By Steffen Koehler
Despite the digital revolution of the past several decades, radio-frequency (RF) signals continue to play a large role in communications systems from military, to wireless, to cable television applications.
Recently there has been increasing reliance on relatively high RF frequencies as data bandwidths increase and communications bands become congested.
Electronic warfare threats and countermeasure systems, as well as radar sensor systems, continue to move to high-frequency microwave and millimeter-wave bands, yet the basic architecture of microwave and millimeter-wave systems has remained largely unchanged-until recently.
Frequency-converter subsystems located very close to the antenna apertures usually generate or downconvert microwave signals.
Dictating this architecture are the losses and dispersion inherent in metallic transmission lines such as coax cables, which prevent high microwave or millimeter-wave signals from being transmitted very far without unacceptable signal degradation.
Still, it may not always be convenient or desirable to colocate up/down-converter modules with antennas in the wingtip of a jet fighter jet. As a result, designers must often make compromises in RF performance, size, weight, and power.
Optical fiber offers an alternative that is freeing system designers from the architectural constraints that metallic cables impose. The carrier frequency of light on an optical fiber at 1550-nanometer wavelength is approximately 192,000 GHz, so the effects of dispersion for microwave signal sidebands modulated onto an optical carrier are insignificant over distances of hundreds of meters to kilometers.
Optical fiber is also extremely low cost, lightweight, is immune to electromagnetic interference and crosstalk, and has RF transmission loss of less than 0.5 decibels per kilometer.
Consequently, it is an ideal transmission medium for RF signals. Optical fiber, moreover, is enabling RF system designers to move frequency converters away from antenna apertures and permitting sharing of scare electronic resources between several apertures.
The challenge in exploiting optical fiber for RF transmission lies in getting the RF signals on and off the fiber without degrading the signals.
Two options are commercially available for converting RF signals into optical modulated carriers: direct modulation of a laser diode (via its electrical drive current) or external modulation of a continuous-wave laser with an optical modulator device.
Because directly modulated lasers tend to have relatively high frequency chirp, high noise, and low dynamic range at high frequencies, native fiber-optic transport of microwave signals as high as 18 GHz or beyond requires external modulation to achieve the required signal-to-noise ratio and linearity.
This is usually done using LiNbO3 modulators because of the maturity of the technology and the low optical insertion loss.
Until recently, these optical components were still relatively bulky and expensive, and they required a working knowledge of fiber-optic technology to assemble a fiber-optic transmitter subsystem. Now, however, advances in optical packaging technology have enabled the manufacture of small, integrated, low-chirp laser/modulator modules with bandwidths as high as 18 GHz, such as the PB9020 module from Phasebridge Inc. in Pasadena, Calif. (see Fig. 1).
The most widespread deployment of fiber-optic analog RF links is for cable television signals, where it transfers more than 100 RF subcarriers from 50 MHz to approximately 1 GHz from a head end to multiple local distribution nodes, spanning distances as far as 70 kilometers or more.
RF fiber links are also being deployed to meet the ever-increasing military bandwidth demands of RF systems in airborne, maritime, and terrestrial applications.
On airplanes and ships, fiber also has the significant advantage over copper cable of being extremely lightweight. With the proliferation of antennas on modern airplanes and ships, this can translate to a significant weight savings, and therefore a very significant savings in fuel costs and the extension of mission profiles.
Commercial airliners now have satellite links for in-flight television and data, and these signals are backhauled from the antennas to a communications hub via optical fiber.
Military airplanes have a host of sensors and antennas for all manner of real-time battlefield communication, and the U.S. military is actively pursuing photonic RF links as a replacement to metallic cabling in new aircraft designs, primarily for weight savings.
A modern warship can have hundreds of antennas, most of which require high-bandwidth backhaul. Finally, satellite antennas now operate up to 40 GHz in the Ka band, and antennas in terrestrial, airborne, and maritime environments will need to transport such signals to their points of use.
