JP2019046973A - Dispersion compensator and wavelength sweeping light source using the same - Google Patents

Abstract

To provide a dispersion compensator capable of continuously compensating group velocity dispersion in a desired wavelength band with a little elements, and a wavelength sweeping light source using the dispersion compensator.SOLUTION: A polarization controller 201 performs control in such a manner that polarized light emitted from a circulator 202 is on an axis which is vertical to an optical axis within a page space, and is transmitted through a polarization beam splitter 203 as it is. A 1/4 wavelength plate 204 is disposed on the optical axis of the light that is transmitted through the polarization beam splitter 203, and disposed in such a manner that a Fast axis of the 1/4 wavelength plate 204 and a deflection axis of the polarized light form 45 degrees. Reflection in a first CFBG 205-1 is capable of giving dispersion that is reverse to dispersion of a resonator, to incident light. Reflection light from the first CFBG 205-1 passes through the 1/4 wavelength plate 204 again, is converted into linearly polarized light of which the polarization axis is rotated at 90 degrees from a polarization axis of the incident light, is reflected by the polarization beam splitter 203, and is made incident to a second CFBG 205-2.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a dispersion compensator and a wavelength swept light source using the dispersion compensator, and more specifically, a dispersion compensator that can be used as a light source for optical coherence tomography (OCT) applications and a wavelength sweep using the same. It relates to the light source.

  One type of OCT is a swept source OCT (SS-OCT) using a wavelength swept light source in which the wavelength is swept continuously over a certain range and the sweep is periodically repeated. SS-OCT is a kind of Fourier domain OCT (FD-OCT), and it is known that a highly sensitive OCT signal can be obtained without using a spectroscope necessary for conventional FD-OCT.

  In SS-OCT, an interference waveform that is an OCT signal can be acquired with a general photodetector or differential detector instead of an array detector required for a spectrometer. Therefore, the speed performance of FD-OCT at the speed of the array detector. Mainly used in 1m band and 1.3m band OCT, where

  What is important in SS-OCT using a wavelength swept light source is the instantaneous laser line width of the wavelength swept light source related to the measurement depth in the depth direction. The distance at which the OCT signal attenuates by 6 dB is determined by the coherence length (the value obtained by dividing the speed of light by the laser line width). In general, it takes a certain amount of time for the laser to oscillate and the line width to narrow, so it is known that the laser line width is in a trade-off relationship with the sweep speed in a wavelength swept light source whose oscillation wavelength constantly changes. (See Non-Patent Document 1). That is, the laser line width is widened with a fast wavelength swept light source.

  Therefore, Fourier domain mode lock (FDML) has been proposed as a technique for improving the trade-off relationship. In FDML, laser light of all wavelengths in the sweep band is always accumulated in the optical path of the resonator, and it is not necessary to start up laser oscillation from scratch with spontaneous emission light. The laser line width can be reduced (see Non-Patent Document 2).

  FIG. 1 shows a basic configuration of a wavelength swept light source 100 using a conventional FDML. The wavelength swept light source 100 includes a ring-type resonator, and an optical connection unit that optically connects the wavelength selection unit 101, the dispersion compensation unit 102, the light extraction unit 103, the optical delay unit 104, and the optical amplification unit 105 to these components. 106. For the optical amplification unit 105, an SOA, a fiber amplifier, a solid amplifier, a dye amplifier, or the like can be used.

  From the broadband light generated by the optical amplification unit 105, only a predetermined narrow spectral region is selectively extracted by the wavelength selection unit 101. The selectively extracted light passes through the dispersion compensation unit 102, the light extraction unit 103, and the optical delay unit 104, is amplified again by the optical amplification unit 105, and enters the wavelength selection unit 101.

The selection wavelength in the wavelength selection unit 101 is sequentially changed, and if the sweep frequency is f _sweep , the light circulating in the resonator passes through the wavelength selection unit 101 again without loss, so that the resonator The time t _round required for one _round needs to be an integral multiple of the reciprocal of this sweep frequency f _sweep . When the wavelength sweep period 1 / f _sweep and the time t _round that circulates in the resonator coincide with each other, the light having a predetermined wavelength selected by the wavelength selection unit 101 goes around the resonator and travels again. When returning to, the wavelength selection unit 101 selects the same predetermined wavelength. Therefore, light can pass through the wavelength selection unit 101 again without loss, and by repeating this, light can continue to circulate in the resonator.

