定义：
采用有源、无源或者混合锁模技术来产生超短脉冲的二极管激光器。

二极管激光器可以用来产生超短脉冲，通过锁模技术或者增益关联技术。采用模式锁定方式产生的脉冲长度在0.5-5ps之间，脉冲重复率在1 GHz到几百 GHz之间。而一些极限情况下，脉冲重复率可以超过1 THz。超快二极管激光器最主要的应用是在光纤通信系统中，激光器用来作为数据发射器或者全光信号处理的脉冲光源。 

目录

1. 锁模二极管激光器中的技术
2. 单片激光器与外腔激光器比较
3. 一些局限性
4. 其它锁模半导体激光器

锁模二极管激光器中的技术 
以下所述的技术都会用于锁模二极管激光器中： 
有源锁模
     有源锁模可以在激光谐振腔中结合光调制器。调制器可以是在未泵浦区域加调制电压或者在放大区域进行电流调制的电吸收      调制器。 
无源锁模
     无源锁模依赖于谐振腔中的可饱和吸收器。这可以简单的是装置中的未泵浦部分。通常施加电偏压来调整吸收器的性质。然      而，这种类型的吸收器的恢复时间相对较长。在某一个端面注入氮离子（N+，N2+）可以缩短恢复时间。这将会引入晶体缺      陷，载流子可以在该处复合。

     吸收器通常位于谐振腔末端，但是也可以放置在谐振腔的某一位置这样不同的脉冲可以在吸收器中相遇（对撞脉冲锁模）。 
混合锁模
    混合锁模（图1）是有源和无源锁模的结合。采用这种技术可以在外部控制脉冲时序，通有源锁模二极管激光器中一样，同时      可以得到类似无源锁模二极管激光器中的较短的脉冲长度。 

图1：两个外腔混合锁模二极管激光器的示意图。（a）采用了调制的注入电流和注入氮离子的饱和吸收器，（b）吸收区域被调制。 
许多情况需要采用多片半导体结构，包含增益区域，同时还有吸收器或者调制区域，有时还有其它的部分，例如，重复率控制装置。 

单片激光器与外腔激光器比较 
当脉冲重复率低于10 GHz时，通常采用外腔装置，因为单片装置太长。激光器谐振腔的一段为半导体芯片再加上体光学元件合成一个整体，至少包含一个对准棱镜（例如，GRIN棱镜）和端镜。 
另外一种技术方案是在由单模光纤绕成的环形激光谐振腔中加入半导体芯片。环形腔通常非常长，可以采用光纤器件。因此半导体器件可以是光纤耦合的半导体光放大器（SOA）。 
外腔激光器具有如下优势： 

1. 脉冲重复速率可以处于比较大的范围，并且可以通过移动端反射镜或者采用光纤绕制在光电换能器上的谐振腔来对其进行调谐。 
2. —可以加入光滤波器来稳定辐射波长，或者将衍射光栅作为端镜（参考外腔二极管激光器）。 
3. 即使脉冲重复速率很高，这时需要采用谐波锁模，外腔结构装置也很占优势，因为它们具有更低的激光器噪声，例如，在定时抖动方面。 

因此，在单片激光器不适用的领域，外腔锁模激光器可以与锁模光纤激光器相比拟。然而，采用基本的锁模方式单片式装置结构更加紧凑，加工成本更低，并且辐射的脉冲非常稳定。 

一些局限性 
尽管锁模二极管激光器结构很紧凑坚固，很适宜于应用到光纤通信系统中，装置也存在一些局限性，使其无法发挥全部的性能潜力： 

1. 脉冲能量是有限的，通常小于1 pJ。平均输出功率通常小于1 mW。由于上能态寿命很短，超快半导体激光器不适宜于脉冲重复速率小于1 GHz的情况，同步泵浦情况除外。 
2. 半导体的增益带宽与脉冲长度匹配，约为几十个飞秒，但是得到的脉冲通常要更加长，至少几百个飞秒，甚至皮秒量级。形成脉冲的动力学过程比较复杂，例如，非线性相位变化与增益饱和相关，得到最优结果很难。 
3. 得到的脉冲质量没有锁模光纤激光器中得到的脉冲质量好。尤其是存在一些由抗反射涂层中的瑕疵产生的附加的脉冲。另外，脉冲通常为啁啾脉冲，也就是说它们不是带宽受限的。 
4. 定时抖动和其它脉冲参数的噪声比其它锁模激光器的要高。这是低功率的一个后果。 

其它锁模半导体激光器 
二极管激光器不是唯一可以实现锁模的半导体激光器。还有垂直外腔表面发射激光器，它是采用光泵浦的。这一装置可以产生更高的脉冲能量，平均输出功率可以达到几个瓦特，并且脉冲重复速率在一个很大的范围内工作，从几个 GHz到几十个 GHz。这一激光器可以应用到高脉冲重复率的情况，是锁模掺杂绝缘体激光器的竞争对手。
 

Definition: diode lasers which are actively, passively or hybrid mode-locked for generating ultrashort pulses

More general terms: mode-locked lasers, diode lasers

Diode lasers can be used for generating ultrashort pulses – either with various techniques of mode locking, as discussed in this article, or with gain switching. Typically, pulses with durations between 0.5 and 5 ps and pulse repetition rates between 1 GHz and hundreds of gigahertz are generated with mode locking. In extreme cases, the repetition rate can even be above 1 THz [8]. The main application of ultrafast diode lasers is in optical fiber communications systems, where such lasers function as pulse sources of fast data transmitters or for all-optical signal processing.

