Photonic multi-shape UWB pulse generation using a semiconductor optical amplifier-based nonlinear optical loop mirror

We propose and demonstrate a scheme to implement photonic multi-shape ultra-wideband(UWB) signal generation using a semiconductor optical amplifier(SOA) based nonlinear optical loop mirror(NOLM).By employing the cross phase modulation(XPM) effect,cross gain modulation(XGM),or both,multi-shape UWB waveforms are generated including monocycle,doublet,triplet,and quadruplet pulses.Both the shapes and polarities of the generated pulses are flexible to adjust,which may be very useful in UWB pulse shape modulation and pulse polarity modulation.     

All-optical switchable UWB pulses generation,modulation and transmission

We proposed and demonstrated all-optical polarity- and shape-switchable ultrawideband (UWB) Gaussian pulses generation schemes using a single polarization interferometer (PI) or a semiconductor optical amplifier (SOA) cascaded by a PI. When an optical signal with dark return-to-zero (RZ) format propagates directly through a single PI, a pair of polarity-reversed UWB monocycle pulses and a positive UWB doublet pulse is acquired respectively only by adjusting the polarization controllers, without changing the bias of the intensity modulator. If a SOA is connected at the input of the PI to firstly implement the first-order differential function to the input dark RZ pulse utilizing gain saturation effect of the SOA, both polarity- and shape-switchable UWB Gaussian monocycle and doublet pulses are all acquired from the cascaded system. Moreover, if a nonreturn-to-zero control signal is introduced into the SOA, utilizing the cross-gain modulation (XGM) effect of the SOA and the differential characteristic of PI, all-optical shape modulation to UWB signals can be realized, and the polarity of the modulated UWB pulses can also be switched. Also, UWB signal transmission characteristics in fiber link were investigated.     

Photonic generation of ultrawideband signals by exploiting gain saturation of dark pump pulse with double undershoots in a highly nonlinear fiber

We have proposed and theoretically demonstrated an alternative photonic scheme to generate ultrawideband (UWB) doublet pulses based on gain saturation effect in one piece of highly nonlinear fiber (HNLF). A flexible format swap between UWB doublet and monocycle pulses can be successfully realized by properly tuning the optical tunable delay line (ODL). Moreover, the key parameters for UWB pulse, including the center frequency (Fc), 10 dB bandwidth (BW_10dB), and fractional bandwidth (FBW), are also investigated with Fc being Einstein shift when the doublet pulse transforms into the monocycle pulse. Finally, our proposed scheme exhibits good performance that the obtained UWB pulse can have a FBW of > 100% at the Fc of ~ 7 GHz and UWB-over-fiber technology is also implemented without dispersion management.     

一种基于非线性光纤环镜开关特性的超短光孤子产生方法

提出了一种利用非线性光纤环镜的开关特性将连续波同时转化为亮孤子和暗孤子的新方法 ,即让连续波和另一波长的调制脉冲串共同耦合入光纤环镜 ,交叉相位调制使得一部分连续波被环镜透射 ,其余部分被反射 ;再让透射波和反射波分别在反常色散光纤和正常色散光纤中传输 ,自相位调制和群速度色散之间的相互作用使得透射波和反射波分别演化为亮、暗孤子。数值计算表明 ,该方法不仅可产生脉宽比调制脉冲窄、重复频率比调制脉冲高的亮孤子和暗孤子 ,而且几乎可将全部的连续波能量转化为孤子能量。     

Filter-free ultrawideband generation based on semiconductor optical amplifier nonlinearities

A filter-free scheme for ultrawideband (UWB) generation with single semiconductor optical amplifier (SOA) is proposed and demonstrated. A pair of polarity-reversed optical pulses is generated due to cross gain modulation (XGM) in the pump-probe scheme, whereas the amplified pump pulse becomes sharp at its leading edge such that the power peak is shifted forward. Hence, the combination of the pump and probe signals at the SOA output is quasi-monocycle shape. Our scheme is an improved scheme to avoid exploiting optical filters and time-delay devices. The generated UWB radio frequency spectrum shows good stability when the input probe power varies from −10 dBm to 6 dBm, the probe wavelength varies in the whole C-band, and the bias current varies from 100 mA to 240 mA. Although the generated quasi-monocycle deviates from a standard Gauss monocycle to some extent, the frequency spectrum conforms to the UWB regulation. Two SOAs with different XGM dynamics are compared in generating UWB signals.     

Photonic ultra-wideband monocycle pulses generation using semiconductor optical amplifier and electro-absorber in parallel

A scheme for photonic generation of ultra-wideband （UWB） pulses using a semiconductor optical amplifier （SOA） and an electro-absorber （EA） in parallel is proposed and numerically demonstrated. By adjusting the time delay between two pump signals incident into the SOA and the EA, we can obtain monocycle pulses with reversed polarities and different bandwidths. The proposed method is flexible in pulse shaping and easy in practical optimization.     

