Fibre laser power boost

Opt. Lett. 35, 94–96 (2010)

Credit: FRIEDRICH SCHILLER UNIVERSITY

A femtosecond fibre laser with an average output power of 830 W has been demonstrated by researchers in Germany and Denmark. The researchers say that, to their knowledge, this is the highest average power ever reported for a femtosecond laser.

“One of the main challenges was to suppress detrimental nonlinear effects that occur inside the fibre amplifiers,” says Tino Eidam from the Friedrich Schiller University in Jena, Germany. “We solved this by using specially designed large-mode-area fibres in combination with the chirped pulse amplification technique.”

Eidam and his colleagues used two large-core photonic crystal fibres as pre-amplifiers, and a low-numerical-aperture step index large-mode-area fibre as the main amplifier. Chirped pulse amplification allows the laser pulses to be stretched in time using dielectric reflection gratings. The pulses, now less intense, are amplified and compressed back into femtosecond pulses. The group is now working on increasing the average power to beyond a kilowatt and the pulse energy to around 1 mJ. This will require new fibre designs to reduce the nonlinearities, and large cores that can deliver sufficient higher-order mode suppression.

An ultrafast switch for X-rays

Nature Phys. 6, 69–74 (2009)

Researchers in the USA have used light to control the transmission of X-rays through a medium, creating an ultrafast X-ray switch. “X-rays are transmitted through the absorbing medium, neon gas, only when an intense optical laser field is present,” explains Linda Young from the Argonne National Laboratory in Illinois. “This phenomenon is reversible and ultrafast, so we have essentially created an ultrafast X-ray switch.”

One of the main challenges was to overlap the co-propagating X-ray pulses (250 fs in duration and ∼30 μm in length) and light pulses (∼300 fs in duration and 50 μm in length) as they travelled through the 20-mm-long neon gas cell. Spatial overlap was accomplished by knife-edge scans through the interaction region. Temporal overlap of the two light bullets was roughly set using a fast photodiode — sensitive to both X-ray and optical radiation — to place the two pulses within ∼3 ps of each other.

“We plan to use laser-induced X-ray transparency as a tool for making direct, quantitative and all-photonic measurements of femtosecond X-ray pulse durations,” says Young. “Other possibilities include the investigation of this effect in solid-state systems.”

Few-cycle pulse generation

Opt. Lett. 34, 3851–3853 (2009)

Sub-two-cycle laser pulses of 4.6 fs in duration have been generated using soliton self-compression of 41-fs pulses from a Ti:Sapphire laser in a short (<5 mm) highly nonlinear photonic crystal fibre.

“We showed that, for a properly chosen fibre length, the input pulse energy and duration can be adjusted to obtain sub-two-cycle pulses directly at the fibre output without the need for additional — usually complex — phase-compensation devices,” says Helder Crespo from the University of Porto, Portugal. The output pulses were collimated and characterized in the temporal domain using a near-dispersionless optical set-up based on octave-spanning, and with double-chirped mirrors designed by researchers at the Massachusetts Institute of Technology in the USA.

The group, which also includes researchers from the Engineering Institute in Porto and the University of Siena in Italy, believes that this technique shows promise for the generation of even shorter pulses. “We hope to ultimately obtain carrier–envelope phase-stabilized pulses approaching the single-cycle limit,” says Crespo.

VUV pulse manipulation

Opt. Express 17, 23443–23448 (2009)

Researchers in Japan have observed ultrafast switching of vacuum-ultraviolet (VUV) light caused by saturable absorption of a solid metal target. The group used VUV light pulses from a free-electron laser (FEL) source, and tin as a target material for the saturable absorber. The researchers observed a strong nonlinear change in absorption of the tin film for wavelengths of 51 and 61 nm from a VUV-FEL. The change in transmission was very rapid, and the ratio between before- and after-switching was approximately 1:100.

“We want to use this technology for pulse shortening of extreme-ultraviolet laser pulses,” says Hitoki Yoneda from the University of Electro-Communications in Tokyo. “The saturable absorber can also be used for spatial mode cleaning of the FEL. Although the present VUV-FEL laser operates at the saturation mode, it is not a single-mode operation both for transverse and longitudinal modes. We therefore have a chance to improve pulse shortening using this saturable absorber.”

Extending supercontinuua

Opt. Lett. 34, 3337–3339 (2009)

Credit: KANSAS STATE UNIVERSITY

Using a method called double optical gating, researchers in the USA have obtained supercontinuum spectra in the extreme-ultraviolet region, including in the 'water window'. This broad spectrum supports a 16-attosecond pulse, which is shorter than one atomic unit of time (24 attoseconds).

“Double optical gating functions as a special shutter that opens for less than one optical period of the driving laser, 2.6 fs, thereby gating the attosecond pulse generation process,” explains Zenghu Chang from Kansas State University. The method combines the advantages of two powerful gating methods: two-colour gating and polarization gating. Two-colour gating doubles the spacing between adjacent attosecond pulses in a train, which reduces the requirement of the shutter speed for selecting a single pulse. Polarization gating relies on the fact that attosecond emission occurs much more efficiently when the driving laser is linearly polarized. If the polarization state of the driving laser is shaped in such a way that only a short portion of the pulse is linearly polarized, then it serves as a shutter. “The main advantage of double optical gating is that it works with relatively long-pulse driving lasers,” says Chang. “For example, 8-fs pulses were used in the experiments, but the shutter opening time was less than 2.6 fs. In other words, an 8-fs laser was used to do the work of a half-cycle laser.”