Application Note 90 April 2002 Current Sources for Fiber ...

1 Application Note 90AN90-1 April 2002 Current Sources for Fiber Optic LasersA Compendium of Pleasant Current EventsJim Williams, Linear Technology CorporationINTRODUCTIONA large group of Fiber optic lasers are powered by DCcurrent. laser drive is supplied by a Current source withmodulation added further along the signal path. Thecurrent source, although conceptually simple, constitutesan extraordinarily tricky design problem. There are anumber of practical requirements for a Fiber optic currentsource and failure to consider them can cause laser and/or optical component Criteria for Fiber Optic laser Current SourcesFigure 1 shows a conceptual laser Current source.

2 Inputsinclude a Current output programming port, an outputcurrent clamp and an enable command. laser Current isthe sole output. This block diagram is deceptively practice, a laser Current source must meet a number ofpractical requirements, some quite subtle. The key to asuccessful design is a thorough understanding of indi-vidual system requirements. Various approaches suitdifferent sets of freedoms and constraints, although allmust address some basic features must be included to prevent laser andoptical component damage. The laser , an expensive anddelicate device, must be protected under all conditions,including supply ramp up and down, improper controlinput commands, open or intermittent load connectionsand hot plugging.

3 Detailed Discussion of Performance IssuesIt is useful to expand on the above cursory discussion toclarify design goals. As such, each previously called outissue is treated in greater detail Power SupplyThe available power supply should be defined. A single rail5V supply is presently the most common and tolerances, typically 5%, must be accounted distribution voltage drops may result in surpris-ingly low rail voltages at the point of load. Occasionally,split rails are available, although this is relatively , split rail operation can complicate laser pro-tection, particularly during supply sequencing. See LaserProtection Features below for additional Current CapabilityLow power lasers operate on less than 250mA.

4 Higherpower types can require up to Voltage ComplianceCurrent source output voltage compliance must be able toaccomodate the laser s forward junction drop and anyadditional drops in the drive path. Typically, voltage com-pliance of is adequate., LTC and LT are registered trademarks of Linear Technology TO LASERTYPICALLY 0 TO 250mAOR 0 TO TO 5V = OFF0V = ON(THIS FUNCTION CAN BEBUILT INTO CIRCUIT BLOCK)IOUT CLAMPTYPICALLY 0V TO PROGRAMTYPICALLY 0V TO F01 Figure 1. Conceptual laser Current Source isDeceptively Simple. Practical System Issues andLaser Vulnerability Necessitate Careful DesignThere are two basic sets of concerns for laser currentsources: performance and protection.

5 Performance is-sues include the Current source s magnitude and stabilityunder all conditions, output connection restrictions, volt-age compliance, efficiency, programming interface andpower Note 90AN90-2 EfficiencyHeat build up in Fiber optic systems is often a concern dueto space limitations. Accordingly, Current source effi-ciency can be an issue. At low Current , linear regulation isoften adequate. Switching regulator based approachesmay be necessary at higher ConnectionIn some cases, the laser may float off ground; otherapplications require grounded anode or cathode opera-tion. Grounding the anode seemingly mandates a negativesupply but single rail operation can be retained if switchingregulator techniques are Current ProgrammingOutput Current is set by a programming port voltage.

6 Thevoltage may be derived from a potentiometer, DAC orfiltered PWM. Typically, a range of 0V to correspondsto 0A to 250mA or 0A to Set point accuracy is usuallywithin , although better tolerances are readily achiev-able. Output Current stability, discussed below, is consid-erably Current source should be well regulated against line,load and temperature changes. Line and load inducedvariations should be held well within , with typicaltemperature drifts of Judicious component choicecan considerably improve these source noise, which can modulate laser output,must be minimized. Typically, noise bandwidth to 100 MHzis of interest. A linearly regulated Current source hasinherently low noise and usually presents no regulator based Current Sources require specialtechniques to maintain low ResponseThe Current source does not need fast transient responsebut it cannot overshoot the programmed Current underany circumstances.

7 Such overshoots can damage thelaser or associated optical Discussion of laser Protection IssuesOvershootAs noted above, outputs overshooting the nominal pro-grammed Current can be destructive. Any possible combi-nation of improper control input or power supply turn on/off characteristics must be accounted for. Also, any spu-rious laser Current under any condition is that portions of the Current source circuitry may haveundesired and unpredictable responses during supplyramp up/down, complicating enable line allows shutting the Current source outputoff. The enable line can also be used to hold Current outputoff during supply ramp up, preventing undesired can be tricky because the enable signal circuitry maybe powered by the same supply that runs the laser .

8 Theenable signal must reliably operate independent of powersupply turn-on profile. Optionally, the enable function canbe self-contained within the Current source, eliminatingthe necessity to generate this Current ClampThe output Current clamp sets maximum output Current ,overriding the output Current programming voltage controlled input can be set by a potentiom-eter, DAC or filtered laser ProtectionAn unprotected Current source s output rises to maximumvoltage if the load is disconnected. This circumstance canlead to hot plugging the laser , a potentially destructiveevent. Intermittent laser connections can produce similarundesirable results. The Current source output shouldlatch off if the load disconnects.

9 Recycling power clearsthe latch but only if the load has been preceding discussion dictates considerable care whendesigning laser Current Sources . The delicate, expensiveload, combined with the uncertainties noted, should pro-mote an aura of thoughtful The following circuitexamples (hopefully) maintain this outlook while simulta-neously presenting practical, usable circuits. A variety ofapproaches are shown, in keeping with the broad area ofNote 1. For Fools Rush in Where Angels Fear to Tread. An essay oncriticism, A. Pope. Note 90AN90-3application. The designs can be directly utilized or serve asstarting points for specific Current SourceFigure 2, a basic laser Current source, supplies up to250mA via Q1.

10 This circuit requires both laser terminals tofloat. The amplifier controls laser Current by maintainingthe 1 shunt voltage at a potential dictated by the pro-gramming input. Local compensation at the amplifierstabilizes the loop and the F capacitor filters inputcommands, assuring the loop never slew limits. Thisprecaution prevents overshoot due to programming inputdynamics. The enable input turns off the Current source bysimultaneously grounding Q1 s base and starving theamplifier s + input while biasing the input high. Thiscombination also insures the amplifier smoothly ramps tothe desired output Current when enable switches low.