Chapter 3 A Single-Fiber Optical UPSR Ring Network Using DSB-SC Modulation
3.3 Experimental Setup and Results
Our experimental setup is shown in Fig. 3.2. Two continuous-wave lasers (CW1, CW2) with 200 GHz channel spacing were used in the experiment. Their center frequencies were shifted 25GHz away from a typical 50GHz ITU grid, so that the USB’s and LSB’s can sit right on the 50GHz ITU grid if fc is chosen to be 25 GHz (see Fig. 3.3). The wavelengths of CW1 and CW2 after the 25GHz offsetting were 1554.33 and 1555.95 nm, respectively. Both lasers were respectively externally modulated by a 10Gb/s NRZ signal using a single-electrode LiNbO3 modulator. The modulated signals were then combined via a WDM filter and sent to a dual-drive LiNbO3 modulator. To achieve DSB-SC modulation (in this case, the “suppressed carrier” is actually a “suppressed modulated carrier (SMC)”) at both wavelengths, the dual-drive modulator was modulated by a 17 dBm microwave oscillator at 25 GHz and biased at its minimum optical power point. The half-wave DC voltage of the push-pull modulator was 4.5 V. To minimize the modulated optical carrier and the microwave harmonic distortion (HD), the microwave driving power and the delay between the microwave driving signals at the two electrodes were carefully tuned.
The method presented in Fig. 3.2, which uses only one microwave module, can be applied to a lab environment when there are multiple DSB-SC wavelengths. In Fig.
3.2, the two 50/100GHz interleavers were used to simulate the two interleavers in Fig.
3.1. No MUX or DEMUX is used in this experiment, which does not affect the experimental result. The 50/100GHz interleaver has a 0.5dB bandwidth of 35 GHz, and suppresses adjacent channel by more than 15dB. For demonstration purpose, two optical switches (SW1 and SW2) were used to select either LSB or USB to pass through the interleavers. From the optical spectrum at the output of the dual-drive modulator, it can be seen that the suppression of the modulated optical carrier and the 2nd harmonic distortion were both more than 30 dB. The optical spectra at the output of the second interleaver show that the adjacent USB or LSB channel suppression was more than 35 dB due to the cascaded interleaver/de-interleaver pair (the upper and lower spectra show predominantly the USB’s and LSB’s when SW1 or SW2 was
on, respectively). The adjacent channel crosstalk was actually 10-15 dB lower than the amplified spontaneous emission (ASE) power and should cause negligible system penalty.
The effect of the Rayleigh back-scattering was also examined, and the results are shown in Fig. 3.4. The optical spectrum shown in the inset (a) is the transmitted USB’s with a launched power per channel of 0 dBm. Note that the LSB’s were not suppressed as deep as that shown in Fig. 3.2 because the DSB-SC signal passed through only one interleaver. The optical spectrum in the inset (b) is the reflected USB/LSB’s after 50 km of conventional single-mode fiber transmission. In this spectrum, the Rayleigh backscattering (RB) signals and spontaneous Brillouin scattering (SpBS)-induced shifted frequencies (shifted by ±11GHz) were observed [40]. The measured Rayleigh reflection is -35.3 dB. After the interleaver, the USB’s were suppressed another 19~24 dB, while the LSB’s power remained essentially the same, as shown in Fig. 3.4(c). If we add the LSB suppression due to the transmitting-end E-MUX (~10 dB) with 100GHz ITU-grid spacing, the received RB-reflected LSB power level could be as low as -70 dBm (10 dB lower than what is shown in Fig. 3.4(c)). When there is a fiber-cut-induced air-interface back reflection, the reflected LSB is about 20 dB higher than that in the RB signal, as shown in Fig.4(d), i.e., having a power level around -50 dBm. In both cases, the reflected LSB power should be way below a normally received westbound LSB signal (-20 ~ -25 dBm), and therefore resulted in a <1dB system power penalty [41].
Using the experimental setup shown in Fig. 3.2, we measured the BER performance and obtained a less than 1 dB system power penalty at a BER of 10-9, for both the LSB and USB signals (at 1555.95 nm and 1554.33 nm). Note that the results obtained were based on an optical band pass filter (OBPF) with a 1-dB bandwidth of ~100GHz (see Fig. 3.3), and a commercial 10Gb/s PIN receiver.
3.4 Discussion
An optimum microwave driving power needs to be chosen in order to maximize
optical carrier suppression and minimize 2nd harmonics. In this section, we discuss how this optimum driving power can be obtained.
For a dual-drive MZM biased at minimum optical carrier output, with a finite extinction ratio and unbalanced microwave drive amplitudes at the two electrodes, the suppression ratios of the optical carrier and the 2nd harmonic distortion relative to the fundamental signal are given by
amplitude of the optical carrier, the fundamental modulating signal and its 2nd harmonics, respectively;
π level of unbalance), Vπ is the RF switching voltage of the modulator at 25GHz,
1 1 +
= − δ
γ δ is the parameter specifying the effect of extinction ration δ (the DC extinction ratio of the DSB-SC modulator was 40 dB, therefore γ=0.9802), and
)
Jn(x is Bessel function of the first kind with order n.
In our experiment, the unbalanced driving level between the two electrodes of the DSB-SC modulator was x=~1 dB due to the insertion loss of the phase shifter. For a 25 GHz tone with a power level of 17 dBm (measured before the 7-dB-loss microwave power divider), we obtain a calculated α =0.5161 (V1=1 V, and Vπ
=6.0868 V) for the arm with larger power. The derived Vπ of 6.0868at25GHzis calculated by fitting the measured 2nd harmonic suppression level with Bessel function using equation (3-2), as illustrated by [43]. It should be noted that the optical carrier suppression is very sensitive to the DC bias voltage. As observed in this experiment, at the maximum carrier suppression point, a 0.04V voltage change could make a difference in the carrier power by as much as 10 dB. Based on the data given
above, the calculated optical carrier suppression and 2nd harmonic levels can be obtained from (3-1) and (3-2), and are closely matched with the measured data, as shown in Fig. 3.5.
Fig. 3.1 Typical node configuration in a single-fiber O-UPSR network. The node is composed of a BD-OADM and a DSB-SC transmitter. USB is east-bound and LSB is west-bound.
Fig. 3.2 Experimental setup. The resolution bandwidth of the measured optical spectra is 0.01 nm. SMC: suppressed modulated optical carrier, HD1 and HD2: 2nd harmonic distortion.
WDM Dual-driveMZM Interlaever Interlaever
LSB USB LSB USB
Fig. 3.3 The relative frequency arrangement of the two externally modulated lasers and their respective double-sideband signals. Note that the OBPF in an ideal case should be narrow enough to filter just one USB or LSB signal.
Fig. 3.4 Experimental setup to investigate the impact of back-reflections: (a) the transmitted spectrum, (b) the reflected spectrum from 50km fibers, the additional “R”
in USB1R, SMC1R, etc., stands for Rayleigh backscattered signals, (c) the reflected spectrum after the interleaver through 50km SMF, and (d) the reflected spectrum after the interleaver with an air interface.
XT measurement
Termination With low reflection
With 50km fibers (c)
Open to air w/o fibers (d)
Fig. 3.5 Measured (solid symbols) and calculated (solid and dashed lines) power suppression (relative to sideband power) of the optical carrier and the second harmonics (HD). Also shown is the operating point of this experiment.