Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2002-03-18
2002-12-31
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S014000, C359S566000
Reexamination Certificate
active
06501883
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of components for optical telecommunications and more particularly concerns a method and a corresponding apparatus for recording optical gratings in a photosensitive medium with an enhanced control of the characteristics of the grating.
BACKGROUND OF THE INVENTION
Phase masks are widely used for the fabrication of UV-induced fiber Bragg gratings since their first reports (see for example K. O. Hill, B. Malo, F. Bilodeau, D. C. Johnson, and J. Albert, “
Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask
” Appl. Phys. Lett., pp.1035-1037 (1993); U.S. Pat. No. 5,367,588 (Hill et al.); D. Z. Anderson, V. Mizrahi, T. Erdogan, and A. E. White, “
Production of in-fibre gratings using a diffractive optical element
” Electron. Lett., pp.566-568 (1993); and U.S. Pat. No. 5,327,515 (Anderson et al.). The use of such a diffractive element renders easy the mass production of fiber Bragg gratings as the mask acts somewhat as a master replicated onto a large number of fiber Bragg gratings. However, a typical writing setup with a phase mask is not flexible and allows the fabrication of only one type of fiber Bragg gratings, that is, the one with the specifications prescribed by the phase mask.
The fiber is characterized by an effective index n
eff
that is modified by the UV radiation. A fiber Bragg grating is mainly characterized by the period p of the index modulation in the core of the fiber, along its axis. The fiber Bragg grating reflects light having a wavelength &lgr;
B
(the Bragg wavelength) given by:
&lgr;
B
(
z
)=2
p
(
z
)
ñ
eff
(
z
), (1)
Where ñ
eff
is the slowly varying effective index of the fiber inside the grating, z is the position along the grating and the dependence of the parameters over z indicates that both the period and the slowly varying effective index are not necessarily uniform along the grating. There is an interest in the control of the Bragg wavelength along a grating. This can be done by controlling the period of the grating along the fiber.
Translating a UV-beam along the phase mask is a convenient way to achieve long gratings (J. Martin, and F. Ouellette, “
Novel writing technique of long and highly reflective in-fibre gratings” Electron. Lett
., pp.911-812 (1994)). In particular, it allows a fine control of the apodisation, that is the strength of the grating, along the fiber axis.
Several techniques based on a phase mask but with enhanced flexibility have been proposed over the past few years. One of the most straightforward way to modify the grating period is by stretching the fiber, such as taught in K. C. Byron, and H. N. Rourke, “
Fabrication of chirped fibre gratings by novel stretch and write technique
” Electron. Lett., pp.60-61 (1995) and K. Sugden, I. Bennion, A. Molony, and N. J. Copner, “
Chirped gratings produced in photosensitive optical fibres by fibre deformation during exposure
” Electron. Lett., pp.440-442 (1994). There is also suggested in Y. Painchaud, A. Chandonnet, and J. Lauzon, “
Chirped fibre gratings produced by tilting the fibre
” Electron. Lett., pp.171-172 (1995) and U.S. Pat. No. 5,903,689 (PAINCHAUD et al.) to adjust the period by controlling the angles of both the phase mask and the fiber with respect to the UV-beam axis.
Referring to U.S. Pat. No. 6,072,926 (COLE et al.) and M. J. Cole, W. H. Loh, R. I. Laming, M. N. Zervas, and S. Barcelos, “
Moving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase mask
” Electron. Lett., pp.1488-1490 (1995), it is known to adjust the period by moving the phase mask. For the fine tuning of the Bragg wavelength, Cole proposed a lateral displacement of the phase mask during a writing process involving a scan of the UV beam. Excellent results have been obtained but the adjustment range is limited to about 1 nm.
FIG. 1
(PRIOR ART) shows the limit of the grating period adjustment when the UV beam diameter is 350 &mgr;m: the reflectivity decreases as a function of the detuning which corresponds to a decrease in the writing efficiency. The adjustment range increases as the UV beam size decreases. Cole also proposed a displacement of the phase mask at variable velocity for the adjustment of a chirp in the grating period.
On another hand, Prohaska, described in U.S. Pat. No. 5,351,321 (SNITZER) and J. D. Prohaska, E. Snitzer, S. Rishton, and V. Boegli, “
Magnification of mask fabricated fibre Bragg gratings
” Electron. Lett., pp.1614-1615 (1993) a technique for controlling the period of a Bragg grating over a large range (several nanometers) by using a magnifying lens along the UV beam axis. The right side of
FIG. 2
(PRIOR ART) shows the interference fringes at the output of a phase mask when a convergent UV beam is incident at the input surface. By placing a fiber at a distance q from the output surface of the phase mask, a grating will be photo-imprinted having a period p given by:
p=
&Lgr;/2
·M,
(2)
where
M
=
1
-
q
z
f
,
(
3
)
is the magnification factor, &Lgr; is the phase mask period, q is the distance between the output surface of the phase mask and the fiber core and z
f
is the distance between the output surface of the phase mask and the focal plane, that is the plane where the beam would be focalized. The distance z
f
also corresponds to the radius of curvature of the wavefront at the phase mask.
Oppositely, the left side of
FIG. 2
(PRIOR ART) illustrates the interference fringes at the output of a phase mask when a collimated beam is incident. In this case, the period of the grating is independent of the distance between the phase mask and the fiber.
The technique described by Prohaska allows an adjustment of the Bragg wavelength over a large range (several nanometers). However, the optical characteristics of the resulting grating are degraded: the photo-induced grating is slanted (blazed) in a spatially-dependent manner. Such a slanted fringes inside the grating causes a spatial dependence of the diffraction efficiency and increases significantly the polarization dependent loss and polarization mode dispersion. Another drawback is the need for uncommonly large lenses when a long grating is to be photo-induced, making the method more costly and unpractical.
There is therefore a need for a fabrication techniques for Bragg gratings or the like alleviating the above mentioned drawbacks of the prior art.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method of recording optical gratings in a photosensitive medium that is versatile and commercially practical.
It is another object of the present invention to provide an apparatus adapted to carry out such a method.
It is a preferable object of the invention to provide such a method that enables the recording of long gratings over a large wavelength range.
It is another preferable object of the invention to provide a method and apparatus for recording superimposed grating components in a photosensitive medium.
Accordingly, the present invention provides a method for recording an optical grating along a waveguiding axis in a photosensitive medium. The method includes:
a) providing a phase mask proximate the photosensitive medium along the waveguiding axis;
b) projecting a light beam through a portion of the phase mask to generate a light beam with a modulated intensity profile. The light beam with a modulated intensity profile impinges on the photosensitive medium to locally record therein a portion of the optical grating, having a characteristic period;
c) moving the light beam along the waveguiding axis of the photosensitive medium to successively record portions of the optical grating therealong; and
d) concurrently to the moving of the light beam:
i) moving the phase mask in a direction parallel to the moving of the light beam. The moving of the phase mask is adjusted relative to the moving of the light beam to locally tune the characteristic period of each portio
Chotard Hélène
Mailloux Alain
Painchaud Yves
Darby & Darby
Teraxion Inc.
Ullah Akm E.
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