JP2022139361A - Mode synchronous laser - Google Patents

Abstract

To provide a mode synchronous laser capable of generating high power optical pulses using a durable element that replaces a damage threshold limited saturable absorber.SOLUTION: A mode synchronous laser 100 includes a polarization-maintaining resonator 10, an optical amplifier 20 arranged in the resonator 10, and a transmission adjustment portion 30 that is arranged in the resonator 10 and provided with a saturable absorption characteristic by having a second optical fiber 31 which is a polarization-maintaining adjustment waveguide having bending loss.SELECTED DRAWING: Figure 1

Description

The present invention relates to a mode-locked laser using an optical fiber as a resonator, and more particularly to a mode-locked laser that generates high-intensity short pulses.

Mode-locked lasers are widely used in fields such as measurement and medicine. In general, mode-locked lasers are known to generate ultrashort pulse trains using a saturable absorber (see Patent Document 1 and Non-Patent Document 1, for example). A saturable absorber has a characteristic that the higher the intensity of the irradiated light, the more saturated the absorption and the higher the transmittance. Therefore, among the noise components emitted from the fiber amplifier, those components with strong peaks survive and become the seeds of oscillating optical pulses. As the saturable absorber, a carbon nanotube (CNT: carbon nanotube) or the like, which is a carbon material, is generally used. However, CNTs and the like are limited by relatively low damage thresholds, so extremely high light output cannot be expected.

Polarization maintaining in fiber lasers is important for practical applications that require high environmental stability. Material-based polarization-maintaining lasers suffer from long-term stability and durability with respect to optical damage. A Kerr effect-based (hereinafter also referred to as Kerr-based) approach can be used to solve these problems, but most Kerr-based fiber lasers are non-polarization-maintaining. As Kerr-based fiber lasers, interference type PM-F8 (for example, see Non-Patent Document 2), interference type PM-F9 (for example, see Non-Patent Document 3), PM-NPR using nonlinear polarization rotation (See, for example, Non-Patent Document 4). However, PM-F8 is not easy to self-start, PM-F9 solves the problem of PM-F8 but requires a complex Faraday polarization control component, and PM-NPR has a polarizer between a pair of polarizers. It contains a fiber section that produces nonlinear polarization rotation and has environmental instability such as temperature and mechanical fluctuations.

Instead of using a saturable absorber, we used polarization dependent loss (PDL) caused by bending a single mode optical fiber (SMF) and placing a polarizer at the exit. There are mode-locked lasers (see, for example, Non-Patent Documents 5 and 6).

JP 2015-118348 A

U. Keller, "Recent developments in compact ultrafast lasers." Nature 424, 831-838 (2003). J. W. Nicholson, et al. "A polarization maintaining, dispersion managed, femtosecond figure-eight fiber laser," Opt. Express 14, 8160-8167 (2006) N. Kuse, et al. "All polarization-maintaining Er fiber-based optical frequency combs with nonlinear amplifying loop mirror." Opt. Express 24, 3095-3102 (2016) J. Szczepanek, et al. "Nonlinear polarization evolution of ultrashort pulses in polarization maintaining fibers." Opt. Express 26, 13590-13604 (2018) Q. Wang, et al. "Polarization dependence of bend loss for a standard singlemode fiber." Optics express 15.8 (2007): 4909-4920. H. Jiang, et al. "Laser mode locking using a single-mode-fiber coil with enhanced polarization-dependent loss." Optics Letters 45.10 (2020): 2866-2869.

The present invention has been made in view of the above background art, and provides a mode-locked laser that can generate high-power optical pulses by using a highly durable element that replaces the saturable absorber limited by the damage threshold. intended to provide

To achieve the above object, a mode-locked laser according to the present invention comprises a polarization-maintaining resonator, an optical amplifier disposed in the resonator, and a polarized wave having bending loss disposed in the resonator. and a transmission adjustment section having a saturable absorption characteristic by having a holding type adjustment waveguide.

In the above-described mode-locked laser, the transmittance decreases due to the bending loss of the adjustment waveguide in the transmission adjustment section, but the transmittance increases due to the nonlinear optical effect as the light intensity increases. In other words, when the light intensity is low, the light confinement (guiding) effect of the waveguide for adjustment is low due to the influence of bending, and the light leaks to the outside. The confinement effect is increased and light leakage is suppressed. In this way, since the saturable absorption characteristic is realized only by the adjustment waveguide, ultrashort pulses can be generated without using a saturable absorber such as CNT. Therefore, limitations such as long-term durability and damage threshold of saturable absorbers such as CNTs can be eliminated, and high-power optical pulses can be generated over a long period of time. A mode-locked laser having the above-described transmission adjustment section realizes a short-pulse laser with high stability and long life, and is useful for various applications. For example, it can be used as a light source for deep tissue imaging (TPM, CARS, SRS, OCT, etc.) and laser micromachining. Such mode-locked lasers can achieve high durability, low cost, small size, and the like.

