Showing papers on "Single-mode optical fiber published in 2009"
TL;DR: In this paper, the authors give a tutorial overview of OFDM and highlight the aspects that are likely to be important in optical applications, and discuss the constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless.
Abstract: Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.
1,761 citations
TL;DR: A fiber-optical switch that is activated at tiny energies corresponding to a few hundred optical photons per pulse is demonstrated by simultaneously confining both photons and a small laser-cooled ensemble of atoms inside the microscopic hollow core of a single-mode photonic-crystal fiber.
Abstract: We demonstrate a fiber-optical switch that is activated at tiny energies corresponding to a few hundred optical photons per pulse. This is achieved by simultaneously confining both photons and a small laser-cooled ensemble of atoms inside the microscopic hollow core of a single-mode photonic-crystal fiber and using quantum optical techniques for generating slow light propagation and large nonlinear interaction between light beams.
442 citations
TL;DR: A dual sensing fiber-optic hydrophone that can make simultaneous measurements of acoustic pressure and temperature at the same location has been developed and showed that the sensor is not susceptible to viscous heating, is able to withstand high intensity fields, and can simultaneously acquire acoustic waveforms while monitoring induced temperature rises.
Abstract: A dual sensing fiber-optic hydrophone that can make simultaneous measurements of acoustic pressure and temperature at the same location has been developed for characterizing ultrasound fields and ultrasound-induced heating. The transduction mechanism is based on the detection of acoustically- and thermally-induced thickness changes in a polymer film Fabry-Perot interferometer deposited at the tip of a single mode optical fiber. The sensor provides a peak noise-equivalent pressure of 15 kPa (at 5 MHz, over a 20 MHz measurement bandwidth), an acoustic bandwidth of 50 MHz, and an optically defined element size of 10 microm. As well as measuring acoustic pressure, temperature changes up to 70 degrees C can be measured, with a resolution of 0.34 degrees C. To evaluate the thermal measurement capability of the sensor, measurements were made at the focus of a high-intensity focused ultrasound (HIFU) field in a tissue mimicking phantom. These showed that the sensor is not susceptible to viscous heating, is able to withstand high intensity fields, and can simultaneously acquire acoustic waveforms while monitoring induced temperature rises. These attributes, along with flexibility, small physical size (OD approximately 150 microm), immunity to Electro-Magnetic Interference (EMI), and low sensor cost, suggest that this type of hydrophone may provide a practical alternative to piezoelectric based hydrophones.
268 citations
TL;DR: In this paper, a comprehensive study of the strain and temperature-sensing characteristics of singlemode-multimode-singlemode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers was carried out.
Abstract: We present a comprehensive study of the strain and temperature-sensing characteristics of single-mode-multimode-single-mode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers. A detailed theoretical study of the structures in terms of the refractive index distribution, effect of dopant and their concentrations, and the variation of core diameter has been carried out. Our study shows that for the SMS structure with a GeO2-doped MMF there exists a critical wavelength on either side of which the spectrum shows opposite spectral shift with a change in temperature/strain, whereas for structures with a P2O5-doped MMF it shows monotonic red shift with increasing temperature/strain. It has been found that the critical wavelength shifts toward higher wavelengths with decreasing ldquoqrdquo value/doping concentration. Using different MMFs, both the red and blue spectral shifts have been observed experimentally. It has also been found that the SMS structure has higher sensitivity toward this critical wavelength. The study should find application in designing strain-insensitive high-sensitive temperature sensors or vice versa.
262 citations
Patent•
05 May 2009
TL;DR: In this paper, a singlemode optical fiber with bending losses of 0.15 dB/turn or less for a radius of curvature of 5 millimeters is presented, at a wavelength of 1550 nanometers.
Abstract: The present invention embraces a single-mode optical fiber that, at a wavelength of 1550 nanometers, has bending losses of 0.15 dB/turn or less for a radius of curvature of 5 millimeters.
