定义：
以载流p-n结为增益介质的半导体激光器。

激光二极管是电泵浦的半导体激光器，其中增益是由p-n结或p-i-n结构中的电流产生的。在这种异质结构中，电子和空穴会发生复合，释放的能量为光子能量的倍数。
这一过程可以是自发的，也可以由入射光子激发，结果会得到光学放大，如果在激光器谐振腔中采用光学反馈就可以实现激光器振荡。在词条半导体激光器中有更详细的关于半导体中激光放大过程的描述。 
二极管激光器是采用一个或多个激光二极管的激光器。 
大部分半导体激光器都是基于激光二极管的基础上的，但是也有一些光泵浦的半导体激光器，它们不需要二极管结构，因此不属于二极管激光器。 

目录 

1. 激光二极管的种类
2. 辐射波长
3. 辐射带宽和波长调谐
4. 功率转化效率
5. 光束质量和光束整形
6. 光束合束
7. 脉冲产生
8. 噪声特性
9. 器件寿命
10. 应用
11. 相关器件

激光二极管的种类 
激光二极管通常为边发射激光器，镀膜的或者不镀膜的半导体薄膜的端面可以形成激光器谐振腔。它们通常为双异质结结构，可以将产生的载流子限制在很窄的区域，同时也可以作为光场的波导（双层限制）。
电流也限制在同一个区域，有时需要采用隔离栅。这样可以得到较低的阈值功率和高的效率。有源区通常很薄，可以看作是一个量子阱。有时会采用量子点。 
目前的LDs通常是表面发射类型，即发射方向垂直于薄膜表面，增益是由多个量子阱提供的。 
有很多不同类型的LDs，工作在不同的输出功率、波长、带宽和其它性质下： 

1. 小的边发射LDs产生几毫瓦到0.5个瓦特之间的光束，并且光束质量很高。输出光束可能发射到自由空间或者耦合到单模光纤中。这种激光器可以是折射率导引（利用波导结构使光在LD中传播）或者增益导引（通过偏性放大使光束非常窄）。 
2. 类似于具有短的谐振腔的分布反馈激光器（DFB激光器）或者分布布拉格反射激光器（DBR激光器）的小的LDs可以实现单频工作，有是的还具有波长可调谐性。 
3. 外腔二极管激光器包含激光二极管作为长的激光谐振腔的增益介质，还具有其它的光学元件，例如激光反射镜或者衍射光栅。它们也是波长可调的，并且辐射带宽很小。 
4. 大面积激光二极管（也称为宽条形激光二极管，宽条形激光器或者高亮度激光二极管）的输出功率有几瓦特。其光束质量比低功率LDs低很多，但是比二极管线阵高。锥形大面积激光器具有更好的光束质量和亮度。 
5. 平板耦合光波导激光器（SCOWLs）在一个较大的波导结构中包含一个多量子阱增益区域，可以产生具有衍射极限的瓦特量级的光束，形状为圆形。 
6. 高功率二极管线阵包含一列大面积发射器，产生较低光束质量的几十瓦特的光束。除了其功率更高之外，它的亮度比大面积LD要低。 
7. 大功率堆积二极管线阵（参阅二极管堆）是多个二极管线阵堆积在一起的，可以产生几百甚至几千瓦特的超高功率。 
8. 表面发射半导体激光器（VCSELs）通常产生几毫瓦高光束质量的光。还有外腔发射半导体激光器（VECSELs）可以产生更高功率但是仍然具有很好光束质量的光。 

激光二极管可以将光束辐射到自由空间中，也有很多LDs存在光纤耦合的形式。后者在应用中特别方便，例如，作为光纤激光器和光纤放大器的泵浦光源。 
几乎所有的电泵浦半导体激光器都是激光二极管，量子级联激光器是一个例外。其它的半导体激光器是利用光学泵浦，因此不需要p-n结，它们可以由不掺杂的半导体材料得到。 

