ACES Int'l
Secrets & Tips
for Fiber Optics (Page 2)
Figure 2 shows a planar heterojunction LED in a cross sectional view. In order to achieve a concentration of light emission in a small area, a manufacturing method was incorporated to confine current to the desired area. Since individual layers are grown across the entire area of the wafer, a separate process was used to confine the current. First an n-type tellurium-doped layer is grown on a zinc-doped p-type substrate. Here’s where it gets interesting... Before any additional layers are grown, a hole is etched through the n-layer and just into the substrate. The diameter of the hole defines the ultimate light-emitting area. Next, a p-type layer of AlGaAs is grown. this layer is doped such that its resistivity is quite high: this impedes carrier flow in a horizontal direction, but vertical flow is not impeded since the layer is so thin. This ensures that current flow from the substrate will be confined to the area of the etched hole.
The next layer to be grown is the p-type active layer. The aluminum-gallium mix of this layer gives it an energy gap corresponding to 850 nm wavelength photons. The actual P-N junction is then formed by growth of n-type tellurium-doped aluminum-gallium-arsenide. The doping and aluminum-gallium mix of this layer is set to give it a larger energy gap than the p-layer just below it. This makes it essentially transparent to the 850 nm photons generated below. A final cap layer of gallium arsenide is grown to enable ohmic contact by the top metal. The end result is an 850 nm planar LED of small emission area. The radiation pattern is still lambertian, however, and that can cause problems when coupling this device to an optical fiber. One might expect that when a fiber with a core diameter equal in area to the emission area of the LED is placed right down onto the surface of the LED a good coupling would result. But, since the emission pattern is lambertian, high order mode rays would not be launched into the fiber. This has to do with the numerical aperture NA of the fiber, the cone of acceptance, and the critical angle of incidence. Higher mode emissions will not be coupled into the fiber and loss will occur.
One way to compensate for the effects of these lambertian emissions is to focus the light produced by the LED by means of collimation. By placing a spherical lens over the emitting area, this collimating effect will convert high order modes to low order modes (see Figure 3).
