What Are The Structure And Use Of Laser Diodes?

The Laser Diode is a light source semiconductor laser, also known as a laser diode, invented in the 1960s. LASER is an acronym for "Light Amplification by Stimulated Emission of Radiation ", often referred to as LD. Because it can produce light with exactly the same wavelength and phase properties, high coherence is its biggest feature. Let's find out.

1. Physical structure

A layer of photoactive semiconductor is placed between the junctions of the light-emitting diode, and its end face has a partial reflection function after polishing, thus forming an optical resonator. In the case of forward bias, the LED generation emits light and interacts with the optical resonator, thereby further stimulating the emission of single-wavelength light from the junction, the physical properties of which are dependent on the material.

In the VCD machine, the semiconductor laser diode is one of the core components of the laser head, which is mostly composed of a double heterostructure gallium aluminum-arsenic (AsALGA) ternary compound, is a near-infrared semiconductor device with a wavelength of 780 ~ 820 nm and a rated power of 3 ~ 5 MW. In addition, there is a visible light (such as red) semiconductor laser diode, which is also widely used in VCD machines and barcode readers.

The shape and size of the laser diode are shown in Figure 1. There are three types of internal structures.

As can be seen from Figure 2, the laser diode includes two parts: the first part is the laser emission part (which can be represented by LD), its role is to emit laser, as shown in Figure 2 electrode (2); The second part is the laser receiving part (can be represented by PD), its role is to accept and monitor the laser emitted by the LD (of course, if the output of the LD is not monitored, the PD part can be used), as shown in Figure 2 electrode (3); These two parts share a common electrode (1), so the laser diode has three electrodes.

The laser diode has the advantages of small size, lightweight, low power consumption, simple drive circuit, convenient modulation, mechanical shock resistance, and vibration resistance, but it is extremely sensitive to overcurrent, overvoltage, and electrostatic interference, therefore, in use, pay special attention not to make its working parameters exceed its maximum allowable value, the method can be used as follows:

(1) The laser diode is driven by a DC constant current source.

(2) Series current-limiting resistors and parallel bypass capacitors on the laser-electrode circuit.

(3) Because the temperature of the laser diode will increase the current value flowing through it, the necessary heat dissipation measures must be used to ensure that the device works within a certain temperature range.

(4) In order to avoid the breakdown damage caused by the laser diode due to excessive reverse voltage, the silicon diode can be fast in reverse parallel at both ends.

2. Detection method

(1) Resistance measurement method: Remove the laser diode and measure its positive and reverse resistance values with a multimeter R×1k or R×10k. Normally, the forward resistance value is between 20 and 40 kω, and the reverse resistance value is infinity. If the positive resistance value is measured to exceed 50kΩ, it indicates that the performance of the laser diode has declined. If the measured forward resistance value is greater than 90 kω, it indicates that the diode has been seriously aged and can no longer be used.

(2) Current measurement method: Use a multimeter to measure the voltage drop at both ends of the load resistor in the laser diode drive circuit, and then estimate the current value flowing through the tube according to Ohm's law, when the current exceeds 100mA, if the laser power potentiometer is adjusted, and the current does not change significantly, the laser diode can be judged to be seriously aging. If the current surges out of control, it means that the optical cavity of the laser diode has been damaged.

3. Matters needing attention

⑴The laser emitted by the laser diode may cause damage to the human eye. When the diode is working, it is strictly prohibited to directly look at its end face, not to look directly at the laser through the lens, and not to observe the laser through the mirror.

⑵The device needs a suitable drive power supply, the instantaneous reverse current should not exceed 2uA, and the reverse voltage should not exceed 3V, otherwise, it will damage the device. Measures to prevent inrush current when the power supply is on and off. When testing the driver circuit with an oscilloscope, disconnect the power supply and then connect the oscilloscope probe. If the probe is tested with power on, the device may be damaged by an inrush current.

⑶The device should be stored or worked in a clean environment.

⑷ Working at a higher temperature will increase the threshold current, lower conversion frequency, and accelerate the aging of the device. When adjusting the optical input, the optical power meter should be detected to prevent exceeding the large rated output.

⑸If the output power is higher than the specified parameter, it will accelerate the aging of the component.

⑹The machine needs to fully heat or use under refrigeration conditions, and the temperature of the laser diode is strictly controlled below 20 degrees to ensure life.

⑺ The diode is an electrostatic sensitive device, which can be taken only when the human body is in good condition. Anti-static can be used by anti-static bracelets.

⑻ The output wavelength of the laser is affected by the working current and heat dissipation, so it is necessary to maintain good heat dissipation conditions and reduce the temperature of the tube core when working. A heat sink is added to prevent the laser diode from rising too high during operation.

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