CN102306897B - Ultra narrow linewidth low noise high power single frequency fiber laser - Google Patents

Abstract

The invention discloses a single-frequency fiber laser with ultra-narrow line width, low noise and high power, which includes a single-mode semiconductor laser pump source, a polarization-maintaining wavelength division multiplexer, a coupling output polarization-maintaining fiber grating, and a high-reflection polarization-maintaining fiber grating , wave plate, high-gain fiber, low-reflection narrow-linewidth fiber grating, dichroic mirror, heat sink, sealed air chamber, fiber fixture, and polarization-maintaining fiber isolator; the common end of the polarization-maintaining wavelength division multiplexer and the coupling output protection Polarization-maintaining fiber grating connection, the coupled output polarization-maintaining fiber-bragg grating and high-reflection polarization-maintaining fiber-optic grating are written on the same polarization-maintaining fiber or on two polarization-maintaining fibers respectively, and the fast and slow axes are in the same direction when connected, and the high-reflection-maintaining The polarizing fiber grating is connected to the high-gain fiber through the wave plate, and the high-gain fiber is connected to the low-reflection narrow-linewidth fiber grating and the dichromatic mirror in sequence. A single-frequency fiber laser capable of producing ultra-narrow linewidth and polarization-maintaining output.

Description

A single-frequency fiber laser with ultra-narrow linewidth, low noise and high power

technical field

The invention relates to a fiber laser, in particular to a single-frequency fiber laser with ultra-narrow line width, low noise and high power. The line width of the laser is less than hundreds of Hz, and the output power is as high as hundreds of mW.

Background technique

Ultra-low-noise single-frequency lasers have broad application prospects in ultra-high-precision cutting-edge fields such as coherent optical communications, optical atomic clocks, gravitational wave detection, and quantum secure communications because of their narrow spectral linewidth and excellent laser coherence characteristics. To improve the detection distance or accuracy of fiber laser, it is necessary to use laser with high coherence performance, so the laser is required to have an ultra-narrow linewidth, such as a single-frequency fiber laser with a linewidth of several hundred Hz and a mW level is required as a seed source. Generally, it is difficult to achieve high power (> 100mW) and ultra-narrow linewidth (< 200Hz) single-frequency laser output with silica-doped fibers.

At present, the ultra-narrow linewidth single-frequency fiber laser is studied, and the rare earth ion highly doped quartz fiber is used as the laser medium. The short straight FP cavity or DBR cavity structure can only output a few mW single-frequency laser, and the multi-component glass fiber is used as the laser medium. The gain medium of a single-frequency laser can realize a single-frequency fiber laser with an output power of more than 100 mW and a line width of less than 2 KHz. A single-frequency fiber laser with a wavelength of less than 2 KHz and a wavelength of 1.5 μm [Optics Express, 2010, 18(2): 1249]. In 2004, Alexander University and NP Photonics applied for a rare earth-doped phosphate glass single-mode fiber laser [patent number: US 6816514 B2] and a high-power narrow-linewidth single-frequency fiber laser in the research of ultra-narrow linewidth single-frequency fiber laser. Laser [publication number: US 2004/0240508 A1] two patents, which are based on (30-80) P ₂ O ₅ -(5-30) L ₂ O ₃ (L ₂ O ₃ : Al ₂ O, B ₂ O ₃ , Y ₂ O ₃ , La ₂ O ₃ and their mixtures)-(5~30) MO (MO: BaO, BeO, MgO, SrO, CaO, ZnO, PbO and their mixtures) the rare earth doped matrix Heterophosphate glass single-mode optical fiber, and claims a partial cavity structure. In 2008, South China University of Technology applied for a low-noise narrow-linewidth high-power single-longitudinal-mode fiber laser [Patent No.: 200810220661.X] patent for ultra-narrow linewidth single-frequency fiber laser research. A polarization maintaining rare earth doped phosphate glass single mode optical fiber is claimed. The narrow-linewidth single-frequency fiber lasers provided by the above documents and invention patents all have spatial hole-burning effects in their gain fibers, and can only achieve kHz-level linewidth output. The application of the present invention proposes a new type of resonant cavity structure, which solves the spatial hole burning effect in the gain fiber through polarization rotation technology. The fiber grating plus the total reflection mirror constitutes an intracavity filter and is connected with a pair of polarization-maintaining fiber gratings at the front and rear respectively. Constructing a folded compound resonant cavity is beneficial to improving the cavity life of the resonant cavity, so as to achieve the purpose of reducing the laser line width (up to hundreds of Hz).

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the prior art and provide a single-frequency fiber laser with ultra-narrow linewidth, low noise and high power. Single-frequency fiber laser with narrow linewidth (less than hundreds of Hz) and polarization-maintaining output.

The purpose of the present invention is achieved through the following technical solutions:

A single-frequency fiber laser with ultra-narrow linewidth, low noise and high power, including a single-mode semiconductor laser pump source, a polarization-maintaining wavelength division multiplexer, a coupling output polarization-maintaining fiber grating, a high-reflection polarization-maintaining fiber grating, a wave plate, High-gain fiber, low-reflection narrow-linewidth fiber grating, dichroic mirror, heat sink, sealed air chamber, fiber fixture and polarization-maintaining fiber isolator, the single-mode semiconductor laser pump source and polarization-maintaining wavelength division multiplexer The pump input end is connected, and the common end of the polarization-maintaining wavelength division multiplexer is connected to the coupling output polarization-maintaining fiber grating, and the coupling output polarization-maintaining fiber grating and the high-reflection polarization-maintaining fiber grating are both written on the same polarization-maintaining fiber or They are respectively written on two polarization-maintaining fibers and the fast and slow axes are in the same direction when connected. The high-reflection polarization-maintaining fiber grating is connected to the high-gain fiber through a wave plate, and the high-gain fiber is connected to a low-reflection narrow-linewidth fiber grating and a dichromatic mirror in sequence. connection, wherein the coupled output polarization-maintaining fiber grating, high-reflection polarization-maintaining fiber grating, wave plate, high-gain fiber, low-reflection narrow-linewidth fiber grating and dichromatic mirror together form a single-frequency laser resonator, and are fixed and packaged at an automatic temperature In the controlled heat sink, the entire single-frequency laser resonator is packaged with a sealed air chamber on the heat sink at the same time, and the pigtail of the entire single-frequency laser resonator is fixed on the front shell surface of a sealed air chamber by an optical fiber clamp. The single-frequency laser generated by the laser resonator is coupled and output through the signal end of the polarization-maintaining wavelength division multiplexer, and then output through a polarization-maintaining fiber isolator; the core of the high-gain fiber is doped with high-concentration luminescent ions, and the The luminescent ions are a combination of one or more of lanthanide ions and transition metal ions, and the doping concentration of the luminescent ions is greater than 1×10 ¹⁹ ions/cm ³ and uniformly doped in the fiber core.

In the above-mentioned single-frequency fiber laser with ultra-narrow linewidth, low noise and high power, the high-gain fiber is a common rare-earth-doped phosphate single-mode glass fiber or a polarization-maintaining rare-earth-doped phosphate single-mode glass fiber.

The above-mentioned single-frequency fiber laser with ultra-narrow linewidth, low noise and high power has a core composition of phosphate glass, which is composed of 70P ₂ O ₅ -8Al ₂ O ₃ -15BaO-4La ₂ O ₃ -3Nd ₂ O ₃ ; The gain per unit length of the high-gain fiber is greater than 1 dB/cm, and the fiber length is 0.5 to 5 cm.

In the aforementioned ultra-narrow linewidth, low-noise, and high-power single-frequency fiber laser, the wave plate is a quarter wave plate or a three-quarter wave plate.

In the aforementioned ultra-narrow linewidth, low-noise, and high-power single-frequency fiber laser, the fast axis or slow axis of the high-reflection polarization-maintaining fiber grating forms an included angle of 45 ^° with the axis of the wave plate.

The above-mentioned single-frequency fiber laser with ultra-narrow linewidth, low noise and high power, the fast axis or slow axis center reflection wavelength of the coupled output polarization maintaining fiber grating is the same as the slow axis or fast axis center reflection wavelength of the high reflection polarization maintaining fiber grating The wavelength is matched, that is, the fast axis center reflection wavelength reflection spectrum of the coupling out polarization maintaining fiber grating is located in the slow axis center reflection wavelength reflection spectrum of the high reflection polarization maintaining fiber grating, or the slow axis center reflection wavelength reflection spectrum of the coupling out polarization maintaining fiber grating The spectrum is located within the reflection spectrum of the fast axis center reflection wavelength of the high reflection polarization maintaining fiber grating.

The above-mentioned single-frequency fiber laser with ultra-narrow linewidth, low noise and high power, the slow-axis or fast-axis central reflection wavelength of the polarization-maintaining fiber grating is the laser output wavelength, the 3dB reflection spectral width is less than 0.15 nm, and the central wavelength reflection wavelength is less than 0.15 nm. The rate is 10-90%; the fast axis or slow axis central reflection wavelength of the high reflection polarization maintaining fiber grating is the laser output wavelength, the 3dB reflection spectrum width is greater than 0.15 nm, and the central wavelength reflectivity is greater than 90%; the low reflection narrow The central reflection wavelength of the linewidth fiber grating is the laser output wavelength, the 3dB reflection spectral width is less than 0.15 nm, and the central wavelength reflectance is 10-50%.

The above-mentioned single-frequency fiber laser with ultra-narrow linewidth, low noise and high power, the dichroic mirror is formed by coating a thin film directly on the end face of the low-reflection narrow-linewidth fiber grating after grinding and polishing, and the thin film has a great influence on the laser signal. The wavelength reflectance is greater than 95%, and the transmittance to the pump wavelength is greater than 90%.

The above-mentioned single-frequency fiber laser with ultra-narrow linewidth, low noise and high power, the lanthanide ions are Er ³⁺ , Yb ³⁺ , Tm ³⁺ , Gd ³⁺ , Tb ³⁺ , Dy ³⁺ , Ho ³⁺ or Lu ³⁺ ; the transition metal ion is Cu ²⁺ , Co ²⁺ , Cr ³⁺ , Fe ²⁺ or Mn ²⁺ .

The dichroic mirror is coated with a thin film on the surface of the cavity mirror or directly coated with a thin film on one end face of the low-reflection narrow-linewidth fiber grating after grinding and polishing. The reflectivity of the thin film to the laser signal wavelength is greater than 95%, and the pump Pu wavelength transmittance greater than 90%.

The connection between the coupled output polarization-maintaining fiber grating and the high-reflection polarization-maintaining fiber grating can be written on the same polarization-maintaining fiber, or can be directly butt-coupled after optical fiber fusion or grinding and polishing of the corresponding fiber end face. And their fast and slow axes need to be aligned, and the distance between the two grating areas is less than 5cm. The corresponding wavelength of the quarter-wave plate is the laser output wavelength. The core diameter of the high-gain optical fiber is 3-10 μm, and the cladding diameter is 60-800 μm.

The invention utilizes the high doping and high gain characteristics of the phosphate glass core material, designs and manufactures the phosphate glass single-mode fiber as the laser medium material, adopts a short straight cavity structure, utilizes the coupling output polarization maintaining fiber grating, low reflection and narrow line width The frequency selection function of the fiber Bragg grating, under the continuous pumping of the pump light source, the linearly polarized light reflected by the fast axis (or slow axis) of the high-reflection polarization-maintaining fiber Bragg grating is rotated by a quarter-wave plate to right-handed circularly polarized light in the It is amplified in the high-gain fiber core, and after filtered and reflected by a filter composed of a low-reflection narrow-linewidth fiber grating and a total reflection cavity mirror, it becomes right-handed circularly polarized light opposite to the original propagation direction, and then returns to high The gain fiber core is further amplified. At this time, the circularly polarized light forms a virtual ring light path in the cavity, and then passes through the quarter-wave plate to become slow-axis (or fast-axis) linearly polarized light. The slow axis (or fast axis) of the fiber Bragg grating is transmitted and partially reflected, and a part of the light is transmitted and output to form a polarization-maintaining single-frequency laser. axis) the same polarization rotation as linearly polarized light reflected, right-handed circularly polarized light amplification, filtering, total reflection reverse, right-handed circularly polarized light re-amplification, also form another virtual ring light path in the cavity, and finally repolarized The linearly polarized light rotated to the fast axis (or slow axis) is then reflected again by the fast axis (or slow axis) of the highly reflective polarization-maintaining fiber Bragg grating, thus forming a resonance period containing two virtual rings, which can effectively solve the high Hole burning in gain fibers due to traveling waves. In addition, the low-reflection narrow-linewidth fiber grating, the high-reflection polarization-maintaining fiber grating, and the coupled output polarization-maintaining fiber grating together form a resonant cavity in the cavity, and together with the original resonant cavity, they form a straight-cavity composite cavity, thus significantly The lifetime of photons in the cavity is enhanced, and the purpose of reducing the laser line width is achieved. In particular, it not only combines the high gain per unit length characteristics of rare earth-doped phosphate glass single-mode fiber, and realizes single longitudinal mode selection by coupling out polarization-maintaining fiber grating and narrow linewidth of low-reflection narrow-linewidth fiber grating, but also can utilize low The longitudinal mode selected by the ultra-short inner cavity and the longitudinal mode selected by the outer cavity, which are composed of reflective narrow linewidth fiber grating and total reflection dichroic mirror, can achieve finer single frequency selection. Based on the above-mentioned technical advantages, the present invention can realize single-frequency laser output with ultra-narrow line width, low noise, high power and polarization maintenance.

Compared with the prior art, the technical effect of the present invention is: a centimeter-level high-gain rare-earth-doped phosphate glass single-mode fiber is used as the gain medium of the laser, and is composed of a coupling output polarization-maintaining fiber grating and a high-reflection polarization-maintaining fiber grating The front and rear cavity mirrors of the resonant cavity structure, under the continuous excitation of the single-mode semiconductor laser pump source, the highly doped rare earth particles in the fiber core are reversed to generate signal light for stimulated emission. The linearly polarized light generated by coupling out the polarization-maintaining fiber grating and the high-reflection polarization-maintaining fiber grating rotates and forms circularly polarized light, thus forming a resonant period containing two virtual ring optical paths, which can effectively solve the problem of high-gain fibers due to The hole-burning effect caused by traveling waves is beneficial to improve the noise characteristics of single-frequency lasers. In addition, the low-reflection narrow-linewidth fiber grating, the high-reflection polarization-maintaining fiber grating, and the coupled output polarization-maintaining fiber grating together form a resonant cavity in the cavity, which together with the original resonant cavity constitutes a straight-cavity composite cavity. The increase can significantly improve the lifetime of photons in the cavity and achieve the purpose of reducing the laser linewidth, thereby ensuring the realization of ultra-narrow linewidth (hundred Hz level) single-frequency laser. In particular, it not only combines the high gain per unit length characteristics of rare earth-doped phosphate glass single-mode fiber, and realizes single longitudinal mode selection by coupling out polarization-maintaining fiber grating and narrow linewidth of low-reflection narrow-linewidth fiber grating, but also can utilize low The longitudinal mode selected by the ultra-short inner cavity and the longitudinal mode selected by the outer cavity of the ultra-short inner cavity composed of reflective narrow linewidth fiber grating and total reflection dichroic mirror coincide to achieve finer single frequency selection, and mode hopping is not easy to occur. Based on the above-mentioned technical advantages, the present invention can realize the technical effects of low noise, ultra-narrow line width and no mode-hopping single-frequency laser output.

Description of drawings

Fig. 1 is a schematic diagram of the principle of a single-frequency fiber laser of the present invention;

Figure 2 is a schematic diagram of the positions of the quarter-wave plate principal axis and the fast and slow axes of the high-reflection polarization-maintaining fiber grating;

Figure 3 is a schematic diagram of the design of the reflection spectrum of the fiber grating.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and examples. It should be noted that the protection scope of the present invention is not limited to the range expressed in the examples.

As shown in Figure 1, an ultra-narrow linewidth, low-noise, and high-power single-frequency fiber laser includes a single-mode semiconductor laser pump source 1, a polarization-maintaining wavelength division multiplexer (PM-WDM) 2, a coupled output polarization-maintaining fiber grating 3, High reflection polarization maintaining fiber grating 4, quarter wave plate 5, high gain fiber 6, low reflection narrow linewidth fiber Bragg grating 7, dichroic mirror 8, heat sink 9, sealed air chamber 10 and fiber holder 11, polarization maintaining Fiber isolator 12. Among them, the coupling output polarization maintaining fiber grating 3, high reflection polarization maintaining fiber grating 4, quarter wave plate 5, high gain fiber 6, low reflection narrow linewidth fiber grating 7 and dichroic mirror 8 form a single frequency fiber laser cavity 13. The single-mode semiconductor laser pump source 1 is connected to the pump input end of the polarization-maintaining wavelength division multiplexer 2, the common end of the polarization-maintaining wavelength division multiplexer 2 is connected to the coupling output polarization-maintaining fiber grating 3, and the coupling output polarization-maintaining fiber The grating 3 is then connected to the high-reflection polarization-maintaining fiber grating 4, connected to the high-gain fiber 6 via the quarter-wave plate 5, and then connected to the low-reflection narrow-linewidth fiber grating 7 and the dichromatic mirror 8, and polarization-maintaining wavelength division multiplexing The signal end of the user 2 is connected to the polarization maintaining fiber isolator 12 , the single-frequency fiber laser cavity 13 is fixed in the heat sink 9 with automatic temperature control, and the heat sink is packaged in a sealed air chamber 10 through the fiber clamp 11 . The temperature of the heat sink 9 is generally set at any temperature value within the range of -30 to 70°C, generally set at 25°C, and its control accuracy is less than 0.1°C. The laser wavelength is tuned by controlling the temperature of the heat sink 9 . The high-gain optical fiber 6 is a rare-earth-doped phosphate glass single-mode optical fiber, the matrix composition of the fiber core is phosphate glass, and the rare-earth doping ions in the fiber core are erbium, ytterbium, or erbium-ytterbium co-doped.

The centimeter-scale high-gain optical fiber 6 is used as the gain medium of the laser, and the front and rear cavity mirrors of the resonator structure are composed of the coupling output polarization-maintaining fiber Bragg grating 3 and the high-reflection polarization-maintaining fiber Bragg grating 4, and the coupling output polarization-maintaining fiber Bragg grating 3 slow axis (or The reflection spectrum of the fast axis) is located in the reflection spectrum of the fast axis (or slow axis) of the high-reflection polarization-maintaining fiber Bragg grating 4, and the central reflection wavelength coincides; among them, the coupled output is 3dB of the slow axis (or fast axis) of the polarization-maintaining fiber Bragg grating 3 The reflection spectral width is less than 0.15 nm, and the central wavelength reflectance is 10-90%; the 3dB reflection spectral width of the fast axis (or slow axis) of the high-reflection polarization-maintaining fiber grating is greater than 0.15 nm, and the central wavelength reflectance is greater than 90%. The pump light adopts a single-mode semiconductor laser pump source 1 forward pumping mode, and is input from the pump end of the polarization maintaining wavelength division multiplexer 2 and output through the coupling polarization maintaining fiber grating 3, high reflection polarization maintaining fiber grating 4, four The quarter-wave plate 5 is coupled to the core of the high-gain fiber 6 in the laser cavity to pump highly doped rare earth ions to invert the number of particles and generate signal light stimulated emission. The quarter-wave plate is used to 5. Rotate the linearly polarized light generated by coupling out the polarization-maintaining fiber grating 3 and the high-reflection polarization-maintaining fiber grating 4 to form circularly polarized light, which is composed of a low-reflection narrow-linewidth fiber grating 7 and a total reflection dichroic mirror 8 After filtering and reflecting after the filter, it becomes right-handed circularly polarized light opposite to the original propagation direction, and then returns to the high-gain fiber 6 core to be further amplified, thus forming a resonance cycle composed of two virtual ring optical paths. Finally, the formed linearly polarized light is output via the polarization-maintaining fiber grating 3 . Among them, the coupled output polarization-maintaining fiber grating 3 and the high-reflection polarization-maintaining fiber grating 4 are connected by fusion splicing or end-to-face connection, and their fast and slow axes must be aligned; the high-reflection polarization-maintaining fiber grating 4 and the quarter-wave plate 5 are connected by Tight butt joint connection, the end face of the high-reflection polarization-maintaining fiber grating 4 must be ground and polished, and the direction of its fast axis (or slow axis) must form an angle of 45 ^° with the main axis of the quarter-wave plate 5, as shown in Figure 2 The connection between the high-gain fiber 6 and the low-reflection narrow-linewidth fiber grating 7 adopts fusion splicing or end face grinding and polishing; the low-reflection narrow-linewidth fiber grating 7 and the dichroic mirror 8 form a function of longitudinal mode selection and filtering The modules are connected by the method of grinding and polishing the end face of the optical fiber and closely connecting with the cavity mirror. The central wavelength reflection spectrum of the low reflection narrow linewidth fiber Bragg grating 7 is located in the slow axis (or fast axis) reflection spectrum of the coupled output polarization maintaining fiber Bragg grating 3, the reflection spectrum width is less than 0.15 nm, and the central wavelength reflectance is 10-50%; The color mirror 8 is coated with a thin film on the surface of the cavity mirror or directly coated with a thin film on the end face of the low-reflection narrow linewidth fiber grating 7 after grinding and polishing. The reflectivity of the thin film to the laser signal wavelength is greater than 95%, and the pump wavelength is transmitted The rate is greater than 90%. Combined with the high gain per unit length characteristic of the high-gain fiber 6, and the narrow linewidth of the coupled output polarization-maintaining fiber Bragg grating 3 and the low-reflection narrow-linewidth fiber Bragg grating 7, a single longitudinal mode selection must be satisfied. The condition that the longitudinal mode selected by the ultra-short inner cavity and the longitudinal mode selected by the outer cavity coincides with each other by the grating 7 and the total reflection dichroic mirror 8, so as to realize the high-power laser polarization-maintaining output with a narrower linewidth and a single frequency.

