Novel Fiber Optic Devices & Subsystems

Pulsed & High Power Fiber Lasers:

High power fiber lasers and amplifiers have been extensively studied in the past few decades and a phenomenal increase in the peak (MW) as well as average power levels (kW) have been demonstrated. However, the power scaling of such fiber lasers is limited by several factors including fiber non-linear effects, occurrence of self pulsing and thermal mode instability. Our research is focused on developing high power CW and pulsed fiber laser sources. Master Oscillator Power Amplifier (MOPA) is usually a reliable and scalable approach, where a low power highly stable laser source (CW or pulsed) output is amplified using multiple amplifier stages to achieve the desired power levels. Q-switch fiber lasers is used to generate high power peak pulses by modulating the loss of laser cavity. Mode lock fiber laser is used to generate ultra-short (ps) pulsed by phase locking the longitudinal modes of the cavity.

Thermal mode Instability The large-mode area (LMA) double-clad fibers are used to increase the surface area of the fiber, thereby increase the threshold for non-linear effects such as SBS and SRS. However, the LMA fibers have been reported to exhibit an instability in the transverse modes of the fiber beyond a certain threshold average power, where the power from fundamental mode starts transferring from FM to HOM and vise-versa. The instability or exchange in power among the transverse modes does not reflect as a power fluctuation at the output. However it severly degrades the beam quality. The present work is towards the design of a multi-stage MOPA with a high average power to achieve power levels at which the thermal mode instability threshold have been reported before. The design of the high power amplifier is presented which includes four amplification stages where the parameters at each stage is optimised using RP-Fiber Power software such that the output of the final stage is > 500W.

Pulsed lasers Pulsed fiber lasers have a variety of applications such as laser processing, surgery, optical communications etc. The requirements of pulse characteristics and repetition rate are dependent on the application. There are 2 major methods for generation of laser pulse trains.

OAM Using higher order modes (HOMs) in a fiber which exhibit large effective modal area is a possible solution for addressing the above limitations. One such example is a class of modes known as Orbital angular momentum (OAM) modes which have higher mode area compared to Gaussian beams and hence expected to increase the threshold power required for the onset of non-linear mechanisms as well as thermal mode instability.

Optical beams with helical wavefronts have Poynting vector component in the azimuthal direction which results in orbital angular momentum of per photon in the direction of beam propagation, being the intrinsic charge of the OAM beam. The pitch and handedness of the helix determines the intrinsic charge type (positive/negative) of the OAM beam. In optical fibers, OAM modes can be realized by superposition of degenerate vector modes with necessary phase shift. Combining and modes with 900 shift i.e., = gives circularly polarized OAM mode with topological charge 1. Circular polarization is a manifestation of spin angular momentum (SAM) and hence in fiber OAM and SAM exists together.

Research Objectives:

  1. Generation of OAM beams:
  • Understand the properties of OAM beams by generating with SPP.
  • Generation of composite vector OAM beams.
  • Generation of OAM modes in fiber using fused fiber coupler technique.

2.Propagation and of OAM beams:

  • Understand the propagation dynamics of composite OAM beams in free-space.
  • Study the propagation dynamics of OAM modes in optical fiber of different index profiles.
  • Estimate the purity of OAM beams in fiber with Ring technique.

3.Amplification of OAM beams

  • Study the amplification of OAM beams of different topological charge.
  • Estimate the purity of amplified OAM beams for different topological charge.

Q-switched lasers High energy pulses with short durations find applications in material processing, laser based ranging and distributed sensing. Q-switching is a commonly used technique to generate such laser pulses by introducing a suitable loss-modulator in the laser cavity. The goals of this research work include the development of a complete simulation model of Q-switched fiber ring laser, experimental implementation of Q-switched laser in 1550 nm and 1064 nm wavelength range, and validation of simulation model against experimental results..

Mode-locked lasers Mode-locking technique is used to generate ultra-short pulses (ps-fs regime) at higher repetition rates (>MHz) for applications in communications. Mode-locking is realized by modulating the loss of the laser cavity at a repetition rate equal to the cavity round-trip frequency, thus phase-locking the longitudinal modes of the cavity. When a phase relationship is established between the modes, the output of the cavity is a pulse train of short pulse-widths and the spectrum of the output is a comb of frequencies..

Distributed Fiber Sensors:
Structural health monitoring (SHM) has become an essential part of capital-intensive structures such as bridges, dams, oil/gas pipelines and aircrafts for early detection of abnormalities thereby reducing maintenance cost and extending their lifetime. Monitoring of strain and temperature are quite helpful for SHM of these structures. Distributed sensors are advantageous to monitor these large structures. Optical fiber based sensors are capable of distributed sensing where a single strand of optical fiber is used as sensor. We use Brillouin scattering which is a non-linear phenomenon for distributed measurement of strain and temperature.

