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Space division multiplexing (SDM) has generated much interest lately as a potential means for enhancing the capacity of optical transmission systems. During the past several years, we have observed tremendous research efforts in the development of SDM amplifiers with an aim to increase core and mode counts and to improve amplification properties and pump conversion efficiencies. We report on the recent development of multicore fiber amplifiers suitable for amplifying space division multiplexed signals. Multicore fiber amplifiers with different number of cores, and pumping schemes have been developed to pump the cores individually or through a common cladding. We will report on the structures of SDM amplifiers, their optical properties, and discuss prospects for further development.

Sterilization of medical equipment using e-beam or gamma radiation produces high sterility assurance but may lead to alterations in materials being treated. Thus, color centers developed in optical fibers result in added attenuation. In this work a series of specialty single-mode (SM) and multimode (MM) optical fibers (13 in total) were exposed to gamma and e-beam sterilization doses of 32 kGy, and the radiation-induced attenuation was investigated. The optical fibers selected for the study had different core chemical compositions, including pure silica and also SiO2 doped with Ge, F, P and Al. The fibers also differed in the clad diameters (80 and 125 μm), numerical aperture (0.1 – 0.21), coating types (carbon, acrylate, polyimide) and the cutoff wavelength (SM fibers only, 930 – 1470 nm). Effects of all these features on the radiation-induced attenuation were analyzed primarily in the framework of medical applications. The radiation-induced optical losses can be moderated by subsequent thermal treatment of the fibers, and such option was also investigated. Our results showed that while the radiation-induced losses may be high (102 – 104 dB/km), sterilization using e-beam and gamma radiation can be acceptable for many medical applications that deploy 3 m or less fiber lengths. After gamma or e-beam sterilization, 3 meters length of SM fibers with pure SiO2 and Ge doped core transmit >98% of optical power at 1550 and 1310 nm. In contrast, similar length of SM fibers with P and Al doped cores exhibit very low transmission and cannot be used after ionizing sterilization. Among the MM fiber, F-doped core fibers display much higher transmission than Ge-core fibers, especially at 850 nm. The clad diameter, coating type and bend insensitive features in the refractive index profile did not show much effect on the radiation-induced loss. While gamma and e-beam radiation caused similar changes in the attenuation spectra, gamma radiation caused ~25% higher loss increase than e-beam at the same 32 kGy dose. Since our data showed that postradiation annealing at 100°C for 24 hours results in only ~30% recovery of the radiation induced loss, it may not be practical to implement such annealing scheme for medical applications.

We report on the excitation and polarization preserved propagation of a very large effective-area (Aeff ∼ 2240 μm2) higher-order-mode in an optical fiber. A laser signal operating in the 1 μm wavelength region is transported in a Bessel-like LP0,4 mode over a 10 m long section of the polarization-maintaining higher-order-mode fiber. We observe that the light propagates through the fiber with >10 dB polarization-extinction-ratio as the fiber is coiled into circular loops of 40 cm diameter.

We report on the recent development of multicore fiber amplifiers suitable for amplifying space division multiplexed signals. We have constructed 7-core amplifiers, where the cores could be pumped individually or simultaneously through the cladding, and studied the amplification and noise properties of these amplifiers. In the core-pumped amplifier, the net average gain was 25 dB, and noise figure was less than 4 dB. Using side-coupled cladding pumping, gain over 25 dB was obtained in each of the cores over a 40-nm bandwidth covering the C-band. We have further studied numerically the amplification and noise properties of a few-moded single/multicore erbium doped fiber amplifier with a doped region that extends beyond the few-mode core. We show that when excited by a multimoded pump, this can provide nearly uniform gain in C-band for modes such as, LP01, LP11, LP21, and LP02 over a wide range of pump power. Differential modal gain (DMG) between LP01, and LP02 was ~ 0.5 dB for a small signal gain of 32 dB at 1550 nm signal wavelength. The dependence of modal gain on the size of the doped core has also been studied.

