Liquid crystals over the last two decades have been successfully used to infiltrate fiber-optic and photonic structures initially including hollow-core fibers and recently micro-structured photonic crystal fibers (PCFs). As a result photonic liquid crystal fibers (PLCFs) have been created as a new type of micro-structured fibers that benefit from a merge of “passive” PCF host structures with “active” LC guest materials and are responsible for diversity of new and uncommon spectral, propagation, and polarization properties. This combination has simultaneously boosted research activities in both fields of Liquid Crystals Photonics and Fiber Optics by demonstrating that optical fibers can be more “special” than previously thought. Simultaneously, photonic liquid crystal fibers create a new class of fiber-optic devices that utilize unique properties of the photonic crystal fibers and tunable properties of LCs. Compared to „classical” photonic crystal fibers, PLCFs can demonstrate greatly improved control over their optical properties. The paper discusses the latest advances in this field comprising PLCFs that are based on nanoparticles-doped LCs. Doping of LCs with nanoparticles has recently become a common method of improving their optical, magnetic, electrical, and physical properties. Such a combination of nanoparticles-based liquid crystals and photonic crystal fibers can be considered as a next milestone in developing a new class of fiber-based optofluidic systems.
Liquid Crystal Photonic Crystal Fibers (LC-PCFs) known also as Photonic Liquid Crystal Fibers (PLCFs) are advanced
specialty fibers that benefit from a combination of "passive" photonic crystal fiber host microstructures infiltrated with
"active" liquid crystal guest materials and are responsible for a diversity of new and uncommon spectral, propagation,
and polarization properties. This combination has simultaneously reinvigorated research in both fields of Liquid Crystals
Photonics and Fiber Optics by demonstrating that optical fibers can be more "special" than previously thought.
Simultaneously, photonic liquid crystal fibers create a new class of optical waveguides that utilizes unique guiding
properties of the micro-structured photonic crystal fibers and attractive tunable properties of liquid crystals. Comparing
to the conventional photonic crystal fibers, the photonic liquid crystal fibers can demonstrate greatly improved control
over their optical properties.
The paper describes basic physics including guiding mechanisms, spectral properties, polarization phenomena, thermal,
electrical and optical controlling effects as well as innovative emerging technology behind these developments. Some
examples of novel LC-PCFs highly tunable photonic devices as: attenuators, broadband filters, polarizers, waveplates,
and phase shifters recently demonstrated at the Warsaw University of Technology are also presented. Current research
progress in the field indicates that a new class of emerging liquid crystals tunable photonics devices could be expected.
In this paper we present a new method of depolarization of light by using an electrically-controlled liquid crystal rotator
in combination with a solid-state crystal with high birefringence. In general, anisotropic crystals depolarize partially
temporary coherent light depending on their birefringence and path length of the light passing through them as well as
on the azimuth of the optical beam i.e. the angle between the electric vector of the light wave and the birefringence axis
of the crystal. Electrically-controlled liquid crystal rotator is a useful tool to introduce changes in azimuth and in the
same to control depolarization of the light passing through the crystal placed behind the polarization rotator. Some
experimental results of degree of polarization measurements for different light sources as a superluminescent diode and
a laser diode are presented. Totally depolarization of light was achieved for the superluminescent diode and lithium
niobiate crystal with the liquid crystal electrically controlled rotator.
The paper presents construction of a simple fiber-optic color sensor designed for recognition of objects from a set comprising an a prori defined, limited number of colors. Color sensing is based on diffusive reflectivity of examined surfaces measured in several separate spectral channels. Number and positions of channels depend on predetermined set of colors. The channels are fed by selected LED's sequentially, i.e. in Time-Division Multiple Access mode. Collected serially analogue signals are converted onto digital data enabling some processing variants. In order to extend the performance capability of such a sensor and to enable measurements of others colors, a simple neural network was applied.
