Bragg grating variable optical attenuator

An attenuator capable of controllably attenuating at least two wavelengths of light is configured to have an optical waveguide section having at least two Bragg gratings disposed therein in series. The Bragg gratings have a sloped reflectivity response within a predetermined wavelength range. The sloped response is a function of refractive index variations within each of the grating elements. By compressing or expanding at least one of the gratings, the periodicity is modified so as to shift its central wavelength within a predetermined wavelength range.

FIELD OF THE INVENTION 
This invention relates generally to a Bragg grating disposed within a 
waveguide and more particularly, the principal focus of this invention 
relates to but is not limited to Bragg gratings embedded in the core of an 
optical fibre. 
BACKGROUND OF THE INVENTION 
Various constructions of optical filters are known however so-called Bragg 
filters are gaining popularity. One type of a Bragg filter, which has been 
developed for sensing stress and/or temperature changes in structures is 
incorporated or embedded in the core of an optical fiber by a method 
disclosed, for instance, in U.S. Pat. No. 4,807,850. As is discussed in 
this patent permanent periodic gratings of this kind can be provided or 
impressed in the core of an optical fibre by exposing the core through the 
cladding to the interference pattern of two coherent beams of ultraviolet 
light that are directed against the optical fibre symmetrically to a plane 
normal to the fiber axis. This results in a situation where the material 
of the fiber core has permanent periodic variations in its refractive 
index impressed therein by the action of the interfering ultraviolet light 
beams thereon, with the individual grating elements (i.e. the periodically 
repetitive regions of the core exhibiting the same refractive index 
behavior) being oriented normal to the fiber axis so as to constitute the 
Bragg grating. The embedded Bragg grating of this kind reflects the light 
launched into the fiber core for guided propagation therein, in a 
propagation direction; only that light having a wavelength within a very 
narrow range dependent on the grating element periodicity is reflected 
back along the fibre axis opposite to the original propagation direction, 
while being substantially transparent to light at wavelengths outside the 
aforementioned narrow band so that it does not adversely affect the 
further propagation of such other light. In effect, this type of grating 
creates a narrow notch in the transmission spectrum, and by the same token 
a similarly narrow peak in the reflection spectrum. In the applications 
for which this kind of Bragg filter has been developed, any stresses or 
temperature changes encountered at the location of the structure in which 
the Bragg filter is embedded affect the grating and change its 
periodicity, or the index of the refraction of the material of the core, 
or both so that the position of the central wavelength in the spectrum is 
shifted, thus providing an indication of the stress or temperature changes 
existing or taking place in the structure being monitored at the location 
of the grating. Although the significance of these applications of Bragg 
gratings are formidable, further development in this area is disclosed in 
U.S. Pat. No. 5,007,705 (hereafter referred to as the '705 patent) that 
relates to a different aspect or use of these earlier discovered 
principles. In the '705 patent various means are disclosed for 
intentionally shifting the reflection wavelength response of a Bragg 
grating. By deliberately varying the period of the grating or altering the 
index of refraction in a predetermined manner, by external forces or 
actions on the fibre section containing the grating in a controlled 
manner, a variable light filtering element is provided. Furthermore, 
tuning a grating by various means such as the application of heat, 
compression, or stretching are all known. 
One useful application of the principals described heretofore, can be found 
in U.S. Pat. No. 5,446,809 in the name of Fritz et al. who discloses an 
optical fiber wavelength selective optical switch, utilizing tunable Bragg 
fibre gratings. The fiber wavelength selective switch has one or more 
1.times.N input optical couplers and utilizes a plurality of in-line Bragg 
fiber gratings in series along multiple parallel paths. For a given 
wavelength of light to pass through a particular grating, the grating must 
be detuned. By providing a plurality of Bragg gratings in series, each 
designed to reflect a different wavelength, and having means for 
controlling or shifting the response of each grating individually, signals 
can selectively be passed through a fibre or can be reflected backwards in 
a binary on-off fashion. 
