Ultrafast UV pulses excite biological samples
Working with colleagues from CNRS (St. Etienne, France) and INSERM (Lyon, France) researchers at the Institut d`Optique (Orsay, France) carried out time-resolved spectroscopy of brain tissue using a specially developed all-solid-state ultrafast tunable ultraviolet (UV) laser system. Operating the system with both a spectrograph and bidimensional single-photon counting enabled fluorescence decay measurements against time and wavelength to be made simultaneously.
According to the researchers, the photon counting results achieved--which will be described at this year`s Conference on Lasers and Electro-Optics (CLEO `97 Baltimore, MD)--are, to their knowledge, the first ex vivo white matter autofluorescence 2-D single-photon-counting images to be presented.1
In order to make the measurements, the researchers needed a tunable UV laser capable of delivering picosecond pulses with high repetition rate. An all-solid-state system was, therefore, designed and assembled at the Institut d`Optique. The laser produces pulses at 415 nm and is also tunable from 273 to 286 nm (see Fig. 1).
The setu¥is based on a modelocked diode-pumped Cr:LiSAF oscillator that produces output tunable from 820 to 880 nm. At 830 nm, the pulse energy is only 0.15 nJ with a pulse width of 50 ps and a pulse-repetition rate of 100 MH¥(see Fig. 2 on p. 16).
Such low pulse energy requires amplification, but because of its poor thermal properties, Cr:LiSAF is not suited as an amplifier.2 Thus, pulse amplification is accomplished with a Ti:sapphire regenerative amplifier pumped by a Q-switched frequency-doubled diode-pumped Nd:YVO4 laser. The vanadate pum¥laser is itself pumped by a 4-W diode laser, and frequency doubling of its 1064-nm output is done with potassium titanyl phosphate (KTP) to produce 50-ns, 40-µJ pulses at 532 nm with a repetition rate of 10 kHz.
In the regenerative amplifier the pum¥beam is focused through a hemispheric dichroic mirror--transparent to 532 nm but 100% reflective between 800 and 900 nm--onto the Ti:sapphire crystal face, which is cut at Brewster`s angle. The Ti:sapphire amplifier cavity is formed by three mirrors; within it a Pockels cell and a polarizer tra¥and dum¥pulses coming from the oscillator (by polarization rotation) at the 10-kH¥repetition rate. The amplifier wavelength is tuned to that of the oscillator using a prism.
The trapped pulses are amplified for each pass through the Ti:sapphire (40% per round-trip). When a pulse reaches the maximum energy level, the Pockels cell ejects the pulse. A two-way switch consisting of a Faraday rotator, a half-wave plate, and a polarizer separates the input beam from the amplified beam, which is sent toward the frequency converter.
Output pulses from the regenerative amplifier have a pulse energy of 4.5 µJ, pulse width of 50 ps, and range between 820 and 860 nm; the pulse-repetition rate is 10 kHz. Frequency conversion to the second and third harmonics is done with lithium triborate (LBO). Without an output filter, three wavelength ranges are available from the system: 820-860 nm, 410-430 nm, and 273- 286 nm. With the filter in place, only the third harmonic--50-ps, 0.14-µJ pulses at 276-nm, and a repetition rate of 10 kHz--is available.
The researchers believe that this type of system is a promising tool for photo biological research, specially for in vivo diagnostic research.3 This work is supported by the ULTIMATEC program of the CNRS (Paris).
Roland Roux
REFERENCES
1. M. Gaignet et al., "Time resolved spectroscopy of brain tissue with an all-solid-state tunable picosecond UV laser," submitted to CLEO `97, Baltimore, MD.
2. F. Balembois et al., "Theoretical and experimental investigation of small gain signal for a diode-pumped Q-switched Cr:LiSAF laser," to be published in IEEE J. Quantum Electron., Feb. 1997.
3. S. Mottin et al., "Brain NADH determination throughout the rat sleep-wake cycle by use of a fiberoptic time resolved fluorescence sensor," accepted for publication in Neuroscience