Laser beam fine focus analyzer

 

 

 

 

 

 

 

 

 

 

 

 

Darmin Catak – s011480

 

David Bue Pedersen – s031950

Preface

 

 

First and foremost we would like to acknowledge The Department of Manufacturing Engineering, the Technical University of Denmark, and CALM without whom this project would not be possible.

 

CALM is an abbreviation of Centre for applied laser micro manufacture, and the experiments within this project have been conducted in laboratories provided by CALM and IPL (Department of Manufacturing Engineering) at DTU (Technical University of Denmark). The laser used for the experiments is a fiber laser also provided by the Institute of Mechanical Engineering and Manufacturing, built by NKT research, also a CALM partner.

 

As well as providing the laser and laboratory, they have provided us with the necessary finances and all other equipment that has been needed to conduct the experiments and build our device.

 

Also we would like to thank Mr. Peter Carøe Nielsen, Mr. Jacob Nielsen and professor Mr. Fleming O. Olsen for their supervision and patience during the creation and completion of this project.

 

We would also like to thank Peter, Jakob and Flemming for giving us the opportunity to undertake this interesting project, and for providing us with the necessary literature and materials needed to understand and grasp the task that was given to us.

 

 

 

Denmark, Kgs. Lyngby, January 31 2007

 

 

 

______________________ ____________________________

Darmin Catak – s011480 David Bue Pedersen – s031950

Abstract

 

A method for monitoring and analyzing a laser beam is being researched. In this report one will be presented with the challenging task and the considerations that must be taken in order to monitor and construct a tool for analyzing the laser beam from an industrial laser.

 

Based on a known method presented in the 1983 by Lim G.C. and Steen W.M, Dept. of Metallurgy, Imperial College, London SW7, GB, it will be shown that it is possible to monitor a laser beam with a high resolution using a relatively simple approach.

 

Though this method back in 1983 was proven to provide inline feedback in the form of a beam intensity profile, the method was at the stage of discovery suffering from the lack of tools for strong numerical data processing and data logging.

 

Today these tools are readily available at an affordable price. An improved analyzing method based on the method of 1983 will be examined through a series of experiments. These will serve as a reference in the final development of a laser beam analyzer.

 

By using off-the-shelf electronic components a simple, yet advanced control system as well as driver software for the beam analyzer will be created which will aid in collecting analytical data through experiments. This will result in much better and more accurate datasets, than what is possible to achieve by collecting analytical data manually.

Through extensive mathematical analysis using numerical mathematical tools there will be developed a script based on existing source provided by Mr. Peter Carøe Nielsen. The script will be written in MatlabÒ by MathworksÒ and will have the ability to automatically process data from the analyzer during experiments.

 

This data processing procedure gives a good picture of how the laser beam propagates. These data then serve as an aid in the monitoring and assessing the quality of the laser beam.

 

Furthermore a mechanical structure will be created. It will contain all the subsystems the analyzer is made of. It will provide the ability to measure the laser beam from different angles around the path of propagation, and it will be designed in a manner so that it can be used to measure most industrial or experimental lasers.

 

The laser beam analyzer will be designed with ease-of–use in mind so that it can be operated after a short introduction. It will be able to be connected to any personal computer, and it will be able to execute measurement series unassisted by a human operator, thereby eliminating a possible source of error.

 

Finally the construction of the laser beam analyzer will be expected to be much cheaper in comparison to the purchase of most existing laser beam analyzers on the market today.

