1 Measurement of beam pattern)

1.1 What is beam profiler?

Beam profiler is a device that measures the beam diameter and spatial intensity distribution of a laser beam. They are characteristics of a laser beam, which means the laser beam behavior.

For example, the condensing characteristics of a laser with the same intensity and beam diameter are changed by the different spatial intensity distribution, which can’t obtain the same behavior. Moreover, the strictly designed laser resonator is also difficult to accurately predict the beam characteristics obtained due to the surrounding environment such as manufacturing errors of the optical elements and the temperature.
Therefore, measurement of the beam characteristics is important in an application using a laser beam, so a beam profiler is available for the measurement.

There are mainly 2 measuring methods: camera-based type and scanning-based type (Figure 1).
The camera-based beam profiler measures the entire beam at one time with a 2D optical sensor, so it can measure the beam pattern efficiently. In addition, it can measure continuous-wave lasers and pulsed lasers.

The scanning-based beam profiler sequentially measures the laser beam intensity using a single photodetector. The profiler is more reasonable than the camera-based one, and it can measure the pattern of continuous-wave lasers. Both methods can measure the NFP (near field pattern) and F.F.P. (far field pattern) of the beam.
We will describe the definition of the beam diameter and each beam profile measuring method here.

(Figure: Advantages & Disadvantages of Camera-based beam profiler and Scanning-based beam profiler )

Advantages Disadvantages
Camera-based beam profiler
・CCD camera
・Short measuring time
・Available for measuring pulsed lasers
・Available for distinguishing complex beam patterns (flat-top or doughnut))
・Expensive
・Unavailable for measuring small beam diameter
*Kokyo’s beam profilers can easily measure small beam diameter at a reasonable price
・Unavailable for measuring large beam diameter
*Kokyo’s beam profilers can easily measure large beam diameter at a reasonable price )
Scanning-based beam profiler
・Pinhole
・Slit
・Knife-edge
・Easy to measure small beam diameter ・Long measuring time
・Unavailable for measuring pulsed lasers
・Unavailable for distinguishing complex beam patterns

1.2 Definition of beam diameter

The laser beam has a light intensity distribution in a plane orthogonal to the propagation direction. Beam light has no clear boundary, so its beam diameter is necessary to be defined. There are several definitions for beam diameter; the most common definition is 1/e2 as the maximum light intensity. It is described by a Gaussian beam, a centrosymmetric intensity distribution. The intensity distribution of a Gaussian beam with light intensity is expressed by the following equation.

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Figure1(a) shows a profile and an intensity profile of a Gaussian beam represented by the above equation. The beam diameter is W0. In addition to this definition, the full width at half maximum (full-width half-maximum, FWHM) is also commonly used.

The above definition cannot be easily applied to an asymmetric beam pattern. For a beam with a complex pattern, a range including 86.5% of the total light intensity (Fig. 1 (b)) [1] is defined as a beam diameter. The ISO standards define the level at which the value of the second moment of intensity distribution is 1/e2 as the beam diameter. More complex beam patterns and difficult-to-define beam diameter follow the standard.

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Table 1 Definition of the beam diameter)
 (a) Gaussian beam 
(b) Beam with complex patterns)

 

1.2.1. Details of beam diameter

Click here for beam diameter

1.3 Beam pattern measurement with a camera-based beam profiler

1.3.1 CCD camera-based beam profiler

CCD camera-based beam profiler measures the light intensity of the whole beam with a 2D optical sensor at once. This allows the profiler to measure more efficiently than a scanning-based beam profiler: it analyzes the obtained data for measurement of beam diameter, beam profiler, and M2. [2]

To detect beam light, a semiconductor device called a charged coupled device (CCD) is available. It arranges the optical sensor in a two-dimensional array. When the light is irradiated onto the CCD, its intensity distribution is recorded pixel by pixel. The obtained intensity information is fetched in a computer as digital data and then reconstructed as a two-dimensional or three-dimensional intensity distribution image via software. (Fig. 2)The spatial resolution is determined by the pixel size of the CCD. The typical pixel size is approximately 10 mm, so a beam diameter from 100 μm to 10 μm is measurable.
As the reading speed of the charge stored in the CCD is faster, the measurement time can be faster. However, it is limited by the monitor frame rate (30 ~ 60 fps) and the data transfer rate from the CCD to the PC.

A semiconductor laser can’t directly measure a beam profile of 1.3 μm or 1.5 μm used in optical communication because the typical CCD detects a wavelength of about 1.1 μm. For the out-of-band wavelength, an image converter for converting infrared light into visible light and a pyroelectric image sensor is used.

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Figure 2 CCD camera-based beam profiler 

1.4 Beam pattern measurement with a scanning-based beam profiler

The scanning-based beam profiler has the following characteristics: it measures the intensity of a beam light passing through a shield such as a pinhole or a slit in a plane orthogonal to the optical axis. Measuring the light intensity while scanning the shield along with its cross-section measures the spatial intensity distribution of the beam light. It takes more time than the camera-based one but allows a convenient measurement. In addition to that, attenuation of the laser beam is not necessarily required. With a photodetector that matches the wavelengths for measurement, any wavelength beams can be measured. We describe the details of each method below.

