Far L-band Single Channel High Speed Downstream Transmission

using Injection-locked Quantum-dash Laser for WDM-PON

M. Talal A. Khan

, E. Alkhazraji

, A. Ragheb

, H. Fathallah

2,3

and M. Z. M. Khan

1,*

Optoelectronics Research Lab, Electrical Engineering Department, King Fahd University of Petroleum and Minerals,

Dhahran-31261, Saudi Arabia

KACST-TIC in Radio Frequency and Photonics for the e-Society (RFTONICS), Riyadh, Saudi Arabia

Department of Electrical Engineering, King Saud University, Riyadh, Saudi Arabia

Keywords: Quantum Dash Laser Diode, Passive Optical Networks, Wavelength-division Multiplexing, Optical

Communication, External Modulation, Injection-locking.

Abstract: We demonstrate an externally modulated single channel 64 Gbit/s DP-QPSK transmission based on injection-

locked Fabry-Pérot broadband quantum-dash laser at far L-band ~1621 nm wavelength. A receiver sensitivity

of -16.7 dBm has been observed after 10 km SMF transmission, with power penalty of ~2 dB, under the FEC

threshold. We also propose that these novel quantum-dash laser diode could be a route towards next generation

100Gbit-PONs as a unified upstream and downstream transmitters.

1 INTRODUCTION

Over the next few years, mobile data traffic will

increase nearly eight times globally compared to the

scenario in 2015 (

Cisco 2016). The relentless

continuous growth at the user end demands a

development of higher bandwidth and capacity

applications. This will put the transmission capacity

limitations in existing wavelength division

multiplexing (WDM) based optical access networks

in near future. To encounter the demand of broad-

band services and high transmission rate per channel,

next generation passive optical networks (NG-PON)

like 10Gbit-PONs, 100Gbit-PONs, and 400Gbit-

PONs, have been considered as a promising solution.

In addition, looking down the road, the existing C–

band communication window would be exhausted

and hence calls for exploring far L– and U– bands for

upstreaming and down streaming in NG-PON

technologies, for cost effective and flexible optical

systems.

Recently, an injection-locking scheme has been

found to be a potential candidate for realizing next

generation colorless WDM-PONs. A single channel

Fabry-Perot laser diode (FP-LD) (

Nguyen et al., 2010)

has been demonstrated as a sustainable transmitter in

multi-Gbit/s WDM-PON, however, the directly

modulated transmission data rate is found to be

limited to 4 Gb/s over 25 km single mode fiber (SMF)

(

Chi et al., 2012). On other hand, an externally

modulated FP-LD laser achieved a transmission rate

of 42.5 Gb/s/channel over 100 km SMF (

Silva et al.,

2010

). Moreover, a directly modulated semiconductor

optical amplifier (SOA) (

Gay et al., 2014) has also been

proposed by employing a comb-generator with a

transmission capacity of 28 Gb/s over 100 km SMF

for colorless PON. A new study has been proposed on

injection-locked directly modulated weak resonant

cavity FP-LD (WRC-FPLD) transmitting data at 25

Gb/s over 28 channels in near L-band (1582 nm)

(

Cheng et al., 2014) which is capable of ~13 nm L–

band wavelength tunability via low power optical

injection. WRC-FPLD would require a wideband

spectrum in order to support multiple channels at the

optical line terminal (OLT) and reasonable coherence

for modulation bandwidth at the same time. However,

these schemes limit the expansion of number of users

in the optical systems because of typical narrow band

nature of the FP-LDs (

Xu et al., 2012). Hence, a

broadband laser diode (BLD) (

Lee et al., 2010), with

several longitudinal modes in stimulated emission

could be a possible candidate, to meet up with the

future requirements. Moreover, such source also

enable large scale production and deployment of

Khan M., Alkhazraji E., Ragheb A., Fathallah H. and Khan M.

Far L-band Single Channel High Speed Downstream Transmission using Injection-locked Quantum-dash Laser for WDM-PON.

DOI: 10.5220/0006156100900094

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 90-94

ISBN: 978-989-758-223-3

transceivers in optical network units (ONU) and

optical line terminals (OLT). Besides, most of the

aforementioned work is reported on direct

modulation scheme which shows trade-off between

the limited bandwidth and simple device structure

cost-effective PON

In this paper, we report an externally modulated

64 Gb/s single channel dual polarization quadrature

phase shift keying (DP-QPSK) down transmission in

the far L-band to each user based on injection-locked

broadband quantum-dash (Qdash) laser as a potential

WDM transmitter. This type of Qdash laser is capable

of broadband stimulated emission, hence which can

deploy as a transmitter in WDM-PONs. The external

seeding source is placed in the OLT and could be

shared among all the users for down streaming.

