大面积激光二极管(broad-area laser diodes)

定义：
辐射区域具有非常不对称形状的激光二极管。

大面积激光二极管（也称为宽条形或者宽发射器激光二极管，单发射器激光二极管和高亮度二极管激光器）是边发射激光二极管，其中位于前表面的发射区域的形状为宽条形（见图1），尺寸为1 μm × 100 μm。由于发射器是非对称的，光束的性质在两个方向上也是不同的： 

1. 在垂直方向，宽度很小，因此容易实现单模传导，且得到衍射极限的光束质量，因此[[M2因子|M2因子]]稍大于1。由于孔径很小，在该方向的光束发散角很大，光束发散叫的半角为370 mrad，对应的FWHM角范围为25°。由于发散很快，也称该方向为快轴。 
2. 在长度方向（慢轴方向），长条的宽度可以是50,100,200μm甚至更大，因此在该方向上光具有多个空间模式。因此，光束发散角远大于同尺寸衍射极限的光束，尽管其远小于快轴方向的发散角。光束质量会下降，对于100 μm的长条，M2在20左右。另外，在水平方向，光束可能有多个峰，强度分布与入射电流有关。 

图1：大面积激光二极管。 
长条越宽，能够得到的功率越高，但是在慢轴方向的光束质量越差。技术上发展的方向是得到越来越高的功率，即使采用的长条很窄的情况下，但是这受限于其前端面很高的光强和相关的热学问题。
特殊的端面钝化技术可以实现更高的功率。孔径宽度为100μm时，商用器件的输出功率通常为几瓦甚至10瓦。 

图2：大面积激光二极管快轴和慢轴方向输出光束半径的演化。在快轴最开始光束半径非常小，但是迅速增大。 
光束形状的机器不对称以及在快轴方向上很大的发散角需要对大面积激光器的输出进行仔细的对准。常用的方法是采用圆柱形的快轴准直透镜，具有很大的数值孔径，可与二极管端面相比拟。
这种透镜可以在快轴方向准直光束，避免光束半径变得过大。第二个圆柱形透镜在更远处可以用来准直慢轴光。通过选取合适焦距的透镜，可以得到圆形光束，但是由于在两个方向上具有不同的光束质量，因此具有不同的发散角。 
激光谐振腔通常都是单色的，半导体芯片端面的电介质涂层提供反射。在较少见的情况下，外置激光谐振腔的一端会采用抗反射涂层。（参阅外腔二极管激光器） 
大面积激光二极管常用来泵浦固态激光器。一个器件具有200μm宽的发射器可以辐射的功率为2-4W，因此泵浦激光器可以辐射功率约为2W。激光二极管通常放置在热电冷却器，可以在几纳米的范围内调谐辐射波长，因此辐射峰值与激光晶体的吸收峰值匹配。 

特殊性质 
有些二极管线阵具有特殊的性质： 

1. 将快轴准直透镜集成到激光器装置中。由于这样外装置的对准公差很小，因此是有利的。还有的激光二极管模块具有两个方向的内置光束准直器。 
2. 内置的布拉格光栅可以稳定辐射波长，使辐射光谱变窄。采用外置的布拉格光栅也能够实现相同的效果。 
3. 还有拉锥二极管激光器，其中在传播方向上的宽度显著增大，因此有源区面积也会显著增大。光束质量和亮度要比在整个有缘区域具有峰值宽度的激光二极管高。但是，输出具有很强的像散，与工作功率有关。 

与二极管线阵对比 
将多个大面积发射器结合起来得到一个器件就是二极管线阵，可以得到几十瓦甚至大于100W的光功率。但是，二极管线阵的亮度比单个发射器的激光器要低，虽然其输出功率更高，因为其光束质量更低。
因此，设计二极管泵浦的激光器要比简单的将大面积二极管泵浦要简单。即使是高功率激光器，包括高功率光纤激光器和放大器，采用多个大面积激光器代替二极管线阵也是由很多优势的。其中一个优势为大面积二极管激光器可以开或者关，而不会缩短寿命。

Definition: laser diodes with a strongly asymmetric shape of the emitting region

Alternative terms: broad stripe emitters, multimode single emitters, high-brightness diodes

More general term: laser diodes

Broad area laser diodes (also called broad stripe, multimode single emitters or broad emitter laser diodes, single-emitter laser diodes, and high brightness diode lasers) are edge-emitting laser diodes where the emitting region at the front facet has the shape of a broad stripe (see Figure 2), with dimensions of e.g. 1 μm × 100 μm. Due to the asymmetry of the emitter, the beam properties are also completely different for the two directions:

• In the vertical (short) direction, the height (e.g. 1 μm) is small enough to obtain single-mode guidance and thus an essentially diffraction-limited beam quality with an M² factor only slightly above 1. Because of the small aperture size, the beam divergence in this direction is relatively high, with a beam divergence half-angle of e.g. 370 mrad, corresponding to an FWHM angular range of 25°. Due to that fast divergence, this is called the fast axis direction.
• In the long direction (slow axis direction), the stripe width may be e.g. 50, 100, 200 μm, or even larger, so that the light is distributed over many spatial modes in this direction. As a result, the beam divergence is much larger than for a diffraction-limited beam with that size, although still significantly smaller than for the fast axis direction. (Typical values are around 5–10° FWHM.) The beam quality in terms of focusability is reduced; the M² factor might be of the order of e.g. 20 for a 100-μm stripe. Furthermore, the beam profile may be multi-peaked in the horizontal direction, and the shape of the intensity pattern may depend on the injection current.

