TDLAS Tunable Diode Laser Absorption Spectroscopy（ Tunable laser diode Spectral absorption).The theoretical basis of TDLAS technology is Beer Lambert law. When a laser beam passes through the measured gas, when t...
TDLAS Tunable Diode Laser Absorption Spectroscopy（ Tunable laser diode Spectral absorption).
The theoretical basis of TDLAS technology is Beer Lambert law. When a laser beam passes through the measured gas, when the laser wavelength is at a certain absorption spectral line position of the measured gas, the gas molecule will absorb photons and transition to the high energy level, which is shown by the gas absorption of the laser, making the laser power attenuate.
TDLAS technology makes use of the narrow linewidth and output wavelength of tunable semiconductor laser changing with current and temperature to scan some absorption peaks of gas molecules. Semiconductor laser used in this technology light source Its spectral width is far less than the broadening of gas absorption spectrum, so TDLAS technology is a high-resolution absorption spectrum technology.
CO2 has a group of dense absorption peaks near the position of 2004 nm. Therefore, the light source selected for this experiment is 2004 nm DFB Laser , used to scan the absorption peak of CO2.
The laser is installed in the control box, and its output pigtail is connected to the reflective collimator. The output light of the collimator is reflected by the golden mirror after 20 m optical path, and then returns to the parabolic mirror. The parabolic mirror focuses the laser on the focus, and the photodetector is placed at the focus to receive the optical signal.
The photoelectric detector converts the optical signal into an electrical signal and inputs it to the Oscilloscope 。 The temperature current wavelength tuning curve of the 2004nmDFB laser used in this experiment is:
At constant temperature, the output wavelength increases with the increase of LD current; When the LD current is fixed, the output wavelength increases with the increase of temperature.
The LD current is scanned from the Start point to the End point. In the scanning process, when the laser output wavelength passes through the CO2 absorption peak position in the scanning process, the absorption depression will appear.
Before finding the absorption peak, set the current range to a larger one, and then slowly narrow the current scanning range after finding the absorption peak. The tunable temperature range of the laser is 10-50 ℃. If the initial temperature is 25 ℃, the temperature range can be divided into two sections: 10-25 ℃ and 25-50 ℃. Slowly adjust the temperature to find the absorption peak within 10-25 ℃. If no absorption peak is found, the absorption peak may be within 25-50 ℃. After finding the absorption peak, slowly narrow the current scanning range.
The absorption peak is found by slowly adjusting the temperature control and scanning current range, and the absorption depression in the sawtooth signal is observed on the oscilloscope. After the absorption depression is found, the scanning range of temperature and current is not adjusted subsequently.
take Photodetector The sent electrical signal is connected to the PREAMP IN interface of the control box, DAC OUT and TRIG OUT are connected to the oscilloscope. On the premise of no signal distortion, increase the sine wave amplitude and gain multiple, and observe the second harmonic after demodulation.
In this experiment, reflective collimator and parabolic mirror are used. Parabola has an important property: the light emitted by the point light source at the focus of the parabola is reflected by the parabola, and the reflected light is parallel to the axis of symmetry of the parabola. After the optical fiber is connected to the reflective collimator, the light rays shoot from the focus to the paraboloid, and the outgoing light is parallel light. Thanks to the excellent optical properties of the parabola, the reflective collimator has a wide working range, and the paraboloid mirror can also focus the laser on the sensitive area of the photodetector.
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