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ONET1141L: APC operation

Yukihiro Araya
Prodigy 10 points
Part Number: ONET1141L

Always Thank you for supprts.

I have a question about the APC operation of ONET1141L.

I am currently using the ONET1141L to evaluate pulse driving of Laser Diode.

1. Please explain the APC control operation in APC mode (in closed loop operation).

    I think APC control is controlling Bias current by LD light intensity (=photodiode current).

    Does the APC have control to increase or decrease the BIAS current?

    (Does APC provide control to increase as well as to decrease BIAS current?)

    We have confirmed that when the duty cycle of the modulation current is changed, the BIAS current increases or decreases in accordance with the change in light intensity.

    (Bias current decrease, Duty down --> Bias current increase)

    I have determined that it is working correctly as an APC control, is that correct?

2. Please explain about Bias current control in APC OFF mode (in open loop operation).

    When the duty cycle of the modulation current was changed in the APC OFF mode (in open loop operation), we confirmed that the BIAS current increased or decreased according to the duty cycle change, just as in the APC mode (in closed loop operation).

    (Bias current decrease, Duty down --> Bias current increase)

    It seems that the operation in APC OFF mode (in open loop operation) is not correct.

    (I suspect that the Bias current will not change?)

    Do you have any advice for this phenomenon?

Thank you.

  • 0
    TI__Guru* 93670 points

    Greetings,

    Your understanding of the APC loop is correct. In open-loop mode, bias current is set through registers 4 and 5. 

    Given your observations, i am thinking perhaps please make sure OLENA bit is set to 1 and PDPOL is set correctly.

    Regards  ,, Nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 10 points

    Thank you for reply.

    I am relieved about your reply that my understanding of the APC loop behavior is correct.

    Following your reply, I rechecked the settings I was evaluating and confirmed that the settings were correct.

    I confirmed that I had set the following and again checked the ONET1141L's operation in open loop mode.

        Mod=Rrg3:FF/Reg2:03(Modulation current max)

        Bias=REG5:5B/REG4:00

        POL(Reg0/Bit2):0 (pin22=OUT+, pin21=OUT-)

        OLENA(Reg0/Bit4):1 (=Open loop) and 0 (=Closed loop)

        PDRNG1,0(Reg0/Bit6,5): 00 or 01 or 1x

    As a result, we confirmed that in open loop, the optical output waveform does not change even if the PD current range (PDRNG1,0(Reg0/Bit6,5)) setting is changed. (Attached file 1/2)

    I believe this result indicates that the APC loop is disconnected in the Open loop.

    In open-loop operation, I thought that the Bias current would not change with the duty cycle.

    However, both open-loop and closed-loop operation resulted in the Bias current increasing or decreasing depending on the duty cycle.

    Is this open loop operation correct?

    (See attached for confirmation results.)

    ONET1141L open loop operation results of reevaluation.pdf

    BR

  • 0 in reply to Yukihiro Araya
    TI__Guru* 93670 points

    Greetings,

    Thanks for sending this detailed report.

    Please note ONET1141L data sheet page 8. In open loop operation the bias current is set directly by the 10-bits BIASC registers 4 &5 as you have noted. I think the main difference i see is that you are using a 199MHz clock or 200Mbps data rate. This device has not been characterized to operate at this data rate. Therefor device may not perform as you would expect.

    Regards ,, Nasser 

  • 0 in reply to Nasser Mohammadi
    Prodigy 10 points

    Thank you for reply.

    I understand the content of your reply.
    It is true that the ONET1141L datasheet says that it is designed at data rates from 1Gbps up to 11.3Gbps, but I was considering the possibility of running it at less than 1Gbps (300MHz to 500MHz).
    I would like to examine more, including the peripheral circuits.

    Thank you for your very kind answer.
    If I have any more questions, I will contact you.

    BR