CN113948953B - Cascade pumped erbium doped laser - Google Patents

Abstract

The present disclosure provides a cascade pumped erbium doped laser comprising: the resonant cavity comprises a third reflecting mirror, a Q switch, an erbium-doped crystal and an output mirror which are sequentially arranged; the cascade gain unit is used for generating pumping light with a first wavelength, pumping the erbium-doped crystal and comprises pumping gain modules which are respectively symmetrically arranged at two sides of the erbium-doped crystal and form a set angle with the erbium-doped crystal; the doping concentration of the erbium-doped crystal is changed in a gradient and alternative mode, after being pumped by the pumping light with the first wavelength, the erbium-doped crystal generates light with the second wavelength in the resonant cavity, and the loss in the resonant cavity is regulated through the Q switch, so that the giant pulse laser with the second wavelength is obtained.

Description

Cascaded Pumped Erbium-Doped Lasers

technical field

The disclosure relates to laser technology and its application field, in particular to a cascaded pumped erbium-doped laser.

Background technique

In recent years, the laser beam output at the same time in the medium and long wavelengths has broad application prospects in the fields of laser target indication, laser radar, and photoelectric countermeasures. Photons with a wavelength of 2.8 μm undergo nonlinear frequency conversion technology to obtain idler light with a wavelength of 9-12 μm and signal light with a wavelength of 3.65-4.06 μm. The beams of these two bands correspond to the long-wave and medium-wave window regions of atmospheric propagation respectively. Therefore, obtaining a mid-infrared laser with a wavelength of 2.8 μm with high beam quality is the basis for obtaining simultaneous output laser beams of medium and long wavelengths.

At present, the following laser systems are commonly used to obtain lasers with a wavelength of 2.8 μm: (1) quantum cascade lasers; (2) electrically excited hydrogen fluoride; (3) overtone CO lasers; (4) nonlinear frequency conversion technology; (5) ) Erbium-doped laser. Among them, the fifth type has obvious advantages compared with the first four types:

(1) Compared with quantum cascade lasers, it can obtain high peak power through Q-switching technology;

(2) Compared with electro-excited hydrogen fluoride, it has the advantages of large gain of gain medium per unit length, compact structure, no chemical pollution, and long life;

(3) Compared with the over-frequency CO laser, it still has a higher light-to-light efficiency at room temperature;

(4) Compared with nonlinear frequency conversion technology, it has no nonlinear damage problem, and can learn from solid-state laser heat dissipation technology and resonant cavity design technology to obtain high-power laser output.

Therefore, erbium-doped lasers are expected to become high-quality pump sources for medium and long-wave integrated OPOs.

The integrated OPO with medium and long wavelengths has more urgent application requirements for laser sources with high beam quality, high peak power and high average power at 2.8 μm and its nearby bands. Erbium-doped lasers are one of the main ways to realize light sources in the 2.8μm and nearby bands. However, it is difficult for the erbium-doped laser realized by the current technical route to meet the beam characteristic requirements of high beam quality and high average power at the same time.

Contents of the invention

In view of this, the main purpose of the present disclosure is to provide a cascaded pumped Erbium-doped laser, in order to partially solve at least one of the above-mentioned technical problems.

In order to achieve the above object, as an aspect of the present disclosure, a cascaded pumped erbium-doped laser is provided, including: a resonant cavity, including a third mirror, a Q switch, an erbium-doped crystal, and an output mirror arranged in sequence; A coupled gain unit, used to generate pumping light with a first wavelength to pump the erbium-doped crystal, including pumping gains symmetrically arranged on both sides of the erbium-doped crystal and at a set angle with the erbium-doped crystal module; the doping concentration of the erbium-doped crystal changes alternately in a gradient, and after being pumped by the pumping light of the first wavelength, light of the second wavelength is generated in the resonant cavity, and the Q switch in the resonant cavity is adjusted Loss to obtain the second wavelength giant pulse laser.

