Michelle Y. Sander

Photonic ADC: overcoming the bottleneck of electronic jitter

Accurate conversion of wideband multi-GHz analog signals into the digital domain has long been a target of analog-to-digital converter (ADC) developers, driven by applications in radar systems, software radio, medical imaging, and communication systems. Aperture jitter has been a major bottleneck on the way towards higher speeds and better accuracy. Photonic ADCs, which perform sampling using ultra-stable optical pulse trains generated by mode-locked lasers, have been investigated for many years as a promising approach to overcome the jitter problem and bring ADC performance to new levels. This work demonstrates that the photonic approach can deliver on its promise by digitizing a 41 GHz signal with 7.0 effective bits using a photonic ADC built from discrete components. This accuracy corresponds to a timing jitter of 15 fs - a 4-5 times improvement over the performance of the best electronic ADCs which exist today. On the way towards an integrated photonic ADC, a silicon photonic chip with core photonic components was fabricated and used to digitize a 10 GHz signal with 3.5 effective bits. In these experiments, two wavelength channels were implemented, providing the overall sampling rate of 2.1 GSa/s. To show that photonic ADCs with larger channel counts are possible, a dual 20-channel silicon filter bank has been demonstrated.

Compact, stable 1 GHz femtosecond Er-doped fiber lasers

We demonstrate a high-repetition-rate soliton fiber laser that is based on highly doped anomalously dispersive erbium-doped fiber. By splicing an 11 mm single-mode fiber to the erbium-doped fiber, the thermal damage of the butt-coupled saturable Bragg reflector (SBR) is overcome. The laser generates 187 fs pulses at a repetition rate of 967 MHz with a measured long-term stability of more than 60 h.

Nonintrusive phase stabilization of sub-two-cycle pulses from a prismless octave-spanning Ti:sapphire laser.

Carrier-envelope (CE) phase-stabilized sub-two-cycle pulses are generated from a 500 MHz compact prismless octave-spanning laser without extracavity nonlinear optical processes distorting the laser output. The necessary f and 2f spectral components are generated intracavity and coupled out independently from the main pulse through specially designed cavity mirrors, resulting in a 55 dB CE beat note (100 kHz resolution bandwidth). The in-loop CE phase error (integrated from 2.5 mHz to 10 MHz) is 67 mrad, equivalent to a timing jitter between carrier and envelope of 28 as at 790 nm.

Dynamics of Dispersion Managed Octave-Spanning Titanium:Sapphire Lasers

An extensive one-dimensional laser model based on dispersion managed mode locking is presented that accurately describes the pulse dynamics of octave-spanning titanium:sapphire lasers generating sub-two-cycle pulses. By including detailed characteristics for the intracavity elements (mirrors and output coupler), it is demonstrated that the spectral output and temporal pulse shape of these lasers can be predicted quantitatively in very good agreement with experimental results.

Kerr-lens mode locking with minimum nonlinearity using gain-matched output couplers

Broadband Kerr-lens mode locking is demonstrated at greatly reduced mode-locking strength with a gain-matched output coupler that compensates for gain filtering. Already at very low pump powers, slightly above the cw lasing threshold, we are able to initiate robust mode locking and generate <8 fs output pulses from a Ti:sapphire laser with good beam quality. Because dielectric coatings offer flexible design capabilities, this approach is applicable to various lasers with different gain media to extract pulses covering the full gain spectrum with minimum saturable absorber action.

Carrier-envelope phase dynamics of octave-spanning dispersion-managed Ti: sapphire lasers.

The carrier-envelope phase dynamics of few-cycle octave-spanning Ti:sapphire lasers are analyzed based on a numerical one-dimensional dispersion-managed laser model. The dominant contribution to the carrier-envelope phase shift with respect to intracavity energy arises from the asymmetric impact of self-steepening on pulse formation and laser output. We show that this term is larger by a factor of four than the energy-dependent round trip phase and is thus more significant than in the corresponding result for conventional soliton lasers. Frequency shifts due to the Raman effect are studied and found to be of minor impact for octave-spanning lasers.

Octave-spanning, dual-output 2.166 GHz Ti:sapphire laser

A self-referenced octave-spanning Ti:sapphire laser with 2.166 GHz repetition rate is demonstrated. The laser features both direct generation of octave-spanning spectra and a dual-output design for non-intrusive carrier-envelope (CE) phase-stabilization. Only a few percent of total power containing 1f and 2f spectral components is coupled out through a specially designed laser mirror and generates a >50 dB CE beat note in 100 kHz resolution bandwidth without perturbing the main output that still delivers octave-spanning spectra and 750 mW of output power.

10 GHz femtosecond pulse interleaver in planar waveguide technology

Coherent pulse interleaving implemented in planar waveguide technology is presented as a compact and robust solution to generate high repetition rate frequency combs. We demonstrate a 10 GHz pulse train from an Er-doped femtosecond fiber laser that is coupled into waveguide interleavers and multiplied in repetition rate by a factor of 16. With thermal tuning of the chip elements, we achieve optical and RF sidemode suppression levels of at least -30 dB.

Octave Spanning 1 GHz Ti:sapphire Oscillator For HeNe CH4-based Frequency Combs and Clocks

In this paper, we demonstrate a greatly improved octave spanning 1 GHz Ti:sapphire laser (Fortier, 2006) using the most broadband double-chirped mirror pairs, optimized Kerr-Lens modelocking (KLM) and an optimized output coupler. As a result the laser generates, at 9 W of pump power, 0.6 W-1 W of output power with an output spectrum of more than one octave as measured on a linear scale. The spectrum corresponds to a Fourier limited pulse of 3.5 fs duration. Second harmonic generation with this output in 1mm BBO directly generates 1f-2f beatnotes for carrier-envelop phase stabilization with >55 dB signal-to-noise (SNR) in 100 kHz bandwidth, and by difference frequency generation (DFG) in a 5 mm long PPLN radiation at 3.39 mum is generated. The 3.39 mum radiation is strong enough to result in a beatnote with a single frequency HeNe reference laser of 30 dB. This laser will serve as the clockwork of a HeNe CH4-based molecular clock (Foreman, 2005) with a measured Allan variance approaching 10~14 in 100 s and as an absolute femtosecond laser frequency comb for an optical arbitrary waveform generator.

Modeling of Octave-Spanning Sub-Two-Cycle Titanium:Sapphire Lasers: Simulation and Experiment

It is shown that a one-dimensional temporal laser model under optimized intracavity dispersion settings can quantitatively predict the spectral output and temporal pulse shape of octave-spanning, sub-two-cycle Ti:sapphire lasers.