Precise control method of the hottest laser diode

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Precise control method of laser diode emission

in many applications that use light to control a process, it is very important to maintain constant brightness. In some systems, a simple LED or laser diode is used to generate a light source to provide illumination, but even after initial calibration, the light source will degrade over time. As the LED ages, its "current to light" emission ratio will decrease. Finally, the brightness will also decrease. If you want to always maintain the emission energy set at the factory, you need a control circuit to monitor the emission and control the current supplied to the optical transmitter to keep the output constant. Such a configuration can be applied to photometry to obtain accurate brightness; It can also be used in control applications for precise optical positioning of servo devices; It can also be used in test equipment for optical reference. Figure 1 is a block diagram of such a system

Figure 1 Schematic diagram of conventional system

photodiode 101

the structure of silicon photodiode is similar to that of PN junction diode, but the p layer is very thin. The thickness of the p layer is adjusted according to the wavelength of the light to be detected. Photodiodes also have a capacitance, which, like non photodiodes, is proportional to the reverse bias voltage applied to the photodiode. The range of typical values is PF. The photodiode has two terminals -- cathode and anode

photodiodes can be used in forward mode (current flows from anode to cathode) or reverse mode (current flows from cathode to anode). When a photodiode is used in the reverse mode (the anode is negative), it is highly linear with the illumination at a given frequency, which is a benefit. Because it is much easier to build a control circuit when it is regarded as a linear relationship. 1.1 under the condition of strictly abiding by the transportation, storage, device and application rules of the experimental machine, there is only leakage current (also known as "dark current") in the prototype design, and the amplifier will become overloaded. This condition draws the current limited by the resistor from the bottom of the transistor whose initial state is saturated

once the current begins to flow through the transistor, the LED or laser diode will start emitting light. The photodiode converts part of the light into current, which flows through RG. As the current increases, the voltage drop formed on RG also increases. On March 17, 2015, when the voltage approaches Vbias (ground in Figure 2), the loop will close and maintain the correct drive for the transistor to maintain the current in the LED, thereby maintaining a constant brightness level (or current in the photodiode). This forms the basis of circuit DC analysis

Figure 2 prototype control circuit

if you have a simple operational amplifier as shown above, it will be easy to create an accurate brightness for many different application devices. Even when the light emitter is aging, the control loop can maintain a constant brightness by adjusting the current in the LED

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