Pulse equalization in q-switched gas lasers

Abstract

A Q-switched gas laser device with binary pulse equalization technology includes a gas laser, a sensor, and electronic circuitry. A Q-switch switches the laser resonator between high-loss and low-loss states to generate a pulsed laser beam. The sensor receives a measurement of the pulsed laser beam representing the laser pulse energy. The electronic circuitry operates the Q-switch to (a) repeatedly switch the laser resonator between the high-loss and low-loss states to set a repetition rate of the laser pulses in the pulsed laser beam, (b) adjust a loss level in the low-loss state based on the pulse energy measurement to achieve a target laser pulse energy, and (c) adjust a duration in the low-loss state to achieve a target laser pulse duration. By adjusting both the pulse energy and duration, uniform pulse energy is achieved over a wide repetition rate range, and, if necessary, uniform pulse duration is achieved.

Description

[Technical Field]

[0001] Field of Invention

[0002] This application claims priority to U.S. Provisional Application No. 63 / 281,044, filed November 18, 2021, the disclosure of which is incorporated herein by reference in its entirety.

[0003] This invention relates to Q-switched gas lasers, such as Q-switched carbon dioxide (CO2) and carbon monoxide (CO) lasers. Particularly, this invention relates to the variation of laser pulse energy and laser pulse duration in the output of a Q-switched gas laser after pulse repetition rate modification. [Previous Technology]

[0004] Background of the Invention

[0005] The laser gain medium of a gas laser is a gas mixture. The optically active entity providing the laser effect is an atom, ion, or molecule. These optically active entities are typically powered by a radio frequency (RF) or direct current (DC) high-voltage electric field that generates a population inversion within the optically active entity through a gas discharge. Many gas lasers have been replaced by solid-state lasers in commercial applications due to their generally higher efficiency, smaller size, lower cost, and simpler operation. However, certain types of gas lasers remain popular and are preferred solutions for certain laser applications. For example, carbon dioxide lasers and carbon monoxide lasers see substantial use in industrial processes such as laser machining, where their infrared (IR) wavelengths and high average power are advantageous.

[0006] Carbon dioxide lasers can deliver IR laser radiation with wavelengths ranging from about 9 micrometers (µm) to about 11 micrometers, while carbon monoxide lasers can deliver IR laser radiation with wavelengths ranging from about 4.5 micrometers to about 6.0 micrometers. Average power of up to about 8 kilowatts can be obtained using both carbon dioxide and carbon monoxide lasers.

[0007] Many laser machining applications require pulsed laser radiation. In the case of carbon dioxide and carbon monoxide lasers, pulsed laser beams can be generated by switching the gas discharge in the gain medium on and off. However, the rise and fall times of the laser pulses generated by this technique are governed by the kinetics of the energy transfer process in the gas mixture and are typically about 10 to 200 microseconds (µs). These rise and fall times result in a total laser pulse duration 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 in many laser machining applications that require a small heat-affected zone in the irradiated material. When the laser pulse duration is tens of microseconds or more, thermal diffusion of the irradiated material during a single laser pulse causes the heat-affected zone to grow significantly away from the irradiated location. Even for applications that do not require shorter laser pulses, the modulation of the gas discharge has the additional disadvantage of optical instability due to associated thermal changes.

[0008] Q-switching is a technique for generating laser pulses by modulating the cavity loss of a laser resonator. Q-switching switches the laser resonator between a high-loss state (low quality factor) and a low-loss state (high quality factor) while maintaining stable pumping of the laser gain medium. Laser pulse generation is achieved by first operating the laser resonator in a high-loss state to prevent laser action. Without laser action, the pumping of the gain medium causes a large amount of energy to accumulate in the laser gain medium. Next, the resonator loss suddenly drops to a low value that enables laser action. After a period of accumulation, the cyclic laser power rapidly increases in this low-loss state and quickly depletes the stored energy. This results in a laser pulse, typically lasting in the nanosecond range. Most commonly, Q-switching is performed periodically to generate a series of laser pulses characterized by a pulse repetition rate.

[0009] The Q-switch can be active or passive. In an active Q-switch, the laser resonator includes an active loss element, such as an acousto-optic modulator (AOM) or an electro-optic modulator (EOM), which is controlled to redirect or dedirect radiation from the resonator to switch the laser resonator between high-loss and low-loss states. Active Q-switches are typically used to generate laser pulses with pulse repetition rates ranging from hundreds of kilohertz (kHz) to one kilohertz or lower. Active Q-switches can be used to generate a single laser pulse as needed.

[0010] Laser processes using pulsed laser beams typically have an optimal pulse repetition rate. This optimal pulse repetition rate depends on the process. Therefore, the most common pulsed laser systems can operate within a range of pulse repetition rates, sometimes ranging from a single beam to hundreds of kilohertz. It is not uncommon for laser machining processes to require changes in the pulse repetition rate during the processing of individual components, thus necessitating dynamic adjustment of the pulse repetition rate. In the case of Q-switched lasers, adjusting the pulse repetition rate means changing the duration of the high-loss state of the laser resonator. At least for relatively high pulse repetition rates, this change affects the amount of energy accumulated in the gain medium before switching to a low-loss state. Consequently, adjusting the pulse repetition rate may be accompanied by changes in the laser pulse energy. [Summary of the Invention]

[0011] Summary of the Invention

[0012] Disclosed herein is a Q-switch gas laser device and related method with a binary pulse equalization technique for both laser pulse energy and laser pulse duration. The device and method of the present invention utilize an active Q-switch configured to equalize laser pulse energy and duration over a wide pulse repetition rate range. One equalization mechanism directly affects the laser pulse energy, while another equalization mechanism directly affects the laser pulse duration. The pulse-energy equalization mechanism adjusts the loss of the laser resonator in the low-loss state, while the pulse-duration equalization mechanism adjusts the duration of the low-loss state. For example, when the Q-switch is an AOM (Alternating Optical Oscillator), the pulse-duration equalization mechanism adjusts this diffraction amount based on maintaining a certain amount of diffraction, even in the low-loss state, to achieve the desired pulse energy. For comparison, conventional Q-switches are completely closed during the low-loss state to minimize the loss of the laser resonator.

