JP7740680B2 - Mode-locked laser - Google Patents

Description

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

Mode-locked lasers are widely used in fields such as measurement and medicine. It is generally known that mode-locked lasers use saturable absorbers to generate ultrashort pulse trains (see, for example, Patent Document 1 and Non-Patent Document 1). Saturable absorbers have the property that the stronger the irradiated light intensity, the more saturated the absorption becomes and the higher the transmittance. Therefore, among the noise components emitted from a fiber amplifier, components with strong peaks survive and become the seeds of oscillating optical pulses. Carbon materials such as carbon nanotubes (CNTs) are generally used as saturable absorbers. However, CNTs and other materials are limited by their relatively low damage threshold, making extremely high optical output unfeasible.

Polarization maintenance in fiber lasers is important for practical applications requiring high environmental stability. Material-based polarization-maintaining lasers have problems with long-term stability and durability against optical damage. To solve these problems, Kerr-effect-based (hereinafter also referred to as Kerr-based) techniques can be used, but most Kerr-based fiber lasers are non-polarization-maintaining. Previously proposed Kerr-based fiber lasers include the interferometric PM-F8 (see, e.g., Non-Patent Document 2), the interferometric PM-F9 (see, e.g., Non-Patent Document 3), and the PM-NPR (see, e.g., Non-Patent Document 4) using nonlinear polarization rotation. However, the PM-F8 is not easily self-starting, the PM-F9 solves the problems of the PM-F8 but requires complex Faraday polarization control components, and the PM-NPR includes a fiber section that generates nonlinear polarization rotation between a pair of polarizers, making it susceptible to environmental instability such as temperature and mechanical fluctuations.

Instead of a saturable absorber, there is a mode-locked laser that utilizes polarization-dependent loss (PDL) that occurs when a single-mode optical fiber (SMF) is bent and a polarizer is placed at the exit (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 was made in consideration of the above-mentioned background art, and aims to provide a mode-locked laser that can generate high-power optical pulses using a highly durable element that replaces a saturable absorber that is limited by its damage threshold.

To achieve the above objective, the mode-locked laser according to the present invention comprises a polarization-maintaining resonator, an optical amplifier located within the resonator, and a transmission adjustment section located within the resonator and having a polarization-maintaining adjustment waveguide with bending loss, thereby imparting saturable absorption characteristics.

In the above-described mode-locked laser, the transmittance is reduced in the transmission adjustment section due to bending losses in the adjustment waveguide, but as the optical intensity increases, the transmittance increases due to the nonlinear optical effect. In other words, at low optical intensities, the bending of the transmission adjustment section reduces the optical confinement (waveguiding) effect of the adjustment waveguide, resulting in light leakage. However, as the optical intensity increases, the nonlinear optical effect enhances the optical confinement effect, suppressing light leakage. Because saturable absorption characteristics are achieved solely with the adjustment waveguide, ultrashort pulses can be generated without using saturable absorbers such as CNTs. This eliminates limitations such as the long-term durability and damage threshold of saturable absorbers such as CNTs, enabling the generation of high-power optical pulses over extended periods. A mode-locked laser equipped with the above-described transmission adjustment section realizes a highly stable, long-life short-pulse laser, making it useful for a variety of 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, and compact size.

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 is a polarization-maintaining second optical fiber that is forcibly bent. By configuring the resonator with the first optical fiber and the adjustment waveguide with the second optical fiber, it is possible to achieve a smaller size and lighter weight.

In another aspect of the present invention, in the above-described mode-locked laser, the transmission adjustment section exhibits a first bending loss equal to or greater than a predetermined value when light of a relatively low first intensity passes through, and exhibits a second bending loss less than a predetermined value when light of a relatively high second intensity passes through. In this case, when light of a relatively low first intensity passes through the transmission adjustment section, the first bending loss becomes relatively large, making it possible to actively generate leakage light.

In yet another aspect of the present invention, the second optical fiber has a nonlinear optical effect, and when light of a second intensity passes through it, the refractive index is higher than when light of a first intensity passes through it. In this case, the nonlinear optical effect increases the refractive index within the fiber core when the light intensity is strong, improving the waveguiding ability and reducing bending loss.

