Beam profiler

Abstract

An M.sup.2 value beam profiling apparatus and method is described. The M.sup.2 value beam profiler comprises an optical axis defined by a focussing lens assembly and a detector, wherein the focussing lens acts to create an artificial waist within an optical field propagating along the optical axis. The beam profiler also comprises a multiple blade assembly having a first set of blades located at an artificial waist position and a second set of blades longitudinally separated along the optical axis from the artificial waist position. The multiple blade assembly therefore provides a means for selectively passing the blades through the location of the optical axis. Employing these measured widths allows for the M.sup.2 value of the optical field to be determined.

Claims

1. An M.sup.2 value beam profiler comprising: an optical axis defined by a focussing lens assembly and a detector, wherein the focussing lens acts to create an artificial waist within an optical field propagating along the optical axis; a multiple blade assembly having a first set of blades located at an artificial waist position and a second set of blades longitudinally separated along the optical axis from the artificial waist position, wherein the multiple blade assembly provides a means for selectively passing the blades through the location of the optical axis and comprises ten or more blades that are longitudinally separated along the length of the assembly.

2. An M.sup.2 value beam profiler as claimed in claim 1 wherein the multiple blade assembly also provides a means for successively passing the blades through the location of the optical axis.

3. An M.sup.2 value beam profiler as claimed in claim 1 wherein the ten or more blades are equally spaced along the length of the assembly.

4. An M.sup.2 value beam profiler as claimed in claim 1 wherein the multiple blade assembly comprises a rotatable multiple blade assembly.

5. An M.sup.2 value beam profiler as claimed in claim 4 wherein the blades are mounted upon a rotatable shaft.

6. An M.sup.2 value beam profiler as claimed in claim 5 wherein the rotatable shaft defines an axis of rotation for the rotatable multiple blade assembly.

7. An M.sup.2 value beam profiler as claimed in claim 5 wherein the blades are mounted upon the rotatable shaft and occupy a unique rotational position.

8. An M.sup.2 value beam profiler as claimed in claim 6 wherein the rotatable multiple blade assembly further comprises a reference that provides a means for determining the rotational orientation of the rotatable multiple blade assembly relative to the axis of rotation.

9. An M.sup.2 value beam profiler as claimed in claim 1 wherein the multiple blade assembly comprises a mechanical actuator.

10. A method of profiling an output field from a laser the method comprising propagating the output field along an optical axis; focussing the output field to form an artificial waist; locating ten or more blades longitudinally separated along the length of the optical axis by locating a first set of blades at an artificial waist position and locating a second set of blades longitudinally separated along the optical axis from the artificial waist position; measuring the width of the output field at the ten or more positions along the optical axis by selectively passing the ten or more blades through the optical axis to prevent the propagation of the output field; and employing the measured widths to determine the M.sup.2 value of the output field.

11. A method of profiling an output field as claimed in claim 10 wherein measuring the width of the output field further comprises successively passing the blades through the location of the optical axis.

12. A method of profiling an output field as claimed in claim 10 wherein the ten or more blades are equally spaced along the optical axis.

13. A method of profiling an output field as claimed in claim 10 wherein the first and second set of blades are separated by a distance greater than or equal to the Rayleigh length of the output field.

14. A method of profiling an output field as claimed in claim 10 wherein the first and second sets of blades are passed through the optical axis by rotation of a shaft.

15. A method of profiling an output field as claimed in claim 10 wherein the first and second sets of blades are passed through the optical axis by the translational movement of a mechanical actuator.

16. A method of profiling an output field as claimed in claim 10 wherein the measured widths of the output field are employed to calculate a beam waist value for the output field.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings in which:

(2) FIG. 1 presents a schematic illustration of the propagation of a uniform Gaussian profiled optical field and a non-uniform Gaussian profiled optical field through a focusing lens;

(3) FIG. 2 presents a schematic representation of a beam profiler known in the art;

(4) FIG. 3 presents a schematic representation of an alternative beam profiler known in the art;

(5) FIG. 4 presents a schematic (a) side view and (b) rear view of a beam profiler in accordance with an embodiment of the present invention;

(6) FIG. 5 presents a flow chart of the methodology involved in operating the beam profiler of FIG. 4;

(7) FIG. 6 presents a schematic experimental trace obtained by the beam profiler of FIG. 4;

(8) FIG. 7 presents a schematic representation of an alternative multiple blade assembly; and

(9) FIG. 8 presents a schematic representation of a further alternative multiple blade assembly.

