Femtosecond laser inscription of waveguides and Bragg gratings in transparent cyclic olefin copolymers

Fiber Bragg grating inscription assisted by a spatial light modulator

In this Letter, we proposed a new technique for point-by-point fiber Bragg grating (FBG) writing in a static fiber by using a spatial light modulator to control the position of the focal point inside the fiber core. Various types of short-length FBGs (uniform, phase-shifted, and apodized) were demonstrated by this inscription technique. Moreover, the capability to tailor the transverse dimension of a grating pitch from a single point to more complex shapes, such as a wide plane covering a whole fiber core or a transverse ring, was shown.

Birefringence in Injection-Molded Cyclic Olefin Copolymer Substrates and Its Impact on Integrated Photonic Structures

This contribution quantifies the birefringence within injection-molded cyclic olefin copolymer plates and discusses its impact on the mechanical properties of the plates. It also focuses on the impact of birefringence on integrated waveguides and Bragg gratings and provides fabrication guidelines for such structures. The anisotropy in all three dimensions of the workpiece is examined by means of polarimetry and a prism coupler. It is found that the birefringence is inhomogenously distributed within the workpieces, whereas the maximum birefringence not only varies locally, but also depends on the observation direction. Overall, a maximum birefringence of 10 × 10-4 is found at the plate's surface near the injection gate. The anisotropy then reduces exponentially towards the center of the workpiece and saturates at 1.8 × 10-4, in a depth of 0.4 mm. Thus, the birefringence strongly affects near-surface photonic structures. It is found that, depending on their orientation and the local birefringence of the substrate, waveguides and Bragg gratings fabricated with comparable parameters behave completely differently in terms of polarization-dependent optical attenuation, cross-sectional intensity distribution and Bragg reflection signal. For example, the support of the TM mode can vary between total loss and an optical attenuation of 0.9 dB × cm-1. In consequence, this study underlines the importance of quantifying the birefringent state of an injection-molded cyclic olefin copolymer workpiece if it is supposed to serve as a substrate for integrated photonic structures. The study furthermore demonstrates that birefringence effects can be omitted by burying the photonic structures deeper into the volume of the thermoplastic.

Integration of Bragg gratings in aerosol-jetted polymer optical waveguides for strain monitoring capabilities

We demonstrate and discuss the integration of Bragg gratings in aerosol-jetted polymer optical waveguides, produced in the optical assembly and connection technology for component-integrated bus systems (OPTAVER) process. By using a femtosecond laser and adaptive beam shaping, an elliptical focal voxel generates different types of single pulse modification by nonlinear absorption in the waveguide material, which are arranged periodically to form Bragg gratings. Integration of a single grating structure or, alternatively, an array of Bragg grating structures in the multimode waveguide yields a pronounced reflection signal with typical multimodal properties, i.e., a number of reflection peaks with non-Gaussian shapes. However, the main wavelength of reflection, located around 1555 nm, is evaluable by means of an appropriate smoothing algorithm. When loaded by mechanical bending, a pronounced Bragg wavelength shift of this reflected peak up to 160 pm is detected. This demonstrates that the additively manufactured waveguides can be used not only for signal transmission but also as a sensor.

UV-Femtosecond-Laser Structuring of Cyclic Olefin Copolymer

We report on the laser ablation of cyclic olefin copolymer using an amplified ultrashort pulsed laser in the ultraviolet spectral range. In addition to a high ablation depth per laser-structured layer up to 74 μm at a fluence of 22 J cm−2, an excellent mean roughness Ra of laser-patterned surfaces down to 0.5 μm is demonstrated. Furthermore, with increasing fluence, increasing ablation efficiencies up to 2.5 mm³ W⁻¹ min⁻¹ are determined. Regarding the quality of the ablation, we observed steep ablation flanks and low debris formation, though for fluences above 10.5 J cm−2 the formation of troughs was observed, being attributed to multiple reflections on the ablation flanks. For comparison, laser ablation was performed under identical conditions with an infrared laser wavelength. The results highlight that UV ablation exhibits significant advantages in terms of ablation efficiency, surface roughness and quality. Moreover, our results show that a larger UV focus spot accelerates the ablation process with comparable quality, paving the way for high-power UV ultrashort pulsed lasers towards an efficient and qualitative tool for the laser machining of cyclic olefin copolymer. The production of complex microfluidics further underlines the suitability of this type of laser.

Sapphire Photonic Crystal Waveguides with Integrated Bragg Grating Structure

This contribution demonstrates photonic crystal waveguides generated within bulk planar sapphire substrates. A femtosecond laser is used to modify the refractive index in a hexagonal pattern around the pristine waveguide core. Near-field measurements reveal single-mode behavior at a wavelength of 1550 nm and the possibility to adapt the mode-field diameter. Based on far-field examinations, the effective refractive index contrast between the pristine waveguide core and depressed cladding is estimated to 3·10−4. Additionally, Bragg gratings are generated within the waveguide core. Due to the inherent birefringence of Al2O3, the gratings exhibit two distinct wavelengths of main reflection. Each reflection peak exhibits a narrow spectral full width at a half maximum of 130 pm and can be selectively addressed by exciting the birefringent waveguide with appropriately polarized light. Furthermore, a waveguide attenuation of 1 dB cm−1 is determined.

Direct femtosecond laser inscription of Bragg gratings in Ho3+/Pr3+ co-doped AlF3-based glass fibers for a 2.86 µm laser

In this Letter, we report the fabrication of fiber Bragg gratings (FBGs) in home-made Ho³⁺/Pr³⁺ co-doped single-cladding fluoroaluminate (AlF₃) glass fibers and its application in watt-level lasing at the mid-infrared (MIR) wavelength of 2.86 µm. The FBGs were inscribed using an 800 nm femtosecond (fs) laser direct-writing technique. The FBG properties were investigated for different pulse energies, inscription speeds, grating orders, and transversal lengths. A second-order FBG with a high reflectivity of 99% was obtained at one end of a 16.5-cm-long gain fiber. Under 1150 nm laser pumping, this fiber yielded a power exceeding 1 W at 2863.9 nm with an overall laser efficiency of 17.7%. The fiber laser showed a FWHM bandwidth of 0.46 nm and long-term spectral stability.

Ultrashort pulsed laser backside ablation of fused silica

We report on the fabrication of rectangular microchannels with vertical sidewalls in fused silica by laser backside ablation. A 515 nm femtosecond laser is focused by an objective with a NA of 0.5 through the sample on the glass/air interface, allowing processing from the backside into the bulk material. Experimental investigations reveal a logarithmically increasing depth of the channels with an increasing number of scans, while keeping the focal position fixed. A certain number of scans has to be applied to generate rectangular shaped channels while their depth can be controlled by the applied fluence from 2.64 µm to 13.46 µm and a corresponding ablation roughness R_a between 0.20 µm and 0.33 µm. The channel width can be set directly via the number of parallel ablated lines demonstrated in a range from 10 µm to 50 µm. By adjusting the focal position after each scan the channel depth can be extended to 49.77 µm while maintaining a rectangular channel geometry. Finally, concentric rings are ablated to demonstrate the flexibility of the direct writing process.