CN105356216A - All-fiber narrow-linewidth single frequency green laser - Google Patents

Abstract

The invention provides an all-fiber narrow-linewidth single-frequency green laser, which includes a pump source, a broadband fiber grating, a back-end band collimator of the broadband fiber grating, a nonlinear crystal, and a front-end band collimator of a high-gain optical fiber doped with ytterbium ions. Straightener, ytterbium-doped high-gain fiber, narrow-band polarization-maintaining fiber grating, polarization-maintaining filter, polarization-maintaining optical isolator, micro precision temperature-controlled furnace and heat sink. The invention adopts short linear resonant cavity and inner cavity frequency doubling structure of high-gain optical fiber, uses centimeter-level high-gain optical fiber as laser working material, and selects the reflection peak corresponding to the slow axis of polarization-maintaining fiber grating to overlap with the reflection peak of broadband fiber grating , and then generate a narrow-linewidth linearly polarized fundamental-frequency laser; a centimeter-length nonlinear crystal is placed in a fundamental-frequency laser resonator operating at a high power density to perform resonance-enhanced frequency doubling. The present invention can achieve high power, narrow linewidth, High conversion efficiency, high stability single-frequency green laser output, the device structure is very compact, simple and easy to operate.

Description

An all-fiber narrow-linewidth single-frequency green laser

technical field

The invention relates to the technical field of lasers, in particular to an all-fiber narrow-linewidth single-frequency green laser with an inner cavity frequency-multiplied laser linewidth up to the order of kHz.

Background technique

Single-frequency laser refers to operating in a single longitudinal mode state, which has many advantages such as narrow output spectral linewidth, long coherence length, and low noise. In particular, single-frequency green lasers have very broad application prospects in the fields of optical parametric oscillation (OPO), holographic imaging, biomedicine, atom cooling and trapping. However, in the absence of a gain medium in the green band that can directly lase the laser, frequency doubling is the most powerful means to obtain a short-wavelength green light source, that is, directly using the second harmonic generation (SHG) green light of the 1.0 μm band laser laser.

The current research work on single-frequency green lasers is focused on: based on traditional solid-state lasers with relatively wide spectral widths or single-frequency fiber lasers (linewidth 0.1~10MHz) as the fundamental frequency light source, using an external cavity single-pass frequency doubling structure, but The harmonic conversion efficiency of the mode is generally relatively low. For example: Samanta et al. used a high-power continuous single-frequency fiber laser and a lithium tantalate crystal to obtain a single-frequency green light output with a power of 9.64W and a conversion efficiency of 32.7% [Opt.Lett., 2009, 34(10)]. In addition, extra-cavity resonance enhancement structures can be used. For example: Ou et al. used a 10mm-length KTP crystal and an extracavity resonance enhancement structure to achieve a continuous green light output with a power of 560mW and a conversion efficiency of 85% [Opt.Lett., 1992, 17(9)]. In addition, an intracavity resonant frequency doubling structure can also be used, that is, a nonlinear crystal is placed in the laser resonator to obtain high-efficiency frequency doubling green light. In these resonance enhancement methods, although high output power and conversion efficiency can be achieved, the device structure is complex, the system stability is poor, the cost is high, and it is not fully optical fiber.

The related patents are: (1) Shanxi University applied for the patent of single-frequency intracavity frequency-doubling laser in 2008 [Publication No.: CN101355224A], using laser crystal and cavity mirror to form a ring resonator, and placing the frequency-doubling crystal in the cavity , to realize the intracavity frequency doubled single-frequency laser output, but the single-frequency laser required by it does not have the characteristics of all-fiber, narrow linewidth, and the structure is more complicated. (2) Hefei Hengrui Optoelectronics Technology Co., Ltd. applied for the patent of frequency-doubled green fiber laser in 2014 [Publication No.: CN104242039A], using semiconductor saturable absorption mirror SESAM, double-clad ytterbium-doped fiber, lithium niobate crystal and The broadband fiber grating constitutes a laser resonator, which realizes the output of small frequency-doubled green fiber laser, but the required green laser does not have narrow linewidth and single-frequency output characteristics.

Contents of the invention

The object of the present invention is to overcome the problems existing in the prior art and provide an all-fiber narrow-linewidth single-frequency green laser that generates a kHz linewidth. The technical problem to be solved is to overcome the disadvantages of the existing green lasers such as wide output line width, low conversion efficiency and complex structure.

