G.E. Betts

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.

Optically sampled analog-to-digital converters

Optically sampled analog-to-digital converters (ADCs) combine optical sampling with electronic quantization to enhance the performance of electronic ADCs. In this paper, we review the prior and current work in this field, and then describe our efforts to develop and extend the bandwidth of a linearized sampling technique referred to as phase-encoded optical sampling. The technique uses a dual-output electrooptic sampling transducer to achieve both high linearity and 60-dB suppression of laser amplitude noise. The bandwidth of the technique is extended by optically distributing the post-sampling pulses to an array of time-interleaved electronic quantizers. We report on the performance of a 505-MS/s (megasample per second) optically sampled ADC that includes high-extinction LiNbO/sub 3/ 1-to-8 optical time-division demultiplexers. Initial characterization of the 505-MS/s system reveals a maximum signal-to-noise ratio of 51 dB (8.2 bits) and a spur-free dynamic range of 61 dB. The performance of the present system is limited by electronic quantizer noise, photodiode saturation, and preliminary calibration procedures. None of these fundamentally limit this sampling approach, which should enable multigigahertz converters with 12-b resolution. A signal-to-noise analysis of the phase-encoded sampling technique shows good agreement with measured data from the 505-MS/s system.

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,. >

Linearized modulator for suboctave-bandpass optical analog links

The spurious-free dynamic range of a suboctave externally modulated optical analog link can be improved (to >140 dB Hz/sup 4/5/) with modest received optical power and little noise figure penalty by using a simple linearized modulator. Its design consists of two standard Mach-Zehnder interferometric modulators in series, and so can operate at microwave frequencies. This paper develops standardized measures of linearized modulator performance, and uses them to evaluate the modulator; link examples are also given. A simple experiment verifies the basic theoretical predictions. >

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.

Microwave analog optical links using suboctave linearized modulators

We report experimental links at frequencies up to 4 GHz which use two integrated optical Mach-Zehnder interferometric modulators in series to reduce third-order intermodulation distortion. This design improves the intermodulation-free dynamic range (up to 88 dB with 1 MHz noise bandwidth is demonstrated) with no noise figure penalty relative to a link using a standard modulator. Detector current is low (<2 mA), which is necessary for the detector to provide adequate linearity.

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.

Effects of crosstalk in demultiplexers for photonic analog-to-digital converters

Time interleaving of samples digitized by a parallel array of analog-to-digital (A/D) converters provides a means of increasing the sampling rate beyond that possible with a single A/D converter. For time-interleaved photonic A/D converters, optical demultiplexers can be used to advantage. Both time-division and wavelength-division demultiplexers must yield low crosstalk between the parallel output channels in order to yield accurate A/D conversion. An analysis predicts the level and form of the resulting errors. The analytical results compare well with experiment.

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>>