A continuous QCL driver based on DSP for CO and N2O detection

Abstract

A kind of continuous QCL driver based on DSP CO and N2O detection, including DSP master unit, power supply system, drive waveform generation circuit, voltage-controlled constant current source circuit, overcurrent protection circuit, current detection and optimization control circuit and temperature control circuit;DSP master unit is used to control DAC and the signal required for DDS to generate drive waveform, while controlling ADC to carry out analog-digital conversion, and integrated Fuzzy-PID algorithm is optimized to output drive current;Drive waveform generation circuit generates drive voltage signal with two modulation depths, improves the lower limit of the detection of two gases;Current detection and optimization control circuit uses ADC to sample drive voltage signal, to ensure the stability of QCL laser wavelength.The present application improves the situation that only one kind of gas absorption can be optimized when multiple gas detection, ensures high signal transmission accuracy, and achieves more accurate current control effect.

Description

Technical Field

[0001] This invention relates to a driver for a continuous QCL (quantum cascade laser) for detecting CO and N2O in the atmosphere, belonging to the field of CO and N2O detection technology. Background Technology

[0002] Nitrogen gas (N2O) is a crucial greenhouse gas. While its average concentration in the atmosphere is low, its greenhouse effect is significant. Furthermore, N2O exhibits no obvious degradation mechanism in the troposphere and has a long lifetime. Copper (CO) is also a major source of global warming and air pollution, a key parameter in atmospheric observation, and is considered an indirect greenhouse gas. Therefore, researching detection technologies for CO and N2O in the atmosphere is of great importance.

[0003] Quantum cascade lasers (QCLs) are novel unipolar semiconductor devices based on the principles of electron interband transitions and phonon-assisted resonant tunneling within semiconductor quantum wells. Unlike the electron-hole recombination stimulated emission mechanism of traditional PN junction semiconductor lasers, the stimulated emission process in QCLs involves only electrons, and the selection of the lasing wavelength can be achieved through bandgap trimming of the potential well and barrier in the active region. QCLs have spearheaded a revolution in semiconductor laser theory and mid-infrared and THz semiconductor light sources, making them ideal light sources for trace gas monitoring and free-space communication.

[0004] Quantum cascade lasers are mid-infrared monopole light sources based on inter-subband electronic transitions, and their operating principle is quite different from that of conventional semiconductor lasers. Their lasing scheme utilizes the separated electronic states caused by quantum confinement effects within a semiconductor heterojunction layer perpendicular to its nanometer-thickness. Population inversion is generated between these excited states. The active region of this laser consists of multiple cascaded stages of coupled quantum wells (typically more than 500 layers), achieving multiphoton output through single-electron injection.

[0005] The emergence of quantum cascade lasers (QCLs) pioneered the development of mid- and far-infrared semiconductor lasers using wide-bandgap materials, marking a new milestone in the history of mid- and far-infrared semiconductor laser development. The first mid-infrared QCL capable of continuous operation at room temperature was successfully developed in 2002, and its emergence greatly promoted the practical application of mid-infrared quantum cascade lasers (MIR-QCLs).

[0006] The luminescence characteristics of a laser are closely related to the detection limit, sensitivity, and operational stability of a detection system. Therefore, developing a high-performance QCL driver is beneficial for the stable and efficient operation of the laser, thereby ensuring the performance indicators of the entire detection system. Generally, for different operating modes, there are two types of QCLs: pulsed drivers and continuous drivers. The former has a current pulse duration of several microseconds or even several nanoseconds, while the latter typically has a driving current duration of milliseconds or more.

[0007] While commercially available QCL drivers offer excellent performance and ease of use, their generally large size and high cost hinder miniaturization and integration into trace gas detection systems. Many researchers have developed smaller, single-board drivers to meet the needs of their detection systems. In 2014, Liu Fei et al. designed a high-precision QCL driver for a micro laser-driven UAV, incorporating a feedback voltage-controlled constant current source module and a protection circuit module. In the same year, Zheng Mingjie designed a QCL driver for a non-contact gas identification electronic nose system, including a DSP-based control module and a negative feedback voltage-controlled constant current source module, and used a PID control algorithm to stabilize the current. Compared to commercially available drivers, these drivers have slightly fewer features, but their miniaturization, low cost, and suitability for their specific system performance requirements make them easier to integrate and more practical.

[0008] Currently, there is an urgent need in the market for a miniaturized driver that integrates protection for the QCL and provides a highly stable drive current. Summary of the Invention

[0009] This invention addresses the shortcomings of existing QCL driver technology by providing a DSP-based continuous-flow QCL driver for CO and N2O detection. This driver meets the measurement requirements for CO and N2O, protects the QCL (quantum cascade laser), and provides a highly stable drive current. It is used for detecting CO and N2O in the atmosphere.

