JP2018022041A - Wavelength swept light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide highly reproducible SS-OCT measurement by a wavelength-swept light source in which the center wavelength and sweep wavelength range of wavelength sweep are stable over a long time.SOLUTION: Provided is a wavelength-swept light source including a gain medium coupled at one end to a wavelength filter constituted from a diffraction grating and an end face mirror via an optical deflector and facing an output coupling mirror at other end, and constituting an optical resonator between the end face mirror and the output coupling mirror. The optical deflector is provided with a wavelength monitor for causing an incident light that is entered to be deflected in a direction to traverse the groove of the diffraction grating, detecting light of a specific wavelength out of the output light and outputting a wavelength monitor signal in the wavelength-swept light source that varies the wavelength of output light. The wavelength monitor signal is measured at wavelength sweep time, and the optical deflector is feedback controlled so that the time of day when the light of specific wavelength appears is constant.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a wavelength swept light source, and more particularly, to a wavelength swept light source using an optical deflector using KTN, which relates to a wavelength swept light source having high reproducibility of the center wavelength of the wavelength sweep and the swept wavelength range.

  The wavelength-swept laser light source can be applied to, for example, optical coherence tomography (OCT), which is known as a measurement technique that can acquire a tomographic image of a living body under the surface with micron order spatial resolution. There are a plurality of methods for acquiring a tomographic image in OCT. Among them, OCT (SS-OCT: Swept Source-OCT) using a wavelength-swept laser light source that sweeps the wavelength of a light source has a great advantage in high-speed image acquisition. In SS-OCT, the wavelength of the light source is continuously swept. By performing the wavelength sweep of the light from the light source, a single detector can be used to detect signals of each wavelength in a time division manner. That is, it is possible to perform wavelength division according to time and complete a single detector.

  An OCT light source using a wavelength sweep type laser light source is required to change the wavelength at high speed, to narrow the width of the instantaneous oscillation spectrum, and to have a wide wavelength sweep range. In SS-OCT, when high-speed wavelength sweeping is possible, dynamic analysis such as high-speed image processing, blood flow observation, and oxygen saturation concentration change can be performed. In SS-OCT, the observable depth of the living body surface can be increased by narrowing the line width of the instantaneous oscillation spectrum. In SS-OCT, a high resolution image can be acquired by sweeping a wide range of wavelengths.

  Further, in clinical diagnosis, in order to obtain a clear tomographic image without being affected by the movement of an observation target, a wavelength sweep type laser light source that operates stably for a long time with the above high speed is required.

  An external resonator type laser light source is known as a laser light source capable of high-speed and wide-band scanning. A general external resonator type laser light source includes a gain medium such as a semiconductor and an external mirror, which constitute an external resonator. Then, only a specific wavelength is oscillated using a band pass filter provided between the gain medium and the external mirror. The bandpass filter continuously sweeps the laser oscillation wavelength by continuously changing the transmission wavelength. The transmission wavelength of the band-pass filter is controlled to change by the functions of the wavelength dispersion element and the optical deflector provided in the laser resonator. Examples of the wavelength dispersion element include a diffraction grating and a prism. Examples of the optical deflector include a polygon mirror, a galvanometer mirror, a MEMS (Micro Electro Mechanical System), an acousto-optic deflector, and an electro-optic deflector.

One of the electro-optic deflectors, potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1), K _1-y Li _y Ta _1-x Nb _x O ₃ (0 <x <1, 0 <y <1): The KTN optical deflector using a crystal (hereinafter collectively referred to as KTN) can be deflected at high speed and can be operated at a low voltage. A high-speed wavelength swept light source is realized by combining a KTN optical deflector and a diffraction grating.

  FIG. 1 shows a configuration of a wavelength swept light source using a conventional KTN optical deflector. This wavelength tunable light source has a configuration in which the oscillation wavelength is switched by changing the traveling direction of light using a KTN optical deflector, and is disclosed in Non-Patent Document 1, for example. This variable wavelength light source uses an optical semiconductor amplifying element (SOA) as a gain medium. Hereinafter, the configuration and operation of the variable wavelength light source will be described.

