Laser Tweezer Instruments

We have extensive experience providing both off-the-shelf and custom electronics for laser tweezer systems. The most commonly needed items that we provide are shown below, but please feel free to contact us about your own special requirements.

Tweezer Spot Detector

This is a two channel instrument for use in two dimensional position sensing applications such as 'laser tweezers'. Each channel provides analog X and Y outputs that vary as the position of the light spot moves across the face of a 4 segment photodiode. The system includes a main analog box (shown above), and a photodetector head stage for each channel.

Features

• Low noise analog electronics
• X and Y analog position outputs
• "SUM" output: the sum of the signals from all four segments
• "ADC" output: a separate filtered, amplified signal appropriate for an ADC input
• front panel polarity switches (helpful when optical geometry is changed)
• "INJECT" input: injects a signal to X or Y outputs (for feedback applications)
• "OFFSET" control: offsets X or Y outputs (for feedback applications)
• Rack mounted or benchtop enclosure (3.5" x 19" x 16")
• Power: available for U.S. or European line voltages.

For more information about the tweezer spot detector, click here

Head Stage

The head stage is the most important element in the laser tweezer electronics system, from a noise standpoint. Our head stage is active, meaning that it contains dc powered amplifiers that convert the diode's photocurrent into a boosted voltage before it is transmitted over the cable to the main unit.

The photodetector head stage can be modified for the spot size and range of motion anticipated, but the standard unit is based on a detector made up of (4) 1.5mm x 1.5mm square segments, and is a silicon device with sensitivity over the visible range. We can also provide a germanium detector for sensitivity at longer wavelengths.

Features

• Transconductance: 1 kohm to 100 Mohm by six position switch (on switched gain model)
• Risetime: 2 us settling within 90% at 1k, 10k settings
• 3" x 1.25" x 1" overall
• (4) 1.5mm x 1.5mm square detector segments
• Single 4 foot long cable with DB-9P to main unit

Feedback Controller

This instrument "closes the feedback loop" by providing adjustable control of the signals that control the deflection of the optical beam. The amount of direct and integral feedback can be varied with front panel potentiometers, an offset signal can be injected, and an adjustable clamp keeps the mirrors or AO modulators from being overdriven.

Inputs

• "IN": This is the main input, and is (typically) a voltage signal proportional to the detected position of a bead or bright spot of light. It has the same gain as the input marked "10", and also has a 10 k ohm input impedance.
• "10k": This is a second input having the same input impedance and gain as the first, and provides a way of injecting a position signal into the feedback unit for purposes such as modulating the bead/spot position. This signal is summed with the one labeled "IN"
• "100k": This input has 1/10th the voltage gain of the first two inputs. It is a convenient way of moduating the bead/spot position with a lower gain, which is often desirable because voltage sources such as signal generators and DAC outputs are typically scaled inappropriately high for direct 1x use.
• "TTL": This is a logic signal that allows electronic selection of the A or B integral value, when the A|TTL|B switch is in the middle (TTL) position.
• "TEST": This input is mostly useful for debugging. A signal injected here will drive the output directly (without gain, integration, offset, etc.).

Controls

• "DIRECT": This potentiometer sets the gain of the direct (sometimes called "proportional") signal that goes to the output. The gain range is 0.0x to 10x for the inputs "IN" and "10k", and 0.0x to 1.0x for input "100k". The setting of this pot. is modified by the setting of the next two switches.
• "ADJ" | "1x" Switch: Selects a fixed gain of 1.0 or the variable gain set by "DIRECT".
• "ON" | "OFF": Sets whether or not the direct signal (or the selected fraction of it) goes to the output.

Outputs

• "OUT": This is the main output, and is typically connected to the actuator that deflects the light beam (often an AOD).
• "10x/100x": This is a scaled-up version of the main output that is useful for monitoring the drive signal on an oscilloscope or ADC input.

For More Information

Please call us at 215-387-5456, or send email.