1. Introduction

Lightwave transport systems have been developed with high expectations for future communications due to such advantages of optical fiber with low transmission loss and large bandwidth, as well as transparent characteristics for signal transmission [1–3]. However, lightwave transport systems for very-short-reach (VSR) end user applications have not been addressed. With the rapid development of information technology, the increasing demands for broadband services raise the needs for high bandwidth, not only for the single-mode fiber (SMF)-based primary networks, but also for the plastic optical fiber (POF)-based in-house ones. SMF, in particular, has already established an undisputable position to distribute high quality signals. As a widespread medium, SMF offers better performances than POF in terms of attenuation and dispersion. However, when the SMF is deployed toward in-house networks, installation cost and convenience are concerned issues needed to be solved. The SMF with relative much smaller core size requires trained persons, high precision devices, and higher cost to install and maintain. POF alternatively has much larger core size, relative smaller bending radius, and easier to be installed characteristics. In result, the in-house connection becomes a critical bottleneck to successfully deliver high quality signals from the central office (CO) to the client premises. To overcome the challenge, a new kind of in-house network medium is required to replace the existed coaxial cable or twisted pair. Recently, graded-index POF (GI-POF) has been developed with good transmission performance and lower cost [4,5]. This flash product is a promising candidate to solve the fiber-to-the-home (FTTH) last mile problem. Clear advantages are offered by using the GI-POF for in-house networks: its large core diameter and small bending radius considerably eases coupling and splicing, as well as its ductility and flexibility simplifies installation in customer locations [6–8]. As a result, GI-POF is an ideal medium to integrate fiber backbone networks and in-house ones. In this paper, an integration of FTTH and GI-POF in-house networks based on injection-locked vertical cavity surface emitting lasers (VCSELs) and direct-detection technique is proposed and experimentally demonstrated. In the proposed networks, RF signals are transmitted from the CO to the client premises, and then to the dedicated rooms. With the assistance of injection locking technique at the CO and a tunable optical band-pass filter (TOBPF) at the client premises, the optical carrier and one of the sidebands are eliminated before detecting. Optical signal with only one optical sideband format is processed by optical devices, and the baseband (BB) data is obtained directly from the residual optical sideband [9]. In traditional lightwave transport systems, the optical signal at the receiving site is detected by a broadband photodiode (PD) to convert it into RF signal, and then the RF signal is demodulated by a group of high-bandwidth RF devices. In such way, the bandwidth of systems is limited by the bandwidth of RF devices; and expensive RF devices increase the cost of systems. The proposed networks used two wavelengths for 5.8 and 10 GHz worldwide interoperability for microwave access (WiMAX) signals transmission. Error free transmissions with sufficient low bit error rate (BER) values are achieved over a combination of 20-km SMF and 50-m GI-POF transport. Signal qualities meet the WiMAX demand with data signals of 20Mbps/5.8GHz and 70Mbps/10GHz, respectively. Different from current POF networks, which concentrate on short-distance POF transmission only, our novel proposed network is the first one to deliver widespread signals to the client premises. This proposed network is shown to be a prominent one not only presents its economy in the last mile end user application, but also reveals its convenience to be installed.

2. Experimental setup

Figure 1 illustrates the schematic diagram of the integrated FTTH and GI-POF in-house networks over a combination of 20-km SMF and 50-m GI-POF transport. The CO consists of four VCSELs, two optical circulators (OCs), and two signal generators. Four VCSELs (VCSEL1-4) were selected with wavelengths of 1312.32 (λ₁), 1317.12 (λ₂), 1312.46 (λ₃), and 1317.26 (λ₄) nm. Data rate of 20 Mbps mixed with 5.8 GHz RF carrier was directly modulated into the VCSEL1, and data rate of 70 Mbps mixed with 10 GHz RF carrier was directly modulated into the VCSEL2. As to the light injection, light is injected through a 3-port OC with an injection power level of 4 dBm per optical channel. For λ₁ (VCSEL1) injection-locked, λ₃ (VCSEL3) is coupled into the port 1 of OC1, the injection-locked VCSEL1 is coupled into the port 2 of OC1, and the port 3 of OC1 is multiplexed by a 2×1 optical combiner. The modulated lightwaves are transmitted through two fiber links: 20 km SMF (with a dispersion coefficient of 17ps/nm⋅km) and 50-m GI-POF (Chromis Fiberoptics GigaPOF50SR-PC-SM, with core diameter = 50±5μm, numerical aperture = 0.185±0.015, macro-band loss <0.25 dB for 10 turns on a 25-mm radium quarter circle, and long-term bend radium = 5.0 mm). The attenuation value of GI-POF has reached <20 dB/km at 1.3 µm, and the bandwidth-length product has reached 1.1 GHz⋅km. Over a combination of 20-km SMF and 50-m GI-POF transmission, the combined optical wavelengths are passed through a TOBPF to pick up the upper sideband of the transmitted optical signal, adjusted by a variable optical attenuator (VOA), directly detected by a broadband PD, and finally fed into a BER tester for BER analysis. It may be convenient to transmit the 20 and 70 Mbps signals directly at BB, and can be predicted to obtain the same transmission performances. Nevertheless, no wireless broadcasting is possible in such manner. For FTTH applications at the premises, the RF carrier is necessary for sending signal wirelessly to integrate the optical networks and the wireless radio (as shown in Fig. 2 ). Since no RF carrier is existed for BB transmission, yet it cannot integrate the optical networks and the wireless radio.

