Fiber laser apparatus, optical fiber resonator, and method for manufacturing the same

Abstract

The present invention provides a fiber laser device, an optical fiber resonator, and a method for manufacturing the same, which can obtain stable laser oscillation in a single longitudinal mode. [Solution] The fiber laser apparatus 10 includes an optical fiber resonator 12, which has an amplification fiber 21 and fiber Bragg gratings 22 and 23. The amplification fiber 21 is a fluoride optical fiber having a core and cladding. Rare earth elements are added to the core of the amplification fiber 21. The fiber Bragg gratings 22 and 23 are formed at both ends of the amplification fiber 21 by a phase mask method using a phetosecond laser. The fiber Bragg gratings 22 and 23 selectively reflect light of a single frequency.

Description

Technical Field

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[0001] The present invention relates to a fiber laser device, an optical fiber resonator, and a method for manufacturing the same.

Background Art

[0002] A fiber laser device using a Fabry-Perot type optical fiber resonator having a fluoride optical fiber made of fluoride glass such as ZBLAN as a gain medium, that is, an amplification fiber, is known (see Non-Patent Document 1). In the fiber laser device described in Non-Patent Document 1, dielectric multilayer mirrors are provided as reflection portions opposed to both end faces of the amplification fiber, and light of a specific wavelength is reflected by each dielectric multilayer mirror. Thereby, light is reciprocated in the core of the amplification fiber doped with a rare earth element to cause laser oscillation.

[0003] A technique for forming a fiber Bragg grating using a femtosecond laser beam having a wavelength of 800 nm in a fluoride optical fiber made of ZBLAN, which is a fluoride glass, is known from Non-Patent Document 2.

Prior Art Documents

[0005] Incidentally, fluoride optical fibers can be doped with rare earth elements that cannot be used in silica-based glass optical fibers, allowing for selection of laser light wavelengths across a wide wavelength range including the visible light region, thus offering greater flexibility in wavelength selection. However, in laser devices using fluoride optical fibers configured as described in Non-Patent Document 1, the resonator length becomes longer and the free spectral range (FSR) narrows due to the configuration in which a dielectric multilayer mirror is placed outside the amplification fiber. As a result, there is a problem in that laser oscillation in a single longitudinal mode tends to become unstable due to the presence of multiple longitudinal modes within the reflection band of the dielectric multilayer mirror. In other words, the laser oscillation mode is prone to changing (mode hopping), or simultaneous oscillation in multiple longitudinal modes (multimode oscillation).

[0006] The present invention has been made in view of the above circumstances, and aims to provide a fiber laser device, an optical fiber resonator, and a method for manufacturing the same that can obtain stable laser oscillation in a single longitudinal mode. [Means for solving the problem]

[0007] The optical fiber resonator of the present invention comprises an optical fiber having an amplification fiber region in which rare earth elements are added to a fluoride optical fiber, and a pair of fiber Bragg gratings formed at each position of the optical fiber flanking the amplification fiber region, which selectively reflect light of a single frequency.

[0008] The fiber laser apparatus of the present invention comprises the above-mentioned optical fiber resonator and an excitation light source unit that has a light source that outputs excitation light and incidents the excitation light from the light source into the amplification fiber region.

[0009] The present invention provides a method for manufacturing an optical fiber resonator, comprising the steps of: preparing an optical fiber having an amplification fiber region in which rare earth elements are added to a fluoride optical fiber; and forming a pair of fiber Bragg gratings that reflect light of a single frequency from the light stimulated to emit from the amplification fiber by irradiating each position of the optical fiber fringes generated by phetosecond laser light onto the amplification fiber region. [Effects of the Invention]

[0010] According to the present invention, a fiber Bragg grating that selectively reflects light of a single frequency is formed at both ends of an amplification fiber made of a fluoride optical fiber doped with rare earth elements, thereby constituting an optical fiber resonator, and thus stable laser oscillation in a longitudinal single mode can be obtained. [Brief explanation of the drawing]

[0011] [Figure 1] This is an explanatory diagram showing the configuration of a fiber laser apparatus according to an embodiment. [Figure 2] This is a schematic diagram illustrating a fiber Bragg grid formed on an amplification fiber. [Figure 3] This is an explanatory diagram showing the irradiation state of phetosecond laser light onto an amplification fiber using the phase mask method. [Modes for carrying out the invention]

[0012] In Figure 1, the fiber laser apparatus 10 comprises an optical fiber resonator 12, an excitation light source unit 14, and a delivery unit 16. The optical fiber resonator 12 consists of an amplification fiber 21 provided as an amplification fiber region in an optical fiber 20, and a pair of fiber Bragg gratings (hereinafter referred to as FBGs) 22 and 23 formed at both ends of the amplification fiber 21, respectively, and generates laser light with excitation light.

