JP2008047730A - Wavelength-variable light source and optical fault imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a wavelength-variable light source to be compact and simplified structure. <P>SOLUTION: A light source 10 includes: a semiconductor laser 11 serving as a light amplification means; a wavelength selecting means in which the light from the semiconductor laser 11 is made incident and the light whose wavelength corresponds to the incident position is selectively reflected; and a deflection means 13 of a relative displacement means to carry out the relative displacement of the light from the semiconductor laser 11 and the wavelength selecting means. As the wavelength selecting means, a Bragg grating 15 is used which is constituted so that the lattice spacing changes along the direction of the relative displacement. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an external resonator type wavelength tunable light source having a variable oscillation wavelength, and an optical tomographic imaging apparatus that acquires an optical tomographic image of a measurement target by OCT (Optical Coherence Tomography) using the wavelength tunable light source.

  Conventionally, as a light source capable of sweeping a wide wavelength width at high speed, a wavelength dispersive element such as a diffraction grating is inserted in the optical path of an external resonator as shown in FIGS. Alternatively, a method of switching the wavelength of light returning to the optical amplifier at high speed by changing the angle of light incident on the element at high speed has been devised.

  The Littrow type light source shown in FIG. 13 converts the light emitted from the semiconductor laser 111 into parallel light by the collimator lens 112, and then enters the diffraction grating 113 to emit light having a specific wavelength out of wavelength-dispersed diffracted light. Only the laser beam is fed back to the semiconductor laser 111 to select the oscillation wavelength. By rotating the diffraction grating 113, the wavelength of the returning light is changed, and wavelength sweeping is performed (for example, see Patent Document 1).

  The Littman type light source shown in FIG. 14 converts the light emitted from the semiconductor laser 111 into parallel light by the collimator lens 112, then enters the diffraction grating 113, and reflects the wavelength-dispersed diffracted light by the mirror 114. Then, only the light having a specific wavelength orthogonal to the reflection surface of the mirror 114 is re-incident on the diffraction grating 113 through the reverse optical path and fed back to the semiconductor laser 111 to select the oscillation wavelength. By rotating the mirror 114, the wavelength of the returning light changes, and wavelength sweeping is performed.

  13 and 14, when the optical amplifier is a point light source such as a semiconductor laser, the end face of the optical fiber (not shown) optically coupled to the semiconductor laser 111 or the semiconductor laser 111 is accurately set. It is necessary to continue to capture. If there is an inclination error in the diffraction grating or mirror, the converging position of the feedback light will occur, and the amount of feedback light to the semiconductor laser will fluctuate, resulting in the wavelength stability and output stability of the output laser light. Damaged.

  The light source shown in FIG. 15 is intended for high-speed wavelength sweep. The mirror 114 in FIG. 14 is replaced with a polygon mirror 125, and two lenses 124a and 124b for relay are provided between the diffraction grating 113 and the polygon mirror 125. Provided. In the light source shown in FIG. 15, the wavelength of the feedback light is changed by the rotation of the polygon mirror 125, and wavelength sweeping is performed (see, for example, Patent Document 2).

  Patent Document 3 describes a light source using an optical filter configured so that the wavelength of light to be transmitted continuously changes along the scanning direction of light emitted from the light deflection element. . In this configuration, the wavelength selected by the optical filter changes with scanning, and wavelength sweeping is performed by returning light of that wavelength by the mirror.

  In Patent Document 4, light emitted from a semiconductor element is reflected by a diffraction grating arranged at one end of an external resonator, and an incident angle to the diffraction grating is changed by a oscillating mirror arranged in an intermediate optical path. A light source that selects and changes the oscillation wavelength is described. In this light source, a chirped Bragg grating that reflects the emitted light from the semiconductor element and transmits the oscillation light to the outside is arranged at the other end of the external resonator, and the resonator length is further increased by this chirped Bragg grating. To adjust.

On the other hand, an optical tomographic imaging apparatus (SS-OCT apparatus) using SS-OCT (Swept source OCT) measurement is known as an important application of a wavelength tunable light source capable of wavelength sweep as described above. The optical tomographic imaging apparatus divides the coherence light emitted from the light source into measurement light and reference light, and then combines the reflected light and the reference light when the measurement light is applied to the measurement object, and reflects the reflected light. An optical tomographic image is acquired based on the intensity of the interference light between the light and the reference light. In the SS-OCT apparatus, the reflected light and the reference light are interfered with each other while the frequency of the light emitted from the light source is changed with time, and the obtained interference light is detected, and a predetermined frequency is determined from the interferogram in the optical frequency domain. The reflection intensity at the depth position of the measurement object is detected, and a tomographic image is generated using this.
JP-A-9-64439 US 2005/0035295 Specification Japanese Patent Laid-Open No. 2006-24876 US Pat. No. 5,956,355

  By the way, in the light source using the wavelength dispersion element such as the diffraction grating shown in FIGS. 13 to 15, it is necessary to increase the diameter of the light beam incident on the wavelength dispersion element in order to ensure the wavelength resolution, resulting in a large optical system. As a result, there is a problem that the size of the apparatus is increased.

