Contents


Introduction

The III-V group has developed expertise in design, growth, fabrication and test of semiconductor laser diodes over the past 6 years. The spectrum from 660nm to 1.6µm has been covered using both internally and externally supplied source wafers. Variously the laser output characteristics have been optimised for high power, low threshold, specific wavelength, single transverse mode and single longitudinal mode. These lasers have found applications ranging from telecommunications to sensing and machining.


The facilities available for laser fabrication in addition to the growth capability include optical and e-beam lithography, e-beam evaporation for metallisation, wet and dry etching of III-V compounds, plasma enhanced chemical vapour deposition, oxide sputtering for isolation and coating, furnacing and scribing.


Current commercial activities are in the areas of (a) high power (>1W) in the spectral regions around 808 and 980nm and (b) in single transverse mode lasers around 980nm with typical powers of 100mW. We specialise in custom products as for example in user defined arrays, special wavelengths and other optimised output characteristics either as direct sales or via contract research. Research activities include high brightness lasers using etched facets, low divergence 980nm lasers, development of vertical cavity lasers at 660nm, mode selection using novel processing techniques and the use of microcavity effects to improve the laser characteristics.




High Power Lasers

Work has concentrated on the spectral regions around 980nm using strained InGaAs quantum wells and around 808nm using either narrow GaAs or InGaAlAs wells. The lasers are bonded epitaxial side down on SLM1 (C type) submounts and temperature dependent characteristics measured on a thermoelectric cooled stage. Figures 1 and 2 exhibit typical characteristics of nominal 1.2W devices based on individual 120µm aperture coated lasers which have threshold of 180mA and front facet slope efficiencies of 0.8W/A for a 1mm long device. These class 4 lasers have applications in localised heat generation for medical and machining and in optical pumping. Higher powers are available either via wider apertures or with multistripe arrays. Lasers can be supplied either as coated bars or as mounted devices. Contact Brian Corbett for further information on your specific requirements.



Fig 1. LI characteristic of 120µm aperture at 1.5A, 20°C



Fig 2. Spectral emission at 1.5A, 20°C



Low Threshold Lasers

Single transverse mode output from a semiconductor laser is required to optimise the coupling of the light into an optical fibre. The higher order transverse modes are eliminated through the use of a narrow ridge to restrict the current injection and provide weak index guiding. The ridge is fabricated either by wet chemical etching or by reactive ion etching. This technology is used to produce single transverse mode lasers around 980nm. Figures 3 and 4 show typical characteristics of 4µm ridges with front facet slope efficiencies of 0.75W/A and threshold of 25mA. Lasers can be supplied either as coated bars or as mounted devices. Contact Brian Corbett for further information on your specific requirements.



Fig 3. LI characteristic of single transverse mode ridge laser at 20°C



Fig 4. Spectral characteristic of ridge laser at 150mA



Other laser configurations

A number of research areas in semiconductor lasers are being actively pursued with both industrial, government and EU support. Three examples are mentioned below. These aim in various ways to optimise the longitudinal mode spectrum, improve the brightness or reduce the threshold. Further information of these or any other configurations being pursued may be obtained from Brian Corbett.


Slots - A cost effective single frequency laser

A technique to achieve single longitudinal mode operation for InP based lasers (wavelengths 1.3 - 1.6µm) has been developed. The introduction of controlled defects along the ridge of a conventional ridge waveguide structure converts a multimode laser into a single mode (figures 5 and 6). Side mode suppression ratios of 30dB have been obtained. Research is being undertaken to extend the range of temperatures and currents without mode hopping.



Fig 5. Spectral emission from ridge laser



Fig 6. Spectral emission from ridge laser with well defined slots


Curved facets - A high brightness laser source

The brightness of a wide stripe (high power) laser is limited by the non-linear effects which cause filamentation of the laser beam. An unstable resonator can be formed if the laser mirrors can be made non-parallel. We have developed reactive ion etching processes to allow the definition of laser mirrors of any desired curvature. This process has been used to achieve a laser with 600mW pulsed output and a focal spot of 11µm.



Fig 7. Power characteristic of etched facet unstable resonator under 0.1% pulsed operation


Microdisk - A low threshold source

As the dimension of a laser resonator is reduced the mirror losses become more significant. The use of total internal reflection along a circular surface is the basis of achieving a small resonator with high Q. We have fabricated this microdisk structure in a novel manner by transferring the epitaxial layers to a glass substrate thereby achieving improved optical confinement for the resonator modes. These disks have been optically pumped at 77K to achieve a record low threshold pump power of 5µW (see figure 8)



Fig 8. Emitted lasing light intensity (1.5µm) from an 8µm single strained quantum well as a function of pump power at 77K.

 

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