Testing of laser didoe

[chaitanya2k6er]

Hai everyone. I had a laser diode part no: ADL-63201TL i.e with a photo diode parllel to it for the feed back. How can i test it and come to a conclusion that it is working properly? Suggest me some possible ways to test the laser diode. Please find the attached datasheet.

 

[klystron]

You must control ( keep within specifucation) the lazer diode current based on the intensity as sensed by the sensing diode

 

[chaitanya2k6er]

You must control ( keep within specifucation) the lazer diode current based on the intensity as sensed by the sensing diode

Thanks for your reply. Please find the attachment. As the laser operating current is 87mA with 2.2V drop across it and monitoring current 0.08mA typically, i have rigged up a circuit as shown with laser current 85mA. As per the specifications, monitoring current should be 0.08mA, i have connected a 6.2K resistor in series with photo diode such that 0.5v should be developed across it. When i power on the circuit, the same is achieved and with in no time the voltage across laser diode is getting increased and the current through photo diode is getting decreased. Is this laser diode working properly? As the output power is 20mW i have not connected any heat sink to the laser diode.

 

[FvM]

Operating the laser diode at a low current should be safe. For full power output, closed loop feedback is required. If you read the data sheet carefully, you'll notice that the monitor diode sensitivity can vary over a wide range (PD current at rated LD power is 0.03..0.5 mA). This means that a calibrated optical power meter is required to adjust the laser diode output power. Without it, you run at risk to set an output power above the LD rating that may damage the device.

 

[chaitanya2k6er]

Operating the laser diode at a low current should be safe. For full power output, closed loop feedback is required. If you read the data sheet carefully, you'll notice that the monitor diode sensitivity can vary over a wide range (PD current at rated LD power is 0.03..0.5 mA). This means that a calibrated optical power meter is required to adjust the laser diode output power. Without it, you run at risk to set an output power above the LD rating that may damage the device.

What if i set operating current to the typical value i.e 87mA, without using in the feed back loop. As per the specifications laser diode has to work properly at 85mA. But i dont find it working properly as i have mentioned in the previous post. Before conducting this experiment i have driven the laser with laser driver. I dont find it working. So inorder to isolate the problem, i have checked the laser in this way. Is this the correct process to check a laser?

 

[FvM]

I don't read anything indicating "not working" in your previous post. You reported unstable operation, which can be expected under the said conditions. It may be the case, that the LD has been already damaged by overheating or exceeded optical power rating, hopefully not.

As per the specifications laser diode has to work properly at 85mA.

As the output power is 20mW i have not connected any heat sink to the laser diode.

I think, you misunderstood several points in the datasheet.
- 87 mA is specified as typical operating current with 20 mW optical output. It's not guaranteed to keep the diode power below 22 mW maximum rating.
- As you can easily calculate, the electrical input will be about 180 mW. The maximum case temperature is specified with 40°C. This means, it's impossible to achieve specified operation without a heatsink.

- - - Updated - - -

An additional point about LD behaviour in open loop. Not shown in your short form datasheet, but the laser threshold is also strongly temperature dependent. This leads to a reduction of optical output, or even stop of laser operation with increasing temperature, see the characteristic of a red 30 mW LD from Mitsubishi.

In so far, the observed behaviour, is pretty understandable.

 

[Tunelabguy]

Just out of curiosity, what sort of drive do the cheap shirt-pocket laser pointers use? Open loop or closed loop?

 

[chaitanya2k6er]

I don't read anything indicating "not working" in your previous post. You reported unstable operation, which can be expected under the said conditions. It may be the case, that the LD has been already damaged by overheating or exceeded optical power rating, hopefully not.



I think, you misunderstood several points in the datasheet.
- 87 mA is specified as typical operating current with 20 mW optical output. It's not guaranteed to keep the diode power below 22 mW maximum rating.
- As you can easily calculate, the electrical input will be about 180 mW. The maximum case temperature is specified with 40°C. This means, it's impossible to achieve specified operation without a heatsink.

- - - Updated - - -

An additional point about LD behaviour in open loop. Not shown in your short form datasheet, but the laser threshold is also strongly temperature dependent. This leads to a reduction of optical output, or even stop of laser operation with increasing temperature, see the characteristic of a red 30 mW LD from Mitsubishi.

In so far, the observed behaviour, is pretty understandable.

In the datasheet it was mentioned as at 20mW power output, operating current is 87mA, operating voltage is 2.2V, monitoring current is 0.08mA. I dont understand how come with 87mA and 2.2V power will be around 20mW?

 

[FvM]

The data refer to about 10% optical efficiency.

 

[HTA]

Here you find some idea how to design and test **broken link removed** .

Enjoy your design work!

 

[Roger Miller]

I don't have much idea about this but I have found a video on how to test 980nm laser diode, may be it is helpful for all.