होम Journal of Semiconductors 35 km amplifier-less four-level pulse amplitude modulation signals enabled by a 23 GHz...

35 km amplifier-less four-level pulse amplitude modulation signals enabled by a 23 GHz ultrabroadband directly modulated laser

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खंड:
41
पत्रिका:
Journal of Semiconductors
DOI:
10.1088/1674-4926/41/3/032304
Date:
March, 2020
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आप पुस्तक समीक्षा लिख सकते हैं और अपना अनुभव साझा कर सकते हैं. पढ़ूी हुई पुस्तकों के बारे में आपकी राय जानने में अन्य पाठकों को दिलचस्पी होगी. भले ही आपको किताब पसंद हो या न हो, अगर आप इसके बारे में ईमानदारी से और विस्तार से बताएँगे, तो लोग अपने लिए नई रुचिकर पुस्तकें खोज पाएँगे.
Journal of Semiconductors

PAPER

35 km amplifier-less four-level pulse amplitude modulation signals
enabled by a 23 GHz ultrabroadband directly modulated laser
To cite this article: Yaoping Xiao et al 2020 J. Semicond. 41 032304

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ARTICLES

Journal of Semiconductors
(2020) 41, 032304
doi: 10.1088/1674-4926/41/3/032304

35 km amplifier-less four-level pulse amplitude modulation
signals enabled by a 23 GHz ultrabroadband directly
modulated laser
Yaoping Xiao1, 2, Yu Liu1, 2, †, Yiming Zhang1, 2, Haotian Bao1, 2, and Ninghua Zhu1, 2
1State

Key Laboratory on Integrated Optoelectronics, Institution of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

2School

of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: The 4-level pulse amplitude modulation (PAM4) based on an 23 GHz ultrabroadband directly modulated laser (DML)
was proposed. We have experimentally demonstrated that based on intensity modulation and direct detection (IMDD) 56 Gbps
per wavelength PAM4 signals transferred over 35 km standard single mode fiber (SSMF) without any optical amplification and
we have achieved the bit error rate (BER) of the PAM4 transmission was under 2.9 × 10–4 by using feed forward equalization
(FFE).
Key words: directly modulated laser; PAM4; FFE; IMDD
Citation: Y P Xiao, Y Liu, Y M Zhang, H T Bao, and N H Zhu, 35 km amplifier-less four-level pulse amplitude modulation signals
enabled by a 23 GHz ultrabroadband directly modulated laser[J]. J. Semicond., 2020, 41(3), 032304. http://doi.org/10.1088/16744926/41/3/032304

1. Induction
As the significant development of data center, real-time
video, and online social networks, there are rapid growth of
the transmission capacity in the optical fiber communication
system[1]. To deal with the large capacity in the optical transmission 400G Ethernet (400 ; GbE) has been discussed for this
standardization[2]. One attractive candidate for this 400 GbE is
to adopt 56 Gbps/λ because such high bit rate can greatly reduce the number of lanes in optical transmission and it can
significantly reduce power consumption[3]. Currently, in order to improve the bit rate per wavelength, various modulated formats based on IMDD have been proposed to meet
such low power consumption, such as discrete multi-tone
(DMT)[4], non-return to zero (NRZ), orthogonal frequency division multiplexing (OFDM)[5], PAM[6−8] and carrier-less amplitude and phase (CAP)[9]. For DMT modulation format, the advantage of DMT is that it can use low cost optical devices.
However, the high complex digital signal processing (DSP)
will be required and the linearity requirement is more stringent than PAM[8]. The greatest advantage of NRZ modulation
format is simple modulation and high nonlinear tolerance,
whereas its signal bandwidth is large so it is limited by bandwidth of optical devices. So compared with above modulation formats, PAM4 is a better solution due to its half signal
bandwidth, flexible implementation and simple structure[10].
However, compared to NRZ modulation format, PAM4 is sensitive to the linearity of components both at receiver and
transmitter sides, so we should employ equalization schemes
to mitigate the impact of devices’ non-linearity. Because of
the low dispersion characteristic, the major challenge of band
Correspondence to: Y Liu, yliu@semi.ac.cn
Received 13 AUGUST 2019; Revised 26 SEPTEMBER 2019.
©2020 Chinese Institute of Electronics

