A.D. Cohen

Dynamic digital holographic wavelength filtering

This paper describes the theory and results of a new generic technology for use in optical telecommunications and wavelength division multiplexing (WDM): dynamic digital holographic wavelength filtering. The enabling component is a polarization-insensitive ferroelectric liquid crystal (FLC) spatial light modulator (SLM) in conjunction with a highly wavelength-dispersive fixed diffractive element. The technology has been used to perform demultiplexing of single or multiple WDM signals, dynamic erbium doped fiber amplifier (EDFA) gain equalization and channel management, and used to tune an erbium-doped fiber laser (EDFL) functioning as a high power, very narrow linewidth WDM source.

Dynamic holographic spectral equalization for WDM

We report a new technique for active management of wavelength division multiplexing (WDM) channels in an optically amplified telecommunications system, based on a polarization-insensitive holographic in-line wavelength filter. Results are presented for the control, amplification and equalization of five WDM channels spaced by 4 nm. Input signal power variations of 8.5 and 2.0 dB are reduced to 0.4 and 0.3 dB, respectively, by switching between two equalising holograms in 5 ms, with signals experiencing gains of up to 3.3 dB while undergoing equalization. An unused channel and amplified spontaneous emission noise from the erbium-doped fiber amplifier (EDFA) have also been suppressed by greater than 18 dB. This is the first demonstration of active holographic spectral equalization for WDM.

Active management of 100-GHz-spaced WDM channels

We demonstrate active management of 8 nm/spl times/0.8 nm wavelength-division multiplexing (WDM) channels with arbitrary dropping/passing and channel equalization. The optical/add-drop-(de)multiplexing functionality is achieved using a polarization-insensitive spatial-light-modulator-based holographic filter. Finer resolution (50 GHz) and spectral shaping can be straight forwardly attained.

Applications of FLC SLM dynamic holograms to WDM network components

This paper presents an overview of ferroelectric liquid crystal spatial light modulator (FLC SLM) dynamic digital holographic filtering technology and its application to WDM network components. Early work demonstrated a dynamically tunable filter and, in conjunction with an erbium-doped fibre amplifier (EDFA), a fibre laser[1]. The hologram design algorithm was subsequently expanded to yield a multiple variable amplitude passband filter, or spectral equaliser, for four[2] or six[3] WDM channels. Latest results for eight-channel equalisation are presented in this article, together with a demonstration of flattening the spectral shape of an individual passband. The paper concludes with a discussion of future work, including the redesign of filter parameters to meet ITU standards, and further improvements in computerised hologram design to achieve finer tuning resolution.

Fast polymer spatial light modulators for dynamic holographic WDM network components

The provision of a range of WDM network component functionality by ferroelectric liquid crystal (FLC) spatial light modulators (SLM) has already been demonstrated, but reconfigurration speed is limited to the order of 100 kHz by liquid crystal response time. A future generation of fast spatial light modulators might be based on a (chi) (2) material, yielding a device response time limited only by electronic RC product. This paper presents device modeling and characterisation of single-pixel SLMs based on an approximately 2 micrometer film of commercially available nonlinear optical polymer (NLOP) in an asymmetric Fabry-Perot (AFP) cavity. Polymer films were poled at a low field of approximately 35 V/micrometer and a 10-V modulating signal applied. Intensity modulation of 0.3% was provided by a gold/gold etalon device at 633 nm, while 1.4% modulation was provided by a hybrid device having an ITO-coated dielectric rear mirror and gold front mirror. Transmitted intensity modulation of the former device around 1.55 micrometer was approximately 0.1% and had a 3-dB bandwidth of 10.8 nm. Modelled behaviour has been extrapolated to yield two significant performance indicators: (1) the diffraction efficiency (eta) of a multiple-pixel 1-D AFP NLOP-SLM is largely phase modulation-dependent, having a maximum value of approximately 13.5% cf. approximately 40% for a binary phase FLC-SLM; (2) the 3-dB bandwidth of (eta) ca. 1.55 micrometer is approximately 5 nm in this configuration, but an approximately 190 nm-thin cavity hybrid device could have an (eta) 3dB approximately 30 nm.

Dynamic holographic telecommunications components based on spatial light modulation with pixelated χ(2) polymers in a Fabry-Pe´rot cavity

The provision of a range of wavelength division multiplexing (WDM) network component functionalities by ferroelectric liquid crystal (FLC) spatial light modulators (SLMs) has already been demonstrated in both transmissive and reflective arrangements. The FLC-SLM reconfiguration rate is limited to ~100 kHz by the liquid crystal response time. x(2) nonlinear optical polymers (NLOPs) might form the material basis of a future generation of faster and less costly SLMs for WDM applications, with response time limited only by electronic RC product. We present a theoretical analysis and the experimental characterization of single-pixel SLMs based on an ~2-?m film of commercially available x(2) NLOP in an asymmetric Fabry-Perot (AFP) cavity. Rigorous modeling enables optimization of the generic pixel design and specific pixel parameters, with the design guidelines and constraints dictated by the context of wideband WDM operation ~1.55 ?m. Polymer films are poled at a low field of ~35 V/?m and a 10-V modulating signal is applied. A modulation depth of 1.4% is provided by an AFP device of optimal pixel structure. Further characterization reveals a 3-dB width of transmitted intensity modulation ~10.8 nm near 1.55 ?m. The modeled behavior is extrapolated to yield two significant performance indicators. One indicator is that the holographic diffraction efficiency n of a full-scale pixelated AFP NLOP-SLM will be principally phase-modulation-dependent if the polymer Pockels coefficient r13 is of the order of 5 pm/V. For the preferred reflective geometry, the diffraction efficiency has a maximum value of ~13.5% cf. ~40% for a binary phase FLC-SLM. The second indicator is that for experimental devices, the 3-dB bandwidth of n~1.55 ?m will be only ~5 nm, but a~190 nm-thin cavity-optimized structure will have y3dB of ~30 nm.

Active holographic spectral equalization and channel management for WDM

In this paper, we present a new technique for active management of the gain, based on a polarization-insensitive diffractive ferroelectric liquid crystal (FLC) in-line optical filter. The technique is scaleable to tens of channels and is potentially low-cost in volume production.

Dynamic Holographic Multi-channel Equalisation for Concatenated Amplifier Systems in WDM Networks

We report equalisation and control of up to eight WDM channels amplified by a non-gain-flattened EDFA. The technique can be applied to concatenated optical amplifiers.