Dispersion-compensated Fiber Link Research Articles

Cost-effective demonstration of fiber-optic frequency transfer.

In this paper, we demonstrate a cost-effective solution for fiber-optic frequency transfer. By employing the commercially available small form pluggable (SFP) transceivers and other components for telecom, the scheme is compatible with the existing telecom networks. An experiment testbed based on the common round trip frequency transfer is carried out to investigate the corresponding performance in detail. Compared with the conventional analog electro-optic interconversion scheme, the optical modulation and demodulation employing SFP transceiver has an inferior performance in terms of additive phase noise. However, the scheme enables a much larger operation range of receiving optical power for an optimal system performance, which indicates the certain capability of adapting to different fiber transmission links. On the basis of the established testbed, fiber-optic frequency transfer with a frequency of 1 GHz is demonstrated over a 50 km dispersion-compensated fiber link. Furthermore, the additive phase noise of -84dBc/Hz @1 Hz and -130dBc/Hz @10 kHz is achieved. Simultaneously, the Allan deviation (ADEV) of 9.6×10-14/s and 8.4×10-17/104s is reached, respectively. The scheme can provide a relatively feasible solution to implement fiber-optic frequency transfer over the existing telecom network infrastructure.

Wideband Microwave Frequency Distribution for Multi-Access Along a Single Fiber Link

In this paper, a fiber-optic signal access method is proposed for stable wideband microwave frequency distribution. Incorporating with the tapped counter-propagating reference signals and variable optical delay lines (VODLs), a stable wideband microwave frequency signal can be obtained at any access point (AP) along the main fiber link. Moreover, all the phases of received wideband microwave frequency can be aligned. In this way, the whole system complexity can be dispersed as the central station only stabilizes the main fiber link delay, and the access ends (ANs) operate independently. Experimentally, a multi-access wideband microwave frequency distribution is demonstrated over a 50 km dispersion-compensated fiber link. A system figure of merit is investigated in detail. The results show that the fiber-optic wideband microwave frequency transmission system can operate for the frequency over 40 GHz. The additive phase noises of -125 dBc/Hz for the remote end (RN), and -131 dBc/Hz for the AN, respectively, are achieved at the 1 kHz offset frequency for a 1 GHz frequency signal. Thanks to the active delay stabilization, the Allan deviation (ADEV) of 1.3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> /s and 2.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> /100 s for the RN, and the one of 8.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> /s and 4.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> /100 s for the AN, respectively, are acquired.

Peak-to-average power ratio reduction of transmission signal of all-optical orthogonal time/frequency domain multiplexing using fractional Fourier transform

We examine a peak-to-average power ratio (PAPR) reduction effect in an optical fiber link using a fractional Fourier transform (FrFT). Fractional OFDM (FrOFDM) based on FrFT in place of discrete Fourier transform is a multiplexing approach that can balance the pros and cons of subcarriers in the time and frequency domains while keeping its orthogonal condition. A careful investigation of the PAPR behavior of a FrOFDM signal confirms that its PAPR can be minimized at a point where a Nyquist pulse train is formed by a time-lens effect. With emphasis on this low-PAPR characteristic of a Nyquist pulse train, the signal quality degradation owing to fiber nonlinearity can be mitigated. A 1.0-dB signal quality improvement after propagation in a dispersion-compensated fiber link is demonstrated in a simulation for a 12×10-Gbaud∕s 16 QAM FrOFDM signal in comparison with that of a conventional OFDM.

Remote two-color optical-to-optical synchronization between two passively mode-locked lasers.

Using balanced detection in both the radio frequency (RF) and the optical domain, we remotely synchronize the repetition rate of a Ti:sapphire oscillator to an Er-doped fiber oscillator through a 360 m length-stabilized dispersion compensated fiber link. The drift between these two optical oscillators is 3.3 fs root mean square (rms) over 24 hours. The 68 MHz Er-doped fiber oscillator is locked to a 476 MHz local RF reference clock, and serves as a master clock to distribute 10 fs-level timing signals through stabilized fiber links. This steady remote two-color optical-to-optical synchronization is an important step toward an integrated femtosecond fiber timing distribution system for free-electron lasers (FELs); it does not require x-ray pulses, and it makes sub-10-fs optical/x-ray pump-probe experiments feasible.

Placement of optimized semiconductor optical amplifier in fiber optical communication systems

This paper deals with the placement of semiconductor optical amplifier for 10 Gb/s non-return to zero format in single mode and dispersion-compensated fiber link. We investigated post-, pre- and symmetrical power compensation methods for different positions of the SOA in fiber link. The effect of increase in signal input power for the three power compensation methods is compared in terms of eye diagram, bit error rate, eye closure penalty and output received power. It is found that the post-power compensation method is superior to pre- and symmetrical power compensation methods. Further, the effect of variation in length of the standard single mode fiber and dispersion-compensated fiber for three power compensation configurations has also been observed. The impact of eye closure penalty is observed for large transmission distances for different signal input powers.

40-Gb/s tandem electroabsorption modulator

In this letter, we have developed a tandem electroabsorption modulator with an integrated semiconductor optical amplifier that is capable of both nonreturn-to-zero and return-to-zero (RZ) data transmission at 40 Gb/s. The tandem modulator consists of a broad-band data encoder and a narrow-band pulse carver. The pulse carver is able to produce 5-ps pulses with more than 20 dB of extinction. The on-chip semiconductor optical amplifier provides up to 8.5 dB of fiber-to-fiber gain and enables the modulator to be operated with zero insertion loss. Devices have been realized with greater than 40-GHz bandwidth, and 13-dB dynamic extinction for a 2.5-V swing. For optimized designs bandwidths of nearly 60 GHz: have been realized. Using these devices penalty free RZ data transmission over a 100-kin dispersion compensated fiber link has been demonstrated with a received power sensitivity of -29 dBm.

