Optical Single Sideband Research Articles

A linearized optical single-sideband modulation RoF link with tunable optical carrier-to-sideband ratio

In this paper, an optical single-sideband (OSSB) modulation radio over fiber (RoF) link with tunable optical carrier-to-side-band ratio (OCSR) and simultaneous third-order intermodulation distortion (IMD3) and power fading suppression is proposed. In the proposed link, a phase-modulated signal and an optical carrier are polarization multiplexed using a Sagnac loop. The periodic power fading caused by chromatic dispersion is avoided by filtering the lower sideband. By adjusting the polarization state, the IMD3 is suppressed and a tunable OCSR can be achieved. Compared with the traditional Mach–Zehnder modulator (MZM) based link, the spurious free dynamic range (SFDR) of the proposed link is improved by more than 18 dB with different RF frequencies. In the broadband characteristic test, the proposed link exhibits lower error vector amplitude (EVM) and higher adjacent channel power ratio (ACPR) in a larger input RF power range.

Simultaneous RF Self-Interference Cancellation, Local Oscillator Generation, Frequency up- and down-Conversion in an Integrated In-Band Full-Duplex 5G RF Transceiver Front-End

Wideband radio-frequency (RF) self-interference cancellation (SIC), local oscillator (LO) generation, frequency up-, and down-conversion are all crucial for an in-band full-duplex (IBFD) 5G RF transceiver front-end. In this paper, a microwave photonic approach enabled by only one optical dual-polarization IQ modulator (DP-IQMZM) is proposed to simultaneously realize wideband RF SIC, LO generation, frequency up- and down-conversion. Note that, the LO signal with high spectral purity and low phase noise is produced by a dual-loop optoelectronic oscillator (OEO) using a self-polarization stabilization structure, and the generated LO signal is applied for frequency up- and down-conversion. Besides, the optical single-sideband signal is produced at the output of this 5G RF transceiver for uplink transmission from the remote antenna unit (RAU) to the baseband unit (BBU), so the power fading induced by fiber dispersion could be completely suppressed. A proof-of-concept experiment is performed, and the LO signal with the central frequency of 10 GHz, the phase noise performance of −108.97 dBc/Hz at a frequency offset of 10 kHz is achieved. A 5 × 20 MHz Long Term Evolution (LTE) 16/64-ary quadrature amplitude modulation-orthogonal frequency division multiplexing (16/64QAM-OFDM) signal with the central frequency of 12.6 GHz is down-converted to 2.6 GHz. Meanwhile, the cancellation ratio of 30.22 dB and 26.26 dB could be achieved when the bandwidth of the self-interference signal is 150 MHz and 300 MHz, respectively. Moreover, a 3 × 20 MHz LTE 64QAM-OFDM signal with a central frequency of 2.6 GHz is up-converted to 12.6 GHz, and the electrical signal-to-noise ratio of the generated RF signal can achieve 31.34 dB. The measured error vector magnitude performance shows that the generated down- and up-converted signal both meet the 3^rd Generation Partnership Project (3GPP) requirements for 5G systems.

Optimizing DC restoration in Kramers-Kronig optical single-sideband receivers.

Kramers-Kronig optical single-sideband receivers remove the signal-signal beat interference (SSBI) that occurs when detecting a signal that has electrical signals mapped onto its optical field at the transmitter; such signals support electronic dispersion compensation without the need for a coherent receiver. To use the full range of the analog-to-digital converter's (ADC) range, it is best to a.c.-couple the photocurrent, to remove its DC content; however, the DC must be restored digitally before the KK algorithm is applied. Recent publications have concentrated on perfectly determining the restored DC's required level from the signal, with a view this is optimal for lowering error rates. In this paper, we investigate signal-signal beat interference (SSBI) cancellation in a single photodiode receiver using Kramers-Kronig receiver algorithm, with large variations in optical carrier-to-signal power ratio (CSPR) and DC offset level. Through simulations and experiments, we find a strategy to optimize the signal quality without the need of an extensive search for the DC offset value. We also find that a theoretically perfect determination of the original DC level does not provide best signal quality especially for low CSPRs; in order to achieve maximum cancellation of signal-signal beat interference, the level of the restored DC has an optimum value that depends on the optical CSPR. We define a digital CSPR, which is the value of the CSPR in the digital domain after DC restoration. Our measurements show that we simply need to bias the signal upwards and make the minimum signal above zero by 0.1% of the r.m.s. signal amplitude when the optical CSPR is low. For higher values of optical CSPR, the optimal digital CSPR is about 2-dB lower than the optical CSPR, and the optimal DC offset can be calculated from this digital CSPR. We find that the boundary between our low optical CSPR region and high optical CSPR region depends on the noise level in the system.

