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Showing papers on "Pulse duration published in 1991"


Journal ArticleDOI
01 Feb 1991-Science
TL;DR: A high-speed streak camera detector was used to determine the duration of these x-ray pulses, and computer simulations of the plasmas agree with the experimental results, predicting that brighter and more efficient x-rays are obtained by the use of more intense laser pulses.
Abstract: A high-temperature plasma is created when an intense laser pulse is focused onto the surface of a solid. An ultrafast pulse of x-ray radiation is emitted from such a plasma when the laser pulse length is less than a picosecond. A high-speed streak camera detector was used to determine the duration of these x-ray pulses, and computer simulations of the plasmas agree with the experimental results. Scaling laws predict that brighter and more efficient x-ray sources will be obtained by the use of more intense laser pulses. These sources can be used for time-resolved x-ray scattering studies and for the development of x-ray lasers.

339 citations


Journal ArticleDOI
TL;DR: A direct measurement of the amplitude and the phase of a femtosecond light pulse is performed for the first time to the authors' knowledge, and the time dependence of the field can be easily obtained by a Fourier transform.
Abstract: A direct measurement of the amplitude and the phase of a femtosecond light pulse is performed for the first time to our knowledge The measurement is made in the frequency domain, and the time dependence of the field can be easily obtained by a Fourier transform The technique relies on a pulse synthesis scheme to unravel the frequency dependence of the phase A mask filters the spectrum, which gives rise to a pulse with a measurable temporal profile related to the frequency dependence of the phase In particular, with a rectangular slit the time delay of the synthesized pulse is the first derivative of the phase with respect to the frequency of the original pulse at the central frequency of the filter The amplitude of the spectrum is obtained from the power spectrum

233 citations


Journal ArticleDOI
TL;DR: A model of electromagnetic stimulation predicts the transmembrane potential distribution along a myelinated nerve axon and the volume of stimulated tissue within a limb and the dependence of threshold stimulus strength on pulse duration and axon diameter.
Abstract: A model of electromagnetic stimulation predicts the transmembrane potential distribution along a myelinated nerve axon and the volume of stimulated tissue within a limb. Threshold stimulus stregth is shown to be inversely proportional to the square of the axon diameter. It is inversely proportional to pulse duration for short pulses and independent of pulse duration for long ones. These results are also predicted by dimensional analysis. Two dimensionless numbers, Sem, the ratio of the induced transmembrane potential to the axon's threshold potential, and Tc/T, the ratio of the pulse duration to the membrane time constant, summarise the dependence of threshold stimulus strength on pulse duration and axon diameter.

157 citations


Patent
20 Sep 1991
TL;DR: In this article, an ultrasonic transmission system for transmitting sound waves, focussed on the tissue to be treated, by way of a coupling medium, is described, where the ultrasonic signal produced at the focus of the system comprises brief pulses having at least one rarefaction phase with a negative sonic pressure amplitude with a value greater than 2×105 Pa.
Abstract: A device for generating sonic signal forms for limiting, preventing or regressing the growth of pathological tissue comprises an ultrasonic transmission system for transmitting sound waves, focussed on the tissue to be treated, by way of a coupling medium. An ultrasonic signal produced at the focus of the system comprises brief pulses having at least one rarefaction phase with a negative sonic pressure amplitude with a value greater than 2×105 Pa. The ultrasonic signal is radiated with a carrier frequency exceeding 20 kHz, a sonic pulse duration T of less than 100 μs and a pulse recurrence rate of less than 1/(5T). The device produces controlled cavitation in the tissue to be treated.

150 citations


Journal ArticleDOI
TL;DR: Using GaAs epilayers with arsenic precipitates (GaAs:As) as the photoconductive material in a broad-band optoelectronic terahertz beam system, the authors in this paper have generated and detected freely propagating, sub-picosecond electromagnetic pulses.
Abstract: Using GaAs epilayers with arsenic precipitates (GaAs:As) as the photoconductive material in a broad‐band optoelectronic terahertz beam system, we have generated and detected freely propagating, subpicosecond electromagnetic pulses. The receiver signal gave a measured integrated pulse width of 0.71 ps. Fast photoconductive rise times have been achieved which are characteristic of good mobility GaAs. In addition, the material exhibits a short ‘‘effective’’ carrier lifetime of several ps due to the embedded, closely spaced (about 20 nm) arsenic precipitates.

