MICROMACHINING: Ultrafast pulses create waveguides and microchannels

Multiphoton interaction of femtosecond pulses with glass allows waveguides to be fabricated anywhere within the material.

Philippe Bado

In the early 1990s, researchers at the University of Michigan (Ann Arbor, MI) determined that laser-matter interaction for femtosecond pulses is fundamentally different from the interaction resulting from longer pulses—a finding that opened the way to generalized fine laser micromachining. Machining with laser pulses of very short duration essentially eliminates heat flow to surrounding material. Thermally induced substrate degradations that commonly occur with most standard machining techniques are prevented. The material-removal mechanism changes from melt expulsion for microsecond and nanosecond pulses to vaporization or sublimation for femtosecond pulses. Debris generation and surface contamination are significantly reduced. In addition, the threshold nature of ultrashort-pulse interaction with matter permits ultrafast lasers to cut and drill materials to feature sizes smaller than the wavelength of light. When working with dielectrics, the exactness of the ablation threshold increases both the precision and reproducibility.

Femtosecond micromachining workstation was used to direct-write localized index-of-refraction changes within a glass substrate, creating a three-dimensional optical waveguide.

Over the last couple of years, numerous groups have evaluated the use of ultrashort pulses for ultraprecision micromachining. The machining of most materials has been studied, with the bulk of this international work directed at metal removal. Significant commercial successes have resulted; for example, femtosecond lasers are now the preferred tools to repair lithographic photomasks. However, available femtosecond lasers are complex and expensive and produce low average power (although all of this is changing). For the foreseeable future, femtosecond-based machining will be restricted mainly to applications that cannot be addressed with other machining technologies. One such application is glass micromachining—a much more subtle endeavor than machining of metals. Using the proper laser parameters, glass can be either removed by ablation or physically modified without being ablated.

Ultraviolet photosensitivity of glass

It is well known that the properties of glass can be changed by exposure to light. Ultraviolet (UV) lasers—either continuous-wave or nanosecond pulsed—are routinely used in holographic setups to direct-write Bragg gratings into fused-silica fiberoptic devices. These devices are used in fiber communication systems for gain flattening, wavelength selection, multiwavelength pump combiners, Raman filters, compensation of group-velocity dispersion, and so on.

The UV-based direct-write method, however, has some inherent limitations. In particular, many types of glass are not sufficiently photosensitive to yield significant index-of-refraction change upon UV exposure. Irradiation with UV light causes an increase of the refractive index owing to some poorly understood changes in the glass, including densification, increases in tension, and—in germanosilicate glasses—the formation of germanium-related glass defects.

The best UV direct-write results have been obtained with high-pressure cold hydrogen-soaked germanium-doped fibers. These devices age—that is, their refractive-index change relaxes with time, especially when subjected to high temperatures. Additional manufacturing steps, such as annealing at elevated temperatures, are required to stabilize these changes in index. Also, the UV photosensitivity band is very close to the absorption edge of most glasses, and thus the UV penetration depth is small. This fact has precluded using UV direct-write techniques to produce subsurface or three-dimensional structures in bulk glass.

Direct-write of index change

Femtosecond micromachining has been proposed to circumvent the shortcomings associated with the UV-based manufacture of glass. The linear absorption of near-infrared (IR) light for transparent glass is negligible, as the absorption edge for most glass is in the UV. However, the very high intensities associated with femtosecond pulses enable nonlinear absorption through multiphoton processes.

Large localized increases in refractive-index change have been observed in a wide variety of glasses subjected to high-intensity femtosecond pulses. This localized effect allows for the fabrication of two- or three-dimensional waveguiding structures through a direct-write process by translation of the sample with respect to the focal point (see photo on p. 73 and on cover). This idea has been applied to the fabrication of optical waveguides in silicate, borosilicate, chalcogenide, and fluoride glasses.¹ The manufacturing threshold was found to vary with the glass formulation. Adjusting focal spot and laser intensity allows fabrication of waveguides that are by design either single-mode or multimode. The quality of the resulting waveguides is excellent (see Fig. 1). Due to the deterministic nature of the femtosecond-matter interaction, the waveguide walls are well defined and optically smooth. The internal scattering losses are very low.

All waveguides created in this manner have been found to be stable at room temperature. Scanning the laser spot or translating the sample along an axis perpendicular to the propagation direction of the incident light beam provides the most flexibility for writing planar patterns. For example, more-complex structures such as Y-junction splitters and long-period gratings have been produced with this femtosecond direct-write configuration.^2,3

Although the exact physical mechanisms responsible for IR photosensitivity are still under investigation, the process is certainly initiated by multiphoton absorption and therefore exhibits a highly nonlinear dependence on the beam intensity. Recent experimental results have shown that structural modifications can be confined to submicron size regions smaller than the focused spot when exposed to intense femtosecond pulses. Optical storage based on this process has been demonstrated.

In contrast to standard waveguide-manufacturing methods such as physical-vapor deposition or ionic exchange, the femtosecond approach is not restricted to the glass surface. The absence of linear absorption provides a three-dimensional capability. Multiple patterned layers can be created by simply changing the depth of focus (writing perpendicular to the waveguide) or by moving the sample laterally (writing parallel to the waveguide). Closely spaced parallel waveguides have been fabricated that demonstrate coupling of light from one waveguide to its neighbor or neighbors.

Direct-write of active waveguides

One of the most exciting developments associated with the femtosecond direct-write approach is its capability to write active waveguides—namely, waveguides that exhibit gain when activated by the appropriate source. Researchers at Clark-MXR in collaboration with Kim Winick of the University of Michigan have used a femtosecond micromachining workstation to direct-write active waveguides deep inside neodymium-doped glass substrates. The resulting waveguides were 1 cm long. It was found that the waveguide machining process does not significantly affect the dopant distribution. These devices exhibit gain of approximately 3 dB/cm at 1062 nm for input pump power levels of approximately 350 mW at a pump wavelength of 514 nm. This amount of gain should be sufficient to support lasing when high reflectors are affixed to the ends of the waveguide.⁴

FIGURE 1. Output of a waveguide formed by femtosecond micromachining is captured on a charge-coupled device to confirm its single-mode nature.

These first direct-write active waveguides operate in the near-IR, a region where other techniques are available to manufacture waveguides—at least at or near the surface. But this new waveguide-manufacturing approach is also compatible with doped glasses designed to operate at longer IR wavelengths. It is generally difficult to build waveguide lasers in oxide glasses, because these glasses have high phonon energy and hence large rates of nonradiative decay at the long-wavelength lasing transitions. These problems can be partially circumvented by using nonoxide glasses such as fluorozirconates or chalcogenides. Unfortunately, there are not many good methods available for making waveguides in nonoxide glass. The femtosecond direct-write approach holds promise. Waveguides have, in fact, already been written in fluoride and chalcogenide glass using femtosecond lasers.

Ablation makes microchannels

Above a certain threshold, the change in the index of refraction is replaced by an ablation process. The deterministic nature of the femtosecond machining provides high precision and repeatability. Due to the multiphoton nature of the interaction, the ablation process can be conducted on the sample surface or within its bulk.

FIGURE 2. Electrokinetic microfluidic system contains integrated detection. The microchannel, including the "Y" mixing region, is fabricated by ablation. The precise machining threshold provides high-quality wall finish. The waveguides are fabricated by index change. The active waveguide can be a laser with integrated Bragg gratings, an optical amplifier, or an amplified-spontaneous-emission source.

The same micromachining workstation can be used for ablation and for direct-writing index changes. This inherent dual capability can be used to fabricate complex miniaturized devices. For example, the usefulness of microfluidic systems—which have found applications in many types of biological assays, such as nucleic-acid separation, enzymes essays, immunoassays, and drug-screening—can be greatly increased by integrating the detection system with the analytical section on a single glass substrate (see Fig. 2). By placing all the key components on a single chip, the complexity and fluid loss associated with multichip designs are avoided.

Femtosecond laser machining is now starting to make inroads in the industrial world. Its advantages are especially apparent when working in transparent media. A commercially available femtosecond workstation can be used not only to fabricate passive or active waveguides in glass, but to create high-quality fluidic microchannels. Such machining technology will play an important commercial role in the manufacture of microfluidic systems, as well as in the optical-communication component industry.

REFERENCES

1. K. Miura et al., Appl. Phys. Lett. 71, 3329 (1997).
2. D. Homoelle et al., Opt. Lett. 24, 1311 (1999).
3. Y. Kondo et al., Opt. Lett. 24, 646 (1999).
4. Y. Sikorski et al., "Optical waveguide amplifier in Nd doped glass written with near-IR femtosecond laser pulses," to appear in Electron. Lett.

PHILIPPE BADO is vice president of technology at Clark-MXR Inc., 7300 West Huron River Dr., Dexter, MI 48130; e-mail: [email protected].