Q-switched laser damage of infrared nonlinear materials

Q-switched laser-damage thresholds have been determined for six materials (proustite-Ag3AsS3, pyrargyrite-Ag3SbS3, cinnabar-HgS, silver thiogallate-AgGaS2, tellurium-Te, and gallium arsenide-GaAs) of interest for nonlinear optics in the medium infrared. Four TEM00mode lasers were employed with outputs at wavelengths of 694 nm, 1.06, 2.098, and 10.6 μm. Damage has been found to be confined to the surface of the crystals and occurs for radiation intensities between 3 and 75 MW/cm². Particular care is needed in the cutting and polishing of tellurium crystals if a high-damage threshold is to be achieved.     

Stimulated Raman scattering of picosecond light pulses in hydrogen, deuterium, and methane

Experimental results are presented on stimulated Raman scattering of short pulses of approximately 100 ps duration in H2, D2, and CH4, both in capillary waveguides and in a tight focusing geometry. Experimentally determined thresholds are in good agreement with calculation. Low thresholds (< 20 muJ) are observed in CH4and preliminary results using a mode-locked dye laser as pump indicate a useful source of tunable short pulse radiation in the near infrared.     

Large purely refractive nonlinear index of single crystal P-toluenesulphonate (PTS) at 1600 nm

Z-scan measurements at 1600 nm on single-crystal PTS (p-toluene sulfonate) with single, 65 ps pulses gave a complex nonlinear refractive index coefficient of n₂=2.2(±0.3)×10^-12 cm²/W at 1 GW/cm² and α₂<0.5 cm/GW. This is the first highly nonlinear, organic material to satisfy the conditions imposed by the figures of merit     

Study of the Nd: Glass laser radiation

The Nd:glass laser has become one of the most useful sources of light pulses a few picoseconds in duration. In this paper, we review the results of an extensive study of the time and spectral structure of the Nd:glass laser radiation. The time structure was studied by means of two-photon fluorescence (TPF) patterns: these were scanned by a very thin (28-μ) cell containing the fluorescent dye, the fluorescence being monitored by a photomultiplier. When the Nd:glass laser isQswitched by a rotating mirror or when it is free running, we find TPF patterns fully consistent with a model where the laser emission has the character of Gaussian noise (thermal light), i.e., a model where the modes are randomly phased. When the laser was simultaneouslyQswitched and mode locked we made two observations : 1) the TPF patterns show that the ultra-short pulses observed previously have an overall duration of ∼8 ps, but also possess an internal substructure containing peaks 0.4-0.8 ps in duration; 2) the spectral width of these pulses is <20 cm^-1at the beginning of theQ-switched train and expands to ∼80 cm^-1in the middle of the train. This rapid spectral broadening during pulse buildup is attributed to self-phase modulation in the laser glass matrix due to a nonlinear index n2which we evaluate asn{2}= (2 pm 1) times 10^{-22}m²/V²(or1.8 times 10^{-13}esu). Gain limiting due to this effect and self-focusing become very important at power densities above 1 GW/cm², presenting a serious limitation on the energy density (J/cm²), which one may hope to extract from Nd: glass laser systems.     

Efficient third-harmonic generation of a picosecond laser pulsewith time delay

A new configuration based on the polarization-mismatching scheme with time delay for efficient frequency tripling conversion is proposed in this paper. The calculated results showed that the requirement for the efficient frequency tripling conversion of a 1-ps laser pulse is not only the optimization of peak intensity of the second-harmonic pulse, but also the optimization of the pulse duration ratio and temporal difference between the o-polarization second-harmonic pulse and the e-polarization first-harmonic pulse due to group-velocity mismatch among the interacting pulses. With the proposed scheme the group velocity mismatch can be compensated. Overall energy conversion efficiency increases from 55% to 75% under the optimized conditions at the intensity of 6 GW/cm². The temporal shape of the third-harmonic pulse with 1 ps pulse duration has no subpulses. The optimization of the efficient frequency tripling conversion for intensities of over 75 GW/cm² is also described. The results showed that the maximum tripling energy conversion efficiency is close to 80% with the optimized doubler and tripler     

Tunable alexandrite lasers

Wavelength tunable laser operation has been obtained from the solid-state crystal alexandrite (BeAl2O4:Cr³⁺) over the continuous range from 701 to 818 nm. The tunable emission was observed at room temperature and above in a homogeneously broadened, vibronic, four-level mode of laser action. In this mode the laser gain cross section increases from7 times 10^{-21}cm²at 300K to2 times 10^{-20}cm²at 475K, which results in improved laser performance at elevated temperatures. Efficient 2.5 percent, low-threshold (10 J) operation has been obtained with xenon-flashlamp excitation of the 6 mm diameter × 76 mm length laser rods. Output pulses of greater than 5 J and average power outputs of 35 W have been demonstrated, limited by the available power supply. The emission is strongly polarizedEparallelb, with a gain that is 10 times that in the alternate polarization. The 262 μs, room-temperature fluorescence lifetime permits effective energy storage andQ-switched operation. TunableQ-switched pulses as large as 500 mJ have been obtained with pulsewidths ranging between 33 and 200 ns depending on the laser gain. Laser action has also been demonstrated on the high-gain (3 times 10^{-19}cm²emission cross section)Rline at 680.4 nm and is also polarizedEparallelb. This three-level mode is analogous to the lasing in ruby except that the stimulated emission cross section in alexandrite is ten times larger than for ruby.     

