CN114204396A - All-solid-state blue-green laser based on thulium-doped ion crystal - Google Patents

Abstract

An all-solid-state blue-green laser based on a thulium-doped ion crystal includes an excitation source, a gain medium, a nonlinear frequency-doubling crystal, a cavity mirror system and a tuning element. The laser output direction of the excitation source is followed by a plane mirror, a gain medium, a tuning element, and a plano-concave cavity mirror; if it is at a certain angle with the previous optical path, a nonlinear frequency-doubling crystal and a plane-concave cavity mirror are placed in order in the direction of the reflected light path of the plano-concave cavity mirror. A concave cavity mirror; the plane mirror, the plano-concave cavity mirror and the plano-concave cavity mirror together form a folded resonant cavity. The pump light emitted by the excitation source is injected into the gain medium through collimation and focusing, and the output 1.85-2.1μm mid-infrared laser is converted by a nonlinear frequency-doubling crystal to output a blue-green laser in the 462.5-525nm band. The laser has the advantages of rich emission wavelength, tunable laser wavelength, high output power, high stability, simple structure and small size.

Description

All-solid-state blue-green laser based on thulium ion crystal

technical field

The invention relates to a laser, in particular to an all-solid-state blue-green laser based on a thulium-doped ion crystal.

Background technique

The blue-green laser in the 462-525nm band is in the low-loss window of seawater, and has important applications in underwater detection, transmission, sensing and communication. In particular, the blue light at the wavelength of 486.1 nm forms the Fraunhofer dark line in the solar radiation spectrum due to the absorption of hydrogen atoms. Using this wavelength for laser transmission or communication can effectively avoid environmental noise and improve the signal-to-noise ratio. Blue laser has the advantages of short wavelength and small spot, which can significantly increase the information capacity in optical storage, and has become a hot spot in high-density optical storage research. In addition, blue-green laser also corresponds to the absorption band of oxygenated hemoglobin, and also has important application prospects in laser intravascular irradiation therapy instruments. At present, the existing technology mainly uses semiconductor technology and nonlinear optical effects to obtain blue-green laser, such as the use of the sum frequency between the wavelengths of neodymium ions (LaserPhysics, 21, 2011), the up-conversion of thulium-doped crystals (CN109873292A), the thulium-doped Optical fiber quadruple obtained (CN103311782A). However, the blue-green lasers obtained by the existing technology often have shortcomings such as low efficiency and complex systems, and it is difficult to meet the requirements of the modern information society for high-efficiency, high-integration blue-green lasers. Therefore, there is an urgent need for a method that can obtain high-efficiency, high-power, high-integration blue-green lasers to meet the needs of high-efficiency underwater information transmission, topographic detection optical storage, and medical treatment.

The transition of thulium ion energy level ³ F ₄ - ³ H ₆ can emit laser light around 2 μm, and combined with nonlinear optical crystals PPLN, LBO, KTP, etc., the output of blue-green laser can be obtained. At present, thulium-doped crystals have obtained 100-watt output power in the 2μm band (Appl Phys B (2009) 94:195–198, Laser Phys. 29 (2019) 115004 (4pp)), and then using frequency doubling technology, it can be obtained Blue-green laser with higher conversion efficiency and higher output power. Compared with thulium-doped fibers, thulium-doped crystals are rich in varieties and have various mid-infrared emission wavelengths, which is conducive to the realization of blue-green laser emission of various wavelengths after frequency doubling. Compared with optical fibers, the emission spectrum of thulium-doped crystals is wider, which is conducive to the tunability of laser output wavelengths and the generation of ultrafast lasers. In addition, the crystal also has excellent thermal properties and damage threshold, and has weak nonlinear effects under high-power pumping, and has significant advantages in generating large-energy, short-pulse, and high-peak power lasers.

