Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
  • H01S3/1123—Q-switching
  • H01S3/117—Q-switching using intracavity acousto-optic devices
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
  • H01S3/22—Gases
  • H01S3/223—Gases the active gas being polyatomic, i.e. containing two or more atoms
  • H01S3/2232—Carbon dioxide (CO2) or monoxide [CO]
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/02—Constructional details
  • H01S3/03—Constructional details of gas laser discharge tubes
  • H01S3/0315—Waveguide lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/07—Construction or shape of active medium consisting of a plurality of parts, e.g. segments
  • H01S3/073—Gas lasers comprising separate discharge sections in one cavity, e.g. hybrid lasers
  • H01S3/076—Folded-path lasers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
  • H01S3/0813—Configuration of resonator
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
  • H01S3/0813—Configuration of resonator
  • H01S3/0816—Configuration of resonator having 4 reflectors, e.g. Z-shaped resonators
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/097—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser
  • H01S3/0975—Processes or apparatus for excitation, e.g. pumping by gas discharge of a gas laser using inductive or capacitive excitation

Definitions

• the present invention relates in general to Q-switched pulsed lasers.
• the invention relates in particular to Q-switched high-power pulsed carbon dioxide (CO 2 ) lasers.
• High power Q-switched pulsed C02 lasers have been manufactured in folded waveguide and folded free space Gaussian mode designs. Such a Q-switched CO 2 lasers have optical characteristics attractive for a number of specialized, high-value applications in materials processing, such as via hole drilling, and printed circuit board (PCB) polymer ablation.
• a high-power Q-switched CO 2 laser is also useful as an oscillator in a master- oscillator power- amplifier (MOPA) arrangement for use in plasma-EUV (extreme ultraviolet) radiation generation for photolithography.
• MOPA master- oscillator power- amplifier
• Such a laser combines very high peak instantaneous optical power, for example, about 10 kilowatts (kW) or greater, with modest average power, for example, about 10 watts (W) or greater, in a compact package.
• One common method of Q-switching a high power CO 2 laser involves use of an intra-cavity electro-optic (E-O) modulator. This provides very fast, for example, tens of nanoseconds (ns), optical switching, resulting in a laser with short optical pulses, for example about 100 ns FWHM (full width at half maximum), and high pulse repetition rate, for example about 100 kilohertz (kHz) or greater.
• E-O intra-cavity electro-optic
• CdTe single-crystal cadmium telluride
• CdTe for E-O Q-switching CO2 lasers presents two key issues.
• the first is high component costs which include the cost of the CdTe crystal itself and the cost of a high-speed, high-voltage modulator driver for driving the CdTe crystal. These component costs result in a cost of the finished pulsed CO 2 laser many times higher than that of a continuous wave (CW) CO 2 laser of comparable average power.
• CW continuous wave
• An alternate, relatively inexpensive approach to Q-switching a CO 2 laser involves scanning a resonator end-mirror or a resonator fold-mirror such that the mirror sweeps reciprocally from one completely misaligned position to another through an optimally aligned position.
• This approach is described in U.S. Patent Application No. 11/638,645, filed March 13, 2006, assigned to the assignee of the present invention and incorporated herein by reference.
• a problem with this approach is that the mirrors are preferably scanned at a characteristic resonant frequency to provide a suitable combination of scan- angle and sweep-speed.
• a laser in accordance with the present invention comprises a laser resonator having a gaseous gain-medium therein including CO 2 .
• An excitation arrangement is provided for exciting the CO 2 including gain-medium.
• An acousto-optic (AO) Q-switch is located in the laser resonator and includes an AO material transparent at a fundamental wavelength characteristic of CO 2 .
• the AO material is preferably transparent at wavelengths between about 9 micrometers and about 11 micrometers. One preferred wavelength is about 10.6 micrometers. One preferred AO material is germanium.
• the laser resonator preferably has output coupling greater than about 50 percent.
• the laser resonator is terminated by first and second mirrors, and the second mirror is partially transparent to the fundamental wavelength and provides the output coupling.
• the inventive laser is capable of delivering a train of pulses having a peak power of at least about 10 kW and an average power of at least about 10 W.
• a single RF power supply is utilized to power the electrodes of the laser exciting the laser gas as well as to drive the AO cell.
• Figure 1 illustrates a preferred form an acousto-optic (AO) cell suitable for
• Figure 2 illustrates a carbon dioxide laser system Q-switched with an AO cell.
• Figure 3 is a timing diagram illustrating the operation of a carbon dioxide laser of the type illustrated in Figure 2.
• Figures 4, 5 and 6 are oscilloscope tracings of the input RF power and output light pulses from a carbon dioxide laser of the type illustrated in Figure 2.
• Figure 7 is a plot of average output power and beam divergence as a function of pulse repetition frequency (PRF) of a carbon dioxide laser of the type illustrated in Figure 2.
• Figure 8 is a plot of the far field profile of a beam generated by a carbon dioxide laser of the type illustrated in Figure 2.
• PRF pulse repetition frequency
• Figure 9 is a schematic diagram of a laser system wherein a single power source is used to supply RF energy to both the laser and the AO cell.
• Figure 10 is an illustration of one form of an RF tap which can be used to divert some of the energy generated by the RF power source to the AO cell.
• FIG 11 is a timing diagram illustrating the operation of a carbon dioxide laser of the type illustrated in Figure 2 but modified in accordance with Figure 9.
• Figures 12 and 13 are oscilloscope tracings of the input RF power and output pulses from a carbon dioxide laser of the type illustrated in Figure 2 but modified in accordance with Figure 9.
• the subject invention relates to a carbon dioxide laser that uses an acousto-optic (AO) cell to achieve Q-switching instead of a more conventional electro-optic Q-switch.
• AO cells have been used in the prior art for Q-switching other lasers. This is a very common technique used with near-IR solid-state lasers, such as Nd:YAG. At these shorter wavelengths (1-2 ⁇ m compared to 10 microns for CO 2 lasers), there exist several candidate AO materials with low optical loss, such as quartz and TeO4. However, the only AO material that is useful at present in the 9-11 ⁇ m band is single crystal germanium (Ge).
• germanium is a generally good optical material, it none-the- less has relatively high IR absorption, typically about 1-2% per centimeter in the 9 to 11 micron band.
• the Ge AO crystal through which the laser beam passes is typically 5cm wide in order to obtain approximately 80% deflection efficiency of the laser beam propagated through the AO cell at reasonable acoustic power levels.
• the resulting optical loss through such a crystal is therefore about an order of magnitude higher than the loss associated with CdTe.
• the detrimental optical lensing effects caused by this absorption are offset by the relatively high thermal conductivity of Ge over CdTe.
• Ge has a relatively high refractive index of 4 so anti-reflection coatings on its surfaces are necessary for inter-cavity laser operation.
• Ge has the advantage of superior optical quality uniformity across the crystal when compared with CdTe. It is also available in much larger sizes than CdTe.
• germanium has been successfully and commonly used, for over 15 years, in conjunction with CO 2 lasers for controlling the direction of the beam outside the laser cavity. It has been most commonly used as an optical beam deflector external to a laser's optical cavity in high speed CO 2 laser beam scanning systems for commercial marking and hole drilling applications.
• An AO cell has also been used as a frequency shifter external to a laser's optical cavity in CO 2 laser radar systems.
• a high gain CO 2 laser has a long gas-discharge gain medium composed of numerous narrow ceramic waveguides having either square, round or elliptical cross sections, arranged in a multiple folded geometry so as to be connected optically in series.
• the high gain can be also obtained by super pulsing the laser discharge thereby offering the option of reducing the number of folds.
• the use of optical folding can yield a total laser gain length of several meters long.
• each waveguide channel or pass is on the order of 45 centimeters.
• Insertion of an AO modulator into such a high gain laser' s resonator having such high output coupling will increase the total cavity loss by a much lower percentage then if the AO cell was inserted into a low gain (i.e., short) CO 2 laser. Consequently the laser efficiency of an AO Q-switched high gain CO 2 laser is not as bad as one would experience with a short laser (i.e., a low gain laser) because the loss contributed by the AO modulator will be substantially less than the laser's output coupling. This results in a smaller fraction contribution by the AO cell optical losses to the overall cavity losses. For a CO 2 laser having a 75% output coupling mirror, the 15% to 20% contributed by the AO modulator loss (for example) is sufficiently small so as to be acceptable.
