Fundamental Principles .1 Emitter Element

The fundamental characteristics of the components described here is the conversion of an electric current into an electromagnetic wave (light), or the reverse.

Under the heading light is understood the electromagnetic spectrum from near to the ultra-violet range ( λ = 0.3µm) through the visible range (0.38µm < λ < 0.78 µm) up to the infra-red region ( λ = 1.2 µm).

Important modern emitter components are light emitting diodes (LED,IRED) and solid state laser diodes as emitter components; receiver components are photodiodes (P-N diodes, PIN-diodes), photo-transistors and lateral effect diodes (PSD).

5.1.1.1 Light emitting diodes (LED, IRED)

Light emitting diodes are basically semiconductors components, which consist of a PN junction. When a voltage is applied in the forward direction of a PN junction the electrons are excited and move easily into the p-side. Gallium Arsenide is the semiconductor ma-terial mainly used to produce light emitting diodes, which have a high efficiency. Gallium Arsenide has a wavelength of λ = 0.9µm. This wavelength lies near the infra-red region, for that reason GaAs is suitable for infrared diodes (IRED) with a high quantum efficiency (efficiency of a luminous source).

An important representative of indirect semiconductor is GaP;by doping with Nitrogen (N) or Zinc Oxide (ZnO) recombination of electron-hole pairs is achieved with emission of light ,which is related to these impurities. The rest of the energy is lost as heat. It can be seen from this that the efficiency as a luminous source( quantum efficiency) of an impure or extrinsic semiconductor is less than for an intrinsic semiconductor.

Through the choice of semiconductor and by doping with equipotential recombination centres it is possible to adjust the wavelength. The quantum efficiency of light emitting diodes in the visible spectrum is very much less compared to that of an IR-Diode.

Providing the power dissipation limit and the maximum junction temperature of the semi-conductor crystal are taken into consideration the light emitting diode can be modulated with a high impulse current I

d; the associated momentary radiated power is many times that produced in continuous operation. Diagram 5.1 shows a typical maximum permiss-ible pulse current for given duty cycles and known pulse widths t_i.

Infrared light emitting diodes have typical rise and fall time in the region of 400ns to 1µs and are therefore suitable for optical modulation.

One distinguishes between component types with an optical glass window and those with an optical system. The first have a very large aperture angle (diagram 5.2).

window cryswindow crystaltal

These components indicate a relatively small radiated intensity, but with the addition of an optical system they demonstrate a well defined radiated intensity distribution (direc-tivity characteristic). For reflex photoelectric sensors, whichrequire as far as possible a parallel radiation distribution, light emitting diodes with an optical glass window are particularlysuitable. In the case of components fitted with a lens the radiated intensity is relatively high and the aperture angle is small (Diagram5.3).

Another area of application for these components can be found in the direct detection sensors for middle and smalldetection ranges, equally they can be adapted for use with light guides.

Diagram 5.1 : Pulse Loading

Diagram 5.2: LED with optical glass window;

left: sketch of package, right: Intensity Distribution

Diagram 5.3: LED with lens;

left: sketch of package,

right: radiated intensity distribution.

Lens Crystal

window crystal

5.1.1.2. Semiconductor Laser diode

In the simplest example a semiconductor laser consists of highly doped p-n junction made from Gallium Arsenide (intrinsic semiconductor).

Two important effects give the semiconductor laser it's typical property of emitting coherent light, they are the so called induced emission and the optical resonator in the semiconductor crystal. Coherence means that the wave trains of light have the same frequency and have a rigid phase relationship to each other. As opposed to the spontaneous emission of light emitting diodes in the case of induced emission the recombination process is started by the external influence of light with the correct frequency.

For example an electron can start to emit at the moment when the influencing light wave oscillation rises, in this way all the emission processes are automatically coherent.

Amplification also occurs, in that a weak primary radiation induces a strong secondary radiation. An optical feedback must be provided to maintain this process. An optical resonator, which is tuned exactly to the transition frequency fulfills this requirement, since a standing fundamental frequency wave is produced, which is again a fundamental condition for induced emission.

