Optical Microwave-Signal-Channelization Technique Analyzed

December 1, 2008

Office of Naval Research

In a recent study, a proposed technique for optical frequency-band channelization of a microwave signal or other wide-band radio signal in a receiver was analyzed and compared with prior electronic and acousto-optical techniques. In frequency-band channelization, which is commonly used in wide-band radio receivers, a received signal is divided into narrow frequency bands (channels) and the signal in each channel is detected by a relatively narrow-band sub-receiver. In principle, frequency-band channelization offers benefits of reduced noise bandwidth and processing gain in the narrowband sub-receivers, leading to increased sensitivity, such that the wide-receiver could have a sensitivity approaching that of a narrow-band receiver. The quality of the channelization ultimately determines the performance of the receiver.

Heretofore, channelization has been effected, variously, by use of electronic filters or acousto-optical Bragg cells. For processing received signals having exceptionally large bandwidths (necessitating the use of many channels), optical frequency-band channelization offers potential advantages of reductions of size, weight, and power consumption.

The figure shows the functional blocks of two frequency- channelizing -and -digitizing microwave receiver systems: a traditional one utilizing electronic frequency-channelization filters and a proposed one utilizing the optical frequency-channelization technique that was analyzed. For each system, a photonic microwave link is shown as an appropriate means of coupling the radio-frequency (RF) signal from the antenna to the receiver in a typical setting in which there is significant electrical noise and/or the antenna is distant from the receiver. Significantly, for the purpose of the study, the photonic link in each system was considered to include an electronic-to-optical (E/O) converter in the form of either an electro-optical phase modulator (φM) or a Mach-Zehnder interferometric intensity modulator (MZM).

In the traditional system, the channelization and down-conversion of the RF signal are performed entirely in the electronic domain: Channelization is effected by a bank of electrical filters, while down-conversion is effected by mixing of the channelized RF signals with signals from electronic oscillators (LOs). If this system were required to be capable of handling hundreds of channels simultaneously, then the size, weight, and power demand could be so large as to render the system impractical. Therefore, in practice, it is common to provide for switching a group of fewer channels onto a channelizer at a given time.

In the proposed system, the channelization would occur entirely in the optical domain and down-conversion would occur partly in the optical and partly in the electronic domain:

• Channelization would be effected by narrow-band passive optical filters.
• Down-conversion would be effected by mixing of the channelized optical signals and optical local oscillator (OLO) signals in photodetectors constituting the optical-to-electronic (O/E) converters of the sub-receivers.

Ideally, the optical modulator should be linear in the optical electric field so as not to generate spurious optical-field components, but such a modulator does not yet exist. In the study, it was noted that because the optical channelizer in the proposed system would direct only a portion of the optical spectrum onto the photodetector in each channel, spurious optical-field components that might cancel at the photodetector in the absence of channelization might not cancel in the presence of channelization. Accordingly, effects of non-linearities in the φM or MZM were considered in the analysis and found to impose limits on attainable performance. Hence, further, it was concluded that mitigation of nonlinearities is necessary for the development of high-performance channelizers.

This work was done by Matthew S. Rogge, Vincent J. Urick, and Frank Bucholtz of the Naval Research Laboratory for the Office of Naval Research.

ONR-0007

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