Features

• Complete, Preassembled, and Pre-Aligned System
• Typical 5% Accuracy for Beam Quality Measurements
• Measures M², Divergence, Waist Diameter, Rayleigh Range, and Astigmatism
• Compatible with CW and Quasi-CW Pulsed Laser Sources with Repetition Rate ≥ 300 kHz
• Compliant with ISO11146 International Standards for Beam Quality Measurements
• Complete, Ready-to-Use System with Compact and Flexible Design
• Live Data Readout via TCP/IP to a DataSocket Server to Allow Easy Access by Other Programs

Thorlabs’ M2SET M² Meter is designed for automated M² beam quality analysis. This system allows the measurement of the M² (times diffraction limit) factor of a laser beam and its reciprocal beam quality, K=1/M², together with the divergence, waist diameter, Rayleigh range, and astigmatism of the beam.

The M² Meter is available as a complete, ready-to-use system, preassembled to meet the requirements of most general applications. The system consists of a scanning slit beam profiler* fixed on a 150 mm long translation stage, two mounted Ø1" adjustable mirrors used to direct the laser beam into the beam profiler, an iris diaphragm to aid with beam alignment, and a lens mounted in a flip mount for easy insertion and removal of the lens from the beam path. All of these components are mounted on a 24” x 6” (600 x 150 mm) breadboard. If you would like to purchase an M² Meter with a different length translation stage, please contact Tech Support for information and pricing. Each component of the M²-Meter system is also available separately. An additional scanning slit beam profiler can be purchased to extend the wavelength range of your M² Meter.

The height and horizontal position of the iris diaphram and focusing lens, as well as the position of the second mirror along the beam path, are all pre-aligned to the center of the entrance aperture of the scanning slit beam profiler. Alignment of an input laser beam can be achieved entirely through adjustments of the two Ø1" mirrors. A manual containg a detailed description of this procedure can be viewed by clicking on the red document icon next the part numbers below.

If you already own one of Thorlabs' scanning slit beam profilers or require a different travel range, you can construct an M² Meter by ordering the BP1M2 Extension Set and the appropriate optical components. The BP1M2 Extension set consists of a translation stage and mounting accessories. Additional components to build the optical setup, such as focusing lenses, holders, and beam alignment mirrors, are listed on the Configurations tab and must be purchased separately. See the Tutorial tab for guidance in selecting the correct focal length of the focusing lens and the correct length of the translation stage.

Thorlabs' Beam software package is included with both the M2SET-BP209 M² analysis system or the BP1M2 extensions sets (see the Software tab to download a copy).

Functionality
To truly characterize a laser beam, more than a single profile must be measured along the propagation axis of the focused beam. According to the ISO11146 standard, at least five beam measurements in the linear divergence region and another five measurements in the waist region need to be taken. The M2SET system can easily make all the required measurements.

During operation, the profiler automatically moves step-wise along the beam path and analyzes the beam at various positions. The beam quality is determined by fitting a curve to the beam diameters measured as a function of profiler position. To perform the tests, the laser beam must be aligned with the translation path of the stage and focused within a certain divergence angle. The pre-mounted and pre-aligned system makes this process much simpler for the user.

After the measurement of a focused beam is complete, the focusing lens can be flipped out of the beam so that the parameters of the original unfocused laser beam can be measured. By combining both results, the complete parameter set for the unfocused beam is derived, including the beam waist diameter, waist position, and Rayleigh range.

The supplied Thorlabs Beam software (see the Software tab) has an easy-to-operate Graphical User Interface (GUI) that fully supports automated M² measurements as well as basic beam profiling measurements. The same software package is used to operate our scanning slit beam profilers when they are not integrated into an M² measurement system. Reports can be stored to a file or transmitted to another application, such as LabView, via DataSocket.

Thorlabs also offers a CCD beam profiler and extension set for M² measurements using the CCD profiler.

*Note: The knife-edge mode of the BP209 beam profilers cannot be used for M² measurements.

• For an explanation of how the lens focal length and translation stage length restrict the measurable beam diamters in the M² meter, see the Tutorial tab.
• Can be extended by using a longer post in the mirror periscope assembly.

Click to Enlarge

Select your operating wavelength on the x-axis and go up to the curve representing waist diameter. Then read out the maximum acceptable initial beam diameter at the vertical axis. For a waist diameter at the measurement limit (20 µm, red curve) an operating wavelength of 633 nm yields a maximum beam diameter of 8.1 mm. Choosing a somewhat higher waist diameter of (e.g. 30 µm, green curve) is recommended in order to relax the demands on the Beam Profiler. This will allow it to operate at a higher measurement speed and achieve better accuracy at the expense of the maximum allowed input beam diameter, which drops down to 5.4 mm at 633 nm. Further limitations on the input beam diameters are imposed by the 20 mm usable lens diameter and the 9 mm Beam Profiler aperture.

Determining the Focal Length of the Focusing Lens and the Travel Length of the Stage

What is M²
The M² value is the ratio of a laser beam’s divergence to the divergence of a diffraction-limited beam of the same waist size. It is used as an industry standard for beam quality¹. Simply put, it can be thought of as a measure of how “purely” TEM₀₀ a beam is. An M² value of 1, for example, is a pure TEM₀₀ beam. Anything higher indicates that the beam is not strictly a TEM₀₀ beam (by definition, M² >1). Because of this mode mixing, a laser will have a waist size that is larger than if it's beam were a pure TEM₀₀ mode. The M² value shows how much “larger” a beam is for a fixed divergence if it is mixed with non-TEM₀₀ modes. Not only will mixing modes cause the minimum waist size to increase, but it will also change the beam diameter (d(z)) as the beam propagates, which effects the intensity gradient of the beam. For applications sensitive to mode matching or intensity gradients (e.g., frequency doubling or trap depths for laser cooling and trapping, respectively), the M² value is of significant importance.

