Click to view Principle of cavity ring down technology - AOL Laboratory ⑥Utilize the Cavity ring down System, we successfully measured the absorption peak of 10ppm CH4 gas at 1653.7nm. At the same time, the cavity mirror refl...
Click to view Principle of cavity ring down technology - AOL Laboratory ⑥
Utilize the Cavity ring down System, we successfully measured the absorption peak of 10ppm CH4 gas at 1653.7nm. At the same time, the cavity mirror reflectivity calculated by cavity ring down time is consistent with the cavity mirror index, which further verifies the availability and reliability of the cavity ring down system.

The following figure shows the CRDS measurement structure of Xiaoxiao Photon: 1653 nm DFB laser diode Driven by Thorlabs ITC4005, the diode operating temperature is controlled at 40 ℃, and the current is 37~45mA. Connect the output laser to the AOM to turn off the laser input when the ring down is triggered, so as not to affect the ring down result. The adjustable collimator (CFC8A-C) is used to output the laser collimation space passing through the AOM. By adjusting the distance between the collimator and the resonator, as well as the distance between the lens and the fiber in the collimator, the waist radius and waist position after the laser collimation output conform to the theoretical calculation value.
The resonator is composed of a pair of high reflective mirrors with a curvature radius of 1m and a reflectivity of>99.99%. The rubber ring is used to seal the air chamber with a length of 0.5m. The air chamber has two input ports and one output port, which can be used to control the input and output of different gases. PD detector is used to detect the output optical signal and transmit it to the oscilloscope in real time. Set trigger out in the oscilloscope. When the PD detector detects that the voltage signal reaches the set threshold, it outputs a trigger signal to the AFG. An inverted pulse signal is preset in the AFG. Once triggered, it will output 0V level to the AOM Driver for 100us, and then return to 5V level again, waiting for the next trigger.Under the control of the 100us 0V level, The AOM Driver will not provide 100M RF signals to the AOM, The AOM will be in the closing stage. Use an oscilloscope to record this ring down waveform, which can average about 100 times. Save a waveform for fitting analysis.下图是37mA时(此时激光波长不在CH4的吸收峰上),腔衰荡信号的一次拟合分析,拟合函数为:y = a*exp(-x/t)+c,参数t就是拟合的衰荡时间。从图中可以看到,拟合的t为5.864us。由于我们使用的高反镜是1550nm中心的,在1653.7nm处的反射率达不到99.99%的指标,用5.864us的衰荡时间可以推出,1550nm高反镜在1653.7nm的发射率约为99.97%,符合1550nm高反镜在1653.7nm的反射率。为了达到更高的测量灵敏度,可以更换1650nm中心的 High mirror 。 However, the CH4 concentration measured this time is 10ppm, and 99.97% has an obvious absorption peak.Take 1mA as the measurement interval, collect the ring down curves one by one from 37 to 45mA (the absorption peak is within this current range as confirmed by direct absorption method in advance), fit the ring down time respectively, draw the absorption curve of CH4 as follows, and it can be seen that 10ppm absorption has been very obvious.