Repetition Rate Multiplication Pulsed Laser Source Based on a

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Repetition rate multiplication pulsed laser source based on a micro-ring resonator Weiqiang Wang, Wenfu Zhang, Sai T. Chu, Brent E. Little, Qinghua Yang, Leiran Wang, Xiaohong Hu, Lei Wang, Guoxi Wang, Yishan Wang, and Wei Zhao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00129 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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Repetition rate multiplication pulsed laser source based on a micro-ring resonator Weiqiang Wang†,‡,#, Wenfu Zhang†,‡,#,*, Sai T. Chu§, Brent E. Little†,‡, Qinghua Yangǁ, Leiran Wang†,‡, Xiaohong Hu†, Lei Wang†, Guoxi Wang†,‡, Yishan Wang†, and Wei Zhao†,‡ †

State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics,

Chinese Academy of Sciences, Xi’an 710119, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

§

Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China

ǁ

School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, China

Abstract We demonstrate a stable high-Q micro-ring resonator (MRR) based pulsed laser source with adjustable repetition rate from 49 GHz to 735 GHz, corresponds to repetition rate multiplication of up to 15 times the free spectral range (FSR) of the MRR. The repetition rate multiplication is realized by temporal multiplexing multiple pulses in the MRR through simply tuning of the fiber cavity length. Thus the repetition rate of the pulsed laser source breaks the frequency limitation of the previous dissipative four wave mixing based mode locked lasers whose repetition rate is equal to the FSR of the built in comb filter. This high quality chip-based repetition rate multiplicable pulsed laser source is an effective approach to on-chip ultra-high speed optical clock frequency multiplication systems. Keywords: Ultrafast lasers, Dissipative four wave mixing, Integrated optics, Nonlinear Nanophotonics, Mode-locked laser, Micro-cavity.

High repetition rate laser sources have attracted great research interests in optical communications,1 millimeter-wave wireless communication,2 frequency combs,3 high-resolution photonic analog-to-digital converters,4 optical spectroscopy,5 frequency metrology6 and optical arbitrary waveform generation,7 where a mode locked fiber laser has been extensively used to generate stable pulses trains. However, the repetition rate of common fundamental mode-locked lasers is limited to around 10 GHz as it is a function of the laser cavity length which must be kept relatively long to provide sufficient gain to produce mode-locking.8, 9 A method to increase the repetition rate of the fiber laser is with dissipative four-wave-mixing (DFWM). It was first introduced by Yoshida et al.,10 where 115 GHz repetition rate pulses were generated using an intra-cavity Fabry-Pérot (FP) filter. Since then, ultrahigh repetition rates passive mode-locked lasers have been reported by inserting different kinds of optical comb filters in the laser cavity, such as with a fiber Bragg grating,11-13 a programmable optical processor,14 a silicon micro-ring resonator (MRR),15, 16

or Mach-Zehnder interferometer (MZI).17, 18 However, these high repetition rate mode-locked lasers generally suffer from

super-mode instability10 where the common method to control the instability in these approaches is by reducing the laser cavity length or adding an additional bandpass filter. A more effective method to limit the instability is by introducing a high-Q nonlinear MRR which has been extensively applied for broad optical frequency comb generation19-22 and filter-driven four-wave mixing (FD-FWM) based mode-locked laser. 23, 24 However, the repetition rate of the reported FD-FWM based mode locked lasers is equal to the FSR of the MRR in the loop and cannot be applied to applications where clock multiplication is required. An alternative method to increase the repetition rate is with repetition rate multiplication schemes. The most direct method is using a temporal multiplexing technique which is widely used in optical temporal division multiplexing (OTDM)

