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材料的吸收截面、发射截面、荧光寿命、热导率等光谱和热动力学特性对激光器的输出性能有极大影响。国内外的许多研究人员都对Fe:ZnS/ZnSe的吸收和发射光谱进行了独立的测试。图 1所示的是MYOUNG等人的测试结果。在常温下,Fe:ZnS和Fe:ZnSe有着宽的吸收带和发射带,Fe:ZnSe的吸收截面和发射截面峰值分别在3μm和4.3μm附近,而Fe:ZnS的吸收和发射截面峰值要略小于Fe:ZnSe。宽的吸收带使得Fe:ZnS/ZnSe激光器在抽运源的选择上范围较广,而宽的发射带非常有利于获得宽调谐的激光输出[17]。
Fe2+离子的荧光寿命是影响Fe:ZnS/ZnSe的激光性能的另一个重要参量,由于存在多声子淬灭效应,Fe2+离子的荧光寿命随温度升高而迅速下降。许多研究人员都对Fe:ZnS/ZnSe在不同温度下的荧光寿命做了较为系统的测试,由于被测的Fe:ZnS/ZnSe材料的制备质量及掺杂工艺有所差别,得到的数据也有所差异。图 2为MIROV等人的测试数据[11]。在100K温度下Fe:ZnSe的荧光寿命约为57μs,随着温度的升高,荧光寿命迅速降低,300K时,Fe:ZnSe的荧光寿命降为0.4μs,Fe:ZnS的荧光寿命要明显低于Fe:ZnSe,在接近0K时,其荧光寿命约为6.2μs。
激光介质的散热对于固体激光器,尤其是高功率高能量固体激光器至关重要。热导率κ是影响激光介质内热分布的重要参量,以传统的棒状激光介质为例,激光棒的中心温度T0与棒外表面的温度Tr, 0之差可以用下式表示[18]:
$ {T_0} - {T_{{\rm{r, 0}}}} = {P_{\rm{a}}}/(4{\rm{ \mathsf{ π} }}\kappa L) $
(1) 式中,Pa为激光棒内需要散的总热量,L为棒的长度。由(1)式可知,在抽运功率和激光介质尺寸确定的条件下,热导率越高,激光棒内的温差越小,而温差的大小又直接决定了热应力和热致双折射,因此热导率的大小对于固体激光器的热效应有极大影响,进而影响激光器的输出效率和光束质量。ZnSe和ZnS的热导率会随着温度的上升而下降。俄罗斯的LUGUEVA等人将ZnS/ZnSe样品密封在一个真空容器中,测试并计算了4K~400K温度下的ZnS/ZnSe的热导率[19],如图 3a所示。在低温下ZnS/ZnSe的热导率要明显高于常温下的热导率。山东大学的YANG博士在其博士论文里面通过测量ZnSe的热扩散系数,计算了298K~573K范围内ZnSe的热导率[20],如图 3b所示。对比两人的计算结果,在300K附近ZnSe的热导率的值是比较一致的,掺杂Fe2+离子后,Fe:ZnS/ZnSe的热导率会随着掺杂浓度的增高而降低。
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激光材料是激光技术的核心和基础,ZnS/ZnSe高温挥发严重且存在相变,其中ZnSe的相变温度约为1425℃,ZnS的相变温度约为1020℃,所以无法用传统的提拉法单晶生长技术来获得高质量晶体。目前ZnS/ZnSe的制备方法主要有热压法和气相沉积法,其中,气相沉积法又分为物理气相沉积法(physical vapor deposition,PVD)和化学气相沉积法(chemical vapor deposition,CVD)[21]。
热压法制备ZnS/ZnSe主要包括粉末的制备和高温挤压成形两个过程,其中ZnS/ZnSe粉末的制备需要先用还原氨(NH2·NH2)产生ZnS/ZnSe·N2H4沉淀,然后将其与醋酸反应并通过加热去除醋酸盐杂质。将制备好的粉底放入真空炉(一般会加入氮气)中加热、加压,从而使微晶粒子挤压和再分布,排出粒子中的微气泡。热压法制备ZnS/ZnSe的主要问题是在粉底的提纯时会引入杂质,真空热压后不可避免存在微气孔,并且热压中模具与衬底分离时的巨大应力会在ZnS/ZnSe表面产生微裂纹,这些都会增加ZnS/ZnSe的散射损耗。
PVD法通过控制气化温度来提纯原材料,其制备ZnS/ZnSe的主要流程包括:先将粉末状ZnS/ZnSe(固相)在真空中加热到气相,当气相的ZnS/ZnSe遇到温度较低的衬底后会在衬底上沉积成晶体(固相)。PVD法工艺较为简单,但制备出来的ZnS/ZnSe的纯度较差。
CVD法是目前制备高质量ZnS/ZnSe的主要方法,这种方法将原材料制成多种高纯度的气相物质,并使它们在一定的温度和压力下进行气相反应,气相反应生成物沉积在衬底上,从而形成一定尺寸的晶体。利用此方法制备ZnS/ZnSe的主要流程包括:Zn蒸汽和H2S(制备ZnSe为H2Se)气体以Ar为载气输送到一定的温度和压力的反应室,在衬底上实现下列气相反应, 反应生成的ZnS/ZnSe分子在衬底上长成ZnS/ZnSe多晶体。
