JP5296814B2 - Wavelength swept light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problems that, in a wavelength sweeping light source of the prior art using a polygon mirror, an oscillation wavelength is changed approximately linearly in the shape of a convex upward slightly with respect to time, in a wavelength sweeping light source of SS-OCT, on the other hand, it is required to change the wavelength as a wavenumber (inverse of wavelength) is changed linearly on a time axis, in the wavelength sweeping light source of the prior art, such a wavelength change cannot be implemented, nonlinear distortion occurs in a resultant OCT image and a point image cannot be kept sharp. <P>SOLUTION: In a wavelength sweeping light source utilizing an electro-optical deflector including KTN or the like, a part of oscillation output is made incident to an interferometer comprising a difference between two optical path lengths, and on the basis of an electric signal including an AC component of interference light intensity, a VCO input signal in PLL circuit pull-in operation is applied to a gain control voltage of a high voltage amplifier. Regardless of nonlinearity caused by a diffraction grating equation, wavelength sweep of which the wavenumber is changed linearly with respect to the time can be implemented. Variation or nonlinearity in control voltage couple deflection angle characteristics caused by nonuniformity, temperature distribution, change or individual variation in materials of the electro-optical deflector is also canceled. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a light source that can be used in an optical apparatus, an electronic apparatus, and the like. More specifically, the present invention relates to a wavelength swept light source that can be used for imaging using wavelength swept light.

  Imaging technology using optical devices is spreading not only to consumer electronic devices such as cameras, printers and facsimiles, but also to the medical field. In order to non-invasively image a tomography inside a living body, X-ray imaging using X-rays and diagnosis using ultrasonic waves are already widely used. In the method using X-rays, due to the problem of exposure, the use frequency and the use site are greatly limited, and the resolution is limited to the resolution of the same magnification photographing of the film. In the method using ultrasonic waves, there is no problem of exposure, so there is no limitation of use like X-ray, but the resolution is usually only about 1 cm. Therefore, imaging at the cell level size is impossible.

  In the medical field, a new technique capable of generating a tomographic image of the living body under the skin with micron order resolution has been desired. As a technique for realizing this, optical coherence tomography (hereinafter referred to as OCT), which has been developed since the 1990s, is known.

  OCT uses the principle of a Michelson interferometer, and uses low-coherence light as a light source to irradiate the living body with this low-coherence light. Based on the interference light between the reference light and the reflected light from the living body, an image of the living body under the skin is obtained. OCT has been put to practical use as an indispensable diagnostic instrument in ophthalmology for retinal diagnosis.

FIG. 5 is a diagram for explaining the basic principle of OCT (Non-Patent Document 1). In the following, only the outline of the basic principle will be briefly described. From the low-coherence light source 1, low-coherence light having a coherence length Δl _C is supplied as incident light to the living body 4. The outgoing light 6 from the light source 1 enters the beam splitter 2 and is divided into two equal parts. One of the halved lights 7 travels to the movable mirror 3, is reflected by the movable mirror 3, and travels again to the beam splitter 2 as reference light 8. The other light 9 of the bisected light is reflected on the reflection surfaces A, B, or C at different depths inside the living body 4 to obtain signal lights 11a, 11b, and 11c, respectively. Each signal light interferes with the reference light 10 through the beam splitter 2. By this interference, the reflected light whose wavefront is distorted due to excessive scattering in the living body is removed, and only the reflected light maintaining the original plane wave is selectively detected.

Here, the light reflected by the reflecting surface A causes interference when the movable mirror 3 is at the position A ′. At this time, if the distance between the center of the beam splitter 2 and the movable mirror 3 is L _R , and the distance between the center of the beam splitter 2 and the reflecting surface A is L _S , the reference light and the signal light satisfy the relationship of the following formula: And an electric signal is obtained from the photodetector 5.
| L _R −L _S | <Δl Formula _C (1)
The above expression is established in a one-to-one correspondence with the reflecting surface A and the position A ′ of the movable mirror, the reflecting surface B and the position B ′ of the movable mirror, and the reflecting surface C and the position C ′ of the movable mirror. Therefore, by continuously moving the movable mirror 3 at a constant speed v, the reflected light intensity distribution under the living body surface along the optical axis (z-axis) direction in the living body 4 is measured with a spatial resolution of Δl _C. be able to. By scanning the light incident on the living body in the x direction with a scanning mirror or the like, a reflected light intensity distribution on the surface of the living body in the xz plane is obtained, and this is the final OCT image. The configuration of the Michelson interferometer in FIG. 5 can use an optical fiber coupler, so that an inspection apparatus that can be used in clinical settings is realized.

  The OCT having the configuration shown in FIG. 5 is called a time domain (TD) OCT (hereinafter referred to as TD-OCT) because image data is acquired in time series by moving a movable mirror. In TD-OCT, since it is necessary to move a movable mirror having a mass, the scanning speed is limited. However, it is often difficult to completely fix the living body itself depending on the state of the examination, and it is preferable to perform scanning in as short a time as possible to obtain necessary image data. Further, when scanning is performed up to a depth where a blood vessel exists in a living body, it is difficult to acquire an image due to scattering by red blood cells moving in the blood vessel. For this reason, OCT has been requested to acquire information in the depth direction as quickly as possible.

