Intracavity frequency conversion in solid-state laser resonators with end pumping.

Abstract

A method for intracavity frequency conversion includes generating an intracavity laser beam (192) that circulates within a laser cavity (110) by end-pumping a solid-state gain medium (120) within the laser cavity (110) with a pump laser beam (190), and generating a frequency-converted laser beam (196) by frequency-converting a portion of the intracavity laser beam (192) within a nonlinear crystal (130) located within the laser cavity (110). The method controls the output power and at least one output beam parameter of the frequency-converted laser beam (196) by adjusting (a) the pump power and (b) the cavity loss imposed on the intracavity laser beam (192).

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Provisional Application No. 63 / 332,983, filed April 20, 2022, the entire contents of which are incorporated herein by reference.

[0002] FIELD OF THE DISCLOSURE The present invention relates to intracavity frequency conversion of laser radiation in an end-pumped solid-state laser resonator, and more particularly to controlling the power and beam parameters of intracavity laser radiation in the presence of thermal lensing effects in the laser gain medium, thereby controlling the power and beam parameters of the frequency converted laser radiation. [Background technology]

[0003] Background of the Disclosure The gain medium of a solid-state laser or laser amplifier is a solid-state host material doped with optically active ions capable of generating or amplifying laser radiation when excited. The host material is generally glassy or crystalline, and the optically active ions are typically rare earth or transition metal ions such as neodymium, erbium, ytterbium, or titanium. The gain medium can be in the form of a bulk crystal / glass or optical fiber. Most bulk gain media are shaped as rods or slabs. Compared to gas lasers, solid-state lasers have many advantages, including simplicity of operation, efficiency, reliability, compactness, and lower cost.

[0004] Typically, solid-state laser gain media are optically pumped, i.e., optically active ions are optically excited to provide the population inversion required for lasing action. Most solid-state laser gain media are pumped by a laser beam. Diode lasers are a particularly popular choice for pump laser sources due to their many advantages, such as efficiency, compactness, long lifetime, and low cost. Some systems utilize arrays of laser diodes to provide the required pump powers, as high as hundreds of watts or even kilowatts.

[0005] In the case of laser-pumped bulk gain media, several different pump geometries are possible: in end pumping, the pump laser radiation is co-propagating or counter-propagating with the intracavity laser radiation, which is generated in the bulk gain medium and circulates within the laser resonator; in side pumping, the pump laser radiation is directed into the gain medium through a face that is parallel to the propagation direction of the output laser beam, such that the propagation direction of the pump laser radiation is approximately perpendicular to the propagation direction of the intracavity laser radiation.

[0006] End pumping allows good spatial overlap between the pump laser radiation and the intracavity laser radiation, minimizing the pump energy lost to the portion of the bulk gain medium that does not participate in the lasing action, thereby resulting in a larger laser gain. End pumping is also an advantageous geometry for thermal management, since the side surfaces of the bulk gain medium can be in contact with cooling elements without interfering with the propagation path of either the pump or output laser radiation. However, at high pump powers, end pumping generates a thermal lens within the bulk gain medium in the path of the laser radiation. Thermal lensing is primarily due to the thermo-optic effect, which is the temperature dependence of the refractive index of the gain medium and the thermal expansion of the gain medium. The optical design of the laser resonator can be optimized to accommodate a given magnitude of thermal lensing.

[0007] Many different wavelengths of laser radiation can be generated by solid-state lasers, but frequency conversion of the originally generated laser radiation may be necessary to reach certain wavelengths, especially in the ultraviolet (UV) spectral range. The laser beam may undergo frequency conversion in a nonlinear crystal through harmonic generation, sum frequency mixing, or difference frequency mixing. In intracavity frequency conversion, a nonlinear crystal is placed inside the laser resonator used to generate the laser beam to be frequency converted. Intracavity frequency conversion benefits from the high power of the intracavity laser beam circulating in the laser resonator.

[0008] Any given laser application has certain requirements regarding laser power and laser beam parameters. Typically, the beam parameters that are specified are: (a) the size of the beam waist, (b) the location of the beam waist, and (c) the beam divergence angle or beam quality factor M. 2 . The power and beam parameter specifications can be very strict. Some laser applications rely on the laser power being adjustable, sometimes in conjunction with adhering to strict beam parameter specifications. Although it may be possible to adjust the output power by adjusting aspects of the internal operation of the laser device, such adjustments often have other consequences as well. For example, the output power of a laser device based on a solid-state laser resonator with end pumping can be adjusted by adjusting the pump laser power. However, when pump-induced thermal lensing is not negligible, the beam parameters of the intracavity laser beam are affected as well, since the thermal lensing makes the gain medium an element in the laser resonator with optical power. To avoid such problems, laser power adjustments are often accomplished by simply attenuating the output laser beam, for example, using an acousto-optical modulator (AOM) or an electro-optical modulator (EOM). Summary of the Invention [Means for solving the problem]

[0009] (Summary of the Disclosure) Disclosed herein is an advantageous scheme for controlling the power and beam parameters of a frequency converted output laser beam of a solid-state laser resonator with end pumping and intracavity frequency conversion. The control scheme is applicable to scenarios with non-negligible thermal lensing effects in the solid-state gain medium. The control scheme is based on the inventors' discovery that independent adjustment of pump power and resonator losses facilitates independent control of the power and beam parameters of the intracavity laser beam over a wide power and beam parameter range. This is a result of both the pump laser beam and the intracavity laser beam contributing to thermal lensing effects in the laser gain medium. Independent control of the power and beam parameters of the intracavity laser beam is equivalent to independent control of the power and beam parameters of the frequency converted laser beam. In less capable control schemes based on adjustment of only one of the pump power and resonator losses, the beam parameters of the intracavity laser beam are directly coupled to its power, and this coupling transfers, at least to some extent, to the frequency converted laser beam. The control scheme does not require an AOM or EOM for attenuation of the frequency converted laser beam. Especially when the frequency converted laser beam is ultraviolet, such modulators can be expensive and themselves can further modify the beam parameters.