For example, when the sweep frequency is 200 kHz, the circulation time t _round of the resonator is 5 μs, and the optical path length L _cav of the resonator is 1500 m. If the resonator is composed of an optical fiber and the refractive index of the optical fiber is approximately 1.46, the physical length of the resonator is 1027 m.

  For this reason, in order to accumulate light without loss in the resonator at all wavelengths within the sweep band, the resonator length needs to be invariant with respect to each wavelength so that the circulation times are equal. . However, FDML has a resonator length of several kilometers, and is usually constituted by a waveguide such as an optical fiber as described above. Therefore, the wavelength dependence of the resonator length occurs due to the refractive index dispersion of the optical fiber. Therefore, the wavelength dependence of the resonator length occurs due to the refractive index dispersion of the optical fiber.

When the center wavelength of the wavelength swept light source is 1050 nm, the group delay amount generated when making a round of the resonator calculated from the D parameter expressing the dispersion is 1000 nm and 1100 nm with respect to the center wavelength as shown in FIG. It becomes about 2000 ps and -1600 ps. A minus sign indicates that it arrives fast. Approximating the dispersion curve of FIG. 2 using the following quadratic function and obtaining the first and second order dispersion coefficients D ₂ and D ₃ ,

D ₂ is −38.68 ps / nm, and D ₃ is +0.106 ps / nm ² . The value of the first-order dispersion rate D ₂ indicates that an optical path length difference of about 11.6 mm occurs in the resonator length every time the wavelength is different by 1 nm. Since the transmission band varies with time in the wavelength selection unit 101, the gate time at each wavelength is expressed by the following equation (see Non-Patent Document 2).

Δλ _filter is a 3 dB bandwidth of the transmission band, and Δλ _range is a sweep bandwidth. When the transmission bandwidth is 0.3 nm and the sweep bandwidth is 100 nm, τ _gate is 4.8 ns. If 1050 nm, which is the center wavelength of the sweep band, is used as a reference for the group delay 0, light of wavelength λ can circulate the resonator only N times satisfying the following relationship.

  When the left side is equal to or longer than the gate time, the transmission band wavelength when the light of that wavelength reaches the wavelength selection unit is at another wavelength, and the light is blocked. For example, at 1000 nm, N = 2.5 is obtained, and light can only circulate the resonator only twice. Since the laser line width decreases in inverse proportion to the number of times of resonator circulation, narrowing of the FDML laser line width is limited in a resonator in which dispersion exists. As a result, at the wavelengths at both ends of the sweep band, the laser beam cannot be efficiently stored in the resonator, so that the line width is increased and the oscillation band is limited. For this reason, it is necessary to perform group delay compensation by performing dispersion compensation in the dispersion compensation unit 102 shown in FIG.

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)

  However, by performing dispersion compensation, it is possible to improve the performance of the wavelength swept light source by aligning the resonator length for each wavelength, but there are problems that cost reduction and continuous dispersion compensation are difficult.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a dispersion compensator capable of continuously compensating group velocity dispersion in a desired wavelength band with a small number of elements. It is another object of the present invention to provide a wavelength swept light source using the above.

  In order to solve the above problems, the present invention is a dispersion compensator, which outputs a polarized light having a predetermined polarization axis from input light, and is output from the polarization controller An optical circulator to which polarized light is input, and a polarization beam splitter disposed so as to transmit one of two mutually perpendicular polarized lights, reflect the other at 90 degrees, and transmit the polarized light output from the optical circulator The polarized light that has been transmitted through the polarizing beam splitter is input, the polarized light output from the optical circulator and the quarter wavelength plate arranged so that the fast axis forms 45 degrees, and transmitted through the quarter wavelength plate. The first CFBG to which circularly polarized light is input and the reflected light from the first CFBG pass through the quarter-wave plate and are input to the polarizing beam splitter. Reflected polarized light is characterized by comprising a second CFBG inputted to.

