All-optical clock recovery from RZ-format data by using a ... - IEEE Xplore

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 20, NO. 9, SEPTEMBER 2002

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All-Optical Clock Recovery From RZ-Format Data by Using a Two-Section Gain-Coupled DFB Laser Weiming Mao, Yuhua Li, Member, OSA, Mohammed Al-Mumin, and Guifang Li, Member, IEEE

Abstract—High-speed all-optical clock recovery using a two-section gain-coupled distributed feedback laser is demonstrated operating in the coherent and the incoherent modes. It is found that the coherent mode has a much better performance compared with the incoherent mode. The performance of the coherent clock recovery scheme at 12 and 40 Gb/s, including wavelength and polarization insensitivity, timing jitter, and phase noise; power penalty; sensitivity; dynamic range; locking bandwidth; detuning range of the injection wavelength; and lockup time are described in detail. A comparison between the performances of the two modes of operation is also presented. Index Terms—Distributed feedback (DFB) laser, injectionlocked oscillators, laser modes, optical clock recovery, semiconductor optical amplifier (SOA).

I. INTRODUCTION

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N DIGITAL communications, a clock must be used as the timing basis to sample the received signal. Because the clock should be locked in both phase and frequency with respect to the received data signal, it must be derived from the received signal. The process of deriving a synchronized clock signal from a received signal, instead of transmitting it along with the data, is known as clock recovery. Clocking signals can be extracted and recovered from a data signal through electrical [1], [2], optical [3]–[14], or hybrid techniques [15], [16]. The clock recovery circuit is a critical component of an optical receiver, which also consists of a photodiode, amplifiers, and a decision circuit. The receiver performance is characterized by measuring the bit error rate (BER), defined as the probability of incorrect identification of a bit by the decision circuit of the receiver. The average optical power corresponding to a ) is a measure of receiver sensitivity. certain BER (e.g., The receiver sensitivity can be degraded if the recovered clock has phase noise or timing jitter. The best sampling time corresponds to the situation in which the signal level difference between 1 and 0 b is maximal. If a bit is not sampled at its center, the sampled value is reduced by an amount that depends on the is a random variable, the reduction in timing jitter . Since the sampled value is also random. The BER increases as a result of timing jitter of the recovered clock. The BER can be mainManuscript received March 12, 2002; revised May 29, 2002. W. Mao was with the School of Optics/CREOL, University of Central Florida, Orlando, FL 32816-2700 USA. He is now with CENiX Incorporated, Allentown, PA 18106 USA. Y. Li and G. Li are with the School of Optics/CREOL, University of Central Florida, Orlando, FL 32816-2700 USA (e-mail: [email protected]). M. Al-Mumin was with the School of Optics/CREOL, University of Central Florida, Orlando, FL 32816-2700 USA. He is now with the College of Technological Studies, Shuwaikh 70453, Kuwait. Digital Object Identifier 10.1109/JLT.2002.802214

tained by increasing the received optical power. This increase is the power penalty induced by timing jitter. In general, the root mean square (rms) value of the timing jitter should be below 10% of the bit period for a negligible power penalty [17]. The all-optical clock recovery system is an important component for high-speed optical time-division multiplexing (OTDM) systems, where the aggregate data rate is above the state-of-the-art electronic speed limit. To perform high-speed all-optical switching and regeneration, a synchronized optical pulse train is required to provide timing signals. All-optical clock recovery has been demonstrated using self-pulsating laser diodes [3]–[7], passively mode-locked semiconductor laser [8]–[10], and fiber lasers using fiber and semiconductor nonlinearities [11]–[14]. Of the various techniques demonstrated, self-pulsating lasers are of significant interest because they have such advantages as compactness, high speed, and wide-locking bandwidth. In this paper, a comprehensive description of all-optical clock recovery from return-to-zero (RZ) format data using a two-section (TS) gain-coupled distributed feedback (DFB) laser is presented. In Section II, the modes of operation and mechanisms of clock recovery are explained in detail. Section III describes the experimental setup used to demonstrate the all-optical clock recovery schemes. In Section IV, important characteristics of clock recovery, including phase noise or timing jitter, dynamic range, and lockup time are presented. Finally, the performance of two operating modes is compared at 40 Gb/s in Section V. II. MECHANISM AND THEORY A. Modes of Operation All-optical clock recovery using TS–DFB lasers can operate in two modes [18], [19]. In the first mode of operation, the incoming data is carried on a wavelength that is at least 1 nm away from the free running wavelength of the TS–DFB laser [20], as shown in Fig. 1. The injected data signal optically modulates the carriers in the first section of the TS–DFB laser. As a result, the optical mode belonging to the first section is intensity modulated. This modulation will generate two sidebands separated from the first mode by the clock frequency. The optical mode belonging to the second section of the TS–DFB is then injection locked to one of these sidebands if they both are close enough in frequency and that sideband has sufficient power. Once locked, the beating frequency of the two optical modes is exactly the clock frequency of the data. We shall call this mode of clock recovery “incoherent clock recovery” since it relies on intensity-induced carrier density fluctuations. Because