[0010] The present control scheme adds versatility and control to laser devices based on solid-state laser resonators with end pumping and intracavity frequency conversion. Using this control scheme, the power and beam parameters of the frequency converted laser beam can be adjusted to meet various specifications without any need for hardware reconfiguration. Active stabilization of pump power and / or resonator losses can be employed to maintain the required power and beam parameters in the presence of environmental changes, UV degradation of optical elements, and other sources of noise, fluctuations, and drift. The control scheme can also be used to change the power and / or beam parameters of the frequency converted laser beam during the process. For example, the frequency converted laser power can be stepped up or down (within a certain range) while maintaining the same beam parameters.

[0011] In one aspect, a method for intracavity frequency conversion includes: (a) generating an intracavity laser beam circulating in a laser cavity by end-pumping a solid-state gain medium in the laser cavity with a pump laser beam having a pump power, (b) imposing losses on the intracavity laser beam, (c) frequency converting a portion of the intracavity laser beam in a nonlinear crystal located in the laser cavity to generate a frequency-converted laser beam having an output power, and (d) adjusting the pump power and losses to control the output power and at least one output beam parameter of the frequency-converted laser beam, the at least one output beam parameter being selected from the group consisting of a size of the beam waist, a location of the beam waist, a beam divergence angle, and a beam quality factor.

[0012] In another aspect, a laser apparatus with intracavity frequency conversion includes a laser resonator having a solid-state gain medium, a nonlinear crystal, and an adjustable loss element arranged to impose an adjustable loss on the laser resonator. The laser apparatus further includes a pump laser for generating a pump laser beam having a pump power. The pump laser is arranged to end-pump the gain medium to generate an intracavity laser beam that circulates in the laser resonator. The intracavity beam undergoes partial frequency conversion in the nonlinear crystal to generate a frequency-converted laser beam having an output power. The laser apparatus also includes one or more sensors for monitoring an output power and at least one output beam parameter of the frequency-converted laser beam, and a controller configured to control the output power and the at least one output beam parameter by adjusting the pump power and the loss according to the monitored values ​​of the output power and the at least one output beam parameter. The at least one output beam parameter is selected from the group consisting of a size of a beam waist, a location of the beam waist, a beam divergence angle, and a beam quality factor. [Brief description of the drawings]

[0013] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0014] [Figure 1] FIG. 1 illustrates a laser device with intracavity frequency conversion in a solid-state laser resonator having an end-pumped solid-state gain medium according to an embodiment.

[0015] [Diagram 2] FIG. 2 shows an example of pump and intracavity beams incident on the end facets of the gain medium of the device of FIG.

[0016] [Diagram 3] FIG. 3 illustrates the effect of thermal lensing within the gain medium on intracavity beam caustics in one embodiment of the resonator of the apparatus of FIG.

[0017] [Figure 4] FIG. 4 illustrates the relationship between outcoupling and intracavity power in one embodiment of the apparatus of FIG.

[0018] [Diagram 5] FIG. 5 illustrates the contribution from intracavity power to thermal effects in the gain medium for the device embodiment of FIG.

[0019] [Figure 6] FIG. 6 demonstrates the influence of intracavity power on intracavity beam parameters through the effect of thermal lensing in the apparatus embodiment of FIG. 1 of FIG.

[0020] [Figure 7] FIG. 7 is similar to FIG. 6 but zoomed in on the waist diameter.

[0021] [Figure 8] FIG. 8 is a flow chart for an intra-cavity frequency conversion method applicable to a solid-state laser resonator with an end-pumped gain medium, according to an embodiment.

[0022] [Figure 9] FIG. 9 illustrates a laser apparatus with intracavity frequency conversion in a solid-state laser resonator having an end-pumped polarized solid-state gain medium, a photodiode, and a polarized output coupler according to an embodiment.

[0023] [Figure 10]FIG. 10 illustrates the simultaneous power and beam waist diameter control achieved using an embodiment of the apparatus of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Detailed Description of the Disclosure Referring now to the drawings, where like components are designated by like numbers, FIG. 1 illustrates one laser apparatus 100 with intracavity frequency conversion in a solid-state laser resonator 110 having an end-pumped solid-state gain medium 120. The resonator 110 includes a solid-state gain medium 120, a nonlinear crystal 130, and an adjustable loss element 140. The apparatus 100 includes the resonator 110, a pump laser 160, and a controller 170. In operation, the pump laser 160 generates a pump laser beam 190 that is incident on an end face 122 of the gain medium 120. The pump beam 190 induces laser action by energizing the gain medium 120, thereby generating an intracavity laser beam 192 that circulates within the resonator 110. The intracavity laser beam 192 undergoes partial frequency conversion within the nonlinear crystal 130. That is, a portion of the intracavity beam 192 is converted to a different frequency in the nonlinear crystal 130, which leads to the generation of a frequency-converted laser beam 196. The loss element 140 imposes an adjustable loss on the intracavity beam 192 in the resonator 110, for example, by outcoupling or absorbing a portion of the intracavity beam 192. The controller 170 controls the power and one or more beam parameters of the intracavity beam 192 by setting the power of the pump beam 190 and the loss imposed by the loss element 140, thereby also controlling the power and one or more beam parameters of the frequency-converted beam 196. The controlled beam parameters may include one or more of the size of the beam waist, the location of the beam waist, the beam divergence angle, and the beam quality factor.

[0025] Apparatus 100 may be configured to facilitate any one of several different types of frequency conversion in nonlinear crystal 130, including harmonic generation, sum-frequency mixing, and difference-frequency mixing. In embodiments of apparatus 100 configured for sum-frequency or difference-frequency mixing, intracavity beam 192 may mix with another laser beam 194 within nonlinear crystal 130 to generate a frequency-converted beam 196. Such embodiments of apparatus 100 may include an external laser 180 that generates beam 194.

