JP7659211B2 - Wavelength conversion device - Google Patents

Description

The present invention relates to a wavelength conversion device, and more particularly to a technique for efficiently maintaining constant the output of a nonlinear optical element constituting a wavelength conversion device.

The periodically poled structure of nonlinear optical elements is a structure for quasi-phase matching, which is a method for obtaining a high nonlinear optical effect by inverting the crystal orientation for each coherence length of the fundamental wave and the wavelength-converted wave and compensating for the amount of phase mismatching by inverting the sign of the nonlinear constant. It 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 special nonlinear optical crystals.

In general, the refractive index of a nonlinear optical material has a temperature dependency, and in order to strictly satisfy the quasi-phase matching condition in a second-order nonlinear optical element, it is necessary to keep the temperature of the element constant. Usually, a temperature measuring device such as a thermistor or thermocouple is provided on or near the second-order nonlinear optical element to monitor its resistance value, and a mechanism is provided to keep the element at a constant temperature using a temperature regulator such as a heater or Peltier element before operation.

However, due to two problems, it has been difficult to stabilize a wavelength conversion optical device using a second-order nonlinear optical element using only a conventional mechanism for controlling a temperature regulator so as to keep the monitored value of a temperature sensor constant.

One of the challenges is that a temperature sensor such as a thermistor or thermocouple can monitor only the average temperature of the entire second-order nonlinear optical element, and not the temperature of the waveguide portion itself that produces the nonlinear optical effect. Therefore, simply monitoring the temperature of the temperature sensor may not strictly ensure operation at the optimum temperature.

For example, if the environmental temperature (outside air temperature) of the device changes, even if temperature control is performed to keep the element or a temperature sensor installed in its vicinity constant, the core through which light propagates, located on the surface of the element, has three sides that are not in contact with the base substrate and are in contact with an air layer, so that the optimal operating point will shift slightly due to changes in the environmental temperature.

In addition, when strong excitation light is incident on the waveguide, heat is generated due to optical absorption of the excitation light incident on the waveguide. However, this heat is localized in the waveguide, and it is difficult to correctly detect a shift in the optimal operating point due to this localized heat generation simply by monitoring the element or a temperature sensor installed in its vicinity.

The second issue is the fluctuation in the amount of coupling of light input and output to the second-order nonlinear optical element. Input and output light is usually propagated through optical fiber, but in order to couple it to the second-order nonlinear optical element, the light is output from the fiber into space and then coupled to the second-order nonlinear optical element using a lens or other device. When a slight optical axis shift occurs in this spatial coupling system due to changes in the environmental temperature, etc., the ratio of input light coupled to the second-order nonlinear optical element and the ratio of output light coupled from the second-order nonlinear optical element to the fiber fluctuate. This also causes the output of the wavelength-converted light to fluctuate.

In response to this, Patent Document 1 describes adjusting the temperature of a nonlinear optical element so as to maximize the intensity of the second harmonic wave obtained by wavelength conversion.

JP 2015-225127 A

However, simply compensating for the wavelength characteristics to be at the optimum value does not solve the two problems at the same time, and output stabilization is not possible.

An object of the present invention is to provide a wavelength conversion device capable of operating a nonlinear optical element at an appropriate temperature and stabilizing the output light thereof.

In order to achieve this object, one aspect of the wavelength conversion device of the present invention is a wavelength conversion device having a nonlinear optical element and performing wavelength conversion, comprising: optical power adjustment means for adjusting the power of light input to the nonlinear optical element; monitor light detection means for converting output light from the output of the nonlinear optical element into an electrical signal and feeding it back to the optical power adjustment means; control means for setting an adjustment parameter corresponding to the optical power adjusted by the optical power adjustment means so that the power of light output from the nonlinear optical element is a constant value; and temperature adjustment means for adjusting the temperature of the nonlinear optical element, wherein the control means sets a plurality of different temperatures using the temperature adjustment means, and sets the temperature corresponding to the smallest adjustment parameter among the adjustment parameters set corresponding to each of the plurality of set temperatures as the target temperature when the temperature adjustment means adjusts the temperature of the nonlinear optical element.

According to the above configuration, in the wavelength conversion device, the nonlinear optical element can be operated at an appropriate temperature, and the output light can be stabilized.

