定义：
利用半导体作为增益介质的激光器。

半导体激光器是采用半导体增益介质的激光器，其中在导带具有很高载流子浓度的情况下，会实现带间受激辐射，产生增益。 
半导体中增益产生的物理机制如图1所示（正常的带间跃迁情况）。不存在泵浦光的情况下，大部分的电子处于价带。泵浦光束中光子能量只要稍大于带隙能量，就可以将电子激发到导带中的较高态，然后迅速衰变到导带中接近导带底的态上。
同时，价带中产生的空穴往价带顶移动。导带中的电子和这些空穴重新复合，产生能量接近于带隙能量的光子。这一过程也可由具有合适能量的入射光子激发。对这一过程的定量描述可以采用价带导带电子的费米-狄拉克分布来得到。 
大多数半导体激光器都是激光二极管，是由电流泵浦n型掺杂和p型掺杂半导体材料复合区域。但是，也有光学泵浦的半导体激光器，其中载流子由被吸收的泵浦光产生，还有量子级联激光器，利用了带间跃迁。 

图1：半导体增益的物理解释。
常见的半导体激光器材料为（以及其它光电器件）： 

1. GaAs（砷化镓） 
2. AlGaAs（铝镓砷） 
3. GaP（磷化镓） 
4. InGaP（铟镓磷） 
5. GaN（氮化镓） 
6. InGaAs（铟镓砷） 
7. InGaNAs（铟镓氮砷） 
8. InP（磷化铟） 
9. GaInP（磷化镓铟）

这些都是直接带隙半导体，间接带隙半导体（例如，硅）不能进行强的、有效的光辐射。由于激光二极管的光子能量接近于带隙能量，因此不同带隙能量的组分可以辐射不同的波长。
对于三元和四元半导体化合物来说，带隙能量可以连续的在很大范围内变化。例如，在AlGaAs（AlxGa1−xAs）中，提高铝的含量（即增大x）可以提高带隙能量。 
大多数半导体激光器是工作在近红外光谱区域，还有一些产生红光（例如，在GaInP激光笔）或者蓝光或者紫光（采用氮化镓）。对于中红外辐射，有硒化铅（PbSe）激光器（铅盐激光器）和量子级联激光器。 
除了以上提到的无机半导体，有机半导体化合物也可以用到半导体激光器中。对应的技术还不是特别成熟，但是发展还在继续，为了找到一种大规模生产这种激光器的方法。
到目前为止，只存在光学泵浦的有机半导体激光器，但由于各种原因采用电泵浦很难达到很高的效率。 

目录

1. 半导体激光器的类型
2. 典型特点和应用
3. 脉冲输出
4. 调制和稳定

半导体激光器的类型 
有很多种类的半导体激光器，具有不同的参数并且应用到不同的领域： 

1. 小的边发射激光二极管产生几个毫瓦的输出功率（或者达到0.5W），光束具有很高的光束质量。它们可以用作激光笔，CD播放器和光纤通信中。 
2. 外腔二极管激光器包含激光二极管作为长的激光腔中的增益介质。它们通常是波长可调的，辐射线宽也很小。 
3. 单片和外腔的低功率激光器都可以进行锁模产生超短脉冲。 
4. 大面积激光二极管产生几个瓦特的输出功率，但是光束质量非常差。 
5. 高功率二极管线阵包含一列大面积发射器，产生光束质量差的几十瓦的输出功率。 
6. 高功率堆积二极管线阵包含二极管线阵的堆积，产生成百上千瓦的极高功率。 
7. 边发射激光器（VCSELs）辐射激光的方向垂直于薄片，光束质量很高，约为几毫瓦。 
8. 光学泵浦的表面发射外腔半导体激光器（VECSELs）可以产生几瓦的输出功率，光束质量很高，甚至可以实现锁模。 
9. 量子级联激光器工作在带内跃迁（而不是带间跃迁），通常辐射中红外光，有时在太赫兹区域。它们可以用于痕量气体分析。 

