151W monolithic diffraction-limited Yb-doped photonic bandgap fiber laser at ~978nm.

We demonstrate that the strong 4-level Yb emission in a fiber laser can be almost completely suppressed in an Yb all-solid double-clad photonic bandgap fiber, resulting in highly efficient high-power monolithic Yb fiber lasers operating at the 3-level system. We have achieved single-mode continuous wave laser output power of ~151W at ~978nm with an efficiency of 63% with respect to the launched pump power in a practical monolithic fiber laser configuration for the first time. The demonstrated power in this work are setting new records for diffraction-limited double-clad fiber lasers operating at ~978nm.

Highly efficient cladding-pumped single-mode three-level Yb all-solid photonic bandgap fiber lasers.

Efficient cladding-pumped three-level Yb fiber lasers are difficult to achieve due to the competing four-level system and necessary high inversions. We demonstrate an efficiency of ∼62.7% versus a coupled pump, a record for cladding-pumped fiber lasers with a single-pass pump. 84 W at ∼978 nm with ∼1.12M2 was achieved, a record power for flexible fibers. Amplified spontaneous emission was suppressed by >40 dB. The efficiency is quantum-limited ∼94% versus an absorbed pump. This is made possible by the use of a photo-darkening-free Yb phosphosilicate core and recent progress in single-mode large-core all-solid photonic bandgap fiber designs, which provide the necessary large core-to-cladding ratio and suppression of the four-level system.

Efficient 240W single-mode 1018nm laser from an Ytterbium-doped 50/400µm all-solid photonic bandgap fiber.

Lowering the quantum defect by tandem pumping with fiber lasers at 1018nm was critical for achieving the record 10kW single-mode ytterbium fiber laser. Here we report the demonstration of an efficient directly-diode-pumped single-mode ytterbium fiber laser with 240W at 1018nm. The key for the combination of high efficiency, high power and single-mode at 1018nm is an ytterbium-doped 50µm/400µm all-solid photonic bandgap fiber, which has a practical all-solid design and a pump cladding much larger than those used in previous demonstrations of single-mode 1018nm ytterbium fiber lasers, enabling higher pump powers. Efficient high-power single-mode 1018nm fiber laser is critical for further power scaling of fiber lasers and the all-solid photonic bandgap fiber can potentially be a significant enabling technology.

Highly efficient ytterbium-doped phosphosilicate fiber lasers operating below 1020 nm.

Highly-efficient high-power fiber lasers operating at wavelength below 1020 nm are critical for tandem-pumping in >10 kW fiber lasers to provide high pump brightness and low thermal loading. Using an ytterbium-doped-phosphosilicate double-clad leakage-channel fiber with ~50 µm core and ~420 µm cladding, we have achieved ~70% optical-to-optical efficiency at 1018 nm. The much larger cladding than those in previous reports demonstrates the much lower required pump brightness, a key for efficient kW operation. The demonstrated 1018 nm fiber laser has ASE suppression of ~41 dB. This is higher than previous reports and further demonstrates the advantages of the fiber used. Limiting factors to efficiency are also systematically studied.

Extending mode areas of single-mode all-solid photonic bandgap fibers.

Mode area scaling of optical fiber is highly desirable for high power fiber laser applications. It is well known that incorporation of additional smaller cores in the cladding can be used to resonantly out-couple higher-order modes from a main core to suppress higher-order-mode propagation in the main core. Using a novel design with multiple coupled smaller cores in the cladding, we have successfully demonstrated a single-mode photonic bandgap fiber with record effective mode area of ~2650µm(2). Detailed numeric studies have been conducted for multiple cladding designs. For the optimal designs, the simulated minimum higher-order-mode losses are well over two orders of magnitudes higher than that of fundamental mode when expressed in dBs. To our knowledge, this is the best higher-order-mode suppression ever found in fibers with this large effective mode areas. We have also experimentally validated one of the designs. M(2)<1.08 across the transmission band was demonstrated.

Polarizing ytterbium-doped all-solid photonic bandgap fiber with ~1150µm(2) effective mode area.

