Radek Helan

Stabilization of semiconductor lasers by fiber Bragg gratings

We present methods of improvement of wavelength stability and tuneability of semiconductor laser diodes in fiber laser interferometers by fiber Bragg gratings (FBGs). We developed simulation method to calculation of arbitrary fiber grating (apodized, chirp etc.) with high precision by combination of methods based on layered dielectric media (LDM) and transfer matrix. On the basis of our simulations and measurements of the commercially available fiber gratings we designed a special 100 mm long fiber Bragg grating with apodization. We expect the application of the FBG to improvement of the linewidth and mode-hop free tuning range of semiconductor lasers at the wavelength 760 nm to increase resolution of fiber laser interferometer based on these diodes. We built the absolute fiber laser interferometer with Vertical Cavity Surface Emitting Laser (VCSEL) to easy employ FBG to stabilize wavelength and control the tuning range. First set up is presented.

Stabilization of semiconductor lasers by fiber Bragg gratings for absolute laser interferometry

We present application of methods for calculation of parameters of apodized fiber Bragg gratings (FBG). We used combination of methods based on layered dielectric media (LDM) and the transfer matrix. On the contrary to the other calculation techniques the LDM method is based on sequence of thin films of dielectric media assembled in the direction of wave propagation. The combination of the LDM method and the transfer matrix method can be used to the calculation of arbitrary fiber gratings with high precision. For this designed apodized FBG we calculated the phase mask to manufacture by interference patterns. The phase mask and the fixation bottom of the fiber were made by e-beam lithography to achieve highly precise stability during manufacturing. We present the set-up of this system for writing FBG by pulsed UV laser with wavelength 266nm. Measurement of commercially available FBG with comparison to our calculated FBG is presented. We put together the absolute fiber laser interferometer where Vertical Cavity Surface Emitting Laser (VCSEL) is used to easy employment of FBG to stabilization and control the tuning range of the wavelength. The first set-up is presented.

Optical fiber sensors for measurement strain and vibration

We present optical fiber sensors to measurement strain and vibration. The sensors are based on fiber Bragg gratings (FBG). We prepared construction of strain sensors with respect to its implementation on the outer surface of concrete structures and with compensation of potential temperature drifts. These sensors are projected with look forward to maximal elongation and strength which can be applied to the sensor. Each sensor contains two optical fibers with FBGs. One FBG is glued into the sensor in points of fixation which are in the line with mounting holes. This FBG is prestressed to half of measurement range, than the stretching and pressing can be measured simultaneously by one FBG. The second FBG is placed inside the sensor without fixation to measure temperature drifts. The sensor can be used to structure health monitoring. The sensors to measurement vibration are based on tilted fiber Bragg grating (TFBG) with fiber taper. The sensor uses the TFBG as a cladding modes reflector and fiber taper as a bend-sensitive recoupling member. The lower cladding modes (ghost), reflected from TFBG, is recoupled back into the fiber core via tapered fiber section. We focused on optimization of TFBG tilt angle to reach maximum reflection of the ghost and taper parameters. In this article we present complete set-up, optical and mechanical parameters of both types of sensors.

Preparation and measurement of TFBG based vibration sensor

We present vibration fiber sensor set up based on tilted fiber Bragg grating (TFBG) and fiber taper. The sensor uses the TFBG as a cladding modes reflector and fiber taper as a bend-sensitive recoupling member. The lower cladding modes (ghost), reflected from TFBG, is recoupled back into the fiber core via tapered fiber section. We focused on optimization of TFBG tilt angle to reach maximum reflection of the ghost and taper parameters. Comparative measurements were made using optical spectrum analyzer and superluminiscent diode as broadband light source. We present dependence between intensity of recoupled ghost mode and sensor deflection.

Design and fabrication of fiber Bragg gratings for vertical cavity surface emitting laser diodes

We simulated chirped and apodized fiber Bragg gratings (fiber gratings with modulation of the amplitude and with modulation of the spacing) the same as uniform fiber gratings. Results of our simulations are presented. Apodization of fiber Bragg gratings (FBG) has many advantages in comparison to other types of FBGs. Main improvement is suppressing of the side lobes in the grating spectral properties. The difference of side lobes suppression for uniform and Gaussian apodized grating is more than 110dB. On the basis of these results we designed a special fiber Bragg grating with apodization especially for using to stabilize semiconductor lasers for high precision laser applications with central wavelength 760nm. We present application of methods for calculation of parameters of apodized fiber Bragg gratings (FBG). We used combination of methods based on layered dielectric media (LDM) and the transfer matrix. On the contrary to the other calculation techniques the LDM method is based on sequence of thin films of dielectric media assembled in the direction of wave propagation. The combination of the LDM method and the transfer matrix method can be used to the calculation of arbitrary fiber gratings with high precision. For this designed apodized FBG we calculated the phase mask to manufacture by interference patterns. The phase mask and the fixation bottom of the fiber were made by e-beam lithography to achieve highly precise stability during manufacturing. We present the set-up of this system for writing FBG by pulsed UV laser with wavelength 266nm. Measurement of commercially available FBG with comparison to our calculated FBG is presented.

Comparison of Methods for Fiber Bragg Gratings Simulation

This paper discusses methods for calculation of arbitrary fiber Bragg gratings (FBGs). Aim of the work is compare known methods and choose proper method for implementation grating simulation into the education. There were compared especially two suitable methods. The first method is based on coupled mode theory, the second on layered dielectric media. Both methods can be used in combination with transfer matrix method. Combination with transfer matrix brings fast and accurate method for calculation of arbitrary grating with or without apodization and chirp profiles.

Miniaturized GPS position logger for tracking of small mammals

The satellite tracking of animal individuals remains a signiücant source of knowledge in the area of zoology. The greatest limitation is the size and weight of the satellite position loggers that prevents the usage of satellite telemetry on three-fourths of mammal and bird species. We present an architecture and design of a miniaturized GPS logger with the weight below ten grams that would be suitable for the studies targeting demanding study groups such as the only European flying fox. Our design combines the GPS logging with traditional telemetry signaling, typically used for manual radio tracking, allows for radio-transmitted position data retrieval and the 2–3 days of battery life. The performance has been verified within a field testing.

Preparation and measurement of FBG-based length, temperature, and vibration sensors

We present system of structure health measurement by optical fiber sensors based on fiber Bragg gratings. Our system is focused to additionally install to existing buildings. We prepared first set-up of the system to monitoring of the nuclear power plant containment shape deformation. The presented system can measure up to several tens of sensors simultaneously. Each sensor contains optical fiber grating to measurement of change of length and the other independed fiber grating to monitor the temperature and the other ineligible effects.

First setup of the optical fiber measuring system to monitoring structure health of nuclear power plant

We present system of structure health measurement by optical fiber sensors based on fiber Bragg gratings. Our system is focused to additionally install to existing buildings. We prepared first set-up of the system to monitoring of the nuclear power plant containment shape deformation. The presented system can measure up to several tens of sensors simultaneously. Each sensor contains optical fiber grating to measurement of change of length and the other independed fiber grating to monitor the temperature and the other ineligible effects.

Design and simulation of fiber Bragg gratings with 760 nm central wavelength

We present the simulation method to calculation of arbitrary fiber grating (apodized, chirp etc.) with high precision by combination of methods based on layered dielectric media (LDM) and transfer matrix. On the contrary to the other calculation techniques the LDM method is based on sequence of thin films of dielectric media assembled in the direction of wave propagation. The critical parameter of fiber gratings, the bandwidth can be narrowed by higher refractive index change, longer gratings or by multiplying fiber gratings. On the basis of our simulations and measurements of the commercially available fiber gratings we designed a special 100 mm long fiber Bragg grating with apodization. Finally, we simulated parameters of fiber gratings arrays-multiple fiber gratings. We expect the application of FBGs to improvement of the linewidth and mode-hop free tuning range of semiconductor lasers at the wavelength 760 nm to increase resolution of fiber laser interferometer based on these laser diodes.