$ \left\{ \begin{array}{l} {\rm{Zn + }}{{\rm{H}}_{\rm{2}}}{\rm{S}} \to {\rm{ZnS + }}{{\rm{H}}_{\rm{2}}}\\ {\rm{Zn + }}{{\rm{H}}_{\rm{2}}}{\rm{Se}} \to {\rm{ZnSe + }}{{\rm{H}}_{\rm{2}}} \end{array} \right.{\rm{}} $
(2) 目前,ZnS/ZnSe材料的制备存在的主要问题是在制备过程中易混入杂质以及微小气孔难以完全排除。原料提纯技术的改进和热等静压等烧结技术的引入使得ZnS/ZnSe的气孔和杂质大幅度减少,但目前也到了一个新的瓶颈期,因此,寻找新的制备技术解决气孔和杂质的问题是目前Fe:ZnS/ZnSe激光器的研究热点。近期,中国科学院上海光学精密机械研究所的研究人员提出高温诱导陶瓷定向生长技术,希望能够获得与单晶质量相媲美的陶瓷(多晶)材料,基本原理为:首先将单晶和陶瓷通过光胶键合,形成多层复合结构,然后通过高温处理,达到陶瓷增益层单晶化势垒,使陶瓷沿单晶方向定向生长,并通过合理控制诱导温度和时间,实现陶瓷增益层完全单晶化。目前,他们已经成功利用单晶YAG诱导Yb:YAG陶瓷,实现Yb:YAG陶瓷与YAG单晶的无键合面高质量平面波导结构材料制备,并获得了数十瓦激光输出,最近正在将陶瓷定向生长技术的研究范围从近红外拓宽到中红外波段。
Fe:ZnS/ZnSe中红外固体激光器研究进展
Progress in Fe: ZnS/ZnSe middle-infrared solid-state lasers
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摘要: 高性能中红外激光在军事对抗、生物安全、环境科学等领域有重要的应用价值。Fe:ZnS/ZnSe具有长输出波长、宽吸收带和发射带的特点,是实现高性能、宽调谐3μm~5μm激光输出的最有效激光介质。介绍了Fe:ZnS/ZnSe的光谱和热动力学特性,评述了Fe:ZnS/ZnSe在低温和常温下激光输出性能方面的最新进展,分析了Fe:ZnS/ZnSe激光器在功率、能量提升以及室温运转方面面临的科学挑战。
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关键词:
- 激光器 /
- 固体激光器 /
- 中红外波段 /
- Fe:ZnS/ZnSe /
- 高性能
Abstract: High-performance mid-infrared laser has important application value in the field of military confrontation, biological safety and environmental sciences. Fe:ZnS/ZnSe is considered to be the most effective laser medium to obtain 3μm~5μm mid-infrared lasers with high performance and wide tuning range because of long output wavelength, wide absorption band and emission band. The spectral and thermodynamic characteristics of Fe:ZnS/ZnSe are introduced. The latest development of Fe:ZnS/ZnSe at low temperature and room temperature is reviewed. Scientific challenges of Fe:ZnS/ZnSe laser in power, energy enhancement and room temperature operation are also analyzed.-
Key words:
- lasers /
- solid-state lasers /
- middle-infrared band /
- Fe:ZnS/ZnSe /
- high performance
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