Analysis of Optical Single Sideband Modulators for Radio Over Fiber Link with and without Second Order Sidebands

The analysis of Optical Single Sideband (OSSB) generation with and without second-order sidebands for Radio over Fiber (RoF) is presented. The performance of systems based on hybrid coupler with distinct phase angle and Dual-Parallel Dual-Drive Mach-Zehnder Modulator (DP-DDMZM) is compared using simulation. Impact of parameters like Single-Mode Fiber (SMF) length, Radiofrequency (RF) amplitude, and laser linewidth on Signal-to-Noise Ratio (SNR) has been investigated. It has been observed that eliminating one of the second-order sidebands with 120° hybrid coupler improves peak SNR in comparison to 90° hybrid coupler with both second-order sidebands and DP-DDMZM-based system without second-order sidebands irrespective of Continuous Wave (CW) laser linewidth.

High Modulation Efficiency and Dynamic Range Optical Single Sideband Modulation Without Gain Penalty in Nonlinear Distortion Suppression

The optical single sideband (OSSB) modulation with tunable optical carrier to sideband ratio (OCSR) is a promising and popular technique in wideband analog signal transmission and microwave signal processing, which can improve modulation efficiency and combat the fiber chromatic dispersion. However, in all reported schemes for achieving OSSB modulation with tunable OCSR, the spurious free dynamic ranges (SFDR) of the systems are all limited by the third-order intermodulation distortion (IMD3). In this paper, our object is to improve the SFDR and suppress the IMD3 in a novel OSSB modulation with tunable OCSR. The proposed system consists of a primary channel and a secondary channel. In primary channel, an OSSB modulation with tunable OCSR is implemented. In secondary channel, the optical carrier and first-order sidebands are perfectly cancelled out, which can suppress the IMD3 without gain penalty at the same received optical power. In this way, a large-SFDR OSSB modulation with tunable OCSR is realized. Experimental results exhibit that the IMD3 gets effective cancellations without gain penalty under unchanged total received power, and the output microwave signal power obtains about 11-dB improvement by optimizing the OCSR. A greatly increased SFDR of over 128 dBHz4/5 is achieved in the proposed system.

Microwave Photonic Phase-/Delay-Tunable Mixer Based On OSSB-PolM With Ultralow Mixing Spurs

In this paper, a microwave photonic (MWP) phase-/delay-tunable mixer with ultralow mixing spurs based on optical single-sideband polarization modulation (OSSB-PolM) is proposed and theoretically investigated which is verified by simulations. In the proposed structure, by passing the OSSB-PolM signal through a polarization controller (PC) and an optical polarizer (Pol.), a carrier-suppressed optical single-sideband (CS-OSSB) modulated signal is generated. In the up-converter mode, optical carrier, intermediate frequency (IF) even- and (4n+3) odd-order sidebands, n is an integer, local oscillator (LO) even- and (4n+1) odd-order sidebands are suppressed without using any optical/electrical filter. Also, in the down-converter mode, optical carrier, radio frequency (RF) and LO even- and (4n+3) odd-order sidebands are suppressed. By beating sidebands of the pure CS-OSSB signal in a photodetector, RF up-converted (or IF down-converted) signal is generated with ultralow mixing spurs. Simulation results show that the sideband suppression ratio (SSR) for both cases of the up-/down-converted signals are >45 dB in the electrical domain. Effects of the phase and power imbalances of the practical non-ideal RF hybrid couplers on the performance of the proposed MWP mixer are investigated. It is shown that with ±3 degrees phase and ±1.5 dB power imbalances, the SSR decreases to 25.6 dB and 15.6 dB, respectively. Closed-form expression for the conversion efficiency is derived which is verified by simulations. Besides, simulations show that the spurious-free dynamic range (SFDR) of the proposed MWP mixer is 117.67 dB.Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2/3</sup> (with −160 dBm/Hz noise floor). In addition, optical frequency conversion and optical phase-shifting functions are combined and a full-range phase-tunable MWP mixer is proposed and investigated. Besides, optical frequency conversion function and optical true-time delay (TTD) are combined and a delay-tunable MWP mixer is introduced and investigated. It can find many applications in MWP systems, such as in MWP TTD beamformers for broadband squint-less array antennas.