辐射波长 
激光二极管的辐射波长主要由激光活性半导体材料的带隙所决定：即光子能量接近于带隙能量。由于存在非常多的半导体材料，因此该激光器可以覆盖很大的光谱区域。
尤其是存在很多三元或者四元半导体混合物，可以通过改变混合物组分在很大范围内调节半导体混合物的带隙能量。例如，例如在AlxGa1−xAs中提高铝的含量可以提高带隙能量，因此辐射波长变短。表1给出了典型的材料系统的概括。

表1：各种类型激光二极管的辐射波长 

大多数激光二极管辐射的光在近红外光谱区域，也有的辐射可见光（尤其是红色或者蓝色）或者中红外光。 

辐射带宽和波长调谐 
大多数LDs辐射光束的光谱带宽在几个纳米。该带宽来自于多个纵的（也可能是横的）谐振腔模式（多模激光二极管）的同时振荡。其它的LDs工作在单谐振腔模式上（参阅单频工作），尤其是分布反馈激光器，因此辐射带宽窄很多，一般线宽在MHz范围。
采用外腔和利用参考腔的窄带光学反馈（参阅激光器稳定）可以进一步使线宽变窄。 
多模LDs的辐射波长（光谱的中心频率）通常与温度有关，温度没提高1 K，波长通常提高约0.3 nm，由于峰值增益与波长有关。（温度会影响价带和导带的粒子数分布。）
因此如果激光晶体的吸收带宽很窄（例如，只有几个纳米），需要稳定固态激光器中二极管泵浦的p-n结处的温度。也可以通过改变p-n结处的温度来调谐辐射波长。 
单模二极管辐射波长的温度系数低很多。如果应用到扫描光谱学中时，波长通常是工作在激光间隙进行扫描的。在每个脉冲作用时温度都会升高，引起光学频率减小。外腔激光器的波长可以通过旋转激光腔中的衍射光栅来改变。 

功率转化效率 
激光二极管具有很高的电光转化效率，典型值为50%，有时甚至大于60%。（关于如何提高高功率LDs的效率使其大于70%的项目目前也在发展。）效率主要受一些因素的限制，例如电阻，载流子泄漏，散射，吸收（尤其是在掺杂区域）和自发辐射。
当激光二极管辐射波长在940-980 nm波长范围时效率是最高的（可以用来泵浦掺镱高功率光纤装置），而808 nm的二极管效率则不是很高。 

光束质量和光束整形 
有些低功率LDs可以辐射相对较高光束质量的光束（尽管其光束发散角比较大需要在准直过程中仔细处理）。
但是，大多数高功率LDs具有相对比较低的光束质量，还有其他不是很理想的性质，例如光束发散角大，光束半径和两个垂直方向光束质量很高的不对称性，还有像散现象。光束整形光学中找到合适的设计非常重要，尺寸要小，易于加工和排列，保持光束质量并且避免产生干涉条纹，消除像散，具有较低损耗等。通常二极管激光光束整形光学涉及到准直透镜，孔径和变形棱镜。 

光束合束 
由于激光二极管辐射的光是线偏振的，因此可以采用一个偏振分束器将两二极管输出光合在一起，这样就可以得到具有两倍于单个二极管功率、与单个二极管光束质量相同的非偏振的光（偏振复用）。还可以采用二色性反射镜将两个波长稍微不同的LDs的光束合在一起（参阅光谱合束）。光束合成更系统的方案可以将很多发射器发出的具有很好光束质量的光合在一起。 

脉冲产生 
最常见的LDs的工作模式为连续光工作，但很多LDs也可以产生光学脉冲。大部分情况下，脉冲产生的原理是增益开关，即通过开光泵浦电流调制光学增益。小的二极管也可以是锁模的，产生皮秒甚至飞秒脉冲。锁模激光二极管可能是外腔装置或者单片式的，后者通常包含工作在不同电流下的部分。 

