Light source device

Abstract

This light source device comprises: a first mode-locked laser light source that emits first output light; a second mode-locked laser light source that emits second output light; a first frequency shifter that sets the repetition frequency of the first output light to a first drive frequency; and a second frequency shifter that sets the repetition frequency of the second output light to a second drive frequency. The first drive frequency is different from the second drive frequency. The first drive frequency is different from the value of the repetition frequency of the first output light when the first frequency shifter is not operating.

Description

light source device

[0001] The present disclosure relates to a light source device.

[0002] Conventionally, a light source called an optical frequency comb or optical comb is known. An optical frequency comb is a laser light source that emits laser light whose pulse waveforms are evenly spaced on the time axis and whose spectrum is evenly spaced on the frequency axis. Hereinafter, this laser light source will be referred to as an optical comb laser or optical comb laser light source. Furthermore, the light emitted from this laser light source will be referred to as optical comb laser light.

[0003] A method of preparing a first optical comb laser that emits a first optical comb laser beam and a second optical comb laser that emits a second optical comb laser beam that has a slightly different repetition frequency from the first optical comb laser beam, and measuring by interfering the first optical comb laser beam with the second optical comb laser beam, is called a dual comb (see, for example, non-patent document 1).

[0004] In a dual-comb system, the beat is generated by the interference of two optical comb laser beams. The important point about dual-comb systems is that the spectrum of the optical comb laser beam before interference is in the THz range, which is the frequency of light, while the spectrum after interference is in the MHz range, which is the radio frequency range.

[0005] Conventional detectors have a response frequency below GHz, so they cannot physically detect optical signals in the THz range. Therefore, conventionally, detectors could not be used directly to examine the wavelength of light; instead, a spectrometer was used to separate the light into wavelengths before the detector was used. This resulted in the drawback of taking a long time to sweep the wavelength, making it impossible to perform spectrum measurements in a short time.

[0006] However, dual combs can downconvert light to the MHz range, eliminating the need for a spectrometer and enabling faster spectral measurements than conventional methods. In addition, because they can directly measure optical information, they can achieve highly sensitive and accurate measurements. This has led to dual combs being used in a wide variety of measurements, including spectrometry, distance measurement, and frequency measurement.

[0007] I. Coddington et. al, “Dual-comb spectroscopy”, Optica, Apr. 14, 2016, Vol. 3, Issue 4, pp. 414-426 (2016)Z. Wang et.al, “A III-V-on-Si ultra-dense comb laser”, Light Science & Applications, Dec. 3, 2016, Vol. 6, Issue 5

[0008] To change the repetition rate of an optical comb laser, the optical path length of the resonator is generally changed, which is inversely proportional to the repetition rate. In a free-space laser source, the components can be moved freely, so the repetition rate can be changed by, for example, moving the mirrors at the ends of the resonator.

[0009] However, if the laser light source is fixed in a housing or integrated on a semiconductor substrate, the repetition rate cannot be changed by moving the components as described above, which means that once the repetition rate is determined, it cannot be changed to a desired repetition rate.

[0010] In spectroscopic measurements using a single optical comb laser, the target information is detected and calculated based on the original repetition frequency, so there is no need to change the repetition frequency. However, in the case of a dual comb, the difference in the repetition frequencies of the two optical comb laser beams becomes important, so in some cases it may be necessary to change the repetition frequency. If the repetition frequency cannot be changed, there is a problem of reduced measurement accuracy.

[0011] Therefore, the present disclosure provides a light source device that can suppress a decrease in measurement accuracy when used for measurement.

[0012] A light source device according to one aspect of the present disclosure includes a first mode-locked laser light source that emits first output light, a second mode-locked laser light source that emits second output light, a first frequency shifter that sets the repetition frequency of the first output light to a first drive frequency, and a second frequency shifter that sets the repetition frequency of the second output light to a second drive frequency. The first drive frequency is different from the second drive frequency. The first drive frequency is different from the value of the repetition frequency of the first output light when the first frequency shifter is not operating.

[0013] According to the present disclosure, when used for measurement, it is possible to suppress a decrease in measurement accuracy.

[0014] FIG. 1A is a diagram schematically illustrating a time change in the electric field of optical comb laser light. FIG. 1B is a diagram schematically illustrating a frequency spectrum of optical comb laser light. FIG. 2A is a diagram schematically illustrating a frequency spectrum of two optical comb laser lights in a dual comb. FIG. 2B is a diagram schematically illustrating a frequency spectrum of light after interference in a dual comb. FIG. 2C is a diagram schematically illustrating a time change in the electric field of light after interference in a dual comb. FIG. 3 is a diagram schematically illustrating a configuration of a light source device according to an embodiment. FIG. 4A is a diagram illustrating an example in which two laser light sources included in a light source device according to an embodiment are integrated on different semiconductor substrates. FIG. 4B is a diagram illustrating an example in which two laser light sources included in a light source device according to an embodiment are integrated on the same semiconductor substrate. FIG. 5A is a top view schematically illustrating a laser light source included in a light source device according to an embodiment. FIG. 5B is a cross-sectional view in the xz plane schematically illustrating the laser light source shown in FIG. 5A. FIG. 6A is a diagram illustrating the frequency characteristics of a laser light source included in a light source device according to an embodiment. 6B is a diagram showing the frequency characteristics of the output light when a drive frequency is set to the same value as the design value of the repetition frequency of the output light from the laser light source included in the light source device according to the embodiment. FIG. 6C is a diagram showing the frequency characteristics of the output light when a drive frequency is set to a value different from the design value of the repetition frequency of the output light from the laser light source included in the light source device according to the embodiment.

