Abstract: Although laser-type photoelectric switches and commonly used incoherent optical photoelectric switches belong to the same category of photoelectric switches, laser-type photoelectric switches are made of highly coherent semiconductor lasers. Compared with the latter, they have significant advantages such as longer detection distance, higher detection position accuracy, and a wider range of selectable wavelengths, ranging from visible light to infrared with more than ten varieties. Therefore, they have a very broad range of new applications. Keywords: Semiconductor laser; Photoelectric switch; Photodiode I. Introduction Semiconductor lasers are devices that generate laser light using semiconductor crystal materials. Like other lasers, they have the characteristics of good coherence, strong directionality, small divergence angle, and high brightness. They also have unique advantages such as small size, high efficiency, and convenient modulation. Therefore, this product has wide applications in integrated optics, fiber optic communication, fiber optic sensing, laser ranging, optical storage, laser printing, and military engineering. Using semiconductor lasers to make photoelectric switches is a successful application of their photoelectric sensing technology. This article will give a brief introduction to the simple working principle of semiconductor laser-type photoelectric switches, their unique characteristics, and application examples. II. Working Principle of Laser-Type Photoelectric Switches 1. Working Principle of Semiconductor Laser Diodes To understand the working principle of laser-type photoelectric switches, we will first briefly introduce the working principle and characteristics of semiconductor laser diodes. The basic structure of a semiconductor laser diode is shown in Figure 1. A pair of parallel planes perpendicular to the PN junction plane constitute a Fabry-Perot resonant cavity. These can be cleavage planes of the semiconductor crystal or polished planes. The other two sides are relatively rough to eliminate the laser effect in directions other than the main direction. Light emission in semiconductors is usually caused by carrier recombination. When a forward voltage is applied to the PN junction of a semiconductor, the PN junction barrier is weakened, forcing electrons to enter the P region from the N region through the PN junction, and holes to enter the N region from the P region through the PN junction. These non-equilibrium electrons and holes injected near the PN junction will recombine, thereby emitting photons with a wavelength of λ, as shown in the following formula: Where: h—Planck's constant; c—speed of light; Eg—bandgap of the semiconductor. The phenomenon of light emission due to the spontaneous recombination of electrons and holes is called spontaneous emission. When photons generated by spontaneous emission pass through a semiconductor, they can be excited to recombine with previously emitted electron-hole pairs, producing new photons. This phenomenon, where photons induce the recombination of excited charge carriers to emit new photons, is called stimulated emission. If the injected current is large enough, a charge carrier distribution opposite to the thermal equilibrium state will form, i.e., population inversion. When a large number of charge carriers in the active layer are inverted, a small number of photons generated by spontaneous emission will produce induced radiation due to the reciprocating reflection from the two ends of the resonant cavity, causing frequency-selective resonance positive feedback, or gain at a certain frequency. When the gain is greater than the absorption loss, coherent light with good spectral lines—laser light—can be emitted from the PN junction. This is the simple principle of a laser diode. With the development of technology and processes, semiconductor laser diodes currently in use have complex multilayer structures. Figure 2 shows the structure of a red semiconductor laser diode from Sanyo Corporation of Japan. Figure 3 is a cross-sectional view of a low-power laser tube. As can be seen from the figure, the laser chip is attached to a heat sink for heat dissipation, and a PIN photodiode is sealed on the tube socket near the lower part of the laser chip. Figure 4 shows the appearance of a common laser diode. As can be seen from the figure, the low-power laser tube has three pins. This is because a photodiode is also encapsulated inside the tube to monitor the working current of the laser tube. Common parameters of semiconductor laser diodes include: (1) Wavelength: the working wavelength of the laser tube. Currently, the wavelengths of laser tubes that can be used as photoelectric switches are 635nm, 650nm, 670nm, 690nm, 780nm, 810nm, 860nm, 980nm, etc. (2) Threshold current Ith: the current at which the laser tube begins to generate laser oscillation. For general low-power laser tubes, its value is about tens of milliamperes. The threshold current of laser tubes with strained multi-quantum-well structures can be as low as below 10mA. (3) Working current Iop: the driving current when the laser tube reaches its rated output power. This value is important for designing and debugging laser driving circuits. (4) Vertical divergence angle θ⊥: the angle at which the light-emitting band of the laser diode opens in the direction perpendicular to the PN junction. It is generally about 15˚～40˚. (5) Horizontal divergence angle θ∥: The angle at which the light-emitting band of the laser diode opens in the direction parallel to the PN junction, generally around 6˚～10˚. (6) Monitoring current Im: The current flowing through the PIN tube when the laser tube is at its rated output power. 