Tunable Diode Laser Absorption Spectroscopy Based Temperature Measurement with a Single Diode Laser Near 1.4 μm

The appropriate selection of spectral absorption lines is essential for temperature measurement in TDLAS [41]. H₂O is one of the main products of alkane fuel combustion and contains a large number of spectral absorption lines in the near-infrared band. Compared to lasers and detectors in the mid-infrared band, diode lasers and detectors in the near-infrared band are mature and inexpensive and do not require operation at low temperatures [42,43], which is beneficial to practical applications. In addition, lasers in the optical fiber communication band (1.25 µm to 1.65 µm) can be transmitted over long distances. Selecting absorption lines in this band can keep the instrument away from the test site, to avoid the impact of the on-site interference on the TDLAS system. Based on the above analysis, H₂O spectral absorption lines in the optical fiber communication band will be selected in this research.

According to the HITRAN 2012 database [44,45], a large number of H₂O absorption lines exist near 1.3 µm in the overtone region. Many of them have a spectral line intensity of ~10⁻⁴ cm⁻²atm⁻¹. The selection principle of absorption lines can be considered from the following aspects. Firstly, the selected absorption lines cannot be interfered with by other absorption lines. Secondly, the absorption coefficient of the selected lines should meet the test requirements. When the effective optical path length was determined, the absorption rate mainly depends on the absorption coefficient. Therefore, to obtain a high signal-to-noise (SNR) ratio, when the optical path length is less than 10 cm, the absorption coefficient should not be lower than 10⁻⁴ cm⁻¹. Finally, to ensure the high sensitivity of the system for temperature measurement, the energy difference between the low transition states of the two selected spectral lines should not be lower than 700 cm⁻¹.

In this experiment, two H₂O absorption lines located at 7153.749 cm⁻¹ (L1) and 7154.354 cm⁻¹ (L2) were selected as the target lines for temperature measurement using double-line thermometry. The absorbance of H₂O, methane (CH₄), carbon monoxide (CO), and CO₂ which may be present in the test environment at different temperatures was analyzed at concentrations of 30%, 30%, 5%, and 30%, respectively. It can be found from Figure 1 that CH₄ and its combustion products of CO and CO₂ do not interfere with the target absorption lines. Furthermore, wide fluctuations in temperature can cause changes in spectral line broadening, so the effect of temperature on spectral linewidth must be considered. If the absorption lines are too close to each other, it will lead to undesirable test results. The trends of the absorbance of the target absorption lines at different temperatures in Figure 1 were obtained from simulations based on the Voigt line function. It can be found that the target absorption lines in the range of 500 to 2500 K do not interfere with each other, which proves the applicability of the target absorption lines for temperature measurements.

According to the principle of double-line pyrometry, the ratio of the intensity of the two absorption lines is the same as the ratio of the spectral absorbance integral value, as shown in Equation (1):

where A_i is the integral value of the spectral absorbance corresponding to the different absorption lines (i = 1, 2); I₀ and I_t are the light intensities before and after crossing the area to be measured, respectively; P is the pressure [atm]; T is the temperature [K]; L is the effective light path length [cm]; X is the gas concentration; S_i is the spectral intensity of the different absorption lines (i = 1, 2) [cm⁻²atm⁻¹]; R is the absorption line intensity ratio. In addition, the measurement sensitivity of temperature can usually be expressed as the differentiation of the ratio of spectral line intensity R against the temperature T, as shown in Equation (2):

where h, c, and k are Planck constant [J·s], lightspeed [m/s], and Boltzmann constant [J/K], respectively; ΔE″ is the low state energy level difference [cm⁻¹].

