JP7744610B2 - Wavelength conversion module manufacturing method and manufacturing device - Google Patents

Description

The present invention relates to a method and apparatus for manufacturing a wavelength conversion module.

Wavelength conversion technology is used in various application fields, such as optical signal wavelength conversion in optical communications, optical processing, medicine, and bioengineering. In optical communications systems, wavelength conversion technology is also used, for example, in wavelength conversion devices that perform wavelength conversion operations using difference frequency generation (described below) and amplification operations using the parametric effect. Focusing on materials used for wavelength conversion, periodically poled lithium niobate (PPLN) waveguide elements made of lithium niobate (LiNbO ₃ ), a second-order nonlinear material with a large nonlinear constant, are widely used in light sources due to their high wavelength conversion efficiency.

The second-order nonlinear optical effect utilizes a wavelength conversion mechanism in which light of wavelength _λ1 and light of wavelength _λ2 are input into a second-order nonlinear medium to generate a new wavelength _λ3 . The wavelength conversion expressed by the following equation is called sum frequency generation (SFG).
1/λ ₃ = 1/λ ₁ + 1/λ ₂ Equation (1)
Furthermore, wavelength conversion that satisfies the following equation, which is obtained by modifying equation (1) with λ ₁ =λ ₂ , is called second harmonic generation (SHG).
λ ₃ = λ ₁ /2 Equation (2)
Furthermore, wavelength conversion that satisfies the following equation is called difference frequency generation (DFG).
1/λ ₃ =1/λ ₁ -1/λ ₂ Equation (3)
The wavelengths _λ1 , _λ2 , and _λ3 used in difference frequency generation according to the above formula (3) are called pump light, signal light, and idler light, respectively. Furthermore, an optical parametric oscillator can be configured that generates _λ2 and _λ3 that satisfy formula (3) by inserting a nonlinear medium into the resonator _and inputting only λ1.

In recent years, improvements in wavelength conversion efficiency have made it possible to use second-order nonlinear effects to amplify light in the communications field. This type of optical amplifier, known as a phase-sensitive amplifier (PSA), is capable of amplifying input light without degrading the signal-to-noise ratio, and is expected to replace erbium-doped fiber amplifiers as an optical amplifier for long-distance transmission.

Two amplification operations are known for PSA. One utilizes degenerate parametric amplification, in which signal light and pump light with half the wavelength of the signal light are input to a second-order nonlinear medium and the signal light is amplified (Non-Patent Document 1). The other utilizes non-degenerate parametric amplification, in which a pair of signal light and idler light, together with pump light with a wavelength that is the sum frequency of the signal light and idler light, are input and the signal light and idler light are amplified (Non-Patent Document 2). The pair of signal light and idler light is generated by the DFG mechanism described above.

When wavelength conversion technology is used in the communications field, DFG and optical parametric amplification (OPA) are the two main mechanisms that utilize the second-order nonlinear effect mentioned above. In DFG and OPA mechanisms, the signal light and idler light are in the 1.55 μm communications wavelength band, so the pump light is in the 0.78 μm band.

Figure 1 shows the basic configuration of a wavelength conversion module. Figure 1(b) shows a top view (x-y plane) of wavelength conversion module 100, and Figure 1(a) shows a partially simplified cross-sectional view (y-z plane) of module 100 taken along line Ia-Ia parallel to the y-axis. Wavelength conversion module 100 receives fundamental light through optical window 105-1 and generates second harmonic generation (SHG) light, a type of sum frequency light, through optical window 108-2. Wavelength conversion module 100 includes, within metal housing 101, a wavelength conversion element 102, a temperature control element 103, and a metal plate 104 that supports the wavelength conversion element. Metal housing 101 is equipped with optical windows 105-1 and 105-2 for fundamental light and optical windows 108-1 and 108-2 for SHG light. The wavelength conversion element 102 includes lenses 106 before and after the wavelength conversion element 102, and further includes optical filters 107-1 and 107-2 and other filter elements for separating the fundamental wave light and the SHG light. The wavelength conversion element 102 is, for example, a PPLN element.

While Figure 1 shows a configuration including a wavelength conversion element for generating SHG light, modules with mechanisms such as PSA and OPA can also be realized by inputting two light beams of different wavelengths into the wavelength conversion element through one of the modules' optical windows, 105-1 for the fundamental wave and 108-2 for the SHG light. Furthermore, in the wavelength conversion module 100 of Figure 1, the arrows indicate the light propagation direction when fundamental wave light is input through optical window 105-1 and SHG light is output through optical window 108-2. However, SHG light can also be generated from fundamental wave light even if the light propagation direction is opposite to that of Figure 1. Fundamental wave light can also be input through optical window 105-2 and SHG light can be output through optical window 108-2, and the wavelength conversion module 100 has a symmetrical structure between input and output.

As shown in Figure 1, the wavelength conversion module 100 is designed to input and output multiple light beams with different wavelengths. Devices incorporating such modules have often been configured with spatial optical components that utilize bulk optical components due to their ease of availability. In recent years, advances in the development of optical fiber communication devices have led to widespread availability of optical fiber-type optical components and optical waveguide-type optical components for communication wavelength bands. Devices incorporating wavelength conversion modules are also beginning to adopt pigtail-type modules, which emphasize easy optical coupling with optical fibers. Pigtail-type modules are compact, do not require optical alignment, and are versatile enough to be used in wavelength conversion, PSA, and OPA configurations. To promote wider adoption, a manufacturing method and apparatus capable of producing pigtail-type modules at low cost are needed.

The PPLN element 102 of the wavelength conversion module 100 in Figure 1 utilizes quasi-phase matching (QPM). In a PPLN element with a single-period QPM structure, the phase-matched wavelength is determined once the period of the repeating structure is determined. Because the refractive index of nonlinear optical materials is temperature-dependent, the PPLN element must be set to and controlled at an optimal temperature to achieve highly efficient wavelength conversion.

T. Umeki, O. Tadanaga, A. Takada, and M. Asobe, “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides,” Optics Express Vol.19, No. 27, pp.6326-6332, 2011 T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto, and H. Takenouchi, “First demonstration of high-order QAM signal amplification in PPLN-based phase sensitive amplifier,” Optics Express Vol.22, No. 3, pp.2473-2482, 2014

However, when modularizing PPLN elements, there was a problem of the phase-matched wavelength fluctuating during various processes, from obtaining the characteristics of the individual element to fixing all the fibers to the module. The process of attaching the fibers to the module required tedious and complicated steps that required skilled workers, making it unsuitable for mass production. The present invention provides a manufacturing method and manufacturing apparatus for pigtail-type modules that are highly suitable for mass production.

