Power penalty assessment in optically preamplified receivers witharbitrary optical filtering and signal-dependent noise dominance

The assessment of the power penalty of optically preamplified receivers with signal-dependent noise (SDN) dominance is often accomplished by neglecting the influence of the optical filtering of the amplified spontaneous emission (ASE) noise on the signal-ASE beat noise. In this paper, it is shown that the optical filtering of the ASE noise can have a strong impact on the signal-ASE beat noise and remarkably affect the power penalty, even for optical filter bandwidths five times wider than the signal bandwidth. A simple expression to analytically evaluate the power penalty due to optical filtering, which describes reasonably well the influence of the optical filter on the signal-ASE beat noise variance, is proposed. The accuracy of the new expression is investigated, in the case of assessment of the optical filter detuning impact on receiver performance and the case of optical filter bandwidth optimization, and its predictions are satisfactory in comparison with rigorous estimates. Two new expressions of power penalty due to extinction ratio and to eye closure are also presented. It is shown that the power penalty due to eye closure depends on the extinction ratio and vice versa. Our results show also that the power penalty due to eye closure is remarkably dependent on the eye closure asymmetry     

Impact of crosstalk in an arrayed-waveguide multiplexer onN×N optical interconnection

The impact of crosstalk in an arrayed-waveguide N×N wavelength multiplexer is investigated precisely in relation to its application to wavelength-routing N×N all optical networks. In such systems multiple crosstalk light which has the same wavelength as the signal results in signal-crosstalk beat noise. We confirm that the noise is Gaussian and obtain the relation between crosstalk and power penalty. It is shown that the crosstalk must be less than -38 dB for a 16×16 system to keep the power penalty below 1 dB at a bit error rate of 10^-9     

Optimum optical power splitting ratio of decision drivenphase-locked loop in BPSK optical homodyne receiver

In a BPSK optical homodyne receiver that utilizes a decision-driven phase-locked loop, the splitting ratio of the received power and that of the local oscillator power are very important parameters in achieving high receiver sensitivity. This paper determines the optimum setting of these parameters considering the influence of the relative intensity noise of the local oscillator and the thermal noise of the preamplifier. The optimum splitting ratio of the local oscillator power to the Q-arm is found to be 0.5. The splitting ratio of the received power to Q-arm is obtained as a function of laser linewidth. The optimum setting of the received power and the local oscillator power Is independent of the relative intensity noise of the local oscillator, the thermal noise of the preamplifier and the bit rate, At the optimum splitting ratios, required beat linewidth is obtained as 1.3×10 ^-3/T_b(τ/T_b≪1) and 2.99×10 ^-3/τ(τ/T_b≫1), where T_b is the bit duration and τ is the loop propagation delay time. We show that the total power penalty of 0.8 dB from the shot noise limit can be realized with the relative intensity noise of -170 dB/Hz and equivalent input noise current of 10 pA/√(Hz), even if an imperfect balanced receiver is utilized; quantum efficiency ratio of the twin-photodetector is 0.96, propagation time difference T/T_b is 0.01. To confirm the theoretical model, a BPSK homodyne detection experiment is performed and good agreement is found between theoretical and experimental results     

Effects of In-Band Crosstalk in Wavelength-Locked Fabry–PÉrot Laser-Diode-Based WDM PONs

We investigated the impact of in-band crosstalk on the system's performance in wavelength-locked Fabry-Perot laser-diode (FP-LD)-based wavelength-division-multiplexed passive optical networks (WDM PONs). As expected, wavelength-locked FP-LD, which was injected by using an incoherent broadband light source, was more tolerant to in-band crosstalk than distributed-feedback (DFB) laser diode. A 1-dB power penalty in the wavelength-locked FP-LD-based PON system was observed when the crosstalk-to-signal ratio was ~-9 dB. We also compared the measured power penalties with the calculated power penalties. It has been shown that the in-band crosstalk-induced power penalty in wavelength-locked FP-LD-based WDM PON could be estimated properly by taking into account both effects of power addition and signal-crosstalk beat noise.     

