1. Practical Optics Class Opti696D, Fall 2008 1 Laser Bar Code Scanner Chunyu Zhao

2. Practical Optics Class Opti696D, Fall 2008 2 OPTI696D requirement System overview: –Describe the class of systems, stating the key metrics –Explain the principles of how the system works –Identify key subsystems, relate system performance to subsystem requirements. –Summarize the current state of technology for this type of system Analysis: –Disassemble the system and show key components and subsystems –Discuss features of this system and surmise design decisions

3. Practical Optics Class Opti696D, Fall 2008 3 Outline Barcode basics and laser scanners How a laser scanner works Scan engine: –Optical design and analysis –Manufacturing laser scanners in volume State of the art scanning technology

4. Practical Optics Class Opti696D, Fall 2008 4 Barcode basics: Type of barcodes 1D barcode –UPC –Code 39 –Code 128 –etc 2D barcode –PDF 417 –MaxiCode –etc

5. Practical Optics Class Opti696D, Fall 2008 5 Barcode basics: Benefit of using barcodes For retailers –Quickly identifying fast and slow selling items to help stocking decisions –Repositioning a given product within a store to move more profitable items to occupy the best space, –Historical data can be used to predict seasonal fluctuations very accurately. For shipping companies –Keep track of packages from start to destination

6. Practical Optics Class Opti696D, Fall 2008 6 1D laser scanners Handheld, single line Hand-free, multi-line Fixed mount, multi-line Scan engine

7. Practical Optics Class Opti696D, Fall 2008 7 Key metrics Size Working range Poor quality barcode reading capability Width of field/scan angle Pitch, roll and yaw angles Barcode contrast Ambient light level

8. Practical Optics Class Opti696D, Fall 2008 8 How a laser scanner works Signal Processing: Hardware and Software A laser spot is scanned across the bar code symbol that is to be read. The light reflected from the symbol is directed to a photodiode where it is converted from optical energy to electrical current. The signal is processed through both hardware and software, and the information it carries is extracted.

9. Practical Optics Class Opti696D, Fall 2008 9 Scanning the Bar Code When the laser is scanned across the bar code, the reflected signal is the convolution of the laser spot and bar code symbol. Simply stated, the convolution is the area of the overlap of the two waveforms. In order to make finding the transition point from bar to space more easy to find, the signal is differentiated.

10. Practical Optics Class Opti696D, Fall 2008 10 Noise Corrupted Signal

11. Practical Optics Class Opti696D, Fall 2008 11 Sources of Noise Internal Sources –Thermal noise of the electronic components. –Intrinsic noise of the preamplifier. External Sources –Printed noise on the symbol. –Speckle noise created by the laser. –Sunlight. –Fluctuating ambient light. –EMI, RFI and power supply noise.

12. Practical Optics Class Opti696D, Fall 2008 12 System performance defining factors The characteristics that define how well a scanner will decode are –Depth of Modulation laser focus other optical components –Signal Amplitude optical alignment signal blockage laser focus optical AGC –Noise Amplitude field of view optical alignment laser focus

13. Practical Optics Class Opti696D, Fall 2008 13 Retro or non-retro system Retro system: Scan mirror is part of the collection optics, so the FOV follows the laser spot. Small FOV, therefore less ambient light noise Small photodiode, so noise from PD is small. Need better alignment. Non-retro system: Scan mirror is NOT part of the collection optics, so the FOV is fixed and cover the whole scanning field, and it’s BIG. More ambient light noise. Need a big photodiode, thus the noise is huge, so the working range is reduced. Alignment is easier. Laser beam Collection FOV Scanning Collection FOV Laser beam Scanning

14. Practical Optics Class Opti696D, Fall 2008 14 The Design Process To design a scanner you need to do the following things: –extract size and performance information from the customer or marketing spec –develop an optomechanical configuration –calculate optical field of view and photodiode size –develop a laser profile to meet the performance requirements –perform sensitivity study and tolerance analysis –develop inspection criteria for manufacturing

15. Practical Optics Class Opti696D, Fall 2008 15 The most important part: laser beam profile Laser Profile –Ideally, the cross section of the laser beam should be a Delta function. –In reality, the beam size is finite and expands as it propagates due to diffraction. –A small spot diameter is required to read high density bar codes. A large spot area is needed to minimize speckle noise and poorly printed symbols. Trade-off needs to be made.