  Further, the light source using the polygon mirror shown in FIG. 15 requires an optical system for bringing the center of the reflection surface of the polygon mirror and the diffraction grating into a conjugate position, which also increases the size of the apparatus. .

  In the configuration in which the optical filter and the mirror described in Patent Document 3 are combined, the optical amplifier exit end face and the mirror are in a conjugate position. In this respect, the optical system is stable, but the optical filter and the mirror are combined. For this reason, the following restrictions (1) and (2) occur, making installation difficult. (1) Since the optical filter reflects light having a wavelength other than the selected wavelength to be transmitted, it is necessary to arrange the optical filter so as to be inclined with respect to the optical axis so that the reflected light from the optical filter does not return to the optical amplifier. (2) Since the exit end face of the optical amplifier and the mirror are at the conjugate position, the mirror is disposed at the beam waist position. To increase the wavelength resolution, the optical filter is placed within the focal depth of the beam at the beam waist position, that is, the mirror. It is necessary to arrange in the vicinity. In practice, it is technically very difficult to manufacture a band-pass filter having a sub-nm wavelength width and a continuous selection wavelength change using a dielectric multilayer film as described in Patent Document 3. Have difficulty.

  Accordingly, the present invention has been made in view of the above circumstances, and an object thereof is to provide a tunable light source having a compact and simplified configuration, and an optical tomographic imaging apparatus including the tunable light source.

  The wavelength tunable light source according to the present invention includes an optical amplifying unit, a wavelength selecting unit that selectively reflects light having a wavelength corresponding to the incident position, and the incident position changes. And a relative moving means for relatively moving the light from the light amplifying means and the wavelength selecting means, and the wavelength selecting means is configured such that the grating interval changes along the direction of the relative movement. It is a Bragg grating.

  Here, the “Bragg grating” is capable of selectively reflecting only light having a wavelength that matches the Bragg diffraction condition based on the grating interval of the diffraction grating formed inside.

  When the wavelength tunable light source is one that sweeps the wavelength, it is preferable that the grating interval of the Bragg grating is continuously changed. However, depending on the application, it may be changed stepwise. .

  As the optical amplifying means, for example, a semiconductor laser can be used. In that case, one of the emission end faces of the semiconductor laser can be used as a reflection face constituting the end of the resonator.

  The wavelength tunable light source preferably further includes an optical system that optically conjugates the exit surface of the optical amplification means and the Bragg grating.

  The relative moving means may be a deflecting means for deflecting light from the optical amplifying means, or a moving means for moving the Bragg grating.

  Here, the deflecting means may be a rotating polarizing mirror such as a polygon mirror, or a reciprocating deflecting mirror such as a galvano mirror or a torsion mirror. Moreover, as a moving means, you may use parallel moving means, such as a linear motor, for example.

  The relative movement means rotates the rotating body around a rotation center axis substantially parallel to the optical axis of the light from the light amplification means, and the Bragg grating moves the rotation body to the rotation center axis. May be provided so that the lattice spacing changes in the circumferential direction centering on the.

  The Bragg grating may be configured such that a monotonous change in the wavelength of reflected light from the Bragg grating occurs a plurality of times while the rotating body makes one round.

  It is preferable that the wavelength of reflected light from the Bragg grating is swept by the relative movement, and the sweep direction is always one direction.

  Here, the “sweep direction” means the direction of wavelength change, that is, the direction of changing from a short wavelength to a long wavelength, or the direction of changing from a long wavelength to a short wavelength.

  Furthermore, the optical tomographic imaging apparatus according to the present invention includes the wavelength tunable light source, a light dividing unit that divides the light emitted from the wavelength tunable light source into measurement light and reference light, and the measurement light irradiates the measurement target. A combining means for combining the reflected light from the measurement object and the reference light when detected, and an interference light for detecting interference light between the reflected light and the reference light combined by the combining means It is characterized by comprising detection means and image acquisition means for acquiring a tomographic image of the measurement object from the interference light detected by the interference light detection means.

  According to the wavelength tunable light source of the present invention, the incident position of the light from the optical amplifying means on the Bragg grating can be changed along the changing direction of the grating interval of the Bragg grating. Therefore, the Bragg grating based on the grating interval is used. Only light having a wavelength that matches the diffraction conditions can be selectively reflected by the Bragg grating and its wavelength can be changed. Compared with a conventional light source using a planar diffraction grating, the wavelength tunable light source of the present invention can achieve the same effect with a small number of components, and it is not necessary to increase the beam diameter, so it is compact and simplified. A wavelength tunable light source having the configuration can be realized.

  In the wavelength tunable light source according to the present invention, when the optical system is further provided with an optically conjugate positional relationship between the exit surface of the optical amplifying means and the Bragg grating, the Bragg is not necessary even if the adjustment accuracy of each optical component is low. The return light from the grating can be stably returned to the exit surface of the optical amplifying means, and the stability of the output from the optical amplifying means can be ensured.

