V. Balaswamy

A simplified architecture for high efficiency cascaded Raman fiber lasers

We propose a new, all-passive architecture for high-efficiency cascaded Raman conversion. We demonstrate this with a high-power, fifth-order cascaded Raman converter (from 1117nm to 1480nm) with output power of 64W and conversion efficiency of 60%.

A Diode Drive Mechanism for ``Always Resonant'' Pumping with Laser Diodes without Wavelength Locking

We demonstrate a simple to implement, drive scheme for standard laser diode modules (without wavelength locking) used for pumping rare-earth doped lasers and amplifiers. This scheme enables an “always-resonant” mode of operation. The deleterious effect accompanying power/current tuning - drifts of emission wavelength of the diodes from the peak absorption band of the gain medium is completely avoided. In this work, we demonstrate the drive mechanism and its performance in a fiber amplifier. We anticipate this scheme to have significant impact in enabling a cost-effective solution which achieves an optimal balance of efficiency, nonlinearity and reliability in laser systems.

Observation of a rainbow of visible colors in a near infrared cascaded Raman fiber laser and its novel application as a diagnostic tool for length resolved spectral analysis

In this work, we report and analyse the surprising observation of a rainbow of visible colors, spanning 390nm to 620nm, in silica-based, Near Infrared, continuous-wave, cascaded Raman fiber lasers. The cascaded Raman laser is pumped at 1117nm at around 200W and at full power we obtain ∼100 W at 1480nm. With increasing pump power at 1117nm, the fiber constituting the Raman laser glows in various hues along its length. From spectroscopic analysis of the emitted visible light, it was identified to be harmonic and sum-frequency components of various locally propagating wavelength components. In addition to third harmonic components, surprisingly, even 2nd harmonic components were observed. Despite being a continuous-wave laser, we expect the phase-matching occurring between the core-propagating NIR light with the cladding-propagating visible wavelengths and the intensity fluctuations characteristic of Raman lasers to have played a major role in generation of visible light. In addition, this surprising generation of visible light provides us a powerful non-contact method to deduce the spectrum of light propagating in the fiber. Using static images of the fiber captured by a standard visible camera such as a DSLR, we demonstrate novel, image-processing based techniques to deduce the wavelength component propagating in the fiber at any given spatial location. This provides a powerful diagnostic tool for both length and power resolved spectral analysis in Raman fiber lasers. This helps accurate prediction of the optimal length of fiber required for complete and efficient conversion to a given Stokes wavelength.

Simple modules for high efficiency conversion of standard ytterbium doped fiber lasers into octave spanning continuous-wave supercontinuum sources

We have demonstrated a ~34 W continuous wave supercontinuum using the standard telecom fiber (SMF 28e). The supercontinuum spans over a bandwidth of ~1000 nm (>1 octave) from 880nm to 1900 nm with a substantial power spectral density of >1mW/nm from 880-1350 nm and ~50-100mW/nm in 1350-1900 nm. The distributed feedback Raman laser architecture was used for pumping the supercontinuum which ensured high efficiency Raman conversions and helped in achieving a very high efficiency of ~44% for supercontinuum generation. Using this architecture, Yb laser operating at any wavelength can be used for generating the supercontinuum and this was demonstrated by using two different Yb lasers operating at 1117nm and 1085 nm to pump the supercontinuum.

Raman based power combining and wavelength conversion of high power ytterbium fiber lasers

In this work, we demonstrate an architecture to perform Raman-based power combining and simultaneous wavelength conversion of two independently controlled high-power Ytterbium doped fiber lasers operating at different wavelengths into a single laser line at the 1.5-micron band. Specifically, we have been able to achieve an in-band output power of ∼99W with a conversion of ∼64% of the quantum limited efficiency. This power combining is illustrated for different cases of the input wavelengths of the Ytterbium fiber laser. In each case, we have been able to demonstrate a power combining of >87 W in the final 1.5-micron band, with more than 85% of the fraction of the power residing in the final desired band.

Effect of intensity noise on stimulated Raman scattering in high power fiber lasers and amplifiers

Stimulated Raman Scattering is one of the primary nonlinearities limiting power scaling of high power fiber lasers and amplifiers. Conversion of signal light into the Raman generated Stokes component can result in several undesirable consequences such as temporal instabilities, spurious pulsing, beam quality degradation, focusing errors etc. most of which are catastrophic for the laser system. In this work, we investigate a novel and important aspect of Raman scattering in fiber lasers and amplifiers. Owing to intrinsic intensity noise at high powers, we demonstrate that the actual Raman gain can be substantially higher than the expected values calculated from mean powers. Using a physical model for intensity noise, we demonstrate very interesting connections between the laser bandwidth, dispersion in fiber and the actual Raman gain. This work enables further power scaling by providing design methodologies to control Raman scattering in fiber lasers and amplifiers.