Design of detection and receiving front end of the

Digital laser warning system detection and reception front-end design

Introduction

after decades of development of laser technology, laser weapons have moved from theory to practice. The emergence of a large number of laser weapons has brought an increasingly serious threat. The detection and prediction of laser attack by laser warning equipment is the basic means of laser countermeasures. How to detect and judge the laser attack signal from the complex environment is the primary task faced by the warning system. With the development of digital technology and the continuous improvement of processing speed, it is possible to use digital users to select products that are more suitable for their own equipment, such as FPGA, to process alarm signals

the detection and reception front-end designed in this paper for the digital laser warning system is mainly to detect the incoming laser signal in a certain band. It has high sensitivity, and the minimum detectable laser energy reaches 1 W, and the dynamic range reaches 100 dB. In the design, a high-sensitivity laser detection diode is used to obtain the pulse current of the laser incoming signal. Under the action of the minimum pulse, the equivalent bandwidth of the system is more than 400 MHz. Working through a large bandwidth and low-noise operational amplifier, it is necessary to point out that the amplification and current voltage conversion are carried out in the transconductance amplification mode, and then the pulse signal that can be recognized by the digital circuit is obtained after amplification and shaping, Thus, the intensity and other information of the incoming laser signal can be judged according to the pulse width. Because the narrow pulse is sensitive to the capacitance in the system, the size of each level of capacitance is determined by ADS simulation. The simulation and test results show that the receiving front end has high detection sensitivity, large dynamic range, and can provide accurate incoming signal characteristics for post-processing digital systems. The method of using broadband transconductance operational amplifier circuit instead of traditional special triode to amplify weak narrow pulse is proposed in the system, which has the characteristics of wide band (500 MHz) and low cost. It provides a good broadband scheme for amplifying weak ns level and below narrow pulse current signals. At the same time, the system has simple structure, strong adaptability to the environment and easy maintenance

1 design and Simulation of the front-end scheme of detection and reception

according to the requirements of the back-end digital system, the incoming laser signal needs to be transformed into a digital pulse that can be processed by the digital system through the photoelectric detection diode. Because the photodiode generates a narrow pulse current under the action of laser signal, the minimum output current of the photodiode is 10 Na pulse current, and the pulse width is 10 ns. According to the effective frequency, the frequency band of the amplification circuit needs to be greater than 400 MHz. In order to meet this requirement, an operational amplifier with a large bandwidth of 500.mhz is used as the amplification, and the conversion between current and voltage is completed to obtain the pulse voltage. Because the pulse signal cannot reach the voltage required by the digital signal when the incoming signal is low or too large, it needs to be shaped and amplified in order to meet the standard of the conventional voltage of the digital system (the high voltage is 5 ~ 3.3 V, and the low voltage is 2.1 ~ 0 V). The system adopts the method of amplifying the received signal until the subsequent amplification circuit is saturated to realize the digital voltage shaping. The overall scheme is shown in Figure 1, and the waveform requirements after the amplifier are shown in the figure after each stage; Finally, the photocell detection current is changed into the form of pulse voltage, and the pulse width represents the amount of laser energy

because the minimum signal bandwidth of the system is very wide and the pulse current is weak, it is sensitive to the capacitive components in the circuit. In order to obtain the specific parameter values, the transient simulation method is used in ads (ad-vated design system) software for system simulation. Figure 2 is the topology diagram of the simulation circuit. According to the minimum detection requirements of the system and the principle characteristics of the photocell, the model of pulse current source, resistance and capacitance in parallel is used in ADS software to replace the actual model of the photocell generating pulse current under the action of laser. As shown in Figure 3, the circuit parameter setting is to adopt the current pulse with 10na output current and 10ns width under the minimum incoming laser energy of 1 W. the corresponding port current simulation waveform is shown in Figure 4. In the system, the amplifier adopts an operational amplifier with low noise and high gain bandwidth product (500 MHz) to realize amplification. The output waveform of the system under the action of different light energy of the incoming laser is simulated. In the simulation, pay attention to turning off the main motor source. In the model of laser probe photocell, the change of current pulse size and current pulse width is used to express. The results are shown in figures 5 to 7. It can be seen from the results that the digital pulse can be obtained by this amplification method, and the output pulse width is proportional to the incoming laser power. The system can not only judge whether there is an incoming laser, but also calculate the energy of the incoming laser by taking into account the recently acquired Austrian company mold matic and the Dutch Bosch sprang

2 realization of receiving front-end circuit

according to the above simulation results, gd3561 photodetector produced by No. 44 of China Power Group is selected as the laser detection diode, with a minimum detectable energy of 1 W and a minimum response time of 2.5 ns. The first stage adopts the form of transconductance amplification. Both the device and the amplification intermediate stage use BB's broadband and low-noise operational amplifier chip opa656. The gain bandwidth product reaches 500 MHz, the voltage establishment time of 8 ns, and the input noise is 18 NV/Hz; The shaping circuit adopts ad8611, which has a very short delay time of 4 ns. See Figure 8 for the specific circuit of the system

the test adopts a 10000 w laser light source with a wavelength of 1.3 M. through the optical attenuator, the attenuation of 100 dB can reach the minimum power required by the system of 1 W. the output end uses a high-speed oscilloscope tds460 to capture the output pulse signal. The results are shown in Figure 9 ~ Figure 12

Figure 9 shows the instantaneous voltage pulse at both ends of the photocell under the action of 1 W laser energy. It can be seen that under the action of minimum power, the generated voltage and current pulse time is short and the peak value is small; Figure 10 shows the digital pulse waveform after the pulse is amplified by the system, which meets the requirements of digital circuit processing. Figure 11 shows the action result of 1 MW, and Figure 12 shows the action result of 1 W energy. It can be seen that the rectangular pulse signal that can be identified by the digital circuit is obtained after the amplification of the system, and the strength of the signal is proportional to the output pulse. The dynamic range of the system reaches 100 dB

3 conclusion

the test results show that the detection receiving front end has an incoming laser signal of at least 1 W, the dynamic range reaches 100 dB, and the generated pulse waveform meets the processing level of digital circuit. It can provide accurate detection signals for subsequent digital circuits such as FPGA circuits. The system uses a broadband transconductance operational amplifier circuit to amplify the weak narrow pulse signal, and uses the simulation method in ADS software to deal with the key problem of the narrow pulse sensitive to capacitance, which provides a good scheme for providing weak narrow pulse current signal amplification. The amplification system has a bandwidth of 500 MHz, and the narrow pulse width that can be amplified can be narrower than ns level. At the same time, it is cheap and cost-effective. In the actual test, the false alarm rate is low, the detection speed is fast, and the noise produced by the detection and reception front end is low. The system is simple in structure and easy to maintain. It can be used not only for laser attack detection, but also for laser security system