1.

Introduction

In a remote monitoring system, data gathering sites are distributed over broad areas typically up to $1\mathrm{km}$ away from the central server station. The remote sites extract information from the environment, including high data rate information such as video. The extracted information is then transferred back to a central server station for analysis. One important remote monitoring application example is the mobile radar sensing technology, where the radar antennas are placed on radar trucks and distributed throughout a surveillance field. In a typical setting, radar trucks are distributed between $100\mathrm{m}\text{to}1\mathrm{km}$ distance from the central processing truck, and the data rate is $2.5\mathrm{Gbps}$ per channel.¹

Existing solutions for this type of communication system have significant drawbacks. For example, electronic cables are heavy and bulky, therefore fiber optics is preferred. Vertical cavity surface emitting laser (VCSEL) based optical transmitters have limited range and reliability, and suffer from wavelength shifts with temperature.² Also, dense wavelength division multiplexing passive optical network (DWDM-PON) is often too expensive.³

We propose a PON-like^{4, 5} low-cost communication architecture utilizing superluminescent light emitting diodes (SLEDs), coarse wavelength division multiplexers (CWDMs), and reflective semiconductor amplifiers (RSOAs). The SLED is used as a low-cost broadband source,⁶ while the CWDM multiplexers are used to slice the entire spectrum into four different $20\text{-}\mathrm{nm}$ -wide sources for each individual remote site. The utilization of a CWDM multiplexer instead of a DWDM multiplexer further reduces the cost and reduces the environmental (primarily temperature) constraints, both of which are important in a remote sensing application. Finally, the RSOA was chosen to modulate and amplify the remote data, since this device is relatively stable in a wide range of temperatures,^{7, 8} reducing the requirements for environmental control at the remote site. However, commercially available RSOA are suitable for use at data rates up to only $1.25\mathrm{Gbps}$ ; therefore, we utilized the pre-emphasis technique from microwave circuits to improve speed to $2.5\mathrm{Gbps}$ . We have demonstrated that the proposed architecture provides data transfer over $1\text{-}\mathrm{km}$ transmission distance for all four CWDM channels and promises bit error rate (BER) of better than ${10}^{-12}$ to enable Ethernet connections.

2.

Proposed Architecture

The proposed PON architecture is shown in Fig. 1 and works as follows. the central office (central server station) is shown on the left side. Four remote monitoring sites (optical network units) are on the right side. The latter contain the RSOA as modulator and amplifier, RSOA driver circuit, and the monitoring devices (sensors). The light source for the optical link is a SLED placed in the central server station. This eliminates the need for multiple light sources, such as VCSELs or laser banks. Furthermore, the SLED is a robust device and not as sensitive to high temperatures as laser sources. This ensures a lower overall cost for the optical system due to relaxed temperature control requirements.

Fig. 1

Proposed architecture for the communication link.

The SLED output is coupled via a circulator into a single-mode fiber. The remote end of this fiber is connected to a demultiplexer that slices the broadband optical signal into four standard $20\text{-}\mathrm{nm}$ -wide CWDM channels. The $20\text{-}\mathrm{nm}$ -wide optical sources are transmitted through a second fiber to the RSOAs, with total fiber lengths between SLED and RSOAs of up to $1\mathrm{km}$ . The RSOA modulates the entire $20\text{-}\mathrm{nm}$ -wide light source and reflects the signal back through the fiber, through the CWDM multiplexer, and back into the circulator. The circulator redirects the reflected signal into the receiver section of the central server station. The channels are separated again with another CWDM and fed into optical receivers. Finally, the data from all the remote sites are collectively analyzed by a central processor.

3.

Experimental Setup

To demonstrate our proposed architecture, we built an experimental setup as shown in Fig. 2. For experimental purposes, we only populated one channel. The server site uses a SLED (DenseLight DL-BX9-CS5107A, Singapore) to provide the optical power needed for data transmission ( $8\text{-}\mathrm{mW}$ total) and an APD-TIA receiver, which works at data rates up to $2.5\mathrm{Gbps}$ (OEMarket RX957, Sydney, Australia). The remote site contains the RSOA as the modulator (SOA-RL-OEC-1550 from CIP Technologies, IP switch, United Kingdom); the bit-error-rate tester as the data source [Anritsu (Richardson, Texas) 1632A BERT]; and a RSOA driver circuit, which is comprised of several key modules, as can be seen in Fig. 2.

Fig. 2

(a) Constructed experimental system setup. (b) Waveform of signal from BERT. (c) Waveform with overshoot after pre-emphasis driver. Different levels of pre-emphasis have different output waveforms.

