JP2013182915A - Light source device and optical interference tomographic imaging apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that it is difficult for a conventional variable wavelength light source to perform high-speed wavelength sweeping because wavelength is changed by causing reciprocating motion of a Fabry-Perot filter.SOLUTION: A light source device includes: a member for emitting light having a wavelength width; a resonator that amplifies the light emitted from the member; and a wavelength selection element that exists including a part of an optical path in the resonator. The wavelength selection element changes an optical path length of the optical path by rotation thereof.

Description

  The present invention relates to a light source device and an optical coherence tomography apparatus using the light source device.

  A light source capable of changing an emitted wavelength (hereinafter sometimes abbreviated as a wavelength tunable light source) is used in various fields.

Applications of the wavelength tunable light source in the inspection apparatus include a laser spectrometer, a dispersion measuring instrument, a film thickness measuring instrument, and an optical coherence tomography apparatus (hereinafter sometimes referred to as an OCT apparatus).
Etc.

  The OCT apparatus irradiates light on an object, continuously changes the wavelength of the irradiated light, and causes interference between reference light and reflected light returning from different depths of the object. Then, a tomographic image of the object is obtained by analyzing the frequency component included in the time waveform of the intensity of the interference light.

An OCT apparatus using a wavelength tunable light source (Swept Source OCT apparatus, hereinafter sometimes referred to as SS-OCT apparatus)
Since a spectroscope is not used, it is expected to obtain a tomogram with a high SN ratio with little loss of light. When using the SS-OCT apparatus, the faster the wavelength sweep speed of the wavelength variable light source, the shorter the acquisition time of the tomographic image, and the easier it is to observe the living tissue as it is in the living body. Patent Document 1 discloses a wavelength variable light source having a semiconductor gain medium and a wavelength selection element. The wavelength tunable light source disclosed in Patent Document 1 can selectively emit light having a desired wavelength by using a Fabry-Perot filter cavity as a wavelength selection element and controlling the optical path length of the cavity.

US Pat. No. 6,339,603

  Here, in the wavelength tunable light source disclosed in Patent Document 1, since the control of the optical path length of the cavity is performed by reciprocating the Fabry-Perot filter, it is difficult to change the optical path length at a high speed. Sweep was difficult.

  A light source device according to the present invention includes a member that emits light having a wavelength width, a resonator that amplifies the light emitted from the member, and a wavelength selection element that includes a part of the optical path in the resonator. And the wavelength selection element changes an optical path length of the optical path by rotating.

  Since the light source device according to the present invention only needs to rotate the wavelength selection element in order to change the wavelength to be selected, the wavelength to be selected can be changed at a high speed, and as a result, the wavelength can be swept at a high speed. it can.

It is the figure which showed the light source device which concerns on Embodiment 1 of this invention. It is the figure which showed an example of the wavelength selection element in embodiment of this invention. It is the figure which showed the other example of the wavelength selection element in embodiment of this invention. It is the figure which showed the further another example of the wavelength selection element in embodiment of this invention. It is the figure which showed the light source device which concerns on Embodiment 2 of this invention. It is the figure which showed the light source device which concerns on Embodiment 3 of this invention. It is the figure which showed the optical coherence tomography apparatus concerning Embodiment 4 of this invention. It is the figure which showed the wavelength selection element in Example 4 of this invention. It is the figure which showed the wavelength selection element in Example 5 of this invention.

  Hereinafter, although embodiment of this invention is described, this invention is not limited to these.

  A light source device according to an embodiment of the present invention includes a member that emits light having a wavelength width, a resonator that amplifies light emitted from the member, and a wavelength that includes a part of an optical path in the resonator. And a selection element. The wavelength selection element rotates to change an optical optical path length of the optical path. When the optical path length of the optical path of the resonator changes, the wavelength of the light emitted from the light source device changes. Therefore, the light source device according to the embodiment of the present invention can be used as a wavelength tunable light source. In addition, the light source device according to the embodiment of the present invention can change the wavelength of the resonator at high speed because the optical path length of the resonator is changed by rotational movement in one direction.

  In addition, the light source device according to the embodiment of the present invention changes the wavelength by wavelength-dispersing white light using a diffraction grating and irradiating the wavelength-dispersed light on a striped reflection mirror provided on a rotating disk. The optical path in the resonator can be shortened compared to the light source device to be made. As a result, the number of round trips (number of laps) of light traveling back and forth between the resonators can be increased, so that the coherence distance can be increased.

  Hereinafter, several forms of the light source device according to the embodiment of the present invention will be specifically described.

(Embodiment 1)
The light source device according to Embodiment 1 will be described with reference to FIG. FIG. 1 is a cross-sectional view obtained by cutting along a plane including a direction in which light is emitted (X-axis direction) in the light source device according to the present embodiment.

