Axel Schülzgen

PbS quantum-dot-doped glasses for ultrashort-pulse generation

We investigate the use of PbS quantum-dot-doped glasses as saturable absorbers for ultrashort-pulse lasers by means of absorption bleaching experiments and numerical analysis of the pulse shaping process using the Haus’ master equation. We explain the mode-locking mechanism and the limitations of these absorbers. The generation of transform-limited fs pulses is predicted by soliton mode locking initiated by the absorption saturation of higher excited states of the quantum-dot saturable absorber.

Phase locking and in-phase supermode selection in monolithic multicore fiber lasers.

We report a compact multicore fiber laser that utilizes an all-fiber approach for phase locking and in-phase supermode selection. By splicing passive coreless fibers of controlled lengths to both ends of an active 19-core fiber, we demonstrate that the fundamental in-phase supermode can be selectively excited with a completely monolithic fiber device, instead of conventional free-space and bulk optics, to achieve phase-locked operation for a multiemitter laser device.

Whispering-gallery-mode microring laser using a conjugated polymer

We observed laser emission in whispering gallery modes using a microring composed of a semiconducting polymer poly[2,5-bis-(2′-ethylhexyloxy)-p-phenylenevinylene coated on an etched fiber under transient and quasisteady-state pumping conditions. The threshold for laser oscillation was 1 mJ/cm2 (0.1 MW/cm2) and 30 μJ/cm2 (300 MW/cm2) for nanosecond and femtosecond excitation, respectively. The laser output showed superlinear dependence on the excitation energy above the threshold. The demonstration of lasing under quasisteady-state pumping shows the possibility to develop electrically pumped polymer lasers.

Hole-Assisted Few-Mode Multicore Fiber for High-Density Space-Division Multiplexing

A seven-core few-mode multicore fiber in which each core supports both the LP01 mode and the two degenerate LP11 modes has been designed and fabricated for the first time, to the best of our knowledge. The hole-assisted structure enables low inter-core crosstalk and high mode density at the same time. LP01 inter-core crosstalk has been measured to be lower than -60 dB/km. LP11 inter-core crosstalk has been measured to be around -40 dB/km using a different setup. The LP11 free-space excitation-induced crosstalk is simulated and analyzed. This fiber allows multiplexed transmission of 21 spatial modes per polarization per wavelength. Data transmission in LP01/LP11 mode over 1 km of this fiber has been demonstrated with negligible penalty.

Multicore fiber sensor for high-temperature applications up to 1000°C

A novel high temperature sensor based on customized multicore fiber (MCF) is proposed and experimentally demonstrated. The sensor consists of a short, few-centimeter-long segment of MCF spliced between two standard single-mode fibers. Due to interference effects, the transmission spectrum through this fiber chain features sharp and deep notches. Exposing the MCF segment to increasing temperatures of up to 1000°C results in a shift of the transmission notches toward longer wavelengths with a slope of approximately 29  pm/°C at lower temperatures and 52  pm/°C at higher temperatures, enabling temperature measurements with high sensitivity and accuracy. Due to its compact size and mechanical rigidity, the MCF sensor can be subjected to harsh environments. The fabrication of the MCF sensor is straightforward and reproducible, making it an inexpensive fiber device.

Generation of 9.3-W multimode and 4-W single-mode output from 7-cm short fiber lasers

We generate 9.3-W continuous-wave 1535-nm multimode output from a 7.0-cm short-length Er-Yb codoped phosphate fiber laser. A slope efficiency of 29% is obtained at pump powers below 27 W. Very high output power per unit fiber length of 1.33 W/cm is achieved. From another 7.1-cm Er-Yb codoped fiber laser, 4.0-W single-transverse-mode output with M/sup 2//spl ap/1.1 is generated.

Femtosecond pulsed laser micromachining of glass substrates with application to microfluidic devices.

We describe a technique for surface and subsurface micromachining of glass substrates by using tightly focused femtosecond laser pulses at a wavelength of 1660 nm. A salient feature of pulsed laser micromachining is its ability to drill subsurface tunnels into glass substrates. To demonstrate a potential application of this micromachining technique, we fabricate simple microfluidic structures on a glass plate. The use of a cover plate that seals the device by making point-to-point contact with the flat surface of the substrate is necessary to prevent the evaporation of liquids in open channels and chambers. Methods for protecting and sealing the micromachined structures for microfluidic applications are discussed.

Room-temperature gain at 1.3 μm in PbS-doped glasses

We report on room-temperature optical gain at the ground exciton transition of PbS quantum-dot-doped glasses while optical pumping into the next-higher exciton resonance. The material gain in the quantum dots is as large as 80 cm−1. The dot-size selective excitation provides tunability of the optical gain. This is demonstrated by tuning the gain from 1317 to 1352 nm by changing the pump wavelength from 900 to 980 nm.

Generation of watt-level single-longitudinal-mode output from cladding-pumped short fiber lasers

We generate as much as 1.6 W of continuous-wave 1550 nm single-longitudinal-mode output from a cladding pumped Er-Yb codoped phosphate fiber laser. This power is to our knowledge among the highest in single-longitudinal-mode fiber lasers. The narrowband fiber Bragg grating output coupler is demonstrated to be an effective element for providing the single-longitudinal-mode selection.

Single-frequency fiber oscillator with watt-level output power using photonic crystal phosphate glass fiber

Utilizing phosphate glass fiber with photonic crystal cladding and highly doped, large area core a cladding-pumped, single-frequency fiber oscillator is demonstrated. The fiber oscillator contains only 3.8 cm of active fiber in a linear cavity and operates in the 1.5 micron region. Spectrally broad, multimode pump light from semiconductor laser diodes is converted into a single-mode, single-frequency light beam with an efficiency of about 12% and the oscillator output power reached 2.3 W.