Wavelength conversion device and control method
Patent Information
- Application Number
- JP2025541190
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2023-08-21
- Publication Date
- 2025-02-27
Abstract
Description
Wavelength conversion device and control method
[0001] The present disclosure relates to a wavelength conversion device and a control method.
[0002] Wavelength conversion techniques using difference frequency generation (DFG) in a nonlinear optical medium are known, and can convert light in a wavelength band used in optical fiber communications, for example, into light in another wavelength band all at once.
[0003] Therefore, such wavelength conversion technology can be applied to optical routing technologies such as optical routing in wavelength division multiplexing (WDM) systems or wavelength collision avoidance in optical routing, and is therefore positioned as one of the key technologies for building large-capacity optical communication networks.
[0004] Japanese Patent Application Laid-Open No. 2020-76834
[0005] 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
[0006] In conventional wavelength conversion techniques, there is room for improvement in terms of control for stably operating wavelength conversion devices using nonlinear optical media.
[0007] One non-limiting object of the present disclosure is to simplify control for stably operating a wavelength conversion device using a nonlinear optical medium.
[0008] Therefore, a wavelength conversion device according to one aspect of the present disclosure comprises a wavelength converter using a nonlinear optical medium, a wavelength separation filter that separates a portion of the output light generated in the wavelength converter by parametric fluorescence in response to input of excitation light to the nonlinear optical medium, a detection circuit that detects the intensity of the light separated by the wavelength separation filter and the slope of the change in the intensity, and a control circuit that individually controls the intensity of the excitation light and the temperature of the wavelength converter based on the intensity and the slope.
[0009] Furthermore, a control method according to one aspect of the present disclosure includes separating, using a wavelength separation filter, a portion of output light generated in a wavelength converter by parametric fluorescence in response to input of excitation light to the wavelength converter using a nonlinear optical medium; detecting, using a detection circuit, the intensity of the light separated by the wavelength separation filter and the slope of the change in the intensity; and individually controlling, using a control circuit, the intensity of the excitation light and the temperature of the wavelength converter based on the detected intensity and slope.
[0010] FIG. 1 is a block diagram showing an example of the configuration of a wavelength conversion device according to a first embodiment; FIG. 2 is a diagram showing an example of the relationship between the wavelengths (or frequencies) of pump light, WDM signal light, and converted light of the WDM signal light; FIG. 3 is a graph showing an example of change in wavelength conversion band with respect to change in operating temperature of an optical parametric amplifier (OPA) module; FIG. 4 is a graph showing an example of change in intensity (left vertical axis) and slope (right vertical axis) with respect to temperature change for light of a specific wavelength when the temperature of the wavelength conversion device changes while the pump light power is constant; and FIG. 5 is a block diagram showing an example of the configuration of a wavelength conversion device according to a second embodiment.
[0011] Hereinafter, embodiments will be described in detail with reference to the drawings. In order to avoid redundant description and to simplify and clarify the description, elements or parts with the same numbers in the drawings refer to the same or similar elements or parts unless otherwise specified.
[0012] [Summary] Nonlinear optical devices and electro-optical devices are being developed to generate and modulate coherent light across the ultraviolet, visible, infrared, and terahertz ranges, which can be applied to fields such as optical signal wavelength conversion in optical communications, optical modulation, optical measurement, optical processing, medicine, and bioengineering.
[0013] Various materials have been researched and developed as examples of nonlinear optical and electro-optical media used in such optical devices. For example, oxide compounds such as lithium niobate (LN:LiNbO3) are promising materials because they have higher second-order nonlinear optical and electro-optic constants than other materials.
[0014] An example of an optical device that utilizes the high nonlinearity of LN is a wavelength conversion element that utilizes second harmonic generation (SHG), difference frequency generation (DFG), and / or sum frequency generation (SFG) using periodically poled lithium niobate (PPLN).
[0015] In wavelength conversion using DFG, signal distortion can be compensated for by utilizing the fact that the converted light after wavelength conversion is in a phase conjugate relationship with the signal light. For example, the signal light before conversion to phase conjugate light may experience dispersion in an optical transmission path (e.g., optical fiber) and nonlinear signal distortion due to nonlinear optical effects in the optical fiber. When signal light with dispersion and signal distortion is converted to phase conjugate light, the converted signal light may experience dispersion and nonlinear signal distortion that are opposite to those before conversion in the optical transmission path through which it propagates after conversion.
[0016] Therefore, for example, by converting signal light into phase conjugate light approximately at the midpoint of the optical transmission line, the dispersion and nonlinear signal distortion occurring in the signal light before conversion can be offset and reduced by the inverse dispersion and nonlinear signal distortion occurring after conversion. In this way, wavelength conversion technology is positioned as one of the technologies that can reduce dispersion and nonlinear signal distortion.
[0017] By the way, when a wavelength conversion element with excellent wavelength conversion efficiency is used for wavelength conversion to phase conjugate light, an amplifier of signal light called optical parametric amplification can be constructed by transferring energy from pump light power to signal light. For example, a phase-sensitive amplifier, which has amplification characteristics according to the phase relationship between pump light and signal light, is expected to be a technology that enables optical amplification with low noise.
[0018] Optical waveguide devices are effective in improving wavelength conversion efficiency in PPLN because wavelength conversion efficiency is proportional to the power density of light propagating through the nonlinear optical medium, and the formation of a waveguide structure makes it possible to confine light within a limited range.
[0019] Therefore, various optical waveguides using nonlinear optical media are being investigated, such as ridge-type optical waveguides, which have the advantages of high optical damage resistance, long-term reliability, and easy device design, because they can utilize the bulk properties of the crystal.
