The per-subscriber data rate of the optical access network has been increased by an order of magnitude in every 5 to 6 years during the last 30 years. If this trend continues, it will be necessary to introduce faster than 10-Gb/s services within the next 10 years or so. The wavelength-division-multiplexed passive optical network (WDM PON) has been considered as one of the most promising solution for providing such a large capacity to each subscriber. However, for the prevalent commercial deployment, it is crucial to improve the competitiveness of WDM PON by increasing its operating speed, maximum reach, and spectral efficiency. In addition, for the cost-effectiveness, it is imperative to implement the optical network units (ONUs) located at the customer premises by using colorless optical transmitters such as reflective semiconductor optical amplifiers (RSOAs). However, it is difficult to increase the operating speed of the RSOA-based WDM PON to be faster than 10 Gb/s since the intrinsic modulation bandwidth of RSOA is typically less than 3 GHz.
The primary objective of this dissertation is to develop a practical and cost-effective method for increasing the operating speed and spectral efficiency of RSOA-based WDM PON for the use in the next-generation optical access network. To achieve this objective, we propose and develop (1) a practical ultra-dense 12.5-Gb/s RSOA-based WDM PON having with a spectral efficiency of 1 bit/s/Hz, (2) RSOA-based WDM PON operating at 20 Gb/s by using the polar return-to-zero (RZ) 4-ary pulse amplitude modulation (PAM) format and direct-detection receiver, (3) RSOA-based WDM PON operating at 28 Gb/s by using the electrical and optical equalization techniques, and (4) ROSA-based WDM PON operating at 40 Gb/s by using the newly proposed offset polarization-division-multiplexing (PDM) technique and cost-effective coherent receivers.
Recently, there have been growing interests in ultra-dense WDM PON to provide broadband services to a large number of subscribers cost-effectively. There have also been some efforts to develop high-speed $(\geq 10 Gb/s/channel)$ ultra-dense WDM PONs for the use in future optical access networks. However, most of these networks may not be cost-effective enough for the practical deployment due to their use of the expen-sive I/Q modulators, tunable lasers, and digital coherent receivers. For the development of the cost-effective ultra-dense WDM PONs, it is essential to implement the ONUs colorless, utilize the single-fiber bidirectional transmission, avoid the use of expensive external modulators, and minimize the use of optical amplifiers. Thus, we propose and demonstrate a cost-effective 12.5-GHz spaced ultra-dense WDM PON operating at the per-wavelength speed of 12.5 Gb/s (i.e., spectral efficiency: 1 b/s/Hz) for both downstream and upstream links. This network satisfies all the requirements described above. In this network, the downstream signals are transmitted by using electroabsorption-modulated lasers (EMLs) in the 4-PAM format and direct-detection PIN receivers, while the upstream signals are transmitted by utilizing directly-modulated RSOAs with cost-effective self-homodyne coherent receivers. To further enhance the cost-effectiveness of the pro-posed network, an optical frequency comb generator is utilized to provide the seed light for colorless RSOAs instead of a large number of tunable lasers. The results show that we can achieve the error-free transmission of 12.5-Gb/s signals, spaced at 12.5-GHz, even after the 20-km long standard single mode fiber (SSMF) transmission. The crosstalk-induced penalties are negligible for the downstream and upstream signals. The inter-symbol-interference (ISI) induced penalties are measured to be less than 1.1 dB and 4.2 dB for the downstream and upstream signals, respectively.
To accommodate the incessant rise in data consumption, it is essential to increase the operating speed of the optical access network continuously. However, it would be extremely difficult to increase the operating speed of the conventional time-division-multiplexed passive optical network (TDM PON) to be faster than 10 Gb/s. Thus, for the realization of the high-speed optical access network operating at the speed faster than 10 Gb/s, WDM PON has been considered as one of the most promising solution. However, it is still too ex-pensive for the massive commercial deployment due to the extra costs involved in utilizing WDM transmit-ters operating at many different wavelengths. To solve this problem, there have been substantial efforts to develop the cost-effective WDM PON operating at the speed much faster than 10 Gb/s by using colorless optical transmitters such as RSOAs. However, the modulation bandwidth of RSOA is typically less than 3 GHz due to its limited carrier lifetime. To overcome this bandwidth limitation, we propose the polar RZ 4-PAM format and evaluate the possibility of realizing high-speed RSOA-based WDM PON operating at the per-wavelength speed of 20 Gb/s by using cost-effective direct-detection receivers. For the high-speed oper-ation of RSOA, the RZ format is superior to non-return-to-zero (NRZ) format due to its temporally confined pulse shape. Thus, we first attempt to increase the operating speed of RSOA to 20 Gb/s by using the RZ 4-PAM format together with an electrical equalizer and optical delay interferometer which is used as an optical equalizer. However, in the case of using 20-Gb/s RZ 4-PAM signal, waveform is still distorted and transmis-sion distance is limited to be only ~10 km. To improve the transmission performance of 20-Gb/s upstream signal, we propose to use the polar RZ 4-PAM format which has the up-down symmetry in its waveform. As a result, we transmit the 20-Gb/s upstream signal generated by directly modulating an RSOA, which has a 3-dB bandwidth of only 3.2 GHz, over 20 km of SSMF and detect it by using a cost-effective direct-detection receiver.
