Circuit design of a high scanning frequence and fast response laser methane remote sensor

To solve the problem of slow detection of laser methane sensors based on tunable diode laser absorption spectroscopy (TDLAS), a methane detection system with a high detection rate was proposed. Focusing on the hardware circuit design of the system which consists of a laser driver circuit, laser temperature control circuit, optical amplifier and filter circuit and main control circuit based on FPGA+ARM, this circuit is designed to achieve a laser scanning frequency of 50 kHz. A voltage-controlled current source is designed to drive the laser diode (LD). A TEC controller is designed to keep the operating temperature of the laser diode stable. Since the photocurrent can be very weak, to ensure the quality of signals from the photodiode (PD), a signal collection circuit consisting of the transimpedance amplifier, gain-controlled amplifier and low-pass filter was designed. An experiment is carried out to test system measurement capabilities. The results show the circuit can reach a response time of 2.5 ms while ensuring a measurement error of less than 4.3%.


Introduction
As an energy gas, methane is widely used in various fields, but it can be harmful as a flammable and explosive gas and one of the major greenhouse gases, which make it important to monitor methane in a certain situation [1] .For now, methane sensors mainly employ catalyst combustion, electrochemical, or semiconductor, which has some disadvantages such as slow response speed, short lifespan and low measuring range [2] .As a new type of methane sensor, tunable diode laser absorption spectroscopy (TDLAS) is an effective approach for the concentration of gases based on optics, avoiding the drawbacks of traditional methane sensors.TDLAS has the characteristics of high measurement accuracy, fast response speed, non-contact measurement and strong resistance to interference with non-target gases, which now is widely used in the fields of environmental protection and safety monitoring [3][4] .
Due to the good propagation characteristics of the laser, TDLAS technology can achieve a good application in the remote measurement of methane.With the development of laser technology, much research on methane remote sensor systems has been proposed in recent years.A large number of devices based on TDLAS have come out.However, most of the research focuses on improving the measurement distance and accuracy.Therefore the existing detection systems still have the disadvantage of low scanning frequency which is generally below 10 KHz, which limits the detection rate of the system [5][6][7][8] .In practical applications, for example, on-vehicle sensors, the laser spot can move very fast on reflectors, but the existing systems make it difficult to meet the high-speed gas detection needs.In order to improve the detection speed of the system, a new circuit was designed.It can achieve a laser scanning frequency of 50 kHz, which greatly improves the detection speed and broadens the application scene of the laser methane detection system.

Principle of as detection
TDLAS technology is based on Lambert-Beer law.When a laser beam of a certain wavelength passes through a uniform gas, its intensity decreases due to the absorption of the laser light by the gas molecules.The Lambert-Beer law quantifies the change in intensity of monochromatic light before and after it passes through the gas [9][10] .
where I0 is the initial light intensity before passing through the light.It is the intensity of the laser after passing through the gas.ν is the laser emission frequency.C is the concentration of the gas to be measured.L is the path length of the laser through the gas to be measured.a(ν) is the absorption coefficient of the gas to be measured and it can be expressed as follows.
where P is the gas pressure; S(T) is the intensity of characteristic spectral lines, which is determined by gas temperature; ϕ(υ) is a line function, which is related to the gas temperature and pressure.Its integral in the frequency domain is 1.The gas integral concentration can be expressed as: With temperature T, gas pressure P and the intensity of the gas absorption spectrum S (T), the gas integral concentration to be measured can be calculated.The schematic of the detection principles is shown in Figure 1.For laser detection of methane, the absorption intensity near the wavelength of 1653.7 nm is large and the rest of the interference gases such as H2O and CO2 in this band have much smaller absorption intensity than the methane, so the impact on methane detection can be ignored [11] .Therefore this circuit is designed based on a 1653 nm laser.

Hardware circuit design of the system
The main functions of the circuit are to drive the laser and control its current, amplify and filter the photocurrent signal from PD, collect it and transmit the concentration data to the host computer.The circuit needs to drive the laser to emit a sawtooth wave with a frequency of 50 kHz.High-speed DAC and ADC are used to complete the emission and collection of signals.Due to the requirement of a high detection rate, the circuit adopts FPGA (AG1280) as the main controller and ARM (STM32G071) as the assistant controller.FPGA is used to control the high-speed DAC and ADC and sends the raw data to ARM.The data is digitally filtered by ARM.FPGA and ARM are communicated via SPI.

Circuit design of power supply
As a measurement system, it requires a low power supply ripple and noise.According to the power supply requirements of the loads of the circuit, the system power supply structure is designed, as shown in Figure 2.An LDO regulator AP7361C is used to make a pretreatment to the power supply.The laser diode requires a large and stable driving current.A high-performance, 300mA LDO regulator VRH3601NTX is used.The operational amplifiers of the amplifying and filtering circuit require both positive and negative power supply, so a low noise power supply LTC3260 chip is used, which includes an inverting charge pump with both positive and negative LDO regulators.The ADC and DAC need a large operating current and are supplied by ADP123.A dual DC/DC converter TPS62410 which provides two independent output voltage rails is used to power the FPGA and MCU (channel 1) and TEC controller (channel 2).The core voltage for FPGA is supplied by a 1.2 V LDO LP3986.