[0020] A ridge-type optical waveguide exemplarily has a core formed on a base substrate according to a waveguide pattern and has a step-type refractive index profile (see, for example, Non-Patent Document 1). Note that a ridge-type optical waveguide can operate even if the top and sides of the core are air layers (i.e., the refractive index is 1).
[0021] Furthermore, the nonlinear optical medium may be provided with a periodically poled structure (hereinafter abbreviated as "periodically poled structure") for quasi-phase matching. In the periodically poled structure, for example, the crystal orientation is inverted for each coherence length between the fundamental wave and the wave obtained by wavelength conversion of the fundamental wave, and the sign of the nonlinear constant is inverted, thereby making it possible to gradually or stepwise compensate for the amount of phase mismatch. A nonlinear optical medium having a periodically poled structure has great practical value in that it can perform a wide range of wavelength conversion from the mid-infrared region to the visible region without using a special nonlinear optical crystal.
[0022] On the other hand, since nonlinear optical materials have refractive indices that are temperature dependent, the quasi-phase matching condition in second-order nonlinear optical elements may not be satisfied due to temperature changes in the nonlinear optical material. Therefore, second-order nonlinear optical elements can be controlled so that their temperature is kept constant.
[0023] For example, a temperature sensor such as a thermistor or thermocouple is provided on or near the second-order nonlinear optical element to monitor its resistance, etc. The temperature of the second-order nonlinear optical element can be kept constant by feedback-controlling a temperature regulator such as a heater or Peltier element so that the difference between the monitored result and a target value is minimized.
[0024] However, a mechanism for controlling a temperature regulator to keep the monitored value by a temperature sensor constant may not be sufficient to precisely stabilize the operation of a second-order nonlinear optical element. For example, the temperature that can be monitored by a temperature sensor such as a thermistor or a thermocouple is the average temperature of the entire second-order nonlinear optical element, not the temperature of a local part (e.g., an optical waveguide) that produces a nonlinear optical effect.
[0025] Therefore, when a temperature is monitored using a temperature detector, it may be difficult to control the temperature at which the second-order nonlinear optical element operates to a desired temperature at which stable operation is possible. For example, as described above, a second-order nonlinear optical element having a ridge-type optical waveguide can operate even when there is an air layer above and on the sides of the core, and therefore may be affected by changes in the environmental temperature (e.g., the outside air temperature) depending on the area of the part in contact with the air layer.
[0026] Therefore, if the temperature of the second-order nonlinear optical element is controlled so that the monitored temperature by a temperature sensor installed at or near the second-order nonlinear optical element is constant, the operating point of the second-order nonlinear optical element may shift from the intended operating point depending on changes in the environmental temperature.
[0027] Furthermore, when the power of the pump light incident on the optical waveguide is increased in order to improve the conversion efficiency or gain of optical parametric amplification, the optical waveguide may generate heat due to optical absorption of the pump light incident on the optical waveguide.
[0028] Since this heat generation is localized to the optical waveguide, the monitoring accuracy may be reduced when using a temperature sensor installed in or near the nonlinear optical element, which may prevent the nonlinear optical element from meeting the desired operating conditions for stable operation.
[0029] As an alternative to temperature control using a temperature detector, the application of the technology described in Patent Document 1 is being considered. Patent Document 1 describes a method of separating two wavelengths of light generated by parametric fluorescence in a wavelength converter, and controlling the temperature of the wavelength converter based on the difference in light intensity of the two separated wavelengths.
[0030] However, in this technology, since multiple wavelengths of light are separated using an optical branching coupler and an optical bandpass filter to control the temperature of the wavelength converter, the number of optical components used for temperature control increases, which can complicate the configuration and operation of the control system.
[0031] Therefore, in some non-limiting embodiments described below, a technique that can simplify control for stable operation of a wavelength converter using a nonlinear optical medium will be described.
[0032] For example, a non-limiting example of a wavelength conversion device that includes a wavelength converter using a nonlinear optical medium and is capable of performing wavelength conversion, phase conjugation, and parametric amplification of converted light by utilizing parametric fluorescence from the nonlinear optical medium is shown below.
[0033] [First Embodiment] Fig. 1 is a block diagram showing an example of the configuration of a wavelength conversion device 10 according to the first embodiment. The wavelength conversion device 10 shown in Fig. 1 illustratively includes a pumping light source 101, an erbium-doped optical fiber amplifier (EDFA) 102, a first wavelength converter 103, and a second wavelength converter 104.
[0034] The EDFA 102 is optically connected to the output of the pump light source 101, and the first wavelength converter 103 is optically connected to the output of the EDFA 102. The second wavelength converter 104 exemplarily has one of two input ports optically connected to the output of the first wavelength converter 103.
[0035] The wavelength conversion device 10 also includes, for example, a thermoelectric cooler (TEC) 105, which is an example of a temperature regulator, and an optical branching coupler 106 optically connected to the output of the second wavelength converter 104. The optical branching coupler 106 may also be referred to as a "tap." The wavelength conversion device 10 also includes, for example, a wavelength separation filter 107 optically connected to one of the branched outputs of the optical branching coupler 106, and an optical intensity detector 108 optically connected to the output of the wavelength separation filter 107.
[0036] The wavelength conversion device 10 also includes, for example, a multiplier 109 electrically connected to the output of the optical intensity detector 108, an AC signal source 110 electrically connected to each of the multiplier 109 and the wavelength separation filter 107, and a controller 111. The controller 111 is, for example, electrically connected to each of the EDFA 102, the TEC 105, the optical intensity detector 108, and the multiplier 109.
[0037] The pumping light source 101 generates pumping light using, for example, a semiconductor laser, and outputs the generated pumping light to the EDFA 102. The EDFA 102 amplifies the pumping light incident from the pumping light source 101, and outputs the amplified pumping light to the first wavelength converter 103. The amplification gain of the EDFA 102 (in other words, the power of the pumping light) can be varied by control from the controller 111, for example.
