This is the gauntlet; a place where things are tortured in ways that only an engineer could appreciate.
Today’s victim is a 1.0W green laser module, manufactured by Suzhou Daheng under the brand name “DHOM”.
As far as Chinese laser manufacturers go, Suzhou Daheng is about one rung lower than CNI in terms of quality. Although US companies like Coherent blow these guys out of the water, both are still reputable nonetheless. As far as Chinese lasers themselves go, this one seems a bit conservatively rated; a nice change from the “1000MW 532nm laser cat toy burning module” that’s not too uncommon on dealextreme and the like.
More after the break…
- Wavelength: 532nm
- Output Power: <1000mW
- Transverse Mode: TEM00
- Divergence: <1.5mRad
- Beam Diameter: <2.0mm
- Cooling Mode: TEC
- Modulation: Analog 0~10K
- Expected Lifetime: 10,000hrs
- IR leak: <1mW
Let’s start off by identifying the transverse electromagnetic mode and divergence. The following photo was taken at f/36, 1/2000th shutter and a 4x neutral density filter. For all you camera buffs out there, that should give you an idea how bright this spot actually is.
The laser mode hops dramatically as it warms up, but it eventually settles on what I can best describe as TEM11. While theTEM mode isn’t up to spec, that’s not too uncommon in high power lasers like this; getting a TEM00 beam at this intensity is quite hard to do. Fortunately, the mode is circular and will not have a fast axis when focused with a lens.
Beam diameter at the output was measured to be 3mm, where it then expanded to 20mm after transversing 20 feet. This works out to be a divergence of about 2.7 milliradians, which is not great, but not terrible either. Certainly a far cry from the stated divergence of 1.5mRad though.
P = I2R
Next, let’s test the output power. As expected with a high power DPSS system, it takes a little while for this laser to stabilize.
At about 800 seconds into operation though this beam settles around 1.45 watts. That’s pretty good! It far exceeds the manufacturer’s <1000mW rating, even if it must operate at a higher order mode to do so. This is pure green power mind you, since this laser was found to leak no discernible amount of infrared light.
f = 1 / 2π√LC
Time to put the laser’s modulation capabilities to the test. Supposedly, this one can be analog-modulated up to about 10kHz, but is this true?
The following setup will be used to verify this rating. It’s pretty much just a green LED rigged to the input of my scope.
At 500Hz, the modulation is nice and square. A wee bit of baseline offset but nothing too concerning.
At 5000 Hz we’re starting to see a little bit of slewing. The fall time is still nice and quick though, so no streaking should be observed when using the laser to draw an image.
At 10,000Hz the beam’s modulation becomes more of a sawtooth than a square, and falling edge starts to slur quite a bit. I’d say this is about the maximum frequency at which I’d like to modulate this thing at.
At 20kHz all hell breaks loose, while at 50k there’s no longer any light to be seen.
Vout = 1.25 * (1+(R2/R1))
Everybody loves teardown photos, so let’s have a look-see inside the power supply.
It seems to be nothing more than a linear current regulator piggy-backed on a switch mode converter. Although the switch-mode PSU looks like a rather high quality one, I’ve measured the 5V bus to be a bit high at 5.13V. Though it’s higher than I would have liked, that 5.13V rail is actually very stable and has no meaningful high-frequency noise.
A top-down look reveals a few recognizable ICs.
Operationally it seems rather simple. The modulation input is compared to a TL431 reference voltage and the current supplied to the laser diode is varied accordingly. That NAND gate makes sure that power is only given to the laser diode when the thermistor inside the laser head says it’s safe to do so, while another voltage reference’d op-amp circuit makes sure that the KTP crystal’s thermoelectric cooler/heater is maintaining a certain temperature. Unfortunately none of these trimmers are labeled…
[In case anybody's wondering, that big potentiometer is actually my modification to the supply. It replaces the inconvenient key-switch that was once there].
No user serviceable parts inside
While it appears to be built a bit better than the PSU everything in the laser head is cemented with white epoxy; leaving no room for adjustment. Though it’s not atypical for Chinese lasers to be constructed like this, it’s impossible to move those crystals a few microns to coax the laser into TEM00 emission. On the bright side, this epoxy completely protects that KTP and ND:YVO4 from the elements.
- Wavelength: 532nm ++
- Output Power:
- Transverse Mode:
- Beam Diameter:
- Cooling Mode: TEC ++
- Modulation: Analog 0~10K ++
- Expected Lifetime: 10,000hrs ??
- IR leak: <1mW ++
Overall I’d say it’s an average laser; not wonderful, but not craptastic either. If your goal is to make a laser harp or a projector then this module should work fine, however if your goal is holography or high-speed long-distance communication this isn’t the module for you. At $1200 though, it’s still a good deal as far as the green ones go.
That’s all folks; feel free to tweet ideas for future victims to @irfp260.