Apparatus and method for measuring frequency sweep amount of optical frequency comb

Abstract

To provide a frequency sweep amount measuring device for an optical frequency comb that reduces its size and allows measurement of an amount of all comb mode frequency sweeps at low cost, and a frequency sweep amount measuring method for the same.SOLUTION: A frequency sweep amount measuring device for an optical frequency comb comprises: an unbalanced Mach-Zehnder interferometer that outputs output light in which an optical frequency comb and an optical frequency comb delayed by a time τ are caused to interfere; two band-pass filters that transmit an N-th comb mode and an M-th comb mode, respectively; two photodetectors that detect light output from the two band-pass filters; and a signal processing apparatus that calculates an amount of comb mode frequency sweep from signals from the two photodetectors. The signal processing apparatus calculates the amount of change in an N-th comb mode frequency δφN and the amount of change in an M-th comb mode frequency δφM according to relationships of δνN=1 / τ×(δφN / 2π) and δνM=1 / τ×(δφM / 2π) by using the amount of change in the N-th comb mode δνN and the amount of change in the M-th comb mode δνM, in a phase of a waveform of the interference, and calculates an amount of sweep δνK in an arbitrary comb mode frequency.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an apparatus and method for measuring the frequency sweep amount of an optical frequency comb. [Background technology]

[0002] An optical frequency comb is a type of ultrashort pulse mode-locked laser, and has a comb-like spectrum that is discrete and evenly spaced. By treating each comb mode of an optical frequency comb as a CW laser light source and sweeping the comb mode frequency, it becomes possible to measure distances using light (Light Detection and Ranging, LiDAR), shape measurement, etc. (see, for example, Non-Patent Document 2).

[0003] The comb mode frequency can be swept by, for example, a method of sweeping excitation light from a CW laser (Non-Patent Document 1), a method using a microheater attached to a microresonator that generates an optical frequency comb (see Non-Patent Document 3), a method using a piezoelectric element attached to a resonator (see Non-Patent Document 4), a method of controlling the comb mode frequency with an electro-optic modulator (see Non-Patent Document 5), or a combination of these methods. A method of measuring the comb mode frequency sweep amount includes a method of measuring the comb mode frequency sweep amount by interfering a CW laser with a comb mode and measuring the frequency difference with a photodetector. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] N. Kuse,et al.,Optics Letters,45,pp.927-930(2020) [Non-Patent Document 2] J.Riemensberger.et al.,Nature,581,164(2020) [Non-Patent Document 3] C. Joshi, et al., Optics Letters, 41, pp. 2565-2568 (2016) [Non-Patent Document 4] J. Liu,et al.,Nature,583,pp.385-390(2020) [Non-Patent Document 5] C. Wang,et al.,Nature,562,pp.101-104(2018) Summary of the Invention [Problem to be solved by the invention]

[0005] When performing measurements such as LiDAR and shape measurement, it is necessary to measure the comb mode frequency sweep amount of all comb modes used. Conventional methods for measuring the comb mode frequency sweep amount, such as by interfering a CW laser with a comb mode and measuring the frequency difference, can only measure the frequency sweep amount of a single comb mode. In order to measure the comb mode frequency sweep amounts of all comb modes used, multiple conventional comb mode frequency sweep amount measuring devices must be prepared, which is large in size and costly.

[0006] In view of the above problems, an object of the present invention is to provide an apparatus and method for measuring the frequency sweep amount of an optical frequency comb that is capable of measuring all comb mode frequency sweep amounts while reducing size and cost. [Means for solving the problem]

[0007] A first aspect of the present invention is an apparatus for measuring a frequency sweep amount of an optical frequency comb, the apparatus comprising: an unbalanced Mach-Zehnder interferometer that inputs an optical frequency comb to be frequency swept and outputs output light obtained by interfering the optical frequency comb with an optical frequency comb delayed by a time τ; a first optical bandpass filter that transmits only the Nth comb mode of the output light and a second optical bandpass filter that transmits only the Mth comb mode (where N ≠ M); two photodetectors that detect the light output from the first optical bandpass filter and the second optical bandpass filter, respectively; and a signal processing device that calculates a comb mode frequency sweep amount of the optical frequency comb from signals from the two photodetectors, the signal processing device calculating a change amount δφ in the Nth comb mode of the phase of the waveform of the interference obtained by the signals from the two photodetectors. N and δφ in the Mth comb mode M Using δν N =1 / τ×(δφ N / 2π), δν M =1 / τ×(δφ M / 2π), the change in comb mode frequency δν N and δv M Calculate the change amount δν N and δv M The sweep amount of any comb mode frequency from δν K The gist of the present invention is to calculate the following:

[0008] In the first aspect of the present invention, the signal processing device calculates the change amount Δν of the comb mode frequency of the Nth comb mode and the Mth comb mode. N and δv M Using δν K = δν N +(KN) / (NM)×(δν N -δν M ) the sweep amount δν K may be calculated.

