ADN2872 Просмотр технического описания (PDF) - Analog Devices

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ADN2872

3.3 V Dual-Loop, 50 Mbps to 3.3 Gbps Laser Diode Driver

Analog Devices 

ADN2872

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, ITH, and slope in the linear

region beyond the threshold current, referred to as slope

efficiency, LI.

ER =

P1

P0

P1

P1 + P0

PAV =

2

PAV

ΔP

ΔI

LI = ΔP

ΔI

P0

ITH CURRENT

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.

MPD

OPTICAL COUPLING

INPUT

BIAS

SHA

Gm

Φ1

BIAS

VCC

Φ2

1.2V

CURRENT

VBGAP

ERSET

IEX IPA

HIGH

SPEED

SWITCH

PAVSET

MOD

Φ2

SHA

Φ2

MOD

CURRENT 100

2

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), ITH

and LI, by varying IBIAS. APC control is performed by measuring

the MPD current, IMPD. 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 IMPD × RPAVSET to the BGAP

voltage, VBGAP. If IMPD falls, the bias current is increased until

IMPD × RPAVSET equals VBGAP. Conversely, if the IMPD increases, IBIAS

is decreased.

Modulation Control Loop

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

by monitoring the IMPD changes. During the ERCL, IMPD is

temporarily increased by ΔIMOD. The ratio between IMPD and

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

increased to decrease settling time.

During ERCL, switching in ΔIMOD causes a temporary increase

in average optical power, ΔPAV. However, the APC loop is disabled

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

disturb the optical eye. When ΔIMOD is switched into the laser

circuit, an equal current, IEX, is switched into the PAVSET resis-

tor. The user sets the value of IEX; this is the ERSET setpoint. If

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

and increases IMPD and, therefore, ΔIMOD accordingly until ΔIMPD

is equal to IEX. The previous control cycle status of the IBIAS and IMOD

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.

Rev. 0 | Page 10 of 20

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