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Optics Communications 266 (2006) 495–499 www.elsevier.com/locate/optcom
Improvement of radio-on-multimode fiber systems based on light injection and optoelectronic feedback techniques Hai-Han Lu a
a,*
, Guan-Lin Chen a, Yao-Wei Chuang a, Chia-Chin Tsai b, Chien-Pen Chuang b
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 optoelectronic 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 enormous 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 realizing ROF transport systems [1–4]. In previous studies, distributed feedback (DFB) laser diode is used as an optical source in ROF transport systems. In ROF transport systems, 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 characteristic, DFB laser diode has many advantages over VCSEL. *
Corresponding author. Tel.: +886 2 27712171x4621; fax: +886 2 87733216. E-mail address:
[email protected] (H.-H. Lu). 0030-4018/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.05.009
For the comparison of ROF–DFB and ROF–VCSEL, it can be expected that the performance of ROF–DFB is better 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, systems’ performance can be further improved by using optoelectronic feedback technique [7–10]. In this paper, we proposed and demonstrated a potentially low-cost radioon-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.
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2. Experimental setup The experimental system configuration of our proposed radio-on-MMF transport systems based on VCSELs injection-locked and optoelectronic feedback techniques is present 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 initially 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 circulator (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 splitter. 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 transmission 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 demodulator. The fundamental signal and intermodulation distortion 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 signal 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 injection 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 I n ¼ G P þ k loop ½P ðt sÞ P av ð1Þ ot eV sn oP 1 2 pffiffiffiffiffiffiffi ¼ G PP i cosðhÞ ð2Þ Pþ ot sp sg rffiffiffiffiffi oh 1 1 Pi ¼ df þ aðG Gsi Þ sinðhÞ ð3Þ ot 2 sg P
11 Mbps Signal Generator 2.4GHz
.
OC
1
VCSEL1 Signal Generator 5.8 GHz
. 2
.
VCSEL2
3
Optical Splitter
54 Mbps
PD1 MMF Patchcord
PD2 Optical Electrical
Spectrum Analyzer
RF Splitter
Vector Signal Analyzer
MMF RF Splitter RF TBPF
DeModulator
BER Tester
Fig. 1. Experimental system configuration of our proposed radio-on-MMF transport systems based on VCSELs injection-locked and optoelectronic feedback techniques.
H.-H. Lu et al. / Optics Communications 266 (2006) 495–499 10
Received RF Power (dBm)
0
Response (dB)
0
-10 free running -20
-10 dBm injection -10 dBm injection and feedback
-20
-40
54 dB -60
-80
-100
-30 0
2
4
6
8
10
3.4
4.8
5.8
Frequency (GHz) 0
ð4Þ
where g0 is the gain coefficient. Out-of-phase carrier reinjection 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), respectively. 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 running, 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 departure from a perfectly modulated carrier, departure from ideal amplitude gives rise to a proportional increase in
-20
-40 60 dB -60
-80
-100
2.4
(b)
3.4
4.8
5.8
Frequency (GHz) 0
Received RF Power (dBm)
where n is the carrier density, I is the slave pumping current, V is the laser active volume, sn is the carrier lifetime, G is the gain, P is the photon density, kloop is the feedback coefficient, s is the delay of the feedback loop, Pav is the average photon density, sp is the photon lifetime, sg is the cavity transit time, Pi 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 optoelectronic 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 f0 can be stated in [13]
Received RF Power (dBm)
Fig. 2. The frequency response of the VCSEL2.
g0 P 4p2 sp
2.4
(a)
Frequency (GHz)
f02 ¼
497
-20
-40 72 dB -60
-80
-100
(c)
2.4
3.4
4.8
5.8
Frequency (GHz)
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 with 10 dBm light injection and feedback.
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
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H.-H. Lu et al. / Optics Communications 266 (2006) 495–499 free running -10 dBm injection -10 dBm injection and feedback 4
EVM (%)
3
2
1
0 0
0.5
1
1.5
2
2.5
3
3.5
MMF Length (km)
(a)
free running -10 dBm injection -10 dBm injection and feedback 6 5
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 optoelectronic 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; with 10 dBm light injection, the received optical power is 23.8 dBm; with 10 dBm light injection and feedback, 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 optoelectronic feedback techniques are simultaneously employed. Most injection locking experiments involving data transmission have the follower laser (VCSEL2) modulated. In this experiment, VCSEL1 is modulated. It has been previously predicted that in such a configuration, there should free running
EVM (%)
4
-10 dBm injection -10 dBm injection and feedback
3 10
2
10-6
1
0
0.5
1
1.5
2
2.5
3
BER
10-7
0
(b)
-5
10-9
MMF Length (km)
10-10
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).
10-11 -35
-32
(a)
-29
-26
-23
-20
-17
-20
-17
Received Optical Power ( dBm ) free running -10 dBm injection -10 dBm injection and feedback 10 -5 10 -6 10 -7
BER
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 injection, as well as with 10 dBm light injection and feedback
10-8
10 -8 10 -9 10 -10 10 -11 -35
(b)
-32
-29
-26
-23
Received Optical Power ( dBm )
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).
H.-H. Lu et al. / Optics Communications 266 (2006) 495–499
be a significant attenuation of the data [14]. In this experiment, 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. Optoelectronic feedback technique causes out-of-phase carrier reinjection, and thereby increases laser resonance frequency. The lasers resonance frequency f0 can be approximated by [15,16] 1=2 3G f0 ffi ðI b I th Þ ð5Þ 4p2 q where q is the electron charge, Ib is the bias current, and Ith is the threshold current. It shows that f0 increases with a decreased threshold current. Optoelectronic feedback technique increases laser resonance frequency, leading to threshold current reduction, finally resulting in higher optical power launched into the fiber. The higher optical power we get, the lower modulation suppression we obtain. Optoelectronic feedback technique is employed as a compensation 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.
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Acknowledgment The authors thank the financial support from the National Science Council of the Republic of China under Grant NSC 94-2215-E-027-001. References [1] B. Masella, X. Zhang, IEEE Photon. Technol. Lett. 18 (2006) 301. [2] M.G. Larrode, A.M.J. Koonen, J.J.V. Olmos, A. Ng’Oma, IEEE Photon. Technol. Lett. 18 (2006) 241. [3] M. Bakaul, A. Nirmalathas, C. Lim, D. Novak, R. Waterhouse, IEEE Photon. Technol. Lett. 17 (2005) 2718. [4] J. Capmany, B. Ortega, A. Martinez, D. Pastor, M. Popov, P.Y. Fonjallaz, IEEE Photon. Technol. Lett. 17 (2005) 471. [5] L. Chrostowski, X. Zhao, C.J. Chang-Hasnain, R. Shau, M. Ortsiefer, M.C. Amann, IEEE Photon. Technol. Lett. 18 (2006) 367. [6] H.H. Lu, P.C. Lai, W.S. Tsai, IEEE Photon. Technol. Lett. 16 (2004) 1215. [7] P. Devgan, D. Serkland, G. Keeler, K. Geib, P. Kumar, IEEE Photon. Technol. Lett. 18 (2006) 685. [8] F.Y. Lin, J.M. Liu, IEEE J. Quantum Electron. 39 (2003) 562. [9] F.Y. Lin, J.M. Liu, Opt. Commun. 221 (2003) 173. [10] S. Rajesh, V.M. Nandakumaran, Opt. Commun. 213 (2006) 113. [11] M. Attygalle, Y.J. Wen, IEEE Photon. Technol. Lett. 18 (2006) 478. [12] P. Saboureau, J.P. Foing, P. Schanne, IEEE J. Quantum Electron. 33 (1997) 1582. [13] W.I. WayBroadband Hybrid Fiber/coax Access System Technologies, vol. 4, Academic Press, San Diego, 1998, p. 122. [14] E.K. Lau, M.C. Wu, in: IEEE International Topical Meeting Microwave Photonics MC-29, 2004, p. 142. [15] E. Goutain, J.C. Renaud, M. Krakowski, D. Rondi, R. Blondeau, D. Decoster, Electron. Lett. 32 (1996) 896. [16] S. Lindgren, H. Ahlfeldt, L. Backlin, L. Forssen, C. Vieider, H. Elderstig, M. Svensson, L. Granlund, L. Andersson, B. Kerzar, B. Broberg, O. Kjebon, R. Schatz, E. Forzelius, S. Nilsson, IEEE Photon. Technol. Lett. 9 (1997) 306.