Narrow-linewidth thulium-doped fiber laser oscillator

Abstract

The invention discloses a narrow-linewidth thulium-doped fiber laser oscillator, which is characterized in that a saturable absorption fiber has certain absorption on signal light wavelength and is arranged between a first high-reflectivity fiber bragg grating and a second high-reflectivity fiber bragg grating, and when the laser gain exceeds the loss of a laser resonant cavity formed by the first high-reflectivity fiber bragg grating, the second high-reflectivity fiber bragg grating and a low-reflectivity fiber bragg grating, a thulium-doped fiber is generated; laser oscillation is formed and output through the low-reflection fiber bragg grating; the first and second high-reflectivity fiber gratings form an etalon effect; an etalon effect is formed between the second high-reflectivity fiber bragg grating and the low-reflectivity fiber bragg grating and between the first high-reflectivity fiber bragg grating and the low-reflectivity fiber bragg grating; signal light oscillated between the first high-reflectivity fiber bragg grating and the second high-reflectivity fiber bragg grating can form standing waves, and a dynamic grating is formed in the saturable absorption fiber. Under the action of the resonant cavity vernier effect and the saturable absorption dynamic optical fiber, signal light oscillates in a narrow linewidth mode between the first high-reflectivity fiber bragg grating and the second high-reflectivity fiber bragg grating, and the narrowed signal light is injected into the thulium-doped gain optical fiber through the second high-reflectivity fiber bragg grating, so that the laser maintains narrow spectral linewidth operation.

Description

Technical Field

[0001] The present invention relates to the field of lasers, and in particular to a narrow linewidth thulium-doped fiber laser oscillator. Background Art

[0002] Narrow linewidth lasers in the 1.7 - 2.1 μm band have important applications in fields such as spectroscopy, coherent detection, high-resolution imaging, and quantum communication. Most of the narrow linewidth high-power lasers in this band are achieved through a single-frequency or single-frequency phase-modulated seed source combined with a master oscillator-power amplifier (MOPA) structure. However, the MOPA system itself has a relatively complex structure, and the cost of the single-frequency seed source in this band is very high, resulting in a high cost of the entire MOPA system, which poses an obstacle to practical applications.

[0003] On the other hand, in the MOPA structure, it is difficult for the low-power seed light to fully saturate the laser gain during the amplification process, which inevitably leads to problems such as a decrease in the optical signal-to-noise ratio.

[0004] For a relatively simple and low-cost laser oscillator, the control of the laser spectral linewidth can be achieved by adding a narrowband filter, introducing a composite cavity, or constructing a saturable absorber dynamic grating, etc., to introduce different losses to lasers of different frequencies (wavelengths). However, as the laser gain increases, the above frequency selection measures are difficult to provide sufficient discrimination for different frequencies, and thus it is impossible to maintain a narrow laser spectral linewidth at high power. Therefore, a method that can enable a fiber laser oscillator to maintain a narrow spectral linewidth at a higher power has important application value.

[0005] References

[0006] [1] X. Wang, X. Jin, W. Wu et al., 310-W single frequency Tm-doped all-fiber MOPA, IEEE Photonics Technology Letters, 27(6), 677 - 680(2015).

[0007] [2] C. Ren, Y. Shen, Y. Zheng et al., Widely-tunable all-fiber Tm doped MOPA with >1kW of output power, Opt. Express 31, 22733 - 22739(2023). Summary of the Invention

[0008] The present invention provides a narrow linewidth thulium-doped fiber laser oscillator. The present invention combines the Vernier effect of a multi-grating resonator and the dynamic grating effect formed by the saturable absorption process of the doped fiber on the oscillating signal light to compress the laser spectral linewidth in the sub-cavity formed by two high-reflection fiber gratings. The narrow linewidth signal light in the sub-cavity then forms self-injection feedback on the main cavity composed of the high-reflection fiber grating and the low-reflection fiber grating, enabling the laser to maintain a narrow spectral linewidth when operating at a relatively high power. Details are described below:

[0009] A narrow linewidth thulium-doped fiber laser oscillator, the laser comprising: a first high-reflection fiber grating, a saturable absorption fiber, a pump source, a pump coupling device, a second high-reflection fiber grating, a thulium-doped gain fiber, a low-reflection fiber grating, and a cladding light stripper.