[0026] In the embodiment depicted in FIG. 1, the path of the intracavity beam 192 in the resonator 110 is defined by four cavity mirrors 112 forming a rectangular ring resonator. Many other layouts are possible. However, the ring resonator layout is advantageous for intracavity frequency conversion in some scenarios, at least because of the possibility of restricting the propagation of the intracavity laser radiation to one direction. When the intended purpose of the device 100 is harmonic generation in the nonlinear crystal 130, the unidirectional propagation through the nonlinear crystal 130 ensures that the frequency converted laser radiation generated in the nonlinear crystal 130 is contained in a single laser beam, which is usually preferred. The resonator 110 may include a photodiode 150, such as a Faraday rotator and a wave plate, that ensures the unidirectional propagation of the intracavity beam 192 in the resonator 110. When the intended purpose of apparatus 100 is sum or difference frequency mixing, the propagation direction of laser beam 194 ensures that the frequency converted laser radiation generated in nonlinear crystal 130 is contained in a single laser beam, regardless of whether resonator 110 is a ring resonator or a linear standing wave resonator. However, a ring resonator may be preferred for other reasons, e.g., to minimize power noise in frequency converted beam 196.

[0027] Although not depicted in FIG. 1 , the resonator 110 may include a series of nonlinear crystals 130, for example, two serially arranged nonlinear crystals 130. In one embodiment of the present invention, the first nonlinear crystal 130 converts a portion of the intracavity beam 192 to its second harmonic. This second harmonic beam then undergoes sum-frequency mixing with an unconverted portion of the intracavity beam 192 in the second nonlinear crystal 130, thereby generating a third harmonic of the intracavity beam 192. In the following, unless otherwise stated, it is assumed for simplicity that there is only one nonlinear crystal 130 in the resonator 110. However, the discussion is easily extended to embodiments with multiple nonlinear crystals 130.

[0028] The frequency converted beam 196 may propagate to the intracavity beam 192 in the nonlinear crystal 130 at an angle, as dictated by the phase matching condition. Depending on the magnitude of this angle, the frequency converted beam 196 may pass by one of the cavity mirrors 112 (or another optical element that defines the resonator 110), thereby leaving the resonator 110. It is also possible to separate the frequency converted beam 196 from the intracavity beam 192 by dispersion, for example using a nonlinear crystal 130 with Brewster cut input and output faces. Alternatively, the apparatus 100 may include a dichroic beam splitter or another optical element (not shown in FIG. 1 ) that extracts the frequency converted beam 196 from the resonator 110.

[0029] FIG. 2 shows pump beam 190 and intracavity beam 192 incident on end faces 122 of gain medium 120. Gain medium 120 is bulk crystal or glass. In the depicted embodiment, gain medium 120 is a cubic slab with square end faces 122. Gain medium 120 may have different shapes. For example, gain medium 120 may be a slab with oval end faces 122, or gain medium 120 may be a rod with circular end faces 122. Intracavity beam 192 may be at least approximately Gaussian. Pump beam 190 may be approximately Gaussian, may have an approximately flat-top intensity distribution, or may have an intensity distribution in between. The transverse profile of pump beam 190 and / or intracavity beam 192 may be elliptical rather than circular.

[0030] Regardless of the shapes of the gain medium 120, the pump beam 190, and the intracavity beam 192, the transverse extent 290D of the pump beam 190 is preferably less than the corresponding transverse extent 220L of the gain medium 120 so as not to waste pump energy. In a typical mode of operation, the transverse extent 292D of the intracavity beam 192 is less than the transverse extent 290D of the pump beam 190. The transverse extents 290D and 292D are each 1 / e of the corresponding transverse intensity profile. 2 In one embodiment, the transverse extent 220L of the gain medium 120 is in the range of 1 to 10 millimeters, the transverse extent 290D of the pump beam 190 is in the range of 100 to 3,000 micrometers (μm), and the transverse extent 292D of the intracavity beam 192 is in the range of 50 to 3,000 μm or 50% to 100% of the transverse extent 290D.

[0031] The pump beam 190 induces a thermal lensing effect in the gain medium 120. The power of this thermal lens is an increasing function of the power of the pump beam 190. The intracavity beam 192 also contributes to the thermal lensing effect in the gain medium 120. The resulting thermal lensing induced by the combination of the pump beam 190 and the intracavity beam 192 affects the beam parameters of the intracavity beam 192 during its propagation through the resonator 110.

[0032] 3 is a plot illustrating the effect of thermal lensing in the gain medium 120 on the caustics of the intracavity beam 192 in one embodiment of the resonator 110. FIG. 3 plots the caustics of the intracavity beam 192 for two different powers of thermal lensing in the gain medium 120. Each caustic has a magnitude of 1 / e 2 is shown as the beam radius w versus the location z along the propagation path of the intracavity beam 192 within the resonator 110. At any given location z, the distance between the upper and lower curves of the same caustic is 1 / e 2 Beam diameter 2w: 3.7 meters -1 (m -1 ) results in a caustic 310 characterized by a waist w0. The nonlinear crystal 130 is advantageously positioned at the waist of the intracavity beam 192. -1 Increasing the thermal lensing by about 8% results in caustics 320. As the thermal lensing is increased, the beam diameter increases in gain crystal 120 and decreases in nonlinear crystal 130. In nonlinear crystal 130, this decrease is characterized by a waist w0' that is about 20% below waist w0. The location z0 of the waist remains the same in this example.

[0033] Consider an embodiment in which the apparatus 100 is configured to generate a frequency converted beam 196 as the second harmonic of the intracavity beam 192. In this embodiment, the local frequency conversion efficiency scales with the square of the intensity of the intracavity beam 192. As a consequence, the frequency conversion efficiency is approximately inversely proportional to the fourth power of the beam radius. Applying the example of FIG. 3 to this second harmonic embodiment and assuming that the nonlinear crystal 130 coincides with the waist of the intracavity beam 192 (as depicted), the frequency conversion efficiency can be calculated as 3.7 to 4.0 m. -1 3. The increase in power of the thermal lens to 310 results in more than a doubling of the frequency conversion efficiency as long as the power of the intracavity beam 192 is the same (or greater) with respect to caustic 320 as with caustic 310.

[0034] In embodiments where the intracavity beam 192 undergoes sum or difference frequency mixing with the beam 194, the local frequency conversion efficiency is linearly proportional to the intensity of the intracavity beam 192. However, the spatial overlap between the beams 192 and 194 within the nonlinear crystal 130 must be considered as well. Typically, it is optimal for both the beams 192 and 194 to form waists within the nonlinear crystal 130, and for these two waists to be co-located and have approximately the same transverse size. The apparatus 100 may include a lens 184 that focuses the beam 194 to form a waist within the nonlinear crystal 130. Thermal lenses within the gain medium 120 affect both the intensity of the intracavity beam 192 within the nonlinear crystal 130 and the spatial overlap between the beams 192 and 194.

