High-bandwidth balanced photoreceiver suitable for 40-gb/s RZ-DPSK modulation formats

A novel fully packaged balanced photoreceiver designed for 40-Gb/s differential phase shift keying (DPSK) applications at 1.55-/spl mu/m wavelength is presented. Based on InP : Fe substrate, two waveguide-integrated photodiodes with spot size converters are monolithically integrated with a distributed amplifier. This configuration leads to a high symmetry of the two optical inputs and avoids interconnection parasitics. The RF output of the photoreceiver is dc-coupled, but the design enables an interfacing to subsequent electronics without external bias-tee by using a negative bias supply at the source terminal of the integrated amplifier. A reduction of system costs and required space will be obtained by this concept. Mounted in a butterfly package, a 3-dB cutoff frequency of 42 GHz is achieved. Return-to-zero (RZ)-DPSK experiments exhibit excellent operation of the balanced photoreceiver over a large dynamic range of optical input power up to +10 dBm. The high-power capability and the clear eye opening at low input power demonstrates the suitability for high performance core and metro systems.     

Continuous-phase frequency-shift keying with external modulation

A high-speed external modulation scheme for optical continuous-phase frequency-shift keying (CPFSK) is presented. In external frequency-shift keying (FSK) modulation using single-sideband modulation technology, an optical upper/lower sideband (USB/LSB) component with respect to a carrier frequency is selectively generated. However, the FSK-modulated signal has phase discontinuities at the timings of frequency shifts. To overcome the problem, we propose a synchronous control technique that enables continuous phase modulation. In the external CPFSK modulation, the USB/LSB is allowed to shift to the other state when their phases coincide with each other. It is shown that CPFSK with a zero-to-peak frequency deviation of 0.5/spl times/(bit rate) is achieved with this synchronous control. Occupant bandwidth is less than half that of the externally modulated incoherent FSK with phase discontinuities. At a modulation speed of 10 Gb/s, higher order sidelobes are highly suppressed by more than 20 dB at the 20-GHz offset, comparing to a conventional binary phase-shift keying (BPSK). By the use of a Mach-Zehnder interferometer with balanced photodetection, receiver sensitivity is 3-dB greater than that of an on-off keying, as well as a BPSK. In this paper, we report on the experimental demonstration of CPFSK modulation/demodulation. A six-channel wavelength-division-multiplexed, 10-Gb/s CPFSK modulation/demodulation was successfully demonstrated.     

电力光纤通道的PS-QPSK调制

高频谱偏振相干光高效率的优势在电力光纤传输中得到了广泛应用,然而,这种传输机制对外界干扰极其敏感。为减少光纤传输损伤,提高信道信噪比,高效的调制格式是改善传输信道性能和延长传输距离的有效途径。通过对传统QPSK星座图的旋转,提出了偏振切换正交相移键控(PS-QPSK),在接收侧采用相干检测技术可以实现高性能的信号解调。对于电力主干光纤传输系统,采用PS-QPSK的调制方式,在光信噪比和传输距离方面都有所改善。在误码率为3.8×10-3的信道中信噪比提升近1d B,50GHz的主干光纤中最大覆盖范围较传统方法增加了30%。     

Forward error correction based on block turbo code with 3-bit soft decision for 10-Gb/s optical communication systems

The first experimental demonstration of a forward error correction (FEC) for 10-Gb/s optical communication systems based on a block turbo code (BTC) is reported. Key algorithms, e.g., extrinsic information, log-likelihood ratio, and soft decision reliability, are optimized to improve the correction capability. The optimum thresholds for a 3-bit soft decider are investigated analytically. A theoretical prediction is verified by experiment using a novel 3-bit soft decision large scale integrated circuit (LSI) and a BTC encoder/decoder evaluation circuit incorporating a 10-Gb/s return-to-zero on-off keying optical transceiver. A net coding gain of 10.1 dB was achieved with only 24.6% redundancy for an input bit error rate of 1.98/spl times/10/sup -2/. This is only 0.9 dB away from the Shannon limit for a code rate of 0.8 for a binary symmetric channel. Superior tolerance to error bursts given by the adoption of 64-depth interleaving is demonstrated. The ability of the proposed FEC system to achieve a receiver sensitivity of seven photons per information bit when combined with return-to-zero differential phase-shift keying modulation is demonstrated.     

