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Need to modulate a 200 mW laser diode to 2 Gbps. The diode has typical threshold of 50mA and typical forward current of 500mA.

Chips such as: ADN2871 by Analog devices; SY882322 by Micrel; and M02067 by Mindset Tech. can provide as much as 80mA bias and 100 mA of drive current for modulation anywhere from 2 to 4 Gbps. Is there a simple way to supply the additional current (e.g. through a bias tee) to achieve full output power from the laser and full modulation to 2 Gbps? Thanks.

Lasers1
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  • A bias tee could increase the average power, but it would not increase the optical modulation amplitude (OMA). Is that what you want? – The Photon Feb 18 '15 at 23:19
  • When you say "full modulation" I assume you mean you want near 100% modulation depth (aka very high extinction ratio). – The Photon Feb 18 '15 at 23:22
  • Can you translate 2 Gbps into Hz. ? – George Herold Feb 19 '15 at 03:43
  • Thank you all. Yes, I would like to get the full output power of the laser. It is very difficult to get full extinction ratio from diode lasers. 10-15% MER would be OK. Lets say 2 GHz. – Lasers1 Feb 19 '15 at 05:34

1 Answers1

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I don't know any IC that can drive such a high power laser at that rate.

If this is for a bench-top test, you should be able to build a driver with a power amplifier, a current source, and a bias tee.

schematic

simulate this circuit – Schematic created using CircuitLab

The power amplifier should be wideband, from maybe 1 to 1000 MHz, and it will need to deliver at least 4 W to generate 400 mA modulation amplitidue (800 mA peak-peak). Maybe consider Minicircuits ZFL-1000, for example.

Depending on the laser input impedance you might also need to add a series resistor in front of it for impedance matching. This resistor could be absorbing 80% of the input power, so you might have to trade off rf performance, resistor power rating, and impedance matching.

The current source should include compliance voltage limit in case of reverse connection, and you might also want a slow-start feature (although it's not likely to be needed on such a high-power laser).

The bias tee could be maybe Mini Circuits ZFBT-4R2GW+, although that has a max current of 500 mA so there's no margin from your requirement.

You'll find that as you increase the extinction ratio, the jitter performance will become very bad. So yo won't ever achieve "full modulation". You'll have to experiment with bias current and modulation amplitude to find a good trade-off between modulation and jitter.

The Photon
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