JP2018040622A - Method of evaluating wavelength swept light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem of an SNR evaluation method for a wavelength swept light source in prior art for which improvements for increasing the performance of an SS-OCT device is desired for reasons of insufficiency, that a wavelength swept signal light includes a fundamental wave and harmonic components of repetition frequency of wavelength sweep, but the relationship spectrum characteristics of the fundamental wave and harmonic components and whether an evaluation parameter in a conventional frequency swept electric signal is appropriate for the wavelength swept light source are unknown, and the stability of the wavelength swept light source cannot be accurately evaluated as long as only noise parameters of the fundamental wave are relied as in the case of the SNR evaluation of an electric signal generator.SOLUTION: In a wavelength swept light source evaluation method of the present invention, the output light of a wavelength swept light source is converted into an electric signal and then, with the intensity fluctuation of each of a fundamental wave component of wavelength swept repetition frequency and its harmonic wave component taken into account, the stability of the light source is determined on the basis of the average of intensity fluctuations. The present invention can also be implemented as a wavelength swept light source evaluation system. It can also be implemented as a program invention for carrying out the wavelength swept light source evaluation method.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a method for evaluating a wavelength swept light source. Wavelength swept light sources are widely used in optical equipment and electronic equipment using imaging technology using light. For example, it is most often used in consumer electronic devices such as cameras, printers, and facsimiles, and has spread to the medical field. Optical coherent tomography (OCT), which can acquire information in the depth direction with high resolution using low coherent light, has been attracting attention in order to non-invasively image a tomography inside a living body. It was. In recent years, OCT has also been used in the field of molecular (molecular) imaging for elucidating biological events at the molecular and cellular levels in vivo. As will be described later, the wavelength swept light source is an important key element that supports the OCT technology, and greatly affects the performance of the OCT apparatus. Therefore, the performance of the wavelength swept light source and the evaluation method of the wavelength swept optical signal are important.

  In the above-described molecular imaging, a method of detecting target molecules with high sensitivity using optical information is the main force, and an OCT apparatus is used. Here, an overview of the configuration and operation of the OCT apparatus is as follows. In order to collect three-dimensional information with an OCT apparatus, it is necessary to scan a light beam output from a low-coherence light source called a super luminescence diode (SLD) in the horizontal and vertical directions. For this scanning, in the OCT apparatus, the light beam output from the SLD light source through the collimator is first separated into reference light and measurement light by a beam splitter. The separated measurement light is mechanically scanned in the horizontal and vertical directions using a biaxial galvanometer mirror. The scanned measurement light is reflected by each layer of the measurement object T input through the objective lens, and returns to the beam splitter again. The measurement light from the measurement object is merged again with the reference light reflected and returned by the movable mirror in the beam splitter, and enters the photo detector (PD). The signal processing apparatus accompanying the OCT apparatus detects the intensity and time shift of the measurement light based on the interference phenomenon that occurs when the measurement light and the reference light merge, and derives the spatial positional relationship (three-dimensional information). Yes. OCT apparatuses that acquire tomographic images using low coherence interference include time domain OCT (TD-OCT) and Fourier domain OCT (FD-OCT: Fourier Domain Optical Coherence tomography). The FD-OCT is further classified into a spectral domain OCT (SD-OCT: Spectral Domain Optical Coherence Tomography) and a wavelength sweep OCT (SS-OCT: Swept-Source Optical Coherence Tomography). SS-OCT using a wavelength swept light source is particularly excellent in terms of high-speed response. Since the responsiveness of SS-OCT is largely limited by the performance of the light source, development of various types of high-speed broadband light sources is accelerating.

FIG. 10 is a diagram schematically illustrating the basic configuration and operation of the SS-OCT apparatus. In the SS-OCT apparatus 20, for example, an optical signal 26 in which the optical frequency (or wave number) is linearly swept with respect to time is supplied from the optical frequency (wavelength) sweep light source 21 to the living body 24. As the optical frequency sweep light source 21, for example, a wavelength variable laser is used. Further, the position of the mirror 23 is fixed. If the distance between the center of the beam splitter 22 and the mirror 23 is L _R , and the distance between the center of the beam splitter 22 and the living body surface 31 is L _S , each element is arranged so that L _R = L _S. Has been.

At this time, the difference in optical frequency between the reference light 28, each reflected light 29b from the reflecting surface 32 in the living body, and each reflected light 29c from the reflecting surface 33 is constant regardless of time. If the difference between these optical frequencies is f ₂ and f ₃ , beat frequencies f ₂ and f ₃ corresponding to the reflective surface 32 and the reflective surface 33 are mixed due to interference between the reference light 28 and the reflected light 29b and 29c. Signal light is obtained. When this signal light is subjected to Fourier transform, reflected light intensity having peaks at beat frequencies f ₂ and f ₃ can be obtained. If it is optical frequency sweep linearly from the light source 21, and the beat frequency f _2, f _3, the depth d _2, the depth d ₃ is directly proportional to. In the living body, reflected light is generated from various places. Therefore, the reflected light intensity distribution along the optical axis (z-axis) direction can be obtained by Fourier transforming the interference light. If a beam scan is performed also in the x-axis direction, an OCT tomographic image in the xz plane can be obtained.