The $3\text{-}\mathrm{dB}$ frequency of the RSOA is only about $1.25\mathrm{GHz}$ . To achieve high quality data transmission at $2.5\mathrm{Gbps}$ , we used pre-emphasis, which is a technique that increases the amplitude of high frequency components of the signal, so that the entire system can achieve a higher bandwidth. The actual implementation of pre-emphasis is done in the time domain by adding an overshoot to the signal during transition; waveforms before and after application of pre-emphasis are shown in Figs. 2 and 2. Translated into frequency domain, the overshoot has the effect of amplifying the high frequency components. Overall, an RSOA is a low pass filter that rolls off around $1.25\mathrm{GHz}$ , but by integrating a pre-emphasis driver (which is a high pass filter) into the system, the overall frequency response is improved, allowing higher transmission data rates. In our RSOA driver circuit [Fig. 2], we used a standard pre-emphasis driver (Maxim3982, Maxim, Sunnyvale, California) after which the signal is further amplified by an rf amplifier [Marki (Morgan Hill, California) A08058]. Finally, the amplified rf signal is combined with a dc bias current in the bias-tee (Marki BT-0018) and coupled into the RSOA.

To evaluate the sensitivity of the optical system, a variable optical attenuator (VOA) [Thorlabs (Trenton, New Jersey) VOA50-APC] is used to attenuate the optical signal, which is then coupled into the APD-TIA receiver. The electrical output signals are fed back into the BERT to analyze the BER.

4.

Experimental Results

The pre-emphasis driver provides four different levels of pre-emphasis, as shown in Fig. 2. Therefore, we first determined the optimal level of pre-emphasis for the system. Then, since several components are wavelength dependent, we studied the performance of each individual channel. Finally, we evaluated the effect of long distance transmission for the system. The performance of the optical link was evaluated with a pseudorandom bit sequence (PRBS) pattern with a length of $2^{15}-1$ to represent radar data. The SLED output was measured as $-0.72$ , 1.76, 2.79, and $1.55\mathrm{dBm}$ in the 1511, 1531, 1551, and $1571\text{-}\mathrm{nm}$ CWDM channels, respectively. Power received by the RSOA is $5\mathrm{dB}$ below those levels caused by attenuation in the system. The RSOA was operated without temperature control with a bias current of $60\mathrm{mA}$ .

4.1.

Performance Comparison of Pre-Emphasis Levels

The four different levels of pre-emphasis are 2, 4, 8, and $14\mathrm{dB}$ . The performances of the four pre-emphasis levels were compared to a communication link with no pre-emphasis.

To analyze the performance of different pre-emphasis levels, the experimental system was set with a fiber length of $100\mathrm{m}$ , and the $1551\text{-}\mathrm{nm}$ channel was activated. The eye diagrams and the sensitivity measurement results are shown in Fig. 3: 8- and $4\text{-}\mathrm{dB}$ pre-emphasis are very similar and show the best performance, with a sensitivity improvement of about $6\mathrm{dB}$ (at ${10}^{-9}$ BER) compared to the case without pre-emphasis. 2- and $14\text{-}\mathrm{dB}$ pre-emphasis are only slightly better than no pre-emphasis. A pre-emphasis of $2\text{-}\mathrm{dB}$ level is not sufficient, while a pre-emphasis of $14\mathrm{dB}$ overcompensates and leads to intersymbol interference (ISI), which can be seen as line doubling in the eye diagram. The sensitivity of the system using the optimal pre-emphasis level of $8\mathrm{dB}$ was measured as $-31.5\mathrm{dBm}$ .

Fig. 3

Eye diagram and sensitivity measurement results for (a) different pre-emphasis levels, (b) different channels; and (c) different transmission lengths.

5.

Conclusion

We develop a working prototype for a low-cost, high-speed RSOA-based optical connector for remote monitoring applications. We demonstrate that a $2.5\text{-}\mathrm{Gbps}$ optical fiber communication link can be achieved with a SLED as light source and a directly modulated RSOA for data transmission on a CWDM channel roster. The concept of pre-emphasis for driving the RSOA is introduced to boost the data rate from the manufacturer specified $1.25\mathrm{Gbps}\text{to}2.5\mathrm{Gbps}$ . Low BER data transmission is demonstrated for all four CWDM channels with a power penalty in the $1511\text{-}\mathrm{nm}$ channel due to the present bandwidth limitations of optical components. Transmission over $100\mathrm{m}$ and $1\text{-}\mathrm{km}$ distance is demonstrated, showing capability for in remote monitoring applications.