Ria G. Becker

Cladding mode coupling in highly localized fiber Bragg gratings: modal properties and transmission spectra

The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs considerably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications.

Ultrashort pulse inscription of tailored fiber Bragg gratings with a phase mask and a deformed wavefront [Invited]

We report on the inscription of chirped fiber Bragg Gratings (FBGs) with a phase mask and a deformed wavefront using a femtosecond laser. A qualitative model is developed to predict the behavior of the resulting grating period for a deformed wavefront. In addition the quantitative change of the period was simulated based on a ray optical solution of the diffraction behind the phase mask. For deforming the wavefront experimentally a cylindrical tuning lens was used. Tilting of the lens increased the higher order aberrations like coma and spherical aberration, which leads to chirped FBGs. A chirped FBG with a FWHM bandwidth of 2.5 nm could be realized. The change of the resulting fiber Bragg grating period was measured using a side diffraction setup yielding good agreement with the measured spectra.

CGH-based real-time analysis of fiber Bragg gratings in few mode LMA fibers

We report on a new approach for experimentally analyzing transversal mode coupling in few mode fiber Bragg gratings (FBGs). Utilizing a spatial light modulator (SLM) enables real-time modulation of the incident spatial light distribution coupled into the fiber. Thus, individual transversal fiber modes can be excited, allowing for effort-less mode switching and mode rotation. Simultaneously, the transmitted and reflected modes are analyzed. We use this setup to demonstrate LP mode discrimination by asymmetric FBGs which favor reflection of a designated LP11 mode orientation for few mode large mode area fibers.

Efficient volume Bragg gratings in various transparent materials induced by femtosecond laser pulses

Volume Bragg gratings (VBGs) are optical devices that consist of a periodic refractive index structure in a bulk and have a wide range of applications such as frequency stabilizer of semiconductor lasers or as beam combiner for fiber or solid state lasers. Furthermore, chirped VBGs can be used for pulse compression in CPA laser systems. Typically, VBGs are fabricated in photo-thermo-refractive glass by exposure to an interference pattern of an UV-laser ([1]). Due to the restriction to this special glass the method is very limited in its application with respect to monolithic integration of VBGs. This can be overcome by making use of femtosecond laser pulses for the inscription of VBGs in non-photosensitive glass. Due to the high intensity of focused fs pulses a refractive index change can be obtained even in non-photosensitve glass through the process of nonlinear absorption. This process is additionally the cause for higher reflection orders one can see although a single period grating was inscribed ([2]) and opens the door for using one grating at different wavelengths without the need to turn the grating for wavelength adaption.

Tailored fiber Bragg gratings inscribed with a phase mask and a deformed wave front by ultrashort pulses

We report on the ultrashort pulse inscription of chirped fiber Bragg gratings (FBG) using a constant phase mask and a deformed wave front. To analyze the influence of wave front deformations on the grating period, a numerical model was developed. It is based on a ray optical solution of the diffraction of an arbitrary wave front at a phase mask with a constant period, where the wave front is described by Zernike polynomials. Generally, a plane wave front generates an interference pattern with half the phase mask period. A defocus changes the grating period, while higher order aberrations like coma or spherical aberration yield a chirp along the inscribed grating. Additionally, the grating period depends on the distance of phase mask and fiber. To experimentally deform the wave front of the inscription beam, a cylindrical tuning lens was introduced to the setup. Due to decentering and tilting of the tuning lens, higher order aberrations were generated. Furthermore the fiber was tilted with respect to the phase mask. A chirped FBG with a FWHM bandwidth of 6.2 nm was realized.

Orientation dependence of higher order mode reflections in femtosecond pulse written fiber Bragg gratings

Ultrashort pulse lasers allow for the inscription of fiber Bragg gratings largely independent of the fiber geometry. Here, we investigate how the orientation of a reflected higher order mode depends on the FBGs cross-section.

Variable ultrafast fiber Bragg grating inscription with a phase mask and a deformed wave front

Femtosecond inscribed fiber Bragg gratings (FBGs) have become more and more an integral part for monolithic fiber lasers in high power systems [1,2]. These gratings overcome the requirements for photosensitive fibers in conventional UV-fabricated FBGs. Thus, the FBGs can be written directly into the active rare-earth doped fiber avoiding additional losses due to splices. One well established approach to inscribe gratings in fibers is the phase mask scanning technique [3]. However, this method is limited in the flexibility due to the fixed period of the phase mask.

Femtosecond laser induced fiber Bragg gratings in active and multi mode fibers

Active Yb-doped large mode area (LMA) fibers allow for power scaling of lasers with excellent beam quality up to the kW regime [1]. However, the demand for compact, robust and cheap high power sources pushes research towards integrated, monolithic fiber setups. Most prominently, intra-core cavity mirrors can be realized with fiber Bragg gratings (FBG). However, in contrast to single mode fibers, FBGs in large core few mode fibers are challenging for two reasons: Firstly, conventional fabrication of FBGs in fibers with large cross-section is time-consuming and costly. Secondly, multimode interaction in few mode fibers leads to modal instabilities and polarization effects [2].

Monolithic narrow linewidth polarization-maintaining thulium fiber laser using femtosecond laser written fiber Bragg gratings

We have demonstrated an all-fiber thulium laser system that, without any intracavity polarizing elements or freespace components, yielded a stable polarization extinction ratio (PER) of ~18 dB. The system is based on singlemode polarization-maintaining silica fiber and its cavity is formed from each a high and low reflectivity femtosecond laser written fiber Bragg grating resonant at 2054 nm. The output of the fiber is not only highly polarized, but maintains a narrow linewidth of 78 pm at its maximum output power of 5.24 W. The high PER without any polarizing elements in the cavity is of great interest and makes the systems useful for spectral beam combining and other applications which require polarization dependent optical elements.

All-fiber single-mode PM thulium fiber lasers using femtosecond laser written fiber Bragg gratings

A polarization-maintaining (PM), narrow-linewidth, continuous wave, thulium fiber laser is demonstrated. The laser cavity is formed from two femtosecond-laser-written fiber Bragg gratings (FBGs) and operates at 2054 nm. The laser output possesses both narrow spectral width (78 pm) and a high polarization extinction ratio of ~18 dB at 5.24 W of output power. This laser is a unique demonstration of a PM thulium fiber system based on a two FBG cavity that produces high PER without any free-space elements. Such a narrow linewidth source will be useful for applications such as spectral beam combining which often employ polarization dependent combining elements.