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How to choose the right laser beam profiler model given my beam size for laser beam profiling? Beam size is often taken at 1/e2 for a Gaussian beam. It is important to understand that a 1mm beam size for instance, does not mean that there is 0% energy outside a circle of 1mm of diameter. The tail of the beam, although of small intensity, is necessary in order to compute accurate ISO standard measurements on the beam.
Minimum beam size measurable: The accuracy of the size measurement depends on the number of illuminated pixels. 15 pixels will give very high accuracy. Under 12 pixels, the accuracy is acceptable. It is not recommended to profile a laser beam with less than 10 illuminated pixels. Therefore, the minimum measurable beam size of a laser beam profiler is pixel_ pitch (µm) x ~10. For example: the CinCam CMOS 1201 laser beam profiler has a pixel pitch of 5.2µm. Therefore, the minimum beam size recommended is >52µm. The smallest pixel pitch available is 2.2µm (see CinCam CMOS 1204 laser beam profiler and CinCam CMOS PICO laser beam profiler)
Maximum beam size measurable: The active area of the laser beam profiler sensor will define the maximum beam size measurable. A rule of thumb is to take ~75% of the length in one direction. For example: The CinCam CMOS Nano 1.001 laser beam profiler has an active area of 11.3 x 11.3 mm. Therefore, the maximum beam size measurable is ~8.5mm.
The largest CMOS sensor is the CinCam CMOS Nano 1.001 laser beam profiler with 11.3 x 11.3 mm active area. The largest CCD sensor is the CinCam CCD 3501 laser beam profiler with 36 x 24 mm active area.
Is my laser continuous-wave or pulsed, and if so, what is its repetition rate? Some sensors have a rolling shutter which means all the pixels are not read at the same time but rather in a row-by-row fashion. For CW lasers, a rolling or global shutter is suitable. However for pulsed lasers with a repetition rate <1kHz, a global shutter is necessary. Pulsed lasers with a repetition rate >1kHz or >>1kHz, a global shutter or rolling shutter is suitable as such frequency will be ‘seen’ as CW by the laser beam profiler. In other words, the sensor will not see the difference.
What is my beam power and what OD or attenuation do I need? Whether using an absorptive or reflective type ND filter, laser beam profilers allows the maximum peak power of~1W. For power greater than 1W, a attenuation unit can be added directly to the laser beam profiler (works on all CMOS, CCD and InGaAs models thanks to a large spectral range of 190 nm to 2000 nm). The attenuator is based on two uncoated fused silica wedges and is designed for pre-attenuation of high intensity laser beams. The principle is based on the polarization effect by reflection on an optical surface. The s-pol. and p-pol. parts of the laser beam have different reflection factors. Orthogonal arrangement of the wedges compensates the polarization effect and allows neutral attenuation of the laser beam
You can use the prism attenuator up to intensities of 2GW/cm2 for pulse wave and 25kW/cm2 for continuous wave. It is possible to combine with neutral density filters for final power adjustment to the beam profiler sensitivity level. The high performance optical design in compact housing allows precise beam attenuation.
3 models are available: 5W, 100W or 200W.
In laser science, the parameter M²,M^2, also known as the beam quality factor, represents the degree of variation of a beam from an ideal Gaussian beam. It is calculated from the ratio of the beam parameter product (BPP) of the beam to that of a Gaussian beam with the same wavelength. It relates the beam divergence of a laser beam to the minimum focussed spot size that can be achieved. For a single mode TEM00 (Gaussian) laser beam, M²,M^2, is exactly one.
The M², M^2, value for a laser beam is widely used in the laser industry as a specification, and its method of measurement is regulated as an ISO Standard (11146-1 and 11146-2).
M² is useful because it reflects how well a collimated laser beam can be focused to a small spot, or how well a divergent laser source can be collimated. It is a better guide to beam quality than Gaussian appearance because there are many cases in which a beam can look Gaussian, yet have an M² value far from unity. Likewise, a beam intensity profile can appear very “un-Gaussian”, yet have an M² value close to unity.
The value of M² is determined by measuring D4σ or “second moment” width. Unlike the beam parameter product, M² is unitless and does not vary with wavelength.
The quality of a beam is important for many applications. In fiber-optic communications beams with an M² close to 1 are required for coupling to single-mode optical fiber.
M²,,M2, determines how tightly a collimated beam of a given diameter can be focused: the diameter of the focal spot varies as M², M2, and the irradiance scales as 1/M4. For a given laser cavity, the output beam diameter (collimated or focused) scales as M, and the irradiance as 1/M². This is very important in laser machining and laser welding, which depend on high fluence at the weld location.
Generally, M², M2, increases as a laser’s output power increases. It is difficult to obtain excellent beam quality and high average power at the same time due to thermal lensing in the laser gain medium.
Siegman’s proposal became popular because of its simplicity, but experimentally it isn’t so straightforward, and some uncertainties arise from these principles. For example, if you want to measure the waist radius in the lab, how can you be sure that your measurement device is positioned exactly at the focus?
And how far do you need to go to be in the far field to measure the divergence? Are these two data points enough? The folks at the International Organization for Standardization, or ISO, decided to put an end to all this confusion, so they wrote a norm explaining how to measure and calculate M2, M2, properly: ISO 11146.
The ISO norm explains a method to calculate M², M2, from a set of beam diameter measurements, in a way that minimizes sources of error. Here are the main steps:
The ISO norm also states a few extra rules about the measurement of diameters (especially when using array sensors such as CCD or CMOS sensors):