The use of terahertz pulses for imaging has opened new possibilities for scientific and industrial applications in the terahertz frequency range. In this article, we describe the technology necessary to perform terahertz "T-ray" imaging, novel imaging techniques, and commercial applications of T-ray imaging.

T-ray imaging

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

We introduce a traffic model for circuit switched all-optical networks (AONs) which we then use to calculate the blocking probability along a path for networks with and without wavelength changers. We investigate the effects of path length, switch size, and interference length (the expected number of hops shared by two sessions which share at least one hop) on blocking probability and the ability of wavelength changers to improve performance. Our model correctly predicts unobvious qualitative behavior demonstrated in simulations by other authors.

Models of blocking probability in all-optical networks with and without wavelength changers

A theory of noise in mode-locked lasers is developed that applies to additive pulse mode-locked and Kerr lens mode-locked systems. Equations of motion are derived for pulse energy, carrier linewidth, frequency pulling, and timing jitter. The effect of gain fluctuations, mirror vibrations, and index fluctuations are determined. Measurements that can determine all four fluctuation spectra are described. Experimental data in the literature are compared with theory. >

Noise of mode-locked lasers

Organic materials for nonlinear optics

We describe a modulator-driven, erbium-fiber ring laser that produces chirp-free pulses with width adjustable over the range of 5 to 100 ps. A high-finesse Fabry-Perot etalon with a free spectral range equal to the 2.5-GHz laser repetition rate eliminates unwanted laser ring modes and stabilizes the pulse amplitude. Using this laser, we have extended the error-free distance of a series of soliton transmission experiments by 2000-3000 km over previous results with a mode-locked semiconductor laser.

Harmonically mode-locked fiber ring laser with an internal Fabry-Perot stabilizer for soliton transmission.

Colliding-pulse mode-locked lasers are an excellent source of ultrashort pulses but exhibit timing jitter that can seriously degrade the temporal resolution in experiments that require synchronization to an external source. We describe a method that synchronizes the colliding-pulse mode-locked laser to a high-stability external oscillator by actively controlling the cavity length of the laser and reduces the absolute timing jitter from ~25 ps rms to only 1.7 ps over the 1-kHz bandwidth, which contains most of the laser phase noise. By using this method, the repetition rate of the laser pulses is held constant at 100 MHz.

Timing-jitter stabilization of a colliding-pulse mode-locked laser by active control of the cavity length.

The timing jitter and spurious amplitude modulation of colliding-pulse mode-locked (CPM) lasers were measured The absolute jitter (the jitter of the laser alone) varied between 5 and 10 ps RMS in a 50-500-Hz bandwidth The smallest measured relative jitter (timing fluctuations between the CPM and a radio-frequency (RF) synthesizer synchronized to the CPM) was 18 ps RMS in a 2-Hz to 1-kHz bandwidth Separate from the jitter, spurious modulation in the CW pump laser mixes with the CPM pulse train to produce a set of discrete amplitude-modulated sidebands in the power spectrum of the CPM output The frequencies of these sidebands change with cavity length, and the sidebands can be eliminated by operating the pump laser in a single longitudinal mode >

Timing jitter and pump-induced amplitude modulation in the colliding-pulse mode-locked (CPM) laser

Spurious sidebands observed in the noise spectrum of colliding-pulse mode-locking dye lasers result from coupling of the longitudinal mode beating modulations of the (argon-ion) pump laser with the dye laser. We achieve a 50-dB amplitude noise reduction at the mixing frequencies by inserting a single-frequency etalon in the argon-laser cavity. Furthermore, we find the amplitude noise of the colliding-pulse mode-locking laser to be entirely dominated by the (plasma discharge) noise of the argon laser.

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Amplitude noise reduction of 50 dB in colliding-pulse mode-locking dye lasers

Electro‐optic sampling is performed on a high‐speed silicon multiplexer integrated circuit using a gain‐switched semiconductor laser and an external probe tip fabricated from GaAs. An approximate electrostatic model is used to calculate the dependence of the electro‐optic modulation on the height of the probe tip above the circuit, the geometry of the circuit, and the dielectric constants of the probe tip material and the passivation layer (if any) on the circuit. The measured variation of the electro‐optic modulation with probe tip height is in good agreement with the prediction of the model.

George T. Harvey

Papers

Harmonically mode-locked fiber ring laser with an internal Fabry-Perot stabilizer for soliton transmission.

Timing-jitter stabilization of a colliding-pulse mode-locked laser by active control of the cavity length.

Timing jitter and pump-induced amplitude modulation in the colliding-pulse mode-locked (CPM) laser

Amplitude noise reduction of 50 dB in colliding-pulse mode-locking dye lasers

Electro-optic sampling of high-speed silicon integrated circuits using a GaAs probe tip