Our investigations into the polarization properties of mechanically-stressed monolithic Nd:YAG oscillators have led to two important discoveries. First is the applicability of mechanical stress as a means for rapidly tuning the output frequency of a single-mode monolithic oscillator. The second, quite unexpected, discovery is that the polarization emitted by such a Nd:YAG oscillator is strongly dependent on both the polarization and precise wavelength of the (nominally 800 nm wavelength) pump radiation.
Performance characteristics and reliability of the GaAs/AlGaAs one-watt laser diode are reported. The broad area gain-guided lasers have threshold current in the 400 mA range. The farfield radiation angles parallel and perpendicular to the junction are 13 deg and 25 deg, respectively. With a special-designed package which has a built-in thermo electric cooler the emission wavelength can be tuned accurately. These performances are stable during a long term operation except for 1 to 2 nm wavelength shift due to the increase of operation cur-rent in an Automatic Power Control(APC) mode. The extrapolated mean time to failure of 100-500 mW laser diodes are 20,000 hours at room temperature. A similar lifetime can be expected for the one-watt laser at operating temperatures lower than room temperature.
The sensitivity of a monolithic, unidirectional nonplanar ring laser to optical feedback depends on the polarization eigenmodes of the resonator, which are complicated functions of the resonator geometry. Resonator geometries that increase the loss difference between the two directions of propagation around the ring improve the resistance of the lasers to optical feedback. Measurements on Nd:YAG nonplanar ring lasers having two different ring geometries are discussed. Currently, 1% optical feedback produces 10% reduction in useful output power for the more feedback-resistant oscillator, a tenfold improvement over previous designs.
Diode-pumped injection seeding lasers produced by Lightwave Electronics Corporation in 1987 and early 1988 have frequency drift of up to 50 MHz per hour, due to temperature drift. Thus they are not useful as wavelength standards for spectroscopy. Units with better long term stability will be introduced in 1988.
A Nd:YAG laser oscillator with a radially variable reflectivity mirror has been designed and tested for operation in high power, single longitudinal mode by the injection seeding technique. Design criteria are outlined and results obtained with an oscillator / amplifier system are presented
The construction and operation of a long pulse, single-mode Nd:YAG laser is described. The laser is configured around an injection-seeded, unstable oscillator/single amplifier and produces 80 ns pulses at 10 Hz with pulse energies of over 300 mJ. When frequency doubled, the infrared is converted to a 56 ns pulse at 532 nm with a Fourier-limited linewidth of 8 MHz. This approach brings the Nd:YAG source to within the bandwidth domain normally dominated by visible CW lasers, but at a considerably higher power, thus providing an attractive alternative for applications in high resolution spectroscopy and non-linear studies.
Enhanced output performance from dye lasers and nonlinear frequency doubling and mixing accessories is obtained when pumped with nearly transform limited Q-switched pulses from an injection seeded Nd:YAG oscillator/amplifier. Measurements are reported under a variety of conditions, showing output energy enhancement up to a factor of two. This improvement is explained from the basic theory of the processes involved. Examples of experimental applications that take advantage of the high power, high spectral brightness, and long-term stability of this system are described.
A narrow-line laser has been developed for testing of narrow band optical filters. The frequency doubled Ti:sapphire laser, pumped at 20 Hz by a frequency doubled Nd:YAG laser, is tunable between 450 - 470 nm and line-narrowed to 6 mÅ with dichroic mirrors, a four plate birefringent filter, and a high finesse etalon. Although the laser was designed with sufficient selectivity to discriminate against adjacent axial modes, spatial hole burning in the standing wave resonator allows two axial modes to oscillate.
With the advent of new nonlinear materials and single frequency pump sources, there is renewed interest in Optical Parametric Oscillators (OPO's). We have used a single-mode diode-laser-pumped monolithic Nd:YAG nonplanar ring laser that is both amplified and frequency doubled, to pump a monolithic MgO:LiNbO3 pulsed singly resonant OPO. The OPO signal output was temperature tuned from 834 nm to 958 nm, producing an idler tuning from 1.47 μm to 1.2 μm. Efforts toward a cw all-solid-state doubly resonant OPO are also described.
