In order to demonstrate how BDFA can be used in laser spectroscopy, a setup that enables photothermal spectroscopy (PTS) to be performed was built. PTS can be used to detect and quantify gas samples by observing indirect effects of light–matter interaction. Energy absorbed by gas is turned to heat, leading to a local rise of the gas temperature and change of the gas refractive index. The refractive index change can be measured using, e.g., an interferometric setup [
28,
29]. Because the photothermal signal is proportional to the absorbed energy, an optical amplifier can be used to improve the performance of PTS. This was previously demonstrated using photoacoustic spectroscopy (a technique which is fundamentally similar to PTS) performed in a more convenient telecom wavelength region (below 1.6 µm, where erbium-doped fiber amplifiers can be used) [
30,
31,
32,
33,
34]. Here, we show BDFA-enhanced PTS of methane using the setup schematically shown in
Figure 8. The light emitted from a 1651 nm DFB laser diode was amplified with the BDFA and entered the gas sample (enclosed in a glass cell) through a 90:10 fiber coupler. The DFB laser diode was current modulated with a sinewave (
fm = 0.5 kHz) to produce periodic changes of the refractive index inside the gas cell. These changes were measured using an interferometer formed between two fiber couplers (90:10 and 50:50). Additional saw-tooth modulation (1 Hz) was added to provide a wavelength scan across the target transition. The light from the 1550 nm DFB laser diode was used as a probe beam. An acousto-optic frequency shifter/modulator (AOM) driven at
fa = 200 MHz was placed in one arm of the interferometer, enabling heterodyne detection of the photothermal signal: Changes of the optical length in one arm were detected as changes of the phase of the heterodyne signal. The main advantage of heterodyne-based detection [
35] compared to homodyne-based sensing is that a heterodyne-based setup does not require the arms of the interferometer to be stabilized [
36,
37]. A high-speed lock-in amplifier (UHFLI from Zurich Instruments) was used to perform phase demodulation of the beat note at 200 MHz, as well as subsequent signal demodulation (filtering) at 2 ×
fm (this helps with removing background signals due to, e.g., non-resonant absorption in the setup).
Figure 9a shows the PTS spectra measured for a 100 mm-long sample containing 4.29% methane (balanced with nitrogen, total pressure of 740 torr; the concentrations was determined based on direct absorption measurement and spectral fitting with data from the HITRAN database [
38]). Four different powers at the output of BDFA were used (the optical power was measured after the collimator and before the gas cell). 2
f PTS spectra are baseline-free and their amplitude clearly depends on the optical power.
A linear dependence between the signal to noise ratio (SNR) and the optical power level was also confirmed, as shown in
Figure 9b. For this measurement, a shorter gas cell (25 mm) was used (with a methane concentration of approximately 5.35%). The signal was measured as the 2
f PTS amplitude at the transition center and the noise was calculated as the standard deviation of the signal recorded when the sample was removed from the setup. Based on this measurement (i.e., SNR ≈ 52 for the incident power of ~120 mW), we could calculate the noise equivalent concentration, which was shown to be approximately 19.3 ppm × m × Hz
−1/2, corresponding to the noise equivalent absorbance of ~7.2 × 10
−4 Hz
−1/2. This result is comparable to the sensitivity demonstrated previously with a similar method [
35]. We expect that a significantly better detection limit could be obtained if photothermal spectroscopy was performed inside a hollow core fiber [
36,
37].