We report a method for the characterization of the higher-order modes in polarization-maintaining multimode optical fibers using a Bragg grating. We evaluate a 400 µm diameter, polarization-maintaining higher-order-modes fiber by inscribing a Bragg grating, resonant at the wavelength λ ∼ 1067 n m . By comparing the reflection spectra of the light polarized along the slow and the fast axes of the fiber and using the derived mathematical relations, we estimate the birefringence and the polarization-extinction ratio of the various higher-order modes in the fiber. This represents a simple and attractive approach for a detailed investigation of the multimode polarization-maintaining fibers that are emerging as an invaluable tool for advanced manufacturing, defense, bio-imaging, and telecommunications systems.

This paper describes the physics and properties of a novel optical fiber that would be attractive for building high-power fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian-shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra-short pulses - a capability that is practically nonexistent in conventional fibers. Particularly interesting is the tact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter-length fibers (as in high-power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm 2 , and propagate signals with negligible mode distortions over up to 50-meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.

We demonstrate a new high efficiency architecture for cascaded Raman fiber lasers based on a single pass cascaded amplifier configuration. Conversion is seeded at all intermediate Stokes wavelengths using a multi-wavelength seed source. A lower power Raman laser based on the conventional cascaded Raman resonator architecture provides a convenient seed source providing all the necessary wavelengths simultaneously. In this work we demonstrate a 1480nm laser pumped by an 1117nm Yb-doped fiber laser with maximum output power of 204W and conversion efficiency of 65% (quantum-limited efficiency is ~75%). We believe both the output power and conversion efficiency (relative to quantum-limited efficiency) are the highest reported for cascaded Raman fiber lasers., 9 pages, 7 figures, replaced with an updated version

We demonstrate a 1480 nm cascaded Raman fiber laser with a new high efficiency architecture providing a record output power of 204 W. We achieve this through multiple Raman shifts of a high power 1117nm Yb-doped fiber laser in a single pass configuration mediated at all intermediate wavelengths using a seed source comprising a low power conventional 1480nm Raman laser. The conversion efficiency from 1117nm to 1480nm is ~65% (for a quantum limited efficiency of 75%). Enhancement in efficiency is achieved by elimination of excess optical loss present in the conventional cascaded Raman resonator based architecture.

We demonstrate resonant grating couplers with the broadest bandwidth (565 nm) reported to date, using microbend gratings in dispersion-optimized few-mode fibers. Control of modal dispersion enables the demonstration of spectrally flat (< 0.25-dB ripple) coupling. This enables a compact (1 cm), broad-band (110 nm) variable optical attenuator, which is highly cost-effective, since device assembly simply involves pressing an as-drawn fiber between two corrugated blocks.

This letter describes a novel structure and crosstalk characteristics of a graded-index multicore fiber with seven cores arranged in a hexagonal array. A total of 70 Gb/s (7 × 10 Gb/s) is transmitted over a single 100-m seven-core multimode fiber (MMF) using tapered multicore fiber connectors and 850-nm vertical-cavity surface-emitting lasers, demonstrating high-speed multicore MMF transmissions for parallel optical data links.

Beams with polarization singularities have attracted immense recent attention in a wide array of scientific and technological disciplines. We demonstrate a class of optical fibers in which these beams can be generated and propagated over long lengths with unprecedented stability, even in the presence of strong bend perturbations. This opens the door to exploiting nonlinear fiber optics to manipulate such beams. This fiber also possesses the intriguingly counterintuitive property of being polarization maintaining despite being strictly cylindrically symmetric, a prospect hitherto considered infeasible with optical fibers.

We generate radially-polarised beams, for the first time with a novel, phase-engineered fiber that possesses these spatial patterns as stable eigenmodes. With conversion-efficiencies >99.7%, this is an attractive alternative to complex free-space techniques used currently.

