Hybrid microstructured fibers, utilizing both air holes and high index cladding structures, provide important advantages over conventional fiber including robust fundamental mode operation with large core diameters (>30μm) and spectral filtering (i.e. amplified spontaneous emission and Raman suppression). This work investigates the capabilities of a hybrid fiber designed to suppress stimulated Brillouin scattering (SBS) and modal instability (MI) by characterizing these effects in a counter-pumped amplifier configuration as well as interrogating SBS using a pump-probe Brillouin gain spectrum (BGS) diagnostic suite. The fiber has a 35 μm annularly gain tailored core, the center doped with Yb and the second annulus comprised of un-doped fused silica, designed to optimize gain in the fundamental mode while limiting gain to higher order modes. A narrow-linewidth seed was amplified to an MI-limited 820 W, with near-diffraction-limited beam quality, an effective linewidth ~ 1 GHz, and a pump conversion efficiency of 78%. Via a BGS pump-probe measurement system a high resolution spectra and corresponding gain coefficient were obtained. The primary gain peak, corresponding to the Yb doped region of the core, occurred at 15.9 GHz and had a gain coefficient of 1.92×10-11 m/W. A much weaker BGS response, due to the pure silica annulus, occurred at 16.3 GHz. This result demonstrates the feasibility of power scaling hybrid microstructured fiber amplifiers
We report >100 W output power by coherently combining two, three, and four high power lasers in all-fiber and all-passive configurations using Yb fiber laser cavities operating at a wavelength of 1080 nm with polarization maintaining fibers and 2×2 fused fiber couplers. We present the power and number scaling characteristics of the laser arrays and compare their beam combining efficiencies in different array configurations. We experimentally show beam combining efficiency degradation with increase of laser power and number of lasers. In addition, we discuss the effect of fiber nonlinearity on beam combining efficiency.
An overview of recent fiber based component developments is presented that lead to robust all-fiber lasers and optical
amplifiers. Critical issues impacting the integration of high power rare earth doped optical amplifiers are presented.
The optical performance of key fiber components is reported with emphasis on temperature effects of a monolithically
integrated multi-Watt, single mode, Polarization Maintaining (PM), Ytterbium (Yb) doped double clad optical amplifier.
Performance of an all-fiber optical amplifier design that integrates two fiber coupled laser diode pumps and one PM Yb
double clad feed through fiber directly in to a combiner is reported.
KEYWORDS: Optical amplifiers, Fiber lasers, Fiber amplifiers, Signal attenuation, Diodes, High power fiber amplifiers, Polarization, Phase modulation, High power fiber lasers, Laser optics
An overview of fiber based components is presented that lead to robust all-fiber lasers and optical amplifiers suitable for
space environments. Critical issues impacting the monolithic integration of high power rare earth doped fiber based
optical lasers and amplifiers are presented. Optical performance of various all fiber components are reported with
emphasis on temperature effects of fiber based pump combiners with polarization maintaining (PM) fiber feed throughs.
The all fiber pump combiner is a key component that enables efficient high power pump coupling into double clad rare
earth doped optical fibers. A new pump combiner design that injects two fiber coupled pumps plus one PM signal fiber
directly into a single polarization maintaining double clad fiber is reported.
KEYWORDS: Phase shift keying, Optical components, Photodetectors, Phase modulation, Signal processing, Modulation, High power fiber amplifiers, Optical amplifiers, Phase measurement, Signal to noise ratio
We report high power phase locked fiber amplifier array using the Self-Synchronous Locking of Optical Coherence by
Single-detector Electronic-frequency Tagging technique. We report the first experimental results for a five element
amplifier array with a total locked power of more than 725-W. We will report on experimental measurements of the
phase fluctuations versus time when the control loop is closed. The rms phase error was measured to be λ/60. Recent
results will be reported. To the best of the authors' knowledge this is the highest fiber laser power to be coherently
combined.
We report high power phase locked fiber amplifier array using the Self-Synchronous Locking of Optical Coherence by
Single-detector Electronic-frequency Tagging technique. We report the first experimental results for a five element
amplifier array with a total locked power of more than 725-W. We will report on experimental measurements of the
phase fluctuations versus time when the control loop is closed. The rms phase error was measured to be λ/60. Recent
results will be reported. To the best of the authors' knowledge this is the highest fiber laser power to be coherently
combined.