In a specific aspect of the present invention, in the above-described mode-locked laser, the resonator includes a polarization-maintaining first optical fiber, and the adjustment waveguide includes a forcibly bent polarization-maintaining second optical fiber. Fiber. The size and weight of the device can be reduced by forming the resonator from the first optical fiber and forming the adjustment waveguide from the second optical fiber.

According to another aspect of the present invention, in the above-described mode-locked laser, the transmission adjustment section exhibits a first bending loss greater than or equal to a predetermined value when light having a relatively low first intensity passes through, and has a relatively high first bending loss. When two intensities of light pass through, it exhibits a second bending loss that is less than a predetermined. In this case, in the transmission adjustment section, when light having a relatively low first intensity passes through, the first bending loss becomes relatively large, and light leakage can be positively caused.

In yet another aspect of the invention, the second optical fiber has a non-linear optical effect such that when light of the second intensity passes through it, the refractive index increases relative to when light of the first intensity passes therethrough. In this case, due to nonlinear optical effects, the refractive index in the fiber core increases for high light intensities, improving the guiding ability and reducing bending losses.

In yet another aspect of the invention, the second optical fiber is a PANDA fiber and is wound in one of the slow axis direction and the fast axis direction of the PANDA fiber. In this case, bending loss can be controlled by the bending radius and the number of loops.

In yet another aspect of the invention, the radius of curvature of the loop of the second forcedly bent optical fiber is constant.

In still another aspect of the present invention, the second optical fiber is a PANDA fiber, and uses polarized waves in either the slow axis direction or the fast axis direction of the PANDA fiber.

In still another aspect of the present invention, the second optical fiber uses slow-axis polarized waves.

In yet another aspect of the invention, the first optical fiber is a PANDA fiber. In this case, polarization can be maintained easily and reliably.

In still another aspect of the present invention, the following conditional expression is satisfied with respect to the V value, which is the normalized frequency parameter of the first or second optical fiber.
V1 < V2
However, the value V1 is the V value of the first optical fiber, and the value V2 is the V value of the second optical fiber.
Note that the V value is defined by the following formula.
V=πdNA/λ
where the value d is the core diameter of the fiber of interest, the value NA is the numerical aperture of the core of the fiber of interest, and the value λ is the frequency used for the fiber of interest. Fiber of interest means either the first or the second optical fiber.

In still another aspect of the present invention, the resonator is formed in a ring shape by the first optical fiber. In this case, for example, a reflector becomes unnecessary, and maintenance becomes almost unnecessary.

1 is a conceptual diagram illustrating a mode-locked laser according to a first embodiment; FIG. FIG. 2 is a diagram illustrating a polarization-maintaining fiber used in the mode-locked laser of FIG. 1; (A) and (B) are diagrams for explaining bending directions of polarization-maintaining fibers, and (C) and (D) are diagrams for explaining directions of polarization used in polarization-maintaining fibers. (A) and (B) are diagrams showing analysis results based on the finite element method with respect to bending loss of an optical fiber. FIG. 5 is a typical cross-sectional view of the polarization-maintaining optical fiber analyzed in FIG. 4; FIG. 2 is a conceptual diagram illustrating the structure of a transmission adjustment unit incorporated in the mode-locked laser of FIG. 1; (A) is a diagram for explaining the refractive index, light intensity distribution, etc. around the core when the second optical fiber of the transmission adjustment unit is bent; It is a figure explaining the influence of a nonlinear optical effect, etc. (A) is a conceptual diagram illustrating the relationship between light intensity and transmittance when the second optical fiber is bent, and (B) is a light intensity and bending loss when the second optical fiber is bent. is a conceptual diagram for explaining the relationship between. FIG. 4 is a diagram showing analysis results based on the finite element method with respect to the Kerr effect; FIG. 10 is a diagram showing the results of an intensity scan when a transmission adjuster is provided; It is a figure explaining the mode locking laser of 2nd Embodiment. It is a figure explaining the mode locking laser of 3rd Embodiment.

[First Embodiment]
A mode-locked laser according to a first embodiment of the present invention will be described below with reference to FIG.