  Therefore, a Fourier domain (FD) -OCT (hereinafter referred to as FD-OCT) has been proposed in which the interference signal is Fourier-transformed to obtain the reflected light intensity along the optical axis. In FD-OCT, a spectroscope that decomposes signal light from a living body into light of each wavelength is disposed in front of the photodetector 5 in FIG. 5, and an interference spectrum is detected by a detector element array of the photodetector. . That is, an interference spectrum is obtained by a parallel detector in which a large number of detector element elements corresponding to each wavelength are arranged. The spectrum detected by the parallel detector is Fourier transformed to obtain a reflected light intensity portion distribution along the optical axis. However, in FD-OCT, a parallel detector having a large number of detector element elements is required. In many cases, it is necessary to detect signals of respective wavelengths simultaneously in more than 1000 detector element elements. Such a parallel detector is only realized by a silicon CCD or CMOS in a wavelength band of 1.1 μm or less, and it is difficult to obtain a parallel detector at a longer wavelength.

  Therefore, at present, FD-OCT can be applied to retinal diagnosis and the like in which visible light can be used, but cannot be applied to OCT for tissues such as skin that require operation in a longer wavelength region. In addition, when applied to tomographic imaging of blood vessels, absorption by hemoglobin cannot be ignored unless light on the long wavelength side is used up to about 1.3 μm due to the scattering of red blood cells by hemoglobin. On the other hand, when the wavelength of the light source reaches about 1.5 μm, the absorption by water becomes remarkable. If it exceeds 1.6 μm, it will be difficult to obtain a photodetector. For these reasons, there has been a demand for using a light source in the 1.3 μm band in order to use OCT on the skin and the like.

  Accordingly, a newly attracting attention is a swept source (SS) -OCT (hereinafter referred to as SS-OCT) which transforms the FD-OCT and sweeps the frequency of the light source. In SS-OCT, signal light obtained by irradiating a living body with light from a low-coherence light source as in FD-OCT is split by a spectroscope to generate a plurality of wavelength signals at one time. Regularly sweeps the wavelength. 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, in FD-OCT, wavelength division is performed according to a spatial position by a spectroscope, whereas in SS-OCT, wavelength division is performed according to time, and a single detector can be used. Since a parallel detector having a large number of detector element elements is not required and there is no restriction on the selection of detectors, a light source of 1.3 μm band can also be used.

FIG. 6 is a schematic diagram for explaining the principle of SS-OCT (Non-Patent Document 1). In SS-OCT, an optical signal 26 in which the optical frequency is swept linearly with respect to time is supplied from the optical frequency (wavelength) sweep light source 21 to the living body 24. As the optical frequency sweep light source 21, for example, a wavelength variable laser is used. In the SS-OCT shown in FIG. 6, the position of the mirror 23 is fixed. When the distance between the center and the mirror 23 of the beam splitter 22 L _R, the distance between the center and the biological surface 31 of the beam splitter 22 and L _S, are arranged the elements such that L R ₌ L _S.

At this time, the difference in optical frequency between the reference light 28 and the reflected lights 29b and 29c from the reflecting surface 32 and the reflecting surface 33 in the living body is constant regardless of time. If the difference between these optical frequencies is f ₂ and f ₃ , beat frequencies f ₂ and f ₃ corresponding to the reflective surface 32 and the reflective surface 33 are mixed due to interference between the reference light 28 and the reflected light 29b and 29c. Signal light is obtained. When this signal light is Fourier transformed, the reflected light intensity at beat frequencies f ₂ and f ₃ is obtained. If it is optical frequency sweep linearly from the light source 21, and the beat frequency _f 2, _{f 3,} the depth _{d 2,} the depth _{d 3} is directly proportional to. In the living body, reflected light is generated from various places. Therefore, the reflected light intensity distribution along the optical axis (z-axis) direction can be obtained by Fourier transforming the interference light. If beam scanning is also performed in the x-axis direction, an OCT image in the xz plane can be obtained.

  In SS-OCT, the photodetector 25 only needs to detect signal light mixed with interference light having different beat frequencies with a single detection element, so that no parallel detector is required as in conventional FD-OCT. It is possible to use a 1.3 μm band swept light source suitable for diagnosing skin and the like. SS-OCT is being put to practical use in areas other than ophthalmic practice because of its stable configuration using optical fiber couplers, high-speed image acquisition by eliminating the need for movable mirrors, and ease of use of various photodetectors. It has been.

International Publication WO2006 / 137408 Specification

Masamitsu Haruna “Progress of Optical Coherence Tomography”, Applied Physics Vol. 77, No. 9, p1085-1922, 2008

  In the SS-OCT described above, the sweep and light wavelength sweep light source is one of the important components. As a light wavelength sweeping light source in the prior art, for example, a light source using a polygon mirror has been used.

FIG. 7 is a diagram showing a configuration of a wavelength swept light source using a conventional polygon mirror. The wavelength swept light source 100 includes a polygon mirror 120 and a laser oscillator including a diffraction grating 106 and a gain medium 101 arranged in a Littrow configuration. The laser oscillator includes condensing lenses 102 and 111 at both ends of the gain medium 101, and output light 113 is obtained from the output coupling mirror 112. Incident light from the gain medium 101 is reflected by the reflecting surface A of the polygon mirror 120 and enters the diffraction grating 106 at an incident angle θ (110) that satisfies the following condition of the diffraction grating equation.
2Λsin θ = mλ Formula (2)
In the above equation, Λ is the pitch of the diffraction grating, λ is the oscillation wavelength, and m is the diffraction order.

