Position Sensing Detectors

Zoom

Click to Enlarge
The shaded area in the chart above shows the min and max input power levels as a function of wavelength. Make sure the incident beam power is as close to the max level for the best resolution and noise figure possible. Going over the max level will saturate the sensor and give erroneous results.

Features

• Lateral Effect Position-Sensing Detector
• Insensitive to Beam Shape and Power Density
• Compatible with SM05 (0.535"-40) Lens Tubes
• Compact Footprint

The PDP90A Position Sensor utilizes a silicon photodiode-based 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 provides 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 0.750 μm_rms. Resolution is directly related to the optical input power by the following equation:

where
ΔR is the Resolution,
L_x is the resistance length, 10 mm,
e_n is the output noise voltage, <300 µV_rms, and
V_o is the SUM output voltage level, 4 V max.

Therefore, at the maximum power level, the resolution is 0.75 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.

Controller
The PDP90A is compatible with the KPA101 controller, available separately below. The KPA101 K-Cube™ Beam Position Aligner is a part of Thorlabs' new and growing Kinesis^® line of high-end, compact motion controllers. It interfaces with our range of Quadrant and Lateral Effect Sensor Heads and can be used either to measure the position of the beam on the sensor (in Open-Loop mode) or to generate a signal that can be used as the input for an automated beam steering element (in Closed-Loop mode). When combined with one or more KPC101 K-Cube Piezo Controller and Strain Gauge Readers, this unit is ideal for such closed-loop beam-steering applications. See the full web presentation for more information.

Click to Enlarge

• Resolution is dependent on input optical power. Specification assumes photocurrent is 40 µA.
• Across whole 15 kHz bandwith of detector.
• The difference between the physical center and electrical center of the detector.
• Spots smaller than Ø0.2 mm could increase the position detector error.
• Specified by the active area of the pincushion.
  

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, a diagram of which is shown to the right. This sensor moves the anodes to the four corners of the sensors, reshaping 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 Δx, Δy, 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 to the light beam's position, and L_x and L_y are the resistance lengths of the sensor. For the PDP90A sensor, L_x = L_y = 10 mm. Note that the resistance lengths are not the same as the active area. The active area is depicted by the gray region in the diagram above.

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 position-sensing detector. Position resolution (ΔR) is a function 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 resistance 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 0.750 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.

Click to Enlarge
PDP90A Packaging

Smart Pack

• Reduce Weight of Packaging Materials
• Increase Usage of Recyclable Packing Materials
• Improve Packing Integrity
• Decrease Shipping Costs

Thorlabs' Smart Pack Initiative is aimed at waste minimization while still maintaining adequate protection for our products. By eliminating any unnecessary packaging, implementing packaging design changes, and utilizing eco-friendly packaging materials for our customers when possible, this initiative seeks to improve the environmental impact of our product packaging. Products listed above are now shipped in re-engineered packaging that minimizes the weight and the use of non-recyclable materials.^b As we move through our product line, we will indicate re-engineered packages with our Smart Pack logo.

• Travel-based emissions reduction calculations are estimated based on the total weight reduction of packaging materials used for all of 2013’s product sales, traveling 1,000 miles on an airplane, to provide general understanding of the impact of packaging material reduction. Calculations were made using the EPA’s shipping emissions values for different modes of transport.
• Some Smart Pack products may show a negative weight reduction percentage as the substitution of greener packaging materials, such as the Greenwrap, at times slightly increases the weight of the product packaging.

Zoom

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 K- or T-Cube™
• Compatible with SM05 (0.535"-40) Lens Tubes
• 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. 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.

Controller
The PDQ80A and PDQ30C are compatible with the KPA101 controller, available separately below. The KPA101 K-Cube™ Beam Position Aligner is a part of Thorlabs' new and growing Kinesis^® line of high-end, compact motion controllers. It interfaces with our range of Quadrant and Lateral Effect Sensor Heads and can be used either to measure the position of the beam on the sensor (in Open-Loop mode) or to generate a signal that can be used as the input for an automated beam steering element (in Closed-Loop mode). When combined with one or more KPC101 K-Cube Piezo Controller and Strain Gauge Readers, this unit is ideal for such closed-loop beam-steering applications. See the full web presentation for more information.

