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Development in high-speed optical transmission systems

1st April 2013


Even in the current economic downturn, the internet continues to experience exponential growth in data traffic. In order to accommodate and promote this growth we need not just higher capacity systems but ones that significantly reduced acost-per-bit-transmitted'. Harry J R Dutton and Andros Payne report.

Dense wavelength division multiplexing (DWDM) can deliver a massive increase in system capacity because it carries many optical channels at different wavelengths (frequencies) on the same strand of fibre. In DWDM systems minimum cost is achieved by using the smallest number of the fastest channels possible. To reduce system cost we need to increase the speed of individual optical channels. To do this many challenges must be faced:

€ Impairments of the fibre (such as anon-linearities' and apolarisation mode dispersion') that were relatively unimportant at 2.5Gbps become significant at 10Gbps. At 40Gbps they become critical.

€ Lasers that work well at lower speeds often cannot meet the challenge of a more rigorous high-speeds.

€ Every time you double the link speed you halve the sensitivity of your receiver. At very high speeds we need either significantly better receivers or higher power transmitters or (preferably) both.

When you increase the speed of an optical channel to 10Gbps you discover that you need to change the way data is represented (encoded) on the fibre. Almost all systems of 2.5Gbps and below transmit their data using a very basic form of encoding. You turn the laser ON to send a a1' bit and OFF to send a a0' bit. This system, (called aNON-Return-to-Zero', NRZ) is simple and low cost but not conducive to extremely high-speed operation. For speeds of 10 Gbps and above there are three related but distinct techniques are available:

RZ Coding: RZ (return to zero) encoding is used by most long-distance 10Gbps systems. Instead of having the laser turned ON for a whole bit period you send a shorter pulse (typically half of a bit period).

Solitons: Solitons are extremely short, (around 1 pico-second) high-power pulses of light. As a Soliton travels on a fibre, the intensity of the light modifies (reduces) the refractive index of the fibre. This has the effect of slowing down the beginning of a pulse and speeding up the end of it such that the pulse retains its shape over extremely long distances (over 10000km).

Solitons solve the problem of chromatic dispersion. Because different wavelengths (within the same optical pulse) travel at a different speeds within the fibre, pulses aspread out' or disperse during their journey. If we do nothing to counter the effects of dispersion, after a certain distance, pulses merge into one another and the receiver has no way of distinguishing between them.

Solitons are not yet a commercial technology because they require special fibre, frequent amplification and have problems with amplifier noise.

€ Dispersion managed (DM) solitons. These are high-power RZ pulses which are longer than Solitons (typically between 7ps and 10ps) and which (unlike true Solitons) disperse as they travel on the fibre. This dispersion is amanaged' by constructing the optical link to include lengths of fibre with the opposite dispersion characteristics to those of standard fibre. These characteristics are balanced over the whole link so that the total dispersion is zero. DM Solitons have most of the advantages of Solitons without many of the drawbacks.

Finding a suitable laser source for RZ systems has been difficult. Pulses need to have both a high peak power and good spectral characteristics. Semiconductor lasers are not a good source of RZ pulses:

€ Power. Semiconductor lasers produce a relatively constant power level when they are aON'. So if you send a pulse for only half of a bit time then you halve the average power transmitted! To correct this a apost amplifier' is typically used.

€ Chirp: If you modulate the drive current the laser produces a rapid shift in wavelength immediately after it is turned ON. This widens the spectrum produced and causes significant problems with dispersion.

€ Extinction ratio: In order to avoid problems with chirp people typically operate semiconductor lasers in the ON state all of the time. A a1' is sent as a high-power pulse and a a0' as a low power one. This means that we have to distinguish between different power levels at the receiver rather than just making the decision alight' or ano light'. This reduces the sensitivity of the receiver and is reflected in shorter transmission distances.

A possible solution to this is the aself-pulsating' laser. Such lasers produce short, high-power pulses with NO chirp and an extinction ratio better than 1000 to 1. They are suitable for all kinds of RZ and Soliton transmission. Until recently these were made only in small quantities for laboratory experiments. However, these devices are becoming available soon at reasonable cost.

ENQUIRY No 72

Harry J R Dutton and Andros Payne are with GigaTera Inc, Dietikon, Switzerland. www.gigatera.com





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