Photonic crystal fibers are a new class of optical fibers. Their artificial crystal-like microstructure results in a number of unusual properties. They can guide light not only through a well-known total internal reflection mechanism but using also photonic bandgap effect. In this paper different properties possible to obtain in photonic crystal fibers are reviewed. Fabrication and modeling methods are also discussed.

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Photonic Crystal Fibers

Solitons are waveforms that preserve their shape while propagating, as a result of a balance of dispersion and nonlinearity. Soliton-based data transmission schemes were investigated in the 1980s and showed promise as a way of overcoming the limitations imposed by dispersion of optical fibres. However, these approaches were later abandoned in favour of wavelength-division multiplexing schemes, which are easier to implement and offer improved scalability to higher data rates. Here we show that solitons could make a comeback in optical communications, not as a competitor but as a key element of massively parallel wavelength-division multiplexing. Instead of encoding data on the soliton pulse train itself, we use continuous-wave tones of the associated frequency comb as carriers for communication. Dissipative Kerr solitons (DKSs) (solitons that rely on a double balance of parametric gain and cavity loss, as well as dispersion and nonlinearity) are generated as continuously circulating pulses in an integrated silicon nitride microresonator via four-photon interactions mediated by the Kerr nonlinearity, leading to low-noise, spectrally smooth, broadband optical frequency combs. We use two interleaved DKS frequency combs to transmit a data stream of more than 50 terabits per second on 179 individual optical carriers that span the entire telecommunication C and L bands (centred around infrared telecommunication wavelengths of 1.55 micrometres). We also demonstrate coherent detection of a wavelength-division multiplexing data stream by using a pair of DKS frequency combs-one as a multi-wavelength light source at the transmitter and the other as the corresponding local oscillator at the receiver. This approach exploits the scalability of microresonator-based DKS frequency comb sources for massively parallel optical communications at both the transmitter and the receiver. Our results demonstrate the potential of these sources to replace the arrays of continuous-wave lasers that are currently used in high-speed communications. In combination with advanced spatial multiplexing schemes and highly integrated silicon photonic circuits, DKS frequency combs could bring chip-scale petabit-per-second transceivers into reach.

Microresonator-based solitons for massively parallel coherent optical communications

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

We introduce a traffic model for circuit switched all-optical networks (AONs) which we then use to calculate the blocking probability along a path for networks with and without wavelength changers. We investigate the effects of path length, switch size, and interference length (the expected number of hops shared by two sessions which share at least one hop) on blocking probability and the ability of wavelength changers to improve performance. Our model correctly predicts unobvious qualitative behavior demonstrated in simulations by other authors.

Models of blocking probability in all-optical networks with and without wavelength changers

A full-vectorial imaginary-distance beam propagation method based on a finite element scheme is newly formulated and is effectively applied to investigating the problem of leakage due to a finite number of arrays of air holes in photonic-crystal holey fibers (HFs). In order to treat arbitrarily shaped air holes and to avoid spurious solutions, a curvilinear edge/nodal hybrid element is introduced. Furthermore, in order to evaluate propagation characteristics of not only bound modes but leaky modes in HFs, an anisotropic perfectly matched layer is also employed as a boundary condition at computational window edges. It is confirmed from numerical results that the propagation loss increases rapidly with increasing wavelength, especially for HFs with one ring of smaller air holes, and that the propagation loss is drastically reduced by adding one more ring of air holes to the cladding region.

Full-vectorial imaginary-distance beam propagation method based on a finite element scheme: application to photonic crystal fibers

A low-loss and highly birefringent polarization maintaining photonic crystal fiber has been fabricated. The fiber loss and modal birefringence at 1550 nm were 1.3 dB/km and 1.4×10-3, respectively.

Optical properties of a low-loss polarization-maintaining photonic crystal fiber

Soliton data transmission at 7.2–10 Gbit/s over one million kilometres has been successfully achieved by using a new technique that incorporates synchronous shaping and retiming using a high speed optical modulator. This technique enables the Gordon–Haus limit, the accumulation of amplified spontaneous emission, and the effect of interaction forces between adjacent solitons to be overcome. This technique is analogous to the synchronously pumped laser technique and can extend the soliton transmission distance almost infinitely.

10 Gbit/s soliton data transmission over one million kilometres

A high-speed optical transmission experiment was successfully demonstrated in an ultra low-loss polarisation maintaining photonic crystal fibre. The fibre loss and modal birefringence at 1550 nm were 1.3 dB/km and 1.4/spl times/10/sup -3/, respectively. A 10 Gbit/s bi-directional optical signal was successfully transmitted through the 1.5 km fibre.

High-speed bi-directional polarisation division multiplexed optical transmission in ultra low-loss (1.3 dB/km) polarisation-maintaining photonic crystal fibre

It is shown by computer runs and simple analysis that one hundred million km soliton transmission is possible by means of soliton transmission controls in the time and frequency domains. This means that limitless transmission is possible. The key to success is to reduce noise by the synchronous modulation technique which can also reduce timing jitter and nonlinear interaction forces. The accumulated noise converges to a fixed low level even after limitless transmission.

Infinite-distance soliton transmission with soliton controls in time and frequency domains

A 160 Gbit/s WDM (20 Gbit/s/spl times/8 channels) soliton data signal was successfully transmitted over 10000 km through the use of in-line synchronous modulation and optical filtering. A four-segment dispersion decreasing configuration was used to reduce the soliton interaction and the dispersive waves. Polarisation scrambling and phase modulation at the input were also employed to extend the transmission distance.

K. Suzuki

Papers

Optical properties of a low-loss polarization-maintaining photonic crystal fiber

10 Gbit/s soliton data transmission over one million kilometres

High-speed bi-directional polarisation division multiplexed optical transmission in ultra low-loss (1.3 dB/km) polarisation-maintaining photonic crystal fibre

Infinite-distance soliton transmission with soliton controls in time and frequency domains

160 Gbit/s WDM (20 Gbit/s × 8 channels) soliton transmission over 10000 km using in-line synchronous modulation and optical filtering