JP2019114721A - Wavelength sweeping light source - Google Patents

Abstract

To provide a wavelength sweeping light source realizing stable FDML operation by controlling the sweep frequency of a wavelength selector according to resonator length change.SOLUTION: A beam sampler 251 divides a part of the light entering from the fiber ring side, and a part of the light entering from the grating 234 and the optical deflector 233 side, and the divided light permeates a bandpass filter 254 and enters photodetectors 255, 256. When the difference Δt of peak time becomes smaller than a reference value (when the reference value is zero, Δt<0), it means that the optical length of the fiber ring becomes short and the circulation cycle has been shortened, and thereby the wavelength sweeping frequency fis increased so that the Δt becomes zero. Meanwhile, when the phase difference Δt becomes larger than a reference value (when the reference value is zero, Δt>0), it means that the optical length of the fiber ring becomes long and the circulation cycle has been lengthened, and thereby the wavelength sweeping frequency fis decreased so that the Δt becomes zero.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a wavelength swept light source in which a narrow spectrum is instantaneously swept periodically on the wavelength axis.

  As a method of optical coherence tomography (OCT), swept source OCT (SS-OCT) using a wavelength-swept light source in which a narrow laser spectrum is periodically swept on the wavelength axis is used. is there. SS-OCT is known as a form of Fourier domain OCT (FD-OCT) that does not require a spectrometer.

  Spectral domain OCT (SD-OCT), another form of FD-OCT, separates the interference signal and obtains an OCT signal as a spatial light intensity distribution. In this case, a high-resolution array detector is used for light detection. It will be necessary. The OCT signal here is a signal including tomographic information, and the spatial frequency of the intensity distribution changes according to the delay difference between the two light waves at the time of interference.

  On the other hand, in SS-OCT, since the spectrum of a narrow linewidth light source is swept directly on the wavelength or frequency axis in time, it can be regarded as being temporally dispersed, and a general photodetector detects a time waveform As an OCT signal. The depth information corresponds to the raw frequency of the interference signal. Therefore, it is mainly used in OCT of longer wavelength than near infrared such as 1 μm band or 1.3 μm band in which the imaging speed of SD-OCT is limited by the speed of the array detector.

  What is important in SS-OCT using a wavelength-swept light source is the instantaneous laser linewidth of the wavelength-swept light source associated with the measurement limit depth in the depth direction. The inverse of the instantaneous line width is proportional to the coherence length, and the OCT signal attenuates by 6 dB when the depth of the reflection point inside the observation object corresponds to 1⁄4 of the coherence length, so it is one index that represents the dynamic range in the depth direction. Used.

  Generally, it takes a certain amount of time for the laser to oscillate and the line width to be narrowed, so in the case of a wavelength swept light source where the oscillation wavelength constantly changes, the laser line width has a trade-off relationship with the sweep speed. It is reported (refer nonpatent literature 1). That is, in the case of a wavelength sweeping light source having a high sweeping frequency, the time change of the wavelength is fast, the rounding time permitted for each wavelength is short, and the number of turns is also reduced, so that the laser line width becomes wide.

  A technique for breaking this trade-off relationship is the Fourier domain mode lock (FDML) laser (see Non-Patent Document 2). The basic components of the FDML laser are an optical amplifier, a wavelength selector, an optical delay, and an optical extractor that extracts part of the optical power from the resonator. The components are optically connected by light propagation such as free space or a waveguide. The wavelength selector determines an instantaneous wavelength and wavelength change, and the wavelength sweep is realized by continuously changing the wavelength.

  The optical delay delays the light by a time corresponding to one wavelength sweep period of the wavelength selector or an integral multiple thereof. This makes it possible to synchronize the sweep operation of the wavelength selector and the optical circulation, and to store laser light of all wavelengths within the sweep band in the resonator. Therefore, even when the sweep frequency is high, the laser line width can be narrowed by the cumulative filtering effect in the wavelength selector without reducing the number of light resonators.

  FIG. 1 shows a configuration example of a conventional ring-type wavelength swept light source. The ring-shaped wavelength swept light source includes a fiber ring and a wavelength filter 140 that constitute a ring resonator in which the optical amplifier 110, the isolator 120, the polarization controller 130, the wavelength filter 140, the optical coupler 160, and the optical delay fiber 170 are disposed. The driver unit 150 drives the light deflector 145.

