AK2571 Datasheet, PDF (21/52 Pages) Asahi Kasei Microsystems – Single-Chip Automatic Power/Temperature Control for WDM Laser Diodes

AKM Confidential
AK2571
4.5 ATC Feedback function
The ATC Feedback function compensates for wavelength shifts caused by aging. R_CTRL_USER.Atc_fb = “1” enables the
function, “0” disables it. Turning on this function changes the target temperature according to LD aging error from the PD
monitor voltage. Its value (R_ATC_CMPNST) is calculated from (E_ATC_FB_SET). This function assumes that there is a
first order function between the LD aging error and wavelength increases and uses this function to compensate for this
shift caused by aging. When using this function, please note this assumption carefully. Figure 4-8 indicates the block
diagram for this function.
Figure 4-8 ATC Feedback Block
Initial current set
Iini
+
I-DAC
current step
(ΔIstep)
I-DAC output
current
Iidac
LD set current
External
Ild
current amp
Gi (e.g. G=4)
LD module
Target temperature
LD ageing current
(R_APC_CMPNST)
Parameter
Kc
shift value
(R_ATC_CMPNST)
Temperature shift step
ΔTstep = 0.03℃ (typ)
Set by E_ATC_FB_SET
Initial Target
Temperature set value
(E_ATC_TRGT)
-
PID control
Target temperature
(R_TMPRT_TRGT)
Function of wavelength and
ageing current: Kw
e.g. Kw = 0.01nm/mA
Function of wavelength and
temperature: Kt
e.g. Kt = -0.1nm/℃
Its operation is described below
1) With LD aging error engaged, initiate the APC compensation circuit. LD compensation current (Digital) is added to
the I-DACs selected by R_APC_CMPNST.
2) Calculate the shift value of the target temperature (R_ATC_CMPNST) from the compensation current and the
parameter stored in E_ATC_FB_SET (=Kc).
3) Shift the target temperature (R_TMPRT_TARGET) which is the initial target temperature (E_ATC_TRGT) minus
the shift value of target temperature (R_ATC_CMPNST).
Kc as above is calculated by the expression below.
If the shift value of wavelength is ∆λ1, Compensation current (Analog) is ∆Ild,
∆λ1= Kw*∆Ild
(1)
If the rate of analog output current per one step of I-DACs is ∆Istep, the gain of external current amplifier is Gi,
The value of compensation current (Digital)
R_APC_CMPNST = ∆Ild / Gi / ∆Istep
(2)
The shift value of target temperature
R_ATC_CMPNST = Kc*R_APC_CMPNST
(3)
If the rate of temperature shift per one step of R_ATC_CMPNST is ∆Tstep, the value of wavelength shift by the shift of
target temperature is ∆λ2,
∆λ2=Kt*R_ATC_CMPNST*∆Tstep
(4)
In that sense the value of Kc to make ∆λ1=∆λ2, for compensation of the wavelength shift.
Kc=Kw*Gi / Kt* ∆Istep / ∆Tstep
(5)
For example: If Kw=0.01nm/mA, Gi=4, Kt=0.1nm/°C, ∆Istep=0.08mA/step, ∆Tstep=0.03°C/step, Kc=1.07.
Since this function does not actually watch the wavelength, care must be exercised when setting these values.
Parameter values of Kc can be selected from 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, 1.0, 1.125, 1.25 and 1.5.
<Rev. 0.6E Preliminary>
-21-
2001/11