Optical interferometer

Abstract

To provide an optical interferometer which can suppress increase of a noise floor.SOLUTION: The optical interferometer includes: a light source 20; a branch unit 30 for branching an optical signal output from the light source 20 into a measurement signal and a reference signal; a sending-receiving unit 40 for sending the measurement signal to an outer space and receiving the measurement signal when the measurement signal returns from the outer space; an interference unit 60 for causing the measurement signal received by the sending-receiving unit 40 and a reference signal to interfere with each other and generating an interference signal; and a delay length adjusting unit 50 for adjusting a reference signal delay length as the delay length of a reference signal from the branch unit 30 to the interference unit 60.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to optical interferometers. [Background technology]

[0002] In optical interferometers used in FMCW LiDARs that detect objects around a vehicle, it is known that back reflection from optical windows, fiber output ends, optical IC (integrated circuit) output ends, etc. can degrade distance measurement performance. FMCW stands for Frequency Modulated Continuous Wave. LiDAR stands for Light Detection And Ranging. For example, when back scattered light returns to the light source, the amplitude and phase noise of the output light increase, and the line width increases.

[0003] Patent Document 1 proposes a method of mitigating the effects of backscattering by using a polarizer such as a quarter-wave plate to make the polarization of the output light from the light source and the polarization of the backscattered light orthogonal to each other, thereby preventing the backscattered light from passing through the polarizer and returning to the light source. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Pat. No. 1,135,382 Summary of the Invention [Problem to be solved by the invention]

[0005] In an optical interferometer, the light output from a light source is divided into a measurement signal and a reference signal, and distance measurement and the like are performed using these interference signals. In an optical interferometer, in addition to the backscattered component returning to the light source, there is also a backscattered component that follows the same path as the measurement signal and interferes with the reference signal. This may cause the noise floor to rise and the signal-to-noise ratio to decrease, resulting in performance degradation such as a decrease in the detection distance, so it is necessary to suppress the rise in the noise floor caused by the interference between the backscattered component and the reference signal. However, Patent Document 1 does not propose any measures against this interference.

[0006] In view of the above, an object of the present disclosure is to provide an optical interferometer capable of suppressing an increase in the noise floor. [Means for solving the problem]

[0007] In order to achieve the above object, according to one aspect of the present disclosure, an optical interferometer includes a light source (20), a branching section (30) that divides an optical signal output by the light source into a measurement signal and a reference signal, a transmitting / receiving section (40) that transmits the measurement signal to external space and receives the measurement signal returned from the external space, an interference section (60) that generates an interference signal by causing the measurement signal and reference signal received by the transmitting / receiving section to interfere with each other, and a delay length adjustment section (50) that adjusts the reference signal delay length, which is the delay length of the reference signal from the branching section to the interference section.

[0008] This makes it possible to adjust the reference signal delay length to approach the delay length of the backscattered light, thereby reducing the effect of backscattering on the interference signal and suppressing an increase in the noise floor.

[0009] The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and specific components described in the embodiments described below. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing a configuration of an optical interferometer according to a first embodiment. [Diagram 2]FIG. 13 is a diagram illustrating a configuration of a delay length adjustment unit. [Diagram 3] FIG. 2 is a diagram illustrating frequencies of a reference signal and a received signal. [Figure 4] FIG. 4 is a diagram illustrating the amplitude of a beat signal. [Diagram 5] 1 is a power spectrum of a beat signal. [Figure 6] 1 is a power spectrum of a beat signal. [Figure 7] FIG. 1 is a diagram showing the slope height of the noise floor. [Figure 8] FIG. 4 is a diagram showing the angle of emitted light. [Figure 9] FIG. 11 is a diagram for explaining a delay length adjusting method in the second embodiment. [Figure 10] FIG. 13 is a diagram illustrating the relationship between chirp rate and beat frequency resolution. [Figure 11] FIG. 4 is a diagram showing center frequencies of a measurement signal and an adjustment signal. [Figure 12] FIG. 13 is a diagram illustrating a configuration of a delay length adjustment unit in the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other will be denoted by the same reference numerals.

