JP2013051332A - Light source device and imaging device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light source device capable of simultaneously achieving increase in a wavelength sweeping speed and reduction in an oscillation spectral line width.SOLUTION: A light source device has: an optical resonator having an optical gain medium for amplifying light; a disperse element dispersing light emitted from the optical gain medium depending on a wavelength; and a wavelength selector reflecting or transmitting light having a specific wavelength that went through the disperse element. While the light emitted from the optical gain medium goes and comes back in the optical resonator, a plurality of times of wavelength selection are performed at the wavelength selector. Thus, light in an overlapped wavelength band that is obtained by the plurality of times of wavelength selection is emitted.

Description

  The present invention relates to a light source device capable of changing an oscillation wavelength and an imaging device using the same.

  As light sources, particularly laser light sources, various types having variable oscillation wavelengths have been used in the fields of communication networks and inspection devices.

  In the field of communication networks, high-speed wavelength switching is required, and in the field of inspection equipment, high-speed and wide-range wavelength sweeping is desired.

  Applications of the wavelength tunable (sweep) light source in the inspection apparatus include a laser spectrometer, a dispersion measuring instrument, a film thickness measuring instrument, and a wavelength sweep type optical interference tomography (SS-OCT) apparatus.

  Optical interference tomography is tomographic images of specimens using optical interference, and research in the medical field has become popular in recent years because of the micron-order spatial resolution and non-invasiveness. Imaging technology.

  In wavelength-swept optical interference tomography, spectral interference is used to obtain depth information, and no spectroscope is used, so that it is expected to acquire an image with a high SN ratio with little loss of light.

  In the case of configuring a medical imaging apparatus to which the SS-OCT technology is applied, the image acquisition time can be shortened as the sweep speed increases, and the spatial resolution of the tomographic image can be increased as the wavelength sweep width increases. These parameters are important.

  Specifically, when the wavelength sweep width is Δλ, the oscillation wavelength is λ0, and the refractive index of the specimen is n, the depth resolution is

It is represented by Therefore, in order to increase the depth resolution, the wavelength sweep width needs to be expanded, and a broadband wavelength sweep light source is required.

  On the other hand, in the SS-OCT apparatus, it is desired that a deep structure of the specimen can be detected, that is, a long coherence distance can be realized. For this reason, as the performance of the light source of the SS-OCT apparatus, it is desirable that the oscillation spectral line width is narrower.

  Specifically, when the oscillation spectral line width is δλ, the coherence distance (coherence length) is

It is represented by Therefore, in order to widen the measurement range in the depth direction of the specimen, it is necessary to narrow the oscillation spectrum line width, and a wavelength swept light source having a narrow line width is required.

  Under these circumstances, an external resonator type wavelength swept light source in which an optical gain medium and a wavelength variable mechanism are incorporated in a resonator is mainly used as a light source used in the SS-OCT apparatus. Among such light sources, Patent Document 1 discloses a method of changing the wavelength by moving a slit as a wavelength variable mechanism.

  In this slit movement method, the light emitted from the optical gain medium is incident on the diffraction grating as parallel and diffracted at different angles for each wavelength. At this time, only a specific wavelength can be selected by cutting out only the diffracted light diffracted at a certain angle by the slit. Laser oscillation can be performed by returning the selected light to the optical gain medium with a mirror to form a resonator. Wavelength tuning is realized by moving the slit mechanically with a motor to change the selected wavelength.

  In addition, the wavelength sweep speed can be changed depending on the rotational speed of the disk by forming the slit along the circumference on the rotating disk. A rotating disk used for a linear encoder or the like can be used. Since the rotating disk is smaller in weight than a polygon mirror, it can be rotated relatively easily at a high speed.

JP 2008-98395 A

  In the wavelength sweeping light source using the slit moving method, it is necessary to increase the rotation speed of the disk on which the slit is formed in order to realize faster wavelength sweeping, but the rotation speed of the disk is limited to the rotation speed of the motor. Therefore, it is difficult to further increase the speed by this method.

  Another possible method is to increase the speed by reducing the diffraction angle by the diffraction grating and shortening the passage time of the slit with respect to the spatial spread width of the light beam, but the disadvantage of widening the wavelength selection width by the slit. It will occur.

