Short solid-state laser

Abstract

A solid-state laser has a stable resonator (2) that has a first doped YAG material (3) as an active laser medium, a second doped YAG material (4) as a saturable absorber, a first end mirror (5), which is formed by a coating of the first doped YAG material (3) on the side distant from the second doped YAG material (4), and a second end mirror (6), which is formed by a coating of the second doped YAG material (4) on the side distant from the first doped YAG material (3), wherein pump radiation (1) for pumping the laser medium can be radiated in through the first end mirror (5). A coating (8) arranged between the first doped YAG material (3) and the second doped YAG material (4) is provided that is at least 50% reflective for the pump radiation (1) and at least 5% reflective for the laser radiation (7). The laser radiation (7) has at least substantially a single longitudinal mode.

Description

[0001]

[0002] SHORT SOLID-STATE LASER

[0003] The invention relates to a solid-state laser with a resonator, which has a first doped YAG material as the active laser medium for generating laser radiation, a second doped YAG material as the saturable absorber, a first end mirror which is formed by a coating of the first doped YAG material on the side remote from the second doped YAG material, and a second end mirror which is formed by a coating of the second doped YAG material on the side remote from the first doped YAG material, wherein pump radiation for pumping the laser medium can be irradiated through the first end mirror, wherein a coating is provided which is arranged between the first doped YAG material and the second doped YAG material and which is at least 50% reflective for the pump radiation and at least partially transparent for the laser radiation,where the reflection of the laser radiation by the coating is at least 5%.,

[0004] Short solid-state lasers are usually designed as microchip lasers, which have a monolithic resonator, i.e., the resonator components are bonded together. These are usually passively Q-switched lasers used to generate pulsed laser radiation, with the saturable absorber often formed by a SESAM. Microchip lasers are typically pumped by laser diodes. The dimensions of such a microchip laser depend on the materials used and the configuration. Examples of suitable laser media include Yb:YAG, Nd:YVO4, or Nd:YAG.

[0005] A solid-state laser in which the laser medium is Nd:YAG and the saturable absorber is Cr 4+ :YAG is formed, for example, from Zayhowski JJ and Wilson AL “Short- pulsed Nd : YAG / Cr4+ : YAG passively Q-switched microchip lasers", OSA / CLEO 2003. A first end mirror is formed by a coating of the first doped YAG material on the side remote from the second doped YAG material and a second end mirror is formed by a coating of the second doped YAG material on the side remote from the first doped YAG material, wherein pump radiation for pumping the laser medium can be irradiated through the first end mirror. Various possible configurations are mentioned, which lead to different pulse widths and pulse energies. In the shortest configuration, the length of the laser medium is 1 mm and the length of the saturable absorber is also 1 mm, resulting in a pulse width of 169 ps and a pulse energy of 29 pj.

[0006] Other lasers with Nd:YAG as active laser medium and Cr 4+:YAG as a saturable absorber are disclosed in Rakesh Bhandari and Takunori Taira, "Palm-top size megawatt peak power ultraviolet microlaser", Optical Engineering 52 (7) , 076102 (July 2013), in EP 3 694 062 A1 and in EP 3 667 838 A1. In the first of these aforementioned publications, wavelength reduction is carried out at the laser output, in the second of the aforementioned publications, pulse width compression is carried out at the output, and in the third of these publications, the resonator is designed as an unstable resonator. Other possible doped YAG materials for the active laser medium are Yb:YAG and Er:YAG.

[0007] Microchip lasers which use an yttrium vanadate, especially Nd, as the active laser medium. 3+:YVO4, differ significantly in their parameter ranges from microchip lasers, which use a doped YAG material as the laser medium. The achievable pulse energies are considerably lower (in the nJ range). Significantly shorter resonator dimensions are achievable, for example, a few tens of pm, with even shorter pulse durations, down to the 20 ps range, being possible. One such microchip laser is described, for example, in Eva Mehner et al., "Sub-20-ps pulses from a passively Q-switched microchip laser at 1 MHz repetition rate", OPTICAL LETTERS, VOL. 39, No. 10 / May 15, 2014, pages 2940-2943. A SESAM is used as a saturable absorber in such microchip lasers. It is stated that the SESAM is provided with a highly reflective coating for the pump radiation to prevent pre-saturation of the SESAM by the pump radiation.

