445nm Direct Injected Laser Diode
This webpage is about the recent developments relating to 445nm laser diodes found in new DLP projectors.
Disclaimer:
* All of this information is provided as 'best effort' by myself, and I cannot guarantee any of these measurements.
* These diodes are class IV lasers, and must be used with extreme care. Under NO circumstances should they be incorporated into a hand-held laser pointer, unless correct measures have been taken to reduce the output power to <5mw (putting a safety interlock on a laser pointer does not make it safe nor legal!).
Specs
- The internal laser diode is isolated from the case, so issues with common cathode/anode do not apply to this diode. The ground pin has been clipped short, leaving the 2 power pins as seen in the picture at the top of the page.
In the projector, these diodes were run at 1.44A, at a 39% duty cycle, on a 8.2ms period (120Hz)
Additionally, there is a 16% 'warm up' period where they are run at 0.24A, before each pulse, which is just above the threshold current for the diode. Please see the section 'In Projector Testing' for updated information!
- The average output power, after the internal projector lens, was 0.88W (per diode) as measured on a thermal power meter designed for properly averaging pulsed measurements. This means means the diodes are being run at 2.25W during the 'on' period.
- The diodes are passively cooled with a heatsink/fan, and run about 40C in normal operation.
- The 'stock' lens in the projector is a biconvex glass lens, broadband AR coated, roughly 6mm in diameter. It does not give stellar performance when used to collimate the diodes, giving a spot size roughly 1" in diameter at 10ft.
- When colliminated using a moderate quality triple element lens, such as the A-HGL-905-3 from Aixiz gives a spot that is approximately 2mm by 5mm at the lens, and 3.5mm by 15mm at 20ft, giving a divergence of approximately 2mRad of divergence on the 'fast' axis and 0.2mRad in the 'slow' axis.
- It has been discovered that at low powers the diode runs single longitudinal mode, which makes it suitable for holography. For more information see Wler's blog
At aproximately 30C I took the flowing spectrum of the output, the multimode (longitudinal modes) characteristics of the diodes can clearly be seen as the power being divided into 2 large modes and several smaller ones. The current for this test was 300ma, well above the threshold current for the diode. The wavelength of this (and all laser diodes) shifts with varying temperature and current, shifting to longer wavelengths with increasing temperature/current. That said, at 30C the diode I tested was actually 442nm. All of my spectrometers are currently in the wrong county so I cannot take any further measurements to comment on the wavelength spread across different diodes or variation with temperature.
In Projector Testing
Before removing these diodes from the projector, I added a 1 ohm current sense resistor in series with one of the diodes. I then measured the voltage across it with an oscilloscope, and used ohms law to deduce the current through the diode (1V across the sense resistor equals 1A of current through the diode) so the trace below shows the diode current at 500mA/division:
NOTE1-I this trace is inverted (ie, the diode shows negative current because I had the clipped the probe in backwards).
NOTE2-I am not sure if my projector was in economy mode while this test was taken, if it was then the power may be slightly higher than what I measured.
From the above data it can be seen that the diode is being run at 1.44A for 3.2ms, 1.22A for 1.3ms, and is run at approximately 20ma the rest of the time (this current was too small to accurately measure with my equipment, it is in the range of 5-50mA). I measured the average power from the projector to be 0.88W (per diode). I then tried to go back and figure out what the powers were at the 1.44A and 1.22A currents, by assuming that the output power scales linearly with current past the threshold current -ie, if plotted in a line, power = (current-200mA)*arbitrary constant. From this I concluded that at 1.44A there is 1.7W of output power, and at 1.22A there is 1.4W of power, which match within 10% of the CW measurements I took.
More Information
I can be contacted at contact@krazerlasers.com
