Rayleigh Backscattering Effects Research Articles

Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs Incorporating P2MP Flexible Optical Transceivers and Advanced Passive Remote Nodes

Driven by a large number of emerging diversified services, in the 5G and beyond era, concurrent direct inter-ONU and upstream communications inside a PON-based mobile access network are highly desirable to provide dynamic, ultra-dense, and fast ONU-to-ONU (without involving an OLT) and ONU-to-OLT connections. To cost-effectively deliver highly dynamic and low latency direct inter-ONU communications, this paper proposes and experimentally demonstrates novel concurrent direct inter-ONU and upstream communications in an upstream 27 km, >62.47 Gbit/s IMDD PON. For supporting inter-ONU communications between a large number of ONUs, an advanced passive remote node is also proposed. Based on different passive optical components, this remote node can be implemented using two approaches, which can, respectively, reduce the inter-ONU signal power losses by >12.2 dB and >16.6 dB (for 128 ONUs) in comparison with existing inter-ONU communication techniques’ remote nodes. In each ONU and OLT, a single pair of cascaded IFFT/FFT-based point-to-multipoint (P2MP) flexible optical transceivers are employed to simultaneously and dynamically establish multiple ONU-to-ONU and ONU-to-OLT communications according to actual users’ requirements. Experimental results show that the proposed network has excellent robustness against various transmission system impairments, including chromatic dispersion, the Rayleigh and Brillouin backscattering effects, and the channel interference effects. For each ONU, dynamic channel allocation can be made without compromising its overall performance.

Effect of Rayleigh backscattering on the multiwavelength random fiber laser bandwidth

We investigated the effect of Rayleigh backscattering on the bandwidth of a multiwavelength random fiber laser (MWRFL). This MWRFL is based on an erbium-doped fiber amplifier (EDFA) as the gain medium and a Lyot filter as the wavelength-selective device. A high number and stable lasing line is generated with the assistance of a nonlinear polarization rotation effect induced by highly nonlinear fiber (HNLF) and optimized by a polarization controller. Longer single-mode fiber (SMF) produces wider multiwavelength bandwidth as compared to shorter SMF lengths, as 10 km, 3 km, and 0.1 km obtained 6.24 nm, 5.76 nm, and 2.75 nm wavelength bandwidth, respectively. When the SMF is removed from the setup, no Rayleigh backscattering is provided in the cavity, which led to a narrower multiwavelength bandwidth of 2.69 nm. Subsequently, the alteration of EDFA power, half-wave plate (HWP) angle, and quarter-wave plate (QWP) angles affected the degradation of multiwavelength performance. The best output spectrum is achieved at an EDFA power of 18 dBm, a HWP angle of 60°, and a QWP angle of 105°. The laser stability over a period of 120 min was excellent with a peak power deviation of 1.33 dB.

Stable Triple-Wavelength Random Fiber Laser Based on Fiber Bragg Gratings

We demonstrate a generation of three lasing wavelengths with the assistance of Rayleigh backscattering as the stabilizer of peak power variations. The proposed laser consists of a combination of the semiconductor optical amplifier (SOA) and erbium-doped fiber amplifier (EDFA) as the amplifying media. Three fiber Bragg gratings are employed as the selective wavelength selectors at 1544, 1547 and 1550 nm. At 110 mA SOA current and 18 dBm EDFA output power, a flattened output spectrum with 0.9 dB peak power variation is attained. In terms of stability, the maximum peak power fluctuation for the individual laser is 0.24 dB within 120 minutes observation period. Without the Rayleigh backscattering effect, the peak power flatness is severely degraded. This shows that the weakly distributed photons can be utilized as peak power stabilizers in fiber laser systems.

Distributed Poloidal Magnetic Field Measurement in Tokamaks Using Polarization-Sensitive Reflectometric Fiber Optic Sensor.

Determination of the poloidal magnetic field distribution in tokamaks is of prime importance for the successful operation of tokamaks. In this paper, we propose a polarization-sensitive reflectometry-based optical fiber sensor for measuring the spatial distribution of the poloidal magnetic field in tokamaks. The measurement method exploits the Rayleigh backscattering and Faraday magneto-optic effect in optical fibers. The former is an intrinsic property of optical fibers and enables distributed polarization measurements, while the latter arises in the presence of a magnetic field parallel to the optical fiber axis and rotates the polarization state of the light. When an optical fiber is looped around a toroidal section of the vacuum vessel, the local polarization rotation of the light is proportional to the local poloidal magnetic field in the tokamak. The proposed method is discussed theoretically and experimentally using the results from JET. The obtained magnetic field measurement shows a good agreement with that of the internal discrete coils. A potential solution to recover the magnetic field data from the noise-affected region of the optical measurement is proposed and is demonstrated through simulations using the JET magnetic field configuration.

Stable Radio Frequency Transfer Over Free Space by Passive Phase Correction

The frequency transmission over free-space link can effectively overcome the dependence of existing optical fiber time-frequency transmission technology on optical fiber network. In this paper, we demonstrate a passive phase correction technique over a 124 m long indoor atmospheric radio frequency (RF) dissemination. This scheme simplifies the system, and suppresses the effects of Rayleigh backscattering. We verify the feasibility of the scheme by transmitting 1 GHz RF signals over the atmospheric link. After timing fluctuation compensation, the relative frequency stability can reach 6.8 × 10−15 for 1 s and 3.2 × 10−17 for 103 s, the RMS timing fluctuation is 186 fs in 5000 s. It can be proved that the scheme is very effective in eliminating phase noise caused by atmospheric link.

THEORETICAL ANALYSIS OF SINGLE-FIBER TRANSMISSION OVER WDM-PON SYSTEMS

In this paper, a rigorous theoretical analysis on optical beat interferometric (OBI) noises caused by the Rayleigh backscattering (RB) and Fresnel reflection (FR) effects on the performance of a single-fiber single-wavelength WDM-PON (wavelength division multiplexing-passive optical network) transmission system has been presented. Two categories of PON are evaluated: a conventional and a long-reach WDM-PON system. The conventional WDM-PON took place in an access network for a typical transmission distance of 20 km without any optical amplification at the remote node (RN). In this case, the gain of the optimal reflective optical network unit (ONU) will depend on the relative RB power and also the FRs magnitudes and locations along the optical fiber channel/transmission link. However, in the cases of the long-reach WDM-PON deployed at a transmission distance of 100 km or further, the optical amplifiers are indispensable to enhance the optical power budget and to prevent the effect of fiber nonlinearities that may occur at high power injection levels. Although the presence of an optical amplifier (OA) in a long-reach WDM-PON will actually give some additional gain to the whole system, as a result it also produces the worse RB and FR effects to the system itself. A crosstalk to signal (C/S) ratio is used as a key performance indicator of whether the transmission system is in good working order or not. Therefore, this study may provide insight and relevant information in terms of transmission optimization for WDM-PON systems and developments in the future. To educate on the current technologies and developments surrounding conventional and long-reach WDM-PON, a brief overview is provided in the background part of the introduction.

A novel wavelength reused bidirectional RoF-WDM-PON architecture to mitigate reflection and Rayleigh backscattered noise in multi-Gb/s m-QAM OFDM SSB upstream and downstream transmission over a single fiber

A novel m-QAM Orthogonal Frequency Division Multiplexing (OFDM) Single Sideband (SSB) architecture is proposed for centralized light source (CLS) bidirectional Radio over Fiber (RoF) – Wavelength Division Multiplexing (WDM) – Passive Optical Network (PON). In bidirectional transmission with carrier reuse over the single fiber, the Rayleigh Backscattering (RB) noise and reflection (RE) interferences from optical components can seriously deteriorate the transmission performance of the fiber optic systems. These interferometric noises can be mitigated by utilizing the optical modulation schemes at the Optical Line Terminal (OLT) and Optical Network Unit (ONU) such that the spectral overlap between the optical data spectrum and the RB and RE noise is minimum. A mathematical model is developed for the proposed architecture to accurately measure the performance of the transmission system and also to analyze the effect of interferometric noise caused by the RB and RE. The model takes into the account the different modulation schemes employed at the OLT and the ONU using a Mach Zehnder Modulator (MZM), the optical launch power and the bit-rates of the downstream and upstream signals, the gain of the amplifiers at the OLT and the ONU, the RB-RE noise, chromatic dispersion of the single mode fiber and optical filter responses. In addition, the model analyzes all the components of the RB-RE noise such as carrier RB, signal RB, carrier RE and signal RE, thus providing the complete representation of all the physical phenomena involved. An optical m-QAM OFDM SSB signal acts as a test signal to validate the model which provides excellent agreement with simulation results. The SSB modulation technique using the MZM at the OLT and the ONU differs in the data transmission technique that takes place through the first-order higher and the lower optical sideband respectively. This spectral gap between the downstream and upstream signals reduces the effect of Rayleigh backscattering and discrete reflections.

