Edward I. Ackerman

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

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.

Signal-to-Noise Performance of Two Analog Photonic Links Using Different Noise Reduction Techniques

We demonstrate two analog photonic links that use different noise reduction techniques to achieve high gain and low noise figure without electronic amplification. Both links use a high-power, low-noise master oscillator power amplifier as the optical source, a balanced-bridge dual-output LiNbO3 Mach-Zehnder modulator with a record low Vpi = 1.33 V at 12 GHz, and either one or two high-power rear-illuminated photodetectors. In the first link, both outputs of the quadrature-biased modulator are used to illuminate two photodetectors configured for laser noise cancellation, yielding record high gain (> 17.0 dB) and low noise figure (< 6.9 dB) across the 6-12 GHz band. The second link uses low biasing to maximize the signal-to-noise ratio in one of the two modulator outputs, and thus requires only one photo-detector. This link has lower gain (> 12.7 dB) but also record low noise figure (< 5.7 dB) across this same frequency band.

Interfaces for high speed fiber-optic links: analysis and experiment

An analysis of directly and externally modulated fiber-optic links is presented. The theoretical analysis is based on the signal flow diagram of the interface circuits to the laser diode, Mach-Zehnder electro-optic modulator, and p-i-n photodiode. System parameters such as gain, noise figure, two-tone intermodulation distortion, and spurious free and compression dynamic range are expressed as a function of frequency and operating point of the laser and external modulator. Two directly and externally modulated fiber-optic links were designed and fabricated to verify the analytical models. The direct modulation FO link was developed at the Ku-band (11.6-12.4 GHz), whereas the external modulation link was at L-band (875-925 MHz). Spurious-free dynamic ranges of 95.8 dB Hz/sup 2/3/ and 113 dB Hz/sup 2/3/, respectively, were achieved. The predictions based on the analytical models match the measured results. >

High-speed fiber-optic links for distribution of satellite traffic

Low-loss fiber-optic links are designed for distribution of data and the frequency reference in large-aperture phased-array antennas based on the transmit/receive-level data mixing architecture. In particular, design aspects of a fiber-optic link satisfying the distribution requirements of satellite data traffic are presented. The design is addressed in terms of reactively matched optical transmitter and receiver modules. Analog and digital characterization of a 50-m fiber-optic link realized using these modules indicates the applicability of this architecture as the only viable alternative for distribution of data signals inside a satellite at present. It is demonstrated that the design of a reactive matching modules enhances the link performance. A dynamic range of 88 dB/MHz was measured for analog data over 500-1000-MHz bandwidth. >

RF-Over-Fiber Links With Very Low Noise Figure

Previously published analytical models for the noise figure of an amplifierless fiber-optic link fail to predict the measured performance, with a discrepancy of 1.1 dB at 1 GHz that increases to 2.3 dB by 12 GHz. We use an equivalent circuit to derive the effect of an additional source of noise not accounted for in earlier models: thermal noise arising from loss in the modulators traveling-wave electrodes. The electrode thermal noise has a frequency dependence matching that of the links noise figure, such that predictions using the improved model match the measured 1-12 GHz performance of a link with record low noise figure to within ~ 0.4 dB across this band.

Broad-band external modulation fiber-optic links for antenna-remoting applications

We present a 6-12 GHz external modulation fiber-optic link with a spurious-free dynamic range (SFDR) of 65.5 dB MHz. This result validates an analytical model for external modulation link performance which we have updated from a previously published model to account for the use of a traveling-wave external modulator. Using the revised model, we compare the expected performance of two different link architectures.

Microwave photonics: Past, present and future

In this paper we highlight some of the notable advances in microwave photonics over the last ~45 years and present some speculations about where we may go in the future. Some personal reminiscences are also included.

Demonstration of a single-aperture, full-duplex communication system

The ability to simultaneously transmit and receive (STAR) through the same aperture is a key capability that will enable full duplex use of communications channels. We report the first demonstration of a single-aperture STAR system, which was enabled by a recent advance in microwave photonics: a new type of optical link that has high transmit- to-receive (T/R) isolation over a broad bandwidth. We measured 14 key performance metrics on a prototype system; key among these was T/R isolation. T/R isolation > 100 dB is projected for the technique; isolation as high as 85 dB was measured, which is the highest reported to date.

Interfaces for high-speed fiberoptics links

An analysis of directly and externally modulated fiber-optic (FO) links is presented. The theoretical analysis is based on the signal flow graph of the interface circuits to the laser diode, the Mach-Zehnder electrooptic modulator, and the p-i-n photodiode. The system parameters, such as gain, noise figure, two-tone intermodulation distortion, and dynamic range, are expressed as a function of frequency. FO link analytical models are compared with the experimental results obtained on a custom-designed directly modulated FO link at 12 GHz and an externally modulated FO link at 900 MHz.<<ETX>>

Microwave Photonic Links with Gain and Low Noise Figure

We review reports of microwave photonic links with gain >0 dB in the absence of electronic amplifiers, and their corresponding noise figures.