Stretched flexible hollow fiber
The standard way of compressing high power laser pulses down to durations in the few-cycle regime is to utilize spectral broadening in gas-filled hollow waveguides. Up to recently, the design criteria of hollow fiber compressor schemes were as follows: 1) The peak power of the input pulses should not exceed the level of critical power of self focusing, 2) The peak intensity should be below the threshold of photoionization and 3) The length of the fiber is limited to ~ 1 m because of technical reasons. This posed a limitation to the achievable spectral broadening and hence the corresponding pulse compression capability of these schemes. In order to overcome these limitations, we introduced a new concept and use stretched flexible hollow fibers.
With this new approach, virtually arbitrary fiber lengths can be realized. Therefore an essential former limitation ceased to apply and the design strategy for the optimal fiber parameters could be reformulated. As a result, it was shown that the long stretched flexible hollow fibers are superior to hollow fibers of standard size in terms of both the achievable spectral broadening and the transmission. It was verified that the novel fiber assembly exhibits superior waveguiding properties. As a test the transmission and the beam propagation behind the fiber were evaluated. The measured data approached the theoretical values within the tolerance of the measurement.
In order to demonstrate the potential of the novel approach, the operation of a 3 m hollow fiber (ID = 320 µm) was tested. 71 fs long pulses with an energy of 1.1 mJ were launched into the fiber. Applying an Ar fill of 500 mbar, an over 20-fold spectral broadening was achieved. Temporal compression of the pulses was accomplished by 12 reflections on chirped mirrors with a bandwidth of 1.5 octaves. The compressed pulses were characterized by a SHG-FROG device yielding a pulse duration of 4.5 fs.
Compression of multi-mJ pulses to 4.3 fs duration in long hollow fibers
In collaboration with the Laboratoire d'Optique Appliquée (LOA, Palaiseau, France) and the Leibniz University Hannover our unique stretched flexible hollow fiber technology was used to generate carrier-envelope phase (CEP) stabilized sub-2-cycle light pulses at an unprecedentedly high energy of ~3mJ. The circularly polarized output of a double-CPA Ti:sapphire amplifier system (LOA) producing 23 fs pulses of 8 mJ energy at 1 kHz repetition rate was spectrally broadened in a 2-m long stretched flexible hollow fiber of 450 µm inner diameter. The waveguide was filled with helium in a pressure gradient scheme. At an optimal pressure of 1.8 bar an over-octave-spanning spectrum was achieved at 3.5 mJ pulse energy with excellent beam quality. The spectral width supports a transform-limited pulse duration of 2.5 fs which corresponds to the duration of a single optical cycle at the central wavelength of 746 nm.
The pulses were compressed by a combination of chirped mirrors and fused silica wedges and characterized by a single-shot SHG FROG device. The retrieved pulse duration was 4.3 fs. During the measurements the CEP was stabilized at an RMS error level of 360 mrad.
T. Nagy, P. Simon:
Single-shot TG FROG for the characterization of ultrashort DUV pulses Opt. Express 17, 8144 (2009)
T. Nagy, P. Simon:
Generation of 200-mJ, sub-25-fs deep-UV pulses using a noble-gas-filled hollow fiber Opt. Lett. 34, 2300 (2009)
T. Nagy et al. Flexible hollow fiber for pulse compressors Appl. Opt. 47, 3264 (2008)
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Laser-Laboratorium Göttingen e.V. (LLG)
Head of the Department
Dr. Peter Simon
"Short Pulses / Nanostructures"
Contact person for
Nano Structure Technology:
Dr. Jürgen Ihlemann