Numerical analysis of UFMC and FBMC in wavelength conversion for radio over fiber systems using semiconductor optical amplifier

The 5G networks promise to use advanced access techniques and modulation formats to achieve new requirements of the huge number of users such as higher data, high speed, and low latency. Millimeter-wave (MMW) signal and new modulation schemes such as universal filtered multicarrier (UFMC) and filtered bank multicarrier (FBMC) are playing a fundamental role to accomplish these requirements. UFMC and FBMC are classified under multicarrier modulation formats which are considered as a suitable modulation scheme in 5G and replacing a well-known modulation format of filtered orthogonal frequency division multiplexing (OFDM) which was used in 4G. This interest in using UFMC and FBMC in 5G wireless systems is the reduction of out-of-band (OOB) spectrum in UFMC and reduction of sidelobe suppression in FBMC. The contribution of this paper is to propose a model of wavelength conversion using a semiconductor optical amplifier (SOA) for both schemes UFMC and FBMC- off-set quadrature amplitude (OQAM). In addition, we analyze and compare their performance in terms of bit error rate (BER) and error vector magnitude (EVM) for a wavelength conversion application. Finally, we investigate the main disadvantage of OFDM which is the high Peak to Average Power Ratio (PAPR) for both schemes, and compare between them. Photonic switching of a 4 Gbps 16-QAM UFMC and FBMC-OQAM signal centered at 50 GHz is performed. The optical single-sideband (OSSB) signal is generated with 18.85 dB of sideband suppression ratio (SSR). The results show that the best values of BER and EVM can be obtained when the injection current (IC) is 0.7–0.9A. For converted signal using FBMC-OQAM, it has power penalty ∼ 2.5 dB at BER threshold 1 × 10-3and has also ∼ 2 dB power penalty compared to the UFMC scheme at EVM threshold 5.6%. In addition, the wavelength switching system with FBMC and UFMC modulation schemes achieves the same PAPR.

Performance Analysis Of Optical Modulation Techniques Based On Reduction Of Second Order Harmonics And Chromatic Dispersion on OFDM Signals

ABSTRACT This work explains the theoretical and mathematical investigations of the transmission performance of an optical Single Sideband (SSB) modulation technique generated by a Mach Zehnder Modulator (MZM) with a 90° and 120° phase shift using hybrid coupler (Modified SSB) in addition to the Vestigial Sideband technique (VSB). The problem addressed in this paper is the effect of non-linearity of the transmitter system and the chromatic dispersion in single mode fiber due to the presence of second- and higher order harmonics. The non-linearity of the transmitter generates second and higher order harmonics, which affects the system performance leading to reduction in the data rate and increased distortion. The transmission performance of SSB, M-SSB and VSB is evaluated based on the extinction ratio of the MZM, different transmission lengths of the fiber, optical signal modulation index and the receiver sensitivity. A mathematical model has been developed for all the systems representing the modulation techniques till the second order. The baseband modulation consists of an RF signal of frequency 18 GHz and an Orthogonal Frequency Division Multiplexing (OFDM) signal with 4-QAM modulation technique converted to optical signal, using the MZM and transmitted over a SMF of distance of 75 Km. Our results show that the M-SSB technique proves to be more immune toward transmitter nonlinearity and chromatic dispersion as compared to its counterparts. In addition to this, it improves the receiver sensitivity by 6.56 dBm. The results generated by software simulation are in tandem with the mathematical model.

Performance optimization of RSOA based mm-wave radio-over-fibre access network

In this paper, a Reflective Semiconductor Optical Amplifier based, Radio-over-Fibre access network configuration has been proposed to feed future millimeter-wave radio systems. The system architecture combines several approaches to overcome the challenges of millimeter-wave signal transmission. Reflective semiconductor optical amplifier modulator realizes a colorless and relatively cost-effective Remote Antenna Unit. The same optical carrier is used for both downlink and uplink. Optical single-sideband modulation is used at the downlink, which is robust against chromatic dispersion, but the complex realization of this modulation format is not possible at the Remote Antenna Unit. Optical intermediate frequency transmission is applied at the uplink direction, and the required local oscillator signal originates from the central station. The critical element is the reflective optical amplifier, as it compensates for the optical loss and works as an external intensity modulator. The operation of the reflective optical amplifier is modeled by multisection rate and wave equation-based description. The amplification and modulation behaviors of an available reflective optical amplifier are also measured. The experimental work validated the colorless operation and the quality of the modulation versus bias current and input optical power. Finally, system simulation was realized. The uplink and downlink power budgets were balanced, and optimal values for the optical coupling rate and RSOA bias current have been selected.