  The spontaneous emission light having a broad spectrum output from the optical amplifier 110 having an optical amplification effect in a wavelength range equal to or more than the sweep bandwidth of the light source passes through the isolator 120 and the polarization controller 130 and is free space optical system by the circulator 141 The wavelength filter 140 is introduced. The light is collimated by a collimator lens 142 disposed at the fiber output end, passes through a half-wave plate 143 and a polarizing beam splitter 144, and enters a potassium tantalate niobate (KTN) light deflector 145 and a diffraction grating 146.

  At this time, the polarization beam splitter 144 is disposed so that light having a polarization state compatible with the polarization dependency of the KTN light deflector 145 is incident, and the polarization beam splitter 144 is used by using the polarization controller 130 and the half-wave plate 143. The polarization state is optimized to maximize the light power transmitted through the Here, instead of using the polarization controller 130, the half-wave plate 143 and the polarization beam splitter 144, the ring fiber including the optical delay fiber 170 is used as a polarization maintaining fiber (PM fiber), and the light circulating around the fiber ring is KTN light It is also possible to maintain the polarization state compatible with the polarization dependency of the light source 145.

  In the wavelength filter 140, the selected wavelength is determined according to the voltage waveform that the driver device 150 outputs to the KTN light deflector 145. The reflected light wavelength-selected by the KTN light deflector 145 and the diffraction grating 146 is returned from the circulator 141 back to the fiber ring following the same path as the incident path.

  A part of the light returned to the fiber ring by the circulator 141 is extracted from the fiber ring by the light branching element 160, and this is the output light of the wavelength selective light source. The remaining light retraces the fiber ring, passes through the optical delay fiber 170, and is fed back to the optical amplifier 110.

  The wavelength filter 140 performs a wavelength selection operation according to the drive voltage waveform from the driver device 150. Here, a sine wave with a frequency of 200 kHz is applied to the KTN light deflector 145. That is, the filter wavelength 140 is swept periodically, and the length of the optical delay fiber 170 is the same as or an integral multiple of the wavelength sweep period of the wavelength filter 140 for the time required for the light to travel around the entire resonator. Thus, when the light of the wavelength selected by the wavelength filter 140 makes a round of the fiber ring and then passes through the wavelength filter 140 again, it coincides with the timing for selecting the same wavelength of the next sweep of the wavelength filter 140, and again with the minimum loss. It can receive filtering effect. This phenomenon occurs at all wavelengths within the swept band.

Patent No. 4751389

R. Huber, M. Wojtkowski, K. Taira, JG Fujimoto, and K. Hsu, “Amplified, frequency swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express 13, 3513-3528 (2005 ) R. Huber, M. Wojtkowski, and J. G. Fujimoto, “Fourier Domain Mode Locking (FDML): A New Laser Operating Regime and Applications for Optical Coherence Tomography,” Opt. Express 14, 3225-3237 (2006)

  The operating condition of this FDML laser is that the sweeping frequency of the wavelength selector coincides with the free spectral range (FSR) determined by the cavity rounding time of light or an integer multiple of FSR. Since the sweep frequency of a general OCT wavelength swept light source is tens to hundreds of kHz, it is necessary to have an optical path length of several kilometers in order to obtain an equivalent FSR. An optical fiber is preferred when providing an optical delay of this length.

  However, when a long optical fiber is used, there is a problem that the resonator length is easily changed, which causes desynchronization between the circulation period of the ring resonator and the wavelength sweeping period of the wavelength filter. In an optical fiber having a length on the order of km, even with a slight temperature change, the change in length causes a desynchronization that can not be ignored for FDML operation. Therefore, in the conventional FDML laser, it is necessary to control the temperature of the entire resonator including a long optical fiber in order to realize stable operation and narrow laser line width.

  In general, temperature control requires a large output in proportion to the volume to be controlled, but the FDML laser needs to be stabilized within a very narrow temperature fluctuation range, so a large-scale temperature control device is also required. Therefore, the conventional FDML laser has a problem in stable operation and cost reduction.

  The present invention has been made in view of such problems, and the object of the present invention is to control the sweep frequency of the wavelength selector in accordance with the change of the resonator length, thereby achieving the wavelength sweep that achieves stable FDML operation. It is to provide a light source.

