Measurement device and measurement method

Abstract

A measurement device (201) comprises: a first mode-locked laser light source (11) that emits first output light; a first frequency shifter (71) that sets a repetition frequency of the first output light to a first drive frequency; a detector (51) on which the first output light is incident; and a feedback circuit (61) that determines a set value of the first drive frequency on the basis of a first signal output from the detector (51) when the first mode-locked laser light source (11) is driven while sweeping the first drive frequency by means of the first frequency shifter (71).

Description

Measuring device and measuring method

[0001] The present disclosure relates to a measurement device and a measurement method.

[0002] Patent Document 1 discloses an optical frequency comb device that includes a gain medium and a saturable absorber disposed between two mirrors arranged on an optical waveguide. The optical frequency comb device disclosed in Patent Document 1 includes a control unit that fixes one of the repetition rate and the carrier envelope offset frequency of light output from the end of the optical waveguide and changes the other.

[0003] International Publication No. 2022 / 118647

[0004] To stabilize the repetition frequency, it is necessary to supply a signal having a drive frequency equal to the repetition frequency to the light source. However, since the repetition frequency also changes when the drive conditions of the light source, such as the current value, change, it is necessary to check the repetition frequency each time before measurement. Checking the repetition frequency requires components that are not required for measurement, such as an RF (Radio Frequency) spectrum analyzer, which complicates the configuration of the measurement device.

[0005] Therefore, the present disclosure provides a measurement device and a measurement method that can perform measurements with high accuracy using a simple configuration.

[0006] A measurement device according to one aspect of the present disclosure includes a first mode-locked laser light source that emits first output light, a first frequency shifter that sets a repetition rate of the first output light to a first drive frequency, a detector onto which the first output light is incident, and a feedback circuit that determines a set value of the first drive frequency based on a first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency using the first frequency shifter.

[0007] A measurement method according to one aspect of the present disclosure includes sweeping a drive frequency using a frequency shifter that sets the repetition frequency of output light emitted by a mode-locked laser light source to the drive frequency; driving the mode-locked laser light source while sweeping the drive frequency to cause the output light to be incident on a detector; and determining a set value of the drive frequency based on a signal output from the detector.

[0008] Furthermore, one aspect of the present disclosure can be realized as a program that causes a computer to execute the measurement method, or as a computer-readable non-transitory recording medium storing the program.

[0009] According to the present disclosure, accurate measurements can be performed with a simple configuration.

[0010] FIG. 1A is a diagram schematically illustrating the time change of the electric field of the optical comb laser light. FIG. 1B is a diagram schematically illustrating the frequency spectrum of the optical comb laser light. FIG. 2 is a diagram schematically illustrating the frequency spectra of two optical comb laser lights and interference light in a dual comb. FIG. 3A is a top view schematically illustrating a laser light source provided in a measurement device according to an embodiment. FIG. 3B is a cross-sectional view of the laser light source shown in FIG. 3A in the xz section. FIG. 4A is a diagram illustrating the frequency spectrum of the optical comb laser light. FIG. 4B is a diagram illustrating the frequency spectrum of the optical comb laser light when HML (Hybrid Mode Locking) is performed. FIG. 5A is a diagram illustrating the frequency spectrum of the optical comb laser light when the intensity of the high-frequency signal provided by HML is low. FIG. 5B is a diagram illustrating the frequency spectrum of the optical comb laser light when the intensity of the high-frequency signal provided by HML is high. FIG. 6A is a diagram for explaining the pulse interval of the interference light. FIG. 6B is a diagram illustrating the pulse interval of the interference light when the difference between the repetition frequency and the drive frequency is large when HML is not performed. FIG. 6C is a diagram illustrating the pulse interval of the interference light when the difference between the repetition frequency and the drive frequency is small when HML is not performed. FIG. 7 is a diagram illustrating the relationship between the difference between the repetition frequency and the drive frequency and the pulse interval when HML is not performed. FIG. 8 is a diagram illustrating the relationship between the difference between the repetition frequency and the drive frequency and the light intensity when HML is not performed. FIG. 9 is a diagram illustrating an example of the configuration of the measurement apparatus according to the first embodiment. FIG. 10 is a diagram illustrating another example of the configuration of the measurement apparatus according to the first embodiment. FIG. 11 is a flowchart illustrating the operation of the measurement apparatus according to the first embodiment. FIG. 12 is a diagram illustrating an example of the configuration of the measurement apparatus according to the second embodiment. FIG. 13 is a diagram illustrating another example of the configuration of the measurement apparatus according to the second embodiment. FIG. 14 is a diagram illustrating another example of the configuration of the measurement apparatus according to the second embodiment. FIG. 15 is a flowchart illustrating an example of the operation of the measurement apparatus according to the second embodiment. FIG. 16 is a flowchart illustrating another example of the operation of the measurement apparatus according to the second embodiment. Fig. 17 is a flowchart showing another example of the operation of the measuring apparatus according to embodiment 2. Fig. 18 is a flowchart showing another example of the operation of the measuring apparatus according to embodiment 2.Fig. 19 is a flowchart showing another example of the operation of the measuring apparatus according to embodiment 2. Fig. 20 is a flowchart showing another example of the operation of the measuring apparatus according to embodiment 2.

[0011] (Summary of the Present Disclosure) (First Aspect) A measurement device according to a first aspect of the present disclosure includes a first mode-locked laser light source that emits first output light, a first frequency shifter that sets a repetition frequency of the first output light to a first drive frequency, a detector into which the first output light is incident, and a feedback circuit that determines a set value of the first drive frequency based on a first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency by the first frequency shifter.

[0012] As a result, by sweeping the first drive frequency, the value of the repetition frequency of the first output light can be confirmed based on the first signal output from the detector. By confirming the value of the repetition frequency of the first output light, an appropriate setting value for the first drive frequency when performing HML can be determined, thereby improving measurement accuracy. Since an RF spectrum analyzer is not required, the configuration can be simplified. Therefore, according to this aspect, a measurement device that can perform accurate measurements with a simple configuration can be realized.

[0013] (Second Aspect) The measurement device according to the first aspect of the present disclosure may further include a second mode-locked laser light source that emits second output light, and a second frequency shifter that sets the repetition frequency of the second output light to a second drive frequency, the second output light may further be incident on the detector, and the feedback circuit may further determine a setting value for the second drive frequency based on the first signal.

[0014] This allows the value of the repetition frequency of the second output light to be confirmed based on the first signal output from the detector. By confirming the value of the repetition frequency of the second output light, an appropriate setting value for the second drive frequency when performing HML can be determined, thereby improving measurement accuracy. Since an RF spectrum analyzer is not required, the configuration can be simplified.

[0015] (Third Aspect) In the measurement device according to the second aspect of the present disclosure, the first signal may be a signal representing intensity of interference light generated by interference between the first output light and the second output light.

[0016] As a result, the frequency of the interference light is several orders of magnitude lower than the frequency of the output light before interference, making detection easier. For example, the detector can detect not only the light intensity but also the time interval between pulses. Furthermore, the interference light generated by the interference between the first output light and the second output light can be used for measurement, making detection easier and improving measurement accuracy. Furthermore, the time required for measurement can be shortened.

[0017] (Fourth Aspect) In the measurement device according to the second or third aspect of the present disclosure, the first mode-locked laser light source and the second mode-locked laser light source may be integrated on different semiconductor substrates.

[0018] This allows the measurement device to be made smaller.

[0019] (Fifth Aspect) In the measurement device according to the second or third aspect of the present disclosure, the first mode-locked laser light source and the second mode-locked laser light source may be integrated on the same semiconductor substrate.

[0020] This allows the measurement device to be further miniaturized.

[0021] (Sixth Aspect) In the measurement device according to any one of the first to fifth aspects of the present disclosure, the first frequency shifter may include a saturable absorber and a signal generator that supplies a radio frequency signal to the saturable absorber, the frequency of the radio frequency signal may be the first drive frequency, and the first mode-locked laser light source may include a resonator that includes a gain medium and the saturable absorber.

[0022] This makes it possible to easily sweep the drive frequency by adjusting the frequency of the high frequency signal.

[0023] (Seventh Aspect) In the measurement device according to the sixth aspect of the present disclosure, after the feedback circuit determines the set value of the first drive frequency, the signal generator may supply the high-frequency signal having a frequency of the set value of the first drive frequency determined by the feedback circuit, with an intensity higher than an intensity of the high-frequency signal during the sweep of the first drive frequency.

[0024] By reducing the intensity of the high-frequency signal during the sweep of the first drive frequency, the frequency range in which HML is possible and that includes the repetition frequency is narrowed, thereby enabling the repetition frequency to be determined with high accuracy.

[0025] (Eighth Aspect) In the measurement device according to any one of the first to seventh aspects of the present disclosure, the first mode-locked laser light source may be an optical frequency comb laser light source.

[0026] As a result, the optical frequency comb laser light source emits output light with a highly stable repetition frequency, thereby improving measurement accuracy and long-term measurement reliability.

[0027] (Ninth Aspect) The measurement device according to the first aspect of the present disclosure may further include a second mode-locked laser light source that emits second output light, and a second frequency shifter that sets the repetition frequency of the second output light to a second drive frequency, and the second drive frequency may be fixed.

[0028] By fixing the second drive frequency, the configuration of the measurement device can be simplified and the cost can be reduced.

[0029] (Tenth Aspect) In the measurement device according to any one of the first to ninth aspects of the present disclosure, the feedback circuit may determine the set value of the first drive frequency based on intensity information of the first signal.

[0030] This eliminates the need for an RF spectrum analyzer, simplifying the configuration. Therefore, according to this aspect, a measurement device that can perform measurements with high accuracy with a simple configuration can be realized.

[0031] (Eleventh Aspect) In the measurement device according to the third aspect of the present disclosure, the feedback circuit may determine the set value of the first drive frequency and the set value of the second drive frequency based on time information of the first signal.

[0032] This eliminates the need for an RF spectrum analyzer and a high-speed photodetector, simplifying the configuration. Therefore, according to this aspect, a measurement device that can perform measurements with high accuracy with a simple configuration can be realized.

[0033] (Twelfth Aspect) In the measurement device according to any one of the second to fifth aspects of the present disclosure, the feedback circuit may confirm the value of the repetition frequency of the first output light based on the first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency, and then confirm the value of the repetition frequency of the second output light based on the second signal output from the detector when the second mode-locked laser light source is driven while sweeping the second drive frequency, and may determine the set value of the first drive frequency and the set value of the second drive frequency based on the value of the repetition frequency of the first output light and the value of the repetition frequency of the second output light.

[0034] This eliminates the need for an RF spectrum analyzer, simplifying the configuration. Therefore, according to this aspect, a measurement device that can perform measurements with high accuracy with a simple configuration can be realized.

[0035] (Thirteenth Aspect) In the measurement device according to the third aspect of the present disclosure, the feedback circuit may confirm the value of the repetition frequency of the first output light based on the first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency, and then detect a difference between the repetition frequency of the first output light and the repetition frequency of the second output light, and may determine the set value of the first drive frequency and the set value of the second drive frequency based on the difference.

[0036] This eliminates the need for an RF spectrum analyzer and a high-speed photodetector, simplifying the configuration. Therefore, according to this aspect, a measurement device that can perform measurements with high accuracy with a simple configuration can be realized.

[0037] (14th Aspect) In the measurement device according to any one of the second to fifth aspects of the present disclosure, the feedback circuit may determine the set value of the first drive frequency and the set value of the second drive frequency so that the difference between the repetition frequency of the first output light and the repetition frequency of the second output light becomes a predetermined value.

[0038] This eliminates the need for an RF spectrum analyzer, simplifying the configuration. Therefore, this aspect makes it possible to realize a measurement device that can perform accurate measurements with a simple configuration. Furthermore, by setting the difference in repetition frequency to a predetermined value, it becomes possible to change the measurement mode depending on the situation, such as shortening the measurement time and / or improving the measurement accuracy. This broadens the scope of use of the measurement device.

[0039] (Fifteenth Aspect) A measurement method according to a fifteenth aspect of the present disclosure includes sweeping a drive frequency by a frequency shifter that sets a repetition frequency of output light emitted by a mode-locked laser light source to the drive frequency, driving the mode-locked laser light source while sweeping the drive frequency to cause the output light to be incident on a detector, and determining a set value of the drive frequency based on a signal output from the detector.

[0040] This allows accurate measurements to be performed using a measurement device with a simple configuration.

[0041] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0042] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0043] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0044] Furthermore, in this specification, the numerical ranges are not expressions that express only the strict meaning, but also expressions that include a substantially equivalent range, for example, a difference of about several percent.

[0045] In this specification, the terms "above" and "below" do not refer to the upward direction (vertically upward) and downward direction (vertically downward) in absolute spatial recognition, but are used as terms defined by a relative positional relationship based on the stacking order in a stacked configuration. Furthermore, the terms "above" and "below" are used not only when two components are arranged with a gap between them and another component exists between them, but also when two components are arranged closely together and are in contact with each other.

[0046] In this specification, the x-axis, y-axis, and z-axis represent the three axes of a three-dimensional Cartesian coordinate system.

[0047] Furthermore, in this specification, ordinal numbers such as "first" and "second" do not refer to the number or order of components unless otherwise specified, but are used for the purpose of avoiding confusion and distinguishing between components of the same type.

[0048] [Optical Comb Laser] First, the time variation and frequency spectrum of the electric field of optical comb laser light will be described with reference to FIGS. 1A and 1B.

[0049] FIG. 1A is a diagram showing an example of the temporal change in the electric field of an optical comb laser beam. In FIG. 1A, the horizontal axis represents time, and the vertical axis represents the electric field of the optical comb laser beam. The optical comb laser beam is an example of output light emitted by a mode-locked laser light source and is also called an optical frequency comb laser beam. In this specification, the optical comb laser beam may be simply referred to as laser beam.

[0050] As shown in FIG. 1A, the optical comb laser light has a repetition period T ｒｅｐ It is formed from a train of optical pulses generated at a repetition period T ｒｅｐ is, for example, 1 ps or more and 100 ns or less. The full width at half maximum of each optical pulse is represented by Δt. The full width at half maximum of each optical pulse Δt is, for example, 10 fs or more and 100 ps or less.

[0051] In a laser resonator, the envelope of the light pulse propagates at a group velocity v ｇ and the phase velocity v of the waves propagating within the light pulse. ｐ The group velocity v ｇ and the phase velocity v ｐ Due to the difference between the wavelengths of adjacent optical pulses and the wavelength of the laser beam, when two adjacent optical pulses are overlapped so that their envelopes coincide, the phase of the waves in these optical pulses shifts by Δφ, which takes a value between 0 and 2π. The repetition period of the optical pulse train is T ｒｅｐ = L / v ｇ is expressed by

[0052] 1B is a diagram showing a frequency spectrum of the optical comb laser light, where the horizontal axis represents frequency and the vertical axis represents intensity of the optical comb laser light.

[0053] As shown in Figure 1B, the optical comb laser light has a comb-like frequency spectrum formed by a number of discrete equally spaced lines. The frequencies of the discrete equally spaced lines correspond to the resonant frequencies of the longitudinal modes in the laser resonator. The repetition frequency, which corresponds to the spacing between two adjacent equally spaced lines in the optical frequency comb, is f ｒｅｐ = 1 / T ｒｅｐ The repetition frequency f ｒｅｐis, for example, 10 MHz or more and 1 THz or less. The circumferential length L of the laser resonator is 30 cm, and the group velocity v ｇ is the speed of light in a vacuum (= 3 × 10 ８ m / s), the repetition period T ｒｅｐ becomes 1 ns, and the repetition frequency f ｒｅｐ becomes 1 GHz.

[0054] The full width at half maximum of the optical comb laser light is Δf=1 / Δt. The full width at half maximum Δf of the optical comb laser light is, for example, 10 GHz or more and 100 THz or less. Assuming that the equally spaced lines exist up to near zero frequency, the frequency of the equally spaced line closest to zero frequency is called the carrier envelope offset frequency. The carrier envelope offset frequency is f ＣＥＯ =(Δφ / (2π))f ｒｅｐ The carrier envelope offset frequency f ＣＥＯ is a function of the repetition frequency f ｒｅｐ The carrier envelope offset frequency f ＣＥＯ If we define f as the 0th mode, then the nth mode in the optical comb laser is f ｎ = f ＣＥＯ +nf ｒｅｐ The electric field of the optical comb laser light shown in FIG. 1A is expressed by the nth mode frequency f ｎ The amplitude and phase of the electric field at E ｎ and φ ｎ Then, E(t) = ΣnE ｎ exp[-i(2πf ｎ t+φ ｎ ) ]

[0055] Repetition frequency f ｒｅｐ and the carrier envelope offset frequency f ＣＥＯ can change slightly due to disturbances such as vibrations or temperature changes to the optical comb laser source. This is because vibrations can change the circumferential length L of the laser cavity, or temperature changes can change the group velocity v via changes in the refractive index of the laser cavity. ｇ and the phase velocity v ｐ This is because the repetition frequency f ｒｅｐand the carrier envelope offset frequency f ＣＥＯ are much lower than the frequency of light. Even a slight change in these frequencies can affect the identification of the mode frequencies in the optical comb laser light. For this reason, the optical comb laser source must have a repetition rate of f ｒｅｐ and the carrier envelope offset frequency f ＣＥＯ A modulator element may be incorporated to stabilize the

[0056] The modulation element may include, for example, a piezoelectric element for suppressing vibration, a temperature adjustment element for suppressing temperature changes, or a Peltier element. The modulation element may be provided outside the optical comb laser light source. The repetition frequency f ｒｅｐ In order to stabilize the carrier envelope offset frequency f, for example, a portion of the optical comb laser light is detected, and based on the detected portion, feedback is performed using a piezoelectric element and / or a Peltier element so that the circumferential length L of the laser resonator is kept constant. ＣＥＯ In order to stabilize the nonlinearity in the laser resonator, for example, a portion of the optical comb laser light is detected using a nonlinear optical effect, and based on the detected portion, feedback is performed by adjusting the intensity of the excitation light input to the laser resonator so that the nonlinearity in the laser resonator remains constant.

[0057] [Dual Comb] Next, the principle of dual comb will be briefly explained with reference to Figure 2. Dual comb uses two optical comb laser beams. In contrast, a technique using one optical comb laser beam is sometimes called single comb.

[0058] 2 is a diagram showing the frequency spectrum of two optical comb laser beams and interference light in a dual comb. In the first optical comb laser beam, the nth mode frequency f １ｎ is f １ｎ = f ＣＥＯ１ +nf ｒｅｐ１ In addition, the second optical comb laser light has the nth mode frequency f ２ｎ is f ２ｎ = f ＣＥＯ２ +nf ｒｅｐ２ It is expressed by f ＣＥＯ１is the carrier envelope offset frequency of the first optical comb laser light. ＣＥＯ２ is the carrier envelope offset frequency of the second optical comb laser light. ｒｅｐ１ is the repetition frequency of the first optical comb laser beam. ｒｅｐ２ is the repetition frequency of the second optical comb laser light. ｒｅｐ１ and f ｒｅｐ２ is slightly different, and f ｒｅｐ２ = f ｒｅｐ１ +δf ｒｅｐ The following relationship holds: δf ｒｅｐ is f ｒｅｐ１ Specifically, δf ｒｅｐ is f ｒｅｐ１ δf is 1% or less, but is not limited to this. ｒｅｐ is f ｒｅｐ１ It may be 0.5% or less, 0.1% or less, 0.05% or less, or 0.02% or less. ｒｅｐ is, for example, 1 Hz or more and 10 MHz or less.

[0059] In the mode of the first optical comb laser light and the mode of the second optical comb laser light shown in Figure 2, interference occurs between the closest modes, resulting in a frequency spectrum as shown in Figure 2 being detected as interference light. The detected interference light is a beat generated by the interference between the first optical comb laser light and the second optical comb laser light. The frequency interval of the interference light is δf, which is the difference between the repetition frequency of the first optical comb laser light and the repetition frequency of the second optical comb laser light. ｒｅｐ This becomes:

[0060] The time waveform of the interference light will be described in detail later with reference to FIG. 6A, but it is the same pulse waveform as in FIG. 1A. The pulse interval of the interference light is δT ｒｅｐ δT ｒｅｐ = 1 / δf ｒｅｐ Therefore, the time required for one measurement in spectroscopy or distance measurement is the pulse interval δT ｒｅｐ That is, 1 / δf ｒｅｐ Therefore, the number of measurements per second (sampling rate) is δf ｒｅｐTherefore, there is a trade-off between measurement time and sampling rate.

[0061] Furthermore, the longer the time taken for one measurement, the more accurate the measurement. Therefore, there is a trade-off between measurement accuracy and sampling rate. ｒｅｐ By adjusting , it is possible to obtain greater accuracy or a higher sampling rate.

