Nd: Y AG gain medium

Well-known as a laser material for many years, neodymium doped yttrium aluminium garnet, Nd:YAG is still overwhelmingly the most commonly used flashlamp pumped solid-state laser material [3.2]. The YAG host lattice is robust enough to allow conventional rod fabrication techniques, has high thermal conductivity and can be of good optical quality. Ions such as neodymium within the YAG lattice typically exhibit fluorescence of narrow spectral linewidth, such that the corresponding laser transitions can benefit from a high stimulated emission cross-section. In consequence, Nd:YAG oscillators can have low pump power thresholds that are suitable for excitation by laser- diodes.

Nd^+ dopant ions substitute with Y^+ ions during the crystal growing process to become incorporated randomly into the lattice, such that dopant level of neodymium is restricted to <1.5% (atomic) as crystals of higher neodymium concentration suffer from strain, concentration quenching and reduced thermal conductivity. For the cw end- pumped lasers a doping level of nominally 1.0% was selected, though uniformity of doping throughout the boule is tO.1%, to give good optical quality and low laser phase-front distortion in the crystal. Nd:YAG with a higher percentage of neodymium, nominally 1.3% atomic, was selected for the transversely pumped laser described in chapter 4, as pumping efficiency is more critically dependent upon absorptivity for this laser than for the end-pumped counterpart.

An important consideration in the selection of quality laser material was the degree of birefringence observed when the Nd:YAG boule cross-section was placed between crossed polarisers and illuminated by a tungsten lamp. Axially symmetric strain induced birefringence was visible in the boule, along with distortions at the core induced by the Czochralski growth process. For lasers which have intracavity polarisation selective elements, such as the twisted mode cavity laser and the linearly

Chapter 3 Cw end-pumped Nd: Y AG laser page 33 polarised laser described in section 3.7.2, good laser performance is dependent upon selection of material with a minimum of birefringence.

3 . 2 . 1 Spectroscopy of Nd:YAG

The active ion in Nd:YAG is the trivalent neodymium ion, for which a diagram of the energy levels is given in figure 3.1 [3.1]. Pumping with laser-diodes emitting around 809 nm excites Nd^+ ions from the ground state manifold to the pump bands of the 4p5/2 and % g/2 manifolds. These excited ions emit non-radiatively, decaying rapidly to the metastable '^p3/2 level which has a fluorescence lifetime of ~230 ps [3.32]. Split into two levels, R% and R2, of slightly differing energy, this manifold contains the upper levels for all the common lasing transitions in Nd:YAG. A full listing of the possible transitions, the lasing wavelengths and the branching ratios is given by Koechner [3.2].

The laser transition with the highest transition probability is the 1.064 pm line, from the upper R2 level of the ^P3/2 manifold to the ^In/2 state. Rapid thermal replenishment from the Ri level, <10 ns, ensures that the Boltzmann distribution determines that 39% of all the ions in ^p3/2 state exist in the R2 level at room temperature [3.3]. The short lifetime of <10 ns of the lower level is due to rapid decay of the neodymium ions to the ground state, though as the lower ^In/2 level is essentially unpopulated, the 1.064 pm transition can be considered for most purposes as a four-level laser transition [3.3]. The parameters relevant to laser action in Nd:YAG at 1.064 pm, are summarised below in table 3,1,

Chapter 3 Cw end-pumped Nd: Y AG laser page 34

Nd atom density (1% atomic), Ntot 1.4 * 10^® cm“3 [3.2]

Stimulated emission cross-section at 1.064 pm, Ge 7.4 * 10-19 cm2 [3.31]

i

Fluorescence lifetime of'^p3/2 state, i2 230 ps [3.32] ?

Fraction of ions in ^P3/2 state in level 2 _{39% [3.12]}

at room temperature, fz

Refractive index of NdtYAG at 1.064 pm, nL 1.82 [3.2]

1

Table 3.1 - Spectroscopic laser parameters of Nd: YAG

Although the 946 nm transition is not considered in this work, it is perhaps worth noting that the laser model for this wavelength is significantly different from the 1.064 pm transition in two aspects. Firstly, the upper laser level is the Ri level in the ^p3/2 manifold, rather than the ; R^ level in the case of the 1.064 pm transition. More significantly, the lower laser level for the 946 nm transition is in the ground state manifold, As there is significant thermal population of the lower level, the 946 nm NdrYAG laser acts as a quasi-3-level system.

3 . 2 . 2 Diode-array pump absorption in Nd:YAG

The absorptivity of 1% doped Nd:YAG across the spectrum that is accessible to GaAlAs laser-diode pumps is shown in figure 3.2 [3.1]. The dominant feature is a peak centred at 808.5 nm of approximately 1 nm width, at which the maximum absorptivity is -0.8 mm l

The laser-diode array selected to pump our cw laser was a Spectra-Diode Labs SDL- 2430-H2, which emitted on several longitudinal modes with an overall spectral bandwidth of -2 nm, as shown in figure 2.12. To obtain the optimum pump efficiency of the Nd:YAG laser, the laser-diode wavelength was temperature tuned to obtain maximum pump absorption by maximising the overlap, or convolution, of the laser-

diode spectrum and the Nd:YAG absorption peak at 808.5 nm. The percentage of "A

Chapter 3 Cw end-pumped Nd:YAG laser page 35 pump light transmitted by a sample of Nd:YAG measured as a function of laser-diode temperature is shown in figure 3.3. For the laser-diode operating at 100 mW, the maximum absorptivity of 0.70 mm'l was found to occur at 15 °C. It should be noted that this absorptivity is an approximate figure and that each of the laser-diode’s

longitudinal modes experiences a different absorptivity such that a single exponential # decay term cannot strictly be used to express the overall transmitted intensity.

Changes in laser-diode temperature necessitated corrections in the laser-diode drive- current to maintain a constant output power, as the threshold current for the laser-diode was a function of temperature.

It is worth noting that in laser designs that require maximum absorption, there may be a role for index-guided laser-diodes. The emission from an index-guided laser-diode is normally single longitudinal mode, and as such emits with a spectral width that is narrow compared to the absorption peak at 808.5 nm. Though index guided laser- diodes are generally of lower power than gain guided devices, the higher absorption efficiency is of benefit in pumping lasers, such as microchips, which have a short length of crystal in which to absorb the pump light.