# What is current modulated laser driver? What does it do to the laser?

The title is actually one of the many questions I have. I've started to work on a project that includes lasers. I've worked with lasers and fiber optics before and I even constructed a simple constant current driver and used it in a fiber ring laser system. So I am not that unfamiliar with the concepts. However, all these were quite basic. Okay now let me ask my questions.

1) I've been told that I need a current modulated laser driver to use one of my 1550nm laser diodes. (LUCENT D2570)

https://www.koheron.com/photonics/drv200-laser-diode-driver

Here's the link of the product. What is this current modulation? What does it do? What does that 5MHz mean? What is its effect? Does it make the CW laser pulsed? Because my advisor said it had to be 5MHz to obtain 200ns pulses. If it does make the laser pulsed, how does that happen?

2) I also have a 1310nm pulsed laser diode.(NEC NDL7503P1) It is a coaxial laser with 4 pins. When I first started working on this project it was activated with a DC source which was simply connected to laser anode and cathode with alligators. I was also told that I need the same driver given in the link above. The question is, how does this diode work with that simple connection? Does the DC source act as a constant current supplly? Since it says the laser is pulsed, does it generate pulses with this connection or does it mean that it can generate pulses when with a suitable driver? If so, what kind of driver is that? Is it the one in the link? How do you make it pulsed? Is it CW now without the driver?

I know some of these questions are silly or requires very basic knowledge but the guy working on electronics left. Therefore, I now have to work on both optical part and electronic part. I know don't know much about this topic but before coming here believe me I tried everything to learn these.

Rigby.

• IMO it is modulated. You modulate the beam with an information, like audio, video,...The frequency response characteristics always shows sine wave amplitude vs. frequency, so 5MHz sine wave, not pulsed. – Marko Buršič Jan 31 '18 at 17:51
• The LUCENT D2570 is quite a sophisticated laser - do you know why you need to use that particular part? It seems to me (guessing) that the driver you link does not match well with the laser given the spec of the laser and what it can be used for. – Andy aka Jan 31 '18 at 18:02
• @Andyaka There is no specific reason. It was avaliable in our inventory at that time. Given driver was for another 1550 nm laser diode which was replaced by this one since the previous one had very short pins that made it very difficult to put on a zif socket. – Rigby Feb 1 '18 at 16:17
• I'm hoping this question has been answered to your satisfaction already, but I wanted to point out (since I didn't see it here explicitly) that current modulation is also used to produce coherent sidebands in your laser. E.g. sinusoidally modulating the input current at say, 5MHz on your 1550nm laser, will produce output at 1550nm + 5MHz and 1550nm - 5MHz. (Actually at every integer multiple of 5MHz.) The intensity at every sideband is related to the Bessel functions. – nivk Nov 9 '19 at 1:39

The resistance of a semiconductor laser is low and strongly related to temperature and too much current flowing can easily damage the laser.

The electronics driving the laser controls (adjusts) the voltage such that the desired current flows through the laser by measuring the voltage across a sensing resistor. This is a constant current source.

If the optical output of the laser is to be modulated - its amplitude is changed with time - this is accomplished by changing the voltage supplying the laser (and measuring the current flowing through the laser) then the device driving the laser is called a current modulated laser driver. The driver can only work so fast - this is the bandwidth of the modulation.

You would likely drive the input to the electronics module with a modulating voltage that varies between off (actually a voltage that corresponds to a current which is just about the lasing threshold) and the maximum desired optical output. You'll need a device that can do this at the rate you require - some sort of pulse generator.

Because the driver has a finite bandwidth the rise/fall times of the optical output will be limited. (Higher bandwidth sharper edges).

You'll need another controller to measure and control the temperature of the D2570 - $\lambda$ is sensitive to laser temperature. The D2570 has a pins for an on-board TEC and pins for a thermistor to monitor the temperature.

Connecting a laser to a constant voltage supply with croc clips is a bad idea.

• So as far as I understand a current modulated laser driver turns the laser on and off (I get that it's not exactly off). The frequency value given there is then means how fast that operation occurs and as a result we get a pulsed laser from a CW laser. Then that 200 ns long pulse concept also became clear. Thank you very much. I can control the temperature. I was just curious about other things so I forgot to mention about that. For the other laser, I too think that it's dangerous but I have no idea why and how they did that. – Rigby Feb 1 '18 at 16:08
• the current modulated laser driver takes a voltage input and drives current thru the laser. You control the input and you make the low input such that the current thru the laser is just at threshold and a high input is less than the maximum current rating of the laser. It's not a pulsed laser in the usual sense (ie Q switched) - you have a cw laser that is on off modulated. – D Duck Feb 2 '18 at 9:03