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A full-duplex lightwave transmission system employing innovative VCSEL-based PM-to-IM converters to deliver intensity-modulated CATV/phase-modulated RoF/intensity-remodulated RoF signals over two 40-km SMFs links is proposed and demonstrated. To be the first one of employing VCSEL-based PM-to-IM converters in full-duplex lightwave transmission systems, the downstream light is successfully intensity-remodulated with RoF signal for up-link transmission. Good performances of CNR/CSO/CTB are achieved for downstream CATV signal transmission, and low BER values are obtained for both downstream and upstream RoF signals transmissions. Our proposed systems present brilliant performances in delivering hybrid CATV and RoF signals. Such a full-duplex lightwave transmission system would be very attractive for fiber trunk applications to provide broadband integrated services.
© 2014 Optical Society of America
1. Introduction
Broadband optical access networks are promising candidates to transmit diverse multimedia services over long-haul fiber links. Amplitude modulated subcarrier multiplexed (SCM) systems employing the 1550 nm optoelectronic technology through standard single-mode fiber (SMF) transmission are attractive for fiber optical cable television (CATV) transmission systems due to high video qualities and large number of distribution nodes. Recently, CATV integrating with radio-over-fiber (RoF) transmission systems have attracted much attentions [1–3], owing to low transmission loss and large bandwidth characteristics of the optical fiber. The performances of full-duplex CATV/RoF lightwave transmission systems are evaluated by parameters such as carrier-to-noise ratio (CNR), composite second-order (CSO), composite triple-beat (CTB), and bit error rate (BER). Since these parameters are seriously affected by noises and distortions, yet it is important to suppress the noises and distortions in full-duplex CATV/RoF lightwave transmission systems. The main difference between the intensity modulation (IM) and the phase modulation (PM) schemes is that the latter employs phase shift to record the signal state instead of employing intensity variation [4–6]. PM scheme, which has constant power operation feature, is expected to have good transmission performances in full-duplex CATV/RoF lightwave transmission systems. Similar with other PM scheme, the phase-modulated lightwave transmission systems also need a PM-to-IM converter to transform the phase-modulated signal into the intensity-modulated signal before been detected by a photodiode (PD). Traditionally, such a PM-to-IM conversion process is achieved by passing the optical signal through a delay interferometer (DI) [7]. Although the PM schemes can greatly promote the overall transmission performances; the sophisticated and expensive DI will be a serious restriction in promoting such full-duplex lightwave transmission systems. To overcome this drawback several PM-to-IM converters have been developed based on a fiber Bragg grating (FBG) tilt filter or an optical band-pass filter (OBPF) [8–10]. To pick up one of the sidebands (−1 or + 1 sideband) of the phase-modulated optical signal, a PD can directly detect the received optical signal. To consider the flexibility, however, the reflection spectra of the FBG-based and OBPF-based converters are fixed and not flexible. In parallel with utilizing FBGs or OBPFs to compose PM-to-IM converters, utilizing stimulated Brillouin scattering (SBS) effect to boost up one of the sidebands of the phase-modulated optical signal is another method to achieve the conversion [11]. However, these schemes require a high power seeding carrier to generate SBS effect. This will significantly complicate the transport systems and will boost up the overall cost. In [12,13], optical single sideband (OSSB) modulation schemes have been developed using optical injection locked semiconductor lasers. To utilize this founding in PM schemes and to overcome the constraint in the FBG-based and OBPF-based converters, an innovative vertical cavity surface emitting laser (VCSEL)-based PM-to-IM converter is proposed. In result, the optical signal can be detected directly by a PD. It is not necessary to use the sophisticated and expensive DI, or fixed and not flexible FBG-based and OBPF-based converters. In this paper, a full-duplex lightwave transmission system employing innovative VCSEL-based PM-to-IM converters at the receiving sites to deliver intensity-modulated CATV, phase-modulated RoF, and intensity-remodulated RoF signals over two 40-km SMFs links is proposed and experimentally demonstrated. The VCSEL is employed to amplify one of the optical sidebands, in which resulting in a PM-to-IM conversion. To be the first one of employing VCSEL-based PM-to-IM converters in such full-duplex lightwave transmission systems, the downstream light is successfully intensity-remodulated with RoF signal for up-link transmission. The performances of intensity-modulated CATV/phase-modulated RoF/intensity-remodulated RoF full-duplex lightwave transmission systems are measured and analyzed by parameters like CNR, CSO, CTB, and BER. Good performances of CNR/CSO/CTB are obtained for CATV signal, and low BER values are achieved for phase-modulated and intensity-remodulated RoF signals. With the assistance of VCSEL-based PM-to-IM converter, the phase-modulated signal is successfully transformed into the intensity-modulated signal, in which leading to direct detection of optical signal. This proposed system not only presents its advancement in broadband applications but also reveals its flexibility to meet the demands in optical access networks.