[0009] A second aspect of the present invention is a method for measuring a frequency sweep amount of an optical frequency comb, comprising the steps of: inputting an optical frequency comb into an unbalanced Mach-Zehnder interferometer, and outputting output light obtained by interfering the optical frequency comb with an optical frequency comb delayed by a time τ; independently outputting an Nth comb mode and an Mth comb mode of the output light; detecting the Nth comb mode and the Mth comb mode and converting them into electrical signals; and measuring an amount of change δφ in the Nth comb mode of the phase of the interference waveform obtained by the electrical signal by sweeping the comb mode frequency of the optical frequency comb. N and the change in the Mth comb mode, δφ M and measuring the measured change amount δφ N and the change amount δφ M Using δν N =1 / τ×(δφ N / 2π), δν M =1 / τ×(δφ M / 2π), the change in comb mode frequency δν N and δv M and calculating the calculated change amount δν N and δv M The sweep amount of any comb mode frequency from δν K The present invention also includes a step of calculating:

[0010] In the second aspect of the present invention, the calculated change amount Δν of the comb mode frequency between the Nth comb mode and the Mth comb mode is N and δv M Using δν K = δν N +(KN) / (NM)×(δν N -δν M ) the sweep amount δν K The method may further include the step of calculating: Effect of the Invention

[0011] According to the present invention, it is possible to provide an apparatus and method for measuring the frequency sweep amount of an optical frequency comb that can measure all comb mode frequency sweep amounts while reducing the size and cost. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing an example of the configuration of an apparatus for measuring a frequency sweep amount of an optical frequency comb according to an embodiment of the present invention; [Diagram 2] 13 is a graph showing the time change of an interference signal observed by a photodetector after passing through MZIs of the Nth comb mode and the Mth comb mode. [Diagram 3] 11 is a graph showing frequency sweep amounts of the 0th and −16th comb modes obtained by actual measurement using the frequency sweep amount measurement device according to the present embodiment. [Figure 4] 13 is a graph showing the frequency sweep amount of the fourth comb mode obtained by actual measurement and calculation using the frequency sweep amount measurement device according to the present embodiment. [Diagram 5] Figure 4 shows the difference between the measured and calculated frequency sweeps of the fourth comb mode. [Figure 6] 1 is a flowchart illustrating a method for measuring a frequency sweep amount of an optical frequency comb according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Next, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings relating to the embodiment, the same or similar parts are given the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each member, etc. are different from the actual ones. Therefore, the specific thickness and dimensions should be determined with reference to the following description. In addition, it goes without saying that the drawings include parts with different dimensional relationships and ratios.

[0014] Furthermore, the embodiments are merely examples of devices and methods for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention does not specify the configuration, arrangement, layout, etc. of each component to those described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims.

[0015] (Embodiment) An apparatus for measuring the frequency sweep amount of an optical frequency comb according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 shows an example of the configuration of an apparatus for measuring the frequency sweep amount of an optical frequency comb according to an embodiment of the present invention. As shown in Fig. 1, an apparatus 10 for measuring the frequency sweep amount of an optical frequency comb according to this embodiment includes an interferometer 101, optical bandpass filters (BPFs) 102 and 103, photodetectors 104 and 105, optical directional couplers 106 and 107, analog-to-digital converters (ADCs) 117 and 118, and a signal processing device 119.

[0016] The interferometer 101 is a two-wavelength unbalanced Mach-Zehnder interferometer, and is composed of a first optical fiber 108, a second optical fiber 109, a first optical directional coupler 106, and a second optical directional coupler 107, which have different optical path lengths. The optical path length difference between the first optical fiber 108 and the second optical fiber 109 is assumed to be known. Although Fig. 1 shows the interferometer 101 composed of optical fibers, it may be composed of space or an optical waveguide.

[0017] The optical frequency comb 110 may be any type of optical frequency comb, such as one generated by an optical fiber, one generated by an electro-optical modulator, one generated by a micro-resonator, etc. In addition, the method for sweeping the comb mode frequency of the optical frequency comb 110 may be any method, such as a method using excitation light from a CW laser (see Non-Patent Document 1), a method using a micro-heater (see Non-Patent Document 3), a method using a piezoelectric element (see Non-Patent Document 4), a method using an electro-optical modulator (see Non-Patent Document 5), or a combination of these.

