Marco A. Krumbügel

Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating

We summarize the problem of measuring an ultrashort laser pulse and describe in detail a technique that completely characterizes a pulse in time: frequency-resolved optical gating. Emphasis is placed on the choice of experimental beam geometry and the implementation of the iterative phase-retrieval algorithm that together yield an accurate measurement of the pulse time-dependent intensity and phase over a wide range of circumstances. We compare several commonly used beam geometries, displaying sample traces for each and showing where each is appropriate, and we give a detailed description of the pulse-retrieval algorithm for each of these cases.

Measurement of the intensity and phase of ultraweak, ultrashort laser pulses

We show that frequency-resolved optical gating combined with spectral interferometry yields an extremely sensitive and general method for temporal characterization of nearly arbitrarily weak ultrashort pulses even when the reference pulses is not transform limited. We experimentally demonstrate measurement of the full time-dependent intensity and phase of a train of pulses with an average energy of 42 zeptojoules (42 x 10(-21) J), or less than one photon per pulse.

Measurement of 10-fs laser pulses

We report full characterization of the intensity and phase of /spl sim/10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks.

FREQUENCY-RESOLVED OPTICAL-GATING MEASUREMENTS OF ULTRASHORT PULSES USING SURFACE THIRD-HARMONIC GENERATION

We demonstrate what is to our knowledge the first frequency-resolved optical gating (FROG) technique to measure ultrashort pulses from an unamplified Ti:sapphire laser oscillator without direction-of-time ambiguity. This technique utilizes surface third-harmonic generation as the nonlinear-optical effect and, surprisingly, is the most sensitive third-order FROG geometry yet.

Ultrafast optical switching by use of fully phase-matched cascaded second-order nonlinearities in a polarization-gate geometry.

We show that cascaded second-order nonlinear-optical processes can occur in a convenient polarization-gate beam geometry. Our arrangement uses type II phase matching, and both individual second-order processes (upconversion and downconversion) are simultaneously phase matched. This geometry can be applied to efficient ultrafast optical switching. With a beta-barium borate crystal and lightly focused 250-fs, 7.3-microJ pulses, we achieve a switching efficiency of 15% and an on-off ratio of 3 x 10(4) on a pulse-length-limited time scale.

Temporal characterization of mid-IR free-electron-laser pulses by frequency-resolved optical gating

We performed what we believe are the first practical full-temporal-characterization measurements of ultrashort pulses from a free-electron laser (FEL). Second-harmonic-generation frequency-resolved optical gating (FROG) was used to measure a train of mid-IR pulses distorted by a saturated water-vapor absorption line and showing free-induction decay. The measured direction of time was unambiguous because of prior knowledge regarding free-induction decay. These measurements require only 10% of the power of the laser beam and demonstrate that FROG can be implemented as a pulse diagnostic simultaneously with other experiments on a FEL.

Direct ultrashort-pulse intensity and phase retrieval by frequency-resolved optical gating and a computational neural network

Ultrashort-laser-pulse retrieval in frequency-resolved optical gating has previously required an iterative algorithm. Here, however, we show that a computational neural network can directly and rapidly recover the intensity and phase of a pulse.

Amplified ultrafast optical switching by cascading cascaded second-order nonlinearities in a polarization-gate geometry

Abstract We demonstrate amplified ultrafast all-optical switching of 250-fs pulses using two different types of cascaded second-order nonlinearities (CSNs) simultaneously in a simple polarization-gate geometry using a type II crystal. The first set of CSN processes has the effect of rotating the input-pulse polarization, yielding polarization gating. This signal pulse is then amplified by a second (different) set of CSN processes involving second-harmonic generation of the gate followed by a stimulated parametric down-conversion process. Using these “cascaded cascaded” second-order processes, we observe amplified switching with 320% efficiency and an on-off ratio of 6 × 10 5 in a 1-mm-thick crystal of BBO.

Frequency-resolved optical gating using cascaded second-order nonlinearities

We demonstrate a method that yields intuitive traces, unambiguous pulse measurement, and sufficient sensitivity to measure unamplified pulses, and it is simple to implement. It uses FROG cascaded X(2) effects for the optical nonlinearity, in this case, upconversion followed by downconversion.

Frequency-resolved optical grating using surface third-harmonic generation

Summary form only given. Frequency-resolved optical grating (FROG) based on polarization-gate (PG FROG) and second-harmonic generation (SHG FROG) have been used successfully to characterize ultrashort optical pulses from the infrared to the ultraviolet. We extend this FROG technique with the use of third-harmonic generation (THG FROG) on the surface of virtually any substrate that transmits the fundamental wavelength. Unlike SHG FROG where the presence of the time-reversal ambiguity resulted in traces that are somewhat unintuitive, the third-order process of THG is unambiguous therefore the resulting THG FROG trace is as appealing as the popular PC FROG. We used a femtosecond Ti:sapphire laser oscillator.