Single-frequency fiber lasers have unique properties such as ultra-narrow linewidth, adjustable frequency, ultra-long coherence length, and ultra-low noise. The FMCW technology on microwave radar can be used for ultra-high-precision coherent detection of ultra-long-distance targets. Change the market's inherent concepts of fiber sensing, lidar and laser ranging, and continue to carry out the revolution in laser applications to the end.
Application in optical fiber sensing:
Ultra-narrow linewidth fiber lasers can be applied to distributed fiber sensing systems to detect, locate and classify targets as far away as 10 kilometers. Its basic application principle is frequency modulated continuous wave technology (FMCW), which can provide low-cost, fully distributed sensor security protection for nuclear power plants, oil/gas pipelines, military bases and national defense borders.
In FMCW technology, the laser output frequency is constantly changing around its center frequency, and part of the laser light is coupled into a reference arm with a fixed reflectivity. In a heterodyne coherent detection system, the reference arm acts as a local oscillation The role of the LO (LO). Acting as a sensor is another very long optical fiber, please see Figure 2. The laser light reflected from the sensing fiber is mixed with the reference light from the local oscillator to produce an optical beat frequency, which corresponds to the time delay difference it has experienced. The remote information on the sensing fiber can be obtained by measuring the beat frequency of the photocurrent on the spectrum analyzer. The distributed reflection on the sensing fiber can be the simplest Rayleigh backscatter. Through this coherent detection technology, signals with a sensitivity as low as -100db can be easily detected.
At the same time, since the beat signal of the photocurrent is proportional to the reflected light signal and the power of the reference light from the local oscillator, and the reference light also has the function of amplifying the signal light, this sensing technology can achieve other current Any optical fiber sensing technology can not achieve ultra-long-distance dynamic measurement. External factors that interfere with the sensing fiber, such as pressure, temperature, sound, and vibration, will directly affect the reflected laser light, thereby realizing the detection of these external environments.
However, for any set of coherent FMCW technology system, the most critical part is to need a light source with a long coherence length to achieve high spatial accuracy and large measurement range. Optical library communication thinks what you think, and tailors a variety of ultra-narrow-line fiber lasers for you. These lasers benefit from the patented technology of the United States, the frequency is absolutely single, and the coherence length can reach tens of kilometers, which is the most ideal light source in FMCW technology. The fiber laser equipped with optical library communication has the longest sensing distance of more than 10 kilometers, while the detection distance of DFB laser diodes on the market is only a few hundred meters. Since only one such laser and photodetector can monitor the changes of ultra-long-distance sensing parts, the sensing system can upgrade the current security standards at a very low cost, which can be widely used in a wide range of applications. , Long-distance homeland security and military fields.
Laser pointer and military ranging:
At present, the military's ISR (intelligence, surveillance, reconnaissance) integrated platform is usually equipped with an electro-optical imaging system, which can generally image at long distances and accurately locate the movement of small targets, such as launch vehicles and tanks. However, due to the impact of the terrain accuracy of the imaging system, the system generally cannot transmit the precise position of the target to these command platforms to direct the weapon to the target. In fact, the military has always had a huge demand for low-cost, ultra-long-distance (several hundreds of kilometers), and ultra-high-precision (less than 1 meter) laser target indication/ranging in terms of ISR systems.
At present, the measurement distance of a general commercial laser rangefinder is 10-20 kilometers, which is limited by its dynamic range and measurement sensitivity, and cannot meet the requirements of the military ISR system. At present, most laser rangefinders are based on the principle of optical time domain reflection of pulsed lasers. They are composed of fast photodetectors and simple analyzers, which directly detect the light pulse signals reflected from the target. The measurement accuracy is usually 1 -10 meters, which is limited by the pulse width of the laser (relative to the 3-30nm long laser pulse). The shorter the laser pulse, the higher the measurement accuracy, and the bandwidth of the laser measurement will also be greatly improved. This will undoubtedly increase the detection noise, thereby reducing the dynamic measurement distance. Since the photocurrent signal is linearly proportional to the energy of the reflected light signal, these enhanced noises limit the sensitivity of the detection signal. Because of this, the longest measurement distance of the current military laser rangefinder is only 10-20 kilometers.
Based on the principle of FMCW technology, the 1550nm ultra-narrow linewidth fiber laser can be widely used in laser target indication and laser ranging for hundreds of kilometers, so that the ISR platform can be built at a very low cost. A set of ultra-long-distance laser indication/ranging is composed of laser, collimator and receiver, and signal analyzer. The frequency of the narrow linewidth laser is linearly and rapidly modulated. The remote information can be obtained by measuring the signal light reflected from the target and mixing the reference light to generate a photocurrent. In the FMCW technology system, the line width or coherence length of the laser determines the distance and sensitivity of the measurement. The fiber laser line width provided by Optical Library Communication is as low as 2Khz, which is 2-3 orders of magnitude lower than the line width of the best semiconductor laser in the world. This important feature can achieve laser indication and distance measurement of hundreds of kilometers, and the accuracy is as high as 1 meter or even less than 1 meter.
The laser indicator/measurement instrument made of this fiber laser has many advantages over most current laser indicator/measurement instruments based on pulsed lasers, including very long dynamic distance, very high measurement sensitivity, and human Eye-safe, small size, light weight, stable and firm, easy to install, etc.
Doppler Lidar:
Generally speaking, coherent radar systems require pulsed laser light sources, and in order to generate heterodyne or homodyne signals for Doppler sensing, these lasers must also work at a single frequency. However, traditionally, such lasers are generally composed of three parts: sub-laser, main laser, and complicated circuit control. Among them, the sub-laser is a high-power pulsed laser oscillator, the main laser is a low-power but very stable continuous laser, and the electronic control part is mainly used to control and maintain the sub-laser's single-frequency oscillation. There is no doubt that this traditional single-frequency pulsed laser is too bulky, and faces great challenges in durability and robustness, and cannot be scaled up because it requires frequent and troublesome calibration of sensitive discrete optical components. At the same time, it must be matched that the seed signal from the main laser can be smoothly coupled into the sub-laser.
The single-frequency, all-fiber Q-switched pulsed fiber laser can satisfy the ultra-strong and compact Doppler lidar system. This novel laser can work alone with a local oscillator, it can also be frequency-locked for pulse operation, and it can also be used as a seed source for injection of lasers through the local oscillator. The reflected Doppler frequency shift can be easily read by checking the photocurrent generated by the mixing of the reference light and the signal light. The continuous wave fiber laser of Optical Library Communication is your ideal seed source laser. It has a high degree of compatibility with our all-fiber pulsed fiber laser. All optoelectronic devices are integrated in a small and light box, which is very suitable for field work. Due to the natural waveguide structure of the fiber, the fiber laser does not require optical alignment and adjustment at all. At the same time, unless through complex nonlinear frequency conversion, current crystal solid-state lasers generally cannot directly output the 1550nm wavelength that is safe for the human eye. This makes our erbium-doped fiber lasers more attractive and thus becomes one of the best light sources for lidar.