One aspect of the present invention is a method for manufacturing an optical module including a wavelength conversion element and a temperature control element fixed to the wavelength conversion element via a metal plate, the method comprising the steps of: setting the wavelength conversion element to an initial temperature; aligning an input fiber so that the transmitted light of fundamental light is maximized; correcting the current temperature of the wavelength conversion element, which includes sweeping the wavelength of the fundamental light over a predetermined wavelength range to determine the peak wavelength at which the level of wavelength-converted light is maximized; calculating the corrected temperature of the wavelength conversion element based on the operating wavelength of the wavelength conversion element, the peak wavelength, the current temperature, and the temperature dependence coefficient of the phase-matching wavelength; and resetting the wavelength conversion element to the corrected temperature; realigning the input fiber so that the level of the wavelength-converted light is maximized at the corrected temperature; and fixing the adjusted input fiber to the metal housing of the optical module.

Another aspect of the present invention is a method for manufacturing an optical module including a wavelength conversion element and a temperature control element fixed to the wavelength conversion element via a metal plate, and connected to a fundamental wave input fiber, the method comprising the steps of: setting the wavelength conversion element to an initial temperature; aligning the output fiber so that the transmitted light of the fundamental wave light is maximized; correcting the current temperature of the wavelength conversion element, which includes sweeping the wavelength of the fundamental wave light over a predetermined wavelength range to determine the peak wavelength at which the level of the wavelength-converted light is maximized; calculating the corrected temperature of the wavelength conversion element based on the operating wavelength of the wavelength conversion element, the peak wavelength, the current temperature, and the temperature dependence coefficient of the phase-matching wavelength; and resetting the wavelength conversion element to the corrected temperature; re-aligning the output fiber so that the level of the wavelength-converted light is maximized at the corrected temperature; and fixing the adjusted output fiber to the metal housing of the optical module.

We provide a manufacturing method and manufacturing equipment for pigtail-type wavelength conversion modules that are suitable for mass production.

1 is a diagram showing the basic configuration of a wavelength conversion module using a PPLN element. 1A to 1C are diagrams illustrating an outline of a manufacturing process for a wavelength conversion module including a PPLN element. 10A and 10B are diagrams showing changes in phase matching curves before and after fixing a wavelength conversion element. 10A and 10B are diagrams illustrating the temperature dependence of the phase-matched wavelength before and after fixing the element. 10A to 10C are diagrams illustrating changes in operating wavelength through the manufacturing process of a wavelength conversion module. 10A and 10B are diagrams illustrating a procedure for connecting four PTFs to a wavelength conversion module. 1 is a diagram showing an apparatus for implementing a wavelength conversion module manufacturing method according to the present disclosure. FIG. 2 is a flow chart illustrating the steps of a method for manufacturing a wavelength conversion module according to the present disclosure. 1A and 1B are diagrams illustrating the principle of determining the corrected temperature of a wavelength conversion element. 10 is a diagram showing another configuration of an apparatus for carrying out a wavelength conversion module manufacturing method. 10 is a diagram showing another configuration of an apparatus for carrying out a wavelength conversion module manufacturing method. FIG. 10 is a flowchart illustrating another procedure of the method for manufacturing the wavelength conversion module. FIG. 10 is a diagram illustrating the configuration of initial data in the manufacturing method of the present disclosure.

The disclosed method for manufacturing a wavelength conversion module provides an efficient procedure for operating a PPLN element at a desired operating wavelength while simultaneously attaching an optical fiber to the module. By automating most of the processes through computer control, the problem of element temperature fluctuations specific to PPLN elements during the manufacturing process is addressed, improving the mass productivity of pigtailed modules. The disclosed method for manufacturing a wavelength conversion module also incorporates aspects of a wavelength conversion module manufacturing apparatus that includes a computer. Below, we will discuss the specific issues associated with wavelength conversion modules that include PPLN elements, followed by a detailed description of the disclosed method for manufacturing a wavelength conversion module, as well as the configuration, operation, and procedures of the manufacturing apparatus.

FIG. 2 is a diagram showing an overview of the manufacturing process for a wavelength conversion module including a PPLN element. The manufacturing process begins with a chip selection process 11 for PPLN elements. In this process 11, chips containing PPLN waveguides to be used for modularization and having wavelength conversion performance according to the intended function are selected. This chip selection process 11 is parallel to a chip data acquisition process 15. In this process 15, chip data is acquired for each chip or each product type, including at least a predetermined operating wavelength λ _init corresponding to the phase-matching wavelength and an initial temperature T _init corresponding to the operating wavelength. The operating wavelength varies depending on the product type depending on the intended function, but can be the same for products of the same type. Products of the same type may also be set to different operating wavelengths λ _init within a certain wavelength range.

Once the operating wavelength λ _init is determined for a PPLN device, the principle is to operate it consistently at the same operating wavelength λ _init even after modularization. The initial temperature T _init means the temperature to be set for the device to achieve phase matching at the operating wavelength λ _init . Therefore, in this chip data acquisition process 15, there is a one-to-one correspondence between the operating wavelength λ _init and the initial temperature T _init .

Following step 11 in FIG. 2 , step 12 is performed, in which the chip is incorporated into a module housing and fixed. During modularization, in order to stably hold the wavelength conversion element 102 and uniformly control the temperature of the entire element, the wavelength conversion element 102 is fixed to a metal plate 104 that serves as a support, as shown in FIG. 1 . When the wavelength conversion element 102, such as a PPLN element, is fixed to the metal plate 104, stress changes occur inside the wavelength conversion element 102 due to the difference in thermal expansion coefficient between the element and the metal plate. This stress change causes a change in the refractive index of the wavelength conversion element 102 due to the photoelastic effect, piezoelectric effect, Pockels effect, etc. Therefore, once the wavelength conversion element 102 is mounted in a module in chip fixing step 12, the actual operating wavelength of the wavelength conversion element 102 deviates from the initial temperature T _init acquired in step 15. In other words, even if the wavelength conversion element is mounted in a module in step 12 and then set to the initial temperature T _init acquired in chip data acquisition step 15, a phase matching state cannot be obtained. This issue is further detailed in FIGS.

Following chip fixing step 12 are step 13, in which a pigtail fiber (hereafter abbreviated as PTF for simplicity) is set in the module, aligned, and the chip temperature is searched for and optimized, and step 14, in which the PTF is fixed to the module. The manufacturing method of the wavelength conversion module disclosed herein, described below, corresponds to steps 13 and 14. In the manufacturing method of the wavelength conversion module disclosed herein, chip data including the initial temperature and other data acquired in chip selection step 11 is updated to the corrected temperature and other data in PTF fixing step 16.

FIG. 3 shows the change in the phase matching curve before and after the wavelength conversion element is fixed. In FIG. 3, the horizontal axis represents the wavelength (nm) of the input fundamental light, while the vertical axis represents the transmitted light output spectrum 10 of the fundamental light and the SHG light output spectra 20 and 21 in arbitrary units (au). The transmitted light output spectrum 10 of the fundamental light is obtained by inputting the fundamental light through the optical window 105-1 in FIG. 1 and outputting it from the optical window 105-2 on the opposite side of the module. The SHG light output spectrum is obtained as output from the optical window 108-1. The SHG light output spectra 20 and 21 represent the phase matching curves obtained when the chip alone is set to an initial temperature T _init and when the chip is fixed to a metal plate and then set to the same initial temperature T _init , respectively. That is, they correspond to the phase matching curve 20 obtained in steps 11 and 15 in FIG. 2 and the phase matching curve 21 obtained after the chip fixing step 12 in FIG. 2 is completed. The peak wavelength of the phase matching curve 20 is the operating wavelength λ _init .

As is clear from Fig. 3, even if the initial temperature is set to T _init , the phase matching curve changes significantly before and after the wavelength conversion element is fixed. Conversely, after the chip is fixed to the metal plate, in order to achieve a phase matching state having a peak at the operating wavelength λ _init , the chip temperature must be corrected from the initial temperature T _init .

Figure 4 is a diagram explaining the temperature dependence of the phase-matching wavelength before and after fixing the wavelength conversion element. The horizontal axis shows the set temperature (°C) of the chip, and the horizontal axis shows the peak wavelength of the SHG light, i.e., the phase-matching wavelength (nm). The phase-matching wavelength has a roughly linear temperature dependence, and before and after fixing the wavelength conversion element chip, it changes from the temperature dependence curve 22 obtained in the selection process 11 to the temperature dependence curve 23 after the completion of the chip fixing process 12. The slope of the temperature dependence curve corresponds to the temperature dependence coefficient (nm/°C), and the temperature dependence coefficient also changes before and after chip fixing. Note that for the purpose of explanation, the change in temperature dependence before and after chip fixing in Figure 4 is slightly exaggerated.

As mentioned above, the phase-matched wavelength obtained by setting the chip to the initial temperature T _init varies before and after the wavelength conversion element is fixed to the module. As shown in Figure 3, the phase-matched curve, which has a bandwidth of only about 0.5 nm, shifts, making it impossible to obtain SHG light of the desired wavelength even when a fundamental light beam with an operating wavelength λ _init is input. Not only does this prevent the desired wavelength conversion function from being achieved, but the PTF alignment process is also impossible without obtaining SHG light. Therefore, it is necessary to determine the "corrected temperature" that achieves phase matching at the operating wavelength λ _init as efficiently as possible after the wavelength conversion element is fixed to the module. To determine the "corrected temperature" of the wavelength conversion element, the element temperature must be changed. To efficiently search for the "corrected temperature," it is also necessary to avoid prolonging the time it takes for the element's characteristics to stabilize. Furthermore, stress changes occurring inside the wavelength conversion element can also be caused by impacts during the fiber welding and fixation process. It is also necessary to consider the possibility that fluctuations similar to the phase-matched wavelength fluctuations during chip fixation described above may occur each time the fiber is fixed to the module housing.

The wavelength conversion module manufacturing method disclosed herein solves or at least alleviates the above-mentioned problems and significantly improves mass productivity by automating the module assembly process, which previously relied on skilled workers.

FIG. 5 is a diagram illustrating the change in the variation of the phase-matched wavelength through the manufacturing process of a wavelength conversion module. In the chip selection step 31, a specific type of wavelength conversion element is selected at the target operating wavelength λ _init . In the actual selection process, there is a certain adjustment variation range at the operating wavelength λ _init . In the chip selection step 31, an initial temperature T _init corresponding to the operating wavelength λ _init is determined. In step 32 after the wavelength conversion element chip is fixed to the metal plate, the actual operating wavelength at which the chip is in a phase-matched state when set to the initial temperature T _init varies as described in FIG. 4. Since the amount of variation due to chip fixation also varies, the actual operating wavelength variation when set to the initial temperature T _init increases.

In the manufacturing method of the wavelength conversion module of the present disclosure, in the module assembly stage 33 including the attachment of the PTF to the module, the “corrected temperature” that realizes the phase matching state at the operating wavelength λ _init is calculated and readjusted, so that the variation of the actual operating wavelength is kept within a certain range.

The following explanation will use as an example a wavelength conversion module with two optical windows on one side (input side) and two optical windows on the other side (output side) of the wavelength conversion element 102, as shown in Figure 1, for a total of four ports (two inputs and two outputs). A fundamental wave in the 1535 nm band is input to the input port, and transmitted light of the input light and SHG light in the 775 nm band are output from the output port. Four PTFs are connected sequentially to the four ports to complete the pigtail-type module. Depending on the function to be realized, it is also possible to input light of two different wavelengths from the input port.

Figure 6 is a diagram outlining the procedure for connecting four PTFs to a wavelength conversion module. Wavelength conversion module 100 is a simplified version of the wavelength conversion module in Figure 1. Port 1 corresponds to the fundamental light port of optical window 105-1, and port 3 corresponds to the fundamental light port of optical window 105-2. Port 2 corresponds to the SHG light output port of optical window 108-1, and port 4 corresponds to the SHG light output port of optical window 106-2. In the manufacturing method described below, the fundamental port is used as the input port for the fundamental light and the output port for the transmitted light of the fundamental light.