2. Semiconductor laser photoelectric switch The laser photoelectric switch consists of a laser transmitter and a receiver, used to complete tasks such as target detection, counting, positioning, travel control, and remote control. Its general principle block diagram is shown in Figure 5. Figure 5 shows the transmitting and receiving circuit of the semiconductor laser photoelectric switch made with dedicated remote control integrated circuit chips BA5104, BA5204 (Editor's note: BA5104/5204 is no longer produced, and the replacement model is SM5031/5032) and M6938. In Figure 6(a), BA5104 is the transmitter chip, K1～K8 are the control input terminals, and pull-up resistors are connected internally. When any key is pressed, the clock circuit connected to pins OSC1 and OSC2, along with the external 455kHz crystal and capacitors C1 and C2 forming an oscillation circuit, begins to oscillate. The internal circuit then divides the frequency to generate a 38kHz carrier frequency. The BA5104 encodes the data input from pins C1, C2, and K1 to K8, and outputs it serially from pin D0. After amplification by a Darlington transistor D1581, it drives the semiconductor laser diode (LD) to output a modulated carrier pulse laser signal. Potentiometer W adjusts the laser diode's operating current to keep it within its rated operating current. The LED pin is the output for displaying the emission status; a high-level output illuminates the LED. Pins C1 and C2 are user-defined inputs, allowing for four encoding settings. The LD semiconductor laser diode in the diagram is a key component of the photoelectric switch transmitter. The receiving circuit is shown in Figure 6(b). When the receiving and demodulation module M6938 receives the laser signal, it amplifies, selects the frequency, and demodulates the pulse. It then outputs a low level, which is inverted by VT1 and applied to pin D1 of BA5204. After comparison and decoding by this circuit, corresponding control signals are output from pins 3 to 10 of BA5204. When any key from K1 to K6 on the transmitter is pressed, the corresponding HP pin of BA5204 outputs a high level; releasing the transmitter key outputs a low level. CP1 and CP2 are self-locking inverted level outputs. When one of the keys K7 or K8 is pressed, the level output from the corresponding CP pin of BA5204 flips; each press flips the level once. C1 and C2 are user code inputs. There are four encoding methods, which must be consistent with the settings of C1 and C2 pins of BA5104. Capacitor C1, resistor R2, and the internal circuit of pin 13 of BA5204 form an oscillation circuit with a frequency of 38kHz. The AS1117 is a low-dropout regulator with an output of 3.3V. Figure 7 shows a laser-type photoelectric switch circuit composed of a microcontroller and a phase-locked loop (PLL) circuit. The microcontroller programs the encoding and modulation functions, greatly simplifying the circuit and improving stability, which is beneficial for mass production. Furthermore, semiconductor laser tubes are sensitive to surges, i.e., instantaneous overvoltages and overcurrents. Using a microcontroller, in addition to software-implemented encoding and modulation functions, software delay can be used to achieve slow start-up of the laser circuit, effectively preventing damage to the laser tube caused by electrical surges. The IRM8881V in Figure 7(b) amplifies the received laser signal and has demodulation capabilities, while the PLL acts as a decoder. By adjusting the voltage-controlled oscillation frequency of the PLL to the encoding frequency determined by the software, when the receiver receives the encoded and modulated signal emitted by the laser, and after amplification, demodulation, and decoding by the receiver, a low-level signal is output at pin 8 of the LM567, resulting in a high-level output at the collector of the PNP transistor, thus completing the reception of the transmitted signal. The output form of the receiver can be of various forms, such as being driven by PNP or NPN transistors, or driven by relays, etc. The output state can be either normally open or normally closed. The detection methods include through-beam, diffuse reflection, specular reflection and fiber coupling. The laser-type photoelectric switch has all the functions of a general photoelectric switch, and also has the following significant advantages. (1) The detection distance can be as long as hundreds of meters to several kilometers. Semiconductor laser light sources are highly coherent light sources, and therefore have strong directionality. After collimation by an optical system, the divergence angle can be easily limited to within 0.2 mrad, as shown in Schematic 8. The size of the laser spot can be approximately calculated by the following formula: Where, d — spot diameter (mm); L — detection distance (m); θ — divergence angle (mrad). If a laser beam is projected to a distance of 500m, the spot diameter can be approximately 100mm. It can be seen that the spot is not large, and there is still a large energy distribution within this range, which is sufficient to drive the receiver to work. At the same time, using a high-power laser tube can easily achieve target detection at a distance of several kilometers. Currently, the effective range of commonly used infrared photoelectric switches using incoherent light sources is generally limited to tens of meters. This is because when infrared light shines at a distance of 20 to 30 meters, the diameter of the radiated beam becomes several tens of centimeters or more, and the energy becomes highly dispersed. Therefore, using laser-type photoelectric switches will greatly expand the applications of photoelectric switches, such as wide-area boundary alarms, remote control switches, remote demolition, and long-distance control. Figure 9 shows a laser-type photoelectric switch with an effective range of up to 600 meters. For such laser-type photoelectric switches operating at long detection distances, target alignment is easily achieved if a visible light source, such as a red laser tube, is used as the light source; however, if an infrared laser is used as the light source, night vision devices or infrared test cards should be used for alignment. (2) High-precision positioning control and detection of tiny targets can be achieved. Since semiconductor lasers are highly directional coherent light sources, after being collimated or focused by an optical lens, the beam is very thin. Therefore, a small aperture can be used to obtain a laser beam with a diameter of 0.1mm to 0.5mm. With this thin collimated beam, high-precision positioning control, position detection, and wire diameter measurement can be achieved, and targets with diameters as small as a needle tip can be detected. In particular, after coupling and collimating the semiconductor laser beam through a single-mode fiber with a core diameter of 5mm to 9mm, the detection accuracy can be improved to the micrometer level, and targets as small as the diameter of a human hair can be easily detected. It is worth mentioning that after the collimated laser beam is passed through a cylindrical optical lens, a straight beam can be emitted with a line width of 0.5mm, as shown in Figure 10. Using this line light source, material positioning and cutting in assembly line operations, target positioning in medical CT scans, and detection of the straightness and unevenness of objects can be achieved, with a wide range of applications. In addition, a cross-shaped laser beam can be emitted after passing a collimated laser beam through a certain grating sheet, which also has unique applications in detection and positioning. (3) To adapt to different usage requirements, there are multiple wavelengths of laser light sources available. In the past one or two decades, semiconductor laser technology has made great strides. Commercially available semiconductor laser tubes have four types in the visible light band with wavelengths of 635nm, 650nm, 670nm, and 690nm, and several types in the near-infrared band with wavelengths of 780nm, 810nm, 830nm, 850nm, 860nm, 910nm, and 980nm. According to the usage requirements, any one of them can be flexibly selected as the laser power supply for photoelectric switches. Generally speaking, if the visible light band is selected, it is more convenient to install, debug, and align. If the near-infrared band is selected, a night vision device or infrared test card must be used in conjunction with the installation and debugging process. When using models with wavelengths shorter than 860nm, although the laser beam is not visible, a red light can still be clearly seen in the laser tube window, a phenomenon known as "red burst." Therefore, when using photoelectric emission sensors for perimeter alarms, it is advisable to select 910nm or 980nm lasers that do not exhibit "red burst" to avoid revealing the target. Sometimes, the loss in the transmission medium must also be considered. For example, green lasers are rarely absorbed in water, while other wavelengths are easily absorbed, causing severe energy attenuation. Therefore, a 532nm green laser is most suitable for underwater operation. In short, the wide wavelength range and rich variety of semiconductor lasers available today provide users with ample choice. III. Application Examples Laser-type photoelectric switches can be applied to all areas where traditional infrared photoelectric switches are used, and they also have unique functions. 