[0015] (Summary of the Present Disclosure) (First Aspect) A light source device according to a first aspect of the present disclosure includes a first mode-locked laser light source that emits first output light, a second mode-locked laser light source that emits second output light, a first frequency shifter that sets the repetition frequency of the first output light to a first drive frequency, and a second frequency shifter that sets the repetition frequency of the second output light to a second drive frequency. The first drive frequency is different from the second drive frequency. The first drive frequency is different from the value of the repetition frequency of the first output light when the first frequency shifter is not operating.

[0016] In this way, the repetition frequencies (also referred to as peak frequencies) of the first output light and the second output light are set to the first drive frequency and the second drive frequency, respectively. Here, the first drive frequency is a value different from the value of the repetition frequency of the first output light when the first frequency shifter is not operating (hereinafter referred to as the design value). The design value of the repetition frequency of the first output light is determined based on the optical path length of the resonator of the first mode-locked laser light source, and is therefore a value determined depending on the manufacturing conditions of the first mode-locked laser light source, etc. According to the light source device of this aspect, it is not necessary to match the first drive frequency to the design value, so by adjusting the first drive frequency, the difference between the repetition frequency of the first output light and the repetition frequency of the second output light can be set to a desired value. In this way, since the light source device of this aspect is capable of changing the repetition frequency, when used for measurement, it is possible to suppress a decrease in measurement accuracy.

[0017] Furthermore, with the light source device according to this aspect, it is not necessary to slightly differ the optical path lengths of the resonators of the first and second mode-locked laser light sources in order to set the difference between the repetition frequencies of the first and second output light to a predetermined value. For example, two mode-locked laser light sources having the same optical path length can be used as the first and second mode-locked laser light sources. This broadens the range of light sources that can be used as mode-locked laser light sources. Furthermore, the tolerance for errors in the optical path length due to process errors during the manufacturing of the mode-locked laser light sources is increased, thereby improving yield.

[0018] (Second Aspect) In the light source device according to the first aspect, the first frequency shifter may include a first saturable absorber and a first signal generator that supplies a first high-frequency signal to the first saturable absorber, the frequency of the first high-frequency signal may be the first drive frequency, and the first mode-locked laser light source may include a first resonator that includes a first gain medium and the first saturable absorber.

[0019] This makes it possible to easily adjust the first drive frequency by adjusting the frequency of the first high-frequency signal.

[0020] (Third Aspect) In the light source device according to the first or second aspect, the second drive frequency may be different from the value of the repetition frequency of the second output light when the second frequency shifter is not operating.

[0021] In this way, since it is not necessary to match the second drive frequency to a design value, the difference between the repetition frequency of the first output light and the repetition frequency of the second output light can be set to a desired value by adjusting the second drive frequency. Since both the first drive frequency and the second drive frequency can be adjusted, it is possible to adjust the difference in repetition frequency to be more suitable for measurement. Therefore, when the light source device according to this aspect is used for measurement, it is possible to suppress a decrease in measurement accuracy.

[0022] (Fourth Aspect) In the light source device according to any one of the first to third aspects, the second frequency shifter may include a second saturable absorber and a second signal generator that supplies a second high-frequency signal to the second saturable absorber, the frequency of the second high-frequency signal may be the second drive frequency, and the second mode-locked laser light source may include a second resonator that includes a second gain medium and the second saturable absorber.

[0023] This makes it possible to easily adjust the second drive frequency by adjusting the frequency of the second high-frequency signal.

[0024] (Fifth Aspect) In the light source device according to any one of the first to fourth aspects, the first mode-locked laser light source and the second mode-locked laser light source may be disposed on different semiconductor substrates.

[0025] This allows the light source device to be miniaturized. Furthermore, when the first mode-locked laser light source and the second mode-locked laser light source are manufactured using a semiconductor process, errors in the optical path length are likely to occur due to process errors. Therefore, the effect of the light source device according to this aspect, which has a wider tolerance for errors in the optical path length, is more effective.

[0026] (Sixth Aspect) In the light source device according to any one of the first to fourth aspects, the first mode-locked laser light source and the second mode-locked laser light source may be integrated on the same semiconductor substrate.

[0027] This allows the light source device to be further miniaturized.

[0028] Seventh Aspect In the light source device according to any one of the first to sixth aspects, at least one of the first drive frequency and the second drive frequency may be switchable between a plurality of values.

[0029] This makes it possible to adjust the difference between the repetition frequency of the first output light and the repetition frequency of the second output light to a value more suitable for measurement. Therefore, when the light source device according to this aspect is used for measurement, it is possible to suppress a decrease in measurement accuracy.

[0030] (Eighth Aspect) In the light source device according to any one of the first to seventh aspects, the first mode-locked laser light source and the second mode-locked laser light source may each be an optical frequency comb laser light source.

[0031] 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.

[0032] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] In this specification and the drawings, the x-axis, y-axis, and z-axis represent the three axes of a three-dimensional Cartesian coordinate system.

