Conventional Mode-locked Laser Research Articles

Transient dynamics in a common singly resonant optical parametric oscillator

The dynamics of a common optical parametric oscillator (OPO) is demonstrated for the first time with dispersive Fourier transform (DFT) technique. The singly resonant MgO:PPLN-based optical parametric oscillator is synchronously pumped with an Yb-doped fiber amplifier, and delivers a femtosecond signal wave with a spectrum tuned from 1284 to 1517 nm. Then a DFT device is built and the birth and extinction dynamics of the single- and dual-wavelength signal wave of the OPO is recorded. Compared with conventional mode-locked lasers, the dynamics evolution of the OPO is entirely attributed to the nonlinear effects in the nonlinear crystal different from the atomic transitions in the active media, which gives rise to novel features in the starting progress of the OPO. Furthermore, a new way of energy transfer is discovered for the first time during the evolution of dual-wavelength signal wave. The results are believed to provide a unique insight into evolution in OPOs and are instructive for in-depth understanding of OPO and other nonlinear devices and materials.

Synthesis and dissociation of soliton molecules in parallel optical-soliton reactors

Mode-locked lasers have been widely used to explore interactions between optical solitons, including bound-soliton states that may be regarded as “photonic molecules”. Conventional mode-locked lasers normally, however, host at most only a few solitons, which means that stochastic behaviours involving large numbers of solitons cannot easily be studied under controlled experimental conditions. Here we report the use of an optoacoustically mode-locked fibre laser to create hundreds of temporal traps or “reactors” in parallel, within each of which multiple solitons can be isolated and controlled both globally and individually using all-optical methods. We achieve on-demand synthesis and dissociation of soliton molecules within these reactors, in this way unfolding a novel panorama of diverse dynamics in which the statistics of multi-soliton interactions can be studied. The results are of crucial importance in understanding dynamical soliton interactions and may motivate potential applications for all-optical control of ultrafast light fields in optical resonators.

Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser

The ability to generate phase-stabilized trains of ultrafast laser pulses by mode-locking underpins photonics research in fields, such as precision metrology and spectroscopy. However, the complexity of conventional mode-locked laser systems has hindered their realization at the nanoscale. Here we demonstrate that GaAs-AlGaAs nanowire lasers are capable of emitting pairs of phase-locked picosecond laser pulses with a repetition frequency up to 200 GHz when subject to incoherent pulsed optical excitation. By probing the two-pulse interference spectra, we show that pulse pairs remain mutually coherent over timescales extending to 30 ps, much longer than the emitted laser pulse duration (≤3 ps). Simulations performed by solving the optical Bloch equations produce good quantitative agreement with experiments, revealing how the phase information is stored in the gain medium close to transparency. Our results open the way to phase locking of nanowires integrated onto photonic circuits, optical injection locking and applications, such as on-chip Ramsey comb spectroscopy.

Rainbow Stars—Multi-Color Laser Emission for Science and Art

Rainbow Stars—Multi-Color Laser Emission for Science and Art

Highly Linear Broadband Photonic-Assisted Q-Band ADC

A highly linear broadband photonic-assisted analog-to-digital converter (ADC) based on high-frequency optical sampling utilizing a dual output Mach–Zehnder modulator operating with signal frequencies up to 50 GHz is presented. The pulses employed in the optical sampling were generated by a cavity-less pulse source operated at 10 GHz in preference to conventional mode-locked lasers. The optical sampling front-end greatly extends the operational frequency range of the Nyquist limited electronic digitization back-end. The performance of the sampling system is characterized with 7.1 effective number of bits (ENOBs) at 40 with 5 GHz fully accessible bandwidth, and greater than 99 dB·Hz2/3 spurious free dynamic range for the 30–40 GHz frequency range. Furthermore, more than 8 ENOB was achieved by reducing the effective bandwidth to 1 GHz with a digital filter, demonstrating the additional advantage of using a higher sampling rate compared to previous demonstrations. A new figure of merit of photonic-assisted sub-sampled ADCs is also presented accompanied with a comparison to previous implementations.

Single-cycle-pulse passively-mode-locked laser with inhomogeneously broadened active medium

Generation of single-cycle pulses from a laser passively mode locked by the technique of coherent mode locking is theoretically demonstrated for the case when resonant lines of both the gain and absorbing media are inhomogeneously broadened. In contrast to conventional mode-locked lasers, for which the inhomogeneous nature of line broadening sets a severe limitation on their performance, here, the stable single-cycle pulse operation is shown to persist even when the spectral widths of the inhomogeneously broadened lines of the amplifier and the absorber become nearly as wide as the central resonance frequency of their two-level transitions. This study is aimed at showing the potential for application of the coherent-mode-locking technique for quantum-dot lasers, which are typically characterized by a great amount of inhomogeneous broadening.

160-GHz 1.55-$\mu{\rm m}$ Colliding-Pulse Mode-Locked AlGaInAs/InP Laser With High Power and Low Divergence Angle

A monolithic ~1.55-μm colliding-pulse mode-locked AlGaInAs/InP laser with a three-quantum-well active layer incorporating a passive far-held reduction layer has been demonstrated. The device emits pulses at 162 GHz, with a pulsewidth of 0.98 ps, a pulse energy of 0.13 pJ, and a time-bandwidth product of 0.52, while demonstrating a low divergence angle (12.7° × 26.3°) with a twofold improvement in butt coupling efficiency to a flat cleaved single-mode fiber, compared to the conventional mode-locked lasers.