2. Experimental setup
The experimental configuration of the proposed full-duplex lightwave transmission systems employing innovative VCSEL-based PM-to-IM converters to transmit intensity-modulated CATV/phase-modulated RoF/intensity-remodulated RoF signals is shown in Fig. 1. Channels 2-78 (55.25-547.25 MHz) generated from a multiple signal generator (MATRIX SX-16) are symmetrically and directly fed into a distributed feedback laser diode (DFB LD), with a relative intensity noise (RIN) value of −165 dB/Hz, located in each side of the system for intensity-modulated CATV signals. The DFB LDs outputs, with central wavelengths of 1549.32 (λ1) in left-hand side and 1554.85 (λ2) nm in right-hand side, are fed into two phase modulators (EOSPACE’s, operational from DC to 20 GHz and beyond) for phase-modulated RoF signals. A 2.5-Gbps data stream mixed with a 20-GHz RF carrier to generate the 2.5Gbps/20GHz RoF signal, and the resulting RoF signal is applied to the phase modulator. When the lightwave is modulated by a phase modulator driven by a large RoF signal, multiple optical sidebands will be generated. However, the higher order sidebands are a kind of noise in the system and will degrade the transmission performance since they contain the same information as that in the firstly order sidebands but their power is much smaller. To eliminate the effect of higher order sidebands, we use an appropriate RoF signal to drive the phase modulator, only the first-order sidebands (−1 and + 1 sidebands) are generated, and the peak of the first-order sidebands is 20 GHz away from the optical carrier. The optical signal is firstly amplified by a C-band erbium-doped filter amplifier (EDFA) which has an output power of 16 dBm and a noise figure of around 3 dB, at an input power of 0 dBm. Subsequently, the signal is transmitted through a 40-km SMF link via two optical circulators (OCs; OC1 and OC2). Over a 40-km SMF transport, the received optical signal is split by a 1 × 2 optical splitter. Part of the optical signal is went through the OC3 combined with FBG1 (λc = 1549.32 nm), and received by a CATV receiver. For the better performance of the CATV receiver, the received optical power level needs to be kept at −3 ~ + 3 dBm [14]. All CATV parameters (CNR, CSO, and CTB) are measured and analyzed by using an HP-8591C CATV analyzer. The function of the OC3 combined with FBG1 is to pick up the optical carrier of the phase-modulated optical signal. Another part of the optical signal is passed through a VCSEL-based PM-to-IM converter. Such a VCSEL-based PM-to-IM converter is composed of an OC combined with a VCSEL. The optical spectra before [Fig. 1 insert (i)] and after [Fig. 1 insert (ii)] the VCSEL-based PM-to-IM converter are given in Figs. 2(a) and 2(b), respectively. Following with the VCSEL-based PM-to-IM converter outport, the optical signal is passed through the OC4. The outport of OC4 connected with FBG2 (λc = 1549.32 nm) has the optical signal only with the upper sideband [Fig. 1 insert (iii); as shown in Fig. 2(c)], and another outport of OC4 has the optical signal only with the optical carrier [Fig. 1 insert (iv); as shown in Fig. 2(d)]. The OC4 combined with FBG2 plays a role of separating the optical carrier and the upper sideband of the phase-modulate optical signal. The optical signal went through the FBG2 is directly detected by a PD, and fed into a BER tester (BERT) for BER analysis. In here, to obtain proper outcomes, the FBG1 and FBG2 are selected with very high rejection (reflectivity ~99.9%).
Fig. 1 Experimental configuration of our proposed full-duplex lightwave transmission systems employing innovative VCSEL-based PM-to-IM converters.
