Mar 6, 2016 - Lobanov, A. F. Suchkov, I. V. Kholin, and A. Yu. Chugunov, Kvanto- vaya Elektron. (Moscow) 4, 1761 (1977) [Sov. J. Quantum Electron. 7,.
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Mode locking in a neodymium glass laser with a plasma mirror
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1983 Sov. J. Quantum Electron. 13 1637 (http://iopscience.iop.org/0049-1748/13/12/L24) View the table of contents for this issue, or go to the journal homepage for more
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Mode locking in a neodymium glass laser with a plasma mirror P. Hfibek.V. KubeCek, and M. Vrbova Czech Technical University, Prague, Czechoslovakia
(Submitted June 8, 1983) Kvantovaya Elektron. (Moscow) 10, 2508-2510 (December 1983) An investigation was made of a superradiantly triggered neodymium glass laser system emitting quasicontinuously trains of 20 + 4 psec pulses. Stimulated Brillouin backscattering is probably responsible for the plasma reflectivity causing mode locking. PACS numbers: 42.55.Rz The use of a plasma mirror in various experimental systems (such as high-power neodymium glass lasers and TEA CO2 lasers)1"4 provides a simple method of generation of ultrashort high-power laser and plasma pulses. However, an incomplete understanding of the dynamics of such systems makes it difficult to design, optimize, and use them. We investigated a superradiantly triggered neodymium glass laser system (Fig. 1). The superradiance from amplifiers 1 was focused by a lens 4 on the surface of a tilted target 5. The target was heated and evaporated by the incident radiation, giving rise to an ionization plasma. When the power density of superradiance was sufficiently high, a plasma appeared very quickly and was accompanied by rapid changes in the intensity of the radiation reflected from it. This plasma caused injection of a modulated optical signal into an 8-m laser cavity consisting of a mirror 2, a lens 4, and a plasma mirror on the surface of the target 5. The injected signal was further shaped by passing through the amplifiers 1 (because of dispersion and saturation of the active medium in the amplifiers) and by a reflection from the plasma. The time structure of the laser radiation was governed by the simultaneous influence of the amplifying medium and of the target plasma on the optical field inside the cavity. The optical fields incident on and reflected by the plasma mirror were separated by wedges 6. The energy emerging from the amplifiers and the reflectivity of the plasma mirror were determined using calorimeters 8. Vacuum photodiodes 7 together with an oscilloscope made it possible to record the time structure of the beam (Fig. 2a). The shapes of the individual pulses were observed using an EOK-2M streak camera fitted with a Hamamatsu television readout system and a time analyzer. Mirrors denoted by 3 in Fig. 1 were used to create several beams with different time delays relative to the
triggering of the streak camera. High-contrast pulses of 20 + 4 psec duration were observed. This duration was comparable with the resolution of the streak camera and of the recording system. In fact, the duration of the pulses was either close to 20 psec or even less. The total energy extracted from the amplifiers varied from 10 to 45 J, depending on the target material.5 The energy of a single pulse was of the order of 1-10 mJ. Measurements were made of the reflectivity of the plasma mirror (defined as the ratio of the energy, reflected by the plasma into focusing optics to the energy incident on the target) and it was found to be about 0.01 for all the investigated target materials. The beam diameter on the target material was estimated to be 100// and the peak power density reaching the plasma mirror was 1012 W/cm 2 or even more. We shall propose the following description of the dynamics of the laser system in the case of quasicontinuous mode locking. The target plasma is heated by the short laser pulse and expands during the relatively long periods between the pulses. This creates an expanding moderately dense plasma, which is convenient for stimulated Brillouin backscattering. The peak power density estimated from our results suggests that the gain of such scattering is ~ 10" 12 cm7W.
2 0 psec
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FIG. 1 Experimental setup: l)-9) see text; 10) readout system; 11) television monitor; 12) television camera; 13) X-Yplotter; 14) polarizer. 1637
Sov. J. Quantum Electron. 13 (12), Dec. 1983
FIG. 2. Temporal structure of a beam obtained using a carbon target: a) pulse train observed using a vacuum photodiode and an oscilloscope (each division on the abscissa is 50 psec and each division on the ordinate is 0.5 V); b) streak camera outout signal.
This value is in agreement with the experimental and theoretical results reported in Ref. 6 on the assumption that the plasma temperature is of the order of 100 eV and that the characteristic size of the region occupied by the plasma is 30 fi. Moreover, the reflectivity of the plasma mirror measured by us is also in agreement with Ref. 6. The energy stored in the active medium of the amplifiers is extracted in the form of short pulses. The quasicontinuous operating regime corresponds to the saturation value of the gain
