JP2013210560A - Optical axis adjustment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical axis adjustment method that enables favorable high-frequency response characteristics.SOLUTION: There is provided an optical axis adjustment method in which an axis of an optical fiber 22 (optical component) is adjusted relative to that of a light receiving element 12. The optical axis adjustment method includes the steps of: inputting signal light intensity-modulated by a single frequency through the optical fiber 22 into the light receiving element 12; and adjusting a relative positional relationship between the optical fiber 22 and the light receiving element 12 so that, of harmonic signals contained in an electric signal output from the light receiving element 12, the intensity of even-numbered order harmonic ones relative to a fundamental wave signal contained therein is a prescribed value or less.

Description

  The present invention relates to an optical axis adjustment method.

  The optical axis adjusting device is a device for combining various optical components and aligning and fixing each other's optical axes. By welding after matching the optical axis of the optical component with high accuracy, the light transmission efficiency of the optical device can be improved, and the light transmission distance can be increased along with noise reduction. For example, Patent Document 1 discloses a method for adjusting an optical axis between an optical fiber and an optical element.

JP-A-7-230025

  As a method of adjusting the optical axis between an optical component such as an optical fiber and a light receiving element, signal light emitted from a light source such as a laser diode is incident on the light receiving element via the optical component, and the photocurrent flowing through the light receiving element is changed. There is a method to do while monitoring. In such an optical axis adjustment method, the direct current output from the light receiving element is usually monitored, and the optical axes of the optical component and the light receiving element are adjusted.

  However, in the method of adjusting by monitoring the direct current, the adjustment that directly evaluates the high-frequency response characteristic cannot be performed. An object of this invention is to provide the optical axis adjustment method which can make a high frequency response characteristic favorable in view of said subject.

  An optical axis adjustment method according to the present invention is an optical axis adjustment method of an optical component with respect to a light receiving element, and the step of inputting signal light intensity-modulated at a single frequency to the light receiving element via the optical component; Among the harmonic signals with respect to the fundamental signal included in the electrical signal output from the light receiving element, the relative relationship between the optical component and the light receiving element is set so that the even-order harmonic intensity is not more than a specified value. Adjusting the positional relationship. According to the optical axis adjustment method of the present invention, high frequency response characteristics can be improved.

  In the optical axis adjustment method, the relative positional relationship between the optical component and the light receiving element may be adjusted such that even-order harmonic distortion is 10% or less.

  The adjustment of the relative positional relationship adjusts the relative positional relationship between the optical component and the light receiving element so that the intensity of the odd-numbered harmonics of the harmonic signal is equal to or higher than a specified value. A process may be included.

  In the optical axis adjusting method, the relative positional relationship between the optical component and the light receiving element may be adjusted so that odd-order harmonic distortion is 30% or more.

  The adjustment of the relative positional relationship is such that the even-order harmonic intensity of the harmonic signal and the odd-order harmonic intensity of the harmonic signal are equal to or less than a specified value. A step of adjusting a relative positional relationship between the optical component and the light receiving element may be included.

  In the optical axis adjustment method, the relative positional relationship between the optical component and the light receiving element may be adjusted so that both the even-order harmonic distortion and the odd-order harmonic distortion are 10% or less. .

  In the optical axis adjustment method, the average intensity of the signal light may be varied, the positional relationship may be adjusted a plurality of times, and an optimal one may be selected. In the optical axis adjustment method, the voltage applied to the light receiving element may be varied, and the positional relationship may be adjusted multiple times to select the optimum one. In the optical axis adjustment method, the wavelength of the signal light may be varied, the positional relationship may be adjusted multiple times, and an optimal one may be selected.

  Prior to the adjustment of the positional relationship, the relative positional relationship between the optical component and the light receiving element is adjusted so that the light intensity detected by the light receiving element exceeds a predetermined reference value. You may further include the process to do.

  According to the optical axis adjustment method of the present invention, high frequency response characteristics can be improved.

FIG. 1 is a schematic diagram for explaining a measurement system. 2A to 2C show the measurement results of the response waveform, and FIGS. 2D to 2F show the measurement results of the frequency characteristics. 3A and 3B show the measurement results of the response waveform, and FIGS. 3C and 3D show the measurement results of the frequency characteristics. FIG. 4 is a flowchart for explaining the optical axis adjustment method according to the first embodiment. FIG. 5 is a flowchart for explaining the optical axis adjustment method according to the second embodiment.

