Double-blind frequency-resolved optical gating
Well established ultrafast measurement techniques such as frequency-resolved optical gating and its simplified version GRENOUILLE can only measure one unknown ultrashort laser pulse at a time.
In modern optics experiments, ultrashort laser pulses have been used in a great variety of engineering application and scientific research, for example, biomedical engineering, material science, nonlinear spectroscopy, ultrafast chemistry, etc.
In all such cases, measuring more than one pulse simultaneously is required to completely characterize the experiment and understand its results in order to eventually understand the underlying science of the process under study.
Early attempts to solve the two-pulse measurement problem were made by Trebino and Kane and co-workers beginning in 1995.
[1][2] They took advantage of the fact that FROG and its variations involved crossing two replicas of the pulse to be measured in a nonlinear-optical medium (where one gated the other in time) and measuring the spectrum of the product of the two pulse electric fields vs. delay.
Without extra information about the pulses, such as the spectra, non-trivial ambiguities are found by the Blind FROG retrieval algorithm.
The attosecond-laser-pulse community, however, finds the Blind FROG approach useful due to the specific mathematical form used in the retrieval algorithm in this case.
On the other hand, for more common, longer pulses, improvements were required.
This second FROG trace contains the extra information required to retrieve both pulses essentially uniquely (with only trivial ambiguities, such as the zeroth-order phase and the first-order spectral phase, which corresponds to the pulses’ average arrival time).
The modification required to turn a Blind PG FROG into Double Blind PG FROG is the addition of a pair of crossed polarizers and a spectrometer.
This term, of course, also has the form of a FROG trace produced by PG XFROG.
is not the correct one, but it is an improved version of it, since trace 1 contains information about
as the unknown) and runs trace 2 with standard XFROG algorithm to produce a better version of
It typically takes 3-5 cycles to converge depending on the complexity of the pulse pair.
Pulse pairs with Time Bandwidth Products (TBP) ranging from 1 to 6 and also different wavelengths have been measured and retrieved experimentally using DB PG FROG.
[5][6] These measurements demonstrated that the DB FROG retrieval algorithm is capable of ignoring experimental noise and various inevitable non-physical details in the recorded traces and that it returns the correct retrieved pulse.
In addition to experimental work, numerical simulations have also shown that the DB FROG retrieval algorithm is extremely robust and reliable.
In the case of Polarization-Gate geometry, the advantage is the infinite phase-matching bandwidth which makes the system alignment insensitive.
On the other hand, a disadvantage of PG geometry is the requirement of high-quality polarizers (calcite polarizers work fine) which could be expensive and introduce non-negligible distortion into the pulse.
This distortion could be removed by numerically back propagating the pulse through the polarizer.
DB FROG is promising and, although not in widespread use, it is a subject of active current research.