  The oscillating light is diffracted by the diffraction grating 106 at the same emission angle θ as the incident angle, and reciprocates in the optical path between the diffraction grating 106 and the output coupling mirror 112. Since the polygon mirror 120 rotates at a constant speed in the direction 121, the incident / reflection angle of the oscillation light on the reflection surface A of the polygon mirror 120 changes periodically with rotation. Therefore, the oscillation wavelength λ determined by the equation (2) changes with rotation according to the incident angle θ to the diffraction grating 106. Since the polygon mirror 120 rotates at a constant rotation speed, the incident (reflection) angle θ to the diffraction grating 106 changes at a constant speed. Therefore, the oscillation wavelength λ of the wavelength swept light source 100 changes substantially linearly with respect to time.

  FIG. 8 is a diagram showing the time variation of the oscillation wavelength obtained by the conventional wavelength swept light source using the polygon mirror. As indicated by the solid line 40, the oscillation wavelength is slightly convex with respect to the time t on the horizontal axis and changes in a substantially linear (linear) manner. However, the time change profile of the wavelength required for SS-OCT is different from the time change in which the oscillation wavelength changes linearly as obtained by a wavelength swept light source using a polygon mirror. .

  Here, the basic principle of OCT described with reference to FIG. 5 will be referred to again. In OCT, in order to linearly scan the inside of a living body in the optical axis (z-axis) direction with good linearity, the delay times of the reference beams 7 and 8 need to change linearly. In other words, the position of the movable mirror 3 moves at a constant speed from A ′ to C ′ over the entire movable range, so that the living body is also scanned at a constant speed. Such an operation is most ideal from the viewpoint of the linearity of the OCT image. If the mirror does not move at a constant speed, the resulting OCT image will be non-linearly distorted in the optical axis (z-axis) direction.

  Further, consider the case where the above-mentioned ideal movement of the movable mirror at constant speed is adapted to the SS-OCT configuration of FIG. When the mirror position is Fourier transformed, it becomes the reciprocal of the position. The reciprocal of the position corresponds to the wave number. Therefore, a wavelength swept light source whose wavelength changes so that the wave number (reciprocal of wavelength) changes linearly with time is ideal. Otherwise, the beat frequency corresponding to one reflecting surface in the living body is not single, and as a result, the sharpness of the OCT image is impaired.

  In FIG. 8, the wavelength change when the wave number changes linearly with respect to time is simultaneously shown as a desirable wavelength change by a broken line 41. Compared with the wavelength change in the case of the wavelength swept light source of the prior art using a polygon mirror, the curve is convex downward and the rate of change increases as the oscillation wavelength becomes longer (upward on the vertical axis in FIG. 8). A suitable wavelength change profile is appropriate.

  However, when a polygon mirror is used, since the polygon mirror has a large moment of inertia, it is extremely difficult to control the rotation speed by a method other than a constant rotation speed. Further, according to the prior art using the polygon mirror shown in FIG. 8, the upward convex and substantially linear wavelength change is a shape derived from the sin term of the diffraction grating equation of Equation (2), and is usually The direction of the curve cannot be changed even if the above means is used.

  As described above, the wavelength swept light source of the prior art cannot realize a wavelength change suitable for SS-OCT, and has a problem that a sharp OCT image cannot be obtained.

  The present invention has been made in view of the above problems, and can eliminate the distortion of linearity in the depth direction of the OCT image, obtain a sharp OCT image, and can be applied to SS-OCT. An object is to provide a possible wavelength swept light source.

  According to the first aspect of the present invention, in the light source whose output wavelength periodically changes with time, an oscillator unit including an electro-optic deflector and two optical paths provided with a difference between optical path lengths from the oscillator unit An interferometer that propagates a part of the oscillation output light and outputs an electrical signal representing an interference light intensity including an AC component having a frequency proportional to the optical path length difference and the wave number change rate of the oscillation output light; An error signal generating circuit for generating a feedback signal for keeping the frequency of the alternating current component constant, and a corrected control voltage supplied to the electro-optic deflector by operating the feedback signal on a gain with respect to a lamp voltage signal And a feedback control unit having a control voltage generation circuit for generating the wavelength sweeping light source.

As an electro-optic deflector, potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1): KTN) or lithium-doped (K _1-y Li _y Ta _1-x Nb) _It is preferable to use xO ₃ (0 <x <1, 0 <y <1).
The invention according to claim 2 is the wavelength swept light source according to claim 1, wherein the error signal generation circuit is input with the electrical signal and is proportional to a deviation amount of the frequency of the electrical signal from a predetermined frequency. The phase lock loop circuit outputs the feedback signal, and the control voltage generation circuit is a high voltage amplifier that amplifies the ramp voltage signal when the feedback signal is applied as a gain control signal.

  The invention according to claim 3 is the wavelength swept light source according to claim 1 or 2, wherein the oscillator unit includes a gain medium, a diffraction grating on which light from one end of the gain medium is incident, and the diffraction grating And an end mirror to which the diffracted light of the incident light is directly incident, and is configured by a resonator in which the gain medium and the end mirror are optically connected via the diffraction grating. , Between the gain medium and the diffraction grating, and disposed on an optical path formed by the resonator.

