Light sources and detectors

Most light sources and detectors are electronic devices built from the same semiconductor materials as are used in transistors and integrated circuits. The design of these devices is a separate study and will not be considered here. Instead, our view will be restricted to the characteristics which are of interest to the user.

Lasers

The most common form of laser diode is called an injection laser diode (ILD) or just injection diode (ID). The word injection is not of interest — it merely refers to part of the process occurring inside the semiconductor material. A laser provides a light of fixed wavelength which can be in the visible region around 635 nm or in any one of the three infrared windows. The light has a very narrow bandwidth, typically only a few nanometers wide. This ensures that chromatic dispersion is kept to a low value and this, together with fast switching, allows high data transmission rates.

As the laser device itself is barely visible to the unaided eye, it must be contained in some form of package. Two typical examples are shown in Figure 14.1.

Lasers for visible light

The light is launched via a lens system to allow it to be concentrated into a beam. Visible laser light finds applications in bar code readers, CD players, medical and communication systems. They are usually fitted with a built-in light detector so that they can receive reflected information as in the case of the bar code reader.

Lasers for 850 nm use

These can be packaged in either of the ways illustrated in Figure 14.1 depending on their application. The fact that their output is not visible allows for use in security, ranging, automotive and industrial and military applications. They also provide the light source for short and medium range fiber communications.

Lasers for singlemode communications

Successful launching into singlemode fibers requires very high precision and this is achieved by optimizing the position of an attached pigtail which can then be connected to the main fiber run by any desired method.
A photoelectric cell is also included as a monitoring device to measure the output power. This provides feedback to allow for automatic control of the laser output power.
The output power of a laser is affected by any change in its temperature, generally decreasing in power as the temperature increases. Some laser modules include a temperature sensor to combat this problem. It provides internal temperature information which is used to control a thermo-electric cooler like a small refrigerator, to maintain the temperature. The temperature stability is also improved by bolting the laser package to some form of heat sink such as the instrument casing.

Laser safety

Both visible and infrared light can cause immediate and permanent damage to the eyes. The shorter wavelengths cause damage to the retina and the longer wavelengths attack the cornea, in neither case can medical science offer remedy once the damage is done. Permanent loss of eyesight in less than a second by exposure to light we can't even see — it doesn't seem fair somehow.
It is extremely important that we take sensible precautions.
Never look into:

• A live laser source
• An unknown light source
• Any fiber until you have ascertained that it is safe. Check it yourself even if trusted colleagues say 'its OK we've just checked it out'. They may be talking about a different fiber or they may have made a mistake.

If an instrument such as a live fiber detector is used, make sure it is working.

Beware of concentrating the light by instruments such as will happen when checking a cleave or the end condition of a connector with a microscope.
Laser classifications are based on an international standard titled Radiation Safety of Laser Products, Equipment Classification, Requirements and User's Guide, referred to as IEC standard 825. Additional national standards apply in each country.

The IEC 825 classification has used four classes of laser based on the accessible emission limit (AEL). Every laser must carry a warning label stating the class of laser as shown in Figure 14.2. It is the responsibility of the manufacturer to

determine the classification of the laser and they do so by measuring the wavelength, output power and the pulsing characteristics.

IEC classifications

Class 1: Safe under reasonably foreseeable conditions of operations. Note that it doesn't say 'safe under any conditions'.

Class 2: Visible lasers with light output within the visible spectrum of 400–700 nm. There is an assumption here that the blink reflex will close the eyes within a fraction of a second and hence provide protection. Prolonged exposure will cause damage.

Class 3a: Safe for viewing by the unaided eye either visible or infrared light but possibly unsafe when viewed with instruments.

Class 3b: Direct viewing is hazardous but reflected light is normally OK. Note the normally. Not to be viewed with instruments.

Class 4: Horribly dangerous. Even reflections are hazardous and the direct beam can cause fires and skin injury. Not normally used for communications.

Control measures

For classes 2, 3 and 4, control measures are employed such as interlocks, keys, laser 'on' warning lights, remote switching, prevention of reflections across walkways. The precautions depend on the situation, use and power of the laser. The appropriate national standards as well as IEC 825 should be consulted for guidance.