257 citations
TL;DR: In this paper, the authors present the data on the present degree of purity of chalcogenide glasses for fiber optics, on their methods of production and on the properties, which are essential for their actual application, are generalized.
Abstract: The data on the present degree of purity of chalcogenide glasses for fiber optics, on their methods of production and on the properties, which are essential for their actual application, are generalized. The content of limiting impurities in the best samples of chalcogenide glasses is 10–100 ppb wt.; of heterophase inclusions with size of about 100 nm is less than 103 cm−3. On the basis of chalcogenide glasses the multimode and single mode optical fibers are produced with technical and operation characteristics sufficient for a number of actual applications. The minimum optical losses of 12–14 dB/km at 3–5 µm are attained in the optical fiber from arsenic-sulfide glass. The level of losses in standard chalcogenide optical fibers is 50–300 dB/km in 2–9 µm spectral range. The factors, affecting the optical absorption of glasses and optical fibers, are analyzed, and the main directions in further development of chalcogenide glasses as the materials for fiber optics are considered.
236 citations
Patent•
29 Oct 2009
TL;DR: In this article, a modular, compact and widely tunable laser system for the efficient generation of high peak and high average power ultrashort pulses is proposed, where the output of the fiber amplifiers can be expanded by using optimized pulse shapes, as well as dispersively broadened pulses.
Abstract: A modular, compact and widely tunable laser system for the efficient generation of high peak and high average power ultrashort pulses. Modularity is ensured by the implementation of interchangeable amplifier components. System compactness is ensured by employing efficient fiber amplifiers, directly or indirectly pumped by diode lasers. Peak power handling capability of the fiber amplifiers is expanded by using optimized pulse shapes, as well as dispersively broadened pulses. Dispersive broadening is introduced by dispersive pulse stretching in the presence of self-phase modulation and gain, resulting in the formation of high-power parabolic pulses. In addition, dispersive broadening is also introduced by simple fiber delay lines or chirped fiber gratings, resulting in a further increase of the energy handling ability of the fiber amplifiers. The phase of the pulses in the dispersive delay line is controlled to quartic order by the use of fibers with varying amounts of waveguide dispersion or by controlling the chirp of the fiber gratings. After amplification, the dispersively stretched pulses can be re-compressed to nearly their bandwidth limit by the implementation of another set of dispersive delay lines. To ensure a wide tunability of the whole system, Raman-shifting of the compact sources of ultrashort pulses in conjunction with frequency-conversion in nonlinear optical crystals can be implemented, or an Anti-Stokes fiber in conjunction with fiber amplifiers and Raman-shifters are used. A particularly compact implementation of the whole system uses fiber oscillators in conjunction with fiber amplifiers. Additionally, long, distributed, positive dispersion optical amplifiers are used to improve transmission characteristics of an optical communication system. Finally, an optical communication system utilizes a Raman amplifier fiber pumped by a train of Raman-shifted, wavelength-tunable pump pulses, to thereby amplify an optical signal which counterpropogates within the Raman amplifier fiber with respect to the pump pulses.
227 citations
TL;DR: In this paper, the authors proposed coherent optical single carrier transmission (COSC) using overlap frequency domain equalization (OFDE) for the chromatic dispersion (CD) compensation.
Abstract: In this paper, we present coherent optical single carrier transmission (COSC) using overlap frequency domain equalization (OFDE) for the chromatic dispersion (CD) compensation. Residual inter-symbol interference is suppressed by time domain equalization with small tap size. In single polarization transmission, 25-Gb/s COSC transmission with OFDE is demonstrated over 800 km to 4320 km of single-mode fiber (SMF) without optical dispersion compensation. The measured results show that the proposed configuration improves the transmission performance by setting adequate overlapped size of FFT windows and that it is suitable for long-haul optical communication systems.