  In the wavelength tunable light source 100, the gain medium 101 is disposed between the condenser lens 103 and the collimating lens 102. The gain medium 101 passes through a collimating lens 102 and an electro-optic deflector 106, and is coupled to a wavelength filter including a diffraction grating 104 and a direct incident end mirror 108. The condenser lens 103 is opposed to the output coupling mirror 105. An optical resonator having both ends of the output coupling mirror 105 and the end mirror 108 is configured. Output light 107 is obtained from the output coupling mirror 105 by the laser action of the optical resonator. The wavelength of the output light 107 is changed by changing the light traveling direction by the electro-optic deflector 106 and changing the incident angle θ to the diffraction grating 104 which is a wavelength dispersion element. In other words, the oscillation wavelength in the optical resonator can be selected by changing the incident angle by deflecting the diffraction grating 104 in the direction crossing the groove.

  The wavelength of the output light 107 is selected by the voltage of the control voltage source 109 connected to the electro-optic deflector 106. The voltage applied to the electro-optic deflector 106 is controlled to change the electric field in the x-axis direction (direction perpendicular to the optical axis (z-axis) of the output light 107) in FIG. That is, a change in refractive index is induced in the electro-optic deflector 106 due to a change in the electric field applied to the electro-optic deflector 106. As a result, when the light beam output from the gain medium 101 to the diffraction grating 104 passes through the electro-optic deflector 106, the light beam is bent toward a higher refractive index, and the incident angle of the light beam on the diffraction grating 104 changes. In this way, by changing the voltage applied to the electro-optic deflector 106, a high-speed wavelength change is realized without the intervention of a movable part.

  However, in the wavelength tunable light source using the conventional KTN optical deflector, when the KTN optical deflector is deflected for a long time, the operation state of the KTN optical deflector gradually changes, and the optical output of the wavelength swept light source, There was a problem that the sweep wavelength band and the coherence length were reduced. Therefore, in the wavelength tunable light source described in Patent Document 1, in order to ensure long-term stability of the light source, a direct current in addition to an alternating current (AC) voltage for wavelength sweeping is applied as a voltage applied to the KTN optical deflector. A bias voltage of (DC) is superimposed to make the amount of electrons constant throughout the interior of the KTN crystal. It has been reported that this enables stable operation for 72 hours or more.

JP2015-142111A International Publication No. 2006/137408

Y. Okabe, et al. "200 kHz swept light source equipped with KTN deflector for optical coherence tomography," Electron. Lett. 48, 201-202 (2012). S. H. Yun, et al. "High-speed optical frequency-domain imaging," Opt. Express 11, 2953-2963 (2003). Y. Yasuno, V. D. Madjarova, S. Makita, and K. Chan, "swept-source optical coherence tomography for in vivo investigation of human anterior eye segments," Opt. Express 13, 10652-10664 (n.d.).

  When a voltage is applied to the dielectric crystal, the amount of current in the crystal is proportional to the dielectric constant of the crystal in the space charge control state (see, for example, Patent Document 2). The amount of electrons trapped in the crystal is affected by the balance between the electrons supplied by the current and the electrons excited from the trap site by heat or light. When the temperature control element (for example, Peltier element) provided in the KTN crystal has a large fluctuation in the outside air temperature that cannot be controlled, and the crystal temperature of the KTN crystal changes, the dielectric constant also changes. As a result, the amount of current in the crystal also varies, and the amount of electrons supplied into the crystal varies. As a result, the amount of trapped electrons in the crystal varies. Since the internal electric field also changes due to fluctuations in the amount of trapped electrons, the refractive index distribution also changes. The light deflection of the KTN crystal utilizes the fact that the traveling direction accumulates and changes according to the refractive index distribution when light passes through the crystal having the refractive index distribution. Accordingly, the center wavelength of the wavelength sweep and the sweep wavelength range vary.

  In SS-OCT, the light output from the light source is branched into two, and the interference signal between the reflected light from the object to be measured and the reference light is measured. The frequency of the interference signal changes according to the optical path length difference between the reflected light and the reference light, and the depth and position of the reflection point in the object to be measured can be obtained. A tomographic image can be formed by two-dimensionally scanning the incident position on the object to be measured and connecting the reflection point information at each point.

  When the refractive index of the object to be measured is n and the physical distance to the reflection point is L, an interference signal obtained by SS-OCT measurement corresponds to an optical path length of n × L. Furthermore, if the refractive index of the object to be measured has wavelength dependence and is expressed as n (λ), the interference signal to be measured is n (λ) × L. At this time, when the center wavelength of the wavelength swept light source changes, the value of n (λ) × L to be measured changes according to the wavelength. Further, according to Non-Patent Document 2, the spatial resolution δz in the depth direction of the object to be measured in SS-OCT measurement is as follows:

It is. λ ₀ is the center wavelength during wavelength sweeping. When the wavelength sweep band Δλ of the wavelength swept light source changes, the spatial resolution changes for each measurement according to Δλ. Therefore, even in the acquired SS-OCT image, the sharpness changes for each image. That is, due to changes in the center wavelength and wavelength sweep band of the wavelength swept light source, a constant OCT image cannot be obtained even if there is no change in the object to be measured, and stable SS-OCT measurement is hindered.

