A GRATING DESIGN

Abstract: The present invention provides an optical device incorporating a sampled grating structure having a chirped sampling period. The present invention may alternatively be defined as a method of producing a grating structure in a photosensitive optical waveguide, the method comprising the step of irradiating the device with UV light at an intensity sufficient to induce refractive index variations in the waveguide in a manner to produce a sampled grating structure, and wherein the radiation is controlled in a manner to effect chirping of the sampling period. The invention may alternatively be defined as providing a group delay dispersion compensator device comprising a sampled grating structure a having chirped sampling period. It has also been recognised that a method of producing a zero dispersion WDM channel can be provided.

Description: A GRATING DESIGN Field of the Invention The present invention relates broadly to a grating structure, method of writing the grating structure and devices incorporating such gratings . The present invention will be described herein with reference to grating structures for non-linear group delay dispersion compensation. However, it will be appreciated that the invention does have broader applications, such as for engineering of phase response of a fibre Bragg grating device . Background of the Invention Grating structures are widely used in optical waveguides for example as filters or as compensators for linear group delay dispersion. In many systems non-linear group delay dispersion, i.e. second and higher order group delay dispersion, plays a significant role. Therefore, it is desirable that a compensator structure be provided that can compensate for non-linear group delay dispersion in such systems. Summary of the Invention The present invention provides an optical device incorporating a sampled grating structure having a chirped sampling period. The optical waveguide may be in the form of an optical fibre. Alternatively, the optical waveguide may be in the form of a planar waveguide. The present invention may alternatively be defined as a method of producing a grating structure in a photosensitive optical waveguide, the method comprising the step of irradiating the device with UV light at an intensity sufficient to induce refractive index variations in the waveguide in a manner to produce a sampled grating structure, and wherein the radiation is controlled in a manner to effect chirping of the sampling period. The method may further comprise a step of applying an apodisation function during the UV-inducing of the refractive index variations to produce a smooth grating profile. This can help to avoid ripples. The photosensitive optical waveguide may comprise an optical fibre or a planar optical waveguide. The invention further provides an optical waveguide incorporating a sampled grating structure a having chirped sampling period. The invention may alternatively be defined as a method of compensating for group delay dispersion in an optical signal, comprising transmitting the optical signal through a sampled grating structure having a chirped sampling period. The invention may alternatively be defined as providing a group delay dispersion compensator device comprising a sampled grating structure a having chirped sampling period. Having made this invention, it has been recognised that a method of producing a zero dispersion WDM channel can be provided, the method comprising the steps of: filtering a narrow band optical signal from an input broad band optical signal using a square reflection band filter; using a sampled grating structure having a chirped sampling period to compensate for dispersion of the narrow band optical signal in the reflection band filter. It is noted here that the terms "narrow band" and "broad band" are not intended to be limited to a particular range, but rather to indicate the relative breadth of one when compared with the other. Further, the present invention provides a device for producing a zero dispersion WDM channel, the device comprising a square reflection band filter for filtering a narrow band optical signal from an input broad band optical signal, and following the optical filter, a sampled grating structure having a chirped sampling period for compensating for dispersion of the narrow band optical signal in the square reflection band filter. The device may comprise a circulator having a plurality of ports, the square reflection band filter being located at one of the ports for filtering the square amplitude narrow band optical signal from the input broad band optical signal entering the circulator at an input port, and the sampled grating structure being located at another port of the circulator to compensate for dispersion in the square band filter, the circulator further comprising an output port for outputting the dispersion- compensated narrow band optical signal. The invention has applications for both planar and cylindrical waveguides such as optical fibres. Preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Brief Description of the Drawings Figure 1A shows a typical refractive index profile of a grating produced by UV-induced refractive index variations . Figure IB shows a portion of the profile shown in Fig. 1A on an expanded length scale to more clearly show the refractive index variations in the grating. Figure 2 is a schematic drawing illustrating direct UV writing techniques. Figure 3 is a schematic drawing illustrating interferometric UV writing techniques . Figure 4A shows a refractive index profile of a sampled grating. Figure 4B shows a portion of the profile shown in Fig. 4A on an expanded length scale to more clearly show the refractive index variations in the grating. Figure 5A shows a refractive index profile of a grating embodying the present invention. Figure 5B shows a portion of the profile shown in Fig. 5A on an expanded length scale to more clearly show the refractive index variations in the grating. Figure 6 is a plot illustrating group delay dispersion of an apodised grating embodying the present invention. Figure 7A shows an apodised refractive index profile of a grating embodying the present invention. Figure 7B shows a portion of the profile shown in Fig. 7A on an expanded length scale to more clearly show the refractive index variations in the grating. Figure 8 shows a plot illustrating group delay dispersion of a WDM channel . Figure 9 is a schematic drawing of an optical device embodying the present invention. Figure 10 shows a plot illustrating the resulting group delay dispersion of the optical device of Figure 9. Detailed Description of the Preferred Embodiments In Figures 1A and IB, a typical refractive index profile 10 of a grating produced by UV-induced refractive index variations in a photosensitive waveguide material is shown. The profile is substantially sinusoidal, with a spatial period Λ. For Bragg gratings, typical spatial periods will be of the order of parts of micrometers such that the Bragg condition is fulfilled for a particular wavelength. Typically, the wavelengths of optical signals utilised in optical devices are between 1200 and 1600 nm. The refractive index profile 10 is achieved by utilising interference of UV light beams for UV-inducing the refractive index variations in a photosensitive material, either through direct writing techniques (see Figure 2) or interferometric techniques (see Figure 3) . In sampled gratings the amplitude of the refractive index variation (e.g. sinusoidal variation) is varied periodically, resulting in a refractive index profile 40 as illustrated in Figures 4A and 4B. A typical sampling period length would be of the order of millimeters. From the above it follows that whilst the spatial period of the grating, which is typically of the order of parts of micrometers, is a parameter which is experimentally difficult to control and/or manipulate, the sampling spatial period is experimentally relatively easy to control and/or manipulate. As illustrated in Figures 5A and 5B, the refractive index profile 50 of a sampled grating for which the sampling period has been chirped, the spatial period of the sinusoidal "envelope" 52 due to the sampling function decreases along the length of the grating. Importantly, the period of the grating Λ 2 remains constant throughout the entire length of the grating, thereby placing no special demands on the writing of the short period structure. Only the relatively "long" period of the sampling function needs to be varied. It is noted here that for illustrative purposes the sampling period lengths of Figures 5A and 5B have been set to higher values as they would typically be in a real system. In Figure 6, the group delay dispersion 60 of an example sampled grating written with a chirped sampling period is shown. The sampling function is: {cos[(K 0 + AK(z))z] + cos[(^ 0 " Δ*T(z))z]} / 2 = COS[K Q Z]COS[AK(Z)Z] . Furthermore, an apodisation function has been applied in the form of a function which monotonically decreases from a starting value at the beginning of the grating to zero at the end of the grating. The refractive index profile 62 of the resulting grating is shown in Figures 7A and 7B. It will be appreciated that the group delay dispersion shown in Figure 6 can be utilised to compensate for non- linear group delay dispersion, for example for non-linear group delay dispersion in a WDM channel. In Figure 8, the group delay dispersion 80 of a WDM channel is illustrated. The group delay dispersion is substantially inverse to the group delay dispersion 60 of the example grating structure (see Figure 6) and it will be appreciated by a person skilled in the art that through appropriate selection of the sampling function and apodisation function, group delay dispersion in WDM channels can be compensated using a sampled grating for which the sampling period has been chirped. In Figure 9, an optical device 90 comprises a circulator 92 having a sampled grating structure 91 with a chirped sampling period at one port 94 and a grating filter 96 optimised for "square" reflection band amplitude response at another port 98. An incoming broad band optical signal 100 entering the circulator at an input port 102 will initially propagate to the grating filter 96, of which a narrow band signal (not shown) within the square reflection band is reflected back into the circulator 92. The narrow band signal is then reflected at the sampled grating 91 having the chirped sampling period, whereby an output signal 106 leaving the circulator 92 at an output port 108 will be a narrow band optical signal with substantially zero group delay dispersion within the square-shaped amplitude "channel". In other words, the group delay is substantially constant within the square- shaped amplitude channel, as shown in Figure 10, portion 110 of graph 112. It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive, For example, apodisation functions other than the one described could be used during the writing of the sampled grating with a chirped sampling period.

