445nm, 1W Blue Laser Diodes
And so it happened that I got three of those diodes that grow inside the Casio data projectors A140 and M140 like peanuts in their tin - ready to be picked. These little beasts come in handy TO-18 cans and produce an incredible 1W of pure blue laser light. However since they can only be obtained at reasonable price by dissecting the light engine of one of the aforementioned projectors, no data sheet exists for those laser diodes. There has been some investigation about them, and it seems that the ones harvested from the A140 are different to those from the M140. There are a lot of numbers on the net about them, and I will not repeat them here since many of them, especially the maximal optical power the are -in my eyes- pure speculation. According to the net, the M140 diodes are supposed to have about twice the max. optical power, they have a lower threshold current and a higher slope efficiency.
In this small article I show my measurements on one A140 and two M140 diodes. For each diode I plot the optical power vs. current at temperature between 0°C and 50°C, I compare the threshold curent and slope efficiency.
I have three diodes that I benchmark: Diode 1 is from an A140 projector (via ebay), and diodes 2, 3 are from an M140 (DTR'S Laser Shop).
|Diode #||Source ||Power|
|1||A140 ||1 W |
In the picture below you can see the diode from the A140 projector on the left and the M140 on the right. I supplied a small current to the A140 diode, just enough to make the junction light up. You can see that the submount (grey block) and the top of the die are contacted by three bonding wires. In contrast the M-diode has four each, suggesting better current handling capabilities. The submount of the M-diode is also physically wider.
I power the lasers with a custom constant current driver capable of delivering up to 1.2A, resolution 0.25mA. The diode is mounted on an aluminium block and fitted with a three element glass lens, anti reflection coated in the blue spectrum. The diode holder is temperature controlled by means of a peltier cooler. Note that although I am able to set the temperature of the holder rather precisely, this does not mean that the temperature of the die is at the exact same temperature. So I expect some degrading in efficiency for higher drive currents. But this reflects the configuration of a normal setup when the diode is mounted on a heat sink. I varied the temperature from 0°C to 50°C in steps of 5°C. Power measurement was done with a photodiode that I calibrated to my calibration free power meter which can only handle 100mW.
First, let's look at the diodes at room temperature (25°C). I varied the current from 100mA to 1.2A in 1mA increments:
For low currents, the laser diode behaves like an LED, that is it is emitting some light, but this light has a rather wide spectrum and only low power. When increasing the current above a certain limit called threshold current, optical power increases rapidly and about linear with the driving current. The slope of this increase is called the slope efficiency given in [W/A]. For larger currents, the laser die heats up due to the dissipated power and the curve gradually flattens.This effect is most prominent for diode 1.
|Diode||Source ||Threshold ||Slope |
|1||A140||203.1 mA||1.116 W/A|
|2||M140||146.4 mA|| 1.232 W/A|
|3||M140||140.2 mA|| 1.229 W/A|
Threshold currents are considerably lower for the M-diodes, about 60mA less. And at the same time the slope efficiency is higher by about 10%. Combined this leads to a power increase of 20% at 1.2A. The A-diode shows a strange behaviour at the onset of laser operation: There is a small buckle where the efficiency is higher than the linear fit. As a result the threshold calculated via the fit does not coincide with the first increase of light output. The same behaviour , although much weaker, can bee seen for the other diodes as well. I have absolutely no explanation for this.
Temperature dependance of output power
I varied the temperature to monitor the effect on the power output.
Observation: For currents above the threshold the output power decreases by about 2 mW/°C.
Temperature dependance of the threshold current and the slope efficiency
From the above diagrams (currents up to 400mA) I calculated the threshold curent and the slope efficiency at various tamperatures. This allows to plot both parameters versus temperature:. For my diodes I plotted the thresholdcurrent normalized to 25°C, i.e. Ithreshold / Ithreshold @25°C
Although the threshold is different for the A140 and M140 diodes, the normalized shift with temperature is basically the same. As already outlined above, the efficiency is considerably lower for the A-diode, and it does not particularly like the higher temperatures either. But given the variance for the M-diodes, in the range 0..40°C the drift with temperature is essentially identical (approx. -2.5mW/(A K)) for all diodes.
- From a pure electrical point of view the two diodes are indistinguishable: The forward voltage versus current, and the drift of VF vs. temperature (measured, but not shown here) are identical.
- Threshold of the M-dopde is lower by about 60mA, slope efficiency is higher by roughly 10%.
- Tempco of threshold and slope are found to be identical within statistical variation.
- Roithner has a datasheet for dismounted blue laser diodes