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A narrow linewidth optical frequency comb (OFC) based on a directly modulated microcavity laser with external optical feedback is investigated numerically and demonstrated experimentally. Based on the numerical simulations with rate equations, the evolution of the optical and electrical spectra is presented for the direct-modulated microcavity laser with increased feedback strength, and the linewidth property is improved at suitable feedback conditions. The simulation results also show good robustness for the generated OFC in terms of feedback strength and phase. Moreover, the OFC generation experiment is performed by combining with the dual-loop feedback structure to suppress the side mode, and an OFC with a side-mode suppression ratio of 31 dB is realized. Thanks to the high electro-optical response of the microcavity laser, a 15-tone OFC with a frequency interval of 10 GHz is obtained. Finally, the linewidth of each comb tooth is measured to be around 7 kHz under the feedback power of 47 µW, which indicates an enormous compression of approximately 2000 times compared with the free-running continuous-wave microcavity laser.

We theoretically and numerically study optical modes in regular-polygonal microcavities with non-uniform gain and loss, where high quality (Q) whispering-gallery-like modes typically appear as superscar states. High Q superscar modes can be described by the propagating plane waves in an effective rectangle formed by unfolding the periodic orbits and exhibit regular and predictable spatial field distributions and transverse-mode spectra. With non-uniform gain and loss, anti-Hermitian coupling between the transverse modes with close frequencies occurs according to the mode coupling theory, which results in novel mode properties such as modified mode spectra and field patterns, and the appearance of exceptional points. Numerical simulation results are in good agreement with the theoretical analyses, and such analyses are also suitable for other kinds of high Q microcavities with non-uniform gain and loss. These results will be highly useful for studying non-Hermitian physics in optical microcavities and advancing the practical applications of microcavity devices.

A tri-mode micro-square laser under optical feedback is proposed and demonstrated to generate chaos with the broadband flat microwave spectrum. By adjusting lasing mode intensities, frequency intervals, and optical feedback strength, we can enhance the chaotic bandwidth significantly. The existence of two mode-beating peaks makes the flat bandwidth much larger than the relaxation oscillation frequency. Effective bandwidth of 35.3 GHz is experimentally achieved with the flatness of 8.3 dB from the chaotic output spectrum of the tri-mode mode laser under optical feedback.

We propose and demonstrate a circular-side octagonal microcavity (COM) semiconductor laser with a spatially distributed current injection for manipulating the lasing modes. There are two types of high-quality-factor whispering-gallery (WG) modes with distinct field patterns in a COM: the four-bounced quadrilateral modes and the eight-bounced octagonal modes. By designing two separated p-electrodes, the COM laser is divided into two regions that are pumped independently to select specific modes for lasing. The two types of WG modes lase simultaneously when the two regions are injected with equivalent currents. Degeneracy removal of the quadrilateral modes is observed in both simulation and experiment when the two regions are injected with inequivalent currents. The quadrilateral modes are suppressed when one of the two regions is un-injected or biased with a negative current, and single-octagonal-mode lasing is realized. The results show that the lasing modes can be efficiently manipulated with the spatially distributed current injection considering the distinct field patterns of different WG modes in the microcavities, which can promote the practical application of the microcavity lasers.

We numerically demonstrate the generation of wide-band laser chaos with flat power spectrum in a 2D circular-side hexagonal resonator (CSHR) microlaser subject to long-cavity optical feedback. The bandwidth and flatness of the chaotic power spectrum are investigated under different bias currents and optical feedback rates. Under low bias current, the bandwidth under an optimized optical feedback rate increases obviously as raising bias current and the power spectrum flatten simultaneously. Under high bias current, the optimized bandwidth gradually tends toward stabilization, with corresponding flatness less than 5 dB. We compare the chaotic power spectra with small-signal modulation response (SSR) curves under different bias currents. It can be concluded that wide-band and flat SSR indicates wide-band and flat chaotic power spectrum. This work argues that we can enhance laser chaos by using a laser device with wide-band and flat SSR and simple optical feedback configuration, which is significantly beneficial to synchronization-based applications including chaos communication and key distribution

