This guide covers the basic optical transmitter and receiver types that were current at around 1999 - 2001, so the products referenced will be obsolete by now. However, the physics involved will still be relevant today. I have also included a little about fiber types, just to complete the work.
The posts are split into the following:
- Laser Diodes
- Wavelength
- Laser Receivers
- Fiber Types
1. Laser Diodes
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 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, (at the data rates we are talking about) 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.
To compensate, 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 for very high data rates, 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.
Moving on from our discussion of laser types, lets take a look at wavelength and where these lasers are used.
2. Wavelength
There is a whole spectrum of wavelengths used for a multitude of purposes out there, but the wavelengths most often used in networking are:
There is also the 1480nm wavelength and this is used a great deal in data communications, but generally as a part of an EDFL pump amplifier. That is really outside of the scope of this document but does bear mentioning.
850nm
850nm lasers are cheap to make and use cheap POF (Plastic Optical Fiber) or multimode fiber. This makes them ideal for directly modulated applications where cost is an issue. The drawback for this type of laser is the short distance it is able to transmit over. These types of links tend to be short distance fiber Ethernet and similar.
1310nm
The 1310nm wavelength sits between two absorption bands making it really useful for single signals, rather than multiplexed signals, but it can work well with both single and multi mode fiber.
1550nm
With 1550 nm we have a larger spectrum to work with allowing us to create a large number of channels (1470nm - 1610nm). Matched with single mode fiber, DWDM (Dense Wave Division Multiplexing) systems are capable of carrying over 160 channels of very high data rates over long distances. This is made possible by the ability to tune lasers as discussed in the previous post.
To Compare
Wavelength Fiber Type Data Rate Distance
850nM POF 1Gb/s 100m
850nm multi mode 1Gb/s 500m
1310nm multi mode 10Gb/s 1km
1310nm single mode 40Gb/s 10km+
1550NM single mode 100Gb/s 100Km+
It should be noted that these are very approximate and not absolute. As technology marches onward, these numbers will change, and are most likely out of date now. For instance, if we look at the current Ethernet
standards:
Standard Fiber Distance wavelength
1000Base-SX mmf 550m 770-860nm
1000Base-LX* smf 5km 1310nm
1000Base-ZX smf 100km 1550nm
*There is also a mmf version that has a range of 550m.
... and then there is SDH, ATM, Metro Ethernet, CWDM and DWDM long haul systems to name just a few fiber systems out there and they are all specified differently, but the above information is good 'rule of thumb' stuff.
3. Receivers.
There are generally two types of receiver used in optical communication systems:
- PIN Diodes (Positive insulator negative)
- ADPs (Avalanche photo Diode
PIN diodes are simple to use, require little circuitry, and are cheap, but have no 'gain' so one photon received = one electron in the circuit. APDs on the other hand have gain (~100) due to their structure and materials used, but they are expensive and require a high voltage circuit to bias them. That is the short version.
For the long version of this answer, there is google and wikipedia and a whole slew of other white papers from folks like Fujitsu and JDSU who manufacture these devices and know a great deal about them.
In terms of their construction, they are very similar:
The 'secret sauce' in most cases for these devices is the lens.
So where are they used?
When I was working with these devices, the rule of thumb was this:
PIN 1310nm short distance 2-3Gb/s mmf
APD 1310nm long distance 10Gb/s+ smf
APD 1550nm any 10Gb/s+ smf
I suspect this has changed now due to advances in manufacturing and technology in general.
And so that brings us to fiber which I will cover in the next post.
4. Fiber Types
In simple terms, fiber is the pipe we push the light through; it is the optical equivalent of the electrical wire and just like wire there are certain things about fiber that we need to know.
First of all, fiber is not loss-less. As our light signal travels down our fiber, it becomes attenuated and distorted, just like and electrical signal on a wire. The numbers involved are different of course, but still, we have to design around these issues.
Fiber is also made of glass, and generally about the thickness of a hair. This makes it very hard to join or splice with other fibers. The glass fiber is clad in another material (the cladding) that keeps the light from escaping the fiber, by a process called total internal reflection.
The fiber and cladding are then protected by another outer layer, the buffer, and then the plastic jacket forming the outer layer.
There are two (main) types of fiber for networking - single mode fiber and multi mode fiber. I was going to write a whole lot about this, but two things come to mind; like wiring, we want to keep the fiber cost low, and out of the two, multi mode fiber is less costly than single mode fiber. So we use mmf for short haul lower data rates (850nm, 1310nm) and we use smf for high data rates and long distances (1550nm)
I said there was two things didn't I? The second thing is this - Wikipedia has a great page on fiber and I encourage you to read it.
And that wraps up my quick discussion on optical networking components. I hope that was useful information.