Picosecond soliton transmission using concatenated gain-distributed nonlinear amplifying fiber loop mirrors

Stable picosecond soliton transmission is demonstrated numerically by use of concatenated gain-distributed nonlinear amplifying fiber loop mirrors (NALMs). We show that, as compared with previous soliton transmission schemes that use conventional NALMs or nonlinear optical loop mirror (NOLM) and amplifier combinations, the present scheme permits significant increase of loop-mirror (amplifier) spacing. The broad switching window of the present device and the high quality pulses switched from it provide a reasonable stability range for soliton transmission. Soliton-soliton interactions can be reduced efficiently by using lowly dispersive fibers.     

Optical pulse compression and amplitude noise reduction using a non-linear optical loop mirror including a distributed Gires-Tournois etalon

We propose and analyse numerically a novel dispersion-imbalanced non-linear optical loop mirror (NOLM) scheme allowing simultaneous compression and amplitude noise reduction of chirped ultrashort-pulsed signals. An all-fibre distributed Gires-Tournois etalon (DGTE) made of two uniform fibre Bragg gratings is inserted asymmetrically in the loop, and is used both to dispersion imbalance the NOLM and to compress the pulses. The results show that the output pulses are accompanied by side lobes, which originate from the no-uniformity of the DGTE dispersion spectrum. We analyse the formation of these side lobes and show that, in the case of highly chirped input pulses, side lobes are strongly reduced by the NOLM architecture. In this case, the device operates well even when the signal optical spectrum extends over an entire free spectral range of the DGTE. We believe that this work will be useful for the design of all-optical regeneration schemes for highly chirped data streams in optical transmission systems.     

High-Order Ultrawideband Pulse Generation from NRZ-DPSK Signals

A simple method to generate ultrawideband （UWB） doublet and triplet from nonreturn-to-zero （NRZ） differential phase shift keying （DPSK） signals is proposed and experimentally demonstrated. The proposed configuration consists of a Mach-Zehnder modulator （MZM） to generate NRZ-DPSK signals, a section of single-mode fibre to form a microwave bandpass filter, which is used to generate doublet pulses, and a Gaussian optical bandpass filter （OBF）, which serves as a frequency discriminator to generate higher-order UWB pulses. A pair of polarity- reversed triplet pulses is achieved by locating the optical carrier at the positive and negative linear Mopes of the OBF, where the OBF detuning is 0.12nm and -0.2 nm, respectively. The spectra of the pair of UWB triplets have a central frequency of 5 GHz and 5.6 GHz, and have a -10 dB bandwidth of 6.9 GHz and 8.1 GHz, respectively. The UWB pulses remain doublet shape when the light wavelength is located at the peak of the OBF. The spectrum of the doublet has a central frequency of 5.6 GHz and a -10 dB bandwidth of 6.9 GHz.     

Photonic generation of ultra-wideband doublet pulse using a semiconductor-optical-amplifier based polarization-diversified loop

We propose and demonstrate a novel scheme of ultra-wideband (UWB) doublet pulse generation using a semiconductor optical amplifier (SOA) based polarization-diversified loop (PDL) without any assistant light. In our scheme, the incoming gaussian pulse is split into two parts by the PDL, and each of them is intensity modulated by the other due to cross-gain modulation (XGM) in the SOA. Then, both parts are recombined with incoherent summation to form a UWB doublet pulse. Bi-polar UWB doublet pulse generation is demonstrated using an inverted gaussian pulse injection. Moreover, pulse amplitude modulation of UWB doublet is also experimentally demonstrated. Our scheme shows some advantages, such as simple implementation without assistant light and single optical carrier operation with good fiber dispersion tolerance.     

Optimum splitting ratio for amplifier noise reduction by an asymmetric nonlinear optical loop mirror

We theoretically and experimentally analyze the influence of the splitting ratio and the input power on the noise reduction capability of an asymmetric nonlinear optical loop mirror (NOLM) for different input noise levels. An easy method to calculate the optimum parameters for noise reduction is also presented. The best noise reduction is found at NOLM input powers at which the nonlinear transfer function has a slope close to zero. Additionally, the splitting ratio of the NOLM has to be adapted to its input noise level to suppress amplitude fluctuations effectively. Since the noise reduction by the NOLM is due to the Kerr nonlinearity, which has a timescale below a few femtoseconds, the noise reduction is applicable to short pulses in the picosecond and femtosecond range. This makes the NOLM applicable as an optical regenerator in an optical data transmission system at high bit rates, such as 160 GBit/s.     

相移干涉计量中相移器的原路相干相位检测标定

A new all-optical clock extraction scheme based on an asymmetric nonlinear optical loop mirror (NOLM) is proposed and demonstrated experimentally. The proposed scheme can operate at a relatively low switching power (only 0.3 Pπ is required for the clock pulse, where Pπ means the required switching power for the maximum switching in the asymmetric NOLM) and a relatively low input clock to data ratio (2～2.5). The proposed scheme is promising for clock extraction in the future ultrahigh-speed self-clocked OTDM network.     