[0013] We have realized that the equalization extension of both pulse energy and duration can achieve a pulse repetition rate range for uniform pulse energy. Although it is possible to achieve uniform pulse energy within a limited range of relatively high pulse repetition rates using only a pulse-energy equalization mechanism, this pulse-duration equalization mechanism helps to extend this range to lower pulse repetition rates. Therefore, the binary pulse equalization technique of the present invention is useful even if the goal is simply to maintain uniform laser pulse energy over a wide range of pulse repetition rates, regardless of the laser pulse duration. This pulse-duration equalization mechanism further enables the desired laser pulse duration to be achieved over a wide range of pulse repetition rates, thereby providing ultimate laser pulse control.

[0014] In one aspect, a Q-switched gas laser device with pulse equalization technology includes a gas laser, a sensor, and an electronic circuit. The gas laser includes a laser resonator with a Q-switch operable to switch between a high-loss state and a low-loss state to generate a pulsed laser beam. The sensor is configured to obtain a measurement of the pulsed laser beam representing a laser pulse energy. The electronic circuit is communicatively coupled between the Q-switch and the sensor and configured to operate the Q-switch to (a) repeatedly switch the laser resonator between the high-loss and low-loss states to set a repetition rate of the laser pulses of the pulsed laser beam, (b) adjust a loss level of the low-loss state based on the measurement 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.

[0015] 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 resonator of the Q-switched gas laser between high-loss and low-loss states to generate a pulsed laser beam. The method further includes: equalizing the laser pulse energy and laser pulse duration of the pulsed laser beam by repeating the following steps: (a) sampling the pulsed laser beam to obtain a measurement representing the laser pulse energy, (b) adjusting a loss level of the low-loss state based on the measurement representing the laser pulse energy to approximate a target laser pulse energy, and (c) adjusting a duration of the low-loss state to approximate a target laser pulse duration.

Implementation Method

[0029] Referring now to the accompanying drawings, in which similar components are indicated by the same element symbols, FIG1 illustrates a Q-switched gas laser device 100 with binary pulse equalization technology. The device 100 includes a gas laser 110, a sensor 150, and electronic circuitry 160. The gas laser 110 includes a laser resonator 116 with a Q-switch 118. For example, the Q-switch 118 is an AOM or an EOM.

[0030] Essentially, the gas laser 110 also includes a gas gain medium 120. In one embodiment, the gas laser 110 is a carbon dioxide or carbon monoxide laser and the gain medium 120 correspondingly includes carbon dioxide or carbon monoxide. At least when the gas laser 110 is a carbon dioxide or carbon monoxide 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) carbon dioxide or carbon monoxide molecules therein. In other embodiments, the gas laser 110 may be a different type of gas laser with a long upper-state lifetime. The gas laser 110 may include electrodes 122 and 124 located on opposite sides of the gain medium 120 to generate a discharge in the gas gain medium 120. Electrode 122 is coupled to a high-voltage RF source, and electrode 124 is grounded.

[0031] In the embodiment illustrated in FIG1, resonator 116 is a linear resonator with two end mirrors 112 and 114 through which laser radiation 190 propagates back and forth. End mirror 114 is an output coupler with a partial reflectivity, for example, in the range of 10% to 90%. End mirror 112 may be a highly reflective mirror with at least 99% reflectivity. In other embodiments of device 100, resonator 116 is a linear resonator defined by two or more mirrors, wherein the propagation path of laser radiation 190 is folded. In still other embodiments, resonator 116 is a ring resonator.

[0032] Q-switch 118 is placed in the propagation path of the cyclic laser radiation 190. Electronic circuit 160 controls the operation of Q-switch 118 to pulse the laser radiation 190, causing the gas laser 110 to output a pulsed laser beam 192. More specifically, electronic circuit 160 operates Q-switch 118 to repeatedly switch resonator 116 between high-loss and low-loss states to set the repetition rate of the laser pulses 192P of the laser beam 192. By changing the rate at which Q-switch 118 switches resonator 116 between high-loss and low-loss states, electronic circuit 160 can change the pulse repetition rate of the laser beam 192. In other words, electronic circuit 160 can control Q-switch 118 to achieve a laser pulse 192P period T within a numerical range. Electronic circuit 160 can receive a repetition rate input 188 indicating the desired repetition rate of the laser pulses 192P. Electronic circuit 160 can receive a repetition rate input 188 from an external control system or user.

[0033] Electronic circuit 160 is also configured to control and modify the losses caused by Q switch 118 when the resonator 116 is in a low-loss state, in order to achieve the target energy of laser pulse 192P. Q switch 118, sensor 150 and electronic circuit 160 are configured in an active feedback loop to apply servo control of the energy of laser pulse 192P. Device 100 splits a portion 196 of laser beam 192 and guides the laser beam portion 196 to sensor 150. Sensor 150 measures the pulse energy E (or related parameter) of the laser pulse of laser beam portion 196 to obtain a measure (or another parameter representing it) of the energy of laser pulse 192P. Sensor 150 transmits this pulse energy measurement to electronic circuit 160, and electronic circuit 160 adjusts the operation of Q switch 118 accordingly to achieve the target pulse energy within at least a certain tolerance.

