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As the title says, I would like to add an external Bias tee to modulate a high frequency signal into a laser diode, because the laser driver I have chosen does not have a built-in modulation circuit. I have bought the MLD203P1 laser diode driver from Thorlabs. This laser driver does not have the modulation circuit, so I decided to add an external bias-tee to it. The typical application of the MLD203P1 is seen below.

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What I know so far is that a bias-tee consists of an inductor and a capacitor, which is in parallel and with one DC-input, one AC-input and one AC+DC output. So I think this should be the solution. This is seen below.

enter image description here

I just only need a modulation range of 10 kHz to 20 MHz. I have also read the datasheet of the MLD203P1 and don't see a problem if I attach the external bias to the LDA (Laser diode Anode). Although, I will add a current limit resistor between the LDA and the DC input of the Bias Tee.

So I wonder if this is working or has anyone tried it with another laser driver? And is this the right path I am following, to get a modulation signal of the laser diode?

Datasheet of MLD2031P1: https://www.thorlabs.com/drawings/8c0acf99c70bcbeb-392580F3-E400-B9B9-C9E56C95CCB14E62/MLD203P1-DataSheet.pdf

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  • \$\begingroup\$ Have you considered asking Thorlabs directly if they can provide a laser driver with modulation capabilities, otherwise your idea of the bias-T should be fine. Thorlabs may also provide application notes for their products that you can read and obtain more information on methods to add modulation ... \$\endgroup\$ – citizen Aug 26 '20 at 7:44
  • \$\begingroup\$ This laser driver attempts to maintain light output at a constant level set by a potentiometer. Your attempt at modulation will be a disturbance to this level control loop. While the loop frequency response may be low, you may still run into problems at 10 kHz. \$\endgroup\$ – glen_geek Aug 26 '20 at 13:37
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The main problem with your proposed solution is that you will also modulate the photodiode (PD) signal fed back to the MLD203 and this easily give unexpected results.

Ideally, you should interrupt the cathode circuit of the laser diode with an inductor and AC modulate the cathode with a capacitor but the MLD203P1 is pretty light on information in this area so maybe go for a MAXIM chip and read this MAXIM document about interfacing.

This chip might be a better solution: -

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Or maybe this one: -

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Here are some other offerings from MAXIM that are worth investigating.

One more consideration is the photodiode - it will not be calibrated to a specific light output when you get it it and I would suggest you get hold of a cheapish light meter to ensure that the monitor current for a given light output power is known.

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  • \$\begingroup\$ Thank you for your response. I am going to use the thorlabs LPS-785-FC lasermodule. The thing is , is the datasheets of MAXIM does not give the noise specs specifically. I think I have to take laser noise source into account. What about that? \$\endgroup\$ – Kwok Aug 26 '20 at 12:24
  • \$\begingroup\$ There's very little you can do about laser noise source if it's present. I'd be much more inclined to be concerned about the device that is receiving the modulated laser signal - it is likely to be a more significant noise source in the signal path. \$\endgroup\$ – Andy aka Aug 26 '20 at 13:22
  • \$\begingroup\$ I have read some information of the MAXIM about what you have suggested. I see almost everything with xx gbps, but what does it mean and how is it related to the modulation frequency? My requirements for modulation are 10 KHz to 20 MHz range with a sine wave. And as far as I see, this is only for square wave form, I think. Have you some experience with using those chips? I am namely new to laserdiode drivers. \$\endgroup\$ – Kwok Aug 27 '20 at 8:19
  • \$\begingroup\$ The Gbps is the maximum switching rate. They will work at lower rates with the appropriate value of inductor. I have used some maxim chips for this type of application operating in the Mbps area. Modulating the laser light with a sinewave is not going to be effective due to laser light level non-linearities. You should digitize the sinewave and transmit it as data then recover at the receiving end. \$\endgroup\$ – Andy aka Aug 27 '20 at 8:22
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Have you looked at the Thorlabs T1G bias-T PCB? I use these with constant current drivers to modulate DFB lasers and superluminescent diodes. There's a decent set of instructions linked on the product page - you attach it to the output of the driver and then to the device you want to drive/modulate. It will definitely do the modulation bandwidth you are after. I can't get the link at the moment as their website is down.

I'm surprised that Thorlabs didn't suggest this to you. From the previous comments - are you expecting to modulate in constant power mode? You need to be using the driver in constant current mode and then the bias-T superimposes the modulation waveform on the DC current.

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If you're really stuck with needing to use the Thorlabs driver device, you may consider the possibility of adding the bias-T on the Cathode side of the LD. This way you provide a DC path for the LD bias to ground and you modulate the Cathode instead. The LD pin of the driver would be connected to the LD Anode to bias the LD. Otherwise change driver device... Take note though, over the modulation bandwidth of 10KHz to 20 MHz you will have some significant variation in inductor reactance so your modulation depth will taper at lower frequencies relative to the higher frequencies... but that's another subject.

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  • \$\begingroup\$ "The LD pin of the driver would be connected to the LD Anode" . You mean the pin 'LDA' of the MLD203P1 to the LD Anode right? \$\endgroup\$ – Kwok Aug 27 '20 at 8:13
  • \$\begingroup\$ Yes, that's right, however please note the comment above by @glen_geek regarding the control feedback loop. The driver may attempt to correct any modulation you superimpose on the LD bias current, because its main purpose is to keep LD output more or less at a constant value (like a regulator would do). So, I think even though there are some potentially helpful comments and suggestions on this forum, you should double-check with Thorlabs directly, and see if your bias-T idea is actually going to work as intended. They may quickly reply to you and say NO it will not work. \$\endgroup\$ – citizen Aug 27 '20 at 8:33
  • \$\begingroup\$ So better to check with Thorlabs first ! In the end it is YOU that will be responsible for the final working circuit... \$\endgroup\$ – citizen Aug 27 '20 at 8:33
  • \$\begingroup\$ I have checked it yesterday with Thorlabs. According to them, they don't think it will work. Is there then another way to have a Continuous wave (sine wave) laser driver with automatic power control with modulation (another alternative for what Andy aka has suggest)? \$\endgroup\$ – Kwok Aug 28 '20 at 9:34
  • \$\begingroup\$ Does Thorlabs offer a continuous wave laser driver with automatic power control with modulation ? They should have proposed an alternative solution, if it were available ... Otherwise, sorry, it is beyond the scope of your initial question now, after confirmation of Thorlabs, maybe you should consider Maxim alternatives, and ask Maxim for advice on your specific application. \$\endgroup\$ – citizen Aug 28 '20 at 13:18

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