Impact of finite bandwidth on PRBS-modulated optical spectra.

Applying a pseudo-random binary sequence (PRBS) to phase modulators is a recent development in the broadening of optical spectra of laser sources to defeat stimulated Brillouin scattering (SBS). The theoretical underpinning of this method relies on alternating the phase of the optical signal between its native value and an out-of-phase value (i.e., imposing a binary phase shift of 0 or π in a pseudo random manner) to prevent coherent buildup of the SBS acoustic grating. In real physical systems, realizing such a binary shift is impossible due to the finite response times of the electronics and electro-optic components. The influence of these effects is investigated in this work, specifically the finite bandwidth of the electronic PRBS generator and the frequency-dependent response of the phase modulator, on the resultant temporal waveforms of the PRBS signal and its RF optical spectra. It is found that the optimal SBS suppression in real systems is achieved when the phase modulator is driven at a voltage beyond V π, even though driving at V π has been deemed as ideal. Moreover, both the SBS suppression and spectral broadening are always weaker than what is predicted by analytical results since any degradation of the sharp edges of the PRBS signal result in loss of high-frequency content in the RF spectrum of the PRBS signal. Lastly, it is noticed that the typical ways of measuring spectral width are not relevant when applied to the complex PRBS RF optical spectrum. An 'equivalent spectral width' is defined to accurately quantify the correlation between spectral width an SBS suppression.

Cavity requirements for realizing high-efficiency, Tm/Ho-doped, coaxial fiber laser systems.

Coaxial fiber lasers, consisting of a Ho-doped core surrounded by a Tm-doped ring, are studied via experiments and numerical simulations. Previous simulations indicated that coaxial fiber lasers have the potential to reach power conversion efficiencies of up to 54%, but experiments have yielded much lower efficiencies. To understand this difference, a wavelength dimension is added to the previous model. The new simulations explain the discrepancies with the experiments via spectral gain competition and lead to a path to optimize experimental efficiencies. Specifically, an output coupler is spectrally designed to optimize the efficiency of coaxial fiber systems, and a path towards realizing the predicted 54% efficiency is presented. The results indicate that the Tm/Ho coaxial fiber laser has the potential to be a compact and efficient means of producing 2100 nm radiation.

Three fiber designs for mitigating thermal mode instability in high-power fiber amplifiers.

An improved fiber amplifier model for simulating thermal mode instability (TMI) in high-power fiber amplifiers is developed. The model is applied to reveal new physics regarding the thermal physics that is critical to the TMI process, which are not the glass volume or the cooling method, but rather the transit path length of the quantum-defect-defined thermal peak in the fiber amplifier. The new physics and model analysis are applied to create a set of design rules to guide the development of new fiber types specifically for intrinsically mitigating TMI. These rules and the improved model are applied to three new fiber concepts for mitigating TMI in high-power fiber amplifiers. All three fiber types are shown to substantially increase the TMI threshold, up to a factor of 2 in some cases. In addition, all three new fiber classes offer ways to simultaneously increase the core diameter and the TMI threshold.

Tm/Ho-doped fiber laser systems using coaxial fiber.

Coaxial fiber lasers are studied via numerical simulations as an alternative to conventional cladding-pumped fiber lasers. The coaxial fiber consists of a Ho-doped core surrounded by a Tm-doped ring. When pumped at 805 nm, this fiber type resulted in a 54% power conversion efficiency, defined as the ratio of output signal power to total input pump power. The performance of this laser was numerically compared to conventional Tm/Ho doped fiber lasers under the same pump conditions. Simulations of a Tm:fiber laser pumping a Ho:fiber laser yielded a maximum 43% power conversion efficiency, while simulations of a Tm/Ho co-doped fiber laser yielded a maximum power conversion efficiency of 34%. These results demonstrate that the coaxial fiber laser has the potential for a significant efficiency improvement over conventional methods in addition to being a more compact system.

Analysis of nonlinear optical and dynamic gain effects of moderate-power, pulse-position-modulated, erbium-doped fiber amplifiers for deep-space applications.

Lasers for use in deep-space applications such as interplanetary optical communications employ multiwatt resonantly pumped dual-clad erbium-doped fiber amplifiers and the pulse-position modulation scheme. Nonlinear optical effects and dynamic gain effects often impair their performance and limit their operational range. These effects are analyzed theoretically and numerically with a time-dependent two-level propagation model, respectively. Self-phase modulation and stimulated Raman scattering are found to limit the usable data format space. In operational regimes free from nonlinear effects, dynamic gain effects such as the variation in the output pulse energy and square-pulse distortion are quantified. Both are found to primarily depend on the symbol duration and can be as large as 28% and 21%, respectively.

Optimization of resonantly cladding-pumped erbium-doped fiber amplifiers for space-borne applications.

Lasers for use in space-borne applications require ultrahigh efficiency due to limited heat dissipation and power generation capacity. In particular, interplanetary optical communication systems require high-efficiency, moderate-power (>4 W) optical transmitters in the 1600 nm wavelength range. Resonantly pumped dual-clad erbium-doped fiber lasers are best suited for this purpose. Parametric numerical optimizations are performed using a two-level propagation model modified to include spatial effects specific to large-mode-area fibers. Propagation loss mechanisms are found to be limiting factors due to the relatively low cross-sections and low quenching-free doping densities of erbium. Although experimental reports have demonstrated efficiencies up to 33%, simulation results indicate that over 53% power-conversion efficiency can be achieved using commercial fibers, and over 75% can be achieved using custom fibers employing propagation-loss mitigation strategies.