All-optical inverter based on long-wavelength vertical-cavity surface-emitting laser

An all-optical inverter using transverse mode switching in a 1.5-/spl mu/m vertical-cavity surface-emitting laser (VCSEL) was demonstrated theoretically and experimentally. The fundamental mode in a single-mode VCSEL was strongly suppressed when external light was injected into an LP02 high-order mode, resulting in an on-off ratio of over 20 dB. The polarization dependence of the static optical input--output characteristics of inverter operations was as low as /spl sim/13%. The VCSEL-based optical inverter operated with a 5-Gb/s nonreturn-to-zero (NRZ) input signal. A theoretical result using two-mode rate equations is in good agreement with experimental results.     

Error probability of DPSK signals with cross-phase modulation induced nonlinear phase noise

The error probability is derived analytically for differential phase-shift keying (DPSK) signals contaminated by both self- and cross-phase modulation (SPM and XPM)-induced nonlinear phase noise. XPM-induced nonlinear phase noise is modeled as Gaussian distributed phase noise. When fiber dispersion is compensated perfectly in each fiber span, XPM-induced nonlinear phase is summed coherently span after span and is the dominant nonlinear phase noise for typical wavelength-division-multiplexed (WDM) DPSK systems. For systems without or with XPM-suppressed dispersion compensation, SPM-induced nonlinear phase noise is usually the dominant nonlinear phase noise. With longer walkoff length, for the same mean nonlinear phase shift, 10-Gb/s systems are more sensitive to XPM-induced nonlinear phase noise than 40-Gb/s systems.     

Single-mode 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP quantum well vertical cavity surface emitting lasers

The 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP vertical cavity surface emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current varies from 1.8 to 1.1 mA and the slope efficiency falls less than /spl sim/35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75/spl deg/C. The VCSELs continuously operate up to 105/spl deg/C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is estimated to be 10.7 GHz with modulation current efficiency factor of /spl sim/5.25GHz/(mA)/sup 1/2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25/spl deg/C to 70/spl deg/C. We also accumulated life test data up to 1000 h at 70/spl deg/C/10 mA.     

A PO-CI/MC-CDMA scheme for high modulation styles

Through analyzing the theoretical spreading principle, it has been proved in this paper that the benefit of pseudo-orthogonal carrier interferometry (PO-CI) spreading code is not supported when complex signal modulation (e.g., quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM)) types are employed. On this basis, a novel and feasible structure for this problem is brought forward. Within the structure mentioned, instead of complex modulation patterns, pulse amplitude modulation (PAM) combined with PO-CI spreading code is utilized. This allows us to maintain the throughput increase of a multi-carrier code division multiple access (MC-CDMA) system with minimal loss in performance and no bandwidth expansion. __________ Translated from Journal of Beijing University of Posts and Telecommunications, 2007, 30(4): 14–18 [译自: 北京邮电大学学报]     

Phase-diversity homodyne detection of multilevel optical modulation with digital carrier phase estimation

This paper describes a phase-diversity homodyne receiver that which can cope with multilevel modulation formats. The carrier phase drift is estimated with digital signal processing (DSP) on the homodyne-detected signal, entirely restoring the complex amplitude of the incoming signal. Our DSP-based phase-estimation scheme consists of a simple and demultiplexable architecture that allows the system to reach significantly higher performance than conventional optical delay detection. Since the whole optical signal information is preserved with our receiver, various kinds of postprocessing of the received signal become possible. For example, we can demultiplex wavelength-division/optical time-division multiplexed channels and compensate for group velocity dispersion of fibers as well as the nonlinear phase noise in the electrical domain. We also experimentally evaluate the performance of our receiver. Our offline bit-error rate experiments show the feasibility of transmitting polarization-multiplexed 40-Gb/s quadrature phase-shift keying signals over 200 km with channel spacing of 16 GHz, leading to spectral efficiency of 2.5 b/s/Hz.     

