Optical amplifier

Abstract

This optical amplifier (100) is an optical parametric amplifier that has two different phase matching wavelengths (λ1, λ2), and determines the temperature of a secondary nonlinear element (106) on the basis of the spectral intensity of spontaneous emission light at three different wavelengths (λ1, λ2, λ3) and performs feedback control of a temperature adjuster (113). It is also possible to perform feedback control of the output level of excitation light on the basis of the ASE spectral intensity for each of the two different phase matching wavelengths. The secondary nonlinear element can be maintained at an optimum operating temperature without being affected by the temperature detection precision of a temperature sensor.

Description

[Technical Field]

[0001] This invention relates to an optical amplifier used in optical communication systems and laser devices. [Background technology]

[0002] In optical communication systems, erbium-doped fiber amplifiers (EDFAs) are widely used to relay signals that have been attenuated by propagation through optical fibers. EDFAs enable the amplification and relaying of optical signals with a simple configuration, significantly reducing the cost of optical relay.

[0003] The diversification of information and communication technology services in recent years has led to a demand for further increases in transmission capacity in optical communication systems. According to Shannon's communication theory, frequency utilization efficiency, which is given by the ratio of transmission capacity per unit frequency band, is determined by the upper limit of the signal-to-noise (S / N) ratio. To increase frequency utilization efficiency, transmitting at high optical power is effective. In reality, however, due to the nonlinear optical effects of optical fibers, a phenomenon occurs where the S / N ratio of the optical signal deteriorates, and this S / N ratio deterioration is called the nonlinear Shannon limit. To further increase transmission capacity, it is necessary to expand the frequency band used for optical communication.

[0004] The wavelength range that can be amplified by EDFAs, which are widely used in current optical communication systems, is limited to the C-band (1530-1565nm) and L-band (1565-1625nm). If wavelength bands other than those mentioned above can be used, it will be possible to significantly increase the transmission capacity of optical communications.

[0005] One method for achieving optical amplification that is not limited by the wavelength band of the aforementioned EDFA is to use parametric amplification using a second-order nonlinear optical medium. A typical method is to use an optical waveguide made of periodically polled lithium niobate (PPLN) as the second-order nonlinear optical medium. For example, Non-Patent Literature 1 shows that broadband optical amplification operation is possible using PPLN by utilizing difference frequency generation, which is a second-order nonlinear optical effect. With the method using PPLN, it can be assumed that there is almost no degradation of signal quality due to nonlinear optical effects.

[0006] Figure 1 shows the basic configuration of an optical parametric amplifier using a PPLN waveguide (Non-Patent Literature 2). The optical parametric amplifier (OPA) 500 uses PPLN waveguide elements 133 and 134 having the same phase-matched wavelength (1550 nm). A fundamental wave light in the 1550 nm band is generated from a laser light source 131 used in optical communication, and the fundamental wave light is amplified using an EDFA 132 to obtain sufficient power to obtain a nonlinear optical effect. The amplified fundamental wave light 123 is incident on the first PPLN waveguide element 133, and a second harmonic (SH light) is generated. The signal light 120 and SH light 122 from the optical fiber 130 are incident on the second PPLN waveguide element 134, and non-degenerate parametric amplification is performed, so that the amplified signal light 121 is output. Simultaneously with optical amplification, a differential frequency generation (DFG) process outputs wavelength-converted light (idler light) corresponding to the frequency difference between the signal light 120 and the SH light 122. The OPA500 functions as an optical amplifier if only the amplified signal light 121 is extracted, and as a wavelength converter if only the wavelength-converted light is extracted.

[0007] Figure 2 illustrates the optical parametric amplification and DFG processes on the wavelength axis. Here, the phase matching curve is explained using the DFG process, but the same principle applies to the optical parametric amplification process, only the wavelength of the target light differs. The fundamental wave light 30 is a single-wavelength laser beam output from the laser light source 131 in Figure 1. The second harmonic generation (SHG) process of the PPLN waveguide element generates the excitation light, SH light. The phase matching bandwidth 32 for the SH light is narrower than that for the DFG process described later, but the linewidth of the laser beam of the fundamental wave light 30 is sufficiently wider. Here, the fundamental wave wavelength λ0 (frequency: ω0) is 1545 nm, and the excitation light wavelength λp (frequency: 2ω0) is 772.5 nm. This section describes the wavelength conversion bandwidth of a PPLN waveguide element when the value is m.

[0008] By inputting excitation light and signal light 10, a converted light 20 is generated by the DFG process in the PPLN waveguide. For example, if the wavelength λs (frequency: ωs) of the signal light 10 is 1540 nm, then, according to the relationship 2ω0-ωs, a converted light 20 with wavelength λc = 1550 nm is generated. It is formed by folding the signal light along the wavelength axis, centered at the fundamental wavelength λ0 = 1545 nm. Converted light is generated.

[0009] In a PPLN waveguide, the quasi-phase matching (QPM) condition is satisfied among the three waves: excitation light, signal light, and converted light. That is, when the effective refractive indices in the waveguide at the respective wavelengths of the excitation light, signal light, and converted light are np, ns, and nc, respectively, it has a polarization inversion structure with an inversion period Λ that satisfies the following equation. np / λp-ns / λs-nc / λc=1 / Λ Equation (1) Even if the signal light wavelength is changed, the same conversion efficiency can be obtained between the converted light and the excitation light as long as equation (1) is satisfied. Specifically, for example, if the signal light wavelength λs is changed from 1540 nm to the shorter wavelength side 1539 nm, then according to the 2ω0-ωs relationship, the longer wavelength side 155 1 nm conversion light is generated. When the wavelengths of the signal light and the conversion light change in this way, the effective refractive indices ns and nc also change. In the bandwidth of about the phase matching curve 31 of the DFG in FIG. 2, the refractive index dispersion is generally linearly and monotonically decreasing. Around the fundamental wavelength λ0 Due to the linear refractive index dispersion, as ns increases, nc decreases by the same amount, and the relationship of Equation (1) can be satisfied even when the wavelength of the signal light is changed. In a parametric amplifier using a PPLN waveguide, a wide wavelength conversion band 31 as shown in FIG. 2 can be obtained for the DFG process.

[0010] If the bandwidth is further widened, the dispersion characteristics of the material of the PPLN waveguide element are no longer linear, and the increase amount of ns and the decrease amount of nc are not exactly the same. As a result, the conversion efficiency gradually decreases, and the band of the phase matching curve 31 is restricted. When the fundamental optical wavelength λ0 of the pump light (1545 nm in this example) is made to coincide with the phase matching wavelength of the PPLN waveguide element, when the PPLN waveguide length is 45 mm, a band of about 60 nm can be obtained around the fundamental optical wavelength. Amplification with a wider band than that of a general EDFA is possible. Furthermore, as described in Non-Patent Document 1, it is known that by detuning the phase matching wavelength and the pump light wavelength, the shape of the amplification band can be changed, and optical amplification in an even wider band becomes possible.

Prior Art Documents

Non-Patent Documents

[0011]

Non-Patent Document 1

[0012] As described later, by using a PPLN waveguide element having two phase-matching wavelengths, an optical amplifier that can utilize a broadband region exceeding the phase-matching band 31 for DFG in FIG. 2 can also be configured. Although optical parametric amplification using two phase-matching wavelengths is expected to have broadband characteristics, there were the following problems for application to optical communication.

[0013] Since the refractive index of the nonlinear optical material has temperature dependence, it is necessary to keep the temperature of the PPLN waveguide element constant. However, there is a limit to the accuracy of the element temperature that can be detected by the temperature sensor, and it was difficult to maintain the OPA in the optimal operating state. Also, there was a problem that the coupling amount of each light fluctuated with temperature between the spatial optical components used in combination with the PPLN element and the PPLN element, and the levels of the amplified signal light and wavelength-converted light fluctuated.

[0014] The present invention has been made in view of these problems, and its objective is to provide a novel configuration that can always operate an OPA having two phase-matched wavelengths at an optimal operating temperature and stabilize its output light. [Means for solving the problem]

[0015] One aspect of the present invention is an optical amplifier comprising: a second-order nonlinear optical element having two phase-matched wavelengths λ1 and λ2 (λ1 < λ2), which optically parametrically amplifies input light including multiple signal lights by a first excitation light corresponding to λ1 and a second excitation light corresponding to λ2; a temperature controller for adjusting the temperature of the second-order nonlinear optical element; a demultiplexer for separating light separated from the output light of the second-order nonlinear optical element into different wavelength components λ1, λ2, and λ3; a photodetector for detecting the light intensity of the wavelength component λ1 and outputting a first electrical signal; and a photodetector for detecting the light intensity of the wavelength component λ2. A photodetector outputs a second electrical signal, and a third electrical signal is generated by detecting the light intensity of the λ3 wavelength component. The device comprises a photodetector that outputs a signal, and a feedback control circuit that controls the temperature controller so that the difference between either the first or second electrical signal and the third electrical signal is within a predetermined range, wherein the difference corresponds to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element.

[0016] Another aspect of the present invention is an optical amplifier comprising: a second-order nonlinear optical element having two phase-matched wavelengths λ1 and λ2 (λ1 < λ2), which optically parametrically amplifies input light including multiple signal lights by a first excitation light corresponding to λ1 and a second excitation light corresponding to λ2; a temperature controller for adjusting the temperature of the second-order nonlinear optical element; a demultiplexer for separating light separated from the output light of the second-order nonlinear optical element into different wavelength components λ1, λ2, and λ3; a photodetector for detecting the light intensity of the λ1 wavelength component and outputting a first electrical signal; and a photodetector for detecting the light intensity of the λ2 wavelength component. A photodetector detects and outputs a second electrical signal, and a third one detects the light intensity of the λ3 wavelength component. The system comprises a photodetector that outputs an electrical signal, a second demultiplexer that separates the wavelength component of wavelength λ3 from the input light separated on the input side of the second-order nonlinear optical element, a photodetector that detects the light intensity of the wavelength component of λ3 and outputs a fourth electrical signal, and a feedback control circuit that controls the temperature controller so that the difference between the third electrical signal and the fourth electrical signal is within a predetermined range, wherein the difference corresponds to the gain of the signal light of wavelength λ3 from the second-order nonlinear optical element. It is characterized by.

[0017] Another aspect of the present invention is an optical amplifier comprising: a first second-order nonlinear optical element having a phase-matching wavelength λ1 and optically parametrically amplifying input light including a plurality of signal lights with a first excitation light corresponding to λ1; a first temperature controller for adjusting the temperature of the first second-order nonlinear optical element; and connected to the output side of the first second-order nonlinear optical element and having a phase-matching wavelength λ2 (λ1 < λ2) The plurality of signal lights are optically parametrically amplified by a second excitation light corresponding to λ2. A second second-order nonlinear optical element; a second temperature controller for adjusting the temperature of the first second-order nonlinear optical element; a first demultiplexer for separating light separated from the output light of the first second-order nonlinear optical element into wavelength components of different wavelengths, λ1 and λ3; a second demultiplexer for separating light separated from the output light of the second second-order nonlinear optical element into wavelength components of different wavelengths, λ2 and λ3; and a first electrical signal for detecting the light intensity of the λ1 wavelength component from the first demultiplexer. A photodetector is used to detect the light intensity of the λ2 wavelength component from the second demultiplexer, and the second electric A photodetector that outputs a signal and the light intensity of the λ3 wavelength component from the first demultiplexer are detected. A photodetector outputs a third electrical signal, and the light intensity of the λ3 wavelength component from the second demultiplexer The device comprises a photodetector that detects a temperature and outputs a fourth electrical signal, and a feedback control circuit that controls the first temperature controller so that the difference between the first electrical signal and the third electrical signal is within a predetermined range, and controls the second temperature controller so that the difference between the second electrical signal and the fourth electrical signal is within a predetermined range, wherein the difference corresponds to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element.

[0018] A further aspect of the present invention is an optical amplifier comprising: a first second-order nonlinear optical element having a phase-matching wavelength λ1 and optically parametrically amplifying input light including a plurality of signal lights with a first excitation light corresponding to λ1; a first temperature controller for adjusting the temperature of the first second-order nonlinear optical element; and a phase-matching characteristic of the phase-matching λ1 connected to the output side of the first second-order nonlinear optical element. A second second-order nonlinear optical element has a gradient (λ1 < λ2) in the vicinity of which the gain characteristics are attenuated on the short-wavelength side, and uses a second excitation light corresponding to λ2 to optically parametrically amplify input light containing multiple signal lights; a second temperature controller for adjusting the temperature of the first second-order nonlinear optical element; a first demultiplexer for separating light separated from the output light of the first second-order nonlinear optical element into wavelength components of different wavelengths λ1 and λ3 (λ3 < λ1); a second demultiplexer for separating light separated from the output light of the second second-order nonlinear optical element into wavelength components of different wavelengths λ2 and λ4 (λ2 < λ4); and a first electrical signal is output by detecting the light intensity of the λ1 wavelength component from the first demultiplexer. A photodetector is used to detect the light intensity of the λ2 wavelength component from the second demultiplexer, and the second electric A photodetector that outputs a signal and the light intensity of the λ3 wavelength component from the first demultiplexer are detected. A photodetector outputs a third electrical signal, and the light intensity of the λ4 wavelength component from the second demultiplexer The device comprises a photodetector that detects a temperature and outputs a fourth electrical signal, and a feedback control circuit that controls the first temperature controller so that the difference between the first electrical signal and the third electrical signal is within a predetermined range, and controls the second temperature controller so that the difference between the second electrical signal and the fourth electrical signal is within a predetermined range, wherein the wavelength λ3 is located near a slope portion where the gain characteristics attenuate on the short-wavelength side of the phase matching characteristics of λ1, and the wavelength λ4 is located near a slope portion where the gain characteristics attenuate on the long-wavelength side of the phase matching characteristics of λ2, and the difference corresponds to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element. [Effects of the Invention]

[0019] This invention makes it possible to operate an OPA with two phase-matched wavelengths at the optimal operating temperature at all times and stabilize its output light. [Brief explanation of the drawing]

[0020] [Figure 1] This diagram shows the configuration of an optical parametric amplifier using a PPLN waveguide. [Figure 2] This diagram illustrates the optical parametric amplification and DFG processes on the wavelength axis. [Figure 3] This diagram shows the configuration of an optical parametric amplifier with band-separated signal light. [Figure 4] This diagram shows a simplified OPA configuration with two phase matching characteristics. [Figure 5] This diagram illustrates the operation of an OPA with two phase matching characteristics. [Figure 6] This figure shows the configuration of the OPA having two phase matching characteristics according to Embodiment 1. [Figure 7] This figure shows the ASE spectrum of a PPLN element with two phase-matched wavelengths. [Figure 8] This figure shows the temperature dependence of ASE spectral intensity at three different wavelengths. [Figure 9] This figure shows the configuration of the OPA having two phase matching characteristics according to Embodiment 2. [Figure 10] This figure shows the temperature dependence of ASE intensity and gain at three different wavelengths. [Figure 11] This figure shows the configuration of the OPA having two phase matching characteristics according to Embodiment 3. [Figure 12] This figure shows the configuration of the OPA with the monitor wavelength changed by modifying Embodiment 3. [Figure 13] This figure shows the monitor wavelength in the optical amplifier of a modified example of Embodiment 3. [Modes for carrying out the invention]

[0021] The optical amplifier disclosed herein is an optical parametric amplifier having two different phase-matched wavelengths, in which the temperature of a second-order nonlinear element is determined based on the spontaneous emission (ASE) spectral intensity at three different wavelengths, and feedback control is performed to a temperature controller. Feedback control can also be performed on the output level of the excitation light based on the ASE spectral intensity for each of the two different phase-matched wavelengths. The second-order nonlinear element can be maintained at an optimal operating temperature without being affected by the temperature detection accuracy of the temperature sensor. Fluctuations in the coupling amount between the spatial optical component and the second-order nonlinear element are also stabilized by feedback control on the output level of the excitation light. One of the three different wavelengths can be a wavelength located midway between the two phase-matched wavelengths.

[0022] The following sections first describe the basic configuration of an OPA with two different phase-matched wavelengths. Next, the problems with conventional OPA configurations are described in more detail. After that, the configuration and operation of the optical amplifier disclosed herein are described.

[0023] Similar to the DFG process explained in Figure 2, in the parametric amplification process, not only is the signal light amplified, but the converted light is positioned at a point where the signal light is folded back around the fundamental wavelength λ0 of the excitation light. This occurs when the input signal light streams are on both the longwave and shortwave sides of the fundamental wavelength. As a result, the converted light from one side is generated in the signal wavelength band of the other side, causing the signal light and converted light to mix. Therefore, a configuration is needed to handle the signal light and converted light separately.

[0024] Figure 3 shows the configuration of an optical parametric amplifier with band-separated signal light. The OPA600 includes a wavelength separator 135 to separate the signal light 120 into short-wavelength signal light 10-1 and long-wavelength signal light 10-2. The separated signal light is optically parametric amplified by the corresponding PPLN waveguide elements 134-1 and 134-2. When the amplified signal light 11-1 and 11-2 are combined again using the wavelength multiplexer 136, it is necessary to cut the converted light 20-1 and 20-2 and combine only the signal light. In Figure 3, the configuration for generating the SH light, which is the excitation light, is omitted.