Although the prior art describes a plurality of modes and means for varying 
the output response of a Bragg grating within an optical fibre, there 
remains a need for useful application of this technology that provides a 
non-binary response. For example, there remains a need for a variable 
attenuator or equalizer that utilizes Bragg grating technology. 
Therefore, it is an object of this invention to provide an optical 
equalizer that incorporates Bragg gratings. 
It is yet a further object of the invention to provide a multi-channel 
equalizer. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided, an attenuator capable 
of controllably variably attenuating at least two wavelengths of light 
independently, comprising: an optical waveguide section having a plurality 
of Bragg grating elements disposed therein in series, at least two Bragg 
grating elements having an inclined or sloped reflectivity response within 
a predetermined wavelength range that is determined by the periodicity and 
refractive index variations of the grating elements; and, means for so 
applying at least to one grating element of the optical waveguide section 
an external influence to modify the periodicity of the at least one 
grating element to shift its central wavelength within a predetermined 
wavelength range. 
In accordance with the invention, there is provided, an attenuator capable 
of controllably attenuating at least two wavelengths of light 
independently, comprising: an optical waveguide section having a plurality 
of Bragg grating elements disposed therein in series, at least two Bragg 
grating elements having a sloped reflectivity response within a 
predetermined wavelength range that is determined at least in part by 
refractive index variations of the grating elements; and means for so 
applying at least to one grating element of the optical waveguide section 
an external influence to modify a periodicity of the at least one grating 
element to shift its central wavelength within a predetermined wavelength 
range. 
In accordance with the invention, there is further provided, a wavelength 
selective optical attenuator for attenuating an input signal having a 
predetermined wavelength, comprising: an optical waveguide having an input 
port and an output port; at least one optical reflective element located 
along said optical waveguide between the input port and the output port, 
said reflecting element reflecting a predetermined wavelength band of 
light centred at a predetermined central wavelength, said central 
wavelength being at a base wavelength when said reflective element is not 
detuned, said base wavelength including a predetermined offset 
corresponding to the predetermined wavelength of said input signal, said 
optical reflective element having a reflectivity along its length that 
varies such that one end of the grating is substantially more reflective 
than another end; and, tuning means, attached to said reflective element 
for detuning said central wavelength away from said base wavelength and 
towards the wavelength of the input signal so as to attenuate said 
corresponding wavelength of said input signal.

DETAILED DESCRIPTION 
As was described heretofore, U.S. Pat. No. 5,446,809 discloses an optical 
switch shown in FIG. 1, wherein Bragg gratings are used as controllable, 
selective, transmissive/reflective binary elements that are capable of 
transmitting a predetermined wavelength of light or alternatively 
reflecting that wavelength in dependence upon the period of the grating. A 
piezo-electric transducer is coupled to each grating; and when a 
particular transducer is energized it stretches the grating, changing its 
period, and thus changes its reflectivity response by shifting it in 
wavelength. In this patent the Bragg gratings operate in a binary "on-off" 
manner to either reflect a particular wavelength of light or transmit that 
wavelength of light. 
Referring generally now to FIGS. 2a to 2c, an equalizer circuit is shown 
wherein Bragg gratings or elements are utilized in a controllable manner 
to attenuate a multiplexed input optical beam comprising three signals of 
three wavelengths, .lambda.1, .lambda.2, and .lambda.3. In the drawing, 
the three signals on the left side are unequal in amplitude, .lambda.2 
having the least intensity, then .lambda.1 followed by .lambda.3 having 
the highest intensity. In many instances it is preferred to have the 
optical signals equalized, wherein their amplitudes are as close as 
possible to one another. One use of the equalizer of this invention is 
with rare-earth doped optical fiber amplifiers. 
One limitation of any rare-earth doped optical fibre amplifier is unequal 
gain over a range of frequencies or optical channels of interest, as well 
as for various input signal strengths (i.e. different saturation levels). 
