According to the different generation mechanism of short-wave infrared lasers, there are three types of short-wave infrared lasers, namely semiconductor lasers, fiber lasers and solid-state lasers. Among them, solid-state lasers can be divided into solid-state lasers based on optical nonlinear wavelength conversion and solid-state lasers that directly generate short-wave infrared lasers from laser working materials.
Semiconductor lasers use semiconductor materials as laser working materials, and the output laser wavelength is determined by the band gap of the semiconductor materials. With the development of materials science, energy bands of semiconductor materials can be tailored to a wider range of laser wavelengths through energy band engineering. Therefore, multiple short-wave infrared laser wavelengths can be obtained with semiconductor lasers.
he typical laser working material of short-wave infrared semiconductor laser is phosphor material. For example, an indium phosphide semiconductor laser with an aperture size of 95 μm has output laser wavelengths of 1.55 μm and 1.625 μm, and the power has reached 1.5 W.
Fiber laser uses rare-earth-doped glass fiber as the laser medium and semiconductor laser as the pump source. It has excellent characteristics such as low threshold, high conversion efficiency, good output beam quality, simple structure, and high reliability. It can also take advantage of the wide spectrum of rare earth ion radiation to form a tunable fiber laser by adding selective optical elements such as gratings in the laser resonator. Fiber lasers have become an important direction in the development of laser technology.
1.Solid-state laser
The solid-state laser gain media that can directly generate short-wave infrared lasers are mainly Er: YAG crystals and ceramics, and Er-doped glass. The solid-state laser based on Er:YAG crystal and ceramics can directly output 1.645μm short-wave infrared laser, which is a hot spot in the research of short-wave infrared laser in recent years [3-5]. At present, the pulse energy of Er: YAG lasers using electro-optic or acousto-optic Q-switching has reached a few to tens of mJ, a pulse width of tens of ns, and a repetition frequency of tens to thousands of Hz. If a 1.532 μm semiconductor laser is used as the pump source, It will have great advantages in the field of laser active reconnaissance and laser countermeasures, especially its stealth effect on typical laser warning devices.
Er glass laser has compact structure, low cost, light weight, and can realize Q-switched operation. It is the preferred light source for active detection of short-wave infrared laser. However, due to the four shortcomings of Er glass materials: First, the central wavelength of the absorption spectrum is 940 nm or 976 nm, which makes lamp pumping difficult to achieve; Second, the preparation of Er glass materials is difficult and it is not easy to make large sizes; Third, Er glass The material has poor thermal properties, and it is not easy to achieve repetitive frequency operation for a long time, let alone continuous operation; fourth, there is no suitable Q-switching material. Although the research of short-wave infrared laser based on Er glass has always attracted people’s attention, due to the above four reasons, no product has come out. Until 1990, with the emergence of semiconductor laser bars with wavelengths of 940 nm and 980 nm, and the emergence of saturated absorption materials such as Co2+:MgAl2O4 (cobalt-doped magnesium aluminate), the two major bottlenecks of pump source and Q-switching were broken. The research on glass lasers has developed rapidly. Especially in recent years, my country’s miniature Er glass laser module, which integrates semiconductor pump source, Er glass and resonant cavity, weighs no more than 10 g, and has a small batch production capacity of 50 kW peak power modules. However, due to the poor thermal performance of Er glass material, the repetition frequency of the laser module is still relatively low. The laser frequency of the 50 kW module is only 5 Hz, and the maximum laser frequency of the 20 kW module is 10 Hz, which can only be used in low frequency applications.
The 1.064 μm laser output by the Nd:YAG pulsed laser has a peak power of up to megawatts. When such a strong coherent light passes through some special materials, its photons are inelastically scattered on the molecules of the material, that is, the photons are absorbed and produced relatively Low-frequency photons. There are two types of substances that can achieve this frequency conversion effect: one is nonlinear crystals, such as KTP, LiNbO3, etc.; the other is high-pressure gas such as H2. Place them in the optical resonant cavity to form an optical parametric oscillator (OPO).
OPO based on high-pressure gas usually refers to a stimulated Raman scattering light parametric oscillator. The pump light is partially absorbed and generates a low-frequency light wave. The mature Raman laser uses a 1.064 μm laser to pump high-pressure gas H2 to obtain a 1.54 μm short-wave infrared laser.