  According to a second aspect of the present invention, in the dispersion compensator according to the first aspect, between the second CFBG and the polarization beam splitter, the polarization output from the optical circulator and the fast axis form 45 degrees. And the reflected light from the second CFBG passes through the second quarter wave plate and is input to the polarizing beam splitter, and the polarizing beam splitter. And a third CFBG to which the polarized light that has passed through is input.

  The invention according to claim 3 is a dispersion compensator, which transmits the optical circulator and one of two perpendicularly polarized light beams output from the optical circulator and reflects the other at 90 degrees. And a first quarter-wave plate arranged such that the polarized light transmitted through the polarization beam splitter is input and the polarization output from the optical circulator and the fast axis form 45 degrees. And the first CFBG to which the circularly polarized light transmitted through the first quarter-wave plate is input, and the reflected light from the first CFBG passes through the first quarter-wave plate and is Polarized light that is input to the polarizing beam splitter, reflected at 90 degrees by the polarizing beam splitter, is input, and the polarized light output from the optical circulator and the fast axis are arranged to form 45 degrees. A quarter wavelength plate, a second CFBG to which circularly polarized light transmitted through the second quarter wavelength plate is input, and reflected light from the second CFBG is the second quarter wavelength plate. Is transmitted to the polarization beam splitter, the polarized light transmitted through the polarization beam splitter is input, and the third 1 / is arranged so that the polarization output from the optical circulator and the fast axis form 45 degrees. And a third CFBG to which circularly polarized light transmitted through the third quarter-wave plate is input.

  According to a fourth aspect of the present invention, there is provided a wavelength swept light source, an optical amplifying unit that generates broadband light including a predetermined wavelength band, and selectively transmits light in a predetermined wavelength band from the broadband light at a predetermined sweep frequency. The wavelength compensator for transmission and the light transmitted through the wavelength selector are input, and the light output from the dispersion compensator is given a predetermined delay amount. A resonator including an optical delay unit that is input to the optical amplification unit, and a light extraction unit that extracts a part of the light propagating through the resonator.

  In the present invention, a desired dispersion compensation effect can be obtained with a small number of elements by adopting a multipath configuration using polarization rotation. Further, compared to a configuration in which CFBGs having different center wavelengths are cascade-connected, group velocity dispersion can be easily compensated continuously in a desired wavelength band. As a result, the number of times that light in the entire wavelength band circulates in the FDML resonator can be increased, and the laser line width can be narrowed instantaneously.

It is a figure which shows the basic composition of the wavelength sweep light source 100 using the conventional FDML. It is a figure which shows the group delay amount for every wavelength produced when making a round the resonator calculated from D parameter which shows dispersion | distribution in case a center wavelength is 1050 nm. It is a figure which shows the structure of the multipath dispersion compensation part using the chirp fiber Bragg grating (CFBG) which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the multipath dispersion compensation part using CFBG which concerns on Embodiment 2 of this invention. It is a figure which shows the structure of the multipath dispersion compensation part using CFBG which concerns on Embodiment 3 of this invention.

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

(Embodiment 1)
FIG. 3 shows a configuration of a multipath dispersion compensation unit using a chirped fiber Bragg grating (CFBG) according to an embodiment of the present invention. The dispersion compensation unit of the present invention can be used as the dispersion compensation unit 102 of the wavelength swept light source 100 as shown in FIG.

  A length is required to compensate the group delay of the dispersion rate with one CFBG over a wide wavelength band, but it is difficult to produce a CFBG having a length that enables such dispersion compensation. Therefore, in the present invention, a plurality of CFBGs having a wide wavelength band but a small amount of dispersion are used. For example, by using three CFBGs, the dispersion amount compensated by each CFBG becomes 1/3 of the total dispersion amount, and the necessary length of each CFBG can be suppressed. In addition, in the configuration of the first embodiment shown in FIG. 3, three paths can be realized by two CFBGs 205-1 and 205-2, and the number of elements can be suppressed.

