Optical multi-image encryption based on frequency shift

We present a novel multi-image encryption and decryption algorithm based on Fourier transform and fractional Fourier transforms. Lower frequency parts of the original images are selected, frequency shifted and encrypted by using double phase encoding in fractional Fourier domains. Multiple images are encrypted together into a single one. A simple optical setup is given to implement the proposed algorithm. This scheme has features of enhancement in decryption accuracy and high optical efficiency. Numerical results have been given to verify the validity and efficiency of the proposed scheme.     

Experimental opto-digital synthesis of encrypted sub-samples of an image to improve its decoded quality

We develop an experimental protocol to visualize decrypted images that otherwise would have been barely recognizable, while keeping the standard security levels. This image deterioration is partly due to the natural speckle noise as well as the practical limitations arising from the optical elements composing the setup. This protocol is based on an optical image synthesis with digital holography using enlarged sub-samples of an entire image together with a multiplexing technique. We implement the process using a Mach-Zehnder interferometer with a joint transform correlator encrypting architecture. As a result, we get smaller speckle patterns on the final assembled image and a spatial frequency enhancement with respect to the decoded image obtained with the conventional procedures.     

Optical bistability by two laser beams in photorefractive crystals

For a nonlinear medium, a given incident laser beam may produce different transmitted light waves. This phenomenon (the so-called optical bistability) corresponds to multiple solutions of the boundary value problems of the nonlinear Helmholtz equation. Optical bistability can be useful in the design of optical switches. Devices that display this behavior could potentially play a major role in the development of optical communication systems and computing. In this article we present experimental results concerning the optical bistability in photorefractive BaTiO₃:Fe crystal. Two laser beams were used to interact with the photorefractive crystal which resulted in the bistability of the intensity of transmitted wave. This was achieved without the application of any optical resonator. High contrast optical bistability is found experimentally in the pump-ratio dependence of the output intensity.     

Impact of channel baud-rate on logarithmic digital backward propagation in DP-QPSK system with uncompensated transmission links

We numerically investigate the impact of channel baud-rate on the performance of logarithmic step-size based spilt-step Fourier method (SSFM). This algorithm is used to implement digital backward propagation (DBP) to efficiently compensate fiber chromatic dispersion (CD) and non-linearities (NL). The DBP method is implemented in N-channel dual-polarization quadrature phase shift keying (DP-QPSK) transmission over 2000 km standard single mode fiber (SMF) with no in-line optical dispersion compensation. We investigate the same-capacity and same-bandwidth transmission systems with 56 Gbit/s/ch (14 GBaud), 112 Gbit/s/ch (28 GBaud) and 224 Gbit/s/ch (56 GBaud). Each system has the bandwidth occupancy of 500 GHz with a total transmission capacity of 1.12 Tbit/s. Moreover, we have also compared the multiple channel transmission performance with single channel transmission to quantify the impact of inter-channel (cross-phase modulation ‘XPM’ and four-wave mixing ‘FWM’) and intra-channel (self-phase modulation ‘SPM’) non-linearities. The logarithmic step-size based DBP algorithm (L-DBP) depicts efficient mitigation of CD and NL impairments. The benefit of the logarithmic step-size is the reduced complexity and computational time for higher baud-rate transmission systems.     

Modal analysis and cutoff frequencies of a doubly clad plasma loaded waveguide having a lemniscates of Bernoulli-type core cross-section

The modal dispersion characteristics of EM waves in a new unconventional plasma loaded doubly clad waveguide having a shape of the lemniscates of Bernoulli-type core cross-section have been studied analytically. The proposed waveguide has three parts namely the core with slighter high index, the inner cladding with plasma and the outer cladding with air. Now using the necessary orthogonal coordinates for the proposed structure and imposing the boundary conditions under the weak guidance condition, the modal characteristics equation has been obtained considering variations in plasma width and plasma frequency, the dispersion curves and cutoff frequencies have been obtained and presented in this paper. It is noted that as the width of plasma layer increases, the cutoff frequency also increases considerably in all considered cases. This shows that using plasma width as a new parameter we can control any particular mode on our wish.     

