beam profilers (original) (raw)

Definition: devices for measuring the intensity profile of a laser beam

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Contents

What are Beam Profilers?

A beam profiler (beam analyzer, mode profiler) is a diagnostic device for laser beam characterization which can measure the whole optical intensity profile of a laser beam, i.e., not only the beam radius but also the detailed shape.

Beam profilers are used in various ways; a qualitative impression of a beam profile can help during alignment of a laser, whereas measurements of the beam radius at different locations along the beam axis (the caustic) allow one to calculate the _M_2 factor or the beam parameter product, quantitatively characterizing the beam quality.

intensity profile of a Gaussian beam and a multimode beam

Figure 1: Intensity profiles of a Gaussian beam (left) and a multimode laser beam (right). The latter exhibits more complicated variations of the intensity. Such multimode beams can be generated in lasers where the fundamental resonator modes are substantially smaller than the pumped region in the gain medium.

Beam quality monitoring with appropriate laser beam diagnostics can be important for many laser applications such as laser material processing; the quality of drilled holes, e.g., may be achieved more consistently if the beam quality is monitored.

Camera-based Beam Profilers

Many beam profilers are based on some type of digital camera. For the visible and near-infrared spectral region, CMOS and CCD cameras are the most common. CMOS devices are less expensive, but CCDs generally have a better linearity and lower noise. A resolution (given by the pixel size) of the order of 5 μm is possible with both CCDs and CMOS cameras, so that the beam radius may be as small as 50 μm or even less. The active area may have dimensions up to several millimeters, so that very large beams can be handled.

camera-based laser beam profiler

Figure 2: A laser beam profiler for _M_2 measurements, consisting of a CCD camera mounted on a motorized translation stage. The photograph has been kindly provided by Gentec Electro-Optics.

Different wavelength regions require different sensor types. Silicon-based sensors are a good choice for wavelengths in the visible and near-infrared spectral region up to roughly 1 or 1.1 μm, whereas InGaAs-based detectors can be used up to ≈ 1.7 μm. For still longer wavelengths, e.g. for the beam characterization of CO2 lasers, pyroelectric and microbolometer infrared cameras are suitable. These are fairly expensive. Their comparatively low responsivity may not be a disadvantage, given the high output power of such lasers. For ultraviolet lasers, CCD and CMOS arrays can be used in conjunction with UV conversion plates, converting the radiation to longer wavelengths which do not damage the arrays.

The spatial resolution of a camera sensor is an important quantity. With silicon sensors, pixel sizes well below 10 μm are possible, allowing to measure beam diameters down to the order of 50 μm. InGaAs detectors have substantially larger pixels with a width of e.g. 30 μm, whereas pyroelectric arrays don't get far below 100 μm. The consequence of a low spatial resolution is that the beam sizes have to be kept large, which also leads to a long Rayleigh length. For that reason, more space is required for a complete _M_2 measurement. The number of pixels is also of practical importance; a larger number allows one to measure beam diameters in a larger range.

When used with narrow-linewidth laser radiation, camera-based systems are particularly sensitive to artifacts caused by the high temporal coherence. A careful optical design (without windows, causing parasitic reflections) is required for suppressing such artifacts and/or for eliminating their effects on the measured data.

Most cameras are very light sensitive — often much more than required. The laser beam then has to be attenuated (see below) before hitting the camera. Some imaging optics (e.g. beam expanders or beam reducers for expanding the range of allowed beam radii) may also be used, so that the camera records a beam profile as it occurs at some other location (the imaged plane). This also allows good shielding against ambient light. However, the optics should not, of course, introduce excessive optical aberrations.

The recorded beam profile may be displayed on a computer screen, possibly together with measured parameters such as beam radius, beam position, ellipticity, and statistical information, or Gaussian fits. The software may allow to select between different methods of determining the beam radius, such as the D4σ method or a simple ($1/e^2$) criterion; see the article on beam radius for details on such criteria.

Scanning Beam Profilers Based on Slits, Knife Edges, or Pinholes

There are also beam profilers which scan a beam profile with one or several pinholes, with a slit, or with a knife edge. In any case, some structured mechanical part (which is often fixed on a rotating part) is quickly moved through the beam, while the transmitted power is recorded with a photodetector and some electronics. A computer (a PC or a built-in microprocessor) is used to reconstruct the beam profile from the measured data and to display it on a screen. For example, the transmitted power versus position of a knife edge may essentially be differentiated to obtain a one-dimensional intensity profile of the beam, whereas a moving slit directly provides the intensity profile.

scanning slit profiler

Figure 3: A scanning slit beam profiler. The PC screen displays the obtained scans in two directions as well as the reconstructed beam profile. The photograph has been kindly provided by Ophir-Spiricon.

