Unlike flash lamps that emit in a very large spectral domain, laser diodes have a short bandwidth (around 3 nm) centered on a peak wavelength. For flash lamps, a large range of radiation means high losses in optical coupling to the solid-state medium that has a specific absorption spectrum. Nd:YAG presents a peak of absorption centered at 808 nm with a short bandwidth of about 2,5 nm. This means for lamp-pumping most of the radiation is converted into heat by non-radiation transitions in the laser crystal that cause cooling difficulties. Moreover, the UV radiation can cause optical and chemical damages.

Since the peak wavelength of the laser diode depends on the bandgap energy, it is possible to choose exactly this wavelength by controlling the semi-conductor materials. GaAlAs material is used and providing emission within the range between 750 and 900 nm. Variation of the Al concentration can tune the output wavelength (to get a junction emitting at 808 nm, the Al proportions are Ga 0.91, Al 0.09.

Compared with the broad spectral output of lamps, the bandwidth of laser diodes is very small that leading to a great efficiency of the pump process.

The diode wavelength is strongly dependent on the temperature and on the current. By increasing the junction temperature, the band gap energy and the wavelength changes. Moreover the wavelength depends on the current through the diode due to a thermal effect: the higher the current, the higher the wavelength. The properties of the active medium of the diode allow a wide range of its output wavelengths (around 10 nm). By choosing the adequate temperature and current the emission can be tuned to exact wavelength corresponding to the absorption band.

The efficiency of high power laser diode is about 35% so 65% of the electrical power is converted into heat. Therefore elaborate heat removal is required for holding an operating temperature.

There are a lot of possibilities:

Laser diodes systems are supplied with a low voltage DC power and the diodes are operated electrically in series. Moreover the power supply must deliver a high current. The power supply device must stabilize the current. A fault in the power supply can be lethal for the diode so the system must limit the current. Also, reverse voltage and electrostatic voltage discharges can destroy the diode junction. Since high power laser diodes are expensive, operation must be done with a lot of precautions to avoid the risk of electrostatic or optical damage.

The lifetime of high power laser diodes has been dramatically increased thanks to large progress in semi-conductor design. The long life expectancy of the laser is one of the most important reasons for their use to pump solid-state lasers. More than 10 000 hours can be expected for CW high power diodes. This number could be contrasted to the lifetime of lamps systems, which is about 1000 hours in CW mode.

Nevertheless, it should be noticed that the phenomena of degradation are numerous and complex. There can occur spontaneous catastrophic failure or a continuous degradation of output power over thousands of hours. The reason for this is the high electrical and optical power density inside the diodes.

The YAG is a crystal with attractive properties: it is optically transparent at 1064 nm, chemically stable and has good mechanical properties. The neodynium ions Nd³⁺ are implemented instead of Y³⁺, that means that is no charge compensation required, and as the size of the two types of ions differs very few, only small strain is induced in the crystal.

Nd:YAG laser is a four level system, the main level transition occurs at 1064 nm but many other transitions with cross section exist.

In CW mode the pump light causes heating in the rod. This heating is caused by several reasons:

We consider the case where the heat generated within the laser rod by pump light absorption is removed by a coolant flowing along the cylindrical rod surface. With the hypothesis of uniform internal heat generation and cooling along the cylindrical surface of an infinitly long rod, the heat flow is strictly radial, and end effects and the small variation of coolant temperature in axial direction can be neglected. The radial temperature distribution in a cylindrical rod with thremal conductivity K, in which heat is uniformly generated at a rate Q per unit volume, is obtained from the one-dimensional heat conduction equation.

d²T/dr² + dT/(rdr) + Q/K=0

The solution of this differential equation gives the steady-state temperature at any point along a radius of length r. With the boundary condition T(r₀) for r=r₀, where t(r₀) is the temperature at the rod surface and r0 is the radius of the rod, it follows that :

T(r) = T(r₀) + (Q/4K)(r₀²*r²)

The temperature profile is parabolic, with the highest temperature at the center of the rod. The temperature gradient inside the rod are not a function of the surface temperature T(r₀) of the rod.

Close loop circulating water system seems the best solution for a DPSS Laser, indeed most of the compact DPSS laser producers use a similar system.

Moreover this system is relatively compact. A large number of cooling liquid is available. But water presents the advantage not to decompose and to be optically stable under light expositions. Moreover, it presents a high specific heat and a high thermal conductivity. Still, chemical interactions between water and materials present in the cooling loop are not very well investigated. It is preferable to banish any plastic components with softeners and any parts with metals (other than stainless steel) that can be easily oxydated.

This is a schematic of a close loop circulating water system :

The filter will retain fluid impurities before laser cooling. In case the flow rate decrease or the fluid temperature increase, the security will switch off the laser. Water in the tank is cooling thanks to another cooling system. Deionized water circulate in the loop thanks to a pump.

In side-pumped geometry, the diode arrays are placed along the length of the laser rod and pump the active material perpendicularly to the direction of propagation of the laser resonator mode.

Thanks to this system high output power can be achieved (in comparison with end-pumped system, which has a higher optical efficiency but a lower output power).

For instance, we can consider a simple set-up, the laser materials (Nd :YAG) is side-pumped with a diode array coupled to the rod without intervening optics.

Nd:YAG rod pumping

Picture at back shows energy level of ion Nd3+ (low concentrated) in YAG crystal (Yttrinium Aluminium Garnet). Ions are excited to high energy levels thanks to the pumped process, then they fall to a lower level by a non-radiation transition. Laser emission is produced by 1,06m m transition in infrared range.