The main parameters of wavenumber (v), line strength (S), lower state energy (E″), air-broadened half-width (γ_air), and self-broadened half-width (γ_self) for the target absorption lines are listed in Table 1. The wavenumber difference between the target absorption line pairs is greater than 0.605 cm⁻¹, which can be covered simultaneously in a single scan period by a continuous wave distributed feedback (CW-DFB) diode laser. Therefore, the laser wavenumber can be calibrated using the absorption peak position. The energy level difference between the two absorption lines is ΔE″ = E₁ − E₂ > 700 cm⁻¹, which can meet the test requirements. At room temperature and standard pressure, the absorption coefficient of the target absorption lines is about 10⁻⁷ cm⁻¹, which is much lower than 10⁻⁴ cm⁻¹ in the test region. Therefore, when the absorption coefficient in the air is much smaller than the region of interest, the interference caused by H₂O in the environment can be ignored, which can ensure a high test accuracy of the system.

Figure 2 shows the variation trend of the spectral intensity of the two H₂O absorption lines with temperature. In the temperature range of T < 400 K, the intensity of selected absorption lines is much lower than 10⁻⁴ cm⁻²atm⁻¹, so the H₂O absorption in the environment can be neglected. When the temperature is greater than 500 K, the spectral line intensity S(T) > 2.7 × 10⁻⁴ cm⁻²atm⁻¹, which can ensure that the measured signal has a high SNR and meet the temperature measurement requirements. According to the sensitivity calculation Equation (2), it can be known that the greater the energy level difference ΔE″ produces the higher the temperature sensitivity. As shown in Figure 3, the sensitivity value of this TDLAS system is greater than 0.47 in the entire temperature range (500 to 2500 K), which ensures high sensitivity for the test system to temperature measurement.

The schematic of the reported TDLAS system is depicted in Figure 4. A CW-DFB diode laser (NEL NLK1E5GAAA) emission at 1397.80 nm was chosen as the excitation source. The laser was driven by a laser controller (Healthy Photon DFB-2000) whose drive signal was derived from a triangular wave generated by a signal generator. The laser beam was collimated and sent to the burning area of the flat flame (McKenna standard burner). The collimator (f = 30 mm) was fixed on the z-axis stage by a clamp, and fine adjustment down to ~10 µm can be achieved by an adjustable platform. A gold-coated reflector (Thorlabs CM508) was used to refract the laser beam back to a photodetector for twice absorption. The signal from the photodetector (Thorlabs DET20C/M) was finally collected by a data acquisition card (DAQ, Healthy Photon USB2066) and uploaded to the computer. A narrow-band filter (Thorlabs FB1400-12) was placed in front of the photodetector to reduce disturbances such as the spontaneous emission spectrum generated during the combustion process. The laser beam was adjusted to pass through the center of the flat flame burner as it travels across the combustion area to ensure an adequate absorption path.

The McKenna burner [46,47,48] was chosen in the experiment to generate a flat flame for the test. The certified gas CH₄ and air were used as the fuel and accelerant for the flat flame burner, respectively. The flow rates of CH₄ and air were controlled separately by two mass flow meters to achieve the change of the burner equivalence ratio. The mass flow meter has a systematic error of about 3%, which affects the accuracy of the equivalence ratio setting slightly. The flat flame burner is capable of producing a disc-shaped flame [40] with a burning area of approximately 60 mm in diameter. The Cartesian coordinate system was constructed with the center of the flat flame burner as the coordinate origin. The direction along which the flame burns was defined as the z-direction.

The output wavenumber of the diode laser can be controlled using temperature tuning and current tuning. In this experiment, the CW-DFB laser can cover the two selected absorption lines simultaneously in a single scanning period, and its scanning range was shown in Figure 5. The wavelength tuning coefficient of the current was 0.023 cm⁻¹/mA. In the investigations, the laser temperature was set to 18.5 °C, and the current range of the sawtooth wave was from 25 to 90 mA. The two absorption peaks correspond to currents and powers of 49 mA, 72 mA, 12.55 mW, and 18.16 mW, respectively. However, the long-term operation of the laser and environmental changes can cause a wavenumber shift, which affects the accuracy of the TDLAS system. Usually, an additional device of etalon [49], wavelength meter [50], etc., are used to monitor the changes in laser wavenumbers. In this manuscript, the relative positions of target absorption lines were used to determine the interval of scanning wavenumbers. Therefore, the wavelength tuning coefficient for the scanning current can be corrected timely. According to this coefficient, the trend of the absorption ratio as a function of time can be obtained. The system sampling rate determines the resolution of wavenumber calculation, and the resolution of wavenumber increases with the increase in the sampling rate. In this experiment, the sampling rate of the system was set to 1 MSa/s, which is about 1000 times that of the triangular wave signal.

