JP2013110328A - Wavelength sweeping light source device and optical tomographic image pickup device including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength sweeping light source device in which the resonator length can be shortened, the SN ratio can be enhanced by suppressing mixture of ASE light into the output light, and a reflection film having an accurate shape can be formed by inhibiting the reflection film from being peeled off during manufacture, and to provide an optical tomographic image pickup device including the light source device.SOLUTION: In the wavelength sweeping light source device configured so that a resonator consisting of a partial reflection mirror and a scanner mirror causes laser oscillation of light having a wavelength selected by the scanner mirror, the scanner mirror is constituted of slit-like mirrors of equal width formed, at an equal interval, on a substrate of disc shape along the circumference of the disc shape. The slit-like mirrors are formed by removing the parts becoming the mirrors of a light-shielding film on a reflection film deposited on the substrate of disc shape.

Description

  The present invention relates to a wavelength swept light source device and an optical tomographic imaging apparatus including the light source device, and more particularly to a wavelength swept light source device capable of sweeping an oscillation wavelength and an optical tomographic image capturing apparatus including the light source device.

As light sources, particularly laser light sources, various types of 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.
As one application of a wavelength tunable (sweep) light source in an inspection apparatus, a laser spectrometer, a dispersion measuring instrument, a film thickness measuring instrument, and a wavelength swept optical tomography (hereinafter referred to as SS-OCT). There is a device.
SS-OCT captures a tomographic image of a specimen using low coherence light interference. This is an imaging technique that has been actively researched in the medical field in recent years because of the micron-order spatial resolution and non-invasiveness.

Currently, OCT is capable of obtaining a tomographic image with a resolution in the depth direction of several microns and a depth of several mm, and is used for ophthalmic photography, dental photography, and the like.
Among these, SS-OCT sweeps the oscillation wavelength (frequency) of the light source in time. This falls into the category of Fourier domain (FD) OCT.
Similarly, the spectral region (spectrum domain: SD) OCT that falls into the category of FDOCT requires a spectroscope that separates interference light, whereas SS-OCT does not use a spectroscope, so the loss of light is small and high. Image acquisition of S / N ratio is also expected.

When configuring a medical imaging apparatus to which the SS-OCT technology is applied, the image acquisition time can be shortened as the sweep speed is increased, and the depth spatial resolution of the tomographic image can be increased as the wavelength sweep width is increased. These parameters are important because they are possible.
Specifically, when the wavelength sweep width Δλ, the oscillation wavelength λ0, and the refractive index of the specimen are n, the depth resolution δL is

δL = 2ln2 / π × λo ² / nΔλ (Formula 1)

It is represented by
Therefore, in order to increase the depth resolution, it is necessary to increase the wavelength sweep width, 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 δλ, the oscillation wavelength λ0, and the refractive index of the specimen are n, the coherence distance (coherence length) L is

L = λo ² / nδλ (Formula 2)

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.
In addition, when the wavelength sweep type optical interference tomography is used for acquiring a tomographic image of a living body, most of the light irradiated inside the living body is diffused, and the rate of being coupled to the interferometer as backscattered light is extremely −40 dB to −50 dB. Small.
Therefore, in order to enable signal detection at a deeper position, a high-power wavelength swept light source 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.