  The relative movement means rotates the rotating body around a rotation center axis substantially parallel to the optical axis of the light from the light amplification means, and the Bragg grating moves the rotation body to the rotation center axis. In the case where the grating spacing is changed in the circumferential direction centering on, a high-speed wavelength sweep is possible.

  If the Bragg grating is configured so that the monotonous change in the wavelength of the reflected light from the Bragg grating occurs multiple times during the rotation of the rotating body, a faster wavelength sweep is possible. become.

  If the wavelength of the reflected light from the Bragg grating is swept by the relative movement and the sweep direction is always one direction, the optical amplifier depends on the wavelength sweep direction and the spectrum of the output light Even if there is a large difference in shape, the spectral shape of the output light can be made constant.

  In addition, the optical tomographic imaging apparatus of the present invention can include a wavelength variable light source having a compact and simplified configuration.

  Hereinafter, embodiments of a wavelength tunable light source and an optical tomographic imaging apparatus including the wavelength tunable light source according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical tomographic imaging apparatus 1 according to the first embodiment of the present invention. The optical tomographic imaging apparatus 1 acquires, for example, a tomographic image of a measurement target such as a living tissue or a cell in a body cavity by the above-described SS-OCT measurement.

  The optical tomographic imaging apparatus 1 divides the light L0 emitted from the light source unit 10 into the measurement light L1 and the reference light L2 while emitting the laser light L0 while sweeping the oscillation wavelength at a constant period. The light splitting means 3, the optical path length adjusting means 20 for adjusting the optical path length of the reference light L2 split by the light splitting means 3, and the probe for guiding the measurement light L1 split by the light splitting means 3 to the measuring object S 30, a combining unit 4 that combines the reflected light L <b> 3 reflected by the measuring object S when the measuring light L <b> 1 is irradiated from the probe 30 and the reference light L <b> 2, and a combining unit 4. The interference light detection means 40 for detecting the interference light L4 between the reflected light L3 and the reference light L2, and the tomographic image of the measuring object S is obtained by frequency analysis of the interference light L4 detected by the interference light detection means 40. Image capture And it means 50.

  The light source unit 10 is an external resonator type tunable light source including a semiconductor laser 11 as an optical amplifying unit, a Bragg grating 15 as a wavelength selecting unit, and a deflecting unit 13 as a relative moving unit. Details of the light source unit 10 will be described later.

  The light splitting means 3 is composed of, for example, a 2 × 2 optical fiber coupler, and splits the light L0 guided from the light source unit 10 through the optical fibers FB0 and FB1 into the measurement light L1 and the reference light L2. The light splitting means 3 is optically connected to the two optical fibers FB2 and FB3, respectively, the measuring light L1 is guided by the optical fiber FB2, and the reference light L2 is guided by the optical fiber FB3. The light dividing means 3 in this embodiment also functions as the multiplexing means 4.

  The probe 30 is optically connected to the optical fiber FB2, and the measurement light L1 is guided from the optical fiber FB2 to the probe 30. The probe 30 is inserted into a body cavity from a forceps port through a forceps channel, for example, and is detachably attached to the optical fiber FB2 by an optical connector 31.

  The optical probe 30 includes a cylindrical probe outer cylinder 32 whose tip is closed, and one optical fiber 33 disposed in the inner space of the probe outer cylinder 32 so as to extend in the axial direction of the outer cylinder 32. And a prism mirror 34 that deflects the light L1 emitted from the tip of the optical fiber 33 in the circumferential direction of the probe outer cylinder 32, and the light L1 emitted from the tip of the optical fiber 33 is arranged on the outer circumference of the probe outer cylinder 32. A rod lens 35 that collects light so as to converge on the measurement target S as a body to be scanned and a motor 36 that rotates the prism mirror 34 about the axis of the optical fiber 33 as a rotation axis are provided.

  On the other hand, the optical path length adjusting means 20 is disposed on the side of the optical fiber FB3 where the reference light L2 is emitted. The optical path length adjusting unit 20 changes the optical path length of the reference light L2 in order to adjust the position at which tomographic image acquisition for the measurement target S is started, and the reference light L2 emitted from the optical fiber FB3 is changed. A reflection mirror 22 to be reflected, a first optical lens 21a disposed between the reflection mirror 22 and the optical fiber FB3, and a second optical lens 21b disposed between the first optical lens 21a and the reflection mirror 22. have.

  The first optical lens 21a has a function of converting the reference light L2 emitted from the core of the optical fiber FB3 into parallel light and condensing the reference light L2 reflected by the reflection mirror 22 onto the core of the optical fiber FB3. ing. Further, the second optical lens 21b condenses the reference light L2 converted into parallel light by the first optical lens 21a on the reflection mirror 22, and makes the reference light L2 reflected by the reflection mirror 22 into parallel light. have.

  Therefore, the reference light L2 emitted from the optical fiber FB3 is converted into parallel light by the first optical lens 21a, and is condensed on the reflection mirror 22 by the second optical lens 21b. Thereafter, the reference light L2 reflected by the reflection mirror 22 becomes parallel light by the second optical lens 21b, and is condensed on the core of the optical fiber FB3 by the first optical lens 21a.