In semiconductor lasers the optical resonator is produced by the parallel planes of the end surfaces of the Gallium Arsenide crystal in which the p-n junction is formed. The reflection at these cleavage planes is about 30% and therefore large enough to achieve the required feedback effect. The remainder of the light passes out of the crystal at both ends (Diagram 5.4).

Diagram 5.4: GaAs Semiconductor Laser

Metal Contact

In contrast to light emitting diodes the emission spectrum of semiconductor lasers is very much smaller, as a result of the induced emission and the amplification in the resonator.

The spectrum of the laser is different to that of light emitting diode, which has a continu-ous spectrum, in that the spectrum in most cases consists of discrete spectrum lines,

pro-duced by a large number of natural oscillations of the fundamental frequency together.

By the use of special light guide techniques the spectrum can be

compressed to prac-tically a single line.

(Diagram 5.5)

Semiconductor lasers are very sensitive to changes in temperature. The threshold current has a temperature coefficient of typically 1.5%/°C. Especially by falling temperatures is this effect critical, here the laser characteristic curve is very steep, because of this the diode reaches a region of high power, which results in damage. For this reason it is necessary to provide sufficient

temperature stabilisation for the crystal. A further possibility is to control the output power and keep it constant. Many semiconductor lasers have an integrated monitor diode to enable the output power to be regulated.

Semiconductor laser diodes have typical rise times and fall times in the region of 1ns to 5ns, which makes them especially suitable for high frequency optical modulation. In the case of laser diodes the beam forming exit slit is very small compared to that of the usual light emitting diode, so that by the inclusion of suitable optics almost parallel radiation can be produced. As well as laser diodes with integrated monitor diode, there are com-plete laser units with diode and optics available (Diagram 5.6).

Diagram 5.6: Laser diode with Optics

Optics Laser

Diagram 5.5: Spectrum of an :LED and a Laser

5.1.2 Receiver Elements

5.1.2.1 Photodiodes (p-n and PIN diodes)

The task of the photodiode is the conversion of a received optical signal into an electric current. In light emitting diodes a radiating recombinations process is brought about by the injection of charge carriers in the p-n junction, in the photodiode the opposite process occurs (Diagram 5.7).

Diagram 5.7: Method of operation of a photodiode

Due to the different carrier concentrations in p- and n-regions a, so called, space charge region is produced, without any external influence, which is free from moving charge carriers.

Penetrating photons lead to the production of electron hole pairs close to the p-n

junction. Carrier pairs, which are produced in the space charge region, are separated by the electric field present and at the same time transported to the other side. Holes are attracted to the p-region and electrons to the n-region. In this way a photocurrent (drift current) flows in the reverse direction, without an external voltage being applied. Hole-pairs, which are created outside of the space charge region must first diffuse into the space charge region, there separated and contribute eventually to the photocurrent (diffusion current).

While in the case of the drift current the separation and transport of the carrier pairs takes place quickly in the case of the diffusion current the carrier pairs must first reach the space charge zone by the comparatively slow diffusion process. By means of a suitable internal construction of the photodiode the type of photocurrent and therefore the dynamic behaviour of the device can be controlled.

Optical Compensation

blue red infra-red Contact

Oxide

Space Charge Region RLZ

Metal contact N-Region N+ -Region

Photons of different Wavelengths

p + - region

In the case of the, so called, PN-diode the space charge zone is very small. Charge carrier pairs are formed outside of the space charge zone, mainly in the border region.

For this reason the diffusion current is predominant, so that PN diodes are distinguished by a relatively low frequency limit and a large rise time. On the other hand the so called Dark Current is relatively small. Hence the PN-diodes are particularly suitable for

measuring very low levels of illumination. PN-diodes have rise times and fall times in the range of 1µs to 3µs and junction capacities from 100pF to 1nF.

In the case of PN-diodes with wide space charge regions the resulting small junction capacitance C

j together with a selected load resistance produces a low-pass

characteristic, which influences significantly the frequency behaviour of the system.

