Generation of Different Modulation Formats Using Sagnac Fiber Loop With One Electroabsorption Modulator

We demonstrate a technique for performing optical time-division demultiplexing with simultaneous all-optical timing extraction using a single electroabsorption modulator (EAM). The technique is based on time-dependent transmission of the all-optically controlled EAM using a counterpropagation scheme between the data and clock optical pulses. The extracted timing is used in a feedback configuration to recover the optical clock. The bit error ratio measurement for demultiplexing from 40 Gb/s (4/spl times/10 Gb/s, 2/sup 31/-1 pseudorandom bit sequence) to 10 Gb/s showed an error-free operation with a 3.2-dB power penalty. The recovered clock-timing jitter is less than 200 fs.

Broadband optical frequency comb (OFC) generation based on a single electro-absorption modulator (EAM) is proposed. The EAM is driven by a radio frequency (RF) multi-frequency signal generated by a multiplication coupler composed of an electrical power splitter and an arithmetic circuit. Thus the number of comb-lines of the generated OFC can be increased. A complete theoretical model of OFC generation by an EAM driven by nth power of the RF source is established, and the performance of the OFC is analyzed by using OptiSystem software. The results show that, the number of comb-lines of the OFC is positively correlated with the number of multiplication of the RF source signal. The frequency spacing of the comb-lines is twice the frequency of the RF source signal and is tunable by adjusting the frequency of the RF source signal. Increasing chirp factor and modulation index of EAM could increase the number of comb-lines of the generated OFC. The amplitude of the RF source signal had little impact on the flatness of the OFC and the average OFC power. The scheme developed is not only simple and low-cost, but also can produce a large number of comb-lines.Graphical

Simultaneous modulation of several WDM channels using a single electro-absorption modulator

In this paper, a tunable and switchable erbium-doped fiber laser (EDFL) based on parallel dual Lyot filters cascaded with a Sagnac loop is proposed. The filter can be regarded as a composite filter, whose structure is composed of cascaded different filters to ensure the presented tuning range and flexibility of parameter configuration. The wavelength resolution and polarization stability of the filter are enhanced by the synergistic effect of the multistage polarization-maintaining fibers (PMFs) and polarization controllers (PCs). The experimental results show that when the pump power is 250 mW and the lengths of the PMFs used are 4 m, 4 m, and 8 m respectively, the wavelength spacing can be adjusted by tuning the three PCs in the filter, allowing the laser to be reconfigured to alter the polarization state, thereby achieving continuous excitation of 1 to 7 wavelengths in the range from 1530.52 nm to 1568.04 nm. In addition, the output wavelength is adjustable over a range of ∼37.52 nm. The side-mode suppression ratios for the output wavelengths of this laser all exceed ∼41.60 dB. The proposed EDFL has good tunability, switchability, and stability.

Analogue fibre-optic transmission of full-duplex 120 Mbit/s QPSK data, plus multichannel video, is demonstrated over 25 km using a single electroabsorption modulator as simultaneous optical transmitter and receiver at the remote terminal.

Simultaneous CW generation of over 24 wavelength channels with 100GHz spacing at 1550nm has been demonstrated at room temperature in a Raman ring laser with Fabry-Perot. A single EAM was employed to simultaneously modulate 19 of the output channels at 10GHz.

We propose and experimentally demonstrate a stable and tunable PT-symmetric single-longitudinal-mode (SLM) fiber ring laser using a nonreciprocal Sagnac loop. To suppress multiple-longitudinal-mode oscillation, the Sagnac loop only including a 3-dB optical coupler (OC) and a polarization controller (PC) is incorporated into the fiber ring cavity, which induces nonreciprocal light transmission and coupling between the frequency-degenerate clockwise (CW) and counterclockwise (CCW) resonator modes. The two light paths traveling along the CW and CCW directions in the Sagnac loop are defined as the gain loop and loss loop, respectively. By adjusting the polarization states of the two light waves, when the gain and loss coefficients are larger than the coupling coefficient, the PT symmetry is broken, singlemode lasing is thus generated in the fiber laser cavity.

A novel photonic scheme for the simultaneous generation of microwave amplitude shift keying (ASK) and frequency shift keying (FSK) signal based on gain switching in Vertical Cavity Surface Emitting Laser (VCSEL) is proposed, for the first time. In this approach, gain switched VCSEL is utilized to produce optical frequency comb (OFC) and the linearly polarised OFC signal is splitted into two orthogonal polarizations using polarisation controller and polarisation beam splitter. Two electro absorption modulators (EAM) are employed to modulate each polarization by 1Gbps data. Microwave FSK signals at 2/4 GHz and 6/8 GHz with 1 Gbps data are detected at the photodetector. Moreover, microwave ASK signals are also generated from each polarization branch at 2 GHz and 4 GHz. Simulations are performed to substantiate the proposed system. The influence of VCSEL modulating signal power and EAM reverse bias on FSK output power is also discussed. Results predict a maximum sideband suppression ratio of 33 dB, for a modulating signal current and EAM reverse bias voltage of 15 mA and 1.5 V respectively. The receiver sensitivity of the FSK signal at 2/4 GHz for a 20 km fibre transmission is found to be −16.3 dBm with a Q factor of 6, at a BER of 10−9. This scheme offers a simple, cost-effective approach and requires a low modulating current and its applications include radar, wireless and optical communications.

