Delay Interferometer Research Articles

Simulation and Comparison of Advanced Modulation Formats for Wavelength Reuse in High-SpeedWDM-PON System

In recent years, the innovation in devices and schemes of physical layer plays an important role in promoting the development of WDM-PON (Wavelength Division MultiplexingPassive Optical Network).Among the key technologies adopted in the WDM-PON system, advanced modulation formats at downstream and remodulations at upstream attract the most attention. DPSK (Differential Phase Shift Keying Modulation), FSK(Frequency-Shift Keying), IRZ(Inverted Return-to-Zero) and Manchester are four of the most common formats and each of them was studied before, but, however, nobody compared and analyzed them together.So, we will do the job by numeralsimulation in this paper. During our simulation, the 40-Gbit/s FSK is generated through the demodulation of two DPSK signals and 40-Gbit/s Manchester signal bases of employing a delay interferometer (DI) and an optical delay line as format conversion device to convert RZ-DPSK(Return to Zero-Differential Phase Shift Keying Modulation) coding to Manchester coding, both of the two format generation schemes are novel.Meanwhile,this paper emphasizes the input optical power and dispersion tolerance, which both related to the sensitivity of the system. Analyze the sensitivity with DPSK, FSK, IRZ and Manchester, and then compare the four advanced modulation formats.

Dispersion Tolerance for Optical Duobinary Transmitters based on a Mach-Zehnder Modulator and an Optical Filter

We theoretically investigated dispersion tolerance of an optical duobinary transmitter employed a Mach-Zehnder modulator and an optical filter. Compared to the optical transmitter based on a Mach-Zehnder modulator and an optical delay interferometer by optimizing the applied voltage for improving the dispersion tolerance, the demonstrated duobinary transmitter provides improved receiver sensitivity and higher dispersion tolerance while maintaining narrow spectral bandwidth.

All-optical wavelength conversion of nonreturn-to-zero signals based on a nonlinear polarization switch and a delayed interferometer

A novel all-optical wavelength converter for nonreturn-to-zero systems based on a nonlinear polarization switch (NPS) and a delayed interferometer (DI) is proposed and demonstrated experimentally. The performance of the proposed wavelength converter is compared with the performance of a conventional wavelength converter that only incorporates a NPS. Results show that the quality of the converted signal is significantly improved by inserting a DI after the NPS. The transmission performance of the wavelength-converted signal is also improved by the DI.

A fast-scanning Fourier transform 2D IR interferometer

Ultrafast two-dimensional infrared spectroscopy (2D IR) allows for the characterization of vibrational couplings and chemical dynamics. The fastest method of acquiring a Fourier transform 2D IR data set involves spectrally dispersing the signal field onto an infrared array detector. However, use of this method carries disadvantages, including the high cost of IR arrays and the decrease of signal intensity due to dispersion. As an alternative, we demonstrate a readily implemented full time-domain 2D IR detection method in which data from a pulsed laser source is rapidly acquired by scanning an interferometer delay at constant velocity. The stage's position is determined with high accuracy on a shot-to-shot basis by quadrature detection of HeNe tracer interference fringes.

Signed frequency offset measurement for direct detection DPSK system with a chromatic dispersion offset

We demonstrated a method for the measurement of signed frequency offset between optical source and delay interferometer (DI) for 10 Gb/s DPSK signals based on asynchronous delay-tap sampling technique with a chromatic dispersion (CD) offset. The demodulated DPSK signals show asymmetrical property and amplitude shoulder appears on the waveforms with frequency offset and a fixed CD offset together. The delay-tap sampling scatter plots also show the asymmetry related to the asymmetrical signal distortion. Our proposed method cannot only realize the measurement of the magnitude of frequency offset but also the polarity. The measurement range is from -2 GHz to +2 GHz and the sensitivity can reach ±100MHz. The simulation and experimental results are demonstrated and in good agreement.