We experimentally studied the coherent beam combining characteristics of fiber laser arrays in all-fiber passive
configurations using polarization maintaining fibers. In addition, we simulated the coherent performance by including
fiber nonlinearity. The beam combining performance is affected by both optical feedback and the laser cavity length
difference. In addition, Kramers-Kronig and n2 induced nonlinearity plays an important role for the coherent phase
locking. We describe the scalability of the coherent array to high power via scaling of laser power and fiber count. We
show coherently combined output powers of 27.4 Watts and 12.2 Watts at 1083 nm in 2-laser and 4-laser arrays.
The first theory for two novel coherent beam combination architectures that are the first
electronic beam combination architectures that completely eliminate the need for a separate
reference beam are presented. Experimental results demonstrating the coherent addition of a
3 by 3 array of fiber amplifiers with a total phase locked power of 100-W are also described.
A novel high accuracy all electronic technique for phase locking arrays of optical fibers is demonstrated. We report the first demonstration of the only electronic phase locking technique that doesn't require a reference beam. The measured phase error is λ/20. Excellent phase locking has been demonstrated for fiber amplifier arrays.
In this paper we present advances made in the development and fabrication of highly efficient, large-mode area fibers for eye-safe wavelengths (1.55 μm, 2.0 μm). LMA Er/Yb co-doped and Tm doped fibers have been successfully fabricated, with 25 μm core and 250 to 300 μm clad diameters, that are suitable for nanosecond pulsed amplification in LIDAR applications as well as high power CW amplification. Manufacturing challenges for these novel fibers are discussed. Measured and modeled data, for both types of fibers, are presented. The development of non-PM and PM-LMA fibers for eye-safe applications is expected to spur rapid progress in power scaling at these wavelengths, similar to that witnessed by the industry at 1.06 μm.
We report a novel coherent beam combining technique. This is the first actively phase locked optical fiber array that eliminates the need for a separate reference beam. In addition, only a single photodetector is required. The far-field central spot of the array is imaged onto the photodetector to produce the phase control loop signals. Each leg of the fiber array is phase modulated with a separate RF frequency, thus tagging the optical phase shift for each leg by a separate RF frequency. The optical phase errors for the individual array legs are separated in the electronic domain. In contrast with the previous active phase locking techniques, in our system the reference beam is spatially overlapped with all the RF modulated fiber leg beams onto a single detector. The phase shift between the optical wave in the reference leg and in the RF modulated legs is measured separately in the electronic domain and the phase error signal is feedback to the LiNbO3 phase modulator for that leg to minimize the phase error for that leg relative to the reference leg. The advantages of this technique are 1) the elimination of the reference beam and beam combination optics and 2) the electronic separation of the phase error signals without any degradation of the phase locking accuracy. We will present the first theoretical model for self-referenced LOCSET and describe experimental results for a 3 x 3 array.
Stimulated Brillouin Scattering (SBS) is a polarization-dependent, nonlinear process that is often the limiting factor for high-power fiber laser applications. We report the results of experiments measuring the SBS thresholds and the SBS gain bandwidths in several passive optical fibers. Fibers with nearly identical mode-field diameters and loss coefficients from different manufacturers were selected. Observations from these experiments indicate that the SBS gain coefficient for fibers from different manufacturers varied significantly resulting in a 70% deviation in SBS threshold. Also, polarization-maintaining (PM) fiber exhibited a significant increase in the SBS threshold for a linearly polarized pump beam that is launched into the PM fiber at 45° relative to the fiber's slow axis. This increase in threshold was not mirrored in non-PM fiber. These results suggest that the polarization multiplier in the SBS threshold equation may be highest when a PM fiber is used with the appropriate launch conditions, rather than a non-PM conventional single-mode fiber. We will present the experimental results and a theoretical model demonstrating the polarization dependent gain properties in both PM and non-PM fiber.
Optoelectronic devices based on nonlinear optic (NLO) polymers, with electro-optic (EO) coefficients in excess of 100 pm/V at 1.06 μm and dielectric constants of < 3, have demonstrated 100+ GHz data rates with less than 4 volt operating voltages. This has gained interest from the space based applications community, since in addition to being tolerant to a space environment, electro-optic devices for space applications will also need to operate at high data rates and at low operational powers. We have investigated various NLO polymers for core materials as well as passive polymers with various conductivities, both ionic and electronic, suitable for use as optical cladding layers in NLO polymer based opto-electronic devices. Our goal was to find materials that would be tolerant to irradiation as well as maximizing the nonlinearity of the NLO core material, thus minimizing the total applied poling voltage, and minimize the optical absorption loss. Using a cladding material that is more conductive than the NLO core material, the majority of the applied poling voltage is dropped across the core, thus maximizing the EO coefficient with minimum applied voltage or power. We found, however, that it is necessary to balance the optical and electromagnetic properties of the materials with their processability and compatibility.
