The increasing demand for broadband mobile communications has generated interest in exploring new frequency bands
and modifying network structures. In such systems, photonic technologies can bring both cost reduction as well as an
increase in performance, mainly due to the low-loss properties of optical fibers. An optical source capable of producing
tunable, high-quality microwave/mm-wave signals would be of great interest not only in such communications systems,
but in fiber sensors and numerous other applications as well.
One potentially cost-effective method to fabricate such a system is via optical heterodyning. In this approach, the
difficulties in generating a high-quality signal are two-fold. The first issue is in maintaining a specific frequency
difference (i.e. microwave signal) between the lasers for an extended period of time. The second is in narrowing the
inherent linewidth of the laser from the MHz values typically produced by conventional semiconductor lasers, down to
values practical for a communications system. Both of the above requirements are facilitated by the newly developed
doped-fiber, external cavity laser (DFECL), which offers relatively stable single-longitudinal-mode operation in addition
to narrow linewidth operation.
This paper will demonstrate frequency locking of a DFECL using a delay-line discriminator. The RF linewidth, initially
10-15MHz, is reduced to levels conducive to optical PLL locking. Optical power levels are approximately -3 dBm and
unamplified microwave power output levels are typically -35 dBm, depending on photodetector responsivity. Carrier-to-noise
ratios are generally 40-45 dB. The physical mechanisms underlying the observed laser dynamics are discussed,
including laser-to-fiber alignment and thermal fluctuations.
Photonic true-time delay (TTD) beamforming has been considered a promising technique for wideband phased-array
antenna (PAA) systems. An efficient way to achieve optical TTD beamforming is through the use of fiber Bragg grating
(FBG) delay lines. In an FBG-based TTD beamforming system, a microwave signal carried by an optical carrier is
required, which is usually obtained by modulating the microwave signal on the optical carrier using an external
modulator. In this paper, we propose a novel approach to economically generate high-frequency microwave signals
using two wavelengths from two phase-locked laser diodes through optical heterodyning. Since no optical modulator is
required, the cost is significantly reduced. In addition, since the system uses only two wavelengths, the power-penalty
problem caused by chromatic dispersion is minimized. In the proposed approach, the two phase-locked wavelengths are
generated using an optical phase-locked loop (OPLL). A TTD beamforming system using an OPLL in combination with
FBG-based delay lines to achieve tunable time delays is investigated. Experimental results are provided.