We have experimentally compared the performances of optical millimeter-wave generation or up-conversion using external modulators based on different modulation schemes. The generated or up-converted optical millimeter wave using the optical carrier suppression (OCS) modulation scheme shows the highest receiver sensitivity, highest spectral efficiency, and smallest power penalty over long-distance delivery. Moreover, the OCS modulation scheme has a simple configuration and low-frequency bandwidth requirement for both electrical and optical components. Employing an OCS modulation scheme, 16-channel dense wavelength-division multiplexing signals at 2.5-Gb/s per channel have been up-converted to a 40-GHz carrier simultaneously.

Optical millimeter-wave generation or up-conversion using external modulators

This paper is divided into two major parts. Following a brief introduction that establishes some definitions and assumptions, Section II updates our earlier study on the limits of the RF performance of optical links. Section III reviews progress since our 1997 review paper in the development of devices enabling link performance closer to these limits, including (but not limited to): 1) cascade lasers that permit broad-band direct modulation links with gain >0 dB; 2) injection-locked edge- and surface-emitting lasers at 1300 and 1550 nm with modulation frequency responses as great as 40 GHz; 3) modulators with improved performance, especially electroabsorption modulators that now have switching voltages as low as 0.36 V, or handle optical powers as great as 60 mW, or have bandwidths as great as 50 GHz (but not all three of these in one device yet); and 4) high-speed photodetectors with high saturation currents, e.g., a 20-GHz device with a saturation current of 90 mA and a 55-GHz device with saturation at 50 mA. We conclude in Section IV by summarizing the component developments necessary for higher performance RF-over-fiber links, i.e.: 1) semiconductor lasers (for direct modulation) that have higher slope efficiency and bandwidth and lower relative intensity noise (RIN) at reasonable bias current levels; 2) continuous wave (CW) lasers (for external modulation) with higher fiber-coupled power and lower RIN; 3) higher frequency lower loss external modulators with more linear transfer functions and lower V/sub /spl pi// that can withstand larger CW optical powers; and 4) photodetectors with higher responsivity and bandwidth that respond linearly even when illuminated by greater average optical powers.

Limits on the performance of RF-over-fiber links and their impact on device design

Analog Optical Links presents the basis for the design of analog links. Following an introductory chapter, there is a chapter devoted to the development of the small signal models for common electro-optical components used in both direct and external modulation. However this is not a device book, so the theory of their operation is discussed only insofar as it is helpful in understanding the small signal models that result. These device models are then combined to form a complete link. With these analytical tools in place, a chapter is devoted to examining in detail each of the four primary link parameters; gain, bandwidth, noise figure and dynamic range. Of particular interest is the inter-relation between device and link parameters. A final chapter explores some of the trade offs among the primary link parameters.

Analog Optical Links: Theory and Practice

The integration of optical and wireless systems is considered to be one of the most promising solutions for increasing the existing capacity and mobility as well as decreasing the costs in next-generation optical access networks. In this paper, several key enabling technologies for hybrid optical-wireless access networks are described, including optical millimeter-wave (mm-wave) generation, upconversion, and transmission in a downlink direction, and full-duplex operation based on wavelength reuse by using a centralized light source in an uplink direction. By employing these enabling technologies, we design and experimentally demonstrate an optical-wireless testbed that is simultaneously delivering wired and wireless services in the integrated optical-wireless and wavelength-division-multiplexing passive-optical-network access networks. The actual applications consisting of 270-Mb/s uncompressed standard-definition TV signal and 2.5-Gb/s data channels for downstream are successfully transmitted over a 25-km fiber and a 10.2-m indoor wireless link with less than a 1.5-dB power penalty. The results show that this integrated system is a practical solution to deliver superbroadband information services to both stationary and mobile users.

Key Enabling Technologies for Optical–Wireless Networks: Optical Millimeter-Wave Generation, Wavelength Reuse, and Architecture

The authors correct an error in their original article (see ibid., vol.54, no.2, p.906-20, Feb. 2006).

Corrections to "Limits on the Performance of RF-Over-Fiber Links and Their Impact on Device Design"

An efficient, high-dynamic range microwave mixer implemented using an analog fiber-optic link with an overdriven optical modulator to produce the local oscillator requirement has been demonstrated and analyzed. This approach is compared with the heterodyned lasers approach to generating the local oscillator requirement and it is shown that increased conversion efficiency and more frequency and phase stable upconverted or downconverted signals are obtainable. Aside from RF link loss, which can be compensated with preamplification, a conversion loss below 5 dB has been demonstrated out to 10 GHz.

Efficient microwave frequency conversion using photonic link signal mixing

Efficient frequency conversion into and out of the millimeter wave frequency band has been demonstrated using photonic link signal mixing with cascaded optical modulators. By adjusting the modulator bias point and RF drive power to the modulator introducing the local oscillator signal at f/sub LO/=8.8 GHz, frequency conversions from f/sub s/ to f/sub LO//spl plusmn/f/sub s/, sf/sub LO//spl plusmn/f/sub s/, and 4f/sub LO//spl plusmn/f/sub s/ with respective losses of 4.8, 6.3, and 7.5 dB have been demonstrated. The direct phase noise measurement of the optical RF signal at 2f/sub LO/=17.6 GHz with 1 kHz offset shows -89 dBc/Hz, limited by the RF drive source.

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A photonic-link millimeter-wave mixer using cascaded optical modulators and harmonic carrier generation

Microwave photonic link technology suitable for shipboard RF antenna applications is presented. High performance narrowband (sub-octave) direct modulation links designed for shipboard GPS and UHF communications antenna remoting are described along with a novel broadband (multi-octave) photonic link linearization approach.

Microwave fiber optic links for shipboard antenna applications

Techniques and systems for reducing nonlinear distortions in an output optical beam from an optical transmitter by using both electrical pre-distortion compensation and optical compensation.

Hybrid Optical and Electrical Fiber Optic Link Linearizer

The phase and amplitude stability of directly and externally modulated analog fiber optic links are investigated and factors which limit link stability are discussed. Link gain, phase noise, and phase delay fluctuation measurements have been performed on fiber-optic links which are currently being considered for Navy shipboard antenna remoting and phased array antenna applications. Measurements indicate that good amplitude and adequate phase stability can be obtained using either directly or externally modulated analog photonic links provided certain design considerations are met.

Richard Joseph Orazi

Papers

Efficient microwave frequency conversion using photonic link signal mixing

A photonic-link millimeter-wave mixer using cascaded optical modulators and harmonic carrier generation

Microwave fiber optic links for shipboard antenna applications

Hybrid Optical and Electrical Fiber Optic Link Linearizer

Phase and amplitude stability of broadband analog fiber optic links