It has been recently shown and reported for the first time at this meeting, that Excimer pumping of a single-mode, short-cavity, grazing-incidence, longitudinally-pumped pulsed dye laser is feasible. In this paper the key concepts upon which this latest development is based are presented and are in a somewhat unusual form. This manuscript describes five specific dye laser examples. The five examples represent a progression from the simplest type of dye laser to the single-mode version mentioned above. The examples thus serve as a tutorial introduction to potential users of dye lasers. The article is organized into five sections or STEPS, each of which describes a different pulsed dye laser. Since the subtle points about dye lasers are best appreciated only after one actually attempts to build a working model, a PROCEDURES category is included in which details about the construction of the particular form of laser are given. As one reads through this category, think of it as looking over the shoulder of the laser builder. The NOTES category which follows is a brief but essential discussion explaining why various components and procedures are used, as well as how laser performance specifications are obtained. This subsection can he viewed as a discussion with the laser builder concerning the reasons for specific actions and choices made in the assembly of the example laser. The last category contains COMMENTS which provide additional related information pertaining to the example laser that goes beyond the earlier annotated discussion. If you like, these are the narrator's comments. At the end of the article, after the five sequential forms of the laser have been presented, there is a brief summation.
We demonstrate a means of generating narrowband, tunable laser pulses near 650 nm with continuously adjustable pulse width from 0.2 to 3 nsec. The system utilizes a short-cavity single-mode Littman oscillator which is quenched by a second short-cavity dye laser. Both are pumped by the same 6-nsec full-width-at-half-maximum (FWHM) frequency-doubled single-mode Nd:YAG laser. We also demonstrate a polarization technique in the three-stage amplification of these pulses to an energy of 20 mJ with minimum temporal broadening.
The output spectral characteristics of fully saturated pulsed dye amplifiers for multimode input waves having amplitude modulation (AM) are modeled and compared with experimental measurements. The AM input in our experiments was generated by combining the outputs of two single-mode dye lasers. The spectral output of the saturated dye amplifiers consisted of the input frequencies and several sidebands spaced by the input difference or beat frequency. For our experimental system with kiton red dye amplifiers, up to 20% of the output intensity was contained in these sidebands. Sideband production was observed to decrease as the beat frequency increased with a characteristic half-width of approximately 10 GHz.
Very high spectral quality flashlamp-pumped dye lasers are described, in which spectral narrowing is achieved by injection-locking of pulsed amplifying cavities to the wavelength of low power single-mode cw lasers. With the help of a fast electronic stabilization of the cavity length, we obtain a reliable complete injection. Peak powers up to 12 kW in 300 ns at a bandwith of 6 MHz are achieved over a 150 nm tuning range between 572 and 722 nm. The repetition rate is 10 Hz, limited mainly by the circulating pumps. Applications of these lasers are found in high resolution coherent Raman spectroscopy and nonlinear difference frequency mixing.
A commercially available pulsed dye laser equipped with Hansch oscillator and intracavity etalon has been converted into a single-mode laser with less than 100 MHz bandwidth by extending the pulse duration to 50 ns and modifying the optical elements. In particular an intracavity etalon of high finesse provides for emission of a single longitudinal mode, although strong mode-pulling occurs. Further, λ/4 plates have been used in order to avoid the effect of spatial hole-burning.
Third-and fifth-order sum-and difference frequency conversion of pulsed dye laser generates radiation in the vacuum ultraviolet at wavelengths λVUV 1 = 60 - 200 nm. The generated VUV light is of narrow spectral width (ΔE = 0.01 - 1 cm-1)anau high spectral intensity (0.03 - 104 W; 108 - 3.1013 photons/pulse). Because of their spectral brightness these VUV light sources are a powerful tool for VUV spectroscopy of atoms, molecules and solid states.
Our goal is to maximize conversion efficiency of sum-frequency mixing in an energy scalable system. We analyze 130.2-nm generation via two-photon-resonant four-wave mixing in Hg vapor for unfocused light beams emphasizing an understanding of efficiency-limiting processes. Our calculations, based on our measurements of Hg dipole matrix elements, indicate that mixing efficiencies of 5-10% should be possible.
