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Highly efficient and compact acousto-optic modulator of a fibre Bragg grating is reported for the first time. The device is composed of millimetre scaled components and a 1 cm grating inscribed in a four air holes birefringent suspended core fibre. The reflection of the orthogonal polarization modes is tuned by a sinusoidal electrical signal at the resonance frequency of 469 kHz. A significant modulation depth of 45% is achieved at a maximum drive voltage of 10 V. The demonstrated 4 cm long all-fibre modulator is 60% shorter compared to previous similar setups, indicating new possibilities for stable and fast switching of fibre-integrated photonic devices.

Sensing at small dimensions in biological and medical environments requires miniaturized sensors with high sensitivity and measurement resolution. In this work a small optical fiber probe was developed to apply the Vernier effect, allowing for enhanced temperature sensing. Such effect is an effective way of magnifying the sensitivity of a sensor or measurement system in order to reach higher resolutions. The device is a multimode silica Fabry-Perot interferometer structured at the edge of a tapered multimode fiber by focused ion beam milling. The Vernier effect is generated from the interference between different modes in the Fabry-Perot interferometer. The sensor was characterized in temperature, achieving a sensitivity of -654 pm/°C in a temperature range from 30°C to 120°C. The Vernier effect provided a temperature sensitivity over 60-fold higher than the sensitivity of a normal silica Fabry-Perot interferometer without the effect. The temperature resolution obtained was 0.14°C, however this value was limited by the resolution of the OSA and can be improved further to less than 0.015°C.

Based on favorable properties with respect to beam quality, efficiency and applicability, fiber-integrated lasers replace costly bulk lasers in various application fields, such as in academia, industry, medical technology and life sciences. Additionally, rare-earth doped fibers exhibit broad gain regions, providing huge potential to develop broadly tunable fiber lasers for spectroscopy. Recently a fiber-integrated tuning concept based on an FBG array as discrete spectral filter was demonstrated, exhibiting a unique flexibility to tailor spectral and temporal emission properties. In this work, we present the prospects of this concept to address Differential Absorption LIDAR (DIAL) for environmental sensing. With tunable multi-wavelength operation for enhanced measurement speeds, and customized spectral emission lines to probe specific molecules, it may provide a fast and cost-efficient solution with excellent usability.

Two multi-path interferometers were developed using cleaved silica microspheres. A microsphere on top of a singlemode fiber tip was cleaved with a focused ion beam. The asymmetry introduced in the structure generates a new set of optical paths due to random reflections inside the microsphere. The obtained reflection spectrum presents a random-like interferometric behavior with strong spectral modulation of around 3 dB amplitude. Two distinct regions can be observed when a fast Fourier transform is applied. The first involves two cavities at a lower frequency and the second region involves a band of frequencies that is originated by the random interferometric reflections. These two spectral characteristics can be separated using low-pass and high-pass filters, respectively. A correlation method was used to obtain a temperature response from the two-cavity component. A similar structure was also created in a microsphere of multimode fiber. The microsphere was cleaved by polishing the structure with a certain angle. The interference between the different optical paths can be seen as the superposition of several two-wave interferometers, which can be discriminated through signal processing. Temperature sensing was also explored with this structure. The sensitivity to temperature is more than 3-fold for smaller cavities. Moreover, a sensitivity enhancement is also verified if a correlation method is used.

We demonstrate a new technique to generate a continuous-wave supercontinuum based on the stimulated Raman effect in an Yb-based ring laser configuration. Continuously pumping this ring cavity with a maximum of 19W optical power, we were able to excite up to 6 Stokes orders and achieved wavelengths up to 1600nm. Due to the feedback mechanism of the ring cavity the generated spectrum does not exhibit plain and separated stokes peaks but the fundamental Raman nature of the spectrum is altered additional nonlinear effects. This results in a dense coverage and an almost complete excitation of the wavelength range from the laser wavelength to the highest stokes wavelength and hence in a continuous-wave supercontinuum. Since the main mechanism of broadening is the Raman effect, we do not rely on anomalous dispersion and modulation instability as typically required to seed continuous wave supercontinuum generation.

