Optical frequency comb and measurement equipment

Abstract

An optical frequency comb device (10A) is provided with the following: an optical waveguide (13w); a mirror (13m1) disposed at a first position on the optical waveguide (13w); a mirror (13m2) disposed on the waveguide (13w) at a second position that is different from the first position; a gain medium (13g) and a saturable absorber (13sa), which are disposed between the mirror (13m1) and the mirror (13m2); and a control unit that fixes one of the carrier envelope offset frequency and repetition frequency of the optical frequency comb output from an end of the optical waveguide (13w), and that varies the other.

Description

[Technical Field]

[0001] The present disclosure relates to optical frequency comb devices and metrology devices. [Background technology]

[0002] The optical frequency characteristics of an object can be investigated by irradiating the object with light and obtaining the frequency spectrum of the light transmitted through or reflected by the object. Conventionally, to obtain the frequency spectrum of high-frequency light, a light source with fluctuating intensity and a diffraction grating or prism were used to disperse the light. This limited the accuracy of the frequency spectrum obtained.

[0003] However, optical frequency comb technology is making it possible to obtain precise optical frequency spectra. An optical frequency comb refers to a comb-like frequency spectrum formed from multiple discrete, equally spaced longitudinal modes. In this specification, laser light having an optical frequency comb is referred to as "optical frequency comb laser light" or simply "optical frequency comb." Non-Patent Documents 1 to 3 disclose optical frequency comb devices that generate optical frequency combs. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] AL Gaeta et al., “Photonic-chip-based frequency combs”, Nature Photonics, 2019, Vol. 13, pp. 158-169 [Non-patent document 2] J. Riemensberger et al., “Massively parallel coherent laser ranging using a soliton microcomb”, Nature, 2020, Vol. 581, pp. 161-170 [Non-patent document 3] Z. Wang et al., “A III-V-on-Si ultra-dense comb laser”, Light: Science & Applications, 2017, Vol. 6, e16260 Summary of the Invention [Problem to be solved by the invention]

[0005] In order to apply optical frequency combs to various applications, it is necessary to be able to change the frequency with high precision.

[0006] The present disclosure provides an optical frequency comb device and the like that can change frequency with high precision. [Means for solving the problem]

[0007] An optical frequency comb device according to one embodiment of the present disclosure includes an optical waveguide, a first mirror disposed at a first position of the optical waveguide, a second mirror disposed at a second position of the optical waveguide different from the first position, a gain medium and a saturable absorber disposed between the first mirror and the second mirror, and a control unit that fixes one of the repetition frequency and the carrier envelope offset frequency of an optical frequency comb output from an end of the optical waveguide and changes the other.

[0008] Another aspect of the present disclosure provides an optical frequency comb device comprising: an optical waveguide; a first mirror disposed at a first position of the optical waveguide; a second mirror disposed at a second position of the optical waveguide different from the first position; a gain medium and a saturable absorber disposed between the first mirror and the second mirror; and a signal generator that supplies a high-frequency signal to the saturable absorber, wherein the signal generator changes the frequency of the high-frequency signal to change the repetition frequency of an optical frequency comb output from an end of the optical waveguide.

[0009] A measurement device according to one aspect of the present disclosure includes an optical frequency comb device according to the above aspect, an emission unit that emits the optical frequency comb toward an object, an optical detection unit into which reflected light of the optical frequency comb by the object is incident, and a calculation unit that calculates the distance to the object or the speed of the object based on the detection result by the optical detection unit. [Effects of the Invention]

[0010] According to an optical frequency comb device according to an aspect of the present disclosure, it is possible to change the frequency with high precision. [Brief explanation of the drawings]

[0011] [Figure 1A] FIG. 1A is a diagram showing a schematic diagram of the change over time in the electric field of the optical frequency comb laser light. [Figure 1B] FIG. 1B is a diagram schematically illustrating the frequency spectrum of optical frequency comb laser light. [Figure 2A] FIG. 2A is a top view diagram illustrating a schematic of an optical frequency comb laser source consisting of a resonator including a gain medium integrated on a semiconductor substrate. [Figure 2B] FIG. 2B is a cross-sectional view schematically illustrating the optical frequency comb laser light source at the position indicated by the line IIB-IIB in FIG. 2A. [Figure 2C] FIG. 2C is a cross-sectional view that schematically shows a propagation path of light in the cross section shown in FIG. 2B. [Figure 3] FIG. 3 is a diagram showing the principle of distance measurement in FMCW. [Figure 4] FIG. 4 is a diagram showing an optical frequency comb on the frequency axis before and after modulating the carrier envelope offset frequency. [Figure 5] FIG. 5 is a diagram showing an optical frequency comb on the frequency axis before and after modulating the repetition rate. [Figure 6] FIG. 6 is a diagram schematically illustrating an example of an optical frequency comb device according to the first embodiment. [Figure 7] FIG. 7 is a diagram illustrating an example of an optical frequency comb device according to the second embodiment. [Figure 8] FIG. 8 is a diagram illustrating an example of an optical frequency comb device according to the third embodiment. [Figure 9] FIG. 9 is a diagram schematically illustrating an example of an optical frequency comb device according to a modification of the embodiment. [Figure 10] FIG. 10 is a diagram schematically illustrating the configuration of a measurement device according to the fourth embodiment. [Figure 11] FIG. 11 is a diagram schematically illustrating the configuration of a detection unit of a measurement device according to the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] (Summary of the Disclosure) An optical frequency comb device according to one embodiment of the present disclosure includes an optical waveguide, a first mirror disposed at a first position of the optical waveguide, a second mirror disposed at a second position of the optical waveguide different from the first position, a gain medium and a saturable absorber disposed between the first mirror and the second mirror, and a control unit that fixes one of the repetition frequency and the carrier envelope offset frequency of an optical frequency comb output from an end of the optical waveguide and changes the other.

[0013] This prevents both the repetition rate and the carrier envelope offset frequency from changing simultaneously, allowing the frequency of the optical frequency comb to be precisely adjusted to the desired frequency. Frequency modulation is one example of frequency modulation.

