定义：
在一个方向传播的光束。

大多数情况下，激光器辐射的光都是激光光束。即辐射光束在一个特定方向传播，光功率集中在几个平方毫米的小区域上。 
激光光束与高斯光束很接近，高斯光束光强的横剖面可由高斯方程描述，在传播方向上其宽度会发生改变。光束很宽时尺寸改变很小，而紧聚焦光束的尺寸则变化很快。 

图1：高斯光束焦点附近电场分布。这里光束半径稍大于波长，光束发散角很大。 
通常激光光束具有很高的空间相干性，因此光束质量很高。所以，激光光束具有很好的聚焦性能，可以形成具有很低光束发散角的准直光束。 
激光光束的光学带宽通常很小，因此时间相干性也很高。很高的相干性的一个缺点就是容易形成斑纹图样。 
激光光束在透明介质中传播时功率几乎不变，在吸收或散射介质中会快速衰减。非各向同性介质（介质折射率是局部变化的）也会使激光光束形状发生畸变。
这可能是由于增益介质中的热透镜产生的热效应引起的。 
一些激光器工作于连续模式，但是激光光束包含一系列脉冲，通常为每秒几百万甚至几十亿个脉冲（参阅脉冲重复速率）。 
大多数激光光束是线偏振的，即电场在与传播方向垂直的某一个方向振荡。而有些激光器辐射的激光的偏振态是不确定的。 
功率足够高的可见光激光光束在空气中传播时是可见的。因为光束在空气中传播会受到尘埃粒子以及密度涨落的散射，散射的光到达眼睛，于是光束就是可见的。
当激光光束射向漫散射物体时，例如白屏，可以在屏幕上看到一个更亮的点，因为大多数的光功率都散射到了这一个点上。 

近场和远场激光光束 
近场激光光束是指束腰（焦点）附近的区域。而远场则是指远离束腰时的光束截面，即距焦点的距离远大于有效瑞利长度。
远场强度截面能够给出光束发散角，而如果在近场需要得到光束发散角只能测量波前。
由于实际中很难直接得到远场光束，需要采用聚焦透镜得到焦平面上的强度截面，从而可以得到缩小版的远场图样。 

实验表征激光光束 
可以采用许多装置的技术来表征激光光束： 

• 可以采用功率计测量激光光束的总功率。还可以用能量计来测量脉冲激光器的脉冲能量。 
• 可以采用光束质量分析仪测量强度截面（辐照剖面），该仪器需要采用照相机。一些激光光束存在“热点”，即该区域强度更高，有可能产生损伤现象。
• 伴线结构，晕圈等其它现象会引起光束不均匀。光束强度截面的形状在传播过程中也会发生改变。 
• 有些装置不是测量整个光束截面，而是仅测量光束半径或直径。光束半径的定义可以为半高半宽值，也可能是强度变为峰值强度的 1/e2时对应的半径。这在高斯光束中有讲述。 
• 波前传感器，例如Shack-Hartmann类型，可以用来测量光束波前的形状。 
• 光束质量（M2因子）可以由多个强度截面计算得出，或者由某一位置的强度和波前计算得出。 
• 激光光束存在光束指向涨落，可以在一个合适位置监测光束中的某一点（强度分布的中心）来测量该涨落。 
• 偏振态可由通过不同指向的偏振器后的透射功率得出，也可以由旋光测定法得出。 

在激光器应用中，采用合适的方法进行束流诊断是非常必要的，因为遇到的许多问题都能采用这种装置检测出来。

Definition: light beams propagating dominantly in one direction

More general term: light beams

More specific terms: collimated beams, Gaussian beams, diffraction-limited beams, flat-top beams

In most cases, a laser emits light in the form of a well directed laser beam. This means that the light dominantly propagates in a certain direction, typically with most of the optical power concentrated to a small area of the order of a square millimeter.

Laser beams are often close to Gaussian beams, where the transverse profile of the optical intensity can be described with a Gaussian function, the width of which varies along the propagation direction.

Figure 1: Snapshot of the electric field distribution around the focus of a Gaussian beam. In this example, the beam radius is only slightly larger than the wavelength, and the beam divergence is strong.

Generally, laser beams exhibit a high degree of spatial coherence, which is related to a high beam quality. As a result, one obtains good focusability and the potential to form collimated beams with very low beam divergence.

When a laser beam hits some object (for example a workpiece in laser material processing), the arriving optical intensity is also called the irradiance.

The propagation of Gaussian beams can be calculated with a set of relatively simple equations. In cases with non-ideal beam quality, one can use a generalized set of equations which also involves the so-called beam quality factor M². In this case, the equations cannot predict the detailed evolution of beam profile, but only of the beam radius based on the second moment of the intensity profile (D4σ method).

