Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to light emitting diode devices adapted to
give stabilized light output.
One of the primary light sources used in fiber optic
co~munications s~stems is the light emitting diode (LED) which consists
essentially of a p-n junction in which injection of current carriers causes
electron-hole recombination with an associated spontaneous photon emission when
the device is forward biased.
To transmit information, bias current to the LED is modulated by
a modulating signal to give a corresponding variation in light output. In
order that the light output is an accurate analog of the modulating signal,
the LED operating characteristic, i.e. optical OUtpllt level and linearity (for
analog operation) must be stabilized against aging and temperature-related
variation.
To stabilize LED output, the output level is monitored and bias
current adjusted in order to restore the particular light characteristic of
interest.
In the past, monitoring was achieved by tapping a small,
predetermined fraction of light from the output fiber. In one method, a
tapping fiber is wrapped around and fused with the output fiberq In another
method the output fiber is merely sharply bent in order to couple light out of
the fiber. In both cases it is found that the light in the monitoring fiber
tracks light in the output fiber only to within about 10%. Consequently the
level of light launched into the output Fiber cannot be accurately assessed.
Since for both of these arrangements light for monitoring
purposes is taken from output light propagating along the output fibre, the
light output desirable for transmission of inFormation is somewha~ decreased.
Consequently, for a particular transmission length, greater amplification is
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required to compensate for ~he correspondlng reduction in signal to noise
ratio.
In its broadest aspect, this invention proposes ~onitoring the
level of side emitted light from the active region oF a Burrus-type LED the
side emission being a predetermined fraction of LED top emission ~rom the
active region and thus providing an analo~ of the top emission. Monitoring
means ~or monitoring the side emission can be spatially separated from the LED
or can be integrally fabricated with the LED. In the latter case a forward
biased p-n light emitting junction in the LED structure is reverse-biased to
function as a light sensitive junction in the monitoring means. The intensity
level of LED side emission tracks LED top emission very closely. In a fibre
optic system, the Burrus-type LED top emission is normally launched into an
output fibre. Side emission, being of an appreciably lower intensity, has
hitherto been neglected.
In both of the cases described, an electrical analog of the
monitored side emission light is used to modify operating currents directed to
the LED. With an appropriate feedback loop, either the peak optical power
Ifor digital systems) or the r.m.s. power tfor analog systems) can be
stabilized. In addition, for analog systems, the linearity can also be
stabilized.
Embodiments of the invention will now be described by way of
example with reference to the accompanying drawings in which:-
Figure 1 is a sectional view showing an electro-optic device
according to the invention,
Figure 2 shows a feedback network for stabilizing LED light
output as a function of modulating current;
Figure 3 is a cross sectional view showing a monolithic
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electro-optic device according to the invention.
Referring firstly to Figure 1, there is shown a Burrus-type light
emitting diode (LED) 10 having a dou21e heterostructure layered construction.
A 0~2 to 1 micron active layer 12 of p-type GaAlAs is flanked by a 4 micron
confining layer 14 of n-type GaAlAs and a 1.5 micron confining layer 16 of
p-type GaAlAs. A substrate layer 18 of n-type GaAs is etched to produce a well
20 extending ~o the confining layer 14. Over the outer surface of the
substrate is deposited a 2500A contact layer 22 of Au/Ge. On the undersurface
of the confining layer 16 is chemically vapour deposited a SiO2 insulating
layer 24 except at a 75 micron diameter window 26 where the exposed confining
layer is subjected to a zinc skin diffusion step. The zinc diffused region 28
has a contact layer 30 of 70A units of Cr and 2500A of Au applied to it.
Subsequently, a 5-10 micron gold contact layer 32 is plated over the bottom of
the device to function as a heat sink. Connecting leads (not shown) are then
soldered to the contact layers 22 and 32.
The LED is mounted against one wall of a hermetically sealed
chamber 34 which protects the LED from damaging atmospheric conditions. The
connecting leads and an output optical fiber 38, whose end is secured into the
well 20 by epoxy 40, exit the chamber 34 through hermetically sealed
feedthroughs (not shown~ in the chamber walls. When current is directed
through the LED via contacts 22 and 32, spontaneous light emission occurs. A
major part of this light is emitted perpendicularly to the LED active layer as
top emission. Since the top conflning layer 14 is very thin, only a small
proportion of the top emission is absorbed before the light is launched
into the end of optical fibre 38.
The spontaneous emission is emitted in all directions, a portion
of the light tending to be guided at the junction plane. Typically, this light
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travels through over 200 microns mainly along the active layer 12 but also
along the confining layers 14 and 16. Consequently a large proport1On of the
total side emission fronn the LED active rlegion is absorbed and side emission
actually emitted from the LED chip is an order oF magnitude less than that
available as top e~ission. As illustrated by J.C. Dyment et al (IEEE
Transactions on Electron Devices, Vol. E.D. 24 No. 7, July l977), typically the
top emission ranges from 0 - 25 mW for a correspond;ng rise in side em;ssion
from 0 - 800 microwatts with operating current varying from 0 - 600 mA.
