定义：
一种锁模技术，利用的是主动调制腔内损耗或者相位变化。

在词条锁模中介绍了通过锁模来产生超短脉冲的方法。这里讨论的是主动锁模，利用周期性调制谐振腔损耗或者往返的相位差。调制可采用声光调制器或者电光调制器，马赫曾德尔集成光学调制器，或者半导体电吸收调制器。如果将调制与谐振腔结合，可以产生超短脉冲。 

图1：主动锁模激光器的示意图

主动锁模的原理很容易理解就是调制谐振腔损耗。在合适时刻的脉冲经过调制器后的损耗最小（如图2）。然而，脉冲两侧仍然会存在一定的损耗，在腔内往返一周后脉冲显著缩短，直至脉冲缩短效应由其他能使脉冲展宽的效应抵消（例如，增益变窄）。 

在一些简单的情况下，稳态的脉冲长度可以通过Kuizenga-Siegman理论计算。脉冲长度通常在皮秒量级，与调制信号强度等参数只有很小的联系。联系来自于调制器的脉冲缩短效应对于更短的脉冲变小，而其它能使脉冲变长的效应变大。 

图2：主动锁模激光器功率和损耗随时间的变化曲线

调制器使脉冲两侧的损耗变大，有效缩短脉冲。脉冲长度与脉冲周期比如图所示的比例要小得多，调制器的脉冲缩短效应通常比较弱。 

频率调制锁模 
还有的情况下，主动锁模采用周期性的相位调制，尽管这会得到啁啾脉冲。这种技术也称为频率调制锁模，尽管称为相位调制锁模可能更为合适。有些频率调制锁模激光器具有某种不稳定性：两个工作模式之间存在随机耦合，其中脉冲在相位延迟最小或最大值处通过调制器。这种双稳定性有时可通过色散或者非线性效应移除。 

同步反馈方法 
为了稳定工作，在谐振腔中的往返时间需要与调制器信号的周期严格相等（或者是其整数倍），这样脉冲每次经过调制器都经历最小的损耗。激光器谐振腔与驱动信号之间很小的频率不匹配都会产生非常强的时间抖动，甚至产生混沌行为。 

调制器驱动器和激光器的同步可以通过经过调整激光器装置或者采用反馈回路自动改变调制频率或者激光器谐振腔的损耗。常用的一种技术是再生锁模。其中，调制器信号不是由自由运行的或者稍微修正的电子振荡器产生，而是来自于探测到的脉冲列的强度调制信号本身。这种方法对于得到可调谐脉冲重复速率非常重要，通常用于锁模光纤激光器和激光二极管中。 

主动锁模得到更高的脉冲重复频率 
由于几何限制，通过缩短激光器谐振腔很难得到非常高的脉冲重复速率。一种解决方法是谐波锁模，其中多个脉冲在谐振腔中振荡。调制器频率是往返频率的整数倍。该方法的另一种为有理数谐波锁模，其中调制频率是往返频率乘以两个整数的比值。 

与被动锁模比较 
与被动锁模相比，主动锁模能够产生更长的脉冲。但是其缺点就是需要光学调制器，电子驱动器和同步的方法。然而，当需要将脉冲列与一些电子信号合成时，主动锁模是最好的方法，或者需要同时采用多个激光器时。因此，主动锁模通常用于光纤通信中。

主动锁模(active mode locking)英文版

Definition: a technique of mode locking, based on active modulation of the intracavity losses or the round-trip phase change

More general term: mode locking

Opposite term: passive mode locking

The general aspects of the generation of ultrashort pulses by mode locking are discussed in the article on mode locking. This article addresses active mode locking, which involves the periodic modulation of the resonator losses or of the round-trip phase change. This can be achieved e.g. with an acousto-optic or electro-optic modulator, a Mach–Zehnder integrated-optic modulator, or a semiconductor electroabsorption modulator. If the modulation is synchronized with the resonator round trips, this leads to the generation of ultrashort pulses in the form of a regular pulse train: an output pulse is obtained each time when the circulating pulse hits the output coupler.

Figure 1: Schematic setup of an actively mode-locked laser.