The temperature of the flat flame burner at the equivalence ratios (φ) of 1.0, 1.1, 1.2, 1.3, and 1.4 was measured by this TDLAS method with a scanning rate of 1 kHz. The CH₄ flow rate was set to 1.733 L/min, and the airflow rate was set to 16.50 L/min, 14.96 L/min, 13.70 L/min, 15.00 L/min, and 11.80 L/min, respectively.

Figure 6 shows the measured signal using the TDLAS system when the equivalence ratio of CH₄ was 1.0 and the height (z) was 15 mm. The transmitted light intensity I_t in the time domain was captured by the photodetector after the laser beam passes through the burning region. The initial light intensity I₀ was obtained by baseline fitting with It. According to the positions of the absorption peaks of H₂O, the wavenumber calibration of the light intensity signals can be carried out. Based on Beer–Lambert law the absorbance of the two lines was obtained as a function of wavenumber. Using the Voigt multi-peak fitting function, the absorbance was fitted, and the results were shown in Figure 6b. Comparing the fitted absorbance curve with the test data, the residual was shown in Figure 6c, and the standard deviation was 0.02. The integrated area A corresponding to the absorption peak was obtained, respectively, and the corresponding temperature values can then be derived using the principle of double-line thermometry [26,27,28].

By gradually changing the height of the laser beam across the burning region in the z-axis direction, the flame temperatures were measured as shown in Figure 7a. It can be found that the flame temperature firstly increased with increasing height and then gradually stabilized at ~2000 K. Compared with the values obtained using the CARS method and theoretical calculation [51], it was found that the relative errors of the TDLAS results were less than 4% and were all distributed within the error band. Subsequently, the laser height was fixed at 15 mm to measure the flame temperature distribution at different equivalent ratios. The obtained results in Figure 7b showed that the flame temperature decreases as the equivalent ratio increases. CH₄ fuel in the rich combustion state did not burn sufficiently, so less heat was released, and the temperature decreased. The temperature measured by TDLAS for different combustion equivalent ratios followed the same trend as the temperature measured by CARS technique and theoretical calculation [47]. The error bar was as small as 70 K at the same equivalent ratio, which may be caused by the flame jitter in the combustion process.

To further verify the performance of such TDLAS technique, temperature measurement was performed on a scramjet model engine. The schematic diagram is shown in Figure 8, with the laser beam passing through the tail flame region directly. Kerosene was used as the fuel for the scramjet model engine, and the combustion products included H₂O, CO, and CO₂. The combustion process in a scramjet model engine is much more violent than in a flat flame burner. Therefore, to guarantee the test stability, the reflector was removed, and the frequency of the triangular wave was increased to 3 kHz. Figure 9 shows the trend of temperature variation in the tail flame region of the scramjet model engine. With the decrease in fuel equivalence ratio, the tail flame temperature gradually decreased from 2250 K to 900 K. The TDLAS system was able to respond quickly to temperature changes by adjusting the fuel equivalent ratio in 1.5 s intervals. Compared to the McKenna flat flame burner, the temperature testing results from the scramjet model engine have suffered high jitter noise, which can be attributed to the strong turbulent flow field in the tail flame region. This TDLAS system had shown an excellent response capability and a wide temperature measuring range during the measurement of the scramjet model engine.