Laser Diode Readers: The Precision of Coherent Light in Barcode Scanning |
Executive Summary |
This article provides a comprehensive exploration of laser diode readers in barcode scanning applications, focusing on the unique advantages and design considerations that distinguish them from LED-based systems. We examine how the coherent, collimated beam of a laser enables long-range scanning, high resolution, and superior performance on challenging surfaces. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real patent disclosures from industry leaders including U.S. Philips Corporation, Symbol Technologies, and other pioneering companies. We explore the dual-facet nature of laser diodes as both light sources and detectors, the automatic power control (APC) loop that maintains constant optical output, the integration of laser drive and signal processing on single chips, and the practical challenges of invisible beams and safety regulations. The article covers both the fundamental principles and the practical implementation details that make laser-based readers the preferred choice for demanding applications. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting laser diode readers for barcode reading applications. |
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Chapter 1: The Coherent Advantage |
The laser diode represents a fundamental shift in barcode reading technology. Unlike an LED, which emits light in a broad, divergent beam, a laser diode produces a coherent, collimated beam that can be focused to a very small spot over a long distance. This allows a laser reader to read barcodes from much greater distances than an LED-based system and to resolve finer details, making it suitable for high-density barcodes. |
The laser beam is generated by a semiconductor laser chip with two reflective end faces that form an optical cavity. Light bounces back and forth between these faces, building up intensity until a coherent beam is emitted from the front facet. The key property of this beam is its coherence: the light waves are all in phase, which allows them to be focused to a diffraction-limited spot. This is the source of the laser's superior resolution and range. |
The early adoption of laser diodes in barcode readers was driven by the desire for miniaturization. Gas lasers like helium-neon tubes were bulky and power-hungry. Semiconductor laser diodes offered a much smaller footprint and lower power consumption, but they had a critical drawback: they typically emitted invisible infrared light, making it difficult for the operator to aim the reader. This challenge drove innovations in aiming beam technology that are still used today. |
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Chapter 2: The Dual-Facet Laser Diode |
A key innovation in laser diode readers is the dual-facet nature of the laser chip. The laser chip has two end faces, both of which emit light. The front facet emits the main beam that is directed to the barcode. The rear facet emits a smaller beam in the opposite direction, which is used for monitoring and control. |
In a conventional laser diode package, the rear facet beam is directed onto a monitor photodiode that is part of a feedback loop. The monitor photodiode is mounted within the casing, rearwardly of the laser chip and radially offset from the optical axis. The photodiode 'watches' the rearwardly-directed beam and generates a current proportional to the laser's output power. This current is used by a current controller to adjust the forward current to the laser chip, maintaining a constant optical output. |
What makes the laser diode reader truly innovative is that the monitor photodiode can serve two functions simultaneously. The Symbol Technologies patent describes an arrangement where the laser diode unit's monitor photodiode is used not only for laser power control, but also as the photoreceiver for the reflected light from the barcode. A collector outside the laser diode unit collects the reflected light and directs it to the same monitor photodiode, eliminating the need for a separate discrete photoreceiver. This simplifies the reader design and reduces cost. |
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Chapter 3: Automatic Power Control --- The Feedback Loop |
The automatic power control (APC) loop is essential for safe and reliable laser operation. Laser diodes are sensitive devices; their output power can vary with temperature, aging, and manufacturing tolerances. The APC loop maintains the optical output at a constant, safe level by adjusting the forward current. |
The APC loop works as follows: the monitor photodiode produces a photocurrent that is proportional to the laser output power. This photocurrent is fed back to a controller, which compares it to a reference and adjusts the laser drive current accordingly. If the laser output drops, the photocurrent drops, and the controller increases the drive current. If the output rises, the controller reduces the drive current. |
The Symbol Technologies patent describes the importance of this feedback system: 'the laser diode unit has a casing bounding an interior, including laser chip means in the interior of the casing and operative for emitting a forwardly-directed laser beam along an optical path toward a symbol for reflection therefrom to generate reflected light and for emitting a rearwardly-directed laser beam, and monitor photodiode means in the interior of the casing ... operative for outputting a signal descriptive of the output power of laser chip means'. |
Quarton inc., a manufacturer of laser modules for barcode readers, explains the principle in their product documentation: 'The Laser output power must be stable especially in safety reason. But the Laser power is related to the Junction Temperature (Tj) of the LD. It means that as the different Tj, the Laser power will be increased or decreased. An APC (automatic power control) circuit is used to compensate the varying Laser output power to constant'. Their modules feature an integrated APC driver circuit that 'enables the laser power output safe and constant'. |
