The Pump Laser 4.1 Introduction
4.4 Laser ions and solid-state hosts
This section briefly introduces the properties of the transition metals and lanthanides (rare earths) which make them suitable active laser ions when doped into solid-state laser hosts. The first section describes the spectroscopic properties of solid-state laser materials in general followed by more details on the Nd ion when doped in the hosts YAG and YLF.
4.4.1 Solid-state laser ions
As mentioned previously, the elements in the fifth period of the periodic table after the element lanthanum produce triply charged ions with sharp transitions when doped in a host due to the shielding of the 4f electrons by the outer 5s and 5p electrons. These sharp features lend themselves to diode pumping where the emission spectrum of the diode is considerably narrower than that of lamp pumped sources. This makes diodes more efficient pumps of laser ions. However, diode lasers can only be used if the emission wavelength of the diode matches an absorption of a laser ion. The fortunate overlap between the absorption feature of Nd doped materials and the emission from AlGaAs lasers at 809 nm has been the reason why so much of the work on diode pumped lasers has involved the Nd ion.
With work progressing on diode lasers for attainment of wavelengths from the far infrared to possibly the blue region of the spectrum, the possibility of pumping other ions is increasing. Erbium has been made to lase at 1.5 and 2.9 jim when diode pumped at around 970-980 nm with InGaAs diodes [32], though this requires the use of Ytterbium as a sensitiser for the 1.5 jim transition due to the three level nature of the transition. Other laser ions which have been diode pumped are Thulium and Holmium [33], where these ions have absorptions close to 800 nm and can be pumped by high power AlGaAs lasers. They are not as efficient as Nd as they do not possess a four level laser structure.
The transition metal elements are also candidates for diode pumping. These elements occur in the fourth period of the periodic table, and lose the outer 4s and possibly some 3d electrons in bonding to give the outer electron configuration \s^2s^2p^3s^'ip^3d”'
where n<10. As the 3d electrons reside outside the ion core they interact strongly with the crystal field and can therefore result in broad transitions. These transition metal ions are then capable of producing 'vibronic' or tunable lasers. Of particular note are the Ti:AI2O3 and CrtLiCaAIFg and CriLiSrAlFs lasers. The trivalent titanium ion consists
Ch. 4 : The Pump Laser
lasers to date. With its main absorptions in the green it has yet to be pumped directly by a diode laser but has been pumped by frequency doubled diode pumped Nd lasers. The Cr lasers are of current interest for diode pumping as the pump absorption peak at around 630-650 nm is rapidly becoming accessible for direct diode pumping. Diodes are available at these wavelengths but at the moment the power available is small, a few tens of mW.
We now turn our attention to the Nd ion, and in particular the hosts YAG and YLF, which were of interest in this work.
4.4.2 Nd:YAG
The material neodymium doped yttrium aluminium garnet has been the mainstay of solid-state lasers for 3 decades. As such the technology of its growth and fabrication is well established and large, high optical quality samples are readily available. Being a rare earth ion, Nd doping of YAG produces a narrow spectral linewidth and the lasing transitions therefore benefit from a high stimulated cross-section (~ 6.5 x 10“ cm^ for 1 % atomic doping [34]). The doping density is constrained to be < 1.5 % (atomic) due to strain and concentration quenching. The pump absorption around 800 nm is shown in fig. 4.2. The peak at 809 nm (width ~1 nm) is amenable to AlGaAs laser diodes which transfers the ions from the ^^1 9 /2 ground state to the "^F5 /2 and % 9 /2 manifolds. Rapid
non-radiative decay to the metastable '^F3 /2 (lifetime -230 |is) results in the 1.064 jam
transition to the short lived ^ In/2 lower state behaving as a four level laser transition.
The upper state lifetime is of sufficient length that efficient energy storage can be obtained with the currently available diode laser pulse lengths of 200-400 \is.
1.4 1.2- A h O - s s S 0.8-
§
1
< 0.4 4 0.6 - 0.2 - 790 800 780 810 820 830 Wavelength / nm fig. 4.2 Absorption spectrum of 1 % doped Nd:YAG near 800 nm.'v;l
Ch. 4 : The Pump Laser
powers, however, it is susceptible to thermal lensing and thermally-induced bi refringence. Both of these can be partially compensated for by proper cavity design but are sources of loss and aberration. The birefringence is particularly a problem when any intracavity polarisation selection is required, such as for electro-optic Q-switching. The high thermal fracture limit means that these lasers can be, and are, used for high average power applications
4.4.3 Nd-.YLF
The material Ytrrium Lithium Fluoride is anisotropic with the scheelite structure. This anisotropy results in the emission and absorption properties being polarisation dependent. The emission at 1 p.m then consists of two lines, 1.047 |im in the %
polarisation being stronger than the orthogonal a polarisation at 1.053 pm. The 1.053 pm radiation matches the line centre of Ndiglass and Nd:YLF lasers are often used as seeders for high power glass amplifier systems [34].
The absorption near 800 nm for Nd:YLF is polarisation dependent but is wider and stronger than in YAG which, as is found in this work, is beneficial in end pumping as a tighter confined gain mode can be produced. The emission cross section has been measured to be about 75 % that of Nd:YAG [35], but combined with a longer upper state lifetime (-480 ps) the cross-section lifetime product is higher for YLF implying lower thresholds and a greater energy storage capacity. It can therefore be pumped with pump pulses of twice the duration as in YAG while retaining the same storage efficiency.
The natural birefringence dominates over thermally induced birefringence reducing depolarisation losses, and the thermal lensing is much less than in YAG due to partial cancellation between the negative dnldT and the stress induced index change. A dis advantage it has compared with YAG is a less advanced growth technology resulting in large, high quality samples not being as readily available. Additionally, the thermal fracture limit is less and so YLF cannot be operated at as high average powers as can YAG material. However, as is demonstrated in this work, for lower average powers, particularly Q-switched operation, the spectroscopic and material properties of YLF are superior to YAG.