[0034] The property measured by sensor 150 can be the average energy of laser beam portion 196 and can be obtained as an average of several or many laser pulses 192P. In one implementation, sensor 150 is a thermopile sensor, a photoconductive or photovoltaic semiconductor sensor, or a bolometric sensor. Alternatively, sensor 150 can be fast enough to measure the energy of pulses in individual laser beam portions 196. Apparatus 100 may include one or more beam splitters to obtain laser beam portions 196 from laser beam 192.

[0035] Electronic circuitry 160 is further configured to control the duration of the low-loss state of resonator 116 defined by Q-switch 118 when the duration of laser pulse 192P needs to be adjusted. Electronic circuitry 160 may make this adjustment based on a pre-calibration relationship between (a) a measured value of the duration of laser pulse 192P, or (b) the pulse repetition rate, the target pulse energy, the pulse energy measurement obtained from sensor 150, and one or more of the target pulse duration.

[0036] Although not shown in FIG1, electronic circuit 160 may receive target pulse energy and target pulse duration from an external system or user. Electronic circuit 160 may include one or more of discrete electronic components, integrated circuits, microprocessors, and computers equipped with software.

[0037] Certain embodiments of the device 100 include a sensor 152 and guide a portion 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 portion 198 and transmits this pulse-duration measurement to 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 at least within a certain tolerance. For example, the sensor 152 is a high-speed optical detector that records the waveform of individual laser pulses 192P.

[0038] The device 100 may include one or more beam splitters to split a portion of the laser beam 192 to the sensor 150 and, if necessary, also to the sensor 152. In the embodiment illustrated in FIG1, the device 100 includes (a) a beam splitter 130 that splits a portion 194 of the laser beam 192 toward the sensors 150 and 152, and (b) a beam splitter 132 that splits the portion 194 into laser beam portions 196 and 198. The beam splitter 132 may be omitted in specific embodiments that do not include the sensor 152. Without departing from this scope, the device 100 may implement other methods to obtain the laser beam portion 196 from the laser beam 192, and, if necessary, the laser beam portion 198.

[0039] Electronic circuitry 160 may include controller 162 and driver 164. Driver 164 generates an electrical drive signal 182 and supplies drive signal 182 to Q-switch 118 to modulate resonator loss. For example, when Q-switch 118 is AOM, driver 164 is an RF driver and drive signal 182 is a high-voltage RF signal. Command signal 180 received from controller 162 governs drive signal 182 generated by driver 164. Command signal 180 defines the repetition rate of laser pulse 192P, the loss of resonator 116 in a low-loss state, and the duration of the low-loss state. For any given pulse repetition rate, controller 162 generates command signal 180 based at least in part on pulse-energy measurements obtained from sensor 150 and, if necessary, also on pulse-duration measurements obtained from sensor 152. The generation of command signal 180 by controller 162 is further based on the desired pulse repetition rate, for example, defined by repetition rate input 188.

[0040] In one embodiment, device 100 is configured as a master oscillator power amplifier (MOPA). In this MOPA embodiment, gas laser 110 is the master oscillator and device 100 further includes a laser amplifier 170 that amplifies the laser beam 192. In this MOPA embodiment, sensors 150 and 152 (if included) may be configured to sample the laser beam 192 before or after amplification by amplifier 170. Sampling the laser beam 192 after amplification is generally advantageous, as shown in FIG1. ​​If the beam splitting occurs after amplification, the portion of the laser beam 192 split towards the sensor(s) may be insignificant. If the beam splitting is performed before amplification, a larger portion of the laser beam 192 must be split, and the impact on the final power delivered by the laser beam 192 to the application (e.g., laser machining) may be significant. Furthermore, since the amplified laser beam 192 is the output of MOPA, sampling of the amplified laser beam 192 provides a more direct assessment of the actual output, and the control applied by the electronic circuit 160 can compensate for the effects induced in the amplifier 170.

[0041] Figure 2 is a timing diagram of a prior art solution 200 for a laser resonator equipped with an AOM Q-switch. Figure 2 illustrates three curves 210, 220, and 230 regarding the generation of the laser pulse 232. Curve 210 depicts the time evolution of the command signal 212 supplied to the RF driver for driving the AOM. Curve 220 depicts the time evolution of the RF drive signal 222 supplied from the RF driver to the AOM. The RF drive signal 222 is defined by the command signal 212. The value VC of the command signal 212 switches between two values: Vhigh and zero. When VC = Vhigh, the RF voltage VRF of the drive signal 222 has an amplitude ∆Vhigh, and the correlation diffraction of the AOM imparts losses to the resonator, causing the laser resonator to be in a high-loss state. When VC is zero, VRF is zero, causing the AOM to turn off, thereby causing the laser resonator to be in a low-loss state.

[0042] Graph 230 depicts the temporal evolution of the laser power PL circulating in the laser resonator. In scheme 200, laser pulse 232 is generated by command pulse 214 in command signal 212, which drops to VC=0. Command pulse 214 has a duration ∆t between the leading edge of time t1 and the trailing edge of time t2. The duration ∆t of command pulse 214 defines the duration of the low-loss state of the laser resonator. Acoustic and optical delays cause a delay T_delay from time t1 to laser pulse 232. The acoustic delay corresponds to the generation of sound waves in the AOM optical element that propagate completely through the laser radiation path 190 from the sensor before time t1. The optical delay corresponds to the laser radiation caused by spontaneous emission, which is "gradually built up" by stimulated emission during multiple round trips through the energized resonator. In the scenario depicted in Figure 2, the acoustic delay is longer than the optical delay, so laser pulse 232 is generated after time t2.

