100-nm tunable femtosecond Cr:LiSAF laser mode locked with a broadband saturable Bragg reflector.

We report broad tunability of a femtosecond (fs) diode-pumped Cr:LiSAF laser mode-locked with a broadband saturable Bragg reflector (SBR). The SBR had seven pairs of AlxOy(n∼1.5) and Al0.17Ga0.83As(n∼3.5) layers in its Bragg stack, enabling a 250 nm reflectivity bandwidth around 850 nm. A 6-nm-thick strained In0.15Ga0.85As quantum well placed between Al0.17Ga0.83As cladding layers was used as a broadband saturable absorber in the 800-920 nm wavelength range. The laser was pumped by 6 single mode diodes-four at 640 nn and two at 660 nm. Mode locking was self-starting and fs pulses could be continuously tuned from 800 nm to 905 nm by an intracavity birefringent filter with an out-of-plane optic axis. The pulse widths varied from 70 fs to 255 fs as the laser was tuned. The laser had an 85.5 MHz repetition rate and the output power varied from 80 mW to 180 mW with tuning.

Improving thin-film manufacturing yield with robust optimization.

A novel robust optimization algorithm is demonstrated that is largely deterministic, and yet it attempts to account for statistical variations in coating. Through Monte Carlo simulations of manufacturing, we compare the performance of a proof-of-concept antireflection (AR) coating designed with our robust optimization to that of a conventionally optimized AR coating. We find that the robust algorithm produces an AR coating with a significantly improved yield.

Chirally-coupled-core Yb-fiber laser delivering 80-fs pulses with diffraction-limited beam quality warranted by a high-dispersion mirror based compressor.

We demonstrate a high-energy femtosecond laser system that incorporates two rapidly advancing technologies: chirally-coupled-core large-mode-area Yb-fiber to ensure fundamental-mode operation and high-dispersion mirrors to enable loss-free pulse compression while preserving the diffraction-limited beam quality. Mode-locking is initiated by a saturable absorber mirror and further pulse shortening is achieved by nonlinear polarization evolution. Centered at 1045 nm with 39-MHz repetition rate, the laser emits 25-nJ, positively chirped pulses with 970-mW average power. 6 bounces from double-chirped-mirrors compress these pulses down to 80 fs, close to their transform-limited duration. The loss-free compression gives rise to a diffraction-limited optical beam (M2 = 1.05).

Visible wavelength astro-comb.

We demonstrate a tunable laser frequency comb operating near 420 nm with mode spacing of 20-50 GHz, usable bandwidth of 15 nm and output power per line of ~20 nW. Using the TRES spectrograph at the Fred Lawrence Whipple Observatory, we characterize this system to an accuracy below 1m/s, suitable for calibrating high-resolution astrophysical spectrographs used, e.g., in exoplanet studies.

Phase distortion ratio: alternative to group delay dispersion for analysis and optimization of dispersion compensating optics.

We present a new approach for designing dispersion-engineered optics based on a simple unitless spectral quantity we call the phase distortion ratio (PDR). In contrast to minimizing the group delay dispersion (GDD) deviation from the ideal, minimizing the PDR is optimal in the sense that it minimizes the fraction of pulse energy lost to phase distortions. As an example, a mirror system optimized via PDR is empirically found to result in significantly better compression of single-cycle pulses than a system designed in terms of GDD. In the context of coupling pulse trains to cavities, minimizing the PDR of the cavity is shown to maximize throughput.

Diode-pumped passively mode-locked GHz femtosecond Cr:LiSAF laser with kW peak power.

We report a single-mode diode-pumped, passively mode-locked Cr:LiSAF laser with gigahertz (GHz) repetition rate and kilowatt peak power. A low-loss saturable Bragg reflector with low modulation depth and optimized dispersion compensation mirrors enables the generation of stable, cw mode-locked, sub-100-fs pulses at 1 GHz repetition rate around 865 nm when pumping with four or six laser diodes. Using a 0.25% output coupler and a total absorbed pump power of 0.9 W from six laser diodes, 55 fs, 110 pJ pulses are produced. This corresponds to 1.8 kW of peak power, which to our knowledge represents a record result for GHz Cr:colquiriite lasers.