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Fig. 2. Schematic diagram demonstrating mechanism of coherent clock recovery. Fig. 1. Schematic diagram demonstrating mechanism of incoherent clock recovery. The first and second mode belong to the front and back sections of the TS–DFB, respectively. The sidebands are generated through carrier modulation from the injected data signal containing the clock. BPF: Bandpass filter. AMP: Optical amplifier.

tional component that is needed for the coherent scheme (Fig. 2) is a probe laser. B. Theory of Operation

the incoming data needs to optically modulate the carrier of the TS–DFB laser, strong injection, which is on the order of a few milliewatts, is required especially for high-speed clock recovery. Since typical values of optical power at the receiver are on the order of tens of microwatts, some form of optical amplification will be necessary. As expected, this mode of operation possesses the advantage of wavelength and polarization insensitivity (if the TS–DFB laser is properly designed). The drawback of the incoherent scheme is that the recovered clock will inevitably retain residual data patterns since one of the optical modes is intensity modulated by the data signal. In addition, strong injection may cause instability of the TS–DFB laser. In the second mode of operation, the incoming data is carried on approximately the same wavelength as the free-running wavelength of the TS–DFB laser [18]. The mechanism for clock recovery for this mode of operation is essentially injection locking [21]. The two mutually coherent optical modes contained in the incoming data generated from the clock signal lock the frequencies and the phase difference of the two modes in the two sections of the DFB laser. As a result, the output intensity of the DFB laser becomes the recovered clock signal. We shall call this mode of clock recovery “coherent clock recovery” since it relies on coherent interaction between the optical fields of the incoming data and those inside the DFB laser. Sensitivity for this mode is on the order of a few microwatts. It is wavelengthand polarization-sensitive. However, we can deploy wavelength conversion to render coherent clock recovery wavelength- and polarization-insensitive. The principle of operation, shown in Fig. 2, is straightforward. By inserting a semiconductor optical amplifier (SOA) and a probe laser, an incoming signal at any arbitrary wavelength is converted onto the probe laser, which operates near or on the same wavelength of the TS–DFB laser. The wavelength-converted signal then recovers the clock signal coherently, insensitive to the wavelength of the original incoming signal. Furthermore, if the SOA is polarization-insensitive and a polarization-maintaining fiber is used between the SOA and the TS–DFB laser, the scheme also becomes polarization-insensitive. Compared to the incoherent scheme (Fig. 1), the only addi-

Since the underlying mechanism of clock recovery using TS–DFB lasers is essentially optical injection locking, it is reasonable to begin with a theoretical analysis of injection locking in DFB lasers. An injection-locked laser is characterized by the Van der Pol equations [22] for coupled oscillators. The photon number , the carrier number , and the instantaneous phase between the slave and master laser satisfy the difference following coupled differential equations [23]: (1) (2) (3) is where is the photon lifetime, is the velocity of light, is the inthe group refractive index, is the cavity length, jected photon number, is the bias current, is the spontaneous is the frequency difemission lifetime, ference between the master and slave lasers, is the linewidth is the gain, where enhancement factor, and is the differential gain, and and are the carrier density and the carrier density at transparency, respectively. In the steady state of injection locking, the two lasers must oscillate at the same frequency. The phase of the slave laser relative to that of the master laser is locked at a constant value . This means . The stationary solutions of (1)–(3) . and are found by further setting . Therefore, (4) is defined as the injection ratio. where , (4) can be written By introducing the constant as (5)

