An optical fiber sensor consisting of short sample of experimental Hi-Bi optical fiber integrated between two single mode (SM) optical fibers is proposed and experimentally demonstrated. The experimental fiber consists of core with diameter equals 8.5 μm and the two side Stress Applied Part (SAP) of diameter 25 μm. The SAPs are made of Al-Ladoped silica. The birefringence of the fiber is 3.19·10<sup>-4</sup>. We measured spectral distribution of intensity at the output of the SM fiber as function of the analyzer's adjustment angle and the suitable position of the analyzer was determined. Then we applied the bodies with weights up to 100 g on the fiber for two significant positions of SAPs: SAPs were next to each other and above each other. Evaluation of the measurements were done by the peak-to-peak amplitude of optical power difference (in dB) of two measurement for which we determine the applied force (weight). The sensitivities were determined to be 0.081 dB/g for SAPs next to each other and 0.037 dB/g for second position of SAPs, respectively. From the results it follows that the sensor could be used not only for determination of lateral force but also for determination of applied force position.
This paper reports on utilization of a single holographic/diffraction optical element (HOE/HDE) recorded in a photopolymer-based holographic material for monitoring the change of linear dimensions of an object which is in direct contact with the formed HOE. The holographic optical element – transmission grating, is produced by recording of interference pattern generated by two collimated beams from Argon ion laser operated at wavelength 514 nm into a thin photopolymer holographic film. The size of a grating spacing Λ was proposed with respect to range of the investigated deformations in order to be able to observe an effect of external mechanical stress on the grating. The proper experimental conditions for recording of the proposed transmission grating at selected wavelength were determined, first. This was done via recording of several quasi-harmonic phase diffraction gratings at different exposures and comparing their diffraction efficiencies. The chosen grating was then firmly attached to a transparent test specimen made of polycarbonate which was subjected to mechanical stress. The change of the specimen’s dimension was monitored, and at the same time the grating was illuminated by a probing beam from a laser diode module (639 nm) and the displacement of the first diffraction order was recorded. The dependence of the position of the diffraction order, i.e. spatial displacement as function of strain ε provides information on change of grating spacing Λ which is required for analyzing possibilities of utilization of the holographic material for sensor applications.
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.
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 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 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.
Optical homogeneity of materials intended for optical applications is one of the criterions which decide on an appropriate application method for the material. The existence of a refractive index inhomogeneity inside a material may disqualify it from utilization or by contrary, provide an advantage. For observation of a refractive index inhomogeneity, even a weak one, it is convenient to use any of interferometric methods. They are very sensitive and provide information on spatial distribution of the refractive index, immediately. One can use them also in case when the inhomogeneity evolves in time, usually due to action of some external fields. Then, the stream of interferograms provides a dynamic evolution of a spatial distribution of the inhomogeneity. In the contribution, there are presented results of the analysis of interferograms obtained by observing the creation of a refractive index inhomogeneity due to illumination of thin layers of a polyvinyl-alcohol/acrylamide photopolymer and a plate of photorefractive crystal, lithium niobate, by light and a refractive index inhomogeneity originated at the boundary of two layers of polydimethylsiloxane. The obtained dependences can be used for studying of the mechanisms responsible for the inhomogeneity creation, designing various technical applications or for diagnostics of fabricated components.
The photorefractivity of iron-doped lithium niobate crystal is utilized for creating a negative lens-like structure. The refractive index inhomogeneity acting as a lens is formed due to illumination of the crystal by an optical field with Gaussian spatial distribution of intensity modified by a proper cylindrical lens. Blue line (488 nm) of an argon ion laser is used as a source of light for inducing the inhomogeneity within the crystal. Imaging properties of the photorefractive inhomogeneity are discussed in terms of wave and ray optics approaches and they are practically demonstrated by means of coherent light coming from a He–Ne laser (633 nm). Finally, the focal lengths of the “lens” are calculated using both the lens equation and the formula resulting from analysis of the phase curvature of the wave of light, which passed through the refractive index inhomogeneity.
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.
A setup for observing formation of light-induced diffraction grating in LiNbO3:Fe in real time is presented. The utilization of this setup allows monitoring the temporal dependence of the diffraction efficiencies of several diffraction orders and observing the temporal evolution of the grating by means of the Mach-Zehnder optical interferometer at the same time. The part of the setup responsible for imaging of the space-time distribution of the refractive index changes in a crystal is advantageous to those making long-period gratings in various photorefractive materials because of easy and direct readout of both the grating spacing and the amplitude of the refractive index modulation. These two parameters are well controlled, and the recording of the grating can be stopped after desired values are reached.
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.
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 contribution presents results of the experimental investigation of the photorefractive effect in LiNbO<sub>3</sub> crystals obtained
from light diffraction observed on photorefractive records of periodic optical field and obtained from interference images of
refractive index modulation in the records. These results are compared with results following from equations describing the
process of the photorefractive record formation in cases when various mechanisms of generation of the internal electric
current responsible for photorefractive effect are taken into account. The comparison between the calculated and measured
dependences shows that the modulation of the refractive index is not caused by
the electro-optical effect (as the standard model assumes) but by the redistribution of carriers trapped at the donor centres and
by change of their polarisability.
Short analyze and some results of interference imaging of refractive index distribution in photorefractive records in the
LiNbO<sub>3</sub> sample are presented. The presented result shows that an easily readable image can be obtained when refractive
index is modulated only in direction parallel with interference fringes. For this purpose, it is useful to use Mach-Zehnder
interferometer what allows adjusting the interference fringes to desired position. Obtained images of records of a light
field with aperiodic distribution of intensity show that refractive index profiles do not correspond to profile of light
intensity or intensity of internal electric field (which is formed by the sample illumination). It is more like the profile of
distribution of carrier concentration trapped on the donors or profile of square of the electric field when gradient of
refractive index is parallel or perpendicular to c axis of the crystal, respectively. This result leads to necessity of
modifying explanation of mechanism of photorefractive record creation. At least in investigated LiNbO<sub>3</sub>:Fe.