  In order to solve the above-mentioned subject, one mode of the present invention is a wavelength sweeping light source, and it is predetermined among the light amplification parts which emit the light of a predetermined wavelength band, and the lights emitted from the light amplification part. A wavelength selection unit that reflects light whose wavelength changes at the wavelength sweep frequency of the light beam, and the light reflected by the wavelength selection unit is split into two, and one branched light is fed back to the optical amplification unit; A resonator unit including: a first light branching unit that outputs the output light as an output light; and an optical delay unit disposed between the light amplification unit and the wavelength selection unit; A stabilization mechanism unit that controls the wavelength selection unit to adjust in accordance with a change in the optical path length of the delay unit, and from each of the light emitted from the wavelength selection unit and the light incident on the wavelength selection unit A beam splitter for splitting a part, and the wavelength selector Measuring the light intensities of the same predetermined wavelength of the first light split by the beam splitter unit from the incident light and the second light split by the beam splitter unit from the light emitted from the wavelength selection unit The time difference between the light detection unit and the peak time of the light intensity signal of the first light output from the light detection unit and the light intensity signal of the second light is the maximum light intensity of the output light, or And a feedback control unit that adjusts the wavelength sweep frequency so as to have a time difference when the predetermined value is reached.

  In another aspect of the present invention, when the feedback control unit further delays the peak time of the light intensity signal of the first light with respect to the peak time of the light intensity signal of the second light. The wavelength sweep frequency is lowered, and when the peak time of the light intensity signal of the first light is temporally advanced with respect to the peak time of the light intensity signal of the second light, the wavelength sweep frequency is increased. It is characterized by

  Another aspect of the present invention is characterized in that the resonator unit holds the polarization state of propagating light.

  According to another aspect of the present invention, the wavelength selector further includes a KTN light deflector and a diffraction grating.

  The present invention can realize stable FDML operation without using a temperature controller by controlling the sweep frequency of the wavelength selector according to the change of the resonator length as a synchronization method of the FDML laser.

It is a figure which shows the structural example of the conventional ring-type wavelength sweeping light source. It is a figure which shows the structural example of the wavelength sweeping light source which concerns on one Embodiment of this invention. Intensity signal (a) of light incident from the fiber ring side when the circulation period of the resonator and wavelength sweep period of the wavelength filter coincide, and intensity signal of light incident from the diffraction grating 234 and light deflector 233 side It is a figure which shows the time change of (b). It is a figure which shows the time change of intensity signal (a) and intensity signal (b) when the circulation period of a resonator and the wavelength sweep period of a wavelength filter shift | deviate.

  Hereinafter, embodiments of the present invention will be described in detail.

FIG. 2 shows an exemplary configuration of a wavelength swept light source according to an embodiment of the present invention. The wavelength swept light source of one embodiment of the present invention shown in FIG. 2 is an FDML laser using a ring resonator. The configuration of the FDML laser unit includes an optical amplifier 210, an isolator 220, a wavelength filter 230, an optical coupler 260, a fiber ring forming a ring resonator in which an optical delay fiber 270 is disposed, and an optical deflector 233 provided with the wavelength filter 230. It consists of the driver device 240 to drive. In addition, a polarization maintaining fiber (PM fiber) is used for a ring fiber including the optical delay fiber 270 so as to maintain the polarization state of the circulating light. In the present invention, in addition to the FDML laser portion, the wavelength filter 230 is set so that the difference between the looping period of the fiber ring and an integral multiple of one or more of the wavelength sweep periods of the wavelength filter 230 becomes zero or a predetermined value. A stabilization mechanism 250 is provided to adjust the wavelength sweep frequency f _F.

The stabilization mechanism 250 includes a beam sampler 251, mirrors 252 and 253, a band pass filter 254, photodetectors 255 and 256, and a feedback controller 257. The reflectance of the beam sampler 251 is, for example, about 5%, and the band pass filter 254 is, for example, a narrow band filter having a center frequency f _c and a bandwidth of about 1 nm.

  In the FDML laser unit, the spontaneous emission light having a broad spectrum is output from the optical amplifier 210 having an optical amplification effect in a wavelength range equal to or more than the sweeping bandwidth of the wavelength swept light source. It is introduced into the wavelength filter 230 which is a space optical system. The light is collimated by a collimator lens 232 disposed at the fiber output end, and then enters a potassium tantalate niobate (KTN) light deflector 233 and a diffraction grating 234. At this time, the ring fiber including the optical delay fiber 270 is a polarization maintaining fiber (PM fiber), and the light circulating around the fiber ring maintains the polarization state compatible with the polarization dependency of the KTN light deflector 233.