  Further, the optical path length adjusting means 20 has a base 23 to which the second optical lens 21b and the reflecting mirror 22 are fixed, and a mirror moving means 24 for moving the base 23 in the optical axis direction of the first optical lens 21a. is doing. The optical path length of the reference light L2 is changed by moving the base 23 in the direction of arrow A.

  The multiplexing means 4 is composed of a 2 × 2 optical fiber coupler as described above, and combines the reference light L2 whose optical path length has been changed by the optical path length adjusting means 20 and the reflected light L3 from the measuring object S to be light. It is configured to emit light toward the interference light detection means 40 via the fiber FB4.

  The interference light detection unit 40 detects the interference light L4 between the reflected light L3 combined by the combining unit 4 and the reference light L2. In the apparatus of this example, the interference light detection means 40 is guided to the light detector 40a and the light detector 40b by dividing the interference light L4 by the light splitting means 3, and the calculation means 41 performs balance detection. have. By this mechanism, the influence of light intensity fluctuation can be suppressed and a clearer image can be obtained.

  The computing means 41 is connected to an image acquisition means 50 comprising a computer system such as a personal computer, for example, and the image acquisition means 50 is connected to a display device 60 comprising a CRT or a liquid crystal display device. The image acquisition unit 50 acquires reflection information at each depth position of the measurement target S by performing a Fourier transform on the interference light L4 detected by the interference light detection unit 40. Then, the image acquisition unit 50 acquires a tomographic image of the measurement target S using the intensity of the reflected light L3 at each depth position of the measurement target S, and this tomographic image is displayed on the display device 60.

  Here, the detection of the interference light L4 and the generation of the image in the interference light detection means 40 and the image acquisition means 50 will be briefly described. Details of this point are described in “Mitsuo Takeda,“ Optical Frequency Scanning Spectrum Interference Microscope ”, Optical Technology Contact, 2003, Vol41, No7, p426-p432”.

When the measurement light L1 is irradiated onto the measurement object S, interference fringes with respect to each optical path length difference l when the reflected light L3 from the depth of the measurement object S and the reference light L2 interfere with each other with various optical path length differences. S (l) is the light intensity I (k) detected by the interference light detection means 40.
I (k) = ∫ ₀ ^∞ S (l) [1 + cos (kl)] dl (1)
It is represented by Here, k is the wave number, and l is the optical path length difference. Formula (1) can be considered to be given as an interferogram in the optical frequency domain with the wave number k as a variable. For this reason, in the image acquisition means 50, the spectral interference fringes detected by the interference light detection means 40 are subjected to frequency analysis by Fourier transform, and the light intensity S (l) of the interference light L4 is determined, thereby measuring the measurement object S. It is possible to acquire distance information from the start position and reflection intensity information and generate a tomographic image.

  Next, an operation example of the optical tomographic imaging apparatus 1 having the above configuration will be described. First, when the movable stage 23 moves in the direction of arrow A, the optical path length is adjusted so that the measuring object S is positioned in the measurable region. Thereafter, the light L0 is emitted from the light source unit 10, and the light L0 is split into the measurement light L1 and the reference light L2 by the light splitting means 3. The measurement light L1 is guided into the body cavity by the probe 30 and irradiated to the measurement object S. Then, the reflected light L3 from the measurement object S is combined with the reference light L2 reflected by the reflection mirror 22 by the combining means 4, and the interference light L4 between the reflected light L3 and the reference light L2 is detected by the interference light detecting means 40. Is done. The detected signal of the interference light L4 is subjected to frequency analysis in the image acquisition means 50, whereby a tomographic image is acquired.

  If the probe 30 is rotated and the measurement light L1 is scanned in a one-dimensional direction with respect to the measurement target S, information in the depth direction of the measurement target S can be obtained at each portion along the scanning direction. A tomographic image of a tomographic plane including this scanning direction can be acquired. In addition, by scanning the measuring object S1 with the measurement light L1 in a second direction orthogonal to the scanning direction, a tomographic image of a tomographic plane including the second direction can be further acquired. Is possible.

  Hereinafter, a detailed description of the light source unit 10 will be described. The light source unit 10 includes a semiconductor laser 11, a collimating lens 12 disposed on the emission end face 11 b side of the semiconductor laser 11, a deflecting unit 13, a condenser lens 14, a Bragg grating 15, and an emission end face of the semiconductor laser 11. And an optical coupling lens 16 disposed on the 11a side. The exit end face 11b is provided with an antireflection film (AR coating).

  The Bragg grating 15 has a diffraction grating formed therein, and selectively reflects only light having a wavelength that matches the Bragg diffraction condition based on the grating interval. Further, the Bragg grating 15 of the present embodiment is a chirped Bragg grating configured such that the lattice spacing continuously changes along one of the directions orthogonal to the optical axis of incident light. Therefore, the wavelength of light reflected by the Bragg grating 15 is selected according to the incident position. Further, the Bragg grating 15 has a width equal to or larger than the incident beam diameter in a direction perpendicular to both the one direction and the optical axis direction of the incident light (a direction perpendicular to the paper surface of FIG. 1).

