Analog optical links are finding increased application in commercial and military systems ranging from radio-over-fiber applications, antenna remoting, and optical signal processing. As the performance of an analog link improves with received photocurrent, optical amplifiers — predominantly erbium-doped fiber amplifiers (EDFAs) — have been readily incorporated into a variety of systems. It is known that the addition of an optical amplifier (EDFA) raises the electrical noise floor in both digital and analog applications due to the presence of amplified spontaneous emission (optical) noise. To mitigate this additional noise in systems employing EDFAs prior to modulation, dual-output optical modulators and balanced detection are frequently employed. This technique has been utilized alone to achieve the first multi-gigahertz bandwidth analog optical link with a noise figure

While pre-modulation amplification and balanced detection may be utilized to approach shot noise-limited performance in an analog link, other techniques may be applied to increase the link gain (increase the received photocurrent), while also decreasing the effect of noise from the EDFA; in particular, arrayed receivers — those employing multiple optical paths post-modulation and multiple photodiodes.

In these architectures, the desired output RF photocurrent is recovered from each photodiode individually; the individual photocurrents are then coherently combined in the electrical domain. Conceptually, the operation of these receivers is analogous to a phased-array antenna operating in receive mode, where the outputs of multiple antenna elements are combined in-phase to increase the signal-to-noise ratio of the received signal as compared to that from a single element. These phased receivers (with no intentional filtering) could find widespread application in analog systems utilizing freespace optical links.

(a) Intensity-modulated direct detection Analog Link employing a 4-channelreceiver (EDFA=erbium-doped fiber amplifier); (b) single-channel referencelink architecture.
The performance of arrayed receivers in which each optical (receiver) path employs an EDFA is analyzed. The analysis derives general expressions for two of the primary RF performance metrics — gain and noise figure — of an analog link utilizing the arrayed receiver concept. This analysis links the performance of the EDFAs utilized in the system to the RF noise figure. Specifically, this is achieved by extending the concept of the noise penalty associated with an EDFA — defined for a highly compressed amplifier — to all regimes of amplifier operation; in particular, the linear amplification regime.

To verify the analysis, two 4-channel optoelectronic receivers were constructed. The general receiver architecture is shown in (a) in the figure. The output of a single distributed feedback laser (DFB) is modulated with a single-output low-Vn Mach-Zehnder intensity modulator (MZM). The modulator output is subsequently split evenly using a 1×4 optical coupler. Each output of the coupler is amplified using a commercial ~30 mW EDFA and the input RF modulation of each channel is recovered via direct detection with a photodiode (~12 GHz or ~3 GHz). The photodiode outputs are then combined either with a 1×4 RF power combiner (10 MHz - 2 GHz), or via hard wiring with a custom-built combining circuit. Prior analysis compared the performance of the arrayed receiver to that of a single reference link. The reference link architecture utilized in this work is shown in (b) in the figure.

This work was done by Jason D. McKinney, Vincent J. Urick, Frank Bucholtz, and Carl Villarruel of the Naval Research Laboratory; and Christopher Sunderman of Global Strategies Group. For more information, download the Technical Support Package (free white paper) at under the Photonics category. NRL-0034

This Brief includes a Technical Support Package (TSP).
Analysis of Analog Photonic Links Employing Multiple-Channel (Arrayed) Receivers

(reference NRL-0034) is currently available for download from the TSP library.

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This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine.

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