The invention utilizes the high doping and gain characteristics of the ytterbium-doped high-gain optical fiber, and adopts a short linear cavity and an inner cavity frequency doubling structure. Utilizing the frequency selection function of the slow axis of the broadband fiber grating and the narrow-band polarization-maintaining fiber grating, under the continuous pumping of the pump source, a narrow linewidth linearly polarized fundamental frequency laser of the order of kHz is generated in the laser resonator; The linear crystal is placed in the laser resonator to increase the fundamental frequency optical power density in the frequency doubling crystal, so that the fundamental frequency laser generation process and the nonlinear second harmonic generation process exist in the resonator simultaneously, thereby obtaining high-efficiency frequency doubling green light. By carefully fabricating the reflection spectral width of the narrow-band polarization-maintaining fiber grating, adjusting the crystal coupling parameters and controlling the length of the laser resonator, an all-fiber, compact narrow-linewidth single longitudinal mode (single-frequency) green laser output can be realized.

In order to achieve the above object, the specific technical solutions adopted in the present invention are as follows.

An all-fiber narrow-linewidth single-frequency green laser, which is composed of a pump source, a broadband fiber grating, a collimator or a burning ball (optional) at the back end of the broadband fiber grating, a nonlinear crystal, and ytterbium-doped ions The front end of the high-gain fiber is equipped with a collimator or a burning ball (optional), a high-gain fiber doped with ytterbium ions, a narrow-band polarization-maintaining fiber grating, a polarization-maintaining filter, a polarization-maintaining optical isolator, a micro precision temperature-controlled furnace, and a heat sink. . The pigtail of the pumping source is connected to the broadband fiber grating, the rear end of the broadband fiber grating with a collimator or burning ball (optional) is connected to one end of the nonlinear crystal, and the other end of the nonlinear crystal is connected to The front end of the ytterbium-doped high-gain fiber is connected with a collimator or a burning ball (optional). The input ends of the filters are connected, the output end of the polarization maintaining filter is connected with the input end of the polarization maintaining optical isolator, and the output end of the polarization maintaining optical isolator is used as the output port of the single-frequency green laser.

Further, the pumping method of the all-fiber narrow-linewidth single-frequency green laser adopts forward pumping, backward pumping, bidirectional pumping or a combination thereof.

Further, the pumping source is a semiconductor laser, fiber laser or other solid-state laser, with single transverse mode or multi-transverse mode output, and its output state is continuous or pulsed. The pumping wavelength range is 800-1200nm, and the pumping power is greater than 20mW. The specific wavelength is selected according to the rare earth luminescent ion type and energy level structure.

Further, the ytterbium-doped high-gain optical fiber has a gain per unit length greater than 1dB/cm at a wavelength of 1.0 μm; its specific use length is determined according to the laser output power, line width, and reflection spectrum of the narrow-band fiber grating. Choose, the general use length is 0.5~30cm.

Further, the broadband fiber grating has a high transmittance to the pump light wavelength, and the transmittance is between 85% and 99.9%; it is highly reflective to both the 1.0 μm band of the fundamental frequency laser and the green light wavelength, and the reflectance is 80 ~99.9%.

The rear end of the broadband fiber grating is equipped with a collimator or a burning ball (optional), that is, a collimator with a short working distance (0.5~50cm) is directly made on the end face of the grating fiber, or the end face of the grating fiber is directly burned In the shape of a microsphere, it acts as a focusing lens.

Further, the nonlinear crystal is periodically poled crystal lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ); or birefringent crystal LBO, BBO, BIBO, etc. The number of nonlinear crystals used is one for single crystal frequency doubling, or more than one (combination selection between the same or different types of crystals) for multiple crystal cascaded frequency doubling. The length of the crystal is 0.5~10cm.

Further, the front end of the ytterbium-doped high-gain fiber is provided with a collimator or a burning ball (optional), that is, a collimator with a short working distance (0.5-50cm) is directly fabricated on the end face of the high-gain fiber, or directly The end face of the high-gain fiber is fired into a microsphere shape, which acts as a focusing lens.

Further, the narrow-band polarization-maintaining fiber grating is partially transmissive (selective reflection) to both the 1.0 μm band of the fundamental frequency laser and the wavelength of green light, and the reflectivity range at the central wavelength is between 20% and 90%. In between, it also serves as the rear cavity mirror and output coupling component of the single-frequency laser resonator.