  First, an experiment conducted by the inventor will be described. FIG. 1 is a schematic diagram for explaining a measurement system. As shown in FIG. 1, the measurement system used in the experiment is provided with an optical connector 20 holding an optical fiber 22 and an optical inlet 18 (receptacle) into which the optical connector 20 is inserted on a side wall, for example, a PD (photodiode). An ROSA (receiver optical sub-assembly) type light receiving device 100 having a package 10 having a light receiving element 12 inside is used. For example, signal light output from a light emitting element 32 which is an LD (laser diode) is incident on the optical fiber 22. The light emitting element 32 is supplied with a driving current intensity-modulated by a single frequency sine wave from the AC power supply 30. As a result, the light emitting element 32 outputs signal light whose intensity is modulated with a sine wave having a single frequency. As an example of the single frequency, when the light receiving element 12 is for 10 Gbit / sec, 1 GHz corresponding to 1/10 of the light receiving element 12 can be cited.

  The light receiving device 100 includes a transimpedance amplifier (hereinafter referred to as TIA) 14 and a lens 16 in addition to the light receiving element 12 inside the package 10. The signal light incident on the optical fiber 22 passes through the optical fiber 22, is collected by the lens 16, and is received by the light receiving element 12. The light receiving element 12 converts the received signal light into a current signal by photoelectric conversion. The current signal is output to the TIA 14. The TIA 14 converts the impedance of the input current signal, amplifies it, and outputs it as a voltage signal. The direct current component of the current signal output from the light receiving element 12 is measured by the direct current ammeter 36.

  An electrical spectrum analyzer (hereinafter referred to as ESA) 38 is connected to the subsequent stage of the TIA 14. The voltage signal output from the TIA 14 is measured by the ESA 38.

  Next, specific contents of the experiment conducted by the inventor will be described. First, the inventor changed a part of the measurement system shown in FIG. 1 and connected an oscilloscope instead of the ESA 38 at the subsequent stage of the TIA 14. Then, the relative positional relationship between the optical fiber 22 and the light receiving element 12 is changed in the X-axis, Y-axis, and Z-axis directions, and the signal light is intensity-modulated with a single frequency sine wave incident on the optical fiber 22. The response waveform was measured with an oscilloscope. Here, the TIA 14 has a function of determining whether the light receiving element 12 is used as a linear operation type (hereinafter referred to as a linear type) or a saturation operation type (hereinafter referred to as a limiting type). The response waveform measured by the oscilloscope is different between the case where the TIA 14 functioning as the linear type is used and the case where the TIA 14 functioning as the limiting type is used. Next, the inventor connects the ESA 38 after the TIA 14 as in the measurement system shown in FIG. 1 with the relative position of the optical fiber 22 and the light receiving element 12 fixed at the position where the response waveform is measured. Then, the frequency characteristics were measured by ESA38.

  First, measurement results of response waveforms and frequency characteristics when the light receiving element 12 is used as a limiting type will be described. FIG. 2A to FIG. 2C show response waveform measurement results when the relative positions of the optical fiber 22 and the light receiving element 12 are set to three different positions. FIG. 2D to FIG. 2F show the measurement results of the frequency characteristics at the above three positions.

  In FIG. 2A, the rise time and the fall time of the data signal are fast and the waveform distortion is small (that is, the waveform symmetry is good). From this, it can be seen that the alignment of the optical axes of the optical fiber 22 and the light receiving element 12 is a desirable state. As shown in FIG. 2D, the frequency characteristics in this case are output of even-order harmonics such as the second-order harmonic (f2) and the fourth-order harmonic (f4) of the harmonic signal. Is suppressed. On the other hand, odd-order harmonics such as the third-order harmonic (f3) and the fifth-order harmonic (f5) are output greatly, and the intensity of the fundamental wave (fo) and the intensity of the odd-order harmonics are The difference is getting smaller.

  On the other hand, in FIG. 2B, the waveform distortion is small, but the rise time and fall time of the data signal are slow. In the frequency characteristic in this case, as shown in FIG. 2 (e), the output of the even-order harmonics of the harmonic signal is suppressed. The output of the odd-order harmonics is smaller than that in the case of FIG. 2D, and the difference between the intensity of the fundamental wave and the intensity of the odd-order harmonics is large.