[0010] Wherein, the pump source emits pump light within the absorption band of the thulium-doped gain fiber, and the pump light enters the thulium-doped gain fiber through the pump coupling device; the thulium-doped gain fiber absorbs the pump light and generates gain within the 1.7 - 2.2 μm emission band of thulium ions. The laser resonator is composed of the first high-reflection fiber grating, the second high-reflection fiber grating, and the low-reflection fiber grating. The reflectivity of the first high-reflection fiber grating to the signal light is close to 1, the second high-reflection fiber grating has a relatively low transmittance in the range of 1% - 10% for the signal light, and the low-reflection fiber grating has a relatively high transmittance for the signal light and serves as the output mirror of the resonator. The saturable absorption fiber has a certain absorption of the signal light wavelength and is placed between the first high-reflection fiber grating and the second high-reflection fiber grating. When the laser gain exceeds the loss of the laser resonator composed of the first high-reflection fiber grating, the second high-reflection fiber grating, and the low-reflection fiber grating, laser oscillation is formed and output through the low-reflection fiber grating. Since both the first high-reflection fiber grating and the second high-reflection fiber grating have a relatively high reflectivity to the signal light, a Fabry-Perot effect is formed between them; a Fabry-Perot effect is also formed between the second high-reflection fiber grating and the low-reflection fiber grating, and between the first high-reflection fiber grating and the low-reflection fiber grating. Only the laser whose frequency simultaneously satisfies the resonance conditions of these three can oscillate in the cavity with a relatively low loss, that is, the Vernier effect, while other frequencies will suffer greater losses, thus playing a role in restricting the laser linewidth. Further, the signal light oscillating between the first high-reflection fiber grating and the second high-reflection fiber grating will form a standing wave, and a dynamic grating is formed in the saturable absorption fiber, that is, the ground-state particles in the saturable absorption fiber at the antinode of the standing wave are excited and no longer exhibit absorption to the frequency of this standing wave, and the light of other frequencies is lost when passing through the saturable absorption fiber. Under the action of the resonator Vernier effect and the saturable absorption dynamic fiber, the signal light can only oscillate with a narrow linewidth between the first high-reflection fiber grating and the second high-reflection fiber grating, and the narrowed signal light is then injected into the thulium-doped gain fiber through the second high-reflection fiber grating, enabling the laser to maintain narrow spectral linewidth operation.

[0011] Among them, the thulium-doped gain fiber can be a single-mode fiber or a multi-mode fiber, a single-clad fiber or a double-clad fiber. The type of fiber and the corresponding pumping method are selected according to the required power, as long as the thulium ions can provide laser gain and form laser oscillation in the 2μm band.

[0012] The saturable absorption fiber can be a thulium-doped fiber, a holmium-doped fiber, or a thulium-holmium co-doped fiber. The fiber type is selected according to the actual working wavelength, as long as it can provide appropriate absorption for the signal light generated by the thulium-doped gain fiber.

[0013] Among them, the pump source can be a semiconductor laser, or a fiber laser, a solid laser, or other forms of lasers, and its laser mode can be a fundamental transverse mode or a multi-transverse mode. As long as its emission wavelength is within the absorption band of the thulium-doped gain fiber, it can enable the thulium-doped gain fiber to generate laser gain.

[0014] The pump coupling device may be a wavelength division multiplexer (WDM), a signal pump combiner, or a direct fusion coupling method, and the corresponding coupling method and device are selected according to the form of the pump source and the transverse mode.

[0015] The beneficial effects of the technical solution provided by the present invention are:

[0016] 1. The oscillator structure generates high-power 2μm-band lasers with narrow spectral linewidth. Compared with the commonly used master oscillator-power amplifier structure, the laser oscillator structure has high intracavity power, more complete gain saturation, and higher output optical signal-to-noise ratio;

[0017] 2. The saturable absorption fiber in the present invention is placed outside the active sub-cavity formed by the second high-reflection fiber Bragg grating and the low-reflection fiber Bragg grating, which reduces the signal light power borne by the saturable absorption fiber, avoids the saturable absorption fiber being over-saturated and bleached, which causes the dynamic grating to fail, and enhances the spectral control capability;

[0018] 3. The present invention combines the vernier effect of the multi-subcavity etalon and the dynamic grating frequency selection effect, and has a stronger spectral linewidth control capability, which can enable the laser to maintain narrow linewidth operation at higher power. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the optical path of a narrow-linewidth thulium-doped fiber laser oscillator.

[0020] In the accompanying drawings, the components represented by the reference numerals are as follows:

[0021] 1: The first high-reflection fiber Bragg grating; 2: Saturable absorption fiber;

[0022] 3: Pump source; 4: Pump coupling device;

[0023] 5: The second most reflective fiber Bragg grating; 6: Thulium-doped gain fiber;

[0024] 7: Low-reflection fiber Bragg grating; 8: Cladding light stripper. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention are described in further detail below.