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 Q1, Q2, Q3, and Q4 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 related 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, rather than the geometric center of the beam. For these types of beams, a lateral sensor might be better suited.

Typical Auto-Alignment Schematic

Example Auto-Alignment Setup

A basic auto-alignment schematic is shown to the right. It consists of a PDQ80A Photodiode Sensor, a KPA101 Beam Position Aligner, two KPC101 Piezo Controller and Strain Gauge Readers, a previous-generation ASM003 two-axis piezo mirror mount (alternative mounts can be found here), 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 the beam's power density.

Click to Enlarge
Example Auto-Alignment Setup

The experimental setup shown above was created based on the schematic to the right. An LDM635 red laser diode module emitting at 635 nm serves as the laser source. The light is incident on a previous-generation ASM003 turning mirror mount, which is mounted on an MBT616D flexure stage (center of photo). The turning mirror's X and Y motions are controlled using two KPC101 units (not shown). 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 (right of photo), which is controlled by the KPA101 Auto-Aligner (not shown).

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 wheel on our K-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

Click to Enlarge
PDP90A Packaging

Smart Pack

• Reduce Weight of Packaging Materials
• Increase Usage of Recyclable Packing Materials
• Improve Packing Integrity
• Decrease Shipping Costs

Thorlabs' Smart Pack Initiative is aimed at waste minimization while still maintaining adequate protection for our products. By eliminating any unnecessary packaging, implementing packaging design changes, and utilizing eco-friendly packaging materials for our customers when possible, this initiative seeks to improve the environmental impact of our product packaging. Products listed above are now shipped in re-engineered packaging that minimizes the weight and the use of non-recyclable materials.^b As we move through our product line, we will indicate re-engineered packages with our Smart Pack logo.

• Travel-based emissions reduction calculations are estimated based on the total weight reduction of packaging materials used for all of 2013’s product sales, traveling 1,000 miles on an airplane, to provide general understanding of the impact of packaging material reduction. Calculations were made using the EPA’s shipping emissions values for different modes of transport.
• Some Smart Pack products may show a negative weight reduction percentage as the substitution of greener packaging materials, such as the Greenwrap, at times slightly increases the weight of the product packaging.

Zoom

Click to Enlarge
Back and Top Views of the KPA101 K-Cube
(See the Pin Diagrams Tab for More Information)

Features

• Compact Footprint: 60.0 mm x 60.0 mm x 49.2 mm
• Auto Alignment of Beam to Center of Sensor when in Closed-Loop Mode
• Beam Position Measurement when in Open-Loop Mode
• OLED Menu and Position Display for Complete On-Unit Control
• Voltage Outputs for Sum, Difference, and Feedback Signals
• Full Kinesis^® or APT™ Software Control Suite (See Motion Control Software Tab for Details)
• Software Compatible with other Kinesis and APT Controllers for Integrated Systems Development
• Multi-Axis Expansion Using USB Controller Hubs (Sold Separately)
• Magnetic, Clip-On Optical Table Mounting Adapter Included

The KPA101 K-Cube™ Beam Position Aligner is a part of Thorlabs' new and growing Kinesis^® line of high-end, compact motion controllers. It interfaces with our range of Quadrant and Lateral Effect Sensor Heads and can be used either to measure the position of the beam on the sensor (in Open-Loop mode) or to generate a signal that can be used as the input for an automated beam steering element (in Closed-Loop mode). When combined with the KPC101 K-Cube Piezo Controller and Strain Gauge Reader, this unit is ideal for such closed-loop beam-steering applications. Please contact Tech Support for details on how the KPA101 can be used with sensors from other manufacturers.

Embedded software allows this unit to communicate with position sensing detectors (PSDs) either using the on-unit menu button, display, and control wheel or using external trigger signals. In addition to the on-unit controls, USB connectivity provides simple PC-controlled operation with two available software platforms: our new Kinesis software package or our legacy APT™ (Advanced Positioning Technology) software package.

