Delivering high peak powers from fiber lasers is limited by the accumulation of nonlinear effects due to the high optical intensities and the long interaction lengths of fibers. Peak power scaling at 2 μm is limited by modulation instability (MI), which is not found for 1 μm sources. This work investigates the performance of a spectrally broadband, nanosecond pulsed thulium-doped fiber laser. The average power and pulse energy performance of the single-mode amplifier delivers >20 W and ~280 μJ. A variable spectral filter is incorporated to study the onset of MI and subsequent spectral broadening as a function of seed linewidth. It is observed that MI-induced spectral broadening is enhanced for larger linewidths. However, when the seed linewidth is increased beyond >10 nm, this trend is reversed. A fiber amplifier model including MI (treated as degenerate four-wave mixing) simulates a parametric gain bandwidth of ~900 GHz for this amplifier configuration, which is equivalent to ~11.5 nm at the 1960 nm center wavelength. Therefore, the decrease in spectral broadening for seed linewidths <10 nm is due to a reduced overlap with the MI gain bandwidth. The capability to scale 2 μm fiber lasers to high powers is strongly dependent on the spectral quality of the seed. Any power initially located within the MI gain bandwidth will degrade performance, and must be considered for power scaling.
Due to the intrinsic absorption edge in silica near 2.4 μm, more exotic materials are required to transmit laser power in the IR such as fluoride or chalcogenide glasses (ChGs). In particular, ChG fibers offer broad IR transmission with low losses < 1 dB/m. Here, we report on the performance of in-house drawn multi-material chalcogenide fibers at four different infrared wavelengths: 2053 nm, 2520 nm and 4550 nm. Polymer clad ChG fibers were drawn with 12.3 μm and 25 μm core diameters. Testing at 2053 nm was accomplished using a > 15 W, CW Tm:fiber laser. Power handling up to 10.2 W with single mode beam quality has been demonstrated, limited only by the available Tm:fiber output power. Anti-reflective coatings were successfully deposited on the ChG fiber facets, allowing up to 90.6% transmission with 12.2 MW/cm2 intensity on the facet. Single mode guidance at 4550 nm was also demonstrated using a quantum cascade laser (QCL). A custom optical system was constructed to efficiently couple the 0.8 NA QCL radiation into the 0.2 NA ChG fiber, allowing for a maximum of 78% overlap between the QCL radiation and fundamental mode of the fiber. With an AR-coated, 25 μm core diameter fiber, >50 mW transmission was demonstrated with > 87% transmission. Finally, we present results on fiber coupling from a free space Cr:ZnSe resonator at 2520 nm.
This work studies the accumulated nonlinearities when amplifying a narrow linewidth 2053 nm seed in a single mode Tm:fiber amplifier. A <2 MHz linewidth CW diode seed is externally modulated using a fiberized acousto-optic modulator. This enables independent control of repetition rate and pulse duration (>30 ns). The pulses are subsequently amplified and the repetition rate is further reduced using a second acousto-optic modulator. It is well known that spectral degradation occurs in such fibers for peak powers over 100's of watts due to self-phase modulation, four-wave mixing, and stimulated Raman scattering. In addition to enabling a thorough test bed to study such spectral broadening, this system will also enable the investigation of stimulated Brillouin scattering thresholds in the same system. This detailed study of the nonlinearities encountered in 2 μm fiber amplifiers is important in a range of applications from telecommunications to the amplification of ultrashort laser pulses.
Thulium and holmium have become the rare earth dopants of choice for generating 2 micron laser light in silica fiber. The majority of Tm:fiber lasers are pumped with high power diodes at 790nm and rely upon cross-relaxation processes to achieve optical-to-optical efficiencies of 55-65%. Tm:fiber lasers can also be pumped at <1900nm by another Tm:fiber laser to minimize quantum defect, reaching efficiencies >90%. Ho:fiber lasers are similarly pumped by Tm:fiber lasers at 1900-1950nm, with <70% typical efficiency. In this work, Tm:fiber and Ho:fiber lasers are in-band pumped using the same experimental setup to directly compare their performance as 2 micron sources.
