13 Lasers | discovery

D47 - Laser Export LCM-DTL-374QT 355nm laser manual C29-May-09.pdf

13. Lasers

INSTRUMENT REPAIRS & MODIFICATIONS (IN ALPHANUMERIC ORDER)

# Action Status Problem Manufacturer Model Function

1. Repair Fixed Low power PhotoSynergy ? 532nm laser 200mW

2. Restore Ongoing Low power/No HG Laser Export LCS-DTL-374QT 355nm laser

3. Restore FIXED PSU blew ? ? HeNe ???nm

1. PhotoSynergy DPSS 532nm laser

Manufacturer: PhotoSynergy, UK
Model: S500

Type: YV04 810nm > 1064nm + 532nm

Mode: CW

Wavelength: 532nm (810nm, 1064nm, 532nm)
Output power: 532nm ≤500mW

Polarity: unknown

REPAIRED 11/05/21 LIBS6 P.17

I bought this because it was only £40. I did not expect it to work and if it did, I expected low power and hoped I could repair it which proved to be the case.

The top of the laser says 500mW max but lasers never reach the power stated on the label and there is no documentation to say what the power level should be.

2. Russian DPSS 355nm laser

Manufacturer: Laser Export, Russia
Model: LCS-DTL-374QT

Type: YV04 810nm > 1064nm + 532nm > 355nm

Mode: Q-switched

Wavelength: 355nm (810nm, 1064nm, 532nm)
Output power: 355nm 30µJ 10ns / 355nm avg 50mW / 810nm, 532nm, 1064nm avg 420mW

Polarity: unknown

INITIAL REPAIR 04/03/24 LIBS7 P.171

I did not expect this to work, assuming it to be worn out or even just plain broken. It wasn't very expensive and I bought it out of curiosity as it is rare to find a 355nm source.

When it arrived there was a gaping hole in the back panel that I'd seen in the auction photos but could not fully make out. Something big was loose inside the box. This turned out to be a Pentium PC PSU that had once been GLUED to the case bottom using what looked like black potting compound.

The entire PSU cable loom was loose out in view, with just a few connections made with Molex connectors whose wire crimps were crudely attached with bare strands of wire visible between the crimp and the wire insulation, possibly torn from the weight of the loose PSU. Unused PSU wires were cropped and hanging around with heatshrink covering the bare ends, some of which was loose. The PSU mains lead was chopped off just inside the case. Immediately it appeared the unit had been hacked.

I traced the PSU wiring and found +3.3V, +5V and +12V leads plugged into a separate Russian pcb using Molex connectors. This pcb contains power supply filters and drivers for two LDs (Laser Diodes) and two TECs (Thermo Electric Coolers) on the laser heatsink assembly.

The PC Power Enable line and multiple supplies from the PSU plug into a front panel which has an Atmel AVR AT90S6535 8-bit µC: https://ww1.microchip.com/downloads/en/DeviceDoc/doc1041.pdf

The laser section is on top of a big heatsink cooled by two 12V fans, but no lead was present to power them. I connected a 12V mains adaptor to the laser heatsink fans and found they worked.

A permanent umbilicus attaches the controller to the laser, including an SMA RF coax cable which is usually indicative of a Q-switch, as was a button on the front panel labelled 'PUMPING ON/OFF'. The other end of the RF cable mates with a SMA connector halfway down the laser body.

On top of the lid is an orange LED that seems to be on regardless of output wavelength. The controller displays an error if the lid is absent, i.e. LED connector not mated, see below for its exact location:

Having paid just £45 for it together with 3 more items of which 2 worked: an analogue signal to 2-channel scope buffer and a transistor driven mains relay box, I really had nothing to lose by seeing if it would do anything. I repaired the bad connections and with the lid off, propped up the PSU, attached an RCD and applied mains. To my surprise it appeared to function with responsive front panel controls.

Depressing 'PUMPING ON/OFF' illuminated its orange LED and '0.30' flashed on and off the LED display when set to mW. I fed the laser into a Laser Precision Rjp-735 energy probe coupled to a RJ-7610 energy radiometer but no laser output was evident. When I fed the output into a Rkp-575 power probe coupled to a Rk-5710 power meter it indicated 10mW.

Intrigued, I measured the wavelengths with my Stellarnet spectrometer, first through a B-cube attenuator and then with just a cosine collector lens on the FO offset to the beam. It detected a significant peak at 808nm, a tiny peak at 1064nm and no 532nm or 355nm. Having a 16-bit detector, the spectrometer has a maximum amplitude of 65k units. With the 808nm peak maximised at 64k, the 1064nm peak was around 30 times smaller at ~2k.

Unhappy with the loose PSU and its mess of wires, I decided to secure it to the case before investigating further. First I removed the black potting compound from the PSU and case. I then opened the PSU & rotated its mains plug so I could rotate the PSU 180° to hide its unused wires inside the case wall cavity. I didn't have a 90° plug so I repaired the chopped mains lead too.

