CN101414729B - Self-mode-locking laser - Google Patents

Abstract

In the setting method of the self-mode-locked laser of the present invention, the pump light is incident to the laser resonator cavity from one end face of the laser crystal, and the light intensity at the center of the pump spot is smaller than the light intensity around it by using a beam shaping system, that is, there is For the concave Gaussian beam, the laser crystal acts as a laser gain crystal and a mode-locking device at the same time. A region similar to an aperture is formed in the center of the laser crystal. By adjusting the pump beam, the size of the aperture region can be controlled. The crystal Kerr effect and soft aperture work together to complete the start and maintenance of mode-locked laser operation, realize continuous mode-locked ultrashort pulse output or realize Q-switched mode-locked pulse output. It solves the problems that the current ultrashort pulse laser has a large system, many peripheral devices, high precision for debugging, and cannot be started automatically, and improves efficiency and reduces costs. The invention has high application value.

Description

A self-mode-locked laser

technical field

The invention relates to a self-mode-locked laser, in particular to a self-starting self-mode-locked laser.

Background technique

Kerr lens mode locking is currently one of the most effective methods for obtaining ultrashort pulses from solid-state lasers. The simplest specific device is to use the nonlinear refractive index of the laser crystal as the soft aperture formed by the Kerr medium and the gain distribution inside the laser material (see Spence D.E.etc., Opt.Lett.16, p42), but because the laser crystal The nonlinear refractive index itself is generally relatively small, and is limited by the saturation light intensity of the laser crystal itself. Only when the peak power density of the oscillation pulse reaches a certain intensity can an effective gain stop be formed in the crystal, and then in the cavity A non-linear loss that decreases with increasing light intensity is formed to maintain continuous mode-locked operation. In practical applications, people mostly use a four-mirror cavity, place a crystal with a large nonlinear refractive index at the beam focusing position, and place a hard stop at the beam waist, so as to obtain the effect of a fast saturable absorber.

In addition, since the Kerr lens mode-locking is not enough to self-start the continuous mode-locking program, it is necessary to achieve self-starting by perturbing the cavity or increasing the intensity fluctuation caused by the nonlinearity of the system, such as regenerative feedback, synchronous pumping, and resonant Mounting piezoelectric transducers on the cavity mirror and driving with semiconductor saturable absorbers (see W. Kirchner, "Solid State Laser Engineering"), these methods will undoubtedly make the entire laser system variable while starting the mode-locked operation. It is too large and complex, which also increases the difficulty of system debugging.

The pumping light of the traditional end-pumped laser is a flat-top Gaussian beam, and the purpose is to obtain the Gaussian TEM00 mode output. For this reason, various coupling systems have been designed. However, it is difficult for this ideal flat-top Gaussian pump beam to form an effective self-starting mode-locking effect in laser crystals. Although there have been some theoretical proofs that a gain stop can be formed in laser crystals, it has not been fully realized experimentally. , even if occasionally reported, is not accepted because of its poor reproducibility (see G. Cerullo, Opt. Lett. 19, p1040).

Contents of the invention

The object of the invention is: solve the shortcoming of above-mentioned Kerr lens mode-locked laser, propose a kind of setting method of self-mode-locked laser and laser thereof: do not need diaphragm and the crystal with large nonlinear refractive index in the laser cavity, There is no need for any mode-locking starting device, only by changing the light intensity distribution of the pump beam, an effective mode-locking mechanism is formed in the laser crystal, and the self-starting of continuous mode-locking operation is realized. The mode-locking operation realized by this method is not Limited by the saturation intensity of the laser crystal itself, ultrashort pulse laser can be obtained under the condition of very low pump light intensity.

The technical solution of the present invention is: the setting method of the self-mode-locked laser uses a beam shaping system to make the light intensity at the center of the pump spot smaller than the light intensity around it, that is, the central light intensity of the pump light is sunken, and the pump The pump light enters the laser resonator from one end face of the laser crystal. The laser crystal is used as the laser gain crystal and at the same time as the mode-locking device, thereby forming an area similar to an aperture in the center of the laser crystal. By adjusting the pump beam, the The size of the aperture area can be controlled, and the crystal Kerr effect and the soft aperture work together to complete the start and maintenance of the mode-locked operation. Its pump sources include various pump sources such as diode lasers and all-solid-state lasers. Q mode-locked pulse output.

