Reduction of dispersion-induced distortion in SCM transmission systems by using predistortion-linearized MQW-EA modulators

Abstract

The application of low-chirp MQW (multiquantum well)-EA (electro-absorption) modulators to subcarrier multiplexing (SCM) optical transmission systems is studied. The authors show that a third-order predistortion circuit is feasible for compensating the nonlinearity of this type of modulator. The degree of frequency chirping per optical intensity modulation depth of the modulator is theoretically determined to be about 1.4 MHz. A 42-channel AM-SCM signal transmitter with the predistortion circuit realizes both composite second-order distortion (CSO) and composite triple beat distortion (CTB) values under 057 dBc after 200-km-long transmission. The authors confirm dispersioninduced distortion of the MQW-EA modulator is as small as that of the LiNbO3 modulator. Carrier-to-noise ratios (CNR's) of 45.2 dB for channel 1 and 43.8 dB for channel 42 were measured after 100 km transmission. It is found that the deterioration of a CNR is caused by the cascaded erbium doped fiber amplifiers (EDFA's), Rayleigh backscattered power and the optical phase noise. Theoretical CNR's show that the output power of the modulator should be higher to improve CNR.

Large Signal Analysis of Multiquantum Well Electroabsorption Modulators

“…Comparison of Figures 1 and 2 and inspection of Figure 3 clearly shows that, except at or nearby the values of the normalized bias voltages where the second-order intermodulation achieves its minimum, the second-order intermodulation is dominant. These trends are in good agreement with the measured and calculated observations reported in Iwai et al [12] and Yun et al [13].…”

“…This result agrees well with the predictions made from Eqs. (16)-(18) and the results reported in Iwai et al [12] and Yun et al [13]. However, for large values of the normalized amplitudes of the input RF, the rate of increase of the second-and third-order intermodulation decreases.…”

“…This is the typical performance of a third-order nonlinearity. These results are in excellent agreement with the calculations reported in Iwai et al [12] and Yun et al [13] using a third-order polynomial representation of the MQW-EAM transfer function. This confirms that while the analyses presented in Iwai et al [12] and Yun et al [13] may be applicable under small and large signal conditions, their accuracy deteriorates as the signal amplitudes increase and, therefore, can not be used to predict the large signal performance of the MQW-EAM.…”

“…(1) can not be used for predicting the harmonic and intermodulation performance of the MQW-EAM, with non-integer values of α, excited by multisinusoidal signals. Recourse to Taylor series expansion around the bias voltage V b is, therefore, necessary [12,13]. By truncating this Taylor series after the third term [12] and [13], calculations can be performed for the second-and third-order intermodulation distortion.…”

“…Recourse to Taylor series expansion around the bias voltage V b is, therefore, necessary [12,13]. By truncating this Taylor series after the third term [12] and [13], calculations can be performed for the second-and third-order intermodulation distortion. However, by virtue of their derivation, these calculations are valid only under relatively small modulating voltage amplitudes and can not be used for predicting the large signal performance of the MQW-EAM.…”