Patent Document 1 discloses a wavelength tunable light source including an optical amplifying medium and a reflector using a diffraction grating outside the optical amplifying medium. FIG. 5 shows a light source device described in Patent Document 1. In FIG. 5A, the light emitted from the optical amplifying medium 201 having a predetermined wavelength gain width becomes a parallel light flux through the collimator lens 202.
This parallel light beam enters the diffraction grating 203 and is dispersed in the diffraction wavelength. The diffracted and wavelength-dispersed light beam is condensed by the condensing lens 205 on the rotating disk 207 with a spread corresponding to each wavelength.
In the rotating disk 207, slits 209 are arranged radially.
On the opposite side of the rotating disk 207 from the diffraction grating, a reflecting mirror 210 that reflects the light beam transmitted through the slit and functions as one of a pair of resonators is disposed.
When one of the slits 209 comes to a position where light is condensed with a spread corresponding to each wavelength, a part of the condensed light is transmitted there, and the wavelength is selected at this time. The transmitted light beam is reflected by the reflection mirror 210, passes through the slit 209 again, returns to the optical amplification medium 201 through the forward path, and is optically amplified.
Reference numeral 212 denotes another transflective reflector in the pair of resonators, which is coupled to a fiber (not shown) and output to the OCT apparatus.
FIG. 5B is another light source device described in Patent Document 1.
In the apparatus of FIG. 5B, a rotating disk 231 in which a slit-like reflecting film 232 is arranged in radial light is arranged instead of the rotating disk 207 and the reflecting mirror 210. When one of the slit-like reflective films 232 comes to a position where light is condensed with a spread corresponding to each wavelength, the condensed light is reflected there. At this time, the wavelength is selected, and returns to the optical amplifying medium 201 through the forward path to be optically amplified.
Reference numeral 212 denotes another transflective reflector in the pair of resonators, which is coupled to a fiber (not shown) and output to the OCT apparatus.

JP 2008-98395 A

In the case of configuring a medical image pickup apparatus using a wavelength swept light source, it has been described in [Background Art] that it is preferable to shorten the image acquisition time as the sweep speed increases.
It takes 0.01 seconds to capture a two-dimensional tomographic image at 100 KHz in the depth direction (Z) and 1000 sampling points in the horizontal direction (X). It can be seen that a three-dimensional image can be acquired in 10 seconds if the number of sampling points is 1000 in the vertical direction (Y).
The wavelength swept light source using the rotating disk disclosed in Patent Document 1 as a wavelength selection filter is a wavelength swept light source capable of relatively high-speed wavelength sweeping, but has the following problems.
That is, in the apparatus using the transmission type slit shown in FIG. 5A, both the slit 209 and the reflection mirror 210 need to be disposed within the focal point of the condenser lens 205. It becomes longer and the resonator length cannot be shortened much.
For this reason, when the wavelength is swept at a high speed, the number of round trips in the resonator is reduced, the coherency is deteriorated, and the observation depth of the OCT image is reduced.
Further, in the apparatus using the slit-shaped reflective film shown in FIG. 5B, the ASE light is often mixed in the output light, the SN ratio is not good, and the slit-shaped reflective film is in the middle of the pattern forming process. It is easy to peel off, and it becomes difficult to form a large number of accurately shaped reflective films.

In view of the above problems, the present invention can shorten the resonator length and can suppress the mixing of ASE light into the output light and improve the SN ratio.
An object of the present invention is to provide a wavelength swept light source device capable of forming a reflective film having an accurate shape while suppressing the reflection film from being peeled off during manufacture, and an optical tomographic imaging apparatus including the light source device.

The wavelength swept light source device of the present invention is
A partially reflective mirror,
A semiconductor optical amplification medium;
A collimating lens for collimating the light emitted from the semiconductor optical amplification medium;
A diffraction grating that gives different angular dispersion to the collimated light flux depending on the wavelength; and a condenser lens that collects the light flux given angular dispersion by the diffraction grating at different positions depending on the wavelength;
A scanning mirror that scans the focal plane of the condenser lens and reflects light of some wavelengths;
A wavelength swept light source device configured such that light of a wavelength selected by the scanning mirror is laser-oscillated by a resonator including the partial reflection mirror and the scanning mirror,
The scanning mirror is composed of slit-shaped mirrors of equal width formed on the disk-shaped substrate at equal intervals along the circumference of the disk-shaped substrate,
The slit-like mirror is formed by removing a portion of the light-shielding film that serves as the mirror on a reflective film formed on the disk-shaped substrate.
An optical tomographic imaging apparatus of the present invention includes a light source unit including the above-described wavelength swept light source device,
A subject measurement unit that irradiates the sample with light from the light source unit and transmits reflected light from the subject; and
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 subject 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 acquires an optical tomographic image of the subject based on the light detected by the light detection unit;
It is characterized by having.