A mode-locked laser 100 of the first embodiment shown in FIG. 1 is a passive mode-locked laser, and includes a resonator section 10, an optical amplifier section 20, a transmission adjustment section 30, an isolator 40, and an output coupler 50. The mode-locked laser 100 includes a resonator section 10, an optical amplifier section 20, a transmission adjustment section 30, an isolator 40, and an output coupler 50, which are joined by fusion or the like. The illustrated mode-locked laser 100 is an example of a ring-type fiber laser with unidirectional operation. Mode-locked laser 100 is an all-fiber polarization-maintaining mode-locked laser.

The resonator 10 of the mode-locked laser 100 is formed in a ring shape by the first optical fiber 11 . Forming the resonance section 10 with the first optical fiber 11 enables reduction in size and weight. Further, by forming the resonance part 10 in a ring shape, for example, a reflector is not required, and maintenance becomes almost unnecessary. The first optical fiber 11 is a polarization maintaining optical fiber (PMF: Polarization Maintaining Fiber).

Polarization-maintaining optical fiber (PMF) uses the photoelastic effect and structural changes to create birefringence with different effective refractive indices in the vertical and horizontal directions of the core, thereby enhancing the property of maintaining the polarization plane of propagating light. Optical fiber. A polarization-maintaining fiber has a clear refractive index difference between the vertical and horizontal cross-sections of the optical fiber so that polarization does not interfere between the vertical and horizontal directions. There are two types of polarization-maintaining fibers: the stress-imparted type, in which birefringence is imparted using the photoelastic effect, and the structural type, in which the effective refractive index is changed vertically and horizontally in the core. In the stress-applying type, a stress-applying material (SAP: Stress Applying Parts) having a thermal contraction rate much larger than that of the clad material is inserted in one direction of the clad cross section so as to sandwich the core. Polarization-maintaining fibers are generally of the stress-imparting type. (Bow-tie) type, elliptical elliptical jacket fiber, and polarization-maintaining photonic crystal fiber. In this embodiment, a case where the polarization-maintaining fiber shown in FIG. 2 is a PANDA type will be described. By using a PANDA fiber, which is a PANDA-type polarization-maintaining fiber, polarized waves can be easily and reliably maintained.

As shown in FIG. 2, the PANDA fiber PF has a core 1, a clad 2, a stress-applying portion 3, and a covering portion 4. As shown in FIG. The PANDA fiber PF has a structure in which a core 1 is arranged at the center of the fiber, a clad 2 is arranged around the core 1, and two circular stress-applying parts 3 are arranged on both sides of the core 1. A covering portion 4 covers the cladding 2 and protects the inside of the fiber. The refractive index of core 1 is higher than that of clad 2 . The stress-applying portion 3 is formed of a silica glass rod doped with, for example, B ₂ O ₃ in order to increase the coefficient of linear expansion. The stress-applying portion 3 has a higher coefficient of linear expansion than the clad portion of pure silica glass, and stress is applied to the core along the X-axis in the drawing due to tensile strain occurring in the stress-applying portion 3. . Note that the X-axis of FIG. 2 indicates the slow axis of the PANDA fiber PF, and the Y-axis indicates the fast axis of the PANDA fiber PF.

The optical amplification section 20 has a gain fiber 21 and an excitation section 22 . The optical amplifier 20 is arranged in the resonator 10 . The optical amplifying section 20 can be replaced with an optical amplifying element using another amplifying medium such as a semiconductor optical amplifier, a fiber Raman amplifier, or the like.

The gain fiber 21 is a polarization-maintaining optical fiber doped to provide amplification. Specifically, the gain fiber 21 is a doped fiber doped with a rare earth element such as erbium (Er), and amplifies light circulating in the resonator 10 . A gain fiber 21 is connected in-line to the first optical fiber 11 .

The excitation unit 22 has an excitation light source 22a and a multiplexing coupler 22b. The pumping unit 22 supplies pumping light PL to the gain fiber 21 . The excitation light source 22a is composed of, for example, a semiconductor laser, and outputs excitation light with a wavelength of 980 nm, for example. The multiplexing coupler 22b does not prevent the light with a wavelength of 1550 nm from propagating and circulating in the first optical fiber 11, for example. The pumping light introduced into the resonator 10 via the pumping unit 22 excites the dopant added to the doped fiber of the gain fiber 21 to enable stimulated emission at the wavelength of the resonant light for output.

The transmission adjustment section 30 is arranged in the resonance section 10 and has a saturable absorption characteristic by having an adjustment waveguide AW having a bending loss. Specifically, the adjustment waveguide AW is a polarization-maintaining optical fiber that has a saturable absorption characteristic by forcibly bending the second optical fiber 31 . By forming the adjustment waveguide AW from the second optical fiber 31, reduction in size and weight can be achieved.

