Microring-based reconfigurable optical pulse shaper

Optik 127 (2016) 6150–6154

Contents lists available at ScienceDirect

Optik journal homepage: www.elsevier.de/ijleo

Original research article

Microring-based reconfigurable optical pulse shaper Xiaowei Dong ∗ , Mengzhen Xu College of Information Engineering, North China University of Technology, Beijing 100144, China

a r t i c l e

i n f o

Article history: Received 23 February 2016 Accepted 18 April 2016 Keywords: Optical pulse shaping Microring (MR) resonator Flat-top waveform Triangular waveform

a b s t r a c t Based on the amplitude and phase-shift characteristics of microring (MR) resonator, a novel reconfigurable optical pulse shaping approach is proposed. By adjusting the variable attenuators to set the proper relative weights, a transform-limited Gaussian input optical pulse can be shaped into flat-top waveform or triangular waveform flexibly. In contrast to the conventional pulse shaping method, only positive real relative weights are used in the proposed pulse shaper, which not only facilitates the reconfigurable adjustment, but also is robust to the wavelength detuning and temporal duration variations of the input pulse. © 2016 Elsevier GmbH. All rights reserved.

1. Introduction Pulses with the time scales of picosecond and femtosecond have attracted considerable attentions in many applications. The mode-locked lasers are by far the most common sources of ultrashort pulses. But the temporal shapes generated by mode-locked lasers are typically Sech2 or Gaussian, which are unsuitable for some special applications [1]. For example, ultra-short pulse with flat-top waveform is highly desirable for nonlinear optical switching system. And odd-symmetry Hermite-Gaussian pulse, which consists of two consecutive ␲ phase-shifted pulses, is important for optical coding and highorder soliton excitation in dispersion-managed fiber links [2]. In order to obtain a customized optical pulse waveform, pulse shaping and processing technologies are indispensable. The well-known approach is the 4f Fourier transform setup [3], in which an appropriately designed amplitude or phase mask is used to reshape the spectrum of incident pulse. However, devices employed by the 4f system are bulky, lossy, and expensive. This has prompted recent efforts on the implementation of waveguide-based optical shaping elements. For example, optical pulse shaper based on fiber Bragg grating (FBG) [4,5], superstructured Bragg gratings [6], or long-period-grating(LPG) co-directional coupler [7] have been demonstrated. Recently, we have also reported ultrashort optical pulse shapers on the basis of complex-modulated LPG [8] or double-phase-shifted FBG [9]. However, optical pulse shapers mentioned above are usually non-adjustable, which make their output waveforms cannot be modified once they are fabricated. Therefore, in this paper, we present a novel reconfigurable optical pulse shaping approach based on the amplitude and phase-shift characteristics of microring (MR) resonator. Compared with the waveformcontrollable optical pulse shaper using arrayed waveguide grating [10] or modulator-assisted FBGs array [11], the pulse shaper proposed in this paper is much more compact and easier to be realized. 2. Structural model and theoretical backgrounds Fig. 1 illustrates a schematic diagram of microring (MR)-based reconfigurable optical pulse shaper. Utilizing the amplitude and phase-shift induced by the MR-resonators and adjusting the variable attenuators (VA) to set the proper relative weights,

∗ Corresponding author. E-mail address: [email protected] (X. Dong). http://dx.doi.org/10.1016/j.ijleo.2016.04.072 0030-4026/© 2016 Elsevier GmbH. All rights reserved.

X. Dong, M. Xu / Optik 127 (2016) 6150–6154

6151

Fig. 1. Schematic of Microring-based reconfigurable optical pulse shaper.

