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Features

• Lateral Effect Position-Sensing Detector
• Insensitive to Beam Shape and Power Density
• SM05 Lens Tube Compatible
• Compact Footprint

The PDP90A Position Sensor utilizes a pincushion tetra lateral sensor to accurately measure the displacement of an incident beam relative to the calibrated center. These devices are ideal for measuring the movement of a beam, the distance traveled, or as feedback for alignment systems. 

The large detection surface allows a beam diameter of <9 mm, however, our recommended range is 0.2 to 7 mm. Unlike quadrant sensors which require overlap in all quadrants, the lateral sensor will provide positional information of any spot within the detector region, independent of beam shape, size and power distribution. The PDP90A has a very low noise figure of <2 mV_pp (<300 µV_rms), which corresponds to a detection error of 0.675 μm_rms. Resolution is directly related to the optical input power by the following equation,

where,
ΔR is the Resolution,
L_x is the detector length, 9 mm,
e_n is the output noise voltage, <300 µV_rms, and
V_o is the Sum output voltage level, 4 V max.

Therefore, for the maximum power level, the resolution will be 0.675 µm. More detailed technical information can be found on the Tech Info tab.

Each PDP90A lateral position-sensing detector is packaged with a 8-32 to M4 adapter to provide compatibility with either imperial or metric mounting posts. A controller, TQD001, is available separately below.

* Resolution is dependent on input optical power. Specification assumes photocurrent is 40 µA.
** Across whole 15 kHz bandwith of detector.

2D Lateral Effect Position-Sensing Detectors

Overview

The 2D lateral effect sensors provide an accurate way to measure displacement - movements, distances, or angles – as well as feedback for alignment systems such as mirror control, microscope focusing, and fiber launch systems. These sensors work by proportionally distributing photocurrent using resistive elements to determine position and are usually referred to as lateral effect sensors. There are two different types of lateral effect sensors available Duo lateral and Tetra lateral.

Duo lateral sensors provide a resistive layer on both the anode and cathode photodiode connections. This isolates the x and y positional information of the sensor and allows the sensor to be highly linear and very accurate. However, the added resistive layers significantly increase manufacturing cost.

Tetra lateral sensors use a single resistive layer with a common cathode and an anode on each side of the detection area. This makes them very inexpensive to manufacture, however the linearity decreases as the spot moves away from the center. This is caused by the physical location of the anodes along the sides of the sensor, specifically in the corners where the anodes approach each other. Thorlabs uses a variation of the Tetra lateral sensor commonly called a “pin cushion” sensor. This sensor moves the anodes to the four corners of the sensors and reshapes the sensor area, to produce linearity comparable to a duo lateral sensor at a significantly lower cost.

Calculating the Position

The PDP90A includes circuitry to decode the Dx, Dy, and SUM signals as follows:

From these formulas, the actual distances can be calculated with the following equations:

Where x and y are the distances from the center of the sensor. Lx and Ly are the resistance lengths of the active sensor region. The PDP90A sensor is 9 mm x 9 mm, so then Lx = Ly = 9 mm.

Positional Errors

Unlike quadrant style photodiodes, lateral effect sensors are independent of beam shape and size. They will provide positional information of the centroid of the spot as long as the light is located on the sensor. If part of the light moves off the sensor, it will cause a shift in the centroid making the beam measurement unreliable.

Positional errors will also be caused by ambient light levels. For best results the PDP90A should be operated in a dark location. However, light baffling using lens tubes and irises to narrow the field of view (FOV) will limit ambient light level errors.

Position Resolution

Photo of a PDP90A sensor with 2 SM05L10 Lens Tubes
and 1 SM05D5 Iris

Position resolution is the minimum detectable displacement of a light spot incident on the PSD. Position resolution (ΔR) is a factor of both the resistance length (L_x or L_y), or the x and y length, and the signal to noise ratio (S/N).  The signal to noise ratio for this system can be defined as the SUM output signal level (V_o) divided by the output voltage noise (e_n). The PDP90A output noise is <2 mV_pp, or <300 µV_rms.

where,
ΔR is the Resolution,
L_x is the detector length,
e_n is the output noise voltage, and
V_o is the Sum output voltage level.