[0010] The DSP-based CO and N2O detection continuous QCL driver of the present invention adopts the following technical solution:

[0011] The QCL driver includes a DSP main control unit, a power supply system, a drive waveform generation circuit, a voltage-controlled constant current source circuit, an overcurrent protection circuit, a current detection and optimization control circuit, and a temperature control circuit.

[0012] The DSP main control unit is used to control the digital-to-analog converter (DAC) and direct digital frequency synthesizer (DDS) to generate the signals required for the drive waveform, while controlling the analog-to-digital converter (ADC) to perform analog-to-digital conversion, and integrating a Fuzzy-PID control algorithm to optimize the output drive current;

[0013] The power supply system is used to supply power to various circuits and has soft-start protection. It provides soft-start protection for the voltage-controlled constant current source circuit to reduce damage to the QCL laser during power-on and power-off.

[0014] The driving waveform generation circuit is used to generate driving voltage signals with two modulation depths, so that the absorption modulation coefficients m of CO and N2O are both 2-2.4, thereby improving the detection limits of the two gases.

[0015] A voltage-controlled constant current source circuit is used to convert a drive voltage signal into a current signal using the principle of deep negative feedback, and output a highly stable drive current.

[0016] The overcurrent protection circuit, driven by a high-side drive, cuts off the circuit at a nanosecond speed when an overcurrent occurs, ensuring the safety of the QCL laser.

[0017] The current detection and optimization control circuit uses an analog-to-digital converter (ADC) to sample the drive voltage signal and optimizes it using an adaptive Fuzzy-PID algorithm in the DSP to ensure that a highly stable drive current can be output under different environments, thereby ensuring the stability of the QCL laser wavelength.

[0018] The temperature control circuit performs high-precision temperature control on the built-in semiconductor cooler (TEC) of the QCL laser to ensure the temperature stability of the QCL laser during operation.

[0019] The key features of this invention are the dual-modulation drive waveform generation circuit, the nanosecond-level overcurrent protection current, and the current optimization control based on Fuzzy-PID.

[0020] The power supply system includes:

[0021] An AD-DC converter converts 220V AC voltage into 24V DC voltage.

[0022] The LDO linear regulator module with soft-start function steps down the 24V DC voltage to a lower ripple 18V DC voltage and applies it directly to the two ends of the QCL laser, and eliminates power spikes at the power-on and power-off points, thus protecting the QCL laser.

[0023] The DC-DC switching power supply module steps down the 24V DC voltage to 5V and 12V DC voltage.

[0024] The driving waveform generation circuit includes:

[0025] The DAC (Digital-to-Analog Converter) is connected to the DSP (Digital-to-DSP) main control unit and is used to generate the DC voltage signal and low-frequency sawtooth wave signal required by the constant current source.

[0026] The DDS direct digital frequency synthesizer, connected to the DSP main control unit, is used to generate two high-frequency sine wave signals;

[0027] The digital potentiometer accepts two high-frequency sine wave signals generated by the DDS and generates two sine wave signals with different amplitudes.

[0028] A high-speed multiplexer is used to select and output two sinusoidal signals with different amplitudes.

[0029] The adder adds two sinusoidal signals with different amplitudes after passing through a high-speed multiplexer to a low-frequency sawtooth wave signal generated by a DAC, producing a drive signal with dual modulation depth.

[0030] The constant current source circuit and overcurrent protection circuit include:

[0031] A high-precision operational amplifier controls the MOSFET gate voltage by receiving feedback current voltage, enabling the current stability of the constant current source to reach the nanoampere level.

[0032] MOSFET, a high-power enhancement-mode N-channel metal-oxide-semiconductor field-effect transistor, has its gate voltage controlled by an operational amplifier to enable the constant current source to operate in the constant current region.

[0033] The sampling resistor is a 2Ω high-precision metal film non-inductive resistor.

[0034] Transient voltage suppressor diode (TVS): When static electricity occurs, ultra-high voltage will quickly break it down, and the current will directly pass through its branch to ground, thus protecting the QCL laser.

[0035] Schottky diodes release surge voltage on CW-QCL and absorb reverse current;

[0036] Tantalum capacitors eliminate the inductive effect of CW-QCL and prevent voltage surges in the PN junction of QCL lasers;

[0037] A buffer resistor reduces the input impedance of the MOSFET gate, thereby reducing oscillation.

[0038] The current sensing amplifier obtains the current magnitude by detecting the voltage difference across the sampling resistor, amplifies it by 10 times, and then sends the signal to the voltage comparator.

