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In this paper, we proposed and demonstrated all-optical clock recovery at 10GHz with switchable wavelengths. Very stable clock signals corresponding to the bit rate of the injection data have been obtained by injecting 10Gbit/s 231-1 PRBS data signals into the ring cavity. Wavelength switching among eight wavelengths is achieved by merely tuning the delay time of the intra-cavity optical delay line. The proposed clock recovery method is experimentally demonstrated to be insensitive to the polarization changes of the input data.
©2005 Optical Society of America
1. Introduction
Optical clock recovery is a key technology for all-optical 3R regeneration (re-amplifying, re-shaping and re-timing). Because clock signal should be locked in both phase and frequency with respect to the received data signal, it must be extracted from the received signal. In recent years, a number of optical timing extraction techniques suitable for high-speed operation have been demonstrated [1–14]. They include optical phase-locked loops, self-pulsating distributed feedback (DFB) lasers, synchronized mode-locked fiber ring lasers and passively mode-locked laser diodes. The optical phase-locked loop technique may be the most practical method for clock recovery [1–3]. Yet this technique is complex and costly as it needs internal signal source and feedback control electronics. All optical clock recovery using a passively mode-locked laser diode has a simple configuration and can yield high quality recovered clock signals [4]. However, the method requires an intra-cavity saturable absorber in order to perform mode-locking and the recovered clock signal exhibits a narrow locking range. Clock recovery based on the self-pulsating DFB laser is another promising approach owing to its simple structure and high-speed operation [5–7]. However, the wavelength of the recovered clock signal is difficult to tune. On the other hand, optical clock recovery in a fiber ring laser actively mode-locked by the optical data stream can permit tuning and switching of the wavelength of the recovered clock signal through designing a suitable structure of the laser cavity. Besides, clock division has also been obtained using this technique. In an earlier work, it was shown that high-speed optical pulses with switchable wavelengths could be generated by optical injection into a ring cavity which used a semiconductor optical amplifier (SOA) as both the gain medium and the mode-locking element and cascaded fiber Bragg gratings (FBG) as the wavelength selecting element. In this letter, all optical clock recovery at 10GHz with switchable wavelengths is reported using the same experimental configuration as described in [15]. This proposed method of all-optical clock recovery retains the same advantages as explained in [15], i.e., reduced influence of environmental perturbation on the signal stability, simple laser configuration and convenient wavelength switching among a predetermined set of standard wavelengths. In fact, the wavelength of the recovered clock signals can span the entire gain spectrum of the SOA by incorporating FBGs with the appropriate reflection wavelengths. Furthermore, the proposed clock recovery cavity can be made such that it is independent of the polarization and wavelength of the incoming data signal in the whole gain spectrum of the SOA if the SOA is designed appropriately. Our results also show that the side-mode-suppression ratio (SMSR) of the clock signals recovered in this scheme is better than 28dB and the repetition rate is stable and remains unchanged during the process of wavelength switching.
2. Principle and experimental setup
The experimental configuration for all-optical clock recovery based on semiconductor fiber ring laser is shown schematically in Fig. 1. The 10Gbit/s return-to-zero(RZ) optical data stream at 1548.9nm is generated by modulating a tunable CW laser source using an electrical absorption modulator and then gating the optical pulses according to the input pseudorandom data by a LiNbO3 Mach-Zehnder modulator. The clock recovery cavity is composed of the 1.55µm-SOA, a polarization independent isolator, a polarization controller, eight cascaded FBGs, two optical circulators, one 10:90 coupler and an electrically controlled optical delay line. The optical data signal is launched into the ring cavity through an optical circulator and is blocked by the optical isolator after it passes through the SOA. The optical isolator and circulators ensure the unidirectional operation of the ring cavity. The power of the input optical data can be adjusted by a variable optical attenuator. The net gain of the cavity decides whether the clock signal can oscillate or not when the fundamental or harmonic frequencies of the cavity are adjusted to match the frequency of the input data signal. The synchronization between the input signal and the output clock pulses is realized by cross-gain-modulation in the SOA and harmonic active-mode-locking of the semiconductor fiber ring laser. The oscillating wavelength of the clock recovery cavity is determined by the cascaded FBGs. The peak reflection wavelengths of the FBGs are λ1=1548.7nm, λ2=1549.5nm, λ3=1550.4nm, λ4=1551.1nm, λ5=1552.0nm, λ6=1552.8nm, λ7=1553.8nm, and λ8=1554.7nm with a peak reflectivity of about 92% and a 3-dB bandwidth of 0.18nm–0.2nm. Because the physical reflection locations of the different wavelengths in the eight cascaded FBGs are different, each of the eight wavelengths can be selected one after another at the same frequency by designing an appropriate distance between adjacent gratings and adjusting the optical delay line properly. Besides, the electrically controlled optical delay line can also automatically compensate the dispersion-induced optical cavity length change during the process of wavelength switching. The recovered clock signal is coupled out through the 90:10 coupler for measurements by the optical spectrum analyzer and a photodetector connected to a digital sampling oscilloscope or the RF spectrum analyzer.
3. Results and discussion
By injecting 10Gbit/s data pulses into the ring laser and adjusting the bias current of the SOA, stable clock signals at each of the eight wavelengths have been successfully recovered one after another by carefully tuning the optical delay line. Figure 2(a) shows the eye diagram of the input 10Gbit/s 231-1 PRBS optical data. Figure 2(b) shows a typical recovered clock train at 10GHz when the SOA is biased at 155mA and the average power of the input optical data is 6.5dBm. Figure 2(c) and Fig. 2(d) show respectively the corresponding optical spectrum and RF spectrum of the recovered clock signal. The average optical pulsewidth and root-mean-square timing jitter measured by the oscilloscope are 37ps and 1.5ps respectively. The wavelength of the recovered clock signal is 1551.1nm and the 3dB spectral width is about 0.24nm. Thus the time bandwidth product of the recovered optical pulse train is over 1.1, far from the transform limit. Narrower and better quality output clock signals can be obtained by using fiber Bragg gratings with a broader and flatter wavelength response and by compensating the chirp produced in the SOA [16]. Soliton shaping effect can also be exploited through_introducing SMF and DSF fiber into the cavity to compress the pulses, which can help to obtain the short optical pulses with transform limit. However, the use of long fiber will increase the instability of the ring cavity due to small vibration, temperature fluctuations and the change of the environmental condition. The SMSR is better than 28dB for all of the 8 switchable wavelengths. In a systems application the output is likely to be filtered before being used. We have tried to use optical filter to choose the mode-locked wavelength and find that the filtering will not affect the output of the recovered clock signals since the mode-locking mechanism can effectively suppress the competition between the different wavelengths for the cavity gain. The RF noise is about 20dB below the clock signal. A harmonic signal at 20GHz is observed, but its amplitude is 10dB below that of the main peak at 10GHz. The RF spectrum contains no obvious pedestal. There exist obvious amplitude noise in the recovered clock signals. The amplitude noise may arise from the gain fluctuations, spontaneous emission fluctuations and especially the independent supermodes oscillating. Supermodes oscillate simultaneously and compete with each other, thus a fluctuation of the pulse amplitude is generated which appears as a noise in the RF spectrum. The ultra-fast gain saturation characteristics of the SOA and some other proposed methods can be use to suppress the noise in the ring cavity [17–20]. As a comparison, Figure 2(e) shows the RF spectrum of the 10GHz clock signal generated by the combination of CW laser and external modulator. Obviously, in this case the RF noise level is much lower below the clock signal. When the polarization of the input data is changed by adjusting the polarization controller, there is no obvious change in the recovered optical clock signal because the SOA used in the experiment has a small polarization sensitivity of only 0.5dB. Hence the proposed clock recovery method is insensitive to the polarization changes of the input data. Meanwhile, when the wavelength of the injected data signal is changed by tuning the CW DFB laser source which have 8 different output wavelengths from 1547.72nm to 1558.98nm, the recovered clock signals remain unchanged. So the proposed clock recovery method is also experimentally confirmed to be insensitive to the wavelength of the input data signal. When the bias current to the SOA is reduced, the intensity of the recovered clock signal will gradually weaken and then disappear because the total gain of the ring cavity provided by the SOA is below the threshold level. But if the bias current to the SOA is increased over 180mA, the output optical signal will develop a CW component because some non-mode-locked wavelengths will start to oscillate with the increasing of the gain in the cavity. At the same time, the pulsewidth of the recovered clock signal will broaden since the relative modulation depth of the SOA driven by the injected data is reduced. Similar behavior can be observed by changing the optical power of the input data while keeping the bias current to the SOA constant. In particular, if we turn off the injection signal, the recovered clock signal will disappear completely. The corresponding optical spectrum and RF spectrum indicate that the 8 different wavelengths are now competing for the cavity gain in a random fashion, as none of the modes can be mode-locked because the mode-locking mechanism is absent. The wavelength switching characteristics of the recovered clock signals are investigated by adjusting the optical delay line and the results are shown in Fig. 3, which shows clearly that the output wavelength can be switched among all those allowed by the FBGs. Indeed the range of switched wavelengths can be extended to cover the whole gain region of the SOA (about 40nm) by including FBGs whose reflection wavelengths span the entire gain region. The results for all the wavelengths are similar during the process of wavelength switching. Naturally, the switching speed of the clock pulses from one wavelength to another is restrained by the speed of the mechanical motion of the delay line, which has a typical response time on the order of seconds. The extinction ratio, which can be defined as the ratio of maximum level to minimum level in the autocorrelation trace of the clock signal, is an important figure in evaluating the quality of the recovered clock signal. Figure 4 shows the measured typical extinction ratio of the 8 different wavelengths of the recovered clock signals when SOA is biased at about 155mA and the injected optical power of the optical data signal is about 6.5dBm. The extinction ratio is sensitive to the bias current of the SOA and the optical power of the injecting data signals. When the injection optical power or the SOA-biased current is changed from the optimal values of 155mA and 6.5dBm respectively, the extinction ratio of the recovered clock signal will be reduced. The extinction ratio of the recovered clock signal needs to be improved for the practical application. Lastly, the effectiveness of the proposed clock recovery scheme when the input RZ data contain many zeros is evaluated by changing the density of the zeros in the input PRBS signal and then measuring the RF power of recovered clock signal. The measured power levels are shown in Fig. 5, which shows clearly that the proposed clock recovery scheme can still function well even when the density of zeros in the input signal is as high as 75%.
Fig. 5. RF power of 10GHz input data signal (▪) and recovered clock signal (▴) versus the density of zeros in the input data signal.
4. Conclusion
A simple all optical clock recovery scheme based on an actively mode-locked fiber ring laser employing a 1.55µm semiconductor optical amplifier which plays the roles of both a gain medium and an optically controlled mode-locker and eight cascaded fiber Bragg gratings playing the role of the wavelength selecting element has been proposed and demonstrated. Very stable clock signals corresponding to the bit rate of the injection data have been obtained by injecting 10Gbit/s 231-1 PRBS data signals into the laser cavity. Wavelength switching among eight wavelengths is achieved by merely tuning the delay time of the intra-cavity optical delay line while other parameters remain unchanged. The repetition frequency of the recovered clock signal remains unchanged during the wavelength-switching process because the optical delay line can also automatically compensate the changes in the effective cavity length induced by dispersion during wavelength switching. In principle, the wavelength of the recovered optical clock signals can cover the entire gain region of the SOA by cascading fiber Bragg gratings of the appropriate reflection wavelengths. The clock recovery scheme is experimentally demonstrated to be insensitive to the density of zeros (up to 75%) in the input PRBS data. Hence it may be useful in high-speed all optical signal processing.
Acknowledgments
This work was supported by the Research Grants Council of Hong Kong SAR Government under Grant CUHK4169/01E and Direct Grant #2050302. Also, very good comments and useful suggestions from reviewers are gratefully acknowledged.
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