Tweezer Spot Detector Manual

This is a manual for the Current Designs Laser Tweezer Spot Detector, which is an analog electronic instrument designed to make it easier for labs to set up laser tweezer systems.

The Tweezer Spot Detector is a two channel device, meaning that it generates X, Y position data for two spots of light.

This manual is divided into sections on:

1. Front Panel Items
2. Rear Panel Items
3. Head Stage

This is a draft document and our goal is to make it into something which answers questions and provides guidance, so please contact us with suggestions and questions.

Front Panel Items

Note that there are two identical sections on the front panel, corresponding to the two channels of the device. Each channel contains the differencing electronics to take signals from a head stage and convert them into X and Y position signals. Each channel also provides other signal conditioning features which have been found to be useful in typical laser tweezer applications, and these are described below.

Below is a picture of the right hand side of a Tweezer Spot Detector, showing the controls for a single channel:

OFFSET This potentiometer control gives the user a way of shifting the X signal by a constant positive or negative amount. This pot gives about +/- 0.5V of range on the output, and this range doesn't change when the transimpedance of the head stage is changed. So its possible to add (or subtract) amounts that range in magnitude from much less than the spot's signal to much more than it.

In a system with a feedback loop, this control can be used to change the X position of the spot. On the other hand, in an open loop situation this control might be used to shift the apparent position of the spot, as monitored on a scope or adc input.

The INJECT input, and selector switch A signal applied to this input is added to the X (or Y) signal from the head stage, and will appear at the X (or Y) output. If the position of the spot is not being controlled in a feedback loop, then the injected signal won't affect the spot position-- it will only affect the output signal. But in a closed loop system the inject signal is one way that the position of the spot can be controlled (in one axis) electronically.

X and Y Outputs, and their polarity switches If the detector is viewed as having four segments labeled A through D in this arrangement:

Then the X and Y signals (with a sign controlled by the POS vs NEG switch) will be:

Y = (A + B) - (C + D) [+ inject] X = (A + C) - (B + D) + offset [+ inject]

The absolute voltage range of this signal is approximately +/-12 volts (it is set by the internal power supply voltage rails and a small voltage loss in the op amp output stages). But what really sets the size of the Y signal (and X, too, similarly) is the combination of the optical power incident on the detector and the transimpedance selected on the head stage, which is discussed below.

A signal gain of 10.0 is applied between the head stage outputs and the final X,Y outputs, in two stages of 3.32x each.

The polarity switches are provided for simplicity when changing optical configurations, which can otherwise lead to confusing differences in directions.

The SUM output Here the sum output is calculated as:

SUM = A + B + C + D

Obviously, this is a measure of the total light power incident on the detector, so it can be used as a guide in setting the gain (more properly, the transimpedance). In some cases it might also be used to normalize the X and Y signals (dividing them by the power). Normalizing the direction signals might make sense if fluctuations are anticipated in the optical power. But its also worth noting that this can multiply noise in the output if the fluctuations are not very small compared to the optical power or, looked at another way, if the optical power is low.

The way the X and Y signals are calculated, as differences between close detector segments, tends to reduce common-mode light variations, especially when the spot is well centered. It follows from this that it is good practice to keep the spot centered if its practical to do so.

The ADC output This output is almost the same as the X output, but has another stage of gain that can adjusted by an internal potentiometer to scale it appropriately for a particular ADC. Also (optionally) this output goes through a programmable bandwidth filter to provide anti-aliasing.

The gain, relative to the X output, can be varied from 1.0 to 11.0 by adjusting internal resistor R44 using a small screwdriver. This gain is set to 1.0 by default.

The Head Stage Connection The head stage for each channel is connected to this 9 pin D-type connector on the front panel.

Note- It is a good idea to turn off the main power switch when disconnecting or connecting the head stage, to protect the head stage circuitry from momentary asymmetries in the power supply rails.

There are four signals in this connector (A, B, C, and D) and also the +/- 15 volt power lines to the head stage op amps.

The TO SERVO output The X signal from one of the two channels can be directed to a servo through this connector. "A" is the left channel, "B" is the right. There is only one of these outputs, not one for each channel as is the case for all of the other outputs.

Rear Panel Items

The rear panel is relatively simple, with just two items:

Power Entry Module

Power to the Tweezer Spot Detector comes through the power entry module on the rear panel. This unit has a power switch, a filter to remove high frequency noise, and a fuse assembly. If the green LED on the front panel does not come on when the main switch is turned on, unplug the power cord and check the fuses here.

The "ADC Out" Bandwidth Thumbwheel Switch (Optional)

If your instrument has the programmable filter option installed, this switch will be on the rear panel. It controls the low-pass cutoff frequency of the filter, which affects only the output labeled "ADC".

Head Stage

The first gain stage in our system is in the small "head stage", which contains both a four quadrant diode detector and four transimpedance amplifiers. In a system without an active head stage, a very low level current signal is carried from the detector to the first amplifier, which might make such a system prone to interference from things like motors and high voltage switching devices.

There is only control on the head stage: the transimpedance switch. This switch controls the sensitivity of the system to light, in transimpedance units of Volts per Amps, or ohms. The higher the transimpedance, the greater the voltage output that will appear for a given light input.

The only electronic filtering in the head stage is there to keep the amplifiers stable and is outside the range of the system's 100kHz bandwidth.