Optics in 1997 Ultrafast Technology

Ranka Figure 1. (a) Spectral dependence of the generated two-pho­ton current for a GaAsP photodiode measured with an 80-MHz train of 100-fsec pulses (circles). The calculated spectral response of a 100-μm-thick BBO crystal is shown by the solid line. (b) Interfero­metric autocorrelation measurement of a 6-fsec pulse with a GaAsP photodiode (circles) and a 15-mm-thick BBO crystal (line). (c) Range of the two-photon response for various photodiode materials.

including the pulsewidth and spectral dependence of the two-photon response, the lower limits of measurable pulse duration due to the inherent dispersion of the device, and the dynamic range of input intensities over which the quadratic dependence is maintained.

We have performed such an experimental characteri­zation6 of an "off-the-shelf" GaAsP diffusion diode (Hamamatsu G1117) for use in measuring pulses in the near infrared from modelocked laser systems such as Ti:sapphire. In Figure la we plot the measured spectral dependence of the two-photon response of the diode for a 100-fsec pulse train (circles). The response can be seen to be relatively uniform over a 250-nm range. For com­parison with a second-harmonic crystal of comparable sensitivity to the photodiode, the calculated spectral response of a 100-mm-thick B-barium borate (BBO) crystal for Type-I phasematching is shown (line). Figure lb shows the interferometric autocorrelations of a 6-fsec pulse measured using a 15-μm BBO crystal (line) and the photodiode (circles). These measurements demonstrate that the dispersive pulse broadening in the active region is minimal and that the bandwidth of the two-photon response is sufficient to accommodate such short pulses.

At present, commercially available photodiodes should allow for autocorrelation measurements of puls­es ranging from 400 nm-10 μm (see Fig. 1c). We believe that, in many applications, photodiodes will replace second-harmonic crystals for use in ultrashort pulse-autocorrelation measurements as a result of their numerous advantages including sensitivity, broad-band response, low cost, compactness, and elimination of phase-matching requirements.

References 1. E.P. Ippen and C.V. Shank, Ultrashort Light Pulses, S.L.

Shapiro, ed. (Springer-Verlag, New York, N.Y., 1977), pp. 89-92 .

2. Y. Takagi et al., "Multiple- and single-shot autocorrelator based on two-photon conductivity in semiconductors," Opt. Lett. 17, 658 (1992).

3. D.T. Reid et al., "Light-emitting diodes as measurement devices for femtosecond laser pulses," Opt. Lett. 22, 233 (1997).

4. W. Rudolph et al., "Femtosecond autocorrelation measure­ments based on two-photon photoconductivity in ZnSe," Opt. Lett. 22, 313 (1997).

5. A . M . Weiner, "Effect of group velocity mismatch on the measurement of ultrashort optical pulses via second har­monic generation," IEEE J. Quantum Electron. 19, 1276 (1983).

6. J.K. Ranka et al., "Autocorrelation measurement of 6-fs pulses based on the two-photon-induced photocurrent in a GaAsP photodiode," Opt. Lett. 22, 1344 (1997).

20 Femtosecond Visible Pulses Go Tunable by Noncollinear Parametric Amplification T. Wilhelm and E. Riedle, Institut fur Medizinische Optik, Ludwig-Maximilians-Universität München, München, Germany.

T he past few years have revolutionized ultrafast spec­troscopy and femtosecond technology. Based on the

modelocked Ti:sapphire laser and on amplifiers using the same active material, extremely powerful and stable sys­tems are now available with pulse lengths below 10 fsec. These sources operate at a fixed center wavelength and

Wilhelm Figure 1. Sub-20-fsec pulses tunable across the visible.

50 Optics & Photonics News/December 1997

Optics in 1997 Ultrafast Technology

their broad spectrum cannot be tuned. However, many spectroscopic experiments need wide tunability. One way to achieve this goal is the conversion of the fixed frequency pulsed light in optical parametric amplifiers (OPAs).1 These devices have considerably matured in the last few years, but have one basic disadvantage when generating ultrashort pulses. Due to the group velocity mismatch (GVM) between the three interacting optical waves, the pulses of different colors prop­agate at different speeds in the nonlinear optical medium of finite size, and consid­erable pulse lengthening results.

An elegant way to solve this principal problem of collinear parametric amplifica­tion was reported by Gale and coworkers who used noncollinear phasematching in a synchronously pumped optical parametric oscillator (OPO) to generate ultrashort visi­ble pulses.2 In the noncollinear geometry, only the projection of the idler group veloc­ity (v i d l e r ) on the signal beam (v s i g n a l ) deter­

mines the pulse lengthening by GVM. So when these two velocities are made identi­cal, no lengthening occurs. Three problems still remain in a synchronously pumped OPO: A very strong pump laser is needed, the tunability of the system is limited by the reflectivity of the resonator mirrors, and the system is complicated to align.

An alternative approach that avoids these problems has been reported.3 A sin­gle stage noncollinear parametric ampli­fier is pumped by the frequency doubled output of a kHz regenerative Ti:sapphire amplifier (65 fsec pulse length). The OPA is seeded by a high quality white light continuum generated in a sapphire plate. As a result, sub-20 fsec pulses can be pro­duced throughout the whole visible spec­trum with a pulse energy of up to 11 μJ. Sample spectra are shown in Figure 1 together with an autocorrelation mea­surement and a schematic of the non­collinear arrangement. The unique com­bination of ultrashort, tunable pulses and high efficiency is obtained in a 2-mm-long BBO crystal. The system is uncom­plicated, stable, and easy to align.

Since the setup does not require extremely short pump pulses, it can be used with most commercial regenerative ampli­fiers and even traditional dye amplifier sys­tems. It is therefore an extremely useful tool for any experiment where the fastest decay and reaction rates in nature are observed. Due to its simplicity, independently tunable stages can readily be implemented in paral­lel to provide totally synchronized pulses at various wavelengths.

References 1. A.P. Piskarskas, "Optical parametric generators: Tunable,

powerful & ultrafast," Opt. Phot. News 8 (7), 24 (1997). 2. G.M. Gale et al., "Sub-20-fs tunable pulses in the visible

from an 82-MHz optical parametric oscillator," Opt Lett. 20, 1562 (1995).

3. T. Wilhelm et al., "Sub-20-fs pulses tunable across the vis-able from a blue-pumped single pass noncollinear paramet­ric converter," Opt. Lett 22, 1494 (1997).

Optics & Photonics News/December 1997 51