噪声特性 
不同种类的二极管具有不同的噪声特性。当大于弛豫振荡频率时（通常几个GHz），强度噪声通常是接近于量子极限的。但是，有些工作在低温下的低功率LDs甚至具有很显著的振幅压缩，即强度噪声可以小于散粒噪声极限。所有的半导体激光器中，强度噪声都与相位噪声相互关联。 
如上所述，线宽值是非常不同的。多模LDs具有很多与跳模有关的附加噪声。不同模式的噪声可以是非常反相关的，因此单个模式的强度噪声比总功率的噪声强很多。这会得到一个很重要的结果就是，如果二极管线阵在某一孔径被截断或者被光谱滤波，那么强度噪声反而会增加。 二极管驱动器也会对激光器噪声有贡献，因此即使电流涨落非常快，它也会转化成产生光的强度和相位涨落。 

器件寿命 
当工作条件比较适宜时，二极管激光器的寿命在上万个小时。但是有很多其它的因素会使寿命缩短，例如工作在过高的温度（例如，由于冷却不充足），电流或电压太高，例如来自于静电放电或者不合理设计的激光驱动器。 
还有一些失败的工作模式，包括光学灾变损伤（COD）（在几毫秒或者更短时间内器件完全损坏）和稳步下降。除了工作条件之外，还有很多设计因素会影响寿命。
例如，如果采用无铝的有源区可以得到更好的稳定性和寿命，在光学表面的特定涂层（或者一些附件的半导体涂层）也非常有用。有些改进的二极管设计并没有被公开，是由于制造者要保持技术上的优势。 
为了提高器件寿命，LDs通常工作在较低的电流下（输出功率也较低）。适当降低功率由于具有较低的结电压，可以提高电光转换效率，然而过度降低则会降低效率。 

应用 
激光二极管可以应用到很多方面。下面给出一些重要的例子： 

1. 低功率的具有较高光束质量的单模LDs可以用作在CD-ROMs,DVDs,蓝光Discs和全息数据存储介质中进行数据存储和读取。这些激光器可以工作在不同的光谱区域，从红外光到蓝光和紫光区域，波长越短，存储密度越大。 
2. 单模LDs在光纤通信中应用非常广泛，尤其是作为数据发射器。有些情况下，数据调制可直接由驱动电流来实现。 
3. 单模LDs通常可以应用到光谱学中，采用尺寸非常小的低功率测量器件。 
4. 小的红光激光二极管（参阅红光激光器）可以作为激光笔。 
5. 采用调制的低功率二极管激光器可以进行距离测量。激光打印机、扫描仪和条形码读取器采用的也是类似的激光器。 
6. 大面积激光二极管，二极管线阵和二极管堆可用作固态激光器的二极管泵浦。光纤耦合的大面积LDs可以作为光纤放大器的泵浦光源。 
7. 二极管线阵的辐射可以进行医学诊断（例如治疗前列腺肿大）和皮肤病治疗。 
8. 高功率的二极管堆可以直接用于材料加工，这时不需要高的光束质量，例如用于表面加固，焊接和锡焊。与其它高功率激光器相比，它们更加简单，并且具有更高的电光转化效率。 

在销量方面，光学数据存储和通信是占主导的，占第三位的是固态激光器的泵浦光源，它的销量比前两个领域小一个数量级。 

相关器件 
LDs通常以激光二极管模块的形式存在，模块还包含了其它的用于光束整形、冷却和保护、以及波长转化的器件。 
半导体光放大器（SOA）装置类似于LD，但是其端反射被抑制。如果没有入射信号的情况下，这一器件可以作为超发光二极管（SLD），通过放大的自发辐射产生光。这时光谱是非常平滑，非常宽的。 
发光二极管（LEDs）采用与LDs相同的机制产生光子，但是通常没有显著的光放大（激光增益）。共振腔LEDs一定程度上利用了受激辐射，因此是介于LEDs和SLDs之间的。

Acronym: LD

Definition: semiconductor lasers with a current-carrying p–n junction as the gain medium