A novel bidirectional RoF link with compensated SBS and RB for 16-QAM OFDM based mm-wave downlink and uplink vector signal generation and transmission on a single fiber

A novel bidirectional RoF link with compensated SBS and RB for 60 GHz OFDM-based 16-QAM downlink and uplink vector signal generation and co-propagation over a single fiber is proposed. At the central station (CS), 60 GHz optical mm-wave is generated and modulated in the optical single-sideband modulation (OSSB) technique by the 16-QAM OFDM downlink vector signal. At the base station (BS), the 60 GHz downlink signal is detected by a high-speed photodetector (PD). The 60 GHz electrical mm-wave signal is transmitted to the user end by an antenna. The uplink carrier is extracted from the unmodulated sideband of the mm-wave downlink vector signal. This reduces the complexity of BS and makes it free from the laser source. The 16-QAM OFDM-based mm-wave uplink signal is received from the user end through an antenna and down-converted to a 10 GHz uplink vector signal. The uplink signal modulates the extracted blank carrier in the OSSB technique. The uplink and downlink vector signals are co-propagated through the standard single bidirectional fiber. Although the carrier reuse over a single fiber reduces the fiber capacity by half, it generates non-linear backscattering such as Rayleigh Backscattering (RB) and Stimulated Brillouin Scattering (SBS). A theoretical analysis is made on the backscattered power and its effect on the fiber transmission. The compensation of SBS by maintaining the SBS threshold and mitigation of RB is carried out by appropriate modulation technique. In addition, downlink and uplink signal with different data rate is transmitted through the compensated bidirectional RoF link, and the transmission performance is analyzed. Based on the analysis, the theoretical model for simultaneous downlink and uplink signal transmission over a single fiber is proposed and verified by the software simulation. The compensated bidirectional RoF link shows up a better performance up to 75 km with negligible power penalty.

50 Gb/s Transmission using OSSB-MultiCAP Modulation and a Polarization Independent Coherent Receiver For Next-Generation Passive Optical Access Networks

In this paper, a spectrally efficient version of multiband Carrierless Amplitude Phase modulation (MultiCAP) based on Optical Single-Sideband (OSSB) techniques is proposed for its use in high capacity access links. The proposed system consists of four 2.5 GBd OSSB-MultiCAP bands with quadrature amplitude modulation and uses an insensitive polarization receiver to avoid optical polarization issues and adjustments at the receiver side. This scheme has been experimentally evaluated and can provide an aggregated transmission rate of 50 Gb/s over 50 km of standard single mode optical fiber using only 10G electronic and photonic devices in C-band with a sensitivity of -23.2 dBm and a measured optical power budget of 25.2 dB. 40 Gb/s transmission over 50 km with a sensitivity of -27.5 dBm and a measured power budget of 32.5 dB is also demonstrated.

True Time Delay Optical Beamforming Network Based on Hybrid Inp-Silicon Nitride Integration

We demonstrate a broadband and continuously tunable 1×4 optical beamforming network (OBFN), based on the hybrid integration of indium phosphide (InP) components in the silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) platform. The photonic integrated circuit (PIC) comprises a hybrid InP-Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> external cavity laser, a pair of InP phase modulators, a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> optical single-sideband full carrier (SSBFC) filter followed by four tunable optical true time delay lines (OTTDLs), and four InP photodetectors. Each OTTDL consists of eight cascaded thermo-optical micro-ring resonators (MRRs) that impose tunable true time delay on the propagating optical signals. The OBFN-PIC is designed to facilitate the steering of a microwave signal with carrier frequency up to 40 GHz over a continuous set of beam angles. We evaluate the performance of the OBFN-PIC to handle and process microwave signals, measuring the link gain, the noise figure (NF), and the spurious-free dynamic range (SFDR) parameters. Moreover, we assess its beamforming capabilities assuming that the OBFN-PIC is part of a wireless system operating in the downlink direction and feeds a multi-element antenna array. Using microwave signals at 5 and 10 GHz with quadrature amplitude modulation (QAM) formats at 500 Mbaud, we evaluate the performance of the OBFN-PIC under various configurations. We show that error-free performance can be achieved at both operating frequencies and for all the investigated beam angles ranging from 45° to 135°, thus validating its potential for high-quality beamforming performance.

