ADN2870 Просмотр технического описания (PDF) - Analog Devices

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ADN2870

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

Analog Devices 

ADN2870

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

PO

P1

PAV =

P1 + PO

2

PAV

∆P

∆I

LI =

∆P

∆I

PO

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 lasers

changing LI characteristics.

Single-loop compensation schemes use the average monitor

photodiode current to measure and maintain the average

optical output power over temperature and laser aging. The

ADN2870 is a dual-loop device, implementing both this

primary average power control loop and, additionally, a

secondary control loop, which maintains constant optical

extinction ratio. The dual-loop control of average power and

extinction ratio implemented in the ADN2870 can be used

successfully both with lasers that maintain good linearity of LI

transfer characteristics over temperature and with those that

exhibit increasing nonlinearity of the LI characteristics over

temperature.

Dual Loop

The ADN2870 uses a proprietary patented method to control

both average power and extinction ratio. The ADN2870 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 ADN2870.

ERSET

MPD

INPUT

Gm

Φ2

IEX

1.2V

VBGAP

IPA

PAVSET

OPTICAL COUPLING

BIAS

SHA

Φ1

BIAS

CURRENT

VCC

MOD

SHA

Φ2

HIGH

SPEED

SWITCH

Φ2

MOD

CURRENT 100

2

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

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

loop) and the ERCL (extinction ratio control loop), 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 Ith and LI by varying

Ibias. APC control is performed by measuring MPD current,

Impd. This current is bandwidth-limited by the MPD. This is

not a problem because the APCL must be low frequency since

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 LD, and

changes Imod as LI changes. During the ERCL, Imod is

temporarily increased by ∆Imod. The ratio between Imod and

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

increased in order to decrease settling time.

During ERCL, switching in ∆Imod causes a temporary increase

in average optical power, ∆Pav. However the APC loop is dis-

abled during ERCL, and the increase is kept small enough so as

not to disturb the optical eye. When ∆Imod is switched into the

laser circuit, an equal current, Iex, is switched into the PAVSET

resistor. The user sets the value of Iex; this is the ERSET setpoint.

If ∆Impd is too small, the control loop knows that LI has

decreased and increases Imod and, therefore, ∆Imod accordingly

until ∆Impd is equal to Iex. The previous time state values of

the bias and mod settings are stored on the hold capacitors

PAVCAP and ERCAP.

The ERCL is constantly measuring the actual LI curve, therefore

it compensates for the effects of temperature and for changes in

the LI curve due to laser aging. Thus the laser may be calibrated

once at 25°C and can then automatically control the laser over

temperature. This eliminates expensive and time consuming

temperature calibration of the laser.

Rev. 0 | Page 11 of 20

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