  The invention according to claim 4 is the wavelength swept light source according to claim 1 or 2, wherein the oscillator unit includes a gain medium and a diffraction grating on which light from one end of the gain medium is incident, and the gain medium And the diffraction grating are optically connected to each other, and the electro-optic deflector is disposed between the gain medium and the diffraction grating on an optical path formed by the resonator. It is characterized by being.

  As described above, according to the present invention, it is possible to provide a wavelength swept light source that realizes a wavelength change suitable for SS-OCT. A part of the oscillation output light is incident on an interferometer having a difference between the two optical path lengths, and the operation of the feedback control unit that keeps the AC component frequency of the obtained electrical signal constant makes the wavelength change linear with respect to the wave number A wavelength sweep voltage that varies with time is generated and applied, and the linearity of the OCT image is greatly improved, and a sharp OCT image can be obtained.

FIG. 1 is a diagram showing a first configuration of a wavelength swept light source according to the present invention. FIG. 2 is a diagram for explaining the configuration and operation of a deflector used in the wavelength swept light source of the present invention. FIG. 3 is a block diagram showing the configuration of the feedback control unit of the wavelength swept light source of the present invention. FIG. 4 is a diagram showing a second configuration of the wavelength swept light source of the present invention. FIG. 5 is a diagram for explaining the basic principle of OCT. FIG. 6 is a diagram schematically showing the principle of SS-OCT. FIG. 7 is a diagram showing a configuration of a wavelength swept light source using a conventional polygon mirror. It is the figure which showed the time change of the oscillation wavelength obtained by the wavelength swept light source using the polygon mirror of a prior art.

  The present invention relates to a wavelength swept light source using an electro-optic deflector including KTN, etc., and a part of the oscillation output is incident on an interferometer having a difference between two optical path lengths, and the intensity of the generated interference light Is converted into an electrical signal. Based on this electric signal, a voltage controlled oscillator (VCO) input signal at the time of pull-in operation of the phase lock loop is fed back to the gain of the high voltage amplifier. Even if there is nonlinearity derived from the diffraction grating equation described later and instability of the deflection control characteristic of the electro-optic deflector itself, wavelength sweep in which the wave number changes linearly with respect to time can be realized. The wavelength swept light source of the present invention has a wave number that changes linearly with respect to time even when there is non-linearity with respect to the control voltage of the deflection angle due to non-uniformity of the material constituting the electro-optic deflector, temperature change or individual variation. Thus, the oscillation wavelength can be swept.

  Hereinafter, the configuration of the oscillator unit of the wavelength swept light source of the present invention will be described first. Next, the configuration and operation of the electro-optic deflector will be described. Further, the configuration and operation of the feedback control unit that supplies the control voltage to the electro-optic deflector will be described in detail.

  FIG. 1 is a diagram showing a first configuration of a wavelength swept light source according to the present invention. The wavelength swept light source 200 includes an oscillator unit 200a and a feedback control unit 200b. The oscillator unit 200a is a laser oscillator having a configuration in which the diffraction grating 106 and the end surface mirror 110 are arranged in a Littman manner. The gain medium 101 is disposed between the first condenser lens 111 and the second condenser lens 102. The gain medium 101 is coupled to a wavelength filter including an electro-optic deflector 103, a diffraction grating 106, and a direct incident end mirror 110 through a second condenser lens 102. The gain medium 101 and the end mirror 110 are optically connected via the diffraction grating 106 to form a resonator. Further, the first condenser lens 111 is opposed to the output coupling mirror 112 and constitutes an optical resonator having the output coupling mirror 112 and the end face mirror 110 as both ends as a whole. From the output coupling mirror 112, output light 113 is obtained by the laser action of this optical resonator. The electro-optic deflector 103 is disposed between the gain medium 101 and the diffraction grating 106 and on the optical path formed by the resonator.

  In the wavelength swept light source of the present invention, there is no particular limitation between the incident angle θ from the side facing the condenser lens 102 to the diffraction grating 106 and the incident angle φ from the side facing the end mirror 110. However, from the viewpoint of obtaining a stronger filter effect, in the above-described wavelength filter, the incident angle θ from the side facing the condenser lens 102 to the diffraction grating 106 is equal to the incident angle φ from the side facing the end mirror 110. It is preferable that the absolute value is set larger than that. As a result, the diffraction grating outgoing light beam 108 is expanded as compared with the diffraction grating incident light beam 107 to the diffraction grating 106, and is reflected by the end mirror 110 as a thick light beam having a small divergence angle. Therefore, the selected wavelength width of the wavelength filter can be narrowed. The oscillation wavelength is swept by deflecting the diffraction grating incident light beam 107 through the control voltage source 104 supplied from the feedback control unit 200b and applied to the electro-optic deflector 103.

  That is, the incident angle θ to the diffraction grating 106 is changed by the deflection by the electro-optic deflector 103. By changing the voltage 104 supplied from the feedback control unit 200b and applied to the electro-optic deflector 103, the wavelength can be swept at high speed without the intervention of the movable unit. Details of the operation of the feedback control unit 200b will be described later. Next, an electro-optic deflector suitable for use in the wavelength tunable light source of the present invention will be described.

  Recently, a new phenomenon has been found in specific electro-optic effect crystals. In this electro-optic effect crystal, charges are injected into the crystal in accordance with an electric field generated by applying a voltage. As a result, a space charge distribution formed by the injected charges or a trap charge distribution generated by trapping the injected charges in the electro-optic crystal is generated in the crystal. A non-uniform electric field distribution due to the charge distribution causes a refractive index gradient, and a phenomenon occurs in which the path of the light beam orthogonal to the gradient is bent.