Gain evaluation of compact designing on Er/Yb codoped germanate glass channel waveguide

Bent waveguide structures (U- and F-bend) based on UV-sensitive Er³⁺/Yb³⁺ codoped germanate glass substrates have been designed to achieve high-gain C-band amplification. Using simulated-bend method, the optimal radius for curved structure is offered to be 1.90 cm with loss coefficient of 0.0015 dB/cm, as the substrate size is minimally schemed. In the wavelength range of 1528–1559 nm, obvious gain enhancement for the bent structure waveguides is anticipated, while, for the F-bend waveguide, the internal gain at 1533.8 nm wavelength is derived to be 22.55 dB, which is much higher than the value of 14.06 dB in the U-bend waveguide, and over three times higher than that of the straight one, after compensating both the bend loss and the transition loss. The simulation results indicate that the bent structure designing is beneficial in attaining high optical gain in Er³⁺/Yb³⁺ codoped germanate glass substrates, which assures that long-period grating can be applied to implement practical C-band gain-flattened amplification.     

Design and tolerance analysis of optical interleaver based on retardant crystals

A set of recursion equations was developed for the design of an optical interleaver based on retardant crystals and a third-order optical interleaver with maximally flat passband was designed accordingly. The fabrication tolerance for the retardant crystals was simulated based on crosstalk consideration. Given crosstalk requirement of −25 dB, the tolerance for the crystal thickness and axis orientation was obtained as 0.91 μm and 0.51°, respectively.     

The system design and assemble for the high resolution telescope system with segmented aperture

A design method along with its results, for a space optical system with high resolution and wide field of view, is described. Such optical systems can be used in the infrared as well as visible configurations. The proposed design is based on an on-axis Ritchey-Chrétien system with corrected lens element while the primary mirror is a segmented aperture. Here the on-axis concept allows wide-field enabling a variety of observations designed for the multi-object spectrometer instruments, optimized for low scattering and low emission of light. The use of segmented mirrors in the optical system allows adopting any method for its fabrication purposes. Segment results are discussed and the image quality of the design based on these results is evaluated in this paper.     

An adaptive reservation scheme for optical network

In this paper, the various Optical Burst Switching (OBS) reservation schemes have been examined in order to reduce data loss caused by either channel scheduling or resources. A novel multi-service OBS edge node with synchronized bandwidth reservation mechanism (SRM) has been proposed, which enables high-speed network transport nodes to dynamically reserve bandwidth needed for active data burst flows. The performance of the proposed mechanism is evaluated by means of NS-2 simulation. The results show that the packet delay is kept within the constraint for each traffic flow and the performance metrics such as burst loss rate, throughput and fairness are remarkably improved.     

Study on optical properties of aggregated ultra-small metal nanoparticles

We study the optical properties of dimer and septamer aggregates (in forms of being separated or merged) of ultra-small silver nanoparticles (a few nanometers in diameter) by calculating their extinction spectra and optical field distributions using the discrete dipole approximation method. Characteristics of their extinction spectra are identified due to different resonance modes dependent on the incidence states of light. Specifically, as polarization of the incidence light is perpendicular to the center-to-center lines of the nanoparticles, interaction of neighboring nanoparticles is in the “repulsive coupling mode”, which results in blue-shift of the resonance peak with decrease of the nanoparticle interspacing. While, whenever there is a projection of the polarization of incident light on the center-to-center lines of the nanoparticles, the “attractive coupling mode” between the nanoparticles dominates in their resonant interaction, which results in red-shift of the resonance peak with decrease of the nanoparticle interspacing. It is also shown that optical interaction of the ultra-small nanoparticles is only effective when their interspacings are approximately less than 15 nm, and particularly prominent when Coulomb interactions of the electronic charges of plasmons are active within a few nanometers.