A Description of the Tools Used and Data Collected During the Study of Underpowered Lasers
Underpowered Helium Neon Laser
Variable Power Supply: Hewlett Packard 6284A DC Power Supply
Laser Pumping System: Laser Drive Inc. Model (134)006-1013738
Recommended Operation: Input DC 10-14V at 0.9A, Output DC 1250V at 4.0mA
Preliminary Observation of Operation Status: (input values unless otherwise noted)
When the DC voltage supplied is 12V the ammeter reads a value of 0.41A and operation is normal (i.e.: continuous and steady). For 10V there is normal operation at 0.50A and for 8V the operation is again normal and at 0.63A. Lowering the voltage to around 6.8V the operation begins to alternate between normal operation and an ordered flickering state. During periods of normal operation the current is near 0.61A, but during periods of flickering the current drops to around 0.41A.
Current maxima occurs at 0.79A for around 7.0V. "High" voltage current local minima occurs at 0.41A for 17.5V and "low" voltage current minima is obviously zero, and this starts around 1.59V. There exists a definite hystresis in current levels with respect to direction of change of voltage. Bringing up the voltage from zero the current jumps to 0.20A when the voltage reaches 6.0V, but as one decreases the voltage from above the current settle to around 0.15A at a voltage of 4.9V, which results in inoperation of the laser.
From 20V down to around 7V laser operation is steady for the most part (if one leaves the voltage too high for very long the state of operation becomes unstable, but this was not studied in any further detail here). Near 6.5V the laser goes into a state of alternating between ordered flickering and normal operation. The length between pulses for 6.5V and 0.22A is 10 to 20 ms. Pulsewidths at these levels are from 250 to 450 microseconds. From near 6.5V to 5V the laser is in an ordered flickering state, though at the low end of these voltages the state will decay into alternating between ordered flickering and inoperation after some amount of time.
When putting no limit on the voltage but lowering the current the voltage drops to around 6.1V as one lowers the current through 0.41A and ordered flickering takes place. Down to 0.17A the flickering becomes less ordered as short periods of inoperation intermittantly occur. These are only noticed due to the change in the sound of the laser as they occur. Down to 0.16A the alternating between ordered flicker and inoperation is more definite. The periods of inoperation become much longer until the transient dies down and the laser is left in a state of inoperation. At this point the voltage settles down to 5.0V. Further lowering of the current leads to a minimum setting of 1.65V and 0.02A in a state of inoperability.
It seems that for this setup leaving the voltage at 6.5V and the current at 0.20A produces a state of alternating between ordered flicker and inoperation that is condusive to the study of the structure of the alternating state (i.e.: the transient before settling into inoperability is of a time scale much greater than that of the alternating).
Underpowered Diode Laser
A diode laser was hooked up to a vaiable DC source and analyzed with a grating spectrometer. The recommended voltage for this particular laser was 3.0V. The mode structure displayed at this level changed very little as one lowered the voltage to around 2.7V. After this a definite shift in modes was noticed down to around 2.3V. At this point the intensity had died down enough to no longer be detectable. Raising the voltage from 3.0V to 6.0V seemed to have little or no effect on the mode structure.