JP2010175271A - Optical tomographic image display system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image of high resolving power using a light source having no expanse of a spectrum even if high speed scanning is performed in a narrow band in an optical tomographic image display system. <P>SOLUTION: A gain element 40 and a wavelength variable filter 50 are used as a wavelength scanning type light source 10. The natural emission light of the gain element 40 is projected on the wavelength variable filter 50 to oscillate it by a selected wavelength. The optical tomographic image display system is constituted using this wavelength scanning type light source 10. The enhancement of the resolving power of the optical tomographic image display system is achieved by scanning the selected wavelength of the wavelength variable filter 50 at a high speed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical tomographic image display system for observing an image of an internal structure under a surface of an object or an image of a subepidermal layer tomography of a biological tissue.

  In recent years, with advances in medical technology such as endoscopic treatment, a method for non-intrusive and real-time diagnosis of a pathological tissue is desired. Conventionally, for example, an electronic endoscope using a CCD, imaging using CT, MRI, and ultrasonic waves are used as diagnostic methods. The electronic endoscope is limited to observation of the surface of a living body, and the latter diagnostic imaging system has a technical limit in observing with a resolution of micron order. Optical coherence tomography technology has attracted attention as a technology that complements such methods.

  There are two types of coherent tomography techniques, time domain tomography and frequency domain tomography, and there are two types of frequency domain tomography: a spectrometer type and a wavelength scanning light source type. The wavelength scanning light source type irradiates a living body with light, continuously changes the wavelength of the irradiation light, and interferes the reference light and reflected light returning from different depths in the living body with an interferometer. This is a system for obtaining a tomographic image by analyzing a frequency component of a signal. This technology is expected as an advanced system because it can construct a tomographic image with extremely high resolution from frequency analysis of signals from inside the object. The wavelength scanning light source type is suitable for actual use such as an endoscope because it has high measurement sensitivity and is resistant to dynamic noise. Here, the wider the wavelength scanning band of the irradiated light, the higher the frequency analysis band, so that the resolution in the depth direction increases.

  As a light source for a tomographic image display device, a wavelength scanning light source capable of scanning a single wavelength at high speed is required. An external resonator type laser light source is generally used as a narrow-spectrum and wide-band wavelength scanning light source.

  Next, the oscillation principle of the external resonator type laser will be described. In a general external resonator type laser, as shown in FIG. 1, a gain medium 101 such as a semiconductor laser and an external mirror 104 are used as external resonators, and a collimating lens 102 and an optical bandpass filter 103 are provided therebetween. Yes. Here, laser oscillation is obtained at the point where the characteristics of the bandpass filter 103 and the external resonator mode coincide. Here, when the selected wavelength of the bandpass filter 103 is shifted, the oscillation mode changes to the adjacent external resonator mode in which the loss is the smallest among the external resonator modes. This is called a mode hop.

  Patent Document 1 discloses a multimode laser light source that uses an optical fiber loop, lengthens the external resonator length, and eliminates the effects of mode hops. A laser light source using an optical fiber loop is formed by including a semiconductor amplifier in an optical fiber loop, and a wavelength variable optical filter for selecting a wavelength is provided in a part of the loop.

  In the oscillation of the multimode laser light source, as shown in FIG. 2A, a plurality of external resonator longitudinal modes included in the filter resonate simultaneously. FIG. 2B shows filter characteristics, and FIG. 2C shows output characteristics. In the multimode laser light source, the mode near the center of the filter with the least oscillation loss is resonantly amplified, and the line width of the output extracted from the laser light source is larger than the characteristics of the optical filter as shown in FIG. Narrow.