  In FIG. 2C, the rise time and fall time of the data signal are slow and the waveform distortion is large. In the frequency characteristic in this case, as shown in FIG. 2 (f), even-order harmonics of the harmonic signal are output largely. The output of the odd-order harmonics is smaller than that in the case of FIG. 2D, and the difference between the intensity of the fundamental wave and the intensity of the odd-order harmonics is large.

  From this result, when the light receiving element 12 is used as a limiting type, the inventor can reduce the waveform distortion by aligning so that the even-order harmonics of the harmonic signal have a small intensity, thereby reducing the high-frequency response. We found new knowledge that the characteristics can be improved. In addition, the waveform distortion can be reduced by adjusting the intensity of the even-order harmonics so that the intensity of the odd-order harmonics is increased, and the rise time and fall time of the data signal can be reduced. I found new knowledge that it was possible to improve the high-frequency response characteristics.

  Next, measurement results of response waveforms and frequency characteristics when the light receiving element 12 is used as a linear type will be described. 3A and 3B show the response waveform measurement results when the relative positions of the optical fiber 22 and the light receiving element 12 are set to two different positions. FIG. 3C and FIG. 3D are measurement results of frequency characteristics at the two positions.

  In FIG. 3A, the response waveform is a clean linear shape. From this, it can be seen that the alignment of the optical axes of the optical fiber 22 and the light receiving element 12 is a desirable state. In the frequency characteristics in this case, as shown in FIG. 3C, neither the even-order harmonics nor the odd-order harmonics of the harmonic signal are output.

  On the other hand, in FIG. 3B, the response waveform is distorted. In the frequency characteristic in this case, as shown in FIG. 3D, even-order harmonics and odd-order harmonics of the harmonic signal are output.

  From this result, in the case where the light receiving element 12 is used as a linear type, the inventor performs alignment so that the intensity of at least one of the even-order harmonic and the odd-order harmonic of the harmonic signal becomes small, We have found new findings that response waveform distortion can be reduced and high frequency response characteristics can be improved. In addition, by aligning both the even harmonics and odd harmonics of the harmonic signal, the response waveform distortion can be reduced and the high frequency response characteristics can be improved. I found a new finding.

  Therefore, examples based on the new knowledge obtained by the above experiment will be described below.

  The first embodiment is an optical axis adjustment method suitable when the light receiving element is used as a limiting type. FIG. 4 is a flowchart for explaining the optical axis adjustment method according to the first embodiment. The optical axis adjustment method according to the first embodiment uses the measurement system shown in FIG. 1 and will be described with reference to FIG. As shown in FIG. 4, first, in order to use the measurement system described in FIG. 1, the optical connector 20 holding the optical fiber 22 and the optical inlet 18 into which the optical connector 20 is inserted are provided. Is prepared (step S10).

  Next, a drive current that is intensity-modulated with a single frequency sine wave is input to the light emitting element 32 using the AC power supply 30. Thereby, the signal light intensity-modulated with a single frequency sine wave is emitted from the light emitting element 32, and this signal light is made incident on the optical fiber 22 (step S12).

  Next, the current signal output when the light receiving element 12 receives the signal light via the optical fiber 22 is measured by the ESA 38 connected via the TIA 14, and the intensity of the fundamental signal and the intensity of the harmonic signal are measured. Is monitored (step S14).

Next, the relative positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted while monitoring the intensity of the fundamental wave signal and the intensity of the harmonic signal. Specifically, the optical fiber is designed so that the intensity of the even harmonics of the harmonic signal is not more than a specified value and the intensity of the odd harmonics is not less than the specified value while keeping the intensity of the fundamental signal large. The relative positional relationship between 22 and the light receiving element 12 is adjusted (step S16). Here, as the specified value, for example, a value of harmonic distortion can be used, and the value can be arbitrarily determined according to the use of the light receiving element 12. As an example, the even-order harmonic distortion (HD _even ) shown in the following equation 1 is 10% or less, and the odd-order harmonic distortion (HD _odd ) shown in the following equation 2 is 30% or more. The positional relationship between the optical fiber 22 and the light receiving element 12 can be adjusted. V ₁ in Equations 1 and 2 is the effective voltage of the fundamental wave component, and V ₂ , V ₃ , V ₄ ... Are the effective voltages (V ₂ ) of the second-order harmonics, Harmonic effective voltage (V ₃ ), fourth-order harmonic effective voltage (V ₄ ), and other harmonic component effective voltages.

