Transform Limited Ultrashort Laser Pulses

As a consequence of the time-bandwidth uncertainty principle, ultrashort laser pulses carry significant bandwidth. If the spectral components that fall under the bandwidth of the laser pulse are time coincident (figure 1.1), the pulse is said to be at its transform limit. A transform-limited pulse implies that the pulse duration is minimized.

Figure 1.1: The temporal relationship between selected Fourier components of a transform-limited pulse.

Measuring Chirped Pulses

Material properties such as dispersion alter the (phase) relationship between the spectral components of light, effectively separating the blue and red components of the pulse in time (figure 1.2). This effect is known as chirp, and for an ultrashort pulse, serves to elongate the pulse in time. Although autocorrelation can measure the duration of an ultrashort pulse, it is unable to characterize the phase relationship between different spectral components. The primary reason for this is that autocorrelators utilizes a single element photodetector that effectively integrates over the spectral profile of the pulse.

Figure 1.2: The temporal relationship between selected Fourier components of a positively chirped pulse.

Frequency-Resolved Optical Gating (FROG)

Fortunately, techniques exist that monitor the spectral profile of a pulse as a function of time. These techniques allow for the complete reconstruction of the electric field. Of these techniques, frequency-resolved optical gating (FROG) is arguably the most straightforward and easiest to implement. As mentioned earlier, FROG allows for full phase retrieval of the input field without the ambiguity associated with autocorrelation. Our Amplitude and Phase Characterization of Ultrashort Laser Pulses application note presents a simple, low dispersion, easy-to-use, general purpose FROG device that can readily be implemented on the Newport Long Scan Autocorrelator platform as described in Application Note 27.

Self-diffraction FROG Geometry

Self-diffraction FROG (SD FROG) geometries are for amplified systems. The SD FROG trace gives an intuitive picture of the pulse (i.e. the direction of time is preserved), which is a highly desirable feature for real-time laser alignment. Moreover, the geometry is identical to the Long Scan Autocorrelator where the nonlinear medium is a thin piece of glass (<200 µm), thus making this geometry cost effective and straightforward to implement. On the downside, SD FROG requires relatively high peak powers not readily available from most ultrafast oscillators, so its use is restricted to amplified systems.

Diagram of SD FROG Setup

Second Harmonic FROG Geometry

Second harmonic FROG (SHG FROG) geometries are the only real option for ultrafast oscillators. An SHG FROG is simply a spectrally resolved autocorrelator. Although the sign of the spectral phase is lost in the measurement, efficient algorithms exist which can deduce the order and magnitude of the spectral phase where the sign can be determined if need by performing additional measurements. (Rick Trebino, "Frequency-Resolved Optical Gating: The Measurement of Ultrafast Laser Pulses", Norwell, MA, Kluwer Academic Publishers (2000)) If planning to use the standard kit with an oscillator, a BBO crystal used to generate the second harmonic – such as Newport model 90015087 (covering input wavelength range of 700-2000 nm) and/or 90015088 (480-700 nm) – must also be purchased separately.

Diagram of SHG FROG Setup

Reference Software

The data acquisition and display software included with this product has been tested in the experimental setup described in the application note and is designed to work only with specific spectrometers. Third-party freeware software is available for interpretation and analysis of complex FROG traces. Please contact us to discuss a custom version of this kit or to get additional information related to acquiring FROG analysis software.