Asymmetric Nonlinear Optical Loop Mirror for All Optical Clock Extraction in Self-clocked Optical Transparent OTDM Network

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非线性光纤环形镜的脉冲透过特性研究

研究了光纤反常色散区非线性环形镜（NOLM）的脉冲透过特性，得到NOLM周期性透过率函数 第一极大值处透过率、压缩比和对应的孤子阶数与环长之间的函数关系图.通过比较长环和 短环NOLM对无啁啾、啁啾脉冲的透过率和压缩比特性，得出了长环有利于脉冲整形而短环有 利于脉冲压缩的结论. 关键词： 非线性环形镜 透过率 压缩比 孤子阶数     

Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier

We propose a novel scheme to generate ultrawideband (UWB) monocycle pulses based on cross-phase modulation (XPM) of a semiconductor optical amplifier (SOA). The proposed system consists of a SOA and an optical bandpass filter (OBF). Due to the XPM, a continuous wave (CW) probe signal is phase modulated by another optical Gauss pulse in the SOA. The OBF will convert the phase modulation to intensity modulation. A pair of polarity-reversed UWB monocycle pulses is achieved by locating the probe carrier at the positive and negative linear slopes of the OBF. Both cases conform to the UWB definition of the Federal Communications Commission.     

Optical ultra-wideband pulse generation and distribution using a dual-electrode Mach-Zehnder modulator

<正>A novel approach to generate and distribute ultra-wideband(UWB) pulses in optical domain is investigated. In this proposed scheme,a dual-electrode Mach-Zehnder modulator(DE-MZM) is biased at its quadrature point so as to realize the linear response.Then the intensity of output optical field can be assumed to the subtraction of two input Gaussian pulses.If the input Gaussian pulses are with the same sharp parameters but different time delays,a quasi-monocycle-waveform UWB signal can be generated.If the input Gaussian pulses are with different amplitudes and full-width at half-maximum(FWHM),a quasi-doublet-waveform UWB signal can be generated.A transmission of the UWB signals through a 25-km single mode fiber is carried out successfully.The results in both temporal and frequency domains are also presented.     

Power-efficient impulse radio ultrawideband pulse generator based on the linear sum of modified doublet pulses

We propose a new and power-efficient impulse radio ultawideband (IR-UWB) pulse design concept. The proposed concept is based on a linear sum of modified doublet pulses. The proposed concept is both simulated and experimentally demonstrated. The experimental demonstration employs a photonic scheme that generates the designed pulse using two main steps, mainly optical shaping and differential detection. The optical shaping is performed using a single electro-optic modulator biased in the nonlinear portion of its transfer function, and the differential detection is performed using a balanced photodetector. The generated IR-UWB pulse is fully Federal Communications Commission compliant, even in the highly power-restricted global positioning system band. The proposed optical scheme has potential to be integrated on a compact optical chip and thus suitable for reliable, low-cost, high-speed, short-range UWB wireless access, such as in-building networks.     

All-optical ultrawideband monocycle generation utilizing gain saturation of a dark return-to-zero signal in a semiconductor optical amplifier

We demonstrate a simple and compact scheme to generate ultrawideband (UWB) monocycle pulses utilizing gain saturation of a dark return-to-zero (RZ) signal in a semiconductor optical amplifier (SOA). When optical pulses with a dark RZ format propagate through the SOA, the output power at the rising edge will be overamplified compared with that at other durations due to the gain unsaturation. As a result, the output pulses are monocyclelike. The UWB frequency spectra at different injected currents, different input pulse widths, and different input wavelengths are analyzed. Our experiments show that the monocycle generation has good tolerance to the SOA bias current and the input signal wavelength.     

Highly nonlinear optical fibre loop mirror for reshaping femtosecond pulses from a soliton-compressed electro-absorption modulator source over a wide repetition rate and wavelength range

Highly nonlinear fibre (HNF) applied to a nonlinear optical fibre-loop mirror (NOLM) proves very effective for high quality femtosecond pulses shaping. The HNF exhibiting about four times larger nonlinearity than either dispersion flattened fibre or dispersion-shifted fibre excites higher-order soliton propagation enabling significant pulse compression, while shortening the loop length required for efficient switching, which advantageously reduces both the detrimental higher-order chromatic dispersion effects and sensitivity to environmental perturbations. A short NOLM consisting of a 60 m HNF demonstrates effective reshaping and compression of 254–394 fs pulses from a fibre-soliton compressed electro-absorption modulator source to produce pedestal-free and transform-limited 186–243 fs pulses over a wide wavelength (1550–1570 nm) and repetition-rate (6.8–18 GHz) range.     

Group-velocity-dispersion-compensated femtosecond optical parametric amplifier

A tilted pulse front geometry is known to allow group-velocity-dispersion (GVD) compensated propagation of short pulses of different wavelengths in dispersive media. This scheme is shown to be applicable to an optical parametric amplifier (OPA) pumped at 450nm. A numerical calculation shows that a certain combination of the tilt of the pulse front and the angle between the interacting beams provides optimum GVD compensation for each wavelength. The parameters providing broadband tunability are identified. The tunability of the GVD-compensated OPA pumped with 55fs pulses was experimentally demonstrated between 545nm and 645nm. This revised version was published online in November 2006 with corrections to the Cover Date.