Ronald D. Esman

Nonlinearities in p-i-n microwave photodetectors

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, Poissons 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.

Fiber-optic prism true time-delay antenna feed

Experimental results for an optical-control technique for implementing a true time-delay function for array antennas are reported. A microwave signal is transmitted on a wavelength-tunable optical carrier through a fiber-optic prism-a set of nominally equal-delay fibers with differing net dispersion-to photodetectors that feed each antenna element. The relative interelement time-delay (beam angle) adjustment is accomplished by tuning the optical carrier wavelength. Measured antenna patterns of a two-element array clearly demonstrate beam steering and true time-delay operation over a two-octave bandwidth of 2-8 GHz.<<ETX>>

Design considerations for high-current photodetectors

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.

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

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.

Performance of a time- and wavelength-interleaved photonic sampler for analog-digital conversion

We experimentally demonstrate the sampling of microwave signals using a novel time- and wavelength-interleaved pulse train derived from a mode-locked fiber laser. Experimental results are presented indicating a modulator limited bandwidth of 18 GHz and a laser relative intensity noise limited effective number of bits of /spl sim/7 when tested for use in a hybrid photonic analog-digital converter architecture.

True time-delay fiber-optic control of an ultrawideband array transmitter/receiver with multibeam capability

A true time-delay beamformer based on a fiber-optic dispersive prism is developed and characterized. The beamformer is used to control an ultrawideband time-steered array antenna, which is a significant improvement over inherently narrowband phased-array antennas. The time-steered transmitter antenna consists of eight broadband spiral elements in a sparsely-populated array. In transmit mode the bandwidth is microwave-component limited to 2-18 GHz. The transmitter shows an unprecedented performance with >100/spl deg/ azimuth steering and no observed squint over the full frequency range. We also extend the beamformer functionality and demonstrate, we believe for the first time, fully-independent dual-beam dual-frequency ultrawideband antenna transmitter operation. Furthermore, the beamformer is shown to be capable of controlling the transmitter under pulsed operation with microwave pulse-widths as short as 75 ps. In the phase-steered receive mode, the antenna is component-limited to two elements and a frequency range of 6-16 GHz. However, we can still demonstrate squint-free receiver steering over >70/spl deg/ azimuth over the full available frequency range. >

Wideband efficiency improvement of fiber optic systems by carrier subtraction

Carrier subtraction from a weakly modulated optical signal is proposed, analyzed, and demonstrated to improve fiber-optic system efficiency. For constant photodetector current, an amount of carrier suppression results in an equal amount of RF gain. Improvement of 9-13 dB in link loss from 1 to 25 GHz is measured using a Fabry-Perot filter operating in the reflection mode. Measured induced harmonic distortion agrees with predicted values.<<ETX>>

Effects of high space-charge fields on the response of microwave photodetectors

We present simulation results, based on a one-dimensional p-i-n photodetector model, to explain the reductions in photodetector response observed at high optical powers. The nonlinear effects are attributed to the redistribution of the internal electric field due to space-charge effects created by the photocarriers. The corresponding modification of the position-dependent charge velocities results in high-frequency response reduction, roll-off reduction, and pulse narrowing. Electric field redistribution leads to two dimensional effects, which must be included for accurate modeling of milliampere photocurrents.<<ETX>>

Optically amplified downconverting link with shot-noise-limited performance

Experimental results of a 12-GHz downconverting optical link are presented that demonstrate shot-noise-limited performance at a maximum total photoreceiver current of 72 mA. A balanced photoreceiver is utilized to reject both laser intensity noise and noise added by an erbium-doped fiber amplifier. The receiver was constructed of four commercial 2-GHz photodetectors, each capable of detecting greater than 20 mA of average photocurrent with less than I-dB compression in received RF power.

Optical single-sideband suppressed-carrier modulator for wide-band signal processing

We develop and demonstrate a technique for implementing a wide-band optical single-sideband suppressed-carrier modulator. The technique relies on a dual-electrode traveling wave modulator for sideband suppression and a Sagnac fiber-loop interferometer for carrier suppression. The implementation uses all commercially available components and demonstrates >40 dB optical carrier suppression, and as much as 60 dB unwanted signal sideband rejection across a 6-20 GHz microwave frequency range. The modulator is used to demonstrate broad-band spectral shifting- and inversion across the 3-18 GHz frequency range.