The adoption of light emitting diodes by lighting industry is inevitable. They provide many advantages, but one of them is very interesting from the point of view of using liquid crystals: it is the small etendue of these sources. Rather well collimated and small diameter white light beams can be obtained. Here is where liquid crystal lenses can have an important application. We report the development of an optical element that can dynamically stretch broad band beams in one or in orthogonal or in both directions. Thus, we can start from a narrow circular beam and obtain a rectangular shaped beam to fit perfectly the size and the exposure requirements in home, office, architectural or automobile applications. The basics of its operation as well as of its optical performance data will be reported.
We describe the electro-optical behavior of polymer stabilized liquid crystal (PSLCs)
networks used for the development of electrically variable focus lenses. We start with a short review of
mechanisms influencing the performance of those lenses, including the most important one : the light
scattering. Then the role of the polymer chain morphology in electrically controllable molecular
reorientation and formation of orientation defects in PSLCs is investigated. We use two non mesogene
monomers, with respectively one and two functionalities, to create two different degrees of cross-linking in
PSLCs. By using optical polarimetry and scattering experiments, we investigate the defect formation in
those PSLCs, outline the presence of 3D orientational defects and show that the PSLCs with higher crosslinking
demonstrate better electro-optical reversibility.
We present a waveguide with in-core Bragg grating and electro-optic liquid crystal cladding. The electric field induced
reorientation of liquid crystal allows the reflection of guided light of desired wavelengths and polarizations at desired
spatial positions.
We present a fiber long period grating based sensor for biological applications. Inscribed by
using point-by-point method, the LPG has a resonant wavelength close to the emission wavelength of
biological objects of interest. The part of the light, emitted from sensing area, is collected by both fiber core
and cladding, while the majority of light is coupled into the cladding. Back coupling of light of certain
wavelength can selectively be achieved from fiber's cladding to core thanks to the LPG. Such selective
back coupling allows the increase of efficiency of remote detection of biological objects of interest, using
excitation source and detector on the same side of the fiber.
The variable optical attenuator (VOA) is an important part of agile optical telecommunication systems. VOAs built on so called free space technologies carry important drawbacks in terms of mechanical reliability, size and optical loss. Evanescent field approach have been used to design VOAs with very low insertion loss. Thermo-optic modulation mechanism was mainly used to control the attenuation level, which unfortunately requires from 10 to 100 times more electrical power compared to above mentioned free space architectures. This power consumption issue may be very challenging in high count arrays of VOAs.
At the same time, liquid crystals (LC) have been proved to require very low electrical power for operation.
In the present work, we report the creation of evanescent field modulation based VOA with extremely low insertion loss (below 0.1dB) and low electrical power consumption by removing a portion of the original fiber's cladding and replacing it by a specifically synthesized composite LC material, which have an ordinary refractive index lower than the glass one. The initial orientation of LC molecules provides an effective refractive index of the electro-optic cladding that is equal to the refractive index of the original silica cladding. We then create a LC molecule reorientation by the so-called Fredericksz effect by applying to the LC material an electrical voltage. This reorientation changes the refractive index around the depressed cladding area and brings to the partial leakage of the guided radiation into the cladding area, achieving thus attenuation levels above 50dB. Measured maximum electrical power consumption of the VOA is in the microwatt range.
In contrast with conventional polarization maintaining fibers, a simple Ge-doped single-mode fiber is used to generate tunable femtosecond soliton pulses. Soliton self-wavelength-shift up to 200 nm is achieved in a 17 m long fiber. The generated “monocolored” soliton pulses have quasi-ideal sech spectral shapes. A high contrast optical switching scheme is proposed as an example of potential application of the soliton self-frequency shift.
We present the optical performances of a compact variable optical attenuator (VOA) developed at Photintech. The presented VOA’s operation principle is based on the guided wave evanescent field manipulation. This approach allows the cost effective fabrication of VOAs with extremely low insertion loss, below 0.1 dB. The access to the evanescent portion of the guided radiation is achieved by removing a portion of the original waveguide’s cladding and replacing it by a thermo-optic composite polymer material. By changing the temperature of the thermo-optic material we create a partial leakage of the guided radiation into the replaced cladding area, attenuating thus the guided radiation. It is well known however, that the application of polymer materials for optical component fabrication creates significant birefringence due to the shrinkage and thermal stresses. As a result, the polarisation dependence of such devices is relatively high.
We apply a specific cladding geometry and heating electrode (pending patent of Photintech inc), which ensures axial compensation of the birefringence, providing thus very small polarisation dependence. The design provides also a high dynamic range operation (above 50dB). An in-house designed electronic board allow the VOA operation in three different regimes: direct driving, constant output power or constant attenuation coefficient with a precision as better as 0.1dB. The developed VOA device can be used in agile optical networks, for such applications as dynamic gain equalisation, dynamic channel equalisation, optical transmitter power control and receivers protection in the telecommunication systems.
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