07 Apr 2009
Femtosecond source's "extremely fast" wavelength tuning could find applications in multiphoton microscopy and optical coherence tomography.
A femtosecond soliton source with fast and broad spectral tunability has been developed by researchers in Argentina. The source, which comprises a Ti:sapphire laser and a highly nonlinear photonic-crystal fibre, can be tuned from 850 nm to 1000 nm with nearly constant pulse width and average power (Optics Letters 34 842).
"The laser can be tuned simply by controlling the power launched into the fibre, which has two advantages," Martin Masip, a researcher at the University of Buenos Aires, told optics.org. "First, the tunability is controlled by an electronic device, which results in a robust scheme with no mechanical movable parts. Second, the wavelength can be changed extremely fast."
The key to the laser's tuning performance is the use of solitons generated in the photonic-crystal fibre. At the low-power coupling regime, solitons can be tuned over a broad range of wavelengths from 850 to 1000 nm. The solitons generated in the fibre maintain almost constant pulse and spectral widths regardless of input power.
"The original motivation was to have a bright light source to face up to single-particle spectroscopy. We were looking for a white-light source for spectroscopy and a pulsed source for multiphoton microscopy," explained Masip. "Our device can be tuned in tens of nanoseconds, which means a custom-made pseudo-CW spectrum in the infrared can be built easily. What's more, a simple modulation of the voltage could give rise to a pulse train with a different colour for each one of the pulses."
In the set-up, a photonic-crystal fibre measuring 75 cm in length is pumped with a Ti:sapphire laser that provides 37 fs pulses at a repetition rate of 94 MHz and a wavelength of 830 nm. Average power ranging from 1 to 10 mW is pumped into the fibre, controlled by an acousto-optic modulator (AOM).
"The use of an AOM provides a tunable source with a 10 ns set-time across the entire spectra," commented Masip. "Our measurements for the central wavelength are limited by the detection system to below 1000 nm; however, simulations predict soliton tuning can be achieved for a much wider spectral range."
Masip and colleagues are planning to investigate several applications for their soliton source, ranging from multiphoton microscopy and optical coherence tomography to Coherent anti-Stokes Raman spectroscopy or second-harmonic generation.