A scheme to generate a flat optical frequency comb (OFC) is proposed and experimentally demonstrated based on a directly modulated distributed feedback (DFB) laser cascaded with a polarization modulator (PolM). In the proposed scheme, the DFB laser is optically injection-locked by a tunable laser source and directly modulated by a radio frequency (RF) signal, which is amplified by a microwave power amplifier. The optical signal is then sent to PolM via a polarization controller (PC) and modulated by the amplified and phase-shifted RF signal from the same source. The optical signal is finally received and measured by an optical spectrum analyzer (OSA) after transmitting through another PC and a polarizer. Here, the OFC with their power variation within 3 dB is desired, and four OFCs with 6, 6, 5, and 4 comb lines are generated using the RF signals with different frequencies, which have a flatness of, respectively, 2.4, 2.5, 0.7, and 0.6 dB. Here, the number of comb lines is decreased, which is due to the RF signal power decrease while its frequency is raised.
An innovative approach to realise high chip rate in OCDMA transmission system is proposed and experimentally investigation, the high chip rate is achieved through a 2-D wavelength-hopping time-spreading en/decoder based on the supercontinuum light source. The source used in the experiment is generated by high nonlinear optical fiber (HNLF), Erbium-doped fiber amplifier (EDFA) which output power is 26 dBm, and distributed feed-back laser diode which works in the gain switch state. The span and the flatness of the light source are 20 nm and 3 dB, respectively, after equalization of wavelength selective switch (WSS). The wavelength-hopping time-spreading coder can be changed 20 nm in the wavelength and 400 ps in the time, is consist of WSS and delay lines. Therefore, the experimental results show that the chip rate can achieve 500 Gchip/s, in the case of 2.5 Gbit/s, while keeping a bit error rate below forward error correction limit after 40 km transmission.
An optical approach to generating chirped microwave signal using a photonic microwave delay-line filter (PMDLF) with a quadratic phase response is proposed and demonstrated. In this scheme, a narrow band Gaussian pulse is used as the original signal. In order to eliminate the need for a wideband original microwave chirped-free signal, a mixer and a radio frequency signal are used to up-convert the spectrum of the original signal and the dispersion curve is tuned to minimize the attenuation caused by the fiber dispersion. Then the required frequency response can be reconstructed by a nonuniformly spaced PMDLF. Since the majority of the power of the original signal can bypass the filter, the power of the generated chirped microwave signal will be increased. A reconstruction example of a desired frequency response with a central frequency of 10 GHz is provided, and the generation of the corresponding chirped microwave signal is demonstrated by numerical simulations.
KEYWORDS: Modulation, Radio optics, Hybrid fiber radio, Single mode fibers, Linear filtering, Signal detection, Single sideband modulation, Dispersion, Radio over Fiber, Signal generators
A full-duplex radio over fiber system with optimum optical carrier to sideband ratio (OCSR) of 0 dB and wavelength reuse for uplink are proposed. At the central office, single-sideband modulation with OCSR larger than 0 dB is realized based on optical injection. At the base station, a notch filter with the notch depth corresponding to the OCSR of the transmitted signal is used to achieve the optimum OCSR of 0 dB. The reflected weakly modulated carrier is reused as the carrier for upload signals. A simulation is carried out to verify the proposed system. Results show that the download 60-GHz-RF signal is almost not affected by the fiber chromatic dispersion, and the power penalty after transmission over a 50-km single-mode fiber is negligible in uplink and is only 1.27 dB in downlink.
An innovative approach for security-enhanced optical stealth transmission in a synchronous digital hierarchy network is proposed and experimentally investigated. The security enhancement is achieved through a signal modulation format, so-called polarization-modulated-code-shift-keying, which is implemented with two superstructured fiber Bragg gratings-based optical-code-division multiple-access encoders and a polarization modulator. The proposed modulation format can provide a constant energy level for both bits 0’s and 1’s, which avoids secure vulnerability of single-stealth-user with on-off-keying modulation before coupling into the host channel and after the cascade of filters. Moreover, a self-made cost-effective gain-switched distributed feedback laser with relatively narrow spectrum is first employed as a stealth optical source, which greatly reduces the system cost and complexity. The stealth signal is recovered and detected asynchronously. The experimental results show high secure performance and robustness against eavesdropping, while keeping a bit error rate below forward error correction limit.