[0018] The optical frequency comb 110 propagates through the optical fiber 116 and enters the interferometer 101, then splits into two at the first optical directional coupler 106, and propagates through the first optical fiber 108 and the second optical fiber 109, which have different optical path lengths, in the interferometer 101. The optical frequency comb 112 that propagates through the second optical fiber 109 is delayed by a time τ, and then merges with the optical frequency comb 111 that propagates through the first optical fiber 108 at the second optical directional coupler 107, and is output as output light 113. The optical frequency comb 110, the optical frequency comb 111, and the optical frequency comb 112 are all the same light, but the optical frequency comb 112 is delayed by the time τ compared to the optical frequency comb 111, and the output light 113 is light resulting from interference between the frequency comb 111 and the optical frequency comb 112.

[0019] In the second optical directional coupler 107, the optical path is branched into two, and a first optical bandpass filter (first BPF) 102 and a second optical bandpass filter (second BPF) 103 are connected to each of the two branched optical paths. The first optical bandpass filter 102 and the second optical bandpass filter 103 are set to transmit only different comb modes of the output light 113. In this embodiment, the first optical bandpass filter 102 extracts an N-th comb mode 114 from the output light 113, and the second optical bandpass filter 103 extracts an M-th comb mode 115 from the output light 113 (where N≠M). That is, the first optical bandpass filter 102 passes only the output light in a frequency band corresponding to the N-th comb mode 114. The second optical bandpass filter 103 passes only the output light in a frequency band corresponding to the M-th comb mode 115.

[0020] The first photodetector 104 and the second photodetector 105 detect interference signals of the Nth comb mode 114 and the Mth comb mode 115. The signals obtained by the first photodetector 104 and the second photodetector 105 are digitized by analog-to-digital converters (ADCs) 117 and 118, and a signal processor 119 performs the following calculation processing.

[0021] The first photodetector 104 and the second photodetector 105 measure the time change of the interference signal between the Nth comb mode 114 and the Mth comb mode 115. The output light 113 is a superposition of the optical frequency comb 111 and the optical frequency comb 112 delayed by time τ. Therefore, when the time change of the interference signal between the Nth comb mode 114 and the Mth comb mode 115 is plotted on a graph as in FIG. 2, the occurrence of interference can be observed.

[0022] When the comb mode frequency of the optical frequency comb 110 is shifted, the frequencies of all comb modes of the optical frequency comb 110 are shifted simultaneously, and the phase of the interference waveform shown in Fig. 2 changes in response to the change in the comb mode frequency. As described below, the amount of shift in the comb mode frequency can be calculated by measuring the change in phase of the interference waveform shown in Fig. 2 before and after shifting the comb mode frequency of the optical frequency comb 110.

[0023] Let the comb mode frequency of the Nth comb mode 114 be ν N , the comb mode frequency of the Mth comb mode 115 is ν M The amount of sweep of the comb mode frequency of the Nth comb mode when the comb mode frequency of the optical frequency comb 110 is shifted is expressed as δν N , the sweep amount of the comb mode frequency of the Mth comb mode is δν M Then, δν N , δν M can be calculated from the following formula:

[0024]

number

[0025]

number

[0026] Here, the comb mode frequency ν of the nth comb mode of the optical frequency comb is n teeth,

[0027]

number

[0028]

number

[0029]

number

[0030]

number

[0031]

number

[0032] Figure 3 shows the frequency sweep amount of the 0th and -16th comb modes obtained by actual measurement. Here, the 0th and -16th comb modes are the orders of the comb modes when counting the excitation light from the CW laser as the reference. Figure 4 shows the frequency sweep amount of the -4th comb mode obtained by actual measurement and calculation using Equation 7. The vertical axis of the graphs shown in Figures 3 and 4 shows the frequency sweep amount, and the horizontal axis shows the time it takes to reach the frequency after sweeping the frequency. In Figure 4, there is no difference between the measured and calculated values ​​of the frequency sweep amount of the -4th comb mode. Figure 5 shows the difference between the measured and calculated values ​​of the frequency sweep amount of the -4th comb mode shown in Figure 4. For a frequency sweep amount of about 60 GHz, the error is about 10 MHz, and it can be said that the sweep amount of any comb mode frequency can be obtained using Equation 7.

[0033] An example of a method for measuring the frequency sweep amount of an optical frequency comb according to this embodiment will be described with reference to the flowchart shown in FIG.

[0034] In step S601, optical frequency comb 110 is input to unbalanced Mach-Zehnder interferometer 101, and output light 113 is output, which is produced by interfering optical frequency comb 111 with optical frequency comb 112 delayed by time τ.