  The polarization controller 201 converts the light into linearly polarized light and makes the polarized light enter the circulator 202. The incident polarized light is output from the end face of the optical fiber connected to the optical circulator 202 to the lens 206-1 and collimated. The polarization controller 201 performs control so that the polarization axis of polarized light emitted from the optical circulator 202 is on an axis perpendicular to the optical axis in the drawing and is transmitted through the polarization beam splitter 203 as it is.

  A quarter-wave plate 204 is disposed on the optical axis of the light transmitted through the polarization beam splitter 203, and the Fast axis of the quarter-wave plate 204 and the polarization deflection axis are disposed at 45 degrees. As a result, the polarized light that has passed through the quarter-wave plate 204 becomes circularly polarized light. The circularly polarized light emitted from the quarter-wave plate 204 is collected by the lens 206-2 and is incident on the first CFBG 205-1.

  The reflection at the first CFBG 205-1 can be reflected at a deeper portion with a longer wavelength by gradually increasing the grating period of the CFBG from the incident end, and the incident light has a dispersion opposite to the dispersion of the resonator. Can be given. In addition, the birefringence inside the CFBG is sufficiently small and the length is several tens of centimeters, so it is considered that the polarization state is not affected, and the polarization state of the reflected light remains circularly polarized. The same applies to reflection by a second CFBG 205-2, which will be described later.

  The reflected light from the first CFBG 205-1 passes through the quarter wavelength plate 204 again via the lens 206-2 and is converted into linearly polarized light whose polarization axis is rotated 90 degrees from the polarization axis of the incident light. Thereby, in the return path, it is reflected by the polarization beam splitter 203 and enters the second CFBG 205-2 via the lens 206-3. The light reflected by the second CFBG 205-2 is reflected again in the direction of the first CFBG 205-1 by the polarization beam splitter 203 because the polarization state is preserved.

  The polarized light reflected by the polarizing beam splitter 203 is converted into circularly polarized light by the quarter wavelength plate 204 and is incident on the first CFBG 205-1 again via the lens 206-2. The circularly polarized light reflected by the first CFBG 205-1 passes through the quarter wavelength plate 204 again to become linearly polarized light that is further rotated by 90 degrees, and is transmitted through the polarization beam splitter 203.

  That is, in the first embodiment, the light incident from the optical circulator 202 passes through the CFBG in the order of the first CFBG 205-1, the second CFBG 205-2, and the first CFBG 205-1 and is then dispersed by the optical circulator 202. Output from the compensation unit 102. In this configuration, it is possible to compensate for the same amount of dispersion as the three CFBGs by passing the two CFBGs 205-1 and 205-2 a total of three times.

(Embodiment 2)
FIG. 4 shows the configuration of a multipath dispersion compensation unit using CFBG according to the second embodiment of the present invention. In this configuration, it is possible to realize the same dispersion compensation amount as in the case of using five CFBGs by using the three CFBGs 305-1 to 305-3 on the same principle as in the first embodiment.

