Gadi Lenz

Large Raman gain and nonlinear phase shifts in high-purity As 2 Se 3 chalcogenide fibers

Third-order Kerr nonlinearities and Raman gain are studied experimentally in high-purity As2Se3 optical fibers for wavelengths near 1.55 μm. Kerr nonlinear coefficients are measured to be nearly 1000 times higher than those for silica fibers. In pulsed mode, nonlinear phase shifts near 1.2-π rad are measured in fibers only 85 cm long with peak pulse powers near 3 W. However, there are nonlinear losses near 20% for nonlinear phase shifts near π. By use of a cw optical pump, large Raman gains nearly 800 times that of silica were measured. In the cw case there were losses in the form of index gratings formed from standing waves at the exit face of the fiber. Discrete Raman amplifiers and optical regenerators are discussed as possible applications.

Large Kerr effect in bulk Se-based chalcogenide glasses

High-speed optical communication requires ultrafast all-optical processing and switching capabilities. The Kerr nonlinearity, an ultrafast optical nonlinearity, is often used as the basic switching mechanism. A practical, small device that can be switched with ~1-pJ energies requires a large Kerr effect with minimal losses (both linear and nonlinear). We have investigated theoretically and experimentally a number of Se-based chalcogenide glasses. We have found a number of compounds with a Kerr nonlinearity hundreds of times larger than silica, making them excellent candidates for ultrafast all-optical devices.

Optical all-pass filters for phase response design with applications for dispersion compensation

Lossless all-pass optical filters are introduced, which can approximate any desired phase response while maintaining a constant, unity amplitude response. Architectures using cascade and lattice structures based on ring resonators and cavities defined by reflectors are discussed. Two applications are presented: 1) for fiber dispersion compensation and 2) for compensation of the nonlinear phase response of narrow bandpass optical filters such as thin-film or Bragg grating filters. All orders of dispersion can be compensated in principle, and the filters are periodic so multiple channels can be compensated with a single device. The architectures are very compact compared to alternatives such as chirped Bragg gratings.

Integrated all-pass filters for tunable dispersion and dispersion slope compensation

New integrated optical all-pass filters are presented that can be used for tunable dispersion compensation, dispersion slope compensation, and as building blocks in tunable bandpass filters. The dispersion slope compensation capability is demonstrated using ring resonators in /spl Delta/=2% Ge-doped silica planar waveguides. In addition, a tunable four-stage filter with free-spectral range (FSR)=25 GHz, a passband width of 14 GHz (0.56/spl times/FSR), and D=1800 ps/nm is reported.

A tunable dispersion compensating MEMS all-pass filter

A tunable dispersion compensating filter based on a multistage optical all-pass filter with a microelectromechanical (MEM) actuated variable reflector and a thermally tuned cavity is described. A two-stage device was demonstrated with a tuning range of /spl plusmn/100 ps/nm, 50-GHz passband and a group delay ripple less than /spl plusmn/3 ps. The device has negligible polarization dependence and is suitable for single or multiple channel compensation. An off-axis, two-fiber package with an excess loss <2 dB/stage avoids the need for a circulator. By cascading four stages, a passband to channel spacing ratio of 0.8 is obtained that allows both 40 Gb/s nonreturn-to-zero (NRZ) and return-to-zero (RZ) signals to be compensated.

Multistage dispersion compensator using ring resonators

A compact, multichannel dispersion-compensating filter is demonstrated with D=-4200 ps/nm, a +/-5-ps group delay ripple, <3-dB loss, and a 4.5-GHz passband width out of a 12.5-GHz free spectral range. We show that multistage designs can achieve a substantial increase in passband width and peak dispersion for a given group-delay ripple compared with single-stage designs. The dispersion-compensation effectiveness was demonstrated in a 320-km, seven-channel nonlinear system simulation for OC48 signals.

Implications of fiber grating dispersion for WDM communication systems

For high bit-rate dense wavelength-division multiplexed (DWDM) applications fiber grating dispersion for the transmitted adjacent channels is shown to be detrimental and ultimately leads to a penalty. We consider design criteria for fiber grating filters in DWDM systems using both Gaussian pulses and super-Gaussian pulses that approximate square pulses that are more common in nonreturn-to-zero (NRZ) systems.

Efficient wide-band and tunable midspan spectral inverter using cascaded nonlinearities in LiNbO 3 waveguides

We report on efficient (-7-dB fiber-to-fiber), wide-band (over 70 nm), tunable, and excess-noise-free mid-span spectral inverters based on cascaded second-order nonlinearities in periodically poled LiNbO/sub 3/ waveguides. We demonstrate their performance in a 4/spl times/10 Gb/s transmission over 150 km of standard single-mode fiber.

Pulse compression using fiber gratings as highly dispersive nonlinear elements

Pulse compressors rely on two separate sections. The first section is for bandwidth generation through self-phase modulation and chirp linearization through normal dispersion. In conventional compressors this first section consists of a normal dispersion fiber of appropriate length. The second section is for compensating this linear chirp through anomalous dispersion, typically a prism pair or grating pair. In this way a transform-limited input pulse is compressed into an almost-transform-limited pulse. This scheme is quite different from chirped fiber gratings that are used in reflection to compensate existing chirp: no extra bandwidth is generated and nonlinear effects are not necessary. We propose a scheme for optical pulse compression utilizing an apodized fiber grating in transmission as the nonlinear dispersive element for the first section of the compressor. Near the band edge, on the long-wavelength side of the stop band of the grating, the normal quadratic dispersion is orders of magnitude greater than in a standard optical fiber. Therefore the first section of the compressor may be scaled down in length and the constraints placed on these systems may be relaxed. In this paper we discuss the limitations and the design of such fiber-grating compressors. Analysis and numerical simulation show efficient pulse compression. Further numerical simulation reveals that sufficiently far from the band edge the fiber grating can be modeled as an effective homogeneous medium obeying the nonlinear Schrodinger equation.

Strong self-phase modulation in planar chalcogenide glass waveguides

Single-mode planar waveguides were fabricated from chalcogenide glass compounds with large Kerr nonlinearities. Strong self-phase modulation of subpicosecond pulses along with low linear and nonlinear absorption losses demonstrates the potential for ultrafast, low-power, all-optical processing applications.