  In the SS-OCT apparatus, the photodetector 25 only needs to detect signal light in which interference light with different beat frequencies is mixed with a single detection element, and thus a parallel detector required for TD-OCT is not necessary. Furthermore, it is possible to use a 1.3 μm-band swept light source that is suitable for diagnosis of skin and the like and is easily available because there is no scattering of red blood cells by hemoglobin. The SS-OCT system is practical in the field of molecular imaging including ophthalmic medical care because of its stable configuration using optical fiber couplers, the ability to acquire high-speed images by eliminating the need for movable mirrors, and the ease of use of various photodetectors. Is being promoted. Here, let us consider the conditions necessary for the wavelength swept light source in the SS-OCT apparatus.

  Referring to Non-Patent Document 1, the SS-OCT interference signal s (t) is expressed as follows.

Here, t is time, k (t) is the wave number of the light source light, A (k) is the electric field intensity of the light source light with respect to the wave number of the light, and z is the distance from the reference plane to the reflecting surface being observed. , C indicate the speed of light, P _s indicates the reflected light power on the sample side to be combined, P _r indicates the reflected light power on the reference side to be combined, and γ (2z / c) indicates the coherence function. .

  Here, a case where the wave number of the output light of the wavelength swept light source changes linearly with respect to time, that is, a light source having a wave number linear is considered. The wave number k (t) at this time is expressed by the following equation.

Here, k ′ represents the rate of change with respect to time k when k ′ = Δk / Δt, where Δk is the sweep wavelength width of the wavelength swept light source, and Δt is the time for sweeping by Δk, and k ₀ is t = The value of k when 0.
When the expression (2) is applied, the interference signal s (t) of the expression (1) is expressed as follows.

Further, when the equation (3) is Fourier transformed, it is expressed as the following equation.

However, the following expression represents the Fourier transform, and the subscript f represents the frequency.

When a point spread function (PSF) to be described later is obtained, the value of k ₀ that is an offset does not matter, so k ₀ = 0.

According to Equation (4), the Fourier transform of the interference signal s (t) has a shape obtained by Fourier transforming the square of the electric field strength A ² (t), and the center frequency is represented by the following equation.

As shown in Expression (4), a signal obtained by Fourier transforming the interference signal s (t) is referred to as a point spread function (or point spread function: PSF). If A ² (t) is a symmetric monomodal function such as Gaussian, the frequency at the peak position of the PSF is the center frequency of the interference signal after Fourier transform expressed by Equation (6).

As described above, when a light source whose wave number of the output light of the wavelength swept light source is linearly changed as expressed by the equation (2) is used, the position z of the reflecting surface in the observation target is the interference signal s ( It can be obtained by measuring the position of PSF which is a signal obtained by Fourier transforming t). In particular, if the square A ² (t) of the electric field strength is a symmetric monomodal function such as Gaussian, the peak position of the PSF corresponds to the position z of the reflecting surface. The equation for calculating the position z of the reflecting surface is as follows.

  From the principle of acquiring an image from the interference light in SS-OCT as described above, the frequency of the interference signal increases in proportion to the distance z from the reference plane as shown in FIG. Even if there are multiple reflection surfaces within the observation target, frequency components corresponding to the distance from the reference surface to each reflection surface are included in the interference signal, so by performing frequency analysis (Fourier transform), multiple reflection surfaces Can be measured simultaneously. The detection of the reflecting surface in SS-OCT is performed for each wavelength sweep of the light source. Accordingly, the shorter the wavelength sweep time of the wavelength swept light source, that is, the higher the wavelength sweep frequency, the more tomographic images with less blur can be obtained even if the observation object moves faster.

As the wavelength sweep swept light source to realize the acquisition of less tomographic image of such blurring, in recent _{years, KTN (KTa 1-x Nb} x O 3) crystal or _{KLTN (K 1-y Li y} Ta 1-x Nb x O 3 A high-speed wavelength swept light source in which an optical deflector is incorporated in an external resonator by taking advantage of the high-speed control of the refractive index in the crystal has attracted attention. For example, as disclosed in Patent Document 1, a KTN optical deflector is a key device for realizing high speed in a medical optical tomographic imaging system. Among optical deflectors that change the traveling direction of light, optical deflectors using KTN crystals or KLTN crystals (referred to as KTN deflectors) have no moving parts, unlike galvanometer mirrors, polygon mirrors, MEMS mirrors, etc. It is a solid element. A KTN crystal is known as a substance having a large electro-optic effect, the refractive index of which greatly changes when a relatively low voltage is applied. Further, when Ti or Cr is used as an electrode for applying a driving voltage, charges can be injected into the KTN crystal. By utilizing the internal electric field generated by this injected charge, a high-speed and wide-angle optical deflector can be realized. Therefore, in applications where general optical components such as lenses, prisms, and mirrors need to move at high speed, these optical components can be replaced with KTN.

  As described above, the high speed of the swept wavelength light source is necessary for obtaining an OCT tomographic image without blurring. In addition, the signal-to-noise ratio (SNR) of the OCT signal is important for obtaining a clear image. Index. For example, when various noises such as light source noise and electric noise enter the measured interference signal s (t), the noise also remains in k (t) obtained from the interference signal s (t). In the wavelength-swept light source of the SS-OCT apparatus, a linear scan is actually performed with respect to the wavelength of light. Therefore, in the data processing of the SS-OCT apparatus, it is necessary to convert the data into data whose wave number changes linearly with respect to time so as to correspond to the equation (2). This conversion process is called rescaling process.