1

0.8

(b)

(a)

0.5

Phase /π

Normalized intensity

1

0.6 0.4

-0.5

0.2 0 1.535

0

1.54

1.545

1.55

1.555

1.56

-1 1.535

1.565

1.54

1.545

1.55

1.555

1.56

1.565

Wavelength /μm

Wavelength /μm

Fig. 2. (a) Transmission and (b) phase response of MR-resonator with normalized loss  = 0.8 and different transferring coefficient t = 0.9 (dash); t = 0.8 (solid blue); t = 0.7 (dotted). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

each replicas of input optical pulse are coherently overlapped and shaped into different output pulse shapes (including flat-top and triangular waveforms). 2 2 Assuming a transform-limited Gaussian optical √ pulse Ain (t) = exp(−t /0 ) with the carrier wavelength 0 is input, where its full width half maximum (FWHM) is 0 × 2 ln 2. The amplitude and phase-shift induced by the resonators MR1, MR2 and MR3 can be described by the transmission functions [12]: Tm =

tm − m exp(jm ) 1 − tm m exp(jm )

(m = 1, 2, 3)

(1)





with tm is the normalized coupling coefficient; m = 2/ neff × 2Rm is the round-trip phase shift of MR-resonator; neff is the effective refractive index;  is light wavelength; Rm is the MR-resonator radius; m = exp(−˛m × 2Rm ) is the normalized round-trip loss. Due to the high refractive index contrast of silicon-on-insulator (SOI) technology, wavelength-scale MR-resonator can be fabricated with very small bend loss. Assuming all of the MR-resonators having the same radius R = 10 ␮m and normalized loss  =0.8 and by setting the width and height of the top silicon waveguides, the suitable effective index can obtained at 1550 nm wavelength [13]. Fig. 2 gives the transmission and phase responses of MR-resonator with different transferring coefficients t. As can be seen, although the phase shifts of smaller or greater than  are induced at the condition of t >  or t < , the transmission reduces to zero and an exact  phase-shift appears across the central resonance at the critical-coupling condition (t = ). This means that the first derivative is obtained when the wavelength of input optical pulse coincides with the central resonance of the MR-resonators. Based on the general principle that any desired temporal waveform could be synthesized as a linear superposition of a Gaussian pulse and its successive time derivatives [14], we anticipate that a flat-top waveform and a triangular waveform can be well approximated by only three temporal terms if their relative weights can be adjusted properly. 3. Analyses and discussions On the basis of theoretical analyses above, the MR-based pulse shaping capabilities are investigated. We assume a transform-limited Gaussian optical pulse with the full width half maximum (FWHM) 10 ps is input. When the MR-resonators operates at the critical-coupling condition and the wavelength of input optical pulse coincides with the central resonance of the MR-resonators, the output pulse waveforms at each arm of MZI are depicted. Compared with the input replica in Fig. 3(a), when the transform-limited Gaussian passes through one MR-resonator (MR1), a Hermite-Gaussian pulse, which consists of two consecutive pulses with similar waveform is obtained, as shown in Fig. 3(b). However, if the transform-limited Gaussian pulse passes through two MR-resonators (MR2 and MR3), the output pulse is made of one main peak and two symmetric side-lobes, as shown in Fig. 3(c).

X. Dong, M. Xu / Optik 127 (2016) 6150–6154 1

(a)

0.8

Normalized intensity

Normalized intensity

1

0.6 0.4 0.2 0 -60

-40

-20

0

20

40

60

0.8

1

(b)

Normalized intensity

6152

0.6 0.4 0.2 0 -60

-40

-20

Time /ps

0

20

40

60

0.8

(c)

0.6 0.4 0.2 0 -60

-40

-20

0

20

40

60

Time /ps

Time /ps

Fig. 3. Output pulse waveforms corresponding to (a) upper arm; (b) middle arm; (c) lower arm of MZI.

1

1

(b) Normalized intensity

Normalized intensity

(a) 0.8 0.6 0.4 0.2 0 -60

-40

-20

0

20

40

60

0.8 0.6 0.4 0.2 0 -60

Time /ps

-40

-20

0

20

40

60

Time /ps

Fig. 4. Transform-limited Gaussian input optical pulse is shaped into (a) flat-top waveform; (b) triangular waveform. Solid line: = 0 nm; dotted line: = 0.1 nm.

