Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-11-16
2003-02-04
Nguyen, Khiem (Department: 2839)
Optical waveguides
With optical coupler
Input/output coupler
C385S127000
Reexamination Certificate
active
06516118
ABSTRACT:
BACKGROUND
1. Field
The present invention relates to the field of optical fibers.
More specifically, the present invention relates to the design of an optical fiber suitable for producing energy-dissipative and wavelength-selective filters capable of acting as doped-fiber amplifier gain equalizing filters.
2. Description of the Related Art
The field of optical fibers has already given rise to a vast amount of literature.
An optical fiber is a waveguide. It is called a monomode fiber when only a single light mode propagates through it. This mode is called the fundamental mode.
Those skilled in the art know that certain dopants (germanium, fluorine, etc.) inserted into the fiber are sensitive to light radiation (ultraviolet radiation in the case of the dopants mentioned) and that the physicochemical reactions caused by this radiation may modify the refractive index of the photosensitive part.
A periodic perturbation of the refractive index of the photosensitive part of the fiber constitutes a Bragg grating.
The spectral response of short-pitch Bragg gratings produced in an optical fiber results from the coupling of the copropagative fundamental mode into counterpropagative cladding modes and into the guided coupling corresponds to the part which is reflected in the firection opposite to that of the fundamental mode (it is also called reflection or reflected power). The counterpropagative cladding modes are modes propagating in the cladding which surrounds the core of the fiber. If the grating is coated with a material whose index is close to that of silica, the cladding modes then become radiative modes. All the latter leak to the outside of the fiber (See, V. Mirahi and J. E. Sipe, “Optical properties of photosensitive fiber phase gratings”, IEEE, Journal of Lightwave Technology, Vol. 11, No. 10, October 1993; E. Erdogan, “Fiber Grating Spectra”, IEEE, J.L. Technology, Vol. 15 No. 8, 1997).
Moreover, those skilled in the art know that the fact of tilting the lines of the grating with respect to the axis of the fiber promotes coupling into the radiative modes to the detriment of guided counterpropagative coupling.
Such gratings have already been produced in a fiber having a photosensitive core in order to make the gain of doped-fiber amplifiers uniform. the results are presented in reference (See, R. Kashyap, R. Wyatt and R. J. Campbell, “Wideband gain flattened erbium fibre amplifier using a photosensitive fibre blazed grating”, Elect. Lett., Vol. 29, pp 154-165, 1993). An amplified spontaneous emission (ASE) spectrum has been equalized to ±0.5 dB at 35 nm. The reflected power problem is very briefly discussed. In reference, Erdogan shows that the change in the power reflected by the grating as a function of the angle of tilt of the lines is oscillatory (E. Erdogan, J. E. Sipe, “Tilted fiber phase gratings”, J. Opt. Soc. Am. A, Vol. 13, No. 2, 1996). He has shown that the amplitude of these oscillations is smaller in the case of the writing of gratings into a fiber in which part of the cladding is as photosensitive as the core (see, L. Brilland, D. Pureur, J. F. Baton and E. Delevaque, “Slanted gratings UV-written in photosensitive cladding fibre”, Elect. Letters, Vol. 35, No. 3, pp 234-236, 1999). Tilted gratings written into a fiber with a photosensitive cladding have already been used to equalize the 1 543.5 nm to 1 561.5 nm erbium gain band (see, I. Riant, L. Gasca, P. Sansonetti, G. Bourret, J. Chesnot, “Gain equalization with optimized slanted Bragg grating on adapted fibre for multichannel long-haul submarine transmission”, Optical Fiber Communication Conference, ThJ6, San Diego, February 1999).
However, as far as the inventors are aware, despite the extensive research conducted in the field, no gain equalizing filter composed of tilted Bragg gratings produced in a fiber with a photosensitive core and photosensitive cladding has yet been developed in order to make the gain of fiber amplifiers uniform over a wavelength range of more than 20 nm.
SUMMARY
The objective of the invention is to improve the known state of the art by proposing such a device.
This objective is achieved within the context of the present invention by means of an optical fiber whose core and part of the cladding are photosensitive to radiation, for example ultraviolet radiation, and in which fiber the difference in refractive index (&Dgr;n) between the core and the cladding is less than 3×10
−3
.
According to another advantageous feature of the present invention, the normalized frequency parameter V is less than 1.5 at a wavelength of 1.55 &mgr;m.
According to another advantageous feature of the present invention, the radius b of the photosensitive cladding is at least 1.5 times greater than the radius of the core a.
According to another advantageous feature of the present invention, the tilt of the lines of the Bragg grating with respect to the axis
0
—
0
of the fiber is great than 3°.
According to another advantageous feature of the present invention, the fiber comprises three to five Bragg gratings having different spectral characteristics.
REFERENCES:
patent: 5852690 (1998-12-01), Haggans et al.
patent: 6278817 (2001-08-01), Dong
E. Erdogan, J.E. Sipe, “Tilted fiber phase gratings”, J. Opt. Soc. Am. A, vol. 13, No. 2, 1996.
L. Brilland, D. Pureur, J.F. Baton and E. Delevaque, “Slanted gratings UV-written in photosensitive cladding fibre”, Elect. Letters, vol. 35, No. 3, pp. 234-236, 1999.
I. Riant, L. Gasca, P. Sansonetti, G. Bourret, J. Chesnot, “Gain equalization with optimized slanted Bragg grating on adapted fibre for multichannel long-haul submarine transmission”, Optical Fiber Communication Conference, ThJ6, San Diego, Feb. 1999.
G.W. Yoffe, P.A. Krugg, F. Ouelette, “Temperature-Compensated optical fiber Bragg gratings”, OFC 95.
V. Mirahi and J.E. Sipe, “Optical properties of photosensitive fiber phase gratings”, IEEE, Journal of Lightwave Technology, vol. 11, No. 10, Oct. 1993.
E. Erdogan, “Fiber Grating Spectra”, IEEE, J.L. Technology, vol. 15 No. 8, 1997.
R. Kashyap, R. Wyatt and R.J. Campbell, “Wideband gain flattened erbium fibre amplifier using a photosensitive fibre blazed grating”, Elect. Lett., vol. 29, pp 154-165, 1993.
Bayon Jean-François
Brilland Laurent
Delevaque Eric
Pureur David
Blakely & Sokoloff, Taylor & Zafman
Nguyen Khiem
SA Highwave Optical Technologies
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