[0038] The first wavelength converter 103 generates, for example, by SHG, which is one of the nonlinear optical effects, the second harmonic of the pump light incident from the EDFA 102, i.e., light having a frequency twice the frequency of the pump light (in other words, half the wavelength of the pump light). Note that light equivalent to the second harmonic may be referred to as "SH light" for convenience.
[0039] The first wavelength converter 103 that generates SH light may also be referred to as an SHG module 103, and illustratively includes a first spatial optical system 131, a first PPLN waveguide 132, and a second spatial optical system 133.
[0040] The first spatial optical system 131 spatially guides, for example, pump light incident from the EDFA 102 to the input port of the first wavelength converter 103 to the first PPLN waveguide 132. The first PPLN waveguide 132 is an example of a second-order nonlinear optical element that uses a nonlinear optical medium with a periodically poled structure, and generates SH light in response to the pump light by SHG as described above.
[0041] The second spatial optical system 133 spatially guides the SH light generated in the first PPLN waveguide 132 to the output port of the first wavelength converter 103. The output port of the SHG module 103 is illustratively connected to the first input port of the second wavelength converter 104, and the SH light output from the SHG module 103 is input to the second wavelength converter 104.
[0042] The second wavelength converter 104 can operate, for example, as an OPA that utilizes parametric fluorescence, which is one of the nonlinear optical effects, and amplifies the signal light (e.g., WDM signal light) input through the second input port by the SH light input through the first input port.
[0043] The second wavelength converter 104 functioning as an OPA may be referred to as an OPA module 104. The OPA module 104 illustratively includes a third spatial optical system 141, a first dichroic mirror 142, a second PPLN waveguide 143, a fourth spatial optical system 144, and a second dichroic mirror 145.
[0044] The third spatial optical system 141, for example, combines the SH light input from the SHG module 103 to the first input port with the signal light input to the second input port, and spatially guides the combined light to the second PPLN waveguide 143.
[0045] For example, the signal light passes through the first dichroic mirror 142 and is coupled to the second PPLN waveguide 143, and the SH light is reflected by the first dichroic mirror 142 and is coupled to the second PPLN waveguide 143. Therefore, the spatial optical system 141 and the dichroic mirror 142 may be understood to constitute, for example, a "dichroic mirror type multiplexer."
[0046] The second PPLN waveguide 143 is an example of a second-order nonlinear optical element using a nonlinear optical medium with a periodically poled structure. The periodically poled structure of the second PPLN waveguide 143 satisfies, for example, a quasi-phase matching condition between the input signal light and SH light and the output converted light. Therefore, the second PPLN waveguide 143 can parametrically amplify the signal light using the SH light as pump light, for example.
[0047] The fourth spatial optical system 144 outputs the amplified signal light from the output light of the second PPLN waveguide 143 to the optical branching coupler 106. Illustratively, the amplified signal light passes through the second dichroic mirror 145 and is output to the optical branching coupler 106, and the SH light is reflected by the second dichroic mirror 145 and emitted to the outside of the OPA module 104. Therefore, the spatial optical system 144 and the dichroic mirror 145 may be understood to constitute, for example, a "dichroic mirror type demultiplexer."
[0048] The OPA module 104 is thermally coupled to the TEC 105 , and the temperature of the OPA module 104 can be controlled by, for example, the controller 111 controlling the TEC 105 .
[0049] The SHG module 103 and the OPA module 104 may each contain, for example, LiNbO3, LiTaO3, LiNbO4, or LiTaO5. (x) Ta (1-x) A nonlinear optical medium containing O3 (0≦x≦1) or at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive may be used.
[0050] The optical branching coupler 106 is illustratively a tap having a branching ratio of approximately 1 / 100 to 1 / 10, and branches off a portion of the output light from the OPA module 104 (e.g., approximately 1% to 10% of the light intensity) and outputs it to the wavelength separation filter 107.
[0051] The remaining light intensity is output as output light from the OPA module 104. Note that the output light from the OPA module 104 may include, for example, amplified signal light, converted light from the signal light, and amplified spontaneous emission (ASE) light, which will be described later.
[0052] The wavelength separation filter 107 is, for example, a band-pass filter (BPF) type optical filter that transmits light of a specific wavelength in the wavelength band of the branched light from the optical branching coupler 106 to the light intensity detector 108 as an example of monitor light, and blocks light of other wavelengths.
[0053] The transmission bandwidth of the wavelength separation filter 107 is, for example, about 1 nm, but is not limited to this, and the transmission bandwidth of the wavelength separation filter 107 is not limited as long as the filter has a characteristic in which a change in the amount of light appears in the output in response to the oscillation of the central transmission wavelength, which will be described later.
[0054] In this example, the wavelength separation filter 107 is, by way of non-limiting example, a fiber Bragg grating (FBG) type optical filter, and its central transmission wavelength can be electrically tunable. For example, the wavelength separation filter 107 can periodically change its central transmission wavelength (in other words, oscillate according to the frequency of the AC signal) in response to an AC signal supplied from an AC signal source 110.
[0055] The light intensity detector 108 detects, for example, the intensity of light transmitted through the wavelength separation filter 107. Here, as described above, when the central transmission wavelength of the wavelength separation filter 107 is vibrated by an AC signal, the amount of light transmitted through the wavelength separation filter 107 periodically fluctuates in response to the vibration.
[0056] In other words, the monitor light output from the wavelength separation filter 107 is frequency modulated by the AC signal. Therefore, a modulated signal component corresponding to the frequency of the AC signal appears in the output (e.g., a DC voltage signal) of the optical intensity detector 108.