Semi-guiding high-aspect-ratio core (SHARC) fiber amplifiers with ultra-large core area for single-mode kW operation in a compact coilable package.

A new class of optical fiber, the SHARC fiber, is analyzed in a high-power fiber amplifier geometry using the gain-filtering properties of confined-gain dopants. The high-aspect-ratio (~30:1) rectangular core allows mode-area scaling well beyond 10,000 µm2, which is critical to high-pulse-energy or narrow-linewidth high-power fiber amplifiers. While SHARC fibers offer modally dependent edge loss at the wide "semi-guiding" edge of the waveguide, the inclusion of gain filtering adds further modal discrimination arising from the variation of the spatial overlap of the gain with the various modes. Both methods are geometric in form, such that the combination provides nearly unlimited scalability in mode area. Simulations show that for kW-class fiber amplifiers, only the fundamental mode experiences net gain (15 dB), resulting in outstanding beam quality. Further, misalignment of the seed beam due to offset, magnification, and tilt are shown to result in a small (few percent) efficiency penalty while maintaining kW-level output with 99% of the power in the fundamental mode for all cases.

Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package.

A new class of optical fiber is presented that departs from the circular-core symmetry common to conventional fibers. By using a high-aspect-ratio (~30:1) rectangular core, the mode area can be significantly expanded well beyond 10,000 µm2. Moreover, by also specifying a very small refractive-index step at the narrow core edges, the core becomes "semi-guiding," i.e. it guides in the narrow dimension and is effectively un-guiding in the wide mm-scale dimension. The mode dependence of the resulting Fresnel leakage loss in the wide dimension strongly favors the fundamental mode, promoting single-mode operation. Since the modal loss ratios are independent of mode area, this core structure offers nearly unlimited scalability. The implications of using such a fiber in fiber laser and amplifier systems are also discussed.

Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering.

We present, for the first time to our knowledge, an explicit experimental comparison of beam quality in conventional and confined-gain multimode fiber lasers. In the conventional fiber laser, beam quality degrades with increasing output power. In the confined-gain fiber laser, the beam quality is good and does not degrade with output power. Gain filtering of higher-order modes in 28microm diameter core fiber lasers is demonstrated with a beam quality of M(2)=1.3 at all pumping levels. Theoretical modeling is shown to agree well with experimentally observed trends.

Polarization-insensitive high-dispersion total internal reflection diffraction gratings.

We report the first experimental realization of total internal reflection (TIR) diffraction gratings. Performance of less than 0.7-dB insertion loss (IL) for both TE and TM polarizations and 0.5-dB polarization-dependent loss (PDL) are predicted over a 50-nm spectral bandwidth with simultaneous fabrication tolerances on the depth and the duty cycle of binary gratings of +/-5% and +/-14%, respectively. Nineteen gratings were fabricated that met these specifications, yielding IL and PDL values less than 0.6 and 0.2 dB, respectively, across the entire 50-nm bandwidth. Measurements made under the Littrow configuration resulted in high efficiency and low PDL across a 100-nm bandwidth, with up to 100% diffraction efficiency within the experimental measurement error.

High-efficiency, high-dispersion diffraction gratings based on total internal reflection.

We report a new class of high-dispersion immersed diffraction gratings for which the reflective nature of the diffraction is provided by the phenomenon of total internal reflection (TIR) regardless of grating tooth shape. Thus, the component can be fabricated from a single dielectric material and requires no metallic or dielectric film layers for high reflection diffraction efficiency. With the absence of metallic absorption, diffraction efficiencies of these TIR gratings can reach more than 99% for 15-20-nm spectral bandwidths, making them suitable for many laser-based technologies.

Optical measurement of depth and duty cycle for binary diffraction gratings with subwavelength features.

We describe a new and unique method for simultaneous determination of the groove depth and duty cycle of binary diffraction gratings. For a near-normal angle of incidence, the +1 and -1 diffracted orders will behave nearly the same as the duty cycle is varied for a fixed grating depth. The difference in their behavior, quantified as the ratio of their respective diffraction efficiencies, is compared to a look-up table generated by rigorous coupled-wave theory, and the duty cycle of the grating is thus obtained as a function of grating depth. Performing the same analysis for the orthogonal probe-light polarization results in a different functional dependence of the duty cycle on the grating depth. By use of both TE and TM polarizations, the depth and duty cycle for the grating are obtained by the intersection of the functions generated by the individual polarizations. These measurements can also be used to assess qualitatively both the uniformity of the grating and the symmetry of the grating profile. Comparison with scanning electron microscope images shows excellent agreement. This method is advantageous since it can be carried out rapidly, is accurate and repeatable, does not damage the sample, and uses low-cost, commonly available equipment. Since this method consists of only four fixed simple measurements, it is highly suitable for quality control in a manufacturing environment.