Further, the polarization-maintaining filter completely passes the wavelength of green light, and filters out light other than the 1.0 μm wavelength band of the fundamental frequency laser and the wavelength of the pump light, that is, only single-frequency green light can pass through.

Further, the miniature precision temperature-controlled furnace is a thermoelectric cooler TEC, a heating resistance wire or other precision temperature adjustment devices, through precise temperature control, the nonlinear crystal placed therein can work at the best temperature matching point.

Further, the broadband fiber grating, the ytterbium-doped high-gain fiber and the narrow-band polarization-maintaining fiber grating are fixed and packaged together on a metal heat sink to effectively manage the heat and ensure the stability of the single-frequency laser output power and working wavelength. reliability.

Compared with the prior art, the advantages and beneficial effects of the present invention are: the present invention uses centimeter-scale ytterbium ion-doped high-gain optical fibers, centimeter-length nonlinear crystals as laser working medium and inner cavity frequency doubling crystals. The fundamental-frequency laser resonator consists of high-gain fiber, broadband fiber grating, and narrow-band polarization-maintaining fiber grating to form a short linear cavity DBR structure. The broadband fiber grating and narrow-band polarization-maintaining fiber grating constitute the front and rear mirrors of the short F-P cavity. Under the continuous pumping of the pump source, the rare earth luminescent ions in the high-gain fiber core doped with ytterbium ions undergo population inversion, and stimulated emission signal light (fundamental frequency light) is generated. Under the feedback of the resonant cavity mirror , the signal light oscillates back and forth multiple times and is amplified multiple times. The reflection peak corresponding to the slow axis of the polarization-maintaining fiber grating is selected to overlap with the reflection peak of the broadband fiber grating to realize linearly polarized laser light, that is, to generate linearly polarized single-frequency fundamental-frequency laser light. Since the length of the resonant cavity is only a few centimeters, the longitudinal mode interval in the cavity can reach GHz. When the 3dB reflection spectrum of the narrow-band polarization-maintaining fiber grating is narrowed to 0.08nm, only a single longitudinal mode fundamental frequency laser operation can be realized in the laser cavity. With the continuous increase of the pump source power, the linewidth of the single-frequency laser is continuously narrowed, and finally a single-frequency fundamental-frequency laser with a linewidth of the order of kHz can be produced. By placing the nonlinear crystal in the resonant cavity, the power density of the fundamental-frequency light in the cavity is relatively high. Due to the effect of the cavity mirror, the single-frequency fundamental-frequency laser and the single-frequency green light pass through the crystal multiple times to form a resonance-enhanced frequency-doubled green light. output, a higher harmonic conversion efficiency can be obtained. Adjust the temperature of the micro-precision temperature-controlled furnace to make the crystal work at the best temperature matching point, and finally realize the high-power, narrow-linewidth, and high-stability single-frequency green laser output of all fiber optics. The device has a compact structure, is simple and easy to operate, and is easy to control and operate.

Description of drawings

Fig. 1 is the back-end belt collimator structure schematic diagram of broadband fiber grating described in the example;

Fig. 2 is a schematic diagram of the back-end burning ball structure of the broadband fiber grating described in the example;

Fig. 3 is a schematic diagram of the principle of the all-fiber narrow-linewidth single-frequency green laser in the embodiment of the example.

In the figure: 1—pump source, 2—broadband fiber grating, 3—back end of broadband fiber grating with collimator or burning ball (optional), 4—nonlinear crystal, 5—Ytterbium-doped high-gain fiber Front end with collimator or burning ball (optional), 6—Ytterbium-doped high-gain fiber, 7—narrowband polarization-maintaining fiber grating, 8—polarization-maintaining filter, 9-polarization-maintaining optical isolator, 10—miniature precision temperature control Furnace, 11—Heat sink.

detailed description

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 scope expressed in the examples.

As shown in Figure 1, the rear end of the broadband fiber grating used in the present invention has a collimator 3, which is to grind the end face of the grating fiber into an angle of 8 degrees, and coat it with a collimator that acts on the 1.0 μm wave band of the fundamental frequency laser and the green light wavelength. The anti-reflection coating is then packaged together with a microlens to make a collimator, and its working distance is 0.5~50cm to ensure the operation requirements of the short resonant cavity structure.

As shown in Figure 2, the rear end of the broadband optical fiber grating used in the present invention burns the ball 3, namely the end face of the optical fiber grating is fired into a microsphere shape by heating sources such as hydrogen oxyflame and electrode discharge, and by controlling the shape of the microsphere, The size and other parameters play the role of focusing lens.

