Getting Practical

From oemagazine May 2004
01 May 2004
By Arnd Krueger and Philippe Féru

Nearly a decade has passed since the combination of mode-locked titanium-doped sapphire (Ti:sapphire) technology and diode pumping enabled the development of the first all-solid-state, ultrafast laser systems. This development spurred tremendous growth in ultrafast applications, particularly in biology and photochemistry. More recently, the advent of one-box Ti:sapphire femtosecond laser amplifiers has opened up applications in precision materials processing. The latest solid-state systems are helping to meet the needs of the ever-diversifying base of applications.

The challenge with Ti:sapphire is that it must be pumped at wavelengths only available from other solid-state lasers such as neodymium-doped vanadate or yttrium lithium fluoride. Such demands add to system size and complexity. A laser capable of Ti:sapphire performance with direct diode pumping would provide enormous advantages in a number of applications. Recent work with ytterbium-doped tungstate (Yb:KGW) has yielded diode-pumped ultrafast systems capable of generating sub-500-fs pulses with peak powers in the gigawatt range.

Mode Locking

At the heart of any ultrafast laser system is a mode-locked oscillator. Mode locking is based on continuous wave (CW) operation in broadband lasers (see figure 1). If the relative phases of the longitudinal cavity modes are locked together, spatial and temporal interference results in a single pulse circulating within the cavity. The more modes involved, the narrower the resulting pulse width. In fact, the pulse width is inversely proportional to the spectral bandwidth. Every time the circulating pulse reaches the output coupler, part of it escapes the cavity. The repetition rate, or cavity frequency, is inversely proportional to the cavity length, and typically in the range of 80 MHz.

Figure 1. Cavity modes locked in phase constructively interfere at some point and destructively interfere elsewhere, resulting in a pulse that travels around the cavity at the speed of light.

In Ti:sapphire oscillators, active regenerative mode locking uses an acousto-optic deflector as a fast intracavity gate, modulating the cavity loss at the cavity frequency. This active technique reliably operates over the full Ti:sapphire tuning range and at pulse widths from the femtosecond to the picosecond regime.

In passive mode locking, an intracavity optical element creates higher loss for CW operation, thereby favoring mode-locked output. The first commercialized technique was based on an optical Kerr lens induced in the laser crystal, combined with an intracavity slit or pinhole. This technique required an active starter mechanism, and its nonlinear dependence on the peak intensity limits its use to shorter pulse widths.

A more recent commercial technique is based on the saturable Bragg reflector (SBR), a semiconductor-based cavity mirror containing a quantum well that absorbs at the laser wavelength, causing a loss in CW operation. This absorption saturates at high fluence, however, eliminating this loss in mode-locked operation. Because it is reliable and completely self-starting, this technique is being increasingly used, particularly in non-tunable industrial lasers.

Mode-locked oscillators generate pulse energies at the nanojoule level, but many applications in basic research as well as femtosecond materials processing require higher pulse energies. A two-stage system incorporating an ultrafast oscillator and an optical amplifier meets this demand. Briefly, the two most common amplifier types use either regenerative cavity or multi-pass configurations, with regenerative amplifiers being more widespread in the ultrafast field (see oemagazine, October 2003, p. 28). In a regenerative Ti:sapphire amplifier, the laser crystal sits inside a cavity and is pumped by a visible Q-switched or CW solid-state laser. An optical gate allows a single pulse from the oscillator to enter this amplifier cavity. The pulse traverses the cavity, gaining energy each time it passes through the laser crystal. When the gain is saturated, the optical gate deflects the pulse out of the cavity.

Commercial regenerative Ti:sapphire amplifiers produce pulse energies ranging from a few microjoules to a few millijoules at 800 nm, depending on their repetition rate—typically in the kilohertz range—and the properties of the pump laser. The TEM00 amplifier cavity defines the beam characteristics of the output pulse train. Users tend to favor regenerative amplification for applications in which high beam quality and stability are important, such as optical parametric amplifier (OPA) pumping.

To amplify very short pulses or to reach higher pulse energies, the most common method is to use multi-pass amplifiers. A multi-pass Ti:sapphire amplifier uses routing mirrors to direct the oscillator pulse a few times through an amplifier crystal. However, with no cavity to define the beam characteristics, the output beam quality is generally inferior compared to that of a regenerative amplifier.

The high peak power generated by an ultrafast amplifier necessitates the use of pulse stretching. The seed pulses are stretched temporally before they enter the amplifier, then recompressed after amplification. Both stretcher and compressor typically include a single diffraction grating, which by spectral dispersion generates different path lengths and time delays for the different spectral components of the ultrafast pulse.

Tuning Pulses

Many applications in basic research, such as time-resolved spectroscopy, require the ability to tune the center wavelength of ultrafast pulses. Although it is possible to tune the output of an ultrafast Ti:sapphire oscillator, we obtain much wider tunability from an optical parametric oscillator. In the case of a Ti:sapphire amplifier, a related device—an OPA—provides extended tunability.

These devices rely on a nonlinear process called optical parametric downconversion in a nonlinear crystal such as beta barium borate. In this process, a high-energy photon is split into two lower energy photons at the so-called signal and idler wavelengths (1/λpump = 1/λsignal + 1/λidler). Phase matching determines the wavelength of the signal and corresponding idler. In commercial devices, adjusting either the temperature or the angle of the nonlinear crystal accomplishes wavelength tuning. In an OPA, part of the input pulse generates a white-light continuum, which then seeds the OPA. Commercial OPA products also include an integrated module for frequency mixing or harmonic generation of signal and idler, thereby enabling wavelength tunability from 240 nm to 10 µm.

Obviously, ultrafast laser sources are relatively complex, compared to most other commercial laser types. This complexity has undoubtedly limited their use, particularly in industrial and medical applications. Fortunately, in recent years laser manufacturers have made significant strides in streamlining ultrafast systems, first with one-box tunable ultrafast oscillators and later with one-box Ti:sapphire ultrafast amplifiers. The latter development prompted end users to seriously explore the benefits of high-energy femtosecond pulses in materials processing. Ultrafast pulses can remove material with virtually no peripheral thermal damage, enabling feature sizes that are much smaller than those achievable with any other type of laser.

New Materials

As a gain material, Yb:KGW offers significant advantages for ultrafast amplifier applications. The material has a broad emission spectrum and thus supports femtosecond output. It can deliver higher output powers than Ti:sapphire. Most important, Yb:KGW can be directly pumped by diode lasers at 940 or 980 nm, eliminating the challenge of pumping the oscillator and the amplifier with two complex intracavity frequency-doubled, solid-state lasers, as required in the case of a Ti:sapphire system.

Figure 2. The Yb:KGW femtosecond amplifier includes a sealed oscillator module, a stretcher/compressor, an amplifier module, and an optional second-harmonic generator. Fiber-coupled diode lasers provide pump power to the system.

Our Yb:KGW-based integrated femtosecond amplifier system contains an oscillator and an amplifier, each in a sealed module and connected by a standard pulse stretcher (see figure 2). The oscillator uses a single laser rod and the cavity mirrors include dispersion-compensation coatings, eliminating the need for additional optics such as a prism pair to compensate for the positive dispersion of the rod itself. The use of a single SBR mirror delivers self-starting and reliable mode-locked output. Finally, the rod is pumped with two single-emitter diode lasers located remotely in the system controller and fiber-coupled into the oscillator. This 80-MHz oscillator generates 100 mW of average power at a center wavelength of 1048 nm and with a pulse duration of 150 fs.

The output pulses are stretched in time by a standard single-grating stretcher before reaching a high-speed switcher that acts as a gate into the regenerative amplifier cavity. A single rod of Yb:KGW, collinearly pumped at both ends with the fiber-coupled output of two high-power diode bars, provides the gain in the amplifier.

Figure 3. The output of a Yb:KGW femtosecond amplifier has a TEM00 mode profile created through the use of the regenerative cavity for amplification.

The final system output power is up to 4 W at 1048 nm, delivered in a TEM00 beam (see figure 3). With repetition rates of up to 7 kHz and pulse widths shorter than 500 fs, the system generates peak powers in the gigawatt range, suitable for high-throughput, precision materials processing. An optional second-harmonic generator integrated into the laser head delivers an output power of more than 1.5 W at 524 nm, which is suitable for pumping one or more femtosecond OPAs, each allowing for wavelength tuning from 265 nm to 10 µm. Going forward, well-established techniques for pulse width reduction will allow for significantly shorter system pulse widths. Because of the visible pump wavelength, generating this wide tuning range requires one less frequency conversion step than an OPA pumped by a Ti:sapphire amplifier at 800 nm.

Simple, robust, and high power, amplifiers based on diode-pumped Yb:KGW could come to dominate the ultrafast market in the next few years. Ti:sapphire systems still have a role to play, however, because they can achieve the shortest possible pulse widths. For this reason, laser manufacturers continue to invest in Ti:sappphire technology.

In the world of ultrafast lasers and amplifiers, recent years have seen an emphasis on system simplicity and enhanced ease of use. This is in marked contrast to earlier times when laser designers strove for advances in raw performance, such as shorter pulse widths. As simpler, more reliable systems come into the marketplace, ultrafast lasers will continue to find their way into new applications. oe

Arnd Krueger, Philippe Féru

Arnd Krueger is director of the ultrafast group and Philippe Féru is product manager at Spectra-Physics, Mountain View, CA. 

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