[0038] 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.

[0039] (Embodiment) [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.

[0040] 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.

[0041] 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.

[0042] 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

[0043] 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.

[0044] As shown in FIG. 1B, the optical comb laser light has a comb-like frequency spectrum formed by a plurality 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 interval between two adjacent equally spaced lines in the frequency spectrum of the optical comb laser light, is f ｒｅｐ = 1 / T ｒｅｐ The repetition frequency fｒｅｐ is, 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.

[0045] 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 the 0th mode as f, 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 ) ].

[0046] 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

[0047] 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 part of the optical comb laser light is detected, and based on the detected part, 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.

[0048] [Dual Comb] Next, the principle of the dual comb will be briefly explained with reference to FIGS. 2A and 2B.

[0049] 2A is a diagram showing the frequency spectra of two optical comb laser beams 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 ＣＥＯ１ is the carrier envelope offset frequency of the first optical comb laser light. ＣＥＯ２is 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.

[0050] 2B is a diagram showing a schematic diagram of the frequency spectrum of light after interference in the dual comb. As a result of interference between the closest modes of the first and second optical comb laser beams shown in FIG. 2A, the frequency spectrum shown in FIG. 2B is detected as interference light. The detected interference light is a beat generated by the interference between the first and second optical comb laser beams. The frequency interval of the interference light is δf, which is the difference between the repetition frequency of the first and second optical comb laser beams. ｒｅｐ This becomes:

[0051] 2C is a diagram showing a time waveform of light after interference in a dual comb. The interference waveform is a pulse waveform like that in FIG. 1A, and the pulse interval 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.

[0052] 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.

[0053] [Configuration] Next, the configuration of the light source device according to the embodiment will be described with reference to Fig. 3. Fig. 3 is a diagram schematically showing the configuration of the light source device 1 according to the present embodiment.

[0054] 3, the light source device 1 includes two laser light sources 10 and 20. The light source device 1 also includes repetition frequency shifters 14 and 24. In the example shown in Fig. 3, the repetition frequency shifters 14 and 24 are included in the resonators of the laser light sources 10 and 20, respectively.

[0055] The laser light source 10 is an example of a first mode-locked laser light source that emits output light 15, which is an example of a first output light. Specifically, the laser light source 10 is an optical frequency comb laser light source. The laser light source 10 has a first resonator including a first mirror 11, a second mirror 12, and a gain medium 13 and a repetition frequency shifter 14 provided between the first mirror 11 and the second mirror 12. The gain medium 13 and the repetition frequency shifter 14 are provided on the optical path between the first mirror 11 and the second mirror 12. The repetition frequency shifter 14 is an example of a first frequency shifter and sets the repetition frequency of the output light 15 to a first drive frequency. In this embodiment, the first drive frequency is different from the design value of the repetition frequency of the output light 15. Furthermore, the first drive frequency is switchable between multiple values. In this case, all of the multiple values ​​are different from the design value of the repetition frequency of the output light 15, but one of the multiple values ​​may be the design value of the repetition frequency of the output light 15.

[0056] The repetition frequency of the output light 15 is the repetition frequency f ｒｅｐ１The design value of the repetition frequency of the output light 15 is the value of the repetition frequency of the output light 15 when the repetition frequency shifter 14 is not operating. The design value of the repetition frequency of the output light 15 can be measured by detecting the output light 15 emitted from the laser light source 10 when the laser light source 10 is operated without operating the repetition frequency shifter 14. The design value of the repetition frequency of the output light 15 is a value determined based on the optical path length L1 of the resonator of the laser light source 10. Specifically, the design value of the repetition frequency of the output light 15 is a value obtained by dividing the speed of light c by the optical path length L1. The optical path length L1 corresponds to the value obtained by multiplying the resonator length, which is the distance between the first mirror 11 and the second mirror 12, by the refractive index of the portion through which light propagates.

[0057] The laser light source 20 is an example of a second mode-locked laser light source that emits output light 25, which is an example of second output light. Specifically, the laser light source 20 is an optical frequency comb laser light source. The laser light source 20 has a second resonator including a first mirror 21, a second mirror 22, and a gain medium 23 and a repetition frequency shifter 24 provided between the first mirror 21 and the second mirror 22. The gain medium 23 and the repetition frequency shifter 24 are provided on the optical path between the first mirror 21 and the second mirror 22. The repetition frequency shifter 24 is an example of a second frequency shifter and sets the repetition frequency of the output light 25 to a second drive frequency. In this embodiment, the second drive frequency is different from the design value of the repetition frequency of the output light 25. Furthermore, the second drive frequency is switchable between multiple values. In this case, all of the multiple values ​​are different from the design value of the repetition frequency of the output light 25, but one of the multiple values ​​may be the design value of the repetition frequency of the output light 25.

[0058] The repetition frequency of the output light 25 is the repetition frequency f ｒｅｐ２The design value of the repetition frequency of the output light 25 is the value of the repetition frequency of the output light 25 when the repetition frequency shifter 24 is not operating. The design value of the repetition frequency of the output light 25 can be measured by detecting the output light 25 emitted from the laser light source 20 when the laser light source 20 is operated without operating the repetition frequency shifter 24. The design value of the repetition frequency of the output light 25 is a value determined based on the optical path length L2 of the resonator of the laser light source 20. Specifically, the design value of the repetition frequency of the output light 25 is a value obtained by dividing the speed of light c by the optical path length L2. The optical path length L2 corresponds to the value obtained by multiplying the resonator length, which is the distance between the first mirror 21 and the second mirror 22, by the refractive index of the portion through which light propagates.

