drive circuitry for leds and laser

Presented By :- Diwaker PantME (REGULAR- 2011)NITTTR CHD

April 12, 2023 1Presented by :- Diwaker

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To achieve the reliable and secure communication using optical fiber it is essential that all the components within the transmission system are compatible so that their individual performances, as far as possible enhance the overall system performance.

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Prior to the discussion of the transmitter circuits , we will discuss the differences and limitations between LED and ILD.

There are certain parameters on the basis of which we can discuss the limitations of Optical Sources.

1. Power Considerations 2.Linearty 3.Thermal Consideration 4.Response

5.Spectral Width

April 12, 2023Presented by :- Diwaker Pant 3

1. Power Considerations


Fig. 2 Light Output Vs. Input Current

Optical power coupled into a fiber from a LED can be 10 to 20 dB below that obtained a with a typical Laser diode.

Power advantage gained with the ILD is a major factor in the choice of the source.

2. Linearity Considerations

The linearity of the optical output power against current characteristics is an important consideration with both ILD and LED.

Source non-linearity can cause severe distortion of the transmitted signal.

An alternative approach to obtaining a linear source characteristic is to operate an injection laser in the light generating region above its threshold.


3. Thermal Considerations

The variation of injection laser threshold current with the device junction temperature can cause a major operating problem.

Hence any significant increase in the junction temperature may cause loss of lasing and a subsequent dramatic reduction in the optical output power.

In general we can say that LEDs are less temperature dependent as compare to ILDs.


4. Response Considerations

The speed of response of the two types of optical source is largely dictated by their respective radiative emission mechanisms.

Spontaneous emission for LEDs and Stimulated Emission for Laser diode are responsible for their response.

In general we can say that ILDs have fast response as compare to LEDs.


5. Spectral Width

Incoherent emission from an LED usually displays a spectral line-width of between 20 and 50 nm when operating in the 0.8 to 0.9 μm wavelength range.

LEDs have wide spectral line-width as compare to ILDs.

Wide line width of the LEDs causes material dispersion within the fiber.

Use of the injection laser greatly reduces the effect of material dispersion within the fiber.


As we know that ILDs are more powerful and having high speed over LEDs but LEDs are also used because LEDs are very easy to operate as compare to ILDs.

Drive circuits are circuit configurations that may be used to convert the information voltage signal at the transmitter into a modulation current suitable for an LED source.

We divided the drive circuitry into two parts namely

1. Analog Transmission drive circuitry

2.Digital Transmission drive circuitry


Reason for drive circuitry..

For analog transmission the drive circuit must cause the light output from an LED source to follow accurately a time varying input voltage waveform in both amplitude and phase.

It is important that the LED output power responds linearly to the input voltage or current.

Unfortunately, this is not always the case because of inherent nonlinearities within LEDs which create distortion in the signal.

So suitable compensation techniques are used.


Two possible high speed drive circuitry are given below-:


CE Configuration Drive circuit Darlington transistor pair Drive Circuit

Common emitter amplifier which converts an

input base voltage into a collector current.

Darlington transistor pair is used to reduce the impedance of the source or kind of current amplification.

Darlington transistor pair is used to drive the high radiance LEDs at freq. of 70 MHz.


Another simple drive circuit configuration is differential

amplifier operated over its linear region which directlly modulates the LED.


Differential amplifier drive circuit continued…

The LED operating point is controlled by a reference voltage Vref.

The transistor T3 is used as current generator and T1, T2 limit the maximum current through the device.

R1 and R2 provides a current feedback and is used for reducing transimpedance of driver.

Normally the value of R1 and R2 are equal.


Linearization Methods for LED drive circuits

1.Complementary Distortion Technique


• Non-linear devices are used for distortion compensation.

•Pre-distortion compensation & Post- distortion Compensation.

Linearization Methods for LED drive circuits…

2.Negative Feedback Compensation Technique


• The optical output is detected and compared with the input waveform, amount of compensation depends on gain of feedback loop.

Linearization Methods for LED drive circuits…

3.Selective Harmonic Compensation Technique


•The input signal is divided into equal parts which are phase shifted with respect to each other.

The operation of the LED for binary digital transmission requires the switching ON and OFF of a current in the range of several tens to several hundreds of milli-amperes.


This single-stage circuit provides current gain.

Small Voltage drop across the switch when transistor is in saturation.

The maximum current flow through the LED is limited by the resistor R2.

Biasing is provided by the resistor R3

A simple drive circuit for binary digital transmission consisting of a CE saturating switch

Low- impedance drive circuit consisting of an emitter follower with compensating matching network.


The capacitor used here is known as speed up capacitor.

Pre-emphasis is accomplished by use of speed up capacitor.

Resistor R3 Capacitor C together forms a Compensating Matching Network.

The current amplification is produced by Emitter Follower circuit.

Now days drive circuitry with common logic interface are also available. For example


Fig.(a) Block schematic showingthe interfacing of the LED drive circuit with logic input levels.

Fig.(b) A simple TTL-compatible LED drive circuit employing a Texas Instruments’ 74S140 line driver.

Fig.(b) A TTL shunt drive circuit using a commercially available integrated circuit.

A number of configurations which we have used for LED can be used for ILD with certain modifications.

As we know that ILD is a threshold device so its drive current requirements are different than LED.

For example when digital transmission is considered, the laser is usually given a substantial applied bias in the off state and this biasing is called Pre-biasing.

Benefits of Pre-biasing are as follows-: (1).Reduction in switch on delay. (2).Reduction in junction

heating. (3).Easy compensation for change in ambient temperature and

device aging.



A shunt drive circuit for ILD

Here we have used FET for high speed laser operation.

Sufficient voltage is maintained in series with the laser using the resistor R2 and the compensating capacitor C such that the FET is biased into its active or pinch-off region.

FET works as an inverter here.

Using a stable gallium arsenide MESFETs, a rate of 1Gbit per second can be achieved.


An ECL-compatible high-speed laser drive circuit

Two differential amplifiers are connected in parallel for providing modulation current amplitude for laser.

Input stage is ECL compatible , by the help of T1 (Emitter Follower) , T2 as a current source with the Zener

diode for adjusting the signal levels for ECL operation.

IE is provided by optical feedback control.

Optical feedback control is required to ensure continuous LASER operation as threshold level of LASER is sensitive to temperature and aging.


Conclusion

We conclude that drive circuits are very important part of

an optical fiber communication system.

By the proper use of drive circuits the output power of sources can be maintained.

April 12, 2023 Presented by :- Diwaker Pant 25

REFFERENCESOptical Fiber Communications by John M.

Senior 3rd edition, Pearson Publication

Optical Fiber Communication by Gerd Keiser 3rd edition, Mc.Graw Hill.

www.wikipedia.com


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drive circuitry for leds and laser

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