Pulse equalization in Q-switched gas lasers

Abstract

A Q-switched gas laser apparatus with bivariate pulse equalization includes a gas laser, a sensor, and electronic circuitry. The Q-switch switches a laser resonator between a high-loss state and a low-loss state to generate a pulsed laser beam. The sensor obtains a measurement of the pulsed laser beam indicative of the laser pulse energy. The electronic circuitry operates the Q-switch to (a) repeatedly switch the laser resonator between a high-loss state and a low-loss state to set a laser pulse repetition rate of the pulsed laser beam, (b) adjust a loss level of the low-loss state based on the pulse energy measurement to achieve a target laser pulse energy, and (c) adjust a duration of the low-loss state to achieve a target laser pulse duration.

Description

[Technical field]

[0001] (Priority) This application claims priority to U.S. Provisional Patent Application No. 63 / 281,044, filed November 18, 2021, the disclosure of which is incorporated by reference in its entirety.

[0002] (Technical field of the invention) The present invention relates to Q-switched gas lasers, such as Q-switched carbon dioxide (CO2) lasers and Q-switched carbon monoxide (CO) lasers. The present invention is particularly related to the variation of laser pulse energy and laser pulse duration at the output of a Q-switched gas laser in response to changes in pulse repetition rate. [Background technology]

[0003] (Discussion of Background Art) The laser gain medium of a gas laser is a gas mixture. The optically active entities that provide the laser action can be atoms, ions or molecules. These optically active entities are usually energized by a high voltage electric field (either radio frequency (RF) or direct current (DC)), which creates a gas discharge, thereby providing population inversion in the optically active entities. Many gas lasers have been supplanted in commercial applications by solid-state lasers due to their generally higher efficiency, smaller size, lower cost and simpler operation. However, certain gas lasers remain popular and are the preferred solution for some laser applications. For example, CO2 and CO lasers find significant use in industrial processes where infrared (IR) wavelengths and high average power are advantageous, such as laser processing.

[0004] CO2 lasers can deliver IR laser radiation in the wavelength range of about 9 micrometers (μm) to about 11 μm, while CO lasers can deliver IR laser radiation in the wavelength range of about 4.5 μm to about 6.0 μm. Average powers of up to about 8 kilowatts can be obtained with CO2 and CO lasers.

[0005] Many laser processing applications require that the laser radiation be pulsed. In the case of CO2 and CO lasers, a pulsed laser beam can be generated by turning a gas discharge on and off in the gain medium. However, the rise and fall times of the laser pulses generated by this technique are determined by the kinetics of the energy transfer process in the gas mixture and are typically on the order of 10 microseconds (μs) to 200 μs. These rise and fall times result in total laser pulse durations in the range of tens to hundreds of microseconds, which is too long for some applications. In particular, these laser pulse durations are too long for many laser processing applications that require a small heat-affected zone in the irradiated material. When the laser pulse duration is tens of microseconds or longer, the temperature diffusion in the irradiated material during a single laser pulse causes the heat-affected zone to expand significantly in the direction away from the irradiated location. Even for applications that do not require shorter laser pulses, modulation of the gas discharge has the additional disadvantage that the associated temperature fluctuations lead to optical instabilities.

[0006] Q-switching is a technique for generating laser pulses by modulating the intracavity losses of a laser resonator. Q-switching switches the laser resonator between a high-loss state (low Q) and a low-loss state (high Q) while maintaining stable pumping of the laser gain medium. Laser pulses are generated by first operating the laser resonator in a high-loss state to prevent lasing. In the absence of lasing, pumping of the gain medium results in the accumulation of a large amount of energy in the laser gain medium. The resonator losses then suddenly drop to a low value that allows lasing. After a build-up time, the circulating laser power increases rapidly in this low-loss state and the stored energy is quickly depleted. The result is the generation of laser pulses with durations typically in the nanosecond range. Most commonly, Q-switching is performed periodically to generate a train of laser pulses characterized by a pulse repetition rate.

[0007] Q-switching can be active or passive. In active Q-switching, the laser resonator contains an active loss element, e.g., an acousto-optic modulator (AOM) or an electro-optic modulator (EOM), which is controlled to divert or not divert radiation from the resonator to switch the laser resonator between high-loss and low-loss states. Active Q-switching is typically used to generate laser pulses with pulse repetition rates ranging from hundreds of kilohertz (kHz) down to 1 kHz or lower. Active Q-switching can be used to generate single laser pulses on demand.

[0008] Laser processes performed with pulsed laser beams usually have an optimum pulse repetition rate. The optimum pulse repetition rate is process dependent. Therefore, most versatile pulsed laser systems are capable of operating at a range of pulse repetition rates, sometimes spanning from single shot to hundreds of kilohertz. It is not uncommon for laser processing processes to involve varying the pulse repetition rate during each single part process, requiring on-the-fly adjustment of the pulse repetition rate. In the case of Q-switched lasers, adjusting the pulse repetition rate amounts to changing the duration of the high-loss state of the laser cavity. At least for relatively high pulse repetition rates, such a change can affect the amount of energy stored in the gain medium before switching to the low-loss state. As a result, adjustment of the pulse repetition rate can occur concomitantly with the laser pulse energy being changed. Summary of the Invention [Means for solving the problem]

[0009] (Summary of the invention) Disclosed herein is a Q-switched gas laser apparatus and related methods with bivariate pulse equalization of both laser pulse energy and laser pulse duration. The apparatus and methods are configured to use active Q-switching to equalize laser pulse energy and duration over a wide range of pulse repetition rates. One equalization mechanism directly affects the laser pulse energy, whereas the other equalization mechanism directly affects the laser pulse duration. The pulse energy equalization mechanism adjusts the loss of the low-loss state of the laser cavity, whereas the pulse duration equalization mechanism adjusts the duration of the low-loss state. For example, when the Q-switch is an AOM, the pulse duration equalization mechanism is based on maintaining some diffraction even in the low-loss state and adjusting this degree of diffraction to achieve the desired pulse energy. For comparison, a conventional Q-switch is turned off completely during the low-loss state to minimize losses in the laser cavity.