For the PDP90A:

For best results V_o should be maximized to 4 V resulting in a position resolution of 0.675 µm. To do this, monitor the SUM output voltage of the sensor and adjust the optical intensity until the output is approximately 4 V. Increasing above 4 V will saturate the system causing unreliable readings. The provided operational software will indicate the voltage level on a slide bar. If the SUM is saturated the slide bar color will turn red. Back off the intensity until it switches to green. This will be equivalent to a 4 V SUM output.

Smart Pack

• Minimize CO₂ Footprint
• Reduce Non-Recyclable Packing Materials
• Improve Packaging Integrity
• Saves on Shipping Costs

Thorlabs' Green Initiative is aimed at reducing waste. To that end, we have begun to re-engineer our product packaging. Products listed above are now shipped in re-engineered packaging that minimizes the use of non-recyclable materials.

Over the course of a year, the re-engineered packaging across our product line is projected to reduce our CO₂ footprint by more than 16,184 kg and reduce packaging by over 13,893 pounds! As we move through our product line, we will label re-engineered packages with our green Smart Pack sticker (above).

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Features

• Segmented Quadrant Position-Sensing Detector
• Versions for Visible or NIR
• Ideal for Automatic Alignment
• Compatible with Four Channel Position Sensing System or a Single Channel T-Cube
• SM05 Lens Tube Compatible
• Compact Footprint

Thorlabs' offers a wide variety of coherent light sources compatible with both quadrant detectors. A selection of appropriate lasers is outlined on the Suggested Lasers tab. Sample position sensing arrangements are shown in the Application Idea tab. For questions about specific light detection arrangements contact tech support.

The PDQ80A and PDQ30C detectors are segmented, position-sensing, quadrant detectors for precise path alignment of light in the 400 to 1050 nm or 1000 to 1700 nm range, respectively. A 6-pin Hirose connector on each detector outputs signals proportional to the incident beam's power distribution (and thus position). For best results, quadrant detectors should be used with powers under 1 mW (linear regime of sensor) and beam diameters within their specified range.

Each quadrant detector is packaged with an 8-32 to M4 adapter to provide compatibility with either imperial or metric mounting posts. A controller, TQD001, is available separately below.

Segmented-Quadrant Position-Sensing Detectors

The segmented-quadrant position sensors consists of four distinct yet identical quadrant-shaped photodiodes that are separated by a ~0.1 mm gap and together form a circular detection area capable of providing a 2D measurement of the position of an incident beam. When light is incident on the sensor, a photocurrent is detected by each sector (labeled A, B, C, and D as shown in the figure to the right). From these signals difference signals can be determined using an appropriate A/D converter. The sum of all four signals is also determined for normalization purposes. The normalized coordinates (X, Y) for the beam's location are given by the following equations.

If a symmetrical beam is centered on the sensor, four equal photocurrents will be detected, resulting in null difference signals, and hence, the normalized coordinates will be (X, Y) = (0, 0). The photocurrents will change if the beam moves off center, thereby giving rise to difference signals that are directly proportional to the beam displacement from the center of the sensor.

These sensors are very accurate and are ideal for auto-alignment applications. However, care should be taken in the shape and density distribution of the incident beam. These sensors are sensitive to these two parameters. A beam that does not have a Gaussian power distribution will be centered based on the power not on the geometric center of the beam. For these types of beams, a lateral sensor might be better suited.

TQD001 Quadrant Detector Reader and PDQ80A Photodiode Sensor in an Auto-Alignment Setup

A basic auto-alignment schematic is shown below. It consists of a PDQ80A Photodiode Sensor, a TQD001 quadrant detector, two TPZ001 piezo drivers, a piezo-actuated 2-axis turning mirror mount (Item# ASM003), a laser source, and a computer. Together, the system is used to position and maintain the laser beam so that it is located at the center of the detector array with respect to power density.