Wideband Doppler frequency shift measurement and direction discrimination based on optical single sideband modulation with a fixed low-frequency reference signal

A wideband Doppler frequency shift (DFS) measurement and direction discrimination method based on optical single sideband (OSSB) modulation with a fixed low-frequency reference signal is proposed and experimentally demonstrated. In the proposed approach, the transmitted signal and the fixed low-frequency reference signal are applied to the lower and upper Mach–Zehnder modulators (MZMs) of a dual-drive dual-parallel MZM (DD-DPMZM) respectively, for realizing OSSB modulation independently. The echo signal is applied to a conventional MZM for optical double sideband (ODSB) modulation. A low-speed photodetector (PD) is used to implement optical-to-electrical conversion and generate three low-frequency beating signals. Both the value and direction of the DFS can be simply obtained by analyzing the frequencies of the three low-frequency beating signals. The proposed scheme has a simple structure, a wideband operating frequency range, and a low requirement of the receiving circuit. Experiments demonstrate that wideband DFS measurement and direction discrimination within 5∼18 GHz with a measurement error of less than ±0.06 Hz are achieved by using a 20 MHz reference signal.

A photonic transmission link with enhanced dynamic range by incorporating phase shifters in dual drive dual parallel Mach–Zehnder modulator

Abstract A linearization scheme is proposed for microwave photonic link to enlarge spurious free dynamic range using a dual-electrode dual parallel Mach–Zehnder modulator (MZM). This scheme employs phase control method to improve performance of the link by adjusting phase of radio frequency (RF) signals and bias voltages of optical modulator. Optical single sideband modulation is achieved through sub-modulators of dual parallel MZM which increases efficiency of the link. The simulated results show that third order intermodulation distortion is suppressed by 28 dB when the input RF signals are 9.1 and 9.5 GHz and noise floor is at −161 dBm/Hz. The spurious free dynamic range is also improved by 12.6 dB.

QAM Vector mm-Wave Signal Generation Based on Optical Orthogonal Polarization SSB Scheme By a Single Modulator

Millimeter-wave (mm-wave) has become an attractive technique in the fifth generation (5G) because it meets the demand of explosive increase in data traffic. In this study, we propose and demonstrate the generation of quadrature amplitude modulated (QAM) vector mm-wave signal employing optical orthogonal polarization single sideband (SSB) modulation, which employs a single polarization-division multiplexing MachZehnder modulator (PDM-MZM). Based on our scheme, there is no inter-band crosstalk and inter-polarization signal-signal beat interference (SSBI). In our scheme, the two sub-modulators in PDM-MZM both work within their linear modulation range. After PDM-MZM, the two optical SSB signals are orthogonally polarized. By using an interleaver (IL), the low sideband (LSB) and upper sideband (USB) optical signals in the two polarization directions are extracted respectively. In addition, IL also suppresses the unwanted components of the opposite orthogonally polarized signal. We demonstrate the generation of two independent 28 GHz 12 Gbaud quadrature-phase-shift-keying (QPSK) and 6 Gbaud 16-ary quadrature amplitude modulation (16-QAM) signals. 65 km single-mode fiber (SMF) transmission is achieved with the bit-error-rate (BER) below 20% forward-error-correction (FEC) soft-decision threshold of 2.4 102 without dispersion compensation. We also demonstrate the generation of two independent 28GHz 1 Gbaud 64-QAM signals and transmission SMF over 10-km with the BER below 20% FEC soft-decision threshold of 2.4 102. Since the LSB and USB signals are on opposite polarization sides, the inter-band crosstalk and the inter-polarization SSBI can be significantly reduced. Because SSB modulation scheme is deployed, our proposed QAM vector signal generation system is robust to fiber dispersion.

Multiplexing scheme for digital signal processing-assisted coherent radio over fiber system

In this paper, a multiplexing scheme, involving optical tandem single-sideband modulation, orthogonal optical single-sideband multiplexing, and digital signal processing (DSP)-assisted optical coherent detection, is proposed. Based on the design, a coherent radio-over-fiber (RoF) optical transmission system is established, which improves channel capacity and spectrum efficiency. Phase-modulated radio frequency signals with different bit rates are carried on an optical single-carrier, transmitted, received after 40 km transmission, and recovered by the DSP algorithm. System performances for all binary phase-shift keying channels are verified by simulation and experiment. Thus, the proposed multiplexing scheme is feasible to improve channel capacity and spectrum efficiency without increasing the system complexity. The work in this paper brings potential applications to coherent RoF systems and hybrid optical wireless communications for future broadband wireless access networks.