The occurrence of this phenomenon requires a secondary electro-optic effect that occurs in proportion to the refractive index change or the square of the electric field. In addition, the deflection phenomenon appears only with practical values of applied voltage and current when a crystal exhibiting this effect has a large dielectric constant and small mobility. As a typical example of this type of crystal, potassium tantalate niobate (KTa _1-x Nb _x O ₃ (0 <x <1): KTN) or lithium-doped (K _1-y Li _y Ta) _1-x Nb _x O ₃ (0 <x <1, 0 <y <1)) is known.

  In such a crystal, all parts in the crystal are responsible for the deflection action. The light beam is emitted from the crystal in response to the accumulated deflection of the action in each part of the light beam propagation path. That is, the amount of deflection obtained is proportional to the light propagation length in the crystal. In this respect, the operation mechanism is completely different from that of the conventionally used prism type optical deflector. As a result of the unique deflection mechanism, the deflection operation is fast and the deflection angle range is large. Details of such an electro-optic deflector are disclosed in Patent Document 1.

  FIG. 2 is a diagram for explaining the configuration and operation of an electro-optic deflector used for the wavelength-swept light source of the present invention. FIG. 2 illustrates the basic configuration and operation of the deflector viewed from the deflection surface. On the opposing surface of the electro-optic crystal 301, an electrode 302 and a ground electrode 303 are formed, respectively. Incident light 305 propagates along a central optical axis 308 that passes between these two electrodes. Here, when a voltage is applied to the electrode 302 by the control voltage source 304, the light beam in the crystal follows the deflecting optical path 307 bent toward the negative electrode (the ground electrode 303 in FIG. The light is emitted from the crystal 301 as the incident light 306.

  When the polarization optical path 307 is observed on the “crystal side (inside)” of the emission end face A, it appears as if a light beam is emitted from the polarization center 310 located at the crystal center. That is, the light beam appears to rotate around the polarization center 310 due to the deflection action. When this is observed with respect to the outgoing light 306 “outside” the crystal, it now appears to rotate around the exit center 309 due to the deflection action. Such an exit center 309 approaches the exit end face side due to the refraction action at the exit end face A, and is located at L / (2n) from the exit end face A when the crystal length is L. Here, n is the refractive index of the crystal.

  The amount of deflection obtained in the electro-optic deflector is proportional to the crystal length. However, when trying to lengthen the crystal, it becomes more difficult to ensure the uniformity of the crystal. Further, when the crystal is lengthened, the apparent power required for the control voltage source 304 increases because the capacitance increases. As a result, the high-speed driving of the electro-optic deflector is hindered. In this type of electro-optic deflector, instead of using the crystal of the actually required length, an effect equivalent to increasing the crystal length can be obtained by using the folding of the optical path by internal reflection. it can.

  As described above, the electro-optic deflector using KTN or the like has the characteristics that the deflection operation is performed at a high speed and the deflection angle range is large. On the other hand, there is a problem of uniformity of crystal material, individual variation, variation of electrification and injection characteristics of two electrodes, and non-uniformity of trap charge. is there. Such nonlinearity changes depending on the crystal, the applied voltage, and the sweep frequency, so that correction in an open loop is generally difficult.

  In the wavelength swept light source of the present invention, a control voltage supplied to the electro-optic deflector is supplied from the feedback control unit in order to sweep with a constant oscillation wavelength width from the minimum oscillation wavelength to the maximum oscillation wavelength.

  In general, in an electro-optic deflector using KTN or the like, the optical path in the electro-optic deflector is deflected to the lower side of the y-axis in FIG. 1, where δ is the change in the incident angle to the diffraction grating received by the electro-optic deflector. When δ becomes negative, the control voltage becomes a positive voltage. For simplicity, the desired control voltage will be described assuming that the relationship between the control voltage 104 and δ is linear. That is, it is assumed that the control voltage 104 and δ are in a directly proportional relationship.

  In the case of the configuration shown in FIG. 1, by applying a control voltage 104 to KTN such that the rate of change of δ increases as δ becomes a positive value, the time change of the oscillation wavelength becomes convex downward. Wavelength sweep profile can be obtained. In other words, the control voltage may be controlled so that the rate of change of the oscillation wavelength increases as the oscillation wavelength is swept to the longer wavelength side. In the present invention, the feedback control unit 200b detects the rate of change of the wave number from the interference light generated from part of the oscillation output light, and sequentially corrects the ramp signal based on this rate of change information to correct it to a desired control voltage. I do. Due to the action of the feedback control unit 200b, the longer the oscillation wavelength is, the faster the oscillation wavelength changes, and the time variation of the oscillation wavelength becomes a downwardly convex shape. At this time, in the wavelength sweep light source of the present invention, the sweep wavelength is controlled so that the wave number linearly changes on the time axis. In addition, by controlling with a feedback loop including an electro-optic deflector, the above-described nonlinearity of the deflection angle of the counter voltage is also corrected. Therefore, when the present invention is applied to SS-OCT, a linear OCT image having a good linearity in the depth direction of the living body of the object can be obtained.

