JP5463473B2 - Optical tomographic image display system - Google Patents

Description

  The present invention relates to an optical tomographic image display system using a mode-locked laser oscillator.

  In recent years, with the advancement of medical techniques such as endoscopic treatment, a diagnostic method that performs 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. An optical tomographic image display system (also referred to as an optical coherence tomography system or OCT) has attracted attention as a technique for complementing such a method.

  There are two types of OCT: time-domain OCT (TD-OCT) and frequency-domain OCT (FD-OCT). Also in FD-OCT, spectrometer type (SD-OCT) and wavelength scanning light source There are two types (SS-OCT). In the case of time domain OCT, broadband light is applied to a living body, and interference components of reflected light from the light are extracted. However, with this method, reflected light from different depths overlaps with the interference light. Therefore, only signal light from a specific depth could not be detected with high sensitivity.

  On the other hand, the wavelength scanning type OCT irradiates a living body with light having a narrow spectral wavelength, continuously changes the wavelength of the irradiation light, and generates a 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 causing interference and analyzing the frequency component of the interference 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. SS-OCT using a wavelength scanning light source is suitable for practical 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.

  In particular, SS-OCT is advantageous for tomography of a site that requires infrared light having a wavelength of 1 μm or more, and for applications in an endoscope using an optical fiber. On the other hand, in ophthalmology and cardiovascular tomography, in order to remove the influence of the movement of a living body or to photograph a large area in a short time, it is required to photograph at higher speed. Therefore, a light source having a high scanning speed is required.

  A conventional wavelength scanning light source scans a wavelength by mechanically deflecting light using a polygon mirror, a MEMS mirror, or the like to change a filter wavelength selection characteristic in an external resonator. With such a light source, high-speed scanning of 100 kHz or more has to use a complicated mechanism, and it has been difficult to realize at low cost. In addition, there is a drawback that reliability is low due to mechanical operation.

  Patent Documents 1 and 2 propose a wavelength-tunable mode-locked laser that can change the oscillation wavelength by changing the frequency of the clock signal for mode-locking by placing a dispersion element in an external resonator. Yes. The mode-locked laser is a laser that modulates a laser at a frequency that is the same as or an integral multiple of the frequency interval of the external resonator mode, and repeatedly oscillates the laser at that frequency. By using a wavelength tunable method using chromatic dispersion for the external resonator type laser, a mechanical mechanism can be eliminated and high-speed scanning of 100 kHz or more can be realized. This has a simple configuration and enables high-speed wavelength tuning.

Next, the conditions for mode-locked oscillation and the principle of variable wavelength will be described. First, in general, a frequency corresponding to the frequency fc of a clock signal for modulating the light intensity inside the resonator (hereinafter referred to as a clock frequency or a mode-lock frequency) fc and a repetition time corresponding to the time that light propagates in the external resonator. When the frequency F matches, stable mode-lock oscillation is realized. The clock frequency fc can be freely changed from a waveform synthesizer, a function generator, or the like. On the other hand, the repetition frequency F is a frequency corresponding to a propagation time _{TL in} which light propagates in one round or one round trip in the external resonator of the laser. Propagation time T _L is _{L when} the resonator length of the external resonator is L.
T _L = nL / c (1)
Where n is the refractive index and c is the speed of light.
The relationship between F and T _L is
F = m / T _L (2)
However, m (integer)> 0
There is a relationship. In a normal mode-locked laser, the gain characteristic of a gain medium such as a semiconductor optical amplifier inserted in the external resonator in FIG. 1A is shown by a curve A, and the spectrum determined by the external resonator length is shown at the same time. , Fc and F coincide with each other, broadband oscillation is assumed within the gain characteristic range as shown in FIG. The mode interval of this spectrum is determined by FSR (= c / L).