202 citations
TL;DR: A chain of four Tm-doped fibers amplified a single-frequency, 2040 nm diode laser to 608 W with M(2)=1.05+/-0.03, to be known as the highest power obtained from any single- frequencies, single-mode fiber laser.
Abstract: A chain of four Tm-doped fibers amplified a single-frequency, 2040 nm diode laser to 608 W with M2=1.05±0.03, limited by available pump power. Stimulated Brillouin scattering limits were investigated by splicing different lengths of passive fiber to the output of the final amplifier stage. Integrated rms phase noise above 1 kHz was less than λ/30, suggesting the possibility of further scaling via coherent beam combining. To our knowledge, this is the highest power obtained from any single-frequency, single-mode fiber laser.
192 citations
TL;DR: An electrically-pumped hybrid silicon microring laser fabricated by a self-aligned process with compact structure and small electrical and optical losses is demonstrated and application as on-chip optical interconnects is discussed.
Abstract: We demonstrate an electrically-pumped hybrid silicon microring laser fabricated by a self-aligned process. The compact structure (D = 50 μm) and small electrical and optical losses result in lasing threshold as low as 5.4 mA and up to 65 °C operation temperature in continuous-wave (cw) mode. The spectrum is single mode with large extinction ratio and small linewidth observed. Application as on-chip optical interconnects is discussed from a system perspective.
186 citations
TL;DR: In this paper, the authors developed a field-coupling model for propagation in graded-index MMF, analogous to the principal-state model for polarization-mode dispersion in single-mode fiber.
Abstract: Power-coupling models are inherently unable to describe certain mode coupling effects in multimode fiber (MMF) when using coherent sources at high bit rates, such as polarization dependence of the impulse response. We develop a field-coupling model for propagation in graded-index MMF, analogous to the principal-states model for polarization-mode dispersion in single-mode fiber. Our model allows computation of the fiber impulse response, given a launched electric-field profile and polarization. In order to model both spatial- and polarization-mode coupling, we divide a MMF into numerous short sections, each having random curvature and random angular orientation. The model can be described using only a few parameters, including fiber length, number of sections, and curvature variance. For each random realization of a MMF, we compute a propagation matrix, the principal modes (PMs), and corresponding group delays (GDs). When the curvature variance and fiber length are small (low-coupling regime), the GDs are close to their uncoupled values, and scale linearly with fiber length, while the PMs remain highly polarized. In this regime, our model reproduces the polarization dependence of the impulse response that is observed in silica MMF. When the curvature variance and fiber length are sufficiently large (high-coupling regime), the GD spread is reduced, and the GDs scale with the square root of the fiber length, while the PMs become depolarized. In this regime, our model is consistent with the reduced GD spread observed in plastic MMF.
TL;DR: The fabricated photonic lantern has a coupling loss for a MM to SM tapered transition of only 0.32 dB which proves the feasibility of the technology and demonstrates efficient and reversible coupling between a MM fiber and a number of SM fibers.
Abstract: We demonstrate the fabrication of a high performance multi-mode (MM) to single-mode (SM) splitter or "photonic lantern", first described by Leon-Saval et al. (2005). Our photonic lantern is a solid all-glass version, and we show experimentally that this device can be used to achieve efficient and reversible coupling between a MM fiber and a number of SM fibers, when perfectly matched launch conditions into the MM fiber are ensured. The fabricated photonic lantern has a coupling loss for a MM to SM tapered transition of only 0.32 dB which proves the feasibility of the technology.
TL;DR: The proposed refractometer with improved sensitivity for low SRI measurement together with the tip-reflection sensing feature, is a good candidate for sensing in chemical and biological applications.