1.3-/spl mu/m AlGaInAs strain compensated MQW-buried-heterostructure lasers for uncooled 10-gb/s operation

We have successfully fabricated 1.3-/spl mu/m AlGaInAs strain-compensated multiple-quantum-well (MQW) buried-heterostructure (BH) lasers by narrow-stripe selective metalorganic vapor-phase epitaxy. Based on the optimization of AlGaInAs strain compensated MQW and the Al-oxidation-free BH process, we obtained a low-threshold current of 12.5 mA and a relaxation frequency of more than 10 GHz at 85/spl deg/C for Fabry-Perot lasers. For distributed feedback lasers, we demonstrated a 10-Gb/s operation and transmission of over 16 Km for a single mode fiber at 100/spl deg/C. Furthermore, a record-low 25.8-mA/sub p-p/ modulation current for a 10-Gb/s modulation at 100/spl deg/C was demonstrated with shorter cavity and high grating-coupling coefficient. A median life of more than 1/spl times/10/sup 5/ h at 85/spl deg/C was estimated after an aging test of over 5000 h for these lasers. These superior characteristics at high temperatures were achieved by the combination of the high differential gain of AlGaInAs strain compensated MQW and the BH structure.     

Benefit of SPM-based all-optical reshaper in receiver for long-haul DWDM transmission systems

The effectiveness of a self-phase modulation (SPM)-based all-optical reshaper with optically time-division-demultiplexing receiver was experimentally investigated using 42.7-Gb/s carrier-suppressed return-to-zero (CS-RZ) signals. We have confirmed that this scheme is quite effective to suppress the waveform degradation due to optical signal bandlimitation. We have demonstrated 80% spectral efficiency without using polarization demultiplexing by using the all-optical reshaper. We have also demonstrated 50-GHz-spaced 55/spl times/42.7 Gb/s signals transmission over 2500 km, using an optically bandlimited CS-RZ signal and the SPM-based all-optical reshaper in receiver without using polarization demultiplexing. A Q-factor improvement of about 1.5 dB was obtained by using the all-optical reshaper.     

High-capacity undersea long-haul systems

This paper reviews technologies and techniques that have been used in deployed long-haul wavelength division multiplexing (WDM) systems and emerging technologies that could be used for the next generation of cost-reduced systems. The overview of current generation technologies starts with a discussion of modulation formats, focusing on the superior properties of the chirped return to zero on-off shift keying (CRZ-OOK) modulation format. The use of 10-Gb/s CRZ-OOK modulation format together with advanced fiber types, more powerful forward error correction (FEC), and broadband erbium-doped fiber amplifiers resulted in the deployment of dense WDM systems with capacities per fiber in terabits per second range and trans-Pacific reach. Demand for the systems with large design capacity led to further development of broadband optical amplifiers. Laboratory demonstrations successfully expanded transmission into the full C-band and later in the C and L transmission bands. The current market conditions dictate the need for reducing the first cost of an installed system rather than reaching record capacity per fiber. Reducing first cost can be achieved by reducing the amount of optical amplifiers in the cable by utilizing an excess performance margin provided by available RZ-OOK technology. Further improvements would be possible if more powerful FEC and modulation formats with better receiver sensitivity are used. For example, the RZ differential phase-shift keying (RZ-DPSK) modulation format with 3-dB better receiver sensitivity and better nonlinear tolerance to large amounts of accumulated dispersion is a very promising technology. This paper will review long-haul transmission results using RZ-DPSK and will compare the transmission properties of RZ-DPSK signals versus RZ-OOK signals. Due to superior receiver sensitivity, the RZ-DPSK modulation format can be an enabling technology for 40-Gb/s per channel transoceanic transmission.     