典型特点和应用 
半导体激光器的一些特点包括： 

1. 适当电压进行电学泵浦和高效率尤其在高功率二极管激光器中可以实现，并且可以作为高效固态激光器的泵浦光源（参阅二极管泵浦激光器）。 
2. 采用不同的装置可以得到很宽的波长范围，覆盖了可见光、近红外光和中红外光谱区域。这些装置还可以被波长调谐。 
3. 小的激光二极管可以实现快速的开关和调制光功率，因此可以用作光纤链路中的发射器。 

以上的特性使半导体激光器为最重要的一种激光器。它的应用非常广泛，包括光学数据传输，光学数据存储，测量，光谱学，材料加工，泵浦固态激光器（参阅二极管泵浦激光器）和其它各种的医学治疗。 

脉冲输出 
大部分半导体激光器产生的都是连续输出。由于它们的能量存储能力较差（很短的上能态寿命），半导体激光器非常适合采用Q开关得到脉冲产生，但是准连续光工作可以得到更大的功率。
并且，半导体激光器利用锁模或增益开关可以产生超短脉冲。短脉冲的平均输出功率最多为几个毫瓦，光学泵浦的边发射外腔半导体激光器（VECSELs）除外，后者可以产生平均输出功率为几瓦的皮秒脉冲，其重复速率为GHz量级。 

调制和稳定 
上能态寿命短的好处就是半导体激光器可以由非常高的频率进行调制，对于VCSELs，调制频率可达几十GHz。这在光学数据传输中应用比较多，还可以用在光谱学，将激光器稳定到参考腔上等。

Definition: lasers based on semiconductor gain media

More general term: solid-state lasers

More specific terms: laser diodes, optically pumped surface-emitting semiconductor lasers, quantum cascade lasers

Semiconductor lasers are solid-state lasers based on semiconductor gain media, where optical amplification is usually achieved by stimulated emission at an interband transition under conditions of a high carrier density in the conduction band.

The physical origin of gain in an optically pumped semiconductor (for the usual case of an interband transition) is illustrated in Figure 1. Without pumping, most of the electrons are in the valence band. A pump beam with a photon energy slightly above the band gap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band. At the same time, the holes generated in the valence band move to the top of the valence band. Electrons in the conduction band can then recombine with these holes, emitting photons with an energy near the bandgap energy. This process can also be stimulated by incoming photons with suitable energy. A quantitative description can be based on the Fermi–Dirac distributions for electrons in both bands.

Note that this process works well only for a semiconductor with a direct band gap. In indirect band gap semiconductors (e.g. silicon), the conduction band electrons in the holes acquire substantially different wavenumbers, and that does not allow for optical transitions due to problems with momentum conservation.

Most semiconductor lasers are laser diodes, which are pumped with an electrical current in a region where an n-doped and a p-doped semiconductor material meet. However, there are also optically pumped semiconductor lasers, where carriers are generated by absorbed pump light, and quantum cascade lasers, where intraband transitions are utilized.

Figure 1: Physical origin of gain in a semiconductor.

Common materials for semiconductor lasers (and for other optoelectronic devices), all having a direct band gap, are

• GaAs (gallium arsenide)
• AlGaAs (aluminum gallium arsenide)
• GaP (gallium phosphide)
• InGaP (indium gallium phosphide)
• GaN (gallium nitride)
• InGaAs (indium gallium arsenide)
• GaInNAs (indium gallium arsenide nitride)
• InP (indium phosphide)
• GaInP (gallium indium phosphide)

As the photon energy of a laser diode is close to the bandgap energy, compositions with different bandgap energies allow for different emission wavelengths. For the ternary and quaternary semiconductor compounds, the bandgap energy can be continuously varied in some substantial range. In AlGaAs = Al_xGa_1−xAs, for example, an increased aluminum content (increased x) causes an increase in the bandgap energy.