[0035] In step S602, the Nth comb mode 114 and the Mth comb mode 115 of the output light 113 are output independently using the optical bandpass filters 102 and 103, respectively.

[0036] In step S603, the Nth comb mode and the Mth comb mode are detected by the photodetectors 104 and 105 and converted into electrical signals.

[0037] In step S604, the amount of change Δφ in the phase of the interference waveform in the Nth comb mode due to sweeping the comb mode frequency of the optical frequency comb 110 is N and δφ of the phase of the interference waveform in the Mth comb mode M Measure.

[0038] In step S605, the measured change amount Δφ N and δφ M Using the above, the change in comb mode frequency δν N and δv M Calculate.

[0039] In step S606, the calculated change amount Δν N and δv M Using the above, the sweep amount δν of an arbitrary comb mode frequency is calculated according to the relationship in Equation 7. K Calculate.

[0040] The frequency sweep amount of any comb mode of an optical frequency comb can be calculated by measuring the frequency sweep amount of two comb modes among the multiple comb modes of the optical frequency comb. Any two comb modes can be used to measure the frequency sweep amount, but the greater the difference between the measured and calculated frequency sweep amounts of any comb mode, as shown in Figure 4, the smaller the difference between the two comb modes.

[0041] As described above, the present invention naturally includes various embodiments not described here. Therefore, the technical scope of the present invention is defined only by the invention-specifying matters related to the scope of the claims appropriate from the above description. [Explanation of symbols]

[0042] 10. Frequency sweep measurement device 101 Interferometer 102 First optical bandpass filter 103 Second Optical Bandpass Filter 104 First photodetector 105 Second Photodetector 106 1st optical directional coupler 107 Second optical directional coupler 108 First Optical Fiber 109 Second Optical Fiber 110, 111, 112 Optical frequency comb 113 Output light 114 Nth Comb Mode 115 Mth Com Mode 116 Optical Fiber 117, 118 Analog-to-digital converter 119 Signal Processing Device

Claims

1. An unbalanced Mach-Zehnder interferometer that receives an optical frequency comb to be frequency swept and outputs an output light obtained by interfering the optical frequency comb with the optical frequency comb delayed by a time τ; a first optical bandpass filter that transmits only the N-th comb mode of the output light and a second optical bandpass filter that transmits only the M-th comb mode of the output light (where N≠M); two photodetectors for detecting light output from the first optical bandpass filter and the second optical bandpass filter, respectively; a signal processing device that calculates a comb mode frequency sweep amount of the optical frequency comb from signals from the two photodetectors; The signal processing device is provided with a change amount δφ in the phase of the waveform of the interference obtained by the signals from the two photodetectors in the N-th comb mode. Ｎ and δφ in the Mth comb mode Ｍ Using [0010] [0025] The change in comb mode frequency δν Ｎ and δv Ｍ Calculate the change amount δν Ｎ and δv Ｍ The sweep amount of any comb mode frequency from δν Ｋ The frequency sweep amount measuring device for an optical frequency comb is characterized by calculating

2. The signal processing device calculates the change amount δν Ｎ and δv Ｍ Using [0030] According to the relationship, the sweep amount δν Ｋ 2. The apparatus for measuring a frequency sweep amount of an optical frequency comb according to claim 1, wherein:

3. inputting an optical frequency comb into an unbalanced Mach-Zehnder interferometer and outputting output light in which the optical frequency comb and the optical frequency comb delayed by a time τ are interfered with each other; outputting the N-th comb mode and the M-th comb mode of the output light independently; detecting and converting the Nth comb mode and the Mth comb mode into electrical signals; The amount of change δφ in the phase of the interference waveform obtained by the electrical signal in the Nth comb mode by sweeping the comb mode frequency of the optical frequency comb. Ｎ and the change amount δφ in the Mth comb mode Ｍ and measuring The measured change amount δφ Ｎ and the change amount δφ Ｍ Using [0045] [0050] The change in comb mode frequency δν Ｎ and δv Ｍ and calculating The calculated change amount δν Ｎ and δv Ｍ The sweep amount of any comb mode frequency from δν Ｋ and calculating A method for measuring a frequency sweep amount of an optical frequency comb, comprising:

4. The calculated change amount δν of the comb mode frequency of the Nth comb mode and the Mth comb mode Ｎ and δv Ｍ Using [006] According to the relationship, the sweep amount δν Ｋ 4. The method for measuring a frequency sweep amount of an optical frequency comb according to claim 3, further comprising the step of calculating:

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