[0012] (First embodiment) A first embodiment will be described. In this embodiment, a case where an optical interferometer is used in an FMCW LiDAR will be described, but the optical interferometer may be used for other purposes.

[0013] 1, the optical interferometer includes a housing 10, a light source 20, a branching section 30, a transmitting / receiving section 40, a delay length adjusting section 50, an interference section 60, a light receiving section 70, a signal processing section 80, an optical IC 90, and a substrate 100. The light source 20 to the substrate 100 are disposed in the space inside the housing 10.

[0014] The light source 20 is a laser light source composed of a semiconductor optical amplifier or the like. The light source 20, the branching section 30, the interference section 60, and the light receiving section 70 are arranged in an optical IC 90 formed by a semiconductor process, and the optical IC 90 and the signal processing section 80 are arranged on a substrate 100 composed of silicon or the like. The light source 20 is connected to the branching section 30 by a waveguide formed in the optical IC 90, and the optical signal output by the light source 20 is input to the branching section 30.

[0015] The branching unit 30 divides the optical signal output by the light source 20 into a measurement signal and a reference signal. The measurement signal is an optical signal for detecting an object, and the reference signal is an optical signal for distance measurement using the FMCW method. The branching unit 30 is composed of a coupler or the like that branches a waveguide, and is provided so as to divide the waveguide extended from the light source 20 into two.

[0016] Of the two waveguides branched by the branching unit 30, one of the waveguides is extended to the end face of the optical IC 90 and is connected to the end face of the substrate 100 by a waveguide on the substrate 100, and a measurement signal emitted from the end face of the substrate 100 is input to the transmitting / receiving unit 40. The other waveguide is connected to the interference unit 60 via the delay length adjustment unit 50, and the reference signal is propagated through this waveguide and input to the interference unit 60. The delay length adjustment unit 50 is connected to the substrate 100 by, for example, an optical fiber cable, and the branching unit 30 and the interference unit 60 are connected to the delay length adjustment unit 50 by the waveguide on the optical IC 90, the waveguide on the substrate 100, and this optical fiber cable. The reference signal is propagated through the waveguide extended from the branching unit 30 to the end face of the substrate 100 and the optical fiber cable on the input side of the delay length adjustment unit 50, and input to the delay length adjustment unit 50. The reference signal that has passed through the delay length adjustment unit 50 propagates through the optical fiber cable on the output side of the delay length adjustment unit 50 and the waveguide that extends from the end face of the substrate 100 to the interference unit 60, and is input to the interference unit 60.

[0017] The transmitting / receiving unit 40 transmits a measurement signal to the external space of the housing 10 and receives the measurement signal returned from the external space of the housing 10. The transmitting / receiving unit 40 is composed of a MEMS (Micro Electro Mechanical Systems) mirror or the like.

[0018] An opening is provided in the housing 10, and this opening is covered by a plate-shaped optical window 11 made of a material that transmits optical signals, such as glass (SiO2). A measurement signal input from the branching unit 30 to the transceiver unit 40 is reflected by a MEMS mirror provided in the transceiver unit 40, passes through the optical window 11, and is irradiated into the external space of the housing 10. Then, the transceiver unit 40 oscillates the MEMS mirror, whereby the measurement signal is scanned in the external space of the housing 10.

[0019] When a measurement signal reflected by an object in the external space of the housing 10 passes through the optical window 11 and returns to the inside of the housing 10, this measurement signal is reflected by a MEMS mirror provided in the transceiver unit 40 and is irradiated onto the end face of the substrate 100. A waveguide connected to the interference unit 60 is exposed on this end face, and the optical signal received by the transceiver unit 40 is input to the interference unit 60.

[0020] As indicated by arrow A1, backscattering of the measurement signal occurs at the optical window 11, and a portion of the measurement signal irradiated from the transceiver unit 40 to the optical window 11 is reflected at the optical window 11, received by the transceiver unit 40, and input to the interference unit 60. In other words, if the optical signal received by the transceiver unit 40 is defined as the received signal, the received signal includes the measurement signal that has passed through the optical window 11 and returned to the inside of the housing 10, and the measurement signal reflected by the optical window 11. The measurement signal that has passed through the optical window 11 and returned to the inside of the housing 10 is defined as a first received signal, and the measurement signal reflected by the optical window 11 is defined as a second received signal.

[0021] The delay length adjustment unit 50 adjusts the delay length of the reference signal from the branching unit 30 to the interference unit 60 (hereinafter referred to as the reference signal delay length). Specifically, the reference signal delay length is the length of the optical path along which the reference signal propagates from the branching unit 30 to the interference unit 60. The delay length adjustment unit 50 has an optical path along which an input optical signal is output after being reflected by a plurality of folding mirrors, and changes the positions of at least some of the plurality of mirrors to change the reference signal delay length.

[0022] 2, the delay length adjustment unit 50 includes mirrors 51, 52, 53, and 54, and input light is output after being reflected in order by the mirrors 51 to 54 as indicated by the arrow A2. The mirrors 52 and 53 are movable mirrors, and the delay length adjustment unit 50 adjusts the reference signal delay length by changing the positions of the mirrors 52 and 53 as indicated by the arrows A3 and A4.

[0023] Specifically, the delay length adjustment unit 50 shortens the reference signal delay length by moving the mirror 52 closer to the mirror 51 and the mirror 53 closer to the mirror 54. Alternatively, the delay length adjustment unit 50 lengthens the reference signal delay length by moving the mirror 52 away from the mirror 51 and the mirror 53 away from the mirror 54.

[0024] For example, the delay length adjustment unit 50 includes a rail-like member (not shown), and the mirrors 52 and 53 are arranged on the rail-like member. The delay length adjustment unit 50 includes a mechanism that combines a motor, gears, etc. (not shown), and the positions of the mirrors 52 and 53 are changed by sliding the mirrors 52 and 53 on the rail-like member using this mechanism.

[0025] In addition, when the reference signal delay length is adjusted before the shipment of the optical interferometer, the positions of the mirrors 52 and 53 may be adjusted manually. For example, the mirrors 52 and 53 may be manually slid on the above-mentioned rail-like member, and the mirrors 52 and 53 may be fixed after the reference signal delay length is adjusted. In addition, for example, the mirrors 52 and 53 may be fixed with screws, a shim plate may be sandwiched between the mirrors 52 and 53 and the fixed member, and the positions of the mirrors 52 and 53 may be changed by changing the thickness of the shim plate. The reference signal generated by the branching unit 30 passes through the delay length adjustment unit 50 and is input to the interference unit 60.

[0026] The interference unit 60 generates an interference signal by causing interference between the measurement signal and the reference signal received by the transmission / reception unit 40. The interference unit 60 includes a coupler or the like that merges two waveguides, and is provided to merge a waveguide connected from the branching unit 30 via the delay length adjustment unit 50 with a waveguide connected from the end face of the substrate 100 facing the transmission / reception unit 40, and to multiplex the light propagating through these waveguides. The waveguides merged by the interference unit 60 are connected to the light receiving unit 70, and the interference signal is input to the light receiving unit 70.

[0027] The light receiving unit 70 converts the interference signal generated by the interference unit 60 into an electric signal, and is composed of a photodetector, etc. The light receiving unit 70 is connected to the signal processing unit 80 by electric wiring formed on the substrate 100, and the output signal of the light receiving unit 70 is input to the signal processing unit 80.

[0028] The signal processing unit 80 measures the distance to an object that reflects the measurement signal in the external space of the housing 10, based on the interference signal converted into an electric signal. The signal processing unit 80 is composed of an IC and the like arranged on the substrate 100.

[0029] The following describes FMCW distance measurement performed by the signal processor 80. The light source 20 outputs an optical signal whose frequency changes over time. Here, we will explain the case where the frequency of this optical signal changes in a triangular wave shape that repeats monotonous increase and decrease.