The spatial resolution of scanning systems can be as high as a few micrometers, or even close to a single micrometer (particularly for scanning pinholes or slits), suitable for the characterization of small-diameter beams. An important advantage of the concept of scanning is that the photodetector used does not need to have a spatial resolution, so that detectors for very different wavelength regions can easily be used. Also, it is easier to obtain a large dynamic range, compared with, e.g., a camera. The powers which can be handled may range from microwatts to watts. Beam attenuation before the detector is easily achieved because the required optical quality is much lower than that for a camera system.

Scanning beam profilers, particularly those based on a slit or a knife edge, are most suitable for beam profiles which are not too far from Gaussian because the recorded signal is usually integrated in one spatial direction, so that the reconstruction of complicated (more structured) beam shapes is not perfect.

Some scanning beam profilers can also be used for pulsed laser beams such as those from Q-switched lasers. However, this works only for high enough pulse repetition rates; note that the minimum repetition rate may depend on the beam diameter.

Important Issues to Observe

Various requirements need to be assessed when selecting a beam profiler for a particular application:

Beam Attenuation

In many cases – particularly for camera-based systems –, it is necessary first to attenuate the power of a laser beam before sending it into the beam profiler. Some systems use an optical attenuator (e.g. a wedged neutral density filter) in transmission; the weak reflection e.g. from a high-quality glass plate may also be utilized.

Although attenuation may appear as a trivial task, inappropriate methods can cause a number of problems. Some examples are:

Aspects of convenience may also matter. For example, it is helpful if the electronics can automatically adjust the required attenuation factor.

Frequently Asked Questions

What is a beam profiler?

A beam profiler, also called a beam analyzer, is a diagnostic device that measures the complete optical intensity profile of a laser beam, including its shape and radius.

What are the main types of laser beam profilers?

The two main types are camera-based profilers, which use a CCD or CMOS sensor to directly capture the 2D beam image, and scanning profilers, which reconstruct the profile by moving a slit, pinhole, or knife-edge through the beam.

For what laser wavelengths are beam profilers available?

Different sensor materials are used for different wavelengths. Silicon-based sensors are for visible and near-infrared light (up to ~1.1 μm), InGaAs sensors for up to ~1.7 μm, and pyroelectric or microbolometer cameras for longer infrared wavelengths.

Why is it often necessary to attenuate a laser beam before measurement?

Many beam profilers, especially camera-based types, are very light-sensitive. Attenuation is required to reduce the laser power to a level that prevents saturation or damage to the detector.

What are the risks of using improper beam attenuation?

Excessive optical power can damage a camera sensor. Improper attenuation can distort the measurement. For example, poor quality optics, thermal lensing in absorbing filters, or interference effects can degrade the beam profile before it is measured.

How is a beam profiler used to measure beam quality?

By measuring the beam radius at multiple positions along the beam's propagation axis (the 'caustic'), a beam profiler provides the data needed to calculate the ($M^2$) factor, a quantitative measure of beam quality.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 39 suppliers for beam profilers. Among them:

CNI Laser

⚙ hardware

beam profilers

CNI offers various beam profilers, based on different wavelength range and spot size:

It is widely used in scientific research and industrial fields.

Edmund Optics

⚙ hardware

beam profilers

Edmund Optics offers Coherent® Lasercam™ beam profilers as well as an own series of beam profilers, designed to measure a wide range of laser beam sizes, providing information to optimize laser system operation. These laser beam profilers feature high resolution and large area sensors to ensure accurate profiling of both small and large laser beams.

Gentec Electro-Optics

⚙ hardware

beam profilers

When it comes to characterizing a laser beam in the UV to near-IR range, a beam profiler is the go-to solution.

Thanks to its unique combination of high pixel density and large sensor size, the BEAMAGE series of laser beam diagnostics instruments has the double advantage of accurately characterising both very small beams of only a few tens of microns to larger beams of several millimeters in width, thus effectively covering most applications in one package.

The simple software is highly intuitive and yet includes many great functions, useful both for basic or advanced users. Furthermore, the calculations done by the software are ISO-compliant and give the user the most accurate beam characterization available, all this in a lightweight environment that doesn’t take days to fully master.

Femto Easy

⚙ hardware

beam profilers

Femto Easy offers various types of BeamPro beam profilers with different specifications:

They all come with a powerful and user-friendly software.

Kokyo

⚙ hardware

beam profilers

Kokyo's beam profiler (developd by ourselves) supports your laser beam measurements:

Kokyo offers different packages:

The software (LaseView) also offers a free trial during 7 days.

Bibliography

[1] ISO Standard 11554, “Optics and photonics — Lasers and laser-related equipment — Test methods for laser beam power, energy and temporal characteristics”

(Suggest additional literature!)

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