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

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

In yet another aspect of the present invention, the second optical fiber is a PANDA fiber, and polarization is used in either the slow axis direction or the fast axis direction of the PANDA fiber.

In yet another aspect of the present invention, the second optical fiber uses slow axis polarization.

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

In yet another aspect of the present invention, the following conditional expression is satisfied regarding the V value, which is a normalized frequency parameter of the first or second optical fiber.
V1<V2
where 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.
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. The fiber of interest means the first or second optical fiber.

In yet another aspect of the present invention, the resonator is formed in a ring shape using the first optical fiber. In this case, for example, a reflector is not required, and maintenance is almost entirely unnecessary.

1 is a conceptual diagram illustrating a mode-locked laser according to a first embodiment. 2 is a diagram illustrating a polarization-maintaining fiber used in the mode-locked laser of FIG. 1. FIG. 1A and 1B are diagrams for explaining the bending direction of a polarization-maintaining fiber, and 1C and 1D are diagrams for explaining the direction of polarization used in the polarization-maintaining fiber. 1A and 1B are diagrams showing the results of an analysis based on the finite element method regarding bending loss of an optical fiber. FIG. 5 is a typical cross-sectional view of the polarization-maintaining optical fiber analyzed in FIG. 4. 2 is a conceptual diagram illustrating the structure of a transmission adjusting section incorporated in the mode-locked laser of FIG. 1. FIG. 1A is a diagram illustrating the refractive index and light intensity distribution around the core when the second optical fiber of the transmission adjusting section is bent, and FIG. 1B is a diagram illustrating the influence of the nonlinear optical effect in the second optical fiber that is forcibly bent. FIG. 1A is a conceptual diagram illustrating the relationship between light intensity and transmittance when the second optical fiber is bent, and FIG. 1B is a conceptual diagram illustrating the relationship between light intensity and bending loss when the second optical fiber is bent. FIG. 10 is a diagram showing the analysis results based on the finite element method regarding the Kerr effect. 10A and 10B are diagrams showing the results of an intensity scan when a transmission adjustment unit is provided. FIG. 10 is a diagram illustrating a mode-locked laser according to a second embodiment. FIG. 10 is a diagram illustrating a mode-locked laser according to a third embodiment.

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

The mode-locked laser 100 of the first embodiment shown in Figure 1 is a passive mode-locked laser and includes a resonator 10, an optical amplifier 20, a transmission adjuster 30, an isolator 40, and an output coupler 50. The mode-locked laser 100 includes the resonator 10, the optical amplifier 20, the transmission adjuster 30, the isolator 40, and the output coupler 50, which are joined together by fusion splicing or the like. The illustrated mode-locked laser 100 is an example of a ring-type fiber laser that operates in one direction. The mode-locked laser 100 is an all-fiber polarization-maintaining mode-locked laser.

In the mode-locked laser 100, the resonator 10 is formed in a ring shape using the first optical fiber 11. By forming the resonator 10 using the first optical fiber 11, it is possible to achieve a smaller and lighter laser. Furthermore, by forming the resonator 10 in a ring shape, for example, a reflector is not required, and maintenance is almost entirely unnecessary. The first optical fiber 11 is a polarization-maintaining optical fiber (PMF).

Polarization-maintaining optical fiber (PMF) utilizes the photoelastic effect and structural changes to create birefringence, where the effective refractive index differs between the vertical and horizontal directions of the core, enhancing the polarization-maintaining properties of propagating light. Polarization-maintaining fiber has a clear difference in refractive index between the vertical and horizontal directions of the optical fiber cross section, preventing interference between vertical and horizontal polarizations. Polarization-maintaining fibers include stress-applying fibers, which use the photoelastic effect to create birefringence, and structural fibers, which change the effective refractive index between the vertical and horizontal directions of the core. Stress-applying fibers sandwich the core with stress-applying parts (SAPs) that have a significantly higher thermal contraction rate than the cladding material in one direction of the cladding cross section. Stress-applying fibers are commonly used in polarization-maintaining fibers, including the PANDA (Polarization-Maintaining and Absorption-Reducing) type, which uses a round stress-applying part; the bow-tie type, which uses an elliptical jacket fiber; and polarization-maintaining photonic crystal fiber. In this embodiment, we will explain the case where the polarization-maintaining fiber shown in Figure 2 is a PANDA type. By using a PANDA fiber, which is a PANDA type polarization-maintaining fiber, polarization can be maintained easily and reliably.

As shown in FIG. 2 , the PANDA fiber PF has a core 1, a cladding 2, a stress-applying portion 3, and a coating 4. The PANDA fiber PF has a structure in which the core 1 is located at the center of the fiber, the cladding 2 is located around the core 1, and two circular stress-applying portions 3 are located on either side of the core 1. The coating 4 covers the cladding 2 and protects the inside of the fiber. The refractive index of the core 1 is higher than that of the cladding 2. The stress-applying portion 3 is formed of a silica glass rod doped with, for example, B ₂ O ₃ to increase its linear expansion coefficient. The stress-applying portion 3 has a linear expansion coefficient larger than that of the cladding portion made of pure silica glass. Tensile strain occurs in the stress-applying portion 3, which applies stress to the core along the X-axis in the figure. The X-axis in 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 amplifier 20 includes a gain fiber 21 and a pumping unit 22. The optical amplifier 20 is disposed within the resonator 10. The optical amplifier 20 can be replaced with an optical amplifier element using another amplification medium, such as a semiconductor optical amplifier or a fiber Raman amplifier.

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

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

The transmission adjustment section 30 is disposed within the resonator section 10 and has an adjustment waveguide AW with bending loss, thereby providing saturable absorption characteristics. Specifically, the adjustment waveguide AW is a polarization-maintaining optical fiber that is given saturable absorption characteristics by forcibly bending the second optical fiber 31. By forming the adjustment waveguide AW from the second optical fiber 31, it is possible to achieve a smaller size and lighter weight.

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. This allows the bending loss to be controlled by the bending radius and the number of loops. Figure 3(A) shows the second optical fiber 31 bent in the slow axis direction (X-axis), and Figure 3(B) shows the second optical fiber 31 bent in the fast axis direction (Y-axis). The second optical fiber 31 uses polarization in either the slow axis direction or the fast axis direction of the PANDA fiber PF. In particular, the second optical fiber 31 uses polarization in the slow axis direction. Figure 3(C) shows the second optical fiber 31 using polarization P in the slow axis direction (X-axis), and Figure 3(D) shows the second optical fiber 31 using polarization P in the fast axis direction (Y-axis). The transmission adjustment unit 30 uses the second optical fiber 31 in four patterns, combining one of the two bending patterns shown in Figures 3(A) and 3(B) with one of the two polarization patterns shown in Figures 3(C) and 3(B).

Figures 4(A) and 4(B) show the results of calculations of bending loss in optical fibers using COMSOL Multiphysics (registered trademark: COMSOL, Inc.) based on the finite element method (FEM). Figures 4(A) and 4(B) show the mode distribution within a polarization-maintaining optical fiber. Figure 4(A) shows the optical fiber bent in the slow axis direction (X-axis) as shown in Figure 3(A), and Figure 4(B) shows the optical fiber bent in the fast axis direction (Y-axis) as shown in Figure 3(B). Figure 5 shows a typical cross-section of a polarization-maintaining optical fiber drawn in COMSOL Multiphysics. In Figure 5, region A1 is the matching layer used to calculate bending loss, and regions A2 to A5 correspond to the core 1, cladding 2, stress-applying portion 3, and coating portion 4 of the PANDA fiber PF shown in Figure 2, respectively. As shown in Figures 4(A) and 4(B), when the optical fiber is bent, the mode spreads in the bending direction, and light leaks out in the direction of the arrow LE.

As shown in FIG. 6, the transmittance adjustment unit 30 is, for example, a second optical fiber 31 wound around a groove 91 formed in a cylindrical metal shaft member 90. It is preferable that the radius of curvature of the loop of the forcibly bent second optical fiber 31 is constant. The method of bending the second optical fiber 31 into a coil shape can be changed as appropriate. It may be wound inside the shaft member 90, or it may not use the shaft member 90. The radius of curvature of the second optical fiber 31 is, for example, 5 mm to 20 mm. The number of turns of the second optical fiber 31 is, for example, 1 to 10.

The saturable absorption characteristics of the transmission adjustment unit 30 are described below. Forcibly bending the second optical fiber 31 changes the internal refractive index structure, making it more susceptible to light leakage. In the transmission adjustment unit 30, the nonlinear optical effect causes a change in refractive index depending on the light intensity. When the intensity of the propagating light is high, the increase in the nonlinear refractive index shifts the propagation trajectory toward the center of the optical fiber core. On the other hand, when the intensity of the propagating light is low, the decrease in the nonlinear refractive index shifts the propagation trajectory toward the outside of the optical fiber core. From another perspective, when a large curvature occurs in the fiber and the angle of incidence at the boundary between the core and cladding is smaller than the critical angle, the light is not totally reflected, and some of the light is radiated into the cladding, resulting in bending loss. When the intensity of the propagating light is high, the nonlinear optical effect increases the refractive index of the core, suppressing leakage into the cladding. As a result, when ordinary light of relatively low intensity is incident, the transmission adjustment unit 30 reduces the optical fiber's confinement effect, causing light to leak to the outside and increasing bending loss. On the other hand, when high-intensity light is incident on the transmission adjusting section 30, the nonlinear optical effect relatively enhances the optical fiber's confinement effect, suppressing light leakage and reducing bending loss. In other words, phenomena such as nonlinear bending loss and light intensity-dependent bending loss occur. In other words, the transmittance of the transmission adjusting section 30 increases as the light intensity increases, and it functions like a saturable absorber.

7A is a diagram illustrating the refractive index and light intensity distribution 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. The solid line K1 indicates the light intensity distribution when the second optical fiber 31 is bent, and the 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, as shown in FIG. 7A, the refractive index within the second optical fiber 31 changes from the refractive index _ns to the refractive index _nb due to the influence of the bending, and the light intensity distribution shifts slightly toward the outside of the core. When the second optical fiber 31 is forcibly bent, the bending has little effect on the transmittance when the light intensity is high. However, when the light intensity is low, the transmittance is more susceptible to the influence of the bending, resulting in bending loss.

The second optical fiber 31 has a nonlinear optical effect, and when high-intensity light (relatively strong second-intensity light) passes through it, the refractive index increases more than when low-intensity light (relatively weak first-intensity light) passes through it. This nonlinear optical effect increases the refractive index in the fiber core when high-intensity light is incident, improving waveguiding ability and reducing bending loss. The nonlinear optical effect is an optical effect caused by nonlinear polarization induced when high-intensity light is incident on a material. Examples of nonlinear optical effects related to refractive index change include the Kerr effect and the Pockels effect. In this embodiment, the refractive index change n = n ₀ + n ₂ I, which depends on the light intensity due to the Kerr effect, a third-order nonlinear process, is utilized. Here, n ₀ is the linear refractive index, n ₂ 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 change in refractive index can be appropriately applied.

The response speed of the Kerr effect of silica glass is 10 fs or less, so the method proposed in this invention has a similar response speed. The response speed of conventional saturable absorbers is approximately 500 fs for CNTs and approximately 300 fs for graphene. Compared to the response speed of conventional saturable absorbers, the response speed when using the second optical fiber 31 is significantly improved.

Figure 7(B) is a diagram illustrating the effects of nonlinear optical effects in a forcibly bent second optical fiber 31. The right axis in Figure 7(B) represents light intensity. In Figure 7(B), the solid line L1 represents the refractive index without the nonlinear optical effect, and the dashed line L2 represents the refractive index with the nonlinear optical effect. The dashed-dotted line M1 represents the intensity distribution of high-intensity light (relatively strong second-intensity light), and the dashed-two-dot line M2 represents the intensity distribution of low-intensity light (relatively weak first-intensity light). As shown in Figure 7(B), in the transmission adjustment unit 30, when the light intensity increases, the nonlinear optical effect, specifically the Kerr effect, narrows the light intensity distribution in the core diameter direction, confining the light within the fiber core. However, when the light intensity decreases, the light intensity distribution widens in the core diameter direction, resulting in light leakage from the fiber core. The transmission adjustment unit 30 is designed to actively generate light leakage when the light intensity is low, thereby creating a state equivalent to that of a saturable absorber.

