Improvement of radio-on-multimode fiber systems based on light
injection and optoelectronic feedback techniques
Hai-Han Lu a,*, Guan-Lin Chen a, Yao-Wei Chuang
a, Chia-Chin Tsai b, Chien-Pen Chuang b
a Institute of Electro-Optical Engineering, National Taipei
University of Technology, 1, Section 3, Chung-Hsiao East
Road,
Taipei 10608, Taiwan, ROC b Institute of Industrial
Education, National Taiwan Normal University, Taiwan,
ROC
Received 10 April 2006; received in revised form 5 May 2006;
accepted 8 May 2006
Abstract
A radio-on-multimode fiber (MMF) system based on vertical-cavity
surface-emitting lasers (VCSELs) injection-locked and optoelec-
tronic feedback techniques is proposed and demonstrated. Injection
locking and optoelectronic feedback achieves large frequency
response of the VCSEL, resulting in good performances of
intermodulation distortion to carrier ratio (IMD/C), error vector
magnitude (EVM), and bit error rate (BER). Using VCSELs as optical
sources in radio-on-MMF systems are very attractive, as they are
relatively simple to fabricate and potentially low-cost. Such a
proposed radio-on-MMF system is suitable for the short-haul
microwave optical links. 2006 Elsevier B.V. All rights
reserved.
Keywords: Injection-locked; Optoelectronic feedback;
Radio-on-multimode fiber; Vertical-cavity surface-emitting
laser
1. Introduction
Recently, radio-on-fiber (ROF) transport systems, in which
micro-cells in a wide area connected by optical fibers and radio
signals transmitted over optical fiber links among central station
(CS) and base stations (BSs), have attracted much attentions. This
is because of the low loss and enor- mous bandwidth of the optical
fiber, the increasing demand for capacity, and the benefit it
offers in terms of low-cost deployment, all of which make it an
ideal candidate for real- izing ROF transport systems [1–4].
In previous studies, dis- tributed feedback (DFB) laser diode is
used as an optical source in ROF transport systems. In ROF
transport sys- tems, vertical-cavity surface-emitting laser (VCSEL)
can be used to replace DFB laser diode due to its
low-cost [5]. Owing to the limitation of the inherent
linearity character- istic, DFB laser diode has many advantages
over VCSEL.
For the comparison of ROF–DFB and ROF–VCSEL, it can be expected
that the performance of ROF–DFB is bet- ter than that of the
ROF–VCSEL. But, DFB laser diode is a high-cost optical source. For
a practical implementation of ROF transport systems, it is
necessary to develop low-cost systems. The feasibility of employing
injection-locked VCSELs in radio-on-multimode fiber (MMF) transport
systems was demonstrated previously [6]. However, sys- tems’
performance can be further improved by using opto- electronic
feedback technique [7–10]. In this paper, we proposed and
demonstrated a potentially low-cost radio- on-MMF system for IEEE
802.11a/b applications based on VCSELs injection-locked and
optoelectronic feedback techniques. Injection locking and
optoelectronic feedback achieves large frequency response of the
VCSEL. VCSEL with injection-locked exhibits an increment in
frequency response, and the optoelectronic feedback can further
enhance it [11]. Good performances of intermodulation
distortion to carrier ratio (IMD/C), error vector magnitude (EVM),
and bit error rate (BER) were obtained in our proposed radio-on-MMF
systems.
0030-4018/$ - see front matter 2006 Elsevier B.V. All
rights reserved.
doi:10.1016/j.optcom.2006.05.009
* Corresponding author. Tel.: +886 2 27712171x4621; fax: +886 2
87733216.
E-mail address:
[email protected] (H.-H. Lu).
www.elsevier.com/locate/optcom
2. Experimental setup
The experimental system configuration of our proposed radio-on-MMF
transport systems based on VCSELs injec- tion-locked and
optoelectronic feedback techniques is pres- ent in Fig. 1.
