Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR SWITCHING OPTICAL SIGNALS AND A THERMO-
OPTICAL SWITCH
The invention pertains to a method for switching an optical signal from an
input port of a digital thermo-optical (TO) switch to at least one output port
of the switch, with a temperature difference being imposed between at least
two output ports.
Such a method is known from N. Keil, et ai., "(2x2) digital optical switch
realised by low cost polymer waveguide technology," Electronics Letters,
Vol. 32, No. 19 (1 August 1996), pp. 1470-1471. In this document a
thermo-optical switch is described which has four waveguides (or, more
accurately, waveguide channels), two input ports and two output ports, and
four electrodes for heating the waveguides. Switching a signal from one of
the input ports to one of the output ports is effected via selective heating
of
the waveguides.
In addition to this 2x2 switch there are, int. al., 1 x2 (e.g.) "Y branched"}
thermo-optical switches. In one embodiment a "single mode" signal is
switched from the input port to one of the output ports by heating only one
of the output ports, in other words, by inducing a temperature difference,
and consequently a difference in index of refraction) between the output
ports. In the most common switches the signal will travel through the
waveguide which has the highest index of refraction. In the case of
polymeric digital thermo-optical switches that means the waveguide which
has the lowest temperature (and therefore the highest density).
After switching, the switch is in the envisaged position and the temperature
difference is maintained. In actual practice, an electrode serving as a
CONFIRMATION COpY
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heating member is driven at a certain voltage, which voltage is kept
constant (in other words, the switch is step-function driven).
In thermo-optical switches a phenomenon may be encountered which
occurs in particular when a thermo-optical switch has been in the same
state for a long period of time (days to years), which phenomenon is best
described as "remanent induced junction asymmetry."
To illustrate this phenomenon use is made of a 1x2 ("Y-branched")
polymeric digital thermo-optical switch such as is depicted schematically in
Figure 1. This switch 1 comprises an input port 2, a left-hand output port 3,
a right-hand output port 4, and electrodes 5 (shown here in a finer version
than the electrodes customary in actual practice) for heating the output
ports 3, 4.
A signal S2 in the input port 2, is sent to the left-hand output port 3. In
that
case electrode 5 of the right-hand output port 4 is heated. At an isolation
(i.e., the difference in power of the signals travelling through the
respective
output ports of the switch) of 20.0 dB, '! % of the signal S2 is guided to the
right-hand output port 4, while 99% of S2 is guided to the left-hand output
port 3 (10 log (99I1 ) = 20.0 dB). When the electrode 5 of the right-hand
output port 4 is no longer driven, the switch ideally will return to the
ground
state. In this state the switch functions as a passive splitter which divides
the signal S2 equally (50%I50%; isolation 0 dB) between the two output
ports 3, 4.
It was found that thermo-optical switches which are in the same state for a
long time fail to revert to the aforesaid ground state after the driven
electrode has been switched off (or fail to do so within an acceptable period
of time), reverting instead to an asymmetric state in which over half of the
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S2 signal is still guided to, in this example, left-hand output port 3. The
switch thus continues to have a certain bias for its previous state. Even
when electrode 5 of the left-hand output port 3 is driven at the
aforementioned voltage and causes a temperature difference between the
output ports, this asymmetry is not overcome (rapidly) enough, and it is no
longer possible to achieve the prescribed isolation.
In such cases switching can still be carried out by an extra increase in the
voltage over the electrode, but this requires intensive monitoring of the
'10 switch (which is subject to practical objections) and comparatively
complex
driving means. Moreover, when the voltage is increased more than once,
the asymmetry may be subject to resonant rise and finally reach a level at
which the switch will be rendered unsuitable for use.
in a number of TO switch applications switching takes place only under
(comparatively) rare conditions. Good examples of this are protection and
redundancy, where the switch will always guide a signal to one and the
same optical fibre, except when there is a problem, such as damage to the
fibre. 1n such a case the switch will be turned and the signal guided to a
second, undamaged fibre. If proper switch action in such an application
cannot be guaranteed, the whole redundancy system is pointless. In other
words) the value of such a system is greatly dependent on the reliability of
the switch.