Example 1

The high-gain fiber 6 is a rare-earth-doped phosphate glass single-mode fiber, which is used as the gain medium of the fiber laser. The length can be selected according to the laser output power of the device and the reflection spectrum width of the coupling output polarization-maintaining fiber grating 3. In this example, it is 1.0 cm, generally 0.5~10 cm. The high-concentration luminescent ions doped in the core of the high-gain fiber 6 are erbium and ytterbium, and the doping concentrations of erbium and ytterbium rare earth ions are 2.5×10 ²⁰ ions/cm ³ and 5.0×10 ²⁰ ions/cm ³ , generally It should be greater than 1×10 ¹⁹ ions/cm ³ . The fiber core diameter is 3-10 μm, which is 6.0 μm in this example. The core matrix composition of the high-gain optical fiber 6 is phosphate glass, and its composition is: 70P ₂ O ₅ -8Al ₂ O ₃ -15BaO-4La ₂ O ₃ -3Nd ₂ O ₃ . Rare earth ions are uniformly doped at a high concentration in its core. High-gain optical fiber 6 is made of preform by drilling method and tube-and-rod method: first, the cladding glass is processed into a glass rod with a diameter of 30 mm, and then a circular hole with a diameter of 1.44 mm is drilled at the center of the glass rod , and then polish the inner surface of the glass hole; secondly, process the core glass into a round rod with a diameter of 1.44mm, and then polish the outer surface of the round rod; thirdly, insert the core glass rod into the cladding glass rod In the hole in the center, an optical fiber preform is assembled; finally, the assembled optical fiber preform is drawn in a high-temperature furnace in an optical fiber drawing tower, and the rare earth-doped phosphate glass single-mode optical fiber finally drawn is High Gain Fiber 6.

As shown in Figure 3, the central reflection wavelength of the slow axis (or fast axis) of the coupled output polarization-maintaining fiber grating 3 in this example is the laser output wavelength of 1548.92 nm. 0.15 nm, the central wavelength reflectance is 10-90%, and the central wavelength reflectance is 55% in this case. The central reflection wavelength of the fast axis (or slow axis) of the high-reflection polarization-maintaining fiber grating 4 coincides with the central reflection wavelength of the slow axis (or fast axis) of the coupled output polarization-maintaining fiber bragg grating 3, which is also the laser output wavelength of 1548.92 nm in this example. The 3dB reflection spectrum width is greater than 0.15 nm, and the central wavelength reflectance is greater than 90%. The coupled output polarization-maintaining fiber grating 3 and the high-reflection polarization-maintaining fiber grating 4 form front and rear cavity mirrors of the resonator. The central reflection wavelength of the low-reflection narrow-linewidth fiber Bragg grating 7 also coincides with the central reflection wavelength of the slow axis (or fast axis) of the polarization-maintaining fiber Bragg grating 3. In this example, the laser output wavelength is 1548.92 nm, and the reflection spectral width is less than 0.15 nm, the central wavelength reflectance is 10-50%, in this example it is 20%. The dichroic mirror 8 is coated with a thin film on the surface of the cavity mirror or directly coated with a thin film on the end face of the low-reflection narrow-linewidth fiber grating 7 after grinding and polishing. Greater than 95%, the transmittance to the pump wavelength of 980 nm is greater than 90%. The low-reflection narrow-linewidth fiber grating 7 and the dichromatic mirror 8 form a functional module with longitudinal mode selection and filtering functions. By designing the reflection spectral width of the slow axis (or fast axis) of the polarization-maintaining fiber grating 3, controlling the cavity length of the entire laser cavity, and adjusting the distance between the grid region of the low-reflection narrow linewidth fiber grating 7 and the dichromatic mirror 8 , can achieve only a single longitudinal mode laser output, and no mode hopping and mode competition. Before the laser power is saturated, as the pump power increases, the laser linewidth will continue to narrow, and finally an ultra-narrow linewidth polarization-maintaining output on the order of hundreds of Hz can be achieved. As long as the central reflection wavelength of the slow axis (or fast axis) of the polarization-maintaining fiber grating 3 is selected to be the design laser wavelength value, the ultra-narrow linewidth single-frequency fiber laser with the required wavelength can be realized. Among them, the coupled output polarization-maintaining fiber grating 3 and the high-reflection polarization-maintaining fiber grating 4 are connected by fusion splicing or end-to-face connection, and their fast and slow axes must be aligned; the high-reflection polarization-maintaining fiber grating 4 and the quarter-wave plate 5 are connected by Tight butt joint connection, the end face of the high-reflection polarization-maintaining fiber grating 4 must be ground and polished, and the direction of its fast axis (or slow axis) must form an angle of 45 ^° with the main axis of the quarter-wave plate 5, as shown in Figure 2 The connection between the high-gain fiber 6 and the low-reflection narrow-linewidth fiber Bragg grating 7 adopts fusion splicing or end surface grinding and polishing; the end surface of the low-reflection narrow-linewidth fiber Bragg grating 7 near the gate area 1-2 mm needs to be ground and polished to The low-reflection narrow-linewidth fiber grating 7 and the dichroic mirror 8 are connected by grinding and polishing the end face of the fiber and closely connecting the cavity mirror.