• Fig. 1 schematically illustrates the functioning of a AO modulator with an RF signal applied to the acoustic transducer (i.e., the switch S in Figure 1 is closed.
• ⁇ is typically 77.1 mrad.
• ⁇ 10.6 microns
• the Bragg angle ⁇ is approximately 38.5 mrad for an acoustic frequency of 40 MHz in Ge.
• the intensity of the acoustic wave (Pac) determines the amount of diffracted laser power (P 0 ⁇ P 1 ⁇ ) where ⁇ is the percentage of the P 1 laser power that is diffracted at angle ⁇ .
• ⁇ is approximately 80% at 10.6 microns with an acoustic frequency of 40 MHz and a power of approximately 30 watts in Ge.
• the un-diffracted power of the through-put beam is Pi ⁇ (P 1 -P 0 ).
• the percentage of the diffracted beam " ⁇ " can be calculated from:
• Pac is the acoustic power
• the AO cell is oriented with the deflection plane parallel to the waveguide polarization axis.
• the cell is rotationally adjusted so that the intra-cavity beam enters the Ge crystal at the Bragg angle.
• lasing is suppressed by applying RF power to the AO cell, resulting in a beam deflection of twice the Bragg angle.
• RF power to the AO cell is switched off briefly, with the length of the pulse tail being determined by the RF off time.
• Fig. IB schematically illustrates the functioning of the AO modulator with no RF power applied to the acoustic transducer (i.e., the switch S is opened).
• the switch S i.e., the switch S is opened.
• optical absorption i.e., can be 1 to 2 % per cm at 9 to 11 microns
• the laser gain cell consists of a sealed, RF-excited waveguide laser tube, with a total of nine waveguide U-shaped waveguide slots (3.2 mm x 3.2 mm cross section), arranged in a multiple fold geometry.
• the slots are typically defined by channels machined into a ceramic block.
• Each of two large mirrors Mi and M 2 at each end of the laser housing provided three folded reflections.
• the output coupler Mo had a 25 % reflectivity (or 75 % coupling), and a ZnSe thin-film polarizer (TFP) 20 provided both resonator polarization and a vacuum window.
• TFP ZnSe thin-film polarizer
• Fig. 2 For simplicity only a 5 pass folded resonator configuration is shown in Fig. 2.
• Mo is the outcoupling mirror.
• Mirrors Mi and M 2 are two rectangular beam folding mirrors and mirror M 3 is located external to the hermetically sealed laser housing and is the high reflectivity mirror of the laser cavity. Additional information about folded waveguide lasers is disclosed in the following commonly owned patents, each of which is incorporated by reference: U.S. Patent Nos. 6,788,722 and 6,798,816.
• the gain cell was mounted on a stable base structure.
• a standard 5.0 cm wide Ge acousto-optic beam deflector (Intra- Action # ADM-406B1) was then aligned next to the polarizer 20 at the end of the tube, and rotationally oriented to be at the Bragg angle, which is approximately 38 mrad at the 40 MHz acoustic center frequency and at an IR wavelength of 9.25 microns.
• a piano end reflector (M3) was positioned behind the AO cell to complete the resonator. The AO cell is inserted outside the hermetically sealed laser tube housing containing the CO 2 gas mixture between the end mirror M 3 and the thin film polarizer 20.
• a standard 100 MHz power supply 22 was used to provide approximately 1 kW of RF power to the electrodes 24 of the CO 2 discharge through switch S 2 .
• a standard 40 MHz supply 30 was used to drive the AO cell with approximately 30 W max power. This latter unit was gated by an external pulse generator to provide the desired optical pulse repetition frequency (PRF).
• PRF optical pulse repetition frequency
• Synchronizing the opening and closing of switches Sl and S2 of Fig. 2 offers flexibility in the pulse formats that can be delivered by the laser.
• FIG. 3 A shows the RF power to the laser.
• Figure 3B illustrates the power to the AO cell.