Vacuum ultraviolet laser output energies of 0.25 mJ (2 MW cm-2 unfocused) at 130 nm have been achieved using four-wave sum-frequency mixing in mercury. High energy conversion efficiencies of 2.7 percent have been demonstrated in a collimated beam geom1etry over 1 meter interaction lengths. The 130 nm radiation is tunable over 2.5 cm-1 and the pulse length is 2.2 ns. The experimental facility assembled to produce this efficient VUV laser will be described, and comparison between experimental measurements and theory are provided.
Nearly transform limited pulses of Lyman-α radiation at 1216 Å have been produced by sum frequency generation in 0.1 to 10 torr of mercury vapor. The input beams, consisting of photons at 3127 Å and 5454 Å, originate in 1 MHz bandwidth ring-dye laser oscillators. The beams are amplified in pulsed-dye amplifiers pumped by the frequency doubled output of a Nd:YAG laser. The 3127 A photons are tuned to be resonant with the two-photon 6IS to 71S mercury transition. The VUV radiation can be tuned by varying the frequency of the third non-resonant photon. We have also observed difference frequency generation at 2193 Å and intense fluorescence from the 61P state at 1849 Å. A dramatic reduction of the 1849 Å radiation occurs when the 5454 Å beam is added to the cell. We have studied the intensity and linewidth dependance of the output radiation on input beam intensity, mercury density, and phase matching buffer gas pressure and composition.
We show that extraction of energy in the form of a train of subnanosecond pulses from a discharge-pumped KrF excimer amplifier is an efficient means of enhancing the intensity of the laser radiation. The effect of interpulse time separation and pulse width are investigated experimentally. Intensity enhancements of up to 7x above steady-state, with extraction efficiencies in excess of 60% of the steady-state value, were obtained by amplifying a train of 0.4-nsec full-width-at-half-maximum (FWHM) pulses separated by 5 nsec.
Stable operation of a 1.2 As pulse, high energy, electron beam pumped XeCl laser at a bandwidth of 225 MHz has been achieved. This bandwidth is obtained without external injection by utilizing the self-regenerating feedback from a portion of the broadband laser output that is filtered through a series of Fabry-Perot etalons in a retro-ring configuration. The laser is a positive-branch confocal unstable resonator with a typical narrowband output of -20 J. The laser comes under narrowband control after -200 nsec and has a wavelength tuning accuracy of ± 50 MHz. Polarization locking achieved by this system is quite efficient, producing typical locking ratios of S-to P-polarization of > 100:1. The narrowband output is analyzed with a 1.5 GHz confocal etalon and a streak camera.
Recent progress in the generation of ultraviolet laser radiation has raised several intriguing possibilities for performing very precise spectroscopic measurements in the far ultraviolet, by exciting multiphoton transitions using UV light generated from single-mode lasers. We have recently begun to apply some of these methods to the spectroscopy of the simplest neutral molecule, H2. Both two-and three-photon excitation have been used to determine some of the intervals from the ground state to the lowest electronically excited states with considerably higher accuracy than has previously been achieved. The transitions are observed using resonant multiphoton ionization in a collisionless supersonic beam, collimated to reduce the Doppler width. Several vibrational bands of the E,F4-X transition have been measured using two-photon excitation near 220 nm. The observed spectra were free of power broadening, and had linewidths limited only by residual Doppler broadening in the collimated molecular beam. These intervals were measured to 0.01 cm-1, and much higher accuracy is possible. In the three-photon measurements, we studied transitions to the B(2pσ) and C (2pπ) states. These results were somewhat less precise because shifts arising from the AC Stark effect and from a blue shift that dep-ends strongly on number density, arising from the interference of three-photon excitation and third harmonic generation. A few of the many prospects for future work are discussed briefly.