More general term: semiconductor lasers

More specific terms: broad area laser diodes, high brightness laser diodes, diode bars, diode stacks, tapered laser diodes, Fabry–Pérot laser diodes, photonic crystal surface-emitting lasers

Laser diodes are electrically pumped semiconductor lasers in which the gain is generated by an electrical current flowing through a p–n junction or (more frequently) a p–i–n structure. In such a heterostructure of a bipolar interband laser, electrons and holes can recombine, releasing the energy portions as photons. This process can be spontaneous, but can also be stimulated by incident photons, in effect leading to optical amplification, and with optical feedback in a laser resonator to laser oscillation. The article on semiconductor lasers describes more in detail how the laser amplification process in a semiconductor works.

Diode lasers are lasers based on one or several laser diodes, and possibly auxiliary parts, e.g. for cooling.

Most semiconductor lasers are based on laser diodes, but there are also some types of semiconductor lasers which do not require a diode structure and thus not belong to the category of diode lasers. In particular, there are quantum cascade lasers and optically pumped semiconductor lasers. The latter can be made of undoped semiconductor materials which cannot conduct significant electrical currents.

Types of Laser Diodes

Figure 1: Schematic setup of an edge-emitting low-power laser diode. The waveguide and the output beam emerging at one edge of the wafer die are shown, but not the electrode structures.

Most laser diodes (LDs) are built as edge-emitting lasers, where the laser resonator is formed by coated or uncoated end facets (cleaved edges) of the semiconductor wafer. They are often based on a double heterostructure, which restricts the generated carriers to a narrow region and at the same time serves as a waveguide for the optical field (double confinement). The current flow is restricted to the same region, sometimes using isolating barriers. Such arrangements lead to a relatively low threshold pump power and high efficiency. The active region is usually quite thin – often so thin that it acts as a quantum well. In some cases, quantum dots are used.

Some modern kinds of LDs are of the surface-emitting type (see below), where the emission direction is perpendicular to the wafer surface, and the gain is provided by multiple quantum wells.

There are very different kinds of LDs, operating in very different regimes of optical output power, wavelength, bandwidth, and other properties:

• Small edge-emitting LDs generate between a few milliwatts and up to roughly half a watt of output power in a beam with high beam quality. The output may be emitted into free space or coupled into a single-mode fiber (→ fiber-coupled diode lasers). Such lasers can be designed to be either index guiding (with a waveguide structure guiding the laser light within the LD) or gain guiding (where the beam profile is kept narrow via preferential amplification on the beam axis).
• For small LDs made as Fabry–Pérot laser diodes or more specifically as distributed Bragg reflector lasers (DBR lasers) with short resonators, one may achieve single-frequency operation. More reliable single-frequency operation can be achieved with distributed feedback lasers (DFB lasers). In some cases one also achieves substantial wavelength tunability.
• External cavity diode lasers contain a laser diode as the gain medium of a longer laser resonator, completed with additional optical elements such as laser mirrors or a diffraction grating. They are often wavelength-tunable and exhibit a small emission linewidth.
• Broad area laser diodes (also often called broad stripe laser diodes or wide stripe lasers) generate up to a few watts of output power. The beam quality is significantly poorer than that of lower-power LDs, but better than that of diode bars (see below).
• High brightness laser diodes are laser diodes which are optimized for a particularly high radiance (brightness). Different technologies may be used, and such lasers are available on quite different power levels.
• Slab-coupled optical waveguide lasers (SCOWLs), containing a multi-quantum well gain region in a relatively large waveguide, can generate a watt-level output in a diffraction-limited beam with a nearly circular profile.
• High-power diode bars contain an array of broad-area emitters, generating tens of watts with poor beam quality. Despite the higher power, the brightness is lower than that of a broad area LD.
• High-power stacked diode bars (→ diode stacks) are stacks of multiple diode bars for the generation of extremely high powers of hundreds or thousands of watts.
• Monolithic surface-emitting semiconductor lasers (VCSELs) typically generate a few milliwatts with high beam quality. There are also external-cavity versions of such lasers (VECSELs) which can generate much higher powers with still excellent beam quality. Finally, there are photonic crystal surface-emitting lasers, which can combine the performance of a VECSEL with a monolithic setup and electrical pumping..

