What are DPSS or Diode Pumped Solid State Lasers?
Diode Pumped Solid State (DPSS) Lasers are solid-state lasers pumped with laser diodes. They deliver very short nanosecond pulses with high peak power and repetition rate with little thermal input. These lasers deliver a wide variety of different wavelengths with very good beam quality. The gain medium of these lasers is glasses or crystals which are doped with impurities such as neodymium, chromium, erbium, or other ions. This gain medium is placed between two mirrored surfaces. The mainly used laser crystals in this laser are Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG), Neodymium-doped yttrium orthovanadate (Nd:YVO4,) and Potassium Titanyl Phosphate (KTP). The KTP crystal is sensitive to optical damage at extremely high output levels. DPSS lasers are optically pumped with diode modules having a longer lifetime. Since the laser diodes have low noise intensity, the diode-pumped lasers also have very low noise. Figure 1 represents the internal diagram of a DPSS laser.
Figure 1: Internal diagram of a DPSS Laser
DPSS lasers are continuous wave lasers that can also work in pulsed mode by Q-switching. These lasers are sensitive to temperatures. They have a more compact design with small laser heads than flashlamp-pumped lasers and they are very efficient, reliable, and flexible. They also require very less maintenance. The possibility of harmonic generations in this laser allows the wavelength emission from ultraviolet to infrared. The commonly used DPSS laser is 532 nm green laser. DPSS lasers utilize the narrow linewidth property of diode lasers together with their high brightness. This leads to maximum absorption causing increased efficiency.
Working of DPSS Lasers
The gain medium containing the laser crystal is placed between a resonator cavity formed using two mirrors. One of the mirrors is fully reflecting and the other one partially reflecting. The laser light from the laser diode is focused on the gain medium with a focusing lens. In the case of a DPSS laser with green emission, an Aluminium Gallium Arsenide (AlGaAs) diode module pumps an Nd:YAG laser at a wavelength of 808 nm causing lasing emission at a wavelength of 1064 nm. The laser light is transmitted through the partially reflecting mirror and comes out through the output coupler. This wavelength is frequency doubled using a nonlinear optical process in the KTP crystal to generate a 532 nm wavelength. Figure 2 represents the schematic diagram of a DPSS laser.
Figure 2: Schematic diagram of a DPSS Laser
Laser light can modify the optical properties of a material system. In frequency doubling also called second harmonic generation, photons of frequency ω interact with a nonlinear medium or crystal with a second-order nonlinear optical susceptibility χ(2) combine to form photons of double the frequency which is 2ω. The second-order nonlinear optical interactions can occur only in non-centrosymmetric crystals, i.e., crystals that do not display inversion symmetry. As the fundamental beam with frequency ω propagates through the nonlinear crystal, its intensity gets depleted and the intensity of the second harmonic wave with frequency 2ω starts to grow. The second harmonic generation converts the output of a fixed-frequency laser to a different spectral region. For example, the Nd:YAG lasers operate in the near-IR range at a wavelength of 1.06 µm. The second harmonic generation converts this wavelength to 0.53 µm which is in the middle of the visible spectrum.
Figure 3: Geometry and Energy level diagram of second harmonic generation
Here, in figure 3, two photons of frequency ω are converted to a photon of frequency 2ω. For frequency doubling, the phase-matching condition has to be satisfied and it is given by,
∆k = k2 – 2k1 = 0
where k1 and k2 are the wavenumbers of the fundamental and the second harmonic beam respectively. By angle tuning the crystal, the phase-matching condition is achieved.
Diode-pumped solid-state lasers are optically pumped using laser diodes. Different types of laser diodes are used for this purpose. Small edge-emitting laser diodes are used to pump low-power lasers with power up to 200 mW. The beam quality of these lasers is diffraction-limited. Broad area laser diodes are used to end pump solid-state lasers with few watts of output power. They have an asymmetric beam quality. High-power lasers are pumped with high-power diode bars having more than 100 watts of output power. The beam quality of these lasers is also asymmetric and poor. Their radiance is lower than lower-power diodes. Beam shapers are used to make the beam profile symmetric so that the light beam can be used to pump huge lasers and to couple light into a fiber. For even higher power like multiple kilowatts, diode stacks can be used to side-pump the lasers. These lasers also have lower beam quality and radiance. The main disadvantage of diode pumping is that it requires a high cost per watt of pump power.
Energy level diagram of DPSS Lasers
DPSS laser is a four-level laser system as shown in Figure 4. In the case of a Diode Pumped Nd:YAG laser, atoms or molecules in the ground state E0 absorb the 808 nm wavelength from the laser diode and get excited to the energy level E3 where the lifetime is less. So, they make a non-radiative transition to the energy level E2.
Figure 4: Energy level transitions in a Diode pumped Nd:YAG Laser
E2 is a metastable state where the lifetime is high. This helps to develop population inversion between energy levels E2 and E1. Therefore, lasing takes place between these two levels emitting a wavelength of 1064 nm. And then the atoms make a non-radiative transition to ground state E0 from E1. The output wavelength of DPSS lasers varies from diode to diode.
Applications of DPSS lasers
Diode-pumped solid-state lasers are used for a variety of applications like medicine, dentistry, military, industry, research, etc. These lasers are used to create bright and colorful displays for sporting events, concerts, and other huge gatherings. They are used in scientific research to study the properties of materials and molecules. They are used for industrial applications such as alignment and positioning, laser cutting, welding, drilling, cladding, hardening, trimming, etc. These lasers produce highly focused intense light beams that can cut and weld materials from metals to plastics.
DPSS lasers find applications in the medical and dental fields too. In these fields, they are used for different procedures such as laser therapy, laser surgery, and laser-assisted dentistry. These lasers can remove tissue or repair damage without causing much damage to the surrounding areas.
They are used for military and aerospace applications too. In the military, they are used for communication, long-range audio spying, defensive countermeasures, target designation, and weapon research. For example, the DPSS laser is used in the mine hunting system to detect and classify mines from the sea floor to the surface. This laser system is towed underwater from a helicopter and it gives real-time sonar images to operators to see and record mine-like objects. DPSS lasers are used as range finders in Lunar Reconnaissance Orbiter (LRO) satellites. Accurate maps of lunar surfaces in preparation for NASA’s return to the moon is one of the goals of LRO. The ground station contains DPSS lasers for determining the range of the satellite and spacecraft position. The moon's orientation and gravity are better understood because of this technology, which is important for studying the moon's structure and navigating it.
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