Hongjun Cao

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.

MOCVD growth of InNxAs1-x on GaAs using dimethylhydrazine

InNAs/GaAs multiple-quantum-well samples were grown by MOCVD on (100) n + -GaAs substrates at 500 °C and 60 Torr using uncracked dimethylhydrazine (DMHy). Quantum well layers were grown using trimethylindium, tertiarybutylarsine, and 95-97.5% of DMHy in the vapor phase, while GaAs buffer, barrier, and cap layers were grown using trimethylgallium and arsine. The crystalline quality and solid phase composition were evaluated using high-resolution X-ray diffraction analysis. The nitrogen content in InNAs wells was determined to be 18%. Surface morphology was investigated by atomic force microscopy (AFM) and field emission microscopy (FEM). Photoluminescence measurements confirm that the bandgap energy of InNAs is significantly lower than that of InAs. The peak emission wavelength of ∼6.5 μm at 10 K is the longest reported so far for dilute nitride semiconductors.

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.

Highly unidirectional Y-junction-coupled S-section quantum-dot ring lasers

Fabrication and characterization of Y-junction-coupled S-section InAs/InGaAs/GaAs quantum-dot ring lasers with high unidirectionality is reported. Stable unidirectional operation of the ring lasers is ensured by a new design that suppresses the unwanted counterpropagating modes more effectively than it was possible in the previous S-section-racetrack design.

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.

Monolithic passively mode-locked lasers using quantum-dot or quantum-well materials grown on GaAs substrates

In this work, the optical characteristics of monolithic passively mode-locked lasers (MLLs) fabricated from 1.24-&mgr;m InAs dots-in-a-Well (DWELL), 1.25-&mgr;m InGaAs single quantum well (SQW), and 1.55-&mgr;m GaInNAsSb SQW structures grown using elemental source molecular beam epitaxy (MBE) are reported. 5 GHz optical pulses with sub-picosecond RMS jitter, high pulse peak power (1W) and narrow pulse width (< 10 ps) were demonstrated in monolithic two-section InAs DWELL passive MLLs. With the 42% indium InGaAs SQW MLL, a record high-temperature performance for a monolithic passively mode-locked semiconductor laser is found. Compared with the typical operating range of the InAs DWELL devices (<60°C), the operation is in excess of 100 °C. The first 1.55-&mgr;m GaInNAsSb SQW MLL operates at a repetition rate of 5.8 GHz and has a 3-dB bandwidth of 170 kHz in the RF spectrum indicating respectable jitter.

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.

Y-Junction-Coupled S-Section InAs/InGaAs/GaAs Quantum-Dot Ring Lasers with High Unidirectionality

Fabrication and characterization of Y-junction-coupled S-section InAs/InGaAs/GaAs quantum dot ring lasers with high unidirectionality is reported. The new design suppresses the unwanted counterpropagating modes more effectively than in the previous S-section-racetrack design.