All-frequency PMD compensator in feedforward scheme

We present the architecture of a broad-band polarization mode dispersion (PMD) compensator in a feedforward PMD compensation scheme. It is comprised of four stages. The net effect of the first two stages is equivalent to a frequency dependent polarization rotation that aligns all PMD vectors into a common direction. The third stage compensates the frequency dependent variable differential group delay (DGD) and the last stage compensates for the isotropic dispersion created by the first three stages. In Stokes space formulation, we describe the algorithm to find the required rotation angles of each stage using the PMD concatenation rule, and show that these rotation angles can be synthesized using all-pass filters (APF) in a compact integrated optics circuit. Through numerical simulations, we show significant improvement of the signal quality as a result of the compensation.     

Three-dimensional analysis of scattering losses due to sidewall roughness in microphotonic waveguides

We present a three-dimensional (3-D) analysis of scattering losses due to sidewall roughness in rectangular dielectric waveguides valid for any refractive-index contrast and field polarization. The analysis is based on the volume current method and uses array factors to introduce significant mathematical simplifications to better understand the influence of individual waveguide parameters on scattering losses. We show that the typical two-dimensional (2-D) analyses can substantially overestimate scattering losses in small waveguides and that scattering losses exhibit considerable polarization dependence. We produce scattering-loss estimates for a wide variety of waveguides and provide guidelines for design of waveguide cross sections that are less sensitive to sidewall roughness.     

Noise of the stretched pulse fiber laser. II. Experiments

For pt.I see ibid., p.649, 1997. The noise characteristics of the stretched pulse fiber ring laser are determined from the power spectrum of the current of a detector illuminated by the output pulse train. Amplitude fluctuations as low as 0.05% of the pulse energy and jitter <80 fs (measurement time=0.09 s) are obtained with no external stabilization. These are the lowest noise levels reported to date for a passively mode-locked laser. The experimental results are compared with the noise theory of the companion paper. Good agreement is obtained between theory and experiment. Comparison with theory also shows that the jitter at frequencies higher than 60 Hz is due to amplified spontaneous emission (quantum) fluctuations when the measurement time is on the order of 0.1 s     

Deterministic approach to first- and second-order PMD compensation

The authors show that three concatenated first-order polarization mode dispersion (PMD) segments are sufficient to compensate any first- and second-order PMD present in a transmission fiber. They determine analytically the required individual rotation matrix of the polarization rotators and the required differential group delay for the variable delay line in the compensator.     

Injection locking of a passively mode-locked laser

We present and analyze a model of a passively mode-locked laser, which is subjected to injection of a coherent pulse train. The model, based on soliton perturbation theory, predicts the possibility of coherent injection locking, and gives analytical estimates for the locking ranges. The ability to simultaneously lock the timing and the phase of the output pulses of a mode-locked laser to an injected, coherent pulse train is shown. The interesting limiting case of phase locking the mode-locked laser to a constant amplitude external injection, is addressed with implications for future coherent soliton communication systems     

A variational coupled-mode theory for periodic waveguides

A simple variational theorem for the dispersion relations and propagation constants of periodic waveguides is derived. The trial fields used in the variational formula incorporate all the Floquet components correct to first order. This yields a propagation constant in which errors enter only to fourth order in the trial field. The analysis yields a new coupled mode formulation which is shown to yield excellent agreement with exact analytic solutions (in l-D), and numerical simulations (in 2-D), for high-index contrast structures     

Measuring timing jitter with optical cross correlations

The use of optical cross correlations for characterizing timing jitter is investigated. Applications, limitations, and correspondence to radio frequency measurements are presented and clarified. The probability density function of the timing jitter of semiconductor mode-locked lasers is deconvolved from the cross-correlation measurements with the aid of pulse characterization techniques.     

Symmetric three-guide optical coupler with nonidentical center and outside guides

To obtain maximum power transfer efficiency from one outside guide to another in a three-guide coupler, the differences among the propagation constants of the three propagating modes must be equal, i.e.,beta_{A} - beta_{C}must equalbeta_{C} - beta_{B}. This is not the case when all three guides are identical. By increasing either the width or index of the center guide relative to the outside guides, the differences between the propagation constants can be made equal. This can result in a significant increase in power transfer from one outside guide to the other. With equal spacing between the propagation constants of the modes, overall power transfer efficiency is limited only by radiation losses at the input and output of the three-guide coupler, due to the imperfect match between the mode shape of the input and output guides and the shape of the linear combination of the three modes of the coupler which best matches it. For couplers with reasonable guide spacing to guide width ratiosSand reasonable confinement, this loss is usually small, and power transfer efficiencies for power transfer from one outside guide to another can approach 100 percent. For example, with correctly adjusted β's, power transfer efficiency in a three-guide coupler withS = 1can be as high as 99.8 percent. Even withS = 0.5, efficiencies can be as high as 97.8 percent.     

Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20 ps pulses at the fundamental repetition frequency of 0.78 MHz.