Fiber laser coherent synthesis has achieved significant results in China

Recently, a number of academicians and experts from the Chinese Academy of Sciences and the Ministry of Education’s colleges and universities and domestic research institutions engaged in optoelectronics, lasers, information, computer applications and other technical fields have investigated and identified the “kilowatts” developed by the Institute of Optoelectronic Science and Engineering of the National University of Defense Technology in Changsha. Fiber laser coherent synthesis test system".

The Expert Group’s appraisal comments pointed out: “The project has achieved the first international kilowatt-level coherent composite output of fiber lasers and the system output power has reached 1.5 kilowatts. This system is complex and technically difficult. It has significant innovations in the theory and technology of fiber laser coherent synthesis. It is a major breakthrough in the field of high-power laser coherent synthesis. The comprehensive level of the system has reached the international advanced level, and the technologies of coherent composite output power, wide-spectrum and multi-wavelength laser coherent synthesis are at the leading position in the world."

This indicates that Chinese scientists have obtained major independent innovation achievements in the field of fiber laser coherence synthesis theory and technology.

It is understood that in nature, the coherence of light (ie, "consistency") is an objective reality. The physics study found that only light beams with the same direction of vibration, the same frequency, and a constant phase difference can be called coherent light with “step-by-step”. The light source can be called a coherent light source. When coherent light waves overlap, an "interference phenomenon" occurs. This interference may be constructive or destructive.

In an ordinary light source, atomic emission processes are all spontaneous emission processes, and the photons emitted by each atom are different in frequency, vibration direction, and initial phase. Therefore, the brightness, directionality, and monochromaticity of light are different from each other. Therefore, ordinary light sources do not have spatial and temporal coherence.

Because the laser is generated by stimulated radiation, its light waves have the same frequency, direction, vibration state and strict phase relationship. The laser needs a laser to produce. The laser produced by the laser has three important characteristics: high brightness, good directivity, and good coherence. Lasers with different wavelengths (1 to 14 μm) have different uses. For example, a laser with a wavelength of 2.0 μm is widely used in weather monitoring, laser ranging, and laser radar. The 2.8-μm-wavelength laser can be used in biological and medical applications. In the field, the 3.0-5.0 micron band laser has strong atmospheric penetrating ability, and has great potential for application in laser guidance, laser remote sensing and other fields. As high-brightness lasers have a wide range of applications in many fields such as the national economy and national defense, laser technology has been listed as one of the cutting-edge technologies of the National Medium- and Long-Term Scientific and Technological Development Planning Outline (2006-2020).

In an interview, the reporter learned that fiber lasers are lasers that use glass fibers doped with rare earth elements as laser media. Fiber lasers have the advantages of high conversion efficiency, good beam quality, compact structure, and the ability to obtain high beam quality laser output, which has become an important direction for the next generation of high-energy lasers following chemical lasers.

However, the output power of a single fiber laser is limited due to the "non-linear" effect, the thermal effect, and the damage of the laser element that are objectively existing between the laser and the material.

Therefore, in order to realize high power and high beam quality, building a modular fiber laser "combination" and coherently synthesizing it is the most ideal solution, which has become a research hotspot in the global laser technology field. More than 40 institutions in the world are exploring how to synthesize high-power lasers with small and medium-power lasers. In 2009, the United States Air Force Laboratory (AFRL) achieved a coherent synthesis of five 100-watt fiber laser amplifiers with an output power of 725 watts.

Faced with this worldwide problem in the field of laser technology, academic leaders of the new solid-state laser laboratory of the National Defense Science and Technology University, leader of the Innovation Team of the Ministry of Education and chief expert of the National “973” Project Liu Zejin led the high-energy laser technology research team. Invented a phase control method for fiber laser coherent combination based on random parallel gradient descent and single frequency jitter (abbreviated as “optimization algorithm”). It was found in the experiment that a wide-spectrum (spectral line width of about 30 GHz) laser can also achieve partially coherently synthesized physics. Phenomenon, proposed a non-single-frequency laser coherent synthesis of the technical route, clarifying the broad spectrum, multi-wavelength laser coherent synthesis of the physical mechanism.

They developed nine-hundred-hundred-half watt-class fiber laser amplifiers. When the total input power is 1900 watts, the coherent composite power can reach 1,560 watts. This output power is currently the highest level in the world known in this field.

The new scientific discovery of the partial coherent combination of wide-spectrum and multi-wavelength lasers using an optimization algorithm breaks the previous understanding that only narrow-spectrum lasers can achieve coherence.

They further studied the physical mechanism of the discovery and laid a theoretical foundation for the development of a new generation of high-power fiber laser systems. His research results were published on the cover artwork in the Optics Letters, a famous academic journal in the field of international optics published in the United States in 2009, which has aroused widespread concern in the international laser science community.

They and the American scholars independently proposed the technical scheme of random co-gradient descent optimization algorithm for fiber laser coherent synthesis, and separately proposed a single-frequency jitter phase control method for fiber laser coherent synthesis. Both methods were successfully implemented. “Synchronous automatic alignment of laser beams with different wavelengths” solves the technical difficulties of phase control in coherent synthesis and obtains high-quality stable laser output, enabling the current active phase control methods for fiber laser coherent synthesis to reach four kinds, resulting in an international laser field. Peer's approval.

At the same time, Liu Zejin et al. also proposed a coherent composite beam quality evaluation method, which solved the problem that conventional beam quality evaluation standards could not describe the coherent composite beam well and was accepted and adopted by the academic community. The book “Introduction to High-power Fiber Lasers” published by the American Directive Energy Association uses this method, calling it “the best test method for the beam quality of an aperture-filled coherent array.” The Optoelectronic Technology Group of the General Armament Department recommends it as a coherent synthesis in China. Laser beam quality evaluation criteria.