Fiber amplifiers with a robust monolithic seed coupling and very high peak power in a near diffraction-limited beam are increasingly demanded by many industrial applications in laser materials processing. A tapered all-solid rod-type fiber amplifier scheme is proposed. The principle of this approach is the use of a local adiabatic taper to provide a monolithic signal path and selectively excite the fundamental mode in highly multimode fiber. A large mode area fiber is used to scale up the peak power and suppress the nonlinear effects. The powder-sintering technology (REPUSIL) was employed to achieve rod-type fibers with excellent refractive index homogeneity. In this work, we present a double-clad Yb3+-doped rod-type REPUSIL fiber with a core/clad diameter of 45µm/190µm and a core/clad numerical aperture of 0.09/0.19.This developed fiber has a relatively low Al3+-concentration to reduce diffusion during tapering and an optimized outer cladding material to reduce the taper process temperature. Finally it is demonstrated that the diffusion phenomenon is successfully eliminated and the near-diffraction limited beam quality during the amplification process is maintained. First experiments with improved Yb3+-doped rod-type amplifiers delivered 2ns pulses with peak powers of 210kW for the non-tapered rod and 140kW for the tapered rod (limited by facet damage). For the tapered fiber, the beam quality was between 1.3 and 1.7, significantly improved compared to the beam quality of the non-tapered fiber (M2 = 3.3 ~ 4.5). Future work will concentrate on adopting endcaps to protect the fiber facets from damage while scaling up the peak power.

usc Refereed/Peer-reviewed Focused ion beam technology is combined with dynamic chemical etching to create microcavities in tapered optical fiber tips, resulting in fiber probes for temperature and refractive index sensing. Dynamic chemical etching uses hydrofluoric acid and a syringe pump to etch standard optical fibers into cone structures called tapered fiber tips where the length, shape, and cone angle can be precisely controlled. On these tips, focused ion beam is used to mill several different types of Fabry-Perot microcavities. Two main cavity types are initially compared and then combined to form a third, complex cavity structure. In the first case, a gap is milled on the tapered fiber tip which allows the external medium to penetrate the light guiding region and thus presents sensitivity to external refractive index changes. In the second, two slots that function as mirrors are milled on the tip creating a silica cavity that is only sensitive to temperature changes. Finally, both cavities are combined on a single tapered fiber tip, resulting in a multi-cavity structure capable of discriminating between temperature and refractive index variations. This dual characterization is performed with the aid of a fast Fourier transform method to separate the contributions of each cavity and thus of temperature and refractive index. Ultimately, a tapered optical fiber tip probe with sub-standard dimensions containing a multi-cavity structure is projected, fabricated, characterized and applied as a sensing element for simultaneous temperature and refractive index discrimination.

Refereed/Peer-reviewed Optical fibers are promising tools for performing biological and biomedical sensing due to their small cross section and potential for multiplexing. In particular, fabricating ultra-small sensing devices is of increasing interest for measuring biological material such as cells. A promising direction is the use of interferometric techniques combined with optical fiber post-processing. In this work we present recent progress in the development of Fabry-Perot micro-cavities written into optical fiber tapers using focused ion beam (FIB) milling. We first demonstrate that FIB milled optical fiber microcavities are sensitive enough to measure polyelectrolyte layer deposition. We then present new results on the fabrication and optical characterization of serially-multiplexed dual cavity micro-sensors. Two cavities were written serially along the fiber with two different cavity lengths, producing a total of four reflecting surfaces and thus six possible interferometric pairs/cavities. By using fast Fourier transform it is possible to obtain de-multiplexed measurements for each cavity. This will be particularly important for bioassays where positive and negative controls are required to be measured within close spatial proximity.