[0030] In the FMCW method, a beat signal generated by interference between a received signal and a reference signal is detected, and the distance and speed of the object are calculated based on the frequency of the beat signal. The first received signal is phase shifted with respect to the reference signal depending on the distance between the optical interferometer and the object, and is Doppler shifted depending on the relative speed of the object with respect to the optical interferometer. The frequency of the beat signal reflects the frequency difference between the reference signal and the first received signal due to the phase shift and Doppler shift. The signal processing unit 80 converts the signal transmitted from the light receiving unit 70 into the frequency domain to detect the frequency of the beat signal, and calculates the distance and speed of the object using the detected frequency.

[0031] In Figure 3, the solid line indicates the frequency of the reference signal. When the measurement signal is reflected by an object whose relative speed to the optical interferometer is zero, the frequency of the first received signal becomes as shown by the dashed and dotted line. That is, the first received signal has a phase delay with respect to the reference signal depending on the distance between the optical interferometer and the object. When the measurement signal is reflected by an object approaching the optical interferometer, the frequency of the first received signal becomes higher than the reference signal, as shown by the dashed and dotted line.

[0032] When the relative velocity of the detected object is zero, the beat frequency f beat is expressed as Equation 1.

[0033]

number

[0034] When an object has a relative velocity to the optical interferometer, the Doppler shift f d is expressed as Equation 2.

[0035]

number

[0036] The beat signal generated by the interference unit 60 has a beat frequency f up , f down The component of beat frequency f is detected. up is the frequency difference between the reference signal and the received signal when the frequencies of the reference signal and the received signal are monotonically increasing. The beat frequency f down is the frequency difference between the reference signal and the received signal when the frequencies of the reference signal and the received signal are monotonically decreasing. diff is the beat frequency f beat and Doppler shift f d is a parameter corresponding to the beat frequency f beat and Doppler shift f d is the beat frequency f up , f down It is calculated as follows using:

[0037]

number

[0038]

number

[0039]

number

[0040]

number

[0041] The intensity of the beat signal generated by the interference between the first received signal and the reference signal is as follows. Let E Signal , E Local Then, the light energy E Signal , E Local is expressed as Equation 7 and Equation 8.

[0042]

number

[0043]

number

[0044]

number

[0045] If the optical energy of the interference signal between the first received signal and the reference signal is E1, then E1(t) = E Signal (t)+E Local Since the frequency of the beat signal is (t), the intensity of the beat signal detected by the signal processing unit 80 is expressed by the following formula 10.

[0046]

number

[0047]

number

[0048]

number

[0049] In contrast, in Equation 12, by setting the frequency of the cosine wave component, which is the differential component of the phase noise, to 0, that is, dis L =dis B As f L =f B It is believed that noise due to backscattering can be reduced by doing so. L Dis B In this embodiment, the signal processing unit 80 transforms the beat signal into the frequency domain, and sets the reference signal delay length based on the slope shape of the spectrum obtained by transforming the beat signal into the frequency domain.

[0050] Specifically, in the first adjustment after starting the optical interferometer, the signal processing unit 80 sets the reference signal delay length to a distance longer or shorter than the current distance, and transmits a signal corresponding to the set distance to the delay length adjustment unit 50.

[0051] The delay length adjustment unit 50 changes the positions of the mirrors 52 and 53 in response to a signal transmitted from the signal processing unit 80. That is, when the signal processing unit 80 sets the reference signal delay length to a distance longer than the current one, the delay length adjustment unit 50 moves the mirrors 52 and 53 so that the optical path length in the signal processing unit 80 becomes longer than the current one. On the other hand, when the signal processing unit 80 sets the reference signal delay length to a distance shorter than the current one, the delay length adjustment unit 50 moves the mirrors 52 and 53 so that the optical path length in the signal processing unit 80 becomes shorter than the current one.