Laser diodes may emit a beam into free space, but many LDs are also available in fiber-coupled form. The latter makes it particularly convenient to use them, e.g., as pump sources for fiber lasers and fiber amplifiers.

Emission Wavelengths

The emission wavelength of a laser diode is essentially determined by the band gap of the laser-active semiconductor material: the photon energy is close to the band gap energy. In quantum well lasers, there is also some influence of the quantum well thickness. A variety of semiconductor materials makes it possible to cover wide spectral regions. In particular, there are many ternary and quaternary semiconductor compounds, where the bandgap energy can be adjusted in a wide range simply by varying the composition details. For example, an increased aluminum content (increased x) in Al_xGa_1−xAs causes an increase in the bandgap energy and thus a shorter emission wavelength. Table 1 gives an overview on typical material systems.

Table 1: Emission wavelengths of various common types of laser diodes.

Note that there some laser diodes operating outside the spectral regions given in the table. For example, InGaN lasers may be optimized for longer emission wavelengths, reaching the green spectral region, although typically with lower performance. Besides, there are e.g. lead salt diodes for generating mid infrared light.

Most laser diodes emit in the near-infrared spectral region, but others can emit visible (particularly red or blue) light or mid-infrared light.

Emission Bandwidth and Wavelength Tuning

Most LDs emit a beam with an optical bandwidth of a few nanometers. This bandwidth results from the simultaneous oscillation of multiple longitudinal (and possibly transverse) resonator modes (multimode laser diodes). Some other kinds of LDs, particularly distributed feedback lasers, operate on a single resonator mode (→ single-frequency operation), so that the emission bandwidth is much narrower, typically with a linewidth in the megahertz region. Further linewidth narrowing is possible with external cavities and particularly with narrowband optical feedback from a reference cavity (→ frequency-stabilized lasers).

The emission wavelength (center of the optical spectrum) of multimode LDs is usually temperature sensitive, typically with an increase of ≈ 0.3 nm per 1 K temperature rise, resulting from the temperature dependence of the gain maximum. (The temperature influences the thermal population distributions in the valence and conduction band.) For that reason, the junction temperature of LDs for diode pumping of solid-state bulk lasers has to be stabilized, if the absorption bandwidth of the laser crystal is narrow (e.g. only a few nanometers wide). It is also possible to tune the emission wavelength via the junction temperature.

The emission wavelength also reacts to changes of the drive current – essentially just because of the induced temperature change. The current dependence thus depends on details like how the laser diode is cooled.

Single-mode diodes can have a significantly smaller temperature coefficient of the emission wavelength, as the resonance frequencies react less to temperature changes than the optical gain does. For applications in scanning laser absorption spectroscopy, the wavelength is sometimes scanned by operating the laser intermittently. The temperature then rises during each current pulse and causes the optical frequency to fall. The wavelength of external-cavity lasers can also be tuned, e.g. by rotating the diffraction grating in the laser cavity.

Voltage–Current Characteristics

Laser diodes have voltage–current characteristics like other semiconductor diodes. A substantial current flows only above a certain critical voltage, which depends on the used material system. (The critical voltage is roughly proportional to the bandgap energy of the material and also depends somewhat on the device temperature.) Above the critical voltage, the current rises rapidly with increasing voltage.

Figure 2: Current vs. applied voltage for an 808-nm laser diode. For a current of 1.2 A, as needed for the nominal output power of 1 W, the required voltage is roughly 1.8 V. (For comparison, the photon energy for 808 nm is 1.53 eV.) Note that this curve is shifted to the right for increasing device temperature; one then obtains a higher current for the same voltage.