According to the present invention, the resonator length can be shortened, the mixing of ASE light into the output light can be suppressed, and the SN ratio can be improved.
It is possible to realize a wavelength swept light source device and an optical tomographic imaging apparatus including the light source device that can form a reflective film with an accurate shape by suppressing the reflection film from being peeled off during manufacturing.

The schematic diagram explaining the structural example of the wavelength swept light source device which can sweep the oscillation wavelength in embodiment and Example 1 of this invention. The figure explaining the structure of the slit-shaped mirror in embodiment and Example 1 of this invention. The figure explaining the structural example of the wavelength swept light source device in Example 2 of this invention. The figure explaining the optical tomographic imaging apparatus in Example 4 of this invention. The figure explaining an example of the wavelength sweep light source in a prior art example.

Next, a configuration example of a wavelength swept light source device capable of sweeping the oscillation wavelength in the embodiment of the present invention will be described with reference to the schematic diagram of FIG.
In FIG. 2, an optical amplifier 26 as an optical gain medium has a gain at the oscillation wavelength of laser oscillation, and uses, for example, a semiconductor optical amplifier (SOA).
Light emitted from one end face of the optical amplifier 26 is converted into parallel light by the collimator lens 25. The parallel light is incident on a diffraction grating 24 that gives different angular dispersion depending on the wavelength, and is diffracted at different angles according to the wavelength of the light.
The diffracted light is condensed by a condensing lens 23, and a reflection wheel 21 in which a large number of long and narrow slit-shaped mirrors 22 constituting a scanning mirror are formed on the circumference 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 22 and is incident on the optical amplifier 26 again.
On the other hand, the light output from the other end face of the optical amplifier 26 is collimated by the collimator lens 27. A part of the parallel light is reflected by the partial reflection mirror 28 and is incident on the optical amplifier 26 again.
Therefore, the slit mirror 22 and the partial reflection mirror 28 constitute a resonator, and light having a wavelength selected by the slit mirror oscillates as an external resonator light source. And the light which permeate | transmitted the partial reflection mirror 28 is taken out as an optical output.
The position of the slit mirror 22 is mechanically moved to scan the focal plane of the condensing lens, and the wavelength of reflected light changes, so that wavelength tunability can be realized.

FIG. 1 is a diagram illustrating the structure of the slit mirror 22.
The slit-shaped mirror 22 is formed by sequentially forming a reflective film 12 and a light-shielding film 13 on a disk-shaped substrate 11 and removing a portion of the light-shielding film 13 that becomes a mirror.
As the substrate 11, a glass, a metal such as aluminum, a resin such as polycarbonate, or the like processed into a disk shape can be used.
The reflective film 12 is formed by forming a metal having a high reflectance at the wavelength to be used by sputtering or vacuum deposition.
The material of the reflective film is a material having high reflectivity, good adhesion to the substrate / light-shielding film, and excellent weather resistance, and is one of Al, Au, Ag, Cu, and Rh. A gold alloy, a silver alloy, an aluminum alloy, a copper alloy, a rhodium alloy, or the like, which is a metal or an alloy containing the metal as a main component, is preferable.
Any material that absorbs light can be used for the light shielding film 13, but a material having a higher etching rate in reactive dry etching with tetrafluoromethane than the reflective film is preferable in order to form the light shielding film 13 with a stable and accurate shape. In particular, silicon, germanium, or the like that is easy to dry-etch is suitable.

