Tapered Amplifier Control Electronics

Steck Lab
Department of Physics
University of Oregon

Page last updated Jun. 04, 2013
Mail questions or comments to pmartin@uoregon.edu

To operate our tapered amplifier (TA), we use three homebuilt circuits: a current supply which is modified from the standard laser diode driver we use in our lab, a temperature controller, and a protection circuit inside the TA's housing (all three can be traced to freely available circuits designed by Todd Meyrath at UT Austin). An external high current (DC +12) supply is necessary for operation, and we also use a laser cutter to etch our control box's front panels, but beyond this, this page is meant to fully document the materials and features of each of the three aforementioned circuits. The temperature and current controller sections include gerber files for ordering printed circuit boards (PCBs) (we use Advanced Circuits $33 each offer), but since the protection circuit is so simple (and it is more cost-effective to etch it in house), we include only the schematic and .pdf etching mask. Altogether, our TA control unit costs ~$835, using current vendor pricing without bulk discounts.

File format notes:

TA current supply

To operate the tapered amplifier, we use an altered version of our lab's generic laser diode current driver, which itself is a descendent of Todd Meyrath's design from UT Austin. Our supply is based around the high-voltage high-current OPA549 operational amplifier, which is capable of continuously outputing 8A. Additionally, care has been taken to attempt to avoid some of the many ways a sensitive tapered diode can be destroyed. To monitor current, we use a 0.1Ω sense resistor which is rated for 2W, and thus limits our current output to ~4.5A. If more current is required, the sense resistor can be reduced, but this increases the noise of the supply. While we don't go into too much detail of the inner-workings of the circuit, we have some basic instructions on how to adjust the trimpots on our calibration page.

We use the 5-pin, PCB-mounted connector by LEMO as our current output; the matching male plug is LEMO P/N PAG.M0.5GL.AC52G, and can be purchased at Mouser.

We have additionally performed some noise measurements on our current supply.

Features of our TA current supply include:

Operational note: The driver has two manual switches, one disengages the output shorting relay and the other enables current to flow within the circuit. These are labeled on the front panel as "diode bypass" and "enable", respectively. To safely operate the diode, the former should be switched on (off) first (last) when powering on (off) the laser diode. We typically leave the shorting switch in the disengaged position (up), and use the enable switch to turn the laser on and off. Using the shorting switch to directly turn on and off the diode may cause transients that ultimately kill the laser.

The relevant documents are:

Current supply circuit

Total price (as of 02/2013): $302.96

TA temperature controller

Our temperature control circuit is simply an interface for Wavelength Electronics' WTC3243 servo chip (we also use the associated heat sink and +12V cooling fan). This chip monitors the temperature via a 50 kΩ thermister and supplys current to a thermoelectric cooler (TEC) to match a manual set point. Currently, we can adjust the sepoint from about 15°C to 43°C, but changing a pair of resistor values is all that is necessary to alter the controller's range.

Additionally, we use a second temperature transducer—the design supports either an AD520 or an LM35, but we exclusively use the AD590—to allow us to view the current temperature via an LCD screen or a more accurate analog BNC output. This chip can be installed in the copper block near the thermistor in a couple of different ways:

To calibrate the AD520's output, ice- and boiling-water baths are used to set the zero-point and gain, respectively. BNC outputs are included to monitor the TEC current, the servo's error signal (difference between thermistor reading and temperature set point), and, as mentioned, the AD590 readout.

The relevant documents are:

Temperature controller circuit

Total price (as of 02/2013): $318.23

TA control box construction

We enclose the temperature and current controllers together in an electronics box from Parmetal. This rack-mounted aluminum box has an alodine coating for RF shielding and is attached to the ground of our input 120V AC power connection. Power supplies are placed inside, with the exception of an external high current supply (used to drive TA, TEC, and cooling fans), which is brought into the rear of the box with binding posts. The boxes come with an external front panel, onto which the PCBs mount directly, and an internal front panel, which is placed behind the PCBs to shield them from the power supplies mounted in the back of the box. (See photos for detailed view of box and its connections.) The current controller is near the bottom lid of the box, so to prevent shorting should the board flex and contact the lid, we place heavy-duty packing tape on the inside of the lid (we choose not to use standoffs to allow for easy access to the backside of the board).

The external front panel is black annodized and etched using a 60W laser cutter to produce professional-quality labeling (we use a VersaLaser VLS series to quickly etch outlined text since vector images do not need to be rastarized). We provide CNC .igs files for machining the panel as well as .ai and .pdf files for laser etching. They are labeled for 780 nm TA diodes, but this can easily be altered or removed in the Adobe Illustrator file.

The power supplies inside the box have annodized aluminum chassis, and we use lock-tooth washers to bite through and connect them to ground via a grounding strip and jumper (see photo). We also sand the faces of the chassis that contact the rear of the box so that the enclosing box is grounded. (Incidentally, mounting hardware for power supplies and cooling fans is not included in the documentation.) Since the TA temperature and current controllers were adapted from our generic laser-diode controllers, we have a legacy +5V DC power supply in the electronics box that isn't really necessary. Currently, the only purpose for this bulky 1.5A supply is to power the LCD readouts on each board. In future realizations of these control electronics, the 24V supply can easily be regulated down to 5V (with a little heat) or a cheaper switching power supply option can be used instead.

Finally, we note again that an external power supply (not included in the documentation below) is necessary to supply high current at +12V to the OPA549 amplifier, and the TEC driving stage of the WTC3243 temperature chip. We have installed +12V cooling fans that also operate using this supply and aid in dumping heat from these two ICs (we have four fans, but this is probably overkill, so the bill of materials calls for only two). We currently use a 10-amp supply to power two TA setups, operating at around 2A of current each.

The relevant documents are:

TA control box

Total price (as of 12/2012, not including annodization, machining, etching, or mounting hardware): $215.33

TA protection circuit

We place a protection circuit inside the tapered amplifier's housing as a final safeguard against accidental electrocution. This circuit has two separate low-pass filters to remove any pickup that occurs between the control box and the TA, and, in addition, there are several diodes to suppress effects of any reverse-biasing or large transients.

Since this is a fairly simple circuit, we etch a single-sided PCB in house. The ferrite beads used are Panasonic EXC-ELSA35 (Mouser P/N 667-EXC-ELSA35). We use axially-terminated components for simplicity, but this protection circuit could be miniaturized by using surface mount components.

The relevant documents are:

TA protection circuit