Stellan Jacobsson

Single photomask, multilevel kinoforms in quartz and photoresist: manufacture and evaluation

Kinoforms manufactured in photoresist by photolithographic techniques using a single, ten-level, grey scale photomask, exposed in a specially designed laser exposure system, are described. Kinoforms designed for uniform as well as for partial Gaussian beam illumination are discussed. The highest measured diffraction efficiency was 55%. Photoresist kinoforms were transferred into quartz substrates by reactive ion etching. The highest measured diffraction efficiency for the resulting all-quartz kinoforms was 53%.

Optically pumped far infrared lasers

Abstract The far-infrared (FIR) region of the electromagnetic spectrum is commonly thought of as the wavelength region ≈ 30μm-≈2 mm. Thus, the FIR wavelength region is located between the more familiar areas of microwaves and optics. Primarily due to the lack of FIR sources and detectors, the FIR region is difficult to access and therefore relatively unexplored and unused. The FIR source problem is presently under attack from neighbouring disciplines; from the microwave side by extending the frequency operating range of classical electron tube oscillators (e.g. backward wave oscillators) and semiconductor devices (e.g. IMPATT and quantum well oscillators) and from the optical side primarily by optically pumped molecular gas lasers. The FIR technology evolution accelerated in the mid 60s with the discovery of the discharge pumped hydrogen cyanide laser, lasing at a handful of lines located at about 330μm wavelength. However, the most important step towards a useful coherent FIR source was the discovery of the optically pumped FIR laser in 1970. In optically pumped FIR lasers a molecular gas (e.g. methyl fluoride methyl alcohol, formic acid) is pumped by an external laser, usually a carbon dioxide laser. The FIR laser transitions typically takes place between adjacent rotational levels in an excited vibrational state. Today, optically pumped FIR lasers cover the full FIR region by more than one thousand discrete laser lines observed in hundreds of FIR laser media. FIR output powers on the order of 1–100 mW are available from a vast number of laser transitions. Despite the rapid development of semiconductor FIR oscillators the optically pumped FIR laser is still the only practical unit that bridge the full frequency-gap between microwaves and optics. The fact that FIR lasers are considered as local oscillators in space born applications, indicate that FIR laser technology has matured considerably. This survey paper discusses optically pumped FIR lasers from the engineers point of view: principles of operation, design and characteristics.

Replication of continuous-relief diffractive optical elements by conventional compact disc injection-molding techniques

Continuous-relief diffractive optical elements have been replicated by use of conventional compact disc injection-molding techniques. Two continuous-relief microstructures, a blazed grating and a fan-out element, were chosen to evaluate the replication process. Original elements were fabricated by direct-write electron-beam lithography. Optical measurements and atomic force microscopy were used for investigating the replication fidelity.

Partially illuminated kinoforms: a computer study

The properties of kinoforms being fully and partially illuminated during synthesis are compared by computer simulation. Partial illumination during synthesis results in smoother intensity patterns produced by the kinoforms. A new iteration technique which uses a Gaussian beam that moves during kinoform synthesis is proposed.

Linear cascade VCSEL arrays with high differential efficiency and low differential resistance

We have fabricated monolithic linear cascade vertical-cavity surface-emitting laser (VCSEL) arrays intended for low loss fiber-optic radio-frequency links. Arrays with up to four VCSELs connected in series, with a spacing of 250 /spl mu/m, were fabricated on an insulating substrate. A thick layer of BCB was used to facilitate series connection through contact metallization and enable high frequency operation. The threshold current is 0.6 mA, independent of the number of VCSELs in the array. Differential efficiency, maximum output power, voltage, and differential resistance scale linearly with the number of VCSELs. For a four VCSEL array, we achieve a high differential efficiency of 160%, a maximum output power of 24 mW, and a low differential resistance of 220 /spl Omega/. Basic link measurements, using a series connected VCSEL array, a parallel fiber ribbon, and a parallel connected detector array revealed a differential current gain exceeding unity in the low frequency limit.

Computer generated phase holograms (kinoforms) for millimeter and submillimeter wavelengths

Computer generated Fourier transform phase holograms, known as kinoforms, have been synthesized, manufactured and their performance evaluated at a wavelength of 3 mm (100 GHz). The kinoforms were synthesized to give a prescribed far-field intensity distribution and manufactured by milling the computed kinoform surface relief into a Teflon plate, using a numerically controlled milling machine. The measured diffraction efficiencies exceed 50 percent. Millimeter-wave kinoforms can be used in various quasi-optical applications,e.g. distributing a local oscillator signal to an array of detector elements in heterodyne receivers.

Birefringence measurements of crystalline polymers using a far‐infrared interferometer

It is demonstrated that far‐infrared interferometry can be used successfully to measure birefringence of crystalline polymer samples thick enough to be opaque at visible wavelengths. The use of a strong, optically‐pumped laser as a source of radiation allows absorbing plastics to be measured. An injection molded, high‐density polyethylene specimen showed a birefringence of about 2%, whereas a compression‐molded sample of the same material showed no measurable birefringence.

Single Photomask, Multilevel Kinoforms: Manufacture And Evaluation

Kinoforms manufactured in photoresist with photolithographic techniques using a single, ten-level, grey-scale photomask, exposed in a specially designed laser exposure system, are described. Kinoforms designed for uniform as well as for partial Gaussian beam illumination are investigated. Measured diffraction efficiencies exceeded 50 percent.

A New Kinoform Manufacturing Process

Kinoforms were manufactured by transferring computer-calculated phase-data into a photo-resist layer applied to a glass substrate. This transfer was carried out with conventional photolithographic techniques, using one single photomask exposed in discrete grey scale levels. The photomasks were generated in a specially designed PC controlled exposure system. We present kinoforms manufactured in ten levels with a pixel width of 20 μm. About SO% of the totally transmitted power was diffracted into the desired intensity distribution, and a few percent were undeflected at λ = 632.8 nm.