Diode pumped solid state laser
Encyclopedia
Diode-pumped solid-state (DPSS) lasers are solid-state laser
s made by pumping
a solid gain medium, for example, a ruby
or a neodymium-doped YAG
crystal
, with a laser diode
.
DPSS lasers have advantages in compactness and efficiency over other types, and high power DPSS lasers have replaced ion laser
s and flashlamp-pumped lasers in many scientific applications, and are now appearing commonly in green and other color laser pointer
s.
s.
High power lasers use a single crystal, but many laser diodes are arranged in strips (multiple diodes next to each other in one substrate) or stacks (stacks of substrates). This diode grid can be imaged onto the crystal by means of a lens
. Higher brightness (leading to better beam profile and longer diode lifetimes) is achieved by optically removing the dark areas between the diodes, which are needed for cooling and delivering the current. This is done in two steps:
The beams from multiple diodes can also be combined by coupling each diode into an optical fibre, which is placed precisely over the diode (but behind the micro-lens). At the other end of the fiber bundle, the fibers are fused together to form a uniform, gap-less, round profile on the crystal. This also permits the use of a remote power supply.
Each single strip diode typically has an active volume of:
and depending on the cooling technique for the whole bar (100 to 200) µm distance to the next laser diode.
The end face of the diode along the fast axis can be imaged onto strip of 1 µm height. But the end face along the slow axis can be imaged onto a smaller area then 100 µm. This is due to the small divergence (hence the name: 'slow axis') which is given by the ratio of depth to width. Using the above numbers the fast axis could be imaged onto a 5 µm wide spot.
So to get a beam which is equal divergence in both axis, the end faces of a bar composed of 5 laser diodes, can be imaged by means of 4 (acylindrical) cylinder lenses onto an image plane with 5 spots each with a size of 5 mm x 1 mm. This large size is needed for low divergence beams. Low divergence allows paraxial optics, which is cheaper, and which is used to not only generate a spot, but a long beam waist inside the laser crystal (length = 50 mm), which is to be pumped through its end faces.
Also in the paraxial case it is much easier to use gold or copper mirrors or glass prisms to stack the spots on top of each other, and get a 5 x 5 mm beam profile. A second pair of (spherical) lenses image this square beam profile inside the laser crystal.
In conclusion a volume of 0.001 mm³ active volume in the laser diode is able to saturate 1250 mm³ in a Nd:YVO4 crystal.
green laser pointer
. A powerful (>200 mW) 808 nm wavelength infrared
GaAlAs laser diode pumps a neodymium-doped yttrium aluminium garnet
(Nd:YAG)
or a neodymium-doped yttrium orthovanadate (Nd:YVO4) crystal which produces 1064 nm wavelength light from the main spectral transition of neodymium
ion. This light is then frequency doubled using a nonlinear optical
process in a KTP
crystal, producing 532 nm light. Green DPSS lasers are usually around 20% efficient, although some lasers can reach up to 35% efficiency. In other words, a green DPSS laser using a 2.5 W pump diode would be expected to output around 500-900 mW of 532 nm light.
In optimal conditions, Nd:YVO4 has a conversion efficiency of 60%, while KTP has a conversion efficiency of 80%. In other words, a green DPSS laser can theoretically have an overall efficiency of 48%.
In the realm of very high output powers, the KTP crystal becomes susceptible to optical damage. Thus, high-power DPSS lasers generally have a larger beam diameter, as the 1064 nm laser is expanded before it reaches the KTP crystal, reducing the irradiance from the infrared light. In order to maintain a lower beam diameter, a crystal with a higher damage threshold, such as LBO, is used instead.
Blue DPSS lasers use a nearly identical process, except that the 808 nm light is being converted by an Nd:YAG crystal to 946 nm light (selecting this non-principal spectral line of neodymium in the same Nd-doped crystals), which is then frequency-doubled to 473 nm by a beta barium borate
(BBO) or lithium triborate
(LBO) crystal. Because of the lower gain for the materials, blue lasers are relatively weak, and are only around 3-5% efficient. In the late 2000s, it was discovered that bismuth triborate (BiBO) crystals were more efficient than BBO and LBO and do not have the disadvantage of being hygroscopic
, which degrades the crystal if it is exposed to moisture.
Violet DPSS lasers at 404 nm have been produced which directly double the output of a 1,000 mW 808 nm GaAlAs pump diode, for a violet light output of 120 mW (12% efficiency). Initially, these lasers out-performed gallium nitride (GaN) direct 405 nm Blu-ray diode lasers. As direct 405nm diode technology progressed (primarily for use in Blu-ray disc writers) output powers of greater than 500mW have become possible, exceeding the output powers possible from directly doubled 404nm DPSS lasers. Further, the frequency-doubled violet lasers have a considerable infrared component in the beam, resulting from the pump diode.