Absolute atomic bound-bound two-photon cross sections have been measured using a single-frequency laser, eliminating the usual uncertainties about unresolved temporal fluctuations. The cross sections were measured using the technique of two-photon excited fluorescence. For the 3p3 P2,1,0+2P3P2 transition in atomic oxygen at 226 nm the integrated cross section (obtained by s6afining the laser frequency through the two-photon resonance) is 1.87 ±0.60 x 10-35 cm4. Relative fine structure cross sections and energy spacings have been measured by Doppler-free spectroscopy. Absolute and relative two-photon cross sections agree well with ab initio calculations. Comparison with similar measurements carried out with multimode lasers yields information about multimode laser photon statistics. The experimental integrated cross section is shown to be proportional to the product of the atomic cross section and G(2)(0) the second-order intensity correlation at time zero of the laser field. It is assumed that G(2)(0)=1.0 for the single-frequency laser source. Five other laser 226 configurations have values of G(2)(0) ranging from 0.8 ±0.2 Until multimode lasers are well enough understood to allow predictions of G"2) (0 (especially after frequency conversion processes such as doubling and Raman shifting), single-frequency lasers will be the best source of reliable multiphoton cross section measurements.
A tunable vuv-xuv laser system with good pulse energy (>100mJ in the visible) and narrow bandwidth (<210MHz in the xuv) was built in order to do ultra-high resolution photoionization spectroscopy. The capabilities of this system have been demonstrated with a study of isotope shifts and hyperfine splitting in Kr Rydberg levels.
Stark level-crossing spectroscopy is a new application of pulsed, single-frequency lasers which accesses highly excited vibrational states in a polyatomic molecule. The fluores-cence lifetime of a single rovibrational level in the first excited electronic state (S1) is recorded as it is Stark tuned across highly excited states of the ground electronic state (S0). Resonantly enhanced nonradiative decay causes dips in the S1 lifetime as the S1 state passes through resonance with S0 states, permitting the highly excited So level structure to be mapped out. Stark level-crossing spectra of d2-formaldehyde are presented.
With the help of high spectral quality (linewidth , 6 MHz) injection-locked flashlamp-pumped dye lasers, high resolution Coherent Anti-Stokes Raman Spectroscopy (CARS) has been applied to the study of the Q-branches of 02 and CO2. The Q-branch linewidths of 02 were obtained from the first nine transitions (J2 = 1 to 17) recorded at pressures from 0.094 to 1 atm at room temperature. A least squares program was employed to fit calculated spectra to experimental spectra by adjusting different parameters. The collisional broadening coefficients we have obtained are consistent with available Raman data. We have also studied the Q-branch of the fundamental v1 band of CO2 in the Fermi resonance region near 1285.5 cm-1. The spectrum was recorded at a pressure of 5 kPa (37.5Torr) at room temperature and it was possible to resolve this band for the first time. We measured the line positions and collisional widths for the transitions with J values from 8 to 38. Our experimental line positions are in excellent agreement with those derived from molecular constants given in the literature. The observed linewidths agree for most of the lines within 5 % with the values calculated from a recently developed model.
Preliminary results of a high resolution spectroscopic study of the pressure dependence of the Raman vibrational Q-branch spectrum of pure CO are reported. Measurements are made at room temperature over the pressure range 0.5 to 6 atm. The technique of quasi-cw inverse Raman spectroscopy utilizing a pulsed single-frequency laser source is employed. This approach gives enhanced sensitivity compared to earlier work which employed cw lasers, allowing extension of that work to higher accuracy, higher J states, and higher pressure. The goal of this work is to test the accuracy of a modified exponential-gap rate law model which is used to predict the pressure dependent spectra.
We have used an injection-locked Nd:YAG laser/dye laser combination together with a frequency-narrowed argon-fluoride laser to simultaneously observe coherent anti-Stokes Raman scattering (CARS) and stimulated. Raman pumping of oxygen. At high power densities, strong Raman pumping occurs leading to significant distortion of the CARS spectrum and a broadening of the Raman-pumped lineshapes. A simple saturation model has been used to fit these lineshapes with reasonable accuracy.
