JP2009244082A - Light source and optical tomographic imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light source that is compact, can be manufactured at low costs, and has high light utilization efficiency and high-speed wavelength sweeping, and has high wavelength selectivity. <P>SOLUTION: The linear resonator type light source 1 includes: an optical amplification medium SOA2; a Fabry-Perot resonator 5 that allows light emitted from the optical amplification medium to enter as scattered light 7, transmits light having a specific wavelength, and reflects light having a wavelength other than the specific one; and a concave mirror 6 as a feedback means. The Fabry-Perot resonator 5 has two reflection surfaces 3, 4 arranged with a slope to the optical axis Z of the linear resonator to prevent the reflected light having a specific wavelength from entering a scattering center O<SB>1</SB>of the scattered light 7. The feedback means feeds back the transmitted light so that light transmitted through the Fabry-Perot resonator 5 converges to the scattering center O<SB>1</SB>. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light source that can emit light of a specific wavelength, and an optical tomographic imaging apparatus that acquires an optical tomographic image of a measurement target using the light source.

  Conventionally, as a wavelength tunable light source capable of changing the wavelength sweep of output light, a wavelength swept laser light source using a Fabry-Perot tunable filter (hereinafter referred to as FP-TF) based on a Fabry-Perot resonator is known. It has been. The FP-TF has a function of transmitting only light of a wavelength selected by interference and reflecting light of other wavelengths, and is configured to change the selected wavelength. This type of light source using FP-TF forms a round-trip optical path from a ring resonator type (for example, refer to Patent Document 1) that forms a loop-shaped optical path by an optical fiber or the like from the configuration of the optical path of the resonator. Can be roughly divided into linear resonator types (see, for example, Non-Patent Document 1).

  FIG. 7 shows a configuration of a ring resonator type laser light source 70 using FP-TF. The laser light source 70 forms a ring-shaped optical path resonator by the optical fiber F20, and an SOA (Semiconductor Optical Amplifier) 71 as an optical amplification medium in the resonator, and a wavelength selection and wavelength sweeping element. FP-TF72 is arranged. The FP-TF 72 can change the selected wavelength by changing the distance between the reflecting surfaces used for interference using the control means 73. The laser light oscillated by this resonator is output by an optical coupler 74 provided in a part of the ring-shaped optical path.

  Since the ring resonator type laser light source can oscillate in both the clockwise and counterclockwise directions on the ring-shaped optical path, when both the left and right modes oscillate simultaneously, mode competition between the oscillation modes in both directions Happens. At this time, when only the oscillation mode in one direction is viewed, the output of the oscillation light becomes unstable. In order to avoid this, it is desirable to oscillate in only one of the left and right directions. As shown in FIG. 7, light traveling in one direction is allowed to pass through the optical path of the ring resonator type and light in the opposite direction is blocked. It is desirable to arrange the optical isolators 75a and 75b.

  As such an optical isolator, generally, an optical isolator composed of a Faraday element, a magnet for applying a magnetic field to the Faraday element, and a polarizing element is widely used. As the Faraday element, a material made of a magnetic garnet crystal such as a YIG (yttrium-iron-garnet) crystal or a Bi-substituted garnet crystal is known.

  Next, FIG. 8 shows a configuration of a linear resonator type laser light source 80 using FP-TF. The laser light source 80 includes an SOA 81, a lens 82, an FP-TF 83, and a mirror 84 that are arranged on a straight line. The FP-TF 83 includes two parallel plate-like transparent electrodes 85a and 85b each having a reflection surface formed inside, and a liquid crystal 86 sandwiched therebetween, and changes the voltage applied to the liquid crystal 86 by the power source 87. Thus, the refractive index of the liquid crystal is changed, and thereby the wavelength transmitted through the FP-TF 83 is changed.

  In the laser light source 80 shown in FIG. 8, the light emitted from the SOA 81 is converted into convergent light by the lens 82 and then enters the FP-TF 83, and only the light having the selected wavelength passes through the FP-TF 83, and is not the selected wavelength. Light is reflected. The transmitted light is reflected by the mirror 84 constituting the resonator end, returns through the same optical path, passes through the FP-TF 83 and the lens 82, enters the SOA 81 again, and reaches the end surface 81a opposite to the lens 82 of the SOA 81. Reflected and amplified by the SOA 81. The light reflected by the FP-TF 83 is excluded from the optical path of the resonator and is not fed back to the SOA 81 because the FP-TF 83 is arranged to be inclined with respect to the optical axis of the incident light. As described above, in the linear resonator type laser light source 80, the light of the selected wavelength reciprocates in the resonator and oscillates.

