Mathias Westlund

Increase of the SBS threshold in a short highly nonlinear fiber by applying a temperature distribution

We evaluate numerically and experimentally the stimulated Brillouin scattering (SBS) threshold increase for a short, highly nonlinear GeO/sub 2/-doped fiber by applying different temperature distributions along the fiber. The temperature coefficient for the Brillouin frequency downshift is measured to 1.2 MHz//spl deg/C. A threefold SBS threshold increase is obtained for a 100-m long highly nonlinear fiber with a 140/spl deg/C temperature gradient. The proposed scheme is implemented in a wavelength converter based on fiber optical four-wave mixing (FWM). The SBS suppression scheme shows negligible influence on the FWM efficiency as well as the wavelength conversion bandwidth. The temperature coefficient for the zero dispersion wavelength is measured to 0.062 nm//spl deg/C.

High-performance optical-fiber-nonlinearity-based optical waveform monitoring

An all-optical waveform sampling system with simultaneous submilliwatt optical signal sensitivity (20-dB signal-to-noise ratio) and subpicosecond temporal resolution over more than 60-nm optical bandwidth is demonstrated in this paper. The optical sampling was implemented by four-wave mixing in a 10-m highly nonlinear fiber using a sampling pulse source with a sampling pulse peak power of only 16 W. The sampling performance was evaluated in terms of sensitivity, temporal resolution, and optical bandwidth with respect to fiber length, sampling pulse source wavelength offset from the zero-dispersion wavelength of the highly nonlinear fiber, sampling pulse peak power, and walk-off due to chromatic dispersion. This paper also presents a summary of the available methods to achieve polarization-independent optical sampling as well as a brief summary of the available sampling pulse sources viable for optical sampling.

Software-synchronized all-optical sampling for fiber communication systems

This paper describes a software-synchronized all-optical sampling system that presents synchronous eye diagrams and data patterns as well as calculates accurate Q values without requiring clock recovery. A synchronization algorithm is presented that calculates the offset frequency between the data bit rate and the sampling rate, and as a result, synchronous eye diagrams can be presented. The algorithm is shown to be robust toward poor signal quality and adds less than 100-fs timing drift to the eye diagrams. An extension of the software synchronization algorithm makes it possible to automatically find the pattern length of a periodic data pattern in a data signal. As a result, individual pulses can be investigated and detrimental effects present on the data signal can be identified. Noise averaging can also be applied. To measure accurate Q values without clock recovery, a high sampling rate is required in order to establish the noise statistics of the measured signal before any timing drift occurs. This paper presents a system with a 100-MHz sampling rate that measures accurate Q values at bit rates as high as 160 Gb/s. The high bandwidth of the optical sampling system also contributes to sampling more noise, which in turn results in lower Q values compared with conventional electrical sampling with a lower bandwidth. A theory that estimates the optically sampled Q values as a function of the sampling gate width is proposed and experimentally verified.

0.5-Tb/s eye-diagram measurement by optical sampling using XPM-induced wavelength shifting in highly nonlinear fiber

In this letter, we report on a novel all-optical sampling scheme using cross-phase modulation-induced wavelength shifting and optical filtering. Up to 500-Gb/s optical signal eye-diagram measurements have been demonstrated for the first time with a temporal resolution of 0.7 ps. Signal operational wavelength range covering basically the whole erbium-doped fiber amplifier gain range (1535-1569 nm) with temporal resolutions equal to or less than 1 ps was also demonstrated in the experiment. These results show that the sampling system is suitable for directly monitoring and evaluating ultrahigh bit-rate optical time-division multiplexed data at or above 160 Gb/s.

40-Gbaud 16-QAM transmitter using tandem IQ modulators with binary driving electronic signals

We propose a novel 16-quadrature amplitude modulation (QAM) transmitter based on two cascaded IQ modulators driven by four separate binary electrical signals. The proposed 16-QAM transmitter features scalable configuration and stable performance with simple bias-control. Generation of 16-QAM signals at 40 Gbaud is experimentally demonstrated for the first time and visualized with a high speed constellation analyzer. The proposed modulator is also compared to two other schemes. We investigate the modulator bandwidth requirements and tolerance to accumulated chromatic dispersion through numerical simulations, and the minimum theoretical insertion attenuation is calculated analytically.

Mode locking a 1550 nm semiconductor disk laser by using a GaInNAs saturable absorber

Passive mode locking of an optically pumped, InP-based, 1550 nm semiconductor disk laser by using a GaInNAs saturable absorber mirror is demonstrated. To reduce material heating and enable high-power operation, a 50 microm thick diamond heat spreader is bonded to the surface of the gain chip. The laser operates at a repetition frequency of 2.97 GHz and emits near-transform-limited 3.2 ps pulses with an average output power of 120 mW.

All-Optical Waveform Sampling in High-Speed Optical Communication Systems Using Advanced Modulation Formats

We review techniques to characterize optical data with very high fidelity using the principle of optical sampling based on four-wave mixing in a highly nonlinear fiber. This approach results in sub-ps time resolution without artifacts in the impulse response and also excellent sensitivity allowing statistical analysis. In particular we describe implementations aimed at differentially phase encoded data (such as DQPSK) with all-optical balanced detection, formats requiring the complete signal field recovery (such as QAM) with phase-sensitive sampling, and signals requiring real-time recovery with all-optical real-time sampling. The all-optical approach to characterize optical waveform is shown to be very attractive and practical way to capture the details of various kinds of data and is, in principle, also scalable to extremely high baud rates.

Coherent All-Optical Phase and Amplitude Regenerator of Binary Phase-Encoded Signals

The performance of future ultralong-haul communication systems exploiting phase-encoded signals is likely to be compromised by nonlinear phase noise generated during signal transmission. One potential way to mitigate against nonlinear phase noise is to use phase-sensitive amplifiers (PSAs) that have been demonstrated to help remove such phase noise as well as to provide simultaneous signal amplitude noise suppression when operated in saturation. Recently, we have shown that a PSA-based signal regenerator based on degenerate four-wave mixing could be implemented in a network-compatible manner in which only the (noisy) signal is present at the device input (black-box operation). However, this scheme was tested only with relatively high-frequency deterministic perturbations applied to the signal. Here, we address both theoretically and experimentally the important issue of how such a regenerator works with more realistic random broadband amplitude/phase noise distributions. Good regenerative performance is demonstrated and our study also illustrates an additional unique feature of PSA-based regenerators-namely error correction for differentially encoded signals when placed in front of a DPSK receiver. Furthermore, we present a simplified regenerator implementation providing highly stable operation and representing a significant further step toward a practical device.

Third-order dispersion compensation using a phase modulator

Compensation of third-order dispersion in a fiber-optic transmission system using a phase modulator is studied both theoretically and experimentally. A sinusoidal signal is used as modulation function, where the amplitude and phase delay are optimized. The 2-ps input pulses (160-Gb/s compatible) were transmitted through a 626-km fiber link, where the characteristic oscillating tail was measured with a 1.6-ps resolution optical sampling system. When applying the phase modulation, the oscillating tail was significantly suppressed. The pulses were also used in a 160-Gb/s transmission experiment, where the eye diagrams were measured with the sampling system. Numerical simulations and practical experiments showed excellent agreement.

Simple scheme for polarization-independent all-optical sampling

We propose and demonstrate a simple, single-path technique for polarization-independent all-optical sampling in fiber. The residual polarization dependence was only 0.3 dB and no polarization-dependent waveform distortion was observed.