 

Table of contents

Preface........................................................................................................................... 1

Abstract.......................................................................................................................... 3

1. Background................................................................................................................ 7

2. Project definition......................................................................................................... 8

3. Main objectives.......................................................................................................... 8

4. Theory...................................................................................................................... 10

4.1 The beam analyzer explained............................................................................ 10

4.2 An introduction to laser optics............................................................................ 11

5. The Laser platform................................................................................................... 14

5.1 The 20 W fiber laser........................................................................................... 14

5.2 The Optical system............................................................................................ 15

5.3 The NC system.................................................................................................. 15

6. Initial experiments:................................................................................................... 16

6.1 Experimental setup............................................................................................ 17

6.3 Initial experimental considerations:.................................................................... 17

6.4 Resolution parameters:...................................................................................... 17

6.5 Theoretical minimal rotational speed.................................................................. 20

6.6 Equipment used................................................................................................. 22

6.6.1 The DC motor:............................................................................................ 22

6.6.2 The reflective rod and fitting........................................................................ 22

6.6.3 The detector................................................................................................ 22

6.6.4 The beam dump.......................................................................................... 24

6.6.5 The vision system....................................................................................... 24

6.6.6 The oscilloscope:........................................................................................ 24

6.7 Other equipment used during the initial experiments......................................... 25

6.8 Experimental approach...................................................................................... 26

6.9 Experimental procedures................................................................................... 26

6.10 Commonly routines and commonly used parameters:.................................... 28

6.11 Experimental overview:.................................................................................... 29

6.12 Experiment 1:................................................................................................... 30

6.13 Experiment 2:................................................................................................... 31

6.14 Experiment 3:................................................................................................... 31

6.15 Experiment 4:................................................................................................... 32

6.16 Data post processing....................................................................................... 34

6.16.1 Needed data.............................................................................................. 34

6.16.2 Determining the Gaussian........................................................................ 34

6.16.3 Calculating the beam propagation path..................................................... 37

6.17 Presentation of data from the initial experiments............................................. 39

6.17.1 Experiment 1............................................................................................. 39

6.17.2 Experiment 2............................................................................................. 41

6.17.3 Experiment 3............................................................................................. 45

6.18.4 Experiment 4............................................................................................. 46

6.18 Conclusion of the initial experiments................................................................ 47

7 Designing the beam analyzer................................................................................... 49

7.1 The first design concepts:................................................................................. 50

7.2 The final design:................................................................................................. 53

7.2.1 The turntable and horizontal plate............................................................... 53

7.2.2 The ball bearing unit.................................................................................... 54

7.2.3 The back plate............................................................................................ 56

7.2.4 Reflector rod holder.................................................................................... 56

7.2.5 The reflector rod......................................................................................... 58

7.2.6 The inverted T-frame skeleton................................................................... 59

7.2.7 Spindle and linear motion rail....................................................................... 59

7.2.8 Strength of the turntable plane.................................................................... 60

7.3 Electronic components and mechatronics........................................................ 63

7.3.1 The power supply....................................................................................... 63

7.3.2 Stepper motor............................................................................................. 63

7.3.3 Stepper motor controller............................................................................. 65

7.3.4 Brushless motor and controller................................................................... 68

7.3.5 Beam analyzer controller software:............................................................ 70

7.3.6 REV counting mechanism........................................................................... 73

8 The finished laser beam analyzer............................................................................. 75

9 Economic overview................................................................................................... 77

10 Validating the laser beam analyzer......................................................................... 77

10.1 Measurement set 1.......................................................................................... 77

10.1.1 Measurement data 6A:.............................................................................. 78

10.1.2 Measurement data at 5.5A........................................................................ 80

10.1.3 Measurement data at 5A........................................................................... 82

10.2      Conclusion of the validation experiments.................................................... 84

10.3 Hunting the distortion problem......................................................................... 85

11 Error identification................................................................................................... 87

11.1 Human errors................................................................................................... 87

11.2 Reflector rod.................................................................................................... 87

11.3 Rotational speed.............................................................................................. 87

11.4 Cladding mode................................................................................................. 87

12 Conclusion.............................................................................................................. 89

13 Future work............................................................................................................. 90

14 References............................................................................................................. 91

15 Appendices............................................................................................................. 92

15.1 The documentation CD-ROM.......................................................................... 92

15.2 Matlab script..................................................................................................... 92