Next, the principle of variable wavelength will be described. In a mode-locked laser, when a dispersive element is included in an external resonator, the external resonator length L depends on the oscillation wavelength λ, and the propagation time _TL also depends on the wavelength λ. Therefore, the external resonator length L and the propagation time T _L are functions of the wavelength λ as follows.
L = L (λ)
T _L = T _L (λ)
Therefore, the repetition frequency F also depends on the oscillation wavelength λ. FIG. 2A shows a spectrum in which oscillation is possible in this case, and simultaneously shows a gain characteristic of the semiconductor optical amplifier as a curve A. FIG. In this case, when modulation driving is performed at a certain clock frequency fc,
fc = F (λ) (3)
Stable mode-locked oscillation occurs at a wavelength λ that satisfies the above. At other wavelengths λ, the condition of the expression (3) is not satisfied, and thus oscillation does not occur. Therefore, if the clock frequency fc is, for example, fc ₁ , only the light having the spectrum shown in FIG. 2B oscillates. When fc ₂ by changing the clock frequency, it is possible to oscillate the light of the spectrum shown in Figure 2 (c). Furthermore, by continuously changing the clock frequency fc, it is possible to continuously change the oscillation frequency of the laser light. In this case, the longitudinal mode interval is determined by FSR (= c / L (f)).

  Non-Patent Document 1 reports a laser light source that can sweep a wavelength at high speed using a dispersion optical fiber as a dispersion element based on this principle.

Japanese Patent No. 3432457 Japanese Patent No. 3814495

Yuichi Nakazaki and Shinji Yamashita, "Fast and wide tuning range wavelength-swept fiber laser based on dispersion tuning and its application to dynamic FBG sensing," 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 8310

  However, the wavelength sweep laser of Non-Patent Document 1 uses a long dispersion-compensating optical fiber of, for example, 100 m as a dispersion element. However, when the external resonator length is long, the resonator mode becomes dense and the spectrum that can oscillate is shown in FIG. Therefore, when the clock frequency fc is changed, as shown in FIGS. 3B and 3C, the selectivity of the resonator mode that is mode-locked during the wavelength sweep is lowered, and as a result, a plurality of adjacent modes, That is, it oscillates in multiple modes. In addition, when the resonator length is long, the number of times the light circulates in the resonator during the wavelength sweep decreases, and the resonance amplification of the laser becomes weak, which also causes the oscillation spectrum to widen. Become. Therefore, when the wavelength of the mode-locked laser is scanned at a high speed, the high coherence required for wavelength scanning optical coherence tomography, that is, the narrow oscillation spectrum line width cannot be maintained, and the spectrum becomes thick. there were. Further, as described above, if the wavelength selectivity is deteriorated or the resonance amplification is weakened, it becomes a factor for narrowing the wavelength sweep range. Therefore, when this laser light source is applied to an optical tomographic image display system, there is a drawback that the depth of image and the resolution are lowered.

  The present invention has been made in view of the problems of such a conventional distributed mode-locked laser, which can change the wavelength and realize a high-speed and high-coherence laser light source. Therefore, it is a technical object to provide a high-resolution optical tomographic image display system.

  In order to solve this problem, an optical tomographic image display system of the present invention includes a wavelength scanning type light source that includes a wavelength tunable laser and periodically scans an oscillation wavelength of light, and outputs light from the wavelength scanning type light source. An interference optical meter that branches into reference light and irradiation light to the object, generates interference light between the reflected light from the object and the reference light, and receives the interference light obtained from the interference optical meter to obtain a beat signal A light receiving element, and an A / D conversion unit that performs A / D conversion on the output from the light receiving element, and a signal processing unit that forms a tomographic image of the object by Fourier transforming the interference signal. The wavelength scanning light source includes an external resonator, a wavelength dispersion element having a pair of opposing diffraction gratings provided inside the external resonator, and a semiconductor optical amplifier that amplifies the light in the external resonator; , Modal mode added to the semiconductor optical amplifier Has a mode-locked signal generator for changing the frequency of click signal, a, is to vary the wavelength by modulating the mode-locked signal.

  Here, the resonator length of the external resonator of the wavelength scanning light source may be 10 m or less.

  Here, the A / D conversion unit of the signal processing unit may perform A / D conversion at a sampling frequency that is an integral multiple of the frequency of the mode-locked signal or a fraction of an integer.

  Here, the signal processing unit may sample the A / D conversion unit using a mode lock signal as an external clock.

  Here, the mode-lock signal generator may change the frequency of the mode-lock signal with a function corrected so that the laser oscillation frequency of the wavelength scanning light source sweeps linearly in terms of time.

