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Shack-Hartmann Wavefront Sensors, 1.3 Megapixel Resolution
Includes Post-Mounting Adapter
A WFS150-5C Mounted
The Shack-Hartmann sensor consists of a lenslet array and a camera. When a wavefront enters the lenslet array, a spotfield is created on the camera; each spot is then analyzed for intensity and location. Using this method, Shack-Hartmann wavefront sensors can dynamically measure the wavefronts of laser sources or characterize the wavefront distortion caused by optical components. In addition, they can provide real-time feedback for adaptive optics systems and are included for this reason in Thorlabs' Adaptive Optics Kits. For more details on the theory of Shack-Hartmann wavefront sensing, see the SH Tutorial tab above.
Thorlabs' High-Resolution Shack-Hartmann Wavefront Sensors, which incorporate CCD cameras with 1.3 megapixel resolution, provide accurate measurements of the wavefront shape and intensity distribution of beams. These wavefront sensors are available with either a chrome-masked microlens array for use in the 300 - 1100 nm range or an AR-coated microlens array for use in the 400 - 900 nm range. The former has a lenslet pitch of 150 µm whereas the latter is available with a lenslet pitch of either 150 or 300 µm. These three offerings allow the end user to select a system that offers high spatial resolution, enhanced contrast, or high wavefront accuracy. Please note that calibration of the microlens-camera pair is required; to purchase a new lenslet array for a previously purchased Shack-Hartmann Wavefront Sensor, please contact Technical Support for a quotation on the microlens array and calibration service.
If your application would benefit from a fast wavefront sensor, please see our line of Shack-Hartmann wavefront sensors with frame rates up to 1120 Hz. For more information about choosing the appropriate Shack-Hartmann wavefront sensor for a particular application, see the Selection Guide tab above.
Shack-Hartmann Kits with Two Microlens Arrays
Adaptive Optics Kits
Microlens Array Specifications
How a Shack-Hartmann Wavefront Sensor Works
A Shack-Hartmann wavefront sensor uses a lenslet array to divide an incoming wavefront into an array of smaller beams. Each beam is focused onto a CMOS camera that is placed at the focal plane of the lenslet array, as shown in the figure to the left. If a uniform, planar wavefront is incident on the Shack-Hartmann sensor, each lenslet forms a spot along the optical axis of the lenslet. This yields a regularly spaced grid of spots on the detector.
A distorted wavefront, however, will cause some lenslets to focus with the spots displaced from the optical axis. Therefore, the light imaged on the sensor will consist of some regularly spaced spots mixed with displaced spots and missing spots. This information can be used to calculate the shape of the wavefront that was incident on the microlens array. Shack- Hartmann type wavefront sensors can be used to characterize the performance of optical systems. In addition, they are increasingly used in applications where real-time monitoring of the wavefront is used to control an adaptive optic with the intent of removing the wavefront distortion before creating an image.
Wavefront Distortion and Spot Displacement
As discussed above, each microlens of the lenslet array collects the light falling onto its aperture and generates a single spot at the detector plane. The figure below is a detail of a wavefront incident on a single microlens. The spot positions will be directly behind the lenses (shown in green) only if the incident wavefront is flat and parallel to the plane of the lenslets. For a wavefront which is distorted in the region of the microlens, the spot positions will be deviated in the X and Y direction (as shown by the red dot) so that every spot lies away from the optical axis z of its associated microlens by an angle θ. This angle θ is the same as the angle between the distorted wavefront and the planar wavefront, as shown in the figure.
Parameters Affecting Shack-Hartmann Performance
Four parameters that influence the performance of a Shack-Hartmann wavefront sensor are the number of lenslets that cover the detector active area, the dynamic range, the measurement sensitivity, and the lenslet focal length. The number of lenslets restricts the maximum number of Zernike coefficients that a reconstruction algorithm can reliably calculate. When selecting the number of lenslets required, consider the amount of distortion being modeled (i.e., how many Zernike coefficients are needed to effectively represent the true wave aberration).
