Help identify this puzzling electro-optical device? - Page 1
Forum section: electronics / general

author: nmccoy
author_rank: Newbie
timestamp: September 30, 2018, 02:37:17 am
content: I started dissecting a flash-pumped crystal laser recently. I can identify pretty much everything on the inside (mirrors, active shutter, fiber-coupling optics, laser cavity with flash tube, etc), but one part is really puzzling me.The laser cavity (shiny box at the center of the assembly) is mounted to this peculiar block, with many pictures below. I carefully took it apart hoping to find a model number hiding somewhere, but no dice.What I know:* Made of a hard insulating plastic, possibly delrin.* A coupleelectrical components potted on the inside.* Box has asingle SMA connection.It was routed to to the outside of the laser assembly with an SMA extension, so it's probably relevant.* SMA connection has aDC resistance of ~0.2 ohms. Below the reasonable bounds of my multimeter.*Resonant frequency of ~250 kHzwhen stimulated with a function gen and measured with a scope. At this frequency, input attenuation is ~0 dB.* Strangemetal nub is electrically relevant, has a voltage ~50x the input voltageat the resonant frequency.*Metal nub electrically contacts the aluminum laser cavity.The nub has some give to it, sticking out of the plastic when the cavity is removed, but recesses back down as force is applied. This doesn't affect the resonant frequency (but electrically contacting it with my finger does).* There'snothing interesting on the inside of the cavitynear the nub. It's just hollow and has the flash tube.Quite an information dump, but I'm quite stuck here (and so are other people I've asked about this). I want to know the purpose of it, and what it's role is in this laser.Scope shot shows a frequency sweep from 100 kHz to 20 Mhz (log scale over time, 14 seconds), amplitude 0.5 vpp. Yellow channel is the input at the SMA. Pink channel is me holding a probe on the metal nub. Note the 50x difference in channel scalings. Resonant peak is ~250 kHz.Any semi-relevant information or suggestions of further tests would be appreciated

author: amyk
author_rank: Super Contributor
timestamp: September 30, 2018, 03:16:01 am
content: My guess:https://en.wikipedia.org/wiki/Mode-locking#Active_mode-locking

author: nmccoy
author_rank: Newbie
timestamp: September 30, 2018, 07:07:12 am
content: Quote from: amyk on September 30, 2018, 03:16:01 amThat was one of my initial guesses, too. I also considered some sort of q-switching mechanism that could vary the gain peizoelectrically or otherwise. However, it's nowhere near the beam path, so I can't figure out how that would work.I decided to rip the darn potting out. It turns out....it's just a tiny transformer that connects to the ball with a crimp.This gives me more questions than answers.Is this a sensor?* What would it even sense? Parasitic coupling from the flash tube?* The transformer would step down the voltage by ~50x in this case, which doesn't sound super useful unless there's some strange impedance matching thing going on....or is this a control?* What would controlling the voltage here even do? Could a fast-changing high voltage help strike the xenon lamp?

author: DTJ
author_rank: Super Contributor
timestamp: September 30, 2018, 07:17:19 am
content: Step up xfmr to trigger something?

author: edpalmer42
author_rank: Super Contributor
timestamp: September 30, 2018, 07:39:17 am
content: So the metal nub is just a spring-loaded, captive ball bearing that pushes against the laser cavity? Is the cavity grounded or floating? If floating, it reminds me of the trigger mechanism on a xenon flash tube. No electrical contact with the gas, but use a pulse transformer to send a high-voltage pulse to an electrode that's wrapped around the tube and it triggers the flash.Ed

author: nmccoy
author_rank: Newbie
timestamp: September 30, 2018, 08:04:17 am
content: Quote from: edpalmer42 on September 30, 2018, 07:39:17 amYep, conductive captive ball pushes against an otherwise floating laser cavity. I just started going down that thought path, and it looks like Fig 4 straight out of here:http://www.excelitas.com/downloads/dts_triggertransformers_final%20071004.pdf. Inductance and winding ratio are approximately what I've measured as well.However, I've already taken out the lamp and there's no trigger electrode. The inner reflector surrounding it is also a non-conductive ceramic of some sort.On second thought, the whole cavity is filled in a liquid cooling loop. If the fluid was slightly conductive, that may be enough to work as a trigger. Weird, but seems the most likely at this point.

author: edpalmer42
author_rank: Super Contributor
timestamp: September 30, 2018, 05:25:42 pm
content: Quote from: nmccoy on September 30, 2018, 08:04:17 amThe entire (metal) laser cavity is the trigger electrode. The laser has some voltage across the electrodes, but not enough to break down the gas. The trigger pulse is capacitively coupled to the gas via the metal cavity. Electrically, the trigger pulse is connected to the same ground as the electrodes in the tube. The capacitive coupling completes the circuit.Ed