M Squared Lasers (United Kingdom)

We are developing an atom interferometer that measures horizontal accelerations to form a component of a quantum inertial measurement unit. Benefitting from the inherent stability of atom interferometry, this points to a navigation system that offers long-term accuracy without recourse to GPS. To aid compactness and transportability, the sub-components of the quantum sensor are mounted in a 19-inch rack. This includes an optical system of two titanium-sapphire lasers that delivers all of the optical pulses required to operate a highly sensitive atom interferometer using 87Rb atoms. Active compensation systems that allow operation of the quantum sensor in the noisy environment of a moving vehicle are described. Finally, we give an account of recent field trials on a variety of test platforms.

Abstract The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.

An ultrafast semiconductor disk laser (SDL) and a single frequency continuous SDL, respectively named Dragonfly and Infinite are currently under development at M Squared lasers with the aim of offering an alternative to Titanium-sapphire based systems. Such SDLs-based systems have the potential to be easy-to-use, low-cost and maintenance free tools for markets including nonlinear microscopy and quantum technologies. Introducing the SDLs technology into the nonlinear microscopy market requires reproducing the performances of currently employed systems: pulses with 1-W average power, duration below 200fs and a typical repetition rate of 80MHz. Such a low repetition rate is particularly challenging for SDLs which are limited by their short carrier lifetime and preferably operate at GHz repetition rates. With Dragonfly, a repetition rate of 200 MHz has been found as a good compromise to balance the mode-locking instabilities while reducing the repetition rate. By adding external pulse compression and spectral broadening stages, pulses as short as 130fs and an average power of 0.85W have been achieved. Developing low-footprint, single-frequency narrow-linewidth CW SDLs could enable quantum technologies to move from the lab to successful commercialisation. In this context, we have been developing a variant of the Infinite system suitable for Sr atom cooling. About 1W and a sub-MHz linewidth at 461nm are required. As direct emission is not an option, an SDL emitting at 922nm followed by an M Squared Lasers SolTiS ECD-X doubler is currently under development. The SDL oscillator delivered > 1W at 922nm with an RMS frequency noise < 150kHz.

We report the first optical parametric oscillator synchronously pumped by a SESAM modelocked vertical external-cavity surface-emitting laser (VECSEL). As a nonlinear medium, we use a periodically poled MgO:PPLN crystal. The VECSEL operates at a wavelength of 982 nm and a repetition rate of 198 MHz. The pump radiation is converted to signal and idler wavelengths tunable in the ranges of 1.4-1.8 µm and 2.2-3.5 μm, respectively, simply by a change of the poling period and crystal temperature. The signal pulses have a duration between 2 ps to 4 ps and an average output power up to 100 mW.

Numerous applications would benefit from a compact laser source with tunable, continuous-wave emission in the deep ultraviolet (wavelengths <250 nm); however, very few laser sources have been demonstrated with direct emission in this spectral region and options are generally limited to pulsed, fixed wavelength sources or complex and impractical setups for nonlinear frequency mixing of the emission of several infrared lasers in various external enhancement cavities. Here, we propose an all-solid-state, continuous-wave, tunable laser with emission between 224 and 226 nm via intracavity frequency tripling in an AlGaInP-based semiconductor disk laser (SDL). Output power up to 78 μW is achieved in continuous wave operation, with a tuning range over 350 cm−1. AlGaInP-based SDLs may be designed to emit anywhere between ∼640–690 nm such that wavelengths between 213 and 230 nm may be targeted for specific applications using a similar set-up. An in-depth study of the nonlinear conversion has been carried out to understand the limitations of the set-up, namely large walk-off angles for phase-matching in the nonlinear crystals, and the potential for increasing the output power to several milli-Watts. This is, to the authors’ knowledge, the first implementation of intracavity frequency tripling in a visible SDL and the shortest wavelength emitted from an SDL system.

An external ring cavity quantum cascade laser operating at ∼5.2 μm wavelength in a continuous-wave regime at the temperature of 15 °C is demonstrated. Out-coupled continuous-wave optical powers of up to 23 mW are observed for light of one propagation direction with an estimated total intra-cavity optical power flux in excess of 340 mW. The uni-directional regime characterized by the intensity ratio of more than 60 for the light propagating in the opposite directions was achieved. A single emission peak wavelength tuning range of 90 cm−1 is realized by the incorporation of a diffraction grating into the cavity.

Current stand-off hyperspectral imaging detection solutions that operate in the mid-wave infrared (MWIR), nominally 2.5 – 5 μm spectral region, are limited by the number of absorption bands that can be addressed. This issue is most apparent when evaluating a scene with multiple absorbers with overlapping spectral features making accurate material identification challenging. This limitation can be overcome by moving to the long wave IR (LWIR) region, which is rich in characteristic absorption features, which can provide ample molecular information in order to perform presumptive identification relative to a spectral library. This work utilises an instrument platform to perform negative contrast imaging using a novel LWIR optical parametric oscillator (OPO) as the source. The OPO offers continuous tuning in the region 5.5 – 9.5 μm, which includes a number of molecular vibrations associated with the target material compositions. Scanning the scene of interest whilst sweeping the wavelength of the OPO emission will highlight the presence of a suspect material and by analysing the resulting absorption spectrum, presumptive identification is possible. This work presents a selection of initial results using the LWIR hyperspectral imaging platform on a range of white powder materials to highlight the benefit operating in the LWIR region compared to the MWIR.

Electrically pumped vertical external cavity surface emitting laser is passively mode-locked at record-low repetition rate of 216 MHz demonstrating potential peak power scalability. A quantum dot saturable absorber is used to achieve stable operation.

Using quantum well gain materials, ultrafast VECSELs have achieved higher output powers (2.1 W) and shorter pulses (60 fs) than any other semiconductor laser. Quantum dot (QD) gain materials offer a larger inhomogeneously broadened bandwidth, potentially supporting shorter pulse durations. We demonstrate the first femtosecond QD-based VECSEL using a QD-SESAM for modelocking, obtaining 63 mW at 3.2 GHz in 780-fs pulses at 960 nm. In continuous wave operation we obtained 5.2 W using an intra-cavity diamond heat spreader, which has been the highest output power from a QD-VECSEL so far. Further power scaling is thus expected also for modelocked operation.