Active mode-locked laser resonator, and optical coherence tomography apparatus using the same

Abstract

To achieve narrowing of the spectral linewidth of the laser. [Solution] An active mode-locked laser resonator comprising a gain medium, a modulator, and a linear chirp fiber diffraction grating connected in an orientation used in the anomalous dispersion region, forming a ring resonator, wherein the product of the third-order dispersion coefficient β3 of the linear chirp fiber diffraction grating and the wavelength range Δλ is 100 to 10000 ps 3 That is the case.

Description

[Technical Field]

[0001] This disclosure relates to an active mode-locked laser resonator and an optical coherence tomography apparatus using the same. [Background technology]

[0002] To provide a mode-locked laser light source device capable of realizing laser light with a narrow oscillation spectral distribution, a mode-locked laser light source device is known that includes: a semiconductor optical amplifier that generates carriers when an injection current is injected and amplifies the laser light pulse by the consumption of carriers, and generates phase modulation equivalent to self-phase modulation dependent on the laser light pulse intensity by a change in carrier density; a sweep modulation unit that makes the oscillation wavelength of the laser light pulse emitted from the semiconductor optical amplifier variable; a ring resonator that feeds back the laser light pulse modulated by the sweep modulation unit to the semiconductor optical amplifier to produce a laser oscillation phenomenon; and a dispersion compensator used in an anomalous dispersion region and changes the feedback time of the laser light pulse depending on the wavelength of the laser light pulse guiding the ring resonator (see Patent Document 1 below). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2013-92544 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, further narrowing of the laser's spectral linewidth is required for use in optical coherence tomography (OCT) systems.

[0005] This disclosure may provide an active mode-locked laser resonator capable of narrowing the spectral linewidth of a laser, and an optical coherence tomography apparatus using the same. [Means for solving the problem]

[0006] The active mode-locked laser resonator of this disclosure is an active mode-locked laser resonator comprising a gain medium, a modulator, and a linear chirp fiber diffraction grating connected in an orientation used in an anomalous dispersion region, comprising a ring resonator, wherein the product of the third-order dispersion coefficient β3 of the linear chirp fiber diffraction grating and the wavelength range Δλ is 100 to 10000 ps 3 That is the case. [Effects of the Invention]

[0007] This disclosure potentially enables narrowing of the spectral linewidth of lasers. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram showing the main components of the active mode-locked laser resonator and optical coherence tomography apparatus using the same according to the present disclosure. [Figure 2] This figure shows a linear chirp fiber diffraction grating. [Figure 3] This figure shows the conditions that were changed during the simulation. [Figure 4] This diagram shows the oscillation line width (spectrum) of the lasers in the comparative example and Examples 1-4. [Figure 5] This figure shows the coherence functions of the light sources in the comparative example and Examples 1-4. [Figure 6] This figure shows an example of an application of the active mode-locked laser resonator of this disclosure. [Modes for carrying out the invention]

[0009] The active mode-locked laser resonator and optical coherence tomography apparatus using the same described herein will be explained with reference to the drawings. Figure 1 is a schematic diagram showing the main components of the active mode-locked laser resonator and optical coherence tomography apparatus using the same described herein.

[0010] In FIG. 1, an active mode-locked laser resonator 1 (hereinafter, also simply referred to as laser resonator 1) constitutes a ring resonator including a gain medium 20, a modulator 30, and a linear chirped fiber diffraction grating 10 connected via an optical circulator 40.

[0011] The gain medium 20 may be, for example, a semiconductor optical amplifier.

[0012] The semiconductor optical amplifier may have a waveguide structure. One end face of the waveguide structure may be an incident end face, and the other end face of the waveguide structure may be an emission end face. An injection current is injected into the waveguide structure to generate carriers in the waveguide structure. The carriers are consumed by the stimulated emission phenomenon caused by an optical pulse incident on the incident end face of the waveguide structure. As a result, the pulse of the laser light is amplified, and the pulse of the laser light is emitted from the emission end face.

[0013] The pulse of the laser light emitted from the emission end face of the waveguide structure may be guided to the modulator 30 via an optical isolator 50 as an optical element that allows light to pass only in one direction and blocks the return light.

[0014] The modulator 30 is used for active mode locking and has a function of modulating the intensity or phase of the pulse of the laser light incident on the modulator 30. Specifically, an electro-optic modulator (EOM) that modulates the intensity may be used.

[0015] The optical circulator 40 may be, for example, one having three ports as shown in the figure. An emission light guide fiber for guiding the pulse of the laser light output from the modulator 30 to the optical circulator 40 may be connected to the first port 401 of the optical circulator 40.

[0016] A linear chirp fiber diffraction grating (LC-FBG) 10 is connected to the second port 402 of the optical circulator 40. Figure 2 shows this linear chirp fiber diffraction grating. As shown in Figure 2, the linear chirp fiber diffraction grating 10 has a linearly changing grating period, resulting in linearly different reflection positions for low-frequency and high-frequency components in a pulse. By adjusting the chirp rate and length of the grating, it is possible to adapt it to specific dispersion characteristics and bandwidths. Such a linear chirp fiber diffraction grating 10 also functions as a reflector and a wavelength-selective filter.

[0017] In a linear chirp fiber diffraction grating, to achieve desired characteristics, for example, by utilizing a two-beam interference lithography method using ultraviolet light, the desired grating period can be chiped by gradually changing the intersection angle of the intersecting light, and the period can be continuously changed by exposing the grating while changing its position along the optical fiber.

[0018] The linear chirp fiber diffraction grating 10 has both normal dispersion and anomalous dispersion characteristics depending on its orientation, and its use in the normal dispersion region and anomalous dispersion region is changed by how the second port of the optical circulator of the linear chirp fiber diffraction grating is connected. In other words, this linear chirp fiber diffraction grating can be used in the normal dispersion region where long-wavelength pulse components are reflected first and short-wavelength pulse components are reflected later, and in the anomalous dispersion region where short-wavelength pulse components are reflected first and long-wavelength pulse components are reflected later.

[0019] In this disclosure, the linear chirp fiber diffraction grating 10 is connected in an orientation used in an anomalous dispersion region, where short-wavelength pulsed light components are reflected first and long-wavelength pulsed light components are reflected later.

[0020] The third port 403 of the optical circulator is connected to a feedback optical fiber that returns the laser pulse light reflected by the linear chirp fiber diffraction grating 10 back to the gain medium 20.

[0021] The laser light pulses output from the transmission end face 102 of the linear chirp fiber diffraction grating 10 may be guided via the isolator 51 to the optical system 500 of a subsequent optical coherence tomography (OCT) device or an appropriate interferometer. The optical coherence tomography device may be, for example, an SS-OCT device and may be used in ophthalmic equipment.

[0022] By using such a ring resonator, the laser spectral linewidth can be narrowed by using the integrated linear chirp fiber diffraction grating in the anomalous dispersion region. However, further narrowing of the laser spectral linewidth is required for use in optical coherence tomography (OCT) systems.

[0023] By the way, dispersion occurs in a ring resonator using an optical fiber as described in this disclosure. The propagation constant β(ω) of light propagating through the fiber is: β(ω)=n EFF (ω)ω / c=β0+(ω-ω0)β1+1 / 2×(ω-ω0) 2 β² + 1 / 6 × (ω - ω0) 3 β3+…, It can be expanded using a tailoring method as shown below. Here, n EFF ω0 is the transmission refractive index of the mode, ω0 is the central optical frequency of the propagating mode, and β is the central optical frequency of the mode. n This represents the nth order variance coefficient. For example, the first order variance coefficient β1 represents the group delay (its reciprocal is the group velocity), and the second order variance coefficient β2 represents the group velocity variance (GVD).

[0024] Here, the relationship between the variance parameter D and the second-order variance coefficient β2 is given by D = -2πc / λ 2 The relationship β²[ps / nm / km] exists, and the case where D is positive (β² is negative) is called anomalous dispersion. Note that the above units are assumed for fibers with very long optical path lengths, but in the case of the linear chirp fiber diffraction grating 10, the length is limited and known, so it is acceptable to use [ps / nm] which reflects the known length by integrating it.

[0025] The third-order dispersion coefficient (TOD) β3 represents the wavelength dependence of the group velocity dispersion (GVD) of a pulse. In conventional linear chirped fiber diffraction gratings, the third-order dispersion coefficient has been treated as 0 (zero), and in fact, the third-order dispersion coefficient was also 0 in terms of the performance of the diffraction grating. However, in ultrafast lasers, as the pulse width becomes shorter, the spectral bandwidth becomes wider. In the propagation of this broadband pulse, the influence of the third-order dispersion coefficient becomes greater.

[0026] In recent years, for example, the manufacturing technology of linear chirped fiber diffraction gratings including the above-mentioned two-beam interference exposure method has improved, and there has emerged room to achieve characteristics designed for the third-order dispersion coefficient. If the third-order dispersion coefficient can be intentionally controlled, it may be possible to achieve a shorter pulse width while maintaining the waveform symmetry. Therefore, the following simulations were conducted to examine the third-order dispersion coefficient suitable for narrowing the spectral linewidth of the laser.

[0027] Specifically, it was examined whether it is possible to narrow the spectral linewidth of the laser by changing the third-order dispersion coefficient β3 of the linear chirped fiber diffraction grating.

[0028] The following conditions were set using commercially available optical simulation software. Central optical frequency: 1.0 GHz Pulse width: 500 ps Dispersion parameter D: 10 ps / nm Product of the third-order dispersion coefficient β3 and the wavelength range Δλ: 0 - 10000 ps 3 In an experimental example using a manufactured product of a linear chirped fiber diffraction grating with a third-order dispersion coefficient β3 of 0, it has been confirmed that the difference between this simulation and the experimental results is within a range that is not a problem in practical applications.

[0029] Here, as a condition for changing the third-order dispersion coefficient β3, the product of the third-order dispersion coefficient β3 and the wavelength range Δλ [ps 3The ] setting was applied. The wavelength range Δλ was set to 100 nm. In addition, the linear chirp fiber diffraction grating has a specific finite length. As a result, the third-order dispersion coefficient β3[ps] of the linear chirp fiber diffraction grating was set. 3 The length dimension disappears when the sum of [nm] (where a finite length is already considered) × wavelength range Δλ [nm] is calculated, and ps 3 It is expressed as follows. Therefore, it is possible to express the coefficient of variance in such units.

[0030] Figure 3 is a table showing the values ​​obtained by varying the product of the cubic dispersion coefficient β3 and the wavelength range Δλ. As shown in this condition table, the values ​​were varied in the comparative example and Examples 1-4. Below, we compare the simulation results of the comparative example and each example.

[0031] Figure 4 shows the oscillation line width (spectrum) of the lasers of Examples 1 to 4 in comparison with the comparative example. The horizontal axis represents wavelength, and the vertical axis represents intensity in dB. This is important for evaluating interference signals in dB in optical coherence tomography (OCT) and imaging.

[0032] In this figure, in all of Examples 1 to 4, a narrowing of the laser spectral linewidth was observed compared to the comparative example.

[0033] Figure 5 shows the coherence functions of the light sources in Examples 1 to 4 in comparison with the comparative example. The horizontal axis represents the optical path difference (OPD), and the vertical axis represents the coherence function. The coherence function on the vertical axis is normalized at OPD=0. The coherence function decreases as OPD increases, meaning that the coherence of light decreases with distance. The coherence of light is closely related to the oscillation linewidth; it decreases rapidly in light sources with a wide oscillation linewidth and attenuates gradually in narrow-band light sources.

[0034] In this figure, in all of Examples 1 to 4, suppression of the decay of the coherence function was confirmed compared to the comparative example.

[0035] In Figure 5, we attempt a quantitative evaluation of the suppression of coherence function decay. Using -1 dB (corresponding to a value of 0.8 on the vertical axis of this figure) as a guideline for coherence function decay, approximately β3·Δλ = 0 ps 3 So, OPD = 2 mm, β3·Δλ = 100 ps 3 So, OPD = 3 mm, β3·Δλ = 500 ps 3 So, OPD = 5 mm, β3·Δλ = 1000 ps 3 So, OPD = 6 mm, β3·Δλ = 10000 ps 3 Therefore, OPD = 10 mm. Consequently, β3·Δλ = 100 ps 3 In the above, suppression of attenuation was observed. Furthermore, in SS-PCT, coherence at OPD:4~6mm is often important, and β3·Δλ = 500~1000ps 3 This has yielded extremely effective results.

[0036] As described above, the product of the third-order dispersion coefficient β3 of a linear chirp fiber diffraction grating and the wavelength range Δλ is 100 to 10000 ps 3 It was confirmed that in this case, it is possible to narrow the spectral linewidth of the laser. In particular, even in OCT applications, β3·Δλ = 500~1000 ps 3 It was confirmed to be even more effective.

[0037] Here, in order to achieve a high value of β3·Δλ, in addition to the method of designing the grating using the above manufacturing technology, a modified example is shown in Figure 6. In this figure, only the number of linear chirp fiber diffraction gratings differs from that in Figure 1, so the explanation of the other configurations is omitted.

[0038] In Figure 6, in the ring resonator constituting the active mode-locked laser resonator of this disclosure, a linear chirp fiber diffraction grating 11 is further connected by an optical circulator 41 in a direction that utilizes it in the anomalous dispersion region. In other words, the linear chirp fiber diffraction grating 11 is used as an additional chirp mirror. This also makes it possible to achieve a high value of β3·Δλ.

[0039] Furthermore, in the application example shown in Figure 6, there is no restriction on connecting even more linear chirp fiber diffraction gratings 11 with the optical circulator 41 in a manner that utilizes them in the anomalous dispersion region.

[0040] Other methods for achieving a desired cubic dispersion coefficient β3 of a linear chirp fiber diffraction grating include connecting multiple linear chirp fiber diffraction gratings in series and using multiple linear chirp fiber diffraction gratings with different dispersion characteristics. Those skilled in the art will consider these methods to be achievable with a reasonable number of trials. Furthermore, the cubic dispersion coefficient β3 can also be actively controlled by heating and cooling the linear chirp fiber diffraction grating and controlling its temperature.

[0041] Several of the embodiments described above will be noted again below. [1] An active mode-locked laser resonator, A gain medium and a modulator, A ring resonator is formed, comprising a linear chirp fiber diffraction grating connected in an orientation used in the anomalous dispersion region, An active mode-locked laser resonator in which the product of the third-order dispersion coefficient β3 of the linear chirp fiber diffraction grating and the wavelength range Δλ is 100 to 10000 ps3. [2] The product of the third-order dispersion coefficient β3 and the wavelength range Δλ is 500 to 1000 ps3. [1] Active mode-locked laser resonator. [3] An active mode-locked laser resonator according to [1] or [2], wherein a plurality of linear chirp fiber diffraction gratings are connected in the ring resonator in the orientation used in the anomalous dispersion region. [4] An optical coherence tomography apparatus comprising an active mode-locked laser resonator as described in any one of [1] to [3].

[0042] This concludes the explanation provided in this disclosure. However, the new technologies described herein can be realized in various other forms, and parts of the content may be omitted, modified, or replaced without departing from the spirit of this disclosure. The embodiments and variations thereof shown in this disclosure are also included in the scope and spirit of this disclosure and shall be treated as equivalent and comparable to the technologies protected under the claims. [Explanation of Symbols]

[0043] 1. Active mode-locked laser resonator 10 Linear chirp fiber diffraction grating 11 Linear chirp fiber diffraction grating 20 Gain medium 30 Modulators 40 Light Circulator 41 Light Circulator 50 Optical Isolators 51 Optical Isolator

Claims

1. An active mode-locked laser resonator, A gain medium and a modulator, A ring resonator is formed, comprising a linear chirp fiber diffraction grating connected in an orientation used in the anomalous dispersion region, The third-order dispersion coefficient β of the linear chirp fiber diffraction grating ３ The product of this and the wavelength range Δλ is 100 to 10000 ps ３ This is an active mode-locked laser resonator.

2. The aforementioned tertiary dispersion coefficient β ３ The product of this and the wavelength range Δλ is 500 to 1000 ps ３ That is, The active mode-locked laser resonator according to claim 1.

3. The active mode-locked laser resonator according to claim 1, wherein a plurality of linear chirp fiber diffraction gratings are connected in the ring resonator in an orientation used in the anomalous dispersion region.

4. An optical coherence tomography apparatus comprising an active mode-locked laser resonator as described in claim 1.

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