Rolf Broennimann

Laser Direct Writing of Single-Mode Polysiloxane Optical Waveguides and Devices

Using a custom-built laser direct writing system, single-mode polymer optical waveguides, and devices for board-level optical interconnects were fabricated. A novel photopatternable polysiloxane material was developed that combines low-loss, simple, and large-area processability, and reliability during manufacturing and system operation. The polysiloxane waveguides were designed with quadratic cross sections of 5.5 × 5.5 μm2 and a refractive index contrast of 0.0086 between core and cladding polymer for single-mode operation at the wavelength of 1.3 μm. A straight waveguide propagation loss of 0.28 dB/cm was achieved. A wide range of passive optical devices, including Y-splitters, directional couplers, and Mach-Zehnder interferometers were successfully fabricated and characterized. The results prove that the presented combination of material and process technology is a viable implementation for short distance board-level optical links.

Defect evolution during catastrophic optical damage of diode lasers

We present an analysis of the catastrophic optical damage effect that is artificially provoked in 808 nm emitting broad area diode lasers by single current pulses. The kinetics of the sudden degradation process, monitored with a nanosecond temporal resolution, is linked to the damage pattern observed. This involves in situ tracing of emission power and hot-spot motion within the cavity as well as the verification of the resulting defects by defect spectroscopy and cathodoluminescence mapping. A complementary model is presented which explains the shape of the observed defect pattern. The combination of unidirectional energy transfer to defects by laser light within the laser cavity, spatially isotropic defect growth, and the presence of shadowing effects explain the complex damage pattern observed in the gain material, including effects of defect branching. The study is made with standard industrial devices making the findings directly applicable for device testing and performance improvements.

Optical fiber Bragg gratings for tunnel surveillance

We report on application tests of novel sensor elements for long term surveillance of tunnels. The sensors are made of glass fiber reinforced polymers (GFRP) with embedded optical fiber Bragg gratings. The tests were made in a tunnel near Sargans in Switzerland and we will present strain and temperature data of more than one year of operation of the sensor elements. Two sensor types were tested. First, GFRP rockbolts with a diameter of 22 mm were produced. They have a load-bearing function as anchors for tunnel or mine roofs and in addition measure distributed strain fields and temperature with embedded optical fiber Bragg grating arrays. Rockbolts are key elements during construction and operation of tunnels. Data about strain inside the rockbolts can support decision about precautions to be taken and reveal information about the long term movement of the rock. Second, thin and flexible GFRP wires of 3 mm in diameter were found to be robust and versatile sensors not only for tunnel surveillance but for many civil engineering applications where they can be attached or embedded (e.g., in concrete). The fabrication of both sensor types and solutions for the connection of the embedded fiber sensors to a fiber cable will be presented. Moreover, laboratory and tunnel data of functionality and long term stability tests will be discussed and compared.

Application of fiber optical and resistance strain gauges for long-term surveillance of civil engineering structures

We report on applications of surveillance and test systems for civil engineering structures. The system key elements are optical-fiber Bragg grating sensors and conventional resistance strain gauges. A recently built stay cable bridge with a world novelty of two carbon-fiber-reinforced-polymer cables was equipped with both types of sensors. The sensor system on the bridge is now operational for ten months and the bridge is open for traffic for 4 months. Results of the bridge surveillance are presented. To monitor a large concrete structure, the electrical power dam of Luzzone in the Swiss Alps, a prototype sensor rod was designed. First measurements with a sensor rod embedded in a concrete test prism are discussed. Several redundant measurements are made to compensate for temperature drift and to monitor the reliability of the measurement chain.

Applications of distributed fiber Bragg grating sensors in civil engineering

We report on civil engineering applications of wavelength multiplexed optical-fiber Bragg grating arrays produced directly on the draw tower for testing and surveying advanced structures and material like carbon fiber reinforced concrete elements and prestressing tendons. We equipped a 6 m X 0.9 m X 0.5 m concrete cantilever beam reinforced with carbon fiber lamellas with fiber Bragg grating sensors. Static and dynamic strain levels up to 1500 micrometers /m were measured with a Michelson interferometer used as Fourier spectrometer with resolutions of about 10 micrometers /m for all sensors. Comparative measurements with electrical resistance strain gauges were in good agreement with the fiber optic results. We used the fiber sensors in two different arrangements: some Bragg grating array elements measured the local strain while others were configured in an extensometric way to measure moderate strain over 0.1-1 m.

Fiber optic Bragg grating sensors embedded in GFRP rockbolts

Rockbolt anchors for tunnel or mine roofs are key elements during construction and operation. We report on the fabrication of glass fiber reinforced polymer (GFRP) rockbolts with embedded fiber optical Bragg grating sensors and their first field application in a test tunnel. Optical fibers and in-fiber Bragg grating sensors were embedded in GFRP rockbolts during a continuously ongoing pultrusion process on an industrial production machine. Depending on their outer diameter the rods equipped with fiber sensors serve as measuring rockbolts or as extensometric sensors for the motion of boulders in the tunnel roof. The adhesion and force transfer of different fiber coatings were tested by push-out experiments. By temperature and strain cycle tests the performance of the rockbolt sensors was evaluated. We will present these results and the measurements made during a first installation of fiber optical rockbolt sensors in a tunnel.

Structurally embedded fiber Bragg gratings: civil engineering applications

In civil engineering it is of interest to monitor long-term performance of structures made of new lightweight materials like glass or carbon fiber reinforced polymers (GFRP/CFRP). In contrast to surface applied optical fiber sensors, embedded sensors are expected to be better protected against rough handling and harsh environmental conditions. We report on two recently done fiber optical sensor applications in civil engineering. Both include structurally embedded fiber Bragg grating (BG) arrays but have different demands with respect to their operation. For the first application fiber BGs were embedded in GFRP rockbolts of 3 - 5 m in length either of 3, 8, or 22 mm diameter. The sensor equipped rockbolts are made for distributed measurements of boulder motion during tunnel construction and operation and should withstand strain up to 1.6%. Rockbolt sensors were field tested in a tunnel near Sargans in Switzerland. For a second application fiber BGs were embedded in CFRP wires of 5 mm diameter used for the pre- stressing cables of a 56 m long bridge near Lucerne in Switzerland. The permanent load on the cable corresponds to 0.8% strain. Due to the embedded sensors, strain decay inside the cable anchoring heads could be measured for the first time during loading and operation of the cables. For both applications mechanical and thermal loading tests were performed to assess the function of these new elements. Also, temperature and strain sensitivity were calibrated. Reliability studies with respect to stress transfer, fiber mechanical failure, and wavelength shift caused by thermal BG decay as well as monitoring results of both applications are presented.

Fiber optical Bragg grating sensors embedded in CFRP wires

Based on the example application of Emmenbridge, a newly built steel-concrete-composite bridge in Switzerland with 47 m long built-in carbon fiber reinforced polymer (CFRP) prestressing cables, we will present and analyze the process chain leading to a reliable surveillance of modern civil engineering structures with embedded fiber optical Bragg gratings. This consists first in the embedding of optical fibers and in-fiber Bragg gratings in long CFRP wires in an industrial environment, including fiber optical monitoring of the curing process. Then, various qualifying tests were done: annealing experiments for determining optical lifetime of the Bragg gratings used, dynamic and static tensile tests for estimating their mechanical lifetime under operation, push-out experiments to check adhesion of fiber/coating/matrix interfaces, and performance tests to determine strain and temperature sensitivity of the embedded Bragg gratings. Finally, the prestressing cables were equipped with the CFRP sensor wires and built into the bridge.

Application of optical fiber sensors on the power dam of Luzzone

The hydro-electric power dam of Luzzone in Switzerland has been heightened by 17 m to 225 m. Distributed fiber optical sensors, based on wavelength multiplexed Bragg gratings, have been embedded in the concrete to monitor temperature and strain evolution. The sensors were designed to make them suitable for embedding and operation in this harsh environment. Data taken during curing and the first year afterwards are presented.

Reliability and durability of fiber grating sensors in structural monitoring applications

There is strong interest to develop fiber-optical sensing systems for long term surveillance and structural monitoring. Although many detection schemes have been proposed, industrial acceptance of optical fibers as validated replacement of other sensors is limited. Low cost manufacturability, reliability, and long term stability are very important for usability in concrete and composite material structures. Lifetime for major structures in civil engineering of 50 - 100 years are very demanding on the sensors and require accurate aging models and test data to demonstrate their reliability and durability. Acceleration factors of several orders of magnitude can be achieved under reasonable testing conditions depending on temperature, mechanical stress, humidity, chemical environment and activation energy of the damaging process. We report on accelerated aging tests and failure mechanisms of optical fibers and Bragg gratings at elevated temperature, humidity and mechanical stress. Aging behavior is discussed and results from field measurements of large civil structures are presented.