UWB Impulse Radio Transmitter Using an Electrooptic Phase Modulator Together With a Delay Interferometer

A simple ultra-wideband (UWB) impulse radio transmitter is experimentally demonstrated using an electrooptic phase modulator (PM) and a delay interferometer (DI). By applying an electrical nonreturn-to-zero (NRZ) baseband signal to the PM, a pair of optical Gaussian pulses with opposite polarities is generated at the two output ports of the DI. By properly setting the delay between these two polarity-reversed optical Gaussian pulses using an optical delay line (ODL) component, an optical UWB monocycle pulse is generated. Pulse amplitude modulation of the UWB monocycle pulse can also be achieved by driving the PM with an NRZ baseband signal at a flexible bit rate. Experimental results verify that the proposed scheme can be used to simultaneously generate and amplitude modulate the UWB monocycle pulse. Furthermore, both the shape and polarity of the generated UWB monocycle pulse can be easily controlled by appropriately adjusting the delay of the ODL. Pulse polarity modulation of the generated UWB monocycle can also be realized by electrically switching the bias voltage of the DI.

Simultaneous all-optical multi-channel RZ and CSRZ to NRZ format conversion

We propose and demonstrate multi-channel all-optical return-to-zero (RZ) and carrier-suppressed RZ (CSRZ) to non-return-to-zero (NRZ) format conversions using a same fiber delay-interferometer (DI). The conversions are based on simultaneous constructive interference induced by the periodical comb-like filtering characteristic of the DI. By using a DI with free spectral range of 40 GHz, 8 channel RZ and CSRZ signals at 20 Gb/s can be converted to corresponding NRZ signals at the same time. Furthermore, a duplicate output can be obtained for multi-channel CSRZ input. Bit error ratio measurements show 4 and 3.5 dB penalties for the proposed multi-channel RZ and CSRZ to NRZ conversions respectively. Further transmission for the converted NRZ signals verifies the good performance of the proposed all-passive multi-channel format converter. The potential of multi-channel RZ signals with mixed duty-cycle to NRZ conversions is also investigated.

Directly Modulated Laser Driven by Low-Bandwidth Duobinary Signals

We report on the generation of 10-Gb/s nonreturn-to-zero signals using a directly modulated laser driven by 9B/10B line-coded, 3.5-GHz-bandwidth duobinary signals. The ternary frequency-modulated signals from the laser are converted into binary intensity-modulated signals by a delay interferometer (DI). Two schemes, one using the constructive port of the DI and the other using the destructive port, are demonstrated through experiment.

10-Gb/s Operation of RSOA Using a Delay Interferometer

We report on the use of a delay interferometer (DI) to transmit 10-Gb/s optical signals from a 1.5-GHz-bandwidth reflective semiconductor optical amplifier (RSOA). The DI acts as a two-tap optical equalizer, compensating for the severe bandwidth limitation imposed by the RSOA. We demonstrate the successful transmission of 10.7-Gb/s nonreturn-to-zero signals over 50-km standard single-mode fiber with a 0.3-dB penalty.

Photonic True-Time Delay Beamforming Based on Polarization-Domain Interferometers

In this paper, we propose a novel photonic true-time delay beamforming system for phased array antennas. The system relies on tunable delay lines which are based on Mach-Zehnder delay interferometers (MZDIs) with tunable coupling ratio. As the MZDIs are implemented on the polarization domain, a single optical source and a single piece of polarization maintaining fiber are required. The proposed implementation is theoretically assessed and beam squinting is investigated by simulation. A proof-of-concept experiment that validates the operation principle of the proposed delay lines is presented.

DPSK Regeneration at 40 Gb/s and Beyond Using a Fiber-Sagnac Interferometer

We analyze the experimental demonstration of a phase-incoherent regeneration scheme for differential phase-shift keying signals at 40 Gb/s. The scheme is based on decoding of the input signal by a 1-bit delay interferometer and subsequent optical remodulation using a fiber-Sagnac interferometer. Power penalty improvement up to 3 dB is reported for phase, amplitude, and amplified spontaneous emission noise loaded signals. Simulation results are in agreement with the experimental findings, and reveal the capability of extending the method at ultrahigh data rates.

Semiconductor optical amplifier pattern effect suppression for return-to-zero data using an optical delay interferometer

An optical delay interferometer (ODI) is employed to suppress the pattern effect manifested on a 10 Gb/s return-to-zero (RZ) data stream when amplified by a semiconductor optical amplifier (SOA) operated in deep gain saturation. The experimental results verify the competence of the scheme to confront the problem for this signal format by achieving a far better performance than that with the SOA alone.

Signed and Accurate Measurement of Phase Offset in Optical DPSK Demodulator

Abstract-A signed and accurate phase offset estimation method of the delay interferometer (DI) for the direct detection differential phase-shift keying system based on a delay-tap sampling technique has been demonstrated. A chromatic dispersion offset module was introduced to help the phase offset measurement. The proposed method can not only measure the value but also distinguish the polarity of phase offset of DI. The measurement sensitivity can reach ±4° and measurement range can reach ±75°. The simulation and experimental measurement results are in close agreement with each other.