In this work we investigate light beam propagation in twisted nematic liquid crystalline film. In the linear case (for low power of the light beam) the diffraction and walk-off of the light beam is observed. Due to the optical reorientation nonlinearity light beam is self-focusing and finally spatial solitary wave is created. The direction of light beam propagation is also changing with increasing the nonlinear effect. The samples were filled with 6CHBT nematic liquid crystals and we measured the propagation of light beam at the distance of few millimeters. Nonlinear self-focusing was observed for a light power of order of few tenths of milliwats. The experimental results are in a good agreement with theoretical predictions and numerical simulation. The proposed configuration of our cell can be applied to switching of the light beam in low power all-optical systems.
One of the main problem of colorimetry is sensitivity of the optical fiber sensor for displacement of examined object. It means that the sensor also works as proximity or distance measurement sensor. In order to omit a problem of cross-sensitivity of the sensor we have analyzed several configurations of optical fibers inside sensor head. Results of the tests of several types of sensor head are presented. The best solution of fiber-optic head for colorimetric application is discussed as well. In conclusions some other applications of the optimized optical fiber sensor head are shown.
One of many hazards in industry is presence of airborne dust. Recently, we have proposed a "dust measuring sensor," operating on light scattering effect and constructed in space-optics technique. Tests of the device performed in normal operating conditions in a coal mine underground showed it worked properly and met requirements of that application except of zero-drift stability. The unsolved problem was dirtying of optical elements of the device during work by dust. Operating conditions of such a device are cruel -- humidity, elevated temperature, vibrations, and over-all contact with dust are harmful for optics. Thus, achieving reliable indications of the sensor is really a challenge. In the present work we test the idea to solve this problem by replacing bulk optics in the sensing area by optical fibers. First laboratory examination and operational tests of the fiber-optic sensor are also reported.
Initial results of fiber-optic non-invasive sensing of ignition and combustion process in the inner combustion engine are presented. An optical signal from a polarimetric fiber sensor is studied. Optical sensing is immune to high voltage of the ignition system and to some extent to temperature gradients arising close to the engine.
The paper presents propagation properties of elliptical fibers with liquid crystal core under the influence of temperature, hydrostatic pressure and external electric field. Initial data on transmission losses of the liquid crystal fiber in the 0.5 - 2.0 μm spectral range is also presented. The scattering loss was found to be around 0,2 dB/cm for elliptical-core (4x18 μm) liquid crystal fibers that is in accordance with other measurements, whereas a significant increase to about 5 dB/cm was observed in liquid crystal capillaries.
Optical imaging of objects inside high scattering medium by use of light instead of X-rays is called optical tomography. There are several types of optical tomography methods. One of them is direct transillumination with laser light in order to collect shadow images as a starting point for tomography procedure leading to three-dimensional object imaging. In this paper we present a modification of a direct transillumination optical tomography method with neural processing to measure a position of the object placed inside water solution of Intralipid, i.e. inside liquid, which simulates optical properties of a living tissue. As a light source a laser diode emitting red light beam (λ ≈ 670 nm) is used. Detection system of transillumination images is based on scanning of the optical field by photodiode. Scattered field measurements for opposite directions of object illumination, are processed by the neural net. In this approach all the time-consuming computations are conducted while training the artificial neural network. Results of object position measurement obtained in works leading to the paper show that neural processing is helpful in better accuracy of the object positioning, than direct image processing.
Initial results of study a lateral stress, static and dynamic, on a light propagation in a high birefringent fiber are presented. A permanent birefringence in optical fiber arises due to large thermal stress in the core. Ambient parameters like temperature and hydrostatic pressure influence the stress anisotropy and thus the light propagation. The parameters can be assumed as uniform and the optical response does not depend on the position of the fiber birefringent axes. For a stress (lateral pressure) the optical response of a fiber reveals an azimuth dependence on mutual direction of an exerted pressure and birefringent axes. With dynamic perturbation, even slowly varying, some additional effects like strain can not be avoided. The effective optical response is composed and strongly depends on the direction of stress.