[0062] [Laser Light Source] Next, an example of the configuration of a mode-locked laser light source will be described with reference to FIGS. 3A and 3B.

[0063] Fig. 3A is a top view schematically showing the laser light source 10 provided in the measurement device according to this embodiment. Fig. 3B is a cross-sectional view schematically showing the laser light source 10 at the position indicated by line IIIB-IIIB in Fig. 3A. For reference, mutually orthogonal x-axis, y-axis, and z-axis are shown schematically, but this is for the sake of convenience and does not limit the orientation during use. Note that Fig. 3B omits some of the diagonal shading representing the cross section.

[0064] The laser light source 10 is an example of a mode-locked laser light source that emits the output light 30. Specifically, the laser light source 10 is an optical frequency comb laser light source.

[0065] 3A and 3B, the laser light source 10 includes a resonator 20, a semiconductor substrate 21, and a protective layer 22. In the illustrated example, the surface of the semiconductor substrate 21 is parallel to the xy plane. As shown in FIG. 3B, the semiconductor substrate 21 includes a high refractive index layer 21a made of, for example, Si, and a SiO ２ The protective layer 22 has a laminated structure in which the high refractive index layer 21a and the low refractive index layer 21b such as SiO are laminated in this order in the z-axis direction. The refractive index of the high refractive index layer 21a is higher than the refractive index of the low refractive index layer 21b. The semiconductor substrate 21 does not necessarily have to include the high refractive index layer 21a. The protective layer 22 is made of, for example, SiO ２ or an organic insulating material such as BCB (benzocyclobutene). The protective layer 22 is provided so as to cover the upper surface of the low refractive index layer 21b.

[0066] The resonator 20 is an example of a laser resonator having a predetermined optical path length. As shown in FIG. 3A, the resonator 20 includes a semiconductor layer 20s, an intermediate layer 20i, an optical waveguide 20w, and a first mirror 20m. １ and the second mirror 20m ２ As shown in FIG. 3B, the resonator 20 includes an n-doped layer 20d. １ and p-doped layer 20d ２ and 20d ３ The resonator 20 also includes a gain medium 20g and a saturable absorber 20sa. Parts of the semiconductor layer 20s function as the gain medium 20g and the saturable absorber 20sa, respectively.

[0067] 3A, the planar shape of the semiconductor layer 20s in the resonator 20 is represented by a dotted line. The semiconductor layer 20s is tapered at both ends. The tip of the tapered portion overlaps the intermediate layer 20i in top view. The intermediate layer 20i is also tapered at both ends. The tip of the tapered portion overlaps the optical waveguide 20w in top view. This allows light passing through the semiconductor layer 20s to be efficiently propagated to the optical waveguide 20w via the intermediate layer 20i.

[0068] As shown in FIG. 3B , the optical waveguide 20w in the resonator 20 is embedded in a low-refractive-index layer 21b in the semiconductor substrate 21. The optical waveguide 20w may be provided on the low-refractive-index layer 21b. The optical waveguide 20w may be formed from a high-refractive-index material such as SiN. The refractive index of the optical waveguide 20w is higher than the refractive index of the low-refractive-index layer 21b in the semiconductor substrate 21 and the refractive index of the protective layer 22. This allows light to propagate through the optical waveguide 20w by total internal reflection.

[0069] The intermediate layer 20i is formed of, for example, a-Si (amorphous silicon). As long as the refractive index satisfies the following condition: optical waveguide 20w < intermediate layer 20i < semiconductor layer 20s, the intermediate layer 20i may be formed of other semiconductors. Without the intermediate layer 20i, the refractive index of the semiconductor layer 20s is greater than the refractive index of the optical waveguide 20w, making it difficult for light that has passed through the semiconductor layer 20s to pass through the optical waveguide 20w.

[0070] First mirror 20m１ and the second mirror 20m ２ The first mirror 20m may be formed of, for example, a distributed Bragg reflector. In a distributed Bragg reflector, light is reflected by Bragg reflection due to a periodic structure of the refractive index. １ The second mirror 20m is provided at the end of the optical waveguide 20w. ２ is the first mirror 20m in the optical waveguide 20w. １ The first mirror 20m is provided at the end opposite to the first mirror 20m. １ and the second mirror 20m ２ The first mirror 20m reflects the light that has propagated through the optical waveguide 20w. １ and the second mirror 20m ２ The first mirror 20m may be made of, for example, metal, as long as it has a reflecting function. １ and the second mirror 20m ２ may be a reflective element made of a semiconductor such as a loop mirror.

[0071] Second mirror 20m ２ is the first mirror 20m １ Specifically, the reflectance of the first mirror 20m is lower than that of the first mirror 20m. １ The reflectivity of the first mirror 20m is substantially equal to 100%. ２ The reflectance of the first mirror 20m is equal to or greater than a predetermined value. １ The predetermined value is, for example, 50%, and as an example, ２ The reflectivity of the second mirror 20m can be, for example, 70% or 90%. ２ The second mirror 20m reflects most of the light propagating through the optical waveguide 20w, but transmits a portion of the light. ２ The light transmitted through the filter becomes output light 30, which is optical comb laser light.

[0072] As shown in FIG. 3B, the semiconductor layer 20s includes an n-doped layer 20d １ and p-doped layer 20d ２ and 20d ３ The n-doped layer 20d is sandwiched between the １ and p-doped layer 20d ２ and 20d３ The positional relationship may be reversed.

[0073] Semiconductor layer 20s, n-doped layer 20d １ and p-doped layer 20d ２ and 20d ３ is buried in the protective layer 22. １ The bottom surface of the n-doped layer 20d may be in contact with the surface of the semiconductor substrate 21. １ may be in contact with the intermediate layer 20i.

[0074] The semiconductor layer 20s may be formed of, for example, a III-V semiconductor material, which may include at least one material selected from the group consisting of InGaAlP, InGaAs, GaInAsP, GaInAsSb, InP, GaN, GaAs, InGaAs, AlGaAs, and AlInGaN.

[0075] The semiconductor layer 20s includes a gain medium 20g and a saturable absorber 20sa. The gain medium 20g is a part of the semiconductor layer 20s and is an n-doped layer 20d. １ and p-doped layer 20d ２ The saturable absorber 20sa is a part of the semiconductor layer 20s, and is sandwiched between the n-doped layer 20d. １ and p-doped layer 20d ３ It is the part sandwiched between and.

[0076] n-doped layer 20d １ is an n-type semiconductor layer. １ is formed by doping an n-type impurity into the same III-V semiconductor material as the semiconductor layer 20s. The n-type impurity may be, for example, a tetravalent element such as Si or a hexavalent element such as selenium (Se).

[0077] p-doped layer 20d ２ and 20d ３ is a p-type semiconductor layer. ２ and 20d ３The p-doped layer 20d is formed by doping a p-type impurity into the same III-V semiconductor material as the semiconductor layer 20s. For example, a divalent element such as zinc (Zn) can be used as the p-type impurity. ２ and p-doped layer 20d ３ and have the same composition, for example.

[0078] p-doped layer 20d ２ and p-doped layer 20d ３ The p-doped layer 20d is separated from the p-doped layer 20d. ２ and 20d ３ Each of the p-doped layers 20d is attached with a different electrode (not shown). ２ A current is injected into the p-doped layer 20d via an electrode. ３ The n-doped layer 20d １ By applying this voltage, a reverse bias voltage is applied between the p-doped layer 20d, which is a part of the semiconductor layer 20s. ３ The portion of the saturable absorber 20sa that contacts the first mirror 20m functions as the saturable absorber 20sa. The saturable absorber 20sa may be formed using carbon nanotubes. １ and the second mirror 20m ２ It may be integrated with.

[0079] p-doped layer 20d ２ Similarly, the n-doped layer 20d １ An electrode (not shown) is also attached to the n-doped layer 20d. １ and p-doped layer 20d ２ A part of the semiconductor layer 20s into which charges are injected from the electrodes attached to each of the first and second mirrors 20m functions as a gain medium 20g from which light is stimulated and emitted. The stimulated and emitted light is guided to the first mirror 20m via the intermediate layer 20i and the optical waveguide 20w. １ and the second mirror 20m ２The light is repeatedly reflected between the first mirror 20m and the second mirror 20m. That is, the light is amplified by passing through the gain medium 20g many times. The amplified light becomes a mode-locked optical pulse train by the saturable absorber 20sa. Only the wavelength corresponding to the optical path length of the resonator 20 (i.e., the resonator length multiplied by the refractive index) is amplified. As a result, the resonator 20 emits optical comb laser light as output light 30. The resonator length of the resonator 20 is determined by the first mirror 20m. １ and the second mirror 20m ２ is the distance between

[0080] [HML (Hybrid Mode Locking)] Next, hybrid mode locking (HML) will be described.

[0081] HML is a technology for stabilizing the repetition frequency of an optical comb laser beam. Specifically, a frequency shifter is used to set the repetition frequency of the optical comb laser beam to a predetermined drive frequency. For example, the frequency shifter includes a signal generator that supplies a high-frequency signal. In the case of the laser light source 10 shown in FIGS. 3A and 3B, a signal generator (not shown) supplies a high-frequency signal to the saturable absorber 20sa. The frequency of this high-frequency signal is the drive frequency of the frequency shifter.

[0082] FIG. 4A shows the frequency spectrum of the optical comb laser light. Specifically, FIG. 4A shows a graph of the electrical signal output from the detector, observed with an RF spectrum analyzer, after the optical comb laser light is incident on the detector without HML. The peak frequency of approximately 2.9617 GHz in the graph shown in FIG. 4A corresponds to the repetition frequency of the optical comb laser light. The resonator length of the laser light source can vary depending on environmental conditions such as temperature changes and operating conditions. As a result, as shown in FIG. 4A, the peak of the output electrical signal has a wide half-width.

[0083] 4B shows the frequency spectrum of the optical comb laser light when HML is performed. Specifically, FIG. 4B shows a graph of the electrical signal output from the detector, observed with an RF spectrum analyzer, after HML is performed and the optical comb laser light is incident on the detector. In HML, the repetition frequency value when HML is not performed, i.e., the high-frequency signal of approximately 2.9617 GHz shown in FIG. 4A, is supplied to the saturable absorber. Hereinafter, the repetition frequency value when HML is not performed may be referred to as the "repetition frequency reference value."

