Optical add-drop multiplexer for optical communication networks

The optical add/drop multiplexer has a pair of nominally identical interference filters, one of which carries out the function of extracting a carrier from a wavelength division multiplexed stream and the other the insertion function. The filters are arranged in parallel planes and are secured to opposite faces of a transparent plate so that the stream exits the device after having undergone reflection by both filters. The plate is mounted on a support which can be rotated in either direction to vary the tuning wavelength of the filters.

SPECIFICATION 
FIELD OF THE INVENTION 
Our present invention relates to optical communication networks and, more 
particularly to an optical add-drop multiplexer, i.e. a device for the 
extraction of a carrier, modulated by a certain information signal, from a 
stream of optical signals comprising multiple carriers, and for the 
insertion of the information into the stream of another carrier, modulated 
by a different information signal. 
BACKGROUND OF THE INVENTION 
Add-drop multiplexers are used, for instance, in nodes of a wavelength 
division multiplexing communication network, to extract from the 
multiplexed stream a channel containing the information directed to a user 
or to users connected to the node, letting the remaining channels pass 
through unaltered, and to add to the multiplexed stream a new channel 
which transports the information generated within the node. In practice, 
to simplify network management, it is advantageous that the dropped 
channel and the added channel are allocated the same wavelength position, 
which is therefore unambiguously associated with the node itself. Another 
possible application of such devices is in routing nodes of reconfigurable 
optical networks, to re-route certain information streams as a result of 
changed traffic conditions or to remedy a failure downstream of the node. 
Add-drop multiplexers can be considered four-port devices, with two ports 
for the input/output of the overall stream and two ports for the 
input/output of the individual carrier. Hereinafter the ports related to 
the overall stream shall respectively be called the input port of the 
output port, and those related to the individual carrier shall be called 
the "drop" and respectively the "add" (or insertion) port. 
To manufacture add-drop multiplexers, the use of non-absorbing interference 
filters has been proposed. These filters, as is well known, comprise a 
succession of dielectric layers having such refraction indices and 
thicknesses that the filters transmit a certain portion of the spectrum of 
the incident radiation and reflect the remaining portion. A simple 
inclined interference filter, placed in a suitable system for collimating 
and re-focusing the optical beams, in principle could directly perform the 
functions required of the device. The filter receives on one face the 
multiplexed stream and reflects all wavelengths except the one to be 
extracted, which exits from the opposite face. This face receives the 
wavelength to be re-inserted, which in turn is transmitted through the 
filter and is combined with the multiplexed stream. However, a device 
using a single filter has a poor extinction ratio in reflection. Thus a 
non-negligible fraction of the power associated with the extracted channel 
is transferred to the output port, thereby creating interference phenomena 
between this signal and the one at the same wavelength that is present at 
the add port, and these phenomena lead to severe degradation in the 
performance of the system. Moreover filter tuning is hard to achieve, 
since the variation of the filter inclination required to vary the 
transmitted band also causes the angular displacement of the reflected 
beam with respect to an optical collecting system. 
The paper "Multilayer add-drop multiplexers in a self healing WDM ring 
network", presented by A. Hamel et al. at the OFC '95 Conference, San 
Diego (USA), 26 Feb.-3 Mar. 1995, paper TuQ2, describes a device utilizing 
two interference filters, one to drop and the other to add a channel. This 
device has a good extinction ratio, according to the data reported in the 
paper. However, the actual structure of the device is not described, nor 
is the possibility of filter tuning mentioned. 
OBJECT OF THE INVENTION 
The object of the present invention is to provide an add-drop multiplexer 
which has a good extinction ratio in reflection and is tunable. 
SUMMARY OF THE INVENTION 
The add-drop multiplexer according to the invention comprises a pair of 
nominally identical interference filters, one of which carries out the 
drop function and the other the add function. The two filters are arranged 
in parallel planes and are secured to opposite faces of a transparent 
body, so that the stream including the plurality of carriers leaves the 
device after having undergone reflection by both filters. The body is in 
turn mounted on a support which is hinged on an axis passing through the 
barycenter of the multiplexer and parallel to the plane of the filters, 
and which is associated with means which cause its rotation in either 
direction to vary the tuning wavelength of the filters. 
EP-A 0 153 722 in the name of Oki Electric Industry Company, Limited, 
published on 4 Sep. 1985, describes a device for multiplexing and 
demultiplexing multiple wavelengths (or for transmitting and receiving 
multiple wavelengths) which, in case of operation on two wavelengths, has 
two parallel interference filters applied to two opposite faces of a 
transparent support. However, the filters are tuned on different 
wavelengths, so the device cannot be used to extract and re-insert given 
carrier. The extraction of a carrier from a transiting stream and the 
re-insertion into the stream of the same carrier are also made impossible 
by the fact that the device incorporates the generators and detectors for 
all carriers to be multiplexed or demultiplexed. Finally, this device too 
is not tunable.