As shown in Figure 3, the all-fiber narrow-linewidth single-frequency green laser includes: pump source 1, broadband fiber grating 2, the rear end of broadband fiber grating with collimator 3 or burning ball (optional), nonlinear crystal 4. Front end of Yb-doped high-gain fiber with collimator 5 or burning ball (optional), Yb-doped high-gain fiber 6, narrowband polarization-maintaining fiber grating 7, polarization-maintaining filter 8, polarization-maintaining isolator 9, Micro precision temperature control furnace 10, heat sink 11. The pumping source 1 is a single-mode semiconductor laser, and its output state is continuous output. The pump wavelength range is 980nm, and the pump power is 1W. The gain coefficient per unit length of the ytterbium-doped high-gain fiber 6 at the fundamental frequency laser wavelength is 10dB/cm. The length used is selected according to the power of the fundamental frequency laser, the size of the line width, and the reflection spectrum of the narrow-band fiber grating. In this example, the length used is 1cm. Among them, the broadband fiber grating 2 has a high transmittance to the pump light wavelength, and the transmittance is 99%; the reflectance to both the 1.0 μm band of the fundamental frequency laser and the green light wavelength is 99%. Wherein the rear end of the broadband fiber grating has a collimator 3, and the working distance is 2cm. Among them, the nonlinear crystal 4 is a periodically poled lithium niobate crystal (LiNbO ₃ ), the quantity used is 1 piece, and the length used is 1 cm. The front end of the ytterbium-doped high-gain fiber has a collimator 5, and the working distance is 2 cm. Among them, the reflectivity of the narrow-band polarization-maintaining fiber grating 7 to the 1.0 μm band of the fundamental frequency laser and the wavelength center of the green light is 65%. Wherein, the broadband fiber grating 2, the high-gain fiber 6 and the narrow-band polarization-maintaining fiber grating 7 are fixed and packaged together in a copper block for heat dissipation.

The ytterbium-doped high-gain fiber 6 is used as the gain medium of the laser, and the front and rear cavity mirrors of the short F-P cavity structure are composed of the broadband fiber grating 2 and the narrow-band polarization-maintaining fiber grating 7 . The central reflection wavelength of the slow axis of the narrowband polarization-maintaining fiber grating 7 is within the gain spectrum of the laser working medium, and the reflection peak corresponding to the slow axis overlaps with the reflection peak of the broadband fiber grating 2 . By precisely controlling the central wavelength of the narrow-band polarization-maintaining fiber grating 7, the 3dB reflection spectrum, the length of the gate area, the length of the nonlinear crystal 4 and other parameters, the length of the entire laser resonator cavity is controlled below 3.5cm, so that the narrow-band polarization-maintaining fiber grating The reflection spectral width of 7 is less than 0.08nm, that is, the longitudinal mode spacing is close to the bandwidth of the fiber grating. It can be obtained that there is only a single longitudinal mode fundamental frequency laser oscillation and operation in the laser cavity, and there is no mode hopping, mode competition and other phenomena. By placing the periodically poled lithium niobate crystal 4 in the resonant cavity, the fundamental frequency laser generation process and the nonlinear second harmonic generation process exist in the resonant cavity at the same time. Due to the effect of the cavity mirror, the single frequency fundamental frequency laser and the single frequency The green light passes through the crystal multiple times to form a resonance-enhanced frequency-doubling green light output.

The present invention uses a short F-P linear resonant cavity structure based on the ytterbium-doped high-gain optical fiber 6, combined with quasi-phase matching technology to perform internal cavity resonance frequency multiplication on the periodically poled lithium niobate crystal 4. The pumping method adopts forward pumping, that is, the pumping source 1 injects the pumping light, which is respectively coupled into the fiber core of the high-gain optical fiber 6 in the laser resonator through the broadband fiber grating 2 and the nonlinear crystal 4, so that the ytterbium The number of ions undergoes particle population inversion to generate a laser signal of stimulated radiation (fundamental frequency light). Under the feedback of the front and rear cavity mirrors, the fundamental frequency light oscillates back and forth multiple times and is effectively amplified. With the continuous increase of pump power, The linewidth of the single-frequency fundamental-frequency laser will be continuously narrowed, and finally a linearly polarized single-frequency fundamental-frequency laser with a linewidth of the order of kHz can be produced. Place the periodically poled lithium niobate crystal 4 in the resonant cavity, and the fundamental frequency laser and single-frequency green light will pass through the crystal multiple times. By adjusting the temperature of the precision temperature-controlled furnace to 39°C, higher harmonics can be obtained. Conversion efficiency and narrow linewidth single-frequency green laser output. Based on the above method, high power, narrow linewidth (kHz order), high polarization extinction ratio (greater than 20dB), and high stability all-fiber single-frequency green laser output can be realized, and the device structure is simple, compact and practical.