Wavelength exchange in a highly nonlinear dispersion-shifted fiber:theory and experiments

With a suitable arrangement of two pumps and two signals with respect to the zero-dispersion wavelength of a fiber, simultaneous wavelength exchange between two signals can be realized by four-wave mixing in the fiber. We have demonstrated near-complete wavelength exchange between two signals at 1573.4 and 1579.9 nm with two 0.25-W pumps in a 1-km-long highly nonlinear dispersion-shifted fiber. We also have evaluated the bit-error-rate performance of wavelength exchange with a 10-Gb/s signal, and obtained a power penalty of less than 1 dB for the exchanged signal     

Boundary effects on the optical properties of InGaN multiple quantum wells

We examine the issues of spontaneous and piezoelectric polarization discontinuity on the optical properties of 3.0-nm-thick indium gallium nitride (InGaN) multiple quantum wells (MQWs). A quench of band-edge emission from the cap GaN layer is observed when the photoexcitation source is changed from a 355- to a 248-nm laser. The interband transitions from the InGaN wells exhibit a linear dependence on the 1) spectral blue shift of /spl sim/8.5/spl times/10/sup -18/ meV /spl middot/ cm/sup 3/ and 2) change of the internal field of /spl sim/3/spl times/10/sup -14/ meV /spl middot/ cm/sup 2/ with the injected carrier density up to N/sub inj//spl sim/10/sup 19/ cm/sup -3/ at 77 K. These observations are attributed to the redistribution of photogenerated carriers in the InGaN wells due to the polarization discontinuity at the QW interface and the surface band bending effect. By incorporating an additional boundary condition of surface Fermi-level pinning into the Poisson equation and the band-structure analysis, it is shown the emission from the InGaN-GaN MQWs is dominant by the recombination between the high-lying subbands and the screening of internal field effects.     

Characterization of Si implants in p-type GaN

Si ion implantation into p-type GaN followed by rapid thermal annealing (RTA) in N/sub 2/ has been performed. X-ray diffraction analyses indicate that ion-implanted damage remains even with 1050/spl deg/C, 60 s RTA. By varying implantation and postimplantation annealing conditions, we could convert carrier concentration from p-type 3 /spl times/ 10/sup 17/ cm/sup -3/ into n-type 2 /spl times/ 10/sup 17/ cm/sup -3/ /spl sim/2 /spl times/ 10/sup 19/ cm/sup -3/. It was found that typical activation energies of Si implants in p-GaN are lower than 10 meV. Such activation energies are smaller than those observed from epitaxially grown Si-doped GaN films. A deep donor level with activation energy of 60 meV was also found from some samples. Photoluminescence studies show that the peak appears at 372 nm might be related to implantation-induced defects. It was also found that a green emission band could be observed from Si-implanted GaN. It was shown that such a green emission is related to the yellow band observed from epitaxially grown Si-doped GaN. The transport properties of these Si-implanted samples were also studied.     

Multiple-wavelength emission from In/sub x/Ga/sub 1-x/As-GaAs quantum wells grown on a nanoscale faceted GaAs substrate by molecular beam epitaxy

Multiple-wavelength photoluminescence (PL) spanning a 160-nm range from 980 to 1140 nm (77 K) has been obtained from In/sub x/Ga/sub 1-x/As-GaAs quantum wells (QWs) with varying In composition x on a nanoscale faceted (nanofaceted) GaAs substrate grown by molecular beam epitaxy. Five nanofaceted regions which consist of periodic [100]-(n11) (n = 3 or 1) facets along [011~] with different periods were prepared on a single substrate by interferometric lithography and selective growth of GaAs. The pattern period p was varied from infinity (large-area unpatterned) to 210 nm while the lateral width of the (n11) facet region was kept constant at /spl sim/180 to 200 nm within each period. A 5-nm-thick In/sub 0.23/Ga/sub 0.77/As layer was deposited on this multiple-period nanofaceted single GaAs surface in a single-run growth. Orientation-dependent migration and incorporation (ODMI) of In atoms [mass transport of incident In atoms from the (n11) to adjacent [100] facets] results in a variation of x of the In/sub x/Ga/sub 1-x/As layer section on the [100] facet as the width of the [100] facet was changed from /spl sim/20 to /spl sim/200 nm. ODMI induces a higher x on the [100] facet for smaller p. The PL exhibits a polarization dependence which is more pronounced for decreasing p [i.e., the width of [100] facet]. Consistent variations of the PL peak energy and linear polarization along the pattern direction confirm that ODMI results in a variation of the In composition and imply that the In/sub x/Ga/sub 1-x/As layer on a [100] facet has characteristics of a quantum wire as its width is decreased to /spl sim/20 nm for p = 210 nm. A possible application of nanopatterned growth to wavelength-division-multiplexing transmitters is discussed.     