[0043] The electronic circuit 160 is capable of operating the Q switch 118 according to the prior art solution 200. However, the prior art solution 200 does not provide pulse equalization technology, and the energy and duration of the laser pulse 192P change with the pulse repetition rate.

[0044] The set of exemplary laser pulses illustrated in Figure 3 are generated by a specific embodiment of device 100, which implements Q switch 118 as AOM and operates only as described in prior art solution 200. The laser pulse waveforms are all recorded by sensor 152, which acts as a high-speed optical detector to detect and record the instantaneous laser power changing over time. In the embodiment of Figure 3, the duration ∆t of command pulse 214 is 1.0 microsecond. Figure 3 illustrates the resulting laser pulse waveforms recorded at pulse repetition rates of 1 kHz, 5 kHz, 10 kHz, 25 kHz, 50 kHz, 75 kHz, and 100 kHz. Although the pulse repetition rate of the laser pulse waveform hardly changes in the range from 1 kHz to 25 kHz, a substantial change occurs when the pulse repetition rate increases above 25 kHz. In particular, for pulse repetition rates above 25 kHz, the pulse energy decreases significantly with increasing pulse repetition rate. Furthermore, the shape and duration of the laser pulse change when the pulse repetition rate increases above 25 kHz. Laser pulses generated at pulse repetition rates in the range of 1 to 25 kHz consist of a main pulse and a prominent tail 310 immediately following it. As the pulse repetition rate increases above 25 kHz, the tail 310 gradually disappears and the duration of the laser pulse decreases accordingly.

[0045] Figure 4 illustrates another set of exemplary laser pulses, which are generated by the same embodiment of the device 100 as in Figure 3, but using a shorter command pulse 214 different from the prior art solution 200. This technique is hereinafter referred to as "pulse trimming". In the embodiment of Figure 4, the duration ∆t of the command pulse 214 is 0.8 microseconds. Figure 4 illustrates the resulting laser pulse waveforms recorded at pulse repetition rates of 1 kHz, 5 kHz, 10 kHz, 25 kHz, 50 kHz, 75 kHz, and 100 kHz. Reducing the command pulse 214 from 1.0 microseconds to 0.8 microseconds substantially eliminates the tail 310 of all measured pulse repetition rates. However, the laser pulse energy is still significantly reduced when the pulse repetition rate increases above 25 kHz. Therefore, although pulse trimming helps to achieve a more uniform laser pulse duration and shape within a pulse repetition rate range, pulse trimming does not equalize the laser pulse energy.

[0046] Figure 5 further explores the effect of command pulse duration on laser pulse energy and duration at a fixed pulse repetition rate. Figure 5 illustrates laser pulse waveforms obtained with different command pulse durations at a fixed pulse repetition rate. Each laser pulse waveform in Figure 5 is labeled with the corresponding duration ∆t of command pulse 214. Starting with the longest command pulse duration of 1.1 microseconds, the laser pulse waveform exhibits a substantial columnar tail. The main pulse has a width of approximately 0.2 microseconds, and the columnar tail extends the total laser pulse duration to approximately 0.5 microseconds. As the command pulse duration decreases from 1.1 microseconds, this tail is truncated and substantially eliminated when the command pulse duration reaches 0.7 microseconds. When the command pulse duration decreases below 0.7 microseconds, the laser pulse duration further decreases, resulting in a reduction in the width of the main pulse. Furthermore, the peak power of the laser pulse decreases.

[0047] Figure 5 demonstrates that, at least at certain pulse repetition rates, laser pulse energy is highly sensitive to the duration of the low-loss state. Figure 5 also shows that, at least at certain pulse repetition rates, only a fairly narrow range of command pulse durations provides a clean, tailless pulse while retaining the full energy of the main pulse. In other words, there exists an "optimal" command pulse duration, and even relatively small deviations from this optimal command pulse duration can have a significant impact. It has been found that the optimal command pulse duration increases with increasing pulse repetition rate, at least for pulse repetition rates above a certain threshold rate.

[0048] Figures 3, 4, and 5 together demonstrate the strong coupling relationship between the repetition rate, energy, and duration of laser pulses generated by, for example, a Q-switched gas laser 110. The nature of these relationships depends on their position in a three-dimensional parameter space unfolded from the pulse repetition rate, energy, and duration. Furthermore, these relationships are sensitive to other parameters, including the properties of the resonator 116, the gain medium 120, and the pumping effect of the gain medium 120.

[0049] The flowchart in Figure 6 illustrates a method 600 for binary equalization of laser pulses generated by a Q-switch gas laser. Method 600 can be applied to device 100 and overcomes the problems illustrated in Figures 3 to 5 in providing uniform laser pulse energy and duration over a wide pulse repetition rate range. Method 600 requires operating the Q-switch according to a more advanced scheme than the prior art solution 200. Hereinafter, method 600 is discussed in the context of device 100. Method 600 includes steps 610 and 620 performed in parallel.

[0050] In step 610, electronic circuit 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 in the explanation of FIG1. ​​In step 620, electronic circuit 160 cooperates with sensor 150, and as needed with sensor 152, to equalize the energy and duration of laser pulse 192P within the range of pulse repetition rate.

