We present a novel method for three-dimensional optical splitter that have U-grooves, which are used for fiber alignment, within a fused silica glass using near-IR femtosecond laser pulses. The fiber aligned optical splitter has a low insertion loss, less than 4 dB, including an intrinsic splitting loss of 3 dB and excess loss due to the passive alignment of a single-mode fiber. The output field pattern is presented, demonstrating the splitting ratio of the optical splitter is approximately 1:1. Finally, we demonstrate the utility of the femtosecond laser writing of periodic patterns by fabricating the submicron line and dot patterns inside the silica glass, which is applicable to 3-D optical memory.
© 2005 Optical Society of America
Although over the past few years the change in refractive index induced by ultraviolet (UV) light in glasses has been investigated, UV-photosensitive glasses were limited due to the requirement for doping with germanium. Owing to ultrashort pulses with high peak power density, the interaction of a femtosecond (fs) laser with matter causes many nonlinear physical phenomenon, such as multi-photon absorption, photo-darkening, plasma and so on. The fs laser can sensitize a wide variety of glasses, such as fused silica glasses and chalcogenide glasses, etc. In recent years, using infrared fs lasers to induce a change in refractive index by the multi-photon absorption process in transparent materials has been widely investigated. The application of the fs laser provides a new technique for making three-dimensional integrated photonic structure in a variety of glasses. This technique has been applied to fabricate photonic structures, such as passive optical waveguides [1–2], gratings [3–5], rare earth-doped waveguide amplifiers , and couplers [7–9].
Here we report the fabrication of waveguides and optical devices using a Ti:Sapphire laser. The pulse width was 100 fs, the wavelength was 800 nm, and the repetition rate was 1 kHz. The laser beam was guided into a microscope and focused by a 20× objective (NA, 0.42) into the core. The glasses were placed on a computer-controlled stage. The average power of the laser beam was controlled by neutral density filters inserted between the laser and the microscope objective. Using 1 kHz pulse trains of 100 fs laser pulses, the optical splitter and U-grooves for the passive fiber alignment are simultaneously obtained. The fiber aligned optical splitter, directly written by fs laser pulses in a fused silica glass, is described and characterized. The excess loss due to the passive alignment of the fibers is 0.3 dB, and the total insertion loss of the optical splitter is less than 4 dB. Moreover, the output field pattern is presented, demonstrating the splitting ratio of the optical splitter at approximately 1:1. Finally, the line and dot gratings with the period of 2 µm, directly written inside of the fused silica glass by fs laser pulses with pulse energy of 320 nJ and a 50× microscope objective, are described and applied to optical memory.
2. Experiments and results
2.1 Waveguide fabrication
When a femtosecond laser pulse is tightly focused inside a transparent material, the laser intensity at the focus becomes high enough to induce nonlinear absorption through a combination of multiphoton absorption, tunneling ionization, and avalanche ionization. If the absorption deposits enough energy in the material, permanent structural changes are produced. These structural changes are confined to the focal volume because of the nonlinear nature of the absorption. By scanning the laser focus of a continuous pulse train inside the sample, the refractive index can be changed in regions of any desired three-dimensional shape. We have used this technique to write single-mode waveguides and an optical splitter in fused silica glass.
Using 1 kHz pulse trains of 100 fs laser pulses focused by a 0.42 NA microscope objective, the waveguides were written inside a slab of transparent material about 30 µm beneath the surface of the sample with laser power of 300, 400, and 500 nJ as shown in Fig. 1. We translate the sample at a speed of 50 µm/s in a direction perpendicular to the axis of the fs laser beam. It can be observed that the diameter of the cross section increases with the rising pulse energy of the fs laser beam. One important parameter for device design is the change in refractive index which can be achieved using a given laser irradiation. The refractive-index change of the waveguides is determined by the coupling of a He:Ne laser into the waveguides. The NA of a step-index waveguide is related to the induced index change (Δn) by for small Δn, where n is the refractive index of the glass. As the pulse energy was 300–500 nJ, the refractive-index changes were 0.006-0.01. Because the refractive-index change depends on the pulse energy and speed of the sample, we can control the irradiation conditions to create differing refractive-indices and core diameters in the waveguides. A waveguide propagation loss of ~0.86dB/cm at a wavelength of 1550 nm was measured. The core diameters of waveguides in this figure were controlled by changing the average power of the writing laser.
2.2 Optical device fabrication
A schematic diagram of the 1×2 optical splitter is presented in Fig. 2. The length of the splitter is 5 mm, and the separation of the two branches is 0.25 mm. The optical splitter was fabricated by fs laser pulses inside a fused silica glass with a pulse energy of 400 nJ and a scan speed of 50 µm/s. The relative coupling into the two branches depends on their splitting angle, and in this case the radius of the curved waveguides was 30 mm, resulting in equal amounts of light into the two branches. In addition to fabrication of the optical devices, the optical interconnection between fibers and optical waveguides is essential for low-cost packaging of multichannel planar lightwave circuit (PLC)-type optical devices . In the study of optical devices technology, passive alignment has become a critical issue. So, we machined U-grooves for the passive alignment in the one-input and two-output ports of the splitter with a pulse energy of 30 µJ and scan speed of 500 µm/s. It is quite obvious that the U-grooves, machined with femtosecond pulses, are very clean as shown in Fig. 3. The U-groove size in each waveguide of the optical splitter is 126µm-width, 87µm-depth, and 700µm length within tolerance of ±0.5µm. The roughness of the bottom surface on the U-groove was measured around ±0.3µm by using a 3D Surface Profiling System (SIS-2000). These results confirm the highly precise alignment of the optical fibers and PLC optical devices.
Fiber aligned one-input and two-output channels of the U-grooved optical splitter are shown in Fig. 4. Our packaging technique is to insert and directly align the single mode optical fiber and the waveguide of the optical splitter with engraved U-grooves, directly formed using the fs laser micromachining technique. This packaging technique requires neither the use of optical fiber array blocks in the active alignment nor difficult etching processes such as reactive ion etching through photo lithography. One major advantage of this method is that it substantially obviates one or more of the limitations and disadvantages of the conventional techniques, which are time-consuming and considerably high in cost. The loss is less than 4 dB for two channels, including an intrinsic splitting loss of 3 dB. This means that the excess loss is less than 1 dB. Note that the excess loss is the sum of the propagation loss of the waveguide (0.86 dB/cm), the radiation loss of the 1×2 optical splitter, and the coupling loss (0.3 dB) between the optical splitter and a single-mode fiber. To examine the guiding properties of the optical splitter, we coupled a 1550 nm laser beam into the input channel of the optical splitter and imaged the output onto a CCD camera. Figure 5 shows the far-field pattern of the optical splitter’s output, demonstrating that the splitting ratio of the optical splitter with a length of 5 mm is approximately 1:1.
2.3 Optical memory
In addition to waveguides, we also fabricate submicron line and dot patterns using an 800 nm Ti:Sapphire laser. When the laser pulses were focused inside the fused silica glass, a modification of the optical properties was observed along the optical axis of the laser pulses. The visible laser damage can be formed only inside the focused region because nonlinear optical processes, such as multiphoton absorption, occur in regions with high optical intensity above the damage threshold. Modification of the sample is visible in a transmitted light optical microscope. Figure 6 shows the microscope image of line and dot patterns directly written inside fused silica glass by fs laser pulses with pulse energy of 320 nJ and a 50× microscope objective as the focusing lens. The line and dot patterns are separated by 2 µm and the line width and dot diameter are 500 nm, respectively. In Fig. 6(a), each line represents the optical modification inside the glass in the region where laser pulses were focused. The scan speed was 10 µm/s, and only a single scan was performed for each line. Like as shown in Fig. 6(b), submicron dot patterning is applicable to optical memory and photonic crystal. We created 3-D dot patterns written by a single shot of femtosecond laser beam. It consists of three layers as shown in Fig. 7. Each layer displays the letter of I, C, and U, respectively. The conditions of fs laser processing are same as shown in Fig. 6(b). Femtosecond laser submicron line and dot patterning has the potential to fabricate the Bragg gratings, optical memory, and photonic crystal.
We have demonstrated that an optical splitter and U-grooves, which are used for the passive fiber alignment, can be simultaneously fabricated in a fused silica glass through the use of near-IR femtosecond laser pulses. The output optical field pattern of the optical splitter was observed, and a refractive-index change of 0.006-0.01 was obtained with the NA method. The fiber aligned optical splitter has a low insertion loss, less than 4 dB, including an intrinsic splitting loss of 3 dB and excess loss due to the passive alignment of a single-mode fiber. In addition, submicron line and dot patterns were written inside fused silica glass through the use of fs laser pulses, demonstrating its ability to fabricate 3-D optical memory. In conclusion, the fs micromachining technique is a novel means of fabricating silica PLC devices; it is simple and produces accurate passive alignment. Future research could include an investigation into the fabrication of photonic bandgap structures, such as three-dimensional Bragg grating and photonic crystals.
This work is supported by OIRC from the Information and Communications University.
References and links
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