Observation of a cascaded process in intracavity terahertz optical parametric oscillators based on lithium niobate.

Cascaded difference frequency generation has been observed in intracavity optical parametric oscillators based on bulk lithium niobate and producing nanosecond pulses of terahertz radiation. Two idler waves are generated, namely: the primary idler wave associated with the parametric down conversion process itself; and a secondary idler wave, due to difference frequency generation. Experimental investigations of the frequency, temporal evolution, propagation direction, intensity, phase matching and oscillation threshold of the generated down-converted waves are reported. The overall generation efficiency for the terahertz radiation is enhanced, thereby overcoming the Manley-Rowe limit. Advantages of the present approach over schemes based on periodically poled lithium niobate are identified.

Production of degenerate polarization entangled photon pairs in the telecom-band from a pump enhanced parametric downconversion process.

The design and implementation of a novel source of degenerate polarization entangled photon pairs in the telecom band, based on a cavity enhanced parametric downconversion process, is presented. Two of the four maximally entangled Bell states are produced; the remaining two are obtainable by the addition of a half wave plate into the setup. The coincident photon detection rate in the A/D basis between two detectors at the output of the device revealed the production of highly entangled states, resulting in quantum interference visibilities of 0.971 ± 0.041 (ϕ = 0 state) and 0.932 ± 0.036 (ϕ = π state) respectively. The entangled states were found to break the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality by around 6 standard deviations. From the measured coincidence counting rates and the optical system losses, an entangled photon pair production rate of 8.9 × 10(4) s(-1) mW(-1) pump was estimated.

Stable, continuous-wave, intracavity, optical parametric oscillator pumped by a semiconductor disk laser (VECSEL).

We report relaxation oscillation free, true continuous-wave operation of a singly-resonant, intracavity optical parametric oscillator (OPO) based upon periodically-poled, MgO-doped LiNbO3 and pumped internal to the cavity of a compact, optically-excited semiconductor disk laser (or VECSEL). The very short upper-laser-state lifetime of this laser gain medium, coupled with the enhancing effect of the high-finesse pump laser cavity in which the OPO is located, enables a low threshold, high efficiency intracavity device to be operated free of relaxation oscillations in continuous-wave mode. By optimizing for low-power operation, parametric threshold was achieved at a diode-laser power of only 1.4 W. At 8.5 W of diode-laser power, 205 mW of idler power was extracted, indicating a total down-converted power of 1.25 W, and hence a down-conversion efficiency of 83%.

Insanity defense pleas in Baltimore City: an analysis of outcome.

OBJECTIVE: The authors studied all defendants in Baltimore City's circuit and district courts who pleaded not criminally responsible, Maryland's version of the not guilty by reason of insanity plea, during a 1-year period. The study was designed to compare the perception that the insanity plea is misused to actual outcome data. METHOD: The cohort of defendants who pleaded not criminally responsible in both the circuit and district courts during calendar year 1991 was identified. Data on demographic characteristics, crimes committed, diagnoses, and psychiatrists' opinions on criminal responsibility were collected. Trial outcome data were obtained through a search of the circuit and district court computer systems. RESULTS: Of the 60,432 indictments filed in the two courts, 190 defendants (0.31 per 100 indictments) entered a plea of not criminally responsible. All but eight defendants (0.013 per 100 indictments) dropped this plea before trial. For these eight cases, both the state and the defense agreed that the defendant should be found not criminally responsible, and the plea was uncontested at trial. The remaining defendants had their charges dropped before trial, remained not competent to stand trial at the time of the study, or withdrew their pleas of not criminally responsible before trial. CONCLUSIONS: There were no trials that contested the plea of not criminally responsible. The state and defense agreed with each other for all of the defendants who actually retained the plea at trial. The perception that the insanity defense is overused and misused is not borne out by data.

Quantitative analysis of cardiac left ventricular variables obtained by MRI at 3 T: a pre- and post-contrast comparison.

Short-axis cine images are acquired during cardiac MRI in order to determine variables of cardiac left ventricular (LV) function such as ejection fraction (EF), end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV) and LV mass. In cardiac perfusion assessments this imaging can be performed in the temporal window between first pass perfusion and the acquisition of delayed enhancement images in order to minimise overall scanning time. The objective of this study was to compare pre- and post-contrast short-axis LV variables of 15 healthy volunteers using a two-dimensional cardiac-gated segmented cine true fast imaging with steady state precession sequence and a 3.0 T MRI unit in order to determine the possible effects of contrast agent on the calculated cardiac function variables. Image analysis was carried out using semi-automated software. The calculated mean LV mass was lower when derived from the post-contrast images, relative to those derived pre-contrast (102 vs 108.1 g, p<0.0001). Small but systematic significant differences were also found between the mean pre- and post-contrast values of EF (69.4% vs 68.7%, p<0.05), EDV (142.4 vs 143.7 ml, p<0.05) and ESV (44.2 vs 45.5 ml, p<0.005), but no significant differences in SV were identified. This study has highlighted that contrast agent delivery can influence the numerical outcome of cardiac variables calculated from MRI and this was particularly noticeable for LV mass. This may have important implications for the correct interpretation of patient data in clinical studies where post-contrast images are used to calculate LV variables, since LV normal ranges have been traditionally derived from pre-contrast data sets.

The Hong-Ou-Mandel interferometer: a new procedure for alignment.

Two major requirements in the construction of the Hong-Ou-Mandel interferometer are the alignment and length balancing of two optical paths. A new method is presented for meeting these requirements that requires no custom optics or expensive equipment. Using this method, a two photon interferometer sourced by degenerate noncollinear parametric photon pairs was aligned and the optical paths were balanced to within an average of 11.6 microm, yielding two-photon interference features with visibilities of approximately 0.9. The method is applicable to arbitrary noncollinear emission angles, including nondegenerate downconversion situations where the signal and idler emission angles differ.

Use of flow graphs in the analysis of optical cavities.

Signal flow graphs, which are used widely in microwave circuit theory, are applied to the analysis of optical cavities. Flow graphs describing common optical components and systems are derived, and toplogical and analytical methods of solution are summarized. The technique is used to analyze the influence of an etalon on the modes of a laser cavity.

Efficient ultraviolet LiB(3)O(5) optical parametric oscillator.

We report what is to our knowledge the first optical parametric oscillator (OPO) generating coherent laser radiation with high optical-to-optical conversion efficiency in the near ultraviolet. The OPO, pumped at 307.8 nm by a pulsed injection-seeded XeCl excimer laser, employs a crystal of LiB(3)O(5) in a noncritical phase-matching geometry to generate tunable output over the range 381-387 nm in the near ultraviolet and 1.5-1.6 microm in the near infrared. We report internal energy conversion efficiencies as high as 28% for the signal wave at 385.5 nm and 7% for the idler wave at 1.53 microm, leading to a total internal conversion efficiency of 35%. Studies of oscillation threshold, external and internal conversion efficiency, spectral linewidth, and temporal variation of output pulses are also reported. Temperature tuning of the LiB(3)O(5) OPO under the noncritical phase-matching condition is demonstrated for the first time to our knowledge.

Continuous-wave singly resonant pump-enhanced type II LiB(3)O(5) optical parametric oscillator.

We report what is to our knowledge the first demonstration of a pump-enhanced singly resonant continuous-wave optical parametric oscillator. The nonlinear material used was lithium triborate cut for noncritical phase matching along the z axis, and the device was pumped by a single-frequency argon-ion laser. The oscillation threshold was ~1.0 W at 514.5 nm. For 3.4 W of pump power, we obtained single-frequency output powers of 500 mW in the nonresonant wave, which we temperature tuned over 14 nm.

Application of a continuously tunable, cw optical parametric oscillator for high-resolution spectroscopy.