[0013] The excitation light source unit 14 consists of a light source 24 that outputs laser light as excitation light, an optical fiber region 20a of the optical fiber 20, and an optical system 26 consisting of lenses for injecting light into the optical fiber region 20a. The optical fiber region 20a is connected to one end of the amplification fiber 21 on the FBG 22 side. The excitation light source unit 14 injects excitation light from the light source 24 into the end of the optical fiber region 20a via the optical system 26. This causes the excitation light to be injected into the amplification fiber 21 via the optical fiber region 20a and FBG 22. Alternatively, the excitation light from multiple light sources may be injected into the amplification fiber 21 using a combiner.

[0014] In this example, the delivery section 16 consists of an optical fiber region 20b, which is provided as a region of the optical fiber 20, and the optical fiber region 20b is connected to one end of the amplification fiber 21 on the FBG23 side. The laser light generated in the optical fiber resonator 12 propagates through the optical fiber region 20b and is output.

[0015] The optical fiber region 20a of the excitation light source unit 14 and the optical fiber region 20b of the delivery unit 16 are made of, for example, silica-based glass and are fusion-spliced ​​to the ends of the amplification fiber 21, which is a fluoride optical fiber described later, to form a single optical fiber 20 together with the amplification fiber 21. Note that the optical fiber region 20a and optical fiber region 20b may also be made of fluoride optical fibers. Furthermore, the optical fiber region 20a of the excitation light source unit 14 may have rare earth elements added, similar to the amplification fiber 21 described later.

[0016] In Figure 2, the amplification fiber 21 in this example has a layered structure in which a cladding 21b with a lower refractive index than the core 21a surrounds the core 21a, and is configured as a single-mode optical fiber. For example, the core 21a has a diameter of several μm, and the cladding 21b has an outer diameter of about 125 μm. This amplification fiber 21 is a fluoride optical fiber formed from fluoride glass. The fluoride glass used in the fluoride optical fiber is not particularly limited, but ZBLAN glass (ZrF4-BaF2-LaF3-AlF3-NaF) with ZrF4 as the main component, or AlF3-based glass with AlF3 as the main component can be preferably used. Note that the amplification fiber 21 may also be a double-clad fiber.

[0017] The core 21a of the amplification fiber 21 is doped with rare earth elements. The rare earth elements doped into the core 21a of the amplification fiber 21 can include Pr (praseodymium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Nd (neodymium), Dy (dysprosium), etc., and are selected according to the wavelength of the laser light to be generated, taking into consideration the type of fluoride glass. The cladding 21b may also be doped with rare earth elements in the same way as the core 21a.

[0018] Excitation light from the excitation light source unit 14 is incident on the core 21a. As the excitation light propagates through the core 21a, electrons of rare earth element ions in the amplification fiber 21 are excited. When these excited electrons return to their ground state, they spontaneously emit light, and this spontaneously emitted or stimulated light stimulates the emission of light from other excited rare earth element ions. The wavelength of the excitation light is preferably one that efficiently causes the population inversion necessary for laser oscillation with respect to the rare earth elements added to the amplification fiber 21. For example, if the amplification fiber 21 is made of ZBLAN glass and Pr is added to its core 21a, a wavelength of around 440 nm is preferred for the excitation light.

[0019] FBG22 and 23 selectively reflect light of a specific single frequency (single wavelength) among the light propagating through the amplifying fiber 21. Thereby, the optical fiber resonator 12 is caused to oscillate in a single longitudinal mode (single frequency mode). As schematically shown in FIG. 2, FBG22 is formed as a refractive index modulation region where the refractive index varies (increases and decreases) at a predetermined period in the axial direction of the core 21a of the amplifying fiber 21. This FBG22 is formed so as to selectively reflect light of a specific single frequency among the light emitted by the ions of the rare earth element added in the core 21a. FBG23 is formed as a refractive index modulation region in the core 21a in the same manner as FBG22, and is formed as a refractive index modulation region where the refractive index varies in the axial direction of the core 21a so as to reflect light of the same wavelength as FBG22. The periods of the refractive index modulation of FBG22 and 23 are determined based on the effective refractive index of the amplifying fiber 21 so that the wavelength of the light to be reflected becomes the center wavelength of the reflection band.

[0020] The rare earth element may or may not be added to the core 21a in which FBG22 and 23 are formed. Also, FBG22 and 23 may be formed at the positions of the optical fibers 20 so as to sandwich the amplifying fiber 21. Therefore, FBG22 and 23 may be formed in the optical fiber regions 20a and 20b composed of silica glass fibers.

[0021] The reflectivities of FBG22 and 23 with respect to the above-mentioned specific single wavelength light are adjusted so that, for example, FBG22 is approximately 100% and FBG23 has a lower reflectivity (for example, 60%) than FBG22. In the amplifying fiber 21, the specific wavelength light is repeatedly reflected between FBG22 and FBG23. FBG22 and 23 are Fabry - Perot type optical fiber resonators that repeatedly reflect light of a specific single frequency between them to form a standing wave.

[0022] The optical fiber resonator 12 is adjusted to satisfy the laser oscillation condition such that the gain of optical amplification by stimulated emission when light reciprocates between the FBG 22 and the FBG 23 is greater than the loss in the optical fiber resonator 12. The loss in the optical fiber resonator 12 is the loss due to light transmitting through the FBGs 22 and 23 and exiting the optical fiber resonator 12, or the loss due to absorption and scattering of light in the amplification fiber 21 between the FBGs 22 and 23. As a result, the stimulated emission light is amplified by repeatedly reciprocating in the core 21a of the amplification fiber 21 in its axial direction to become laser light.

[0023] The FBGs 22 and 23 preferably have a high reflectivity at the wavelength at which laser oscillation is desired and a low reflectivity at the wavelength at which laser oscillation is not desired. For this reason, at the peak of the reflection spectra of the FBGs 22 and 23, it is preferable to adjust to satisfy the laser oscillation condition as described above and suppress the bandwidth of the reflection spectra to be equal to or less than the FSR (resonance frequency interval) of the optical fiber resonator 12. Specifically, it is preferable that the bandwidth of the reflection spectra of the FBGs 22 and 23 be 10 times or less of the FSR, more preferably 2 times or less, and particularly preferably 1 times or less. By suppressing the bandwidth of the reflection spectra of the FBGs 22 and 23 to be equal to or less than the FSR, modes other than the longitudinal mode for which laser oscillation is desired do not satisfy the laser oscillation condition, and thus single longitudinal mode oscillation that prevents mode hopping can be obtained. The bandwidth of the reflection spectra of the FBGs 22 and 23 can be adjusted by the lengths of the FBGs 22 and 23 and the modulation depth of the refractive index change.

[0024] The spacing between FBG22 and 23 (the length of the amplification fiber 21) is set to be approximately equal to or less than the attenuation length at the wavelength of the excitation light. From the viewpoint of stably obtaining single longitudinal mode oscillation, it is preferable to increase the FSR of the optical fiber resonator 12 and, as described above, to keep the bandwidth of the reflection spectrum equal to or less than the FSR. Therefore, the spacing between FBG22 and 23 needs to be such that a gain that satisfies the laser oscillation conditions can be obtained, but from this viewpoint, it is preferable to make it as short as possible. Since FBG22 and 23 are formed in the optical fiber 20, the spacing between them can be made sufficiently short, so stable single longitudinal mode oscillation can be easily realized. The spacing between FBG22 and 23 can be, for example, several centimeters to about 30 centimeters.

[0025] FBG22 and 23 are formed by irradiating the core 21a of the amplification fiber 21 with interference light (interference fringes) generated from femtosecond laser light. Note that in fluoride optical fibers, it is not necessary to add photosensitive materials such as Ge that induce photo-induced refractive index changes, nor is it necessary to add rare earth elements to the core 21a. In this example, femtosecond laser light is used to reliably induce an irreversible refractive index change in the core 21a, but other types of light that can reliably induce an irreversible refractive index change may also be used.

[0026] As described above, in the optical fiber resonator 12, FBGs 22 and 23 are formed on the core 21a of the amplification fiber 21 as reflective sections that reflect light of a specific single frequency. Therefore, there is no need to provide any external components to the amplification fiber 21, such as a mirror. Accordingly, this configuration using FBGs 22 and 23 is advantageous for miniaturizing the optical fiber resonator 12 and, by extension, the fiber laser device 10, which uses a fluoride optical fiber as the amplification fiber 21.

[0027] FBG22 and 23 are formed using femtosecond laser light, for example, by the phase mask method. As shown in Figure 3, one end of the amplification fiber 21 is placed at a predetermined distance from a phase mask 37, which has grooves formed at a predetermined pitch (period). In this state, femtosecond laser light from a femtosecond laser device 38 is irradiated onto one end of the amplification fiber 21 via the phase mask 37. This forms interference fringes on the core 21a at one end of the amplification fiber 21 by interfering the +1st order diffracted light and -1st order diffracted light of the femtosecond laser light diffracted by the phase mask 37, and the interference light of the femtosecond laser light, whose intensity is modulated in the axial direction of the core 21a, is irradiated onto one end of the amplification fiber 21. This irradiation with femtosecond laser light forms FBG22 on the core 21a, in which the refractive index fluctuates in the axial direction. Similarly, when forming FBG23, the interference light of the femtosecond laser light generated in the same way is irradiated onto the other end of the amplification fiber 21 to form FBG23.