The principle of active mode locking by modulating the resonator losses (AM mode locking) is easy to understand. The action of the modulator has two different important effects:

• A pulse with the “correct” timing can pass the modulator at times where the losses are at a minimum (see Figure 2). It is thus favored against any other radiation circulating in the resonator. As the pulse will in the steady state saturate the laser gain such that its round-trip gain is zero, other circulating radiation will have a negative round-trip gain and will thus die out sooner or later.
• Still, the wings of the pulse experience a little attenuation, which effectively leads to (slight) pulse shortening in each round trip: The round-trip gain is slightly negative for the wings and slightly positive for the pulse center. As a result, the pulses get shorter and shorter, until the pulse shortening is offset by other effects (e.g. gain narrowing or chromatic dispersion) which tend to broaden the pulse.

Figure 2: Temporal evolution of optical power and losses in an actively mode-locked laser. The modulator causes increased losses for the pulse wings, effectively shortening the pulses. As the pulse duration relative to the pulse period is typically much smaller than shown, the pulse-shortening effect of the modulator is usually very weak.

In simple cases, the pulse duration achieved in the steady state can be calculated with the Kuizenga–Siegman theory; it simply results from the balance of pulse shortening in the modulator with pulse broadening by gain narrowing. The pulse duration is typically in the picosecond range and is only weakly dependent on parameters such as the strength of the modulator signal. This weak dependence arises from the fact that the pulse-shortening effect of the modulator rapidly becomes less effective for shorter pulse durations, whereas other effects which lengthen the pulse (e.g. gain narrowing and chromatic dispersion) then become more effective.

Frequency Modulation Mode Locking

Somewhat less obviously, active mode locking also works with a periodic phase modulation (instead of amplitude modulation) e.g. in a Pockels cell, even though this leads to chirped pulses. This technique is called FM mode locking (FM = frequency modulation), although the term phase modulation mode locking would seem to be more appropriate. Some FM mode-locked lasers exhibit an instability: they exhibit random hopping between two operation modes, where the pulses pass the modulator at times where either a minimum or a maximum of the phase delay is reached. This kind of bistability is sometimes removed by dispersive and nonlinear effects.

Synchronization; Regenerative Feedback Method

For stable operation, the round-trip time of the resonator must fairly precisely match the period of the modulator signal (or some integer multiple of it), so that a circulating pulse can always pass the modulator at a time with minimum losses. Even a small frequency mismatch between the laser resonator and the drive signal can lead to a strong timing jitter or even to chaotic behavior, since the obtained “pulling force” on the pulse timing is quite weak.

Synchronization between the modulator driver and the laser can be achieved either by careful adjustment of a stable laser setup, or by means of a feedback circuit which automatically adjusts either the modulation frequency or the length of the laser resonator (and thus its round-trip time). A frequently used technique is regenerative mode locking (also called mode locking with regenerative feedback) [5]. Here, the modulator signal is not generated by a free-running or slightly corrected electronic oscillator, but rather is derived from the detected intensity modulation of the pulse train itself. Such schemes are particularly important for achieving tunable pulse repetition rates, and are often applied to mode-locked fiber lasers and laser diodes.

Active Mode Locking with Higher Pulse Repetition Frequencies

Due to geometric constraints, it can be difficult to reach very high pulse repetition rates by making the laser resonator very short. A solution can be harmonic mode locking, where multiple pulses circulate in the laser resonator. The modulator frequency is then an integer multiple of the round-trip frequency. A variation of the method is rational harmonic mode locking, where the modulation frequency is the round-trip frequency times the ratio of two integers.

Comparison with Passive Mode Locking

Compared with passive mode locking, active mode locking typically generates longer pulses. This is essentially because the pulse shortening effect of the modulator becomes less and less effective as the pulses get shorter. In contrast, a saturable absorber as used for passive mode locking can provide a faster loss modulation as the pulses get shorter.

A further disadvantage of active mode locking is the need for an optical modulator, the electronic driver and (in most cases) means for synchronization (see above).

On the other hand, active mode locking can be the natural solution when pulse trains are required which are synchronized with some electronic signal, or when many lasers need to be operated in synchronism. Therefore, active mode locking is often used in the context of optical fiber communications.

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