The second optical fiber 31 is, for example, a PANDA fiber PF. The second optical fiber 31 is bent and wound in either the slow axis direction or the fast axis direction of the PANDA fiber PF. Thereby, the bending loss can be controlled by the bending radius and the number of loops. 3A shows the case where the second optical fiber 31 is bent in the slow axis direction (X-axis direction), and FIG. 3B shows the case where the second optical fiber 31 is bent in the fast axis direction (Y-axis direction). Shown when bent. The second optical fiber 31 uses polarized waves in either the slow axis direction or the fast axis direction of the PANDA fiber PF. In particular, it is assumed that the second optical fiber 31 uses polarized waves in the slow axis direction. FIG. 3C shows a case where the second optical fiber 31 is polarized in the slow axis direction (X-axis direction), and FIG. A case where a polarized wave P in the axial direction) is used is shown. The transmission adjuster 30 is a 4-wave pattern combining either one of the two bending patterns shown in FIGS. 3A and 3B and one of the two polarization patterns shown in FIGS. 3C and 3B. The second optical fibers 31 are used in the same pattern.

FIGS. 4(A) and 4(B) show results of bending loss of an optical fiber calculated using COMSOL Multiphysics (registered trademark: COMSOL) based on the finite element method (FEM). . 4(A) and 4(B) show the mode distribution in a polarization-maintaining optical fiber. FIG. 4A shows the case where the optical fiber is bent in the slow axis direction (X-axis direction) as shown in FIG. 3A, and FIG. shows the case of bending in the fast axis direction (Y-axis direction). FIG. 5 is a typical cross-sectional view of a polarization-maintaining optical fiber drawn in COMSOL Multiphysics. In FIG. 5, area A1 is a matching layer for calculating bending loss, and areas A2 to A5 are the core 1, cladding 2, stress-applied portion 3, and covering portion 4 of the PANDA fiber PF shown in FIG. handle. As shown in FIGS. 4A and 4B, when the optical fiber is bent, the mode spreads in the bending direction, and light leaks out in the direction of arrow LE.

As shown in FIG. 6, the transmission adjuster 30 is formed by winding a second optical fiber 31 along a groove 91 formed in a cylindrical shaft member 90 made of metal, for example. The radius of curvature of the loop of the second optical fiber 31 that is forcibly bent is preferably constant. The method of bending the second optical fiber 31 to form a coil can be changed as appropriate. A configuration in which it is wound inside the shaft member 90 may be used, or a configuration in which the shaft member 90 is not used may be used. The radius of curvature of the second optical fiber 31 is, for example, 5 mm to 20 mm. Also, the number of turns of the second optical fiber 31 is, for example, 1 to 10 turns.

The saturable absorption characteristics of the transmission adjuster 30 will be described below. By forcibly bending the second optical fiber 31, the internal refractive index structure changes, and light leakage is likely to occur. In the transmission adjuster 30, a nonlinear optical effect causes a refractive index change depending on the light intensity. When the intensity of propagating light is high, the nonlinear refractive index rises and the propagation trajectory moves closer to the center of the optical fiber core. On the other hand, when the intensity of propagating light is low, the propagation trajectory moves outside the core of the optical fiber due to the decrease in the nonlinear refractive index. From a different point of view, the fiber has a large curvature and the angle of incidence on the boundary between the core and the clad becomes smaller than the critical angle. When the intensity of the generated and propagating light is high, the nonlinear optical effect increases the refractive index of the core and suppresses leakage to the cladding. As a result, when ordinary light of relatively low intensity is incident, the optical fiber confinement effect is reduced in the transmission adjustment section 30, the light leaks to the outside, and the bending loss increases. On the other hand, when high-intensity light is incident, in the transmission adjusting section 30, the confinement effect of the optical fiber is relatively increased due to the nonlinear optical effect, light leakage is suppressed, and bending loss is reduced. In other words, phenomena such as nonlinear bending loss and light intensity dependent bending loss occur. That is, the transmission adjuster 30 increases in transmittance as the light intensity increases, and functions like a saturable absorber.

FIG. 7A is a diagram for explaining the refractive index, light intensity distribution, etc. around the core when the second optical fiber 31 is bent. In _FIG . 7A, the solid line J1 indicates the refractive index nb when the second optical fiber 31 is bent, and the dashed line J2 indicates the refractive index _ns when the second optical fiber 31 is not bent. A solid line K1 indicates the light intensity distribution when the second optical fiber 31 is bent, and a dashed line K2 indicates the light intensity distribution when the second optical fiber 31 is not bent. When the second optical fiber 31 is forcibly bent, the refractive index in the second optical fiber 31 changes from the refractive index _ns to the refractive index nb due to the bending, as shown in _FIG . The intensity distribution shifts slightly towards the outside of the core. In the second optical fiber 31 that is forcibly bent, when the light intensity is high, the transmittance is less affected by bending, but when the light intensity is low, the transmittance is affected by the bending. bending loss occurs.

The second optical fiber 31 has a nonlinear optical effect, and when high intensity light (relatively strong second intensity light) passes through, low intensity light (relatively weak first intensity light) The refractive index increases more than when . Due to this nonlinear optical effect, the refractive index in the fiber core increases for high light intensities, improving the guiding ability and reducing bending losses. A nonlinear optical effect is an optical effect caused by nonlinear polarization induced when high-intensity light is incident on a substance. As a nonlinear optical effect, there is a phenomenon in which a refractive index change occurs. Examples of nonlinear optical effects related to refractive index change include the Kerr effect and the Pockels effect. In this embodiment, a refractive index change n=n ₀ +n ₂ I that depends on light intensity due to the Kerr effect, which is a third-order nonlinear process, is used. where _n0 is the linear refractive index, n2 is the _second -order nonlinear refractive index, and I is the light intensity. In other words, when a strong optical electric field (E) propagates through a medium, the Kerr effect causes a refractive index change n=n ₀ +n ₂ <E ² > that depends on the light intensity. The nonlinear optical effect is not limited to the Kerr effect, and any nonlinear optical effect that causes a refractive index change can be applied as appropriate.

Since the Kerr effect response speed of quartz glass is less than 10 fs, the method proposed in the present invention has a similar response speed. Regarding the response speed of the conventional saturable absorber, it is about 500 fs for CNT and about 300 fs for graphene. Speed has improved significantly.

FIG. 7B is a diagram for explaining the influence of the nonlinear optical effect in the second optical fiber 31 that is forcibly bent. The axis on the right side of FIG. 7B represents the light intensity. In FIG. 7B, the solid line L1 indicates the refractive index without nonlinear optical effect, and the dashed line L2 indicates the refractive index with nonlinear optical effect. In addition, the dashed-dotted line M1 shows the intensity distribution of high intensity light (relatively strong second intensity light), and the two-dotted chain line M2 shows the intensity distribution of low intensity light (relatively weak first intensity light). indicates As shown in FIG. 7B, in the transmission adjustment section 30, when the light intensity increases, the light intensity distribution narrows in the core radial direction due to the nonlinear optical effect, specifically, the Kerr effect. is confined, but as the light intensity decreases, the light intensity distribution widens in the core radial direction, and light leaks out from the core of the fiber. The transmission adjustment unit 30 has a structure that positively causes leakage light when the light intensity is low, thereby creating a state equivalent to a saturable absorber.

FIG. 8A is a conceptual diagram illustrating the relationship between light intensity and transmittance when the second optical fiber 31 is bent. FIG. 8B is a conceptual diagram illustrating the relationship between the light intensity and the bending loss when the second optical fiber 31 is bent. The symbol ΔT in the figure indicates the modulation depth, the symbol I _sat indicates the saturation intensity, the symbol α _ns indicates the unsaturated loss, and the symbol α ₀ indicates the background absorption loss. Regarding the modulation depth ΔT, the saturation intensity I _sat , the unsaturated loss α _ns , and the background absorption loss α ₀ , reference is made to Document 1 and Non-Patent Document 1 below (Document 1: J. Jeon, et al. Numerical study on the minimum modulation depth of a saturable absorber for stable fiber laser mode locking." JOSA B, 2015, 32(1): 31-37.). The modulation depth ΔT is defined as the difference between the background absorption loss α ₀ and the unsaturated loss α _ns . As shown in FIGS. 8A and 8B, in the forcibly bent second optical fiber 31, when the light intensity is low, the transmittance is low due to the bending loss, and when the light intensity is high, Bend loss effects are reduced. That is, the transmission adjustment section 30 exhibits a first bending loss equal to or greater than a predetermined value when light with a relatively low first intensity passes through, and exhibits a first bending loss greater than a predetermined value when light with a relatively high second intensity passes through. A second bend loss less than a predetermined is indicated. As a result, when light having a relatively low first intensity passes through the transmission adjusting section 30, the first bending loss becomes relatively large, and it is possible to positively cause leakage light. In other words, in the transmission adjuster 30, the bending loss (corresponding to the first bending loss) causes light leakage to the clad with respect to the light of predetermined intensity below the oscillation level. Also, when light of a relatively high second intensity passes through, the second bending loss is relatively small, corresponding to the saturable absorption characteristic required to generate a short pulse. In other words, the interaction between the bending loss and the nonlinear optical effect can reduce the leakage of light having an intensity equal to or higher than the oscillation level. In this embodiment, the bending loss for low intensity light is about 3 dB to 10 dB.

