Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to laser and LED packages for use in
fiber optic communication systems. A typical package for use in a fiber
optic system basically comprises a laser diode of the double
heterostructure type, mounted within a housing. High temperature
resulting from normal laser operation both accelerates aging and causes
fluctuation of light output. To retard aging, the laser has a heat sink
which is mounted upon, and cooled by, a thermoelectric cooler. Also
housed within such a package are a temperature sensor for monitoring the
temperature of the heat sink and an avalanche photodiode for monitoring
output from the laser. Electrical outputs from the sensor and the
photodetector control the drive current applied to the cooler and the
laser respectively. Leads to electrical elements within the housing are
taken through a housing wall at hermetic feedthroughs. In addition, an
optical fiber also mounted in a hermetic feedthrough in the housing wall
has a bared end portion anchored in a mass of cured epoxy resin in a
position and orientation in which the fiber end receives light from the
laser front facet.
Epoxy resin is not an ideal material for anchoring an
optical fiber. For one thing, the room temperature curing time is long;
furthermore, the curing reaction is irreversible. Consequently, to remove
the fiber it must be broken out of the cured resin with the attendant risk
of leaving detritus on both the fiber and laser. Fusible alloys represent
a possible alternative material for anchoring a bared fiber, such material
offering one advantage that it can be melted and solidified repeatedly.
Fusible alloys are, however, not ideal for this purpose since, to melt the
alloy heat must be applied so very close to the fiber and other elements
inside the package that damage may result. Thus if a soldering iron is
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used and contacts the fiber it can cause breakage and contamination.
Moreover, it is difficult to control the temperature of miniature, heat
sensitive parts such as the laser near a soldering iron tip.
It is now proposed that a thermoelectric device operable in
one mode as a cooler and operable in another mode as a heater be
substituted for the laser cooler of prior optical source packages. The
thermoelectric device can be used in its heating mode to melt the fusible
alloy prior to anchoring the fiber or when removing the fiber from the
package and in its cooling mode to cool the laser during normal operation.
According to one aspect of the invention there is provided
an electro-optic package comprising: a housing, a thermoelectric device
within the housing and, mounted to the thermoelectric device, a light
emitting source and a mass of fusible alloy, the thermoelectric device
operable in one mode as a heater for melting the fusible alloy and
operable in another mode as a cooler for cooling the light emitting
device.
Preferably a portion of an optical fiber is anchored in the
mass of fusible alloy in a position in which light emitted from the light
emitting device is incident on an end surface of the fiber. The fusible
alloy preferably has a melting point in the range 90-100C. The alloy can
have a composition containing indium to give low melting point. The light
emitting device, for example a laser diode or light emitting diode,
preferably has a heat sink bonded to a surface of the thermoelectric
device.
Preferably the operating modes of the thermoelectric device
are related to current polarity through the device.
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The housing preferably also contains a temperature sensor
forming part of a feedback circuit controlling said thermoelectric device.
According to another aspect of the invention there is
provided a method for anchoring an optical fiber in an electro-optic
package housing a light emitting device, the package also housing a
thermoelectric device operable in a first mode as a heater and operable in
a second mode as a cooler to cool a light emitting device mounted thereon,
the method comprising operating the thermoelectric device in said first
mode while contacting the thermoelectric device with a low melting point
fusible alloy to deposit molten fusible alloy on the thermoelectric
device, leading one end of an optical fiber into the housing, manipulating
the end portion to obtain a predetermined positional relationship between
the fiber end surface and the light emitting device with part of the fiber
end portion contacting the molten fusible alloy, and then discontinuing
heating by said thermoelectric device whereby to solidify the fusible
alloy and anchor the fiber end portion.
An embodiment of the invention will now be described by way
of example with reference to the accompanying drawings in which:-
Figure 1 is a plan view of a package according to the
invention;
Figure 2 is a longitudinal sectional view of the Figure 1package; and
Figure 3 shows a circuit schematic drawing of the Figure l
package.