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where the locking-half bandwidth is given by (6) From (5), the locked phase can be determined as a function of as the frequency detuning (7) Now we can apply the above results to the TS–DFB lasers for clock recovery applications. Assuming that injection locking of two optical modes are independent, the phase of the recovered clock relative to the incoming data is determined by (8) and ( and ) are the phase (frequency) where differences between the two optical modes of the free-running TS–DFB lasers and the two optical modes corresponding to the and are the clock signal in the injected data signal. locking-half bandwidths of the two optical modes. This equation suggests the following two observations. First, a change in injection power will result in a change in and ) and thus causes a relalocking bandwidths ( tive phase shift between the recovered clock signal and the clock signal contained in the injected data signal. Second, a change in and ) would cause a phase the frequency detuning ( shift of the recovered clock signal. The frequency detuning is determined by both the wavelength and the bit rate of the injection signal. Since the bit rate drift is much smaller than the wavelength shift, which can be on the order of hundreds of megahertz, the performance of the recovered clock is mainly dominated by the stability of the wavelength of the probe lasers with respect to that of the TS–DFB lasers. One way to reduce the phase shift caused by wavelength drift is to balance the injection ratios for the two sections. In so doing, is equal to ; consequently, the optical phases of the two locked modes drift in the same direction by the same amount. Therefore, their effects on the phase of the recovered clock cancel each other. Phase shifts of the recovered clock can be clearly observed in simulations when the wavelength drifts. Fig. 3 is the simulated relative phase shift (the absolute phase shift divided by ) caused by a 500-MHz wavelength drift as a function of the power ratio between the carrier and clock component demonstrating the two observations described previously. For a given ratio, the relative phase shift is calculated by comparing the phases of the recovered clock before and after the 500-MHz wavelength change of the injection signal. In this simulation, a sinusoidally modulated optical signal is injected into the TS–DFB laser. The optical injection ratio, defined as the ratio of the average injected power to the average output power of the TS–DFB laser, is 13 dB. The amplified spontaneous emission (ASE) of the SOA is not taken into consideration. The clock component of the injection signal locks the mode belonging to the first section, and the carrier locks the mode belonging to the second section. The average power or carrier power of the injection signal is kept constant while the power

Fig. 3. Relative phase shift (the absolute phase shift divided by 2 ) caused by a wavelength drift of 500 MHz versus the ratio of carrier power to clock power.

ratio between the carrier and clock component is changed. The phase shift decreases with the increase of the power ratio initially. It reaches a minimum at 22 dB (corresponding to the balance of injection ratios for the two sections) and then sharply increases. This sharper slope is a result of the smaller static due to the weaker clock component. Inlocking range creasing the injection power can also reduce the phase shift and ). through increasing the static locking ranges ( The best way to reduce the phase shift is to integrate monolithically the probe laser and TS–DFB lasers on a single chip. Since they are on the same chip, the wavelength difference is stable and is insensitive to ambient temperature variations. To obtain high-quality clock signals, it is very important to balance the injection ratios for the two sections. Typically, the optical carrier is much stronger than the sidebands, especially after the cross-gain modulation (XGM) wavelength conversion. Therefore, it is necessary to have the sideband to lock the mode belonging to the first section and the carrier to lock the mode to the second section, since the absorption at the first section of the TS–DFB laser will introduce attenuation. III. EXPERIMENTAL SETUP In this section, we describe details of the experimental setup used to demonstrate all-optical clock recovery using TS–DFB lasers. The 12-Gb/s RZ data was generated by two cascading LiNbO modulators, as shown in Fig. 4(a). The first analog LiNbO modulator (UTP Extended Frequency Response Analog Modulator) modulated the output of a DFB laser at 1560 nm with a clock signal. The resultant optical clock signal was then modulated with a second LiNbO digital external modulator (UTP OC-192) driven by a pseudorandom . bit stream (PRBS) with a pattern length of Fig. 4(b) shows the scheme of generating 40-Gb/s RZ-format data. Data pulses were generated with an actively mode-locked fiber laser at a 10-GHz repetition rate (an actual rate of 9.6 GHz). Because of the relatively slow response of the electronic feedback loop for pulse stabilization, the output pulse train has a timing jitter of approximately 1 ps. The pulse train was then encoded with 10-Gb/s PRBS data through an external modulator. long. A 40-Gb/s data stream The data pattern length was was generated through a two-stage optical time-division multiplexer. The length difference of two arms is approximately 30 m

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IV. EXPERIMENTAL RESULTS FOR COHERENT CLOCK RECOVERY In this section, we describe the results of the all-optical clock recovery demonstration using TS–DFB lasers. Since the coherent scheme has a much better performance than the incoherent scheme, our discussion shall focus on coherent clock recovery. The performance of these two operating modes will be compared in Section V. A. Wavelength and Polarization Insensitivity

Fig. 4. Experimental setup for generating RZ-format data at (a) 12 Gb/s and (b) 40 Gb/s. PC: Polarization controller. MZI: Mach–Zehnder intensity modulator. MFL: Mode-locked fiber laser.