The multi-passband microwave photonic filter (MPF) with flexible passband selection based on a second-order Solc-Sagnac interferometer and dispersive medium is proposed and experimentally demonstrated. The second-order Solc-Sagnac interferometer is used as the spectrum slicer of the MPF, and by adjusting polarization controllers (PCs) inside the interferometer, the passband can be flexibly selected. Two output port of the optical coupler (OC) are connected with two sections of polarization maintaining optical fiber (PMF) and two PCs to form the second-order Solc-Sagnac interferometer. PMF with different length results in different FSR of the interferometer pattern of Sagnac interferometer and thus different frequency of the MPF’s passband. By adjusting PCs in the second-order Solc-Sagnac interferometer, the visibility of the corresponding interference patterns can be changed, and as a result the passband of the MPF can be selected. In our experiment, by carefully adjusting the PCs in the Sagnac loop, multi-passband MPF with three selectable passbands has been implemented, whose passbands are located at the frequency of 0.219 GHz, 0.451 GHz, and 0.651 GHz, respectively when the length of two PMFs are 1 m and 2 m. In addition, we can also obtain MPF with flexible switching between single passband, double passbands and three passbands, respectively. And by designing the length of the PMFs in the second-order Solc-Sagnac interferometer, passbands at higher frequency can be realized, e.g. in our experiment, by using PMFs with the length of 2 m and 4 m, MPF with switchable passbands at 0.435 GHz, 0.877 GHz, 1.318 GHz has been realized, and the suppression ratio of the MPF is more than 25 dB.

Eight-wavelength EDF laser combining tilted fiber grating with sampled fiber grating

Code division multiplexing (CDM) is proposed as a modulation format for single user communication over highly dispersive fading channels. This modulation format uses well-known spread spectrum technology together with newly developed multiuser detection techniques to produce a modulation and demodulation technique for non-spread data transmission. It is shown that for frequency selective fading channels with large delay spreads, the CDM modulation and demodulation formats developed in this paper represent a superior option to both single carrier modulation formats such as phase shift keying (PSK) as well as multicarrier formats such as orthogonal frequency division multiplexing (OFDM). Hence, CDM is an attractive modulation format for future wireless systems requiring high data rates such as wireless Internet applications and wireless video transmission.

We present a colorless transmitter based on a reflective electroabsorption modulator (EAM) monolithically integrated with a semiconductor optical amplifier (SOA). A semi-insulating buried heterostructure (SI-BH) technology is used to define the device waveguide structures. The integration of the SOA allows to fully compensate for insertion losses. We observe up to +2 dB net gain in the fiber-coupled output power, for 90 mA SOA bias current. We demonstrate 25 Gb/s operation with non-return-to-zero (NRZ) modulation format, with a dynamic extinction ratio (DER) up to 14 dB for 2.6 V peak-to-peak EAM driving voltage. Colorless operation is demonstrated over 15 nm spectral range in the C-band. We achieved 10 km dispersion limited transmission distance at 25 Gb/s NRZ with a penalty of 3 dB at 10−3 bit error rate (BER).

A photonic versatile waveform generator based on phase modulation in a Sagnac loop is proposed and experimentally verified. A continuous‐wave light is modulated by a sinusoidal signal in a phase modulator to generate optical sidebands. Combined with a polariser, the Sagnac loop is used to manipulate the carrier of the modulated signal. Then, a fibre Bragg grating is applied to attenuate one of the first‐order sidebands. By properly choosing the modulation index and the polarisation state of the light wave output from the Sagnac loop, versatile waveform sequences can be generated. In the demonstrating experiment, full‐duty‐cycle sawtooth, triangular and square waveform sequences are successfully generated. The proposed photonic microwave waveform generator features the advantages of a simple structure, low cost, being free from bias drift and tunability in repetition rate.

A novel fiber grating sensing technique utilized for vibration detection is proposed and experimentally demonstrated. A single wavelength laser is formed by polarization controllers and polarization maintaining fiber (PMF) to construct the high birefringence (Hi-Bi) Sagnac loop, PMF combined with erbium-doped fiber, single mode fiber and polarizer. The cantilever glued to an FBG is used as a sensor probe, and the linear edge of Sagnac ring laser are used to demodulate fiber optic seismic vibration signal. Sagnac ring theory and its edge effects are described in detail, and numerical simulation and experiment show that the vibration signal detection system is adjusted periodically in a range from L1 to L1 + L2, sensitivity goes up to 38.2 W/nm of discrimination, the linearity is 0.9996, and the dynamic range is 4070 dB, these meeting the requirements for vibration detection technical parameters.

A switchable multi-wavelength thulium-doped fiber laser (TDFL) using a four-mode fiber based Sagnac loop filter was proposed and demonstrated. The Sagnac loop, incorporating a 4.8 m long four-mode step-index fiber, acting as a comb filter was analyzed theoretically and experimentally. By adjusting two polarization controllers, eight stable single-wavelength operations and five stable dual-wavelength operations were obtained with easy switching between the different operation modes. The wavelength range covered during the single-wavelength switchable operation with a signal-to-noise ratio of up to 55 dB was ∼28.51 nm. In dual-wavelength switchable operation, the maximum and minimum wavelength spacings were ∼21.78 nm and ∼1.50 nm, respectively. Due to the abundant wavelength resources, the proposed TDFL has great potential in wavelength division multiplexing free-space optical communications, optical fiber sensing, and photonic generation of microwave signals.