[0025] When utilizing the entire amplification bandwidth centered around the wavelength λ0 of the fundamental light, broadband amplification is possible. However, a configuration like the OPA500 shown in Figure 3 is required. The OPA configuration in Figure 3 has two problems. First, because the long wave and short wave sides are amplified separately, two PPLN waveguides for optical amplification are required, increasing the number of components and making the configuration complex. Second, because the signal must pass through the wavelength separator 135 before optical amplification, the noise figure of the amplifier increases by the amount of transmission loss. If the noise figure increases, it becomes impossible to transmit the signal over long distances while maintaining the quality of the optical signal, no matter how wide the amplification bandwidth is. Excess noise on the input side of the optical amplifier must be suppressed as much as possible, and the noise figure must be improved.

[0026] Figure 4 shows a simplified OPA configuration with two phase matching characteristics. The OPA700 simplifies the OPA configuration in Figure 3 by using a second-order nonlinear optical medium such as PPLN that has two phase matching characteristics. As will be described later, the PPLN waveguide element 134 has two phase matching wavelengths λ1 and λ2 in a single element. Two SH lights 123-1 and 123-2 with wavelengths (λ1 / 2 and λ2 / 2) corresponding to the phase matching wavelengths λ1 and λ2 are combined by the multiplexer 137 and supplied to the PPLN waveguide element 134. A configuration that splits the signal light 120 into two paths, a short-wavelength side and a long-wavelength side, is unnecessary, and optical parametric amplification is possible with only a simplified BPF filter 138.

[0027] Figure 5 illustrates the amplification operation of an OPA with two phase-matching characteristics. The input signal optical spectrum, output optical spectrum, and amplification gain characteristics with two phase-matching characteristics are schematically shown. The two phase-matching wavelengths λ1 and λ2 are the wavelengths at both ends of the amplification band containing the input signal light 10, with λ1 representing the shorter wavelength and λ2 representing the longer wavelength. By injecting the respective second-harmonic excitation light corresponding to the phase-matching wavelengths λ1 and λ2 into the PPLN waveguide element, a combined band 31 of parametric amplification bands 31-1 and 31-2 centered on λ1 and λ2 is obtained. On the output side, converted light 20-1 and 20-2 appear outside the bandwidth of the amplified signal light 11. As shown in Figure 4, it is sufficient to remove the wavelength-converted light component at the output side of the PPLN waveguide element 134 using a simple BPF filter 138; there is no need to separate the signal light 120 into long-wavelength and short-wavelength sides at the input side. Since no loss occurs at the input side of the optical amplifier, there is no degradation of the noise figure.

[0028] As mentioned above, the OPA700, which has two phase matching characteristics as shown in Figure 4, had problems with the accuracy of temperature detection and the variation in the coupling amount with spatial optical components. In general, the refractive index of nonlinear optical materials is temperature-dependent. In order to strictly satisfy the pseudo-phase matching condition in a second-order nonlinear optical element at a specific wavelength, it is necessary to keep the temperature of the element constant at the optimal operating temperature. Normally, a temperature sensor such as a thermistor or thermocouple is installed in or near the second-order nonlinear optical element and its resistance value is monitored. After providing a mechanism to keep the element at a constant temperature using a temperature regulator (TEC: ThermoElectric Cooler) 113 such as a heater or Peltier element, the OPA The 700 is operated. Temperature stabilization of the element by the TEC113 is performed by a control signal from a temperature control circuit, which is not shown in Figure 4.

[0029] However, conventional stabilization mechanisms that control the TEC to keep the monitored temperature from the temperature sensor constant remained insufficient due to the two aforementioned problems. Temperature sensors such as thermistors and thermocouples can monitor the average temperature of the entire second-order nonlinear optical element, not the temperature of the waveguide portion itself that produces the nonlinear optical effect. Simply monitoring the temperature detected by the temperature sensor sometimes made it impossible to operate the second-order nonlinear optical element at the optimal temperature.

[0030] For example, if the ambient temperature (outside temperature) of a device changes, the temperature detected by a temperature sensor installed on or near the element may be inaccurate. In second-order nonlinear optical elements such as PPLN waveguide elements, the top and sides of the chip, which are not in contact with the base substrate, are in contact with the air layer and are affected by changes in ambient temperature. If the ambient temperature changes, a difference will occur between the temperature at the temperature sensor location and the actual temperature in the core located on the surface of the PPLN waveguide element where light propagates. Even if temperature control is performed based on the temperature detected by the temperature sensor, the actual temperature of the core of the PPLN waveguide element will deviate from the optimal operating temperature for each individual PPLN element at a given operating wavelength.

[0031] When strong excitation light is injected into a waveguide core to obtain high conversion efficiency or high-gain optical parametric amplification, heat is generated due to the absorption of the excitation light injected into the waveguide. This heat generation is localized within the waveguide core, and it has been difficult to accurately detect the actual temperature of the core under such localized conditions by simply monitoring the element or a temperature sensor placed near it.

[0032] Furthermore, temperature fluctuations in the amount of light coupled to and from the second-order nonlinear optical element cannot be ignored. The coupling of light between the PPLN waveguide element 134 and the module is usually performed via an optical fiber. In Figure 4, the PPLN waveguide element 134 is configured as a module with a PPLN chip mounted on a substrate, and the waveguide core on the chip and the optical fiber are coupled by a spatial optical component. In Figure 4, the signal light 120 is coupled to the waveguide core of the PPLN chip via lenses 140, 142 and a dichroic mirror 141 in the module 134. The SH light 122 is also coupled to the waveguide core of the PPLN chip via lenses 143, 142 and a dichroic mirror 141 in the module 134. In configurations including these spatial optical components, minute optical axis misalignments occur due to changes in ambient temperature, causing fluctuations in the coupling ratio of the signal light and SH light input to the second-order nonlinear optical element. Similarly, spatial optical components such as lenses are used on the output side of module 134, which also causes the coupling rate of the amplified signal light 121 to the output fiber to fluctuate.

[0033] The optical amplifier disclosed herein determines the temperature of a second-order nonlinear element based on the ASE spectral intensity at three different wavelengths, and maintains the second-order nonlinear element at an optimal operating temperature without being affected by the temperature detection accuracy of a temperature sensor. Embodiments of the optical amplifier of the present invention will be described below with reference to the drawings.

[0034] [Embodiment 1] Figure 6 shows the configuration of the optical amplifier (OPA) with two phase matching characteristics according to Embodiment 1. The optical amplifier 100 is an OPA using a PPLN waveguide element 106, which uses a second-order nonlinear optical medium such as PPLN having two phase matching characteristics, similar to the conventional optical amplifier 700 shown in Figure 4. Signal light 120 is input to the PPLN waveguide element 106, and the optically parametrically amplified signal light 121 is output. Similar to the conventional optical amplifier 700, two excitation lights 123-1 and 123-2 of different wavelengths from two excitation light sources 101 and 102 are combined in a multiplexer 137 and supplied to the PPLN waveguide element 106 as combined SH light 122. The PPLN waveguide element 106 has two phase matching wavelengths λ1 as shown in Figure 5. It has λ1 and λ2, and a parametric amplification bandwidth 31-1, 31-2 centered on λ1 and λ2. It has a combined amplification gain bandwidth 31.

[0035] The differences from the conventional OPA700 shown in Figure 4 are as follows: The intensity of the spontaneously radiated amplified light (ASE) spectrum at at least three different wavelengths λ1, λ2, and λ3 is measured. It is equipped with a mechanism for outputting light. Furthermore, it is equipped with a mechanism for determining the shape of the gain characteristics from the detected ASE spectral intensity, estimating the temperature of the PPLN waveguide element 106, and providing feedback control to the excitation light level and temperature controller 113. Below, the configuration and operation of the optical amplifier 100 will be explained, focusing on these differences.

[0036] The output side of the optical amplifier 100 is equipped with an optical branching coupler 107 that extracts a portion of the amplified signal light 121 as monitor light. The monitor light from the optical branching coupler 107 is input to a wavelength demultiplexer 108 and is demultiplexed into three different wavelengths λ1, λ2, and λ3. Light is converted into electrical signals by corresponding photodetectors 109, 110, and 111. Electrical signals V from each photodetector M1 , V M2 , V M3This is provided to the control unit 112, such as a processor. The control unit 112 sends a control signal S to the variable optical attenuators (VOAs) 103 and 104, which adjust the power level of each SH light 123-1 and 123-2. VOA1 S VOA2 The control unit 112 further outputs a control signal S to the TEC 113 which controls the temperature of the PPLN waveguide element 106. TEC They also supply it.

