Optics and Laser Technology: Wenjing Xu, Xinlu Gao, Mingyang Zhao, Mutong Xie, Shanguo Huang
Optics and Laser Technology: Wenjing Xu, Xinlu Gao, Mingyang Zhao, Mutong Xie, Shanguo Huang
Optics and Laser Technology: Wenjing Xu, Xinlu Gao, Mingyang Zhao, Mutong Xie, Shanguo Huang
a r t i c l e i n f o a b s t r a c t
Article history: A full duplex radio over fiber system with frequency quadrupled millimeter-wave signal generation
Received 3 September 2017 based on polarization multiplexing using a single-drive intensity modulator is proposed and experimen-
Received in revised form 30 November 2017 tally demonstrated. At the central station, downstream signal modulated on the single-drive intensity
Accepted 9 January 2018
modulator is polarization multiplexed with pure optical carrier by a polarization beam combiner (PBC)
before transmitted over the single mode fiber to the base station. The single-drive intensity modulator
is biased at the maximum transmission point to generate optical carrier and two second-order sidebands.
Keywords:
At the base station, by simply adjusting the difference angle between the principle axis of polarizer and
Microwave photonics
Millimeter-wave signal generation
one principle axis of PBC to 135°, a frequency quadrupled millimeter-wave signal is generated. In addi-
Double sideband modulation tion, when the difference angle between the principle axis of polarizer and one principle axis of PBC is
adjusted to 90°, the original pure optical carrier is recovered, which is wavelength reused to provide light
source for the uplink to deliver upstream signal. A proof-of-concept experiment is performed. Pure opti-
cal carrier and 40 GHz millimeter-wave signal with 20 dB optical harmonic suppression ratio are
obtained. The power penalties of the bidirectional links are less than 0.3 dB after transmitted over
10.5 km single mode fiber. The measured power fluctuation of the generated millimeter-wave signal is
less than 1 dB in one hour, showing the proposed scheme is relatively stable for long-distance transmis-
sion system.
Ó 2018 Elsevier Ltd. All rights reserved.
https://doi.org/10.1016/j.optlastec.2018.01.035
0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.
268 W. Xu et al. / Optics and Laser Technology 103 (2018) 267–271
direction easily [16], which simplify the system greatly with no 2. Operation principle
optical filter. However, the frequency multiplication factor of the
existed systems is only 1 [17–18], i.e., when 10 GHz RF signal is The conceptual configuration of the full duplex RoF system is
applied at the transmitter, the generated RF signal of the receiver shown in Fig. 1. The laser diode (LD) is split into two orthogonal
is 10 GHz, which cannot make full use of the large bandwidth polarization directions (i.e., x-pol and y-pol) by a polarization
mm-wave signal. Therefore, how to increase the frequency multi- beam splitter (PBS). A polarization controller (PC1) set before PBS
plication factor while not increase the system complexity is still is used to adjust the power splitting ratio c (0 < c < 1) of the x-
a problem. pol and y-pol directions flexibly. The x-polarized optical carrier is
In this paper, a full duplex RoF system with frequency quadru- fed into a MZM driven by RF signal carrying downlink baseband
pled mm-wave signal generation based on polarization multiplex- signal Data1, and the MZM is biased at the maximum transmission
ing using a single-drive intensity modulator is experimentally point to suppress the odd-order sidebands. While the y-polarized
demonstrated. At the CS, the continuous wave is split into two optical carrier is keep unmodulated. Considering the characteris-
orthogonal polarization directions (i.e., x and y). The x-polarized tics of the Bessel function of the first kind, the optical field at the
optical carrier is sent into a single-drive intensity modulator driven output of MZM can be mathematically expressed as:
