Help On CNI 10W Green laser source

UPDATE: Problem solved. New Temp values is added to the CPU on the driver. all is now running perfect. :-)

Hallo.

I need help to get some info on a CNI OEM-W laser source. It's a DPSS 10W 532nm.

The sources are total new.

I have a funny thing on the source:

On +5w input "CW mode" all perfect - 10W output perfect "flicker" free output. But as soon i add modulation (PWM/TTL) is "flicker" random on the output beam.

But if i turn it down to run about 50% output, it is flicker free output again.

If some one have an overview of the trimmer on the driver i would like to have it asap.

Believe its a wrong Temperature set point on the TEC's.

But all Input is welcome.

I have attached some pics

Poul.

Attachment 56218
Attachment 56219

As a general rule, DPSS lasers do not respond well to pulse-width modulation. If you are using a high frequency PWM scheme to try to simulate a true analog response, you will have *great* difficulty doing so with this laser. You would be much better off using analog modulation (assuming this is for a projector).

If your application *requires* pulse-width modulation, then you should probably consider purchasing a direct-diode laser. (520 nm vs 532 nm)

You are correct in that the problem lies in the TEC, but the problem is more complicated than just a temperature setpoint. The issue is that thermal changes from the TEC happen much slower than thermal changes caused by the output of the pump diode. So the TEC controller ends up constantly chasing the setpoint. In fact, the people who initially developed the DPSS laser design felt that it could never be modulated for just this reason. (And when you consider just how temperature-dependent the frequency-doubling process is, it's frankly amazing that we have been able to modulate DPSS lasers - at least a little bit.)

In my experience, most DPSS lasers will start showing similar symptoms to what you are reporting when the modulation speed exceeds 10KHz. The fact that your laser operates normally with +5 volts DC applied to the driver tells me that the laser is working correctly. As for the case when it works better at lower power - by reducing the power output below 50% you are dramatically reducing the magnitude of the temperature spikes when the pump diode turns on. This helps to smooth out the thermal curve so the laser output is not as erratic.

Adam

If you can, buffer your PWM and run it into a RC low pass filter with a pulldown . Really you should use a tailored active filter with output clamping, but that is beyond the scope of this post. The TE board on a 10 watt system may be smart enough to have an "anticipator" circuit looking at the incoming modulation. If it sees a smooth waveform it will probably do better,
calculating the minimalist RC circuit to do this without adding excessive lag is a math and bench measurement exercise. One assumes a minimum R of 470-1000 Ohms to protect various parts.
:th_Jttesur:
In other words, take your PWM and make it analog.
:th_Jttesur:
There are purely PWM input DPSS Green lasers in the world. These work with digital pulses of constant energy per pulse through the pump diode and provide a signal from the modulator to the TEC driver. Basically your PWM gates a pulse train inside the laser diode driver . Generally these come from very large, very professional laser hardware companies with large RD staffs, and are not off the shelf. There isn't much demand for them, either, so they are expensive. Done correctly, and respecting the 256 microsecond upper storage lifetime of ND:YAG, they have VERY nice modulation. However, these drivers are rarer then hens teeth. They never caught on in the laser show industry due to patent issues. It is not an easy board to design, as you need specialized power supply monitoring, a DC bias current, and really sophisticated diode protection circuits. Not to mention rather brilliant characterization of the doubling crystal.

Steve

Hallo.

Update: We have been in contact with CNI Engineer for the last 4 days. They are fast responding to solve the problem. :-) Good service CNI.

Yes. The system was ok in CW mode. over 10W out. But then with modulation it was unstable.

Who did they make the driver:
The driver design is well made. They have a ARM processor controlling all on the board.
They have a Dual H-bridge for the TEC's, so they can heat and cool both the Laser Diode and the Crystal. But they also made it quite nice, compared to many other drivers i have worked with in the past.
They made a LD and Crystal set points for CW mode and for TTL Modualtion (4 set points totally). Know it from many other systems it's comprimise to find a set point value where both CW and Mod. so it can work stable.
But here the have 4 set points, so the ARM looks at the in coming signal. If it's a static 0/5V signal or its modulating. The ARM changes to match the right temp set point. Normally these CW / Mod. set points are quite close in temp i notes, and the driver is relative fast to moved the temp. from CW Mode values to TTL Mod. Mode values, and of cause back again.
All values is correction be sending commands to the ARM CPU. So we have made a USB-RS232 modul + a Level converter to 3.3V to match the voltage for the communication. A good Friend of mine have made a Pyhton Scribt there calculate the dec to HEX values and also calc the Check Sum and send the values to the driver, so it's much faster to manual calc the values one at the time. A huge time saver for finding the right settings.
You have 2 Analog test points, where we can meassure the NTC DC values so we know what is going on with the regulation of both TEC's. Here you can also see how fast the temp setting is reguleted in.
When all 4 settings are correct. you Won't notes the temp shift the tecs makes, all is really stable less then 1 mV be 0.00X Volt on both Tec Circuits.
Really surprised who good and precise the driver is runing from CNI. Compared to what we have seen just 5 years ago.

/Poul Moeller.

I see you've solved the problem, but I have a lingering question: How fast is your modulation input signal, and what is the duty cycle?

Adam

Quote:

---

Originally Posted by buffo

I see you've solved the problem, but I have a lingering question: How fast is your modulation input signal, and what is the duty cycle?

Adam

---

The test frame we did see the flicking on was duty cycle about 80% "on" time and 20% "off" time. with a repeat rate in only mS. So not super fast.
But i will do some checking with diffende frequences up to 30Khz to make sure all is total stable, with the settings i have now.