 

Fig. 1 The schematic diagram of the integrated FTTH and GI-POF in-house networks over a combination of 20-km SMF and 50-m GI-POF transport.

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Fig. 2 FTTH applications/at the premises.

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3. Experimental results and discussions

The resonance frequency, ${\omega }_R$, of the injection-locked laser is given by [10]:

The optical spectrum of directly modulated VCSEL1 with optical carrier and two sidebands is present in Fig. 3(a) (double sideband (DSB) format). Figure 3(b) shows the optical spectrum of injection-locked VCSEL1 locked at λ₃. To study the injection locking behavior, the wavelength of VCSEL3 is tuned relative to VCSEL1 by using a temperature controller. One key feature of injection locking is that the injection-locked laser is forced to oscillate at the injection frequency instead of the original free-running frequency. Therefore, the frequency component at the injection frequency becomes dominant. The injection locking behavior happens as an injection laser (VCSEL3) is slightly detuned to frequency lower than that of the injected laser (the upper sideband of VCSEL1). It means that negative frequency detuning is employed to achieve an injection locking behavior [11]. As the upper sideband of VCSEL1 (1312.38 nm) is injection-locked, its optical spectrum shifts a slightly longer wavelength (1312.46 nm). When an injection-locked laser with negative frequency detuning is modulated by an RF signal, its upper sideband will be amplified [12,13]. An injection locking enhances the intensity of the upper sideband and produces the optical spectrum with near DSB format. Similar optical single sideband (SSB) modulation using direction modulation with light injection technique has been proposed [12]. By employing optical SSB modulation scheme, the RF power degradation induced by fiber dispersion can be suppressed; however, it cannot be cancelled. For our proposed approach, the RF power degradation can be avoided even when the optical carrier and two sidebands are transmitted. It reveals an outstanding one with better performance. To convert near DSB format into only one optical sideband one, the TOBPF at the premises is aligned so that only the upper sideband is picked up (as shown in dash-line window of Fig. 3(b)). This TOBPF is worth employing since a group of expensive high-bandwidth RF devices for RF signal demodulation are not required. At the premises, the optical signal with only one optical sideband (upper sideband) format for direct-detection is shown in Fig. 3(c).

 

Fig. 3 (a) The optical spectrum of directly modulated VCSEL1 (DSB format). (b) The optical spectrum of injection-locked VCSEL1 locked at λ₃. (c) The optical signal exhibits only one optical sideband (upper sideband, λ₃) for direct-detection.

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The measured BER curves of 20Mbps/5.8GHz (VCSEL1) and 70Mbps/10GHz (VCSEL2) data channels are present in Fig. 4(a) and (b) , respectively. It can be seen that the receiver sensitivity of systems without TOBPF (DSB and near DSB; RF demodulation) is worse than that of systems with TOBPF (only one optical sideband; direct-detection). At a BER of 10⁻⁹, there exist large power penalties of 6.2 dB (Fig. 4(a)) and 6.3 dB (Fig. 4(b)) between the back-to-back cases and the free-running ones (DSB) due to fiber chromatic dispersion in the SMF and modal dispersion in the GI-POF. And at a BER of 10⁻⁹, there exist small power penalties of 2.9 dB (Fig. 4(a)) and 3.3 dB (Fig. 4(b)) between the back-to-back cases and the 4 dBm injection ones (only one optical sideband); improvements of 3.3 and 3 dB receiver sensitivities are achieved. The receiver sensitivities improvements can be attributed to the use of direct-detection technique to cancel the RF power degradation induced by fiber chromatic dispersion. Moreover, at a BER of 10⁻⁹, there exist little power penalties of 0.6 dB (Fig. 4(a)) and 0.5 dB (Fig. 4(b)) between the 4 dBm injection (only one optical sideband) cases and the 4 dBm injection (near DSB; without TOBPF) ones. It is obvious that the improvement results are limited. The main function of direct-detection technique is to cancel the RF power degradation induced by fiber chromatic dispersion. Nevertheless, since the transmission media are the combination of SMF and GI-POF, yet modal dispersion is one of the factors that degrade the transmission performances. However, error free transmissions are still achieved to demonstrate the possibility of establishing an integration of FTTH and GI-POF in-house network over a combination of 20-km SMF and 50-m GI-POF links. The amount of modal dispersion in time (ps) can be expressed as [14]:

 

Fig. 4 (a) The measured BER curves of 20 Mbps/5.8 GHz data channel (VCSEL1). (b) The measured BER curves of 70 Mbps/10 GHz data channel (VCSEL2).

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4. Conclusions

An integration of FTTH and GI-POF in-house networks with the assistance of injection-locked VCSELs and direct-detection technique is proposed. We have experimentally demonstrated the feasibility of networks and obtained good BER performances over a combination of 20-km SMF and 50-m GI-POF links. Since our proposed network does not use sophisticated and expensive RF devices in premises, it reveals a prominent one with simpler and more economic advantages than those of networks with RF signal demodulation. Our proposed architecture is suitable for the SMF-based primary and GI-POF-based in-house networks.