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 light intensity and bending loss when the second optical fiber 31 is bent. In the diagram, the symbol ΔT indicates modulation depth, the symbol I _sat indicates saturation intensity, the symbol α _ns indicates unsaturated loss, and the symbol α ₀ indicates background absorption loss. The modulation depth ΔT, saturation intensity I _sat , unsaturated loss α _ns , and background absorption loss α ₀ are based on the following references: Literature 1 and Non-Patent Document 1 (Literature 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 Figures 8A and 8B, in the forcibly bent second optical fiber 31, when the optical intensity is low, the transmittance is reduced due to the influence of bending loss, and when the optical intensity is high, the influence of bending loss is reduced. That is, the transmission adjusting unit 30 exhibits a first bending loss equal to or greater than a predetermined value when light having a relatively low first intensity passes through it, and exhibits a second bending loss less than a predetermined value when light having a relatively high second intensity passes through it. As a result, when light having a relatively low first intensity passes through the transmission adjusting unit 30, the first bending loss becomes relatively large, thereby actively generating leakage light. That is, the transmission adjusting unit 30 causes light leakage into the cladding due to bending loss (corresponding to the first bending loss) for light having a predetermined intensity below the oscillation level. Furthermore, when light having a relatively high second intensity passes through it, the second bending loss becomes relatively small, corresponding to the saturable absorption characteristic required to generate short pulses. That is, the interaction between bending loss and nonlinear optical effects can reduce leakage of light having an intensity above 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 up to 5%. In this case, as the optical intensity increases, the loss may increase to 4.8 dB to 6.8 dB. In the mode-locked laser 100, the gain is very high, at 10 dB or more, and is designed to be approximately 5 dB to 10 dB. The loss in the Kerr effect waveguide (the transmission adjustment section 30 that utilizes the Kerr effect and bending loss characteristics) is greater than that of conventional saturable absorbers. However, low loss is not essential. In a laser system, the gain medium provides gain that exceeds the loss in the entire cavity. In a fiber laser system, the gain is very high, typically 10 dB or more. Therefore, the loss in the Kerr effect waveguide is typically designed to be 5 dB to 10 dB. For example, in the case of this embodiment, if the fiber coil exhibits a loss of 80%, and the optical intensity increases and this loss increases to 75%, the resulting loss is an intensity-dependent loss (so-called modulation depth) of 5%. In mode-locked laser systems, especially fiber lasers, a modulation depth of 5% or less is considered sufficient.

The mode-locked laser 100 satisfies the following conditional expression (1) regarding the V value, which is a normalized frequency parameter of the first or second optical fiber 11 or 31.
V1<V2 ... (1)
Here, the value V1 is the V value of the first optical fiber 11, and the value V2 is the V value of the second optical fiber 31.
The V value is defined by the following formula.
V=πdNA/λ... (2)
where d is the core diameter of the fiber of interest, NA is the numerical aperture of the core of the fiber of interest, and λ is the frequency used for the fiber of interest. The fiber of interest refers to the first or second optical fiber 11, 31. In this case, the radius of curvature of the second optical fiber 31 in the transmission adjustment unit 30 does not need to be made too small to increase bending loss in the second optical fiber 31 and cause light leakage, and the number of turns can be reduced. 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.

Conventional polarization-maintaining fibers with a normal V value have low bending loss at operating wavelengths such as 1550 nm. To achieve the loss required for a Kerr effect waveguide, a loss of, for example, 5 dB, or 70%, is required, and conventional polarization-maintaining fibers for 1550 nm must be bent at an extremely small diameter. This embodiment proposes the use of conventional fibers designed for short wavelengths with a low V value. For example, polarization-maintaining fibers for 1064 nm can be used at longer wavelengths such as 1550 nm to achieve appropriate bending loss at a safe bending diameter.

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

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

The operation of the mode-locked laser 100 is described below. In the mode-locked laser 100, light propagating counterclockwise in the B1 direction circulates, resonating and amplifying, narrowing down to a specific mode. The pump light source 22a of the pump unit 22 supplies pump light PL, for example, with a wavelength of 980 nm, to the gain fiber 21. In the gain fiber 21, the pump light PL excites the dopant added to the doped fiber of the gain fiber 21, causing stimulated emission at the wavelength of the output resonant light (for example, a wavelength of 1550 nm), amplifying the light. The gain fiber 21 also amplifies the light circulating through the resonator 10. The transmission adjustment unit 30 has saturable absorption characteristics, allowing the center portion of a pulse with high optical intensity to pass, while the wings of a pulse with low optical intensity are strongly absorbed, resulting in pulse shortening. As the light circulates through 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 the output optical fiber 52.

(Example)
Figure 9 shows the results of calculations using COMSOL Multiphysics based on the finite element method for the Kerr effect. Figure 9 shows the relationship between incident peak power and transmittance in a polarization-maintaining optical fiber. The typical cross-sectional view of a polarization-maintaining optical fiber drawn in COMSOL Multiphysics is the same as Figure 5. As shown in Figure 9, when the light intensity is weak, there is no Kerr effect and the transmittance is low due to the influence of bending loss. As the light intensity increases, the influence of the Kerr effect increases and the transmittance increases. As described above, by combining the Kerr effect and bending loss, it is possible to impart characteristics similar to those of a saturable absorber to a polarization-maintaining optical fiber.

Figure 10 shows the results of an intensity scan (I-scan) when the transmission adjustment unit 30 (coil-shaped second optical fiber 31) of this embodiment is provided. Regarding the I-scan, reference was made to the following literature 2 (literature 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 Figure 10 was calculated using equation (1) in literature 1. As shown in Figure 10, when the transmission adjustment unit 30 is provided, the transmittance exhibits light intensity dependence, and saturated absorption characteristics can be observed.

In the mode-locked laser 100, the transmission adjustment section 30 exhibits a lower transmittance than normal due to bending losses in the second optical fiber 31, which serves as the adjustment waveguide AW. However, as the optical intensity increases, the transmittance increases due to the nonlinear optical effect. In other words, when the optical intensity is low, the bending of the transmission adjustment section 30 reduces the optical confinement (waveguide) effect of the second optical fiber 31, causing light to leak to the outside. However, as the optical intensity increases, the nonlinear optical effect increases the optical confinement effect, suppressing light leakage. In this way, saturable absorption characteristics are achieved using only the second optical fiber 31, enabling ultrashort pulses to be generated without using a saturable absorber such as a CNT. This eliminates limitations on the long-term durability and damage threshold of saturable absorbers such as CNTs, enabling the generation of high-power optical pulses over long periods of time. The mode-locked laser 100 equipped with the transmission adjustment section 30 realizes a highly stable, long-life short-pulse laser, making it useful for a variety of 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, compactness, etc. For example, compared to conventional endoscopic technology, the use of the mode-locked laser 100 enables faster measurement times, less invasiveness, and deeper reach in deep tissue imaging. Furthermore, compared to conventional microscopic technology, the use of the mode-locked laser 100 allows for deeper penetration into tissue and allows for clearer images to be obtained.

Second Embodiment
The 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.

The mode-locked laser 100 of the second embodiment shown in Figure 11 comprises a resonator 10, an optical amplifier 20, a transmission adjuster 30, and an output coupler 50. The mode-locked laser 100 of this embodiment is an example of a bidirectional ring-type fiber laser. Light output from the pump light source 22a branches into counterclockwise C1 and clockwise C2, and in the resonator 10, the light propagates in both counterclockwise C1 and clockwise C2 directions.

The operation of the mode-locked laser 100 of this embodiment is described below. In the mode-locked laser 100, light propagating counterclockwise C1 and clockwise C2 circulates, resonating and amplifying, and narrowing to a specific mode. The pump light source 22a of the pump unit 22 supplies pump light PL, for example, with a wavelength of 980 nm, to the gain fiber 21. In the gain fiber 21, the pump light PL excites the dopant added to the doped fiber of the gain fiber 21, causing stimulated emission at the wavelength of the output resonant light (for example, a wavelength of 1550 nm), amplifying the light. The gain fiber 21 also amplifies the light circulating through the resonator 10. The transmission adjustment unit 30 has saturable absorption characteristics, allowing the center portion of a pulse with high optical intensity to pass, while the wings of a pulse with low optical intensity are strongly absorbed, resulting in pulse shortening. As the light circulates through the resonator 10, it narrows 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 the output optical fiber 52.

Third Embodiment
The 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.

The mode-locked laser 100 of the third embodiment shown in Figure 12 comprises a resonator 10, an optical amplifier 20, a transmission adjuster 30, a total reflection mirror 60, and an output mirror 70. The total reflection mirror 60 is a folding mirror that substantially reflects the light propagating through the resonator 10. The output mirror is a partially transparent mirror with a reflectivity of, for example, 40% to 70%, and outputs laser light OB, which is a portion of the amplified light and has been narrowed down to a mode that matches the resonance conditions, to the outside as a high-power ultrashort pulse. The mode-locked laser 100 of this embodiment is an example of a linear fiber laser. The mode-locked laser 100 has a configuration in which the resonator 10 is sandwiched between the total reflection mirror 60 and the output mirror 70, and the light is resonated and amplified between the total reflection mirror 60 and the output mirror 70.

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

Furthermore, in the above embodiment, the configuration of the mode-locked laser 100 can be modified as appropriate, and for example, a polarization controller may be provided in the resonator section 10.

In addition, in the above embodiment, the resonator 10 includes the first optical fiber 11. However, the mode-locked laser 100 can also be applied to other waveguide-type optical devices that use optical fibers. Examples of waveguide-type optical devices include PLCs (Photonics Lightwave Circuits), silicon photonics waveguides, and semiconductor waveguides (InP, GaAs, InGaAsP, etc.).

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

1...Core, 2...Cladding, 3...Stress-applying portion, 4...Coating portion, 10...Resonator portion, 11...First optical fiber, 20...Optical amplifier portion, 21...Gain fiber, 22...Pumping portion, 22a...Pumping light source, 22b...Wave-combining coupler, 30...Transmission adjustment portion, 31...Second optical fiber, 40...Isolator, 50...Output coupler, 51...Output port, 52...Optical fiber, 60...Total reflection mirror, 70...Output mirror, 90...Axial member, 91...Groove, 100...Mode-locked laser, AW...Adjustment waveguide, BO...Laser light, P...Polarization, PF...PANDA fiber, PL...Pumping light

Claims (10)

a polarization-maintaining resonator;
an optical amplifier section disposed in the resonator section;
a transmission adjusting section disposed in the resonator section and having an adjusting waveguide that is a polarization-maintaining optical fiber having bending loss, thereby giving the transmission adjusting section saturable absorption characteristics;
Equipped with
A mode-locked laser, wherein the transmission adjustment section exhibits a first bending loss equal to or greater than a predetermined loss when light of a relatively low first intensity passes through, and exhibits a second bending loss less than the predetermined loss when light of a relatively high second intensity passes through. 2. The mode-locked laser according to claim 1, wherein the resonator includes a polarization-maintaining first optical fiber, and the adjustment waveguide is a polarization-maintaining second optical fiber in which nonlinear bending loss is generated by forcibly bending the optical fiber. 3. The mode-locked laser according to claim 2 , wherein the second optical fiber has a nonlinear optical effect, and a refractive index increases when the light of the second intensity passes through the second optical fiber compared to when the light of the first intensity passes through the second optical fiber. A mode-locked laser according to any one of claims 2 and 3, wherein the second optical fiber is a PANDA fiber and is bent and wound in either the slow axis direction or the fast axis direction of the PANDA fiber. A mode-locked laser according to any one of claims 2 to 4, wherein the radius of curvature of the forcibly bent loop of the second optical fiber is constant. The mode-locked laser according to any one of claims 2 to 5, wherein the second optical fiber is a PANDA fiber and uses polarization in either the slow axis direction or the fast axis direction of the PANDA fiber. The mode-locked laser of claim 6, wherein the second optical fiber uses polarization in the slow axis direction. A mode-locked laser according to any one of claims 2 to 7, wherein the first optical fiber is a PANDA fiber. 9. The mode-locked laser according to claim 2, wherein the V value, which is a normalized frequency parameter of the 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 formula.
V = πdNA/λ
however,
d: Core diameter of the first or second optical fiber NA: Numerical aperture of the core of the first or second optical fiber λ: Frequency used in the first or second optical fiber A mode-locked laser as described in claim 2, wherein the resonator is formed in a ring shape by the first optical fiber.