12-THz frequency difference measurements and noise analysis of anoptical frequency comb in optical fibers

A span up to 50 THz of optical frequency comb (OFC) has been obtained by self-phase modulation in an optical fiber. The coherent nature of the process was verified by heterodyne-detecting the sidebands offset by up to 12 THz from the carrier. The signal-to noise ratio (SNR) of the beat signal between a sideband at 12 THz offset and another single-mode laser was 32 dB in a 1-MHz bandwidth. Although the linewidth of each beat signal was maintained within a few megahertz, phase noise pedestal power increased with the offset frequency     

Performance analysis of an asymptotically quantum-limited opticalDPSK receiver

An optical, direct-detection differential phase-shift keying (DPSK) receiver whose error probability is quantum-limited as the transmitting laser linewidth vanishes is analyzed. The receiver design is based on a binary equiprobable hypothesis test with doubly stochastic point process observations, the conditional random rates of which depend on the transmitting laser phase noise, which is modeled as a Brownian motion. The receiver structure consists of a simple delay-and-sum optical preprocessor followed by a photoelectric converter and an integrate-and-dump circuit. Upper and lower bounds on the receiver bit error rate are derived by developing bounds on the conditional rates of the point process, and it is shown that the error probability bounds converge to the true value as the transmitting laser linewidth decreases. Bounds on the power penalty are computed for parameters corresponding to existing semiconductor injection lasers, and are seen to be less than the limiting power penalty for the balanced DPSK receiver     

Efficient Power‐Saving 10‐Gb/s ONU Using Uplink Usage‐Dependent Sleep Mode Control Algorithm in WDM‐PON

We propose and demonstrate an efficient power‐saving optical network unit (ONU) based on upstream traffic monitoring for 10‐Gb/s wavelength division multiplexed passive optical networks (WDM‐PONs). The power‐saving mode controller uses a μ‐processor and traffic monitoring modules followed by the proposed power‐saving processes to operate the sleep mode ONU. The power consumption of the ONU is effectively reduced from 19.3 W to 6.4 W when no traffic from the users is detected. In addition, we design a power‐saving mechanism based on a cyclic sleep mode operation to allow a connectivity check between the optical line terminal and ONU. Our calculation results show that the WDM‐PON ONU reduces the power consumption by around 60% using the proposed mechanism.     

Colorless ONU implementation for WDM-PON using direct-detection optical OFDM

A novel architecture for the colorless optical network unit (ONU) is proposed and experimentally demonstrated with direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM). In this architecture, polarization-division multiplexing is used to reduce the cost at ONU. In optical line terminal (OLT), quadrature amplitude modulation (QAM) intensity-modulated OFDM signal with x-polarization at 10 Gbit/s is transmitted as downstream. At each ONU, the optical OFDM signal is demodulated with direct detection, and y-polarization signal is modulated for upstream on-off keying (OOK) data at 5 Gbit/s. Simulation results show that the power penalty is negligible for both optical OFDM downstream and the on-off keying upstream signals after over 50 km single-mode fiber (SMF) transmission.     

A self-seeded reflective SOA-based optical network unit for optical beat interference robust WDM/SCM-PON link

We have proposed and experimentally demonstrated a novel optical source configuration that uses a self-seeded reflective semiconductor optical amplifier (RSOA) as an optical network unit (ONU) in a wavelength division multiplexed/subcarrier multiplexing (SCM)-passive optical network (PON). A fiber Bragg grating (FBG) was used to obtain a single longitudinal mode of the RSOA. The proposed ONU configuration is simple, cost-effective, and effective regardless of wavelength allocation. Additionally, it would be robust for optical beat interference (OBI) noise. As the ONU is composed only of an RSOA and FBG, the self-seeded RSOA, due to the strong self-injection caused by the reflection of the FBG, has a broad optical spectrum. A self-homodyne apparatus method was performed in order to demonstrate the robustness of the self-seeded RSOA in OBI. To confirm the validity of the proposed scheme, a 16-quadrature amplitude modulation transmission experiment was performed in a 10-, 20-km optical access link with an SCM frequency of 1 GHz. An error vector magnitude of less than 4% for 2 Msps was successfully obtained through the transmission experiment.     

Performance of homodyne detection of binary PSK optical signals

This study evaluates the performance of an optical receiver for binary phase shift keyed (BPSK) signals in the presence of noise originating from the photodetectors and the phase fluctuations of the optical sources. Analysis of the homodyne detection process shows that the performance is degraded by two effects: One due to the phase error fluctuations of the recovered carrier and the other due to reduction of the energy per bit available for data recovery. The resulting power penalty can be minimized by dividing in an optimal way the received optical signal between the carrier recovery and the data recovery circuits of the receiver. The minimum penalty thus obtained depends on the 3-dB linewidth and on the transmission rate. For example, a penalty of 0.5 dB, relative to the quantum limit of 9 photon bit needed to achieve a BER of 10^-9, imposes a minimum transmission rate of about 180 Gbit/s when the optical source has a 3-dB linewidth of 20 MHz.     

