Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-02-10
2003-04-08
Nguyen, Thong (Department: 2812)
Optical: systems and elements
Diffraction
From grating
C359S569000, C359S570000, C359S574000, C359S575000, C359S015000, C359S034000, C385S037000
Reexamination Certificate
active
06545808
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical elements for the fabrication of diffraction gratings. More particularly, it relates to a non-uniform or apodized phase mask used for recording Bragg gratings or other high spatial frequency grating structures into an optical medium, the gratings having an apodized profile.
BACKGROUND OF THE INVENTION
Already known in prior art, there is a uniform phase mask, which is a phase diffraction grating usually recorded in photoresist and then etched in a bulk substrate as a one-dimensional periodic surface-relief pattern. Phase masks are generally used in transmission to record Bragg gratings in the core of photosensitive optical fibers or on optical waveguides. The use of a phase mask significantly decreases the coherence requirements on the writing laser in comparison to holographic direct writing techniques. Ultraviolet (UV) light is usually used to imprint a Bragg grating into fibers or in photoresist, and therefore the phase mask material is preferably transparent to UV illumination.
The phase mask technique is described in U.S. Pat. No. 5,367,588 (HILL et al.). Hill discloses a method of fabricating Bragg gratings in the interior of a photosensitive optical waveguide comprising disposing a silica glass phase grating mask adjacent and parallel to the optical waveguide, and applying a single collimated light beam through the mask to said medium. The grating on the mask is so designed that the power in the transmitted zero diffraction order beam is minimized (in general to less than 5% of the light diffracted by the mask) and the diffracted power in the plus and minus first transmitted diffraction orders is maximized (each typically contain more than 35% of the incident light) over the entire grating surface. When illuminated at normal incidence with coherent or partially coherent light, the phase mask produces in its near-field a self-interference between the first order diffracted beams. This interference fringe pattern will have a spatial frequency that is twice the spatial frequency of the phase mask, and will be photo imprinted in the photosensitive medium placed in close proximity of the phase mask grating.
A variation of the above phase mask geometry, which uses off-normal illumination and the interference of the transmitted zero and minus-first diffraction orders, is also possible with small period phase masks and is disclosed in U.S. Pat. No. 5,327,515 (ANDERSON et al.) and U.S. Pat. No. 5,413,884 (KOCH et al.). In this case, the phase mask and the generated interference pattern have the same periodicity. If the phase mask period &Lgr; has a size within the interval 0.5&lgr;<&Lgr;<1.5&lgr;, where &lgr; designates the readout wavelength in the medium, the phase mask generates, at incidence angles close to the Bragg angle, only the transmitted zero and minus-first diffraction orders. Since no diffraction orders have to be suppressed, the fabrication tolerances for the grating shape are less severe compared to the previous approach. This off-axis geometry has been proposed in the Anderson patent for the recording of Bragg gratings in an optical medium, and the Koch patent discloses a method of making such a mask.
Traditional phase masks comprise a uniform one-dimensional grating structure of constant period and relief depth, allowing for a constant diffraction efficiency of the mask. This generates a uniform periodic modulation of the refractive index in the photosensitive medium. The resulting device acts as a Bragg filter. Typically the Bragg spectral response will be a narrow-band reflectivity peak of nearly 100% at the design wavelength, accompanied by a series of sidelobes at adjacent wavelengths. The calculated reflectivity spectrum of a uniform Bragg grating is shown in
FIG. 1
(prior art). It is important to reduce the reflectivity of the sidelobes in the Bragg response in devices where cross-talk cannot be tolerated, such as in devices for wavelength demultiplexing applications. Bragg gratings with reduced sidelobes in their spectral response can be achieved by recording an index modulation with a spatially varying modulation amplitude (see for example M. Matsuhara and K. O. Hill, Appl. Opt., 13, pp. 2886-2888 (1974) and I. Bennion et al., “UV-written in-fibre Bragg gratings”, Tutorial review, optical and Quantum Electronics 28, pp. 93-135 (1996)). This generates a Bragg grating with a spatially varying coupling efficiency, which is called apodized. The condition for an efficient sidelobe suppression is that the modulation level decreases continuously to zero at both limits of the Bragg grating. Appropriate apodization profiles for this task are for example Gaussian, Blackman or Hamming functions or one period of the cos
2
function (see for example D. Pastor et al., Journal of lightwave Technology, vol 14, no 11, pp. 2581-2588 (1996)).
Different approaches have been investigated to realize Bragg gratings generating a spectral response without sidelobes, with or without the use of a phase mask. Most of these approaches are variations based on three methods which will be described and discussed below.
The first of these methods concerns the use of a beam-profile shaping filter. In this approach, the intensity profile of the recording beams is appropriately shaped by means of absorptive, diffractive or Fabry-Perot apodizing filters, in order to generate the desired index modulation in the recording medium. Beam-shaping filters can be placed in the single beam incident on a uniform phase mask or in the two recording beams required for direct holographic writing in the fiber. However, with the beam-shaping filter technique, two successive recording steps with different filters have to be applied, in order to generate a narrow-bandwidth spectral Bragg response without sidelobes. The first step produces the periodic index modulation profile, whereas the second step compensates variations of the average refractive index in the recording medium, which is necessary to generate a single reflected Bragg wavelength (B. Malo et al. “Apodised in-fibre Bragg grating reflectors photo imprinted using a phase mask”, Electr. Lett. Feb. 2, 1995, Vol. 31, No. 3, pp. 223-224).
The main drawback of the beam-shaping filter technique is that it is time consuming, difficult to use and therefore not appropriate for industrial production. It implies having to perform two exposure steps. The first exposure with apodized beams will induce the desired index modulation profile, but without a second exposure the local average refractive index will also vary, introducing an unwanted chirp. It is therefore necessary to expose a second time with a compensated beam to equalize the average refractive index. This second exposure step needs very precise alignment with respect to the first exposure. In addition, it is difficult to calibrate, since the response of the recording medium changes after the first exposure. Therefore, the final response of the Bragg grating is difficult to control in practice.
A second approach to write a Bragg grating having a reflection response with reduced apodized secondary maxima is the scanning beam technique. Variable exposure energies can be achieved by scanning focused beams or a slit mask in the recording beams with respect to the recording medium. The exposure energy is locally controlled by varying either the scanning speed or the optical power of the recording beams. The technique can be applied to recording with a single uniform phase mask or to direct holographic writing. As with the apodizer technique, two successive recording steps are required to generate a selective apodized Bragg grating (see J. Martin and F. Ouellette, “Novel writing technique of long and highly reflective in-fibre gratings”, Electr. Lett., 30, 811-812 (1994)). An interesting implementation of this technique has been proposed in M. J. Cole et al. “Moving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase mask”, Electr. Lett., 31, 1488-1490 (1995). It is shown t
Capolla Nadia
Ehbets Peter
Poirier Michel
Xu Zhihong
Boutsikaris Leo
Institut National d'Optique
Nguyen Thong
LandOfFree
Phase mask with spatially variable diffraction efficiency does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Phase mask with spatially variable diffraction efficiency, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phase mask with spatially variable diffraction efficiency will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3010890