• Figure 3C is intended to show the effect of the AO cell at the beam location within the cell. More specifically, and as discussed below, there is a delay between the time the power is applied to the transducer in the cell and the time the acoustic wave reaches the laser beam.
• FIG. 3D shows the laser output.
• Fig. 3 A at time ti the RF power to the laser's RF power supply is turned on.
• the termination of the acoustic wave initiated at time tl reaches the laser beam causing the laser to switch to a low loss state as in Figure IB. Consequently the laser begins to oscillate and create a pulse of light.
• the acoustic wave initiated at time t 2 reaches the laser beam in the cell and again causes a high loss state within the laser because light is being diffracted out of the resonator path. This results in the tail of the q-switched laser pulse, which would normally end at time 'V'. to be is curtailed at time 'V'.
• the cut-off time of the laser's pulse tail is controlled by the duration of "tix". i-e-, the time duration the AO cell is turned off.
• Figs. 4 and 5 Several oscilloscope traces illustrating typical Q-switched operation are shown in Figs. 4 and 5 for a 9 fold waveguide CO 2 laser.
• the upper trace (Figs. 4A and 5A) is the modulator RF drive signal at approximately 40 MHz.
• the 40 MHz signal was captured by using an in-line directional coupler.
• the lower trace in each Figure (Figs. 4B and 5B) is the Q-switched laser pulse.
• the sweep speed of Fig. 4 is 400 nanoseconds per major division and for Fig. 5 it is 10 microseconds per major division.
• the RF power to the AO cell was maintained at 30 W to maintain a high loss state to allow the laser to achieve a large over population of the upper laser level.
• the RF drive to the cell was then gated off for approximately an 800nsec interval.
• the Q-switched laser pulse occurred approximately 2,250 nanoseconds later.
• a large part of this delay is due to the finite transit time for the acoustic wave in the Ge crystal to reach the intra-cavity laser beam and the transit time it takes the acoustic waves to propagate across the diameter of the laser beam.
• the acoustic velocity in Ge is 5500 m/s, while the beam diameter of the laser used in the experiment was approximately 2 mm across.
• Fig. 4B shows that the resulting Q-switched laser pulse has a width on the order of 110 ns FWHM, which is comparable to that obtained with traditional electro-optic Q-switching.
• the tail of the pulse tends to roll off for a considerable longer time, rather than being quickly cut off as in EO Q-switching. This is again due to the finite acoustic transit time in the Ge crystal. It should be also noted that this relatively long transit time makes the use of acousto-optic cells in simultaneously laser Q-switching and cavity-dumping operation unattractive because the resulting optical pulse duration will be on the order of 100 ns, as opposed to less than 20 ns for the EO cavity dumped version.
• Fig. 5 illustrates Q-switched operation at a 5OkHz PRF.
• the oscilloscope trace of Fig. 6 shows a Q-switched pulse at a faster sweep speed, i.e., at 100 nsec per major division..
• a broad range of pulse formats can be commanded with AO cells. This includes pulse bursts synchronized with pulsed RF driving the laser discharge, coupled with external pulse triggering from a user interface so as to provides AO Q-switching the same flexibility as with the EO Q-switching.
• a plot of average output power with PRF from 1OkHz to 100 kHz and its corresponding two dimensional (x and y axes) far-field beam divergence are provided in Fig. 7.
• power supply 22 provides the high power input for the electrodes while power supply 30 provides a lower power input for the AO cell.
• power supply 22 operated at a frequency of 100MhZ and power supply 30 operated at 40MHz. Since both supplies generate RF power, it would be possible to operate the laser with a single power supply at a single frequency. This approach can simplify the laser and significantly reduce costs. Using one power supply will somewhat reduce the freedom to vary the shape of the pulses.
• Fig. 9 schematically illustrates the basic physical arrangement for a laser system wherein only power supply is used to excite the discharge and activate the Q-switch utilizing the invention. The output from the power supply 50 is passed through a single switch "S".
• the RF signal out of switch S is passed through a chain of transistor power amplifiers 52 to obtain the power desired to be delivered to the laser's discharge.
• the output power from the amplifier P is typically about 1 kW.
• the AO cell can be, for example, the IntraAction Corp.
