The development of silicon-compatible optical components that simultaneously amplify and process a broad range of wavelength channels is critical for future data communication technology based on photonic chips. Until now, such devices have only been able to amplify a single wavelength channel. Now, using nanoscale silicon waveguides designed for the purpose, Foster et al. have achieved broadband amplification. The key is the exploitation of a nonlinear optical effect known as four-wave mixing. This process can also be used for other all-optical functions previously only possible in extended lengths of optical fibre. Phase-matched four-wave mixing can take place with high efficiency in a suitably designed silicon waveguide — this advance could allow for the implementation of dense wavelength channels for optical processing in an all-silicon photonic chip. Developing an optical amplifier on silicon is essential for the success of silicon-on-insulator (SOI) photonic integrated circuits. Recently, optical gain with a 1-nm bandwidth was demonstrated using the Raman effect1,2,3,4,5,6,7,8,9, which led to the demonstration of a Raman oscillator10,11, lossless optical modulation12 and optically tunable slow light13. A key strength of optical communications is the parallelism of information transfer and processing onto multiple wavelength channels. However, the relatively narrow Raman gain bandwidth only allows for amplification or generation of a single wavelength channel. If broad gain bandwidths were to be demonstrated on silicon, then an array of wavelength channels could be generated and processed, representing a critical advance for densely integrated photonic circuits. Here we demonstrate net on/off gain over a wavelength range of 28 nm through the optical process of phase-matched four-wave mixing in suitably designed SOI channel waveguides. We also demonstrate wavelength conversion in the range 1,511–1,591 nm with peak conversion efficiencies of +5.2 dB, which represents more than 20 times improvement on previous four-wave-mixing efficiencies in SOI waveguides14,15,16,17. These advances allow for the implementation of dense wavelength division multiplexing in an all-silicon photonic integrated circuit. Additionally, all-optical delays18, all-optical switches19, optical signal regenerators20 and optical sources for quantum information technology21, all demonstrated using four-wave mixing in silica fibres, can now be transferred to the SOI platform.

Broad-band optical parametric gain on a silicon photonic chip

Several kinds of nonlinear optical effects have been observed in recent years using silicon waveguides, and their device applications are attracting considerable attention. In this review, we provide a unified theoretical platform that not only can be used for understanding the underlying physics but should also provide guidance toward new and useful applications. We begin with a description of the third-order nonlinearity of silicon and consider the tensorial nature of both the electronic and Raman contributions. The generation of free carriers through two-photon absorption and their impact on various nonlinear phenomena is included fully within the theory presented here. We derive a general propagation equation in the frequency domain and show how it leads to a generalized nonlinear Schrodinger equation when it is converted to the time domain. We use this equation to study propagation of ultrashort optical pulses in the presence of self-phase modulation and show the possibility of soliton formation and supercontinuum generation. The nonlinear phenomena of cross-phase modulation and stimulated Raman scattering are discussed next with emphasis on the impact of free carriers on Raman amplification and lasing. We also consider the four-wave mixing process for both continuous-wave and pulsed pumping and discuss the conditions under which parametric amplification and wavelength conversion can be realized with net gain in the telecommunication band.

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Nonlinear optical phenomena in silicon waveguides: Modeling and applications

An applications-oriented review of optical parametric amplifiers in fiber communications is presented. The emphasis is on parametric amplifiers in general and single pumped parametric amplifiers in particular. While a theoretical framework based on highly efficient four-photon mixing is provided, the focus is on the intriguing applications enabled by the parametric gain, such as all-optical signal sampling, time-demultiplexing, pulse generation, and wavelength conversion. As these amplifiers offer high gain and low noise at arbitrary wavelengths with proper fiber design and pump wavelength allocation, they are also candidate enablers to increase overall wavelength-division-multiplexing system capacities similar to the more well-known Raman amplifiers. Similarities and distinctions between Raman and parametric amplifiers are also addressed. Since the first fiber-based parametric amplifier experiments providing net continuous-wave gain in the for the optical fiber communication applications interesting 1.5-/spl mu/m region were only conducted about two years ago, there is reason to believe that substantial progress may be made in the future, perhaps involving "holey fibers" to further enhance the nonlinearity and thus the gain. This together with the emergence of practical and inexpensive high-power pump lasers may in many cases prove fiber-based parametric amplifiers to be a desired implementation in optical communication systems.

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Fiber-based optical parametric amplifiers and their applications

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

We analyze the scalability of diffraction-limited fiber lasers considering thermal, non-linear, damage and pump coupling limits as well as fiber mode field diameter (MFD) restrictions. We derive new general relationships based upon practical considerations. Our analysis shows that if the fiber's MFD could be increased arbitrarily, 36 kW of power could be obtained with diffraction-limited quality from a fiber laser or amplifier. This power limit is determined by thermal and non-linear limits that combine to prevent further power scaling, irrespective of increases in mode size. However, limits to the scaling of the MFD may restrict fiber lasers to lower output powers.

Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power

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.

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

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.

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

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.

Software-synchronized all-optical sampling for fiber communication systems

When there is a need to accurately characterize optical waveforms and, it is not surprising that some of the best, albeit only recently established, techniques to do this rely on all-optical phenomena. Some basic reasons why all-optical sampling holds great promise as a very useful tool well into the foreseeable future are that there are no ringing phenomena with associated waveform distortion as in electronic sampling due to impedance mismatch, and that the time resolution can be made extremely high (<< 1 ps) while yet also offering high sensitivity for e.g. eye diagram (a superposition of all '1' and '0' in a data sequence that is widely used in telecommunications testing) and statistical analysis. In this paper, we review recent developments in optical fiber-based sampling of optical waveforms. In particular, we describe the state-of-the-art in terms of the various performance measures as well as their trade-offs.

Mathias Westlund

Papers

Fiber-based optical parametric amplifiers and their applications

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

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

Software-synchronized all-optical sampling for fiber communication systems

Nonlinear optical fiber based high resolution all-optical waveform sampling