[0008] Another microchip laser with Nd 3+:YVO4 as the active laser medium and a SESAM for forming a saturable absorber are disclosed in EP 3 167 516 B1. Between the absorber layer of the SESAM and the laser crystal is a reflection layer for the pump radiation, which is at least partially transparent to the laser beam, e.g., 30%.

[0009] A solid-state laser of the type mentioned above is disclosed in EP 4 120 014 A1. A wide variety of materials are mentioned for the active laser material, including Nd:YAG and Yb:YAG. A coating is arranged between the laser medium and the absorber. This coating is described as being highly reflective for the pump radiation and anti-reflective for the laser radiation, although a partially reflective design for the laser radiation is also mentioned. The resonator is designed as an unstable resonator. Very high pulse energies can be achieved. A pulse energy of 13.2 mJ is mentioned, with the pulse duration and pulse repetition rate given as 476 ps and 10 Hz.

[0010] The object of the invention is to provide an advantageous solid-state laser of the type mentioned above that enables relatively high pulse energies with short pulse durations. According to the invention, this is achieved by a solid-state laser having the features of claim 1.

[0011] Advantageously, such high pulse energies can be achieved that the desired material processing or tissue ablation is possible without amplification of the pulse energy.

[0012] In the solid-state laser of the invention, which has a first doped YAG material as the active laser medium and a second doped YAG material as the saturable absorber, a coating is provided between the first doped YAG material, i.e. the active laser medium, and the second doped YAG material, i.e. the saturable absorber, which coating is at least 50% reflective for the pump radiation and at least partially transparent for the laser radiation, wherein a coating is provided between the first doped YAG material and the second doped YAG material, which coating is at least 50% reflective for the pump radiation and at least partially transparent for the laser radiation, wherein the reflection of the laser radiation by the coating is at least 5%.The resonator is designed as a stable resonator, wherein the length of the resonator is less than 2 mm and the length of the second doped YAG material forming the saturable absorber is more than twice the length of the first doped YAG material forming the active laser medium.

[0013] It has been found that in a laser having a first doped YAG material as the laser medium and a second doped YAG material as the saturable absorber, the pulse width can be further reduced by such a coating between the first doped YAG material and the second doped YAG material. Presaturation of the saturable absorber formed by the second doped YAG material due to irradiation with pump light is at least reduced.

[0014] Preferably, the solid-state laser is designed as a microchip laser. The laser thus has a monolithic resonator, i.e., the components of the resonator are firmly bonded to one another.

[0015] In a preferred embodiment of the invention, the reflection of the laser radiation by said coating is at least 10%. However, the reflection of the laser radiation by the coating should be small enough to maintain purely pulsed operation of the solid-state laser.

[0016] Purely pulsed operation of the solid-state laser is understood to mean that essentially no laser radiation is emitted between the individual pulses, i.e. the intensity of the laser radiation in the middle between two pulses is in any case less than 0.1 b of the intensity of the laser radiation in the maximum of a respective pulse.

[0017] Because the coating reflects the said part of the laser radiation, the first doped YAG material forms a type of "sub-cavity" with this coating and the first end mirror. This makes it possible to achieve an increase in the intensity of the laser radiation in the laser medium. This results in an effect similar to a higher amplification of the laser medium or an increase in the emission cross section c. This means that the pulse rate can be increased. For a certain desired total energy, the energy emitted per pulse can be reduced, thereby reducing damage problems caused by the laser radiation. This is particularly advantageous with Yb:YAG because this laser medium has a comparatively very low emission cross section.

[0018] However, if the reflection of the laser radiation by the coating were too great, a continuous emission of laser radiation could occur in the laser medium, which is undesirable. In specific embodiments, it has proven expedient for the reflection of the laser radiation by the coating to be less than 50%, preferably less than 30%, and particularly preferably less than 20%, in order to ensure purely pulsed operation of the solid-state laser.

