1.2 The Basic Principles and Components of Solid-State Lasers
1.2.6 Pumping Configurations
There are two dominant configurations to optically pump bulk solid-state lasers, namely side-pumping and end-pumping (Koechner, 1999). Figure 1.5 depicts the two pumping schemes. The top of Figure 1.5 shows the side (left) and front (right) view of a side- pumped circular laser rod, while the end-pumped scheme is shown at the bottom. The pump light is indicated by the red arrows, the excited area of the crystal is shaded red, and the laser mode is depicted by the yellow lines (side view, left) and yellow circles (front view, right).
In the side-pumped scheme, the gain medium is pumped approximately perpendic- ular to the laser resonator mode (top, Figure 1.5). In this scheme most of the crystal is filled with pump light and therefore the whole crystal volume is excited, with most pump light being absorbed, and therefore most excited ions being near the outer edge of the crystal. However, energy is extracted only from the excited ions which spatially overlap with the laser resonator mode in the centre. The energy deposited outside this region is wasted and increases the heat load, contributing to the thermal stresses in the crystal. The fact that most of the energy delivered by the pump is deposited in the outer region of the crystal, where it’s not being extracted again as laser light, makes this scheme less efficient. The higher absorption at the edge of the crystal results in the gain being higher in the outer region than in the centre, where the laser mode is. This can cause higher order modes to lase. The additional lasing of the higher
CHAPTER 1. INTRODUCTION TO END-PUMPED SOLID-STATE LASERS
pump light gain medium
laser mode Side-Pumped
End-Pumped
Figure 1.5: A schematic diagram of side-pumped (Top) and End-Pumped (Bottom) schemes. The pump light is indicated by the red arrows, the excited area in the crystal is shaded red, and the resonator mode is indicated by the yellow lines.
order modes then degrades the beam quality of the laser. Special care thus needs to be taken to prevent these higher order modes from lasing by designing the resonator to force the laser to operate only on the fundamental TEM00 mode. This can be done by inserting an aperture into the resonator cavity which only transmits the fundamental mode, or increasing the losses for the higher order modes in some other fashion. The side-pumped scheme is often used for diode-pumped, multi-kilowatt lasers.
In the end-pumped scheme, also known as longitudinal pumping, the gain medium is pumped co-linearly with the resonator mode (bottom, Figure 1.5). End-pumping tends to be more efficient than side-pumping since the pumped (and therefore excited) volume is mostly restricted to the region where the laser mode is. This leads to higher gain, allowing the generation of shorter Q-switched pulses (Pachotta, 2009). The good spatial overlap of the resonator mode and pumped volume in the gain medium also lowers the heat load. Since the sides of the crystal need not be accessible for pumping, the crystal surface can be in direct contact with a cooled mount, simplifying cooling significantly. The drawback is that end-pumping requires a pump source with sufficient beam quality in order that the pump beam is sufficiently collimated over the length
CHAPTER 1. INTRODUCTION TO END-PUMPED SOLID-STATE LASERS of the crystal, or at least the absorption length thereof. Another drawback is that most of the pump light is absorbed at the end of the crystal, where there is no contact with a cooling surface. The heat deposited subsequently leads to thermal stresses and thermal lensing. This is discussed in greater detail in Sections 2.3.3 to 2.3.6. Despite this, end-pumped lasers often achieve better beam quality than similar side-pumped lasers.
In the work reported, only end-pumped configurations are considered.
Chapter 2
High-Power Diode-End-Pumped Nd:YLF Laser
2.1 Introduction
A relatively short time after the first laser was demonstrated in 1960 (Maiman, 1960), the first reported Neodymium laser (Nd:CaWO4) was demonstrated at Bell Labora- tories by L.F. Johnson and K. Nassau (Johnson & Nassau, 1961). Three years later J.E. Geusic, H.M. Marcos and L.G. van Uitert demonstrated what was to be the most widely used solid-state laser, namely the Neodymium-doped Yttrium Aluminium gar- net (Nd:YAG) laser (Geusic et al., 1964). Since then Nd lasers using many different host materials have been demonstrated (Weber, 2001).
The first Neodymium-doped Yttrium Lithium Fluoride (Nd:YLF) laser was demon- strated in 1982 by T.M. Pollak et al. (Pollak et al., 1982). This could have been a particularly attractive material for use in high-power end-pumped solid-state lasers if not for its low thermal fracture limit.
The YLF crystal’s weak thermal lens on theσ-polarisation enables the construction of lasers with diffraction-limited beams, while the long upper laser level lifetime of 525µs (Ryan & Beach, 1992) of Neodymium in YLF crystals support efficient pulsed operation. In addition, the natural birefringence of YLF eliminates thermally induced depolarisation in high-power applications.
However, the relatively low thermal conductivity of Nd:YLF, which can lead to
CHAPTER 2. HIGH-POWER DIODE-END-PUMPED ND:YLF LASER
Figure 2.1: A mounted Nd:YLF rod, fractured from thermally induced stress (Bollig et al., 2005).
thermal runaway effects, and its low thermal fracture limit has made power scaling difficult (Bernhardi, 2008). Furthermore, the astigmatic thermal lens of YLF also requires special consideration when designing a resonator that compensates for this (Hardman et al., 1999).
In this chapter a state of the art high-power diode-end-pumped Nd:YLF laser is presented. The aim of this project was to demonstrate a Nd:YLF laser with high av- erage output power delivered in a diffraction limited beam, while being highly efficient in both continuous and pulsed mode. The subsequent good results were achieved by addressing the issues mentioned above in several ways.