  An object of the present invention is to provide highly reproducible SS-OCT measurement by using a wavelength swept light source in which the center wavelength of the wavelength sweep and the swept wavelength range are stable for a long time.

  In the present invention, in order to achieve such an object, in one embodiment, one end face is coupled to a wavelength filter including a diffraction grating and an end mirror through an optical deflector, and the other end face is coupled to output. A wavelength swept light source including a gain medium facing a mirror and constituting an optical resonator between the end facet mirror and the output coupling mirror, wherein the optical deflector diffracts incident light incident thereon; In a wavelength swept light source that deflects in the direction crossing the grooves of the grating and varies the wavelength of the output light, the wavelength swept light source includes a wavelength monitor that detects a specific wavelength of the output light and outputs a wavelength monitor signal. The wavelength monitor signal is measured during sweeping, and the optical deflector is feedback-controlled so that the time at which the light of the specific wavelength appears is constant.

  According to the present invention, it becomes possible to keep the center wavelength and the swept wavelength range of the wavelength sweep light source constant and perform highly reproducible SS-OCT measurement without being affected by the wavelength dispersion of the object to be measured. Can be executed.

It is a figure which shows the structure of the wavelength sweep light source using the conventional KTN optical deflector. It is a figure for demonstrating the principle of operation of a wavelength monitor. It is a figure which shows the change of the deflection center by DC bias in the deflection | deviation of a KTN optical deflector. It is a figure which shows the relationship between the time change of a wavelength sweep, and the DC bias voltage applied to a KTN optical deflector. It is a figure which shows the structure of the wavelength sweep light source concerning Example 1 of this invention. It is a figure which shows the structure of the wavelength sweep light source concerning Example 2 of this invention. 10 is a diagram for explaining the operation of a wavelength monitor in Embodiment 2. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
The output light of the wavelength swept light source used for SS-OCT measurement is branched, one is used for OCT measurement, and the other is input to the wavelength monitor. The light input to the wavelength monitor is received by the photodetector (PD) through a wavelength selection element that transmits and selects only a specific wavelength in the sweep wavelength range. As such a wavelength selection element, a grating, a prism, a dielectric multilayer mirror, or the like can be used. As a method of monitoring the wavelength, the signal output from the PD is observed with an oscilloscope or the like, and the appearance time of light of a specific wavelength is observed.

  When the PD detects the light selected by the wavelength selection element, as shown in FIG. 2, the PD outputs a wavelength monitor signal at time t1 with reference to a trigger signal synchronized with the applied voltage during the wavelength sweep. Although the wavelength monitor signal does not have information on the wavelength of the detected light, only the specific wavelength λ1 is detected by the wavelength selection element, so that the specific wavelength λ1 is output at time t1 during the wavelength sweep. I can know that. Since the grating, prism, and dielectric multilayer mirror can change the wavelength to be transmitted and selected depending on the incident angle to these elements, the specific wavelength detected by the PD can be changed.

  As described above, the wavelength swept light source has a wavelength selection element in the optical resonator. For example, a wavelength selection element can be formed by using an electro-optic crystal such as a KTN crystal as an optical deflector and a combination with a diffraction grating. The diffraction angle is determined according to the wavelength and angle of light incident on the diffraction grating, and laser oscillation occurs at the wavelength fed back to the gain medium in the optical resonator. At this time, the oscillation wavelength range is determined according to the deflection angle range of the optical deflector. Accordingly, when the center of the deflection angle or deflection width changes, the laser oscillation range or the center wavelength changes.

  In the present embodiment, the time at which a specific wavelength appears during the wavelength sweep is detected from the wavelength monitor. The voltage and temperature applied to the electro-optic crystal of the wavelength sweep light source are feedback controlled so that the time appearing during the wavelength sweep is constant.

According to Non-Patent Document 3, the current i (t) from the PD is such that the reflection point of the object to be measured has a depth at which the optical path length difference Z ₀ between the reflected light and the reference light is

It becomes. Here, k (t) = 2π / λ (t), and the time waveform i (t) detected by the PD is determined by the time change λ (t) of the wavelength of the output light from the wavelength swept light source. The Here, it is obvious that i (t) also changes if the wavelength change λ (t) of the output light from the wavelength swept light source changes.

  Therefore, by making the center wavelength and the swept wavelength range of the wavelength swept light source constant, the frequency in the interference waveform measured as a time waveform can also be stabilized. By stabilizing the center wavelength and the swept wavelength range of the wavelength swept light source, the effective reflection point can be prevented from changing due to the wavelength dispersion of the object to be measured in SS-OCT measurement. As a result, highly accurate measurement can be performed stably.

  FIG. 3 shows the change of the deflection center due to the DC bias in the deflection of the KTN optical deflector. In the KTN optical deflector, electrodes 202 and 203 are formed on opposing surfaces of the KTN crystal 201, and the optical axis is set perpendicular to the electric field applied from the control voltage source 204. By changing the DC bias voltage, the incident light 211 can be changed from the central axis 212 of the emitted light at the time of the low DC bias to the central axis 213 of the emitted light at the time of the high DC bias. When the amount of electrons supplied into the crystal changes due to a change in the dielectric constant of the electro-optic crystal, the refractive index distribution in the crystal changes and the central axis of the emitted light also changes. Can be compensated. Making the central axis of the outgoing light from the KTN optical deflector constant means making the center of the incident angle to the diffraction grating constant when used in the wavelength swept light source, and the center at the time of wavelength sweeping. This leads to stabilization of the wavelength.

  FIG. 4 shows the relationship between the time change of the wavelength sweep and the DC bias applied to the KTN optical deflector. This shows that when the dielectric constant of the KTN crystal is constant, the time change of the wavelength sweep can be controlled by changing the applied DC bias voltage and controlling the central axis of the deflection angle of the emitted light. When the dielectric constant of the KTN crystal changes, the time change of the wavelength sweep shifts in the time direction with the change of the central axis of the deflection angle. In SS-OCT measurement, an interference waveform is acquired at a time when wavelength sweep is performed, and measurement is performed using a constant wavelength sweep width and a center wavelength. Therefore, if the time change of the wavelength sweep is shifted in the time direction, the time for capturing the interference waveform must be shifted according to the amount of the shift, and the processing becomes complicated.

  Therefore, if the time shift of the wavelength sweep is canceled by controlling the voltage applied to the KTN optical deflector, the same time change of the wavelength sweep can always be obtained. At this time, it is not necessary to control the time for capturing the interference waveform of the SS-OCT measurement, and the wavelength sweep range used for the SS-OCT measurement is always constant. It is possible to prevent the point from changing and to perform highly accurate measurement.

  FIG. 5 shows a configuration of the wavelength swept light source according to the first embodiment of the present invention. The light source for SS-OCT measurement includes a wavelength variable light source 300 in the 1.3 μm band. The wavelength swept light source 300 includes a semiconductor optical amplification (SOA) chip and a KTN optical deflector as gain media. The KTN optical deflector comprises a 4.0 × 3.2 × 1.2 mm KTN crystal and an electrode made of Ti / Pt / Au deposited on a 4.0 × 3.2 mm surface, with an electrode spacing of 1.2 mm. The temperature is adjusted so that the dielectric constant of the KTN crystal is 17,500 in the cubic region, and then a control voltage is applied to the KTN crystal. The incident light has a diameter of 1.0 mm and is linearly polarized light parallel to the internal electric field generated by voltage application, and propagates between the electrodes along the optical axis perpendicular to the electric field.

The outgoing light from the KTN optical deflector passes through the collimator lens and then enters the diffraction grating having a score of 1200 mm ⁻¹ . The incident angle θ to the diffraction grating was set so that the center wavelength was 1310 nm when the KTN optical deflector was driven under the following voltage application conditions. Of the light diffracted by the diffraction grating, it oscillates at a wavelength that is fed back into the laser resonator.

  The applied voltage conditions to the KTN optical deflector are as follows. First, in the trap filling time, a DC voltage of 300 V having the same positive and negative values is alternately applied for 60 seconds to set the KTN crystal to the initial trap filling state. Here, the positive and negative are alternately applied, but only a positive voltage or only a negative voltage may be applied. Subsequently, an AC voltage of 20 kHz (amplitude 300 V) superimposed with a DC bias voltage of −200 V is applied to operate as a high-speed deflector. The light output from the wavelength swept light source 300 has a center wavelength of 1310 nm and a wavelength sweep width of 100 nm.