On the other hand, an optical tomographic imaging apparatus using SS-OCT (Swept source OCT) measurement is known as an important application of the wavelength tunable laser light source 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 an optical tomographic imaging apparatus based on SS-OCT measurement, a time waveform of a signal corresponding to a temporal change in wavelength is measured while temporally changing the wavelength of light emitted from a light source, and spectral interference obtained thereby. A reflected light intensity distribution corresponding to the depth position is acquired by performing frequency analysis represented by Fourier transform on the intensity signal by a computer.
Japanese Patent No. 2648417 “Tunable Light Source Using a Liquid-Crystal Fabry-Perot Interferometer”, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.3, No.6, JUNE 1991

  As described above, the ring resonator type laser light source requires an optical isolator because of its optical path configuration. Conventional optical isolators generally use a Faraday element made of a magnetic garnet crystal or the like. This element has good characteristics with respect to light with a wavelength of 1.3 to 1.55 μm, which is usually used in optical communication. For example, when an optical tomographic imaging apparatus is used to measure a living body, light with a shorter wavelength is used. In this case, various problems occur. Specifically, if light having a wavelength shorter than 1.3 to 1.55 μm is used, a magnetic crystal having good characteristics has not been put into practical use, and an expensive material must be used for the optical isolator. However, since the Verde constant of the Faraday element is reduced, the applied magnetic field must be increased, resulting in an increase in size of the apparatus, an increase in light scattering rate, and a decrease in light utilization efficiency.

  In addition, since the resonator length of the ring resonator type inevitably increases, the size of the resonator increases and the optical saturation time until the ASE light generated by the SOA reaches laser oscillation increases. is there. Usually, since the wavelength sweep speed of the wavelength sweep laser is determined by the light saturation time, it is difficult for a light source having a long resonator length to be swept at a high speed.

  Therefore, it is conceivable to use a linear resonator type light source. However, the linear resonator type light source shown in FIG. 8 uses a liquid crystal for the FP-TF, so the response speed is low. There is a problem that it is difficult.

  Further, in the resonator as shown in FIG. 8, a lens 82 is used immediately before the FP-TF 83 in order to convert divergent light emitted from the SOA 81 into convergent light and make it incident on the FP-TF 83. Usually, the lens is an element having aberration, and it is difficult to design and manufacture the lens so that it has no aberration in the entire wavelength band of light emitted from the wavelength tunable light source. When a beam with aberration is incident on the FP-TF, the distortion of the wavefront in the FP-TF cannot be sufficiently reduced, so that there is a problem that high finesse resonance, that is, high wavelength selectivity cannot be realized. Arise.

  In view of the above problems, the present invention is a light source that can be manufactured in a small size and at low cost, and can realize high light utilization efficiency, high-speed wavelength sweep, and high wavelength selectivity, and optical tomographic imaging including the light source An object is to provide an apparatus.

  The light source of the present invention is a linear resonator type light source. The light amplifying medium and the light emitted from the light amplifying medium are incident as divergent light, transmit light having a specific wavelength, and reflect light other than the specific wavelength. Then, the reflected light is transmitted through the Fabry-Perot resonator having two reflecting surfaces arranged to be inclined with respect to the optical axis of the linear resonator so that the reflected light does not enter the divergence center of the divergent light, and the Fabry-Perot resonator. Feedback means for returning the transmitted light so that the light converges to the center of divergence is provided.

  Here, "the light emitted from the optical amplifying medium is incident as diverging light and the Fabry-Perot resonator" means that the light emitted from the optical amplifying medium is directly diverged to the Fabry-Perot resonator without passing through other members. This includes both the case where the light is incident and the case where the light emitted from the optical amplifying medium is incident as divergent light to the Fabry-Perot resonator via another member.

  Further, the above-mentioned “optical axis of the linear resonator” means an optical axis when light that can reciprocate through the linear resonator enters the Fabry-Perot resonator. For example, when the optical path of the linear resonator is not a straight line and a part thereof is a bent shape or a bent shape, the direction of the optical axis differs depending on the portion of the linear resonator. The “optical axis of the linear resonator” is considered as described above.

  The “feedback means” may be any means that feeds back the transmitted light so that the light transmitted through the Fabry-Perot resonator converges to the divergence center. You may comprise with this optical member. As the feedback means, for example, a reflecting means such as a concave mirror or a reflective diffractive optical element can be used.

  In the light source of the present invention, the two reflecting surfaces may be configured such that a reflecting surface having a curved shape centering on the divergence center is inclined with respect to the optical axis of the linear resonator.

  Furthermore, the optical tomographic imaging apparatus of the present invention comprises the light source of the present invention, a light splitting means for splitting the light emitted from the light source into measurement light and reference light, and when the measurement light is irradiated to the measurement object. Combining means for combining the reflected light from the measurement object and the reference light, interference light detecting means for detecting the interference light between the reflected light combined by the combining means and the reference light, and the interference light detecting means And an image acquisition means for acquiring a tomographic image of the measurement object based on the interference light detected by.

  Since the light source of the present invention adopts a linear resonator type configuration rather than a ring resonator type, an optical isolator composed of a Faraday element or the like is not required, which is a problem with conventional ring resonator type light sources. It is possible to avoid an increase in the size and cost of the device, a decrease in the light utilization efficiency, and an increase in the resonator length, and it can be manufactured in a small size and at a low cost, realizing a high light utilization efficiency. It becomes possible. In addition, since the light source of the present invention can realize a linear resonator type light source without using liquid crystal, high-speed wavelength sweep is possible. Further, in the conventional linear resonator type light source, an optical element with aberration such as a lens is necessary to make the light incident on the Fabry-Perot resonator into convergent light or parallel light. Is a device that makes divergent light incident on the Fabry-Perot resonator and includes feedback means that feeds back the light transmitted through the Fabry-Perot resonator so that it converges to the divergent center. An element is not required, resonance of high finesse can be realized, and high wavelength selectivity can be realized.