  For this reason, it is conceivable to narrow the wavelength selection width by using a narrower slit. However, there is a limit in the manufacturing process to manufacture the slit itself narrower.

  Therefore, there is a limit to the production of a narrower slit, and the wavelength selection width is naturally limited. That is, since there is a trade-off relationship between speeding up the wavelength sweep and narrowing the oscillation spectrum, it is actually difficult to sweep the wavelength at high speed while satisfying a narrow spectral line width.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a light source device that can simultaneously realize speeding up of the wavelength sweep and narrowing of the oscillation spectrum line width.

A light source device provided by the present invention includes an optical resonator including an optical gain medium that amplifies light, and
A light source device having a dispersion element that disperses light emitted from the optical gain medium according to a wavelength, and a wavelength selection unit that reflects or transmits light of a specific wavelength that has passed through the dispersion element,
While the light emitted from the optical gain medium makes one round trip in the optical resonator, the wavelength selection unit performs wavelength selection a plurality of times, and the light in the overlapping wavelength band obtained by the plurality of wavelength selections. Is emitted.

  In the light source device of the present invention, the wavelength selection unit performs wavelength selection a plurality of times while the light reciprocates once in the resonator. And the light of the overlapping wavelength band obtained by multiple times of wavelength selection is radiate | emitted. This narrows the oscillation line width of the oscillation spectrum.

The schematic diagram which shows an example of the light source device of this invention Schematic diagram showing an example of a conventional light source device Schematic diagram illustrating the light source device according to the first embodiment of the present invention. Schematic diagram illustrating a light source device according to a second embodiment of the present invention. Schematic diagram illustrating an imaging apparatus according to a third embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described in comparison with the prior art with reference to the drawings.

  FIG. 2 is a schematic diagram showing an example of a conventional light source device.

  In the light source device shown in this figure, an optical amplifier 201 as an optical gain medium has a gain at the oscillation wavelength of laser oscillation, and uses, for example, a semiconductor optical amplifier (SOA). . Output light from one end face of the optical amplifier 201 is converted into parallel light by the collimator lens 202. The parallel light is incident on the diffraction grating 203 and diffracted at different angles depending on the wavelength of the light. The diffracted light is condensed by the condenser lens 204, and a slit mirror 205 is provided at the focal position of the condensed light. Since the focal position differs depending on the wavelength, only a specific wavelength is reflected according to the position of the slit mirror 205 and is incident on the optical amplifier 201 again.

  Therefore, the light of this wavelength functions as an external resonator type light source and oscillates. Since the wavelength of the reflected light changes by mechanically moving the position of the slit mirror 205, the wavelength can be varied.

  Further, part of the light emitted from the other end face of the optical amplifier 201 is reflected by the partial reflection mirror 207 via the collimator lens 206 and part of the light is transmitted. The light transmitted by the partial reflection mirror 207 is taken out as output light.

<Narrowing of oscillation spectrum in the present invention>
FIG. 1 is a schematic view showing an example of a light source device of the present invention. In the light source device shown in this figure, output light from one end face of an optical amplifier 101 constituted by a semiconductor optical amplifier or the like is incident on a diffraction grating 103 as a dispersion element via a collimator lens 102.

  The diffracted light is condensed by the condenser lens 104, and the slit mirror 105 is disposed at the focal position.

  A part of the light emitted from the other end face of the optical amplifier 101 is reflected by the partial reflection mirror 107 via the collimator lens 106 and a part of the light is transmitted. The light transmitted through the partial reflection mirror 107 is extracted as output light, and so far the configuration is the same as that of the conventional light source device shown in FIG.

  Here, as shown in FIG. 2A, the slit mirror 105 is disposed slightly inclined with respect to the optical axis, and the light collected by the condenser lens 104 is reflected at a certain angle by the slit mirror 105. Here, the slit mirror 105 functions as a wavelength selection unit.

  Further, the light reflected by the slit mirror 105 is incident again on the slit mirror 105 by the concave mirror 108.

  At this time, as shown in FIG. 2B, the light is incident on the slit mirror 105 with a slight shift so that only a part of the incident light hits the slit mirror 105.

