The common feature of all optical sensors is that they modulate an optical signal in response to a given measurand. In the design of a light modulated sensor system there are several light sources which are available. The main classes will be described briefly here.
The principle sources are:
1 the incandescent sources;
2 the light emitting diode (LED); and 3 the laser.
5.2.1.1 The Incandescent Source
Incandescent sources produce ’white’ light and can be used for simple intensity modulated sensors and for excitation of phosphorus and photoluminescent material. The tungsten filament lamp is the most commonly used broadband sources.
The filament is heated by passing a current through it which radiates energy over a broad wavelength spectrum. The lifetime of commercially available sources are in the range 100-100,000 hours which is dependent on the operating current.
Incandescent sources cannot be directly modulated due to the delay between current change and lamp output change. Only the chopper can be used as a modulator as the other techniques are dependent on wavelength. This limits modulation to 20kHz.
5.2.1.2 The Light Emitting Diode (LED)
The LED is most commonly used in multimode fibre systems. The output from the LED is incoherent and cannot therefore be used for interferometric sensors. LEDs are relatively inexpensive and can be modulated directly by varying their current drive. LEDs are made from a doped semiconductor p-n junction and when current is injected into the junction under forward bias, electron-hole recombination occurs and photons are emitted. This spontaneous recombination process is called injection luminescence. The wavelength of the emitted photons is a function of the energy gap between the semiconductor valence and conduction bands. This may be controlled by choice of semiconductor materials and dopant concentrations which has led to a wide range of wavelengths being available from 650nm in the visible part of the spectrum to 1.3/zm in the infra-red.
The output spectrum of a LED is around 2% of its nominal centre wavelength giving a typical full width half maximum value of 50nm.
If used within specification, LEDs degenerate very slowly with time and are considered to have failed when the output drops to 67% of the original value for a given current. Specified lifetimes are in excess of 106 hours.
5.2.1.3 The Laser
There are a wide range of lasers commercially available each with various characteristics. Lasers are classified based on the medium in which the lasing action takes place. Hence, there are solid-state lasers, gas lasers, dye lasers and semiconductor lasers. Of these, the HeNe laser and semiconductor lasers have established themselves at the forefront in optical applications. Gas lasers and the injection laser diode can be
used for both multi and single mode optical fibre sensors. In multimode sensors they are used when an LED cannot provide sufficient power. Single mode sensors are interferometric and therefore require a coherent optical source which can only be provided by a laser. However where a highly coherent and stable continuous wave source is required a single mode HeNe laser is normally used. HeNe lasers generate output powers in the range 0.1 to lOOmW; 5mW is usually adequate for single mode sensors at wavelength of 633 nm.
These two sources will now be reviewed in more detail. (a) The HeNe Laser
The HeNe is commonplace in any optics laboratory to demonstrate reflection, diffraction and interference phenomena. It is used extensively in commercial applications including bar code scanners, alignment systems and holography.
Gas lasers consist of an optically resonant cavity filled with a medium which exhibits optical gain. In order for lasing to take place stimulated emission must dominate over spontaneous emission and absorption. This is achieved when the upper energy level population is greater than that of the lower level. This situation is described as population inversion and is brought about by an input of energy into the laser medium. Because the laser is made up of stimulated emission all the photons emitted have the same phase and this gives a highly coherent source.
(b) Semiconductor Lasers
Semiconductor lasers are, generally, favoured because they can be directly current driven and are cheaper than gas lasers. Semiconductor lasers, also known as injection laser diodes (ILD), like the LED, are formed from a p-n junction which emits light
when current is injected under forward bias. In fact at low injection currents the ILD behaves as an LED emitting light by spontaneous emission. However, the p-n junction of the ILD is heavily doped which means that larger currents cause a population inversion of charge carriers to occur. In this situation a spontaneously emitted photon may generate stimulated emission of another photon and if the injection current is sufficient, optical gain takes place. The p-n junction is contained in an optically resonant cavity to form a laser. At low currents the optical output is dominated by spontaneous emission and the ILD behaves like an LED with a wide spectrum. Above a threshold current, stimulated emission dominates which causes lasing and a consequential reduction in spectral width.
ILDs can either be multi or single mode devices. Single mode lasers have high spatial coherence and are therefore suitable for single mode interferometric sensors. For multimode systems where coherence is not relevant, the high launch powers of the ILD can make it an attractive device.
5.2.1.4. Conclusions
The device to be designed in this research programme was to utilise phase modulation. Therefore, a coherent source is a prime requisite and therefore eliminates both incandescant sources and LEDs. Planning ahead, it would make good sense to incorporate both the light source and processing electronics in a single unit. Consequently a semiconductor laser would seem appropriate given the relatively bulky nature and added expence of HeNe lasers. Both these factors would be of importance if such a light modulated accelerometer is to gain market acceptance. However, at the onset of the project a HeNe was available and was, therefore, to be used in the initial development of the device.