[0039] The voltage comparator generates an output signal by comparing the input voltage and a threshold voltage. If the input voltage is greater than the threshold voltage, it outputs a 5V high level; otherwise, it outputs a 0V low level. Its rail-to-rail output directly drives CMOS or TTL levels.

[0040] Operational amplifiers are used in voltage follower circuits to ensure that the input and output voltages are consistent.

[0041] The high-side drive unit controls the on / off state of the constant current source by controlling the switching of the switching transistor. It consists of an NPN transistor, a switching MOSFET, and a pull-up resistor. When an overcurrent occurs, the constant current source is turned off; otherwise, the MOSFET is turned on normally and the constant current source works normally.

[0042] The sampling resistor is used to measure the voltage difference and determine whether an overcurrent has occurred. A 100mΩ metal film non-inductive resistor is selected.

[0043] The current detection and optimization control circuit (Fuzzy-PID controller) includes:

[0044] The fuzzy control module consists of fuzzification, fuzzy inference, defuzzification, and a rule base. It uses the error quantity e(t) and the error rate of change ec as input linguistic variables, and the parameter change ΔK as the input linguistic variable. P ΔK I and ΔK D As the output of the fuzzy controller, the K of the PID controller P K I K D The three parameters are dynamically tuned;

[0045] A PID controller is used to optimize the output current.

[0046] QCL G1(s), as the controlled object, is considered as the optimized output drive current.

[0047] The aforementioned Fuzzy-PID controller, compared to the traditional PID controller, incorporates fuzzy control theory, leveraging its robustness and excellent dynamic characteristics. This allows the PID parameters to adapt to different current ranges, achieving more precise current control.

[0048] The basic working principle of the QCL driver described above is as follows:

[0049] The DSP main control unit controls the DAC and DDS to generate a low-frequency sawtooth wave and two high-frequency sine waves. These two sine waves are added to the low-frequency sawtooth wave after passing through a digital potentiometer and a high-speed multiplexer, outputting a drive voltage signal with dual modulation depth. This signal enters the voltage-controlled constant current circuit and passes through a high-precision sampling resistor to ensure the accuracy of the output drive current. The negative feedback circuit structure ensures that the output current remains in a highly stable state. The nanosecond-level overcurrent protection circuit uses a current-sensing amplifier to detect the voltage across the sampling resistor. After passing through a high-speed comparator, the signal is transmitted to the high-side drive unit, which then controls the switching of the switching transistor to control the on / off state of the constant current source, achieving nanosecond-level high-speed overcurrent protection. The Fuzzy-PID control algorithm allows the PID parameters to adapt to different current ranges, achieving more precise drive current control. In the temperature control circuit, a temperature control chip integrating a PI control algorithm injects current into the TEC based on the temperature value returned by the thermistor to achieve precise temperature control.

[0050] This invention employs a dual-modulation drive signal generation circuit to ensure that the absorption spectra of both CO and N2O gases are at their optimal modulation depth. It introduces a nanosecond-level overcurrent protection circuit that can change the threshold current. With "high speed" as the core design principle, it uses high-side drive technology to enable the constant current source circuit to quickly cut off the circuit to protect the laser when an "overcurrent" phenomenon occurs. To achieve more precise current control, Fuzzy-PID composite control theory is introduced.

[0051] This invention has the following characteristics:

[0052] (1) By using segmented modulation drive signals with two tuning depths, the absorption modulation coefficients of CO and N2O gases are both around 2.2, which improves the situation where only one gas absorption can be optimized when detecting multiple gases.

[0053] (2) The nanosecond-level overcurrent protection circuit uses a high-speed operational amplifier, a high-speed comparator, a high-speed current detection amplifier and a high-side drive unit to ensure high signal transmission accuracy and short propagation delay time, so that the constant current source circuit can quickly cut off the circuit to protect the laser when an "overcurrent" phenomenon occurs.

[0054] (3) The Fuzzy-PID control algorithm combines fuzzy control theory and PID algorithm, so that the PID parameters can adapt to different current ranges and achieve a more accurate current control effect. Attached Figure Description

[0055] Figure 1 This is a schematic diagram illustrating the composition principle of the DSP-based CO and N2O detection continuous QCL driver of the present invention.

[0056] Figure 2 This is a schematic diagram of the power supply system in this invention.

[0057] Figure 3 This is a schematic diagram of the driving signal generation circuit with dual modulation depth according to the present invention.

[0058] Figure 4 This is a schematic diagram of the constant current source circuit and nanosecond-level overcurrent protection circuit in this invention.

[0059] Figure 5 This is a block diagram of the current detection and optimization control circuit (Fuzzy-PID controller) in this invention.