Laser specifications

Wavelength

The wavelength quoted is only a typical value. So if we want to buy a laser for the 1300 nm window, the one offered may well be quoted as 1285–1320 nm and the actual frequency will fall somewhere between these limits. Sometimes it would just be sold as 1300 nm (nominal).

Rise and fall time — Figure 14.3
This is a measure of how quickly the laser can be switched on or off measured between the output levels of 10% to 90% of the maximum. A typical value is 0.3 ns.

Threshold current — Figure 14.4

This is the lowest current at which the laser operates. A typical value is 50 mA and the normal operating current would be around 70 mA.

Spectral width — Figure 14.5

This is the bandwidth of the emitted light. Typical spectral widths lie between 1 nm and 5 nm. A laser with an output of 1310 nm with a spectral width of 4 nm, would emit infrared light between 1308 nm and 1312 nm.

Operating temperature

No surprises here. Typical values are –10°C to +65°C and therefore match the temperature ranges of fibers quite well.

Voltages and currents

The specifications also list the operating voltages and currents of the monitor detector, the cooler current and the thermistor resistance. These are generally only of interest to the equipment designer or the repair technician.

Output power
The output power may be quoted in watts or in dBm.

LEDs — light emitting diodes

LEDs can provide light output in the visible spectrum as well as in the 850 nm, 1350 nm and the 1500 nm windows. Compared with the laser, the LED has a lower output power, slower switching speed and greater spectral width, hence more dispersion. These deficiencies make it inferior for use with high speed data links and telecommunications. However it is widely used for short and medium range systems using both glass and plastic fibers because it is simple, cheap, reliable and is less temperature dependent. It is also unaffected by incoming light energy from Fresnel reflections etc. Although the lower power makes it safer to use, it can still be dangerous when the light is concentrated through a viewing instrument. Typical packages are shown in Figure 14.6.

PIN diodes

A PIN diode is the most popular method of converting the received light into an electronic signal. Their appearance is almost identical to LEDs and lasers. Indeed the diagrams in Figure 14.6 would serve equally well for PIN diodes if the labels were changed. They can be terminated with SMA, ST, SC, biconic and a variety of other connectors or a pigtail.
It may be of interest to have a brief look at its name. It uses a semiconductor material, either germanium or silicon. The pure semiconductor material is called an intrinsic semiconductor — this is the I in the name. To make it work, we have to add a controlled amount of impurity into the semiconductor to change its characteristics. The semiconductor is converted into two types, one called P-type semiconductor and the other called N-type. These are arranged either side of the I material to make an I sandwich. Hence P-I-N or PIN diode. The theory of its operation will not be considered further.
While we can still buy straightforward PIN diodes, it is more usual for it to have an amplifier built into the module to provide a higher output signal level.
Avalanche diode also called an avalanche photo diode or APD
Higher output signals can be achieved by an avalanche diode. It uses a small internal current to generate a larger one in the same way that a snow-ball rolling down a mountainside can dislodge some more snow which, in turn, dislodges even more snow and eventually gives rise to an avalanche.
They have the advantages of a good output at low light levels and a wide dynamic range — it can handle high and low light levels. However there are a number of disadvantages which tend to outweigh the benefits. It has higher noise levels, costs more, generally requires higher operating voltages and its gain decreases with an increase in temperature.

Light receiver specifictions

Wavelength
This is quoted as a range e.g. 1000 nm to 1600 nm, or by stating the frequency that provides the highest output e.g. peak wavelength = 850 nm.

Dynamic range or optical input power

Dynamic range is the ratio of the maximum input power to the lowest. It is quoted in decibels e.g. 21 dB. The optical input power is the same information expressed in watts. e.g. 1 μW to 125 μW.

Responsivity
A measure of how much output current is obtained for each watt of input light. e.g. 0.8AW–1. This means that the current will increase by 0.8 amps for every watt of increased light power.

Response time
This is the rise and fall time that we saw in Figure 14.3. It determines the fastest switching speed of the detector and hence limits the maximum transmission rate e.g. tr or tf = 3.5 ns.

Bit rate or data rate or bandwidth

These are both measures of the maximum speed of response to incoming signals and is therefore determined by the response time above.

Kevin M Contreras H
CI 18.255.631
CRF
http://www.kiet.edu/ensite/downloads/Introduction%20to%20Fiber%20Optics%20-%20John%20Crisp.pdf