SPECIFIC DESCRIPTION 
As can be seen in FIG. 1, the device comprises two nominally identical 
interference filters 11, 12, i.e. filters having the same spectral 
responses in reflection and transmission. The filters are parallel to each 
other and rigidly rotatable together around the overall barycentre of the 
device. The wavelength division multiplexed stream present on the input 
port (I) reaches a face of filter 11, which transmits towards the drop 
port (E) one of the channels (whose wavelength depends on the inclination 
of the filter). The other channels are reflected towards filter 12 from 
which they are reflected again, thus passing to the output port (U). 
Filter 12 receives on the opposite face the channel to be re-inserted (add 
port A) which is transmitted and introduced into the multiplexed stream 
present on the output port. Owing to the way the ports are arranged, it 
can be noted that the stream passes to the output port after having 
undergone reflection by both filters. Filter 12 too, being nominally 
identical to filter 11, will transmit a substantial fraction of the 
residual power associated to the extracted channel. In this way the 
extinction ratio in reflection is doubled (in dB) with respect to a 
structure utilizing a single filter which, as stated above, could carry 
out the functions of an add-drop multiplexer. Moreover, the rigid rotation 
of the two filters allows tuning of their spectral responses, while 
maintaining the parallelism between the beams present on the drop and 
output ports. Thus such beams can still be easily focused onto respective 
fibers. 
FIG. 2 is a more detailed depiction of a preferred embodiment of the 
device, indicated as a whole as 10. The elements described with reference 
to FIG. 1 are indicated with the same references characters. Ports I, A, 
E, U are made of optical fibers terminating at the device. As can be seen, 
filters 11, 12 are borne by a plate 13 of transparent material. Such an 
arrangement can be obtained by direct deposition of the dielectric layers 
or by gluing with adhesives having a suitable refraction index. The 
assembly constituted by plate 13 and filters 11, 12 is mounted on a 
support 14 hinged on a vertical axis 15 passing through the barycenter, 
and can rotate in both directions, as indicated by arrow F. The rotation 
of support 14 can be actuated by a micrometer screw device 16 through a 
lever 17. 
The beams present at ports I, A are conveyed towards device 10 by 
respective graded index lenses 18, 19, and the beams leaving device 10 and 
directed to ports E, U are collected by respective graded index lenses 20, 
21. Device 10 with the rotation control means and lenses 18-21 are mounted 
on a common support, which is not shown. 
The inclination angle of plate 13 with respect to the axes of lenses 18-21 
in a position corresponding essentially to the central tuning value of the 
filter must take into account two contrasting requirements. On the one 
hand a pronounced inclination improves filter tuning and increases the 
spatial separation between the beams directed to ports E, U, as is 
required to allow the beams to be coupled in the respective focusing 
lenses 20, 21 (whose diameters are typically of the order of 3 mm). On the 
other hand, the device is somewhat dependent on polarization and the 
dependence increases as the angle of incidence of the beam increases. More 
particularly, in the case of a polarization component perpendicular to the 
incidence plane, the reflected band is much narrower than in the case of 
the parallel component and, since the state of polarisation of the stream 
arriving at the device cannot be guaranteed a priori, this can represent a 
limitation to the performance of the device. It has been found that an 
inclination angle of about 25.degree. represents a good compromise between 
the various requirements. 
As regards the extent of rotation of device 10, and thus the amplitude of 
the filter tuning interval, it is necessary to consider the typical 
tolerances of graded index lenses with respect to parallel translations of 
an input beam. It has been found that, with the mean inclination angle 
stated above, rotations of about .+-.3 degrees, corresponding to a tuning 
interval of over 10 nanometers, can be suitable. 
To reduce dependence on the polarization of the signals of the incident 
stream, device 100 shown in FIG. 3 can be used. In FIG. 3, the elements 
identical to those shown in FIG. 2 are denoted by the same reference 
numerals. 
In this embodiment, interference filters 11, 12 are secured to opposite 
faces of a body 103 made of transparent material and comprising a pair of 
blocks 104, 105 each of which carries a respective filter 11, 12. The two 
blocks are joined together by means of a half-wave plate 106 which has 
bi-refringence axes at 45.degree. with respect to the incidence plane of 
the radiation. The wavelength division stream present on input port I, 
which is randomly polarized, with a certain ratio between the components 
parallel and perpendicular to the incidence plane of the stream itself, 
arrives on a face of filter 11, which as before transmits towards 
extraction port E one of the channels (whose wavelength depends on the 
inclination angle of the filter) and reflects the other channels towards 
filter 12. In the path between the two filters 11 and 12, the reflected 
stream crosses plate 106 which rotates the polarization plane of the 
stream by 90.degree., so that in the stream impinging on filter 12 the two 
polarization components are mutually exchanged with respect to the stream 
impinging on filter 11. In this way, the different responses of the 
filters to the polarization parallel and orthogonal to the incidence plane 
are compensated. Since the presence of half-wave plate 106 reduces the 
dependence of the device on polarization, the stream can be sent onto the 
device with higher angles of incidence. 
In respect of the channel dropped through filter 11 and the one added 
through filter 12, it should be remembered that the diversity in the 
filter response to the two polarization states is less noticeable in 
transmission; moreover, only one carrier is involved and therefore 
bandwidth problems are not critical. 
It is evident that what is described herein is given solely by way of 
non-limiting example and that variations and modifications are possible 
without departing from the scope of the invention.