in the first stage and 15 m in the second stage. The polarizations of the 40-Gb/s pulses were aligned through a polarization controller in each stage. The basic scheme of coherent clock recovery has already been shown in Fig. 2. The Alcatel SOA used in the experiment has an active region of length 800 m, width 1.2 m, and thickness 0.2 m. It was biased at 145 mA and provided a 23-dB fiber-to-fiber gain. The polarization sensitivity of the SOA is approximately 0.3 dB. This SOA was used to convert the wavelength of the incoming date through XGM. Using wavelength conversion, the clock recovery operation is both wavelength- and polarization-insensitive because the arbitrary incoming wavelength and polarization is converted to the wavelength and polarization of the probe which are locally aligned with those of the self-pulsing laser. The probe power was set to be 10 dBm. The converted signal was selected out by a filter and then injected into the two-section gain-coupled DFB laser with an output wavelength of 1543.3 nm. The two sections were biased at 39.5 mA and 41.0 mA for 40 Gb/s, and 30.2 mA and 65.3 mA for 12 Gb/s, respectively. The in-fiber injection power was approximately 200 W. The recovered clock was detected by a 45-GHz photodetector followed by a 50-GHz amplifier (HP83051A) and then investigated by a RF spectrum analyzer (HP8565E) with a phase-noise measurement capability. In the time domain, the waveforms were measured by a 50-GHz digital sampling oscilloscope. An 11-GHz optical receiver followed by a 100-MHz bandpass filter and a 35-GHz optical receiver (Discovery DSCR 404) followed by a 400-MHz narrow-band amplifier were used for detecting 12-GHz and 40-GHz clock signals, respectively. BER measurement was also performed at 12 Gb/s.

Polarization-insensitive operation of coherent clock recovery using TS–DFB lasers was achieved by wavelength conversion. Among the schemes proposed for wavelength conversion, XGM in SOAs has the simplest structure. The efficiency of wavelength conversion depends on both the probe and pump powers. If a high extinction ratio is desired, we should use lower probe powers. However, for clock recovery, a strong clock component, rather than the extinction ratio, is desired. The strength of the clock component depends not only on the extinction ratio but also on the average power of the output at probe wavelength. In other words, the power excursion of the output probe should be maximized for a given pump power. Fig. 5 shows the power of the 40-GHz clock component as a function of the probe power measured experimentally under different pump levels. In general, the optimal value of probe power is approximately equal to the pump power. To insure reasonable sensitivity (low pump power) as well as large dynamic range, a moderate value of 100 W was selected as the probe power. Even though the achievable extinction ratio for XGM wavelength conversion is usually small ( 10 dB), it is quite suitable for clock recovery, where the most important factor is the strength of the clock component rather than the extinction ratio. Although cross-phase modulation (XPM) can achieve a higher extinction ratio, XPM is not suitable for this particular application since the carrier is well suppressed in the phase modulation. A relatively stronger carrier is also essential since it experiences attenuation as it passes thought the first section of TS–DFB laser, as described in Section II. Fig. 6 shows the optical spectra of the wavelength-converted 40-Gb/s signal as well as the recovered clock signal. The converted signal has a very strong carrier. To prevent the TS–DFB laser from instability due to excessive injection power, a tunable filter was tuned so that the optical carrier was attenuated by approximately 6 dB with respect to the clock component. The carrier-attenuated signal has a carrier to the clock component ratio of approximately 17 dB. No carrier attenuation was performed at 12 Gb/s for two reasons. First, the conversion efficiency is much higher at 12 Gb/s than at 40 Gb/s. Second, the carrier and clock components are too close to attenuate the carrier only. The recovered clock has two equal intensity optical modes (intensity difference 0.5 db), which means a modulation index of almost 100% was achieved. Clock recovery has been achieved in a wide wavelength range from 1530 to 1565 nm. The shorter wavelength limitation is a result of low conversion efficiency for wavelength up-conversion because SOA gain shifts to longer wavelengths due to carrier depletion. However, this is not a fundamental limit because one can design the operating wavelength of the self-pulsating

MAO et al.: ALL-OPTICAL CLOCK RECOVERY FROM RZ-FORMAT DATA BY USING A TWO-SECTION GAIN-COUPLED DFB LASER

Fig. 5. Optical power of the clock component as a function of the probe power measured under different pump levels.

Fig. 6. laser.

Optical spectra of the signals injected to and output from the TS–DFB

two-section gain-coupled DFB laser at a shorter wavelength (e.g., 1530 nm for the band). To verify the polarization insensitivity of the clock recovery circuit, the polarization state of the input data was scrambled randomly with a polarization controller. No observable performance degradation was observed, owing to the 0.3-dB polarization sensitivity of the SOA. We also investigated the possibility of launching the input data and probe signal from opposite sides of the SOA. This is important since optical filtering of the target wavelength from the original wavelength can be avoided. This makes it possible to integrate SOA, continuous-wave (CW) laser, and TS–DFB laser monolithically, which is a robust way to stabilize the wavelengths relative to each other. Clock recovery has also been achieved using wavelength conversion in the counter-propagating configuration. However, the performance is much worse compared with the co-propagating geometry. The main reason is that the conversion efficiency in the counter-propagating configuration is lower since the input (pump) and target (probe) waves are no longer traveling together in the SOA. Fig. 7 shows the optical spectra of the converted signal at 40 Gb/s for both the co- and counter-propagating geometry, when the pump and probe power input to the SOA were set at 80 W and 100 W for both cases. The clock component for the counter-propagating geometry is approximately 5 dB lower than that for the

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Fig. 7. Optical spectra of the wavelength-converted signal with co- and counter-propagating schemes.

