detected light intensity values ​​shared from the light intensity detectors 108a and 108d at time intervals shorter than the polarization scrambling time constant Tsc (also called the data acquisition interval Td) (data acquisition interval Td < polarization scrambling time constant Tsc < mean calculation integral time Ti). 【0054】 The ratio at which the monitor light of wavelength λ is separated into X-polarization and Y-polarization by PBS121 at time t (referred to here as the X-polarization separation ratio Rx(λ,t) and the Y-polarization separation ratio Ry(λ,t), respectively) changes rapidly in time within a shorter time than the polarization scramble time constant Tsc, while satisfying Rx+Ry=1. The instantaneous value of the total gain G(λ,t) is the sum of the gains obtained by weighting the X-polarization gain Gx(λ,t) and Y-polarization gain Gy(λ,t) by the X-polarization separation ratio Rx(λ,t) and the Y-polarization separation ratio Rx(λ,t), respectively (i.e., G(λ,t)=Pout(λ,t) / Pin(λ,t)=Rx(λ,t)・Gx(λ,t) + Ry(λ,t)・Gy(λ,t)). Here, λ is either λH or λL. 【0055】Referring to Figure 9, the method for controlling the polarization diversity wavelength converter 30 will be explained. As shown in Figure 9, intensity data acquired at time intervals Td shorter than the polarization scramble time constant Tsc will change rapidly, reflecting the polarization separation ratio (Rx and Ry) which fluctuates rapidly in time within a time shorter than the polarization scramble time constant Tsc. For example, when Rx changes from 0 (time t) to 1 (time t+Δt), Ry simultaneously changes from 1 (time t) to 0 (time t+Δt). Therefore, the estimation formula for the deviation between the maximum and minimum values ​​of G(λ,t) is |Gx - Gy|. On the other hand, considering the average value calculated over an integration time Ti longer than the polarization scramble time constant Tsc, similar to the second embodiment, the polarization scrambler 131 makes the time average of the X-polarization and Y-polarization separation ratio 1:1, so Rx = Ry = 1 / 2. Therefore, the estimation formula for the total gain G(λ,t) calculated from the intensity data averaged over integration time Ti is (Gx + Gy) / 2. 【0056】 In this embodiment, when polarization-scrambled monitor light is introduced such that the time average of the separation ratio of X-polarization to Y-polarization is 1:1, the total gain G(λ), calculated from the ratio of the light intensity Pin(λ) detected by the light intensity detector 108a and the light intensity Pout(λ) (total light intensity of X-polarization and Y-polarization) detected by the light intensity detector 108d, is controlled so that the time average (estimation formula is ((Gx + Gy) / 2)) becomes the target gain value Gt. At the same time, the time fluctuation of the total gain G(λ) calculated from the detected values ​​(deviation of the total gain G(λ) calculated from the detected values ​​within a time interval Td = maximum value - minimum value (estimation formula is |Gx - Gy|)) is controlled to be minimized, thereby equalizing the gain of the X-polarization and the gain of the Y-polarization. 【0057】 In this way, by simultaneously executing a control that sets the average value of the total gain G(λ) (total gain of X-polarization and Y-polarization) calculated from the detected values ​​to Gt, and a control that sets the deviation of the total gain G(λ) calculated from the detected values ​​to zero, both the gain of X-polarization and the gain of Y-polarization can be set to the desired gain value Gt (Gx = Gy = Gt). The target of such control is the set temperature of the temperature controller or the excitation light intensity, and can be executed in combination, for example, as follows. 【0058】 The control targets at wavelength λH are the intensity of the X excitation light from excitation light source 100 and the intensity of the Y excitation light from excitation light source 101. Gx(λH) is adjusted by controlling the intensity of the X excitation light, and Gy(λH) is adjusted by controlling the intensity of the Y excitation light. At this time, the control is performed as follows: The sum of the intensity of the X excitation light and the intensity of the Y excitation light is controlled so that the total gain G(λH) = 1 / 2{Gx(λH) + Gy(λH)} averaged over integration time Ti becomes Gt. The difference between the intensity of the X excitation light and the intensity of the Y excitation light is controlled so that the time fluctuation (deviation) of the total gain |Gx(λH) - Gy(λH)|, measured at high speed over time interval Td, is always zero. 