  When the two reflecting surfaces of the light source of the present invention are configured so that the curved reflecting surfaces centering on the divergence center are inclined with respect to the optical axis of the linear resonator, the two reflecting surfaces are arranged. Unlike the case where the plane is simply configured as a plane, the optical path length between two reflecting surfaces of the light beam traveling near the optical axis of the diverging light and the optical path length between the two reflecting surfaces of the light beam traveling near the periphery of the diverging light Therefore, the wavelength selectivity of the light transmitted through the Fabry-Perot resonator can be increased.

  The optical tomographic imaging apparatus of the present invention includes the light source of the present invention and acquires a tomographic image using light emitted from the light source, so that the apparatus can be manufactured at a low cost while suppressing the enlargement of the apparatus. Therefore, it is possible to perform measurement by high-speed wavelength sweep using light having high light utilization efficiency and high wavelength selectivity.

  Embodiments of a light source and an optical tomographic imaging apparatus including the light source according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical tomographic imaging apparatus 100 according to an embodiment of the present invention. The optical tomographic imaging apparatus 100 uses a Mach-Zehnder interferometer to acquire 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 100 divides the light L emitted from the light source unit 110 into the measurement light L1 and the reference light L2 while emitting the laser light L while sweeping the oscillation wavelength at a constant period. A light splitting means 101, an optical path length adjusting means 120 for adjusting the optical path length of the reference light L2 split by the light splitting means 101, and a probe for guiding the measurement light L1 split by the light splitting means 101 to the measuring object S 130, the combining means 104 for combining the reflected light L3 reflected by the measuring object S and the reference light L2 when the measuring light L1 is irradiated from the probe 130 to the measuring object S, and the combining means 104. Interference light detecting means 140 for detecting the interference light L4a and L4b between the reflected light L3 and the reference light L2, and the measurement object S being disconnected based on the interference light detected by the interference light detecting means 140. And an image acquisition unit 150 for acquiring an image.

  The light source unit 110 is a linear resonator type wavelength tunable laser device that emits light L while sweeping the oscillation wavelength at a constant period, and includes the light source according to the embodiment of the present invention. The detailed configuration of the light source unit 110 will be described in detail later. The light L from the light source unit 110 is output by the optical fiber F1 and enters the light splitting means 101 connected to the optical fiber F1.

  The light splitting means 101 is composed of, for example, a 2 × 2 optical fiber coupler, and splits the light L guided from the light source unit 110 through the optical fiber F1 into the measurement light L1 and the reference light L2. The light splitting means 101 is optically connected to the two optical fibers F2 and F3, respectively. The measurement light L1 is guided by the optical fiber F2, and the reference light L2 is guided by the optical fiber F3.

  A circulator 102 is connected to the optical fiber F2, and optical fibers F4 and F5 are connected to the circulator 102. A probe 130 that guides the measurement light L1 to the measurement object S is connected to the optical fiber F4.

  The optical probe 130 is inserted into a body cavity via a forceps channel of an endoscope, for example, and is detachably attached to the optical fiber F4 by a connector (not shown). The measuring light L1 emitted from the probe 130 is irradiated to the measuring object S. Then, the reflected light L3 reflected by the measuring object S is guided by the optical fiber F4, enters the circulator 102, is emitted from the circulator 102 to the optical fiber F5 side, and is coupled to the optical fiber F5. Incident on means 104.

  On the other hand, a circulator 103 is connected to the optical fiber F3, and optical fibers F6 and F7 are connected to the circulator 103. An optical path length adjusting unit 120 that changes the optical path length of the reference light L2 in order to adjust the tomographic image acquisition region is connected to the optical fiber F6.

  The optical path length adjusting means 120 has a collimating lens 120a for collimating the reference light L2 emitted from the optical fiber F6 and two orthogonal reflecting surfaces, and these two reflecting surfaces are collimated by the collimating lens 120a. The reflection mirror 120b that reflects the reference light L2, and the optical terminator that returns the reference light L2 reflected by the two reflection surfaces of the reflection mirror 120b to the reflection mirror 120b again and travels in the same optical path as the incident light. 120c. The reflecting mirror 120b is fixed on a movable stage (not shown). By moving the movable stage, the reflecting mirror 120b moves in the optical axis direction of the collimating lens 120a (the direction of arrow A in FIG. 1). Thus, the optical path length of the reference light L2 is changed. The optical path length adjusting unit 120 is not limited to the above configuration, and other configurations may be adopted as long as the optical path length of the reference light L2 can be changed.

  The reference light L2 whose optical path length has been changed by the optical path length adjusting means 120 is guided by the optical fiber F6, enters the circulator 103, is emitted from the circulator 103 to the optical fiber F7 side, and is connected to the optical fiber F7. Is incident on the combining means 104.