Laser light often have a small optical bandwidth, so that the temporal coherence is also high. An often unwanted consequence of the high level of coherence is the tendency to form laser speckle patterns.

The optical power of a laser beam may hardly change during propagation in a transparent medium, or quickly decay in an absorbing or scattering medium. Inhomogeneous media (i.e., media with a locally varying refractive index) can also distort the shapes of laser beams. This can happen due to e.g. thermal effects such as thermal lensing in a gain medium.

Some lasers emit continuously, but a laser beam can also consist of a fast sequence of pulses, with many millions or even billions of pulses per second (→ pulse repetition rate). The light distribution may then be described as a regular sequence of a kind of “light bullets”.

Laser light is often linearly polarized, i.e., the electric field oscillates in a certain direction perpendicular to the propagation direction. Some lasers, however, emit light with an undefined, fluctuating polarization state.

Is a laser beam visible from the side?

A laser beam of visible light with sufficiently high power may be visible when propagating in air. This is because a tiny portion of the optical power is scattered by dust particles and/or density fluctuations in the air and can therefore reach the observing eye. When the laser beam hits some diffusely scattering object, such as a white screen, a much brighter spot is seen on that screen, since most of the optical power is scattered at this point.

Near Field and Far Field of Laser Beams

The near field is understood to be the region around the beam waist (focus). The far field concerns the profile far from the waist, i.e., in a distance from the focus which is large compared with the effective Rayleigh length. The far field intensity profile reveals details of the beam divergence, which in the near field can be obtained only with wavefront measurements. As it is often not practical to access the far field directly, one may use a focusing lens (or mirror) to obtain an intensity profile in its focal plane which reveals a scaled-down version of the far field pattern.

Limitations for the Focusing of Laser Beams

Laser beams can be used for transmitting optical energy to rather small spots or with low divergence over large distances. However, there are limitations to this, which involve the optical wavelength, the beam quality and the transverse size of the used focusing or collimation optics. Some of these limitations are discussed in the following, always assuming propagation of the beam through a homogeneous medium (i.e., with no added beam distortions on the way).

If a laser beam is focused to a spot (beam waist) with beam radius w₀, it exhibits a certain beam divergence angle which is inversely proportional to the waist beam radius and proportional to the optical wavelength in the M² factor:

The used focusing optics must be able to handle that amount of divergence; a limitation for that results from the limited numerical aperture of the optics. Also, if the focus has to have a large distance from the focusing optics and/or the waist radius is small or the beam quality is low, the beam radius in the focusing optics will necessarily be quite large. A correspondingly large open aperture of the optics is required. Such factors can in practice set a lower limit to the achievable radius in the focus, or an upper limit to the tolerable M² factor.

In some cases, one optimizes the beam radius in the focus such that the radius at a certain distance from the focus is as small as possible. For that purpose, one has to choose the waist beam radius such that the effective Rayleigh length equals the mentioned transmission distance. The resulting radius at the distant position will then be ≈1.41 times the waist beam radius.

If the sent-out light does not have to have its focus at the focusing optics, one should place the focus in the middle between the optics and the distant point. Ideally, the effective Rayleigh length will then be half the transmission distance. The beam radius at the distance spot will be identical to that at the focusing optics, and ≈1.41 times larger than in the beam focus.

For example, if a Gaussian laser beam at 1064 nm should be sent to the moon (distance 380 000 km) in order to illuminate a small spot there, the ideal radius in the focus will be 8 m, such that the Rayleigh length is 190 000 km. The radius in the focusing optics and on the moon will then be 11.3 m. If the used sending telescope cannot be that large, the illuminated spot on the moon will be larger.

When creating a beam focus in some distance from some focusing optics with a limited open aperture, the possible waist radius will increase if that distance is increased. Furthermore, there is a maximum to that distance; in the extreme case, the distance equals the effective Rayleigh length of the beam, and the resulting waist beam radius will be smaller than the beam radius at the focusing optics by the square root of 2 (≈1.41). One can thus easily calculate the Rayleigh length based on the initial radius and thus the maximum focusing distance.

Experimental Characterization of Laser Beams

There are various devices and techniques for characterizing a laser beam in various respects; see the article on laser beam characterization.

For many laser applications, it is essential to have proper means for beam diagnostics, as many possible problems can be identified with such instruments.

Switching of Laser Beams

If a laser beam is unwanted at certain times – for example, for safety reasons –, one may in principle simply turn off the laser. This is not always practical, however. In some cases, one applies some kind of beam shutter, with which the beam can be blocked when necessary. The switching can also be automated, e.g. to automatically lock a beam when a laser enclosure is opened. For periodic switching, there are optical choppers.

Bibliography