Table 1 Optimal relative weights. Figures

Relative weights [VA0, VA1, VA2]

Fig. 4(a) Fig. 4(a) Fig. 5

Solid line:[1.0, 0.8, 0.39]; Dotted line: [0.2, 1.0, 0.2] Solid line:[1.0, 0.55, 0.7]; Dotted line: [0.0, 0.4, 1.0] Red line: [1.0, 0.80, 0.39]; Blue line: [0.80, 0.80, 0.39] Black line: [1.0, 0.64, 0.39]; Green line: [1.0, 0.8, 0.312] Red line: [1.0, 0.72, 0.39]; Blue line: [1.0, 0.82, 0.39] Red line: [1.0, 0.50, 0.70]; Blue line: [1.0, 0.60, 0.70]

Fig. 6(a) Fig. 6(b)

By adjusting the variable attenuators (VA) to set the proper relative weights of aforementioned pulses at each arm, output pulse with different shapes can be obtained flexibly. Fig. 4 shows the results corresponding to the flat-top waveform and triangular waveform, respectively. Due to the practical fabrication deviation, it is difficult to align the input optical pulse to the central resonance of MR-resonators accurately. However, the reconfigurable optical pulse shaper based on MRresonators is quite robust to wavelength detuning . The desired pulse shape can be again synthesized by readjusting the variable attenuators (VA). Only slight time-duration reduction for the flat-top optical pulse and very small fluctuations in the rising-falling slope parts of the triangular optical pulse are observed for the wavelength detuning  = 0.1 nm (shown with the dotted curve in Fig. 4). The optimal relative weights for these designs above are given in Table 1. In contrast to the conventional pulse shaping method, only positive real relative weights are used in the proposed pulse shaper, which not only facilitate the reconfigurable adjustment, but also is tolerant to the variation of relative weights [15]. As an example, we consider that each relative weight is deviated 20% with respect to its optimal value for synthesizing the flat-top optical pulse. Simulation results are shown in Fig. 5. In the worst case for each deviation, the fluctuations are induced at the flat-top region are less than 5% of the signal’s normalized intensity. In addition, an advantaged feature of the proposed pulse shaping is that the desired pulse waveform can be accurately obtained even if the temporal duration of the input pulse varying over a given range. Moreover, only one relative weight needs to be modified. To illustrate this feature, the targeted flat-top and triangular waveforms have been generated from a 5-ps and a 15-ps Gaussian input pulses, as shown in Fig. 6. And the results given in Table 1 demonstrate that, correspondingly to the variations of input pulse duration, the relative weight in the middle arm decreases or increases while leaving the other two relative weights in the upper and lower arms remain.

X. Dong, M. Xu / Optik 127 (2016) 6150–6154

1.05

(a) Normalized intensity

Normalized intensity

1 0.8 0.6 0.4 0.2 0 -60

6153

1

(b)

0.95 0.9 0.85 0.8 0.75

-40

-20

0

20

40

0.7 -30

60

-20

-10

0

10

20

30

Time /ps

Time /ps

Fig. 5. Influences of variations in relative weights to the output pulse waveform. Red line: 1, 0.8, 0.39 (optimal value); blue line: 0.8, 0.8, 0.39; black line: 1, 0.64, 0.39; green line: 1, 0.8, 0.312. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

1

(a) Normalized intensity

Normalized intensity

1 0.8 0.6 0.4 0.2 0 -60

-40

-20

0

Time /ps

20

40

60

0.8

(b)

