Peter J. Winzer

Sensitivity enhancement of optical receivers by impulsive coding

A theory for the signal-to-noise ratio (SNR) of optical direct-detection receivers employing return-to-zero (RZ) coding (and possibly optical preamplification) is developed. The results are valid for both signal-independent noise limited and signal-dependent noise limited receivers, as well as for arbitrary optical pulse shapes and receiver filter characteristics. Even if the same receiver bandwidth is used, RZ coding is seen to perform better than nonreturn-to-zero (NRZ) coding. Asymptotic expressions for the SNR in case of very high and very low receiver bandwidths show that the full sensitivity enhancement potential of RZ coding is exhausted at fairly moderate duty cycles. A realistic example taking into account intersymbol interference (ISI) shows that a receiver sensitivity gain (compared to NRZ coding) of, e.g., 3.2 dB can be obtained in a signal-independent noise limited receiver with a bandwidth of 80% of the data rate, using a duty cycle of three.

Design of bidirectionally pumped fiber amplifiers generating double Rayleigh backscattering

Optimum pumping schemes for bidirectionally pumped distributed fiber amplifiers are evaluated that simultaneously minimize the beat noise between signal and amplified spontaneous emission, as well as between signal and double Rayleigh backscattering. A way of adding a constraint on signal power to describe operation in a strong Kerr fiber nonlinearity regime is included. We show that one can find an optimum Raman net gain, as well as a percentage of forward to total Raman pumping maximizing the signal-to-noise ratio at the end of a transmission line using bidirectionally Raman-pumped transmission fibers.

Fiber coupling efficiency for random light and its applications to lidar

Employing the van Cittert-Zernike theorem of classical coherence theory, we derive a general expression for the efficiency with which quasi-monochromatic random light can be coupled to an optical fiber by means of a lens. For the important case of a source with Gaussian intensity distribution, we obtain and discuss the dependence of the coupling efficiency to single-mode fibers on the lens-to-fiber coupling geometry as well as on the ratio of lens size to speckle size. A specialization to the emerging fields of both incoherent and coherent fiber-based lidar applications shows that a maximum coupling efficiency of approximately 42% can be obtained for a monostatic system.

Electronic predistortion and fiber nonlinearity

We compare the impact of fiber nonlinearities in systems using electronic predistortion (EPD) to systems using in-line optical dispersion compensation. We show that EPD is significantly more susceptible to fiber nonlinearities

Dependence of optically preamplified receiver sensitivity on optical and electrical filter bandwidths-measurement and simulation

In this letter, we investigate both experimentally and by means of simulations, the dependence of receiver sensitivity on the optical filter bandwidth as well as on the bandwidth of the detection electronics for the optical noise limited direct detection case. The experiment is in good agreement with simulations employing advanced Gaussian noise statistics. Bandwidth optimization is performed both for nonreturn-to-zero and return-to-zero coded signals, yielding a measured sensitivity only 1.4 dB off the quantum limit at a data rate of 10 Gb/s.

Minimum length of a single-mode fiber spatial filter

Using the concept of leaky modes, we derive the minimum length of a single-mode fiber required to act as a spatial-mode filter of given quality. The degree of filter action is defined by the ratio of power carried by the fundamental mode to that carried by the leaky modes.

Choice of MUX/DEMUX filter characteristics for NRZ, RZ, and CSRZ DWDM systems

This paper investigates the influence of filter bandwidth and flank steepness of both multiplexing and demultiplexing filters in dense wavelength division multiplexed systems (spectral efficiency 0.8 b/s/Hz) in the presence of coherent wavelength division multiplexing (WDM) crosstalk. Using a recently introduced technique for the statistically reliable performance prediction of systems impaired by coherent WDM crosstalk, this paper presents numerical results for nonreturn-to-zero (NRZ), 33% duty-cycle return-to-zero (RZ), and 67% duty-cycle carrier-suppressed return-to-zero signals. This paper confirms that steep filter flanks are generally preferable, both in terms of optical signal-to-noise ratio penalty and in terms of filter bandwidth tolerance.

Alignment tolerances for plane-wave to single-mode fiber coupling and their mitigation by use of pigtailed collimators

We discuss the efficiency with which coherent plane waves can be coupled to single-mode fibers in the presence of deterministic or stochastic misalignments of the fiber relative to the focal point of a lens. We point out how the alignment demands can be relaxed by means of graded-index-lens fiber-pigtailed collimators.

Amplified spontaneous emission in pulse-pumped Raman amplifiers

We discuss amplified spontaneous emission (ASE) generated in Raman amplifiers that are counter-pumped with trains of pulses. Our experimental and theoretical results show that if the peak power of the pump pulses is too high, the ASE output from the amplifier can be significantly higher than that from a continuous-wave pumped amplifier providing the same gain. This effect places a lower limit on the duty cycle of pump pulses one can use for a given level of Raman gain. Furthermore, we report an additional ASE enhancement if there is insufficient walkoff between the pump pulses and copropagating ASE to average the effects of higher frequency pump intensity noise. As a result, less pump intensity noise can be tolerated when pulse-pumping a fiber having a zero-dispersion wavelength located midway between the pump and signal wavelengths.

Relative impact of multiple-path interference and amplified spontaneous emission noise on optical receiver performance

In this paper we compare the impact of MPI and ASE noise on Q-factor measurements when using an optically-preamplified receiver. We performed measurements on a 10-Gb/s data stream for two transmission formats: return-to-zero with a 50% duty cycle and a nonreturn-to-zero.