Intracavity frequency conversion in solid state laser resonators using end pumping

Abstract

An intra-cavity frequency conversion method includes end-pumping a solid state gain medium (120) in a laser resonator (110) with a pump laser beam (190) to produce an intra-cavity laser beam (192) circulating in the laser resonator (110), and frequency-converting a portion of the intra-cavity laser beam (192) in a nonlinear crystal (130) located in the laser resonator (110) to produce a frequency-converted laser beam (196). 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 resonator loss exerted on the intracavity laser beam (192). Using the contribution of the pump laser beam (190) and the intracavity laser beam (192) to the thermal lens effect in the gain medium, the control scheme enables control of the output power and output beam parameters independently of each other.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 332,983, filed on April 20, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to intracavity frequency conversion of laser radiation in an end-pumped solid-state laser resonator. In particular, the present invention relates to controlling the power and beam parameters of the intracavity laser radiation in the presence of thermal lensing in the laser gain medium, thereby controlling the power and beam parameters of the frequency-converted laser radiation. Background Art

[0004] The gain medium of a solid-state laser or laser amplifier is a solid matrix material doped with optically active ions that are capable of producing or amplifying laser radiation when excited. The matrix material is usually glassy or crystalline, and the optically active ions are usually rare earth or transition metal ions, such as neodymium, erbium, ytterbium, or titanium. The gain medium can be in the form of bulk crystals / glass or optical fibers. Most bulk gain media are in the form of rods or plates. Solid-state lasers have many advantages over gas lasers, including simple operation, high efficiency, high reliability, compactness, and low cost.

[0005] Typically, solid-state laser gain media are optically pumped, i.e., optically active ions are excited by light to provide the population inversion required for laser 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 high efficiency, compactness, long lifetime, and low cost. Some systems utilize arrays of laser diodes to provide the required pump powers, up to hundreds of watts or even kilowatts.

[0006] In the case of laser pumping of bulk gain media, several different pumping geometries can be employed. In end pumping, the pump laser radiation propagates in the same or opposite direction as the intracavity laser radiation generated in the bulk gain medium and circulating in the laser resonator. Side pumping requires directing the pump laser radiation into the gain medium through a plane parallel to the propagation direction of the output laser beam, so that the propagation direction of the pump laser radiation is approximately perpendicular to the propagation direction of the intracavity laser radiation.

[0007] End pumping allows good spatial overlap between the pump laser radiation and the intracavity laser radiation and minimizes the pump energy loss in the part of the bulk gain medium that does not participate in the lasing action, resulting in greater laser gain. End pumping is also a favorable geometry for thermal management, because 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 laser radiation or the output laser radiation. However, at high pump powers, end pumping can produce a thermal lens in the bulk gain medium in the path of the laser radiation. The thermal lens is mainly due to the thermo-optic effect, i.e., 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 size of the thermal lens.

[0008] Although solid-state lasers can generate laser radiation at many different wavelengths, the initially generated laser radiation may need to be frequency converted to reach certain wavelengths, particularly in the ultraviolet (UV) spectral range. Laser beams can be frequency converted in nonlinear crystals by 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.

[0009] Any given laser application has certain requirements for laser power and laser beam parameters. Typically, the specified beam parameters are (a) beam waist size, (b) beam waist position, and (c) beam divergence or beam quality factor M. 2 . Power and beam parameter specifications can be very strict. Some laser applications rely on the adjustability of laser power, sometimes in combination with adherence to strict beam parameter specifications. While the output power of a laser device can be adjusted by adjusting the internal operation of the laser device, such adjustments often have other consequences. 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 the pump-induced thermal lensing is not negligible, the beam parameters of the intracavity laser beam will also be affected, since the thermal lensing makes the gain medium an element in the laser resonator with optical power. To avoid such problems, laser power adjustment is often achieved by simply attenuating the output laser beam using, for example, an acousto-optic modulator (AOM) or an electro-optic modulator (EOM). SUMMARY OF THE INVENTION

[0011] 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 utilizing end pumping and intracavity frequency conversion. The present control scheme is applicable to situations where there is non-negligible thermal lensing in the solid-state gain medium. The control scheme is based on our discovery that independent adjustment of the pump power and the resonator loss facilitates independent control of the power and beam parameters of the intracavity laser beam over a wide power and beam parameter range. This is because both the pump laser beam and the intracavity laser beam contribute to the thermal lensing in the laser gain medium. Independently controlling the power and beam parameters of the intracavity laser beam is equivalent to independently controlling the power and beam parameters of the frequency converted laser beam. In a less powerful control scheme based on adjusting only one of the pump power and the resonator loss, the beam parameters of the intracavity laser beam will be directly coupled to its power, and this coupling will be transferred to the frequency converted laser beam at least to some extent. The present control scheme does not require an AOM or EOM to attenuate the frequency converted laser beam. In particular, when the frequency converted laser beam is ultraviolet, such a modulator can be expensive and may itself further modify the beam parameters.

[0012] The present control scheme adds versatility and controllability to laser devices based on solid-state laser resonators utilizing end pumping and intracavity frequency conversion. With this control scheme, the power and beam parameters of a frequency converted laser beam can be customized to meet a range of specifications without reconfiguring the hardware. Active stabilization of pump power and / or resonator losses can be employed to maintain the desired power and beam parameters in the face of environmental changes, UV degradation of optical components, and other sources of noise, fluctuations, and drift. The control scheme can also be used to change the power and / or beam parameters of a frequency converted laser beam during a process. For example, the frequency converted laser power can be increased or decreased (within a certain range) while maintaining the same beam parameters.

[0013] In one aspect, an intracavity frequency conversion method comprises the following steps: (a) end-pumping a solid-state gain medium in a laser resonator with a pump laser beam having a pump power to generate an intracavity laser beam circulating in the laser resonator; (b) applying a loss to the intracavity laser beam; (c) frequency converting a portion of the intracavity laser beam in a nonlinear crystal located in the laser resonator to generate a frequency-converted laser beam having an output power; and (d) 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. The at least one output beam parameter is selected from the group consisting of a beam waist size, a beam waist position, a beam divergence angle, and a beam quality factor.

[0014] In another aspect, a laser device with intracavity frequency conversion includes a laser resonator with a solid-state gain medium, a nonlinear crystal, and an adjustable loss element arranged to apply adjustable loss to the laser resonator. The laser device also 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 circulating 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 device also includes one or more sensors for monitoring the 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 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 beam waist size, a beam waist position, a beam divergence angle, and a beam quality factor. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention by way of illustration 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.

[0016] Figure 1 A laser device with intracavity frequency conversion in a solid-state laser resonator with an end-pumped solid-state gain medium is shown according to an embodiment.

[0017] Figure 2 Shows the incident Figure 1 Embodiment of a device with a pump beam on the end face of the gain medium and an intracavity beam.

[0018] Figure 3 The thermal lensing effect in the gain medium is shown Figure 1 Figure 4. Effect of caustics of an intracavity beam in an embodiment of a resonator of a device.

[0019] Figure 4 Shows Figure 1 Relationship between output coupling and intracavity power in one embodiment of the device.

[0020] Figure 5 Shows Figure 4 of Figure 1 Contribution of intracavity power to thermal effects in the gain medium in device embodiments.

[0021] Figure 6 Shows Figure 4 of Figure 1 The intracavity power in the device embodiment affects the intracavity beam parameters through the thermal lens effect.

[0022] Figure 7 and Figure 6 Similar, but with enlarged waist diameter.

[0023] Figure 8 is a flow chart of an intracavity frequency conversion method applicable to a solid-state laser resonator with an end-pumped gain medium according to an embodiment.

[0024] Fig. 9 A laser device with intracavity frequency conversion in a solid-state laser resonator with end pumping, polarization solid-state gain medium, optical diodes, and polarization output couplers according to an embodiment is shown.

[0025] Fig.10 Shows the use of Fig. 9 Embodiments of the apparatus enable simultaneous power and beam waist diameter control. DETAILED DESCRIPTION OF THE INVENTION

[0027] Referring now to the drawings, wherein like components are represented by like numerals, Figure 1 A laser device 100 is shown having intracavity frequency conversion in a solid-state laser resonator 110 having an end-pumped solid-state gain medium 120. The resonator 110 includes the solid-state gain medium 120, a nonlinear crystal 130, and a tunable loss element 140. The device 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 excites the gain medium 120 to induce lasing, thereby generating an intracavity laser beam 192 that circulates in the resonator 110. The intracavity beam 192 undergoes partial frequency conversion in the nonlinear crystal 130. That is, a portion of the intracavity beam 192 is converted to a different frequency in the nonlinear crystal 130, thereby generating a frequency-converted laser beam 196. The loss element 140 applies adjustable loss to the intracavity beam 192 in the resonator 110, for example by coupling out or absorbing portions of the intracavity beam 192. The controller 170 sets the power of the pump beam 190 and the loss applied by the loss element 140 to control the power and one or more beam parameters of the intracavity beam 192, 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 a beam waist size, a beam waist position, a beam divergence angle, and a beam quality factor.

[0028] The device 100 can be configured to facilitate any of several different types of frequency conversion in the nonlinear crystal 130, including harmonic generation, sum frequency mixing, and difference frequency mixing. In embodiments of the device 100 configured for sum frequency or difference frequency mixing, the intracavity beam 192 can be mixed with another laser beam 194 in the nonlinear crystal 130 to produce a frequency converted beam 196. Such embodiments of the device 100 can include an external laser 180 that generates the beam 194.

[0029] exist Figure 1 In the illustrated embodiment, 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 in some cases for intracavity frequency conversion, at least because it can restrict the propagation of the intracavity laser radiation to one direction. When the intended purpose of the device 100 is to generate harmonics 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 generally preferred. The resonator 110 may include an optical diode 150, such as a Faraday rotator and a wave plate to ensure the unidirectional propagation of the intracavity beam 192 in the resonator 110. When the intended purpose of the device 100 is sum or difference frequency mixing, the propagation direction of the laser beam 194 ensures that the frequency converted laser radiation generated in the nonlinear crystal 130 is contained in a single laser beam, regardless of whether the resonator 110 is a ring resonator or a linear standing wave resonator. However, a ring resonator may be preferred for other reasons, for example, to minimize power noise in the frequency converted beam 196.

[0030] although Figure 1 Although not shown in the figure, the resonator 110 may include a series of nonlinear crystals 130, such as two nonlinear crystals 130 arranged in series. In one embodiment thereof, the first nonlinear crystal 130 converts part of the intracavity beam 192 into its second harmonic. This second harmonic beam is then sum-frequency mixed with the unconverted part of the intracavity beam 192 in the second nonlinear crystal 130 to produce the third harmonic of the intracavity beam 192. In the following, unless otherwise stated, for simplicity, it is assumed that there is only one nonlinear crystal 130 in the resonator 110. However, the discussion can be easily extended to embodiments with multiple nonlinear crystals 130.

[0031] The frequency converted beam 196 can propagate at an angle to the intracavity beam 192 in the nonlinear crystal 130, as determined by the phase matching condition. Depending on the size of the angle, the frequency converted beam 196 can pass through one of the cavity mirrors 112 (or another optical element defining the resonator 110) and thereby exit the resonator 110. The frequency converted beam 196 can also be separated from the intracavity beam 192 by dispersion, for example using a nonlinear crystal 130 with Brewster cut input and output faces. Alternatively, the device 100 may include a dichroic beam splitter or another optical element ( Figure 1 ), for extracting the frequency-converted light beam 196 from the resonator 110.

[0032] Figure 2 A pump beam 190 and an intracavity beam 192 are shown incident on an end face 122 of a gain medium 120. The gain medium 120 is a bulk crystal or glass. In the illustrated embodiment, the gain medium 120 is a cubic plate with a square end face 122. The gain medium 120 can have different shapes. For example, the gain medium 120 can be a plate with a rectangular end face 122, or the gain medium 120 can be a rod with a circular end face 122. The intracavity beam 192 can be at least approximately a Gaussian beam. The pump beam 190 can be approximately a Gaussian beam, can have an intensity profile that is approximately a flat top, or can have an intensity profile that is somewhere in between. The cross-section of the pump beam 190 and / or the intracavity beam 192 can be elliptical instead of circular.

[0033] Regardless of the shapes of the gain medium 120, the pump beam 190, and the intracavity beam 192, the lateral extent 290D of the pump beam 190 is preferably smaller than the corresponding lateral extent 220L of the gain medium 120 to avoid wasting pump energy. In a typical operating state, the lateral extent 292D of the intracavity beam 192 is smaller than the lateral extent 290D of the pump beam 190. Each of the lateral extents 290D and 292D may represent 1 / e of the corresponding lateral intensity distribution. 2 In one embodiment, the lateral extent 220L of the gain medium 120 is in the range of 1 to 10 millimeters, the lateral extent 290D of the pump beam 190 is in the range of 100 to 3000 micrometers (μm), and the lateral extent 292D of the intracavity beam 192 is in the range between 50 and 3000 μm or between 50% and 100% of the lateral extent 290D.

[0034] The pump beam 190 causes a thermal lens 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 lens effect in the gain medium 120. The resulting thermal lens caused by the combination of the pump beam 190 and the intracavity beam 192 affects the beam parameters of the intracavity beam 192 during propagation through the resonator 110.

[0035] Figure 3 is a diagram 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. Figure 3 The caustics of the intracavity beam 192 are plotted for two different powers of the thermal lens in the gain medium 120. Each caustic is shown as 1 / e 2 The relationship between the beam radius w and the position z along the propagation path of the intracavity beam 192 in the resonator 110. At any given position z, the distance between the top and bottom curves of the same caustic represents the 1 / e value at this position z. 2 Beam diameter 2w. 3.7m -1 (m -1 ) produces a caustic 310 characterized by a waist w0. The nonlinear crystal 130 is advantageously located at the waist of the intracavity beam 192. The thermal lens is increased by about 8% to 4.0 m -1 Caustics 320 are generated. As the thermal lens increases, the beam diameter increases in the gain crystal 120 and decreases in the nonlinear crystal 130. In the nonlinear crystal 130, this decrease is characterized by a waist w0' that is approximately 20% smaller than the waist w0. In this embodiment, the waist position z0 remains unchanged.

[0036] Consider an embodiment in which the apparatus 100 is configured to generate the frequency converted beam 196 as the second harmonic of the intracavity beam 192. In this embodiment, the local frequency conversion efficiency is proportional to the square of the intensity of the intracavity beam 192. Therefore, the frequency conversion efficiency is approximately inversely proportional to the fourth power of the beam radius. For this second harmonic embodiment, the local frequency conversion efficiency is proportional to the square of the intensity of the intracavity beam 192. Figure 3 In the embodiment, assuming that the nonlinear crystal 130 coincides with the waist of the intracavity beam 192 (as shown in the figure), the thermal lens power is from 3.7m -1 Increase to 4.0m -1 This results in more than doubling the frequency conversion efficiency, as long as the power of the intracavity beam 192 is the same (or greater) for the caustics 320 than for the caustics 310 .

[0037] In embodiments where the intracavity beam 192 is sum-frequency or difference-frequency mixed 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 in the nonlinear crystal 130 must also be considered. Generally, the best case is that the two beams 192 and 194 form a waist in the nonlinear crystal 130, and the two waists are co-located and have approximately the same lateral dimensions. The device 100 may include a lens 184 that focuses the beam 194 to form a waist in the nonlinear crystal 130. Thermal lensing in the gain medium 120 affects the intensity of the intracavity beam 192 in the nonlinear crystal 130 and the spatial overlap between the beams 192 and 194.

[0038] The power of the intracavity beam 192 depends directly on the power of the pump beam 190 and the resonator loss imposed by the loss element 140. Each of the pump power and the resonator loss also indirectly affects the intensity distribution of the intracavity beam 192 in the nonlinear crystal 130 due to the thermal lens effect in the gain medium 120. Even though the power and beam parameters of the intracavity beam 192 are coupled through the thermal lens effect in the gain medium 120, it turns out that the two degrees of freedom provided by the power of the pump beam 190 and the resonator loss imposed by the loss element 140 allow the power and beam parameters of the intracavity beam 192 to be controlled independently of each other.

[0039] Figures 4 to 7 The relationship between resonator loss, intracavity power, thermal lens effect and intracavity beam parameters is further explored. Figures 4 to 7 The specific relationships shown in are exemplary and relate to embodiments of the apparatus 100 in which the gain medium 120 is a neodymium-doped yttrium vanadate (Nd 3+ :YVO4) crystal, the pump beam 190 is a continuous wave (cw) beam with a power of 105 watts, the resonator 110 implements the optical diode 150 as a Faraday rotator based on potassium terbium fluoride (KTF) crystal and a half-wave plate, and the loss element 140 is a polarization output coupler combined with a rotatable half-wave plate.

[0040] Figure 4 The relationship between output coupling and intracavity power is shown. Figure 4 The measured intracavity power 410 (solid circles) of the intracavity beam 192 is plotted versus the output coupling percentage. As used herein, “intracavity power” refers to the power of the intracavity beam 192 circulating in the resonator 110 . Figure 4The output coupled power 420 (open circles) of the output coupled laser beam is also plotted versus the output coupling percentage. The output coupling percentage represents the resonator losses. In a practical implementation, there will be other small losses due to various unavoidable imperfections, such as light leakage through the cavity mirror 112 and absorption in the Faraday rotator, and due to frequency conversion in the nonlinear crystal 130. As the output coupling percentage increases to couple out more laser power, the intracavity power drops significantly. Figure 4 Based on an experiment, the experiment explored a range of output coupling percentages from 2% to 16%. This output coupling percentage range resulted in a range of intracavity powers from 1175 Watts (W) down to 325 W.

[0041] Figure 5 The contribution of the intracavity power to the thermal effects in the gain medium 120 is shown. Figure 5 The measured optical power 510 (filled circles) of the thermal lens in the gain medium 120 is plotted versus the power of the intracavity beam 192. The optical power 510 is measured by evaluating the beam parameters of a leakage beam through one of the mirrors 112 and comparing the measured beam parameters with simulated beam parameters obtained for different values ​​of the optical power 510. In addition, Figure 5 The measured heat load 520 (open circles) on the gain medium 120 is plotted versus the intracavity power. A thermoelectric cooler (TEC) heats or cools the gain medium 120 as needed to maintain a constant operating temperature. The electrical power required by the TEC to maintain this constant operating temperature is used as a measure of the heat load. Figure 5 Draw Figure 4 Thermal lens optical power 510 and thermal load 520 within the intracavity power range achieved in .

[0042] The thermal lens optical power 510 generally increases with the intracavity power. At the lowest evaluated intracavity power of 325 watts, the thermal lens power is approximately 3.7 m -1 At the highest evaluated intracavity power of 1175 watts, the thermal lens power is about 4.0 m -1 This indicates that the intracavity power contributes to the thermal lensing effect in the gain medium 120. However, at least in this embodiment, the power of the intracavity beam 192 contributes less to the thermal lensing effect than the contribution of the pump beam 190.

[0043] The heat load 520 is not monotonically dependent on the intracavity power, but is relatively stable over the intracavity power range between 550 and 900 Watts, and then increases in both directions away from this range. This behavior suggests that the intracavity beam 192 heats the gain medium 120 via more than one heating mechanism. At higher intracavity powers (above 900 Watts), the increased heat load is probably mainly due to self-absorption of the intracavity beam 192. At lower intracavity powers (below 550 Watts), the increased heat load is due to nonradiative relaxation and spontaneous emission, with higher population inversion.

[0044] Next, Figure 6 The influence of intracavity power on intracavity beam parameters is demonstrated through thermal lens effect. Figure 6 A measured beam diameter 610 (open circles) of the intracavity beam 192 in the gain medium 120 and a measured beam diameter 620 (filled circles) of the intracavity beam 192 at the beam waist are plotted. Figure 6 Draw for Figure 4 The beam diameter 610 and 620 of the intracavity power range achieved in FIG. 6A and 6B increase with increasing intracavity power, while the beam waist diameter 620 of the intracavity beam 192 decreases.

[0045] Figure 7 The beam waist diameter of 620 was enlarged for improved clarity. As the intracavity power increased from 375 to 1175 W, the waist diameter decreased from about 545 to about 460 microns, a 15% reduction.

[0046] Reference again Figure 3 , caustics 310 and 320 are from Figures 4 to 7 1. The caustic line 310 corresponds to an intracavity power of 325 watts and the caustic line 320 corresponds to an intracavity power of 1175 watts.

[0047] Figures 3 to 7 The effects of resonator losses on the power and beam parameters of the intracavity beam 192 in the resonator 110 are demonstrated by way of example. The intracavity power depends on the resonator 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. Therefore, the power and beam parameters of the frequency converted beam 196 depend on the pump power and the intracavity power, and the dependence on these two parameters is coupled to each other. Surprisingly, the nature of these dependencies allows the power and beam parameters of the frequency converted beam 196 to be independently controlled over a fairly wide dynamic range using two degrees of freedom (i.e., pump power and resonator losses).

