J.C. Macfarlane

Nonlinear magnetic flux response in high temperature superconductors

Abstract We have spectrum analysed the voltage induced in a pick-up coil wound around a cylindrical bulk sample of YBa 2 Cu 3 O 7−δ when driven by a weak low frequency AC magnetic field, superimposed on a weak DC magnetic field, at 77 K. The harmonics generated by the highly nonlinear response are explained quantitatively in great detail by employing a critical state model with a field independent pinning force density, where intergranular vortices sweep in and out of the Josephson weak link regions during each AC cycle. The symmetry relation for the local flux density, which relates the first and second half of an AC cycle, is broken in the presence of a DC magnetic field, causing even harmonics to appear. This suggests high-sensitivity applications in magnetometry.

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.

Novel electromagnetic effects in high‐temperature superconductors

We report the first observations at liquid‐nitrogen temperatures of parametric inductance behavior, signal up‐conversion, and the nondestructive read‐out of persistent supercurrent states in a superconductor. These observations have been made using a toroidal transformer with a core of the high‐temperature superconductor YBa2Cu3O6.8. The effects arise from the field‐dependent behavior of the magnetic susceptibility of this superconductor.

Electromagnetic shielding properties of yttrium barium cuprate superconductor

Abstract Measurements of the attenuation of alternating magnetic fields by disc-shaped samples of yttrium barium cuprate superconductor at 77K are reported. The effectiveness of this material in magnetic shielding applications is examined as a function of sample density, and its stability against atmospheric corrosion is demonstrated.

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.

Comparison between the constant-potential steps of two point-contact Josephson junctions

Microwave‐induced steps in the current‐potential characteristics of two niobium point‐contact junctions have been compared using a SQUID voltmeter with a resolution of less than 10−12 V. It has been established that the uncertainty in step potential at the 1‐mV level is less than 2 in 109.

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.

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.

High-frequency phase-modulated interferometer

Abstract In optically pumped submillimetre lasers, the laser molecule is excited from a rotational level of the ground vibrational state to a specific rotational level of an excited vibrational state. A population inversion can be achieved not only for the directly excited transition but also for rotational transitions immediately below this transition. With appropriate cavity adjustment, a formic acid laser has been operated simultaneously on three such transitions. When a non-linear detector is used, a low-frequency mixing product at approximately 1 MHz is produced. This phenomenon has been applied to produce phase modulation of a Mach-Zehnder interferometer. An accurate measurement of the refractive index of Mylar has been made with this instrument.