  The multiplexing means 104 is made of, for example, a 2 × 2 optical fiber coupler, and combines the reflected light L3 guided by the optical fiber F5 and the reference light L2 guided by the optical fiber F7. Interference light is generated by the multiplexing in the multiplexing means 104, and the interference light is divided into two by the multiplexing means 104 to become two interference lights L4a and L4b, which are respectively emitted to the optical fibers F8 and F9. The interference lights L4a and L4b respectively guided by the optical fibers F8 and F9 enter the interference light detection means 140.

  The interference light detection means 140 is detected by the light detection unit 140a that detects the interference light L4a, the light detection unit 140b that detects the interference light L4b, the interference light L4a detected by the light detection unit 140a, and the light detection unit 140b. And calculating means 140c for amplifying the difference from the interference light L4b and performing balance detection to obtain an interference signal. By this mechanism, the influence of light intensity fluctuation can be suppressed and a clearer image can be obtained.

  The interference light detection unit 140 is connected to an image acquisition unit 150 including a computer system such as a personal computer. The image acquisition unit 150 is connected to a display unit 160 including a CRT or a liquid crystal display unit. The image acquisition means 150 acquires reflection information such as the intensity of the reflected light L3 at each depth position of the measurement object S by Fourier transforming the interference signal output from the interference light detection means 140, and uses this information. A tomographic image of the measurement object S is generated. The display device 160 displays the tomographic image acquired by the image acquisition unit 150.

  Here, the detection of the interference light L4 and the generation of the image in the interference light detection means 140 and the image acquisition means 150 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 140.
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 150, the spectral interference fringes detected by the interference light detection means 140 are subjected to Fourier transform to perform frequency analysis, and the light intensity S (l) of the interference light L4 is determined. 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 100 having the above configuration will be described. First, the optical path length adjustment unit 120 adjusts the optical path length so that the measuring object S is positioned within the measurable region. Thereafter, the light L is emitted from the light source unit 110, and the light L is split by the light splitting means 101 into the measurement light L1 and the reference light L2. The measurement light L1 is guided into the body cavity by the probe 130 and irradiated to the measurement object S. Then, the reflected light L3 from the measurement object S is combined with the reference light L2 by the combining means 104, and the interference lights L4a and L4b between the reflected light L3 and the reference light L2 are detected by the interference light detecting means 140 to generate an interference signal. As issued. The interference signal is frequency-analyzed by the image acquisition means 150 to acquire a tomographic image.

  In addition, if the measurement light L1 is scanned in the one-dimensional direction with respect to the measurement target S by rotating the probe 130 or the like, information in the depth direction of the measurement target S is obtained at each portion along the scanning direction. Therefore, 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.

  In the above example, the configuration example of the optical tomographic imaging apparatus using the Mach-Zehnder interferometer has been described. However, the present invention is not limited to this, and another example such as a Michelson interferometer or a Fizeau interferometer is used. An optical tomographic imaging apparatus using this kind of interferometer is also possible.

  Hereinafter, embodiments of the light source of the present invention applicable to the light source unit 110 will be described in detail. FIG. 2 shows the configuration of the light source 1 according to the first embodiment of the present invention. The light source 1 shown in FIG. 2 has a linear resonator type (also referred to as a linear resonator type) in which laser oscillation is performed by reciprocating light in an optical path. The light source 1 includes a SOA (Semiconductor Optical Amplifier) 2, a Fabry-Perot resonator 5 having two reflecting surfaces 3 and 4, and a concave mirror 6, which are sequentially arranged along the optical path.

The SOA 2 corresponds to the optical amplification medium of the present invention, can emit light of a predetermined wavelength band, and has a function of amplifying incident light and emitting it. In the SOA 2 of this embodiment, a highly reflective coat (HR coat) is applied to the end face 2a opposite to the Fabry-Perot resonator 5, and this face functions as one resonator end of the linear resonator. In the SOA 2, divergent light 7 is emitted from the end surface opposite to the end surface 2 a, and the divergent light 7 enters the Fabry-Perot resonator 5. In the light source 1, the divergent center O ₁ of the divergent light 7 is a light emitting point on the exit end of the SOA 2.

  The Fabry-Perot resonator 5 transmits only light having a specific wavelength, which is a selected wavelength, and reflects light other than the specific wavelength due to interference of light occurring on the two reflecting surfaces 3 and 4. It functions as.

  Here, a Fabry-Perot resonator used as wavelength selection means will be described. The Fabry-Perot resonator has a transmitted light spectrum distribution having a transmission peak that is periodic and determined by the distance between the two reflecting surfaces and the reflectance, and the light spectrum that cannot be transmitted is reflected. The free spectrum range value (hereinafter referred to as FSR) increases in inverse proportion to the interval between the reflecting surfaces, and the interval between transmission peaks is repeated at an interval determined by FSR. Further, the greater the reflectance, the narrower the transmission peak width. Finesse is obtained by dividing the FSR by the full width at half maximum of the transmission peak spectrum, and the higher the finesse light source, the higher the wavelength selectivity.