  The light source device according to the present embodiment includes a member 101 that emits light having a wavelength width, resonators (102, 103) that amplify light emitted from the member 101, and one of the optical paths of the resonators (102, 103). And a wavelength selection element 104 that includes the portion. Then, the wavelength selection element 104 rotates about the direction parallel to the optical path of the resonator (102, 103) (X′-axis direction) as an axis, and the optical path length of the optical path of the resonator (102, 103) by rotating. It is characterized by changing.

  Here, the direction parallel to the optical path of the resonator (102, 103) is defined as a range from −5 ° to + 5 ° from the optical path of the resonator (102, 103).

  The resonators (102, 103) are constituted by two reflectors (102, 103) as shown in FIG. 1, and light is amplified by reciprocating between these two reflectors (L1). At least one of these two reflectors can transmit light (L2, L3). In FIG. 1, L indicates the direction of light.

  As the wavelength selection element 104 rotates, the optical path length of the optical path of the resonator (102, 103) changes, so that the wavelength of light selected by the wavelength selection element 104 changes. The light of the wavelength selected by the wavelength selection element 104 is transmitted through at least one of the two reflectors (102, 103) and emitted. That is, as the wavelength selection element 104 rotates, the wavelength of light (at least one of L2 and L3) emitted from the light source device also changes.

  Thus, since the light source device according to the present embodiment changes the wavelength of the emitted light by rotating the wavelength selection element 104, the wavelength of the emitted light can be changed at high speed.

  In the light source device according to the present embodiment, a collimator lens 105 that collimates light may be provided between the member 101 that emits light having a wavelength width and one of the resonators 102, or the wavelength width. A condensing lens 106 for condensing light may be provided between the member 101 that emits light having the wavelength and the wavelength selection element 104. By providing the condensing lens 106, it is possible to condense in a narrow region of the wavelength selection element 104, which is preferable because the wavelength selectivity is high (FIG. 1).

(Wavelength selection element)
The wavelength selection element according to the first embodiment is a rotating body that rotates about a direction parallel to the optical path of the resonator (102, 103). Then, the wavelength of the light to be selected is changed by changing the optical path length of the optical path of the resonator (102, 103) by rotating the rotating body. In other words, the light source device according to the first embodiment includes a wavelength selection element and a rotation mechanism that rotates the wavelength selection element. Further, the optical path length of the wavelength selection element is not uniform in the circumferential shape passing through the portion existing in the optical path of the wavelength selection element with the rotation axis of the wavelength selection element as the center, that is, at least a part in the circumferential direction Can be said to be different.

  Examples of the wavelength selection element in the present embodiment include the following three forms.

(Example 1 of wavelength selection element)
An example of the wavelength selection element according to the present embodiment will be described with reference to FIG. 2A is a diagram showing a part of the wavelength selection element in FIG. The right side of FIG. 2A shows the wavelength selection when the wavelength selection element 104 in FIG. 1 is viewed in the X′-axis direction from 103 constituting a part of the optical resonator. It is the figure which showed a part of element. As shown in FIG. 2A, the wavelength selection element according to this example rotates around a rotation axis indicated by the center point of the X mark (the depth direction of the paper surface is the X ′ axis direction at the center point of the X mark). It has a disk 201. The optical part of the circumference of the disk 201 and the part existing in the optical path of the resonator when the center point of the mark is centered is not uniform in the circumferential direction, that is, in the circumferential direction However, at least a part of the optical path length is different. This circumferential portion exists in the optical path of the resonator. Note that the rotating body does not have to be a disk shape as long as it rotates, and a rotating body having an arbitrary shape can be used. In addition, an antireflection film may be formed on the main surface of the disk in the optical path of the resonator, if necessary, or antireflection processing may be performed. By forming the antireflection film or performing the antireflection treatment, it is possible to suppress the disturbance of the wavelength of the light swept by the light source device in the present embodiment.

  In the wavelength selection element described in this example, the wavelength selection element 104 shown on the right side of FIG. 2A toward the paper surface is shown in FIG. 2B when viewed from the B direction (upward toward the paper surface). It has a structure like this. That is, Fabry-Perot filters 202, 203, 204... Having an inclination from A to A ′ are provided. Here, the Fabry-Perot filter is a structure in which two half mirrors (a mirror that transmits part of light and reflects part of the light, the same applies hereinafter) face each other through an air gap, or optically This structure is provided on both sides of the transparent member, and is the same unless otherwise specified.

  Here, the Fabry-Perot filter 203 exists including a part of the optical path of the light (L4) amplified by the resonators (102, 103). Of the light irradiated to the Fabry-Perot filter 203, the wavelength corresponding to the position irradiated to the Fabry-Perot filter is selected. Specifically, the light (L5) irradiated to the position where the thickness of the Fabry-Perot filter is small has a short optical optical path length in the resonators (102, 103), and therefore the selected wavelength is short. On the contrary, the light (L6) irradiated to the position where the thickness of the Fabry-Perot filter is large has a long optical optical path length in the resonators (102, 103), and therefore the selected wavelength is long.