[0084] As shown in Figure 4B, the peak of the electrical signal output from the detector has a narrow half-width and a steep peak. This indicates that the repetition frequency of the optical comb laser light is stable. Thus, in HML, the repetition frequency can be stabilized by driving the optical frequency comb laser light source at a drive frequency set to the same value as the repetition frequency reference value. A stable repetition frequency can improve measurement accuracy.

[0085] In order to perform HML, it is important to check the value of the repetition frequency when HML is not performed (i.e., the repetition frequency reference value). Note that the set value of the drive frequency and the repetition frequency reference value do not need to match perfectly, but HML can only be performed appropriately within a predetermined range that includes the repetition frequency reference value. For this reason, it is necessary to check the repetition frequency reference value.

[0086] As described above, an RF spectrum analyzer or a frequency counter can be used to confirm the value of the repetition frequency. However, these are unnecessary components for measurement, which increases the complexity of the measurement device. Furthermore, since the repetition frequency can change depending on the surrounding environment, operating conditions, etc., it is desirable to confirm the repetition frequency each time a measurement is performed. Therefore, there is a need for a technology that can confirm the value of the repetition frequency without performing HTML with a simple configuration.

[0087] Specifically, the measurement device according to the present disclosure includes a feedback circuit that checks the repetition frequency value when HML is not performed based on the signal output from the detector by sweeping the drive frequency with the frequency shifter. The feedback circuit determines the setting value of the drive frequency to be used during measurement based on the checked repetition frequency value. The specific configuration and operation of the measurement device according to the present disclosure will be described later.

[0088] [Applicable Range of HML] Next, the applicable range of HML when the drive frequency is swept will be described. In the following, an example will be described in which a frequency shifter that sweeps the drive frequency includes a saturable absorber and a signal generator that supplies a high-frequency signal to the saturable absorber.

[0089] The frequency shifter sweeps the drive frequency, which is the frequency of the high-frequency signal supplied to the saturable absorber, within a predetermined range. When the drive frequency matches the repetition frequency reference value, a steep peak is observed, as shown in Figure 4B. Below, we will explain the case where the drive frequency is swept from the repetition frequency reference value.

[0090] 5A and 5B show the frequency spectrum of the optical comb laser light when the intensity of the high-frequency signal supplied by the HML is low, and 5B shows the frequency spectrum of the optical comb laser light when the intensity of the high-frequency signal supplied by the HML is high. In each of FIGS. 5A and 5B, the horizontal axis represents the difference between the repetition frequency reference value and the driving frequency, and the vertical axis represents the signal intensity.

[0091] The example shown in Figure 5A is a graph showing the case where the driving frequency of the high-frequency signal is supplied to the saturable absorber while being shifted by 100 kHz from the reference repetition frequency. As shown in Figure 5A, even when the difference is shifted up to 500 kHz, a steep peak is observed, and there is no problem in performing HML. On the other hand, when the difference is 600 kHz, distorted frequency components appear. This indicates that HML is not functioning.

[0092] The example shown in Figure 5B shows a graph in which the drive frequency of the high-frequency signal is supplied to the saturable absorber while being shifted by 500 kHz from the reference repetition frequency. As shown in Figure 5B, even when the difference is shifted up to 2 MHz, a steep peak is observed, and there is no problem in performing HML. On the other hand, when the difference is 2.5 MHz, distorted frequency components appear. This indicates that HML is not functioning.

[0093] 5A and 5B show examples in which the drive frequency is greater than the reference repetition frequency, but the same applies when the drive frequency is less than the reference repetition frequency. In other words, in the example shown in Fig. 5A, HML can be performed appropriately when the drive frequency is within a range of ±500 kHz of the reference repetition frequency. In the example shown in Fig. 5B, HML can be performed appropriately when the drive frequency is within a range of ±2 MHz of the reference repetition frequency.

[0094] From the above, it can be seen that in order to properly perform HML, the set value of the drive frequency must be within a predetermined range that includes the repetition frequency reference value. This range is called the HML application range. It can also be seen that the size of the HML application range changes depending on the strength of the high-frequency signal. Specifically, the higher the strength of the high-frequency signal, the wider the HML application range, and the lower the strength of the high-frequency signal, the narrower the HML application range. To confirm the repetition frequency reference value, the strength of the high-frequency signal is lowered and the HML application range is narrowed. This can improve the accuracy of confirming the repetition frequency reference value.

[0095] [Use of Time Waveform] Next, a method for checking the repetition frequency reference value using the time waveform will be described, taking the dual comb described with reference to FIG. 2 as an example.

[0096] 6A is a diagram for explaining the pulse interval of the interference light. In FIG. 6A, the horizontal axis represents time, and the vertical axis represents signal intensity. σ Ｔ represents fluctuations on the time axis, and is also called timing jitter. ｒｅｐ represents the pulse interval of the interference light, and the difference δf between the repetition frequency of the first optical comb laser light and the repetition frequency of the second optical comb laser lightｒｅｐ is the reciprocal of

[0097] 6B is a diagram illustrating the pulse interval of the interference light when the difference between the repetition frequency reference value and the drive frequency is large. FIG. 6C is a diagram illustrating the pulse interval of the interference light when the difference between the repetition frequency reference value and the drive frequency is small. For simplicity, the following description will be given of a case where HML is not performed on the first optical comb laser beam, but is performed on the second optical comb laser beam. Note that FIG. 6A corresponds to a case where HML is not performed on either the first or second optical comb laser beam.

[0098] Specifically, in FIG. 6B, the difference between the repetition frequency reference value of the second optical comb laser light and the driving frequency is within ±0.1% of the repetition frequency reference value. For example, when the repetition frequency reference value is 3 GHz, the driving frequency is within the range of 3 GHz ±3 MHz. Ｔ Since HML is performed, σ shown in FIG. Ｔ That is, σ1 Ｔ <σ Ｔ In addition, δT1, which represents the pulse interval, ｒｅｐ is δT shown in FIG. ｒｅｐ Although a larger example is shown, it may be smaller.

[0099] In FIG. 6C, the difference between the repetition frequency reference value of the second optical comb laser beam and the driving frequency is smaller than that in the example shown in FIG. 6B. For example, the difference between the repetition frequency reference value of the second optical comb laser beam and the driving frequency is 0. Ｔ Since HML is performed, σ shown in FIG. Ｔ That is, σ2 Ｔ <σ Ｔ δT2 represents the pulse interval ｒｅｐ is δT shown in FIG. ｒｅｐ becomes the same as

[0100] When HML is performed at a drive frequency outside the HML application range, the result is the same as in Fig. 6A. The HML application range is the repetition frequency reference value ±500 kHz.

[0101] Fig. 7 is a diagram showing the relationship between the difference between the repetition frequency reference value and the drive frequency and the pulse interval. Fig. 7 is a graph of the states shown in Figs. 6A to 6C. Specifically, the graph is not an actual measurement of the pulse interval, but a conceptual representation based on the results of visual observation of signal fluctuations. The pulse interval can vary depending on the magnitude of timing jitter.

[0102] As shown in FIG. 7, outside the HML application range, the pulse interval is δT ｒｅｐ Within the HML application range, the fluctuation amount of the pulse interval becomes small, while the center of the fluctuation becomes δT depending on the difference between the repetition frequency reference value and the drive frequency. ｒｅｐ In the example shown in FIG. 7, when the drive frequency is smaller than the repetition frequency reference value (i.e., when the difference is a negative value), the center of the fluctuation of the pulse interval is δT ｒｅｐ When the driving frequency is greater than the reference repetition frequency (i.e., the difference is a positive value), the center of the fluctuation of the pulse interval is δT ｒｅｐ When the driving frequency is equal to the reference value of the repetition frequency, the center of the fluctuation of the pulse interval is δT ｒｅｐ becomes equal to

[0103] In this way, by sweeping the drive frequency within a predetermined range based on the behavior of the pulse interval inside and outside the HML application range, it is possible to confirm the repetition frequency. For example, when the drive frequency is outside the HML application range, the center value of the fluctuation of the pulse interval is estimated based on the signal, and δT ｒｅｐ In addition, by sweeping the driving frequency within the HML application range, the central value of the fluctuation of the pulse interval becomes δT ｒｅｐ The drive frequency when it becomes equal to can be regarded as the repetition frequency reference value.

[0104] The sweep range of the drive frequency can be determined based on specifications such as the optical path length of the resonator of the optical frequency comb laser source. The repetition frequency reference value may vary depending on the ambient environment and operating conditions, but the range of variation is not large. Therefore, the sweep range can be a range that includes the repetition frequency determined as a specification value based on the specifications of the optical frequency comb laser source. Alternatively, the sweep range can be a range that includes the repetition frequency reference value confirmed during a previous measurement.

[0105] [Using Light Intensity] Next, a method for checking the repetition frequency reference value using light intensity will be described. The following will be explained using a single comb as an example, but the same can be applied to a dual comb.

[0106] 8 is a diagram showing the relationship between the difference between the repetition frequency reference value and the drive frequency and the light intensity. The HML application range is, for example, the repetition frequency reference value ±500 kHz.

[0107] As shown in FIG. 8 , outside the HML application range, the light intensity is approximately constant at 330 μW or 331 μW. This light intensity value is approximately the same as the light intensity without HML, as represented by the square plot in the figure. In contrast, within the HML application range, the light intensity varies depending on the difference between the repetition frequency reference value and the driving frequency. In the example shown in FIG. 8 , when the driving frequency is smaller than the repetition frequency reference value (i.e., when the difference is negative), the light intensity tends to form a downward convex graph, whereas when the driving frequency is larger than the repetition frequency reference value (i.e., when the difference is positive), the light intensity tends to form an upward convex graph. Within the HML application range, the light intensity graph is approximately point-symmetrical with respect to the point where the difference is zero.

[0108] In this way, by sweeping the drive frequency within a predetermined range based on the behavior of the light intensity inside and outside the HML application range, it is possible to confirm the repetition frequency. For example, the range in which the light intensity fluctuates can be regarded as the HML application range, and the frequency at the center of that HML application range can be regarded as the repetition frequency reference value. Alternatively, since the shape of the light intensity graph is roughly point symmetric, the frequency at the center of that point symmetry can be regarded as the repetition frequency reference value.

[0109] As described above, the measurement device according to the present disclosure can confirm the repetition frequency reference value based on the signal obtained from the detector when the optical frequency comb laser light source is driven while sweeping the drive frequency. Specifically, the measurement device can confirm the repetition frequency reference value based on the pulse interval or optical intensity of the signal's time waveform. The measurement device determines the setting value of the drive frequency to be used for measurement based on the confirmed repetition frequency reference value.

