目录

• 光纤和体激光器装置的技术挑战 
• 1.1 基本激光器和光学特性
• 1.2 高功率装置需要考虑的问题
• 1.3 超短脉冲产生
• 1.4 光学反馈灵敏度
• 1.5 坚固性和成本
• 1.6参数不确定性
• 一些参考

光纤和体激光器装置的技术挑战 
下面从几个常用的角度概括的对比两种激光器。 

基本激光器和光学特性 
稀土掺杂光纤单位长度的增益和泵浦吸收是中等强度的，受限于玻璃中的掺杂浓度，需要使用相对较长的增益介质，得到不同的结果，如下所述。 
光纤激光器的长度很长以及小的有效模式面积会产生很强的效应，例如非线性克尔效应，尽管熔融石英（光纤最常用的材料）的非线性系数相对较小。尤其是超短脉冲产生过程中，还有单频激光器和放大器中，附加的非线性效应非常不利。该效应通常会限制得到的脉冲长度或输出功率。 
大多数光纤激光器具有很高的增益和谐振腔损耗。因此它们不会受到其它腔内成分的附加损耗的影响。例如，光纤激光器谐振腔中可能包含光栅对用于波长调谐，但是不会犹豫引入谐振腔损耗而降低效率。但是，体激光器中很小的谐振腔损耗式它们可以用于腔内倍频，得到很低的相位噪声。 
许多光纤装置存在的问题是不能控制双折射。这不仅会使偏振态由线偏振变成椭偏振，还会改变随温度和弯曲的变化。因此当温度改变时，许多光纤激光器需要重新调整偏振控制器。在实验室应用中可以接受，但是商业产品器件中则不能允许这种情况。消除这一问题并不总是采用保偏光纤，因为有些特殊光纤和光纤装置不存在保偏的种类，并且非线性偏振旋转得到的模式锁定在保偏光纤中不能工作。 

高功率装置需要考虑的问题 
光纤和体激光器都能产生几千瓦的功率。光纤装置的功率转化效率很高，比体激光器大很多。但是，光纤装置需要泵浦光源具有很高的光束质量和亮度，与体激光器的泵浦二极管相比，单位瓦特泵浦光源的成本更高。 
光纤的几何结构和其波导效应会消除主要的热效应（例如热透镜效应），因此即使在非常高功率时也能得到很好的光束质量。体激光器则更加困难，但是有些体激光器（尤其是薄片激光器）有很大的潜力得到更高的单模功率。 
非线性效应会限制高功率光纤装置的性能，尤其是脉冲产生过程。脉冲光纤装置中的峰值功率限制来自于自聚焦效应。光纤中的拉曼效应也很强，是高功率连续波光纤激光器中的一个限制因素。单频工作时，受激布里渊散射会引入一些限制。所有这些限制在体激光器装置中都很小，因为其非线性效应小了几个量级。 — 短脉冲产生 光纤和体激光器都可以Q开关产生纳秒脉冲。体激光器通常能得到最短的脉冲，因为其单位长度的增益比光纤装置更小。另外，光纤装置的峰值功率也受限制。 高功率纳秒光纤激光器系统通常需要种子激光器和光纤放大器，因此平均功率很高，但是峰值功率还是受限。理论上来说，可以放大很短的种子脉冲，但是需要使种子脉冲足够长来保持峰值功率足够低。 

超短脉冲产生 
光纤激光器采用稀土掺杂光纤，与稀土掺杂激光晶体相比，提供更高的放大带宽。光纤类型使采用的玻璃无需考虑其低热导性（由于其几何结构和波导效应）和他们很低的激光截面（光纤中可以得到很高的增益）。因此，光纤激光器有很宽的波长调谐性，并且通过无源模式锁定可以产生很短的光脉冲。但是，锁模光纤激光器不能完全将其增益带宽用于脉冲长度，因此附加的非线性和高阶模式色散效应的影响。 
可以采用非线性偏振旋转使光纤激光器模式锁定，该技术非常有效，并且比体激光器中的附加脉冲模式锁定更简单。但是，这种激光器由于双折射的热变化通常不能长时间稳定。 
具有高脉冲重复速率（千赫兹）的光纤激光器，通常需要谐波锁模，这会使激光器装置比体激光器复杂很多。 