The Kinesis Software features .NET controls which can be used by 3rd party developers working in the latest C#, C++, Matlab, Labwindows/CVI, Visual Basic, LabVIEW™, or any .NET compatible languages to create custom applications. Our legacy APT software allows the user to quickly set up complex move sequences with advanced controls made possible via the ActiveX^® programming environment. For example, all relevant operating parameters are set automatically by the software for Thorlabs stage and actuator products. For more details on both software packages, please see the Motion Control Software, Kinesis Tutorials, and APT Tutorials tabs.

The unit has a highly compact 60.0 mm x 60.0 mm x 49.2 mm footprint, allowing it to be positioned close to the system for added convenience when using the top panel controls. Tabletop operation also allows minimal drive cable lengths for easier cable management. Each unit contains a front-located power switch that, when turned off, saves all user-adjustable settings. Please note that this switch should always be used to power down the unit. For convenience, a 1.5 m long Type A to Type Micro B USB 3.0 cable is included with the KPA101 cube.

Operation
The KPA101 K-Cube can be controlled by the manual interface on the top of the unit or via a USB connection to a computer running our new Kinesis software package or our legacy APT (Advanced Positioning Technology) software package. Both interfaces allow the auto aligner to be operated in either an open- or closed-loop mode. In both modes, the K-Cube generates a left-minus-right X difference (X DIFF) signal, a top-minus-bottom Y (Y DIFF) difference signal, and a sum (SUM) signal that are displayed on the target screen.

When in open-loop mode, the X DIFF and Y DIFF difference signals shown on the target screen are not output to the SMA connectors on the back of the unit. Instead, the X DIFF and Y DIFF SMA outputs, which represent X OUT and Y OUT position demand signals, will be fixed at zero or the last closed-loop value. This mode allows for manual alignments on the detector. Once roughly aligned, the unit can be set to closed-loop mode to begin automated alignments of the system.

In the closed-loop mode, a DSP processor inside the KPA101 runs two independent feedback loops that output X DIFF and Y DIFF position demand signals. These signals will be present at the SMA connectors on the back of the unit for use as the inputs to the beam steering elements being used to center the beam on the detector. The proportional, integral, and differential (PID) constants can be adjusted to fine tune the response of the feedback loops to changes in the target position. Floating-point arithmetic allows for a greater range of PID coefficients to be used in the control loop, resulting in higher precision and lower noise (see Appendix B of the manual for details). This increases the range of actuators that can be stabilized using the auto aligner. Furthermore, the unit incorporates a notch filter that can be used to counteract the natural resonance of the specific mechanical system in which the unit is being used. This can stabilize actuators that are prone to mechanical resonances, such as piezo mirror gimbal mounts.

When the unit is controlled via the USB interface, the open-loop output of the unit is also exported digitally to the computer. Please note that stable closed-loop operation can only be achieved with continuous wave (CW) lasers. The KPA101 K-Cube is not recommended for use with pulsed lasers.

Optical Table Mounting Plate
Each unit comes with a mounting plate that clips onto the base of the module. The plate contains two magnets for temporary placement on an optical table and two counterbores for 1/4"-20 (M6) cap screws for a more permanent placement on the tabletop. Please see the Mounting Options tab for how to mount the plate.

Power Supply Options
The KPA101 K-Cube, which does not ship with a power supply, can be powered using either a TPS002 power supply or a KCH301 or KCH601 Controller Hub and Power Supply. The TPS002 power supply plugs into a standard wall outlet and provides +15 VDC, -15 VDC, and +5 VDC for up to two K-cubes or T-Cubes. In contrast, the KCH301 and KCH601 each consist of two parts: a hub that can support up to three (KCH301) or six (KCH601) standard footprint K-Cubes, and a power supply that plugs into a standard wall outlet and powers the hub and all connected cubes.

The USB Controller Hub's single USB connection provides USB connectivity to all the K-Cubes and T-Cubes connected to the hub. In addition, when the KPA101 is used in the closed-loop mode with beam-steering controllers such as the KPC101 Piezo Controller and Strain Gauge Reader, the hub is especially useful since it allows for direct communication between the cubes connected on it. As a result, the feedback signals generated by the KPA101 in the closed-loop mode can be sent directly to the KPC101 piezo controllers being used to direct the beam steering elements.

K-Cube Mounting Options

Two options are available to securely mount our K-Cube controllers onto an optical table. An optical table mounting plate, provided with every K-Cube, allows for a single controller to be attached to an optical table. Alternatively, three- and six-port USB controller hubs are offered (sold separately) that can mount and power our K-Cube controllers. These options are described in further detail below.

Optical Table Mounting Plate
Each K-Cube unit comes with a mounting plate that clips onto the base of the controller, as shown in the animation below. The plate contains two magnets for temporary placement on an optical table and two counterbores for 1/4"-20 (M6) cap screws for a more permanent placement on the tabletop. 

Kinesis USB Controller Hubs
Multiple units can be mounted and connected to a single PC by using the KCH301 or KCH601 USB Controller Hubs. They each consist of two parts: the hub, which can support up to three (KCH301) or six (KCH601) K-Cubes™ or T-Cubes™, and a power supply that plugs into a standard wall outlet. K-Cubes simply clip into place using the provided on-unit clips, while current- and previous-generation T-Cubes require the KAP101 Adapter Plate, shown in the animation above. The hub vastly reduces the number of USB and power cables required when operating multiple controllers.

K-Cube vs. T-Cube™ Feature Comparison
Feature	KPA101 K-Cube	TPA101 T-Cube
Kinesis Software Compatibility
APT Software Compatibility
Kinesis USB Controller Hubs Compatibility		Requires KAP101 Adapter
TCH002 T-Cube USB Controller Hubs Compatibility	N/A
OLED Position Display		N/A
Power Switch		N/A
Bidirectional SMA Trigger Porta	2	N/A
SMA SUM, X Diff, and Y Diffa
Computer Connectiona	USB 3.0 Micro B (USB 2.0 Compliant)	USB 2.0 Micro B (USB 2.0 Compliant)
Included Mounting Plate
Size (L x W x H)	60.0 mm x 60.0 mm x 49.2 mm (2.36" x 2.36" x 1.94")	60.0 mm x 60.0 mm x 49.2 mm (2.36" x 2.36" x 1.94")
On-Unit Menu
Open Loop Mode
Closed Loop Mode
Monitor Mode
Automatic Loop Switching		N/A
Screen Brightness		N/A

• Please see the Pin Diagrams tab for details.

Introducing Thorlabs' Kinesis^® Motion Controllers

A major upgrade to the former-generation T-Cubes™, the growing K-Cube™ line of high-end controllers provides increased versatility not only through the new Kinesis software, but through an overhaul and updating of their physical design and firmware.

Every K-Cube controller includes a digital display. In addition to basic input and output readouts, the KPA101 OLED display hosts a number of menu options that include monitor, closed loop, open loop, and automatic closed/open loop switching. The on-unit wheel and menu button are used to scroll through the available options. Each unit contains a front-located power switch that, when turned off, saves all user-adjustable settings as well as two bidirectional SMA trigger ports that accept or output a 5 V TTL logic signal.

Please see the table to the right for a full comparison of the features offered by our new KPA101 K-Cube and previous-generation TPA101 T-Cube PSD controllers.

Click to Enlarge
KPA101 K-Cube Kinesis PSD Controller

Kinesis USB Controller Hubs
Complementing our K-Cubes are our Kinesis USB 2.0 controller hubs. With two versions available for three or six  K- or T-Cubes, these USB hubs are designed specifically for communication between multiple controllers and the host control PC. These hubs are backward compatible with our T-Cubes. 

K-Cubes simply clip into place using the provided on-unit clips, while current- and previous-generation T-Cubes require the KAP101 Adapter Plate, shown in the animation to the below right. The hub vastly reduces the number of USB and power cables required when operating multiple controllers.