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Method for Line Coding for Directly Modulated Multi Level Optical Transmission

IP.com Disclosure Number: IPCOM000018577D
Original Publication Date: 2003-Jul-24
Included in the Prior Art Database: 2003-Jul-24
Document File: 7 page(s) / 196K

Publishing Venue

IBM

Abstract

A method is proposed to enable multilevel Pulse-Amplitude Modulation (PAM) transmission for directly modulated optical data transmission links. The method is based on a novel class of codes specially suited for this application.

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Method for Line Coding for Directly Modulated Multi Level Optical Transmission

Optical links take a growing share of short-distance data transmission for transmitting data within a large system. Traditionally, optical transmission systems use binary non-return to zero (NRZ) coding as shown in Fig. 1. Increasing the data throughput requires multi-level Pulse Amplitude Modulation (PAM) where more than one bit is transmitted per symbol. This, however, is difficult, because unlike their electrical counterparts, optical links suffer from the nonlinear dynamics of the laser in the transmitter. Looking at a typical eye-diagram as displayed in Fig.1 reveals that the rising edge of the signal shows significant overshoot. Worse, the amount of overshoot is dependent on many factors such as temperature, bit rate, bias current and modulation amplitude. Falling edges, on the contrary, do not show a large amount of underershoot.

Fig. 1 Measured optical 12.5 Gb/s NRZ eye diagram of a typical VCSEL (PRIOR ART)

The origin of this nonlinear behavior can be found in the basic properties of the laser. This effect is present in all diode lasers be it a VCSEL or an edge emitter. A very

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simplified explanation is given next: In a conventional diode laser a dc current is used for pumping. This current will excite carriers into the conduction band. This results in the necessary population inversion. In general carriers which transit from the conduction to the valence band will emit a photon. In edge emitting lasers and VCSEL the recombination time constant of carriers are about three orders of magnitude larger than the corresponding photon time constants. Typical values are 2.7 ns and 2 ps respectively. Assuming the diode laser is modulated with a current pulse. A sharp increase in current (rising pulse edge) will result in an increase in the electron density. Consequently, the photon density and the optical output power will increase. However, due to carrier recombination the steady state value of the electron density is lower than the peak value reached initially. This explains the overshoot of the optical pulse for the rising edge transition. For the falling edge of the electrical pulse no significant undershoot is observed. A decrease in current leads to a decrease in the electron density which results in a decrease in generated photons and finally a decrease in optical output power.

The observed overshot is not that disturbing for NRZ modulation; however, the situation is more critical for muli level modulation (PAM). Using straight forward PAM-3 encoding results in a data eye shown in Fig. 2. The upper portion of the data eye is significantly closed due to the overshoot of a transmission from the lowest (0) level to the mid (1) level. Fig. 2 shows measured data using the commercial 10Gb/s VCSEL shown in Fig. 1

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FIG. 2 Measured 12.5 Gb/s optical eye diagram, modulation:...