Peak equalization of rational-harmonic-mode ... - OSA Publishing

Peak equalization of rational-harmonic-modelocking fiberized semiconductor laser pulse via optical injection induced gain modulation Jung-Jui Kanga, Yu-Chan Linb, Chao-Kuei Leea, and Gong-Ru Linb a Department of Photonics, National Sun Yat-sen University, 70 Lien-Hai Rd.,Kaohsiung 804, Taiwan R. O. C. Graduate Institute of Photonics and Optoelectronics, and Department of Electrical Engineering, National Taiwan University, No.1 Roosevelt Rd. Sec. 4, Taipei 10617, Taiwan R. O. C. Corresponding and Reprint E-mail: [email protected]

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Abstract: Optical injection induced gain modulation of a semiconductor optical amplifier (SOA) is demonstrated to equalize the peak intensity of pulses generating from the rational-harmonic-mode-locking (RHML) SOA based fiberized semiconductor laser. This is achieved by adjusting the temporal shape of the injected optical signal generated from a MachZehnder intensity modulator, in which the DC biased level exceeding Vπ and the electrical pulse amplitude of 1.5Vπ are concurrently employed. Numerical simulation on the injected optical signal profile and the SOA gain during the inverse-optical-pulse injection induced gain modulation process are also demonstrated. After a peculiar inverse-optical-pulse injection, each pulse in the 5th-order RHML pulse-train experiences different gain from temporally varied SOA gain profile, leading the pulse peak to equalize one another with a minimum standard deviation of 2.5% on the peak intensity variation. The optimized 5th-order RHML pulse exhibits a signal-to-noise suppression ratio of 20 dB and a reduced variation on temporal spacing from 11 to 4 ps. The clock amplitude jitter is compress from 35.3% to 7.3%, which is less than the limitation up to 10% for 5th order RHML generation. ©2008 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.4370) Nonlinear optics, fibers; (140.3510) Laser, fiber; (250.5980) Semiconductor optical amplifiers.

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Received 9 Oct 2008; revised 28 Nov 2008; accepted 1 Dec 2008; published 12 Jan 2009

19 January 2009 / Vol. 17, No. 2 / OPTICS EXPRESS 850

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1. Introduction Fiber lasers generating ultra-short pulse at high-repetition-rate with sub-harmonic frequency source have emerged as a key component for the high-bit-rate optical time-divisionmultiplexing (OTDM) communication system. Active mode-locking of the fiber laser has become the leading technology lately due to its flexibility in repetition-rate control and wavelength detuning. However, the active harmonic mode-locking (HML) frequency is usually limited by the bandwidth of modulator, electronic components, and signal generator. To overcome these drawbacks, the rational-harmonic-mode-locking (RHML) technology has been investigated by applying low frequency synthesizer, then slightly detuning its frequency away from the harmonic longitudinal mode of the HML fiber laser to approach repetition-rate multiplication. Previously, Yoshida et al. and Lin et al. have individually proposed the generation of RHML pulse up to 200 GHz and 40th order [1, 2]. Zhu et al. further demonstrated the high-quality pulse-train up to 80 Gbit/s, which is doubling by injecting a 40GHz RHML pulse-train into an external fiber loop mirror [3]. Later on, Zhao simulated that the RHML pulsewidth could be shortened with increasing RHML order [4]. Moreover, the wavelength tunable RHML fiber laser was also investigated by controlling the length of dispersion compensation fiber in the ring cavity and concurrently using Fabry-Perot semiconductor modulator as both the mode-locker and the tunable filter [5, 6]. However, the high-order RHML inevitably leads to an output pulse-train of inequivalent peak amplitudes, since the frequency-multiplied RHML pulses experiences different gain in fiber laser cavity when modulating by a slightly deviated HML frequency. The inequivalent pulse amplitude results in a serious problem and restricts the RHML fiber laser for versatile applications. Thus, several configurations were proposed to equalize the pulse amplitude, including the use of a semiconductor optical amplifier in a loop mirror [7], a nonlinear optical loop mirror [8], and a nonlinear polarization rotation [9]. In particular, some special frameworks proposed using both mode-locker and pulse-amplitude equalizer to generate high-order RHML without external configuration. For example, the RHML pulse amplitude equalization can be