  Here, 120 denotes a band of light dispersed according to the wavelength by the diffraction grating 104 and condensed on the rotating disk. Reference numeral 151 denotes light that is first wavelength-selected by the slit mirror 105, and 151 a denotes light that is reflected by the concave mirror 105. Reference numeral 152 denotes light that has been wavelength-selected for the second time by the slit mirror 151 in the reflected light 151a. Since the slit mirror 105 is arranged on a disk and the disk rotates, when the wavelength is selected for the second time, the position moves compared to the first time, so that a part of the reflected light 151a is partly reflected. The light is selected 152.

  Considering this in the wavelength region, the result is as shown in FIG.

  In FIG. 2 (C), the horizontal axis indicates the respective wavelength bands for the light 151 that was wavelength-selected for the first time and the light 152 that was wavelength-selected for the second time. And W2. The vertical axis represents the light intensity. Then, oscillation of the light 152 in the wavelength band where the first wavelength selection and the second wavelength selection overlap is generated.

  From FIG. 2C, it is understood that the wavelength band is narrower than the light 152 selected for the second time and the light 151 selected for the first time, and the oscillation spectrum can be narrowed. The

  Here, the center wavelength of the wavelength selection band of the first reflected light by the slit mirror 105 is λ1, the center wavelength of the second wavelength selection band is λ2, the center wavelength at the nth selection is λn, and a slit as a wavelength selection unit It is preferable that | λ1-λn | <Δλ is satisfied, where Δλ is a wavelength range in which wavelengths are selected by the mirror at a time.

  As described above, while the light emitted from the optical gain medium makes one round trip in the optical resonator, the wavelength selection unit selects the wavelength a plurality of times, and the light in the overlapping wavelength band obtained by the plurality of wavelength selections is obtained. By emitting the light, the wavelength selection width by the slit mirror 105 can be narrowed even when the slit mirror 105 is wide. For this reason, it becomes possible to achieve narrowing of the oscillation spectral line width.

<Other forms that can be adopted>
In the above, a semiconductor optical amplifier (SOA) has been described as an example of an optical gain medium. However, as an optical amplification medium, a rare earth-doped (ion-doped) optical fiber containing erbium, neodymium, or the like, or a dye in an optical fiber is used. Those that are added and amplified with a dye can be employed.

  The rare earth-doped optical fiber is suitable for obtaining good noise characteristics with high gain. In the dye-doped optical fiber, the choice of the variable wavelength is increased by appropriately selecting the fluorescent dye material or its host material.

  The semiconductor optical amplifier is preferable because it is small and can be controlled at high speed. As the semiconductor optical amplifier, both a resonator type optical amplifier and a traveling waveform optical amplifier can be used. As a material constituting the semiconductor optical amplifier, a compound semiconductor constituting a general semiconductor laser or the like can be used. Specifically, compounds such as InGaAs, InAsP, GaAlSb, GaAsP, AlGaAs, and GaN are used. A semiconductor can be mentioned. The semiconductor optical amplifier can be appropriately selected and employed from among those having a gain center wavelength of, for example, 840 nm, 1060 nm, 1300 nm, and 1550 nm according to the use of the light source.

  As an optical resonator that can be used in the present invention, a ring resonator, a σ-type resonator, or the like can be employed in addition to the linear resonator described above. Examples of the linear resonator include an optical resonator having a pair of parallel planes (a so-called Fabry-Perot resonator), and a resonator having a linear shape with the end face of the optical fiber as a mirror.

  As the ring-type resonator, in addition to a resonator using an optical fiber, a resonator using an optical system that propagates light in the air or in a vacuum using a slab waveguide or a mirror can be adopted.

  In the present invention, a dispersion element that disperses according to the wavelength of light may be a diffraction grating (transmission type, reflection type), a prism, or a combination of a diffraction grating and a prism.

  As a wavelength selection part, it can comprise using the rotary body provided with the reflection part, for example.

  The rotating body has a disc shape, and a rotating mirror including a plurality of slits functioning as reflecting portions in the periphery of the disc or a polygon mirror can be adopted.

