JP2019106488A - Wavelength sweeping light source - Google Patents

Abstract

To provide a wavelength sweeping light source for achieving a stable FDML operation by controlling a sweeping frequency of a wavelength selector in accordance with a resonator length change.SOLUTION: Reference light output from a laser light source 251 is reflected by a dichroic mirror 255, and input from a fiber collimator 232 to a fiber ring. The reference light that has gone around the fiber ring is reflected by the dichroic mirror 255, and reflected by a polarization beam splitter 253, and made incident to a photodetector 256. When optical path length of the fiber ring including an optical delay fiber 270 changes due to a temperature change, a phase difference Δφ changes. When the phase difference Δφ is negative (Δφ<0), a cycle period becomes short, and then a sweeping frequency Fis increased such that the phase difference Δφ becomes zero. On the other hand, when the phase difference Δφ is positive (Δφ>0), the cycle period becomes long, and then the sweep frequency Fis decreased such that the phase difference Δφ 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 for a time corresponding to one 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 a 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 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 a desynchronization between the loop period of the ring resonator and the sweep 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 problems, the present invention is a wavelength sweeping light source, and an optical amplification unit for emitting light of a predetermined wavelength band, and a predetermined wavelength sweeping of light emitted from the optical amplification unit A wavelength selection unit that reflects light whose wavelength changes according to frequency, and the light output from the wavelength selection unit is branched into two, one branched light is fed back to the optical amplification unit, and the other branched light is output light A resonator unit including a first light branching unit for outputting as a light source and a light delaying unit disposed between the light amplifying unit and the wavelength selecting unit; and a wavelength sweeping frequency of at least A stabilization mechanism unit that controls the wavelength selection unit to adjust in accordance with a change in optical path length, and a light source unit that outputs reference light intensity-modulated at the wavelength sweep frequency; Light and the reference light are combined and output from the optical delay unit. An optical multiplexing / demultiplexing unit that demultiplexes the reference light, a light detection unit that measures the light intensity of the reference light demultiplexed by the optical multiplexing / demultiplexing unit, and the light of the reference light output from the light detection unit A feedback control unit that adjusts the wavelength sweep frequency such that the phase difference between the intensity signal waveform and the intensity modulated signal waveform of the light source unit becomes the phase difference when the output light has a maximum value or a predetermined value; And a stabilization mechanism unit including the

  In another aspect of the present invention, the feedback control unit lowers the wavelength sweep frequency when the light intensity signal waveform of the reference light is delayed in time with respect to the intensity modulation signal waveform of the light source unit, The wavelength sweeping frequency may be increased when the light intensity signal waveform of the reference light temporally advances with respect to the intensity modulated signal waveform of the light source unit.

  Another aspect of the present invention is characterized in that the reference light has a frequency which does not enter the amplification band of the light amplification section.

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

  Another aspect of the present invention is characterized in that the wavelength selector 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 Embodiment 1 of this invention. It is a figure which shows the phase change of the intensity signal of the reference light which circulated around the fiber ring, and the drive signal of a KTN light deflector. It is a figure which shows the structural example of the wavelength sweeping light source which concerns on Embodiment 2 of this invention.

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

(Embodiment 1)
FIG. 2 shows a configuration example of the wavelength swept light source according to the first embodiment of the present invention. The wavelength swept light source of Embodiment 1 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 first embodiment, in addition to the FDML laser unit, the sweep period of the wavelength filter 230 is adjusted so that the difference between the loop period of the fiber ring and an integral multiple of one or more of the sweep periods of the wavelength filter 230 is zero. A stabilization mechanism 250 is provided.

  The stabilization mechanism 250 of the first embodiment includes a laser light source 251 for outputting reference light, an intensity modulator 252 for modulating the intensity of reference light, and a polarization beam splitter 253 for reflecting only the reference light circulating around a fiber ring. Wave plate 254, dichroic mirror 255, photodetector 256 for detecting the intensity of reference light circulating around the fiber ring, delay line 257, phase difference between the intensity signal of reference light circulating around the fiber ring and the drive signal of the KTN light deflector 233 And a feedback controller 259. The wavelength of the light output from the laser light source 251 is set so that only the light output from the laser light source 251 can be reflected by the dichroic mirror 255 and does not enter the amplification band of the optical amplifier 210.

  The reference light output from the laser light source 251 is linearly polarized light having a predetermined polarization axis, and the intensity modulator 252 has an intensity so as to be synchronized with the sweep period of the wavelength filter 230 by the drive signal output from the driver 240. Modulated. In addition, the polarization beam splitter 253 transmits the reference light output from the intensity modulator 252, and the light having a polarization axis rotated by 90 degrees with respect to the polarization axis of the reference light output from the intensity modulator 252 is in a predetermined direction. Arrange to reflect to.

  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 reference light output from the laser light source 251 is transmitted through the polarization beam splitter 253, converted into circularly polarized light by the 1⁄4 wavelength plate 254, reflected by the dichroic mirror 255, and transmitted from the fiber collimator 232 It is input to the ring. The reference light circulating around the fiber ring is reflected by the dichroic mirror 255, converted by the quarter-wave plate 254 into linearly polarized light rotated 90 degrees with respect to the polarization axis of the reference light output from the intensity modulator 252, and polarized The light is reflected by the beam splitter 253 and enters the photodetector 256. The light intensity signal output from the photodetector 256 is input to the phase difference meter 258 together with the drive signal output from the driver device 240.

  At this time, the drive signal output from the driver device 240 is input to the phase difference meter 258 via the delay line 257 for compensating for the difference in arrival time to the phase difference meter 258 due to the difference in circuit configuration. The feedback controller 259 adjusts the wavelength sweeping frequency of the driver device 240 as follows according to the phase difference detected by the phase difference meter.

FIG. 3 shows the phase change of the intensity signal of the reference light circulating around the fiber ring and the drive signal of the KTN light deflector. A curve (a) indicated by a solid line represents the intensity signal of the reference light, and a curve (b) indicated by a broken line represents a drive signal. Curve (a) of the phase phi _a phase difference between the phase phi _b of the curve _{(b) Δφ (= φ a} -φ b) , the optical coupler 250 ring resonator comprising an optical delay fiber 270 at a predetermined temperature The delay line 257 is set so as to have a predetermined reference value, eg, zero, when the intensity of the output light output from the signal is maximum or has a predetermined value.

When the optical path length of the fiber ring including the optical delay fiber 270 changes due to temperature change, the phase of the curve (a) representing the intensity signal of the reference light changes, and the phase difference Δφ takes other than the reference value. When the phase difference Δφ becomes smaller than the reference value (Δφ <0 when the reference value is zero), this means that the optical path length of the fiber ring becomes short and the circulation period becomes short, so Δφ becomes zero. The sweep frequency F _F is increased to On the other hand, when the phase difference Δφ becomes larger than the reference value (if the reference value is zero, Δφ> 0), this means that the optical path length of the fiber ring becomes longer and the circulation period becomes longer. Lower the sweep frequency F _F to be zero. Thus, by adjusting the frequency of the drive signal, that is, the sweep frequency F _F according to the value of the phase difference Δφ, the optical coupler 250 can be synchronized by synchronizing the loop period of the ring resonator with the sweep period of the wavelength filter. Can be adjusted so that the intensity of the output light output from can be maximum or a predetermined value.

  Here, some explanation will be added about adjusting the intensity of the output light output from the optical coupler 250 to the maximum or a predetermined value.

  In FDML, when the sweep period is gradually changed, the intensity of the output light changes dramatically at the boundary of the sweep period in which the time of an integral multiple of one or more of the sweep period matches the circulation time of the resonator. 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 sweep period coincides with the round trip time of the resonator, the output light intensity with the longest coherence length is measured in advance, and the intensity is set as the target. As a value, by adjusting or controlling the sweep period, 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 light intensity output from the optical coupler 260 fluctuates due to some effect (such as a change in birefringence due to a change in tightening strength to the bobbin of the optical fiber due to a change in temperature) Since the fluctuation can be suppressed, it can be resistant to environmental changes such as temperature.

  On the other hand, if it is necessary to maintain the maximum output light intensity or the output light intensity desired by the user, the sweep cycle is adjusted or controlled to achieve such power.

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

  As a method of adjusting the 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 sweep period, 2) There is a method of selecting a large cycle of the light intensity and then repeating 1) and 2) many times with the cycle as a center until the light intensity becomes maximum (or maximum). The parameters of this adjustment method include the initial value of the sweep period, and the range of the change period around the current sweep period. When the sweep cycle is discretely changed at equal intervals, the interval of the cycle is also set as a parameter.

Second Embodiment
The structural example of the wavelength sweeping light source which concerns on FIG. 4 at Embodiment 2 of this invention is shown. The configuration of the FDML laser portion is the same as that of the first embodiment, and only the stabilizing mechanism 280 differs from the first embodiment. The stabilization mechanism 280 of the second embodiment includes a laser light source 281 that outputs reference light, an intensity modulator 282 that modulates the intensity of reference light, an optical coupler 283 that combines light propagating through a fiber ring and reference light, fiber ring Coupler 284, which divides the reference light from the light propagating through the light, a photodetector 285 which detects the intensity of the reference light circulated around the fiber ring, a delay line 286, an intensity signal of the reference light circulated around the fiber ring, and the KTN light deflector 233 And a phase difference meter 287 that measures the phase difference between the

The stabilization mechanism 280 places the optical couplers 283 and 284 in the vicinity of the input / output end of the optical delay fiber 270, and outputs the reference light output from the laser light source 281 and modulated by the optical intensity modulator 282 to the fiber ring from the optical coupler 283. The reference light having been input and propagated through the optical delay fiber 270 is branched by the optical coupler 284 and detected by the photodetector 285. Based on the phase difference Δφ between the intensity signal of the reference light measured by the phase difference meter 287 and the drive signal of the KTN light deflector 233, the drive signal of the driver device 240, that is, the sweep frequency _FF is adjusted. The adjustment method of the sweep frequency F _F will be described in the case where the reference value of the phase difference Δφ is zero for the sake of simplicity. As in the first embodiment, when the phase difference Δφ is negative (Δφ <0), it means that the optical path length of the optical delay fiber 270 becomes short, that is, the optical path length of the fiber ring becomes short and the orbiting time becomes short. Therefore, the sweep frequency F _F is increased so that Δφ becomes zero. On the other hand, when the phase difference Δφ is positive (Δφ> 0), it means that the optical path length of the optical delay fiber 270 becomes longer, that is, the optical path length of the fiber ring becomes longer and the orbiting time becomes longer. Lower the sweep frequency F _F so that As described above, by adjusting the frequency of the drive signal, that is, the sweep frequency F _F according to the value of the phase difference Δφ, one or more of the circulation period of the ring resonator and the sweep period of the wavelength filter can be obtained as in the first embodiment. By synchronizing with the integral multiple, it is possible to adjust the intensity of the output light output from the optical coupler 250 to be the maximum or a predetermined value. As the adjustment method, the method described in Embodiment 1 can be used.

  In the first and second embodiments, an example in which a ring resonator type configuration is used for the FDML laser unit is shown, but in the present invention, the FDML laser unit is configured using a wavelength filter including an optical deflector and a diffraction grating. If it is, it may be a configuration other than the ring resonator type. 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.

  Further, the intensity modulators 252 and 282 are not required when the laser light sources 251 and 281 are directly modulated. The configuration consisting of the polarization beam splitter 253 and the quarter wave plate 254 can be replaced by a beam sampler and an isolator. The signals input to the intensity modulators 252 and 282 and the phase difference meters 258 and 287 may use the drive signal of the driver device 240 as it is, or a signal obtained by multiplying the drive signal by an integer or a drive signal or its integer. It may be a spike wave or the like in which the double signal and the position of the peak coincide.

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, 283, 284 Optical coupler 170, 270 Optical delay fiber 250, 280 Stabilizing mechanism 251, 281 Laser light source 252, 282 Intensity modulator 253 Polarizing beam splitter 254 Quarter-wave plate 255 Dichroic mirror 256, 285 Photodetector 257, 286 delay line 258, 287 phase difference meter

Claims (5)

A light amplification unit that emits light of a predetermined wavelength band;
A wavelength selection unit that changes a selected wavelength at a predetermined wavelength sweep frequency from light incident from the light amplification unit;
A first light branching unit that branches the light output from the wavelength selection unit into two, feeds one branched light back to the optical 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 light source unit for outputting a reference beam intensity-modulated at the wavelength sweep frequency;
An optical multiplexing / demultiplexing unit that multiplexes the light input to the optical delay unit and the reference light and demultiplexes the reference light output from the optical delay unit;
A light detection unit that measures the light intensity of the reference light split by the light combining / splitting unit;
The phase difference between the light intensity signal waveform of the reference light output from the light detection unit and the intensity modulation signal waveform of the light source unit is the phase difference when the output light has a maximum value or a predetermined value. And a feedback control unit for adjusting the wavelength sweeping frequency, and a stabilization mechanism unit.   The feedback control unit lowers the wavelength sweep frequency when the light intensity signal waveform of the reference light is temporally delayed with respect to the intensity modulated signal waveform of the light source unit, and the light intensity signal waveform of the reference light is The wavelength sweeping light source according to claim 1, wherein the wavelength sweeping frequency is increased when temporally advancing with respect to the intensity modulated signal waveform of the light source unit.   The wavelength swept light source according to claim 1 or 2, wherein the reference light has a frequency which does not enter the amplification band of the light amplification unit.   The wavelength swept light source according to any one of claims 1 to 3, wherein the resonator unit holds a polarization state of propagating light.   The wavelength swept light source according to any one of claims 1 to 4, wherein the wavelength selection unit includes a KTN light deflector and a diffraction grating.

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