Unidirectional Laser Research Articles

Isolator-free unidirectional thulium-doped fiber laser

We report the first demonstration of a unidirectional, isolator-free 2-mmthulium-doped fiber (TDF) laser, relying on the properties of the theta cavity (ring resonator with S-shaped feedback). The core pumped theta cavity TDF laser provides sub-Watt output power with a slope efficiency of 25%, a 2 dB flat tuning range of 1900–2050 nm, and a linewidth of 0.2 nm, and achieves the extinction ratio of 18– 25 dB (depending on the feedback value) between the favored and suppressed lasing directions. It is shown that these characteristics are competitive with, if not superior to, those of conventional ring cavities. The simulation results of the linear and Kerr-nonlinear theta cavities are also presented, explaining certain unexpected features of the laser behavior and establishing the importance of the doped fiber nonlinearity on the spectral shaping of the emitted signal.

Unidirectional hybrid silicon ring laser with an intracavity S-bend.

We demonstrate a hybrid silicon ring laser with an internal amplifying S-bend that couples a fraction of the counter-clockwise circulating light into the the clockwise direction. The device supported single-mode, unidirectional laser oscillation at certain bias conditions. A spatial field distribution model is derived to describe the unidirectional operation. A unidirectional clockwise laser output with a suppression ratio up to 18.6 dB over the counter-clockwise mode was achieved.

Integrated Microwave Photonic Isolators: Theory, Experimental Realization and Application in a Unidirectional Ring Mode-Locked Laser Diode

A novel integrated microwave photonic isolator is presented. It is based on the timed drive of a pair of optical modulators, which transmit a pulsed or oscillating optical signal with low loss, when driven in phase. A signal in the reverse propagation direction will find the modulators out of phase and, hence, will experience high loss. Optical and microwave isolation ratios were simulated to be in the range up to 10 dB and 20 dB, respectively, using parameters representative for the indium phosphide platform. The experimental realization of this device in the hybrid silicon platform showed microwave isolation in the 9 dB–22 dB range. Furthermore, we present a design study on the use of these isolators inside a ring mode-locked laser cavity. Simulations show that unidirectional operation can be achieved, with a 30–50-dB suppression of the counter propagating mode, at limited driving voltages. The potentially low noise and feedback-insensitive operation of such a laser makes it a very promising candidate for use as on-chip microwave or comb generators.

A self-injected, diode-pumped, solid-state ring laser for laser cooling of Li atoms.

We have constructed a solid-state light source for experiments with laser cooled lithium atoms based on a Nd:YVO4 ring laser with second-harmonic generation. Unidirectional lasing, an improved mode selection, and a high output power of the ring laser were achieved by weak coupling to an external cavity which contained the lossy elements required for single frequency operation. Continuous frequency tuning is accomplished by controlling two piezoelectric transducers (PZTs) in the internal and the external cavities simultaneously. The light source has been utilized to trap and cool fermionic lithium atoms into the quantum degenerate regime.

Unidirectional Lasing From a Spiral-Shaped Microcavity of Dye-Doped Polymers

We experimentally demonstrate fabrication of a spiral-shaped polymer microdisk cavity by femtosecond laser direct writing via two-photon polymerization of dye-doped resins. This spiral microresonator supports highly directional emission from the notch with a high-Q factor exceeding 1.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and a low lasing threshold at room temperature. Our numerical simulation results reveal that clockwise propagating high-Q whispering gallery-like modes may couple to the counterclockwise modes and give the directional output. Benefiting from the low cost and good chemical compatibility of polymer materials, the fabrication of such spiral-shaped polymer microlasers with high-Q and unidirectional emission holds great potentials for use as elements of integrated organic optoelectronic devices.

Cavity spatial mode-locking and high controllability of radial output coupling for circular/square plasmonic nano-resonator lasers.

We proposed and investigated a novel output coupling scheme for a circular and a square plasmonic nano-ring laser based on a T-shaped radial coupler that is easier to realize than a tangential coupler. The amount of coupling efficiency is shown to be highly controllable from a few percent to tens of percents. This is due to the fact that the standing-wave lasing mode pattern will rotate to give the minimal cavity loss at the T-coupler's location, making the amount of output coupling surprisingly low and hence, controllable. For a non-circular cavity, other symmetry-breaking and geometry-induced scattering could result in separate mode-pattern locking. These give a few main ways to control and optimize the coupling efficiency: via widening/narrowing or rotating the T-coupler's waveguide, or, for the case of a non-circular cavity, via shifting the location of the T-coupler. We observed increased unidirectional lasing induced by either rotating the waveguide or shifting it (for non-circular cases). We simulated the coupling using Maxwell's equations based on the multi-level multi-electron FDTD (MLME-FDTD) method to realistically model the lasing and output coupling behaviors of such plasmonic semiconductor lasers.

