Pre-amplified Receiver Research Articles

All-optical amplitude-phase transmultiplexing of RZ-OOK and RZ-BPSK to RZ-QPSK by polarization-insensitive XPM using a nonlinear birefringent AlGaAs waveguide

Polarization-insensitive (PI) phase-transmultiplexing (PTM) of a 10-Gb/s return-to-zero ON-OFF keying (RZ-OOK) pump and a 10-Gb/s RZbinary phase-shift keying (RZ-BPSK) probe to 20-Gb/s RZ-quadrature-PSK (RZ-QPSK) has been successfully demonstrated for the first time in a passive, birefringent AlGaAs waveguide, utilizing PI cross-phase modulation (PI-XPM). For differential QPSK (DQPSK)-detection, a 10 − 9-BER pre-amplified receiver sensitivity penalty of ≈ 2.5 dB for the in-phase component and ≈ 4.9 dB for the quadrature component were found. The penalties were relative to the FPGA-precoded RZ-DQPSK baseline for a pump-probe detuning of ≈ 12 nm, when the probe state of polarization was scrambled and the pump was launched off-axis into the waveguide.

Data Transfer From RZ-OOK to RZ-BPSK by Polarization-Insensitive XPM in a Passive Birefringent Nonlinear AlGaAs Waveguide

The polarization-insensitive data transfer of 10-Gb/s return-to-zero ON-OFF keying (RZ-OOK) to RZ-binary phase-shift keying (RZ-BPSK) has been successfully carried out for the first time, in a passive birefringent nonlinear Al0.18Ga0.82As waveguide, utilizing polarization-insensitive cross-phase-modulation (PI-XPM). A 10-9 bit-error-rate (BER) pre-amplified receiver sensitivity penalty of ≈2.2 dB relative to baseline RZ-BPSK was measured for PI-XPM, when the RZ-OOK pump was polarization-scrambled and the RZ-probe was launched at 40°. The probe's optimal launch angle for PI-XPM was significantly influenced by the anisotropy of the Al0.18Ga0.82As waveguide.

Scalable modulator for frequency shift keying in free space optical communications

Frequency shift keyed (FSK) modulation formats are well-suited to deep space links and other high loss links. FSK's advantage comes from its use of bandwidth expansion. I.e., FSK counteracts power losses in the link by using an optical bandwidth that is greater than the data rate, just as pulse position modulation (PPM) does. Unlike PPM, increasing FSK's bandwidth expansion does not require increased bandwidth in electronic components. We present an FSK modulator whose component count rises logarithmically with the bandwidth expansion. We tested it with four-fold bandwidth expansion at 5 and 20 Gbit/s. When paired with a pre-amplified receiver, the required received power was about 4 and 5 dB from the theoretical best for such receivers. We also tested the FSK transmitter with a photon counting receiver.

Demonstration of Record Sensitivities in Optically Preamplified Receivers by Combining PDM-QPSK and M-Ary Pulse-Position Modulation

We present the principle, implementation, and performance of a recently introduced high-sensitivity modulation format based on the combined use of polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK, or PQ) and m-ary pulse-position modulation (m-PPM). This novel modulation format, termed PQ-mPPM, offers high receiver sensitivity in optically preamplified receivers. We study the sensitivity of the PQ-mPPM format both analytically and experimentally, and compare it to common modulation formats such as PDM-QPSK, m-PPM, differential phase-shift keying, and polarization-switched QPSK. The bandwidth expansion factor of this format is also discussed. A record sensitivity of 3.5 photons per bit at BER = 10-3 is experimentally demonstrated at 2.5 Gb/s with a novel pilot-assisted digital coherent-detection scheme, outperforming PDM-QPSK by about 3 dB.