15.2.1 Gauss_fit.m............................................................................................... 92

15.2.2 beam_propagation.m................................................................................ 95

15.2.3 Support functions for the matlab script..................................................... 95

15.3 Stepper motor controller schematics............................................................... 97

15.4 Stepper motor controller firmware code.......................................................... 98

15.5 Stepper motor data sheet.............................................................................. 102

15.6 Detector datasheet........................................................................................ 103

15.7 SKF 61819 ball bearing datasheet................................................................. 104

15.8 Parts and costs.............................................................................................. 105

15.9 Technical drawings........................................................................................ 107

15.9.1 Turntable overview................................................................................. 107

15.9.3 Back plate............................................................................................... 108

15.9.3 Turntable plate........................................................................................ 109

15.9.4 Reflector rod fitting.................................................................................. 110

15.9.5 Turntable................................................................................................. 111

15.9.6 Detector holder....................................................................................... 112

15.10 Optocoupler holder.................................................................................. 114

15.9 Tolerance chart.............................................................................................. 115

 

1. Background

 

 

In 1983 an article describing a laser beam analyzing method was published^[1]. The force of the method seemed to be the simplicity of the experimental setup and the ability of monitoring the intensity profile and spot size of a high power laser directly in the focus of the laser beam.

 

 

When using high power laser beams for industrial or research purposes it is very important that the process is monitored either in-process or offline.^[2]

 

Depending on the laser process, different characteristics of the beam itself need to be maintained. For example in laser welding or cutting, it is needed that the beam penetration is very narrow and deep. To make sure this is possible a finely formed beam with large depth of focus is needed.^[3] This is just one of the processes that requires monitoring, to make sure that the quality of the process, in this case the weld or cut, is kept. In other processes different characteristics are needed, but the monitoring is still very important if the quality is to be insured.

 

General perception of the laser beam is that its profile always stays constant, and that the laser system will always produce perfectly coherent results. However this is not true. Also written in the 1983-article is mentioned that it has been proven that if this consistency is to be kept then frequent analyzing of the beam is needed.

 

The basic principles behind the beam analyzing method and the optics involved will be further explained throughout the report.

2. Project definition

 

            The project forming the foundation for this rapport has the purpose of developing a beam analyzer that can measure the beam propagation of an industrial or experimental fiber laser.

 

The laser beam analyzer must be based on the principles described in the article from 1983 ^[1]. The final goal is to be able to describe a series of beam parameters such as parameters that describe the beam propagation path, and the intensity distribution over the beam.

 

            To construct the measuring equipment following work must be undertaken:

 

-      The underlying theory must be understood. This theory will form the knowledge base of the math and physics that will be used throughout the project.

 

-      Replication of the experimental setup from 1983 is needed for a proof-of-concept, and these initial analytical data will be used to define the further design requirements for the beam analyzer.

 

-      A data processing method will be developed. This will consist of the development of a numerical MatlabÒ script that will be used to process data from the beam analyzer.

 

-      A prototyping phase will be undertaken where possible beam analyzer designs are examined after which a final design is to be selected.

 

-      The manufacturing of the parts used in the beam analyzer will be ordered. Some parts is produced by the project team, and assembly of the laser beam will be undertaken.

 

-      Experimental work on the developed beam analyzer will be done for validation.

 

-      Finally the project will be documented in the form of the report you are currently reading.

 

 

 

 

 

 

3. Main objectives

 

 

The main objective of this project is to be able to measure the spot size at the focal point for lasers with a spot size down to of 10 µm. Only one geometrical binding has been requested by the IPL/CALM group. It is that the beam analyzer must be able to fit underneath a laser system with 50-70 mm clearance from the substructure to the laser nozzle.