transmission is no longer waveform distortion. The dispersion mainly comes from bandwidth limitation totally can be
compensated by FFE and employing (decision feedback equalization) DFE will not have significant improvement[11].
The data centers are extremely sensitive to power consumption, footprint and cost. In order to satisfy the requirements, we should consider the reliability and power consumption of devices. The power consumption of electro-absorption modulated laser (EML) is larger compared to the DMLs,
which will cause the larger power consumption in the data
centers. Besides, it is difficult for us to characterize the reliability of EMLs due to its increased complexity[12]. IEEE 802.3 Beyond 10 km Optical PHYs Study Group is in the developments
stage of 50, 100, 200, 400 Gbps Ethernet in which DML has
been considered as a suitable candidate due to its low cost,
high linearity, and simple configuration[13]. The performance
and reliability of DML play a significant role in our PAM transmission system, since the DML made in our laboratory owns
a low threshold current and high linearity so we can set the bias-current at a relative low level which will not introduce reliability and low extinction ratio (ER) problem[14]. Based on the
above features, we are able to adopt a lower complexity DSP.
In our previous work, we have experimentally demonstrated that PAM4 transmission system over 40 km SSMF with
the complex electrical equalization processing of DFE and
FFE, which will result power consumption in the practical application[15]. So in this paper, we further extend the investigation using a ultrabroadband DML module with 3 dB bandwidth up to 23 GHz and low threshold voltage operated at Oband. We experimentally demonstrated 56 Gb/s PAM4 transmission over 35 km SSMF only by employing a simple DSP of
different sizes of FFE to figure out the equalization requirements of limited bandwidth system without light amplification, optical chromatic dispersion (CD) compensation or other optical processing. This paper is organized as follows. We

2

Journal of Semiconductors

doi: 10.1088/1674-4926/41/3/032304

(a) 10
1310.19 nm

−10
50 dB

Normalized power (dB)

0

−20
−30
−40
−50
−60
−70

Fig. 1. (Color online) Assembly schematic of proposed DML.

first illustrate the performance of the DML module Finally, an
experiment of 56 Gbps with various transmission distance
based on this DML module has been carried on.

1304

3. Experiment setup and results
The experimental setup is shown in Fig. 3, which shows
the transmission of the single channel 56 Gbps PAM-4 signal
based on DML over 35 km SSMF. Four copies of pseudo-random bit sequences (PRBS) of length 215–1 was generated by

1308 1310 1312
Wavelength (nm)

1314

(b) −21

−33

2. Analysis of the performance of DML
S21

20 GHz

1316

20 mA
30 mA
40 mA
50 mA

20 GHz

−45

−57

−69
0

5

10

15 20 25 30
Frequency (GHz)

35

40

45

0

10

20

30

70

80

90

(c) 16

12
Power (mW)

In our laboratory, we have packaged a batch of DMLs
and the paper[15] selected a DML with higher threshold current compared this paper. As we all know the lower thresholdcurrent will not introduce serious low ER problems because
the DML is able to enter the linear region by using lower direct bias current. Fig. 1 shows the assemble picture of the proposed DML in which the chips operating in O-band were fabricated by nanoimprint lithography technology. The DML chips
was packed in a butterfly housing with high-frequency coaxial connector and seven pins. Moreover, the high-reflection
and anti-reflection films were coating on the front and rear
fronts to emit high power laser[15].
Fig. 2(a) shows the center wavelength of the DML
(1310.19 nm) is in the range of zero dispersion point, which is
able to greatly reduce the complexity of DSP. Besides, the
Fig. 2(a) also shows the side mode suppression rations
(SMSR) is able to reach at 50 dB, which it is advantageous for
DML to work in single longitudinal mode (SLM). As shown in
Fig. 2(b), the degradation reaches at 3-dB at 23 GHz. We can
reduce the degradation of bandwidth in the high frequency
range[16] by setting wiring bonding optimally. From the
Fig. 2(b), we can see that when the current was set at 20 mA,
the bandwidth of the DML is 20 GHz, but when the current is
larger than 30 mA, the bandwidth of this DML was almost
stable at 23 GHz, so during the experiment, considering the
optimal linearity and ER problem, the range of 30 to 40 mA
was adopted as optical driving current for the DML module.
Fig. 2(c) shows the P–I–V characteristics of this DML module,
it has a threshold current of 5 mA when the temperature was
set at 25 °C, besides, the lower threshold current allows the
DML to enter the linear region with lower direct current,
which plays a significant role in improving the ER. The maximum optical power can be extended to 15 mW when the biascurrent was set at 80 mA, and larger output power is advantageous for signal to noise ratio (SNR) of long PAM-4 distance
transmission.

