定义：
辐射光谱很窄的单频激光器。

一些激光器应用需要激光器线宽很窄，也就是窄光谱。窄线宽激光器都是指单频激光器，也就是激光器值存在一个谐振腔模式，相位噪声很低，因此光谱纯度很高。通常这类激光器强度噪声很低。 

窄线宽激光器类型 
最重要的几种窄线宽激光器如下： 

1. 半导体激光器，分布反馈激光二极管（DFB激光器）和分布布拉格反射激光器（DBR激光器），最重用的都是应用于1500或者1000nm区域。典型的工作参数为输出功率为几十毫瓦（有时大于100毫瓦），线宽为几个MHz。 
2. 半导体激光器得到的线宽可能更窄，例如采用包含窄带光纤布拉格光栅的单模光纤拓展谐振腔，或者采用外腔二极管激光器。采用这种方法，可以实现几 kHz甚至小于1kHz的超窄线宽。 
3. 小的分布反馈光纤激光器（谐振腔由特殊光纤布拉格光栅组成）可以产生几十毫瓦的输出功率，线宽为几kHz范围。 
4. 具有非平面环形谐振腔的二极管泵浦固态体激光器也可以得到几kHz的线宽，同时输出功率相对较高，在1W量级。尽管典型波长为1064nm，其它的波长区域，例如1300或者1500nm也可能实现。 

激光器窄线宽的主要影响因素 
为了实现激光器具有很窄的辐射带宽（线宽），在激光器设计时需要考虑以下因素： 

1. 首先，需要实现单频工作。这很容易通过采用小增益带宽的增益介质和短的激光器谐振腔（得到大的自由光谱范围）来实现。目标应该是长时间稳定的单频工作，不存在跳模。 
2. 其次，需要最小化外界噪声的影响。这需要稳定的谐振腔设置（单色），或者特殊保护装置来避免机械振动。电泵浦激光器需要采用低噪声的电流或者电压源，光学泵浦的激光器的泵浦光源则需要具有低的强度噪声。另外，需要避免所有的反馈光波，例如采用法拉第隔离器。理论上外界噪声影响小于内部噪声，例如增益介质中的自发辐射。这在噪声频率高时很容易实现，但是当噪声频率低时则对线宽的影响最重要。 
3. 第三，需要优化激光器设计使激光器噪声最小，尤其是相位噪声。最好采用高的腔内功率和长谐振腔，尽管在这种情况下稳定的单频工作更难实现。 

对系统优化需要了解不同噪声源的重要性，因为根据占据主要地位的噪声源的不同需要采用不同的测量方式。例如，根据Schawlow-Townes方程最小化的线宽没有必要最小化实际的线宽，如果实际线宽是由机械噪声决定的。 

噪声特征和性能指标 
窄线宽激光器的噪声特征和性能指标都是都是比较琐碎的问题。在词条线宽中讨论了不同的测量技术，尤其是线宽在几kHz或更窄时对测量要求很高。另外，仅考线宽值不能给出全部的噪声特征；需要给出完整的相位噪声光谱，还有相对强度噪声信息。线宽值至少需要和测量时间结合考虑，或者与考虑长时间的频率漂移的其它信息结合起来。
 
当然不同的应用要求不同，在不同的实际情况中考虑需要达到什么水平的噪声性能指标。 

窄线宽激光器的应用 

1. 一个很重要的应用是在传感领域，例如压力或者温度光纤传感器，各种干涉仪传感，利用不同的吸收LIDAR来探测追踪气体，采用多普勒LIDAR测量风速。有些光纤传感器需要激光器线宽为几kHz，而在LIDAT测量中，100kHz线宽就足够了。 
2. 光学频率测量需要光源线宽非常窄，需要采用稳定技术来实现。 
3. 光纤通信系统则对线宽的要求相对宽松，主要用于发射器或者用于探测或者测量。

Definition: single-frequency lasers with a narrow optical emission spectrum

More general terms: lasers

More specific term: frequency-stabilized lasers

A number of laser applications (see below) require lasers with a very small optical linewidth, i.e., with a narrow optical spectrum. The term narrow-linewidth lasers usually applies to single-frequency lasers, i.e., lasers oscillating on a single resonator mode with low phase noise and thus with high spectral purity. Typically, such lasers also exhibit low intensity noise.

