Combustion Analysis with CARS – It Really is Rocket Science

The basic concepts of CARS are shown in figure 2. The molecular sample is irradiated with three laser frequencies (wavelengths), called the pump, the Stokes, and the probe, that interact with the sample through a third order non-linear optical mechanism to generate a fourth wavelength, the CARS, which is sent out as a “laser-like” coherent signal. When the difference between the two frequencies, the pump and the Stokes, corresponds to a difference between two energy levels in the probed molecules, the signal strength is resonantly enhanced by multiple orders of magnitude. Through such resonant enhancement a spectrum of the sample’s energy levels can be efficiently obtained using a single-laser shot where the intensity of each spectral peak depends both on the species number density, and on the populations of each species’ internal energy levels. In this way CARS tells you how much of each chemical species is present in the gaseous sample while the shape of the spectrum tells you the local Boltzmann temperature by consisting of pure-rotational or rotational-vibrational transitions. 

CARS with a Single Ultrafast Amplifier

Since the goal is to map as many transitions as possible via impulsive excitation, CARS today is usually performed with ultrafast lasers pulses. Bohlin explains, “We want a femtosecond duration pulse for the pump/Stokes beam as this has a broad spectral bandwidth covering many of the energy levels of interest simultaneously. The short pulse duration < 50 fs can be considered impulsive for most diatomic- and triatomic species, and it provides the most efficient form of exciting the molecules. We then need a tunable narrowband probe pulse, i.e., a picosecond duration pulse, so that the CARS spectrum – the signal in figure 2 - consists of well-resolved spectral features of our various molecular targets. And if you can get both pulses from the same laser source, these become automatically synchronized at the measurement location, which greatly simplifies the setup, it also reduces the signal beating between closely spaced transitions shot-to-shot, and it increases signal-to-noise.”

In his laboratory at Delft University of Technology (the Netherlands) Bohlin and his students built a CARS analysis setup based on a Coherent Astrella “one-box” femtosecond amplifier configured for 35 fs output pulses. The high (several millijoules) pulse energy of this amplifier enables them to split the output and use part of it directly as the broadband pump/Stokes pulse. The other part is then used to generate a picosecond duration pulse with a device called a Second Harmonic Bandwidth Compressor (SHBC). After the SHBC a home-built pulse shaper is used to tune the pulse duration in the regime of ~3-15 picoseconds.

With this as their basic CARS engine, Bohlin and colleagues have successfully performed a variety of different studies on combustion flames and systems, perfecting techniques such as pure-rotational CARS with in-situ monitoring of the excitation efficiency, developing space-time CARS, cascaded CARS, and using cutting-edge concepts including self-phase modulation to generate and expand the necessary laser wavelengths “behind the windows”, in the combustion chamber itself.

Emphasizing Simplicity, Precision and Accuracy

Perhaps the most important aspects of Bohlin’s CARS system, are its stability and relative simplicity compared to more traditional approaches. And over the last few years Bohlin has made innovative improvements toward those ends. He states, “We wanted a universally applicable method that we could take to the combustion site if necessary, rather than a lab-bound method which could only be used on smaller engines that could be brought into the lab. You obviously can’t bring even a test rocket engine into a typical research lab. The use of a single laser source with the simplicity and stability of Astrella has played a huge role in making a portable system a reality.”

In terms of CARS performance, he has been purposefully targeting measurements with the world’s best precision and accuracy. Bohlin explains, “The history of laser spectroscopy shows that measuring experimental parameters with greater detail doesn’t just put decimal points on some numbers. Rather it often reveals important new science.” As an example of his approach, just by using the 400 nm output of the SHBC pumped by Astrella rather than the more usual 532 nm CARS laser wavelength, he has reduced the point spread function in his CARS imaging, from 40 microns to 20 microns.

He is now re-establishing a “canonical burner” for H₂ flames where the standardized performance details will be quantified with unprecedented accuracy and precision. By looking at things that are fundamental to hydrogen flame propagation in greater fidelity, such as steep thermal gradients and diffusion problems, he is checking old theories and assumptions based on measurements made in the 1990’s.

Several of the recent advances in CARS combustion analysis deployed by Bohlin merit closer examination.

CARS with In-Situ Referencing of the Excitation Efficiency

Another important development from the Bohlin group is their innovative polarization-sensitive coherent imaging spectrometer, which allows for the simultaneous recording of resonant and non-resonant CARS signals on the same detector frame [2]. He explains, “This detection scheme can be used to acquire in-situ information about the impulsive excitation efficiency, which has been an unknown in all previous femtosecond spectroscopies. Although the complexity level is quite high for this new protocol to become widely employed, it provides a unique pathway to go beyond the currently established accuracy and precision levels of CARS, which is already considered to be the benchmark for gas-phase diagnostics. With this innovation we have the chance to become truly calibration free, and I see a clear prospect of this method to push the performance of scalar determination to the ultimate dream-limit of delivering ±1% precision and ±1% accuracy.” 

Cascaded CARS – Highly Sensitive to Number Density

Cascaded CARS takes the whole CARS concept one step further so that the stimulated CARS signal itself becomes the probe pulse generating a higher order CARS signal from the sample! With multi-species targets as found in combustion systems, this might be expected to produce spectra of unusable complexity. However, Bohlin’s group have demonstrated that the spectra are actually perfectly amenable to computer analysis [3]. But why use a technique guaranteed to generate signals that are both weak and rather complex to analyze? He explains, “The signal strength is incredibly sensitive to number density – it scales as (number density)₄. So it really supports our mission to push the sensitivity of CARS species measurements. It can for instance be used as a powerful laser diagnostic to quantify even tiny fluctuations in bulk mixture composition under well-mixed conditions. The ability to precisely determine, understand, and control a combustible mixture preparation process has always been very important for designers of efficient, clean-burning reaction engines.” He notes that the signal strength is also very sensitive to laser intensity, so the high stability of Astrella is critically important here in terms of pulse energy as well as the temporal and spectral profiles of the pulse.