One of many hazards in mining industry is presence of airborne dust on underground boards. Hazards caused by dust generated and spread in mines are of the two types: (1) health risk for miners from airborne dust produced from rocks, coal, soluble minerals (pneumoconiosis, toxicity), (2) danger of explosion of carbon dust. Dust particles produced in mines underground range from 0 to about 400 micrometers, have irregular shapes and prevailingly are strongly light absorbing. It is assumed that the most health-risky are particles between 1 μm and 5 μm in size. They are not visible with naked eyes, so their control and measurement need technical equipment. As a standard in polish mines, gravimetric measurement method is used at present. This method works well in post-event evaluation of total health-risk factor, but is not much useful for instantaneous risk warning. In order to recognize and possibly prevent the dust risk as it appears, other methods have to be used, like optical method. Looking towards this demand, an experimental optical dust sensor is demonstrated. The sensor is based on light scattering effect by dust particles, as usual do devices of this type. Originality of this solution lies in construction details of the sensor. Scattering is a complex function of dust kind, size, shape and concentration. Moreover, operating conditions of such a device are cruel -- humidity, elevated temperature, vibrations, and over-all contact with dust -- are harmful for optics. Thus, to achieve reliable indications of the sensor is really a challenge. This paper describes optical construction attempting to overcome difficulties in obtaining dust concentration sensor intended for mining industry and similar applications. First laboratory and operational tests are also reported.
Optical tomography system utilizing two laser diode module lasing at 685 nm and 785 nm wavelengths is presented. During one scanning procedure about 8000 electronic transmission images of optically nonhomogeneous object are registered by us of CCD camera. Special prepared software based on modified Radon transform is applied in order to reconstruct of 3D structure of the object. Some examples of advantages of two-wavelength system in comparison with other systems are presented as well. As conclusions some specific industrial applications of the two-wavelength optical tomography system are discussed.
This paper presents results of tests performed on a fiber optic system of liquid crystalline transducer for hydrostatic pressure monitoring based on properties of colorimetry. The system employs pressure-induced deformations occurring in liquid crystalline (LC) cells configured in a homogeneous Frederiks geometry. The sensor is compared of a round LC cell placed inside a specially designed pressure chamber. As a light source we used a typical diode operating at red wavelength and modulated using standard techniques. The pressure transducer was connected to a computer with a specially designed interface built on the bas of advanced ADAM modules. Results indicate that the system offers high response to pressure with reduced temperature sensitivity and, depending on the LC cell used, can be adjusted for monitoring of low hydrostatic pressures up to 6 MPa. These studies have demonstrated the feasibility of fiber optic liquid crystal colorimeter for hydrostatic pressure sensing specially dedicated to pipe- lines, mining instrumentation, and process-control technologies.
The paper presents experimental construction of a liquid-crystalline Bragg filter. The filter is designed as a lowcost WDM decoupler but can also function as demultiplexer in polarisation-coded transmission or may be used as a variable, electrically controllable bandpass filter. Theoretical basis, principle of operation and results of experimental examination of the device are given.
The paper presents initial studies of potential application of polarimetric sensing in flow control by the effect of vortex shedding (vosh). Discussion on geometrical factors of the flow tube for arising of a turbulence and oversimplified analysis of the response of the high birefringent (Hi-Bi) fiber on the flow are presented. Some features of the polarimetric sensing of the fluid flow are exemplified.
Nonlinear optical properties of chiral liquid crystalline media are in scope of our interest since several years. Our previous observations in the subject were already reported, where the effect of light interaction with chiral nematic structure was examined by reflected-probe-beam technique. The present contribution is a continuation of that work. Now the concept of modulation of light by light is presented as an example of potential application of previously achieved results. This paper just considers the use of reflected- probe-beam arrangement in combination with optical fiber technique for 'in line' controlling of high-intensity light beams. The idea utilizes nonlinear optical effect in combination with selective light reflection in chiral liquid crystals. The preliminary results of experimental examination of the 'all optical modulator' are demonstrated. Also some practical aspects and application problems are defined.