  According to the present invention having such features, wavelength scanning is possible by continuously sweeping the clock signal frequency of the injected current when injecting the current into the semiconductor optical amplifier which is a gain medium for mode locking. It becomes. In the present invention, a relatively large dispersion value can be obtained with a short external resonator length as compared with the case of using a dispersion compensating fiber. Therefore, by shortening the external cavity length, the number of adjacent modes of the laser oscillation spectrum can be reduced, and the laser light with a narrow spectrum can be oscillated. By using this light source, it is possible to improve the depth of the tomographic image.

FIG. 1 is a diagram showing the oscillation principle of a mode-locked laser. FIG. 2 is a diagram showing a laser oscillation state when the clock frequency is changed using a dispersion element of a mode-locked laser. FIG. 3 is a diagram showing an oscillation state of the mode-locked laser using the dispersion element when the resonator length of the dispersion element is long. FIG. 4 is a diagram showing an overall configuration of the optical tomographic image display system according to the first embodiment of the present invention. FIG. 5 is a diagram showing the configuration of the wavelength scanning light source used in this embodiment. FIG. 6 is a view showing an optical portion of the wavelength scanning light source. FIG. 7 is a diagram illustrating waveforms of the modulation signal and the mode lock signal according to the present embodiment. FIG. 8 is a diagram showing laser oscillation spectra at times t ₁ and t ₃ and times t ₂ and t ₄ . FIG. 9 is a diagram showing the relationship between the light pulse of the laser light source, the interference signal, and the sampling timing. FIG. 10 is a diagram showing the relationship between the optical pulse of the laser light source, the interference signal, and the sampling timing. FIG. 11 is a diagram showing an overall configuration of an optical tomographic image display system according to the second embodiment of the present invention. FIG. 12 is a diagram showing changes in the oscillation frequency with respect to the time axis. FIG. 13 is a diagram showing the relationship between the modulation signal and the mode lock signal.

(First embodiment)
FIG. 4 is a block diagram showing the overall configuration of the optical tomographic display system using the wavelength scanning light source according to the first embodiment of the present invention. In this figure, a wavelength-tunable mode-locked laser light source that oscillates an optical signal in a certain frequency range is used as the wavelength scanning light source 10. The output of the wavelength scanning light source 10 is given to one end of the optical fiber 11. A collimating lens 12 and a reference mirror 13 are provided at the other end of the optical fiber 11. In addition, a coupling portion 14 is provided at an intermediate portion of the optical fiber 11 to cause another optical fiber 15 to approach and interfere. One end of the optical fiber 15 is provided with a collimating lens 16 that converts an optical signal obtained from the wavelength scanning light source 10 through the coupling unit 14 into parallel light, and a scanning mirror 17 that scans the light. The scanning mirror 17 changes the reflection angle of parallel light by rotating within a certain range about an axis perpendicular to the paper surface. The converging lens 18 is disposed at a position where the reflected light is received, condenses the light to the measurement site, and scans (scans) in the horizontal direction. Here, the optical distance L ₁ from the coupling portion 14 to the reference mirror 13 is set equal to the optical distance L ₂ from the coupling portion 14 to the surface of the measurement site. 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 portion 14, the collimating lens 12, the reference mirror 13, the collimating lens 16, the scanning mirror 17, and the focusing 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 signal processing unit 22 obtains a tomographic image signal by Fourier transforming a light reception signal obtained from the interferometer, as will be described later.

  Next, the wavelength scanning light source 10 used in the first embodiment of the present invention will be further described. FIG. 5 is a diagram showing the overall configuration of the wavelength scanning light source 10. In FIG. 5, on the side of the mirror 31, a pair of diffraction gratings 32 and 33 are arranged in parallel and inclined as shown, with their diffraction surfaces facing each other. In the present embodiment, the wavelength dispersion element is constituted by a pair of diffraction gratings 32 and 33, and the resonator lengths on the short wave side and the long wave side of the wavelength scanning range are made different to generate a delay amount corresponding to the wavelength. . A lens 34 and a semiconductor optical amplifier (hereinafter referred to as SOA) 35 for converging light passing through the external resonator are provided on the side of the diffraction grating 33. The SOA 35 is configured as a reflecting surface at the right end in the figure, and the mirror 31 and the right end of the SOA 35 constitute an external resonator. The end face of the SOA 35 can be used as an external resonator by providing a mirror on the outside of the SOA 35 without using the reflecting face. A part of the light is output from the SOA 35 to the outside through the lens 36 as laser light.