For fibreoptic systems purposes where long lifetime is desired
the drive current is usually maintained below 150 mA and it has been
ascertained that in the 0 - 150 mA range, the proportion of side emission to
top emission emitted from the LED active region is substantially uniform. In
one example, side emission tracked top emission within 1% over the current
range of 5 to 150 mA and was within 0.5% over the current range of 50 to 110
mA.
Mounted on another wall of the chamber 34 and spaced from the
nearest side surface of the LED 10 is a PIN photodiode 42. The PIN photodiode
is of a conventional form having metal contact regions 4~ and 4fi and n- and
p-type silicon semiconductor regions 48 and 50 respectively. The top contact
44 which is separated from p-type region 50 by an oxide layer 52 has a ~indow
53 through which incident photons pass, the photons penetrating to the p-n
jun~tion region to render the junction region conducting through
electron-hole pair generation when reverse bias is applied. The PIN
photo-diode 42 receives light from one face of the LED. As indicated
previously, the light intensity provides an accurate analog of output light
launched into the fibre 38. If the output of the LED decreases through aging
cr because of telnperature fluctuations, the top emission from the LED is
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reduced. The monitored light is used to generate a control signal which is
applied via a feedback loop to a LED driver clrcuit to preserve constant
optical power output and also to improve linearity.
Referring to Figure 2, the feedback circuit shows schematically
the LED 10 and photodiode 42~ Photodiode output is amplified at an amplifier
54 before feeding into a comparator 56 for comparison with a reference voltage
corresponding to a preset peak optical power (digital) or r.m.s, optical power
(analog). The comparator output is fed to a current generating network 58 for
modifying DC bias current 59 or modulation current 60, respectively.
For analog systems, the comparator outpuk can also be fed to an
automatic gain control unit 62 (broken line). An LED operating level and
corresponding photodiode current are first determined. The gain control unit 62
is then set to apply negative feedback to the current generating network 58
proportional to excursions of the photodiode current output from that operat~ng
level. The negative feedback improves linearity so ensuring that output light
launched into fibre 38 is a more accurate representation oF the impressed AC
modulation 60.
Referring now to Figure 3 there is shown an electro-optic deYice
which is functionally similar to Figure 1 but in which an LED 6~ and a
photodiode monitor 66 are fabricated side-by-side as a unitary structure. A
particular advantage of this device is that the LED p-n junction is grown
simultaneously with, and as an extension oF, the photodiode monitoring
junction. As shown, a GaAs substrate 68 has epitaxially grown thereon GaAlAs
c~nfining and active layers 70, 72 and 74. The chip is formed with a contact
layer 76a, 76b, the contact layer and the substrate 68 being etched through to
produce a well 78. On a lower surface, a chemically Yapour deposited
insulating layer 80 of SiO2 is photo-defined to produce windows 82. A
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Cr/Au contact layer 84 is vacuum deposited through the windows and gold
heatsinks 86, 87 are electroplated over the chip lower surface. Leads ~8
connect contact 76a and heatsink 86 into an operating current generating
circuit shown schematically as current generator 90 and variable resistor 92.
Leads 94 connect contact 76b and heatsink 87 into a monitoring c;rcuit shown
schematically as hattery 96 and signal level indicator 98.
Electrical isolation between the forward biased p-n Junction
region of the emitter and the reverse biased p-n junction region of the
detector is produced by etching a well 75 through to the top confining layer
70.
Although not shown, it will be understood that the output of
signal level indicator 98 is used to control a feedback circuit similar to one
of those shown in Figure 2 for stabilizing optical power output level or
linearity.
In operation, when a current is directed through the LED 64 via
the DC bias generating circuit 90, top emission takes place into the well 78
and side emission propagates through a guiding layer comprising the active
layer 74 and adjacent parts of the confining layer 70 and 72. In a p-n
junction region above window 82 which is subject to a reverse bias voltage of
about 4 to 5V, incident photons, which are absorbed energize electron-hole
pairs in a depletion region causing a current flow which is recorded at the
level detector 98. Part of the emitted light impinges on the detector after
radiation across the gap between emitter and detector mesas defined by well 753
Further light is guided around the well 75.
In one example of a monolithic emitter-detector fabricated, a
current range of between 100 and 200 mA directed through the emitter produced a
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monitored current of between 50 and 100 microamps at the level dekector 98.
Tracking was found to be within 1%.
As shown, the detector contact 85 is larger in area than the
emitter bottom contact 84 so as to increase the size of the region over which
electron-hole pair recombination takes place in the detector. However, a
symmetrical device can be fabricated with a detecting section identical with
the emitting section shown in Figure 3. This permits the device to be used as
a dual source LED if preferred to an emitter-detector combination device.
Although not illustrated~ a structure having a single central well can be
fabricated characterized by a bottom contact pattern including an internal spot
contact for the emitter and an external ring contact for the detector. Some
variation from symmetry will be required, in practice~ in order to permit
soldering of connecting leads and the provision of a heatsink for the emitter.
Although the invention is described in terms of a double hetero-
structure, it will be clear to those skilled in the semiconductor LED art that
single heterostructures or homojunction devices can be fabricated in a similar
manner. In addition, although the embodiments described have an LED
fabricated from GaAlAs, other III - V compounds can be used.