[0019] In order to achieve a short pulse duration, it is preferred that the length of the resonator is less than 1.5 mm.

[0020] By designing the laser according to the invention with the saturable absorber more than twice as long as the length of the laser medium, in particular in connection with the design of the previously mentioned sub-cavity, a certain wavelength selection for the laser light can be achieved, as will be explained in more detail below.

[0021] The laser radiation of the laser according to the invention has at least substantially only a single longitudinal mode. In this context, "at least substantially" means that more than 95% of the energy of the laser radiation is contained in this mode.

[0022] Further advantages and details of the invention are explained below with reference to the accompanying drawing. The single figure shows a schematic representation of an exemplary embodiment of a solid-state laser according to the invention.

[0023] In particular, the proportions are shown schematically. For the sake of clarity, the coatings are shown considerably thicker than they actually are.

[0024] Pump radiation 1, which is indicated by an arrow in the figure, is radiated into a resonator 2 of the solid-state laser from a pump radiation source (not shown in the figure), which is conventionally formed by a laser diode or a laser diode array. The resonator 2 has a first doped YAG material 3 as the active laser medium. The resonator 2 has a second doped YAG material 4 as a saturable absorber. The first doped YAG material 3 is provided with a coating on the side remote from the second doped YAG material 4, which coating forms a first end mirror 5 of the resonator 2. The pump radiation 1 is radiated into the first doped YAG material 3 by this first end mirror 5. For this purpose, the first end mirror 5 is designed to be highly transparent for the pump radiation. The first end mirror 5 is designed to be highly reflective for the laser radiation produced.

[0025] The second doped YAG material 4 is provided with a coating on the side remote from the first doped YAG material 3, which forms a second end mirror 6 of the resonator 2. In the exemplary embodiment, the laser radiation 7 is coupled out through this second end mirror 6, which is indicated in the figure by an arrow. For example, the second end mirror 6 is designed to be approximately 50% reflective and approximately 50% transmissive for the laser radiation 7. An advantageous range for the transmission can be between 30 and 70%, preferably between 40 and 60%.

[0026] The first doped YAG material 3 in the exemplary embodiment is Yb:YAG. Other doped YAG materials can also be used as the active laser medium, as is known per se, for example, Nd:YAG or Er:YAG.

[0027] In the exemplary embodiment, the wavelength of the continuously irradiated pump radiation is 940 nm, which is particularly suitable for Yb:YAG.

[0028] The pump radiation is also laser radiation. To distinguish it from the laser radiation emitted by the solid-state laser, the radiation used to pump the laser is always referred to as pump radiation in this document. The second doped YAG material 4 in the exemplary embodiment is Cr 4+ :YAG. Other doped YAG materials can also be used as saturable absorbers, as is well known, e.g., V:YAG. For example, the combination of Nd:YAG with V:YAG or Cr 4+ :YAG is appropriate.

[0029] Between the first doped YAG material and the second doped YAG material is a coating 8 that is at least 50%, preferably at least 75%, and particularly preferably at least 90% reflective of the pump radiation. A value of more than 95% is even more preferred. In the exemplary embodiment, the reflection of the coating 8 for the pump radiation is approximately 98%.

[0030] Coating 8 is partially transmissive and partially reflective for laser radiation. The reflection of coating 8 for laser radiation is at least 5%, preferably at least 10%, and in the exemplary embodiment, approximately 15%.

[0031] Due to the partial reflection of the laser radiation by the coating 8, a "sub-cavity" for the laser radiation is formed between the first end mirror 5 and the coating 8. Thus, there is no saturable absorber in this sub-cavity. By forming such a sub-cavity, an overall increase in gain is achieved. The laser therefore begins to lase even with a lower excitation of the active laser medium. The pulse rate is therefore higher. For a certain desired output energy of the laser, the energy per laser pulse can therefore be lower. This reduces the problems caused by damage. However, the reflection of the laser radiation by the coating 8 should be small enough to maintain purely pulsed operation of the solid-state laser. If the reflection were too high, continuous lasing could be triggered in the active laser medium.