The rear panel is secured to the extruded case sides using 4 countersunk screws which are obscured by the plastic fascia covering the panel. A previous owner had pulled this up at the corners to expose them, clearly to open the box. I drilled access holes for the screws and Superglued the fascia back down.

The case has a thick aluminium plate below the PSU with a square hole cut in it next to a case vent that I assume was for the original PSU. The PC PSU had been glued to this base but the PSU fan outlet didn't fully align with the hole. I now corrected this by fabricating a 90° bracket to secure the PSU over it, tapping a couple of holes in the false aluminium base to secure the bracket.

While I was at it, I replaced the needlessly long yellow safety interlock lead with a wire link.

Excellent teardown of similar Laser Export LCM-DTL-347QT:

https://krazerlasers.com/lasers/347QT/

The above site includes an explanation of operation and indicates the presence of an AOM (Acousto-Optic Modulator) based Q-switch inside the laser. This is driven by the RF output from a small aluminium enclosure to the rear left of the PSU, from which an SMA connector protrudes at the rear of the case.
I traced the wiring to it, opened its lid and looked inside but could see no indication of damage.

Power to the AOM driver comes from the PSU +3.3V line via the driver pcb independent of all other circuitry as it just provides 2 LF 100µF 16V electrolytics and 2 accompanying HF MLC caps underneath.

3.3V seemed a bit low so I looked on the web for an idea of the driving voltage for an AOM but could not find any, probably because companies that make them don't want you to know how to do it.

A wire from the front panel µC leads into the AOM driver with a 5V 10kHz pulse train on it consisting of a 2µs wide logic 0 negative pulse occuring every 100µs, tying up with the kHz measurement display on the front panel, which can be altered with the parameter value MIN/MAX buttons that seem to be associated with the selected display: mW, µJ, kHz. Power (and frequency) is modified by altering the PWM distance between the 2µs pulses. The power settings were by default set to max.

Following this signal through the AOM driver pcb, on the far RHS of the AOM pcb is a metal can that houses a 28.67MHz crystal, and the RF circuitry somehow increases this to produce the 78.8MHz signal which is turned off during the 2µs logic 0 of the 10kHz input. The 78.8MHz signal feeds into the input of the large Mitsubishi M76643 RF AMP IC at the top of the pcb. The output of the RF AMP goes to the SMA connector that leads to the laser AOM cell. There is also a resistor/inductor feedback at this point, which is why when the RF cable is absent the front panel indicates this with an error number.

Below, tracing the signals (the output is driving an external 14W 50R RF load on the SMA connector):

Left, blue probe is 10kHz signal and yellow probe is RF amp pin 1 input (78.8MHz switched by 10kHz).

Right, yellow probe is RF amp pin 1 input and blue probe is RF amp pin 5 output (no signal).

Tracing signals on the M76643 RF AMP IC indicated pin 3: output amp bias was 0V. On the pcb this comes from a SOT-89 to the left of the white label **'б.3.09/14 051', see photo.

**The '6' is actually Russian letter 'be':

https://en.wikipedia.org/wiki/Be_(Cyrillic)#:~:text=Beta's%20numeric%20value.-,Form,corresponds%20in%20the%20Latin%20alphabet.

The SOT-89 is labelled 'F5 32' but no SMD index I have [E42] lists a device with a pinout that matches the pcb layout wiring. The most obvious candidate is a voltage regulator but I could not find one with this SMD label.

Assuming the SOT-89 to be faulty I removed it using low melting point ChipQik solder and hooked up an Amrel PPS1202 programmable PSU in its place. With the +3.3V supply applied as before I slowly increased the voltage on the RF AMP bias pin and a signal appeared on its output that mimiced its input, indicating the RF AMP was functional.

Only now did it occur to me to try the removed SOT-89 on my Peak Atlas DCA Pro component analyser which revealed to be a fully operational bog standard +5V regulator!

The only logical conclusion is the amateurishly added Pentium PSU was of course not the original fitted to the laser controller. I assume the original PSU became faulty at some point and **whoever previously owned it was unable to determine the voltage that fed into the AOM so cautiously wired in the lowest available voltage (**the other instruments originated at the University of St Andrews in Fife, Scotland, so I assume this did too). Why they didn't use the +5V supply when a 10kHz 5V signal was evident, is beyond me. Nobody powers logic with a voltage lower than its input signal as this is often met with that device's demise. Fortunately the RF circuitry is analogue and no damage resulted.

In fact, the M76643 is designed to run off +12.5V and is rated to 7W at 78.88MHz.

The DCA Pro indicated a dropout voltage of 1.54V at its test current of 3.56mA, meaning to obtain a +5V output it would require 5V + 1.54V = +6.6V minimum.

