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A novel scheme, for both baseband and millimeter-wave band gigabit data transmission in radio-over-fiber system, is proposed and experimentally demonstrated by using cascaded injection-locked Fabry-Pérot laser diodes. It was able to improve suppression ratio of carrier suppressed signal using the cascaded injection-locking. The suppression ratio improvement of the optical carrier suppressed signal of 20dB was verified. Applying this mechanism, 60-GHz millimeter-wave carrier of enhanced signal quality could be accomplished. Its peak power and phase noise were obtained as −40dBm and −103.5dBm/Hz respectively, which was suitable for 60-GHz data transmission. In addition, a successful bidirectional transmission of 1.25-Gbps wired and wireless data was achieved by adopting remodulation technique using a gain-saturated reflective semiconductor optical amplifier for uplink.
©2009 Optical Society of America
1. Introduction
In these days, wireless access network services have been evolved from voice and simple message to multimedia. In addition, these services have pursued liberation from temporal and spatial restrictions due to spotlight of the ubiquitous network [1–7]. To satisfy these trends, radio-over-fiber (RoF) system, the convergence between wireless network and conventional optical link, has been proposed as various system models and received attractiveness as one of the most competitive future network [7–10].
To support more broadband services of wireless, subcarrier frequency of each subscriber has become higher and higher and this tendency has been spreading from microwave to millimeter-wave (mm-wave) band. Therefore, it has been attractive issue for the RoF system how to generate the mm-wave carriers with superior signal quality and efficient cost [7–10]. These studies have been divided into two categories; one is based on single-sideband (SSB) modulation [11–15] and the other is rooted in carrier suppressed optical signal technique [16–21]. Both methods have been suggested in order to avoid dispersion-induced carrier suppression, fatal drawback of the mm-wave carrier based network [22,23]. Generally, the SSB based method have been less preferred due to its low signal-to-noise ratio (SNR) characteristic as well as requirement of local oscillator which has the same frequency to desired mm-wave carrier and therefore induces cost problem. On the other hand, the carrier suppressed optical signal technique is free from the dispersion-induced carrier suppression phenomenon and able to generate desired mm-wave carrier using just half of reference frequency. Moreover, the SNR property of the carrier suppressed optical signal is much better than that of the SSB signal.
In the carrier suppressed optical signal based technique, suppression ratio (SR) of the carrier suppressed optical signal should be satisfied with such level of which can generate the mm-wave carrier of enough RF peak power. It is because the RF power of the mm-wave carrier is determined by sidemode optical power of the carrier suppressed optical signal and this is inverse-proportion to the SR of the carrier suppressed optical signal under the same received optical power. In addition, the SNR of the mm-wave carrier is proportion to the SR of the carrier suppressed optical signal [17–21,28]. However, it is difficult to achieve high SR using a single-electrode Mach-Zehnder modulator (MZM). There are many techniques to accomplish the carrier suppressed optical signal of high SR and these can be categorized as two schemes by following; one is implemented by dual-electrode MZM [17–20] and the other is realized by optimized input modulation depth of the carrier suppressed optical signal [21]. The dual-electrode MZM based technique required a lot of RF control devices such as RF phase shifter and mixer, and phase control of RF local oscillator signal should be accurate to get high suppressed optical carrier signal [17]. For cascaded dual-electrode MZM scheme [19], this problem would be more serious because those RF control devices should be worked with both MZMs. Optical upconversion technique [20] should be operated with local oscillator signal of full swing modulation index (2Vπ for MZM) to get high suppression ratio. Optimization technique of the modulation index [21] would need some optimization circuits or modules to implement and this could increase complexity. Meanwhile, optical injection locking based techniques, which were implemented by Fabry-Pérot laser diode (FP-LD), have been also proposed to improve the SR [24–27]. However, those schemes only focused on the signal quality of generated mm-wave carrier in terms of phase noise and peak power, and validity of the data transmission for mm-wave band [24–27]. In addition, they should be based on a specific slave laser that has the same longitudinal mode spacing to desired mm-wave carrier frequency. Practically, this point would be the main drawback in terms of flexibility. Also, signal quality of the generated mm-wave carrier might be deteriorated by injection locked optical of unwanted sidemode pair [27]. Dual parallel structure of two injection-locked FP-LDs was proposed to solve these problems [28]. In this scheme, phase noise of generated mm-wave carrier, which could deteriorate signal quality of wireless data, was sensitive to polarization state of each optical sidemode of the carrier suppressed optical signal. Therefore, it is very difficult to minimize the phase noise using polarization controller.