  The light output from the wavelength swept light source 300 is branched into two by a coupler 302 connected to the optical fiber 301. One optical output is output as wavelength-swept light for SS-OCT measurement, and the other is used for wavelength monitoring. The wavelength monitoring light is input from the coupler 302 to the fiber collimator 304 via the circulator 303. The light is extracted as parallel light into the space by the fiber collimator 304 and is incident on the diffraction grating 305 as a spectroscopic element. The diffraction grating 305 is arranged so that the first-order diffracted light having a wavelength of 1310 nm is recombined with the fiber collimator 304. The light recombined with the fiber collimator 304 is incident on the photodetector (PD) 306 via the circulator 303.

  When the PD 306 detects light selected by the wavelength selection element including the diffraction grating 305, the PD 306 outputs a wavelength monitor signal to the oscilloscope 307. The oscilloscope 307 measures the peak value of the received light, that is, the peak value of the specific wavelength λ1 (= 1310 nm) selected by the wavelength selection element. The time at which this peak value appears, that is, the time t1 at the time of wavelength sweep, was 16.17 μs from the falling edge when the falling edge of the trigger signal on the rectangle synchronized with the applied voltage was used as a reference. This time information is fed back to the wavelength sweep light source 300.

  The DC bias voltage of the KTN optical deflector in the wavelength sweep light source 300 is controlled so that the time at which the specific wavelength detected by the wavelength monitor appears always appears constant during the wavelength sweep.

  The wavelength swept light source 300 is operated in an environment where the ambient temperature changes greatly, so that the operation fluctuation of the optical deflector and the change of the center wavelength and the wavelength sweep range of the wavelength swept light source accompanying it occur. Along with this, it was confirmed that the time at which light having a wavelength of 1310 nm was detected fluctuated from the time of 16.17 μs. The DC bias voltage applied to the KTN optical deflector is feedback controlled in a range of −200 V to −205 V so that the time for detecting light having a wavelength of 1310 nm is constant at 16.17 μs.

  As a result, the time waveform detected by the PD of the wavelength monitor became the same during SS-OCT measurement. Further, when the feedback control was not performed, the measurement depth in the object to be measured varied by 0.2%, but when the feedback control was performed, the variation could be suppressed to 0.04%. In the wavelength sweep type SS-OCT measurement, a depth of several millimeters is measured with a resolution of about several μm, and therefore, fluctuations in measurement can be suppressed to about 1/10 of the resolution by feedback control using a wavelength monitor.

  In Example 1, control was performed so as to obtain a desired oscillation wavelength as a wavelength swept light source by changing the DC bias voltage applied to the KTN optical deflector. On the other hand, the temperature of the KTN crystal of the KTN optical deflector may be controlled to change the dielectric constant, and the deflection angle of the emitted light may be changed to control to obtain a desired oscillation wavelength. Further, the KTN crystal of the KTN optical deflector is irradiated with ultraviolet light, the intensity of the ultraviolet light is changed to change the trap electron density, and the deflection angle of the outgoing light is changed to obtain a desired oscillation wavelength. Control may be performed.

  FIG. 6 shows a configuration of a wavelength swept light source according to the second embodiment of the present invention. The light source for SS-OCT measurement includes a 1.3 μm band wavelength variable light source 400. The configuration of the wavelength swept light source 400 is the same as that of the first embodiment.

  The light output from the wavelength swept light source 400 is branched into two by the coupler 402 connected to the optical fiber 401. One optical output is output as wavelength-swept light for SS-OCT measurement, and the other is used for wavelength monitoring. The wavelength monitoring light is input from the coupler 402 to the dielectric multilayer filter 408. Only light having two wavelengths of 1280 nm and 1340 nm is transmitted by the dielectric multilayer filter 408 and is incident on the photodetector (PD) 406.

  The PD 406 outputs a wavelength monitor signal to the oscilloscope 407 when detecting the light selected by the dielectric multilayer filter 408 which is a wavelength selection element. The oscilloscope 407 measures two peak values of the received light. The time when the peak value appears is fed back to the wavelength swept light source 400.