Over a 35 nanometer gain bandwidth, erbium doped fibre amplifiers (EDFAs) 
typically exhibit a 10 to 15 dB small-signal gain variation. In long 
chains of cascaded EDFAs small spectral gain variation can result in 
unacceptable large difference in received optical power and therefore, it 
is preferable to lessen even small spectral variation in gain. 
To date, several gain equalization and flattening techniques have been 
proposed and described in a variety of prior art references. For example, 
gain clamping with enhanced inhomogeneous saturation is described by 
V.S.da Silva et al in Proc. OFC'93. paper THD2, P.174, 1993. One of the 
limitations of this method is the requirement that fibre be cooled to 77 
K. The use of passive internal/external filters has been explored by M. 
Tachibana, et al in IEEE Photonics Technol. Lett. 3, no. 2, 118, 1991, by 
M. Wilkinson et al. in Electron. Lett. 28, no. 2, p. 131, 1992, and by 
Kashyap et al in Electron. Lett. 29, no. 2, P.154, 1993, and as well by 
Grasso et al, in Proc. OFC'91, paper FA3, p. 195, 1991. Another attempt to 
provide a doped optical fibre amplifier that is suitable for use over a 
range of frequencies is described in U.S. Pat. No. 5,245,467 entitled 
Amplifier with a Samarium-erbium Doped Active Fibre, issued Sep. 14, 1993 
in the name of Grasso et al. Although the invention described in the 
patent works well at particular wavelengths and for particular signal 
strengths, it has been found to be limited at other wavelengths. However, 
a major limitation with most of these devices and methods is the 
requirement for bulk optics and non-standard components. The use of 
external active acousto-optic filters has been explored by S. F. Su et. al 
in IEEE Photonics Technol. Lett. 4, no. 3, p.269, 1992; the drawback with 
this proposal is that it requires bulk optics, and is complex in design. 
The exemplary circuit shown in FIG. 2a in conjunction with the responses 
shown FIGS. 3b and 3c can be used to offset the unequalized gain over a 
wavelength region exhibited from commercially available erbium doped 
amplifiers; or, can be used to vary or offset the relative strengths of 
particular wavelengths prior to amplification so as to obtain a relatively 
equalized signal after amplification. Although only three Bragg gratings 
102, 104, and 106 are shown, other gratings can be included and chained in 
series tuned to other wavelengths of interest. 
Referring specifically now to FIG. 2a, a circuit is shown wherein three 
Bragg gratings 102, 104, and 106 are shown in series written into an 
optical fiber 101. Tuning means in the form of individually controllable 
piezo-electric transducers 102a, 104a, and 106a are coupled to the 
respective gratings each for changing the period of a grating in 
dependence upon a control signal. A tuning control circuit 108 can be 
preprogrammed or programmed in real-time for issuing appropriate control 
signals to the transducers. Each of the Bragg gratings 102, 104, and 106 
are designed to have a different period, and are consequently designed to 
affect a different channel. In one embodiment, the gratings are chirped, 
appodized gratings having a sloped or inclined reflectivity response as 
shown in FIG. 3b. Therefore the refractive index difference .DELTA.n 
between adjacent regions within each of the gratings varies along the 
length of the chirped grating so as to provide a sloped reflectivity 
response over a predetermined wavelength region. Of course, other methods 
of forming reflective/transmissive elements having a variable reflectively 
response can be envisaged. One way of fabricating the grating is to 
provide a standard linear chirp using a mask; when exposing the fibre an 
amplitude mask can be utilized so that one side of the grating is exposed 
more so than the other, resulting in a periodicity that varies in 
refractive index rather than in distance, providing a variable masked 
chirp. The effective .DELTA.n can be varied by varying the width of 
.DELTA.n sections. Another means of providing a sloped region is by using 
a very short hydrogen loaded grating, for example one having a 2 nm 
bandwidth, can be utilized; by providing such a grating, tuning can be 
achieved by utilizing the falling or rising edge of the gratings 
reflectivity response. By tuning or wavelength shifting these gratings a 
variable reflectivity response and consequently a variable amount of 
attenuation is provided. Another means of achieving a chirp in a grating 
is by stretching or bending a grating having a non-varying period in 