[0052] The delay length adjustment unit 50 moves the mirrors 52 and 53, thereby changing the reference signal delay length and changing the spectral shape of the beat signal. In the second and subsequent adjustments, the signal processing unit 80 sets the reference signal delay length based on the slope height of the noise floor after the spectral shape of the beat signal has changed and the previous slope height of the noise floor, i.e., before the spectral shape of the beat signal changed. The slope height here is the difference between the amplitude value or power in a specific frequency band of the average spectrum and the ideal noise floor level.

[0053] When the slope height after the change is larger than before the change, the signal processing unit 80 sets the reference signal delay length to a distance changed in the opposite direction from the previous time, and transmits a signal corresponding to the set distance to the delay length adjustment unit 50. On the other hand, when the slope height after the change is smaller than before the change, the signal processing unit 80 sets the reference signal delay length to a distance changed in the same direction as the previous time, and transmits a signal corresponding to the set distance to the delay length adjustment unit 50. The amount of change in the reference signal delay length is set according to, for example, the absolute value of the difference between the current slope height and the previous slope height.

[0054] The spectrum of the beat signal is, for example, as shown in Fig. 6. The average of such a spectrum is, for example, as shown in Fig. 7. The solid line in Fig. 7 indicates the average of the spectrum of the beat signal including the actual noise floor, and the dashed line indicates the ideal noise floor level. The reference signal delay length is set based on the slope height indicated by the arrow A5, for example.

[0055] In this way, the signal processing unit 80 sets a new reference signal delay length based on the change in the interference signal relative to the change in the reference signal delay length. The signal processing unit 80 repeats this process to bring the slope height of the spectrum closer to 0. This adjustment process of the reference signal delay length may be performed before or after the shipment of the optical interferometer. When the adjustment process of the reference signal delay length is performed after the shipment, for example, the ranging process and the adjustment process of the reference signal delay length are alternately performed while the optical interferometer is running.

[0056] As shown in FIG. 8, the transmitting / receiving unit 40 scans a predetermined detection range by changing the angle of the emitted light and irradiating it on the optical window 11. The delay length and the amount of light of the backscattering at the optical window 11 vary depending on the angle of the emitted light. For example, in the emitted light shown by the arrows A6 to A10 in FIG. 8, the delay length of the emitted light shown by the arrows A6 and A10 is the longest, and the delay length of the emitted light shown by the arrow A8 is the shortest. The amount of backscattering light is the maximum for the emitted light shown by the arrows A8, and the minimum for the emitted light shown by the arrows A6 and A10. When the slope height of the noise floor is brought close to 0 by the signal processing unit 80, the reference signal delay length is adjusted in accordance with the delay length of the emitted light with a large amount of light. For example, in FIG. 8, the reference signal delay length is adjusted in accordance with the delay length of the emitted light shown by the arrow A8.

[0057] As described above, in this embodiment, the delay length adjustment unit 50 can adjust the reference signal delay length to approach the delay length of the backscattered light, thereby reducing the effect of backscattering on the interference signal and suppressing an increase in the noise floor.

[0058] Furthermore, according to the above embodiment, the following effects can be obtained.

[0059] (1) A signal processing unit 80 is provided that sets the reference signal delay length based on a change in the interference signal relative to a change in the reference signal delay length. This makes it possible to efficiently bring the reference signal delay length closer to the delay length of the backscattered light.

[0060] (2) The signal processor 80 sets the reference signal delay length based on the slope of the spectrum obtained by converting the interference signal into the frequency domain, which makes it possible to directly control the noise floor level.

[0061] (3) The signal processing unit 80 sets the reference signal delay length so that the slope height of the spectrum obtained by transforming the interference signal into the frequency domain approaches 0. This makes it possible to reduce the noise floor to near the theoretical value.

[0062] Second embodiment The second embodiment will be described. This embodiment is different from the first embodiment in that the method of adjusting the reference signal delay length is changed, and other aspects are the same as the first embodiment, so only the differences from the first embodiment will be described.

[0063] The signal processor 80 of this embodiment sets the reference signal delay length based on the center frequency of the interference signal. Specifically, when the center frequency of the spectrum of the beat signal is detected as shown in Fig. 9, the reference signal delay length is set so as to bring this center frequency closer to DC, i.e., 0 Hz, as shown by the arrow A11.