Techniques for Mode Locking of Diode Lasers

The following techniques can be used for mode locking of diode lasers:

Active Mode Locking

Active mode locking can be accomplished with an optical modulator in the laser resonator. This is usually either an electroabsorption modulator in the form of an unpumped region with some modulated voltage, or an amplifying section where the drive current is modulated.

Passive Mode Locking

Passive mode locking relies on a saturable absorber in the resonator. This can simply be an unpumped section of the device. It is common to apply an electrical bias for adjusting the absorber properties. However, the recovery time of that kind of absorber is fairly long. Shorter recovery times are achieved e.g. by implanting nitrogen (N⁺ or N²⁺) ions from one facet. This introduces crystal defects, where carriers can recombine.

The absorber is often placed at a resonator end, but it can also be placed somewhere within the resonator so that different pulses can meet in the absorber (colliding pulse mode locking).

Hybrid Mode Locking

Hybrid mode locking (Figure 1) is a combination of active and passive mode locking. With this approach it is possible to control the pulse timing externally, as in an actively mode-locked laser, while also achieving a shorter pulse duration as in a passively mode-locked device.

Figure 1: Setups of two external-cavity hybrid mode-locked diode lasers. Version (a) uses a modulated injection current and a nitrogen-implanted saturable absorber, whereas version (b) has a modulated absorber section.

In most cases, a multisegment semiconductor structure is required, containing not only a gain section, but also an absorber or modulator section, and possibly additional means for e.g., repetition rate control.

Monolithic versus External-cavity Lasers

For pulse repetition rates roughly below 10 GHz, an external cavity setup is usually required, as a monolithic device would become too long. The laser resonator usually has the semiconductor chip on one end and is completed with bulk-optical elements, at least a collimation lens (e.g. a GRIN lens) and an end mirror.

Another technical approach is to incorporate the semiconductor chip into a ring laser resonator made of optical single-mode fiber. In the latter case, the resonator is typically much longer, and allows the use of fiber-optic components. The semiconductor device may then be a fiber-coupled semiconductor optical amplifier (SOA).

External-cavity lasers have various advantages:

• The pulse repetition rate can be chosen in a wide range, and can easily be tuned e.g. by moving the end mirror, or with a fiber resonator by stretching a piece of fiber with a piezo transducer.
• It is possible to insert an optical filter for fixing the emission wavelength, or use a diffraction grating as the end mirror (Littrow configuration; see the article on external-cavity diode lasers).
• Even for higher pulse repetition rates, where harmonic mode locking is required, external-cavity devices can be advantageous, because they have a potential for lower laser noise, e.g. in the form of timing jitter. However, the nurse properties are at least substantially better than for gain-switched diode lasers.

Therefore, mode-locked external-cavity diode lasers sometimes compete with mode-locked fiber lasers in areas where monolithic laser diodes would not be suitable. On the other hand, a monolithic setup with fundamental mode locking can be very compact, much cheaper to manufacture, and can exhibit very robust pulse emission.

Limitations

Although mode-locked diode lasers can be very compact and robust devices, which fit particularly well into optical fiber communications systems, they are subject to various limitations, which do not allow them to reach the full performance potential of, e.g., mode-locked fiber lasers:

• The pulse energy is fairly limited – often far below 1 pJ. Average output powers are often below 1 mW. Due to the short upper-state lifetime, ultrafast semiconductor lasers are generally not suitable for lower repetition rates of e.g. well below 1 GHz, except with synchronous pumping.
• Although the gain bandwidth of semiconductors would be compatible with pulse durations of a few tens of femtoseconds, the pulse durations achieved are usually much longer – at least hundreds of femtoseconds, and often picoseconds. The pulse formation dynamics are relatively complicated, e.g. due to nonlinear phase changes associated with gain saturation, and difficult to optimize.
• The pulse quality is normally not as good as e.g. for mode-locked fiber lasers. In particular, there are often additional satellite pulses, caused e.g. by imperfections of the anti-reflection coating. Also, the pulses are often chirped, i.e., they are not bandwidth-limited.
• The timing jitter and the noise of other pulse parameters are higher than for other mode-locked lasers. This is partly a consequence of the low power level.

Other Mode-locked Semiconductor Lasers

Diode lasers are not the only semiconductor lasers which can be mode-locked. There are also vertical external-cavity surface-emitting lasers, which can be optically pumped. This approach has a potential for much higher pulse energies and average output powers up to several watts, and has also been demonstrated to work in a wide range of pulse repetition rates from a few gigahertz to tens of gigahertz. Such lasers can in some areas, particularly for high pulse repetition rates, even rival mode-locked doped-insulator bulk lasers.

Bibliography