Fiber lasers provide the perfect basis to develop broadly tunable lasers with high efficiency, excellent beam quality and user-friendly operation as they are increasingly demanded by applications in biophotonics and spectroscopy. Recently, a novel tuning scheme has been presented using fiber Bragg grating (FBG) arrays as fiber-integrated spectral filters containing many standard FBGs with different feedback wavelengths. Based on the discrete spectral sampling, these reflective filters uniquely enable tailored tuning ranges and broad bandwidths to be implemented into fiber lasers. Even though the first implementation of FBG arrays in pulsed tunable lasers based on a sigma ring resonators works with good emission properties, the laser wavelength is tuned by a changing repetition rate, which causes problems with applications in synchronized environments. In this work, we present a modified resonator scheme to maintain a constant repetition rate over the tuning range and still benefit from the advantages of FBG arrays as filters. With a theta ring cavity and two counter propagating filter passes, the distributed feedback of the FBG array is compensated resulting in a constant pulse round trip time for each filter wavelength. Together with an adapted gating scheme controlling the emission wavelength with a modulator, the tuning principle has been realized based on a Ytterbium-doped fiber laser. We present first experimental results demonstrating a tuning range of 25nm, high signal contrast and pulse durations of about 10ns. With the prospect of tailored tuning ranges, this pulsed fiber-integrated laser may be the basis to tackle challenging applications in spectroscopy.

Over the past years, Thulium (Tm) -doped fiber lasers in the 2μm region have gained a lot of interest due to many potential applications in materials processing and biophotonics. Based on the broad gain regions spanning from 1800nm to 2100nm, they offer the perfect basis to implement broadly tunable and user-friendly light sources like they are increasingly demanded in spectroscopic applications. Recently, a novel tuning mechanism based on a fiber Bragg grating (FBG) array as versatile spectral filter has been reported. This concept combines unique spectral freedom for customized tuning ranges and ultrabroad bandwidths with a fiber-integrated setup in order to maintain the advantages of the waveguide geometry. In this work, we demonstrate such a dispersion tuned and pulsed fiber laser in the Tm domain around 1950nm using a modulator and a discrete FBG array to control the emission wavelength. In order to comply with the demands of potential applications in biophotonics, for the first time, this tuning concept is realized in a polarization maintaining (PM) configuration ensuring linearly polarized output. While a simple FBG array is employed containing five gratings inscribed in PM fiber, we also outline the prospect to implement FBGs fabricated in a standard single mode fiber. The emission characteristics of the system are investigated showing pulse durations down to 11ns and a good spectral signal contrast. In order to highlight the prospect for tunable high-power operation, we have also implemented an amplification stage scaling the average power to more than 25W.

An ytterbium-doped fiber laser mode-locked by the interaction of a fiber Bragg grating and longitudinal acoustic waves in a suspended core fiber is experimentally investigated. An optimized design of an acousto-optic modulator is also proposed. The results indicate output pulses with a width of less than 550 ps at a repetition rate of 10 MHz. The reduction of the power consumed by the transducer and the grating length points out to more efficient, compact and fast acousto-optic modulators for mode-locked all-fiber lasers.

Regeneration of fibre Bragg gratings under application of a high temperature annealing process in a high birefringent polarisation maintaining fibre of type Panda was investigated. During the annealing process, a distinct nonlinearity and hysteresis of the birefringence with temperature was observed. After the temperature process, the birefringence between slow and fast axis at room temperature was nearly doubled, which is in agreement with observations of other researchers. The hysteresis in birefringence might be explained by the crossing of the transition temperature of the stress applying parts and the relief of in-frozen mechanical and thermal stresses.

Focused ion beam milling is used on chemically etched tapered fiber tips to create fiber Bragg gratings. These fiber Bragg gratings are based on a modulation of silica and external medium. This leads to a wide and structured spectrum obtained due to imperfections and the inherent structure of the tip. The fiber Bragg gratings presented are very short and have a length of 27 μm and 43 μm and are milled on the tapered fiber tip. They are characterized in the high temperature range 350-850oC and a sensitivity of 14.4 pm/K is determined.

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.

Sapphire based optical fibers provide an attractive basis for ultra-high temperature stable optical sensor elements. Fiber Bragg gratings can be inscribed in such fibers by means of femtosecond-laser pulses with a wavelength of 400 nm in combination with a two-beam phase mask interferometer. We have investigated crystalline optical fibers as well as structured sapphire-derived all glass optical fibers with aluminum content in the core of up to 50 mol%. The reflection properties, the index modulation and the attenuation effects will be discussed. Results concerning the temperature and strain sensitivity for use as sensor elements at high temperatures will be presented.

We report on the fluorine incorporation in powder based materials for the fabrication of Al and Al/Yb co-doped silica glasses. The achieved maximum Fluorine concentration of 1.55 mol% SiF 4 corresponds to a refractive index decrease of -8 x 10 -3 . Simultaneously, the Tg of the doped material is reduced by about 200 K compared to pure silica. Moreover, the fluorine doping is also eminently suitable for the direct refractive index adjustment in active doped silica glass materials (e.g. Al/Yb or Al/Tm). The index matching with pure silica is possible to date up to 2.7 mol% Al 2 O 3 and 0.1 mol% Yb 2 O 3 . The additional influence on the blue shift of the UV transmission will also be discussed.

We report on the development of large-core Yb-doped fibers with up to 100 μm core diameter and present first experimental results for high peak power amplification. The material for core and pump cladding was fabricated by Powder Sinter Technology. Using a high Al concentration we achieved a numerical aperture (NA) of 0.21 of the pump cladding and a core NA below 0.1. The rod-type fiber exhibits high pump absorption. Using a 0.55 m short fiber sample as the main amplifier in a 3-stage ns pulsed fiber Master Oscillator Power Amplifier system we achieved 3 ns output pulses with 360 kW peak power and 2 mJ pulse energy. We observed suppressed Stimulated Raman Scattering with respect to the signal pulses, which offers the possibility of further power scaling of such fiber amplifier systems.

We report on two low-loss 7:1 single-mode to multi-mode fiber coupler-designs generating more than 5 kW of output power. These all-glass fiber-optical devices have the objective to keep the brightness at its theoretical maximum and the heat load at its technical minimum. To the best of our knowledge, regarding all-fiber geometrical combined power generation, beam quality and heat stability, the presented results had never been reported before.

Here we report on the inscription of fiber Bragg gratings with two beam interferometry and deep ultraviolet femtosecond laser exposure. This allows to target first order Bragg grating reflections from 2000 nm down to 660 nm without the need for germanium doping or hydrogen loading. The grating in the visible region is remarkable because it bases on a femtosecond laser induced structural size around 110 nm.

In this work we present a novel highly Ge doped microstructured fiber design dedicated for fiber Bragg grating (FBG) inscription and longitudinal strain sensing. Three series of the reported fiber differentiated by air-hole diameters were drawn and presented. After numerical analysis of the propagation conditions (with effective refractive index, loss and mode area calculated) in the real structures, the fibers were subjected to femtosecond FBG inscription. We show the resulting typical FBG spectra, as well as measure the longitudinal strain sensitivity of the fabricated samples and its dependence on the microstructure geometry. An increase of approx. 4% in the Bragg wavelength strain sensitivity was noticed for an increase of the large air-hole diameter of approx. 10%.

A miniaturized design of a fibre-optic based total temperature probe is presented which is intended to be used for aircraft engine testing. The FBG-based probes were experimentally characterized in the harsh environment of an open jet test bench at air stream velocities up to Mach numbers of 0.8 and they showed similar performance as conventional sensing techniques. The reduction of the size of the transducer was possible by using small sized FBG-based temperature sensors. Due to the smaller probe design and thinner signal transmission cables, less distortion of the air stream is expected with the fibre-optic sensor, when compared to conventional instrumentation. This allows a higher density of instrumented sensors and thus a higher spatial resolution of the measurement. This is, to our knowledge, the first demonstration of total temperature measurements at high air stream velocities with FBG-based transducers.

The regeneration of fibre Bragg gratings (FBG) is supposed to be seeded by the relief of the in-frozen core cladding stress of the fibre. Earlier results lead to the assumption that the diffusion process of small atoms or molecules through the glass matrix is sufficient to seed regeneration of an FBG. To prove this theory, a regeneration experiment was performed with hydrogen loaded, pristine and out-diffused SMF28 fibers, respectively. For the FBG in an out-diffused fibre, the fibre was stored at room temperature after hydrogen loading to get rid of the total amount of hydrogen before FBG inscription. The FBG inscription experiments confirmed that the diffusion of hydrogen changes the fibre intrinsic stresses. From all three kinds of SMF28 fibers, one FBG was annealed with a stepwise temperature profile for regeneration. Only the FBG in hydrogen loaded fibre regenerated, what implies that the formation of hydroxyl groups is necessary for the regeneration process. The reduction of the in-frozen stress due to the hydrogen diffusion, in contrast, was not sufficient to seed regeneration.

A combination of focused ion beam milling and chemical etching is proposed for the creation of Fabry-Perot cavities in microwires. Both simple cavities and cantilevers are created on 15 μm-diameter microwires and characterized in temperature. The cantilever structure shows sensitivity to vibration and is capable of measuring frequencies in the range 1 Hz – 40 kHz.

Microstructured optical fibers (MOF) sometimes also referred to as photonic crystal fibers (PCF) have been a subject of extensive research for over a decade. This is mainly due to the fact that by changing the microstructure geometry (e.g. distribution and size of the air-holes) fiber properties can be significantly modified to better fit specific applications. In this manuscript we present a novel fiber design with three large air-holes neighboring the core and report on how the air-hole diameter influences the effective refractive index strain sensitivity. As direct measurement of the effective refractive index change may be complex and challenging, we propose to use fiber Bragg gratings (FBG) in our sensing set up. The Bragg wavelength is a function of the effective refractive index, hence the external strain changes can be monitored through the Bragg wavelength shift with a simple optical spectrometer. Furthermore we also include an analysis of the fibers temperature sensitivity.

We demonstrate a spatially distributed measurement technique to observe temperature changes along a pumped Ybdoped fiber. The technique is based on an array of fiber Bragg gratings acting as temperature sensor line. The Bragg gratings are inscribed directly into the actively doped fiber core using high intensity laser pulses and an interferometric setup. We studied the temperature evolution in differently doped fibers under pumping conditions and find different effects contributing to the observed temperature increase. To avoid an additional heating caused by pump-induced photodarkening, we measure the spatially resolved temperature profile during pumping along a photodarkening-reduced, cerium co-doped fiber.

We demonstrate three-level laser operation at 976 nm of a large-core Yb-doped aluminosilicate fiber, which is fabricated by powder-sinter technology and shows a very homogeneous refractive index profile. The investigated fiber has a core diameter of 126 μm and a numerical aperture of 0.18, well-matched to standard fiber coupled pump diodes. The core composition has been optimized to reduce photodarkening effects. Multimode and single mode operation with multiple Watts output power is presented for this fiber making it useful for the realization of high brightness fiber coupled pump sources.

Photonic crystal fibers (PCF), sometimes also referred to as microstructured fibers (MSF), have been a subject of extensive research for over a decade. This is mainly due to the fact that by changing the geometry and distribution of the air holes the fiber properties can be significantly modified and tailored to specific applications. In this paper we present the results of a numerical analysis of the influence of the air-hole distribution on the sensitivity of the propagated modes’ effective refractive index to externally applied longitudinal strain. We propose an optimal strain sensitive fiber design, with a number of fibers drawn and experimentally evaluated to confirm the theoretical results. Furthermore as the direct measurement of the effective refractive index change may be complex and challenging in field environment, we propose to use fiber Bragg gratings (FBG) in our sensing set-up. As the Bragg wavelength is a function of the effective refractive index, the external strain changes can be monitored through the Bragg wavelength shift with a simple optical spectrometer. Moreover, since the PCF is also optimized for low-loss splicing with standard single mode fiber, our novel sensor head can be used with standard off-the-shelf components in complex multiplexed sensing arrays, with the measured signal transmitted to and from the sensor head by standard telecom fibers, which significantly reduces costs.

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.

In the present work, the use of a single mode fiber (SMF) with high Germanium doped core as temperature sensor is studied. The fiber core consists of a 1.1 μm highly germanium doped step index waveguide surrounded by a pedestal of 3.5 μm diameter. The outer diameter of the fiber is 125 μm. A short stub of ~2 mm is used in the fabrication of the interferometer. The highly germanium doped fiber is spliced between two standard SMF. In one of the splices both fibers are ideally aligned, in the other splice a small misalignment between the fibers is done. An annealing process is made for 5 hours at 850°C which results in a good operation stability up to 700°C. A wavelength shift as a function of temperature of 76 pm/°C is reported. To demonstrate the interferometer efficiency, a fiber Bragg grating is written in the highly germanium doped core and tested for temperature response. A temperature sensitivity of 13pm/°C was demonstrated. The interferometer fabrication requires only a few and easy steps. Due to the standard splices made between the fibers, the device is robust. We believe that the sensor may be used under harsh environmental conditions, since it shows a high sensitivity and a small size in combination with great robustness.

We have investigated the possibility to use a step-chirped fiber Bragg grating array for a time-delay-spectrometer. This concept allows a simultaneous spectral and temporal characterization of pulsed light in the nanosecond regime which can also be applied for the investigation of single pulses. A spectral resolution in the 100 picometer range has been achieved and used for the characterization of laser pulses in the nanosecond range.

Fiber Bragg gratings (FBG) are one of the most successful fiber optic technologies with very interesting perspectives for application in fiber optic sensing. It has been already reported that the possibility of its fabrication in novel microstructured fibers (MSF), creating a unique 3D structure, can significantly improve their performance and sensing properties. In this paper we present the results of FBG inscription in a dual-mode highly birefringent (HB) MSF with enhanced polarimetric strain sensitivity of the second order mode, as its mode maxima are closer to the cladding air-holes, where the strain distribution during fiber elongation is the highest. We perform an analysis and comparison of the FBG reflection and transmission characteristics, showing the effects of power coupling to cladding radiation modes. Furthermore we present the results of temperature and longitudinal strain sensitivities of the particular modes visible in the grating reflection spectrum followed by conclusions with reference to our previously reported results of polarimetric strain and temperature measurements of a similar fiber design.

In a highly Ge doped photosensitive fiber, co doped with phosphorus and fluorine and with a doped inner cladding, we observed, during annealing experiments, an anomalous variation of the FBG strength as a function of temperature and time. This characteristic only occurred when the fiber had been loaded with hydrogen. During a stepwise temperature increase from room temperature to 1200°C, the gratings showed, after an initial thermal decay, in two distinct temperature ranges a significant increase in reflectivity with time. One range with an increase in reflectivity was found between 850°C and 1000°C. The behaviors of the FBGs in this range were very similar to that known from chemical composition or regenerated gratings. Another temperature range with increasing reflectivity was found between 500°C and 700°C, followed by a strong decrease in reflectivity at temperatures between 750°C to 800°C. Both regimes of regeneration might be associated with changes in the core and doped inner cladding of this type of fiber.

Small sections of suspended twin-core fiber are used in reflection configurations to create two parallel Fabry-Perot cavities. Situations where both cores are excited and where only one core is excited are analyzed and compared. When both cores are excited, two parallel and equivalent cavities are formed and an interference pattern with higher visibility is obtained. The structure is also characterized with respect to temperature and a sensitivity of 12.4 pm/K is achieved.