Oliver Lux

Intrinsically stable high-power single longitudinal mode laser using spatial hole burning free gain

A fundamental advantage of lasers is their ability to produce a large number of photons in a single optical mode, yet this is achieved in only a minor fraction of devices due to the instability mechanism called spatial hole burning. Here, we exploit the spatial hole burning free nature of a stimulated scattering gain medium to demonstrate single longitudinal mode (SLM) operation in a generic standing wave cavity. A continuous wave diamond Raman oscillator with multi-Watt-level output power and a frequency stability of 80 MHz is demonstrated without use of additional mode-selective elements. Mode stability is addressed by considering the coupling of the Stokes power with thermally induced optical path length changes in the gain medium. The results foreshadow a novel approach for greatly extending the power and wavelength range of SLM laser sources, and with potential advantages for achieving sub-Poissonian intensity noise and sub-Schawlow–Townes linewidths.

High-power continuous-wave Raman frequency conversion from 1.06 µm to 1.49 µm in diamond

We report continuous-wave beam conversion from 1.06 to 1.49 µm in a diamond Raman laser operating on the second Stokes shift. High power (114 W) and high conversion efficiency (44%) is achieved using a single cavity that is highly resonant at the first Stokes wavelength but has high output coupling at the second Stokes wavelength (89%). An analytical model was developed for external-cavity Raman lasers operating in steady-state, revealing that optimization of second Stokes output is markedly different to first Stokes and that there is a direct and proportional relationship between the second Stokes output coupling and the pump depletion in the diamond, which we have confirmed by experiment. This technology shows promise for power scaling beyond the capabilities of current fiber lasers operating in the applications-rich 1.5-1.6 µm wavelength range.

The North Atlantic Waveguide and Downstream Impact Experiment

Multi-aircraft and ground-based observations were made over the North Atlantic in fall 2016 to investigate the importance of diabatic processes for midlatitude weather. The North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) explored the impact of diabatic processes on disturbances of the jet stream and their influence on downstream high-impact weather through the deployment of four research aircraft, each with a sophisticated set of remote-sensing and in situ instruments, and coordinated with a suite of ground-based measurements. A total of 49 research flights were performed, including, for the first time, coordinated flights of the four aircraft; the German High Altitude and LOng Range Research Aircraft (HALO), the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) Dassault Falcon 20, the French Service des Avions Francais Instrumentes pour la Recherche en Environnement (SAFIRE) Falcon 20, and the British Facility for Airborne Atmospheric Measurements (FAAM) BAe 146. The observation period from 17 Sep to 22 Oct 2016 with frequently occurring extratropical and tropical cyclones was ideal to investigate midlatitude weather over the North Atlantic. NAWDEX featured three sequences of upstream triggers of waveguide disturbances, their dynamic interaction with the jet stream, subsequent development, and eventual downstream weather impact on Europe. Examples are presented to highlight the wealth of phenomena that were sampled, the comprehensive coverage and the multi-faceted nature of the measurements. This unique dataset forms the basis for future case studies and detailed evaluations of weather and climate predictions to improve our understanding of diabatic influences on Rossby waves and downstream impact of weather systems affecting Europe.

Single longitudinal mode diamond Raman laser in the eye-safe spectral region for water vapor detection

We report a narrowband and tunable diamond Raman laser generating eye-safe radiation suitable for water vapor detection. Frequency conversion of a tunable pump laser operating from 1063 to 1066 nm to the second order Stokes component in an external standing-wave cavity yielded 7 W of multimode output power in the wavelength range from 1483 to 1488 nm at a conversion efficiency of 21%. Stable single longitudinal mode operation was achieved over the whole tuning range at low power (0.1 W), whereas incorporation of a volume Bragg grating as an output coupler enabled much higher stable power to be attained (0.5 W). A frequency stability of 40 MHz was obtained over a minute without active cavity stabilization. It was found that mode stability is aided via seeding of the second Stokes by four-wave mixing, which leads to a doubling of the mode-hopping interval. The laser was employed for the detection of water vapor in ambient air, demonstrating its potential for remote sensing applications.

