Numerical modeling of Dy3+-doped aluminosilicate fiber lasers for yellow light emission.

Numerical simulations of D y 3+-doped aluminosilicate fiber lasers for yellow light emission are presented. The 4 F 9/2→6 H 13/2 laser transition emitting at approximately 580 nm has been developed experimentally with 445 nm diode pumping and shows promise for higher output power in both silicate and in particular fluoride glass hosts. In this report, we focus on accumulating the published spectroscopic data in order to quantify cross relaxation (CR) in each of these hosts and use it to estimate its role in the laser dynamics. The model involves calculation of the branching ratios, and radiative and nonradiative decay rates and compares well with reported experimental results. We show the important role of the background losses on previous laser performance and the relatively strong increase in the laser threshold as a result of CR despite the moderately low D y 3+ concentrations that have been experimentally tested.

High-efficiency fluoroindate glass fiber laser.

We report the high-efficiency operation of a 3.05 µm dysprosium-doped fluoroindate glass fiber laser that is in-band pumped at 2.83 µm using an erbium-doped fluorozirconate glass fiber laser. The demonstrated slope efficiency of the free-running laser of 82% represents approximately 90% of the Stokes efficiency limit; a maximum output power of 0.36 W, the highest for a fluoroindate glass fiber laser, was recorded. Narrow-linewidth wavelength stabilization at 3.2 µm was achieved by utilizing a first-reported, to the best of our knowledge, high-reflectivity fiber Bragg grating inscribed in the Dy3+-doped fluoroindate glass. These results lay the foundation for future power-scaling of mid-infrared fiber lasers using fluoroindate glass.

Scattering under the Rayleigh-Debye-Gans condition for transparent glass ceramics.

We reviewed the various theoretical relations that quantify light scattering under the Rayleigh-Debye-Gans (RDG) approximation and applied these relations to calculate scattering within transparent glass ceramics (TGCs) composed of large nanocrystals within a glass matrix. For a more realistic picture of scattering, we included material dispersion of the crystals and glasses across the transparency range of these materials by way of the Sellmeier equation. We first selected a number of crystal-glass sets that are near-index-matched in the visible and near-IR to fulfill one of the RDG criterion. We found that the various forms of scattering under the RDG approximation differ significantly across the visible and near-IR. We also found that the inclusion of material dispersion significantly changes the trends in the calculated scattering cross section across the studied wavelength range. Overall, we found that calculation of the scattering cross section is highly dependent on the chosen theoretical relation and that the inclusion of material dispersion is vital to better understand scattering loss in this new class of optical materials.

Picosecond pulse formation in the presence of atmospheric absorption.

Mode-locked mid-infrared (MIR) fiber laser research has been dominated by the generation of pulses in the picosecond regime using saturable absorbers (SAs) and more recently frequency shifted feedback (FSF). Despite the significant emphasis placed on the development of materials to serve as the SAs for the MIR, published pulse durations have been substantially longer than what has been reported in the near-infrared (NIR). In this report we present experimental data supporting the view that the majority of demonstrations involving SAs and FSF have been limited by the presence of molecular gas absorption in the free-space sections of their cavities. We show that the pulse duration is directly linked to the width of an absorption-free region of the gaseous absorption profile and that the resulting optical spectrum is nearly always bounded by strong absorption features.

Ultrafast 3.5 µm fiber laser.

We report, to the best of our knowledge, the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 µm. The laser uses dual-wavelength pumping and nonlinear polarization rotation to produce 3.5 µm wavelength pulses with minimum duration of 580 fs at a repetition rate of 68 MHz. The pulse energy is 3.2 nJ, corresponding to a peak power of 5.5 kW.

Long wavelength operation of a dysprosium fiber laser for polymer processing.

We demonstrate operation of a mid-infrared dysprosium-doped fiber laser with emission at 3388 nm, representing the longest wavelength yet achieved from this class of laser, to the best of our knowledge. Oscillation far removed from the Dy3+ gain peak around 3 µm is achieved through the design of a high feedback optical cavity employing a directly inscribed fiber Bragg grating as the output coupler. Laser performance is characterized by a slope efficiency with respect to injected pump power of 38% and maximum output power of 134 mW, an improvement of at least three orders of magnitude over prior attempts at long wavelength Dy3+ fiber laser operation. This wavelength coincides with a maximum in the absorption coefficient of PMMA, which we exploit for preliminary demonstration of the utility of this source in polymer processing.

Fiber-based sources of coherent MIR radiation: key advances and future prospects (invited).