PICTURE 1
The typical application of shortwave infrared GV system is long-distance imaging at night. The laser illuminator should be a short-pulse short-wave infrared laser with high peak power, and its repetition frequency should be consistent with the frame frequency of the strobed camera. According to the current status of short-wave infrared lasers at home and abroad, diode-pumped Er: YAG lasers and OPO-based 1.57 μm solid-state lasers are the best choices. The repetition frequency and peak power of the miniature Er glass laser still need to be improved.3. Application of short-wave infrared laser in photoelectric anti-reconnaissance
The essence of short-wave infrared laser anti-reconnaissance is to irradiate the enemy’s optoelectronic reconnaissance equipment working in the short-wave infrared band with short-wave infrared laser beams, so that it can obtain wrong target information or cannot work normally, or even the detector is damaged. There are two typical short-wave infrared laser anti-reconnaissance methods, namely the distance deception interference to the human eye-safe laser rangefinder and the suppression damage to the short-wave infrared camera.
1.1 Distance deception interference to human eye safety laser rangefinder
The pulsed laser rangefinder converts the distance between the target and the target by the time interval of the laser pulse going back and forth between the launching point and the target. If the rangefinder detector receives other laser pulses before the reflected echo signal of the target reaches the launch point, it will stop timing, and the converted distance is not the actual distance of the target, but smaller than the actual distance of the target. False distance, which achieves the purpose of deceiving the distance of the rangefinder. For eye-safe laser rangefinders, short-wave infrared pulse lasers of the same wavelength can be used to implement distance deception interference.
The laser that implements the distance deception interference of the rangefinder simulates the diffuse reflection of the target to the laser, so the laser peak power is very low, but the following two conditions should be met:
1) The laser wavelength must be the same as the working wavelength of the interfered rangefinder. An interference filter is installed in front of the rangefinder detector, and the bandwidth is very narrow. Lasers with wavelengths other than the working wavelength cannot reach the photosensitive surface of the detector. Even the 1.54 μm and 1.57 μm lasers with similar wavelengths cannot interfere with each other.
2) The laser repetition frequency must be high enough. The rangefinder detector responds to the laser signal reaching its photosensitive surface only when the range is measured. To achieve effective interference, the interference pulse should at least squeeze into the rangefinder wave gate 2 to 3 pulses. The range gate that can be achieved at present is on the order of μs, so the interfering laser must have a high repetition frequency. Taking a target distance of 3 km as an example, the time required for the laser to go back and forth once is 20 μs. If at least 2 pulses are entered, the laser repetition frequency must reach 50 kHz. If the minimum range of the laser rangefinder is 300 m, the repetition frequency of the jammer cannot be lower than 500 kHz. Only semiconductor lasers and fiber lasers can achieve such a high repetition rate.
1.2 Suppressive interference and damage to short-wave infrared cameras
As the core component of the short-wave infrared imaging system, the short-wave infrared camera has a limited dynamic range of response optical power of its InGaAs focal plane detector. If the incident optical power exceeds the upper limit of the dynamic range, saturation will occur, and the detector cannot perform normal imaging. Higher power The laser will cause permanent damage to the detector.
Continuous and low peak power semiconductor lasers and fiber lasers with high repetition frequency are suitable for continuous suppression interference of short-wave infrared cameras. Continuously irradiate the short-wave infrared camera with a laser. Due to the large-magnification condensing effect of the optical lens, the area reached by the laser diffused spot on the InGaAs focal plane is severely saturated, and therefore cannot be imaged normally. Only after the laser irradiation is stopped for a period of time, the imaging performance can gradually return to normal.
According to the results of many years of research and development of laser active countermeasure products in the visible and near-infrared bands and multiple field damage effectiveness tests, only short-pulse lasers with a peak power of megawatts and above can cause irreversible damage to TV cameras at a distance of kilometers away. damage. Whether the damage effect can be achieved, the peak power of the laser is the key. As long as the peak power is higher than the detector damage threshold, a single pulse can damage the detector. From the perspective of laser design difficulty, heat dissipation and power consumption, the repetition frequency of the laser does not necessarily have to reach the frame rate of the camera or even higher, and 10 Hz to 20 Hz can meet actual combat applications. Naturally, shortwave infrared cameras are no exception.