  FIG. 3 is a block diagram showing the configuration of the feedback controller 200b of the wavelength swept light source of the present invention. A part of the oscillation output light 113 shown in FIG. 1 is extracted by the light beam tap 121 and given to the feedback control unit 200b. The feedback control unit 200b includes an optical interferometer unit that provides a difference between two optical path lengths, and an electrical signal processing unit other than that.

  The optical interferometer unit includes a semi-transparent mirror 201, a first reflecting mirror 202, a second reflecting mirror 203, a first photodetector 204 and a second photodetector 205. Each reflecting mirror has two orthogonal reflecting surfaces, and emits the reflected light in the opposite direction parallel to the incident light, as shown in FIG.

  In the optical interferometer unit, the combined light from the light beam tap 121 is first branched in two directions at point a of the semi-transparent mirror 201 and directed to the first reflecting mirror 202 and the second reflecting mirror 203, respectively. The light traveling on the first optical path via the first reflecting mirror 202 is further branched at the point b of the semi-transparent mirror 201 and detected by the first photodetector 204 and the second photodetector 205, respectively. . Similarly, the light traveling on the second optical path via the second reflecting mirror 203 branches at the point b of the semi-transparent mirror 201 and is detected by the first photodetector 204 and the second photodetector 205, respectively. The Therefore, in the first detector 204, an electric signal representing the intensity level of the interference light of the light that has respectively traveled along the two optical paths of the first optical path and the second optical path is obtained. These two electrical signals are always in opposite phase to each other. Similarly, the second detector 205 can also obtain an electric signal representing the intensity level of the interference light of the light that has passed the two optical paths, the first optical path and the second optical path.

  Here, an optical path length difference corresponding to a delay time of T is set between the first optical path and the second optical path, as shown in FIG. Compared with the first optical path, the second optical path passing through the second reflecting mirror 203 is a distance (light speed × T / 2) corresponding to the T / 2 delay time from the semi-transparent mirror 201 in the x-axis direction of FIG. Only stretched.

  Both of the electrical signals representing the intensity of the interference light output from the two photodetectors correspond to the DC fluctuation component corresponding to the change in the oscillation light intensity due to the gain wavelength dependency of the gain medium and the delay time T. The signal includes an AC signal corresponding to the optical path length difference. However, the phases of the AC components of the two electrical signals are shifted from each other by π, and the maximum instantaneous amplitude of the AC component of one electrical signal corresponds to the minimum instantaneous amplitude of the AC component of the other electrical signal. ing. Therefore, each electric signal from the two photodetectors is input to the subtracting circuit 206, and an electric signal having only an AC component in which the DC component is removed and the amplitude is doubled is obtained. The frequency of the AC signal depends on the optical path length difference, and the frequency increases as the optical path length difference increases.

Here, the fact that the frequency of the AC component of the electrical signal representing the intensity of the interference light obtained by the photodetector represents the rate of change of the wave number k which is the reciprocal of the oscillation wavelength λ in the oscillator unit will be briefly described below. To do. The intensity of the interference light obtained by the photodetector is expressed by the following equation.
S (t) ∝I (t) + I (t) cos (2πν (t) T + φ) Equation (3)
Here, I (t) is the light intensity. φ is a phase change received by the semi-transparent mirror, and there is always a difference of π between the interference lights detected by the two photodetectors. Therefore, the difference signal between the two photodetectors is expressed by the following equation, and the DC component is removed.
ΔS (t) ∝ 2I (t) cos (2π ν (t) T + φ) Equation (4)
In Expression (4), ν (t) represents a temporally changing optical frequency output from the wavelength swept light source, and is rewritten as follows using the relationship between the optical frequency and the wavelength.
2πν (t) = 2πc / λ (t) = ck (t) Equation (5)
Here, c is the speed of light, and k (t) = 2π / λ (t) is nothing but the wave number.

  The frequency of ΔS (t) that vibrates with time is (dν / dt) T = (c / 2π) (dk / dt) T. Here, dν / dt is the change rate of the optical frequency, and dk / dt is the change rate of the wave number. If these change rates are constant, ν (t) and k (t) change linearly with time. That is, a constant frequency of ΔS (t) is a necessary and sufficient condition because the wave number changes linearly with respect to time.

  Therefore, when the lamp voltage signal is applied to the electro-optic deflector to sweep the oscillation wavelength approximately linearly, the frequency of the AC component of the electrical signal obtained from the photodetector is detected, and the AC component By performing the operation of keeping the frequency constant, a wavelength sweep in which the wave number changes linearly with respect to time can be realized. The operation of keeping the frequency of the AC component constant is performed by an electric signal processing unit described below.

  Referring to FIG. 3 again, the output electric signal ΔS (t) from the subtraction circuit 206 is input to the phase lock loop (PLL) circuit 207 in order to keep the frequency of the AC signal component constant. The PLL circuit 207 is generally well known for applications such as oscillator frequency synchronization used with a master clock oscillator and a voltage controlled oscillator (VCO). In general, an AC signal synchronized with the oscillation frequency is input, and a control voltage reflecting the fluctuation of the AC signal is supplied to the VCO to operate to maintain a predetermined oscillation frequency.

