Nd:YLF Mode-Locked Pulsed Lasers
An increasing number of applications, including new illumination techniques in fluorescence optical microscopy, require a reliable high average-power laser source that enables efficient frequency conversion to ultraviolet and visible wavelengths. Several variants of the diode-pumped solid state laser have been developed, and of these, the Nd:YLF (neodymium: yttrium lithium fluoride) laser produces the highest pulse energy and average power in the repetition rate ranging from a single pulse up to approximately 6 kHz. This tutorial explores the operation of a Nd:YLF multi-pass slab laser side-pumped by two collimated diode-laser bars.
Upon initialization of the tutorial, photons are generated within a slab-shaped crystal of neodymium-doped yttrium lithium fluoride (labeled (Nd:YLF Crystal). The other components adjacent to the Nd:YLF gain module are a pair of transversely mounted and offset Diode Laser bars that provide pump energy to the laser element. External high reflectors (labeled Mirror in the tutorial), closely coupled at opposite ends of the slab, direct the photons to make multiple passes (five in this case) through the slab, increasing the gain prior to exit from the crystal to the external components of the resonator. Photons leaving the gain module are reflected by a plane mirror to a plano-concave Spherical Tuning Mirror, and then through an Acousto-Optic Modulator that allows Q-switching for operation in pulsed mode rather than continuous output mode. After reflection from the plano-convex cylindrical high reflector (High Reflection Mirror) at the end of the housing, the photons travel again through the Q-switch and return to the crystal slab, where a proportion of them achieve a trajectory that allows them to exit as the laser beam through the flat Output Coupler.
Two sliders are provided beneath the tutorial window that control basic functions. The left-hand slider, entitled Pulse Width allows variation of the laser beam pulse width, which simulates adjustment that would be made to the acousto-optic modulator when operating the laser in the Q-switched regime. The values of pulse width can be varied in the tutorial from 50 nanoseconds up to 300 nanoseconds. The right-hand slider (Applet Speed) enables control of the speed at which the tutorial runs.
One of the advantages of the side-pumped laser configuration illustrated in the tutorial is its simplicity, reliability, and physical and thermal tolerance. End-pumped diode systems produce a significant brightness level at high efficiency, but have greater complexity and are not easily scaled to high average-power output. The side-pumped slab design is capable of producing high average and peak power, and in combination with its beam quality, is suited to higher harmonic generation that can be utilized to produce green or ultraviolet wavelengths in addition to the native 1047-nanometer infrared radiation of the Nd:YLF crystal.
Nd:YLF lasers are likely to find increased application in providing pump light for operation of other lasers, such as the Ti:Sapphire pulsed lasers that are currently the preferred illumination source in most multiphoton imaging techniques in fluorescence microscopy. A factor that limits the more widespread use of multiphoton imaging systems is that argon-ion lasers are generally used to pump the Ti:Sapphire laser, and these are large, inefficient, and very expensive to purchase, maintain, and operate.
In addition to the potential utilization of the Nd:YLF laser to provide pump light for other lasers, it has been applied directly by some investigators in all-solid-state multiphoton systems, utilizing the mechanism of three-photon excitation. The 1047-nanometer wavelength of the Nd:YLF laser is somewhat longer than can be readily attained with a Ti:Sapphire laser, and improves viability in live cell studies, as well as making imaging possible from greater depths within specimens. When used as the excitation source, the diode laser-pumped system requires an external pulse compressor to obtain the extremely short pulses required to optimize the excitation.
The combination of mirrors utilized in this laser design results in an astigmatic resonator that compensates for the elliptical thermal lensing of the gain module crystal, and produces a beam with nearly equal vertical and horizontal quality parameters. The physical layout of the components facilitates the incorporation of the acousto-optic modulator for Q-switching duties. Each of the diode-laser bars are coupled directly to the Nd:YLF crystal through a single fiber aspherical lens intended to minimize the divergence of the pump light in the plane perpendicular to the linear crystal. By being offset on opposite sides of the slab, the diode bars create a sheet-like region of maximum gain throughout the length of the crystal. The pump faces of the crystal have segmented high-reflection dielectric coatings that cause the pump light to make two passes through the material for increased absorption and efficiency with respect to power input to the gain element.
Multiple passes through the gain element of the resonator are achieved in the laser by the external mirrors at the ends of the crystal, but can alternatively be accomplished by deposition of segmented coatings directly on the end facets of the Nd:YLF crystal. Alignment for multiple passes of the light in this manner increases gain, and also improves the extraction efficiency in fundamental-mode (TEM(00)) operation by reducing the effective aperture.