1306

8

4

0

40 50 60
Current (mA)

Fig. 2. (Color online) (a) Measured optical spectrum of DML. (b) Frequency response of DML. (c) Measured P–I curve of DML.

arbitrary waveform generator (Keysight M8195A) operating at
65 GSa/s with 23 GHz analog bandwidth. In order to reduce
the inter-symbol interference (ISI), the roll-off coefficient of
root rising cosine was set at 0.35 by its attached software.
The signal was amplified by electric amplifier (EA), then this signal directly drives the DML. The output of DML was launched
into SSMF, and the received optical power (ROP) was controlled by a variable optical attenuator (VOA). Then the signal was detected by a P–I–N type photodiode with a transimpedance amplifier (TIA). The signal was sampled at 80 GSa/s
by a real-time digital sampling oscilloscope (Keysight
DSOZ634A) with 32 GHz analog bandwidth. The stored signal was processed by FFE with different tap numbers offline
compensating ISI induced by bandwidth limitation. It is
worth to note that the network has not adopt any optical amplification.

Y P Xiao et al.: 35 km amplifier-less four-level pulse amplitude modulation signals enabled ......

Journal of Semiconductors

doi: 10.1088/1674-4926/41/3/032304

3

Bias

AWG

EA

TIA
SSMF
VOA
DML

PD
Upsampling

AWG: Arbitary waveform generator
SSMF: Standard single mode fiber
VOA: Variable optical attenuator
PD: Photodetector
TIA: Transimpedance amplifier

Lass pass filtering
Downsampling
Real-time
scope

FFE
BER count

10−1

BTB
25 km
35 km

10−2

3.8 × 10−3

−2.8
35 km 56 Gbps

−2.9
−3.0

−3

10

2.4 × 10−4

lg (BER)

BER

Fig. 3. (Color online) Experimental setup of single wavelength PAM-4 signal transmission.

−3.1
−3.2
−3.3

10−4

−3.4
−5

10

−14

−12

−10
−8
−6
Received power (dBm)

−3.5

−4

Fig. 4. (Color online) BER performances versus ROP for different distance.

6

8 10 12 14 16 18 20 22 24 26 28 30
FFE tap number

Fig. 5. (Color online) BER versus FFE tap number.
With FFE
Without FFE

10−1

10−2
BER

We tested that the BER performances versus ROP for the
proposed scheme. The Fig. 4 shows the BER against the ROP
for BTB, 25 km, 35 km distance. From this figure, it can be
seen that the BER performance degrades as the ROP decreases and the back to back (BTB) PAM-4 transmission and
25 km PAM-4 transmission have the some change tendency.
From the Fig. 4 the BTB PAM4 transmission and 25 km PAM4
transmission almost have the same BER of different ROP and
this phenomenon can be explained by the fact that a small
amount of negative chirp in distributed feedback (DFB) laser
can reduce ISI[17]. Besides, when the ROP is –6 dBm, BER for
BTB transmission will be 0 after FFE equilibrium but BER for
25 km transmission system is to be 0 after FFE equilibrium
with the ROP is –4.2 dBm, so for the ROP greater than
–6 dBm of the BTB transmission and the ROP greater than
–4.2 dBm of the 25 km transmission have not been displayed
in the figure. Considering sensitivities of photodiode, the ROP
should be greater than –12 dBm and in order to achieve all
the 56 Gbps PAM-4 transmission cases. The BTB PAM-4 transmission has satisfied HD-FEC 3.8 × 10–3 KP4-FEC 2.4 × 10–4
with ROP greater than –9 and –7 dBm, the 25 km PAM-4 transmission meets HD-FEC 3.8 × 10–3 and KP4-FEC 2.4 × 10–4 with
the ROP greater than –9 and –8 dBm and the 35 km PAM-4
transmission meets HD-FEC 3.8 × 10–3 and KP4-FEC 2.4 × 10–4
with the ROP greater than –7.7 and –6 dBm, respectively.
The Fig. 5 plots BER curves versus tap number of 35 km
56 Gbps PAM4 transmission with ROP of –5.7 dBm. As expected, BER reduces with the FFE Tap number increase. From