(a)

(b) Fig. 8. Temporal waveforms of the recovered (a) 12-GHz and (b) 40-GHz clock signals.

co-propagating geometry. The counter-propagating geometry also suffers from relatively higher ASE noise. B. Timing Jitter and Phase Noise The timing jitter of the recovered clock signal was measured directly by a digital sampling oscilloscope. The oscilloscope was triggered by a clock signal from the pattern generator, which was synchronized to the PRBS data signal. Thus, only synchronized signals were displayed. Fig. 8 shows the temporal wave-

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Fig. 10. BER measurement for the back-to-back configuration as a baseline and a configuration using optically recovered clock. The power penalty for optical clock recovery is approximately 1 dB.

Fig. 9. Timing jitter and phase-noise performance of the 40-GHz clock recovery. (a) Phase noise of 40-GHz original clock and the clock recovered from coherent and incoherent schemes. (b) RF spectrum of 40-GHz coherent clock recovery.

forms of the recovered clock at 12 Gb/s and 40 Gb/s. The pulses are stable in amplitude and time. The rms jitters measured by the oscilloscope, including the contribution of the detection system, were less than 2 ps for 12 Gb/s and 1.1 ps for 40 Gb/s. The timing jitter measured with the oscilloscope matches well with that computed from phase-noise measurement. Taking clock recovery at 40 Gb/s, for example, the phase noises of the original clock signal and the recovered clock signal measured by an RF spectrum analyzer (HP8565E) are shown in noise of the recovered clock signal is almost Fig. 9(a). The identical to that of the original clock signal injected into the clock recovery circuitry. The white noise of the recovered clock signal is approximately 10 dB higher than that of the original clock signal. The integrated timing jitter over a 200-MHz bandwidth is 0.9 ps. The recovered clock has a carrier-to-noise ratio (CNR) of approximately 40 dB as shown in Fig. 9(b). No pedestal and no low-frequency noise components are observed in the RF spectrum. A weak 10-GHz component (35 dB lower than the clock signal) and its high-order harmonic signals are observed because of the nonideal multiplexing of the 40-Gb/s data from a 10-Gb/s signal. C. Power Penalty A comprehensive way to characterize the performance of a recovered clock system is to measure the power penalty. First, the quality of data detection was investigated for the back-to-back situation where the clock signal from a pattern generator was

used to trigger the bit error tester. The sensitivity (BER ) of the test bed was 7.75 dBm. This relatively high power level of sensitivity was due to the fact that no limiting amplifier was used. Next, optical clock recovery was performed and the electrically converted clock signal was used to trigger the bit error tester. Since the TS–DFB laser is not packaged, it is sensitive to environmental conditions and small vibration, and temperature fluctuations will cause burst errors. To reduce the effect of environmental fluctuations, the measurement time in these exBER). periments was 10 s (1 bit error corresponds to . The data pattern length was Fig. 10 shows the BER at 12 Gb/s as a function of the received power for both cases. The penalty for clock recovery circuit compared with the back-to-back situation is only 1 dB. It is believed that this penalty results from environmental instabilities and therefore can be reduced significantly if the TS–DFB laser is packaged. D. Sensitivity and Dynamic Range Clock recovery has been achieved at both 12 and 40 Gb/s when the input power was as low as 50 W. Increasing the input power can increase the CNR of the recovered clock for both 12 and 40 Gb/s. For example, the CNR of the recovered 12 GHz clock was 35 dB at an input power of 50 W. An increase in the input power improves the wavelength conversion efficiency and then increases the clock component of the converted signal. As a result, the quality of the recovered clock can be improved. A CNR of 42 dB was obtained when the input was increased to 500 W. The dynamic range of the clock recovery scheme is measured with the help of a BER tester so that only results for 12 Gb/s are available. The variation in signal power affects the performance of clock recovery in two ways. First, it causes the efficiency of wavelength conversion to change and thus changes the carrier to the clock ratio of the converted signal. Second, it changes the average power of the converted signal. Both of these changes lead to changes in the injection condition (mainly the injection ratio into the TS–DFB), which, in turn, induce a power-dependent phase shift of the recovered clock with respect

