Time of flight light detection and ranging (TOF LiDAR) is the main detection technology solution used in the mainstream LiDAR products at present. Smog has a certain interference effect on the laser, which affects the ranging accuracy of TOF LiDAR system. This paper focuses on the accuracy of TOF LiDAR ranging under smog conditions and the error analysis. A vertical cavity surface emitting laser is selected to emit a pulsed laser through a smog-filled smog chamber, a CMOS sensor is used to receive the reflected light, and the experimental data are collected using the host computer software. The collected data are then extracted from the effective area and processed with algorithms such as average filtering. Finally, the image of each pixel with the measured distance and the error situation are obtained. The effect of smog is simulated by increasing the system delay time to obtain the simulated image under smog conditions. The results obtained from the TOF LiDAR smog experiment are compared with the simulation results, and it can be seen that the smog has a significant attenuation effect on the laser transmission, with the attenuation ratio up to 63.99%, and the range error under the smog condition decreases by 53.55%, and the accuracy of the range measurement is improved.
Reducing carbon emissions is a global priority due to human impact on atmospheric pollution and the greenhouse effect. Achieving carbon peak and neutrality requires real-time monitoring of CO2 concentrations. However, developing high-sensitivity, portable, and anti-jamming gas detection solutions is challenging. Among spectroscopic techniques, Tunable Diode Laser Absorption Spectroscopy (TDLAS) is highly sensitive for detecting CO2 concentrations. This paper elaborates on the principles of TDLAS for detecting CO2 concentrations and proposes a noise reduction algorithm to meet diverse environmental requirements. Simulations were performed using software to simulate CO2 absorption spectra at approximately 1.57866535μm under high-intensity noise (0.1mW - 1mW). Based on this simulation, we applied the Wavelength Modulation Spectroscopy (WMS) technique to calculate the ratio of the output differential signal's second harmonic intensity to the first harmonic S2f/1f(T) and output power to reduce light intensity influence and improve concentration inversion linearity. The weighted convolutional moving average filtering was utilized to optimize WMS denoising, utilizing weight transfer to make the process more precise and reliable. After analyzing various window functions, it was concluded that a window length of 9 would be the most optimal. The algorithm improved the signal-to-noise ratio (SNR) by 22.435% under these conditions. When the noise level increased fourfold from the original signal, the algorithm enhanced the SNR by 59.514%, enabling reliable CO2 monitoring even under challenging conditions.
Wavelength-swept laser is one of the most critical common components of fiber Bragg grating (FBG) sensors. However, a fast, stable, integrated and low-cost multi-channel wavelength-swept laser array is still unavailable. In this article, a multi-channel simultaneous wavelength-swept DFB laser array based on the reconstruction-equivalent-chirp (REC) technique is proposed and manufactured. The REC technology simplifies the fabrication process and greatly reduces the cost of the laser. The laser array contains 4 lasers in parallel with integrated heating resistance for thermal tuning to broaden the wavelength-tuning range. Each single-wavelength DFB laser introduces a π-phase shift structure, which is used to improve the single-mode performance of the laser. Meanwhile, the active multiplexer responsible for coupling and an optical amplifier (SOA) responsible for compensating the coupling loss of the laser are also integrated on the same chip. The driving circuits of the laser use FPGA to control the DAC chip to obtain precise current output and realizes continuous linear output of wavelength by changing the injection current size. The packaged laser module can realize a continuous wavelength-swept range of 2.4 nm per channel with the SMSR over 40 dB, achieving a simultaneous sweep of four channels with good scanning linearity and scanning speed. The work of this paper realizes the integration of linear wavelength-swept light sources, which creates the conditions for a low-cost, small-volume multi-channel sensing system in the future.
A weak double-peak fiber Bragg grating (FBG) temperature sensor is proposed and demonstrated. Wavelength-swept tunable laser is regarded as one of the most popular demodulation methods for fiber Bragg grating (FBG) sensors. However, due to the limitations of the existing tunable laser technologies, a fast, compact, stable and low-cost tunable laser for FBG sensors is still unavailable, which will become one of the major barriers for more widespread applications of FBG sensors. To further improve the efficiency and accuracy of the FBG interrogation system, a FBG temperature sensor is proposed and demonstrated by using tunable laser and a weak double-peak FBG. Since the reflection of the weak double-peak FBG has two main reflection peaks and relatively wide bandwidth, it is convenient to track the two characteristic peaks to accurately obtain the wavelength shift during the alteration of ambience temperature. A proof-ofconcept experiment is also conducted to verify the theory. By demodulating a weak double-peak FBG in the temperature experiment, a sensor sensitivity of 10.17 pm/ °C is measured for the proposed interrogation system.
A multi-longitudinal mode (MLM) laser sensor system based on software radio demodulation is proposed, which realizes the simultaneous measurement of vibration and temperature. The software demodulation is realized through the SDR (Software Defined Radio) technique due to the applied vibration can be analyzed as the superposition of a series of frequency modulation (FM) signals. Furthermore, the absorption of erbium-doped fiber (EDF) decreases with the increase of temperature, so the output power of the laser can be measured as another sensing signal. As a result, by demodulating these two sensing signals, the vibration from 10 Hz to 2 kHz can be successfully measured and the sensitivity of the temperature is around 0.011 dBm/°C.
An innovative demodulation system for multilongitudinal mode fiber laser sensor has been proposed. By using a bandpass filter and a low-speed analog-to-digital converter (ADC), the high-frequency sensing signal can be downconverted and sampled simultaneously when the unaliasing condition is satisfied. Since the MLM fiber laser sensor could generate a wideband electrical signal after optical-to-electrical conversion, it is convenient to filter the signal to meet the unaliasing condition by a tunable bandpass filter while keeping the sample rate unchanged. Moreover, each tone of the beating frequency signal has the full information for demodulation of measurand. The demodulation system only needs a bandpass filter and a low-speed ADC which reduces the cost of the system and make the system more stable. A proof-of-concept experiment is conducted to verify the proposed scheme. Eventually by demodulating a beat frequency in 1.625GHz, a sensitivity of -5.87kHz/°C is achieved in a fiber laser sensing system with a sample rate of 500MHz.