It should be noted that when used with older versions of the TPZ001 T-Cubes (i.e., Rev. 1. The revision number is displayed on the LED screen when the T-Cube is booted), the piezo cubes must be connected to the Quad Detector Reader using two external SMA connectors even if a TCH002 hub is being used; if Rev. 2 or higher TPZ001 T-Cubes are used with a TCH002 hub, the SMA to SMA cables are not needed. However, regardless of revision number, SMA to SMA cables are needed if the TCH002 hub is not used.

Typical Auto-Alignment Setup

Thermal variations in a lab cause misalignment of components. Optical tables are susceptible to bowing dependent on the different coefficients of thermal expansion of top and bottom plates. Mechanics such as mirror mounts exhibit drift from temperature variation. By using a quadrant detector in a closed-loop system, one can minimize the effects of thermal variation on an experiment's alignment.

The experimental setup shown to the right was created based on the schematic above. An LDM635 red laser diode module outputting light at 635 nm serves as the laser source. The light is incident on an ASM003 turning mirror mount, which is mounted on a MBT616 flexure stage (center of photo). The turning mirror's x and y motions are controlled using two TPZ001 piezo actuators (these are the first and third T-Cubes shown on the TCH002 hub at the bottom of the picture). Please note that the piezo elements are meant for small beam alignment adjustments. The turning mirror directs light to a BP150 pellicle beamsplitter. The light transmitted by this beamsplitter will continue on to the rest of the experimental setup (not shown) while the reflected light is directed towards the PDQ80A Quadrant Sensor (back right of photo), which is controlled by the TQD001 T-Cube Quadrant Reader (located between the two TPZ001 piezo actuators on the TCH002 hub).

When using a Quadrant Detector, choosing an appropriate alignment laser is essential. The laser's wavelength, beam diameter, and power all have a significant effect on the performance of the system. While there are countless sources that are suitable for use with our quadrant detectors, the lasers and optical components below have been selected for their cost-effectiveness and ease of use.

Photodiodes, such as those used in our quadrant detectors, should have less than 1 mW of incident light on them for linear responsivity. Beam splitters are commonly used in auto-alignment systems and reduce the amount of light on the detector. Depending on the optical setup, the laser's power may need to be further reduced, which can be accomplished using the power adjustment knob on our T-Cube lasers.

The suggested fiber patch cables are each 2 m long and are matched to the fiber type in the laser source. While we offer many different collimators, the ones suggested here are part of our Adjustable Collimator line and provide an appropriately-sized beam diameter for use with our Quadrant Detectors. The collimators can be mounted numerous ways, but the VC1 (VC1/M) is a versatile solution suitable for most applications.

PDQ80A Compatible

• Wavelength Range: 400 - 1050 nm
• Input Beam Diameter: 1 - 3.9 mm
• Max Incident Power (for Linear Response): 1 mW

PDQ30C Compatible

• Wavelength Range: 1000 - 1700 nm
• Input Beam Diameter: 0.2 - 0.5 mm
• Max Incident Power (for Linear Response): 1 mW

Smart Pack

• Minimize CO₂ Footprint
• Reduce Non-Recyclable Packing Materials
• Improve Packaging Integrity
• Saves on Shipping Costs

Thorlabs' Green Initiative is aimed at reducing waste. To that end, we have begun to re-engineer our product packaging. Products listed above are now shipped in re-engineered packaging that minimizes the use of non-recyclable materials.

Over the course of a year, the re-engineered packaging across our product line is projected to reduce our CO₂ footprint by more than 16,184 kg and reduce packaging by over 13,893 pounds! As we move through our product line, we will label re-engineered packages with our green Smart Pack sticker (above).

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Features

• Auto Alignment of Beam to Center of Sensor* when in Closed-Loop Mode
• Beam Position Measurement When in Open-Loop Mode
• LED Cross Hair Position Display
• USB and Manual Interfaces
• Flexible Software Suite
• Compatible with T-Cube Hub System
• Sum, Difference, and Position Demand Analog Outputs

The TQD001 T-Cube Quadrant Detector Reader interfaces with our range of Quadrant (PDQ80A and PDQ30C) and Lateral Effect (PDP90A) Sensor Heads and can be used either to measure the position of the beam on the sensor or to generate a signal that can be used as the feedback input for an automated beam steering element. The signal generated can be used to steer the beam to the center of the sensor head. When combined with the TPZ001 Piezo Driver T-Cubes, this unit is ideal for such closed-loop beam-steering applications. Please call tech support for details on how the TQD001 can be used with sensors from other manufacturers.