Short-pulse oscillator development for the Nd:glass laser-fusion systems

This paper describes the development of an actively mode-locked andQ-switched (AMQ) Nd:YAG oscillator system that generates a single pulse at 1.06 μm in the range of 100 ps to 1 ns, with energy ofsim 250 muJ, and with good stability and reliability. A stable single axial modeQ-switched oscillator is also described that generates a long smooth pulse out of which pulses in the ns range are sliced. Synchronization of these two oscillators is also discussed.     

Characteristics of 10.6-µ chirped pulses

Q-switched CO2laser pulses generated with an offset rotating mirror have been found to be frequency-swept (chirped). Chirp rates as high as 120 MHz/μs have been observed.     

Experiments on transverse mode locking of circular cylindrical optical beams and certain flat beams

Transverse mode locking of the Laguerre forms with proper magnitudes and phases in an optical beam with circular cylindrical symmetry can produce a beam with time-varying spot size and intensity. The effect is demonstrated by locking two modes of the cylindrically symmetric set, TEM00qand TEM10qof an He-Ne 6328-Å laser with relative amplitudes 1 and 0.6, respectively. Verification of the time-varying spot was by sampling of the beam with an iris moved about the beam at constant radius. The shape and phase of the detected pulses agreed with predictions for the mode-locked beam. For a flat strip beam, the rectangular TEM00q, TEM20q, and TEM40qmodes were locked with relative amplitudes 1, 0.5, and 0.3, respectively. A beam with time-varying width resulted and was sampled with a slit.     

Subnanosecond laser pulses from a grazing incidence dye laser

Dye laser pulses with a duration of 160-350 ps and a spectral width of 0.06 nm were obtained by pumping a grazing incidence dye laser with aQ-switched Nd : YAG laser. The pulse shortening was due to a cavity transient effect which has also been observed in N2laser-pumped dye lasers.     

7A11--Laser induced emission of electrons, ions, and X rays from solid targets

We have first studied the spatial distribution of electrons and ions emitted from a solid target when hit by a focused,Q-switched laser beam. The particle emission is highly anisotropic. In a second experiment we have analyzed the ion energy for various targets such as beryllium, carbon, and molybdenum. For each target element, evidence for multicharged ions has been shown as a function of laser intensity. With a laser flux at the target surface of6 times 10^{10}W/cm², the maximum ion energy obtained is about 2 keV. Finally, we have shown that particle emission is accompanied by X-ray emission.     

Simultaneous determination of the spectral and temporal properties of tunable, single, picosecond pulses from a short cavity dye laser

We report simultaneous quantitative spectral and temporal measurements of the output pulses from a single-axial mode rhodamine 6G short cavity (1-8 mum) dye laser. The dye laser was pumped at 10 Hz with single second harmonic (532 nm) pulses from a mode-locked Nd⁺³:YAG laser. Spectral measurements made with a high resolution spectrograph and temporal measurements made with an ultrafast streak camera indicate the production of Fourier transform limited pulses for near threshold operation. Output pulses of 7-14 ps (FWHM) are observed from the short cavity dye laser when pumped with 15-30 ps duration 532 nm pulses. The output pulse delay and jitter are also investigated. A theoretical model for the temporal evolution of the pulses is presented and compared with the experimental results.     

Nonlinear photoemission from organic cathodes excited by a pulsed near-ultraviolet laser

High current density electron beams with fast modulation capability can be generated using laser-driven photoemission. Organic film cathodes having a real intermediate energy level can work in a poor vacuum environment and with near ultraviolet (∼ 350 nm) illumination where temporal formats from CW to picosecond pulse are available. Measured nonlinear photoemission characteristics of several organic films are presented and compared to the response of a rate equation model. Pulse distortions for 5 ns and 5 ps pulses also have been computed. Current densitities above 30 A/cm²for 5 ns, 337 nm pulses have been measured: an increase up to 3 kA/cm³for 5 ps pulses is predicted by the model.     

Optical power limiting and stabilization based on a novel polymercompound

Optical power limiting and stabilization based on the two-photon absorption (TPA) mechanism is performed in a polymer solution excited by ~810 nm and ~7-ns laser pulses. The solute is a novel polymer, a poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EBO-OPPE). Using 1-cm path-length EHO-OPPE solution in chloroform of d₀=0.03 mol of repeat unit/liter as the nonlinear absorptive medium, the dynamic transmission changes from T=0.92 to 0.28 when the input intensity of the ~810-nm laser beam is increased from I₀=15 to 600 MW/cm² . The measured nonlinear absorption coefficient is 14.5 cm/GW. Optical power stabilization is demonstrated at an average input intensity level of I₀≈400 MW/cm² with a Δ≈±25% peak-power fluctuation of the laser pulse. After passing through the nonlinear medium, the output peak-power fluctuation is reduced to Δ≈±8%. The spectral-width effect of the input laser beam on the nonlinear absorption of the EHO-OPPE solution is investigated. For three different spectral structures of the input laser beam (single narrow spectral line, multiple spectral lines, and broad spectral band), measured values of TPA cross section for EHO-OPPE are σ₂=66, 80, and 101×10^-20 cm⁴/GW, respectively. This means that EHO-OPPE is one of the best known nonlinear absorptive materials for power limiting purposes     