  Therefore, when the disc 201 rotates in a direction from A ′ to A (in the direction of the black arrow in FIG. 2B) in a state where the Fabry-Perot filter 203 is irradiated with light, the selected wavelength is changed from a short wavelength to a long wavelength. Change to wavelength. Then, the light irradiated to the position where the Fabry-Perot filter 203 has the largest thickness is then irradiated to the position where the Fabry-Perot filter 204 has the smallest thickness, and the short-wavelength light is selected again, and the wavelength gradually increases. The light of will be selected.

  In this way, by using the wavelength selection element 104 as shown in this example, light having a short wavelength to a long wavelength is selected, and thus the light source device according to this embodiment can be used as a wavelength tunable light source.

  In this example, the Fabry-Perot filter is provided on the 103 side among the planes of the disk, but may be provided on the 102 side plane. Or you may provide in the inside of a disk, without providing in the surface of a disk.

  In this example, the configuration in which Fabry-Perot filters 202, 203, 204,... Are provided on the disc 201 is not particularly limited as long as the optical path length varies depending on the thickness. An example of the Fabry-Perot filter is a Fabry-Perot etalon.

(Example 2 of wavelength selection element)
Another example of the wavelength selection element according to this embodiment will be described with reference to FIG. In the wavelength selection element described in this example, the wavelength selection element 104 shown on the right side of FIG. 2A toward the paper surface is shown in FIG. 3A when viewed from the B direction (upward toward the paper surface). It has a structure like this. That is, Fabry-Perot filters 301, 302, 303,... Having a refractive index gradient from A to A ′ are provided. Of the Fabry-Perot filters, the refractive index is smaller on the A side and the refractive index is larger on the A ′ side.

Also in the wavelength selection element of this example, the wavelength according to the position irradiated to the Fabry-Perot filter among the lights irradiated to the Fabry-Perot filter 302 is selected. Specifically, the light (L5) irradiated to the position where the refractive index of the Fabry-Perot filter is small (n ₀ −Δn (Δn> 0)) has a short optical optical path length in the resonator (102, 103). The wavelength selected is short. On the contrary, the light (L6) irradiated to the position where the refractive index of the Fabry-Perot filter is large (n ₀ ) has a long optical optical path length in the resonators (102, 103), and therefore the selected wavelength is long.

  Therefore, in the same manner as in Example 1, when the Fabry-Perot filter 302 is irradiated with light, the disc 201 is selected when it rotates in the direction from A ′ to A (the direction of the black arrow in FIG. 2B). The wavelength changes from a short wavelength to a long wavelength. Then, the light irradiated to the position where the refractive index of the Fabry-Perot filter 302 is the highest is next irradiated to the position where the refractive index of the Fabry-Perot filter 303 is the lowest, and light having a short wavelength is selected again, and gradually Long wavelength light is selected.

  In this way, by using the wavelength selection element 104 as shown in this example, light having a short wavelength to a long wavelength is selected, and thus the light source device according to this embodiment can be used as a wavelength tunable light source. In this example, the Fabry-Perot filter is provided on the 103 side among the planes of the disk, but may be provided on the 102 side plane. Or you may provide in the inside of a disk, without providing in the surface of a disk.

  In this example, the configuration in which Fabry-Perot filters 301, 302, 303,... Are provided on the disc 201 is not particularly limited as long as the optical path length varies depending on the thickness. An example of the Fabry-Perot filter is a Fabry-Perot etalon.

(Example 3 of wavelength selection element)
Still another example of the wavelength selection element according to the present embodiment will be described with reference to FIG. In the wavelength selection element described in this example, when the wavelength selection element 104 shown on the right side of FIG. 2A is viewed from the B direction (upward toward the page), the structure as shown in FIG. It has become. That is, Fabry-Perot filters 401, 402, 403,... Having a refractive index gradient from A to A ′ are provided. The Fabry-Perot filter used in this example has a structure having grooves, and the pitch of the grooves is wide in the direction in which the wavelength selection element rotates. That is, the groove pitch is wider toward the A side, and the groove pitch is narrower toward the A ′ side. Therefore, when the refractive index of the material constituting the Fabry-Perot filter is larger than the refractive index in air, the refractive index is smaller on the A side of the Fabry-Perot filter (n ₀ −Δn (Δn> 0)), and A ′ The refractive index increases toward the side (n ₀ ). Note that the refractive index shown here is not the refractive index at a specific position of the Fabry-Perot filter, but the average refractive index in the region irradiated with light.

Also in the wavelength selection element of this example, the wavelength according to the position irradiated to the Fabry-Perot filter among the lights irradiated to the Fabry-Perot filter 402 is selected. Specifically, the light (L5) irradiated to the position where the refractive index of the Fabry-Perot filter is small (n ₀ −Δn (Δn> 0)) has a short optical optical path length in the resonator (102, 103). The wavelength selected is short. On the contrary, the light (L6) irradiated to the position where the refractive index of the Fabry-Perot filter is large (n ₀ ) has a long optical optical path length in the resonators (102, 103), and therefore the selected wavelength is long.

