Generation and direct measurement of giant chirp in a passively mode-locked laser.

We evaluate the shape and chirp of nanosecond pulses from a fiber laser passively mode locked with a nanotube-based saturable absorber by using a synchronously scanning streak camera and a monochromator to directly measure the pulse spectrogram. We show that the stable sech(2) output pulse possesses a predominantly linear chirp, with a residual quartic phase and low noise. Comparison with analytical mode-locking theory shows a good quantitative agreement with the master equation mode-locking model.

Time-resolved spectroscopy of hemoglobin and its complexes with subpicosecond optical pulses.

Subpicosecond optical pulses have been used to study the photolysis of hemoglobin conplexes. Photodissociation of carboxyhemoglobin is found to occur in less than 0.5 picosecond. In hemoglobin and oxyhemoglobin a nondissociative excited state recovery in 2.5 picoseconds is observed.

Subpicosecond spectroscopy of bacteriorhodopsin.

Subpicosecond pulses have been used to study the ultrafast dynamics of the photochemistry of bacteriorhodopsin. An optically induced absorption that appears in about 1.0 picosecond at physiological temperatures has been resolved in time. The data can be interpreted in terms of the photochemical formation of bathobacteriorhodopsin and provide support for an excitation mechanisms involving molecular rearrangement in the protein induced by electron redistribution in the chromophore.

CMOS-compatible dual-output silicon modulator for analog signal processing.

A broadband, Mach-Zehnder-interferometer based silicon optical modulator is demonstrated, with an electrical bandwidth of 26 GHz and V(pi)L of 4 V.cm. The design of this modulator does not require epitaxial overgrowth and is therefore simpler to fabricate than previous devices with similar performance.

Time-resolved studies of Nd:YAG laser-induced breakdown. Plasma formation, acoustic wave generation, and cavitation.

The use of high intensity ultrashort pulsed laser radiation to produce optical breakdown is an important approach for the surgical treatment of intraocular structures. We have investigated the transient properties of Nd:YAG laser induced breakdown in a saline model using time-resolved spectroscopic techniques. Spatially resolved pump and probe techniques are applied to study the dynamic behavior of the plasma formation, acoustic wave generation, and cavitation processes which accompany the optical breakdown. Measurements of plasma shielding and luminescence indicate that the laser induced plasma forms on a subnanosecond time scale and has a lifetime of several nanoseconds. An acoustic transient is generated at the breakdown site and propagates spherically outward with an initial hypersonic velocity, then loses energy and propagates at sound velocity. Transient heating following the plasma formation produces a liquid-gas phase change and gives rise to cavitation or gas bubble formation. This gas bubble expands rapidly for several microseconds, then slows to reach its maximum size and finally collapses.

Integrated mode-evolution-based polarization splitter.

A mode-evolution-based polarization splitter suitable for high-index-contrast systems and directly integratable with a recently reported on-polarization rotator is described and its performance verified through both finite-difference time-domain and eigenmode expansion simulations. For a device length of 200 microm, greater than 22 dB of extinction is obtained across a 1.45-1.75-microm bandwidth.

Three-dimensional photonic devices fabricated in glass by use of a femtosecond laser oscillator.

Three-dimensional photonic waveguide devices are fabricated in glass by use of femtosecond pulses from an extended-cavity laser oscillator. Three-dimensional devices, including a symmetric three-waveguide directional coupler and a three-dimensional microring resonator, are fabricated and tested. Waveguides can be fabricated at depths of approximately 1 mm inside a glass substrate, thus demonstrating the capability of achieving dramatic increases in device density. These results demonstrate the potential to fabricate new classes of devices that are not possible in two dimensions.

Automatic feedback control of an Er-doped fiber laser with an intracavity loss modulator.

Suppression of Q-switching instabilities with an actively controlled intracavity loss modulator is demonstrated in an Er-doped waveguide laser that is mode locked with a slow saturable absorber at repetition rates of as much as 100 MHz. By automatic gain control in the feedback loop, stable mode locking is achieved over the entire parameter range of the laser. This approach renders laser stabilization independent of the characteristics of the gain medium and intracavity power. The pulse-shaping dynamics is not affected by the presence of the intracavity loss modulator.

220-fs erbium-ytterbium:glass laser mode locked by a broadband low-loss silicon/germanium saturable absorber.

