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Figure 1.1  PDP90A Sensor with Two SM05L10 Lens Tubes
and One SM05D5 Iris

Features

• Lateral Position-Sensing Detector 
  • Wavelength Range: 320 - 1100 nm
  • Insensitive to Beam Shape and Power Density
• Segmented-Quadrant Position-Sensing Detector 
  • Available Wavelength Ranges: 400 - 1050 nm or 1000 - 1700 nm
  • Ideal for Automatic Alignment Applications
• Compatible with SM05 (0.535"-40) Lens Tubes
• Compact Footprint: 2.00" x 1.20" x 0.65" (50.8 mm x 30.5 mm x 16.5 mm)
• KPA101 Position-Sensing Detector Auto Aligner Available Separately Below

Thorlabs offers Position-Sensing Detectors (PSDs) that provide precise position measurements of incident beams. The PDP90A detector is a lateral effect PSD and utilizes a silicon photodiode-based tetra lateral sensor to accurately measure the displacement of an incident beam relative to the calibrated center. The detector is sensitive to wavelengths from 320 - 1100 nm and is ideal for measuring the movement of a beam, the distance traveled, or as feedback for alignment systems. The PDQ80A and PDQ30C detectors are segmented-quadrant PSDs and are sensitive to light in the 400 - 1050 nm or 1000 - 1700 nm range, respectively. Their detectors are comprised of four quadrant-shaped photodiodes, and the position of an incident beam is measured by comparing the intensity in each quadrant. These detectors are highly accurate and are ideal for auto-alignment applications. Please see the Operation tab for more information.

These PSDs have a compact design measuring 2.00" x 1.20" x 0.65" (50.8 mm x 30.5 mm x 16.5 mm) and feature internal SM05 (0.535"-40) threads, allowing for easy integration into SM05 lens tube systems. The detectors are also packaged with an 8-32 to M4 adapter to provide compatibility with either imperial or metric mounting posts.

All of the PSDs on this page are compatible with the KPA101 controller, available separately below. The KPA101 K-Cube^® Beam Position Aligner is a part of Thorlabs' 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.

2D Lateral Effect Position-Sensing Detectors

Overview

Figure 4.1  Lateral Effect Sensor Diagram

The 2D lateral effect sensor provides 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: 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, such as the PDP90A, 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 in Figure 4.1. 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 Figure 4.1.

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

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.

Segmented-Quadrant Position Sensing Detectors

Figure 4.2  Segmented-Quadrant PSD Diagram

The segmented-quadrant position sensors consist 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 Figure 4.2). 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. Therefore, 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 because these detectors are sensitive to both parameters. A beam that does not have a Gaussian intensity 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.

PDP90A Pin Diagrams

Photodiode Detail

Female 6-Pin Hirose Connector, Detector Out

PDQ80A and PDQ30C Pin Diagrams

Photodiode Detail

Female 6-Pin Hirose Connector, Detector Out

Figure 6.2  Typical Auto-Alignment Schematic

Example Auto-Alignment Setup

A basic auto-alignment schematic is shown in Figure 6.2. 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.

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Figure 6.1  Example Auto-Alignment Setup

The experimental setup shown in Figure 6.1 was created based on Figure 6.2. 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 Figure 6.1). 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 Figure 6.1), which is controlled by the KPA101 Auto-Aligner (not shown).

When using a Quadrant Detector, choosing an appropriate alignment laser, such as the KLS635 or KLS1550, 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(/M) is a versatile solution suitable for most applications.

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

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

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Figure 8.2  PDP90A Packaging

In support of Thorlabs' company values, we are taking steps to implement more sustainable product packaging, including:

• Reducing the Weight of Packaging Materials
• Increasing the Usage of Recyclable Packing Materials

Thorlabs' sustainability 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. Figure 8.2 demonstrates the use of recycled paper in place of plastic bubble wrap in this product line. For more information on our efforts toward a greener future, please visit our Sustainability page.