CN1295823C - A laser rod thermal lens effect compensation apparatus and compensation method - Google Patents

Abstract

The present invention discloses a laser rod thermal lens effect compensating device and a compensating method. The present invention comprises a concave lens and a convex lens, wherein the concave lens and the convex lens are arranged behind a laser rod, the center of the concave lens is a fixed distance away from the end surface of the laser rod, and the convex lens is provided with a position moving mechanism; an optical axis of the concave lens, an optical axis of the convex lens and an optical axis of the laser rod are coincident. The focal distance of a thermal lens when the laser rod achieves thermal equilibrium at different pumping mean power is determined by calculation or measurement; a regulating distance between the convex lens and the concave lens is calculated so that the convex lens moves by the corresponding regulating distance to the direction in which the distance between the convex lens and the concave lens is reduced; in this way, the phenomenon that the beam divergence angle of the laser device is enlarged because of a thermal lens effect generated by the laser rod at any pumping power can be fully compensated, the damage to working laser material because of a thermal self-focusing phenomenon can be fully avoided, and laser output power can not be greatly influenced; besides, the single-pulse running of the laser device can not be influenced.

Description

A laser rod thermal lens effect compensation device and compensation method

technical field

The invention relates to a thermal lens effect of a laser rod in a solid-state laser, in particular to a compensation device and a compensation method for the thermal lens effect of a laser rod.

Background technique

Existing solid-state lasers absorb pump light energy during operation, except for a small part that is output in the form of laser, most of the energy is converted into heat and deposited in the laser working material to cause temperature rise, and the pumping and heat conduction are repeated continuously Under the action, the temperature gradient in the working substance increases continuously until the thermal equilibrium is reached when the heating power is equal to the heat taken away by the cooling liquid. The existence of a temperature gradient in the working substance makes the working substance with a completely uniform refractive index become a lens-like medium, and the light beam is focused after passing through the working substance, commonly known as thermal self-focusing. In the case of symmetrical pumping, the cylindrical laser rod becomes a spherical lens, and the distance from its main plane to the focal point is called the focal length of the thermal lens. Thermal self-focusing not only increases the divergence angle of the laser beam rapidly, but more seriously, it will generate a real focus inside the working material, which will cause laser damage inside the material. In order to reduce the impact of thermal lens effect on laser devices, it is widely used to grind the end surface of the working material into a concave surface for compensation, but it can only compensate for the specific thermal lens effect under a specific pump power. For this reason, people try to find additional additional compensating device to realize the compensation of thermal lens effect, disclose a kind of novel thermal stable cavity in No. A compensation lens driven by a stepping motor is arranged between the medium (equivalent to the laser rod in the present invention) and the output mirror. The compensation lens is driven by the stepping motor to move back and forth to adjust the position of the compensation lens according to the power of the solid laser medium. The applied optical principle of the compensation device is to keep the distance between the laser medium and the compensation lens as the sum of the focal length of the thermal lens and the focal length of the compensation lens, even if the thermal lens and the compensation lens form a Keplerian telescope, thereby achieving compensation The thermal lens effect of the solid laser medium makes the unstable cavity into a stable cavity, and the high-power laser output has better stability and beam quality. But above-mentioned compensating device also has following problem: the compensating method of this compensating device is to make the distance of rod and lens equal to " sum of thermal lens focal length and lens focal length " promptly thermal lens and lens form Keplerian telescope, because the thermal effect of rod is only It is impossible to fully compensate the single lens; the compensation device can only be used when the focal length of the thermal lens is very small, and the "sum of the focal length of the thermal lens and the focal length of the lens" can be used for the actual laser device. When the focal length of the thermal lens is large (such as several meters) , "the sum of the focal length of the thermal lens and the focal length of the lens" is quite large, which is difficult to be used in practical laser devices; in addition, the laser cannot perform a single operation after adding a lens in the cavity.

Contents of the invention

The technical problem to be solved by the present invention is to provide a laser rod thermal lens effect compensation device and compensation method in view of the above-mentioned current state of the art, which can completely realize the thermal self-focusing effect of the laser rod in the solid-state laser under different pump powers. compensate.

The technical solution adopted by the present invention to solve the above technical problems is: a laser rod thermal lens effect compensation device, which includes a concave lens and a convex lens arranged behind the laser rod, the focal length of the concave lens is smaller than that of the convex lens Focal length value, the center of the concave lens and the end face of the laser rod are a fixed distance, the convex lens is provided with a position movement mechanism, the optical axis of the concave lens, the optical axis of the convex lens and the The optical axes of the laser rods mentioned above coincide with each other.

The maximum distance that the convex lens moves is the focal length of the convex lens minus the focal length of the concave lens.

The concave lens is a thin spherical concave lens, and the convex lens is a thin spherical convex lens.

