1、,Lecture 9,5.6 Hole-Burning and The Lamb Dip in Doppler-Broadened Gas Laser,I. Doppler Effect,5.6 Hole-Burning and The Lamb Dip in Doppler-Broadened Gas Laser,II. Hole-Burning,Assume laser oscillate at,5.6 Hole-Burning and The Lamb Dip in Doppler-Broadened Gas Laser,III. Lamb Dip,5.7 Multimode Laser
2、 Oscillation and Mode Locking,I. Mode Competition in Homogeneous Broadening Laser,A) Mode Selection,The ideal homogeneous broadened laser can oscillator only at a single frequency close to,Self gain saturation,Gain saturation from others,B) Multimode Oscillation due to spatial hole burning,5.7 Multi
3、mode Laser Oscillation and Mode Locking,Longitudinal mode spatial competition,Gas Laser, rapid movement eliminates spatial hole burning,Single mode only,In solid state Laser, spatial hole burning exist,Multimode possible,II. Multimode Oscillation in Inhomogeneous Broadening Laser,5.7 Multimode Laser
4、 Oscillation and Mode Locking,when,and,compete to each other,Mode competition occurs at oscillation modes share the same part of inversion particles, for example:,III. Methods for Mode Selecting,5.7 Multimode Laser Oscillation and Mode Locking,Transverse Mode Selection- select fundamental mode depre
5、ss high order modes,TEM00,1. Resonator parameter selection,Metastable cavity,3. Small hole iris selection,2. Unstable resonator selection,Suit for high gain media,B) Longitudinal Mode Selection- select single longitudinal frequency,5.7 Multimode Laser Oscillation and Mode Locking,1. Short Length Res
6、onator,2. Traveling Wave Resonator,No spatial hole burning exists,3. FP Etalon,4. Hybrid Resonator,Fox-Smith interferometer,IV. Frequency Stabilization,5.7 Multimode Laser Oscillation and Mode Locking,Passive frequency stabilization,Feed back through electric circuits,Feed back through Geometry Prop
7、erties,稳定度,复现性,Active frequency stabilization,A) 稳定性的度量,B) 稳定方法,1. Lamb Dip Method,5.7 Multimode Laser Oscillation and Mode Locking,2. Zeeman Frequency Stabilization,5.7 Multimode Laser Oscillation and Mode Locking,Zeeman Effect,原谱线,左旋光,右旋光,5.7 Multimode Laser Oscillation and Mode Locking,3. Saturat
8、ion Absorption,5.7 Multimode Laser Oscillation and Mode Locking,The principle is the same as Lamb dip frequency stabilization,4. Frequency Stabilization in Passive Resonator,5.7 Multimode Laser Oscillation and Mode Locking,5.7 Multimode Laser Oscillation and Mode Locking,V. Mode Locking,指在激光器内不同振荡纵模
9、之间实现位相锁定，以期获得规则的超短脉冲序列的专门技术。,不锁模时，在激光工作物质增益线宽内往往会产生多个或大量纵模的同时振荡，它们相互之间的位相关系对时间来说是随机变化的，彼此之间不 能产生持续的相干作用；与此相应,输出激光实际上是由一系列不规则的宽度较宽而高度较低的杂乱脉冲组合而成。,锁模时，众多纵模之间保持同步振荡和彼此之间相互“干涉”作用，导致输出激光呈现为一系列规则的脉冲系列。,5.7 Multimode Laser Oscillation and Mode Locking,Suppose all modes have a same amplitude and initial pha
10、se is zero. We start with the field has the maximum amplitude at z=0 and t=0. Then after a time interval T, when,they all have the maximum amplitude again. But if their initial phase is fluctuation, then the amplitude will also fluctuation.,5.7 Multimode Laser Oscillation and Mode Locking,Lets talk
11、about more general case, suppose,and,5.7 Multimode Laser Oscillation and Mode Locking,1),2),3),VI. Mode Locking Methods,5.7 Multimode Laser Oscillation and Mode Locking,1、主动锁模,振幅调制锁模：腔内插入损耗调制器，以周期 T 调制振幅,位相调制锁模：腔内插入电光调制器，以角频率 调制相位,2、被动锁模：腔内插入饱和吸收染料，通过非线性吸收调节腔内损耗,3、同步泵浦锁模：用已锁定的激光器作为子脉冲，去泵浦另一台激光器并实现锁模
12、,4、自锁模：利用增益介质自身非线性效应实现锁模,5.8 Giant Pulse (Q-switch) Laser,调Q技术的出现和发展，是激光发展史上的一个重要突破，它是将激光能量压缩到宽度极窄的脉冲中发射，从而使光源的峰值功率提高几个数量级的技术。通常我们将这种高峰值功率的窄脉冲叫做巨脉冲。,普通：,调Q：,通过某种方法使谐振腔的损耗因子按照规定的程序变化，在泵浦激励刚开始时，具有高的损耗因子，以便积累较高水平的反转粒子数目。然后在适当时候，突然降低损耗因子，受激辐射在短时间内便可迅速增强，形成一个巨脉冲输出。,I. 工作原理,5.8 Giant Pulse (Q-switch) Laser,II. Q调制方法,凡能使谐振腔损耗因子发生突变的元件都能用作Q开关,1、电光调Q,电光效应：改变偏振,2、声光调Q,声光效应：改变光传播方向,3、被动调Q,饱和吸收体：吸收与光强有关,
