What is a CO2 Laser?
CO2 lasers (carbon dioxide lasers) are lasers that have a CO2 Based gaseous mixture as the gain medium. The gaseous mixture usually consists of carbon dioxide (CO2), helium (He), nitrogen (N2), xenon (Xe), hydrogen (H2), and possibly some water vapor. CO2 lasers typically operate from 9 to 11 μm and produce an output power from tens of watts to many kilowatts. The power conversion efficiency of CO2 laser is above 10% and it can be pumped electrically by DC supply or AC supply (50/60 Hz). CO2 lasers can operate in both continuous wave (CW) and pulsed wave modes. In pulsed mode they can work as nanosecond, picosecond or nanosecond lasers. CO2 lasers are ideal for galvo scanners, plastic, wood, and leather cutting, digital converting, polypropylene coating & film cutting, industrial welding & cutting, and medical applications.
Construction of C02 Lasers
Construction of CO2 lasers
CO2 lasers consist of a quartz tube that contains the gas mixture N2:CO2: He (typically in the ratio, 15%:15%:70%). The tube has a fully silver coated mirror at one end and a partially silver coated mirror at the other end that forms the optical cavity. A power supply terminal is available to give the required supply voltage. The output power of the laser is linearly proportional to the tube diameter. The length of the tube can be several meters, and its diameter is usually in centimeters. CO2 lasers are either air-cooled or water-cooled.
How does a CO2 laser work?
Energy levels of CO2 lasers
When an electrical supply is given to the laser, energetic electrons are discharged in the tube and travel through the gas mixture (helium, nitrogen, and CO2). The ground state electrons of nitrogen atoms absorb energy from energetic electrons and move to an excited energy state (higher energy level). Now, collisions occur between the nitrogen atoms (in an excited state) and CO2 atoms. So, the excited electrons of nitrogen will give energy to the ground state electrons of CO2. As a result, the ground state electrons of CO2 move to a higher energy state (001) (a metastable state with a lifetime of 10-3 s), whereas the excited electrons of nitrogen will fall into the ground state.
Likewise, millions of ground state electrons of CO2 are excited to the metastable state by the nitrogen atoms. Hence, the population inversion is achieved. The state excited state 001 is known as the asymmetric stretch state of CO2 atoms. After some time (10-3 s), the electrons of CO2 atoms in the metastable state (001) will fall spontaneously into the energy level 020 (Bending state) or energy level 100 (symmetric stretch state) and release photons with wavelength 9.6 µm (Bending state) or 10.6 µm (symmetric stretch state).
These emitted photons reflect back and forth between the reflective surfaces and strikes more electrons in the metastable state that leads to the release of more photons (stimulated emission). Again, these newly emitted photons reflect back and forth millions of times between the reflective surfaces and strikes more ions which results in the release of even more photons. When the photons' activity becomes so intense at a certain point, a strong beam of laser light is obtained.
The helium gas is used to conduct the heat to the walls of the tube as well as to decrease the population in the lower levels (010). The other constituents hydrogen (H2), water vapor, and/or xenon (Xe) help to reoxidize the carbon monoxide (formed in the discharge) to the carbon dioxide.
Specifications of CO2 Lasers
Wavelength: Represents the wavelength of laser light emitted from a CO2 laser. These lasers are available from the deep infrared range (9.3 µm) to the IR range (11 µm). The wavelength is represented in a micrometer (µm).
Operation mode: Represents the operation mode. These lasers are available both in continuous wave (CW) and pulsed mode operation. The pulse laser is also called a q-switched laser.
Laser color: Infrared
Gain medium type: Gas
Laser gain medium: CO2 gas
Power: Represents the power output of the CO2 laser. It can have the range from some tens of watts to many kilowatts.
Pulse width: Represents the pulse duration of the CO2 laser working in pulsed mode of operation. It can be in nanoseconds (10−9 s) or picoseconds (10−12 s) or femtoseconds (10−15 s) range.
Pulse energy: Represents the pulse energy of the CO2 laser working in pulsed mode of operation. Usually, it is in range from mJ to J (joule) range.
Ultrafast laser: The pulse laser or Q-switched laser with pulse width is in nanosecond (10−9 s) or picosecond (10−12 s) or femtosecond (10−15 s) range is called as ultrafast laser.
Beam quality factor (M2): It is the measure of beam quality. As per ISO Standard 11146, the beam quality is defined as the beam parameter product (product of beam radius in a focus and the far field beam divergence) divided by λ / π. Typically, a diffraction-limited beam has the M2 factor of 1 and is a Gaussian beam. Small values of M2 is not physically possible.
Polarization: Represents the polarization mode of light. The CO2 lasers can be horizontally/vertically polarized.
Beam diameter: Represents the beam diameter in mm.
Beam divergence: Represents the divergence of the laser beam. Usually, it is represented in milliradian (mrad).
Dimension: Represents the mechanical dimension of the CO2 laser in length (mm) x width (mm) x height (mm).
Weight: Represents the weight of the laser system in Kg.
Package: Usually, it is available in a benchtop package.
Cooling method: Represents the cooling method used in the CO2 laser. It can be air/water cooled.
Power supply: It can be a DC/AC power supply. It is represented in Volts (V).
Operating temperature: Represents the safe operating temperature of the CO2 laser system. It is represented in Degree Celsius.
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