光学反馈灵敏度 
用于材料加工的高功率光纤装置通常对光学反馈很敏感。一个原因是这种装置通常是主振荡功率放大器，因此背向反射光返回到种子激光器中时也会被放大。另外，有效模式面积相对比较小，因此容易损坏光纤端口。可以采用一个法拉第隔离器阻止背向反射光，但是通常不实际，尤其是在高功率时。 
很多情况系下，不采用隔离器，这时需要避免工作区域的垂直入射保证避免背向反射。但是，这又会限制灵活性和处理质量。 

坚固性和成本 
简单的光纤激光器可以由相对便宜的器件组成，并且需要的机械器件很少。理想情况下光纤激光器应该只由光纤制作，不涉及到空气。这也是可能的，需要使用光纤器件间的熔接等，光纤激光器比体激光器便宜很多，也更小。输出光可以方便的通过光纤连接器与其它器件连接，无需对准过程。 
但是，光纤激光器谐振腔中都会包含空气，例如，当特定体光学元件需要放置在激光器谐振腔中时，或者采用光纤模式面积不同并且没有用于适宜于各种模式面积的锥形光纤时。这种情况下，对单模光纤的对准程度要求很严格，有时由于光纤端口处的光强很高，光纤激光器比体激光器更不坚固，成本也可能不低。 
光纤装置通常不需要维修。但是如果其中存在一个缺陷，就很难确定其位置再替换。但是，如果采用光纤连接器可以很方便的更换光纤器件。 
发展光纤激光器系统的成本通常很高，需要的时间更长。部分原因是很难获得部分器件，有时由于光纤激光器（尤其是超短激光器）的工作原理比体激光器复杂很多。另外，在体光学装置中各个器件很容易结合一起，或者从系统插入、移除。 

参数不确定性 
激光器系统发展过程中，一个困难是不能知道增益介质相关参数的准确值。 
对体激光器中的激光晶体来说，它包含有限的参数，例如掺杂浓度，光谱数据和几何参数。而普通的晶体材料这些参数的不确定性很小，有些不常见材料的光谱数据通常不确定。 
而稀土掺杂光纤中，情况就没有这么理想。首要原因是光纤中玻璃材料的成分不是确定的。例如，即使光纤制造者也无法确定锗硅酸盐光纤纤芯中的锗含量，因此用户更加不知道具体成分。另外，光纤具有额外的参数，例如，纤芯直径、折射率分布曲线，它们在不同样品中都是不确定的。 

一些参考 
下面是一些参考在哪些领域光纤激光器或者体激光器更适用： 
 

• 光纤激光器适用于产生高平均功率高光束质量的光。尤其是在不常用波长区域也适用，即在不能采用晶体或者体玻璃实现的波长区域。光纤激光器在一些难实现激光的机制中占优势，例如低增益跃迁或上转换。但是，体激光器在有些光谱区域需要，例如，在700-1000 nm区域没有一种光纤激光器可以取代很宽的可调谐钛蓝宝石激光器。
• 采用Q开关或者模式锁定，体激光器能得到更高的脉冲能量和峰值功率。
• 体激光器可以使用光束质量很差的泵浦光源。极端情况下，边泵浦棒状激光器可以由气体放电灯泵浦。当需要很高峰值功率，中等重复速率的脉冲时，采用体激光器非常有利。
• 当需要的非常稳定的线偏振时也采用体激光器（由于某些原因不能采用保偏光纤）。
• 产生超短脉冲时，体激光器可以得到很高的峰值功率和高脉冲质量（平滑的光谱形状，低啁啾和低背景光）。
• 在制造成本方面，当不需要很高峰值功率、偏振、辐射带宽、脉冲质量等装置中，光纤激光器更好。但是，对以上参数要求更加严格的情况倾向于体激光器，因为光纤装置需要复杂的附加装置或者特殊器件。并且，光纤装置的高发展成本在销量较低的情况下也是一个问题。

以上讨论的几点可以看出体激光器和光纤激光器各有其优缺点，因此根据实际需要选取适合的激光器类型。

It is natural to compare different laser concepts with each other, in particular when they are used in overlapping application areas. This is particularly the case for two different kinds of solid-state lasers: bulk lasers and fiber lasers. There have been claims that fiber lasers, which have shown tremendous progress in recent years, will eventually replace most bulk lasers, since they could reach the same or better performance at a lower price. However, it is then overlooked that they also face a number of challenges, and depending on the specific goals these may outweigh the advantages. The outcome of such a comparison can thus depend strongly on various details of the specific requirements.

In recent years, global fiber lasers sales showed a significantly stronger growth than those of traditional solid-state lasers. In the area of laser material processing (including areas like laser marking and micro material processing), in 2020 they have already reached a market share around 50%. Less important application areas are in the military domain, fiber-optic sensors and medical lasers.