[0057] The multiplier 109 synchronously detects the modulated signal component in the output of the optical intensity detector 108 by multiplying the output of the optical intensity detector 108 by a signal synchronized with the AC signal of the AC signal source 110. In other words, the multiplier 109 performs lock-in detection (or first-order differential detection) of the output of the optical intensity detector 108 using a reference signal having a frequency that is one time the frequency of the AC signal supplied to the wavelength separation filter 107.
[0058] The modulated signal component is demodulated by this lock-in detection to obtain a differential signal corresponding to the first-order differential of the intensity change, in other words, a signal indicating the slope of the intensity change. Therefore, the multiplier 109 may be understood to correspond, for example, to a "differential signal generator" that generates a differential signal corresponding to the first-order differential of the intensity change, or a "slope detector" that detects the slope of the intensity change.
[0059] The AC signal source 110 exemplarily supplies an AC signal, which is an example of a periodic signal that periodically changes the central transmission wavelength of the wavelength separation filter 107 as described above, to the wavelength separation filter 107. The frequency of the AC signal may be selected based on, for example, the response characteristics in lock-in detection, which will be described later, and is exemplarily a frequency on the order of 10 kHz.
[0060] It should be noted that the above-mentioned light intensity detector 108, multiplier 109, and AC signal source 110 may be understood to constitute a non-limiting example of a "detection circuit" that detects the above-mentioned light intensity and the slope of the change in light intensity.
[0061] The controller 111 is an example of a control circuit, and exemplarily controls the temperature of the TEC 105 and the amplification gain of the EDFA 102 individually based on the optical intensity obtained by the optical intensity detector 108 and the slope of the optical intensity change obtained by the multiplier 109. The expression "individually control" may be understood to include an embodiment in which either the temperature of the TEC 105 or the amplification gain of the EDFA 102 is the object of control, and an embodiment in which both are the object of control.
[0062] For example, the controller 111 individually controls the temperature of the TEC 105 and the amplification gain of the EDFA 102 to keep the intensity and the slope of the intensity change constant, in other words, to minimize the changes in the intensity and the slope.
[0063] The temperature of the OPA module 104 is controlled in response to the temperature control of the TEC 105, and the power of the pump light input to the SHG module 103 is controlled in response to the amplification gain control of the EDFA 102. Therefore, the SH light power input to the OPA module 104 is controlled in response to the amplification gain control of the EDFA 102. For example, proportional-integral-differential control (PID control) may be applied to the control by the controller 111.
[0064] (Operation Example) Next, an operation example of the above-mentioned wavelength conversion device 10 will be described. In the wavelength conversion device 10, the pump light output from the pump light source 101 is subjected to power control by the EDFA 102 and then input to the SHG module 103. In the SHG module 103, SH light of the pump light is generated by SHG in the first PPLN waveguide 132. The SH light of the pump light is input to the OPA module 104.
[0065] For example, SH light and WDM signal light as an example of input signal light are input to the OPA module 104. In the OPA module 104, the WDM signal light and pump light are multiplexed by a dichroic mirror multiplexer and enter the PPLN waveguide 143.
[0066] The PPLN waveguide 143 generates converted light from the WDM signal light by, for example, DFG. Here, the frequency of the pump light is 2ω0, and the frequency corresponding to one of the wavelengths constituting the WDM signal light is ω s and respectively, the DFG in the PPLN waveguide 143 causes the frequency 2ω0-ω s The converted light is generated by dividing the phase of the pump light by Φ p , the phase of the signal light is Φ s When these are expressed as follows, the phase of the pump light Φ p With the phase as the reference, p -Φ s The phase conjugate light of the signal light is generated as follows:
[0067] If the wavelength (frequency: ω0) twice that of the pump light is defined as the fundamental wavelength, the signal light of multiple wavelengths contained in the WDM signal light is generated as converted light of a wavelength folded back around the fundamental wavelength as the central wavelength axis. As the converted light is generated, energy is transferred from the pump light to the WDM signal light, and the WDM signal light is amplified.
[0068] The converted light generated by the PPLN waveguide 143 is output to the dichroic mirror demultiplexer together with the WDM signal combined with the pump light. In the dichroic mirror demultiplexer, the pump light is demultiplexed from the output light of the PPLN waveguide 143, and the remaining light (e.g., amplified WDM signal light + converted light of the WDM signal light + ASE light) is output to the optical branching coupler 106 as output light of the OPA module 104.
[0069] The optical branching coupler 106 branches off a small portion of the output light from the OPA module 104 and outputs it to the wavelength separation filter 107, and outputs the remaining output light as output light from the wavelength conversion device 10. When "converted WDM signal light" is extracted from the output of the optical branching coupler 106, the wavelength conversion device 10 functions as a wavelength converter and a phase conjugate converter. On the other hand, when "amplified WDM signal light" is extracted from the output of the optical branching coupler 106, the wavelength conversion device 10 functions as an optical parametric amplifier.
[0070] 2 is a diagram showing an example of the relationship between the wavelengths (or frequencies) of pump light, signal light, and converted light of the signal light. As a non-limiting example, the fundamental wavelength λ (frequency: ω) is 1545 nanometers (nm), and the pump light wavelength λ p The wavelength conversion (band) of the PPLN waveguide 143 when the wavelength (frequency: 2ω0) is set to 772.5 nm will be described with reference to FIG.
[0071] The length of the PPLN waveguide 143 in the longitudinal direction (element length) is 42 millimeters (mm), as a non-limiting example. Also, Fig. 2(b) shows an example of the wavelength conversion band in the case where the signal light is WDM signal light, and its explanation will be given later.
[0072] When pump light and signal light are input to the PPLN waveguide 143, converted light of the signal light is generated by the DFG of the PPLN waveguide 143. As shown in FIG. s (Frequency: ω s ) is 1540 nm, the converted light of the signal light has a frequency difference between the pump light and the signal light, 2ω0-ω s , that is, wavelength λ c When considered in terms of wavelength domain, the converted light is generated by folding back the signal light with the fundamental wave wavelength λ0 as the central wavelength axis.
[0073] In the PPLN waveguide 143, the quasi-phase matching condition is satisfied among the three waves of light, namely, the pump light, the signal light, and the converted light. For example, the effective refractive indices of the pump light, the signal light, and the converted light in the optical waveguide are respectively set as n p , n s , n c , the PPLN waveguide 143 has a polarization inversion structure with an inversion period Λ that satisfies the following formula 1: p / λ p -n s / λ s -n c / λ c = 1 / Λ (1)
[0074] When Equation 1 is satisfied, the wavelength λ of the signal light s Even if the frequency of the pump light and the frequency 2ω0-ω s For example, the same conversion efficiency can be obtained between the converted light of the signal light wavelength λ s (Frequency: ω s ) is set to 1539 nm, then 2ω0-ω s Thus, converted light with a wavelength of 1551 nm is generated.
[0075] At this time, the effective refractive index of the signal light is n s and the effective refractive index of the converted light n c The effective refractive index of the signal light n s The effective refractive index of the converted light n c Therefore, the signal wavelength λ sEven if the above is changed, the formula 1 can be satisfied, and the wavelength conversion band can be widened.
[0076] Here, such a band shape before and after wavelength conversion (in other words, a spectral shape) is satisfied when the operating temperature of the OPA module 104 is appropriate, but if the operating temperature deviates from the appropriate temperature, the band shape may change. In the case of a temperature change, the effective refractive index n p , n s , n c Since each of these factors changes, the wavelength conversion band obtained may change accordingly.
[0077] 3 is a graph showing an example of the change in wavelength conversion band with respect to the change in operating temperature of the OPA module 104. As a non-limiting example, the graph shows a case where the reference temperature used in wavelength band conversion is set to ΔT=0.0°C, and the temperature is changed within temperature ranges of ±0.1°C and ±0.2°C from the reference temperature. Note that in FIG. 3, the light intensity of the wavelength conversion band under the above-mentioned conditions is shown normalized.
[0078] One possible method to control the operating temperature of the OPA module 104 is to monitor the converted light of one wavelength of the WDM signal light, but the temperature dependency of the optical intensity differs depending on the wavelength of the converted light, making it difficult to simply identify the temperature relationship (in other words, the direction of temperature increase or decrease) in temperature control.
[0079] Although a method of monitoring the entire band of the converted light to control the operating temperature of the OPA module 104 may be considered, this may increase the number of components and complicate the control. Furthermore, if the power of the signal light input to the OPA module 104 fluctuates, the intensity of the converted light also fluctuates in response to this fluctuation, which may further complicate the control if the OPA module 104 is assumed to receive optical input from an external source.
[0080] Therefore, in this embodiment, the operating temperature of the OPA module 104 is controlled by utilizing the characteristics of the PPLN waveguide 143. For example, light in a wavelength region having a specific bandwidth, which is converted from the excitation light by parametric fluorescence, is used as the monitor light.
[0081] Parametric fluorescence is a spontaneous parametric process in which pump light incident on a nonlinear optical medium is converted into light of two frequencies lower than the original frequency due to the presence of spontaneous emission (ASE) light from the nonlinear optical medium, even without input of signal light. For example, when pump light of frequency 2ω is incident on a second-order nonlinear optical medium, the pump light is converted into light of two frequencies ω1 and ω2, which satisfy the equation ω1 + ω2 = 2ω0.
[0082] As described above, the wavelength separation filter 107 is a BPF-type optical filter that transmits light of a specific wavelength and blocks light of other wavelengths. The light of the specific wavelength may be, for example, light of a wavelength that does not overlap with the band of converted light of WDM signal light (hereinafter sometimes referred to as the "converted light band") within the band of light generated by parametric fluorescence in the OPA module 104. Alternatively or additionally, the light of the specific wavelength may be light of a wavelength that does not overlap with the band of WDM signal light.
[0083] For example, a wavelength close to the converted optical band of the WDM signal light is set to a specific wavelength (frequency: ω BPF 2B, when the wavelength band of the input WDM signal light is set to 1528 to 1544 nm, the WDM signal light is wavelength-converted to converted light in the wavelength band of 1546 to 1562 nm.
[0084] In this case, the central transmission wavelength (λ BPF ) may be set to a wavelength that does not overlap with either the wavelength bands 1528 to 1544 nm or 1546 to 1562 nm and is close to the converted light band 1546 to 1562 nm, for example, a wavelength of 1563 nm on the long wavelength side where WDM signal light is not allocated. This wavelength of 1563 nm is a non-limiting example of a wavelength in a band where neither WDM signal light nor its converted light is allocated.
[0085] With the central transmission wavelength of the wavelength separation filter 107 set as described above, the light intensity and the gradient of the change in light intensity due to parametric fluorescence are detected from the output light of the wavelength separation filter 107 by the light intensity detector 108 and the multiplier 109.
[0086] Then, the pumping light power of the pumping light source 101 and the temperature of the OPA module 104 are individually controlled by the controller 111 so that the detected light intensity and the slope of the change in light intensity become constant. This control makes it possible to keep the spectral shape of the conversion band of the WDM signal light constant, or to minimize changes in the spectral shape.
[0087] 4 is a graph showing an example of changes in intensity and slope with temperature for light of a specific wavelength (e.g., near 1545 nm) in the ASE light spectrum when the temperature of the wavelength conversion device 10 changes while the pump light power is constant. In FIG. 4, reference numeral 401 indicates the change in intensity with temperature (left vertical axis), and reference numeral 402 indicates the change in slope with temperature (right vertical axis). The change in intensity is normalized. The temperature is as shown in FIG. 3.
