Thursday, July 7, 2011

Meeting Jay Sharping - Fiber Optical Parametric Oscillators

Hey everyone!

Meeting time! I recently went to the dinner and meeting hosted by the N-Cal OSA chapter. This month the guest of honor was Professor Jay Sharping from the University of California at Merced. 

We were joined by a whole crowd of optics locals. Dinner with this group of people is always fun. I get to hear much about what else is going on with other groups and areas of optics. Here’s Jay (front) with Rick Rairden, Olaf Korth, Yujun Deng, Eric Hansotte and I’m-not-sure-who-that-is? I forced everyone to stop their discussion so I could click this pic and they kindly obliged! :) I couldn’t manage to get the whole table in one click. Need one of those 180 degree panoramic viewing cameras!

Fig.1. Dinner with Jay Sharping (front)
Afterwards we drove over to Xerox PARC where Jay talked about his work at UC Merced. UC Merced is a relatively new school, started around 2005, located about 90 minutes South-East from the Bay area. Jay is on their Physics faculty and heads the Applied Photonics Research Group.

Most of his research revolves around Optical Parametric Oscillators. OPOs are essentially used as tunable light sources. An OPO uses the principles of nonlinear optics i.e. the nonlinear influence of the electric field of light on the dielectric polarization of a material such as nonlinear crystals or fibers. 

In an OPO, an input pump frequency is passed through an optical non-linearity such as a nonlinear crystal or fiber to output a pulse at a different frequency. You can change the output frequency by tuning the phase matching properties of the nonlinear material. Jay’s team works specifically in the area of OPOs that use photonic crystal fiber to provide the non-linearity.  Photonic crystal fibers show similar nonlinear effects as ordinary optical fiber, along with the benefit of greater chi(3) nonlinearity. Here and here are two of his papers that elaborate a little more.

My interest in OPOs and femto second/pico second lasers stems from their use in two-photon microscopy, CARS (Coherent Anti-stokes Raman Spectroscopy) and other such non-linear imaging modalities.

Two-photon emission results from the absorption of two photons which are at twice the wavelength of absorption of a fluorescent dye. i.e. If your dye would fluoresce on the absorption of one photon at 350nm, instead it will fluoresce the same emission w/l if it is bombarded with two photons, each at 700nm. So effectively one ends up using much longer w/ls for two photon excitation. Longer w/ls penetrate deeper, so one can image deeper. Longer w/ls are also usually safer for living tissue. The nonlinear emission process is generally localized to the center of the beam where the intensity is maximum, hence it naturally imparts some amount of sectioning. Non-linear emission processes are less efficient for longer pulse widths (or continuous beams) since they require high peak intensities which can typically be achieved only in short pulse widths.

Pulsed lasers are commonly used as sources for non-linear imaging when a narrow band of wavelength tunability is sufficient. In cases where a broad range of tunable wavelengths are required, OPOs offer significant advantages. 

Jay showed some good results using their Fiber-OPO for CARS imaging. I also liked his one-line description of CARS, “If you tune two frequencies to coincide with the Raman mode of a material, you can efficiently excite that mode and use it to image!” That’s succinct! Well, CARS sure makes for useful images! 

OPOs do have other uses in probing material properties, quantum states, etc. I haven't written much about that here. But you can look up his work at UC Merced as well as his earlier collaborators, the Gaeta Group at Cornell for more details if you are interested.

Cheers and thanks for reading!

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