Electro-optic devices

#This invention relates to light emitting diode devices adapted to give stabilized light output.

#One of the primary light sources used in fiber optic communications systems 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 output level and linearity (for analog operation) must be stabilized against aging and temperature-related variation.

#Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

#FIG. 1 is a sectional view showing an electro-optic device according to the invention;

#FIG. 2 shows a feedback network for stabilizing LED light output as a function of modulating current;

#Referring firstly to FIG. 1, there is shown a Burrus-type light emitting diode (LED) 10 having a double 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 to the confining layer 14. Over the outer surface of the substrate is deposited a 2500 A contact layer 22 of Au/Ge. On the undersurface of the confining layer 16 is chemically vapour deposited a SiO.sub.2 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 70 A units of Cr and 2500 A 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 confining 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 travels through over 200 microns mainly along the active layer 12 but also along the confining layers 14 and 16. Consequently a large proportion of the total side emission from the LED active region is absorbed and side emission actually emitted from the LED chip is an order of magnitude less than that available as top emission. As illustrated by J. C. Dyment et al (IEEE Transactions on Electron Devices, Vol. E.D. 24 No. 7, July 1977), typically the top emission ranges from 0-25 mW for a corresponding rise in side emission from 0-800 microwatts with operating current varying from 0-600 mA.