A Radio-on-Hybrid WDM Transport System Based on
Mutually Injection-Locked FP LDs
Wen-I Lin, Hai-Han Lu*, Shah-Jye Tzeng, Ardhendu Sekhar Patra, and Wan- Lin Tsai
Institute of Electro-Optical Engineering, National Taipei University of Technology
*E-Mail: hhlu@ntut.edu.tw
Abstract: A radio-on-hybrid wavelength-division-multiplexing (WDM) transport system employing mutually
injection-locked Fabry-Perot laser diodes (F-P LDs) is proposed and demonstrated. System performances
evaluated by carrier to noise ratio (CNR), composite second order (CSO), composite triple beat (CTB), bit error
rate (BER) for simultaneous transmission of CATV/LAN/ITS are improved. Our proposed systems are
relatively simple and potentially low cost due to the use of mutually injection-locked F-P LDs as broadband
light source (BLS).
OCIS codes: (060, 2330), (060, 2360)
1. INTRODUCTION
Radio-on-fiber (ROF) has many advantages in the field of fiber optics and wireless radio communication systems. It
has provided a promising technique in broadband services such as local area network (LAN) and intelligent transport
system (ITS) due to its large bandwidth and low attenuation [1]. Hybrid wavelength-division-multiplexing (WDM)
transport systems are very useful in OC-48/OC-192, M-ary quadrature amplitude modulation (M-QAM), and CATV
[2]. ROF and hybrid WDM were linked together to build up a radio-on-hybrid WDM transport system which can
interoperate with broadband ROF distribution networks [3]. Such a system has multiple number of distributed
feedback laser diodes (DFB LDs) which are wavelength selected for each channel and controlled to operate at a
specific wavelength, this process will increase the cost and complexity of system. As the number of WDM channels
increase, both reducing the cost of light sources and controlling the wavelength of all channels are required.
Fabry-Perot (F-P) LDs light sources are proposed for cost-effective solution to reduce the installation and
management cost [4]. In this paper, a four-wavelength radio-on-hybrid WDM transport system based on mutually
injection-locked F-P LDs is proposed and demonstrated. Two wavelengths are employed for CATV channels
transmission, one wavelength is employed for 2.4GHz/LAN signal transmission, and one wavelength is employed for
5.8GHz/ITS signal transmission. The characteristic of this proposed system is the use of mutually injection-locked
F-P LDs [5], which are relatively simple and cost-effective compared with other demonstrated scheme. F-P LDs have
been used as broadband light source (BLS) with fifteen modes (1532–1544 nm) and with a flatness of <4 dB. System
performances were studied over an 80-km standard single-mode fiber (SMF) transport. Good performances of carrier
to noise ratio (CNR), composite second order (CSO), composite triple beat (CTB) for CATV signal; bit error rate
(BER) values for 2.4GHz/LAN and 5.8GHz/ITS signals were obtained in our proposed systems.
2. EXPERIMENTAL SETUP
The experimental configuration of our proposed radio-on-hybrid WDM transport systems based on mutually
injection-locked F-P LDs is shown in Fig. 1. The transmitting site comprises of one BLS, four optical band-pass filters
(BPFs), and four external modulators. The BLS is split into four optical channels using four optical BPFs with 3-dB
bandwidth of 0.4 nm. Four wavelengths of 1535.6 (λ1), 1537.2 (λ2), 1538.8 (λ3), 1540.4 (λ4) nm are filtered out
and fed into four separate external modulator. CATV signals (channels 2-40 and 41-78) generated from a multiple
signal generator are fed into two external modulators with wavelengths of λ1 and λ2. A data rate of 11 Mbps is mixed
with 2.4 GHz microwave carrier, and a data rate of 20 Mbps is mixed with 5.8 GHz microwave carrier to generate data
signals. The resulting data signals are supplied to two external modulators with wavelengths of λ3 and λ4. The all
modulated signals are coupled with an optical coupler and transmitted through two 40 km SMF spans with the help of
two erbium-doped fiber amplifiers (EDFAs) and two variable optical attenuators (VOAs). After transmission over a
fiber link of 80 km, the combined optical signal is passed through an optical tunable BPF to select the appropriate
wavelength and detected by a broadband photodiode (PD). The output of PD is applied to an HP-8591C CATV
analyzer, demodulator, and spectrum analyzer for system performance evaluation. CNR, CSO, and CTB CATV RF
parameters are measured using an HP-8591C CATV analyzer. 11 and 20 Mbps data signals are demodulated and fed
into a BER tester for BER analysis after demodulation.
