The fiber optics telecommunications area includes such technologies as fiber optical cables and fiber optical networks. Fiber optical networks carry a great variety of everyday information, such as conversations, data communications (e.g., fax messages), computer-to-computer data transfers, cable television, the Internet, and so forth. Such information signals are transported between cities as well as from place to place within cities. Due to the rapidly increasing amounts of such communication traffic, the increased capacity of fiber optical cables is more and more necessary, compared to the lower capacities of older metallic wire cables.
An optical fiber cable is typically composed of a bundle of individual optical fibers. One single optical fiber can carry thousands of data and communication signals on a single wavelength of light. That same single optical fiber can also carry multiple wavelengths of light, thus enabling it to carry many, many multiple optical signals at the same time. Each wavelength alone can carry data transferring at a rate over 10 Gbit/s.
To maintain communications over such optical networks, it is necessary to perform a variety of sensitive analyses, such as measuring the optical power, wavelength, and the optical signal-to-noise ratio of the optical signals at each of the wavelengths traveling through the optical fiber. Such analysis is carried out by an analytical tool called an optical spectrum analyzer (“OSA”). The OSA performs optical spectrum analysis (also referred to as “OSA”), which, as indicated, is the measurement of optical power as a function of wavelength.
OSA is typically performed by passing an optical signal to be analyzed through a tunable optical filter. “Tunable” means that the filter can be adjusted to resolve or pick out the individual components (wavelengths) of the optical signal.
Three basic types of filters are widely used to make OSAs: diffraction gratings, Fabry-Perot (“FP”) filters, and Michelson interferometers. A tunable FP filter (“TFPF”) has many advantages for OSA. Principal among these are its relatively simple design, small size, fast speed, ease of control, and its great sensitivity for distinguishing optical signals that are very closely spaced (i.e., signals that have frequencies or wavelengths that are very nearly the same.)
However, as compared with a diffraction grating with the same 3-dB bandwidth (which is defined as the magnitude of wavelength or frequency difference between the left and right spectral positions at 3-dB down from the peak position), the transmission profile of a TFPF has a relatively “broad skirt”. The broad skirt means that beyond the 3-dB bandwidth (“BW”) spectrum position, for example, the TFPF has a relatively slow decay of the rejection ratio to optical signals that are nearby in frequency or wavelength to the signals of interest. Such a broad skirt can be a considerable disadvantage for TFPFs when used to measure the optical-signal-to-noise-ratio (“OSNR”) of signals of a wavelength division multiplexing (“WDM”) system. This can allow signals from nearby, or adjacent, wavelengths to leak through and raise the “noise” floor artificially. The relatively broad skirt admits cross talk from adjacent WDM channels, thereby limiting the FP OSA's dynamic range (“DR”) for OSNR measurements.
In contrast, with the same 3-dB BW the transmission profile of a diffraction grating has a much steeper skirt, but it is not so sensitive at distinguishing optical signals that are very closely spaced as compared with a TFPF. Theoretically, every optical filter admits cross talk from adjacent WDM channels. Due to its steeper skirt, a diffraction grating has much smaller cross talk than the FP filter with the same 3-dB BW.
Thus, a considerable need remains for methods and apparatus that can greatly enhance the DR for OSNR measurements of a FP filter-based OSA. In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.