Finally the beam analyzer must be developed with simplicity in mind and the budget of the laser beam analyzer must not exceed 2,000 $

 

Experiments conducted in the preliminary experimental work will define further demands and requirements that need to be abided for the beam analyzer to perform as desired. Final tests on the finished beam analyzer will show if these demands have been met.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4. Theory

 

            After a brief explanation of the working principle of the laser beam analyzer, the theory and optics that is used throughout the report will be explained

 

4.1 The beam analyzer explained

 

            The mechanism of the laser beam analyzer described in the 1983 article^[1] is very similar to what is seen in a supermarket bar-code reader. A laser beam is by a mechanism scanned over a detector. The detector delivers an output voltage representing the intensity of the light hitting the detector window. A barcode reader uses the intensity peaks received between the black lines in the bar code to read a binary code. The laser beam analyzer uses the output signal to measure the intensity distribution of the laser beam.

 

In a barcode reader a low powered laser diode < 5 mW are scanned over the barcode by a rotating mirror, and it is the reflections from the barcode that provides the signal. In the laser beam analyzer, the laser beam is reflected by a highly reflective rod rotating at high velocity trough the focus of the laser beam. As the rod travels trough the laser beam it is scanned directly over the detector that reads the intensity profile of the signal.

 

4.2 An introduction to laser optics

 

The model that most people have in mind when thinking of optics is a very simplified model descending back to childhood experiences with a magnifying glass used to focus the sunrays. The model states that a parallel bundle of sunrays are focused in an angle defined by the lens and that there is one clearly defined focal point in where the area of the beam spot is close to zero. This model is extremely simplified compared to what happens in reality. Light from a laser never propagates truly parallel, and diffraction sets a limit on how small a spot can be focused^[4].

 

The beam close to the focal point has the shape seen on figure 4.1. The beam waist is defined as the smallest radii of the beam throughout the path of propagation.^[5] Even though this is called the focal point, it is easily seen that the beam radii changes slowly close to focus, and that the intensity of the beam in this area is almost as high at the waist position. This focal range is also called the Rayleigh Length^[6].

 

The following optical parameters will be discussed:

            Beam waist:                [w₀]

            Rayleigh length:                      [z_r]

            Angle of divergence:   [q]

            Beam quality:              [M²]

 

Figure 4.2.1 profile of the laser beam

 

 

 

Another common statement is the fact that a beam can be parallel. There is no such thing as a parallel beam. A beam bundle that starts as being parallel will further down the beam path slowly diverge due to diffraction. Diffraction is the phenomenon that a parallel beam bundle will spread as it propagates, much like the phenomenon that occurs when observing the backscattered waves as they roll back in a spreading formation after a wave front hits an offshore structure in the water. Furthermore there is a relation between wavelength and diffraction. For shorter wavelengths the diffraction is decreased.^[7]

 

The diffraction phenomenon is linked to another important parameter: The angle of divergence. This angle explains how hard the laser beam is focused and is defined from classic ray tracking. A hard focused beam has a high angle of divergence, and can focus to a smaller spot size than a beam with a smaller angle of divergence. The limit for how hard a laser beam can be focused is called the diffraction limitation phenomena^[8].

 

The parameters described so far can now be used to describe one of the most important parameters in the world of lasers. This is the M² value^[9]. This value is defined as the parameter for quantifying the beam quality of laser beams, and its form is seen in formula 4.1:

 

(Formula 4.2.1)

 

Where [q] is the angle of divergence, [l] is the wavelength of the laser and [w₀] is the beam waist.

 

 

The relation between M² and beam quality is that for higher quality M² ® 1. This can be explained in the following way. Assume that there are two lasers. Laser #1 with M² ≈ 1 and laser #2 with M² ≈ 2. They must be focused to the same spot size with same diameter of the focus lens. To obtain this, laser #2 must be focused harder than laser #1 This affects the position of the waist as it will be moved closer to the lens for laser #2 than laser #1.

