What is a Laser Beam Profiler?
A laser beam profiler captures, displays, and records the spatial intensity profile of a laser beam at a particular plane transverse to the beam propagation path. Since there are many types of lasers — ultraviolet, visible, infrared, continuous wave, pulsed, high-power, low-power — there is an assortment of instrumentation for measuring laser beam profiles. No single laser beam profiler can handle every power level, pulse duration, repetition rate, wavelength, and beam size.
How Does a Laser Profiler Work?
Laser profilers are laser displacement sensors that collect height data across a laser line rather than a single point. This enables 2D/3D measurements such as height difference, width, or angle to be performed using a single sensor. In addition to height data, sensors also collect intensity data to provide a stable solution for inline measurement and inspection. The lineup includes a wide range of sensors to support a variety of applications and industries.
Applications of Laser Profiler
- Laser cutting:
- Nonlinear optics
- Laser monitoring
- Laser and laser amplifier development
Laser Beam Profiler Method
1. Knife-edge Scanning
In this method, the beam is aimed at a sensor behind a spinning drum with several open sections that allow the laser to pass through. Each opening has a precise knife-edge that “cuts” through the laser. Sensors detect exactly what portion of the beam reaches past the knife-edge each time to determine shape, width and intensity. Knife-edge scanners are typically very accurate and can stand up to high intensities.
2. Camera-based Measurements
Digital camera-based profilers are extremely easy to use and interface effortlessly with software. They capture two-dimensional laser readings as pixels, making them able to detect the beam’s complete structure and intensity for analysis. Cameras can handle both pulsed and continuous-wave lasers equally well. The only downside of most models is the pixel limitations that make small micron readings difficult.
3. Scanning Slit and Pinhole Profilers
Though technically two different systems, these measurement methods operate similarly: by allowing a small amount of light through a tiny aperture and capturing one-dimensional profiles of the laser. The advantage of these methods is their precision; even submicron measurements are no problem. The difficulty that arises, however, is attempting to work out two-dimensional images. This requires extensive repositioning with extreme care, and this can take a lot of time.
The beam width is the single most important characteristic of a laser beam profile. At least five definitions of beam width are in common use: D4σ, 10/90 or 20/80 knife-edge, 1/e2, FWHM, and D86. The D4σ beam width is the ISO standard definition and the measurement of the M2 beam quality parameter requires the measurement of the D4σ widths. The other definitions provide complementary information to the D4σ and are used in different circumstances. The choice of definition can have a large effect on the beam width number obtained, and it is important to use the correct method for any given application. The D4σ and knife-edge widths are sensitive to background noise on the detector, while the 1/e2 and FWHM widths are not. The fraction of total beam power encompassed by the beam width depends on which definition is used.
Beam quality parameter, M2
The M2 parameter is a measure of beam quality; a low M2 value indicates good beam quality and ability to be focused to a tight spot. The value M is equal to the ratio of the beam's angle of divergence to that of a Gaussian beam with the same D4σ waist width. Since the Gaussian beam diverges more slowly than any other beam shape, the M2 parameter is always greater than or equal to one. Other definitions of beam quality have been used in the past, but the one using second moment widths is most commonly accepted.
Beam quality is important in many applications. In fiber-optic communications beams with an M2 close to 1 are required for coupling to single-mode optical fiber. Laser machine shops care a lot about the M2 parameter of their lasers because the beams will focus to an area that is M4 times larger than that of a Gaussian beam with the same wavelength and D4σ waist width before focusing; in other words, the fluence scales as 1/M4. The rule of thumb is that M2 increases as the laser power increases. It is difficult to obtain excellent beam quality and high average power (100 W to kWs) due to thermal lensing in the laser gain medium.