Datasheet

ManualsBrandsANALOG DEVICES ManualsPMICsPMIC - laser controller Laser diode driver LFCSP 24 VQ Surface-mount

ADN2872

Rev. 0 | Page 10 of 20

THEORY OF OPERATION

Laser diodes have a current-in to light-out transfer function, as

shown in Figure 23. Two key characteristics of this transfer

function are the threshold current, I

, and slope in the linear

region beyond the threshold current, referred to as slope

efficiency, LI.

OPTICAL POWER

CURRENT

ΔP

ΔI

ER =

P1 + P0

LI =

ΔP

ΔI

08013-005

Figure 23. Laser Transfer Function

DUAL-LOOP CONTROL

Typically, laser threshold current and slope efficiency are both

functions of temperature. For FP and DFB type lasers, the

threshold current increases and the slope efficiency decreases

with increasing temperature. In addition, these parameters vary

as the laser ages. To maintain a constant optical average power

and a constant optical extinction ratio over temperature and

laser lifetime, it is necessary to vary the applied electrical bias

current and modulation current to compensate for the laser

changing LI characteristics.

Single-loop compensation schemes use the average monitor

photodiode (MPD) current to measure and maintain the

average optical output power over temperature and laser aging.

The ADN2872 is a dual-loop device, implementing both this

primary average power control loop and a secondary control

loop, which maintains a constant optical extinction ratio. The

dual-loop control of the average power and extinction ratio

implemented in the ADN2872 can be used successfully with

both lasers that maintain good linearity of LI transfer characteris-

tics over temperature, and with those that exhibit increasing

nonlinearity of the LI characteristics over temperature.

Dual Loop

The ADN2872 uses a proprietary patented method to control both

average power and extinction ratio. The ADN2872 is constantly

sending a test signal on the modulation current signal and

reading the resulting change in the MPD current as a means of

detecting the slope of the laser in real time. This information is

used in a servo to control the ER of the laser, which is done in a

time-multiplexed manner at a low frequency, typically 80 Hz.

Figure 24 shows the dual-loop control implementation on the

ADN2872.

08013-039

ERSET

MPD

INPUT

PAVSET

OPTICAL COUPLING

BIAS

SHA

MOD

SHA

MOD

CURRENT

1.2V

BGAP

100 2

BIAS

CURRENT

HIGH

SPEED

SWITCH

Figure 24. Dual-Loop Control of Average Power and Extinction Ratio

A dual loop is made up of an average power control loop

(APCL) and the extinction ratio control loop (ERCL), which are

separated into two time states. During Time Φ1, the APC loop

is operating, and during Time Φ2, the ER loop is operating.

Average Power Control Loop

The APCL compensates for changes in the laser diode (LD), I

and LI, by varying I

BIAS

. APC control is performed by measuring

the MPD current, I

MPD

. This current is bandwidth limited by the

MPD. This is not a problem because the APCL must be low

frequency and the APCL must respond to the average current

from the MPD. The APCL compares I

MPD

× R

PAVS E T

to the BGAP

voltage, V

BGAP

. If I

MPD

falls, the bias current is increased until

MPD

× R

PAVS ET

equals V

BGAP

. Conversely, if the I

MPD

increases, I

BIAS

is decreased.

Modulation Control Loop

The ERCL measures the slope efficiency, LI, of the laser diode

by monitoring the I

MPD

changes. During the ERCL, I

MPD

temporarily increased by ΔI

MOD

. The ratio between I

MPD

and

ΔI

MOD

is a fixed ratio of 50:1, but during startup, this ratio is

increased to decrease settling time.

During ERCL, switching in ΔI

MOD

causes a temporary increase

in average optical power, ΔP

. However, the APC loop is disabled

during ERCL, and the increase is kept small enough so as not to

disturb the optical eye. When ΔI

MOD

is switched into the laser

circuit, an equal current, I

, is switched into the PAVSET resis-

tor. The user sets the value of I

; this is the ERSET setpoint. If

ΔI

MPD

is too small, the control loop knows that LI has decreased,

and increases I

MPD

and, therefore, ΔI

MOD

accordingly until ΔI

MPD

is equal to I

. The previous control cycle status of the I

BIAS

and I

MOD

settings are stored on the hold capacitors, PAVCAP and ERCAP.

The ERCL is constantly measuring the actual LI curve; it compen-

sates for the effects of temperature and for changes in the LI

curve due to laser aging. Therefore, the laser can be calibrated

once at 25°C so that it can then automatically control the laser

over temperature. This eliminates the expensive and time

consuming temperature calibration of a laser.