A diode-pumped solid-state laser (DPSSL) is a
solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid 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 ...
made by
pumping a solid
gain medium
The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a ...
, for example, a
ruby
A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sa ...
or a
neodymium-doped YAG crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
, with a
laser diode
file:Laser diode chip.jpg, The laser diode chip removed and placed on the eye of a needle for scale
A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a di ...
.
DPSSLs have advantages in compactness and efficiency over other types, and high power DPSSLs have replaced
ion laser
An ion laser is a gas laser that 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 helium–neon lasers, the e ...
s and
flashlamp-pumped lasers in many scientific applications, and are now appearing commonly in green and other color
laser pointer
A laser pointer or laser pen is a small handheld device with a power source (usually a battery) and a laser diode emitting a very narrow coherent low-powered laser beam of visible light, intended to be used to highlight something of interest by ...
s.
Coupling
The wavelength of 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 lamp
High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge 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 tu ...
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
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
. 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
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infini ...
micro-lenses.
# The partially collimated beams are then
image
An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
d 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 fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
, 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:
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 than 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:YVO
4 crystal.
Common DPSSL processes
![YAG2](https://upload.wikimedia.org/wikipedia/commons/6/69/YAG2.svg)
The most common DPSSL in use is the 532 nm
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
green
laser pointer
A laser pointer or laser pen is a small handheld device with a power source (usually a battery) and a laser diode emitting a very narrow coherent low-powered laser beam of visible light, intended to be used to highlight something of interest by ...
. A powerful (>200
mW) 808 nm wavelength
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
GaAlAs laser diode pumps a neodymium-doped
yttrium aluminium garnet
Yttrium aluminium garnet (YAG, Y3 Al5 O12) is a synthetic crystalline material of the garnet group. It is a cubic yttrium aluminium oxide phase, with other examples being YAlO3 (YAP) in a hexagonal or an orthorhombic, perovskite-like form, and ...
(Nd:YAG) or a
neodymium-doped yttrium orthovanadate
Neodymium-doped yttrium orthovanadate (Nd:YVO4) is a crystalline material formed by adding neodymium ions to yttrium orthovanadate. It is commonly used as an active laser medium for diode-pumped solid-state lasers. It comes as a transparent blue-t ...
(Nd:YVO
4) crystal which produces 1064 nm wavelength light from the main spectral transition of
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
ion. This light is then ''frequency doubled'' using a
nonlinear optical
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...
process in a
KTP crystal, producing 532 nm light. Green DPSSLs are usually around 20% efficient, although some lasers can reach up to 35% efficiency. In other words, a green DPSSL using a 2.5 W pump diode would be expected to output around 500-900 mW of 532 nm light.
In optimal conditions, Nd:YVO
4 has a conversion efficiency of 60%, while KTP has a conversion efficiency of 80%. In other words, a green DPSSL 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 DPSSLs 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
lithium triborate
Lithium triborate (LiB3O5) or 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 secon ...
(LBO), is used instead.
Blue DPSSLs 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
Barium borate is an inorganic compound, a borate of barium with a chemical formula BaB2O4 or Ba(BO2)2. It is available as a hydrate or dehydrated form, as white powder or colorless crystals. The crystals exist in the high-temperature α phase and ...
(BBO) crystal or 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
Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs na ...
(BiBO) crystals were more efficient than BBO or LBO and do not have the disadvantage of being
hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
,
which degrades the crystal if it is exposed to moisture.
Yellow DPSSLs use an even more complicated process: An 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 DPSSLs 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
DPSSLs and diode lasers are two of the most common types of solid-state lasers. However, both types have their advantages and disadvantages.
DPSSLs 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 often makes them less desirable. In fact, single-mode operation is essential in some applications, such as
optical drive
In computing, an optical disc drive is a disk drive, disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives ...
s.
On the other hand, diode lasers are cheaper and more energy efficient. As DPSSL crystals are not 100% efficient, some power is lost when the frequency is converted. DPSSLs 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. DPSSLs also require a more complex construction.
Diode lasers can also be precisely modulated with a greater frequency than DPSSLs.
Neodymium-doped solid state lasers continue to be the laser source of choice for industrial applications. Direct pumping of the upper Nd laser level at 885-nm (rather than at the more traditional broad 808-nm band) offers the potential of improved performance through a reduction in the lasing quantum defect, thereby improving system efficiency, reducing cooling requirements, and enabling further TEM00 power scaling. Because of the narrow 885-nm absorption feature in Nd:YAG, certain systems may benefit from the use of wavelength-locked diode pump sources, which serve to narrow and stabilize the pump emission spectrum to keep it closely aligned to this absorption feature. To date, high power diode laser locking schemes such as internal distributed feedback Bragg gratings and externally aligned volume holographic grating optics, VHG’s, have not been widely implemented due to the increased cost and assumed performance penalty of the technology. However, recent advancements in the manufacture of stabilized diode pump sources which utilize external wavelength locking now offer improved spectral properties with little-to-no impact on power and efficiency.
Benefits of this approach include improvements in laser efficiency, spectral linewidth, and pumping efficiency.
References
External links
Sam's laser FAQ
{{Solid-state laser
Semiconductor lasers
Solid-state lasers