Yellow DPSS lasers use an even more complicated process: A 808 nm pump diode is used to generate 1,064 nm and 1,342 nm light, which are summed in parallel to become 593.5 nm. Due to their complexity, most yellow DPSS lasers are only around 1% efficient, and usually more expensive per unit of power.
Another method is to generate 1,064 and 1,319 nm light, which are summed to 589 nm. This process is more efficient, with about 3% of the pump diode's power being converted to yellow light.
DPSS lasers generally have a higher beam quality and can reach very high powers while maintaining a relatively good beam quality. Because the crystal pumped by the diode acts as its own laser, the quality of the output beam is independent of that of the input beam. In comparison, diode lasers can only reach a few hundred milliwatts unless they operate in multiple transverse mode. Such multi-mode lasers have a larger beam diameter and a greater divergence, which makes them less desirable. In fact, single-mode operation is essential in some applications, such as optical drives.
On the other hand, diode lasers are cheaper and more energy efficient. As DPSS crystals are not 100% efficient, some power is lost when the frequency is converted. DPSS lasers are also more sensitive to temperature and can only operate optimally within a small range. Otherwise, the laser would suffer from stability issues, such as hopping between modes and large fluctuations in the output power. DPSS lasers also require a more complex construction.
Diode lasers can also be precisely modulated with a greater frequency than DPSS lasers.
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...
s made by pumping
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
a solid gain medium, for example, a ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...
or a neodymium-doped YAG
Nd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...
crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
, with a laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
.
DPSS lasers have advantages in compactness and efficiency over other types, and high power DPSS lasers have replaced ion laser
Ion laser
An ion laser is a gas laser which uses an ionized gas as its lasing medium.Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions...
s and flashlamp-pumped lasers in many scientific applications, and are now appearing commonly in green and other color laser pointer
Laser pointer
A laser pointer or laser pen is a small portable device with a power source and a laser emitting a very narrow coherent low-powered beam of visible light, intended to be used to highlight something of interest by illuminating it with a small bright spot of colored light...
s.
Coupling
The wavelength of the laser diodes is tuned by means of temperature to produce an optimal compromise between the absorption coefficient in the crystal and energy efficiency (lowest possible pump photon energy). As waste energy is limited by the thermal lens this means higher power densities compared to high-intensity discharge lampHigh-intensity discharge lamp
High-intensity discharge lamps are a type of electrical lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. This tube is filled with both gas and metal salts. The gas facilitates the...
s.
High power lasers use a single crystal, but many laser diodes are arranged in strips (multiple diodes next to each other in one substrate) or stacks (stacks of substrates). This diode grid can be imaged onto the crystal by means of a lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
. Higher brightness (leading to better beam profile and longer diode lifetimes) is achieved by optically removing the dark areas between the diodes, which are needed for cooling and delivering the current. This is done in two steps:
- The "fast axis" is collimated with an aligned grating of cylindrical micro-lenses.
- The partially collimated beams are then imagedImagingImaging is the representation or reproduction of an object's outward form; especially a visual representation .- Imaging methodologies and technologies :...
at reduced size into the crystal. The crystal can be pumped longitudinally from both end faces or transversely from three or more sides.
The beams from multiple diodes can also be combined by coupling each diode into an optical fibre, which is placed precisely over the diode (but behind the micro-lens). At the other end of the fiber bundle, the fibers are fused together to form a uniform, gap-less, round profile on the crystal. This also permits the use of a remote power supply.
Some numbers
High power laser diodes are fabricated as bars with multiple single strip laser diodes next to each other.Each single strip diode typically has an active volume of:
| 1 µm | 2 mm | 100 µm |
| Height | Depth | Width |
| fast axis | optical axis | slow axis |
and depending on the cooling technique for the whole bar (100 to 200) µm distance to the next laser diode.
The end face of the diode along the fast axis can be imaged onto strip of 1 µm height. But the end face along the slow axis can be imaged onto a smaller area then 100 µm. This is due to the small divergence (hence the name: 'slow axis') which is given by the ratio of depth to width. Using the above numbers the fast axis could be imaged onto a 5 µm wide spot.
So to get a beam which is equal divergence in both axis, the end faces of a bar composed of 5 laser diodes, can be imaged by means of 4 (acylindrical) cylinder lenses onto an image plane with 5 spots each with a size of 5 mm x 1 mm. This large size is needed for low divergence beams. Low divergence allows paraxial optics, which is cheaper, and which is used to not only generate a spot, but a long beam waist inside the laser crystal (length = 50 mm), which is to be pumped through its end faces.
Also in the paraxial case it is much easier to use gold or copper mirrors or glass prisms to stack the spots on top of each other, and get a 5 x 5 mm beam profile. A second pair of (spherical) lenses image this square beam profile inside the laser crystal.
In conclusion a volume of 0.001 mm³ active volume in the laser diode is able to saturate 1250 mm³ in a Nd:YVO4 crystal.
Common DPSS processes
The most common DPSS laser in use is the 532 nm wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
green laser pointer
Laser pointer
A laser pointer or laser pen is a small portable device with a power source and a laser emitting a very narrow coherent low-powered beam of visible light, intended to be used to highlight something of interest by illuminating it with a small bright spot of colored light...