  That is, first, the polarization control unit 301 controls the polarization so that the polarized light emitted from the optical circulator 302 is transmitted through the polarization beam splitter 303. The linearly polarized light that has been collimated by the lens 306-1 and passed through the polarization beam splitter 303 is circularly converted by the first quarter-wave plate 304-1 arranged so that the Fast axis and the polarization deflection axis form 45 degrees. It is converted into polarized light, collected by the lens 306-2, and incident on the first CFBG 305-1. The circularly polarized light reflected by the first CFBG 305-1 is converted into linearly polarized light rotated 90 degrees from the polarization axis of the incident light by the first quarter wavelength plate 304-1 and is incident on the polarization beam splitter 303. . The linearly polarized light rotated by 90 degrees from the polarization axis of the incident light is reflected by the polarization beam splitter 303, and the second quarter-wave plate 304-2 arranged so that the Fast axis and the polarization deflection axis form 45 degrees. Is converted into circularly polarized light, condensed by the lens 306-3, and incident on the second CFBG 305-2. The circularly polarized light reflected by the second CFBG 305-2 is converted into linearly polarized light rotated 180 degrees from the polarization axis of the incident light by the second quarter wavelength plate 304-2, and is incident on the polarization beam splitter 303. . The linearly polarized light rotated by 180 degrees from the polarization axis of the incident light passes through the polarizing beam splitter 303, is condensed by the lens 306-4, and is incident on the third CFBG 305-3. The polarized light reflected by the third CFBG 305-3 enters the polarizing beam splitter 303, passes through the polarizing beam splitter 303, is converted into circularly polarized light by the second quarter wavelength plate 304-2, and is converted into the second CFBG 305. -2. The circularly polarized light reflected by the second CFBG 305-2 is converted into linearly polarized light rotated by 270 degrees from the polarization axis of the incident light by the second quarter wavelength plate 304-2, and is incident on the polarization beam splitter 303. The light is reflected by the polarization beam splitter 303 and enters the first CFBG 305-1 through the first quarter-wave plate 304-1. The circularly polarized light reflected by the first CFBG 305-1 is converted to linearly polarized light rotated 360 degrees from the polarization axis of the incident light by the first quarter wave plate 304-1 and transmitted through the polarization beam splitter 303. The light enters the optical circulator 302.

  Thus, the order of passing through the CFBG is the first CFBG 305-1, the second CFBG 305-2, the third CFBG 305-3, the second CFBG 305-2, and the first CFBG 305-1.

(Embodiment 3)
FIG. 5 shows a configuration of a multipath dispersion compensation unit using a CFBG according to Embodiment 3 of the present invention. This configuration represents the configuration of the polarization-independent dispersion compensation unit 102. In the third embodiment, there is no polarization controller arranged in the first and second embodiments, and light that is not subjected to polarization control is incident on the optical circulator 402. Therefore, the light output from the optical circulator 402 is separated into two-axis polarized light perpendicular to the optical axis and perpendicular to each other by the polarization beam splitter 403, and the polarization component parallel to the paper surface travels straight, and the polarized light perpendicular to the paper surface. The component is reflected at 90 degrees. When the axis parallel to the paper surface is the P axis and the vertical axis is the S axis, the P-polarized light passes through the CFBG in the order of the first CFBG 405-1, the second CFBG 405-2, and the third CFBG 405-3.

  That is, when the light emitted from the optical circulator 402 is collimated by the lens 406-1 and enters the polarizing beam splitter 403, the P-polarized light is transmitted through the polarizing beam splitter 403. The P-polarized light is converted into circularly polarized light by the first quarter-wave plate 404-1 arranged so that the Fast axis and the polarization axis form 45 degrees, condensed by the lens 406-2, and first CFBG 405. -1. The circularly polarized light reflected by the first CFBG 405-1 is converted into S-polarized light rotated 90 degrees from the polarization axis of the incident light by the first quarter-wave plate 404-1, and is incident on the polarization beam splitter 403. . The S-polarized light is reflected by the polarization beam splitter 403 at 90 degrees, converted to circularly polarized light by the second quarter-wave plate 404-2 arranged so that the Fast axis and the polarization axis form 45 degrees, and the lens 406 -3 and is incident on the second CFBG 405-2. The circularly polarized light reflected by the second CFBG 405-2 is converted into P-polarized light rotated by 90 degrees from S-polarized light by the second quarter wave plate 404-2, and is incident on the polarization beam splitter 403. The P-polarized light is transmitted through the polarization beam splitter 403, converted into circularly polarized light by the second quarter-wave plate 404-3 arranged so that the Fast axis of the wave plate and the polarization axis form 45 degrees, and the lens 406 -4 and is incident on the third CFBG 405-3. The circularly polarized light reflected by the third CFBG 405-3 is converted by the third quarter wave plate 404-3 to S-polarized light rotated 90 degrees from the P-polarized light, and is incident on the polarizing beam splitter 403 to 90 degrees. The light is reflected and enters the optical circulator 402.