Like all members of the T-Cube family, the footprint has been kept to a minimum [60 mm x 60 mm x 47 mm (2.4" x 2.4" x 1.8")], and the unit can be mounted directly to the optical table next to the detector and steering elements under control, thereby minimizing drive cable lengths while providing a convenient location to control the experiment manually via the top panel controls.

Operation
The TQD001 T-Cube can be controlled by the manual interface on the top of the unit or via a USB connection to a computer running the included apt™ software or ActiveX command modules. Using either interface, the TQD001 operates in either an open- or closed-loop mode. The open-loop mode is used to measure the position of the beam on the detector. When in this mode, the T-Cube generates a left-minus-right X difference signal, a bottom-minus-top Y difference signal, and a sum signal. In the closed-loop mode, a DSP processor inside the TQD001 runs two independent feedback loops that generate X and Y position demand outputs for use as the input to the beam steering element being used to center the beam on the detector. If the unit is being controlled manually, the beam position information (open-loop mode) or feedback signal (closed-loop mode) is available via SMA connectors on the side of the unit. When the unit is controlled via the USB interface, the open-loop output of the unit is also exported digitally to the computer.

Power Supply Options
The TQD001 T-Cube, which does not ship with a power supply, can be powered using either a TPS002 power supply or a TCH002 T-Cube Hub and Power Supply. The TPS002 power supply plugs into a standard wall outlet and provides +15, -15, and +5 VDC for up to two T-Cubes. The TCH002 consists of two parts: a hub that can support up to six standard footprint T-Cubes and a power supply that plugs into a standard wall outlet and powers the hub, which in turn powers all of the T-Cubes connected to the hub. The hub's single USB connection provides USB connectivity to all the T-Cubes plugged into the hub. In addition, when the TQD001 is intended to be used in the closed-loop mode with beam-steering controllers such as the TPZ001 Piezo Driver T-Cubes, the TCH002 is especially useful since the hub allows for the direct communication between the T-Cubes connected to the hub. As a result, the feedback signals generated by the TQD001 in the closed-loop mode can be sent directly to the TPZ001 piezo controllers being used to direct the beam steering elements.

The TQD001 Software Overview

The APT (Advanced Positioning Technology) family covers a wide range of motion controller products ranging from small low powered single channel optomechanical motor drivers (the 'Cube' drivers) to high power multi-channel modular 19" rack nanopositioning systems (the APT Rack System).

All controllers in the APT family share a common software platform, the APT System Software. The software CD supplied with all controllers contains an installation of this system software, together with a wealth of support information in the form of handbooks, help files, tutorial videos, FAQs and other relevant information on using and programming these Thorlabs products.

By providing this common software platform, Thorlabs has ensured that users can easily mix and match any of the APT controllers in a single application while only having to learn one single set of software tools. In this way it is perfectly feasible to combine any of the controllers from the low powered single axis to the high powered multi-axis systems and control all from a single PC based unified software interface.

The APT System Software allows two methods of usage - graphical user interface utilities (supplied) for direct interaction and control of the controllers out of the box, and a set of programming interfaces that allow custom integrated positioning and alignment solutions to be easily programmed in the development language of choice.

Detailed information on both usage modes is provided on the CD, but see the tutorial section for an overview of the APT Server, APTUser and APTConfig utilities, and details of the extensive developer support information included on the CD.

Of particular interest is the inclusion on the software CD of a range of software video tutorials (see the Video Tutorials tab). These videos illustrate some of the basics of using the APT System Software from both a non-programming and a programming point of view. There are videos that illustrate usage of the supplied APT utilities that allow immediate control of the APT controllers out of the box. There are also a number of videos that explain the basics of programming custom software applications using Visual Basic, LabView and Visual C++. 

Click here to go direct to the Thorlabs Download Area to access the full APT software CD. Experiment with the software using the simulator mode - refer to the Tutorial Videos for the APTConfig utility to learn how to select simulator mode.