1. Security Alert Applications: Figure 11 illustrates an example of large-scale security alerts. Assuming a square or rectangular area (other shapes can also be addressed) needs to be monitored, as shown, plane mirrors coated with optical reflective material can be placed at the corners of the three sides of the square, forming a 45° angle with the laser beam. After the laser transmitter and receiver are powered on, the mirrors are adjusted so that the receiver can receive the laser signal. Thus, when a moving target passes through the laser beam path and blocks it, the receiver outputs a signal to the microcomputer monitoring system, achieving a security alarm. In practical applications, a dual-beam laser system is typically used in appropriate locations to avoid false alarms caused by falling leaves, birds, cats, or dogs. This alarm system is suitable for residential communities, airports, farms, power plants, prisons, border defense facilities, and has a wide range of applications. For some critical facilities, an independent transmitter-receiver system can be installed on each side, with each receiver connected to the monitoring system. This way, once an alarm signal occurs, it can be immediately identified which side is alarmed, gaining time for timely handling and eliminating the need for mirror maintenance. 2. Detection of Vehicles Exceeding Height and Size Limits: Railway and transportation departments have strict requirements on the height and width of objects loaded on vehicles to prevent damage to roads and surrounding traffic facilities or major traffic accidents. Laser-type photoelectric switches can easily detect vehicles exceeding height and size limits. Figure 12 shows a schematic diagram of a train vehicle detection device. A magnetic conductor is installed approximately 100 meters from the detection system in the direction of oncoming traffic. When the locomotive's wheels pass the magnetic conductor, the device sends an activation signal to the detection system, initiating its operation. When a vehicle passes the detection system, if an object blocks the laser beam (dual-beam system), the receiver will issue an alarm signal. If a software program based on my country's vehicle structure is added to statistically analyze axles and vehicles, it can identify which vehicle is exceeding height and size limits. However, in this case, two magnetic sensors need to be installed at certain intervals to provide auxiliary information such as wheel spacing and vehicle speed. 3. Application in Textile Systems: In the textile industry, numerous spinning machines operate. A spinning machine, often over ten meters wide, has many spindles in motion. If a yarn breaks, the machine must be stopped immediately for the operator to reconnect the broken end. Laser-type photoelectric switches can effectively achieve online detection of broken yarn. A transmission-type laser emitter and receiver are installed below the yarn path, perpendicular to the direction of yarn movement. When a broken yarn falls, it blocks the laser beam path, triggering an alarm signal from the receiver, stopping the spinning machine and allowing for reconnection. 4. Remote Laser Power Supply in Special Environments: In environments with high concentrations of flammable and explosive atmospheres, such as those in the oil and mining industries, the power supply is strictly limited, and sparks are absolutely prohibited. Therefore, the power supply for all detection instruments has stringent requirements. Using lasers for remote power supply in such environments is undoubtedly a novel and safe option. Figure 15 shows a structural block diagram of a low-power sensor power supply system. A semiconductor laser beam with a modulation frequency of around 10kHz, after being collimated by an optical lens, is coupled to a 125mm core diameter multimode fiber and transmitted over a long distance to a site with a flammable or explosive atmosphere. The photocell in the receiving system receives the laser pulses emitted from the fiber end, generating an alternating voltage. This voltage is then stepped up by a transformer and rectified by a full-bridge rectifier to output a voltage of 3V–5V. Laser pulses of tens of milliwatts can output hundreds of microamps or even milliamps of current, which is sufficient for driving low-power CMOS circuits. For even higher output current, a more powerful laser can be used. To improve rectification efficiency, a bridge circuit composed of Schottky diodes with a forward voltage drop of only about 0.3V is recommended. As shown in Figure 13, the output section of the receiver in this system differs from commonly used photoelectric switches, but this can be considered a functional variation. Semiconductor laser-type photoelectric switches can also be used in many other applications, such as remote-controlled blasting, landslide prediction, vibration detection of bridges when vehicles pass, and laser target shooting, which will not be listed here. IV. Conclusion Laser-type photoelectric switches are a powerful supplement to the functions of infrared photoelectric switches that use incoherent light sources, and greatly broaden the application range of photoelectric switches. They are an excellent alternative to the latter. It is believed that they will have an increasingly broad application prospect in the future and will play a more important role in the fields of detection, industrial control, security detection and other fields. References: [1] Ming Hai, Zhang Guoping et al. Optoelectronic Technology [M]. University of Science and Technology of China Press, 1998. [2] Jiang Jianping. Semiconductor Lasers [M]. Electronic Industry Press, 2000. [3] Mou Bin, Liang Baqi. Semiconductor Laser Remote Control [J]. Electronic World, 2001, No.3. [4] SANYO, LASER DIODE [R]. SANYO, 2002. [5] SANYO [R]. SANYO, 2002/10 - 2003/9