4

3.8 × 10−3

10−3
2.4 × 10−4
10−4
−13 −12 −11 −10 −9 −8 −7
Received power (dBm)

−6

−5

Fig. 6. (Color online) BER performance with FFE and without FFE
versus ROP for 35 km.

this graph, we can learn that BER will stable at around 3.5 ×
10–4 with FFE tap coefficients reaches at 27, considering the
DSP complexity and BER performance, 27 taps were fixed during the off-line processing. Fig. 6 presents the BER against different ROP of PAM4 35 km transmission system with FFE and
without FFE, respectively. We can see that the BER of the system has greatly decreased greatly after FFE equilibrium, When
the ROP is lower, the role of FFE equilibrium is more obvious
but the effect of FFE equilibrium has degraded as the ROP decreased due to the sensitivity of the P–I–N photodiode. Fig. 7
shows that the eye diagram of different distance (BTB, 25 km,
35 km) of 56 Gbps. After the 25 and 35 km transmission, we

Y P Xiao et al.: 35 km amplifier-less four-level pulse amplitude modulation signals enabled ......

Journal of Semiconductors

2

1

1

0
−1

0

1

2

3

4
Time

5

6

BTB before FFE

0
−1
−2

7

Real amplitude

2

1

−2

0

1

2

3

4
Time

5

6

−2

7

25 km before FFE

0
−1

2

1

1

1

−1
−2

0

1

2

3

4
Time

5

6

7

BTB after FFE

Real amplitude

2

0

0
−1
−2

0

1

2

3

4
Time

5

6

25 km after FFE

0

1

2

3

4
Time

5

6

7

[2]
[3]

35 km before FFE

2
Real amplitude

Real amplitude

doi: 10.1088/1674-4926/41/3/032304

2
Real amplitude

Real amplitude

4

7

[4]

0
−1
−2

0

1

2

3

4
Time

5

6

7

35 km after FFE

Fig. 7. (Color online) The eye diagram performance for different distance.

are able to see the horizontal eye opening and vertical eye
opening becoming worse showing that the BER of the system is terrible. After the FFE Equilibrium we can see that the
horizontal eye opening and vertical eye opening have been improved and it also makes lower jitter time, which can obviously show that the FFE equilibrium have better equilibrium
effect and It effectively reduces the bit error rate.

4. Conclusion
In this paper, we have experimentally demonstrated that
the 56 Gbps single wavelength PAM-4 signal transmission
system transferred over 35 km SSMF only using the different
size of FFE equilibrium without any optical amplification. We
experimentally demonstrated that the BER of BTB, 25 km,
35 km PAM-4 transmission system is below the 7% FEC limit
of 3.8 × 10–3 with the receiver sensitivity of –8 dBm and is
below the KP4-FEC 2.4 × 10–4 with the receiver sensitivity of
–6 dBm. Our DMLs with high bandwidth play a significant
role in reducing ISI induced by device bandwidth, which significantly help to reduce the complexity of the algorithm. The
DML made in our laboratory with good linearity is suitable
for PAM-4 modulation due to high nonlinear tolerance of
PAM-4 modulation. Besides, our DML also owns high output
power and low threshold current playing an important role in
simplifying transmission system, which will be a low cost
choice in the practical application.

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]
[14]

[15]

Acknowledgments
This work was supported by National Key Research and Development Program of China (No. 2018YFB2201101) and the
National Natural Science Foundation of China (Nos. 61635001
and 61575186).

[16]

[17]

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Y P Xiao et al.: 35 km amplifier-less four-level pulse amplitude modulation signals enabled ......