  When various noises as described above enter the interference signal s (t), there is a problem that the conversion accuracy of the interference signal is lowered when rescaling is performed. Since the PSF peak is lowered due to the reduced conversion accuracy of the interference signal, there is a problem that the SNR in the OCT tomographic image is lowered. Furthermore, since the SNR of the tomographic image is lowered, there is a problem that the sensitivity to the incident light intensity to the observation object is lowered. Therefore, in order to improve the performance of the SS-OCT apparatus, it is necessary to accurately evaluate the SNR in the wavelength swept light source, and the SNR evaluation method is extremely important.

  When measuring the SNR of wavelength-swept light in a wavelength-swept light source, the optical signal output from the wavelength-swept light source is converted into an electric signal by a photodiode (envelope detection), as in the evaluation of an electrical signal generator. After that, the converted electric signal was evaluated. For example, as shown in Non-Patent Document 2 and Non-Patent Document 3, a frequency domain measuring device such as an RF spectrum analyzer is used to evaluate phase noise (phase fluctuation) near the fundamental wave spectrum, Methods were used to evaluate the noise floor away from the wave spectrum and to evaluate the intensity fluctuation of the fundamental spectrum.

JP 2012-074597 A

Masahiro Ueno, et al. Improvement of coherence distance and SNR of 200 kHz KTN wavelength swept light source by widening KTN optical deflector, Science Technology, 2,014, CPM2014-44, pp.7-12 J. A. Barns et al., “Characterization of frequency stability”, IEEE Trans. Instrum. Meas., IM-20, no. 2, 1971, pp. 105-120 M. Shinagawa et al., “Jitter analysis of high-speed sampling systems”, IEEE J. Solid-State Circuits, SC-25, no. 1, 1990, pp. 220-224

  However, the SNR evaluation method for the wavelength swept light source of the prior art is still not sufficient, and improvement has been desired for improving the performance of the SS-OCT apparatus. As the wavelength swept light source, an optical functional device such as an electro-optical crystal or a semiconductor laser is used. Further, the wavelength sweep signal light includes a fundamental wave component and its harmonic component when the repetition frequency of the wavelength sweep is a fundamental wave. It was not clear whether the relationship between the spectrum characteristics of the fundamental wave component and the harmonic wave component, and whether the conventional evaluation parameters were valid as they were. Therefore, if only the various noise parameters of the fundamental wave are evaluated as in the case of evaluating the SNR of an electric signal generator, the instability of the wavelength swept light source cannot be evaluated sufficiently accurately.

  The present invention has been made in view of such problems of the prior art, and the object of the present invention is to establish a more accurate evaluation method for the stability of a wavelength-swept light source, and to reduce SSNR reduction. The object is to provide an environment for realizing the apparatus.

In order to achieve the above object, according to the present invention, in the system for evaluating the stability of the wavelength swept light from the wavelength swept light source, the wavelength swept light is input and the wavelength swept light is inputted. A photoelectric conversion unit that outputs an electrical signal that is photoelectrically converted from light, a fundamental wave having a wavelength sweep repetition frequency of the electrical signal, and intensity fluctuations of one or more harmonics of the fundamental wave are obtained. A frequency domain signal intensity measuring unit; a dispersion calculating unit for determining a dispersion value of intensity fluctuation at each frequency of the fundamental wave and the one or more harmonics; and an average calculating unit for determining an average value of the dispersion values. This is a system characterized by that.
The invention according to claim 2 is the system according to claim 1, further comprising: a stability determination unit that determines a command for correcting a control parameter of the wavelength-swept light source based on the average value from the average calculation unit. It is further provided with a feature.

  A third aspect of the present invention is the system according to the first or second aspect, wherein the signal intensity measurement unit performs time domain intensity fluctuation in synchronization with the fundamental wave and the one or more harmonics. A frequency setting unit for setting the measurement frequency as required is further provided.

  According to a fourth aspect of the present invention, in the apparatus for evaluating the stability of the wavelength swept light from the wavelength swept light source, the light that receives the wavelength swept light and outputs an electrical signal that is photoelectrically converted from the wavelength swept light. An electrical converter; a fundamental wave having a wavelength sweep repetition frequency of the electrical signal; and a signal intensity measuring part in a frequency domain for obtaining intensity fluctuations of one or more harmonics of the fundamental wave; and the signal intensity measuring part A frequency setting unit that sets a measurement frequency of the signal strength measurement unit so as to obtain a time-domain intensity fluctuation in synchronization with the fundamental wave and the one or more harmonics, the fundamental wave, and the fundamental wave An apparatus comprising: a dispersion calculating unit that obtains a dispersion value of each intensity fluctuation of one or more harmonics; and an average calculating unit that obtains an average value of the dispersion values.

  The invention of claim 5 is the apparatus of claim 4, wherein the stability of the wavelength swept light source associated with the dispersion value at the fundamental and at least one harmonic of the wavelength swept light source. A data storage unit including reference data, a stability determination unit that determines a command for correcting the control parameter of the wavelength-swept light source based on the reference value of the stability and the average value from the average calculation unit; It is further provided with a feature.