[0060] Figure 4 In the middle: 1. High-precision operational amplifier, 2. MOSFET, 3. Sampling resistor, 4. Transient voltage suppressor diode (TVS), 5. Schottky diode, 6. Tantalum capacitor, 7. Buffer resistor, 8. Current sense amplifier, 9. Voltage comparator, 10. Operational amplifier, 11. High-side drive unit, 12. Sampling resistor. Detailed Implementation

[0061] In this invention, a continuous quantum cascade laser (CW-QCL) is selected to meet the measurement requirements of CO and N2O. This type of laser is expensive, requires a large driving current, and its wavelength is easily affected by heat generation, leading to wavelength drift. To protect the laser and provide a highly stable driving current, a DSP-based driver for the CW-QCL is designed.

[0062] like Figure 1 As shown, the DSP-based CO and N2O detection continuous QCL driver of this invention includes a DSP main control unit, a power supply system with soft-start protection, a drive signal generation circuit with dual modulation depth, a voltage-controlled constant current source circuit, a nanosecond-level overcurrent protection circuit, a current detection and optimization control circuit, and a high-precision temperature control circuit. In this invention, the DSP main control unit, as the core of the QCL driver, controls the digital-to-analog converter (DAC) and direct digital frequency synthesizer (DDS) to generate the signals required for the drive waveform, simultaneously controls the analog-to-digital converter (ADC) to perform analog-to-digital conversion, and integrates a Fuzzy-PID control algorithm to optimize the output drive current. The following provides a detailed description of each part.

[0063] 1. DSP main control unit

[0064] The DSP, as the main control unit, is the core of the entire QCL driver. It controls the digital-to-analog converter (DAC) and direct digital frequency synthesizer (DDS) to generate the signals required for the drive waveform, and integrates a Fuzzy-PID algorithm to optimize the control of the output current.

[0065] The DSP main control unit controls the DAC and DDS to generate a low-frequency sawtooth wave and two high-frequency sine waves. These two sine waves are then added to the low-frequency sawtooth wave after passing through a digital potentiometer and a high-speed multiplexer, outputting a drive voltage signal with dual modulation depth. This signal enters the voltage-controlled constant current circuit, where a high-precision sampling resistor ensures the accuracy of the output drive current. The negative feedback circuit structure ensures the output current remains highly stable. The nanosecond-level overcurrent protection circuit uses a current-sensing amplifier to detect the voltage across the sampling resistor. After passing through a high-speed comparator, the signal is transmitted to the high-side drive unit, which then controls the switching of the switching transistor to control the on / off state of the constant current source, achieving nanosecond-level high-speed overcurrent protection. The Fuzzy-PID control algorithm allows the PID parameters to adapt to different current ranges, achieving more precise drive current control. In the temperature control circuit, a temperature control chip integrating a PI control algorithm injects current into the TEC based on the temperature value returned by the thermistor to achieve precise temperature control.

[0066] 2. Power supply system

[0067] The power supply system mainly supplies power to various circuits and can provide soft-start protection for the constant current source circuit, reducing damage to the laser during power-on and power-off. The drive waveform generation circuit can generate drive voltage signals with two modulation depths, making the absorption modulation coefficient m of CO and N2O about 2.2 (2-2.4), thus improving the detection limit of the two gases.

[0068] like Figure 2 As shown, the power supply system includes the following parts:

[0069] AD-DC converts 220V AC voltage to 24V DC voltage;

[0070] An LDO (Linear Regulator Module) with soft-start function steps down a 24V DC voltage to a lower ripple 18V DC voltage, which is then applied directly to the laser. It also eliminates power spikes during voltage increases and decreases, thus protecting the laser.

[0071] DC-DC, a switching power supply module, steps down 24V DC voltage to 5V and 12V DC voltage, with higher efficiency and greater power, to supply power to other parts of the entire system.

[0072] In this invention, at the front end of the power supply system, an AC-DC power module is used to convert 220V AC voltage into 24V DC voltage. The back end of the power supply is divided into two parts: one part is a linear regulator (LDO) module with soft-start function, which steps down the 24V to a 18V DC voltage with lower ripple and directly applies it to both ends of the laser; the other part is a switching power supply (DC-DC) module, which steps down the 24V to 5V and 12V, with higher efficiency and greater power, to supply power to the other parts of the entire system.

[0073] 3. Dual-modulation-depth drive signal generation circuit

[0074] The dual-modulation drive signal can make the absorption modulation coefficients of both CO and N2O reach about 2.2, thus achieving simultaneous optimization of the absorption of two gases in multi-gas detection.