JP-A-2005-347668

  In the wavelength scanning laser light source disclosed in Patent Document 1, a fiber is used to increase the external resonator length and oscillate in multiple modes. When the selected wavelength of the tunable filter is swept, the longitudinal mode that has been suppressed is excited as the wavelength moves, and the process in which the oscillating mode is suppressed is continuously repeated. At this time, if the wavelength sweep is performed at a high speed and the filter is swept at a rate equivalent to or faster than the time for which the excited light circulates the resonator, the number of times of passing through the gain element is reduced. Therefore, it moves to the adjacent mode group one after another without obtaining a sufficient gain, that is, resonance amplification. As a result, as shown in FIG. 2D, there is a drawback that the output is saturated to the same extent as the filter width or has a thicker line width than that. That is, as shown in FIG. 3, there is a drawback in that the spectral width of the laser light source increases as the scanning speed increases. Thus, conventionally, an optical tomographic image display system using a wavelength scanning light source that realizes a narrow line width, a wide band, and a high-speed variable has not been realized.

  The present invention pays attention to the above-mentioned problems, and by using a wavelength scanning light source capable of scanning at a high speed over a wide band with a narrow oscillation spectrum line width, an optical tomography capable of displaying an image at a wide measurement depth. An object is to provide an image display system.

  In order to solve this problem, an optical tomographic image display system according to the present invention includes a wavelength scanning light source that periodically scans the oscillation wavelength of light, and irradiates the reference light and the object with light from the wavelength scanning light source. An interference optical meter that branches into light and generates interference light between reflected light from the object and reference light, a light receiving element that receives the interference light obtained from the interference optical meter and obtains a beat signal, and measurement of the object An optical tomographic image display comprising: a signal processing unit that calculates a tomographic image of the object by Fourier-transforming a signal obtained based on an output from the light receiving element when a reflector is disposed at a position; In the system, the wavelength scanning light source includes a gain element having a gain in an oscillation wavelength band and a wavelength variable filter that periodically scans a selected wavelength, and the spontaneous emission light of the gain element is the wavelength variable filter. In Isa is, by amplifying incident again to the gain element light of a selected wavelength, and is characterized in that outputs light obtained by scanning the oscillation wavelength.

  Here, the wavelength scanning frequency of the wavelength scanning light source may be 10 kHz or more.

  Here, the wavelength tunable filter of the wavelength scanning light source may be a reflective wavelength tunable filter that selectively reflects the wavelength of the naturally radiated light incident from the gain element and inputs the reflected light to the gain element again.

  Here, the wavelength tunable filter of the wavelength scanning light source is a transmissive wavelength selection filter, and is disposed on the side of the wavelength tunable filter, and transmits light having a wavelength selected through the wavelength tunable filter again. You may make it further have a mirror which reflects so that it may inject into a wavelength tunable filter.

  Here, the wavelength tunable filter of the wavelength scanning light source is a transmission-type wavelength selection filter, and is disposed between the gain element and the wavelength tunable filter, and the natural radiated light emitted from the gain element is tunable. You may make it further comprise the optical circulator which injects into the filter and returns the light which permeate | transmitted the said wavelength variable filter to the said gain element again.

  According to the present invention having such characteristics, the oscillation wavelength can be continuously scanned at a high speed by continuously changing the selection wavelength of the wavelength tunable filter. Since the wavelength selection characteristic of the wavelength tunable filter does not change even when the scanning speed increases, it is possible to scan at a high speed while maintaining a narrow band. Therefore, it is possible to provide an optical tomographic image display system having high interference signal detection sensitivity and high internal depth.

FIG. 1 is a diagram showing a configuration of a conventional general external resonator type laser. FIG. 2 is a diagram showing the characteristics of the multimode laser light source. FIG. 3 is a diagram showing the relationship between the scanning speed and the spectral width of the multimode laser light source. FIG. 4 is a diagram showing an overall configuration of a wavelength scanning optical tomographic image display system according to an embodiment of the present invention. FIG. 5 is a graph showing temporal changes in the frequencies of the reference light and the reflected light. FIG. 6 is a block diagram of the wavelength scanning light source according to the present embodiment. FIG. 7 is a diagram showing an example of a wavelength tunable filter of the wavelength scanning light source according to the embodiment of the present invention. FIG. 8 is a diagram showing another example of the wavelength scanning light source according to the present embodiment. FIG. 9 is a graph showing the oscillation state of the wavelength scanning light source according to this embodiment. FIG. 10 is a diagram showing the relationship between the scanning speed and the spectral width of the wavelength scanning light source according to this embodiment. FIG. 11 is a diagram showing the relationship between the wavelength tunable filter and the gain element of the present embodiment.

  FIG. 4 is a block diagram showing the overall configuration of the optical tomographic image display system according to the embodiment of the present invention. In this figure, a wavelength scanning light source 10 is a wavelength scanning light source that oscillates an optical signal in a certain range, for example, 220 to 250 THz, and its output is given to an optical fiber 11. A collimating lens 12 and a reference mirror 13 are provided at the other end of the optical fiber 11. Further, an intermediate branch portion 14 is provided at the middle portion of the optical fiber 11 to allow another optical fiber 15 to approach and interfere. At one end of the optical fiber 15, a collimator lens 16 that converts the optical signal obtained from the wavelength scanning light source 10 through the coupling / branching unit 14 into parallel light, and a scanning mirror 17 that scans the light are provided. A lens 18 for irradiating the measurement object with light is provided at a position where the reflected light of the scanning mirror 17 is received. The scanning mirror 17 rotates within a certain range about an axis perpendicular to the paper surface, thereby changing the reflection angle of parallel light and scanning (scanning) the light focusing position in the horizontal direction. A photodiode 20 is connected to the other end of the optical fiber 15 via a lens 19. The photodiode 20 is a light receiving element that receives the reflected light from the reference mirror 13 and the interference light of the light reflected by the measurement site to obtain the beat signal as an electrical signal. Here, the optical fibers 11 and 15, the coupling / branching unit 14, the collimating lens 12, the reference mirror 13, the collimating lens 16, the scanning mirror 17, and the lens 18 constitute an interference optical meter.

  The output of the photodiode 20 is input to the signal processing unit 22 via the amplifier 21. The wavelength scanning light source 10 generates a trigger signal at the start of light scanning, and its output is given to the signal processing unit 22. The signal processing unit 22 is provided with a low-pass filter (LPF) 31 for removing the aliasing distortion of the optical interference signal as shown in the figure, and this output is given to the A / D converter 32. The A / D converter 32 performs A / D conversion at predetermined equal time intervals and outputs the output to the CPU 33. The CPU 33 holds this data, performs a Fourier transform by a Fourier transform circuit 34 based on an algorithm to be described later, and outputs the obtained image to the monitor 36. The D / A converter 35 generates a sawtooth image sweep signal based on the trigger signal output from the A / D converter 32 and outputs it to the monitor 36.

  Next, the principle of optical coherence tomography measurement using a wavelength scanning light source will be described. The target object is irradiated with coherent light whose optical frequency continuously and periodically changes from the light source, and the backscattered light reflected from the inside of the object or the subsurface layer of the living body is caused to interfere with the reference light using an interference optical meter. By measuring the intensity distribution of the interference light and measuring the change in the intensity distribution corresponding to the change in the optical frequency, a tomographic image along the depth direction can be constructed. Further, by scanning a spatial beam in one dimension and two dimensions on the object, a two-dimensional and three-dimensional tomographic image can be constructed, respectively.