  According to a sixth aspect of the present invention, in the method for evaluating the stability of the wavelength swept light from the wavelength swept light source, the fundamental wave having a wavelength swept repetition frequency of the electrical signal photoelectrically converted from the wavelength swept light. A step of tuning, a step of measuring an intensity of an electric signal of a tuning frequency that is a frequency of the fundamental wave, obtaining an intensity fluctuation, a step of obtaining a variance of the intensity fluctuation of the tuning frequency, and the tuning frequency, Instead of the fundamental wave, sequentially tuning to one or more harmonics of the fundamental wave to obtain intensity fluctuations in each of the one or more harmonics and a dispersion value of the intensity fluctuations; and Determining the dispersion value and an average value of the dispersion value in each of the one or more harmonics.

  The invention according to claim 7 is the method according to claim 6, wherein the reference data of the stability of the wavelength-swept light source associated with the dispersion value in the fundamental wave and the one or more harmonics, and Determining whether to change the control parameter of the wavelength-swept light source based on the average value.

  The invention according to claim 8 is a program comprising computer-executable instructions for executing the steps according to claim 6 or 7.

  As described above, according to the present invention, an environment for realizing an SS-OCT apparatus in which a more accurate evaluation method for the stability of a wavelength-swept light source is established and an SNR is suppressed can be provided.

FIG. 1 shows the configuration of a measurement system for evaluating the stability of a wavelength swept light source using KTN. FIG. 2 is a diagram showing a typical spectrum of wavelength swept light from the wavelength swept light source. FIG. 3 is a diagram schematically showing an enlarged spectrum in the vicinity of one frequency component of the wavelength swept light from the wavelength swept light source. FIG. 4 is a diagram illustrating a measurement example of intensity fluctuation of the harmonic component of the wavelength swept light. FIG. 5 is a diagram illustrating a measurement example of intensity fluctuation of another harmonic component of the wavelength sweep light. FIG. 6 is a diagram conceptually illustrating the process in the long sweep light source evaluation method of the present invention. FIG. 7 is a diagram showing the configuration of a system that uses the wavelength-swept light source evaluation method of the present invention and feeds back a stability measurement result to the wavelength-swept light source. FIG. 8 is a flowchart showing a procedure for feeding back the stability measurement result to the wavelength sweep light source using the wavelength sweep light source evaluation method of the present invention. FIG. 9 is a diagram showing data obtained by actually measuring the relationship between the current value of the SOA driving the semiconductor laser and the average value of the intensity fluctuation dispersion. FIG. 10 is a diagram schematically illustrating the basic configuration and operation of the SS-OCT apparatus.

  In the evaluation method of the wavelength swept light source of the present invention, after converting the output light of the wavelength swept light source into an electric signal, the intensity of the fundamental wave component of the repetition frequency to be swept in wavelength and the intensity fluctuations of its harmonic components are taken into consideration. The stability of the light source is determined based on the fluctuation average. The present invention can also be implemented as a wavelength swept light source evaluation system or evaluation apparatus. Further, the present invention can also be implemented as a computer program invention for performing a wavelength-swept light source evaluation method.

  The inventors have found that the fundamental wave component and its harmonic component in the output light of the wavelength swept light source using KTN behave differently from the frequency sweep signal of the electric signal in the intensity fluctuation. We propose a method for evaluating the SNR in consideration of the behavior of the characteristic intensity fluctuation in the wavelength swept output light from the wavelength swept light source, and a method for evaluating the stability of the wavelength swept light source.

  FIG. 1 is a diagram showing a configuration of a measurement system for evaluating the stability of a wavelength swept light source using KTN. The measurement system 10 includes a wavelength swept light source unit 1, a photoelectric converter 2, and a signal intensity measuring device 3 in the frequency domain. Although not shown in FIG. 1, it is also possible to provide a configuration that can automate the stability evaluation of the wavelength swept light source by further providing a communication interface. The optical signal to be evaluated output from the wavelength swept light source unit 1 is converted into an electric signal (envelope detection) by the photoelectric converter 2 such as a photodiode. The converted electric signal is obtained for each frequency component by a signal intensity measuring device in the frequency domain. The frequency domain signal strength measuring instrument is typically a spectrum analyzer. The photoelectric signal converter may include a mixer function for performing frequency conversion so as to match the band of the spectrum analyzer to be used.

  FIG. 2 is a diagram showing a typical spectrum of wavelength swept light from the wavelength swept light source. The converted electrical signal from the photoelectric converter 2 includes a fundamental wave component and a higher-order harmonic component that is an integer n times (n ≧ 2) of the fundamental wave component, assuming that the repetition frequency for wavelength sweeping is the fundamental wave. When the converted electric signal is measured using a signal intensity measuring device 2 in a frequency domain such as an RF spectrum analyzer, spectrum characteristics as schematically shown in FIG. 2 are shown. A fundamental wave component located on the leftmost side and a plurality of harmonic components located on the right side of the fundamental wave component on the frequency axis and gradually decreasing in intensity are observed.