[0075] like Figure 3 As shown, the dual-modulation waveform generation system includes the following components:

[0076] The DSP, as the main control unit, controls the DAC and DDS to generate the signals required for the drive waveform;

[0077] A DAC (Digital-to-Analog Converter) generates the DC voltage signal and low-frequency sawtooth wave signal required by the constant current source.

[0078] DDS, a direct digital frequency synthesizer, generates two high-frequency sinusoidal signals;

[0079] The digital potentiometer accepts two high-frequency sine wave signals generated by the DDS and generates two sine wave signals with different amplitudes.

[0080] A high-speed multiplexer allows two sinusoidal signals with different amplitudes to be selected and output back and forth after passing through the high-speed multiplexer;

[0081] The adder adds two sinusoidal signals with different amplitudes after passing through a high-speed multiplexer to a low-frequency sawtooth wave signal generated by a DAC, producing a drive signal with dual modulation depth.

[0082] The DSP main control unit controls the DAC to generate the DC voltage signal and low-frequency sawtooth wave signal required for the constant current source, and controls the DDS to generate two high-frequency sine wave signals. After receiving the two high-frequency sine wave signals generated by the DDS, the digital potentiometer generates two sine wave signals with different amplitudes. After passing through a high-speed multiplexer, the output can be selected back and forth. The two sine wave signals with different amplitudes after passing through the high-speed multiplexer are added to the low-frequency sawtooth wave signal generated by the DAC to generate a drive signal with double modulation depth.

[0083] 4. Constant current source circuit and overcurrent protection circuit

[0084] The current source circuit utilizes the principle of deep negative feedback to convert the drive voltage signal into a current signal, outputting a highly stable drive current. The overcurrent protection circuit is another core component of the driver. It employs high-side drive technology to cut off the circuit at nanosecond speeds when an overcurrent occurs, ensuring the safety of the laser.

[0085] like Figure 4 As shown, the voltage-controlled constant current source and nanosecond-level overcurrent protection include the following parts:

[0086] The high-precision operational amplifier 1 controls the gate voltage of the MOSFET by receiving feedback current voltage, so that the current stability of the constant current source reaches the nanoampere level;

[0087] MOSFET2, a high-power enhancement-mode N-channel metal-oxide-semiconductor field-effect transistor, has its gate voltage controlled by an operational amplifier to enable the constant current source to operate in the constant current region.

[0088] The sampling resistor 3 is crucial because its precision determines the degree of change in the feedback voltage, significantly impacting the output current accuracy of the constant current source. Furthermore, since continuous-current drivers operate under high current conditions for extended periods, the sampling resistor requires high precision; therefore, a 2Ω high-precision metal film non-inductive resistor is selected.

[0089] Transient voltage suppressor diode (TVS)4: When static electricity occurs, ultra-high voltage will quickly break it down, and the current will directly pass through its branch to ground, which can effectively protect the laser's safety.

[0090] Schottky diode 5 releases surge voltage on CW-QCL and absorbs reverse current;

[0091] Tantalum capacitor 6 has good high-frequency characteristics, which can eliminate the inductive effect of CW-QCL and prevent voltage surges in the laser PN junction;

[0092] The buffer resistor 7 reduces the input impedance of the MOSFET gate, thus reducing oscillation.

[0093] The current sensing amplifier 8 obtains the current magnitude by detecting the voltage difference across the sampling resistor, and after amplifying it by 10 times, the signal enters the voltage comparator.

[0094] Voltage comparator 9 generates an output signal by comparing the input voltage and a threshold voltage. If the input voltage is greater than the threshold voltage, it outputs a 5V high level; otherwise, it outputs a 0V low level. Its rail-to-rail output can directly drive CMOS or TTL levels.

[0095] Operational amplifier 10 is used in a voltage follower circuit to ensure that the input and output voltages are consistent.

[0096] The high-side drive unit 11 utilizes high-side drive technology to control the on / off state of the constant current source by controlling the switching of the switching transistor. It mainly consists of an NPN transistor, a switching MOSFET, and a pull-up resistor. When an overcurrent occurs, the constant current source is turned off; conversely, the MOSFET is normally turned on and the constant current source operates normally.

[0097] Sampling resistor 12 is used to measure the voltage difference and determine whether an overcurrent has occurred. A 100mΩ metal film non-inductive resistor is selected.

