Femtosecond Pulse Shaping


3.  Prior results


3.1 Temporal pulse shaping


The ultrafast optics laboratory at SDSU is ready to undertake the experimental endeavors proposed here.  The lab includes a Ti:Sapphire oscillator (capable of generating 10-100 fs pulses), a multipass amplifier, computers, electronics, assorted optics and mounts.  At 200-300 mW  output, the oscillator permits a range of experiments without amplification. There are two operational pulse shapers currently in the lab.  The first is a temporal pulse shaper.  It is a grating-based 4f system using a liquid crystal spatial light modulator (SLM) consisting of two 128 arrays sandwiched between polarizers (Cambridge Research SLM-256).  This device can shape both the amplitude and phase of our femtosecond pulses.  The outgoing temporal structure can be conveniently monitored using a SPIDER pulse characterization apparatus.  For a SPIDER review, see [Anderson LPL].  With this system we have demonstrated, for example, the ability to compress complicated pulses back to the transform limit in a single step (an experiment that was highlighted in Optics & Photonics News’ “Highlights from the year in Optics”) [Anderson OPN].


Figure 3. SDSU in femtosecond laser light.


3.2 Spatial pulse shaping

Figure 4.  Femtosecond Laguerre-Gauss vortex beams in the infrared (1st column), second harmonic (2nd column), and theoretical prediction (3rd column).  From top to bottom, the vortex charge is 5, 10, 15, and 20.



The second SLM is our spatial mode shaper.  It is a 2D liquid crystal display (Hamamatsu X8267-17, $25k, obtained with a Blasker Grant from the San Diego Foundation). This SLM is reflective and has high transmission and diffraction efficiency, improving the overall throughput of the device.  It is a phase-only device, but can cleverly be made to control amplitude as well through the use of blazed diffraction patterns, as shown in Figure 3.


3.3 SHG vortices


This device has been employed by Dr. Anderson’s MS student to create femtosecond optical vortices and study their behavior in nonlinear second harmonic generation (SHG) [presented at OSA Annual 2009].  The results are shown in Figure 4. 


Figure 5. Modulated vortex pattern and resultant beam profile.


3.4 Modulated vortices


A BS student of Dr. Anderson used the 2D SLM to create femtosecond modulated vortices [to be presented at OSA Annual this year].  Modulated vortices have an azimuthally varying spiral phase, which results in a “flower” pattern as shown in Figure 5.  (Note: for clarity, not shown is the actual phase pattern employed to generate this flower which consisted of a spiral phase combined with a blazed grating).



 Anderson, M.E., A. Monmayrant, S.-P. Gorza, P. Wasylczyk, I.A. Walmsley, “SPIDER:A decade of measuring ultrashort pulses,”  Laser Physics Letters 5, 259-266 (2008).


Anderson, Matthew E., Josh Thornes, and Phillip Poon, “Compressing femtosecond laser pulses non-iteratively,” Optics & Photonics News, “Optics in 2004,” 15, 43, (December 2004).