  In contrast, the S-polarized component of the incident light passes through the CFBG in the order of the third CFBG 405-3, the second CFBG 405-2, and the first CFBG 405-1. That is, the light beam is first reflected by the polarizing beam splitter 403 at 90 degrees and incident on the third CFBG 405-3, and the reflected light is converted to P-polarized light and transmitted through the polarizing beam splitter 403 and incident on the second CFBG 405-2. Is done. The reflected light of the second CFBG 405-2 is converted into S-polarized light, reflected by the polarization beam splitter 403 at 90 degrees, and incident on the first CFBG 405-1. The reflected light of the first CFBG 405-1 is converted into P-polarized light, passes through the polarization beam splitter 403, and enters the optical circulator 402.

  Since the two polarization states pass through a common path, polarization dispersion can be ignored.

  This configuration uses three polarization-independent optical circulators 402 and three CFBGs, and has the same amount of dispersion compensation as when CFBG is passed three times, but is compact because no polarization controller is required. .

DESCRIPTION OF SYMBOLS 100 Wavelength sweep light source 101 Wavelength selection part 102 Dispersion compensation part 103 Light extraction part 104 Delay part 105 Optical amplification part 106 Optical connection part 201, 301 Polarization controller 202, 302, 402 Optical circulator 203, 303, 403 Polarization beam splitter 204, 304, 404 1/4 wave plate 205, 305, 405 CFBG
206, 306, 406 Lens

Claims (4)

A polarization controller that outputs polarized light having a predetermined polarization axis from the input light;
An optical circulator to which the polarized light output from the polarization controller is input;
A polarizing beam splitter arranged to transmit one of the two axes of polarized light perpendicular to each other, reflect the other at 90 degrees, and transmit the polarized light output from the optical circulator;
A quarter wave plate that is input so that the polarized light transmitted through the polarizing beam splitter is input, and the fast axis is 45 degrees with the polarized light output from the optical circulator;
A first CFBG that receives circularly polarized light transmitted through the quarter-wave plate;
Reflected light from the first CFBG passes through the quarter-wave plate and is input to the polarization beam splitter, and second CFBG to which polarized light reflected at 90 degrees by the polarization beam splitter is input;
A dispersion compensator characterized by comprising: A second quarter-wave plate disposed between the second CFBG and the polarizing beam splitter so that the polarization output from the optical circulator and the fast axis form 45 degrees;
Reflected light from the second CFBG passes through the second quarter-wave plate and is input to the polarization beam splitter, and third CFBG to which polarized light transmitted through the polarization beam splitter is input;
The dispersion compensator according to claim 1, further comprising: An optical circulator,
A polarization beam splitter disposed so as to transmit one of two perpendicularly polarized light beams output from the optical circulator and reflect the other at 90 degrees;
Polarized light that has passed through the polarizing beam splitter is input, and the polarized light output from the optical circulator and a first quarter-wave plate arranged so that the fast axis forms 45 degrees;
A first CFBG that receives circularly polarized light transmitted through the first quarter-wave plate;
Reflected light from the first CFBG passes through the first quarter-wave plate and is input to the polarization beam splitter, and polarized light reflected at 90 degrees by the polarization beam splitter is input, and the optical circulator A second quarter-wave plate arranged so that the polarization output from and the fast axis form 45 degrees;
A second CFBG that receives circularly polarized light transmitted through the second quarter-wave plate;
The reflected light from the second CFBG passes through the second quarter-wave plate and is input to the polarizing beam splitter, and the polarized light transmitted through the polarizing beam splitter is input and output from the optical circulator. A third quarter-wave plate arranged so that the polarization and the fast axis form 45 degrees;
A third CFBG that receives circularly polarized light transmitted through the third quarter-wave plate;
A dispersion compensator characterized by comprising: An optical amplifying unit that generates broadband light including a predetermined wavelength band;
A wavelength selector that selectively transmits light in a predetermined wavelength band from the broadband light at a predetermined sweep frequency;
The dispersion compensator according to any one of claims 1 to 3, wherein light transmitted through the wavelength selection unit is input, and the light output from the dispersion compensator is given a predetermined delay amount to the optical amplification unit. A resonator including an optical delay unit for input; and
A light extraction portion for extracting a part of the light propagating in the resonator;
A wavelength-swept light source comprising: