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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 |
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«Technology Transfer Office : Sponsored Projects : Dartmouth College |
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Phone: (603) 646-3027 |
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Fax: (603) 646-3670 |
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