[0032] Depending on the specific configuration, the reflection of the laser radiation by the coating required to achieve such purely pulsed operation of the solid-state laser is less than 50%, less than 30%, or less than 20%. In the exemplary embodiment, the reflection is approximately 15%.

[0033] The pulse length t p of the individual laser pulses in the passively Q-switched laser is t p = 1 , 76 * 2 * T R / AR .

[0034] T R is the round-trip time in the cavity and thus proportional to the length of the resonator 2 . AR is the modulation depth e of the saturable absorber , related to the intensity and thus corresponds to the absorption of the saturable absorber (if other losses in the absorber material are neglected).

[0035] In order to obtain a short pulse length, the length s of the resonator , measured parallel to the axis of the laser beam , from the outer surface of the first end mirror 5 to the outer surface of the second end mirror 6 , is less than 2 mm, particularly preferably less than 1.5 mm .

[0036] The length of the first doped YAG material 3 is, particularly when using Yb:YAG, advantageously more than 0.05 mm, preferably more than 0.1 mm. The length of the second doped YAG material 4 is, particularly when using Cr 4+ :YAG, advantageously more than 0.3 mm, preferably more than 0.5 mm.

[0037] The length of the second doped YAG material 4 forming the saturable absorber is more than twice the length of the first doped YAG material 3 forming the active laser medium. As a result, the sub-cavity formed by the coating 8 is less than one-third as long as the resonator 2. Thus, the greater mode spread in the sub-cavity allows for advantageous wave selection, i.e., the wavelength at which the laser starts operating and thus emits the laser radiation. In an advantageous embodiment of the invention, the length of the second doped YAG material 4 can be more than three times the length of the first doped YAG material 3.

[0038] In the exemplary embodiment, the length of the first doped YAG material 3 is approximately 0.25 mm, and the length of the second doped YAG material 4 is approximately 0.8 mm. The length s of the resonator in the exemplary embodiment is approximately 1 mm to 1.1 mm.

[0039] The doping of the first doped YAG material 3 is approximately 10% in the exemplary embodiment and the doping of the second doped YAG material 4 is such that the transmission T o is 68%.

[0040] In the case of using Yb:YAG as the first doped YAG material 3, the wavelength of the emitted laser radiation is 1,030 nm. The laser radiation has at least substantially a single longitudinal mode, ie at least more than 95% of the energy of the laser radiation is contained in a single longitudinal mode.

[0041] The coating 8 is designed in the manner of a Bragg coating, i.e., two materials with different refractive indices are applied alternately in a plurality of layers, for example 24, as is known per se. In order to withstand high laser radiation energy, silicon dioxide and hafnium oxide can be used. The absorption of the laser radiation during one pass through the coating 8 is advantageously below 1 kΩ.

[0042] The coating 8 is applied to one of the two doped YAG materials 3, 4. The other doped YAG material 3, 4 is bonded thereto, for example by diffusion bonding.

[0043] The resonator is thus monolithic, i.e., its components are firmly bonded together. It is therefore a microchip laser.

[0044] In principle, it is also conceivable and possible to clamp components of the solid-state laser against each other.

[0045] By means of a laser according to the invention, pulse lengths of less than 200 ps, ​​preferably less than 150 ps or even less than 100 ps can be achieved (where the pulse length is determined as usual as the FWHM of the power).

[0046] In case of using Nd : YAG as active laser medium, the configuration of the laser will result in the resonator being longer than in Yb : YAG, so that longer pulses are obtained compared to Yb : YAG and also several longitudinal modes can be obtained.

[0047] The resonator is designed as a stable resonator. The two end mirrors 5, 6 are planar, and the mode-shaping element is thus formed by the thermal lens.

[0048] With a laser according to the invention, pulse energies of more than 10 pj, preferably more than 30 pj, can be achieved.