[0051] Step 620 includes steps 630 and 632. In step 630, sensor 150 samples laser beam 192 to obtain a measurement representing the energy of laser pulse 192P, as described above in the explanation of FIG1. ​​Step 630 may implement step 634: obtaining this pulse-energy measurement by averaging a plurality of laser pulses 192P. In step 632, electronic circuitry 160 adjusts the loss level of the resonator 116 in a low-loss state 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 repeated in an active feedback loop to achieve the target pulse energy. In an exemplary scenario, step 620 achieves a laser pulse energy within 10% of the target pulse energy, wherein the laser pulse energy is evaluated as the energy of a single laser pulse or the average energy of a plurality of laser pulses.

[0052] Figure 7 is a timing diagram of one embodiment 700 of Q-switching gas laser 110 used by method 600. Embodiment 700 is a variation of embodiment 200, which equalizes both enable pulse energy and pulse duration. Figure 7 illustrates embodiment 700 for a specific embodiment of Q-switching with AOM. Embodiment 700 is readily adaptable to other types of Q-switching, such as EOM. Graphs 720 and 730 of Figure 7 provide a more detailed embodiment of step 632. Graph 720 depicts the time evolution of the RF drive signal 722 supplied to the AOM by electronic circuitry 160. Compared to the RF drive signal 222 of embodiment 200, the RF drive signal 722 is not necessarily zero during the low-loss state of resonator 116. Instead, the RF drive signal 722 generally has a non-zero amplitude ∆Vlow during the low-loss state, and electronic circuitry 160 adjusts the value of ∆Vlow based on the pulse-energy measurement obtained by sensor 150 in step 630.

[0053] Graph 730 depicts the time evolution of the laser power PL circulating in resonator 116. When the AOM is driven by RF drive signal 722, the RF drive signal 722 decreases from amplitude ∆V high to non-zero amplitude ∆V low, resulting in the generation of laser pulse 732. Compared to laser pulse 232 generated when the RF drive signal decreases all the way to zero (refer to Figure 2), laser pulse 732 has a lower peak power P peak and a lower pulse energy E. The reduction in pulse energy E (and peak power P peak) results in non-zero diffraction loss for the AOM during the low-loss state of resonator 116. In step 632, electronic circuit 160 adjusts ∆V low as needed to achieve the target pulse energy. For example, when the pulse-energy measurement obtained from sensor 150 indicates that the energy of laser pulse 192P exceeds the target pulse energy, electronic circuit 160 may increase ∆V low to increase the loss of resonator 116 during the low-loss state. Conversely, when the pulse-energy measurement value obtained from sensor 150 indicates that the energy of laser pulse 192P is less than the target pulse energy, electronic circuit 160 can reduce ∆V low.

[0054] 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 a target pulse duration. Graphs 720 and 730 of Figure 7 provide a more detailed embodiment of step 642. In this embodiment, electronic circuitry 160 adjusts the duration ∆t of the RF drive signal 722 with amplitude ∆V low. In another embodiment, electronic circuitry 160 reduces the duration ∆t to prevent the laser pulse 732 from having a tail, or increases the duration ∆t to maximize the energy in the main pulse of the laser pulse 732. In yet another embodiment, electronic circuitry 160 adjusts the duration ∆t to achieve a target pulse duration, for example, at a certain full width of the half-maximum (FWHM) pulse duration.

[0055] In one embodiment of step 620, step 640A precedes step 642, wherein sensor 152 samples laser beam 192 to obtain a measurement representing the duration of laser pulse 192P, as described above in the explanation of FIG1. ​​Step 640A may implement step 644: obtaining this pulse-duration measurement from a single laser pulse 192P, or by averaging several single pulse durations. When step 620 includes step 640A, step 642 is based on the pulse-duration measurement obtained in step 640A. This embodiment may repeatedly execute steps 640A and 642 in an active feedback loop. The pulse-duration feedback loop performed by steps 640A and 642 may be faster than the pulse-energy feedback loop performed by steps 630 and 632, especially when the laser pulse duration is obtained from measurements of a single or only a few laser pulses 192P. In an exemplary scenario, step 620 achieves a laser pulse duration within 10% of the target pulse duration.

[0056] In another specific embodiment of step 620, step 640B precedes step 642, wherein electronic circuit 160 either calculates the desired duration ∆t of the low-loss state of resonator 116 or retrieves the desired duration ∆t from a lookup table. Electronic circuit 160 can calculate the desired duration ∆t from a pre-calibration function relationship between (a) the target pulse duration and (b) the duration ∆t and the pulse repetition rate, and, if necessary, also calculate one or more other parameters of gas laser 110 and / or target pulse energy. For example, by adjusting the duration ∆t in a manner linearly dependent on the period T of laser pulse 192P (equivalent to the reciprocal of the pulse repetition rate), it is possible to achieve a constant laser pulse duration within the pulse repetition rate range. Alternatively, the pre-calibration duration ∆t, the pulse repetition rate, and, if necessary, one or more other parameters of gas laser 110 and / or target pulse energy as a function of the target pulse duration, can be listed in a lookup table included in electronic circuit 160.

[0057] In one scenario, step 610 includes step 612 of changing the repetition rate of the laser pulse 192P. In this scenario, method 600 performs step 632 and, if necessary, step 642 in response to the repetition rate change generated in step 612 in order to minimize the changes in laser pulse energy and laser pulse duration caused by the repetition rate change.