The laser resonator is a linear cavity pumped from an end face, an X-shaped cavity pumped from one end face of a laser crystal, and a folded cavity pumped from a double end face. All resonant cavities that are pumped from the end face of a laser crystal can be used Method to achieve ultrashort pulse laser.

The coupling mirror can be used to focus the pump beam output by the fiber directly, so that the light intensity at the center of the spot of the focused beam can be reduced; the output of the diode laser can also be directly adjusted to make the transverse mode of the diode laser meet the requirements, as long as it is By changing the light intensity distribution of the pumping beam, which is different from the traditional flat-topped Gaussian beam, an effective mode-locking mechanism is formed in the laser gain medium, all of which belong to the technical solution of the present invention.

Laser crystals can be tunable laser crystals doped with transition metal ions Cr ³⁺ , Ti ³⁺ , Co ^{2+ ,} etc., or Nd ³⁺ , Ho ³⁺ , Yb ³⁺ , Tm ²⁺ , Er ³⁺ Gain media doped with rare earth ions, etc., as long as the laser crystal can be pumped from the end face of the crystal, this scheme can be used to realize ultrashort pulse laser operation.

Other mode-locking devices for the purpose of enhancing mode-locking, such as hard diaphragms, nonlinear saturable absorbers Cr ⁴⁺ :YAG, GaAs, SESAM, etc., can be installed in the resonant cavity.

For wide-gain laser crystals, group velocity dispersion compensation devices can be installed inside or outside the cavity to obtain ultrashort pulses with switching limits; The velocity dispersion has little influence on the pulse width, and the dispersion compensation device can no longer be added.

Single-sided pumped X-cavity femtosecond laser, equipped with a beam shaping system that makes the pump beam incident on the laser crystal a flat-top Gaussian beam with a concave light intensity at the center, and the 527nm output frequency of Nd:YLF The beam is shaped, the laser crystal in the X-cavity is Ti: sapphire, the input mirror M2 of the X-cavity is coated with 750-850nm broadband high reflection and 527nm high transmission, and the other three cavity mirrors M ₁ , M ₃ of the X-cavity, M ₄ coating is 750-850nm broadband high reflection, the output of X cavity passes through the prism that is the group velocity dispersion compensation device and then output through output mirror M ₅ , M ₅ broadband 750-850nm transmittance is 3.6%, realizes self-starting Continuous mode-locked operation, output femtosecond pulsed laser with tunable wavelength.

The single-end pumped Z-type folded cavity picosecond laser is equipped with a beam shaping system including a coupling mirror that makes the pump beam incident on the laser crystal a flat-top Gaussian beam with a concave light intensity at the center, inside the Z-type cavity The laser crystal _is Nd:YVO ₄ , the plane input mirror M ₁ of the Z-type folded cavity is coated with a high reflection of _1342nm and a high transmission of 808nm; Reflection at 1342nm; the plane output mirror _M4 of the laser has a transmittance of 10% for 1342nm, and the coupling mirrors are two plano-convex focusing lenses.

The beneficial effects of the present invention are: at present, the method of the present invention has been used to realize a complete self-starting continuous mode-locking picosecond pulse sequence in experiments, and complete continuous mode-locking has been realized when the pump power is less than 1 watt It is running, and the output power and pulse width remain stable for a long time, completely self-starting, without any external disturbance, which proves that this method is completely effective and feasible. Single-side-pumped X-cavity femtosecond lasers and Z-folded-cavity picosecond lasers are especially good implementations.

Although this kind of self-mode-locked laser has been realized in experiments, the mode-locking mechanism has yet to be further explored, and the mode-locking mechanism of the Kerr lens is actually being studied. However, the mode-locked laser realized by the method of the present invention simplifies the entire laser system, does not require any additional disturbance measures, greatly reduces the mode-locked threshold, and has a wide range of operating wavelengths. This method can be used for all wavelengths within the stimulated radiation range of crystals Achieve ultrashort pulse laser output.

