Integrated Tunable Flat Lenses (TuneFuL)

This proposal aims to revolutionise one of the fundamental components used in optics : the semiconductor laser. These components are used in a huge array of applications from high speed optical communications - they power the internet, to laser machining, ultra sensitive gas sensors, the defence industry and scientific research. Common to all these applications is the requirement to focus, control or shape the beam of light coming from the laser, this is conventionally done with an external lenses which are often many 10's of times the size of the laser, which is typically a few mm^3 in volume, and often much more expensive. Allied to this problem is that once the beam is focused or controlled it is very difficult and expensive to alter it. Moreover, for many applications such as sensing and defence one may want to scan the laser beam across a field of view to interrogate or acquire information from a wide area. Finally, the spectral purity of the laser is critically important in many of these applications where it essential that there is only one single emitted wavelength. This proposal aims to solve all these problems in a low cost, mass market way by creating a flat, electronically controllable lens which can be patterned onto the emitting facet of the laser. This proposal will apply exciting ideas from the emerging field of optical nanoantennas to create tunable nanoantenna arrays configured as flat lenses. They will be first developed as standalone devices and then integrated with a range of semiconductor lasers to create electronically tunable output beams for focusing and steering applications and simultaneous control of the spectral purity of the laser.

This proposal will have numerous direct impacts for industry and thus society in general. In terms of market sizes a recent Optoelectronics Industry Development Association (OIDA) survey estimated the global optoelectronic related market to be $745Billion dollars in 2008 and within this the laser market is $6Billion and Optical Communications $21.9Billion. The improvements and cost reductions this proposal will produce could have major impacts on these markets. For example in optical communications each laser needs to be packaged and the light from the laser coupled to an optical fibre. This is a time consuming, expensive process that could be dramatically simplified and thus reduced in cost with an electronically controllable lens. Lasers come in many different wavelengths and we are targetting two important ones, first 1550nm which is used for optical communications, secondly Mid InfraRed lasers in the 3-5um range. Mid-infrared laser sources are required for a wide range of important and diverse applications. These include environmental monitoring (e.g. hydrocarbon sensing), healthcare (e.g. eye surgery), and defence and security (e.g. heat-seeking missile countermeasures). Semiconductor laser sources, such as quantum well optically pumped semiconductor lasers (OPSL), strained quantum well laser diodes, and quantum cascade lasers (QCLs) are gradually emerging from the research phase, but several obstacles continue to block the widespread adoption of this technology. One key obstacle is the total cost of the laser system which often has to include expensive and specialist optics to shape and optimise the emitted beam. Here again an electronically controlled, integrated flat lens could radically reduce costs.
The relatively poor beam quality of these semiconductor lasers is primarily due to two major factors. As with all semiconductor lasers, the emitting area of these sources is small, resulting in a relatively large beam divergence (several tens of degrees). For applications that utilise a long optical path, for example missile countermeasures, expensive and specialist optics have to be incorporated to negate this intrinsic beam divergence. Secondly, many applications require significant power levels (hundreds of miliwatts) and broad area lasers (BALs) are an attractive means of achieving these powers. However, the emission properties of BALs are determined by the complex interplay between the laterally extended waveguide (typically 10s or 100s of microns wide) and the nonlinear, local interaction of the intense light field with the semiconductor active medium, in which gain and refractive index are strongly coupled (leading to filamentation). Overall this leads to a broad optical spectrum with a complex mode structure, and an output beam that is irregular, multi-lobed, highly elliptical in the cross section, and dependent on the drive current and temperature. The addition of an integrated, tunable laser facet with controllable spectral reflectivity can counteract many of the above issues, resulting in much reduced cost, very high performance lasers for many different applications.