[0058] In a specific embodiment of the electronic circuit 160 including a controller 162 and a driver 164, the execution of scheme 700 requires a driver 164 to generate an RF drive signal 722 (an embodiment of drive signal 182). The driver 164 generates the RF drive signal 722 according to a command signal 180 generated by the controller 162. A graph 710 in FIG7 illustrates the time evolution of the command signal 712. The command signal 712 is an embodiment of a command signal that causes the driver 164 to generate the RF drive signal 722. The command signal 712 is a voltage VC that alternates between a high value Vhigh and a low value Vlow. When VC = Vhigh, VRF has an amplitude ∆Vhigh. When VC = Vlow, VRF has an amplitude ∆Vlow. The value of Vlow is a signal value that governs the value of ∆Vlow. Each laser pulse 192P is generated after a command pulse 714 characterized by the signal value Vlow. The value of Vlow controls the value of ∆Vlow, thereby controlling the energy of the laser pulse 192P.

[0059] The controller 162 can set the repetition rate of the laser pulse 192P by setting the period of the leading edge 714L of the successive command pulses 714 to the reciprocal of the desired pulse repetition rate. In this case, the controller 162 adjusts the duration ∆t of the low-loss state of the resonator 116 by adjusting the time position of the trailing edge 714T of the command pulses 714.

[0060] Command signal 712 contains both timing information (times t1 and t2) and a variable voltage Vlow. Alternatively, controller 162 may generate a command signal with two independent components: a timing signal and a variable DC voltage. The timing signal may be a transistor-transistor-logic (TTL) signal containing a TTL pulse similar to command pulse 714 but with a constant signal value Vlow. The variable DC voltage may be an analog signal, such as a DC voltage with a dominant amplitude ΔVlow or the value Vlow of RF drive signal 722. Commercial RF drivers are often configured for digital control rather than analog control. The command signal may be a digital signal containing digital encoding time t1, delay Δt, voltage value ΔVhigh, and voltage value ΔVlow.

[0061] Figure 8 illustrates an embodiment of laser pulse 192P, which is generated when the device 100 is operated using the binary pulse equalization technique according to scheme 700, and the dynamic adjustment of both the laser pulse energy and the laser pulse duration is implemented based on the measurements of sensors 150 and 152. For comparison, Figure 8 also illustrates an embodiment of laser pulse, which is generated when the device 100 is operated according to the prior art scheme 200 without pulse equalization technique. Figure 8 depicts oscilloscope tracks of six laser pulse waveforms measured by sensor 152. Tracks 810, 820, and 830 are obtained using the prior art scheme 200 at pulse repetition rates of 10 kHz, 50 kHz, and 100 kHz, respectively. Therefore, for tracks 810, 820, and 830, Vlow remains zero and the duration ∆t remains unchanged. Trajectories 812, 822 and 832 were also obtained at pulse repetition rates of 10 kHz, 50 kHz and 100 kHz respectively, but they used the pulse equalization technique according to scheme 700 and dynamically adjusted both the signal value Vlow and the duration ∆t.

[0062] The measured pulse energies for tracks 810, 820, and 830 were 704 microjoules (µJ), 542 microjoules, and 355 microjoules, respectively, with corresponding FWHM pulse durations of 105 nanoseconds (ns), 118 nanoseconds, and 117 nanoseconds. Clearly, without pulse equalization, the pulse energy changes significantly as the pulse repetition rate increases from 10 kHz to 100 kHz. In contrast, when implementing the pulse equalization technique according to scheme 700, it can be seen from tracks 812, 822, and 832 that all three pulse repetition rates yield substantially the same laser pulse waveform. With pulse equalization, the pulse energies measured at 10 kHz, 50 kHz, and 100 kHz were 250 microjoules, 254 microjoules, and 262 microjoules, respectively. Furthermore, each of these three pulses had an FWHM pulse duration of 99 nanoseconds.

[0063] Figures 9A and 9B illustrate embodiments of laser pulse energy and duration for three different pulse repetition rates of 10 kHz, 50 kHz, and 100 kHz, with and without binary pulse equalization technology. Data set 910 is obtained without pulse equalization technology, i.e., ∆V low = 0 and ∆t remains unchanged. Data sets 920 and 930 are obtained using binary pulse equalization technology according to scheme 700, and ∆V low and ∆t are dynamically adjusted based on measurements obtained from sensors 150 and 152. In the case of data set 920, the target pulse energy E target is 340 microjoules and the target pulse duration w target is 99 nanoseconds. In the case of data set 930, the target pulse energy E target is 250 microjoules and the target pulse duration w target is 117 nanoseconds.

[0064] Data set 910 shows that, without pulse equalization, the laser pulse energy decreases rapidly with the pulse repetition rate decreasing by a factor of two from 10 kHz to 100 kHz. Furthermore, the laser pulse duration is highly sensitive to the pulse repetition rate in a negligible manner. In contrast, as shown in data sets 920 and 930, the binary pulse equalization technique is very effective. The target pulse duration is achieved for all three pulse repetition rates, and the pulse energy variation is minimal. For each of the two different target pulse energies, 340 and 250 microjoules, the measured laser pulse energy is within 5% of the target pulse energy.

[0065] Figure 10A illustrates an RF driver 1000, which can be implemented as driver 164 in a specific embodiment of device 100 where Q switch 118 is AOM. Figure 10B illustrates an exemplary command signal 1080 controlling the RF driver 1000. The RF driver 1000 includes an RF oscillator 1010, a mixer 1020, and an amplifier 1040. The RF oscillator 1010 generates an RF signal. The mixer 1020 receives this RF signal and further receives the command signal 1080 at the "IF" port of the mixer 1020. The command signal 1080 is a variable analog voltage VVA composed of successive instances of the command signal 712 to define the repetition rate Vlow and duration ∆t of the laser pulse 192P (refer to Figure 7). The command signal 1080 is generated using an embodiment of controller 162. The mixer 1020 modulates the RF signal from the RF oscillator 1010 according to the command signal 1080. Amplifier 1040 amplifies this modulated RF signal to generate an RF drive signal 1082 for driving the AOM Q switch. RF drive signal 1082 is equivalent to a successor instance of RF drive signal 722. RF driver 1000 may further include a low-pass filter 1030 to filter out undesirable harmonics generated by mixer 1020.