  By sufficiently narrowing the distance between the two reflecting surfaces constituting the Fabry-Perot resonator to increase the FSR, only one transmitted light spectrum distribution can exist in the optical gain band of the SOA. Furthermore, by sufficiently increasing the reflectance of the reflecting surface, the width of the transmitted light spectrum distribution can be sufficiently narrowed, and only one light spectrum distribution that is substantially narrow within the optical gain band of the SOA is Fabry-Perot resonance. It is possible to set so that it can pass through the vessel. As a result, the light source 1 can oscillate with only a sufficiently narrow light spectrum distribution.

  Furthermore, it is possible to change the position of the center wavelength of the transmitted light spectrum distribution by changing the distance between the two reflecting surfaces. When the interval between the reflecting surfaces is increased, the transmitted light spectrum is shifted to the longer wavelength side, and when the interval between the reflecting surfaces is decreased, the transmitted light spectrum is shifted to the shorter wavelength side. By continuously changing the interval between the two reflecting surfaces 3 and 4, it is possible to continuously sweep the laser oscillation light spectrum distribution having a sufficiently narrow light spectrum distribution.

  For example, by changing the interval between the reflecting surfaces 3 and 4 in a sine wave manner in the direction of enlarging and reducing, the oscillation wavelength of the light source 1 alternately repeats the shift toward the long wavelength side and the shift toward the short wavelength side, The light source 1 can be made a variable wavelength light source by continuously sweeping one oscillation wavelength.

In the Fabry-Perot resonator 5 of the present embodiment, as shown in FIG. 2, the two reflecting surfaces 3 and 4 are formed of curved mirrors arranged at substantially equal intervals. Further, the two reflecting surfaces 3 and 4 are disposed to be inclined with respect to the optical axis Z of the linear resonator so that the light reflected by the Fabry-Perot resonator 5 does not enter the divergence center O ₁ of the divergent light 7. . That is, the line N perpendicular to the center of the reflecting surface 4 and the optical axis Z of the linear resonator do not coincide with each other and form an inclination angle θ. Here, the optical axis Z of the linear resonator means the optical axis of light that passes through the Fabry-Perot resonator 5, is reflected by the concave mirror 6, and passes through the Fabry-Perot resonator 5 again. In this embodiment, since the diverging light 7 is assumed to be symmetrical in the vertical direction of the paper in FIG. 2, the optical axis Z of the linear resonator is the direction of the light beam passing through the center of the diverging light 7.

In FIG. 2, θ is illustrated at a large angle for the sake of clarity. However, in order to prevent the light reflected by the Fabry-Perot resonator 5 from entering the diverging center O ₁ of the diverging light, the reflection is performed. The surfaces 3 and 4 may be inclined at a minute angle, and the angle θ can take a very small value. By tilting the two reflecting surfaces 3 and 4, it is possible to prevent light other than the specific wavelength reflected by the Fabry-Perot resonator 5 from entering the divergence center O ₁ . Therefore, light other than the specific wavelength travels on the optical path off the optical axis of the linear resonator and is not amplified by the SOA2.

Preferably, the two reflecting surfaces 3 and 4 are arranged such that a reflecting surface having a curved surface around the divergence center O ₁ , for example, a spherical reflecting surface is inclined with respect to the optical axis of the linear resonator. Since the divergent light 7 is a light beam having a spread, the divergent light 7 includes a light beam that travels in the vicinity of the center of the light beam of the divergent light 7 (a light beam in the same direction as the optical axis Z) and the vicinity of the periphery of the light beam of the divergent light 7 Light rays (light rays that form an angle with the optical axis Z). If the two reflecting surfaces 3 and 4 are configured as a plane, the light path length between the light beams traveling near the center and the light beams traveling near the periphery differs between the two reflecting surfaces and passes through the Fabry-Perot resonator. The wavelength selectivity of light decreases. On the other hand, in this embodiment, by making the curved surface as described above, the optical path lengths between the reflecting surface 3 and the reflecting surface 4 of the light beam traveling near the center and the light beam traveling near the periphery are made substantially equal. Therefore, the wavelength selectivity of light transmitted through the Fabry-Perot resonator can be increased.

Further, the reflecting surfaces 3 and 4 are preferably curved surfaces having substantially the same shape as the wavefront of the diverging light 7. In this case, the optical path length from the divergence center O _{1 of} each light beam included in the diverging light 7 to the reflecting surface 3 can be made equal, and similarly, the light beam reflected from the divergence center O _{1 of} each light beam included in the diverging light 7 is reflected. The optical path lengths to the surface 4 can be made equal, and as a result, the optical path lengths from the reflecting surface 3 to the reflecting surface 4 of each light beam can be made equal. Thereby, the width of the transmission peak spectrum of the Fabry-Perot resonator 5 can be made extremely narrow, and very high wavelength selectivity can be realized.

  As shown by the arrow B in FIG. 2, the reflecting surfaces 3 and 4 are configured such that the interval can be changed by the control means 8. By changing the interval between the reflecting surfaces 3 and 4, the wavelength of light that can be transmitted through the Fabry-Perot resonator 5 can be changed.

The concave mirror 6 is another resonator end of the linear resonator and functions as a feedback means for feeding back the transmitted light so that the light transmitted through the Fabry-Perot resonator 5 converges to the divergence center O ₁ . It is. Schematically, the concave mirror 6 can be considered as a curved surface centered on the divergence center O ₁ , for example, a spherical mirror arranged so that its central axis coincides with the optical axis Z of the linear resonator.

More specifically, the concave mirror 6 has a surface shape perpendicular to each light beam constituting the diverging light 7 incident on the concave mirror 6. With this shape, the light reflected by the concave mirror 6 can travel in the reverse direction on the same optical path as the light incident on the concave mirror 6 and can be returned to the divergence center O ₁ . More strictly, the concave mirror 6 is preferably a curved surface having substantially the same shape as the wavefront of the diverging light 7.

  If the diverging light 7 from the SOA 2 is non-axisymmetric light with respect to the optical axis Z, the concave mirror 6 has a non-axisymmetric shape corresponding to the non-axisymmetric light, for example, an analog It is necessary to have a morphic shape, and it is preferable that the reflecting surfaces 3 and 4 have a non-axisymmetric shape, for example, an anamorphic shape.

An operation example of the light source 1 will be described. From the divergence center O _{1 of the} SOA 2, divergent light including light having a specific wavelength and light having a wavelength other than the specific wavelength is emitted and enters the Fabry-Perot resonator 5. Among them, light having a wavelength other than the specific wavelength is reflected by the Fabry-Perot resonator 5 and becomes convergent light. However, since the reflecting surfaces 3 and 4 are arranged to be inclined, the reflected light is shown by a dotted line in FIG. It does not enter the divergence center O ₁ and is not amplified by the SOA 2.

On the other hand, light of a specific wavelength incident on the Fabry-Perot resonator 5 is transmitted through the Fabry-Perot resonator 5, reflected by the concave mirror 6, becomes convergent light, travels in the same optical path in the opposite direction, and passes through the Fabry-Perot resonator 5 again. Then, the light is incident so as to converge on the divergence center O ₁ . Light of a specific wavelength and re-entering the divergence center O ₁ travels through the SOA 2, is reflected by the end face 2a. Thereafter, the same thing is repeated, and light of a specific wavelength oscillates in the resonator.

  Then, by changing the distance between the reflecting surfaces 3 and 4 by the control means 8, the specific wavelength that can be transmitted through the Fabry-Perot resonator 5 can be changed, and the wavelength-swept laser light can be obtained.

  Since the light source 1 has a linear resonator type configuration, the problems of the conventional ring resonator type light source can be avoided. Since the light source 1 has a compact configuration with a small number of parts, the light source 1 can be manufactured at low cost, has little light loss, and can achieve high light utilization efficiency. In addition, the light source 1 does not use an optical element such as a lens that causes aberration as compared with the laser light source shown in FIG. 8, so that high wavelength selectivity can be realized, and since no liquid crystal is used, high-speed wavelength sweeping is achieved. Is possible.

  In order to externally output the oscillation light in the light source 1, for example, the concave mirror 6 may be a partial reflection mirror instead of a total reflection mirror, and a part of the light incident on the concave mirror 6 may be transmitted and output to the outside. .

  Or you may make it perform an external output using an optical fiber like the light source 10 shown in FIG. Compared to the light source 1 shown in FIG. 2, the light source 10 shown in FIG. 3 uses an SOA 12 instead of the SOA 2, and an optical fiber F 10 is connected to the opposite side of the Fabry-Perot resonator 5 of the SOA 12 via a coupling lens 11. The arrangement point is different. Differences between the light source 10 shown in FIG. 3 and the light source 1 will be mainly described.

  The left end face of the SOA 12 of the light source 10 shown in FIG. 3, that is, the end face opposite to the Fabry-Perot resonator 5 is not coated with high reflection. Therefore, the light source 10 emits light of a specific wavelength that has exited the SOA 12, reciprocated through the Fabry-Perot resonator 5 and re-entered the SOA 12, and exits from the left end face of the SOA 12 in FIG. Is incident on. The optical fiber F10 can be composed of, for example, a single mode fiber. The end face F10a opposite to the lens 11 of the optical fiber F10 is provided with a highly reflective coating, and this end face F10a functions as one resonator end of the linear resonator. The highly reflective coating on the end face F10a can transmit a part of the incident light, and the transmitted light becomes output light to the outside.

  By using the optical fiber F10 in this manner, the position and direction of the output light to the outside can be freely set regardless of the position of the SOA 12, and the degree of freedom in device design can be improved. In addition, when an optical fiber is used for the waveguide means, there are advantages that assembly adjustment is easier and high stability can be ensured than when a normal bulk element is used. Although the light source 10 has a lens 11, the laser light source shown in FIG. 8 has a lens 82 disposed between the SOA 81 and the FP-TF 83, whereas the light source 10 has the SOA 12 and the Fabry-Perot resonator 5. Is different in that no lens is arranged. In the laser light source shown in FIG. 8, since the convergent light is incident on a Fabry-Perot resonator having a parallel reflecting surface, finesse is lowered and wavelength selectivity is lowered. Compared with this, in the light source 10 shown in FIG. 3, since the divergent light is incident on the Fabry-Perot resonator having the curved surface described above, the wavelength selectivity can be improved.