  A wavelength selection part can also be set as the structure provided with another reflection member which reflects the light reflected in the reflection part of the rotary body toward this reflection part. Alternatively, the two reflecting portions of the rotating body may be reflection surfaces constituting the optical resonator, and a pair of dispersive elements may be provided with an optical gain medium interposed therebetween.

  Hereinafter, the present invention will be described in detail with specific examples.

  FIG. 3 shows a schematic diagram of the light source device of this embodiment.

  The semiconductor optical amplifier 301 has a gain between a wavelength of 800 nm and 880 nm, and output light from one end face of the semiconductor optical amplifier 301 is converted into parallel light by a collimator lens 302.

  The parallel light is incident on the diffraction grating 303 and is diffracted at different angles depending on the wavelength of the light. The number of grating lines of the diffraction grating 303 is 1200 lines / mm.

  The diffracted light is condensed by the condenser lens 304, and a slit mirror 305 is provided at the focal position of the condensed light. The slit width of the slit mirror 305 is 3 μm.

  The light reflected by the slit mirror 305 is reflected by the concave mirror 306 and is incident on the slit mirror 305 again.

  A large number of slit mirrors 305 are arranged at regular intervals along the circumference of the rotating disk 307. The drive unit 308 drives the rotating disk 307 along its central axis at a constant speed.

  Further, part of the light emitted from the other end face of the semiconductor optical amplifier 301 is reflected by the partial reflection mirror 310 via the collimator lens 309 and part of the light is transmitted. An external resonator is formed between the concave mirror 306 and the partial reflection mirror 310, and the resonator length is 100 mm. The light transmitted through the partial reflection mirror 310 is extracted to the outside as output light.

  The operation in this embodiment will be described below.

  The light beam emitted from the semiconductor optical amplifier 301 by driving the semiconductor optical amplifier 301 is incident on the diffraction grating 303 via the collimator lens 302. At this time, the diffracted light from the diffraction grating 303 is condensed through the condenser lens 304.

  Only the light of the wavelength component corresponding to the diffraction angle is reflected by the slit mirror 305 disposed on the rotating disk 307. At this time, the diffracted light is spatially spread on the surface of the rotating disk 307 with a size of 1 mm, and light having a wavelength width of 0.24 nm is reflected by the slit mirror 305 having a slit width of 3 μm.

  This reflected light is incident on the concave mirror 306 and is reflected in a direction inverted by 180 ° from the incident direction. The reflected light from the concave mirror 306 is incident on the slit mirror 305 again, but is arranged so that only a part of the incident light strikes the slit mirror 305.

  Therefore, only a part of the wavelength component included in the incident light is reflected again by the slit mirror 305, and the wavelength width of the reflected light is narrowed.

  In the present embodiment, when the light reflected by the concave mirror 306 is incident on the slit mirror 305 again, half of the incident light hits the slit mirror 305, so the wavelength width of the light reflected again by the slit mirror 305 is 0.12 nm. It became.

  By returning this reflected light to the semiconductor optical amplifier 301, an external resonator is formed between the concave mirror 306 and the partial reflection mirror 310 to cause laser oscillation.

  Then, according to the rotation of the rotating disk 307, light having different wavelengths reflected by the diffraction grating 303 is selected and the oscillation wavelength changes. When the gain band of the semiconductor optical amplifier 301 is exceeded, oscillation does not occur, and when the diffracted light can pass through the next slit mirror 305 by rotation, the light is reflected again by the slit mirror 305 and oscillates. Therefore, the oscillation wavelength changes in a sawtooth waveform.

  As described above, the wavelength is selected twice by the slit mirror 305 while the light makes one round trip in the resonator. At the time of the second wavelength selection, only a part of the light hits the slit mirror 305, so that only a part of the wavelength component is reflected again by the slit mirror 305, and the selected wavelength range is narrowed. Since the same effect as that reflected by the very narrow slit mirror 305 is obtained, the wavelength selection width by the slit mirror 305 can be narrowed even if the slit mirror 305 is wide. For this reason, it becomes possible to achieve narrowing of the oscillation spectral line width.

  FIG. 4 shows a schematic diagram of the light source device of this embodiment.