Orthogonally polarized RF optical single sideband generation with integrated ring resonators

We review recent work on narrowband orthogonally polarized optical RF single sideband generators as well as dual-channel equalization, both based on high-Q integrated ring resonators. The devices operate in the optical telecommunications C-band and enable RF operation over a range of either fixed or thermally tuneable frequencies. They operate via TE/TM mode birefringence in the resonator. We achieve a very large dynamic tuning range of over 55 dB for both the optical carrier-to-sideband ratio and the dual-channel RF equalization for both the fixed and tunable devices.

Improvements on the performance of subcarrier multiplexing/wavelength division multiplexing based radio over fiber system

Radio over fiber (RoF) techniques are good candidates to create the backbone of the next generation of wireless networks. Many parameters affect RoF communications such as amplified spontaneous emission noise (ASE), four-wave mixing nonlinearity (FWM), the modulation, channel spacing, switching voltage, and phase shifter. In this paper, we propose an improved model of RoF communication systems using subcarrier multiplexing/wavelength division multiplexing (SCM/WDM) technique with unequal channel spacing and 1-km Erbium-doped fiber amplifier (EDFA). Simulation results confirmed that we could obtain the lowest bit error rate and noises when the EDFA is placed at 1 km from the transmitter by using optical single-sideband (OSSB) modulation at frequencies 193.1 THz, 193.2 THz, 193.35 THz, and 193.6 THz.

Peak-to-Average Power Ratio Reduction of Carrier-Suppressed Optical SSB Modulation: Performance Comparison of Three Methods

We compare the performances of three previously proposed methods to reduce the peak-to-average power ratio (PAPR) of the carrier-suppressed optical single-sideband (OSSB-SC) signal. PAPR of OSSB-SC signal becomes high due to the peaky Hilbert-transformed signal which is used for spectral suppression. Nonlinear phase shifts induced by high PAPR degrade OSSB-SC signal during fiber transmission. Previously, we proposed peak folding, peak clipping, and high-pass Hilbert transform methods to reduce the PAPR of OSSB-SC modulation. In this study, we numerically compare the effectiveness of proposed methods in a 10 Gbit/s non-return-to-zero (NRZ)-coded 100-km single-channel transmission link. Due to the reduced PAPR, peak folding and peak clipping can increase the self-phase modulation (SPM) threshold of the studied system by 2.40 dB and 2.63 dB respectively. The high-pass Hilbert transform method improves the SPM threshold by more than 9 dB.

Analog Low-Latency Kramers-Kronig optical Single-Sideband Receiver

Kramers-Kronig (KK) receivers have been shown to be able to cancel the signal-signal beat interference that occurs upon direct-photodetection in optical single-sideband systems, where the transmitted optical field is proportional to the modulation signal (Field-Modulated Direct-Detection systems). They can support complex modulation formats, and being single-sideband, electronic dispersion compensation, without the need for a coherent receiver. All current research has used digital-signal processing (DSP) to implement the KK algorithm. Radio-frequency KK receivers using analog processing were first introduced in the 1960's, and offer an alternative to DSP, particularly in their approximate form. Analog processing is inherently real-time and offers the lowest latency (processing delay). In this paper, we show that with minor changes, analog processing can also be used with signals from a photodetector. We have built an experimental receiver, using standard parts that have functionality that approximates to the desired algorithm. In particular, we show that a differential pair of bipolar transistors can approximate a square-root function, that the Hilbert transform can be approximated by an 8-tap transmission-line delay, and that a commercial analog multiplier chip can be used for a squaring and subtraction function. The system is demonstrated with 16-QAM at 500 Mbit/s with the photocurrent emulated in software and the KK algorithm performed in hardware. Finally, software band-limits the signal, down-converts it to form a constellation, and calculates symbol error rates. We show that the analog KK reduces the symbol error rate by at least a factor of ten, enabling low carrier-to-signal power ratios to be used with standard forward error correction. Much higher data rates could be supported using custom-designed microwave ASICs, which could be incorporated in packaged receivers. This would enable miniaturized receivers using standard DSP to work with dispersion-limited links. Interestingly, half the benefit could be gained by simply re-biasing the differential stage of a standard photoreceiver.