When the second optical fiber 31 is bent, the background absorption loss is, for example, 5 dB to 7 dB and the modulation depth is 5% maximum. In this case, the loss can be 4.8 dB to 6.8 dB at higher light intensities. The mode-locked laser 100 has a very high gain of 10 dB or more, and is designed to be about 5 dB to 10 dB. The loss in the Kerr effect waveguide (the transmission adjustment section 30 that utilizes the properties of the Kerr effect and bending loss) is greater than that of conventional saturable absorbers. However, low loss is not essential. In a laser system, the gain medium provides a gain that exceeds the loss across the cavity. In fiber laser systems, the gain is very high, typically above 10 dB. Therefore, the loss in Kerr effect waveguides is typically designed to be 5 dB to 10 dB. For example, in the case of the present embodiment, if the fiber coil exhibits a loss of 80% and the light intensity increases to 75% this loss will result in an intensity dependent loss (so-called depth of modulation) of 5%. In a mode-locked laser system, especially a fiber laser, a modulation depth of 5% or less is sufficient.

The mode-locked laser 100 satisfies the following conditional expression (1) with respect to the V value, which is the normalized frequency parameter of the first or second optical fibers 11 and 31 .
V1<V2 (1)
However, the value V 1 is the V value of the first optical fiber 11 and the value V 2 is the V value of the second optical fiber 31 .
Note that the V value is defined by the following formula.
V=πdNA/λ (2)
where the value d is the core diameter of the fiber of interest, the value NA is the numerical aperture of the core of the fiber of interest, and the value λ is the frequency used for the fiber of interest. A fiber of interest means the first or second optical fiber 11 , 31 . In this case, there is no need to make the radius of curvature of the second optical fiber 31 in the transmission adjustment section 30 too small, and the number of turns can be reduced because the bending loss in the second optical fiber 31 increases and light leakage occurs. can be done. In this embodiment, the V value of the first optical fiber 11 is, for example, 1.8 to 2.0, and the V value of the second optical fiber 31 is, for example, 2.2 to 2.4.

Typical V-value conventional polarization-maintaining fibers have low bend losses at operating wavelengths, eg, 1550 nm. To achieve the required loss in a Kerr effect waveguide, for example, 5 dB or 70% loss would be required, and a conventional 1550 nm polarization-maintaining fiber would require very small diameter bends. In this embodiment, we propose to use a conventional fiber designed for short wavelengths and low V values. For example, polarization-maintaining fiber for 1064 nm can be used at longer wavelengths such as 1550 nm to achieve adequate bend loss with safe bend diameters.

Returning to FIG. 1, the isolator 40 is an in-line isolator arranged in the first optical fiber 11 . In the resonator 10, light propagates only in the forward direction of the isolator 40, which is the counterclockwise direction B1 in the example of FIG.

The output coupler 50 is an optical coupler arranged in the first optical fiber 11 . A laser beam BO formed by the mode-locked laser 100 is output to the outside through an output optical fiber 52 connected to an output port 51 of an output coupler 50 .

The operation of the mode-locked laser 100 will be described below. In the mode-locked laser 100, the light propagating in the counterclockwise direction B1 circulates, is amplified while resonating, and is narrowed down to a specific mode. The excitation light source 22 a of the excitation unit 22 supplies the gain fiber 21 with excitation light PL having a wavelength of 980 nm, for example. In the gain fiber 21, the dopant added to the doped fiber of the gain fiber 21 is excited by the pumping light PL, stimulated emission occurs at the wavelength of the resonant light for output (for example, wavelength 1550 nm), and the light is amplified. Also, the gain fiber 21 amplifies the light circulating in the resonator 10 . Since the transmission adjustment unit 30 has a saturable absorption characteristic, the central portion of the pulse with high light intensity passes through, but both wings of the pulse with low light intensity undergo strong absorption, resulting in shortening of the pulse. As the light circulates around the resonator 10, it is narrowed down to a mode that matches the resonance conditions. In the output coupler 50 , the laser light BO formed by the mode-locked laser 100 is output to the outside as a high-power ultrashort pulse via an output optical fiber 52 .