[0037] The two phase matching characteristics of a second-order nonlinear element can be realized, for example, by connecting multiple periodic structures in a multi-stage dependent configuration within the periodic polarization reversal structure of the waveguide of the PPLN waveguide element 106. Alternatively, it can be realized in a multi-quasi-phase matching (QPM) element that incorporates a periodic polarization reversal structure with a certain fundamental period and applies long-period spatial periodic phase modulation to this structure. In the optical amplifier 100 of this embodiment, the wavelengths of the excitation lights 123-1 and 123-2, which are second-harmonic (SH) light, are set to the 780 nm band. DFB lasers and the like can be used as the excitation light sources 101 and 102, and can directly output 780 nm band SH light. Alternatively, the excitation light sources 101 and 102 may be a combination of a fundamental wave light source and a PPLN waveguide element using an SHG process. The control unit 112 is constructed using electronic components including a microcontroller. A processor, CPU, digital signal processor (DSP), FPGA, etc., can also be used as the control unit 112.

[0038] In the optical amplifier 100, the ASE spectral strength is strong at three different wavelengths λ1, λ2, and λ3. The degree is detected. Wavelengths λ1 and λ2 correspond to the two phase-matching wavelengths of the PPLN waveguide element 106, and are the wavelengths at both ends of the signal optical band to be amplified. That is, the phase-matching wavelength λ1 on the short wavelength side is the first measurement wavelength, and the phase-matching wavelength λ2 on the long wavelength side is the second measurement wavelength. The third measurement wavelength λ3 is approximately the wavelength midway between λ1 and λ2 on the wavelength axis, as will be described later. Optical amplification The input signal light to the device 100 is not limited to a wavelength division multiplexing (WDM) signal arranged in the range λ1 < λ < λ2. In the optical amplifier 100, PPLN is used based on the ASE spectral intensities of three different wavelengths λ1, λ2, and λ3. Feedback control is applied to the power level of the excitation light input to the waveguide element and the operating temperature of the nonlinear optical device. Below, we will explain the feedback control operation that maintains the second-order nonlinear element at its optimal operating temperature and suppresses fluctuations in the coupling ratio with the spatial optical component, without detecting the actual temperature of the second-order nonlinear element.

[0039] Figure 7 shows the ASE spectrum of a PPLN waveguide element with two phase-matched wavelengths. ASE is noise that is always present on the output side in the excited state when excitation light is input to an optical parametric amplifier. Normally, in optical parametric amplifiers, the phase-matched wavelength is not used for optical amplification of the signal light. This is because, at the phase-matched wavelength, the amplified signal light and the wavelength-converted light for the signal light are of the same wavelength, and the level fluctuates violently due to interference between the two. In the optical amplifier of this disclosure, the ASE spectral intensities of the two phase-matched wavelengths, which are not normally used, are utilized for feedback control for the stabilization operation of the optical amplifier. Note that the ASE spectrum output from the optical amplifier directly reflects the gain characteristics of the optical amplifier. That is, the shape of the ASE spectrum in the excited state when only excitation light is input, as shown in Figure 7, is similar in shape to the gain characteristic profile of the OPA when signal light is input, and corresponds to the gain characteristic profile shown in Figure 5. The ASE spectral characteristics shown in Figure 7 directly reflect the amplification gain characteristics when signal light is input to the optical amplifier, and the ASE intensity can be converted into an electrical signal to provide information representing the gain of the optical amplifier.

[0040] Figure 7 shows the ASE spectrum (±0°C) when excitation light of wavelengths λ1 / 2 and λ2 / 2 is input to a PPLN waveguide element that has two phase-matching wavelengths, λ1 and λ2, similar to the PPLN waveguide element whose gain characteristics are shown in Figure 5, and is adjusted to the optimal operating temperature. Figure 7 also shows the variation in the ASE spectrum when the temperature of the PPLN waveguide element is shifted from the optimal operating temperature within a range of -0.4 to +0.4°C, indicated by dotted lines. Dotted line 31-1 is due to the phase-matching characteristics of λ1 on the short wavelength side. The dotted line 31-2 shows the temperature variation of the ASE spectrum, due to the phase matching characteristics of λ2 on the longer wavelength side. This shows the temperature variation of the ASE spectrum. The solid line spectrum is the composite spectrum of two ASE spectra combined. Except for the vicinity of the center of the entire bandwidth, the ASE spectrum from one phase matching characteristic and the composite spectrum overlap. In Figure 7, λ1 is set to 1530n With m and λ2 set to 1602 nm, the two ASE spectra are at their respective phase-matched wavelengths. This shows the case where they have equal strength.

[0041] If the actual temperature of the PPLN waveguide element deviates from the optimal operating temperature, the phase matching characteristics, which were centered at λ1 and λ2, change, and the overall composite spectral shape also changes significantly. Specifically, if the actual temperature of the PPLN waveguide element deviates to the positive side from the optimal operating temperature, both phase matching characteristics will shift towards the center of the entire bandwidth, resulting in an ASE spectrum with a raised central portion. Conversely, if the actual temperature of the PPLN waveguide element deviates to the negative side from the optimal operating temperature, both phase matching characteristics will move away from each other in opposite directions towards the edges of the entire bandwidth, resulting in an ASE spectrum with a concave central portion.

[0042] The inventors focused on the fact that, as shown in FIG. 7, the shape of the ASE spectrum characteristically changes according to the actual temperature of the PLN waveguide device, and thus the actual temperature of the PPLN waveguide device can be estimated from the information on the shape of the spectrum. The device temperature detected by a temperature sensor in the prior art was not sufficiently accurate. In contrast, the spectrum shape, particularly at the center, of the PPLN waveguide device having two phase matching wavelengths changes unidirectionally according to the amount of deviation from the optimum operating temperature. Instead of measuring the actual device temperature, it was considered that the actual temperature of the device could be estimated from the intensity difference between the ends (λ1, λ2) and the center (λ3) of the spectrum. Also, by keeping the shape of the spectrum synthesized from the two phase matching characteristics constantly flat, the relative intensity difference between the WDM signals between wavelengths can be directly stabilized.

[0043] Furthermore, regarding the variation in the coupling ratio of the spatial optical components, the gain variation of the optical amplifier can be grasped from the intensity change of the ASE spectrum at the ends (λ1, λ2) of the spectrum. To address the problems of the prior art optical parametric amplifier having two phase matching characteristics, the optical amplifier 100 in FIG. 6 employs a configuration in which the output ASE spectrum is monitored at three wavelengths.

[0044] In the optical amplifier 100 of FIG. 6, the two pump lights (SH lights) 123-1 and 123-2 are each output from a light source, their intensities (power levels) are adjusted by the VOAs 103 and 104, combined by the combiner 105, and input to the PPLN waveguide device 106. The parametrically amplified signal light or ASE in the state where there is no signal light is partially tapped by the optical branching coupler 107 and then demultiplexed by the wavelength demultiplexer 108 into the respective wavelength components of λ1, λ2, and λ3. are demultiplexed into the respective wavelength components.

[0045] The photodetectors 109, 110, and 111 corresponding to the three wavelengths generate electrical signals representing the ASE intensity at each wavelength, that is, the monitor voltages V M1 、V M2 、V M3The signal is output and sent to the control unit 112. Here, the control unit 112 performs control that feeds back to the control signals of VOA 103 and 104 so that the electrical signals, i.e., the monitor voltages, at phase-matching wavelengths λ1 and λ2 are at predetermined target values. Through this control, it is possible to control the ASE intensity, i.e., the gain of the optical amplifier 100, to always be the same at the two points of wavelengths λ1 and λ2. The predetermined target values ​​of the electrical signals, i.e., the monitor voltages, will be described later with reference to Figure 8.

[0046] In the control of the VOA at wavelengths λ1 and λ2 described above, in the gain characteristics shown in Figure 5, only the gain bands 31-1 and 31-2 corresponding to one phase matching characteristic move up and down along the vertical axis, and the shape of the overall composite spectrum cannot be stabilized. If the actual operating temperature of the PPLN waveguide element 106 fluctuates due to changes in the external environment, the shape of the overall composite spectrum will change as shown in Figure 7. Therefore, the ASE spectrum is used to control the TEC113 as well in order to stabilize the shape of the overall spectrum. In the optical amplifier 100 in Figure 6, the third wavelength λ3 is set to 1564 nm, and the ASE intensity between wavelengths λ1 and λ3 is The control unit 112 performs feedback control on the TEC 113 of the PPLN so that the difference is zero or within a certain range, thereby stabilizing the spectral shape.

[0047] Figure 8 shows the temperature dependence of ASE spectral intensity at three wavelengths. The horizontal axis represents temperature (°C), and the vertical axis represents the ASE output intensity in dB relative to each other. λ1, λ2, λ3 The temperature dependence of the ASE spectral intensity around the optimal operating temperature is shown for each wavelength. In this example, the optimal operating temperature of the PPLN waveguide element is 50°C. At wavelengths λ1 and λ2, which are at both ends of the signal light amplification band, the behavior of the ASE intensity with respect to temperature changes is generally consistent because they both correspond to phase-matching wavelengths. On the other hand, at wavelength λ3, which is midway between the two phase-matching wavelengths, the ASE intensity fluctuates in a roughly monotonically increasing manner with respect to temperature changes. Therefore, by monitoring the difference in ASE intensity between wavelengths λ1 and λ3, the actual temperature of the core portion of the PPLN waveguide can be determined. However, it is possible to determine whether the temperature is high or low relative to the optimal operating temperature. By controlling the TEC113 so that the ASE intensity difference between wavelengths λ1 and λ3 is within a certain range, the temperature of the core portion of the PPLN waveguide element can be kept close to the optimal operating temperature, and the spectral shape can also be stabilized. The ASE intensity difference between wavelengths λ1 and λ3 is determined by the electrical signal V at wavelength λ1. M1 And the electrical signal V with wavelength λ3 M3 It can be calculated as the difference between the two electrical signals. The difference between the two electrical signals is the electrical signal V with wavelength λ1. M 1 and the electrical signal V with wavelength λ3 M3 The difference can be the true value of the two values, or the difference can be the logarithmic value (converted to dB).

[0048] The temperature dependence information of the ASE spectral intensity at the three wavelengths shown in Figure 8 is data that can be obtained in advance for each lot of PPLN waveguide elements, for example. At the element's optimal operating temperature (e.g., 50°C), the detection voltage of the ASE output at the three wavelengths can be acquired in advance using the same photodetector as in Figure 6. The detection voltage of the ASE output at the optimal operating temperature is set to the target value V of the monitoring voltage at phase-matching wavelengths λ1 and λ2. Target This can be achieved. The monitor voltage V obtained by the optical amplifier 100 in Figure 6 M1 , V M2 The target value is V Target The control should be set so that this occurs. Furthermore, the ASE intensity difference between wavelengths λ1 and λ3, i.e., the monitor voltage V M1 , V M3 By controlling the TEC113 so that the ratio remains within a certain range, the PPLN waveguide element is controlled to its optimal operating temperature of 50°C.

[0049] Therefore, the optical amplifier of this disclosure has two phase-matched wavelengths λ1 and λ2 (λ1 < λ2), and λ1 The first excitation light 123-1 corresponds to λ2 and the second excitation light 123-2 corresponds to λ2. The system includes a second-order nonlinear optical element 106 that optically parametrically amplifies an input light 120 containing multiple signal lights, a temperature controller 113 that adjusts the temperature of the second-order nonlinear optical element, and a device that separates the light from the output light of the second-order nonlinear optical element into different wavelength components λ1, λ2, and λ3. A demultiplexer 108 separates the light, and a light that detects the light intensity of the wavelength component λ1 and outputs a first electrical signal. Detector 109 and a photodetector that detects the light intensity of the λ2 wavelength component and outputs a second electrical signal. 110 and a photodetector 111 that detects the light intensity of the λ3 wavelength component and outputs a third electrical signal. The system includes a feedback control circuit 112 that controls the temperature controller so that the difference between either the first or second electrical signal and the third electrical signal is within a predetermined range, and the difference is assumed to correspond to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element.

[0050] In the optical amplifier 100 of this embodiment, the control unit 112 controls the TEC 113 of the PPLN waveguide element so that the difference in ASE intensity between wavelengths λ1 and λ3 is within approximately 0.1 dB. As a result, the phase matching wavelength of the PPLN matches λ1 and λ2, and the PPLN waveguide element is controlled to operate at its optimal temperature of 50°C.

[0051] As explained above, the ASE intensity at three points, λ1, λ2, and λ3, is monitored, and the PPLN waveguide... The intensity (power) of the SH excitation light and the temperature of the PPLN waveguide element were fed back to stabilize the element's parametric amplification gain at 18 dB. As a result, the variation in amplification gain was stabilized to within 0.5 dB across the entire amplification band between λ1 and λ2 where the signal light was positioned.

[0052] In the optical amplifier of Embodiment 1 described above, the actual element temperature is estimated from the spectral shape based on the ASE intensity difference between wavelengths λ1 and λ3. However, the exact same control is possible even if the element temperature is estimated based on the ASE intensity difference between wavelengths λ2 and λ3.

[0053] In the embodiments described above, the phase matching wavelengths were set to 1530 nm and 1602 nm in parametric amplification, but the invention is not limited to these examples, and the phase matching wavelength can be any wavelength. Furthermore, in the above description, it was assumed that the phase matching wavelength is exactly twice the wavelength of the excitation light (SH light), and that the fundamental frequency of the SH light and the phase matching frequency coincide on the frequency axis. However, the phase matching wavelength may not be exactly twice the excitation light (SH light) wavelength, but rather a detuned phase matching wavelength may be used. For example, the phase matching wavelength λ1 can be detuned to the longer wavelength side, and the phase matching wavelength λ2 can also be detuned to the longer wavelength side, respectively, with respect to the fundamental wavelength of the SH light.

[0054] In the above explanation, the two excitation light sources used were light sources that directly output light in the 780 nm wavelength band. However, it is also possible to combine a light source that outputs fundamental wave excitation light with a PPLN waveguide element having phase-matched wavelengths (λ1, λ2) and generate SH light excitation light using the SGH process of the PPLN waveguide element.

[0055] In this embodiment, a VOA is used as the power adjustment mechanism for adjusting the intensity of the excitation light, but the invention is not limited to this, and the output intensity of the excitation light may be adjusted using an optical amplifier such as an EDFA. Alternatively, an electrical signal that directly drives the excitation light sources 101 and 102 that generate SH light may be used. Therefore, the control signal S may be adjusted according to the excitation light intensity adjustment mechanism. VOA1 S VOA2 This can be in various forms, such as voltage signals or current signals.

[0056] In the optical amplifier 100 of this embodiment, wavelengths λ1, λ2, and λ3, which are used to monitor the ASE intensity for stabilization control, cannot be used for optical amplification of the signal light. Wavelengths λ1 and λ2 are phase-aligned. It is a convergent wavelength point where the signal light and the wavelength-converted light overlap, and it is not possible to place the signal light at wavelengths λ1 and λ2 in the first place. On the other hand, the third wavelength λ3 is near the center of the combined gain band. The wavelength is 1564 nm, and it is possible to position the signal light there. However, instead of temperature detection by a temperature sensor, control will be based on the shape of the ASE spectrum, so it will not be used for amplifying the signal light. There will be a gap around wavelength λ3 that is large enough to monitor the ASE intensity. It is necessary to prepare a spectral band (for example, 1-2 channels of WDM signals, or a width of about 100 GHz).

[0057] The third wavelength λ3 used to control the shape of the ASE spectrum is 1564 nm. Not limited to, a position where no signal light or wavelength-converted light is generated, shifted significantly to a shorter wavelength than λ1. Alternatively, a wavelength shifted significantly to the longer wavelength side than λ2 may be used. That is, ASE in Figure 7. In the intensity wavelength profile, the slope of the left (short wavelength) end of the phase matching characteristic of λ1 This should be done in the vicinity of the section, or near the inclined portion at the right (long wavelength) end of the phase matching characteristic of λ2. It is also possible to do this. When setting the wavelength λ3 in this way, it is not necessary to prepare a blank spectral band. Furthermore, if the wavelength λ3 is set to be near the left end of the phase matching characteristic of λ1, then the wavelength λ3 in Figure 8 is The temperature characteristic of ASE intensity is monotonically decreasing. The same is true when wavelength λ3 is near the right end of the phase matching characteristic of λ2. Therefore, the third wavelength λ3 is intermediate between λ1 and λ2. It is sufficient if the point is located near the slope where the gain characteristic attenuates on the short-wavelength side of the phase matching characteristic of λ1, or near the slope where the gain characteristic attenuates on the long-wavelength side of the phase matching characteristic of λ2.

[0058] Second-order nonlinear optical elements are made of LiNbO3, LiTaO3, or LiNb (x) Ta (1-x)O3 (0 ≤ x ≤ 1), or a selection from the group consisting of Mg, Zn, Sc, and In. It can be said that it contains at least one type as an additive.