6(a) shows the stage of connecting the first input PTF 202-1 to port 1, where a detector 203 for the fundamental wave 212 and a detector 206 for the SHG light 213 are used. In the process of connecting the first input PTF, initial data including the initial temperature T _init acquired during chip selection is used. The configuration of a specific manufacturing apparatus is shown in FIG. 7, and the flow of the manufacturing method is explained in FIG. 8.

Figure 6(b) shows the stage of connecting the second input PTF 202-2 to port 3, with input PTF 202-1 already connected to port 1. A detector 210 for fundamental light and a detector 206 for SHG light 213 connected to input PTF 202-1 are used. Figure 10 shows the specific configuration of the manufacturing equipment, and the flow of the manufacturing method is generally the same as Figure 8. Please note that in the process of connecting the second and subsequent PTFs, updated data including the corrected temperature obtained in the process of connecting the first input PTF is used.

Figure 6(c) shows the stage where the third output PTF 211-1 is connected to port 4, with two input PTFs 202-1 and 202-2 already connected to ports 1 and 3. A wavelength swept light source (SW) 201 is connected to input PTF 202-1, and a fundamental light detector 210 is connected to input PTF 202-2. Figure 11 shows the configuration of a specific manufacturing apparatus, and the flow of the manufacturing method is explained in Figure 12. Updated data including the corrected temperature is used.

Figure 6(d) shows the stage where the fourth output PTF 211-2 is connected to port 2; three PTFs are already connected to the wavelength conversion module 100. The wavelength swept light source 201 is connected to the input PTF 202-2, and the fundamental light detector 210 is connected to the input PTF 202-1. The manufacturing method flow is generally the same as Figure 12. Updated data including the corrected temperature is used.

Below, the manufacturing procedures will be explained in detail, divided into the manufacturing method when connecting a PTF for fundamental wave light input and the manufacturing method when connecting a PTF for SHG light output.

Figure 7 is a diagram showing the configuration of an apparatus for implementing the wavelength conversion module manufacturing method of the present disclosure. The manufacturing apparatus has a configuration for connecting a PTF to the wavelength conversion module 100 and for searching for and determining a correct operating temperature for operating the wavelength conversion element at the target operating wavelength. The manufacturing apparatus includes a wavelength-sweepable light source 201 that supplies fundamental light, a detector 206 that detects SHG light 213 and converts it into an electrical signal, and a detector 207 that detects transmitted fundamental light 212 and converts it into an electrical signal. The manufacturing apparatus also includes an aligner 203 that adjusts the position of the PTF at the fiber connection point to align the optical axis between the waveguide in the chip and the core of the PTF, and a lens aligner 204 that adjusts the position of lens 205-1. The above-mentioned elements are controlled by a computer 208. Although not shown, the computer 208 includes a processor, memory, and an A/D converter that converts the electrical signals from the detectors 206 and 207 into digital data. The computer 208 can be connected to an external storage device or network, not shown.

In the wavelength conversion module manufacturing method disclosed herein, initial data for the wavelength conversion element acquired in advance during the chip selection process is read from a memory or the like and used in the manufacturing method flow described below. The initial data is updated during the manufacturing method flow. The initial data can be stored in the memory of computer 208, an external storage medium, or on a network.

Figure 13 is a diagram illustrating the configuration of initial data for wavelength conversion elements in the manufacturing method of the present disclosure. (a) in Figure 13 shows the initial data acquired in the chip selection process. (b) shows the data updated in the manufacturing method flow described below. As an example, the data for the wavelength conversion element may include the element identification number, operating wavelength, initial temperature, temperature dependence coefficient of the phase matching wavelength, wavelength conversion efficiency, loss element characteristics, etc. A file in a format such as a table is saved in the computer's memory, and this file is read from the control program for the manufacturing method. The PTF installed in the module is identified by the element identification number, etc., and the wavelength of the light source 201 and the set temperature of the temperature control element (temperature regulator) inside the module are controlled.

Figure 8 is a flow diagram illustrating the steps of the method for manufacturing a wavelength conversion module according to the present disclosure. The flow diagram shown in Figure 9 corresponds to the flow for connecting the input PTF 202-1 for inputting the first fundamental wave light shown in Figure 6(a). The explanation will also refer to the configuration of the device in Figure 7. Each step in the flow is explained using step numbers S100 to S116. Please note that the description of each step in Figure 8 is simplified.

In S100, the wavelength conversion module, the input PTF 202-1 to be connected, the aligners 203 and 204, etc. are set in the adjustment jig of the manufacturing equipment.

In S101, initial data is read into the computer 208, and the fundamental wave light from the light source 201 is set to an operating wavelength. In addition, the wavelength conversion element is set to an initial temperature T _init in the initial data. After a certain temperature stabilization time has passed, the wavelength conversion element reaches the initial temperature T _init , and the initial temperature T _init becomes the "current temperature." The "current temperature" is updated each time the "step of correcting the current temperature" is repeated, as will be described later.

The detector 207 aligns the input PTF 202-1 so that the level of the transmitted light 212 of the fundamental light is maximized. The alignment to maximize the transmitted light level is performed by aligners 203 and 204 and can be controlled by a processor using an appropriate control algorithm. The fundamental light from the end face of PTF 202-1 is optically coupled to the wavelength conversion element in the module housing via lens 205-1. As shown as transmitted light 10 in Figure 3, the transmitted light level of the fundamental light has almost no wavelength dependence and very little temperature dependence. It is sufficient to set the light source wavelength to a wavelength close to the operating wavelength. Note that this alignment step is performed on the fundamental light.

Next, in S102, the light source 201 is controlled to sweep the wavelength of the input light within a predetermined wavelength range centered on the fundamental light. The predetermined wavelength range can be determined so that the peak wavelength of the SHG light is observed, taking into account the phase-matching wavelength variation of the SHG light expected in the wavelength conversion element due to the chip fixing process. At this time, the SHG light detector 206 acquires data on the wavelength-dependent characteristics 21 of the SHG light level corresponding to the swept wavelength range, as shown in Figure 3.