  In addition, other values other than the harmonic distortion value may be used. For example, a ratio value of what percentage with respect to the intensity of the signal light output from the light emitting element 32 may be used. As an example, the total value of the even-order harmonic intensities is equal to or less than the ratio value determined with respect to the intensity of the output signal light of the light-emitting element 32, and the total value of the odd-order harmonic intensities is the light-emitting element 32. The positional relationship between the optical fiber 22 and the light receiving element 12 may be adjusted so as to be equal to or greater than a ratio value determined with respect to the intensity of the output signal light.

  It is desirable to measure the intensity of the higher harmonics, but for example, the intensity of the second to fifth harmonics has a large contribution to the response waveform, so that the second to fifth harmonics are measured. The position of the optical fiber 22 and the light receiving element 12 may be adjusted by measuring the intensity. As a simpler method, the positions of the optical fiber 22 and the light receiving element 12 may be adjusted by measuring the intensity of the second and third harmonics.

  By performing the adjustment in step S16, the optical axes of the optical fiber 22 and the light receiving element 12 can be brought into a desired alignment state as described with reference to FIGS. 2 (a) and 2 (d). When the alignment of the optical axes of the optical fiber 22 and the light receiving element 12 is completed, the optical entrance 18 is fixed to the package 10 at the adjusted position (step S18).

  As described above, in the optical axis adjustment method according to the first embodiment, the signal light whose intensity is modulated at a single frequency is input to the light receiving element 12 through the optical fiber 22 (optical component). Of the harmonic signals with respect to the fundamental signal included in the electrical signal output from the light receiving element 12, the intensity of the even-order harmonics is not more than a specified value, and the intensity of the odd-order harmonics is not less than the specified value. Thus, the relative positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted. By using such a method, when the light receiving element 12 is used as a limiting type, the distortion of the response waveform can be reduced and the rise time and fall time of the data signal can be shortened as described with reference to FIG. High frequency response characteristics can be improved.

As shown in FIGS. 2 (a) to 2 (f), when the intensity of even-order harmonics is small, the distortion of the response waveform is small. Therefore, in adjusting the positional relationship between the optical fiber 22 and the light receiving element 12, the even-order harmonic distortion (HD _even ) is preferably 10% or less, and more preferably 5% or less. Preferably, it is more preferably 3% or less.

As shown in FIGS. 2A to 2F, when the intensity of the odd-order harmonics is large, the rise time and the fall time of the data signal are shortened. Therefore, in adjusting the positional relationship between the optical fiber 22 and the light receiving element 12, the odd-order harmonic distortion (HD _even ) is preferably 30% or more, and more preferably 40% or more. Preferably, it is more preferably 50% or more.

  In the first embodiment, the positional relationship between the optical fiber 22 and the light receiving element 12 so that the even-order harmonic intensity of the harmonic signal is equal to or less than the specified value and the odd-order harmonic intensity is equal to or greater than the specified value. Although the case of adjusting is shown as an example, it is not limited to this case. By adjusting the relative positional relationship between the optical fiber 22 and the light receiving element 12 so that at least the intensity of the even-order harmonics of the harmonic signal is equal to or less than the specified value, FIG. As shown, the distortion of the response waveform can be reduced and the high frequency response characteristics can be improved.

  Example 2 is an optical axis adjustment method suitable for the case where the light receiving element is used as a linear type. FIG. 5 is a flowchart for explaining the optical axis adjustment method according to the second embodiment. The optical axis adjustment method according to the second embodiment is also described with reference to FIG. 1 because the measurement system shown in FIG. 1 is used. As shown in FIG. 5, first, in order to use the measurement system described in FIG. 1, the optical connector 20 holding the optical fiber 22 and the optical inlet 18 into which the optical connector 20 is inserted are provided. Is prepared (step S20).

  Next, a drive current that is intensity-modulated with a single frequency sine wave is input to the light emitting element 32 using the AC power supply 30. Thereby, the signal light intensity-modulated with a single frequency sine wave is emitted from the light emitting element 32, and this signal light is made incident on the optical fiber 22 (step S22).

  Next, the current signal output when the light receiving element 12 receives the signal light via the optical fiber 22 is measured by the ESA 38 connected via the TIA 14, and the intensity of the fundamental signal and the intensity of the harmonic signal are measured. Is monitored (step S24).