[0014] Furthermore, for example, the control unit may include a current source that supplies a current to the gain medium, and a signal generator that supplies a high-frequency signal to the saturable absorber.

[0015] This allows the carrier envelope offset frequency to be fixed or changed by adjusting the magnitude of the current supplied to the gain medium, and also allows the repetition rate to be fixed or changed by adjusting the frequency of the high frequency signal supplied to the saturable absorber.

[0016] Furthermore, for example, the current source may change the carrier envelope offset frequency by changing the magnitude of the current, and the signal generator may fix the frequency of the high frequency signal.

[0017] This allows for easy modulation of multiple modes (i.e., frequency components) of the optical frequency comb by varying the carrier envelope offset frequency. Furthermore, since the repetition rate can be fixed, each mode can be easily separated at the detector. This facilitates distance measurement based on frequency-modulated continuous wave (FMCW).

[0018] Furthermore, for example, the current source may fix the magnitude of the current, and the signal generator may change the frequency of the high-frequency signal to change the repetition frequency.

[0019] This makes it possible to perform distance measurement based on FMCW even when the repetition frequency is changed.

[0020] Also, for example, an optical frequency comb device according to one embodiment of the present disclosure may further include a phase modulator disposed between the first mirror and the second mirror, the control unit may further include a voltage source that supplies a voltage to the phase modulator, the voltage source may change the magnitude of the voltage to change the carrier envelope offset frequency, and the signal generator may fix the frequency of the high frequency signal.

[0021] This allows for easy modulation of multiple modes (i.e., frequency components) of the optical frequency comb by varying the carrier-envelope offset frequency. Furthermore, since the repetition rate can be fixed, each mode can be easily separated at the detector. Therefore, distance measurement based on FMCW can be easily performed.

[0022] Also, for example, an optical frequency comb device according to one aspect of the present disclosure may further include a semiconductor substrate, and the optical waveguide, the first mirror, the second mirror, the gain medium, and the saturable absorber may be integrated on the semiconductor substrate.

[0023] This allows for a more compact optical frequency comb device. Furthermore, frequency change is easier than with a device that includes a CW laser light source and a microresonator that resonates the laser light emitted from the CW laser light source to generate an optical frequency comb. In other words, even when changing the frequency of the laser light from a CW laser light source, theoretically, an optical frequency comb with a changed repetition rate or carrier-envelope offset frequency can be obtained. However, it is difficult to maintain resonance between the CW laser light source and the microresonator while changing the frequency of the laser light from the CW laser light source, making it difficult to maintain the accuracy of the frequency change. In contrast, an on-chip optical frequency comb device including a gain medium, such as the optical frequency comb device according to one embodiment of the present disclosure, allows the frequency of the optical frequency comb to be precisely changed to a desired frequency by a control unit.

[0024] Furthermore, for example, the control unit may be integrated on the semiconductor substrate.

[0025] This allows further miniaturization of the optical frequency comb device.

[0026] Another aspect of the present disclosure provides an optical frequency comb device comprising: an optical waveguide; a first mirror disposed at a first position of the optical waveguide; a second mirror disposed at a second position of the optical waveguide different from the first position; a gain medium and a saturable absorber disposed between the first mirror and the second mirror; and a signal generator that supplies a high-frequency signal to the saturable absorber, wherein the signal generator changes the frequency of the high-frequency signal to change the repetition frequency of an optical frequency comb output from an end of the optical waveguide.

[0027] This allows the repetition frequency to be changed with high precision in accordance with changes in the frequency of the high frequency signal.

[0028] Also, for example, an optical frequency comb device according to another aspect of the present disclosure may further include a current source that supplies a current to the gain medium, and the current source may fix the magnitude of the current.

[0029] This prevents both the repetition rate and the carrier envelope offset frequency from changing simultaneously, allowing the frequency of the optical frequency comb to be changed to a desired frequency with high precision.

[0030] In addition, a measurement device according to one aspect of the present disclosure includes an optical frequency comb device according to any of the above aspects, an emission unit that emits the optical frequency comb toward an object, an optical detection unit into which reflected light of the optical frequency comb by the object is incident, and a calculation unit that calculates the distance to the object or the speed of the object based on the detection result by the optical detection unit.

[0031] This allows one of the carrier envelope offset frequency and the repetition frequency to be fixed and the other to be varied, making it possible to easily perform distance measurement based on FMCW.

[0032] Furthermore, for example, the optical frequency comb may be incident on the optical detection unit, and the calculation unit may calculate the distance or the velocity based on a beat signal generated by interference between the reflected light and the optical frequency comb in the optical detection unit.

[0033] This makes it possible to easily calculate not only the distance but also the speed by using the beat signal.

[0034] Furthermore, for example, the light detection unit may include a demultiplexer that separates incident light into frequencies, and a plurality of light receivers that receive the light of each separated frequency.

[0035] This allows detection using wavelength division multiplexing (WDM) etc. Since multiple modes can be detected separately, a wide modulation range can be secured for each mode.

[0036] Furthermore, for example, the emission unit may emit the optical frequency comb in a different direction for each frequency.

[0037] This allows distance and / or speed to be measured at multiple points simultaneously.

[0038] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0039] 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.

[0040] 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.

[0041] Furthermore, in this specification, terms indicating the relationship between elements, such as parallel or coincident, terms indicating the shape of elements, such as rectangle, and numerical ranges are not expressions that only express the strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0042] Furthermore, 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 applied not only to a case where two components are arranged with a gap between them and another component exists between them, but also to a case where two components are arranged closely together and the two components are in contact with each other.

[0043] 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.

[0044] (optical frequency comb) First, with reference to FIGS. 1A and 1B, the time variation and frequency spectrum of the electric field of an optical frequency comb (i.e., optical frequency comb laser light) will be described.

[0045] 1A is a diagram showing an example of temporal changes in the electric field of an optical frequency comb laser beam, where the horizontal axis represents time and the vertical axis represents the electric field of the laser beam.

[0046] As shown in Figure 1A, the optical frequency comb laser light has a repetition period of T rep It is formed from a train of optical pulses generated at a repetition period of T rep is, for example, 100 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 1 ps or less.