[0110] Specific embodiments of the measurement device according to the present disclosure will be described below.

[0111] (Embodiment 1) First, a description will be given of a measurement device according to embodiment 1. The measurement device according to embodiment 1 uses a single comb.

[0112] [Configuration] First, the configuration of the measurement device according to the first embodiment will be described with reference to Fig. 9. Fig. 9 is a diagram showing the configuration of a measurement device 101 according to the present embodiment. As shown in Fig. 9, the measurement device 101 includes a laser light source 11, a detector 51, a feedback circuit 61, and a frequency shifter 71.

[0113] The measurement device 101 shown in Fig. 9 is used to measure the surface shape of a measurement object (not shown) or the distance from the measurement device 101 to the measurement object. Although not shown in Fig. 9, the measurement device 101 may include one or more optical systems for irradiating light toward the measurement object and detecting reflected light from the measurement object. The one or more optical systems include optical fibers, lenses, mirrors, etc.

[0114] The laser light source 11 is an example of a first mode-locked laser light source that emits first output light. In this embodiment, the laser light source 11 is an optical frequency comb laser light source, and is integrated on a semiconductor substrate. Note that an optical frequency comb laser light source integrated on a semiconductor substrate may be called an on-chip comb light source, or simply an on-chip comb. The laser light source 11 outputs output light 31, which is an example of the first output light. The output light 31 is an optical comb laser light. The repetition frequency f of the output light 31 is ｒｅｐ is, for example, 10 MHz or more and 1 THz or less.

[0115] Specifically, as shown in FIG. 9, the laser light source 11 includes a first mirror 41m １ and the second mirror 41m ２ The laser light source 11 has a resonator including a first mirror 41m, a gain medium 41g, and a saturable absorber 41sa. The specific configuration of the laser light source 11 is the same as that of the laser light source 10 shown in FIGS. 3A and 3B. １ , second mirror 41m ２ , the gain medium 41g and the saturable absorber 41sa are connected to the first mirror 20m. １ , second mirror 20m ２ , the gain medium 20g and the saturable absorber 20sa.

[0116] The output light 31 emitted by the laser light source 11 is incident on the detector 51. The detector 51 is, for example, a high-speed photodetector. The detector 51 converts the incident output light 31 into an electrical signal by photoelectric conversion and outputs the electrical signal according to the intensity of the output light 31. The electrical signal is output to a feedback circuit 61.

[0117] The output light 31 emitted by the laser light source 11 may be incident on the detector 51 directly or via an optical element such as an optical fiber or a lens. The detector 51 is a photodetector that is also used for measurement, but may also be a photodetector dedicated to setting the drive frequency (i.e., dedicated to checking the repetition frequency reference value).

[0118] The frequency shifter 71 shifts the repetition frequency f ｒｅｐis set as the drive frequency (first drive frequency). When the frequency shifter 71 operates and the set value of the drive frequency is within the HML applicable range, HML becomes possible. That is, the repetition frequency f ｒｅｐ When the frequency shifter 71 does not operate, or when the set value of the driving frequency is outside the HML application range, the repetition frequency f ｒｅｐ becomes the repetition frequency reference value, and its fluctuation becomes large.

[0119] Since the driving frequency of the frequency shifter 71 is variable, the repetition frequency f ｒｅｐ The sweep is performed, for example, discretely at a predetermined step width within a predetermined range, but may also be performed continuously. The frequency shifter 71 can sweep the repetition frequency f ｒｅｐ The element may be an optical element, a physical element, or an electrical element, as long as it can change the value of the element.

[0120] The feedback circuit 61 determines the set value of the drive frequency based on the signal output from the detector 51 when the laser light source 11 is driven while sweeping the drive frequency of the frequency shifter 71. Specifically, the feedback circuit 61 determines the set value of the drive frequency based on signal intensity information. For example, as described with reference to FIG. 8 , the feedback circuit 61 confirms the repetition frequency reference value from changes in light intensity, and determines the confirmed repetition frequency reference value as the set value of the drive frequency.

[0121] The feedback circuit 61 is realized, for example, by an LSI (Large Scale Integration), which is an integrated circuit (IC). The integrated circuit is not limited to an LSI and may be a general-purpose processor. For example, the feedback circuit 61 may be a microcontroller. The microcontroller includes, for example, a non-volatile memory in which a program is stored, a volatile memory that is a temporary storage area for executing the program, input / output ports, and a processor that executes the program. The feedback circuit 61 may also be a programmable FPGA (Field Programmable Gate Array) or a reconfigurable processor in which the connections and settings of circuit cells within the LSI can be reconfigured. The functions performed by the feedback circuit 61 may be realized by software or hardware.

[0122] As described above, in the measuring device 101 according to this embodiment, the driving frequency is swept, and the repetition frequency f of the output light 31 is obtained based on the signal output from the detector 51. ｒｅｐ The value of the repetition frequency f of the output light 31 can be confirmed. ｒｅｐ By checking the value of , it is possible to determine an appropriate setting value for the drive frequency when performing HML, thereby improving the accuracy of measurement. Since an RF spectrum analyzer is not required, the configuration can be simplified. Therefore, according to this aspect, it is possible to realize a measurement device 101 that can perform accurate measurements with a simple configuration.

[0123] 9 shows an example in which the frequency shifter 71 is provided separately from the laser light source 11, but this is not limiting. Part of the configuration of the frequency shifter 71 may be included in the laser light source 11.

[0124] 10 is a diagram showing the configuration of a measuring apparatus 102 according to another example of the present embodiment. The measuring apparatus 102 shown in Fig. 10 includes a frequency shifter 81 instead of the frequency shifter 71 of the measuring apparatus 101 shown in Fig. 9.

[0125] The frequency shifter 81 includes an RF oscillator 91 and a saturable absorber 41sa, which is the same as the saturable absorber 41sa shown in FIG.

[0126] The RF oscillator 91 is an example of a signal generator that supplies a high-frequency signal to the saturable absorber 41sa. The frequency of the high-frequency signal is the drive frequency (first drive frequency) of the frequency shifter 81. The RF oscillator 91 is realized by an electronic circuit including at least one of, for example, a resistor, an inductor, a capacitor, a transformer, a diode, and a transistor. Part or all of the RF oscillator 91 may be mounted on the substrate on which the laser light source 11 is integrated.

[0127] The RF oscillator 91 can change the frequency of the radio frequency signal. The RF oscillator 91 sweeps the frequency of the radio frequency signal under control of the feedback circuit 61. The RF oscillator 91 can also change the signal intensity of the radio frequency signal. For example, after the feedback circuit 61 determines the set value of the drive frequency, the RF oscillator 91 supplies the saturable absorber 41sa with a radio frequency signal having a frequency equal to the set value of the drive frequency and with an intensity higher than the intensity of the radio frequency signal during the sweep of the drive frequency. In other words, the intensity of the radio frequency signal supplied during the sweep of the drive frequency is lower than the intensity of the radio frequency signal supplied during measurement. This makes it easy to confirm the repetition frequency by sweeping, as described with reference to FIGS. 5A and 5B . Furthermore, since the HML application range is wide during measurement, high-precision measurement using the HML is possible even if the repetition frequency reference value fluctuates depending on the operating conditions.

[0128] The configuration of the frequency shifter 71 is not limited to the frequency shifter 81 shown in FIG. 10 . For example, the frequency shifter 71 may include a phase shifter and a voltage supply circuit that supplies a DC voltage to the phase shifter. The phase shifter is a component whose refractive index changes in response to a change in the DC voltage supplied from an external source. By placing the phase shifter on the optical path of the resonator, the refractive index of a portion of the optical path of the resonator changes. This changes the optical path length of the resonator, making it possible to change the repetition frequency of the output light.

[0129] The frequency shifter 71 may also include an optical chopper. The optical chopper is an optical chopper that can repeatedly switch between a light-blocking state and a light-transmitting state. By placing the optical chopper on the optical path of the resonator, optical pulses that are evenly spaced on the time axis are output as output light. By controlling the timing of switching between the light-blocking state and the light-transmitting state of the optical chopper, the time interval between the optical pulses, i.e., the repetition frequency of the output light, can be changed.

[0130] [Operation] Next, a description will be given of the operation of the measuring device 101 or 102 according to this embodiment. Below, the operation of the measuring device 102 will be described as a representative, but the operation of the measuring device 101 is similar.

[0131] Fig. 11 is a flowchart showing the operation of the measurement apparatus 102 according to this embodiment. The operation shown in Fig. 11 is performed, for example, immediately before measuring the measurement object. The sweep range of the drive frequency is set in advance. The sweep range is set manually by the user or automatically based on the specification information of the laser light source 11 or a previously confirmed reference repetition frequency value. The sweep range is, for example, a range of ±1 MHz of the specification value of the repetition frequency determined from the specification information of the laser light source 11, or a previously confirmed reference repetition frequency value ±1 MHz.

[0132] 11 , the measurement device 102 first performs HML at a drive frequency within the sweep range (S10). For example, at the start of operation, the minimum or maximum value of the sweep range is set as the frequency of the high-frequency signal. Specifically, while the RF oscillator 91 supplies a high-frequency signal of the set frequency to the saturable absorber 41sa, the laser light source 11 is operated to output the output light 31. The detector 51 converts the output light 31 from the laser light source 11 into an electrical signal and outputs the signal to the feedback circuit 61.

[0133] Next, the feedback circuit 61 acquires the light intensity based on the signal output from the detector 51 (S11). The feedback circuit 61 stores the acquired light intensity and frequency in association with each other in a memory.

[0134] If sweeping of the sweep range is not complete (No in S12), the feedback circuit 61 controls the frequency shifter 81 to change the drive frequency (S13). For example, the step size for changing the drive frequency is 10 kHz. After changing the drive frequency, the processes from step S10 to step S13 are repeated until sweeping of the sweep range is complete.

[0135] When the sweep of the sweep range is completed (Yes in S12), the feedback circuit 61 checks the repetition frequency reference value (S14). Specifically, the method described with reference to FIG. 8 is used.

[0136] The feedback circuit 61 determines the set value of the drive frequency based on the confirmed repetition frequency reference value (S15). For example, the feedback circuit 61 determines the repetition frequency reference value as the set value of the drive frequency. Then, the measurement device 102 performs HML using the determined set value.

[0137] As described above, by sweeping the drive frequency, the repetition frequency reference value can be confirmed, enabling highly accurate measurements. Since an RF spectrum analyzer is not required, the configuration of the measuring device 101 or 102 can be simplified. Therefore, according to this embodiment, it is possible to realize the measuring device 101 or 102 that can perform accurate measurements with a simple configuration.

