Mini Laser Mode Analyzer 1 (mLMA1) Version 1

Description and Testing

Version 1.02 (30-Dec-2022)

Copyright © 1994-2022
Sam Goldwasser
--- All Rights Reserved ---

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
  1. This notice is included in its entirety at the beginning.
  2. There is no charge except to cover the costs of copying.


Table of Contents


Preface

Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Copyright © 1994-2016
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.

DISCLAIMER

mLMA1 is not currently intended to replace a stand-along SFPI controller with oscilloscope display. That may come in the future.

Acknowledgement

Thanks to Jan Beck for getting me interested in microcomputer development. If anyone had told me six months ago that I'd be writing code for an Arduino-compatible board - and enjoying it (sort of) - I would have suggested they were certifiably nuts. ;-) Links to his Web information may be found under References.


Introduction

This is the original prototype mLMA1 design and it may be all that's needed. But for the greatly enhanced version, see Mini Laser Mode Analyzer 1 (mLMA1) Version 2.

Mini Laser Mode Analyzer 1 (mLMA1) was originally intended as a proof of concept for implementing a Scanning Fabry Perot Interferometer (SFPI, also known as a laser spectrum analyzer) using any SFPI head that uses low voltage PZT drive. But mLMA1 turned out so well that it could have practical value, especially where a portable low cost instrument is desired for basic checking of the mode behavior of lasers intended for applications like holography. ;-) And several other functions were added.

For use as an SFPI, mLMA1 replaces the normal ramp driver and oscilloscope with a compact Arduino-based system with OLED display. A home-built SFPI head could be constructed that is the size of a small pill bottle, so the entire setup could be fit in a pocket - really. :) Its entire parts cost - excluding that of the low voltage SFPI head :) - is under $10.

 

Photo of mLMA1 Prototype running SFPI (left) and Home-Built Hemispherical Confocal SFPI Head (right)

But is soon became clear that other functions could be added using the same platform. The present functions are:

The Longitudinal mode plotter provides a moving graph like a chart recorder with a full screen (end-to-end) update of about 1 minute.

Photo of mLMA1 Mode Sweep Display of one Polarization Orientation of Melles Griot 05-LHR-911 HeNe Laser

The Laser power meter (POWER) displays the level at an update rate of around 4 samples (which can be averaged) per second.

Photo of mLMA1 Mode Sweep Display of one Polarization Orientation of Melles Griot 05-LHR-911 HeNe Laser

The Hold function locks the display and flashes a "*" in the upper right corner of the screen to indicate that the function isn't updating.

The system described below implements all of these using the Atmega 328 Nano 3.0 PCB and a hand-full of discrete parts but has limited flexibility in terms of scan rate, offset, and magnification. As in none except for trim-pots for input sensitivity. :) The default settings for the SFPI would be acceptable for a laser with an output power of 1 mW or more to display its longitudinal modes in real time. Some minimal controls may be added, though nothing fancy. But a version using the previously implemented ramp driver and photodiode preamp PCB described in the section:

  • Sam's Scanning Fabry-Perot Interferometer Driver 1 (SG-SF1) would have more flexibility. It generates a ramp of up to 60 V p-p along with a sync signal for the Arduino, as well as including a variable gain photodiode preamp.

    While this won't replace a $5,000 instrument, it is more than adequate as-is for determining the basic health of a laser such as whether it is Single Longitudinal Mode (SLM). The display above shows the actual scan of the lognitudinal modes of a 5 mW red HeNe laser. The benefits of mLMA1-V1 include its extreme simplicity: it could be put together in an hour to go with any SFPI head with a low voltage PZT. And with only limited creaping featurism :), it is quite fast. There are no plans to make mLMA1-V1 available as a complete kit with PCB, but if there is any interest, a kit including the Atmega, OLED, all discrete components, and solderless breadboard may be available for around $25.

    mLMA1 consists of 3 parts:

    1. SFPI head using low voltage PZT: Anything that will scan 2 to 3 FSRs with less than 60 V p-p should quality. The one to be used for testing is home-built with a hemispherical-confocal cavity with an effective FSR of approximately 1.75 GHz. For these tests, only a single polarization mode will be supported, but the extension to full dual polarization support is straightforward.

