We present a quadrature phase-shift keying (QPSK) signal demodulator that combines the graphene photodetector with symmetrical electrodes and an optical hybrid based on standing wave interference to demodulate the QPSK signal. Compared with a conventional QPSK demodulator, which contains a hybrid based on multimode interference couplers and two pairs of balanced detectors, the proposed demodulator can greatly decrease the complexity and size. The simulation with the finite-difference time-domain method demonstrates that the demodulator can achieve QPSK signal extraction. We further discuss the effective responsivity, bandwidth, and error vector magnitude of the QPSK constellation diagram. The scheme will provide a feasible approach for realizing high-speed compact QPSK coherent receivers theoretically.
A tunable all-optical microwave generator based on period-one (P1) dynamics of an external optically injected distributed-feedback laser diode (DFB-LD) is proposed and demonstrated. A DFB-LD implements the functions of light source, microwave frequency selection, and optical–optical modulation. To overcome the frequency drift in P1 oscillation process, a set of reference frequencies from a fiber ring laser (FRL) is introduced. Once the P1 oscillation signal and the modes of FRL are coupled, the whole system is closed and a stable frequency determined by P1 oscillation can be established. In the experimental verification, tunable photonic microwave signals with frequencies from 6.42 to 21.36 GHz are obtained. The measured single-sideband phase noise is −83 dBc / Hz at 10-kHz offset, which is operation frequency independent.
KEYWORDS: Fiber lasers, Sensors, Demodulation, Head, Signal to noise ratio, Sensing systems, Laser systems engineering, Temperature metrology, Optical filters, Polarization
In this paper, a customizable, sensitive fiber laser temperature-sensing scheme based on beat frequency demodulation method is proposed. The scheme adopts a ring fiber laser cavity structure and monitors temperature variations by tracking changes in the beat frequency signal (BFS). An erbium-doped fiber amplifier is used to provide gain for the optical fiber laser sensing system, and an ordinary single-mode fiber is used as the sensing head. Sensing fibers of different lengths were heated, and the sensitivities were estimated at -0.177, -0.538, and -1.116 kHz/°C for lengths of 1, 4, and 8 m, respectively. Meanwhile, by measuring the monitored frequency shifts, different sensitivities were obtained, estimated at -0.42 and -1.12 kHz/°C for monitored frequencies of 0.5 and 1.168 GHz, respectively. The BFS bandwidth reaches to 8.0 GHz, the signal-to-noise ratio is approximately 65 dB, and the -3dB linewidth is less than 2.5 kHz.
A parabolic pulse generator based on non-reciprocal nonlinear optical fiber loop mirror is proposed. When an incident triangular optical pulse is split into two branches of the loop mirror, two coherent light fields with linear intensity profile are counter-propagated in the interferometer and suffer different nonlinear phase shift caused by self-phase modulation. Because the intensity of interference field is determined by the amplitudes and phase relationship between two coherent lights, the interference result gives parabolic pulse. This concept is verified by theoretical analysis and simulation. In the experiment, tunable bright and dark parabolic pulses with repetition frequencies of 8, 9, and 10 GHz are simultaneously generated, which agree with the theoretical expectation well and exhibit large operation bandwidth.
A simple scheme of tunable parabolic pulses generation is proposed. Mathematically, a parabolic function is a quadratic function, which is equivalent to a sine-squared function when the independent variable is small enough. Since the transmissivity curve of Mach–Zehnder modulator is exactly a sine-squared function, bright parabolic pulse or dark parabolic pulse can be obtained that a linear drive signal with small voltage is used to map a center range of the peak or valley of the curve to the time domain. Starting from a sinusoidal signal, the second-order approximation triangular signal is simply obtained and used as a drive signal. By setting the bias point appropriately, parabolic pulses with the same or double frequency as the drive signals can be generated. Theoretical analysis and simulation are given. In the experiment, full-duty-cycle parabolic pulses at 2, 3, and 4 GHz and corresponding frequency-doubled pulses are obtained, which are consistent with the theoretical expectations.
An all-optical microwave oscillator is proposed and experimentally demonstrated, in which a distributed feedback laser diode (DFB-LD) acts as light source and optical-optical modulator simultaneously. By employing stimulated Brillouin scattering (SBS) process for oscillation frequency selection and active optical comb filter for side-modes suppression, single-mode photonic microwave signal generation can be achieved through a pure optical oscillation. In the experiment, a stable 10.87-GHz photonic microwave signal is obtained. The signal quality is evaluated by measuring the side-mode suppression ratio and single side-band phase noise of the signal. The corresponding values are 42 dB and −85 dBc / Hz at 10 kHz, respectively. This scheme explores the innovative application potentiality of DFB-LD, which will benefit on-chip photonic microwave systems.