SUMMARY OF THE INVENTION

In view of the deficiencies of the prior art, the present invention provides an all-solid-state blue-green laser based on a thulium ion crystal, which uses the thulium ion crystal to obtain mid-infrared fundamental frequency light around 1.85-2.1 μm, and then uses nonlinear frequency doubling. The technology obtains blue-green laser output around 462.5-525nm, which has the advantages of tunable wavelength, simple structure, high output power, high conversion efficiency and high stability.

The technical solution of the present invention is as follows:

An all-solid-state blue-green laser based on thulium-doped ion crystals, capable of outputting 462.5-525 nm blue-green lasers, is characterized in that it includes an excitation source, a gain medium, a nonlinear frequency-doubling crystal, a cavity mirror system and a tuning element. The cavity mirror system described above includes a plane mirror, a plano-concave cavity mirror, and a plano-concave posterior cavity mirror;

Along the laser output direction of the excitation source, the plane mirror, the gain medium, the tuning element, and the plano-concave cavity mirror are sequentially arranged; those that are at a certain angle with the previous optical path are arranged in order in the direction of the reflected light path of the plano-concave cavity mirror. the non-linear frequency doubling crystal and the plano-concave back cavity mirror; the plane mirror, the plano-concave cavity mirror and the plano-concave back cavity mirror together form a folded resonant cavity;

The plane mirror, the plano-concave cavity mirror and the plano-concave back cavity mirror are respectively optimized for their working wavelengths. The cavity mirror is coated with a dielectric film of 1.85-2.1μm high reflection, 0.925-1.05μm high reflection, 793nm high reflection and 462.5-525nm high transmission; ～1.05μm high reflection and 462.5～525nm high reflection dielectric film.

The excitation source is a laser diode laser with an output wavelength near 793 nm;

The gain medium is a thulium ion crystal;

The nonlinear frequency doubling crystal can be a combination of a first-order frequency-doubling crystal and a second-order frequency-doubling crystal, or a direct output method of a single nonlinear crystal;

When the nonlinear frequency doubling crystal is selected in the combination of a first-order frequency doubling crystal and a second-order frequency doubling crystal, the first-order frequency-doubling crystal is selected from KTP, PPLN or LBO crystal, and the second-order frequency doubling crystal is selected from KTP, PPLN or LBO crystal. The frequency doubling crystal is selected from KTP, LBO or BBO crystal;

When the nonlinear frequency doubling crystal is selected as the direct output mode of a single nonlinear crystal, the nonlinear frequency doubling crystal is a periodically polarized lithium niobate crystal with a multi-period optical superlattice structure. There are various polarization periods in the crystal, which can realize the quadruple frequency of the laser in the same crystal.

The all-solid-state blue-green laser based on the thulium-doped ion crystal of the present invention includes an excitation source, a gain medium, a nonlinear frequency-doubling crystal, a cavity mirror system and a tuning element, and firstly generates a mid-infrared laser output with a wavelength of around 1.85-2.1 μm, and then It is converted into blue-green laser output with a wavelength of 462.5-525 nm by nonlinear frequency doubling technology.

The gain medium is a thulium-doped ion crystal selected from thulium-doped yttrium lithium fluoride, lutetium lithium fluoride, gadolinium lithium fluoride, lanthanum fluoride, lead fluoride, LiCaAlF ₆ , LiSrAlF ₆ , yttrium aluminum garnet, Yttrium aluminate, yttrium calcium aluminate, yttrium oxide, lutetium oxide, scandium oxide and other crystals.

The thulium-doped ion crystal is processed into a rod with a length of 1-30 mm in the light-transmitting direction according to specific use requirements during operation, and the light-transmitting surface is cylindrical, rectangular or other forms.

A tuning element is added in the fundamental frequency laser cavity, so as to realize the output wavelength tunable blue-green laser. Preferably, the tuning element can be selected from the following schemes: prism, grating, birefringent filter or Fabry-Perot etalon.