Some lasers do not only need to have a small linewidth, but also exhibit very high stability of the optical free. See the article on frequency-stabilized lasers.

Types of Narrow-linewidth Lasers

The most important types of narrow-linewidth lasers are the following:

• Among semiconductor lasers, distributed feedback laser diodes (DFB lasers) and distributed Bragg reflector lasers (DBR lasers), operating e.g. in the 1.5- or 1.0-μm wavelength region, are the most common. Typical operation characteristics are an output power of some tens of milliwatts (or possibly somewhat above 100 mW) and a linewidth of several megahertz.
• Significantly smaller linewidths from semiconductor lasers are possible e.g. by extending the resonator with a single-mode fiber containing a narrow-bandwidth fiber Bragg grating, or with other types of external-cavity diode lasers. In such a way, ultra-narrow linewidths of a few kilohertz or even below 1 kHz can be achieved.
• Small fiber lasers in the form of distributed feedback lasers (with the resonator formed essentially by a special fiber Bragg grating) can generate tens of milliwatts of output power with a linewidth in the region of a few kilohertz.
• Higher output powers can be generated with longer distributed Bragg reflector lasers (DBR fiber lasers) or unidirectional fiber ring lasers, and also by using a fiber amplifier.
• Diode-pumped solid-state bulk lasers, e.g. in the form of nonplanar ring oscillators, can also have linewidths of a few kilohertz, combined with relatively high output powers of the order of 1 W. Although a 1064-nm wavelength is typical, other wavelengths e.g. in the 1.3- or 1.5-μm wavelength regions are also possible.

Essential Factors for a Narrow Laser Linewidth

For achieving a narrow emission bandwidth (linewidth) from a laser, several issues of laser design have to be observed:

• First, single-frequency operation needs to be achieved. This is easiest when using a gain medium with small gain bandwidth and a laser resonator with short length (leading to a large free spectral range). The goal should be long-term stable single-frequency operation without mode hopping.
• Second, external noise influences must be minimized. This requires a stable resonator setup (preferably a monolithic one), possibly with special protection against mechanical vibrations. An electrically pumped laser should be operated with a low-noise voltage or current source, and an optically pumped laser should have a pump source with low intensity noise. Furthermore, any optical feedback must be avoided, e.g. by using a Faraday isolator. Ideally, external noise influences will become lower than internal noise, e.g. from spontaneous emission in the gain medium. This is often easily achievable at high noise frequencies, but not at low noise frequencies which are most important for the linewidth.
• Third, the laser design should be optimized so that the laser noise and in particular the phase noise are minimized. A high intracavity optical power and long resonator can be beneficial, although stable single-frequency operation is more difficult to achieve with a longer resonator.

Of course, the design optimization requires that the relative importance of different noise sources is known, because different measures can be required depending on which noise source is dominant. For example, measures which minimize the linewidth according to the Schawlow–Townes equation will not necessarily minimize the actual linewidth, if this is determined e.g. by mechanical noise.

Noise Characterization and Specification

Both the characterization and the specification of the noise of narrow-linewidth lasers are far from trivial issues. Various measurement techniques are discussed in the article on linewidth, and particularly for linewidth values of a few kilohertz or less such measurements are demanding. Furthermore, a linewidth value alone can not be considered a complete noise characterization; it is preferable to have a complete phase noise spectrum, apart from information on relative intensity noise. At least, the linewidth value should be specified together with a measurement time, and possibly with some information concerning frequency drifts for longer time intervals.

Of course, different applications have different requirements, and it should be checked in detail how tight noise specifications should really be demanded in any particular case.

Applications of Narrow-linewidth Lasers

• A particularly important field of application is the area of sensors, e.g. fiber-optic sensors for strain and/or temperature, various types of interferometric sensing, trace gas detection with differential absorption LIDAR (DIAL), or wind speed measurements with Doppler LIDAR. Linewidths of only a few kilohertz are required for some fiber-optic sensors, whereas 100 kHz can be sufficient for, e.g., LIDAR measurements.
• Optical frequency metrology requires sources with very narrow linewidth, often achieved with stabilization techniques.
• Holography requires either continuous-wave or pulsed single-frequency lasers for generating highly coherent light.
• Normally less demanding in terms of linewidth are applications in optical fiber communications, e.g. in transmitters or for test and measurement purposes.

Bibliography