Spectral engineering has been demonstrated for the circular-side square microlasers with an output waveguide butt-coupled to one vertex. By carefully optimizing deformation parameter and waveguide connection angle, undesired high-order transverse modes are suppressed while the mode Q factors and the transverse-mode intervals are enhanced simultaneously for the low-order transverse modes. Dual-mode lasing with pure lasing spectra is realized experimentally for the circular-side square microlasers with side lengths of 16 μm, and the transverse mode intervals can be adjusted from 0.54 to 5.4 nm by changing the deformation parameter. Due to the enhanced mode confinement, single-mode lasing with a side-mode suppression-ratio of 36 dB is achieved for a 10μm-side-length circular-side square microlaser with a 1.5μm-wide waveguide.

We propose and demonstrate an optoelectronic oscillator with a directly modulated AlGaInAs/InP integrated twin-square microlaser for generating wideband frequency-tunable microwave signals with low phase noise. Apart from the relaxation oscillation peak, the modulation response of the twin-square microlaser working at the mutual optical injection state exhibits a significant enhancement around the beating frequency of the lasing modes in the two square cavities owing to the photon-photon resonance. A self-sustaining oscillation can be generated around the modulation response peak with the lowest loop loss occurring at the relaxation oscillation frequency or the beating frequency, depending on the practical state of the twin-square microlaser. High-quality tunable microwave signals ranging from 2.22 to 19.52 GHz are generated with single sideband phase noises below -110 dBc/Hz at the 10 kHz offset frequency and side-mode suppression ratios of approximately 40 dB by tuning the injection currents of the twin-square microlaser.

Mode selection in square resonator semiconductor microlasers is demonstrated by adjusting the width of the output waveguide coupled to the midpoint of one side. The simulation and experimental results reveal that widely tunable single mode lasing can be realized in square resonator microlasers. Through adjusting the width of the output waveguide, the mode interval of the high-Q modes can reach four times of the longitudinal mode interval. Therefore, mode hopping can be efficiently avoided and the lasing wavelength can be tuned continuously by tuning the injection current. For a 17.8-μm-side-length square microlaser with a 1.4-μm-width output waveguide, mode-hopping-free single-mode operation is achieved with a continuous tuning range of 9.2 nm. As a result, the control of the lasing mode is realized for the square microlasers.

Narrow-linewidth and low phase noise photonic microwave generation under sideband-injection locking are demonstrated using an 8-μm-radius AlGaInAs/InP microdisk laser subject to optical injection and optoelectronic feedback. Microdisk laser subject to external optical injection at the period-one state provides the microwave subcarrier seed signal, and the optoelectronic feedback serves as direct current modulation to stabilize and lock the generated microwave signal without using the electrical filter. High-quality photonic microwave signals are realized with the 3-dB linewidth of less than 1 kHz and the frequency tunable range from 8.8 to 17 GHz. Single sideband phase noise of -101 dBc/Hz is obtained at a frequency offset of 10 kHz for the generated 14.7 GHz signal. Furthermore, the dependences of photonic microwave signal on the optical injection and optoelectronic feedback parameters are investigated.

Thermal and high speed modulation characteristics are investigated for a unidirectional-emission microdisk laser with a radius of 7 μm surrounded by BCB-cladding layer, with a threshold current of 1.5 mA at the temperature of 287 K. The lasing spectra under different widths of pulsed current are measured to characterize the temperature rise during the pulse period, and the thermal distribution in the microdisk laser is simulated by the finite-element modeling technique. A temperature rise of 25 K is estimated for the microdisk laser biased at 20 mA. Furthermore, small signal modulation response with 3dB bandwidth up to 20 GHz is obtained for the microdisk laser at the biasing current of 18 mA, and eye-diagrams at the modulation bit rates of 20, 25, and 30 GHz are also measured at the temperature of 287 K.

