L.T. Nichols

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.

Optimizing the ultrawide-band photonic link

Performance of wide-band photonic links (PLs) using Mach-2ehnder modulators (MZMs) is reported. Comparison parameters include loss, noise figure, and spur-free dynamic range (SPDR). The feasibility of a 0-dB noise-figure link even with passive matching is given and the advantages of dual-output MZMs are presented. A new figure of merit is introduced to quantitatively optimize link performance with or without a preamplifier.

A two-channel optical downconverter for phase detection

Experimental results for a two-channel optical downconverter link operating from 2 to 18 GHz are presented. Using low-noise preamplifiers results in a noise figure (NF) of 8.5-14 dB over the frequency range of 2-18 GHz. For the first time, relative phase measurements between optically downconverted signals have been performed. An in-phase/quadrature phase-measurement technique indicates a phase precision of /spl plusmn/2/spl deg/ with as little as -60 dBm radio frequency (RF) received power. Comparing the optical microwave downconverter to an electrical microwave downconverter in terms of phase detection reveals similar performance between the two systems.

Demonstration of a wide-band image rejection microwave downconverter

A photonic image-rejection downconverter capable of down-converting radio-frequency (RF) signals over a range of 5-18 GHz into an IF band 2.1 GHz wide (3.3-5.4 GHz) is presented. RF signals received by the system are first up-converted into the passband of a 21.3-23.4-GHz filter, and then optically down-converted to 3.3-5.4 GHz. The system exhibited better than 60 dB of image rejection over its entire bandwidth and spurious signals resulting from harmonic mixing products were more than 25 dB below the downconverted signal.

Single sideband modulation techniques and applications

Techniques to generate optical single-sideband modulation with and without carrier are presented. Various applications are reviewed including combating dispersion effects, RF frequency translation, and grating characterization.

Photonic remoting of the receiver of an ultra-high dynamic range radar

Fiber optic links have been designed and built to remote the antenna of a radar with ultra-high dynamic range, the AN/SPQ-9B ADM. The links tested successfully in receive configuration without significantly degrading the radars 83 dB SNR. These results demonstrate that photonic technology can meet the phase noise requirements for remoting modern radars.

A four channel angle of arrival detector using optical downconversion

A fiber-optic system for microwave angle of arrival measurements is demonstrated in an anechoic chamber for the first time. The system consists of a four-channel optical downconverter capable of processing received signals from 2-18 GHz. Direction resolution is determined by the antenna spacing, and at 10 GHz is spatially better than 0.1.

Microwave phase detection for angle of arrival detection using a 4-channel optical downconverter

Fiber optic links maintain an advantage over traditional microwave transmit and receive systems in terms of multi-octave operation and RF isolation, and the ability to remote microwave signals great distances. For the first time, a 4-channel optical downconverter capable of detecting the relative microwave phase of incoming microwave signals up to 18 GHz is tested. The phase information can be related to angle of arrival detection. We present the first results of a 4-channel remotable optical downconverter used for phase detection of incoming signals up to 18 GHz. The phase information is related to angle of arrival detection.

Noise effects in polarization maintaining optical amplifiers

Excessive noise generation in optically amplified analog fiber optic systems due to different polarization-maintaining EDFA configurations is investigated. Increases in the effective RIN level of more than 20 dB are predicted and experimentally observed for double-pass configurations.

Dynamic range comparison of ultrawide-band photonic downconverters

Dynamic range of alternative configurations for a single-channel cascaded-MZM downconverter are compared for the first time to our knowledge. Increased RIN in an optically amplified setup may outweigh increased RF power gain in ultrawideband systems.