Claims: The Claims Defining the Invention are as Follows: 1. An optical device incorporating a sampled grating structure having a chirped sampling period. 2. An optical waveguide incorporating a sampled grating structure a having chirped sampling period. 3. A group delay dispersion compensator device comprising a sampled grating structure having a chirped sampling period. 4. A device for producing a zero dispersion WDM channel, the device comprising a square reflection band filter for filtering a narrow band optical signal from an input broad band optical signal, and following the optical filter, a sampled grating structure having a chirped sampling period for compensating for dispersion of the narrow band optical signal in the square reflection band filter. 5. A device in accordance with claim 4, comprising a circulator having a plurality of ports, the square reflection band filter being located at one of the ports for filtering the square amplitude narrow band optical signal from the input broad band optical signal entering the circulator at an input port, and the sampled grating structure being located at another port of the circulator to compensate for dispersion in the square band filter, the circulator further comprising an output port for outputting the dispersion-compensated narrow band optical signal. 6. A method of producing a grating structure in a photosensitive optical waveguide, the method comprising the step of irradiating the device with UV light at an intensity sufficient to induce refractive index variations in the waveguide in a manner to produce a sampled grating structure, and wherein the radiation is controlled in a manner to effect chirping of the sampling period. 7. A method of compensating for group delay dispersion in an optical signal, comprising utilising a sampled grating structure having a chirped sampling period. 8. A method of producing a zero dispersion WDM channel, the method comprising: filtering a narrow band optical signal from an input broad band optical signal using a square reflection band filter; using a sampled grating structure having a chirped sampling period to compensate for dispersion of the narrow band optical signal in the reflection band filter. Dated this 21st day of September 2000 The University of Sydney By their Patent Attorneys GRIFFITH HACK

Inventors: SCEATS MARK, STEPANOV DMITRII YU, SCEATS, MARK, STEPANOV, DMITRII, YU
Applicants: SCEATS MARK, STEPANOV DMITRII YU, UNIV SYDNEY, SCEATS, MARK, STEPANOV, DMITRII, YU, THE UNIVERSITY OF SYDNEY
Application year: 2000
Main classification: G02B5
IPC codes: G02B5, G02B6