Next, in S103, a peak wavelength _λP is determined from the wavelength-dependent characteristics of the SHG light level that has been obtained. This peak wavelength _λP corresponds to the actual phase-matching wavelength at the current temperature of the wavelength conversion element included in the module. This actual phase-matching wavelength is shifted from the operating wavelength λinit at _which the element is originally intended to operate, and the wavelength difference between the peak wavelength _λP and the operating wavelength _λinit is defined as Δλ. In this step, the light source 201 performs a wave sweep multiple times to obtain the average value of the phase-matching curve, thereby obtaining the peak wavelength _λP at which the SHG light output is maximized.

In the state of S103, the wavelength difference Δλ between the peak wavelength λ _P and the operating wavelength λ _init is large, so in many cases, an SHG optical output at a level that allows alignment cannot be obtained. It is difficult to align the input PTF 202-1 so that the level of the SHG light is maximized. Therefore, the current temperature of the wavelength conversion element is corrected to find a "corrected temperature" that reduces the wavelength difference Δλ.

In S104, the "corrected temperature" is calculated from the wavelength difference Δλ. More specifically, the corrected temperature of the wavelength conversion element is calculated based on the operating wavelength λ _{init of} the wavelength conversion element, the above-mentioned peak wavelength λ _P , the current temperature, and the temperature dependence coefficient of the phase matching wavelength included in the initial data.

FIG. 9 illustrates the principle of determining the corrected temperature of a wavelength conversion element. Similar to FIG. 3, FIG. 9(a) illustrates the phase-matching wavelength shift due to the chip fixing process. The wavelength difference Δλ is the peak wavelength difference 24 between the two phase-matching curves 20 and 21. FIG. 9(b) schematically illustrates the relationship between the phase-matching wavelength (horizontal axis) and the corresponding chip temperature (°C). The black circle 26 corresponds to the state in which the phase-matching wavelength is at the operating wavelength λ _init , which is the target state. The black circle 27 corresponds to the state in S203 in which the phase-matching wavelength is at the peak wavelength λ _P at the current temperature. To move from the current state of black circle 27 to the target state of black circle 26, the "current" chip temperature can be corrected by the temperature difference ΔT. Using the temperature dependence coefficient (nm/°C) of the initial data shown in FIG. 13, the "corrected temperature" can be determined based on the operating wavelength λ _init , the peak wavelength λ _P , the current temperature, and the temperature dependence coefficient of the phase-matching wavelength. To avoid the hunting phenomenon, the temperature may be corrected to a state 28 immediately before the target state 26. There are no restrictions on the specific algorithm for determining the corrected temperature, and any general control algorithm may be used. Returning to Fig. 9 again, following S104, the wavelength difference Δλ is compared with a reference value.

In S105, the wavelength difference Δλ is compared with a first threshold value, which is a reference value. Depending on the type of wavelength conversion element, the direction of wavelength shift from the initial temperature at the time of chip fixing may differ, so the absolute value of the wavelength difference Δλ can be compared with a positive first threshold value. If the wavelength difference Δλ is below the first threshold value (Y), the actual phase-matched wavelength is sufficiently close to the operating wavelength λ _init , so the process proceeds to the input PTF realignment step in S108. If the wavelength difference Δλ exceeds the first threshold value (N), the actual phase-matched wavelength is still far from the operating wavelength λ _init , so in S106 the wavelength conversion element is set to a corrected temperature from the current temperature. After waiting for a certain temperature stabilization time, the manufacturing method returns to the step of sweeping the wavelength of the fundamental wave light in S102. At the stage of returning to S102, the "corrected temperature" becomes the "current temperature."

The entire process from S102 to S106 described above can be said to be a process of correcting the current temperature of the wavelength conversion element. The reference value judgment process in S105 can also be performed before S104, which calculates the corrected temperature. Furthermore, in parallel with determining the "corrected temperature," the temperature dependency coefficient can also be updated in S107.

In S107, the temperature dependence coefficient is updated based on the operating wavelength, peak wavelength, current temperature, and corrected temperature. As shown in Figure 9(b), the temperature dependence coefficient of the phase-matching wavelength can be calculated from the wavelength difference Δλ and the temperature difference ΔT. Therefore, the temperature dependence coefficient included in the initial data shown in Figure 13 can be updated, and steps S102 to S106 can be performed using the new temperature dependence coefficient upon return.

In S108, the light source 201 is set to the operating wavelength, and the input PTF 202-1 is re-aligned using the electrical signal from the detector 206 so that the level of the SHG light 213 is maximized. The alignment to maximize the SHG light level is performed by the aligners 203 and 204, and can be performed under the control of the processor using an appropriate control algorithm. Despite the alignment being performed in S101 to maximize the level of the transmitted light 212 of the fundamental wave light, re-alignment is performed in S108 for the following reasons.

First, the alignment position at which the fundamental light output is maximized may differ from the alignment position at which the SHG output is maximized. The guided mode shape of the fundamental light and the guided mode shape of the SHG light may not match depending on the core size. Second, after the temperature is set to the corrected temperature in S106, if S108 is reached after repeating S102 to S105, the chip set temperature will have changed from the initial temperature. This may cause the shape of the wavelength conversion element to change slightly, resulting in a deviation from the optimal position aligned with the fundamental light. For the reasons stated above, in addition to alignment to maximize the level of the transmitted light 212 of the fundamental light in S101, the input PTF 202-1 is realigned in S108 to maximize the SHG output.

Next, in S109, a determination is made as to whether the SHG light level meets a reference value. The reference value is a second threshold value, which is a predetermined SHG light level. If the reference value is equal to or greater than the second threshold value (Y), the process proceeds to S111. If the reference value is less than the second threshold value (N), there is a high possibility that there is some kind of defect in the wavelength conversion element itself or the module assembly process, so further processing is halted.

In S111, the metal housing, lens 205-1, and input PTF 202-1 are fixed together by welding using a YAG laser 209 or other methods. The "corrected temperature" is determined and updated by the above-mentioned steps S102 to S107, or by repeating these steps, and the wavelength conversion element achieves a phase-matched state at the operating wavelength after being fixed to the module. However, the YAG welding in S111 may change the charged state of the PPLN element or cause drift during alignment. Furthermore, the YAG welding and fixing of the input PTF 202-1 may cause the optical axis to shift, resulting in a decrease in optical coupling efficiency.