A novel self-Q-switched and orthogonally polarized dual-wavelength laser has been investigated with Yb³⁺-doped CGB disordered crystals. By slightly inclining output coupler to introduce the Fresnel loss, we realized simultaneously dual-wavelength laser operation at 1052.6 nm in E//b polarization and 1057.7 nm in E//c polarization with a frequency difference of 1.38 THz. Self-Q-switched pulse generation was observed in this free-running laser, originating from the nonlinear reabsorption effect of Yb:CGB as well as the strong storage of inversion population induced by the long excited-state lifetime (~1 ms). Pulse duration of 287 ns was obtained with an output average power of 416 mW and a repetition rate of 35 kHz. The self-Q-switching effect increased the peak power by 100 times the average power. This very simple laser, free from the complexity and high-cost of additional intracavity polarization modulator and Q-switcher, may be applied for constructing miniature, integrated and portable laser system.

The suitability of a terahertz plasmonic sensor for sensing applications is successfully demonstrated using a hybrid planar waveguide composed of a subwavelength plastic ribbon waveguide and a diffraction metal grating. The subwavelength-confined terahertz plasmons on the hybrid waveguide resonantly reflect from the periodic metal structure under phase-matched conditions and perform resonant transmission dips. The resonant plasmonic frequencies are found to be strongly dependent on the refractive indices and thicknesses of analytes laid on the hybrid planar waveguide. Both plastic films with varying thicknesses and granular analytes in different quantities are successfully identified according to the spectral shifts of resonant dips. An optimal refractive index sensitivity of 261 GHz per refractive index unit is achieved. Within localized and enhanced terahertz plasmonic fields, the minimum detectable optical path difference can be reduced to 2.7 μm corresponding to λ/289, and the minimum detectable amount of analytes in powdered form reaches 17.3 nano-mole/mm(2). The sensing technique can be used to detect particles in a chemical reaction or monitor pollutants.

Defected circular resonators laterally confined by a metal layer with a flat side as an emitting window are numerically investigated based on the boundary element method for realizing unidirectional emission microlasers. The results indicate that Fabry-Perot (FP) modes become high Q confined modes in the defected circular resonator with a metallic layer. The mode coupling between the FP mode and chaotic-like mode can result in high Q confined mode for unidirectional emission with a narrow far field pattern.

A novel tunable terahertz notch filter is demonstrated using antiresonant reflecting hollow waveguides with movable metal layers outside dielectric claddings. Based on the Fabry-Perot resonance of the dielectric cladding, multiple deep notches are observed in a broad THz transmission spectrum. Continuous shift of notch frequencies is for the first time experimentally observed by lateral translation of metal layers from dielectric claddings. The measured maximum frequency-tuning-range approached 60GHz, equaling to 50% of the bandwidth of every passband, and a 20dB rejection notch-depth with a linewidth as narrow as 6GHz at frequency of around 0.2THz was also achieved. Numerical simulations match the measurements and verify the spectral-tuning mechanism.

A simple dielectric hollow-tube has been experimentally demonstrated at terahertz range for bio-molecular layer sensing based on the anti-resonant reflecting wave-guidance mechanism. We experimentally study the dependence of thin-film detection sensitivity on the optical geometrical parameters of tubes, different thicknesses and tube wall refractive indices, and on different resonant frequencies. A polypropylene hollow-tube with optimized sensitivity of 0.003 mm/μm is used to sense a subwavelength-thick (λ/225) carboxypolymethylene molecular overlayer on the tube's inner surface, and the minimum detectable quantity of molecules could be down to 1.22 picomole/mm(2). A double-layered Fabry-Perot model is proposed for calculating the overlayer thicknesses, which agrees well with the experimental results.

We demonstrated the 1.33 microm laser performance with Nd:Gd(3)Al(x)Ga(5-x)O(12) (x = 0.94) (Nd:GAGG) laser crystals for the first time. Continuous-wave (cw) output power of 2.45 W was obtained with the optical-optical conversion efficiency of 21.8% and slope efficiency of 23.3%. In the passive Q-switching regime, the highest output power, the shortest pulse width, largest pulse energy and highest peak power were achieved to be 326 mW 18.2 ns, 36.3 microJ and 2.0 kW, respectively, with V(3+):YAG crystal as the saturable absorber.