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Chapter 4: The Invisible Beam Challenge |
The most significant challenge with early laser diode readers was the invisibility of the beam. Most semiconductor lasers emit in the infrared spectrum, typically around 815 nanometers, which is invisible to the human eye. This made aiming the reader difficult, as the operator could not see where the beam was pointing. |
The invisibility problem was addressed by the development of visible laser diodes, such as those emitting at 670 nanometers. However, the laser diodes capable of providing the necessary intensity of illumination were typically gallium arsenide laser material which lases in the infrared. Since a laser beam is very small in diameter, it becomes easy to miss the bar code, which can lead to an unacceptably small rate of successful readings per scan. |
For applications where infrared lasers were still used, the marker beam technique was developed. An optical device described in a patent uses an invisible laser beam to scan the symbol while simultaneously illuminating the symbol with a visible marker beam that is coincident with the laser beam. This 'enables the operator to locate the laser beam with respect to the code symbols, such that the code symbols can be scanned with a high degree of assurance that the laser beam will be reflected from the code symbols'. |
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Chapter 5: The U.S. Philips Fiber-Based Design |
A radically different approach to laser diode barcode reading was pioneered by U.S. Philips Corporation. Their design uses a semiconductor laser diode with a first end face optically coupled to an end of an optical fiber, and a second end face optically coupled to a detector. The bar code is scanned at the free end of the optical fiber, and the information is read by the semiconductor laser diode by way of optical feedback. |
The operation relies on a shift in the laser's luminous flux characteristic caused by feedback from reflected light. When the current intensity exceeds a threshold value, lasing occurs. With feedback, this threshold current is shifted to a lower value. For a given current intensity, more power is output from the laser with feedback than without feedback. |
The reflected light is detected by the laser itself because it is imaged on the end face of the laser. The semiconductor laser diode responds to variations of the light reflected by the bar code by causing variations of the power emitted at the first end face. These laser output variations are detected by the detector, which generates an electrical signal for the data processing system. |
The Philips design eliminates the need for a directional coupler, which is expensive and absorbs part of the energy of the reflected light. It also allows the laser to operate with less energy because the fed back light increases the laser's output, so the transmitted light energy need not be so high. |
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Chapter 6: The Scanning Mechanism |
The laser beam must be scanned across the barcode to create the time-varying signal. This is typically done with a moving mirror. The scanning mechanism is a critical component that affects the scan speed, accuracy, and power consumption. |
A patent from Symbol Technologies describes a scanning mechanism where a support plate carries the laser diode, optics, and circuitry. The support plate is a printed circuit board carrying the laser diode, the electric circuitry, and the optics for forming the beam. The printed circuit board together with the parts mounted thereon forms a unitary structure insertable into position and fixable in the housing after assembly. |
In gun-shaped readers, the scanning is performed by an oscillating mirror coupled to a scanning drive motor. The user aims the reader at the barcode and pulls a trigger, activating the laser and the scanning motor. A combined range scanner described in a patent uses two laser optical illumination systems: a first system optimized for contact operation (up to approximately two feet), and a second system optimized for longer distance scanning (from approximately two to seventeen feet). The two laser scanning systems are integrated into one combined range laser scanner. |
The scanning mirror is driven by a scanning motor. For short range operation, the amplitude of the applied scanning motor driving signal should be greater; for longer range scanning, it should be less. This controls the frequency of received light signals to that of standard signal processing and decoding circuits. |
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Chapter 7: The Unitary Scan Engine |
The ultimate expression of integration is the unitary scan engine, where all components --- laser diode, optics, detector, and electronic circuitry --- are assembled into a single, self-contained unit. The patent from a barcode reader manufacturer describes such a 'unitary structure insertable into position and fixable in said housing after this structure has been assembled'. |
The unitary scan engine offers several benefits. It simplifies manufacturing by reducing the number of assembly steps. It improves reliability by minimizing the number of interconnections and adjustments. It enables the scanner to be used in a variety of housings and configurations. |
The printed circuit board has upper and lower surfaces, with the diode, photodetector and optics attached to the upper surface, and the lower surface disposed upon the flat bottom of the housing. This compact design allows the scanner to be small and lightweight for hand-held operation. |
Chapter 8: Integrated Laser Modules |
For designers who want to simplify their designs, integrated laser modules provide a convenient solution. These modules include the laser diode, collimating lens, and APC driver circuit in a compact package. |