Next, the relative positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted while monitoring the intensity of the fundamental wave signal and the intensity of the harmonic signal. Specifically, the optical fiber 22 and the light receiving element are set so that the intensity of the even-order harmonics and the intensity of the odd-order harmonics of the harmonic signal are not more than a specified value while keeping the intensity of the fundamental signal large. The relative positional relationship with 12 is adjusted (step S26). Here, as the specified value, for example, a value of harmonic distortion can be used, and the value can be arbitrarily determined according to the use of the light receiving element 12. As an example, both the even-order harmonic distortion (HD _even ) shown in Equation 1 and the odd-order harmonic distortion (HD _odd ) shown in Equation 2 are 10% or less. The positional relationship between the optical fiber 22 and the light receiving element 12 can be adjusted.

  Also, other values other than the harmonic distortion value may be used. For example, a ratio value of what percentage may be used with respect to the intensity of the output signal light of the light emitting element 32. As an example, the total value of the intensities of the even-order harmonics and the total value of the intensities of the odd-order harmonics are equal to or less than a ratio value determined with respect to the intensity of the output signal light of the light emitting element 32. The positional relationship between the optical fiber 22 and the light receiving element 12 may be adjusted.

  It is desirable to measure the intensity of the higher harmonics, but for example, the intensity of the second to fifth harmonics has a large contribution to the response waveform, so that the second to fifth harmonics are measured. The positional relationship between the optical fiber 22 and the light receiving element 12 may be adjusted by measuring the intensity. As a simpler method, the intensity of the second and third harmonics may be measured to adjust the positional relationship between the optical fiber 22 and the light receiving element 12.

  By performing the adjustment in step S26, the optical axes of the optical fiber 22 and the light receiving element 12 can be brought into a desired alignment state as described with reference to FIGS. 3 (a) and 3 (c). When the alignment of the optical axes of the optical fiber 22 and the light receiving element 12 is completed, the optical entrance 18 is fixed to the package 10 at the adjusted position (step S28).

  As described above, in the optical axis adjustment method according to the second embodiment, the intensity of the even-order harmonics and the harmonics of the odd-order harmonics among the harmonic signals with respect to the fundamental signal included in the electrical signal output from the light receiving element 12 are determined. The relative positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted so that the intensity is equal to or less than the specified value. By using such a method, when the light receiving element 12 is used as a linear type, the distortion of the response waveform can be reduced and the high frequency response characteristics can be improved as shown in FIG.

  As shown in FIGS. 3A to 3D, when both the even-order harmonic intensity and the odd-order harmonic intensity are small, the distortion of the response waveform is small. Therefore, in adjusting the positional relationship between the optical fiber 22 and the light receiving element 12, both the even-order harmonic distortion and the odd-order harmonic distortion are preferably 10% or less, and preferably 5% or less. More preferably, it is more preferably 3% or less.

In the second embodiment, the relative positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted so that the intensity of the even-order harmonics and the intensity of the odd-order harmonics of the harmonic signal are equal to or less than a specified value. Although the case where it does is shown as an example, it is not restricted to this case. By adjusting the positional relationship between the optical fiber 22 and the light receiving element 12 so that the intensity of at least even harmonics of the harmonic signal is below a specified value, the distortion of the response waveform can be reduced and the high frequency response can be reduced. The characteristics can be improved. In this case, in adjusting the positional relationship between the optical fiber 22 and the light receiving element 12, the even-order harmonic distortion (HD _even ) is preferably 10% or less, and preferably 5% or less. Is more preferable, and it is still more preferable to make it 3% or less.

Further, the positional relationship between the optical fiber 22 and the light receiving element 12 may be adjusted so that the intensity of at least odd-order harmonics in the harmonic signal is equal to or less than a specified value. In this case, when adjusting the positional relationship between the optical fiber 22 and the light receiving element 12, the odd-order harmonic distortion (HD _even ) is preferably 10% or less, and preferably 5% or less. More preferably, it is more preferably 3% or less.

  In the first and second embodiments, the optimum position may be obtained by repeatedly performing steps S12 to S16 in FIG. 4 or steps S22 to S26 in FIG. 5 for a plurality of signal lights having different average intensities. . In this way, by changing the average intensity of the signal light, adjusting the positional relationship between the optical fiber 22 and the light receiving element 12 a plurality of times, and selecting the optimum position, the optical input power is applied to the light receiving element 12. Even when there is dependence, good high frequency response characteristics can be obtained.

