Taking measurements in a scramjet engine is particularly challenging because of the harsh testing environment. Any probe inserted in the flow would generate shock waves, strongly perturbing the flow. Coherent Anti-Stokes Raman spectroscopy (CARS) is a non-intrusive laser-based measurement technique that has been implemented successfully to measure temperature and species concentrations in ducted scramjet engines.
CARS is a third-order, nonlinear optical measurement technique in which three beams are focused and overlapped at the measurement volume, where they interact and generate a coherent signal containing the Raman spectrum of the probed species. Tens of micrometers of displacement are sufficient to prevent the beams from overlapping correctly, causing a strong reduction in the signal-to-noise ratio. Turbulence produces rapid variations in space and time of the medium refractive index; larger eddies, with size comparable to the beam diameter, randomly refract the beam, and smaller eddies diffuse it. Mechanical vibrations may displace the beams, with similar consequences on beam overlap. Beam steering effects are generally tolerable for ducted flow, but they become critical for large free jets or high-pressure combustors. This work focuses on a new strategy, based on beam shaping, that reduces the effect of beam steering on CARS signal intensity. The new method obtains quantitative measurements of temperature, N2, O2, and H2 in a scramjet engine.
Often in CARS setups, the beam energy is limited by breakdown conditions at the measurement volume and excess laser energy is dumped. When shaping the beam, this energy can be used to keep the intensity at the focus constant, preventing any signal loss. The measurement volume is nearly independent of the shaping; the longitudinal dimension is determined by the angle between the beams and by the beams’ size in the plane — the cross section by the smallest beam.
An Nd:YAG laser beam is split in three beams: one to pump a commercial narrowband dye laser, another to pump a homemade broadband dye laser, and the last as a pump/probe beam for CARS signal generation. The two dye laser beams are overlapped on the cart by a dichroic mirror. The three beams are focused at the measurement volume by two spherical lenses. The lens for the 532-nm beam is mounted on a rotating stage; by tilting the lens, it is possible to add astigmatism to the beam, thereby shaping the beam. Rotating the lens, the beam waist stays constant in the plane of the three beams and increases exponentially in the direction orthogonal to the plane. Small angles are sufficient to obtain large axis ratio.
The lens is mounted on a translation stage to compensate for the shift in focus due to tilting the lens. A pair of cylindrical lenses could have alternately been used to achieve a similar effect, had a single lens been used to focus all three beams. Two beam viewing systems are used to image the beams at the focus point: one system collects a portion of the beams before the measurement volume and focuses them on a camera, and another system is placed after the measurement volume and images the beams at the focus point. Images on the two cameras can be compared to determine beam steering effects due to any turbulent flow placed along the beam path.