REV. 0

–6–

AD9660

RECALIBRATE THE BIAS LOOP

POWER-UP

OR LASER

NOT IN USE

BIAS

CAL

TIME

WRITE

LOOP

CAL

TIME

RECALIBRATE WRITE LOOP

LASER POWER

MODULATED

BETWEEN

BIAS AND WRITE LEVELS

DISABLE

BIAS CAL

WRITE CAL

WRITE PULSE

LASER

OUTPUT POWER

CALIBRATED WRITE

CALIBRATED BIAS

WRITE LOOP

HOLD TIME

Figure 4. Normal Operating Mode

voltages to their minimum levels (minimum output current)

when DISABLE = logic HIGH. It is used during initial power-

up of the AD9660 or during time periods when the laser is inac-

tive. When the AD9660 is re-enabled the control loops must be

recalibrated.

Normal operation of the AD9660 involves (in order, see figure):

1. The AD9660 is enabled (DISABLE = logic LOW).

2. The input voltages (BIAS LEVEL and WRITE LEVEL) are

driven to the appropriate levels to set the calibrated laser di-

ode output power levels.

3. The bias loop is closed for calibration (BIAS CAL = logic

HIGH), and then opened (BIAS CAL = logic LOW).

4. The write loop is closed for calibration (WRITE PULSE and

WRITE CAL = logic HIGH) and then opened.

5. While both loops are open, the laser is pulsed between the

two calibrated levels by WRITE PULSE.

6. The bias and write loops are periodically recalibrated as

needed.

7. The AD9660 is disabled when the laser will not be pulsed for

an indefinite period of time.

Control Loop Transfer Functions

I

V

1.85

× (R

once the bias loop is calibrated. When the bias loop is open

tional to the held voltage at BIAS HOLD; the external hold ca-

pacitor on this pin determines the droop error in the output bias

current between calibrations.

I

V

1.85

× (R

once the write loop is calibrated. The current supplied by the write

When the write loop is open (WRITE CAL logic LOW),

The external hold capacitor (WRITE HOLD) determines the

on and off by WRITE PULSE when the write loop is open.

4

3

0

2

1

0

120

20

40

60

80

100

CONSTANT WRITE POWER

BIAS CALIBRATION POWER

0

25

CASE

50

BIAS

MOD

FORWARD CURRENT – mA

Figure 5. Typical Laser Diode Current-to-Optical

Power Curve

The sections below discuss choosing the external components in

the feedback loops for a particular application.

loop’s transfer function to the laser diode/photo diode

combination.

The user should define the maximum laser diode output power

chosen as:

R

1.6 V

1.85

× I

MONITOR MAX

− 50 Ω

.

and WRITE LEVEL inputs.