1 is a diagram showing a configuration of a wavelength conversion device according to an embodiment of the present invention. 1A shows a graph of the wavelength dependency of the second harmonic of the nonlinear optical device 103. FIG. 1B shows a diagram for explaining the monitoring results of the power of the optical power adjusting mechanism 102 and the resetting of the set temperature T. 1 is a diagram showing a wavelength conversion device according to a first embodiment of the present invention; 1A is a diagram showing a method for driving a wavelength conversion device according to a first embodiment of the present invention, and FIG. 1B is a diagram showing a relationship between a temperature of a PPLN waveguide and a current value of an EDFA. FIG. 11 is a diagram showing a wavelength conversion device according to a second embodiment of the present invention. 11A to 11C are diagrams illustrating a method of driving a wavelength conversion device according to a second embodiment of the present invention. FIG. 11 is a diagram showing a wavelength conversion device according to a third embodiment of the present invention. 1A is a diagram showing a parametric amplification process, a SHG process, and a fundamental wave light. FIG. 11 is a diagram showing a wavelength conversion device according to a fourth embodiment of the present invention.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the basic configuration of a wavelength converter 100 including a wavelength converter and an optical parametric amplifier according to an embodiment of the present invention. The wavelength converter 100 includes a second-order nonlinear optical device (element) 103, a temperature regulator 104, an optical power adjustment mechanism 102, a control signal generator 109, an optical branching coupler 105, an optical intensity detector 106 for detecting monitor light, and a PID controller (feedback circuit) 107. In the figure, solid connection lines indicate optical connections, and dashed lines indicate electrical connections. The same applies to the drawings related to the wavelength converter described hereinafter. The optical power adjustment mechanism 102 is constructed of any one of an optical amplifier (EDFA), a variable optical attenuator (VOA), an optical splitter, and an optical detector, or a combination thereof, as will be described in a specific embodiment described later. The nonlinear optical device 103 is provided with a fundamental wave input port for inputting light in the fundamental wave wavelength band at its input end, and a second harmonic output port for outputting light in half the wavelength band of the fundamental wave at its output end.

In the configuration shown in FIG. 1, the second harmonic, which is the output light from the nonlinear optical device 103, is fed back to the optical power adjusting mechanism 102 as monitor light, and the optical power adjusting mechanism 102 adjusts the power (intensity) of the light input to the nonlinear optical device 103 so that the output from the nonlinear optical device 103 is constant (first control). In addition, with regard to the temperature control, the optical power adjusting mechanism 102 performs control to optimize the wavelength characteristics of the optical power of the second harmonic generation (second control).

2A shows a graph of the wavelength dependency of the second harmonic of the nonlinear optical device 103. The wavelength characteristic of the second harmonic changes according to the wavelength as a Sinc function and reaches a maximum value at a predetermined wavelength. The wavelength characteristic has a wavelength dependency that shifts in the wavelength direction when the temperature of the nonlinear optical device 103 changes (T-2ΔT to T+2ΔT).

In the first control, a part of the output from the nonlinear optical device 103 is branched off, and the power is monitored and fed back to the optical power adjusting mechanism 102 via the PID controller 107 so that the output from the nonlinear optical device 103 is constant. The optical power adjusting mechanism 102 then makes the output from the nonlinear optical device 103 constant in order to compensate for the fluctuation of the optical coupling accompanying the change in the environmental temperature, etc. In the embodiment of the present invention, the output from the nonlinear optical device 103 is not necessarily constant by this first control. However, unless the optimum value is set in consideration of the temperature, the pumping light efficiency may deteriorate, and if the pumping light efficiency is too poor, it may go beyond the range of coverage of the optical power adjusting mechanism. It is difficult to stabilize the output from the nonlinear optical device 103 by the first control alone.

In the embodiment of the present invention, in addition to the first control, a second control is performed. This control optimizes the wavelength characteristic of the second harmonic of the nonlinear optical device 103 with respect to the wavelength of the input pump light. This is to set the wavelength of the input pump light to the wavelength at which the wavelength characteristic of FIG. 2A has a maximum value.

For such control to obtain the maximum value of the wavelength characteristic, the PDH method (Pound-Drever-Hall method) is generally used. In the PDH method, a dither signal is generated in the system, and the response signal is demodulated with the dither signal to obtain a function obtained by differentiating the response function. Then, by using this differential function as an error function, it is possible to set the point where the differential function becomes zero, that is, the point where the response function becomes the maximum value. However, when using the PDH method, since the output light that is originally to be stabilized is minutely fluctuated using the dither signal, there is a risk of deteriorating the stability of the output, even though the fluctuation is due to only a specific frequency component. In addition, even if the fluctuation of the output light due to the dither signal is suppressed to an extremely small level, it is difficult to use it together with the first control. In other words, since the first control controls the output light power to be constant, the fluctuation due to the dither signal of the second control cannot be read.