[0066] Figure 11A illustrates another RF driver 1100, configured to receive individual timing and voltage inputs. Figure 11B illustrates an exemplary timing command signal 1180 controlling the timing 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 held at voltage V high, and the other port receives V low from one embodiment of the controller 162. The analog switch 1150 also receives an individual timing command signal 1180 from the controller 162. The timing command signal 1180 is similar to the command signal 1080, except that it is a digital signal that only receives two values, high and low, without carrying information about V low. The analog switch 1150 switches between V high and V low according to the timing command signal 1180, thereby generating the command signal 1080.

[0067] Figure 12 illustrates a controller 1200 for binary pulse equalization and servo control of both laser pulse energy and laser pulse duration. Controller 1200 is a specific embodiment of controller 162 and may be implemented in electronic circuitry 160 together with RF driver 1100. Controller 1200 receives an average power measurement PAVE of laser beam 192 from sensor 150 and a laser pulse waveform of laser pulse 192P from sensor 152. The laser pulse waveform is an instantaneous laser power PINST(t) that changes over time. Controller 1200 processes PAVE and PINST(t) to generate Vlow and timing command signals 1180, respectively, and then transmits Vlow and timing command signals 1180 to RF driver 1100. The processing of PAVE is used to achieve a target pulse energy E target, and the processing of PINST(t) is used to achieve a target pulse duration w target. (There is some degree of coupling between laser pulse energy and laser pulse duration, as described above in the explanation of Figures 3 to 5).

[0068] For the purpose of pulse energy equalization, controller 1200 includes a pulse energy calculator 1210, an adder node 1220, and a proportional-integral-derivative (PID) controller 1230. The pulse energy calculator 1210 calculates the pulse energy E from the average power measurement PAVE and the pulse repetition rate frep. The pulse repetition rate frep can be received from an external source / signal or defined internally by controller 1200. The adder node 1220 evaluates and calculates the difference ΔE between the pulse energy E and the target pulse energy Etarget, and generates an error signal representing ΔE. Then, the PID controller 1230 determines and outputs a new Vlow to reduce ΔE.

[0069] PID control is only one embodiment of the feedback algorithm that can be employed by device 100 and method 600. Alternatively, device 100 and method 600 may employ different feedback algorithms known in the art. Accordingly, the PID controller 1230 may be replaced by another servo controller based on a different principle for minimizing error signals.

[0070] For the purpose of 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 INST(t) to derive the laser pulse duration w. Then, the timing signal generator 1260 compares this measured laser pulse duration w with 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 ∆t, as described above in the explanation of Figures 6 and 7. Then, the timing signal generator 1260 transmits the timing command signal 1180 to the RF driver 1100.

[0071] However, each iteration of the adjustment of V low is based on sampling of a series of laser pulses 192P, while each iteration of the adjustment of the timing command signal 1180 can be based on a single laser pulse 192P and thus can be completed in a shorter time scale.

[0072] The above description based on Figures 7, 10A, 10B, 11A, 11B, and 12 pertains to Q switches based on AOM. By correspondingly changing the drive signal used for the Q switch, these specific embodiments can be modified for other types of Q switches. For example, when Q switch 118 is EOM, the aforementioned RF drive signal is replaced with a DC drive signal.

[0073] The present invention has been described above with reference to a preferred embodiment and other embodiments. However, the present invention is not limited to the specific embodiments described and depicted herein. Rather, the present invention is limited only to the appended claims. [Simplified Explanation of the Diagram]

[0016] The accompanying drawings, which are incorporated in and constitute a part of this patent specification, are schematic diagrams illustrating preferred embodiments of the present invention, and together with the above description of the invention and the following description of the embodiments, are used to explain the principles of the invention.

[0017] Figure 1 illustrates a Q-switch gas laser device with equalization technology for both laser pulse energy and laser pulse duration according to a specific embodiment.

[0018] The timing diagram in Figure 2 illustrates a prior art solution for a laser resonator with an AOM Q switch for a Q switch.

[0019] Figure 3 illustrates a set of exemplary laser pulses generated by a specific embodiment of the device of Figure 1 that implements an AOM Q switch and operates according to the prior art of Figure 2.

[0020] Figure 4 illustrates another set of exemplary laser pulses, which are generated by a specific embodiment of the device of Figure 1 that is the same as that in Figure 3 but different from the prior art solution in Figure 2 in that it implements pulse trimming.

[0021] The data shown in Figure 5 further explores the effect of the duration of the command pulse used to control the Q switch on the laser pulse energy and duration at a fixed pulse repetition rate.

[0022] The flowchart in Figure 6 illustrates a method for binary equalization of laser pulses generated by a Q-switched gas laser according to a specific embodiment.

[0023] Figure 7 illustrates a timing diagram of the scheme used in the method of Figure 6 to Q-switch a gas laser using binary pulse equalization technology, according to a specific embodiment.

[0024] FIG8 illustrates, according to a specific embodiment, an example of laser pulse generation when the device of FIG1 is operated using binary pulse equalization technology according to the scheme of FIG7, and the dynamic adjustment of both laser pulse energy and laser pulse duration is implemented based on measurements from their respective sensors. For comparison, FIG8 also illustrates an example of laser pulse generation without pulse equalization technology.

[0025] Figures 9A and 9B respectively illustrate examples of laser pulse energy and laser pulse duration for three different pulse repetition rates with and without binary pulse equalization technology.

