Application of angular multiplexing technology in excimer laser amplifiers

For excimer laser amplifiers, since the upper energy state storage time of the active medium is very short, the transient energy storage (Em) must be continuously replenished to obtain high-energy output, that is, E=EmT/t, where T is the gain time and t is the gain recovery time. For electron beam pumped excimer laser amplifiers, the gain time can be as long as 200ns, T/t≈100, which means that for a single short pulse to be amplified, only a small part of the energy can be extracted. In order to continuously extract the energy stored in the amplifier, optical multiplexing coding technology is usually used.

Optical multi-path coding technology uses pulse trains to extract energy from the amplifier and makes each pulse work in the best state when passing through the amplifier. The short pulse to be amplified is obtained by beam splitting to obtain multiple pulses, and then a pulse train is obtained through an appropriate encoder. The interval between pulses is generally the same as the amplifier gain recovery time. After the short pulse signal is amplified, it passes through a decoder opposite to the encoder to combine the pulse train into a single short pulse, as shown in Figure 1(a). For short pulse signals above the ns level, the overlap accuracy between pulses can be controlled to tens of ps, that is, the optical path difference between each path is in the cm level. For ultra-short pulses required for fast ignition processes or special spike structures in shaped pulses required for shock wave ignition, special optical path forms must be used, such as the Segnac interferometer structure, which can achieve coherent beam combining by combining geometric splitting, physical splitting and polarization splitting, as shown in Figure 1(c).

Figure 1 Schematic diagram of optical multiplexing encoding and decoding

For large excimer laser systems, the number of pulses to be encoded is more than dozens. In order to extract the amplifier energy and achieve beam combination, angular multi-path encoding technology is used. The basic principle is shown in Figure 2. This encoding technology further separates each pulse at different angles in space on the basis of multi-path encoding technology. In general, the encoding and decoding processes are carried out separately, requiring a large number of beam splitters and reflectors. The laser transmission distance is very long, and the lengthy optical path is very unfavorable for the scale control and stable operation of the system.

Figure 2 Schematic diagram of optical angle multi-channel encoding technology

Since the energy of each laser is very high after passing through the amplifier, the decoding optical path cannot be replaced by optical elements such as dielectric waveguides that transmit signals, so the first thing to consider is to simplify the structure of the encoding optical path. Theoretically, there are many options for the encoding process, such as using optical fiber to achieve beam splitting and necessary time delay, but for high-power excimer laser systems, the laser wavelength is in the deep ultraviolet band, and optical fibers in this band cannot fully meet practical requirements. In addition, in order to reduce the length of the spatial optical path of the encoding, multiple laser oscillator sources can be used, and different oscillator sources act synchronously according to a fixed delay, that is, electrical delay is used to partially realize the function of angular multi-path encoding. This is a technical solution that can be considered when there are many encoding paths and the system scale is kept small.