In the interference optical meter, when the optical path of the two arms from the coupling / branching unit 14, that is, the optical path L1 to the reference mirror 13 and the optical path L2 to the reflecting surface in the object are equal, the beat frequency of the interference light is zero. Next, when the reflected light is reflected from a certain depth z inside the object, if the optical frequency changes with time, the reflected light from the object and the reflected light from the reference mirror 13 are equivalent to the difference in the optical path. A difference occurs in frequency, and a beat occurs in the interference light. Here, for example, it is assumed that the optical frequency of the light source is scanned linearly in terms of time. It is assumed that the surface of the object is at a position where the lengths of the arms of the interferometer are equal, and the reflecting surface of the object is only at a position at a depth z from the surface. The temporal changes in the frequency of the reference light and the frequency of the reflected light (object light) from the object at the combining / branching unit 14 are as shown by straight lines A and B in FIG. Here, the optical frequency is assumed to be scanned over a frequency width Δf = αT [Hz] within a time T [s] at a scanning rate α [Hz / s]. The delay time τ of the object light with respect to the reference light is expressed as follows:
τ = 2 nz / c
It becomes. Therefore, the interference light received by the photodiode 20 has a beat frequency fb = ατ = (Δf / T) (2 nz / c) (1)
Will fluctuate.

  Actually, since the reflected light is continuously generated from different positions along the depth inside the object, the reflected light has different beat frequency components corresponding to each depth. Therefore, the intensity of reflected light from a specific depth corresponding to the beat frequency can be detected by frequency analysis of the intensity change of the interference light. A tomographic image can be constructed by taking the spatial distribution of the reflection intensity.

第１，２項はそれぞれ参照ミラーと、物体からの反射光の直流成分であり、第３項が干渉信号光成分である。これをフーリエ変換することによって、物体中の任意の深さに対応する散乱光強度の関係を得ることができる。
干渉光信号：Ｆ（ｚ）＝ΣＩ_dct［ｋ_m］ｅｘｐ（−ｊ２ｋ_mｚ_n） ・・・（３）
ｋ_m＝ｋ（ｔ_m）＝２π／λ（ｔ_m）＝２πｆ（ｔ_m）／ｃ
上記干渉光信号はｋ空間で均等なサンプリングでフーリエ変換することによって、歪みのない画像が得られる。 Mathematically, this frequency analysis is obtained by Fourier transforming the interference optical signal I _dct represented by the following equation (2).

The first and second terms are the direct current components of the reference mirror and the reflected light from the object, respectively, and the third term is the interference signal light component. By performing a Fourier transform on this, it is possible to obtain a relationship of scattered light intensity corresponding to an arbitrary depth in the object.
Coherent light _{signal: F (z) = ΣI dct} [k m] exp (-j2k m z n) ··· (3)
_{k m = k (t m)} = 2π / λ (t m) = 2πf (t m) / c
The interference light signal is Fourier-transformed with uniform sampling in the k space, thereby obtaining an image without distortion.

  The trigger signal that gives the sampling timing needs to be synchronized with the optical frequency scanned by the wavelength scanning light source 10, and in addition, needs to be equal on the wave number, that is, on the frequency axis.

Here, the resolution δz in the depth direction is expressed by Equation (4), and is proportional to the reciprocal of the scanning range, that is, the higher the scanning range, the higher the resolution.
δz = (2ln2 / π) · (λ ₀ ² / Δλ) (4)
Here, λ ₀ is the center wavelength, and Δλ is the wavelength scanning range.

Next, the coherent length Lc is expressed by the following equation.
Lc = (2ln2 / π) · (C / Δν) (5)
Here, Δν is the dynamic line width, that is, the spectral line width in the middle of the wavelength shift.

  Next, the coherent length Lc represented by Expression (5) corresponds to twice the measurement distance in the depth direction, and becomes wider in inverse proportion to the line width. That is, the wavelength scanning light source 10 is preferably a light source having a wide wavelength scanning range and a narrow line width.

  Next, the wavelength scanning light source 10 according to the present embodiment will be described in detail. As shown in the block diagram of FIG. 6, the wavelength scanning light source 10 according to the present embodiment includes a gain element 40 having a gain in the oscillation wavelength band, a control unit 41 that controls the gain, a wavelength tunable filter 50, and its control. The unit 60 is configured. The wavelength tunable filter 50 is an optical filter for selecting a narrow-band wavelength, and the control unit 60 is for scanning the selected wavelength at high speed.

  In the wavelength scanning light source according to this embodiment, the naturally radiated light from the gain element 40 is incident on the narrow-band tunable filter 50, and a part of the wavelength is cut out by the tunable filter 50, and this light is output by the gain element 40. It oscillates by amplifying. Therefore, the wavelength scanning light source according to the present invention does not use an external resonator.

  The gain element 40 is configured by, for example, an SOA (semiconductor optical amplifier). In this case, the control unit 41 is a current supply circuit that supplies current to the SOA, and can adjust the gain of the SOA according to the current value.

  An example of the wavelength tunable filter 50 and its control unit 60 will be described. As the wavelength tunable filter 50, a diffraction grating 51 can be used as shown in FIG. In this case, the control unit 60 can be configured by means having a function of changing the selection wavelength by changing the incident angle of the light incident on the diffraction grating 51, for example, the polygon mirror 61 and the drive unit 62. In this case, if the Littrow arrangement is used to reflect incident light in the same direction, a reflection type wavelength tunable filter is obtained. The control unit 60 can also be configured by a scanning mirror, a galvano mirror, and its driving unit.

  Another example of the wavelength tunable filter and its control unit will be described. As shown in FIG. 8, the tunable filter 50 can be constituted by a Fabry-Perot etalon 52. The Fabry-Perot etalon 52 includes a pair of mirrors 53 and 54 arranged in parallel. The opposing surfaces of the mirrors 53 and 54 are provided with a highly reflective coating, and the outer surfaces of the mirrors 53 and 54 are provided with a non-reflective coating. An optical fiber 55 and a collimating lens 56 are provided to allow light to enter the etalon filter. A condensing lens 57 and an optical fiber 58 are provided to condense the light that has passed through the Fabry-Perot etalon 52. In this case, the control unit 60 only needs to have a function of controlling the distance between the pair of mirrors of the Fabry-Perot etalon 52. For example, the piezoelectric element 63 and its drive circuit 64 can be used.

  Furthermore, the wavelength tunable filter 50 can be configured to use an interference multilayer filter and select the wavelength by minutely changing the angle of incidence on the interference multilayer filter.

  Next, the operation of the wavelength scanning light source according to this embodiment will be described. FIG. 9A shows an emission spectrum of spontaneous emission light (ASE light) of the gain element 40. FIG. 9B shows the transmittance characteristics of the wavelength tunable filter 50 having a peak in a part of the emission band of spontaneous emission light. When light is incident from the gain element 40, filtered spontaneous emission light is obtained as shown in FIG. By amplifying this light again by the gain element 40, amplified spontaneous emission light is obtained as shown in FIG. 9 (d). By cutting out naturally radiated light with a narrow-band tunable filter in this way, the characteristics of the filter appear as it is even when the wavelength is scanned at high speed, and the spectrum width increases as the scanning speed increases. Absent.

  FIG. 10 is a graph showing the relationship between the wavelength scanning speed and the spectrum width. In the conventional external resonator type tunable filter, the oscillation spectrum width gradually increases as the scanning frequency is increased as shown in FIG. On the other hand, in the wavelength scanning light source according to the present embodiment, the spectrum width is constant regardless of the scanning speed as shown in FIG. Therefore, compared with the conventional characteristic shown in FIG. 3, it is more effective than the conventional one at a scanning speed of 10 to 20 kHz or more. Therefore, the wavelength scanning light source 10 can be suitably used as a light source for an optical tomographic image display system that requires high-speed scanning.

  Next, various arrangement examples of the wavelength tunable filter and the gain element will be described with reference to FIG. In FIG. 11, the control units 41 and 60 are omitted. FIG. 11A shows a case where the wavelength tunable filter 50 is a reflection type. In this case, the output from the gain element 40 is incident on the wavelength tunable filter 50, and the reflected light can be again amplified by the gain element 40 to oscillate.