DETAILED DESCRIPTION

(10) A beam profiler in accordance with an embodiment of the present invention, and generally depicted by reference numeral 15, will now be described with reference to FIGS. 4 to 6. The beam profiler 15 can be seen to comprise a focussing lens assembly 16, a rotatable multiple blade assembly 17 and a signal detection and processing system 18.

(11) In the presently described embodiment, the signal detection and processing system 18 comprises a detector 19, an oscilloscope 20 connected to the detector 19 and a CPU 21 connected to the oscilloscope 20.

(12) An optical axis 22 of the beam profiler 15 is defined by the location of the focusing lens assembly 16 and the detector 19.

(13) The rotatable multiple blade assembly 17 can be seen to comprise a central shaft 23 that defines an axis of rotation 24 for the assembly 17. The axis of rotation 24 can be seen to be parallel to, but offset form, the optical axis 22. Attached to a proximal end of the shaft 23 is a motor 25 that provide a means for rotating the shaft 23 about the axis of rotation 24.

(14) Moving along the shaft 23 from the proximal end there is located a shaft head 26 upon which is mounted a reference aperture 27. The reference aperture 27 provides the beam profiler 15 with a means for determining the rotational orientation of the rotatable multiple blade assembly 17 relative to the axis of rotation 24.

(15) Also located on the shaft 23 are ten blades, as depicted by reference numerals 28, 29, 30, 31, 32, 33, 34, 35, 36, and 37, respectively. In the embodiment presented in FIG. 4(a) the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 are spaced along the shaft 23. The blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 may be equally spaced along the shaft 23.

(16) It should be noted that the detector 19 has been omitted from FIG. 4(b) for ease of understanding of the orientation of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37.

(17) The blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 may be equal in length. Irrespective of the actual length of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 they each must have a length greater than the offset distance between the axis of rotation 24 and the optical axis 22 so as to allow the beam profiler 15 to function correctly.

(18) As well as the ten blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 being spaced longitudinally along the shaft 23 they each also occupy a unique rotational position about the axis of rotation 24. In the presently described embodiment the ten blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 are arranged to form a helical array from one end of the shaft 23 to the other.

(19) Method of Beam Profiling

(20) The operation of the beam profiler 15 will now be described again with reference to FIGS. 4 to 6. In the first instance the beam profiler 15 is deployed by arranging for the output field 3 of the laser to be analysed to propagate along the optical axis 22. The focussing lens assembly 16 acts to focus an output field 3 having a diameter W down to a waist d.sub.0. The output field 3 then propagates along the optical axis 22 so as to be incident on the detector 19.

(21) Deployment of the beam profiler 5 may further comprise adjusting the position of the focusing lens assembly 16 and or the position of one or more of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 such that they form a first set of blades 38 located about the position of the waist z.sub.0 and a second set of blades 39 positioned at a distance z.sub.R that is greater than or equal to a Rayleigh length of an output field 3.

(22) The motor is then employed to rotate the rotatable multiple blade assembly 17 such that each of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 in turn acts to block the output field 3 from reaching the detector 19. A typical experimental trace 40 obtained by the detector 19 and recorded by the oscilloscope 20 is presented in FIG. 6. It can be seen that the transmission profile 40 obtained by the detector 19 comprises ten absorption features 41, 42, 43, 44, 45 and 46, 47, 48, 49, 50 one each corresponding to each of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 passing through the output field 3. The width of each absorption feature 41, 42, 43, 44, 45, 46, 47, 48, 49 and 50 is directly related to the width of the of the output field 3 at the position z along the optical axis corresponding to the longitudinal position of the corresponding blade 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37.

(23) In order to obtain the experimental trace it is necessary for the response speed of the detector 19 to be correlated with the speed of rotation of the rotatable multiple blade assembly 17. Apart from this criterion there is a large flexibility in the choice of detector 19 employed by the signal and detection processing system 18 since all that is required is for the detector 19 to be capable of measuring the presence and absence of the output field as dictated by the rotational position of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37. Indeed a thermal detector could be employed if the speed of rotation of the rotatable multiple blade assembly 17 were slowed to an appropriate value to correlate with its response speed. This flexibility of the choice of detectors means the beam profiler 15 can operate over a greater range of the electromagnetic spectrum e.g. from the ultraviolet region through to the terahertz region, although this may require the use of different focusing lens assemblies.

