Figure 3. Experimental results for 10-m-long HC-PBF.
(a) Second harmonic lock-in output (signal) when pump laser is tuned across the P(9) line of acetylene at 1,530.371 nm. (b) Second harmonic lock-in output when the pump wavelength is fixed to 1,530.53 nm. Black line: pump power is ∼15.3 mW. Red line: pump power is zero (off). The measurements were conducted consecutively for the two pump power levels but the results are displayed in the same panel for easy comparison. (c) Second harmonic signal and the s.d. of the noise as functions of pump power level. Error bars show the s.d. from five measurements and the magnitudes of the error bars are scaled up by 10-fold for clarity reason. The mean probe power level on PD2 is ∼50 μW. The gas concentration used is 10 p.p.m. acetylene balanced by nitrogen, and the experiments were conducted at room temperature. The amplitude of wavelength modulation was set to ∼2.2 times of the absorption linewidth. The time constant of the lock-in amplifier is 1 s with a filter slope of 18 dB Oct−1, corresponding to a detection bandwidth of 0.094 Hz.