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to the data. Therefore, if the receiver is optimized for a particular input power level, a change in the input power will cause degradation in the BER. The BER measurement was performed under the following conditions. The data output port of the patter generator was connected directly to the data input port of the error analyzer. The amplitude of the data output was set to 500 mV. The recovered clock was electrically converted and used for triggering the error analyzer. The autosynchronization function of the analyzer was disabled while varying the power of the input data. The signal wavelength of 1560 nm and the probe power of 10 dBm were used in this experiment. Under the error-free criterion, a dynamic range of approximately 7 dB (from 90 W to 425 W) was achieved. This dynamic range was mainly limited by the phase shift, as described in Section II. E. Locking Bandwidth The free-running frequency of the TS–DFB laser is controlled by its dc-bias currents. This frequency will vary slightly as environmental parameters (temperature, dc bias, etc.) change. Through aging, the device will also experience a drift in the free-running frequency even without any drift in the dc-bias current. Deviations in the data signal frequency can also exist, especially in a wavelength division multiplexing (WDM) network, where data streams may come from a multiple of independent sources. For this reason, it is important that the clock recovery circuitry be able to robustly lock to a range of data frequencies. To demonstrate the frequency-locking bandwidth of the clock recovery circuitry at 12 Gb/s, the BER test was performed while varying the bit rate of the input signal. The measurement conditions were the same as those described in Section IV-D except the autosynchronization was on. With an input data power of 200 W, approximately 100 MHz of detuning was achieved under the error-free criterion. This locking bandwidth was mainly limited by the electrical bandpass filter after the electrical amplifier for the clock signal, which has a 3-dB bandwidth of approximately 100 MHz. When investigated in the frequency domain only where the electrical bandpass filter is absent, the locking range is much bigger. Fig. 11 shows the upper bound and lower bound of clock recovery frequencies achieved for a given average input data power of 200 W. The recovered clock had a CNR of 40 dB throughout this range. There is no broad pedestal observed in the RF spectrum. The clock recovery circuitry maintained stable, low-phase-noise operation across the entire locking range of 600 MHz. F. Detuning Range of the Probe Wavelength Since the mechanism of the clock recovery is essentially optical injection locking, the wavelength detuning range can be predicted using the theory of optical injection locking. 30 dB for both sections in the The injection ratio is current setup. Given these injection ratios, according to [21], the stable locking range is a few gigahertz. To experimentally determine the static wavelength detuning range, the RF spectrum was monitored while varying the wavelength of

Fig. 11. Frequency-locking range. The two RF spectra of the recovered clock shown correspond to the lower and upper limit of the injected clock frequency.

the probe signal from the tunable CW laser. With an input data power of 0.2 mW, a 3-GHz wavelength detuning range was achieved for both 12 Gb/s and 40 Gb/s. This result agrees well with the stable locking range of optical injection locking presented in [21]. This range is much larger than the wavelength fluctuation of temperature-controlled DFB lasers, which is approximately a few hundred megahertz. A larger detuning range is possible with increased input data power. As discussed in Section IV-A, wavelength change in the injection signal will cause phase shift of the recovered clock, which will eventually limit the dynamic wavelength detuning range. To experimentally demonstrate the dynamic wavelength detuning range (at 12 Gb/s), BER measurements were performed while varying the wavelength of the probe signal. The measurement conditions were the same as those in Section IV-D. Under the error-free criterion, more than 500 MHz of wavelength detuning was achieved with an input power of 0.2 mW. G. Lockup Time It is important to measure the time required to achieve clock synchronization since clock lockup time will affect system latency. To investigate the lockup time, the setup was modified as shown in Fig. 12(a) [24]. The 12-GHz optical clock signal was 1543.5 nm and then gated first wavelength-converted onto using a second LiNbO external modulator driven by a data. In so doing, a short series of “1”s followed by a long stream of “0”s can be easily generated. The data signal was terminated for a long temporal period to ensure loss of clock synchronization. When the data signal was reestablished, time was required for the two-section laser to resynchronize to the incoming signal. Fig. 12(b) shows the pulse traces of the injected data signal. Due to the small conversion bandwidth of the SOA, the extinction ratio of the injection data is quite low. The data signal was switched on for 16 b and switched off for 496 b. The average injected data power was 58 W. When the measured signal was averaged on the digital sampling oscilloscope, only clock pulses that were synchronized to the data signal were observed, and clock lockup time could be measured.

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(a)

(b) Fig. 12. Lockup time measurement. (a) Experimental setup. (b) Data pattern of the signal injected to the TS–DFB laser.