While the most common semiconductor lasers are operating in the near-infrared spectral region, some others generate red light (e.g. in GaInP-based laser pointers) or blue or violet light (with gallium nitrides). For mid-infrared emission, there are e.g. lead selenide (PbSe) lasers (lead salt lasers) and quantum cascade lasers.

Apart from the above-mentioned inorganic semiconductors, organic semiconductor compounds might also be used for semiconductor lasers. The corresponding technology is by far not mature, but its development is pursued because of the attractive prospect of finding a way for cheap mass production of such lasers. So far, only optically pumped organic semiconductor lasers have been demonstrated, whereas for various reasons it is difficult to achieve a high efficiency with electrical pumping.

Types of Semiconductor Lasers

There is a great variety of different semiconductor lasers, spanning wide parameter regions and many different application areas:

• Small edge-emitting laser diodes generate a few milliwatts (or up to 0.5 W) of output power in a beam with high beam quality. They are used e.g. in laser pointers, in CD players, and for optical fiber communications.
• External cavity diode lasers contain a laser diode as the gain medium of a longer laser cavity. They are often wavelength-tunable and exhibit a small emission linewidth.
• Both monolithic and external-cavity low-power levels can also be mode-locked for ultrashort pulse generation.
• Broad area laser diodes generate up to a few watts of output power, but with significantly poorer beam quality.
• High-power diode bars contain an array of broad-area emitters, generating tens of watts with poor beam quality.
• High-power stacked diode bars can generate extremely high powers of hundreds or thousands of watts.
• Surface-emitting lasers (VCSELs) emit the laser radiation in a direction perpendicular to the wafer, delivering a few milliwatts with high beam quality.
• Optically pumped surface-emitting external-cavity semiconductor lasers (VECSELs) are capable of generating multi-watt output powers with excellent beam quality, even in mode-locked operation. Electrically pumped photonic crystal surface-emitting lasers promise to reach similar performance.
• Quantum cascade lasers operate on intraband transitions (rather than interband transitions) and usually emit in the mid-infrared region, sometimes in the terahertz region. They are used e.g. for trace gas analysis.

Typical Characteristics and Applications

Some typical aspects of semiconductor lasers are:

• Electrical pumping with moderate voltages and high efficiency is possible particularly for high-power diode lasers, and allows their use e.g. as pump sources for highly efficient solid-state lasers (→ diode-pumped lasers) and even as direct diode lasers.
• A wide range of wavelengths are accessible with different devices, covering much of the visible, near-infrared and mid-infrared spectral region. Some devices also allow for wavelength tuning.
• Small laser diodes allow fast switching and modulation of the optical power, allowing their use e.g. in transmitters of fiber-optic links.

Such characteristics have made semiconductor lasers the technologically most important type of lasers. Their applications are extremely widespread, including areas as diverse as optical data transmission, optical data storage, metrology, laser spectroscopy, laser material processing, pumping solid-state lasers (→ diode-pumped lasers), and various kinds of medical treatments.

Pulsed Output

Most semiconductor lasers generate a continuous output. Due to their very limited energy storage capability (low upper-state lifetime), they are not suitable for pulse generation with Q switching, but quasi-continuous-wave operation often allows for significantly enhanced powers.

Also, semiconductor lasers can be used for the generation of ultrashort pulses with mode locking or gain switching. The average output powers in short pulses are usually limited to at most a few milliwatts, except for optically pumped surface-emitting external-cavity semiconductor lasers (VECSELs), which can generate multi-watt average output powers in picosecond pulses with multi-gigahertz repetition rates.

Modulation and Stabilization

A particular advantage of the short upper-state lifetime is the capability of semiconductor lasers to be modulated with very high frequencies, which can be tens of gigahertz for VCSELs. This is exploited mainly in optical data transmission, but also in spectroscopy, for the frequency stabilization to reference cavities, etc.

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