Ytterbium Tungstate Revolutionizes the Field of High-Power Ultrafast Lasers

Dr. Arnd Krueger and Philippe Féru, Spectra-Physics

A recently commercialized laser material, ytterbium tungstate (Yb:KGW), is the basis for a new generation of high-power femtosecond systems that can be efficiently and directly pumped with diode lasers. These new amplifiers have a simpler and more rugged architecture than traditional Ti:sapphire amplifiers. Moreover, they can deliver higher average powers (up to 4 W) at higher repetition rates (up to 7 kHz) — addressing the demand for higher throughput in the area of femtosecond materials processing applications.

The output of these amplifiers can be used to drive one or more widely tunable visible optical parametric amplifiers (OPAs). Because of their simplicity, reliability and ease of use, they are expected to expand the market for high-power ultrafast laser pulses, in both basic research and industrial applications.

Direct diode pumping

For the past decade or more, most ultrafast laser sources have been based on Ti:sapphire as the active material, with its emission centered at 800 nm. Although the advent of that material in the late ’80s represented a giant advance over earlier dye lasers, Ti:sapphire ultrafast systems are still fairly complex. Because the gain material’s absorption falls within the visible wavelength range, they must be pumped by the green output of intracavity frequency-doubled solid-state lasers. This complexity is even more noticeable in high-energy systems, where the output pulses from a mode-locked Ti:sapphire oscillator are used to seed a Ti:sapphire regenerative amplifier. Both the oscillator and the amplifier need separate green pump lasers.

In recent years, tremendous advances in system design and engineering have enabled all these lasers to be packaged in a single laser head, as in the Spectra-Physics Hurricane. Nonetheless, the complexity of these systems can still be daunting and limiting for those who are not laser experts. Fortunately, this situation has been improved, thanks to ytterbium tungstate and the high-power femtosecond lasers based on this new material.

Ytterbium is a lanthanide element similar to erbium and, thus, has a broad emission spectrum when it is doped into various glasses and crystals. Of the various ytterbium-containing crystals, Yb:KGW is a particularly attractive laser material because it combines a high cross section for stimulated emission with an emission spectrum from ~1020 to ~1060 nm. To laser engineers and many knowledgeable laser users, such a broad emission spectrum indicates the potential for mode-locking with pulse durations in the femtosecond regime. In addition, the absorption spectrum of ytterbium tungstate (Figure 1) shows that it can be efficiently pumped with laser diodes at 940 or 980 nm, wavelengths where high-power laser diodes are readily available with proven reliability.

Figure 1. Pumping efficiency with laser diodes at 940 or 980 nm is illustrated in this normalized spectral absorption cross section of Yb:KGW for the three crystal axes.

Although several low-power lasers based on Yb:KGW have been developed, high-power amplifiers are only now becoming available. One main challenge has been the thermal management of this new material. Intense pumping of most laser materials can produce significant thermal gradients and, hence, thermal stress, which can lead to fracture and failure. Early Yb:KGW-based prototypes were particularly susceptible to this type of damage.

We have addressed the thermal damage issue with a multipronged approach, allowing the full potential of Yb:KGW to be accessed. The steps include: selection of defect-free Yb:KGW crystals with the optimum doping concentration, fabrication of high-quality rods from these crystals, thorough investigation of the thermal properties of Yb:KGW, selection of the optimum rod orientation, design of a proprietary crystal mounting and cooling scheme, and design of a proprietary optical coating that eliminates surface damage.

With the thermal stress problems solved, our engineers were able to design a high-power femtosecond laser system. Some elements are new, whereas others draw on the company’s Ti:sapphire product portfolio and/or the industrial mode-locked Nd:YVO₄ laser line.

The Eclipse amplifier consists of a single self-contained laser head, together with a compact rack-mounted power supply and control unit (Figure 2). The system, which also includes a closed-loop chiller, does not require external cooling water.

Figure 2. The new generation of femtosecond Yb:KGW amplifiers benefit from the simplicity of direct diode pumping, resulting in a compact and reliable package for demanding industrial and scientific applications.

Low-power seed oscillator

Two self-contained units are inside the laser head — an oscillator and a regenerative amplifier — as well as optics for stretching and compressing the pulses, before and after amplification (Figure 3). The mode-locked oscillator is a factory-sealed unit containing a single rod of Yb:KGW. It is end-pumped (at both ends) by the output from two fiber-coupled single-emitter laser diodes that are operated at derated power levels. Located in the control unit, the diodes have expected lifetimes in excess of 10,000 hours but nevertheless are designed to be field-replaceable without need of optical alignment.

Figure 3. The Eclipse Yb:KGW femtosecond amplifier includes a) a sealed oscillator module, b) a stretcher/compressor, c) an amplifier module, d) an optional second-harmonic generator, e) a safety shutter and f) a high-speed switcher electronics rail. The pump diodes are in the control module and fiber-coupled to the oscillator and amplifier modules.

The oscillator generates more than 100 mW at 1048 nm, the fundamental wavelength for Yb:KGW. The output is passively mode-locked by use of a single saturable Bragg reflector optic that serves as one of the cavity mirrors. Sometimes referred to as a semiconductor saturable absorber mirror, this is a high reflector that contains a quantum well structure designed to absorb at the laser wavelength. The saturable Bragg reflector absorption saturates at high laser intensity, forcing the laser to naturally operate in a mode-locked manner. This passive mode-locking is self-starting and simpler than active mode-locking.

The Bragg reflector method is also much more robust than other passive techniques, which usually require some type of starter mechanism and can suffer from pulse dropouts. We have used this technology for some time to mode-lock our industrial-grade Nd:YVO₄ lasers with green and ultraviolet outputs. The oscillator also utilizes chirped mirror technology. The use of these negative-dispersion mirrors simplifies the cavity by eliminating the need for additional optics such as prism pairs to compensate for the positive dispersion introduced by the crystal itself. The mirrors enable the oscillator to deliver transform-limited pulses less than 150 fs in duration at a repetition rate of 80 MHz.

As with Ti:sapphire, the optimum method of producing high pulse energy is to amplify a subset of the oscillator pulses in a regenerative amplifier. It delivers the high-quality TEM₀₀ beam profile required for high-precision femtosecond materials processing or for driving a widely tunable OPA, which is an essential accessory for research applications.

Using standard chirped pulse amplification, the output pulses from the sealed oscillator are stretched in a single grating pulse stretcher before entering the sealed amplifier module. Here, a proprietary high-speed switching scheme using two Pockels cells serves a fourfold purpose: It injects only one preselected pulse at a time into the regenerative amplifier, eliminates background seed pulses for maximum contrast ratio, minimizes intracavity losses for maximum output power and blocks any backreflection that could disturb the seed oscillator. Operated by an electronics rail (Figure 3) specifically designed for driving the Pockels cells at high speed over an extended range of repetition rates, switching is accomplished within 3 ns to prevent multiple pulses from entering the amplifier cavity.

The gain in this cavity is supplied by a single rod of Yb:KGW, pumped at both ends by the output of two 40-W diode bars, each of which is derated to less than 30 W. A telescope conditions the fiber-coupled pump light before it enters the Yb:KGW rod. These high-power ProLite diode modules, located in the rack-mounted control electronics, also can be field-replaced with no optical alignment.

Amplified output

After a number of trips around the amplifier cavity to reach gain saturation, the high-speed switcher deflects the amplified pulse out of the cavity. This pulse is then recompressed in time by a single grating pulse compressor. The result is an amplified output pulse train in a TEM₀₀ beam with an average power of up to 4 W at repetition rates of up to 7 kHz. The pulse width is less than 500 fs, and the contrast ratio exceeds 1000:1. This output can be frequency-doubled by an optional integrated second-harmonic generator, which produces green output at 524 nm with an average power of up to 1.5 W and a contrast ratio as high as 10⁵:1.

Operational simplicity

Direct diode pumping results in a significantly simpler amplifier architecture with better reliability and lower cost of ownership than with Ti:sapphire. Four fiber-coupled pump diodes in the control electronics module replace the two complex, frequency-doubled, solid-state lasers required for pumping ultrafast Ti:sapphire amplifier systems. For operational simplicity, the compact rack-mounted controller has an RS-232 interface for easy integration and remote control from an external computer. The passively mode-locked oscillator serves as the master clock.

A fast photodetector sends the required electronic pulse train to the controller, which uses these signals to drive the two Pockels cells at optimum times appropriate for the user-selected amplifier repetition rate. The pulse rate can be adjusted from single shot to 7 kHz, or custom bursts of pulses can be programmed, which is useful in certain materials processing applications. The system uses additional photodetectors downstream to enable performance monitoring and system self-diagnosis, including an onboard power meter that detects the output level.

Most scientific users of these new amplifiers will seek to obtain tunable high-energy pulses by driving an OPA, which relies on a nonlinear process called optical parametric down-conversion to produce two tunable longer-wavelength beams (called signal and idler) from a shorter-wavelength input. When pumped by one of the frequency-doubled Yb:KGW amplifiers at 524 nm, the signal and idler can be tuned from the red (<650 nm) through the near-IR (>2.6 μm). Frequency conversion of signal and idler allows the OPA to extend tunability even further. An OPA pumped at 524 nm can reach <325 nm in the UV and >10 μm in the IR, with only one additional nonlinear process.

Ultrafast applications

The ability of femtosecond lasers to create small patterns — such as microscopic holes, grooves or surface features — with high precision is widely known. The Eclipse, with its high average power at high repetition rates, meets the industry’s requirement for high throughput. The TEM₀₀ beam (Figure 4) can be focused to a diffraction-limited spot, enabling high spatial resolution. This can be further improved by using the lower diffraction limit of the laser’s optional green output.

Figure 4. The use of the regenerative cavity for amplification results in a TEM₀₀ mode profile, shown here for the IR output at 1048 nm.

When used with one or more OPAs, these amplifiers cover research applications, including time-resolved spectroscopy and molecular dynamic studies, nonlinear optics, solvation effects, semiconductor research, fluorescence up-conversion and advanced telecommunications research such as solitons.

So where does this leave Ti:sapphire? The Yb:KGW amplifiers offer three principal advantages over that technology: direct diode-pumped simplicity and reliability, higher average power and higher repetition rates. However, Ti:sapphire-based systems still can produce shorter pulses than Yb:KGW, which is why continuing and improving our Ti:sapphire product lines is important. We expect Yb:KGW to dominate many applications, but in those where the shortest possible pulse width is a paramount consideration, there is clearly still a role to fill for Ti:sapphire.

To summarize, we have solved the practical problems of using Yb:KGW at high output power, allowing the successful development of high-repetition-rate regenerative amplifiers. The simplicity and other advantages of this new generation of ultrafast amplifier will be a tremendous benefit in both research and industrial ultrafast applications.

Meet the authors

Arnd Krueger is director of the ultrafast group at Spectra-Physics in Mountain View, Calif.

Philippe Féru is product manager for the company’s ultrafast amplifier product line.