The behavior of two nonlinear polymer-modulator materials irradiated by 63.3 MeV protons to a dose of 1 Mrad(Si) is reported. The effects of proton induced ionization and heating in disperse red 1/poly(methylmethacrylate) and in poly(ethylene dioxythiophene)/poly styrene sulphonate/poly(vinyl alcohol) thin films are discussed. Attenuation of the light transmission at an optical wavelength of (lambda) equals 0.6328 micrometers was measured in the polymer films, conductive coatings and their respective substrates. Comparison of these results with a recently reported study of related nonlinear polymer modulator materials exposed to protons is discussed. Conclusions and recommendations regarding the potential application of polymers in the near-Earth space environment are presented.
A nonlinear optic polymer blend of disperse red 1 in poly methyl methacrylate, spin cast on an indium tin oxide coated borosilicate glass substrate and a conductive polymer blend of polyethylene dioxythiophene/poly styrene sulphonate (Baytron P) in poly vinyl alcohol, spin cast on an uncoated borosilicate glass substrate were irradiated by 63.3 MeV proton to a dose of 1 Mrad (Si) of proton particles. The pre and post irradiated optical transmission characteristics of these polymer films over a wavelength range of 400 - 2000 nm, as well as in-situ thermal heating generated by irradiation are presented.
Fiber Bragg gratings were examined in situ while exposed to gamma-ray and proton environments that emulated or exceeded nominal low earth orbit radiation doses. Radiation and temperature induced changes to the reflected Bragg grating amplitude and spectral characteristics were observed in fiber gratings not formed under hydrogen loading. THe irradiations were performed in situ, while the ambient temperature near the Bragg grating was allowed to vary. Shifts in the reflected spectra from the Bragg gratings were observed and attributed to ionization and thermal effects. During irradiation, the spectral shifts were observed to move to longer wavelengths, saturate at low dose and to decrease exponentially following the cessation of radiation.
Experiments were conducted to evaluate the viability of commercial-off-the-shelf (COTS) technology in the design of high-performance optical intersatellite communications links. The optical links were operated at 155, 622 and 2488 Megabits per second (Mbps) and at 1550 nm wavelength. Communications components were evaluated against performance parameters for on-off keyed (OOK) systems. Transmitters, receivers, optical amplifiers, and filters, used to minimize the impact of amplified spontaneous emission (ASE), were characterized individually and within system configurations. Bit error rate (BER) as a function of photons per bit was characterized for several system configurations. Extinction ratio analysis experiments were conducted to determine the limiting factors on the systems performance. Widely varying optical powers in space (for low earth orbit) imply a robust receiver dynamic range requirement. Dynamic range of COTS receivers was examined and resulted in sufficient performance. In addition, gamma radiation tests on fiber amplifiers were also evaluated in a system context.
Four Erbium doped fiber optic amplifiers (EDFAs) were irradiated by gamma-rays to dose levels of 40 Krad(Si) and 100 Krad(Si) at dose rates of 0.25 rad(Si) sec-1 and 1.0 rad(Si) sec-1, respectively. All EDFAs were observed to incur radiation induced permanent decreases to their pre-irradiated signal gains. The paper will discuss the influence of gamma-ray irradiations on EDFA parameters such as gain, noise figure, and integrated amplified spontaneous emission. A brief discussion of how changes to these parameters evolve is presented.
High quality optical fiber to OEIC pigtailing, using non-conventional technology, is required to create a real integrated optical
system for optical communications, computing, signal processing, control, and sensing. In this paper, Physical Optics
Corporation (POC) presents a novel singlemode fiber to singlemode GaAs channel waveguide pigtailing approach. This
pigtailing approach involves two key technologies. First, a fiber end-face lensing technology was used to improve modeprofile
matching between singlemode fiber and singlemode channel waveguide, so fiber to waveguide coupling efficiency
could be improved. Second, resistance layer assisted dual-carrier-soldering (RLADCS) technology was introduced to facilitate
fiber and waveguide chip alignment and fixing, so accurate, convenient, and reliable fiber to optoelectronic integrated cicuit
(OEIC) pigtailing could be achieved. By using radiation hardened fiber and special OEIC, this pigtailing and packaging
technology has potential applications in a space environment. This publication addresses all aspects of this pigtailing
approach, including theoretical analysis, design, fabrication, testing, and measurement results.