by the downstream signal, while the y-polarized optical carrier is
Ex ¼ c½J 0 ðbÞ expðjxo tÞ þ J 2 ðbÞ exp jðxo þ 2xm Þt þ J 2 ðbÞ
keep unmodulated. The single-drive intensity modulator is biased
at the maximum transmission point to generate the optical carrier exp jðxo 2xm Þt ð1Þ
and two second-order sidebands. Then the two orthogonally polar-
where xo and xm are the angular frequency of the optical and elec-
ized signals are combined by a PBC. At the BS, by aligning the prin-
trical field, respectively. J n ðÞ is the n-th order Bessel function of the
ciple axis of a polarizer at an angle of 135° to one principle axis of
first kind, b is the modulation index of the intensity modulator. And
the PBC, the optical carriers with two orthogonal polarization
the unmodulated y-polarized optical carrier can be written as:
directions are canceled out and the frequency quadrupled mm-
wave signal is generated after optical/electrical conversion. On Ey ¼ ð1 cÞ expðjxo tÞ ð2Þ
the other hand, when the other polarizer with its principle axis
Then the two orthogonally polarized signals are polarization
aligned at an angle of 90° to one principle axis of the PBC is
multiplexed by a PBC. If a polarizer with its principle axis aligned
employed, pure optical carrier is recovered, which is then wave-
at an angle of h to one principle axis of the PBC is applied to incor-
length reused to provide light source for the uplink. Experimental
porate Ex and Ey , the optical field at the output of the polarizer can
results show that after transmitted over 10.5 km single mode fiber,
be shown as:
the power penalty of bidirectional 1 Gb/s pseudo random binary
sequence is less than 0.3 dB. Compared with the published RoF Epol ðtÞ ¼ cos hEx þ sin hEy
studies, as no wavelength-dependent devices are required and a
¼ ½c cos hJ 0 ðbÞ þ ð1 cÞ sin h exp jxo t þ c cos hJ 2 ðbÞ
frequency quadrupled 40 GHz mm-wave signal is generated in
the proposed scheme, the system is potentially wideband and ½exp jðxo þ 2xm Þt þ exp jðxo 2xm Þt ð3Þ
cost-effective.
Fig. 1. The schematic diagram of the proposed method and the optical spectra at the corresponding point: (A) the output of MZM1 along x-pol direction, (B) the y-polarized
optical carrier along y-pol direction, (C) the combined signal along h direction, (D) the output after the PL1, (E) the output after the PL2. CS: center station, LD: laser diode, PC:
polarization controller, PBS: polarization beam splitter, MZM: Mach-Zehnder modulator, PBC: polarization beam combiner, SMF: single mode fiber, OC: optical coupler, PL:
polarizer, BS: base station.
W. Xu et al. / Optics and Laser Technology 103 (2018) 267–271 269
As can be seen from Eq. (3), when c cos hJ 0 ðbÞ þ ð1 cÞ sin h ¼ 0, 3. Experimental setup and results
i.e., h ¼ arctan c1r
J0 ðbÞ
, we have
A proof-of-concept experiment setup is configured and shown
cð1 cÞ in Fig. 1. At the CS, a continuous wave signal centered at
Epol ðtÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi J 2 ðbÞ½exp jðxo þ 2xm Þt
1546.32 nm is emitted from laser diode (Southern Photonics
ð1 cÞ2 þ c2 J 20 ðbÞ
TLS150D) before sent into MZM1 (Optilab) driven by 10 GHz RF
þ exp jðxo 2xm Þt ð4Þ signal carrying 1 Gb/s pseudo random binary sequence (PRBS)
downlink Data1 with a word length of 212 1. The MZM1 has a
The photocurrent of the generated mm-wave signal can be
bandwidth of 40 GHz and half wave voltage of 5 V. The 10 GHz
expressed as:
RF signal is generated by the Vector Signal Generator (Agilent
c2 ð1 cÞ2 J22 ðbÞ E3640A). Data1 and Data2 are generated by the Arbitrary Wave-
IðtÞ ¼ REpol ðtÞ Epol ðtÞ ¼ 2R cosð4xm tÞ ð5Þ form Generator (Tektronix AWG70000). The PBS and PBC have a
ð1 cÞ2 þ c2 J 20 ðbÞ
minimum directivity of 50 dB and polarization extinction ratio of
where R is the responsivity of the photodiode. And it is clearly about 20 dB. The output at the optical field of the MZM1 is shown