[0047] The optical frequency comb laser source includes a laser resonator that generates an optical frequency comb laser beam by pumping light or charge injection. The details of the laser resonator will be described later. In the laser resonator, the envelope of the optical 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 may differ. gand phase velocity v p Due to the difference between the wavelengths of adjacent optical pulses and the wavelength of light, 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 is smaller than 2π. The repetition period of an optical pulse train is T rep =L / v g is expressed by

[0048] 1B is a diagram showing the frequency spectrum of an optical frequency comb laser beam, where the horizontal axis represents frequency and the vertical axis represents laser beam intensity.

[0049] As shown in Figure 1B, the optical frequency comb laser light has a comb-like frequency spectrum formed by multiple discrete equally spaced lines. The frequencies of the multiple 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 rep is, for example, 10 MHz or more and 10 GHz or less. The optical path 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 will be 1GHz.

[0050] If the full width at half maximum of the optical frequency comb is Δf, then Δf = 1 / Δt. The full width at half maximum of the optical frequency comb Δf is, for example, between 1 THz and 100 THz. Assuming that the equidistant lines exist up to near zero frequency, the frequency of the equidistant 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 the repetition frequency f repLower than the carrier envelope offset frequency f CEO is the 0th mode frequency, then the nth mode frequency f n is f n =f CEO +nf rep The electric field E(t) of the optical frequency comb laser light shown in Figure 1A is expressed as 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 )].

[0051] (Optical frequency comb device) 2A and 2B, a brief description will be given of an optical frequency comb device integrated on a semiconductor substrate, i.e., an on-chip optical frequency comb device. The optical frequency comb device is also called an optical frequency comb laser source.

[0052] FIG. 2A is a top view of an optical frequency comb laser source having a resonator including a gain medium integrated on a semiconductor substrate. FIG. 2B is a cross-sectional view of the optical frequency comb laser source taken along line IIB-IIB in FIG. 2A. For reference, the mutually orthogonal X-, Y-, and Z-axes are shown for ease of explanation and do not limit the orientation during use. In FIG. 2B, the n-doped layer 13d1, p-doped layers 13d2 and 13d3, low-refractive-index layer 14b, and protective layer 16 are not shaded to indicate a cross section.

[0053] As shown in FIGS. 2A and 2B, the optical frequency comb laser source 10 includes a resonator 13, a semiconductor substrate 14, and a protective layer 16. In the illustrated example, the surface of the semiconductor substrate 14 is parallel to the XY plane. As shown in FIG. 2B, the semiconductor substrate 14 has a layered structure in which a high-refractive index layer 14a, such as Si, and a low-refractive index layer 14b, such as SiO2, are stacked in this order in the Z-axis direction. The refractive index of the high-refractive index layer 14a is higher than that of the low-refractive index layer 14b. The semiconductor substrate 14 does not necessarily include the high-refractive index layer 14a. The protective layer 16 is formed of an inorganic insulating material, such as SiO2, or an organic insulating material, such as BCB (benzocyclobutene). The protective layer 16 is provided to cover the upper surface of the low-refractive index layer 14b.

[0054] The resonator 13 is an example of a laser resonator having a predetermined optical path length. As shown in FIG. 2A, the resonator 13 includes a semiconductor layer 13s, an intermediate layer 13i, an optical waveguide 13w, and mirrors 13m1 and 13m2. As shown in FIG. 2B, the resonator 13 also includes an n-doped layer 13d1 and p-doped layers 13d2 and 13d3. The resonator 13 also includes a gain medium 13g and a saturable absorber 13sa. Portions of the semiconductor layer 13s function as the gain medium 13g and the saturable absorber 13sa, respectively.

[0055] In FIG. 2A, the planar shape of the semiconductor layer 13s in the resonator 13 is represented by a dotted line. The semiconductor layer 13s is tapered at both ends. The tips of the tapered edges overlap the intermediate layer 13i in top view. The intermediate layer 13i is also tapered at both ends. The tips of the tapered edges overlap the optical waveguide 13w in top view. This allows light passing through the semiconductor layer 13s to be efficiently propagated to the optical waveguide 13w via the intermediate layer 13i. In other words, in the optical frequency comb laser light source 10, the optical waveguide 13w, the intermediate layer 13i, and the semiconductor layer 13s each function as a path for transmitting light.

[0056] As shown in FIG. 2B, the optical waveguide 13w in the resonator 13 is embedded in the low-refractive-index layer 14b in the semiconductor substrate 14. The optical waveguide 13w may be provided on the low-refractive-index layer 14b. The optical waveguide 13w may be formed of at least one high-refractive-index material selected from the group consisting of SiN and Si. The refractive index of the optical waveguide 13w is higher than the refractive index of the low-refractive-index layer 14b in the semiconductor substrate 14 and the refractive index of the protective layer 16. This allows light to propagate through the optical waveguide 13w by total internal reflection.

[0057] The intermediate layer 13i is provided between the optical waveguide 13w and the semiconductor layer 13s. The intermediate layer 13i is formed of, for example, a-Si (amorphous silicon). The refractive index of the intermediate layer 13i is higher than that of the optical waveguide 13w and lower than that of the semiconductor layer 13s. The intermediate layer 13i may be formed of other semiconductors as long as the refractive index satisfies the following relationship: optical waveguide 13w < intermediate layer 13i < semiconductor layer 13s. If the intermediate layer 13i were not provided, the difference in refractive index between the semiconductor layer 13s and the optical waveguide 13w would be large, making it difficult for light to propagate from the semiconductor layer 13s, which has a higher refractive index, to the optical waveguide 13w, which has a lower refractive index. This would increase the loss of light during propagation between the semiconductor layer 13s and the optical waveguide 13w. The provision of the intermediate layer 13i reduces the refractive index difference between the layers. Therefore, by providing the intermediate layer 13i, light can be propagated from the semiconductor layer 13s to the optical waveguide 13w with low loss, as shown in Fig. 2C. Fig. 2C is a cross-sectional view schematically showing the propagation path of light in the cross section shown in Fig. 2B. The intermediate layer 13i can be omitted when the difference in refractive index between the semiconductor layer 13s and the optical waveguide 13w is relatively small.

