Measuring the spatiotemporal field of a single ultrafast laser pulse

Although techniques for measuring the temporal intensity and phase of ultrashort laser pulses (i.e., pulses lasting only 1ps or less) are now well established,^1,2 these methods rarely measure the wide range of spatial distortions and couplings that can occur between the space, time, and frequency coordinates. This is a problem because ultrashort laser pulses are used in turn to make measurements. Distortions in pulses degrade most ultrafast measurements, and thus reduce the reliability of the results. The scientific community that uses ultrashort pulses needs a technique that measures the spatial, temporal, and spatiotemporal characteristics of the pulse electric field, E(x,y,t), completely. Many distortions are not evident in separate measurements of the temporal and spatial profiles. The ability to measure E(x,y,t) would also be a powerful diagnostic tool that would allow the study of complex media with time-varying spatial structures.

Numerous techniques have been proposed to measure the spatiotemporal profile of ultrashort pulses, but they are either limited to one spatial coordinate,³ or they average the characteristics of the pulse over time or frequency,⁴ or they require multiple measurements on a train of identical pulses in order to obtain E(x,y,t).⁵ In contrast to such efforts, we have developed the spatially and temporally resolved intensity and phase evaluation device: full information from a single hologram (or, more handily, STRIPED FISH)⁶ that yields E(x,y,t) on a single shot so that an individual ultrashort pulse can be fully characterized in space and time.

The STRIPED FISH technique involves generating multiple holograms, one for each frequency component in the pulse and then combining them to yield E(x,y,w). Specifically, this entails interfering the pulse under test (the ‘signal’ pulse) with a pre-characterized spatially uniform ‘reference’ pulse (see Figure 1) at a small angle, as in standard holography. Then these two pulses pass through a low-resolution 2D diffraction grating, which generates a 2D array of replicas of the incident signal and reference pulses, yielding an array of holograms where the beams cross. The second component of STRIPED FISH, a tilted interference band-pass filter or etalon, separates the wavelengths of the holograms. Finally, we also orient the 2D diffraction grating so that it is rotated slightly about the optical axis. As a result, the hologram array is also slightly rotated. Therefore, each hologram involves pairs of beams of a (uniformly spaced) different wavelength. The resulting quasi-monochromatic holograms, each at a different color, yield the complete spatial field—both intensity and phase—for each color in the pulse.⁷ These holograms can then be combined, using the known spectral phase of the reference pulse, to yield the complete spatiotemporal field of the signal pulse, E(x,y,t). All this information can be captured in a single frame from a digital camera.

The STRIPED FISH method should be able to measure very complex pulses. It is easy to show that it can theoretically measure pulses with space-time-bandwidth products as large as 1,000,000, which corresponds to an extremely complex pulse.

We tested STRIPED FISH by fully characterizing ultrashort pulses from a mode-locked Ti:sapphire laser. We introduced various controlled distortions (such as group delay, group-delay dispersion, and spatial chirp) into the signal pulse. We successfully measured these distortions using our method (see Figure 2). We hope that STRIPED FISH will simplify the measurement in time and space of considerably more complex pulses.

The ability to measure the full spatiotemporal field of ultrashort pulses should play an important role in evaluating and improving the performance of ultrafast lasers, as well as aiding the study of systems with spatial and temporal structure. The STRIPED FISH technique permits such measurement in a single-shot geometry with a simple device.