Radiation induced changes to the polarization states of light propagating in various photonic devices have been observed. This paper reports on the nature of the polarization changes in one particular photonic device -- the acousto optic Bragg cell. A short description of the detection methodology used to quantify transient changes to the polarization states and to the degree of polarization in tellurium dioxide and gallium phosphide Bragg cells are presented. A discussion of the Stokes parameter analysis used to interpret the data is also presented.
Pulsed electron-induced responses observed in a proton exchanged LiNbO3 optical directional coupler waveguide are presented. The proton exchanged coupler was observed to be less sensitive to electron radiation than a previously studied titanium indiffused LiNbO3 directional coupler. A brief comparison of the attenuation and crosstalk behavior experienced by both couplers exposed to comparable electron doses is presented. The responses of the couplers are believed to depend on their respective photorefractivity and on the extent of radiation-induced heating.
Radiation testing of photonic components is not new, however component level testing to date has not completely addressed quantities which are important to system behavior. One characteristic that is of particular importance for optical processing systems is the frequency response. In this paper, we present the results of the analysis of data from an experiment designed to provide a preliminary understanding of the effects of radiation on the frequency response of acousto-optic devices. The goal is to present possible physical mechanisms responsible for the radiation effects and to discuss the effects on signal processing functionality. The experiment discussed in this paper was designed by Sandia National Laboratories (SNL) and performed by SNL and Phillips Laboratory (PL) personnel at White Sands Missile Range (WSMR). In the experiment, a TeO2 slow shear-wave aocusto-optic cell was exposed to radiation from the WSMR linear accelerator. The TeO2 cell was placed in an experimental configuration which allowed swept frequency diffracted power measurements to be taken during radiation exposure and recovery. A series of exposures was performed. Each exposure consisted of between 1 to 800, 1 microsecond(s) ec radiation pulses (yielding exposures of 2.25 kRad(Si) to 913 kRad(Si), followed by recovery time. At low total and cumulative doses, the bandshape of the frequency response (i.e. diffracted power vs. frequency) remained almost identical during and after radiation. At the higher exposures, however, the amplitude and width of the frequency response changed as the radiation continued, but returned to the original shape slowly after the radiation stopped and recovery proceeded. It is interesting to note that the location of the Bragg degeneracy does not change significantly with radiation. In this paper, we discuss these effects, and we discuss the effect on the signal processing functionality.
Experimental data of the first observed radiation-induced transient changes to the bandwidth responses of an acouso-optic (AO) Bragg device is reported. A brief description is presented of a procedure for measuring radiation-induced transient changes to the first-order diffracted beam and the zero-order transmitted beam to determine the diffraction efficiencies of PbMoO4, TeO2, and GaP AO deflectors and modulators. A brief analysis of the effects of radiation-induced color centers and heating effects in a PbMoO4 AO Bragg device is presented. It was determined that a significant portion of the changes to the PbMoO4 swept frequency reponses were due to radiation-induced heating, and that radiation-induced color center effects played a secondary role. Changes to the PbMoO4 diffraction efficiency were induced by CO2 laser heating of the PbMoO4 crystal. The laser-induced changes correlated highly with electron-induced changes indicating alteration of the acoustic diffraction grating which resulted in changes to the bandwidth and bandshape responses. The presence of thermally induced spatial index gradients across the acousto-optic interaction region are believed responsible for these changes.
A brief report on an ongoing study concerned with the responses of acousto-optic modulators (AOMs) and deflectors (AODs) exposed to linearly accelerated electrons, gamma rays, and X rays is presented. The diffracted spatial intensities of PbMoO4, TeO2, and InP devices were observed to undergo displacements and attenuation. Discussions of radiation- induced temperature gradients and color center formation believed to be responsible for these effects are included.
Optical guided wave devices can experience crosstalk and mode-switching in the presence of ionizing radiation. This paper discusses the response of LiTaO3 and LiNbO3:Ti directional coupler waveguides to exposures of linearly accelerated electrons. A comparison of the waveguides in terms of sensitivity to the ionizing radiation is made.
The exposure of birefringent guided wave devices to ionizing radiation can result in nonequivalent signal attenuations over the eigenmode axes and to highly interactive crosstalk between the coupled waveguides. This paper discusses the response of polarization maintaining (PM) fibers and directional coupler waveguides exposed to accelerated electrons, and reports on advances made in delineating and understanding the response mechanisms involved.
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