[0026] Figure 10A illustrates an RF driver that can be implemented in a specific embodiment of the device of Figure 1, where the Q switch is an AOM. Figure 10B illustrates exemplary command signals for controlling this RF driver.

[0027] Figure 11A illustrates another RF driver that can be implemented in a specific embodiment of the device of Figure 1, where the Q switch is an AOM. This RF driver is configured to receive individual timing and voltage inputs. Figure 11B illustrates exemplary timing command signals used to control the timing aspects of the RF driver of Figure 11A.

[0028] Figure 12 illustrates, according to a specific embodiment, a controller for binary pulse equalization under servo control of both laser pulse energy and laser pulse duration.

Claims

1. A Q-switch gas laser device with pulse equalization technology, comprising: a gas laser including a laser resonator with 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; a first sensor for obtaining a first measurement of the pulsed laser beam representing a laser pulse energy; a second sensor for obtaining a second measurement of the pulsed laser beam representing a laser pulse duration; and an electronic circuit communicatively coupled between the Q-switch and the first and second sensors and 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 repetition rate of a plurality of laser pulses of the pulsed laser beam; (b) adjust a loss level of the low-loss state based on the first measurement to achieve a target laser pulse energy; and (c) adjust a duration of the low-loss state based on the second measurement to achieve a target laser pulse duration.

2. The apparatus as claimed in claim 1, wherein, The first and second sensors are configured to sample the pulsed laser beam outside the laser resonator.

3. The apparatus as claimed in 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 as described in claim 3, wherein, The pulse-duration feedback loop is faster than the pulse-energy feedback loop.

5. The apparatus as claimed in claim 3, wherein, The first sensor is a photosensitive detector configured to obtain the first measurement value from an average of a plurality of laser pulses, and the second sensor is an optical detector configured to obtain the second measurement value from a waveform of a single laser pulse.

6. The apparatus as claimed in claim 1, wherein, The Q switch includes an acoustic modulator disposed in the laser resonator.

7. The apparatus as claimed in claim 6, wherein, The electronic circuit includes: an RF driver for generating and supplying an RF signal to the acousto-optic modulator, causing the acousto-optic modulator to introduce losses in the resonator through diffraction when the RF signal has a non-zero amplitude; and 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 a lower amplitude RF signal that puts the laser resonator in the low-loss state, each trailing edge causing the RF driver to generate a higher amplitude RF signal that puts the laser resonator in the high-loss state, and each signal value setting the value of the lower amplitude; wherein the controller is configured to (a) set the repetition rate by setting a time period between successive leading edges to match the reciprocal of a target repetition rate, (b) adjust the signal value based on the first measurement value to achieve the target laser pulse energy, and (c) adjust a duration from the leading edge to the corresponding trailing edge to achieve the target laser pulse duration.

8. The apparatus of claim 1, further comprising a laser power amplifier for amplifying the pulsed laser beam, wherein, The first sensor is configured to obtain the first measurement value from the pulsed laser beam after the pulsed laser beam is amplified by the laser power amplifier.

9. A method for equalizing laser pulses generated by a Q-switched gas laser, comprising the steps of: operating a Q-switch of the Q-switched gas laser to repeatedly switch a laser resonator of the Q-switched gas laser between a high-loss state and a low-loss state to generate a pulsed laser beam; and equalizing the laser pulse energy and laser pulse duration of a plurality of laser pulses of the pulsed laser beam by repeating the following steps: sampling the pulsed laser beam to obtain a first measurement representing the laser pulse energy and obtaining a second measurement representing the laser pulse duration of the laser beam; adjusting a loss level of the low-loss state based on the first measurement to approximate a target laser pulse energy; and adjusting a duration of the low-loss state based on the second measurement to approximate a target laser pulse duration.

10. As in request item 9, wherein, Each instance of the first measurement is obtained by averaging a plurality of laser pulses, and each instance of the second measurement is obtained by a single laser pulse.

11. As in request item 9, wherein: The operational steps include the following: Change the repetition rate of these laser pulses from a first value to a second value that is different from the first value; The equalization step includes: performing the adjustment steps in response to the change step in order to minimize the changes caused by the change step to the laser pulse energy and the laser pulse duration.

12. As in request item 11, wherein, The equalization step includes: within a range of 10%, maintaining the individual or average laser pulse energy at the second value of the repetition rate the same as the individual or average laser pulse energy at the first value of the repetition rate; and within a range of 10%, maintaining the laser pulse duration at the second value of the repetition rate the same as the laser pulse duration at the first value of the repetition rate.

13. As in request item 9, wherein: The operational steps include: generating a command signal comprising 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 resonator into a low-loss state; each trailing edge initiating a transition of the laser resonator into a high-loss state; each signal value defining the loss level of the laser resonator during the low-loss state; and setting a repetition rate of the laser pulses by setting a time interval between successive leading edges to match the reciprocal of a target repetition rate; the loss level adjustment step includes: adjusting the signal value based on the first measurement value; and the duration adjustment step includes: adjusting a duration from the leading edge to the corresponding trailing edge based on the second measurement value.

14. The method of claim 9, further comprising a step of amplifying the pulsed laser beam; wherein, This sampling step is performed on the amplified pulsed laser beam.

15. As in request item 9, wherein, The Q switch is a sound and light modulator.

16. As in request item 15, wherein, The operating steps include: driving the acousto-optic modulator to diffract laser light from the laser resonator, and the loss level generated therefrom being non-zero when in the low-loss state.

17. As in request item 9, wherein, The sampling process is performed outside the laser resonator.

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