Patent Application
20080031563
The accompanying drawings, which are incorporated herein and form part of the specification, help illustrate various embodiments of the present invention. In the drawings, like reference numbers indicate identical or functionally similar elements.
If the thermal environment is known in detail along the full length of the remoting fiber, then the variation in phase due to temperature changes could be corrected for by employing a look-up table approach. In some situations, however, this may be difficult, especially for physically long platforms (e.g., large space based antennas, large UAVs, distributed antennas, etc.). In these types of cases, a more practical solution involves the measurement of the fiber length/phase. As described below, embodiments of the invention enable the measurement of the fiber length/phase.
In step 204, the monitoring signal modulates light produced by a light source. Because the monitoring signal modulates the light, the modulated light produced by the light source “carries” the monitoring signal.
In step 206, the modulated light is injected into one end of an optical fiber.
In step 208, the modulated light, after traveling the length of the optical fiber, is reflected so that it travels along the fiber back towards the light source. In step 210, the reflected light, after traveling through the fiber of interest, is provided to an optical detector, which demodulates the light, thereby producing a phase-shifted monitoring signal.
In step 212, the phase-shifted monitoring signal and the original non-phase-shifted monitoring signal are provided to a mixer. In step 214, the output of the mixer is filtered to produce a DC value. The produced DC value is proportional to the phase shift caused by the fiber of interest. In this manner, the method enables one to measure the phase shift caused by changes in the fiber length.
As further illustrated in
Reflector assembly functions to: (1) separate λRF and λPH; (2) allow λRF to travel to the optical detector 318, (3) send λPH to an optical mirror 322, and (4) direct the mirror-reflected λPH (called λPR) so that the mirror reflected light (λPR) propagates backwards towards the circulator 312. The circulator 312 sends λPR to an optical detector 324, which outputs the “reflected” monitoring RF modulation called fPR. Note that fPR has the same frequency as fPH but its phase is shifted proportionally to twice the fiber-induced phase shift. The two phase-monitoring signals, i.e., the forward and reflected versions, may then be multiplied via the mixer 306. The mixer's output may be passed through a low pass filter 308.
The filter's output is a DC signal, the value of which is directly proportional to twice the overall phase shift. This DC output contains all the phase information needed to correct the link phase/length changes vs temperature; it can be digitized and compared to a lookup table to be used by remoting signal generator to compensate for the phase shifting introduced by fiber 314.
In one embodiment, the “monitoring” frequency (fPH) is selected such that its wavelength is greater or equal to four times (4×) a desired resolution. The reason for the first 2× factor is because the phase change (or delay) is twice the actual phase change (one delay length out and one delay length on the reflected path). The second 2× factor is because only 180° of the signal period can be used. If the signals are in phase there is 0° of phase shift and the DC value is maximum. If they are 180° out of phase, there is 180° of relative phase shift and the DC value is minimum. For any value over 180° of delay the DC value will be approaching the maximum again, thus creating an ambiguity. As an example, consider a temperature range of −100° C. to 100° C. and a fiber length (e.g., brown,
deltaL-fiber=(delay slope)(100 m)(200 C)(effective speed of light), we find that the change in fiber length (deltaL-fiber) is equal to 0.133 m for a fiber with an index of refraction of 1.4682. The frequency required to cover that range would have an RF wavelength 4× or 0.532 m. By taking into account the speed of light through a fiber divided by the wavelength, the input frequency, fPH, is found to be 354 MHz.
Embodiments of the invention can be used for any microwave/digital photonic link and is independent of the actual RF remoting frequency.
While various embodiments/variations of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
Additionally, while the process described above and illustrated in the drawings is shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed simultaneously.