Moreover, we also propose that the scheme could be

used for up-streaming at ONUs thus able to realize

next generation 100Gbit-PONs.

2 RESULTS AND DISCUSSION

The quantum-dash laser diode used in this experiment

was grown on n-type InP. The active region is

composed of a four stack InAs/InGaAlAs Qdash-in-

a-well structure where the layers are separated by

barrier layers of thickness values of 10 nm, 15 nm,

and 20 nm. This intentional chirping the active region

increases the inhomogeneous broadening and hence

provide a broad gain profile and hence ultra-

broadband lasing emission. More details of the

growth and characterization of Qdash laser could be

found in reference (

Khan et al., 2014

Figure 1: Free-running lasing spectra of the Qdash laser.

Insets show L-I characteristic of a free running Qdash laser,

measured at the SMF end the bare device and bare quantum

dash laser.

Figure 2: An injection-locked single FP mode of the

broadband Qdash laser.

In this experiment, a bare 4  800 μm



ridge-

waveguide Qdash laser diode is injected with a DC

current of 0.24 A. The laser device was mounted on a

brass base with p side up configuration. Two probes

are then used where the negative one is connected to

the brass base while the positive probe is placed over

a gold plated clamp with the p side of the laser to

safely probe the current into the laser diode. The total

coupled power from the single laser facet into a

lensed SMF is ~0 dBm, at a fixed temperature of 14

ºC and bias current of 0.24 A. Free running lasing

spectrum of Qdash laser along with fiber end L-I

characteristics under CW operation is shown in Fig. 1

with ~10 nm lasing bandwidth before roll off. The

central lasing wavelength (calculated at the full width

at half maximum) is located at ~1624 nm. The inset

of fig.1 shows the lensed fiber power coupled bare

Qdash laser. This degradation in the laser

performance (both optical power and lasing

bandwidth) under CW is attributed to the non-

optimized growth of the device active region. An

optimized design will enable realization of ultra-

broadband lasing bandwidth of >50 nm and power >

100 mW (

Khan et al., 2013

). When the wavelength of

the tunable master laser source was tuned to one of

longitudinal mode of the Qdash laser while keeping a

fixed external CW injection of 5 dBm, injection-

locking occurs, as shown in setup of Fig. 2. The

output spectra of the injection-locked Qdash laser is

observed at the 2% output of a 2/98 coupler (CP)

using optical spectrum analyzer. The measured single

mode output power and side-mode suppression ratio

(SMSR) are -2.5 dBm and 37 dB, respectively. Next,

the tunability experiment of the injection locked

Qdash laser is performed by fixing the external CW

injection power at 5 dBm and varying the wavelength

Far L-band Single Channel High Speed Downstream Transmission using Injection-locked Quantum-dash Laser for WDM-PON

to lock at different modes. Fig. 3 shows the

measurement results; by proper tuning in the

wavelength range of 1617.5-1628.9 nm with ~1.31

nm tuning steps, tunability of ~12 nm was achieved.

Moreover, short-term stability of injection-locked FP

mode was also conducted and found to be stable

throughout the observation time of 24 minutes. The

injection locked mode was at 1619.68 nm with -5.74

dBm output power. The power fluctuation and

wavelength variation of injection-locked Qdash laser

are 0.56 dB and zero respectively, as shown in fig. 4.

The laser source has performed well in terms of

output power stability and has shown wide tuning

range.

Figure 3: Output spectra of the injection-locked modes at

different wavelengths in the tuning range of 1617.5-1628.9

nm with ~ 1.31 nm tuning steps, and at a fixed CW external

injection of 5 dBm.

Figure 4: Stability characterization in terms of wavelength

variation and output power fluctuation. Initial lasing

wavelength is 1619.68 nm with -5.74 dBm output power.

The down transmission experimental setup is

presented in Fig 5. The setup consisted of Qdash laser

diode, TLS and an external dual polarization IQ (DP-

IQ) modulator. The CW external power of TLS is

fixed at 5 dBm and was injected into port 1 of the

optical circulator (OC) through an isolator. The slave

Qdash laser, on the other hand, was injected into port

2 using a polarization controller (PC), while the

output was taken out at port 3 of the optical circulator.