  The deflecting means 13 deflects the light from the semiconductor laser 11 and changes the incident position of the light on the Bragg grating 15 along the changing direction of the lattice spacing. In the present embodiment, the deflection means 13 is constituted by a reciprocating deflection mirror such as a galvanometer mirror, but may be constituted by a rotary deflection mirror such as a polygon mirror.

  The light emitted from the emission end face 11b of the semiconductor laser 11 is converted into parallel light by the collimator lens 12, then deflected by the deflecting means 13, condensed by the condenser lens 14, and arranged at the beam condensing position. Is incident on the Bragg grating 15. In the Bragg grating 15, only light with a specific wavelength in a narrow band that meets the Bragg diffraction condition based on the grating interval at the incident position is reflected to be returned light. The return light passes through the reverse optical path, returns to the semiconductor laser 11 through the condenser lens 14, the deflecting unit 13, and the collimator lens 12. Then, a laser beam L0 having a narrow-band wavelength oscillated by an external resonator having both ends of the emission end face 11a of the semiconductor laser 11 and the Bragg grating 15 is emitted from the emission end face 11a of the semiconductor laser 11.

In the light source unit 10, the beam scans the Bragg grating 15 along the changing direction of the grating interval by the deflecting means 13, so that the wavelength of the return light continuously changes as λ ₁ ... Λ _n and the semiconductor laser 11 The wavelength of the laser light L0 emitted as oscillation light from λ ₁ also changes continuously as λ ₁ ... Λ _n and can be swept. If a torsion mirror is used as the deflection mirror, the Bragg grating 15 can be scanned at high speed, and wavelength sweeping at several tens of kHz is possible.

  The Bragg grating 15 is arranged in a range where a beam waist position by the condenser lens 14 or a focal depth before and after the beam waist position can be secured. Further, the direction in which the lattice spacing of the Bragg grating 15 changes and the moving direction of the light deflected by the deflecting means 13 are matched. As a result, an optical system with high wavelength resolution is constructed, and the wavelength band of the return light can be narrowed.

The relationship between the wavelength selection width Δλ of the Bragg grating 15 and the thickness d of the Bragg grating 15 is expressed by the following equation (1). Here, λ _{B is the} Bragg wavelength, and n is the refractive index of the Bragg grating 15.

In order to reduce the wavelength selection width Δλ, that is, to increase the wavelength resolution, the Bragg grating 15 is preferably thick. When λ _B = 1 μm, n = 1.5, and Δλ = 0.1 nm, d = 3.3 mm. On the other hand, the relationship between the beam radius w ₀ at the beam waist position and the focal depth z ₀ (distance from the beam waist position in the optical axis direction where the beam diameter becomes √2w ₀ ) is expressed by the following equation (2).

When the beam diameter at the beam waist position is 50 μmφ (that is, w ₀ = 25 μm) and λ _B = 1 μm, z ₀ = 1.96 mm. Therefore, the range in which the focal depth in the optical axis direction can be secured is the beam waist position. Is a total range of 3.9 mm before and after the center. Therefore, a sufficiently small beam diameter can be maintained within the thickness of the Bragg grating 15 having the above d = 3.3 mm.

The divergence angle in the beam diameter is θ = 12.7 mrad = 0.73 ° from the following equation (3).

The expression of the incident angle θ on the Bragg grating 15 and the Bragg wavelength λ _B which is the selected wavelength is expressed by the following expression (4), where the lattice spacing is Λ.

From equation (4), the deviation of the Bragg wavelength λ _B at an incident angle of 0 (normal incidence) to 12.7 mrad is about 0.08 nm, and therefore the substantial spread of the wavelength width of the reflected light due to the spread of the beam diameter is as follows. I can say no. Here, the lattice spacing Λ is calculated as Λ = 1 / (2 × 1.5) assuming that λ _B = 1 μm and θ = 0 (normal incidence).

  The optical system of the light source unit 10 is configured such that the deflecting means 13 is at the pupil position and the Bragg grating 15 side of the condenser lens 14 is telecentric. With this configuration, the principal ray of the light traveling from the condenser lens 14 toward the Bragg grating 15 is parallel to the optical axis of the condenser lens 14 regardless of the scanning position, so that the Bragg grating 15 is aligned with the optical axis of the condenser lens 14. Accordingly, the return light reflected by the Bragg grating 15 can be returned in the same direction as the incident light in the opposite direction.

  The optical system including the collimator lens 12 and the condenser lens 14 of the light source unit 10 is configured such that the emission end face 11b of the semiconductor laser 11 and the Bragg grating 15 are in an optically conjugate positional relationship. With this configuration, even if the adjustment accuracy of each optical component is low, the return light can be stably returned to the semiconductor laser 11.

  According to the light source unit 10 described above, since it is not necessary to increase the beam diameter as compared with a light source using a conventional planar diffraction grating, a simple and compact configuration is realized without increasing the size of the apparatus. In addition, stable wavelength sweeping is possible with high wavelength resolution.

