Laser

Laser Diodes

## 激光器构成

### 谐振腔

Alongitudinal mode(纵模) of a resonant cavity is a particular standing wave pattern formed by waves confined in the cavity. The longitudinal modes correspond to the wavelengths of the wave which are reinforced by constructive interference after many reflections from the cavity's reflecting surfaces. All other wavelengths are suppressed by destructive interference.

A common example of longitudinal modes are the light wavelengths produced by a laser. In the simplest case, the laser's optical cavity is formed by two opposed plane (flat) mirrors surrounding the gain medium (a plane-parallel or Fabry–Pérot cavity). 满足如下条件$$L=q \frac{\lambda}{2}$$即谐振腔的长度$$L$$是介质中的半波长的整数倍，其中$$q$$称为mode order】，实际中$$q$$很大，大约为105 to 106

(1) 激光器的光学谐振腔是什么？
(2) Longitudinal mode-Wiki

### 增益介质

gain medium(也有叫lasing medium激光介质，或者active laser medium，或者工作介质)，is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.

• Certain crystals, typically doped with rare-earth ions (e.g. neodymium, ytterbium, or erbium) or transition metal ions (titanium or chromium); (固体激光)
• Glasses, e.g. silicate or phosphate glasses, doped with laser-active ions;
• Gases, e.g. mixtures of helium and neon (HeNe), nitrogen, argon, carbon monoxide, carbon dioxide, or metal vapors;   (气体激光)
• Semiconductors, e.g. gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN).
• Liquids, in the form of dye solutions as used in dye lasers.   (染料激光)

• 光学和光谱特性
• 具有三能级或四能级系统，从降低阈值和提高效率的角度来衡量能级结构，四能级优于三能级；
• 具有宽的吸收带、大的吸收系数和吸收截面，以利于储能；
• 掺入的激活离子具有有效的发射光谱和大的发射截面；
•  在泵浦光的光谱区和振荡波长处高度透明；
• 在激光波长范围内的吸收、散射等损耗小，损伤阈值高；
• 激活离子能够实现高浓度掺杂，且荧光寿命长；
• 不因泵浦光激发产生色心而导致对光的有害吸收；
• 足够大的尺寸和良好的光学均匀性。
• 物化特性：弹性模量大，热导率高，热膨胀系数小；组分、结构及离子价态稳定；对水、溶剂和环境气氛等的化学稳定性好，具有良好的光照稳定性。
• 热光性：热光稳定性好，热光系数最好接近于零；热光畸变（包括热透镜效应，热应力感生的双轴聚焦，热应力双折射和退偏效应）要小。
• 机械性能：硬度高，自破坏阈值高，抗破坏强度大，易于加工研磨。

(1)激光晶体材料的发展和思考-徐军-2006

3分钟了解单频光纤激光器

## 普朗克黑体辐射公式和爱因斯坦AB系数

Optical Maser(光学激微波): an acronym for microwave amplification by stimulated emission of radiation; an optical device that produces an intense monochromatic beam of coherent light.

## 各种经典激光器

(1)固体激光器

(2)气体激光器

(3)染料激光器

### Ti宝石激光器

Ti蓝宝石激光器是最佳的飞秒激光器，它的吸收和发射都是宽带的，也是tunable laser，650—110 nm，也比tunable dye laser更好，因为 it get ultra-short pulses。

### 铥激光器

(1) 掺铥激光——在激光医疗领域大显身手

## 脉冲激光

### 调Q—纳秒级脉冲

Methods of the cavity Q-switching are divided into active and passive groups：

• Active methods: use modulation devices that change the cavity losses by a given law or in accordance with an external control signal.
• opto-mechanical
• electro-optical
• Passive method: Passive modulating elements are those controlled directly by the radiation field in the laser cavity.

#### Opto-mechanical

opto-mechanical最简单的调Q的modulator，就是a punched disk of an opaque material。Disk旋转轴的方向平行于谐振腔的axis，the disk blocks and opens the path between the mirrors periodically. The Q-switching time provided by such a device cannot be less than a millisecond. This value is considerable larger than the rise time of a giant pulse, which is the main drawback of a disk modulator. 如果想进一步缩短swictching time，可以同样地沿着垂直于cavity axis的方向旋转另一侧的reflector。这里有点类似测气体分子的速率分布的实验，可以分为下面三种形式(方法-123)。注意方法3仅考虑分子经过两个disk的间隔时间小于一个旋转周期的情形。

## 其他

### 小知识

• Quantum defect: In laser science, the term "quantum defect" refers to the fact that the energy of a pump photon is generally higher than that of a signal photon (photon of the output radiation). The energy difference is lost to heat, which may carry away the excess entropy delivered by the multimode incoherent pump. The quantum defect of a laser can be defined as part of the energy of the pumping photon, which is lost (not turned into photons at the lasing wavelength) in the gain medium at the lasing.

### Thin-disk Lasers

The most often used gain medium for thin-disk lasers is Yb:YAG.

(1) Thin-disk Lasers—rp-photonics