Technical Challenges of Fiber and Bulk Laser Devices

In the following, the most important aspects are briefly illuminated in order to permit a fair comparison of the concepts.

Basic Laser and Optical Properties

The moderate gain and pump absorption per unit length of rare-earth-doped fibers, caused by limitations to the doping concentration in glasses, requires the use of relatively long gain media, which has various consequences, as discussed below.

The long length and small effective mode area of fiber lasers lead to strong effects of the Kerr nonlinearity, despite the relatively small nonlinear index of fused silica (the most popular glass material for fibers). Particularly in the context of ultrashort pulse generation, but also for single-frequency lasers and amplifiers, excessive nonlinearity can have very detrimental effects. It often limits the achievable pulse duration or output power.

Most fiber lasers operate with high gain and high resonator losses. This makes them relatively immune to additional losses e.g. from various types of intracavity components. For example, a fiber laser resonator may well contain a grating pair for wavelength tuning without the efficiency being degraded too much by the introduced resonator losses. On the other hand, the small resonator losses of many bulk lasers make those more interesting for intracavity frequency doubling and can lead to lower phase noise.

Many fiber devices are subject to problems with uncontrolled birefringence. It is not only that this often changes the polarization state from linear to elliptical; these changes are unfortunately dependent on temperature and bending. Many fiber lasers therefore may require readjustment of polarization controllers when the temperature changes. This may be acceptable for laboratory use, but often not for commercial devices. One does not always have the option to eliminate such problems by using polarization-maintaining fibers, since special fibers and fiber devices are often not available in this form, and e.g. mode locking with nonlinear polarization rotation would not work with a polarization-maintaining fiber.

Aspects Concerning High-power Devices

Both fiber and bulk lasers can generate multi-kilowatt powers. The power conversion efficiency (wall-plug efficiency) of fiber devices can be very high, and is often significantly better than that of bulk lasers. On the other hand, fiber devices require pump sources with a higher beam quality and brightness, compared with pump diodes for bulk lasers, generally increasing the price per watt of the pump source.

The geometry of fibers and their built-in waveguide effect essentially eliminate disturbing thermal effects (such as thermal lensing) and thus make it possible to achieve an excellent beam quality even at very high power levels. This is generally more difficult with bulk lasers, but some bulk laser concepts (in particular that of the thin-disk laser) have the potential for even higher single-mode powers.

Optical nonlinearities can limit the performance of high-power fiber devices in various ways, particularly in the context of pulse generation. A hard peak power limit in pulsed fiber devices is introduced by nonlinear self-focusing. The Raman effect is also strong in fibers and can be a limiting factor also for high-power continuous-wave fiber lasers. In single-frequency operation, stimulated Brillouin scattering introduces severe limitations. All these limits are much less severe for bulk laser devices, where nonlinearities can be weaker by orders of magnitude.

Short Pulse Generation

Both fiber and bulk lasers can be Q-switched for generating nanosecond pulses. The shortest pulse durations are usually achieved with bulk lasers, as the gain per unit length is smaller for fiber devices. Also, fiber devices are limited in terms of peak power (see above).

High-power nanosecond laser systems based on fibers often contain some seed laser and a fiber amplifier, allowing for very high average power while still being limited in terms of peak power. In principle, very short seed pulses could be amplified, but the seed pulses often have to be made long enough to keep the peak power sufficiently low.

Ultrashort Pulse Generation

Fiber lasers are based on rare-earth-doped glass fibers, which offer a high amplification bandwidth compared with rare-earth-doped laser crystals. The fiber format allows the use of glasses without suffering from their lower thermal conductivity (because of the geometry and the waveguide effect) and their typically lower laser cross sections (because of the ease of obtaining a high gain in a fiber). As a result, fiber lasers can have very broad wavelength tunability and can generate very short optical pulses via passive mode locking. However, mode-locked fiber lasers often cannot utilize the full potential of the gain bandwidth in terms of pulse duration because of the detrimental effects of excessive nonlinearities and higher-order chromatic dispersion.

Fiber lasers can be mode-locked using nonlinear polarization rotation, which is an effective technique and less critical than additive-pulse mode locking of bulk lasers. However, such lasers are usually not long-term stable due to thermal variations of the birefringence (see above).

For fiber lasers with high (multi-gigahertz) pulse repetition rates, one usually requires the technique of harmonic mode locking, which makes the laser setup significantly more complicated than that of a compact bulk laser.