BER impairment due to laser linewidth in OFDM-OOK systems usingdouble-cavity Fabry-Perot demultiplexers

We report on the impact of finite laser linewidth on the bit error rate (BER) performance in directly detected optical frequency-division multiple-access (OFDMA) employing double-cavity Fabry-Perot (FP) filters. Three different double-cavity FP demultiplexers have been considered in this study. Also the laser linewidth impairment on the crosstalk probability mass functions, the crosstalk power penalties and the OFDMA network capacity are calculated. The results demonstrate that, even though these demultiplexing schemes are much superior than single-cavity FP filters for highly coherent sources (laser linewidth lower than a few megahertz), when laser linewidth increases this feature appears to be much less significant due to the severe impairment of the laser linewidth on the BER performance, specially for structures with higher optical filter finesse     

Suppression of optical beat interference-noise in orthogonal frequency division multiple access-passive optical network link using self-homodyne balanced detection

A new technique, which reduces optical beat interference (OBI) noise in orthogonal frequency division multiple access-passive optical network (OFDMA-PON) links, is proposed. A self-homodyne balanced detection, which uses a single laser for the optical line terminal (OLT) as well as for the optical network unit (ONU), reduces OBI noise and also improves the signal to noise ratio (SNR) of the discrete multi-tone (DMT) signal. The proposed scheme is verified by transmitting quadrature phase shift keying (QPSK)-modulated DMT signal over a 20-km single mode fiber. The optical signal to noise ratio (OSNR), that is required for BER of 10⁻⁵, is reduced by 2 dB in the balanced detection compared with a single channel due to the cancellation of OBI noise in conjunction with the local laser.     

Analysis of CNR penalty of radio-over-fiber systems including the effects of phase noise from laser and RF oscillator

The effect of phase noises from a laser and an oscillator on radio-over-fiber (RoF) systems is analyzed and discussed with a power spectral density (PSD) function. A Mach-Zehnder modulator (MZM) and a phase shifter are employed to externally generate an optical single sideband (OSSB) signal since the OSSB signal is tolerable for power degradation due to a chromatic fiber-dispersion effect. It is shown that a carrier-to-noise ratio (CNR) penalty is deeply related to the bandwidth of a receiver filter and the phase noise from a radio frequency (RF) signal oscillator rather than that from a laser in a small differential-delay environment and a direct detection scheme. The CNR penalty due to the increment of the laser linewidth from 10 to 624 MHz is almost 1.1 dB, while the increase of the RF-oscillator linewidth from 1 to 100 Hz results in about a 20-dB penalty at a 30-GHz 10-km transmission in a standard single-mode fiber (SSMF) with a fiber chromatic dispersion of 17 ps/km/spl middot/nm.     

Effects of Upstream Incoherent Crosstalk Caused by ASE Noise from Tx‐Disabled ONUs in XG‐PONs and TWDM‐PONs

A large incoherent crosstalk (IC) caused by amplified spontaneous emission (ASE) noise power from Tx‐disabled optical network units and a differential path loss has been shown to degrade upstream transmission performance in time‐division multiplexing passive optical networks. This paper considers the IC‐induced power penalty of an upstream signal both in an XG‐PON and in a TWDM‐PON. We investigate the degradation of the extinction ratio and relative intensity noise through a simulation and experiments. For the XG‐PON case, we observe a 9.6 dB difference in the level of ASE noise power from Tx‐disabled ONUs (hereafter known simply as ASE noise) between our result and the ITU‐T XG‐PON PMD recommendation. We propose an optical filtering method to mitigate an IC‐induced power penalty. In the TWDM‐PON case, the IC‐induced power penalty is naturally negligible because the ASE noise is filtered by a wavelength multiplexer at the optical line terminal. The results provide design guidelines for the level of ASE noise in both XG‐PONs and TWDM‐PONs.     