We describe the design, construction, and application of what are believed to be the smallest fiber-optic probes used to date during imaging or diagnosis involving low-coherence interferometry (LCI). The probes use novel fiber-optic gradient-index (GRIN) lenses fabricated by a recently developed modified chemical-vapor-deposition (MCVD) process that avoids on-axis aberrations commonly marring MCVD-fabricated GRIN substrate. Fusing GRIN fiber lenses onto single-mode fiber yields automatically aligned all-fiber probes that insert into tissue through hypodermic needles as small as 31-gauge (inner diameter, 127 mum). We demonstrate the use of such probes with LCI by measuring microscopic brain motions in vivo.

We demonstrate a method of generating short pulses at 1350 nm by exciting Cerenkov radiation in a higher-order-mode fiber with a 1064 nm femtosecond fiber laser. We measure a 106 fs, 0.66 nJ output pulse. Cerenkov radiation in fibers allows for energy transfer between a soliton and a dispersive wave, providing an effective and engineerable platform to shift the wavelength of a femtosecond source. With appropriate design of the higher-order-mode fiber, this method of generating short pulses at 1350 nm can be extended to other wavelengths and to higher pulse energies.

We demonstrate soliton self-frequency shift of more than 12% of the optical frequency in a higher-order mode solid, silica-based fiber below 1300nm. This new class of fiber shows great promise for supporting Raman-shifted solitons below 1300nm in intermediate energy regimes of 1 to 10nJ that cannot be reached by index-guided photonic crystal fibers or air-core photonic bandgap fibers. By changing the input pulse energy of 200fs pulses from 1.36 to 1.63nJ we observe Raman-shifted solitons between 1064 and 1200nm with up to 57% power conversion efficiency and compressed output pulse widths less than 50fs. Furthermore, due to the dispersion characteristics of the HOM fiber, we observe redshifted Cerenkov radiation in the normal dispersion regime for appropriately energetic input pulses.

We examined the room temperature radiation-induced optical attenuation of silica-core, polarization-maintaining fibers with boron stress rods. The fibers were exposed to 55 MeV protons to equivalent doses between 100 krad and 1 Mrad at dose rates of 2 to 100 rad/s. Spectra from 1250-1725 nm were recorded during both exposure and ~2,800 h recovery. The spectral response over time indicates two separate populations of defects that affect loss. Explicit kinetic modeling indicates that the growth of the induced loss is consistent with a power law in dose rate, and recovery is consistent with low (1 st or 2 nd ) order annealing with a distribution of activation energies and a single attempt frequency for each population. For predicting the radiation-induced loss in exposure conditions of much lower dose rate and much longer time, our theory and numerical simulations indicate that the best experiments for reducing risk in our extrapolations include higher temperature anneals, and, unavoidably, longer exposures at lower dose rate.

A frequency comb is generated with a Cr:forsterite femtosecond laser, spectrally broadened through a highly nonlinear optical fiber to span from 1.0 to 2.2 ,m, and stabilized using the f-to-2f self-referencing technique. The repetition rate and the carrier-envelope offset frequency are stabilized to a hydrogen maser, calibrated by a cesium atomic fountain clock. Simultaneous frequency measurement of a 657-nm cw laser by use of the stabilized frequency combs from this Cr:forsterite system and a Ti:sapphire laser agree at the 10(-13) level. The frequency noise of the comb components is observed at 1064, 1314, and 1550 nm by comparing the measured beat frequencies between cw lasers and the supercontinuum frequency combs.

We present a source of high power femtosecond pulses at 1550 nm generating compressed pulses at the end of a single mode fiber pigtail. The system generates sub 35 femtosecond pulses at a repetition rate of 50 MHz, with average powers greater than 400 mW. The pulses are generated in a passively modelocked, erbium doped fiber laser, and amplified in a short, erbium doped amplifier. The output of the fiber amplifier consists of highly chirped picosecond pulses. These picosecond pulses are then compressed in standard single mode fiber. While the compressed pulses in the SMF pigtail do show a low pedestal that could be avoided with the use of bulk-optic compression the desire to compress the pulses in SMF is motivated by the ability to splice the single mode fiber to a nonlinear fiber, for continuum generation applications. We demonstrate that with highly nonlinear dispersion shifted fiber (HNLF) fusion spliced directly to the amplifier output, we generate a supercontinuum spectrum that spans more than an octave, with an average power 350 mW. Such a high power, all-fiber supercontinuum source has many important applications including frequency metrology and biomedical imaging.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A phase-locked frequency comb in the near infrared is demonstrated with a mode-locked, erbium-doped, fiber laser whose output is amplified and spectrally broadened in dispersion-flattened, highly nonlinear optical fiber to span from 1100 to >2200 nm. The supercontinuum output comprises a frequency comb with a spacing set by the laser repetition rate and an offset by the carrier-envelope offset frequency, which is detected with the standard f-to-2f heterodyne technique. The comb spacing and offset frequency are phase locked to a stable rf signal with a fiber stretcher in the laser cavity and by control of the pump laser power, respectively. This infrared comb permits frequency metrology experiments in the near infrared in a compact, fiber-laser-based system.

We demonstrate a chromium-doped forsterite femtosecond ring laser that generates 30-fs pulses at a 420-MHz repetition rate with nearly 500 mW of average power. The compact solid-state design and broad spectral output make this laser attractive for telecommunications applications in the 1.3-1.5-micrometre region. Additional spectral broadening of the laser output in highly nonlinear optical fiber leads to octave-spanning spectra ranging from 1.06 to 2.17 micrometre. The octave is reached at a level of 18 dB below the peak. The underlying optical frequency comb can be linked to existing optical frequency standards.

We present a Cr:forsterite ring laser producing 30 fs pulses at a 420 MHz repetition rate. The output is broadened in highly nonlinear fiber yielding spectra covering 1050-2200 nm.

INTRODUCTION:The advantages of Fiber Optic technology have already been demonstrated with terrestrial communicationsystems and various rapid payout deployment and as such became the corner stone for consideration in satellitesand space station applications. Mainly the key advantages that the fiber is providing are high bandwidth and verylow weight with simultaneous immunity to electromagnetic interference and electromagnetic pulse. In additionthe requirement for continuous performance in a radiation environment of low dose level such as space led us tosponsor a development effort for the design of single mode fiber optic cables to withstand the radiation andenvironmental effects of a space deployment.The program was structured in two parallel phases. The first addresses the fiber and related material optimization.The second phase emphasizes the cable design and related material to ensure a twelve year uninterrupted service.FIBER REQUIREMENTS:The single mode fiber design was selected for this effort since it provides the required bandwidth for high speedsignal processing performed on board space vehicles or space stations.The specific requirements on the fiber design are cited in Table 1.FIBER PHASEIn designing the optical fiber to meet the requirements of space deployment it was decided to build on previousexperience and enhance available designs. The effects of hydrogen-doping on improving radiation sensitivitywere systematically studied with Ge-doped and pure silica core optical fibers. The effects of H2-doping wasstudied on four types of single mode optical fiber and their response to radiation sensitivity from -150°C to

A frequency comb is generated with a chromium-doped forsterite femtosecond laser, spectrally broadened in a dispersion-shifted highly nonlinear fiber, and stabilized. The resultant evenly spaced comb of frequencies ranges from 1.1 to beyond 1.8 microm. The frequency comb was referenced simultaneously to the National Institute of Standards and Technology's optical frequency standard based on neutral calcium and to a hydrogen maser that is calibrated by a cesium atomic fountain clock. With this comb we measured two frequency references in the telecommunications band: one half of the frequency of the d/f crossover transition in 87Rb at 780 nm, and the methane v2 + 2v3 R(8) line at 1315 nm.