There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

We monitor the induced phase change produced by a cascaded χ(2):χ(2) process in KTP near the phase-matching angle on a picosecond 1.06-μm-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n2eff≅±2×10−14 cm2/W (±1 × 10−11 esu) for an angle detuning of ±5° from phase match for this 1-mm-thick crystal with a measured deff of 3.1 pm/V For a fixed phase mismatch, this n2eff scales linearly with length and as deff2; however, for the maximum n2eff the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near π/4.

Self-focusing and self-defocusing by cascaded second-order effects in KTP.

We review photonic applications of dielectric whispering-gallery mode (WGM) resonators-tracing the growth of the technology from experiments with levitating droplets of aerosols to ultrahigh-Q solid state crystalline and integrated on-chip microresonators.

Optical resonators with whispering-gallery modes-part II: applications

Cascading is the process by which the exchange of energy between optical beams interacting via second order nonlinearities (χ(2)) leads to various effects such as nonlinear phase shifts, the generation of new beams, all-optical transistor action, the formation of soliton-like (solitary) waves, etc. Here we review the fundamentals of the processes and discuss experimental verification of the effects and various related applications.

χ(2) cascading phenomena and their applications to all-optical signal processing, mode-locking, pulse compression and solitons

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy. Band structure measurements in conjunction with ab initio calculations and photoluminescence spectroscopy show that SnS2 is an indirect bandgap semiconductor over the entire thickness range from bulk to single-layer. Field effect transport in SnS2 supported by SiO2/Si suggests predominant scattering by centers at the support interface. Ultrathin transistors show on–off current ratios >106, as well as carrier mobilities up to 230 cm2/(V s), minimal hyster...

Tin disulfide-an emerging layered metal dichalcogenide semiconductor: materials properties and device characteristics.

An actively mode-locked single-polarization erbium fiber laser modulated at 10 GHz utilizes intracavity soliton formation to produce 1.3-ps pulses, well below the Kuizenga–Siegman limit, without passive mode locking. The observed degree of pulse shortening agrees with the predictions of recently developed soliton laser models. The pulse dropout ratio was measured to be less than 10−12, and the rms amplitude and phase jitter are less than 1.1% and 0.16 ps, respectively.

10-ghz, 1.3-ps erbium fiber laser employing soliton pulse shortening

Measurements are presented of the lowest timing jitter reported to date for a harmonically modelocked Er fibre laser. Phase detection noise measurements demonstrate a timing jitter of <10 fs over a frequency range of 100 Hz-1 MHz.

Phase noise measurements of ultrastable 10 GHz harmonically modelocked fibre laser

Harmonically mode-locked Er-fiber soliton lasers have become a reliable source of high-repetition-rate picosecond pulses in high-speed communications and photonic analog-to-digital conversion systems because of their low-noise, dropout-free operation. We have fabricated such a laser with a strongly dispersion-managed cavity and modeled its operation, and we have found that dispersion management significantly extends the power range over which uninterrupted single-pulse production is attained and dramatically decreases the effects of amplified spontaneous emission on the phase noise of the laser. OCIS codes: 140.3510, 140.4050, 140.3430, 060.5530, 060.7140, 320.7090.

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Dispersion management in a harmonically mode-locked fiber soliton laser.

We study theoretically and experimentally actively modelocked fiber lasers that are used in high repetition rate optical communication systems. Using an innovative numerical technique and a reduced model, we have found that the laser can operate in four different operating regimes when the laser intensity was changed; three of the regimes were experimentally observed in a laser with a sigma configuration. An excellent quantitative agreement between the theoretical and the experimental results was obtained. The use of dispersion management in the sigma laser was found to significantly improve the laser performance.

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Theoretical and experimental study of harmonically modelocked fiber lasers for optical communication systems

Subpicosecond pulses have been generated in an actively modelocked, polarisation-maintaining Er-doped fibre ring laser. A single-polarisation fibre amplifier and an uncompensated polarisation bias section are integrated into the laser. This laser combines the nonlinear pulse-shortening mechanism of polarisation-rotation lasers and the single-polarisation operation, environmental insensitivity, and capacity for active modelocking characteristic of unidirectional single-polarisation ring lasers.

Thomas F. Carruthers

Papers

10-ghz, 1.3-ps erbium fiber laser employing soliton pulse shortening

Phase noise measurements of ultrastable 10 GHz harmonically modelocked fibre laser

Dispersion management in a harmonically mode-locked fiber soliton laser.

Theoretical and experimental study of harmonically modelocked fiber lasers for optical communication systems

Active-passive modelocking in a single-polarisation erbium fibre laser