【0059】On the other hand, the control targets at wavelength λL are temperature controllers 104 and 105. Gx(λL) is adjusted by controlling the operating temperature of the X-polarized OPA-PPLN 102, and Gy(λL) is adjusted by controlling the operating temperature of the Y-polarized OPA-PPLN 103. In this case, the control is performed as follows: The sum of the operating temperatures of OPA-PPLN 102 and OPA-PPLN 103 is controlled so that the total gain G(λL) = 1 / 2{Gx(λL) + Gy(λL)} averaged over integration time Ti becomes Gt. The difference between the operating temperatures of OPA-PPLN 102 and OPA-PPLN 103 is controlled so that the time fluctuation (deviation) of the total gain |Gx(λL) - Gy(λL)| measured at high speed over a time interval Td is always zero. As described in the first embodiment, the period for adjusting the intensity of the excitation light can be, for example, 100 Hz. Furthermore, the temperature adjustment period can be set to, for example, 0.1 Hz. In this way, by making the control period of the excitation light involved in adjusting Gx(λH) and Gy(λH) shorter than the control period of the temperature involved in adjusting Gx(λL) and Gy(λL), it is possible to perform slow-period temperature control while maintaining the relationship Gx(λH) = Gy(λH) = Gt at a short period. Therefore, temperature control that adjusts the difference between Gx(λL) and Gx(λH) by controlling the operating temperature of the X-polarized OPA-PPLN102 is equivalent to temperature control that adjusts the difference between Gx(λL) and Gt (fixed value), thus enabling the adjustment of Gx(λL) by controlling the operating temperature of the X-polarized OPA-PPLN102 as described above. Similarly, temperature control that adjusts the difference between Gy(λL) and Gy(λH) by controlling the operating temperature of the Y-polarized OPA-PPLN 103 is equivalent to temperature control that adjusts the difference between Gy(λL) and Gt (fixed value). Therefore, it is possible to adjust Gy(λL) by controlling the operating temperature of the Y-polarized OPA-PPLN 103 as described above. As a result, stable control of Gx(λL) = Gy(λL) = Gt can be achieved. 【0060】Here, we consider the control in the case where the initial actual values ​​are (Gx,Gy)=(0.8,1.1) as shown in Figure 9. Such values ​​are unknown to the measurer. The total gain G(λ) measured at this time has an average value of 0.95 and a deviation of 0.30. As mentioned above, the estimation formula for the average value of the total gain G(λ) is (Gx+Gy) / 2, and the estimation formula for the deviation is |Gx-Gy|. Therefore, the estimated values ​​of Gx and Gy calculated from the detected values ​​are average value ± deviation / 2. Consequently, if the average value obtained as the detection result is 0.95 and the deviation is 0.30, assuming Gx > Gy, the estimated value (Gx,Gy) = (1.1,0.8), and assuming Gx < Gy, the estimated value (Gx,Gy) = (0.8,1.1). 【0061】 Assuming Gx > Gy is incorrect, if we control the intensity of the X excitation light to decrease and the intensity of the Y excitation light to increase so that the average value (Gx + Gy) / 2 becomes the target gain value Gt (Gt = 1 in this example), the deviation between Gx and Gy will widen. If we continue to control based on this incorrect assumption, in the worst case we will reach Gx = 0.7 and Gy = 1.3. Conversely, assuming Gx < Gy is correct, so if we control the intensity of the X excitation light to increase and the intensity of the Y excitation light to decrease, the deviation between Gx and Gy will decrease, and if we continue to control correctly, Gx and Gy will reach the target gain value Gt = 1, and the deviation between Gx and Gy will become 0. Thus, in this configuration, which performs control based on the mean and deviation, it is crucial to correctly determine the relative magnitudes of Gx and Gy. Therefore, each time control is performed (for example, as explained in the first embodiment, the excitation light intensity is adjusted every 100 Hz, and the temperature control is adjusted every 0.1 Hz), the control continues while determining whether Gx < Gy or Gx > Gy is correct, as follows. When the deviation decreases, the direction of control is correct and the assumption regarding the relative magnitudes of Gx and Gy remains the same. When the deviation increases, the direction of control is reversed, so control is performed based on the assumption that the relative magnitudes of Gx and Gy have been reversed. 【0062】Based on the above, as described above, the controller 111 controls the intensity of the X excitation light from the excitation light source 100 and the Y excitation light from the excitation light source 101, as well as the temperatures of the temperature controllers 104 and 105, so as to simultaneously perform the control to set the average value of the total gain G(λ) calculated from the detected values ​​to the target gain value Gt, and the control to set the deviation of the total gain G(λ) calculated from the detected values ​​to zero. This makes it possible to set Gx and Gy to the target gain value Gt. 【0063】 By correctly controlling the gains of the two monitor wavelengths λH and λL (gains for both X-polarization and Y-polarization), appropriate gain control is achieved across the entire wavelength band used. As described above, the polarization diversity wavelength converter 30 can handle signal light whose polarization state fluctuates independently and randomly over time for each wavelength. Furthermore, the polarization diversity wavelength converter 30 can have a reduced number of components compared to the polarization diversity wavelength converters 10 and 20. Moreover, since the polarization diversity wavelength converter 30 does not have an optical branching coupler 106 and an optical intensity detector 108 before and after the OPA-PPLN 102 and 103, i.e., between the PBS 121 and the PBC 122, it is possible to avoid degradation of polarization characteristics and gain characteristics due to the branching of a portion of the propagating light. 