Low-cost, single-mode diode-pumped Cr:Colquiriite lasers.

We present three Cr3+:Colquiriite lasers as low-cost alternatives to Ti:Sapphire laser technology. Single-mode laser diodes, which cost only $150 each, were used as pump sources. In cw operation, with approximately 520 mW of absorbed pump power, up to 257, 269 and 266 mW of output power and slope efficiencies of 53%, 62% and 54% were demonstrated for Cr:LiSAF, Cr:LiSGaF and Cr:LiCAF, respectively. Record cw tuning ranges from 782 to 1042 nm for Cr:LiSAF, 777 to 977 nm for Cr:LiSGaF, and 754 to 871 nm for Cr:LiCAF were demonstrated. In cw mode-locking experiments using semiconductor saturable absorber mirrors at 800 and 850 nm, Cr:Colquiriite lasers produced approximately 50-100 fs pulses with approximately 1-2.5 nJ pulse energies at approximately 100 MHz repetition rate. Electrical-to-optical conversion efficiencies of 8% in mode-locked operation and 12% in cw operation were achieved.

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.

Robust chirped mirrors.

Optimized chirped mirrors may perform suboptimally, or completely fail to satisfy specifications, when manufacturing errors are encountered. We present a robust optimization method for designing these dispersion-compensating mirror systems that are used in ultrashort pulse lasers. Possible implementation errors in layer thickness are taken into account within an uncertainty set. The algorithm identifies worst-case scenarios with respect to reflectivity as well as group delay. An iterative update improves the robustness and warrants a high manufacturing yield, even when the encountered errors are larger than anticipated.

Efficient optimization of multilayer coatings for ultrafast optics using analytic gradients of dispersion.

A fully analytic method for computing gradients of dispersion (to any order) for a dielectric multilayer coating is developed, and it is demonstrated how group delay gradients can be used to optimize the dispersion of such a filter. The algorithm complexity is linear with the number of layers and quadratic in dispersion order. To our knowledge, this is the first published algorithm for computing exact analytic gradients of dispersion. We show an approximation that speeds up the computation significantly, making it linear in dispersion order. MATLAB and C code implementing the algorithms are made available.

Two-dimensional spectral shearing interferometry for few-cycle pulse characterization.

We present a new method for measuring the spectral phase of ultrashort pulses that utilizes spectral shearing interferometry with zero delay. Unlike conventional spectral phase interferometry for direct electric-field reconstruction, which encodes phase as a sensitively calibrated fringe in the spectral domain, two-dimensional spectral shearing interferometry robustly encodes phase along a second dimension. This greatly reduces demands on the spectrometer and allows for complex phase spectra to be measured over extremely large bandwidths, potentially exceeding 1.5 octaves.

Efficient analytic computation of dispersion from multilayer structures.

We demonstrate an inductive method for computing exact derivatives of reflection phase for layered media by using the transfer-matrix formalism. The algorithm scales linearly with the number of layers. We show a physically realistic approximation that leads to an efficient procedure for accurately computing dispersion significantly faster than with standard finite-difference methods. We discuss the theory behind the approximation and show results for a dispersion-compensating chirped mirror from a Ti: sapphire laser.

Carrier-envelope phase control by a composite plate.

We demonstrate a new concept to vary the carrier-envelope phase of a mode-locked laser by a composite plate while keeping all other pulse parameters practically unaltered. The effect is verified externally in an interferometric autocorrelator, as well as inside the cavity of an octave-spanning femtosecond oscillator. The carrier-envelope frequency can be shifted by half the repetition rate with negligible impact on pulse spectrum and energy.

Ultrabroadband beam splitter with matched group-delay dispersion.

We present a general design strategy for a broadband thin-film beam splitter with matched group-delay dispersion. By taking the substrate dispersion into account in the coating design, any combination of input and output can show the same dispersion for transmission and reflection. As a specific implementation, an ultrabroadband 50:50 beam splitter from 600 to 1500 nm for femtosecond laser applications was designed, fabricated, and characterized.