. A powerful (>200 mW) 808 nm wavelength infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
GaAlAs laser diode pumps a neodymium-doped yttrium aluminium garnet
Yttrium aluminium garnet
Yttrium aluminium garnet is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic and yttrium aluminium perovskite . YAG is commonly used as a host material in various solid-state...
(Nd:YAG)
or a neodymium-doped yttrium orthovanadate (Nd:YVO4) crystal which produces 1064 nm wavelength light from the main spectral transition of neodymium
Neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
ion. This light is then frequency doubled using a nonlinear optical
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
process in a KTP
Potassium titanyl phosphate
Potassium titanyl phosphate or KTP is a nonlinear optical material which is commonly used for frequency doubling diode pumped solid-state lasers such as Nd:YAG and other neodymium-doped lasers. The material has a relatively high optical damage threshold , a great optical nonlinearity and excellent...
crystal, producing 532 nm light. Green DPSS lasers are usually around 20% efficient, although some lasers can reach up to 35% efficiency. In other words, a green DPSS laser using a 2.5 W pump diode would be expected to output around 500-900 mW of 532 nm light.
In optimal conditions, Nd:YVO4 has a conversion efficiency of 60%, while KTP has a conversion efficiency of 80%. In other words, a green DPSS laser can theoretically have an overall efficiency of 48%.
In the realm of very high output powers, the KTP crystal becomes susceptible to optical damage. Thus, high-power DPSS lasers generally have a larger beam diameter, as the 1064 nm laser is expanded before it reaches the KTP crystal, reducing the irradiance from the infrared light. In order to maintain a lower beam diameter, a crystal with a higher damage threshold, such as LBO, is used instead.
Blue DPSS lasers use a nearly identical process, except that the 808 nm light is being converted by an Nd:YAG crystal to 946 nm light (selecting this non-principal spectral line of neodymium in the same Nd-doped crystals), which is then frequency-doubled to 473 nm by a beta barium borate
Beta Barium Borate
Beta barium borate is a crystal frequently used for frequency mixing and other nonlinear optics applications...
(BBO) or lithium triborate
Lithium triborate
Lithium triborate LBO is a non-linear optics crystal. It has a wide transparency range, moderately high nonlinear coupling, high damage threshold and desirable chemical and mechanical properties. This crystal is often used for second harmonic generation of Nd:YAG lasers...
(LBO) crystal. Because of the lower gain for the materials, blue lasers are relatively weak, and are only around 3-5% efficient. In the late 2000s, it was discovered that bismuth triborate (BiBO) crystals were more efficient than BBO and LBO and do not have the disadvantage of being hygroscopic
Hygroscopy
Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed,' somewhat, by an increase in volume, stickiness, or other...
, which degrades the crystal if it is exposed to moisture.
Violet DPSS lasers at 404 nm have been produced which directly double the output of a 1,000 mW 808 nm GaAlAs pump diode, for a violet light output of 120 mW (12% efficiency). Initially, these lasers out-performed gallium nitride (GaN) direct 405 nm Blu-ray diode lasers. As direct 405nm diode technology progressed (primarily for use in Blu-ray disc writers) output powers of greater than 500mW have become possible, exceeding the output powers possible from directly doubled 404nm DPSS lasers. Further, the frequency-doubled violet lasers have a considerable infrared component in the beam, resulting from the pump diode.
Yellow DPSS lasers use an even more complicated process: A 808 nm pump diode is used to generate 1,064 nm and 1,342 nm light, which are summed in parallel to become 593.5 nm. Due to their complexity, most yellow DPSS lasers are only around 1% efficient, and usually more expensive per unit of power.
Another method is to generate 1,064 and 1,319 nm light, which are summed to 589 nm. This process is more efficient, with about 3% of the pump diode's power being converted to yellow light.
Comparison to diode lasers
DPSS and diode lasers are two of the most common types of solid-state lasers. However, both types have their advantages and disadvantages.DPSS lasers generally have a higher beam quality and can reach very high powers while maintaining a relatively good beam quality. Because the crystal pumped by the diode acts as its own laser, the quality of the output beam is independent of that of the input beam. In comparison, diode lasers can only reach a few hundred milliwatts unless they operate in multiple transverse mode. Such multi-mode lasers have a larger beam diameter and a greater divergence, which makes them less desirable. In fact, single-mode operation is essential in some applications, such as optical drives.
On the other hand, diode lasers are cheaper and more energy efficient. As DPSS crystals are not 100% efficient, some power is lost when the frequency is converted. DPSS lasers are also more sensitive to temperature and can only operate optimally within a small range. Otherwise, the laser would suffer from stability issues, such as hopping between modes and large fluctuations in the output power. DPSS lasers also require a more complex construction.
Diode lasers can also be precisely modulated with a greater frequency than DPSS lasers.