  In the wavelength sweep using the Bragg grating, since the incident position in the Bragg grating, that is, the relative position between the Bragg grating and the incident light only needs to be changed, the light source unit of the second embodiment described below is used. As described above, this can also be realized by fixing the incident light and moving the Bragg grating.

  Hereinafter, a light source unit according to another embodiment of the present invention will be described. The light source unit described below can be applied to the optical tomographic imaging apparatus 1 in the same manner as the light source unit 10 shown in FIG. 1, and in this case, the configuration other than the light source unit is the same. It is shown and described, and description of other components is omitted.

  A light source unit 210 according to a second embodiment of the present invention will be described with reference to FIG. Compared with the light source unit 10 of FIG. 1, the light source unit 210 is characterized in that a BG moving means 216 for moving the Bragg grating 15 is provided and the deflecting means 13 is removed. Further, in the light source unit 210, the collimating lens 212 and the condensing lens 214 are used in place of the collimating lens 12 and the condensing lens 14 of the light source unit 10, respectively. The diameter can be made smaller than that of the lens 14.

  The Bragg grating 15 is arranged so that the direction in which the lattice spacing changes is perpendicular to the optical axis of the incident light. Then, the BG moving means 216 moves the Bragg grating 15 in the direction in which the lattice spacing changes, whereby the incident position on the Bragg grating 15 changes, and in this embodiment as well, the light source unit of the first embodiment As in the case of 10, the wavelength can be swept.

  As the BG moving unit 216, a device that moves the Bragg grating 15 at a predetermined speed by installing the Bragg grating 15 on a parallel moving body such as a linear motor can be used.

  FIG. 3 shows a light source unit 211 which is a modification of the light source unit 210 of FIG. The light source unit 211 uses a staircase type Bragg grating 215 in which the grating interval changes in a stepwise manner instead of being continuous, instead of the Bragg grating 15 of the light source unit 210. The wavelength can be changed so that changes stepwise.

The wavelength sweeping of the light source units 10 and 210 of the first and second embodiments is due to the reciprocating motion of the Bragg grating 15 by the deflecting means 13 or the BG moving means 216, as shown in FIG. _The longest wavelength from ₁ to λ _n , and the longest wavelength from λ _n to the shortest wavelength λ ₁ . That is, the sweep direction of the wavelength within the sweep period T is both the short wavelength to the long wavelength and the long wavelength to the short wavelength. For wavelength tunable light sources using semiconductor lasers as optical amplifiers, the document “R. Huber et. Al.“ Three-dimensional and C-mode OCT imaging with a compact, frequency swept laser source at 1300 nm ”OPTICS EXPRESS Vol. 13 , No. 26, 10523-10538 ”, due to the characteristics of the optical amplifier, there is a problem that the shape of the spectrum distribution of the generated light varies greatly depending on the wavelength sweep direction. The light source units of the third and fourth embodiments described below take this point into consideration.

  Next, a light source unit 310 according to a third embodiment of the present invention will be described with reference to FIG. The light source unit 310 includes a semiconductor laser 11, a collimator lens 12, a deflection mirror group 313, a condenser lens 314, and a Bragg grating group 315.

  The deflecting mirror group 313 includes two deflecting mirrors 313a and 313b that function as deflecting means. The deflection mirrors 313a and 313b are both reciprocating deflection mirrors such as galvanometer mirrors, but are arranged so that the deflection directions are substantially orthogonal, and the light deflected by the deflection mirror group 313 is optically scanned two-dimensionally. Become. In the present embodiment, the deflection mirrors 313a and 313b are driven alternately, and after the optical scanning by driving one of the deflection mirrors 313a and 313b, the other deflection mirror is driven to switch the optical path so that the light has a rectangular side. The upper side is circularly scanned in one direction.

  The Bragg grating group 315 includes two Bragg gratings 315a and 315b that function as wavelength selection means. The Bragg gratings 315a and 315b are chirp Bragg gratings configured so that the lattice spacing is continuously changed, similarly to the Bragg grating 15 shown in FIG. Adjacent in parallel so that the directions are opposite to each other. The long side direction of the optical scanning rectangle of the deflecting mirrors 313a and 313b is arranged so as to be the changing direction of the lattice spacing of the Bragg gratings 315a and 315b.

  In the light source unit 310, the light emitted from the semiconductor laser 11 is converted into parallel light by the collimator lens 12, deflected in this order by the deflecting mirror 313a and the deflecting mirror 313b, collected by the condensing lens 314, The light enters the grating group 315 and scans on the Bragg gratings 315a and 315b so as to draw a rectangle.