[0035] The power of the intracavity beam 192 depends directly on both the power of the pump beam 190 and the resonator losses imposed by the loss element 140. Due to the effects of thermal lensing in the gain medium 120, the pump power and the resonator losses each also indirectly affect the intensity distribution of the intracavity beam 192 in the nonlinear crystal 130. It can be seen that even though the power and beam parameters of the intracavity beam 192 are coupled through the effects of thermal lensing in the gain medium 120, the two degrees of freedom provided by the power of the pump beam 190 and the resonator losses imposed by the loss element 140 allow the power and beam parameters of the intracavity beam 192 to be controlled independently of each other.

[0036] 4-7 further explore the relationships between resonator loss, intracavity power, thermal lensing, and intracavity beam parameters. The particular relationships shown in FIGS. 4-7 are exemplary and are intended to illustrate the principles of the present invention and are not to be construed as limiting the scope of the present invention, but are merely illustrative of the principles of the present invention. 3+ This relates to an embodiment of device 100 in which the pump beam 190 is a continuous wave (cw) beam with a power of 105 Watts, the resonator 110 implements the photodiode 150 as a Faraday rotator and half-wave plate based on potassium terbium fluoride (KTF) crystal, and the loss element 140 is a polarized output coupler in combination with a rotatable half-wave plate.

[0037] FIG. 4 illustrates the relationship between outcoupling and intracavity power. FIG. 4 plots the measured intracavity power 410 (black circles) of the intracavity beam 192 as a function of the outcoupling percentage. In this specification, "intracavity power" refers to the power of the intracavity beam 192 circulating in the resonator 110. FIG. 4 also plots the outcoupling power 420 (white circles) of the outcoupling laser beam as a function of the outcoupling percentage. The outcoupling percentage represents the resonator loss. In practical implementations, other small losses exist due to various unavoidable imperfections such as light leakage through the cavity mirrors 112 and absorption in the Faraday rotator, and due to frequency conversion in the nonlinear crystal 130. As the outcoupling percentage increases to outcoupling more laser power, the intracavity power decreases significantly. 4 is based on experiments probing a range of outcoupling percentages from 2% to 16%. This range of outcoupling percentages results in a range of intracavity powers ranging from 1,175 watts (W) down to 325 Watts.

[0038] FIG. 5 illustrates the contribution from intracavity power to thermal effects in the gain medium 120. FIG. 5 plots the measured optical power 510 (black circles) of thermal lensing in the gain medium 120 as a function of the power of the intracavity beam 192. The optical power 510 is measured by evaluating the beam parameters of the leaky beam through one of the mirrors 112 and comparing the measured beam parameters to simulated beam parameters obtained for different values ​​of the optical power 510. In addition, FIG. 5 plots the measured heat load 520 (white circles) on the gain medium 120 as a function of the intracavity power. A thermoelectric cooler (TEC) heats or cools the gain medium 120 as needed to maintain a constant operating temperature. The power required by the TEC to maintain this constant operating temperature is used as a measure of the heat load. FIG. 5 plots the optical power 510 and heat load 520 of thermal lensing for the range of intracavity powers achieved in FIG. 4.

[0039] The optical power 510 of the thermal lens generally increases with intracavity power. At the lowest rated intracavity power of 325 watts, the thermal lens is approximately 3.7 m -1 At the highest rated intracavity power of 1,175 watts, the thermal lens is approximately 4.0 m -1 This demonstrates that the intracavity power contributes to thermal lensing within the gain medium 120. However, at least in this embodiment, the contribution to thermal lensing from the power of the intracavity beam 192 is less than that from the pump beam 190.

[0040] Rather than being monotonically dependent on intracavity power, the heat load 520 is at a relatively stable level for intracavity powers in the range of 550-900 watts, then increases in both directions away from this range. This behavior indicates that the intracavity beam 192 heats the gain medium 120 via more than a single heating mechanism. At higher intracavity powers above 900 watts, the added heat load is presumably due in large part to self-absorption of the intracavity beam 192. At lower intracavity powers below 550 watts, the added heat load is due to non-radiative relaxation and spontaneous emission during high population inversion.

[0041] Next, Figure 6 demonstrates the effect of intracavity power on intracavity beam parameters through the effects of thermal lensing. Figure 6 plots the measured beam diameter 610 (open circles) of the intracavity beam 192 in the gain medium 120 and the measured beam diameter 620 (closed circles) of the intracavity beam 192 at the beam waist. Figure 6 plots the beam diameters 610 and 620 as a function of intracavity power for the range of intracavity powers achieved in Figure 4. The beam diameter 610 in the gain medium 120 increases with intracavity power, while the beam waist diameter 620 of the intracavity beam 192 decreases.

[0042] 7 zooms in on the beam waist diameter 620 for improved clarity. As the intracavity power increases from 375 to 1,175 watts, the waist diameter decreases from about 545 to about 460 μm, a 15% decrease.

[0043] Referring again to Figure 3, caustics 310 and 320 are obtained from modeling the same embodiment of apparatus 100 evaluated in Figures 4-7. Caustic 310 corresponds to 325 watts of intracavity power and caustic 320 corresponds to 1,175 watts of intracavity power.

[0044] 3-7 demonstrate by way of example the influence of the cavity losses on both the power and the beam parameters of the intracavity beam 192 in the cavity 110. The intracavity power depends on the cavity losses and the pump power. Both the intracavity power and the pump power contribute to the thermal lensing effect, which in turn affects the size of the intracavity beam 192 in the nonlinear crystal 130. The frequency conversion in the nonlinear crystal 130 is sensitive to both the power and the size of the intracavity beam 192 in the nonlinear crystal 130. The power and the beam parameters of the frequency converted beam 196 therefore depend on both the pump power and the intracavity power, and the dependencies on these two parameters are mutually coupled. Surprisingly, the nature of these dependencies is such that it is conceivable as possible to independently control the power and the beam parameters of the frequency converted beam 196 over a fairly wide dynamic range using two degrees of freedom, namely the pump power and the cavity losses.

[0045] FIG. 8 is a flow chart for one intracavity frequency conversion method 800 applicable to solid-state laser resonators with end-pumped gain media. Method 800 may be implemented by apparatus 100 and is discussed in this context below. Method 800 controls the power and beam parameters of the frequency-converted laser beam independently of each other by taking advantage of the complex dependence of the power and beam parameters of the frequency-converted laser beam on the pump and intracavity powers. FIGS. 3-7 illustrate aspects of this complex dependence related to the intracavity power.

[0046] Method 800 includes steps 810 and 820. In step 810, apparatus 100 generates a frequency-converted beam 196 using resonator 110 and pump laser 160. Step 820 is a control step. In step 820, controller 170 controls certain aspects of how apparatus 100 performs step 810, such as controlling the power and at least one beam parameter of frequency-converted beam 196 (the power and beam parameter of frequency-converted beam 196 are also referred to as "output power" and "output beam parameter", respectively). The output beam parameter(s) controlled by controller 170 in step 810 include the waist size, waist location, divergence, and beam quality factor M of frequency-converted beam 196. 2 may include one or more of:

[0047] Step 810 includes steps 812, 814, and 816. In step 812, the pump laser 160 energizes the gain medium 120 with the pump beam 190, thereby generating an intracavity beam 192. In step 814, the loss element 140 imposes loss on the resonator 110. In step 816, the nonlinear crystal 130 generates a frequency-converted beam 196 by frequency converting a portion of the intracavity beam 192. In one embodiment of step 816, the intracavity beam 192 undergoes harmonic generation, e.g., second harmonic generation, in the nonlinear crystal 130. In another embodiment of step 816, the intracavity beam 192 mixes with another laser beam in the nonlinear crystal to generate the frequency-converted beam 196 through sum-frequency or difference-frequency mixing. In this embodiment, step 816 may include step 818 of superimposing beam 194 on intracavity beam 192 within nonlinear crystal 130. Step 818 may include generating beam 194 with laser 180. Step 818 may also include focusing beam 194 with, for example, lens 184 to form a waist within nonlinear crystal 130.

[0048] Step 820 includes steps 822 and 824. In step 822, the controller 170 adjusts the power of the pump beam 190 incident on the gain medium 120 in step 812. In step 824, the controller 170 adjusts the cavity loss imposed by the loss element 140 in step 814. Steps 822 and 824 cooperate to control the power and beam parameter(s) of the frequency converted beam 196. By appropriate selection of the pump power and loss in steps 822 and 824, respectively, the controller 170 can adjust the power of the frequency converted beam 196 independently from the beam parameter(s) of the frequency converted beam 196, and vice versa.

[0049] Although the method 800 applies to a symmetric implementation of the resonator 110, such as the resonator of FIG. 3, the method 800 may adjust the size of the waist of the intracavity beam 192 while keeping the location of the waist of the intracavity beam 192 unchanged. The method 800 may also be applied to an asymmetric implementation of the resonator 110, in which the caustics of the intracavity beam 192 evolve differently in two opposite directions away from the gain medium 120. Such an implementation of the resonator 110 may be realized by incorporating one or more other focusing elements that are asymmetrically positioned with respect to the gain medium 120. For an asymmetric implementation of the resonator 110, the waist size and waist location of the intracavity beam 192 are mutually coupled. Thus, the method 800 may be used to adjust the waist size and waist location of the intracavity beam 192 in an asymmetric implementation of the resonator 110. In one scenario, the waist location is the more important parameter, and the method 800 is used to optimize the waist location of the intra-cavity beam 192 .

[0050] The adjustments made to the pump power and losses in step 820 may be based on a pre-calibrated relationship between (a) pump power and losses, and (b) output power and beam parameter(s). Alternatively, or in combination, the adjustments effected by step 820 may be based at least in part on measured properties of the laser beam. Thus, an embodiment of method 800 includes at least one of two monitoring steps 830 and 840. Step 830 monitors the frequency converted beam 196 to obtain a measurement of its power, and optionally also one or more of its beam parameters. Step 840 monitors the intra-cavity beam 192 to obtain a measurement of the intra-cavity power. Step 840 may also evaluate the beam size of the intra-cavity beam 192. In one embodiment, the controller 170 adjusts the pump power and losses in step 820 based at least in part on the measurement of the power of the frequency converted beam 196 obtained in step 830. The pump power adjustments made in step 822 may be further based on measurements of the intra-cavity power obtained in step 840. The adjustments made in step 820 may also be based on measurements of at least one beam parameter of the frequency converted beam 196 obtained in step 830 or on beam size measurements of the intra-cavity beam 192 obtained in step 840.

[0051] Referring again to FIG. 1, the apparatus 100 may include one or more sensors 172 that respond to and measure the frequency converted beam 196 to obtain measurements of the output power and, optionally, one or more output beam parameters, in step 830 of the method 800. The sensor(s) 172 may be implemented in many different ways. The sensor(s) 172 may examine or otherwise detect scattered light emanating from the propagation of the frequency converted beam 196 in the nonlinear crystal 130, or, as shown in FIG. 1, a beam splitter 182 may direct a small portion of the frequency converted beam 196 toward the sensor(s) 172. The beam splitter 182 may be a pick-off mirror or another type of beam splitter.

[0052] In an embodiment, at least one sensor 172 is positioned to respond to the frequency converted beam 196 after processing by one or more downstream optical elements (not shown in FIG. 1 ) outside the resonator 110. For example, such a sensor 172 may respond to the ultraviolet frequency converted beam 196 after passing through one or more downstream optical elements that are exposed to UV degradation. In such an embodiment, the method 800 may advantageously adjust the operation of the resonator 110 to compensate for, for example, noise, fluctuations, and drift caused by the downstream optical element(s).

[0053] The apparatus 100 may include a sensor (not shown in FIG. 1 ) that monitors the intracavity beam 192 to obtain a measurement of the intracavity power in step 840 of the method 800. The sensor may be conveniently positioned in the path of the laser beam leaking out of one of the cavity mirrors 112, and may measure the power of such a leaking beam, for example, to obtain a measurement of the intracavity power. The apparatus 100 may include a sensor that monitors the beam size of the intracavity beam 192 in step 840 of the method 800.

[0054] Steps 810 and 820 may be performed in an active feedback loop along with one or both of steps 830 and 840. In one such scenario, the active feedback loop is used to stabilize the output power, and optionally also the output beam parameter(s), in the presence of environmental changes, UV degradation of optical elements, and / or other sources of noise, fluctuations, and drift.

[0055] Method 800 may also employ either of steps 830 and 840 in conjunction with controller 170 utilizing pre-calibrated relationships between (a) pump power and losses, and (b) output power and beam parameter(s). For example, controller 170 may use the pre-calibrated relationships in one iteration of step 820 to set the power and beam parameter(s) of frequency converted beam 196 to desired values, and then maintain these values ​​of output power and output beam parameter(s) by utilizing feedback from steps 830 and / or 840 in subsequent iterations of step 820.

[0056] The embodiment of the apparatus 100 generating the frequency converted beam 196 of ultraviolet light is susceptible to UV damage of the optical components. In particular, UV degradation of the nonlinear crystal 130 is likely to occur when the frequency converted beam 196 is ultraviolet. In the case of sum and difference frequency mixing with the ultraviolet beam 194, the beam 194 may also contribute to UV degradation of the nonlinear crystal 130 as well. The method 800 may be performed in an active feedback loop with monitoring of the frequency converted beam 196 in step 830 to compensate for the gradual UV degradation of the nonlinear crystal 130. If it becomes necessary to shift the location of the nonlinear crystal 130 to utilize an unexposed portion of the nonlinear crystal 130 for frequency conversion, the controller 170 may perform step 820 to reset the output power and, optionally, the output beam parameter(s) after such crystal shift. The controller 170 may perform this resetting with or without the use of feedback from steps 830 and / or 840.

[0057] Some laser processing tasks utilizing frequency converted laser beam 196 may require changing the output power and / or output beam parameter(s). Such changes may be implemented according to method 800. For example, controller 170 may perform step 820 to change the output power while keeping the output beam parameters substantially the same, or vice versa. Controller 170 may affect such changes according to a pre-calibrated relationship, measurements obtained in steps 830 and / or 840, or a combination thereof.

[0058] In an embodiment of the apparatus 100 configured for sum and difference frequency mixing in the nonlinear crystal 130, the controller 170 may perform step 820 to match the waist size and location of the intracavity beam 192 to the waist size and location of the beam 194 within the nonlinear crystal 130. The apparatus 100 and method 800 are capable of maintaining this matched waist size and waist location in the presence of various sources of noise, fluctuations, and drift. Additionally, the apparatus 100 and method 800 are capable of maintaining this matched waist size and waist location while purposefully varying the output power through suitable coordination between the pump power adjustments in step 822 and the loss adjustments in step 824.

[0059] Method 800 is applicable to both cw and pulsed operation of resonator 110 of apparatus 100. In pulsed embodiments, resonator 110 may further include a Q-switch, such as an AOM, EOM, or saturable absorber. In pulsed embodiments, the pulsed operation of resonator 110 may be synchronized with the pulsed operation of laser 180.

[0060] The method 800 is particularly useful in generating ultraviolet laser radiation. While AOMs and EOMs are reasonably inexpensive in the infrared and even visible spectral range, AOMs and EOMs for ultraviolet radiation can be cost prohibitive and their laser-induced damage can limit the performance of the apparatus 100. When the frequency-converted beam 196 is ultraviolet, the control scheme of the method 800 presents a cheaper alternative to conventional AOM- or EOM-based attenuation of the frequency-converted beam. In one related embodiment of the apparatus 100, the pump laser 160 and the gain medium 120 are configured to generate an infrared intracavity beam 192, and the nonlinear crystal 130 (or a series of nonlinear crystals 130) is configured to partially frequency convert the infrared intracavity beam 192 into an ultraviolet frequency-converted beam 196. The infrared intracavity beam 192 can be mixed with an ultraviolet beam 194 in the nonlinear crystal 130 to generate an ultraviolet frequency-converted beam 196 through sum-frequency or difference-frequency mixing.

[0061] In an embodiment of the apparatus 100 configured for generation of ultraviolet laser radiation through mixing with a beam 194, the gain medium 120 is a neodymium-doped yttrium orthovanadate crystal (Nd 2 O 4 ) that generates an intracavity beam 192 with a wavelength of 1,064 nanometers (nm). 3+ :YVO4) or neodymium-doped yttrium aluminum garnet (Nd 3+ The laser 180 is a frequency-quadrupled Nd:YAG crystal that generates a beam 194 with a wavelength of 266 nm. 3+ :YVO4 or Nd 3+ The laser is a :YAG laser, and the nonlinear crystal 130 is cesium lithium borate (CsLiBO 10 ) crystal, in which intracavity beam 192 undergoes sum-frequency mixing with beam 194 to generate a frequency-converted beam 196 with a wavelength of 213 nm.

[0062] In an embodiment of the apparatus 100 configured for generation of ultraviolet laser radiation without the use of an external laser beam, the gain medium 120 is a Nd 2 O 4 100 MHz gain medium that generates an intracavity beam 192 with a wavelength of 1,064 nm. 3+ : YVO4 crystal or Nd 3+ The resonator 110 includes two nonlinear crystals 130 arranged in series, the first being a YAG crystal. The first nonlinear crystal 130 frequency doubles a portion of the intracavity beam 192 to a wavelength of 532 nm. In the second nonlinear crystal 130, this frequency doubled laser beam undergoes sum frequency mixing with the remaining unconverted component of the intracavity beam 192 to generate a frequency converted beam 196 with a wavelength of 355 nm.

[0063] The method 800 and apparatus 100 may impose cavity losses through various loss mechanisms, including outcoupling a portion of the intracavity beam 192 from the resonator 110, absorbing a portion of the intracavity beam 192 in an absorbing medium, for example using a movable filter with spatially varying transmittance, and adjusting the alignment of the cavity mirrors 112 or another optical element that affects the propagation path of the intracavity beam 192 through the resonator 110.

[0064] 9 illustrates one laser apparatus 900 with intracavity frequency conversion in a solid-state laser resonator 910 having an end-pumped polarized solid-state gain medium 920, a photodiode, and a polarized output coupler. The apparatus 900 is an embodiment of the apparatus 100 that may perform the method 800. The gain medium 920 favors the amplification of a particular polarization component of the intracavity beam 192. The gain medium 920 may be, for example, a Nd 3+ : YVO4 crystal. The resonator 910 includes a Faraday rotator 930, a half-wave plate 932, and a polarizing output coupler 934. The Faraday rotator 930 and the half-wave plate 932 together form an embodiment of the photodiode 150. The half-wave plate 932 further cooperates with the output coupler 934 to form an embodiment of the loss element 140.

[0065] The output coupler 934 couples the polarization components of the intracavity beam 192 out of the resonator 910. The outcoupled polarization components are orthogonal to the polarization components favored for amplification in the gain medium 920. The output coupler 934 can be a polarizing beam splitter. In the embodiment depicted in FIG. 9, the output coupler 934 is a polarizing beam splitter that (a) reflects the favored polarization components of the intracavity beam 192 and (b) transmits the orthogonal polarization components out of the resonator 910 as the outcoupled laser beam 998. The output coupler 934 thereby functions as a cavity mirror that cooperates with the set of other cavity mirrors 912 to define the propagation path of the intracavity beam 192 within the resonator 910.

[0066] The half-wave plate 932 is rotatable and may be mounted on a motorized rotation mount. The controller 170 controls the polarization rotation angle of the half-wave plate 932. For example, the controller 170 controls the rotation angle of the optical axis of the half-wave plate 932 away from alignment with the polarization axis of the gain crystal 920. When the controller 170 sets the polarization rotation angle of the half-wave plate 932 to strictly counteract the polarization rotation imparted by the Faraday rotator 930, minimal cavity loss is achieved. The controller 170 increases cavity loss by rotating the half-wave plate 932 away from a polarization rotation angle that strictly counteracts the polarization rotation imparted by the Faraday rotator 930. In this state, the half-wave plate 932 only partially counteracts the polarization rotation imparted by the Faraday rotator 930, and thus a portion of the intracavity beam 192 is outcoupled by the output coupler 934.

[0067] In addition to a thermal lens in the gain medium, the resonator 910 (and other embodiments of the resonator 110) may include one or more focusing or defocusing elements, such as lenses or one or more curved cavity mirrors, to add optical power to the resonator 910. FIG. 9 depicts one such example, where the resonator 910 includes a focusing lens 940. Compared to an embodiment of the resonator 910 without the lens 940, the lens 940 reduces the waist of the intracavity beam 192 in the nonlinear crystal 130 and shortens the overall path length of the resonator 910. The smaller waist in the nonlinear crystal 130 allows for higher frequency conversion efficiency. The lens 940 may also improve the stability of the resonator 910 and increase the dynamic range of the output power and beam parameters over which they may be adjusted independently of one another.

[0068] In one embodiment of the apparatus 900 implementing the method 800, the controller 170 adjusts the polarization rotation angle of the half wave plate 932 to achieve an outcoupling percentage in the range of 0.05% to 60%. In an embodiment of the resonator 910 implementing the lens 940, stable operation of the resonator 910 with unidirectional propagation of the intracavity beam 192 has been demonstrated for outcoupling percentages as high as 60% through the output coupler 934. It is possible to utilize both losses and pump power adjustments in the apparatus 900 to achieve a wide dynamic range of either the power and beam parameters of the frequency converted beam 196 while keeping the other of the power and beam parameters substantially constant. For example, in a sum frequency mixing scenario, the inventors have demonstrated a dynamic range in the power of the frequency converted beam 196 from 100% down to 10% of the maximum frequency converted power while maintaining substantially identical beam parameters of the frequency converted beam 196.

[0069] 10 illustrates the simultaneous power and beam waist diameter control achieved using an embodiment of apparatus 900 implementing lens 940. In this embodiment, gain medium 920 is a Nd3+ : YVO4 crystal, pump laser 160 is a diode laser, and intracavity beam 192 has a wavelength of 1,064 nm. FIG. 10 shows three cross-sectional images 1010, 1020, and 1030 of intracavity beam 192 at the location of the waist in nonlinear crystal 130. Images 1010, 1020, and 1030 are drawn to the same scale. Each image 1010, 1020, and 1030 was obtained by imaging the leaky beam passing through laser cavity mirror 912, which directs intracavity beam 192 into nonlinear crystal 130. The leaky beam was imaged at a propagation distance from cavity mirror 912 equal to the propagation distance of nonlinear crystal 130. Each image 1010 , 1020 , and 1030 was acquired using a different combination of pump beam 190 power and outcoupling (OC) percentage through output coupler 934 .

[0070] In the case of image 1010, the pump power (P pump ) was 30 watts and the outcoupling percentage was 0.07%. This resulted in an intracavity power (P intra ) and 1 / e of the 614 μm × 605 μm intracavity beam 192 210, resulting in waist 1012 (measured along the major axis by the minor axis). For image 1020, the pump power and outcoupling percentage were both increased slightly to 35 Watts and 0.08%, respectively. These relatively small changes had a significant effect on the size of the beam waist, while the intracavity power was largely unaffected. Specifically, the waist size was reduced to 465 μm by 442 μm (see contour 1022), while the intracavity power was changed relatively small to 2.18 kW. To further reduce the size of the waist, both the pump power and outcoupling percentage were more substantially increased, resulting in image 1030. Here, the pump power was set to 85 Watts and the outcoupling percentage was set to 1.59%. This resulted in a significantly reduced waist size of 269 μm×259 μm (see contour 1032), while the intracavity power again only changed slightly to 2.09 kW.

[0071] 10 demonstrates the ability of the apparatus 900 and method 800 to control the size of the beam waist of the intracavity beam 192 over a wide dynamic range while maintaining a substantially constant intracavity power. With adjustments made to account for the correlation between waist size and frequency conversion efficiency in the nonlinear crystal 130, this capability translates to control of beam parameters of the frequency converted beam 196 independent of its power.