InGaAs focal plane detectors include electron bombardment CCDs based on InGaAs/InP electron migration photocathodes and CMOS later developed. Their saturation and damage thresholds are in the same order of magnitude as Si-based CCD/CMOS, but InGaAs/InP-based detectors have not yet been obtained. Saturation and damage threshold data of CCD/COMS.
According to the current status of shortwave infrared lasers at home and abroad, the 1.57 μm repetitive frequency solid-state laser based on OPO is still the best choice for laser damage to CCD/COMS. Its high atmospheric penetration performance and high peak power short pulse laser The light spot coverage and single pulse effective characteristics are obvious for the soft killing power of the long-distance optoelectronic system equipped with short-wave infrared cameras.
2 .Conclusion
Short-wave infrared lasers with wavelengths between 1.1 μm and 1.7 μm have high atmospheric transmittance and strong ability to penetrate haze, rain, snow, smoke, sand and dust. It is invisible to traditional low-light night vision equipment. The laser in the 1.4 μm to 1.6 μm band is safe for the human eye, and has distinctive features such as a mature detector with a peak response wavelength in this range, and has become an important development direction for laser military applications.
This paper analyzes the technical characteristics and status quo of four typical short-wave infrared lasers, including phosphor semiconductor lasers, Er-doped fiber lasers, Er-doped solid-state lasers, and OPO-based solid-state lasers, and summarizes the use of these short-wave infrared lasers in photoelectric active reconnaissance. Typical applications in anti-reconnaissance.
1) Continuous and low peak power high repetition frequency phosphor semiconductor lasers and Er-doped fiber lasers are mainly used for auxiliary lighting for long-distance stealth surveillance and aiming at night and suppressing interference to enemy short-wave infrared cameras. High-repetition short-pulse phosphor semiconductor lasers and Er-doped fiber lasers are also ideal light sources for multi-pulse system eye safety ranging, laser scanning imaging radar and eye safety laser rangefinder distance deception interference.
2) OPO-based solid-state lasers with a low repetition rate but with a peak power of megawatts or even ten megawatts can be widely used in flash imaging radar, long-distance laser gating observation at night, short-wave infrared laser damage and traditional mode remote human eyes Safety laser ranging.
3) The miniature Er glass laser is one of the fastest growing directions of short-wave infrared lasers in recent years. The current power and repetition frequency levels can be used in miniature eye safety laser rangefinders. In time, once the peak power reaches the megawatt level, it can be used for flash imaging radar, laser gating observation, and laser damage to short-wave infrared cameras.
4) The diode-pumped Er:YAG laser that hides the laser warning device is the mainstream development direction of high-power short-wave infrared lasers. It has great application potential in flash lidar, long-distance laser gating observation at night, and laser damage.
In recent years, as weapon systems have higher and higher requirements for the integration of optoelectronic systems, the small and lightweight laser equipment has become an inevitable trend in the development of laser equipment. Semiconductor lasers, fiber lasers and miniature lasers with small size, light weight and low power consumption Er glass lasers have become the mainstream direction of the development of short-wave infrared lasers. In particular, fiber lasers with good beam quality have great application potential in night-time auxiliary lighting, stealth surveillance and aiming, scanning imaging lidar, and laser suppression interference. However, the power/energy of these three types of small and lightweight lasers is generally low, and can only be used for some short-range reconnaissance applications, and cannot meet the needs of long-range reconnaissance and counter reconnaissance. Therefore, the focus of development is to increase the laser power/energy.
OPO-based solid-state lasers have good beam quality and high peak power, and their advantages in long-distance gated observation, flash imaging radar and laser damage are still very obvious, and the laser output energy and laser repetition frequency should be further increased. For diode-pumped Er:YAG lasers, if the pulse energy is increased while the pulse width is further compressed, it will become the best alternative to OPO solid-state lasers. It has advantages in long-distance gated observation, flash imaging radar, and laser damage. Great application potential.
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Post time: Mar-02-2022