【0064】 (Fourth Embodiment) A polarization diversity wavelength converter according to a fourth embodiment of the present disclosure will be described with reference to Figure 10. In the polarization diversity wavelength converter 40 of this embodiment, without using monitor light, when there is no signal light input, the X excitation light and Y excitation light are turned on / off, and a portion of the light output from the polarization beam combiner (PBC) 122 is branched by the optical branching coupler 106d, and the detected intensity values ​​of two wavelengths (λL and λH) of light from the light intensity detector 108d are used. Here, the two wavelengths (λL and λH) of light are parametric fluorescence output when excitation light is input to the OPA-PPLN 102 or 103 when there is no signal light input to the polarization diversity wavelength converter 40, and are a continuous spectrum that covers the entire range of the monitor light wavelength from λL to λL' and the signal light wavelength shown in Figure 3. 【0065】The control of the operation of the controller 111 in the polarization diversity wavelength conversion device 40 of this embodiment will now be described. The controller 111 receives intensities Pout(λL,t) and Pout(λH,t) from the light intensity detector 108d with the output of the X excitation light turned on and the Y excitation light turned off. 【0066】 The controller 111 adjusts the intensity of the X-polarization light output from the excitation light source 100 so that Gx(λH) becomes the target gain value Gt, using the X-polarization gain Gx(λH) = αPout(λH,t). The controller 111 also adjusts the operating temperature of the OPA-PPLN 102 by controlling the temperature controller 104 so that Gx(λL) and Gx(λH) are equalized (to make the shape of the gain spectrum constant), using the X-polarization gain Gx(λL) = αPout(λL,t). Note that the intensity of parametric fluorescence is proportional to the gain. By pre-determining the proportionality constant between the intensity of parametric fluorescence and the gain, the intensity of parametric fluorescence can be used as the gain value, as shown in the above equation. 【0067】 Next, the controller 111 turns off the output of the X excitation light and turns on the Y excitation light, and receives the intensities Pout(λL,t) and Pout(λH,t) from the light intensity detector 108d. 【0068】 The controller 111 adjusts the intensity of the Y excitation light output from the excitation light source 101 so that Gy(λH) becomes the target gain value Gt, with the Y polarization gain Gy(λH) = αPout(λH,t). The controller 111 also adjusts the operating temperature of the OPA-PPLN 103 by controlling the temperature controller 105 so that Gy(λL) and Gy(λH) are equalized (so that the shape of the gain spectrum is made constant), with the Y polarization gain Gy(λL) = αPout(λL,t). 【0069】As explained above, in situations where there is no signal light input, such as during maintenance, it becomes possible to control the polarization diversity wavelength converter 40 such that Gx(λL)=Gx(λH)=Gy(λL)=Gy(λH)=Gt in the OPA-PPLN 102 and OPA-PPLN 103. As a result, the polarization diversity wavelength converter 40 maintains a state that can handle signal light in which the polarization state fluctuates independently and randomly over time for each wavelength. 【0070】 In addition, similar to the first embodiment, the controller 111 may control the excitation light sources 100 and 101 and the temperature controllers 104 and 105 so that the gain difference between Gx(λL) and Gx(λH) and the gain difference between Gy(λL) and Gy(λH) in the polarization diversity wavelength converter 10's OPA-PPLN 102 and OPA-PPLN 103 are within a predetermined range ΔG (Gx(λL)=Gx(λH)+ΔG and Gy(λL)=Gy(λH)+ΔG), and Gx(λH)=Gy(λH)=Gt. 【0071】 This disclosure provides a polarization-independent wavelength conversion device that can be applied, for example, to a communication system, according to one embodiment of this disclosure. 【0072】10, 20, 30, 40 Polarization diversity wavelength converter 100 X-polarization excitation light source (corresponding to the first excitation light source) 101 Y-polarization excitation light source (corresponding to the second excitation light source) 102 X-polarization OPA-PPLN (corresponding to the first wavelength converter) 103 Y-polarization OPA-PPLN (corresponding to the second wavelength converter) 104 X-polarization temperature controller (TEC) (corresponding to the first temperature controller) 105 Y-polarization temperature controller (TEC) (corresponding to the second temperature controller) 106 Optical branching coupler 108 Light intensity detector (108bx corresponds to the first light intensity detector, 108cx to the second light intensity detector, 108by to the third light intensity detector, 108cx to the fourth light intensity detector, 108d to the fifth light intensity detector, and 108a to the sixth light intensity detector) 111 Controller 121 Polarizing beam splitter (PBS) 122 Polarizing beam combiner (PBC) 130 Monitor light source 131 Polarization scrambler 132 Optical multiplexer coupler 142 Dichroic mirror multiplexer 143 PPLN waveguide 145 Dichroic mirror demultiplexer 200 Optical branching coupler 201 Wavelength filter 202 Time-resolved light intensity detector