An original idea of hydrostatic pressure monitoring is presented. The paper reports latest result of liquid crystal-based fiber optic devices for hydrostatic pressure sensing in both low pressure and high pressure regions. In the low-pressure region, the fiber optic transducer based on properties of colorimetry employs pressure-induced deformations occurring in liquid crystalline cells, configured in a homogenous Frederiks geometry. However, in the high pressure region we propose another approach based on a liquid crystal-core anisotropic fiber acting as a sensing medium due to pressure-induced changes in its birefringence. These fibers unite the unique advantages of liquid crystals and quality of fiber technology that can be used for instant pressure monitoring.
The paper presents initial studies of the fiber flowmeter realized by polarimetric sensing with highly birefringent polarization-maintaining optical fibers. In the studied system one fiber is applied as the both bluff body and sensing element. The flow velocity is evaluated from the amplitude modulated signal of the frequency of vortex shedding. The polarimetric sensing based on polarization interference presents the advantage of sensitivity compared to the interferometry measurement but without the expense of its complex arrangement.
Optical fiber sensor output signals are usually electronically detected and transmitted to the control unit through many communications channels. 1'he signals may be also transformed by microprocessor and transmitted through one channel with high transmission rate. Optical fiber sensor network, for example for environmental monitoring [1], may have so many sensors that such method of data transmission is not sufficient. Hence an idea of optical fiber sensor signal processing by use of an optoelectronic neural system and then transmission of such prepared data to the control unit with help an ordinary transmission link seems to be very promising. In general, there are several thousands types of optical fiber sensors with different types of output signals. Most of them. belonging to either intensity or phase sensors, have similar output signals (Fig. 1) The characteristics shown in Fig. I may be more or less linear and symmetrical with different slopes but they are almost the same for such different variables like pH, temperature proximity, strain, angular movement etc. [2]. Such output signals cannot be directly applied for neural processing without additional transformation by use for example two liquid crystal cells (Fig. 2). One of them (LC I in Fig.2) should have threshold transmission characteristics. The second one (LC II) changes transmission level according to requirements of neural weight. In our work we apply only one liquid crystal cell for each sensor with especiall designed electronic control system playing role of smart pixel in neural processing.
Interaction of intense laser light with chiral nematic structure was examined by use of a reflected-probe-beam experimental technique. The technique is sensitive to structural changes in periodic media and appears to be as valuable for chiral nematics (CHNs) investigation as roentgenographic diffraction for crystal lattices. The probe-beam reflexes for various pitch-to-pump-laser- wavelength ratio including the selective reflection region, various glancing angles of the probe beam, and various CHN- layer thicknesses are presented. In the experiment a great care was taken to distinguish optical nonlinearity of the collective nature from that of thermal or photochemical origin. The results we have obtained point out the optical field-CHN structure coupling rather than the non-collective mechanism of observed effects.
A polished fiber half coupler with a nematic liquid crystal as a multimode planar waveguide overlay has been investigated. Experimental results show an existence of optical bistability with contrast approximately 40 percent for laser power approximately 340mW at (lambda) equals 514 nm. Observation of the optical bistability is explained as a result of an optically induced reorientational effect in a liquid crystalline layer.
Polarimetric fiber optic sensors belong to phase sensors, in which phase shift between two electric field components of beam passing through a high birefringent optical fibers responsible for a change in polarization state of the light. The paper presents polarimetric fiber sensor for dynamic strain measurements. In order to measure the strain in several points of the rail a relatively long, of length of few meters, measuring fiber was used. It causes problems with coherence of light passing through the fiber and hence the compensation of the birefringence was introduced.
An original idea of hydrostatic pressure monitoring based on a new kind of anisotropic fibers with a liquid crystalline core is presented. The liquid crystalline-core optical fiber giving a possibility of birefringence control unite the unique advantages of liquid crystals and quality of fiber technology what can be used for continuous pressure monitoring. Preliminary results of experimental studies of light propagation by optical fibers with liquid crystalline cores under hydrostatic pressure conditions are reported.