Effect of optical amplifier noise on laser linewidth requirementsin long haul optical fiber communication systems with Costas PLLreceivers

The impact of optical amplifier noise is analyzed in investigating the performance of optical long-haul PSK homodyne communication systems with Costas phase locked loop (PLL) receivers. The laser linewidth requirement for an optically amplified system becomes relaxed in comparison with the system with no optical amplifier, owing to the fact that the effect of incomplete phase tracking becomes less important as a larger signal power is demanded to maintain a fixed bit-error rate. Also, it is found that the power splitting ratio regarding the power distributions for the I-arm and the Q-arm of a Costas loop can vary in a wide range without having much influence on the performance of an optically amplified system. As a matter of fact, the power penalty induced by incomplete phase tracking for a system with a large number of cascaded optical amplifiers is mainly due to the finite phase error and not due to the power splitting ratio, and this may fail a previously-reported method for finding the required laser linewidth by assigning a certain amount of power penalty that is due to the power splitting ratio     

High-stability 1.5 μm external-cavity semiconductor lasers forphase-lock applications

Applications using phase-locked semiconductor lasers, such as homodyne detection, require lasers with narrow linewidth and high-frequency stability. The design and operating characteristics of two 1.5 μm external-cavity semiconductor lasers built for such applications are described. The measured beat linewidth is 4 kHz, and the spectral density of relative frequency noise deviates significantly from the intrinsic white spectrum only at frequencies below 4 kHz. It is estimated that this frequency jitter will induce approximately 1.1° RMS phase error in a second-order homodyne optical phase-lock loop that is optimized for the present beat linewidth     

可调谐单频非平面环形腔固体激光器

研究了单块晶体成腔的单频非平面环形腔(NPRO)固体激光器,在1.83W的808nm抽运功率下输出激光1.01W,斜率效率达到60%。采用拍频的方法对激光线宽进行了测试,激光线宽小于2kHz。通过抽运电流反馈控制使弛豫振荡峰得到超过30dB的抑制;通过对激光晶体的温度调节和压电陶瓷电压调节实现了激光器频率的慢调谐和快调谐,温度慢调谐变化10℃时激光频率变化范围超过15GHz;压电陶瓷快调谐范围超过±200MHz,在大于200MHz的范围内响应时间达到45μs。     

Noise filtering with the nonlinear optical loop mirror

The operation of a nonlinear optical loop mirror as a passive and signal polarization independent noise filter, is investigated experimentally. Spontaneous emission not within the signal spectrum is rejected leaving a receiver noise limited solely by signal-spontaneous beat noise. The receiver power penalty caused by spontaneous emission (@BER=10^-9 (PRBS)) was improved from 3 dB to 0.4 dB by propagating a signal through the loop. A comparison between this nonlinear filtering: technique and a variable band-pass filter is also made     

Damping factor influence on linewidth requirements for optical PSKcoherent detection systems

Spectral linewidth requirements for optical phase-shift-keying (PSK) coherent detection systems are found to depend on the phase-locked loop (PLL) parameters. Until now, the damping factor of the PLL has been assumed to be 1/√2 when deriving the required spectral linewidth of a light source, because it is at this value that an electrical PLL offers near optimum performance in many cases. By increasing the PLL damping factor above 1/√2, it is shown that there exists a maximum value of the required linewidth that achieves a received optical power penalty of 1 dB at a bit error rate of 10^-10. The required beat linewidths so obtained are 50% larger than previously reported results (which assume a damping factor of 1/√2). As for PLL frequency acquisition performance, it is shown that raising the camping factor above 1/√2 does not seriously affect the hold-in limit or the pull-in limit. It is also shown that the normalized loop gain that optimizes PLL performance is roughly one half the normalized loop gain at which the PLL oscillation commences     

Analysis of laser phase noise to intensity noise conversion bychromatic dispersion in intensity modulation and direct detectionoptical-fiber transmission

Laser phase noise conversion to intensity noise due to fiber chromatic dispersion is analyzed by deriving the noise power spectral density. Theory predicts that the phase-modulation-amplitude-modulation conversion noise is a principal limiting factor of the gigabit-per-second nonregenerative transmission using an external modulator when the linewidth of the laser transmitter is above several tens of megahertz and the total chromatic dispersion of fibers exceeds several thousand picoseconds per nanometer. This fact is confirmed by the 2.4-Gb/s transmission experiments using multiple inline Er-doped fiber amplifiers. The system penalty due to this noise in the intensity modulation and direct detection (IM-DD) optical transmission using an external modulator is evaluated