[0010] We have realized that equalization of both pulse energy and duration extends the range of pulse repetition rates over which uniform pulse energy can be achieved. Whereas it may be possible to achieve uniform pulse energy over a limited range of relatively high pulse repetition rates by using only the pulse energy equalization mechanism, the pulse duration equalization mechanism facilitates the extension of this range to lower pulse repetition rates. Therefore, the present bivariate pulse equalization is useful even when the objective is simply to maintain uniform laser pulse energy over a wide range of pulse repetition rates, regardless of the laser pulse duration. The pulse duration equalization mechanism further enables one to achieve a desired laser pulse duration over a wide range of pulse repetition rates, thereby providing ultimate laser pulse control.

[0011] In one aspect, a Q-switched gas laser apparatus with pulse equalization includes a gas laser, a sensor, and electronic circuitry. The gas laser includes a laser resonator having a Q-switch operable to switch the laser resonator between a high-loss state and a low-loss state to generate a pulsed laser beam. The sensor is configured to obtain measurements of the pulsed laser beam indicative of laser pulse energy. The electronic circuitry is communicatively coupled between the Q-switch and the sensor and is configured to operate the Q-switch to (a) repeatedly switch the laser resonator between a high-loss state and a low-loss state to set a laser pulse repetition rate of the pulsed laser beam, (b) adjust a loss level of the low-loss state based on the measurements obtained by the sensor to achieve a target laser pulse energy, and (c) adjust a duration of the low-loss state to achieve a target laser pulse duration.

[0012] In another aspect, a method for equalizing laser pulses generated by a Q-switched gas laser includes operating a Q-switch of the Q-switched gas laser to repeatedly switch a laser cavity of the Q-switched gas laser between a high-loss state and a low-loss state to generate a pulsed laser beam, the method further including equalizing the laser pulse energy and the laser pulse duration of laser pulses of the pulsed laser beam through the repeated steps of (a) sampling the pulsed laser beam to obtain measurements indicative of laser pulse energy, (b) adjusting a loss level of the low-loss state based on the measurements indicative of the laser pulse energy to approach a target laser pulse energy, and (c) adjusting the duration of the low-loss state to approach a target laser pulse duration. [Brief description of the drawings]

[0013] BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate generally 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 Q-switched gas laser apparatus with equalization of both laser pulse energy and laser pulse duration, according to an embodiment.

[0015] [Diagram 2] FIG. 2 is a timing diagram of a prior art scheme for Q-switching a laser cavity incorporating an AOM Q-switch.

[0016] [Diagram 3] FIG. 3 shows a set of exemplary laser pulses generated by an embodiment of the apparatus of FIG. 1 implementing an AOM Q-switch and operated according to the prior art scheme of FIG.

[0017] [Figure 4] FIG. 4 shows another set of exemplary laser pulses generated by an embodiment of the apparatus of FIG. 1 implementing pulse trimming in the same manner as the embodiment of FIG. 3, but in a departure from the prior art scheme of FIG. 2.

[0018] [Diagram 5] FIG. 5 displays data further demonstrating the effect of command pulse duration controlling the Q-switch on laser pulse energy and duration at a fixed pulse repetition rate.

[0019] [Figure 6] FIG. 6 is a flow chart of a method for bivariate equalization of laser pulses produced by a Q-switched gas laser, according to an embodiment.

[0020] [Figure 7] FIG. 7 is a timing diagram of a scheme used by the method of FIG. 6 to Q-switch a gas laser with bivariate pulse equalization, according to an embodiment.

[0021] [Figure 8] Figure 8 shows examples of laser pulses generated when operating the apparatus of Figure 1 with bivariate pulse equalization according to the scheme of Figure 7 and implementing dynamic adjustment of both laser pulse energy and laser pulse duration based on measurements from the respective sensors according to an embodiment. For comparison, Figure 8 also illustrates examples of laser pulses generated without pulse equalization.

[0022] [Figure 9] 9A and 9B show examples of laser pulse energy and laser pulse duration, respectively, with and without bivariate pulse equalization at three different pulse repetition rates.

[0023] [Figure 10] Figure 10A illustrates an RF driver that may be implemented in an embodiment of the apparatus of Figure 1 where the Q-switch is an AOM, according to an embodiment, and Figure 10B illustrates an example command signal for controlling this RF driver.

[0024] [Figure 11] FIG 11A illustrates another RF driver that may be implemented in an embodiment of the apparatus of FIG 1 in which the Q-switch is an AOM, according to an embodiment. The RF driver is configured to receive separate timing and voltage inputs. FIG 11B illustrates an example timing command signal for controlling the timing aspects of the RF driver of FIG 11A.

[0025] [Figure 12]FIG. 12 illustrates a controller for bivariate pulse equalization with servo control of both laser pulse energy and laser pulse duration, according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Detailed Description of the Invention Referring now to the Figures, in which like components are designated with like numerals, Figure 1 illustrates one Q-switched gas laser apparatus 100 with bivariate pulse equalization. Apparatus 100 includes a gas laser 110, a sensor 150, and electronic circuitry 160. Gas laser 110 includes a laser cavity 116 having a Q-switch 118. Q-switch 118 may be, for example, an AOM or EOM.

