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Femtosecond micromachining in transparent bulk materials using an anamorphic lens

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A unique anamorphic lens design was applied to a circular 780nm femtosecond laser pulse to transform it into an elliptically shaped beam at focus. This lens was developed to give an alternative method of micromachining bulk transparent materials. The challenge for femtosecond laser processing is to control the nonlinear affect of self-focusing, which can occur when using a fast f-number lens. Once the focused spot is dominated by self-focusing the predicted focused beam becomes a filament inside the bulk, which is an undesirable effect. The anamorphic lens resolves this self-focusing by increasing the numerical aperture (NA) and employing an elliptical beam shape. The anamorphic lens was designed to furnish a 2.5μm by 190μm line at focus. Provided the pulse energy is high enough, transparent bulk material will be damaged with a single femtosecond laser pulse. Damage in this text refers to visual change in the index of refraction as observed under an optical microscope. Using this elliptical shape (or line), grating structures were micro-machined on the surface of SiC bulk transparent substrate. SiC was chosen because it is known for its micromachining difficulty and its crystalline structure. From the lack of self-focusing and using energy that is just above the damage threshold the focused line beam generated from the anamorphic lens grating structures produced a line shape nearly identical to the geometrical approximation. In this paper we discuss a new method of writing gratings (or other types of structures) in bulk transparent materials using a single femtosecond laser pulse. We will investigate the grating structures visually (inspected under an optical microscope) and also by use of an atomic force microscopy (AFM). In addition, we test the grating diffraction efficiency (DE) as a function of grating spacing, d.

©2007 Optical Society of America

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Figures (7)

Fig. 1.
Fig. 1. Zemax analytic views of the anamorphic lens used spread the focused beam elliptically. Top is a Zemax solid layout displaying each lens and their relative position in the lens tube (on the left is a 100mm focal in x, middle is a 50mm in y, and on the right is the spherical lens), and Bottom is a plot of the ray distribution through focus ± 400μm, where at defocus = 0μm is the anamorphic line shape used for laser processing.
Fig. 2.
Fig. 2. Shown here is the geometrical Zemax theoretical M2 (top chart), a zoomed in theoretical chart with linear fit (middle chart), and the experimental (bottom chart), which were used to determine the NA of each axis.
Fig. 3.
Fig. 3. Optical setup for the anamorphic lens micromachining experiment.
Fig. 4.
Fig. 4. (Top) SiC grating view with an optical microscope using Nomarski DIC for (left) semi-insulating SiC on a 10X magnification; (right) 50X magnification. (Bottom left) shows the surface of another SiC sample, and (bottom right) shows just 5μm below the surface. Image processing was performed in order to better resolve the modified surface lines.
Fig. 5.
Fig. 5. AFM results of a 5.0μm (micron) wide and a 10nm (nano-meter) region showing a raised surface modification on semi-insulating SiC material.
Fig. 6.
Fig. 6. This figure depicts the input 632.8nm HeNe beam and the resulting SiC diffraction pattern of the first order. On the right is the 0-order beam and on the left is the first-order diffracted beam.
Fig. 7.
Fig. 7. DE verses grating spacing.

Tables (2)

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Table 1. SiC sample characteristics for the semi-insulating type. These values come from the vendor, Intrinsic Corp.

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Table 2. NA values for theoretical and experimental results.

Equations (2)

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NA = D 2 f = n sin ( θ ) ,
DE = 2 P 1 10 ( ND 0 ND 1 ) P 0 100 .
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