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communication systems. However, it is a great challenge to synchronize the pulse trains, and this will induce extra inter-pulse jitter. Perfect repetition rate multiplication can be realized by the temporal Talbot or self-imaging effect through propagating a periodic temporal signal in a dispersive medium under the first-order dispersion conditions.25 Further, repetition rate demultiplication could also be realized by introducing a suitable periodic temporal phase modulation to the original signal and carefully controlling the amount of dispersion.26 However, both these two schemes mentioned above require extra optical systems out of the laser cavities to modify the repetition rates of the pulse sources. They increase the system complexity and make these systems loose their attractiveness for portable and robust device operation. An intra-laser cavity method is by harmonic mode-locking (HML), where the pulse energy is quantized by the peak-power-limiting effect. Generally, much higher pump power is needed for passive HML lasers to boost its repetition rates to tens of GHz.27, 28 Both the intra-cavity noise fluctuation and the chance of the pulse drop in-out increase along with the increase of the harmonic order.8, 9, 29 This prevents boosting the laser repetition rate beyond 100 GHz. Although pulse sources at repetition rates beyond 100 GHz can be realized by employing active HML schemes,30 a stable radio frequency source is needed and it restricts the dimension and cost for integration applications of the active HML lasers. In this paper, we report a simple and stable pulsed laser source with repetition rate multiplication. The laser is based on the FD-FWM configuration where the repetition rates can be adjusted by tuning the fiber laser cavity length. At each selected repetition rate, multiple equally spaced pulses simultaneously circulate in the MRR. The number of pulses circulating in the MRR is determined by the length of the main cavity, which is subsequently allowed to be tuned. In the experiment, stable mode-locking with adjustable repetition rate from 49 GHz to 735 GHz at increments of 49 GHz, the FSR of the MRR used in the loop, is realized. The super-mode instability10-18 is effectively suppressed by reducing the ratio of the MRR resonance linewidth over the main cavity FSR to 1.41, the smallest to the best of our knowledge. The proposed experiment provides a novel, multiplicable, and extremely high frequency pulsed source for future ultrahigh speed optical communication network, on-chip optical interconnection and optical signal processing, as well as microwave or millimeter-wave generation and high-resolution photonic analog-to-digital converters.

Figure 1. Schematic of the MRR and the waveguide dispersion characteristics. (a) Schematic of the four-port nonlinear MRR which is fabricated on a high index doped silica glass platform. (b)

Measured FSR and the calculated group-velocity dispersion curve of the waveguide. Inset is the input-drop transmission spectrum of the MRR


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for both transverse electric (TE) and transverse magnetic (TM) polarization. Fig. 1(a) shows the schematic of the nonlinear high-Q MRR used in the laser circuit. The waveguide core is low-loss, high-index (n=1.6) doped silica glass surrounded by SiO2 cladding. The device is fabricated by a CMOS compatible process. The core film is deposited using chemical vapor deposition. Then the device patterns are printed in photoresist using in-line stepper and etched by reactive ion etching.31, 32 Both the bus and the ring waveguides have the same cross-section of 2 µm × 3 µm. The radius of the ring is 592.1 µm yielding an FSR of 49 GHz in the C-band. The measured Q-factor of the MRR at around 1550 nm is about 1.45 10 , corresponding to a full-width at half-maximum (FWHM) of 133 MHz. Fig. 1(b) shows the

measured FSR in the wavelength range of 1520 nm to 1580 nm, and we extract the widely used dispersion parameter from the measured ∆FSR, 33 where

2

∆

(1)

Where is the wavelength, c is the speed of light in vacuum, and n() is the refractive index. The group velocity

dispersion (GVD) at 1550 nm is -33.65 ps2/km corresponding to ∆FSR of 102 kHz. Mode transformers are added at every

port of the MRR to reduce the coupling loss to less than 1.0 dB per facet with standard single mode fiber.

Figure 2. Schematic of the repetition rate multiplication pulsed laser source. (a) Experimental setup of the proposed laser. The MRR in (a) is pigtailed using a 250 µm pitch fiber array. The laser cavity length is accurately adjusted by the free space DL. The absorption index of the EDF is about 110 dB/m@1530 nm to provide enough gain for mode-locking. (b) Illustration of the repetition rate multiplication in


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the case of K=6. (c) Illustration of the Vernier effect of the two cavities in the case of K=2. The red lines show the laser cavity resonances. The black lines and the dotted blue lines are the resonant modes of the MRR and the main cavity respectively. LD, Laser Diode; WDM, Wavelength Division Multiplexer; ISO, Isolator; PC, Polarization Controller; MRR, Micro-Ring Resonator; DL, Delay Line. The experimental setup of the demonstrated multi-repetition rate laser source is shown in Fig. 2(a). The laser system is composed of an isolator that forces a unidirectional propagation in the laser loop, a polarization controller to maintain single polarization operation, a high-precision free space delay line for accurately adjusting the laser cavity length, two 980/1550 nm wavelength-division-multiplexed couplers, a ~25 cm length single mode high concentration erbium doped fiber (EDF) (LIEKKI® Er110-4/125, nLIGHT), two high power 980 nm single mode laser diodes as EDF pump sources, and a monolithic integrated high-Q MRR. The laser output is measured at the through port of the MRR to minimize the main cavity length. In term of frequency, the main laser cavity of 2.2 meters corresponds to frequency spacing 94 MHz. This relatively large mode spacing ensures only a single mode exists in the main cavity for each MRR resonance thus suppressed the super-mode instability. The proposed laser consists of two cavities, a high-Q MRR with optical path length LR and the main laser cavity with optical path length LMC. During the fundamental mode locking operation where only one single pulse is circulating in the MRR, the relationship between the cavity path lengths is simply that LMC must be an integer

multiple of LR for it to produce a stable pulse train with repetition rate equals to the FSR of the MRR (λ /L!). Higher repetition rates can be achieved utilizing temporal multiplexing by fine tuning the ratio between LR and LMC to allow multiple pulses to circulate in the MRR while satisfying the resonance conditions for both the main and the MRR cavities. One can fit K pulses in the MRR when L"# = I + J⁄KL! , with I, J and K integers and

provided that J/K is irreducible. When this condition is satisfied, it allows K pulses to couple into the MRR at different time slot within one round trip. Figure 2(b) shows an illustration for the case of K=6, in which 6 pulses are uniformly distributed in the MRR and subsequently the repetition rate is increased by 6-fold. In the frequency domain, the oscillating frequencies of the proposed laser are selected through the Vernier effect which has been widely used to realize integrated optical filter34 and wavelength tunable laser.35 For L"# = I ∗ L! , the resonant wavelengths of the MRR also satisfy the resonance condition of the main optical loop cavity corresponding to the

fundamental mode locking operation. However, for L"# = I + J⁄KL! , the coincident resonances of the two cavities happen every K FSR of the MRR. So the FSR of the proposed laser cavity is K-fold of the FSR of the MRR. Figure 2(c) presents the calculated oscillation spectra for the case of K=2. In the experiment, we first set the forward and backward pump LDs output power to 200 mW and carefully adjusted the main laser cavity length through tuning the DL. Once LMC equals an integer times LR, a stable pulse train with repetition rate 49 GHz is observed. The laser output power is about 2 dBm. Fig. 3(a) shows the measured autocorrelation (AC) trace in black with FWHM pulse width of 5.04 ps and the corresponding optical spectrum is shown in the inset. The calculated AC trace is also plotted in Fig. 3(a) (red dashed line) under the condition that of all the experimental optical spectra are fully coherent and in-phase with each other. The calculated AC trace matches the tested AC trace very well, indicating the pulse circulating in the cavity is transform limited or chirp-free. In order to further verify the stability of the mode locked laser, the laser output is tested by a 59 GHz oscilloscope (DPO75902SX, Tektronix) after being detected by a 50 GHz bandwidth

photodetector (PD, XPDV2120R, U2T). The measured waveform is shown in Fig. 3(b) with inset (I) showing the waveform for a period of 10 µs and inset (II) showing the overlapped eye diagram using both edge triggered modes. The waveform is sine like attributable to the bandwidth limitation of the measurement system. The


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extended waveform in Insert (I) shows an almost constant amplitude indicating only a single main mode is oscillating in each MRR resonance. Furthermore, the Fourier transform (FT) of the measured waveform in Fig. 3(c) shows only a single oscillation at 49 GHz up to the bandwidth limit of the oscilloscope of 59 GHz.

Figure 3. Experimental results for the fundamental frequency operation pulsed laser source. (a) Measured AC trace (black solid line) and calculated AC trace (red dashed line) from the corresponding to the optical spectrum (inset in 3(a)). (b) Waveform tested by an ultra-high bandwidth oscillator (59 GHz bandwidth, 200 GHz sampling rate) using a high speed photodetector (50 GHz). Inset (I) in (b) is the waveform at 10 µs period. Inset (II) is the eye diagram using both edge triggered modes. (c) The Fourier-transform spectrum through the 10 µs waveform data. The inset shows the enlarged spectrum around 49 GHz. The blue dashed line denotes the 49 GHz frequency position. Multiplication of the fundamental repetition rate of the laser can be obtained by increasing the forward and backward pump powers to 350 mW and fine tuning the delay line in the circuit. We are able to observe and record optical pulse trains with repetition rates from one- to fifteen-fold of the FSR of the MRR. Fig. 4(a)-4(d) present the laser output optical spectra with 147, 294, 441, and 735 GHz frequency spacing. The relevant AC traces are shown in Fig. 4(e)-4(h) (blue lines) with the calculated AC traces (red dashed line) under the assumption that all the oscillation modes are coherent. The measured AC traces agree very well with the calculated AC traces, which indicates that the output pulses are nearly chirp-free or transform-limited. The pulse widths (FWHM) are 0.928, 0.806, 0.744 and 0.656 ps, respectively, for the 3-, 6-, 9-, and 15-fold multiplications. It should be noted that the noise


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figure of the generated pulse train increases along with the boost of the repetition rate. The noise floor is mainly caused by the asymmetric laser spectra which can’t fully transform to pulse train. The asymmetric characteristic of the spectra is induced by the oscillation frequencies selection mechanism and the asymmetric cavity gain spectrum envelope of the proposed laser. Moreover, in the laser cavity, the energy of each pulse drops dramatically along with increase of the repetition rate while the amplified spontaneous emission (ASE) noise grows, as is shown in the optical spectrum. This resulted in the pulses energy fluctuations and noise floor increasing along with the increase in the repetition rate, which the same as the high order harmonic mode-locked lasers [29]. Fortunately, most of the ASE noise can be naturally filtered out when extracting part of the energy at the MRR drop port as the laser output. The FT spectra of the AC signals are shown in Fig. 4(i)-4(l) respectively. The spectra shows beat notes at the corresponding multiple pulse repetition rate, which suggests stable phase locking of our pulsed laser source.

Figure 4. Experimental results of repetition rate multiplication mode-locked lasers. (a)-(d) Optical spectra of the pulses and (e)-(h) corresponding measured and calculated AC traces with 3-, 6-, 9- and 15-fold (corresponding LMC=(521+1/3)LR, (521+1/6)LR, (521+1/9)LR and (521+1/15)LR, respectively) of the fundamental frequency of the MRR (49 GHz). (i)-(l) The FT spectra of the corresponding AC traces.


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Figure 5. Single main cavity mode oscillating in each MRR resonance. (a) Optical spectrum of 294 GHz pulsed laser source tested by 10 MHz resolution BOSA. Inset in (a) is the enlarged optical spectrum. (b) RF spectrum of the laser source tested by a 3 GHz PD. Insets in (b) are the RF waveform of the laser output and the linewidth test photocurrent power spectra along with Lorentzian fitting curve respectively. In order to obtain a better understanding of the lasing mode characteristic, a Brillouin high resolution OSA (10 MHz resolution, BOSA 300, Aragon Photonics) is used to observe the details of the mode-locked laser frequency mode. Fig. 5(a) shows the optical spectrum of the 294 GHz mode-locked laser with the BOSA. It can be seen in the insert of Fig. 5(a) that there is only a single main cavity mode in each MRR resonance. This further confirms the single mode operation of the proposed laser. The RF spectrum of the laser output is also tested using a 3 GHz bandwidth photodetector (PD) and an electrical spectrum analyzer ESA. The left inset in Fig. 5(b) is the waveform, where there are no observable beating nor slow modulation. To characterize the phase noise, the linewidth of a single lasing frequency is also measured using a delayed self-heterodyne method with a delay line of 25 km and a 150 MHz modulation frequency acoustic-optical modulator (AOM). The right inset in Fig. 5(b) shows the photocurrent power spectra having less than 50 kHz (FWHM) linewidth. It is important to point out that a single main cavity mode within each MRR resonance is a necessary condition for stable pulse train lasing. In previous reports,10-18 tens or hundreds of meters HNLF are needed in their laser cavities to enhance the FWM effect. The ratio of the comb filter linewidth to the laser modes spacing is up from hundreds to thousands and these lasers suffer from serious super-mode instability. Thus instability hindered the practical applications of the DFWM based laser sources until the FD-FWM configuration laser was introduced.23, 24 The success of the FD-FWM configuration laser is to integrate the comb filter and nonlinear components into a nonlinear MRR and shorten the laser cavity until only a single cavity mode can oscillate in each MRR resonance linewidth. In addition, the ability to control single cavity mode oscillation serves as an important role for repetition rate multiplication mode locked laser. When multiple main cavity modes are allowed to oscillate within the MRR resonance, the optical field will be modulated by mode-beating leading to Q-switching (or gain-switching). In the previous reported FD-FWM laser, 23, 24 a ratio of ~2.34 between the MRR resonance linewidth and the external cavity FSR is small enough for stable mode-locking. But it is quite easy to induce dual main cavity mode lasing within each MRR resonance


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when tuning the laser cavity length. Then the laser turns into a dual mode lasing state in which the pulse train is modulated by the beating of the two main cavity modes.24 In this case the pulses can’t be uniformly distributed in the MRR and the laser repetition rate can’t be multiplied at a stable mode-locking state. So further reducing the ratio is crucial for realizing repetition rate multiplicable lasers. To get an in-depth understanding of the repetition rate multiplication characteristic, we numerically simulate the dynamic evolution of the proposed laser source using the extended nonlinear Schrödinger equation (NLSE).18, 23 The light propagates through the components in the cavity in sequence and the effect of each component is considered through its transformation function. For the FD-FWM configuration, the MRR is not only used as a comb filter but also a nonlinear component.23 So the MRR is equivalent to a high fineness comb filter and a segmented waveguide. The length of the equivalent waveguide +,- is equal to the propagation distance of the photon within the photon storage time ./ of the MRR,

+,- = 01 ∗ ./ = 01 ∗ 2 ⁄3

(2)

Where 01 is the group velocity of the waveguide and ω is the optical angular frequency. The optical field transmitted in the

waveguide is enhanced by the MRR with a factor FE,21, 36 6

4 = 5

5

(3)

@A BCDE F

7878|6 |:/ ∙ 8

where L, G, I and JK are the MRR perimeter, the linear propagation loss coefficient, the coupling efficiency of the ring and straight waveguides and the propagation constant of the waveguide, respectively. FE is ~10 for the MRR used in our experiment. The optical field propagating within each fiber and the equivalent waveguide is modeled by LMN,O LN

+P

Q L MN,O

L O

1

= PR|4 ∗ ST, .| ST, . + U1 +

7 L

VW LX

Z

Y ST, . ST, .

(4)

where ST, . designates the slowly varying envelope of the optical fields. FE is equal to 10 in the equivalent waveguide while

1 for all fibers. The variables . and z represent the time and propagation distance, respectively. and R are the second order

dispersion coefficient and the cubic refractive nonlinearity coefficient, respectively. [1 is the bandwidth of the gain spectrum

and g is the saturable gain of the gain fiber,

\ = \] ∗ ^_` a>Mbc< ⁄acdX

(5)

where \] , a>Mbc< and acdX denote the small-signal gain coefficient, the pulse energy and gain saturation energy that relies on pump power, respectively.

The Vernier effect of the equivalent high fineness comb filter and the main cavity modes is considered in our simulations. The transfer function of these two filters are both regarded as ring filter and use the same expression,37

e=f

86 ∗ Kg/

78786 ∗ Kg

f

(6)

Where h = 2 ∗ + ⁄ (L denotes the optical path length of the ring filters and the wavelength) is the light phase change of one round trip in the ring filter. The length of the main cavity is around 521 fold of the MRR perimeter.

Figure 6. Simulation results. (a) and (b) show the calculated optical spectrum and AC trace (red line) of 6-FSR repetition rate case together with the


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relative experiment results, respectively. (c) and (d) present the calculated optical spectrum and AC trace of 20-FSR repetition rate, respectively. Eq. (4) is solved using the Runge-Kutta algorithm for EDF and split-step Fourier algorithm for SMF fiber and the equivalent waveguide, respectively. In the calculation, the dispersion and nonlinear effects of the EDF are neglected. The simulations start from weak white noise and run until the field becomes self-consistent after a finite number of traversals of the cavity. The

parameters used in the simulation are consistent with the experiments, c = 3 10i m/s, \] =110 dB/m, [1 = 20 jk,

acdX = 40 lm and + = 0.25 k for EDF. G = 0.2 SMF. j1 = 1.64, G = 0.06 uv/wk, R =

no

, R = 1 q 87 ∙ Jk87 , = 21.67 `t /Jk and L = 1.7 m for

Dp 110 q 87 ∙ Jk87 ,

= 33.65 `t /Jk and L = 0.21 m for waveguide. Fig.

6(a) and 6(b) present the calculated optical spectrum and AC trace of the case of 6-FSR repetition rate after 300 round trips, respectively. The theoretical results match with the experiment results quite well. The spectral envelope is triangular-like which is attributed to mode-locked mechanism of DFWM based laser source.10, 23 In principle, the repetition rate could be further increased if the main cavity length has enough adjustable accuracy. However, the highest repetition rate is practically limited by the spectral interaction of these two cavities and the gain bandwidth as well as the dispersion characteristics of the laser cavity. First, the ratio of the MRR resonance linewidth over the external cavity FSR determines the side modes suppression ability of Vernier effect. With increase of the repetition rate, the frequency filtering of the unwanted spectral components will be degraded. Once the side modes can’t be suppressed completely in the cavity, a fundamental frequency modulation will be imposed on the pulses train as shown in Fig. 6(c) and 6(d). Second, the gain bandwidth determines the highest repetition rate that the laser can reach. The gain bandwidth is determined by the gain characteristic of the EDF, pump power, wavelength dependent cavity loss as well as the birefringence-induced spectral filtering effect.18 Meanwhile, the repetition rate is also affected by the intra-cavity fiber dispersion. The fiber dispersion will broaden the pulses because of the negligible nonlinear effect in the intra-cavity fiber. The stability of the laser will be destroyed when two adjacent pulses overlapped with each other. According to our experimental results, the optical spectral bandwidth of the output pulses is about 4.5nm under current pump power. The transform limited pulse width at the output port of the MRR is 560 fs under sech2 assumption. The ~2 m fiber will broaden the pulse width to 713 fs. According to Nyquist criterion, the repetition rate could reach 882 GHz without mutual disturbance of the pulses train under ideal conditions. Furthermore, the noise figure increases with the repetition rate. The decrease of signal to noise ratio will further reduce the maximum repetition rate of the laser according to the Shannon’s theorem. Our experimental results almost reach the theoretical upper limit of the repetition rate. Increasing the gain bandwidth with a dispersion compensated fiber cavity and a higher Q factor MRR can significantly boost the laser repetition rate. We believe pulsed sources with repetition rates beyond THz could be reached by further optimizing the laser design in the future. In conclusion, we have experimentally demonstrated and theoretically analyzed a chip based pulsed laser source with repetition rate multipliable from 49 GHz to 735 GHz in steps of 49 GHz. The repetition rate multiplication is realized by temporal multiplexing technology with multiple equally spaced pulses circulating in the MRR. The repetition rate is adjusted by simply tuning of the laser cavity length through a high-precision optical delay line. To obtain a stable optical pulses train, the laser cavity is shortened and only a single laser cavity mode oscillates in each MRR resonance. Our proposed laser can serve as an ideal ultra-high speed multiplication clock source for future on-chip connection and optical signal process. In addition, it also has important potential applications in future microwave and millimeter wave technology.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ORCID W. Wang: 0000-0002-1157-0370