A Novel Return-to-Zero FSK Format for 40-Gb/s Transmission System Applications

In this paper, we present the generation, the detection and the performance evaluation of a novel return-to-zero frequency shift keying (RZ-FSK) format for 40 Gb/s transmission. Non-return-to-zero (NRZ) FSK signal is generated by using two continues-wave (CW) lasers, one Mach-Zehnder modulator (MZM) and one Mach-Zehnder delay interferometer (MZDI). A RZ-FSK signal is successfully generated by cascading a dual-arm MZM, which is driven by a sinusoidal voltage at half of the bit rate. The demodulation can be simply achieved on 1 bit rate through one MZDI or an array waveguide grating (AWG) demultiplexer with balanced detection. By numerical simulation, two types of frequency modulation schemes using MZM or PM, and impact of the frequency tone spacing (FTS) of the generated FSK signal are discussed. The proposed scheme shows that the novel frequency modulation format offers a few transmission advantages comparing with than that of the other traditional modulation formats, such as RZ and differential phase-shift keying (DPSK), under varying dispersion management. The performance analysis of RZ-FSK signal in a 4 × 40 Gb/s WDM transmission system is introduced. We experimentally demonstrate, transparent wavelength conversion based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) and in a highly nonlinear dispersion shifted fiber (HNDSF) for a 40 Gb/s RZ-FSK signal, clearly validating the feasibility of all optical signal processing of high-speed RZ-FSK signal. Moreover, we investigate the receiver power penalty for the RZ-FSK signal after a 100 km standard single-mode fiber (SMF) transmission link with matching dispersion compensating fiber (DCF), under the post-compensation management scheme. Since the frequency modulation format is orthogonal to intensity modulation and vector modulation (polarization shift keying), it can be employed in the context of the combined modulation format to decrease the data rate or enhance the symbol rate. It can also be utilized in the orthogonal label-switching as the modulation format for the payload or the label.

Theory and design of two-dimensional spectral shearing interferometry for few-cycle pulse measurement

We provide a detailed discussion of our recently developed pulse characterization technique for few-cycle pulses, called two-dimensional spectral shearing interferometry (2DSI). Based on spectral phase interferometry for direct electric-field reconstruction (SPIDER), 2DSI is relatively simple to implement, but requires no interferometer delay or scan calibration. We show simulations of 2DSI in the presence of noise and find that it retains the favorable noise performance of spectral shearing methods, performing identically to standard SPIDER for a given measurement time. The optimal choice of experimental parameters is discussed, with results applicable to any spectral shearing method. Experimental considerations when building and operating a 2DSI are provided, with results shown for a 4.9 fs pulse, verifying the accuracy and precision of the 2DSI.

Line coding to enhance the performance of 10-Gb/s CPFSK-ASK directly modulated signals

The major drawback of frequency modulation (FM)-based directly modulated laser (DML) is its non-uniform FM response at low frequency range which gives rise to a severe pattern-dependent performance degradation. In this paper, we investigate the use of line coding to deplete the low-frequency spectral contents of the signal and thus to alleviate the degradation. We examine various line codes (8B/10B, 5B/6B, 7B/8B, 9B/10B, and 64B/66B) with continuous-phase frequency-shift keying/ amplitude-shift keying (CPFSK/ASK) signals generated using a DML and a delay interferometer. Experimental demonstrations are performed with a long pseudorandom bit sequence length of 2(20)-1 and the bandwidth expansion by each code is taken into consideration. The results show that among the five codes we tested, 9B/10B code outperforms the other codes in terms of receiver sensitivity an dispersion tolerance. We demonstrate successful transmission of 10-Gb/s CPFSK-ASK signals over 65-km standard single-mode fiber with a bandwidth expansion of only 11.1%.

All-Optical Regeneration of 40 Gb/s Constant Envelope Alternative Modulation Formats

We experimentally investigate the all-optical regeneration of constant envelope alternative modulation format signals at 40 Gb/s. The regeneration scheme is based on hard-limiting amplification in a saturated semiconductor optical amplifier (SOA) that can remove the signal intensity noise. Indeed a significant improvement of the signal quality is reported for either polarization shift keying (PolSK) and frequency shift keying (FSK) signal formats, both detected using simple schemes based on direct detection. The regeneration solution, however, is found to be quite limited for non-return-to-zero (NRZ) differential phase shift keying (DPSK) (demodulated using a 1-bit delay interferometer). Indeed, SOA saturation generates a detrimental nonlinear phase noise that can override the reduction of intensity noise.