Sensitivity (αmin) is a function of the minimum detectable spot displacement (δymin), as described by the equation:
αmin = δymin / f
where f is the focal length of the microlens. Dynamic range, θmax, is a measure of the maximum extent of phase that can be measured:
αmax = δymax / f = (d / 2) / f
where d is the diameter of the microlens. Both of these equations were derived using the small angle approximation. αmin is the minimum detectable wavefront slope that can be measured by the wavefront sensor. The minimum detectable spot displacement δymin depends on the pixel size of the detector, the accuracy of the centroid algorithm, and the signal to noise ratio of the sensor. αmax is the maximum wavefront slope that can be measured by the wavefront sensor and corresponds to a spot displacement of δymax, which is equal to the lenslet radius.
A Shack-Hartmann sensor's measurement accuracy (i.e., the minimum wavefront slope that can be measured reliably) depends on its ability to precisely measure the displacement of a focused spot with respect to a reference position. A conventional algorithm will fail to determine the correct centroid of a spot if it partially overlaps another spot or if the focal spot of a lenslet falls outside of the area of the sensor assigned to detect it (spot crossover). Special algorithms can be implemented to overcome these problems, but the limit the dynamic range of the sensor. The dynamic range of a system can be increased by using a lenslet with either a larger diameter or a shorter focal length. Increasing the dynamic range by increasing the lenslet diameter decreases the number of Zernike coefficients available to represent the wavefront. Conversely, increasing the dynamic range by shortening the focal length decreases the sensor's sensitivity. Ideally, a lenslet with the longest focal length that meets both the dynamic range and measurement sensitivity requirements should be used.
The Shack-Hartmann wavefront sensor is capable of providing information about the intensity profile as well as the calculated wavefront.
Selecting a Shack-Hartmann Wavefront Sensor
Thorlabs offers two different cameras for a variety of wavefront sensing applications. The wavefront sensors on this page feature a high-speed CMOS camera capable of reaching frame rates up to 1120 Hz (microlens array dependent). Thorlabs also offers a line of Shack-Hartmann wavefront sensors with a CCD camera. Each camera type is available with one of three microlens arrays offering flexibility in wavelength range, spatial resolution, spot contrast, and wavefront accuracy.
CCD sensors exhibit lower noise and higher image uniformity, but provide much slower frame rates compared to CMOS-based wavefront sensors. The high frame rate of the CMOS detector enables more wavefront measurements per second and thus can detect faster wavefront fluctuations, an important feature for sensors used in high-speed adaptive optics systems.
The CCD-based wavefront sensors have a measurement speed of 15 fps that is independent of the spot count (i.e. independent of the microlens array pitch). By contrast, the measurement speed of the wavefront sensors with CMOS cameras will decrease as the spot count increases. The plots below provide a comparison of performance of the CMOS sensor when used with the 150 µm and 300 µm pitch microlens arrays.
The WFS20 CMOS-based wavefront sensors support three different measurement modes. In the normal measurement mode, the entire spotfield image is transmitted to the PC. This mode can also be used with 2X binning, reducing the amount of data that needs to be transfered to the PC and increasing the measurement speed. In this mode, the spot location is still reported based on the real camera pixel array. Alternatively, the sensor can be used in a "High-Speed Mode" where the wavefront calculations are carried out in the control box and only the centroid locations are transmitted to the PC. This greatly decreases the amount of data that needs to be tranferred to the PC and provides the fastest measurement speeds. However, since the spotfield image is not transferred to the PC, it is harder to detect certain errors, such as those caused by camera saturation.
Choosing a Microlens Array
Each Shack-Hartmann Wavefront Sensor is available with 3 different microlens arrays. The table to the lower right details the features of the microlens included with each item.
MLA150M-5C Microlens Features
This microlens array includes a chrome mask that prevents light from passing between the microlenses. This leads to a higher contrast in the spot field but will considerably increase the amount of back reflections. This microlens array can be used over an extended wavelength range of 300 nm to 1100 nm. The array features a 150 µm lens pitch, which offers a larger number of spots and thus a higher spatial resolution of the wavefront, and a wider wavefront dynamic range because of their shorter focal length.