[0059] The optical amplifier 100 of this disclosure estimates the actual temperature of a PPLN waveguide element using the ASE spectral intensity without using a temperature sensor, and provides feedback control to the TEC. This avoids the accuracy problems of temperature detection by temperature sensors. Furthermore, by using the ASE intensity at phase-matching wavelengths λ1 and λ2 and providing feedback to the excitation light intensity, it can also respond to coupling degree fluctuations of spatial optical components.

[0060] In the following embodiment 2, the third wavelength λ3 can be used for optical amplification of the signal light, resulting in a more wavelength-utilizing effect. Present a configuration with good efficiency.

[0061] [Embodiment 2] Figure 9 shows the configuration of two optical amplifiers (OPAs) with phase matching characteristics according to Embodiment 2. The optical amplifier 200 has a configuration that is generally the same as the optical amplifier 100 of Embodiment 1, the difference being that the ASE intensity measurement at the third wavelength λ3 is performed on the input and output sides of the optical amplifier. This is done in both cases. In the optical amplifier 100 of Embodiment 1, because the ASE intensity was being monitored, the signal light could not be placed at the monitoring wavelength of λ3. Optical amplifier 2 In 00, the signal light intensity of λ3 is compared at the input and output sides of the optical amplifier. The gain is determined, and the shape of the gain spectrum is understood. The differences from Embodiment 1 are described below. The wavelengths λ1, λ2, and λ3 are 1530 nm, 1602 nm, and 1564 nm, respectively. This is the same as in the case of Embodiment 1.

[0062] The structural differences between optical amplifier 200 and optical amplifier 100 are that the input side of the PPLN waveguide element 106 has a second optical branch coupler 114, a third wavelength λ3 demultiplexer 115, and a photodetector 11 It is equipped with 1-1. On the output side of the optical amplifier, a third wavelength λ3 photodetector 111- It is equipped with 2, and measures the signal light intensity of λ3 on both the input and output sides of the optical amplifier. , detected voltage value V M3f and V M3f The control unit 112 measures the detected voltage value V. M3f oh Call V M3f From this, the gain in the λ3 signal light can be calculated.

[0063] As explained by the temperature dependence of the ASE spectral intensity at the three wavelengths in Figure 7, the profiles of the ASE spectrum and the gain spectrum of the optical amplifier are identical. The ASE spectral characteristics shown in Figure 7 directly reflect the amplification gain characteristics when signal light is input to the optical amplifier. Therefore, when the PPLN waveguide element is at its optimal operating temperature and has a nearly flat ASE spectrum (±0°C), the ASE intensity at phase-matching wavelengths λ1 and λ2 and wavelength λ3 are identical. If the relationship with the gain of the optical amplifier is known in advance, the gain of the signal light measured at λ3 will be described later. It is sufficient to control it to a predetermined gain value.

[0064] Figure 10 shows the temperature dependence of ASE intensity and gain at three wavelengths. In Figure 10, the horizontal axis represents temperature (°C), the left vertical axis represents the ASE output in dB for wavelengths λ1 and λ2, and the right vertical axis represents the gain in dB for wavelength λ3. In this example, the optimal movement of the PPLN waveguide element is shown. The operating temperature is 50°C. Figure 9 shows the relationship between the ASE output at wavelengths λ1 and λ2 and the gain at wavelength λ3 at the optimal operating temperature of 50°C. There is a one-to-one correspondence between the ASE intensity and the gain of the optical amplifier. Since there is a relationship, feedback control should be applied to the TEC113 so that the measured gain value of the signal light at wavelength λ3 falls within a certain range of variation, centered around a predetermined gain value of 17.8 dB at 50°C at wavelength λ3. The gain of the signal light at wavelength λ3 is measured as described above. Output voltage value V M3f and V M3fIt is obtained from the difference. The measured signal light at wavelength λ3 It should be noted that controlling the gain to a predetermined gain value is equivalent to controlling it based on the shape of the ASE spectrum and estimating the device temperature in Embodiment 1. This boils down to the fact that the ASE spectrum profile and the gain profile of the optical amplifier are similar in shape.

[0065] Therefore, the optical amplifier of this disclosure has two phase-matched wavelengths λ1 and λ2 (λ1 < λ2), and λ1 Multiple signal lights are generated by a first excitation light corresponding to λ2 and a second excitation light corresponding to λ2. The system includes a second-order nonlinear optical element 106 that optically parametrically amplifies the input light, a temperature controller 113 that adjusts the temperature of the second-order nonlinear optical element, a demultiplexer 108 that separates the light separated from the output light of the second-order nonlinear optical element into different wavelength components λ1, λ2, and λ3, a photodetector 109 that detects the light intensity of the λ1 wavelength component and outputs a first electrical signal, and a photodetector 109 that detects the light intensity of the λ2 wavelength component A photodetector 110 detects the light intensity of a minute and outputs a second electrical signal, and the light of the wavelength component λ3. A photodetector 111-2 detects intensity and outputs a third electrical signal, and a second demultiplexer 1 separates the wavelength component λ3 from the input light separated at the input side of the second-order nonlinear optical element. 15 and a photodetector 111-1 that detects the light intensity of the λ3 wavelength component and outputs a fourth electrical signal. The device also comprises a feedback control circuit 112 that controls the temperature controller so that the difference between the third electrical signal and the fourth electrical signal is within a predetermined range, wherein the difference corresponds to the gain of the signal light of the second-order nonlinear optical element at wavelength λ3.

[0066] For wavelengths λ1 and λ2, feedback control is performed on VOA103 and 104, as in Embodiment 1. That is, the detection voltage of the ASE output at the optimal operating temperature of 50°C in Figure 10 is set to the target value V of the monitoring voltage for phase-matched wavelengths λ1 and λ2. TargetThis can be done. The monitor voltage V obtained by the optical amplifier 200 in Figure 9 M1 , V M2 The target value is V Target To achieve this, the intensity of each excitation light can be controlled by the corresponding VOA. In this case, at wavelengths λ1 and λ2, the gain will be controlled to 18 dB, as is clear from Figure 10.

[0067] The configuration of the optical amplifier 200 in Embodiment 2 is based on the gain of the third wavelength λ3 described above. By providing feedback to TEC113, the ASE of the third wavelength λ3 in Embodiment 1 is achieved. This allows for control similar to that used when acquiring spectra. It avoids the accuracy issues of temperature detection by temperature sensors, and allows the use of wavelength λ3 for signal amplification, thus improving bandwidth utilization efficiency. The gain increases. In the entire amplification band where the signal light is positioned between phase-matched wavelengths λ1 and λ2, the in-band variation of the gain was stabilized to within 0.5 dB.

[0068] In Embodiments 1 and 2, the optical amplifiers were realized using a single PPLN waveguide element with two phase matching characteristics. Two phase matching characteristics can also be achieved by combining two PPLN waveguide elements, each having one phase matching characteristic. Generally, fabricating a PPLN waveguide element with two phase matching characteristics in a single element is costly. Embodiment 3 presents an example of an optical amplifier configuration using two PPLN waveguide elements.

[0069] [Embodiment 3] Figure 11 shows the configuration of the optical amplifier (OPA) with two phase matching characteristics according to Embodiment 3. The basic configuration of the optical amplifier 300 is the same as that of the optical amplifier 100 in Embodiment 1 and the optical amplifier 200 in Embodiment 2, but it uses two stages of PPLN waveguide elements with one phase matching characteristic connected in cascade. The first PPLN waveguide element has a phase matching wavelength of λ1 in the preceding stage. Sub-element 106-1, followed by a second PPLN waveguide element 106-2 having a phase-matched wavelength of λ2. The PPLN waveguide elements are arranged as follows. Each PPLN waveguide element is equipped with a corresponding TEC 113-1, 113-2. The excitation light, SH light 123-1, 123-2, is supplied to the corresponding PPLN waveguide element via the VOA without being combined. Similar to Embodiment 1, the ASE intensity or gain of the signal light is measured for three different wavelengths λ1, λ2, and λ3, which is conventionally done. This solves the problems in optical amplifier technology.

[0070] The amplification bandwidth of the signal light to be amplified is shared by two separate PPLN waveguide elements with different phase matching characteristics. Therefore, measurements are performed at the corresponding phase matching wavelength and a third wavelength on the downstream side of each element. Specifically, the wavelength components of λ1 and λ3 are monitored downstream of the first PPLN waveguide element 106-1, and the wavelength components of λ2 and λ3 are monitored downstream of the second PPLN waveguide element 106-2.

