Theoretical analysis of combining efficiency and beam quality degradation in a misaligned diffractive coherent combining system.

We theoretically investigate the combining efficiency and combined beam quality degradation induced by beam array misalignment in a coherent combining system based on diffractive optical elements. Theoretical model is established based on the Fresnel diffraction. We consider pointing aberration, positioning error and beam size deviation in array emitters as typical misalignments, and discuss their influences on beam combining by this model. The statistical analysis results and the accurate fitting curves of the degradation have been given based on the repetitive simulations with normal distributed random misalignments. According to the results, the combining efficiency is affected greatly by the pointing aberration and position error of the laser array, while the combined beam quality is just affected by the pointing aberration generally. Based on calculation with a series of typical parameters, the standard deviations of the laser array's pointing aberration and position error are required to less than 15µrad and 1µm respectively to maintain an excellent combining efficiency. If we only concentrate on the beam quality, the pointing aberration need to be less than 70µrad.

Brillouin gain spectrum characterization in an acoustic anti-guided delivery fiber for high power narrow linewidth laser.

The Brillouin gain spectrum (BGS) provides key information for stimulated Brillouin scattering (SBS), such as the Brillouin frequency shift (BFS), Brillouin spontaneous linewidth, and Brillouin gain coefficient. In this study, we theoretically investigate the field distributions and BGS characterization of Ge-doped, Al-doped, and Al/Ge co-doped fibers. Additionally, we analyzed and compared the relationship between the BGS and acoustic refractive index. In particular, we demonstrate the crucial role played by acoustic modes in anti-waveguide structures. The simulation results show that the Brillouin gain coefficient decreases with a decreasing acoustic index in the fiber core region. Furthermore, we experimentally measure the SBS threshold and BGS of the Al/Ge co-doped fiber to examine the validity of the numerical model. The simulated and experimental results are consistent.

Simulation and Design of Circular Scanning Airborne Geiger Mode Lidar for High-Resolution Topographic Mapping.

Over the last two decades, Geiger-mode lidar (GML) systems have been developing rapidly in defense and commercial applications, demonstrating high point density and great collection efficiency. We presented a circular scanning GML system simulation model for performance prediction and developed a GML system for civilian mapping. The lidar system used an eye-safe fiber laser at 1545 nm coupled with a 64 × 64 pixels photon-counting detector array. A real-time data compression algorithm was implanted to reduce half of the data transmission rate and storage space compared to the uncompressing situation. The GML system can operate at aircraft above-ground levels (AGLs) between 0.35 km and 3 km, and at speeds in excess of 220 km/h. The initial flight tests indicate that the GML system can operate day and night with an area coverage of 366 km2/h. The standard deviations of the relative altimetric accuracy and the relative planimetric accuracy are 0.131 m and 0.152 m, respectively. The findings presented in this article guide the implementation of designing an airborne GML system and the data compression method.

High power pump and signal combiner for backward pumping structure with two different fused fiber bundle designs by means of pretapered pump fibers.

We report on two high power backward (6 + 1) × 1 pump and signal combiners with different optical designs based on end-pumping technique, each achieving more than 95% coupling ratio of signal light, 98% coupling ratio of pump light, and a total pump power of 2.95kW. Both designs solve the problem of non-uniform arrangement in asymmetric fiber bundles by the pretreatment of the pump fibers and signal fiber. Asymmetric fiber bundle in backward pump and signal combiners means the pump input fibers and signal output fiber have different cladding diameters. Two 3D simulation models were built to calculate the parameters of the fused fiber bundles for matching the mode field diameter between the signal input and the output fibers. Both optical designs of backward combiners were developed based on the beam propagation method and confirmed by experimental results.

Optimization and visualization of phase modulation with filtered and amplified maximal-length sequence for SBS suppression in a short fiber system: a theoretical treatment.

We use a model to investigate both the temporal and spectral characteristics of a signal lightwave which has been spectrally broadened through phase modulation with a maximal-length sequence (MLS), which is a common type of pseudo-random bit sequence. The enhancement of the stimulated Brillouin scattering (SBS) threshold of the modulated lightwave in a fiber system is evaluated by numerically simulating the coupled three-wave SBS interaction equations. We find that SBS can build up on a nanosecond-level time scale in a short fiber, which can reduce the SBS suppressing capability of MLS modulation waveforms with GHz-level clock rate, if the sub-sequence ("run") lengths with the same symbol (zero or one) of the MLS extend over several nanoseconds. To ensure the SBS buildup is perturbed and thus suppressed also during these long sub-sequences, we introduce a low-pass filter to average the signal over several bits so that the modulation waveform changes gradually even during long runs and amplify the RF modulation waveforms to the level required for sufficient spectral broadening and carrier suppression of the optical signal. We find that the SBS suppression depends non-monotonically on the parameters of the filtered and amplified MLS waveform such as pattern length, modulation depth, and the ratio of low-pass filter cutoff frequency to clock rate for maximum SBS mitigation. We optimize the SBS suppression through numerical simulations and discuss it in terms of the temporal and spectral characteristics of the lightwave and modulation waveform using derived analytical expressions and numerical simulations. The simulations indicate that the normalized SBS threshold reaches a maximum for a RMS modulation depth of 0.56π and a ratio of filter cutoff frequency to clock rate of 0.54 and that MLS9 is superior to other investigated patterns.

Investigation of combining-efficiency loss induced by a diffractive optical element in a single-aperture coherent beam combining system.

Filled-aperture geometries can be obtained using a diffractive optical element (DOE) in the coherent beam combining (CBC) architecture. Minimizing the beam deviation is crucial to maintain single-aperture output and reduce the combining-efficiency losses. In this study, we developed a theoretical model for investigating the combining-efficiency losses with beam deviation in a DOE-based CBC architecture. The beam deviations induced by the DOE-mount-tilt error, emitter-incident angular error, and DOE-groove-tilt error are discussed theoretically in detail and verified experimentally. The combining-efficiency losses caused by the three error sources are calculated. Meanwhile, the combining-efficiency losses affected by the beam size and the DOE period are analyzed. For an 11-channel CBC architecture with a DOE period of 50 µm and a beam size of 30 mm, the maximum combining-efficiency losses caused by the three error sources were 3.2%, 1.87%, and 36.41%, respectively, whereas those in case of a DOE period of 20 µm and a beam size of 10 mm were 14.34%, 8.58%, and 25.29%, respectively. We found that the combining-efficiency loss is most sensitive to the DOE-groove-tilt error.

Instantaneous response and suppression of SBS process in a short fiber system with binary sequence phase modulation.

We study the instantaneous response of stimulated Brillouin scattering (SBS) in a fiber system phase modulated by a binary sequence waveform. The buildup time constant of SBS kinetics is investigated analytically and experimentally. A series of binary sequences with adjustable dwell time and sequence period is constructed in order to examine both buildup and suppressing processes for SBS in 15 m short fiber. For a fiber system with SBS buildup time constant within several nanoseconds, the Stokes intensity can be effectively suppressed by implementing a binary sequence phase modulation with a dwell time close to or even less than the buildup time constant. Stokes intensity can be suppressed in several sequence periods, which exhibit a damped-oscillation-like trend.

Simulation and experimental study of laser-induced thermal deformation of spectral beam combination grating.

The multilayer dielectric (MLD) grating is a critical device for combining multiple laser beams into a single beam in a spectral beam combining (SBC) system. We established a theoretical thermal deformation model of the laser-irradiated MLD grating. Thermal deformation on the surface of the grating is simulated according to a series of parameters including the laser irradiation time, laser power density, and substrate size. To verify the model, we exposed a 960 l/mm, 50×50×1.5 mm3 grating to a laser power density of 3.61 kW/cm2 and observed the temperature change. We used a Twyman-Green interferometer to measure the interference fringes on the grating surface. Based on the Fourier-transform method and a Zernike polynomial fitting method, the real-time grating surface profile is reconstructed. The results show that substrate thickness increase or area decrease can reduce thermal deformation, the average decreases are 18.3% and 19.9%, respectively. The discussion and analysis of the grating thermal deformation are potentially valuable for designing grating to decrease the thermal deformation and improve the combined beam quality of a SBC system.

1.27 kW, 2.2 GHz pseudo-random binary sequence phase modulated fiber amplifier with Brillouin gain-spectrum overlap.

We present a 2.2 GHz modulated, 1.27 kW output power, monolithic fiber amplifier based on pseudo-random binary sequence (PRBS) phase modulation. The spectral line spacing of maximizing the threshold enhancement factor (plateau of trend) was found by theoretical simulation. The spectral line spacing was adjusted to 12.7 MHz by a pattern length of n = 9, which is close to the plateau of trend in the proposed architecture. A 2.2 GHz low-pass radio frequency filter was used to control the FWHM of the seed. A four-stage Yb-doped fiber amplifier chain was established to boost a distributed Bragg reflector (DBR) laser and a distributed feedback (DFB) diode laser to 1.2 kW and 1.27 kW with a backward reflectively of <1‰, which shows a good suppression of SBS effect.