The 1.85-2.1μm laser output from the gain medium is converted into a blue-green laser with a wavelength of 462.5-525nm through a nonlinear frequency doubling crystal. According to the choice of the nonlinear frequency doubling crystal, there are two preferred implementation methods as follows:

According to the present invention, the first method is to select two nonlinear frequency doubling crystals respectively, and firstly convert the 1.85-2.1 μm fundamental frequency laser into 0.925-1.05 μm first-order frequency-doubling light through the first-order frequency-doubling crystal, and then pass the second-order frequency doubling crystal. The frequency doubling crystal converts it into 462.5～525nm blue-green laser.

In method 1, KTP, PPLN or LBO crystal is selected for the first frequency doubling crystal, and KTP, LBO or BBO crystal is selected for the second frequency doubling crystal.

In the first method, the cavity mirror system includes M ₁ mirror, M ₂ mirror and M ₃ mirror, and the coating is optimized for its working wavelength. M ₁ mirror is coated with 1.85-2.1μm high reflection and 793nm high transmission dielectric film, M ₂ mirror is coated with 1.85~2.1μm high reflection, 0.925~1.05μm high reflection, 793nm high reflection and 462.5~525nm high transmission Dielectric film, M ₃ mirror is coated with 1.85~2.1μm high reflection, 0.925~1.05μm high reflection and 462.5~525nm high reflection dielectric film.

In method 1, the laser workflow is:

[1] The pump light emitted by the excitation source is incident on the gain medium through the M ₁ mirror to generate a fundamental frequency laser of 1.85-2.1 μm;

[2] The 1.85-2.1μm fundamental frequency laser is reflected into the first-order frequency doubling crystal through the M ₂ mirror to generate 0.925-1.05μm first-order frequency-doubling light;

[3] 0.925～1.05μm first-order frequency-doubling light is incident into the second-order frequency-doubling crystal, thereby generating 462.5～525nm blue-green laser light;

[4] The 462.5-525nm blue-green laser is reflected by the M ₂ mirror, and then output through the M ₂ mirror.

According to the present invention, the second method is to select a single nonlinear frequency doubling crystal, which can directly convert the 1.85-2.1 μm fundamental frequency laser output by the gain medium into a blue-green laser with a wavelength of 462.5-525 nm.

In the second method, the nonlinear frequency doubling crystal is a periodically polarized lithium niobate crystal with a multi-period optical superlattice structure, and the same crystal has multiple polarization periods, which can realize the laser beam laser in the same crystal. Quadruple frequency.

In method 2, the cavity mirror system includes M ₁ mirror, M ₂ mirror and M ₃ mirror, and its coating optimization is the same as that in method 1.

In Method 2, the laser workflow is:

[1] The pump light emitted by the excitation source is incident on the gain medium through the M ₁ mirror to generate a fundamental frequency laser of 1.85-2.1 μm;

[2] The 1.85～2.1μm laser is reflected into the nonlinear frequency doubling crystal through the M ₂ mirror to generate 462.5～525nm blue-green laser;

[3] After 462.5～525nm blue-green light oscillates in the cavity composed of M ₂ and M ₃ , the final laser is output through the M ₂ mirror.

In the above-mentioned all-solid-state blue-green laser based on thulium ion crystal, the refrigeration system is water cycle refrigeration, air-cooled or semiconductor refrigeration, which can effectively dissipate the heat generated during the operation of the laser and ensure normal operation.

In the above-mentioned all-solid-state blue-green laser based on thulium ion crystal, the control system is a current control and stabilization system, which can ensure the stability of the output power of the pump laser, thereby improving the working stability of the blue-green laser.

Compared with the existing blue-green laser, the present invention has the following beneficial effects:

1. The variety of thulium-doped crystals is rich, and the mid-infrared emission wavelengths are diverse, which is conducive to the realization of blue-green laser emission of various wavelengths after frequency doubling;

2. The thulium-doped crystal has a wide emission spectrum, and with the tuning element, the output wavelength of the laser can be adjusted within a certain range;

3. Thulium-doped crystals have excellent thermal properties and damage thresholds, and have significant advantages in generating large-energy, short-pulse, high-peak power, and high-repetition lasers;

4. The crystal can achieve high-concentration thulium ion doping, which can effectively reduce the size of the laser gain medium used, which is beneficial to the miniaturized application of the laser.