The mid-infrared (MIR) represents a large portion of the electromagnetic spectrum that is progressively being exploited for an enormous number of applications. Thermal imaging cameras, dental and skin resurfacing lasers, and narcotics detectors at airports are all mainstream examples involving the MIR, but potential applications of MIR technologies are much larger. Accessing the unique opportunities afforded by the MIR is critically dependent on the specific characteristics of MIR emitting sources that become available. In this review, we survey an important enabling technology to the opening up of MIR science and applications, namely that driven by fiber-based sources of coherent MIR radiation. In this review paper, we describe many of the key advances in the innovation and development of such sources over the past few decades and discuss many of the underlying science and technology issues that have resulted in specific recent source achievements, especially in light of new applications enabled by these new source capabilities. We also discuss a few specific anticipated future needs and some potentially disruptive approaches to future MIR fiber source development.

Electronically tunable picosecond pulse generation from Ho3+-doped fluoride fiber laser using frequency-shifted feedback.

In this Letter, we demonstrate electronically tunable picosecond (ps) pulse emission from the 5I6-5I7 transition of the Ho3+ ion by using an acousto-optic tunable filter. The holmium- and praseodymium-codoped ZBLAN fiber laser produced sub-50 ps pulses over a 100 nm tuning range, critically reaching a longest wavelength of 2.94 µm, which overlaps with the peak absorption of liquid water. Measured pulse energies of 8.1 nJ well exceed those expected from picosecond solitonic operation, suggesting possible application in ablative medicine. Furthermore, we present harmonically mode-locked operation of the oscillator, which indicates the possibility of expanding the capabilities of mid-infrared frequency shifted-feedback lasers through the ability to achieve higher pulse repetition rates.

Towards subdiffraction imaging with wire array metamaterial hyperlenses at MIR frequencies.

We describe the fabrication of metamaterial magnifying hyperlenses with subwavelength wire array structures for operation in the mid-infrared (around 3 µm). The metadevices are composed of approximately 500 tin wires embedded in soda-lime glass, where the metallic wires vary in diameter from 500 nm to 1.2 µm along the tapered structure. The modeling of the hyperlenses indicates that the expected overall losses for the high spatial frequency modes in such metadevices are between 20 dB to 45 dB, depending on the structural parameters selected, being promising candidates for far-field subdiffraction imaging in the mid-infrared. Initial far-field subdiffraction imaging attempts are described, and the problems encountered discussed.

Ultrafast mid-infrared fiber laser mode-locked using frequency-shifted feedback.

We demonstrate ultrashort pulse generation from a fluoride fiber laser co-doped with holmium and praseodymium. To date, the majority of work focused on short pulse generation from this class of fiber laser has employed loss modulators in the cavity, both real and artificial. In this Letter, we alternatively employ a frequency shifting element: an acousto-optic modulator (AOM) in the cavity. This results in mode-locked output of sub-5 ps pulses with 10 nJ of energy at a center wavelength of 2.86 µm and a pulse repetition frequency of 30.1 MHz, equating to a peak power of 1.9 kW. Additional experimental investigation of the relationship between frequency shift and cavity round trip offer insight into the complex underlying dynamics. As a complementary mode-locking technique to conventional loss modulation, this method of pulse-formation may greatly expand the design flexibility of pulsed mid-infrared fiber lasers.

Directly diode-pumped mid-infrared dysprosium fiber laser.

We report the demonstration of a Dy3+-doped ZBLAN fiber laser directly diode-pumped at 800 nm utilizing a codoping scheme with Tm3+ serving as the donor ion. In this initial demonstration, a modest output power of 12 mW is generated with a slope efficiency of 0.3% at an emission wavelength of 3.23 µm. Energy transfer dynamics are investigated through excited state lifetime analysis and comparison to resonant pumping of the Dy3+ upper state. The likely presence of an energy transfer upconversion process is identified, which has an undesirable effect on 3 µm class lasers but may be of significant benefit for future 4 µm systems.

Dysprosium-doped ZBLAN fiber laser tunable from 2.8 µm to 3.4 µm, pumped at 1.7 µm.

We demonstrate a mid-infrared dysprosium-doped fluoride fiber laser with a continuously tunable output range of 573 nm, pumped by a 1.7 µm Raman fiber laser. To the best of our knowledge, this represents the largest tuning range achieved to date from any rare-earth-doped fiber laser and, critically, spans the 2.8-3.4 µm spectral region, which contains absorption resonances of many important functional groups and is uncovered by other rare-earth ions. Output powers up to 170 mW are achieved, with 21% slope efficiency. We also discuss the relative merits of the 1.7 µm pump scheme, including possible pump excited-state absorption.

Emission beyond 4 µm and mid-infrared lasing in a dysprosium-doped indium fluoride (InF3) fiber.

