We have demonstrated a simple technique to make continuous wave (CW), tunable visible lasers. In this experiment, the output from a CW, tunable Raman fiber laser (RFL) is frequency doubled using LBO crystal in a single pass configuration. By tuning the output wavelength of RFL from 1.1-1.3um visible light in green to red wavelengths are generated. The efficiency and output power can be substantially improved by placing the crystal within a resonator cavity and pumping using a polarized RFL with narrower linewidth. Also, by tuning the operating wavelength of Yb laser (1060-1100 nm) broadband visible wavelength tunability can be achieved.
KEYWORDS: Supercontinuum sources, Fiber couplers, Continuous wave operation, Cladding, Optical filters, Collimators, High power fiber lasers, High power lasers, High power fiber coupled lasers, Optical pulse shaping, Femtosecond pulse shaping
Femtosecond pulse shaping, a widely used technology, enables the generation of light sources with arbitrary amplitude, phase and polarization in the ultrafast regime. This technology has seen applications in fiber and nonlinear optics, OCT, confocal microscopy, bandpass filtering etc. However, these shapers work primarily at low optical powers under the 100mW level, limited by in and out coupling optics, shaper configurations and optical design of the shaper. Recently, another exciting field of research has been high power fiber laser sources. Various high power fiber sources based on a variety of nonlinear phenomena such as high power supercontinuum sources, Raman lasers etc., have been demonstrated. However, owing to 10s of W class optical powers involved, Fourier shaping in this field has not been utilized effectively thereby limiting many potential applications. Here, we demonstrate a scalable design for a high power Fourier shaper in 4-f configuration capable of handling 20 W of CW lasers with a working bandwidth of over 450nm between 1-1.5 micron connecting the two very important Yb and Er emission windows. Our design implements a transmissive geometry thereby isolating input and output beams which is otherwise provided by fiber coupled circulators, a component unavailable at high power levels for a broadband source. Cladding mode stripping is effectively implemented to heat-sink the uncoupled laser light to ensure high power operations feasible. The design also takes accounts of modifications in fiber coupled collimators and amplitude masks to conform with the demands of high power fiber laser technology.
We recently reported the highest average power (70 W) from an octave spanning (880nm to >1900nm) CW supercontinuum source module constituted of standard telecom fiber and which can be pumped using an Ytterbium laser source operating at any wavelength. Since many applications demand a spectrally stable and repeatable supercontinuum, we have investigated the spectral stability of this supercontinuum source over an extended period of operation (over 15minutes). The overall change in spectral profile was investigated as a function of time and power cycling of the source. This experiment was carried out at 3 different wavelengths of the Ytterbium fiber laser pumping the supercontinuum and at 3 different output power levels. The RMS value for the spectral change was used as the metric for comparison. It was observed that the changes are small (within 1-dB) over the duration of the continuous run. We attribute this change in spectral profile with time, to the rise in temperature of fiber which reduces the nonlinear coefficient of fiber and can be potentially controlled by better heat sinking the fiber spool. By allowing the fiber to cool down to ambient temperature through power cycling tests, the spectral change was observed to be very small at < 0.4dB. The standard deviation of output power fluctuations measured using a fast photodetector (over several seconds of acquisition, at 1 us time interval) was ~3%. These results show that our supercontinuum source offers excellent spectral and power stability over an extended period of operation.
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.
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.
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.
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.
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.
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.
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