There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Although silicon has dominated solid-state electronics for more than four decades, a variety of other materials are used in photonic devices to expand the wavelength range of operation and improve performance. For example, gallium-nitride based materials enable light emission at blue and ultraviolet wavelengths1, and high index contrast silicon-on-insulator facilitates ultradense photonic devices2,3. Here, we report the first use of a photodetector based on graphene4,5, a two-dimensional carbon material, in a 10 Gbit s−1 optical data link. In this interdigitated metal–graphene–metal photodetector, an asymmetric metallization scheme is adopted to break the mirror symmetry of the internal electric-field profile in conventional graphene field-effect transistor channels6,7,8,9, allowing for efficient photodetection. A maximum external photoresponsivity of 6.1 mA W−1 is achieved at a wavelength of 1.55 µm. Owing to the unique band structure of graphene10,11 and extensive developments in graphene electronics12,13 and wafer-scale synthesis13, graphene-based integrated electronic–photonic circuits with an operational wavelength range spanning 300 nm to 6 µm (and possibly beyond) can be expected in the future. A graphene-based photodetector with unprecedented photoresponsivity and the ability to perform error-free detection of 10 Gbit s−1s data streams is demonstrated. The results suggest that graphene-based photonic devices have a bright future in telecommunications and other optical applications.

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Graphene photodetectors for high-speed optical communications

An overview of analog microwave photonics will be presented. The performance requirements for externally-modulated analog microwave photonic links will be reviewed with specific emphasis placed on modulator efficiency, laser noise, detected photocurrent, and link linearity.

http://ieeexplore.ieee.org/iel5/6387506/6403300/06403360.pdf

Microwave photonics

Broadband and low loss capability of photonics has led to an ever-increasing interest in its use for the generation, processing, control and distribution of microwave and millimeter-wave signals for applications such as broadband wireless access networks, sensor networks, radar, satellite communitarians, instrumentation and warfare systems. In this tutorial, techniques developed in the last few years in microwave photonics are reviewed with an emphasis on the systems architectures for photonic generation and processing of microwave signals, photonic true-time delay beamforming, radio-over-fiber systems, and photonic analog-to-digital conversion. Challenges in system implementation for practical applications and new areas of research in microwave photonics are also discussed.

Microwave Photonics

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Higher carrier frequencies will allow for fast transmission of huge amounts of data as needed for new emerging applications. Despite the tremendous hurdles that have to be overcome with regard to sources and detectors, circuit and antenna technology and system architecture to realize ultrafast data transmission in a scenario with extensive transmission loss, a new area of research is beginning to form. In this article we give an overview of emerging technologies and system research that might lead to ubiquitous THz communication systems in the future.

A Review on Terahertz Communications Research

The nonlinearities in p-i-n photodetectors have been measured and numerically modeled. Harmonic distortion measurements were made with two single-frequency offset-phased-locked Nd:YAG lasers which provide a source dynamic range greater than 130 dB and a 1 MHz-50 GHz frequency range. Carrier transport is analytically described by three coupled nonlinear differential equations, Poisson's equation and the hole and electron continuity equations. These equations are numerically solved to investigate and isolate the various nonlinear mechanisms. The numerical solution incorporates diffusion since our treatment includes carrier generation in the highly doped p-region of the device. This p-region absorption and carrier-dependent carrier velocities associated with a perturbed electric field (due to space-charge and loading effects) are shown to dominate photodetector nonlinear behavior. The numerical model was extended to predict that maximum photodetector currents of 100 mA should be possible in 20 GHz bandwidth devices before a sharp increase in nonlinear output occurs. In addition, second harmonic distortion improvements of 40-60 dB may be possible if photodetectors can be fabricated with strictly-depleted absorbing regions.

Nonlinearities in p-i-n microwave photodetectors

Fundamentals of Microwave Photonics

This paper outlines the design considerations for gigahertz-bandwidth, high-current p-i-n photodiodes utilizing InGaAs absorbers. The factors being investigated are photodetector intrinsic region length, intrinsic region doping density, temperature effects, illumination spot size, illumination wavelength, frequency, and illumination direction. Space-charge calculations are used to determine optimal device geometry and conditions which maximize saturation photocurrent. A thermal model is developed to study the effects of temperature on high-current photodetector performance. The thermal and space-charge model results are combined to emphasize the importance of thin intrinsic region lengths to obtain high current. Finally, a comparison between surface-illuminated p-i-n structures and waveguide structures is made to differentiate between the problems associated with achieving high current in each structure and to outline techniques to achieve maximum performance.

Design considerations for high-current photodetectors

The performance of a dc to 3 GHz externally modulated link utilizing balanced high-power photodetection is presented. Nonlinearity measurements of high power photodiodes show 1 dB compression currents in excess of 55 mA and an output third-order intercept point of +32 to +34 dBm. These high current photodetectors permit the use of high power lasers as external modulator sources for low noise fiber optic links with only small degradations in the predicted link dynamic range. An externally modulated link with a 240 mW Nd:YAG laser, a dual-output 4-V V, (dc) modulator, and balanced 100 mA total photocurrent yielded a link noise figure from 15.5 to 17.5 dB, a spur-free dynamic range of 119.5 dBHz/sup 2/3/, and a 1-dB compression dynamic range of 168.4 dBHz.

Keith J. Williams

Papers

Microwave photonics

Nonlinearities in p-i-n microwave photodetectors

Fundamentals of Microwave Photonics

Design considerations for high-current photodetectors

Photodetector nonlinearity limitations on a high-dynamic range 3 GHz fiber optic link