Coherent random lasing from liquid waveguide gain channels with biological scatters

A unidirectional coherent random laser based on liquid waveguide gain channels with biological scatters is demonstrated. The optical feedback of the random laser is provided by both light scattering and waveguide confinement. This waveguide-scattering-feedback scheme not only reduces the pump threshold but also makes the output of random laser directional. The threshold of our random laser is about 11 μJ. The emission spectra can be sensitively tuned by changing pump position due to the micro/nano-scale randomness of butterfly wings. It shows the potential applications of optofluidic random lasers for bio-chemical sensors on-chip.

Parametric interference effect in electron-nucleus scattering in the field of two pulsed laser waves

Electron scattering on a nucleus in a field of two unidirectional pulsed laser waves is considered. The parametric interference effect is studied, which manifests in electron scattering within the plane formed by both the direction of laser-wave propagation and the initial electron momentum (the interference region). In this kinematics the electron emits and absorbs photons of both waves in a correlated manner. The distribution of the differential cross section of the final-electron energy for the process of electron-nucleus scattering in the field of two pulsed waves is considered. This distribution in the interference region differs qualitatively and quantitatively from the corresponding distribution in any other geometry. The appearance of the parametric interference effect may be experimentally verified by measuring the energy spectrum of final electrons in the framework of modern research projects, which use sources of pulsed laser radiation (XFEL, ELI, PHELIX).

PT-symmetric microring laser-absorber.

The lasing and coherent perfect absorption (CPA) properties of PT-symmetric microrings with mixed index and gain gratings, externally coupled to a bus waveguide, are theoretically investigated. For a complex grating at the PT-symmetry breaking point, perfect unidirectional (either clockwise or counterclockwise) laser emission can be realized, but the grating does not discriminate longitudinal modes and CPA can not be simultaneously achieved. Above the grating the PT-symmetry breaking point, single-mode emission, and simultaneous CPA can be obtained, with unbalanced and controllable excitation of clockwise and counterclockwise modes in the ring.

Reconfigurable directional lasing modes in cavities with generalized PT symmetry.

We introduce a new family of generalized PT-symmetric cavities that involve gyrotropic elements and support reconfigurable unidirectional lasing modes. We derive conditions for which these modes exist and investigate a simple electronic circuit that experimentally demonstrates their feasibility in the radio-frequency domain.

TE-mode nonreciprocal propagation in passive Fe50Co50–InGaAsP/InP semiconductor optical isolators

We report on optical isolation of TE-mode passive Fe50Co50/InGaAsP/InP semiconductor optical isolators for integration with semiconductor ring lasers and unidirectional lasing devices. The target optical isolation was set at 5 dB/mm for unidirectional semiconductor ring lasers. We calculated the optical isolations for devices with waveguide widths, w, of 1.5, 2, and 2.5 µm, and Al2O3 buffer layer thicknesses, d, of 15–40 nm. The optical isolations were calculated to be 13.2 dB/mm for w = 1.5 µm, 5.7 dB/mm for w = 2 µm, and 2.9 dB/mm for w = 2.5 µm, all with d = 30 nm, which satisfy the target optical isolation. On the basis of calculations, we fabricated TE-mode passive Fe50Co50/InGaAsP/InP semiconductor optical isolators and characterized their optical isolations. We obtained optical isolations of 1.3 dB/mm for w = 2.5 µm and 3.3 dB/mm for w = 2 µm. We compared them with the calculated results and discussed the origin of the differences. The experimentally obtained optical isolations in this study and our investigation of their dependence on the device structure will lead the way to practical unidirectional semiconductor ring lasers and should be useful for realizing low-operating-current unidirectional semiconductor ring lasers for all-optical flip-flop memories using semiconductor optical isolators.

Unidirectional Lasing Emerging from Frozen Light in Nonreciprocal Cavities

We introduce a class of unidirectional lasing modes associated with the frozen mode regime of nonreciprocal slow-wave structures. Such asymmetric modes can only exist in cavities with broken time-reversal and space inversion symmetries. Their lasing frequency coincides with a spectral stationary inflection point of the underlying passive structure and is virtually independent of its size. These unidirectional lasers can be indispensable components of photonic integrated circuitry.

Tailoring the directivity of both excitation and emission of dipole simultaneously with two-colored plasmonic antenna

We report a two-colored plasmonic antenna which can control the directivity of the excitation and emission light independently and simultaneously. By carefully tuning the phase difference of the constituting elements of the antenna, unidirectional fluorescence emission and laser light scattering can be obtained. In particular, the direction of the maximum emission and minimum scattering can be tailored in the same direction resulting improvement of signal to noise ratio in single molecule experiment. A two-dipole model is applied to describe the phenomena. The radiation and scattering pattern can be further tuned by varying the antenna structure.

