How to Design a High Performance Optical Receiver?

The structure of an optical receiver is simple: consisting of just a photodiode to produce the electrical current and an amplifier. But do not be fooled: it is far more complex to design a really high performance optical receiver. So we will talk about some of the criteria of actual receivers.

The are a bunch of factors involved in the process. You have to take into count of signal current noise, noise from the photodiode such as from an avalanche diode, noise from the amplification electronics, thermal noise, dark current and signal to noise ratio for high data bit rate.

How to choose the correct photodiode?

Two major types of photodiodes are commercially available for optical receiver applications: PIN photodiode and Avalanche photodiode.

PIN photodiode

PIN photodiode is an extension of PN diode. But PN diode has many vital flaws. PN diode has too small depletion region which makes the received optical power must be fairly high to generate sufficient current. The second flaw is PN diode's slow response which limits it only to kilohertz applications.

PIN photodiode solved these limitations on PN diode. The depletion region has been made as large as possible and most of the photons absorbed within the depletion region. And the inclusion of the intrinsic layer decreases the function capacity which raises the switching speed and the photon capture area.

The benefit of the improved design is a more efficient opto-electro conversion and faster speed.

Avalanche photodiode (APD)

In a PIN photodiode, each absorbed photon produces one electron hole pair which sets one electron flowing in the external circuit.

But in a Avalanche photodiode, a few incident photons result in many carriers being produced and an increased external current. How does an Avalanche diode achieve this?

This is produced by the phenomenon called avalanche multiplication. What this does is a strong electric field will accelerate current carriers so much that they knock valence electrons out of the semiconductor lattice and with a high enough bias voltage an avalanche of carriers will result.

While all those are good, there are also a dark side on this. While the carriers are amplified, the uneven nature of the multiplication introduces noise as well.

As a conclusion, although avalanche photodiodes are non-linear and fairly unstable, they are very similar to normal silicon photodiodes except that they require a slightly lower operation voltage to achieve good multiplication.

Other critical performance parameters of a photodiode

For your reference, some of the most critical parameters are listed below.

Responsivity

Photodiode responsivity is the ratio of generated current to incident light power. This is usually expressed in Amp/Watt. Sometimes this is also referred to as quantum efficiency.

Dark Current

Dark current is the current produced by the photodiode when there is no incident light at all. The dark current includes current generated by background radiation and the saturation current of the semiconductor junction. Dark current is a source of noise when it is being used in optical communication systems.

Noise-equivalent power

Noise equivalent power is the minimum input optical power needed to generate photocurrent. This equals to the rms noise current in a 1Hz bandwidth.