non-uniform manner. For example, an optical fibre 400 having gratings 410 
shown in prior art FIG. 4 are shown being bent over a form 420 having 
non-uniform bend radius. By doing so, the period of the grating is changed 
in such a manner as to vary it along its length, thereby chirping the 
grating The authors of this prior art reference, Variable- spectral 
response optical waveguide Bragg grating filters for optical signal 
processing by K. O. Hill et al., Optics Letters/Vol. 20, No. 12, Jun. 15, 
1995 disclose the usefulness of non-uniformly bending a grating, in 
relation to signal processing applications, for example for dispersion 
compensation; however, we have found a particularly useful feature that is 
less related to the processing of optical signals. The advantage of such a 
scheme is as follows; a substantially long grating with a uniform period 
typically has a very narrow reflectivity response; thus, when the grating 
is un-bent, it can be hidden or stored between adjacent channels 
essentially "tucked away". In the instance where an adjacent channel or 
wavelength range is to be attenuated, the grating can be bent and thereby 
chirped so that range of reflectivity broadens, in a manner similar to an 
opening curtain. Of course by stretching or compressing the grating its 
effective wavelength range shifts. Thus the grating response can be 
compressed, expanded, and/or shifted. 
Standards and specifications are provided by the telecommunications 
industry regarding the minimum allowable or acceptable channel spacing 
between two transmission channels. In some instances where it is desired 
to have a plurality of channels within a small wavelength region, this 
spacing labeled G on FIG. 3a, is relatively small. In order to provide a 
grating having reflectivity response that conveniently lies within this 
space G in one mode of operation, and that can be shifted to have a 
reflectivity response that coincides with the wavelength or channel of 
interest .lambda.1, .lambda.2, or .lambda.3 in a variable manner so as to 
attenuate a particular channel by predetermined amount, the grating must 
have predetermined characteristics. For example, the grating of interest 
must be designed to have a slope of the reflectivity response that is 
suitable for a particular application. FIG. 3c shows an embodiment wherein 
grating couplets are provided, each grating of a couplet of gratings, 
being tunable over substantially the same range, and wherein the couplet 
can share a same space G for convenient storage when no attenuation is 
required. In another embodiment compressive actuation means are also or 
alternatively coupled to each grating, or each other grating; in this 
arrangement one grating of a couplet can be used to attenuate an adjacent 
higher wavelength, while the other grating of a couplet can be utilized to 
attenuate an adjacent lower wavelength signal, by stretching of the 
gratings and compressing the other of the gratings a predetermined amount. 
It should be noted, that although the gratings shown are preferably 
impressed within an optical fibre, other optical waveguide structures can 
conveniently be utilized; for example a plurality of gratings can be 
written into a slab waveguide wherein heating elements can be used to 
control the wavelength reflectivity response of the gratings. 
Turning now to FIG. 2b, a circuit is shown for equalizing an input beam of 
light. The circuit is similar in many respects to that of FIG. 2a, however 
includes a feedback circuit for providing information relating to the 
input beam after it has been attenuated. A tap, 112 taps, for example, 5% 
of the attenuated beam and couples this light to a circuit 116 for further 
processing. The tapped light is wavelength demultiplexed into three 
channels. The intensity of signals representing a 5% portion of 
wavelengths .lambda.1, .lambda.2, and .lambda.3 are measured by circuit 
116 and converted to electrical signals by photodetectors (not shown). A 
tuner control circuit 114 in response to these electrical signals 
continuously and dynamically varies the response of the gratings 102, 104, 
and 106 by providing an appropriate voltage to the actuators 102a, 104a, 
and 106a. The circuit of FIG. 2c operates in a similar manner, however the 
tuner 124 provides appropriate voltages to six Bragg gratings, 102, 102b, 
104, 104b, 106, and 106b. 
Of course, numerous other embodiments may be envisaged, without departing 
from the spirit and scope of the invention.