The main focus of our work is on two objectives: Long range static measurements and short range dynamic measurements.
Long range static measurements:
Brillouin-based distributed optical fiber sensing systems have been one of the most widely attracted, particularly the Brillouin optical time-domain analysis (BOTDA) measurement scheme as they provide high precision distributed temperature and strain measurements along hundreds of thousands of measurement positions over large structures. Multitudes of features that we are focusing are as follows.

    • We have studied the acousto-optic interaction that leads to Brillouin process numerically and explored the multiple features in the Brillouin gain spectrum for temperature and strain discrimination.
    • We have implemented Golay complementary pairs for high spatial resolution in long range applications. We are working on pulse On/OFF time of the Golay sequence for improving the performance.

Short range dynamic measurements:
The objective is to detect dynamic strain variations at frequencies of kHz order over few tens of meters with cm-order spatial resolution and to demonstrate simultaneous multi-point dynamic strain sensing. The application related to this is vibration monitoring on an aircraft. We use Brillouin Optical Correlation Domain Analysis (BOCDA) technique for dynamic strain measurements which has proved to provide measurements with spatial resolution at the sub-meter levels. The salient results are as follows.

  • We have proposed and demonstrated proof of concept of external phase modulation based BOCDA for simultaneous multi-point dynamic sensing.
  • The ongoing work is on dynamic strain sensing.

Fiber Bragg Grating

Fiber Bragg grating (FBG) is an optical fiber with periodic variations in refractive index of the core along the optical propagation direction. When a broadband light is transmitted from one end of the FBG, FBG reflects a narrow band of light due to the refractive index variations in core, and transmits all other wavelengths. The reflected wavelength, termed as Bragg wavelength (λB), depends on two parameters - grating period (Λ) and effective refractive index of fiber (neff).In the Grating Fabrication facility at IITM, Phase mask technique is used to fabricate the FBGs with different Bragg wavelengths such as 650 nm, 1060 nm, and different Bragg wavelengths in between 1530-1575 nm.

 

Structural Health Monitoring using FBGs
Fiber Bragg grating based technique uses feature-guided waves to detect anomalies or defects in plate structures with transverse bends. Ongoing research is on Structure health monitoring (SHM) of composites, which plays a vital role in Aerospace industry. Defect identification is a part of SHM. Composites which are light weight and stiff may give little or no warning before failing. These material failures like delamination will cause severe damage. Using guided waves and embedding FBG in laminates will help to detect the material defects at earlier stage thereby avoiding severe damage. Interrogation with guided waves are attractive as it provides defect identification and localization. FBGs are widely adapted in SHM due to their rugged wavelength-encoded transduction mechanism, immunity to EM interference, ability to sense multiple parameters like temperature and strain, compact in size and less cost compared to conventional sensors.

Detection of partial discharges in power transformers using FBG sensors:
The primary goal of any power distribution network is to guarantee uninterrupted power supply to the consumer. One of the critical components in such a power distribution network is the power transformer which is susceptible to breakdown primarily due to improper insulation design or poor quality of insulation structure or casing. As such, condition monitoring of transformer insulation forms an integral step in ensuring consistent operation of the power equipment. Partial discharges (PD), which consists of highly localized electric discharges occurring in the insulation serve as early indicators of such insulation degradation. Our research is focussed on using FBG sensors to detect the acoustic waves emitted from PD. Acoustic signals are captured in an in-house developed optical receiver using the tunable laser based interrogation technique. We plan to classify PD exploiting the fact that each type of PD exhibits a specific acoustic spectral signature. We have also carried out investigations on Cross Recurrence Plot Analysis based technique for localizing discharges.

Detecting Onset of Combustion Instability in Gas Turbines Through Fiber Optic Sensors: Combustion instability arises due to the coupling between unsteady heat release and acoustics of the chamber. Due to this coupling, high amplitude oscillations occur at the resonant acoustic mode of the combustor and . These oscillations are often self-sustaining and damaging to the system. In addition to causing extreme annoyance, combustion instability can result in hardware damage and can reduce the efficiency of the combustion system. Several approaches have been proposed to address this issue by finding precursors which can sense features that help to detect and characterize combustion instability While most of the existing methods utilize a single sensor to predict instability, most robust approach so far uses data from heterogeneous (e.g. pressure and chemi-luminescence) sensors and models spatio-temporal co-dependence among time series from the sensors to generate a data-driven precursor which is uniformly applicable across multiple experiment protocols with various premixing levels.

Along this line, we are proposing a method where the chemi-luminescence sensor may be replaced by fiber optic bundle and the pressure sensor can be replaced by the FBG which offers advantages like immunity to electromagnetic interference, small size and multiplexing capability. In short, we are planning to provide a complete fiber optic solution for predicting combustion instability. Ongoing work is mainly to overcome the challenge faced by sensors in high temperature environment of the combustor.