The embodiment of the present invention solves the above problem and performs both the first control and the second control. In FIG. 1, the control signal generator 109 commands the temperature regulator 104 to change the temperature in the range of T-2ΔT to T+2ΔT in ΔT intervals. Here, T is the set temperature of the nonlinear optical device 103, and ΔT is the amount of temperature change. At this time, as described above in FIG. 2(a), the wavelength characteristic of the second harmonic shifts in the wavelength direction. Therefore, the power of the output light of the second harmonic fluctuates according to the temperature. On the other hand, since the power (intensity) of the monitored output light of the second harmonic is stabilized to a constant value by the above-mentioned first control, it is not possible to detect the fluctuation according to the temperature change of the second harmonic. Therefore, in the embodiment of the present invention, the set temperature T of the nonlinear optical device 103 is reset by monitoring the power of the optical power adjustment mechanism 102, including the influence of the temperature fluctuation in the nonlinear optical device 103, by feedback. Then, the temperature regulator 104 controls the temperature of the nonlinear optical device 103 with this set temperature T as a target value.

FIG. 2B is a diagram for explaining the monitor result of the power of the optical power adjusting mechanism 102 and the resetting of the set temperature T. As described above, the temperature regulator 104 sets five temperatures, namely the target temperature T at that time, temperatures T-2ΔT and T-ΔT lower than the target temperature T, and temperatures T+ΔT and T+2ΔT higher than the target temperature T, and obtains the value of the monitor power of the optical power adjusting mechanism 102 according to the temperature of the nonlinear optical device 103 at that time. Here, at the temperature at which the generation efficiency of the second harmonic is the highest, the monitor power of the optical power adjusting mechanism 102 is the lowest value. In other words, the pumping light rate is the best. The control signal generator 109 sets the temperature at which this monitor power is the lowest value as the new set temperature T. In the example shown in FIG. 2B, the temperature T+ΔT is set as the new set temperature T. Then, the control signal generator 109 controls the temperature regulator 104 to perform temperature control at the newly set temperature T. This allows the wavelength conversion device to always operate at the optimum temperature and stabilize its optical output light.

In the following, some specific examples based on the above-described embodiment of the present invention will be described.

Example 1
3 is a block diagram showing the configuration of a wavelength converter 300 including a wavelength converter and an optical parametric amplifier according to a first embodiment of the present invention. The wavelength converter and the optical parametric amplifier according to the first embodiment of the present invention includes a light source 101a for pumping a fundamental wave, an optical amplifier (EDFA) 102a, a nonlinear optical device 103, a temperature regulator 104, an optical branching coupler 105, an optical intensity detector, a proportional-integral-differential (PID) amplifier, and a nonlinear optical amplifier (NIR) 106.
The control circuit 100 includes a proportional-derivative (PID) controller 107, a current controller 108a, and a control signal generator 109.

In this wavelength converter and optical parametric amplifier, the nonlinear optical device 103 includes a dichroic mirror type splitter that splits the fundamental pump light, and includes a PPLN waveguide having a periodically poled structure that satisfies quasi-phase matching between the input fundamental pump light and the output second harmonic pump light. That is, when the effective refractive indexes in the waveguide of the fundamental pump light and the second harmonic pump light are nf and np, respectively, np/λp-2nf/λf=1/Λ (Equation 1)
The polarization inversion structure has an inversion period Λ that satisfies the above.

The temperature of the PPLN waveguide of the nonlinear optical device 103 is controlled by a temperature regulator 104 so as to satisfy Equation 1.

The operation of the wavelength conversion device 300 will be described. In this embodiment, the current controller 108a controls the current value (adjustment parameter) for the pumping light source for driving the EDFA 102a, thereby adjusting the input power of the fundamental pumping light to the PPLN waveguide, and stabilizing the power of the second harmonic pumping light output from the PPLN waveguide to the optical fiber. The fundamental pumping light output from the fundamental pumping light source 101a is amplified by the EDFA 102a and then input to the nonlinear optical element (PPLN waveguide) 103. The second harmonic pumping light generated inside the PPLN waveguide and the residual component of the fundamental pumping light are output from the PPLN waveguide and split by a dichroic mirror type splitter, and the second harmonic pumping light is coupled to the output fiber. A part of the second harmonic pumping light is split by the optical splitter 105 and converted by the optical intensity detector 106 into an electrical signal proportional to the optical intensity. The electric signal is fed back to the current controller 108a for driving the EDFA 102a via the PID controller 107. This controls the output power of the second harmonic pump light to a set target value. As described later with reference to Figs. 4(a) and (b), the temperature of the PPLN waveguide is changed at intervals of ΔT in the range of T-2ΔT to T+2ΔT, the current value (adjustment parameter) of the EDFA 102a is monitored, and a set temperature T is set. Note that the period of the current value control to the EDFA 102a needs to be set sufficiently short compared to the period of the temperature control of the PPLN waveguide by the temperature regulator 104. This is because if the period of the current value control is not short enough, it will be affected by the output power fluctuation of the second harmonic due to the temperature fluctuation of the PPLN waveguide.