Sensitivity to Optical Feedback

High-power fiber devices for material processing are often very sensitive to optical feedback. One reason is that such devices are often master oscillator power amplifiers, so that the backreflected light is even amplified on its way back to the seed laser. In addition, the effective mode area is fairly small, so that fiber ends are easily destroyed. It is possible to use a Faraday isolator for preventing backreflected light from reaching the fiber, but this is not always practical, particularly at high power levels.

In many cases, one does not use an optical isolator, and then has to ensure that back-reflections are avoided e.g. by never allowing for normal incidence on a workpiece. This, however, can restrict the flexibility and possibly the processing quality.

Robustness and Cost

Simple fiber laser setups can be made from relatively cheap components, and they need fewer mechanical components. Ideally, a fiber laser setup should be made with fibers only, not involving any air space. Where this is possible, and processes such as fusion splicing of fiber components can be largely automated, fiber lasers can be significantly cheaper and smaller than bulk lasers. The output may then conveniently be delivered to a fiber connector, which allows easy connection to other devices without any alignment procedures.

However, the use of air spaces in fiber laser resonators often cannot be avoided, e.g. when certain bulk optical elements need to be inserted into the laser resonator, or fibers with very different mode areas are used and tapered fibers for mode area adaptation are not available. In such cases, the tight alignment tolerance of single-mode fibers and sometimes the high optical intensities at fiber ends may make the setup less robust than that of a bulk laser, and the cost may also not be low.

Maintenance is often not required by fiber devices. In the case of a defect, however, an all-fiber setup can make it relatively difficult to locate and exchange the defect part. On the other hand, it is easy to exchange fiber-optic components if fiber connectors are used.

The cost for developing a fiber-based laser system is often higher (as discussed in a Photonics Spotlight article), and the required time is longer. This is partly because it can be difficult to procure special parts, and sometimes (particularly for ultrafast lasers) because the operation principles of fiber lasers are more complicated than those of bulk lasers. Also, it is easier to combine different components in a bulk-optical setup, or to insert or remove parts for diagnostic reasons.

Uncertainties in Parameters

During the development of a laser system, difficulties can arise if relevant parameters of the gain medium are not known precisely.

For laser crystals as used in a bulk laser, there is a limited set of parameters such as doping concentration, spectroscopic data and geometrical parameters. For common crystal materials, the uncertainties are usually small, whereas spectroscopic data are often uncertain for less common materials.

For rare-earth-doped fibers, the situation is generally less satisfactory. A first reason is that a fiber consists of glass material where the composition is less well defined. Even the manufacturer may not know exactly the amount of germanium in the core of a germanosilicate fiber, for example, and often the exact composition is at least not revealed to customers. In addition, a fiber has additional parameters such as the core diameter or refractive index profile which can be uncertain and vary between different samples.

Some Guidelines

Some general guidelines for determining in which areas fiber lasers or bulk lasers may be stronger are as follows:

• Fiber lasers are suitable for generating very high average powers with high beam quality. This holds particularly for unusual wavelengths, where no good bulk crystals or glasses are available. Fiber lasers are clearly superior for difficult lasing schemes such as low-gain transitions or upconversion. However, bulk lasers are required for some other spectral regions; for example, there is no fiber laser to replace a broadly tunable Ti:sapphire laser in the region of 700–1000 nm.
• Bulk lasers have a higher potential for high pulse energies and peak powers either with Q switching or with mode locking.
• Bulk lasers can utilize pump sources with very poor beam quality. In the most extreme case, a side-pumped rod laser can be pumped with discharge lamps. That can be advantageous e.g. when very high peak powers in pulses with moderate repetition rates are required.
• Bulk lasers are often preferable when a stable linear polarization is required (and polarization-maintaining fibers can not be used for some reason).
• For ultrashort pulse generation, bulk lasers make it easier to achieve a high peak power and a high pulse quality in terms of smooth spectral shape, low chirp and low background.
• In terms of fabrication cost, fiber lasers will often be superior for devices with low demands on peak power, polarization, emission bandwidth, pulse quality, etc. However, more stringent demands of such type can often favor a bulk laser, as a fiber device would require complicated additional measures or very special parts. Also, the often higher development cost of fiber-based devices can be a problem in cases with small sales numbers.

The points discussed show clearly that both bulk and fiber lasers have significant advantages and disadvantages, so that one or the other concept can be superior depending on the particular circumstances.