Gain-Saturated Reflective Semiconductor Optical Amplifier Cascaded Electroabsorption Transceiver for Enhanced Remote Node Performance Moon-Ki Hong and Sang-Kook Han Department of Electrical and Electronic Engineering Yonsei University Seoul, Korea
[email protected] device operated in reverse bias condition, it is possible for EAT to be a photo detector as well as modulator [7]-[11]. But, distortion products might be generated when the wireless data is modulated by EAT due to its nonlinear property, and these components can degrade the system performance as interference noises. So, it is very important to linearize EAT. Besides, DICS phenomenon which can be one of the serious drawbacks for high RF frequency band should be suppressed [12].
Abstract—A novel remote node (RN) scheme based on gain saturated reflective semiconductor optical amplifier (GS-RSOA) cascaded electroabsorption transceiver (EAT) is presented for linearization and reduction of dispersion induced carrier suppression (DICS) for uplink transmission. The help of gain saturation property of RSOA, the transfer curve of EAT can be linearized and transmission performance can be robust from DICS. By using the proposed scheme, carrier to interference ratio (CIR) was enhanced by 10.7dB at the 10GHz frequency band and 33.4dB gain of 5.65GHz RF frequency was measured for 75.6km data transmission.
In this paper, a novel RN scheme using GS-RSOA cascaded EAT is introduced to improve the linearity for the upstream data transmission and suppress DICS. Linearization can be accomplished by modifying the transfer curve of EAT to be linear. In order to this mechanism, output power of EAT is selectively controlled by GS-RSOA. In addition, using the proposed scheme, DICS problem can be overcome because of negative chirp property of GS-RSOA. Besides, the efficiency of uplink transmission can be improved by amplification function of RSOA.
I. INTRODUCTION In these days, wireless data transmission service is being changed from voice and simple message to multimedia, so data capacity has become much larger and larger. In addition, the number of wireless mobile subscribers is explosively increased. In order to satisfy these trends, radio frequency needs to be higher than the present band - for example, millimeter wave. However, in this frequency, it is difficult to transmit wireless data without the fading which would be occurred by various obstacles such as buildings, cars, and so on. Radio-over-fiber (RoF) system utilizing optical link can be one of the best way to solve this hardship because it can support reliable, fading-free, and broadband data transmission [1]-[3]. In RoF system, wireless data is optically modulated in the central office (CO) and transmitted through optical fiber. Then, this signal is electrically demodulated in the RN and delivered to each subscribers using antenna [4]-[6].
II. OPERATION PRINCIPLE Operation principle of the proposed scheme is illustrated in Fig. 1. There are two different light sources at the CO. The wavelengths of each light source are individually distributed for downlink and uplink data transmission. Downstream data is modulated to the wavelength λD, and coupled with the wavelength λU for uplink and transmitted to the RN through optical fiber. Then at the RN, downlink signal is converted to wireless data by EAT and delivered to subscribers using antenna. Simultaneously, wireless data which is sent from subscribers is optically modulated by EAT and transmitted to the CO through fiber. Finally at the CO, uplink data is filtered by optical bandpass filter which has the center wavelength of λU and then demodulated at the receiver.
There are many elements to construct the whole RoF system. Especially, EAT is very attractive device as electrical to optical converter in the RN because EAT is robust from frequency chirping, and has lower operation bias voltage compared to electrooptic modulator such as Mach-Zehnder modulator (MZM). Moreover, EAT is much smaller than MZM, so it is easy to integrate the RN by using an EAT. Finally, since EAT is a pn junction semiconductor based
At this time, linearization for uplink can be achieved by cascading GS-RSOA to the EAT at the RN. In other words,
This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R01-2005-000-10176-0).
1-4244-1168-8/07/$25.00 ©2007 IEEE.
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A, B in the Fig. 2 and its spectrum is represented in insets of the figure. 1545nm and 1551nm were utilized for downlink and uplink, respectively. To improve the efficiency of downlink transmission, EDFA was connected to the output of 3-dB optical coupler. And tunable optical bandpass filter was used for extracting of uplink optical carrier λD. In the RN part, EAT was installed and two tone RF carriers of 10GHz and 10.01GHz which have the RF power of 0dBm were modulated and delivered to GS-RSOA through circulator. To achieve maximum linearity for EAT itself, bias voltage was set to be -1.6V. In order to gain saturated operation, RSOA was biased to 65mA and input optical power of RSOA was set to -4dBm. Finally, the proposed scheme was compared to the case of EAT only used at the RN. EDFA and tunable optical attenuator were added to the end of optical bandpass filter for objectivity of uplink receiver.
the transfer curve of EAT can be linearized by utilizing the proposed scheme. To modify the transfer curve of the EAT as illustrated in the bottom of Fig. 1, the output power of EAT should be selectively increased or decreased for linearization. In detail, if the output of EAT is large, optical power needs to be decreased and vise versa. It is generally known that GSRSOA satisfies this requirement. Therefore, by cascading GSRSOA to the EAT, the linearity of EAT can be improved.