Claims (10)

1. An all-fiber narrow-linewidth single-frequency green laser, which includes a pump source (1), a broadband fiber grating (2), a collimator or a burning ball at the back end of the broadband fiber grating (3), a nonlinear Crystal (4), Ytterbium-doped high-gain optical fiber front end with collimator or burning ball (5), Ytterbium-doped high-gain optical fiber (6), narrow-band polarization-maintaining fiber grating (7), polarization-maintaining filter (8) , a polarization-maintaining optical isolator (9), a miniature precision temperature-controlled furnace (10), and a heat sink (11); wherein the pigtail of the pump source is connected to the front end of the broadband fiber grating, and the rear end of the broadband fiber grating has The collimator or burning ball is connected to one end of the nonlinear crystal, the other end of the nonlinear crystal is connected to the front end of the Yb-doped high-gain fiber with a collimator or burning ball, and the back end of the Y-doped high-gain fiber is connected to the narrowband One end of the polarization-maintaining fiber grating is connected, the other end of the narrow-band polarization-maintaining fiber grating is connected to the input end of the polarization-maintaining filter, the output end of the polarization-maintaining filter is connected to the input end of the polarization-maintaining optical isolator, and the polarization-maintaining optical isolator The output port of the single-frequency green laser is used as the output port of the single-frequency green laser; the micro precision temperature control furnace enables the nonlinear crystal placed therein to work at the best temperature matching point through precise temperature control; the broadband fiber grating, doped Ytterbium ion high-gain fiber and narrow-band polarization-maintaining fiber grating are fixed and packaged on the heat sink together. 2. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the pump source (1) is a semiconductor laser or a fiber laser, with single transverse mode or multi-transverse mode output, Its output state is continuous or pulse type. 3. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the pumping wavelength range of the pumping source (1) is 800-1200nm, and the pumping power is greater than 20mW; The above pump source (1) adopts forward pumping, backward pumping, bidirectional pumping or a combination thereof. 4. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the broadband fiber grating (2) has a high transmittance to the pump light wavelength, and the transmittance is 85% to 99.9%; Both the fundamental frequency laser 1.0μm band and the green wavelength are highly reflective, and the reflectivity is 80~99.9%. 5. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the rear end of the broadband fiber grating has a collimator or a burning ball (3), which is on the end face of the grating fiber Directly make a collimator with a short working distance of 0.5~50cm, or directly burn the end face of the grating fiber into a microsphere shape, which acts as a focusing lens. 6. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the nonlinear crystal (4) is a periodically poled crystal lithium niobate (LiNbO ₃ ), lithium tantalate ( LiTaO ₃ ); or one of birefringent crystal lithium triborate (LBO), barium metaborate (BBO), bismuth borate (BIBO), the number of nonlinear crystals used is one single crystal or more than two A crystal, the length of use is 0.5~10cm. 7. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the front end of the ytterbium-doped high-gain fiber has a collimator or a burning ball (5), which is The end face of the optical fiber is directly made into a collimator with a short working distance of 0.5~50cm, or the end face of the high-gain optical fiber is directly fired into a microsphere shape, which acts as a focusing lens. 8. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the gain per unit length at a wavelength of 1.0 μm is greater than 1 dB/cm in the high-gain fiber (6) doped with ytterbium ions ;Using length is 0.5~30cm. 9. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the narrow-band polarization-maintaining fiber grating (7) is suitable for both the 1.0 μm band of the fundamental frequency laser and the green wavelength Partial transmission is selective reflection, and the reflectance at the center wavelength of the two is 20-90%. 10. The all-fiber narrow-linewidth single-frequency green laser according to claim 1, characterized in that: the polarization-maintaining filter (8) completely passes the wavelength of green light, and the 1.0 μm band of the fundamental frequency laser and the pump Light outside the wavelength of the pump light is filtered out.