  FIG. 3 is a diagram schematically showing an enlarged spectrum in the vicinity of one frequency component peak in the wavelength swept light from the wavelength swept light source. Referring to the spectrum of FIG. 3, three fluctuation components can be defined for the wavelength swept light from the wavelength swept light source (Non-Patent Document 2, Non-Patent Document 3). First, the peak intensity of a certain frequency component that varies with time is called intensity fluctuation, and can be evaluated by measuring the temporal fluctuation of the peak value of the spectrum. Second, noise power existing in a frequency region slightly away from the frequency indicating the peak value is called phase noise or phase fluctuation, and represents a temporal variation in frequency. Phase noise is also called jitter that represents temporal fluctuations in the waveform. Third, noise in a region that is further away from the frequency region of the phase noise and in which the noise power is constant with respect to the frequency is called a noise floor. These three noises (instabilities) can be evaluated for each harmonic.

  The above three definitions of fluctuation are also used for continuous wave (CW) and frequency sweep wave in the signal generator of the electric signal. Since the characteristic of each harmonic is very similar in the electric signal, only the fundamental wave is used. I should have evaluated. However, a wavelength-swept light source using KTN uses a mechanism such as a polarization mechanism using a KTN electro-optic crystal, a semiconductor laser oscillator, and optical amplification in order to realize wavelength sweep. Therefore, there may be various nonlinearities and distortions different from the frequency sweep signal of the electrical signal not derived from the optical signal. Based on this point, the inventors paid attention to the difference in behavior of three noises and fluctuations in the fundamental wave and the harmonic wave of the electric signal converted from the wavelength swept light.

  Here, an evaluation example regarding intensity fluctuation of a wavelength swept light source using a KTN electro-optic crystal and a semiconductor laser will be described. For example, when the repetition frequency of the wavelength sweep is 200 kHz, a spectrum having a peak characteristic is measured on a frequency corresponding to a harmonic that is an integral multiple of 200 kHz. The peak value of a certain frequency was measured for a certain period of time using the zero span function of the RF spectrum analyzer. The zero span function is a function for measuring only the signal component set to the measurement center frequency with the sweep bandwidth of the spectrum analyzer set to 0 Hz. The zero span function can obtain a time waveform of a specific tuning frequency in a spectrum analyzer, and can measure a signal waveform in the time domain for each tuning frequency. With the zero span function, the intensity fluctuation among the three fluctuations shown in FIG. 3 can be measured.

  FIG. 4 is a diagram illustrating a measurement example of intensity fluctuation of the harmonic component of the wavelength swept light. The horizontal axis represents time (seconds), the vertical axis represents power (dBm), and the peak intensity of the 28th harmonic (5.6 MHz) when the repetition frequency of the wavelength sweep is 200 kHz is 50 seconds. It is the result of observation. FIG. 4 shows that the width of the intensity fluctuation is 1 dB or less, and the power level of the harmonic component at 5.6 MHz is very stable even if the wavelength sweep is repeated at a repetition frequency of 200 kHz.

  FIG. 5 is a diagram illustrating a measurement example of intensity fluctuation of another harmonic component of the wavelength sweep light. As in FIG. 4, the horizontal axis represents time (seconds), the vertical axis represents power (dBm), and the peak of the 34th harmonic (6.8 MHz) when the repetition frequency of the wavelength sweep is 200 kHz. It is the result of observing the intensity of the value for 50 seconds. In contrast to the measurement result of the 5.6 MHz component in FIG. 4, the maximum intensity fluctuation of 12 dB is measured, and there is a difference of about 10 times in the magnitude of fluctuation between the two harmonics. As can be seen from FIGS. 4 and 5, unlike the signal generator for electrical signals, the wavelength fluctuation optical signal from the wavelength sweep light source has different intensity fluctuation characteristics for each harmonic component. Therefore, in order to more accurately evaluate the stability of the wavelength-swept optical signal from the wavelength-swept light source, it became clear that an approach different from the evaluation of only the fundamental wave in the frequency-swept signal of the electrical signal is necessary. .

  The wavelength swept light source evaluation method of the present invention includes evaluation of a plurality of frequency components in the spectrum, as will be described below. Since the level of intensity fluctuation differs depending on the harmonic component, the wavelength swept light source was evaluated in consideration of intensity fluctuation with respect to the fundamental wave and one or more harmonic components. When the phase noise and noise floor were evaluated at each harmonic, there was no significant difference in the characteristics between the harmonics. Therefore, the method for evaluating the wavelength swept light source of the present invention considering only intensity fluctuation will be described below. .

Embodiment 1:
When evaluating the stability of wavelength-swept light from a wavelength-swept light source, the intensity fluctuation characteristics of each harmonic component differ as described above, so the intensity fluctuation is evaluated using only the fundamental wave or a single harmonic component. Doing so is likely to lead to false results. In order to correctly evaluate the performance of the wavelength swept light source, it is necessary to evaluate the spectrum of a plurality of frequency components.

