Abstract
When femtosecond laser pulses are focused inside bulk transparent materials, the intensity in the focal volume becomes high enough to produce permanent structural modifications. This technique has been applied to fabricate three-dimensional photonic structures for optical data storage, waveguides, gratings, and couplers inside a wide variety of transparent materials. We report micro-fabrication of optical elements in glasses with femtosecond laser pulses, including fabrication of couplers, Bragg gratings, and zone plates.
I. Introduction
In recent years, micromachining or microfabrication by femtosecond laser pulses in transparent materials has received much attention. When femtosecond laser pulses are focused inside bulk transparent materials, the intensity in the focal volume can become high enough to cause nonlinear absorption, which leads to localized modification in the focal volume, while leaving the surface unaffected. Recent demonstrations of three-dimensional micromachining of glass using femtosecond laser pulses include waveguides, couplers, gratings, binary data storage, lenses and channels [1]. Direct writing of optical devices using femtosecond laser pulses in glass has potential applications in the telecommunication and optical signal processing industries. The most important feature of the microfabrication technique in glass is its ability for three-dimensional (3-D) integration of optical or photonic devices inside transparent materials: Sequential direct writing of individual devices inside bulk transparent materials simply results in the integration of a 3-D photonic signal-processing system. Although such an approach lacks in the speed of mass production compared with the conventional lithographic process, the ability of 3-D integration is priceless in that 3-D integration of photonic devices is quite difficult to achieve by the conventional lithographic process or other related techniques. The current prosperity of the semiconductor industry relies on the progress of microelectronics technologies and no one can deny the possibility that the present 3-D microfabrication techniques may make some contributions to the industrial fabrication of integrated photonic circuitry.

Fig. 1(a) Schematic of a three-dimensional directional coupler. (b) Near-field patterns of coupler output when coupling to a broadband light source (supercontinuum).

II. Waveguide and Coupler
The induction of permanent refractive-index change at the laser focal point has been reported in bulk glasses to the order of 10-2 to 10-4. By translation of the sample with respect to the focal point, fabrication of waveguides and couplers has been demonstrated. Filamentation of femtosecond laser pulses induces permanent refractive-index change in silica glass [2]. Filamentation occurs due to a balance between the Kerr self-focusing of the laser pulse and the defocusing effect of the high-intensity plasma generated in the self-focal region. Filamentation induces refractive-index change. The location of a filament coincided with that of the refractive-index change. The region of refractive index change was 10 to 500 micrometers long due to the high numerical-aperture (NA) of the focusing lens used on the silica glass. By translation of the sample parallel to the optical axis, the 2-mm straight waveguide was fabricated.
We fabricated a directional coupler containing a 2-mm-long straight waveguide and a curved waveguide connected to a different straight section [3]. The straight section was parallel to the straight waveguide and was separated by a 4-µm center-to-center distance. Another straight waveguide, which length is L mm, was then fabricated parallel to the straight waveguide. The curved waveguide was fabricated from one end of the straight section. The position of the filament in the curved section was determined by computer-controlled positioning along an arc of radius 17 mm. Coupling properties were investigated by focusing a He-Ne laser beam having a wavelength of 632.8 nm into the straight waveguide. Experimental results show that the splitting ratios of the directional couplers with L = 1 mm and L = 0.5 mm are approximately 1:1 and 1:0.5, respectively. By use of a 2-mm coupler, a beam can be divided into 1:1.
Producing real three-dimensional photonic devices is one of the salient features of femtosecond micromachining. We demonstrate the realization of three-dimensional directional couplers using filamentation [3]. Figure 1 (a) shows the schematic of a three-dimensional directional coupler consisting of three waveguides: a 2-mm-long straight waveguide (waveguide I) and two curved waveguides that are connected to straight sections (waveguide II and III). The curved waveguides have arc radii of 17 mm. The straight sections of waveguide II and III were parallel to the straight waveguide at a 4 µm center-to-center separation. Lengths of the straight sectors in waveguide II and waveguide III were 0.5 mm and 1.0 mm. The broadband supercontinuum was coupled to the straight waveguide and near-field output patterns were monitored using the color CCD camera. Figure 1 (b) shows the near-field output patterns of the coupler. Spectra are different at the output because the coupling properties are dependent on wavelength. The technique for fabricating spectroscopic couplers in glass could thus have potential applications in wavelength division, including in spectral filters. Beams of different wavelengths from different lasers can also be coupled into a single waveguide.

Fig 2(a) Fabrication of volume gratings in silica glass. (b) Optical images of the fabricated gratings with the period of 5 mm and thickness of 150 mm. (c) Schematic of diffraction pattern. The fabricated grating with the period of 3 mm and thickness of 150 mm.

III. Volume Grating
Figure 2 (a) shows the schematic of the fabrication of a volume grating. When a 150-µm long filament was scanned for 300 µm in the plane perpendicular to optical axis at the speed of 1 µm/s, a layer of refractive-index change with a thickness of 2 µm was induced. We stacked 60 layers with a sample displacement of 3 µm. Figures 2 (b) and (c) show an optical image of the fabricated grating and its magnified image, respectively. The maximum diffraction efficiency of 74.8 % was obtained with TE (electric field perpendicular to the plane of incidence) polarization of the He-Ne laser beam when the grating was fabricated using the 0.10 NA focusing lens and with a period of 3 µm. The typical profile and image of the diffraction pattern are shown in Fig. 2 (d). The angle between the transmitted zero-th-order beam and the first-order diffraction beams was 12.2 degrees in the case of the period of 3 µm. Volume gratings act as mirrors to reflect beams in silica glass.

Fig 3(a) Optical image of the fabricated Fresnel zone plate by embedding the two-dimensional array of voids. (b) Intensity distribution in the primary focal point when a cw-He-Ne laser beam at the wavelength of 632.8 nm transmitted through the zone plate.

IV. Fresnel Lens
Tightly focusing femtosecond laser pulses with high NA lenses produce submicron-damage inside a wide variety of transparent materials. The damage appears as cavities or voids with diameters of only 200 nm to 1µm, surrounded by densified material. We are presenting in this section the report of the fabrication of a lens by embedding voids inside silica glass.
Our designed Fresnel lens has the primary focal length of 3 mm at the wavelength of 632.8 nm. The radius of the first odd zone was designed to be 43.5 µm. The size of the zone plate was 400 µm ¥ 400 µm. In this condition, the radius of the outer zone is 200 µm, which corresponds to the included number of the odd zone plate is eleven.
The laser pulses were tightly focused by an objective lens with a NA of 0.55 to create the voids inside silica glass. We fabricated the Fresnel zone plate by embedding voids at the depth of 300 µm beneath the sample of silica glass. The sample was displaced dot by dot in the plane perpendicular to the laser propagation axis by steps of 1 µm with a computer-controlled motor stage. Figure 3 (a) shows an optical image of the fabricated Fresnel zone plate by embedding the two-dimensional array of voids. The voids were embedded only in the even zones.
We investigated the focusing properties of the fabricated Fresnel zone plate. The beam incident on the lens is diffracted and converges in on the primary focal spot on the optical axis. Figure 3 (b) shows an intensity distribution in the primary focal point when the He-Ne laser beam at the wavelength of 632.8 nm was incident on the zone plate. The measured spot size was 7.0 µm and agreed with the theoretical value of 6.1 µm. The measured diffraction efficiency was 2.0 %.
V. Conclusion
In conclusion, we present microfabrication of optical devices by femtosecond laser pulses in silica glass. This technique enables us to make integrated optical circuits in bulk silica glass. The technique has potential applications in the integration of a variety of micro optical elements inside a small volume in bulk materials.
References
1. K. Itoh and W. Watanabe, “Toward nano- and microprocessing in glass with femtosecond laser pulses,” RIKEN Review, no.50, 90-94, 2003.
2. K. Yamada et al, “In situ observation of photoinduced refractive index changes in filaments formed in glasses by femtosecond laser pulses,” Opt. Lett., vol. 26, 19-21, 2001.
3. W. Watanabe, et al, “Wavelength division using three-dimensional couplers fabricated by filamentation of femtosecond laser pulses,” Opt. Lett., vol. 28, 2491-2493, 2003.
4. K. Yamada, et al, “Volume grating induced by a self-trapped long filament of femtosecond laser pulses in silica glass,” Jpn. J. Appl. Phys., vol. 42, 6916-6919, 2003.
5. W. Watanabe, et al, “Fabrication of Fresnel zone plate embedded in silica glass by femtosecond laser pulses,” Opt. Express, vol. 10, 978-983, 2002.