Here, the semiconductor laser 11 is allowed to emit light when the light is scanning the change direction of the lattice spacing, and the semiconductor laser 11 is not allowed to emit light when the light is scanned in a direction orthogonal to the change direction of the lattice spacing. For example, the wavelength sweep in the light source unit 310 is as shown in FIG. That is, the Bragg grating 315a and Bragg grating 315b, since the direction of the chirp are arranged in the opposite direction, returning light is always longer wavelength from the short wavelength so as to change from the shortest wavelength lambda ₁ to the longest wavelength lambda _n Become one way to. Therefore, according to the light source unit 310, the spectral shape of the output light can be made constant even if the optical amplifier has a large difference in the spectral shape of the output light depending on the wavelength sweep direction. The direction of the wavelength sweep is not limited to the above direction, and may be from a long wavelength to a short wavelength.

  Next, an optical tomographic imaging apparatus according to a fourth embodiment of the present invention will be described with reference to FIG. Compared with the light source unit 210 in FIG. 2, the light source unit 410 basically includes the Bragg grating 415a (see FIG. 2) in which the Bragg grating 15 and the BG moving unit 216 in FIG. 7) and the BG rotating means 416.

  The BG rotating unit 416 rotates the rotating body 415 around a rotation center axis 417 substantially parallel to the optical axis of the light from the semiconductor laser 11 and functions as a relative moving unit. The Bragg grating 415a is a ring-shaped chirped Bragg grating provided on the rotating body 415 so that the lattice spacing changes in the circumferential direction around the rotation center axis 417, and functions as a wavelength selection means. The Bragg grating 415a rotates integrally with the rotating body 415 by the BG rotating means 416.

  In the light source unit 410, the light emitted from the semiconductor laser 11 is converted into parallel light by the collimator lens 212, collected by the condenser lens 214, and then incident on the Bragg grating 415a. As the rotator 415 rotates, light is incident on portions of the Bragg grating 415a having different lattice intervals, the wavelength of the return light is changed, and wavelength sweeping is performed.

  If the lattice spacing of the Bragg grating 415a changes smoothly with respect to the rotation direction of the rotating body 415, the wavelength of the return light changes smoothly with the rotation of the rotating body 415, and the oscillation wavelength is swept. Can do. The high-speed rotation of the rotating body 415 enables high-speed wavelength sweep.

  Further, the monotonous change in the lattice spacing of the Bragg grating 415a occurs multiple times during the rotation of the rotating body 415 so that the monotonous change in the wavelength of the reflected light from the Bragg grating 415a occurs multiple times. In this case, wavelength sweeping is performed a plurality of times during one round, so that faster wavelength sweeping is possible. In that case, in consideration of the characteristics of the optical amplifier that the difference in the shape of the spectrum distribution of the light generated by the wavelength sweeping direction described above occurs, the wavelength sweeping direction by the rotation of the rotating body 415 becomes the same. It is desirable to be configured as described above.

  The Bragg grating 415a provided on the disk may also be configured such that the lattice spacing changes stepwise like the Bragg grating 215 shown in FIG.

Next, an example of a method for manufacturing a Bragg grating will be described with reference to FIGS. First, for the sake of simplicity, a method for manufacturing a Bragg grating with a constant lattice spacing will be described. As shown in FIG. 8, parallel recording light 70 emitted from a single light source is split into two beams by a light splitting means 71 such as a prism. Mirrors 72 and 73 are arranged in each divided optical path so that the optical paths of the respective beams are bent and intersected again, and a photoconductor 74 is arranged at this intersection. At this time, the photoconductor 74 is arranged so that the normal line coincides with the bisector of the intersection angle of the two beams. With such an arrangement, interference fringes are formed by interference between the two beams inside the photosensitive member 74, and a diffraction grating is formed inside the photosensitive member 74 as conceptually shown in FIG. A grating can be produced. When the light L having a predetermined wavelength width the photoreceptor 74 is incident, as shown in FIG. 9 (B), made to reflect only light L _B selected wavelengths which the Bragg grating has.

If the recording wavelength (wavelength of recording light) at this time is λ and the crossing angle is φ, the lattice interval Λ formed on the photoreceptor 74 is given by the following equation (5).

Considering the case of normal incidence, λ _B = nλ / sin (φ / 2) from the equations (4) and (5). Here, φ / 2 is an angle formed by the normal line of the photoconductor 74 and the beam incident on the photoconductor 74, so φ / 2 <π / 2. Therefore, sin (ψ / 2) <1. Further, n> 1. From the above, the selection wavelength (Bragg wavelength) λ _B is longer than the recording wavelength λ. When the recording wavelength is 532 nm, φ / 2 = 53 ° may be used to obtain a selection wavelength of about 1000 nm of a light source used in a general OCT apparatus.

  As a method for manufacturing the chirped Bragg grating as shown in FIG. 1, a method of irradiating the photosensitive member with a slit beam while gradually changing the incident angle of the beam to the photosensitive member 74 is considered. Batch recording is possible using the method described.

  First, as shown in FIG. 10, a prism 75 whose cross section is triangular and whose apex angle gradually changes and a laminated type photoconductor 76 such as a photopolymer are prepared. The surface facing the apex angle of the prism 75 is disposed so as to be in close contact with the incident surface of the photosensitive member 76, and the recording light 70 divided into two beams is incident at an equal angle from both sides of the prism 75. Since the incident angle to the photosensitive member 76 gradually changes along with the change in the apex angle of the prism 76, the interval between the interference fringes formed on the photosensitive member 76 also changes gradually, and the lattice interval gradually changes. Chirped Bragg gratings can be made.

