Optical filter for telecommunications

An optical filter comprising two ferrules containing a respective duct internally housing a respective portion of optical fibre; the two ferrules are inserted axially aligned within an alignment bush, between said two ferrules there being inserted a vitreous support on which a layer of optically active filtering material is deposited. A method for constructing said optical filter is also described.

This invention relates to an optical filter for telecommunications. 
An optical filter is a passive element which is interposed between one or 
more sources (emitters) and a receiver so that the receiver receives only 
certain components of the optical spectrum emitted by the source. This 
element can be used in optical communication networks in proximity to the 
receiver, along the line, or wherever it is required to discriminate a 
certain spectral component from the others. The use of this element 
enables the same optical carrier to be used to transmit a certain number 
of signals (frequency-division multiplexing), which are then suitably 
selected as required. The filter transmission characteristics can be 
low-pass (those optical spectrum components above a certain wavelength are 
filtered out), high-pass (those optical spectrum components below a 
certain wavelength are filtered out), or band-pass (a relevant band within 
the optical spectrum can be transmitted). 
In the current state of the art an optical filter is used based on the 
Fabry-Perot principle. This comprises two perfectly parallel reflecting 
surfaces, the wavelength (or inversely the frequency) of the passing 
radiation depending on the distance between said two surfaces. 
The aforesaid optical filter consists of two mutually facing ferrules 
rigidly locked by an external guide body of ceramic material to which they 
are rigidly fixed. The two portions of optical fibre between which the 
optical filter is interposed are respectively housed in a respective 
ferrule. 
Such a structure has certain drawbacks. If for example during its operating 
life that line portion on which it bears is subjected to significant 
temperature change, the expansion consequent on this alters the distance 
between the two reflecting surfaces, hence varying the typical radiation 
frequency at which the filter operates. 
In addition its construction is very delicate and complicated, 
necessitating the installation and dimensional refinement of at least four 
elements (a reflecting element and a spacer element each side). In this 
type of optical filter each ferrule is smoothed and lapped at only one 
end, a uniquely pig-tail configuration being provided. This makes its use 
and insertion into an existing optical network not easy and indeed often 
complicated, hence limiting the effective scope of its application. 
The object of the present invention is to provide an optical filter which 
eliminates said drawbacks by allowing easy and reliable use. 
This object is effectively attained by an optical filter comprising two 
ferrules provided with a respective duct housing internally a respective 
portion of optical fibre, and into which the two ferrules are inserted 
axially aligned within an alignment bush, characterised in that between 
said two ferrules there is inserted a vitreous support on which a layer of 
optically active material is deposited. 
Characteristics and advantages of the present invention are described in 
detail hereinafter by way of non-limiting example with reference to the 
accompanying drawings, in which: 
FIG. 1 is a side section through an optical filter according to the present 
invention; 
FIG. 2 is a schematic illustration of an optical filter according to the 
present invention inserted between an emitter and a receiver; 
FIG. 3 is a section through an alignment coupling for optical networks into 
which an optical filter according to the present invention is inserted; 
FIG. 4 is a section through a second alignment coupling as heretofore; 
FIG. 5 is a section through a third alignment coupling in male-female 
configuration.

With reference to FIG. 1, an optical filter according to the present 
invention shown in its mounted position is indicated overall by 11. 
Two ferrules 12 and 13, for example of ceramic material, house within a 
rectilinear duct provided along their axis two lengths of optical fibre 14 
and 15. Interposed between the two ferrules 12 and 13 and glued to one of 
them by means of adhesive there is a vitreous support 16 provided on its 
glued surface with a deposit of optically active material 17. The vitreous 
support 16 has dimensions such as to completely interfere with the optical 
fibre 14 and 15 but not with its positioning between the ferrules 12 and 
13 within an alignment bush 18. 
The alignment bush can be of ceramic, metal, plastic or other material. 
FIG. 3 shows an alignment coupling indicated overall by 30 in female-female 
configuration. It consists of a pair of casings 32 having at their facing 
ends two flanges 33 for their fixing together. Each of the two casings 32 
comprises axial cylindrical countersunk recesses 34 in which a filter 36 
formed in accordance with the present invention is positioned. The 
reference numeral 35 indicates the three alignment bushes included in the 
optical filter in this case. The arrangement comprising three bushes 
allows better alignment of the two optical fibres. 
FIG. 4 shows a second configuration of a female-female alignment coupling, 
here indicated overall by 40, and comprising two casings 42 having two 
flanged ends 43 for their fixing together. Again in this second 
configuration there are three bushes 45 containing the optical element 46 
formed in accordance with the present invention. 
FIG. 5 shows an alignment coupling in male-female configuration, indicated 
overall by 50 and consisting of two casings 51 and 52. These comprise 
fixing flanges indicated respectively by 57 and 58. At its non-flanged 
free end the casing 51 comprises a female termination indicated by 53, 
whereas at its opposite free end the casing 52 comprises a male 
termination indicated by 54. An optical filter 56 formed in accordance 
with the present invention is present within countersunk recesses, it 
being inserted this time into two alignment bushes 55. 
The optical filter according to the present invention is constructed by the 
following steps: 
inserting a respective optical fibre portion into each of two ferrules; 
smoothing and lapping at least one end of each ferrule; 
depositing a layer of optically active filtering material onto a vitreous 
support having a cross-section slightly less than that of the ferrules but 
greater than the cross-section of the optical fibre; 
by means of a suitable adhesive, gluing said vitreous support to a lapped 
end of a ferrule, in such a manner that the vitreous support totally 
interferes with the cross-section projected onto it of the optical fibre 
contained within the ferrule; 
aligning the two ferrules and bringing them into mutual proximity by 
inserting them into a bush. 
After these construction steps, as shown in FIG. 2 the filter 11 is 
connected at one end to any type of optical signal emitter 21, and at the 
other end to an optical signal receiver and measuring device 22. In this 
manner the signal attenuation by the filtering element due to misalignment 
and other constructional defects can be immediately checked and measured. 
By rotating the two ferrules relative to each other the position can now 
be determined for which this attenuation is a minimum. It should be noted 
that such mutual rotation is very small (of the order of degrees). 
When the minimum attenuation position has been determined, the ferrules are 
securely fixed to the bush in said position by a glue, for example epoxy 
resin. 
The fact that the filtering element according to the present invention 
consists of a deposit of optically stable material makes the element 
insensitive to any temperature variation of the environment in which it is 
used. Both ends of each ferrule can be smoothed and lapped (lapping 
oblique at one end and flat at the other or oblique or flat at both), so 
as to strongly reduce optical signal reflection. 
The obliqueness of the positioning of the vitreous support with its 
filtering deposit is an essential characteristic of the present invention, 
in that this expedient avoids the harmful phenomenon of retroreflection of 
the optical signal. This hence prevents part of the signal reaching the 
filtering element being reflected rearwards if this latter is wrongly 
inclined, and re-entering the original fibre to seriously compromise 
signal integrity. 
By varying the composition and type of material deposited on the vitreous 
support the filter can be arranged to operate in various different ways. 
Hence a band-pass filter (the traditional case) can be formed, as can be 
equally easily a high-pass or a low-pass filter. 
From the constructional viewpoint the optical filter of the present 
invention allows a much wider range of application than the traditional 
optical filter.