The interference effect in electron scattering on a nucleus in the field of two pulsed laser waves of circular polarization

Electron scattering on a nucleus in the field of two unidirectional pulsed laser waves of circular polarization is considered. The interference effect was detected if an electron scatters in the plane determined by the direction of laser wave propagation and the initial electron momentum. In this kinematics the electron emits and absorbs photons of both waves in a correlated manner. Broadening of partial differential cross sections is studied near the interference outgoing angle of the electron. It is shown that the partial probability of stimulated processes in the interference region may be five orders of magnitude greater than the corresponding probability of processes in any other geometry. The results may be experimentally verified, for example, in the framework of the FAIR (Facility for Antiproton and Ion Research, Darmstadt, Germany) project.

Resonant Interference Effect in Scattering of an Electron by an Electron in the Field of Two Pulsed Laser Waves

Resonant scattering of an electron by an electron in the field of two unidirectional pulsed laser waves is studied theoretically. The interference kinematical region is considered. Stimulated emission and absorption of external waves' photons by electrons is correlated in the interference region. Resonance condition in the interference region is specified. Analytical expressions for the amplitude and differential cross section of the process for waves' circular polarization are obtained in the first order of the perturbation theory. The cross section contains resonant peaks, their altitude and width are determined by external waves parameters. Considerable effect of wave polarization on the character of stimulated emission and absorption of external waves' photons is found. It is demonstrated that the differential interference cross section of scattering of nonrelativistic electrons by each other in the field of two pulsed light waves of the optical frequency is approximately ten times greater in the magnitude than the corresponding one in the external field absence. Obtained results can be verified experimentally, for example, at the SLAC National Accelerator Laboratory (Stanford, USA) or in the frame of the FAIR Project (Darmstadt, Germany).

Unidirectional III-V microdisk lasers heterogeneously integrated on SOI

We demonstrate unidirectional bistability in microdisk lasers electrically pumped and heterogeneously integrated on SOI. The lasers operate in continuous wave regime at room temperature and are single mode. Integrating a passive distributed Bragg reflector (DBR) on the waveguide to which the microdisk is coupled feeds laser emission back into the laser cavity. This introduces an extra unidirectional gain and results in unidirectional emission of the laser, as demonstrated in simulations as well as in experiment.

Linearly aligned superradiant Bose-Einstein condensates diffracted by a single short laser pulse

Multiorder bidirectional superradiant Bose-Einstein condensates (BECs) were generated in a straight line by an irradiation of a single unidirectional short laser pulse along the long axis of a cigar-shaped sodium BEC in a magnetic trap. The probabilities of the diffracted BECs as a function of the laser intensity were well explained by the square of the Bessel functions and it was estimated that the intensity of the end-fire beam was 25$%$ of the laser intensity. The backward diffractions disappeared at pulse duration longer than 5 \ensuremath{\mu}s because of energy conservation. The probability for the $+$first-order diffraction grew exponentially with pulse duration when the backward diffractions disappeared. We observed the linearly aligned diffracted BECs along the propagation direction of the laser beam, regardless of the aspect ratio of the condensates. This fact indicates that the end-fire beam is triggered by the small backreflection from the vacuum window.

Hybrid III-V-on-Silicon Microring Lasers

ABSTRACTHybrid silicon laser is a promising solution to enable high-performance light source on large-scale, silicon-based photonic integrated circuits (PICs). As a compact laser cavity design, hybrid microring lasers are attractive for their intrinsic advantages of small footprint, low power consumption and flexibility in wavelength division multiplexing (WDM), etc. Here we review recent progress in unidirectional microring lasers and device thermal management. Unidirectional emission is achieved by integrating a passive reflector that feeds laser emission back into laser cavity to introduce extra unidirectional gain. Up to 4X of device heating reduction is simulated by adding a metal thermal shunt to the laser to “short” heat to the silicon substrate through buried oxide layer (BOX) in the silicon-on-insulator (SOI) substrate. Obvious device heating reduction is also observed in experiment.

Teardrop Reflector-Assisted Unidirectional Hybrid Silicon Microring Lasers

We study directional bistability in hybrid silicon microring lasers and demonstrate a unidirectional laser. Unidirectional emission is achieved by integrating a passive reflector that feeds laser emission back into the laser cavity to introduce an extra unidirectional gain. We show that the length of the passive reflector is a critical parameter in determining the lasing behavior.

Thermal Analysis of Multi-Pass Laser Irradiation on Fused Silica

A rough ground fused silica surface can be ground in a ductile mode by ultra-precision grinding after repairing the surface and subsurface micro cracks (SSMC) by CO2 laser irradiation. In this paper, 2D finite element thermal analysis of unidirectional multi-pass laser irradiation on fused silica was conducted, and the simulation results were compared with the thermal analysis and experiments results of single pass laser irradiation. Thermal analysis results show that the SSMC on the ground fused silica can be repaired and surface roughness can be decreased simultaneously by unidirectinal laser raster scan with a power of 10.5 W, a scan velocity of 0.2 m/s and a scan spacing of 40 μm.