(Example)
FIG. 9 shows results calculated using COMSOL Multiphysics based on the finite element method for the Kerr effect. FIG. 9 shows the relationship between incident peak power and transmittance in a polarization-maintaining optical fiber. A typical cross-sectional view of a polarization-maintaining optical fiber drawn on COMSOL Multiphysics is the same as FIG. As shown in FIG. 9, when the light intensity is low, there is no Kerr effect and the transmittance is low due to bending loss. As the light intensity increases, the effect of the Kerr effect increases and the transmittance increases. As described above, by combining the Kerr effect and the bending loss, the polarization maintaining optical fiber can have the same characteristics as the saturable absorber.

FIG. 10 shows the results of an intensity scan (I-scan) when the transmission adjuster 30 (coiled second optical fiber 31) of this embodiment is provided. Regarding the I-scan, reference is made to the following Document 2 (Document 2: W. Zhao, et al. "All-Fiber Saturable Absorbers for Ultrafast Fiber Lasers." in IEEE Photonics Journal, vol. 11, no. 5 , pp. 1-19, Oct. 2019, Art no. 7104019, doi: 10.1109/JPHOT.2019.2941580). The modulation depth ΔT shown in FIG. 10 was obtained by referring to the formula (1) in Document 1. As shown in FIG. 10, when the transmission adjuster 30 is provided, the transmittance exhibits light intensity dependence, and saturated absorption characteristics can be observed.

In the mode-locked laser 100, the transmittance in the transmission adjustment section 30 is lower than it should be due to the bending loss of the second optical fiber 31, which is the adjustment waveguide AW. rises. In other words, when the light intensity of the transmission adjustment unit 30 is low, the light confinement (guiding) effect of the second optical fiber 31 is low due to the influence of bending, and the light leaks to the outside. The light confinement effect is increased, and light leakage is suppressed. In this way, since the supersaturated absorption characteristic is realized only by the second optical fiber 31, an ultrashort pulse can be generated without using a saturable absorber such as CNT. Therefore, limitations such as long-term durability and damage threshold of saturable absorbers such as CNTs can be eliminated, and high-power optical pulses can be generated over a long period of time. The mode-locked laser 100 having the transmission adjustment section 30 realizes a short-pulse laser with high stability and long life, and is useful for various applications. For example, it can be used as a light source for deep tissue imaging (TPM, CARS, SRS, OCT, etc.) and laser micromachining. Such a mode-locked laser 100 can achieve high durability, low cost, small size, and the like. For example, compared to conventional endoscopic techniques, the mode-locked laser 100 allows for faster measurement times, less invasiveness, and greater depth in deep tissue imaging. Also, compared to conventional microscopy techniques, the mode-locked laser 100 can penetrate deeper into tissue and acquire clearer images.

[Second embodiment]
A mode-locked laser according to the second embodiment will be described below. Note that the mode-locked laser according to the second embodiment is a modification of the first embodiment, and parts that are not particularly described are the same as those of the first embodiment.

A mode-locked laser 100 according to the second embodiment shown in FIG. The mode-locked laser 100 of this embodiment is an example of a bi-directional operation ring-type fiber laser. Light output from the excitation light source 22a is branched into counterclockwise C1 and clockwise C2, and in the resonator 10, the light propagates in both the counterclockwise C1 and clockwise C2 directions.

The operation of the mode-locked laser 100 of this embodiment will be described below. In the mode-locked laser 100, the light propagating in the counterclockwise direction C1 and the clockwise direction C2 circulates, is amplified while resonating, and is narrowed down to a specific mode. The excitation light source 22 a of the excitation unit 22 supplies the gain fiber 21 with excitation light PL having a wavelength of 980 nm, for example. In the gain fiber 21, the dopant added to the doped fiber of the gain fiber 21 is excited by the pumping light PL, stimulated emission occurs at the wavelength of the resonant light for output (for example, wavelength 1550 nm), and the light is amplified. Also, the gain fiber 21 amplifies the light circulating in the resonator 10 . Since the transmission adjustment unit 30 has a saturable absorption characteristic, the central portion of the pulse with high light intensity passes through, but both wings of the pulse with low light intensity undergo strong absorption, resulting in shortening of the pulse. As the light circulates around the resonator 10, it is narrowed down to a mode that matches the resonance conditions. In the output coupler 50 , the laser light BO formed by the mode-locked laser 100 is output to the outside as a high-power ultrashort pulse via an output optical fiber 52 .

[Third Embodiment]
A mode-locked laser according to the third embodiment will be described below. The mode-locked laser according to the third embodiment is a modification of the first embodiment, and parts that are not particularly described are the same as those of the first embodiment.