  The modulation signal unit 37 gives a predetermined sawtooth signal to the mode lock signal generation unit 38 as a modulation signal. The mode lock signal generator 38 adds a signal whose frequency continuously changes based on the modulation signal to the bias tee 39 as a mode lock signal. A current source 40 is also connected to the bias tee 39, and the injected current is modulated and applied to the SOA 35 as an intensity modulation signal.

Next, the positional relationship between the pair of diffraction gratings 32 and 33 constituting the dispersion device and the mirror 31 will be described with reference to FIG. Here, when the grating pitch of the diffraction gratings 32 and 33 is a, and the incident angle and the reflection angle to the diffraction grating 33 are θ ₁ and θ ₂ , the following relationship holds.
λ = a (sin θ ₁ + sin θ ₂ ) (4)
Here, the shortest wavelength of the wavelength scanning light source is λ ₁ , and the longest wavelength is λ ₂ . Here the incident angle theta ₁ is constant, the reflection angle theta ₂ is because it depends on the wavelength, the wavelength lambda _1, each reflection angle theta ₂ in the case of lambda ₂ s _{θ 2 (λ 1), θ} 2 (λ 2) And In the case of the shortest wavelength, the length between the mirror 31 and the diffraction grating 32 is L ₄ , and the distance between the diffraction grating 33 and the reflective surface of the SOA 35 is L ₅ as shown in FIG. At this time, the external resonator length L (λ ₁ ) of the shortest wavelength λ ₁ is expressed by the following equation.
L (λ ₁ ) = L ₃ / cos θ ₂ (λ ₁ ) + L ₄ + L ₅ (5)
In the case of the longest wavelength λ ₂ , the external resonator length L (λ ₂ ) is expressed by the following equation.
L (λ ₂ ) = L ₃ / cos θ ₂ (λ ₂ ) + L ₄ + L ₃ sin θ ₁ (tan θ ₂ (λ ₂ )
−tan θ ₂ (λ ₁ )) + L ₅ (6)
The difference between the round trip of the external resonator length is expressed by the following equation.
2ΔL = L (λ ₁ ) −L (λ ₂ )
= 2L ₃ {(1 / cos θ ₂ (λ ₁ ) −1 / cos θ ₂ (λ ₂ ))
_{-Sinθ 1 (tanθ 2 (λ 2} ) -tanθ 2 (λ 1))} ··· (7)

Here, the grating pitch a of the diffraction gratings 32 and 33 is 0.74 μm, the wavelength λ ₁ is 1260 nm, λ ₂ is 1360 nm, L ₃ is 100 mm, and θ ₁ is 72 deg. When light parallel to the x-axis as shown is incident on the diffraction grating 33 and the incident angle and theta _1, wavelength lambda _1, the reflection angle (diffraction angle) of the diffraction grating 33 with respect to lambda ₂ of the light beam theta ₂ ( λ ₁ ) and θ ₂ (λ ₂ ) are as follows.
θ ₂ (λ ₁ ) = 49 deg
θ ₂ (λ ₂ ) = 62 deg
The change ΔL of the external resonator length is 2ΔL = −270 mm.
Is required.

From this dispersion value, the delay amount Δτ is expressed by the following equation.
Δτ = ΔL / c = -900psec
When the delay amount Δτ is divided by the change Δλ (100 nm) from the oscillation wavelength 1260 nm to 1360 nm, a dispersion value D of −9 psec / nm is obtained. This is almost equal to the dispersion value of the dispersion-type optical fiber shown in Non-Patent Document 1, that is, the dispersion value when an optical fiber having a dispersion per unit length of about -100 psec / nm / km is used for 100 m. For example, in order to obtain | −90 ps / nm / km × 100 m | = 10 ps / nm necessary for variable dispersion referred to in Non-Patent Document 1, when a pair of diffraction gratings is used as in this embodiment, A dispersion element having the same dispersion value can be obtained with a length of about one thousandth. Therefore, in the following configuration, a laser light source having a variable range of 100 nm with an oscillation wavelength from 1260 nm to 1360 nm can be realized with a resonator length of, for example, 5 m or less.

  In FIG. 4, the signal processing unit 22 includes an A / D conversion unit 51 that performs A / D conversion on the output from the amplifier 21, a Fourier transform circuit 52 that performs Fourier transform on the A / D conversion output, and the Fourier transformed signal as image information. The CPU 53 is configured. The output of the CPU 53 is given to a monitor 54 that displays an image.