  Accordingly, in the same manner as in Example 1, when the Fabry-Perot filter 402 is irradiated with light and the disc 201 rotates in the direction from A ′ to A (the direction of the black arrow in FIG. 2B), it is selected. The wavelength changes from a short wavelength to a long wavelength. Then, the light irradiated to the position where the refractive index of the Fabry-Perot filter 402 is the highest is next irradiated to the position where the refractive index of the Fabry-Perot filter 403 is the lowest, and light having a short wavelength is selected again, and gradually Long wavelength light is selected.

  In this way, by using the wavelength selection element 104 as shown in this example, light having a short wavelength to a long wavelength is selected, and thus the light source device according to this embodiment can be used as a wavelength tunable light source. In this example, the Fabry-Perot filter is provided on the 103 side among the planes of the disk, but may be provided on the 102 side plane. Or you may provide in the inside of a disk, without providing in the surface of a disk.

  In this example, the configuration in which Fabry-Perot filters 401, 402, 403,... Are provided on the disc 201 is not particularly limited as long as the optical path length varies depending on the thickness. An example of the Fabry-Perot filter is a Fabry-Perot etalon.

(Member emitting light having a wavelength width)
The member that emits light having a wavelength width in the present embodiment is not particularly limited as long as it is not a member that emits monochromatic light but a member that emits light having a wavelength width.

  It is preferable to give a predetermined angle to the antireflection film and the light traveling direction on both end surfaces so that the both end surfaces in the light emitting direction of the light emitting member do not function as a resonator.

  In the present embodiment, the member that emits light having a wavelength width is preferably a member that emits at least light having a wavelength of 800 nm to 1350 nm, and more preferably 800 nm to 900 nm, or 1250 nm to 1350 nm. The emitted light is preferably a coherent light beam.

  A typical example of a member that emits light having a wavelength width is a semiconductor optical amplifier (hereinafter sometimes abbreviated as SOA). In addition, a rare earth-doped (ion-doped) optical fiber containing erbium, neodymium, or the like, an optical fiber added with a dye, and amplified by the dye can be used.

  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. As a member that emits light having a wavelength width in this embodiment, an SOA that is small and capable of high-speed control is preferable. As the SOA, both a resonator type optical amplifier and a traveling waveform optical amplifier can be used. As a material constituting the SOA, a compound semiconductor constituting a general semiconductor laser can be used. Specifically, a compound semiconductor such as InGaAs, InAsP, GaAlSb, GaAsP, AlGaAs, or GaN can be used. Can be mentioned. The SOA 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.

(Embodiment 2)
A light source device according to Embodiment 2 will be described with reference to FIG. In the second embodiment, differences from the first embodiment will be described, and description of the same points will be omitted.

  In the light source device according to the second embodiment, the rotation axis of the wavelength selection element 104 in the light source device according to the first embodiment is θ ° (−5 ° ≦ θ <0 ° from the optical path of the resonator (102, 103), or (0 ° <θ ≦ + 5 ° or less) in a tilted form (FIG. 5A). With this configuration, the light amplified by the resonator (102, 103) forms a resonator between one of the resonators (102 or 103) and the wavelength selection element 104. This can be suppressed. Further, by providing the aperture stop 107 between the condenser lens 106 and the wavelength selection element 104, the reflected light from the reflection mirror 103 constituting the resonator returns to the member 101 that emits light having a wavelength width. Can be suppressed.

(Embodiment 3)
A light source device according to Embodiment 3 will be described with reference to FIG. In the third embodiment, differences from the first embodiment will be described, and description of the same points will be omitted.

  In the light source device according to the third embodiment, the wavelength selection element 104 ′ has an axis in an arbitrary direction in a plane perpendicular to the X axis as shown in FIG. 6A (in FIG. 6A, the depth of the paper surface). Rotate as direction (shown as Y axis). An enlarged view of the wavelength selection element 104 'is shown in FIG. The wavelength selection element 104 ′ is a polygon mirror 601, and a Fabry-Perot filter 602 is provided on each surface. Further, a reflection mirror 103 ′ is provided between the Fabry-Perot filter 602 and the surface of the polygon mirror 601 so as to form a resonator together with 102. The reflection mirror 103 ′ may be a part of the Fabry-Perot filter 602 or a part of the polygon mirror 601. Further, the Fabry-Perot filter 601 has a smaller thickness in the rotating direction, that is, from C to C ′.

  At this time, since the optical path length of the optical path of the resonator (102, 103 ') changes due to the rotation of the wavelength selection element 104', the wavelength of the light selected by the wavelength selection element 104 'changes. The light of the wavelength selected by the wavelength selection element 104 'is transmitted through at least one of the two reflectors (102, 103') and emitted. That is, as the wavelength selection element 104 'rotates, the wavelength of the light (L2') emitted from the light source device also changes.

  As described above, in the light source device according to the present embodiment, the wavelength of the emitted light is changed by rotating the wavelength selection element 104 ′ so that the wavelength of the emitted light is changed at a high speed as in the other embodiments. Can be changed.