A mode-locked laser 100 according to the third embodiment shown in FIG. 12 includes a resonator 10, an optical amplifier 20, a transmission adjuster 30, a total reflection mirror 60, and an output mirror . The total reflection mirror 60 is a folding mirror and substantially reflects the light propagating through the resonator 10 . The output mirror is, for example, a partially transmitting mirror having a reflectance of 40% to 70%. Output to the outside as The mode-locked laser 100 of this embodiment is an example of a linear fiber laser. A mode-locked laser 100 has a configuration in which a resonator section 10 is sandwiched between a total reflection mirror 60 and an output mirror 70, and light is resonated and amplified between the total reflection mirror 60 and the output mirror 70. FIG.

〔others〕
Although the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. For example, various wavelengths can be used for the excitation laser light used in the mode-locked laser 100 .

Further, in the above embodiment, the configuration of the mode-locked laser 100 can be changed as appropriate. For example, a polarization controller may be provided in the resonator 10.

In the above embodiment, the resonator 10 includes the first optical fiber 11, but the mode-locked laser 100 can also be applied to other waveguide-type optical devices using optical fibers. . Waveguide optical devices include, for example, PLCs (Photonics Lightwave Circuits), Silicon Photonics Waveguides, and semiconductor waveguides (InP, GaAs, InGaAsP, etc.).

In the above embodiment, the adjustment waveguide AW of the transmission adjustment unit 30 is configured to have the second optical fiber 31 that is forcibly bent, but other waveguide type optical devices can be used. Such an adjustment waveguide AW is obtained by bending an optical path based on a straight waveguide to generate bending loss without stress. In this case, the adjustment waveguide AW has a bending loss greater than that of the resonator. Waveguide optical devices include, for example, PLC, silicon photonics waveguides, and semiconductor waveguides (InP, GaAs, InGaAsP, etc.).

DESCRIPTION OF SYMBOLS 1... Core 2... Clad 3... Stress application part 4... Coating part 10... Resonator part 11... First optical fiber 20... Optical amplification part 21... Gain fiber 22... Pumping part 22a... Pumping Light source 22b Multiplexing coupler 30 Transmission adjustment unit 31 Second optical fiber 40 Isolator 50 Output coupler 51 Output port 52 Optical fiber 60 Total reflection mirror 70 Output mirror 90... Shaft member 91... Groove 100... Mode-locked laser AW... Adjustment waveguide BO... Laser light P... Polarized wave PF... PANDA fiber PL... Pumping light

Claims (11)

a polarization-maintaining resonator;
an optical amplifier arranged in the resonator;
a transmission adjustment section arranged in the resonance section and provided with a saturable absorption characteristic by having a polarization-maintaining adjustment waveguide having bending loss;
A mode-locked laser with a 3. The mode-locked laser of claim 2, wherein the resonator includes a polarization-maintaining first optical fiber, and the tuning waveguide is a forcibly bent polarization-maintaining second optical fiber. . The transmission adjuster exhibits a first bending loss equal to or greater than a predetermined value when light having a relatively low first intensity passes through, and exhibits a first bending loss less than a predetermined value when light having a relatively high second intensity passes through. 3. The mode-locked laser of any one of claims 1 and 2, exhibiting a second bending loss of . 4. The second optical fiber of claims 2 and 3, wherein the second optical fiber has a non-linear optical effect such that when the light of the second intensity passes through, the refractive index increases more than when the light of the first intensity passes. Mode-locked laser according to any one of clauses. The mode-locked laser according to any one of claims 2 to 4, wherein the second optical fiber is a PANDA fiber, and is wound in one of the slow axis direction and the fast axis direction of the PANDA fiber. . The mode-locked laser according to any one of claims 2 to 5, wherein the radius of curvature of the forcibly bent loop of the second optical fiber is constant. 7. The mode-locked laser according to claim 2, wherein said second optical fiber is a PANDA fiber, and polarized waves in either one of the slow axis direction and the fast axis direction of said PANDA fiber are used. 8. The mode-locked laser according to claim 7, wherein said second optical fiber uses polarization in said slow axis direction. A mode-locked laser according to any one of claims 2 to 8, wherein said first optical fiber is a PANDA fiber. 10. The mode-locked laser according to claim 2, wherein the V value, which is the normalized frequency parameter of said first or second optical fiber, satisfies the following conditional expression.
V1 < V2
however,
V1: V value of the first optical fiber V2: V value of the second optical fiber The V value is defined by the following equation.
V=πdNA/λ
however,
d: core diameter of the fiber of interest NA: numerical aperture of the core of the fiber of interest λ: frequency used for the fiber of interest 3. The mode-locked laser according to claim 2, wherein said resonator is formed in a ring shape by said first optical fiber.

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