[0026] Example 1

[0027] The embodiment of the present invention provides a narrow-linewidth thulium-doped fiber laser oscillator, which includes: a first high-reflection fiber Bragg grating 1, a saturable absorption fiber 2, a pump source 3, a pump coupling device 4, a second high-reflection fiber Bragg grating 5, a thulium-doped gain fiber 6, a low-reflection fiber Bragg grating 7, and a cladding light stripper 8;

[0028] The first high-reflection fiber Bragg grating 1 is inscribed on a passive optical fiber with a core and inner cladding diameter of 25μm / 400μm respectively (hereinafter referred to as "25 / 400 fiber"), and has a reflectivity of >99.5% (23dB) for laser light with a wavelength of 1980nm; the saturable absorption fiber 2 is a holmium-doped fiber, also 25 / 400 fiber, with a length of 1.5m; the pump source 3 is a fiber-coupled output semiconductor laser with a wavelength of 793nm, a fiber core diameter of 105μm, and a power of 140W; the pump coupling device 4 is a pump / signal combiner. The pigtail matches the pump source 3, the saturable absorption fiber 2, and the second high-reflection fiber Bragg grating 5. The second high-reflection fiber Bragg grating 5 is also inscribed on a 25 / 400 fiber and has a 2% transmittance for 1980nm laser light. The thulium-doped gain fiber 6 is a 25 / 400 double-clad active fiber with a cladding absorption coefficient of 3dB / m for 793nm pump light and a length of 4m. The low-reflection fiber Bragg grating 7 is inscribed on a 25 / 400 fiber and has a 20% reflectivity (transmittance ~80%) for 1980nm laser light. The pigtail of the cladding light stripper 8 matches the low-reflection fiber Bragg grating 7. These components are fusion-connected.

[0029] The 793nm pump light emitted by the pump source 3 passes through the pump coupling device 4 and the second high-reflection fiber Bragg grating 5, then enters the cladding of the thulium-doped gain fiber 6. Upon passing through the core of the thulium-doped gain fiber 6, it is absorbed by the thulium ions therein, generating laser gain in the 2μm band. The first high-reflection fiber Bragg grating 1, the second high-reflection fiber Bragg grating 5, and the low-reflection fiber Bragg grating 7 all provide feedback from the 1980nm laser. The laser resonant cavity can be considered to consist of the composite feedback formed by the first high-reflection fiber Bragg grating 1 and the second high-reflection fiber Bragg grating 5, and the low-reflection fiber Bragg grating 7. When the laser gain exceeds the resonant cavity loss, laser oscillation occurs and is output through the low-reflection fiber Bragg grating 7. Residual 793nm pump light that was not completely absorbed is stripped away by the cladding light stripper 8, maintaining the purity of the output spectrum. When the laser frequency (wavelength) simultaneously satisfies the resonance conditions between the three pairs of gratings, namely, the first high-reflection fiber Bragg grating 1 and the second high-reflection fiber Bragg grating 5, the second high-reflection fiber Bragg grating 5 and the low-reflection fiber Bragg grating 7, and the first high-reflection fiber Bragg grating 1 and the low-reflection fiber Bragg grating 7, the highest feedback is obtained, giving the system an advantage in the competition. Due to the vernier effect between the resonant frequencies of the three pairs of gratings, the actual effective longitudinal mode spacing of the resonant cavity is amplified. Since the second high-reflection fiber Bragg grating 5 has a certain transmittance for the 1980nm pump light, part of the oscillating laser light propagating from right to left enters the saturable absorption fiber 2 after passing through the second high-reflection fiber Bragg grating 5, and then propagates from left to right after being reflected by the first high-reflection fiber Bragg grating 1, thus forming a standing wave in the saturable absorption fiber 2. At the antinode of the standing wave (i.e., the position where the light intensity is maximum), the holmium ions in the saturable absorption fiber are excited and the fiber is bleached. However, the light intensity at the node of the standing wave is almost zero, and a large number of holmium ions are still in the ground state, and they still show obvious absorption for frequencies other than the laser frequency that constitutes the standing wave. Therefore, the saturable absorption fiber 2 shows different losses for lasers of different frequencies (wavelengths), thus forming a dynamic grating: there is almost no loss for light at the center frequency that generates the dynamic grating, while light of other frequencies will inevitably pass through the nodes of the existing dynamic grating when passing through the saturable absorption fiber 2, suffering absorption loss, thus playing a frequency selection role. Under the experimental parameters given in the embodiment of the present invention, 120W of 793nm pump light can generate 52W of 1980nm laser light with a spectral linewidth of 3.1GHz.

