Laser diode pumped solid-state lasers

Narrow linewidth emission can be obtained from lasers which consist of some form of host material, doped with a rare earth ion [8], such as Neodymium (Nd) and Holmium

(Ho). The rare earth ions have the outer electron configuration 5s^5p^4f”5d^6s^ where n varies through the fifth period of the periodic table from n=l at caesium to n=13 at ytterbium. When these elements are used as dopants, they can form triply charged ions where the outer 5d and 6s electrons are removed in the bonding process. The electrons

of interest are those in the unfilled 4f level which give rise to the excited states. Because the 5s^5p^ shell is filled and resides outside the 4f shell, the 4f electrons are shielded from the effects of crystal field fluctuations and remain relatively sharp transitions. It is this spectroscopic fact that leads to one of the main advantages of using laser diodes as pump sources as opposed to the previously more conventional flashlamps.

Flashlamp pumped solid-state lasers are prone to instabilities and broad linewidths, mainly due to the 'technical noise’ incurred by the necessity for water cooling. Additionally, a large amount of heat is often deposited in the rod due to the quantum defect involved between the absorbed broadband pump radiation and the laser transition, and also direct heating of the crystal by absorption of short wavelength light. This results in thermal focusing and birefringence caused by thermal stresses in the rod. This either increases losses or reduces the level of power at which the laser can be operated. These problems are substantially reduced, or even removed, by the use of laser diodes as the pump source. Laser diodes emit either a single longitudinal mode (index guided) or multiple longitudinal modes (gain guided) when well above threshold. However, even the multi-longitudinal mode output of laser diodes, with an effective linewidth of a few nanometres, is considerably narrower than the broadband emission of flashlamps. The narrow output emission of laser diodes allows selective excitation of the laser ions with a substantially reduced amount of heat being absorbed by the rod. This vastly reduces the cooling requirements and the associated technical noise. The problems of thermal focusing and birefringence are also greatly reduced. These advantages were recognised early on in the field of recombination radiation from semiconductors.

Possibly the first use of semiconductors for pumping a laser was an LED pumped Nd:CaW0 4 laser by Newman in 1963 [8]. The first use of a laser diode for pumping

was in 1964, not long after the first laser diodes were demonstrated [1,2], using a CaF2:U^+ laser [9], where a side pumping set up was used and both rod and pump

Ch. 4 : The Pump Laser

neodymium doped materials as the absorption bands around 800 nm were well situated for pumping with GaAlAs lasers. However, until recently the laser diodes available were inadequate in terms of reliability, ease of handling, operational lifetime and output power. The ever advancing improvements in laser diodes have begun to allow the early optimism placed in laser diode pumped solid-state lasers to be realised.

Developments in laser diode pumped solid-state lasers, in particular miniature Nd lasers, have been the subject of a number of reviews [10]. Interest has concentrated on Nd doped materials because the major advances in high power diode lasers have been with GaAlAs alloys. A lot of work has dealt with Nd:YAG [11,12] as it was an established material with excellent material and spectroscopic properties, but a number of other Nd doped hosts have been made to lase with laser diode pumping, e.g. YLF [13], YAIO [14], glass [15], YV0 4 [1 6], MgO-.LiNbOs [17]. In doped materials,

concentration quenching of the upper state lifetime limits the doping density and absorption. Higher absorption can be obtained in stochiometric Nd materials, where it is part of the chemical composition rather than a dopant [18]. Although Nd is the most commonly used dopant ion, it has not been the only one, e.g. Holmium[19], Thulium[20] and Erbium[21] have also been used.

End pumping of the lasers is more efficient as the pumped mode and lasing mode are collinear and exhibit a greater degree of mode overlap. However a limited amount of power can be coupled in this way. To use higher pumping powers side pumping with laser bars and stacks has been the usual approach [22]. Recently, new approaches are showing that a substantial amount of power can in fact be coupled in using the more efficient end pumping techniques [23]. For higher peak powers the diode pumped lasers can be gain switched [24], Q-switched [25], and mode-locked [26]. Higher average powers can be obtained by using slabs, as opposed to laser rods, where heat removal is easier [27].

Diode pumped lasers have other advantages over lamp pumped lasers, e.g. compact size, higher efficiency, better beam quality and frequency stability, the latter two of which can be important for non-linear optical applications. The intrinsic frequency stability due to the reduced technical noise of diode pumped lasers is particularly evident in monolithic lasers, in particular the MISER developed by Byer's group at Stanford where a unidirectional ring cavity is achieved by using the Faraday rotation in the YAG itself [28]. The Schalow-Townes limit, which applies to a free running laser, was surpassed by Shoemaker et al [29] by using Pound-Drever (or Drever-Hall) locking which uses phase stabilisation techniques. The stability obtained of ~lm H z is less than the shot noise limit. This sort of stability is required for gravity wave experiments. Laser diode pumped lasers can have frequency versatility by harmonic generation in

Ch. 4 : The Pump Laser

elsewhere, optical parametric oscillation. Applications of diode pumped lasers include lidar, space communications and injection seeding of high power systems.