Claims

1. A polarization diversity wavelength converter comprising: a polarization separator and a polarization combiner; a first wavelength converter disposed between the polarization separator and the polarization combiner; a first excitation light source that supplies excitation light to the first wavelength converter; a first temperature regulator that adjusts the temperature of the first wavelength converter; a second wavelength converter disposed between the polarization separator and the polarization combiner; a second excitation light source that supplies excitation light to the second wavelength converter; a second temperature regulator that adjusts the temperature of the second wavelength converter; a light intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength related to wavelength conversion in the polarization diversity wavelength converter; and a controller, wherein the controller is connected to the first excitation light source and the second excitation light source, and is connected to the first temperature regulator and the second temperature regulator.

2. A polarization diversity wavelength converter comprising: a polarization separator and a polarization combiner; a first wavelength converter having a nonlinear optical medium disposed between the polarization separator and the polarization combiner; a first excitation light source supplying excitation light to the first wavelength converter; a first temperature controller adjusting the temperature of the first wavelength converter; a second wavelength converter having a nonlinear optical medium disposed between the polarization separator and the polarization combiner; a second excitation light source supplying excitation light to the second wavelength converter; a second temperature controller adjusting the temperature of the second wavelength converter; an optical intensity detector outputting the intensity of light of a first wavelength and the intensity of light of a second wavelength related to the parametric effect in the polarization diversity wavelength converter; and a controller, wherein the controller controls the first excitation light source and the second excitation light source so that the gain of the first wavelength converter and the gain of the second wavelength converter are equal, based on the intensity of light of a first wavelength and the intensity of light of a second wavelength. A polarization diversity wavelength converter configured to control the first temperature controller and the second temperature controller.