Optical emission from rare-earth-doped fluoride fibers has thus far been limited to less than 4 µm. We extend emission beyond this limit by employing an indium fluoride (InF3) glass fiber as the host, which exhibits an increased infrared transparency over commonly used zirconium fluoride (ZBLAN). Near-infrared pumping of a dysprosium-doped InF3 fiber results in broad emission centered around 4.3 µm, representing the longest emission yet achieved from a fluoride fiber. The first laser emission in an InF3 fiber is also demonstrated from the 3 µm dysprosium transition. Finally, a frequency domain excited state lifetime measurement comparison between fluoride hosts suggests that multiphonon effects are significantly reduced in indium fluoride fiber, paving the way to more efficient, longer wavelength lasers compared to ZBLAN fibers.

Direct inscription of Bragg gratings into coated fluoride fibers for widely tunable and robust mid-infrared lasers.

We report the development of a widely tunable all-fiber mid-infrared laser system based on a mechanically robust fiber Bragg grating (FBG) which was inscribed through the polymer coating of a Ho3+-Pr3+ co-doped double clad ZBLAN fluoride fiber by focusing femtosecond laser pulses into the core of the fiber without the use of a phase mask. By applying mechanical tension and compression to the FBG while pumping the fiber with an 1150 nm laser diode, a continuous wave (CW) all-fiber laser with a tuning range of 37 nm, centered at 2870 nm, was demonstrated with up to 0.29 W output power. These results pave the way for the realization of compact and robust mid-infrared fiber laser systems for real-world applications in spectroscopy and medicine.

Tunable dysprosium laser.

We report the demonstration of a tunable dysprosium laser. The experiment employed in-band pumping of a Dy3+-doped fluoride fiber and a simple resonator design involving a ruled diffraction grating. The laser produced tuning between 2.95 and 3.35 µm, limited by the availability of optics.

Highly efficient mid-infrared dysprosium fiber laser.

A new, highly efficient and power scalable pump scheme for 3 µm class fiber lasers is presented. Using the free-running 2.8 µm emission from an Er3+-doped fluoride fiber laser to directly excite the upper laser level of the H13/26→H15/26 transition of the Dy3+ ion, output at 3.04 µm was produced with a record slope efficiency of 51%. Using comparatively long lengths of Dy3+-doped fluoride fiber, a maximum emission wavelength of 3.26 µm was measured.

Versatile and widely tunable mid-infrared erbium doped ZBLAN fiber laser.

We report on a long wavelength emitting rare earth doped fiber laser with the emission centered at 3.5 µm and tunable across 450 nm. The longest wavelength emission was 3.78 µm which is the longest emission from a fiber laser operating at room temperature. In a simple optical arrangement employing dielectric mirrors for feedback, the laser was capable of emitting 1.45 W of near diffraction limited output power at 3.47 µm. These emission characteristics complement the emissions from quantum cascade lasers and demonstrate how all infrared dual wavelength pumping can be used to access high lying rare earth ion transitions that have previously relied on visible wavelength pumping.

Ultrafast pulses from a mid-infrared fiber laser.

Ultrafast laser pulses at mid-infrared wavelengths (2-20 µm) interact strongly with molecules due to the resonance with their vibration modes. This enables their application in frequency comb-based sensing and laser tissue surgery. Fiber lasers are ideal to achieve these pulses, as they are compact, stable, and efficient. We extend the performance of these lasers with the production of 6.4 kW at a wavelength of 2.8 µm with complete electric field retrieval using frequency-resolved optical gating techniques. Contrary to the problems associated with achieving a high average power, fluoride fibers have now shown the capability of operating in the ultrafast, high-peak-power regime.

Stable, self-starting, passively mode-locked fiber ring laser of the 3 µm class.

We report a passively mode-locked Ho(3+)Pr(3+)-doped fluoride fiber laser, producing 6 ps pulses at a repetition rate of 24.8 MHz, with a peak power of 465 W. For the first time, a ring cavity was demonstrated in a fluoride fiber laser arrangement which was essential to the generation of stable and self-starting mode-locked pulses.

Aim at the bottom: directly exciting the lower level of a laser transition for additional functionality.

We introduce the concept of directly exciting the lower level of a laser transition in addition to the upper laser level for the provision of new possibilities for light emission from a fiber. In a first demonstration, using diode laser light at 1150 and 1950 nm, we respectively excite the upper and lower laser level of the 5I(6)→5I(7) transition (2.9 µm) of Ho3+-doped ZBLAN, demonstrating a power-scalable arrangement that can switch between free-running and superluminescent spectral output. The spectral composition of the gain-switched pulse derived from modulating the upper laser level pump light depends entirely on the degree of lower laser level excitation at 1950 nm.