In multi-wavelength optical fiber communication systems (usually referred to as "wavelength division multiplexed" or "WDM" systems), efficient wavelength-selective means for tapping electromagnetic radiation (to be referred to herein as "light", regardless of wavelength) from the optical fiber could be advantageously used in a variety of functions, e.g., as a wavelength monitor, channel monitor, demultiplexer, amplifier monitor, or in a feedback loop with an optical amplifier.
U.S. Pat. No. 5,061,032 to G. Meltz et al. discloses an optical fiber tap that comprises a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. The patent also discloses that ". . . the angle of the external path that results in the constructive interference is peculiar to the respective central wavelength .lambda.".
The tap of the '032 patent has some shortcomings. For instance, due to the relatively large (exemplarily .gtoreq.22.degree.) blaze angle that is required to achieve the desired redirection of the light guided in the fiber core to light in space outside of the fiber, the arrangement is subject to undesirable polarization effects, i.e., the fraction of light that is redirected by the grating depends on the polarization of the incident guided light. Whereas for low blaze angles (&lt;10.degree.) the polarization dependent difference in the amount of redirected light is at most about 0.54 dB, this difference increases rapidly with increasing blaze angle, being about 2.86 dB and about 6.02 dB for blaze angles of 22.degree. and 30.degree., respectively. Furthermore, as those skilled in the art will appreciate, the fraction of redirected light decreases with increasing blaze angle, for a given index change. See, for instance, T. Erdogan et al., J. of the Optical Society of America-A, Vol. 13(2), p. 296 (1996).
In view of the many important potential uses of an efficient, wavelength-selective fiber tap, it would be desirable to have available a fiber tap that is substantially free of the shortcomings of the above discussed prior art tap. This application discloses such a tap.