0.6 0.4 0.2 0 -60

-40

-20

0

20

40

60

Time /ps

Fig. 6. Pulse shaping capabilities corresponding to 5 ps (red line) or 15 ps (blue line) Gaussian input pulse; (a) flat-top waveform; (b) triangular waveform. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4. Conclusions In this paper, a novel MR-based reconfigurable optical pulse shaping approach is proposed. By adjusting the variable attenuators to set the proper relative weights, output optical pulse with different shapes (including flat-top and triangular waveforms) can be obtained from Gaussian input pulse. Due to only positive real relative weights are used, the proposed optical pulse shaper is robust to the wavelength detuning and temporal duration variations of the input pulse. And the MRresonators have the merits of small size and compatibility with silicon photonic integration [16]. These advantaged features make the reconfigurable pulse shaper proposed in this paper promising in future high-speed optical communications and all-optical signal-processing. Acknowledgments Project supported by the talents of North China University of Technology (CCXZ201307) and the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD201304001). References [1] Frabcesca Parmigiani, Leif Katsuo Oxenlowe, Michael Galili, et al., All-optical 160Gbit/s retiming system using fiber grating based pulse shaping technology, J. Lightwave Technol. 27 (9) (2009) 1135–1141. [2] R. Slavik, Y. Park, M. Kulishov, R. Morandotti, J. Azana, Ultrafast all-optical differentiators, Opt. Express 14 (2006) 10699–10707. [3] Andrew M. Weiner, Ultrafast optical pulse shaping: a tutorial review, Opt. Commun. 284 (2011) 3669–3692. [4] Chao Wang, Jianping Yao, Fourier transform ultrashort optical pulse shaping using a single chirped fiber Bragg grating, IEEE Photonics Technol. Lett. 21 (19) (2009) 1375–1377. [5] Miguel A. Miguel, Victor Garcia-Minoz, Miguel A. Muriel, Grating design of oppositely chirped FBGs for pulse shaping, IEEE Photonics Technol. Lett. 19 (6) (2007) 435–437. [6] P. Petropoulos, M. Ibsen, A.D. Ellis, D.J. Richardson, Rectangular pulse generation based on pulse reshaping using a superstructured fiber Bragg grating, J. Lightwave Technol. 19 (5) (2001) 746–752. [7] Josa Azana, Mykola Kulishov, Optical pulse shaping capabilities of grating-assisted codirectional coupler, Microelectron. J. 36 (2005) 289–293.

6154

X. Dong, M. Xu / Optik 127 (2016) 6150–6154

[8] Dong Xiao-wei, Liu Wenkai, Ultrashort optical pulse shaper based on complex-modulated long-period-grating coupler, Chinese Phys. B 22 (2) (2013), 024210. [9] Dong Xiao-wei, Guo Pan, Optical pulse shaping based on double-phase-shifted fiber Bragg grating, Optoelectron. Lett. 11 (2) (2015) 0100–0102. [10] D. Miyamoto, K. Mandai, I. Kurokawa, S. Takeda, T. Shioda, H. Tsuda, Waveform-controllable optical pulse generation using an optical pulse synthesizer, IEEE Photonics Technol. Lett. 18 (5) (2006) 721–723. [11] Jingyuan Chen, Peili Li, Dynamic optical arbitrary waveform shaping based on optical modulators of single FBG, Opt. Express 23 (16) (2015) 20675–20685. [12] John E. John, Vincent Wong, Aaron Schweinsberg, Rober W. Boyd, Deborah J. Jackson, Optical transmission characteristics of fiber ring resonators, IEEE J. Quantum Electron. 40 (6) (2004) 726–730. [13] Meng Xiong, Oskars Ozolins, Yunhong Ding, Bo Huang, Yi An, Haiyan Ou, Christophe Peucheret, Xinliang Zhang, Simultaneous RZ-OOK to NRZ-OOK and RZ DPSK to NRZ-DPSK format conversion in silicon microring resonator, Opt. Express 20 (25) (2012) 27263–27272. [14] Y. Park, M. Kulishov, R. Slavik, J. Azana, Picosecond and sub-picosecond flat-top pulse generation using uniform long-period fiber gratings, Opt. Express 14 (26) (2006) 12670–12678. [15] Mohammad H. Asghari, Jose Azana, Proposal and analysis of a reconfigurable pulse shaping technique based on multi-arm optical differentiators, Opt. Commun. 281 (2008) 4581–4588. [16] Shang Wang, Berkehan Ciftcioglu, Hui Wu, Microring-based optical pulse-train generator, Opt. Express 18 (18) (2010) 19314–19323.