M-ary pulse-position modulation and frequency-shift keying with additional polarization/phase modulation for high-sensitivity optical transmission

We present a new class of optical modulation formats based on the combination of m-ary pulse-position modulation (m-PPM) or m-ary frequency-shift keying (FSK) with additional polarization and/or phase modulation, which is applied on the information carrying pulses in the case of m-PPM or on the information carrying frequency carriers in the case of m-FSK. We describe the principle and implementation of this class of optical modulation formats, and formulate their theoretical receiver sensitivities in optically pre-amplified receivers. Pilot-assisted frequency-domain equalization, similar to that used in coherent optical orthogonal frequency-division multiplexing (CO-OFDM), is used for reliable channel estimation and compensation. CO-OFDM also allows m-FSK to be implemented with high spectral efficiency. As a particular format in this class, m-PPM in combination with polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK), termed as PQ-mPPM, offers superior receiver sensitivity in optically pre-amplified receivers at bit error ratios (BERs) around the thresholds of common forward-error correction codes. Record receiver sensitivities of 3.5 photons per bit (ppb) at BER = 10(-3) and 2.7 ppb at BER = 1.5 × 10(-2) are experimentally demonstrated at 2.5 Gb/s and 6.23 Gb/s using PQ-16PPM and PQ-4PPM, respectively. We further demonstrate the transmission of a 6.23-Gb/s PQ-4PPM signal over a 370-km unrepeatered ultra-large-area-fiber span with 71.7-dB total loss budget.

Impact of Raman Amplification on a 2-Tb/s Coherent WDM System

The impact of hybrid erbium-doped fiber amplifier (EDFA)/Raman amplification on a spectrally efficient coherent-wavelength-division-multiplexed (CoWDM) optical communication system is experimentally studied and modeled. Simulations suggested that 23-dB Raman gain over an unrepeatered span of 124 km single-mode fiber would allow a decrease of the mean input power of ~6 dB for a fixed bit-error rate (BER). Experimentally we demonstrated 1.2-dB Q-factor improvement for a 2-Tb/s seven-band CoWDM with backward Raman amplification. The system delivered an optical signal-to-noise ratio of 35 dB at the output of the receiver preamplifier providing a worst-case BER of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> over 49 subcarriers at 42.8 Gbaud, leaving a system margin (in terms of Q -factor) of ~4 dB from the forward-error correction threshold.

Macroscopic Differential Phase Shift Quantum Key Distribution Using an Optically Pre-Amplified Receiver

Since it was noted that quantum computers could break public key cryptosystems based on number theory, extensive studies have been undertaken on quantum cryptography, which offers unconditionally secure communication based on quantum mechanics. We investigate a quantum key distribution (QKD) scheme using macroscopic coherent light with optically pre-amplified direct differential detection. A transmitter “Alice” sends a series of two macroscopic nonorthogonal coherent states that partially overlap due to quantum noise. A receiver “Bob” amplifies and receives it with direct differential detection followed by a thresholding process. To avoid difficulties in detection, our scheme uses conventional direct differential photodetection, not single-photon detection or homodyne detection as in previous QKD protocols. System performance assuming some eavesdropping is evaluated, the results of which suggest that our scheme is usable for short or medium distance.

Bit Patterning in SOAs: Statistical Characterization Through Multicanonical Monte Carlo Simulations

We present a simulation tool based on the Multicanonical Monte Carlo (MMC) method to characterize the statistical properties of bit patterning in semiconductor optical amplifiers (SOAs). Our tool estimates the conditional probability density functions (PDFs) of marks and spaces of the received signal. We introduce an experimental technique to directly measure the conditional PDFs of the received marks and spaces using a high bandwidth sampling scope. We demonstrate that predictions from our simulation tool match the experimental data. We measure the bit error rate (BER) of a SOA-based preamplified receiver, where the SOA operates in the nonlinear regime, and demonstrate that our simulation tool can predict the measured BER.

Spectrum-Sliced Wavelength Division Multiplexed Systems with Optical Preamplifiers

The effect of dispersion on a spectrum-sliced wavelength division multiplexed system with an optical preamplifier receiver is investigated using a theoretical model based on the statistics to improve understanding of spectrum slicing in a realistic situation, where dispersion significantly impacts the signal in transmission. The results show that the optically preamplified receiver delivers increased transmission capacity and a substantially improved power budget compared to a pin receiver. The results are obtained using the saddlepoint approximation and compared to the customary Gaussian approximation, which is found to be reasonably accurate in predicting the optimum bandwidth but conservative in sensitivity predictions.