  Next, a light source 20 according to a second embodiment of the present invention will be described with reference to FIG. The light source 20 substantially has a configuration in which the SOA 12 and the Fabry-Perot resonator are connected by an optical fiber in the light source 10 shown in FIG. The light source 20 includes an optical fiber F10, an optical coupling lens 11, an SOA 12, an optical coupling lens 13, a waveguide optical fiber F11, and an optical fiber F11 arranged in order along the optical path. And a wavelength selection unit 15 connected to the end. Among these, the lenses 11 and 13 and the SOA 12 are configured as a module 14.

  The optical fibers F10 and F11 function as an optical waveguide, and can be constituted by, for example, a single mode fiber. A high reflection coat (HR coat) is applied to the end face F10a opposite to the lens 11 of the optical fiber F10, and this end face F10a functions as a resonator end. A part of the light incident on the optical fiber F10 is configured to be output to the outside from the end face F10a.

  The wavelength selection unit 15 includes a Fabry-Perot resonator and the feedback means of the present invention. FIG. 5 shows an enlarged cross-sectional view of the wavelength selection unit 15. As shown in FIG. 5, the wavelength selection unit 15 includes a ferrule 16 having an approximately cylindrical shape and an optical fiber F <b> 11 inserted therein, a fixed reflecting portion 17 attached to the tip of the ferrule 16, and a distance from the fixed reflecting portion 17. , The actuator 19 that moves the moving reflector 18 in a substantially cylindrical shape, and the holder 29 that holds the ferrule 16 and the actuator 19.

The fixed reflecting portion 17 includes a curved reflecting surface 23 disposed so as to face the end face of the optical fiber F11, and a thin and substantially cylindrical outer peripheral member 21 for supporting the reflecting surface 23 and fixing it to the ferrule 16. Consists of. The inside of the outer peripheral member is a cavity, and the divergent light 27 travels from the divergent center O ₂ on the end face of the core of the optical fiber F11 through the outer peripheral member, and enters the reflecting surface 23.

  The movable reflecting portion 18 is a thin wall for integrating the reflecting surface 24 facing the reflecting surface 23, the concave mirror 26 located a predetermined distance away from the reflecting surface 24 in the optical axis Z direction, and the reflecting surface 24 and the concave mirror 26. The outer peripheral member 22 has a substantially cylindrical shape.

The divergence center O ₂ , the reflection surface 23, the reflection surface 24, and the concave mirror 26 respectively correspond to the divergence center O ₁ , the reflection surface 3, the reflection surface 4, and the concave mirror 6 of the light source 1 shown in FIG. The positional relationship of the arrangement is the same as that of the divergence center O ₁ , the reflective surface 3, the reflective surface 4, and the concave mirror 6.

That is, the reflecting surface 23 and the reflecting surface 24 constitute two reflecting surfaces of the Fabry-Perot resonator 25, and a reflecting surface having a curved shape centering on the divergence center O ₂ is set as the optical axis Z of the linear resonator. They are arranged at an angle. The inclination angles of the reflecting surface 23 and the reflecting surface 24 are set so that the light reflected by the Fabry-Perot resonator 25 does not enter the diverging center O ₂ of the diverging light 27. Further, the concave mirror 26 is a light transmitted through the Fabry-Perot resonator 25 is to converge the diverging center O _2, serves as a feedback means for feeding back the transmitted light.

  The actuator 19 is driven in the direction of the optical axis Z (the direction of the arrow C in FIG. 5) by a control means (not shown), whereby the moving reflecting portion 18 moves in the direction of the optical axis Z and reflects from the reflecting surface 23. The distance between the surfaces 24 is changed, and the wavelength of light that can be transmitted through the Fabry-Perot resonator 25 can be changed.

In the light source 20, the resonator end of the linear resonator includes an end face F10a of the optical fiber F10 and a concave mirror 26. In the light source 20, light having a wavelength other than the specific wavelength out of the light emitted from the SOA 12 is reflected by the Fabry-Perot resonator 25 and travels as indicated by a dotted line shown in FIG. 5. This light is collected at the divergence center O ₂ , that is, at a position deviated from the core of the optical fiber F 11, excluded from the optical path of the linear resonator and not guided by the optical fiber F 11, and therefore returned to the SOA 12. There is no amplification.

On the other hand, light of a specific wavelength out of the light emitted from the SOA 12 passes through the Fabry-Perot resonator 25 and is reflected by the concave mirror 26 so as to converge to the divergence center O _2, and thus reenters the core of the optical fiber F 11. Can be guided by the optical fiber F11 and returned to the SOA 12 for amplification. Therefore, also in the light source 20, only light of a specific wavelength is amplified and oscillated. Further, by driving the actuator 19, the oscillation wavelength of the light source 20 can be continuously swept to make the light source 20 a variable wavelength light source.