A scheme of parabolic pulse generation is proposed and verified by simulations and experiments. Based on a multiply operation, a parabolic pulse can be obtained by carving a triangular envelope with a triangular time window. Theoretical analysis shows that an ideal dark parabolic pulse and frequency-doubled parabolic pulse can be generated under the triangular profile with the second-order approximation. The corresponding fitting degrees of the two types of parabolic pulses are beyond 99% and 97%, respectively. In the experiment, the flexibility is demonstrated, which perfectly agrees with the theoretical expectation.
For satellite-to-ground downlink optical communication systems, it is desirable to reduce the size, weight, and power profiles of the space terminals by employing high sensitivity and large collection area ground terminals. Multiaperture coherent receivers utilizing spatial diversity techniques can satisfy this requirement and have many advantages over the single large aperture coherent receivers. The influence of both atmospheric turbulence and weather conditions on the bit error rate performance of multiaperture coherent receivers utilizing maximum ratio combining, equal gain combining, and selection combining techniques is systematically investigated and compared for the satellite-to-ground downlink communication system. The results are useful for the design and adaptive optimization of the multiaperture coherent receiver-based downlink communication systems.
We propose a simple and cost-effective multifrequency optoelectronic oscillator (OEO) which is able to simultaneously generate two or more independent microwave signals by adding parallel filtering branches in the feedback loop. In the experimental demonstration, two signals with frequencies of 20 and 9 GHz are successfully generated. Compared with a conventional OEO, the generated signals have no additional noise and do not interfere with each other. The side-mode suppression by the optical dual-loop configuration is effective for both channels. The measured side-mode suppression ratios are larger than 65 dB, and the phase noises at a 10-kHz frequency offset are −108 and −113 dBc/Hz for 20 and 9-GHz signals, respectively.
A novel photonic technique of millimeter-wave (mm-wave) sub-carrier generation using optical parametric loop mirror
(OPLM) and fiber Bragg grating (FBG) is proposed. Due to the four-wave mixing (FWM) effect and loop reflection
characteristic in an OPLM, a carrier-suppressed modulation signal is able to generate two new harmonic components
with six times frequency spacing of the modulation frequency and the initial two optical tones can be reflected back to
the input port by the OPLM itself, which servers as the first order filtering. At the transmission port of the OPLM, a
matching FBG will give further suppressing on the pump light. Finally, a millimeter-wave sub-carrier will be obtained
after the FBG.
A stable dual-wavelength with a tunable wavelength spacing fiber laser based on a chirped two phase-shifted grating
filter is demonstrated. In the ring cavity, a chirped grating with two π-phase shifts servers as an ultra-narrow dualwavelength
transmission band filter. The grating is attached in a triangular cantilever for chirp rate tuning. A
semiconductor optical amplifier biased in low-gain regime is induced to reduce the gain competition of the two
wavelengths caused by erbium-doped fiber. And a feedback fiber loop working as a mode filter guarantees the laser in a
single-longitudinal-mode operation. Two lasing lines with different wavelength spacing from approximately 0.15 to 0.37
nm are experimentally demonstrated.
A simple and robust photonic radio frequency multiplication with continuously tunable multiplication factor by using a Fabry-Pérot laser diode (FP-LD) is proposed and demonstrated. In a FP-LD, the four-wave mixing effect can create new harmonic components, and the injection-locking behavior is able to lock and amplify one of the sidebands. When the output signal beats on a high speed photodetctor, the multiplication frequencies with a factor more than 41 can be obtained. The tunability is implemented by detuning the injection wavelength. In the experimental demonstration, the frequency components around 20 GHz are generated from a 500 MHz (or 1 GHz) sinusoidal signal. The measured single sideband phase noise is −95 dBc/Hz at a 10 kHz frequency offset.
A simultaneous optical signal extracting and erasing scheme is presented by utilizing four-wave mixing (FWM) in
optical fiber. Two synchronized gate and negation gate pulses are used as control signal, which are coupled with data
stream and produce FWM effect in high nonlinear fiber (HNLF). Through filtering the idler light generated between each
control signal and data stream, simultaneous signal extracting and erasing functions can be obtained. Experimental
results are successfully demonstrated with NRZ data format at 10 Gbit/s. This system is able to operate at higher bit rates
and other data modulation formats.
