Can you excite a laser with a waveform different than square or DC? - Page 1
Forum section: electronics / projects

author: zdelarosa00
author_rank: Regular Contributor
timestamp: December 21, 2018, 02:53:51 pm
content: I was looking at LIDAR sensors datasheets and manuals, and apparently they all use square pulse trains to fire their lasers.According to this: ("Airborne topographic mapping lidars generally use 1064 nm diode pumped YAG lasers, while bathymetric (underwater depth research) systems generally use 532 nm frequency doubled diode pumped YAG lasers because 532 nm penetrates water with much less attenuation than does 1064 nm. Laser settings include the laser repetition rate (which controls the data collection speed). Pulse length is generally an attribute of the laser cavity length, the number of passes required through the gain material (YAG, YLF, etc.), and Q-switch (pulsing) speed. Better target resolution is achieved with shorter pulses, provided the lidar receiver detectors and electronics have sufficient bandwidth."https://en.wikipedia.org/wiki/Lidar#Components).... I understand that you get more distance range from firing a short train of pulses than powering a continuous beam.But the main question is:Do commercial laser diodes permit amplitude/brightness changes?I exactly would like to understand if dimming is possible or the material itself and construction set a minimum threshold for turning it on and of course a maximum voltage amplitude, so the light can't effectively dim of change brightness, and therefore can't output a light wave different thatn square or DC.Maybe they probably can but it's a very small variation.

author: coppercone2
author_rank: Super Contributor
timestamp: December 21, 2018, 03:23:13 pm
content: if it does not use a resonance burst then yes, otherwise you would need to change the system.

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2018, 04:06:15 pm
content: Quote from: zdelarosa00 on December 21, 2018, 02:53:51 pmNormally not, if you drop the current too far, they stop "lasering". There's a reason for the photodiode for optical feedback in the devices.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: December 21, 2018, 06:03:35 pm
content: Apparently this is a fairly common laser diode datasheet. And they don't specify a minimum where voltage and current is.Do the graphs say something else? Or is the characterization of the semiconductor for performance check under not so normal conditions?

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2018, 06:14:25 pm
content: The output power does vary with current, to a point. Some types of lasers can operate over a wider range than others. With the early red laser diodes the gap between where lasing begins and catastrophic optical damage was quite narrow, hence the need for a feedback photodiode and careful current control. Many more modern lasers can operate over a much wider range. For example I have a 445nm blue laser diode that produces a useful beam from under 100mA all the up to over 1A where it produces ~1W of optical power.Diode pumped solid state lasers tend to have a narrower range of operation.

author: zdelarosa00
author_rank: Regular Contributor
timestamp: December 21, 2018, 06:33:16 pm
content: I totally didn't know that.What would the most ideal digitally controlled power driver for the current pump for one like yours?I'm thinking some kind of MOSFET

author: LaserSteve
author_rank: Super Contributor
timestamp: December 21, 2018, 06:37:18 pm
content: Modern laser diodes you reference can be driven in myriad ways and with VERY fast and linear modulation (once above threshold) with a correctly designed current source. They are almost never driven with a voltage source due to the fact that without current limiting they will run away and self destruct. Usually a current limiting resistor is NOT enough to protect them from turn on spikes. You need an active current source and with smaller, sensitive diodes, a soft start power supply for the diode itself.However the ND:YAG lasing material you mention has an upper state storage lifetime of 235 uS, thermal lensing issues that affect beam shape and power during modulation, and massive energy storage capability. So you can do CW, short pulse, long pulse, mode locked, and Q-Switched. Q-switching or short pulsing is like dumping a capacitor into a low inductance load, and pulses on the order of nanoseconds are created. Short pulsing results in a train of low energy self q-switched spikes most times. But generally ND:YAG acts like a leaky integrator with a high turn on threshold, and is only linear over short ranges. It is incredibly cavity design dependent, and there are often intracavity modulators of various sorts. Redesigning a YAG's output usually results in a new cavity design and certainly a change in output coupler transmission. Often we had to change the rod doping, ie the percentage of Nd in the host crystal.Most of the time constants for the physical properties of a ND:YAG crystal are quite long because it is a thermal insulator.Steve

author: james_s
author_rank: Super Contributor
timestamp: December 21, 2018, 06:41:05 pm
content: I have built some modulated drivers from circuits I found online, but these days you can buy cheap driver boards from China that do that. Most are a simple linear circuit using an op amp to drive a mosfet.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 21, 2018, 06:43:42 pm
content: Most small laser diodes are roughly linear above threshold. So a 5 mA laser diode might start lasing on at 1-2 mA and then linearly increase up to 5 mA. The ranges are strongly diode dependent, so some will have a wider range of safe operation than others. Coherence and stability will be pretty poor around threshold -- that is basically making the transition from an LED to a laser. So you can do OK amplitude modulation via current drive over a decent range (maybe a modulation index of 50%), but you can't get nicely shaped pulses that go to zero. Typically you would either do on-off modulation with a square wave or AM with a modulation index less than 1.A big problem is that the threshold and linear slope depend on temperature and unit to unit variation. This is the reason for the photodiode: if you want to get reasonable power out of a cheap laser diode without damaging it, you need to servo the gain to keep the power constant. Otherwise it either won't perform well at high temperature or will destroy itself at low temperature -- especially if the device isn't temperature stabilized. Since the die temperature depends on the drive current, that adds an additional wrinkle to modulation.So a common way to do simple AM modulation is to drive the laser with a bias tee. The DC port is driven by a gain servo from the photodiode to keep the average power correct and a small AC modulation is added through a capacitor.The wavelength also depends on temperature and drive current. There are two effects here: the cavity line shifts with temperature due to index of refraction and cavity length changes. Second, the gain profile changes, causing mode hopping. So if you want single mode operation, you really don't want to be modulating at all -- you want to find a stable operating point and do external modulation.DPSS and especially FD-DPSS solid state lasers are much harder to modulate via pumping due to the extra non-linearity of the solid state gain medium and the second harmonic generation if applicable.

author: Kilrah
author_rank: Unknown
timestamp: December 21, 2018, 07:56:27 pm
content: Quote from: zdelarosa00 on December 21, 2018, 06:03:35 pmIt does! The top left graph shows optical power following current basically dead-on linearly from the threshold current.BUT said threshold current is specced from 17mA typ to 60mA max, and nominal operating current from 75 to 145mA, so as someone said there will be huge sample variation.One could expect that a diode needing a high operating current will also have a high threshold current, but if it's not the case with the specced extreme you could have a diode that could only be modulated between 60 and 75mA, and another between 17 and 145...

author: Benta
author_rank: Super Contributor
timestamp: December 21, 2018, 08:13:45 pm
content: Quote from: ejeffrey on December 21, 2018, 06:43:42 pmejeffrey, very informative and thorough post. Thank You.