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achieved by passing through a intensity modulator forward and backward consecutively [10], by using a dual-drive Mach-Zehnder modulator (MZM) [11], by injecting the pulse-train into a SOA ring cavity [12], or by using a nonlinear (negative) impulse modulation [13, 14]. Besides, Vlachos et al. demonstrated using an external optical pulse-train to inject a SOA fiber ring cavity for approaching 5-GHz HML and 40-GHz RHML pulses, while the concept to equalize pulse-intensity of RHML by optically cross-gain modulation was sketchily presented [15]. Creating a suitable gain shape of SOA should be the key point for equaling amplitude of RHML pulses, however, the principle of the peculiar gain modulation in time domain was not clearly mentioned. Later on, the similar technology was employed to further promote the repetition-rate of the all-polarization-maintaining SOA fiber laser to 50 GHz [16]. Recently, we have introduced a inverse-optical-pulse injected semiconductor optical amplifier (SOA) to overcome the traditional limitation on the modulation bandwidth of the SOA contact electrode, which gives rise to a 20th order RHML pulse-train generation based on the crossgain modulation (XGM). The operation of the proposed technique relies strictly on the optically cross-gain modulation by injecting a CW light with periodic zero power dips into the SOA. In addition, the pulse quality of RHML was also investigated to show a limitation on RHML up to 7th order by weak gain depletion of SOA [17]. In this work, we demonstrate a peculiar gain modulation of SOA to equalize the mode-locking pulse amplitude of such a fiberized semiconductor laser (FSL) in RHML regime. This is approached by adjusting the profile of electrical comb and the biased point of MZM to form a peculiar inverse-opticalpulse for gain-modulating the SOA, such that each RHML pulse experiences different gain to equalize the intensity of pulse. The peculiar gain modulation process is also simulated to explain the relationship between the shapes of optical pulse and the SOA gain. To optimize pulse-amplitude equalization, the suppression ratio of the peak intensity to the noise level, and the precise confinement on the temporal spacing of the RHML pulse-train with corresponding clock amplitude jitter are discussed. 2. Experimental setup The SOA based FSL setup as shown in Fig. 1, which employs the anti-reflection coated SOA (QPhotonics, QSOA-1550) to provide a gain spectrum centered at 1535 nm with a spectral linewidth of 30-50 nm. The SOA is biased at 300 mA with its gain strongly modulated via an external inverse-optical-pulse injection. The inverse-optical-pulse is implemented by nonlinearly driving an external MZM with an electrical comb generator at 1 GHz. The output of the electrical comb seeded with a RF-synthesized sinusoidal wave after 40dB-gain amplification is slightly power-attenuated to obtain a linear transfer from MZM with Vπ of 4.5 V, while the MZM and the tunable laser were set at VDC = 3.2 V and 1555.8 nm, respectively. The MZM generated inverse-optical-pulse is connected with a set of optical intensity controller, consisting of an Erbium-doped fiber amplifier (EDFA) and an optical attenuator (OTTN). The optimized injection power of the inverse-optical-pulse for the RHML FSL is about 5 dBm. A polarization controller (PC) is required at the input port of SOA to release its polarization dependent gain difference of 3 dB.

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Fig. 1 System setup. EAMP; electrical power amplifier; COMB: comb generator; EATTN: electrical attenuator; MZM: Mach-Zehnder modulator; EDFA: Erbium-doped fiber amplifier; OC: optical coupler; ISO: optical isolator; WDM: wavelength-division multiplexer; SOA: semiconductor optical amplifier; PC: polarization controller.

By detuning the polarization state and the repetition frequency of the inverse-opticalpulse for optimizing the gain-modulation depth and mode-locking power, the different RHML condition up to 20th order can be achieved. The Faraday isolator (ISO) is used to ensure the unidirectional propagation and prevent the intra-cavity power dissipation by the inverseoptical-comb circulated in the FSL cavity. An additional fiber-grating based wavelengthdivision multiplexing (WDM) filter is also used to avoid the regenerative amplification of inverse-optical-comb in the FSL. Moreover, there is a trade-off between the average power and pulsewidth of the RHML pulse as shown in Fig. 2.