[0088] 4, when the ASE optical spectrum is broadened to a certain extent in the wavelength region, the intensity and slope of the ASE optical spectrum at a specific wavelength outside the conversion band tend to increase or decrease monotonically with temperature change. On the other hand, when the temperature is constant, the ASE optical spectrum tends to shift in the intensity direction while maintaining its spectral shape in response to an increase (or decrease) in the pump light power.
[0089] Therefore, by keeping the intensity of the ASE light of a specific wavelength and the slope of the intensity change constant, the shape of the ASE light spectrum can be kept constant or changes in the spectrum shape can be minimized. Therefore, the controller 111 performs feedback control individually on the pump light power and the temperature based on the two parameters of the intensity and slope described above.
[0090] For example, the controller 111 stabilizes the gradient of the intensity change of the ASE light by controlling the temperature, and then stabilizes the intensity of the ASE light by controlling the pump light power (for example, the amplification gain of the EDFA 102).
[0091] The controller 111 performs calculations for PID control based on, for example, the optical intensity detected by the optical intensity detector 108 and the lock-in signal (differential signal) detected by the multiplier 109. Then, the controller 111 performs feedback control of the TEC 105 and the EDFA 102 (amplification gain) individually based on the calculation results. This makes it possible to stabilize the optical intensity at about 0.2 dB in the converted optical band of the WDM signal light, for example.
[0092] As described above, according to the wavelength conversion device 10 of the first embodiment, light of a specific wavelength (for example, wavelength λ ) can be extracted from the output light generated by parametric fluorescence in the OPA module 104. BPF The ASE light of the separated wavelengths is separated by a wavelength separation filter 107. The optical intensity and the slope of the optical intensity change of the separated wavelengths are detected by an optical intensity detector 108 and a multiplier 109, respectively, and the temperature and pump light power of the OPA module 104 are individually controlled by a controller 111 based on the detection results.
[0093] This operation makes it possible to control the spectral shape of the band of light converted by parametric fluorescence to a constant value, regardless of whether or not signal light is input to the OPA module 104. Therefore, it becomes possible to operate the wavelength conversion device 10 stably for a long period of time under optimal or suitable operating conditions.
[0094] In addition, in the individual control of the temperature and the pumping light power of the OPA module 104, a specific wavelength (for example, λ BPF ) can be monitored, it is possible to suppress an increase in the number of optical components used for the above-described control, and therefore it is possible to simplify the control system including the controller 111.
[0095] Furthermore, the central transmission wavelength of the wavelength separation filter 107 (in other words, the wavelength λ of the monitor light) BPF ) is set to a wavelength that does not overlap with the converted light band of the WDM signal light, it is possible to realize an appropriate monitoring operation without interfering with the converted light of the WDM signal light.
[0096] In the first embodiment, an FBG-type optical filter is shown as an example of the wavelength separation filter 107 whose central transmission wavelength can be electrically tuned. Alternatively, the wavelength separation filter 107 whose central transmission wavelength can be electrically tuned may be realized by, for example, combining a narrow-band BPF with an optical interferometer and modulating the optical path difference of the optical interferometer.
[0097] In the first embodiment described above, the wavelength of the monitor light (λ BPF However, for example, a wavelength (e.g., 1545 nm) deviating from the conversion light band to the short wavelength side may be selected. However, for example, if it is assumed that signal light is to be placed between the band of WDM signal light and the conversion light band in order to improve the utilization efficiency of wavelength resources, it is preferable to select a wavelength deviating from the short wavelength side to the long wavelength side as the wavelength of the monitor light.
[0098] [Second embodiment] Fig. 5 is a block diagram showing a configuration example of a wavelength conversion device 10A according to a second embodiment. In Fig. 5, components having the same reference numerals as those already described may be understood to be the same as or similar to the already described components unless otherwise specified.
[0099] The wavelength conversion device 10A differs from the configuration illustrated in FIG. 1 in that it additionally includes a control light source 121 and an optical multiplexing coupler 122 provided on the input side of the OPA module 104, and that it is provided with a wavelength separation filter 107A instead of the wavelength separation filter 107.
[0100] The control light source 121 is configured using, for example, a semiconductor laser and generates control light. The wavelength of the control light will be described later in relation to the central transmission wavelength of the wavelength separation filter 107A. The optical multiplexing coupler 122 multiplexes the control light output from the control light source 121 with the signal light input to the OPA module 104.
[0101] The wavelength separation filter 107A is a BPF type optical filter similar to the wavelength separation filter 107 in the first embodiment, and transmits light of a specific wavelength in the wavelength band of the branched light from the optical branching coupler 106 to the light intensity detector 108 as an example of monitor light.
[0102] The transmission bandwidth of the wavelength separation filter 107A is, for example, about 1 nm, as in the first embodiment, but is not limited to this, and the transmission bandwidth of the wavelength separation filter 107A is not limited as long as the filter has a characteristic in which a change in the amount of light appears in the output in response to oscillation of the center frequency of the control light, which will be described later.
[0103] Here, unlike the first embodiment, the wavelength separation filter 107A does not need to have a variable central transmission wavelength, and illustratively has a fixed central transmission wavelength. Alternatively, a filter with a variable central transmission wavelength may be used as the wavelength separation filter 107A, but the central transmission wavelength may not be variable (for example, vibrated).
[0104] 1 , AC signal source 110 is electrically connected to control light source 121 and supplies an AC signal, which is an example of a periodic signal that periodically changes the center frequency of the control light, to control light source 121. This AC signal oscillates the center frequency of the control light input to OPA module 104 via optical multiplexing coupler 122.
[0105] In other words, the wavelength conversion device 10A has a configuration in which, instead of applying frequency modulation for lock-in detection to the transmitted light of the wavelength separation filter 107A, frequency modulation for lock-in detection is applied to the control light input to the wavelength separation filter 107A.
