Daniel A. Nolan

Provably secure and practical quantum key distribution over 307 km of optical fibre

A discrete-variable quantum key distribution system that is capable of distributing a provably-secure cryptographic key over 307 kilometres is demonstrated at a telecom wavelength.

4 × 20 Gbit/s mode division multiplexing over free space using vector modes and a q-plate mode (de)multiplexer.

Vector modes are spatial modes that have spatially inhomogeneous states of polarization, such as, radial and azimuthal polarization. In this work, the spatially inhomogeneous states of polarization of vector modes are used to increase the transmission data rate of free-space optical communication via mode division multiplexing. A mode (de)multiplexer for vector modes based on a liquid crystal q-plate is introduced. As a proof of principle, four vector modes each carrying a 20-Gbit/s quadrature phase shift keying signal (aggregate 80 Gbit/s) on a single wavelength channel (λ∼1550  nm) were transmitted ∼1  m over the lab table with <-16.4  dB mode crosstalk. Bit error rates for all vector modes were measured at the 7% forward error correction threshold with power penalties <3.41  dB.

Large effective area fiber

Refractive-index nonlinearities have negligible effect on the performance of short-haul fiber-optic communication links utilizing electronic repeaters. However, in long optical fiber links, nonlinearities can cause severe signal degradations. To mitigate nonlinear effects, new generation of fibers, referred to as large effective-area fibers, have been introduced in recent years. This paper reviews the latest research and development work on these fibers conducted by several research groups around the world. Attention is focused on a class of large effective-area fibers that are based on a depressed-core multiple-cladding design. Transmission properties, including dispersion, dispersion slope, effective area, mode-field diameter, bending loss, polarization-mode dispersion, and cutoff wavelength are discussed. Dispersion-shifted, non-zero dispersion-shifted, and dispersion-flattened designs are addressed. Design optimization, particularly with regard to effective area, bending loss, and polarization-mode dispersion, is elaborated upon. The trade-off between effective-area and bending loss is emphasized. Results for dispersion-shifted and non-zero dispersion-shifted large effective-area fibers with zero polarization-mode dispersion and low bending loss at 1.55 micrometer wavelength are presented.

Mode division multiplexing using an orbital angular momentum mode sorter and MIMO-DSP over a graded-index few-mode optical fibre

Mode division multiplexing (MDM)– using a multimode optical fiber’s N spatial modes as data channels to transmit N independent data streams – has received interest as it can potentially increase optical fiber data transmission capacity N-times with respect to single mode optical fibers. Two challenges of MDM are (1) designing mode (de)multiplexers with high mode selectivity (2) designing mode (de)multiplexers without cascaded beam splitting’s 1/N insertion loss. One spatial mode basis that has received interest is that of orbital angular momentum (OAM) modes. In this paper, using a device referred to as an OAM mode sorter, we show that OAM modes can be (de)multiplexed over a multimode optical fiber with higher than −15 dB mode selectivity and without cascaded beam splitting’s 1/N insertion loss. As a proof of concept, the OAM modes of the LP11 mode group (OAM−1,0 and OAM+1,0), each carrying 20-Gbit/s polarization division multiplexed and quadrature phase shift keyed data streams, are transmitted 5km over a graded-index, few-mode optical fibre. Channel crosstalk is mitigated using 4 × 4 multiple-input-multiple-output digital-signal-processing with <1.5 dB power penalties at a bit-error-rate of 2 × 10−3.

Fiber spin-profile designs for producing fibers with low polarization mode dispersion

Using coupled-mode theory, we develop a theoretical model to analyze the effects of fiber spin profiles on polarization mode dispersion (PMD). Constant, sinusoidal, and frequency-modulated spin profiles are examined, and phase-matching conditions are analyzed. Our analysis shows that PMD can be reduced effectively by use of frequency-modulated spin profiles.

Using the nonseparability of vector beams to encode information for optical communication.

In this work, it is experimentally demonstrated that the nonseparability of vector beams (e.g., radial and azimuthal polarization) can be used to encode information for optical communication. By exploiting the nonseparability of a vector beams space and polarization degrees of freedom using conventional wave plates, it is shown that 2 bits of information can be encoded when applying the identity and three Pauli operators to its polarization degree of freedom. It is also shown that vector beams can be efficiently decoded with as low as 2.7% cross talk using a Mach-Zehnder interferometer that exploits a higher-order Pancharatnam-Berry phase and liquid crystal q-plates.

Fibers with low polarization-mode dispersion

Fibers with low polarization-mode dispersion (PMD) enable high-bit-rate time-division-multiplexed optical communication systems. These fibers are becoming increasingly important with the growing implementation of wavelength-division-multiplexing (WDM) transmission with channel bit rates of 40 Gb/s. Important to the realization of low PMD fibers are the fibers index and stress profiles, polarization-mode coupling, and the evolution of the polarization state with length. This paper reviews recent progress in the understanding of the fabrication and characteristics of these fibers. The paper also discusses the important fiber physical parameters, including the fiber index profile and fiber spinning parameters and their impacts on the realization of low PMD performance.

Single-polarization fiber with a high extinction ratio.

An elliptical-core hole assisted single-polarization fiber was designed, fabricated, and characterized. Numerical modeling based on the vectorial Maxwell equation reveals the dependence of the single-polarization bandwidth on core delta and air-hole size. Several single-polarization fibers based on this design with their single-polarization operating windows centered between 0.9 and 1.5 microm were successfully demonstrated. A correlation between fiber birefringence and single-polarization operating bandwidth is qualitatively confirmed. A single-polarization bandwidth as high as 55 nm was observed. These fibers also show very high extinction ratios of 60 dB or higher at lengths much shorter than 1 m. Other properties such as the dependence on length of the single-polarization operating window were also measured.

Nonlinear fibers for signal processing using optical Kerr effects

This paper reviews nonlinear optical-fiber designs for signal processing using optical Kerr effects. The requirements for designing nonlinear fibers are described first. Then, the design concept is discussed and design examples are shown to illustrate the tradeoffs among the different fiber properties such as effective area, dispersion, and attenuation. Furthermore, fiber designs with distributed Brillouin frequency shift to mitigate the effect of simulated Brillouin scattering (SBS) in nonlinear fibers are discussed in detail. An SBS-threshold increase of 7 dB over conventional nonlinear fibers is experimentally demonstrated.

Low-power high-efficiency wavelength conversion based on modulational instability in high-nonlinearity fiber

Bandwidth and peak efficiency are enhanced for wavelength conversion based on induced modulation instability by use of dispersion-shifted fiber in which the nonlinearity (n(2)/A(eff)) is enhanced by a factor of ~4.5 over that of conventional dispersion-shifted fiber. We experimentally obtain a peak conversion efficiency as high as 28 dB over a 40-nm bandwidth with 600 mW of peak pump power. Considerations for further enhancement of fiber-based wavelength conversion are also discussed.