  The semiconductor optical amplifier 401 has a gain between a wavelength of 800 nm and 880 nm, and output light from one end face of the semiconductor optical amplifier 401 is converted into parallel light by a collimator lens 402.

  The parallel light is incident on the diffraction grating 403 and diffracted at different angles according to the wavelength of the light. The number of grating lines of the diffraction grating 403 is 1200 lines / mm.

  The diffracted light is condensed by the condenser lens 404 and a slit mirror 405 is provided at the focal position of the condensed light. The slit width of the slit mirror 405 is 3 μm.

  In addition, light emitted from the other end face of the semiconductor optical amplifier 401 is incident on the diffraction grating 407 via the collimator lens 406, and the diffracted light is condensed by the condenser lens 408.

  The number of grating lines of the diffraction grating 407 is 1200 lines / mm.

  A slit mirror 409 different from the slit mirror 405 is provided at the focal position of the condensed light. The slit width of the slit mirror 409 is 3 μm.

  A large number of slit mirrors 405 and 409 are arranged at regular intervals along the circumference of the rotating disk 410. The drive unit 411 drives the rotating disk 410 along its central axis at a constant speed.

  An external resonator is formed between the slit mirror 405 and the slit mirror 409, and the resonator length is 100 mm. The 0th-order diffracted light from the diffraction grating 403 is extracted outside as output light.

  The operation in this embodiment will be described below.

  By driving the semiconductor optical amplifier 401, a light beam emitted from one end face of the semiconductor optical amplifier 401 is incident on the diffraction grating 403 through the collimator lens 402. At this time, the diffracted light from the diffraction grating 403 is condensed via the condenser lens 404.

  Only the light of the wavelength component corresponding to the diffraction angle is reflected by the slit mirror 405 disposed on the rotating disk 410. At this time, the diffracted light is spatially spread on the surface of the rotating disk 410 with a size of 1 mm, and light having a wavelength width of 0.24 nm is reflected by the slit mirror 405 having a slit width of 3 μm.

  The reflected light is returned to the semiconductor optical amplifier 401, amplified, and then emitted from the other end face of the semiconductor optical amplifier 401. The emitted light is incident on the diffraction grating 407 via the collimator lens 406, and the diffracted light is collected by the condenser lens 408.

  The condensed light is incident on the slit mirror 409, but only a part of the incident light is disposed on the slit mirror 409. Therefore, only a part of the wavelength component included in the incident light is reflected again by the slit mirror 409, and the wavelength width of the reflected light is narrowed.

  In this embodiment, when the light is incident on the slit mirror 409, half of the incident light hits the slit mirror 409, so that the wavelength width of the reflected light by the slit mirror 409 is 0.12 nm. By returning this reflected light to the semiconductor optical amplifier 401 again, an external resonator is formed between the slit mirror 405 and the slit mirror 409, and laser oscillation occurs.

  As described above, the wavelength is selected twice by the slit mirror 405 and the slit mirror 409 while the light makes one round trip in the resonator. In the second wavelength selection, only a part of the light hits the slit mirror 409, so that only a part of the wavelength component is reflected again by the slit mirror 409, and the selected wavelength range is narrowed. Therefore, it is possible to achieve a narrowing of the oscillation spectrum line width.

  In this embodiment, an example of an optical coherence tomography apparatus using the light source of the present invention is shown.

  FIG. 5 is a schematic diagram of the OCT apparatus of this example.

  The OCT apparatus of FIG. 5 basically includes a light source unit (501 etc.), a sample measurement unit (507 etc.), a reference unit (502 etc.), an interference unit (503), a light detection unit (509 etc.), and an image processing unit. (511). Hereinafter, each component will be described.

  The light source unit includes a variable wavelength light source 501 and a light source control unit 512 that controls the variable wavelength light source. The variable wavelength light source 501 is connected to a fiber coupler 503 that forms an interference unit via an irradiation fiber 510. ing.

  The fiber coupler 503 of the interference unit is configured by a single mode in the wavelength band of the light source, and various fiber couplers are configured by 3 dB couplers.

  The reflection mirror 504 is connected to the reference light optical path fiber 502 to form a reference unit that transmits reflected light from the reference mirror, and the fiber 502 is connected to the fiber coupler 503.

  An inspection light optical path 505 fiber, an irradiation condensing optical system 506, and an irradiation position scanning mirror 507 constitute a measurement unit, and the inspection light optical path 505 fiber is connected to a fiber coupler 503. In the fiber coupler 503, the backscattered light generated from the inside and the surface of the inspection object 514 interferes with the return light from the reference unit to become interference light.

  The light detection unit includes a light receiving fiber 508 and a photodetector 509, and guides interference light generated by the fiber coupler 503 to the photodetector 509.

  The light received by the photodetector 509 is converted into a spectrum signal by the signal processing device 511, and further subjected to Fourier transform to obtain depth information of the test object. The acquired depth information is displayed on the image output monitor 513 as a tomographic image (image).

  Here, the signal processing device 511 can be configured by a personal computer or the like, and the image output monitor 513 can be configured by a display screen of a personal computer or the like.

  A characteristic of the present embodiment is a light source unit, and the wavelength tunable light source 501 is controlled by a light source control unit 512 for its oscillation wavelength and intensity and its temporal change.

  The light source control device 512 is connected to a signal processing device 511 that also controls a drive signal and the like of the irradiation position scanning mirror 507, and controls the variable wavelength light source 501 in synchronization with the drive of the scanning mirror 507.

  The wavelength tunable light source 501 using the light source device of the present invention has a narrow spectral line width during wavelength sweeping, and can acquire an interference image from a position equidistant to a position far from the reference mirror during optical coherence tomography. It becomes. A narrow spectral width of the oscillation wavelength in the wavelength sweep corresponds to a long coherence length. That is, an interference signal can be obtained even if the optical path length difference between the two optical paths constituting the interference optical system is long. That is, the OCT apparatus using the light source device of the present invention having a narrow spectral line width has an effect that it can detect a deep structure of the object to be inspected.

Claims (11)

An optical resonator comprising an optical gain medium for amplifying light;
A light source device having a dispersion element that disperses light emitted from the optical gain medium according to a wavelength, and a wavelength selection unit that reflects or transmits light of a specific wavelength that has passed through the dispersion element,
While the light emitted from the optical gain medium makes one round trip in the optical resonator, the wavelength selection unit performs wavelength selection a plurality of times, and the light in the overlapping wavelength band obtained by the plurality of wavelength selections. A light source device that emits light.   The light source device according to claim 1, wherein the wavelengths of the light selected by the plurality of wavelength selections have different center wavelengths. The wavelength range that is selected by the wavelength selector at one time is Δλ, the center wavelength of the wavelength that is initially selected by the wavelength selector is λ1, and the center wavelength when n times is selected is λn,

The light source device according to claim 2, wherein:   4. The light source device according to claim 1, wherein the dispersive element is a diffraction grating.   5. The light source device according to claim 1, wherein the wavelength selection unit uses a rotating body including a reflection unit.   The light source device according to claim 5, wherein the rotating body has a disk shape and includes a plurality of slits in a peripheral portion of the disk.   The light source device according to claim 5, wherein the rotating body is a polygon mirror.   The light source device according to claim 6, wherein the slit functions as a reflection unit.   The light source device according to claim 5, wherein the wavelength selection unit includes another reflection member that reflects the light reflected by the reflection unit of the rotating body toward the reflection unit.   The light source device according to claim 6, further comprising: a pair of dispersive elements sandwiching the optical gain medium with the two reflecting portions of the rotating body as reflecting surfaces constituting the resonator. A light source unit using the light source device according to any one of claims 1 to 9,
A sample measurement unit that irradiates the sample with light from the light source unit and transmits reflected light from the sample;
A reference unit for irradiating a reference mirror with light from the light source unit and transmitting reflected light from the reference mirror;
An interference unit that causes reflected light from the specimen measurement unit and reflected light from the reference unit to interfere with each other;
A light detection unit for detecting interference light from the interference unit;
An image processing unit that obtains a tomographic image of the specimen based on the light detected by the light detection unit;
An optical coherence tomographic imaging apparatus comprising:

Images