Performance analysis of cross-seeding WDM-PON system using transfer matrix method

Abstract In this paper, a model based on the transfer matrix method is adopted to analyze the effects of Rayleigh backscattering and Fresnel multiple reflections on a cross-seeding WDM-PON system. As part of analytical approximation methods, this time-independent model is quite simple but very efficient when it is applied to various WDM-PON transmission systems, including the cross-seeding scheme. The cross seeding scheme is most beneficial for systems with low loop-back ONU gain or low reflection loss at the drop fiber for upstream data in bidirectional transmission. However for downstream data transmission, multiple reflections power could destroy the usefulness of the cross-seeding scheme when the reflectivity is high enough and the RN is positioned near OLT or close to ONU.

Using single side-band modulation for colorless OFDM-WDM access network to alleviate Rayleigh backscattering effects.

In this investigation, we demonstrate a new colorless orthogonal-frequency-division-multiplexing (OFDM) wavelength-division-multiplexing passive optical network (WDM-PON) system with Rayleigh backscattering (RB) noise mitigation. Here, only a single laser source at the central office (CO) is needed to produce the downstream signal and distributed continuous-wave (CW) carrier, which will then be modulated at the optical networking unit (ONU) to produce the upstream signal. Single side-band (SSB) modulation is used to wavelength-shift the distributed CW carrier, which will be launched into a reflective semiconductor optical amplifier (RSOA) based ONU for directly modulation of 5.15 Gbps OFDM upstream signal. To avoid the radio-frequency (RF) power fading and chromatic fiber dispersion, the four-band OFDM modulation is proposed to generate a 40 Gbps downstream when a Mach-Zehnder modulator (MZM) with -0.7 chirp parameter is used. Hence, the RB circumvention can be centralized in the CO. Moreover, the signal performances of downstream and upstream are also studied and discussed in this measurement.

Hybrid bidirectional radio-over-fiber-based orthogonal frequency division multiple access-passive optical network supporting 60 / 120 GHz using offset quadrate phase shift keying

A hybrid bidirectional orthogonal frequency division multiple access-passive optical network (OFDMA- PON) based on offset quadrate phase shift keying (OQPSK) to support 60- and 120-GHz radio-over-fiber system is proposed. The system can support wired/wireless applications and enable the dynamic bandwidth allocation according to a subscriber's application. It is successfully achieved by using the millimeter waves (MMWs) gen- eration and the carrier-reuse technique. In the proposed scheme, the MMW bands used for downlink (DL) and uplink transmissions are generated at the optical line terminal by the dual-arm Mach-Zehnder modulators. Both 60- and 120-GHz MMWs are obtained for the transmission of the high bit-rate services in source-free optical network units (ONUs), only using a single 15-GHz sinusoidal wave source. The Rayleigh backscattering effect is considered in the proposed OQPSK-based OFDMA-PON. For DL transmission over a 30-km single-mode fiber, the power penalties are less than 0.8 and 1 dB for the OQPSK-OFDM wired data at 10 Gb∕s and the OQPSK- OFDM wireless data at 5 Gb∕s, respectively. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.OE.54 .9.096108)

Spectrally Efficient Enhanced-Performance Bidirectional Coherent PON With Laserless 10 Gb/s ONU [Invited

We present and experimentally characterize a spectrally efficient, bidirectional, ultradense wavelength-division multiplexing passive optical network (PON) (2 × 120 Gb/s at 50 GHz) over 30 km standard single-mode fiber. This is achieved using a 4 GHz grid with partial spectrum overlapping, Nyquist-shaped 16-ary quadrature amplitude modulation, and digital frequency shifting. This scheme is based on a laserless, self-homodyne PON with centralized wavelength control. We achieve reduced impact of the Rayleigh back-scattering effect due to Nyquist-pulse shaping and frequency upshifting. Self-homodyne detection relaxes the laser linewidth requirement and digital signal processing complexity at the optical network unit. To demonstrate the extra benefit of the proposed PON system, we also experimentally and theoretically evaluate the impact of nonlinear Raman crosstalk in video overlay. Compared to a baseband Nyquist signal and baud rate non-return-to-zero equivalent, the proposed system improves the carrier-to-Raman crosstalk ratio of video overlay by 4 and 6 dB, respectively.