Optical binary computer may be as easy to operate as parallel system. For such configuration Boolean logic is not very convenient and therefore neural networks should be introduced. In works leading to the paper we used liquid crystal cells as a standard system of liquid crystalline layer between to conducting electrodes in 'sandwich' geometry. We have used 25 micrometers display cells filled with nematic 6CHBT working on 'twisted nematic' effect. Based on such elements a mode of a simple Hopfield network was set up. More advanced experiments were carried out on a model of neurone with supervised learning. The model consists of four laser diodes pigtailed to the multimode optical fibers with 50 micrometers core diameter. The directional couplers help to control the level of input optical power. Four liquid crystal cells allow to change the transmission level according to superivised learning requirements. All the signals were detected by one photodiode. The presented results of experiments are in excellent agreement with theoretical predictions. An additional study was done to check the possibility to build up a linear neural network with Grossberg layer, a neural network with Kohonen layer, and a counter propagation network with two layers of neurones. We have proved that such models may be set up based on simple liquid crystals cells and optical fiber networks.
An electrically controllable optical power attenuator, operating on a light scattering effect in a liquid crystalline cell was constructed for optoelectronic applications and examined. Its optical parameters and performance are presented and discussed.
The aim of this paper is to introduce an optoelectronic mammoscope that has been developed at Warsaw University of Technology for early breast cancer detection. The idea of the mammoscope is based on a direct comparison of light transmission spectra through breast tissue obtained for two selected wavelengths: 670 nm and 810 nm. As light sources, two types of laser diode are used. The optoelectronic mammoscope is equipped with a specially designed optoelectronic head which decreases light path throughout the breast tissue under examination and, simultaneously increases intensity of the light transmitted through the tissue. At the output, a CCD camera coupled to the IBM PC/486 computer is used for detection of shadow images created by the transmitted light at both wavelengths.
The paper discusses application of high-birefringence (HB) fibers as interconnections in liquid crystal logic systems. Since HB fibers can transmit linearly polarized light for a long distance, they are suitable for optical systems which operate only with polarized light. This paper also addresses problems of HB fibers -- liquid crystal combinations used in optical logic as modulators, polarization converters, and logic gates.
The present study has demonstrated the feasibility of transmitting polarization-coded optical information by optical fibers and their ability to operate with optical logic elements. Several simple arrangements of optical fibers joined with fibers have been examined. Sufficient coupling efficiency in the arrangements examined has not been achieved. The maximum SNR in the guidelines investigated was 10:
Some improvement of technology making the coupling of a fiber to a stripe waveguide more simple and effective is presented. The improvement consists in additional reprocessing of the edgeadjacent area of a substrate slab with a fabricated stripe waveguide. The mentioned area is subjected to a proton exchange process with simultanous drawingup of the slab from meltAfter a supplement process in the edge adjacent area the waveguide has a form of a horn with convergency depending on the mask pattern velocity and the melt temperatureAt the entrance face of the slab the channel takes a form of a rectangle of sizes of about tens micrometers wide and up to a few micrometers deep. The coupling of a fiber to an enlarged channel waveguide has been efficiently simplified and the coupling power is not so sensitive to the offaxial and tilt deflection of the fiber. L Interconnection of QjiCa1 wavides In most integrated optics devices a light beam is guided in a stripe waveguide of a required configuration. Low power electrooptical modulation forces decrease of the waveguide width. In photo-and electronolitography processing the stripes of width up to 2 pm have been manufactured1''2. The depth of about 0. 5 im is required for guiding of one mode. Long distance transmission realize fibers. Geometrical crosssection of the both fiber and stripe differ in size and form espe cially for multimode waveguides and the
In polarization-based logic gates the binary logic values are
represented by two orthogonal linear polarizations of light which can
be interconverted by means of the programmable half-wave devices. We
present a device consisted of a SSFLC-cell, a lithium niobate
birefringent crystal and polarization-maintaining highly birefringent
(HB) optical fibers. Due to the birefringent Li-Ni crystal attached to
the SSFLC cell and two output HB fibers with proper oriented axes both
orthogonal polarizations were spatially separated. In this way we
obtained a non-absorptive binary logic gate which did not loose any
information and enabled signal retrieving in any state. Power losses
of the FLC logic gate were estimated in the order of 20 dB and
contrast ratio between two stable logic states achieved a mean value
5 : 1.
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