UV generation using multiwave Raman mixing in Hz has been investigated using a single-frequency YAG laser system. We report energy conversion efficiencies to the backward 1st Stokes at 683 nm and the 8th anti-Stokes (AS) wave at 192 nm starting with doubled-YAG at 532 nm as the pump laser source. We also report on spectral broadening of the 2nd AS radiation at 226 nm by more than 10 x the Fourier-transform limit of the pump laser. The results indicate that the broadening mechanism is primarily due to the AC Stark shift of the Q(1) Raman transition in H2. We present calculations of the AC Stark shift and the Raman Rabi frequency using ab initio calculations of the polarizabilites. These experiments illustrate the new information that can be obtained when experiments are performed using single-frequency lasers.
We have used two methods to measure the oscillator strength of the transition between the ground and 17992 cm-1 level in 174Yb. The first technique involves exciting the transition with a laser pulse that is nearly time-bandwidth limited, of uniform intensity, and has a reproducible shape from shot to shot. The population left in the excited state after the pulse varies sinusoidally with a period that depends on the integral over time of the electric field amplitude and the transition oscillator strength. These are the Rabi oscillations that are predicted by application of the Schrodinger equation to the two-level atom. The excited-state population is probed using a two-step photoionization to the continuum. The electric field amplitude is determined from the temporal profile and the intensity of the laser pulse. The second method involves observation of the polarization rotation of a set of degenerate sublevels brought about by a time-bandwidth limited light-shift laser. One sublevel (mj = 0) of the J = 1 level at 17992 cm-1 is populated by a linearly polarized laser. A second copropagating light-shift laser, which is linearly polarized at an angle to the first laser, is tuned between 7.5 and 30 GHz off-resonance with the transition. The light-shift laser causes population to be promoted into the mi = ±1 levels by a virtual transition through the ground J = 0, = 0 level. Two linearly polarized photoionizing lasers photoionize the population only from the mi = ±1 levels. The photoion signal oscillates sinusoidally with a period that depends only on the integrated pulse intensity, the laser detuning, and the transition oscillator strength.
We discuss the design of high-performance, single-pass, permanent-magnet Faraday isolators that are simple to construct and use. An isolator, designed for use with Q-switched Nd:YAG laser systems, that is 10 cm long and achieves 41 degrees of rotation at 1.06 μm is described.
Several technologies are available for interferometrically measuring laser wavelength to the precision of a Doppler width. These include instruments based on Fabry-Perot etalons, Fizeau wedges, Michelson interferometers, and other novel schemes. The current state of the art of these instruments is reviewed. Two new pulsed wavemeter designs are presented, the solid Fizeau wedge and the index mismatched interferometer. These instruments exhibit accuracies in the 106 range, and are very compact and inexpensive to fabricate.
We have found that the amount of pulse compression increases as single mode laser intensity approaches the stimulated Brillouin scattering (SBS) threshold in Hexane at 355 nm. Using an avalanche photodiode, transient digitizer, and waveform analysis, we also found that the temporal and spectral structure of SBS pulses replicated multimode pump pulses. However, during pump mode beating intensity spikes of short duration and high power, competing effects (perhaps stimulated Raman scattering) depleted gain from SBS. To our knowledge, this is the first direct temporal measurement of partial pulse replication in SBS due to mode beating.
A UV wavemeter to determine absolute wavelength to one part in 107 at 308 nm has been fabricated and tested. The wavemeter consists of three fixed-plate, air-spaced Fabry-Perot etalons in an evacuated, thermally stabilized enclosure. The interference patterns created by the signal passing through the etalons are imaged onto silicon-diode arrays. This analog output is digitized and stored, and from the estimated wavelength and the known etalon spacing, a microcomputer calculates the interference order of the first etalon. The fractional order is calculated from the digitized image and used to give a better estimate of the absolute wavelength. This procedure is iterated and the order value refined for each subsequent etalon in the series, allowing increasingly accurate determination of the bandwidth and absolute wavelength. The XeCl spectrum can be resolved with an accuracy of ± 30 MHz, given by the inherent resolution of the calibration technique which involves a frequency-doubled ring dye laser and an iodine reference cell. The wavemeter is capable of similar performance at 351 nm (XeF) and 248 nm (KrF).