# Review of Recent Advances in QEPAS-Based Trace Gas Sensing
## 3 QEPAS Sensor with High-Power Excitation Source
### QEPAS Sensor Based on an EDFA-Amplified Diode Laser  
Single-mode diode lasers with a butterfly package or transistor outline (TO) package are usually used in QEPAS sensor system due to their low cost and compactness [47; 48; 49]. However, the output power of the diode laser is low, typically in the range of several milliwatts (\(<\)50 mW), which significantly limits the QEPAS sensor sensitivity. An optical fiber amplifier is usually used for optical signal amplification. With the advantage of high-power output (which easily reaches watt level), it is used extensively in optical communications. In a commercially available optical fiber amplifier, an optical fiber with a single-mode structure (and doped with rare-earth ions such as erbium and ytterbium) is used as the active medium. The fiber is pumped by diode lasers. The optical fiber amplifier usually used is an erbium-doped fiber amplifier (EDFA). EDFA has many obvious advantages, such as fiber compatibility, low noise, high gain, and polarization independence [50; 51]. When an appropriate seed diode laser is injected, an EDFA can be used to obtain an amplification gain of more than 30 decibel (dB). Three operating wavelength bands (L band: 1565-1610 nm, C band: 1520-1570 nm, and S band: 1450-1550 nm) of EDFA are commercially available.  
Recently, a QEPAS sensor system based on an EDFA-amplified diode laser was successfully used in acetylene (C\({}_{2}\)H\({}_{2}\)), hydrogen sulfide (H\({}_{2}\)S), and ammonia (NH\({}_{3}\)) detection [52; 53; 54]. In [52],  
Figure 2: Schematic of the QEPAS sensor. TA: transimpedance amplifier; mR: micro-resonator; ADM: acoustic detection module; L: plano-convex lens; FC: fiber collimator.  
a near-infrared, continuous-wave diode laser with emitting power of 6.7 mW served as the seed laser. The output of this laser was sent to an EDFA for power amplification. The maximum output power of the EDFA was 1500 mW. The emission spectra for the seed laser and the EDFA-amplified diode laser are shown in Figure 3. The amplified diode laser with high output power of 1500 mW had the same signal-to-noise ratio as the seed laser (\(\sim\)30 dB). Due to the fact that with high power intensity it is easy to attain high gain, compared to the seed laser, the line width of the amplified diode laser was compressed (see Figure 3a,b). Narrower line width represents better spectral resolution and is beneficial for improving the sensor's selectivity.  
The schematic of an EDFA-based QEPAS sensor is shown in Figure 4. After being amplified by an EDFA, the laser beam of a diode laser was injected into an acoustic detection module (ADM). For C\({}_{2}\)H\({}_{2}\), H\({}_{2}\)S, and NH\({}_{3}\) sensing, the detection limits of EDFA-QEPAS sensors were 33.2 ppb (parts per billion by volume) [52], 142 ppb [53], and 418.4 ppb [54], respectively. No signal saturation was observed in these investigations, which means that an EDFA with higher output power can be adopted to further improve the QEPAS sensor performance.  
Figure 4: EDFA-based QEPAS sensor system.  
Figure 3: Emission spectra for a seed laser and an EDFA-amplified diode laser.