[0098] When the drive voltage signal is input from the positive input terminal of the op-amp, the gate-source voltage of MOSFET 2 is greater than the turn-on voltage, MOSFET 2 is turned on and operates in the constant current region. At this time, the negative feedback loop is formed, so it can be determined by the virtual short and virtual open characteristics of the op-amp:

[0099] U + =U _

[0100] Therefore, the laser's driving current is directly controlled by the driving voltage U+. The negative feedback control process of this circuit is as follows: As shown in the formula, when ILASER increases, U- also increases, thus (U+-U-) decreases. According to the op-amp characteristics, the output voltage of operational amplifier 1 decreases at this time, that is, the gate voltage of MOSFET 2 decreases. When operating in the constant current region, the gate-source voltage decreases, so IR=ID decreases accordingly (where ID is the current flowing through the source of MOSFET 2, and IR is the current flowing through sampling resistor 3), thereby causing U- to decrease, and ILASER decreases to restore its original value. When ILASER decreases, the principle is the same as above, so the output current can always be maintained in a highly stable state, ensuring safe driving of the CW-QCL. When static electricity occurs, the ultra-high voltage will quickly break down the transient voltage suppressor diode (TVS) 4, and the current will directly pass through its branch to ground, which effectively protects the laser. The Schottky diode 5 is used to release the surge voltage on the CW-QCL and absorb the reverse current. The tantalum capacitor 6 has good high-frequency characteristics, which can eliminate the inductive effect of the CW-QCL and prevent voltage surges in the laser's PN junction. The buffer resistor 7 can reduce the input impedance of the MOSFET2 gate and reduce oscillation.

[0101] A high-precision sampling resistor 12 is designed between the QCL laser and the constant current source circuit. The current magnitude can be obtained by detecting the voltage difference across the resistor using a current-sensing amplifier 8, with a fixed 10x gain output. The voltage signal then enters an ultra-high-speed voltage comparator 9, which can output rail-to-rail and directly drive CMOS or TTL levels. When the input signal is greater than the threshold voltage, it outputs a 5V high level; otherwise, it outputs a 0V low level. Finally, the output signal of the comparator enters the high-side drive unit 11 through a follower 10. The high-side drive technology controls the on / off state of the constant current source by controlling the switching of the switching transistor, which mainly consists of an NPN transistor, a switching MOSFET, and a pull-up resistor. When an overcurrent occurs, the output voltage of the current-sensing amplifier is greater than the comparator threshold voltage, outputting a 5V high level. At this time, the collector of the transistor is forward biased and the emitter is reverse biased, and it is in a saturated conduction state. The gate voltage of the MOSFET decreases rapidly, and the MOSFET turns off, thus turning off the constant current source and protecting the laser. When the drive voltage is less than the threshold voltage, the transistor is in the cutoff state, the gate voltage of the MOSFET is close to 5V, the MOSFET turns on normally, and the constant current source works normally.

[0102] 5. Current detection and optimized control circuit

[0103] The current detection and optimization control circuit adopts the concept of "off-site" isolation, uses a high-speed analog-to-digital converter (ADC) to sample the drive signal, and uses an adaptive Fuzzy-PID algorithm in the DSP to optimize it, so as to ensure that the drive current with high stability can be output under different environments, thereby ensuring the stability of the laser wavelength.

[0104] like Figure 5 As shown, the current detection and optimization control circuit (Fuzzy-PID controller) includes the following parts:

[0105] The fuzzy control module mainly consists of fuzzification, fuzzy inference, defuzzification, and a rule base. It uses the error quantity e(t) and the error rate of change ec as input linguistic variables, and the parameter change ΔK as the input linguistic variable. P ΔK I ΔK D As the output of the fuzzy controller, the K of the PID controller P K I K D The three parameters are dynamically tuned;

[0106] PID controllers, as a traditional control algorithm, have the advantages of simple structure and good control effect, and can achieve the purpose of optimizing output current.

[0107] QCL G1(s), as the controlled object, can be regarded as the optimized output drive current.

[0108] KP, KI, and KD are three control parameters of a PID controller, and their operation is based on theoretical and actual output values.

[0109] To stabilize the output current, only three parameters, KP, KI, and KD, need to be tuned. Fuzzy control theory possesses strong robustness and excellent dynamic characteristics. The fuzzy control module in this invention mainly consists of fuzzification, fuzzy inference, defuzzification, and a rule base. It uses the error quantity e(t) and the error rate of change ec as input linguistic variables, and the parameter change ΔK... P ΔK I ΔK D As the output of the fuzzy controller, the K of the PID controller P K I K D The three parameters are dynamically tuned to eliminate overshoot, ensuring a fast settling time and making the entire system more stable.

[0110] The adopted Fuzzy-PID controller, compared to the traditional PID controller, incorporates fuzzy control theory, leveraging its robustness and excellent dynamic characteristics. This allows the PID parameters to adapt to different current ranges, achieving more precise current control.