Demonstration of Low-Cost Uplink Transmission in a Coherent OCDMA PON Using Gain-Switched Fabry–Pérot Lasers With External Injection

We propose and demonstrate a simple and low-cost uplink transmission scheme for coherent optical code-division multiple access (OCDMA) passive optical networks (PONs), using gain-switched Fabry-Perot (GS-FP) lasers with external injection as local light sources at optical network units. Experimental results based on the two-user 1.25-Gb/s OCDMA system confirm the feasibility of this scheme. The system performance is compared with that of the OCDMA system using a conventional mode-locked laser (MLL). The auto-correlation peak to the maximum wing level (P/W) ratio is also given as the central wavelength of the GS-FP laser varies. The present low-cost scheme is highly preferable for OCDMA PON applications.

Analysis of hybrid mode-locking of two-section quantum dot lasers operating at 15 μm

For the first time a detailed study of hybrid mode-locking in two-section InAs/InP quantum dot Fabry-Pérot-type lasers is presented. The output pulses have a typical upchirp of approximately 8 ps/nm, leading to very elongated pulses. The mechanism leading to this typical pulse shape and the phase noise is investigated by detailed radio-frequency and optical spectral studies as well as time-domain studies. The pulse shaping mechanism in these lasers is found to be fundamentally different than the mechanism observed in conventional mode-locked laser diodes, based on quantum well gain or bulk material.

Semiconductor laser diode produces stabilized optical-frequency combs

Over the past decades, the use of lasers in communications, signal processing, test andmeasurement systems, and spectroscopy has enabled many key advances owing to the wide range of useful characteristics inherent in coherent laser radiation. Further advances are expected from simultaneous deployment of multiple lasers spanning a range of wavelengths. This will allow generation of parallel optical channels, thus increasing the system’s measurement and processing capability. Additional functionality is achieved if each optical-frequency channel is phase-locked to the other channels, i.e., if the relative phase relations among all channels are well established, fixed, and not drifting over long times.1 To construct such a set of frequency combs, one can use a single continuous-wave laser, combined with modulation techniques such as amplitude or phase modulation to create sidebands.2 Alternatively, mode-locked lasers can generate a frequency comb with phase-coherent relationships between all wavelength components. The main drawback of a conventional mode-locked laser is that the frequency comb can drift because of both environmental and background quantum effects.3 We have developed an optical source that is capable of producing a set of widely spaced optical frequencies and suitable for a broad range of communication and signal-processing applications. Our approach is also self-referencing because the opticalfrequency comb is referenced to a secondary optical standard, such as a high-quality etalon. To achieve wide optical-channel spacing and high spectral purity, we engineer the optical cavity so that the optical-frequency comb has simultaneous wide channel spacing and a very narrow linewidth for each frequencyFigure 1. Optical-frequency-stabilized mode-locked laser. SOA: Semiconductor optical amplifier. DCF: Dispersion-compensating fiber. I: Isolator. IML: Intensity modulator. FPE: Fabry-Perot etalon. OPS: Optical-phase shifter. VOD: Variable optical delay. PBS: Polarization beam splitter. PC: Polarization controller. PM: Phase modulator. Cir: Circulator. PS: Phase shifter. PD: Photodetector. PID: Proportional-integral-derivative controller. PDH: PoundDrever-Hall.

5-ps, 10-GHz pulse generation from an all-optical semiconductor switch embedded in a ring cavity

We observed spontaneous generation of nearly transform-limited pulses (5 ps, 10 GHz, 1558 nm) from a semiconductor ring oscillator. This oscillator is completely different from conventional mode-locked lasers in that the pulse width is passively determined by a split delay time inside its ring cavity. This is because the ring cavity includes an all-optical semiconductor switch whose window width is determined by the split delay time.

61-GHz dark-soliton generation and propagation by a fiber Bragg grating pulse-shaping technique

We report the experimental shaping of dark-soliton pulse trains at a several-gigahertz repetition rate by means of a simple sinusoidal fiber Bragg grating used as a highly resolving passive filtering element at the output of a conventional mode-locked laser source. We show that the proper choice of grating parameters allows for the transformation of bright-pulse trains into odd-symmetry dark-pulse trains with cw background. The results of a propagation experiment with a 7-km-long fiber demonstrate, together with numerical simulations, the efficiency of the new shaping method.

230 fs, 25 W pulses from conventional mode-locked laser diodes with saturable absorber created by ion implantation

230 fs, 25 W pulses are obtained from conventional mode-locked AlGaAs laser diodes with highly-reflecting external cavities. Uncoated commercial diodes are used with a region of saturable absorption created by ion implantation. The overall pulse performances are believed to be among the best reported from simple electrically-pumped laser diodes without using additional amplifier sections.

The Michelson resonator free-electron laser. II. Supermode structure and mirror detuning effects

For pt.I, see ibid., vol.29, no.2, p.452-64 (1993). Conventional mode-locked laser theory is applied to the longitudinal mode structure in an RF linac-driven Michelson resonator free-electron laser (FEL). A greatly simplified derivation of the small-signal small-gain FEL coupled mode equations is obtained. These equations are solved numerically in the frequency domain to study the supermode evolution in the presence of mode dependent cavity losses. The results are compared with simulations of the detuned Michelson resonator FEL using a pulse propagation code based on the Maxwell-Lorentz equations of motion. Increasing the interferometer detuning broadens and shifts the cavity detuning curve, narrows the supermode spectrum, and decreases the hypermode decay rates. The practical consequences of each of these effects are discussed. A simple theory describing the dependence of the hypermode decay rates on interferometer detuning, in which the decay rates are abruptly decreased beyond a critical detuning that depends primarily on the slippage length, is outlined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>