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The working principle of Q300 RF CO2 laser

The full name of Q-300 is " Radio Frequency Excitation Diffusion Cooling Slab Waveguide Carbon Dioxide Laser ". The principle of this radio frequency laser uses a radio frequency power supply as the pump source. Two rectangular metal slats are placed in parallel with a distance of a few millimeters between them. Different lasers have different intervals. One of the electrode plates is grounded through the metal shell and is named the negative plate, and the other is connected to the radio frequency power supply through the RF feed-in device, and is named the positive plate. Two total reflection mirrors are installed at both ends of the two metal slats to form a laser resonant cavity for extracting laser light. The positive and negative plates and the laser cavity mirror are installed inside a sealed metal cavity, and the metal cavity is usually made of aluminum alloy. The laser working gas, the positive and negative electrode plates, and the laser cavity mirror are sealed in the metal cavity. The RF power is introduced into the electrode plate through a special feedthrough, and the working gas between the electrode plates is ionized to generate gain material. The laser output is extracted through cavity mirrors installed at both ends of the electrode plate, and the laser beam is output from the window on the sealed cavity. The window is sealed with anti-reflection coated ZnSe lens, which can extract laser energy efficiently, and can seal the cavity in a stable and reliable high vacuum. There are water channels inside the positive electrode plate and the negative electrode plate, and both plates need to be forced to cool with water. The water path of the positive plate needs to be specially designed to ensure that the positive plate is insulated from the outside and smooth water flow. The schematic diagram of the Q-300 RF laser principle is as follows, the schematic diagram does not show the cooling water circuit of the pole plate and the inductor coil connected to the positive and negative plates:

Q-300 metal radio frequency carbon dioxide laser working principle diagram

-Heat dissipation

The injected radio frequency power ionizes the CO2 working gas to produce laser light, and at the same time about 80% of the radio frequency power becomes heat and accumulates in the discharge area. The waste heat accumulated in the discharge area is directly transferred to the metal discharge electrode plate, and the flowing cooling water in the discharge electrode plate removes the waste heat to protect the discharge electrode plate from deformation and ensure the stability of the laser power.

-Beam shaping

The asymmetry of the structure of the resonant cavity of the Q-300 RF CO2 laser in two directions determines the asymmetry of the beam output by the laser when it is transmitted outside the cavity. The beam outside the cavity is an elliptical spot, the spot size in the X and Y directions, the beam divergence angle is different, and there is a side lobe beam in the X direction. The Q-300 laser shaping system first uses a cylindrical lens to transform the beam into a circular beam, then focuses, uses a spatial filter at the focal point to remove the sidelobe beam, and finally collimates. Through the above method, the beam is transformed into a circularly symmetrical quasi-Gausky mode beam.

-polarization

The discharge area of ​​Q-300 is parallel to the direction of the bottom plate of the laser, and its output beam is linearly polarized light, and the polarization direction is parallel to the direction of the bottom plate. If you need to use circularly polarized light, you can add a phase retardation device in the optical path, such as a phase retardation mirror.

-Retro-reflected beam isolation principle

When the metal radio frequency carbon dioxide laser is applied to the processing of highly reflective materials, it is necessary to insert a retroreflective beam isolator in the optical path. The principle is as follows: First, the laser beam is incident on a device that completely transmits P polarized light and completely reflects S light, such as the Brewster window, and then enters a 45-degree phase retardation device, such as a 45-degree phase retardation mirror, and then enters it. Focus lens for laser processing. After being reflected by the highly reflective material, the P light passes through the phase retardation mirror twice and becomes S light. When it enters the Brewster window, it cannot be transmitted, is completely reflected, and cannot return to the laser. The unidirectional optical path transmission device composed of Brewster window and 45-degree phase retarder is a kind of beam isolator.


-Beam transmission optical path construction and optical device protection

Under normal circumstances, the laser beam reaches the material by expanding the beam and then focusing for laser processing. For the optical path mode of the cutting machine, the laser beam passes through two movable 45-degree mirrors and is incident on the focusing mirror for material processing. For the optical path mode of the galvanometer marking machine (re-focus), the laser beam first enters the beam expander, then enters two galvanometer lenses, and then passes through the focusing field lens to achieve precise and rapid processing in a small area. The processing area should have a good exhaust device to ensure that the exhaust gas will not pollute the lens. From the laser exit to the focusing lens, a reliable light path sealing device must be installed. It is necessary to blow air (clean air or nitrogen) into the light path sealing device for protection.

Each device on the optical path should be handled carefully during debugging and installation, and the surface of the device should not be contaminated. At the same time, the lens should not be subjected to excessive mechanical stress during installation.

-RF power supply

Spell Laser ’s Q-300 RF CO2 laser is driven by an 81.36MHz RF power supply. The power circuit uses a 48V DC power supply, and the output power is controlled by a control signal (such as a PWM signal). First, a crystal oscillator is used to generate an 81.36MHz RF signal, and then a high-power RF output is generated through a power amplifier circuit. The radio frequency power supply can provide laser status indication, such as normal operation, temperature, flow, voltage and other signals. By observing the status of the signal light, the status of the laser can be judged.

-Refrigeration system and cooling water temperature setting

Reasonable and appropriate cooling is essential for the normal operation of the Q-300 RF laser. Since most of the injected RF power becomes waste heat, a matched refrigeration system is required to cool the laser. According to the difference of ambient temperature and humidity, the setting range of cooling water temperature is 21~30 degrees, and the flow rate is not less than 6.5L/min. When setting the cooling water temperature, refer to the ambient temperature and humidity to prevent condensation on the laser. If condensation occurs on the laser surface, it will cause irreversible damage to the laser and seriously affect the life of the laser. The cooling water temperature should be higher than the dew point of the air. For example, if the dew point is 22 degrees, the cooling water temperature should be set to 23 degrees or higher. Of course, if the set cooling water temperature is too high, higher than the recommended range, such as 35 degrees, the waste heat generated in the discharge area will not be effectively discharged, resulting in a reduction in laser power and increasing the probability of RF power failure. For refrigeration and cooling water, deionized pure water must be used. If you use tap water, it will cause scaling and corrosion of the waterway, which will cause fatal damage to the laser. Do not operate the laser in the environment near the freezing point. The ice crystals that may exist in the cooling water will impact the laser water circuit and affect the life of the laser. When storing the laser, drain the water in the laser waterway to prevent the water from corroding the waterway during the storage process, or the waterway is frozen and the waterway is ruptured, causing the laser to be scrapped.

-Modulation signal and pulse width modulation method

The radio frequency carbon dioxide laser receives the control signal input from the outside and modulates the output power of the laser. According to the pulse width and frequency of the output signal, switch the RF power supply to obtain the corresponding laser pulse output. For the Q-300 laser, the maximum duty cycle is 60%. The relationship between Q frequency and pulse width is as follows:

Among them, W is the modulation pulse width, the unit is µs, and Q is the modulation frequency, the unit is KHz. For example, at a frequency of 5KHz and a duty cycle of 60%, according to the above formula, calculate W=60%*1000/5=120µs.


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