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Our experimental results reveals that the output RHML pulsewidth is inversely proportional to the intra-cavity feedback power. Figure 2 illustrates the variation of the pulsewidth and the intra-cavity peak power at different intra-cavity coupling ratio, while the mode-locking FSL pulsewidth is slightly shortened from 27 to 18 ps as the intra-cavity coupling ratio increases from 10% to 90%. This result can directly be attributed to the relationship between SOA gain and RHML pulsewidth, as described by [18] 2ln 2 2 g 0 1/ 4 1 , τp = ( 2 ) (1) ( f m Δν )1/ 2 π δ where τ p is the FWHM of the pulse, g0 is the single-pass integrated gain, fm is the modulation frequency, and Δ ν is the homogeneous linewidth. As the intra-cavity coupling ratio increases, #102587 - $15.00 USD

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the SOA gain pumped by constant current is depleted by the inverse-optical-pulse schematically illustrated in Fig. 3(a), which exhibits almost constant power except an narrow window where the power instantaneously drops to zero. Such an injection completely depletes the net gain of SOA with an extremely large duty cycle in one modulation period, however, a transient gain left in the SOA due to the inverse-optical-pulse is sufficient to cause mode-locking of the SOA based fiber laser. According to the above equation, the pulsewidth becomes narrower with decreasing SOA gain, which has been confirmed by both the theoretical model and the experimental results. In our case, an output coupler (OC) with power-splitting ratio of 50% is selected to obtain pedestal-free RHML pulse from the FSL with relatively short pulsewidth and sufficient output power. The gain modulation in time domain is a key factor to approach the pulse-amplitude equalization, which is based on injecting the SOA with the peculiar inverse-optical-pulse generating from the different profile of electrical comb and the biased point of MZM. In general case, to arrive the perfect mode-locking condition, the electrical comb is driven by the optimum modulation power (about -8 dBm) and biased at the rising edge of MZM to form the inverse-optical-pulse, which provides the maximum depletion window and modulation depth. We overthrow the traditional conditions to meet the demand of pulse-amplitude equalization by using a non-symmetrical electrical comb to drive the MZM and the DC biased voltage at the falling edge of its transfer function (see Fig. 3(b)). Such an offset electrical pulse voltage crosses over the linear and nonlinear regions of MZM transfer function, providing a peculiar inverse-optical-pulse, which owns relatively narrow gain-depletion-window identical to the inverse-optical-pulse with a non-constant upper power level, as shown in Fig. 3(a). The deformation of electrical comb required to equalize the RHML pulse amplitude also depends on the power of sinusoidal wave used to trigger the electrical comb, as shown in Fig. 3(c). The optimal output of the electrical comb triggered at sinusoidal-wave power of -15 dBm looks like an inverse Gaussian shape. By choosing a triggered condition beyond the best one to form a non-symmetrical electrical comb profile shown in Fig. 3(c), such that a peculiar inverse-optical-pulse shown in Fig. 3(d) is generated to differentiate the gain for the each circulated RHML pulse within the FSL. 3. Results and discussions 3.1 Simulation on optical-injection induced gain-profile reshaping in SOA In order to achieve RHML operation, the modulation frequency of the RF synthesizer is detuned to satisfy the equation of fm=(n±1/p)f0, where fm, f0, n, p denote the modulation frequency of RF synthesizer, the longitudinal mode spacing in cavity of laser, the harmonic and rational harmonic mode-locking orders, respectively. In more detail, the modulation frequency is slightly deviated from the harmonics of the cavity round-trip frequency, n*f0, by an amount of f0/p (p is an integer number). High-order RHML can only be achieved with severely control on the detuned frequency amount, providing a stable pulse-train repeated at multiplied frequency of p*fm. In comparison with the general SOA mode-locking fiber laser at HML condition, the gain profile optimized for HML is unsuitable for RHML to equal the pulse-amplitude. Hence, the detuning on SOA gain shape in time domain is necessary for equalizing the pulse-amplitude by matching the gain profile with the RHML pulse-train envelope. To obtain the optimized optical injection shape for modulating the SOA gain profile during RHML operation, we have created the model to simulate the optically crossgain-modulated pulse-trains without and with pulse-amplitude equalization, as shown in Fig. 4. The injected profile of the electrical comb and the biased voltage of MZM will determine the waveform of the peculiar inverse-optical-pulse or the inverse-optical-pulse to inject the SOA fiber ring via XGM effect. Fig. 4(a) simulated the waveforms of the electrical comb with the ideally inverse Gaussian shape and the non-symmetrical electrical comb, which are formatted by the correlation of Gaussian and exponential decay functions under the different biased