According to an embodiment of the present disclosure, the pumping gain modules disposed on both sides of the erbium-doped crystal include the same number of alkali metal vapor laser gain modules, which are connected in series.

According to an embodiment of the present disclosure, the alkali metal vapor laser gain module includes: a semiconductor laser, used to output semiconductor laser; a beam shaping module, used to convert the semiconductor laser beam into a flat top beam with uniform energy distribution; off-axis The parabolic mirror is used to reflect and focus the flat-top light beam to increase the power density to obtain pump light; the alkali metal vapor chamber is filled with alkali metal simple substance and buffer gas, and is used to realize alkali metal under the action of the pump light. Atomic laser flipping of particles in the upper and lower energy levels; and a temperature-controlled furnace to control the temperature of the alkali metal vapor chamber.

According to an embodiment of the present disclosure, the cross-sectional diameters of the central aperture of the off-axis parabolic mirror and the alkali metal vapor chamber with a capillary structure in the beam propagation direction are both on the order of mm.

According to an embodiment of the present disclosure, the cascaded gain unit further includes a first reflector and a second reflector disposed respectively corresponding to the pump gain modules on both sides of the erbium-doped crystal, forming a symmetrical Π-shaped folded resonant cavity.

According to an embodiment of the present disclosure, the polarization states of the pump light of the first wavelength and the light of the second wavelength are perpendicular to each other, and polarization coupling is realized on the end face of the erbium-doped crystal.

According to an embodiment of the present disclosure, the resonant cavity is a double concave cavity; the Q switch is an acousto-optic Q switch made of tellurium dioxide.

According to an embodiment of the present disclosure, a plurality of photodetectors are correspondingly arranged in the pumping gain modules on both sides of the erbium-doped crystal for monitoring the gain generation of the alkali metal vapor laser gain module.

According to an embodiment of the present disclosure, the erbium-doped crystal is doped with erbium ions whose concentration gradient changes alternately in the YAP crystal to obtain an erbium-doped crystal; The doping concentration of the doped segment is higher than the doping concentration of the doped segment in the regions on both sides.

Based on the above technical solutions, it can be seen that the cascaded pumped erbium-doped laser of the present disclosure has at least one or a part of the following beneficial effects compared with the prior art:

(1) This disclosure adopts a polarization-coupled end-pumping structure, which can obtain laser beams with better beam quality and higher power than the side-pumping technology; the use of the light polarization of the pump light can also avoid end-pumping. The problem of film layer damage on the crystal end face.

(2) The cascaded pumping technology is adopted, so that the end pump can also achieve mode matching at the level of 40mm and above. The resonant cavity structure of the cascaded pumping can achieve a higher mode matching factor and obtain a medium beam with higher beam quality. infrared laser.

(3) Erbium-doped crystals with cross-changing doping concentration gradients have the characteristics of quasi-fiber heat dissipation, and doped regions and undoped regions appear alternately, which helps to solve the thermal problem in the pumping process.

Description of drawings

FIG. 1 is a schematic structural diagram of a cascaded pumped Erbium-doped laser according to an embodiment of the present disclosure.

FIG. 2 is a schematic diagram of the principle of polarization end-face pumping of the cascaded pumped Erbium-doped laser according to an embodiment of the present disclosure.

Detailed ways

Er ³⁺ ions have absorption peaks near the wavelengths of 650nm, 795nm, and 980nm in the visible-near-infrared region. When pumped by light sources of corresponding wavelengths, laser output at 2.8 μm and nearby wavelengths can be obtained through the cross-relaxation process. The output wavelength of rubidium vapor laser (Rb-DPAL) is 795nm, which is in the 795nm absorption band of Er ³⁺ ions. By virtue of the high beam quality of Rb-DPAL, the Er ³⁺ doped laser can be realized by adopting the end-face cascaded pump structure Mid-infrared laser output with high power and high beam quality.

In order to make the purpose, technical solutions and advantages of the present disclosure clearer, the present disclosure will be further described in detail below in conjunction with specific embodiments and experimental equipment, and with reference to the accompanying drawings.

The present disclosure provides a cascaded pumped erbium-doped laser, as shown in FIG. 1 and FIG. 2 , the cascaded pumped erbium-doped laser includes:

A resonant cavity, including a third mirror, a Q switch, an erbium-doped crystal, and an output mirror arranged in sequence;

The cascaded gain unit is used to generate the pumping light of the first wavelength to pump the erbium-doped crystal, including pumping gains symmetrically arranged on both sides of the erbium-doped crystal and at a set angle with the erbium-doped crystal module;

The doping concentration of the erbium-doped crystal changes alternately in a gradient, and after being pumped by the pumping light of the first wavelength, light of the second wavelength is generated in the resonant cavity, and the loss in the resonant cavity is adjusted through the Q switch to thereby Obtain a second wavelength giant pulse laser.

In the embodiment of the disclosure, as shown in Figure 1 and Figure 2, the solid line represents the pump light of the first wavelength obtained by the cascaded gain unit, in the embodiment of the disclosure, it is a laser with a wavelength of 795nm, which is used to The pump light for pumping the erbium-doped crystal, the dotted line represents the light of the second wavelength, which is the mid-infrared laser of 2.8 μm in the embodiment of the present disclosure.

In the embodiment of the present disclosure, the mirrors M1 , M2 , and M3 are all reflective mirrors, and the mirror M4 is an output mirror of the 2.8 μm laser.

In the embodiment of the present disclosure, the acousto-optic Q-switching technology is adopted, and the Q switch is an acousto-optic Q-switch, and the acousto-optic effect of the crystal is used. When the laser passes through the acousto-optic crystal, the loss in the resonant cavity is very large, and the oscillation cannot be started. , when the energy level particles on the working material accumulate to the maximum, the sound field in the acousto-optic medium is suddenly removed, the loss in the cavity is reduced, and the laser outputs a giant pulse, so that a high peak power laser output can be obtained.

In the embodiment of the present disclosure, the erbium-doped crystal is doped with erbium ions in YAP crystal to obtain Er:YAP crystal. YAP crystal belongs to the orthorhombic crystal system, which has good thermal conductivity, low phonon energy, good absorption characteristics at 795nm and high stimulated emission cross section, and it is easier to obtain high-efficiency laser oscillation. Erbium-doped crystals can output laser light with a wavelength of 2.8 μm and its vicinity after being pumped by light of the corresponding wavelength. The erbium-doped crystal is alternately arranged with doped segments and non-doped segments, and the doping concentration of the doped segments in the middle region is higher than that of the doped segments in the side regions. The reason why the erbium-doped crystals with periodic concentration changes alternately is to make the erbium-doped crystals and undoped erbium-doped crystals appear periodically, improve the effective heat dissipation cross section of the crystals, and help to weaken the thermal lens effect during laser operation.

In the embodiment of the present disclosure, the LD is a semiconductor laser, which outputs semiconductor laser light and is used to pump the alkali metal vapor chamber.

In an embodiment of the present disclosure, the alkali metal vapor chamber is a rubidium (Rb) vapor chamber. The chamber is filled with Rb metal element and buffer gas, and methane is used as the buffer gas. After the vapor chamber is pumped by the focused semiconductor laser, the particle flipping of the upper and lower energy levels of the Rb metal atomic laser can be realized.

In the embodiment of the present disclosure, the beam shaping module is used to transform the laser beam emitted by the LD into a flat-top spot with uniform energy distribution, which is irradiated onto the off-axis parabolic mirror after shaping. As shown in Figure 1, in the six Rb-DPAL gain modules, LD is used as the pump source. However, due to the structure of the LD, the divergence angle of the beam in the fast and slow axis direction is different, the energy distribution is uneven, and the energy density is low. , The beam quality is poor, which limits the practical application effect of LD. Therefore, the lens combination shown in Figure 1 is used to collimate the fast axis direction and slow axis direction of the laser beam emitted by the LD, and at the same time compress the spot size to realize the free space Beam shaping.