A wavelength-swept light source shown in FIG. 2 is configured using the reflection wheel 21 created as described above.
Since this wavelength swept light source uses a reflection wheel, it is possible to use a lens having a short focal length as the condenser lens 23, and the resonator length can be shortened.
In order to obtain a coherence length of 8 mm or more necessary for OCT, the number of times that a photon reciprocates through the resonator is about 10 times.
Therefore, for example, in order to realize a wavelength swept light source capable of performing wavelength sweeping at a sweep speed of 100 KHz, the resonator length needs to be 150 mm or less in order to make the number of times the photons reciprocate the resonator 10 times or more.
The wavelength swept light source of the present invention can be easily configured with a resonator length of 150 mm or less. Furthermore, since the reflection wheel 21 is made of a structure of a reflection wheel 21 on the substrate / a large-area reflection film / a light-shielding film in which a small area is etched into a slit shape, the yield during the production of the reflection wheel 21 is high. Durability is also good.

According to the wavelength swept light source device of the present embodiment described above, since the slit for selecting the wavelength is integrally formed on the reflection mirror by the light shielding film, it is possible to use a condensing lens with a short focal length. it can. Thereby, the resonator can be configured to be short, and an optical output having a long coherence length can be obtained. As a result, it is possible to realize a wavelength swept light source that provides a deep observation depth and does not cause the problem that the reflecting mirror peels off during manufacturing.
Further, by forming the antireflection film on the light shielding film, the mixing of the ASE light into the output light can be further reduced, and the SN ratio can be improved.
Further, by forming the light shielding film with a material having a higher etching rate in reactive dry etching with tetrafluoromethane than the reflective film, the process margin when creating a wavelength selection disk can be widened. This makes it possible to create a wavelength swept light source stably at low cost.
The reflection film is formed of any one of Al, Au, Ag, Cu, and Rh, or an alloy containing the metal as a main component, and the light shielding film is formed of Si or Ge. As a result, the difference between the light amount of the light having the wavelength selected by the wavelength selection disk and the light amount of the light having the wavelength not selected can be increased, and the SN ratio of the output light can be further improved.
By forming the antireflection film from silicon nitride or silicon dioxide, the process margin when creating a wavelength selection disk can be increased and the cost can be reduced, as in the case of forming the material with a high etching rate. Thus, it becomes possible to create a wavelength swept light source stably.

[Example 1]
As Example 1, a configuration example of the wavelength swept light source device shown in FIGS. 1 and 2 will be described.
First, a glass disk having an inner diameter of 25.4 mm, an outer diameter of 63.5 mm, and a thickness of 3 mm is used as the substrate 11, and a gold alloy thin film in which a small amount of aluminum is added to gold is formed on the substrate as a reflective film 12 with a thickness of 200 nm. It was formed by sputtering.
Further, an amorphous silicon film serving as the light-shielding film 13 was formed on the reflective film with a thickness of 3050 nm by sputtering.
On this light shielding film, a positive resist was applied, and using a laser exposure machine, 360 slits having a width of 3 μm and a length of 1000 μm were radially patterned at a radius of 30 mm in increments of 1 degree.
This was developed to form an etching mask, the silicon film was etched by dry etching to form the light-shielding film 13, and a reflective wheel 21 having 360 slit mirrors 22 formed at equal intervals was created.
Here, the thickness of the amorphous silicon film serving as the light shielding film may be selected by using the Fresnel formula or the like so as to reduce the reflectance of the light shielding film.

The wavelength sweep light source shown in FIG. 2 was configured using the reflection wheel 21 created as described above.
The focal length of the condenser lens 23 was 5 mm, the focal length of the collimating lens 25 was 3 mm, and the focal length of the collimating lens 27 was 2 mm.
As the diffraction grating 24, a transmission diffraction grating of 1200 lines / mm was used. As the optical amplifier 26, an SOA having a gain with a wavelength width of 40 nm centered on a wavelength of 850 nm was used. The resonator length of this wavelength swept light source could be as short as about 50 mm. Since the resonator length is short, a high-speed wavelength sweep of 180 kHz with a wavelength sweep width of 830 nm to 870 nm can be performed by rotating the reflection wheel 21 with a spindle motor (not shown) at 30000 rpm.
In addition, since the reflection wheel 21 is made of a structure of a reflection wheel 21 on the substrate / a large-area reflection film / a light-shielding film in which a small area is etched into a slit shape, the yield during the production of the reflection wheel 21 is high, and A product having good durability could be obtained.

[Example 2]
As a second embodiment, a configuration example of a wavelength swept light source device different from the first embodiment will be described with reference to FIG.
In this embodiment, a glass disk having an inner diameter of 25.4 mm, an outer diameter of 63.5 mm, and a thickness of 3 mm is used as the substrate 11, and a gold alloy thin film in which a small amount of aluminum is added to gold is used as the reflective film 12 on this substrate. The thickness was formed by sputtering.
On top of that, amorphous silicon to be the light shielding film 13 was formed by sputtering with a thickness of 2960 nm. Further, silicon nitride (SiN) to be the antireflection film 31 was formed on the light shielding film with a thickness of 120 nm by sputtering.
A positive resist was coated thereon, and 360 slits having a width of 3 μm and a length of 1000 μm were radially patterned at a radius of 30 mm using a laser exposure machine.
This was developed to form an etching mask, and dry-etched with a mixed gas of CF ₄ and oxygen to form a slit-like reflecting mirror 22. In this way, a reflection wheel 21 having 360 slit mirrors 22 formed at equal intervals was produced.
Using the reflection wheel 21 created as described above, the wavelength swept light source shown in FIG.
In the wavelength swept light source of Example 2, the S / N ratio of the light output was improved by the antireflection film 31 suppressing the reflectance of the light shielding part to less than 1%.
As the antireflection film, SiN is used here, but other materials can be used, and silicon dioxide is also preferable.

[Example 3]
As a third embodiment, a configuration example different from the above embodiments will be described.
In this example, the same substrate as in Example 2 was used, and an aluminum alloy in which a slight amount of silicon was added to aluminum was formed as a reflective film 12 on this substrate by sputtering to a thickness of 100 nm.
On top of that, germanium serving as the light shielding film 13 was formed by sputtering with a thickness of 1180 nm. Further, SiN was formed as the antireflection film 31 with a thickness of 125 nm by sputtering.
This was processed in the same manner as in Example 2 to create a reflective wheel 21. A wavelength swept light source shown in FIG. 2 was configured using this reflection wheel 21.
The focal length of the condenser lens 23 was 9 mm, the focal length of the collimating lens 25 was 4 mm, and the focal length of the collimating lens 27 was 3 mm. As the diffraction grating 24, a 600 / mm transmission diffraction grating was used.
As the optical amplifier 26, an SOA having a gain with a wavelength width of 80 nm centered on a wavelength of 1050 nm was used.
The resonator length of this wavelength swept light source was about 70 mm.
By rotating the reflection wheel 21 with a spindle motor (not shown) at 30000 rpm, a high-speed wavelength sweep of 180 kHz with a wavelength sweep width of 1010 nm to 1090 nm was possible.
Further, by using a germanium thin film as the light shielding film, a high-speed wavelength swept light source in the 1 μm band can be realized without impairing the productivity of the reflection wheel 21.

[Example 4]
As Example 4, an optical tomographic imaging apparatus (OCT) provided with a wavelength-swept light source device of the present invention in a light source unit will be described with reference to FIG.
The principle of an optical tomographic imaging apparatus (OCT) can be explained by the configuration of a Twiman-Green interferometer.
Sweeps the wavelength of the light source by causing the reflected light from the test surface consisting of multiple interfaces in the thickness (optical axis) direction of one arm to interfere with the light beam from the reference surface that is the other arm Obtained by Fourier transforming the modulated interference signal obtained by the above.
It was described in [Background Art] that the resolution and detection width in the depth direction of the test surface of OCT depend on the wavelength sweep width and spectrum width of the light source spectrum.
ΔL in equation (1) is the depth resolution, and L in equation (2) is the detection width in the depth direction.