Quarton inc. offers integrated laser modules for barcode readers and other applications. Their green cross-line laser modules, for example, include the laser diode, collimating lens, wavy lens, and APC driver circuit in a brass housing. The APC driver circuit enables the laser power output to be safe and constant. |
These modules are available with different specifications, including different operating voltages, output powers, and line widths. The VLM-520-29 series, for example, is optimized for short-distance applications and provides a green cross-line pattern. The modules feature a patented solid brass structure for shock resistance and better heat transfer. |
The modules are designed for high reliability and meet international safety standards. |
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Chapter 9: The Collection Optics |
The collection of reflected light is a critical aspect of the laser scanner design. The optics must efficiently gather the reflected light from the barcode and direct it onto the photodetector. |
A patent describes an aperture stop in the collection optics that defines a constant illumination of the photodetector over the depth of field. The size and location of the aperture are chosen such that it does not limit the amount of light gathered from a maximum point of the depth of focus while limiting the collection of light from a minimum point of the depth of focus to the photosensitive area of the detector. |
The collection lens focuses the beam within a given depth of focus in front of the lens upon the code and focuses the reflected scattered light upon the detector. This ensures a consistent signal amplitude over a wide range of reading distances. |
Chapter 10: LASER Safety Considerations |
Laser safety is a paramount concern in the design of laser diode readers. The laser beam must be contained within the scanner housing and only emitted through the output window when the scanner is properly aimed and activated. The scanner must also comply with regulatory requirements for laser safety. |
The Philips patent addresses safety by using a laser that emits light with a wavelength greater than 1.3 micrometers. Light whose wavelength exceeds 1.3 um is completely harmless to the human eye, because radiation having this wavelength is absorbed by water. The preferred laser is an InGaAsP diode laser, emitting between 1.3 and 1.5 micrometers. |
Laser products manufactured by Quarton inc. meet international safety standards. The integrated APC circuit ensures the laser power output is safe and constant, preventing the laser from exceeding its safe power limit. |
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Chapter 11: Summary --- Laser Diode Readers in Perspective |
Laser diode readers represent a significant advancement in barcode scanning technology, offering long reading distances, high resolution, and superior performance on challenging surfaces. The coherent, collimated beam of a laser allows it to be focused to a small spot over a long range, making it ideal for industrial and high-density applications. |
We have examined how different companies and technologies have approached the challenges of laser diode reader design: |
Symbol Technologies developed a scanner where the monitor photodiode of the laser diode package serves as the photoreceiver for both laser power control and barcode signal detection. This dual use eliminates the need for a separate discrete photoreceiver, simplifying the design. |
U.S. Philips Corporation developed a bar code reader using a semiconductor laser diode with optical feedback through an optical fiber. The reflected light is detected by the laser itself, eliminating the need for a directional coupler and reducing the required laser power. |
Optoelectronics Co., Ltd. developed a controller where the MPU changes the on/off frequency of the laser diode according to the level of the analog signal, using an amp with frequency-dependent gain to simplify the circuit structure. |
A barcode scanner patent describes a unitary scan engine where the laser diode, optics, and circuitry are mounted on a printed circuit board as a single insertable structure, simplifying manufacturing and assembly. |
Quarton inc. offers integrated laser modules with built-in APC driver circuits, providing a convenient solution for consumer-grade barcode readers with constant and safe laser power output. |
A combined range scanner patent describes a scanner with short-range and long-range laser illumination systems integrated into one device, with a common scanning mirror and different scan amplitudes for each range. |
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The key lessons from our exploration are: |
Laser diodes provide superior range and resolution. Their coherent, collimated beam can be focused to a small spot over a long distance, enabling reading of high-density barcodes at long range. |
The dual-facet laser diode can serve dual functions. The rear facet beam can be used for both power control and signal detection, eliminating the need for a separate photodetector. |
Automatic power control is essential. The APC loop maintains a constant optical output, ensuring safe and reliable operation. |
Invisible beams require aiming aids. Visible marker beams or visible laser diodes are needed to allow the operator to aim the reader accurately. |
Integration is the trend. The laser diode, optics, detector, and electronic circuitry are being integrated into single, self-contained modules and scan engines. |
Laser safety is paramount. Laser products must meet international safety standards, with features like APC circuits and safety-compliant wavelengths. |
In the end, laser diode readers are a testament to the power of coherent light. They bring the precision of a laser to the practical task of reading barcodes, extending range, improving resolution, and unlocking new applications. The art of laser diode reader design lies in the careful balance of power, optics, and electronics, creating a scanner that is safe, reliable, and high-performance in the real world. |
cs@easiersoft.com