K. Y. Cho

10-Gb/s Operation of RSOA for WDM PON

We report on the 1O-Gb/s operation of the reflective semiconductor optical amplifier (RSOA) for the next-generation wavelength-division-multiplexed passive optical network (WDM PON). The bandwidth of the RSOA used in this experiment is merely 2.2 GHz. Nevertheless, a clear eye opening is obtained at 10 Gb/s by using the electronic equalizer processed offline. We investigate the impacts of the networks operating conditions (such as the injection power to the RSOA and the fiber length) on the performances of these equalizers. The results show that the RSOA-based WDM PON is operable at 10 Gb/s and the maximum reach can be extended to >20 km with the help of the forward error correction codes.

25.78-Gb/s Operation of RSOA for Next-Generation Optical Access Networks

We report the 25.78-Gb/s operation of the reflective semiconductor optical amplifier (RSOA) for the next-generation optical access network. For this purpose, we develop a butterfly-packaged RSOA and minimize the electrical parasitics. As a result, the modulation bandwidth of RSOA is improved from 2.2 to 3.2 GHz (which is the fundamental limit imposed by the carrier lifetime). In addition, the slope of the RSOAs frequency response curve is enhanced from -40 to -20 dB/decade. Using this butterfly-packaged RSOA, we have demonstrated the 25.78-Gb/s operation. The receiver sensitivity is measured to be -11 dBm with the help of the electronic equalization and forward-error-correction (FEC) techniques. To evaluate the possibility of implementing the 100-Gb/s passive optical network (PON) by using this RSOA and the coarse wavelength-division-multiplexing (CWDM) technique, we evaluate the BER performance at four different wavelengths in the C-band. The results show that the error-free transmission can be achieved in the wavelength range of 20 nm with a penalty less than 2 dB. Thus, we can realize the 100-Gb/s PON cost-effectively by utilizing the directly modulated RSOAs operating at 25 Gb/s.

Effects of Reflection in RSOA-Based WDM PON Utilizing Remodulation Technique

We investigate the effects of the discrete reflection on the performances of upstream and downstream signals in the wavelength-division-multiplexed passive optical network (WDM PON) implemented in a single-fiber loopback configuration using the reflective semiconductor optical amplifiers (RSOAs). We first analyze the optical beat interference (OBI) noise caused by the discrete reflection, and clarify the relation between the reflection tolerance and the networks operating conditions such as the RSOA gain, the link loss, and the location of the reflection point, etc. The results show that the impact of the reflection can be expressed by using the effective crosstalk level. We then measured the reflection tolerance of the RSOA-based WDM PON, in which the downstream signal operating at 1.25 Gb/s is remodulated by the RSOA at the subscribers site for the transmission of 155-Mb/s upstream signal. The reflection tolerances are measured to be in the range of -42 to - 35 dB for the downstream signals and -29 to -19 dB for the upstream signals, depending on the RSOA gain. These small reflection tolerances are caused by the fact that the reflected light is re-amplified by the RSOA. We also show that the dependence of the reflection tolerance on the RSOA gain can be explained by using the effective crosstalk level. These results are used to evaluate the impacts of the unwanted discrete reflections on the RSOA-based WDM PON.

Long-Reach Coherent WDM PON Employing Self-Polarization-Stabilization Technique

We propose a simple self-polarization-stabilization technique for the wavelength-division-multiplexed passive optical network implemented with reflective semiconductor optical amplifiers (RSOAs) and self-homodyne coherent receivers. By placing a 45° Faraday rotator in front of the RSOA in the optical network unit, the state-of-polarization of the upstream signal becomes orthogonal to that of the linearly polarized seed light at the input of the coherent receiver regardless of the birefringence in the transmission link. Thus, we can achieve the polarization stability of the upstream signal at the input of the coherent receiver. We first implement a self-homodyne receiver by using the proposed self-polarization-stabilization technique and measure its sensitivity by using 2.5-Gb/s binary phase-shift keying signals in the laboratory. The result shows an excellent receiver sensitivity of -46.4 dBm. We also confirm the efficacy of the proposed technique in the transmission experiment over 68-km long link partially composed of installed (buried and aerial) fibers. No significant degradation in the receiver sensitivity is observed during the 10-h experiment despite the large polarization fluctuations occurred in these installed fibers.

