Raghuraman Sidharthan

Mid‐infrared (IR) supercontinuum (SC) lasers are important in applications such as pollution detection, stand‐off detection, and non‐destructive testing. The performance in many applications is limited by the noise level of the supercontinuum laser. High noise typically results in low sensitivities or a need for long integration times. In this paper, a simple technique to reduce the noise of high noise soliton‐based SC sources is introduced by adding a short piece of normal dispersion fiber to force the spectrally distributed solitons to spectrally broaden through self‐phase modulation and thereby overlap to average out the noise. The noise reduction is demonstrated experimentally and numerically using a ZBLAN fiber based mid‐IR SC source and adding a short piece of highly nonlinear arsenic‐sulfide fiber. However, the method is generally applicable to any soliton‐based near‐IR or mid‐IR SC source. Its efficiency is underlined by experimentally comparing it to SC generation in fibers in which a second zero‐dispersion wavelength provides the spectral alignment noise reduction mechanism.

We present an efficient 976 nm fiber laser achieved in a step index large-mode-area (LMA) fiber by 915 nm diode pumping. The 976 nm lasing was enabled using spectroscopic engineering in a fiber design that helps suppress the competing 1030 nm gain. Our study shows, alumino-phospho-silicate (Al:P) is advantageous for shaping a gain spectrum in favor of 976 nm compared to other widely used compositions, hence facilitating otherwise the challenging 976 nm lasing. In addition, a germanium (Ge)-cladding was introduced to achieve LMA and low numerical aperture step-index fiber, permitting a wavelength selective bending technique. By applying the bending technique, an efficient 976 nm lasing was achieved with up to 15 W output power (pump power limited) with 28% slope efficiency in the step-index LMA fiber.

We present mode-selection in a laser and an amplifier from a multicore Ytterbium (Yb) doped step-index fiber with six large mode area and low numerical aperture (NA) cores, fabricated using a combination of chelate delivery system (CDS) and stack and draw technique. The six cores are placed in a ring shape with each core having a diameter of ~19 μm and NA of ~0.068, or V-number of ~3.8 at 1.06 μm. The cores are placed with a 1.1 pitch-to-core ratio, which efficiently promotes supermode generation and creates a good modal separation to facilitate mode selection. Mode selection of the multicore fiber is investigated in a laser as well as an amplifier configuration. With an intracavity pinhole, we are able to select the fundamental inphase supermode for lasing up to 33.9W output power with a 52% slope efficiency. In the amplifier configuration, a Gaussian beam from a seed source is successfully coupled to the fundamental supermode in the multicore fiber. The seed source is amplified to ~43.5W from 0.14 W input power.

We report on demonstration of high energy pulse laser at 2 μm by employing a normal dispersion fiber attained by a waveguide dispersion engineering. The normal dispersion fiber has a W-type refractive index profile, with a Thulium(Tm)-doped core. In contrast to a strong anomalous dispersion at 2 μm in a step-index fiber, the design allows strong waveguide dispersion that can generate normal dispersion at the Tm emission band. Mode area scalability of this fiber is discussed under the normal dispersion requirement. The Tm-doped fiber (TDF) is deployed in a ring cavity to produce mode-locked pulses near 2 μm. Subsequently, the pulse is amplified with more than 27 dB gain through the normal dispersion TDF. The output pulse energy reaches up to ~525 nJ at 1852 nm without pulse breaking due to the engineered dispersion. The achieved pulse energy is record high for all-fiber configuration near 2 μm, to our knowledge.

We report a fabrication route to achieve highly GeO₂-doped silica fibers with GeO₂ molar concentration as high as 74% using the conventional MCVD process. Dispersion control and supercontinuum generation in 2.1 – 2.8µm are demonstrated.