Description of drawings

Fig.1 Schematic diagram of optical path 1(a) and pumping intensity distribution 1(b) of an X-cavity self-mode-locked laser with dispersion compensation

Fig.2 Schematic diagram of the optical path 2(a) and pumping intensity distribution 2(b) of the end-pumped Z-cavity self-mode-locked laser

Figure 3 The continuous mode-locked pulse sequence output from the self-mode-locked laser using the device in Figure 2

In the figure, laser crystal 1, beam shaping system with coupling mirror 2, and LD pump light.

Detailed ways

(1) Use a coupling system to modulate the 527nm pump beam output by Nd:YLF frequency doubling, so that the beam incident on the laser crystal is a flat-top Gaussian beam with a concave central light intensity, as shown in Figure 1 (a), Using the folded cavity shown in Figure 1(b), the laser crystal is Ti:Sapphire, the cavity mirror coating M ₁ , M ₃ , M ₄ : HR (high reflection) 750-850nm, M ₂ : HR750-850nm&HT (high transmission )527nm, M ₅ : T (transmittance) = 3.6% 750-850nm, can realize broadband continuous femtosecond pulse output.

(2) Use a lens group to realize the modulation of the flat-top Gaussian pump beam, so that the pump beam incident on the laser crystal is a beam with a concave central light intensity, as shown in Figure 2 (a), using Figure 2 ( b) The end-pumped Z-fold cavity shown in the figure, the laser crystal is Nd:YVO ₄ , the cavity mirror coating M1: HR1342&HT808nm, M ₂ , M ₃ : HR1342nm, M ₄ : T=10% 1342nm, can achieve 1342nm continuous Picosecond pulse output.

(3) According to the self-mode-locked laser realized in the implementation example (2), the laser crystal can also be Nd:YAG, Nd:YLF, Nd:glass, Nd:GdVO ₄ , Yb:YAG, Yb:KVW, Yb:glass, etc. A variety of doped laser crystals change the wavelength of the pump light according to the absorption wavelength of the crystal, and coat the cavity mirror according to the oscillation wavelength, and can also achieve continuous mode-locked laser output with wavelengths near 1.0 μm and 0.9 μm.

(4) According to the self-mode-locked laser realized in the implementation examples (2) and (3), a group velocity dispersion compensation device can also be added in the cavity to obtain ultrashort pulse laser output with conversion limit.

(5) According to the self-mode-locked laser realized by the implementation example (1), the laser crystal can also be Cr:forsterite, Cr:LiSAF, Cr:LiCAF and other laser crystals with wide gain line widths, as long as the pump is changed according to the absorption wavelength of the crystal According to the wavelength of the pump light, the cavity mirror is coated according to the oscillation wavelength, and the broadband tuned continuous ultrashort pulse laser output within the gain linewidth of these crystals can be realized.

(6) According to the self-mode-locked laser realized by the implementation example (1)-(5), in order to enhance the nonlinear modulation in the cavity, an aperture can also be added near the plane mirror, or a nonlinear saturable absorber can be placed, including Cr4 ⁺ : YAG, GaAs, InGaAs quantum well semiconductor saturable absorber, etc.

(7) In order to extend the wavelength range of the self-mode-locked laser realized according to the implementation examples (1)-(5), nonlinear crystals can also be added inside or outside the cavity to obtain ultrashort pulse laser with higher harmonics.

Single-sided pumped X-cavity femtosecond laser, Figure 1(a): A beam shaping system is used to shape the 527nm beam output by Nd:YLF frequency doubling, so that the pump beam incident on the laser crystal is the central light Strong and concave flat-top Gaussian beam, as shown in Figure 1(b), the laser crystal is Ti:Sapphire, the cavity mirrors M ₁ , M ₃ , and M ₄ are coated with 750-850nm broadband high reflection, and the input mirror M ₂ is coated 750~850nm broadband high reflection and 527nm high transmission, the output mirror _M5 broadband 750~850nm transmittance is 3.6%, the prism pair is a group velocity dispersion compensation device, realizes the self-starting continuous mode-locked operation, and the output wavelength is tunable flying second pulse laser.