Fig. 13(a) shows the temporal waveform of the synchronized clock signal. The increase of the amplitude during the process of synchronization can be observed. After about 10 b, the amplitude starts to stabilize, which implies that stable synchronization has been achieved. This indicates that the clock synchronizes to the incoming data signal in less than 10 b (0.83 ns). This ultrafast locking character makes the clock recovery very suitable for applications in an optical packet-switching network, where ultrafast locking to incoming data packets is required. The clock keeps synchronization for a long period of time. Fig. 13(b) demonstrates that the TS–DFB laser maintains to produce the recovered clock signal after the data is turned off. The synchronization could be kept for at least 32 b (2.67 ns). The lockup time is determined by the dephasing mechanism of the slave laser when the data is turned off. Because of gain clamping, the dephasing time constant is rather long. V. COMPARISON OF TWO OPERATING MODES In Section IV, characteristics of coherent clock recovery have been presented in detail. We now compare the performance of coherent clock recovery and incoherent clock recovery at 40 Gb/s. For incoherent clock recovery, a minimum input power of 2 mW into the TS–DFB laser is required for clock recovery. The RF spectra of the recovered clock signal are shown in Fig. 14(a). The recovered clock has a CNR of approximately 35 dB, which is approximately 5 dB lower than that for the coherent clock recovery. There is a broad pedestal (300–400 MHz) in the RF spectrum. In addition, low-frequency components can also be

Fig. 13. Temporal waveforms of the recovered clock signal corresponding to the input data pattern in Fig. 12(b). (a) Clock synchronization process after receiving first few bits of input data. (b) Holding of the recovered clock signal after data is turned off.

observed near dc in the RF spectrum. This broad pedestal and low-frequency components are a direct consequence of the carrier density fluctuations associated with data signal (rather than the clock signal) in the incoming signal. The waveform of the recovered clock is shown in Fig. 14(b). The phase noises of the injection data and the recovered clock signal with incoherent scheme are also shown in Fig. 9(a). The noise of the recovered clock signal is almost identical to that of the original clock signal injected into the clock recovery circuitry. The white noise of the recovered clock signal is approximately 18 dB higher than that of the original clock signal and approximately 8 dB higher than that for the coherent clock recovery. The integrated timing jitter of the recovered clock over a 200-MHz bandwidth is found to be around 2.4 ps. The fundamental mechanism of both modes of operation is optical injection locking. For the incoherent mode, optical carrier modulation and injection locking occurs in a TS–DFB laser. While optical carrier modulation and injection locking are separated for the coherent mode, the former happens in an SOA and the latter in a TS–DFB laser. Optical carrier modulation in an SOA is more efficient and requires lower input power than in a

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and injection locking occur together in the TS–DFB laser. For the coherent mode, on the other hand, optical carrier modulation and injection locking are separated; the former occurs in an SOA and the latter in the TS–DFB laser. Optical carrier modulation in an SOA isolates the pattern-dependent dynamics in the SOA. As a result, the recovered clock in the coherent mode does not have data-dependent noise. We have demonstrated all-optical clock recovery up to 40 Gb/s with both modes. Different from the clock recovery using mode-locked lasers, the current scheme can work with variable bit rates. The maximum speed of the clock recovery scheme will likely be limited by the speed of wavelength conversion. The two-section DFB lasers can operate from 11 to 120 GHz [25]. At an additional expense of a laser, the coherent mode has a much better performance. The ultrafast locking characteristic ( 1 ns) makes the coherent scheme suitable for applications in an optical packet- switching network, where ultrafast locking to incoming data packets is required. Less than a 1-dB power penalty, which we believe mainly results from the environmental instabilities, can be significantly reduced if the TS–DFB laser is packaged. REFERENCES

Fig. 14. 40-GHz incoherent clock recovery. (a) RF spectra of the recovered clock signal. The residual data pattern is clearly seen at low frequencies and the data-pattern-induced jitter results in a broad pedestal near the recovered clock. (b) Temporal waveform of the recovered clock.

DFB laser since SOA has a long chip length. Furthermore, the strong injection, which is necessary for the incoherent scheme, may cause instability of the TS–DFB laser. As a result, the recovered clock inevitably has residual data patterns since one of the optical modes is intensity modulated. These are the reasons why the performance of the coherent scheme is superior to that of the incoherent scheme. The price for a better performance with the coherent mode is an additional probe laser. VI. CONCLUSION In conclusion, the fundamental mechanism of coherent and incoherent clock recovery using TS–DFB lasers is optical injection locking. However, the two modes of operation are quite different. For the incoherent mode, optical carrier modulation

