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.

Applications of Lasers for Sensing and Free Space Communications: introduction to the feature issue.

This feature issue highlights the latest developments in laser-based sensing and free space communications. In total, 15 papers were published in Applied Optics, including an invited review paper that celebrates the legacy of David L. Fried.

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.

Generation of sub-100 fs ultraviolet pulses from a Kerr-lens mode-locked Ce:LiCAF laser.

We report the direct generation of mode-locked pulses as short as 91 fs from the broad-bandwidth gain medium of LiCaAlF6 (Ce:LiCAF) by combining Kerr-lens mode locking with synchronous pumping. The latter of these schemes, and the broad bandwidth of Ce:LiCAF, resulted in dispersion tuning of wavelength via cavity length in the spectral region of 290 nm; this mechanism facilitated a practical means of estimating intra-cavity dispersion, which was compensated for using a Brewster's-cut prism pair. The pulse duration was measured via split-beam asynchronous cross-correlation using a Ti:sapphire reference laser and a known time reference. From the Ce:LiCAF laser cavity, output powers of 110 mW and a 9% slope efficiency were achieved.

Femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss at wavelengths beyond 4 µm.

We report on the fabrication of, to the best of our knowledge, the first highly reflective fiber Bragg gratings for the 4 µm wavelength range. A second-order grating with a coupling coefficient (κ) of 230m-1, losses <0.25dB/cm, and a bandwidth of approximately 3 nm was inscribed into the core of a passive indium fluoride (InF3) fiber using a femtosecond (fs) laser. Thermal annealing of this grating at a temperature of 150°C for 90 min resulted in the enhancement of κ to 275m-1. Further, we show that InF3 fibers respond very differently to irradiation with fs laser pulses as compared to ZBLAN fibers and that this difference manifests itself in a significantly larger process window for inscription and in the formation of a more complex refractive index profile that is believed to be caused by the larger nonlinearity of InF3. This Letter paves the way to the development of new wavelength stabilized all-fiber mid-infrared lasers beyond 4 µm.

Numerical modelling and optimization of actively Q-switched waveguide lasers based on liquid crystal transducers.

We have recently experimentally demonstrated that a novel liquid crystal-based photonic transducer for sensing systems could be utilized as an active Q-switch in a miniaturised and integrated waveguide laser system. In this paper, we now present a comprehensive numerical modelling study of this novel laser architecture by deriving a set of equations that accurately describe the temporal optical response of the liquid crystal cell as a function of applied voltage and by combining this theoretical model with laser-rate equations. We validate the accuracy of this model by comparing the results with previously obtained data and find them in excellent agreement. This enables us to predict that under realistic conditions and moderate pump power levels of 500 mW, the laser system should be capable of generating peak power levels in excess of 1.1 kW with pulse widths of about 20 ns, corresponding to pulse energies > 20 µJ. We believe that such a low-cost and ultra-compact laser source could find applications ranging from trace gas sensing and LIDAR to material processing.

Optimized laser-written ZBLAN fiber Bragg gratings with high reflectivity and low loss.

We report the direct femtosecond laser inscription of type-I fiber Bragg gratings (FBGs) into the core of soft-glass ZBLAN fibers. We investigate and compare various fabrication methods such as single pass (line by line), double pass, and stacking (plane by plane) to create the highest reflectivity FBGs (99.98%) for mid-infrared (mid-IR) applications. In addition, we experimentally demonstrate how the parameters that influence the coupling coefficient, i.e., refractive index modulation and overlap factor, can be controlled in these gratings to specifically tailor the FBG properties. The performance of the direct-written type-I gratings after 6 h of annealing is further analyzed, and the reflectivity increases by approximately 10 dB. To the best of our knowledge, this is the first demonstration of temperature-stable mid-IR FBGs with highest coupling coefficient (464 m-1) and lowest loss (<0.5 dB/cm) without the use of an expensive phase mask.

Realization of aperiodic fiber Bragg gratings with ultrashort laser pulses and the line-by-line technique.

We demonstrate the fabrication of aperiodic fiber Bragg gratings (AFBGs) for their application as filter elements. Direct inscription was performed by focusing ultrashort laser pulses with an oil-immersion objective into the fiber core and utilizing the line-by-line technique for flexible period adaptation. The AFBGs inscribed allow for the suppression of 10 lines in a single grating and are in excellent agreement with simulations based on the specific design. Applications in astronomy for the suppression of hydroxyl emission lines are discussed.

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.

Compact integrated actively Q-switched waveguide laser.

A miniaturized deformed helix ferroelectric liquid crystal transducer cell was used in combination with a femtosecond laser inscribed active waveguide to realize a compact actively Q-switched laser source. The liquid crystal cell was controlled by a low-voltage frequency generator and laser pulse durations below 40 ns were demonstrated at repetition rates ranging from 0.1 kHz to 20 kHz and a maximum slope efficiency of up to 22%. This novel, integrated and low-cost laser source is a promising tool for a broad range of applications such as trace gas sensing, LIDAR, and nonlinear optics. To the best of our knowledge, this is the first demonstration of an actively Q-switched glass waveguide laser that has a user-variable repetition rate and can be fully integrated.

Emission properties of quantum dots in polymer optical fibres.

CdSe/ZnS core-shell quantum dots have been embedded within microstructured polymer optical fibres. The emission properties of quantum dots within fibres have been explored to show that variation in concentration, sample length and pumping regimes effects the emission characteristics of these quantum dots.