Fiber laser coherent combining technology Domestic technical shortcomings
Semiconductor lasers refer to lasers that use semiconductor crystal materials as working materials. They have the advantages of small size, long life, simple structure, and monolithic integration with integrated circuits. As early as 1970, J.E. Ripper of the Bell Communications Laboratory in the United States observed the experimental phenomenon of 2-channel GaAs lasers generating phase-locked output through evanescent wave coupling. In 1975, IBM's E.M.Philipp2Rutz realized 3-channel GaAs laser coherent synthesis using evanescent wave coupling, and the output power reached 5W. In 1978, D.R.Scifres et al. realized 5-channel GaAs laser phase-locked output. In the 1980s, semiconductor laser array coherent synthesis technology has entered a research climax period. The wide application of evanescent wave coupling, Talbot external cavity coupling and binary optics has led to rapid development of semiconductor laser arrays. In 1994, S. Sanders et al. used Talbot external cavity coupling to obtain 144-channel two-dimensional semiconductor laser phase-locked output with a laser power of 1.4W. Due to the characteristics of the semiconductor laser, the phase of the laser can be directly controlled by current or voltage without auxiliary phase control devices. The coherent synthesis of semiconductor amplifier arrays based on active phase control has gradually attracted people's attention. J.S. Osinski of SDI Company, etc. realized the coherent combination of 4 semiconductor amplifiers, with an average output power of 5W. K.H.No of the original McDonnell Douglas Company has realized the coherent combination of 100, 400 and 900 semiconductor amplifiers. The total output power of the 900 amplifiers during coherent combination is 36W. The system structure is shown in Figure 6. It is worth noting that the control of the coherent synthesis of semiconductor amplifiers is generally static phase distortion. After the system is initialized, the phase of each amplifier will not change within a few days. Although the single-tube semiconductor laser has no advantages over other types of high-energy lasers in terms of output power and beam quality, due to the advantages of low-cost, high-efficiency, all-solid-state, and high-reliability semiconductor lasers, its application prospects in the military field Still generally optimistic. The semiconductor laser coherent synthesis plan is still in progress. The Advanced Research Projects Agency of the United States Department of Defense (DARPA) has successively launched projects such as the coherent synthesis high-power single-mode transmitter (COCHISE) and the high-energy semiconductor laser system architecture (ADHELS) in recent years. The aim is to produce high-brightness lasers of the order of 100kW through the coherent synthesis of high-power and high-efficiency semiconductor laser arrays. The use of phase control technology to lock the phase of each laser is the basic condition for achieving coherent synthesis. Phase control technology can be divided into two categories: active phase control and passive phase control. Looking back and analyzing the development history of coherent synthesis, it is not difficult to find that before the development of fiber lasers into high-power devices and their coherent synthesis technology aroused people’s attention, scholars from various countries have already paid attention to various types of lasers, including gas lasers, chemical lasers, semiconductor lasers, and solid-state lasers. In-depth research has been carried out on coherent combination of various types of laser beams. However, the performance of the experimental results in terms of synthesis efficiency and number scalability is not satisfactory. For passive phase control technologies such as external cavity energy coupling, evanescent wave coupling, and SBS optical phase conjugation, taking CO2 laser coherent synthesis as an example, Antyukhov et al. achieved a coherent synthesis efficiency of 61 lasers using Talbot external cavity energy coupling to 15%. Bahanov et al. used intracavity spatial filtering to achieve 55 laser coherent synthesis. When the output power is 7kW, the synthesis efficiency is about 12%. The highest synthesis efficiency is the 85 laser coherent synthesis achieved by Vasil-tsov using intracavity spatial filtering. The combined efficiency is 40% when the output power is 500W. A.F.Glova et al. obtained through theoretical calculations that the coherent synthesis efficiency of passive phase control does not exceed 50%. In terms of number expansion, a large number of experimental results show that with the increase of the number of lasers, the phase-locking effect of the passive phase control scheme decreases, and even the phase-locked output cannot be achieved. The efficiency of coherent synthesis also decreases with the increase of the number of lasers. In theory, the more common passive phase control of intracavity spatial filtering has the disadvantage that the synthesis efficiency decreases as the number of lasers increases. In addition, the theoretical and experimental results of using the SBS phase conjugation method to achieve chemical laser and solid-state laser output show that the nearly stringent requirements for optical path alignment and the high power density required for excitation to generate the stimulated Brillouin effect hinder its practical application. development of. In addition, because the structure of a single CO2 laser, chemical laser, etc. is relatively complicated, the passive phase control of multiple lasers requires the design of complex spatial optical paths, which makes experimental system design and engineering implementation difficult. Passive phase control coherent synthesis is still in the process of seeking a balance in system complexity, number of lasers, synthesis efficiency and other parameters. For the active phase control coherent synthesis technology, in the early stage of its development, because the active devices with phase control function did not meet the requirements in control accuracy, response speed and automatic control technology, it was not able to develop in the direction of large numbers and high power. At the stage of theoretical discussion and conceptual demonstration. In the 1990s, the active phase control of semiconductor laser amplifiers achieved certain experimental results, but it remained at the level of correcting static aberrations. The combined power of 100-channel and 900-channel amplifier arrays is 7.9W and 36W, which is far less than the magnitude required for industrial processing and tactical use. Generally speaking, the experimental results obtained by the coherent combining technology did not exceed the maximum output power of the corresponding single-link laser at that time, so the effect was not obvious.