R. L. Fork

Negative dispersion using pairs of prisms.

We show that pairs of prisms can have negative group-velocity dispersion in the absence of any negative material dispersion. A prism arrangement is described that limits losses to Brewster-surface reflections, avoids transverse displacement of the temporally dispersed rays, permits continuous adjustment of the dispersion through zero, and yields a transmitted beam collinear with the incident beam.

Compression of femtosecond optical pulses

We describe the generation and measurement of optical pulses as short as 30 fs. The pulses are produced using self‐phase modulation in a short 15‐cm optical fiber followed by a grating compressor.

Optical pulse compression to 8 fs at a 5‐kHz repetition rate

Single amplified 40‐fs optical pulses are compressed to 8‐fs duration at a 5‐kHz repetition rate using self‐phase modulation in a single‐mode optical fiber.

Amplification of 70‐fs optical pulses to gigawatt powers

We report a technique for amplifying pulses as short as 70 fs to gigawatt power levels while retaining the short duration of the incident pulse. Pulse recompression by a dispersive delay line is used to compensate temporal broadening by group velocity dispersion.

Femtosecond optical pulses

Recent advances in generation, amplification, compression, and frequency broadening of femtosecond optical pulses are reviewed. We describe use of colliding pulse mode locking to generate pulses of 65 fs duration and pulse compression to reduce those pulse durations to 30 fs. Amplification of femtosecond pulses to gigawatt powers and frequency broadening to obtain white light continuum pulses while retaining femtosecond pulse durations are also examined.

A Scanning Spherical Mirror Interferometer for Spectral Analysis of Laser Radiation

The results of a theoretical and experimental investigation of a scanning spherical mirror interferometer designed specifically for analysis of laser radiation are given. It is shown that the high degree of spatial coherence and monochromaticity of laser radiation makes it possible to excite individual interferometer modes selectively. A theory of single-mode excitation is presented, and a specific example treated. The mechanical construction of the instrument and the dynamics of the scanning system are described. The capability of the instrument for observation of laser mode separation, amplitudes, and frequency shifts is illustrated by specific experiments. It is shown that the instrument can easily be used in both the visible and infrared. Results of simultaneous observation of beats between laser modes with an rf spectrum analyzer and of the laser optical field with the scanning interferometer are given, and it is shown that the combined system, in certain cases, will permit determination of the relative phases of the laser modes. Experiments illustrating the use of the scanning interferometer for observation of mode competition and the effects of magnetic fields on laser output are recounted, as is also the operation of an active scanning interferometer with which resolving powers of 2.5 × 109 and finesses of 1040 were obtained. Effects of coupling between laser and interferometer are shown, and a polarization isolator for decoupling is described.

Optical frequency filter for ultrashort pulses

We describe an optical filter based on prisms, which provides both spectral filtering and an adjustable correction to group-velocity dispersion. The low losses permit incorporation in a laser oscillator, and the group-velocity correction of the prism sequence has been used with pulses as short as 10 fsec.

Frequency Stabilization of a Gas Laser

The frequency of a gas laser was stabilized by making use of the competition between the oppositely circularly polarized components of a single cavity mode of a laser in an axial magnetic field. For appropriate transitions the coupling between the polarizations leads to a sharp crossover in their intensities as the cavity is tuned across the center of the atomic line. The difference of the intensities of the two polarizations is used as the discriminant for a feedback loop which keeps the cavity tuned to the atomic line center. This discriminant is at least an order of magnitude more sensitive than those previously reported. Two identical lasers and control systems were constructed utilizing the 1.52-microm transition of Ne (2s(2) ? 2p(1), J = 1 ? J = 0), and their relative stability was determined by measurements of their difference frequency. The very short term frequency fluctuations were essentially determined by the free-running stability of the laser and were of the order of +/-2 parts in 10(9) per laser. The longterm fluctuations were considerably reduced by the control loop, and over times of the order of 10 min, the average frequencies (10-sec averages) had standard deviations of 1 part in 10(10) per laser.

Effects of combined RF and optical fields on a laser medium

Modulation of a laser signal at radio frequencies ranging from 28 to 347 MHz is experimentally investigated by subjecting coherently emitting atoms of a Xe-He laser amplifier to simultaneous dc and RF magnetic fields. The sidebands are generated by a coherent nonlinear process that exhibits resonances when the Zeeman splitting of the laser levels equals the frequency of the magnetic field. A perturbational theory that is based on a simplified (j = 1 right arrow j = 0) atomic model adequately describes the dependence of the modulation efficiency on the dc magnetic field, the signal frequency, and atomic parameters. A presently unexplained phenomenon is the absence of the upper sideband.