# Help designing a circuit to drive a 1550nm DFB LD and use it to modulate an RF signal

I need to modulate an RF signal (random sine wave acting as data) using the 1550nm DFB Laser Diode, I need a control circuit to drive the Laser Diode and inject the data to modulate the LD carrier wave. I won't mind if you provide me with the complete circuit just as long as you can explain to me step by step the purpose of each part of it, I'm sure this break down will help those that read this to benefit from this question.

The carrier would be the optical wave and the source signal will be an analog continuous wave, i will be using a function generator to create this wave. This means i will modulate the optical wave using the analog wave and carry the modulated wave through a single mode fiber, at the other end i will have a Photo diode and an oscilloscope to see my original signal.

I will be using a function generator to create the RF signal so there is no restriction to the frequency i can use, it can be low or high. The specs of the LD say that the max operating voltage is 1.7V and the max operating current is 90mA. Since i am not sure what else is needed i can provide more information if necessary,the LD is WSLP-1550-008m-9-DFB.

• What is the threshold current for the laser diode? Probably better if you provided a link to a data sheet. Commented Oct 29, 2016 at 18:54
• You are completely right, just added the datasheet link, the max threshold current is 15mA Commented Oct 29, 2016 at 18:59
• Can you please be a little more clear about what is the signal and what is the carrier? When you say "modulate an RF signal" that implies to me that there is a sinusoidal carrier frequency which you want to modulate using some kind of input signal (maybe data, maybe voice, who knows?) It is not even clear if this is digital or analog. Nor what the transmission medium is (fiber, I assume) etc. Commented Oct 29, 2016 at 19:21
• How you should modulate the laser depends mainly on optical link distance and the type of fiber you're using. Please edit your question to make these requirements clear. Commented Oct 29, 2016 at 19:29
• Duly noted, i have included more specification regarding what i want to do. Let me know if you need something more. Commented Oct 29, 2016 at 20:36

What modulation scheme do you intend to use? (Amplitude modulation, Frequency modulation, Pulse-density modulation, etc.).

What type of waveform is your input signal? (sinewave, pulse, triangle, etc.). Are you trying to convey the input's amplitude, frequency, duty-cycle, and/or phase?

For OOK and other pulse-modulation schemes, the laser-diode/LD current would switch from full off to full on (0mA and 75mA for your LD. The 90mA is an absolute max. spec.), at a frequency or width determined by the modulation signal. For FM, the LD would be driven with a pulse train whose frequency changes according to the modulation signal. For AM, the LD would be biased at 50%, and DC-modulated by the input signal.

For OOK, I would use a simple switched current source:

simulate this circuit – Schematic created using CircuitLab

• I was considering using on/off keying (OOK) to modulate the signal since it seems the most simple way and using a sine wave but assuming I create the circuit with that in mind, does that mean i won't be able to modulate a square wave for example? Commented Oct 29, 2016 at 19:14
• This doesn't answer the question. Requests for clarification should be made in comments. Commented Oct 29, 2016 at 19:26
• I've run the simulation, apparently the voltage in the node entering the LD is 2.4V. Shouldn't that create an issue since the max operating voltage is 1.7v? Commented Oct 29, 2016 at 20:11
• No. The circuit, when 'on', delivers 75mA into D2. Your actual LD will should work fine at that current. Commented Oct 29, 2016 at 20:42
• I have a question about your proposed circuit. I can use this circuit to drive the laser but how or where do i insert a random signal (data) to modulate the laser. Please check the question again since i specified a little more what i need Commented Nov 2, 2016 at 18:08

Cutting straight to the core of the problem, if you can do OOK (on-off keying) then just directly modulate the laser and forget an analogue modulation scheme. You will need to drive the laser at high/low currents greater than the threshold else risk loss of data due to the lasing recovery time needed when you drop below the threshold.

To keep data moving (thus allowing the receiver AGC to keep working) I would manchester-encode the data transmitted. I suppose this is a modulation method but it's digital!

• I understand, although i need to be able to modulate a analog signal using the laser diode since its more like a class project. Commented Oct 29, 2016 at 20:16
• One thing in life I've learnt and that's not to compromise a design to please the egos of the team members. Commented Oct 29, 2016 at 20:38
• I will take your advice, in this case could you provide more details like a circuit i can use. And I assume i would need to transform my analog signal to digital first and then modulate it. Commented Oct 29, 2016 at 20:44
• I thought you said that your signal was compatible with OOK therefore it is digital. To modulate the laser you need a simple constant current source that can be "wobbled" with your modulating signal. You ought to fully specify the signal that will modulate the laser to be sure. Commented Oct 29, 2016 at 21:20