Next, the operation of the present embodiment will be described. As shown in FIG. 7A, the signal modulation unit 37 outputs a sawtooth waveform that changes continuously between times t ₁ to t ₂ , t _{3 to} t ₄ ... To the mode lock signal generation unit 38. To do. The mode-lock signal generator 38 generates a mode-lock signal that is continuously changed while continuously changing the sine-wave clock frequency from the frequencies fc ₁ to fc ₂ in accordance with the modulation signal. This signal is applied to the bias tee 39 so as to be superimposed on a constant bias current as a current signal. As a result, as shown in FIG. 8A, when the frequency fc of the clock signal is fc ₁ , the laser light of 1260 nm pulsates, and when the clock frequency is fc ₂ , the laser light of 1360 nm pulsates. The pulse oscillation repetition frequency corresponds to the frequencies of fc ₁ (240 MHz) and fc ₂ (231 MHz), respectively. FIG. 8A shows the oscillation spectrum of the laser beam at times t ₁ and t ₃ , and FIG. 8B shows the oscillation spectrum of the laser beam at times t ₂ and t ₄ . Here, the repetition period of the modulation signal shown in FIG. 7A can be arbitrarily selected, and since there is no mechanical operation part, even if the period is shortened, the pulse oscillation wavelength of the laser follows this at high speed. The oscillation wavelength can be swept.

  Assuming that the dispersion of the dispersion element shown in FIG. 6 is only the primary dispersion, the oscillation wavelength λ changes linearly when the clock frequency fc is swept linearly. However, since laser oscillation is pulse oscillation corresponding to the clock frequency fc, it is necessary to match the timing with the sampling frequency of the A / D converter 51. That is, the sampling frequency of the A / D conversion unit 51 needs to be an integral multiple of the mode lock frequency or a fraction of an integer.

  9 and 10 show different examples of the interference signal, the sampling train, and the optical pulse. 9A to 9C show the relationship between an interference signal obtained from the photodiode 20, a sampling train, and an optical pulse obtained by laser oscillation. As shown in this figure, by sampling the interference signal in synchronization with the optical pulse, the interference signal can be sampled without any information leakage. As shown in FIG. 10, even when the sampling frequency is ½ of the mode lock frequency, data can be collected without omission. Although not shown, data can be obtained in the same way even in the case of an integral multiple and a fraction of an integer, without leakage of information.

Here, in the dispersion variable wavelength sweep laser, the wavelength variable range Δλ is expressed as follows, where the variable range of the repetition frequency is ΔF, the dispersion value of the dispersion element in the resonator is D, and the order is m. .
Δλ = − (n ₀ / cDmFSR) · ΔF
=-(L / Dm) · ΔF

  Here, L / Dm, which is a coefficient, represents wavelength-variable sensitivity (tilt) with respect to the repetition frequency F. And the selectivity of a wavelength becomes high, so that sensitivity L / Dm is small. On the other hand, if the sensitivity L / Dm is too high, there are many components with close mode spacing, so that the wavelength selectivity is deteriorated, and as a result, the spectrum width during oscillation is widened. The smaller the resonator length L, the larger the dispersion D, or the larger the order m (F is higher), the smaller the sensitivity value. In the dispersion compensating fiber, in order to increase the dispersion value D, the length of the fiber must be increased. Therefore, the resonator length L is increased, and conversely, in order to reduce the resonator length L, the dispersion value D is decreased. Therefore, the range of conditions under which the sensitivity can be set small is limited.

  On the other hand, in the present invention, since the dispersion D and the resonator length L can be set independently, D can be set large and the resonator length L can be shortened to reduce the sensitivity. Thereby, conditions with high wavelength selectivity can be set.

