Chiyu Liu

Large S-section-ring-cavity diode lasers: directional switching, electrical diagnostics, and mode beating spectra

Semiconductor lasers with monolithically integrated ring cavities exceeding 1 cm in perimeter were fabricated and characterized optically and electrically. Directional switching was observed, influenced by S-section seeding of unidirectional operation. The lasing threshold was identified by differential current-voltage measurements, which are also shown to be useful in monitoring directional switching. Mode beating was observed at three radio-frequency bands: 7.6, 15.2, and 22.9 GHz.

Frequency beating between monolithically integrated semiconductor ring lasers

Optoelectronic integrated circuits incorporating a pair of optically independent large-cavity semiconductor ring lasers (SRLs), directional couplers, waveguides, Y-junction mixer, and photodetectors are demonstrated. Counterclockwise and clockwise output beams from the two SRLs are collected separately and mixed prior to detection. Frequency beating between modes of two SRLs is measured. The beat frequency is fine-tuned by an integrated Joule heater, designed for thermal control of the lasing wavelength. No signs of frequency lock-in in the vicinity of zero detuning are observed, which makes this structure a promising candidate for applications in ring laser gyros and optical rotation sensors.

Highly unidirectional InAs∕InGaAs∕GaAs quantum-dot ring lasers

We report fabrication and characterization of semiconductor ring lasers with quantum-dot active region. The InAs∕InGaAs∕GaAs “dots-in-a-well” ridge-waveguide ring lasers are monolithically integrated with coupling waveguides and monitoring quantum-dot photodetectors. The lowest threshold current density for semiconductor ring lasers is demonstrated. When enhanced by a forward biased S-section waveguide, stable unidirectional operation with record suppression ratio of counterpropagating waves exceeding 30dB is achieved.

Unidirectional operation of quantum-dot ring lasers

First monolithically integrated ridge-waveguide InAs/InGaAs quantum-dot ring lasers are fabricated and characterized. Enhanced by a forward biased S-section waveguide, stable traveling-wave (unidirectional) operation with record suppression ratio of counterpropagating waves approaching 30 dB is demonstrated.

Nonlinear mode interaction as a mechanism to obtain slow/fast light in diode lasers

The nonlinear mode interactions (NMIs) in semiconductor lasers produce perturbations of optical parameters in the vicinity of strong-mode frequencies. These perturbations include changes in the complex refractive index, which could either increase or decrease, depending on the frequency detuning. Variation of the refractive index over a small frequency range may produce a rather strong variation of the group index. There are regimes of slow light (large group index), fast light (group index smaller than the phase index), and a range of negative group index. Critically anomalous dispersion conditions occur when the group index changes its sign. Thus, the NMIs in diode lasers and in semiconductor optical amplifiers can be used to obtain slow or fast light. A specific application is considered for gyroscopic laser devices that are sensitive to dispersion. It is shown that the fast light phenomenon can be used to increase the scale factor in a gyro without increasing its size.

Longitudinal mode beating spectra in long-cavity quantumdot semiconductor lasers

The optical and microwave spectra of 5-mm-long quantum-dot diode lasers are investigated. Comparison of observed beating lines with quantum-well lasers shows differences in the group index, linewidth, and line shape.

Nonlinear mode interaction and slow/fast light generation in semiconductor lasers

Nonlinear interactions between optical modes in semiconductor lasers and optical amplifiers perturb optical parameters in the vicinity of a strong mode frequency. This effect is considered in terms of slow and fast light propagation.

Mode beating spectra from integrated twin ring diode lasers

We fabricated 1-cm-long semiconductor ring lasers monolithically integrated with passive waveguides and photodiodes. Longitudinal mode beating and fine tuning is investigated in a single ring and in a system of two traveling-wave ring lasers with opposite lasing directions.

Radio-frequency spectra of long-cavity diode lasers under CW pumping

High-frequency spectra of free-running 5-mm-long triple-quantum-well graded-index separate-confinement heterostructure broad-area diode lasers emitting at ~1 μm are investigated in the range of 1-20 GHz using RF spectrum analyzer. The spectra reveal stable beat lines at ~8 and ~16 GHz, corresponding to single and double mode spacings between adjacent longitudinal modes. A current-dependent peak, varying from 0.6 to 2 GHz, is associated with the relaxation resonance. Measurements of mode beating spectra provide additional characterization of diode laser emission for coherent light applications.

Mode beating resonances in microwave spectra of long-cavity InGaAs/GaAs/AlGaAs GRIN-SCH lasers

High-frequency characterization of diode lasers is an important topic in studies of high-speed modulation, mode locking, Q-switching, mode selection and tuning, etc. Beating between longitudinal modes in typical diode lasers occurs at frequencies too high to be measured directly. Consequently, mode beating was initially investigated in external-cavity diode lasers, with mode spacings of ~100 MHz.1 More recently, beating of adjacent longitudinal modes was observed in lasers with 2-2.5-mm long cavities.2,3 Here, we study mode beating spectra in lasers with cavity length of ~5 mm. For the first time, we have observed beating between second-neighbor modes separated by double spacing.