Programmable High Resolution Broadband Pulse Shaping using a 2-D VIPA-Grating Pulse Shaper with a Liquid Crystal on Silicon (LCOS) Spatial Light Modulator

We demonstrate programmable spectral shaping with simultaneous broad-bandwidth(>40nm) and high-resolution(<4GHz) using a 2-D VIPA-Grating pulse-shaper with a LCOS SLM. The apparatus is capable of scaling to bandwidths of 100s of nm with sub-GHz resolution. © Optical Society of America OCIS codes: (320.5540) Pulse Shaping; (320.7085) Ultrafast information processing Femtosecond pulse shaping is a widely used technique with applications ranging from optical communications to coherent control of quantum processes [1]. The achievable complexity of shaped waveforms in conventional 1-D pulse shapers is usually limited by two reasons: (1) limitations of spectral dispersers, which typically have either broad operation bandwidth but coarse spectral resolution or high spectral resolution but limited operation bandwidth. (2) limited number of control elements, e.g., only up to several hundred pixels in 1-D spatial light modulators (SLMs). Here we overcome these limitations to demonstrate an apparatus which can achieve programmable shaping simultaneously over broad bandwidths with high spectral resolution. Our approach combines a high resolution, narrow band spectral disperser with a low resolution, broadband spectral disperser for two-dimensional (2-D) spectral dispersion within a pulse shaping configuration. The 2-D spectral disperser was demonstrated first for optical communication applications [2] and later applied for massively parallel frequency comb spectroscopy [3]. We have previously demonstrated a 2-D pulse shaper based on this approach to achieve high complexity waveforms with timebandwidth product > 1600 (achieving shaping of 150 fs pulses over a 200 ps temporal aperture) [4]. However, the work in [4] was limited to fixed, photolithographically fabricated masks. Our current work is geared toward achieving programmability by using a 2-D liquid crystal on silicon (LCOS) SLM (which has ~ 2 million pixels) [5]. As the high resolution disperser we use the Virtually Imaged Phased Array (VIPA) which is a side entrance Fabry-Perot etalon which achieves spectral dispersion by multiple beam interference [6]. VIPAs have been shown to achieve sub-GHz spectral resolution in pulse shapers [7]. However as is the case with Fabry-Perot devices, a VIPA has a free spectral range (FSR) which may typically be a few 100GHz; hence for most ultrafast applications in which bandwidths are orders higher, spectral components separated by integral number of FSRs are dispersed to the same spatial location. We overcome this by using a diffraction grating aligned in a perpendicular direction to spatially separate out overlapping FSRs. Also, since now the frequency spread is 2-D, we can use 2-D SLMs which have millions of pixels far exceeding the number of control elements available in 1-D geometries. In our current work we use a 2-D Liquid Crystal on Silicon (LCOS) phase-only SLM [5] to perform spectral amplitude shaping (via programmable polarization rotation plus a polarizer). However, it is possible to use a phase-only device to perform simultaneous amplitude and phase shaping using diffraction effects (for e.g. [8]), which we plan to pursue in future work. Here we would like to contrast our work from other 2-D pulse shaping configurations (for example [8, 9]) which still utilize a single spectral disperser and have not been aimed at increasing the time-frequency complexity. Fig.1. (a) Experimental Setup, (b), (c) Fourier plane images before and after programming a user defined mask. (d), (e) – Corresponding spectra. The inset in (e) shows the zoomed version of the circled section showing finer features made possible due to the VIPA. a223_1.pdf OSA / UP 2010 TuF3.pdf