Another important military application for RF analog photonics is in the distribution of signals to electromagnetically radiating towed decoys. Complex high-frequency signals are produced on the aircraft and are then transmitted via a fiber-optic cable to a decoy that is towed behind the plane. The decoy emits a signal that attracts enemy fire to itself instead of to the aircraft. This application would not be possible using coax cable due to the large distance from platform to decoy and weight of the decoy tether.
The key enabling elements of optical RF links involves the photonic transmitter and receiver modules. While the receiver module typically contains only one photodetector, the transmitter consists of a laser and a lithium niobate modulator in a single package.
This requires precision positioning of the laser, the modulator, the isolator, the focusing optics, and the output fiber. It also requires careful design of the RF path. All components and traces need to be carefully impedance-matched to avoid reflections.
Not only must designers position these components precisely, but they also must make sure components stay that way regardless of the ambient temperature. Since the unit will be remotely mounted at the antenna, this temperature can vary widely.
Typical specifications require a temperature range of -54 to 85 degrees Celsius. Therefore, materials and processes must be selected carefully to match coefficients of expansion between the optical elements and their mounting materials.
With the positions of the laser, modulator, and optical fiber fixed in temperature-compensated mounts, precise positioning of the lenses achieves optical coupling between these components. For the most critical lens positions, Phasebridge has designed a family of brackets with positioning accuracies as low as plus-or-minus 0.3 microns over the temperature range.
A flat-footed positioning bracket translates coarse operator manipulation into precise positioning of the lens (see Fig. 3). The feet of this bracket adjust laterally, and once the lens is in the position, low-shift electrical welds are made using the manipulating tool as a resistance welder. The resulting friction helps to control weld shift. Using these so-called “simple brackets,” the combination of two submicron lap welds results in a precision of roughly 1 micron.
A more precise “compound bracket” augments the simple bracket by providing enhanced precision using spring attenuation (see Fig. 3).
Compound brackets are used in applications requiring extreme precision or post-attachment fine positioning to improve yields. The bracket is first manipulated at the inner feet and welded into position in a procedure similar to the simple bracket. Next, the tools engage the outer feet and reestablish optimum lens position to correct for previous weld shifts at the inner feet. When position has been reestablished, the outer feet are welded into place, and the final weld shifts are minimized by the stiff-spring attenuation provided by the feet of the compound bracket. The compound bracket achieves a positioning accuracy of less than 0.3 microns.
Advances in optical packaging such as these precision lens mounts have made possible a new family of precision optical modules that enable low-cost RF photonic links with extremely compact and robust optical equipment deployed at antenna sites.
Photonic RF advances
The high level of integration permits more efficient optical coupling between laser and modulator chips. This in turn leads to higher transmitter optical output power levels for a given DC input power than previously possible with separate fiber-pigtailed components. With the advent of high-power, low-noise lasers approaching the quantum shot-noise limit and high-saturation photodiodes, RF fiber-optic links can achieve higher signal-to-noise ratios (SNRs) and dynamic ranges than previously possible. Such transmitters eliminate the need for inherently noisy optical amplifiers, and therefore maintain sufficiently high SNR levels such that RF system designers can focus on using the optical transmitter and receiver modules as microwave components.
The SNR levels achieved at 10-milliwatts received optical power, illustrate that SNRs in the range of 150 to 160 dB in a 1-Hz bandwidth are achievable (see Fig. 4). The SNR levels achieved at 1-milliwatt received optical power, on the order of 145 dB, are more typical of earlier technology. It is noteworthy that at power levels in the 1- to 4-milliwatt range, the SNR performance is not a strong function of laser RIN because thermal noise dominates. However, at the higher power levels, laser RIN and shot noise dominate compared to thermal noise, and great benefit is obtained by using low-noise lasers. Such levels of performance are only possible with high optical-coupling efficiency and extremely low-noise lasers. Only hybrid integrated components are capable of maintaining this level of performance from -54 to 85 degrees Celsius.
Steffen Koehler is vice president of marketing at Phasebridge Inc. in Pasadena, Calif. Contact the company online at www.phasebridge.com.