Charles H. Cox

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.

An analytic and experimental comparison of direct and external modulation in analog fiber-optic links

Analytic lumped-element small-signal models of directly and externally modulated analog fiber-optic links are derived. Incremental modulation efficiency is defined and used to compare the performances of these modulation techniques. In experiments to optimize link RF-to-RF gain and noise figure, the measurements obtained agreed with calculations to within approximately=1 dB. The externally modulated link was operated with two different impedance matching circuits. With a low-pass match the bandwidth was 150 MHz, and the link transducer gain was 1 dB; with a bandpass match the bandwidth was 22 MHz, the link transducer gain was 11 dB, and the noise figure was 6 dB. The directly modulated link was operated with a low-pass match. In this case, the bandwidth was 1 GHz, the link transducer gain was -14 dB, and the noise figure was 33 dB. These experimental results were achieved with no amplification,. >

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.

Serrodyne optical frequency translation with high sideband suppression

A Ti:NiNbO/sub 3/ serrodyne optical frequency shifter with 40-dB overall sideband suppression has been demonstrated. This performance can be achieved using either a laser diode or a broadband superluminescent diode as an optical source. The device utilizes a single-polarization waveguide design and a specialized sawtooth drive circuit. >

A down-conversion optical link with RF gain

A down-conversion optical link with a radio frequency (RF) conversion gain of 33 dB was demonstrated at 450 MHz. The gain was increased to 17 dB using photodetector impedance matching. Limitations due to each link element are examined and down-conversion gain up to the millimeter-wave (mm-wave) frequency band is predicted.

High-performance optical analog link using external modulator

An experimental fiber-optic analog link with a noise figure of only 6-dB, a 104-dB intermodulation-free dynamic range (measured using a 10-Hz noise bandwidth), and an RF-to-RF gain of 11 dB at 50 MHz is discussed. The link includes no electronic amplification. It uses a very sensitive bandpass impedance-matched Ti:LiNbO/sub 3/ interferometric modulator and an input optical power of 55 mW at 1.32 mu m.<<ETX>>

Relationship between gain and noise figure of an optical analog link

Experimental confirmation is presented of the relationship between link gain and noise figure by varying the optical attenuation in an external modulation link. When we minimized the optical loss we achieved an amplifierless link gain of 31 dB and noise figure of 4.2 dB at 150 MHz.

Effect of pilot tone-based modulator bias control on external modulation link performance

The dynamic range of an external modulation link in which a pilot tone maintains a Mach-Zehnder modulator at quadrature bias is limited to 64/(/spl pi/ m/sub PT/)/sup 4/, where m/sub PT/ is the modulation depth of the pilot tone.

20 GHz optical analog link using an external modulator

Reported is an experimental fiber-optic analog link with an electrical insertion loss of 34 dB at 20 GHz, a noise figure of 41 dB, and an intermodulation-free dynamic range of 108 dB/Hz/sup 2/3/. The link uses a diode-pumped Nd:YAG laser, a bandpass integrated-optical intensity modulator, and a p-i-n detector; it contains no amplifiers.<<ETX>>

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.