Diode-pumped lasers based on neodymium-doped materials now dominate application areas such as surface marking, wafer inspection, via drilling, and pumping titanium-doped sapphire (Ti:sapphire) laser oscillators and amplifiers. In comparison to their lamp-pumped predecessors, diode-pumped lasers provide advantages such as high reliability, output stability, low maintenance requirements, compact size, and low utilities consumption.
Each of the neodymium-doped materials offers different capabilities, depending on its optical and thermo-mechanical properties. Neodymium-doped vanadate (Nd:YVO4), for example, is well-suited to delivering modest pulse energies at relatively high pulse repetition rates, while neodymium-doped yttrium lithium fluoride (Nd:YLF) is better for providing higher pulse energies at low repetition rates, appropriate for pumping kilohertz-class Ti:sapphire amplifiers. Commercial Nd:YLF laser systems have now been refined to a high level of reliability, performance, and ease of maintenance. This last attribute is particularly important for applications requiring long-term, hands-free operation.
Nd:YLF is characterized by a low degree of thermally induced lensing, strong natural birefringence, two orthogonally polarized lasing lines, andparticularly important to its application in pumping kilohertz Ti:sapphire amplifierslong upper-state lifetimes compared to other neodymium-based materials. Specifically, Nd:YLF laser transitions at 1.047 µm and 1.053 µm have upper-state lifetimes of around 490 µs and 540 µs, respectively, versus 230 µs for neodymium-doped yttrium aluminum garnet (Nd:YAG) and around 90 µs for Nd:YVO4. This substantial difference is the key to the use of Nd:YLF in kilohertz-class, high-pulse-energy lasers.
In a typical Q-switched laser, the optimal pumping time is roughly one to two times the spontaneous lifetime. Nd:YLF can be pumped for about 1 ms before the stored optical gain reaches a plateau due to spontaneous upper-state emission. In contrast, the energy storage in Nd:YAG reaches a maximum after pumping for only about half this time. More optical energy can thus be stored in Nd:YLF than Nd:YAG or Nd:YVO4.
Even in an Nd:YLF laser optimized for 1-kHz operation, the average output power of the laser barely drops even at 15 kHz.
A common misconception exists that the long upper-state lifetime of Nd:YLF renders it an inferior material for use at high repetition rates. In reality, in an Nd:YLF laser optimized for 1-kHz operation, the output power drops only imperceptibly at pumping rates as high as 15 kHz (see figure). Several commercial Nd:YLF lasers exist based on cavities designed for optimum performance at repetition rates of 10 kHz and higher, moreover. The misconception probably arises from the fact that Nd:YAG provides higher gain than Nd:YLF, so that Nd:YAG-based systems operating at higher rep rates can achieve greater powers under similar conditions.
Most applications for Nd:YLF lasers require frequency doubling to deliver visible (527 nm) output. Intra-cavity doubling is preferred because of its high efficiency and compact design. One commercially available diode-pumped, intra-cavity doubled Nd:YLF laser (Evolution; Coherent Inc.; Santa Clara, CA) provides a choice of output powers all the way up to 90 W.
With the typical lifetime for high-power diode arrays now exceeding 10,000 hours, a key design goal has been to create a rugged laser head that delivers on the full reliability potential and compactness of the diode-pumping platform. Side pumping with three diode arrays arranged around the rod at 120° to each other delivers high pump energy with reasonable uniformity, generating a smooth "top hat" profile consisting of multiple transverse modes. The top hat profile is well-suited to pumping Ti:sapphire amplifiers. The modular cavity design permits field replacement of the diode arrays at the end of their useful life, with no special alignment required even when the entire module is removed or swapped. Demanding 24/7 Applications
An interesting application for this type of laser involves the Positron-Electron Project-II (PEP-II) located at the Stanford Linear Accelerator Center (SLAC; Stanford, CA). PEP-II is a joint collaboration of SLAC, Lawrence Berkeley National Laboratory (Berkeley, CA), and Lawrence Livermore National Laboratory (Livermore, CA). One of the main goals of PEP-II research is to investigate the origins of the matter/anti-matter imbalance immediately following the Big Bang. Researchers believe a small imbalance in favor of matter has led to the matter-based universe in which we live. SLAC, the world's largest linear accelerator, includes a pair of asymmetric storage rings that are key components in the PEP-II collision experiments. One storage ring contains electrons that are accelerated to an energy of 9 GeV. The other ring contains a beam of positrons accelerated to 3.1 GeV. The energy asymmetry involved in colliding these particles is crucial to investigating the post-Big Bang imbalance.
Researchers create electrons through the photoelectric effect by irradiating a photocathode with an intense pulse from a Q-switched Ti:sapphire laser. An early-model Q-switched Nd:YLF laser (the Evolution II) capable of generating up to about 6 mJ/pulse pumps the SLAC-built Ti:sapphire laser. The radio frequency (RF) driver for the Q-switch was modified for the application to operate at a sub-harmonic of the RF (2856 MHz) frequency for the accelerator, in order to operate synchronously with the accelerator. In the SLAC application, the Nd:YLF laser runs at a 120-Hz repetition rate with a pulse duration of 150 ns. Because this application only requires a modest pulse energy, the SLAC Nd:YLF laser operates at 50% of its nominal power rating, delivering a pulse energy of 3 mJ/pulse.
The SLAC Ti:sapphire laser includes an electro-optic modulator, which acts as both a Q-switch and a cavity dumper, in conjunction with a polarizer. Just before the arrival of the pump pulse, this modulator is switched on so that it drops the cavity Q to a low level, blocking laser action. Turning off the modulator allows the fast buildup of a Q-switched pulse. Turning on the modulator then allows the pulse to be dumped through the Brewster window, yielding a typical output pulse of a few hundred microjoules with a duration of about 100 ns. An external modulator selects a 2-ns slice from this pulse in order to generate pulses of electrons that match the 2-ns "bunch time" of the accelerator. Depending on the quantum efficiency of the specific photocathode material, the target is irradiated with only 30 to 50 µJ/pulse.
The SLAC laser also includes a birefringent filter used to tune the output wavelength to that appropriate for the photocathode material. This is relevant because the laser system is also used to support a research program designed to measure the spectral response of different photocathode materials.
Physicist Axel Brachmann of SLAC explains the benefits of a high-reliability, all-solid-state laser: "Our operating conditions require a 'set and forget' type of laser system. The laser operates 24 hours, seven days a week over extended periods of time (several months continuously). Moreover, the laser is located in an interlocked location that cannot be readily accessed without disrupting all electron-source activity. The Q-switched Nd:YLF laser continues to meet this high-reliability need. In fact," he continues, "the laser performs to specifications with the only interruptions being routine maintenance activities such as pump-diode replacements, which are typically performed once a year."
The group closely monitors the pulse-to-pulse statistics of the laser, says Brachmann, since significant variations would lead to unacceptable changes in the electron generation process. During a typical 24-hour period, with a mean output of 2.99 mJ, the system operates with a standard deviation of only 0.42%.
As a laser material, Nd:YLF is well suited to kilohertz-repetition-rate applications, for which it delivers higher pulse energies than other neodymium-based materials. In conjunction with diode pumping and ultra-stable laser architectures, this has led to lasers with reliability levels that were unthinkable even just a few years ago. oe
Marco Arrigoni is director of marketing at Coherent Inc., Santa Clara, CA.