I. S. Falconer

An experimental study of a CW optically pumped far infrared formic acid vapour laser

Abstract This paper describes a comprehensive experimental study of the performance of an optically pumped formic acid waveguide laser operating at wavelengths of 394, 419, 433 and 513 μm. The variation of output power with Pyrex waveguide diameter, in the range 30–92 mm, and output coupling fraction, between 5 and 40% per double pass, is studied in detail. The results are fitted to a theoretical model to obtain the gain and saturation intensity as a function of tube diameter. For this 1.5m long laser, pumped by a 33 W CO 2 laser, the optimum diameter and output coupling are found to be ~ 50 mm and ~20% respectively. This laser is capable of giving output powers of the order of 50 mW on all four lines. Output coupling is varied by using a range of aluminium strip gratings on fused quartz substrates with Teflon antireflection coatings on their rear surfaces, for which the transmission properties are accurately calculable. The polarising properties of the strip grating couplers are used to measure the polarisation anisotropy of the laser output. For typical operating conditions the ratio of maximum to minimum power is in the range 1.2–1.9 for the four lines studied. Laser resonator interferometry is used to study the pair of lines with wavelenghts close to 433 μm, which are separated by 56 MHz, and to identify the longer wavelength line as the stronger.

The role of pulse length in target poisoning during reactive HiPIMS: application to amorphous HfO2

In conventional reactive magnetron sputtering, target poisoning frequently leads to an instability that requires the reactive gas flow rate to be actively regulated to maintain a constant composition of the deposited layers. Here we demonstrate that the pulse length in high power impulse magnetron sputtering (HiPIMS) is important for determining the surface conditions on the target that lead to poisoning. By increasing the pulse length, a smooth transition can be achieved from a poisoned target condition (short pulses) to a quasi-metallic target condition (long pulses). Appropriate selection of pulse length eliminates the need for active regulation, enabling stable reactive magnetron sputter deposition of stoichiometric amorphous hafnium oxide (HfO 2 ) from a Hf target. A model is presented for the reactive HiPIMS process in which the target operates in a partially poisoned mode with a distribution of oxide on its surface that depends on the pulse length.

A simple microwave plasma source for diamond deposition

We describe a simple microwave‐produced plasma source that has been constructed for plasma‐assisted chemical vapor deposition of diamond thin films. Microwave power from a 700 W domestic microwave oven magnetron is fed into a water‐cooled cylindrical stainless steel vacuum vessel. A methane/hydrogen gas mixture introduced into the vessel is excited by the microwaves to produce a well‐defined plasma ball which does not interact with the walls of the vessel. The position of the plasma ball in the vessel can be predicted by cold cavity calculations of the axial electric field profile. The vessel has several diagnostic ports which do not alter its resonant condition. Diamond has been deposited on various substrates, placed beneath the plasma ball on a graphite‐capped quartz pedestal. Some of these results are presented and discussed.

Production of highly ionized species in high-current pulsed cathodic arcs

Time resolved optical diagnostic techniques were used to study the production of highly ionized species in aluminum plasma produced by a centered-triggered high-current pulsed cathodic arc. Controlling the spacing between cathode spots enabled a correlation between a reduction in the mean charge state and an increase in the spacing of cathode spots to be observed. As the cathode current was increased, the distances between spots were reduced and these charge states were produced for longer times. Strong cathode spot coupling is proposed as a mechanism for the production of high charge states.

Polarization switching in a strip grating coupled optically pumped submillimeter laser

It is shown that an optically pumped formic acid laser operating at 394 or 433 μm can lase on two sets of nondegenerate orthogonally polarized modes when a strip grating output coupler is used. The laser output polarization can be switched from parallel to perpendicular to the strips by a small cavity length change which is in accordance with calculated reflection phase changes of the grating for the two polarizations. For 30 W pump power ∼5 mW can be obtained at 394 μm in either polarization when the grating strips are oriented perpendicular to the pump polarization.

Observation of enhanced laser emission and new laser transitions in triple cascade operation of an optically pumped cw HCOOH submillimeter laser

We report an investigation of the enhancement of the output power and the observation of new cw lasing transitions for an optically pumped formic acid laser when the resonator length is adjusted so that it is simultaneously resonant for adjacent rotational cascade transitions. Cascade operation was found to be a particularly useful technique for increasing the power of the second transition of a cascade sequence. New cw transitions were observed at 418 μm (the third member of the 394 μm, 406 μm cascade), 447 μm (the third member of the 419 μm, 432 μm cascade), and 460 μm (the third member of the 433 μm, 446 μm cascade).

Annular slot array output couplers for submillimetre lasers

Abstract Arrays of circular and square slots in a metal layer on a transparent substrate have been used as output couplers for an optically pumped formic acid submillimetre laser. These couplers exhibit narrow bandpass transmission, which by suitable choice of dimensions, can provide optimum reflectance at the laser wavelength while also having very high reflectance at the CO 2 pump wavelength. The use of such couplers in place of strip grating couplers produced a 67% increase in submillimetre output power.

Fizeau interferometer system for fast high resolution studies of spectral line shapes

A monochromator∕Fizeau interferometer∕intensified CCD camera system is described that was developed for the measurement of the shape of spectral lines that are rapidly time varying. The most important operating parameter that determines the performance of the instrument is the size of the entrance aperture as this determines both the light throughput and the effective interferometer wavelength resolution. This paper discusses, both theoretically and experimentally, the effect of the finite source area on the instrumental resolution to assist in optimizing the choice of this parameter. A second effect that often produces a practical limit to the quality of the spectra is drift of the interferometer plates. Measurements of the shapes of spectral lines of ions and atoms ejected from the cathode spot of continuous and pulsed cathodic arcs are presented to demonstrate the utility of this instrument.

A novel cw optically pumped submillimeter laser

Abstract Laser operation has been achieved in an optically pumped submillimetre laser that employs a novel cw resonator design which separates the pumping optics from the resonator optics by the use of a Brewster window. This design also allows pumping by an unfocused CO 2 laser beam. The preliminary performance characteristics of the laser are presented.

Simultaneous two wavelength operation of an optically pumped HCOOH laser on orthogonally polarized modes

Simultaneous laser operation in orthogonally polarized EH11 and TE01 waveguide modes is reported for pairs of cascade lines from an optically pumped formic acid vapour laser incorporating a strip grating output coupler. Observations were made for the 394 μm/406 μm and 433 μm/446 μm cascade sequences, the resonator length being adjusted so that the laser was simultaneously resonant on a mode of the primary transition polarized perpendicular to the strips of the output coupler and the same transverse mode of the secondary transition polarized parallel to the coupler strips. Power measurements showed that for the laser configuration used in this study the power of the primary transition was increased only slightly when the cascade lines were simultaneously resonant, whereas the secondary line could only lase under these conditions.