Referring in detail to Figures 1 and 2, a laser package 10
has a semiconductor laser 12, for example, of the GaAlAs double
heterostructure type mounted within a Kovar (RTM) housing 14. The laser
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12 is mounted on a pedestal 16 forming part of a conducting metal heat
sink 18. In front of the pedestal 16 a capillary break 20 extends across
the heat sink 18 and behind the pedestal a ceramic substrate 22 is bonded
to the heat sink. The heat sink 18 is mounted on a thermoelectric device
24 which can be operated both to heat and cool an upper surface 26
contacting the heat sink 18. A suitable subminiature thermoelectric
device 24 is available from Melcor Materials Electronic Products
Corporation under the specification number FS0.6-12-06L. The device 24
ut;lizes the Peltier effect and in both its heating and cooling modes is
characterized by one surface being hot and the reverse surface being cold.
The device, which measures only 0.24 inches x 0.16 inches x 0.11 inches,
incorporates beryllia ceramic plates having the property of high
electrical insulation with good thermal conductivity. To the ceramic
substrate 22 are bonded a photodiode 28 and a temperature sensor 30
incorporating a thermistor. The photodetector 28 and the sensor 30 are
addressed by contact pads 32 printed on the substrate 22. Leads 34 to all
the electrical elements within the package 10 extend from pins 36
hermetically sealed into a wall 38 at feedthroughs 41. In an end wall 40
of the housing is mounted an optical fiber hermetic feedthrough 42. The
feedthrough 42 and the nature of pin feedthroughs 41 are incidental to the
present invention and so will not be described in detail. An end portion
of an optical fiber 44, stripped of protective jacket, enters the package
10 at the optical feedthrough 42 and is anchored in place by a mass of
fused alloy 46 sited on the heat sink 18 in front of the capillary break
20.
A preferred alloy is Indalloy No. 8, having a composition of
44% indium, 42% tin and 14% cadmium. The indium content ensures a
relatively low melting point of 93C.
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As indicated previously the thermoelectric device can be
operated as either a heater or cooler. A suitahle circuit for driving the
thermoelectric device in its heating mode to melt the fusible alloy is
shown in Figure 3. The package 10, indicated by a broken line, houses the
thermoelectric device 24 and temperature sensor 30. The control circuit
which is located outside the housing has a 5 volt supply which together
with a controlling resistor R1 and voltage divider R2/R3 establishes a
predetermined voltage at one terminal of a comparator 48. A variable
resistor R4 and thermistor Rt determine the voltage at the other
terminal of the comparator 48. An output from the comparator is amplified
by emitter follower and common emitter stages 5~ and 52 respectively, and
drives the thermoelectric device to heat its top surface 26. A threshold
temperature in the package is set by resistor R4. The resistance of Rt
increases as the temperature of surface 26 increases until the threshold
is reached at which current to the thermoelectric device is stopped.
In the cooling mode the control circuit is replaced by
another control circuit providing a fixed current of reverse polarity so
as to cool the top surface 26. The thermistor Rt is used in a
feedback circuit to regulate the current supply to the laser 12.
When anchoring the optical fiber 44 to the top surface 26 of
the device 24, the thermoelectric device is heated to about 110C and the
Indalloy which has a melting point of 93C is deposited onto the top
surface by rubbing with a piece of solid, fusable alloy. Flux, which is
usually used to obtain wetting by a fusible alloy is undesirable since, in
normal laser operation at high temperature it can volatilize and
contaminate the laser or other elements in the package. By maintaining
current through the thermoelectric device 24 a pool of molten solder is
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retained on the thermoelectric device top surface. The molten alloy is
prevented from contacting the laser 12 or the fiber end face 54 by the
capillary break 20.
With the optical fiber hermetic feedthrough 42 brazed onto
the package wall 40, a projecting end portion of the fiber extends to just
short of the laser site. The fiber end portion is then manipulated until
the fiber end face 54 is at a position and orientation in which it
receives a predetermined light output from the laser 12. Positioning of
the fiber 44 is performed while the laser 12 is operating, light launched
into the fiber from the laser being monitored at a remote end of the
fiber. The temperature sensor meanwhile ensures that at no time during
the heating cycle does the thermoelectric device top surface 26 reach such
a high temperature that elements within the package 10 might be adversely
affected or that solder used within the device 24 is melted. As soon as
the fiber 44 is correctly positioned, current to the thermoelectric device
is stopped to initiate cooling and subsequent alloy solidification to
anchor the fiber 44 in place.