[0058] 2C, a first portion 13w1 of the optical waveguide 13w that is close to the mirror 13m1 and a second portion 13w2 that is close to the mirror 13m2 function as a light propagation path. The first portion 13w1 is a portion of the optical waveguide 13w between the gain medium 13g and the saturable absorber 13sa and the mirror 13m1. The second portion 13w2 is a portion of the optical waveguide 13w between the gain medium 13g and the saturable absorber 13sa and the mirror 13m2.

[0059] The mirror 13m1 is provided at a first position of the optical waveguide 13w. Specifically, the mirror 13m1 is provided at an end of the optical waveguide 13w. The mirror 13m1 may be formed, for example, from a distributed Bragg reflector. In a distributed Bragg reflector, light is reflected by Bragg reflection caused by a periodic structure of refractive index. The mirror 13m1 reflects light propagating through the optical waveguide 13w. The mirror 13m1 may be formed, for example, from a metal as long as it has a reflective function. The mirror 13m1 may also be a reflective element formed from a semiconductor, such as a loop mirror. The same applies to the mirror 13m2. The mirror 13m2 is provided at a second position different from the first position of the optical waveguide 13w. Specifically, the mirror 13m2 is provided at the end of the optical waveguide 13w opposite the mirror 13m1.

[0060] Mirror 13m2 has a lower reflectivity than mirror 13m1. Specifically, the reflectivity of mirror 13m1 is substantially equal to 100%, while the reflectivity of mirror 13m2 is, for example, 70% to 90%. Mirror 13m2 reflects most of the light propagating through optical waveguide 13w, but transmits some of it. The light that passes through mirror 13m2 becomes optical frequency comb laser light 5.

[0061] 2B, semiconductor layer 13s is sandwiched between n-doped layer 13d1 and p-doped layers 13d2 and 13d3. The positional relationship between n-doped layer 13d1 and p-doped layers 13d2 and 13d3 may be reversed.

[0062] Semiconductor layer 13s, n-doped layer 13d1, and p-doped layers 13d2 and 13d3 are buried in protective layer 16. The bottom surface of n-doped layer 13d1 may be in contact with the surface of semiconductor substrate 14. N-doped layer 13d1 may be in contact with intermediate layer 13i.

[0063] The semiconductor layer 13s may be formed of, for example, a III-V semiconductor material, which may include at least one material selected from the group consisting of ZnSe, InGaAlP, InGaAs, GaInAsP, GaInAsSb, InP, GaN, GaAs, InGaAs, AlGaAs, and AlInGaN.

[0064] The semiconductor layer 13s includes a gain medium 13g and a saturable absorber 13sa. The gain medium 13g and the saturable absorber 13sa are provided between the mirror 13m1 and the mirror 13m2. Specifically, the gain medium 13g and the saturable absorber 13sa are provided on the light propagation path between the mirror 13m1 and the mirror 13m2.

[0065] The gain medium 13g is a part of the semiconductor layer 13s, and is a portion sandwiched between the n-doped layer 13d1 and the p-doped layer 13d2. The saturable absorber 13sa is a part of the semiconductor layer 13s, and is a portion sandwiched between the n-doped layer 13d1 and the p-doped layer 13d3.

[0066] The n-doped layer 13d1 is an n-type semiconductor layer. The n-doped layer 13d1 is formed by doping the same III-V semiconductor material as the semiconductor layer 13s with an n-type impurity. For example, a tetravalent element such as Si or a hexavalent element such as selenium (Se) can be used as the n-type impurity.

[0067] The p-doped layers 13d2 and 13d3 are p-type semiconductor layers. The p-doped layers 13d2 and 13d3 are formed by doping the same III-V semiconductor material as the semiconductor layer 13s with p-type impurities. For example, a divalent element such as zinc (Zn) can be used as the p-type impurity. The p-doped layers 13d2 and 13d3 have, for example, the same composition.

[0068] The p-doped layer 13d2 and the p-doped layer 13d3 are separated from each other. Different electrodes (not shown) are attached to the p-doped layers 13d2 and 13d3, respectively. A current is injected into the p-doped layer 13d2 via the electrodes. A reverse bias voltage is applied between the p-doped layer 13d3 and the n-doped layer 13d1. This voltage application causes a portion of the semiconductor layer 13s that is in contact with the p-doped layer 13d3 to function as a saturable absorber 13sa. The saturable absorber 13sa may be formed using carbon nanotubes. The saturable absorber 13sa may be integrated with the mirrors 13m1 and 13m2.

[0069] Like the p-doped layer 13d2, the n-doped layer 13d1 is also provided with an electrode (not shown). A portion of the semiconductor layer 13s, into which charges are injected from the electrodes attached to the n-doped layer 13d1 and the p-doped layer 13d2, functions as a gain medium 13g, which stimulates the emission of light. This stimulated emission of light is repeatedly reflected between the mirrors 13m1 and 13m2 via the intermediate layer 13i and the optical waveguide 13w. In other words, the stimulated emission of light is amplified by repeatedly passing through the gain medium 13g. The amplified light is converted into a mode-locked optical pulse train by the saturable absorber 13sa. Only the wavelength corresponding to the optical path length of the resonator 13, i.e., the resonator length multiplied by the refractive index, is amplified. As a result, the resonator 13 emits optical frequency comb laser light 5. The optical path length of the resonator 13 is the optical path length between the mirrors 13m1 and 13m2.

[0070] (FMCW) The optical frequency comb device according to the embodiment can be used for FMCW. The principle of FMCW will be explained below with reference to FIG. 3. FIG. 3 is a diagram showing the principle of distance measurement in FMCW. In FIG. 3, the horizontal axis represents time, and the vertical axis represents the frequency of the laser light.

[0071] In FMCW, the laser light emitted from the CW light source is frequency modulated over time. In the case of Figure 3, frequency modulation of ΔF is performed within a time width T. The frequency modulation here is linear modulation. In other words, the rate of change in frequency is constant.

