Nils Hempler

High-power, (AlGaIn)(AsSb) semiconductor disk laser at 2.0 μm

We report a high-power (AlGaIn)(AsSb) semiconductor disk laser emitting around 2 microm. With a diamond heat spreader used for thermal management, a maximum output power of just over 5 W and slope efficiencies of over 25% were demonstrated. The output wavelength was tunable over an 80 nm range centered at 1.98 microm. The beam propagation parameter (M2) was measured to be in the range of 1.1 to 1.4 for output powers up to 3 W.

Pulsed pumping of semiconductor disk lasers.

Efficient operation of semiconductor disk lasers is demonstrated using uncooled and inexpensive 905nm high-power pulsed semiconductor pump lasers. Laser emission, with a peak power of 1.7W, is obtained from a 2.3mum semiconductor disk laser. This is seven times the power achieved under continuous pumping. Analysis of the time-dependent spectral characteristics of the laser demonstrate that significant device heating occurs over the 100-200ns duration of the pumping pulse - finite element modelling of the thermal processes is undertaken in support of these data. Spectral narrowing to below 0.8nm is obtained by using an intra-cavity birefringent filter.

Semiconductor disk laser pumped Cr2+:Znse lasers.

A new flexible pump source, the optically-pumped semiconductor disk laser (SDL), for the Cr(2+):ZnSe laser is reported. The SDL provides up to 6W output power at a free running central wavelength of 1.98 microm. The Cr(2+):ZnSe laser operated at an output power of 1.8W and a slope efficiency of approximately 50% with respect to absorbed pump power whilst maintaining a low output intensity noise figure of <0.14% RMS. The system required no optical isolation even under the situation of significant optical feedback.

Real-time imaging of methane gas leaks using a single-pixel camera

We demonstrate a camera which can image methane gas at video rates, using only a single-pixel detector and structured illumination. The light source is an infrared laser diode operating at 1.651μm tuned to an absorption line of methane gas. The light is structured using an addressable micromirror array to pattern the laser output with a sequence of Hadamard masks. The resulting backscattered light is recorded using a single-pixel InGaAs detector which provides a measure of the correlation between the projected patterns and the gas distribution in the scene. Knowledge of this correlation and the patterns allows an image to be reconstructed of the gas in the scene. For the application of locating gas leaks the frame rate of the camera is of primary importance, which in this case is inversely proportional to the square of the linear resolution. Here we demonstrate gas imaging at ~25 fps while using 256 mask patterns (corresponding to an image resolution of 16×16). To aid the task of locating the source of the gas emission, we overlay an upsampled and smoothed image of the low-resolution gas image onto a high-resolution color image of the scene, recorded using a standard CMOS camera. We demonstrate for an illumination of only 5mW across the field-of-view imaging of a methane gas leak of ~0.2 litres/minute from a distance of ~1 metre.

Single-longitudinal-mode ring diamond Raman laser

Continuous-wave single-longitudinal-mode (SLM) lasers are an important tool for applications exploiting light-matter interactions. However, mature laser technologies using inversion gain media cover only a limited part of the optical spectrum. Therefore, nonlinear frequency conversion is necessary to reach wavelengths outside this range. Conversion using stimulated Raman scattering can be a simple and robust approach [1], and an SLM diamond Raman laser using a 40 W pump laser has been recently reported [2].

Development and commercialization of mode-locked VECSELs

This paper will describe the current state-of-the-art in commercial mode-locked Vertical External Cavity Surface Emitting Lasers (VECSEL) and demonstrate their efficacy in key applications. Based on indium gallium arsenide quantum well gain structures, our systems operate between 920 nm – 1050 nm with >1 W output powers, 200 MHz pulse repetition rate and <1 ps pulse duration. Crucially, the development issues that have been overcome to bring this promising technology to market will be discussed. These include: thermal management challenges, electronic control system development and robust mechanical design requirements. Having the potential to replace more conventional titanium sapphire laser technology where wavelength flexibility can be traded off against a significantly lower cost point and form factor, we will discuss the use of VECSELs in key applications such as nonlinear microscopy.

5W mid-IR optically-pumped semiconductor disk laser

We report multi-watt, TEM00 emission from a 2 mum Sb-based optically-pumped semiconductor disk laser utilising an intra-cavity diamond heat spreader for thermal management. An output power of 5 W and a wide tunability of over 160 nm are achieved.

GaAs-based distributed feedback laser at 780 nm for 87 Rb cold atom quantum technology

The UK Quantum Technology Hub in Sensors and Metrology [1] has the aim of developing integrated, small and practical cold atom systems for a range of sensor and timing applications which includes rotation, magnetism, gravity and atomic clocks. The approach is similar to that pioneered by the chip scale atomic clock [2] where atoms held in microfabricated vacuum chambers have atomic transitions excited and probed by diodes lasers [3] and photodetectors. That system used coherent population trapping for the clock transitions whilst we are aiming to first produce lasers for cooling and trapping ions inside vacuum chambers before microwave pulses or controlled lasers are used to create superposition states, recombine them and measure the interference from the final state populations. For cooling 87Rb atoms, 780.24 nm lasers with linewidths below ∼5 MHz are required whilst the lasers for controlling and measuring superposition states typically external cavity lasers have been used to achieve linewidths from 20 kHz [3] down to a few Hz [4]. Most single mode diode lasers aimed at laser cooling have used DBR gratings with regrowth [5] but this is challenging when using AlGaAs materials due to oxidation.

The UK National Quantum Technology Hub in Sensors and Metrology

The UK National Quantum Technology Hub in Sensors and Metrology is one of four flagship initiatives in the UK National of Quantum Technology Program. As part of a 20-year vision it translates laboratory demonstrations to deployable practical devices, with game-changing miniaturized components and prototypes that transform the state-of-the-art for quantum sensors and metrology. It brings together experts from the Universities of Birmingham, Glasgow, Nottingham, Southampton, Strathclyde and Sussex, NPL and currently links to over 15 leading international academic institutions and over 70 companies to build the supply chains and routes to market needed to bring 10–1000x improvements in sensing applications. It seeks, and is open to, additional partners for new application development and creates a point of easy open access to the facilities and supply chains that it stimulates or nurtures.

Advances in mode-locked semiconductor disk lasers

NonLinear Microscopy techniques, such as Two-Photon Excited Fluorescence and Second Harmonic Generation provide advantages over conventional Confocal Laser Scanning Microscopy. A key element in a NonLinear Microscope is an ultrafast laser which produces short pulses with the high intensities needed for exciting nonlinear processes. Semiconductor Disk Lasers potentially offer an alternative to expensive Ti:Sapphire lasers. The reported 200MHz operation of a modelocked Semiconductor Disk laser is to our knowledge the lowest repetition rate as yet demonstrated.