[0106] Therefore, in the second embodiment, the multiplier 109 multiplies the output of the optical intensity detector 108 by a signal synchronized with the AC signal of the AC signal source 110, thereby performing lock-in detection of the modulated signal component in the output of the optical intensity detector 108.
[0107] In the second embodiment, too, the light intensity detector 108, the multiplier 109, and the AC signal source 110 may be understood to constitute a non-limiting example of a "detection circuit" that detects the light intensity and the slope of the change in light intensity described above.
[0108] The frequency of the AC signal that oscillates the center frequency of the control light may be selected based on the response characteristics in lock-in detection, as in the first embodiment, and is illustratively a frequency on the order of 10 kHz.
[0109] For example, the intensity of the control light output from the control light source 121 may be set to an intensity higher than the ASE light level. By setting the intensity of the control light to an intensity higher than the ASE light level, noise resistance of the light intensity detection and tilt detection can be improved compared to the first embodiment.
[0110] The wavelength (frequency: ω i ) is, for example, the same wavelength as the central transmission wavelength of the wavelength separation filter 107A. In other words, the wavelength of the control light matches the wavelength of the light separated by the wavelength separation filter 107A.
[0111] 2, when the fundamental wavelength λ (frequency: ω) is 1545 nm and the central transmission wavelength of the wavelength separation filter 107 is 1563 nm, the wavelength of the control light is set to 1563 nm. This wavelength 1563 nm is a non-limiting example of a wavelength that does not overlap with the converted optical band of the WDM signal light and is deviated from the converted optical band to the longer wavelength side.
[0112] In the wavelength conversion device 10A having the above-described configuration, the control light that has been frequency-modulated by the AC signal from the AC signal source 110 is input to the wavelength separation filter 107A via the OPA module 104 and the optical branching coupler 106.
[0113] Wavelength separation filter 107A, whose central transmission wavelength is set to the wavelength of the control light, transmits the control light input from optical branching coupler 106 to optical intensity detector 108, which detects the intensity of the control light. In addition, the slope of the change in intensity of the control light is detected by lock-in detection using multiplier 109 and AC signal source 110.
[0114] The controller 111 individually controls (e.g., PID control) the pumping light power of the pumping light source 101 and the temperature of the OPA module 104 based on the detected intensity of the control light and the slope of the intensity change. This control keeps the spectral shape of the converted light band constant or minimizes changes in the spectral shape. For example, the optical intensity in the converted light band can be stabilized at about 0.2 dB.
[0115] In the above-described second embodiment, an example was described in which the wavelength of the control light matches the central transmission wavelength of the wavelength separation filter 107A, but the wavelength of the control light may not match the central transmission wavelength of the wavelength separation filter 107A.
[0116] For example, if the frequency of the control light is represented by ω, the frequency ω of the light converted from the control light by parametric fluorescence in the OPA module 104 is i is expressed by the following equation 2 using the frequency 2ω of the excitation light: i = 2ω0-ω1 (2)
[0117] This frequency ω i The converted light of the wavelength separation filter 107A also contains a modulated component according to the frequency modulation applied by the AC signal source 110, just like the control light (frequency: ω1) before conversion. Therefore, the central transmission wavelength of the wavelength separation filter 107A is the frequency ω i may be set to a wavelength corresponding to
[0118] As a non-limiting example of wavelength arrangement, when the fundamental wave wavelength λ (frequency: ω) is 1545 nm, the wavelength λ of the control light is CONT and the central transmission wavelength λ of the wavelength separation filter 107A. BPF The combination may be either of the following (a) and (b): CONT = 1563 nm; λ BPF =1527nm (b)λ CONT = 1527 nm; λ BPF = 1563 nm
[0119] In other words, the central transmission wavelength of the wavelength separation filter 107A may be set to the wavelength of the control light, or may be set to the wavelength of the light converted by parametric fluorescence of the control light. In yet other words, the wavelength of the control light and the central transmission wavelength of the wavelength separation filter 107A are set to the frequencies ω and ω that satisfy the above-mentioned formula 2. i The wavelengths may be set to either of the two wavelengths corresponding to the respective wavelengths.
[0120] The control light included in the output light of the OPA module 104 may be cut by, for example, an optical filter separately provided on the output side of the optical branching coupler 106. Alternatively, for example, by replacing the optical branching coupler 106 with a WDM filter, the control light may be separated from the output light of the OPA module 104 by the WDM filter and input to the wavelength separation filter 107A.
[0121] [Additional information common to all embodiments] In each of the above-described embodiments, a configuration in which the second harmonic (SH light) of the excitation light is generated by the SHG module 103 has been exemplified, but if a light source capable of outputting light of the wavelength of SH light is applied to the excitation light source 101, the SHG module 103 does not need to be provided.
[0122] Furthermore, in each of the above-described embodiments, an example has been shown in which the pump light power is controlled by controlling the amplification gain of the EDFA 102. Alternatively or additionally, the pump light power may be controlled by, for example, controlling the output light power of the pump light source 101 whose output light power is variable, or by controlling the optical attenuation amount of a variable optical attenuator (VOA) provided between the EDFA 102 and the SHG module 103.
[0123] In addition, if the optical power range of the SH light, which is the second harmonic of the pump light input from the SHG module 103 to the OPA module 104, can be sufficient for the operation of the OPA, the EDFA 102 may be omitted.
[0124] Furthermore, if it is possible to control the output optical power of the pumping light source 101 in addition to the amplification gain control of the EDFA 102, it is possible to expand the controllable range of the pumping light power, for example. Furthermore, if it is possible to control the output optical power of the pumping light source 101 or the attenuation amount of the VOA in addition to the amplification gain control of the EDFA 102, then the control (for example, tracking operation) may be shared according to the individual response speeds, for example.
