James van Howe

Simultaneous spatial and temporal focusing of femtosecond pulses

We experimentally demonstrate the concept of simultaneous spatial and temporal focusing of femtosecond pulses. Our technique has the potential to significantly reduce background excitation which fundamentally limits the imaging depth in scattering biological specimens.

All-optical, wavelength and bandwidth preserving, pulse delay based on parametric wavelength conversion and dispersion

We demonstrate an all-optical tunable delay in fiber based on wavelength conversion, group-velocity dispersion, and wavelength reconversion. The device operates near 1550 nm and generates delays greater than 800 ps. Our delay technique has the combined advantages of continuous control of a wide range of delays from picoseconds to nanoseconds, for a wide range of signal pulse durations (ps to 10 ns), and an output signal wavelength and bandwidth that are the same as that of the input. The scheme can potentially produce fractional delays of 1000 and is applicable to both amplitude- and phase-shift keyed data.

Generation of 3.5 nJ femtosecond pulses from a continuous-wave laser without mode locking

Using time-lens compression in a loop configuration, we generate 516 fs pulses at 3.5 nJ pulse energy from a continuous-wave 1.55 mum source without mode locking. Just as a spatial lens can expand or focus a beam in space, so can a time-lens broaden or compress a pulse in time. By placing a time-lens in a loop, we maximize the efficiency of bandwidth generation by using one time-lens driven at low power to emulate a stack of many lenses. Our system is compact, is all fiber, and allows large tuning of the repetition rate and continuous tuning of the pulse width and center wavelength.

Generation of femtosecond pulses at 1350 nm by Cerenkov radiation in higher-order-mode fiber.

We demonstrate a method of generating short pulses at 1350 nm by exciting Cerenkov radiation in a higher-order-mode fiber with a 1064 nm femtosecond fiber laser. We measure a 106 fs, 0.66 nJ output pulse. Cerenkov radiation in fibers allows for energy transfer between a soliton and a dispersive wave, providing an effective and engineerable platform to shift the wavelength of a femtosecond source. With appropriate design of the higher-order-mode fiber, this method of generating short pulses at 1350 nm can be extended to other wavelengths and to higher pulse energies.

Compensation of self-phase modulation in fiber- based chirped-pulse amplification systems

We demonstrate a simple, all-fiber technique for removing nonlinear phase due to self-phase modulation in fiber-based chirped-pulse amplification (CPA) systems. Using a LiNbO3 electro-optic phase modulator to emulate a negative nonlinear index of refraction, we are able to remove 1.0 π rad of self-phase modulation acquired by pulses during amplification and eliminate nearly all pulse distortion. Our technique is high speed, removes nonlinear phase on a pulse-to-pulse basis, and can be readily integrated into existing CPA systems.

Ultrafast optical delay line by use of a time-prism pair

We demonstrate an all-fiber, programmable, ultrafast optical delay line based on reversible frequency conversion by use of a time-prism pair. Using electro-optic phase modulators to provide the time-prism phase profile, we show a record scanning rate of 0.5 GHz and a delay range of 19.0 ps. Computer modeling suggests that aberration correction in the time-prism system can extend the delay range to 28.0 ps. Finally, limitations and potential improvement of our techniques are discussed.

Ultrafast optical delay line using soliton propagation between a time-prism pair

Through theoretical analysis and experiment, we show that the performance of an ultrafast optical delay line using a time-prism pair is significantly improved when solition propagation is used between time-prisms. The enhancement is most dramatic for short pulses where dispersive pulse broadening in a linear propagation regime between time-prisms is large and limits perfomance. Experimentally, we demonstrate an optical delay line using soliton propagation in an all-fiber configuration allowing us to achieve a scan rate of 0.5 GHz, a delay range of 33.0 ps, no pre- and post-dispersion compensation, and a delay-to-pulse-width ratio of 6.0.

All-Fiber, Wavelength and Repetition-Rate Tunable, Ultrafast Pulse Generation in the 2.0 μm Region Without Mode-Locking

We show 3.0 ps pulses from 1877 nm to 2008 nm at variable repetition rates up to 18 GHz using time-lens compression of a tunable CW laser. The center wavelength is changed by tuning the CW seed laser, and the repetition rate is changed by electronically tuning the drive of the master RF clock. The repetition rate of 18 GHz represents a record speed for pulse generation in this spectral region. This simple all-fiber platform uses standard 1550 nm telecom components, offering a turn-key, flexible, robust alternative to pulse generation in the 2.0 μm region with both wavelength and repetition rate tunability.

Compensation of self-phase modulation in fiber-based chirped-pulse amplification systems

We demonstrate a simple, all-fiber technique for removing nonlinear phase on a pulse-to-pulse basis in a fiber-based CPA system. Removal of 1.0 pi of nonlinear phase is achieved.

Soliton self-frequency shift below 1300 nm in higher-order-mode, solid silica-based fiber

We demonstrate soliton self-frequency shift below 1300 nm in a higher-order-mode optical fiber. This new class of fiber shows great promise of supporting frequency-shifted solitons in intermediate energy regimes unattainable using current fibers.