RU2397586C2 - Pulsed solid-state laser - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: pulsed solid-state laser has a pumping source and a microchip element inside the resonator consisting of diffusion welded YAG:Nd³⁺ and YAG:Cr⁴⁺ crystals, in which the output mirror of the resonator is formed by a dielectric coating on a substrate made from a birefringent crystal whose principal crystallographic axis lies in the plane of the reflecting surface of the output mirror.

EFFECT: design of a pulsed solid-state laser based on a microchip element which generates linearly polarised radiation with sub-nanosecond pulse duration and a fixed polarisation plane.

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Description

The invention relates to laser technology, in particular to pulsed solid-state lasers.

A number of modern pulsed solid-state lasers [1], [2] based on an AIG: Nd ³⁺ crystal, which solve various problems from the fields of nonlinear optics, microsurgery, optical location, material processing, etc., use passive Q-switching of the cavity with a phototropic shutter simultaneously with intracavity nonlinear conversion of the radiation wavelength, which makes it possible to obtain laser pulses with the required wavelength and duration of several nanoseconds due to the small cavity length. But due to the constantly growing requirements for accuracy and pulsed power of laser systems, a pulse duration of less than 1 ns is required. However, in lasers with intracavity wavelength conversion, a nonlinear element must also be located inside the resonator, in addition to the active medium and the phototropic shutter, due to which the minimum possible cavity length is several centimeters, and this determines the extremely short pulse duration of more than 1 ns.

The closest in technical essence to the present invention is a pulsed solid-state laser [3] with a generation wavelength of 1064 nm, including an active medium from an AIG: Nd ³⁺ crystal connected to a passive gate from an AIG: Cr ⁴⁺ crystal by high-temperature diffusion welding into a monolithic microchip -element. The orientation of the passive shutter is such that the direction of the crystallographic axis [100] of the AIG: Cr ⁴⁺ crystal coincides with the direction of radiation propagation. The resonator is formed by dielectric reflective coatings deposited on the ends of the microchip element AIG: Nd ³⁺ / AIG: Cr ⁴⁺ . The dense packing of the active medium, the passive shutter, and the resonator mirrors makes it possible to reduce the cavity length to several millimeters and obtain a laser pulse duration of one hundred to one thousand picoseconds, which at a pulse energy of the order of 1 mJ corresponds to a pulse power of one to ten megawatts. With such a duration and peak power of laser radiation, in particular, the rejection of the intracavity wavelength conversion in favor of a highly efficient non-resonant conversion is possible. The energy and duration of the laser pulse thus obtained, among other possible applications, are optimal for creating a long-range optical locator with a resolution of about 10 cm.

But due to the absence of any anisotropic elements in the cavity of such a laser, in addition to an AIG: Cr ⁴⁺ crystal, the polarization of the output radiation is determined by the internal structure of this crystal and can have one of two different directions coinciding with the crystallographic axes [010] and [001] AIH: Cr ⁴⁺ . Due to the specific location of Cr ⁴⁺ ions in the AIG crystal lattice, the probability of generation of radiation polarized in the direction of any of these axes is the same and equal to 50%, respectively [4]. Consequently, the output polarization of a given laser can change in an uncontrolled manner from one pulse to another between these two orthogonal directions. For most practical applications requiring a strictly defined linear polarization of laser radiation, this is unacceptable.

The objective of the present invention is to provide a pulsed solid-state laser based on the microchip element AIG: Nd ³⁺ / AIG: Cr ⁴⁺ , generating linearly polarized radiation with a subnanosecond pulse duration and an invariable plane of polarization.

The problem is solved due to the fact that in a pulsed solid-state laser, including a pump source and a microchip element located inside the cavity, consisting of AIG: Nd ³⁺ and AIG: Cr ⁴⁺ crystals connected by diffusion welding, the output mirror of the resonator is formed by a dielectric coating deposited on a substrate of a birefringent crystal, the main crystallographic axis of which lies in the plane of the reflecting surface of the output mirror.

The drawing shows a schematic diagram of the proposed device.

The pump source 1 carries out the end pumping of the laser through a blind mirror 2, which is a dielectric coating that reflects laser radiation and transmits pump radiation deposited on the end face of the active crystal 3 connected by diffusion welding with a passive shutter 4. The output mirror 5 is formed by a dielectric coating deposited on birefringent substrate 6, the main crystallographic axis of which lies in the plane of the reflective surface of the output mirror.

The proposed laser operates as follows:

The pump source 1 creates an inverse population in the active crystal 3. When a passive shutter 4 is illuminated due to spontaneous emission in an invertedly populated active medium, the development of a laser pulse begins between the mirrors 2 and 5. The reflection coefficient of the dielectric output mirror 5 on the birefringent substrate 6 depends on the angle between the plane of polarization of the radiation and the main crystallographic axis of the crystal from which the substrate is made. Namely, it is minimal (maximum, depending on the thickness and refractive indices of the layers of the dielectric coating) for radiation whose electric field vector is parallel to the main crystallographic axis, and maximum (minimum) for the orthogonal intensity vector. The maximum reflected transverse resonator modes have a minimum generation threshold and begin to develop earlier in the active medium, and all other modes are suppressed. Thus, the selection of transverse modes in the direction of the plane of polarization occurs, that is, the laser generates pulses of output radiation with the same direction of the electric field vector. The subnanosecond laser pulse duration is preserved, since the birefringent substrate 6 is located outside the resonator 2, 5 and does not increase its length.

In the manufacture of the prototype sample, calculations were performed that show that in order to obtain the largest possible difference (3-4%) between the reflection coefficients for ordinary and extraordinary polarization of radiation at a wavelength of 1064 nm, it is necessary to use a birefringent substrate material with refractive indices n _o and n _e in the range 1.4 ... 1.7. The table shows the reflection coefficients of ordinary R _o and extraordinary R _e waves and their difference R _e - R _o for mirrors formed by a coating of different numbers of alternating quarter-wave layers of Al ₂ O ₃ (n = 1.63) and SiO ₂ (n = 1.45) deposited on a substrate of CaCO ₃ (n _o = 1.64, n _e = 1.48). The table shows that for these materials the optimal number of layers corresponding to the maximum difference R _e -R _o equal to 7.

Number of layers 5 7 9 eleven 13 R _e ,% 23.3 32.2 41.1 49.7 57.7 R _o ,% 19.6 28.3 37.2 46.0 54.3 R _e -R _o ,% 3.7 3.9 3.9 3.7 3.4

The proposed laser can be used in laser systems for industrial, medical, technical and scientific purposes, requiring a laser pulse duration of less than 1 ns with an invariable plane of linear polarization to control the radiation or for its technological application. In addition, the pulse energy of a subnanosecond laser with an invariable plane of polarization based on diffusion-welded AIG: Nd ³⁺ / AIG: Cr ⁴⁺ elements can be multiply increased in a system of polarized decoupled amplification stages, which makes it possible to obtain pulses with peak power in the simplest way known about gigawatts and carry out non-resonant nonlinear wavelength conversion with an efficiency of up to 70%.

Information sources

1. Bradley W. Schilling, Stephen R. Chinn, A.D. Hays, Lew Goldberg and Ward Trussell, "End-pumped 1.5 µm monoblock laser for broad temperature operation", Applied Optics, Vol. 45, No. 25, 6607-6615 (2006).

2. Yuri Yashkir and Henry M. van Driel, "Passively Q-switched 1.57-µm intracavity optical parametric oscillator", Applied Optics, Vol. 38, No. 12, 6607-6615 (1999).

3. US patent No. 5394413 - prototype.

4. AGOkhrimchuk, AVShestakov, “Absorption saturation mechanism for YAG: Cr ⁺⁴ crystals,” Physical Reviews B, Vol. 61, No. 2, 988-995 (2000).

Claims (1)

A pulsed solid-state laser including a pump source and a microchip element located inside the cavity, consisting of AIG: Nd ³⁺ and AIG: Cr ⁴⁺ crystals connected by diffusion welding, characterized in that the output mirror of the resonator is formed by a dielectric coating deposited on a birefringent crystal substrate whose main crystallographic axis lies in the plane of the reflecting surface of the output mirror.