Multicore-Fiber-Enabled WSDM Optical Access Network With Centralized Carrier Delivery and RSOA-Based Adaptive Modulation

We proposed and experimentally demonstrated a wavelength-space division multiplexing (WSDM) optical access network architecture with centralized optical carrier delivery utilizing multicore fibers (MCFs) and adaptive modulation based on reflective semiconductor amplifier (RSOA). In our experiment, five of the outer cores are used for undirectional downstream (DS) transmission only, whereas the remaining outer core is utilized as a dedicated channel to transmit upstream (US) signals. Optical carriers for US are delivered from the optical line terminal (OLT) to the optical network unit (ONU) via the inner core and then transmitted back to the OLT after amplification and modulation by the RSOA in the colorless ONU side. The mobile backhaul (MB) service is also supported by the inner core. Wavelengths used in US transmission should be different from that of the MB in order to avoid the Rayleigh backscattering effect in bidirectional transmission. With quadrature phase-shift keying--orthogonal frequency-division multiplexing (QPSK-OFDM) modulation format, the aggregation DS capacity reaches 250 Gb/s using five outer cores and ten wavelengths, and it can be further scaled to 1 Tb/s using 20 wavelengths modulated with 16 QAM-OFDM. For US transmission, 2.5 Gb/s QPSK-OFDM transmission can be achieved just using a low-bandwidth RSOA, and adaptive modulation is applied to the RSOA to further enhance the US data rate to 3.12 Gb/s. As an emulation of high-speed MB transmission, 48 Gb/s in-phase and quadrature (IQ) modulated popularization division multiplexing (PDM)-QPSK signal is transmitted in the inner core of MCF and coherently detected in the OLT side. Both DS and US optical signals exhibit acceptable performance with sufficient power budget.

Effects of Rayleigh backscattering on the stability of distributed feedback fiber laser sensors

Rayleigh backscattering induces mode hopping of distributed feedback (DFB) fiber lasers in the sensor array, and the critical length related to Rayleigh backscattering limits the size of DFB fiber laser sensor networking. Based on a three-mirror cavity model, the critical length for a DFB fiber laser is derived. It increases nearly exponentially with the coupling coefficient for the ideal π-shifted DFB fiber lasers. The reflectivity of the sub-fiber Bragg grating at the lasing wavelength is the main factor to resist Rayleigh backscattering for a nonideal DFB fiber laser. The corresponding experiments have been carried out, and the critical length of larger than 150 m was achieved for 42-mm-long DFB fiber lasers.

Transfer matrix analysis of backscattering and reflection effects on WDM-PON systems

This paper proposes using power transfer matrix analysis to characterize the effects of Rayleigh backscattering and Fresnel reflection on WDM-PON systems. The modeling of a WDM-PON system can be carried out simply by matrix multiplication of the corresponding matrices for all the building blocks, where all possible guided backward lights and resonant configurations along the optical network can be accounted for. The total sum of all interferences affecting the bidirectional transmission that leads to an optical crosstalk-to-signal (C/S) ratio can be modeled as back-reflections through cascaded two-port networks for the downstream and upstream signals. This approach is simple, robust, efficient, and also accurate. Its accuracy is verified for simple system architectures and then applied to study more complicated cases. The results show its versatility to analyze a wide variety of bidirectional optical transmission systems.

25-GHz-Spaced DWDM-PON With Mitigated Rayleigh Backscattering and Back-Reflection Effects

A novel 25-GHz-spaced dense wavelength-division multiplexing passive optical network (DWDM-PON) is proposed to improve the utilization of wavelength resource and mitigate the Rayleigh backscattering (RB) and back-reflection effects. A colorless optical network unit (ONU) is realized by remodulating the downstream (DS) signal via a gain-saturated reflective semiconductor optical amplifier (RSOA). Due to the simple interconnecting architecture at the remote node (RN), the DS and upstream (US) signals in each feed fiber (FF) are carried on different channels. Therefore, the RB and back-reflection effects are significantly reduced. The feasibility and merits of the proposed system are experimentally demonstrated with the error-free transmission of 10-Gb/s DS and 1.25-Gb/s US signals over a 25-km single-mode fiber.