[0071] Specifically, it is amplified by a first PPLN waveguide element 106-1 having a phase-matched wavelength of λ1. A portion of the signal light is extracted as monitor light by the optical branching coupler 114 and input to the wavelength demultiplexer 115. The photodetectors 109 and 111-1 convert the wavelength components of wavelengths λ1 and λ3 into electrical signals, and the electrical signal V M1 , V M3f The following is output. A portion of the signal light amplified by the second PPLN waveguide element 106-2, which has a phase-matched wavelength of λ2, is extracted as monitor light by the optical branch coupler 107 and input to the wavelength demultiplexer 108. The photodetectors 110 and 111-2 convert the wavelength components of wavelengths λ2 and λ3 into electrical signals, and the electrical signal V M2 , V M3b The output is The wavelengths λ1, λ2, and λ3 were 1530 nm, 1602 nm, and 1564 nm, respectively. This is the same as in Embodiment 1 and Embodiment 2. In the optical amplifier 300 of this embodiment, if the signal light is not positioned at wavelength λ3, the same control as in Embodiment 1 is possible.

[0072] When signal light is placed in all channels between the two phase-matched wavelengths λ1 and λ2, control similar to that in Embodiment 1 can be achieved simply by changing the monitoring wavelength of the ASE spectral intensity, as will be described later.

[0073] The control operation when the signal light is not positioned at the third wavelength λ3 is performed by the ASE spectral intensity. This embodiment is largely similar to Embodiment 1, and is as follows: The control that monitors the ASE intensity of two phase-matched wavelengths λ1 and λ2 and feeds it back to the corresponding VOA is the same as in Embodiment 1. The control unit 112 sends control signals S to VOAs 103 and 104 so that the monitored voltages of phase-matched wavelengths λ1 and λ2 reach predetermined target values, respectively. TEC1 S TE C1 Provide feedback.

[0074] The element temperature is determined based on the shape of the ASE spectrum using the ASE intensity at the third wavelength λ3. The estimation and feedback control to the TEC is performed on the two TECs 113-1 and 113-2 of the optical amplifier 300, respectively. The control unit 112 performs feedback control on TEC 113-1 so that the ASE intensity difference between wavelengths λ1 and λ3 obtained from the output of the first PPLN waveguide element 106-1 is zero or within a certain range. Similarly, the control unit 112 performs feedback control on TEC 113-2 so that the ASE intensity difference between wavelengths λ2 and λ3 obtained from the output of the second PPLN waveguide element 106-2 is zero or within a certain range.

[0075] If signal light is placed in all channels between the two phase-matched wavelengths λ1 and λ2, the configuration of the optical amplifier is exactly the same as in Figure 11, and only the wavelength used to monitor the ASE intensity needs to be changed.

[0076] Figure 12 shows the configuration of an optical amplifier with two phase matching characteristics, which is a modified example of Embodiment 3. It shows optical amplifier 300-1, which modifies part of the configuration of optical amplifier 300 in Embodiment 3, changing only the monitor wavelength. In amplifier 300-1, the wavelength monitored from the output of the preceding first PPLN waveguide element 106-1 is λ1 and a third wavelength λ3, which is on the shorter wavelength side of λ1. The following changes will be made. In addition, the wavelengths monitored from the output of the second PPLN waveguide element 106-2 in the subsequent stage will be λ2 and the fourth wavelength λ4, which is the longer wavelength side of λ2. There are no differences in the results.

[0077] Figure 13 shows the monitoring wavelength of the ASE spectral intensity in the modified optical amplifier of Embodiment 3. Similar to Figure 5, Figure 13 shows the amplification gain characteristics having two phase-matching characteristics, and the two phase-matching wavelengths λ1 and λ2 are the same throughout Embodiments 1-3. In the modified embodiment of Embodiment 3, a third wavelength λ3 is set in the short-wavelength slope portion of the gain characteristic 31-1 due to the first phase-matching wavelength λ1 for the output of the preceding first PPLN waveguide element 106-1. Furthermore, a fourth wavelength λ4 is set in the long-wavelength slope portion of the gain characteristic 31-2 due to the second phase-matching wavelength λ2 for the output of the subsequent second PPLN waveguide element 106-2. ASE spectral intensity can be monitored at all wavelengths λ2, λ3, and λ4, and two Signal light can be placed in all channels between phase-matched wavelengths λ1 and λ2. The operation of the optical amplifier 300-1 is as follows:

[0078] The control that monitors the ASE intensity at two phase-matched wavelengths λ1 and λ2 and feeds it back to the corresponding VOAs 103 and 104 is similar to that of the optical amplifier 300 in Figure 11, where the signal light is not positioned at a wavelength intermediate between the two phase-matched wavelengths λ1 and λ2 described above.

[0079] The control of TEC113-1 by the monitor wavelengths λ1 and λ3 of the first PPLN waveguide element 106-1 in the preceding stage, and the control of TEC113-2 by the monitor wavelengths λ2 and λ4 of the second PPLN waveguide element 106-2 in the subsequent stage, are performed independently of each other.

[0080] For the first PPLN waveguide element 106-1, the ASE intensity at the third wavelength λ3 is... However, as shown in the temperature-dependent characteristics of Figure 7, it fluctuates depending on the temperature of the PPLN waveguide element. For example, if the actual element temperature rises above the optimal operating temperature, the gain band 31-1 shifts to the longer wavelength side (to the right in Figure 7), and the ASE intensity at the third wavelength λ3 decreases. Assuming a wavelength λ3 of 1492 nm, the temperature-dependent characteristics in Figure 8 show that the ASE intensity at wavelength λ3 decreases monotonically downwards. The ASE intensity at λ3 changes depending on the amount of deviation from the optimal operating temperature. The degree changes in one direction. However, the AS of wavelength λ3 at the optimal operating temperature (e.g., 50°C) The E intensity is considerably lower compared to the intensity at phase-matched wavelength λ1. Temperature dependence data is obtained beforehand. Once acquired and the ASE intensity difference or ASE intensity at the monitor wavelengths λ1 and λ3 at the optimal operating temperature is determined, feedback control to the TEC113-1 is possible, similar to Embodiment 1. Monitor voltage V from photodetectors 109 and 111 M1 , V M3 Based on this, the control unit 112 performs feedback control on the TEC113-1 so that the difference in ASE intensity between wavelengths λ1 and λ3 becomes a predetermined value, thereby stabilizing the spectral shape.

[0081] For the second PPLN waveguide element 106-2, the ASE intensity at the fourth wavelength λ4 is also measured. However, as shown in the temperature-dependent characteristics of Figure 7, it fluctuates depending on the temperature of the PPLN waveguide element. If the actual element temperature rises above the optimal operating temperature, the gain band 31-2 shifts to the shorter wavelength side (left side in Figure 7), and the ASE intensity at the fourth wavelength λ4 decreases. Specifically, If the wavelength λ4 is 1648 nm, the temperature-dependent characteristics in Figure 8 show that the ASE intensity at wavelength λ3 decreases monotonically downwards. Depending on the amount of deviation from the optimal operating temperature, the ASE intensity at λ4 changes It changes directionally. However, the ASE intensity at wavelength λ4 at the optimal operating temperature is phase-matched wavelength. The intensity is considerably lower compared to that of length λ². Temperature-dependent data is obtained in advance, and the optimal operating temperature is set. If the difference in ASE intensity or the ASE intensity at the monitor wavelengths λ2 and λ4 is determined, feedback control to the TEC113-2 is possible, similar to Embodiment 1. Monitor voltage V from photodetectors 110 and 114 M2 , V M4 Based on this, the control unit 112 performs feedback control on the TEC 113-2 so that the difference in ASE intensity between wavelengths λ2 and λ4 becomes a predetermined value, thereby stabilizing the spectral shape. Therefore, the optical amplifier 300-1 can place signal light in the entire band between the two phase-matched wavelengths λ1 and λ2.

[0082] The control of TEC using λ1, λ2, λ3, and λ4 described above is based on the fact that the ASE intensity at both ends of the spectrum obtained by combining the two phase-matching characteristics changes unidirectionally according to the amount of deviation from the optimal operating temperature. Instead of measuring the actual element temperature, the actual temperature of the element is estimated by the difference in ASE intensity between the phase-matching wavelengths (λ1, λ2) and the ends of the spectrum (λ3, λ4). It should also be noted that the shape of the spectrum obtained by combining the two phase-matching characteristics is kept constant by the difference in ASE intensity between the wavelength of the phase-matching wavelength and the wavelength at the ends of the spectrum.

[0083] As described above, in order to estimate the actual temperature of the PPLN waveguide element, the wavelength used to monitor the ASE spectrum is not limited to the wavelength midway between the two phase-matching wavelengths λ1 and λ2. It may also be a wavelength shifted significantly to the shorter wavelength side than λ1, or a wavelength shifted significantly to the longer wavelength side than λ2, where no signal light or wavelength-converted light is generated. As shown in Figure 13, in the wavelength profile of the ASE intensity, it can be λ3 near the slope at the left (short wavelength side) end of the phase-matching characteristic of λ1, or λ4 near the slope at the right (long wavelength side) end of the phase-matching characteristic of λ2.