    2. Atmega 328 Nano 3.0 with 0.96" OLED 128x64 pixel display: Firmware written in C generates the ramp driver signal and implements a rudimentery DSO.

    3. Source of 5 VDC power and optional boost converter: The Nano runs on 5 VDC. This can come from a USB port on PC, laptop, or USB backup battery pack, or a 5 VDC power or regulated DC wall adapter. The boost converter, if used, runs from the 5 VDC and provides 30 to 50 VDC for PZT ramp driver transistor.

    The Arduino-compatible Atmega 328 Nano 3.0 for mLMA1 provides the functions of both the ramp driver with an external high voltage transistor powered by a DC-DC boost converter if needed, and Digital Storage Oscilloscope (DSO) using a small OLED graphics display. It should work with home-built SFPI heads using PZT beeper elements as well as commercial ones like the SA-200 and others from Thorlabs. For these, a 5 V ramp may be sufficient eliminating the need for the boost converter and its components.

    The current implementation runs at about 15 scans per second using the 64x128 pixel OLED. Since the display works like a DSO, this is way more than adequate since there is no flicker. Thus, even when scaled up to a large display in the future, the refresh rate should be acceptable. And of course, the $2 Nano is not exactly a stellar performer, so a higher performance microprocessor could be substituted if needed.

    A complete pocket-size SFPI based on mLMA1 could be built into a 1x2x4 inch case. :-)

    This document provides a general descriptions of the the mLMA1 hardware and firmware.


    Specifications

    Wiring Diagram and Parts List

    The diagram below - a sort of a hybrid between an electronic schematic and wiring diagram - shows the parts required for an Atmega328 Nano 3.0 system incorporating all the bells and whistles. Note that bypass capacitors (0.1 uF ceramic in parallel with 22 µF electrolytic) are recommended across the +3.3, +5, and REF pins (not shown). These will both cut down on noise in the signals and prevent erratic behavior like random resetting of the Atmega.

    Most of the half dozen connections on the breadboard are made with the electronic parts themselves or bits of excess wire cut from their leads. But there will be a need for a few insulated jumpers which should use #22-#24 solid hookup wire stripped to fit in the holes.

    The pinout for the OLED may differ depending on where it was purchased. Especially important are VCC and GND, which may be swapped. Connecting it with reverse polarity even for an instant will likely damage or destroy the device. And while the schematic shows VCC for the OLED being 5 V, it will run on 3.3 V and that may be safer if the source of 5 V for the Atmega isn't USB.

    The following has everything except the boost circuit components, which isn't needed to scan more than 1 FSR using the typical PZT beeper element.

    Electronic parts list for mLMA1-V1:

     Qty Description                    Comments
    -------------------------------------------------------------------------------
      1  Atmega 328P NANO 3.0           Arduino board
    
      1  96" 128x64 I2C SSD-1306 OLED   OLED Display
     
      1  6x8 cm Prototyping. board      Should fit dual polarization
    
      1  Super bright LED, 3 mm         Sample indicator, green
      1  Super bright LED, 3 mm         Ramp indicator, blue
    
      2  Resistor, 10K ohm, 1/4 W       Current limiting for LEDs
    
      4  Resistor, 470-510 ohm, 1/4 W   5V-3.3V level conversion
      4  Resistor, 1K ohm, 1/4 W        5V-3.3V level conversion
    
      2  Resistor, 15K ohm, 1/4W        Ramp filter
    
      1  Resistor, 330 ohm, 1/4 W       Photodiode protection
    
      1  Resistor, 1K ohm, 1/4 W        External Trigger input
    
     *2  Trim-pot, 1M ohm               SFPI P and S gain
     *2  Trim-pot, 100K ohm             MODE P and S gain
    
      1  Trim-pot, 1M ohm               Ramp amplitude
    
      2  Capacitor, 0.1 µF              Ramp filter
    
     *2  Capacitor, 100 pF              SFPI P and S PD filter (optional)
     *2  Capacitor, 1 nF                MODE P and S PD filter (optional)
    
      2  Capacitor, 22 µF               5V and 3.3V bypass
      2  Capacitor, 22 µF               REF and PD bypass
    
      1  DIP switch, 4 position         "Piano key" style feature select
    

    * Only 1 of each required if dual polarization is not supported.