136 citations


Patent
24 Jan 1991
TL;DR: In this paper, the first pulse is provided to and circulated in a ring signal delay circuit having a plurality of signal delay elements that are connected in series, intermediate points between the delay elements provide delayed pulses having different delay times.
Abstract: A pulse phase difference encoding circuit provides a digital signal indicating a phase difference between a first input pulse and a second input pulse. The first input pulse is provided to and circulated in a ring signal delay circuit having a plurality of signal delay elements that are connected in series. Intermediate points between the delay elements provide delayed pulses having different delay times. Upon receiving the second input pulse, a selector selects one delay pulse provided by the delay element at which the first input pulse has arrived, and generates a digital positional signal indicating a position of the selected delay element. The number of rounds of circulation of the first input pulse in the ring signal delay circuit is separately counted. According to the number of rounds of circulation of the first pulse and the positional signal, the digital signal indicating the phase difference between the first and second input pulses is formed.

116 citations


Patent
19 Jun 1991
TL;DR: Pulse compensation is disclosed for electronic devices capable of producing a pulse output signal suitable for therapeutic purposes with pulse compensation being automatically provided upon sensing of a change in the conditions being monitored as mentioned in this paper.
Abstract: Pulse compensation is disclosed for electronic devices capable of producing a pulse output signal suitable for therapeutic purposes with pulse compensation being automatically provided upon sensing of a change in the conditions being monitored The electronic device, which preferably includes a micro-computer, has externally accessible pulse width and intensity controls for determining the width and amplitude of pulses then being delivered Upon each sensing of a change in a monitored condition capable of causing user discomfort or possible injury, the intensity of the pulses then being delivered is quickly reduced and then gradually increased toward the full intensity requested by the intensity control Pulse compensation is also provided when the pulse width is varied by the pulse width control to thereby automatically cause a corresponding change in the range of intensities of the pulses then delivered, based upon a nerve fiber strength-duration curve of the pulses, to thereby maintain the stimulation capability of the pulses substantially constant at a percentage of intensity as determined by the intensity control

103 citations


Proceedings ArticleDOI
01 Jun 1991
TL;DR: In this paper, the authors present hydrodynamic calculations and analytical modeling relating to both stress and thermal-induced ablation as a function of laser and tissue properties to illustrate the potential benefits of stress induced ablation.
Abstract: The objective of biomedical applications of lasers is frequently to remove tissue in a controlled manner. However, for ablation induced by thermal- or photo-decomposition, damage to surrounding tissue may be excessive in some instances. Tissue can also be ablated by a hydrodynamic process referred to as front surface spallation, in which a thin layer next to a free surface is heated to levels, below vaporization but, so rapidly that it cannot undergo thermal expansion during laser heating. This generates a stress pulse, which propagates away from the heated region, with an initial amplitude that can be calculated using the Gruneisen coefficient. As the pulse reflects from the free surface, a tensile tail can develop of sufficient amplitude, exceeding the material strength, that a layer will be spalled off, taking much of the laser-deposited energy with it. Because tissue is generally a low strength material, this process has the potential of producing controlled ablation with reduced damage to the remaining tissue. However, to achieve these conditions, the laser pulse length, absorption depth and fluence must be properly tailored. This paper presents hydrodynamic calculations and analytical modeling relating to both stress- and thermal-induced ablation as a function of laser and tissue properties to illustrate the potential benefits of stress induced ablation. Also, guidance is given for tailoring the exposure parameters to enhance front surface spallation.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

86 citations


Patent
07 Nov 1991
TL;DR: In this article, a bipolar diathermy instrument was used for capsulotomy, where the pulse duration can constantly be altered between 3 ms and 11 ms, whereby a pause of 30 ms or 22 ms developed.
Abstract: A pulsed high frequency current is fed to a bipolar diathermy instrument for optimum execution of a section in capsulotomy. The pulse duration can constantly be altered between 3 ms and 11 ms, whereby a pause of 30 ms or 22 ms develops. This pause is sufficient to cool the instrument tip, flushed with sodium chloride, in order to prevent the possibility of injury to adjacent tissue. The pulse repetition frequency is about 15 hz. The device necessary for the purpose includes pulse duration control (31) and HF oscillator (30). The HF current is synchronized by pulses from control (31) with AND gate (35). Amplifier (40) feeds handpiece (21) of the diathermy instrument by transformer (46). The required operation free from grounding is thereby assured.

82 citations


Patent
02 Apr 1991
TL;DR: In this paper, a technique for wound debridement in which a pulsed CO 2 laser beam is caused to impinge upon exposed tissue with individual pulses sufficiently energetic to ablate a thin layer of tissue.
Abstract: A technique is disclosed for wound debridement in which a pulsed CO 2 laser beam is caused to impinge upon exposed tissue with individual pulses sufficiently energetic to ablate a thin layer of tissue. Each pulse has a time duration short enough to avoid deleterious heat penetration but long enough so that it does not cause atmospheric breakdown. The CO 2 laser, with a wavelength in the far infrared region, is operated to produce a pulsed beam with individual pulses having an energy of about one joule per pulse or greater. The beam is focused to produce a beam diameter with a fluence of approximately ten joules per square centimeter at the tissue to be ablated and with a pulse repetition rate of approximately one hundred pulses per second and a pulse duration between one microsecond and ten microseconds.

79 citations


Journal ArticleDOI
TL;DR: It is shown how the addition of pulse frequency modulation to pulse width modulation can improve controller performance in single muscle operation and in costimulation, where the combination of pulse width and stimulus pulsefrequency modulation results in better control of transient responses than with pulses width modulation alone.
Abstract: The closed-loop control of electrically stimulated muscle using simultaneous pulse width and frequency modulation is considered. It is shown how the addition of pulse frequency modulation to pulse width modulation can improve controller performance. In single muscle operation, the combination of pulse width and stimulus pulse frequency modulation results in better control of transient responses than with pulse width modulation alone. In the case of costimulation, the controller modulates the pulse stimulus periods of the antagonists in a reciprocal manner, to ensure stable and fast responses. This feedback controller was evaluated in animal experiments. Step responses with rapid rise times but without overshoot were obtained by the combined modulation. Good steady-state and transient performance were obtained over a wide range of static lengths and commands, under different loading conditions and in different animals. >

Patent
14 Jun 1991
TL;DR: In this paper, the authors describe a high voltage, very short pulse, microwave radiating source using low-cost components, and capable of operating at high pulse repetition frequencies (prf).
Abstract: This invention describes a high voltage, very short pulse, microwave radiating source using low-cost components, and capable of operating at high pulse repetition frequencies (prf). The source is activated by an ordinary video trigger commensurate with driving TTL logic. A trigger will cause a chain of N (where N may be 12 or greater) avalanche transistors connected in a Marx generator configuration to threshold resulting in a 1,200 volt or greater baseband pulse having a rise time of less than 100 ps and a duration of about 3 ns driving the input port of a dipole antenna. The dipole is excited by a balun. This invention achieves very short pulse duration broadband microwave radiation at pulse repetition frequencies as high as 30 kHz or greater.

Journal ArticleDOI
TL;DR: In this article, a Mach-Zehnder interferometer with a pulsed dye-laser light source is used to observe the collapse of laser-produced cavitation bubbles in water near a rigid transparent polymeric boundary.
Abstract: A Mach–Zehnder interferometer with a pulsed dye‐laser light source is used to observe the collapse of laser‐produced cavitation bubbles in water near a rigid transparent polymeric boundary. Vapor bubbles with a maximum radius of about a millimeter are formed using a Q‐switched Nd:YAG laser operating at 1.06 μm with a pulse energy of 4.6 mJ and a pulse duration of 8 ns. The pressure in the liquid around the bubbles is determined directly from the resulting interferograms using a fringe tracing method. Prior to bubble rebound, a region of compressed water develops adjacent to the cavity and a weak acoustic wave is directed towards the rigid boundary.

Patent
16 Aug 1991
TL;DR: An antitachycardia device, either implanted or external, delivers a fixed pulse width truncated exponential waveform defibrillation shock and automatically adjusts the pulse duration based upon the impedance measured or calculated following a delivered shock as mentioned in this paper.
Abstract: An antitachycardia device, either implanted or external, which delivers a fixed pulse width truncated exponential waveform defibrillation shock and automatically adjusts the pulse duration based upon the impedance measured or calculated following a delivered shock. The apparatus operates by measuring or calculating the high voltage system impedance, selecting a pulse width for that impedance value and using a pulse width derived from the selected pulse width for the next defibrillation shock.

Journal ArticleDOI
TL;DR: In this paper, the authors reported the generation of 825 V electrical pulses with 1.4 ps rise time and 4.0 ps duration using a pulsebiased low-temperature grown GaAs photoconductive switch triggered by an amplified femtosecond dye laser.
Abstract: We report on the generation of 825 V electrical pulses with 1.4 ps rise time and 4.0 ps duration using a pulse‐biased low‐temperature‐grown GaAs photoconductive switch triggered by an amplified femtosecond dye laser. Dependence of the pulse shape on both electric field and optical energy is observed and discussed.

Patent
Masahira Akasu1
03 Oct 1991
TL;DR: In this article, an analogue-to-digital converter for converting an electric signal output from the photo-detection circuit to digital data under the timing of the clock pulse signal was proposed.
Abstract: A distance measuring apparatus for measuring a distance to an object of concern includes a clock pulse generator for generating a clock pulse signal at a predetermined frequency, a pulsed laser for emitting a light pulse to the object, a photodetection circuit for detecting an echo light pulse resulting from reflection of the light pulse at the object, an analogue-to-digital converter for converting an electric signal output from the photo-detection circuit to digital data under the timing of the clock pulse signal, and a memory for sequentially storing the digital data output from the analogue-to-digital converter on a time-serial basis, and a processing unit for processing the data stored in the memory to thereby arithmetically determine the distance to the object on the basis of a peak appearing in an echo wavefrom reconstituted from the data, a time lapse from the emission of the light pulse to the appearance of the peak, and the frequency of the clock signal. High accuracy distance measurement can be realized with improved linearity and SN ratio. By synthesizing an echo waveform from the data obtained through a plurality of light pulse emissions, resolution of the distance measurement can be enhanced.

Journal ArticleDOI
TL;DR: In this paper, the conditions for the formation of a light bullet pulse, propagating with nearly constant spatial and temporal parameters in a bulk nonlinear medium, are obtained using an induced phase modulation (IPM) configuration.
Abstract: The conditions for the formation of a light bullet pulse, propagating with nearly constant spatial and temporal parameters in a bulk nonlinear medium, are obtained. A pump and probe pulse configuration using an induced phase modulation (IPM) is used. The IPM effect provides a well-controllable spatial and temporal trapping of the probe pulse. The authors derive analytical expressions for the critical parameters of the system, incorporating input power, pulse duration, pump and probe beam radii, medium parameters, etc. The system parameters analyzed are rather general and include varying pump and probe pulse durations and delay. The constant parameters of the light formation are shown to be limited by the walk-off effect. An analytical balance condition is obtained for the spatial and temporal soliton-like formation. >

Patent
29 Oct 1991
TL;DR: In this paper, a system for producing ultra-high peak power pulses employs a plurality of solid state amplifying materials, such as Nd:glass, alexandrite, and Ti:sapphire, to achieve stretching of a pulse prior to amplification by a factor of approximately between 100 and 10,000.
Abstract: A system for producing ultra-high peak power pulses employs a plurality of solid state amplifying materials (13, 14, 15, 16), such as Nd:glass, alexandrite, and Ti:sapphire, to achieve stretching of a pulse (12) prior to amplification by a factor of approximately between 100 and 10,000. The time-stretched pulse is amplified by many orders of magnitude, illustratively 109. After time-stretching and amplification, the pulse is then compressed to its original duration (11). Pumping of the multiple solid-state elements (13, 14, 15, 16) is performed simultaneously using an alexandrite laser which is tunable between approximately 700 and 800 nm. The pumping energy has a pulse duration which is less than the fluorescence lifetime of the excited solid-state media.

