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Rotary Frog

Ultrashort pulses of light are used for micro-machining, laser eye surgery, medical imaging, observing chemical phenomenon that occur on a femtosecond timescale, generation and detection of terahertz radiation (T-rays), and formation of frequency combs, for example.

FROG (Frequency Resolved Optical Gating) allows the characteristics of ultrashort pulses of light to be determined. The phrase "frequency resolved optical gating" results from a theory that a short gate pulse can be used to obtain a sample from a longer pulse by nonlinear mixing (gating) in a nonlinear crystal material. Since a gate pulse shorter than the pulse to be measured is not usually available, FROG uses the pulse itself for gating. There are different versions of FROG that (i) rely upon different nonlinear gating mechanisms, (ii) generate different kinds of FROG traces (requiring different phase retrieval algorithms), and (iii) have differing strengths and weaknesses. For example, Polarization Gated FROG (PG FROG), Self-Difference FROG (SD FROG), Transient-Grating FROG (TG FROG), Second-Harmonic FROG (SHG FROG), and Interferometric FROG (IFROG) are known.

Second-Harmonic FROG (SHG FROG) is the most popular FROG version and is based upon an X(2) nonlinear crystal that allows the apparatus to achieve a much higher sensitivity than is possible with X(3) crystals. However, phase matching issues require careful treatment to avoid distortion for short pulses. One method for phase matching very weak pulses of laser light is to dither a thick crystal in front of the beam. This method, however, introduces several problems. Since the incident angle is exposed to varying thicknesses of the crystal, dependent on the angle of incidence, the efficiency and the bandwidth of the second harmonic varies. The crystal also requires a supporting geometry with a mechanism to introduce acceleration and deceleration for a continuous acquisition. Control of such continuous change in motion of the crystal is difficult to achieve and requires that the acceleration and deceleration be incorporated into measurements when a velocity control mode is used.

A novel Rotary FROG method developed by Dartmouth inventors passes ultrashort pulses through a thick, rotating crystal and measures the second harmonic using a simple detector, thereby overcoming many of the above limitations. This technology is claimed in the published PCT Application No. PCT/US09/40582. We are seeking an industrial partner interested in its commercialization. (Ref: J441)

Last Updated: 7/24/12