§5.6 Hole-Burning and The Lamb Dip in Doppler- Broadened Gas Laser 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 Assume laser oscillate at III. Lamb Dip

Passive Optical Resonator Homogeneous Broadening Laser §5.7 Multimode Laser Oscillation and Mode Locking I. Mode Competition in Homogeneous Broadening Laser A) Mode Selection 1 2 3 Passive Optical Resonator Homogeneous Broadening Laser Self gain saturation Gain saturation from others The ideal homogeneous broadened laser can oscillator only at a single frequency close to

§5.7 Multimode Laser Oscillation and Mode Locking B) Multimode Oscillation due to spatial hole burning Gas Laser, rapid movement eliminates spatial hole burning Longitudinal mode spatial competition Single mode only In solid state Laser, spatial hole burning exist Multimode possible

Passive Optical Resonator Inhomogeneous Broadening Laser §5.7 Multimode Laser Oscillation and Mode Locking II. Multimode Oscillation in Inhomogeneous Broadening Laser 2 1 Passive Optical Resonator Inhomogeneous Broadening Laser 3 Mode competition occurs at oscillation modes share the same part of inversion particles, for example: when , and compete to each other

Suit for high gain media §5.7 Multimode Laser Oscillation and Mode Locking III. Methods for Mode Selecting Transverse Mode Selection -- select fundamental mode depress high order modes TEM00 1. Resonator parameter selection Metastable cavity 2. Unstable resonator selection Suit for high gain media 3. Small hole iris selection

No spatial hole burning exists Fox-Smith interferometer §5.7 Multimode Laser Oscillation and Mode Locking B) Longitudinal Mode Selection -- select single longitudinal frequency 1. Short Length Resonator 2. Traveling Wave Resonator No spatial hole burning exists 3. FP Etalon 4. Hybrid Resonator L2 L3 Fox-Smith interferometer

Feed back through Geometry Properties §5.7 Multimode Laser Oscillation and Mode Locking IV. Frequency Stabilization 稳定度 A) 稳定性的度量 复现性 B) 稳定方法 Active frequency stabilization Feed back through electric circuits Passive frequency stabilization Feed back through Geometry Properties

§5.7 Multimode Laser Oscillation and Mode Locking 1. Lamb Dip Method 光电接收 选频放大 相敏检测 直流放大 调制升压 整 流 音频振荡 PZT A B C unchange strength compress 频率位置 音频频率 激光功率 振荡频率 PZT电压 腔长变化

§5.7 Multimode Laser Oscillation and Mode Locking 2. Zeeman Frequency Stabilization Zeeman Effect 右旋光 左旋光 原谱线 右旋光 左旋光 右旋 左旋

§5.7 Multimode Laser Oscillation and Mode Locking 电光调制 相敏检测 直流放大 调制升压 整 流 PZT 光电接收 选频放大 音频振荡 unchange strength compress 频率位置 光强差 (右-左) PZT电压 腔长变化

§5.7 Multimode Laser Oscillation and Mode Locking 3. Saturation Absorption 光电接收 选频放大 相敏检测 直流放大 调制升压 整 流 音频振荡 PZT 饱和吸收池 泵浦光 The principle is the same as Lamb dip frequency stabilization

§5.7 Multimode Laser Oscillation and Mode Locking 4. Frequency Stabilization in Passive Resonator 半导体激光器 EOM 光电 接收 稳频系统

§5.7 Multimode Laser Oscillation and Mode Locking V. Mode Locking 指在激光器内不同振荡纵模之间实现位相锁定，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 phase is zero. We start with the field has the maximum amplitude at z=0 and t=0. Then after a time interval T, when t … 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 Let’s talk about more general case, suppose and

§5.7 Multimode Laser Oscillation and Mode Locking 1) 2) 3)

§5.7 Multimode Laser Oscillation and Mode Locking VI. Mode Locking Methods 振幅调制锁模：腔内插入损耗调制器，以周期 T 调制振幅 1、主动锁模 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 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 普通： 调Q： I. 工作原理 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。

§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