[0072] When used to measure the distance to an object, a frequency-modulated laser beam is split into two, one of which is used as a reference beam (solid line in Figure 3) and the other as a measurement beam, and irradiated onto the object. The light reflected by the object (long-wave line in Figure 3) is detected by a photodetector. At this time, there is a frequency difference Δf between the reference beam and the measurement beam (i.e., the reflected beam). This is due to the time difference between the measurement beam's travel time to the object and back. Therefore, the distance to the object can be measured by multiplying the frequency difference Δf by cT / 2ΔF. In other words, the distance is expressed by the following equation (1):

[0073] (1) Distance=Δf×c×T / (2×ΔF) Here, c represents the speed of light. For example, if T = 40 μs, ΔF = 4 GHz, and Δf = 4 MHz, the distance to the target object can be calculated as 6 m using the above formula (1).

[0074] (Two methods of frequency modulation in optical frequency combs) In the measurement device according to the present disclosure, this FMCW-based ranging method is applied to an optical frequency comb device.

[0075] As mentioned above, an optical frequency comb has a repetition rate of f rep and the carrier envelope offset frequency f CEO Therefore, if it is possible to modulate either one of these two frequency parameters, it is possible to measure distance in the same way as with FMCW.

[0076] First, the carrier envelope offset frequency f CEO The case where only the carrier envelope offset frequency f is modulated will be explained using FIG. 4. CEO 4 shows the optical frequency comb on the frequency axis before and after modulation. Specifically, (a) in Fig. 4 shows the optical frequency comb before modulation, and (b) shows the optical frequency comb after modulation. In each of (a) and (b), the horizontal axis represents the optical frequency, and the vertical axis represents the optical intensity.

[0077] The value of the carrier envelope offset frequency before modulation is f CEO Then, at a certain time, the carrier envelope offset frequency is f CEO +δf CEO Accordingly, the nth mode frequency f n is f n =(f CEO +δf CEO )+nf rep Then, by regarding each mode (specifically, the light of each frequency component) as laser light from a CW light source, multiple FMCW modes can be performed simultaneously.

[0078] Next, the repetition rate f rep The case where only the repetition frequency f is modulated will be explained using FIG. 5. rep 5 shows the optical frequency comb on the frequency axis before and after modulation. Figure 5 (a) shows the optical frequency comb before modulation, and (b) shows the optical frequency comb after modulation. In both (a) and (b), the horizontal axis represents the frequency of light, and the vertical axis represents the intensity of light.

[0079] The repetition rate before modulation is f rep Then, at a certain time, the repetition frequency is f rep +δf rep Accordingly, the nth mode frequency f n is f n =f CEO+n(f rep +δf rep ) In this case, as in the case of Figure 4, multiple FMCW modes can be generated simultaneously by regarding each mode as a laser beam from a CW light source.

[0080] There are several methods for modulating the frequency, such as electro-optic modulation (EOM) and modulating the cavity length by temperature control. However, in an on-chip optical frequency comb device with a built-in gain medium, even if the amount of current applied to the gain medium is modulated or the cavity length is modulated by temperature control, the repetition frequency f rep and the carrier envelope offset frequency f CEO This makes it difficult to make the optical frequency comb device function as an FMCW device.

[0081] For this reason, in this disclosure, the repetition frequency f rep and the carrier envelope offset frequency f CEO A modulation unit that modulates one of the two and fixes the other is incorporated into the optical frequency comb device. A specific configuration of the optical frequency comb device according to the embodiment of the present disclosure will be described below.

[0082] (Embodiment 1) First, a configuration example of an optical frequency comb device according to the first embodiment of the present disclosure will be described with reference to FIG. 6. FIG. 6 is a diagram schematically illustrating an optical frequency comb device 10A according to this embodiment. The basic structure of the optical frequency comb device 10A is the same as that of the optical frequency comb laser source 10 shown in FIGS. 2A and 2B. The following description will focus on the differences from the optical frequency comb laser source 10 shown in FIGS. 2A and 2B, and will omit or simplify the description of the commonalities.

[0083] 6, the optical frequency comb device 10A is different in that it newly includes a modulator 21A. The modulator 21A is an example of a controller, and includes a modulation current source 17m and a fixed RF signal generator 18.

[0084] The modulation current source 17m is connected to the gain medium 13g and supplies a current whose magnitude is modulated (hereinafter referred to as a modulation current) to the gain medium 13g. Specifically, the modulation current source 17m is connected to the p-doped layer 13d2 and the n-doped layer 13d1, and a modulation current flows from the p-doped layer 13d2 to the n-doped layer 13d1. This causes the modulation current to be supplied to the gain medium 13g sandwiched between the p-doped layer 13d2 and the n-doped layer 13d1.

[0085] The fixed RF signal generator 18 is connected to the saturable absorber 13sa and supplies a radio frequency signal with a fixed frequency (hereinafter referred to as a fixed RF signal) to the saturable absorber 13sa. Specifically, the fixed RF signal generator 18 is connected to the p-doped layer 13d3 and the n-doped layer 13d1 and supplies a radio frequency voltage signal with a fixed frequency as a fixed RF signal between the p-doped layer 13d3 and the n-doped layer 13d1. This supplies the fixed RF signal to the saturable absorber 13sa sandwiched between the p-doped layer 13d3 and the n-doped layer 13d1.

[0086] The modulation current source 17m modulates the magnitude of the current supplied to the gain medium 13g, thereby modulating the carrier envelope offset frequency. At this time, the modulation of the current magnitude modulates not only the carrier envelope offset frequency but also the repetition frequency.

[0087] In contrast, the fixed RF signal generator 18 fixes the repetition frequency by fixing the frequency of the high-frequency signal. This is because the fixed RF signal causes the saturable absorber 13sa to function as a shutter, controlling the pulse interval within the resonator.

[0088] As a result, an optical frequency comb device 10A that modulates only the carrier envelope offset frequency is realized, enabling distance measurement based on FMCW. More specifically, the optical frequency comb device 10A can simultaneously measure distances at multiple points. Specific distance measurement methods will be described later.

[0089] (Embodiment 2) Next, a configuration example of an optical frequency comb device according to a second embodiment of the present disclosure will be described with reference to FIG. 7. FIG. 7 is a diagram schematically illustrating an optical frequency comb device 10B according to this embodiment. The basic structure of the optical frequency comb device 10B is the same as that of the optical frequency comb laser source 10 shown in FIGS. 2A and 2B. The following description will focus on the differences from the optical frequency comb laser source 10 shown in FIGS. 2A and 2B, and will omit or simplify the description of the commonalities.

[0090] As shown in FIG. 7, the optical frequency comb device 10B is different in that it additionally includes a phase modulator 20 and a modulation section 21B.

[0091] The phase modulator 20 is inserted into the optical waveguide 13w. The phase modulator 20 is provided between the mirror 13m1 and the mirror 13m2. In this embodiment, two phase modulators 20 are inserted into each of the first portion 13w1 and the second portion 13w2.

[0092] The modulation section 21B is an example of a control section, and includes a fixed current source 17, a fixed RF signal generator 18, and a modulation voltage source 19m.

[0093] The fixed current source 17 is connected to the gain medium 13g and supplies a current of a fixed magnitude (hereinafter referred to as a fixed current) to the gain medium 13g. Specifically, the fixed current source 17 is connected to the p-doped layer 13d2 and the n-doped layer 13d1, and a fixed current flows from the p-doped layer 13d2 to the n-doped layer 13d1. This causes the fixed current to be supplied to the gain medium 13g sandwiched between the p-doped layer 13d2 and the n-doped layer 13d1.

[0094] The fixed RF signal generator 18 is connected to the saturable absorber 13sa and supplies an RF signal to the saturable absorber 13sa. The specific connections of the fixed RF signal generator 18 are the same as those in the first embodiment. The modulation voltage source 19m is connected to the phase modulator 20 and supplies a voltage to the phase modulator 20.

[0095] The fixed current source 17 fixes the magnitude of the current supplied to the gain medium 13g, thereby fixing the carrier envelope offset frequency. The fixed RF signal generator 18 also fixes the frequency of the RF signal, thereby fixing the repetition frequency. Therefore, no frequency modulation occurs in this state.

[0096] In response to this, the modulation voltage source 19m modulates the carrier envelope offset frequency by modulating the magnitude of the voltage. Specifically, the refractive index of the phase modulator 20 is changed by the supplied voltage. Because the phase modulator 20 is located within the optical waveguide 13w, the phase of the optical frequency comb is modulated. This modulates the carrier envelope offset frequency of the optical frequency comb. In this case, not only the carrier envelope offset frequency but also the repetition frequency can be modulated, but the repetition frequency is fixed because the frequency of the RF signal is fixed.

[0097] As a result, an optical frequency comb device 10B is realized in which only the carrier envelope offset frequency is modulated, and distance measurement based on FMCW can be performed at the same time.

[0098] The optical frequency comb device 10B may include a temperature adjustment mechanism such as a heater instead of the phase modulator 20. The temperature adjustment mechanism can change the temperature of the optical waveguide 13w according to the voltage. The optical waveguide 13w expands or contracts due to changes in its temperature, thereby changing its refractive index. In other words, the temperature adjustment mechanism can change the refractive index of the optical waveguide 13w, just like the phase modulator 20. Therefore, by connecting a modulation voltage source 19m to the temperature adjustment mechanism and supplying a voltage with a modulated magnitude, the same effect as described above can be achieved. Note that the temperature adjustment mechanism only needs to be able to apply heat to the optical waveguide 13w, and does not have to be located inside the optical waveguide 13w.

[0099] In this embodiment, the current supplied to the gain medium 13g is fixed, making it difficult for the intensity of the optical frequency comb to be modulated. This allows the optical frequency comb device 10B to be used in applications where a constant intensity is required for the optical frequency comb, thereby increasing its versatility.

[0100] (Embodiment 3) Next, a configuration example of an optical frequency comb device according to a third embodiment of the present disclosure will be described with reference to FIG. 8. FIG. 8 is a diagram schematically illustrating an optical frequency comb device 10C according to this embodiment. The basic structure of the optical frequency comb device 10C is the same as that of the optical frequency comb laser source 10 shown in FIGS. 2A and 2B. The following description will focus on differences from the optical frequency comb laser source 10 shown in FIGS. 2A and 2B, and will omit or simplify descriptions of commonalities.

[0101] 8, the optical frequency comb device 10C is different in that it newly includes a modulator 21C. The modulator 21C is an example of a controller, and includes a fixed current source 17 and a modulated RF signal generator 18m.

[0102] The fixed current source 17 is connected to the gain medium 13g and supplies a fixed current to the gain medium 13g. The specific connection of the fixed current source 17 is the same as in the second embodiment.

[0103] The modulated RF signal generator 18m is connected to the saturable absorber 13sa and supplies a frequency-modulated RF signal (hereinafter referred to as a modulated RF signal) to the saturable absorber 13sa. Specifically, the modulated RF signal generator 18m is connected to the p-doped layer 13d3 and the n-doped layer 13d1 and supplies a frequency-modulated high-frequency voltage signal as a modulated RF signal between the p-doped layer 13d3 and the n-doped layer 13d1. This causes the modulated RF signal to be supplied to the saturable absorber 13sa sandwiched between the p-doped layer 13d3 and the n-doped layer 13d1.

[0104] The fixed current source 17 fixes the magnitude of the current supplied to the gain medium 13g, thereby fixing the carrier envelope offset frequency, while the modulated RF signal generator 18m modulates the frequency of the RF signal, thereby modulating the repetition frequency.

[0105] As a result of the above, an optical frequency comb device 10C in which only the repetition frequency is modulated is realized, and distance measurement based on FMCW can be performed.

[0106] In each of the above-described embodiments, the modulation of the magnitude of the current, the frequency of the RF signal, and the magnitude of the voltage is repeatedly performed by linear modulation, as in the case shown in Fig. 3. When modulating the carrier envelope offset frequency, the frequency difference in the nth mode is Δf CEO and is a value independent of n. Therefore, by detecting multiple modes separately using wavelength division multiplexing or the like, distance measurement based on FMCW can be performed for each mode. In other words, it is possible to measure the distance and / or speed of multiple points simultaneously. When detecting multiple modes separately in this way, it is not necessary to consider the overlap of the modulation ranges of each mode, so a wide modulation range can be ensured. Detection using wavelength division multiplexing can be achieved, for example, by using a demultiplexer that separates incident light by frequency and multiple photodetectors that receive the light for each separated frequency.

[0107] On the other hand, when modulating the repetition rate, the frequency difference in the nth mode is n×δf rep and its value depends on n. Therefore, it is possible to distinguish between the modes without having to separate the modes for reception. In this case, the modulation ranges of the multiple modes included in the optical frequency comb are set within ranges that do not overlap with each other.

[0108] Fixed current source 17, modulated current source 17m, fixed RF signal generator 18, modulated RF signal generator 18m, and modulated voltage source 19m are each realized by an electronic circuit including at least one of a resistor, an inductor, a capacitor, a transformer, a diode, a transistor, etc. The electronic circuit may be realized by an integrated IC (Integrated Circuit) element.

[0109] In each embodiment, the optical waveguide 13w, the mirrors 13m1 and 13m2, the gain medium 13g, and the saturable absorber 13sa are integrated on the same semiconductor substrate 14. That is, the optical frequency comb devices 10A, 10B, and 10C are so-called on-chip optical frequency comb devices. This allows the optical frequency comb devices 10A, 10B, and 10C to be miniaturized.

[0110] In each embodiment, the modulation sections 21A, 21B, and 21C are configured separately from the semiconductor substrate 14, but this is not limiting. For example, as shown in FIG. 9, the modulation section 21A may also be integrated on the semiconductor substrate 14. FIG. 9 is a schematic diagram showing an optical frequency comb device 10D according to a modification of the embodiment. FIG. 9 is a modification of the optical frequency comb device 10A shown in FIG. 6, but the modulation section 21B or 21C may be integrated on the semiconductor substrate 14 in the optical frequency comb device 10B or 10C shown in FIG. 7 or 8.

[0111] (Fourth embodiment) Next, a measurement device equipped with an optical frequency comb device will be described with reference to FIG.

[0112] 10 is a diagram schematically illustrating the configuration of a measurement device 100 according to the fourth embodiment. The measurement device 100 shown in FIG. 10 measures the distance to an object 101 and / or the moving speed of the object 101. The measurement device 100 includes an optical frequency comb device 10A, couplers 111 and 112, a circulator 113, an output unit 120, a detector 130, and a signal processing circuit 140. Note that the measurement device 100 may include an optical frequency comb device 10B, 10C, or 10D instead of the optical frequency comb device 10A.

[0113] Couplers 111 and 112 and circulator 113 are connected to an optical fiber that guides optical frequency comb laser light 5 emitted from optical frequency comb device 10A. The optical fiber is configured to connect output 15 of optical frequency comb device 10A to output 120, from circulator 113 to detector 130, and from coupler 111 to coupler 112, respectively.

[0114] As shown in FIG. 6, the output unit 15 is provided at the end of the optical waveguide 13w. Specifically, the output unit 15 is located outside the mirror 13m2 and outputs the optical frequency comb laser light 5 that has passed through the mirror 13m2 to the outside. The output unit 15 includes, for example, a coupling unit for an optical fiber. Specifically, the output unit 15 is a grating coupler, but is not limited to this. The output unit 15 may also be an edge-coupled output.

[0115] The coupler 111 splits the optical frequency comb laser light 5 into measurement light 5a and reference light 5b. The coupler 111 splits light at a predetermined intensity ratio, and the frequency and wavelength of the light remain unchanged after splitting. In other words, both the measurement light 5a and the reference light 5b are optical frequency combs with the same carrier-envelope offset frequency and repetition frequency. The intensity ratio between the measurement light 5a and the reference light 5b is, for example, 9:1, but may also be 1:1. The measurement light 5a passes through the circulator 113 and is emitted from the emission unit 120 toward the object 101. The reference light 5b passes through the coupler 112 and enters the detector 130.

[0116] The output unit 120 is an optical element that outputs the measurement light 5a toward the object 101. The output unit 120 is, for example, a prism or a diffraction grating. The output unit 120 outputs the measurement light 5a in different directions for each frequency. In FIG. 10, multiple solid arrows extending from the output unit 120 toward the object 101 represent the light of each frequency component contained in the measurement light 5a. Each light is reflected by a different part of the object 101. Reflected light 5c generated by this reflection returns to the output unit 120 and is guided toward the coupler 112 by the circulator 113. The circulator 113 is an optical element that restricts the traveling direction of the light and prevents the reflected light 5c guided from the output unit 120 from returning to the coupler 111.

[0117] The coupler 112 couples the reference light 5b and the reflected light 5c and makes the combined light incident on the detector 130. The combination ratio is, for example, 1:1, but is not limited to this.

[0118] The detector 130 is an example of a light detection unit, and receives the reference light 5b and the reflected light 5c, generating and outputting an electrical signal corresponding to the intensity of the incident light. The reference light 5b and the reflected light 5c interfere with each other in the detector 130, generating a beat signal. The detector 130 outputs an electrical signal representing the time waveform of the beat signal. The detector 130 is, for example, a photoelectric conversion element such as a photodiode.

[0119] The signal processing circuit 140 is an example of a calculation unit, and calculates the distance to the object 101 and / or the speed of the object 101 based on the detection result by the detector 130. Specifically, the signal processing circuit 140 calculates the distance and / or the speed based on the beat signal. The method of calculating the distance by the signal processing circuit 140 is as described with reference to FIG. 3. The distance can also be calculated based on a general FMCW method.

[0120] In this embodiment, both the reference light 5b and the reflected light 5c are optical frequency combs, so by performing calculations for each frequency, it is possible to calculate the distance to the location irradiated with light of the corresponding frequency and / or the moving speed of that location. This makes it possible to simultaneously measure the distance to and / or the speed of multiple locations on the object 101. In other words, the measurement device 100 is capable of simultaneous multi-point measurement based on FMCW.