[0049] In the exemplary embodiment, pump radiation with a pump power of 3 W was used for a pulse energy of 30 pj, which was focused to a diameter of 60 - 70 pm in the active laser medium.

[0050] Instead of coupling out the laser radiation at the second end mirror, it could also be provided to couple out the laser radiation at the first end mirror, wherein the pump radiation and the laser radiation can be further separated from each other by a dichroic mirror.

[0051] By using Nd : YAG for the first doped YAG material, laser radiation with a wavelength of 1.3 pm or 1.44 pm could be obtained. This makes eye-safe lasers possible at a higher power than at 1.064 pm. At these wavelengths, V : YAG can advantageously be used as a saturable absorber. Key to the reference numbers: Pump radiation Resonator first doped YAG-

[0052] Material second doped YAG-

[0053] Material first end mirror second end mirror laser radiation coating

Claims

A solid-state laser comprising a resonator (2) which comprises a first doped YAG material (3) as the active laser medium for generating laser radiation (7), a second doped YAG material (4) as the saturable absorber, a first end mirror (5) formed by a coating of the first doped YAG material (3) on the side remote from the second doped YAG material (4), and a second end mirror (6) formed by a coating of the second doped YAG material (4) on the side remote from the first doped YAG material (3), wherein pump radiation (1) for pumping the laser medium can be irradiated through the first end mirror (5), wherein a coating (8) is provided which is at least 50% reflective for the pump radiation (1) and at least is partially permeable,wherein the reflection of the laser radiation (7) by the coating (8) is at least 5%, characterized in that the resonator (2) is designed as a stable resonator, that the length of the resonator (2) is less than 2 mm and the length of the second doped YAG material (4) forming the saturable absorber is more than twice as large as the length of the active, Laser medium forming first doped YAG material (3), wherein the laser radiation (7) has at least substantially a single longitudinal mode. Solid-state laser according to claim 1, characterized in that the coating (8) is at least 75%, preferably at least 90% reflective for the pump radiation (1). Solid-state laser according to claim 1 or 2, characterized in that the reflection of the laser radiation (7) by the coating (8) is at least 10%. Solid-state laser according to one of claims 1 to 3, characterized in that the reflection of the laser radiation (7) by the coating (8) is less than 50%, preferably less than 30%, particularly preferably less than 20%. Solid-state laser according to one of claims 1 to 4, characterized in that the reflection of the laser radiation (7) by the coating (8) is sufficiently small to obtain purely pulsed operation of the solid-state laser.Solid-state laser according to one of claims 1 to 5, characterized in that the length (s) of the resonator (2) is less than 1.5 mm. Solid-state laser according to claim 6, characterized in that the length of the first doped YAG material (3) is more than 0.05 mm, preferably more than 0.1 mm, and the length of the second doped material (4) is YAG material (4) is more than 0.3 mm, preferably more than 0.5 mm. Solid-state laser according to one of claims 1 to 7, characterized in that the first doped YAG material (3) is Yb:YAG or Nd:YAG. Solid-state laser according to one of claims 1 to 8, characterized in that the second doped YAG material (4) is Cr 4+:YAG. Solid-state laser according to one of claims 1 to 8, characterized in that the second doped YAG material (4) is V:YAG. Solid-state laser according to one of claims 1 to 10, characterized in that the pulse length of the laser radiation is less than 200 ps, ​​preferably less than 150 ps, ​​particularly preferably less than 100 ps. Solid-state laser according to one of claims 1 to 11, characterized in that the solid-state laser is a microchip laser. Solid-state laser according to one of claims 1 to 12, characterized in that the pulse energy is more than 10 pj, preferably more than 30 pj. Solid-state laser according to one of claims 1 to 13, characterized in that the two end mirrors (5, 6) of the resonator (2) are planar and the mode-shaping element is formed by the formed thermal lens. Solid-state laser according to one of Claims 1 to 14, characterized in that the length of the second doped YAG material (4) forming the saturable absorber is more than three times as large as the Length of the first doped YAG material forming the active laser medium ( 3 ) .

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