Complete dispersion compensation for 400-fs pulse transmission over 10-km fiber link using dispersion compensating fiber and spectral phase equalizer

We have demonstrated essentially complete dispersion compensation for 400-fs pulses over a 10-km fiber link using dispersion compensating fiber and a programmable femtosecond pulse shaper functioning as a spectral phase equalizer. The pulse shaper impresses adjustable quadratic and cubic phases onto the spectrum and removes all the residual dispersion and dispersion slope in the dispersion compensated fiber link. Our work shows that the pulse shaper technique provides a powerful and convenient tool for programmable fiber dispersion compensation over broad optical bandwidth. This allows distortion-free femtosecond pulse transmission over a fiber link in excess of 10 km without requiring the exact trimming of the dispersion-compensating fiber.

Effects of self-phase modulation on sub-500 fs pulse transmission over dispersion compensated fiber links

The effects of nonlinearity on sub-500 fs pulse transmission over dispersion compensated fiber links using dispersion compensating fiber technique are investigated numerically and experimentally. The pulse broadening and recompression ratio of the 2.5-km transmission link is over 300. The postcompensated and precompensated links are compared when the input pulse energy ranges from 15 to 159 pJ. At high powers, self-phase modulation (SPM) degrades the pulse recompression process and provides an upper bound on the transmitted pulse energy. The SPM effect is stronger in the postcompensated link than in the precompensated link. A dramatic spectral narrowing effect was observed in the postcompensated link. Pulse energies up to tens of pJ, consistent with high quality communication, should be possible for a sub-500 fs pulse in such dispersion compensated links.

Cross-phase modulation in intensity modulation-direct detection WDM systems with multiple optical amplifiers and dispersion compensators

The cross-phase modulation (XPM) effect in intensity modulation-direct-detection (IM)-(DD) optical fiber links with multiple fiber segments with different characteristics and optical amplifiers is investigated theoretically and numerically. A generalized model of the IM induced by an arbitrary number of channels through XPM is derived, compared to simulation results and its validity range is presented. Results show that the XPM-induced IM can be modeled as an intensity modulator driven by the intensity of copropagating waves. The frequency response of the intensity modulator corresponding to each copropagating wave is mainly affected by the walk-off parameter and fiber dispersion. When the walk-off effect is weak the XPM-induced IM is approximately proportional to the square frequency. In single-segment fiber links when the walk-off effect is strong the XPM-induced IM is approximately linearly proportional to the frequency and inversely proportional to the wavelength separation. Both theory and simulation show that the XPM-induced IM in fiber links with multiple optical amplifiers can be enhanced or reduced by properly arranging the dispersion characteristics in each fiber segment. In a nondispersion compensated amplified link and for weak walk-off effect, the total XPM-induced IM increases approximately with the square of the number of fiber segments and of modulation frequency. However, if the dispersion is compensated for within each fiber segment the total XPM-induced IM increases proportionally to the number of fiber segments and to the square frequency. Furthermore, it is shown that in fiber links with a large number of segments placing a single dispersion compensator in the last segment of the link leads to almost the same performance as for non dispersion compensated fiber link and is significantly worse than placing one dispersion compensator in each fiber segment as far as the XPM-induced IM reduction is concerned.

A femtosecond code-division multiple-access communication system test bed

This paper reports comprehensive experimental results on a femtosecond code-division multiple-access (CDMA) communication system test bed operating over optical fiber in the 1.5 /spl mu/m communication band. Our test bed integrates together several novel subsystems, including low-loss fiber-pigtailed pulse shapers for encoding-decoding, use of dispersion equalizing fibers in dispersion compensated links for femtosecond pulse transmission and also in femtosecond chirped pulse amplification (CPA) erbium doped fiber amplifiers (EDFAs), and high-contrast nonlinear fiber-optic thresholders. The individual subsystems are described, and single-user system level experimental results demonstrating the ability to transmit spectrally encoded femtosecond pulses over a 2.5-km dispersion compensated fiber link followed by decoding and high contrast nonlinear thresholding are presented.

Broadband fiber dispersion compensation for sub-100-fs pulses with a compression ratio of 300

We report what is, to our knowledge, the first experimental demonstration of nearly dispersion-free transmission of sub-100-fs pulses over several tens of meters of fiber. 62-fs pulses are broadened initially and recompressed by a ratio of 300 over a 42-m concatenated fiber link consisting of standard single-mode and dispersioncompensating fibers. This dispersion-compensated fiber link is estimated to have a third-order dispersion ~6 times lower than that of dispersion-shifted fiber.

Dispersion compensation for ultrashort pulse transmission using two-mode fiber equalizers

We numerically simulate ultrashort pulse propagation in dispersion compensated fiber links using two-mode equalizing fibers. By choosing a proper length ratio between the conventional single-mode fiber and the compensating two-mode fiber, both the dispersion and the dispersion slope can be eliminated simultaneously. As a result, dispersion compensated propagation of sub-picosecond pulses should be possible for distances in excess of 100 km. Our analysis shows that for sub-picosecond pulses, dispersion-limited propagation distances offered by this technique may significantly exceed those possible with soliton propagation. We also evaluate nonlinearity limits due to self-phase modulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>