We report the use of a smoothly tunable, single-frequency continuous-wave optical parametric oscillator (OPO) for high-resolution spectroscopy. The OPO is based on potassium titanyl phosphate and is resonant for both signal and idler fields, resulting in a device with a very low pump power threshold of 30 mW. The frequency-selective nature of the doubly resonant oscillator ensures that the signal and idler modes can be tuned across the entire phase-match bandwidth without the need for additional intracavity frequency-selective components. Smooth frequency tuning of the output of the OPO is obtained by tuning of the pump laser. To demonstrate the practicality of our OPO we recorded the absorption spectrum of cesium vapor in the 1-microm spectral region.

Low-pump-threshold continuous-wave singly resonant optical parametric oscillator.

We describe a compact all-solid-state continuous-wave singly resonant optical parametric oscillator (SRO) with a minimal pump-power requirement. The SRO is based on periodically poled LiNbO(3) as the nonlinear material and is pumped by a 1-W diode-pumped Nd:YVO(4) minilaser at 1.064 microm . By exploiting the intracavity pumping technique in a 50-mm crystal, we have achieved SRO operation threshold at a diode pump power of only 310 mW.At 1 W of input diode power, the SRO delivers 70 mW of output power in the nonresonant idler at 3.66 microm , at a photon conversion efficiency of 55%. Multiparameter tuning of the SRO yields a signal wavelength range from 1.45 to 1.60 microm and an idler wavelength range from 3.16 to 4.02 microm in the mid infrared. The device is characterized by robust turnkey operation and long-term amplitude-stable performance.

Continuous-wave, singly resonant, intracavity parametric oscillator.

Performance characteristics of a continuous-wave intracavity optical parametric oscillator are described by use of an experimental arrangement comprising a KTP singly resonant oscillator located within a Ti:sapphire laser cavity and analyzed by use of a steady-state model. Internal and external powers, circulating fields, tuning ranges, spectral bandwidths, and amplitude-stability levels are measured and discussed. The nonresonant idler tunes from 2.53 to 2.87 microm, delivers a maximum output power of approximately 0.4W and displays long-term amplitude-stable operation. The total downconverted power approaches the optimum power coupled out of the Ti:sapphire laser in the absence of frequency conversion.

Polychromatic optical parametric generation by simultaneous phase matching over a large spectral bandwidth.

The parametric generation of broad spectral bandwidths by the use of suitable phase-matching geometries is reported. Greater than 100-nm simultaneous bandwidth in the visible is generated in a collimated signal beam from a novel, noncollinear phase-matching geometry in a beta-barium borate optical parametric oscillator, which is pumped by the collimated output of a Q -switched and frequency-tripled Nd:YAG laser. Dispersive cavity tuning of the optical parametric oscillator by use of a rotatable Littrow-mounted grating, with a static crystal and pump configuration, is also described. A tunable bandwidth of >100 nm is also demonstrated.

Highly efficient visible urea optical parametric oscillator pumped by a XeCl excimer laser.

We report the successful operation of a visible optical parametric oscillator (OPO) that uses urea as the nonlinear medium and is pumped in the near field of an injection-seeded pulsed XeCl excimer laser operating at 308 nm. The OPO can generate continuously tunable radiation from 537 to 720 nm with a single set of dichroic mirrors, and conversion efficiencies as high as 37% have been achieved in a 15-mm-long crystal at 90 degrees phase matching. The influence of crystal double refraction (and hence beam walk-off) on the conversion efficiency across the OPO tuning range is discussed, and some oscillator parameters, including spectral linewidth and temporal behavior, are characterized.

Continuous-wave optical parametric oscillator based on periodically poled KTiOPO(4) and its application to spectroscopy.

We report a continuous-wave, doubly resonant optical parametric oscillator (OPO) based on the nonlinear material periodically poled KTiOPO(4) and its application to spectroscopy. The OPO, which is pumped by a diode-pumped frequency-doubled Nd:YLF laser at 523 nm, has a low pump-power threshold of 25 mW and can deliver 10 mW of single-frequency output at 1.65 mum for a pump power of 200 mW. The idler wavelength can be temperature tuned at a rate of 0.73 nm/( degrees )C , and smooth tuning of the output frequency over ~3 GHz is achieved by smooth tuning of the pump laser. We demonstrate the practicality of the OPO by recording the absorption spectrum of methane near 1649 nm.

