Summary form only. The wavelength division multiplexing (WDM) scheme is utilized to increase significantly the transmission rate of optical communication systems. Monolithic arrays of wavelength-graded laser diodes are considered as a compact choice for WDM light sources. Lasers in the visible regime are suitable to be used as emitters in optical short-range data transmission since the attenuation minimum of polymethylmethacrylate (PMMA) fibres lies near 650 nm. We have developed a wavelength-graded GaInP-AlGaInP surface-emitting laser array with horizontal cavities.

Wavelength-graded emission from a visible red GaInP-AlGaInP laser diode array has been achieved with the contradirectional surface-mode coupling technique. The wavelength control is attained by postgrowth adjustment of the thickness of the surface waveguide. The horizontal cavity lasers show both edge and surface emission (beam divergence 0.12/spl deg/). The thermal red-shift of the wavelength is 0.028/spl plusmn/0.002 nm/K. They show single-mode emission with a typical spectral linewidth of 0.09 nm and a sidemode suppression ratio up to 29 dB. The wavelength spacing between the individual lasers is 0.76/spl plusmn/0.08 nm yielding a total range across the array of 5.4 nm (from 681.5 to 686.9 nm).

MBE (molecular beam epitaxy) technique allows the epitaxial growth of different compounds. One of the model materials for optoelectronics are epitaxial layers of III-V semiconductors, mainly GaAs and related compounds. The controlled growth of single crystalline layers on an atomic scale makes it possible to design new materials with optimized electrical and optical characteristics. A solid source MBE system (MOD GEN II) is used for the growth of AlGaAs/GaAs semiconductor lasers and InGaAs quantum well lasers; thus, this machine is further equipped with an indium cell. As doping materials we use silicon for n-doping and carbon for p-doping.

An analysis of semiconductor laser structures with adjustable single-mode emission based on a contradirectionally grating coupled twin-waveguide structure consisting of a semitransparent metal (or ITO)/low-index dielectric waveguide on top of a corrugated active laser waveguide is presented. A coupled mode theory, where the coupling coefficients are expressed in terms of field overlap integrals between the unperturbed fields and on-resonant Floquet-fields, is applied to describe the optical fields of the laser resonators with a high degree of accuracy, although the modulation of the refractive index in the grating region is large. At resonance between the laser mode and the surface mode the grating-coupled radiation losses show a sharp drop with a linewidth comparable to the longitudinal Fabry-Perot-mode spacing of the laser cavity, thus preferring a single longitudinal mode. Intermodal discrimination of up to 20 cm-l and moderate threshold gain of similar to 50 cm(-1) can be obtained. Since the resonance wavelength depends on the optical thickness of the surface waveguide, a simple post-processing adjustment of the emission wavelength with a tuning range in excess of similar to 15 nm can be achieved.

In this article we compare the accuracy of a diode laser densitometer emitting 675.2 nm to that of a commercial He-Ne laser densitometer emitting 632.8 nm for GafChromic MD-55 film readout. A Leksell gamma unit (AB Elekta Stockholm, Sweden) Model B with a 14 and 8 mm collimator at the same isocenter (combined 11 mm collimator) was used to irradiate GafChromic MD-55 films. Dose response curves, dose cross profile and FWHM were measured with a custom-designed diode laser scanning device, emitting light at 675.2 nm. The same data were recorded with a commercial He-Ne laser densitometer (PTW FIPS Plus, Freiburg, Germany), emitting light at 632.8 nm. Both measurements were compared to dose cross profiles of a radiosurgery dose planning program (GammaPlan 5.12, Elekta, Sweden). Compared to the commercial He-Ne laser densitometer, the custom-designed diode laser scanning device showed better agreement with the calculated dose cross profile. For two axes, the full width half maxima (FWHM) of the diode laser scanning device was within 0.1 mm deviation compared to the data calculated by the dose planning program. The FWHM of the commercial He-Ne laser densitometer was less accurate (1.6 and 2.1 mm deviation). Our data show that a diode laser scanning device using a light source emitting 675.2 nm increases the accuracy of a GafChromic MD-55 film readout. This greater accuracy may be related to the diode laser measuring the optical density close to maximum absorption of the GafChromic film MD-55 (671-675 nm).

Single-mode surface emission has been achieved from visible red GaInP/AlGaInP laser diodes by applying the contradirectional surface-mode coupling technique. The emission wavelength (679.4 nm) of the laser structures was adjusted (decreased) in steps of 0.2 nm in an interval of 1.2 nm by reducing the thickness of the waveguide on top of the laser diode. The laser diodes emitted via the surface with a beam divergence of 0.12° and showed single-mode emission both in ac as well as in dc operation with a minimum spectral linewidth of 0.09 nm. The highest side-mode suppression achieved in dc operation was 26 dB.

Radiochromic film as a dosimetry medium offers several advantages in high-resolution radiography. A new technique of readout was developed to measure the optical density distributions of the film in purely directed light. This technique implements radiochromic film dosimetry near the film's absorption maximum by using a single-mode top-surface emitting laser diode (675.2 nm). The effective sensitivity of the film, compared with a helium-neon laser densitometer (632.8 nm), is increased approximately threefold. Good accuracy, high spatial resolution and simple assembly of the readout system is achieved. Beam profiles of the four final collimator helmets of a Leksell Gamma Knife (Elekta Inc., Sweden) were experimentally determined. Measured profiles and full-widths at half maximum are consistent with the computer generated data of the dose planning system (Kula 4.4, Elekta Inc., Sweden). The output factor of the 4 mm collimator (the smallest collimator with the steepest dose gradient), essential for the application of well defined doses, was checked. The measurements established an output factor of 826 +/- 9 that lies 9 +/- 1% lower than the adjusted one.

A five-wavelength surface emitting laser diode array based on surface mode emission is reported. The wavelength control is achieved by postgrowth adjustment of the surface structure. Wavelength spacing between laser groups is 1.1 nm in average and the wavelength reproducibility is ±0.13 nm. The total change in the emission wavelength over the array is 4.78 nm. The single array elements show single mode emission with a full width at half-maximum of 0.08 nm in average and a side mode suppression ratio up to 20 dB. This type of laser diode is of high interest for wavelength division multiplexing applications.

Single-mode and single-beam surface emission (675–680 nm) has been achieved from visible red GaInP/AlGaInP laser diodes by applying the surface mode emission technique. The laser diodes emit a single beam via the surface with a beam divergence of 0.16° and show single-mode emission both in ac as well as in dc operation with a minimum spectral linewidth of 0.07 nm. The highest sidemode suppression achieved at present in ac operation is 19 dB.