ECE 4606 Undergraduate Optics Lab Interface circuitry. Interface circuitry. Outline

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1 Interface circuitry Interface circuitry Outline Photodiode Modifying capacitance (bias, area) Modifying resistance (transimpedance amp) Light emitting diode Direct current limiting Modulation circuits Laser diode Chirp, relaxation oscillation Pedrotti 3, Chapter 6 141

2 Photodiode interfaces Photodiode capacitance In reverse bias (v<0), capacitance decreases as depletion region grows C ε ε rqn 2 = 0 D A ( v v ) BuiltIn Bias Proportional to area, decreases like 1/sqrt of reverse voltage In forward bias (v>0), excess charge near junction appears as capacitance q v Bias 2 n0 = kbt C q e 1 k BT Increases exponentially with forward voltage + v i = v Bias = v Bias = v Bias Photodetection and Measurement, Mark Johnson, pg 25, McGraw-Hill Morals Use smallest area (A) photodiode to reduce C everse bias can further reduce C Forward bias is dramatically slower (large C) everse bias increases dark current and thus noise 142

3 Photodiode interfaces Bandwidth of transimpedance amplifier receivers The time constant of a simple load resistor is L C P. The associated rise-time and bandwidth are 1 P τ = 2.2LCP, B = [ Hz] 2π C L P v Bias 0 For a transimpedance amplifier, an ideal, zero input impedance op-amp would enable infinite bandwidth. The input impedance of a real opamp is the feed-back resistance divided by the open loop gain. in = L A i P v Bias - + F L v o The open-loop gain depends on frequency, approximately like 1/f A GPB f The gain is thus lowest at the highest frequency f = B 1 GBP B B = = 2π C 2π C in P L P Yielding the approximate design rule B GBP 2π in C P A= 1. mplificador_741/amplificador_741.html 2. Photodetection and Measurement, Mark Johnson, pg 28, McGraw- Hill

4 LED interfaces LED temporal behavior Bias current + Carrier density Spontaneous emission Mod. current Non-radiative recombination Two cascaded processes: 1. Injected current creates a non-equilibrium excess density of electrons and holes. Time constant = C. 2. Electrons and holes recombine through spontaneous emission (radiative) or thermal (nonradiative) processes. For high efficiency, radiative recombination rate should be significantly faster than non-radiative processes. In most situations, the C time constant is longer and thus the dominant term. The radiative time constant decreases with increasing carrier density, so for very high-speed operation, a DC bias current can be added if the application allows. Photonics Essentials, Thomas P. Pearsall, pg 124, McGraw-Hill

5 LED interfaces DC LED drive circuit 1. Diode voltage varies rapidly in forward bias, so drive with current instead. 2. Major damage mechanisms a. Exceeding rated current. b. Excess reverse bias 3. LED intrinsic voltage varies 10 to 15% from unit to unit. If you connect LEDs in parallel, one will thus draw all the current (and blow out). Instead, connect LEDs in series. 4. Simplest circuit always use a current-limiting resistor: v B vforward I = Diode forward voltage depends weakly on current, so you may need to iterate to zero in on a desired current and output power. High voltage and large resistance looks more like a current source, reducing dependence on the diode. + v Forward 1V v 0 B 145

6 LED interfaces Modulated LED circuits 2 v B 0 C v( t) v( t) 1 1. ectifying output with reverse bias breakdown protection 2. AC-coupled analog input added to bias v 0 B 1 v( t) Digital driver from low-current source (e.g. CMOS) using BJT controlled through voltage divider. 4. Bidirectional communication using two microcontroller pins. a) Forward bias and emit. b) Charge diode capacitance in reverse bias. c) Detect power by measuring discharge time. 1-3: Photodetection and Measurement, Mark Johnson, pg 125, McGraw-Hill : Dietz, Yerazunis, and Leigh (2003). Very Low-Cost Sensing and Communication Using Bidirectional LEDs

7 LD interfaces LD temporal behavior Photon density Laser output Stimulated emission Cavity loss Drive current Carrier density Spontaneous emission LED Non-radiative recombination Current Wavelength Optical power i th λ DC P DC t delay Broad, LED bandwidth LED-emission t t t [ ns] [ ns] [ ns] 1. Current step from below to above threshold. 2. Carrier inversion established after t delay 3. Carriers increase index, optical path length of cavity and resonant wavelength chirp. 4. High initial carrier density creates high photon density which depletes carriers, reducing photons elaxation oscillation. Photonics Essentials, Thomas P. Pearsall, pg 180, McGraw-Hill

8 LD interfaces Issues in laser diode circuitry LD equivalent circuit 1. Much smaller range of forward current between threshold and facet damage. 2. Much more sensitive to reverse voltage. 3. Unlike LED, even momentary overdrive will damage. 4. Output power depends strongly on temperature and age. 5. DC supply typically uses a. Photodiode in feedback to control power this is typically insufficient to provide true, stabilized optical power but good enough to protect the diode from overload. b. TEC and thermistor in feedback to control temperature 6. Modulated supply includes these features and must also attempt to damp relaxation oscillations etc. 148