Chip-based Brillouin processing for carrier recovery in coherent optical communications

Modern fiber-optic coherent communications employ advanced spectrally-efficient modulation formats that require sophisticated narrow linewidth local oscillators (LOs) and complex digital signal processing (DSP). Here, we establish a novel approach to carrier recovery harnessing large-gain stimulated Brillouin scattering (SBS) on a photonic chip for up to 116.82 Gbit/sec self-coherent optical signals, eliminating the need for a separate LO. In contrast to SBS processing on-fiber, our solution provides phase and polarization stability while the narrow SBS linewidth allows for a record-breaking small guardband of ~265 MHz, resulting in higher spectral-efficiency than benchmark self-coherent schemes. This approach reveals comparable performance to state-of-the-art coherent optical receivers without requiring advanced DSP. Our demonstration develops a low-noise and frequency-preserving filter that synchronously regenerates a low-power narrowband optical tone that could relax the requirements on very-high-order modulation signaling and be useful in long-baseline interferometry for precision optical timing or reconstructing a reference tone for quantum-state measurements.Comment: Part of this work has been presented as a postdealine paper at CLEO Pacific-Rim'2017 and OSA Optic

Physics Potential of the ICAL detector at the India-based Neutrino Observatory (INO)

The upcoming 50 kt magnetized iron calorimeter (ICAL) detector at the India-based Neutrino Observatory (INO) is designed to study the atmospheric neutrinos and antineutrinos separately over a wide range of energies and path lengths. The primary focus of this experiment is to explore the Earth matter effects by observing the energy and zenith angle dependence of the atmospheric neutrinos in the multi-GeV range. This study will be crucial to address some of the outstanding issues in neutrino oscillation physics, including the fundamental issue of neutrino mass hierarchy. In this document, we present the physics potential of the detector as obtained from realistic detector simulations. We describe the simulation framework, the neutrino interactions in the detector, and the expected response of the detector to particles traversing it. The ICAL detector can determine the energy and direction of the muons to a high precision, and in addition, its sensitivity to multi-GeV hadrons increases its physics reach substantially. Its charge identification capability, and hence its ability to distinguish neutrinos from antineutrinos, makes it an efficient detector for determining the neutrino mass hierarchy. In this report, we outline the analyses carried out for the determination of neutrino mass hierarchy and precision measurements of atmospheric neutrino mixing parameters at ICAL, and give the expected physics reach of the detector with 10 years of runtime. We also explore the potential of ICAL for probing new physics scenarios like CPT violation and the presence of magnetic monopoles.Comment: 139 pages, Physics White Paper of the ICAL (INO) Collaboration, Contents identical with the version published in Pramana - J. Physic

Low-loss Ti:sapphire waveguides fabricated by pulsed laser deposition

We report the fabrication and characterisation of Ti:sapphire films epitaxially grown on c-cut sapphire substrates by pulsed laser deposition (PLD). Deposition conditions have been studied extensively and optimised in order to produce high-performance optical waveguides. In particular we have studied the effects of different values of oxygen pressure, background gases and substrate temperature on the resultant surface roughness, composition, crystallinity, fluorescence and waveguide losses. For instance we found that Ti:sapphire films deposited in Ar feature higher fluorescence than those grown in O2 and N2 (see Figure 1) under the same deposition conditions: laser fluence F ~ 3.3 J/cm2, laser repetition rate f = 20 Hz, substrate temperature T ~ 1050°C, gas pressure P ~ 2.10-3 mbar, target-substrate distance d = 4 cm

