A sample of lithium niobate crystal with known thickness is set on a vertical rotary stage, placed in between two crossed plane polarizers and illuminated by a collimated beam of white light. The sample is rotated to an appropriate position, and interference fringes observed behind an analyzer are recorded by a fiber-optic spectrometer. The recorded channeled spectrum is then analyzed, and from measured positions of interference minima (dark fringes) and known crystal’s thickness, the group birefringence of the crystal sample is obtained as a function of wavelength. The fringe order versus position of the fringe within the spectrum is fitted by appropriate dispersion function, and as the result phase birefringence as a function of wavelength is found. The measurement is performed for a sample declared as undoped, a sample doped with 0.025 wt. % of Fe, and a sample doped with 0.025 wt. % of Fe and 0.075 wt. % of Mn in the wavelength ranges (470 to 780) nm and (900 to 1700) nm. A good agreement between group birefringence dispersion obtained from positions of interference minima and known sample’s thickness and that calculated from phase birefringence dispersion given by Sellmeier dispersion model (used for undoped sample) and Cauchy model (used for doped samples) is found.
The birefringence of the lithium niobate samples with different dopants is studied in visible and near-infrared spectral regions using a broadband light source, set of linear polarizers and VIS/NIR optical spectral analyzers. From the interference pattern observed behind the polarizer (one from the pair used) the dispersion of birefringence of the investigated samples is estimated. The results show that the dispersion of birefringence can be used for rapid and effective distinguishing among crystal samples of different origins.
Lab-on-a-fiber-tip-devices combine the different materials and different configurations (for example photonic structures) to optical fiber tips. We propose, fabricated and experimentally demonstrated a fiber tip device, the 3D polymer photonic structure of Fabry-Perot cavity integrated at the end of the optical fiber. Also, we prepared and experimentally demonstrated a fiber tip device with Bragg reflector integrated at the end of the optical fiber. Both structures, the Bragg’s reflector and the Fabry-Perot cavity, were created in IP Dip polymer cylinder with a diameter like a diameter of an optical fiber which is advantageous for integration. Layers of structures consist a material with refractive index 1.5 and air. In addition, a method of transferring and bonding of a photonic structure at the end of the optical fiber is described. In all investigated interval of wavelengths there is good agreement between measured and calculated reflection spectra.
A preparation of polydimethylsiloxane (PDMS) interferometer located at the end of a single-mode optical fiber is presented. For preparation of Fabry-Perot interferometer (FPI) we used the PDMS Sylgard 184 (Dow Corning). During fabrication process of FPI the length of micro-cavity in PDMS can be set to required size. After the setting the length of microcavity was fixed by encapsulating of another layer of PDMS. By measuring the transmission characteristic under the constant conditions of environments we observe a periodic change of signal depending on the wavelength – interference pattern. When the measurand is applied on FPI, PDMS changes its volume and also its refractive index resulting in a change in length of microcavity. The change of length of FPI modifies the interference pattern. For evaluation of influence of measurands change, we chose one maximum (wavelength of the maximum) as a reference wavelength at the reflected spectra. Then we changed the amount of measurand and we observed wavelength shift of the maximum and compared it with the wavelength of reference maximum of reflected spectra. We investigated the sensitivity of PDMS FPI on temperature and pressure. The dependencies to temperature and pressure have linear character. The temperature sensitivity of fabricated PDMS FPI is 5.76 nm/°C. In free spectral range of FPI it is possible to determine the difference of temperature 2.7 °C. After that 2π jump in reflected spectra occurs. The pressure sensitivity is -0.64 nm/kPa and free spectral range of FPI corresponds to 11 kPa.
We created a record of an optical field formed due to interference of a spherical divergent reference wave and signal plane wave in a self-developing photopolymer film using a He-Ne laser and Mach-Zehnder-like interferometric set-up. Due to Bragg reflections the recorded volume holographic grating acts as holographic lens and transforms the reference divergent wave onto a near-collimated one. After successful recording the diffraction efficiency of the lens is found to be 84% at Bragg angle. The lens is used to modify the radiation pattern of a commercially available red emitting LED. The effect of the holographic lens on the LED’s radiation pattern is investigated by far-field measurement of intensity pattern as function of angle for LED without the lens and with applied holographic lens.
We created a record of an interference field of reference plane wave and convergent wave in a self-developing photopolymer film using a He-Ne laser and Mach-Zehnder interferometer. Due to Bragg reflections the record acts as holographic lens and transforms the reference plane wave onto a convergent one. To achieve the best performance of the lens the response of the photopolymer film is found and proper conditions for recording are determined. After successful recording the optical parameters of the lens – focal length and diffraction efficiency are measured and compared with values following from the theory. The formed holographic lens has focal length f = 160 mm and reaches diffraction efficiency of 78% at Bragg angle. The used photopolymer meets criteria for the purpose of creating various holographic optical elements.
We prepared and demonstrated a compact, simple-to-fabricate, air microcavity in polydimethylsiloxane (PDMS), placed at the end of a single-mode optical fiber. The air microcavity creates a Fabry-Perot interferometer. The length of microcavity changes with change of temperature. So the wavelength shift of reference minima (maxima) of interference pattern corresponds to temperature change. For the operation of the sensor broadband light source and low-resolution optical spectral analyzer can be used. The sensor response for change of temperature is fast and occurs within a few seconds. The temperature sensitivity is 6.1 nm/°C. For optical spectral analyzer resolution 0.1 nm the smallest temperature difference possible to determine is 0.017 °C.
