高平均功率光纤激光技术基础：模式

周朴

doi:10.11884/HPLPB201830.180087

高平均功率光纤激光技术基础：模式

doi: 10.11884/HPLPB201830.180087

周朴

国防科技大学前沿交叉学科学院，长沙 410073

基金项目:

全国优秀博士学位论文作者专项 201329

霍英东教育基金会高等院校青年教师基金资助项目 151062

详细信息

作者简介:
周朴(1984—), 男，研究员，博士生导师，主要从事光纤激光与光束合成技术研究；zhoupu203@163.com

中图分类号: O438
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出版历程
- 收稿日期: 2018-03-25
- 修回日期: 2018-04-17
- 刊出日期: 2018-06-15

Fundamentals of high-average-power fiber laser technology: Mode

Zhou Pu

College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China

摘要

摘要: 从具有不同模式特性的光纤激光研究现状出发，指出模式是光纤激光特性的核心参数之一。通过算例给出模式与光束质量之间的关系，引出模式分解技术是准确知晓模式组分和光束质量的关键，介绍常见的模式分解技术。针对模式不稳定效应这一限制光纤激光功率提升的新现象，归纳总结了不同因素对模式不稳定效应产生阈值的影响，梳理了提高阈值的物理原理和实现方法。从高阶模抑制、特定高阶模式和结构光场输出等三个方面介绍了光纤激光模式控制的最新进展。
- 光纤激光 /
- 单模 /
- 多模 /
- 模式分解 /
- 模式不稳定 /
- 模式控制
Abstract: Mode (transverse mode) is one of the key parameters, which could be deduced from the status of the fiber lasers with different mode properties. The relationship between mode and beam quality is analyzed by numerical calculations, based on which it is pointed out that mode decomposition is the key to well understand the mode constitution and beam quality, then the common mode decomposition techniques are introduced. Aimed at mode instability (MI), which is a new phenomenon that prohibits power scaling of fiber laser, various parameters affecting the threshold of MI are summarized, then the physical mechanism and technique to increase the threshold are concluded. The recent progress on mode control of fiber laser is introduced from the viewpoint of high-order-mode suppression and structured light field generation.
- fiber laser /
- single mode /
- multimode /
- mode decomposition /
- mode instability /
- mode control

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图 1 不同V值下不同本征模式的M²因子^[11]

Figure 1. The M² factor of different eigenmode in case of different V number^[11]

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图 2 不同LP_11e模比例α以及不同LP_11e模与LP₀₁模的相位差ψ下LP₀₁与LP_11e相干叠加光束的M²因子^[11]

Figure 2. The M² factor of the beam coherently combined by LP₀₁ mode and LP_11e mode in case of different mode weight α and relative phase difference ψ ^[11]

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图 3 成像法实验结构示意图

Figure 3. Experimental setup for imaging method

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图 4 直接测量法实验结构示意图

Figure 4. Experimental setup for direct measuring method

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图 5 改进型阶跃折射率光纤示意图

Figure 5. Illumination of improved step-index fiber

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图 6 全新机制光纤示意图

Figure 6. Schematic dravings of fibers based on new mechanisms

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图 7 基于光纤激光的结构光场产生

Figure 7. Structured light field generation based on fiber laser

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图 8 基于光纤放大器的结构光场产生

Figure 8. Structured light field generation based on fiber amplifier

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图 9 可定制光场输出的光纤激光器^[147]

Figure 9. Fiber Laser with on-demand output property^[147]

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表 1 公开报道的具有代表性的单模和多模光纤激光器的特性参数

Table 1. Characteristic parameters of representative single-mode and multi-mode fiber lasers

	output power/kW	M²	beam parameter product(BPP)	references	备注
single-mode laser	9.6	~1.2		[4]	for the diffraction limited beam in 1 μm wavelength band，M² = 1，BPP=0.33 mm·mrad [6]
single-mode laser	10	＜1.5		[5]
multi-mode laser	20	15	15 mm·mrad	[6]
multi-mode laser	50	30	10 mm·mrad	[6]
multi-mode laser	100		16 mm·mrad	[7]