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Fig. 3 (a) Illustration of the inverse-optical-pulse and the peculiar inverse-optical-pulse generated by MZM. (b) The transmission curve of MZM. (c) The variations of electrical comb with different driving intensity. (d) The profile of actual-optical -pulse with pulseamplitude equalization.

points of the MZM and the modulation intensity. Theoretically, and the output waveforms of the inverse-optical-pulse and peculiar inverse-optical-pulse also can be numerically modeled via the modified transfer equation of the MZM, as described below.

⎧ ⎡ π (Vin + VDC ) ⎤ ⎫ ⎪ ⎪ ⎨1 + sin ⎢ ⎥⎬ Vπ ⎪⎩ ⎣ ⎦ ⎪⎭ ⎧ ( t − nt0 )2 ⎡ ⎛ ∞ − 2 I in ⎪ Vπ V ⎢ ⎜ = (1 − e τ 0 ) + π ⎨1 + sin ⎢π ⎜ 2 2 ⎪ n =−∞ 2 ⎣⎢ ⎝ ⎩

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⎤⎫ ⎞ ⎟ V ⎥ ⎬⎪ , ⎟ π ⎥⎪ ⎥⎦ ⎭ ⎠ ⎤⎫ ⎪ ⎥ Vπ ⎬ ⎥ ⎦⎥ ⎭⎪

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where Vπ denotes the switching-off voltage required for the MZM, VDC is the DC biased voltage of MZM, Vin is the peak amplitude of the electrical comb, τ0 is the electrical comb pulsewidth, t0 is the repetition period of the electrical comb, Iin is the intensity of the tunable laser input, and the Iout denotes the transmission intensity of the MZM. The Eq. (2) shows the normal inverse-optical-pulse obtained by driving the MZM with an ideally inverse Gaussian waveform in general case. For pulse-amplitude equalization, a peculiar inverse-optical-pulse generated by driving the MZM with the same Gaussian waveform of larger amplitude #102587 - $15.00 USD

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crossing over the nonlinear region of the MZM transfer function, and biasing the MZM with a changed DC offset voltage near the nonlinear region is mandatory. In particular, the gainrecovery process of the SOA is also considered by adding an exponential decay function to cross-correlate with a non-symmetrical comb function, where U(τ) is the Heaviside unit step function defined as U(τ)=1 for τ≥0 and U(τ)=0 for τ5 is difficult to be suppressed within 10% via the peculiar inverse-optical-pulse modulation, and the induced pulse-amplitude equalization will gradually be degraded with increasing RHML order. 5. Conclusion In conclusion, we have demonstrated a new scheme of gain-profile modulating for the pulseamplitude equalization of the optical-injection RHML-FSL. By detuning the shape of the injected optical pulse via a Mach-Zehnder intensity modulator to modulate the SOA gain profile during RHML operation, and the actual experimental results are in good agreement with the numerical simulate, and we also create the optical injection model to explain the relationship between the curve of optical pulse and the gain profile of SOA based on the timedomain analysis. Using the peculiar inverse-optical-pulse injected the FSL, the RHML pulse experiences different gain from the temporally varied gain profile of the SOA to equalize the pulse-amplitude. To quantify the performances before the 5th-order RHML, the signal-tonoise suppression ratio is suppressed to about 20 dB, the timing error of the pulse spacing can be greatly improved from 11 to 4 ps corresponding to the percentage error decreased from 4% to 0.02 %, and the CAJ after pulse-amplitude equalization is degraded from 1.0 % to 7.3 % below the threshold limitation of 10%. Acknowledgments The authors thank the National Science Council of Republic of China for financially supporting this research under grants NSC97-2221-E-002-055, NSC 97-ET-7-002-007-ET and NSC 97-2221-E-110-019. The authors also appreciate Prof. Wood-Hi Cheng’s helpful discussion. Mr. Yu-Chan Lin is a part-time research assistant in the Lab. of fiber laser communication and Si nanophotonics at National Taiwan University.

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