  FIG. 11B shows a case where the wavelength tunable filter 50 is a transmission type. In this case, a mirror 71 is provided outside the wavelength tunable filter 50, and natural radiation emitted from the gain element 40 is incident on the mirror 71 through the wavelength tunable filter 50. The light reflected by the mirror 71 returns to the gain element 40 through the wavelength variable filter 50 again. For this reason, the wavelength selection characteristic can be improved by passing through the wavelength tunable filter 50 twice. In this case, the etalon filter described above is used.

  In FIG. 11C, the wavelength tunable filter is a transmission filter, and the naturally radiated light from the gain element 40 is incident on the wavelength tunable filter 50 via the optical circulator 72. The light transmitted through the wavelength tunable filter 50 returns to the gain element 40 again via the optical circulator 72. Also in this case, the wavelength can be set to a narrow band by the wavelength selection characteristic of the wavelength tunable filter 50.

  In the present invention, the external resonator type laser light source is not used as the wavelength scanning light source, and the natural radiated light of the gain element is cut by a narrow band filter to obtain a narrow band wavelength. Therefore, the spectrum does not spread even when scanned at high speed, and the optical tomographic image can be displayed at a wide measurement depth by applying it to the optical tomographic image display system.

  In the present invention, a wavelength scanning light source that does not spread spectrum even when scanned at high speed is applied to an optical tomographic image display system. Therefore, the present invention can display an optical tomographic image at a wide measurement depth, and is suitably used for an optical tomographic image display system for observing an image of an internal structure below the surface of an object or an image of a subepidermal layer tomographic image of a living tissue. be able to.

DESCRIPTION OF SYMBOLS 10 Wavelength scanning light source 11,15 Optical fiber 13 Reference mirror 17 Scanning mirror 20 Photodiode 21 Amplifier 22 Signal processing part 40 Gain element 41 Control part 50 Wavelength variable filter 51 Diffraction grating 52 Fabry-Perot etalon 60 Control part 61 Polygon mirror 62 Drive Unit 63 Piezoelectric element 63 Drive circuit 71 Mirror 72 Optical circulator

Claims (5)

A wavelength scanning light source that periodically scans the oscillation wavelength of light;
An interference optical meter for branching light from the wavelength scanning light source into reference light and irradiation light to the object, and generating interference light between the reflected light from the object and the reference light;
A light receiving element that receives interference light obtained from the interference optical meter and obtains a beat signal;
A signal processing unit that calculates a tomographic image of the object by Fourier-transforming a signal obtained based on the output from the light receiving element when a reflector is disposed at the measurement position of the object; In the tomographic image display system,
The wavelength scanning light source is:
A gain element having a gain in the oscillation wavelength band;
A wavelength tunable filter that periodically scans the selected wavelength;
The spontaneous emission light of the gain element is incident on the wavelength tunable filter, and light having a selected wavelength is incident on the gain element again to be amplified, thereby outputting light whose oscillation wavelength is scanned. Optical tomographic image display system.   The optical tomographic image display system according to claim 1, wherein a wavelength scanning frequency of the wavelength scanning light source is 10 kHz or more. The wavelength tunable filter of the wavelength scanning light source is:
3. The optical tomographic image display system according to claim 1, wherein the optical tomographic image display system is a reflective wavelength tunable filter that selectively reflects the wavelength of spontaneously emitted light incident from the gain element and inputs the reflected light to the gain element again. The wavelength tunable filter of the wavelength scanning type light source is a transmission type wavelength selection filter,
3. The light according to claim 1, further comprising a mirror that is disposed on a side of the wavelength tunable filter and reflects the light having a selected wavelength that is transmitted through the wavelength tunable filter so as to be incident on the wavelength tunable filter again. Tomographic image display system. The wavelength tunable filter of the wavelength scanning type light source is a transmission type wavelength selection filter,
Arranged between the gain element and the wavelength tunable filter, the natural radiation emitted from the gain element enters the wavelength tunable filter, and the light transmitted through the wavelength tunable filter is fed back to the gain element again. The optical tomographic image display system according to claim 1, further comprising an optical circulator.

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