[0072] 9, the half-wave plate 932 forms part of both the photodiode and the adjustable loss element. In a modification of the device 900, the photodiode is implemented separately from the loss element, and the half-wave plate 932 is used exclusively to control the resonator loss. In addition, the concept of using a rotatable half-wave plate and a polarizing output coupler (e.g., the half-wave plate 932 and the output coupler 934) can be implemented in other embodiments of the resonator 110.

[0073] The outcoupling scheme based on a rotatable half-wave plate is just one example of imposing cavity losses by outcoupling a portion of the intracavity beam 192. Alternatives include incorporating an AOM or EOM in the path of the intracavity beam 192 within the cavity 110. While this modulator-based solution may add cost and complexity, it is a convenient scheme in Q-switching embodiments of the cavity 110 that already implement an AOM or EOM for Q-switching purposes.

[0074] The present invention has been described above in terms of preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Rather, the present invention is limited only by the claims appended hereto.

Claims

1. A method for intracavity frequency conversion, wherein the method is The steps include generating an intracavity laser beam circulating within a laser resonator by end-pumping a solid gain medium within the laser resonator using a pump laser beam having a certain pump power, The steps include imposing a loss on the intracavity laser beam, The steps include generating a frequency-converted laser beam having a certain output power by frequency-converting a portion of the intracavity laser beam within a nonlinear crystal located in the laser resonator, A step of adjusting the pump power and the loss to control the output power and at least one output beam parameter of the frequency-converted laser beam, wherein the at least one output beam parameter is selected from the group consisting of beam waist size, beam waist location, beam divergence angle, and beam quality coefficient. Methods that include...

2. The method according to claim 1, wherein the pump laser beam and the intracavity laser beam each contribute to the thermal lensing effect in the gain medium, and the contribution of the pump laser beam to the thermal lensing effect exceeds the contribution of the intracavity laser beam to the thermal lensing effect.

3. The method according to claim 1 or claim 2, wherein the frequency-converted laser beam is an ultraviolet laser beam.

4. The method according to claim 3, wherein the adjustment step includes stabilizing the output power and the at least one output beam parameter in the presence of ultraviolet degradation of the nonlinear crystal.

5. The method according to claim 3, further comprising shifting the nonlinear crystal in a transverse direction with respect to the propagation direction of the intracavity laser beam through the nonlinear crystal, wherein the adjusting step comprises resetting the output power and / or the at least one output beam parameter after the shift.

6. The method according to claim 1 or 2, wherein the adjusting step includes (a) changing the at least one output beam parameter while keeping the output power constant, or (b) changing the output power while keeping the at least one output beam parameter constant.

7. The method according to claim 1 or 2, wherein the at least one output beam parameter includes the waist size and location of the frequency-converted laser beam.

8. The method according to claim 1 or 2, further comprising obtaining a measurement of the output power by monitoring the frequency-converted laser beam, wherein the adjusting step comprises adjusting the pump power and the losses based at least in part on the measurement of the output power.

9. The method according to claim 8, further comprising obtaining a measurement of the power of the intracavity laser beam by monitoring the intracavity laser beam, wherein the adjustment of the pump power in the adjustment step is further based on the measurement of the power of the intracavity laser beam.

10. The method according to claim 8, further comprising obtaining a measurement of the at least one output beam parameter by monitoring the frequency-converted laser beam, wherein the adjustment of the pump power and the loss in the adjustment step is further based on the measurement of the at least one output beam parameter.

11. The method according to claim 1 or 2, wherein the frequency conversion step includes superimposing a second laser beam onto the intracavity laser beam in the nonlinear crystal, thereby generating the frequency-converted laser beam from a mixture of the intracavity laser beam and the second laser beam.

12. The superimposing step includes focusing the second laser beam onto the waist within the nonlinear crystal, The method according to claim 11, wherein the adjusting step includes matching the waist size and location of the intracavity laser beam to the waist size and location of the second laser beam.

13. The method according to claim 11, wherein at least one of the second laser beam and the frequency-converted laser beam is ultraviolet light.

14. The method according to claim 1 or 2, wherein the imposing step includes externally coupling a portion of the intracavity laser beam from the laser resonator.

15. The method according to claim 14, further comprising optically selecting a polarization component of the intracavity laser beam for amplification in the gain medium, wherein the imposing step includes rotating the polarization of the intracavity laser beam so as to move away from the selected polarization component, the externally coupled portion being a polarization component orthogonal to the selected polarization component.

16. The method according to claim 15, further comprising using a photodiode to restrict the propagation of the intracavity laser beam in the laser resonator to a unidirectional circulation.

17. The method according to claim 16, wherein the photodiode includes a Faraday rotor and a half-wave plate, and the rotating step includes setting the half-wave plate to partially oppose the polarization rotation by the Faraday rotor in each passage of the intracavity laser beam through the photodiode.

18. The method according to claim 14, wherein the imposing step involves externally coupling the portion using an acoustic-optic modulator or an electro-optic modulator.

19. A laser apparatus with intracavity frequency conversion, wherein the laser apparatus is A laser resonator, wherein the laser resonator is Solid-state gain medium and Nonlinear crystals and A controllable loss element is provided to impose a controllable loss on the laser resonator, A laser resonator, A pump laser for generating a pump laser beam having a certain pump power, wherein the pump laser is arranged to end-pump the gain medium to generate an intracavity laser beam circulating within the laser resonator, and the intracavity laser beam undergoes partial frequency conversion within the nonlinear crystal to generate a frequency-converted laser beam having a certain output power, One or more sensors for monitoring the output power and at least one output beam parameter of the frequency-converted laser beam, wherein the at least one output beam parameter is selected from the group consisting of beam waist size, beam waist location, beam divergence angle, and beam quality coefficient, A controller configured to control the output power and the at least one output beam parameter by adjusting the pump power and the losses according to monitored values ​​of the output power and the at least one output beam parameter. A laser device equipped with the following features.

20. The laser apparatus according to claim 19, further comprising a second laser for mixing with the intracavity laser beam by delivering a second laser beam to the nonlinear crystal to generate the frequency-converted laser beam.

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