[0084] Through the control described above, it was possible to stabilize the gain fluctuation within 0.5 dB across the entire bandwidth in which the signal light is placed, without the need to prepare a blank band in the signal light.

[0085] As described in detail above, the optical amplifier of this disclosure makes it possible to operate an OPA with two phase-matched wavelengths at the optimal operating temperature at all times and stabilize its output light. [Industrial applicability]

[0086] This invention can be used in optical communications.

Claims

1. It is an optical amplifier, Two phase-matched wavelengths λ 1 , λ 2 (λ 1 <λ 2 ) has, λ 1 The first excitation light and λ corresponding to 2 A second-order nonlinear optical element that uses a second excitation light corresponding to the above to optically parametrically amplify input light containing multiple signal lights, A temperature controller for adjusting the temperature of the aforementioned second-order nonlinear optical element, The light separated from the output light of the second-order nonlinear optical element is separated into wavelength components of different wavelengths λ 1 , λ 2 and λ 3 by a wavelength demultiplexer, λ 1 A photodetector that detects the light intensity of the wavelength component and outputs a first electrical signal, λ 2 A photodetector that detects the light intensity of the wavelength component and outputs a second electrical signal, λ 3 A photodetector that detects the light intensity of the wavelength component and outputs a third electrical signal, A feedback control circuit controls the temperature controller such that the difference between either the first or second electrical signal and the third electrical signal falls within a predetermined range. Equipped with, The parametric amplification bandwidth corresponding to λ1 and the parametric amplification bandwidth corresponding to λ2 overlap near the center of the entire amplification bandwidth. The third wavelength λ3 is located in the band where the two parametric amplification bands overlap in the entire amplification band. The optical amplifier is characterized in that the difference corresponds to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element.

2. It is an optical amplifier, Two phase-matched wavelengths λ 1 , λ 2 (λ 1 <λ 2 ) has, λ 1 The first excitation light and λ corresponding to 2 A second-order nonlinear optical element that uses a second excitation light corresponding to the above to optically parametrically amplify input light containing multiple signal lights, A temperature controller for adjusting the temperature of the aforementioned second-order nonlinear optical element, Light separated from the output light of the aforementioned second-order nonlinear optical element is separated by a different wavelength λ 1 , λ 2 and λ 3 A demultiplexer that separates the wavelength components, λ 1 A photodetector that detects the light intensity of the wavelength component and outputs a first electrical signal, λ 2 A photodetector that detects the light intensity of the wavelength component and outputs a second electrical signal, λ 3 A photodetector that detects the light intensity of the wavelength component and outputs a third electrical signal, From the input light separated on the input side of the second-order nonlinear optical element, wavelength λ 3 A second demultiplexer separates the wavelength components, λ 3 A photodetector that detects the light intensity of the wavelength component and outputs a fourth electrical signal, A feedback control circuit controls the temperature controller such that the difference between the third electrical signal and the fourth electrical signal falls within a predetermined range. Equipped with, The parametric amplification bandwidth corresponding to λ1 and the parametric amplification bandwidth corresponding to λ2 overlap near the center of the entire amplification bandwidth. The third wavelength λ3 is located in the band where the two parametric amplification bands overlap in the entire amplification band. The difference is the wavelength λ of the second-order nonlinear optical element. 3 An optical amplifier characterized by corresponding to the gain of the signal light.

3. It is an optical amplifier, Phase matching wavelength λ 1 It has, λ 1 A first second-order nonlinear optical element that optically parametrically amplifies input light containing multiple signal lights using a first excitation light corresponding to the first, A first temperature controller for adjusting the temperature of the first second-order nonlinear optical element, Connected to the output side of the first second-order nonlinear optical element, with a phase-matched wavelength λ 2 (λ 1 <λ 2 ) has, λ 2 A second second-order nonlinear optical element that optically parametrically amplifies the plurality of signal lights with a second excitation light corresponding to the second, A second temperature controller for adjusting the temperature of the first second-order nonlinear optical element, The light separated from the output light of the first second-order nonlinear optical element is divided into different wavelengths λ 1 and λ 3 A first demultiplexer that separates the signal into its individual wavelength components, The light separated from the output light of the second second-order nonlinear optical element is divided into different wavelengths λ 2 and λ 3 A second demultiplexer separates the wavelength components, λ from the first demultiplexer 1 A photodetector that detects the light intensity of the wavelength component and outputs a first electrical signal, λ from the second demultiplexer 2 A photodetector that detects the light intensity of the wavelength component and outputs a second electrical signal, λ from the first demultiplexer 3 A photodetector that detects the light intensity of the wavelength component and outputs a third electrical signal, λ from the second demultiplexer 3 A photodetector that detects the light intensity of the wavelength component and outputs a fourth electrical signal, A feedback control circuit controls the first temperature controller so that the difference between the first electrical signal and the third electrical signal is within a predetermined range, and controls the second temperature controller so that the difference between the second electrical signal and the fourth electrical signal is within a predetermined range. Equipped with, The parametric amplification bandwidth corresponding to λ1 and the parametric amplification bandwidth corresponding to λ2 overlap near the center of the entire amplification bandwidth. The third wavelength λ3 is located in the band where the two parametric amplification bands overlap in the entire amplification band. The optical amplifier is characterized in that the difference corresponds to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element.

4. It is an optical amplifier, Phase matching wavelength λ 1 It has, λ 1 A first second-order nonlinear optical element that optically parametrically amplifies input light containing multiple signal lights using a first excitation light corresponding to the first, A first temperature controller for adjusting the temperature of the first second-order nonlinear optical element, Connected to the output side of the first second-order nonlinear optical element, with phase matching λ 1 Near the slope portion where the gain characteristic attenuates on the short-wavelength side of the phase matching characteristic, λ 1 <λ 2 ) has, λ 2 A second second-order nonlinear optical element that optically parametrically amplifies input light containing multiple signal lights by a second excitation light corresponding to the second excitation light, A second temperature controller for adjusting the temperature of the first second-order nonlinear optical element, Light separated from the output light of the first second-order nonlinear optical element is separated by a different wavelength λ 1 and λ 3 (λ 3 <λ 1 A first demultiplexer that separates the wavelength components of the signal, The light separated from the output light of the second second-order nonlinear optical element is separated by a different wavelength λ 2 and λ 4 (λ 2 <λ 4 A second demultiplexer separates the wavelength components of the following: λ from the first demultiplexer 1 A photodetector that detects the light intensity of the wavelength component and outputs a first electrical signal, λ from the second demultiplexer 2 A photodetector that detects the light intensity of the wavelength component and outputs a second electrical signal, λ from the first demultiplexer 3 A photodetector that detects the light intensity of the wavelength component and outputs a third electrical signal, λ from the second demultiplexer 4 A photodetector that detects the light intensity of the wavelength component and outputs a fourth electrical signal, A feedback control circuit controls the first temperature controller so that the difference between the first electrical signal and the third electrical signal is within a predetermined range, and controls the second temperature controller so that the difference between the second electrical signal and the fourth electrical signal is within a predetermined range. Equipped with, The parametric amplification bandwidth corresponding to λ1 and the parametric amplification bandwidth corresponding to λ2 overlap near the center of the entire amplification bandwidth. wavelength λ 3 is, λ 1 It is located near the slope portion where the gain characteristic attenuates on the short-wavelength side of the phase matching characteristic, wavelength λ 4 is, λ 2 It is located near the slope where the gain characteristic attenuates on the longer wavelength side of the phase matching characteristic, The optical amplifier is characterized in that the difference corresponds to the difference in ASE intensity in the spontaneous emission light (ASE) profile of the second-order nonlinear optical element.

5. A first power adjuster for adjusting the intensity of the first excitation light, A second power regulator for adjusting the intensity of the second excitation light and Furthermore, The optical amplifier according to any one of claims 1 to 4, further characterized in that the feedback control circuit is configured to control the first power regulator based on the first electrical signal and to control the second power regulator based on the second electrical signal.

6. The optical amplifier according to claim 5, characterized in that the first power regulator and the first power regulator are composed of either an erbium-doped optical fiber amplifier (EDFA) or a variable optical attenuator, or a combination of an EDFA and a variable optical attenuator.

7. The aforementioned second-order nonlinear optical element is LiNbO 3 , LiTaO 3 , or LiNb (x) Ta (1-x) O 3 The optical amplifier according to any one of claims 1 to 4, characterized in that (0 ≤ x ≤ 1), or contains at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive.

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