J. E. Bjorkholm

Observation of a single-beam gradient force optical trap for dielectric particles

Optical trapping of dielectric particles by a single-beam gradient force trap was demonstrated for the first reported time. This confirms the concept of negative light pressure due to the gradient force. Trapping was observed over the entire range of particle size from 10 μm to ~25 nm in water. Use of the new trap extends the size range of macroscopic particles accessible to optical trapping and manipulation well into the Rayleigh size regime. Application of this trapping principle to atom trapping is considered.

TUNABLE DISTRIBUTED‐FEEDBACK DYE LASER

Efficient broadly tunable dye‐laser action with a narrow line output has been demonstrated with a mirrorless laser system using distributed feedback. The feedback was obtained from a spatial modulation of both gain and index of refraction, induced by pumping a liquid organic dye solution with fringes formed by the interference of two coherent light beams. The device was tunable over 640 A either by varying the angle between interfering pump beams or the refractive index of the dye solvent. Linewidths less than 0.01 A were measured and a peak power of 36 kW was observed.

Phase‐matched three‐wave mixing in silica fiber optical waveguides

We have observed phase‐matched nonlinear mixing in a silica fiber optical waveguide using the dispersion of the guide modes to compensate for bulk dispersion. A tunable dye laser signal is mixed with a fixed‐frequency pump wave to generate output in specific waveguide modes at different frequencies. The technique is useful as a probe of waveguide properties and for the study of the nonlinearities of the medium.

Some effects of spatially nonuniform pumping in pulsed optical parametric oscillators

Previous analyses of the large signal operation of optical parametric oscillators have considered uniform plane-wave pump beams. These analyses are not adequate to describe the behavior of efficient pulsed oscillators pumped by beams having nonuniform intensity profiles and pulse durations that are short enough that the steady-state transverse modes of the oscillator cavity do not become established. This situation is typical of oscillators with plane-parallel cavities pumped by Q -switched lasers. This paper theoretically and experimentally investigates the behavior of such oscillators when pumped with beams having Gaussian intensity profiles. Each of the three basic external optical parametric oscillators are considered. For simplified cases it is shown that the response to a Gaussian pump beam is quantitatively and qualitatively much different than that predicted by the uniform plane-wave calculations. Experimental results in fair agreement with these calculations are presented. More importantly, uniform plane-wave behavior was observed by using an aperture to probe a small section of the pump beam transmitted through the oscillator. This useful technique gives a sensitive indication of the oscillator operation and, at present, it is the most accurate means of measuring thresholds in pulsed oscillators. As a result of our calculations and experiments, previous experimental observations that were puzzling are now well understood.

FREQUENCY CONTROL OF A PULSED PARAMETRIC OSCILLATOR BY RADIATION INJECTION

A singly resonant optical parametric oscillator (SRO) is pumped by a high power, pulsed source and is tuned approximately to a predetermined desired frequency. Radiation of that frequency is injected into a mode of the SRO from a stabilized, low-power injection source. The output of the SRO is a pulsed, high-power signal with frequency equal to the predetermined frequency to a high degree of accuracy.

Atomic-density-dependent losses in an optical trap.

We have observed that two-body collisions between cold sodium atoms confined within a magnetic-molasses optical trap lead to significant atomic-density-dependent trap losses. Such losses set an upper limit to the product of atomic density and confinement time that can be achieved in such a trap.

Distributed-feedback lasers in thin-film optical waveguides

We have investigated some of the properties of distributed-feedback (DFB) lasers made in active thin-film optical waveguides. By optically pumping these guides with spatially modulated light we have obtained narrow-line tunable oscillation. Operation in single-mode waveguides is well behaved and understood. Measurement of the oscillating wavelengths can be used to determine accurately the parameters that characterize the optical waveguide. This DFB measurement technique offers several advantages over the prism-coupler measurement technique for use with active films.

Higher‐Order Distributed Feedback Oscillators

We report the observation of higher‐order disftributed‐feedback (DFB) laser oscillation using sinusoidal susceptibility modulation. For higher‐order DFB oscillation the period of the susceptibility modulation is a multiple m of one‐half the lasing wavelength. We have obtained tunable DFB oscillation for m=2 and m=3 by pumping rhodamine 6G with interference fringes. A discussion of two possible mechanisms which could account for our observations is presented. Higher‐order DFB operation reduces the resolution required for fabricating a DFB laser.

Improvement of optical parametric oscillators by nonresonant pump reflection

Several important characteristics of optical parametric oscillators can be substantially improved by nonresonantly reflecting the pump wave back upon itself after it has passed through the nonlinear medium. Benefits are reduced thresholds, decreased buildup times, and improved efficiencies. Steady-state analyses of doubly and singly resonant oscillators are presented.

Conversion of cw light into a train of subnanosecond pulses using frequency modulation and the dispersion of a near‐resonant atomic vapor

Sinusoidally frequency‐modulated cw laser light is converted into a train of repetitive short pulses by passing it through a nearly resonant atomic vapor. Using a 200‐MHz modulation frequency and sodium vapor, pulses as short as 240 psec, having peak powers of 6.3 times the input power, have been obtained at 5890 A. Calculation and experiment are in good agreement.