The solid line represents optical signal path, and the dash line
represents electrical signal one. The aim is to transmit microwave
signals from CS to BS over MMF transmission. 11-Mbps data stream
was initially mixed with 2.4 GHz microwave carrier to generate the
data signal (IEEE 802.11b), and 54-Mbps data stream was ini- tially
mixed with 5.8 GHz microwave carrier to generate the data signal
(IEEE 802.11a). The resulting microwave data signals were then
combined to directly modulate the VCSEL1. The combined data signals
are injected into the VCSEL2 via a 3-port polarization maintaining
optical cir- culator (OC). The VCSEL2, with a central wavelength
of 1590 nm, exhibits a light output of 7 dBm at a bias
current of 8 mA. The VCSEL1 is coupled into the port1 of OC, the
injection-locked VCSEL2 is coupled into the port 2 of OC, and the
port 3 of OC is separated off by a 1 · 2 optical split-
ter. This 3-port polarization maintaining OC is worth employing due
to excellent optical characteristics including low insertion loss
(0.8 dB) and high isolation (>40 dB). Such a high isolation
ability prevents reflected laser light from getting into the
VCSEL1. VCSEL1 and VCSEL2 have typically two possible
polarizations, however, VCSEL1 and VCSEL2 will have identical
polarizations after passing through the polarization maintaining
OC. Half of the laser output was used for feedback through an
optoelectronic feedback loop, and the other half of the laser
output was used for MMF transmission. In the optoelectronic
feedback loop, fiber span between OC and the pin photodiode1 (PD1)
is a MMF patchcord. The PD1 converts laser light into microwave
data signals to directly modulate the VCSEL2. As to the MMF
transmission part, fiber transmis- sion between OC and the PD2 is
MMF with different length
from 0.5 to 3.5 km. After the optical signal received by the PD2,
the output of the PD2 is separated off by a 1 · 2 RF
splitter, then applied to the spectrum analyzer and the RF tunable
band-pass filter (TBPF) to select the appropriate microwave signal.
Through the RF TBPF, the selected microwave signal is also
separated off by a 1 · 2 RF splitter, then applied to
the vector signal analyzer and the demodu- lator. The fundamental
signal and intermodulation distor- tion terms are investigated by
two-tone signal at 2.4 and 5.8 GHz. The IMD/C value is analyzed by
using a spectrum analyzer. The EVM value is measured by using a
vector sig- nal analyzer under various MMF lengths. 11 and 54 Mbps
data signals are demodulated and fed into a BER tester for BER
analysis after demodulation.
3. Experimental results and discussions
The frequency response of the VCSEL2 for free running, with 10
dBm light injection, as well as with 10 dBm light injection
and feedback is shown in Fig. 2. In the free running case,
the laser resonance frequency is around 2.8 GHz; with 10 dBm light
injection, the laser resonance frequency is increased to 5 GHz;
with 10 dBm light injec- tion and feedback, the laser resonance
frequency is increased up to 8.5 GHz. Injection locking achieves
about 1.8 times (5/2.8 1.8) enhancement in the laser resonance
frequency; the optoelectronic feedback further enhances the laser
resonance frequency up to 3 times (8.5/2.8 3).
The rate equations for laser diode with light injection and
optoelectronic feedback techniques are given by [12]
on
sn
G P þ k loop½ P ðt sÞ
P av ð1Þ
o P
2 aðG G siÞ
Fig. 1. Experimental system configuration of our proposed
radio-on-MMF transport systems based on VCSELs injection-locked and
optoelectronic feedback techniques.
where n is the carrier density, I is
the slave pumping cur- rent, V is the laser active
volume, sn is the carrier lifetime, G is the
gain, P is the photon
density, k loop is the feedback coefficient,
s is the delay of the feedback loop, P av
is the average photon density, sp is the photon
lifetime, sg is the cavity transit time, P i
is the external injection power, h is the phase
difference between salve and master lasers, df is the frequency
detuning, and a is the linewidth enhancement factor.
