S. Arun

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.

Low power generation of equalized broadband CW supercontinua using a novel technique incorporating modulation instability of line broadened pump

Continuous-wave(CW) supercontinuum sources find applications in various domains such as imaging, spectroscopy, test and measurement. They are generated by pumping an optical fiber with a CW laser in the anomalous-dispersion region close to its zero-dispersion wavelength. Modulation instability(MI) sidebands are created, and further broadened and equalized by additional nonlinear processes generating the supercontinuum. This necessitates high optical powers and at lower powers, only MI sidebands can be seen without the formation of the supercontinuum. Obtaining a supercontinuum at low, easily manageable optical powers is attractive for many applications, but current techniques cannot achieve this. In this work, we propose a new mechanism for low power supercontinuum generation utilizing the modified MI gain spectrum for a line-broadened, decorrelated pump. A novel two-stage generation mechanism is demonstrated, where the first stage constituting standard telecom fiber slightly broadens the input pump linewidth. However, this process in the presence of dispersion, acts to de-correlate the different spectral components of the pump signal. When this is sent through highly nonlinear fiber near its zero-dispersion wavelength, the shape of the MI gain spectrum is modified, and this process naturally results in the generation of a broadband, equalized supercontinuum source at much lower powers than possible using conventional single stage spectral broadening. Here, we demonstrate a ~0.5W supercontinuum source pumped using a ~4W Erbium-Ytterbium co-doped fiber laser with a bandwidth spanning from 1300nm to 2000nm. We also demonstrate an interesting behaviour of this technique of relative insensitivity to the pump wavelength vis-a-vis zero-dispersion wavelength of the fiber.

High-power tunable grating-free cascaded Raman fiber lasers

Cascaded Raman lasers enable high powers at various wavelength bands inaccessible with conventional fiber lasers. However, the input and output wavelengths are fixed by the multitude of fiber gratings in the system providing feedback. In this work, we demonstrate a high power, tunable, grating-free cascaded Raman fiber laser with an output power of >30W and a continuous tuning range from 1440nm to 1520nm. This corresponds to the entire in-band pumping region of Erbium doped gain media. Our system is enabled by three novel aspects – A grating free feedback mechanism for Raman lasers, a filter fiber to terminate the Raman cascade at the required wavelength band and a tunable high-power Ytterbium doped fiber laser as input. In this work, the primary system is a novel, cascaded Raman conversion module which is completely color blind to the input pump source and does wavelength band conversion at high efficiency. In addition, the conversion module also provides high spectral purity of greater than 85% at the required wavelength by terminating the cascade using high distributed losses provided by specialty Raman filter fibers. Using a high-power Ytterbium doped fiber laser continuously tuned from 1060nm to 1100nm and Raman filter fiber with distributed loss beyond 1520nm, we achieve a continuously tunable 1440nm to 1520nm laser corresponding to 5th or 6th Raman Stokes shift of the input. To the best of our knowledge, the reported powers at these wavelengths have been the highest for tunable Raman fiber lasers and is currently only limited by the input power.

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.

High power, equalized, continuous-wave supercontinuum generation using cascaded Raman fiber amplifiers

Continuous-wave (CW) super-continua have found applications in various domains such as sensing, spectroscopy, test and measurement and generation of broadly tunable lasers. CW supercontinua are implemented by high power pumping of a fiber in the anomalous dispersion regime, close to its zero-dispersion [1-3]. Pumped by Ytterbium doped fiber lasers, photonic crystal fibers are necessary since conventional silica fibers have zero dispersion only beyond 1310nm [1]. However, this approach has limitations arising from the complexities of using photonic crystal fibers. It is desirable to have an all fiber supercontinuum using standard silica fibers. However, sources at the 1.5micron band, where highly nonlinear fibers with zero dispersion are easily available are limited in power. Further, the power density achieved in such sources is poor in the normal dispersion region (between 1–1.5micron) [2-3]. Here, we demonstrate a novel technique for high power, high efficiency, supercontinua generation using the recently proposed cascaded Raman fiber amplifier architecture [4] for Raman lasers.