Figure 1: Evolution of the beam radii of the output of a broad area laser diode in “fast” and “slow” direction. The beam radius starts from a much smaller value in the fast axis direction, but increases rapidly.

The wavefronts at the output facet are approximately plane in horizontal and vertical direction, but there can be some astigmatism, i.e., a slightly different focus position for the two directions.

Figure 2: Broad area laser diode.

The broader the stripe, the higher is the achievable power, but the worse is the beam quality in the “slow” direction. The technological trend is to obtain higher and higher powers even from narrow stripes, but this is limited by the high optical intensity at the front facet (which can lead to catastrophic failure) and by thermal issues. Special techniques of facet passivation can be used to allow for higher powers. For a 100 μm wide aperture, the output power of a commercial device (e.g. in the 0.8-μm wavelength region) is typically a couple of watts or up to the order of 10 W. Similar devices are also available in other wavelength regions, e.g. around 480 nm (blue) or 1550 nm, but typically with lower performance.

The laser resonator is in most cases monolithic, with reflections from dielectric coatings on the end faces of the semiconductor chip. In less common cases, a gain chip with an anti-reflection coating on one side is used in an external laser resonator (→ external-cavity diode lasers).

Asymmetric Beam Properties

The strongly asymmetric beam profile and the large divergence in the “fast” direction requires special care, e.g. for properly collimating the output of a broad area laser. A common method is the use of a cylindrical “fast axis collimator” lens with high numerical aperture in close proximity to the diode facet. Such a lens collimates the beam in the fast axis direction, before the beam radius becomes too large. A second cylindrical lens at a larger distance may then be used for collimation in the slow axis direction. By choosing lenses with suitable focal lengths, a circular beam can be obtained, which however will have different divergence angles in the two directions due to the different beam quality values.

Figure 3: Stacked beamlets from multiple broad area emitters after a suitable beam shaper – with one column (left) or two columns (right).

Much higher powers (hundreds of watts or even several kilowatts) can be launched into a multimode fiber with given parameters when using multiple (e.g. 10 to 20) broad area emitters, the outputs of which are properly combined with suitable optics (Figure 3). Here, one stacks the elongated emission patterns of such diodes such that overall a roughly quadratic area is filled with radiation, the shape of which fits better to the circular cross section of a multimode fiber core. In case of a large number of emitters, one may arrange the emitters in two columns.

Special Features

Some diode bars are offered with special features:

• A fast axis collimation lens may be integrated into the laser package. This is helpful, since the alignment tolerance for external mounting would be tight. There are also laser diode modules with built-in beam collimation in both directions.
• A built-in Bragg grating can stabilize the emission wavelength and make the emission spectrum significantly narrower. The same could also be achieved with an external volume Bragg grating.
• There are tapered laser diodes, having a region where the width and thus the area of the active region is significantly increased along the propagation direction. Due to a straight region with smaller width, the beam quality and radiance (brightness) achieved are better than in a laser diode with the maximum width along the whole active region. Therefore, such diodes are often called high brightness laser diodes.

Tapered Amplifier Devices

As an alternative to a broad area diode laser, one can use a tapered amplifier either as part of a MOPA device or as an external-cavity diode laser. With that technology, one can obtain similar output powers in conjunction with much better beam quality.

Comparison with Diode Bars

The combination of several broad area emitters in a single device leads to a diode bar, which can emit tens of watts or even more than 100 W of optical power. However, a diode bar has a lower brightness than a single-emitter laser, despite the higher output power, because the beam quality in the horizontal direction is much lower. For that reason, the design of a diode-pumped laser is generally simpler when using broad-area diodes for pumping. Even for high-power lasers, including high-power fiber lasers and amplifiers, pumping with a significant number of broad-area lasers instead of fewer diode bars has some advantages. One of those is that broad-area diode lasers, other than diode bars, can usually be turned on and off fairly often without shortening the lifetime.

Applications of Broad Area Laser Diodes

Broad area laser diodes are often used for pumping solid-state lasers. A device with a 100-μm or 200-μm broad emitter may easily emit several watts. The laser diode is often mounted on a thermoelectric cooler, which makes it possible to tune the emission wavelength within a few nanometers, so that the emission peak can be matched to the absorption maximum of the laser crystal.

One can also use such diodes for pumping fiber lasers. In other cases, diode bars are used, containing multiple broad area emitters.

Another important application area is laser material processing, see the article on direct diode lasers.

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