Bright Spatially Coherent Wavelength-Tunable Deep-UV Laser Source Using an Ar-Filled Photonic Crystal Fiber

We report on the spectral broadening of $\sim 1\mu \mathrm{J}$ 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving $<1\mathrm{m}$ of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8%. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.

Figure 1

(a) Group velocity dispersion of the hollow-core PCF used in the experiments, for different Ar pressures. (b) Experimental UV conversion efficiency at 9.9 bar from 30 fs pump pulses (1 kHz repetition rate) centered at 800 nm, based on the total UV power in the spectral range 240–350 nm. (c) Pressure dependence of UV spectra measured at a constant input pulse energy of $1.5\mu \mathrm{J}$ for a fiber length of 20 cm. The inset shows the measured near-field profile of the UV mode superimposed on a scanning electron micrograph of the PCF core structure; in all cases, the UV light is in the fundamental mode. (d) Power dependence of UV spectra at a constant pressure of 9.9 bar and 20 cm fiber length. The linear color scale applies to both (c) and (d) except that the deep blue region has been removed from (c) in order to distinguish the details more clearly.

Figure 2

(a) Measured and (b) reconstructed second-harmonic frequency resolved optical gating traces [the same color scale as in Fig. 1d] of the residual pump pulse at the fiber output for an input pulse energy of 950 nJ. (c) Intensity and phase of the retrieved 10 fs pulse envelope.

Figure 3

Numerically modeled propagation of a $1.5\mu \mathrm{J}$, 30 fs pulse along a 20 cm length of an Ar-filled PCF at a pressure of 9.9 bar. (a) Temporal domain (left-hand color scale, linear). The inset shows a detailed view of the compressed pulse. The white arrow indicates the dispersive wave in the UV. (b) The evolution of the spectrum (right-hand color scale, dB) when including the shock derivative term. The peak spectral energy density (at 270 nm) is $2.6\mathrm{nJ}/\mathrm{THz}$. $A$ and $N$ indicate regions of anomalous and normal dispersion, respectively, and the red dashed line indicates the zero dispersion wavelength. (c) The same as (b) but with the shock derivative term switched off. The UV generation process (labels $d$ to $f$) is explained in the text.

Figure 4

Numerically simulated spectra for a fiber length of 20 cm. (a) Pressure-tuned spectrum at the fiber output at a constant pump pulse energy of $1.5\mu \mathrm{J}$. (b) Pulse-energy-tuned spectrum at a constant pressure of 9.9 bar. The soliton order appears on the right-hand vertical axis.