  In the first and second embodiments, voltages having different magnitudes are applied to the light receiving element 12, and steps S12 to S16 in FIG. 4 or steps S22 to S26 in FIG. 5 are repeated to obtain an optimum position. Also good. As described above, the voltage applied to the light receiving element 12 is varied, the positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted a plurality of times, and the optimum position is selected, whereby the light receiving element 12 is applied. Even when there is voltage dependence, good high frequency response characteristics can be obtained.

  In the first and second embodiments, the optimum position may be obtained by repeatedly performing steps S12 to S16 in FIG. 4 or steps S22 to S26 in FIG. 5 for a plurality of signal lights having different wavelengths. As described above, the light receiving element 12 is dependent on the wavelength by adjusting the positional relationship between the optical fiber 22 and the light receiving element 12 a plurality of times by selecting the optimum position by changing the wavelength of the signal light. Even in this case, good high frequency response characteristics can be obtained. Examples of wavelengths include 1310 nm, 1490 nm, and 1550 nm used for optical communication.

  In Example 1 and Example 2, prior to step S14 in FIG. 4 or step S24 in FIG. 5, the optical fiber 22 and the light receiving element 12 are arranged so that the light intensity detected by the light receiving element 12 exceeds a predetermined reference value. Pre-alignment may be performed to adjust the relative positional relationship. The predetermined reference value can be arbitrarily determined. This preliminary alignment is the same as the conventional alignment method. The direct current output from the light receiving element 12 is measured by the DC ammeter 36 and the relative positional relationship between the optical fiber 22 and the light receiving element 12 is adjusted. This can be done. This preliminary alignment determines the relative positional relationship between the optical fiber 22 and the light receiving element 12 to some extent. Therefore, the position adjustment in step S16 in FIG. 4 or step S26 in FIG. 5 can be easily performed in a short time. Become.

  In the first and second embodiments, the light receiving device 100 is described using a ROSA type light receiving device having the light inlet 18 into which the optical connector 20 holding the optical fiber 22 is inserted. For example, the present invention is also applicable to a light receiving device in which an optical fiber is directly connected to a package. The optical component is not limited to the optical fiber 22 and may be other optical components.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

10 package 12 light receiving element 14 TIA
16 Lens 18 Optical entrance 20 Optical connector 22 Optical fiber 30 AC power supply 32 Light emitting element 36 DC ammeter 38 ESA
100 Photodetector

Claims (10)

An optical axis adjustment method of an optical component with respect to a light receiving element,
Inputting signal light intensity-modulated at a single frequency to the light receiving element via the optical component;
Among the harmonic signals with respect to the fundamental signal included in the electrical signal output from the light receiving element, the relative relationship between the optical component and the light receiving element is set so that the even-order harmonic intensity is not more than a specified value. Adjusting the positional relationship, and an optical axis adjustment method.   The optical axis adjustment method according to claim 1, wherein the relative positional relationship between the optical component and the light receiving element is adjusted so that even-order harmonic distortion is 10% or less.   The adjustment of the relative positional relationship adjusts the relative positional relationship between the optical component and the light receiving element so that the intensity of the odd-numbered harmonics of the harmonic signal is equal to or higher than a specified value. 3. The optical axis adjustment method according to claim 1, further comprising a step.   4. The optical axis adjustment method according to claim 3, wherein the relative positional relationship between the optical component and the light receiving element is adjusted so that odd harmonic distortion is 30% or more.   The adjustment of the relative positional relationship is such that the even-order harmonic intensity of the harmonic signal and the odd-order harmonic intensity of the harmonic signal are equal to or less than a specified value. 2. The optical axis adjustment method according to claim 1, further comprising a step of adjusting a relative positional relationship between the optical component and the light receiving element.   6. The relative positional relationship between the optical component and the light receiving element is adjusted so that both the even-order harmonic distortion and the odd-order harmonic distortion are 10% or less. Optical axis adjustment method.   7. The optical axis adjustment method according to claim 1, wherein the positional relationship is adjusted a plurality of times with different average intensities of the signal light, and an optimum one is selected.   8. The optical axis adjusting method according to claim 1, wherein the voltage applied to the light receiving element is varied, the positional relationship is adjusted a plurality of times, and an optimum one is selected. .   9. The optical axis adjustment method according to claim 1, wherein the position relationship is adjusted a plurality of times with different wavelengths of the signal light, and an optimum one is selected.   Prior to the adjustment of the positional relationship, the relative positional relationship between the optical component and the light receiving element is adjusted so that the light intensity detected by the light receiving element exceeds a predetermined reference value. The optical axis adjusting method according to claim 1, further comprising a step of:

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