Generation, Propagation and Purity Estimation of Optical Beams carrying Orbital Angular momentum

High power fiber lasers and amplifiers have been extensively studied in the past few decades and a phenomenal increase in the peak (MW) as well as average power levels (kW) have been demonstrated. However, power scaling of such fiber lasers are limited by several factors including fiber non-linear effects and thermal mode instability. Using higher order modes (HOMs) which exhibit large effective modal area in an optical fiber is a possible solution for addressing the above limitations. One such example is a class of modes carrying orbital angular momentum (OAM) known as OAM modes, which have higher mode area compared to the fundamental Gaussian beam and hence are expected to increase the threshold power required for the onset of non-linear mechanisms as well as thermal mode instability. Optical beams carrying OAM typically exhibit helical wavefronts with a Poynting vector component in the azimuthal direction, which results in orbital angular momentum of per lh/2(pi) photon in the direction of beam propagation, l being the intrinsic charge of the OAM beam. The pitch and handedness of the helix determines the intrinsic charge type (positive/negative) of the OAM beams. In optical fibers, OAM modes can be realized by superposition of degenerate vector modes with necessary phase shift. Combining HE(even) and HE(odd) modes with 90(deg) shift gives circularly polarized OAM mode. Circular polarization is a manifestation of spin angular momentum (SAM) and hence OAM and SAM are coupled with each other in optical fiber. SAM can be right (sigma minus) or left (sigma plus) circular. l = 2 OAM modes are shown below.

l = 2, sigma plus

l = -2, sigma minus

l = 2, sigma minus

l = -2, sigma plus

                       

Generation

Here, we experimentally demonstrate an all-fiber technique for the excitation of an OAM mode using a fused fiber coupler. Our approach is based on mode-selective coupling between different modes in two dissimilar fibers. The phase matching condition is studied using the COMSOL Multiphysics® eigen mode solver to estimate the required ratio between the cladding diameter of the SMF and that of the few mode fiber. The schematic diagram of the experimental setup of fused coupler and the generated OAM mode of topological charge is shown in the below figure. Reference: "Orbital angular momentum beam excitation using an all-fiber weakly fused mode selective coupler," S. Pidishety, S. Pachava, P. Gregg, S. Ramachandran, G. Brambilla, and B. Srinivasan, Opt. Lett. 42, 4347-4350 (2017); https://doi.org/10.1364/OL.42.004347 Propagation We use modified polarizing Sagnac interferometer to generate composite OAM beams. We analyze the propagation dynamics of the composite beam with a perturbation. A knife edge is placed immediately after the lens to block a part of the beam as shown in below and the beam profile is captured along the propagation path. It is clearly evident that the truncated beam regained its original intensity structure in the Rayleigh range. To understand the above phenomenon, we experimentally observed the propagation dynamics of the two orthogonal components of the composite beam. It is clearly observed that the +l and -l charge vortex beams rotate in opposite directions, but at the same rate. At the Rayleigh range, the two vortex beams appear at diametrically opposite orientation and the “self-healing” behaviour is observed. Reference: "Investigation of propagation dynamics of truncated vector vortex beams," P. Srinivas, C. Perumangatt, Nijil Lal, R. P. Singh, and B. Srinivasan, Opt. Lett. 43, 2579-2582 (2018); https://doi.org/10.1364/OL.43.002579 Purity Estimation: Purity estimation is an important aspect that needs careful attention while investigating the generation as well as propagation of OAM beams. In our work, we address this need through the demonstration of an optical correlation technique that incorporates both amplitude as well as phase structures of LG modes to accomplish the modal decomposition. In modal decomposition method, any scalar light beam (U) can be represented by a superposition of LGl,p modes with corresponding complex weights Wl,p. The complex weight of a mode is calculated by optically correlating the input beam with the complex conjugate of the mode. The experimental implementation is shown in the below figure. In our experiments, the SLM is partitioned into two equal halves. One half is programmed to generate the required LG beam and the other half is programmed to perform the dot product operation. We demonstrate the use of the optical correlation algorithm for decomposing a composite beam consisting of 10 different LG modes (azimuthal mode indices l = −2 to +2 and radial mode indices p = 0, 1) and the results are shown in the below figure.

Reference: "Modal decomposition of Laguerre Gaussian beams with different radial orders using optical correlation technique," Srinivas Pachava, Awakash Dixit, and B. Srinivasan, Opt. Express 27, 13182-13193 (2019);https://doi.org/10.1364/OE.27.013182

Please use the link to view the video of OAM beams: https://youtu.be/WFAKQlNQ8T4