  FIG. 6 is a diagram conceptually illustrating processing in the wavelength sweeping light source evaluation method of the present invention. In the wavelength-swept light source evaluation method of the present invention, the stability of the light source is taken into consideration by considering the level of linearity of the wavelength (wave number) sweep required in an apparatus (for example, SS-OCT apparatus) that uses the wavelength-swept light source. Consider all harmonic components that fall within the power detection band. In the description of FIG. 6, an example is shown in which the detection band is a band that includes a fundamental wave to a harmonic component that is five times higher. In the evaluation method of the present invention, as shown in FIG. 6, the intensity fluctuations of the peak values of the fundamental wave (n = 1) and 2 to 5 times (n = 2 to 5) harmonics are obtained at a predetermined time. Find it with a spectrum analyzer. The variance of the intensity fluctuation obtained for each frequency component is obtained. An average value of each variance in all frequency components within the detection band is obtained, and statistical processing is performed using this as the fluctuation of the wavelength swept light source. In this way, unlike the evaluation of only a single frequency component as in the prior art, the average value of dispersion considering the intensity fluctuations of a plurality of frequency components is used as an indicator of the stability of the wavelength swept light source. The stability of the swept light source can be more accurately evaluated.

  In the description of FIG. 6, the intensity fluctuation is obtained for all the harmonic components in the detection band. However, the intensity fluctuation appears only in a specific harmonic component depending on the configuration and properties of the wavelength swept light source to be evaluated. If it is known in advance, statistical processing may be performed on a plurality of harmonic components in which instability appears remarkably. When the standard deviation of the intensity fluctuation of the nth harmonic is σn, the dispersion of each harmonic component is

Then, the average value A is represented by Formula (8).

Embodiment 2:
FIG. 7 is a diagram showing a configuration of an evaluation system that feeds back a stability measurement result to the wavelength sweep light source using the wavelength sweep light source evaluation method of the present invention. The evaluation system 70 includes a wavelength swept light source unit 71, a photoelectric conversion unit 72, and a signal intensity measuring device 73 in the frequency domain, which are measurement systems for evaluating the stability of the wavelength swept light source using the KTN shown in FIG. The configuration is the same as that in FIG. The evaluation system 70 further includes a dispersion calculation unit 74, an average calculation unit 76, and a signal intensity measuring device 73 in the frequency domain for measuring intensity fluctuations for implementing the wavelength sweeping light source evaluation method of the present invention shown in FIG. A frequency setting unit 75 for setting is included. Although FIG. 7 shows that the frequency setting in the signal strength measuring device 73 is performed in accordance with a frequency change command from the average calculating unit 76, the present invention is not limited to this. The central processing unit (CPU) that controls the operation of the entire evaluation system 70 may be in any block in the evaluation system 70 other than the average calculation unit 76, or may have a control unit independently. Control from outside the evaluation system 70 not shown in FIG.

  Therefore, according to the present invention, in the system 70 for evaluating the stability of the wavelength swept light from the wavelength swept light source, the photoelectric conversion unit that receives the wavelength swept light and outputs an electrical signal that is photoelectrically converted from the wavelength swept light. 72, a fundamental wave having a wavelength sweep repetition frequency of the electrical signal, and a signal intensity measuring unit 73 in a frequency domain for obtaining intensity fluctuations of one or more harmonics of the fundamental wave, the fundamental wave and the 1 The present invention can be implemented as a system including a dispersion calculation unit 74 that obtains a dispersion value of intensity fluctuations at each frequency of two or more harmonics, and an average calculation unit 76 that obtains an average value of the dispersion values. Preferably, a stability determination unit 77 that determines a command for correcting the control parameter of the wavelength swept light source based on the average value from the average calculation unit may be further included. The signal intensity measurement unit may further include a frequency setting unit 75 that sets a measurement frequency so as to obtain an intensity fluctuation in the time domain in synchronization with the fundamental wave and the one or more harmonics. .

  The evaluation system 70 can further include a stability determination unit 77 that determines the stability of the wavelength swept light source 71 based on the dispersion average value A obtained from the average calculation unit 76. The stability determination unit 77 cooperates with the database 78 in which reference data for determining stability according to the type of the wavelength swept light source and the measurement target is stored, and the average value A of the calculated dispersion is stored in the database. It can be determined whether the reference value is satisfied. When the stability criterion is satisfied, the adjustment of the wavelength swept light source unit 71 is finished. When the stability standard is not satisfied, the current and temperature of the semiconductor optical amplifier (SOA) used in the wavelength swept light source unit 71, the KTN drive voltage, the set temperature of the KTN crystal, and the temperature of the light source housing The parameter for controlling the wavelength swept light source is changed, and the adjustment flow is performed again as described below.

  The evaluation system shown in FIG. 7 except for the wavelength swept light source unit 71 except for the wavelength swept light source unit 71 evaluates the stability of the wavelength swept light from the wavelength swept light source, and evaluates the stability of the wavelength swept light from the wavelength swept light source. It can also be implemented as a device that performs. In this apparatus, the database is not necessarily provided. Next, a flow of a processing procedure for feeding back the stability measurement result to the wavelength swept light source using the evaluation system 70 will be described.

Embodiment 3:
FIG. 8 is a flowchart showing a procedure for feeding back the stability measurement result to the wavelength sweep light source using the wavelength sweep light source evaluation method of the present invention. As shown in FIG. 7, in the evaluation system 70, the optical signal from the wavelength swept light source unit 71 is converted into an electric signal by the photoelectric converter 72 (envelope detection).