Saturation of 1047- and 1064-nm Absorption in Cr4+:YAG Crystals

To investigate the physical mechanism of the saturation process in Cr⁴⁺:YAG crystals we solved the three coupled rate equations which describe the saturable absorber. We experimentally verified this model using two lasers with nanosecond pulses and continuous-wave radiation. We used crystalline and ceramic Cr⁴⁺-doped YAG saturable absorbers with various initial transmissions. The ratio between the ground and the excited-state absorption cross section at 1064 nm was measured to be between 3.8 plusmn 0.2 and 4.7 plusmn 0.2 for crystalline and 3.6 plusmn 0.1 for ceramic Cr⁴⁺:YAG. The ratio between the above named cross sections at 1047 nm was found to be 6.2 plusmn 0.2 for both crystalline and ceramic Cr⁴⁺:YAG. With these results the ground-state and the excited-state absorption cross sections at 1047 nm were calculated to be (9.55plusmn0.01)times10^-19 cm² and (1.54plusmn0.03)times10^-19 cm², respectively     

A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO2laser, area ∼ 8 cm², initiates an explosive reaction between SF6and SiH4. This occurs at a minimum energy of 4 J [full width at half maximum (FWHM)sim 1.5 /mus] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 <p(SiH4)/p(SF6) < 1.8. The spectral and temporal distributions of the emitted chemiluminescence depend sensitively on the fuel to oxidizer ratio, and on the pulse energy; we investigated the range 4 → 20 J. The principal emission is due to S2(B^{3}Sigma-_{u} rightarrow X^{3}Sigma-_{g}). Transitionsupsilon' (0-4) rightarrow upsilon" (2-15)were recorded. In the³Sigma-_{u}state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH4)/(SF6) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH4)/(SF6) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF6(0.2-0.5 percent of initial amount) is enriched in³⁴SF6; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO2laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.     

Amplification of picosecond 10 µm pulses in multiatmosphere CO2lasers

Picosecond pulses, generated by semiconductor switching (2-40 ps), have been regeneratively amplified to an energy density of ∼1.5 J/cm²in a multiatmosphere CO2laser. The characteristics of the amplified pulses have been investigated as a function of wavelength, pulse duration, and the pressure in the gain medium. Using a high-power pulse compression technique, 10 μm pulses as short as 600 fs have been obtained.     

LaserQswitching by a PLZT shutter

The application of a polycrystalline ferroelectric ceramic Pb1-xLax(ZryTiz)1-(x/4)O3(called PLZT) electrooptic shutter is described for controlling the giant pulse emission of a ruby laser. The PLZT material hady = 65percent,z = 35percent, and the value ofxwas in the vicinity of 8 percent. Half-wave retardation switching was not possible with the PLZT shutter when 10-ns rise-time 70-ns-duration voltage pulses (fastQswitching) were applied. The increase insertion loss, due to less than half-wave retardation, resulted in laser pulsewidths that were wider and contained less energy than predicted by fastQ-switching theory. Since the PLZT material was damaged at relatively low power densities of between 5.6 to 8.5 MW/cm², respectively, its use is restricted toQ-switching small output power.     

Pulse compression inside an actively AM mode-locked ND:YAG laser using a liquid Kerr cell

A mode-locked Nd:YAG laser capable of generating pulses substantially shorter than the present limit of 30-50 ps, while retaining the stability of active mode locking, would be a useful development. We present here a simplified theoretical analysis and a preliminary experimental verification of the use of an optical Kerr cell as a passive lossless pulse compressor inside aQ-switched and actively mode-locked YAG laser. The theoretical model indicates possible compression to as short as ∼5 ps in a typical low-power Nd:YAG laser. The preliminary experimental results show substantial pulse compression via the predicted mechanism, though still not below ∼30 ps. The technique appears capable, however, of pulse compression to substantially shorter values without the instabilities, statistical fluctuations, or energy losses associated with saturable-absorber mode locking.     

Optically pumped semiconductor platelet lasers

Tunable CW laser action of platelet semiconductors is reported in both mode-locked and unmode-locked configurations. The gain media are platelets of CdS, CdSe, CdSSe, and InGaAsP cooled to 85 K and longitudinally pumped by an argon-ion laser. Antireflection (AR) coating of the crystal face and external bandwidth restriction have been used to generate pulses as short as 4 ps. The pulses observed are chirped, with nontransform limited time-bandwidth products of about 1.7. The energy conversion efficiency is 20 percent into the TEM00mode, with output powers of over 10 mW from CdS.