MLA150M-7AR and MLA300M-14AR Microlens Features
Both of these microlens arrays are AR coated for the 400 nm to 900 nm wavelength range, making them ideal for applications that are sensitive to back reflections. The MLA150M-7AR microlens array has a 150 µm lens pitch, which offers a larger number of spots and thus a higher spatial resolution of the wavefront, and a wider wavefront dynamic range because of their shorter focal length. The MLA300M-14AR has a 300 µm lens pitch that supports higher wavefront accuracy and sensitivity at the expense of dynamic range and spatial resolution.
Click on the Software button to the right for the latest version of Thorlabs' Shack-Hartmann Wavefront Sensor Software Package. The download includes the software package with a graphical user interface for operating the WFS in standard applications and support for developers who want to extend or adapt the functionality of the device to their special requirements.
*When used with USB 3.0, the WFS150 and WFS300 wavefront sensors operate with reduced speed and increased minimum exposure times.
**This is the Advanced Beta Version, also known as a Release Candidate, of Thorlabs' Wavefront Sensor Software. It is provided as is for users who would benefit from the additional functionality. This software version has undergone preliminary bug testing. If it passes further tests without issue, it will eventually be released as the next official version of the wavefront sensor software.
GUI Display of Measured Wavefront
Software and Graphical User Interface
The software includes a driver package for constructing custom applications with the following software packages:
The WFS kits combine the base camera unit and two interchangeable microlens arrays. The chart below contains the properties of the lens arrays included with each kit, for more details on the lens and camera properties please see the Specs tab above.
How to Interchange Microlens Arrays
The microlens arrays are mounted with a precision patented magnetic holder. They can be easily interchanged and returned to the same position using the pickup tool that is included with the kit, as shown in the photo to the right.
The CAB-DCU-T2 cable is used to replace the USB to Micro Sub-D cable included with the 1.3 megapixel Shack-Hartmann sensors in applications where a trigger is required. For specifications for the trigger input, please see the Specs tab above.
Customers who purchased either a WFS150 or WFS150C wavefront sensor of our superseded earlier generation with fixed lens arrays can upgrade these SH sensors to one of the current WFS150-5C, WFS150-7AR, or WFS300-14AR models. If you order this upgrade service, your old WFS sensor must be sent back to Thorlabs. Please contact your local Tech Support Team for instructions, your choice for the updated model, and other details.
This animation shows how to convert between the right- and left-handed orientations for the KM100WFS. A similar process can be followed for the KM200PM(/M).
Click to Enlarge
The KM200PM mount platform's larger size allows for flexible mounting options.
Thorlabs' KM100WFS and KM200PM(/M) Kinematic Platform Mounts provide kinematic control for our CCD-Based and High-Speed CMOS-Based Shack-Hartmann Wavefront Sensors. Please note that the optical axis of the sensors will not be centered over the optical post. The animation to the right shows how to convert between the left- and right-handed orientations for the KM100WFS. This process is very similar for the KM200PM(/M)
The KM100WFS can accommodate the WFS150 wavefront sensors, and features a similar design to our KM100PM platform. Two M3 through holes are also provided for mounting a WFS150 CCD-based wavefront sensor. Each CCD-based wavefront sensor is shipped with an adapter plate for post mounting; two lengths of M3 screws are included with the KM100WFS so that the wavefront sensor can be mounted with or without the adapter plate attached. The DCU CCD Cameras and the WFS300 and WFS20 sensors can also be mounted on the KM100WFS.
The KM200PM(/M) mount is larger than the KM100WFS(/M), and can mount the WFS150 sensors using one of the #8 (M4) through holes near the front edge of the plate. The larger profile and number of holes provides a variety of mounting options. The platform is secured to the front plate using two 4-40 screws with a 3/32" hex [the KM200PM/M uses M2.5 screws with a 2 mm (5/64") hex]. The two pieces of the mounting platform assembly are held together with two 3-48 screws [5/64" (2 mm) hex]. If desired, the mounting plate can also be removed and replaced with a user-supplied alternative, enabling custom mounting options.