(Embodiment 4)
In Embodiment 4, an optical coherence tomographic imaging apparatus (hereinafter abbreviated as an OCT apparatus) using the light source apparatus according to the present invention described above will be described with reference to FIG.

  The OCT apparatus according to the present embodiment includes at least a light source device 701, an interference optical system 702, a light detection unit 703, and an information acquisition unit 704. The light source device 701 is the light source device according to the present invention described above. is there. Although not shown, the information acquisition unit 704 has a Fourier transformer. Here, the information acquisition unit 704 has a Fourier transformer, and the form is not particularly limited as long as the information acquisition unit has a function of performing Fourier transform on the input data. An example is a case where the information acquisition unit 704 includes a calculation unit, and the calculation unit has a function of performing Fourier transform. Specifically, this is a case where the arithmetic unit is a computer having a CPU, and this computer incorporates an application having a Fourier transform function. Another example is a case where the information acquisition unit 104 includes a Fourier transform circuit having a Fourier transform function. The light emitted from the light source device 701 passes through the interference optical system 702 and is output as interference light having information on the object 712 to be measured. The interference light is received by the light detection unit 703. The light detection unit 703 may be a differential detection type or a simple intensity monitor type. Information on the time waveform of the intensity of the received interference light is sent from the light detection unit 703 to the information acquisition unit 704. The information acquisition unit 704 acquires the peak value of the time waveform of the intensity of the received interference light, performs Fourier transform, and acquires information on the object 712 (for example, information on a tomographic image). Note that the light source device 701, the interference optical system 702, the light detection unit 703, and the information acquisition unit 704 can be arbitrarily provided.

  Hereinafter, a detailed description will be given of the process from when light is oscillated from the light source device 701 until information on a tomographic image of an object to be measured is obtained.

  The light emitted from the light source device 701 that changes the wavelength of the light passes through the fiber 705, enters the coupler 706, and is irradiated into the irradiation light through the fiber 707 for irradiation light and the reference light through the fiber 708 for reference light. Branch off. Irradiation light passes through the collimator 709 to become parallel light and is reflected by the mirror 710. The light reflected by the mirror 710 is irradiated on the object 712 through the lens 711 and reflected from each layer in the depth direction of the object 712. On the other hand, the reference light is reflected by the mirror 714 through the collimator 713. In the coupler 706, interference light is generated by reflected light from the object 712 and reflected light from the mirror 714. The interfered light passes through the fiber 715, is collected through the collimator 716, and is received by the light detection unit 703. Information on the intensity of the interference light received by the light detection unit 703 is converted into electrical information such as a voltage and sent to the information acquisition unit 704. The information acquisition unit 704 obtains tomographic image information by performing Fourier transform on the intensity data of the interference light. The intensity data of the interference light to be Fourier transformed is usually data sampled at equal wave intervals, but it is also possible to use data sampled at equal wavelength intervals.

  The obtained tomographic image information may be sent from the information acquisition unit 704 to the image display unit 717 and displayed as an image. A three-dimensional tomographic image of the object 712 to be measured can be obtained by scanning the mirror 711 in a plane perpendicular to the direction in which the irradiation light is incident. The information acquisition unit 704 may control the light source device 701 via the electric circuit 718. Although not shown, the intensity of light emitted from the light source device 701 may be monitored successively, and the data may be used to correct the amplitude of the interference light intensity signal.

  Next, in the OCT apparatus according to the present embodiment, the wavelength width of the light oscillated by the light source device 701 (hereinafter also referred to as a wavelength sweep width) and the resolution in the depth direction of the object 712 to be observed. The wavelength interval of each data sampled at the above equal wavelength interval (hereinafter also referred to as sampling wavelength interval) will be described. Also, a measurable range in the depth direction when measurement is performed to obtain a tomographic image of the object 712 using the OCT apparatus according to the present embodiment will be described.

  First, the sampling wavelength interval δλ and the maximum value L of the measurable range in the depth direction of the object 712 that can be detected by the OCT apparatus according to this embodiment have the following relationship in principle.

  Here, the center wavelength of the wavelength sweep width oscillated by the light source device 701 (for example, when the wavelength sweep width is 100 nm from 800 nm to 900 nm, the center wavelength is 850 nm) and the wavelength sweep width are λo and Δλ, respectively. And At this time, the resolution δL in the depth direction of the object 712 that can be measured by the OCT apparatus according to the present embodiment is expressed by Expression (2).

If indicated by the embodiment, the maximum value L in the depth direction of the measurable object 712, the central wavelength lambda ₀ 1.15 .mu.m, the sampling wavelength interval δλ and 0.15nm by formula (1), L = 8. 8 mm.

  Further, the resolution δL in the depth direction of the measurable object 712 is δL = 5.5 μm when the wavelength sweep width Δλ is 0.12 μm.

