LINEAR AN90-1 Data Handbook

Application Note 90

April 2002

Current Sources for Fiber Optic Lasers A Compendium of Pleasant Current Events Jim Williams, Linear Technology Corporation

INTRODUCTION Protection features must be included to prevent laser and

optical component damage. The laser, an expensive and

A large group of fiber optic lasers are powered by DC

delicate device, must be protected under all conditions,

current. Laser drive is supplied by a current source with

including supply ramp up and down, improper control

modulation added further along the signal path. The

input commands, open or intermittent load connections

current source, although conceptually simple, constitutes

and “hot plugging.”

an extraordinarily tricky design problem. There are a number of practical requirements for a fiber optic current

Detailed Discussion of Performance Issues

source and failure to consider them can cause laser and/

or optical component destruction. It is useful to expand on the above cursory discussion to clarify design goals. As such, each previously called out

Design Criteria for Fiber Optic Laser Current Sources issue is treated in greater detail below. Figure 1 shows a conceptual laser current source. Inputs

Required Power Supply

include a current output programming port, an output

The available power supply should be defined. A single rail

current clamp and an enable command. Laser current is

5V supply is presently the most common and desirable.

the sole output. This block diagram is deceptively simple.

Supply tolerances, typically ±5%, must be accounted for.

In practice, a laser current source must meet a number of

System distribution voltage drops may result in surpris-

practical requirements, some quite subtle. The key to a

ingly low rail voltages at the point of load. Occasionally,

successful design is a thorough understanding of indi-

split rails are available, although this is relatively rare.

vidual system requirements. Various approaches suit

Additionally, split rail operation can complicate laser pro-

different sets of freedoms and constraints, although all

tection, particularly during supply sequencing. See “Laser

must address some basic concerns.

Protection Features” below for additional comment.

IOUT PROGRAM TYPICALLY 0V TO 2.5V

Output Current Capability

IOUT TO LASER

IOUT CLAMP TYPICALLY Low power lasers operate on less than 250m A. Higher TYPICALLY 0V TO 2.5V 0 TO 250m A

OR 0 TO 2.5A power types can require up to 2.5A.

ENABLE 1V TO 5V = OFF 0V = ON

Output Voltage Compliance

(THIS FUNCTION CAN BE BUILT INTO CIRCUIT BLOCK) Current source output voltage compliance must be able to

AN90 F01

accomodate the laser’s forward junction drop and any

Figure 1. Conceptual Laser Current Source is

Deceptively Simple. Practical System Issues and additional drops in the drive path. Typically, voltage com-

Laser Vulnerability Necessitate Careful Design pliance of 2.5V is adequate. There are two basic sets of concerns for laser current sources: performance and protection. Performance is- sues include the current source’s magnitude and stability under all conditions, output connection restrictions, volt- age compliance, efficiency, programming interface and

power requirements. , LTC and LT are registered trademarks of Linear Technology Corporation.

AN90-1