[0028] In this example, a femtosecond laser device 38 is used as the light source, which outputs a femtosecond laser beam with a wavelength of 400 nm, which is the second harmonic of a femtosecond laser beam with a wavelength of 800 nm. Furthermore, when forming FBGs 22 and 23, a femtosecond laser beam with a pulse width of approximately 100 fs is irradiated 600,000 times (1 kHz × 10 minutes).

[0029] As described above, when an intensity-modulated femtosecond laser beam is irradiated onto the core 21a in the axial direction, the core 21a undergoes refractive index modulation in accordance with the intensity modulation. Specifically, irradiation with the femtosecond laser beam induces a decrease in refractive index. As a result, FBGs 22 and 23 are formed at each end of the amplification fiber 21. The femtosecond laser beam is irradiated onto the amplification fiber 21 by increasing the power density in the direction perpendicular to the fiber axis of the amplification fiber 21 by narrowing the beam diameter in that direction using a cylindrical lens. This strengthens the nonlinear optical effect, allowing for greater refractive index modulation. Furthermore, refractive index modulation is also imparted to the cladding 21b by scanning the beam in the direction perpendicular to the fiber axis.

[0030] Furthermore, it is preferable that FBG22 and 23 have reflection spectra without side lobes. For this reason, it is preferable that FBG22 and 23 be apodized FBGs (apodized FBGs) whose refractive index profile is a Gaussian profile or the like.

[0031] In this example, FBG22 and 23 are formed by the phase mask method, but the method for forming FBG22 and 23 is not limited to this. For example, interference fringes of femtosecond laser light may be irradiated onto each end of the amplification fiber 21 using two-beam interferometry.

[0032] According to the above configuration, excitation light is incident from the excitation light source unit 14 to the core 21a of the amplification fiber 21 via the FBG 22, and the excitation light propagates through the core 21a. At this time, the electrons of the rare earth element ions in the core 21a are excited by the excitation light. Spontaneous emission is produced when the excited electrons transition to the ground state, and stimulated emission occurs when light is incident on the rare earth element ions whose electrons have been excited, thereby amplifying the light.

[0033] FBG22 and 23 selectively reflect light of a specific single frequency, meaning they reflect only light of that specific single frequency. As a result, only light of that specific single frequency is repeatedly amplified and reflected in the optical fiber resonator 12, generating light of the same phase and frequency. When the amplification exceeds the loss, laser oscillation occurs. In this way, the laser light generated by the optical fiber resonator 12 propagates through the low-reflectivity FBG23 to the optical fiber region 20b and is output.

[0034] The optical fiber resonator 12 reduces the reflection bandwidth of FBGs 22 and 23 to approximately the same level as or less than the FSR of the optical fiber resonator 12, thereby limiting the number of longitudinal modes within the reflection bandwidth of FBGs 22 and 23 to about one, and restricting the longitudinal mode that can generate laser light to this one. As a result, a stable single-longitudinal-mode laser beam can be obtained without mode hopping. [Explanation of Symbols]

[0035] 10 Fiber laser device 12. Optical fiber resonator 14. Excitation light source section 21 Amplification Fiber 22, 23 Fiber Bragg grid 37 Phase Mask 38 Femtosecond laser device

Claims

1. An optical fiber having an amplification fiber region in which rare earth elements are added to a fluoride optical fiber, A pair of fiber Bragg gratings are formed at each position of the optical fiber flanking the amplification fiber region, and selectively reflect light of a single frequency. An optical fiber resonator characterized by having the following features.

2. The optical fiber resonator according to claim 1, characterized in that the bandwidth of the reflection spectra of the pair of fiber Bragg gratings is 10 times or less the resonant frequency interval.

3. The optical fiber resonator according to claim 1, characterized in that the pair of fiber Bragg gratings are apodized.

4. An optical fiber resonator according to any one of claims 1 to 3, An excitation light source unit having a light source that outputs excitation light, and which directs the excitation light from the light source into the amplification fiber region. A fiber laser apparatus characterized by comprising the following features.

5. The steps include preparing an optical fiber having an amplification fiber region in which rare earth elements are added to a fluoride optical fiber, The steps include: irradiating interference fringes generated by phetosecond laser light onto each position of the optical fiber flanking the amplification fiber region, thereby forming a pair of fiber Bragg gratings that reflect light of a single frequency from the light stimulated to emit from the amplification fiber region; A method for manufacturing an optical fiber resonator, characterized by having the following features.

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