Journal ArticleDOI
TL;DR: A new alexandrite laser source arrangement designed to measure atmospheric water vapor using the differential absorption lidar technique is described, capable of emitting two pulses at two appropriately selected wavelengths within a single flash lamp discharge.
Abstract: We describe a new alexandrite laser source arrangement designed to measure atmospheric water vapor using the differential absorption lidar technique. This laser is capable of emitting two pulses at two appropriately selected wavelengths within a single flash lamp discharge. A narrow spectral linewidth of Deltalambda < 1 pm is obtained for each pulse by intracavity filtering with a birefringent filter and two Fabry-Perot interferometers. Wavelength commutation between the two pulses is performed by electro-optically tuning the birefringent filter. The temporal separation between the two pulses can be chosen between 50 and 70 micros and each pulse duration is <250-ns (full width at half-maximum). Typical output energies of 50 mJ/pulse at each wavelength are obtained with this laser system at a 10-Hz repetition rate for a 1.3-kW input electrical power.

Proceedings Article
12 May 1991
TL;DR: In this article, the authors investigated self-focusing in the limit in which the beam diameter is significantly less than the pulse length (CT, where T is the pulse duration), where the beam would self-focus to moving singularities in the absence of other nonlinear effects.
Abstract: Self-focusing has been investigated only in the limit in which the beam diameter is significantly less than the pulse length (CT, where T is the pulse duration). In this case, the beam would self-focus to moving singularities in the absence of other nonlinear effects. However, there is an opposite extreme in which the beam diameter is much larger than the pulse length and the pulse disperses before any self-focusing can occur. Short-pulse self-focusing is more related to the second extreme than to the first.

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrated that optically gain switched GaAs vertical lasers (VSELs) with short cavity lengths can generate pulses as short as 4 ps, with delays as long as 20 ps.
Abstract: It is demonstrated that optically gain switched GaAs vertical lasers (VSELs) with short cavity lengths can generate pulses as short as 4 ps, with delays as short as 20 ps. A model of the large-signal response of VSELs to short optical input pulses is presented. The results demonstrate that the critical parameter of determining VSEL response time is the cavity length. A numerical solution to the rate equations is used to calculate the output pulsewidth and delay, given a specific cavity design and set of input conditions. Calculated results are compared to experimental data. Output pulse dependencies on input pulsewidth and power are examined for different cavity designs. The results of the calculations are used to generate design curves for future devices. >

Journal ArticleDOI
01 Jan 1991-Vacuum
TL;DR: In this article, the authors describe two sources of high power ion beams of nanosecond duration, one with energy up to 125 keV, pulse duration from 20 to 200 ns, current density for heavy ions (Al +, Mg +, Fe +, W +, etc.) 1-2 A cm −2 and energy of 12 to 120 J deposited by the beam.

Journal ArticleDOI
Abstract: Soft x‐ray spectra generated on flat Al targets by a 500 fs KrF* laser pulse (intensity 5.3×1015 W/cm2), preceded by a prepulse of the same pulse duration, have been measured as a function of the pulse separation and the prepulse intensity. It was found that not only the total x‐ray emission was much stronger when the prepulse was present but, in particular, lines in the shorter wavelength region had a higher intensity than without prepulse, and were strongly dependent on the time separation between the prepulse and the main pulse.

Journal ArticleDOI
TL;DR: In this paper, the amplitude, timing, and wavelength instabilities in a semiconductor mode-locked laser were studied experimentally and by using an accurate numerical model, and it was shown that these instabilities can occur when the RF drive frequency is tuned to give a minimum average pulsewidth.
Abstract: Amplitude, timing, and wavelength instabilities in a semiconductor mode-locked laser are studied experimentally and by using an accurate numerical model. It is shown that these instabilities can occur when the RF drive frequency is tuned to give a minimum average pulsewidth. It is shown that experimental measurement techniques, such as sampling and averaging, can mask these instabilities. Using this numerical model, it is shown that the wavelength instability is associated with the amplitude and timing instabilities and that the large broadening of the optical spectrum observed experimentally is caused by a cyclic wavelength instability. >

Journal ArticleDOI
TL;DR: The contributions of pulse-induced dipole-dipole interaction to the total pressure acting normal to the membranes of closely positioned pronase treated human erythrocytes during electrofusion was calculated.