A laser diodes is normally not operated by applying a fixed voltage, because the flowing current could then very sensitively depend on that voltage, and could also be substantially affected by the device temperature. There could even be a catastrophic runaway effect: a high current could lead to a strong temperature rise, which could further increase the current, finally destroying the diode. Therefore, in practice one usually uses a laser diode driver which stabilizes a certain current; this means that it automatically adjusts the voltage such that the desired current is obtained. Alternatively, one uses a constant power mode, where the drive current is automatically adjusted for achieving the desired output power.

Note that the current rather than the voltage determines the rate with which carriers are injected into the laser diode. Therefore, there is a strong relation between the flowing current and the emitted optical power. There is essentially no output power below a certain threshold current, and above the laser threshold the output power grows roughly in proportion to the current minus the threshold current.

Power Conversion Efficiency

Diode lasers can reach high electrical-to-optical efficiencies – typically of the order of 50%, sometimes above 60% or even above 70% [7]. At reduced operation temperatures, even around 80% are possible [10]. The efficiency is usually limited by factors such as the electrical resistance, carrier leakage, scattering, absorption (particularly in doped regions), and spontaneous emission. Particularly high efficiencies are achieved with laser diodes emitting e.g. around 940–980 nm (as used e.g. for pumping ytterbium-doped high-power fiber devices), whereas 808-nm diodes are somewhat less efficient.

The highest power conversion efficiency is typically achieved not for the highest output power, but for a somewhat reduced output power, because the required voltage is then lower.

Beam Quality and Beam Shaping

Some low-power LDs can emit beams with relatively high beam quality (even though the high beam divergence requires some care to preserve that during collimation). Most higher-power LDs, however, exhibit a relatively poor beam quality, combined with other non-favorable properties, such as a large beam divergence, high asymmetry of beam radius and beam quality between two perpendicular directions, and astigmatism. It is not always trivial to find the best design for beam shaping optics, being compact, easy to manufacture and align, preserving the beam quality and avoiding interference fringes, removing astigmatism, having low losses, etc. Typical parts of such diode laser beam shaping optics are collimating lenses (spherical or cylindrical), apertures and anamorphic prism pairs.

Beam Combining

As the light emitted by a laser diode is linearly polarized, it is possible to combine the outputs of two diodes with a polarizing beam splitter, so that an unpolarized beam with twice the power of a single diode but the same beam quality can be obtained (polarization multiplexing). Alternatively, it is possible to combine the beams of LDs with slightly different wavelengths using dichroic mirrors (→ spectral beam combining). More systematic approaches of beam combining allow combining a larger numbers of emitters with a good output beam quality. Advances in that field have much contributed to the possibility of using direct-diode lasers e.g. in laser material processing.

Pulse Generation

Although the most common mode of operation of LDs is continuous-wave operation, many LDs can also generate light pulses. In most cases, the principle of pulse generation is gain switching, i.e. modulating the optical gain by switching the pump current. Small diodes can also be mode-locked for generating picosecond or even femtosecond pulses. Mode-locked laser diodes can be external-cavity devices or monolithic, in the latter cases often containing different sections operated with different current.

Noise Properties

Different types of diodes have very different noise properties. The intensity noise is typically close to quantum-limited only well above the relaxation oscillation frequency, which is very high (often several gigahertz). However, some low-power LDs operated at cryogenic temperatures have been demonstrated to exhibit even significant amplitude squeezing, i.e., intensity noise well below the shot noise limit. In all semiconductor lasers, intensity noise is generally coupled to phase noise, making these noise properties strongly correlated.

As mentioned above, linewidth values are very different. Multimode LDs exhibit a lot of excess noise associated with mode hops. Noise in different modes can be strongly anti-correlated, so that the intensity noise in single modes can be much stronger than the noise of the combined power. This has the important consequence that the intensity noise can be increased when the beam e.g. of a diode bar is truncated at an aperture or spectrally filtered.

The laser diode driver can also contribute a lot to the laser noise, because even very fast current fluctuations can be transformed into intensity and phase fluctuations of the generated light.