In the stabilization mechanism 250, the beam sampler 251 reflects a part of the light incident from the fiber ring side and a part of the light incident from the diffraction grating 234 and the light deflector 233 in different predetermined directions. In the present embodiment, the reflected light of the light entering the beam sampler 251 from the side of the diffraction grating 234 and the light deflector 233 passes through the band pass filter 254 and enters the photodetector 255. Reflected light of light entering the beam sampler 251 from the fiber ring side passes through the band pass filter 254 via the mirrors 252 and 253 and enters the photodetector 256. The light intensity signal of the center frequency f _c output from the photodetectors 255 and 256 is input to the feedback controller 257, and the feedback controller 257 measures the time difference at which the peaks of the light intensity signals appear, and according to the time difference The wavelength sweep frequency f _F of the driver device 240 is adjusted as follows.

In FIG. 3, the intensity signal (a) of the light incident from the fiber ring side when the circulation period of the resonator and the wavelength sweeping period of the wavelength filter coincide, and the light is incident from the diffraction grating 234 and the light deflector 233 side. The time change of the light intensity signal (b) is shown. Here, the peak time t _a of the intensity signal (a), the time difference Δt (= t _a -t _b) is a predetermined reference value between the peak time t _b of the intensity signal (b), becomes zero as an example here Thus, the optical path lengths of the beam sampler 251 to the photodetectors 255 and 256 are made equal. The period in which light of a predetermined band at the center frequency f _c is reflected by the diffraction grating 234, ie, the wavelength sweep period is 1 / f _F , and in this case, the cycle period of the resonator is the same 1 / f _F or 1 / f _F is 2 or more integer multiples of f _F.

FIG. 4 shows temporal changes in the intensity signal (a) and the intensity signal (b) when the circulation period of the resonator and the wavelength sweeping period of the wavelength filter deviate from each other. When the optical path length of the fiber ring including the optical delay fiber 270 changes due to a temperature change or the like, the wavelength sweep period remains 1 / f _F but the circulation period of the resonator changes. Therefore, the timing when the light of the center frequency f _c is incident on the diffraction grating 234 and the timing when the diffraction grating 234 can reflect the light of the center frequency f _c deviate from each other. The peak of the intensity signal (b) of the light reflected by the diffraction grating 234 is incident on the diffraction grating 234 as shown in FIG. 4 because the diffraction grating 234 is within the timing at which the light of the central frequency f _c can be reflected. peak of light with center frequency f _c, but in a case where the diffraction grating 234 is deviated from the center frequency f capable of reflecting timing light _c, the difference Δt of peak time takes a value other than zero.

When the peak time difference Δt becomes smaller than the reference value (when the reference value is zero, Δt <0), this means that the optical path length of the fiber ring becomes short and the circulation period becomes short, so Δt is The wavelength sweep frequency f _F is increased to be zero. On the other hand, when the phase difference Δt becomes larger than the reference value (when the reference value is zero, Δt> 0), it means that the optical path length of the fiber ring becomes long and the circulation period becomes long, so Δt is The wavelength sweep frequency f _F is lowered to be zero. As described above, when a deviation occurs between the circulation period of the resonator and the wavelength sweeping period of the wavelength filter, the intensity signal (a), ( The amplitude of b) changes, and the amplitude decreases as the deviation increases. However, in the present invention, as long as the peak time of the intensity signal can be detected, if the peak of the intensity signal can be detected, the change in amplitude is not affected.

In this manner, by adjusting the frequency of the drive signal, that is, the wavelength sweep frequency f _F according to the value of the peak time difference Δt, the loop period of the ring resonator and the wavelength sweep period of the wavelength filter are synchronized. The intensity of the output light output from the optical coupler 260 can be adjusted to a maximum or a predetermined value.

  Here, some explanation will be added about adjusting the intensity of the output light output from the optical coupler 260 to the maximum or a predetermined value.

  In FDML, when the wavelength sweeping period is gradually changed, the intensity of output light is dramatically separated by the wavelength sweeping period where the time of an integer multiple of 1 or more of the wavelength sweeping period matches the round trip time of the resonator. Change. Further, when the output light is described in detail, it is large if the period is a little short and small if the period is a little long, and if the periods coincide, the output light intensity in the middle becomes.

  Since the coherence length is longest when an integral multiple of 1 or more of the wavelength sweep period coincides with the round trip time of the resonator, the output light intensity at which the coherence length becomes longest is measured in advance, By adjusting or controlling the wavelength sweep period (or wavelength sweep frequency) as the target value, the coherence length can be kept at the longest state. Since the output light intensity which is the longest in coherence length is different for each resonator, it is necessary to measure in advance for each resonator.