  In many cases, the light emitted from the SOA is non-axisymmetric. In that case, the concave mirror 6 must also be produced non-axisymmetrically in the light sources 1 and 10 shown in FIGS. On the other hand, in the light source 20 shown in FIG. 4, even if non-axisymmetric light is emitted from the SOA 12, only the light having an axially symmetric component is coupled to the optical fiber F11, so that it passes through the optical fiber F11. As a result, the axially symmetric light can be made incident on the Fabry-Perot resonator 25, and the concave mirror 26 having the axially symmetric shape can be used, so that it can be manufactured at low cost. Furthermore, if a lens that forms non-axisymmetric light from the SOA into axially symmetric light is selected as the lens 13, the light utilization efficiency can be increased. As such a lens 13, for example, an anamorphic lens can be used.

  Although the light source 20 of this embodiment includes lenses 11 and 13 for coupling with an optical fiber, no lens is used in the wavelength selection unit 15. This is due to the fact that the present invention allows divergent light to enter a Fabry-Perot resonator to enable wavelength selection. In the laser light source shown in FIG. 8, the lens 82 is disposed immediately before the FP-TF 83, and the aberration of the lens 82 reduces the finesse and the wavelength selectivity. On the other hand, in the light source 20 shown in FIG. 4, high wavelength selectivity can be realized without the finesse being lowered due to the aberration of the lenses 11 and 13 included in the light source 20. In the conventional linear resonator type light source including the laser light source shown in FIG. 8, a lens is required to allow parallel light or convergent light to enter the Fabry-Perot resonator. In the present invention, a lens for such use is used. Therefore, it is possible to have high wavelength selectivity while reducing the number of lenses as compared with the conventional apparatus.

  Next, a modification of the second embodiment will be described with reference to FIG. In the following description of the modified examples and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 6 shows a configuration of a modified example. The light source 30 of the modification shown in FIG. 6 comprises a resonator end by forming a loop-shaped optical path with an optical fiber. In the example shown in FIG. 6, instead of the optical fiber F10 of the light source 20 shown in FIG. The optical fiber F12 is made of, for example, a single mode fiber. In the light source 30, light incident from one end of the optical fiber F12 can propagate through the loop and be emitted again to one end of the same optical fiber as the incident end. Further, by suitably setting the branching ratio of the ring coupler 32, the oscillated laser beam can be output from the optical fiber end different from the incident end. By using such a loop-shaped optical fiber, it is not necessary to form a highly reflective coating on the end face of the optical fiber, and it can be configured at low cost.

  Note that the configuration in which the resonator end is made of a loop-shaped fiber as shown in FIG. 6 can be applied not only to the second embodiment but also to the configuration shown in FIG.

  The preferred embodiments of the light source and the optical tomographic imaging apparatus according to the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without changing the gist of the invention. Is possible.

  For example, in the above embodiment, an example in which one concave mirror is used as the feedback means has been described. However, the feedback means of the present invention is not limited to this, and a reflective diffractive optical element can be used. The optical member may constitute the feedback means.

1 is a schematic configuration diagram of an optical tomographic imaging apparatus according to an embodiment of the present invention. The block diagram of the light source by the 1st Embodiment of this invention FIG. 3 is a configuration diagram of a light source of a modification according to the first embodiment of the present invention. The block diagram of the light source by the 2nd Embodiment of this invention The expanded sectional view of the wavelength selection unit of the light source by the 2nd Embodiment of this invention The block diagram of the wavelength tunable light source of the modification concerning the 2nd Embodiment of this invention Schematic configuration diagram of a conventional ring resonator type tunable light source Schematic configuration diagram of a conventional linear resonator type tunable light source

Explanation of symbols

1, 10, 20, 30 Light source 2, 12 SOA
3, 4, 23, 24 Reflective surface 5, 25 Fabry-Perot resonator 6, 26 Concave mirror 7, 27 Divergence light 15 Wavelength selection unit 16 Ferrule 17 Fixed reflection unit 18 Moving reflection unit 19 Actuator 100 Optical tomographic imaging device 101 Optical division Means 104 Multiplexing means 110 Light source unit 120 Optical path length adjusting means 130 Probe 140 Interference light detection means 150 Image acquisition means 160 Display device F10, F11 Optical fiber F10a End face L Light L1 Measurement light L2 Reference light L3 Reflected light L4 Interference light O ₁ , O ₂ divergence center S

Claims (3)

A linear resonator type light source,
An optical amplification medium;
Light emitted from the optical amplifying medium is incident as divergent light, transmits light having a specific wavelength, reflects light other than the specific wavelength, and prevents the reflected light from entering the divergent center of the divergent light. A Fabry-Perot resonator having two reflecting surfaces arranged to be inclined with respect to the optical axis of the linear resonator;
A light source comprising: feedback means for returning the transmitted light so that the light transmitted through the Fabry-Perot resonator converges to the divergence center.   2. The light source according to claim 1, wherein both of the two reflecting surfaces are arranged such that a curved reflecting surface centering on the divergence center is inclined with respect to the optical axis of the linear resonator. The light source according to claim 1 or 2,
A light splitting means for splitting light emitted from the 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 based on the interference light detected by the interference light detection unit.

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