The performance of a diode-end-pumped passively Q-switched dual-wavelength Nd:GGG laser operating at 932.9 and 936.5 nm with V(3+):YAG as the saturable absorber was demonstrated for the first time to the best of our knowledge. The maximum dual-wavelength average output power of 150 mW was achieved with a T = 2% output coupler under the absorbed pump power of 2.55 W, corresponding to the optical-to-optical conversion and slope efficiency of 5.9% and 8.0%, respectively. The minimum pulse width was 395 ns with the pulse repetition frequency of 140 kHz, which was attained with a T = 5% output coupler under the absorbed pump power of 2.55 W.

The performance of a diode-pumped passively Q-switched Nd:Gd(0.5)Y(0.5)VO(4) laser at 1.34 microm with V(3+):YAG as the saturable absorber was demonstrated for the first time to the best of our knowledge. The focal lengths of thermal lens in the diode-end-pumped Nd:Gd(0.5)Y(0.5)VO(4) laser for the 1.34 microm transition was experimentally investigated, with the corresponding proportion constant estimated to be approximately1.5x10(4) W/mm. For the passive Q-switching operation, the maximum average output power of 0.96 W was achieved under the pump power of 7.28 W, corresponding to optical-to-optical conversion and slope efficiency of 13.2% and 17.6%, respectively. The minimum pulse width attained was 47.8 ns with the pulse repetition frequency of 76 kHz, with the single pulse energy and peak power estimated to be 8.7 microJ and 182 W, respectively.

We report a diode-pumped passively Q-switched Nd:LuVO4 laserat 1.34 microm using V3+:YAG as saturable absorber. The characteristics of a-cut and c-cut Nd:LuVO4 passive Q-switching operation were studied. The average output power of 1.02 W was obtained under the pump power of 12.88 W, corresponding to the optical conversion efficiency of 8% and slope efficiency of 10% in c-cut Nd:LuVO4 laser. The maximum pulse energy of 17.6 microJ and the highest peak power of 820 W were obtained at pulse width of 21 ns and pulse repetition rate of 22.4 kHz in a-cut Nd:LuVO4 laser.

A high-aspect-ratio metallic rod array is demonstrated to generate and propagate highly confined terahertz (THz) surface plasmonic waves under end-fire excitation. The transverse modal power distribution and spectral properties of the bound THz plasmonic wave are characterized in two metallic rod arrays with different periods and in two configurations with and without attaching a subwavelength superstrate. The integrated metallic rod array-based waveguide can be used to sense the various thin films deposited on the polypropylene superstrate based on the phase-sensitive mechanism. The sensor exhibits different phase detection sensitivities depending on the modal power immersed in the air gaps between the metallic rods. Deep-subwavelength SiO(2) and ZnO nanofilms with an optical path difference of 252 nm, which is equivalent to λ/3968 at 0.300 THz, are used as analytes to test the integrated plasmonic waveguide. Analysis of the refractive index and thickness of molecular membranes indicates that the metallic rod array-based THz waveguide can integrate various biochip platforms for minute molecular detection, which is extremely less than the coherent length of THz waves.

A novel self-Q-switched and orthogonally polarized dual-wavelength laser has been investigated with Yb³⁺-doped CGB disordered crystals. By slightly inclining output coupler to introduce the Fresnel loss, we realized simultaneously dual-wavelength laser operation at 1052.6 nm in E//b polarization and 1057.7 nm in E//c polarization with a frequency difference of 1.38 THz. Self-Q-switched pulse generation was observed in this free-running laser, originating from the nonlinear reabsorption effect of Yb:CGB as well as the strong storage of inversion population induced by the long excited-state lifetime (~1 ms). Pulse duration of 287 ns was obtained with an output average power of 416 mW and a repetition rate of 35 kHz. The self-Q-switching effect increased the peak power by 100 times the average power. This very simple laser, free from the complexity and high-cost of additional intracavity polarization modulator and Q-switcher, may be applied for constructing miniature, integrated and portable laser system.