 

Figure 4.2.1 The relation between beam focusability and M²

 

 

Finally the Rayleigh length will be defined. The Rayleigh length is the distance from the beam waist at which the mode area is doubled. This is expressed with the following relation^[6]:

 

  (Formula 4.2.2)

 

Where [w_r] is the waist at the Rayleigh length and [w₀] is the beam waist.

 

The Rayleigh length for a diffraction limited optical system is given by the expression:

 

     (Formula 4.2.3)

[z_s] is the Rayleigh length, [w₀] is the beam waist and l is the wavelength.

 

 

If it is assumed that M² equals 1, the mode of the beam is a pure TEM00 mode.^[10] This means that the beam spot has the shape as seen on figure 4.3.

 

 

Figure 4.2.3 The shape of a TEM00 beam spot

 

 

The TEM00 mode is called the fundamental mode. The intensity profile across this mode is characterized by being a Gaussian distribution. For higher order modes this does not apply but the lasers used during all the experimental work which forms the basis of this thesis have a small M² value, almost a TEM00 mode and hence a close-to-Gaussian intensity distribution^[11].

 

The theoretically intensity profile for the fiber laser that was used during the experimental work is shown in the lower part of figure 1. It is a Gaussian distribution^[11] following formula 4.4:

 

       (Formula 4.2.4)

 

 

            Where the constants are real as well as: a > 0, b, and c

 

 

 

This completes the optical theory used within the thesis and will be referred to when needed.

5. The Laser platform.

 

5.1 The 20 W fiber laser

 

 

            The test subject in throughout the project is a mid-range 20 W fiber laser. This is suited and used for a wide range of experimental work. It is built into a safety cabinet making it a class I laser system, which means that anybody with a short briefing can use this laser.

 

20 W describes the maximal output power of the laser. Though 20 W isn’t much compared to high-power lasers this laser is far from weak when it comes to power density as the pure mode of the laser gives great focusability^{[figure 4.2.1].}

 

The determining factor for the quality of the fibre lasers are the type op fibre used in the laser. Single mode fibre lasers are known for a very pure and good mode because the output fibre only allows one mode to travel trough. This gives an unmatched M² and makes the beam easy to focus.

 

Specifications provided by the manufacturer^[12]:

            CW Ytterbium doped fiber module

            5-20W output power

Max current 6 A

Effect/Current rating of 3.47W/A.

            Single mode fiber delivery

Diffraction limited output

M²< 1.1 (The CALM laser is a prototype hence M² < 1.2)

Power vs. input current is linear.t

Fixed central wavelength between 1060-1090 nm (The CALM laser has been measured to 1075 nm)

 

 

 

5.2 The Optical system

 

 

            The optical delivery system consists are built up a follows: A SMA connection connects the output fiber from the laser to a collimator. The beam leaves the fiber as it propagates towards infinity at a at a known widening angle. At a desired width the beam are collimated. The Collimator is a convex lens as seen on figure 5.2.1, that is used to redirect the beam so it propagates in a close-to-straight path and the distance from the fiber to the collimating lens is the focal length of the collimating lens^[13].

Figure 5.2.1 Optical system

 

When the beam is collimated it is ready to be focused. This is done by the focusing lens. The specifications of the optical system used is as follows:

 

-      Collimator: Focal length = 35.9 mm (red light), The diameter is oversized.

 

-      Focus lens: Focal length = 75.0 mm (red light), The diameter is oversized.

 

 

-      Estimated minimum waist: 8-9 mm

 

 

5.3 The NC system

           

            The laser is mounted in an NC setup. It is a 3 axis system with movement in the vertical and horizontal plane.

 

The NC table has two vertical axis called X and Y and a horizontal axis called Z. The X- and Y-axis are combined in a base structure and the Z axis is placed on a frame above the former axes. An overview can be seen on picture 5.3.1

 

Figure 5.3.1 Axes of the NC system

 

The resolution of the NC setup is relevant since initial testing will rely on the resolution of all the axes. The resolution is seen in table 5.3.1.