Comparison of DPSK and RZ-DPSK Modulations in Optical Channel with Speed of 10 Gbps

Abstract

<div>This article is devoted to the problematic of error rate and modulations in optical communication. Optic waveguide shows insufficiencies in high speed transfers manifested by corrupted transfer. Although modern technological processes contributed to lowering these insufficiencies, it would be uneconomical to completely reconstruct the optical network infrastructure. The solution is only partial so we approach more economical methods, such as modulations. The article works with a 10 Gbps channel with two modulation types, namely DPSK (Differential Phase Shift Keyed) and RZ-DPSK (Return to Zero-Differential Phase Shift Keyed). These modulations are evaluated and compared according to BER (Bit Error Rate) and Q-factor.<div> </div></div>

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“…They are able to exist within the old infrastructure (fibres of type SMF) without changes to the: dispersion map, resistance to non-linear effects and PMD (Polarization Mode Dispersion), the signal's transition through OADM (Optical Add-Drop Multiplexor), their mutual influencing between 10 Gbit.s -1 and 40 Gbit.s -1 /100 Gbit.s -1 and the option of maintaining the 50 GHz gaping. Increasing the transfer speed from 10 Gbit.s -1 to 40 Gbit.s -1 and more, brings several problems, making the typical solution of the classic amplitude modulation OOK-NRZ (On-Off Keying Non-Return to Zero) unusable for this level [16][17][18][19]. This is why the systems with greater transfer speed utilise the duobinary modulations (DPSK and DQPSK) and phase modulation PSK (Phase Shift Keying).…”

“…The diagram is used for the evaluation of the optical line of some system (in our case the individual lines of DWDM). Some terms -like Q-factor, bit error rate or SNR (Signal to Noise Ratio) -are closely related to the eye diagram [16], [28]. The ideal shape of the eye (10 -40 ) should contain the transmitted "1" and "0".…”

“…(3) In Figure 2 the expected values of BER (µ( ) > "#$ |µ , )/ (µ( ) < "#$ |µ , ) indicated the real values when the particular expectation µ( ) is greater/lesser than the optimal value "#$ or the middle value of the level logic "1"/"0". It indicates that the specific areas overlap and there is problem in differentiating whether a "1" or "0" was received [16], [17] and [22]. This problem of differentiating what was received at/in output could be defined by equations separate for "1" and separate for "0": ,…”

“…Using ROA the OSNR is defined for level i as: , (15) where GRA represents the agent raising OSNR when using ROA (raman Optical Amplifier). From the equation (14) it is obvious that the growing distance results in the decrease of OSNR, but only when EDFA is connected [16], [22]. If a reduction of the drop is needed, there is the option of using the Raman amplification which increases the input power of EDFA according to the provided equation (15).…”

“…By adopting a transfer speed of 10 Gbit.s -1 and creating 100 spectral channels using WDM, transfer speeds of up to 1 Tbit.s -1 within one OF could be achieved. When transferring over long distances -of approximately over 100 km -the signal is weakened (attenuated) and it is necessary to employ optical amplifiers for its subsequent amplification [8], [16]. In such cases the amplifiers used enable a direct amplification of the optical signal -meaning they do no need to convert the optical signal into its electrical form and back again, as is the practise of repeaters [9].…”