[0059] In this embodiment, the optical path length L1 of the laser light source 10 is shorter than the optical path length L2 of the laser light source 20, but this is not limiting. The optical path length L1 of the laser light source 10 and the optical path length L2 of the laser light source 20 may be the same. In this embodiment, the repetition frequency f ｒｅｐ１ and the repetition frequency f of the output light 25 ｒｅｐ２ may vary depending on the operating states of the repetition frequency shifters 14 and 24, respectively.

[0060] Specifically, when the repetition frequency shifter 14 is not operating (when it is off), the repetition frequency f ｒｅｐ１ is the value obtained by dividing the speed of light c by the optical path length L1, that is, the design value of the repetition frequency of the output light 15. On the other hand, when the repetition frequency shifter 14 is operating (when it is on), the repetition frequency f ｒｅｐ１ is deviated from the design value of the repetition frequency and set to the first driving frequency by the repetition frequency shifter 14.

[0061] Similarly, when the repetition frequency shifter 24 is not operating (when it is off), the repetition frequency f ｒｅｐ２ is the value obtained by dividing the speed of light c by the optical path length L2, that is, the design value of the repetition frequency of the output light 25. On the other hand, when the repetition frequency shifter 24 is operating (when it is on), the repetition frequency f ｒｅｐ２is deviated from the design value of the repetition frequency and set to the second driving frequency by the repetition frequency shifter 24.

[0062] The repetition frequency shifters 14 and 24 may be turned on simultaneously, or only one of them may be turned on. The repetition frequency shifters 14 and 24 may be any optical, physical, or electrical element that can change the repetition frequency of the output light 15 and the repetition frequency of the output light 25, respectively.

[0063] 3 shows an example in which the repetition frequency shifter 14 is provided inside the laser light source 10, but a part of the repetition frequency shifter 14 may be provided outside the laser light source 10. Similarly, for the repetition frequency shifter 24, although an example in which the repetition frequency shifter 24 is provided inside the laser light source 20 is shown in FIG.

[0064] For example, each of the repetition frequency shifters 14 and 24 includes a saturable absorber and a signal generator that supplies a high-frequency signal to the saturable absorber. The saturable absorber is disposed on the optical path of the resonator of the laser light source 10. The signal generator is disposed outside the laser light source 10. The specific configuration will be described later.

[0065] Alternatively, each of the repetition frequency shifters 14 and 24 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 externally supplied DC voltage. 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, thereby changing the repetition frequency of the output light.

[0066] Furthermore, each of the repetition frequency shifters 14 and 24 may 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 disposing the optical chopper on the optical path of the resonator, light 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 light pulses, i.e., the repetition frequency of the output light, can be changed.

[0067] In this embodiment, the first driving frequency set by the repetition frequency shifter 14 is different from the second driving frequency set by the repetition frequency shifter 24. The difference between the first driving frequency and the second driving frequency is δf ｒｅｐ That is, by adjusting the drive frequency set by the repetition frequency shifter 14 and the drive frequency set by the repetition frequency shifter 24, δf ｒｅｐ can be adjusted to a value suitable for measurement. Therefore, when the light source device 1 is used for measurement, the accuracy of the measurement can be improved.

[0068] Furthermore, it is not necessary to strictly set the optical path length difference between the optical path length L1 of the laser light source 10 and the optical path length L2 of the laser light source 20 to a predetermined value. For dual comb measurement, the difference between the optical path length L1 and the optical path length L2 is usually required to be a predetermined value greater than 0. However, according to the light source device 1 of this embodiment, even if the optical path length L1 and the optical path length L2 are the same, by adjusting the value of at least one of the first drive frequency and the second drive frequency, δf ｒｅｐ can be set to a value other than 0. This makes it possible to measure the dual comb even if the optical path length differs from the specification value due to a process error or the like. In this way, the tolerance for the difference in optical path length between the optical path length L1 and the optical path length L2 is widened, thereby improving the yield.

[0069] [Optical Comb Laser Integrated on a Semiconductor Substrate] The laser light sources 10 and 20 according to the present embodiment can each be realized as an optical comb laser integrated on a semiconductor substrate.

[0070] 4A is a diagram showing an example in which two laser light sources 10 and 20 included in light source device 1 according to the present embodiment are integrated on different semiconductor substrates. Specifically, as shown in FIG. 4A, laser light source 10 is integrated on semiconductor substrate 17. Laser light source 20 is integrated on semiconductor substrate 27.

[0071] 4A, the laser light source 10 includes a resonator 16 and an output section 19. The resonator 16 and the output section 19 are formed on a semiconductor substrate 17.

[0072] The resonator 16 is an example of a first resonator and includes a first mirror 11, a second mirror 12, a gain medium 13, a saturable absorber 14a, and an optical waveguide 16w. The gain medium 13 and the saturable absorber 14a are examples of a first gain medium and a first saturable absorber, respectively, and are provided on an optical path within the resonator 16. The optical path within the resonator 16 includes an optical waveguide 16w between the first mirror 11 and the second mirror 12. Specific configurations of the first mirror 11, the second mirror 12, the gain medium 13, the saturable absorber 14a, the optical waveguide 16w, and the semiconductor substrate 17 will be described later.

[0073] The output unit 19 outputs the output light 15 to the outside of the laser light source 10. Specifically, the output unit 19 is provided at the end of the optical waveguide 16w. The output unit 19 is located outside the second mirror 12 and outputs light that has propagated through the optical waveguide 16w and passed through the second mirror 12 as the output light 15. Note that, as will be described in detail later, the second mirror 12 has the function of reflecting a portion of the incident light and transmitting another portion. The output unit 19 includes, for example, a coupling unit for an optical fiber (not shown). The output unit 19 is a grating coupler, but is not limited to this. The output unit 19 may also be, for example, an edge coupler.

[0074] The saturable absorber 14a provided on the semiconductor substrate 17 is a part of the repetition frequency shifter 14. In the example shown in Fig. 4A, the repetition frequency shifter 14 includes the saturable absorber 14a and an RF signal generator 14b.

[0075] The RF signal generator 14b is an example of a first signal generator and supplies a first high-frequency signal to the saturable absorber 14a. The frequency of the first high-frequency signal is the first drive frequency. The RF signal generator 14b is realized by an electronic circuit including at least one of a resistor, an inductor, a capacitor, a transformer, a diode, a transistor, etc. Part or all of the RF signal generator 14b may be mounted on the semiconductor substrate 17.

[0076] The laser light source 20 also includes a resonator 26 and an output section 29. The resonator 26 and the output section 29 are formed on a semiconductor substrate 27.

[0077] The resonator 26 is an example of a second resonator and includes a first mirror 21, a second mirror 22, a gain medium 23, a saturable absorber 24a, and an optical waveguide 26w. The gain medium 23 and the saturable absorber 24a are examples of a second gain medium and a second saturable absorber, respectively, and are provided on an optical path within the resonator 26. The optical path within the resonator 26 includes an optical waveguide 26w between the first mirror 21 and the second mirror 22. The first mirror 21, the second mirror 22, the gain medium 23, the saturable absorber 24a, the optical waveguide 26w, and the semiconductor substrate 27 are realized with configurations similar to those of the first mirror 11, the second mirror 12, the gain medium 13, the saturable absorber 14a, the optical waveguide 16w, and the semiconductor substrate 17, respectively.

[0078] The output unit 29 outputs the output light 25 to the outside of the laser light source 20. Specifically, the output unit 29 is provided at the end of the optical waveguide 26w. The output unit 29 is located outside the second mirror 22, and outputs light that has propagated through the optical waveguide 26w and passed through the second mirror 22 as the output light 25. The output unit 29 includes, for example, a coupling unit for an optical fiber (not shown). The output unit 29 is a grating coupler, but is not limited to this. The output unit 29 may also be, for example, an edge coupler.

[0079] The saturable absorber 24a provided on the semiconductor substrate 27 is a part of the repetition frequency shifter 24. In the example shown in Fig. 4A, the repetition frequency shifter 24 includes the saturable absorber 24a and an RF signal generator 24b.

[0080] The RF signal generator 24b is an example of a second signal generator and supplies a second high-frequency signal to the saturable absorber 24a. The frequency of the second high-frequency signal is the second drive frequency. The RF signal generator 24b is realized by an electronic circuit including at least one of a resistor, an inductor, a capacitor, a transformer, a diode, a transistor, etc. Part or all of the RF signal generator 24b may be mounted on the semiconductor substrate 27.

[0081] In the light source device 1 according to the present embodiment, the laser light sources 10 and 20 may be integrated on the same semiconductor substrate 30, as shown in FIG. 4B . FIG. 4B is a diagram showing an example in which the two laser light sources 10 and 20 included in the light source device 1 according to the present embodiment are integrated on the same semiconductor substrate 30. This allows the laser light sources 10 and 20 to be integrated on a single semiconductor substrate 30, making it possible to make the light source device 1 more compact than the example of FIG. 4A . Furthermore, compared to fabricating the laser light sources 10 and 20 separately, it is possible to achieve lower costs due to simplified processes and improved reliability of the device.

[0082] The following describes the configuration of an optical comb laser integrated on a semiconductor substrate, taking the laser light source 10 as an example. The laser light source 20 also has a configuration similar to that of the laser light source 10.

[0083] Fig. 5A is a top view schematically showing the laser light source 10 included in the light source device 1 according to the present embodiment. Fig. 5B is a cross-sectional view schematically showing the laser light source 10 at the position indicated by the line VB-VB in Fig. 5A. 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. 5B omits some of the diagonal shading representing the cross section.

[0084] 5A and 5B, the laser light source 10 includes a resonator 16, a semiconductor substrate 17, and a protective layer 18. In the illustrated example, the surface of the semiconductor substrate 17 is parallel to the xy plane. As shown in FIG. 5B, the semiconductor substrate 17 includes a high refractive index layer 17a made of, for example, Si, and a SiO ２The protective layer 18 has a laminated structure in which the high refractive index layer 17a and the low refractive index layer 17b such as SiO are laminated in this order in the z-axis direction. The refractive index of the high refractive index layer 17a is higher than the refractive index of the low refractive index layer 17b. The semiconductor substrate 17 does not necessarily have to include the high refractive index layer 17a. The protective layer 18 is made of, for example, SiO ２ or an organic insulating material such as BCB (benzocyclobutene). The protective layer 18 is provided so as to cover the upper surface of the low refractive index layer 17b.

[0085] The resonator 16 is an example of a laser resonator having a predetermined optical path length. As shown in Fig. 5A, the resonator 16 includes a semiconductor layer 16s, an intermediate layer 16i, an optical waveguide 16w, a first mirror 11, and a second mirror 12. As shown in Fig. 5B, the resonator 16 includes an n-doped layer 16d. １ and p-doped layer 16d ２ and 16d ３ The resonator 16 also includes a gain medium 13 and a saturable absorber 14a. Parts of the semiconductor layer 16s function as the gain medium 13 and the saturable absorber 14a, respectively.

[0086] 5A, the planar shape of the semiconductor layer 16s in the resonator 16 is represented by a dotted line. The semiconductor layer 16s is tapered at both ends. The tip of the tapered portion overlaps the intermediate layer 16i in top view. The intermediate layer 16i is also tapered at both ends. The tip of the tapered portion overlaps the optical waveguide 16w in top view. This allows light passing through the semiconductor layer 16s to be efficiently propagated to the optical waveguide 16w via the intermediate layer 16i.

[0087] As shown in FIG. 5B , the optical waveguide 16w in the resonator 16 is embedded in a low-refractive-index layer 17b in the semiconductor substrate 17. The optical waveguide 16w may be provided on the low-refractive-index layer 17b. The optical waveguide 16w may be formed from a high-refractive-index material such as SiN. The refractive index of the optical waveguide 16w is higher than the refractive index of the low-refractive-index layer 17b in the semiconductor substrate 17 and the refractive index of the protective layer 18. This allows light to propagate through the optical waveguide 16w by total internal reflection.

[0088] The intermediate layer 16i is formed of, for example, a-Si (amorphous silicon). The intermediate layer 16i may be formed of other semiconductors as long as the refractive index satisfies the following condition: optical waveguide 16w < intermediate layer 16i < semiconductor layer 16s. Without the intermediate layer 16i, the refractive index of the semiconductor layer 16s would be higher than the refractive index of the optical waveguide 16w, making it difficult for light that has passed through the semiconductor layer 16s to pass through the optical waveguide 16w.

[0089] The first mirror 11 and the second mirror 12 may be formed, for example, from a distributed Bragg reflector. In a distributed Bragg reflector, light is reflected by Bragg reflection due to a periodic structure of refractive index. The first mirror 11 is provided at an end of the optical waveguide 16w. The second mirror 12 is provided at an end of the optical waveguide 16w opposite the first mirror 11. The first mirror 11 and the second mirror 12 reflect light propagating through the optical waveguide 16w. The first mirror 11 and the second mirror 12 may be formed, for example, from a metal as long as they have a reflective function. The first mirror 11 and the second mirror 12 may also be a reflective element formed from a semiconductor, such as a loop mirror.

[0090] The second mirror 12 has a lower reflectivity than the first mirror 11. Specifically, the reflectivity of the first mirror 11 is substantially equal to 100%, whereas the reflectivity of the second mirror 12 is equal to or greater than a predetermined value and lower than the reflectivity of the first mirror 11. The predetermined value is, for example, 50%, and the reflectivity of the second mirror 12 can be, for example, 70% or 90%. The second mirror 12 reflects most of the light propagating through the optical waveguide 16w, but transmits a portion of the light. The light that passes through the second mirror 12 becomes output light 15, which is optical comb laser light.

[0091] As shown in FIG. 5B, the semiconductor layer 16s is an n-doped layer 16d １ and p-doped layer 16d ２ and 16d ３ The n-doped layer 16d is sandwiched between the １ and p-doped layer 16d ２ and 16d ３ The positional relationship may be reversed.

[0092] Semiconductor layer 16s, n-doped layer 16d １ and p-doped layer 16d ２ and 16d ３ is buried in the protective layer 18. １ The bottom surface of the n-doped layer 16d may be in contact with the surface of the semiconductor substrate 17. １ may be in contact with the intermediate layer 16i.

[0093] The semiconductor layer 16s 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.

[0094] The semiconductor layer 16s includes the gain medium 13 and the saturable absorber 14a. The gain medium 13 is a part of the semiconductor layer 16s and is an n-doped layer 16d. １ and p-doped layer 16d ２ The saturable absorber 14a is a part of the semiconductor layer 16s, and is sandwiched between the n-doped layer 16d. １ and p-doped layer 16d ３ It is the part sandwiched between and.

[0095] n-doped layer 16d １ is an n-type semiconductor layer. １ is formed by doping the same III-V semiconductor material as the semiconductor layer 16s with an n-type impurity. The n-type impurity may be, for example, a tetravalent element such as Si or a hexavalent element such as selenium (Se).

[0096] p-doped layer 16d ２ and 16d ３ is a p-type semiconductor layer. ２ and 16d ３ The p-doped layer 16d is formed by doping a p-type impurity into the same III-V semiconductor material as the semiconductor layer 16s. For example, a divalent element such as zinc (Zn) can be used as the p-type impurity. ２ and p-doped layer 16d３ and have the same composition, for example.

[0097] p-doped layer 16d ２ and p-doped layer 16d ３ The p-doped layer 16d is isolated from the p-doped layer 16a. ２ and 16d ３ Each of the p-doped layers 16d is attached with a different electrode (not shown). ２ A current is injected into the p-doped layer 16d through the electrode. ３ The n-doped layer 16d １ By applying this voltage, a reverse bias voltage is applied between the p-doped layer 16d, which is a part of the semiconductor layer 16s. ３ The portion of the saturable absorber 14a that contacts the first mirror 11 and the second mirror 12 functions as the saturable absorber 14a. The saturable absorber 14a may be formed using carbon nanotubes. The saturable absorber 14a may be integrated with the first mirror 11 and the second mirror 12.

[0098] p-doped layer 16d ２ Similarly, the n-doped layer 16d １ An electrode (not shown) is also attached to the n-doped layer 16d. １ and p-doped layer 16d ２ A portion of the semiconductor layer 16s, into which charges are injected from electrodes attached to each of the resonators 16, functions as a gain medium 13 from which stimulated emission of light occurs. This stimulated emission of light is repeatedly reflected between the first mirror 11 and the second mirror 12 via the intermediate layer 16i and the optical waveguide 16w. In other words, the light is amplified by passing through the gain medium 13 multiple times. The amplified light becomes a mode-locked optical pulse train by the saturable absorber 14a. Only the wavelength corresponding to the optical path length of the resonator 16 (i.e., the resonator length multiplied by the refractive index) is amplified. As a result, optical comb laser light is emitted from the resonator 16 as output light 15. The resonator length of the resonator 16 is the distance between the first mirror 11 and the second mirror 12.

[0099] [Relationship between the driving frequency of the repetition frequency shifter and the repetition frequency of the output light] Next, the relationship between the driving frequency of the repetition frequency shifter and the repetition frequency of the output light from the corresponding mode-locked laser light source will be described. Here, with reference to Figures 6A to 6C, measurement results of the repetition frequency of the output light 15 when the RF signal generator 14b supplies a high-frequency signal to the saturable absorber 14a for the optical comb laser light source integrated on the semiconductor substrate 17 will be described.

[0100] Fig. 6A is a diagram showing the frequency characteristics of the laser light source 10 included in the light source device 1 according to this embodiment. Fig. 6A shows the frequency characteristics of the output light 15 when the repetition frequency shifter 14 is off, specifically, when no high-frequency signal is supplied from the RF signal generator 14b to the saturable absorber 14a. Since no high-frequency signal is supplied to the saturable absorber 14a, the repetition frequency shown in Fig. 6A (here, approximately 2.95167 GHz) corresponds to the optical path length of the resonator 16. In other words, the repetition frequency shown in Fig. 6A is the design value of the repetition frequency of the output light 15 from the laser light source 10.

[0101] 6B is a diagram showing the frequency characteristics of the output light 15 when the repetition frequency shifter 14 is turned on and the drive frequency is set to the same value as the design value of the repetition frequency of the output light 15 from the laser light source 10 included in the light source device 1 according to this embodiment. Specifically, FIG. 6B shows the repetition frequency when the RF signal generator 14b adjusts the frequency of the high-frequency signal to the repetition frequency equivalent to the optical path length of the resonator 16 (approximately 2.95167 GHz described above) and supplies the high-frequency signal to the saturable absorber 14a.

[0102] 6A, the width of the peak in the frequency spectrum is narrower, indicating that the frequency is stabilized. In other words, the frequency can be stabilized by adjusting the frequency of the high-frequency signal to a repetition frequency equivalent to the optical path length of the resonator 16 (see, for example, Non-Patent Document 2). When a high-frequency signal is supplied, the saturable absorber 14a acts like an electrical shutter, and as a result, the repetition frequency can be synchronized with the frequency of the high-frequency signal.

[0103] 6C is a diagram showing the frequency characteristics of the output light when the repetition frequency shifter 14 is turned on and the drive frequency is set to a value different from the design value of the repetition frequency of the output light 15 from the laser light source 10 included in the light source device 1 according to this embodiment. Specifically, FIG. 6C shows the repetition frequency when the RF signal generator 14b supplies the high-frequency signal to the saturable absorber 14a with the frequency of the high-frequency signal set to a value different from the repetition frequency corresponding to the optical path length of the resonator 16. Specifically, the frequency of the high-frequency signal is set to 2.95147 GHz, and a narrow peak similar to that in FIG. 6B appears at this frequency. Thus, even if the frequency of the high-frequency signal is different from the frequency corresponding to the optical path length of the resonator 16, a steep spectrum can be generated in the frequency spectrum. That is, the repetition frequency can be shifted to a value different from the frequency corresponding to the optical path length of the resonator 16.

[0104] The difference between the frequency of the high-frequency signal, i.e., the drive frequency of the repetition frequency shifter 14 and the design value of the repetition frequency of the output light 15, is within a predetermined range. Specifically, the difference between the drive frequency of the repetition frequency shifter 14 and the design value is within 0.1% of the design value. For example, when the design value is 1 GHz, the drive frequency value is determined so that the difference is within 1 MHz. Specifically, when the design value is 1 GHz, the drive frequency range is 0.999 GHz or more and 1.001 GHz or less. In the example shown in FIG. 6C , the difference between the drive frequency (approximately 2.95147 GHz) and the design value (approximately 2.95167 GHz) is approximately 0.0002 GHz (0.2 MHz), which is within 0.1% of the design value. The difference between the drive frequency and the design value may be within 0.2%, 0.5%, or 1% of the design value.

[0105] The difference between the drive frequency and the design value is the difference δf ｒｅｐ Alternatively, the difference between the drive frequency and the design value may be smaller than the difference δf ｒｅｐ For example, when the optical path lengths of the resonators of the laser light sources 10 and 20 are equal, the difference between the drive frequency and the design value may be equal to the difference δf ｒｅｐThe difference between the drive frequency and the design value is the difference δf ｒｅｐ It may be larger.

[0106] As described above, in the light source device 1 according to the present embodiment, the repetition frequency of the output light 15 or 25 can be adjusted by the repetition frequency shifter 14 or 24. As a result, the difference δf in the repetition frequency suitable for the dual comb is ｒｅｐ This can be easily achieved, thereby improving the accuracy of the measurement.

[0107] Even if the laser light source 10 or 20 is manufactured with a repetition frequency that deviates from the desired repetition frequency due to a process error in a semiconductor process, the laser light source 10 or 20 can be driven so that the frequency of the supplied high-frequency signal becomes the repetition frequency by supplying a high-frequency signal of a predetermined frequency from the RF signal generator 14b or 24b to the saturable absorber 14a or 24a. This widens the tolerance for process errors, thereby improving the yield of the light source device 1. Furthermore, with the light source device 1 according to this embodiment, it is not necessary to actually measure the repetition frequency of the output light when the repetition frequency shifter is off (specifically, the repetition frequency shown in FIG. 6A ), and thus measurement using a dual comb can be easily achieved.

[0108] As another example, the difference in repetition frequency δf ｒｅｐ is the time difference δT of the pulses as shown in FIG. ｒｅｐ The accuracy and / or speed of measurement change depending on the number of pulses. In other words, the difference in repetition frequency can be adjusted, allowing the measurement sampling rate to be changed. For example, lowering the sampling rate enables high-accuracy measurement. In addition, increasing the sampling rate enables high-speed measurement. This allows measurements to be performed according to the application, thereby expanding the range of application of the light source device 1.

[0109] While the light source device according to one or more aspects has 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.

[0110] For example, the second driving frequency may be the same as the design value of the repetition frequency of the output light 25. In other words, the laser light source 20 may be driven at the same driving frequency as the design value. The same applies to the laser light source 10.

[0111] Furthermore, at least one of the first drive frequency and the second drive frequency may be a fixed value, i.e., at least one of the first drive frequency and the second drive frequency may have only one possible value.

[0112] 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.

[0113] The light source device according to the present disclosure can be used for various measurement applications such as distance measurement, displacement measurement, etc. For example, the light source device according to the present disclosure can be used in a displacement meter, a shape inspection device, etc.

[0114] REFERENCE SIGNS LIST 1 Light source device 10, 20 Laser light source 11, 21 First mirror 12, 22 Second mirror 13, 23 Gain medium 14, 24 Repetition frequency shifter 15, 25 Output light 14a, 24a Saturable absorber 14b, 24b RF signal generator 16, 26 Resonator 16d １ n-doped layer 16d ２ , 16d ３ p-doped layer 16i intermediate layer 16s semiconductor layer 16w, 26w optical waveguide 17, 27, 30 semiconductor substrate 17a high refractive index layer 17b low refractive index layer 18 protective layer 19, 29 output section

Claims

1. A light source device comprising: a first mode-locked laser light source that emits first output light; a second mode-locked laser light source that emits second output light; a first frequency shifter that sets the repetition frequency of the first output light to a first drive frequency; and a second frequency shifter that sets the repetition frequency of the second output light to a second drive frequency, wherein the first drive frequency is different from the second drive frequency, and the first drive frequency is different from the value of the repetition frequency of the first output light when the first frequency shifter is not operating.

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

3. The light source device according to claim 1 or 2, wherein the second drive frequency is different from the value of the repetition frequency of the second output light when the second frequency shifter is not operating.

4. The light source device according to claim 3, wherein the second frequency shifter includes: a second saturable absorber; and a second signal generator that supplies a second high-frequency signal to the second saturable absorber, the frequency of the second high-frequency signal being the second drive frequency; and the second mode-locked laser light source includes a second resonator that includes a second gain medium and the second saturable absorber.

5. The light source device according to claim 1 or 2, wherein the first mode-locked laser light source and the second mode-locked laser light source are disposed on different semiconductor substrates.

6. The light source device according to claim 1 or 2, wherein the first mode-locked laser light source and the second mode-locked laser light source are integrated on the same semiconductor substrate.

7. The light source device according to claim 1 or 2, wherein at least one of the first drive frequency and the second drive frequency is switchable between a plurality of values.

8. The light source device according to claim 1 or 2, wherein the first mode-locked laser light source and the second mode-locked laser light source are each an optical frequency comb laser light source.

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