KEYWORDS: Electro optics, Modulators, Computer programming, Code division multiplexing, Electrodes, Receivers, Picosecond phenomena, Signal attenuation, Transmittance, Signal to noise ratio
An electro-optical time gating technique, which is based on an electrical return-to-zero (RZ) pulse driven Mach-Zehnder modulator (MZM) for eliminating multiple access interference (MAI) in optical code-division multiple access (OCDMA) networks is proposed. This technique is successfully simulated in an eight-user two-dimensional wavelength-hopping time-spreading system, as well as in a three-user temporal phase encoding system. Results show that in both systems the MAI noise is efficiently removed and the average received power penalty improved. Both achieve error-free transmissions at a bit rate of 2.5 Gb/s. In addition, we also individually discuss effects of parameters in two systems, such as the extinction ratio of the MZM, the duty cycle of the driven RZ pulse, and the time misalignment between the driven pulse and the decoded autocorrelation peak, on the output bit error rate performance. Our work shows that employing a common MZM as a thresholder provides another probability and an interesting cost-effective choice for a smart size, low energy, and less complex thresholding technique for integrated detection in OCDMA networks.
Realization of a wideband tunable optoelectronic oscillator based on a chirped Mach–Zehnder modulator (MZM) and a chirped fiber Bragg grating is proposed and demonstrated. By simply adjusting the direct-current bias of the chirped MZM, the frequency of the oscillating signal is tuned. A theoretical model is established, then verified by an experiment. A high-purity microwave signal with a tunable frequency from 5.8 to 11.8 GHz is generated. The single-sideband phase noise of the generated signal is −112.6 dBc/Hz at a frequency offset of 10 kHz.
A principle-of-concept demonstration about the effect of filter parameters on the output performance of a highly nonlinear fiber-based all optical thresholding technique (OTT) is proposed. The center wavelength, bandwidth, and rejection parameters of an optical bandpass filter (OBPF) used in fiber-based OTT are discussed. Carefully adjusting each parameter of the OBPF allows a steeper transforming characteristic of optical thresholding, and a fourth-order polynomial power transfer function is achieved, which is the steepest thresholding curve ever reported. To the best of our knowledge, this is the first demonstration of fiber-based OTT focusing on OBPF parameters, which makes the thresholder more flexible and allows customization of thresholding performance to meet requirements in various systems.
A wideband tunable filter with a complex coefficient is proposed and demonstrated. The output of a polarization modulator is filtered by the optical bandpass filter, an optical single sideband modulation will be generated. When it passes through a polarization controller and a polarizer, a complex coefficient will be achieved by tuning the angle of the polarization controller before the polarizer, and the real coefficients are controlled by the optical power of the lasers. As the real coefficients and complex coefficient can be tuned, a two-taps incoherent photonic microwave filter with a tunable range over the entire full free spectral range is realized by adjusting the polarization angle from −90 deg to 90 deg while the shape of filter remains unchanged.
A novel method to measure the fiber length based on optoelectronic oscillator is firstly proposed in the experiment, showing that an optoelectronic oscillator can be used for measuring the fiber length by detecting the space of the oscillation frequency. The shorter the fiber is, the more precisely it can be measured.
An all-optical realization of a two-tap complex-coefficient microwave photonic filter, working under incoherent mode, is proposed and demonstrated. The complex coefficient is realized by simply adjusting the bias voltage applied to the dual-electrode Mach–Zehnder modulator. The real coefficient is controlled by adjusting the power of the two taps. As a result, the frequency response of the filter can be continuously tuned, over a full free spectral range, without changing the shape of the filter.
A novel dispersion compensating fiber with multiple windows is firstly proposed. The proposed DCF is based on hybrid
photonic crystal fiber. This type of DCF gets a multiple negative dispersion windows by the coupling between the hybrid
guiding mode in the edge of each band-gap and the index guiding mode in the cladding deficit area. This is very different
from the ordinary dual-core mechanism. This proposed DCF can be widely used in dispersion compensating system, the
photonic generation of UWB signals, the pulse forming system and the compressing of optical soliton.
A novel highly birefringent photonic crystal fiber with selectively liquid-filled structure in cladding is proposed. The birefringence is numerically investigated with variant structural parameters and refractive index of the infiltrating liquid. The results show that high birefringence can be achieved over a wide wavelength range, and the birefringence is increased because of the symmetry destroyed further in the proposed structure. However, the confinement loss keeps the same order of magnitude even though more materials with high refractive index are filled in the cladding. By varying the refractive index of the infiltrating liquid, the birefringence is shown to be well tuned. It has the potential to be used as optical devices such as all-fiber polarization controllers and high-sensitive sensors.
A novel highly birefringent photonic crystal fiber (PCF) with hybrid cladding is proposed. In this hybrid structural PCF,
some air holes of the cladding are selectively filled with high refractive index material. The increased birefringence
mainly results from that the symmetry is destroyed further in the designed PCF. The birefringence is theoretically
investigated with variant structural parameters and refractive index of the filled material. The plane wave expansion
method (PWE) and full-vector finite-element method (FEM) are respectively employed to investigate the optical
properties of the proposed PCF. The numerical results show that the designed PCF can provide high birefringence and be
well tuned by the structural parameters and refractive index of the filled material. It can be used as all-fiber polarization
controllers and highly sensitive sensors.
A novel multiple-frequency Brillouin fiber-ring laser utilizing a ring cavity is proposed and experimentally
demonstrated. And eight wavelengths can be obtained from this laser. Based on this laser, the mechanism for generation
of high frequency microwave signal is proposed and partly realized. To confirm the feasibility of this method, 11GHz
microwave signal is obtained by the experiment.
The fiber Bragg grating has been widely used in sensors. We have studied the spectral properties of uniform Bragg gratings in photonic crystal fibers under transverse pressure. By the finite element method, the relation between the birefringence and the pressure was simulated in bare photonic crystal fibers. The results show that the birefringence is changed even under weak pressure. A new method based on polarization-dependent loss is presented for measuring pressure. The maximal amplitude of the polarization-dependent loss varies linearly in the pressure range of 0 to 4 MPa with a slope of 0.75 MPa−1.
Two-mode highly birefringent optical fibers support two spatial modes. The two spatial modes can be used to set up two
detectors. The birefringence would change with the variety of environment. Therefore, two-mode highly birefringent
optical fibers can sense two parameters synchronously. A finite element method (FEM) was used to numerically
calculate the distribution of pressure, distribution of temperature and the birefringence variety versus pressure and
temperature in two-mode highly birefringent photonic crystal fibers. Based on the single parameter sensor, the
muti-parameter sensor was analyzed. The results show that the muti-parameter sensor can measure pressure and
temperature synchronously.
Combined DLP algorithm and sampled theory, super-narrow FBG filter for arbitrary shaping spectrum can be
successfully synthesized, and the process of design and fabrication using one uniform mask are demonstrated. A novel
method for a single stage illumination to fabricate dc-free apodized sampled FBG is proposed based on the chirp of
sample period. Above design and fabrication method are testified by simulation and experiment results, and a super-narrowband
triangle shaping FBG filter with 25pm 3dB bandwidth and side lobes suppression beyond 20dB is
fabricated.
Optical en/decoder is one of the key components in Optical Code Division Multiple Access (OCDMA) systems.
Therefore, the improvement of performance of en/decoder is very necessary for improving the whole performance of
OCDMA system. Among all kinds of factors which affecting the performance of en/decoder, the compatibility between
optical source and en/decoder is proved to be an inevitable item. In this paper, as to equivalent phase shift super-structured
FBG (EPS-SSFBG), an advanced en/decoding technology, both optimum pulse width and proper central
wavelength deviation between optical source and EPS-SSFBG based en/decoder are first investigated, which lead to a
novel method of optimizing the compatibility between optical source and en/decoder. The simulation results show that
there exists an optimum pulse width, not the narrowest one, fit for EPS-SSFBG based en/decoder, what's more, it is
proved advisable that proper central wavelength deviation would be beneficial when pulse width is relatively wide.
Simulation and experimental results prove the feasibility and effectivity of the proposed method.
In recent years, more and more optical fiber cable has applied to the long haul trunk, and the optical network has become the backbone of communication network because of the rapid development of optical fiber communication. So higher requirement for management of optical cable network is brought forward. This paper analyzes the technique fo monitoring optical cable, gives an event analysis algorithm based on Gabor transform and minimum description length (MDL) criterion, and analyzes the performance of the algorithm.
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