for my project i am using max3799 IC and VCSEL.I have designed 4.layer PCB board and assembled max3799 IC and VCSEL also all other passive components specified in max3799 IC datasheet. this max3799 IC has 17 registers to modify according to requirements. Using rasp-pi i can successfully access registers and modify them.

for reference i am attaching datasheet of VCSEL and PIC1, PIC2 about forward voltage.

TO drive VCSEL it needs forward voltage of 2V and forward current of 6mA. but i am getting forward voltage of 1.15 V and forward current of 7.8 mA. So, VCSEL is not lasing.

PIC1 shows ideal working condition when VCSEL connected to MAX3799 IC

PIC2 shows my board working condition when VCSEL connected to MAX3799 IC

enter image description hereenter image description here

  • \$\begingroup\$ Forward voltage = supply voltage - Tout-voltage \$\endgroup\$ – kent May 30 '17 at 10:02
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    \$\begingroup\$ How are you measuring forward current? If you measured 7.8 mA current, why do you think the VCSEL is not lasing? \$\endgroup\$ – The Photon May 30 '17 at 16:02
  • \$\begingroup\$ i am measuring current with multimeter. one end of multimeter at BIAS pin and other end to GND \$\endgroup\$ – kent May 30 '17 at 16:35
  • \$\begingroup\$ That is not how you measure current with a multimeter. You need to break the circuit and insert the meter in series with the laser to measure the current. Or measure the voltage across R1 or R2 and calculate from Ohm's law. \$\endgroup\$ – The Photon May 30 '17 at 16:38
  • \$\begingroup\$ See this question for more explanation. \$\endgroup\$ – The Photon May 30 '17 at 16:39

Your VCSEL should have an inbuilt photodiode that provides illumination feedback to the MAX3799 - without that link, the MAX3799 will not properly drive the VCSEL because it receives no feedback.

Look at page 34 of the MAX3799 datasheet and note the feedback method (MD stands for monitor diode I believe).

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  • \$\begingroup\$ MD is option to use as feedback \$\endgroup\$ – kent May 30 '17 at 10:20
  • \$\begingroup\$ If you don't use MD then it is likely that you have to do something with the feedback connections to make the chip think it needs to drive the VCSEL rather than back-off (as I think it is doing). \$\endgroup\$ – Andy aka May 30 '17 at 10:55
  • \$\begingroup\$ are you sure about this in particular nothing mentioned in datasheet \$\endgroup\$ – kent May 30 '17 at 11:08
  • \$\begingroup\$ Have you set up the IBIASMAX register? Have you got a resistor on BMON - there is a fault set if the voltage at this point is too high - this might cause the problem you see. What values do you have for R1 and R2? \$\endgroup\$ – Andy aka May 30 '17 at 11:46
  • \$\begingroup\$ yes, i can access IBIASMAX register and modify. like you said voltage at FAULT pin is HIGH so, i connected FAULT pin to GND. \$\endgroup\$ – kent May 30 '17 at 12:10

Check the effective series resistance of your VCSEL. Unlike edge-emitting laser diodes, values of 20-100 ohms are typical for VCSELs. This means that when combined with the laser resistance your values for R1 and R2 are likely too high to provide the ~100 ohms resistance the driver seems to be designed for.

Second issue, if you measured 7.8 mA current with 1.15 V forward voltage it's likely that either

  • You have damaged your laser somehow and it happened to fail in a semi-short-circuit state.

  • You are driving current into the monitor photodiode rather than the laser. Given the measured values, I think this is more likely than a damaged laser. Double check the pin assignments on your VCSEL and make sure you are connecting the driver to the laser and not the photodiode.

  • You are not using the multimeter correctly. In comments you said you connected the multimeter between the BIAS pin and ground to measure the diode current. This effectively shorts out the bias circuit, drawing maximum current through the laser. To use a multimeter in ammeter mode, you need to break the circuit and put the meter in series with the device whose current you are measuring. Alternately, in this circuit you could just measure the voltage across R1 or R2 and use Ohm's law to determine the laser current.

Based on the new information in the comments, I stand by my previous suggestion: Reduce the value of R1 and R2.

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