  When obtaining a tomographic image using the SS-OCT apparatus, the time for obtaining the tomographic image can be shortened as the wavelength sweeping speed by the light source device increases. Two-dimensional (z-x plane) tomographic image when the wavelength sweep speed by the light source device is 100 KHz and 1000 measurement points are taken in the direction (x direction) perpendicular to the depth direction (z direction) of the object. It takes 0.01 seconds to obtain Further, when taking 1000 measurement points in the z direction and the direction (y direction) perpendicular to the x direction, it takes 10 seconds to obtain a three-dimensional (xyz space) tomographic image. The higher the wavelength sweep speed, the faster the two-dimensional or three-dimensional tomographic image can be obtained. One means for increasing the wavelength sweep speed of the light source device is to shorten the wavelength selection period by the wavelength selection element.

  In Patent Document 1, the wavelength selection is performed by the reciprocating motion of the wavelength selection element, whereas in the present invention, the wavelength selection is performed by the rotational motion. Therefore, the wavelength selection cycle can be shortened. Further, in order to increase the number of times the light reciprocates between the two reflectors constituting the above resonator (between 102 and 103 or between 102 and 103 ′) (number of round trips (number of laps)). By shortening the distance between the two reflectors, the wavelength sweep speed of the light source device can be increased.

  Here, Japanese Patent Laid-Open No. 2008-98395 (Patent Document 2) discloses a wavelength tunable light source that changes the wavelength by rotating a disk. Since the light source device disclosed in Patent Document 2 uses a diffraction grating as a wavelength selection element, it is necessary to provide a design that allows a diffraction angle and a condenser lens. At this time, if the wavelength sweep speed is 200 KHz and the number of round trips (the number of rounds) is 10, the distance between the two reflectors must be 7.5 centimeters or less, which is difficult to realize.

(Use)
The light source device according to the present embodiment is suitably used for a laser spectrometer, a dispersion measuring device, a film thickness measuring device, an SS-OCT device, and the like. It is also used in the communication network field.

  Hereinafter, although the light source device which concerns on the Example of this invention is demonstrated, this invention is not limited to these.

Example 1
In Example 1, an example of a light source device according to the present invention will be described with reference to FIGS. The light source device according to Example 1 has the same device configuration as that described in (Example 1 of wavelength selection element) in the first embodiment. That is, the light emitted from the member 101 that emits light having a wavelength width is amplified by the resonators (102, 103), and the wavelength of the light emitted from the light source device changes as the wavelength selection element 104 rotates. To do. A Fabry-Perot filter 203 constituting a part of the wavelength selection element 104 includes a half mirror 210, a half mirror 211, and a transparent member 212. A plurality of Fabry-Perot filters are provided on the disk and on the circumference (portion of radius r with the rotation axis as the center) existing in the optical path of the resonator (102, 103) (FIG. 2A). ). In FIG. 2A, three Fabry-Perot filters (203, 203, 204) are shown, but many Fabry-Perot filters are arranged on the radius r. Only the Fabry-Perot filter 203 will be described below, but the same applies to other Fabry-Perot filters including 202 and 204.

Then, as shown in FIG. 2 (b), the thickness of the Fabry-Perot filter 203 in the direction in which light is irradiated, a l ₀ at the center (indicated by a dashed line), and most large thickness portion The difference from the portion with the smallest thickness is Δl.

Here, the transmission wavelength of light when the light is irradiated onto the portion where the thickness of Fabry-Perot filter 203 is ₁₀ is λo. The refractive index of the transparent member 211 is n. At this time, since the optical path length between the half mirror 210 and the half mirror 211 is nl ₀ , the wave number N of the light existing between the half mirror 210 and the half mirror 211 is expressed by Expression (3).

When the disk 201 rotates, the distance between the half mirror 210 and the half mirror 211 changes with respect to the light irradiated on the Fabry-Perot filter 203. Here, the Fabry-Perot filter 203 is moved in the direction of the arrow, the distance between the half mirror 210 and the half mirror 211 is to be changed to l _{0 +} .DELTA.l from l _0. At this time, the transmitted wavelength is λo + δλ. At this time, it is assumed that the wave number N does not change. Then, the following equation (4) is established by fully differentiating the equation (3) by δl and δλ.

  By changing Expression (4), a change δλ of the transmission wavelength of the Fabry-Perot filter 203 is expressed as Expression (5).

  Since the change amount δl is proportional to the rotation angle of the disk 201, when the disk 201 rotates, the Fabry-Perot filter 203 moves by its width d, so that one wavelength sweep is realized. When the disk 201 further rotates, wavelength sweeping can be similarly performed in the adjacent Fabry-Perot filter 204.

Here, l ₀ = 4 μm and the initial wavelength λo = 0.84 μm. From the wavelength change δλ and the change δl between the half mirror 210 and the half mirror 211, the above equation (5) is expressed as the following equation (6).