The slave laser relaxation oscillation damping rate
Cf can be derived from the above rate equations. The
opto- electronic feedback increases the stability of the laser
diode when Cf > C0 (damping rate as
laser diode only with light injection), resulting in out-of-phase
carrier re-injection. The laser resonance frequency f 0
can be stated in [13]
f 20 ¼ g 0 P
4p2sp
ð4Þ
where g 0 is the gain coefficient. Out-of-phase
carrier re- injection increase the photon density, in which leading
to an improvement of laser resonance frequency.
Electrical spectra of the received signals for free running, with
10 dBm light injection, as well as with 10 dBm light injection and
feedback are present in Fig. 3(a)–(c), respec- tively. In the
free running case, the IMD/C level is 54 dBc; with 10 dBm
light injection, the residue IMD/C level is 60 dBc; with 10
dBm light injection and feedback, the residue IMD/C level
of 72 dBc is obtained. Compared to the free running
case, 6 dB value improvement of IMD/C is obtained as light
injection technique is employed. However, a huge 18 dB IMD/C value
improvement is achieved as light injection and optoelectronic
feedback techniques are simultaneously employed. The use of light
injection and optoelectronic feedback techniques increases the
resonance frequency of VCSEL, letting system with lower IMD/C
value.
The EVM values at various MMF lengths for free run- ning, with
10 dBm light injection, as well as with 10 dBm light
injection and feedback are shown in Fig. 4(a) and (b),
respectively. EVM represents the depar- ture from a perfectly
modulated carrier, departure from ideal amplitude gives rise to a
proportional increase in
EVM. For the IEEE 802.11a standard (5.8 GHz/54 Mbps), the worst
case EVM should not exceed 5.6%; for the IEEE 802.11b standard (2.4
GHz/11 Mbps), the worst case EVM should not exceed 3.5%. For 2.4
GHz/11 Mbps data signal; in the free running case, the EVM value
can be satisfied at 1.5 km MMF length, with a fiber bandwidth of
3.6 GHz km (2.4 GHz · 1.5 km). With 10 dBm light
injection, the EVM value can be satisfied at 2 km MMF length, with
a fiber bandwidth of 4.8 GHz km (2.4 GHz · 2 km).
With
-30
-20
-10
0
10
Frequency (GHz)
R e s p
o n s e
( d B
)
free running
Fig. 2. The frequency response of the VCSEL2.
-100
-80
-60
-40
-20
0
Frequency (GHz)
R e c e
i v e d
R F P
o w e r
( d B
m )
54 dB
Frequency (GHz)
R e c e
i v e d
R F P
o w e r
( d B
m )
2.4 3.4 4.8 5.8
Frequency (GHz)
R e c e
i v e d
R F P
o w e r
( d B
m )
2.4 3.4 4.8 5.8
(a)
(b)
(c)
Fig. 3. (a) Electrical spectra of the received signal for free
running. (b) Electrical spectrum of the received signal with 10 dBm
light injection. (c) Electrical spectrum of the received signal
with10 dBm light injection and feedback.
10 dBm light injection and feedback, the EVM value can be satisfied
at 3.5 km MMF length, with a fiber bandwidth of 8.4 GHz km (2.4 GHz
· 3.5 km). Compared to the free running case, 4.8 GHz
km fiber bandwidth improvement is obtained as light injection and
optoelectronic feedback techniques are simultaneously employed. For
5.8 GHz/ 54 Mbps data signal; in the free running case, the EVM
value can be satisfied at 1.2 km MMF length, with a fiber bandwidth
of 6.96 GHz km (5.8 GHz · 1.2 km). With 10 dBm light
injection, the EVM value can be satisfied at 1.8 km MMF length,
with a fiber bandwidth of 10.44 GHz km (5.8 GHz ·
1.8 km). With 10 dBm light injection and feedback, the
EVM value can be satisfied at 3 km MMF length, with a fiber
bandwidth of 17.4 GHz km (5.8 GHz · 3 km). Compared to
the free running case, 10.44 GHz km fiber bandwidth improvement is
obtained as light injection and optoelectronic feedback techniques
are simultaneously employed.
