**5. Experimental results**

*Fiber Optics - From Fundamentals to Industrial Applications*

Error free

**80**

**Case**

I II III IV

V VI VII VIII

IX

X **Table 4.**

*Experimental design of one-bit and two-bit delta-sigma digitization.*

32

28

24

20

16

Two-bit

PAM4

16QAM

4096 16,384 16,384

16,384

16,384

16,384

16,384

1.1 × 10−4

16,384

16,384

16,384

16,384

2.8 × 10−5

16,384

4096

4096

4096

1.2 × 10−5

32

28

24

20

16

**Sampling rate (GSa/s)**

**Quantization**

One-bit

OOK

QPSK

1024 2048 8192

512 2048

16,384

2048

2048

2048

4096

512

512

512

16,384

8192

4096

8192

16,384

2048

2048

2048

8192

1024

1024

1024

**Waveform after ADC**

**Coherent transmission**

128 256

2048

256

256

256

1024

128

128

128

**Modulation formats of five DOCSIS channels**

**BER**

In this section, we first demonstrate the experimental results of DP-QPSK and DP-16QAM coherent transmission, and then discuss the results of one-bit and two-bit digitization supported by QPSK/16QAM, respectively. Finally, we will investigate the BER tolerance of delta-sigma digitization.

## **5.1 Coherent transmission**

**Figure 6** shows the results of coherent transmission of DP-QPSK/16QAM, where error vector magnitude (EVM) is plotted as a function of baud rate. Due to the limited bandwidth of low-cost RF amplifiers and the optical IQ MZM, EVM increases with baud rate. **Figure 6(b)** shows the constellations at each baud rate. For DP-QPSK, error free transmission can be achieved for all baud rates. For DP-16QAM, error free transmission is only achievable for 16 and 20 Gbaud. BER values for 24, 28 and 32 Gbaud are labeled in **Figure 6(a)**. In the following discussion, we only present the results of 32 GSa/s delta-sigma digitization. Results of other sampling rates are similar and omitted here for brevity.

#### **Figure 6.**

*Coherent transmission results of low-cost DP-QPSK and DP-16QAM systems, for one-bit and two-bit digitization, respectively. (a) Error vector magnitude (EVM) vs. baud rate. (b) Constellations of QPSK and 16QAM at each baud rate.*