4A and 4B are diagrams for explaining a process of monitoring a current value (adjustment parameter) for driving the EDFA 102a functioning as an optical power adjustment mechanism, and determining (setting) a set temperature Tset as a target for temperature adjustment by the temperature adjuster 104.
This is performed using a control signal generator 109 electrically connected to the temperature regulator 102a and the temperature regulator 104. The control signal generator 109 may be in the form of a microcomputer having a CPU, a ROM, a RAM, and the like.

In this process, in step S41 shown in FIG. 4A, the temperature of the PPLN waveguide is set to Tset-2ΔT, and at that temperature, the current controller 108a obtains a current value for driving the EDFA 102a.
The current value of the EDFA is obtained (S42), then the temperature of the PPLN waveguide is set to Tset and the current value of the EDFA is obtained (S43), the temperature of the PPLN waveguide is further set to Tset+ΔT and the current value of the EDFA is obtained (S44), and the temperature of the PPLN waveguide is further set to Tset+2ΔT and the current value of the EDFA is obtained (S45).

Then, in step S46, as shown in Fig. 4B, the monitored current values of the EDFA 102a at each set temperature are compared, and the temperature at which the current value is at its minimum is set as the new set (target) temperature Tset. In this way, the temperature at which the current value of the EDFA 102a is at its minimum becomes the temperature of the nonlinear optical device (PPLN waveguide) 103 with the best pumping efficiency among the acquired data. By executing the above control, it becomes possible to constantly adjust the temperature of the PPLN waveguide to an appropriate temperature.

In fact, we tried to control the output power of the second harmonic to a constant value with the configuration shown in Fig. 3. The time until the temperature of the PPLN waveguide is changed is set to 2 seconds in consideration of the temperature response time, and ΔT is set to 0.1°C, and control is performed by moving the temperature from the set temperature by 5 points up to ±0.2°C. The current value is fed back to the EDFA 102a at 0.1 second intervals, and control is performed at a speed sufficiently fast compared to the temperature fluctuation, so that adjustment is made so that output fluctuations due to temperature fluctuations do not occur. The target value of the output power of the second harmonic pump light was set to 27 dBm, and continuous operation was performed for 10 hours, and operation stabilization at 27 ±0.1 dB was successfully achieved.

In this embodiment, the temperature was changed by 0.1° C. at each time of the temperature control of the PPLN waveguide, but this is not limited to this value. Also, the temperature was changed by 0.1° C. at five points, but the number of points obtained is not limited to five points, and may be any number of points.

In addition, in this embodiment, the current value (adjustment parameter) of the EDFA 102a is monitored to control the optimal temperature of the PPLN waveguide, but the output power of the EDFA 102a may be monitored, and the temperature of the PPLN waveguide may be updated to the temperature at which the monitored power is at its minimum value.

In this embodiment, a microcomputer is used as the control signal generator 109, but the present invention is not limited to this, and for example, a control circuit such as an FPGA may be used.

Regarding the control time, the temperature change interval was set to 2 seconds, and the current value control interval of the EDFA 102a was set to 0.1 seconds, but this is not limited to these values, and it is sufficient that the current value control interval of the EDFA 102a is sufficiently short compared with the temperature change interval of the PPLN waveguide.

In this embodiment, lithium niobate ( _LiNbO3 ) having a periodically poled structure is used as the nonlinear optical device 103, but this is not limited to this, and nonlinear optical media such as _LiNbO3 , _LiTaO3 , LiNb(x)Ta(1-x) _O3 (0≦x≦1), or any of these containing at least one type selected from the group consisting of Mg, Zn, Sc, and In as an additive, may be used.

Example 2
5 is a block diagram showing the configuration of a second example of a wavelength converter 500 including a wavelength converter and an optical parametric amplifier according to the second embodiment of the present invention. The wavelength converter and the optical parametric amplifier of the second embodiment include a light source 101a for pumping a fundamental wave, an optical amplifier (EDFA) 102b, a variable optical attenuator (VOA) 102c, a first optical intensity detector 106a, a nonlinear optical device 103 and a temperature regulator 104, a second optical branching coupler 105b, a second optical intensity detector 106b, a proportional-integral-derivative (PID) controller 107, a voltage controller 108b, and a control signal generator 109.

The difference from the first embodiment is that the input power of the fundamental pump light is controlled not by the EDFA 102b but by the VOA 102c at the rear stage. The VOA 102c in this embodiment controls the optical attenuation amount by voltage, and as such a VOA 102c, for example, a mechanical VOA that moves an optical filter having a distribution in the optical attenuation level by means of a motor or the like is known. The basic control operation is the same as in the first embodiment, and the temperature of the nonlinear optical device 103 (PPLN waveguide) is constantly swept discretely at intervals of ΔT from T-2ΔT to T+2ΔT.

The monitor power of the second optical power detector 106b is fed back to the voltage controller 108b that drives the VOA 102c so that the output power is kept constant. The optimum temperature of the PPLN waveguide is set to the temperature at which the optical power of the second optical power detector 106b downstream of the VOA 102c is at its minimum, that is, the temperature at which the pumping light rate is the best, by feeding it back to the temperature regulator 104 via the control signal generator 109. By performing such control, it becomes possible to control the output intensity of the second harmonic pumping light to a constant value while keeping the temperature of the PPLN waveguide at the optimum temperature, as in the first embodiment.

6 is a diagram for explaining a process of monitoring a voltage value (adjustment parameter) for driving the VOA 102c functioning as an optical power adjustment mechanism, and determining (setting) a set temperature Tset as a target for temperature adjustment by the temperature regulator 104. This process is executed by a control signal generator 109 electrically connected to the VOA 102c and the temperature regulator 104. The control signal generator 109 can be in the form of a microcomputer having a CPU, ROM, RAM, etc.

6, the temperature of the PPLN waveguide is set to Tset-2ΔT, and the voltage controller 108b acquires the voltage value for driving the VOA 102c at that temperature. Similarly, the temperature of the PPLN waveguide is set to Tset-ΔT, and the voltage value of the EDFA is acquired (S62), the temperature of the PPLN waveguide is then set to Tset, and the voltage value of the EDFA is acquired (S63), the temperature of the PPLN waveguide is then set to Tset+ΔT, and the voltage value of the EDFA is acquired (S64), and the temperature of the PPLN waveguide is then set to Tset+2ΔT, and the voltage value of the EDFA is acquired (S65).
The voltage value of A is obtained (S65).

Then, in step S66, as shown in FIG. 6, the monitored voltage values of the VOA 102c at the respective set temperatures are compared, and the temperature at which the voltage value is the minimum is set as a new set (target) temperature Tset.

In this embodiment, the temperature of the PPLN waveguide is optimized by monitoring the output power from the VOA 102c, but the temperature of the PPLN waveguide may be optimized by monitoring the drive voltage of the VOA 102c.

Example 3
7 is a block diagram showing the configuration of a wavelength converter 700 including a wavelength converter and an optical parametric amplifier according to the first embodiment of the present invention. The wavelength converter and the optical parametric amplifier according to the third embodiment include a light source 101a for pumping a fundamental wave, an optical amplifier (EDFA) 102a, a first bandpass filter 701a, a first optical branching coupler 105a, a nonlinear optical device 103 and a temperature regulator 104, a second optical branching coupler 105b, a second bandpass filter (BPF) 701b, an optical intensity detector, a proportional-integral-derivative (PID) controller 107, a current controller 108a, and a control signal generator 109. Between the fundamental wave output port and the nonlinear optical device 103, a wavelength multiplexing/demultiplexing mechanism is disposed for separating light in a wavelength band of a double wave having a wavelength half that of the fundamental wave light from light in the wavelength band of the fundamental wave light.

In the first and second embodiments, the second harmonic generation process using the nonlinear optical device 103 was dealt with, but in this embodiment, both the second harmonic generation process and the parametric amplification process are caused in one nonlinear optical device 103. In other words, a multi-stage process is used in which the second harmonic pump light generated by inputting fundamental pump light is used as an energy source to parametrically amplify the signal light that is input at the same time. In this embodiment, a method is presented for simultaneously stabilizing the output light power of the parametric amplification and optimizing the temperature of the PPLN waveguide.

In the wavelength converter and optical parametric amplifier of the wavelength converter 700, the nonlinear optical device 103 includes a dichroic mirror type demultiplexer that demultiplexes the fundamental pump light and the signal light from the second harmonic pump light, and includes a PPLN waveguide having a periodically poled structure that satisfies quasi-phase matching between the input signal light, the wavelength converted light, and the second harmonic pump light. That is, assuming that the effective refractive indices in the waveguide of the signal light, the converted light, and the second harmonic pump light are ns, nc, and np, respectively, np/λp-ns/λs-nc/λc=1/Λ (Equation 2)
This corresponds to the case where λs = λc (= λf) in equation (1). The temperature of the PPLN waveguide is controlled by a temperature regulator 104 so as to satisfy equation (2).

Here, the wavelength characteristics of the second harmonic generation process represented by formula (1) and the parametric amplification process represented by formula (2) are shown in FIG. 8. The fundamental light is a single-wavelength laser light output from a single laser light source (FIG. 8(c) showing the fundamental light). The phase matching band for the second harmonic of the PPLN waveguide is narrower than the phase matching band for the parametric amplification, but the line width of the laser light of the fundamental light is sufficiently wider (FIG. 8(b) showing the SHG process). In the parametric amplification process, even if the signal light wavelength is changed, the same conversion efficiency can be obtained between the converted light with a frequency of 2ω ₀ -ω _s and the pump light as long as formula 2 is satisfied. At this time, the effective refractive indexes ns and nc also change, but since nc becomes smaller by the amount that ns becomes larger due to the dispersion of the material, (formula 2) can be satisfied even if the signal light wavelength is changed, and a wide wavelength conversion band is obtained as shown in FIG. 8(a) showing the parametric amplification process, and the two PPLN devices have different phase matching characteristics.

The operation of the wavelength conversion device 700 of the third embodiment will be described. In this embodiment, the current value (adjustment parameter) of the fundamental pumping light source 101a for driving the EDFA 102a is controlled, thereby adjusting the input power of the fundamental pumping light to the PPLN waveguide, and stabilizing the power of the signal light and the wavelength converted light output from the PPLN waveguide to the optical fiber. The fundamental pumping light output from the fundamental pumping light source 101a is amplified by the EDFA 102a, and then unnecessary ASE light is removed by a bandpass filter, and the signal light is multiplexed through the first optical branching coupler 105a. The signal light and the fundamental pumping light are input to a nonlinear optical element (PPLN waveguide), and the wavelength converted light is generated by a parametric amplification process of the second harmonic pumping light and the signal light generated by the fundamental pumping light inside the PPLN waveguide. The residual component of the fundamental pump light, the signal light, the converted light, and the fundamental wavelength band components such as ASE generated in the parametric amplification process, and the second harmonic pump light are output from the PPLN waveguide, and are separated into the second harmonic pump light and the fundamental wavelength band components by a dichroic mirror type splitter. A part of the fundamental wavelength band components is split by the second optical splitter 105b, and the wavelength components near the fundamental pump light wavelength, not including the fundamental pump light, are extracted by the second BPF 701b, and then converted into an electrical signal proportional to the light intensity by the optical intensity detector 106. Here, the reason for extracting the wavelength near the fundamental pump light by the second BPF 701b will be described. When wavelength conversion or optical parametric amplification is performed using the PPLN waveguide of the nonlinear optical device 103, the wavelength (degenerate wavelength) of the fundamental pump light and its nearby wavelengths cannot be used for the signal light. This is because the wavelength-converted light appears in a form that is folded back around the wavelength of the fundamental pump light, and when a signal light near this wavelength is input, the converted light is also output at the same wavelength, resulting in interference and making it impossible to separate. Therefore, since the vicinity of the degenerate wavelength is essentially not used, it has the advantage that even if it is used as a monitor light, it has little effect on other signal lights. Next, in the vicinity of the degenerate wavelength, the fluctuation of the conversion efficiency with respect to temperature is the same as the fluctuation of the SHG efficiency, and the shape is such that the conversion efficiency reaches its peak at the optimal temperature. Therefore, it is possible to control to the optimal temperature in the same manner as the nonlinear optical device 103. However, if residual components of the fundamental pump light wavelength are included, the optical power of the fundamental pump light decreases as the pump light rate improves, and this behavior is not suitable for stabilization control to the maximum output in this case, so the BPF is set to a nearby wavelength that does not include the fundamental pump light component. The electric signal of the monitor output is fed back to the current controller 108a for driving the EDFA 102a via the PID controller 107, and the output power after parametric amplification is controlled to be the set target value. Regarding this output power, the temperature of the PPLN waveguide is changed from T-2ΔT to T+2ΔT at intervals of ΔT to monitor the current value (adjustment parameter) of the EDFA 102a, a process of setting the set temperature T is performed, the set current value of the EDFA 102a is monitored, and feedback is applied to the set temperature of the PPLN waveguide so that the current value becomes the minimum, which is the same as in Example 1. By performing such control, it becomes possible to control the intensity of the output light after parametric amplification to a constant value while maintaining the temperature of the PPLN waveguide at an optimal temperature.

Example 4
9 shows the configuration of a wavelength converter 900 including a wavelength converter and an optical parametric amplifier according to an embodiment of the present invention. The wavelength converter and the optical parametric amplifier of the wavelength converter 900 of the fourth embodiment include a light source 101b for second harmonic pumping, a variable optical attenuator (VOA) 102c, a first optical branching coupler 105a, a first optical intensity detector 106a, a nonlinear optical device 103 and a temperature regulator 104, a second optical branching coupler 105b, a bandpass filter (BPF) 701, a second optical intensity detector 106b, a proportional integral differential (PID) filter 702, and a nonlinear optical device 103 and a temperature regulator 104.
The nonlinear optical device 103 includes a PID (proportional-integral-derivative) controller 107, a voltage controller 108b, and a control signal generator 109. A first wavelength multiplexing/demultiplexing mechanism is disposed at an input end of the nonlinear optical device 103 for multiplexing light in a double wavelength band having a wavelength roughly half that of the signal light with light in the wavelength band of the signal light, and the input end of the nonlinear optical device 103 has an input port for inputting light in the wavelength band of the signal light and an input port for inputting light in the double wavelength band. A second wavelength multiplexing/demultiplexing mechanism is disposed at an output end of the nonlinear optical device 103 for demultiplexing light in the double wavelength band and light in the wavelength band of the signal light.

This embodiment presents a method for inducing a parametric amplification process by inputting second harmonic pump light and signal light into a PPLN waveguide, and simultaneously stabilizing the output light power of the parametric amplification and optimizing the temperature of the PPLN waveguide.

The PPLN waveguide is provided with a dichroic mirror type multiplexer/demultiplexer that multiplexes/demultiplexes the signal light wavelength band and the second harmonic pump light at the input and output, and is provided with a PPLN waveguide having a periodic polarization inversion structure that satisfies quasi-phase matching between the input signal light, the wavelength converted light, and the second harmonic pump light. That is, the PPLN waveguide is provided with a polarization inversion structure with an inversion period Λ that satisfies (Equation 2) where the effective refractive indices in the waveguide for the signal light, the converted light, and the second harmonic pump light are ns, nc, and np, respectively.

The operation of the wavelength converter 900 of the fourth embodiment will be described. In this embodiment, the input power of the second harmonic pump light to the PPLN waveguide is adjusted by controlling the voltage value (adjustment parameter) for driving the VOA 102c, and the output light power of the signal wavelength band output from the PPLN waveguide to the optical fiber is stabilized. The light output from the light source 101b for second harmonic pumping is coupled to the PPLN waveguide via a dichroic mirror after the power is adjusted by the VOA 102c. At this time, a part of the second harmonic pump light is separated by the first optical branching coupler 105a and converted into an electrical signal according to the light intensity by the first optical intensity detector. The signal light is input from the other side of the dichroic mirror and coupled to the PPLN waveguide. In the PPLN waveguide, the amplified signal light and the newly generated wavelength-converted light and ASE light are output from the PPLN waveguide together with the residual component of the second harmonic pump light by the parametric amplification process, and are demultiplexed by the dichroic mirror type demultiplexer. A part of the signal light wavelength band is branched by the second optical branching coupler 105b, and a wavelength component twice the wavelength of the second harmonic pump light, i.e., a wavelength in the vicinity of the fundamental pump light wavelength, is extracted by the BPF 701, and converted into an electric signal proportional to the light intensity by the light intensity detector 106. The electric signal is fed back to the voltage controller 108b for driving the VOA 102c via the PID controller 107. This controls the output power of the signal wavelength band after parametric amplification to be the set target value. The temperature of the PPLN waveguide is changed in intervals of ΔT in the range of T-2ΔT to T+2ΔT, and the voltage value (adjustment parameter) of the VOA 102c is monitored, and a process of setting the set temperature T is performed. The value of the electrical signal from the first optical power detector 106a is fed back to the temperature regulator 104 via the control signal generator 109 so that it becomes the minimum value. By performing such control, it becomes possible to constantly adjust the temperature of the PPLN waveguide to an optimal temperature and stabilize the output power after parametric amplification.

In this embodiment, the optical intensity is adjusted using the VOA 102c, but it is also possible to apply feedback to the electrical signal that drives the light source 101b for second harmonic excitation to stabilize the output optical power after parametric amplification, and to monitor the driving electrical signal to optimally control the temperature of the PPLN waveguide.

The light source 101b for second harmonic excitation may be a laser diode capable of directly outputting the second harmonic excitation light wavelength, or may be a combination of a light source that outputs light with the fundamental excitation light wavelength and a nonlinear optical device that generates the second harmonic.

There are two ways to realize a double wave light source: either prepare a laser that outputs a double wave wavelength, or take the second harmonic of a laser that outputs a fundamental wave wavelength.

The double wave light source is constructed by a laser diode that outputs light with approximately half the wavelength of the signal light, a current controller for driving the laser diode, a VOA, an optical splitter, a photodetector, or a combination of these.

Alternatively, the double wave light source is constructed by a laser diode that outputs light with a wavelength in the wavelength band of the signal light, a voltage controller 108b for driving the laser diode, an optical amplifier 102b, a nonlinear optical device, a VOA 102c, an optical splitter of the second optical branching coupler 105b, a first optical power detector 106a, or a combination of these.

