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.
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, xe2x80x9cBragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase maskxe2x80x9d 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, xe2x80x9cProduction of in-fibre gratings using a diffractive optical elementxe2x80x9d 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 neff 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 xcexB (the Bragg wavelength) given by:
xcexB(z)=2p(z)xc3x1eff(z),xe2x80x83xe2x80x83(1)
Where xc3x1eff 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, xe2x80x9cNovel writing technique of long and highly reflective in-fibre gratingsxe2x80x9d 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, xe2x80x9cFabrication of chirped fibre gratings by novel stretch and write techniquexe2x80x9d Electron. Lett., pp.60-61 (1995) and K. Sugden, I. Bennion, A. Molony, and N. J. Copner, xe2x80x9cChirped gratings produced in photosensitive optical fibres by fibre deformation during exposurexe2x80x9d Electron. Lett., pp.440-442 (1994). There is also suggested in Y. Painchaud, A. Chandonnet, and J. Lauzon, xe2x80x9cChirped fibre gratings produced by tilting the fibrexe2x80x9d 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, xe2x80x9cMoving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase maskxe2x80x9d 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 xcexcm: 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, xe2x80x9cMagnification of mask fabricated fibre Bragg gratingsxe2x80x9d 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=xcex9/2xc2x7M,xe2x80x83xe2x80x83(2)
where                               M          =                      1            -                          q                              z                f                                                    ,                            (        3        )            
is the magnification factor, xcex9 is the phase mask period, q is the distance between the output surface of the phase mask and the fiber core and zf 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 zf 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.
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 portion of the optical grating; and
ii) providing a curvature in the light beam wavefront along the direction of the waveguiding axis, this curvature having a wavefront radius of curvature at a phase mask plane selected to generally optimize an efficiency of the recording of the optical grating for this characteristic period.
In the alternative, the present invention provides another method for recording an optical grating along a waveguiding axis in a photosensitive medium, the method comprising:
a) providing a phase mask proximate to the photosensitive medium along the waveguiding axis;
b) projecting a light beam through a portion of said phase mask to generate a light beam with a modulated intensity profile, said light beam with a modulated intensity profile impinging 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 said moving of the light beam:
i) moving the photosensitive medium in a direction parallel to the moving of the light beam, said moving of the photosensitive medium being adjusted relative to the moving of the light beam to locally tune the characteristic period of each portion of the optical grating; and
ii) providing a wavefront curvature in said light beam along the direction of the waveguiding axis, said curvature having a wavefront radius of curvature in a plane of the phase mask selected to generally optimize an efficiency of the recording of the optical grating for said characteristic period.
In accordance with another aspect of the invention, there is also provided an apparatus for recording an optical grating along a waveguiding axis in a photosensitive medium. The apparatus includes a phase mask provided proximate the photosensitive medium along the waveguiding axis. A light source is also provided, generating a light beam for projection 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.
Light beam moving means are provided for moving the light beam along the waveguiding axis of the photosensitive medium to successively record portions of the optical grating therealong, and phase mask moving means are also included for moving the phase mask in a direction parallel to the moving of the light beam and concurrently thereto. The moving of the phase mask is adjusted relative to the moving of the light beam to locally tune the characteristic period of each portion of the optical grating.
The apparatus finally includes curvature means for providing a curvature in the light beam along the direction of the waveguiding axis. The curvature has a wavefront radius of curvature in a plane of the phase mask selected to generally optimize an efficiency of the recording of the optical grating for the characteristic period of each portion of the optical grating.
Finally, in accordance with a preferred embodiment of the invention, there is provided a method for recording an optical grating along a waveguiding axis in a photosensitive medium, the optical grating comprising a plurality of superimposed grating components each having a characteristic period profile, the method comprising:
a) providing a phase mask proximate the photosensitive medium along the waveguiding axis;
b) for each of the superimposed grating component:
i) projecting a light beam through a portion of said phase mask to generate a light beam with a modulated intensity profile, said light beam with a modulated intensity profile impinging on the photosensitive medium to locally record therein a portion of the optical grating component having a characteristic period;
ii) moving the light beam along the waveguiding axis of the photosensitive medium to successively record portions of the optical grating component therealong; and
iii) concurrently to said moving of the light beam:
1) moving the phase mask in a direction parallel to the moving of the light beam, said moving of the phase mask being adjusted relative to the moving of the light beam to locally tune the characteristic period of each portion of the optical grating component; and
2) providing a curvature in said light beam along the direction of the waveguiding axis, said curvature having a wavefront radius of curvature in a plane of the phase mask selected to generally optimize an efficiency of the recording of the optical grating for said characteristic period.
Advantageously, the present invention allows to locally control the period of an optical grating over a large range (about xc2x12% of the nominal period) while keeping the optical quality of the grating unaffected. Other techniques have been proposed but result in a degradation of the optical performances.
Other aspects and advantages of the present invention will be better understood upon reading a preferred embodiments thereof with reference to the appended drawings.