Description of drawings

1 is a structural diagram of Embodiment 1 of an all-solid-state blue-green laser based on thulium-doped ion crystals of the present invention

2 is a structural diagram of Embodiment 2 of an all-solid-state blue-green laser based on thulium-doped ion crystals of the present invention

3 is a structural diagram of Embodiment 3 of an all-solid-state blue-green laser based on a thulium ion crystal according to the present invention.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention is specifically described below with reference to the accompanying drawings and embodiments. The embodiments herein are only used to explain the present invention, and are not intended to limit the present invention.

Example 1

A thulium-doped scandium oxide crystal is used as the gain medium, and a 500-525 nm tunable blue-green laser with frequency doubling of two nonlinear frequency doubling crystals is used.

The structure is shown in Figure 1, which consists of excitation source 1, gain medium 2, nonlinear frequency doubling crystal 3, cavity mirror system, tuning element M4, refrigeration system, and control system in the order of the optical path.

Excitation source 1 is a laser diode with an emission wavelength of 793 nm. The gain medium 2 is a Tm:Sc ₂ O ₃ crystal with a doping concentration of 5 at.%, which is cut into a 3x3x5 mm square laser rod along the light passing direction c, and the end faces are polished. The nonlinear frequency doubling crystal 3 is divided into a primary frequency doubling crystal 31KTP and a secondary frequency doubling crystal 32LBO.

The laser cavity adopts a folded cavity design and uses three cavity mirrors with coatings. The plane mirror M ₁ is coated with a dielectric film of 2.0～2.1μm high reflection and 793nm high transmission, and the plano-concave cavity mirror M ₂ is coated with 2.0～2.1μm high reflection, 1.0～1.05μm high reflection, 793nm high reflection and 500～525nm high reflection For the transmitted dielectric film, the plano-concave rear cavity mirror M ₃ is coated with a dielectric film of 2.0-2.1μm high reflection, 1.0-1.05μm high reflection and 500-525nm high reflection.

_The tuning element M4 adopts a quartz birefringence filter, which is inserted before the plano-concave cavity mirror _M2 mirror at the Brewster angle, and the wavelength of the fundamental frequency light can be tuned by rotating the angle of the quartz plate, thereby realizing the final output blue-green. Tuning of the laser wavelength.

The excitation source, gain medium, nonlinear frequency doubling crystal, cavity mirror and tuning element are placed on the same collimation axis after calibration. The pump light emitted by the excitation source is collimated and focused and then incident on the gain medium through the M ₁ mirror to generate a 2.0-2.1 μm fundamental frequency laser. The wavelength of the fundamental frequency laser can be tuned by the tuning element M ₄ . The fundamental frequency laser is reflected into the first-order frequency-doubling crystal through the output M ₂ mirror to generate the first-order frequency-doubling light of 1.0-1.05μm. The first-order frequency-doubling light is incident on the second-order frequency-doubling crystal, thereby generating 500-525nm blue-green laser light. Finally, the 500-525nm blue-green laser is reflected by the M ₃ mirror and output through the plano-concave cavity mirror M ₂ .

Example 2

A thulium-doped lanthanum fluoride crystal is used as the gain medium, and a 462.5-500 nm tunable blue-green laser with frequency doubling of a periodically polarized lithium niobate crystal with a multi-period optical superlattice structure is used.

The structure is shown in Figure 2, which consists of excitation source 1, gain medium 2, nonlinear frequency doubling crystal 3, cavity mirror system, tuning element, refrigeration system, and control system in the order of the optical path.

Excitation source 1 is a laser diode with an emission wavelength of 793 nm. The gain medium 2 is a Tm:LaF ₃ crystal with a doping concentration of 4 at.%, which is cut into a 3x3x8 mm square laser rod along the light-passing direction c, and the end faces are polished. The nonlinear frequency doubling crystal is a lithium niobate crystal designed with a special polarization period. It has two polarization periods in the same crystal, which can realize the four-frequency doubling of the 1.85-2.0μm laser in the same crystal. Output blue-green laser.

