Pulse Measurement: Frequency Resolved Optical Gating (FROG)
Ultrafast laser systems, which generate laser pulses with durations of approximately 10 picoseconds or less, have a large number of applications in biochemistry, chemistry, physics, and electrical engineering. Such systems may be used to explore kinetics in proteins or examine carrier relaxation in semiconductors. They are also used as an ultrafast probe in electronic circuits. By using ultrafast diagnostic systems, highly advanced semiconductors, electronic circuitry, and even biomedical products can be developed and tested for commercial applications. Unfortunately, ultrafast laser systems can be difficult to develop and maintain because few diagnostics are available to characterize the ultrashort laser pulses.
Real ultrafast laser pulses are not perfect, smooth pulses. Even though they are short, they exhibit temporal structure in their amplitude and/or phase. The most common structure is called chirp. Chirped pulses can be viewed as changing color. In the case of positive chirp, the wavelengths change from red at the beginning of a pulse to yellow at the end. This chirp is the result of a change in phase of the light during the pulse that can be induced by dispersion or nonlinear optical processes. Pulse chirp is not just detrimental; chirp in ultrafast laser pulses can be specifically designed to excite molecules more efficiently and to generate sculpted quantum states. Chirp also increases the length (duration) of a pulse. Ultrafast laser systems require a diagnostic tool that can measure both the intensity (or amplitude) and the phase of the optical pulses.
Frequency-resolved optical gating, or FROG, measures both the intensity and phase (chirp) of ultrafast laser pulses. Consequently, it is the only true pulse measurement technique available. Other methods, such as MIIPS and SPIDER, are only frequency domain measurement technologies—they only measure the spectral phase of the ultrafast laser pulses. By combining the spectral phase with the pulse spectrum, the pulse shape can be determined. Unfortunately, measuring the pulse spectrum is not as easy as you might think. Amplified spontaneous emission (ASE) from amplified ultrafast laser broadens the measured ultrafast laser pulse spectrum. This broadened spectrum, together with the spectral phase can make the pulses seem shorter than they are. Worse, some ultrafast laser manufacturers use this strategy to make their specifications appear better than they really are. FROG, on the other hand, provides a true pulse measurement independent of the spectral measurement.
FROG characterizes ultrashort laser pulses by interacting one or more pulses in a nonlinear medium. One pulse forms a “gate” that lets a time slice of the other pulse pass to a spectrometer. This signal pulse is spectrally resolved and recorded as a function of the delay between the input pulse and the gate. This record is called a spectrogram or FROG trace. The spectrogram is a plot of signal intensity vs. frequency and time which contains all of the information about the laser pulse. In the chirp example given above, the spectrogram would show that the pulse was red at early times, changing to orange in the middle and yellow at the end of the pulse. The target information, the temporal and spectral profile of the input pulse (intensity and phase), can be obtained from the FROG trace using two-dimensional phase retrieval methods.