Temporal coherence and spectral linewidth of an injection-seeded transient collisional soft x-ray laser

Abstract

The temporal coherence of an injection-seeded transient 18.9 nm molybdenum soft x-ray laser was measured using a wavefront division interferometer and compared to model simulations. The seeded laser is found to have a coherence time similar to that of the unseeded amplifier, ~1 ps, but a significantly larger degree of temporal coherence. The measured coherence time for the unseeded amplifier is only a small fraction of the pulsewidth, while in the case of the seeded laser it approaches full temporal coherence. The measurements confirm that the bandwidth of the solid target amplifiers is significantly wider than that of soft x-ray lasers that use gaseous targets, an advantage for the development of sub-picosecond soft x-ray lasers.

Spectral linewidth of a Ne-like Ar capillary discharge soft-x-ray laser and its dependence on amplification beyond gain saturation

“…The narrow relative linewidth of this laser, λ/λ = (3-4) × 10 −5 , corresponds to a coherence time of ∼2 ps that is much shorter than the pulse duration of 1.2-1.8 ns [2,3,17]. The degree of temporal coherence of the capillary discharge 46.9-nm laser is thus significantly lower than those previously measured in two other types of collisional soft-x-ray lasers (SXRLs), namely the 32.8-nm optical-field-ionization SXRL in a Xe gas medium [23], and the 13.9-nm and 18.9-nm transient collisional soft-x-ray lasers in Ag and Mo plasmas created by laser irradiation of solid targets [25,26]. The difference arises from the fact that while these laser-pumped systems are transient gain systems in which the laser pulse duration of a few picoseconds is relatively close to full temporal coherence, the capillary discharge laser is essentially a quasi-cw laser in which the laser pulse duration is more than two orders of magnitude longer than both the upper-laser-level lifetime and the laser pulse of the transient lasers.…”

“…The absence of observable rebroadening after gain saturation, caused by homogenization of the line profile by collisional broadening, takes place in spite of the fact that capillary discharge lasers operate in a regime in which Doppler broadening greatly dominates the line profile. Since the capillary discharge plasmas have up to two orders of magnitude lower plasma density than those corresponding to laser-pumped collisional soft-x-ray lasers that use solid targets [22,[24][25][26][27], and a higher ion temperatures than that characteristic of optical-field-ionization soft-x-ray lasers [33], it is difficult to find among the current soft-x-ray lasers plasma conditions in which the inhomogeneous component is more dominant. This suggests that the observation of saturation rebroadening in soft-x-ray plasma amplifiers would be an unusual event.…”

“…This results in a very narrow laser line that is difficult to measure because its width is below the resolution of most existing spectrometers at this wavelength. Several measurements of the linewidth of different types of laser-pumped soft-x-ray lasers have been reported [22][23][24][25][26][27]. These measurements were conducted using either a custom-made spectrometer of extremely high resolution [22], or different types of interferometers [23][24][25][26][27].…”

“…This interferometer was previously used to investigate other types of collisional soft-x-ray lasers, including a 32.8-nm injection-seeded optical-field-ionization soft-x-ray laser in a Xe gas medium [23] and injection-seeded and self-seeded transient collisional soft-x-ray lasers in the 18.9-nm line of Ni-like Mo in plasmas created by laser irradiation of solid targets [26].…”

Temporal coherence measurements of a seeded GRIP transient Ni-like Mo soft X-ray laser

“…In order to overcome this problem, we have used another definition for these two quantities, based on quadratic means, which are less sensitive to the choice of the fitting function used to describe the experimental data. This analysis of the data is described in [12].…”