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Spectrally resolved interferometric auto-correlation technique for femtosecond optical measurements.

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Anatoly Masalov
Scientific Center 'Ultratekh', 141700, Dolgoprudny, Moscow dist., Institutsky per. 9, Russia

Sergei Nikitin and Qiang Fu
Quantronix Corporation, 41 Research Way, East Setauket, New York 11733, USA

Abstract: New diagnostic based on spectral resolution of interferometric patterns formed by non-collinear autocorrelator was successfully used to fully characterize femtosecond pulses. Compared to alternative methods, this technique eliminates time-direction ambiguity and improves retrieval reliability.
© 2001 Optical Society of America
OCIS codes: (320.7100) Ultrafast measurements, (190.1900) Diagnostic applications of nonlinear optics

The progress of modern optical ultrafast technologies is closely related to the development of novel ultrafast optical diagnostic methods. The last decade demonstrated an advance from optical correlators to more complicated techniques, such as frequency resolved optical gating (FROG) [1] or spectral phase interferometry for direct E-field reconstruction (SPIDER) [2]. Sub-10-fs pulses were characterized using these methods [3, 4]. When using second harmonic generation (SHG) and single-shot geometry [5], these methods allow non-scanning measurements of low energy pulses. While SPIDER uses interference patterns for non-iterative pulse reconstruction, SHG-FROG requires an iterative algorithm. A new diagnostic reported here, simplifies pulse retrieval procedure and combines the advantage of interferometry with the reliability of FROG.

The diagnostic uses a single-shot autocorrelator [5], modified as shown in Fig.1.

The amplitudes of the input beams are identical: E(t) = A(t)exp(-iw₀t) A very thin nonlinear crystal generates two harmonic beams ~E² and one sum-frequency beam ~E(t)E(t+t), where t is a spatially varying optical delay between the input beams. Usually, only the sum-frequency signal is registered. Here, one of two harmonic beams and the sum-frequency signal are optically relayed from the crystal to the spectrograph entrance slit, where they result in the asymmetric interference pattern:

I_AC(t) = |E(t)² + 2E(t)E(t+t)|²dt(1)

The imaging spectrograph registers two-dimensional "Spectrally Resolved Intrerferometric Auto-Correlation", or SPRINT-AC pattern I_AC(t,w).

Fourier filtering of I_AC(t) gives the following correlation function:

I_INT(t) = A²(t)A(t)A(t+t)exp(2iw₀t)dt + c.c.(2)

An assumption A(t)³ A₀d(t) leads to a very approximate but important relation:

I_INT(t) ~ A(t)exp(2iw₀t) + c.c.(3)

This relation removes time-direction ambiguity and provides the initial guess for the final retrieval using the SPRINT-AC pattern and FROG-like iterative algorithms.

Fig.2 and Fig.3 show a sample SPRINT-AC pattern and the corresponding pulse obtained during the alignment of a commercial laser amplifier.

The efficiency of the SHG process is sufficient to monitor 15-fs pulses directly from a 150 mW oscillator. If necessary, the apparatus can be modified to measure spectrally resolved fringe-resolved autocorrelation or standard SHG-FROG patterns. The improved reliability of this method is helpful in difficult experimental situations, e.g. retrieval of broadband (>50 nm) optical pulses. An accelerated retrieval based on (3) can be very useful for real-time pulse shaping and phase control.

References

D.J. Kane and R. Trebino, " Single-shot measurement of the intensity and phase of an arbitrary ultrashort pulse by using frequency-resolved optical gating," Opt.Lett. 18, 823-825 (1993) .

C. Iaconis and I.A. Walmsley, "Spectral phase interferometry for direct electric-field reconstruction of ultrashort optical pulses," Opt.Lett. 23, 792- 794 (1998).

G. Taft, A. Randquist, M.Murnane, I.P. Christov, H. Kapteyn, K.DeLong, D.Fittinghoff, M. Krumbugel, J.Sweetser and R. Trebino "Measurement of 10-fs laser pulses," IEEE J. Sel. Top. Quantum Electron. 2, 575-585 (1996).

L. Gallman, D.H. Sutter, N. Matushek, G. Steinmeyer, U. Keller, C. Iaconis and I.A. Walmsley "Characterization of sub-6-fs optical pulses with spectral phase interferometry for direct electric field reconstruction," Opt. Lett. 24, 1314-1316 (1999).

R.N. Gyuzalian, S.B. Sogomonian and Z.G. Horvath, "Background-free measurement of time behavior of an individual picosecond laser pulse", Opt. Commun 29, 239-242 (1979).