16. Practical Optics Class Opti696D, Fall 2008 16 Convolution Process Small Spot

17. Practical Optics Class Opti696D, Fall 2008 17 Convolution Process Large Spot

18. Practical Optics Class Opti696D, Fall 2008 18 Convolution Process Very Large Spot

19. Practical Optics Class Opti696D, Fall 2008 19 The Laser Beam Profile: General Requirement Basic requirement - the spot diameter must be no greater than some fixed multiple of the bar code symbol narrow element width over the entire working range. This multiple can range between 2.8 and 3.3 depending on the type and sensitivity of digitizer used. Secondary requirement - ellipticity should be as large as possible to improve speckle noise characteristics and poorly printed symbol readability, and beam pedestal and ripple should be kept to a minimum.

20. Practical Optics Class Opti696D, Fall 2008 20 The Laser Beam Profile The characteristics of the laser beam can be controlled and manipulated using the following parameters; –position and focal length of laser focusing lens –aperture size, shape and aspect ratio –laser divergence angle and astigmatism –rotation of laser (high or low divergence in x axis) –external beam shaping optics

21. Practical Optics Class Opti696D, Fall 2008 21 Laser Beam Profile

22. Practical Optics Class Opti696D, Fall 2008 22 Scanning Optics Flatness of mirrors controls accuracy of laser profile. Curvature will shift waist size and location. Random aberrations will distort overall beam shape. Curvature can be used to add desired ellipticity, if applied to the Y axis.

23. Practical Optics Class Opti696D, Fall 2008 23 Tolerance analysis and error budget Decenter: Lens Barrel vs. Laser Decenter: Lens vs. Barrel Decenter: Phase Plate vs. Barrel Tilt: Lens Barrel vs. Laser A B Lens Barrel

24. Practical Optics Class Opti696D, Fall 2008 24 Error Budget

25. Practical Optics Class Opti696D, Fall 2008 25 Tolerance analysis/sensitivity study: Example 1

26. Practical Optics Class Opti696D, Fall 2008 26 Tolerance analysis/sensitivity study: Example 2

27. Practical Optics Class Opti696D, Fall 2008 27 Collection Optics Collect as much of the laser light reflected from the bar code as possible. Track the position of the laser spot, and keep it in the center of the receiver field of view. Define the size of the optical FOV to be as small as possible.

28. Practical Optics Class Opti696D, Fall 2008 28 Optical Collection Area A large collection area increases the signal received (improving signal to noise ratio) and reduces the effect of speckle noise, but makes the scanner physically bigger and collects more ambient light interference.

29. Practical Optics Class Opti696D, Fall 2008 29 Optical Field of View Alignment has to be maintained between what the laser illuminates and what the photodiode is looking at. A large FOV makes this alignment less critical, but increases the amount of ambient light collected and requires a larger photodiode to do the collecting, both degrading noise performance. A small FOV requires active alignment of the optics or higher tolerance parts, and it may move out of alignment with time or drop and vibration.

30. Practical Optics Class Opti696D, Fall 2008 30 Optical filter Right in front of the photo-detector Let the laser reach the detector and block most of the ambient light

31. Practical Optics Class Opti696D, Fall 2008 31 Manufacturing a scanner Step 1: Focusing the laser module to obtain the desired beam profile Step 2: Install collection and scanning optics, and detector Step 3: Align the collection FOV with scanning beam Inspection DURING and AFTER production!!!

32. Practical Optics Class Opti696D, Fall 2008 32 Laser focusing

33. Practical Optics Class Opti696D, Fall 2008 33 Focusing spec

34. Practical Optics Class Opti696D, Fall 2008 34 Optical alignment Align the collection FOV with the flying laser spot: –For Non-retro system, adjust the center position of the flying spot to the axis of the collection optics –For Retro system, if the collection optics and scan optics are separated, then adjust the scan mirror to maximize the signal; otherwise alignment relies on tight mechanical tolerance

35. Practical Optics Class Opti696D, Fall 2008 35 Final Acceptance Test (FAT)

36. Practical Optics Class Opti696D, Fall 2008 36 State of the art Extended working range: –Double scan beams –Diffraction-free laser beam by using axicon

37. Practical Optics Class Opti696D, Fall 2008 37 Any questions?