Device Lifetime and Reliability

When operated under proper conditions, diode lasers can be very reliable during lifetimes of tens of thousands of hours. However, much shorter lifetimes can result from a number of factors, such as operation at too high temperatures (e.g. caused by insufficient cooling) and current or voltage spikes, e.g. from electrostatic discharge or ill-designed laser drivers.

There are different failure modes, including catastrophic optical damage (COD) (with complete device destruction within milliseconds or less) and steady degradation due to effects like oxidation of the output facet, growth of dislocations in the semiconductor structure, diffusion of metal from electrodes and degradation of heat sinks. Apart from the operation conditions, various design factors strongly influence the lifetime. For example, designs with aluminum-free active regions have been found to have superior reliability and lifetimes, and certain coatings (or just additional semiconductor layers) on optical surface can also be very helpful. The details of some advanced diode designs have not been disclosed by manufacturers in order to maintain a competitive advantage.

In order to improve device lifetimes, LDs are often operated at reduced current levels (and thus output powers). Moderate power reductions can at the same time increase the wall-plug efficiency due to the lower junction voltage, whereas stronger reductions reduce the efficiency.

A high reliability of laser diodes can be achieved not only by optimizing their design, but also by applying testing procedures, e.g. during burn in tests and other types of quality control. See the article on laser diode testing for more details.

Applications

Laser diodes are used in a very wide range of applications. The following list gives some important examples:

• Low-power single-mode LDs with high beam quality are used for data recording and reading on CD-ROMs, DVDs, Blu-ray Discs, and holographic data storage media. Such lasers can operate in different spectral domains from the infrared to the blue and violet region, with the shorter wavelengths allowing for higher recording densities.
• Single-mode LDs including VCSELs are widely used in optical fiber communications, particularly in data transmitters. In some cases, the data modulation is done directly via the drive current.
• Single-mode LDs are also applied in laser spectroscopy (TDLAS) with very compact low-power measurement devices.
• Small red laser diodes (→ red lasers) are used in laser pointers.
• Distance measurements are often done with modulated low-power diode lasers. Similar lasers are used in laser printers, scanners and bar code readers.
• Broad area laser diodes, diode bars and diode stacks are often used for diode pumping of solid-state lasers. Fiber-coupled broad area LDs also serve as pump sources of fiber amplifiers.
• Some kinds of surgery (e.g. treatment of enlarged prostates) and dermatological therapies can be done with radiation from diode bars.
• High-power diode stacks are directly used in laser material processing in cases where a high beam quality is not required, e.g. for surface hardening, welding and soldering. Compared with other high-power lasers, they are simpler and have a much better wall-plug efficiency.

In terms of sales volumes, the applications in optical data storage and telecommunications are very dominating. The third most important application, which is pumping of solid-state lasers, already has sales volumes which are nearly an order of magnitude lower than the previously mentioned sectors.

Low-power laser diodes generate the largest revenues of all laser types – mainly due to applications in communications and data storage. High-power laser diodes have far lower sales numbers and volumes, and are used mainly for displays (with fast growth), medical and military applications. Direct use of high-power laser diodes for material processing (direct-diode lasers) has a small volume so far, but exhibits rapid growth.

Related Devices

LDs are often used in the form of laser diode modules, containing a variety of additional components e.g. for beam shaping and cooling and protection of the LD, and wavelength conversion.

A semiconductor optical amplifier (SOA) has a setup which is similar to an LD, but the end reflections are suppressed. Without an input signal, such a device can act as a superluminescent diode (SLD), generating light via amplified spontaneous emission. The optical spectrum is then smooth and normally much broader.

Light-emitting diodes (LEDs) use the same mechanism of generating photons as LDs, but they usually do not exhibit significant optical amplification (laser gain). So-called resonant-cavity LEDs do exploit some degree of stimulated emission, and are in this sense intermediate between ordinary LEDs and SLDs.

Bibliography