Therefore, in S112, the fundamental light is swept, the peak wavelength _λP of the SHG light is determined, and the corrected temperature is calculated. Furthermore, in S113, the wavelength difference Δλ is judged using a reference value (first threshold value), and if necessary, the current temperature is changed to the "corrected temperature," and the process returns to the sweep of the fundamental light in S112. As explained in S107, the temperature dependency coefficient can also be updated. The steps of S112 and S113 are a simplified version of the series of steps from S102 to S107, and essentially the same steps as S102 to S107 can be performed.

If in S112 it is determined that the wavelength difference Δλ is less than or equal to the first threshold, the manufacturing method proceeds to S113, where the transmission spectrum of the fundamental light and the phase matching curve are obtained.

Next, in S114, the initial data read by the computer 208 is updated with the values that have been changed up to the step S113 described above. Specifically, the initial temperature value is rewritten with the final value of the corrected temperature, as shown in (b) of Figure 13. In addition, the temperature dependence coefficient value is rewritten with the final value of the corrected temperature dependence coefficient. Once the initial data has been updated, the procedure for the method for manufacturing the wavelength conversion module in Figure 8 ends in S116.

Therefore, the present invention can be implemented as a manufacturing method for an optical module 100 including a wavelength conversion element 102 and a temperature control element 103 fixed to the wavelength conversion element via a metal plate 104, comprising the steps of: setting the wavelength conversion element to an initial temperature; aligning the input fiber so that the transmitted light of the fundamental wave light is maximized (S101); correcting the current temperature of the wavelength conversion element, which involves sweeping the wavelength of the fundamental wave light over a predetermined wavelength range to determine the peak wavelength at which the level of the wavelength-converted light is maximized (S103); calculating the corrected temperature of the wavelength conversion element based on the operating wavelength of the wavelength conversion element, the peak wavelength, the current temperature, and the temperature dependence coefficient of the phase-matching wavelength (S104); and resetting the wavelength conversion element to the corrected temperature (S106); realigning the input fiber so that the level of the wavelength-converted light is maximized at the corrected temperature (S108); and fixing the aligned input fiber to the metal housing of the optical module (S111).

The updated data can be used as the initial data for the process of connecting the second and subsequent PTFs to the same wavelength conversion element. In other words, in the connection processes shown in (b) to (d) of Figure 6, by using the "updated data" that reflects the results of the first PTF connection process, optimization of the chip operating temperature can be started from a state very close to the operating wavelength λ _init . The process of connecting the second and subsequent PTFs can be performed in a significantly shorter time than the first PTF connection process shown in Figure 8.

Figure 10 is a diagram showing another configuration of an apparatus for implementing the wavelength conversion module manufacturing method of the present disclosure. The apparatus configuration of Figure 10 shows the stage of connecting the second input PTF 202-2 shown in Figure 6(b) to the optical window 105-2 (port 3), with the input PTF 202-1 already connected to the optical window 105-1 (port 1). A connector-type fundamental light detector 210 and SHG light detector 206 connected to the input PTF 202-1 are used. The manufacturing method flow for connecting the second input PTF using the manufacturing apparatus configuration of Figure 10 is generally the same as that of Figure 8. The differences are that the initial data read in S101 is not the initial data according to Table 1 in Figure 13(a) but the updated data according to Table 2 in Figure 13(b), and that the propagation directions of the fundamental light and SHG light are opposite to those in Figure 7. Therefore, a description of the wavelength conversion module manufacturing method for connecting the second input PTF will be omitted.

Figure 11 is a diagram showing yet another configuration of an apparatus for implementing the manufacturing method of the wavelength conversion module disclosed herein. The apparatus configuration of Figure 11 shows the stage of connecting the output PTF 202-2, which is the third fiber shown in Figure 6 (c), to the optical window 101-1 (port 4), with two input PTFs 202-1 and 202-2 already connected. A connector-type SHG light detector 212 is used on the opposite side of the connection point of the output PTF 211-1. The flow of the manufacturing method for connecting the output PTF using the manufacturing apparatus configuration of Figure 11 is generally the same as Figure 8, except that the fiber to be aligned is the output PTF.

Figure 12 is a flow diagram illustrating the steps of the method for manufacturing a wavelength conversion module of the present disclosure. The flow diagram shown in Figure 12 corresponds to the flow for connecting the first output PTF 211-1, which is the third fiber shown in Figure 6 (c) and is used to output SHG light. The explanation will also refer to the configuration of the apparatus in Figure 11. Each step in the flow will be explained using step numbers S200 to S216. The descriptions of each step in Figure 12 are simplified. As mentioned above, the steps in the flow diagram of Figure 12 are generally the same as those in the flow diagram of Figure 8, and will therefore be explained in a simplified manner.

In S200, the wavelength conversion module, the output PTF 211-1 to be connected, the aligners 203 and 204, etc. are set in the adjustment jig of the manufacturing equipment.

In S201, initial data is read into the computer 208, and the fundamental wave light from the light source 201 is set to an operating wavelength. The wavelength conversion element is set to an initial temperature T _init in the initial data. However, this initial temperature T _init is the initial temperature updated in the step of connecting the first fiber. As in the flow of FIG. 8, the initial temperature T _init becomes the "current temperature." The "current temperature" is updated each time the "step of correcting the current temperature" is repeated, as will be described later.

Furthermore, in S201, the output PTF 211-1 is aligned so that the level of the SHG light detected by the detector 212 is maximized. The alignment to maximize the SHG light level is performed by aligners 203 and 204, and can be controlled by a processor using an appropriate control algorithm. The SHG light from the wavelength conversion element in the module housing is optically coupled to the end face of the output PTF 202-1 via a lens. Even in this flow of connecting the output PTFs shown in FIG. 12, the phase matching curves are misaligned at the current temperature, as shown in FIG. 3, but the input PTFs 202-1 and 202-2 for the fundamental light have already been aligned and fixed. Furthermore, the wavelength conversion element is set to the initial temperature T _init updated in the first fiber connection process. Therefore, in S201, SHG light sufficient for alignment is obtained.

Next, in S202, the light source 201 is controlled to sweep the wavelength of the input light within a predetermined wavelength range centered on the fundamental light. At this time, the SHG light detector 212 acquires data on the wavelength-dependent characteristics 21 of the SHG light level corresponding to the swept wavelength range, as shown in Figure 3.

