定义：电子学控制的可用于从脉冲列中提取单个脉冲的光学开关。

大多数情况下，超短脉冲是由锁模激光器产生的，以脉冲列的形式，脉冲重复速率在10MHz-10GHz量级。由于各种原因（下面有描述），通常需要从单个脉冲列中提取特定的脉冲，例如只允许特定脉冲通过而阻止其它的脉冲。这可以采用一个脉冲选择器来实现，它是一个电子学控制的光学开关。 

脉冲选择器类型 
大多数情况下，脉冲选择器是电光调制器或者声光调制器，与适合的电子学驱动器相结合。如果是电光调制器件，脉冲选择器包括普克尔斯盒和一些偏振光学器件，例如薄膜偏振片；普克尔斯盒调控偏振态，偏振片则根据脉冲的偏振态可使其通过或阻止。 
声光脉冲选择器的原理是施加短的射频脉冲到声光调制器上，将不需要的脉冲反射到别的方向上。反射的脉冲然后通过孔径，而其它的脉冲则被阻止。 
人一种情况下，调制器的速度都是由脉冲列中脉冲的时间间隔决定的（即，由脉冲源的脉冲重复速率决定），而不是由脉冲长度决定。 
脉冲选择器的电子学驱动器需要满足附加的条件。例如，它可以采用光电二极管中产生的信号，感知原始的脉冲列，从而将开关与入射脉冲合成。触发信号可在任意时间输入，电子学装置会在适当的时间作用在开关上使其后面的入射脉冲透过。 

脉冲选择器的应用 
以下是脉冲选择器几个典型的应用： 

• 为了得到高脉冲能量的超短脉冲，常常需要降低脉冲重复速率。这可以通过在种子激光器和放大器之间放置脉冲选择器来实现。放大器只对需要的脉冲有放大作用。阻止的脉冲并不会引起很强的损耗，因为与放大器的平均输出功率相比，种子激光器的平均功率很小，并且剩余的平均功率足以使放大器发生饱和。 
• 在倾腔激光器中，脉冲选择器从强中每隔N圈提取出来脉冲。而其它时间内，脉冲经历很小的光学损耗被放大到很高的能量水平。 
• 脉冲选择器可用在正反馈放大器中用来注入或者提取脉冲。 

脉冲选择器的重要性质 
根据不同的应用要求，需要用到脉冲选择器一些不同的特性： 

• 开关时间（尤其是入射脉冲重复速率高时） 
• 开关的峰值重复速率 
• 透射脉冲的能量损耗 
• 抑制不需要脉冲的程度 
• 光学带宽（尤其是宽带脉冲） 
• 色散（尤其是宽带脉冲，例如，长度小于100fs） 
• 光学非线性（尤其是脉冲峰值功率很高） 
• 开放孔径的尺寸 
• 外尺寸 
• 对准灵敏度（接受角） 
• 电子学驱动器的能力 

Definition: electrically controlled optical switches used for extracting single pulses from a pulse train

Ultrashort pulses are in most cases generated by a mode-locked laser in the form of a pulse train with a pulse repetition rate of the order of 10 MHz – 10 GHz. For various reasons (see below), it is often necessary to pick certain pulses from such a pulse train, i.e., to transmit only certain pulses and block all the others. This can be done with a pulse picker, which is essentially an electrically controlled optical switch.

Operation Principles of Pulse Pickers

A pulse picker is in most cases either an electro-optic modulator or an acousto-optic modulator, combined with a suitable electronic driver. In the case of an electro-optic device, a pulse picker consists of a Pockels cell and some polarizing optics, e.g. a thin-film polarizer; the Pockels cell manipulates the polarization state, and the polarizer then transmits or blocks the pulse depending on its polarization.

The principle of an acousto-optic pulse picker is to apply a short RF pulse to the acousto-optic modulator so as to deflect the wanted pulse into a slightly modified direction. The deflected pulses can then pass an aperture whereas the others are blocked.

In any case, the required speed of the modulator is determined by the temporal distance of pulses in the pulse train (i.e. by the pulse repetition rate of the pulse source), rather than by the pulse duration, which may be far shorter.

The electronic driver of a pulse picker may fulfill additional functions. For example, it may use the signal from a fast photodiode, sensing the original pulse train, in order to synchronize the switching with the input pulses. A trigger signal may then come at any time, and the electronics will act on the switch at the proper time to transmit the next arriving input pulse.

Applications of Pulse Pickers

Some typical applications of a pulse picker are described in the following:

• For obtaining high pulse energies in ultrashort pulses, it is frequently necessary to reduce the pulse repetition rate before amplification. This can be achieved by placing a pulse picker between the seed laser and the amplifier. The amplifier will then act only on the wanted pulses. The blocked pulses do not necessarily constitute a strong energy loss, since the average power of the seed laser may be small compared with the average output power of the amplifier, and the remaining average power can be sufficient for saturating the amplifier.
• In a cavity-dumped mode-locked laser, a pulse picker (then often called cavity dumper) extracts the circulating pulse from the cavity in only every N^th round trip. During all the other round trips, the pulse experiences low optical losses and can be amplified to a high energy.
• A kind of pulse picker is part of any regenerative amplifier, where it is used for injection and extraction of pulses. One may also use an additional pulse because for better suppressing parasitic pulses.

Important Properties of Pulse Pickers

Depending on the application, different properties of a pulse picker can be critical:

• the switching time (particularly for high input pulse repetition rates)
• the maximum repetition rate for the switching
• the insertion loss, i.e., the energy loss of transmitted pulses
• the degree of suppression of unwanted pulses
• the optical bandwidth (particularly for broadband pulses)
• the chromatic dispersion (particularly for broadband pulses, e.g. with durations well below 100 fs)
• the optical nonlinearity (particularly for pulses with high peak powers)
• the size of the open aperture
• the outer dimensions
• the alignment sensitivity (acceptance angle)
• the capabilities of the corresponding electronic driver, e.g. concerning synchronization