These PIN-diodes have a higher frequency limit and a small rise time.

As in the case of light emitting diodes the photodiodes are separated into two important types:

Photodiodes with plane window have a very wide directional characteristic (Diagram 5.8);

therefore they are suitable for measuring intensity of illumination. A narrower and better defined directional characteristic can be obtained by the introduction of an optical system, so that these elements can be used in reflex photoelectric sensors, where this directional characteristic is required.

Photodiodes with integrated lens have a relatively narrow directional characteristic (Dia-gram 5.9). These elements are preferred in direct detection optical sensors with small and medium detection ranges; especially when a possible adaptation for use with light guides is required.

Diagram 5.8: Photodiode with plane window,

left: sketch of package, right: Intensity distribution

Diagram 5.9: Photodiode with lens;

left: sketch of the package right: Intensity distribution.

Radiation Sensitive Surface Cristsal

Lens Cristsal

5.1.2.2 Phototransistors

Phototransistors are basically photodiodes connected to a transistor which amplifies the photo current.

The dynamic behaviour compared to the photodiode is comparatively poor. A photo-transistor has a rise and fall time typically of 20us. The reason for this is to be found in the amplification mechanism, here the junction capacitance, due to the Miller effect, is in-creased by a factor B, because of this the maximum frequency, which can be achieved, is greatly reduced.

As opposed to the photodiode the relationship between the incident radiated power and the resulting photocurrent is not strictly linear in the case of the phototransistor and can vary between 4 % to 20% from the ideal characteristic. Similarly detrimental is the tem-perature dependency.

This large temperature dependency can also be an advantage. Namely, if a optical sensor is made with an IR-light emitting diode and a phototransistor the two temperature

relationships almost cancel each other out.

Phototransistors are available of similar construction and with similar optical properties to photodiodes. Simple and especially small phototransistors have only collector and emitter connections. In addition phototransistors are available with an additional base

connection, which enables the operating point to be adjusted.

5.1.2.3 Position Sensitive Detector (PSD)

An interesting variation of a photodiode is the so called position sensitive detector (PSD).

In principle the PSD is a photodiode with a strip shaped light sensitive surface. Contacts K₁, and K₂ are mounted at each end of the device; the common substrate contact K₀ is connected on the bottom face of the device (Diagram 5.10).

The PSD has, in addition to the blocking layer resistance, a so called cross section resistance R_q , in the longitudinal direction parallel to the light sensitive surface, therefore this resistance lies between K₁ and K₂

If the PSD is radiated with a spot beam of light from a light source a current I

ges is produced. The cross section resistance Rq is divided at this point into two section resistances R_q1 and R_q2. Similarly the current I_ges is divided into two current

components I₁ and I₂, which can be measured at the terminals K₁ and K₂. Of interest is the relationship between the point p

1, radiated by the spot beam, on the light sensitive surface and the components of current I

1 and I

2.

Diagram 5.10: Position sensitive detector

Light sinsitive surface

In diagram 5.10 a normalised abscissa p is inserted of which the points 0 and 1 corres-pond to the ends of the light sensitive surface. The position of the spot of light p₁ can be defined using this abscissa.

The following is true for the component resistances Rq

1 and Rq

From this it can be clearly seen that by measuring the two components of current I₁ and I₂ the position p1 of the spot of light, on the PSD, can be calculated.

The relationship between the penetrating radiated power F

e and the photocurrent is almost linear, as in the case of a conventional photodiode. Here it is interesting to note, that changes or variation in the incident radiated power have theoretically no effect on the position calculation from the above relationship, since these variations effect both components of current I

1 and I

2 in the same proportion and are therefore eliminated from the quotient.

In most cases PIN diodes are preferred, for large surface area position sensitive detectors, in order to keep the rise and fall times small. Depending on the size of the optical surface switching times from 500ns to 50µs are to be found.

In addition to the one dimensional position sensitive diodes described here there are also two dimensional devices.

It is possible with these components to set up a two dimensional coordinate system, which can be used to determine the

position on a surface.

5.2 Methods of Operation of Photoelectric sensors