Abstract: A novel in-fiber structure for power-referenced refractometry with the capability to measure surrounding refractive index (SRI) as low as 1.33 is proposed and demonstrated. A short optical fiber stub containing a weakly tilted Bragg grating is spliced to another fiber with a large lateral offset. The reflection from this structure occurs in two well-defined wavelength bands, the Bragg reflected core mode and the cladding modes. The cladding modes reflect different amounts of power as the SRI changes, while the core-mode reflection from the same weakly tilted FBG remains unaffected by the SRI. The power reflected in the core mode band can be used as a reliable reference to cancel out any possible power fluctuations. The proposed refractometer with improved sensitivity for low SRI measurement together with the tip-reflection sensing feature, is a good candidate for sensing in chemical and biological applications.
TL;DR: Spatially and spectrally resolved imaging of mode content in fibers, or more simply, S2 imaging, is a new measurement technique for analyzing the mode content of large-mode-area (LMA) fibers as mentioned in this paper.
Abstract: Spatially and spectrally resolved imaging of mode content in fibers, or more simply, S2 imaging, is a new measurement technique for analyzing the mode content of large-mode-area (LMA) fibers. It works by spatially resolving the spectral interference that occurs when light in a few-mode fibers scatters into different modes that then propagate with different group delays. A scanning spatial filter in the form of a single-mode fiber probe coupled to an optical spectrum analyzer is utilized to provide both spatial and spectral resolution, and the data are analyzed via the Fourier transform of the optical spectrum. The wealth of data allows for imaging multiple modes simultaneously propagating in the fiber under test as well as quantifying their relative power levels. In addition, the ability to analyze mode images as a function of modal group delay allows distinguishing between discrete scattering at fiber surfaces and distributed scattering that occurs along the length of the LMA fiber. The all-fiber nature of the setup makes the measurement sufficiently stable to measure phase images of the higher order modes (HOMs). Because the method is interferometrically based, even very weak HOMs can be detected.
TL;DR: In this article, a linearly field-modulated, direct-detected virtual single-sideband orthogonal frequency-division multiplexing (VSSB-OFDM) system that employs a tunable frequency gap and an iterative detection technique is proposed as a spectrally efficient format.
Abstract: We analytically and experimentally demonstrate a linearly field-modulated, direct-detected virtual single-sideband orthogonal frequency-division multiplexing (VSSB-OFDM) system that employs a tunable frequency gap and an iterative detection technique. The VSSB-OFDM that uses no frequency gap, which is referred to as the gapless VSSB-OFDM, is proposed as a spectrally efficient format. Compared with the intensity-modulated SSB-OFDM, the gapless VSSB-OFDM saves half the electrical bandwidth (BW), and exhibits better receiving sensitivity and more robust tolerance against fiber chromatic dispersion (CD). Furthermore, by incorporating a tunable frequency gap between the optical carrier and the OFDM data sideband, the calculating burden of the iterative detection is greatly alleviated and the system performance can be flexibly improved within moderate iterations. The width of the optimum frequency gap is found to be ${\sim} {\hbox {0.35}}$ sideband BW, which is reached by trading the levels of signal–signal beat interference and the residual image beat interference. Such a gapped VSSB-OFDM system requires fewer iterations to extract the desired data from the interfered signal and exhibits greater robustness against the carrier-to-signal-power ratio (CSPR) variation, compared with the gapless VSSB-OFDM. In this paper, the analytical model of the proposed gapped VSSB-OFDM system will be addressed. In addition, we also successfully conduct a gapped VSSB-OFDM signal transmission over 1600 km of uncompensated standard single-mode fiber (SSMF) with only ${\sim} {\hbox {3}}$ dB optical SNR (OSNR) penalty, and obtain a significant OSNR sensitivity improvement of ${\sim} {\hbox {8}}$ dB, compared with the gapless VSSB-OFDM, after such a 1600-km fiber link.
TL;DR: In this paper, the authors present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW.
Abstract: High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ~90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.
TL;DR: In this paper, a Mach-Zehnder interferometer with two abrupt singlemode fiber tapers is simulated, constructed, and demonstrated, which has an insertion loss of 5 dB and an extinction ratio over 15 dB.
