We propose and demonstrate a self-reconfiguration network of high-precision time-frequency synchronization. The local oscillator (LO) signal and time reference signal generated by the central site are transmitted to the two-fiber unidirectional ring in opposite directions. The all-optical microwave phase conjugation and the time-frequency domain transform measurement are utilized to eliminate phase fluctuation and time delay introduced by optical fiber. The two-fiber unidirectional ring can automatically switch the working loop when the fiber link is broken and avoid manual intervention, which realizes the self-healing of the network. A 20 GHz frequency signal and 1 pulse per second time signal are transferred along a 7 km fiber ring. At 1000 s averaging time, the long-term frequency stability of the order of 10 to 16 and time stability of the order of ps can be achieved at remote sites. When the network is interrupted, the proposed system can automatically recover the time-frequency synchronization from the failure state within 37 ms.
Dual-comb spectroscopy (DCS) is an emerging spectroscopic tool with the potential to simultaneously achieve a broad spectral coverage and ultrahigh spectral resolution with rapid data acquisition. However, the need for two independently stabilized ultrafast lasers significantly hampers the potential application of DCS. We demonstrate mode-resolved DCS in the THz region based on a free-running single-cavity dual-comb fiber laser with the adaptive sampling method. While the use of a free-running single-cavity dual-comb fiber laser eliminates the need for two mode-locked lasers and their frequency control, the adaptive sampling method strongly prevents the degradation of spectroscopic performance caused by the residual timing jitter in the free-running dual-comb laser. Doppler-limit-approaching absorption features with linewidths down to 25 MHz are investigated for low-pressure acetonitrile/air mixed gas by comb-mode-resolved THz spectroscopy. The successful demonstration clearly indicates its great potential for the realization of low-complexity, Doppler-limited THz spectroscopy instrumentation.
Terahertz dual-comb spectroscopy (THz-DCS) has the potential to be used as universal THz spectroscopy with high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility due to the bulky size, high complexity, and high cost. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fiber laser. While greatly reducing the size, complexity, and cost of the laser source, THz-DCS maintains the spectroscopic performance comparable to a system equipped with dual stabilized fiber lasers, and can be effectively applied to gas spectroscopy.
The reflected intensity change of the Bloch-surface-wave (BSW) resonance influenced by the loss of a truncated onedimensional photonic crystal structure is numerically analyzed and studied in order to enhance the sensitivity of the Bloch-surface-wave-based sensors. The finite truncated one-dimensional photonic crystal structure is designed to be able to excite BSW mode for water (n=1.33) as the external medium and for p-polarized plane wave incident light. The intensity interrogation scheme which can be operated on a typical Kretschmann prism-coupling configuration by measuring the reflected intensity change of the resonance dip is investigated to optimize the sensitivity. A figure of merit (FOM) is introduced to measure the performance of the one-dimensional photonic crystal multilayer structure under the scheme. The detection sensitivities are calculated under different device parameters with a refractive index change corresponding to different solutions of glycerol in de-ionized (DI)-water. The results show that the intensity sensitivity curve varies similarly with the FOM curve and the sensitivity of the Bloch-surface-wave sensor is greatly affected by the device loss, where an optimized loss value can be got. For the low-loss BSW devices, the intensity interrogation sensing sensitivity may drop sharply from the optimal value. On the other hand, the performance of the detection scheme is less affected by the higher device loss. This observation is in accordance with BSW experimental sensing demonstrations as well. The results obtained could be useful for improving the performance of the Bloch-surface-wave sensors for the investigated sensing scheme.
Optical metrology techniques based on dual optical frequency combs have emerged as a hotly studied area targeting a
wide range of applications from optical spectroscopy to microwave and terahertz frequency measurement. Generating
two sets of high-quality comb lines with slightly different comb-tooth spacings with high mutual coherence and stability
is the key to most of the dual-comb schemes. The complexity and costs of such laser sources and the associated control
systems to lock the two frequency combs hinder the wider adoption of such techniques. Here we demonstrate a very
simple and rather different approach to tackle such a challenge. By employing novel laser cavity designs in a mode-locked
fiber laser, a simple fiber laser setup could emit dual-comb pulse output with high stability and good coherence
between the pulse trains. Based on such lasers, comb-tooth-resolved dual-comb optical spectroscopy is demonstrated.
Picometer spectral resolving capability could be realized with a fiber-optic setup and a low-cost data acquisition system
and standard algorithms. Besides, the frequency of microwave signals over a large range can be determined based on a
simple setup. Our results show the capability of such single-fiber-laser-based dual-comb scheme to reduce the
complexity and cost of dual-comb systems with excellent quality for different dual-comb applications.