[0125] As a non-limiting example, assume that the response speed of the pumping light source 101 or the VOA is faster than the response speed of the EDFA 102. In this case, it is possible to share the control operations such that relatively large fluctuations are followed by controlling the amplification gain of the EDFA 102, and relatively small and fast fluctuations are followed by controlling the pumping light source 101 or controlling the attenuation amount of the VOA.
[0126] In addition, although both the temperature and the pumping light power are controlled in the above-described embodiments, either the temperature or the pumping light power may be controlled. For example, in a situation where the light intensity (e.g., the pumping light power) is kept constant or fluctuations in the light intensity are minimized, the temperature of the OPA module 104 may be controlled.
[0127] In contrast, for example, in a situation where the temperature of the OPA module 104 is constant (or temperature fluctuations are minimized), the control target may be the light intensity (e.g., pump light power). However, by controlling both the temperature and the pump light power, the spectral shape can be uniquely determined, including the shift in the light intensity direction, and the control time for stabilizing the spectral shape can be shortened.
[0128] In each embodiment, the optical branching coupler 106 may be replaced with, for example, a WDM filter that separates the signal light or the converted light from the light of the specific wavelength described above (in other words, the monitor light). The wavelength separation filter 107 or 107A having BPF characteristics may be realized by, for example, a combination of a long-pass filter and a short-pass filter.
[0129] Furthermore, in each embodiment, an example has been shown in which the spectral shape of the signal light in the band after conversion by parametric fluorescence is controlled to be constant. Alternatively, the spectral shape of the signal light band before conversion by parametric fluorescence may be controlled to be constant.
[0130] As a non-limiting example, 1527 nm, which is a wavelength that is shorter than the signal light band of 1528 to 1544 nm, is the central transmission wavelength (λ ) of the wavelength separation filter 107 or 107A. BPF) This setting makes it possible to control the "spectral shape of the signal light band" to be constant when operating as an optical parametric amplifier.
[0131] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.
[0132] The present disclosure is expected to be utilized, for example, for stable operation of optical elements using nonlinear optical effects, and is useful in optical communication systems, optical measurement systems, and general amplification technology for optical communication.
[0133] 10, 10A Wavelength converter 101 Pumping light source 102 Erbium-doped optical fiber amplifier (EDFA) 103 First wavelength converter (SHG module) 104 Second wavelength converter (OPA module) 105 Thermoelectric cooler (TEC) 106 Optical branching coupler 107, 107A Wavelength separation filter 108 Optical intensity detector 109 Multiplier 110 AC signal source 111 Controller 121 Control light source 122 Optical multiplexing coupler 131 First spatial optical system 132 First PPLN waveguide 133 Second spatial optical system 141 Third spatial optical system 142 First dichroic mirror 143 Second PPLN waveguide 144 Fourth spatial optical system 145 Second dichroic mirror
Claims
1. A wavelength conversion device comprising: a wavelength converter using a nonlinear optical medium; a wavelength separation filter that separates a portion of the output light generated in the wavelength converter by parametric fluorescence in response to input of excitation light to the nonlinear optical medium; a detection circuit that detects the intensity of the light separated by the wavelength separation filter and the slope of the change in said intensity; and a control circuit that individually controls the intensity of the excitation light and the temperature of the wavelength converter based on the intensity and the slope.
2. A wavelength conversion device as described in claim 1, wherein the wavelength separation filter has a central transmission wavelength that does not overlap with the band of converted light obtained by wavelength-converting the wavelength-multiplexed signal light input to the wavelength converter, and the band of the converted light corresponds to the band of phase conjugate light generated by difference frequency generation of the parametric fluorescence based on a fundamental wavelength that is twice the wavelength of the excitation light.
3. A wavelength conversion device as claimed in claim 1 or 2, wherein the wavelength separation filter is an optical bandpass filter with a variable central transmission wavelength, and the detection circuit comprises: a signal source that supplies to the optical bandpass filter a periodic signal that periodically changes the central transmission wavelength of the optical bandpass filter; a light intensity detector that detects the intensity of the output light of the optical bandpass filter; and a tilt detector that detects the tilt based on a signal synchronized with the periodic signal and the output of the light intensity detector.
4. A wavelength conversion device as claimed in claim 1 or 2, further comprising: a control light source having a variable center frequency of output control light; and an optical multiplexing coupler which multiplexes the control light with the signal light input to the wavelength converter, and the detection circuit comprises: a signal source which supplies to the control light source a periodic signal which periodically changes the center frequency of the control light; a light intensity detector which detects the intensity of the output light of the wavelength separation filter; and a tilt detector which detects the tilt based on a signal synchronized with the periodic signal and the output of the light intensity detector.
5. A wavelength conversion device according to claim 4, wherein the wavelength of said control light coincides with the wavelength of the light separated by said wavelength separation filter.
6. Wavelength ω of the control light i When the optical frequency corresponding to the wavelength of the light separated by the wavelength separation filter is expressed as ω1, ω is expressed as ω for the optical frequency 2ω0 of the excitation light. i 5. The wavelength conversion device according to claim 4, wherein the wavelength satisfies:
7. The nonlinear optical medium is LiNbO3, LiTaO3, LiNb (x) T (1-x) 2. The wavelength conversion device according to claim 1, further comprising O3 (0≦x≦1) or at least one selected from the group consisting of Mg, Zn, Sc and In as an additive.
8. A control method comprising: separating, by a wavelength separation filter, a portion of output light generated in a wavelength converter by parametric fluorescence in response to input of excitation light to the wavelength converter using a nonlinear optical medium; detecting, by a detection circuit, the intensity of the light separated by the wavelength separation filter and the slope of the change in the intensity; and individually controlling, by a control circuit, the intensity of the excitation light and the temperature of the wavelength converter based on the detected intensity and slope.