The Importance of Optical Detectors

Today, I am going to discuss what happens at the other end of a fiber link -- detectors. Optical detectors, as the name implied, can detect the amount of light received. Our very own eyes are a pair of detectors as they can receive light information with the retina and transmit that light data to our brain. In the visible light spectrum, our eyes are great detectors to inspect fiber break or light leakage. However, most fiber works in the invisible wavelength spectrum where human eyes won't be able to see. That is the where the optical detectors come in.

Photoelectric effect

It is impossible to explain how optical detectors work without mentioning the photoelectric effect. 

 

This phenomenon was first observed by German scientist Heinrich Hertz who only published his observations. It was Albert Einstein who later studied this effect and quantified the discrete light energy as photons in one of his famous papers that won him a Nobel Prize in 1921. Vacuum photodiodes and photomultipliers take advantage of this technology and can convert the light signal back to electric signals. One critical parameter for characterizing detector is responsivity. It is the ratio of output electric current to the optical input power, with the unit A/W.

 

In the end, we will compare the responsivity of different detectors and choose wisely based on each application.

 

Vacuum Photodiode and Photomultiplier

A vacuum photodiode (or phototube) is mainly comprised of a cathode and an anode. When the cathode detects photons, electrons are emitted according to the photoelectric effect, and current will go through the circuit since electrons are attracted to the anode. The following sketch shows how vacuum photodiode works2.

 

The limitation of a vacuum tube is that it is physically too big and operates in a wavelength range lower than what fiber communication requires. Another issue is that it also involves much voltage to power it. The typical responsivity of a vacuum photodiode is in the magnitude of mA/W.

 

Photomultiplier, on the other hand, works more efficiently because of its built-in gain mechanism. In addition to the anode and cathode, it also has a series "dynodes" for accelerating the electrons. The following illustration shows the simplified circuitry of a photomultiplier3.

 

Just like in vacuum tube, electrons are radiated after photons got absorbed by the cathode. However, the emitted electrons are attracted by intermediate dynodes which have very high voltage. What is so good about dynodes is that there can be more than one electron gets emitted when only one electron is attracted to it. This is called secondary emission caused by high kinetic energy electrons possess. Each electron now becomes more than one electron after hitting each dynode, causing a series of multiplying which eventually leads to electric signal amplification.

 

The gain at each dynode is about 5, so if there are 3 dynodes in the tube, the total increase will 125 (5x5x5). In reality, there are typically 5 to 10 dynodes in each photomultiplier, so the actual gain is in the magnitude of millions. Photomultiplier tubes are high-speed but also consume hundreds of voltage to power each dynode. It is heavy and big, almost the size of a hand grenade4. Unfortunately, photomultipliers are not suitable for the fiber optic communications.