Another technical solution adopted by the present invention to solve the above technical problems is: a laser rod thermal lens effect compensation method, including the following steps, step 1: a concave lens and a convex lens are arranged behind the output end face of the laser rod, so that the The focal length value of the concave lens is smaller than the focal length value of the convex lens, by adjusting the optical axis of the concave lens and the convex lens to coincide with the optical axis of the laser rod, so that the center of the concave lens and the The end face of the laser rod is a fixed distance, and the distance between the concave lens and the convex lens is adjusted to be the focal length of the convex lens minus the focal length of the concave lens; Step 2: determine that the laser rod is at Thermal lens focal length when thermal equilibrium is reached under different pump average powers; Step 3: Calculate the adjustment distance between the convex lens and the concave lens according to the following formula: $\mathrm{\&Delta;d}=-\frac{{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="C20041002570500081.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}^2}{f-f_1-d_1-L/2\mathrm{no}},$ Among them: Δd- is the adjustment distance of the convex lens movement, n- is the refractive index of the laser rod, f- is the focal length of the thermal lens, f ₁ - is the focal length of the concave lens, d ₁ - is the distance between the concave lens and the output end face of the laser rod, L- is the length of the laser rod, "-" means to adjust along the direction of reducing the distance between the two lenses, and move the convex lens to the direction of reducing the distance between the two lenses by the adjustment distance; step 4: move during the operation of the laser rod The convex lens is dynamically corrected.

The focal length of the thermal lens in the step 2 is obtained by calculation according to the following formula, $f=\frac{2\mathrm{\&kappa;}}{(\mathrm{\&beta;}+P)\&times;\mathrm{wxya}},$ Among them: f-thermal lens focal length, κ-thermal conductivity coefficient of laser material, β-refractive index temperature coefficient of laser material, χ-heating efficiency of laser device, that is, the ratio of heating power to pump average power, Wp-unit volume laser The average pumping power of the working substance, L-the length of the laser rod, P-the stress thermo-optic coefficient of the working substance of the laser. The focal length of the thermal lens in the second step can also be obtained by measurement.

The dynamic correction in step 4 is to move the convex lens so that the far-field light spot output by the laser is close to the near-field light spot.

Compared with the prior art, the present invention has the advantage that it can completely compensate for the phenomenon that the laser beam divergence angle increases caused by the thermal lens effect produced by the laser rod at any pump power, and can completely avoid the thermal self-focusing phenomenon. The damage caused by the laser working material will not have a large impact on the laser output power; when the laser is working in a single pulse, the compensation device is equivalent to a Galileo beam expander telescope, so it will not affect the operation of the laser; by choosing a Concave and convex lenses with different focal length parameters can not be affected by the focal length of the thermal lens, and can fully compensate the thermal lens effect. The compensation device of the present invention can be arranged inside and outside the optical resonant cavity of the laser, and can also be arranged behind the laser rod as an amplifier.

Description of drawings

FIG. 1 is a schematic structural view of Embodiment 1 of the present invention;

Fig. 2 is the structural representation of compensation lens of the present invention;

Fig. 3 is a schematic structural diagram of Embodiment 2 of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Embodiment 1: As shown in Figures 1 and 2, a laser rod thermal lens effect compensation device includes a laser rod 1, and a total reflection mirror 2 and a laser output mirror 3 are respectively arranged at both ends of the laser rod 1 to form the laser rod Optical resonant cavity, a concave lens 4 and a convex lens 5 are arranged in the optical resonant cavity behind the laser rod 1, the center of the concave lens 4 and the end face of the laser rod 1 are a fixed distance d ₁ , the convex lens 5 is provided with a position moving mechanism, and the concave lens 4 is a thin spherical concave lens with a focal length _f1 , the convex lens 5 is a thin spherical convex lens with a focal length _f2 , the focal length value _f1 of the concave lens 4 is smaller than the focal length value _f2 of the convex lens 5, and the maximum distance that the convex lens 5 moves is the focal length of the convex lens 5 The focal length f ₁ of the concave lens 4 is subtracted from f ₂ , and the optical axis of the concave lens 4 , the optical axis of the convex lens 5 and the optical axis of the laser rod 1 coincide.

