Double-blind frequency-resolved optical gating
Double-blind FROG (DB FROG) is a method for measuring two unknown ultrafast laser pulses at the same time. Conventional FROG and its simpler version GRENOUILLE can only measure one pulse at a time, and cross-correlation FROG (XFROG) also measures a single pulse but uses a known reference pulse as the gate.
In many experiments there are two input pulses, such as in Raman spectroscopy, two-color pump-probe setups, or non-degenerate four-wave mixing. To fully understand these experiments, you need to measure both pulses simultaneously.
Early work by Trebino and Kane in 1995 introduced Blind FROG, where the second unknown pulse acts as the gate. The resulting trace is hard to interpret: many different pulse pairs can produce the same trace, and even with extra spectral information, the retrieval can converge slowly. However, Blind FROG can be useful in some contexts.
In 2002, Trebino proposed Double Blind FROG to solve this two-pulse problem. DB FROG uses two FROG traces, and the extra information in the second trace makes it possible to retrieve both pulses uniquely (aside from trivial phase and timing ambiguities).
A common DB FROG setup uses polarization-gate (PG) geometry. To create the second trace, you add crossed polarizers and a spectrometer. Trace 1 is a FROG trace that depends on the two pulses as they gate each other, similar to PG XFROG. Trace 2 is a second trace with the pulses’ roles reversed. Together, the two traces provide enough information to recover both unknown pulses.
The retrieval algorithm is based on XFROG. Start with random guesses for the two pulses. Use trace 1 to retrieve the first pulse while treating the second as the gate. Then swap the roles and use trace 2 to improve the second pulse. Alternate between traces 1 and 2 in cycles (usually 3–5 cycles) until the calculated traces agree with the measurements.
DB FROG works for pulse pairs with time-bandwidth products from 1 to 6 and at different wavelengths. Experiments and simulations show the method is robust to noise and other trace imperfections, and it often converges quickly even if the initial guesses are not perfect.
The PG geometry has the advantage of very broad phase-matching bandwidth, making alignment easier. Its drawback is the need for high-quality polarizers, which can be expensive and may distort the pulse. Any distortion can be corrected by numerically back-propagating the recovered pulse through the polarizer.
Overall, DB FROG is a promising approach for measuring two pulses at once. It is not yet widely used, but it remains an active area of research.
This page was last edited on 3 February 2026, at 11:52 (CET).