    For info on the dual polarization SFPI head using Thorlabs cage parts, see Mini Laser Mode Analyzer 1 (mLMA1) Version 2.

    Printed Circuit Board

    Not likely any time soon unless I'm really really bored. :)

    Atmega/Arduino Pin Assignments

    Here is a list of the Atmega 328 Nano 3.0 external pins used by mLMA1. Not all of these are shown on the wiring diagram since their definitions are in flux:

     Arduino Pin   Physical Pin   Function
    ------------------------------------------------------------------------------
         D3             6         Free_Run (no trigger) if high or open
         D4             7         Interpolate points if high or open
         D5             8         Sample - High during actual scan or capture
         D6             9         Ramp - PWM of scan ramp for SFPI only
         D7            10         Average n (default 8) points if high or open
         D8            11         Dim_Display if high or open
         D9            12         Function_Select 0
        D10            13         Function_Select 1
        D12            15         Dual_Polarization (future)
        D13            16         Same as D5 (Nano_LED, Atmega "L")
    
         A0            20         SFPI_PMode detector input (0 to 5V)
         A1            21         SFPI_SMode detector S-Mode input (0 to 5V)
         A2            22         Trigger input - external trigger only (0 to 5V)
         A3            23         Laser power meter input.
         A6            26         MSweep_PMode detector input (0 to 5V)
         A7            26         MSweep_SMode detectoreinput (0 to 5V)
        +5V            27         +5 VDC input or from on-board regulator
        VIN            30         Optional DC input (+7 to +12 VDC)
        GND           4,29        Ground/Common
    

    CAUTION: The Nano 3.0 can take +12 VDC on VIN since it has an on-board 5 V regulator. But apparently there can be problems when connecting to USB as I found out. Inadvertent ground loops (or something) can result in erasing its brain or damaging the USB chip. Exactly why this occurred is still not clear. The NANO was connected to USB and then the 12 V adapter was plugged in, at which point the USB dropped out, never to be heard from again with this board. The regulated wall adapter was on the same circuit and isolated in any case, so it should not have caused problems. The Atmega microprocessor is still running something so it's not totally dead, thus the suspicion that the problem is the USB chip. But I've been unable to change it so far, even with a programmer. Therefore, it is recommended that only the USB on a PC or laptop, a USB wall adapter, USB backup battery, or 5 V power supply be used.

    Computer and Operating System Requirements

    None.

    Latest Versions of the Firmware

    When the firmware starts up, the initial display will be something like:

    Initial Display after Boot

    Except for the version number. ;-) In a second or so, it will start scanning.

    Installing the Arduino Device Driver

    Before the Atmega board can be used, a Windows device driver must be installed to enable upload of firmware and communications with the mLMA1 GUI.

    There are many ways of doing this - some which may be overly complex, but what I've done for the Atmega 328 Nano 3.0 board is to go to Arduino Software and install the current version of the Arduino IDE (V1.6.9 as of May 2016). (I'm not sure if the board needs to be plugged in to a USB port during this process, but mine was. During the install process, it will ask to install the drivers. Reply "Yes" to all its requests. When the Arduino IDE is started for the first time, go to "Tools", "Board", and select "Arduino Nano". If the Nano is plugged in, its COM port should appear under "Tools", "Port".

    More info on software, drivers, and more at Getting Started with Arduino and Genuino on Windows.

    The more complex installations may be required if you bought the Nano from eBay or off the back of a truck, depending on whether it has the genuine FTDI USB communications chip. And even more complex if it doesn't have the bootloader installed. Links for driver installation may be found under References under "Arduino". Instructions for burning the bootloader may be found in the section: Burning Bootloaders into the Nano or Pro Micro.