[0027] The gas laser 110 also essentially includes a gas gain medium 120. In one embodiment, the gas laser 110 is a CO2 or CO laser, and the gain medium 120 includes CO2 or CO, correspondingly. At least when the gas laser 110 is a CO2 or CO laser, the gas laser 110 further includes one or more electrodes that apply a high voltage electric field through the gain medium 120 to pump (energize) the CO2 or CO molecules therein. In other embodiments, the gas laser 110 can be a different type of gas laser with a long upper state lifetime. The gas laser 110 can include an electrode 122 and an electrode 124 located on either side of the gain medium 120 to generate an electric discharge in the gas gain medium 120. The electrode 122 is connected to a high voltage RF supply, and the electrode 124 is grounded.

[0028] In the example depicted in FIG. 1, the resonator 116 is a linear resonator having two end mirrors, 112 and 114, with the laser radiation 190 propagating back and forth between them. The end mirror 114 is an output coupler with partial reflectivity, for example in the range between 10% and 90%. The end mirror 112 can be a high reflector with a reflectivity of at least 99%. In another example of the device 100, the resonator 116 is a linear resonator defined by more than two mirrors, with the propagation path of the laser radiation 190 folded. In yet another example, the resonator 116 is a ring resonator.

[0029] The Q-switch 118 is disposed in the propagation path of the circulating laser radiation 190. The electronic circuitry 160 controls the operation of the Q-switch 118 to pulse the laser radiation 190 such that the gas laser 110 outputs a pulsed laser beam 192. More specifically, the electronic circuitry 160 determines the repetition rate of the laser pulses 192P of the laser beam 192 by operating the Q-switch to repeatedly switch the resonator 116 between a high-loss state and a low-loss state. The electronic circuitry 160 can vary the pulse repetition rate of the laser beam 192 by varying the rate at which the Q-switch 118 switches the resonator 116 between a high-loss state and a low-loss state. In other words, the electronic circuitry 160 can control the Q-switch 118 to achieve a range of values ​​of the period T between the laser pulses 192P. The electronic circuitry 160 can receive a repetition rate input 188 indicating a desired repetition rate of the laser pulses 192P. The electronic circuitry 160 may receive a repetition rate input 188 from an external control system or a user.

[0030] The electronic circuit 160 is also configured to control and vary the loss imposed by the Q-switch 118 in a low-loss state of the resonator 116 to achieve the target energy of the laser pulse 192P. The Q-switch 118, the sensor 150 and the electronic circuit 160 are arranged in an active feedback loop to exert servo control of the energy of the laser pulse 192P. The apparatus 100 separates a fraction 196 of the laser beam 192 and directs the laser beam fraction 196 to the sensor 150. The sensor 150 measures the pulse energy E (or a related parameter) of the laser pulse of the laser beam fraction 196 to obtain a measurement (or other parameter indicative thereof) of the energy of the laser pulse 192P. The sensor 150 communicates this pulse energy measurement to the electronic circuit 160, which adjusts the operation of the Q-switch 118 accordingly to achieve the target pulse energy within at least some tolerance.

[0031] The property measured by the sensor 150 may be the average energy of the laser beam fraction 196, obtained as an average over several or many laser pulses 192P. In one implementation, the sensor 150 is a thermopile sensor, a photoconductive or photovoltaic semiconductor sensor, or a bolometer sensor. Alternatively, the sensor 150 may be fast enough to measure the energy of individual pulses in the laser beam fraction 196. The apparatus 100 may include one or more beam splitters to obtain the laser beam fraction 196 from the laser beam 192.

[0032] The electronic circuitry 160 is further configured to control the duration of the low-loss state of the resonator 116, as defined by the Q-switch 118, as needed to adjust the duration of the laser pulse 192P. The electronic circuitry 160 may make this adjustment based on (a) a measurement of the duration of the laser pulse 192P, or (b) a pre-calibrated relationship between one or more of the pulse repetition rate, the target pulse energy, the measurement of the pulse energy obtained from the sensor 150, and the target pulse duration.

[0033] 1, the electronic circuitry 160 may receive the target pulse energy and target pulse duration from an external system or user. The electronic circuitry 160 may include one or more of discrete electronic components, integrated circuits, microprocessors, and computers with software.

[0034] An embodiment of the device 100 includes a sensor 152 that directs a fraction 198 of the laser beam 192 to the sensor 152. The sensor 152 obtains a measurement of the duration w of the laser pulse 192P from the laser beam fraction 198 and communicates the pulse duration measurement to the electronic circuitry 160. The electronic circuitry 160 can then adjust the operation of the Q-switch 118 based on the pulse duration measurement. The sensor 152 can complete an active feedback loop that allows servo control of the duration of the laser pulse 192P to achieve a target pulse duration within at least some tolerance. The sensor 152 can be, for example, a high-speed optical detector that records the waveform of each individual laser pulse 192P.

[0035] Apparatus 100 may include one or more beam splitters that separate a fraction of laser beam 192 to sensor 150, and optionally also to sensor 152. In the example depicted in FIG. 1 , apparatus 100 includes (a) beam splitter 130 that separates fraction 194 of laser beam 192 toward sensor 150 and sensor 152, and (b) beam splitter 132 that splits fraction 194 into laser beam fractions 196 and 198. Beam splitter 132 may be omitted in embodiments that do not include sensor 152. Without departing from this scope, apparatus 100 may implement other schemes for obtaining laser beam fraction 196, and optionally laser beam fraction 198, from laser beam 192.

[0036] The electronic circuit 160 may include a controller 162 and a driver 164. The driver 164 generates an electrical drive signal 182 and provides the drive signal 182 to the Q-switch 118 to modulate the resonator loss. For example, when the Q-switch 118 is an AOM, the driver 164 is an RF driver and the drive signal 182 is a high voltage RF signal. The generation of the drive signal 182 by the driver 164 is determined by a command signal 180 received from the controller 162. The command signal 180 defines the repetition rate of the laser pulses 192P, the loss of the resonator 116 in a low loss state, and the duration of the low loss state. For a given pulse repetition rate, the controller 162 generates the command signal 180 based at least in part on pulse energy measurements obtained from the sensor 150 and, optionally, also on pulse duration measurements obtained from the sensor 152. The generation of command signal 180 by controller 162 is further based on a desired pulse repetition rate (eg, as defined by a repetition rate input 188).

[0037] In one embodiment, the apparatus 100 is configured as a master oscillator power amplifier (MOPA). In this MOPA embodiment, the gas laser 110 is the master oscillator, and the apparatus 100 further includes a laser amplifier 170 that amplifies the laser beam 192. In the MOPA embodiment, the sensor 150 and (if included) the sensor 152 may be positioned to sample the laser beam 192 before or after amplification by the amplifier 170. As depicted in FIG. 1, it is generally advantageous to sample the laser beam 192 after amplification. The fraction of the laser beam 192 that is separated off to the sensor(s) may not be significant when the separation is after amplification. When the separation is done before amplification, a larger fraction of the laser beam 192 must be separated, and the impact on the final power of the laser beam 192 delivered to the application (e.g., laser processing) may be significant. Additionally, because amplified laser beam 192 is the actual output of the MOPA, sampling amplified laser beam 192 provides a more direct assessment of the actual output, and the control exerted by electronic circuitry 160 can offset effects induced in amplifier 170.

[0038] 2 is a timing diagram of a prior art scheme 200 for Q-switching a laser resonator equipped with an AOM Q-switch. FIG. 2 shows three graphs 210, 220, 230 for the generation region of a laser pulse 232. Graph 210 illustrates the evolution in time of a command signal 212 supplied to an RF driver for driving the AOM. Graph 220 illustrates the evolution in time of an RF drive signal 222 supplied by the RF driver to the AOM. The RF drive signal 222 is defined by the command signal 212. The value V c V high V c =V high When RF is the amplitude ΔV high and diffraction by the associated AOM introduces cavity losses such that the laser cavity is in a lossy state. cWhen V = 0, the AOM is off. RF is zero, thereby placing the laser cavity in a low-loss state.

[0039] Graph 230 shows the laser power P circulating in the laser resonator. L In scheme 200, a laser pulse 232 is generated at V c =0. The command pulse 214 has a duration Δt between a leading edge at time t1 and a trailing edge at time t2. The duration Δt of the command pulse 214 defines the duration of the low loss state of the laser resonator. The acoustic and optical delays are the delay T from time t1 to the laser pulse 232. delay . The acoustic delay corresponds to an acoustic waveform in the AOM optics that is generated until time t1 and propagates completely through the path of the laser radiation 190 from the transducer. The optical delay corresponds to the laser radiation being initiated by the "build-up" of stimulated spontaneous emission over multiple round trips through the energized resonator. In the scenario depicted in FIG. 2, the acoustic delay is longer than the optical delay, and therefore the laser pulse 232 is generated after time t2.

[0040] The electronic circuitry 160 is capable of operating the Q-switch 118 in accordance with a prior art scheme 200. However, the prior art scheme 200 does not provide pulse equalization, and the energy and duration of the laser pulses 192P vary with the pulse repetition rate.

[0041] FIG. 3 shows a set of exemplary laser pulses generated by an embodiment of the apparatus 100 that implements a Q-switch 118 such as an AOM and is constrained to operate according to the prior art scheme 200. The laser pulse waveforms are recorded by a sensor 152 implemented as a high-speed optical detector that detects and records the instantaneous laser power as a function of time. In the example of FIG. 3, the duration Δt of the command pulse 214 is 1.0 μs. FIG. 3 shows the resulting laser pulse waveforms recorded at each of the pulse repetition rates of 1 kHz, 5 kHz, 10 kHz, 25 kHz, 50 kHz, 75 kHz, and 100 kHz. While the laser pulse waveform changes very little for pulse repetition rates in the range of 1 kHz to 25 kHz, substantial changes occur as the pulse repetition rate increases past 25 kHz. In particular, for pulse repetition rates above 25 kHz, the pulse energy exhibits a strong decrease with pulse repetition rate. In addition, the shape and duration of the laser pulse change when the pulse repetition rate increases beyond 25 kHz. Laser pulses generated at pulse repetition rates ranging from 1 to 25 kHz consist of a main pulse followed by a significant tail 310. As the pulse repetition rate increases beyond 25 kHz, the tail 310 gradually disappears and the laser pulse duration is correspondingly reduced.

[0042] FIG. 4 shows another set of exemplary laser pulses generated by the same embodiment of the apparatus 100 as that of FIG. 3, but using a deviation from the prior art scheme 200 of shorter command pulses 214. This technique is referred to below as "pulse trimming." In the example of FIG. 4, the duration Δt of the command pulse 214 is 0.8 μs. FIG. 4 shows the resulting laser pulse waveforms recorded at each of the pulse repetition rates of 1 kHz, 5 kHz, 10 kHz, 25 kHz, 50 kHz, 75 kHz, and 100 kHz. The reduction of the command pulse 214 from 1.0 μs to 0.8 μs essentially eliminates the tail 310 at all measured pulse repetition rates. However, the laser pulse energy is also dramatically reduced when the pulse repetition rate is increased beyond 25 kHz. Thus, while pulse trimming helps achieve a more uniform laser pulse duration and shape across the range of pulse repetition rates, pulse trimming does not equalize the laser pulse energy.

[0043] FIG. 5 further explores the effect of command pulse duration on laser pulse energy and duration at a fixed pulse repetition rate. FIG. 5 shows the resulting laser pulse waveforms at different command pulse durations at a fixed pulse repetition rate. Each laser pulse waveform in FIG. 5 is labeled by the corresponding duration Δt of the command pulse 214. Starting with the longest command pulse duration of 1.1 μs, the laser pulse waveform exhibits a substantially pedestal-like tail. The main pulse has a width of about 0.2 μs, and the pedestal-like tail extends the total laser pulse duration up to about 0.5 μs. As the command pulse duration decreases from 1.1 μs, this tail is truncated and essentially eliminated when the command pulse duration reaches 0.7 μs. As the command pulse duration decreases past 0.7 μs, the laser pulse duration is further reduced as a result of the width of the main pulse now being reduced. In addition, the peak power of the laser pulse is reduced.

[0044] The data in FIG. 5 demonstrate that, at least in certain pulse repetition rate regimes, the laser pulse energy is sensitive to the duration of the low-loss state. The data in FIG. 5 also show that, at least in certain pulse repetition rate regimes, only a fairly narrow range of commanded pulse durations provides clean pulses without pedestals while maintaining the total energy of the main pulse. In other words, there is an "optimum" commanded pulse duration, and even relatively small deviations from this optimal commanded pulse duration have significant effects. We have found that the optimal commanded pulse duration increases with pulse repetition rate, at least for pulse repetition rates above a certain threshold rate.

[0045] The data in Figures 3, 4 and 5 together demonstrate a strongly coupled relationship between the repetition rate, energy and duration of laser pulses generated by a Q-switched gas laser such as gas laser 110. The nature of these relationships depends on the position in the three-dimensional parameter space that spans the pulse repetition rate, energy and duration. In addition, the relationships are sensitive to other parameters including the characteristics of the resonator 116, the gain medium 120 and the pumping of the gain medium 120.

[0046] FIG. 6 is a flow chart of one method 600 for bivariate equalization of laser pulses generated by a Q-switched gas laser. Method 600 may be applied to apparatus 100 to overcome the challenges illustrated by the data in FIGS. 3-5 to provide uniform laser pulse energy and duration over a wide range of pulse repetition rates. Method 600 involves operating a Q-switch according to a scheme that is more advanced than prior art scheme 200. In the following, method 600 is described within the context of apparatus 100. Method 600 includes steps 610 and 620 that are performed in parallel.

[0047] In step 610, electronic circuitry 160 operates Q-switch 118 to repeatedly switch resonator 116 between high-loss and low-loss states to generate pulsed laser beam 192, as described above with reference to Figure 1. In step 620, electronic circuitry 160 cooperates with sensor 150, and optionally sensor 152, to equalize the energy and duration of laser pulses 192P across a range of pulse repetition rates.

[0048] Step 620 includes steps 630 and 632. In step 630, the sensor 150 samples the laser beam 192 to obtain a measurement indicative of the energy of the laser pulse 192P, as described above with reference to FIG. 1. Step 630 may implement step 634, which obtains a measurement of this pulse energy as an average over multiple laser pulses 192P. In step 632, the electronic circuit 160 adjusts the loss level of the low-loss state of the resonator 116 based on the pulse energy measurement obtained in step 630 to achieve, or at least approach, the target pulse energy. Steps 630 and 632 may be performed iteratively in an active feedback loop to achieve the target pulse energy. In an example scenario, step 620 achieves a laser pulse energy that is within 10% of the target pulse energy, which is evaluated as the energy of a single laser pulse or the average energy of multiple laser pulses.

[0049] FIG. 7 is a timing diagram of one scheme 700 used by the method 600 for Q-switching a gas laser 110. Scheme 700 is a modification of scheme 200 that allows for both equalization of pulse energy and equalization of pulse duration. FIG. 7 shows scheme 700 for an embodiment in which Q-switching is performed using an AOM. Scheme 700 is easily adapted to other types of Q-switches (e.g., EOMs). Graphs 720 and 730 of FIG. 7 provide a more detailed example of step 632. Graph 720 depicts the temporal evolution of an RF drive signal 722 provided by electronic circuitry 160 to the AOM. In comparison to RF drive signal 222 of scheme 200, during the low-loss state of resonator 116, RF drive signal 722 is not necessarily zero. Instead, RF drive signal 722 has a non-zero amplitude ΔV during the low-loss state. low and the electronic circuit 160 determines, based on the pulse energy measurement obtained by the sensor 150 in step 630, ΔV low Adjust the value.

[0050] Graph 730 shows the laser power P circulating through resonator 116. L 7 depicts the time evolution of the amplitude ΔV of the RF drive signal 722 when the AOM is driven by the RF drive signal 722. high to non-zero amplitude ΔV low The drop to 0 results in the generation of laser pulse 732. Compared to laser pulse 232 (see FIG. 2) generated when the RF drive signal is dropped to 0, laser pulse 732 has a lower peak power P peak and has a lower pulse energy E. The pulse energy E (and peak power P peak The decrease in ΔV ) is a result of the AOM imposing non-zero diffraction losses during the low loss state of the resonator 116. In step 632, the electronic circuitry 160 increases ΔV as necessary to achieve the target pulse energy. lowFor example, when the pulse energy measurement obtained from sensor 150 indicates that the energy of laser pulse 192P exceeds the target pulse energy, electronic circuitry 160 adjusts ΔV low In contrast, when the pulse energy measurements obtained from sensor 150 indicate that the energy of laser pulse 192P is below the target pulse energy, electronic circuitry 160 may increase ΔV low can be reduced.

[0051] Step 620 of method 600 also includes step 642, in which electronic circuitry 160 adjusts the duration of the low-loss state of resonator 116 to achieve, or at least approach, the target pulse duration. Graphs 720 and 730 of FIG. 7 provide a more detailed example of step 642. In this example, electronic circuitry 160 adjusts the amplitude ΔV low In another example, the electronic circuitry 160 decreases the duration Δt to prevent the laser pulse 732 from having a tail, or increases the duration Δt to maximize the energy in the main pulse of the laser pulse 732. In yet another example, the electronic circuitry 160 adjusts the duration Δt to achieve a target pulse duration, such as a certain full width at half maximum (FWHM) pulse duration.

[0052] In one embodiment of step 620, step 642 precedes step 640A, in which the sensor 152 samples the laser beam 192P to obtain a measurement indicative of the duration of the laser pulse 192, as described above with reference to FIG. 1. Step 640A may implement step 644, which obtains a measurement of this pulse duration from a single laser pulse 192P or as an average of several single pulse durations. When step 620 includes step 640A, step 642 is based on the measurement of the pulse duration obtained in step 640A. This embodiment may perform steps 640A and 642 iteratively in an active feedback loop. The pulse duration feedback loop implemented by steps 640A and 642 may be faster than the pulse energy feedback loop implemented by steps 630 and 632, especially when the laser pulse duration is obtained from a measurement of a single or very few laser pulses 192P. In an example scenario, step 620 achieves a laser pulse duration that is within 10% of the target pulse duration.

[0053] In another embodiment of step 620, step 642 precedes step 640B, in which the electronic circuit 160 either calculates the desired duration Δt of the low-loss state of the resonator 116 or retrieves the desired duration Δt from a look-up table. The electronic circuit 160 may calculate the desired duration Δt from (a) the target pulse duration and (b) a pre-calibrated functional relationship between the duration Δt and the pulse repetition rate of the gas laser 110, and optionally one or more other parameters of the gas laser 110, and / or the target pulse energy. For example, it may be possible to achieve a constant laser pulse duration over a range of pulse repetition rates by adjusting the duration Δt to be linearly dependent on the period T between the laser pulses 192P (equal to the reciprocal of the pulse repetition rate). Alternatively, the target pulse duration, pulse repetition rate and optionally one or more other parameters of the gas laser 110, and / or the pre-calibrated duration Δt as a function of the target pulse energy of the gas laser 110 may be listed in a look-up table contained in the electronic circuit 160.

[0054] In one scenario, step 610 includes step 612 of varying the repetition rate of laser pulses 192P. In this scenario, method 600 performs step 632 and optionally step 642 in response to the change in repetition rate effected in step 612 to minimize changes to laser pulse energy and laser pulse duration caused by the change in repetition rate.

[0055] In an embodiment in which electronic circuitry 160 includes controller 162 and driver 164, execution of scheme 700 involves driver 164 generating an RF drive signal 722 (an example of drive signal 182). Driver 164 generates RF drive signal 722 according to command signal 180 generated by controller 162. Graph 710 of FIG. 7 illustrates the evolution of command signal 712 over time. Command signal 712 is one example of a command signal that can cause driver 164 to generate RF drive signal 722. Command signal 712 is a high value V high and low values ​​of V low Voltage V alternating between c It is. c =V high When V RF is the amplitude ΔV high V c =V low When V RF is the amplitude ΔV low V low The value of ΔV low Each laser pulse 192P has a signal value V low V low The value of ΔV low , and therefore controls the energy of the laser pulse 192P.

[0056] The controller 162 can set the repetition rate of the laser pulses 192P by setting the period between the leading edges 714L of successive command pulses 714 to the inverse of the desired pulse repetition rate. In this case, the controller 162 adjusts the duration Δt of the low-loss state of the resonator 116 by adjusting the time positioning of the trailing edges 714T of the command pulses 714.

[0057] The command signal 712 includes timing information (time t1 and time t2) and the variable voltage V lowAlternatively, the controller 162 may generate the command signal as two separate components: a timing signal and a variable DC voltage. The timing signal is similar to the command pulse 714, but contains a constant signal value V low The variable DC voltage may be an analog signal, for example, having an amplitude ΔV low The value that determines V low or RF drive signal 722. Often commercial RF drivers are configured for digital control rather than analog control. The command signal is a digitally encoded representation of the time t1, delay Δt, and voltage value ΔV high and voltage value ΔV low The signal may be a digital signal comprising:

[0058] FIG. 8 shows examples of laser pulses 192P generated when operating apparatus 100 with bivariate pulse equalization according to scheme 700 and implementing dynamic adjustment of both laser pulse energy and laser pulse duration based on measurements from sensors 150 and 152. For comparison, FIG. 8 also shows examples of laser pulses generated when operating apparatus 100 according to prior art scheme 200 without pulse equalization. FIG. 8 depicts oscilloscope traces of six laser pulse waveforms measured by sensor 152. Traces 810, 820, and 830 are obtained at pulse repetition rates of 10 kHz, 50 kHz, and 100 kHz, respectively, used in prior art scheme 200. Thus, for traces 810, 820, and 830, V low is kept at 0 and the duration Δt is kept constant. Traces 812, 822 and 832 are also obtained at pulse repetition rates of 10 kHz, 50 kHz and 100 kHz, respectively, but with signal values ​​V low The present invention uses pulse equalization according to scheme 700 with dynamic adjustment of both the period and duration Δt.

[0059] The measured pulse energies for trace 810, trace 820, and trace 830 are 704 microjoules (μJ), 542 μJ, and 335 μJ, respectively, with corresponding FWHM pulse durations of 105 nanoseconds (ns), 118 ns, and 117 ns. It is clear that without pulse equalization, the pulse energy changes dramatically as the pulse repetition rate increases from 10 kHz to 100 kHz. In contrast, when implementing pulse equalization according to scheme 700, essentially identical laser pulse waveforms are obtained for all three pulse repetition rates, as is evident from trace 812, trace 822, and trace 832. With pulse equalization, the measured pulse energies at 10 kHz, 50 kHz, and 100 kHz are 250 μJ, 254 μJ, and 262 μJ, respectively. Additionally, each of the three pulses has a FWHM pulse duration of 99 ns.