Clearly a +5V regulator would not run off a +5V supply, let alone a 3.3V one. I put the SOT-89 back in and substituted the +3.3V supply into the driver pcb with the Amrel PSU, starting at +7V and increasing to +12V, and at some point a green dot appeared on the output of the laser, indicating the presence of 532nm. With a +12V supply the AOM driver output signal was now ~76Vpp.

I replaced the +3.3V that fed into the driver pcb with +12V instead, salvaging the original connector using new crimps. I also twisted all used PSU power leads with their grounds to reduce noise.

Powering up, the laser again produced a green dot. I hooked up the Stellarnet spectrometer and found the 1064nm peak was now 15x bigger at 44k vs 58k for the 808nm (slightly more at 806nm but the latter the same intensity as before: I had to align the spectrometer FO afresh). The intensity of the 532nm was 682 units, 66 times smaller than the 1064nm. The actual values are lower than these because as with my original measurements, I forgot to align the noise floor to zero. There is still no 355nm but this is generated from an additional NLO (1064nm + 532nm > 355nm), which significantly reduces power.

Given the indicated 355nm power of 30µJ is for a 10ns pulse spaced 100µs apart and already knowing the overall 10mW power I measured was far below the maximum average power of 420mW on the label, I wondered if the 355nm was there but the spectrometer simply could not see it.

I fitted a VIS grating to a Bausch & Lomb high intensity monochromator on the laser output and selected 532nm, then fed the B&L output into the PhotonView 350nm-1550nm image intensifier I had bought for a bargain $75 [see Detectors & Visualisers]. This displayed an intense dot in the centre at 532nm confirming laser alignment. I swapped in a UV grating and set it to 355nm. This time I saw a line with a less intense dot on it, confirming 355nm presence.

I bought the laser in 2023 and tested it in 2024. The only information I could find on it was the teardown site (the owner does not respond to enquiries and his links to information on it don't work). Later I found manuals for the LCS version (mine) and LCM version described in the teardown and Sam's Laser FAQ: https://www.repairfaq.org/sam/laserscl.htm#sclle1

A major difference is the LCM (M = Module?) PSU runs off 24V whereas the LCS (S = Standalone?) PSU runs off mains. The LCM also appears to have more intelligence such as an RS-232 link, and its laser LED provides BITE information, e.g. supply voltage status. Internally my LCS runs off 12V. I suspect internally the LCM also runs off a lower supply. However this should not affect operation since the source is a couple of laser diodes that likely run off no more than 2 Volts each.

A week after I got it going I powered it up again and found it no longer produced 1064nm or 532nm and Indicated power is now always zero. Initially the controller indicated it was READY but would not confirm PUMPING. The next day it decided it would PUMP after all. I confirmed the presence of the AOM drive signal but I could not determine any obvious reason for cessation of the harmonics. Maybe the 76Vpp signal fried the AOM? (I doubt it as pockels are similar and they take kV)

CW power is now zero; 1064nm and 532nm now absent. Below right, the 10kHz pulsed 78.8MHz AOM drive
signal is still present (MSO8k top blue Ch2 is SYNC signal from controller front panel, and purple Ch3 is 1MΩ input picking the signal off a T adaptor screwed into the RF SMA connector at the laser).

Three of the pots at the end of the laser are annotated by green marker pen symbols I, II and T.

Powering it up again with PUMPING indicated, I confirmed pot I sets the rear LD drive voltage and II sets the side LD drive voltage. I don't know the function of the T pot to its left (set TEC Temperature?), or the unmarked pot to the far left (set LD current?). I need to see what changes.

I measured rear LD I = 1.727Vf and side LD II = 1.682Vf (HP3478A DMM).

I also found about 1/8th of a turn CW increases the voltage by ~10mV.

I believe the LDs are worn out and need replacing (Sam hints they may be 5W emitters). Since I had nothing to lose given the useless power output levels even when it was working, I detached the side LD as I was aching to see what it looked like, see below. The LDs appear to be custom bars with 6 gold contact wires (only 4 are visible in the photos) and there is no chance of getting exact replacements.

Below: LDs are on PCBs LD pots & measured Vf Side LD removed LD bar has 6 gold wires

Below: LD focused by lens RAP cube combines LDs NPN transistors below the pots may drive the LDs?

Below, at the emission end, part of the beam is reflected onto a large square photodetector on a pcb.

For now I'm stumped by the missing HG, but not too bothered as I've had a lot of fun with it. In 2024 you can get Chinese TO-5 1W LDs for $15. I may buy a couple and see if I can get it to work again; hopefully the unknown pots control LD current.

Below: photos from Sam's website. The similar looking driver pcb suggests the manual's warning that only the original PSU be used is actually saying 'make sure this filter pcb is present', as the original PSU has to be a switcher to be small enough to fit under it (because that's what it looks like to me), so there is likely nothing wrong with using a P4 PC PSU in its place (overkill, in fact).