Multimode Optical Fiber Links Research Articles

Adapting Mach–Zehnder Mesh Equalizers in Direct-Detection Mode-Division-Multiplexed Links

A Mach–Zehnder mesh (MZM), which is comprised of a network of tunable $2\times 2$ Mach–Zehnder interferometers and embedded photodetectors (PDs), can be used to perform arbitrary unitary matrix multiplications in the optical domain and compensate modal crosstalk in short-reach mode-division-multiplexed (MDM) links that use direct detection (DD). MZMs can be adapted using a self-configuration method, proposed by Miller, where multiple low-speed and low-power code sequences are superimposed on parallel high-speed information streams. We show that self-configuration in its original form is a sub-optimal equalization method for high-speed data transmission because adaptation based on detected code strengths is adversely impacted by low measurement signal-to-noise ratios and interference from the high-speed information streams. These impairments prevent the method from accurately tracking the millisecond-timescale modal dynamics of short-reach DD-MDM channels. We propose small modifications to the self-configuration method that can enable the MZM to track up to $10^8$ -fold faster channel dynamics. In particular, we show that replacing continuous equalization of low-power code sequences by periodic equalization of full-power training signals and using special optimization methods can yield faster MZM tuning. We also discuss the tradeoffs between MZM architectures that embed PDs inside the mesh and those that have PDs at the output ports only. Our results indicate that optimally designed MZMs and their associated control methods can increase the information capacity of short-reach multimode optical fiber links.

Combined Data Detection Scheme for Zero-Padded OFDM Signals in MMF Links

In this letter, we propose a receiver scheme for zero-padded orthogonal frequency division multiplexing (OFDM) that combines low complexity from overlap-and-add equalizer and low error rate provided by successive interference cancellation data detection from optimal ordering vertical Bell Laboratories layered space-time (V-BLAST) architecture. Results of numerical simulations on multimode optical fiber links show that the proposed scheme improves the error rate performance of zero forcing (ZF) equalization receiver, reaching results similar to V-BLAST. For example, the proposed scheme can reach 33.9 Gb/s in a 600-m link, whereas the ZF receiver would reach 29.06 Gb/s and cyclic prefix OFDM only 19.37 Gb/s. These results are obtained with a reduction in computational complexity (measured in number of real products) of 86% in detection and 66% in preprocessing with respect to the ZF receiver, and 75% and 97% with respect to the V-BLAST receiver.

Influence of restricted mode excitation on bandwidth of multimode fiber links

It is shown that multimode optical fiber links that use laser-based transmitters may be limited in bandwidth to values less than half those of the over-filled launch bandwidth. A solution to this problem, based on an offset launch, is therefore proposed, which increases the bandwidth to more than the overfilled launch.

Remote displacement measurement using a passive interferometer with a fiber-optic link

Remote displacement measurement is demonstrated using a Fabry-Perot cavity with a multimode optical fiber link. The sensing cavity modulates, as a function of its length, the spectrum of a light-emitting diode (LED). The light returns via the fiber and is analyzed by a tunable reference cavity. A closed-loop control causes the reference cavity to track the sensing cavity length within 2 x 10(-12) m. Displacement range is 2 x 10(-6) m. The reference cavity length is measured interferometrically, using a laser, to obtain the sensing cavity length. Advantages of this sensing technique include compatibility with multimode fiber-optic components, high immunity to optical losses, and large dynamic range.

Multimode integrated optics using dielectric and hollow metallic waveguides

For multimode optical fiber links integrated optical circuits are needed. A first approach has been made by using thick film techniques. We demonstrate an advanced technology using two different materials for the guide. By that we can fabricate high efficient micro optics, like lenses, prisms, beam splitters etc.. To avoid loss within the dielectric guide hollow metallic optical waveguides have been investigated. A 3dB fiber-fiber coupler with an overall loss of 1.3 dB was realized as well as the principle of an integrated prism spectrometer.