The laser cavity adopts a folded cavity design and uses three cavity mirrors with coatings. The plane mirror M ₁ is coated with a dielectric film with high reflection of 1.85-2.0μm and high transmission of 793nm, and the plano-concave cavity mirror M ₂ is coated with high reflection of 1.85-2.0μm, high reflection of 925-1000nm, high reflection of 793nm and high transmission of 462.5-500nm. After passing through the dielectric film, the plano-concave rear cavity mirror M ₃ is coated with a dielectric film of 1.95-2.0 μm high reflection, 925-1000 nm high reflection and 462.5-500 nm high reflection.

Tuning element M ₄ adopts quartz birefringence filter, which is inserted in front of M ₂ mirror at the Brewster angle, and the wavelength of fundamental frequency light can be tuned by rotating the angle of the quartz plate, and then the final output blue-green laser wavelength can be tuned. .

The excitation source, gain medium, nonlinear frequency doubling crystal and cavity mirror are placed on the same collimation axis after calibration. The pump light emitted by the excitation source is collimated and focused and then incident on the gain medium through the M ₁ mirror to generate a 1.85-2.0 μm fundamental frequency laser. After the fundamental frequency laser is reflected by the M ₂ mirror, it is incident on the nonlinear frequency doubling crystal, and the frequency is changed in the periodically polarized lithium niobate crystal, and the blue-green laser of 462.5-500 nm is output. Finally, the 462.5-500nm blue-green laser is reflected by the M ₃ mirror and output through the plano-concave cavity mirror M ₂ .

The excitation source, gain medium and nonlinear frequency-doubling crystal of the above blue-green laser generating device Embodiments 1 and 2 are all effectively cooled by the water circulation system to dissipate the heat generated during the working process. The excitation source regulates the current and output power through the control system.

Example 3

Using thulium-doped yttrium calcium aluminate crystal as gain medium 2, a 486.1nm blue laser with frequency doubling of two nonlinear frequency doubling crystals is used.

The structure is shown in Figure 3, which consists of excitation source 1, gain medium 2, nonlinear frequency doubling crystal 3, cavity mirror system, refrigeration system, and control system in the order of the optical path.

Excitation source 1 is a laser diode with an emission wavelength of 793 nm. The gain medium 2 is a Tm:CaYAlO ₄ crystal with a doping concentration of 4 at.%, which is cut into a 3x3x6 mm square laser rod along the light-passing direction a, and the end faces are polished. The nonlinear frequency doubling crystal 3 is divided into a first-level frequency-doubling crystal 31PPLN and a second-level frequency-doubling crystal 32LBO.

The laser cavity adopts a folded cavity design and uses three cavity mirrors with coatings. The plane mirror M ₁ is coated with a dielectric film with high reflection at 1944.4nm and high transmission at 793 nm, and the plano-concave cavity mirror M ₂ is coated with a dielectric film with high reflection at 1944.4 nm, high reflection at 972.2 nm, high reflection at 793 nm and high transmission at 486.1 nm. The concave cavity mirror M ₃ is coated with dielectric films with high reflection at 1944.4nm, high reflection at 972.2nm and high reflection at 486.1nm.

The excitation source, gain medium, nonlinear frequency doubling crystal, cavity mirror and tuning element are placed on the same collimation axis after calibration. The pump light emitted by the excitation source is collimated and focused and then incident on the gain medium through the M ₁ mirror to generate a 1944.4 nm fundamental frequency laser. The fundamental frequency laser is reflected into the first-order frequency-doubling crystal through the output M ₂ mirror to generate the first-order frequency-doubling light of 972.2nm. The first-order frequency-doubling light is incident on the second-order frequency-doubling crystal, thereby producing a 486.1 nm blue light laser. Finally, the 486.1nm blue laser is reflected by the plano-concave rear cavity mirror M ₃ and output through the plano-concave cavity mirror M ₂ .