The pump light adopts a single-mode semiconductor laser pump source 1 forward pumping mode, and is input from the pump end of the polarization maintaining wavelength division multiplexer 2 and output through the coupling polarization maintaining fiber grating 3, high reflection polarization maintaining fiber grating 4, four The one-half-wave plate 5 is coupled to the core of the high-gain fiber 6 in the laser cavity. Under the continuous pumping of the pumping light source, the highly doped rare earth ions are pumped, the number of particles is reversed, and the signal light of stimulated emission is generated, which is reflected by the fast axis (or slow axis) of the high reflection polarization maintaining fiber grating. The linearly polarized light is rotated by the quarter-wave plate 5 into a right-handed circularly polarized light, which is amplified in the core of the high-gain fiber 6, and passed through a filter composed of a low-reflection narrow-linewidth fiber grating 7 and a total reflection dichroic mirror 8 After post-filtering and reflection, it becomes right-handed circularly polarized light opposite to the original propagation direction, and then returns to the high-gain fiber 6 core to be further amplified. At this time, the circularly polarized light forms a virtual ring light path in the cavity, and then passes through the The quarter-wave plate 5 becomes the slow axis (or fast axis) linearly polarized light, and the slow axis (or fast axis) of the polarization-maintaining fiber grating 3 is coupled to obtain transmission output and partial reflection, and a part of the light transmission output forms polarization maintaining Single-frequency laser, the other part of the reflected linearly polarized light undergoes the same polarization rotation as the linearly polarized light reflected by the fast axis (or slow axis) of the high-reflection polarization-maintaining fiber grating 4, amplification of right-handed circularly polarized light, filtering, and full The re-amplification of the reflected reversed and right-handed circularly polarized light also forms another virtual ring optical path in the cavity, and finally the polarization is rotated to the fast axis (or slow axis) linearly polarized light, and then the high reflection polarization maintaining fiber grating 4 The fast axis (or slow axis) is reflected again, thus forming a resonant cycle containing two virtual rings. Finally, the formed linearly polarized single-frequency laser is output through the coupling output of the polarization-maintaining fiber grating 3, and then input to the front end of the 1550nm polarization-maintaining fiber isolator 12 through the 980/1550nm polarization-maintaining wavelength division multiplexer The polarized fiber isolator 12 isolates the reflected or residual pump light and then outputs a stable, polarization-maintaining, single longitudinal mode fiber laser, while precisely controlling the temperature of the heat sink 9 is conducive to further realizing the stability of the laser wavelength and finally realizing A polarization-maintaining output of ultra-narrow linewidth and low-noise single-frequency fiber laser with an output wavelength of 1548.92 nm was obtained.

Example 2

The high-gain fiber 6 is a rare-earth-doped phosphate glass single-mode polarization-maintaining fiber, which is used as the gain medium of the fiber laser. The length can be selected according to the laser output power of the device and the reflection spectrum width of the coupled output polarization-maintaining fiber grating 3. In this example 1.0cm, generally 0.5~10cm. The high-gain fiber 6 core is uniformly doped with high-concentration luminescent ions of ytterbium, and the doping concentration of ytterbium ions is 7.5×10 ²⁰ ions/cm ³ , generally greater than 1×10 ¹⁹ ions/cm ³ . The core diameter of the high-gain fiber 6 is 8 μm, generally 1-10 μm. The polarization characteristics of the panda-eye structure are designed. The two panda eyes are arranged symmetrically and have the same size. The warp size is 16 μm, generally 10-20 μm is acceptable, and the cladding diameter is 125 μm, generally 125-400 μm is acceptable. The core matrix composition of the high-gain optical fiber 6 is phosphate glass, and its composition is: 70P ₂ O ₅ -8Al ₂ O ₃ -15BaO-4La ₂ O ₃ -3Nd ₂ O ₃ . Because the high-gain fiber 6 has polarization-maintaining characteristics, it has a higher extinction ratio and is insensitive to bending and torsional stress, which is beneficial to eliminate the noise and frequency drift caused by the environmental vibration of the single-frequency fiber laser, thereby further improving the laser signal. Noise ratio, the signal-to-noise ratio can reach 65 dB. The high-gain optical fiber 6 is made of a preform by the drilling method and the tube-and-rod method: first, the cladding glass is processed into a glass rod with a diameter of 35 mm, and then a circular hole with a diameter of 2.24 mm is drilled at the center of the glass rod. Then polish the inner surface of the glass round hole, and then drill two round holes with a diameter of 4.48mm at the design position of the panda eye, and polish the inner surface of the two round holes; secondly, process the core glass into a diameter of 2.24mm mm round rod, and then polish the outer surface of the round rod; again, process another multi-component glass material (its expansion coefficient must be greater than that of phosphate glass) into two circles with a diameter of 4.48mm Rod, polish the outer surface of the two round rods, then insert the core glass rod into the center hole in the cladding glass rod, and insert the two round rod glass rods into the panda eye holes in the cladding glass rod respectively, Assembled into an optical fiber preform; finally, the assembled optical fiber preform is drawn in a high-temperature furnace in an optical fiber drawing tower, and finally a rare earth-doped phosphate glass single-mode optical fiber with polarization-maintaining properties is drawn, which is a high-temperature optical fiber. Gain fiber 6.

In this example, the central reflection wavelength of the slow axis (or fast axis) of the polarization-maintaining fiber grating 3 is the laser output wavelength of 1064.00 nm, and its wavelength can be selected within the range of 1000-1120 nm. The rate is 10 to 90%, and the central wavelength reflectance in this case is 65%. The coupled output polarization-maintaining fiber grating 3 and the total reflection dichroic mirror 8 form the front and rear cavity mirrors of the resonator. The central reflection wavelength of the low-reflection narrow-linewidth fiber Bragg grating 7 also coincides with the central reflection wavelength of the slow axis (or fast axis) of the polarization-maintaining fiber Bragg grating 3. In this example, the laser output wavelength is 1064.00 nm, and the reflection spectral width is less than 0.10 nm, the central wavelength reflectance is 10 to 50%, in this example it is 15%. The dichromatic mirror 8 is coated with a thin film on the surface of the cavity mirror or directly coated with a thin film on the end face of the low-reflection narrow-linewidth fiber grating 7 after grinding and polishing. The material is generally MgO, and the reflectivity of the thin film to the laser signal wavelength is greater than 95. %, the transmittance to the pump wavelength of 976 nm is greater than 90%. The low-reflection narrow-linewidth fiber grating 7 and the dichromatic mirror 8 form a functional module with longitudinal mode selection and filtering functions. By designing the reflection spectral width of the slow axis (or fast axis) of the polarization-maintaining fiber grating 3, controlling the cavity length of the entire laser cavity, and adjusting the distance between the grid region of the low-reflection narrow linewidth fiber grating 7 and the dichromatic mirror 8 , can achieve only a single longitudinal mode laser output, and no mode hopping and mode competition. Before the laser power is saturated, as the pump power increases, the laser linewidth will continue to narrow, and finally an ultra-narrow linewidth polarization-maintaining output on the order of hundreds of Hz can be achieved. As long as the central reflection wavelength of the slow axis (or fast axis) of the polarization-maintaining fiber grating 3 is selected to be the design laser wavelength value, the ultra-narrow linewidth single-frequency fiber laser with the required wavelength can be realized. Among them, the coupling-out polarization-maintaining fiber grating 3 and the quarter-wave plate 5 are closely connected, the end face of the coupling-out polarization-maintaining fiber grating 3 must be ground and polished, and its fast axis (or slow axis) must be in the same direction as the quarter wave plate. One of the main axes of the wave plate 5 is at an angle of 45 ^° , as shown in Figure 2; the high-gain fiber 6 is aligned with the fast and slow axes of the coupled output polarization-maintaining fiber grating 3; the high-gain fiber 6 is aligned with the low-reflection narrow-linewidth fiber grating The connection of 7 adopts welding or end surface grinding and polishing butt joint; the end surface of the low-reflection narrow-linewidth fiber grating 7 close to the gate area 1-2 mm needs to be ground and polished so that the low-reflection narrow-linewidth fiber grating 7 and the dichroic mirror 8 The optical fiber end face is ground and polished and connected with the cavity mirror tightly.

The pumping light adopts 976nm single-mode semiconductor laser pumping source 1. The forward pumping mode is input from the pumping end of 980/1064nm polarization maintaining wavelength division multiplexer 2 and output via coupling polarization maintaining fiber grating 3, quarter A wave plate 5 is coupled into the core of a high gain fiber 6 in the laser cavity. Under the continuous pumping of the pumping light source, the highly doped rare earth ions are pumped, the number of particles is reversed, and the signal light of stimulated emission is generated, which is reflected by the fast axis (or slow axis) of the high reflection polarization maintaining fiber grating. The linearly polarized light is rotated by the quarter-wave plate 5 into a right-handed circularly polarized light, which is amplified in the core of the high-gain fiber 6, and passed through a filter composed of a low-reflection narrow-linewidth fiber grating 7 and a total reflection dichroic mirror 8 After post-filtering and reflection, it becomes right-handed circularly polarized light opposite to the original propagation direction, and then returns to the high-gain fiber 6 core to be further amplified. At this time, the circularly polarized light forms a virtual ring light path in the cavity, and then passes through the The quarter-wave plate 5 becomes the slow axis (or fast axis) linearly polarized light, and the slow axis (or fast axis) of the polarization-maintaining fiber grating 3 is coupled to obtain transmission output and partial reflection, and a part of the light transmission output forms polarization maintaining Single-frequency laser, the other part of the reflected linearly polarized light undergoes the same polarization rotation as the linearly polarized light reflected by the fast axis (or slow axis) of the high-reflection polarization-maintaining fiber grating 4, amplification of right-handed circularly polarized light, filtering, and full The re-amplification of the reflected reversed and right-handed circularly polarized light also forms another virtual ring optical path in the cavity, and finally the polarization is rotated to the fast axis (or slow axis) linearly polarized light, and then the high reflection polarization maintaining fiber grating 4 The fast axis (or slow axis) is reflected again, thus forming a resonant cycle containing two virtual rings. Finally, the formed linearly polarized single-frequency laser is output through the coupling output of the polarization-maintaining fiber grating 3, and then input to the front end of the 1064nm polarization-maintaining fiber isolator 12 through the 980/1064nm polarization-maintaining wavelength division The polarized fiber isolator 12 isolates the reflected or residual pump light and then outputs a stable, polarization-maintaining, single longitudinal mode fiber laser, while precisely controlling the temperature of the heat sink 9 is conducive to further realizing the stability of the laser wavelength and finally realizing A polarization-maintaining output of ultra-narrow linewidth and low-noise single-frequency fiber laser with an output wavelength of 1.06 μm was obtained.