1.4.1 Pinhole Profiler

The pinhole profiler obtains the intensity distribution of the entire beam by two-dimensionally scanning the pinhole in a plane orthogonal to the optical axis of the laser beam (Fig 3). Reducing the pinhole diameter can easily enhance the spatial resolution. A pinhole having a size of about 1% of the beam diameter is generally used. The profiler can measure both the intensity distribution of an arbitrary beam shape and complex beam patterns, which is available for the measurement of continuous-wave lasers without time fluctuation.

 


Fig 3 Pinhole Profiler

1.4.2 Slit Profilers

Like the pinhole profiler, the slit profiler measures the spatial intensity distribution by scanning a slit with a rectangular hole in the beam profile to measure the intensity of passing light through the slit (Fig. 4). One‐dimensional intensity distribution along the scanning direction can be obtained. Scanning from multiple directions can acquire an intensity distribution of two or more dimensions. The profiler is suitable for the measurement of beams with axially symmetric intensity distribution. Narrowing the slit width can enhance the spatial resolution: 0.2μm spatial resolution can measure about 20 μm of the beam diameter.

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Fig 4 Slit Profiler

1.4.3 Knife-edge Profilers

The knife-edge profiler uses a knife or a razor blade as an obstruction and measures the intensity of the light passed through the knife in a plane perpendicular to the optical axis (Fig. 5A). Measuring the intensity while moving the knife along the plane obtains the intensity distribution. Unlike the slit profiler, the light intensity increases or decreases by moving the knife (Fig. 5B). Then, the following relationship between the one-dimensional spatial intensity distribution i (x) and the measured light intensity i ′ (x) is established.
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Dividing the light intensity by displacements of the knife can measure the spatial intensity distribution. The profiler can measure higher spatial resolution (< 0.1 μm), and about 10 μm of the beam diameter.
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Fig 5 (a) Knife-edge profiler,   (b) Relationship between intensity in measurement and intensity distribution

1.5 Measurement of NFP (Near Field Pattern) and FFP (Far Field Pattern)

Measurement of a beam profile near an output end (NFP: near field pattern) and a beam profile sufficiently far from the output end (FFP: far field pattern) is necessary to understand the characteristics of beams from photoelectric devices such as semiconductor lasers and light fibers. We describe how camera-based beam profilers (CCD method) measure these beam profiles. For measuring the micron-level spatial intensity distribution (NFP: Near Field Patterns) at the output end, optical zoom systems like a microscope are available (Fig. 6). An objective lens enhances the output end, while a relay lens forms an image on a CCD. It must form an accurate image for a very small beam. That can be easily solved by a beam splitter, which divides the beam into two and introduces one beam into a CCD for monitoring, positioning it for forming images with a wider field of view.

The measurement of the diffusion angle of semiconductor lasers and the numerical aperture (NA) of light fibers is possible by the FFP measurement. For the measurement, an optical system using an f-θ lens that converts the incidence angle to the focal position is available (Fig. 7).

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Fig 6 Optical system for NFP measurement
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Fig. 7 F.F.P. measurement optical system

2 Measurement of Beam Quality

2.1 What is M2 Beam Quality Measurement System

The beam quality measurement system is a device for measuring laser beam quality. The condensing characteristics of laser beams change depending on the characteristics of the output beam. This is an important parameter in the laser processing field where a certain quality is required. The condensing characteristics of the beam are defined as M2, K, and BPP (beam parameter product) as indicators of the beam quality. A condensed laser beam by a lens determines its theoretical minimum spot diameter by the diffraction limit. However, the disturbing beam intensity profile limits the beam to the diffraction limit. M2 indicates how many times the condensed beam diameter becomes than that of the diffraction limit. If M2 is 1, the beam theoretically obtains the minimum condensed spot. The K is the reciprocal of M2. BPP is expressed by the product of the beam waist radius and the beam divergence angle like M2. BPP mainly indicates the beam quality of semiconductor lasers.

2.2 Definition of M2

Beam with a strong directivity and near-straight propagation such as a laser beam increases the beam diameter gradually as it propagates. Fig 8 shows the propagating beam near the beam waist. Near the beam waist, the beam radii wx, y (z) in the x-axis and y-axis directions are given by the following equation [3].

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w0x, 0y, z0x, 0y, θ0x, 0y represent the beam waist radius, beam waist position, and beam divergence in the x-axis and y-axis. M2x and y are introduced as parameters for determining the beam quality in each axis and have the following properties.

  • M2 must be greater than or equal to 1.
  • In M2 ≡ 1, the beam is a single-mode Gaussian beam.
  • The value of M2 indicates how many times the condensed beam diameter becomes than that of the diffraction limit.)

M2 is defined as follows using the parameters of the laser beam.

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λ shows the wavelengh of laser light.
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Fig 8 Near the beam waist of Centrosymmetric Gaussian beam

2.3 Measurement of M2

As shown above, M2 can be determined by measuring the beam waist radius and the beam divergence. These parameters are obtained by focusing the beam to measure the beam diameter at several points along its optical axis (Fig 9). Measurement at more points is desirable for the reduction of errors. More accurately beam waist position is obtained by taking many measurement points, especially near the beam waist. The M2 of main laser beams is summarized in Table 2 [4].

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Fig 9 Measurement of M2

Table Major Laser Beam Quality

Light sources M2
He-Ne Laser < 1.1
Ion Laser 1.1 ~ 1.3
Semiconductor Laser
(Collimated light, TEM00)
1.1 ~ 1.7
High Power Multi-Mode Laser 3 ~ 4
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