    The Arduino IDE can be used for compiling and uploading, though I prefer UECIDE, below, because compilation and uploading is much faster. For use with the Atmega328 Nano 3.0, either is fine. However the ATtiny and Pro Micro may only be supported by the Arduino IDE. (The latter may come up as Arduino Leonardo though.)


    Loading UECIDE

    UECIDE will work with all versions of the firmware. But the only version of UECIDE I've had success compiling firmware without errors is Version 0.8.8alpha17 though I assume that more recent versions like 0.8.8alpha22 should also be satisfactory. Assuming that, download it from UECIDE: The Universal Embedded Computing IDE. And other versions probably work, they just hate me. :( :) If for some reason 0.8.8alpha22 doesn't work for you, I can provide 0.8.8alpha17, but it probably won't work for you either. ;-)

    The UECIDE files should be unzipped to any convenient location on your computer. UECIDE requires around 160 MB there, and another 600+ MB for support files typically somewhere like c:\users\YourUserID\AppData\Local\UECIDE. This location can be changed in File->Preferences. If doing this after having configured UECIDE, copy all the files to the desired destination first, then change the data directory in File->Preferences. DO NOT delete the original UECIDE directory or the preferences file! :) Otherwise, the configuration information will all be lost.

    Compared to most applications, UECIDE takes forever to start up even on a fast PC. So be patient. That's the bad news. The good news is that compiling and uploading takes literally only a few seconds, much faster than with the Arduino IDE or MPIDE (another one you don't need to know about). Go figure. :)

    The first thing UECIDE will likely do is to tell you that no boards are installed and then open the Plugin Manager. If it does not, do it manually by going to Tools->Plugin Manager. At first the pane along the left will only show the word "Plugins". But after a couple minutes, it should update with a list: Plugins, Libraries, Boards, Cores, Compilers, System. The following are required:

    For each of these click on "Install". Installing the arduino board will probably automagically install the other related files and may take several minutes. Confirm that each entry has a green check mark next to it.

    Close the Plugin Manager and go to "Hardware" or check the status line at the bottom of the window to confirm that the proper Board (Arduino Nano w/ Atmega 328), core (Arduino 1.6.x)), and Compiler (GCC 4.8.1 for AVR) has been selected. Correct it if not.

    Some other quirks of UECIDE that I've found:

    Plug the Atmega board into any available USB port. The power LED should come on. If I (Sam) sent you the Atmega board, it will have been loaded with a version of the mLMA1 firmware and the user LED will be flashing at about 5 Hz rate to let you know it is alive. But by the time you've received it, the firmware will probably be out of date, so reloading will be required in any case. :)

    Assuming the driver has already been installed, go to Hardware->Serial Terminal and select its COM port. Typically, this will be the highest number COM port, or perhaps the only one, since no one uses real COM ports for much of anything anymore.

    UECIDE should remember the configuration settings automatically upon exiting. These are tied to each instance of the UECIDE window, so it's possible to easily deal with multiple totally different board types.


    Uploading the mLMA1 Firmware

    The firmware is provided as a source file which probably has an extension of ".ino" (though the specific name doesn't matter - it's just a text file). However, the name may NOT contain any dashes "-" due to the peculiar restrictions of Java or something. Make a directory with the name of the firmware (without the extension) and put the firmware file there. For example, if the file is named mLMA1_FW_v123.ino, make a directory called mLMA1_fw_v123. and put mLMA1_FW_v123.ino in it. Note that case matters so the name of the directory and name of the firmware file (without the extension) must match case character-by-character exactly. Thus mLMA1_fw_v35.ino is not the same as mLMA1_FW_v35.ino.

    1. Plug the Nano 3.0 board into a USB port. Windows should recognize it with the usual annoying sound of a USB device it recognizes. I've occasionally seen problems using a USB port replicator though these generally are acceptable. But if the board doesn't come up, plug it into a direct USB port.