Fig. 2 (a) The optical spectrum before [Fig. 1 insert (i)] the VCSEL-based PM-to-IM converter. (b) The optical spectrum after [Fig. 1 insert (ii)] the VCSEL-based PM-to-IM converter. (c) The outport of OC4 connected with FBG2 has the optical signal only with the upper sideband [Fig. 1 insert (iii)]. (d) Another outport of OC4 has the optical signal only with the optical carrier [Fig. 1 insert (iv)]. (e) The optical spectrum before [Fig. 1 insert (v)] the OBPF. (f) The optical spectrum after [Fig. 1 insert (vi)] the OBPF.
For the up-link transmission, the optical signal from OC4 outport only with one optical carrier is fed into an intensity modulator. In this experiment, down-link transmission is defined as transmitting signal from left side to right one; whereas up-link transmission is defined as transmitting signal from right side to left one. The 1.25Gbps/10GHz RoF signal is directly fed into an intensity modulator, transmitted by another 40-km SMF link via two OCs (OC5 and OC6), passed through an OBPF, directly detected by a PD, and supplied to a BERT for BER analysis. The OBPF has a 3-dB bandwidth of 0.3 nm and a 40-dB bandwidth of 0.44 nm. The function of the OBPF is to transform the optical signal with double sideband (DSB) format into only one optical sideband format. In the up-link transmission, the 1.25Gbps/10GHz signal is contained in two optical sidebands, while the CATV signal is contained in the optical carrier. When the up-link signal passing through the OBPF, the CATV signal will be removed. The optical spectra before [Fig. 1 insert (v)] and after [Fig. 1 insert (vi)] the OBPF are given in Figs. 2(e) and 2(f), respectively. The upstream signal can be sent back either by the same fiber or by the other fiber to minimize the crosstalk of the downstream signal. In order to minimize the crosstalk, we use two fibers for down-link and up-link transmissions [15].
3. Experimental results and discussions
To verify the feasibility of the VCSEL-based PM-to-IM converter, an optical carrier generated from a DFB LD (λ = 1549.32 nm) is phase-modulated with a 20-GHz sinusoidal signal via a phase modulator, as shown in Fig. 3. Subsequently, the carrier is routed and fed into the VCSEL via an OC. The reflected carrier is again routed by the OC and then fed into an optical spectrum analyzer (OSA). If the upper sideband ( + 1 sideband) of the phase-modulated optical signal is matched with the main mode of VCSEL, then the upper sideband will be amplified, while the lower sideband (−1 sideband) will remain unchanged [12]. The optical spectrum before the VCSEL-based PM-to-IM converter is given in Fig. 3(a). A VCSEL is employed to amplify the upper sideband, and produce the optical spectrum as shown in Fig. 3(b). By applying small signal linear approximation, the injection-locking range can be derived [16]:
whereis the steady-state amplitude of the slave laser under light injection, is the amplitude of the injected signal,is the range of detuning frequencies,is linewidth enhancement factor, is coupling coefficient. In the proposed architecture, the PM signal needs to propagate through a span of 40 km SMF and an optical splitter before been injected into the PM-to-IM converter, so that the injection ratio between the + 1 sideband and the VCSEL lasing wavelength is weak. Such a weak injection ratio will limit the maximum data bandwidth around the subcarrier sideband it can amplify. Nevertheless, more than 0.04 nm or 5 GHz bandwidth is still obtained when the power of the sideband is about −15 dBm. If the upper sideband of the phase-modulated optical signal is not matched with the main mode of VCSEL, then the conversion effect will not work. However, the main mode of the VCSEL can be adjusted to match with the upper sideband of the phase-modulated optical signal by adjusting the VCSEL driving current. Figure 4 presents the optical spectra of the VCSEL under different driving currents. It can be seen that, as the VCSEL driving current changes from 3 mA to 9 mA, the central wavelength of VCSEL shifts from 1548.32 nm to 1551.32 nm. About 3 nm tuning range is obtained to dynamically match with the upper sideband of the phase-modulated optical signal.Figure 5 shows the measured CNR, CSO and CTB values under NTSC channel number. While measuring the CNR, CSO and CTB values, we send the CATV signal in both directions simultaneously. Good CNR/CSO/CTB performances (≥50/61/60 dB) are obtained after a 40-km SMF transport. The power penalties of 2/3/4 dB for CNR/CSO/CTB are present between the back-to-back (BTB) and 40-km SMF case, due to fiber dispersion in SMF. According to the cable passive optical network (PON) performances requirements for 40 km transmission, the CNR/CSO/CTB values at the optical node should be ≥50/60/60 dB. It is clear that systems satisfy the CNR/CSO/CTB performances requirements. Using an OSA to measure the crosstalk level, the crosstalk level from λ1 to λ2 (λ2 to λ1) is less than −45 dB. The lower crosstalk level we obtain in systems, the better CNR/CSO/CTB performances we achieve in systems. The isolation property of the OC is an important characteristic to direct the downstream and upstream carriers to dedicated directions without obvious crosstalk. Since the proposed system does not use sophisticated external modulation scheme, it reveals a prominent one with simpler and more economic advantages than that of external-modulated transmission systems.
In parallel with verifying CATV performance, the measured BER curves of the 2.5Gbps/20GHz data channel are presented in Fig. 6. For CATV on, the received optical power level at a BER of 10−9 is −15.3 dBm for 40-km transmission scenario and is −16.5 dBm for BTB scenario. For CATV off, the received optical power level at a BER of 10−9 is reduced to −16.1 dBm. A power penalty of only 0.8 dB between CATV on and off scenarios is obtained since the 2.5 Gbps data (modulated by 20-GHz RF carrier) is not impacted by the 50-550 MHz low frequency CATV signal. The 2.5 Gbps digital baseband (BB) data is contained in the optical upper sideband, while the 50-550 MHz CATV analog signal is contained in the optical carrier. Almost no interference is observed between the phase-modulated RoF signal and the intensity-modulated CATV signal. In addition, at a BER of 10−9, there exists a small power penalty of 0.4 dB between BTB and 40 km SMF transmission (CATV off) cases due to the use of PM scheme. Since PM scheme is employed in systems, yet noise and distortion induced by systems will be suppressed dramatically, in which leading to significant BER performance improvement. Error free transmission is achieved to demonstrate the feasibility of employing an intensity-modulated scheme to modulate the 50-550 MHz CATV signal and a phase-modulated scheme to modulate the 2.5Gbps/20GHz RoF signal simultaneously.
To evaluate the transmitted upstream RoF signal performance, the measured BER curves of 1.25Gbps/10GHz data channel are shown in Fig. 7. Without OBPF (PD followed by a demodulator), the received optical power levels at a BER of 10−9 are −17 dBm and −13.3 dBm for BTB and 40-km transmission scenarios, respectively. With OBPF, the received optical power levels at a BER of 10−9 are −17.2 dBm and −16.7 dBm for BTB and 40-km transmission scenarios, respectively. A large power penalty improvement of 3.4 dB is obtained in 40-km transmission scenarios when the OBPF is added. This is due to the cancellation of RF power degradation induced by fiber dispersion. Over a 40-km SMF transport, fiber dispersion degrades the performance of systems due to the double sideband (DSB) nature of the optical signal. In only one optical sideband (upper sideband) system, since optical carrier and lower sideband are suppressed before detection, yet the RF power degradation induced by fiber dispersion can be avoided [17]. In conventional RoF lightwave transmission systems, the optical signal at the receiving site is firstly detected by a PD to transform it into RF passband signal, and then the RF passband signal is demodulated by a group of high-bandwidth RF devices. In such way, the bandwidth of systems suffers from the limitation of RF devices, and the expensive RF devices increase the cost of systems. Instead, if only one optical sideband is processed by optical devices, the baseband data can be retrieved directly from the optical sideband. This is shown to be a potential candidate to retrieve the baseband data since expensive RF devices are not employed in systems. Error free transmission is obtained to demonstrate the possibility of establishing an intensity-remodulated RoF signal over a 40-km SMF link.
4. Conclusions
A full-duplex lightwave transmission system employing innovative VCSEL-based PM-to-IM converters to transport intensity-modulated CATV, phase-modulated RoF, and intensity-remodulated RoF signals over two 40-km SMFs links is proposed and demonstrated. To be the first one of employing VCSEL-based PM-to-IM converters in full-duplex lightwave transmission systems, the downstream light is successfully intensity-remodulated with RoF signal for up-link transmission. Good transmission performances of CNR, CSO, CTB, and BER are obtained over two 40-km SMF links. Such a hybrid full-duplex lightwave transmission system would be very attractive for fiber trunk applications; it reveals a prominent one to satisfy the requirements in optical access networks.
Acknowledgment
The authors would like to thank the financial support from the National Science Council of the Republic of China under Grant NSC 100-2221-E-027-067-MY3, NSC 101-2221-E-027-040 -MY3, and NSC 103-2218-E-027-001.
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