Next, in S203, a peak wavelength _λP is determined from the wavelength-dependent characteristics of the SHG light level that has been obtained. This peak wavelength _λP corresponds to the actual phase-matching wavelength at the current temperature of the wavelength conversion element included in the module. This actual phase-matching wavelength is shifted from the operating wavelength λinit _{at which} the element is originally intended to operate, and the wavelength difference between the peak wavelength _λP and the operating wavelength _λinit is defined as Δλ. In this step, the light source 201 performs a wave sweep multiple times to obtain the average value of the phase-matching curve, thereby obtaining the peak wavelength _λP at which the SHG light output is maximized.

In the state of S203, since there is a wavelength difference Δλ between the peak wavelength λ _P and the operating wavelength λ _init , the level of the SHG light is insufficient for accurate alignment. Therefore, the current temperature of the wavelength conversion element is corrected to find a "corrected temperature" that reduces the wavelength difference Δλ.

In S204, the "corrected temperature" is calculated from the wavelength difference Δλ. Specifically, the corrected temperature of the wavelength conversion element is calculated based on the operating wavelength λ _init of the wavelength conversion element, the above-mentioned peak wavelength λ _P , the current temperature, and the temperature dependence coefficient of the phase matching wavelength included in the initial data.

By using the value of the temperature dependence coefficient (nm/°C) of the updated initial data shown in Figure 13(b), the "corrected temperature" can be calculated based on the operating wavelength λ _init , the peak wavelength λ _P , the current temperature, and the temperature dependence coefficient of the phase matching wavelength.

Next, in S205, the wavelength difference Δλ is compared with a first threshold value, which is a reference value. Depending on the type of wavelength conversion element, the direction of wavelength shift from the initial temperature at the time of chip fixation may differ, so the absolute value of the wavelength difference Δλ can be compared with a positive first threshold value. If the wavelength difference Δλ is below the first threshold value (Y), the actual phase-matched wavelength is sufficiently close to the operating wavelength λ _init , so the process proceeds to the output PTF realignment step in S208. If the wavelength difference Δλ exceeds the first threshold value (N), the actual phase-matched wavelength is still far from the operating wavelength λ _init , so in S206 the wavelength conversion element is set to a corrected temperature from the current temperature. After waiting for a certain temperature stabilization time, the manufacturing method returns to the step of wavelength sweeping the fundamental wave light in S202. At the stage of returning to S202, the "corrected temperature" becomes the "current temperature."

The entire process from S202 to S206 described above can also be said to be a process for correcting the current temperature of the wavelength conversion element. The comparison and judgment process with the reference value in S205 can also be performed before S204, which calculates the corrected temperature. Furthermore, in parallel with determining the "corrected temperature," the temperature dependency coefficient can also be updated in S207.

In S207, the temperature-dependent coefficients are updated based on the operating wavelength, peak wavelength, current temperature, and corrected temperature. The temperature-dependent coefficients included in the updated initial data shown in Figure 13(b) are updated, and steps S202 to S206 can be performed upon return using the new temperature-dependent coefficients.

In S208, the light source 201 is set to the operating wavelength, and the output PTF 211-1 is realigned using the electrical signal from the detector 212 to maximize the SHG light level. The alignment to maximize the SHG light level is performed by the aligners 203 and 204, and can be controlled by the processor using an appropriate control algorithm. In addition to the alignment to maximize the SHG light level in S201, the output PTF is realigned in S208, similar to the flow for connecting the input PTF in Figure 8. After the temperature is set to the corrected temperature in S206, if S208 is reached after repeating S202 to S205, the chip set temperature will have changed from the initial temperature. This is because the shape of the wavelength conversion element may change slightly, potentially causing the output PTF to deviate from the optimal position initially aligned in S201.

Next, in S209, a determination is made as to whether the SHG light level meets a reference value. The reference value is a second threshold value, which is a predetermined SHG light level. If the reference value is equal to or greater than the second threshold value (Y), the process proceeds to S211. If the SHG light level is less than the second threshold value (N), there is a high possibility that there is some kind of problem with the wavelength conversion element itself or the module assembly process, so further processing is halted.

In S211, the metal housing, lens, and output PTF 211-1 are fixed together by welding using a YAG laser 209 or other methods. The "corrected temperature" is calculated by performing the above-described steps S202 to S207, or by repeating these steps, and the current temperature is updated with the corrected temperature. By using the "corrected temperature," a phase-matched state is achieved at the target operating wavelength λ _init even after the module is fixed. However, the YAG welding step in S211 may change the charged (pyroelectric) state of the PPLN element or cause wavelength drift during alignment. Furthermore, the YAG welding and fixing of the output PTF 211-1 may cause the optical axis to shift, resulting in a decrease in optical coupling efficiency.

Therefore, in S212, the fundamental light is swept, the peak wavelength _λP of the SHG light is determined, and the corrected temperature is calculated. Furthermore, in S213, the wavelength difference Δλ is compared with a reference value (first threshold value), and if necessary, the current temperature is changed to the "corrected temperature," and the process returns to the sweep of the fundamental light in S212. As described in S207, the temperature dependency coefficient can also be updated in S213. The steps of S212 and S213 are a simplified version of the series of steps from S202 to S207, and essentially the same steps as S202 to S207 may be performed.

If in S213 it is determined that the wavelength difference Δλ is less than or equal to the first threshold, the manufacturing method proceeds to S214, where the transmission spectrum of the fundamental light and the phase matching curve are obtained.

Next, in S215, the initial data read by the computer 208 is updated with the values that have been changed up to the step S213 described above. Specifically, the initial temperature value is rewritten with the final corrected temperature value, as shown in (b) of Figure 13. Once the initial data has been updated, the steps of the method for manufacturing the wavelength conversion module in Figure 12 end in S216.

The present invention therefore provides a method for manufacturing an optical module 100 including a wavelength conversion element 102 and a temperature control element 103 fixed to the wavelength conversion element via a metal plate 104, and connected to a fundamental wave input fiber 211-1, the method comprising the steps of: setting the wavelength conversion element to an initial temperature; aligning the output fiber so that the transmitted light of the fundamental wave light is maximized (step S201); correcting the current temperature of the wavelength conversion element by sweeping the wavelength of the fundamental wave light within a predetermined wavelength range to determine the peak wavelength at which the level of the wavelength-converted light is maximized (step S203); The present invention can also be implemented as a manufacturing method including a correction step including a step S204 of calculating a corrected temperature of the wavelength conversion element based on the operating wavelength of the wavelength conversion element, the peak wavelength, the current temperature, and a temperature dependence coefficient of the phase matching wavelength, and a step S206 of resetting the wavelength conversion element to the corrected temperature, a step S208 of re-aligning the output fiber so that the level of the wavelength-converted light is maximized at the corrected temperature, and a step S211 of fixing the aligned output fiber to a metal housing of the optical module.