Resonances inside metallic nanoslots and along metal–substrate interfaces have been investigated theoretically, which shows the fact that the transmission characteristic of a double periodic metallic nanoslot array could be manipulated by adjusting the structural parameters. A transmission structure over a wide band of infrared and terahertz waves has been proposed, with 30 dB isolation between peaks inside the infrared band and high flexibility in tuning.
An architecture of wavelength division multiplexing (WDM) optical packet switching (OPS) router with all-optical
regenerators is proposed. The regenerator based on phase-sensitive amplifier (PSA) can simultaneously provide phase
and amplitude regeneration for quadrature phase-shift keying (QPSK) or differential quadrature phase-shift keying
(DQPSK) signals. The simulation results show that three regenerators are sufficient for an OPS router in a node with a
nodal degree of five to keep blocking probability below 5% and throughput ratio above 95% under different intensities of
self-similar traffic load.
A fast response suspended core fiber optical gas sensor configuration using side-opening hole and micro-holes array
structure on the thin layers is proposed. The side-opening-hole structure enables a fast filling speed of gases into the
opening-up-hole region, while the micro-holes array on the thin layers ensures that gases could further diffuse into
the other holes columns quickly. Meanwhile, its sensitivity could be tripled in contrast to the previous structures.
Simulation results show that a diffusion limited response time of 12 s could be realized and thus move a step further
toward real-time sensing applications.
A fast response tilted fiber Bragg grating fluid refractometer using an exposed-hole microstructured optical fiber is
proposed and analyzed. The theoretical and simulation results show that a sensitivity of 5.40×10-5 r.i.u within a diffusion
limited response time of 6 s could be achieved. This exposed-hole configuration can be used to construct a fluid
refractometer for achieving a fast response, high sensitive distributed detection.
We report a novel design of photonic crystal fiber (PCF) with a rectangular array of four closely-spaced, highly elliptical
air holes in the core region and a circular-air-hole cladding. The proposed PCF is able to support ultra-wideband singlepolarization
single-mode (SPSM) transmission from the visible band to the near infrared band. With the aid of the inner
cladding formed by the central air holes, one polarization of the fundamental mode can be cut off at very short
wavelengths and ultra-wideband SPSM propagation can be achieved. The inner cladding also suppresses the higher order
modes and allows large air filling fraction in the outer cladding while the proposed fiber remains SPSM, which
significantly reduces the mode effective area and the confinement loss. Our simulation results indicate that the proposed
PCF has a 1540 nm SMSP range with < 0.25 dB/km confinement loss and an effective area of 2.2 m2. Moreover, the
group velocity dispersion (GVD) of the proposed PCF can also be tuned to be flat and near zero at the near infrared band
(~800 nm) by optimizing the outer cladding structure, potentially enabling many nonlinear applications.
Since the advent of slot optical waveguides by Lipson, normally SOI based slot optical waveguides have been under consideration. It
has been found that glass based slot optical waveguide structures, where refractive index contrast ratio is comparatively less can also
play important role in forming complex nano size optical devices. We have made use of power confined inside low index slot region
for a double slot structure, where central high index slab is acting as a cantilever. Novel optomechanical sensor has been proposed
based on variation in power confined inside low index slot region due to the movement of central high index slab under the action of
external force (temperature, pressure, humidity, etc.)
Slot waveguide structure gained attention due to high confinement of power inside low index slot region. The high efield
confinement is dependent upon various geometrical parameters. A double slot structure where two low index slots
of hard material in high index cladding of compressible material is proposed. Power confinement factor dependency
upon distance between low index slots has been numerically computed. Sufficient numerical results obtained lead to the
proposal of opto-mechanical sensor based upon proposed double slot structure.
The path protection with reliability guarantee is investigated under multiple constraints. The key challenge of providing
ample multi-constrained loop-less paths to the detouring as candidates is addressed by a modified relaxation principle of
dominated path. The loops in the new extended sub-paths are explored with an elastic depth, and additive criterions derived from the
path information are adopted to obtain more accurate identification of feasible paths. Our hybrid detouring gives priority to the
end to end disjoint backup paths and uses the residual space of inter-media nodes to store the potential local disjoint subpaths.
The obtained candidate backup paths are selected by appropriate cost functions according to the combined
reliability and the consumed resources. Simulation results demonstrate its superiority in restoration path-finding
capability leads to a significant improvement in the success rate of the QoS path protection.