[0030] Example 2

[0031] In the above embodiment 1, the thulium-doped gain fiber 6 can be a single-mode fiber or a multi-mode fiber, a single-clad fiber or a double-clad fiber; the pump source 3 can be a multi-mode semiconductor laser, a single transverse-mode semiconductor laser or other types of lasers. The type of optical fiber and the corresponding pumping method are selected according to the required power. The pump wavelength can be 793nm, 1570nm or other wavelengths, as long as it corresponds to the pump absorption band of the thulium-doped gain fiber 6. The embodiment of the present invention does not impose any restrictions on this.

[0032] The laser wavelength can also be other wavelengths within the emission band of thulium-doped fiber. Correspondingly, the saturable absorption fiber 2 can be holmium-doped fiber, or thulium-doped or thulium-holmium co-doped fiber. For example, when the laser operates near 1800 nm or at a shorter wavelength, the absorption of holmium ions is very small, while the absorption of thulium ions is relatively obvious. In this case, thulium-doped fiber should be selected to provide the function of saturable absorption for constructing a dynamic grating.

[0033] Correspondingly, for example, if the pump source 1 is a single transverse mode pump source, then the corresponding pump coupling device 2 is a wavelength division multiplexer (WDM) or other types of single-mode coupling devices. The embodiments of the present invention do not limit this.

[0034] Except for the special descriptions of the models of each device in the embodiments of the present invention, the models and specifications of other devices, including the size, numerical aperture, length, doping concentration, etc. of the fiber, are not specifically limited, as long as the devices can perform the above functions.

[0035] Those skilled in the art can understand that the drawings are only schematic diagrams of a preferred embodiment. The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0036] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A narrow linewidth thulium-doped fiber laser oscillator, characterized in that, The laser oscillator includes: a first high-reflection fiber grating, a saturable absorption fiber, a pump source, a pump coupler, a second high-reflection fiber grating, a thulium-doped gain fiber, a low-reflection fiber grating, and a cladding light stripper; Wherein, the pump source emits pump light within the absorption band of the thulium-doped gain fiber, and the pump light enters the thulium-doped gain fiber through the pump coupler; the thulium-doped gain fiber absorbs the pump light and generates gain within the 1.7 - 2.2 μm emission band of thulium ions; The laser resonator is composed of the first high-reflection fiber grating, the second high-reflection fiber grating, and the low-reflection fiber grating; the low-reflection fiber grating has a high transmittance for the signal light and serves as the output mirror of the laser resonator; The saturable absorption fiber has a certain absorption for the signal light wavelength and is placed between the first high-reflection fiber grating and the second high-reflection fiber grating. When the laser gain exceeds the loss of the laser resonator formed by the first high-reflection fiber grating, the second high-reflection fiber grating, and the low-reflection fiber grating, laser oscillation is formed and output through the low-reflection fiber grating; The first high-reflection fiber grating and the second high-reflection fiber grating form an etalon effect; an etalon effect is formed between the second high-reflection fiber grating and the low-reflection fiber grating, and between the first high-reflection fiber grating and the low-reflection fiber grating; the signal light oscillating between the first high-reflection fiber grating and the second high-reflection fiber grating forms a standing wave and forms a dynamic grating in the saturable absorption fiber; Under the action of the resonator vernier effect and the saturable absorption fiber, the signal light oscillates with a narrow linewidth between the first high-reflection fiber grating and the second high-reflection fiber grating, and the narrowed signal light is then injected into the thulium-doped gain fiber through the second high-reflection fiber grating, enabling the laser to maintain narrow spectral linewidth operation.

2. The narrow linewidth thulium-doped fiber laser oscillator according to claim 1, wherein The reflectivity of the first high-reflection fiber grating for the signal light is close to 1, and the second high-reflection fiber grating has a relatively low transmittance in the range of 1% - 10% for the signal light.

3. A narrow linewidth thulium-doped fiber laser oscillator according to claim 1, characterized in that, The thulium-doped gain fiber is a single-mode fiber, or a multi-mode fiber, or a single-clad fiber, or a double-clad fiber.

4. A narrow linewidth thulium-doped fiber laser oscillator according to claim 1, characterized in that The saturable absorption fiber is a thulium-doped fiber, a holmium-doped or thulium-holmium co-doped fiber.

5. A narrow linewidth thulium-doped fiber laser oscillator according to claim 1, characterized in that, The pump source is a semiconductor laser, or a fiber laser, or a solid-state laser, and the laser mode is a fundamental transverse mode, or a multi-transverse mode.

6. The narrow linewidth thulium-doped fiber laser oscillator according to claim 1, characterized in that The pump coupler is a wavelength division multiplexer, or a signal-pump combiner, and the direct fusion coupling method is adopted, and the corresponding coupling method and device are selected according to the form and transverse mode of the pump source.

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