Armen Zohrabyan

Spectral broadening of femtosecond pulses in a single-mode As-S glass fiber.

We report a strong spectral broadening of femtosecond pulses propagating in a single-mode As-S glass fiber of 1.5 m length. The pump pulse spectrum is broadened by a factor of five when the input power is grown up to 16.4 mW. The broadened spectra are nearly symmetric and self-phase modulation is believed to be the dominant nonlinear effect responsible for this process.

Self-written gradient double claddlike optical guiding channels of high stability

Composite photopolymerizable material and self-writing method are developed, allowing the creation of very stable (in time) gradient double-clad optical waveguides with W form of refractive index profile. The guide is formed by the mutual counter diffusion of reactive and nonreactive molecules followed by photopolymerization. After the guide formation by a Gaussian intensity shaped beam, the mixture is simply fixed by uniform irradiation without additional processing. More than 2cm length permanent self-guiding channels are created. The obtained results could be useful for “optical bonding” of dissimilar optical devices.

In-fiber variable optical attenuation with ultra-low electrical power consumption

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 fibers 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.

Low loss and low polarization dependent fiber variable optical attenuators

We present the optical performance 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. Access to the evanescent portion of the guided radiation is achieved by replacing the original waveguide’s cladding with a thermo-optic composite polymer (TOP) material. By changing the temperature of the thermo-optic (TO) material we create guided radiation partial leakage attenuating thus the light. Nevertheless, using polymer materials usually creates significant birefringence due to shrinkage during polymerization or thermal stresses during operation and the polarization 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 polarization dependence. In-fiber design provides also low insertion loss (IL) and high dynamic range operation. Control electronics allow the VOA to operate with a precision better than 0.1dB. The developed VOA can be used in agile optical networks, for applications such as dynamic gain equalisation, dynamic channel equalisation, optical transmitter power control and receivers protection in telecommunication systems.

Light induced gradient polymer stabilized liquid crystals for electrically variable focus lenses: the role of network morphology

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.

Development of fiber long period gratings for biological sensor applications

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 fibers 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.

Dynamic compensation of chromatic and polarization mode dispersion using liquid crystals

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.

Composite Liquid Crystalline Mixtures for Guided Wave Electro–Optic Applications

A low refractive index multifunctional composite liquid crystalline mixture is proposed to address the issues of in-guide tuning of optical properties. As the standard optical telecommunication line is made of fused silica, signal phase and amplitude modulation using liquid crystal requires that its refractive indices match the refractive index of silica, which is low compared to the majority of commercially available mixtures. In the present work, low molecular mass liquid crystals are mixed with fluorinated additives to adjust refractive index below a critical limit (n < 1.4584 at D-line) without impacting significantly on the nematic temperature range (TCN = − 24°C and TNI = 68°C). A method of reducing the surface anchoring energy of liquid crystals to glass surface is also proposed as a non contact alignment method around the reduced diameter optical fibers. The bulk elastic energy around the fiber acts as an alignment mechanism when LC-fused silica molecular interactions are minimized. The combination of these two properties makes possible the in-guide modulation of the light phase and amplitude with the liquid crystal used as an outer-cladding electro-controllable medium, significantly improving insertion losses.

Tunable femtosecond soliton generation in a Ge-doped fiber

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.