  When a staircase type Bragg grating as shown in FIG. 3 is manufactured, a prism 77 whose apex angle changes in a staircase shape as shown in FIG. 11 may be used instead of the prism 75 of FIG.

  The Bragg grating that can be used in the present invention is not limited to the one produced by the above production method, and may be produced by any method.

  The embodiment of the present invention is not limited to the above-described embodiment, and various modifications can be made without changing the gist of the invention. For example, instead of the light source unit 10 of the first embodiment, an optical fiber 511 and a coupling lens 512 are arranged between the collimating lens 12 and the semiconductor laser 11 as shown in FIG. You may employ | adopt the light source unit 510 of the structure connected between 13 by the optical fiber.

  In addition, as an example showing the wavelength sweep, in FIGS. 4 and 6, an example in which the wavelength is linearly changed with respect to time is given. However, the wavelength change with respect to time is not limited to the above example. On the other hand, the reciprocal of the wavelength may change linearly, and can be arbitrarily selected. The wavelength change with respect to time can be arbitrarily selected by appropriately setting the distribution of the lattice spacing of the Bragg grating or the speed change of the relative moving means.

1 is a schematic configuration diagram of an optical tomographic imaging apparatus according to a first embodiment of the present invention. Schematic block diagram of the light source unit concerning the 2nd Embodiment of this invention. The schematic block diagram of the light source unit concerning the modification of this invention The figure which shows the mode of the wavelength sweep of the light source unit of FIG. 1 and FIG. The schematic block diagram of the light source unit concerning the 3rd Embodiment of this invention. The figure which shows the mode of the wavelength sweep of the light source unit of FIG. The schematic block diagram of the light source unit concerning the 4th Embodiment of this invention. Diagram for explaining the manufacturing method of Bragg grating FIG. 9A conceptually shows a Bragg grating, and FIG. 9B shows wavelength selectivity by the Bragg grating. Diagram to explain how to make chirp-Bragg grating Diagram for explaining how to make a staircase Bragg grating The schematic block diagram of the light source unit concerning the modification of this invention Schematic configuration diagram of a conventional wavelength tunable light source Schematic configuration diagram of a conventional wavelength tunable light source Schematic configuration diagram of a conventional wavelength tunable light source

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical tomographic imaging apparatus 2 Optical fiber coupler 3 Optical dividing means 4 Combined means 10 Light source unit 11 Semiconductor laser 11a, 11b Emission end surface 12 Collimating lens 13 Deflection means 14 Condensing lens 15, 415a Bragg grating 20 Optical path length adjusting means 30 Probe 40 Interference light detection means 40a, 40b Photodetector 41 Calculation means 50 Image acquisition means 60 Display device 216 BG movement means 313 Deflection mirror group 315 Bragg grating group 415 Rotating body 416 BG rotation means 417 Rotation center axis L0 Light L1 Measurement light L2 Reference light L3 Reflected light L4 Interference light S Measurement object

Claims (7)

Optical amplification means;
Wavelength selecting means for receiving light from the light amplifying means and selectively reflecting light having a wavelength corresponding to the incident position;
Relative movement means for relatively moving the light from the light amplification means and the wavelength selection means so that the incident position changes,
The wavelength tunable light source, wherein the wavelength selection unit is a Bragg grating configured such that a grating interval changes along the relative movement direction.   2. The wavelength tunable light source according to claim 1, further comprising an optical system that optically conjugates the exit surface of the optical amplification means and the Bragg grating.   3. The wavelength tunable light source according to claim 1, wherein the relative moving means is a deflecting means for deflecting light from the optical amplifying means or a moving means for moving the Bragg grating. The relative movement means rotates the rotating body around a rotation center axis substantially parallel to an optical axis of light from the light amplification means;
2. The wavelength tunable light source according to claim 1, wherein the Bragg grating is provided on the rotating body so that a lattice interval changes in a circumferential direction centering on the rotation center axis.   5. The wavelength according to claim 4, wherein the Bragg grating is configured such that a monotonous change in the wavelength of reflected light from the Bragg grating occurs a plurality of times during one rotation of the rotating body. Variable light source. The relative movement sweeps the wavelength of the reflected light from the Bragg grating,
6. The wavelength tunable light source according to claim 1, wherein the sweep direction is always one direction. The wavelength tunable light source according to any one of claims 1 to 6,
A light splitting means for splitting light emitted from the wavelength tunable light source into measurement light and reference light;
A multiplexing means for multiplexing the reflected light from the measurement object and the reference light when the measurement light is irradiated to the measurement object;
Interference light detection means for detecting interference light between the reflected light and the reference light multiplexed by the multiplexing means;
An optical tomographic imaging apparatus comprising: an image acquisition unit configured to acquire a tomographic image of the measurement target from the interference light detected by the interference light detection unit.

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