Ultrafast waveguide lasers

Mode-locked lasers with repetition-rates in excess of 1 GHz have many applications in areas such as optical sampling, non-linear microscopy, and optical frequency metrology. To date there have been very few demonstrations of such high repetition-rate lasers with sub-picosecond operation and high average power. This thesis deals with the realisation of such compact sources using an integrated-optics platform. Waveguides offer certain key advantages, including a low threshold power, high slope efficiency, compatibility with monolithic devices, and a low mode-locking threshold, making them very promising candidates for such devices. Ultrafast multi-GHz waveguide lasers are described in this thesis, which are compact, mass-producible and low-cost making them very exciting candidates for industrial applications. Mode-locking was demonstrated in an ion-exchanged Yb:phosphate glass waveguide laser with integrated saturable absorber elements. An average output power as high as 80 mW was achieved at a pulse repetition frequency (PRF) of 4.9 GHz, at a wavelength around 1 µm and with pulse durations as short as 740 fs. Using shorter cavity lengths, waveguide lasers with PRFs of 10.4 GHz, 12 GHz and 15.2 GHz were achieved with pulse durations between 757 fs and 824 fs. A Gires Tournois Interferometer (GTI) effect was used to facilitate soliton mode-locking in the waveguides via accurate control of the gap between the waveguide and the output coupling mirror. This is a convenient technique to control the dispersion without introducing any extra elements in the laser cavity. Two further Yb-doped ultrafast laser hosts, RbTiOPO4 and KY(WO4)2, were investigated for their potential as ultrafast waveguide laser sources, having both been previously demonstrated as good bulk ultrafast systems. Laser action was demonstrated for the first time in an (Yb,Nb):RbTiOPO4 planar waveguide laser, fabricated by liquid-phase epitaxy. Ion-beam milling was then used to fabricate the first ever single-mode rib waveguides in (Yb,Nb):RbTiOPO4 fabricated by dry etching techniques but laser action was not possible due to propagation losses of ~3dB/cm. A systematic study of the reactive ion etching of RbTiOPO4 was then carried out to minimise the surface roughness, in an attempt to reduce the propagation losses. The first ever demonstration of single-mode waveguiding in (Yb,Nb):RbTiOPO4 fabricated by reactive ion etching was demonstrated but the propagation losses remained high. Using (Yb,Gd,Lu):KY(WO4)2 as a gain media, efficient laser action was demonstrated in an "inverted-rib" waveguide laser structure fabricated by ion-beam milling. This laser was found to have a threshold power as low as 13 mW and a maximum slope efficiency of 58% and showed characteristics of a pure 3-level laser by lasing at 981 nm. However, further loss reduction is again required in order for efficient ultrafast operation to be obtained in the future. Mode-locked waveguide laser operation was extended to the 1.5µm spectral region based on an ion-exchanged Er,Yb:phosphate glass waveguide laser using a novel SESAM based on a quantum dot in well (DWELL) structure. 2.5 ps pulses at a PRF of 4.8 GHz and an average output power of 9 mW were achieved. With a shorter waveguide sample, a PRF of 6.8 GHz with an average output power of 30 mW and pulse duration of 5.4 ps was achieved. The repetition-rate of the laser was finely tuned by controlling the pump power offering an attractive technique for enabling future frequency-comb stabilisation. This is the highest reported repetition-rate from a mode-locked waveguide laser at 1.5 µm and is also the first ever waveguide laser mode-locked by a quantum dot SESAM. Finally, as an initial step towards further extension to the 2µm spectral region, laser action was demonstrated, for the first time, in an ion-exchanged Tm:glass waveguide laser with a threshold power as low as 44 mW and a maximum slope efficiency of 6.8% around 1.9 µm. Designs for power-scaling of such sources have also been discussed in this thesis

An ion-exchanged thulium-doped germanate glass channel waveguide laser operating near 1.9 micron

Solid-state lasers operating in the eye-safe region near 2 micron are of considerable interest owing to various application areas such as remote sensing, spectroscopy and LIDAR. Thulium-based gain media have several attractive features including a broad emission bandwidth, a broad absorption band near 800 nm that can be diode pumped and the possibility of obtaining a quantum efficiency of up to 200% due to the process of cross-relaxation. Guided-wave devices can offer additional advantages of compactness and integration, as well as lower thresholds and high slope efficiencies if low propagation losses can be obtained. Such devices, when combined with integrated saturable absorber elements, can also be passively modelocked to generate femtosecond pulses with multi-GHz repetition rates

Towards high-power multi-GHz waveguide lasers

There has been a growing interest in the development of laser sources with high (>GHz) pulse repetition rates owing to their potential applications in areas such as nonlinear microscopy, optical sampling, frequency metrology, optical communications, optical arbitrary waveform generation and for the calibration of astronomical spectrographs (astro-combs). Ultrafast lasers based on low-loss waveguide geometry offer a combination of features (low-threshold operation, high efficiency and moderate non-linearities) which make them attractive for development of compact, low-cost, multi-GHz femtosecond sources