  In the present invention, the alternating current component of the interference light intensity from the optical interferometer unit is input to the PLL circuit 207, and an output signal (VCO input signal: VCOin) proportional to the frequency change of the alternating current component is driven to the electro-optic deflector. It is applied as a gain control voltage (Gain) of the high voltage amplifier 211 to be applied. As already described, an electro-optic deflector using KTN or the like requires a drive voltage of about several hundred volts for deflection. Therefore, the drive signal from the ramp signal generator 210 that generates the ramp voltage for wavelength sweep is amplified to a required voltage level by the high voltage amplifier 211 and applied to the electro-optic deflector 104. In the present invention, the output voltage (VCOin) from the PLL circuit 207 circuit is subtracted from the reference voltage 209 by the subtracter 208 and then applied to the gain control input (Gain) of the high voltage amplifier 211. The average gain of the high voltage amplifier 211 is set so that the reference voltage 209 is appropriately set and a desired wave number change rate is obtained.

  When the electro-optic deflector is driven with the lamp voltage from the lamp signal generator 210 as it is, the wavelength changes substantially linearly. That is, it is not possible to realize a wavelength change suitable for SS-OCT in which the wave number changes linearly with respect to time. Therefore, the ramp voltage is not very good at an approximate sweep voltage. In the present invention, interference light is generated based on the combined light from the oscillation output, and the PLL circuit 207 performs a pull-in operation to keep the AC component frequency of the intensity signal of the interference light constant. From the PLL circuit 207, a control voltage (VCOin) proportional to the frequency change of the AC component is obtained. By feeding back the control voltage (VCOin) from the PLL circuit 207 to the gain control voltage (Gain) of the high voltage amplifier 211, the wavelength sweep is performed so that the lamp voltage is corrected and the wave number changes linearly with respect to time. Is done. The feedback control unit 200b feeds back the output of the PLL circuit 207 to the amplification factor of the ramp signal in the high voltage amplifier 211, so that the wave number is linear with respect to time with respect to the sweep voltage of the ramp voltage of a simple sawtooth wave. Thus, voltage correction is performed every moment so as to obtain an oscillation wavelength sweep profile that changes with time.

  More specifically, when the wavelength is swept from the short side to the long side, the deflection angle δ by the electro-optic deflector changes monotonously from negative to positive. In the case of a KTN electro-optic deflector, a sawtooth wave that monotonously decreases from positive to negative is required as the control voltage applied for this purpose.

  When a ramp voltage having a constant negative gradient is applied, the rate of change of voltage is excessive in the vicinity of the short wavelength end at the beginning of the sweep, and therefore the frequency of ΔS (t) is high, and VCOin is the reference voltage 209. It exceeds. As a result, a negative correction signal is supplied from the subtracter 208 to the gain control input (Gain) of the high voltage amplifier 211, thereby reducing the voltage change rate and reducing the frequency of ΔS (t). Acts on direction.

  On the other hand, in the vicinity of the end of the long wavelength at the end of the sweep, the voltage change rate (the magnitude) is too small, and therefore the frequency of ΔS (t) is low, and VCOin is below the reference voltage 209. As a result, a positive correction signal is supplied from the subtracter 208 to the gain control input (Gain) of the high voltage amplifier 211, thereby increasing the voltage change rate (magnitude) and increasing the frequency of ΔS (t). It works in the direction of increasing.

In the laser oscillator having the configuration shown in FIG. 1 and having the Litman arrangement, the oscillation wavelength λ is determined by the following diffraction grating equation.
Λ (sin (θ + δ) + sinφ) = mλ Equation (6)
Here, Λ is the pitch of the diffraction grating, λ is the oscillation wavelength, and m is the diffraction order. θ and φ are the incident angle and the exit angle to the diffraction grating as shown in FIG. δ is an incident angle change amount to the diffraction grating received by the electro-optic deflector. As described in the case of the prior art together with the equation (2), the shape of the change of the wavelength λ is also affected by the sin term including δ on the left side of the equation (6) in the wavelength swept light source of the present invention in FIG. Therefore, even if δ is changed at a constant speed in equation (6), the desired wavelength change in which the wave number changes linearly with respect to time as shown by the broken line 41 in FIG. 8 cannot be realized.

  In addition, as described above, in the electro-optic deflector using KTN or the like, there is a problem of uniformity of crystal material, individual variation, variation of charge injection characteristics of two electrodes, and non-uniformity of trap charge. Therefore, non-linearity remains in the change of the deflection angle with respect to the control voltage. Accordingly, the change of the wavelength λ with respect to δ in the above formula (6) further includes another nonlinear component, and the change of δ with respect to the applied voltage varies depending on the temperature and is difficult to keep constant.

  However, according to the control by the feedback control unit 200b of the present invention, the change in the oscillation wavelength derived from the sin term in the equation (6) is corrected to the change in the oscillation wavelength in which the wave number changes linearly with respect to time. Furthermore, the problem of uniformity of the crystal material inherent in KTN or the like, and non-linearity and instability associated with changes in the deflection angle δ due to individual variations, etc. can be eliminated at the same time.

  FIG. 4 is a diagram showing a second configuration of the wavelength swept light source of the present invention. The wavelength swept light source 400 of FIG. 4 is different only in that the configuration of the oscillator unit in FIG. 1 is replaced from the Littman configuration 200a to the Littrow configuration 400a. Incident light 107 from the gain medium 101 enters the diffraction grating 109, diffracts in the same angular direction, and returns to the gain medium 101. That is, the oscillator unit 400a includes a gain medium 101 and a resonator having the diffraction grating 109 as one end. The configuration of the other feedback control unit 200b may be exactly the same as in FIG. The operation of the feedback control unit 200b of the present invention is the same as in the case of FIG.

