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Optical Design & Engineering

Programmable micro-optics for ultrashort pulses

Spatial light modulators with excellent temporal transfer can shape ultrashort wave packets by digitally mimicking complex phase elements.
19 March 2010, SPIE Newsroom. DOI: 10.1117/2.1201003.002866

We first managed to combine the advantages of low-dispersion layer optics with the near-paraxial design of ultraflat micro-optical elements—and exploit them for high-precision shaping and as diagnostics of femtosecond-laser pulses with durations of a few optical cycles—with thin-film structures fabricated by mask-supported vapor deposition.1 With refractive, reflective, and hybrid micro-axicons (conical lenses) consisting of dielectric, metallic, or compound layers, we demonstrated that ultrashort-pulsed Bessel-like nondiffracting beams (known as ‘X pulses’ or ‘light bullets,’ described by Bessel functions) can be generated easily in linear optical setups.2–4 The unique propagation properties of such beams, including extended depth of focus, high tilt tolerance, axial-field components, and self-reconstruction,5 make them very attractive for applications such as highly robust Shack-Hartmann wavefront sensors,6 multichannel materials processing,7 or 2D autocorrelation.1,8

However, adaptive functionality is urgently required to improve the dynamic measurement range (for instance, by encoding the sub-beams of a wavefront sensor), address individual channels for more flexible processing, or significantly enhance the resolution of interferometric or holographic metrology through the use of additional phase-step procedures. Unfortunately, digital-mirror displays with large numbers of small subapertures are typically limited to amplitude switching and suffer from heat-transfer problems. Liquid-crystal spatial light modulators (LC-SLMs), on the other hand, enable phase and amplitude programming. They were first applied to temporal pulse shaping using Fourier-plane optical processors9 (where pulse duration is not important). However, transmitting LC-SLMs exhibit crucial pulse distortions that become apparent as dispersion in the substrate and liquid-crystal layer. We recently achieved important improvements by introducing new types of liquid-crystal-on-silicon spatial light modulators (LCoS-SLMs). Their substrate dispersion is eliminated by working in reflection (liquid-crystal layer on a mirror).

We systematically analyzed the key problem of pulse transfer as a function of spectral phase and temporal pulse information using a titanium:sapphire laser oscillator with pulse durations of <20fs.10 We showed that (for selected types of LCoS-SLMs) excellent temporal-transfer characteristics can be obtained. We detected small residual phase distortions, which can be explained by Gires-Tournois interference.11 This could be verified by diffraction experiments and spectral-phase interferometry for direct electric-field reconstruction with an extended nonlinear crystal (e.g., LX-SPIDER). Our results agree with theoretical estimates.10 The linear dependence of the LCoS-SLM phase on gray level (voltage signals) allows programming of micro-optical components using calibrated gray-scale maps.

If the maximum phase difference is divided into sufficient steps (say, 256), the resulting resolution allows approximation of phase profiles comparable to dielectric nanolayer structures. Therefore, even fringe-free nondiffracting beams (needle beams1 or—in the pulsed regime—needle pulses) can be shaped that exhibit extraordinarily large aspect ratios. With arrays of addressable, separated femtosecond needle pulses, we demonstrated12 the concept of diffractionless ‘flying images.’13 Figure 1 shows the setup of a programmable (needle) beam-array generator. In Figure 2, we show the radial intensity variation of a femtosecond Bessel beam (part of an array containing 20 illuminated elements) with conical angle θn(n=1, 2, 3) of a micro-axicon (programmed into the phase map) at a distance of z=28mm. The corresponding spatial frequencies vary between 0.009 and 0.0156/μm.

Figure 1. Experimental setup for programming micro-optical functionality into gray-value maps of a liquid-crystal-on-silicon spatial light modulator (LCoS-SLM). The image shows generation of an array of needle beams for multichannel processing.

Figure 2. Variable, ultrashort-pulse Bessel beam generated with a phase-only LCoS-SLM. The spatial frequency depends on the conical angle θn (n = 1, 2, 3) of the micro-axicon programmed in the phase map (titanium:sapphire oscillator, pulse duration 13fs, measuring distance z = 28mm). The spatial frequencies for the angular interval vary between 0.009 and 0.016/μm.

We recently tested the performance of adaptive systems, including LCoS-SLMs, for shaping complex optical fields. Figure 3 shows an example of a propagating femtosecond-wave field. In addition, we explored generating and detecting wave packets carrying a nonzero extrinsic orbital angular momentum.14 We are currently investigating 2D pulse diagnostics with wavefront-dividing Shack-Hartmann-type architectures, in particular novel schemes of spatio-temporal autocorrelators (e.g., nonlinear phase-step Bessel autocorrelators, which extract temporal information from the wobbled fringe envelopes of frequency-converted Bessel pulses15). In summary, LCoS-SLMs are promising high-resolution beam shapers for ultrafast micro-optics.16 They enable 2D spatial shaping and diagnostics of laser pulses. Under optimized conditions, wave packets with pulse durations in the sub-20fs range are transferred with acceptably low distortion. We are also investigating adaptive, nondiffracting wave phenomena like tubular beams, light slices, and phased arrays for high-resolution metrology, multichannel materials processing, and advanced methods of autocorrelation.

Figure 3. Nondiffracting propagation of a pulsed complex pattern (Max Born Institute logo) programmed into the gray-scale map of the SLM (far left). Red: Axial distance. Field of view: 2.5×2.5mm2. The sub-beam diameters (at a distance of z = 3mm) were 90μm (2× Gaussian-waist radius).

Ruediger Grunwald, Martin Bock
Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy
Berlin, Germany

Ruediger Grunwald received his PhD in 1986. He has worked on spectroscopy, laser resonators, and micro-optics. He has been with the Max Born Institute since 1998, where he develops new methods for beam shaping and pulse characterization. He also investigates laser-induced nanostructures and has written a book on thin-film micro-optics.

Martin Bock finished his Master's degree in photonics at the University of Applied Sciences in Wildau (Germany) in 2008. He is currently a PhD student, working on adaptive shaping and diagnosis of femtosecond pulses with spatial light modulators. He is also involved in research on nanostructuring, pulsed vortex beams, statistical methods of spectroscopy, and quantum-interference experiments.