Single fiber colorless symmetric WDM-PON architecture using time interleaved remodulation technique for mitigating Rayleigh backscattering resilience

A time interleaved differential phase shift keying (DPSK) remodulation technique is proposed to mitigate the effect of Rayleigh backscattering (RBS)-induced noise in a single fiber colorless wavelength-division-multiplexing passive optical network (WDM-PON). In order to achieve a cost effective optical network unit (ONU) solution without dedicated laser sources for upstream signals to provide optimum symmetric capacity in a colorless WDM-PON, remodulation becomes the core attraction. Also as the performance of colorless WDM-PON systems suffers from the transmission impairments due to RBS, it is mitigated by using this remodulation scheme. Simulation results show that downstream and upstream signals achieve the error-free performance at 10 Gbit/s with negligible penalty, and enhance the tolerance to RBS-induced noise over a 25 km single-mode fiber.

Bidirectional Hybrid OFDM-WDM-PON System for 40-Gb/s Downlink and 10-Gb/s Uplink Transmission Using RSOA Remodulation

We experimentally demonstrate a bidirectional hybrid orthogonal frequency-division-multiplexing wavelength-division-multiplexing passive optical network (OFDM-WDM-PON) system. The proposed scheme uses OFDM signals occupying different portions of the available signal spectrum for uplink and downlink signal transmissions to reduce the effects of Rayleigh backscattering, chromatic dispersion, and power fading. A 40-Gb/s downlink signal transmission based on a single sideband modulated OFDM signal, and a 10-Gb/s uplink signal transmission using remodulation of a reflective semiconductor-optical-amplifier are realized.

Laser Parameter Variations in a Rayleigh Scattering-Based Raman Fiber Laser With Single Fiber Bragg Grating Reflector

The work presented in this paper details the changes in continuous-wave laser characteristics that were affected by the orientation of pumping separation. The Raman laser was constructed in forward and backward pumping schemes with respect to the 1553.3-nm fiber Bragg grating location. The laser cavity was formed by the induction of Rayleigh backscattering effects in the 51-km fiber length that served as a virtual mirror. From the results obtained, it can be concluded that low threshold operation around 788 mW was satisfied at a coupling ratio of 0 (forward pumping scheme). Moreover, the best output power attainment of 220 mW was realized when 60% of pump powers were delivered via a backward pumping scheme. Thus, the success of this research provides a basis to further understand the principle of backscattered wave interactions along the fiber longitudinal structure.

Overlapped-Subcarrier Multiplexing for WDM Passive Optical Networks: Experimental Verification and Mathematical Analysis

An innovative overlapped-subcarrier multiplexing (O-SCM) technique is proposed for single-feeder wavelength-division multiplexed (WDM) passive optical networks (PONs). Allowing a certain amount of spectral overlap between the uplink and the downlink maximizes the spectrum usage of bandwidth-limited reflective semiconductor optical amplifier (RSOA)-based optical network units (ONUs), while reducing the effect of Rayleigh backscattering. In this paper, we report successful experimental demonstrations up to 2.5 Gb/s symmetrical bit rates over a ~20 km bidirectional PON, using a ~2 GHz bandwidth RSOA. Moreover, we mathematically address the major design issues and tradeoffs. Our simulation results show good agreement with the experiment.

Cross-Seeding Schemes for WDM-Based Next-Generation Optical Access Networks

Rayleigh backscattering (RB) and Fresnel reflection (FR) effects can cause severe degradation on the performance of a WDM-PON system utilizing loop-back schemes that provide colorless upstream transmitters. We proposed cross-seeding schemes to overcome these problems and also to improve the utilization efficiency of the seeding lights. The operation principles of two types of cross-seeding schemes: mutual seeding and shared seeding are discussed in detail for different applications. A generalized formulation of the crosstalk from the RB and FR effects is derived for comparing the performance between a conventional WDM-PON and a cross-seeding WDM-PON. It is found that the cross seeding schemes can be most beneficial for the system with low loop-back gain or low reflection at the distribution fiber or drop fiber. This is especially advantageous for the shared seeding scheme in which the seeding light can be transmitted along a separate fiber with high power or by optical amplification such that the gain of the loop-back transmitter can be kept small. Application examples are used to demonstrate the merits and feasibility of the cross-seeding schemes.