[0048] Figure 8 8 is a flow chart of an intracavity frequency conversion method 800 applicable to a solid-state laser resonator with an end-pumped gain medium. The method 800 may be performed by the apparatus 100 and is discussed below. The method 800 utilizes the complex dependence of the power and beam parameters of the frequency-converted laser beam on the pump and intracavity powers to control the power and beam parameters of the frequency-converted laser beam independently of each other. Figures 3 to 7 Aspects of this complex dependence on intracavity power are illustrated.

[0049] Method 800 includes steps 810 and 820. In step 810, device 100 generates frequency-converted light beam 196 using resonator 110 and pump laser 160. Step 820 is a control step. In step 820, controller 170 controls how device 100 performs certain aspects of step 810 so as to control the power and at least one beam parameter of frequency-converted light beam 196. (The power and beam parameters of frequency-converted light beam 196 are also referred to as "output power" and "output beam parameters," respectively.) The output beam parameters controlled by controller 170 in step 810 may include one or more of waist size, waist position, divergence, and beam quality factor M2 of frequency-converted light beam 196.

[0050] Step 810 includes steps 812, 814, and 816. In step 812, the pump laser 160 excites the gain medium 120 with the pump beam 190, thereby generating an intracavity beam 192. In step 814, the loss element 140 applies loss to the resonator 110. In step 816, the nonlinear crystal 130 frequency converts a portion of the intracavity beam 192 to generate a frequency-converted beam 196. In one embodiment of step 816, the intracavity beam 192 undergoes harmonic generation, such as second harmonic generation, in the nonlinear crystal 130. In another embodiment of step 816, the intracavity beam 192 is mixed with another laser beam in the nonlinear crystal to generate the frequency-converted beam 196 by sum frequency or difference frequency mixing. In this embodiment, step 816 may include step 818 of superimposing the beam 194 on the intracavity beam 192 in the nonlinear crystal 130. Step 818 may include generating the beam 194 by the laser 180. Step 818 may also include focusing beam 194 , such as using lens 184 , to form a waist in nonlinear crystal 130 .

[0051] 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 resonator loss applied by the loss element 140 in step 814. Steps 822 and 824 cooperate to control the power and beam parameters of the frequency-converted beam 196. By appropriately selecting the pump power and loss in steps 822 and 824, the controller 170 is able to adjust the power of the frequency-converted beam 196 independently of the beam parameters of the frequency-converted beam 196, and vice versa.

[0052] When method 800 is applied to a symmetric implementation of resonator 110 (eg Figure 3 When the resonator 110 is asymmetric, the method 800 can adjust the waist size of the intracavity beam 192 while keeping the waist position of the intracavity beam 192 unchanged. The method 800 can 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 can be achieved by incorporating one or more other focusing elements that are asymmetrically positioned relative to the gain medium 120. For an asymmetric implementation of the resonator 110, the waist size and the waist position of the intracavity beam 192 are coupled to each other. Therefore, the method 800 can be used to adjust the waist size and the waist position of the intracavity beam 192 in an asymmetric implementation of the resonator 110. In one case, the waist position is a more critical parameter, and the method 800 is used to optimize the waist position of the intracavity beam 192.

[0053] The adjustments made to the pump power and loss in step 820 may be based on a pre-calibrated relationship between (a) the pump power and loss and (b) the output power and beam parameters. Alternatively, or in combination therewith, the adjustments made by step 820 may be based at least in part on measured laser beam characteristics. Thus, certain embodiments of method 800 include at least one of two monitoring steps 830 and 840. Step 830 monitors the frequency-converted beam 196 to obtain a measure of its power, and optionally, may also obtain one or more beam parameters thereof. Step 840 monitors the intracavity beam 192 to obtain a measure of the intracavity power. Step 840 may also assess the beam size of the intracavity beam 192. In one embodiment, the controller 170 adjusts the pump power and loss in step 820 based at least in part on the measure of the power of the frequency-converted beam 196 obtained in step 830. The pump power adjustments made in step 822 may also be based on the measure of the intracavity power obtained in step 840. The adjustments made in step 820 may also be based on a measurement of at least one beam parameter of the frequency-converted beam 196 obtained in step 830 or a beam size measurement of the intra-cavity beam 192 obtained in step 840 .

[0054] Reference again Figure 1 , the apparatus 100 may include one or more sensors 172 that interrogate the frequency converted light beam 196 to obtain a measure of the output power and optionally a measure of one or more output beam parameters in step 830 of the method 800. The sensors 172 may be implemented in a variety of different ways. The sensors 172 may observe or otherwise detect scattered light originating from the propagation of the frequency converted light beam 196 in the nonlinear crystal 130, or as Figure 1 As shown, beam splitter 182 can direct a small portion of frequency converted light beam 196 to sensor 172. Beam splitter 182 can be a pickup mirror or another type of beam splitter.

[0055] In some embodiments, at least one sensor 172 is positioned to be located adjacent to one or more downstream optical elements ( Figure 1 The method 800 may interrogate the frequency converted light beam 196 after processing by the ultraviolet light sensor 172 (not shown). For example, such a sensor 172 may interrogate the ultraviolet frequency converted light beam 196 after passing through one or more downstream optical components that are subject to ultraviolet degradation. In such an embodiment, the method 800 may advantageously adjust the operation of the resonator 110 to compensate for noise, fluctuations, and drift caused by, for example, the downstream optical components.

[0056] The device 100 may include a sensor ( Figure 1 The sensor monitors the intracavity beam 192 to obtain a measure of the intracavity power in step 840 of method 800. The sensor can be conveniently positioned in the path of a laser beam leaking out of one of the cavity mirrors 112 and, for example, measures the power of such a leaking beam to obtain a measure of the intracavity power. The apparatus 100 can also include a sensor that monitors the beam size of the intracavity beam 192 in step 840 of method 800.

[0057] 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 parameters, in the presence of environmental changes, UV degradation of optical components, and / or other sources of noise, fluctuations, and drift.

[0058] The method 800 may also employ either of steps 830 and 840 in conjunction with the controller 170 utilizing pre-calibrated relationships between (a) pump power and loss and (b) output power and beam parameters. For example, the controller 170 may use the pre-calibrated relationships to set the power and beam parameters of the frequency-converted beam 196 to desired values ​​in one iteration of step 820, and then utilize feedback from steps 830 and / or 840 in subsequent iterations of step 820 to maintain these values ​​of output power and output beam parameters.

[0059] Embodiments of the apparatus 100 that generate the ultraviolet frequency converted beam 196 are susceptible to UV damage of optical components. Specifically, when the frequency converted beam 196 is ultraviolet, ultraviolet degradation of the nonlinear crystal 130 may occur. In the case of sum and difference frequency mixing with the ultraviolet beam 194, the beam 194 may also cause ultraviolet degradation of the nonlinear crystal 130. The method 800 can be performed in an active feedback loop, including monitoring the frequency converted beam 196 in step 830 to compensate for gradual ultraviolet degradation of the nonlinear crystal 130. If it is necessary to move the position of the nonlinear crystal 130 to utilize an unexposed portion of the nonlinear crystal 130 for frequency conversion, the controller 170 can perform step 820 to reset the output power after such crystal shifting, and optionally reset the output beam parameters. The controller 170 can perform this reset with or without feedback from steps 830 and / or 840.

[0060] Some laser processing tasks utilizing the frequency converted laser beam 196 may require changes in output power and / or output beam parameters. Such changes may be made according to method 800. For example, the controller 170 may perform step 820 to change the output power while keeping the output beam parameters substantially the same, or vice versa. The controller 170 may effect such changes based on pre-calibrated relationships, measurements obtained in steps 830 and / or 840, or a combination thereof.

[0061] In an embodiment of the apparatus 100 configured for sum and difference frequency mixing in a nonlinear crystal 130, the controller 170 may perform step 820 to match the size and position of the waist of the intracavity beam 192 with the size and position of the waist of the beam 194 in the nonlinear crystal 130. The apparatus 100 and method 800 are capable of maintaining such matching waist size and waist position in the presence of various noise sources, fluctuation sources, and drift sources. Furthermore, through appropriate coordination between the pump power adjustment in step 822 and the loss adjustment in step 824, the apparatus 100 and method 800 are capable of maintaining such matching waist size and waist position while intentionally changing the output power.

[0062] The method 800 is applicable to both continuous wave and pulsed operation of the resonator 110 of the device 100. In a pulsed embodiment, the resonator 110 may also include a Q switch, such as an AOM, EOM, or a saturable absorber. In a pulsed embodiment, the pulsed operation of the resonator 110 may be synchronized with the pulsed operation of the laser 180.

[0063] The method 800 is particularly useful in generating ultraviolet laser radiation. Although AOMs and EOMs are reasonably priced in the infrared and even visible spectrum, AOMs and EOMs for ultraviolet radiation are cost-prohibitive and their laser-induced damage may limit the performance of the device 100. When the frequency-converted beam 196 is ultraviolet, the control scheme of the method 800 provides a cheaper alternative to conventional AOM- or EOM-based frequency-converted beam attenuation. In a related embodiment of the device 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 the ultraviolet beam 194 in the nonlinear crystal 130 to produce the ultraviolet frequency-converted beam 196 by sum frequency or difference frequency mixing.

[0064] In an embodiment of the apparatus 100 configured to generate ultraviolet laser radiation by mixing with the beam 194, the gain medium 120 is a neodymium-doped yttrium vanadate crystal (Nd 3+ :YVO4) or Nd-doped yttrium aluminum garnet (Nd 3+ :YAG) crystal, generating an intracavity beam 192 with a wavelength of 1064 nanometers (nm), and the laser 180 is a frequency quadrupled Nd 3+ :YVO4 or Nd 3+ :YAG laser, generating a beam 194 with a wavelength of 266nm, and the nonlinear crystal 130 is cesium lithium borate (CsLiB6O 10 ) crystal, wherein the intracavity beam 192 is frequency mixed with the beam 194 to produce a frequency converted beam 196 having a wavelength of 213 nm.

[0065] In an embodiment of the apparatus 100 configured to generate ultraviolet laser radiation without using an external laser beam, the gain medium 120 is Nd 3+ :YVO4 crystal or Nd 3+:YAG crystal, generating an intracavity beam 192 with a wavelength of 1064nm, and the resonator 110 includes two nonlinear crystals 130 arranged in series. The first nonlinear crystal 130 doubles the frequency of a portion of the intracavity beam 192 to a wavelength of 532nm. In the second nonlinear crystal 130, the frequency-doubled laser beam is sum-frequency mixed with the remaining unconverted component of the intracavity beam 192 to generate a frequency-converted beam 196 with a wavelength of 355nm.

[0066] The method 800 and the apparatus 100 can impose resonator losses through various loss mechanisms, including coupling out portions of the intracavity beam 192 from the resonator 110, absorbing portions of the intracavity beam 192 in an absorbing medium using, for example, a movable filter with spatially varying transmission, and adjusting the alignment of the cavity mirror 112 or another optical element that affects the propagation path of the intracavity beam 192 through the resonator 110.

[0067] Fig. 9 A laser device 900 is shown having intracavity frequency conversion in a solid-state laser resonator 910 with end pumping, polarization solid-state gain medium 920, optical diodes, and polarization output couplers. Device 900 is an embodiment of device 100 and can perform method 800. Gain medium 920 facilitates amplification of a specific polarization component of intracavity beam 192. Gain medium 920 is, for example, Nd 3+ :YVO4 crystal. Resonator 910 includes Faraday rotator 930, half-wave plate 932 and polarization output coupler 934. Faraday rotator 930 and half-wave plate 932 together form an embodiment of optical diode 150. Half-wave plate 932 also cooperates with output coupler 934 to form an embodiment of loss element 140.

[0068] The output coupler 934 couples the polarization component of the intracavity beam 192 out of the resonator 910. The coupled polarization component is orthogonal to the polarization component that is beneficial to amplification in the gain medium 920. The output coupler 934 can be a polarization beam splitter. Fig. 9 In the illustrated embodiment, the output coupler 934 is a polarization beam splitter that (a) reflects the favorable polarization component of the intracavity beam 192 and (b) transmits the orthogonal polarization component out of the resonator 910 as an output coupled laser beam 998. The output coupler 934 thus acts as a cavity mirror that cooperates with a set of other cavity mirrors 912 to define the propagation path of the intracavity beam 192 in the resonator 910.

[0069] The half-wave plate 932 is rotatable and can be mounted on an electrically driven rotating bracket. The controller 170 controls the polarization rotation angle of the half-wave plate 932. For example, the controller 170 controls the optical axis rotation angle of the half-wave plate 932 to deviate 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 accurately offset the polarization rotation given by the Faraday rotator 930, minimum resonator loss can be achieved. The controller 170 increases the resonator loss by rotating the half-wave plate 932 away from the polarization rotation angle that accurately offsets the polarization rotation given by the Faraday rotator 930. In this state, the half-wave plate 932 only partially offsets the polarization rotation given by the Faraday rotator 930, so a portion of the intracavity beam 192 is coupled out by the output coupler 934.

[0070] Resonator 910 (and other embodiments of resonator 110) may include one or more focusing or defocusing elements, such as a lens or one or more curved cavity mirrors, to add optical power to resonator 910 in addition to thermal lensing in the gain medium. Fig. 9 One such embodiment is depicted in which the resonator 910 includes a focusing 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 compared to an embodiment of the resonator 910 without the lens 940. The smaller waist in the nonlinear crystal 130 allows for higher frequency conversion efficiency. The lens 940 can also improve the stability of the resonator 910 and extend the dynamic range of the output power and beam parameters, within which the output power and beam parameters can be adjusted independently of each other.

[0071] In one embodiment of the apparatus 900 for performing the method 800, the controller 170 adjusts the polarization rotation angle of the half-wave plate 932 to achieve an output coupling 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 utilizing unidirectional propagation of the intracavity beam 192 has been demonstrated with an output coupling percentage of up to 60% through the output coupler 934. Utilizing the loss and pump power adjustments in the apparatus 900, a wide dynamic range of either the power and beam parameters of the frequency converted beam 196 can be achieved while keeping the other of the power and beam parameters substantially constant. For example, in a sum frequency mixing scenario, we have demonstrated a dynamic range of the power of the frequency converted beam 196 down from 100% to 10% of the maximum frequency converted power while keeping the beam parameters of the frequency converted beam 196 substantially the same.

[0072] Fig.10 900 using an embodiment of the apparatus 900 implementing the lens 940. In this embodiment, the gain medium 920 is Nd3+ :YVO4 crystal, the pump laser 160 is a diode laser, and the wavelength of the intracavity beam 192 is 1064nm. Fig.10 Three cross-sectional images 1010, 1020, and 1030 of the waist position of the intracavity beam 192 within the nonlinear crystal 130 are shown. The images 1010, 1020, and 1030 are depicted at the same scale. Each image 1010, 1020, and 1030 is obtained by imaging the leakage beam passing through the laser cavity mirror 912, which guides the intracavity beam 192 into the nonlinear crystal 130. The leakage beam is imaged at a propagation distance from the cavity mirror 912, which is equivalent to the propagation distance of the nonlinear crystal 130. Each image 1010, 1020, and 1030 is obtained by a different combination of the power of the pump beam 190 and the output coupling (OC) percentage through the output coupler 934.

[0073] In the case of image 1010, the pump power (P pump ) is 30 watts and the output coupling percentage is 0.07%. This results in the intracavity power (P intra ) is 1.96 kilowatts (kW), and the intracavity beam is 1 / e of 192 2 The waist 1012 is 614μm x 605μm (measured along the major x minor axis). For image 1020, the pump power and output coupling percentage are both slightly increased to 35 watts and 0.08%. These relatively small changes have a significant impact on the beam waist size, while the intracavity power is barely affected. Specifically, the waist size is reduced to 465μm x 442μm (see profile 1022), while the intracavity power changes relatively little to 2.18kW. To further reduce the beam waist size, both the pump power and output coupling percentage are greatly increased, resulting in image 1030. Here, the pump power is set to 85 watts and the output coupling percentage is set to 1.59%. This results in a significant reduction in the beam waist size to 269μm x 259μm (see profile 1032), while the intracavity power again changes only slightly to 2.09kW.

[0074] Fig.10 The results demonstrate that the apparatus 900 and method 800 are capable of controlling the beam waist size of the intracavity beam 192 over a wide dynamic range while maintaining a substantially constant intracavity power. This capability translates into controlling the beam parameters of the frequency converted beam 196 independently of the power of the frequency converted beam 196 by making adjustments to account for the correlation between the beam waist size and the frequency conversion efficiency in the nonlinear crystal 130.

[0075] Reference again Fig. 9, half-wave plate 932 forms part of the photodiode and the adjustable loss element. In a variation 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, in other embodiments of the resonator 110, the concept of using a rotatable half-wave plate and a polarization output coupler (e.g., half-wave plate 932 and output coupler 934) can be implemented.

[0076] The output coupling scheme based on a rotatable half-wave plate is only one embodiment of imposing resonator losses by out-coupling a portion of the intracavity beam 192. Alternative schemes include incorporating an AOM or EOM in the path of the intracavity beam 192 in the resonator 110. While this modulator-based solution may increase cost and complexity, it is a convenient solution in Q-switching implementations of the resonator 110 that already implements an AOM or EOM for Q-switching purposes.

[0077] The present invention has been described above according to preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Instead, the present invention is limited only by the appended claims.

Claims

1. An intracavity frequency conversion method, comprising the following steps: end-pumping a solid-state gain medium in a laser resonator with a pump laser beam having a pump power to generate an intracavity laser beam circulating in the laser resonator; applying loss to the intracavity laser beam; frequency converting a portion of the intracavity laser beam in a nonlinear crystal located in the laser resonator to produce a frequency converted laser beam having an output power; as well as The pump power and the loss are adjusted 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 selected from the group consisting of beam waist size, beam waist position, beam divergence angle, and beam quality factor.

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

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

4. The method of claim 3, wherein the adjusting step comprises: The output power and the at least one output beam parameter are stabilized in the presence of ultraviolet degradation of the nonlinear crystal.

5. The method according to claim 3, further comprising: The nonlinear crystal is moved transversely to a through-propagation direction of the intracavity laser beam, and the adjusting step includes resetting one or both of the output power and the at least one output beam parameter after the moving.

6. The method according to any one of the preceding claims, wherein the adjusting step comprises: (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 of any of the preceding claims, wherein the at least one output beam parameter comprises the size and position of the waist of the frequency converted laser beam.

8. The method according to any one of the preceding claims, further comprising: The frequency converted laser beam is monitored to obtain a measure of output power, the adjusting step comprising adjusting the pump power and the losses based at least in part on the measure of the output power.

9. The method according to claim 8, further comprising: The intracavity laser beam is monitored to obtain a measure of power of the intracavity laser beam, wherein the adjustment of the pump power in the adjusting step is also based on the measure of the power of the intracavity laser beam.

10. The method according to claim 8, further comprising: The frequency converted laser beam is monitored to obtain a measure of the at least one output beam parameter, wherein the adjustment of the pump power and the loss in the adjusting step is also based on the measure of the at least one output beam parameter.

11. A method according to any one of the preceding claims, wherein the frequency conversion step comprises: The step of superimposing a second laser beam on the intracavity laser beam in the nonlinear crystal to produce the frequency converted laser beam from a mixture of the intracavity laser beam and the second laser beam.

12. The method according to claim 11, wherein: The superposition step includes: focusing the second laser beam to a waist in the nonlinear crystal; and The adjusting step includes matching the size and position of the waist of the intracavity laser beam to the size and position of the waist of the second laser beam.

13. The method of 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 any one of the preceding claims, wherein the applying step comprises: A portion of the intracavity laser beam is coupled out of the laser resonator.

15. The method according to claim 14, further comprising: A polarization component of the intracavity laser beam is optically selected for amplification in the gain medium, the applying step comprising the step of rotating the polarization of the intracavity laser beam away from the selected polarization component, the coupled-out portion being a polarization component orthogonal to the selected polarization component.

16. The method according to claim 15, further comprising: An optical diode is used to confine the propagation of the intracavity laser beam in the laser resonator to a one-way circulation.

17. The method of claim 16, wherein the optical diode comprises a Faraday rotator and a half-wave plate, and the rotating step comprises arranging the half-wave plate to only partially offset the polarization rotation of the Faraday rotator each time the intracavity laser beam passes through the optical diode.

18. The method of claim 14, wherein the applying step couples out the portion using an acousto-optic or electro-optic modulator.

19. A laser device with intracavity frequency conversion, comprising: A laser resonator, the laser resonator comprising: Solid-state gain media, Nonlinear crystals, and a tunable loss element arranged to impart tunable loss to the laser resonator; a pump laser for generating a pump laser beam having a pump power and arranged to end-pump the gain medium, thereby generating an intracavity laser beam that circulates in the laser resonator and is partially frequency converted in the nonlinear crystal to generate a frequency-converted laser beam having an output power; one or more sensors for monitoring an output power and at least one output beam parameter of the frequency converted laser beam, the at least one output beam parameter selected from the group consisting of a beam waist size, a beam waist position, a beam divergence angle, and a beam quality factor; and A controller is configured to control the output power and the at least one output beam parameter by adjusting the pump power and the loss based on the monitored values ​​of the output power and the at least one output beam parameter.

20. The laser device of claim 19, further comprising a second laser for delivering a second laser beam to the nonlinear crystal to mix with the intracavity laser beam to generate the frequency converted laser beam.

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