As described in detail above, the "corrected temperature" can be determined efficiently when attaching an output PTF to a wavelength conversion module, just as it can be determined when attaching an input PTF to a wavelength conversion module. Once the module, fiber, and alignment member are set in the manufacturing equipment, all processes can be automated under computer control. This provides a method for manufacturing pigtail-type modules that is highly suitable for mass production, covering all processes from attaching fiber to the module.

As already mentioned, Figure 13 is a diagram explaining the data structure of a wavelength conversion element. Table 1 in Figure 13 (a) is the initial data obtained in the chip selection process. Figure 13 (b) is the updated initial data after the first PTF connection procedure has been performed. In the procedure for connecting four PTFs to the wavelength conversion module described in Figure 6, the updated initial data is used in the connection procedures for the second and subsequent PTFs.

The initial data can be updated each time a PTF is connected, or the same updated initial data can be used for the second through fourth PTFs. When the first PTF is connected, the "corrected temperature" is roughly approaching the temperature that achieves phase matching at the desired operating wavelength. When connecting the third and fourth PTFs, the initial data used for the second PTF can be used as is.

The optical module manufacturing method of the present invention described above is not limited to wavelength conversion modules that output SHG light, but can also be used when connecting a PTF to an input light port and an output port for wavelength-converted light. It can also be applied to wavelength conversion modules that use wavelength conversion elements in only one direction. It goes without saying that it can also be applied when connecting a PTF to an OPA module that uses the DFG or OPA mechanisms other than wavelength conversion modules.

As explained in detail above, a manufacturing method for pigtail-type modules that is highly suitable for mass production can be realized.

The present invention can be used in the manufacture of devices for optical signal processing.

Claims (8)

A method for manufacturing an optical module including a wavelength conversion element and a temperature control element fixed to the wavelength conversion element via a metal plate, comprising:
setting the wavelength conversion element to an initial temperature;
aligning the input fiber so that the transmitted light of the fundamental wave light is maximized;
Modifying the current temperature of the wavelength converting element,
a step of sweeping the wavelength of the fundamental wave light within a predetermined wavelength range to determine a peak wavelength at which the level of the wavelength-converted light is maximized;
Calculating a corrected temperature of the wavelength conversion element based on the operating wavelength of the wavelength conversion element, the peak wavelength, the current temperature and a temperature-dependent coefficient of a phase-matching wavelength; and
a modifying step including resetting the wavelength converting element to the modified temperature;
realigning the input fiber so that the level of the wavelength-converted light is maximized at the modified temperature;
and fixing the aligned input fiber to a metal housing of the optical module. Before the realigning step,
determining whether an absolute value of a wavelength difference Δλ between the peak wavelength and the operating wavelength is less than or equal to a first threshold;
repeating the correcting step under the condition that the wavelength difference Δλ exceeds the first threshold value;
2. The manufacturing method according to claim 1, further comprising the step of realigning the input fiber so that the level of the wavelength-converted light is maximized under the condition that the wavelength difference Δλ is equal to or less than the first threshold value. A method for manufacturing an optical module including a wavelength conversion element and a temperature control element fixed to the wavelength conversion element via a metal plate, and connected to a fundamental wave input fiber, comprising:
setting the wavelength conversion element to an initial temperature;
aligning the output fiber so that the wavelength-converted light is maximized;
Modifying the current temperature of the wavelength converting element,
a step of sweeping the wavelength of the fundamental wave light within a predetermined wavelength range and determining a peak wavelength at which the level of the wavelength-converted light is maximized;
Calculating a corrected temperature of the wavelength conversion element based on the operating wavelength of the wavelength conversion element, the peak wavelength, the current temperature and a temperature-dependent coefficient of a phase-matching wavelength; and
a modifying step including resetting the wavelength converting element to the modified temperature;
realigning the output fiber so that the level of the wavelength-converted light is maximized at the modified temperature;
and fixing the aligned output fiber to a metal housing of the optical module. Before the realigning step,
determining whether an absolute value of a wavelength difference Δλ between the peak wavelength and the operating wavelength is less than or equal to a first threshold;
repeating the correcting step under the condition that the wavelength difference Δλ exceeds the first threshold value;
4. The manufacturing method according to claim 3, further comprising the step of realigning the output fiber so that the level of the wavelength-converted light is maximized under the condition that the wavelength difference Δλ is equal to or less than the first threshold value. After the realigning step,
determining whether the level of the wavelength-converted light is greater than or equal to a second threshold;
4. The manufacturing method according to claim 1, further comprising the step of fixing the optical module to a metal housing under the condition that the level of the wavelength-converted light is equal to or greater than the second threshold value. The modifying step includes:
The method of claim 2 or 4, further comprising: updating the temperature-dependent coefficient based on the operating wavelength, the peak wavelength, the current temperature, and the corrected temperature. The manufacturing method described in claim 1 or 3, wherein the initial temperature is either a previously obtained initial temperature corresponding to the operating wavelength of the wavelength conversion element, or the corrected temperature obtained when connecting the input fiber to the optical module. An apparatus for manufacturing an optical module including a wavelength conversion element and a temperature control element fixed to the wavelength conversion element via a metal plate,
a light source that supplies fundamental wave light to the optical module;
a first detector that detects wavelength-converted light from the optical module;
a second detector that detects a fundamental wave transmitted light from the optical module;
an aligner for the target fiber;
a computer including at least a processor and a memory;
The processor:
controlling data conversion of the electrical signals from the light source, the first detector, and the second detector, and the aligner;
Reading element data including at least the operating wavelength, the initial temperature, and the temperature-dependent coefficient of the wavelength conversion element from the memory, the external storage device, or the network; and
configured to update the initial temperature with the corrected temperature;
A manufacturing apparatus for carrying out the method according to any one of claims 1 to 4.