The measured BER curves as a function of the received optical power
for free running, with 10 dBm light injec- tion, as well as
with 10 dBm light injection and feedback
are plotted in Fig. 5(a) and (b), respectively. For 2.4 GHz/
11 Mbps data signal at a BER of 109; in the free running case, the
received optical power is 21.4 dBm; with 10 dBm light
injection, the received optical power is 24.3 dBm; with 10 dBm
light injection and feedback, the received optical power is 30
dBm. Compared to the free running case, 8.6 dB of the received
optical power reduction is achieved when light injection and
optoelec- tronic feedback techniques are simultaneously employed.
For 5.8 GHz/54 Mbps data signal at a BER of 109; in the free
running case, the received optical power is 21 dBm; with10 dBm
light injection, the received optical power is23.8 dBm; with10 dBm
light injection and feed- back, the received optical power
is 29.6 dBm. Compared to the free running case, 8.6 dB of the
received optical power reduction is achieved when light injection
and optoelec- tronic feedback techniques are simultaneously
employed.
Most injection locking experiments involving data trans- mission
have the follower laser (VCSEL2) modulated. In this experiment,
VCSEL1 is modulated. It has been pre- viously predicted that in
such a configuration, there should
0
1
2
3
4
MMF Length (km)
E V M
MMF Length (km)
E V M
free running
-10 dBm injection and feedback
Fig. 4. (a) The EVM values at various MMF lengths (2.4 GHz/11
Mbps). (b) The EVM values at various MMF lengths (5.8 GHz/54
Mbps).
-35 -32 -29 -26 -23 -20 -17
Received Optical Power ( dBm )
10-5
10-6
10-7
10-8
10-9
10-10
10-11
Received Optical Power ( dBm )
10-5
10-6
10-7
10-8
10-9
10-10
10-11
(a)
(b)
Fig. 5. (a) The measured BER curves as a function of the received
optical power (2.4 GHz/11 Mbps). (b) The measured BER curves as a
function of the received optical power (5.8 GHz/54
Mbps).
be a significant attenuation of the data [14]. In this
experi- ment, with 10 dBm light injection technique only, the
modulation suppression is 30 dB. However, with 10 dBm light
injection and optoelectronic feedback techniques simultaneously,
the modulation suppression is decreased to 14 dB. Systems’
transmission performance affected by low modulation suppression
value is limited. Optoelec- tronic feedback technique causes
out-of-phase carrier re- injection, and thereby increases laser
resonance frequency. The lasers resonance frequency f 0
can be approximated by [15,16]
f 0 ffi 3G
1=2
ð5Þ
where q is the electron charge, I b is
the bias current, and I th is the threshold current. It
shows that f 0 increases with a decreased
threshold current. Optoelectronic feedback tech- nique increases
laser resonance frequency, leading to threshold current reduction,
finally resulting in higher opti- cal power launched into the
fiber. The higher optical power we get, the lower modulation
suppression we obtain. Opto- electronic feedback technique is
employed as a compensa- tion scheme to compensate for the
modulation suppression.
4. Conclusion
We proposed and demonstrated a potentially low-cost radio-on-MMF
system for IEEE 802.11a/b applications based on VCSELs
injection-locked and optoelectronic feedback techniques. Good
performances of IMD/C, EVM, and BER were obtained in our proposed
systems. Such a proposed radio-on-MMF system will benefit the
deployment of the short-haul microwave optical link.
Acknowledgment
The authors thank the financial support from the Na- tional Science
Council of the Republic of China under Grant NSC
94-2215-E-027-001.
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