Embodiment 2: As shown in Figure 3, a laser rod thermal lens effect compensation device includes a laser rod 1, and a total reflection mirror 2 and a laser output mirror 3 are respectively arranged at both ends of the laser rod 1 to form an optical resonant cavity of the laser , a concave lens 4 and a convex lens 5 are arranged outside the optical resonant cavity behind the laser rod 1, the center of the concave lens 4 is a fixed distance d ₁ from the end face of the laser rod 1, the convex lens 5 is provided with a position moving mechanism, and the concave lens 4 has Thin spherical concave lens with focal length _f1 , convex lens 5 is a thin spherical convex lens with focal length _f2 , the focal length value _f1 of concave lens 4 is less than the focal length value f2 of convex lens ₅ , the maximum distance that convex lens 5 moves is the focal length f2 of convex lens ₅ minus The focal length f ₁ of the concave lens 4, the optical axis of the concave lens 4, the optical axis of the convex lens 5 and the optical axis of the laser rod 1 coincide, and a second laser rod 1 ' is arranged behind the convex lens 5 as an amplifier. A second concave lens 4' and a second convex lens 5' are arranged at the back of the rod 1', the center of the second concave lens 4' is a fixed distance d ₁ ' from the end face of the second laser rod 1', and the second convex lens 5' A position moving mechanism is provided, the second concave lens 4' is a thin spherical concave lens with a focal length f ₁ ', the second convex lens 5' is a thin spherical convex lens with a focal length f ₂ ', and the focal length of the second concave lens 4' is f ₁ ' in The focal length value f ₂ ′ of the second convex lens 5 ′, the maximum distance that the second convex lens 5 ′ moves is the focal length f ₂ ′ of the second convex lens 5 ′ minus the focal length f ₁ ′ of the second concave lens 4 ′, the second concave lens 4 ′ The optical axis of the second convex lens 5' and the optical axis of the second laser rod 1' coincide with each other.

Embodiment 3: A laser rod thermal lens effect compensation method, including the following steps, step 1: a concave lens 4 and a convex lens 5 are arranged behind the output end face of the laser rod 1, and adjusted by a similar four-dimensional optical adjustment mount The device (not shown in the figure) makes the optical axes of the concave lens 4 and the convex lens 5 coincide with the optical axis of the laser rod 1, so that the center of the concave lens 4 and the output end face of the laser rod 1 are a fixed distance d ₁ , and adjust the concave lens 4 and the convex lens The distance between the 5 is the focal length f ₂ of the convex lens 5 minus the focal length f ₁ of the concave lens 4; Step 2: according to the following formula $f=\frac{2\mathrm{\&kappa;}}{(\mathrm{\&beta;}+P)\&times;\mathrm{wxya}}$ , determine the thermal lens focal length f when the laser rod 1 reaches thermal equilibrium under different average pump powers by calculation, where: f-thermal lens focal length, κ-material thermal conductivity of laser rod 1, β-material refractive index of laser rod 1 Temperature coefficient, χ-the heating efficiency of the laser device, that is, the ratio of the heating power to the average pumping power, Wp-the average pumping power of the laser working substance per unit volume, L-the length of the laser rod 1, P-the stress heat of the laser working substance Light coefficient; Step 3: According to the following formula $\mathrm{\&Delta;d}=-\frac{{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="C20041002570500081.png"\; state="\{\{state\}\}">f</figure-callout>}}_1}^2}{f-f_1-d_1-L/2\mathrm{no}},$ Calculate the adjustment distance Δd between the convex lens 5 and the concave lens 4, and make the convex lens 5 move the adjustment distance Δd in the direction of reducing the distance between the two lenses, where: Δd- is the adjustment distance for the convex lens 5 to move, n- is the refraction of the laser rod 1 f- is the focal length of the thermal lens, f ₁ - is the focal length of the concave lens 4, d ₁ - is the distance between the center of the concave lens 4 and the output end face of the laser rod 1, L- is the length of the laser rod 1, "-" means along The distance between the two lenses is reduced and the direction is adjusted; Step 4: Equipped with a computer, an intelligent laser power supply and a stepping motor, during the operation of the laser rod, the far-field spot and the near-field spot of the laser output are approached by moving the convex lens 5 to perform dynamic correction .

We take Figure 3 as an example to implement a specific scheme: laser rod 1 is a Nd ³⁺ glass laser rod with a size of φ8×200 and a volume of 10cm ³ , and the amplifier set behind the optical resonant cavity of laser rod 1 uses The second laser rod 1' is a Nd ³⁺ glass laser rod with a size of φ12×300 and a volume of 34 cm ³ . After the optical resonant cavity of the laser rod 1, a first concave lens 4 and a first convex lens 5 are arranged. After the amplifier The second concave lens 4' and the second convex lens 5' are arranged, and the concentration of the glass laser rod is N3122 phosphate laser glass with a concentration of 2×10 ²⁰ /cm ³ , the heating efficiency χ=8%, and the temperature coefficient of refractive index β=-4.3 ×10 ^-6 /°C, the stress thermo-optic coefficient is P=5.8×10 ^-6 /°C, and the thermal conductivity is κ=0.56W/m°C. The working repetition frequency of the laser system is 2Hz, the pumping energy per pulse of the oscillator is 200J, the average pumping power density is 40W/cm ³ , the calculated focal length of the thermal lens is 117cm; the pumping energy per pulse of the amplifier is 1000J, and the average pumping power The density is 59W/cm ³ , and the focal length of the thermal lens is calculated to be 53cm. The parameters of the two compensating devices are respectively set as the focal length f ₁ of the first concave lens 4=10cm, the focal length f ₂ of the first convex lens 5=15cm, the focal length f ₁ ′=10cm of the second concave lens 4′, and the focal length of the second convex lens 5′ f ₂ ' = 15 cm.

