ADN2872

Rev. 0 | Page 11 of 20

Operation with Lasers with Temperature-Dependent

Nonlinearity of Laser LI Curve

The ADN2872 ERCL extracts information from the monitor

photodiode signal relating to the slope of the LI characteristics

at the Optical 1 level (P1). For lasers with good linearity over

temperature, the slope measured by the ADN2872 at the Optical 1

level is representative of the slope anywhere on the LI curve.

This slope information is used to set the required modulation

current to achieve the required optical extinction ratio.

0

0.5

3.0

2.5

2.0

1.5

1.0

4.0

3.5

20

40

60

CURRENT (mA)

100

80

RELATIVELY LINEAR LI CURVE AT 25°C

NONLINEAR LI CURVE AT 80°C

0

Figure 25. Measurement of a Laser LI Curve Showing

Laser Nonlinearity at High Temperatures

Some types of lasers have LI curves that become progressively

more nonlinear with increasing temperature (see Figure 25). At

temperatures where the LI curve shows significant nonlinearity,

the LI curve slope measured by the ADN2872 at the Optical 1

level is no longer representative of the overall LI curve. It is

evident that applying a modulation current based on this slope

information cannot maintain a constant extinction ratio over

temperature.

However, the ADN2872 can be configured to maintain near

constant optical bias and an extinction ratio with a laser

exhibiting a monotonic temperature-dependent nonlinearity.

To implement this correction, it is necessary to characterize a

small sample of lasers for their typical nonlinearity by measur-

ing them at two temperature points, typically 25°C and 85°C.

The measured nonlinearity is used to determine the amount of

feedback to apply.

Typically, the user must characterize five to 10 lasers of a particular

model to obtain a good number. The product can then be cali-

brated at 25°C only, avoiding the expense of temperature

calibration. Typically, the microcontroller is used to measure

the laser and apply the feedback. This scheme is particularly

suitable for circuits that already use a microcontroller for control

and digital diagnostic monitoring.

The ER correction scheme, while using the average nonlinearity

for the laser population, supplies a corrective measurement

based on the actual performance of each laser as measured during

operation. The ER correction scheme corrects for errors due to

laser nonlinearity while the dual loop continues to adjust for

changes in the Laser LI.

For more details on maintaining average optical power and

extinction ratio over temperature when working with lasers

displaying a temperature-dependent nonlinearity of LI curve,

contact sales at Analog Devices.

CONTROL

The ADN2872 has two methods for setting the average power

tion ratio can be voltage set using the voltage DAC outputs of a

microcontroller to provide controlled reference voltages to

PAVREF and ERREF. Alternatively, the average power and

extinction ratio can be resistor set using potentiometers at the

PAVSET and ERSET pins, respectively.

VOLTAGE SETPOINT CALIBRATION

The ADN2872 allows an interface to a microcontroller for both

control and monitoring (see Figure 26). The average power at

the PAVSET pin and extinction ratio at the ERSET pin can be

set using the DAC of the microcontroller to provide controlled

reference voltages to PAVREF and ERREF. Note that during

power-up, there is an internal sequence that allows 25 ms before

enabling the alarms; therefore, the user must ensure that the

voltage for PAVREF and ERREF are active within 20 ms.

(V)

AV

CW

CW

MPD

ERSET

P

ER

ER

P

I

R

ERREF













1

1

_

(V)

where:

ER is the desired extinction ratio (ER = P1/P0).

a 1% tolerance and a temperature coefficient of 50 ppm/°C.