When the flow starts (step 80), first, n = 1 is set (step 81). Using a signal intensity measuring device 73 (RF spectrum analyzer) in the frequency domain, the frequency setting unit 75 first sets the measurement frequency to the fundamental wave f ₀ (step 82), and the intensity fluctuation in the fundamental wave f ₀ is measured (step). 83). Here, f ₀ is the sweep repetition frequency of the wavelength swept light in the wavelength swept light source unit 71. Next, a variance value is calculated in the variance calculation unit 74 (step 84). A determination of n ≧ N is performed (step 85), and it is determined whether or not all the harmonic components in the predetermined detection band have been measured. If n ≧ N is false (NO), n = n + 1 and the value of n is incremented by one (step 90). The signal intensity measuring device 73 is set to 2f ₀ which is the second harmonic, and the intensity fluctuation of the second harmonic 2f ₀ is measured and the dispersion value is calculated. Steps 81 to 84 are repeated until the determination of n ≧ N is true (YES), and the variance is calculated until the maximum harmonic Nf ₀ is reached.

  In the flow of FIG. 8, it has been described that the second to Nth harmonics from the fundamental wave are sequentially measured to obtain the dispersion value, but this is all for the detection band shown in FIG. This is an exemplary case of measuring intensity fluctuations for the harmonic components of. For example, when it is known in advance that intensity fluctuation is observed only with a specific harmonic component of the harmonic components, the dispersion value is measured only for some of the harmonics within the detection band. Thus, the average value A may be obtained. In this case, in step 90 in which the value of n is incremented by 1, processing may be added such that steps 82 to 84 are skipped for a specific n.

  When all the harmonic components in the predetermined detection band are measured and the dispersion value is obtained, and the determination of n ≧ N is true (YES), the flow calculates the average value A of dispersion based on the equation (8). Calculate (step 86). The database 78 stores stability reference data corresponding to the type of wavelength-swept light source and the measurement target. The flow proceeds to collation with the database (step 87), and it is determined by the stability determination unit whether the average value A of the variance calculated in step 86 satisfies the reference value in the database. If the calculated average value A of dispersion satisfies a predetermined standard, the adjustment of the wavelength sweep light source unit 71 is terminated (step 91). When the calculated average value A of dispersion does not satisfy a predetermined standard, parameters for controlling the wavelength swept light source such as the SOA current, the KTN drive voltage, and the KTN set temperature used for the wavelength swept light source unit 71 are changed (step) 89). At this time, as shown in FIG. 7, a parameter change command is sent from the stability determination unit 77 to the wavelength swept light source unit 71, and the parameter for controlling the wavelength swept light source can be changed. The parameter change command for feeding back the determination result in the stability determination unit 77 to the control parameter of the wavelength sweep light source unit 71 may be supplied by a control unit different from the stability determination unit 77. The use of the database 78 is not necessarily required, and is included as stability reference data stored in the storage means inside and outside the evaluation system 70 and as a fixed value in the program for executing each step of FIG. May be.

  As the stability reference data, as can be seen from the system configuration of FIG. 7, data relating to light intensity can be used. For example, it may be the upper limit data of the average value A of variance of intensity fluctuations, or may be a variance value. These data may be provided for each type of light source. Therefore, various types of threshold data can be used as long as they are related threshold data for determining the stability of the wavelength swept light source.

  Therefore, in another embodiment of the present invention, in a method for evaluating the stability of wavelength-swept light from a wavelength-swept light source, a fundamental wave having a wavelength-sweep repetition frequency of an electrical signal photoelectrically converted from the wavelength-swept light. The step of tuning, the step of measuring the intensity of the electric signal of the tuning frequency which is the frequency of the fundamental wave, obtaining the intensity fluctuation, the step of obtaining the variance of the intensity fluctuation of the tuning frequency, and the tuning frequency. A step of sequentially tuning to one or more harmonics of the fundamental wave instead of the fundamental wave, and obtaining an intensity fluctuation in each of the one or more harmonics and a dispersion value of the intensity fluctuation; and And a step of obtaining an average value of the dispersion values in each of the one or more harmonics. That. Preferably, based on the reference data of the stability of the wavelength swept light source associated with the dispersion value in the fundamental wave and the one or more harmonics and the average value, the control parameter of the wavelength swept light source is And a step of determining whether or not to change.

  The evaluation system, the evaluation apparatus, and the evaluation method of the present invention can be realized by a computer and a program. For example, an instruction for executing each step of the method for evaluating the stability of the wavelength swept light from the wavelength swept light source is provided. The computer-executable program including the program can be recorded on a recording medium or can be provided through a network.

  As described above, the control parameters for performing feedback include the set temperature of the KTN crystal, the magnitude of the drive voltage applied to the KNT crystal, the drive current / temperature of the laser oscillator and the SOA, the temperature of the entire light source casing, and the like. However, it is not limited to these. In the wavelength sweep light source evaluation method and evaluation system 70 of the present invention, the dispersion of the fundamental wave and the plurality of harmonics of the electrical signal converted from the wavelength sweep light is obtained, and the wavelength sweep light source of the wavelength sweep light source is calculated based on the average value of the dispersion. Assess stability. Based on the average value of the dispersion of the fundamental wave and multiple harmonics, and the conventional technology that evaluates intensity fluctuations etc. only for the fundamental wave in that it performs a feedback operation that changes the control parameters of the wavelength swept light source. Is different.