Compact, continuous-wave, singly resonant optical parametric oscillator based on periodically poled RbTiOAsO4 in a Nd:YVO4 laser.

We describe a compact all-solid-state continuous-wave, singly resonant optical parametric oscillator (SRO) based on periodically poled RbTiOAsO4. The SRO is pumped at 1.064 microm by a Nd:YVO4 laser, which is itself pumped by a 3-W diode laser. Using the intracavity technique produced an oscillation threshold for the SRO of only 1.6 W (diode-laser power). For 3 W of diode pump power some 65 mW was obtained in the (nonresonant) idler (wavelength 3.52 microm). Temperature tuning over the range 10-100 degrees C resulted in tuning ranges of 1.52-1.54 and 3.41-3.54 microm for the signal and the idler waves, respectively. Importantly, relaxation oscillations were absent.

Doubly resonant continuous-wave optical parametric oscillator pumped by a single-mode diode laser.

The performance characteristics of a doubly (signal and idler) resonant continuous-wave optical parametric oscillator based on periodically poled lithium niobate and pumped by a 100-mW single-mode laser diode at 810 nm are reported. Pump power thresholds as low as 16 mW and wavelength tuning over the range 1.15-1.25 microm at the signal and 2.31-2.66 microm at the idler were achieved through variation of crystal temperature, pump wavelength, and grating period. Up to 5 mW of signal output was obtained with the single-mode diode pump, and signal powers of up to 39 mW were obtained when pumping with a 400-mW injection-locked broad-area diode laser.

Doubly resonant optical parametric oscillator formed by index matching cavity mirrors directly onto an uncoated LiB(3)O(5) crystal.

We demonstrate the use of index-matching fluid to contact cavity mirrors directly onto an uncoated LiB(3)O(5) crystal to form a compact, stable resonator for use as an optical parametric oscillator. Specifically, we report on the characteristics of a continuous-wave, doubly resonant, type II phase-matched LiB(3)O(5) optical parametric oscillator formed in this way. Using a single-frequency argon-ion pumping laser operating at 514.5 nm, we measured a pump power threshold of approximately 100 mW and a total external conversion efficiency of approximately 15%. By altering the crystal temperature under noncritical phase matching, we measured coarse frequency tuning over approximately 36 THz, limited only by mirror coating bandwidths. We discuss also the transverse and longitudinal mode properties of the optical parametric oscillator outputs.

Single-frequency, end-pumped Nd:YLF laser excited by a 12-mJ diode-laser array.

We describe the operation of a diode-laser-pumped, end-pumped Nd:YLF laser that utilizes a 12-mJ, 60-W, quasicw diode-laser bar. Multilongitudinal-mode, TEM(00) pulse energies of greater than 2 mJ have been observed, with corresponding Q-switched peak output powers in excess of 118 kW. The incorporation of a prelase Q-switching technique has seen single-longitudinal-mode peak powers in excess of 90 kW continuing to be achieved. The low-threshold operation at 1.539 microm of a singly resonant KTP optical parametric oscillator pumped by this source is also reported.

Frequency-doubled and Q-switched 946-nm Nd:YAG laser pumped by a diode-laser array.

A Q-switched 946-nm Neodymium:yttrium aluminum garnet (Nd:YAG) laser pumped by a diode-laser array has been developed. At room temperature, pulses with 4.9-microj energy and 68.5-ns pulse duration were observed. When the temperature of the Nd:YAG crystal was lowered to 5 degrees C, the duration of the pulses was reduced to 62 ns, giving rise to peak powers of 76 W. By focusing the Q-switched pulses into a 5-mm-long potassium niobate crystal, second-harmonic generation produced blue light pulses at 473 nm of 42-ns duration and 22-W peak power. The pulse repetition rate was kept at 1.5 kHz throughout.