Experiments show that the present invention has the following advantages:

1. The variety of thulium-doped crystals is rich, and the mid-infrared emission wavelengths are diverse, which is conducive to the realization of blue-green laser emission of various wavelengths after frequency doubling;

2. The thulium-doped crystal has a wide emission spectrum, and with the tuning element, the output wavelength of the laser can be adjusted within a certain range;

3. Thulium-doped crystals have excellent thermal properties and damage thresholds, and have significant advantages in generating large-energy, short-pulse, high-peak power, and high-repetition lasers;

4. The crystal can achieve high-concentration thulium ion doping, which can effectively reduce the size of the laser gain medium used, which is beneficial to the miniaturized application of the laser.

The laser has the advantages of rich emission wavelength, tunable laser wavelength, high output power, high stability, simple structure and small size.

Claims (6)

1. The utility model provides an all solid-state bluish-green laser based on thulium ion crystal of mixing, can output 462.5 ~ 525nm bluish-green laser, its characterized in that: comprises an excitation source (1), a gain medium (2) and a non-excitation sourceLinear frequency doubling crystal (3), cavity mirror system and tuning element (M)₄) The cavity mirror system comprises a plane mirror (M)₁) Plano-concave mirror (M)₂) Plano-concave rear mirror (M)₃)；

The plane mirrors (M) are arranged along the laser output direction of the excitation source (1) in sequence₁) Gain medium (2), tuning element (M)₄) Plano-concave mirror (M)₂) (ii) a At an angle to the previous light path in said plano-concave mirror (M)₂) The non-linear frequency doubling crystal (3) and a plano-concave rear cavity mirror (M) are arranged in sequence in the direction of the reflection light path₃) (ii) a The plane mirror (M)₁) Plano-concave mirror (M)₂) Plano-concave rear mirror (M)₃) The folded resonant cavity is formed;

the plane mirror (M)₁) Plano-concave mirror (M)₂) Plano-concave rear mirror (M)₃) Respectively coated at their operating wavelengths, said flat mirrors (M)₁) Plating a dielectric film with high reflection of 1.85-2.1 mu m and high transmission of 793 nm; the plano-concave mirror (M)₂) Plating a dielectric film with high reflection of 1.85-2.1 μm, high reflection of 0.925-1.05 μm, high reflection of 793nm and high transmission of 462.5-525 nm; the plano-concave rear cavity mirror (M)₃) Plating a dielectric film with high reflection of 1.85-2.1 μm, high reflection of 0.925-1.05 μm and high reflection of 462.5-525 nm.

2. The thulium-doped ion crystal based all-solid-state blue-green laser according to claim 1, wherein the excitation source (1) is a laser diode laser with an output wavelength around 793 nm;

3. the thulium-doped ionic crystal-based all-solid-state blue-green laser according to claim 1, wherein the gain medium (2) is a thulium-doped ionic crystal;

4. the thulium-doped ion crystal-based all-solid-state blue-green laser according to claim 1, wherein the nonlinear frequency doubling crystal (3) can be selected from a combination of a first-order frequency doubling crystal and a second-order frequency doubling crystal, or a direct output mode of a single nonlinear crystal;

5. the thulium-doped crystal based all-solid-state blue-green laser according to claim 4, wherein when the nonlinear frequency doubling crystal (3) is selected to adopt a combination of a first frequency doubling crystal and a second frequency doubling crystal, the first frequency doubling crystal is selected from KTP, PPLN or LBO crystal, and the second frequency doubling crystal is selected from KTP, LBO or BBO crystal;

6. the all-solid-state blue-green laser based on thulium-doped ion crystal as claimed in claim 4, wherein when the nonlinear frequency doubling crystal (3) selects the mode of direct output of single nonlinear crystal, the nonlinear frequency doubling crystal is a periodically polarized lithium niobate crystal with a multi-period optical superlattice structure, and has multiple polarization periods in the same crystal, thereby achieving the quadruple frequency of laser in the same crystal.

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