Journal ArticleDOI
Bodil Braren1, Kelly G. Casey1, Roger Kelly1
TL;DR: In this article, the authors discuss experiments in which ∼ 20 ns excimer laser pulses are incident on polymethylmethacrylate in air at 193 or 248 nm and the release process is photographed with a ∼ 1 ns probe pulse.
Abstract: The laser-pulse sputtering of polymers should have two limiting cases. In the one there is ongoing release of particles from the target surface, the particles then form a Knudsen layer (KL), and there is finally an unsteady adiabatic expansion (UAE) (‘effusion’ model). In the other limit, bond-breakage occurs rapidly over a characteristic depth and the resulting gaslike particles then flow out directly in a UAE without a formal KL (‘outflow’ model). To test these idealized gas-dynamic descriptions, we discuss experiments in which ∼ 20 ns excimer laser pulses are incident on polymethylmethacrylate in air at 193 or 248 nm and the release process is photographed with a ∼ 1 ns probe pulse. The results not only give explicit support to the gas-dynamic description of the problem, but also indicate that the KL-UAE model is more appropriate. For example, only this model accommodates the observation that the release process continues for ∼ 6 μs, which is ∼ 500 times the laser pulse length.

Journal ArticleDOI
TL;DR: Broadly tunable and high-power operation of a titanium-doped sapphire laser is obtained with a diode-laser-pumped frequency-doubled Nd:YAG laser as the pump source.
Abstract: Broadly tunable and high-power operation of a Ti-doped sapphire laser is obtained with a diode-laser-pumped frequency-doubled Nd:YAG laser as the pump source. A maximum broadband (FWHM = 25 nm) output pulse energy of 720 microJ at 795 nm in a TEM00 mode is obtained for 1850 microJ of energy of 532-nm pump light. A minimum pulse duration of 7 nsec is obtained from a 40-mm-long cavity. With the use of an intracavity prism, the Ti:sapphire laser is tunable continuously over the 696-1000-nm spectral range (with three different mirror sets).

Patent
10 Oct 1991
TL;DR: In this article, a system and method that uses a digital oscilloscope to digitize a waveform and then loads the digitized waveform into a computer is described, where the computer scans the waveform data locating leading edges by comparing data to a rolling average and a noise threshold and locating tops and bottoms if the pulse is a negative going pulse by compared data to slope criteria until a trailing edge is discovered.
Abstract: The invention is a system and method that uses a digital oscilloscope to digitize a waveform and then loads the digitized waveform into a computer. The computer scans the waveform data locating leading edges by comparing data to a rolling average and a noise threshold and locating tops and bottoms if the pulse is a negative going pulse by comparing data to slope criteria until a trailing edge is discovered. The trailing edge defines the end of the pulse, the immediately preceding leading edge defines the beginning of the pulse and the pulse height and this allows the pulse characteristics to be determined. This discovered pulse is removed from the waveform by setting all data values in the pulse to the pulse baseline value, and the scan for leading edges, pulse tops and a trailing edge is performed again to isolate and characterize another pulse. This cycle of detection, analysis and removal continues until the end of data is encountered before a pulse is discovered.

Patent
07 Jun 1991
TL;DR: In this article, a recording apparatus overwrites an input signal having pulse duration periods and pulse spacing periods to an optical disk by irradiation of a laser beam to form recording marks corresponding to the pulses.
Abstract: A recording apparatus overwrites an input signal having pulse duration periods and pulse spacing periods to an optical disk by irradiation of a laser beam to form recording marks corresponding to the pulse duration periods. The apparatus includes a detector for detecting a leading edge of the pulse duration period and for producing a start signal thereupon, another detector for detecting a trailing edge of the pulse duration period and for producing a stop signal thereupon, a pattern setting circuit for setting a predetermined basic pattern, and a pattern generator for generating the basic pattern from its beginning in response to the start signal and for terminating the generation of the basic pattern in response to the stop signal. The apparatus further includes a circuit for forming a modulated signal using a full or portion of the basic pattern produced from the pattern generator. The laser output is produced in accordance with the modulated signal.

Journal ArticleDOI
TL;DR: In this paper, the authors reported the generation of tunable light pulses with durations as short as 65 fs by pulsed synchronous pumping of a singly resonant optical parametric oscillator.
Abstract: We report the generation of tunable light pulses with durations as short as 65 fs by pulsed synchronous pumping of a singly resonant optical parametric oscillator. The measured wavelength dependence of the pulse duration provides evidence for self-compression by means of chirp reversal of the idler pulses and normal dispersion of the optical parametric oscillator cavity elements.