2 Optical design and measurements
Fig. 1a and b illustrates the complete arrangement of the laser micrometer setup principle. The laser is Coherent laser 0222-200-00EF with 4 mW of power, operates at 635 nm. The laser light falls at an angle of 45° to the facets of a rotating polygon mirror. The polygon mirror is eight facets of Lincoln laser USA with microcontroller PWB 1-2-3060-610-00 P1BB. The microcontroller provides an RPM in the range from 1000 to 10000. The F-theta lens is of Edmond Optics Model no.63-311 with an effective focal length of 165 mm. The collimating lens size is 27 mm with a focal length of 60 mm. The detector system is an 818-BB detector connected to a digital storage oscilloscope (DSO) through BNC. Without any object in the illuminating path, the pulse width is proportional to the lens size. In our case lens size of 27 mm, gives the pulse width of 64.10 µs at 8000 RPM Fig. 2a. When an object is placed, the light in the central portion of the pulse is interrupted causing a dip in the pulse width Fig. 2b. The different pulse sizes are plotted for a number of solid objects from 3 mm to 13 mm Fig. 3. The curves show linearity. The analysis predicts that an RPM value of up to 2000 can be chosen for object size measurements. The slope of the curve gives sensitivity in µs/mm. The reciprocal of this when multiplied with the width of the pulse dip of an unknown object gives the object size in mm.
The linear velocity of the beam spot on scanning line is an important parameter of the laser micrometer. It is shown in Fig. 4 for object size from 3–13 mm with different RPMs. The linear velocity of the scanning line is the velocity of the spot throughout the scan line. It is measured by dividing the object size by the time taken by the line to cross the object. In other words it is an inverse of the slope of the curve in Fig. 3. This is calculated for several values of RPM. It is shown that for RPM values between 2000 and 8000 the velocity of the line is less than 0.87% variation for object size 3–13 mm.The spot size measurement of the scan light is shown in Fig. 5. The measurements are made for a number of observations along the scanning line. The RPM value is kept at 8000. The distance chosen is 165 mm which is the effective focal length of the F-theta lens. The measurement shows a variation of 1.5% of the spot size along the scanning line. The mean spot size is 340.78 µm. Measurement of the spot size away from this location gives a variation of the spot size. For example at 123 mm, the spot size ≈679 µm and at 193 mm it is ≈412 µm.In Fig. 6 we plot the spot size for a number of positions around 165 mm. The spot size measures a constant value at 165±3 mm. The spot size measurements are repeated at RPM of 5000. The measured values are similar to RPM 8000 data in the 165±3 mm positions. This sets a tolerance of the object positions which is ±3 mm to the effective focal length of the F-theta lens in our present setup.In the following we present our calculations with a hole as the object instead of solid object. The pulse shape with the hole as the object is the same without an object but with a decrease width according to the uninterrupted light. It is shown in Fig. 7. The sizes of the pulse versus hole dimensions in mm are shown in Fig. 8. The measurements are done at RPM value of 4000–8000. The curves are linear. The linear velocity calculations are shown in Fig. 9 and spot size measurements are in Fig. 10and Fig. 11 respectively. The beam spot size is same for a number of positions along the length of the line and beam spot size is same at 165±3 mm locations.The present micrometer setup is used to measure dimensions of an electromagnetic undulator [18]. A copper foil runs alternately with a pre-determined gap constitute a copper structure. Two such copper structures kept one above other make the electromagnet undulator. Two copper strips with two adjoining gaps constitute one undulator period. The present undulator period is 10 mm. A precise measurement of the gaps and the solid copper strips define the precision in undulator period. The copper structure is kept vertically upward for the measurement shown in Fig. 12a. The complete measurement setup is shown in Fig. 12b. The copper structure is kept at a distance of 165 mm from F-theta lens. The vertical line from the F-theta lens falls on a large section of the structure thereby producing number of shadows and allowing number of light strips through the structure. The detection section is a lens and a detector. The size of the lens is the crucial parameter that decides the number of solid objects and number of gaps to be detected by the detector. The present lens size is 27 mm. This allows us the simultaneous measurement of two solid copper strips and two adjoining gaps. The corresponding pulse shape in DSO screen is shown in Fig. 12c. The peaks and drops in the voltage denote the gap and the solid copper strip respectively. The measurements are made with two different RPMs at 5000 and 8000. The beam spot size measurement done in Fig. 11 shows that different RPMs provide the same spot size at a fixed distance from the F-theta lens.In Fig. 13, the light falls on two gaps of 2.0 mm and 2.2 mm. There is a variation of 0.2 mm in gap sizes of its self-weight due to vertical standing. For measurements the vertical light is moved horizontally in a length of 45 mm. The complete horizontal length of the copper strip is 70 mm. The measurements are obtained with a maximum deviation of 70 µm with an accuracy of 3 µm. The maximum error in reading 2.2 mm gap is around 3.2% and the error is 3.5% for the 2.0 mm gap. The measurements from the light falling on the solid copper strip are shown in Fig. 14. The measurements are repeated with RPMs of 5000 and 8000. The spot size measurements show a spot size variation at different RPMs (Fig. 6). This introduces measurement difference at the two RPMs. The measurements are done with a maximum error of 13% from the original size.