By utilizing photon energies considerably smaller than the semiconductors’ energy band gap, space-selective modifications can be induced in semiconductors beyond the laser-incident surface. Previously, we demonstrated that back surface modifications could be produced in 500-600 μm thin Si and GaAs wafers independently without affecting the front surface. In this paper, we present our latest studies on trans-wafer processing of semiconductors using a self-developed nanosecond-pulsed thulium fiber laser operating at the wavelength 2 μm. A qualitative study of underlying physical mechanisms responsible for material modification was performed. We explored experimental conditions that will enable many potential applications such as trans-wafer metallization removal for PV cell edge isolation, selective surface annealing and wafer scribing. These processes were investigated by studying the influence of process parameters on the resulting surface morphology, microstructure and electric properties.
Ultra-large mode area thulium-doped photonic crystal fibers (Tm:PCF) have enabled the highest peak powers in 2
micron fiber laser systems to date. However, Tm:PCFs are limited by slope efficiencies of <50% when pumped with 790
nm laser diodes. A well-known alternative is pumping at 1550 nm with erbium/ytterbium-doped fiber (Er/Yb:fiber)
lasers for efficiencies approaching ~70%. However, these 1550 nm pump lasers are also relatively inefficient
themselves. A recently demonstrated and more attractive approach to enable slope efficiencies over 90% in thuliumdoped
step-index fibers is resonant pumping (or in-band pumping). This utilizes a high power thulium fiber laser
operating at a shorter wavelength as the pump. In this manuscript, we describe an initial demonstration of resonant
pumping in Tm:PCF. While the extracted power was still in the exponential regime due to pump power limitations, slope
efficiencies in excess of ~64 have been observed, and there is still room for improvement. These initial results show
promise for applying resonant pumping in Tm:PCF to improve efficiencies and facilitate high power scaling in ultralarge
mode area systems.
Within the past 10 years, thulium (Tm)-doped fiber lasers have emerged as a flexible platform offering high average power as well as high peak power. Many of the benefits and limitations of Tm:fiber lasers are similar to those for ytterbium (Yb)-doped fiber lasers, however the ~2 µm emission wavelength posses unique challenges in terms of laser development as well as several benefits for applications. In this presentation, we will review the progress of laser development in CW, nanosecond, picosecond, and femtosecond regimes. As a review of our efforts in the development of power amplifiers, we will compare large mode area (LMA) stepindex and photonic crystal fiber (PCF) architectures. In our research, we have found Tm-doped step index LMA fibers to offer relatively high efficiency and average powers at the expense of fundamental mode quality. By comparison, Tm-doped PCFs provide the largest mode area and quasi diffraction-limited beam quality however they are approximately half as efficient as step-index fibers. In terms of defense related applications, the most prominent use of Tm:fiber lasers is to pump nonlinear conversion to the mid-IR such as supercontinuum generation and optical parametric oscillators/amplifiers (OPO/A). We have recently demonstrated Tm:fiber pumped OPOs which generate ~28 kW peak power in the mid-IR. In addition, we will show that Tm:fiber lasers also offer interesting capabilities in the processing of semiconductors.
We report on the performance of a prototype pump combiner for use with thulium-doped photonic crystal fiber (PCF). This platform is attractive for “all-fiber” high energy and high peak power laser sources at 2 μm. We will report on the performance of this integrated amplifier in comparison to free space amplification in Tm:PCF. In particular, we carefully look for spectral/temporal modulation resulting from multimode interference between fundamental and higher order transverse modes in the amplifier to evaluate this for ultrashort chirped pulse amplification. The slope efficiency for the all-fiber amplifier is 22.1 %, indicating the need for further improvement. However, an M2 < 1.07 demonstrates excellent beam quality, as well as amplified polarization extinction ratios of ~25 dB.
Optical trapping of single biological cells has become an established technique for controlling and studying
fundamental behavior of single cells with their environment without having "many-body" interference. The development
of such an instrument for optical diagnostics (including Raman and fluorescence for molecular diagnostics) via laser
spectroscopy with either the "trapping" beam or secondary beams is still in progress. This paper shows the development
of modular multi-spectral imaging optical tweezers combining Raman and Fluorescence diagnostics of biological cells.