In this paper, a novel scheme is proposed to generate the mm-wave carrier of high quality as well as to achieve simultaneous baseband and the mm-wave band wireless data transmission. The proposed scheme is implemented by using cascaded structure of injection-locked FP-LDs. To construct bidirectional radio-over-fiber link which is based on the proposed scheme, uplink signal is remodulated by using gain-saturated reflective semiconductor optical amplifier (RSOA) [29,30]. It is experimentally demonstrated and verified for the proposed scheme to analyze the SR enhancement of the carrier suppressed optical signal, the generated mm-wave carrier quality of 60GHz, and transmission characteristics for both baseband and 60-GHz band 1.25-Gbps signal.
2. Operation principle
The proposed scheme is illustrated in Fig. 1 . In order to generate the carrier suppressed optical signal which has RF sidemode spacing of 2LO as shown in inset ‘I’ of Fig. 1, a single-electrode MZM is operated with bias voltage of Vpi, bias point for minimum output optical power of the MZM, and modulates light from CW source using the reference frequency labeled as LO. The modulated signal is passed through a circulator and injected into FP-LD1. A target sidemode for the carrier suppressed optical signal, which is injection-locked using FP-LD1, can be either left or right sidemode of the carrier suppressed optical signal. As represented in inset ‘II’ of Fig. 1, the right sidemode is selected to be locked using FP-LD1 in our scheme. The injection-locked optical signal from FP-LD1 has a peak power of the right sidemode which is a dominant compared to that of other sidemodes. This signal is then passed through another circulator and injected into FP-LD2. At this time, a target sidemode of the carrier suppressed optical signal, which is injection-locked using FP-LD2, is opposite sidemode which is compared to that of FP-LD1. In the proposed scheme, the left sidemode is set to be locked using FP-LD2 as shown in inset ‘III’ of Fig. 1. During the second injection-locking, locked sidemode using FP-LD1 is also suppressed. Nevertheless, it is possible to achieve that most of its peak power can be remained by using an appreciate control of injection-locking operation parameters such as injected optical power. It is possible to cause the locked sidemode from FP-LD1 to have enough-high peak power and to be reflected from FP-LD2 without a serious suppression. Therefore, it is able to neglect to generate the mm-wave carrier and finally, the SR of the carrier suppressed optical signal can be improved.
Direct modulation of the downlink data is preferred to be performed at FP-LD2. If the data were carried on the carrier suppressed optical signal at FP-LD1, this data is also suppressed at FP-LD2 due to the optical injection locking, which is well known as the residual amplitude modulation suppression (RAMS) [31]. This may decrease receiver sensitivity of the downlink data. The carrier suppressed optical signal can be also modulated by the downlink data using both FP-LD1 and FP-LD2 simultaneously. In this case, length of optical delay line, which is proportion to transmission time from FP-LD1 to FP-LD2, should be considered to avoid inter-symbol interference between two modulated optical signals from each FP-LD. Due to its complexity, this issue has been neglected in this work.
From a central office (CO), modulated carrier suppressed optical signal is transmitted to a base station (BS) and splitted into two paths; one is for recovery of the downlink signal using the heterodyne receiving method [32], and the other is utilized to make the uplink signal using the remodulation technique, based on high pass filter characteristic of the gain-saturated RSOA. Finally, the remodulated signal is transmitted back to the CO and recovered.
3. Experiments
Experimental setup of the proposed scheme is illustrated in Fig. 2 . A tunable laser source (TLS) was used as the CW source. Its wavelength and optical power were set to be 1542.92nm and 1.95dBm, respectively. The single-electrode MZM of 40-GHz frequency response was biased at Vpi point of 4.523V and modulated using a local oscillator (LO) of 30-GHz from a RF signal generator. The power of 30-GHz LO signal was amplified to be 20dBm using a low noise amplifier (LNA) in order to achieve maximized modulation efficiency of the used MZM itself, which is proportion to the SR of the carrier suppressed optical signal. Output optical power of the modulated signal was controlled by using an Erdium-doped fiber amplifier (EDFA) followed by a variable optical attenuator (O.A) in order to maintain injection optical power into FP-LD1 for optimized injection-locking operation. In addition, a wavelength detuning of each FP-LD was adjusted to achieve injection-locking operation for target sidemode by using bias current and temperature control of FP-LD1 and FP-LD2 as shown in Fig. 2. 1.25-Gbps PRBS data, from a pulse generator, was utilized to modulate both FP-LD2 and the gain-saturated RSOA for downlink and uplink transmission, respectively. To be suitable for the remodulation, the gain saturated RSOA was biased at 30mA and its input optical power was set to be larger than −20dBm. Peak-to-peak voltages of the modulation signals were set as following; 0.5V for downlink and 2V for uplink, to minimize side effect by incompletely suppressed downlink signal into the remodulation. A 23-km standard singlemode (SSMF) optical fiber was used in transmission. Polarization controllers (PCs) were inserted to maximize coupling efficiencies of both FP-LDs, the MZM, and the RSOA. To recover 60-GHz wireless signal, a LO of 60GHz followed by a LNA was used in the receiver module.
4. Results and discussions
Optical spectra were measured from several point of the experimental setup, which is illustrated as five points from A to E in Fig. 2, to estimate the SR enhancement of the carrier suppressed optical signal. Figure 3 shows its results. As represented in inset ‘C’ of Fig. 3, the SR of the MZM output was at most 5dB. This was improved by 20dB using the proposed scheme as shown in inset ‘E” of Fig. 3. As pointed in the previous section, it was verified that the first locked sidemode (the right sidemode of the carrier suppressed optical signal as shown in inset ‘D’ of Fig. 3) was suppressed by only less than 2dB after the second injection locking.
Mode-partition noise (MPN) from both FP-LDs was the most critical factor to deteriorate the signal quality of the mm-wave carrier in the proposed scheme. Additional phase noise was accumulated into every sidebands of the carrier suppressed optical signal because the MPN has low-frequency-property. Generally, the MPN has been intrinsically suppressed by using the injection-locking operation because it could modify the feature of the multi longitudinal optical source to be a quasi single-mode laser [29]. To analyze this operation, RF spectrum of low frequency band was measured with and without the locking. The phase noise of the mm-wave carrier could also be estimated by verifying low frequency noise of detected signal because the phase noise was proportion to the low frequency noise due to heterodyne receiving method [32]. The low frequency noise spectra were measured as shown in Fig. 4 . When the locking did not work simultaneously in both FP-LDs, the low frequency noise of 15dB was added due to the maximized MPN. An impulse noise around 2.5GHz band was also detected. This was caused by the relaxation oscillation characteristics of the used FP-LDs. Under optimized injection-locking condition, there was no relaxation oscillation peak was observed until 3GHz. This means the frequency responses of used FP-LDs were expanded by injection-locking operation.
RF spectrum and the phase noise of the generated mm-wave carrier are shown in Fig. 5 . A 60-GHz peak power of −40dBm was achieved under received optical power of −24.5dBm. Phase noise of this carrier was −103.5dBm/Hz at 1MHz frequency offset and this was similar to that of the 60-GHz-LO-output.
Stability of the proposed scheme was analyzed by verifying the phase noise under various condition of the injection locking. Most representative parameters were the wavelength detuning between the target sidemode and the longitudinal mode of FP-LD for the locking and the injected total optical power into the FP-LD. Since the optical spectrum of the injected optical signal for both FP-LD 1 and FP-LD 2 were different as illustrated in Fig. 3-C and D, it was very difficult to estimate locking stability using the phase noise of the generated mm-wave carrier with the peak power of the sideband for each injected optical signal. Due to the same reason, several system parameters, such as the external injection total power of FP-LD1 in Fig. 6-(a) and the wavelength detuning in Fig. 6-(b), were fixed as 5dBm and 0GHz, respectively, for simplicity of the analysis. The phase noise of −90dBm/Hz (1-MHz frequency offset) was set as threshold level in order to achieve the mm-wave carrier of suitable signal quality for transmission. Based on these criteria, dynamic ranges of 0.06nm and 15dB, which were estimated as the wavelength detuning and the injected optical power, respectively, could be achieved as illustrated in Fig. 6.
Fig. 6 Phase noise sensitivity analysis for the injection locking parameters. (a) wavelength detuning (b) external total injection power of FP-LD.
In the proposed scheme, each sideband of the carrier suppressed optical signal is optically locked at each FP-LD. This means that if frequency difference between two sideband of the carrier suppressed optical signal is less than the longitudinal mode spacing of used FP-LD, it would be possible to get the mm-wave carrier of the desired frequency. Of course, if the frequency difference between each sideband of the carrier suppressed optical signal is too narrow, locking condition could be unstable and this would deteriorate signal quality of the generated mm-wave carrier. Thus, in case of millimeter-wave generation in the proposed scheme, the locking condition would be fairly stable.
Bit-error-rate (BER) and eye diagram of the bidirectional baseband and the mm-wave band wireless data transmissions were measured and represented in Fig. 7 . In the 23-km transmission of downlink baseband (BB), receiver sensitivity (BER<10−11) of −27dBm was achieved. The 2-dB power penalty was happened in the transmission of the 60-GHz downlink data compared to the BB case by difference of quantum efficiency between both PDs which were utilized to recover the BB and the 60-GHz band data selectively for the convenience of the experiment. In both the BB and the 60-GHz band cases, there were power penalties of 0.5dB between back-to-back (BtoB) and 23-km transmission. This was caused by attenuation and dispersion during signal transmission. It was estimated that these effects were negligible because of the measurement error. In the uplink transmission, 2.5-dB power penalty was verified which was caused by the remodulation process. Incompletely eliminated downlink-baseband signal deteriorated the signal quality of the uplink transmission. In addition, the power penalty of 3.5dB was measured in the 23-km transmission of the uplink data, compared to the BtoB. This was due to the Rayleigh backscattering as well as the attenuation and the dispersion of the fiber. However, the receiver sensitivity of the uplink transmission was still higher than −20dBm. This was the threshold level of used RSOA which to operate in gain-saturation region. Therefore, if the gain-saturation condition of the RSOA was achieved, it was able to construct the uplink transmission link as well as the downlink system.
Fig. 7 BER characteristics of the bidirectional transmission of the proposed scheme. (a) downlink (b) uplink.
The uplink mm-wave wireless signal transmission was not demonstrated in the proposed scheme. That was because the proposed scheme was based on an RSOA for the uplink data transmission. Generally, RSOA has a limitation as a modulator because of its frequency response [29,30,33]. However, the proposed scheme still has a potential ability to transmit the uplink mm-wave wireless signal using down conversion technique [34]. What the downlink data stream is not activated, it would be possible to get the mm-wave carrier at BS by applying time-division-multiplexing (TDM). And this carrier could be used as a local oscillator signal for up-conversion of the uplink mm-wave wireless signal.
Compared to all the other approaches for mm-wave carrier generation, the proposed scheme has several benefits respect to following reasons. In the proposed scheme, each sideband of the carrier suppressed optical signal is independently locked at each FP-LD in order to improve the suppression ratio of carrier suppressed optical signal, and downlink data stream is directly modulated using one of them. There is no need of RF control devices as well as the optimization process of input modulation index of local oscillator signal. Injection locking process is operated for only one sideband of the carrier suppressed optical signal, so unwanted sidemode locking pairs which degrade signal quality of generated mm-wave carrier would not happened. Also, there is no limitation of mode spacing for used FP-LDs to get desired frequency of the mm-wave carrier, so it would be much easier for the proposed scheme to be implemented practically.
7. Conclusion
A novel scheme, which is based on the cascaded structure of injection-locked FP-LDs and the remodulation of the gain-saturated RSOA, have been proposed to generate the mm-wave carrier and implement simultaneous baseband and the mm-wave wireless 1.25-Gbps transmission link. It was experimentally demonstrated that the 60-GHz carrier could be achieved, which had the phase noise of −103.5dBm/Hz at the 1-MHz frequency offset and the RF peak power of −40dBm, by using the received optical power of −24.5dBm. Based on the proposed scheme, simultaneous transmission of the 1.25-Gbps baseband and the mm-wave band wireless data for the downlink and the 1.25-Gbps baseband data transmissions for the uplink were also accomplished in the receiver sensitivity of −21dBm. This scheme would be useful for the application of combined transmission of wireless and wired networks.
Acknowledgments
This work has been supported by Ministry of Knowledge Economy through Electronics and Telecommunications Research Institute (ETRI), Korea and Yonsei University Institute of TMS Information Technology, a Brain Korea 21 program, Korea.
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