Optical pulse distortion and manipulation through polarization effects and chromatic dispersion

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2001.Includes bibliographical references (leaves 112-129).Pulse distortion and shaping mechanisms play a significant role in optical fiber communication and sensing. In this thesis we shall investigate techniques which alleviate pulse deterioration due to polarization effects, and utilize large chromatic dispersion for system performance enhancement. We first demonstrate a method of mitigating polarization mode dispersion (PMD) in fiber optic communication systems. PMD has been a known effect for over a decade. However, it was not an impediment to system performance until recent advances in communication system bit rates. Today, with 10 Gb/s and 40 Gb/s channel rates appearing in new system equipment, PMD prohibits the use of many fiber cables already installed. Current PMD compensation techniques that require feedback control have difficulty meeting the speed and reliability requirements of telecom standards. In the first part of this thesis we investigate alternative compensation schemes which reduce the complexity of the feedback schemes. We next exploit the recent availability of ultra-long length chirped fiber Bragg gratings (FBG). Their enormous chromatic dispersion enables methods of improving current techniques in sensing and high speed optical sampling. In one experiment, we modulate the frequency of a standard distributed Bragg reflector (DBR) laser, and then apply the dispersion of the ultra-long FBG. Picosecond pulses are formed, whose repetition rate is independent of the laser cavity length. Since the gain of the laser is not modulated, the timing jitter is fundamentally limited only by the frequency noise of the laser. Finally, we again utilize the large delay of an ultra-long chirped FBG to implement arbitrary dynamic optical filtering of pulse spectra. In sensing applications such as fiber gyroscopes and optical coherence tomography (OCT), a wide Gaussian spectrum is ideal for low error in the gyro, and high image resolution in OCT. A modelocked fiber laser provides very wide spectra, but the shape can be irregular. We stretch the modelocked pulse temporally with an FBG, and access the frequency components in the time domain. We can then selectively suppress frequencies with an amplitude modulator to synthesize a Gaussian spectrum. Polarization effects and chromatic dispersion will inevitably appear in many optical systems. It is the goal of this thesis to show that their effects can be minimized or utilized for system performance enhancement.by Patrick Chien-pang Chou.Ph.D

Integral Optics: Lecture Notes

An introduction is given to the principles of integrated optics and optical guided-wave devices. The characteristics of dielectric waveguides are summarized and methods for their fabrication are described. An illustration is given of recent work on devices including directional couplers, filters, modulators, light deflectors, and lasers. The textbook reflects the latest achievements in the field of integrated optics, which have had a significant impact on the development of communication technology and methods for transmitting and processing information

Range-resolved optical interferometric signal processing

The ability to identify the range of an interferometric signal is very useful in interferometry, allowing the suppression of parasitic signal components or permitting several signal sources to be multiplexed. Two novel range-resolved optical interferometric signal processing techniques, employing very different working principles, are theoretically described and experimentally demonstrated in this thesis. The first technique is based on code-division multiplexing (CDM), which is combined with single-sideband signal processing, resulting in a technique that, unlike prior work, only uses a single, regular electro-optic phase modulator to perform both range-based signal identification and interferometric phase evaluation. The second approach uses sinusoidal optical frequency modulation (SFM), induced by injection current modulation of a diode laser, to introduce range-dependent carriers to determine phase signals in interferometers of non-zero optical path difference. Here, a key innovation is the application of a smooth window function, which, when used together with a time-variant demodulation approach, allows optical path lengths of constituent interferometers to be continuously and independently variable, subject to a minimum separation, greatly increasing the practicality of the approach. Both techniques are applied to fibre segment interferometry, where fibre segments that act as long-gauge length interferometric sensors are formed between pairs of partial in-fibre reflectors. Using a regular single-mode laser diode, six fibre segments of length 12.5 cm are multiplexed with a quadrature bandwidth of 43 kHz and a phase noise floor of 0.19 mrad · Hz -0.5 using the SFM technique. In contrast, the 16.5 m spatial resolution achieved with the CDM technique points towards its applicability in medium-to-long range sensing. The SFM technique also allows high linearity, with cyclic errors as low as 1 mrad demonstrated, and with modelling indicating further room for improvement. Additionally, in an industrial measurement, the SFM technique is applied to single-beam, multi-surface vibrometry, allowing simultaneous differential measurements between two vibrating surfaces

NASA Tech Briefs, December 1989

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