Semi-analytical approach for performance evaluation of direct-detection OFDM optical communication systems

A semi-analytical method to evaluate the bit error ratio (BER) in direct-detection (DD) optical fibre transmission systems employing orthogonal frequency division multiplexing (OFDM) and optically pre-amplified receivers is proposed. The method considers a Gaussian approach for the signal at the equalizer output and allows evaluating accurately the BER of each OFDM subcarrier for the receiver structure considered and for practical optical and electrical filters shapes, being a powerful tool to perform the optimization of these systems. The results obtained by the proposed method have shown excellent agreement with Monte Carlo estimates for DD-OFDM ultra-wideband radio signals and for two different DD-OFDM signals proposed for long-haul systems.

Gain-controlled semiconductor optical preamplifier for the 100 Gbit/s 40 km Ethernet receiver

A numerical investigation of the performance of an automatic gain-controlled semiconductor optical preamplified receiver for a 4 x 25 Gbits/s wavelength division multiplexing transmission system with a 0-40 km reach is presented. We show that the control scheme acting on the semiconductor optical amplifier (SOA) gain increases the input power dynamic range of the optical receiver, thus allowing the transmission system to operate error free regardless of fiber length. In contrast, a fixed-gain optical receiver shows poor performance that is due to SOA nonlinearity and photodiode overload, which are well captured by the corresponding simulation models. The device represents a practical alternative to the next-generation high-speed Ethernet technology.

Estimation of the Bit Error Rate for Direct-Detected OFDM Signals With Optically Preamplified Receivers

In this paper, we provide a numerical bit error rate (BER) estimation approach for direct-detected orthogonal frequency division multiplexing (OFDM) signals in the presence of optical preamplified receivers. The individual BER of each subcarrier is first computed by considering their electrical signal-to-noise ratio (SNR), and then the ensemble BER is derived simply by taking the average of all the subcarriers' BERs. The calculated BER is verified by the conventional error-counting approach with high precision and is still accurate with higher quadratic-amplitude modulation (QAM) formats, even under the influences of the optical filtering and polarization mode dispersion (PMD) effects. Based on our simulation approach, the required extra power budget for 16- and 64-QAM relative to 4-QAM format are found to be ~3.8 and 8.2 dB, respectively, at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> . Furthermore, we use this approach to compare the receiving sensitivities and PMD tolerances for the previous proposed gapped and interleaved radio-frequency (RF)-tone-assisted OFDM systems. The results show that the gapped OFDM has a better sensitivity while the interleaved OFDM has a more PMD-tolerable capability.

40 Gb/s Field-Modulated Wavelength Converters for All-Optical Packet Switching

We present a high-functionality photonic integrated circuit that performs field-modulated wavelength conversion. This device incorporates an on-chip sampled grating distributed Bragg reflector laser for wide tunability. Wavelength conversion is accomplished using a preamplified semiconductor optical amplifier photodiode receiver interconnected with a traveling-wave modulator to form a high-speed optical gate. This paper discusses the design and performance of this device, as well as its potential for optical packet switching applications. Error-free wavelength conversion is demonstrated at 40 Gb/s with 1-3 dB power penalty compared with back-to-back transmission over 22 nm of input and output tuning. Output extinction in all cases is greater than 9 dB, and conversion efficiency ranges from -2 to -6 dB over the tuning range. This device additionally demonstrates the capability for external 10 Gb/s modulation, which can be used for optical label encoding.

Widely Tunable Separate Absorption and Modulation Wavelength Converter With Integrated Microwave Termination

A widely tunable wavelength converter utilizing a separate absorption and modulation configuration and only dc bias connections is demonstrated. The device integrates an SG-DBR laser with a traveling-wave electroabsorption modulator and an optically pre-amplified receiver and introduces a simplified bias scheme by the inclusion of passive resistor and capacitor circuit elements. We discuss a the design of these passive elements and their compatibility with fabrication of photonic integrated circuits. The device demonstrates over 12 GHz optical bandwidth and error free 10 Gb/s wavelength conversion is achieved with less than 2.5 dB power penalty over 25 nm of output tuning.