• the approximate -13dB RF tap for the AO cell can be obtained by various well known techniques in the RF art , such as a RF directional coupler fabricated by either transmission lines or printed circuit board technologies, or a tapped autotransformer, or by a tapped capacitive bridge or by an inductor/capacitor networks, etc.
• Fig. 10 illustrates a -13dB auto- transformer RF tap approach for coupling a small amount of power from the output of a 1 kW RF power supply to drive an AO cell requiring 5OW of power.
• Either a fixed turn tap (not shown) or a mutual inductive coil tap (as shown in Fig. 10) can be used to couple out the small RF power required to drive the AO cell.
• Such an auto-transformer RF tap technique is well known in the RF art.
• closing the switch S delivers RF power to both the CO 2 laser and the AO cell. Since the AO cell is then turned on, it prevents laser oscillation, thereby causing the overpopulation of the upper laser level to builds up.
• S is opened, thereby turning off the laser and the AO cell at the same time.
• S is opened, the laser discharge is turned off but its optical resonator is in a low loss state because the AO cell is also turned off. Consequently, laser oscillation is initiated as soon as the time delay associated with the acoustic waves propagating from the acoustic transducer pass the location where the laser beam propagates through the AO cell.
• the acoustic velocity in Ge is approximately 5.5x10 5 cm/sec so it takes approximately 1.8 microseconds for the acoustic waves to travel 1 cm.
• Fig. 11 presents a time sequence view of events for the invention.
• the switch S of Fig. 9 is closed thereby providing RF power "P" out of the RF power supply (i.e. assumed to be IkW) as depicted in Fig. 1 IA.
• the time duration of the RF pulse is denoted as T b11 in Figure 1 IA.
• the amplitude of this pulse can be twice the average power of the RF supply's because of the lowered duty cycle (superpulse operation discussed above).
• T D time delay
• the population of the laser's upper state begins to build up and reaches a maximum at time t 2 after which it levels off ( see Fig HD ).
• the RF power to the laser discharge and the AO cell is cut off by closing switch S.
• the pulsed acoustic waves propagating within the AO cell continue to pass through the laser beam for a period equal to TQ and until time U- Since the RF power to the laser discharge is stopped at time t 3 , the population of the upper laser level begins to drop off due to amplified spontaneous emission during the time between t 3 to U-
• the Q-switch becomes transmissive and the laser enters the low loss state.
• the residual radiation contained in the tail of the Q-switched pulse terminates naturally in this example between times t 6 and t ⁇ .
• switch S is closed at time t 6 but the Q-switch remains in the transmissive state until time t 8 due to the time delay TD discussed above. Therefore, in this example, there is no tail clipping.
• the AO cell will switch to the high loss state before the end of the pulse tail, so that the tail can be chopped. Chopping the tail of a pulse can improve performance, particularly for drilling.
• Fig. 12 corresponds to providing experimental data illustrating the actual time sequences after applying RF power simultaneously to the laser's RF discharge and to the AO cell.
• the laser system used was similar to that used to generate the traces in Figs. 4 and 5 but modified to use a single power supply for both the discharge and the AO cell.
• the upper trace (Fig. 12A) is the RF power and lower trace (Fig. 12B) is the laser pulse.
• Fig. 12 corresponds to providing experimental data illustrating the actual time sequences after applying RF power simultaneously to the laser's RF discharge and to the AO cell.
• the laser system used was similar to that used to generate the traces in Figs. 4 and 5 but modified to use a single power supply for both the discharge and the AO cell.
• the time delay for the appearance of the Q-switched pulses, after the RF pulses are turned off is approximately 3.2 microseconds. This delay is the sum of the acoustic propagation time TD plus the time (TL) required for the Q-switch pulse to build up out of the spontaneous emission noise within the lasers optical cavity as shown in Fig 11. Note that in Fig 12B, the long tail of the Q-switch pulse is cut off 2.4 microseconds after the second RF pulse is turned on.
• the time delay (T D ) is the time required for the acoustic wave to intersect the laser beam in the AO cell.
• the PRF and the duration of the RF pulses provided to both the laser discharge and the AO cell, the PRF and the duration of the radiation in the tail in this single RFPS/si ⁇ gle switch, AO Q-switched laser system can be varied to suit the application.

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