A novel photonic technique of 60-GHz millimeter-wave subcarrier generation base on four-wave mixing effect in a semiconductor optical amplifier (SOA) and a dual-frequency Brillouin fiber laser configuration is proposed. In this system, two new harmonic components with six times spacing of the microwave source frequency are created when an optical signal, generated by carrier-suppressed intensity modulation, is launched into the SOA. The two residual modulation sidebands are then suppressed by stimulated Brillouin scattering process, and the leaved idlers provide an millimeter-wave subcarrier signal.
A time- and wavelength-interleaved (T- W-interleaved) optical pulse source based on optoelectronic oscillator is
presented. By utilizing spectral slicing, a multi-loop configuration is formed in OEO system, which can effectively
suppress side-modes. In addition, this configuration is able to generate multi-wavelength optical pluses which can be
arranged T- W-interleaved optical pulse stream at different nodes. In this system, 10-ps-wide optical pulses at 5GHz
repetition rate are generated with 1ps timing jitter (in the 100Hz-1MHz range) and -100dBc/Hz spurious modes
suppression, Along with the optical outputs, 5GHz electrical signal with low phase noise (-113dBc/Hz at 10kHz offset
from the carrier) is also obtained.
A time- and wavelength-interleaved optical pulse source, which is based on spectral slicing and dispersion management,
is proposed and experimentally demonstrated. The pulse source has a simple configuration, and both the repetition rate
and the wavelength channels can be tuned easily.
We demonstrate a tunable chromatic dispersion (CD) compensation technique using a semiconductor optical amplifier
(SOA) and a coil of dispersion compensation fiber (DCF). Based on cross-phase modulation (XPM) in the SOA, the
transient chirp of the received signal can be adjusted by tuning the drive current of the SOA and the power of clock
pulse. In this way, a 10-Gbit/s tunable CD compensation setup, ranging from -40ps/nm to 60ps/nm, is realized without
changing the length of the DCF.
Short locking and unlocking time is essepecially important in the packet clock recovery, as it determines the network
resource utilization. This paper presents a novel 10-GHz all-optical packet clock extractor with ultrafast locking and
unlocking time via self-phase modulation (SPM) effect of the semiconductor optical amplifier (SOA). Low finesse
Fabry-Perot (F-P) filter and a saturated SOA, in cascade, are used to directly extract the packet clock from the data
stream. An optical bandpass filter(OBPF) is used to filter out the red-shifted parts of the packet clock in order to reduce
the locking and unlocking time. We analytically investigate the impact of the locking and unlocking time thanks to the
self-gain modulation (SGM) effect and self-phase modulation(SPM) effect induced by the SOA, and demonstrate 10GHz
clock extraction with the establishing time of 200ps and vanishing time of 600ps experimentally.
We present a compact short pulses source, in which the pulses generated by large-signal directly modulated laser diode
are phase-modulated, and it can compress the pulse from 30ps to 5.3ps with 184fs timing jitter.
We experimentally demonstrate a 4×10-Gb/s optical multicast scheme using a semiconductor optical amplifier (SOA)
and a single optical source, which produce a 10-GHz multi-wavelength pulse-trains used as the probe light of the SOA.
The multicast principle is based on the multi-wavelength conversion (MWC) by means of the interaction of both cross
gain modulation (XGM) and transient cross phase modulation (T-XPM) of the SOA. Because of the nonlinear
amplification of SOA, the 4 output copies are regenerated with respect to the input degraded signal. This scheme is very
simple and allows photonic integration.
We demonstrate a novel 40-Gb/s all-optical serial to parallel converter (AOSPC), which can convert 40-Gb/s serial
optical signals to 4-channel 10-Gb/s parallel optical signals. Moreover, based on the sequential multi-wavelength
conversion the AOSPC shows the regeneration ability.
A phase-modulator-based optoelectronic oscillator (OEO) is utilized to implement a self-starting optical pulse and microwave signal source. This system is able to simultaneously generate a 9.8-GHz optical pulse stream with 141-fs (over 100 Hz to 1 MHz) timing jitter and 6.5-ps pulse width, along with a high spectral purity electrical signal that is locked to the repetition rate of the optical pulses. The measured phase noise is −112 dBc/Hz at 10 kHz away from carrier, and the side modes are perfectly suppressed.
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