[0111] 6. Temperature control circuit

[0112] Temperature is a key factor driving lasers, affecting their emission power and wavelength. Furthermore, CW-QCLs generate significant heat, making heat dissipation and temperature control more challenging. Therefore, the temperature control system in this invention utilizes a commercially available temperature control chip to regulate the laser temperature. A TEC (thermal control device) and a thermistor are integrated within the laser package. The temperature control chip injects current into the TEC based on the temperature value returned by the thermistor. The TEC utilizes the Peltier effect of semiconductor materials; it generates heat when current flows in the forward direction and cools when current flows in the reverse direction. Controlling the current direction controls the TEC temperature, thereby controlling the laser temperature.

[0113] The temperature control circuit uses a semiconductor cooler (TEC) controller as its core to perform high-precision control of the TEC built into the CW-QCL, ensuring the stability of the laser's temperature during operation.

[0114] 7. ADC Acquisition Module

[0115] First, the drive signal is sampled using a high-speed analog-to-digital converter (ADC), and then optimized using an adaptive Fuzzy-PID algorithm in the DSP main control unit to ensure that a highly stable drive current can be output under different environments, thereby ensuring the stability of the laser wavelength.

[0116] Then, the laser temperature return value is collected by the ADC, so that the laser temperature can be observed in real time to see if it is normal. The temperature is adjusted by remotely outputting voltage through the DAC to ensure the accuracy of temperature control, which is beneficial to the safety of the laser.

[0117] The key features of this invention are the dual-modulation drive waveform generation circuit, the nanosecond-level overcurrent protection current, and the current optimization control based on Fuzzy-PID.

[0118] Current and temperature are two key factors driving lasers, both affecting their luminous power and wavelength. Temperature has a more significant impact, and the CW-QCL generates considerable heat, making heat dissipation and temperature control more challenging. Therefore, the temperature control system in this invention utilizes a commercially available temperature control chip to regulate the laser temperature, integrating a PI algorithm for precise temperature control. A TEC (Dielectric Temperature Regulator) and a thermistor are integrated within the laser package. The temperature control chip injects current into the TEC based on the temperature value returned by the thermistor. The TEC utilizes the Peltier effect of semiconductor materials; it heats up when current flows in the forward direction and cools down when current flows in the reverse direction. Controlling the current direction controls the TEC temperature, thereby controlling the laser temperature.

[0119] The main features of this invention are:

[0120] 1. The drive signal generation circuit uses a DAC, DDS, digital potentiometer, and high-speed multiplexer as constant current sources to provide drive signals with dual modulation depths. Two sinusoidal signals with different amplitudes are generated by the DDS and digital potentiometer and added to a low-frequency sawtooth wave to generate segmented modulation drive signals with two tuning depths. This allows the modulation coefficients of both gas absorptions to reach approximately 2.2, improving the situation where only one gas absorption can be optimized in multi-gas detection.

[0121] 2. To achieve nanosecond-level high-speed operation, the overcurrent protection circuit mainly consists of a high-speed operational amplifier, a high-speed comparator, a high-speed current-sensing amplifier, an NPN transistor, and a switching MOSFET. The current-sensing amplifier detects the voltage across the sampling resistor, offering high full-scale accuracy and a large bandwidth. The high-speed comparator has a short propagation delay, allowing rail-to-rail output to directly drive CMOS or TTL levels, and its threshold voltage can be flexibly adjusted. The voltage follower, composed of high-speed operational amplifiers, acts as a buffer, providing isolation and enhancing load-carrying capacity. Finally, the high-side drive unit controls the switching of the switching transistor to control the on / off state of the constant current source, achieving nanosecond-level high-speed overcurrent protection.

[0122] 3. The Fuzzy-PID control algorithm mainly incorporates fuzzy control theory, which enables dynamic tuning of PID control parameters, allowing it to adapt to different current ranges, improve control accuracy, eliminate overshoot, reduce settling time, and achieve more precise current control, making this invention better suited for external atmospheric environments.