[0138] (Embodiment 2) Next, a measurement device according to embodiment 2 will be described. The measurement device according to embodiment 2 uses a dual comb. The following description will focus on differences from embodiment 1, and descriptions of commonalities will be omitted or simplified.

[0139] [Configuration] First, the configuration of a measurement device according to the second embodiment will be described with reference to Fig. 12. Fig. 12 is a diagram showing the configuration of a measurement device 201 according to the second embodiment. As shown in Fig. 12, the measurement device 201 includes laser light sources 11 and 12, half mirrors 43 and 44, detectors 51 and 52, feedback circuits 61 and 62, and frequency shifters 71 and 72.

[0140] The laser light source 12, the detector 52, the feedback circuit 62, and the frequency shifter 72 have the same configurations as the laser light source 11, the detector 51, the feedback circuit 61, and the frequency shifter 71, respectively.

[0141] The laser light source 12 is an example of a second mode-locked laser light source that emits second output light. In this embodiment, the laser light source 12 is an optical frequency comb laser light source and is integrated on a semiconductor substrate. The laser light source 12 outputs output light 32, which is an example of the second output light. The output light 32 is an optical comb laser light. The repetition frequency f of the output light 32 is ｒｅｐ２ is, for example, 10 MHz or more and 1 THz or less.

[0142] Specifically, as shown in FIG. 12, the laser light source 12 includes a first mirror 42m １ and the second mirror 42m ２ The laser light source 12 has a resonator including a first mirror 42m, a gain medium 42g, and a saturable absorber 42sa. The specific configuration of the laser light source 12 is the same as that of the laser light source 10 shown in Figures 3A and 3B. １ , second mirror 42m ２ , the gain medium 42g and the saturable absorber 42sa are respectively connected to the first mirror 20m １ , second mirror 20m ２ , the gain medium 20g and the saturable absorber 20sa.

[0143] In this embodiment, the optical path length of the resonator of the laser light source 11 is different from the optical path length of the resonator of the laser light source 12. For example, the first mirror 41m １ and the second mirror 41m ２ The cavity length, which is the distance between the first mirror 42m and the １ and the second mirror 42m ２ This is different from the resonator length, which is the distance between the output light 31 and the repetition frequency f ｒｅｐ１ and the repetition frequency f of the output light 32 ｒｅｐ２ Since the repetition frequency of the output light 31 and the repetition frequency of the output light 32 are different, the difference δf ｒｅｐ Since δf does not become 0, measurement by dual comb is possible. ｒｅｐTherefore, the optical path length of the resonator of the laser light source 11 and the optical path length of the resonator of the laser light source 12 may be the same.

[0144] The output light 32 from the laser light source 12 is intensity-split by the half mirror 44 and enters both the detector 51 and the detector 52. The half mirror 44 is an example of a beam splitter that splits the intensity of the incident light and emits the light in two different directions. In this embodiment, the half mirror 44 splits the output light 32 into light that enters the detector 51 and light that enters the detector 52. The intensity division ratio by the half mirror 44 is, for example, 50:50, but is not limited to this.

[0145] The detector 52 receives the output light 32 emitted by the laser light source 12. In this embodiment, the light of the output light 32 that has passed through the half mirror 44 is incident on the detector 52. The detector 52 is, for example, a high-speed photodetector. The detector 52 converts the incident output light 32 into an electrical signal by photoelectric conversion and outputs an electrical signal corresponding to the intensity of the output light 32. The electrical signal is output to a feedback circuit 62. The detector 52 is a photodetector that is also used for measurement, but may also be a photodetector dedicated to setting the drive frequency (i.e., dedicated to checking the repetition frequency reference value).

[0146] The frequency shifter 72 shifts the repetition frequency f ｒｅｐ２ is set as the drive frequency (second drive frequency). When the frequency shifter 72 operates and the set value of the drive frequency is within the HML applicable range, HML becomes possible. That is, the repetition frequency f ｒｅｐ２ When the frequency shifter 72 does not operate, or when the set value of the driving frequency is outside the HML application range, the repetition frequency f ｒｅｐ２ becomes the repetition frequency reference value, and its fluctuation becomes large.

[0147] Since the driving frequency of the frequency shifter 72 is variable, the repetition frequency f ｒｅｐ２The frequency shifter 72 can sweep the repetition frequency f of the output light 32. The sweep is performed, for example, discretely at predetermined steps, but may also be performed continuously. ｒｅｐ２ The element may be an optical element, a physical element, or an electrical element, as long as it can change the value of the element.

[0148] The feedback circuit 62 determines the set value of the drive frequency based on the signal output from the detector 52 when the laser light source 12 is driven while sweeping the drive frequency of the frequency shifter 72. Specifically, as described with reference to Fig. 8, the feedback circuit 62 confirms the repetition frequency reference value from changes in the light intensity, and determines the confirmed repetition frequency reference value as the set value of the drive frequency.

[0149] The feedback circuit 62 is realized by an integrated circuit, similar to the feedback circuit 61. The feedback circuit 62 may be configured integrally with the feedback circuit 61. That is, the measuring device 201 only needs to include one feedback circuit that controls each of the frequency shifters 71 and 72.

[0150] The laser light source 11, detector 51, feedback circuit 61, and frequency shifter 71 are all the same as those in the first embodiment, and therefore their explanations are omitted. The present embodiment differs in that not only the output light 31 from the laser light source 11 but also the output light 32 from the laser light source 12 is incident on the detector 51. To distinguish the output light 31 from the output light 32, the output light 31 is considered to be an example of the first optical comb laser light shown in FIG. 2, and the repetition frequency is f ｒｅｐ１ and the carrier envelope offset frequency is f ＣＥＯ１ Let's say.

[0151] The detector 51 receives the output light 31 emitted by the laser light source 11 and the output light 32 emitted by the laser light source 12. Specifically, the light of the output light 32 reflected by each of the half mirrors 43 and 44 enters the detector 51. Furthermore, the light of the output light 31 transmitted through the half mirror 43 enters the detector 51. The output light 31 and the output light 32 that enter the detector 51 interfere with each other and are detected as interference light by the detector 51. That is, the detector 51 converts the interference light between the output light 31 and the output light 32 into an electrical signal by photoelectric conversion and outputs the electrical signal. The electrical signal represents the intensity of the interference light. The electrical signal from the detector 51 is output to the feedback circuit 61. The frequency band of the interference light is several orders of magnitude lower than that of the output lights 31 and 32. For this reason, the detector 51 can be a conventional, inexpensive photodetector rather than a high-speed photodetector. The detector 51 is a photodetector that is also used for measurement, but may be a photodetector dedicated to setting the drive frequency (that is, dedicated to checking the reference value of the repetition frequency).

[0152] The half mirror 43 is an example of a multiplexer that multiplexes two light beams incident from two directions and outputs the multiplexed light in one direction. For example, the half mirror 43 has the same configuration as the half mirror 44. Note that the measurement device 201 may include an optical element capable of splitting or multiplexing light (light synthesis) instead of the half mirrors 43 and 44. For example, the measurement device 201 may include an optical fiber and a photocoupler instead of the half mirrors 43 and 44.

[0153] Furthermore, if interference light is not used to confirm the repetition frequency reference value, the half mirrors 43 and 44 may not be provided. In this case, the detector 51 may detect only the output light 31, and the detector 52 may detect only the output light 32. Furthermore, by changing the positions of the half mirrors 43 and 44, the detector 52 may be configured to detect interference light between the output lights 31 and 32.

[0154] In this embodiment, because the interference light is detected by the detector 51, the feedback circuit 61 can utilize time information of the signal. Specifically, the feedback circuit 61 determines the set value of the first drive frequency based on signal intensity information. For example, as described with reference to FIG. 7 , the feedback circuit 61 confirms the repetition frequency reference value based on the pulse interval, and determines the confirmed repetition frequency reference value as the set value of the first drive frequency.

[0155] In this embodiment, the specific configuration of frequency shifters 71 and 72 is, for example, configured by an RF oscillator and a saturable absorber, similar to embodiment 1. Fig. 13 is a diagram showing the configuration of a measurement apparatus 202 according to another example of this embodiment. The measurement apparatus 202 shown in Fig. 13 includes frequency shifters 81 and 82 instead of the frequency shifters 71 and 72 of the measurement apparatus 201 shown in Fig. 12.

[0156] The frequency shifter 81 has the same configuration as the frequency shifter 81 according to the first embodiment shown in Fig. 10. The frequency shifter 82 also includes a saturable absorber 42sa and an RF oscillator 92 that supplies a high-frequency signal to the saturable absorber 42sa.

[0157] The RF oscillator 92 is an example of a signal generator that supplies a high-frequency signal to the saturable absorber 42sa. The frequency of the high-frequency signal is the drive frequency (second drive frequency) of the frequency shifter 82. The configuration and operation of the RF oscillator 92 are similar to those of the RF oscillator 91. Note that the RF oscillator 92 supplies a high-frequency signal with a different frequency from that of the RF oscillator 91 to the saturable absorber 42sa. As a result, the repetition frequency f ｒｅｐ１ and the repetition frequency f of the output light 32 ｒｅｐ２ This allows measurements based on the dual comb principle to be made possible.

[0158] Furthermore, the detector 52 does not have to be used to confirm the reference value of the repetition frequency of the output light 32. That is, the reference values ​​of the repetition frequencies of the output lights 31 and 32 may be confirmed based on the signal from the detector 51.

[0159] Fig. 14 is a diagram showing the configuration of a measuring device 203 according to another example of this embodiment. The measuring device 203 shown in Fig. 14 differs from the measuring device 202 shown in Fig. 13 in that it does not include a feedback circuit 62. The feedback circuit 61 controls each of the frequency shifters 81 and 82 based on a signal from the detector 51. The specific operation of the feedback circuit 61 will be described later.

[0160] Furthermore, the drive frequency of one of the two frequency shifters may be fixed. For example, in the measurement device 203 shown in Fig. 14, the drive frequency of the frequency shifter 82 may be fixed. By fixing the drive frequency, it is possible to simplify the configuration of the measurement device 203 and reduce costs.

[0161] In this embodiment, the two laser light sources 11 and 12 may be integrated on different semiconductor substrates. This allows the measurement device to be miniaturized. For example, in a measurement device 202 shown in FIG. 13 , the laser light source 11 is integrated on a semiconductor substrate 41, and the laser light source 12 is integrated on a semiconductor substrate 42. By integrating the two laser light sources 11 and 12 on different semiconductor substrates 41 and 42, the degree of freedom in arranging the laser light sources 11 and 12 can be increased.

