Wavelength monitoring optical fibers using detection in the near field
The specification describes a wavelength monitoring system for multiple wavelength communications systems, such as WDM systems, based on the recognition that the mechanism for spatially separating the individual wavelength bands can be achieved within the optical fiber itself. Individual wavelength bands are separated using a series of discrete gratings spaced longitudinally along the fiber core. The wavelength bands are extracted from the fiber core by converting the energy in the selected band from a core-guided mode to a radiation mode. By using a tilted grating, the light in the radiation mode is directed through the cladding and out of the fiber. Spatial resolution of the selected bands can be any desired physical length. An important implication of this is that detection can be made in the near field using inexpensive detecting apparatus. Near field is defined for convenience in this case as the optical field of the radiated energy without any optically modifying elements (collimating devices, diffraction elements, mirrors, etc.). This eliminates nearly all of the optics, and the attendant expense, in prior art monitoring systems.
1. An optical system comprising:
a. a length of optical fiber having a core and a cladding,
b. lightwave signal means for introducing a lightwave signal into the core, the lightwave signal comprising N wavelengths where N is at least three,
c. a plurality of N refractive index gratings within the core, each of the N gratings adapted for selecting one of the N wavelengths, and
d. a plurality of N photodetectors with a photodetector associated with each of the N gratings and wherein the N photodetectors:
1) are located adjacent to and within 0-1 mm of the surface of the optical fiber, and
2) have a length L as measured parallel to the axis of the optical fiber, and adjacent gratings have a center-to-center spacing of at least 1.5L.
2. The optical system of claim 1 further characterized by the absence of optically dispersive means between the photodetector and the optical fiber.
3. The optical system of claim 1 wherein the photodetector contacts the surface of the optical fiber.
4. The optical system of claim 1 further including a narrow band filter incorporated in the photodetector.
5. The optical system of claim 1 wherein the gratings are tilted Bragg gratings that have a grating period less than the vacuum wavelength divided by the mode effective index.
6. The optical system of claim 1 wherein the gratings are long period gratings.
7. The optical system of claim 1 wherein the gratings are chirped gratings.
8. The optical system of claim 1 wherein the photodetectors comprise semiconductor diodes.
9. The optical system of claim 1 wherein the lightwave signal is a WDM signal.
10. The optical system of claim 1 wherein the gratings select less than 10% of the optical power of the signal wavelength.
11. The optical system of claim 1 wherein the signal wavelengths are separated by >20 nm.
12. The optical system of claim 1 wherein the photodetectors provide a monitoring signal and the monitoring signal activates a feedback loop to a source of the signal.
13. The optical system of claim 1 wherein the photodetectors provide a monitoring signal and the monitoring signal activates a feedback loop to a dynamic wavelength or channel dependent attenuator.
14. The optical system of claim 1 wherein the wavelength selection means select channels from a multiplexed signal, and the channels are separated by >10 nm.