Ultrafast Laser Diagnostics

Dec 01, 2013
By Avantes
Volume 28, Issue 12

Even though no current electronics is fast enough to measure femtosecond lasers, an estimation of the pulse duration, peak wavelength, and relative intensity is possible.

The femtosecond time scale is out of reach for even the fastest electronics, so to perform pulse diagnostics other measurement techniques need to be used.

The AvaSoft-Femto add-on turns your Avantes spectrometer into a femtosecond laser diagnostic tool that can provide you with an estimation of the transform limited pulse duration as well as measuring the peak wavelength and relative intensity of the laser output. Using the add-on users can also easily export the measurements to spread sheet programs like Microsoft Excel for record keeping or further analysis.

Figure 1: An AvaSpec-2048L is used for femtosecond laser measurements.
Many femtosecond mode-locked lasers can generate nearly bandwidth-limited pulses, especially when they are based on soliton mode locking. A femtosecond mode-locked Ti:Sapphire laser when fully optimized propagates a soliton-like pulse with a hyperbolic secant squared function intensity distribution in the frequency domain.

By accurately measuring the spectral output of a femtosecond mode-locked Ti:Sapphire and using the time-bandwidth product of different pulse shapes (Gaussian, Hyperbolic Secant Squared, Lorentzian) the lower limit for the pulse duration can be determined.

FWHM Measurement

The full-width at half maximum (FWHM) is accurately measured using an Avantes spectrometer and this is then used in combination with a value of K (that is dependent on the symmetrical shape of the pulse) to calculate the transform limited pulse duration;


Table I: Values for K depending on the pulse shape.
The AvaSoft-Femto software allows for a quick and convenient way to show the transform limited pulse duration according to the pulse shape selected in the software. The complete Femto package also provides a good general purpose spectrometer that can be used for various applications in the lab.

When using AvaSoft-Femto with an Avantes spectrometer that doesn't have an order sorting filter installed, you can also use the system to measure laser lines in the ~300–600 nm range by observing them in the second order. This means 500 nm will show up as 1000 nm on the spectrometer. Lines in the ~200–300 nm range can be seen in the third order (that is, 300 nm will show up at 900 nm on the spectrometer). The spectrometer included in the Ava-Femto package can be used for second and third order measurements.

Typical Configuration for AvaSoft-Femto

The AvaSoft-Femto can be used with virtually any existing Avantes spectrometer provided it covers the required wavelength range.

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