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A new generation of solid-state lasers

02 Jun 2009

A radical rethink of solid-state-laser design is opening up growth applications in analytical instrumentation. Nicolas Féat of Oxxius explains how monolithic resonators are enabling a new class of sources with higher powers and improved stability.

Higher power, narrower linewidth and enhanced stability is a winning combination in many laser applications. Now, thanks to a patented monolithic resonator developed by engineers at Oxxius, France, a unique range of diode-pumped solid-state (DPSS) lasers is set to deliver against all three of these metrics, in turn providing an alternative to traditional gas lasers in the UV and visible. What's more, the compact design of the SLIM product family allows the lasers to be integrated into analytical instruments, opening up new applications in biophotonics and spectroscopy.

What sets the SLIM lasers apart is a proprietary, alignment-free monolithic-resonator (AMR) technology that sees the elements in the cavity assembled into a single ultra-low-loss optical subsystem. Using a process called optical contacting, contact forces (electron sharing, van der Waals') between the end-faces of two crystals create a bond that is extremely robust with time and temperature variations. Dielectric mirrors coated at the end-faces of the crystals complete the monolithic assembly with no moving parts.

With very little alignment required, it is then straightforward to embed the cavity assembly in a laser module. Passive filters in the resonator ensure emission stability, while crystal temperatures are controlled to match these filters to environmental conditions. The resulting lasers make up the SLIM platform with CW emission at 473 nm (up to 50 mW), 532 nm (up to 500 mW) and 561 nm (up to 200 mW).

A decade in the making

Solid-state laser technologies have been replacing gas lasers for more than a decade in a variety of applications (e.g. flow cytometry and confocal microscopy). GaAs laser diodes are widely used beyond 640 nm, 785 nm diodes have been used as Raman spectroscopy sources and 640 nm diodes as He-Ne laser replacements. More recently, reliable GaN products have extended their range from near-UV to blue emission (488 nm laser diodes have been available since 2008). Between 488 and 640 nm, frequency-doubled lasers are an ideal solution for integrators and researchers who wish to do away with the bulk and power consumption of gas-based systems.

In parallel, manufacturers have developed and improved their DPSS laser technologies. Starting with 532 nm, the product range has expanded to more exotic wavelengths, such as 561 or 594 nm, which are of great interest for bioanalysis applications. DPSS lasers are designed with a gain medium (Nd:YAG is the most common), a nonlinear crystal (e.g. KTP), beam-shaping optics and mirrors. Although several resonator designs can be used, these elements are typically glued or soldered to a ceramic base-plate and then thermalized to ensure a stable emission.

As most analytical applications require a very stable laser (i.e. minimal long-term power drift and intensity noise), mechanical stability of the resonator is a must-have specification. Indeed, a cavity-length change of tens of nanometres is enough to trigger mode hops – and with a limited number of longitudinal modes, this directly translates into significant power fluctuations.

While the efforts of several laser manufacturers have finally paid off, the production of stable lasers remains a significant challenge. Careful alignment of various optical elements is essential, increasing the cost and the complexity of manufacture.

Some analytical applications (e.g. Raman spectroscopy, interferometry and holography) have stringent spectral requirements, such as narrow linewidth (less than a few wavenumbers), coherence length (typically greater than 1 m) and wavelength stability (in the order of a few 10s of MHz per minute). Suitable lasers are typically single-frequency devices, which raise the technical difficulty and costs to levels that limit the growth of these applications.

Radical design

Against such obstacles, the monolithic design offers several advantages. The first is improved performance. The absence of air-crystal interfaces reduces the Fresnel reflections that contribute to resonator losses. In addition, reflected light remains in the resonator. This in turn significantly increases the conversion efficiency of the laser. A striking illustration of this is the output power obtained at 561 nm: up to 350 mW has been achieved using a 2 W pump diode, and resonator powers beyond 200 mW are measured routinely.

The monolithic cavity is also instrumental in ensuring single-frequency operation. Since vibrations have no effect on the resonator's length, thermal expansion becomes the sole phenomenon to be managed in this respect. It is therefore possible to predict the behaviour of the laser and to very tightly control the spectral properties. Over time, Oxxius has refined its firmware and is now able to track a given longitudinal mode, guaranteeing wavelength stability to the point that the monolithic microchip approach now gives a spectral purity and frequency stability akin to those of a non-planar ring oscillator.

Another advantage is enhanced reliability, as the monolithic resonator avoids a variety of failure modes. First, as no air is present intracavity, pollutions cannot occur where the IR power density is the highest. Second, the resonator is so efficient that the laser diode is under-driven significantly, thereby increasing lifetime. Third, the removal of the air-crystal interfaces boosts the crystals' lifetime. And finally, given the strength of the bond obtained by optical contacting these crystals, misalignments are not possible.

At the same time, the monolithic design permits volume manufacturing of high-end lasers. The assembly of the monolithic cavity is simple by the standards of optical manufacturing, and relies on a repeatable process and established techniques. Combining these factors makes for predictable performance without the requirement for heavy automation investments.

Such an assembly was a significant challenge to design and develop. Perhaps the biggest hurdle was the painstaking acquisition of the know-how required for the optical contacting of crystals. Although optical contacting is a well known technique – most notably in microscopy – the crystals contacted in this resonator present a significantly smaller area than traditional optics. The training of suppliers was another problem. Although standard crystals are used to make up a resonator, some elements of their manufacture require careful management.

The new solution

Several applications are already benefiting from the use of monolithic solid-state lasers. Primarily, when strict requirements are placed on spectral properties, the SLIM has the potential to become the solution of choice. The laser's spectral purity meets the requirements of Raman spectroscopy, while its frequency stability fits the needs of holography and interferometry alike. Its mode-hop free operation makes it one of the few lasers suitable for frequency-doubling, to generate UV light.

The SLIM is also being used in biomedical research. Last year's Nobel Prize in Chemistry was awarded for work on the engineering of fluorescent proteins. These proteins are excited at 561 nm, and in several applications (such as the sorting of cells in flow cytometry, small-animal imaging and total-internal-reflection fluorescence microscopy) their performance is improved significantly by increasing the power level to 200 mW – which is possible with the SLIM-561.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

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