About TDLAS

Tunable diode laser spectroscopy (TDLAS) has become a relatively ubiquitous technique for measuring temperature and gas phase species concentrations. TDLAS is, quite simply, normal absorption spectroscopy using a narrow band diode laser as the light source rather than a broadband lamp or LED. Just as with any absorption technique, TDLAS produces an average temperature or species concentration over the path defined by the laser through the measurement zone. TDLAS follows Beer’s Law that we are all familiar with from freshman chemistry with slightly different but ultimately equivalent constants:

1) I / I₀(λ) = exp –[S(T,λ)χPL] or equivalently

2) - ln [I / I₀(λ)] = S(T)χPL

Here, I is the transmitted laser light intensity, I₀ is the initial light intensity, λ is the wavelength, S is the temperature and wavelength dependent line strength of the absorber, χ is the partial pressure of the species of interest, P is the total pressure of the environment (therefore χP is the mole fraction of the species of interest) and L is the path length. The first term in the logarithmic equation, -ln(I / I₀), is known as the absorbance. It is a function of wavelength. The function S(T,λ) can be determined from measurable, and, in our case, generally known molecular constants including, E”, the lower state energy level, S₀, the line strength under standard conditions, and certain line broadening constants that determine the lineshape function known as a Voigt profile. A detailed discussion is beyond the scope of this note; however, interested parties are directed to multiple theses of our some time collaborators at Stanford University.

The absorbance of a molecular species is a function of wavelength. In order to measure the concentration of a species, we tune the wavelength of the diode laser by ramping its drive current. Higher current tunes the laser to longer wavelength and also increases its output power . When one tunes over an absorption feature, one observes a dip in the otherwise steadily increasing transmitted laser power as shown. Ratio-metric measurment The integrated area (integrated over wavelength) of this dip is directly proportional to the concentration of the absorbing species. However, in order to calculate a concentration, one needs to know the temperature since the line strength is a strong function of temperature. In many cases, the temperature is well known. However, for all of our applications, the temperature is typically poorly known; moreover, we are typically trying to measure temperature. Temperature can be determined using TDLAS by measuring absorbance on at least two different absorption features in the same species, we nominally use H₂O as the target as it is both a strong absorber and ubiquitous in the combustion environment. Since the integrated area of each absorption feature exhibits a characteristic temperature dependence, the ratio of the absorbances of two well-chosen features is a sensitive function of only temperature since the values of χ, P, and L are the same for both features. (Thus they cancel in the ratio).

Zolo Technologies practices a unique type of TDLAS know as wavelength-multiplexed tunable diode laser spectroscopy (WMTDLAS) that determines temperature by measuring absorbance on more than two absorption features simultaneously. This improves accuracy and increases the temperature range over which accurate measurements can be made. Another advantage of WMTDLAS is that other combustion species such as CO, CO₂, and O₂ (as well as H₂O and temperature) can be measured simultaneously in time and space. This enables the very fast update rates that are required for many aeropropulsion applications. WMTDLAS also allows optimal features to be chosen to increase accuracy and sensitivity for different applications, because, unlike widely tunable diode lasers that can scan perhaps 80 nm, the optimum spectral features for a particular set of measurements may be 100’s of nm apart. (For instance we measure O₂ at 760 nm and water at wavelengths as long as 1900 nm.) A tunable source can not accomplish this feat, and even if it could, we measure at these two wavelengths simultaneously; whereas a tunable source takes a significant amount of time to span the range between two widely spaced wavelengths.

Zolo Technologies, Inc.

About TDLAS