Fig. 1. (a) Schematic of the microscale fiber photoacoustic spectrometer (FPAS). Inset: opto-acoustic-mechanical coupling inside the F–P cavity. The fiber end facet and the membrane are two reflective mirrors, which form the F–P cavity with the silica capillary. The acoustic wave generated during the relaxation of the excited gas molecules by the pump light is confined inside the cavity. The localized acoustic waves drive the membrane to vibrate, which induces the intensity change in the reflected probe light. (b) Signal demodulation scheme for the FPAS. (c) Photograph and (d) microscopic image of the FPAS. (e) SEM image of the fiber end facet with the membrane after the laser patterning. SMF, single mode fiber; PA, photoacoustic; 1f and 2f, the first- and second-harmonic signals.

Fig. 2. (a) Measured response of the FPAS to acoustic waves from 1 to 20 kHz and calculated frequency response based on the equivalent circuit using the lumped elements. The resonant frequency is located at 520 kHz, as observed from the measured noise floor in the absence of an applied acoustic wave. (b) NEP of the FPAS was measured at different frequencies from 1 to 20 kHz. Inset: frequency responses at 5, 10, and 15 kHz. At the frequency of 15 kHz, SNR is estimated to be 54 dB for an acoustic pressure of 280 MPa.

Fig. 3. (a) Schematic diagram of the experimental setup for photoacoustic gas sensing. The EDFA amplified pump laser at 1532.8 nm and the probe light at 1550 nm are combined by the coupler and delivered to the FPAS fiber tip. After filtering off the residual pump light, the probe light is received by a PD. The signal from the PD is analyzed by the lock-in amplifier (LIA). The pump light is wavelength-modulated by the laser controller. (b) Measured second-harmonic (2f) signals for FPASs with different cavity diameters. Inset: pk-pk value of the 2f signal as a function of the cavity diameter. Three samples are tested for each cavity diameter. (c) Relative pk-pk value of the 2f signal in the frequency range from 0.6 kHz to 1 MHz acquired from the experiment and the finite- element simulation. Inset: simulated local photoacoustic waves confined in the cavity through the simulation. (d) Pk-pk value of 2f signal for the pump light at different locations with a step size of 10μm. Inset: schematic of the 2f signal excitation with the pump light inside and outside the cavity. (e), (f) 2f signals for the pump light at locations as indicated by the dashed boxes in panel (d). DAQ, data acquisition unit; SMF, single-mode fiber.

Fig. 4. (a) Dependence of the second-harmonic (2f) signal and the noise floor on the pump power. Inset: 2f signals at different pump powers. (b) Pk-pk value of the 2f signal as a function of the gas concentration. Inset: 2f signals at different gas concentrations. (c) Measured 2f signal to a C2H2 gas concentration of 250 ppm. Inset: noise floor. (d) Temporal response as the C2H2 gas is switched on and off. Inset: response in the duration from 0.7 to 0.8 s.

Fig. 5. (a) Schematic of 2D mapping of the CO2 gas flow. (b) Image of the concentration distribution of the gas flow. (c) Spatial resolution of the FPAS as estimated from the edge-spread function (ESF) and the point spread function (PSF) curves. (d) Schematic of the procedure to monitor the fermentation process of the yeast solution. Photograph of the yeast solution in (e) a test tube and (f) a glass capillary. (g) Temporal variation of the CO2 concentration. The dashed line is the smooth curve with a window of 30 points to clearly show the concentration evolution. Inset: Peak value of the 2f signal for the CO2 released from ∼100nL yeast solution. Exp., experimental.

Fig. 6. (a) Schematic of in vivo monitoring of the dissolved CO2 gas in a rat tail vein. Inset: schematic of the FPAS inserted into the rat tail vein and photograph of the FPAS inserted in a 21G syringe needle. (b) Peak value of the 2f signal from the FPAS versus the concentration of the dissolved CO2 in the water. (c) Temporal response of the FPAS inserted into 5% CO2 dissolved water and the blood sample from the rat tail vein. (d) The bias, or difference of the paired CO2 concentration (CCO2), as a function of the mean value measured by the blood gas analyzer (BGA) and the FPAS. (e) Histogram of the CCO2 values measured by the BGA and the FPAS for seven rat tail blood samples. Real-time in vivo monitoring of the rat intravascular CCO2 in (f) the hypoxia conditions (5% and 10% O2) and (g) the hypercapnic conditions (5% and 10% CO2). The measured data are smoothed with a window of 30 points to show the dynamic concentration change more clearly.