We present the optical fiber sensor based on fused silica capillary as a sensing element spliced between the lead-in and lead-out singlemode (SM) fibers. In the region of the splice the cladding modes of capillary are excited from the fundamental mode of led-in SM fiber. The intermodal interference of the propagating cladding modes results in the formation of the resonant peaks in the transmission spectrum. With the variations of the external refractive index the shift of resonance wavelength of the peak can be observed. The sensitivity for the refractive index values around n=1.33 is observed and using the wet etching technique can be increased, what gives an assumption for its using in a biomedical sensing applications.
Utilization of the photorefractive effect for creation of the waveguiding region may offer rather cheap, easy, "green"
(chemicals-free) and flexible way to fabricate the waveguiding-based devices for integrated optics in proper media. We
present results of a single waveguide fabrication in LiNbO<sub>3</sub>:Fe crystal by means of a single Ar<sup>+</sup> ion laser beam with
special spatial distribution of intensity. The process of the waveguide creation is, in real time, monitored by means of
Mach-Zehnder interferometer. According to interpretation of the resulting interferogram this allows to control the time of
the exposure needed for reaching the desired difference between the refractive indices of the waveguiding and
surrounding regions. The waveguiding properties of the structure are practically demonstrated.
We have measured the dependence of transfer function of endlessly single mode photonic crystal fiber on the bends
radius. The results are confirmed by numerical simulations. New questions on bending insensitivity of the Photonic
Crystal Fibers and effective refractive index approximation of the Photonic Crystal Fiber arise as result.
Various photopolymer materials have recently found a significant number of useful applications in microelectronics and
the PC board industries. Some of these materials have also become attractive optical recording materials for the
recording of holographic devices such as diffractive optical elements and gratings, or as data storage media, for the
fabrication of optical waveguides and photonic processing structures. Ever increasing requirements, driven by
application developments, has lead to the rapid development of newer generations of such materials. As the ever
increasing number of new materials is developed and used, there is a need to characterize the material behavior pre-and
post exposure. In order to produce materials with a desired set of material properties one has to understand the
photochemical processes present during recording. Although in most case emphasis is placed on studying the high
spatial frequency response and the related limitations of such materials, the low spatial frequency response
characteristics can also supply useful information regarding the processes taking place during grating formation. In this
paper we present the experimental results obtained following a detailed examination of the low spatial frequency
response of a photopolymer material in the case of exposure at different recording intensities. The time dependence of
the diffraction efficiency of the grating must be then analyzed using the appropriate diffraction theory of phase gratings.
Furthermore the results of examining the angular scans of the resulting grating diffraction efficiency are presented in
order to specify the condition of the diffraction regime (e.g. thin, thick) for such low spatial frequency gratings.
We present some results of experimental study of modal dispersion measurement using a simple technique for determination of time-delay of optical beams based on low coherence interferometry in Michelson configuration usually used for chromatic dispersion measurement. The time delay observed at experiments is induced by modal dispersion of modes with small difference of their phase constants. For determination of the character of the modes was used a polarisation measurement.
One of the most interesting features of photonic crystal fibres (PCFs) is their unique dispersion. Therefore knowledge of
chromatic dispersion is very important for better utilisation and optimisation of PCF potential. A modified low
coherence Michelson interferometer is described in the contribution. The modified interferometer consists of an arm
with a reference fibre, which has a sputtered mirror at its end face and the other arm contains the fibre under test and the
air line with variable length. The advantage of such setting is that the investigated fibre needs no manipulation (mirror
creating) and that only one arm should be changed when an other fibre is to be measured. A monochromator and
halogen lamp are used as the source that allow measrument of dispersion over a wide spectral range. The optical path
difference between the investigated and the reference fibre is about 15 μm (delay 0.05 ps) and can be readily
distinguished. Besides describing the interferometer set-up also some results of dispersion measurement in samples of
PCF are presented in the contribution.
The LiNbO<sub>3</sub> crystals are well known for their great piezoelectric, electrooptic and photorefractive properties. The letter
properties mentioned can be essential for many applications in photonics. This contribution deals with the generation of
the records of specially structured optical fields which can, under appropriate conditions, behave as the photonic structures. We present the results of the investigation of the light-induced structures and discuss the possibilities of their utilization as the Bragg gratings, wave-guiding structures as well as the data records.
The equipment for intermodal interference investigation is described. The results of an investigation of intermodal
interference within a photonic crystal fibre (from Centaurus Technologies, Sydney) in terms of length and frequency
region are presented. From the measured values the difference of the phase constant of the fundamental and the first
higher order (antisymmetric) modes, as well as the decay constant of the mode in the wavelength region exceeding the
two modes region, are determined.
Intermodal interference in a photonic crystal fibre is measured in fibre samples of different lengths. The measurement was performed for attenuation of the first higher order mode determination. Also, the mode field distributions at the end of short and longer samples were measured. This measurement allows finding field distribution of the second mode.