  Hereinafter, more specific examples will be described.

  In the first configuration of the wavelength swept light source of the present invention shown in FIG. 1, the diffraction grating has a line marking density of 600 l / mm, the incident angle θ is set to 52.31 °, and the incident angle φ is set to 0.04 °. As a result, it operated at a center wavelength of 1.32 μm. The optical path length difference of the optical interferometer unit was set to 3 mm, that is, a value corresponding to the delay time T = 10 ps. The repetition frequency of the sweep operation was 100 kHz, and the feedback control unit was configured so that the frequency of the difference signal ΔS (t) between the two photodetectors was 17.2 MHz. That is, the reference voltage was set to 1.72 V corresponding to the input voltage sensitivity of 10 MHz / V at the VCO frequency in the PLL circuit.

As a result, the change rate of the wave number of the oscillation output light was stabilized in the range of the width of 100 nm around the wavelength of 1.35 μm, and an ideal wavelength change was obtained. At this time, the change rate of the optical frequency during the sweep was 1.72 THz / μs, and the change rate of the wave number was 2π × 57.4 cm ⁻¹ / μs, both of which were kept constant.

  In the second configuration of the wavelength-swept light source of the present invention shown in FIG. 4, the diffraction grating has a line marking density of 300 l / mm, and the incident angle θ is set to 11.7 °, and the center wavelength is 1.35 μm. It worked. As in the first embodiment, the optical path length difference of the optical interferometer unit is set to 3 mm, that is, a value corresponding to the delay time T = 10 ps, and the repetition frequency of the sweep operation is 100 kHz. In this case, the feedback control unit was configured such that the frequency of the difference signal ΔS (t) between the two photodetectors was 16.5 MHz. That is, the reference voltage was set to 1.65 V corresponding to the input voltage sensitivity of 10 MHz / V at the VCO frequency in the PLL circuit.

As a result, the change rate of the wave number of the oscillation output light was stabilized in the range of the width of 100 nm around the wavelength of 1.35 μm, and an ideal wavelength change was obtained. At this time, the change rate of the optical frequency during the sweep was 1.65 THz / μs, and the change rate of the wave number was 2π × 54.9 cm ⁻¹ / μs, both of which were kept constant.

  As described above in detail, according to the present invention, it is possible to provide a wavelength swept light source that realizes a wavelength change suitable for SS-OCT. A part of the output oscillation light is incident on an interferometer with a difference between the two optical path lengths, and the oscillation wavelength is linear with respect to the wave number by the operation of the feedback control unit that keeps the AC component frequency of the electric signal constant A wavelength sweep voltage that changes with time can be generated and applied, and the linearity of the OCT image can be greatly improved and a sharp OCT image can be obtained.

  The present invention can be used for an optical signal processing apparatus. In particular, it can be used for optical coherent tomography.

DESCRIPTION OF SYMBOLS 1 Light source 2, 22 Beam splitter 3, 23, 110 Mirror 4, 24 Living body 5, 25, 204, 205 Photodetector 21, 100, 200, 400 Wavelength sweep light source 31, 32, 33 Reflecting surface 101 Gain medium 102, 111 Condensing lens 103 Electro-optic deflector 104, 304 Control voltage 19, 106, 109 Diffraction grating 110 End mirror 112 Output coupling mirror 120 Polygon mirror 121 Optical beam tap 200a, 400a Oscillator unit 200b Feedback control unit 201 Semi-transparent mirror 202, 203 Reflection Mirror 207 PLL circuit 210 Ramp signal generator 211 High voltage amplifier 300 Electro-optic crystal

Claims (4)

In a light source whose output wavelength changes periodically over time,
An oscillator unit including an electro-optic deflector;
The oscillation output light from the oscillator unit is propagated to two optical paths having a difference between the optical path lengths, and the optical path length difference and the oscillation output light are compared with respect to the interference light generated by the two optical path length differences. An interferometer that outputs an electrical signal representing the intensity of interference light including an alternating current component of a frequency proportional to the rate of change of wave number;
An error signal generation circuit for generating a feedback signal for keeping the frequency of the AC component of the electrical signal constant, and a correction for supplying the feedback signal to the gain of the lamp voltage signal to supply to the electro-optic deflector And a feedback control unit having a control voltage generation circuit for generating a control voltage. The error signal generation circuit is a phase-locked loop circuit that receives the electrical signal and outputs the feedback signal proportional to a deviation amount of the frequency of the electrical signal from a predetermined frequency;
The wavelength sweep light source according to claim 1, wherein the control voltage generation circuit is a high voltage amplifier that amplifies the lamp voltage signal when the feedback signal is applied as a gain control signal. The oscillator unit includes a gain medium, a diffraction grating on which light from one end of the gain medium is incident, and an end face mirror on which the diffracted light of the incident light directly enters the diffraction grating, through the diffraction grating. The gain medium and the end mirror are configured by a resonator optically connected,
3. The wavelength sweep according to claim 1, wherein the electro-optic deflector is disposed between the gain medium and the diffraction grating on an optical path formed by the resonator. light source.   The oscillator unit includes a gain medium and a diffraction grating on which light from one end of the gain medium is incident. The oscillator unit includes a resonator in which the gain medium and the diffraction grating are optically connected. 3. The wavelength swept light source according to claim 1, wherein the deflector is disposed between the gain medium and the diffraction grating on an optical path formed by the resonator.

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