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.

Overview

Laser beam profiling instruments measure the following quantities:

Beam width: There are over five definitions of beam width.
Beam quality: Quantified by the beam quality parameter, M².
Beam divergence: This is a measure of the spreading of the beam with distance.
Beam profile: A beam profile is the 2D intensity plot of a beam at a given location along the beam path. A Gaussian or flat-top profile is often desired. The beam profile indicates nuisance high-order spatial modes in a laser cavity as well as hot spots in the beam.
Beam astigmatism: The beam is astigmatic when the vertical and horizontal parts of the beam focus in different locations along the beam path.
Beam wander or jitter: The amount that the centroid or peak value of the beam profile moves with time.

Instruments and techniques were developed to obtain the beam characteristics listed above. These include:

Camera techniques: These include the direct illumination of a camera sensor. The maximum spot size that will fit onto a CCD sensor is on the order of 10 mm. Alternatively, illuminating a flat diffuse surface with the laser and imaging the light onto a CCD with a lens allows profiling of larger-diameter beams. Viewing lasers off diffuse surfaces is excellent for large beam widths but requires a diffuse surface that has uniform reflectivity over the illuminated surface.
Knife-edge technique: A spinning blade or slit cuts the laser beam before detection by a power meter. The power meter measures the intensity as a function of time. By taking the integrated intensity profiles in a number of cuts, the original beam profile can be reconstructed using algorithms developed for tomography. This usually does not work for pulsed lasers, and does not provide a true 2D beam profile, but it does have excellent resolution, in some cases <1 μm.
Phase-front technique: The beam is passed through a 2D array of tiny lenses in a Shack–Hartmann wavefront sensor. Each lens will redirect its portion of the beam, and from the position of the deflected beamlet, the phase of the original beam can be reconstructed.
Historical techniques: These include the use of photographic plates and burn plates. For example, high-power carbon dioxide lasers were profiled by observing slow burns into acrylate blocks.

, commercial knife-edge measurement systems cost $5,000–$12,000 USD and CCD beam profilers cost $4,000–9,000 USD. The cost of CCD beam profilers has come down in recent years, primarily driven by lower silicon CCD sensor costs, and they can be found for less than $1000 USD.

Applications

The applications of laser beam profiling include:

Laser cutting: A laser with an elliptical beam profile has a wider cut along one direction than along the other. The width of the beam influences the edges of the cut. A narrower beam width yields high fluence and ionizes, rather than melts, the machined part. Ionized edges are cleaner and have less knurling than melted edges.
Nonlinear optics: Frequency conversion efficiency in nonlinear optical materials is proportional to the square of the input light intensity. Therefore, to get efficient frequency conversion the input beam waist must be small, and located within the nonlinear material. A beam profiler can help make a waist of the right size at the right location.
Alignment: Beam profilers align beams with orders of magnitude better angular accuracy than irises.
Laser monitoring: It is often necessary to monitor the laser output to see whether the beam profile changes after long hours of operation. Maintaining a particular beam shape is critical for adaptive optics, nonlinear optics, and laser-to-fiber delivery. In addition, laser status can be measured by imaging the emitters of a pump diode laser bar and counting the number of emitters that have failed or by placing several beam profilers at various points along a laser amplifier chain.
Laser and laser amplifier development: Thermal relaxation in pulse-pumped amplifiers causes temporal and spatial variations in the gain crystal, effectively distorting the beam profile of the amplified light. A beam profiler placed at the output of the amplifier yields a wealth of information about transient thermal effects in the crystal. By adjusting the pump current to the amplifier and tuning the input power level, the output beam profile can be optimized in real-time.
Far-field measurement: It is important to know the beam profile of a laser for laser radar or free-space optical communications at long distances, the so-called "far-field". The width of the beam in its far-field determines the amount of energy collected by a communications receiver and the amount of energy incident on the ladar's target. Measuring the far-field beam profile directly is often impossible in a laboratory because of the long path length required. A lens, on the other hand, transforms the beam so that the far-field occurs near its focus. A beam profiler placed near the focus of the lens measures the far-field beam profile in significantly less benchtop space.
Education: Beam profilers can be used for student laboratories to verify diffraction theories and test the Fraunhofer or Fresnel diffraction integral approximations. Other student laboratory ideas include using a beam profiler to measure Poisson's spot of an opaque disk and to map out the Airy disk diffraction pattern of a clear disk.
Measurements

Beam width

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

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 due to thermal lensing in the laser gain medium.
The M² parameter is determined experimentally as follows:

Measure the D4σ widths at 5 axial positions near the beam waist.
Measure the D4σ widths at 5 axial positions at least one Rayleigh length away from the waist.
Fit the 10 measured data points to, where is the second moment of the distribution in the x or y direction, and is the location of the beam waist with second moment width of. Fitting the 10 data points yields M²,, and. Siegman showed that all beam profiles—Gaussian, flat top, TEM_XY, or any shape—must follow the equation above provided that the beam radius uses the D4σ definition of the beam width. Using the 10/90 knife-edge, the D86, or the FWHM widths does not work.
Complete E-field beam profiling

Beam profilers measure the intensity, |E-field|², of the laser beam profile but do not yield any information about the phase of the E-field. To completely characterize the E-field at a given plane, both the phase and amplitude profiles must be known. The real and imaginary parts of the electric field can be characterized using two CCD beam profilers that sample the beam at two separate propagation planes, with the application of a phase recovery algorithm to the captured data. The benefit of completely characterizing the E-field in one plane is that the E-field profile can be computed for any other plane with diffraction theory.

Power-in-the-bucket or Strehl definition of beam quality

The M² parameter is not the whole story in specifying beam quality. A low M² only implies that the second moment of the beam profile expands slowly. Nevertheless, two beams with the same M² may not have the same fraction of delivered power in a given area. Power-in-the-bucket and Strehl ratio are two attempts to define beam quality as a function of how much power is delivered to a given area. Unfortunately, there is no standard bucket size or bucket shape and there is no standard beam to compare for the Strehl ratio. Therefore, these definitions must always be specified before a number is given and it presents much difficulty when trying to compare lasers. There is also no simple conversion between M², power-in-the-bucket, and Strehl ratio. The Strehl ratio, for example, has been defined as the ratio of the peak focal intensities in the aberrated and ideal point spread functions. In other cases, it has been defined as the ratio between the peak intensity of an image divided by the peak intensity of a diffraction-limited image with the same total flux. Since there are many ways power-in-the-bucket and Strehl ratio have been defined in the literature, the recommendation is to stick with the ISO-standard M² definition for the beam quality parameter and be aware that a Strehl ratio of 0.8, for example, does not mean anything unless the Strehl ratio is accompanied by a definition.