seen that a frequency quadrupled RF signal is generated. On the in Fig. 2(a), and it can be seen that optical carrier and two second-
other hand, when cos h ¼ 0, i.e., h ¼ p=2, Epol ðtÞ becomes: order sidebands are generated. Fig. 2(b) shows the pure optical
carrier along y-pol direction. Then the two optical signals are com-
Epol ðtÞ ¼ Ey ¼ ð1 cÞ exp jxo t ð6Þ
bined by a PBC before transmitted over 10.5 km standard single
It can be seen that the y-polarized optical carrier is ideally mode fiber (SSMF) with a dispersion value of 17 ps/nm/km. At
recovered, and it is then wavelength reused to provide lightwave the BS, a 3 dB optical coupler (OC) is employed to divide the optical
source for the uplink. signal into two same branches. For the upper branch, PL1 with its
principle axis aligned by PC2 at an angle of 135° to one principle 10.5 km SSMF transmission distance cases. For the downlink, 0.3
axis of PBC is employed to suppress the optical carrier shown in dB power penalty is achieved due to the unideal performance of
Fig. 2(c), which shows that the wavelength spacing of the two fiber and insignificant phase fluctuation. For the uplink, only 0.1
second-order sidebands is 0.32 nm (40 GHz), and the optical car- dB power penalty is achieved, which shows that the interference
rier is 20 dB lower than the two second-order sidebands. Then from downlink is negligible and pure optical carrier is recovered
the two second-order sidebands are beaten at a PD which has a to provide light source for the uplink. The eye diagrams of down-
responsivity and 3-dB bandwidth of 0.85 A/W @1550 nm and 40 link and uplink signal at BTB cases keep wide open, which show
GHz, respectively. A 40 GHz LO signal (Agilent E8257D) is applied a good transmission performance.
to down-convert the generated 40 GHz electrical signal to base- In order to explore the nonideal factors that contribute to 20 dB
band to measure the eye diagram and bit error rate (BER) of the optical harmonic suppression ratio (OHDSR) in our system, we
downlink signal. For the lower branch, PL2 with its principle axis simulated the OHDSR versus power unbalance and phase differ-
aligned by PC3 at an angle of 90° to one principle axis of PBC is ence of the optical carrier along the two orthogonal polarization
used to recover the pure optical carrier shown in Fig. 2(d). Com- directions shown in Fig. 4. It can be seen that small power unbal-
pared with the optical carrier in Fig. 2(b), it can be observed that ance or phase difference may cause a relatively large OHDSR degra-
small phase noise is existed, but it is too small to be considered. dation, and when the power unbalance is larger than 1 dB or the
Then it is remodulated by the uplink signal Data2 via MZM2 before phase difference is larger than 10°, the OHDSR is about zero, so
transmitted back to the CS through another 10.5 km SSMF. In addi- the power and phase of the optical carrier in the two orthogonal
tion, in order to ensure that the time delays along the two orthog- polarization directions should be equal as far as possible. In addi-
onal polarization directions are identical, an extra 2 m tion, the system may suffer from the polarization drift accumu-
polarization-maintaining fiber is employed along y-pol direction lated continuously over time. In the experiment, we also
at the CS. investigated the stability of the proposed RoF link by measuring
Fig. 3 shows the bit error rate (BER) versus received optical the power fluctuation of the generated mm-wave signal shown
power of the downlink and uplink at the back to back (BTB) and in Fig. 5 in a time period of one hour, it can be seen that only small
Fig. 3. BER values versus received optical power for downlink and uplink.
Fig. 4. The simulated OHDSR degradation due to (a) power unbalance, (b) phase difference.
W. Xu et al. / Optics and Laser Technology 103 (2018) 267–271 271
References