[0162] Alternatively, the two laser light sources 11 and 12 may be integrated on the same semiconductor substrate. For example, in a measurement device 203 shown in Fig. 14, the laser light sources 11 and 12 are integrated on the same semiconductor substrate 40. This allows the measurement device 203 to be further miniaturized.

[0163] The laser light sources 11 and 12 in the measuring device 202 may be integrated on the same semiconductor substrate 40, and the laser light sources 11 and 12 in the measuring device 203 may be integrated on different semiconductor substrates 41 and 42, respectively. The same applies to the measuring device 201. Furthermore, at least one of the laser light sources 11 and 12 does not have to be integrated on a semiconductor substrate.

[0164] [Operation] Next, a description will be given of the operations of the measuring devices 201, 202, and 203 according to this embodiment. In the following, the operation of the measuring device 202 or 203 will be described as a representative, but the operation of the measuring device 201 is similar.

[0165] First Example First, an operation when using the light intensities of the output lights 31 and 32 will be described with reference to Fig. 15. Fig. 15 is a flowchart showing an example of the operation of the measuring device 202 according to this embodiment.

[0166] In the first example, interference light is not used, and therefore half mirrors 43 and 44 do not need to be provided in the measurement device 202 shown in Fig. 13. Detector 51 detects output light 31, and detector 52 detects output light 32.

[0167] 15 , the measurement device 202 checks the reference value of the repetition frequency of the output light 31 from the laser light source 11 (S20 to S25), and then checks the reference value of the repetition frequency of the output light 32 from the laser light source 12 (S26 to S31). Note that this order of execution is merely an example. For example, the measurement device 202 may check the reference value of the repetition frequency of the output light 32 (S26 to S31) and then check the reference value of the repetition frequency of the output light 31 (S20 to S25). Furthermore, the checks of the reference values ​​of the repetition frequencies of the output lights 31 and 32 may be performed simultaneously in parallel.

[0168] Specifically, first, the measurement device 202 performs HML at one drive frequency within the sweep range (S20). Specifically, while the RF oscillator 91 supplies a high-frequency signal of a set frequency to the saturable absorber 41sa, the laser light source 11 is operated to output the output light 31. The detector 51 converts the output light 31 from the laser light source 11 into an electrical signal, and outputs the signal to the feedback circuit 61.

[0169] Next, the feedback circuit 61 acquires the light intensity based on the signal output from the detector 51 (S21). The feedback circuit 61 stores the acquired light intensity and frequency in association with each other in a memory.

[0170] If sweeping of the sweep range is not complete (No in S22), the feedback circuit 61 controls the frequency shifter 81 to change the drive frequency (first drive frequency) (S23). For example, the step size for changing the first drive frequency is 10 kHz. After changing the first drive frequency, the processes from step S20 to step S23 are repeated until sweeping of the sweep range is complete.

[0171] When the sweeping of the sweep range is completed (Yes in S22), the feedback circuit 61 checks the reference value of the repetition frequency of the output light 31 (S24). Specifically, the method described with reference to FIG. 8 is used.

[0172] The feedback circuit 61 determines the set value of the first driving frequency based on the confirmed repetition frequency reference value (S25). For example, the feedback circuit 61 determines the repetition frequency reference value as the set value of the first driving frequency. Then, the measurement device 202 performs HML on the laser light source 11 using the determined set value.

[0173] Next, the measurement device 202 performs the same processes as steps S20 to S25 on the laser light source 12. Specifically, the measurement device 202 performs HML at one drive frequency within the sweep range (S26). Specifically, while the RF oscillator 92 supplies a high-frequency signal of a set frequency to the saturable absorber 42sa, the laser light source 12 is operated to output the output light 32. The detector 52 converts the output light 32 from the laser light source 12 into an electrical signal and outputs the signal to the feedback circuit 62.

[0174] Next, the feedback circuit 62 acquires the light intensity based on the signal output from the detector 52 (S27). The feedback circuit 62 stores the acquired light intensity and frequency in association with each other in the memory.

[0175] If sweeping of the sweep range is not complete (No in S28), the feedback circuit 62 controls the frequency shifter 82 to change the drive frequency (second drive frequency) (S29). For example, the step size for changing the second drive frequency is 10 kHz. After changing the second drive frequency, the processes from step S26 to step S29 are repeated until sweeping of the sweep range is complete.

[0176] When the sweeping of the sweep range is completed (Yes in S28), the feedback circuit 62 checks the reference value of the repetition frequency of the output light 32 (S30). Specifically, the method described with reference to FIG. 8 is used.

[0177] The feedback circuit 62 determines the set value of the second driving frequency based on the confirmed repetition frequency reference value (S31). For example, the feedback circuit 62 determines the repetition frequency reference value as the set value of the second driving frequency. Then, the measurement device 202 performs HML on the laser light source 12 using the determined set value.

[0178] As described above, by sweeping each of the first drive frequency and the second drive frequency, the repetition frequency reference values ​​of the output beams 31 and 32 can be confirmed, enabling highly accurate measurements. Since an RF spectrum analyzer is not required, the configuration of the measurement device 202 can be simplified. Therefore, according to this embodiment, it is possible to realize a measurement device 202 that can perform accurate measurements with a simple configuration.

[0179] Second Example Next, an operation in a case where the pulse interval of the interference light of the output lights 31 and 32 is used will be described with reference to Fig. 16. Fig. 16 is a flowchart showing an example of the operation of the measuring device 203 according to this embodiment.

[0180] In the second example, similar to the first example, after confirming the reference value of the repetition frequency of the output light 31 from the laser light source 11 (S40 to S45), the reference value of the repetition frequency of the output light 32 from the laser light source 12 is confirmed (S46 to S51). Note that this execution order is merely an example. For example, the measuring device 203 may confirm the reference value of the repetition frequency of the output light 32 (S46 to S51) and then confirm the reference value of the repetition frequency of the output light 31 (S40 to S45).

[0181] First, the measurement device 203 performs HML at one drive frequency within the sweep range (S40). Specifically, while the RF oscillator 91 supplies a high-frequency signal of a set frequency to the saturable absorber 41sa, the laser light source 11 is operated to output output light 31. The laser light source 12 also outputs output light 32. The output lights 31 and 32 are incident on the detector 51 and detected as interference light. The detector 51 outputs a signal representing the intensity of the interference light to the feedback circuit 61.

[0182] Next, the feedback circuit 61 acquires the pulse interval based on the signal output from the detector 51 (S41). The feedback circuit 61 stores the acquired pulse interval and the value of the first drive frequency in association with each other in the memory.

[0183] If sweeping of the sweep range is not complete (No in S42), the feedback circuit 61 controls the frequency shifter 81 to change the drive frequency (first drive frequency) (S43). After changing the first drive frequency, the processes from step S40 to step S43 are repeated until sweeping of the sweep range is complete.

[0184] When the sweeping of the sweep range is completed (Yes in S42), the feedback circuit 61 checks the reference value of the repetition frequency of the output light 31 (S44). Specifically, the method described with reference to FIG. 7 is used.

[0185] The feedback circuit 61 determines the set value of the first driving frequency based on the confirmed repetition frequency reference value (S45). For example, the feedback circuit 61 determines the repetition frequency reference value as the set value of the first driving frequency. Then, the measurement device 203 performs HML on the laser light source 11 using the determined set value.

[0186] Next, the measurement device 203 performs the same processes as steps S40 to S45 on the laser light source 12. Specifically, the measurement device 203 performs HML at one drive frequency within the sweep range (S46). Specifically, the RF oscillator 92 operates the laser light source 12 to output output light 32 while supplying a high-frequency signal of a set frequency to the saturable absorber 42sa. The RF oscillator 91 also supplies a high-frequency signal having a frequency equal to the repetition frequency reference value confirmed in step S44 to the saturable absorber 41sa, thereby operating the laser light source 11 in HML. The output lights 31 and 32 are incident on the detector 51 and detected as interference light. The detector 51 outputs a signal representing the intensity of the interference light to the feedback circuit 61.

[0187] Next, the feedback circuit 61 acquires the pulse interval based on the signal output from the detector 51 (S47). The feedback circuit 61 stores the acquired pulse interval and the value of the second drive frequency in association with each other in the memory.

[0188] If sweeping of the sweep range is not complete (No in S48), the feedback circuit 61 controls the frequency shifter 82 to change the drive frequency (second drive frequency) (S49). After changing the second drive frequency, the processes from step S46 to step S49 are repeated until sweeping of the sweep range is complete. Note that the sweep range may be the same as or different from the sweep range of the laser light source 11.

[0189] When the sweeping of the sweep range is completed (Yes in S48), the feedback circuit 61 checks the reference value of the repetition frequency of the output light 32 (S50). Specifically, the method described with reference to FIG. 7 is used.

[0190] The feedback circuit 61 determines the set value of the second driving frequency based on the confirmed repetition frequency reference value (S51). For example, the feedback circuit 61 determines the repetition frequency reference value as the set value of the second driving frequency. Then, the measurement device 203 performs HML on the laser light source 12 using the determined set value.

[0191] As described above, by sweeping each of the first drive frequency and the second drive frequency, the repetition frequency reference values ​​of the output beams 31 and 32 can be confirmed, enabling highly accurate measurements. Since the detector 51 only needs to detect the interference light, a high-speed photodetector is not required. Furthermore, since an RF spectrum analyzer does not need to be used, the configuration of the measurement device 203 can be simplified. Therefore, according to this embodiment, a measurement device 203 that can perform accurate measurements with a simple configuration can be realized.

[0192] <Third Example> Next, an operation when the second drive frequency sweep is not performed on the laser light source 12 will be described with reference to Fig. 17. Fig. 17 is a flowchart showing an example of the operation of the measuring device 203 according to this embodiment. The following description will focus on differences from the second example, and description of commonalities will be omitted or simplified.

[0193] As shown in Fig. 17, the process of checking the repetition frequency reference value for the laser light source 11 and determining the set value of the first driving frequency (steps S40 to S45) is the same as in the second example shown in Fig. 16. In the example shown in Fig. 17, after the set value of the first driving frequency is determined, the feedback circuit 61 calculates the difference δf ｒｅｐ The difference in repetition frequency δf is obtained (S56). ｒｅｐ is the pulse interval δT ｒｅｐ Since it is the reciprocal of the pulse interval δT obtained in step S41 ｒｅｐ It can be calculated from

[0194] Next, the feedback circuit 61 calculates the difference in repetition frequency δf ｒｅｐSince the repetition frequency of the output light 31 is the first drive frequency of the frequency shifter 81, the feedback circuit 61 determines the repetition frequency reference value of the output light 32 based on the difference δf ｒｅｐ and the repetition frequency of the output light 31, the reference value of the repetition frequency of the output light 32 can be calculated.

[0195] The feedback circuit 61 determines the set value of the second driving frequency based on the confirmed repetition frequency reference value (S58). For example, the feedback circuit 61 determines the repetition frequency reference value as the set value of the second driving frequency. Then, the measurement device 203 performs HML on the laser light source 12 using the determined set value.

[0196] As described above, since it is necessary to sweep only the first drive frequency and not the second drive frequency, the process required to confirm the repetition frequency reference value can be simplified and the time required for this process can be reduced. Therefore, according to this embodiment, it is possible to realize a measurement device 203 that can perform measurements with high accuracy using a simple configuration.

[0197] <Fourth Example> Next, a modification of the operation of the first example will be described as the fourth example. In the first example, the confirmed repetition frequency reference value is determined as the set value of the driving frequency. In this example, however, the desired δf ｒｅｐ The set values ​​of the first drive frequency and the second drive frequency are determined so as to obtain the following. In the following, differences from the first example will be mainly described, and descriptions of commonalities will be omitted or simplified.

[0198] 18 is a flowchart showing an example of the operation of the measurement device 202 according to this embodiment. As shown in FIG. 18, compared to the operation of the first example shown in FIG. 15, the process of determining the first drive frequency (S25) and the process of determining the second drive frequency (S31) are omitted. Instead, in the example shown in FIG. 18, after confirmation of the reference values ​​of the repetition frequencies of the output beams 31 and 32 has been completed, the feedback circuits 61 and 62 determine the difference δf between the desired repetition frequencies. ｒｅｐ The set values ​​of the first drive frequency and the second drive frequency are determined so that:

[0199] For example, the feedback circuits 61 and 62 determine the setting values ​​of the first and second drive frequencies so that the difference between the setting values ​​of the first and second drive frequencies is equal to a desired value and one of the setting values ​​of the first and second drive frequencies is equal to the corresponding repetition frequency reference value. Alternatively, the feedback circuits 61 and 62 may determine the setting values ​​of the first and second drive frequencies so that the sum of the absolute value of the difference between the setting value of the first drive frequency and the repetition frequency reference value of the output light 31 and the absolute value of the difference between the setting value of the second drive frequency and the repetition frequency reference value of the output light 32 is minimized.

[0200] As described above, according to this embodiment, the difference in repetition frequency δf ｒｅｐ can be set to a desired value, it becomes possible to change the measurement mode depending on the situation, such as shortening the measurement time and / or improving the measurement accuracy. Therefore, the range of uses of the measurement device 202 can be expanded.

[0201] <Fifth Example> Next, a modification of the operation of the second example will be described as the fifth example. In the second example, the confirmed repetition frequency reference value is determined as the set value of the driving frequency, whereas in this example, the desired δf ｒｅｐ The set values ​​of the first drive frequency and the second drive frequency are determined so as to obtain the following. In the following, the differences from the second example will be mainly described, and the description of the commonalities will be omitted or simplified.

[0202] 19 is a flowchart showing an example of the operation of the measurement device 203 according to this embodiment. As shown in FIG. 19, compared to the operation of the second example shown in FIG. 16, the process of determining the first drive frequency (S45) and the process of determining the second drive frequency (S51) are omitted. Instead, in the example shown in FIG. 19, after confirmation of the repetition frequency reference values ​​of the output beams 31 and 32 is completed, the feedback circuit 61 determines the difference δf between the desired repetition frequencies. ｒｅｐ The set values ​​of the first drive frequency and the second drive frequency are determined (S61) so that: The specific method for determining the set values ​​is the same as in FIG.

[0203] As described above, according to this embodiment, the difference in repetition frequency δf ｒｅｐ can be set to a desired value, it becomes possible to change the measurement mode depending on the situation, such as shortening the measurement time and / or improving the measurement accuracy. Therefore, the range of uses of the measurement device 203 can be expanded.

[0204] Next, a modified example of the operation of the third example will be described as the sixth example. In the third example, the confirmed repetition frequency reference value is determined as the set value of the driving frequency, whereas in this example, the desired δf ｒｅｐ The set values ​​of the first drive frequency and the second drive frequency are determined so as to obtain the following. In the following, the differences from the third example will be mainly described, and the description of the commonalities will be omitted or simplified.

[0205] 20 is a flowchart showing an example of the operation of the measurement device 203 according to this embodiment. As shown in FIG. 20, compared to the operation of the third example shown in FIG. 17, the process of determining the first drive frequency (S45) and the process of determining the second drive frequency (S58) are omitted. Instead, in the example shown in FIG. 20, after confirmation of the repetition frequency reference values ​​of the output beams 31 and 32 is completed, the feedback circuit 61 determines the difference δf between the desired repetition frequencies. ｒｅｐ The set values ​​of the first drive frequency and the second drive frequency are determined (S61) so that: The specific method for determining the set values ​​is the same as in FIG.

[0206] As described above, according to this embodiment, the difference in repetition frequency δf ｒｅｐ can be set to a desired value, it becomes possible to change the measurement mode depending on the situation, such as shortening the measurement time and / or improving the measurement accuracy. Therefore, the range of uses of the measurement device 203 can be expanded.

[0207] While the measurement device and the measurement method according to one or more aspects have been described above based on the embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the gist of the present disclosure, various modifications conceivable by those skilled in the art to the present embodiments and configurations constructed by combining components of different embodiments are also included within the scope of the present disclosure.

[0208] Furthermore, various modifications, substitutions, additions, omissions, etc. can be made to each of the above-described embodiments within the scope of the claims or their equivalents.

[0209] A measurement device according to the present disclosure can be used for various measurement purposes such as distance measurement, displacement measurement, etc. For example, a measurement device according to the present disclosure can be used as a displacement meter, a shape inspection device, etc.

[0210] 10, 11, 12 Laser light source 20 Resonator 20d １ n-doped layer 20d ２ , 20d ３ p-doped layers 20g, 41g, 42g Gain medium 20i Intermediate layer 20m １ , 41m １ , 42m １ First mirror 20m ２ , 41m ２ , 42m ２ Second mirror 20s Semiconductor layer 20sa, 41sa, 42sa Saturable absorber 20w Optical waveguide 21, 40, 41, 42 Semiconductor substrate 21a High refractive index layer 21b Low refractive index layer 22 Protective layer 30, 31, 32 Output light 43, 44 Half mirror 51, 52 Detector 61, 62 Feedback circuit 71, 72, 81, 82 Frequency shifter 91, 92 RF oscillator 101, 102, 201, 202, 203 Measurement device

Claims

1. A measurement device comprising: a first mode-locked laser light source that emits first output light; a first frequency shifter that sets the repetition frequency of the first output light to a first drive frequency; a detector onto which the first output light is incident; and a feedback circuit that determines the set value of the first drive frequency based on a first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency using the first frequency shifter.

2. The measurement device according to claim 1, further comprising: a second mode-locked laser light source that emits second output light; and a second frequency shifter that sets the repetition frequency of said second output light to a second drive frequency, wherein said second output light is further incident on said detector, and said feedback circuit further determines the set value of said second drive frequency based on said first signal.

3. The measurement device according to claim 2, wherein the first signal is a signal representing the intensity of interference light generated by interference between the first output light and the second output light.

4. The measurement device according to claim 2 or 3, wherein the first mode-locked laser light source and the second mode-locked laser light source are integrated on different semiconductor substrates.

5. The measurement device according to claim 2 or 3, wherein the first mode-locked laser light source and the second mode-locked laser light source are integrated on the same semiconductor substrate.

6. The measurement device according to any one of claims 1 to 3, wherein the first frequency shifter includes: a saturable absorber; and a signal generator that supplies a high-frequency signal to the saturable absorber, the frequency of the high-frequency signal being the first drive frequency; and the first mode-locked laser light source includes a resonator that includes a gain medium and the saturable absorber.

7. The measurement device of claim 6, wherein after the feedback circuit determines the set value of the first drive frequency, the signal generator supplies the high frequency signal having a frequency at the set value of the first drive frequency determined by the feedback circuit, with an intensity higher than the intensity of the high frequency signal during the sweep of the first drive frequency.

8. The measurement device according to any one of claims 1 to 3, wherein the first mode-locked laser light source is an optical frequency comb laser light source.

9. The measurement device according to claim 1, further comprising: a second mode-locked laser light source that emits second output light; and a second frequency shifter that sets the repetition frequency of said second output light to a second drive frequency, wherein said second drive frequency is fixed.

10. The measurement device according to any one of claims 1 to 3, wherein the feedback circuit determines the set value of the first drive frequency based on intensity information of the first signal.

11. The measurement device according to claim 3, wherein the feedback circuit determines the set value of the first drive frequency and the set value of the second drive frequency based on time information of the first signal.

12. The measurement device according to claim 2 or 3, wherein the feedback circuit: confirms the value of the repetition frequency of the first output light based on the first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency; then confirms the value of the repetition frequency of the second output light based on the second signal output from the detector when the second mode-locked laser light source is driven while sweeping the second drive frequency; and determines the set value of the first drive frequency and the set value of the second drive frequency based on the value of the repetition frequency of the first output light and the value of the repetition frequency of the second output light.

13. The measurement device according to claim 3, wherein the feedback circuit confirms the value of the repetition frequency of the first output light based on the first signal output from the detector when the first mode-locked laser light source is driven while sweeping the first drive frequency, and then detects the difference between the repetition frequency of the first output light and the repetition frequency of the second output light, and determines the set value of the first drive frequency and the set value of the second drive frequency based on the difference.

14. The measurement device according to claim 2 or 3, wherein the feedback circuit determines the set value of the first drive frequency and the set value of the second drive frequency so that the difference between the repetition frequency of the first output light and the repetition frequency of the second output light becomes a predetermined value.

15. A measurement method comprising: sweeping a drive frequency using a frequency shifter that sets the repetition frequency of output light emitted by a mode-locked laser light source as a drive frequency; driving the mode-locked laser light source while sweeping the drive frequency, and causing the output light to be incident on a detector; and determining a set value of the drive frequency based on a signal output from the detector.

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