Claims (5)

An apparatus for performing wavelength conversion using a nonlinear optical effect, comprising:
A wavelength conversion device comprising: a nonlinear optical element which generates converted light having a different wavelength from fundamental light; a fundamental wave input port through which light in a wavelength band of the fundamental wave is input to an input end of the nonlinear optical element; a second harmonic output port through which light in a wavelength band that is half the wavelength of the fundamental wave is output from an output end of the nonlinear optical element; a fundamental wave output port through which light in the wavelength band of the fundamental wave is output from an output end of the nonlinear optical element; and a temperature adjustment mechanism which adjusts a temperature of the nonlinear optical element,
a power adjustment mechanism for the fundamental wave light having a monitor output provided on the fundamental wave input port side of the nonlinear optical element ;
a multiplexer for multiplexing signal light is provided before the fundamental wave input port, and a wavelength multiplexing/demultiplexing mechanism for separating light in a double wave wavelength band having half the wavelength of the fundamental wave light from light in the wavelength band of the fundamental wave light is provided between the second harmonic output port and the fundamental wave output port and the nonlinear optical element,
a splitter that separates a part of the fundamental wave and a photodetector that detects the optical intensity of the fundamental wave, the splitter being disposed downstream of the fundamental wave output port;
an optical bandpass filter between the splitter and the photodetector;
having
a feedback circuit that controls the power adjustment mechanism based on a signal from the photodetector to keep the signal light output at a constant value;
a controller that minutely varies a set temperature of the temperature adjustment mechanism, obtains an error from an optimum temperature of the nonlinear optical element based on a monitor output of the power adjustment mechanism, and controls the temperature adjustment mechanism to the optimum temperature at which the second harmonic is most efficiently output;
A wavelength conversion device comprising: 2. The wavelength conversion device according to claim 1 , wherein the power adjustment mechanism is constructed by a combination of an optical amplifier, an optical splitter, and a photodetector , or is constructed by a combination of a variable optical attenuator, an optical splitter, and a photodetector . An apparatus for performing wavelength conversion using a nonlinear optical effect, comprising:
a nonlinear optical element that uses second harmonic light as pump light to generate converted light having a wavelength different from that of the signal light;
a first wavelength multiplexing/demultiplexing mechanism is disposed at an input end of the nonlinear optical element for multiplexing light in a double wavelength band having approximately half the wavelength of the signal light with light in the wavelength band of the signal light;
an input port for inputting light in a wavelength band of a signal light and an input port for inputting light in the wavelength band of the double wave at an input end of the nonlinear optical element;
a second wavelength multiplexing/demultiplexing mechanism is disposed at an output end of the nonlinear optical element, the second wavelength multiplexing/demultiplexing mechanism being configured to demultiplex light in the wavelength band of the signal light from light in the wavelength band of the double wave;
an output port for outputting light in a wavelength band of a signal light and an output port for outputting light in the wavelength band of the double wave at an output end of the nonlinear optical element;
A wavelength conversion device including a temperature adjustment mechanism for adjusting the temperature of the nonlinear optical element,
a power adjustment mechanism provided with a double wave light source and a monitor output of light in the double wave wavelength band on the double wave input port side of the nonlinear optical element ;
a demultiplexer for separating light in the wavelength band of the signal light and a photodetector for detecting the optical intensity of light in the wavelength band of the signal light , the demultiplexer being provided on the output port side of the nonlinear optical element for light in the wavelength band of the signal light;
an optical bandpass filter between the splitter and the photodetector;
a feedback circuit that controls a power adjustment mechanism of the double wave light source based on a signal from a photodetector of light in the wavelength band of the signal light , and maintains a converted optical output of light in the wavelength band of the signal light at a constant value;
A wavelength conversion device characterized by comprising a controller which minutely varies the set temperature of a temperature adjustment mechanism for the nonlinear optical element, obtains an error from the optimum value of the temperature of the nonlinear optical element based on the monitor output of the power adjustment mechanism, and controls the temperature adjustment mechanism to the optimum temperature at which the converted light of the wavelength band of the signal light is most efficiently output. The double wave light source is constructed by combining a laser diode that outputs light having a wavelength approximately half that of the signal light, a voltage controller that is the controller for driving the laser diode, a variable optical attenuator, an optical splitter , and a photodetector, or
The wavelength conversion device of claim 3, wherein the double wave light source is constructed by combining a laser diode that outputs light of a wavelength in the wavelength band of the signal light , a current controller that is the controller for driving the laser diode , an optical amplifier that amplifies the light from the laser diode , a nonlinear optical element that generates light in the double wave wavelength band from the light of the optical amplifier, an optical splitter, and a photodetector. The wavelength conversion device according to any one of claims ₁ to 4, characterized in that the nonlinear optical element is LiNbO3, _LiTaO3 , or LiNb _(x) Ta _{(1-x) O3} (0≦x≦1), or one of them containing at least one element selected from the group consisting of Mg, Zn, Sc, and In as an additive.

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