  Although the output light intensity itself is the target value in the above description, the output light intensity may be divided by the time integral value of the output light intensity in one sweep (a normalized value) as the target value. good. In this case, even if the intensity of the light output from the optical coupler 260 fluctuates due to some effect (such as a change in birefringence due to a change in the tightening strength of the optical fiber to the bobbin due to a temperature change) Since the fluctuation is suppressed, it is possible to construct a control system that suppresses the influence of environmental changes such as temperature.

  On the other hand, if it is necessary to maintain the maximum output light intensity or the user's desired output light intensity, adjust the wavelength sweep period so as to achieve such a predetermined light intensity, Or you can control.

  The wavelength sweep period is adjusted so as to obtain a predetermined output light intensity, such as setting the intensity of the light output from the optical coupler 260 to the output light intensity that maximizes the coherence length or the output light intensity desired by the user. As a method, for example, PID (Proportional-Integral-Differential) control can be considered. In this case, the target value is a predetermined output light intensity, the operation amount is a wavelength sweep period (frequency), and the control amount is an output light intensity.

  As a method of adjusting the wavelength sweep period to keep the output light intensity at maximum, 1) acquire the output light intensity when changing the period within a predetermined range centered on the current wavelength sweep period, 2) Among them, there is a method of selecting a large cycle of light intensity and then repeating 1) and 2) repeatedly about the cycle until the light intensity becomes maximum (or maximum). The parameters of this adjustment method include the initial value of the wavelength sweeping period, and the range of the changing period around the current wavelength sweeping period. When the wavelength sweep period is discretely changed at equal intervals, the interval of the period is also set as a parameter.

  In the embodiment of the present invention shown in FIG. 2, an example in which the ring resonator type configuration is used for the FDML laser portion is shown, but in the present invention, the FDML laser portion is a wavelength filter including an optical deflector and a diffraction grating. The configuration other than the ring resonator type may be used as long as the configuration is used. The wavelength filter 230 may be replaced by a Littrow arrangement, and a total reflection mirror may be added to make it a Littman arrangement. The fiber ring including the optical delay fiber 270 uses the polarization controller, the half-wave plate and the polarization beam splitter in the same manner as the configuration shown in FIG. 1 in place of the PM fiber to change the polarization state of light propagating in the modulator. It may be configured to be held.

DESCRIPTION OF SYMBOLS 110, 210 Optical amplifier 120, 220 Isolator 130 Polarization controller 140, 230 Wavelength filter 141, 231 Circulator 142, 232 Collimator lens 143 Half wave plate 144 Polarized beam splitter 145, 233 KTN light deflector 146, 234 Diffraction grating 150, 240 Driver device 160, 260 Optical coupler 170, 270 Optical delay fiber 250 Stabilization mechanism 251 Beam sampler 252, 253 mirror 254 Band pass filter 255, 256 Photodetector 257 Feedback controller

Claims (4)

A light amplification unit that emits light of a predetermined wavelength band;
A wavelength selection unit for reflecting light of which wavelength changes at a predetermined wavelength sweep frequency among the light emitted from the light amplification unit;
A first light branching unit that divides the light reflected by the wavelength selection unit into two, feeds one branched light back to the light amplification unit, and outputs the other branched light as an output light;
An optical delay unit disposed between the optical amplification unit and the wavelength selection unit;
And a stabilization mechanism unit that controls the wavelength selection unit to adjust the wavelength sweep frequency at least in accordance with a change in an optical path length of the optical delay unit.
A beam splitter unit that divides a part of each of the light emitted from the wavelength selection unit and the light incident on the wavelength selection unit;
The first light split by the beam splitter from light incident on the wavelength selector and the second light split from the light emitted from the wavelength selector by the beam splitter have the same predetermined wavelength A light detection unit for measuring the light intensity respectively;
The time difference between the light intensity signal of the first light output from the light detection unit and the peak time of the light intensity signal of the second light is determined such that the light intensity of the output light has a maximum value or a predetermined value. And a feedback control unit that adjusts the wavelength sweep frequency so as to have a time difference when the wavelength sweeping light source.   The feedback control unit lowers the wavelength sweep frequency when the peak time of the light intensity signal of the first light is delayed with respect to the peak time of the light intensity signal of the second light, The wavelength sweeping frequency is increased when the peak time of the light intensity signal of the first light is temporally advanced with respect to the peak time of the light intensity signal of the second light. Wavelength swept source as described.   The wavelength swept light source according to claim 1, wherein the resonator unit holds a polarization state of propagating light.   The wavelength swept light source according to any one of claims 1 to 3, wherein the wavelength selection unit includes a KTN light deflector and a diffraction grating.

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