Long-reach 10-Gb/s RSOA-based WDM PON employing QPSK signal and coherent receiver.

We demonstrate a long-reach wavelength-division-multiplexed passive optical network (WDM PON) operating at the symmetric rate of 10.3 Gb/s. For the cost-effectiveness, we realize the upstream transmission by utilizing directly-modulated TO-can packaged reflective semiconductor optical amplifiers (RSOAs) and digital coherent receivers. In addition, to overcome the limited modulation bandwidth of this TO-can packaged RSOA (~2.2 GHz) and operate it at 10.3 Gb/s, we utilize the quadrature phase shift keying (QPSK) format and the electronic phase equalization technique. The result shows that we can extend the maximum reach of the 10.3-Gb/s RSOA-based WDM PON to ~80 km without using any remote amplifiers.

Enhanced Operating Range of WDM PON Implemented by Using Uncooled RSOAs

We report on the operable ranges of temperature and wavelength for a 1.25-Gb/s wavelength-division-multiplexed passive optical network (WDM PON) implemented by using uncooled reflective semiconductor optical amplifiers (RSOAs). Both the gain and modulation bandwidth of the RSOA are substantially reduced at high temperatures. As a result, it is difficult to achieve the error- free operation at temperatures above 40degC. In order to mitigate this limitation, we implement a simple bias-control circuit, in which the bias current is controlled stepwise according to the operating temperature of the RSOA. Using this circuit, we extend the operable wavelength range of the RSOA-based WDM PON to ~55 nm in the temperature range of -20degC to 60degC.

Design Issues in RSOA-based WDM PON

We review some issues critical for the design of the wavelength-division-multiplexed passive optical network (WDM PON) implemented by using reflective semiconductor optical amplifiers (RSOAs), including the uncooled operation of RSOAs, the remodulation noise, the reflectionn of SOAs,and the limited modulation bandwidth of RSOAs. In addition, we report our recent research activities to overcome these problems.

Performance of Forward-Error Correction Code in 10-Gb/s RSOA-Based WDM PON

We investigate the performance of the forward-error correction (FEC) code for the 10-Gb/s wavelength-division-multiplexed passive optical network (WDM PON) implemented by using reflective semiconductor optical amplifiers (RSOAs) with extremely limited modulation bandwidth and the electronic equalizers to compensate for the degradations resulting from the use of such RSOAs. We show that the error occurrences in this network strongly depend on the bit pattern and the burst errors are likely to occur. Thus, it is important to use the FEC code capable of correcting the burst errors such as Reed-Solomon (RS) code. In addition, since a significant penalty can be induced by the increased line rate resulting from the use of the FEC code, it is necessary to find the optimum redundancy required to minimize the bit-error rate. We also evaluate the tolerance to the chromatic dispersion of the proposed 10-Gb/s WDM PON implemented by using the RS code with the optimum redundancy.

Chromatic dispersion tolerance of 10-Gb/s WDM PON implemented by using bandwidth-limited RSOAs

We investigate the effect of chromatic dispersion on the 10-Gb/s WDM PON implemented by using bandwidth-limited RSOAs and electronic equalizers.

Transmission of 40-Gb/s QPSK upstream signal in RSOA-based coherent WDM PON using offset PDM technique

We demonstrate the 40-Gb/s upstream transmission in the 60-km reach wavelength-division-multiplexed passive optical network (WDM PON) implemented by using directly modulated reflective semiconductor optical amplifiers (RSOAs) and self-homodyne receivers. It is difficult to operate the RSOA at 40 Gb/s due to its limited modulation bandwidth. To overcome this problem and generate 40-Gb/s upstream signal, we utilize the quadrature phase-shift-keying (QPSK) format and the offset polarization-division-multiplexing (PDM) technique. For this purpose, we install two RSOAs at each ONU and provide the seed light for these RSOAs by polarization-multiplexing the outputs of two lasers with a small frequency offset (20 GHz). This frequency offset is used to separate the polarization-multiplexed seed light by using a simple delay-line interferometer (DLI), instead of the polarization-beam splitter and polarization controller, at the ONU. The separated seed light is modulated by each RSOA at 20 Gb/s in the QPSK format, and then combined again by the DLI before sent back to the central office (CO). The results show that this WDM PON can support the transmission of 40-Gb/s channels spaced at 50 GHz over 60 km without using any remote optical amplifiers.