From microelectronics to photonics, a number of fields have been miniaturized using microfabrication technologies, thus opening the door for a broad range of applications. The field of optofluidics has been transformed by the possibility of realizing microfluidic channels.
Microfluidic patterns and channels are fabricated typically by chemical-etching and lithographic techniques; however these approaches are mainly restricted to the fabrication of 2D patterns on the surface. Indeed for creating a true three-dimensional structure, several layers of glass substrates require to be patterned, fused and etched together.
Femtosecond Laser Irradiation and Chemical Etching (FLICE) enables 3D micromachining, allowing the direct fabrication of buried microchannels with a number of aspect ratios and patterns. This is a maskless technology and permits rapid low-cost prototyping of new devices.
Since it is possible to use the same femtosecond laser for low loss optical waveguides in a 3-D geometry, these femtosecond laser-based technologies may become a comprehensive solution for fabricating microfluidic channels and combining them with optical circuits.
The FLICE Fabrication Procedure
The steps in FLICE Fabrication are given below:
Step 1. Firstly femtosecond laser irradiation is done with intensities below the laser ablation threshold. Figures 1 and 2 show the microchannel design imprinted into the substrate and the result obtained.
Figure 1. Imprinting of microchannel design onto the substrate
Figure 2. Result of microchannel design imprinting
Step 2. Secondly the femtosecond laser modified region is etched using HF acid (highly selective).
Figures 3 and 4 show the etching bath and the result of etching, respectively.
Figure 3. Etching bath
Figure 4. Result of etching
Application Example 1: Mach-Zehnder Interferometer
It is possible to fabricate unbalanced Mach-Zehnder interferometers in a fused silica substrate using the femtosecond laser writing technique. The SHG output at 515nm of 350fs laser pulses has been employed at 1MHz repetition rate, followed by focusing with a 50x 0.6NA objective. Using a pulse energy (at 515nm) of 90nJ and translation speed of 100µm/s, devices were written to a depth of 200µm below the surface of the sample.
Figure 5. Schematic of the unbalanced Mach-Zehdner interferometers fabricated
The formula of the transmitted power as a function of the input wavelength λ in the ideal case is shown below (n is the refractive index and Δs is the difference in the length of the two arms).
The graph shows the measured spectral response of an interferometer with h = 50µm, R = 30mm and L = 18mm. A fringe visibility of over 8dB is observed. The determined fringe periodicity of 25.8nm corresponds with the formula result in the inset (24.6nm), which gives the approximate periodicity of the wavelength fringes in a region around λ0 (assumed to be 1550nm).
Figure 6. Graph showing the measured spectral response of an interferometer with h = 50µm, R = 30mm and L = 18mm
Application Example 2 - 1x2 Vertical and Horizontal Power Splitters
For realizing complicated 3D photonic devices, it is important to fabricate the basic 3D optical elements with good guiding properties. A 1x2 power splitter signifies a core element with specific characteristics such as small size, directly buried in the substrate, and different possible orientations in the volume (Figures 7, 8 and 9).
The material is fused silica and the laser parameters are:
- Wavelength - 515nm
- Pulse length - 350fs
- Repetition rate - 1MHz
- Pulse energy - 90nJ
- Scan speed - 10µm/s
- 50x focusing objective with 0.6N
The basic specifications followed are:
- Intensity mode profiles at 1550nm of the horizontal and vertical splitters
- Distance between the two waveguides: 100µm
- Splitter size: <2.5mm
- Total insertion losses lower than 5 dB @ 1550nm
Figure 7. Scheme of the 1x2 power splitters with two different orientations
Figure 8. Result – horizontal splitter
Figure 9. Result – vertical splitter
Application Example 3 - 3D Directional Coupler for a Micromotor Speed Sensor
An essential issue while applying micrometric motors is the ability to fabricate integrated position sensors of high precision (<100nm) and small size (<10mm). The Laser Zentrum in Hannover developed this micromotor wherein the core device of the speed sensor is a 3D directional coupler fabricated at Politecnico di Milano.
This speed sensor works on interferometric effects and it needs three coupled waveguides to detect both the direction of the motor speed and the motion (Figures 10, 11, and 12).
The material is borosilicate glass and the laser parameters are:
- Wavelength - 1030nm
- Pulse length - 350fs
- Repetition rate - 1MHz
- Pulse energy - 300nJ
- Scan speed - 10mm/s
- 20x focusing objective with 0.4NA
Figure 10. Scheme of the micromotor with the integrated interferometric speed sensor
Figure 11. Scheme of the 3-D direction coupler composed of three femtosecond laser written waveguides.
Figure 12. Intensity mode profiles at the waveguide outputs of the 3-D directional couple
This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.
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