In view of the above there is need for a method for switching thermo-optical
switches in which the increase in remanent induced junction asymmetry
does not occur or occurs only to a very much lower extent. The invention
has for its object to satisfy this need and achieves this when in the method
described in the opening paragraph the (temperature) difference is greater
than or equal to dTm;n at the outset and is subsequently reduced to a
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difference of less than OTmin, with OTm;~ being the minimum temperature
difference required for switching.
It was found that the described remanent induced junction asymmetry of
thermo-optical switches is linked to the heat supplied. In general, it holds
that this asymmetry will increase less rapidly as the supplied power (which
is converted into heat and results in a temperature difference) is lower.
Preferably, the power is reduced to a level where the temperature
difference between the output ports of the switch is lowered to a difference
which is at least 10%) or even at least 20%, less than OTm~n.
Besides reducing the remanent asymmetry, the reduction of the heat
supply is advantageous in itself because the thermal load on the switch
(especially on the electrodes) is reduced and less power is consumed.
Given the action of thermo-optical switches, the possibility of reducing the
power supply to a value which produces a temperature difference of less
than OTmi~ without any substantial change to the switching effectiveness
{isolation) may be deemed remarkable. It was found that thermo-optical
switches (in particular those made of a polymer) exhibit hysteresis, which
means that the temperature difference which is required over the output
ports to increase the isolation between two output ports to above a specific
value (in brief: the temperature difference required for switching) is higher
than the temperature difference at which the isolation drops below said
value (and at which the switching is cancelled).
It should be noted that the answer to the question of whether switching has
been completed or not is dependent on the specifications or the given use
of a switch. If the specifications of a particular switch list an isolation of
15
dB minimally, this means that, in the case of a 1 x2 switch, after completion
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of the switching procedure (which, ordinarily, lasts less than 5 ms) less than
3% of the power of the signal which is supplied to the switch passes
through the output port which is qualified as being in the "ofP' state, while
over 97~t~ of the signal passes through the output port qualified as being in
5 the "on" state.
It will be clear from the above that the term "switched" does not so much
stand for an absolute physical state but rather indicates that the present
switch meets the specification (in this case especially the isolation) of the
switched state. Hence the isolation can be required by the specifications of
the switch or its application. For instance, a switch may allow an isolation
of
30 dB) while 18 dB will suffice for a specific use. In that case, 18 dB will
be
normative in that case.However, it is preferred that at ~Tm;" the isolation is
at least 16 dB, preferably at least 25 dB.
The effects of the present invention are especially noticeable in polymeric
switches (i.e., those switches in which at least the waveguides, but usually
also the cladding(s), consist of an organic polymer). Further, for the method
according to the invention it holds that it is preferred to use switches which
allow an isolation in the switched state of greater (better) than 16 dB, or
even of greater than 25 dB.
As was described above, in the process according to the invention a
(temperature) difference is imposed at the outset which is greater than or
equal to ~Tm;", which difference is subsequently reduced. Preferably, the
difference is reduced as soon as possible after switching, but the time in
question should at least be long enough to guarantee the effect of the
invention. The period of time involved is dependent on the switch itself, but
also on the temperature of the entire switch (the higher this temperature is,
the faster the temperature difference over the output ports of the switch can
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be reduced). In many cases (especially when use is made of polymeric
waveguides) it will be possible to reduce the difference after less than 0,1
to 0,5 seconds. It is preferred, particularly in the case of applications in
which little switching occurs, not to do this until after 5 seconds, or even
after 10 seconds.
For that matter, it holds that the build-up of remanent induced junction
asymmetry manifests itself less strongly as the temperature of the entire
switch is higher. In one embodiment according to the invention the entire
switch is kept at a temperature above 40~C, preferably above 60~C. To this
end the switch may be equipped, e.g., with one or more heating members
and an insulating material.
To reduce the period of time after which the temperature difference can be
reduced and also reinforce the effect of the method according to the
invention further, it is recommended to have a temperature difference at the
outset of more than 20%, or even 50% or 100%, greater than OTm;n.
Shortening the pulse will make the switch less susceptible to switching
conditions out of the ordinary (e.g., alternating high and low frequencies
and/or irregular switching).
The magnitude of OTmin is made up at least of the temperature difference
needed to bring a switch which is in the aforementioned ground state
(isolation 0 dB) to the switched state and an additional temperature
difference needed to quickly overcome the previous switched state (in
milliseconds). OTm", may vary per switch (geometry, materials used, etc.)
and furthermore is dependent on what happened before, so that it is also
possible for ~Tmin to vary in time in one and the same switch.
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It should be noted that it is not possible to select just any high supply of
power to the switch. At a certain high power, susceptibility to polarisation
will come to play an increasingly important role. For that reason preferably
the power used is not such as will create a temperature difference over the
output ports of the switch of 400% greater than OTm;n.
The method according to the invention is depicted schematically in Figure
2. In the polymeric digital thermo-optical 1 x2 switch according to Figure 1 a
signal S2 is presented, and electrode 5 of the right-hand output port 4 is
energised with a voltage of, say, 4 V. As a result, power having a
magnitude A is supplied to electrode 5. This power creates a temperature
difference between the output ports 3 and 4 equal to OTm;n.
After 2 ms 99.91 % of the power of signal S2 is directed to the
comparatively cool left-hand output port 3 (the isolation thus is just above
30 dB, i.e., at a prescribed isolation of >_ 30 dB it is now possible to speak
of a "switched state"). Ordinarily speaking, the supplied power A would
remain unchanged over time. According to the invention) the voltage over
electrode 5 is reduced after 4 seconds to 2,5 V, causing the power to
decrease to B (B ~ 0,6 x power A), whereas the isolation remains just
above 30 dB.
The power C (~ 1,5 x power A) depicted in Figure 2 is an example of the
above-described temperature difference peak. Because of this peak the
voltage can be reduced to B already after one second without the isolation
being adversely affected even in the longer term.
An additional advantage of the method according to the invention is that it
can also be employed with TO switches which are already in use. In many
cases the advantages of the invention, in particular the longer life of the
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switch, are still obtainable by simply replacing or adjusting the drive of the
means for heating the output ports.
For completeness' sake it should be noted that there is a major distinction
between inherent junction asymmetry and induced junction asymmetry.
Inherent asymmetry concerns asymmetry resulting from the switch's
materials and geometry. For instance) in a 1 x2 TO switch consisting of a
waveguide running in a straight line and a branch (in contradistinction to
the "Y-branched" switches where, in a sense, there are two branches), an
isolation of 6 dB can be measured without any of the electrodes being "on."
Induced asymmetry concerns asymmetry resulting from the thermo-optical
effect brought about by the supply of heat and the imposition of a
temperature difference between the branches or output ports. Needless to
say, the two types of asymmetry can be present in a switch simultaneously.
Nor is the invention restricted to one particular type of TO switch; rather,
it
comprises NxM (N and M both are integers) and 1xN switches. In principle,
preference is given to 1 x2 switches (such as the "Solid state 1 x2 optical
switches" or BeamBoxT~" ex Akzo Nobel) and matrices comprising 1x2
switches. Suitable (polymeric) switches are disclosed, int. al, in European
patent applications 95200965.2, 95201460.3, 95201762.2, and
95201761.4.
It should be noted that Japanese patent appication 59 148031 discloses an
optical switch which is operated using a temperature-gradient generating
means and a voltage source for supplying a voltage larger than the steady
ON state voltage at the time only of ON rise. Subsequent reduction of the
voltage to a value below the steady ON state voltage is not disclosed.
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The invention further relates to a digital thermo-optical switch comprising at
least one input port, at least two output ports) elements for imposing a
temperature difference between at least two of the output ports, and means
for driving said elements, which means (preferably an electrical circuit)
drive the elements in such a way that, in response to a stimulus, the
temperature difference at the outset is increased to a value greater than or
equal to ~Tm;n and the difference is subsequently reduced to a difference of
less than OTm;~, with ~Tm;~ being the minimum temperature difference
required for switching. It is preferred that at ~Tm;~ the isolation is at
least 16
dB, preferably at least 25 dB.
Preferably, the temperature difference is reduced to a difference of at least
10% less than ~Tm;n and/or the temperature difference at the outset is at
least 20% greater than ~Tm;n.
Polymeric switches are preferred. Further, if the digital thermo-optical
switch has an isolation which is greater (better) than 16 dB, preferably
greater than 25 dB, the losses in the switch will remain within acceptable
limits and the switch is suitable to be used in virtually any application.