(24) The signal detection and processing system 18 is then employed to determine the widths of the output field 3 at the ten different positions along the optical axis 22. This information can then be used to provide accurate calculations of the M.sup.2 value for the output field 3 as well as other beam parameters e.g. the position of the beam waist z.sub.0. Preferably these calculations involve employing D4 values for the width of the output field 3 at the ten different positions along the optical axis 22 and then fitting these measured data points to equation (3) above.

(25) An alternative embodiment of the beam profiler is presented in FIG. 7, as depicted generally by reference numeral 15b. The difference between the beam profiler 15b and that presented in FIG. 4 is that the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 have been separated into first 38 and second 39 sets of blades. The first 38 and second 39 set of blades are separated by a distance z.sub.R along the length of the shaft 23. It is preferable for the distance z.sub.R to be greater than the Rayleigh length of an output field 3 to be analysed by the beam profiler 15.

(26) It will be appreciated by the skilled reader that the first 38 and second 39 set of blades may comprise more or less than five blades and that each 38 and second 39 need not contain the same number of blades.

(27) In a further alternative embodiment presented in FIG. 8 the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 may be mounted longitudinally along the length of a mechanical actuator 51 instead of on the rotatable shaft 23. The mechanical actuator 51 provides a means for selectively passing each of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37, preferably successively or in turn, through the location of the optical axis 22 e.g. the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 are passed in turn down through the location of the optical axis 22 and then in turn back up through the optical axis 22. This process may then be repeated in a cyclic motion so as to replicate the rotation of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 of the rotatable multiple blade assembly 17 through the optical axis. In this embodiment it is necessary for the response speed of the detector 19 to be correlated with the period of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 passing through the optical axis 22.

(28) It will be appreciated that in all of the above described embodiments the number order of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 may be altered. Furthermore, the relative longitudinal and rotational position of the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 may also be varied. A minimum of two blades are required to exploit the inherent advantages of the invention as described below. However, the upper limit is set be the length of the shaft 23 and the ability to have the blades rotationally separated about the axis of rotation 24. Thus what is important for the operation of the beam profilers 15 and 15b is that the blades 28, 29, 30, 31, 32, 33, 34, 35, 36 and 37 occupy a unique longitudinal position along the shaft 23, a unique rotational position about the axis of rotation 24 and that this information is provided to the signal detection and processing system 18 so as to allow for the analyses of an output field 3 of a laser to be performed.

(29) The beam profilers described above have many advantages over those systems known in the art. In the first instance the beam profiler is significantly quicker than those profilers in the art that employ an automated focusing lens assembly. Results can be achieved in a matter of seconds rather than minutes.

(30) Since the detector is simply required to measure the presence and absence of the output field, as dictated by the rotational position of the blades, then there is no need to employ expensive CCD camera arrays and a reduced requirement to employ power density filters. As a result the beam profiler is significantly cheaper to produce and can be employed over a greater range of wavelengths and powers when compared to profilers known in the art. These reduced operational requirements on the detector also mean that the beam profiler can be employed to analyse CW and pulsed optical fields.

(31) Furthermore, by the appropriate selection of the length of the blades and the offset distance between the optical axis and the axis of rotation the beam profiler can also be employed to operate with a greater range of beam widths than those able to be achieved with the systems known in the art.

(32) The beam profiler is relatively easy to align and so requires less skill and effort on the part of the operator than those systems known in the art.

(33) An M.sup.2 value beam profiling apparatus and method is described. The M.sup.2 value beam profiler comprises an optical axis defined by a focussing lens assembly and a detector, wherein the focussing lens acts to create an artificial waist within an optical field propagating along the optical axis. The beam profiler also comprises a multiple blade assembly having a first set of blades located at an artificial waist position and a second set of blades longitudinally separated along the optical axis from the artificial waist position. The multiple blade assembly therefore provides a means for selectively passing the blades through the location of the optical axis. Employing these measured widths allows for the M.sup.2 value of the optical field to be determined.

(34) The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.