Dipak Chowdhury

Stimulated Brillouin scattering in optical fibers

We present a detailed overview of stimulated Brillouin scattering (SBS) in single-mode optical fibers. The review is divided into two parts. In the first part, we discuss the fundamentals of SBS. A particular emphasis is given to analytical calculation of the backreflected power and SBS threshold (SBST) in optical fibers with various index profiles. For this, we consider acousto-optic interaction in the guiding geometry and derive the modal overlap integral, which describes the dependence of the Brillouin gain on the refractive index profile of the optical fiber. We analyze Stokes backreflected power initiated by thermal phonons, compare values of the SBST calculated from different approximations, and discuss the SBST dependence on the fiber length. We also review an analytical approach to calculate the gain of Brillouin fiber amplifiers (BFAs) in the regime of pump depletion. In the high-gain regime, fiber loss is a nonnegligible effect and needs to be accounted for along with the pump depletion. We provide an accurate analytic expression for the BFA gain and show results of experimental validation. Finally, we review methods to suppress SBS including index-controlled acoustic guiding or segmented fiber links. The second part of the review deals with recent advances in fiber-optic applications where SBS is a relevant effect. In particular, we discuss the impact of SBS on the radio-over-fiber technology, enhancement of the SBS efficiency in Raman-pumped fibers, slow light due to SBS and SBS-based optical delay lines, Brillouin fiber-optic sensors, and SBS mitigation in high-power fiber lasers, as well as SBS in multimode and microstructured fibers. A detailed derivation of evolutional equations in the guided wave geometry as well as key physical relations are given in appendices.

Design concept for optical fibers with enhanced SBS threshold

We propose a criterion to predict the relative value of the stimulated Brillouin scattering (SBS) threshold in single-mode optical fibers with different refractive index profiles. We confirm our results by several representative measurements. We show that with the proper profile design one can achieve more than 3 dB increase in the SBS threshold compared to the standard single-mode optical fiber.

Intrachannel nonlinear penalties in dispersion-managed transmission systems

We have developed a model for intra-channel impairments in dispersion-managed systems and validated it using numerical simulations. We have shown that intrachannel impairments can be reduced using optimal precompensation. We have studied the intrachannel penalty for different modulation formats, and results show that the optimum precompensation is independent of modulation format. The intrachannel penalty decreases as the spectrum of the modulation format broadens.

Accurate modeling of optical multiplexer/demultiplexer concatenation in transparent multiwavelength optical networks

This paper presents an accurate theoretical model for the study of concatenation of optical multiplexers/demultiplexers (MUXs/DMUXs) in transparent multiwavelength optical networks. The model is based on a semianalytical technique for the evaluation of the error probability of the network topology. The error-probability evaluation takes into account arbitrary pulse shapes, arbitrary optical MUX/DMUX, and electronic low-pass filter transfer functions, and non-Gaussian photocurrent statistics at the output of the direct-detection receiver. To illustrate the model, the cascadability of arrayed waveguide grating (AWG) routers in a transparent network element chain is studied. The performance of the actual network is compared to the performance of a reference network with ideal optical MUXs/DMUXs. The optical power penalty at an error probability of 10/sup -9/ is calculated as a function of the number of cascaded AWG routers, the bandwidth of AWG routers, and the laser carrier frequency offset from the channels nominal frequency.

Fiber design considerations for 40 Gb/s systems

In this paper, we review the fundamental advantages and drawbacks of 40-Gb/s systems from a fiber manufacturers perspective. Based on modeling, experimental results, and fundamental understanding, we correlate the fiber design parameters with the expected performance of long-haul systems operating at 40 Gb/s. Nonlinear penalties, dispersion tolerances, modulation formats, polarization-mode dispersion, and Raman amplification are covered. We also present the fiber features required for both metro and submarine networks at this specific data rate.

Coherent frequency-selective polarimeter for polarization-mode dispersion monitoring

Frequency-selective polarimeters measure the state of polarization of the individual spectral components of a modulated optical signal. They can be used either as stand-alone measuring devices or as parts of adaptive polarization-mode dispersion (PMD) compensators. This paper presents a novel frequency-selective polarimeter based on coherent detection, which has superior accuracy compared to previously proposed direct detection-based counterparts. This is due to the high-frequency resolution and power sensitivity of coherent detection, features that minimize the systematic and random error, respectively, in the measurement of the state of polarization of the individual spectral components of the received optical signal. The accuracy of the measurement is independent of the received signal bit rate and modulation format. The proposed frequency-selective polarimeter is studied both theoretically and experimentally. The primary theoretical contribution of this paper is a unified formalism, which allows the modeling of both direct and coherent detection-based frequency-selective polarimeters. Analytical expressions for the output signal of both types of frequency-selective polarimeters are derived. Based on these expressions, a common algorithm is proposed for the evaluation of the Stokes parameters. In addition, an example error signal is used as a metric in order to test the agreement of the theoretical model with the experimental measurements. The successful operation of the coherent frequency-selective polarimeter is demonstrated experimentally for a 10-Gb/s intensity-modulated nonreturn-to-zero (NRZ) optical signal in the presence of first-order polarization-mode dispersion. There is an excellent agreement between theory and experiment.

Design of low-loss and polarization-insensitive reflection grating-based planar demultiplexers

A method for designing polarization-insensitive reflection grating-based planar optical demultiplexer is presented. The grating profile is modified to obtain minimum loss variation over a large (31 nm) bandwidth. Also, the maximum polarization dependent loss variation of only 0.27 dB is obtained over the same spectral range.

High-gain Brillouin amplification : an analytical approach

We derive a closed-form approximate analytical formula for the gain of a Brillouin fiber amplifier operating in the depleted pump regime. We take into account the material loss of the medium, which is a strong effect in optical fibers. We validate obtained results numerically and experimentally. The presented analysis can be used to accurately predict the performance of high-gain Brillouin fiber amplifiers.

Measurement of fiber-cladding diameter uniformity by use of whispering-gallery modes: nanometer resolution in diameter variations along millimeter to centimeter lengths

Variations in fiber-cladding diameter on a nanometer scale were measured along millimeter to centimeter lengths by use of whispering-gallery modes (WGMs) in the elastic scattering of the fiber. The fiber was side coupled with a wavelength-tunable Gaussian beam. The scattered light was imaged approximately 1:1 onto a multichannel photodiode array detector. Based on the WGM wavelength shifts along the fiber, the taper of the fiber claddings diameter was measured on a nanometer/millimeter scale. The fibers surface roughness amplitude (in nanometers) and granular size ( approximately 100 microm) along centimeter-length fibers could also be revealed by use of higher- Q resonances.

Error probability of transparent optical networks with optical multiplexers/demultiplexers

This letter presents an accurate model for the evaluation of the error probability of transparent multiwavelength optical networks with cascaded optical multiplexers/demultiplexers (MUX/DMUXs). The error probability evaluation takes into account arbitrary pulse shapes, arbitrary optical MUX/DMUX and electronic lowpass filter transfer functions, intersymbol interference, and the accurate (non-Gaussian) noise probability density function at the output of the optically preamplified direct-detection receiver. As an example, the model is used to study the cascadability of arrayed waveguide grating routers in conjunction with nonreturn-to-zero pulses.