  The operation of the wavelength monitor in the second embodiment will be described with reference to FIG. As shown in FIG. 7, the peak values of two wavelengths (1280 nm and 1340 nm) are detected with the wavelength sweep from the short wavelength to the long wavelength. The early wavelength monitor signal corresponds to 1280 nm, and the late wavelength monitor signal corresponds to 1340 nm. When the falling edge of the trigger signal on the rectangle synchronized with the applied voltage is used as a reference, the time ((t1 + t2) / 2) at which the two peak values appear from the falling edge is the wavelength sweep. During this period, the DC bias voltage of the KTN optical deflector in the wavelength swept light source 400 is controlled so that it always appears constant.

  In addition, the DC bias voltage of the KTN optical deflector may be controlled so that the time difference between one or both of the two peak values becomes constant with respect to the trigger signal. In addition, the amplitude of the AC voltage among the voltages applied to the KTN optical deflector is feedback-controlled so that the time difference between the appearance times of the two peak values is constant. As a result, during SS-OCT measurement, the time waveform detected by the PD of the wavelength monitor is the same, and measurement with high resolution is possible as in the first embodiment.

100, 300, 400 Variable wavelength light source 101 Gain medium 102 Collimating lens 103 Condensing lens 104 Diffraction grating 105 Output coupling mirror 106 Electro-optic deflector 107 Output light 108 End face mirror 109, 204 Control voltage source 201 KTN crystal 202, 203 Electrode 205 Incident surface 206 Emission surface 211 Incident light 212 Center axis of outgoing light at low DC bias 213 Center axis of outgoing light at high DC bias 214 Wavelength sweep width at low DC bias 215 Wavelength sweep width at high DC bias 301, 401 Optical fiber 302, 402 Coupler 303 Circulator 304 Fiber collimator 305 Diffraction grating 306, 406 Photo detector (PD)
307,407 Oscilloscope 408 Dielectric multilayer filter

Claims (10)

The end face mirror and the output coupling mirror include a gain medium having one end face coupled to a wavelength filter including a diffraction grating and an end face mirror via an optical deflector, and the other end face facing the output coupling mirror. A wavelength swept light source constituting an optical resonator with the optical deflector, wherein the optical deflector deflects incident light in a direction crossing the groove of the diffraction grating to change the wavelength of the output light In the swept light source,
A wavelength monitor that detects light of a specific wavelength from the output light and outputs a wavelength monitor signal,
A wavelength-swept light source characterized by measuring the wavelength monitor signal during wavelength sweep and feedback-controlling the optical deflector so that the time at which the light of the specific wavelength appears is constant. The optical deflector is
An electro-optic crystal;
Two electrodes formed on opposing surfaces of the electro-optic crystal;
A control voltage source that outputs a control voltage for forming an electric field in the electro-optic crystal through the electrode;
Incident light incident along an optical axis substantially perpendicular to the direction of the electric field formed by the control voltage is deflected in a direction parallel to the electric field to vary the wavelength of the output light. The wavelength swept light source according to claim 1.   The wavelength sweep light source according to claim 2, wherein a DC bias voltage is feedback-controlled among control voltages applied to the electro-optic crystal of the optical deflector from the control voltage source.   3. The wavelength swept light source according to claim 2, wherein the temperature of the electro-optic crystal of the optical deflector is feedback controlled.   The wavelength-swept light source according to claim 2, wherein the intensity of ultraviolet light applied to the electro-optic crystal of the optical deflector is feedback-controlled.   6. The wavelength swept light source according to claim 1, wherein the wavelength monitor detects light of the specific wavelength using a spectroscopic element. The wavelength monitor detects light of two wavelengths of the output light,
2. The wavelength swept light source according to claim 1, wherein the optical deflector is feedback-controlled so that a time at a midpoint when the light of the two wavelengths appears is constant.   The wavelength swept light source according to claim 7, wherein the optical deflector is feedback-controlled so that a time difference between the times when the two wavelengths of light appear is constant.   The optical deflector is adjusted such that the time difference between the time of appearance of one or both of the two wavelengths of light is constant with respect to a trigger signal synchronized with an applied voltage applied to the electro-optic crystal of the optical deflector. The wavelength-swept light source according to claim 7, wherein feedback control is performed. The electro-optical _{crystal, KTN (KTa 1-x Nb} x O 3 (0 <x <1): potassium tantalate niobate) crystal, or the addition of lithium to KTN crystal _{KLTN (K 1-y L y} Ta 1 _{-x Nb x O 3 (0 <} x <1,0 <y <1)) wavelength-swept light source according to any one of claims 1 to 9, characterized in that it is crystalline.

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