    2. Use Ctrl-O to open the firmware file. Select the directory. The source code should appear in the same window unless a file is already open, in which case a new window will appear. (If UECIDE thinks it's a firmware directory, it won't even allow you to select the file but will immediately open it. If the name of the directory and file don't match - including case - it will produce an error like "file not found". What a concept? ;-)

    3. Use Ctrl-U to compile and upload the firmware to the board. This typically takes only a few seconds on a PC that is less then 35 years old. :) Near the end, the green status bar will extend nearly all the way to the right and the LEDs on the board will then begin flashing in several different patterns in anticipation of getting new and (hopefully) improved firmware. ;-) The board will be automatically reset and start running the firmware. During this time, confirmation messages similar to the following will appear:

         Compiling...
           * Compiling sketch...
           * Compiling core...
             > arduino
           * Compiling libraries...
           * Linking sketch...
         Compiling done.
         Memory usage
           * Program size: 7532 bytes
           * Memory size: 1092 bytes
           * Compilation took 8.634 seconds
         Uploading firmware...
           * Resetting board...
           * Uploading...
           * Resetting board...
           * Upload Complete
      

    Once the firmware has started, the on-board LED "L" should be flashing at around a 25 Hz rate to let you know it's alive.

    The firmware is retained in non-volatile memory so uploading only needs to be done once - or until a new version is available!

    The firmware may also be compiled without uploading by using Ctrl-R. Since you haven't messed with the code, it should compile without errors. This is slightly faster for testing and doesn't use the board at all so it can be off doing whatever it pleases. :)


    Troubleshooting

    Naturally, all is expected to go smoothly. But if it doesn't, here are some common problems. Some of these may be bugs in the firmware as hard as that is to believe. So, if you find something that cannot be understood or solved based on what's below, contact us for a timely response. "Timely" is defined as no later than when the Sun becomes a red giant. :-)

    Due to the limited resolution of the OLED display - 128x64 pixels, the display is not nice and smooth like that of an analog scope. And the amplitudes of the peaks bounce around quite a bit due to where they fall on the OLED pixel matrix. There are several options (controlled by inputs to selected Digital pins of the Nano) to add interpolation and/or averaging, but what's really needed is an anti-aliasing input filter (either analog or digital) to be able to fainthfully display the narrow spikes of well aligned high resolvance SFPIs. The most easily interpreted (if not nicest) display occurs presently with none of the options enabled since the human eye does a decent job of visualizing what the appearance should be. Improvements may or may not be implenented in for mLMA1. But there should be a mLMA2 at the very least - eventually.


    Firmware Technical Description

    Coming soon, maybe.


    Next Version of the SFPI Head

    In keeping with the small size of the controller, a new SFPI head has been constructed including the dual polarization detector. It uses Thorlabs 16 mm cage parts with a total length of 2.5 inches. A pair of the 43 cm RoC 99.5% mirrors have been installed with a spacing of 1/2 the Roc, resulting in a mode degenerate configuration with an FSR of ~2.32 GHz and a finesse in the 100-200 range at 633 nm. A 4 mm PBS cube with photodiodes attached to its end and side faces behind the rear mirror serves as the dual polarization detector. A 50 mm focal length positive lens helps with the mode matching, though a shorter focal length would probably be better. Eventually a non-polarizing beam-splitter and second dual polarization will be installed in the front to sample the beam for the MODE display and power meter.

    Compact Dual Polarization Scanning Fabry Perot Head (Scale in cm)

    That's a cm scale. :)

    Here is an early trace with no interpolation:

     

    Single and Dual Polarization Scanning Fabry Perot Displays

    Development has been suspended on mLMA1-V1 so all improvements and new and improved bugs will be in mLMA1-V2. :)

    References

    These links open in a single new window or tab.

    UECIDE

    1. UECIDE: The Universal Embedded Computing IDE
    2. UECIDE Beta Programme (Dowload)

    Arduino

    1. Arduino IDE, Refernce, Tutorials, more

    Atmega 328 Nano 3.0

    1. Atmega 328 Nano 3.0
    2. Installing Drivers for an Arduino Nano in Windows
    3. Nano Driver - Windows 7 Instructions
    4. Arduino Nano v3.0 clones (How-to & Review)
    5. How to Burn a Bootloader to Clone Adruino Nano 3.0 - 2

    Jan Beck's Information

    1. Interferometer Project Pages
    2. Github µMD1 GUI Source Code Repository