We demonstrate femtosecond performance of an ultrabroadband high-index-contrast saturable Bragg reflector consisting of a silicon/silicon dioxide/germanium structure that is fully compatible with CMOS processing. This device offers a reflectivity bandwidth of over 700 nm and subpicosecond recovery time of the saturable loss. It is used to achieve mode locking of an Er-Yb:glass laser centered at 1540 nm, generating 220-fs pulses, with what is to our knowledge the broadest output spectrum to date.

Self-starting of passively mode-locked lasers.

The self-starting condition of passively mode-locked lasers, including those locked by additive-pulse mode locking, is reexamined. A quantitative model is developed for a threshold intensity for self-starting based on the hypothesis that spurious reflections have to be overcome by coherent injection signals of sufficient amplitude.

Measurement of pulse asymmetry by three-photon-absorption autocorrelation in a GaAsP photodiode.

Three-photon absorption in a GaAsP photodiode is used for what is believed to be the first time for nonlinear optical autocorrelation at wavelengths from 1.4 to 1.6microm . Theoretical intensity dependence and contrast ratios are achieved, and the ratio of the three- to the two-photon-absorption coefficient is measured. Finally, it is demonstrated that pulse asymmetry can be measured with no direction-of-time ambiguity by use of unbalanced three-photon-absorption autocorrelation.

Group velocity of solitons.

It is shown that, in addition to the well-known phase accumulation of a traveling soliton, which may be interpreted as a change of phase velocity as a result of the Kerr nonlinearity, there is a change in the speed of travel of the envelope, the group velocity. This analysis is extended to dispersion-managed solitons, for which it is shown that the discrepancy between phase- and group-velocity changes is generally smaller.

Transient four-wave mixing and optical pulse compression in the femtosecond regime.

The transient behavior of parametric-scattering four-wave mixing is investigated experimentally with femtosecond laser pulses. Special emphasis is placed on the measurement of parametric pulse durations and the application of this process to the compression of femtosecond optical pulses. Compression factors up to 1.85 have been achieved, yielding high-power pulses of less than 40-fsec duration.

Novel transient scattering technique for femtosecond dephasing measurements.

A novel transient scattering technique is proposed for femtosecond dephasing time measurements. This technique provides several advantages compared with existing techniques for time-domain dephasing measurements, including resolution below the pulse width, insensitivity to rapid energy relaxation, and clear demarcation between homogeneous and inhomogeneous broadening. We report experimental results for the dye Nile blue in solution; the dephasing time T(2) is determined to be less than 20 fsec.

Asynchronous soliton mode locking.

We report a harmonically mode-locked erbium-doped fiber ring laser with short solitons. A phase modulator running asynchronously with the pulses provides pulse start-up, continuous background cleanup, and a new form of pulse timing stabilization. Steady-state operation of picosecond solitons at a repetition rate of 1 GHz was obtained with no need for modulator drive frequency stabilization.

Femtosecond time-resolved reflectometry measurements of multiple-layer dielectric mirrors.

We describe the use of optical pulses as short as 16 fsec for time-resolved reflectometry studies of multiple-layer dielectric mirror coatings. Pronounced pulse-distortion effects have been observed following reflection from broadband dielectric mirrors.

Soliton squeezing at the gigahertz rate in a Sagnac loop.

We demonstrate what is to our knowledge the first all-fiber squeezing experiment. A balanced Sagnac loop is used, and a record 6.1+/-0.2dB of noise reduction below shot noise has been obtained without stabilization. A gigahertz Er-doped fiber laser and a high-power double-clad Er-Yb amplifier have been developed to suppress guided acoustic-wave Brillouin scattering and to make possible the all-fiber configuration.

Subpicosecond solitons in an actively mode-locked fiber laser.

Experimental results are presented for a study of the stability regime of an actively mode-locked polarizationmaintaining fiber ring laser used as a memory. Observations indicate that the pulse widths in the memory can be reduced (by soliton effects) by a factor of approximately 4.4 below the pulse widths predicted by standard active mode-locking theory. Stability regions for the solitons are mapped and compared with theoretical predictions.

Self-starting condition for additive-pulse mode-locked lasers.

We analyze self-starting in additive-pulse mode-locked lasers and find that dynamic gain saturation can play a determining role. A simple condition for self-starting is presented for comparison with experiments on different laser systems.