Adrian Lorenz

Mode resolved bend loss in few-mode optical fibers.

We present a novel approach to directly measure the bend loss of individual modes in few-mode fibers based on the correlation filter technique. This technique benefits from a computer-generated hologram performing a modal decomposition, yielding the optical power of all propagating modes in the bent fiber. Results are compared with rigorous loss simulations and with common loss formulas for step-index fibers revealing high measurement fidelity. To the best of our knowledge, we demonstrate for the first time an experimental loss discrimination between index-degenerated modes.

High temperature sensing with fiber Bragg gratings in sapphire-derived all-glass optical fibers

A structured sapphire-derived all-glass optical fiber with an aluminum content in the core of up to 50 mol% was used for fiber Bragg grating inscription. The fiber provided a parabolic refractive index profile. Fiber Bragg gratings were inscribed by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. Heating experiments demonstrated the stability of the gratings for temperatures up to 950°C for more than 24 h without degradation in reflectivity.

Fiber-based dual-focus time-demultiplexed second harmonic generation microscopy

We present a dual-focus second harmonic generation (SHG) microscopy approach based on stable, compact, and inexpensive fiber technology. One-tenth of the fiber laser output is coupled into a 100 m (≙500  ns) long single-mode fiber and further amplified to achieve two separately guided beams with time-alternating pulse trains. SHG detection is performed sequentially, generating two individual images in one scan. Thus, the configuration allows for imaging of distinct areas within the field of view at twice the repetition rate of the fiber laser but is readily extended to a multiple of the repetition rate with tens of foci.

Fiber Bragg grating inscription in optical multicore fibers

Fiber Bragg gratings as key components in telecommunication, fiber lasers, and sensing systems usually rely on the Bragg condition for single mode fibers. In special applications, such as in biophotonics and astrophysics, high light coupling efficiency is of great importance and therefore, multimode fibers are often preferred. The wavelength filtering effect of Bragg gratings in multimode fibers, however is spectrally blurred over a wide modal spectrum of the fiber. With a well-designed all solid multicore microstructured fiber a good light guiding efficiency in combination with narrow spectral filtering effect by Bragg gratings becomes possible.

Fiber Bragg filters For laser- and multicore fibers

Fiber Bragg gratings (FBGs) have widespread applications in security, information, structural health monitoring, and biophotonics. In telecom applications, FBG inscription has reached a high level of maturity, but remains mainly limited to germanium doped photosensitive single mode fibers. Special applications, like filtering in light harvesting fibers or resonator mirrors for fiber lasers have to deal with special aspects which make the design and realization of FBGs a challenging task. One aspect is the extended wavelength range of these applications. Another aspect is the increasing demand to inscribe fiber Bragg gratings in non-photosensitive germanium-free fibers. Therefore, novel concepts of photosensitivity are proposed. Finally, to increase the amount of captured light the size of the fiber core and the numerical aperture have also to be increased. This goes along with multimode operation and prevents good filtering properties of Bragg gratings.

Single-Mode Multicore Fibers With Integrated Bragg Filters

Fiber Bragg gratings (FBGs) in single-mode fibers, used as narrowband filters, have reached a high maturity level, especially for applications in telecommunications and sensing. In spectroscopic applications, e.g., in Raman spectroscopy, FBG filters can be used to separate laser probing light from generated sensor light which simplifies Raman probes. For this purpose, light from a sample has to be collected with high efficiency into a fiber. The light collection efficiency in single-mode fibers is, however, strongly limited due to the small fiber core area. Therefore, we have investigated the possibility to use single-mode multicore fibers to increase the light collection efficiency while maintaining Bragg filter performance. An optical density filtering effect of 1.5, which is equivalent to a transmission of -15 dB, has been achieved with a FBG in a 61-core fiber. Simulations have been performed in order to clarify the influence of the fiber geometry and refractive index distribution on grating performance. The application of chirped gratings in the multicore fibers improves filter performance in comparison to homogeneous gratings.

Equivalent step-index model of multifilament core fibers

The accuracy of the recently presented1 equivalent step index approximation of multifilament core fibers is analyzed in terms of the effective refractive index, mode field area and bending losses of the fundamental mode. A modified Vparameter for this class of fibers as well as a single-mode condition is proposed. By comparison with a full-vectorial finite element method it is shown that the relative deviation of the effective refractive index and the mode field area are in the magnitude of 1 %. No significant decrease of bending losses is found for multifilament core fibers.

Laser processed preforms for microstructured optical fibers

We present a laser drilling technology eminently suitable for structuring of solid glass preforms for microstructured optical fibers (MOF). This technology allows fiber designs that can not be easily adressed by stack and draw technology. As an example, we present a four ring hexagonal hole structure drilled in a silica rod over a length of 80 mm at ILT. The fiber drawn from this preform was used for absorption measurements and fiber Bragg grating inscription experiments at IPHT. Geometrical aspects are compared to those of a MOF with a similar structure made by the stack and draw technology.