In the embodiment of the present disclosure, the off-axis parabolic mirror can reflect and focus the shaped semiconductor laser, so as to increase the power density of the pump light. The light beam reflected by the off-axis parabolic mirror is directed to the alkali metal vapor chamber and makes the focal point in the chamber.

The temperature-controlled furnace is used to control the temperature of the Rb metal vapor chamber, and is used to provide the working temperature conditions required for the alkali metal laser to work.

Among them, the LD, the beam shaping, the off-axis parabolic mirror and the alkali metal vapor chamber (the Rb vapor chamber in the embodiment of the present disclosure) jointly constitute the gain module of the alkali metal vapor laser. It can be seen from Figure 1 that the pump gain module includes a total of 6 cascaded alkali metal vapor laser gain modules. power density. In this project, through the cascaded gain unit composed of 6 cascaded alkali metal vapor laser gain modules, the Rb-DPAL beam with an average power of 100W (power density > 6kW/cm ² ) is realized in the cavity, and it still works in the linearity of Rb-DPAL The scope of work. The gain unit composed of 6 Rb-DPAL forms a Π-shaped folded resonant cavity, which is distributed at both ends of the Er:YAP crystal to form a symmetrical end-pumped structure. The resonant cavity of the pump gain module is a double concave cavity, so that the beam waist of the Rb-DPAL in the cavity is in the center of the Er:YAP crystal, which facilitates the realization of a high gain region of 2.8 μm.

The cross-sectional diameters of the central aperture of the off-axis parabolic mirror and the rubidium gas chamber of the capillary structure in the propagation direction of the Rb-DPAL beam (the pump light of the first wavelength) are both mm-level, which makes Rb-DPAL have the characteristics of free space transmission At the same time, it also has the effect of pinhole mode limiting, which makes Rb-DPAL have better beam quality, and can generate a Gaussian focused beam with a Rayleigh distance of 10cm in the resonator. The obtained Gaussian focused beam is used to pump the erbium-doped crystal, so that the number of particles in the upper and lower energy levels of the erbium ion 2.8 μm laser can be reversed to form a gain.

In the embodiment of the present disclosure, the resonant cavity is composed of the reflecting mirror M3 and the output coupling mirror M4, and under the action of the Q switch, the loss in the cavity is adjusted to realize the output of the giant pulse light of the second wavelength.

In the embodiment of the present disclosure, the photodetectors 1, 2 and 3 are completely the same, and are used to monitor the working status of the IV, V and VI alkali metal vapor laser gain modules respectively, and its function is to monitor these gain modules Gain generation situation.

In the embodiment of the present disclosure, as shown in FIG. 2 , by controlling the polarization states of the Rb-DPAL beam and the 2.8 μm beam, the two beams whose polarization states are perpendicular to each other are coupled at the end face of the crystal. The 2.8 μm light beam enters the gain medium with P polarization relative to the incident surface of YAP crystal, and the corresponding incident angle is θ _B1 , and the Rb-DPAL enters the gain medium with S polarization relative to the incident surface of YAP crystal, and the corresponding incident angle is θ _B2 . This coupling method takes advantage of the characteristics of the Rb-DPAL polarization output, and only needs to coat the incident surface of the Rb-DPAL laser with an anti-reflective coating, which avoids the problem of damage to the end film layer under high average power and high peak power operation, and realizes Rb - Efficient coupling of DPAL beam and 2.8μm beam.

In the embodiment of the present disclosure, as shown in FIG. 2 , the resonant cavity of the Er-doped laser adopts a double-cavity structure, and at the same time, an acousto-optic Q switch is placed in the cavity to realize a 2.8 μm beam with high peak power. The Q-switch employed in this disclosure is fabricated from tellurium dioxide (TeO ₂ ). The laser transmittance of the acousto-optic Q crystal made of _TeO2 is greater than 99%, and the intracavity loss is modulated by the acousto-optic effect of _TeO2 to obtain a giant pulse of 2.8 μm.

Since the erbium-doped crystal has a self-terminating effect during continuous pumping, the present disclosure adopts a long-pulse pumping method and optimizes the pulse width of the pump light through the experimental results; refer to the finite element analysis of Er-doped crystals with different heat accumulations Analyze the results, optimize the heat dissipation structure of the Er-doped crystal, and obtain the 2.8μm pulsed light output stably for a long time.

The alternative technical solutions of the present disclosure are as follows:

(1) The photodetector can also measure the three Rb-DPAL gain modules on the left to monitor the working status.

(2) The buffer gas of the alkali metal vapor chamber can also use other components, such as helium, ethane, etc.

(3) The Rb-DPAL gain modules are not necessarily required to be 6 in series, but 4 or 8 can be connected in series.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present disclosure in detail. It should be understood that the above descriptions are only specific embodiments of the present disclosure, and are not intended to limit the present disclosure. Within the spirit and principles of the present disclosure, any modifications, equivalent replacements, improvements, etc., shall be included in the protection scope of the present disclosure.

Claims (6)

1. a cascaded pumped erbium-doped laser, characterized in that, comprising: A resonant cavity, including a third mirror, a Q switch, an erbium-doped crystal, and an output mirror arranged in sequence; The cascaded gain unit is used to generate pumping light of the first wavelength, and pump the erbium-doped crystal, including pumping devices symmetrically arranged on both sides of the erbium-doped crystal and at a set angle to the erbium-doped crystal. Gain module, the pump gain module includes the same number of alkali metal vapor laser gain modules connected in series; The erbium-doped crystal is doped with erbium ions whose concentration gradient changes alternately in the YAP crystal, and the erbium-doped crystal is alternately arranged with doped sections and non-doped sections, and the doping concentration of the doped section in the middle region is higher than that in the regions on both sides The doping concentration of the doped segment, after being pumped by the pumping light of the first wavelength, generates light of the second wavelength in the resonant cavity, and the pumping light of the first wavelength and the light of the second wavelength are polarized The states are perpendicular to each other, polarization coupling is realized on the end face of the erbium-doped crystal, and the loss in the resonant cavity is adjusted through the Q switch to obtain the giant pulse laser of the second wavelength. 2. the erbium-doped laser of cascade pumping according to claim 1, is characterized in that, described alkali metal vapor laser gain module, comprises: Semiconductor laser, used to output semiconductor laser; A beam shaping module is used to convert the semiconductor laser beam into a flat top beam with uniform energy distribution; An off-axis parabolic mirror is used to reflect and focus the flat-hat light beam to increase the power density and obtain pump light; An alkali metal vapor chamber, filled with an alkali metal element and a buffer gas, is used to realize the particle flipping of the upper and lower energy levels of the alkali metal atomic laser under the action of the pump light; and Temperature-controlled furnace for controlling the temperature of the alkali metal vapor chamber. 3. the erbium-doped laser device of cascade pumping according to claim 2, is characterized in that, the central aperture of described off-axis parabolic mirror and the cross-sectional diameter of capillary structure alkali metal vapor chamber in beam propagation direction are both mm level. 4. the erbium-doped laser of cascade pumping according to claim 1, is characterized in that, described cascaded gain unit also comprises the first reflection mirror that the pumping gain module setting corresponding to described erbium-doped crystal both sides respectively and the second reflector to form a symmetrical Π-shaped folded resonant cavity. 5 . The cascaded pumped Erbium-doped laser according to claim 1 , wherein the resonant cavity is a double concave cavity; the Q switch is an acousto-optic Q switch made of tellurium dioxide. 6. The erbium-doped laser of cascaded pumping according to claim 1, wherein a plurality of photodetectors are correspondingly arranged in the pumping gain modules on both sides of the erbium-doped crystal for monitoring the alkali Gain generation of a metal vapor laser gain block.

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