[0064] For example, in the first adjustment after starting up the optical interferometer, the signal processing unit 80 sets the reference signal delay length to a distance longer or shorter than the current distance, and transmits a signal according to the set distance to the delay length adjustment unit 50. Then, the delay length adjustment unit 50 changes the positions of the mirrors 52 and 53 according to the signal transmitted from the signal processing unit 80.

[0065] The delay length adjustment unit 50 moves the mirrors 52 and 53, thereby changing the reference signal delay length and changing the center frequency of the spectrum of the beat signal. If the changed center frequency is higher than the previous one, the signal processing unit 80 sets the reference signal delay length to a distance changed in the opposite direction from the previous one. On the other hand, if the center frequency is lower than the previous one, the signal processing unit 80 sets the reference signal delay length to a distance changed in the same direction as the previous one. By repeating this process, the signal processing unit 80 brings the center frequency of the beat signal closer to 0 Hz.

[0066] Note that the higher the chirp rates of the reference signal and the measurement signal, the higher the beat frequency becomes, and the higher the sensitivity of the beat frequency to the delay time becomes. Therefore, by increasing the chirp rate, the adjustment accuracy of the reference signal delay length can be improved. Therefore, when adjusting the reference signal delay length, the chirp rate of the optical signal output from the light source 20 may be set higher than that during distance measurement.

[0067] That is, the chirp rates during distance measurement and adjustment are FMW1 and FMW2, respectively, and the beat frequencies during distance measurement and adjustment are f beat1 , f beat2 If FMW2>FMW1, as shown in Figs. 10 and 11, beat2 >f beat1 For example, if FMW2 = 2FMW1, then f beat2 =2f beat1 In Fig. 10, the solid line and dashed line respectively indicate the frequency of the reference signal and the received signal during distance measurement, and the dashed line and the double-dashed line respectively indicate the frequency of the reference signal and the received signal during chirp rate adjustment.

[0068] This embodiment has the same configuration and operation as the first embodiment, and therefore can obtain the same effects as the first embodiment.

[0069] Furthermore, according to the above embodiment, the following effects can be obtained.

[0070] (1) When the signal processor 80 sets the reference signal delay length, the light source 20 increases the rate of change of the frequency of the output optical signal compared to when the signal processor 80 measures distance. This improves the accuracy of adjusting the reference signal delay length.

[0071] (2) The signal processing unit 80 sets the reference signal delay length based on the center frequency of the interference signal. Since the center frequency corresponds to the optical path length, the required delay amount can be directly derived by referring to the center frequency.

[0072] (3) The signal processing unit 80 sets the reference signal delay length so that the center frequency approaches 0 Hz. This makes it possible to control the time difference between the second received signal and the reference signal by the center frequency corresponding to the optical path length.

[0073] Third embodiment The third embodiment will be described. This embodiment is different from the first embodiment in that the configuration of the delay length adjustment unit 50 is changed, and other aspects are the same as those of the first embodiment, so only the differences from the first embodiment will be described.

[0074] 12, the delay length adjustment unit 50 of this embodiment includes an optical phase shifter 55 in front of mirrors 51 to 54, and an input optical signal passes through the optical phase shifter 55 and is then reflected by the mirrors 51 to 54 and output. The delay length adjustment unit 50 adjusts the reference signal delay length by changing the amount of phase shift by the optical phase shifter 55 in addition to the positions of the mirrors 52 and 53 in response to a signal from the signal processing unit 80.

[0075] In this embodiment, the optical phase shifter 55 is formed in the optical IC 90. However, when the propagation path of the reference signal is configured with an optical fiber, the optical phase shifter 55 may be formed in this optical fiber.

[0076] This embodiment has the same configuration and operation as the first embodiment, and therefore can obtain the same effects as the first embodiment.