In FIG. 4, reference numeral 102 denotes a wavelength-swept light source device according to the present invention, and 106 denotes a fundus that is a subject.
Reference numeral 110 denotes a mirror for one-dimensional scanning of the fundus.
Reference numeral 108 is a reference reflecting surface, 104 is a fiber coupler for combining and interfering the light beam from the subject and the light beam from the reference surface, and 115 is a detector for photoelectrically converting the modulated interference signal.
A computer 116 digitizes the electrically detected signal, calculates a tomographic image by performing a Fourier transform or the like, and a display 117 visualizes the tomographic image.

The light beam from the light source device 102 passes through the fiber 103 and is branched in two directions by the coupler 104. One arm irradiates the retina of the fundus that is the subject through the fiber 105.
Then, the reflected light again passes through the fiber 105 and returns to the fiber coupler 104.
The light beam branched to the other arm passes through the fiber 107 and irradiates the reference mirror 108. The reflected light from the reference mirror 108 passes again through the fiber 107 and returns to the fiber coupler 104.
Thus, the reflected light transmitted from the subject measurement unit and the reflected light transmitted from the reference unit interfere with each other by the fiber coupler 104 (interference unit), and this interference light passes through the fiber 114 and passes through the light detection unit. The photoelectric conversion detector 115 to be configured is entered.
At this time, if the wavelength from the light source device 102 is changed, a modulated interference signal corresponding to the tomographic structure is obtained.
The tomographic signal can be obtained by digitizing this signal and Fourier transforming it by the computer 118 constituting the image processing unit. Since this is a tomographic signal of a point, an optical tomographic image can be detected by measuring a one-dimensional tomographic image by tilting and scanning the mirror 110 and visualizing it on the display 117.
Since the insertion speed is 100 KHz or higher with the light source device described above, an optical tomographic image can be detected at a high speed, and an OCT device having a wide detection depth width and a high detection resolution in the depth direction can be provided.

11: Substrate 12: Reflective film 13: Light shielding film 21: Reflective wheel 22: Slit mirror 23: Condensing lens 24: Diffraction grating 25: Collimating lens 26: Optical amplifier 27: Collimating lens 28: Partial reflection mirror 31: Antireflection film

Claims (6)

A partially reflective mirror,
A semiconductor optical amplification medium;
A collimating lens for collimating the light emitted from the semiconductor optical amplification medium;
A diffraction grating that gives the collimated beam a different angular dispersion depending on the wavelength;
A condensing lens that condenses the light flux given angular dispersion by the diffraction grating at different positions depending on the wavelength;
A scanning mirror that scans the focal plane of the condenser lens and reflects light of some wavelengths;
A wavelength swept light source device configured such that light of a wavelength selected by the scanning mirror is laser-oscillated by a resonator including the partial reflection mirror and the scanning mirror,
The scanning mirror is composed of slit-shaped mirrors of equal width formed on the disk-shaped substrate at equal intervals along the circumference of the disk-shaped substrate,
The wavelength-sweeping light source device, wherein the slit-shaped mirror is formed by removing a portion of the light-shielding film on the reflective film formed on the disk-shaped substrate.   The wavelength-swept light source device according to claim 1, wherein an antireflection film is formed on the light shielding film excluding the portion to be the mirror.   The swept light source device according to claim 2, wherein the antireflection film is formed of silicon nitride or silicon dioxide.   4. The wavelength sweep according to claim 1, wherein the light shielding film is formed of a material having a higher etching rate in reactive dry etching with tetrafluoromethane than the reflective film. Light source device. The reflective film is formed of any one metal of Al, Au, Ag, Cu, Rh, or an alloy containing the metal as a main component,
The wavelength-swept light source device according to claim 4, wherein the light-shielding film is made of Si or Ge. A light source unit comprising the wavelength swept light source device according to any one of claims 1 to 5,
A subject measurement unit that irradiates the subject with light from the light source unit and transmits reflected light from the subject; and
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 subject 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 acquires an optical tomographic image of the subject based on the light detected by the light detection unit;
An optical tomographic imaging apparatus characterized by comprising:

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