Single-end-pumped Z-type folded cavity picosecond laser, Figure 2(a): the laser crystal is Nd:YVO ₄ , the plane input mirror M ₁ , the coating is high reflection 1342nm and high transmission 808nm; plano-concave mirror M ₂ , The radius of curvature of M ₃ is 500mm and 300mm respectively, and the coatings are all highly reflective 1342nm; the transmittance of the flat output mirror M ₄ is 10% for 1342nm, and the coupling mirror is two plano-convex focusing lenses. Since the thickness of the central area of the lens is greater than that of the edge, the lens The focal length gradually increases from the axis area along the radius, and the light intensity in the center area of the spot after the beam is focused by the coupling mirror is smaller than that in the periphery, that is, there is a small depression in the center light intensity, as shown in Figure 2(b), to achieve complete self-starting The 1342nm continuous mode-locked operation outputs picosecond pulse laser.

Claims (2)

1. the X-type chamber femto-second laser of a single face pumping, it is characterized in that being provided with a beam shaping system that makes light intensity centered by the pump beam that incides on the laser crystal that the Flattened Gaussian Beams of depression be arranged, adopt the beam shaping system to make the light intensity of center of pump spot less than the light intensity around it, namely cause central light strength that the Gaussian beam of depression is arranged, make light intensity centered by the light beam that incides on the laser crystal that the Gaussian beam of depression be arranged, pump light by a surface feeding sputtering of laser crystal to laserresonator, laser crystal as the laser gain crystal time again as the locked mode device, in laser crystal, be formed centrally a zone that is similar to diaphragm, by regulating pump beam, just can control the size in diaphragm zone, the Kerr effect of crystal and the acting in conjunction of soft light door screen, finish the startup of mode-locked laser running and keep the ultrashort pulse output of realization continuous locking mold or the mode locking pulse output that realizes transferring Q; From the X-type chamber of an end pumping of laser crystal, the 527nm light beam of Nd:YLF frequency multiplication output is carried out shaping, X-type endovenous laser crystal is Ti: sapphire, the input mirror M in X-type chamber ₂Plated film is the high reflection in 750～850nm broadband and the high transmission of 527nm, and three chamber mirrors of other of X-type chamber are M ₁, M ₃, M ₄Plated film is the high reflection in 750～850nm broadband, the output of X-type chamber through to behind the prism of GVD (Group Velocity Dispersion) compensating device again through outgoing mirror M ₅Output, M ₅Broadband 750～850nm transmitance 3.6% realizes the continuous locking mold running of self-starting, the tunable femtosecond pulse of output wavelength.

2. the Z-type refrative cavity picosecond laser of a single end face pump, it is characterized in that being provided with a beam shaping system that contains coupling mirror that makes light intensity centered by the pump beam that incides on the laser crystal that the Flattened Gaussian Beams of depression be arranged, adopt the beam shaping system to make the light intensity of center of pump spot less than the light intensity around it, namely cause central light strength that the Gaussian beam of depression is arranged, make light intensity centered by the light beam that incides on the laser crystal that the Gaussian beam of depression be arranged, pump light by a surface feeding sputtering of laser crystal to laserresonator, laser crystal as the laser gain crystal time again as the locked mode device, in laser crystal, be formed centrally a zone that is similar to diaphragm, by regulating pump beam, just can control the size in diaphragm zone, the Kerr effect of crystal and the acting in conjunction of soft light door screen, finish the startup of mode-locked laser running and keep the ultrashort pulse output of realization continuous locking mold or the mode locking pulse output that realizes transferring Q; Z-type endovenous laser crystal is Nd:YVO ₄, the plane input mirror M of Z-type refrative cavity ₁, plated film is high reflection 1342nm and high transmission 808nm; Two plano-concave mirror M ₂, M ₃Radius of curvature is respectively 500mm and 300mm, and plated film is high reflection 1342nm; The flat output mirror M of laser ₄To 1342nm transmitance 10%, described coupling mirror is two plano-convex condenser lenses.

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