The BLS is realized by the mutual injection between two F-P LDs with flatness and multiple longitudinal modes
output spectrum. The setup to generate BLS with flat and multiple longitudinal modes spectrum using two F-P LDs
associated with mutual injection is illustrated in Fig. 2. System consists of two F-P LDs, a 50:50 optical coupler, two
optical isolators, two polarization controllers (PCs), and a polarizing beam combiner (PBC). The F-P LDs are set for
1
continuous wave (CW) mode, with a threshold current of 10 mA. F-P LD1 (F-P LD2) injects light into the
injection-locked F-P LD2 (F-P LD1) via a 22 optical coupler. The output of each F-P LD is passed through an optical
isolator, with a-50 dB isolation. Randomly polarized BLS has been realized by polarization multiplexing with PCs and
PBC. The output spectra of F-P LDs are measured using an optical spectrum analyzer.
CATV (CH2 ~ CH40)
Optical
BPF
1535.6 nm
External
Modulator
λ1
CATV (CH41 ~ CH78)
Optical
BPF
Broadband
Light Source
1537.2 nm
External
Modulator
λ2
VOA
Optical
Splitter
VOA
EDFA-Ⅱ
EDFA-Ⅰ
Optical
BPF
1538.8 nm
External
Modulator
λ3
Optical
Coupler
40km SMF
40km SMF
2.4 GHz/11Mbps
Optical
BPF
1540.4 nm
External
Modulator
λ4
HP-8591C
5.8 GHz/20Mbps
PD
BER
Tester
Optical
Tunable BPF
Demodulator
Fig. 1. Experimental configuration of our proposed radio-on-hybrid WDM transport systems.
Isolator
PC
Isolator
PC
F-P LD1
50:50 Coupler
F-P LD2
PBC
Fig. 2. The setup to generate BLS with flat and multiple longitudinal modes spectrum.
3. EXPERIMENTAL RESULTS AND DISCUSSION
The frequency of the injection-locked laser is locked nearly to that of the injection source laser within the locking
range. The spectrum of free running F-P LD is pyramidal and unsuitable for using as multi-wavelength light source.
The central modes of F-P LD1 are injected into the side modes of F-P LD2, and the central modes of F-P LD2 are
injected into the side modes of F-P LD1, within the locking range. The output power level of the side modes are
increased largely at both sides, and the output power level of the central modes are increased limited; so a flat mutually
injection-locked spectrum can be obtained. A flat spectrum of 100 GHz spaced comb with fifteen modes, ranging from
1532 to 1544 nm and with a flatness of <4 dB is achieved, as shown in Fig. 3. After mutually injection-locked and
wavelength filtered out, F-P LD multiple longitudinal modes change into a similar single longitudinal mode
characteristic, and it can be used to transmit signal in radio-on-hybrid WDM architecture.
The measured CNR, CSO and CTB values for CATV channels over an 80-km SMF transport is given Fig. 4.
According to the Fig. 4, CNR >50 dB, CSO <-62 dBc and CTB <-61 dBc are obtained at the end of the radio-on-hybrid
WDM link, in which these values satisfy the fiber optical CATV system requirements (CNR >50 dB, CSO/CTB <-60
dBc). CNR value can be improved by higher EDFA input power level. CSO/CTB performance improvement results
from to the use of half-split-band and WDM techniques. The CSO and CTB performances can be further improved by
using dispersion compensation devices such as chirped fiber grating and photonic crystal fiber [6], [7].
2
5.0
dB/D
1532.00
1538.000nm
1.20nm/D
1544.00
Fig. 3. Flat spectrum due to mutually injection-locked F-P LDs after PBC. Fig. 4. The measured CNR, CSO and CTB values for CATV channels.
The measured BER curves as a function of the received optical power level for 2.42GHz/11Mbps and
5.82GHz/20Mbps data channel are presented in the Fig. 5. At a BER of 10-9, the received optical power levels are
-20.2 (λ3) and -16.5 (λ4) dBm, respectively. The sufficient low BER (<10-9) is leading to an improvement of receiver
sensitivity. Mutual injection locking will reduce the threshold current of F-P LD, thus, increase the optical output
power of F-P LD. The higher optical power is launched into the fiber, the higher signal-to-noise ratio (SNR) value is
obtained. SNR value increases will lead to better BER performance, and results in receiver sensitivity improvement.
Fig. 5. Measured BER curves as a function of the received optical power level.
4. Conclusion
We proposed and demonstrated a radio-on-hybrid WDM transport system based on mutually injection-locked F-P
LDs. Good performances of CNR/CSO/CTB for CATV band, as well as BER for 2.4GHz/LAN and 5.8GHz/ITS
microwave bands were obtained. Signal qualities meet the CATV/LAN/ITS demands; this proposed that such a
radio-on-hybrid WDM transport system has been successfully demonstrated which can inter-operate with CATV,
LAN and ITS applications.
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