Circuit design of LD driving
The circuit drives the current of LD as a sawtooth waveform with a frequency of 50 kHz.The accuracy of the sawtooth waveform will directly affect the signal quality and detection accuracy.An FPGA is utilized to generate voltage waveforms through a high-performance DAC 3PD5651E, which has a 10-bit resolution and a maximum sampling rate of up to 125 MSPS.The sawtooth generated by the DAC cannot directly drive the LD.Therefore, a voltage-controlled current source is used to drive the laser diode, which enables precise control of the current.The op-amp RS8751 with low input offset voltage is used.It has a low-input bias current and low noise, making it no influence impact on the signal.The schematic of the laser diode driving circuit is shown in Figure 3.The driving current can be expressed as:

Circuit design of LD temperature control
The wavelength of light emitted by the laser diode is highly affected by temperature, which can cause wavelength shifts.Therefore it is necessary to ensure that its operating temperature is stable.A Thermoelectric cooler (TEC) is used to regulate its temperature.By controlling the magnitude of current flowing through the TEC and controlling its cooling and heating power, the LD used in this circuit has an internal integrated thermistor and TEC.In order to ensure the precision of temperature control, a monolithic TEC controller ADN8834 is used, which has a small package, high-efficiency architecture, and two zero-drift, rail-to-rail op-amps integrated.ADN8834 thermal feedback loop can be constructed through the internal integration op-amps.The chopper 1 amplifier receives the feedback voltage from the thermistor, converts or regulates the input to a linear voltage output, and inputs it into the chopper 2 amplifier to compare with a temperature setpoint voltage.The Chopper 2 amplifier is used as the PID compensation amplifier.The ADN8834 achieves temperature stabilization through the TEC driving current control.The schematic of the TEC controller is shown in Figure 4.

Circuit design of amplifier and filter
The signal from PD is a current signal, which needs to be converted into a voltage signal that can be collected.Since the received photocurrent signal may be as low as tens of nanoamps, a large gain is required.The circuit needs to have a low noise to ensure signal accuracy.A double-stage amplifier circuit with a transimpedance amplifier and a controlled gain amplifier is used.A second-order low-pass filter is designed to eliminate high-frequency noise.

Transimpedance amplifier (TIA) circuit.
TIA amplifies the photocurrent signal as primary amplification and completes the current-voltage conversion.According to the design requirements, the signal frequency is 50 kHz.The op-amp needs to have a high bandwidth.Besides, the signal can be extremely weak.The circuit noise is strictly required.A High-speed precision op-amp with a 500 MHz bandwidth is used as the TIA op-amp, which has a very low input bias current and noise.It is suitable for the amplification circuit of weak photocurrent signals.The gain of TIA is 10 6 , the output voltage can be expressed as: The circuit schematic is shown in Figure 5 as PART A.

Gain-controlled amplifier circuit.
In practical applications, due to the different types and distances of reflectors, the strength of the received signals may have a large difference, so it is necessary to adjust the gain of the signal in real-time.A digital potentiometer MCP4018 and an op-amp RS8752XM are used to build a gain-controlled amplifier circuit.The MCP4018 has a maximum resistance of 10 KΩ with 128 steps selectable.The RS8752XM combines low power, low noise, high speed and high precision.The schematic is shown in Figure 5 as PART B. The value of the digital potentiometer and resistor R2 determines the gain.The ARM makes real-time adjustments on the digital potentiometer via I 2 C according to the received signal.The gain can be expressed as: where n is 1-128 controlled by the program.

Filter circuit.
The received signal will contain a variety of optical noise, mechanical noise and circuit noise.These noises generally have a high frequency, which is greatly harmful to the signal.Therefore, a second-order low-pass filter is designed to eliminate the effects of these high-frequency noises.The schematic is shown in Figure 6.The cut-off frequency is decided by R8, R9, C2 and C3.
According to the signal frequency of 50 kHz, by setting R8 = 1 K, R9 = 9.2 K, and C2 = C3 = 100 pF, the cut-off frequency can be calculated.
The gain of the filter is Besides, an adder is to superimpose a 1V reference voltage to the signal to ensure the integrity of the converted signal.

Circuit design of high-speed ADC sampling
In order to meet the requirements of high-speed and accurate acquisition of the circuit, a high-speed analog-to-digital converter MS9280 is used, which provides 10-bit accuracy at 50 MSPS data rates and guarantees no missing codes over the full operating temperature range.The voltage data collected by the MS9280 is transmitted in parallel to the FPGA, which receives and processes the data and transmits it to the ARM for concentration calculation.The circuit schematic is shown in Figure 7.

Measurement results and processing
PD, LD and their optical components are connected to the circuit board for experimentation.The reflector is set as ordinary printing paper at a distance of 10 m.Gas clusters are modeled by using N2 and CH4 gas mixtures in gas bags including 2000 ppm•m, 4000 ppm•m and 6000 ppm•m.The gas bag is fixed.The detection system is rotated so that the light path can sweep through the gas bag at a uniform speed.The speed is 9.5 m/s.The width of the gas bag is 0.4 m.The time for the optical path to pass through the gas bag is 42 ms.The measured data are shown in Figure 8.
As the system measures the concentration in the area covered by the laser beam spot when the gas bag enters and leaves the optical path, the measured concentration will slowly rise and fall for the light spot cannot fully cover the airbag.Taking the data when the measured value is stable, it can be seen that the measurement errors of the system at 2000 ppm•m, 4000 ppm•m and 6000 ppm•m are 4.3%, 3.5% and 3.2%.The measurement points are set where the measured concentration is greater than 90% of the target concentration to be the valid points.It takes a response time of less than 2.5 ms to rise from 10% to 90% of the target concentration.

Conclusion
In this paper, given the slow detection rate of the current methane telemetry system, the circuit design of a methane detection system with a 50 kHz laser scanning frequency was completed.The use of a high-precision DAC and voltage-controlled current source achieves a high-precision laser current driving.The TIA circuit, controlled gain amplification and low-pass filter are designed.The sampling is completed by a high-precision ADC to ensure the accuracy of the signal.The use of FPGA and ARM meets the requirements of the system for high-speed acquisition and processing of signals.The circuit designed can achieve a high detection accuracy and response speed and also can be applied to situations with high-speed detection needs.