[0077] (Other embodiments) The present disclosure is not limited to the above-described embodiments, and can be modified as appropriate. The above-described embodiments are not unrelated to each other, and can be combined as appropriate, except when the combination is clearly impossible. In the above-described embodiments, it goes without saying that the elements constituting the embodiments are not necessarily essential, except when they are specifically stated to be essential or when they are clearly considered to be essential in principle. In the above-described embodiments, when the numbers, values, amounts, ranges, etc. of the components of the embodiments are mentioned, they are not limited to the specific numbers, except when they are specifically stated to be essential or when they are clearly limited to a specific number in principle. In the above-described embodiments, when the shapes, positional relationships, etc. of the components are mentioned, they are not limited to the shapes, positional relationships, etc., except when they are specifically stated to be essential or when they are clearly limited to a specific shape, positional relationship, etc. in principle.

[0078] In the third embodiment, the reference signal delay length may be adjusted based on the center frequency of the beat signal as in the second embodiment.

[0079] In the third embodiment, the delay length adjusting section 50 may not include the mirrors 51 to 54, and may adjust the reference signal delay length using only the optical phase shifter 55.

[0080] The transceiver 40 may be formed in the optical IC 90 together with the light source 20, etc. For example, the transceiver 40 may be configured with a diffraction grating formed in the optical IC 90, and light may be emitted from this diffraction grating. Alternatively, the transceiver 40 may be configured with a plurality of waveguides extending from the branching unit 30 to the end face of the optical IC 90, and light may be emitted from the end face of the plurality of waveguides.

[0081] The reference signal delay length may be adjusted to match the delay length of the backscattering component at a location other than the optical window 11. For example, the reference signal delay length may be adjusted to match the delay length of backscattering at the end face of the optical IC 90 or the substrate 100 from which the measurement signal is emitted. Furthermore, when the transceiver 40 is configured with a diffraction grating or a waveguide as described above, the reference signal delay length may be adjusted to match the delay length of backscattering at the emission face of the transceiver 40. Furthermore, when the transceiver 40 is configured to emit the measurement signal from an optical fiber, the reference signal delay length may be adjusted to match the delay length of backscattering at the fiber emission end. [Explanation of symbols]

[0082] 20 light source 30 Branch 40 Transmitter / receiver 50 Delay length adjustment section

Claims

1. 1. An optical interferometer comprising: A light source (20); A branching unit (30) that divides the optical signal output by the light source into a measurement signal and a reference signal; a transceiver unit (40) for transmitting the measurement signal to an external space and receiving the measurement signal returned from the external space; an interference unit (60) that generates an interference signal by causing interference between the measurement signal and the reference signal received by the transmission / reception unit; and a delay length adjustment section (50) that adjusts a reference signal delay length, which is a delay length of the reference signal from the branching section to the interference section.

2. 2. The optical interferometer according to claim 1, further comprising a signal processing unit (80) that sets the reference signal delay length based on a change in the interference signal relative to a change in the reference signal delay length.

3. The optical interferometer according to claim 2 , wherein the signal processing unit sets the reference signal delay length based on a slope shape of a spectrum obtained by converting the interference signal into a frequency domain.

4. The optical interferometer according to claim 3 , wherein the signal processing unit sets the reference signal delay length so that a slope height of the spectrum approaches zero.

5. The optical interferometer according to claim 2 , wherein the signal processing unit sets the reference signal delay length based on a center frequency of the interference signal.

6. The optical interferometer according to claim 5 , wherein the signal processing unit sets the reference signal delay length so that the center frequency approaches 0 Hz.

7. the light source outputs an optical signal whose frequency changes over time; The signal processing unit measures a distance to an object that reflects the measurement signal in the external space based on the interference signal, The optical interferometer according to claim 6 , wherein the light source increases a rate of change of the frequency when the signal processing unit sets the reference signal delay length compared to when the signal processing unit performs the distance measurement.

8. The optical interferometer of claim 1, wherein the delay length adjustment unit has an optical path through which an input optical signal is output after being reflected by a plurality of mirrors (51, 52, 53, 54), and changes the reference signal delay length by changing the positions of at least some of the plurality of mirrors.

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