Separate Absorption and Modulation Mach-Zehnder Wavelength Converter

A monolithic separate absorption and moduation region (SAM) wavelength converter is demonstrated. The transmitter consists of a sampled-grating DBR (SGDBR) laser and a series-push-pull Mach-Zehnder modulator. The preamplified receiver is composed of a flared semiconductor optical amplifier and a quantum-well pin photodetector. Integrated resistors and capacitors are used to minimize microwave losses and remove the need for external bias tees. The design, fabrication, and operation of this photonic integrated circuit is presented. Small-signal response measurements show a device bandwidth in excess of 20 GHz. Operation at 40 Gbps with NRZ data shows less than a 2.5-dB power penalty across the 32-nm laser tuning range with no additional power penalty for conversion to the input wavelength.

Transceive surface coil array for MRI of the human prostate at 4T

A novel torso transceive surface coil array for prostate magnetic resonance imaging (MRI) and spectroscopy (MRS) at 4T is presented. It is shown that with the use of a conformal transceive surface coil array with 50 Omega transmitter amplifiers and receiver preamplifiers, one can perform whole-volume torso imaging while maintaining the high signal-to-noise ratio (SNR) inherent to surface coil designs. Recent theoretical considerations have shown that by focusing the infringing radiofrequency (RF) electromagnetic field, one can achieve increased penetration and signal homogeneity compared to a conventional circularly polarized driving scheme. A variation of this driving scheme particular to the proposed coil design resulted in a twofold increase in SNR in the prostate compared to that achieved with a conventional circularly polarized driving scheme. The novel transceive surface coil array presented is capable of full-volume imaging of the human torso at 4T while maintaining signal penetration in the deep region of the prostate gland.

On the Probability Density Function of an Optical DPSK Signal in the Presence of Intrachannel Crosstalk and ASE Noise

The application of the saddle point integration method for evaluating the probability density function of a DPSK signal in the presence of intrachannel crosstalk and ASE noise is investigated. Numerical results for a 10 Gbit/s optically pre-amplified direct detection receiver show that the shape of the density has an asymmetric two-pronged structure. Moreover, the reliability of a saddle point approximation is also studied, and shown that this technique estimates very well the tails of the density, but lacks accuracy in the central region.

A hybrid AlGaInAs-silicon evanescent waveguide photodetector

We report a waveguide photodetector utilizing a hybrid waveguide structure consisting of AlGaInAs quantum wells bonded to a silicon waveguide. The light in the hybrid waveguide is absorbed by the AlGaInAs quantum wells under reverse bias. The photodetector has a fiber coupled responsivity of 0.31 A/W with an internal quantum efficiency of 90 % over the 1.5 mum wavelength range. This photodetector structure can be integrated with silicon evanescent lasers for power monitors or integrated with silicon evanescent amplifiers for preamplified receivers.

Serial OTDM for 100 Gbit-Ethernet applications

A novel transmitter for 100 Gbit-Ethernet applications is proposed, based on the serial cascade of two 50 Gbit/s inverse-return-to-zero (also known as dark soliton) transmitters based on Mach-Zehnder modulators. The proposed transmitter and demultiplexer system uses commercially available components optimised for 40 Gbit/s applications. A 2.9 dB penalty at 100 Gbit/s is obtained using a single-stage OTDM demultiplexer and a preamplified receiver.

A 5.6-mW 1-Gb/s/pair pulsed signaling transceiver for a fully AC coupled bus

This paper describes a low-power synchronous pulsed signaling scheme on a fully AC coupled multidrop bus for board-level chip-to-chip communications. The proposed differential pulsed signaling transceiver achieves a data rate of 1 Gb/s/pair over a 10-cm FR4 printed circuit board, which dissipates only 2.9 mW (2.9 pJ/bit) for the driver and channel termination and 2.7 mW for the receiver pre-amplifier at 500 MHz. The fully AC coupled multipoint bus topology with high signal integrity is proposed that minimizes the effect of inter-symbol interference (ISI) and achieves a 3 dB corner frequency of 3.2 GHz for an 8-drop PCB trace. The prototype transceiver chip is implemented in a 0.10-/spl mu/m 1.8-V CMOS DRAM technology and packaged in a WBGA. It occupies an active area of 330/spl times/85 /spl mu/m/sup 2/.