In this post, we will discuss laser diodes, and how we use them for data communications.
So what is a laser diode? I’m not going to go too deep with this, as there are some good articles already written and posted that go into a great deal of technical detail about laser diodes and how they work. For the purposes of this article, all we need to know is that it is a semiconductor device, much like an ordinary LED, that generates coherent light at a tightly controlled wavelength.
The other curious fact about laser diodes is that the light they generate, comes out of both ends of the die*. Die is a term used to describe the chip itself. In the case of lasers it is not a silicon chip, but rather a Gallium chip of some sort (Often AlGaAs).
Other pertinent facts are that the wavelength of the light is effected by temperature, so to control the wavelength, we must control the laser temperature.
The other fact to consider is that to generate a coherent takes a little time. This means that for the most part, we cannot simply translate our digital ones and zeros into an on/off function for the laser. Rather we must modulate the light output of the laser to form our data stream.
Quality and stability of the light output are very important factors in lasers for communications. The distance we can transmit data, and the rate at which we can transmit it, are directly connected to the stability and quality of the source.
So how do we control the laser?
Generally speaking we control two things – the laser current and the laser temperature, and both are related. As current flows through the laser, it will generate heat, changing the wavelength (and potentially, the quality) of the light output. You can see how modulating the current directly would cause issues with the laser!
The laser die is mounted on a peltier cooler. We can control the current through the peltier cooler to control the die temperature.
Remember the die also creates light out of both ends? Well the light out of one end goes to our fiber for transmission, but the light at the other end is measured with a sensor to detect the light intensity. This way we can tightly control our laser.
What we have coming out of the laser at this point, is a carrier wave. This is a controlled wavelength of light at a known and constant intensity. What we need is data, so how do we do that?
In order to create ones and zeros, we need to modulate the light from the laser. This can be done in three ways:
- Integrated modulator
- External Modulator
- Direct modulation
The integrated modulator uses another die (usually AlGaAs again) and that die is cemented to the same substrate as the laser. The ‘transparency’ of the modulator die is also controlled by a current and it is this current that modulate with our data signal. In many ways this acts like a shutter, turning the light on and off, although in reality it isn’t that black and white – it tends to limit the light output to 10% for a zero and 90% for a one.
The external modulator works in exactly the same way, except that the modulator is a separate device and has to be connected to the laser.
We mentioned direct modulation of the laser itself above as being a bad idea, and it is, but; for short distance low bandwidth data streams it works just fine and as the technology has progressed, we can see 850nm lasers being directly modulated at 1Gb/s for running over 100-200M.
So just to recap:
- Lasers for data communications are semiconductor devices based on Gallium.
- We control the wavelength by controlling the temperature of the laser
- There are three types of laser:
- Carrier Wave
- Modulator integrated
- Direct modulation