M² measurements require a focused beam with a beam waist exceeding 20 µm. The following guidelines aid in selecting the optimal focal length of the collimating lens and the optimal length for the translation stage for the BP1M2 Series Beam Analysis Extension Kit. The M2SET Series is a complete beam analysis system including focusing lens and stage with parameters that meet the requirements of typical M² beam analysis applications.

Divergent Beams
For divergent beams, a hyperbolic curve fit to the measured data yields reliable and repeatable values for beam waist diameter and position, Rayleigh range, divergence angle, beam pointing direction, waist asymmetry, divergence asymmetry, and astigmatism.

Nearly Parallel Beams
For nearly parallel beams, the divergence angle can be accurately determined by a linear curve fit.

Selecting Focal Length
Diagram A, to the upper right, shows the optimal focal length (f) which depends on the operating wavelength (λ) and the unfocused beam diameter. (See Theory Section below for details.)

To select the appropriate focal length, find your operating wavelength on the X axis of Diagram A and the curve representing your initial (unfocused) beam diameter (1mm to 10mm). Read out the minimal required focal length along the vertical axis. Round it up to the next available focal length (e.g. 50, 100, 150, 200, 300 mm). Lenses with longer focal lengths will generate waist diameters >30µm and relax the measurement requirements for the beam profiler. Refer to the Accessories tab for recommended focusing lenses. Be sure to order the appropriate AR-coating for your wavelength range of interest.

Selecting the Length of the Translation Stage
It is advisable, but not necessary, to make the translation stage length the same as the focal length of the focusing lens. For correct M² detection, the translation range has to be at least five times the Rayleigh length of the focused beam to cover both the beam waist and the neighboring divergent beam propagation. The Rayleigh length depends strongly on the generated waist diameter and also linearly increases with M². Therefore, the translation range needs to be longer for bad beam quality (K << 1, M² >> 1). Diagram B shows the minimal required stage length for M² = 1 depending on the expected waist diameter.

Select your operating wavelength at the X axis of the Diagram B and go up to the curve representing your expected waist diameter. Then read out the minimal required translation range at the vertical axis. Multiply this result with the highest expected M² value of your laser source and round it up to the next available translation stage length (50, 150, or 300 mm). Please note this length is a minimum requirement for M² detection.

If a lens with the optimal focal length is selected according to Diagram A, the resulting waist diameter of 30 µm keeps the required translation range below 50 mm, even for M² values up to 5. If using the short 50 mm stage to carry the Beam Profiler, it needs to be carefully positioned with respect to the beam waist position for every application. Therefore, longer stage lengths of 150 mm or even 300 mm are advisable for universal setups so that they are ready to use for a variety of laser types, beam diameters, wavelengths, M² values, and focal lengths. Also, longer stages allow the entire diverging process on both sides of the beam waist to be analyzed without relocating a the stage to the new focus position.

For these reasons, Thorlabs recommends a translation stage length at least as long as the highest focal length that will be used in the setup. Besides this minimum requirement, more flexibility is added to the setup by selecting a longer stage.

Theory Section

Focal Length
The generated beam waist diameter d₀ must not decrease below the minimal measurable beam diameter (20 µm) of the Beam Profiler. To allow higher measurement speeds, extend it to d_0,min = 30 µm (20 µm diameter requires 1 rev/s for 2 % error).
The beam waist diameter d₀ is:

To fulfill this requirement for each wavelength and the highest focusability (M²=1) the divergence angle θ must not exceed a maximum value θ_max.

Depending on initial beam size d_init, a minimal focal length f can be calculated.

Refer to Diagram A for quick selection help.

Translation Stage Length
For optimal M² detection, the translation range should be at least 5 times the Rayleigh length of the focused beam to cover both the beam waist and the neighboring divergent beam propagation. Minimal translation stage length is:

¹Baida Lu, Bin Zhang, and Shirong Luo, Appl. Opt. 38, 4581 (1999)

Suggested Accessories for Beam Manipulation

Depending on existing lab equipment, additional parts may be required to focus and align the laser beam for M² measurements. As the BP1M2 Beam Quality Analysis Extension Sets do not include any optics, beam alignment and redirection tools to align your laser beam to the M² setup may need to be purchased separately. Some suggestions are listed in the tables below.

M2SET Series Complete M² Beam Analysis System Shipping List

BP1M2 Series M² Beam Analysis Extension Set Shipping List

Software Packages for Thorlabs' Beam Profilers:

There are two complete software packages available for download below.

Software

Version 5.0

Standard full version of software package for 32 bit and 64 bit Windows with driver and graphical user interface for operating the device in standard applications.

OR

Advanced Beta Version*

Version 5.0.561.1304 (April 2, 2013)

Full software version software application with latest bug fixes:

• Calculation cluster in the Thorlabs Beam Library Wrapper adopted to the latest calculation result structure.

*This release candidate software version, which incorporates new features and/or bug fixes as outlined in the change log file on the download page, has been through several rounds of testing and there are currently no significant bugs. Once full testing and documentation is available, this version will become the standard software package. In the interim, Thorlabs is making this complete software file available for those customers whose research endeavors would benefit from the updated features.