E.I. Ackerman

Direct-detection analog optical links

We review state-of-the-art intensity-modulation direct-detection (IMDD) analog optical links, focusing on advances since 1990. We contrast direct and external modulation with respect to gain, noise figure, and dynamic-range performance.

Broadband linearization of a Mach-Zehnder electro-optic modulator

A new analog fiber-optic link architecture uses a standard Mach-Zehnder electro-optic modulator in lithium niobate with a single RF traveling-wave electrode to achieve linearization across greater than an octave bandwidth. A broadband (800-2500 MHz) link with this new architecture has a measured dynamic range of 74 dB (in an instantaneous bandwidth of 1 MHz).

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.

Low Noise Figure, Wide Bandwidth Analog Optical Link

We report achieving a record low noise figure for an amplifierless fiber-optic link — ≤ 15 dB over the frequency range 1.0 – 9.5 GHz — via a combination of a low-VπMach-Zehnder modulator with two antiphase outputs, a high-power laser, and a balanced photodetector pair. We also present a complete model for this link that predicts its measured performance to within 1 dB.

Linearization of a broadband analog optical link using multiple wavelengths

Instead of balancing the RF signal voltage on two modulator electrodes, broadband linearization of a link can be more easily achieved when two wavelengths are simultaneously modulated using a standard single-electrode Mach-Zehnder modulator.

Cascade vertical cavity surface emitting laser arrays

Summary form only given. One simple approach to achieve cascade laser diodes is to lithographically series connect individual devices. We report the first lithographic cascade VCSEL arrays which exhibit > 100% differential quantum efficiency. We present measurements of the light output into a broad area detector versus injection current into the cascade VCSEL arrays. The output power increases approximately linearly with the number of array elements, and arrays with 4 or more VCSELs exhibit > 100% differential quantum efficiency. These initial results indicate that lithographic cascade VCSEL arrays show promise for low cost optical links.

What Do We Need To Get Great Link Performance

It is well known that the RF performance of optical fiber links falls short of what is desired, and often required. These seem to be the facts of life: links using colmmercially available components have at least 20 dB of RF loss, their noise figures are even greater than their loss, and their dynamic ranges are marginal to acceptable [ 11. Further, all three of these performance parameters degrade significantly with increasing frequency. This sitate of affairs is in stark contrast with the fundamental limits of link performance [2], which have shown that low loss, low noise figure, high linearity links should be possible, even up to fairly high frequencies. So why have we not been able to realize links with such performance? We will pursue the answer to this question by hypothesizing a set of link requirements and seeing what level of device performance would be required to meet it. Assume that we desire a link with ,an RF-to-RF gain (GI) of -3 dB over a bandwidth of less than one octave, a noise figure (NF) of 6 dB, and an intermodulation-free dynamic range (IMFDR) of 145 dB in a 1 Hz bandwidth. For the purposes of this discussion we will also limit consideration to amplifierless links using intensity modulation with direct detection (IMDD) and with passive impedance matching at the input and output ends of the link.

Input impedance conditions for minimizing the noise figure of an analog optical link

It has been previously shown that 3 dB is the lowest noise figure attainable for an amplifierless optical link with perfect lossless impedance matching to the RF source. In a prior experimental link with near-perfect impedance matching, dissipative loss in our input matching circuit prevented us from achieving a measured noise figure of less than 4 dB. Investigation of the effects of input impedance mismatch indicates that mismatch can actually lower the noise figure to below 3 dB even in the presence of some dissipative loss in the input circuit. We have verified this theory by using the mismatch effect to reduce the measured noise figure of our link to 2.5 dB at 130 MHz. We believe this is the first demonstration of amplifierless link noise figure of less than 3 dB. We confirmed the validity of our measurement technique by also measuring the noise figure of a 2.5 dB RF attenuator to be 2.5 dB.

Some limits on the performance of an analog optical link

Recent performance improvements in analog optical links have prompted us to analytically investigate the limits to link performance. In this paper these limits are derived and compared to current state-of-the-art link performance.

Analog fiber-optic link technology

Analog photonic components can be used to overcome the electronic bandwidth limitations in backplane, intra-board, and intra-chip applications. The components of an analog optical link are shown. In order for analog photonic components to be practical and appropriate for optical interconnects, methods have been developed to improve the gain, noise figure, and spurious free dynamic range, the figures of merit of a photonic link is defined. Understanding the link design tradeoffs and the performance of the individual components allows the design engineer to develop a photonic system with optimal performance. The state-of-the-art photonic link performance is demonstrated. During this presentation approaches to improve link performance will be reviewed.