Archive for the ‘Uncategorized’ Category

Some intermezzo in repairs and metrology

November 23, 2016

While I was slowly progressing with my Arduino based precision peltier controller (update soon), I stumbled over other things that took some time to sort out.

First, my valuable ultra precision 8-1/2 digit Datron 1281 DMM stopped working. I spent some time checking voltages, ripples, ROM images, but this seemed to be a more subtle problem and threatended to tie me up indefinitely. So I sold in on ebay, and got in return as much such as to afford a brand new Rigol3068. This has only 6-1/2 digits but this is enough for my various applications (temperature and laser current control).

I am very happy with the Rigol3068, it sports also an Ethernet connection which helps me to go beyond GPIB control which gives problems all the time. An advantage is also that it has 240000 counts, which means that I can measure 10K Thermistors (at around 13KOhms or so) with full 6 1/2 resolution in the 20K range, rather than having to use a more typical 100K range which cuts down by a factor of 10 in resolution. So I can safely resolve temperature differences of 1/10000 degrees.

When tinkering around I remembered I had also a faulty Solartron7081 8-1/2 digit voltmeter around and looked again into it. It had intermittent thermal problems and I had repaired a few of them by replacing ILQ74 optocouplers a few years ago. Still, some mysterious 80uV jumps remained without explication, plus an elevated noise level of about an order of magnitude larger than expected. Such things are notoriously difficult to get a handle on.

While at it, I stumbled over the eevblog which adressed these questions in this thread (me calling myself bertik for silly reasons). So I managed to relate these jumps to a faulty auto zero mechanism, which I circumvent by disabling it.

While investigating there was a serious setback: when checking the current setting for the reference Zener, I inadvertently shorted the test pin 301 with the – input of IC 304 (refers to service manual). There was a sense of smoke, which was surprising, since this test pin is supposedly at ground. I checked and alas, there was 17V AC voltage at it! With source impedance of like 2 Ohm… how could this possibly be? Was it the effect of the short? How can 17V AC go straight into the reference voltage section?

After disassembly it turned out that the test pin was wrongly placed by the manufacturer, namely on a via. And indeed there is 17V AC voltage there, fed across half of the board to a few mm near the reference section (!). It enters IC303 which is close by. So that explained it. As a consequence, IC 301 and IC 303 where blown and possibly more.

I decided to by-pass this problem by substituting an (aged!)
+10V/-10V precision reference
VRE102CA of which I had salvaged a few from trash. It can be made fit perfectly into the reference section of the 7081, by attaching the outputs to the reference terminals TP 302/TP 303 and opening the base and collector connections of TR 301 and TR 302. This works as good as before, the reference has like 1ppm/C temperature stability and like 3uV pp noise 0.1-10Hz.

For the time being I’ll leave the reference section like that, since the excessive noise problem is the next pressing one. I will report on any progress in the future.

Solartronrepair

How do I know the noise level? This has to do with another thread on eevblog, namely this thread. There a DIY low frequency noise meter was described, and this is what I wanted anyway, for various reasons. First, for checking out various voltage references that I am going to use for laser drivers. Then, primarily, to measure low intensity noise of the laser driver outputs. I like to get below like 1uA pp noise at about 200mA. So this calls for proper measurement hardware.

Given the valuable tips of very competent people in ths thread, I quickly built my own version. It works fine with a base noise level of 200nV pp at 0.1-10 Hz. I use only half of the dual opamp since I shot the other half ;-(, otherwise it would be in the order of 140nV pp. With this in hand, I characterized differend voltage sources, which is important also to judge the disease of the Solartron7081. Here a few results — more to follow:

VRE102CA (10V): 2.7 – 5uVpp, depending on specimen
VRE102CA (10V), two parallel: 1.9uVpp, 0.33uVrms
MAX6250 (5V): 1.6uVpp 0.32uVrms
Data Precision 8200 Calibrator (10V): 12 uVpp, 1.9 uVrms

Base noise, 200nV pp:

BaseNoise

(100mV corresponds to 1uV).

Dual VRE102CA noise, 1.9uV pp:

VRE102CAdual

The tips in the DIY low frequency noise meter thread were extremely valuable, in particular the issue of leakage current of the input capacitor. I never thought that this could be handled with hobbyist means. But indeed checking out a few caps I had around, there were some with <1 nA leakage current, so this was fine! Still this is a slightly tricky business.

Here are some extra comments:

1) Shielding: an aluminum case with slots on the sides didn't work at all due to overwhelming 50Hz noise. One really needs a kind of watertight shielding, I used a classic Teko case intended for RF applications. Actually two nested ones, the outer one holds battery and supply voltage splitter. I also use switcheable bandwidth control because sometimes I may be interested to go beyond 10Hz.

Low Noise AC Amp

2) Ground loops. When measuring references in existing other setups, one is more or less guaranteed to get ground loops, at least if the scope input is grounded. This shows itself in an elevated noise which does not necessarily looks suspicious. I noticed it when connecting the input to a function generator to track the frequency response (via a 1M/10 Ohm divider).

The solution was to put between amp and scope an extra isolation amp. I use an AD210AN which I had at hand, and I also added an extra gain factor of 10, so the total gain is 10^5. This means 1uV AC input will come to 100mV output.

isolation amp

Now the story continues to go on, in a related direction, since in this thread it was discussed that ultra-cheap Chinese Zener diodes may have noise levels which are lower than the best industrial references. This created a group buy frenzy which reminded me of the one of a few years back about the blue 1W laser diodes – a gold rush type excitement where people buy dozens if not hundreds. Obviously I couldn’t resist to order a dozen or so as well, for potential application to ultra low noise laser drivers.

When these little gadgets arrive, I will conduct tests which are quite similar to what I was doing for laser diodes since years: automated high-precision scans over temperature and current. Let’s see what will come out, stay tuned.

Ultra stable TEC controller as Arduino Shield

May 31, 2016

I didn’t have time for my hobby for a long time, but recently I got around restarting some lingering laser related projects. One thing I felt was needed is an update of my stable laser diode and TEC drivers for holography applications. See here for background. In short, one needs to avoid mode hops by having an exceedingly tight diode current and temperature control. We are talking about stability of the order of 1/1000 degree for extended periods of time. This is in particular critical for ECDL lasers.

This new project was partly motivated by requiring bi-directional drive (so cooling AND heating), so one does not need to run the laser below room temperature. This necessitates a H-bridge PWM drive mode, and this is already close to full digital control. The other motivation was the design of solarfire and dnar and others described in this forum. However that design is not stable enough and also I am not familiar with the microprocessor. So I decided to come up with a stability-improved design, based on the Arduino. It is dirt cheap (whole ready-to-use boards for less than 3 Euro incl shipping..) and there exists lots of useful software for it.

However in order to meet design goals, one has to make some efforts in the hardware and software design. Hardware means precision opamps, voltage references, and <=5ppm/C stable resistors. Fortunately I have a large stockpile of those.
Software means to bring inputs and PWM outputs to like 16bit resolution, which is far more than the usual 10bit ADC input and 8bit PWM output resolutions. Strictly speaking 16bit PWM output is easily possible but then the PWM frequency is a few kHz and this requires an enormous amount of filtering increasing size and also noise pollution. In fact I managed to cook up a software solution for the PWM driver that gives 16bit resolution while still running at 62.5khz! And input wise with suitable oversampling and "dithering” the resolution can increased to 16bit as well.
This avoids expensive 16bit ADC’s.

So all in all, here are the main features of the design that are working as of now:

-full H-bridge for bi-directional TEC drive
-up to +/-3A current
-temperature stability < 1/1000 degree for hours
-noise level about 2/10.000 C
-control via USB serial terminal, optionally LabVIEW
-LCD display
-automatic PID control loop configuration per pushbutton
-saving basic operating data in EEPROM
-ready interface for companion diode driver

So far only a development prototype exists, but I am about finishing
PCBs which could be used as add-on shields for the Arduino Uno.

My plan later is to make an update for the LD driver too, giving another Arduino shield which can be stacked on top. This setup is supposed to be suitable for desktop laser diode controllers.

Finally a miniature version is planned with less power (1.5A), where both TEC and laser drivers are on one board and a stacked-on Arduino Nano is used for control. This setup is supposed to go into compact laser heads.

Here a picture of the LabVIEW interface showing stability of a few 1/10.000 degrees.

Website being moved

April 27, 2015

I was changing provider and so my web site of many years will have a new address:

http://hololaser.kwaoo.me

Some links might not yet work but I will slowly correct it.

Some more diode tests

January 5, 2014

Finally I completed over the holidays a bunch of further diode tests with regard to single mode operation, mostly in ECDL setups. I had collected them over the years but didn’t have enough time to actually do it. Hopefully I’ll be more active again in the future.

In brief, the results are as follows:

  • Mitsubishi ML520G71 300mW/635nm, Opnext HL6388MG 250mW/640nm, Mitsubishi LPC-836 300mW/655nm diodes:
    unsuitable
  • Opnext HL63603TG 120mW/638nm/3.8mm laser diode:
    very good ECDL to 60-70mW
  • Mitsubishi ML520G54 110mW/638nm laser diode:
    very good ECDL to 60-70mW
  • Osram PL520 50mW/520nm laser diode:
    good ECDL to 40mW
  • Further details can be found here. As a spoiler, here a brief movie (2.7MB) of a test of the green Osram, which shows stable single mode operation at around 40mW:
    .

    Happy New Year!

It’s been some while….

June 13, 2013

since my last postings. Well one of the reasons was my professional work, and another was that my workhorse for measurements, my LeCroy 9314M scope, was broken. And my backup scope, a LeCroy 9450, was broken already from the day I rescued it from the scrapyard: the screen went dark after warmup.

So what to do – shelling out close to 1K for a decent 4-channel scope or trying a repair? This question kept me blocked for many months, till I kicked myself and started to see whether I can do something. Two of the 4 channels of the 9314M went bad already a few years before, and now the trigger stopped working, but sometimes it came back, apparently depending on the temperature. This is the worst kind of errors to have! I was afraid that one of the numerous custom ICs from LeCroy went bad, or that there would be some error in the logic board. Nevertheless I gave it a try, partly because the schematics are available in the service manual.

In fact, as silly as it may sound, looking for errors in a fantastically complicated circuitry has something to it. It is a bit like reading a criminal thriller…who is the culprit? How can we use logical reasoning combined with intuition and experience to corner it? And then, can we do something against it? Actually in the past I had successfully repaired quite a number of measuring equipment, from Tex to HP, and given up only on one thing, an intractable Schlumberger Stabilock 4040.

So I undertook this journey for a couple of evenings. First thing was to take everything apart and create extension cables for the front panel, otherwise one never would be able to come close to the live main board. And then finding the way through the SMD circuit with another scope, voltmeters, and most importantly, freeze spray and heat gun. Unfortunately the latter produced most of the time misleading and contradicting results, partly because there were two independent thermal errors. Finally the culprits were found, both very close to hot IC’s which again confirms the expectation that most likely a component fails due to thermal stress. The one responsible for dropping 2 of 4 channels was IC(*) A4/74HCT138 and the one for loosing trigger was diode(*) CR402/SM4004 which ran extremely hot, so that it had partly unsoldered itself, creating in intermittent contact (almost the worst thing to find…). Anyway, to make a long story short, after exchanging these parts the scope worked like new!

And since I was in the mood, I also took on the other scope, LC 9450. There again a thermal error, this time it was the mosfet(*) Q76/IRF830 responsible for emergency blanking the screen. It turned out to leak current at higher temperatures. After exchange all was well!

So now were things back to normal and I restarted activities by investigating a few new diodes for ECDL operation; among them the green 520nm Osram PL520. Stay tuned for results!

(*) refers to schematics in maintenance manuals

Holidays….

February 12, 2012

Holidays are a good time for upgrades, etc, and as always that involves much more work than anticipated. In short, upgrading to the Lion version of Mac OSX for all my computers had some severe fallout. Namely, LabVIEW 8.2 is not supposed to work under Lion, and after upgrading to LabVIEW 11 it turned out that the CIN (code interface node) construct is not any more supported… but that is needed as interface to the IOWarrior chip I am using for controlling my electronic measurement setup (via I2C bus and USB).

So, in effect I decided to scrap the IOWarrior hardware interface alltogether and switch to the Arduino Uno, which is very common, cheap, easy to program and has a lot of support, and in particular, LabVIEW and I2C support. Rewriting the hardware drivers for all my ADC/DAC chips kept me busy for a couple of days, thereby completely overthrowing other projects that I intended to do during the Christmas break… here a pic of my universal laser diode/TEC controller now working with the Arduino and LabVIEW11:

Of course, after all was done, it turned out that not only does LabVIEW 8.2 still work under Lion, but also that CINs still work under LabVEW 11.. well so it goes, but at least I have now a streamlined setup with some hope for future compatibility and extensibility.

Since I now got to appreciate the Arduino, I toy with the idea of building a new, completely Arduino-controlled laser diode controller and analyzer, with all sorts of fancy LCD displays etc… seems great fun ahead 😉

As for other news on the electronics/instrumentation front, the friendly greek friends, whom I sold a few lasers, donated me a nice spectrometer (many thanks to them!) These small, fiber coupled and grating-based spectrometers are sold by Science-Surplus, but they come unaligned. Their site has notes for doing the alignment which are pretty clear to follow, but it still took me a day to get the spectrometer to work. I also extended their LabVIEW interface and it now looks like this:

For some more comments see here.

ECDL update, etc.

March 21, 2010

After I was done with my free-running diode lasers, I was focusing on building a robust ECDL as a prototype and reference model. Some preliminary info is here. Concurrently I was playing with the remodelled commercial ECDL described here. Here a pic of the latest prototype:

ECDL proto

Both lasers work reasonably well, but the main problem ist that at higher powers, the zones of single mode operation of typical DVD diodes becomes very small and thus the lasers can drift out over time. To combat/control this problem, I had developed a noise detector which works very well for free-running, non-ECDL lasers. As for ECDL lasers, I found that they sometimes run multi-mode without the presence of extra noise in the light output. This would be of course an important problem for day-to-day use, and I am spending most of the time with trying to get arund it. It becomes worse with increasing power, and problematic beyond 70-80mW for common diodes.

It seems that one needs to very carefully select a diode, adjust the operation point and the optical properties of the resonator. Particularly well behaved are the diodes that also work well free running, like the ML101J27 and the HL6385DG. It seems that one can typically achieve up to approx 80 and 100mW, resp., in relatively stable single mode operation. But I need to spend more time investigating; there is a lot of variation between individual diodes and the number of parameters in the game is so large that I don’t yet have a clear picture. Moreover I am running out of the Opnext HL6385DG diodes I got moderately priced via a group buy.

Other things I was busy with were finding better collimators; so far I mostly used the familiar Lens-27 from Roithner, which has an extremely high efficiency but a relatively poor beam quality; some holographers had complained about this. I found that the G-650-1 (obtained from here) has a much better profile while having only slightly less power. Some of my findings are here.

Opnext HL6385DG….

June 24, 2009

I just got a few of those 150mW/642nm diodes, which are characterized by “single longitudinal mode” in their data sheet. There was a special price for them during this months; and also a group buy at photonexicon.com.
Obviously of potential interest to holographers…the warm color would be well suited for color holography, and it would work well with optics intended for HeNe lasers such as waveplates.

I immediately ran tests on one of them, with disappointing results at first, but subsequent more careful tests revealed single mode regions at high power and low temperatures, see here. This now seems very promising and I will test more of those diodes, also in ECDL setups – stay tuned.

PS 09.09.09: I found, not unexpectedly, that the HL6385 does fine in an ECDL setup, for details see here.

Open can and blu-ray DVD diodes…

April 4, 2009

I have acquired some more laser diodes for testing their suitability for holography.

The first thing to try out was to measure the mode spectrum of one of those high power “open can” DVD burner diodes, which can do several hundered milliwatts when tortured. Well, the news is –not unexpected– that those diodes are totally unsuitable for holography. see here. Moreover I found the the familiar Sony SLD1236VL and SLD1239JL-54 diodes a little bit better, but not too much.

Summarizing, for all DVD diodes I tested the rule seems to be that they can run single mode at up to 70-90mA, which yields approx 20-30mW. There may be exceptional stable spots higher up, eg. I got approx 50mW with one Rohm diode, for example. The best diode so far was the Mitsubishi ML101J27 which got up to approx 80mW (with some weak other modes; true single mode up to approx 50 mW). Thorlabs sells it as single mode diode, probably for good reason, however there is no mention of this in the data sheet.

There is a number of declared single longitudinal mode diodes, esp from Opnext/Hitachi, some with powers exceeding 100mW. Most likely they would be the way to go, I will try to get a handle on them and do some measurements; unfortunately the more powerful ones are quite pricey. But it seems there is no easy way around that if one aims for more than 30-50mW.

Note added: I now checked a GH04P21A2GE/PHR-803T “blu-ray” diode and the results are here. In a nutshell: totally unsuitable for holopgrapy!

Welcome

January 23, 2009

This is a test to see whether it works… everybody has a blog now and why not me. I get lots of technical questions by email, and it might be easier to have discussions in a blog rather then by mail. So please feel free to drop by, english and german spoken here.

So what happened here during the last two years? Almost nothing, I didn’t have much time for my hobby but plan to be more active again. There is a major incomplete project, namely of stabilized 650nm diode laser modules suitable for holography. I had made a prototype which works well (stable single longitudinal mode at approx 50mW), and next I would build some more of them; but this requires some PCB and mechanical work for which I don’t have time right now.

Moreover I’d be eager to do some holography again, but this requires setting up my optical table which is completely blocked right now by the spectrum measurement apparatus I need for the diode lasers… to bad that I don’t have space for more than one concurrent project.

On the long run, I will try a single mode high power DPSS laser; I have already collected lots of parts like Yag rods and disks, KTP’s, diodes from 2 to 60W, 100A power supplies. All sorts of cooling, temperature stabilization, pump optics design, thermal lensing, green noise, etc, issues will come up and this will be a huge effort as far as I can see. But as a fellow holographer has said…. adversary builds character 😉