ESTABLISHING OPTICAL COHERENCE USING FREE-SPACE OPTICAL LINKS

An apparatus and/or method may be used for distributed synchronization of oscillators at non-collocated stations by means of transmitting and receiving optical signals having frequencies related to a desired oscillator frequency.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

Various embodiments of the invention may relate to the processing of RF signals or other electromagnetic signals. The subsequent discussion will discuss RF signals, but the invention is not necessarily limited to such signals.

To briefly outline the more-detailed discussion below, optical MO distribution may be achieved by simultaneously transmitting two optical sources of closely-spaced frequencies, such that their frequency difference is comparable to radio frequencies of interest. When the two optical frequencies are incident simultaneously on a photodetector, the resulting signal, or “beat tone,” may correspond to an RF signal that, in principle, may be used as an LO.FIG. 1provides an exemplary embodiment of such a system.

FIG. 1shows a set of distributed communication devices (receivers and/or transmitters)101a-101fand102. In an embodiment of the invention,101a-101fmay be devices whose LOs may be synchronized to an MO of a master device102at a frequency Ω. At master device102, an MO may generate a signal at a frequency to which it is desired that the LOs at devices101a-101fmay be synchronized. A first optical source (shown as “Laser1” inFIG. 1) may generate an optical signal of frequency ω1. This optical signal may be fed to an electro-optical (EO) modulator, to which the output signal of the MO may also be fed. The resulting optical signal output of the EO modulator may be filtered using an optical filter. The result may be used to drive a second optical source (shown as “Laser2” inFIG. 1) at a second frequency ω2; this may involve modulation sideband injection. In such a way, the output optical signals from Laser1and Laser2may be heterodyne-injection locked via the EO modulation (a technique for performing such operations is discussed, e.g., in Schneider et al.,Optical Generation of Narrow-line RF by Injection Locking of Modulated DFB Lasers,CLEO, November 2011, incorporated by reference herein; see, also, PCT International Patent Application Publication No. WO 2012/099914, also incorporated by reference herein). The frequencies of the MO and the output optical signals of Laser1and Laser2may, as a result, be related according to Ω=ω1−ω2. The resulting output optical signals of Laser1and Laser2may then be transmitted to the various other devices101a-101f.

In this embodiment, the output optical signals from Laser1and Laser2of master device102may be received at one of the other devices, e.g., device101f.Device101fmay include a photodetector, e.g., a photodiode-based detector, upon which the two optical signals from master device102may be incident. The photodetector may, in response to the incident optical signals, generate a “beat tone” of frequency Ω=ω1−ω2. This may be used to synchronize a LO of device101f(or to act as a synchronized LO of the device, if the MO continues to transmit the optical signals). Similar operations may be performed at devices101a-101e.

In embodiments of the invention, the optical outputs signals of Laser1and Laser2of master device102may be transmitted to the other devices101a-101fvia free-space optical beams, where each beam may be a combination of the output signals. Because radio frequencies represent a tiny fraction of optical frequencies (˜0.05%), there may be negligible net disturbance of the signal by atmospheric effects and dispersion, i.e., both optical beams may typically be identically disturbed as they travel, so all effects of such disturbances may cancel out upon photodetection of the beat tone.

In some embodiments, the master device102may not transmit optical signals directly to every other device101a-101f.This may be due to the fact that, in some environments, it may not be possible to establish line-of-sight (LOS) links from master device102to all of the other devices101a-101f,e.g., due to obstructions. Therefore, it may be useful to provide one or more of devices101a-101fwith the ability to “pass along” optical signals to be used for locking LOs of further devices.

In such an embodiment, a device, such as device101f,may be able to amplify the output of the photodetector and may feed the amplified signal (i.e., the amplified beat tone) to an EO modulator. The EO modulator may receive an input optical signal from a first optical source (“Laser1” of device101f), and the EO modulator may modulate the optical output of Laser1of device101fwith the amplified detected beat tone. The result may be filtered in an optical filter, and the filtered result may be fed to a second optical source (“Laser2” of device101f). That is, a device, such as device101f,may be configured similarly to master device102with respect to transmitting an oscillator signal using two optical signals. As was the case with the output optical signals of Laser1and Laser2of master device102, the output optical signals of Laser1and Laser2may be transmitted to one or more further devices (e.g., if101eis thus outfitted, it may transmit to101a) by transmitting them in one or more free-space optical beams.

The locking of a remote platform's LO, by passing along optical synchronization signals from a first platform receiving optical synchronization signals (where the original optical synchronization signals (ultimately) originated at a master device102), can be achieved in at least two ways. In a first embodiment, the optical source beams may be captured at the device (e.g., device101a-101f) with a photodetector (e.g., a photodiode detector), which may then be followed by amplifying and using the resulting beat tone to drive a EO modulator. The photodetector and the EO modulator may need to be fast, to provide adequate performance. The modulation sideband may then be used to injection lock another pair of optical sources to a frequency offset equal to and coherent with the master oscillator. These lasers' outputs can then be passed along to another platform, and so on. This is the case discussed in one of the above embodiments. In this case, each platform may be equipped with such photodetector and EO modulator to recover the optical beat tone as an electrical signal.

In a further embodiment, the optical source beams may be captured and may, if necessary, be directly amplified, e.g., by one or more optical amplifiers, in the optical domain. These optical references may then be used to injection seed another pair of optical sources that have been tuned to closely match the received signals' wavelengths, which may, in turn, allow this further pair of optical sources to be injection locked to the respective wavelength-matched lasers on the master platform. By keeping the reference signal in the optical domain, the system may not require a high-speed modulator on each platform (however, the MO platform may typically still require one).

To realize the full advantages of distributed coherent platforms, the platforms must be coherent, and such coherence may be provided by embodiments of the present invention. However, accurate knowledge of the relative positions of platforms to within a fraction of the RF wavelength may also be needed, so that the precise signal time delay associated with each platform necessary can be applied, for phased-array beam-forming. This knowledge can be obtained by independent means such as precise GPS systems, or it can be monitored by tracking the phase drift of the beat tones received on each platform, since any relative motion between the platforms would cause a measurable phase change that can be used to accurately and continuously monitor this motion.

FIGS. 2 and 3contain flowcharts showing how various aspects of embodiments of the invention may operate. InFIG. 2, a first optical signal may be generated201and a signal of a desired frequency (e.g., the MO signal) may be generated202. The signal of desired frequency may then be modulated onto the first optical signal203. The result may be used to generate a second optical signal204. The first and second optical signals may then be transmitted205, in order to provide synchronization signals to other devices.

InFIG. 3, first and second optical signals may be received301. The first and second optical signals may have frequencies whose difference may correspond to a desired oscillator frequency. A beat tone may be generated302based on the first and second optical signals. An LO may then be locked to the beat tone303.

Note that, in some embodiments, the LO locked to the beat tone303inFIG. 3may serve as the means by which to generate a signal of desired frequency202, as inFIG. 2. Thus, these methods may be linked together to provide an serial method of synchronizing LOs to an MO.

It is further noted that, although the techniques described herein have been described with a focus on synchronization of local oscillators being used to generate signals used in RF transmitting and/or receiving systems, the invention is not thus limited. It is contemplated that the techniques discussed above may have far broader applications, such as remote synchronization of signals of arbitrary frequencies, for a variety of purposes.

Various embodiments of the invention have now been discussed in detail; however, the invention should not be understood as being limited to these embodiments. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention.