OPTICAL FIBER LIGHT TRANSMISSION SWITCH

COMMUTATEUR DE TRANSMISSION DE LUMIERE POUR FIBRE OPTIQUE

English Abstract

John C. Greenwood - 25
(Revision)
OPTICAL FIBER SWITCH
Abstract of the Disclosure
An optical fiber switching arrangement contains a parallel-
sided cell arranged within an optical fiber at an angle to the
fiber axis. The arrangement includes means for introducing
either a liquid or a vapor into the path of light passing through
the cell. The cell is completely transparent to light when
filled with liquid and is completely reflective to light when
containing vapor.
- 1 -

Claims

John C. Greenwood - 25
(Revision)
WHAT IS CLAIMED IS:
1. An optical switch to control light passing along a
-solid light guide, comprising:
a parallel-sided cell formed in the light guide at an
angle to the axis of the light guide; and
means for filling the cell selectively with a liquid,
or the vapor of the liquid, said liquid having a
refractive index similar to that of the light
guide material such that when the cell is filled
with the liquid the cell is substantially trans-
parent to said light, and when the cell is filled
with the vapor the cell is totally internally
reflective to said light.
2. The switch of claim 1 wherein said liquid filling means
includes a liquid containing reservoir communicating with the
cell and having means for heating and cooling the liquid so as
to expand or contract the liquid.
3. The switch of claim 2 wherein the heating or cooling
means includes a Peltier type device fitted to the reservoir.
4. The switch of claim 1 wherein the cell forms a Fabry-
Perot type etalon filter when filled with the vapor.
5. The switch of claim 1 including a branch light guide
into which the light reflected from the cell is directed.
RAM:ch
August 20, 1976

Description

John C. Greenwood - 25
1~7~857 (Revision)
Background of ~he Invention
This invention relates to optical switch devices, and in
. particular to an optical switch in which a fluid is employed to
optically bridge a gap in an optical fiber or transmission line.
Summary o the Invention
According to the invention there is provided an optical
switch arrangement a~apted to control light passing along a
solid light guide, including a parallel-sided cell formed in the
light guide, and means for filling the cell selectively with a
liquid or the vapor of the liquid. The liquid has a refractive
index similar to that of the light guide material so that the
cell when filled is substantially transparent. The cell is
arranged at an angle to the axis of the light guide such that,
when the cell is filled with the vapor, it is totally internally
reflective to light traveling parallel to the axis of the guide.
Brief Description of the Drawinqs
' Figure 1 is a sectional view of the fluid operated optical
: switch device of this invention; and
Figure 2 is a sectional v.iew of a switching matrix employ-
ing the switches of Fig. 1.
Description of the Preferred Embodiment
Referring to Fig. 1, the switch device is formed in a
portion of an optical fiber or light guide 11 which fiber has a
core 12 and is advantageously provided with a cladding 13.
: ~.
A parallel-sided cell 14 is formed by a diagonal cut in the
fiber core 12 and communicates via a passageway 15 through the
fiber cladding 13 to a reservoir 16 containing a liquid 18.
. The portion of the cell 14 above the liquid meniscus 17 is
filled with the liquid vapor~ The liquid 18 has a refractive
index similar to that of the fiber core 12 at the wavelengths
at which the switch is intended to operate and, advantageously,
- 2 -

~ John C. Greewood - 25
` 107~5~ (~evision)
has a comparatively large thermal volume coefficient of expan-
sion.
-_ When the cell 1~ is illed with vapor, which has a refrac-
tive index substantially equal ~o unity, light -traveling along
the fiber 11 in either direction is totally internally reflected
at the cell 14 i.e. the fiber is 'switched off'. If the cell 14
is then filled with the liquid 18, e.g. by heating the reservoir
16 thus causing the liquid 18 to expand, light is then permitted
to pass along the fiber through the cell 14 in either direction,
i.e. the fiber is 'switched on t,
The liq~lid 18 may be removed from the cell 14 by cooling
the reservoir 16. This heating and cooling may be accomplished
via a Peltier type heater/cooler placed in thermal contact with
the reservoir, or placed in the liquid. This device can employ
a thermocouple junction which is heated or cooled depending on
the direct of an electric current passed through the device.
Alternatively an electromechanical device may be used to control
the liquid.
For most applications the cell 14 should be of sufficient
width to prevent optical interference effects arising from
` diffraction and/or frustrated total internal reflection. In
~; some embodiments, however, the cell 14 may be made sufficiently
thin to operate as a Fabry-Perot etalon type filter which may
be employed for duplex or multiplex switching of light signals
~` 25 of various wavelengths transmitted along the fiber.
Fig. 2 is a schematic cross-section of part of a multi-way
: ,:
switch arrangement. Branch fibers 22 and 23 are placed in
optical contact with a main fiber 21 and adjacent a respective
liquid contacting reservoir 16 (Fig. 1) and can then be filled
with liquid or vapor.
- 3 -

~0768S7 John C. Greenwood - 25
(Revision)
The cells 24 and 25 are each àrranged at an angle of 45 to
the axis of the main fiber 21 such that when either of the cells
--is filled with vapor having a low refractive index, light travel-
ing along the fiber 21 from left to right is reflected along the
correspondin~ branch fiber 22 or 23. For most applications the
cells 24 and 25 should be relatively thick in comparison to the
wavelength of the transmitted light to prevent diffraction as
stated earlier. However, in some applications thin etalon
cells may be used to demultiplex light of various wavelengths
passing along -the fiber 21, or to multiplex light signals from
the branch ~ibers 22 and 23 into the main ~iber 21. The
arrangement shown in Fig. 2 is not of course limited to,two
switches. Thus further switches with or without attendant
branch fibers may be added according to specific application.
In a ~urther embodiment the reservoir and cell may be com-
pletely filled with a volatile liquid which at ambient tempera-
ture is near its boiling point. The cell is then switched by
heating the liquid to cause vaporization and by cooling the
vapor to recondense it to the liquid. In other applications the
reservoirs may be provided with a wlck or a body of absorbent
material which reduces the effective vapor pressure of the
` liquid.
It is to be understood that the foregoing description of
,,
specific examples of this invention is made by way of example
-, 25 only and is not to be considered as a limitation on its scope.
RAM:ch
August 20, 1976