Abstract: A Mach-Zehnder interferometer with two abrupt single-mode fiber tapers is simulated, constructed, and demonstrated. The interferometer has an insertion loss of 5 dB and an extinction ratio over 15 dB. The interferometer is tested as a strain sensor based on the maximum attenuation wavelength shift with a comparable sensitivity (slope: 2000 nm/ epsiv, R 2 = 0.996) with long-period-grating-type sensor and promises low fabrication cost.
TL;DR: This work presents an implementation of a fibre integrated spectrograph for characterizing the spectrum of an ultrafast optical pulse at the single photon level.
Abstract: We demonstrate the implementation of a fiber-integrated spectrograph utilizing chromatic group velocity dispersion (GVD) in a single mode fiber. By means of GVD we stretch an ultrafast pulse in time in order to spectrally resolve single photons in the time domain, detected by single photon counting modules with very accurate temporal resolution. As a result, the spectrum of a very weak pulse is recovered from a precise time measurement with high signal to noise ratio. We demonstrate the potential of our technique by applying our scheme to analyzing the joint spectral intensity distribution of a parametric downconversion source at telecommunication wavelength.
TL;DR: In this paper, the authors proposed abrupt tapers and connector-offset attenuators as an alternative to LPGs as mode-coupling mechanisms to transfer optical power between core and cladding modes in optical fiber.
Abstract: Novel in-line single-mode fiber interferometers- Mach-Zehnder and Michelson-have been designed, fabricated, and tested as refractive index (RI) sensors. Abrupt tapers and connector-offset attenuators are proposed as alternatives to long period gratings (LPGs) as mode-coupling mechanisms to transfer optical power between core and cladding modes in optical fiber. The coupling coefficients between core and cladding modes in the proposed designs were calculated using numerical packages and the devices were subsequently implemented using commercially available fusion splicer. For an abrupt taper, most coupling occurs between the LP01 and LP0 m modes, with the first ten modes accounting for 98% of the incident mode energy. For a connector-offset attenuator, coupling mainly occurs between the LP01 and LP1 m modes, with the first ten LP0 m modes and first ten LP1 m modes accounting for 92% of the incident mode energy. In particular, in the case of connector-offset attenuator, the relative direction between the two connector-offsets was found to be very important to the interference performance. Interference patterns were realized in simulation for the interferometers using both mode-coupling mechanisms. Three interferometers were realized in the experiment using abrupt taper-Mach-Zehnder and Michelson-and connector-offset attenuator-Michelson. They showed large extinction ratios (up to 23 dB) and small insertion losses (smaller than 3 dB). Although it is difficult to make Mach-Zehnder interferometers using connector-offset attenuator pair due to the lack of polarization control in the fusion splicer, some evidence of constructive interference was observed in the experiment. The interferometers were tested as RI sensors using the maximum attenuation wavelength shift. Given that the minimum resolution of optical spectrum analyzer is 10 pm, ~ 10-4 difference of RI can be detected by the proposed interferometric sensors, providing similar performance as LPG-based interferometers at a lower cost and simpler fabrication process.
TL;DR: In this paper, a new ultra-low bending loss single-mode fiber with ring comprising nanometer sized features is designed and manufactured, and a bend radius of 5 mm is demonstrated.
Abstract: A new ultra-low bending loss single-mode fiber with ring comprising nanometer sized features is designed and manufactured. Bending loss less than 0.1 dB/turn at 1550 nm and a bend radius of 5 mm is demonstrated. Other optical parameters of the fiber are fully compatible the standard telecommunications grade single-mode fibers.
TL;DR: In this paper, the authors evaluate the limitation of the effective area for SM operation by taking into account the effects of practical constraints such as fiber bending loss, laser performance, and fiber mechanical reliability on these LMA fiber designs.