Fig. 1 Operation principle of the proposed scheme
In addition, GS-RSOA has a negative chirp parameter [13]-[15]. So optical signals passing through GS-RSOA are experienced self phase modulation (SPM), and their spectrum is changed to asymmetric double sidebands [15]. Therefore utilizing this fact, chromatic dispersion effect for uplink can be reduced and consequently, it is possible to implement the data transmission system which is robust to DICS. Moreover, because of an amplification function of RSOA, the efficiency of data modulation for EAT can be enhanced simultaneously. In the proposed scheme, any electrical devices and complicated operation control do not required. Therefore, it is easy to improve the whole system performance for uplink transmission by using this structure.
Fig. 2 Experimental setup of the proposed scheme
A. Linearity Improvement of EAT In general, most applications of EAT based RN are set to be sub-octave system. In this system, 2nd order harmonic distortion products do not affect the transmission performance because they can be easily filtered out. However, 3rd order intermodulation distortion (IMD3) products need to be suppressed for linearity of transmitter because it is very hard to separate these ones from original carrier. Therefore, purpose of the proposed scheme was decided to reduce these IMD3 products.
III. EXPERIMENTS AND RESULTS From now on, the proposed scheme is analyzed by the experimental demonstration. The experimental setup is described in Fig. 2. When the CO part was constructed, DFBLD and tunable light source (TLS) were used for making the wavelength of downlink and that of uplink, respectively. At this time, the optical spectrum was measured at each point of
Fig. 3 is illustrated the spectrum measurement of RF signals which were modulated by EAT for back to back transmission through RF spectrum analyzer. There was CIR enhancement of 10.7dB for using the proposed scheme compared to EAT only case. As mentioned in section 2, the
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transfer curve of EAT had a nonlinear figure. To linearize this, the output optical power of EAT should be selectively increased or decreased. In order to satisfy this condition, GSRSOA was cascaded to EAT. Fig. 4 is represented the transfer curve of EAT that was used in the experiment. Relate to the bias voltage of -1.6V, output power of EAT should be controlled from +2dB to -2dB. According to the calculation of required gain, device having the gain slope of -0.48 for input optical power was required to be connected to the end of EAT as shown in Fig. 5. When GS-RSOA was operated at 65mA of bias current, its gain slope was -0.481. Under this operation condition, RF carriers of 10GHz and 10.01GHz were increased about 3dB by amplification function of RSOA and IMD3 were suppressed by 7.7dB because of linearization of the EAT. Fig. 5 Calculated required gain of the RSOA
Fig. 3 Measured RF spectrum of the proposed scheme compared to EAT only Fig. 6 SFDR enhancement measurement of the proposed scheme
B. Mitigation of DICS Next, DICS suppression for uplink data transmission was experimentally estimated when the proposed scheme was used. First of all, to make DICS phenomenon, optical fiber of 75.6km was utilized based on the experimental setup of Fig. 2. And then, by using network analyzer, the frequency response of EAT was measured through sweeping the RF modulation frequency of EAT. As indicated in Fig. 7, for EAT only structure, DICS was maximized in the frequency range of 5.65GHz. At this time, it was easily verified that DICS was mitigated by using the proposed scheme. That is, RF gain of 33.4dB for 5.65GHz band was obtained using the proposed structure based RN through 75.6km transmission. Fig. 8 is illustrated the received RF power by varying the transmission distance. As shown in this plot, GS-RSOA cascaded EAT structure kept RF signals from being degraded by dispersion. As mentioned above, this was caused by the asymmetric optical spectrum feature of the output power from GS-RSOA due to its negative chirp parameter. Therefore, chromatic dispersion of uplink data transmission can be suppressed by using the proposed scheme.
Fig. 4 Transfer curve of the EAT used in the experiment
Finally, as illustrated in Fig. 6, the 11dB enhancement of spurious-free dynamic range (SFDR) was experimentally verified which was related to the calculated noise floor of -121dB/Hz by using the proposed scheme.
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for uplink can be improved by utilizing the proposed structure, there is no solution to enhance the photo detection (PD) of EAT for downlink. However, if the bias of EAT is set to be adequate point in which PD efficiency is better than modulation one, sacrificed modulation efficiency can be compensated by amplification of RSOA. Therefore, compared to the EAT only case, it is possible to improve the modulation and PD performances of EAT simultaneously by using the proposed scheme at the RN. REFERENCES [1] [2] Fig. 7 DICS measurement by varying modulation frequency
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Fig. 8 DICS measurement by varying transmission distance
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IV. CONCLUSION AND DISCUSSION In this article, GS-RSOA cascaded EAT scheme was introduced and experimentally demonstrated to improve the linearity and mitigate the DICS for uplink at the RN. Linearization of EAT could be achieved by modify the transfer curve of EAT to be linear, so the output power of EAT should be selectively increased or decreased. This requirement is satisfied by connecting GS-RSOA to the end of EAT. It was experimentally measured that 10.7dB of CIR and 11dB of SFDR were enhanced. In addition, since GS-RSOA has negative chirp parameter, it was possible to suppress the chromatic dispersion which can be generated in the uplink transmission and consequently DICS could be mitigated. For 75.6km transmission, RF gain of 33.4dB was experimentally obtained in the frequency band of 5.65GHz by using the proposed scheme. Moreover, as the help of amplification by RSOA, the modulation efficiency of EAT for uplink could be improved. For these reason, the proposed scheme can be very useful to enhance RN performance in the trends of high capacity and high frequency band for wireless data transmission service.
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EAT has receiving function for downlink as well as modulation for uplink. However, these two functions are trade-off relationship. Nevertheless the modulation efficiency
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