Update

May 14, 2017

Now I am done for the time being with analyzing the ultra low noise Zener diode 2DW23x, including building low noise measurement amplifiers. Some summaries can be found here and here. What remains are long-term burn-in and stability tests which run 24/7 for about half a year.

The motivation was to develop a digitally controlled ultra-low noise laser diode drivers for holography applications. This is a long-term project too. In the meanwhile, some first development prototypes were made, and eben without too much effort they sport already very low noise. The only noteworthy thing is that there was at first a considerable feed-through via the DAC from the I2C digital bus. This was completely eliminated by using an opto-coupler – not just for galvanically isolating the bus but rather for interrupting any communication to the DAC except for a brief moment when the output current needs to be changed.

Here a picture of the current test setup, soon it is time to draft a first PCB version:
LD proto

In the middle there is an Arduino Nano, plus DAC/ADCs for digita/analog interfacing via I2C bus. To the left a low power (1.5A) PWM TEC controller for testing. On the right I used a left-over PCB from my old analog TEC/laser diode driver, for the actual analog current output driver including precision, bulk metal foil current sense resistors.

Arduino-shield high precision TEC controller for laser diode holography

December 10, 2016

Finally, after more than 1/2 year of work this project is completed; well, almost, I still need to tweak software, but I won’t have time for the rest of the year. Here is a quite exhaustive writeup, so nothing needs to be added for now in this blog.

The plan is to develop next another Arduino shield as a companion, very stable laser diode driver. This will be hard since noise and stability requirements will be harder, and not easy to reach given the noisy digital environment. As preparation I have set up an extremely sensitive low noise amplifier for measurements (see previous post).

Moreover, a miniature combo board of TEC and laser diode controller is planned, but only after having gained more experience with the present controllers.

I may sell some of the controllers at a later time once the complete package incl diode driver is done and turnkey useable.

pcb2

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.

More diode tests

October 10, 2011

After a long break I found time to check a few diodes for single longitudinal mode operation.

1) Alleged new, cheap 635/638nm diodes on ebay. These were actually 658nm diodes and also not new. I returned them and so far am still waiting for a refund…..

2) Mitsubishi ML520G71 300mW/635nm laser diode. Unsuitable both in free-running and ECDL mode.

3) HL63133DG 170mW/638nm laser diode. Excellent in ECDL mode, so far the best diode! Clean stable output beyond 100mW can be achieved. See the pic at 635nm and around 100mW:

4) Update: The diode currently sold as Opnext HL45023TG on ebay turned out to be the PL450 from Osram. It has quite similar data, so most wouldn’t complain, but for purposes of single longitudinal mode operation there may be a major difference, potentially. Let’s hope for it, because the diode seems pretty useless for holography. I found it completely unsuitable in free running mode, and (after a lot of tinkering) quite OK in ECDL mode. Initial attempts produced less than than 20mW in single mode operation, but with some optimizations more than 40mW was achieved. And the beam quality is good, especially compared to the common 445nm/1W diodes which do not provide much more power in stable single mode operation.

Here a brief movie (2MB) showing single mode operation from the CCD spectrum analyzer at around 40mW (popping sounds indicate mode jumps):

Further Update: Now after all worked well I built everything “ready to go” into a nice case (see below), and for unknown reasons I can’t get any more stable SLM operation to more than 25-30mW. The only thing that was changed was the collimator, and it seems that the adjustment is extremely critical for stability. Indeed it turns out that the stability depends crucially on how far the focus point, or beam waist, is from the laser; changing it between 1m, 2m, and 5m makes a huge difference, and this amounts to very minor rotations of the collimator.

In order to more systematically investigate the effects of feedback, I made it variable by using a waveplate and a polarizing beam splitter cube; preliminary results show that indeed the feedback must be in a relatively narrow window, but this alone does not guarantee SLM operation…. more work is necessary! Here a pic of the first test setup:

From left to right: grating, beamsplitter, waveplate in rotation mount, diode mount.

Revitalizing the Coherent Sapphire 488-20

September 11, 2011

There was a moderately good ebay deal to be had and so I decided to bring a Coherent Sapphire 488-20 laser head plus driver board back from my trip to the US – along with 20kg of other laser goodies ;-). This is an interesting, though not too powerful optically pumped semiconductor laser (OPSL). The board looked new but the head was used and sold as “probably not up to specs”. Indeed so, I measured 9.6mW instead of 20mW, but I figured out, by some reverse engineering, how to bring it up back to specs, ie, 20mW with clean stable single longitudinal-mode operation. Adding a housing for the driver board and a few analog and digital controls, et voila… system ready for holography! For details see here.