Theoretical study on non-inverted wavelength conversion based on semiconductor optical amplifier and optical band-pass filter

A single semiconductor optical amplifier （SOA） assisted by optical filter is exploited to realize high-speed all-optical wavelength conversion, in which, the filter is employed to optimize output spectrum. We propose a spectrum optimizer which contains a band pass filter （BPF） and a delay interferometer （DI）. Based on this configuration, 80 Gbit/s all-optical non-inverted conversion has been numerically simulated. Furthermore, the quality of output signal has been improved greatly using genetic algorithms to optimize the parameters of the spectrum optimizer. Additionally，the effect of ultra-fast carrier dynamics in SOA on the output signal is analyzed theoretically. The dependence of the signal extinction ratio （ER） and pattern effect （PE） on variations of the BPF’s ER and DI’s ER is also studied.

Development of a Lithium Beam Probe and Measurement of Density Pedestal in JT-60U

A lithium beam probe (LiBP) has been developed for the measurement of electron density profiles with highly spatial and temporal resolutions in JT-60U. Using an electron beam heating ion source with a capability of 10 mA extraction, a 5.5 mA beam has been injected to the plasmas. It corresponds to the equivalent neutral beam current of 2 mA. A spectrum width of the beam emission has been small enough to separate Zeeman splitting. By use of the LiBP, time evolutions of pedestal density profiles during type I and grassy edge localized modes (ELMs) have been obtained for the first time. After a type I ELM crash, the drop of the line-integrated density measured by an interferometer delays by 2 ms later than that of the pedestal density. Comparing the line-integrated density to the line integration of the edge density profile measured by the LiBP, it is found that the recovery from the type I ELM crash is correlated with the reduction of core plasma density. As for grassy ELMs, grassy ELMs have smaller density crashes than that of type I ELMs, which is mainly derived from the narrower ELM affected area.

Optical Coherent Receiver With a Switchable Electrical Dispersion Compensator for 10 Gb/s DPSK Transmission up to 300 km of SSMF in Metro Optical Networks

The purpose of this paper is to demonstrate the potential of an electrical signal processing technique to mitigate the effect of intersymbol interference in metro optical networks, using optical coherent detection. Intersymbol interference in fiber-optic systems can severely degrade the system performance and subsequently limit both the maximum transmission distance and data rate. The main sources of intersymbol interference include chromatic dispersion (CD) and polarization mode dispersion (PMD). The typical reach for 10 Gb/s metro optical systems is on the order of 300 km. To bridge this distance either with or without repeaters, we demonstrate the potential use of 10 Gb/s differential phase-shift keying (DPSK) modulation format in conjunction with an optical coherent receiver with a built-in switchable electrical dispersion compensator. The overall performance of the optical coherent receiver is compared with that of the corresponding direct detection (DD) receiver based on Mach-Zehnder delay interferometer. The optical signal-to-noise (OSNR) required to operate the optical coherent receiver at the enhanced forward error correction (EFEC) threshold of 2.1 X 10-3 is 6 dB/0.1 nm resolution bandwidth. The optical coherent receiver outperforms in absolute required OSNR by 0.5 dB at the EFEC threshold when compared to a DD receiver. A four position switchable CD compensator compensates up to ?5100 ps/nm of CD, using the method of heterodyne detection. The optical coherent receiver has a first-order PMD tolerance of 36.2 ps for 1 dB OSNR penalty. The utility of our optical coherent receiver is demonstrated by doing transmission experiments over 304 km of standard single-mode fiber (SSMF) in both repeatered (multiple spans) and repeaterless (single span) to measure the self-phase modulation (SPM) tolerance of 10 Gb/s nonreturn-to-zero (NRZ)-DPSK. The measured SPM tolerance of NRZ-DPSK over 304 km of SSMF in three spans of ~100 km each is +8.8 dBm fiber launch power for 1.5 dB OSNR penalty. In addition, the best Q factor of ~12 dB is obtained for the fiber launch power of +18 dBm over single repeaterless span of 304 km of SSMF, which gives ~3 dB of system margin at the second generation EFEC threshold of 9.1 dB.