  As will be described later, in the OCT apparatus using the light source device according to the present invention, the coherent length required when the observation depth of the object to be observed is 4 mm is 8 mm, and thus the light source device needs to sweep. The wavelength width is 0.08 nm. Further, it is assumed that this wavelength width is the wavelength resolution of the light source device that sweeps the wavelength, that is, the amount of wavelength change. The interval δl corresponding to this wavelength change amount is 0.38 nm from the equation (6).

Here, if the width d of the Fabry-Perot filter 203 is 500 μm and Δl = 0.4 μm, the moving amount δx of the disk 201 is 0.475 μm.
In order to be able to move 0.475 μm, the radius r of the disk 201 is 31.75 mm. It can be seen from this that the wavelength resolution is 0.08 nm at the rotation angle δθ = 14.96 μrad. It can also be seen that the sweep wavelength width is 80 nm at a rotation angle Δθ = 15.7 mrad.

  Here, about 400 Fabry-Perot filters can be provided on the circumference of the disc 201 having a radius r = 31.75 mm. From this, it is understood that a wavelength sweep speed of 200 KHz can be realized by rotating the disk at a speed of 30000 rpm. Further, if the focal length of the condensing lens 106 is set to about 3 mm, the distance between 102 and 103 constituting the resonator can be within 1.5 cm. Therefore, since the number of round trips is 50, the light source device has a long coherent distance.

  As described above, by using the light source device according to the present embodiment, an OCT device having an observation depth of 4 mm (coherent length 8 mm), a depth resolution of 3 μm, and a wavelength sweep rate of 200 KHz is used. Can be realized.

(Example 2)
In Example 2, another example of the light source device according to the present invention will be described with reference to FIG. The light source device according to Example 2 has the same device configuration as that described in (Example 2 of wavelength selection element) in the first embodiment. The difference from the first embodiment is only the Fabry-Perot filter of the wavelength selection element. Specifically, the Fabry-Perot filter has a refractive index distribution as shown in FIG. Hereinafter, the Fabry-Perot filter in this embodiment will be described with reference to 302, but the same applies to the Fabry-Perot filters 301 and 303.

  The Fabry-Perot filter 302 includes a half mirror 310, a half mirror 311, and a transparent member 312.

The sense 1 between the half mirror 310 and the half mirror 311 is about 4 μm. Then, the refractive index of the transparent member 312 decreases toward the rotation direction of the disk 201. The refractive index n ₀ of the portion with the highest refractive index (the boundary portion between 302 and 303) is 1.5, and the refractive index n ₀ −Δn of the portion with the lowest refractive index (the boundary portion between 301 and 302) is Δn = If 0.1, then 1.4. Therefore, the optical optical path length difference Δn · l is 0.4 μm between the case of passing through the portion with the highest refractive index and the case of passing through the portion with the lowest refractive index. A method for providing the transparent member 312 with a refractive index distribution is not particularly limited, and examples thereof include an ion exchange method and a photochemical method.

  The Fabry-Perot filter in the second embodiment has less lateral shift of the light beam due to multiple reflection at the half mirror 310 and the half mirror 311 compared to the Fabry-Perot filter in the first embodiment, and light is transmitted between the half mirror 310 and the half mirror 311. Easy to confine. As a result, the coherent performance is improved, the filter characteristic is good, and the object to be observed can be observed deeper.

(Example 3)
In Example 3, still another example of the light source device according to the present invention will be described with reference to FIG.

  The light source device according to Example 3 has the same device configuration as that described in (Example 3 of wavelength selection element) in the first embodiment. The difference from the first embodiment is only the Fabry-Perot filter of the wavelength selection element. Specifically, the Fabry-Perot filter has a refractive index distribution as shown in FIG. Also, the difference from the second embodiment is that the Fabry-Perot filter has a sub-wavelength size smaller than or equal to the wavelength of the light in the resonator in order to give the Fabry-Perot filter a refractive index distribution. This is a point realized by a simple structure.

  Hereinafter, the Fabry-Perot filter in the present embodiment will be described by using 402 as a representative, but the same applies to the Fabry-Perot filters 401 and 403.

  The Fabry-Perot filter 402 in this embodiment has a line and space structure as shown in FIG. In this example, the ratio of the line width Q to the pitch width P (line width + space width) in the drawing, called a filling factor, Q / P is 10% smaller in the rotation direction. That is, in Fabry-Perot filter 402, Q / P (MIN) at the boundary between 402 and 401 is 0.9 times Q / P (MAX) with respect to Q / P (MAX) at the boundary between 402 and 403. . In order to prevent diffracted light from being generated in Fabry-Perot filter 402, it is desirable that pitch width P be equal to or less than the wavelength used (for example, 0.8 μm).

  Since this embodiment has a refractive index distribution in the patterning shape of the Fabry-Perot filter, there is an effect that it is stable over a long period of time with less change over time as compared with the second embodiment.

Example 4
In Example 4, in the light source device according to the present invention, as shown in FIG. 5, the wavelength selection element 104 is inclined by θ ° with respect to the optical path (X direction or X ′ direction) of the resonator. The wavelength selection element 104 rotates around an axis inclined by θ ° with respect to the X direction or the X ′ direction.

