定义：
掺杂铬离子的激光增益介质。

铬属于过渡金属族元素。不同电荷态的铬离子都可作为增益介质的激光活性掺杂物： 

1. Cr2+常用在锌化合物中，例如Cr2+:ZnS，Cr2+:ZnSe，Cr2+:ZnSxSe1−x和r2+:CdSe。采用这种晶体的激光器可以辐射的波长在1.9到3.5μm之间，泵浦光波长范围为1.5-1.9μm。除了具有很大的辐射带宽之外，这种激光器还具有较低的阈值泵浦功率，还可以采用二极管泵浦。可以被动锁模这种激光器产生长度小于100fs的脉冲。 
2. Cr3+是红宝石激光器的活性组分（掺铬氧化铝），第一台激光器的增益介质就是紫翠宝石（Cr3+:BeAl2O3），是早期可调谐固态激光器的增益介质。Cr3+常用于以下增益介质中，Cr3+:LiSrAlF6 ，Cr3+:LiCaAlF6 和Cr3+:LiSrGaF6，辐射波长在0.8-09之间μm。（这种晶体被称为氟铝钙锂石。）采用这种介质的被动锁模激光器可以用来产生长度小于10fs的脉冲。与钛蓝宝石激光器相比，这种激光器更便宜，因为它们可以采用红光泵浦光源而不是绿光泵浦源，可以工作在较低的泵浦功率下，因此可以采用二极管泵浦。然而，它得到的输出功率更低（在高温度下，热淬灭效应更高），波长调谐范围更小，最短脉冲长度更长。一些相对较新的材料包括Cr3+:LiInGeO4，Cr3+:LiScGeO4,和 Cr3+:LiInSiO4。其中，Cr3+可以辐射非常长的波长范围，在1.2-1.6μm之间，并且带宽很宽。 
3. Cr4+离子用在例如Cr4+:YAG，Cr4+:MgSiO4及其它硅氧化物中和锗酸盐、磷灰石等晶体中。Cr4+:YAG的辐射范围约为1.35-1.65 μm，Cr4+:MgSiO4的为1.1-1.37 μm.采用Cr4+:MgSiO4钇铝石榴石激光器可以得到长度小于20fs的脉冲，采用的泵浦源为 Cr4+ 激光器。 

由于这些增益介质中很强的电子声子相互作用，掺铬激光器也称作电子振动激光器，具有很大的增益带宽。 
需要注意的是，有些掺铬晶体，尤其是Cr4+:YAG，也可以用作Q开关激光器的饱和吸收器。

Definition: laser gain media doped with chromium ions

More general term: solid-state laser gain media

Chromium (chemical symbol: Cr) is a chemical element belonging to the group of transition metals. Chromium ions of different charge states (2+, 3+, 4+) are used as laser-active dopants of gain media:

Cr²⁺

Cr²⁺ ions are mostly used in zinc chalcogenides such as Cr²⁺:ZnS, Cr²⁺:ZnSe, Cr²⁺:ZnS_xSe_1−x, and Cr²⁺:CdSe. Lasers based on these crystals can emit roughly between 1.9 and 3.5 μm and are typically pumped around 1.5–1.9 μm. Despite this huge emission bandwidth (for which such media are sometimes called “the Ti:sapphire of the infrared”), they can have reasonably low threshold pump powers and can be diode-pumped.

It is possible to passively mode-lock such lasers for generating pulses with durations well below 100 fs [28].

Cr³⁺

Cr³⁺ ions are the active ingredients of ruby (chromium-doped aluminum oxide), the laser medium of the first laser, and alexandrite (Cr³⁺:BeAl₂O₄), an early tunable solid-state laser medium. Cr³⁺ ions are now mostly used in gain media such as Cr³⁺:LiSrAlF₆ (Cr:LiSAF), Cr³⁺:LiCaAlF₆ (Cr:LiCAF) and Cr³⁺:LiSrGaF₆ (Cr:LiSGAF), emitting around 0.8–0.9 μm. (Such crystals are called colquiriites.)

Passively mode-locked lasers based on such media can be used for pulse durations down to roughly 10 fs. Compared with titanium–sapphire lasers, such lasers can be much cheaper, because they use a red rather than a green pump source and can be operated with low pump powers, so that diode pumping is feasible. However, the output powers achievable are lower (partly because of thermal quenching effects at higher temperatures), the wavelength tuning range is smaller, and the minimum pulse duration is larger.

Some rather new materials are Cr³⁺:LiInGeO₄ (Cr:LIGO), Cr³⁺:LiScGeO₄, and Cr³⁺:LiInSiO₄ (Cr:LISO) [21, 23, 25]. Here, Cr³⁺ ions emit in a surprisingly long wavelength range between about 1.2 and 1.6 μm (which is more typical for Cr⁴⁺) and with a very large bandwidth.

Cr⁴⁺

Cr⁴⁺ ions occur in media such as Cr⁴⁺:YAG, Cr⁴⁺:MgSiO₄ (forsterite) and other silicates, and also in germanates, apatites and other crystal types. The emission range is e.g. ≈ 1.35–1.65 μm for Cr⁴⁺:YAG and 1.1–1.37 μm for Cr⁴⁺:MgSiO₄. Pulse durations below 20 fs have been achieved e.g. with Cr⁴⁺:MgSiO₄. Nd:YAG lasers are often used for pumping such Cr⁴⁺ lasers.

Further Remarks

Due to the strong electron–phonon interaction in such gain media, chromium-doped lasers are called vibronic lasers and have a large gain bandwidth.

Note that some chromium-doped crystals, in particular Cr⁴⁺:YAG, are also used as saturable absorbers in Q-switched lasers.

Bibliography