Theory of modal attraction in bimodal birefringent optical fibers.

Nonlinear mode coupling among two beams of different wavelength that copropagate in a bimodal highly birefringent optical fiber may lead to the effect of modal attraction. Under such circumstances, the modal distribution of light at a pump wavelength is replicated at the signal wavelength, nearly irrespective of the input mode excitation conditions of the signal.

Bragg-scattering conversion at telecom wavelengths towards the photon counting regime.

We experimentally study Bragg-scattering four-wave mixing in a highly nonlinear fiber at telecom wavelengths using photon counters. We explore the polarization dependence of this process with a continuous wave signal in the macroscopic and attenuated regime, with a wavelength shift of 23 nm. Our measurements of mean photon numbers per second under various pump polarization configurations agree well with the theoretical and numerical predictions based on classical models. We discuss the impact of noise under these different polarization configurations.

Multichannel Raman polarizer with suppressed relative intensity noise for wavelength division multiplexing transmission lines.

We propose a method of suppressing the relative intensity noise caused by polarization-dependent gain that is inherent to Raman polarizers (RPs). This method involves bit-synchronously scrambling the state of polarization of a pulse (bit) before the pulse enters the RP. The proposed solution works for RPs operating in a depleted regime and is compatible with multichannel configurations.

Silicon Raman polarizer.

We theoretically investigate the polarization properties of Raman amplifiers based on silicon-on-insulator waveguides, and show that it is possible to realize a waveguide Raman polarizer. The Raman polarizer is a special type of Raman amplifier with the property of producing an amplified and highly repolarized beam when it is fed by a relatively weak and unpolarized signal.

Nonlinear repolarization in optical fibers: polarization attraction with copropagating beams.

We propose a type of lossless nonlinear polarizer, novel to our knowledge, a device that transforms any input state of polarization (SOP) of a signal beam into one and the same well-defined SOP toward the output, and perform this without any polarization-dependent losses. At the polarizer output end, the signal SOP appears to be locked to the input pump SOP. The polarizer is based on the nonlinear Kerr interaction of copropagating signal and pump beams in a telecom or randomly birefringent optical fiber.

Exact solution for the gigantic amplification of ultrashort pulses in counterpumped Raman amplifiers.

We exactly solve the initial-boundary value problem of interaction of three waves in the limit when one of these waves is strongly damped. The solution is applied to the characterization of transient effects in Raman amplifiers, with a special emphasis on the possibility of generating Stokes pulses with peak powers that are orders of magnitude higher than the input power of the pump beam.

Theoretical study of polarization attraction in high-birefringence and spun fibers.

Lossless polarizers are conservative nonlinear optical devices that transform unpolarized light into highly polarized light without polarization-dependent losses. The device proposed here consists of an up to 100-m-long segment of nonlinear highly birefringent or unidirectionally spun fiber pumped from the output end by an intense backward-propagating beam. An initially unpolarized (scrambled) signal beam acquires a degree of polarization close to 100% toward the fiber output.

Theory of fiber optic Raman polarizers.

We propose and apply a theoretical description of a Raman amplifier based on the vector model of randomly birefringent fibers to the characterization of Raman polarizers. The Raman polarizer is a special type of Raman amplifier with the property of producing a highly repolarized beam when fed by relatively weak and unpolarized light.

Quasi-parabolic pulses in a coherent nonlinear optical amplifier.

We show that, in a coherent two-level optical amplifier with Kerr nonlinearity and linear loss, any weak seed pulse evolves into a fixed powerful linearly chirped pulse with quasi-parabolic shape. This process is associated with a transition from the incoherent into the coherent amplification regime, thus enabling in practice the generation of pulses with a spectrum wider than the linear gain bandwidth.

Quantum-mechanical calculation of frequency and timing jitter for optical pulses in dispersive fibers with losses.

The noise of spontaneous emission of a short pulse propagating in a fiber with losses is translated into frequency jitter, causing exponential growth of jitter. This is in contrast with a classical treatment in which the jitter remains constant. Absorption-induced frequency noise continuously affects timing through group-velocity dispersion, and the accumulated timing jitter is therefore always larger than when it is evaluated classically. This numerical discrepancy is demonstrated to be considerable and points to the importance of including quantum effects in estimations of noise in high-bit-rate communications.

Quantum noise limits on frequency and timing for optical pulses in a linear high-gain amplifier.

Frequency and timing noise added in a linear optical amplifier of gain G grows as (G-1)/G. Provided that the pulse is initially in a coherent state, the output frequency and timing noise cannot exceed twice the input value. It is also shown that, in a transmission line with zero net gain and loss, the frequency and timing noise added in the lossy medium is equal to or larger than the noise added in the amplifier.