We also attempt to further increase the operating speed of the RSOA-based WDM PON 28 Gb/s, which includes the forward-error correction (FEC) overhead, and enhance the transmission performance. In this experiment, we investigate the benefits of the electrical feed-forward equalizer (FFE) and decision-feedback equalizer (DFE) as well as the required optimum optical equalization characteristics. The results show that the performance of the electrical equalization used at the direct-detection receiver is fundamental-ly limited due to the square-law nonlinearity and lack of the phase information. In addition, the RSOA-based WDM PON becomes unusually sensitive to chromatic dispersion (CD) since the high-frequency com-ponents in the already extremely bandwidth-limited signal are further reduced by CD. Thus, in the case of using only the electrical equalization technique, the signal cannot be recovered properly. However, we can improve the CD tolerance of this network significantly by increasing the modulation bandwidth of RSOA using the optical equalization technique. For example, when we utilize an optical equalizer based on the delay interferometer, the bandwidth of RSOA is significantly improved due to the high-pass filter-like charac-teristics. Thus, we can improve the transmission performance of this network significantly by using both the electrical and optical equalization techniques. In this experiment, we also identify the optimum number of the pulse amplitude modulation levels for the RSOA-based WDM PON operating at the speed of 28 Gb/s. For this purpose, we compare the transmission performances of 28-Gb/s polar RZ PAM-N signals (N=2, 3, and 4). The signal operating at a lower baud rate by increasing the number of modulation levels is benefi-cial to the high-speed modulation of the bandwidth-limited RSOA. However, the higher-level modulation format requires a higher signal-to-noise ratio (SNR) to obtain the same bit-error rate (BER) performance. Thus, there is a tradeoff between the baud rate and the number of modulation levels. The results show that the best receiver sensitivity is achieved by using 2-level polar RZ signal at the back-to-back transmission, while the best performance after 20-km long SSMF transmission is achieved by using the polar RZ 3-PAM signal.
We evaluate the possibility of further increasing the operating speed and maximum reach of the RSOA-based WDM PON by utilizing the PDM technique and cost-effective self-homodyne coherent receiv-ers. For example, we demonstrate the 40-Gb/s upstream transmission in the 60-km reach RSOA-based WDM PON by using these techniques. It is well known that we can double the per-wavelength transmission speed by using the PDM technique. However, this technique is too complex and too expensive for the use in the access network because it not only requires a complicated polarization-control unit at the ONU but also expensive polarization-diversity receivers at the central office (CO). To avoid these problems and generate the 40-Gb/s upstream signal by using the directly modulated RSOAs having a limited modulation bandwidth of only ~3.2 GHz, we utilize the quadrature phase-shift-keying (QPSK) format and the proposed offset PDM technique. For this purpose, we send two seed light in orthogonal polarizations with a small frequency offset (20 GHz) to each ONU. These seed light are separated by using a simple delay interferometer at the ONU and modulated by two RSOAs at 20 Gb/s each in QPSK format. These two modulated QPSK signals are combined again by using delay interferometer and send back to the CO. At the CO, the PDM upstream sig-nals are separated by using a polarization-beam splitter (PBS), and then detect by a pair of cost-effective self-homodyne receivers. From the results, we confirm that it is possible to implement 60-km reach, 40-Gb/s RSOA-based WDM PON cost-effectively. Since the achieved spectral efficiency is 0.8 bit/s/Hz, this network can support a large number of 40-Gb/s channels.
본 논문에서는 경제적인 가입자용 광원인 반사형 반도체 광증폭기(RSOA)를 사용한 차세대 초고속 광가입자망의 구현에 관하여 기술한다. 특히, 10 Gb/s급 WDM PON의 스펙트럼 효율을 제고함으로써 수용 가능한 가입자 수를 획기적으로 증가시킬 수 있는 RSOA 기반 초고밀도 파장분할다중방식 수동형 광가입자망(UD-WDM PON: ultra-dense WDM PON)을 경제적으로 구현하기 위한 기술적 방안을 제시하고, 이러한 광가입자망의 전송성능을 최적화하기 위한 기술적 방안들에 관하여 기술한다. 또한, 급격하게 증가하고 있는 전송수요 증가를 앞으로도 지속적으로 수용할 수 있도록 가입자당 전송속도가 20 Gb/s 급 이상인 초고속 RSOA 기반 WDM PON의 경제적인 구현을 위한 다양한 기술적 방안들을 제시한다. 특히, 제한된 RSOA의 대역폭을 보상하기 위하여 전기적 이퀄라이저 또는 광학적 이퀄라이저를 사용하는 방안과 좁은 변조 대역폭으로 인하여 발생하는 파형 왜곡이나 심볼 간 간섭(ISI: inter-symbol interference) 에 대해 우수한 내성을 갖는 변조방식에 관하여 분석하고, 이와 같은 기술을 적용한 광가입자망의 성능을 평가한다. 마지막으로 RSOA기반 WDM PON의 가입자당 전송속도를 40 Gb/s급 이상으로 증가시키기 위하여 편광분할다중화 기술을 사용하는 방안을 제시하고, 코히런트 수신기술을 적용하여 이와 같은 WDM PON의 전송거리를 대폭적으로 증가시키는 방안에 관하여 기술한다.