Isolator and PC are used to protect the TLS and

improve the IL efficiency, respectively. Then,

injection-locked single channel is fed to a DP-IQ

modulator for DP-QPSK modulation up to 16 Gbaud

data rate. Pre-processing of the signal were performed

using MATLAB. A pseudo random binary sequence

(PRBS) with a length of 2



1is mapped into 4-

QPSK constellation and an 8-bit resolution arbitrary

wave generator (M8195A) with a sampling rate of 65

GSa/s generates four channels multi-level signal, two

channel for individual polarization

The output of four

signals are driven into a quad channel amplifier

having a 3-dB bandwidth of 32 GHz which can

provide 20-dB gain for each channel simultaneously.

Then, these signal are driven into the four RF inputs

of DP-IQ modulator. For signal analysis and

detection, we used Keysight optical modulation

analyzer (OMA N4391A).

Figure 5: Experimental setup of 64 Gb/s DP-QPSK WDM

transmission via injection-locked Qdash laser.

The signal transmission is characterized first in a

back to-back (BtB) configuration and then over a 10

km SMF at 8 and 16 Gbaud transmission which

corresponds to 32 and 64 Gb/s. Fig. 6 presents the

variation of bite error rate (BER) with the received

signal sensitivity. And the insets show the QPSK

constellation diagram for 64 Gb/s after BtB and 10

km transmission. To achieve 3.8  10



FEC

threshold, received power of -18.7 (-19.7) dBm are

required for 64 (32) Gb/s in BtB case. On the other

hand, a receiver sensitivity of -18.4 and -16.7 dBm is

observed for 32 and 64 Gbit/s respectively, after 10

km SMF transmission, with power penalties of ~1.3

dB and ~2 dB respectively, under the FEC threshold.

This might be loss incurred by the fiber due to

extreme far L-band wavelength operation. In

addition, any injection locked mode power variation,

results from the bare Qdash laser and coupled into the

lensed SMF, could also lead to receiver sensitivity

fluctuations. Fig. 7 show the eye diagrams for 64

Gbit/s at -14.4 dBm and -12.5 dBm for 10 km and

BTB transmissions, respectively. A clear open eye

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

with no eye compression further affirms the potential

of Qdash laser as a unified transmitter in WDM-

PONs. In Table I, we summarize our experimental

results in contrast to the recent literature for a single

carrier L-band transmission, to the best of our

knowledge. Hence, deployment of these Qdash laser

diode based transmitters in OLT for down streaming

as well as in ONU for 64 Gb/s upstreaming would

assist a path towards next generation NG-PON.

Figure 6: BER versus received optical power at 32 and 64

Gbit/s under BtB and after 10 km SMF transmission. Insets

show the constellation diagrams for BtB and over 10 km for

64 Gb/s transmission.

(a)

(b)

Figure 7: Eye diagrams for 64 Gbit/s data rate for (a) BtB

and, (b) over 10 km transmission.

Table 1: Comparison of a data transmission rate by a single-

carrier, in L-band, available in literature.

Wavelength

(nm)

Modulation

scheme

Data rate

(Gbit/s)

Type

1583

16-QAM 12 D

1583

16-QAM 20 D

1592

̶ 1.25 D

1595

̶ 1.25 D

1621

DP-QPSK 64 E

Note: ü (Li et al., 2013), ő (Cheng et al., 2014), đ

(Komljenovic et al., 2014), ȶ (Lee et al., 2012), ž:

(This paper), D: Direct modulation, E: External

modulation.

3 CONCLUSION

We have proposed an injection-locked broadband

quantum-dash laser as a potential WDM transmitter

for next generation passive optical networks (NG-

PON). An externally modulated single channel has

been transmitted at 64 Gbit/s using dual polarization

modulator (DP-IQM). Experimental results showed a

receiver sensitivity of -16.7 dBm under BER FEC

limit after 10 km SMF transmission.

ACKNOWLEDGEMENT

This work was funded and supported, in part, by King

Fahd University of Petroleum and Minerals through

SR141002 and KAUST004 grants, and KACST-TIC

in Radio Frequency and Photonics for the e-Society

(RFTONICS).

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PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology