GHz Optical Pulse Train Research Articles

Highly tunable repetition-rate multiplication of mode-locked lasers using all-fibre harmonic injection locking

Higher repetition-rate optical pulse trains have been desired for various applications such as high-bit-rate optical communication, photonic analogue-to-digital conversion, and multi-photon imaging. Generation of multi GHz and higher repetition-rate optical pulse trains directly from mode-locked oscillators is often challenging. As an alternative, harmonic injection locking can be applied for extra-cavity repetition-rate multiplication (RRM). Here we have investigated the operation conditions and achievable performances of all-fibre, highly tunable harmonic injection locking-based pulse RRM. We show that, with slight tuning of slave laser length, highly tunable RRM is possible from a multiplication factor of 2 to >100. The resulting maximum SMSR is 41 dB when multiplied by a factor of two. We further characterize the noise properties of the multiplied signal in terms of phase noise and relative intensity noise. The resulting absolute rms timing jitter of the multiplied signal is in the range of 20 fs to 60 fs (10 kHz–1MHz) for different multiplication factors. With its high tunability, simple and robust all-fibre implementation, and low excess noise, the demonstrated RRM system may find diverse applications in microwave photonics, optical communications, photonic analogue-to-digital conversion, and clock distribution networks.

Polarization-maintained coupled optoelectronic oscillator incorporating an unpumped erbium-doped fiber

A polarization-maintained coupled optoelectronic oscillator (COEO) with its performance significantly improved by a short-length unpumped erbium-doped fiber (EDF) is reported and experimentally investigated. A 10 GHz optical pulse train with a supermode suppression ratio of 61.8 dB and a 10 GHz radio frequency signal with a sidemode suppression ratio of 94 dB and a phase noise of ?121.9 dBc/Hz at 10 kHz offset are simultaneously generated. Thanks to saturable absorption of the 1 m unpumped EDF, which introduces relatively large cavity loss to the undesired modes and noise, the supermode suppression ratio and the phase noise are improved by 9.4 and 7.9 dB, respectively.

Electronically synthesized Nyquist pulses for photonic sampling of microwave signals

We report electronic generation of optical Nyquist pulses using an arbitrary waveform generator (AWG) followed by a Mach Zehnder modulator (MZM), providing a simple, highly stable and flexible technique to perform photonic sampling. Here, we demonstrate the generation of 10 GHz periodic optical Nyquist pulses by synthesizing both all-positive and alternate positive-negative electrical pulse trains using a 25 GHz bandwidth AWG. Biasing the MZM at null ensures the meeting of the Nyquist ISI-free criterion in the optical domain and allows for pulse compression. Moreover, we report the first photonic sampling and demodulation of 1 Gbaud 16- and 32-QAM signals up to 22.5 GHz using 10 GHz optical Nyquist sampling pulse trains.

Supermode noise suppression in an actively mode-locked fiber laser with pulse intensity feed-forward and a dual-drive MZM

The supermode noise in an actively mode-locked fiber laser is suppressed for the first time by a so-called pulse-intensity-feed-forward technique. In this novel scheme, the optical pulse is firstly converted into an electronic signal, and then fed forward to a dual-drive Mach–Zehnder modulator (MZM), where the optical pulse is modulated by the reversed intensity profile of itself. In a 12.5 m long actively mode-locked fiber ring laser experiment, the resulting power limiting shows a stable 10 GHz optical pulse train with a supermode-suppression ratio larger than 81 dB. The phase noise and pulse-to-pulse timing jitter are below −141 dBc Hz−1 at 10 MHz and 22.1 fs, respectively. Compared with the conventional fiber-nonlinearity-based power limiting schemes, our proposal is compact and stable.

Even harmonic pulse train generation by cross-polarization-modulation seeded instability in optical fibers

We show that, by properly adjusting the relative state of polarization of the pump and of a weak modulation, with a frequency such that at least one of its even harmonics falls within the band of modulation instability, one obtains a fully modulated wave at the second or higher even harmonic of the initial modulation. An application of this principle to the generation of an 80 GHz optical pulse train with high extinction ratio from a 40 GHz weakly modulated pump is experimentally demonstrated using a nonzero dispersion-shifted fiber in the telecom C band.

Optical and RF stability transfer in a monolithic coupled-cavity colliding pulse mode-locked quantum dot laser

We report a novel quantum dot based laser design where a stable high-Q master laser is used to injection lock a passively mode-locked monolithic colliding pulse slave laser. Coupling between the crossed orthogonal laser cavities is achieved through a common monolithically integrated saturable absorber, which results in the locking and hence reduction of the timing jitter as well as the long-term frequency drift of the slave laser. A stable 30 GHz optical pulse train is generated with more than 10 dB reduction in the RF noise level at 20 MHz offset and close to 3 times reduction in the 10 dB average optical linewidth of the slave laser.

Eightfold 40–320 Gbit/s phase-coherent multiplexing and 320–40 Gbit/s demultiplexing using highly nonlinear fibers

We experimentally demonstrate 40-320Gbit/s phase-coherent multiplexing based on cross-phase modulation and subsequent 320-40Gbit/s demultiplexing using cross-phase-modulation-induced frequency shift in highly nonlinear fibers. An average penalty of approximately 7dB at a 10(-9) bit-error rate is observed after the nonlinear processes. We also show the generation of a phase-coherent 320GHz optical pulse train. A delay-line interferometer based on differential group delay is used to verify the phase coherence characteristic of the multiplexed 320Gbit/s signal and the generated 320GHz pulses.

Synchronization of over 160 GHz optical pulse train by optical phase-locked loop technique

Synchronization of over 160 GHz optical pulse train by optical phase-locked loop technique

All optical clock division and multiplication by injection mode-locked laser based on SOA

Optical clock division and multiplication were realized with an injection mode-locked fiber ring laser based on semiconductor optical amplifier SOA owing to the relatively long recovery time of carriers in SOA and the rational harmonic mode-locking. Second frequency division and 1.5th frequency multiplication of 10 GHz , second and 3 4 th frequency division of 20 GHz optical pulse trains were realized, respectively, in the experiment.

160 GHz actively modelocked semiconductor laser

A 163.66 GHz optical pulse train was generated by a semiconductor modelocked laser integrated with a high-mesa EA modulator, which enables direct synchronisation with a 81.83 GHz electrical signal. A timing jitter of 0.15 ps and low amplitude modulation noise are achieved.

High-repetition-rate optical pulse generation and control techniques

In this paper, we report the generation of a high-repetition-rate optical pulse trainwith a mode-locked laser diode (MLLD) and its control techniques. We discusskey technologies to increase the mode-locking frequency and reveal that passivemode-locking based on an intracavity semiconductor saturable absorber is apromising way to achieve mode-locking at a very high repetition rateexceeding hundreds of GHz. Generation of a transform-limited 480 GHzoptical pulse train was successfully achieved using a short-cavity (174 µm)MLLD. We also propose and demonstrate two novel techniques, subharmonicoptical synchronous mode-locking and an all-optical down-converting method, tostabilize such an ultrafast MLLD and to evaluate the jitter characteristicsaccurately by all-optical methods without limitations imposed by an electronicdevice’s speed. Stabilization of ultrafast MLLDs (160 and 480 GHz passiveMLLDs) and the jitter measurements were successfully demonstrated.

Transform-limited 480 GHz colliding-pulse modelocked laser diode

Generation of a 480 GHz optical pulse train from a short-cavity colliding-pulse modelocked laser diode is reported. Cavity loss reduction by high-reflection coatings of both laser facets was found to be effective for pulse shortening. The pulsewidth of the laser output was 0.52 ps and the time-bandwidth product was 0.31, indicating that the laser output was nearly transform-limited.

Generation and stabilization of ultrafast optical pulse trains with monolithic mode-locked laser diodes

In this paper, we report on the generation and the stabilization of ultrafast optical pulse trains exceeding 100 GHz from monolithic mode-locked laser diodes (MLLDs) combined with some new techniques such as subharmonic synchronous mode-locking (SSML) and repetition-frequency multiplication (RFM) method. Key subjects to increase the pulse repetition frequencies of the MLLDs such as fast absorption recovery and harmonic mode-locking operation are discussed. 500 GHz optical pulse generation from a short-cavity, graded-index separated confinement heterostructure MLLD and THz-rate pulse generation by harmonic mode-locking are reported. We also demonstrate the stabilization of a 160 GHz MLLD by the SSML with subharmonic-frequency optical pulse injection and reveal that the SSML is very promising as a stabilization technique of the ultrafast MLLD beyond the limitations by the electronic device speed. A method to accurately measure the timing jitter of such ultrafast optical pulse train, all-optical down converting using a nonlinear optical device, is also presented. We also mention another choice for ultrafast optical pulse generation using the MLLD combined with a dispersive medium such as an optical fiber. We demonstrate here the generations of stable 84–256 GHz optical pulse trains by the RFM method of the MLLD stabilized by the SSML.

Fourier synthesis of 257 GHz optical pulse train by phaselocking of three continuous-wave semiconductor lasers

A Fourier-synthesised optical-pulse train with a repetition rate of 257 GHz has been obtained by optically phaselocking three continuous-wave (CW) semiconductor lasers using a semiconductor optical amplifier as a four-wave mixer. Second-harmonic-generation (SHG) correlation traces were obtained to examine the temporal waveforms of the pulse train.

Generation of synchronized subterahertz optical pulse trains by repetition-frequency multiplication of a subharmonic synchronous mode-locked semiconductor laser diode using fiber dispersion

We describe the generation of a synchronized, low-jitter, subterahertz optical pulse train by repetition-frequency multiplication of a subharmonic synchronous mode-locked semiconductor laser diode using the group-velocity dispersion (GVD) of optical fiber. A 49-196 GHz optical pulse train was generated with low intensity fluctuation and with low timing jitter, by adjusting fiber length in accordance with the mode-locking frequency and the fiber's GVD.

Synchronisation of subterahertz optical pulse train from PLL-controlled colliding pulse modelocked semiconductor laser

The synchronisation of a 192 GHz optical pulse train, generated from a colliding pulse modelocked laser diode, to an electrical signal is successfully demonstrated by a phase-locked loop that uses optical modulation sidebands. The synchronisation is confirmed by a synchro-scan streak camera and the timing jitter is estimated to be 0.6 ps from the RF line spectrum of the beat note between the high-order sidebands generated by 40 GHz electroabsorption modulation.

High-repetition-rate optical pulse generation by using chirped optical pulses

A very high repetition rate optical pulse train is generated by using a chirped optical pulse train obtained with a dispersive medium and an optical pulse laser source. Adjusting the group-velocity dispersion (GVD) of the dispersive medium, we can obtain an optical pulse train at a repetition rate which is a multiple of that of the optical pulse laser source initially used. We have successfully generated a 50 to 400 GHz optical pulse train synchronised with an external clock by using a 10 GHz pulse source and optical fibre as the dispersive medium. We have also numerically confirmed this result.

Ultrahigh-speed clock recovery with phase lock loop based on four-wave mixing in a traveling-wave laser diode amplifier

A new phase lock loop (PLL) is proposed and demonstrated for clock recovery from an ultrahigh-speed time-division multiplexed (TDM) optical signal. A traveling-wave laser-diode amplifier (TW-LDA) is used as a phase detector, and the cross-correlation component between the optical signal and an optical clock pulse train is detected as a four-wave-mixing (FWM) signal generated in the TW-LDA. A timing clock from a TDM signal is extracted as a prescaled electrical clock, and this prescaled clock is directly recovered from a randomly modulated TDM optical signal. A prescaled 6.3 GHz clock is successfully extracted from a 100 Gb/s signal using the timing comparison output obtained as the cross-correlation between the optical signal and a short (<10 ps) 6.3 GHz optical clock pulse train in the generated FWM light. A comparison of the PLL phase noise with a previously reported gain modulation method is also shown, and the possibility of the Tbit/s operation of this PLL is also considered in the experiments.

Monolithic integration of MQW modulators on an optical multiplexer on InP for 100 Gb/s transmission

We report on an InP-based optical multiplexer integrated with high-speed multiquantum-well (MQW) electroabsorption modulators for 100 Gb/s transmission. The optical multiplexer is based on a multimode interference (MMI) splitter, a waveguide array, and a taper-type combiner designed using the beam propagation method (BPM). The modulators are integrated on the top of the optical multiplexer. The integrated device is fabricated by three step epitaxy using metal organic vapor phase epitaxy (MOVPE) and by dry etching. The static and dynamic performances are measured, leading to the generation and the modulation of a 100 GHz optical pulse train. This optical pulse train is also coded using a pseudorandom nonreturn to zero (NRZ) pattern.

Generation and coding of a 100 Gbit/s signal by an InP-based optical multiplexer integrated with modulators

The authors demonstrate a novel InP-based optical multiplexer integrated with MQW electroabsorption modulators for 100 Gbit/s transmission. A 100 GHz optical pulse train is generated and coded with a pseudorandom NRZ pattern.