3. The polarization diversity wavelength converter according to claim 1 or 2, wherein the optical intensity detector includes: a first optical intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength input to the first wavelength converter; a second optical intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength output from the first wavelength converter; a third optical intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength input to the second wavelength converter; and a fourth optical intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength output from the second wavelength converter, and the controller is configured to control the first excitation light source and the second excitation light source, and the first temperature controller and the second temperature controller, such that the gain of light of a first wavelength and the gain of light of a second wavelength in the first wavelength converter are equal.

4. The polarization diversity wavelength converter according to claim 1 or 2, wherein the light intensity detector includes a fifth light intensity detector that outputs the intensity of light of a first wavelength and the intensity of light of a second wavelength output from the polarization combiner, and the controller is configured to control the first excitation light source and the second excitation light source, and the first temperature controller and the second temperature controller, based on the intensity of light of a first wavelength and the intensity of light of a second wavelength output from the fifth light intensity detector while the first excitation light source is stopped, and the intensity of light of a first wavelength and the intensity of light of a second wavelength output from the fifth light intensity detector while the second excitation light source is stopped, such that the gain of the first wavelength converter and the gain of the second wavelength converter are equal.

5. A monitor light source that generates monitor light of a first wavelength and monitor light of a second wavelength, wherein the first wavelength and the second wavelength are different from the frequencies of the optical signal input to the polarization diversity wavelength converter; and a polarization scrambler configured to randomly change the polarization state of the monitor light of the first wavelength and the second wavelength, wherein the optical intensity detector includes: a second optical intensity detector that outputs the intensity of the monitor light of a first wavelength and the intensity of the monitor light of a second wavelength output from the first wavelength converter; a fourth optical intensity detector that outputs the intensity of the monitor light of a first wavelength and the intensity of the monitor light of a second wavelength output from the second wavelength converter; and a sixth optical intensity detector that outputs the intensity of the monitor light of a first wavelength and the intensity of the monitor light of a second wavelength input from the polarization scrambler to the polarization separator, wherein the controller is Polarization diversity wavelength converter according to claim 1 or 2, configured to control the first excitation light source and the second excitation light source, and the first temperature controller and the second temperature controller, so that the gain of the first wavelength converter and the gain of the second wavelength converter are equal, based on the intensity of the first wavelength monitor light and the intensity of the second wavelength monitor light output from each of the second, fourth, and sixth light intensity detectors.

6. A monitor light source that generates monitor light of a first wavelength and monitor light of a second wavelength, wherein the first wavelength and the second wavelength are different from the frequencies of the optical signal input to the polarization diversity wavelength converter; and a polarization scrambler configured to randomly change the polarization state of the monitor light of the first wavelength and the second wavelength, wherein the optical intensity detector includes a fifth optical intensity detector that outputs the intensity of the monitor light of the first wavelength and the intensity of the monitor light of the second wavelength output from the polarization combiner; and a sixth optical intensity detector that outputs the intensity of the monitor light of the first wavelength and the intensity of the monitor light of the second wavelength input from the polarization scrambler to the polarization separator, wherein the controller adjusts the gain of the first wavelength converter and the gain of the second wavelength converter to be equal based on the intensity of the monitor light of the first wavelength and the intensity of the monitor light of the second wavelength output from each of the fifth and sixth optical intensity detectors. Polarization diversity wavelength conversion device according to claim 1 or 2, configured to control the first excitation light source and the second excitation light source, and to control the first temperature controller and the second temperature controller.

7. The polarization diversity wavelength converter according to claim 2, wherein the first wavelength light and the second wavelength light related to the parametric effect are the longest wavelength light and the shortest wavelength light among a plurality of wavelengths wavelength-multiplexed in the optical signal input to the polarization diversity wavelength converter.

8. The polarization diversity wavelength conversion device according to claim 2, wherein the first wavelength light and the second wavelength light related to the parametric effect are parametric fluorescence in a state where no optical signal is input.

9. The polarization diversity wavelength conversion apparatus according to claim 3, wherein the first light intensity detector and the second light intensity detector are synchronized, and the third light intensity detector and the fourth light intensity detector are synchronized.

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