Chunte Lu

Stimulated Brillouin scattering suppression through laser gain competition: scalability to high power

We demonstrate stimulated Brillouin scattering (SBS) suppression in a Yb-doped fiber amplifier by seeding with a combination of broad- and single-frequency laser beams that are separated sufficiently to suppress four-wave mixing and to allow for efficient laser gain competition between the two signals. In the experiment, a monolithic fiber configuration was used. With appropriate selection of seed power ratio, we were able to generate single-frequency 1064 nm light with a slope efficiency of 78% while simultaneously suppressing the backscattered Stokes light. We discuss scalability to high power wherein a large thermal gradient can be induced at the output end of the fiber via quantum defect heating, leading to an SBS suppression factor comparable to counterpumping.

Experimental and theoretical studies of phase modulation in Yb-doped fiber amplifiers

We present detailed studies of the effect of sinusoidal phase modulation on stimulated Brillouin scattering (SBS) in ytterbium-doped fiber amplifiers. Based on a time-dependent SBS model, SBS enhancement factor versus pump linewidth for different modulation depths ranging from 0 to π , and modulation frequencies ranging from 30 MHz to 500 MHz were analyzed. In addition, experimental validation of SBS suppression via sinusoidal phase modulation is presented with experimental results agreeing well with the model and simulations. Furthermore, narrow linewidth coherent beam combining (CBC) of 5 sinusoidal phase modulated lasers is demonstrated via LOCSET.

SBS suppression through seeding with narrow-linewidth and broadband signals: experimental results

We present experimental verification of a novel technique to suppress stimulated Brillouin scattering (SBS) in single frequency fiber amplifiers. This technique relies on seeding with a combination of broadband and single frequency laser beams to allow for efficient laser gain competition between the two signals. In the experiment, a monolithic fiber configuration was used. Broadband 1045 nm light and single frequency 1064 nm light were coupled into an Yb-doped gain fiber. With appropriate selection of seed power ratio, we were able to generate an output signal predominantly comprised of 1064 nm light while simultaneously suppressing the back-scattered Stokes light. The slope efficiency for the two-tone amplifier was approximately 78%; slightly below that of a single-tone amplifier. The SBS threshold for the former, on the other hand, was appreciably higher than that of the latter which is in excellent agreement with the theory. In preliminary implementation of this technique at high power, we generated close to 100 W without encountering the SBS threshold. Finally, we show numerically that due to a favorable thermal gradient much higher powers can be obtained.

Extracting fundamental transverse mode operation in broad area quantum cascade lasers

Power scaling in broad area quantum cascade lasers results in the operation of high order transverse modes with a far-field profile consisting of two lobes propagating at large angles relative to the optical axis. We report a method of suppressing the high order transverse modes that can extract the fundamental mode and provide emission along the optical axis. By generating a lateral constriction in the waveguide in the form of short trenches defined by the focused ion beam milling technique, we report broad area devices in which most of the power is contained in a near diffraction-limited beam that provides high brightness.

On-chip unstable resonator cavity GaSb-based quantum well lasers

The focused ion beam milling tool was used to convert a GaSb-based broad area gain-guided quantum well laser device with a standard Fabry-Perot cavity into one with an unstable resonator cavity. A cylindrical mirror was formed at the back facet of the broad area device emitting near 2 μm. Compared to the Fabry-Perot cavity device, where the coherency of the beam is severely disrupted by filamentation, the unstable resonator cavity device exhibits an ∼2× diffraction limited beam. The relatively small penalty in slope efficiency demonstrates that a much higher brightness can be reached in this class of broad area devices.

Distributed loss method to suppress high order modes in broad area quantum cascade lasers

We describe a method where the standard fabrication of broad area quantum cascade lasers is modified to provide a controlled amount of direct contact of device sidewalls with metal. We demonstrate that this provides sufficient levels of distributed losses to suppress the high order transverse modes in favor of the fundamental or near-fundamental transverse mode operation. We observe that the quantum cascade laser power and slope efficiency are degraded by a small amount, resulting in a large increase in brightness to accompany the power scaling.

Improved efficiency in room temperature >3µm diodes using highly strained quantum wells

We present experimental comparison of Type-I diode lasers emitting <3 μm wavelength in room temperature with increased strain in quantum wells (QWs). Due to diminishing hole confinement in the barrier, the performance of mid- IR Type-I diode laser is generally poor. Here we improve the hole confinement using quinary alloy in the barrier in conjunction with highly strained QWs. By using molecular beam epitaxial growth method, we achieve up to 2.3% strain in the QWs. At near room temperature, highly strained laser structure shows approximately 4 times improved laser performance than regular strained laser under the same testing condition. The study demonstrates significant improvement in laser efficiency using highly strained QWs in the GaSb-based type-I mid-infrared laser diodes.

Multi-tone approach to SBS mitigation in narrow linewidth high power fiber amplifiers

We investigate theoretically multi-tone seeding of high power ytterbium-doped amplifiers using a numerical code that solves a two-point boundary problem. A large wavelength separation among the signals is used leading to efficient transfer of power through laser gain while increasing the stimulated Brillouin scattering threshold in the channel of interest. Two-tone and three-tone seeding configurations are considered. For two-tone seeding, it is shown that a combination of narrow linewidth and broadband signals employed in a co-propagating geometry can achieve the same level of SBS suppression as counter-pumping.

Experimental Verification of Two-Tone Amplification in Single Frequency Fiber Amplifiers

We present experimental verification of a novel technique to suppress SBS in narrow linewidth fiber amplifiers. This technique relies on seeding with a combination of broad- and narrow-linewidth laser beams allowing favorable laser gain competition.