G. Lenz

SUBPICOSECOND GAIN AND INDEX NONLINEARITIES IN INGAASP DIODE LASERS

Abstract Studies of active waveguides with short optical pulses provide information about fundamental carrier dynamics and nonlinear processes relevant to high speed modulation, amplification, and switching. We describe recent investigations of the gain and index nonlinearities in InGaAsP optical amplifiers operating in the 1.5 μm region of the spectrum. We have measured the group velocity dispersion, -λd 2 n eff /dλ 2 , for these diodes using time domain reflectometry. We have used a novel heterodyne pump- probe technique to study the gain and refractive index nonlinearities; and we have performed short pulse saturation measurements that confirm predictions derived from these time-domain studies. Nonlinear gain compression is found to be due mainly to carrier heating and two-photon absorption. A small portion of the total response has been attributed to spectral hole burning. The index nonlinearities are dominated by carrier heating and by the instantaneous optical Kerr effect. We discuss how these time domain results, for both the real and imaginary parts of χ (3) , can be related to other experimental determinations of diode nonlinearities. In addition, we include results that go beyond the perturbational limit and show large rapid gain and index nonlinearities for high pump powers. These results will be discussed in the context of all-optical switching in active semiconductor waveguides.

Efficient frequency doubling of a femtosecond fiber laser

We report optimization of a stretched-pulse erbium-doped fiber laser for second-harmonic generation and the evaluation of several nonlinear crystals for this application. With compressed fundamental pulse energies of 2.7 nJ at 31.8 MHz, we achieved 10% conversion efficiency and 86-fs, 771-nm pulses with energies of 270 pJ. Frequency-resolved optical gating was used to analyze both the fundamental and the frequencydoubled pulses.

All-solid-state femtosecond source at 1.55 μm

We report on a stretched-pulse additive-pulse mode-locked erbium-doped fiber laser pumped by a master-oscillator/power- amplifier diode at 980 nm. This laser operates at 1.55 μm and puts out as much as 90 mW of average power at 40 MHz with pulse widths that are prism compensated to less than 90 fs. Amplitude jitter is less than 0.1%.

Carrier heating and spectral hole burning in strained‐layer quantum‐well laser amplifiers at 1.5 μm

We present the first polarization studies of femtosecond gain dynamics in strained‐layer multiple‐quantum‐well laser amplifiers. We observe a response consistent with spectral hole burning when the diode is biased in the absorbing regime. In the gain regime, we show that the carriers are heated by free‐carrier absorption and that there is a measurable delay (∼200 fs) in the thermalization of the hot‐carrier distribution. Subsequent cooling to the lattice temperature follows with a time constant of ∼1 ps.

Heterodyne pump - probe technique for time-domain studies of optical nonlinearities in waveguides.

We describe a novel pump–probe technique in which pump and probe pulses are collinear and have parallel polarizations but are still distinguishable. This new technique is especially useful for femtosecond time-domain studies of waveguides. We present experimental results for bulk (V-groove) semiconductor optical amplifiers at 1.5 μm and compare them with results for orthogonally polarized pump–probe measurements. The comparison demonstrates differences that are due to anisotropy of the group velocity as well as of the waveguide nonlinearity.

Femtosecond time domain measurements of group velocity dispersion in diode lasers at 1.5 mu m

The authors have measured the group velocity dispersion of bulk V-groove semiconductor lasers and multiple quantum well lasers operating at a wavelength near 1.5 mu m. The data yield group velocity dispersions in the range from -0.63 to -0.95 mu m/sup -1/ and indicate that material dispersion is the dominant factor in the diodes. Cross-correlation traces of transmitted femtosecond pulses confirm the measured values of dispersion. >

Variable bandwidth birefringent filter for stable femtosecond lasers

A design for a birefringent filter is described, which is suitable for tunable femtosecond lasers. Using a single plate, which has a steeply diving optic axis, two-octave tunability is attained with negligible deterioration of the stopband rejection. For a specific wavelength region, it means that the filters bandwidth can be changed by a factor of four. Another characteristic of the design is that, for the same bandwidth, the proposed plate is five times thicker than a conventional plate in which the optic axis is parallel to the surface. Thus, etalon effects can be avoided. Tuning characteristics of color center lasers utilizing the new filter are also presented. >

Models for self-limited additive pulse mode-locking

Abstract We analyze Self-Limited Additive Pulse Mode-locked (SLAPM) lasers. SLAPM is APM in which the effective fast saturable absorption rolls over to become fast saturable gain for the pulse peaks, providing an important stabilizing mechanism. The pulses in a SLAPM laser tend to look Gaussian at the top but maintain exponential wings and have time-bandwidth products larger than that of a sech2-shaped pulse. We compare a simple analytic theory with two discrete computer simulations of SLAPM lasers. We show that despite large nonlinear phase shifts per pass in the fiber (close to π) and the lumped nature of the elements in the laser, the analytic model is applicable. Both the simulated pulse parameters and shapes show good agreement with the theory.

Far-field diffraction of truncated higher-order Laguerre-Gaussian beams

A simple analytic expression for the far-field diffraction pattern of a general high-order Laguerre-Gaussian mode from a circular aperture is derived. Aperturing of the fundamental Gaussian beam is shown as a special case, where in the limit of waist size much larger than the aperture the Airy pattern is recovered and in the opposite limit we get a Gaussian intensity pattern. The diffraction pattern is shown to be an infinite series of Bessel functions and is strongly dependent on the ratio of the aperture area to the beam waist area (also known as the truncation parameter). This formulation leads to a more intuitive understanding of the far field of these modes, and the effect of the aperture as a spatial filter is discussed.

FEMTOSECOND DYNAMICS OF THE NONLINEAR ANISOTROPY IN POLARIZATION INSENSITIVE SEMICONDUCTOR OPTICAL AMPLIFIERS

The nonlinear response of polarization insensitive semiconductor optical amplifiers shows that anisotropy may be induced as a function of intensity. With a hetrodyne pump‐probe technique, the nonlinear gain and index components may be polarization resolved as well as time resolved. By extending this measurement technique we measure the dynamic anisotropy directly.