Claims (10)

1. high-power single frequency optical fiber laser of super-narrow line width low noise; It is characterized in that comprising single mode semiconductor laser pumping source (1), protect partial wave division multiplexer (2), be coupled and export polarization-maintaining fiber grating (3), high reflection polarization-maintaining fiber grating (4), wave plate, high-gain optical fiber (6), low narrow linewidth fiber grating (7), dichroic mirror (8), heat sink (9), sealed air chamber (10) and fiber clamp (11) and the polarization maintaining optical fibre isolator (12) of reflecting; Said single mode semiconductor laser pumping source (1) is connected with the pumping input of protecting partial wave division multiplexer (2); The common port of protecting partial wave division multiplexer (2) is connected with coupling output polarization-maintaining fiber grating (3); Said coupling output polarization-maintaining fiber grating (3) all is scribed on same the polarization maintaining optical fibre with high reflection polarization-maintaining fiber grating (4) or is scribed on two polarization maintaining optical fibres respectively and fast and slow axis direction unanimity when being connected; High reflection polarization-maintaining fiber grating (4) is connected with high-gain optical fiber (6) via wave plate (5); High-gain optical fiber (6) is connected with low reflection narrow linewidth fiber grating (7), dichroic mirror (8) more in order; Wherein, Coupling output polarization-maintaining fiber grating (3), high reflection polarization-maintaining fiber grating (4), wave plate (5), high-gain optical fiber (6), low reflection narrow linewidth fiber grating (7) and dichroic mirror (8) are formed single-frequency laser resonant cavity (13) jointly; And fixed sealing is contained in heat sink (9) of automatic temperature-adjusting control; Go up with a sealed gas chamber (10) encapsulation whole single-frequency laser resonant cavity (13) in heat sink (9) simultaneously; The tail optical fiber of whole single-frequency laser resonant cavity (13) is fixed on the preceding end housing face of a sealed gas chamber (10) by a fiber clamp (11); The single-frequency laser that single-frequency laser resonant cavity (13) produces is exported via a polarization maintaining optical fibre isolator (12) via the signal end coupling output of protecting partial wave division multiplexer (2) again; The light emitting ionic of the fibre core doped with high concentration of said high-gain optical fiber (6), said light emitting ionic are one or more assembly in lanthanide ion, the transition metal ions, and said light emitting ionic doping content is greater than 1 * 10 ¹⁹Ions/cm ³And in its fibre core even doping.

2. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1 is characterized in that: said high-gain optical fiber (6) is common rare earth doping phosphoric acid salt single mode glass optical fiber or protects inclined to one side rare earth doping phosphoric acid salt single mode glass optical fiber.

3. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1 is characterized in that: said high-gain optical fiber (6) is rare earth doping phosphoric acid salt single mode glass optical fiber, and its fibre core composition is a phosphate glass, consists of 70P ₂O ₅-8Al ₂O ₃-15BaO-4La ₂O ₃-3Nd ₂O ₃The unit length gain of high-gain optical fiber (6) is greater than 1 dB/cm, and fiber lengths is 0.5～5cm.

4. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1 is characterized in that: said wave plate is quarter-wave plate or 3/4ths wave plates.

5. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1 is characterized in that said high-gain fibre core diameter is 3～10 μ m, and cladding diameter is 60～800 μ m.

6. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1 is characterized in that: the fast axle or the slow axis of said high reflection polarization-maintaining fiber grating (4) become 45 with the axis of wave plate (5) ^oAngle.

7. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1; It is characterized in that: the fast axle of said coupling output polarization-maintaining fiber grating (3) or slow axis foveal reflex wavelength are Wavelength matched with the slow axis or the fast axle foveal reflex of height reflection polarization-maintaining fiber grating (4); Promptly the fast axle foveal reflex wavelength reflectance spectrum of coupling output polarization-maintaining fiber grating (3) is positioned at the slow axis foveal reflex wavelength reflectance spectrum of high reflection polarization-maintaining fiber grating (4), or the slow axis foveal reflex wavelength reflectance spectrum of coupling output polarization-maintaining fiber grating (3) is positioned at the fast axle foveal reflex wavelength reflectance spectrum of high reflection polarization-maintaining fiber grating (4).

8. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1; It is characterized in that: the slow axis of said coupling output polarization-maintaining fiber grating (3) or fast axle foveal reflex wavelength are laser output wavelength; 3dB reflection spectrum width is less than 0.15 nm, and the centre wavelength reflectivity is 10～90%; The fast axle or the slow axis foveal reflex wavelength of said high reflection polarization-maintaining fiber grating (4) are laser output wavelength, and the 3dB reflectance spectrum is wider than 0.15 nm, and the centre wavelength reflectivity is greater than 90%; The foveal reflex wavelength of said low reflection narrow linewidth fiber grating (7) is a laser output wavelength, and 3dB reflection spectrum width is less than 0.15 nm, and the centre wavelength reflectivity is 10～50%.

9. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1; It is characterized in that: said dichroic mirror (8) is for directly plating film formation in a side end face of hanging down after reflecting narrow linewidth fiber grating (7) grinding and polishing; Said film to laser signal wavelength reflectivity greater than 95%, to the pumping wavelength transmissivity greater than 90%.

10. the high-power single frequency optical fiber laser of super-narrow line width low noise according to claim 1 is characterized in that: said lanthanide ion is Er ³⁺, Yb ³⁺, Tm ³⁺, Gd ³⁺, Tb ³⁺, Dy ³⁺, Ho ³⁺Or Lu ³⁺Said transition metal ions is Cu ²⁺, Co ²⁺, Cr ³⁺, Fe ²⁺Or Mn ²⁺

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