Abstract: Step-index (SI) fiber designs are commonly used in achieving large mode area (LMA) and single-mode (SM) operation in fiber lasers. These fibers can either be intrinsically single-moded fibers or few-moded fibers, which can be forced into SM operation through bending. In this paper we evaluate the limitation of the effective area for SM operation by taking into account the effects of practical constraints such as fiber bending loss, laser performance, and fiber mechanical reliability on these LMA fiber designs. It is shown that the effective area of these fibers cannot be arbitrarily scaled up with the size of the fiber core. We also use the modeling result to provide estimated upper limits to the core diameters and corresponding effective areas for conventional SI fiber designs taking into account fiber parameters that are achievable nowadays.
TL;DR: In this paper, the authors proposed a mode-field matched center-launching technique by fusion-splicing a single-mode fiber (SMF) pigtailed transmitter to the MMF, where the splicing condition is optimized to expand the core of SMF slightly so that it can match the mode field distribution of the fundamental mode of MMF.
Abstract: We report that the center-launching technique can be improved to selectively excite the fundamental mode of multimode fiber (MMF). This ldquomode-field matchedrdquo center-launching technique enables us to excite only the fundamental mode in the MMF and, consequently, avoid the inherent limitations imposed by the differential mode delay. We realize this mode-field matched center-launching technique simply by fusion-splicing a single-mode fiber (SMF) pigtailed transmitter to the MMF. The splicing condition is optimized to expand the core of SMF slightly so that it can match the mode field distribution of the fundamental mode of MMF. The results show that, by using this launching technique, we can achieve the transmission characteristics similar to SMF and drastically increase the bandwidth-distance product of MMF. For demonstrations, we have successfully transmitted 10- and 40-Gb/s signals over 12.2 and 3.7 km of MMF, respectively, without using any dispersion compensation techniques. We have also evaluated the robustness of the MMF link implemented by using the proposed launching technique against the mechanical perturbations such as the lateral offset between fiber connectors, fiber bending, and fiber shaking.
TL;DR: In this article, a fiber inline Mach-Zehnder-type core-cladding-mode interferometer fabricated by two-point CO2 laser irradiations on a single-mode fiber was reported.
Abstract: We report a fiber inline Mach-Zehnder-type core-cladding-mode interferometer fabricated by two-point CO2 laser irradiations on a single-mode fiber. The laser irradiations caused efficient light coupling from the core mode to the lower order cladding modes and vise versa. High-quality interference spectra with a fringe visibility of about 20 dB were observed for four different interferometer lengths (5, 10, 20, and 40 mm). The temperature sensitivity of the device with a length of 5 mm was measured to be 0.0817 nm/degC. The sensitivity for refractive index measurement of the device was comparable with a long-period fiber grating of LP04 cladding mode.
TL;DR: A broadband single-mode bi-conical microfiber coupler with a specifically designed transition region that effectively suppresses any higher-order modes present at the input fiber and provides efficient power splitting into the fundamental mode at the two output ports is presented.
Abstract: We present a broadband single-mode bi-conical microfiber coupler (MFC) with a specifically designed transition region that effectively suppresses any higher-order modes present at the input fiber and provides efficient power splitting into the fundamental mode at the two output ports. As a practical example, single-mode 3dB splitting operation over a broad spectral window (400~1700 nm) was demonstrated for a very thin taper waist (~1.5µm) MFC made from conventional telecom optical fibers.
TL;DR: In this article, a single photon source consisting of an InAs quantum dot coupled to a directional-emission photonic crystal (PC) cavity implemented in GaAs was analyzed, and the cavity design also enabled efficient coupling to a higher-order cavity mode for local optical excitation of cavity-coupled quantum dots.
Abstract: We analyze a single photon source consisting of an InAs quantum dot coupled to a directional-emission photonic crystal (PC) cavity implemented in GaAs. On resonance, the dot’s lifetime is reduced by more than 10 times, to 45ps. Compared to the standard three-hole defect cavity, the perturbed PC cavity design improves the collection efficiency into an objective lens (NA = 0.75) by factor 4.5, and improves the coupling efficiency of the collected light into a single mode fiber by factor 1.9. The emission frequency is determined by the cavity mode, which is antibunched to g^(2)(0) = 0.05. The cavity design also enables efficient coupling to a higher-order cavity mode for local optical excitation of cavity-coupled quantum dots.
01 Jan 2009
TL;DR: This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications and the constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless.
Abstract: Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.
TL;DR: The perturbed PC cavity design improves the collection efficiency into an objective lens, and improves the coupling efficiency of the collected light into a single mode fiber, for local optical excitation of cavity-coupled quantum dots.
Abstract: We analyze a single photon source consisting of an InAs quantum dot coupled to a directional-emission photonic crystal (PC) cavity implemented in GaAs. On resonance, the dot's lifetime is reduced by more than 10 times, to 45ps. Compared to the standard three-hole defect cavity, the perturbed PC cavity design improves the collection efficiency into an objective lens (NA=0.75) by factor 6, and improves the coupling efficiency of the collected light into a single mode fiber by factor 1.9. The emission frequency is determined by the cavity mode, which is antibunched to g(2)=0.05. The cavity design also enables efficient coupling to a higher-order cavity mode for local optical excitation of cavity-coupled quantum dots.
TL;DR: A compact in-fiber refractive index (RI) sensor is presented that is based on a microhole created in a conventional single-mode fiber by the use of femtosecond laser micromachining, finding that the relationship between the transmission and the RI is critically dependent on the size of the microhole in the fiber core region.
Abstract: A compact in-fiber refractive index (RI) sensor is presented that is based on a microhole created in a conventional single-mode fiber by the use of femtosecond laser micromachining. The transmission properties of such a device with a microhole of different diameters have been investigated in the wavelength region of 1500-1600 nm and in the RI range of 1.30-1.45. It is found that the relationship between the transmission and the RI is critically dependent on the size of the microhole in the fiber core region. The highest resolution obtained is 6.70×10−5, in the RI range of 1.37-1.42, when the microhole diameter is ~8 μm, close to the fiber core size. The in-fiber RI sensor developed in this work is easy to fabricate and can be used to implement temperature-independent measurements.
TL;DR: A single-mode evanescently coupled multicore fiber consisting of 19 hexagonally arranged cores is investigated and compared to an equivalent hypothetical step-index fiber, showing the high potential of the developed fiber.
Abstract: A single-mode evanescently coupled multicore fiber consisting of 19 hexagonally arranged cores is investigated. Theoretical and experimental results are presented and compared to an equivalent hypothetical step-index fiber. A fundamental mode with an effective area of 465 μm2 and a beam propagation factor M2 of 1.02 was measured, showing the high potential of the developed fiber.
TL;DR: In this article, an optical configuration for target-in-the-loop single-mode fiber amplifier coherent combining through turbulence is presented, with a lambda/15 residual phase error, and theoretical analysis demonstrate that detection subsystem aperture reduction is paramount to lower sensitivity to backward turbulence when using a detector in the laser emitter plane.
Abstract: We report experimental results and theoretical analysis of coherent beam combining with active phase control fiber beam shaping. An original optical configuration for target-in-the-loop single-mode fiber amplifier coherent combining through turbulence is presented, with a lambda/15 residual phase error. The experimental results and theoretical analysis demonstrate that detection subsystem aperture reduction is paramount to lower sensitivity to backward turbulence when using a detector in the laser emitter plane. In this configuration, coherent combining is achieved on a remote scattering surface with sole compensation of the onward turbulence. We also present a numerical model capable of assessing the combining efficiency in the case of high-power multimode large-mode-area (LMA) fiber amplifiers. Preliminary theoretical investigations point out that multiple-transverse-mode combining can result in severe wavefront distortion. In the case of multimode LMA fibers, control of the transverse modes phase relationship has to be achieved to preserve combining efficiency.