Specifications

Reference Design HFRD-25.2 (Rev.8; 01/09) Maxim Integrated Products
Page 26 of 42
operating in continuous-mode operation, the
initialization time is approximately 600ns from
power-on.
10.9 K-Factor Modulation
Compensation
HFRD-25.2 includes a type of K-Factor
modulation compensation for adjusting the
modulation current with changing temperature.
The amount of compensation (KC) applied equals:
⎟
⎠
⎞
⎜
⎝
⎛
⋅
⎟
⎠
⎞
⎜
⎝
⎛
=
4
76,75(
100
hhSRAMIBiasV
KFactorSH
KC
KC is a digital value which is then applied to the
modulation PWM value to increase the
modulation current output. The KC value
translates to a current through the equations
shown in Section 10.4.
10.10 Linear Modulation Temperature
Compensation
Linear compensation of the modulation current is
also provided by the firmware of HFRD-25.2. The
modulation current can be increased in a linear
manner with increasing temperature by using the
KFactorSL (EEPROM 05h) and K2Temp
(EEPROM 06h) settings. Assuming that CalTemp
> K2Temp, the amount of linear modulation
compensation (LINC) applied equals:
()
⎟
⎠
⎞
⎜
⎝
⎛
⋅−=
100
2
KFactorSL
TempKCalTempLINC
LINC is a digital value which is then applied to
the modulation PWM value to increase the
modulation current output. The LINC value
translates to a current through the equations
shown in Section 10.4. Note: A 2
o
C hysteresis is
applied to the temperature used for the linear
modulation compensation.
10.11 Tracking Error Compensation
An automatic power control loop (APC) is
typically used to maintain the average power
output of a laser diode. The APC loop works by
keeping the current of a monitor diode (back
faceted to the laser) constant. The average power
of the laser is well maintained if there is no
change in the responsivity of the monitor diode or
in the coupling efficiency between the laser and
the monitor diode. In practice there are variations
that cause the fiber-coupled average power to
drift, even when the monitor diode current is held
constant. In many cases, the actual output power
will change by ±1dB due to tracking error
between the monitor diode and the laser diode.
HFRD-25.2 includes an optional feature to offset
tracking error variations in laser subassemblies.
Using memory locations TrackErrC (EEPROM
09h) and TETempC (EEPROM 0Ah), a linear
tracking error compensation, with temperature
offset/start point, can be applied. Assuming
CalTemp > TETempC, the APC set point scaling
factor is given by:
()
⎟
⎠
⎞
⎜
⎝
⎛
⋅−=
100
TrackErrC
TETempCCalTempIMDS
The new APC set point (APCSET
NEW
) is then
given by:
APCSET
IMDS
APCSETAPCSET
NEW
⋅
⎟
⎠
⎞
⎜
⎝
⎛
+=
100
Where APCSET = (EEPROM 02h).
Note: TrackErrC is a two’s-compliment number.
A 2
o
C hysteresis is also applied to the measured
temperature used with the tracking error
compensation feature.