[0121] 11, the detector 130 may include a demultiplexer 131 that separates the incident light into frequencies, and a plurality of photoreceivers 132 that receive the light for each separated frequency. The plurality of photoreceivers 132 correspond one-to-one to the plurality of separated frequencies. By separating and receiving the signal for each frequency in this manner, it is possible to receive light of each frequency component with high accuracy, thereby improving the accuracy of distance measurement and speed measurement.

[0122] (Other embodiments) While the optical frequency comb device and measurement device according to one or more aspects have been described based on the embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the spirit 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.

[0123] For example, the modulation unit may not include a modulated current source or a fixed current source. In other words, stimulated emission may be caused by optical excitation of the gain medium, rather than electrical excitation. For example, the modulation unit may include a laser light source instead of a modulated current source or a fixed current source. The laser light source irradiates the gain medium with laser light as excitation light for the gain medium. The laser light source modulates the intensity of the laser light to thereby modulate the carrier envelope offset frequency f CEO In addition, the laser light source can modulate the carrier envelope offset frequency f by fixing the intensity of the laser light source. CEO can be fixed.

[0124] Also, for example, the modulation unit has a repetition frequency f rep and the carrier envelope offset frequency f CEO In other words, the fixed frequency and the modulated frequency do not have to be always the same, and the modulation unit may switch between the fixed frequency and the modulated frequency depending on the situation.

[0125] Specifically, the modulation section has a repetition frequency f rep is fixed, and the carrier envelope offset frequency f CEO The first mode modulates the repetition frequency f rep and modulates the carrier envelope offset frequency f CEO and a second mode in which the first mode is fixed. The modulation section may switch from the first mode to the second mode and from the second mode to the first mode.

[0126] Furthermore, for example, the change in frequency performed by the control unit does not have to be frequency modulation. For example, the control unit may change either the repetition frequency or the carrier envelope offset frequency between two predetermined values.

[0127] 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. [Industrial Applicability]

[0128] The present disclosure can be used as an optical frequency comb device that can change frequency with high precision, and can be used, for example, in devices for detecting, analyzing, and measuring distances to objects. [Explanation of symbols]

[0129] 5 Optical frequency comb laser light 5a measurement light 5b Reference light 5c reflected light 10 Optical frequency comb laser source 10A, 10B, 10C, 10D Optical frequency comb device 13 Resonator 13d1 n-doped layer 13d2, 13d3 p-doped layers 13i middle tier 13g gain medium 13m1, 13m2 mirror 13s semiconductor layer 13sa saturable absorber 13w optical waveguide 13w1 Part 1 13w2 2nd part 14 Semiconductor substrate 14a High refractive index layer 14b Low refractive index layer 15 Output section 16 Protective layer 17 Fixed current source 17m Modulated Current Source 18 Fixed RF Signal Generator 18m Modulated RF Signal Generator 19m Modulated Voltage Source 20 Phase Modulator 21A, 21B, 21C Modulation section 100 Measuring Equipment 101 Object 111, 112 Coupler 113 Circulator 120 Exit part 130 detector 131 Demultiplexer 132 Photoreceiver 140 Signal Processing Circuit

Claims

1. an optical waveguide; a first mirror provided at a first position of the optical waveguide; a second mirror provided at a second position of the optical waveguide that is different from the first position; a gain medium and a saturable absorber disposed between the first mirror and the second mirror; a control unit that fixes one of a repetition frequency and a carrier envelope offset frequency of the optical frequency comb output from the end of the optical waveguide and changes the other. Optical frequency comb device.

2. The control unit a current source that supplies a current to the gain medium; a signal generator that supplies a high frequency signal to the saturable absorber; The optical frequency comb device of claim 1 .

3. the current source varies the carrier envelope offset frequency by varying the magnitude of the current; The signal generator fixes the frequency of the high frequency signal.

3. The optical frequency comb device of claim 2.

4. The current source fixes the magnitude of the current; the signal generator changes the repetition frequency by changing the frequency of the high-frequency signal; 3. The optical frequency comb device of claim 2.

5. further comprising a phase modulator provided between the first mirror and the second mirror, the control unit further includes a voltage source that supplies a voltage to the phase modulator; the voltage source varies the carrier envelope offset frequency by varying the magnitude of the voltage; The signal generator fixes the frequency of the high frequency signal.

3. The optical frequency comb device of claim 2.

6. Further, the semiconductor substrate is provided, the optical waveguide, the first mirror, the second mirror, the gain medium, and the saturable absorber are integrated on the semiconductor substrate; 6. The optical frequency comb device according to claim 1.

7. The control unit is integrated on the semiconductor substrate.

7. The optical frequency comb device of claim 6.

8. an optical waveguide; a first mirror provided at a first position of the optical waveguide; a second mirror provided at a second position of the optical waveguide that is different from the first position; a gain medium and a saturable absorber disposed between the first mirror and the second mirror; a signal generator that supplies a high frequency signal to the saturable absorber; a current source that supplies a current to the gain medium; The current source fixes the magnitude of the current; the signal generator changes the frequency of the high-frequency signal to change the repetition frequency of the optical frequency comb output from the end of the optical waveguide; Optical frequency comb device.

9. An optical frequency comb device according to any one of claims 1 to 8; an emission unit that emits the optical frequency comb toward an object; a light detection unit to which reflected light of the optical frequency comb from the object is incident; a calculation unit that calculates the distance to the object or the speed of the object based on the detection result by the light detection unit, Measuring equipment.

10. The optical frequency comb is further incident on the optical detection unit, the calculation unit calculates the distance or the velocity based on a beat signal generated by interference between the reflected light and the optical frequency comb at the light detection unit. The measurement device according to claim 9.

11. The light detection unit a demultiplexer that separates the incident light into frequencies; a plurality of light receivers that receive light of each separated frequency; The measuring device according to claim 9 or 10.

12. the emission unit emits the optical frequency comb in different directions for each frequency; The measuring device according to any one of claims 9 to 11.

Images