Single-longitudinal-mode diamond Raman lasers in the near-infrared spectral region

Lasers operating in single-longitudinal-mode (SLM) are of great importance for high-precision measurements in nonlinear optics and spectroscopy as well as for applications in remote sensing, laser cooling and the flourishing field of gravitational wave detection. However, stable SLM in standing-wave inversion lasers is impeded by spatial hole burning which causes mode instability and can only be overcome at the expense of power limitations and/or higher complexity of the laser system, e.g. by means of injection-seeding, ring or microchip laser designs. As an alternative approach, we demonstrate that the nonlinear optical process of stimulated Raman scattering provides a spatial hole burning free gain which enables the generation of SLM output that is intrinsically stable [1]. The underlying mechanism was harnessed for the development of two compact Raman laser configurations which were realized as external standing-wave cavities, without use of any mode-selective elements, and containing only the CVD diamond Raman-active gain medium. Efficient frequency conversion of a tunable Yb-fiber-amplified distributed feedback (DFB) laser emitting around 1064 nm to the first-and second-order Stokes components produced SLM output in the near-infrared spectral region at powers up to 7 W, while wavelength tuning over a range of 700 GHz was accomplished by varying the temperature of the DFB pump laser, as depicted in Fig. 1a.

High spectral brightness UV laser for airborne wind-lidar observations

Laser sources employed in light detection and ranging (lidar) systems for the quantification of atmospheric parameters such as wind velocity, temperature or trace gas concentration need to fulfill a large set of strict requirements regarding their power performance as well as their spatial and spectral properties. In particular, the generation of high-energy output pulses in the UV spectral region with excellent spectral purity is mandatory for the precise measurement of wind velocities by means of direct-detection Doppler wind lidar systems. Here, the frequency stability of the laser transmitter must be better than 5 MHz to ensure low systematic errors in wind velocity of about 1 m/s. The realization of reliable, high spectral brightness laser sources is further complicated when operating in severe vibration environments such as on ships or aircraft.

Cascaded continuous-wave Raman frequency conversion in external-cavity diamond lasers

High-brightness continuous-wave (cw) beams at various wavelengths in the near and mid-infrared, visible, and ultraviolet are in demand for numerous applications spanning defence, astronomy, space-science, medicine and remote-sensing. While fiber lasers are an attractive solid-state technology for high-brightness cw beams, their output wavelengths are restricted to 1–1.1 μm (for ytterbium), 2 μm (thulium) and their harmonics. Nonlinear frequency conversion, in OPOs/OPAs and with Raman conversion, is often used as a means for converting beams to a desired wavelength, but thermal loading in the nonlinear crystal typically leads to beam distortions at higher powers, limiting beam brightness.

High power single-longitudinal-mode diamond laser using Hänsch-Couillaud-type stabilization

We report a cavity stabilization scheme to enable single longitudinal mode operation from a diamond Raman laser at increased power. An output power of 7.2 W is demonstrated using a simple standing-wave laser cavity without the addition of frequency-selection elements.

High-brightness and narrow-linewidth diamond Raman lasers

We present our recent advances in the field of Raman frequency conversion using high-optical quality CVD-diamond. Different diamond Raman lasers were developed for efficiently generating multi-Watt output at specific wavelengths from the visible to the eye-safe spectral range, while single-frequency operation was accomplished by exploiting an intrinsic mode stability mechanism.

High-gain 87 cm-1 Raman line of KYW and its impact on continuous-wave Raman laser operation

We report a quasi-continuous-wave external cavity Raman laser based on potassium yttrium tungstate (KYW). Laser output efficiency and spectrum are severely affected by the presence of high gain Raman modes of low frequency (< 250 cm-1) that are characteristic of this crystal class. Output spectra contained frequency combs spaced by the low frequency modes but with the overall pump-to-Stokes conversion efficiency at least an order of magnitude lower than that typically obtained in other crystal Raman lasers. We elucidate the primary factors affecting laser performance by measuring the Raman gain coefficients of the low energy modes and numerically modeling the cascading dynamics. For a pump polarization aligned to the Ng crystallo-optic axis, the 87 cm-1 Raman mode has a gain coefficient of 9.2 cm/GW at 1064 nm and a dephasing time T2 = 9.6 ps, which are both notably higher than for the 765 cm-1 mode usually considered to be the prominent Raman mode of KYW. The implications for continuous-wave Raman laser design and the possible advantages for applications are discussed.