We demonstrate an Yb-doped polarizing all-solid photonic bandgap fiber for single-polarization and single-mode operation with an effective mode area of ~1150µm(2), a record for all-solid photonic bandgap fibers. The differential polarization mode loss is measured to be >5dB/m over the entire transmission band with a 160nm bandwidth and >15dB/m on the short wavelength edge of the band. A 2.6m long fiber was tested in a laser configuration producing a linearly polarized laser output with a PER value of 21dB without any polarizer, the highest for any fiber lasers based on polarizing fibers.

TEM00 mode content measurements on a passive leakage channel fiber.

Mode content measurements with a scanning ring cavity were performed in order to determine the TEM00 mode content of the output beam profile of a resonantly enhanced leakage channel fiber. The measurements were performed at 1.0 and 1.5 µm. In addition, the influence of different bending diameters as well as launching conditions has been investigated. Furthermore, a numerical simulation was used to determine the maximum theoretical TEM00 overlap, if only the fundamental fiber mode is guided. The simulation was also used to analyze how the TEM00 overlap for the case of any additional higher order fiber mode can be determined consistently.

Quantitative mode quality characterization of fibers with extremely large mode areas by matched white-light interferometry.

Quantitative mode characterization of fibers with cores much beyond 50µm is difficult with existing techniques due to the combined effects of smaller intermodal group delays and dispersions. We demonstrate, for the first time, a new method using a matched white-light interferometry (MWI) to cancel fiber dispersion and achieve finer temporal resolution, demonstrating ~20fs temporal resolution in intermodal delays, i.e. 6µm path-length resolution. A 1m-long straight resonantly-enhanced leakage-channel fiber with 100µm core was characterized, showing ~55fs/m relative group delay and a ~29dB mode discrimination between the fundamental and second-order modes.

Ytterbium-doped large-mode-area all-solid photonic bandgap fiber lasers.

Single-mode operation in a large-mode-area fiber laser is highly desired for power scaling. We have, for the first time, demonstrated a 50µm-core-diameter Yb-doped all-solid photonic bandgap fiber laser with a mode area over 4 times that of the previous demonstration. 75W output power has been generated with a diffraction-limited beam and an efficiency of 70% relative to the launched pump power. We have also experimentally confirmed that a robust single-mode regime exists near the high frequency edge of the bandgap. These fibers only guide light within the bandgap over a narrow spectral range, which is essential for lasing far from the gain peak and suppression of stimulated Raman scattering. This work demonstrates the strong potential for mode area scaling of in single-mode all-solid photonic bandgap fibers.

Impact of fiber outer boundaries on leaky mode losses in leakage channel fibers.

In a leakage channel fiber, the desired fundamental mode (FM) has negligible waveguide loss. Higher-order modes (HOM) are designed to have much higher waveguide losses so that they are practically eliminated during propagation. Coherent reflection at the fiber outer boundary can lead to additional confinement especially for highly leaky HOM, leading to lower HOM losses than what are predicted by conventional FEM mode solver considering infinite cladding. In this work, we conducted, for the first time, careful measurements of HOM losses in two leakage channel fibers (LCF) with circular and rounded hexagonal boundary shapes respectively. Impact on HOM losses from coiling, fiber boundary shapes and coating indexes were studied in comparison to simulations. This work, for the first time, demonstrates the limit of the simulation method commonly used in the large-mode-area fiber designs and the need for an improved approach. More importantly, this work also demonstrates that a deviation from circular fiber outer shape may be an effective method to mitigate HOM loss reduction from coherent reflection from fiber outer boundary, even in double-clad fibers, with HOM losses in excess of 20 dB/m measured in the hexagonal LCF with ~50 µm core diameter while keeping FM loss negligible.

Flat-top mode from a 50 µm-core Yb-doped leakage channel fiber.

We demonstrate for the first time a flat-top mode from a 50 µm-core Yb-doped leakage channel fiber (LCF). The flat intensity distribution leads to an effective mode area of ~1880 µm(2) in the straight fiber, an over 50% increase comparing to that of regular LCF with the same core diameter. The flat-top mode was achieved by using a uniform Yb-doped silica glass in the core center with an index of ~2 × 10(-4) lower than that of the silica background. The fiber was also tested in a laser configuration, demonstrating an optical-to-optical efficiency of ~77% at 1026 nm with respect to the pump at 975 nm.

Mode area scaling with all-solid photonic bandgap fibers.

There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~920?m2 can be made with excellent higher order mode suppression.