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[14] H. J. Lee, H. G. Kim, J. Y. Choi, and H. K. Lee, “All-optical clock recovery from NRZ data with simple NRZ-to-PRZ converter based on self-phase modulation of semiconductor optical amplifier,” Electron. Lett., vol. 35, no. 12, pp. 989–990, 1999. [15] M. W. Chbat, P. A. Perrier, and P. R. Prucnal, “Optical clock recovery demonstration using periodic oscillations of a hybrid electrooptic bistable system,” IEEE Photon. Technol. Lett., vol. 3, pp. 65–67, Jan. 1991. [16] X. S. Yao and G. Lutes, “A high-speed photonic clock and carrier recovery device,” IEEE Photon. Technol. Lett., vol. 8, pp. 688–690, May 1996. [17] G. P. Agrawal, Fiber-Optic Communication Systems, 2nd ed. New York: Wiley, 1997, pp. 180–182. [18] W. Mao, X. Wang, M. Al-Mumin, and G. Li, “40 Gb/s all-optical clock recovery using self-pulsation DFB lasers,” in Proc. OFC’2000, pp. 79–80. [19] G. Li and W. Mao, “Nonlinear dynamics for all-optical 3R regeneration,” in LEOS’2000, pp. 521–522. [20] B. Sartorius, C. Bornholdt, O. Brox, H. J. Ehrke, D. Hoffmann, R. Ludwig, and M. Möhrle, “All-optical clock recovery module based on self-pulsating DFB laser,” Electron. Lett., vol. 34, no. 17, pp. 1665–1665, 1998. [21] R. Hui, A. D’Ottavi, A. Mecozzi, and P. Spano, “Injection locking in distributed feedback semiconductor lasers,” IEEE J. Quantum Electron., vol. 27, pp. 1688–1695, June 1991. [22] R. Lang and K. Kobayashi, “External optical feedback effects on semiconductor injection laser properties,” IEEE J. Quantum Electron., vol. QE-16, pp. 347–355, Mar. 1980. [23] F. Mogensen, H. Olesen, and G. Jacobsen, “Locking conditions and stability properties for a semiconductor laser with external light injection,” IEEE J. Quantum Electron., vol. QE-21, pp. 784–793, July 1985. [24] W. Mao and G. Li, “Ultrafast wavelength and polarization insensitive all-optical clock recovery,” presented at the OFC’2001, Anaheim, CA. Paper MG3. [25] M. Al-Mumin, X. Wang, W. Mao, S. A. Pappert, and G. Li, “Optical generation and sideband injection locking of tunable 11–120 GHz microwave/millimeter signals,” Electron. Lett., vol. 36, no. 18, pp. 1547–1548, 2000.

Weiming Mao was born in Zhejiang, China, in 1970. He received the B.S. and M.S. degrees in electronic engineering from Tsinghua University, Beijing, China, in 1992 and 1998, respectively, and the Ph.D. degree in electrical engineering from the University of Central Florida, Orlando, in 2001. Currently, he is with CENiX Incorporated, Allentown, PA, where he is involved in development of high-speed optoelectronic subsystems.

Yuhua Li was born in Hubei Province, China, in 1972. He received the B.S., M.S. and Ph.D. degrees in electronic engineering from Tsinghua University, Beijing, China, in 1994, 1996, and 1999, respectively. From 1999 to 2000, he was a Lecturer with the Department of Electronic Engineering in Tsinghua University. Currently, he is visiting Research Scientist with the School of Optics/CREOL, University of Central Florida, Orlando. His research interests include ultrashort pulse generation and compression, WDM/OTDM transmission systems, all-optical clock recovery, and data regeneration.

Mohammed Al-Mumin was born in Kuwait City, Kuwait, in August 1967. He received the B.S. degree in electronics and electrical engineering and the M.S. degree in microwave and optoelectronics from University College, London, U.K., in 1989 and 1990, respectively. In 1997, he joined the School of Optics/CREOL, University of Central Florida, Orlando, and received the Ph.D. degree in optical sciences and engineering. He is an Assistant Professor with the College of Technological Studies, Shuwaikh, Kuwait. His interests include modeling of semiconductor lasers, dynamics of self-pulsating lasers, injection locking of semiconductor lasers, optical millimeter-wave generation, wireless fiber networks, and high-speed optical communication systems.

Guifang Li (M’94) received the Ph.D. degree in electrical engineering from the University of Wisconsin, Madison. He is an Associate Professor of Optics and Electrical and Computer Engineering at the University of Central Florida, Orlando. He is also an Associate Editor of Optical Networks. His research interests include all-optical signal processing for optical networks, fiber-radio systems, RF photonics, and nonlinear dynamics. Dr. Li is the recipient of the National Science Foundation (NSF) Presidential Young Investigator Award and the Office of Naval Research Young Investigator Award. Currently, he is Director of the NSF IGERT program in Optical Communications and Networking.