Author Contributions #W. Wang and W. Zhang contributed equally to this work.


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Notes The authors declare no competing financial interest.

Acknowledgments. This work was partly supported by National Natural Science Foundation of China (Grant No. 61475188, 61675231, 61635013), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB24030600). We gratefully acknowledge Prof. Yuanshan Liu and Prof. Xiaoping Xie for discussions about the experiments. We also thank Tektronix for providing the ultra-high bandwidth oscilloscope (DPO75902SX) and test introduction.

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(19) Pasquazi, A.; Caspani, L.; Peccianti, M.; Clerici, M.; Ferrera, M.; Razzari, L.; Duchesne, D.; Little, B. E.; Chu, S. T.; Moss, D. J.; Morandotti, R. Self-locked optical parametric oscillation in a CMOS compatible microring resonator: a route to robust optical frequency comb generation on a chip. Opt. Express 2013, 11, 13333-13341. (20) Foster, M. A.; Levy, J. S.; Kuzucu, O.; Saha, K.; Lipson, M.; Gaeta, A. L. Silicon-based monolithic optical frequency comb source. Opt. Express 2011, 19, 14233-14239. (21) Wang, W. Q.; Chu, S. T.; Little, B. E.; Pasquazi, A.; Wang, Y. S.; Wang, L. R.; Zhang, W. F.; Wang, L.; Hu, X. H.; Wang, G. X.; Hu, H.; Su, Y. L.; Li, F. T.; liu, Y. S; Zhao W. Dual-pump Kerr Micro-cavity Optical Frequency Comb with varying FSR spacing. Sci. Rep. 2016, 6, 28501. (22) Haye, P. D.; Beha, K.; Papp, S. B.; Diddams, S. A. Self-Injection Locking and Phase-Locked States in Microresonator-Based Optical Frequency Combs. Phys. Rev. Lett. 2014, 112, 043905. (23) Peccianti, M.; Pasquazi, A.; Park, Y.; Little, B. E.; Chu, S. T.; Moss, D. J.; Morandotti R. Demonstration of a stable ultrafast laser based on a nonlinear microcavity. Nat. Comm. 2012, 3, 765. (24) Pasquazi, A.; Peccianti, M.; Little, B. E.; Chu, S. T.; Moss, D. J.; Morandotti, R. Stable, dual mode, high repetition rate mode-locked laser based on a microring resonator. Opt. Express, 2012, 20, 27355-27362. (25) Azaña, J.; Muriel, M. A. Technique for multiplying the repetition rates of periodic trains of pulses by means of a temporal self-imaging effect in chirped fiber gratings. Opt. Lett., 1999, 24, 1672–1674. (26) Maram, R.; Howe, J. V.; Li, M.; Azaña, J. Noiseless intensity amplification of repetitive signals by coherent addition using the temporal Talbot effect. Nat. Commun. 2014, 5, 5163. (27) Matsas, V. J.; Richardson, D. J.; Newson, T. P.; Payne, D. N. Characterization of a self-starting, passively mode-locked fiber ring laser that exploits nonlinear polarization evolution. Opt. Lett. 1993, 18, 358-360. (28) Tao, S.; Xu, L. X.; Chen, G. L.; Gu, C.; Song, H. Y. Ultra-High Repetition Rate Harmonic Mode-Locking Generated in a Dispersion and Nonlinearity Managed Fiber Laser. J. of Lightw. Tech. 2016, 34, 2354-2356. (29) Collings, B. C.; Bergman, K.; Knox, W. H. Stable multigigahertz pulse-train formation in a short-cavity passively harmonic mode-locked erbium/ytterbium fiber laser. Opt. Lett. 1998, 23, 123-125. (30) Abedin, K. S.; Hyodo, M.; Onodera, N. Active stabilization of a higher-order mode-locked fiber laser operating at a pulse-repetition rate of 154 GHz. Opt. Lett. 2001, 26, 151-153. (31) Little, B. E.; Chu, S. T.; Absil, P. P.; Hryniewicz, J. V.; Seiferth, F.; Gill, D.; Van, V.; King, O.; Trakalo, M. Very high-order microring resonator filters for WDM applications. IEEE Photon. Tech. Lett. 2004, 16, 2263–2265. (32) Little, B. E. A VLSI photonics platform. Opt. Fiber Commun. 2003, 2, 444–445. (33) Lin, G. P.; Chembo, Y. K. On the dispersion management of fluorite whispering-gallery mode resonators for Kerr optical frequency comb generation in the telecom and mid-infrared range. Opt. Express, 2015, 23, 1594-1604. (34) Griffel, G. Vernier Effect in Asymmetrical Ring Resonator. IEEE Photon. Tech. Lett. 2000, 12, 1642-1644. (35) Fujioka, N.; Chu, T.; Ishizaka, M. Compact and Low Power Consumption Hybrid Integrated Wavelength Tunable Laser Module Using Silicon Waveguide Resonators. J. of Lightw. Tech. 2010, 28, 3115-3120. (36) Absil, P. P.; Hryniewicz, J. V.; Little, B. E.; Cho, P. S.; Wilson, R. A.; Jonechis, L. G.; Ho, P. T. Wavelength conversion in GaAs Micro-ring resonators. Opt. Lett. 2000, 25, 554–556. (37) Dominik, G. R. Integrated Ring Resonators. Springer Press: Berlin Heidelberg, 2007; p6.


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Repetition rate multiplication pulsed laser source based on a micro-ring resonator Weiqiang Wang, Wenfu Zhang, Sai T. Chu, Brent E. Little, Qinghua. Yang, Leiran Wang, Xiaohong Hu, Lei Wang, Guoxi Wang, Yishan Wang, and Wei Zhao We demonstrate a stable high-Q micro-ring resonator (MRR) based pulsed laser source with adjustable repetition rate from 49 GHz to 735 GHz, corresponds to repetition rate multiplication of up to 15 times the free spectral range (FSR) of the MRR. The repetition rate multiplication is realized by temporal multiplexing multiple pulses in the MRR through simply tuning of the fiber cavity length. Thus the repetition rate of the pulsed laser source breaks the frequency limitation of the previous dissipative four wave mixing based mode locked lasers whose repetition rate is equal to the FSR of the built in comb filter. This high quality chip-based repetition rate multiplicable pulsed laser source is an effective approach to on-chip ultra-high speed optical clock frequency multiplication systems.


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Repetition Rate Multiplication Pulsed Laser Source Based on a

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