An all-optical regenerator for Quadrature Phase-Shift Keying (QPSK) and Differential Quadrature Phase-Shift Keying
(DQPSK) signals based on Phase-Sensitive Amplifiers (PSAs) is studied through numerical simulations. It is found that
the regenerator can provide significant suppression of both amplitude and phase noise, and that the BER performance of
the regenerated signals can be greatly improved.
The all optical window equivalent-time sampling method using SMIOS and a numerical algorithm used for waveform
recovery are present. A system model based on SOA dynamic model was founded to analyze sampling process. The pulse
broadening in low bandwidth PIN and its response are discussed too. The relationships between sampling windows and
sensitivity due to SOA bias current, aptitude of control pulse and input measured signal wavelength are analyzed
theoretically. The results have shown that the SMIOS sampler system combined with numerical algorithm can measure
short pulses with low error.
The transmission performance of SCI avionic fiber-optic data bus is analyzed in this paper. An analysis on the
relationship between the transmission range and the data rate under certain attenuation and dispersion is presented. A
simulation of the receiver with the M-sequence data and trapezoid pulses as input data stream is discussed in detail, the
relationship between the sensitivity of the receiver and its bandwidth is simulated. The transmission delay due to the
bandwidth of receiver is calculated too. Some operational suggestions on how to apply SCI fiber-optic data bus to the
avionic field are given.
The all optical pulses replicating technique to measure single-shot or low repetition optical pulses and pulses with poor stability based on optical pulse active replicator (OPAR) is present. The OPAR can provide an identical pulse sequence for equivalent-time sampling method. The analysis show replication error of OPAR caused by nonlinear gain and amplified spontaneous emission (ASE) of semiconductor optical amplifier (SOA) could be reduced by pumping light injecting. Bias current of SOA should choose an intermediate value to confine nonlinear gain and ASE simultaneously. This result is validated by experiment. The stability of OPAR and baseline shift in output pulses sequence are discussed too. The results show that OPAR is suitable to generate low-distortion pulses sequence for equivalent-time sampling with high replicating stability.
We here study the transmission performance of differential phase-shift keying (DPSK) modulation compared to the traditional On-Off keying (OOK) modulation in the potential high speed satellite to ground laser communication applications. By modeling the channel from the satellite to ground using widely accepted theoretical model and measured results, we estimate the scintillation-induced received power fluctuation. Through simulations, we show that DPSK systems using direct detection with balanced receivers has a clear advantages in performance over the OOK, especially under significant scintillation in the atmosphere.
We propose a novel method to compensate the nonlinear distortion from an optical pulse replicator based on an active optical fiber loop. It could be used to faithfully recover the input optical pulse shape from the sampled results of the replicated pulse train, despite of the distortion in the loop. Thus, the replicator could be applied to the measurement of optical pulses for optical fiber communication applications. Our proposed post-processing technique is based on modelling the nonlinear amplification behaviour of the optical amplifier in the loop. The methodology of our technique is described in detail, as well as the simulated recovered results using the proposed algorithm.
Based on the analyses of the key limiting factors for duobinary signals in dispersion limited fiber systems, we propose a novel optical receiver design that can further overcome the deterministic timing 'jitter' resulted from the dispersion-induced inter-symbol interference. Our idea is based on over-sampling of the received waveform and a novel decoding scheme. Our technique would provide better estimations of the received bits based on a decision feedback and feedforward scheme that takes advantage of the knowledge of adjacent bits. We study, through extensive simulations, the effectiveness of our novel receiver design in increasing the dispersion tolerance, and our promising results show that our innovative solution can provide significantly improvement over even the best reported schemes thus far.
KEYWORDS: Distortion, Signal to noise ratio, Signal attenuation, Active optics, Interference (communication), Semiconductor optical amplifiers, Optical amplifiers, Analog electronics, Receivers, Optical fibers
We have proposed a simple and convenient gain control scheme of the semiconductor optical amplifier (SOA) utilizing the amplified spontaneous emission (ASE) from the SOA itself. Based upon this scheme, an optical pulse active replicator using a SOA could be used for high-fidelity replication of short analog optical pulses. Taking into account the ASE noise, a numerical model of the SOA is developed to investigate the optical pulse recirculating propagation in the active replicator. Results indicate that the replicator is feasible and effective. Selecting appropriately operating parameters, such as the loss within the loop, the power of signal pulse and the front-end receiver electrical bandwidth, the replicator can generate a sequence of precise periodic optical pulse trains with low distortion and good SNR.
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