The Fruits of Our Labor

February 15, 2019


External optical feedback in vertical-cavity surface-emitting laser (VCSEL) is found to influence its output intensity. We studied the effect of the amount of total output polarisation feedback and polarisation-selective feedback on the output intensities of a VCSEL for a low-resolution sampling and for long temporal duration. A 40 μs resolution time-series correlation analysis is performed for different feedback conditions and the characteristic dynamics is investigated. We found a correlated fluctuation in VCSEL output for a moderate amount of total feedback and polarised feedback. The period of such fluctuations is found to be reduced from total feedback to the polarised feedback of the system

August 05, 2017


One key requirement for many cryptograhic schemes is the generation of random numbers. Sequences of random numbers are used at several stages of a standard cryptographic protocol. One simple example is a Vernam cipher, where a string of random numbers is added to message string to generate encrypted code. C = M ⊕ K. It has been mathematically shown that this simple scheme is unbreakable if key K is as long as M and is used only once. The security of a cryptosystem shall not be based on keeping the algorithm secret but solely on keeping the key secret. The security of a random number generator (RNG) is related to the difficulty of predicting its future sequence values from past values. The quality and unpredictability of secret data is critical to securing communication by modern cryptographic techniques. The generation of such data for cryptographic purposes typically requires an unpredictable physical source of random data. We studied a chaotic circuit which consisted of an inductor, capacitance, diode and thus used for the BB84 protocol. We have studied both pseudo random and true random number generators and evaluated them through various tests like frequency, correlation, NIST etc.

June 01, 2018


We report development of an integrated software module using LabVIEW and experimental setup for free space Optical Communication. The transmitter part of this module has in built option for reading a data from a file, convert it into a code to control a pair of diode lasers by switching them on and off. The receiver part of the software measures their polarization state. The protocol also contains hand-shaking, data transmission and also error correction codes which uses Hamming 7-4 protocol.

October 05, 2016

Atmospheric effects on Quaternary polarization encoding for free space communication

We have simulated atmospheric effects such as fog and smoke in laboratory environment to simulate depolarization due to atmospheric effects during a free space optical communication. This has been used to study noise in two components of Quaternary encoding for polarization shift keying. Individual components of a Quaternary encoding, such as vertical and horizontal as well as 45° and 135°, are tested separately and indicates that the depolarization effects are different for these two situation. However, due to a differential method used to extract information bits, the protocol shows extremely low bit error rates. The information obtained is useful during deployment of a fully functional Quaternary encoded PolSK scheme in free space.

September 24, 2014

Noise Characterization in Free Space Polarization Modulation Communication Using Simulated Atmospheric Conditions in Laboratory

We present a practical scheme for measurement-device-independent polarization shift keying using two state polarization encoding. Most of the previous work on optical free space laser communications through the atmosphere was concentrated on intensity modulated systems. However, polarization modulated systems may be more appropriate for such communication links, because the polarization seems to be the most stable characteristic of a laser beam while propagating through the atmosphere. Thus, a detailed comparison between intensity and polarization modulated systems is of much interest. We analyse the noise in presence of simulated smoke and fog conditions within laboratory and propose a practical scheme for extracting message from the received data. The proposed method uses only two detectors to analyse the polarizations and the practical definition of state of polarization enables a higher signal-to-noise ratio even in presence of depolaration elements in atmosphere such as fog and smoke. The system also takes into account existing imperfections within the experimental setup and hence is more robust.

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