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表 2 模式不稳定阈值的影响因素

Table 2. Factors of influencing mode instability(MI) threshold

first level	second level	third level	conclusions	references
optical fiber	fiber core/cladding diameter		lower core-cladding ratio, higher MI threshold	[45-47]
	numerical aperture of fiber core		numerical aperture decreases, MI threshold increases	[48-49]
	fiber doping	doping concentration	independent of longitudinal distribution of doping concentration	[50]
	fiber doping	doping radius	partially doped area decreases and MI threshold increases	[51-54]
	photodarkening		photodarkening increased, MI threshold decreases	[55-58]
	characteristics of polarization maintaining		MI threshold is irrelevant with polarization maintaining; polarization control causes amplitude modulation, which is relevant	[59-62]
	fiber material		optimization of fiber materials can improve the MI threshold	[63-65]
system	signal light	power of signal light	increasing the signal light power can improve the MI threshold.	[66-67]
		signal noise	relative intensity noise increases，MI threshold decreases	[45]
		initial high-order mode ratio	initial high-order mode components increases，MI threshold decreases	[51, 68]
		wavelength of signal light	MI threshold is related to the signal light wavelength	[69-72]
		linewidth of signal light	MI threshold is affected by relatively wide linewidth	[42, 73-74]
		amplitude modulation of signal light	suppressing signal amplitude modulation helps increasing MI threshold	[61-62]
	cooling capacity		MI threshold is independent of symmetric cooling	[43, 62]
	pump source	pump wavelength	reducing pump absorption cross section can increase MI threshold	[75]
		hybrid pump	increasing the wavelength component deviating absorpting peak can increase MI threshold	[43]
		pump modulation	suppression of pump power/spectrum modulation can increase MI threshold	[61]
	pump modes	pump direction	bidirectional pump and backward pump can increase MI threshold	[43, 76-77]
	pump modes	side pump	multi-point side pumping can increase MI threshold	[43, 77]
	higher-order mode loss		increasing higher-order mode loss can increase MI threshold	[55, 78-81]

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表 3 提高模式不稳定阈值的技术手段

Table 3. Technical measures to increase the mode instability threshold

physical mechanism	implementation method	technical measure	notes	references
increase high-order mode loss	increase high-order mode bending loss	reduce bending radius	bending can lead to changes in the area of the optical fiber module
		reduce numerical aperture of fiber core		[49, 82]
		reduce core diameter	small fiber core is easy to stimulate Raman scattering and other effects
		optimize fiber coiling		[83]
		increase signal wavelength	increasing the wavelength of signal light will lead to increase of quantum loss
	optimize optical fiber design	gain-cut fiber	fiber bending easily leads to more overlap of higher-order modes with doped regions	[84]
		large air hole spacing fiber	increase the threshold by 3 times	[36]
		distributed mode filter fiber	increase the threshold by 1.5 times	[56]
		chirp coupled core fiber	increase the threshold by 2 times	[85-86]
		all-solid photonic band gap fiber		[87-88]
		conical fiber		[89]
increase gain saturation	reduce the core cladding ratio		absorption coefficient decreases, long fibers are required, and nonlinear effects are common	[47]
	change the wavelength of the semiconductor pump		absorption coefficient decreases, long fibers are required, and nonlinear effects are common
	change the pump light injection direction		for large-core fiber, the effect is limited	[76, 90]
	increase injection signal power		the stimulated Raman scattering threshold decreases	[56, 91-92]
	change signal wavelength		unconventional bands need to effectively inhibit ASE	[72, 93-95]
reduce quantum loss	in-band pumping		absorption coefficient decreases, long fibers are required, and nonlinear effects are common

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