Adjust the optical axis of the first concave lens 4, the first convex lens 5 and the second concave lens 4', the optical axis of the second convex lens 5' coincides with the optical axis of the laser system, the first concave lens 4 is the heart distance laser rod 1 output end d ₁ = 30cm, the center of the second concave lens 4' is d ₂ =5cm from the output end face of the second laser rod 1' of the amplifier, when the laser system operates in a single pulse, d=f ₂ -f ₁ =50mm, d'=f ₂ ′- f ₁ ′=50mm, that is, Δd=0, Δd′=0, the compensation device is used as a beam expanding telescope, and its beam expanding magnification is 1.5. When the laser system operates at the repetition frequency of the above working conditions, the adjustments of the compensation devices are calculated to be Δd=-14.1mm and Δd'=-35.1mm respectively, that is, the lens distance of the two compensation devices is reduced from 50mm in the Galileo telescope to 35.9mm and 14.9mm. After the adjustment is completed, the device is operated under the above-mentioned working conditions, and dynamic fine-tuning is performed during the operation until the near-field and far-field spot sizes of the oscillator and amplifier output are the same, which can be considered to have achieved complete compensation.

In the above embodiments, we can also change the moving convex lens to moving concave lens, but at this time the calculation formula should be adjusted accordingly, and the calculation is not as simple as the case of moving the convex lens.

The invention is applicable to various solid-state lasers of different dielectric materials, including continuous or high repetition frequency YAG lasers, and is also applicable to relatively complex "laser oscillation-amplification systems".

Claims (6)

1. A laser rod thermal lens effect compensation device, characterized in that it includes a concave lens and a convex lens arranged behind the laser rod, the focal length of the concave lens is smaller than the focal length of the convex lens, and the focal length of the concave lens There is a fixed distance between the center and the end face of the laser rod, and the convex lens is provided with a position shifting mechanism, the optical axis of the concave lens, the optical axis of the convex lens and the optical axis of the laser rod are three or overlap. 2. A laser rod thermal lens effect compensation device according to claim 1, characterized in that the maximum moving distance of the convex lens is the focal length of the convex lens minus the focal length of the concave lens. 3. A laser rod thermal lens effect compensation device according to claim 1, characterized in that said concave lens is a thin spherical concave lens, and said convex lens is a thin spherical convex lens. 4. A laser rod thermal lens effect compensation method, comprising the following steps, step 1: a concave lens and a convex lens are arranged behind the output end face of the laser rod, so that the focal length of the concave lens is smaller than the focal length of the convex lens value, by adjusting the optical axes of the concave lens and the convex lens to coincide with the optical axis of the laser rod, so that the center of the concave lens and the end face of the laser rod are at a fixed distance, and Adjust the distance between the concave lens and the convex lens to be the focal length of the convex lens minus the focal length of the concave lens; Step 2: determine the thermal lens focal length when the laser rod reaches thermal equilibrium under different pump average powers ; Step 3: Calculate the adjustment distance between the convex lens and the concave lens according to the following formula: $\mathrm{\&Delta;d}=-\frac{f_1^2}{f-f_1-d_1-L/2\mathrm{no}},$ Among them: Δd- is the adjustment distance of the convex lens movement, n- is the refractive index of the laser rod, f- is the focal length of the thermal lens, f ₁ - is the focal length of the concave lens, d ₁ - is the distance between the concave lens and the output end face of the laser rod, L- is the length of the laser rod, "-" means to adjust along the direction of reducing the distance between the two lenses, and move the convex lens to the direction of reducing the distance between the two lenses by the adjustment distance; step 4: move during the operation of the laser rod The convex lens is dynamically corrected. 5. A laser rod thermal lens effect compensation method according to claim 4, characterized in that the focal length of the thermal lens in said step 2 is obtained by calculation according to the following formula, $f=\frac{2\mathrm{\&kappa;}}{(\mathrm{\&beta;}+P)\&times;\mathrm{wxya}},$ Among them: f-thermal lens focal length, κ-thermal conductivity coefficient of laser material, β-refractive index temperature coefficient of laser material, χ-heating efficiency of laser device, that is, the ratio of heating power to pump average power, Wp-unit volume laser The average pumping power of the working substance, L-the length of the laser rod, P-the stress thermo-optic coefficient of the working substance of the laser. 6. A laser rod thermal lens effect compensation method according to claim 4, characterized in that the dynamic correction in step 4 is to move the convex lens so that the far-field light spot and the near-field light spot of the laser output are close to each other.

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