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  • TI Thinks Resolved

ONET4201LD: ONET4201LD

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Part Number: ONET4201LD

Hiii

I want to use this ONET4201LD IC to drive my laser at 1Gbps.

Please find the attached datasheet of laser and let me know whether i can use the same IC or can you suggest some other better performance IC to drive the Laser.2.5Gbps_1550nm_DFB_LD_Module_Jan_09.pdf

  • Hi Sruthi,

    I reviewed the laser spec you had sent. All look good and I believe you should be able to use this laser diode with ONET4201LD.

    I noticed operating voltage of this laser diode is 1.1V to 1.8V. ONET4201 declares fault if MONB/P is >= 1.35V. Therefor we need to operate this laser at <= 1.35V.

    Regards,,nasser

     

  • In reply to Nasser Mohammadi:

    Hiii

    As per your reply, I am not sure whether i can operate the Laser below 1.3V only and get the required optical output power.

    Also i want to know in order to set the required Bias Current or Modulation Current, Is there any another alternative rather than using the resistors.

    Can I use DAC? If any document is there, kindly suggest and also let me know how to interface the Laser Diode to this IC?

    Thanks

  • In reply to Chennuri Sruthi:

    Hi Sruthi,

    I read your laser specification again. Monitor photo diode is 0.5mA typically. ONET4201 is able to handle up to 1.5mA of PD current(three times as much as your laser diode). Using equation 8 of the data sheet, RAPCSET =~ 9.38K. Well above minimum 3.1K ohm minimum requirement.  So with this resistor value, you should be able to operate your laser. However, now I am not sure in your laser specification what operating voltage refers to. Monitor current and threshold current of your laser are well within the ONET4201LD specifications.

    To set required bias and modulation current normally resistors are used. However, there are digital pots as well. However, I cannot comment about their performance since customers normally use regular resistors.

    Regards,, Nasser.

     

  • In reply to Nasser Mohammadi:

    Hiii

    Thanks for the reply.

    As the datasheet is not having more technical information, i have asked the Vendor few questions and they replied me as follows:

    1. The monitor current is >0.05mA when resistance of load is 10 ohm.
    2. The Laser Slope Efficiency is 0.25~0.3 mW/mA.
        The Coupled Power Efficiency is 0.010~0.015 mW/mA for Average Optical Power of 0.4mW.
    3. The Extinction Ratio is >8 dB of transceiver.
    4. Series Resistance of Laser is 510 ohm as per the design.
     
    As per the above information, when i calculate the RAPCSET it is coming as 234.5KOhm and Modulation Current to be set is coming to be 2.3mA.
     
    I am not understanding where i am going wrong. Can you please help?
  • In reply to Chennuri Sruthi:

    Hi Sruthi,

    ONET4201 provides 20-ohm back-termination to minimize jitter caused by reflections. As long as we can meet close to 20-ohm termination, we should be able to use ONET4201LD. Could you please check with the laser vendor regarding series resistance of 510 Ohm. Is this resistance looking into the anode of the laser diode? Please clarify.

    Regards,, Nasser

     

  • In reply to Nasser Mohammadi:

    Dear Nasser,

    Thanks for the reply.

    I have forwarded the query to the Laser Vendor and waiting for the reply. In the Meanwhile, can you please suggest me about the resistors to be used as i have said you already that my calculation is going wrong. Please guide me.

    Regards,

    CH.Sruthi

  • In reply to Nasser Mohammadi:

    Dear Nassar,

    Also as i have already forwarded you the datasheet of the Laser Driver, if driving this laser for 2.5Gbps is complex using ONET4201LD(Selection of Resistors), kindly please take time and suggest me another Driver IC from TI that can ease my application.

    Thanks.

  • In reply to Chennuri Sruthi:

    Dear Nassar,

    The following is the reply that i got from the Laser Vendor.

    "I am sorry to have confused you.

    The 510 ohm is contact with APCSET pin.
    We put 18~24 ohm on your 20 ohm's position."
     
    Now is there any further query that i need to confirm from the Laser Vendor. Kindly let me know.
     
    Regards,
    CH.Sruthi
    Optical Engineer
     
  • In reply to Chennuri Sruthi:

    Hi Sruthi,

    This is what we were expecting. As far as I can see, we should be able to use ONET4201LD with this laser.

    Regards,, Nasser

     

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