[0060] 9A shows example laser pulse energies and FIG. 9B shows example laser pulse durations with and without bivariate pulse equalization for three different pulse repetition rates, 10 kHz, 50 kHz, and 100 kHz. Data set 910 shows the results without pulse equalization, i.e., ΔV low = 0 and Δt is constant. Data set 920 and data set 930 are obtained when bivariate pulse equalization is used according to scheme 700 and ΔV low Δt is obtained when both are dynamically adjusted based on measurements obtained from sensors 150 and 152. For data set 920, the target pulse energy E target is 340 μJ, and the target pulse duration w target For data set 930, the target pulse energy E target is 250 μJ, and the target pulse duration w target was 117ns.

[0061] Data set 910 shows that without pulse equalization, the laser pulse energy decreases rapidly with pulse repetition rate, with a factor of two drop from 10 kHz to 100 kHz. In addition, the laser pulse duration is nontrivially quite sensitive to the pulse repetition rate. In contrast, with reference to data sets 920 and 930, bivariate pulse equalization is highly effective. The target pulse duration is achieved at all three pulse repetition rates, and the pulse energy variations are small. For two different target pulse energies, 340 μJ and 250 μJ, respectively, the measured laser pulse energies are within 5% of the target pulse energies.

[0062] 10A illustrates one RF driver 1000 that may be implemented as driver 164 in an embodiment of apparatus 100 in which Q-switch 118 is an AOM. FIG. 10B illustrates an example command signal 1080 that controls RF driver 1000. RF driver 1000 includes an RF oscillator 1010, a mixer 1020, and an amplifier 1040. RF oscillator 1010 generates an RF signal. Mixer 1020 receives this RF signal and further receives a command signal 1080 at an “IF” port of mixer 1020. Command signal 1080 controls the repetition rate of laser pulses 192P, V low and a variable analog voltage V consisting of successive instances of a command signal 712 (see FIG. 7) for defining a duration Δt. VA 10. The command signal 1080 is generated by an example of the controller 162. The mixer 1020 modulates the RF signal from the RF oscillator 1010 according to the command signal 1080. The amplifier 1040 amplifies the modulated RF signal to generate an RF drive signal 1082 for driving the AOM Q-switch. The RF drive signal 1082 is equal to successive instances of the RF drive signal 722. The RF driver 1000 may further include a low pass filter 1030 that removes undesired harmonics generated by the mixer 1020.

[0063] 11A illustrates another RF driver 1100 configured to receive separate timing and voltage inputs. FIG. 11B illustrates an example timing command signal 1180 that controls the timing aspects of the RF driver 1100. The RF driver 1100 is similar to the RF driver 1000, except that it further includes a digitally controlled analog switch 1150 coupled to the IF port of the mixer 1020. The analog switch 1150 has two analog voltage input ports. One port is for inputting a voltage V high The other is held in V from the controller 62 example. low The analog switch 1150 also receives a separate timing command signal from the controller 162. The timing command signal 1180 is low The analog switch 1150 is similar to the command signal 1080, except that it is a digital signal that does not carry any information about V, but only attains two values, a low value and a high value. The analog switch 1150 switches V according to the timing command signal 1180. high and V low , thereby generating a command signal 1080.

[0064] 12 illustrates one controller 1200 with servo control of both laser pulse energy and laser pulse duration for bivariate pulse equalization. Controller 1200 is an embodiment of controller 162 and may be implemented with RF driver 1100 in electronic circuitry 160. Controller 1200 receives a measurement P of the average power of laser beam 192 from sensor 150. AVE and receives the laser pulse waveform of the laser pulse 192P from the sensor 152. The laser pulse waveform is a function of time P INST (t) is the instantaneous laser power as the time (t). The controller 1200 controls P AVE and P INST By processing (t), V low and timing command signal 1180, and then V lowand timing command signal 1180 to the RF driver 1100. AVE The processing is performed with the target pulse energy E target It serves to achieve P INST The process of (t) is to set the target pulse duration w target (As discussed above with reference to FIGS. 3-5, there may be some coupling between laser pulse energy and laser pulse duration.)

[0065] For pulse energy equalization, the controller 1200 includes a pulse energy calculator 1210, a summing node 1220, and a proportional-integral-derivative (PID) controller 1230. The pulse energy calculator 1210 calculates the pulse energy from the average power measurement P AVE and pulse repetition rate f rep Calculate the pulse energy E from the pulse repetition rate f rep may be received from an external source / signal or may be defined internally in the controller 1200. The summing node 1220 calculates the calculated pulse energy E and the target pulse energy E target The PID controller 1230 then calculates a new V low is determined and output.

[0066] PID control is just one example of a feedback algorithm that may be used by the apparatus 100 and method 600. Alternatively, the apparatus 100 and method 600 may use different feedback algorithms that are known in the art. Thus, the PID controller 1230 may be replaced with other types of servo controllers that use different principles for minimizing the error signal.

[0067] For pulse duration equalization, the controller 1200 includes a waveform analyzer 1250 and a timing signal generator 1260. The waveform analyzer 1250 analyzes the laser pulse waveform P to derive the laser pulse duration w. INST (t). The timing signal generator 1260 then calculates the timing of this measured laser pulse duration w to the target pulse duration w target and adjusts the timing command signal 1180 to minimize the difference between them. Specifically, the timing signal generator 1260 adjusts the duration Δt as described above with reference to Figures 6 and 7. The timing signal generator 1260 then communicates the timing command signal 1180 to the RF driver 1100.

[0068] V low Whereas each iteration of adjustment of the timing command signal 1180 may be based on a sampling of a series of laser pulses 192P, each iteration of adjustment of the timing command signal 1180 may be based on a single laser pulse 192P and therefore may occur on an even shorter time scale.

[0069] The above description based on Figures 7, 10A, 10B, 11A, 11B and 12 relates to Q-switching based on an AOM. These embodiments can be modified to other types of Q-switching by varying the drive signal according to the Q-switch. For example, when the Q-switch 118 is an EOM, the RF drive signal described above is replaced by a DC drive signal.

[0070] The present invention has been described above with respect to preferred and alternative embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Instead, the present invention is limited only by the claims appended hereto.

Claims

1. 1. A Q-switched gas laser with pulse equalization, comprising: a gas laser including a laser cavity with a Q-switch, the Q-switch operable to switch the laser cavity between a high-loss state and a low-loss state to generate a pulsed laser beam; a first sensor for obtaining a first measurement of the pulsed laser beam indicative of laser pulse energy; a second sensor for obtaining a second measurement of the pulsed laser beam indicative of a laser pulse duration; an electronic circuit communicatively coupled between the Q-switch and the first and second sensors, the electronic circuit configured to operate the Q-switch to: (a) repeatedly switch the laser resonator between the high-loss state and the low-loss state to set a laser pulse repetition rate of the pulsed laser beam; (b) adjust a loss level of the low-loss state based on the first measurement to achieve a target pulse energy; and (c) adjust a duration of the low-loss state based on the second measurement to achieve a target laser pulse duration; A Q-switched gas laser device comprising:

2. The apparatus of claim 1 , wherein the first sensor and the second sensor are positioned to sample the pulsed laser beam outside the laser cavity.

3. 10. The apparatus of claim 1, wherein the electronic circuit completes a pulse energy feedback loop between the first sensor and the Q-switch and a pulse duration feedback loop between the second sensor and the Q-switch.

4. The apparatus of claim 3 , wherein the pulse duration feedback loop is faster than the pulse energy feedback loop.

5. 4. The apparatus of claim 3, wherein the first sensor is a photosensitive detector configured to obtain the first measurement as an average over a plurality of the laser pulses, and the second sensor is an optical detector configured to obtain the second measurement as waveforms of individual laser pulses.

6. The apparatus of claim 1 , wherein the Q-switch comprises an acousto-optic modulator located in the laser cavity.

7. The electronic circuit an RF driver for generating and supplying a radio frequency signal to the acousto-optic modulator, the radio frequency signal, when at a non-zero amplitude, causing the acousto-optic modulator to induce losses in the resonator by diffraction; a controller for generating and sending a command signal to the RF driver, the command signal including a series of command pulses each having a leading edge, a trailing edge, and a signal value, each leading edge causing the RF driver to generate the radio frequency signal having a lower amplitude that places the laser resonator in the low-loss state, each trailing edge causing the RF driver to generate the radio frequency signal having a higher amplitude that places the laser resonator in the high-loss state, and each signal value setting a value for the lower amplitude; Including, 7. The apparatus of claim 6, wherein the controller is configured to: (a) set the repetition rate by setting a period between successive leading edges to match the inverse of a target repetition rate; (b) adjust the signal value based on the first measurement result to achieve the target laser pulse energy; and (c) adjust the duration from the leading edge to the corresponding trailing edge to achieve the target laser pulse duration.

8. 10. The apparatus of claim 1, further comprising a laser power amplifier for amplifying the pulsed laser beam, the first sensor being positioned to obtain the first measurement from the pulsed laser beam after amplification by the laser power amplifier.

9. 1. A method for equalizing laser pulses produced by a Q-switched gas laser, the method comprising: operating a Q-switch of the Q-switched gas laser to repeatedly switch a laser cavity of the Q-switched gas laser between a high-loss state and a low-loss state to produce a pulsed laser beam; Equalizing laser pulse energy and laser pulse duration of laser pulses of the pulsed laser beam through a repeating step, the repeating step comprising: sampling the pulsed laser beam to obtain a first measurement indicative of the laser pulse energy and to obtain a second measurement of the laser beam indicative of the laser pulse duration; adjusting a loss level of the low-loss state based on the first measurement to approach a target laser pulse energy; adjusting the duration of the low-loss state to approach a target laser pulse duration based on the second measurement; Do the steps and A method comprising:

10. 10. The method of claim 9, wherein each instance of the first measurement is obtained as an average over a plurality of the laser pulses, and each instance of the second measurement is obtained from an individual laser pulse.

11. the operating step includes changing a repetition rate of the laser pulses from a first value to a second value different from the first value; 10. The method of claim 9, wherein the equalizing step comprises performing the adjusting step in response to the varying step to minimize changes to the laser pulse energy and laser pulse duration caused by the varying step.

12. The equalizing step includes: maintaining individual or average laser pulse energy at the second value of the repetition rate to be the same within 10% as the first value of the repetition rate; maintaining a laser pulse duration at the second value of the repetition rate that is the same as within 10% of the first value of the repetition rate; The method of claim 11 , comprising:

13. The operating step includes: generating a command signal, the command signal including a series of command pulses each having a leading edge, a trailing edge, and a signal value, each leading edge initiating a transition of the laser cavity to the low-loss state and each trailing edge initiating a transition of the laser cavity to the high-loss state, and each signal value defining the loss level of the laser cavity while in the low-loss state; setting the repetition rate of the laser pulses by setting the period between successive leading edges to match the inverse of a target repetition rate; Including, adjusting the loss level includes adjusting the signal value based on the first measurement; 10. The method of claim 9, wherein adjusting the duration comprises adjusting a duration from the leading edge to the corresponding trailing edge based on the second measurement.

14. further comprising the step of amplifying the pulsed laser beam; 10. The method of claim 9, wherein the sampling step occurs on the pulsed laser beam after it has been amplified.

15. The method of claim 9 , wherein the Q-switch is an acousto-optic modulator.

16. 16. The method of claim 15, wherein the operating step includes driving the acousto-optic modulator to diffract laser light out of the laser cavity such that the loss level is not zero in the low-loss state.

17. The method of claim 9 , wherein the sampling step occurs outside the laser cavity.

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