Claims

1. A DSP-based CO and N20 detection continuous QCL driver, characterized in that, The DSP master unit, power supply system, drive waveform generation circuit, voltage-controlled constant current source circuit, overcurrent protection circuit, current detection and optimization control circuit and temperature control circuit are included. The DSP master unit is used for controlling the signal required by the digital-to-analog converter and the direct digital frequency synthesizer to generate the drive waveform, simultaneously controlling the analog-to-digital converter to perform analog-to-digital conversion, and integrating the Fuzzy-PID control algorithm to optimize the output drive current. The power supply system is used for supplying power for each circuit, with soft start protection, and providing soft start protection for the voltage-controlled constant current source circuit to reduce the damage to the QCL laser during power-on and power-off. The drive waveform generation circuit is used for generating a drive voltage signal with two modulation depths, so that the absorption modulation coefficients m of CO and N2O are both 2-2.4, and the detection lower limit of the two gases is improved. The voltage-controlled constant current source circuit is used for converting the drive voltage signal into a current signal by using the deep negative feedback principle, and outputting a high-stability drive current. The overcurrent protection circuit is used for cutting off the loop at a nanosecond level when the overcurrent phenomenon occurs through high-side driving, so as to ensure the safety of the QCL laser. The current detection and optimization control circuit is used for sampling the drive voltage signal by using the analog-to-digital converter, and optimizing in the DSP by using the adaptive Fuzzy-PID algorithm, so as to ensure that a high-stability drive current is outputted under different environments, and the wavelength of the QCL laser is ensured to be stable. The temperature control circuit is used for performing high-precision temperature control on the semiconductor cooler built-in the QCL laser, so as to ensure the stability of the temperature of the QCL laser during operation. The constant current source circuit and the overcurrent protection circuit include: The high-precision operational amplifier is used for controlling the gate voltage of the MOSFET by accepting the feedback current, so that the stability of the current of the constant current source reaches the nanampere level. The MOSFET is a large-power enhancement-mode N-channel metal oxide semiconductor field effect transistor, and the gate voltage thereof is controlled by the operational amplifier, so as to make the constant current source work in the constant current region. The sampling resistor is a high-precision metal film non-inductive resistor with a resistance of 2Ω. The transient suppression diode is used for protecting the safety of the QCL laser when static electricity occurs, and the super-high voltage will quickly break down the transient suppression diode, so that the current directly passes through the branch where the transient suppression diode is located and is grounded. The Schottky diode is used for releasing the surge voltage on the CW-QCL and absorbing the reverse current. The tantalum capacitor is used for eliminating the inductive effect of the CW-QCL, and preventing the voltage of the PN junction of the QCL laser from suddenly changing. The buffer resistor is used for reducing the input impedance of the gate of the MOSFET, and reducing the oscillation. The current detection amplifier is used for obtaining the current size by detecting the voltage difference between the two sides of the sampling resistor, and the signal is amplified by 10 times and then enters the voltage comparator. The voltage comparator is used for generating an output signal by comparing the input voltage and the threshold voltage, and outputting 5V high level when the input voltage is greater than the threshold voltage, and outputting 0V low level when the input voltage is less than the threshold voltage; the rail-to-rail output directly drives the CMOS or TTL level. The operational amplifier is used for the voltage follower circuit, and is used for ensuring that the input and output voltages are consistent. High side drive unit, by controlling the switch of switch tube to control the on-off of constant current source, is composed of NPN triode, switch MOSFET and pull-up resistor, when overcurrent phenomenon occurs, constant current source is disconnected, otherwise MOSFET is normally opened and constant current source works normally. Sampling resistor, voltage difference is measured through sampling resistor, and then it is judged whether overcurrent phenomenon occurs, 100 mΩ metal film non-inductive resistor is selected.

2. The DSP-based CO and N2O detection continuous QCL driver of claim 1, wherein, The power supply system comprises: AD-DC converter, 220V AC voltage is converted into 24V DC voltage; LDO linear voltage regulator module with soft start function, 24V DC voltage is stepped down to 18V DC voltage with lower ripple, which is directly applied to both ends of QCL laser, and no sharp pulse is generated when the power supply is up and down, so as to protect QCL laser; DC-DC switching power supply module, 24V DC voltage is stepped down to 5V and 12V DC voltage.

3. The DSP-based CO and N2O detection continuous QCL driver of claim 1, wherein, The drive waveform generating circuit comprises: DAC digital-to-analog converter connected with the DSP master unit, for generating DC voltage signal and low-frequency sawtooth wave signal required by constant current source; DDS direct digital frequency synthesizer connected with the DSP master unit, for generating two kinds of high-frequency sine wave signals; Digital potentiometer, which receives two high-frequency sine wave signals generated by DDS, generates two sine signals with different amplitudes; High-speed multiplexer, which is used for selecting and outputting two sine signals with different amplitudes back and forth; Adder, which adds two sine signals with different amplitudes after high-speed multiplexer and low-frequency sawtooth wave signal generated by DAC, to generate a drive signal with double modulation depth.

4. The DSP-based CO and N2O detection continuous QCL driver of claim 1, wherein, The current detection and optimization control circuit comprises: Fuzzy control module: composed of fuzzification, fuzzy inference, defuzzification and rule base, with error e(t) and error change rate ec as input language variables, with parameter change ΔK P , ΔK I and ΔK D as output of fuzzy controller, to dynamically set K P , K I , K D three parameters of PID controller; PID controller, which is used to optimize output current; QCL G1(s), as a controlled object, is regarded as the optimized output drive current.

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