  For example, as described above, in order to obtain the same dispersion, the dispersion compensating fiber requires a fiber length of 100 m, whereas in the present invention, the resonator length L is shorter than the conventional one and is 10 m or less, preferably 1 m or less. Can be designed to reduce sensitivity. Further, when the resonator length is short, the number of times the light in the resonator circulates increases in accordance with the effect of increasing the selectivity by decreasing the sensitivity, so that the resonance effect is enhanced and the oscillation spectrum can be further narrowed. That is, as a result, the coherence length can be greatly improved.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. In the first embodiment, since the mode lock frequency changes by the swept frequency, when the sampling timing of the A / D converter 51 is an equal time interval, the timing of the oscillation pulse of the light source during one scan And A / D conversion timing may deviate. For this reason, if the sweep range of the mode lock signal is large, apparently non-linear characteristics are superimposed, and the resolution in the depth direction of the optical tomographic image may deteriorate. In the second embodiment, this problem is solved.

  FIG. 11 is a block diagram showing the overall configuration of the second embodiment. The same parts as those of the above-described embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. In this embodiment, the signal from the mode lock signal generator 38 of the wavelength scanning light source 10 shown in FIG. 4 is shaped and supplied to the A / D converter 51 of the signal processor 22 as an external clock. The A / D converter 51 performs sampling according to the timing of the mode lock signal. By doing so, the sampling timing of the A / D conversion matches, and this problem can be solved.

(Third embodiment)
Next, a third embodiment of the present invention will be described. In this embodiment, when the optical frequency corresponding to the time based on either the timing of the oscillation pulse of the light source or the sampling timing of the A / D conversion changes nonlinearly, the clock frequency is set by the time response corrected by the inverse function. By sweeping, the nonlinearity is canceled out.

As described above, when the mode-lock signal is a sawtooth wave linear with respect to the time axis as shown in FIG. 7A, when there is a high-order dispersion component in the resonator, the time axis as shown in FIG. In contrast, the oscillation frequency f of the laser light changes nonlinearly. Here, the oscillation frequency of the laser beam is detected at equal time intervals, and a function with the change as f = g (t) is obtained. Then, a modulation signal that is an inverse function is generated so that the change in frequency with respect to time is linear, that is, f = −at + b. That is, the following equation t = g ⁻¹ (f)
The mode lock frequency is changed to satisfy This corresponds to modulating the mode-locked signal using a nonlinear modulation signal as shown in FIG. In this way, the oscillation frequency can be changed linearly, and the resolution in the depth direction of the optical tomographic image can be improved.

  INDUSTRIAL APPLICABILITY The present invention is suitable for an optical tomographic image display system for observing an image of the surface of an object, the internal structure and the subepidermal layer of a living tissue by using a wavelength scanning light source that oscillates in a narrow band and can realize high-speed scanning. Can be used.

DESCRIPTION OF SYMBOLS 10 Wavelength scanning light source 11,15 Optical fiber 12,16,19 Collimating lens 13 Mirror 14 Coupling part 17 Scanning mirror 18 Focusing lens 20 Photodiode 21 Amplifier 22 Signal processing part 31 Mirror 32, 33 Diffraction grating 34 Lens 35 Semiconductor optical amplifier 37 Modulation signal section 38 Mode lock signal generation section 39 Bias tee 40 Current source 51 A / D converter 52 Fourier transform circuit 53 CPU
55 Monitor

Claims (5)

A wavelength scanning light source that includes a wavelength tunable laser and 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 has an A / D conversion unit that performs A / D conversion on an output from the light receiving element, and that forms a tomographic image of the object by Fourier-transforming the interference signal;
The wavelength scanning light source is:
An external resonator,
A wavelength dispersion element provided inside the external resonator and having a pair of opposing diffraction gratings;
A semiconductor optical amplifier for amplifying the light in the external resonator;
An optical tomographic image display system comprising: a mode-lock signal generating unit that changes a frequency of a mode-lock signal applied to the semiconductor optical amplifier, and changing a wavelength by modulating the mode-lock signal.   The optical tomographic image display system according to claim 1, wherein a resonator length of an external resonator of the wavelength scanning light source is 10 m or less.   2. The optical tomographic image display system according to claim 1, wherein the A / D conversion unit of the signal processing unit performs A / D conversion at a sampling frequency that is an integral multiple of a frequency of the mode-lock signal and a fraction of an integer. .   The optical tomographic image display system according to claim 1, wherein the signal processing unit samples the A / D conversion unit using a mode lock signal as an external clock.   2. The optical tomographic image according to claim 1, wherein the mode lock signal generator changes the frequency of the mode lock signal with a function corrected so that the laser oscillation frequency of the wavelength scanning light source sweeps linearly in time. Display system.

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