Optical sensing of biomolecules using microring resonators

A biosensor application of vertically coupled glass microring resonators with Q/spl sim/12 000 is introduced. Using balanced photodetection, very high signal to noise ratios, and thus high sensitivity to refractive index changes (limit of detection of 1.8/spl times/10/sup -5/ refractive index units), are achieved. Ellipsometry and X-ray photoelectron spectroscopy results indicate successful modification of biosensor surfaces. Experimental data obtained separately for a bulk change of refractive index of the medium and for avidin-biotin binding on the ring surface are reported. Excellent repeatability and close-to-complete surface regeneration after binding are experimentally demonstrated.     

InGaN-based ultraviolet emitting heterostructures with quaternary AlInGaN barriers

Nitride-based ultraviolet (UV) heterostructures with InGaN quantum wells and AlInGaN barrier layers have been grown by metal-organic chemical vapor deposition on sapphire substrates. The emission band was at 3.307 eV (375 nm) at room temperature (RT) and its full-width at half-maximum was /spl sim/82meV. In addition to the UV band, some blue emission admixture was found in a single-quantum-well (SQW) structure, which the authors attribute to recombination of injected electrons that are not captured into the SQW and enter the p-side of the structure. The authors demonstrate a significant advantage in utilizing multiple-quantum-well (MQW) structures that provide a more effective capture of injected carriers into wells and predominance of UV emission. Temperature-sensitive competition between two emission mechanisms in MQW structures has been observed. Below /spl sim/170 K, the blue impurity-related emission dominated. In the 170-190 K range, an anomalous temperature-induced "blue jump" by over /spl sim/340 meV to UV region occurred, with UV emission dominating above 190 K.     

FPGA-based system for effective IQ imbalance mitigation of RF power amplifiers

To provide an adequate signal integrity to a power amplifier (PA), we propose a digital system for the degradation at the transmitter path, and it is implemented on a field-programmable gate array (FPGA) board. The proposed system offers the following features: A -ary quadrature amplitude modulation (QAM) digital signal generation and in-phase/quadrature (IQ) imbalance mitigation, and by default, it performs as a predistortion model extraction from PA-measured data. The simulations and tests provided are performed to effectively verify the PA linearity by using 256-QAM signals. The nonlinearities are predicted as a reliable solution for linearizing the PA from measurements of AM/AM and AM/PM conversion curves. The performance is evaluated in terms of linearity, computation complexity, and FPGA hardware synthesis according to a dependability compliance of digital signal processing. Finally, the model is validated with input/output data observations to linearize the model with a fitting normalized mean squared error (NMSE) of around  dB, a spurious free dynamic range of 40 dBm, and an adjacent channel power ratio reduction by  dBm, for a class-AB broadband radio frequency PA GaN HEMT of 10 W working at 2.34 GHz.     

Nonlinear Phase Noise Reduction of DQPSK Signals by a Phase-Preserving Amplitude Limiter Using Four-Wave Mixing in Fiber

It is experimentally shown that phase-preserving amplitude regeneration by an all-optical amplitude limiter using saturation of four-wave mixing in a highly nonlinear fiber can enhance differential quadrature phase-shift keying (DQPSK) transmission performance. The limiter suppresses amplitude fluctuations of the signal, by which the nonlinear phase variance caused by self-phase modulation in the transmission fiber is reduced. The 10-GSymbol/s short-pulse DQPSK transmission experiment over densely dispersion-managed fiber shows that the amplitude limitation after an imperfect transmitter increases maximum transmitted signal power by 5 dB.