  FIG. 9 is a diagram showing data obtained by actually measuring the relationship between the current value of the SOA driving the semiconductor laser and the average value A of the intensity fluctuation. The average value A of the intensity fluctuation (dispersion) of the harmonics in the detection band is obtained by changing the drive current of the SOA that drives the semiconductor laser of the wavelength swept light source. It can be seen that the intensity fluctuation (average value of dispersion) depends on the SOA drive current. From the result of FIG. 9, it can be confirmed that the characteristics of the semiconductor laser are related as the cause of the instability of the spectrum of the wavelength swept light source. Therefore, it is possible to optimize the SOA current while monitoring the average value of dispersion as shown in FIG. As a similar light source control parameter, the drive conditions (temperature, drive voltage) of the KTN crystal can be optimized.

  As described above, according to the wavelength sweeping light source evaluation method of the present invention, intensity fluctuations of a spectrum of a plurality of components falling within a specific detection band are observed for a certain period of time, and an average of the intensity fluctuations (dispersion) is used as an index. Thus, the instability of the wavelength swept light source can be more accurately evaluated. The SNR of the OCT image of the SS-OCT apparatus can be improved by using an evaluation method that takes into account the behavior peculiar to the wavelength swept optical signal from the wavelength swept light source.

  The present invention is generally applicable to a wavelength swept light source. In particular, it can be used for a wavelength swept light source applicable to an OCT apparatus.

1, 21, 71 Wavelength sweep light source unit 2, 72 Photoelectric converter 3, 73 Frequency domain signal intensity measurement device 10 Wavelength sweep light source evaluation measurement system 20 SS-OCT device 22 Beam splitter 23 Mirror 24 Living body (measurement object)
25 Photodetector (PD)
70 Evaluation System 74 Distributed Arithmetic Unit 75 Frequency Setting Unit 76 Average Calculation Unit 77 Stability Determination Unit 78 Database

Claims (8)

In a system for evaluating the stability of wavelength swept light from a wavelength swept light source,
A photoelectric conversion unit that receives wavelength-swept light and outputs an electrical signal that is photoelectrically converted from the wavelength-swept light; and
A fundamental wave having a wavelength sweep repetition frequency of the electrical signal, and a signal intensity measuring unit in a frequency domain for obtaining intensity fluctuations of one or more harmonics of the fundamental wave;
A dispersion calculation unit for obtaining a dispersion value of intensity fluctuation at each frequency of the fundamental wave and the one or more harmonics;
An average calculation unit for obtaining an average value of the variance values. The system according to claim 1, further comprising: a stability determination unit that determines a command for correcting a control parameter of the wavelength swept light source based on the average value from the average calculation unit. The signal intensity measurement unit further includes a frequency setting unit that sets a measurement frequency so as to obtain a time domain intensity fluctuation in synchronization with the fundamental wave and the one or more harmonics. The system according to claim 1 or 2. In an apparatus for evaluating the stability of wavelength swept light from a wavelength swept light source,
A photoelectric conversion unit that receives wavelength-swept light and outputs an electrical signal that is photoelectrically converted from the wavelength-swept light; and
A fundamental wave having a wavelength sweep repetition frequency of the electrical signal, and a signal intensity measuring unit in a frequency domain for obtaining intensity fluctuations of one or more harmonics of the fundamental wave;
In the signal strength measurement unit, a frequency setting unit that sets a measurement frequency of the signal strength measurement unit so as to obtain a time-domain strength fluctuation in synchronization with the fundamental wave and the one or more harmonics;
A dispersion calculating unit for obtaining a dispersion value of each intensity fluctuation of the fundamental wave and the one or more harmonics;
An average calculation unit for obtaining an average value of the variance values. A data storage unit including reference data of stability of the wavelength swept light source associated with the dispersion value in the fundamental wave and the one or more harmonics of the wavelength swept light source;
The apparatus further comprises a stability determination unit that determines a command for correcting a control parameter of the wavelength-swept light source based on the stability reference data and the average value from the average calculation unit. Item 5. The apparatus according to Item 4. In the method of evaluating the stability of the wavelength swept light from the wavelength swept light source,
Tuning to a fundamental wave having a wavelength sweep repetition frequency of an electrical signal photoelectrically converted from the wavelength swept light;
Measuring the intensity of an electrical signal having a tuning frequency that is the frequency of the fundamental wave to obtain intensity fluctuation;
Determining a dispersion of intensity fluctuations of the tuning frequency;
Sequentially tuning the tuning frequency to one or more harmonics of the fundamental instead of the fundamental, and obtaining an intensity fluctuation in each of the one or more harmonics and a dispersion value of the intensity fluctuation; ,
Determining the dispersion value of the fundamental and the average of the dispersion values in each of the one or more harmonics. Whether to change the control parameter of the wavelength swept light source based on the reference value of the stability of the wavelength swept light source associated with the dispersion value in the fundamental wave and the one or more harmonics and the average value The method of claim 6, further comprising: determining whether or not.   A computer program comprising computer-executable instructions for performing the steps of claim 6 or 7.

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