Ⅰ Theoretical basis

1. Beer-Lambert law

When a laser beam passes through a gas with a concentration of C, when the wavelength of the laser is the same as the center frequency of a certain absorption line of the gas being measured, the gas molecules will absorb photons and transition to a high energy level, which is manifested as the attenuation of the laser light intensity in the gas absorption band.

2. Wavelength modulation spectroscopy technology

A) Tuning characteristics of lasers

DFB lasers have good monochromaticity, narrow linewidth characteristics and frequency tuning characteristics. DFB lasers can well avoid cross-interference from other background gases, so that the detection system has good measurement accuracy, and are therefore widely used in gas detection.

B) Harmonic detection theory

By adding a high-frequency sinusoidal voltage signal to the driving voltage of the laser, the current is changed, and the output frequency also changes according to the sinusoidal law. By adding a sawtooth voltage to the laser drive, the output wavelength is scanned on both sides of the gas absorption peak, and the harmonic signal is modulated and demodulated using a phase-locked amplifier to measure the gas concentration.

3. Principles of absorption spectrum selection

When performing gas detection, the selection of absorption spectrum is very critical, and the following aspects should be considered

 A) The gas should have a strong absorption peak at the selected spectrum line.

 B) The laser light source technology corresponding to the spectrum line wavelength should be relatively mature.

 C) There is no interference from background gas absorption at the selected absorption spectrum line, or the absorption is relatively weak and can be ignored.

Ⅱ Theoretical basis

1. 1512nm laser

The 1512nm DFB laser launched by Idealphotonics adopts a standard 14PIN butterfly package. The built-in isolator and TEC enable the laser to effectively reduce the impact of drive current, temperature and optical feedback-related factors on the wavelength, making the wavelength more stable. Its good high side mode suppression ratio, high power stability and other characteristics are very suitable for ammonia gas concentration detection and analysis.

 Spectral

 Wavelength Temperature Current Tuning Curve

2. TDLAS laser gas detection integrated control box

This product is a control module for tunable semiconductor laser absorption spectroscopy technology (tunable semiconductor laser absorption spectroscopy technology (TDLAS)). The main functions include: digital laser drive that generates sine wave and triangle wave superposition, adjustable gain, adjustable gain amplifier, 1f/2f digital lock-in amplifier, analog output temperature control unit. The operating parameters and waveforms can be controlled and read by the computer.

 3. 10-meter optical path ultra-small gas absorption cell

The 10-meter optical input and electrical output gas absorption cell is used for various gas spectral analysis and detection. The optical path structure of the gas cell uses the flat wave gas cell (SlimBoss Gas Cell) with excellent optical stability and an auxiliary and high-stability optical packaging structure, which is mainly composed of the gas cell cavity, reflector, standard optical fiber connector, gas inlet and outlet, and shockproof base. The unique suspended optical path design has excellent vibration and temperature stability, can work stably in various complex environments, and is very suitable for online real-time detection of various gases. It has low system noise and can be used for trace gas analysis.

Ⅲ Experimental testing

Operation steps:

1. Connect the power supply and USB cable to the TDLAS laser gas analysis integrated control box

2. Connect the optical fiber input end of the 10-meter optical path ultra-small gas absorption cell to the LASER OUT laser output end of the control box, and the other end to the input end of the detector

3. Use an SMA-BNC line to connect the detector and the PREAMP preamplifier end of the TDLAS laser gas analysis integrated control box

4. Use a BNC-BNC line to connect the oscilloscope and the DACOUT analog output end of the TDLAS laser gas analysis integrated control box

5. Use a BNC-BNC line to connect the oscilloscope and the TRIG OUT trigger end of the TDLAS laser gas analysis integrated control box

6. Turn on the laser and connect the detector power supply

7. 100ppm NH3 was introduced into the gas chamber, the software parameters were adjusted, and the second harmonic signal waveform, amplitude and other information were observed on the oscilloscope.

Process analysis: The control software on the computer was used to adjust the current and temperature to tune the wavelength, so that the laser could scan a certain wavelength range and the output wavelength covered the absorption peak of the gas. The phase-locked amplifier provided a high-frequency sinusoidal modulation signal, so that the laser output frequency was sinusoidally modulated. The light emitted by the laser passed through the gas absorption cell and entered the preamplifier circuit at the PREAMP end through the detector. It was then modulated and demodulated by the phase-locked amplifier and sent to the oscilloscope channel 2 through the DAC OUT analog output end to display the second harmonic signal. During the whole process, we adjusted various parameters in the software and observed the output waveform at the same time to optimize the output waveform.

A) Test results

1. The second harmonic waveform and modulation parameters are as follows:

 Second Harmonic

Software modulation parameters

 2. Verification analysis:

By querying the Hitran database, the absorption lines in the wave number range of 6611.5cm-1 to 6614cm-1 are as follows:

3. Experimental conclusion:

Through testing, we found that when the NH₃  concentration is 100ppm, the second harmonic amplitude can reach 424mV, which shows that our TDLAS analysis system has high test accuracy and good test effect.

Ordering Information

Product model: TDLAS-1512-NH3

Product name: Near infrared TDLAS NH3 ppm level concentration analysis system

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