  If the diameter of the beam waist of the condensing lens 106 is 10 μm, the divergence angle is a half angle of 3.1 °. Therefore, if θ is set to 3.1 ° or more, the reflected light is blocked by the aperture stop 107 and can be prevented from returning to the member that emits light having a wavelength width. Here, it will be described with reference to FIG. 8 that a necessary coherent length can be obtained even when the wavelength selection element is inclined by θ °. FIG. 8 is an enlarged view of a part of the wavelength selection element of FIG.

As described above, light is laterally shifted in the Fabry-Perot filter by the wavelength selection element 104, and this lateral shift amount becomes δx. Here, when θ = 3.1 °, if the refractive index of the Fabry-Perot filter 801 is 1.5, φ = 2.1 ° when the refraction angle is φ, and the interval l between the Fabry-Perot filters 104 is 1 _{If 0} is 4 μm, the lateral displacement amount δx is 0.29 μm. Thereby, it is possible to reciprocate about 30 times before the spot diameter is deviated from 10 μm due to multiple reflection. This number is within a necessary number to realize a coherent length of 8 mm. Therefore, even if the wavelength selection element 104 is tilted by 3.1 ° with respect to the optical path of the resonator, a necessary coherent length can be obtained.

(Example 5)
A fifth embodiment, which is a modification of the fourth embodiment, will be described with reference to FIGS. The light source device according to the fifth embodiment differs from the wavelength selection element 104 in the light source device according to the fourth embodiment in the structure of the Fabry-Perot filter. The Fabry-Perot filter in Example 5 can increase the number of reflections on both sides of the Fabry-Perot filter and further improve the coherent length. As shown in FIG. 9A, the surface (903) of the Fabry-Perot filter 901 that does not contact the disk 201 has a curvature. By setting the surface to an appropriate curvature, the surface of the Fabry-Perot filter 901 that makes contact with the disk 201 (902) and the surface that does not make contact (903) do not resonate and do not shift laterally. FIG. 9B is a diagram viewed from the A direction in the upper right diagram of FIG. In FIG. 9B, d is the rotation direction of the disk 201. As shown in FIG. 9B, it has a wedge shape. Since the wedge has an inclination of about 1/1000 from the parallel surface, the lateral shift in this direction is small, and the number of reflections on both surfaces is 6000 times or more. Since the light source device according to this embodiment has a large coherent length, the OCT device using the light source device according to this embodiment can obtain a high-quality OCT image up to a deeper position of the object to be observed.

(Comparative example)
Here, the variable laser shown in FIG. 10 of Patent Document 1 will be described. The two reflectors constituting the Fabry-Perot cavity 410 in the tunable laser shown in FIG. 10 are 15 μm to 25 μm when the free spectral range (hereinafter referred to as FSR) is set to 45 to 80 nm. Is disclosed, and a wavelength sweep speed of 100 KHz or more can be realized. Here, FSR is generally expressed as a frequency interval by the following equation (7), and equation (8) is obtained by converting equation (7) into a wavelength interval. c is the speed of light, l is the distance between two reflectors constituting the resonator, and n is the refractive index between the two resonators.

  However, when the center wavelength of the insertion wavelength is 840 nm and the FSR is 80 nm, it is known that the interval between the two reflectors constituting the resonator is as small as 4.4 μm. It is considered that it is difficult to perform a high-speed sweep of 200 KHz or more at this narrow interval.

101 A member that emits light having a wavelength width 102 Reflector 103 Reflector 104 Wavelength selection element

Claims (8)

A member that emits light having a wavelength width;
A resonator that amplifies the light emitted from the member;
A wavelength selection element that includes a part of the optical path in the resonator,
The light source device according to claim 1, wherein the wavelength selection element rotates to change an optical optical path length of the optical path.   The light source device according to claim 1, wherein the wavelength selection element rotates about a direction parallel to the optical path.   The light source device according to claim 1, wherein the wavelength selection element rotates around an arbitrary direction in a plane perpendicular to the optical path.   4. The light source device according to claim 1, wherein the wavelength selection element has a configuration in which an inclination is provided in the rotating direction. 5.   The light source device according to claim 1, wherein the wavelength selection element has a configuration in which a refractive index distribution is provided in the rotating direction.   The light source device according to claim 5, wherein the wavelength selection element has a structure having a groove, and a pitch of the groove is wide toward the rotating direction.   The light source device according to claim 1, wherein the member is a member that emits at least light having a wavelength of 800 nm to 1350 nm. A light source device that changes the wavelength of light;
An interference optical system that divides light emitted from the light source device into irradiation light and reference light for irradiating the object, and generates reflected light of the light irradiated on the object and interference light by the reference light;
A light detector that receives the interference light;
In an optical coherence tomography apparatus having an information acquisition unit that acquires information on the object based on a time waveform of the intensity of the interference light,
An optical coherence tomographic imaging apparatus, wherein the light source apparatus is the light source apparatus according to any one of claims 1 to 7.

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