C.B. Su

Characterization of the dynamics of semiconductor lasers using optical modulation

An optical modulation technique for measuring the intrinsic frequency response of semiconductor lasers is described. This technique, which uses an RF-modulated pump laser to create an optical modulation signal to inject into a DC-biased probed laser, offers significant advantages over previous methods such as being affected by electrical parasitics of either the laser to be characterized or the photodetector. The method allows extremely accurate measurements of many important dynamic parameters, including the nonlinear gain coefficients, the amount of spontaneous emission into the guided modes, and the differential carrier lifetime at lasing threshold. >

The relation of doping level to K factor and the effect on ultimate modulation performance of semiconductor lasers

For the first time, K, which is the ratio of the damping factor gamma to the square of the resonance frequency f/sub 0//sup 2/, is shown to depend on the doping level for bulk semiconductor lasers. Since the differential gain is known to depend on the doping level in the active layer, K also depends on the differential gain. The results presented strongly suggest that an effective means to decrease the damping is by increasing the doping level of the active region of the semiconductor laser. Since damping must be reduced in order to increase the maximum damping-limited bandwidth, this result may have important implications for improving the modulation bandwidths of bulk lasers and may be equally significant with respect to damping and the ultimate achievable bandwidth in quantum-well lasers.<<ETX>>

Determination of the gain nonlinearity time constant in 1.3 μm semiconductor lasers

By comparison of the measured K factors (ratio of the damping factor to the square of the resonance frequency) of distributed feedback and Fabry–Perot lasers, it is found that the relaxation time associated with nonlinear gain for 1.3 μm InGaAsP lasers is about 0.1 ps. This short time constant is consistent with spectral hole burning being the dominant process responsible for the nonlinear gain.

Increase in laser RIN due to asymmetric nonlinear gain, fiber dispersion, and modulation

It is shown that the low-frequency relative-intensity-noise (RIN) spectra of a Fabry-Perot laser are adequately described only when the effects of longitudinal mode coupling through the asymmetric nonlinear gain are accounted for. Additionally, for the first time, the authors have included this asymmetric mode coupling to accurately model the translation of the low-frequency noise of a semiconductor laser in the presence of modulation and fiber dispersion. The translation of noise, which determines the signal-to-noise performance in subcarrier multiplexed systems, is also confirmed experimentally.<<ETX>>

High-speed and low-relative-intensity noise 1.3 mu m InGaAsP semi-insulating buried crescent lasers

The dependence of static and dynamic performance on active layer doping concentration in 1.3- mu m InGaAsP semiinsulating buried crescent (SIBC) Fabry-Perot lasers were investigated experimentally. The optical loss in the active region is one of the dominant mechanisms in determining the threshold current for doped active layer lasers. These SIBC lasers have a 3 dB modulation bandwidth of 19 GHz for pulsed operation and 16 GHz for continuous-wave (CW) operation, and a relative intensity noise below -150 dB/Hz for biased current at 120 mA. The doped active lasers show an initial small degradation rate at 65 degrees C operation, which gives an acceptably long operation lifetime. >

The sublinear relationship between index change and carrier density in 1.5 and 1.3 mu m semiconductor lasers

The carrier-induced index change was measured using a novel injection-reflection technique in combination with differential carrier lifetime data. The observed relation between index change and injected carrier density at bandgap wavelength is nonlinear and is approximately given by delta n/sub act/=-6.1*10/sup -14/ (N)/sup 0.66/ for a 1.5- mu m laser and delta n/sub act/=-1.3*10/sup -14/ (N)/sup 0.68/ for a 1.3- mu m laser. The carrier-induced index change for a 1.3- mu m laser at 1.53- mu m wavelength is smaller and is given by delta n/sub act/=-9.2*10/sup -16/ (N)/sup 0.72/.<<ETX>>

Measurements of 50 fs nonlinear gain time constant in semiconductor lasers

The symmetric and asymmetric nonlinear gain in 1.3- mu m semiconductor lasers were measured in the frequency domain by a novel pump-probe technique using an external cavity traveling-wave semiconductor ring laser. A very short time constant of about 50 fs was measured as the dominant process. The data also indicated the presence of a smaller contribution approximately 15% of the magnitude of the dominant process and with a long time constant consistent with the effect of hot carriers.<<ETX>>

Frequency stabilization of a traveling-wave semiconductor ring laser using a fiber resonator as a frequency reference

A fiber resonator with a temperature drift rate of better than 5/spl times/10/sup /spl minus/4/spl deg// C/hour is used for controlling the frequency stability of a 1.3 /spl mu/m wavelength external cavity semiconductor ring laser. A frequency stability of about 600 KHz/hour is achieved.<<ETX>>

Optical homodyne frequency domain reflectometer using an external cavity semiconductor laser

An external cavity traveling-wave semiconductor ring laser with narrow linewidth is used as a light source for research in frequency domain reflectometry. The optical frequency of the laser is linearly chirped by an intra-cavity phase modulator. The time-delayed reflection signal is mixed with a reference signal to produce a microwave frequency that indicates the position of the reflection. For optical fiber measurement, a spatial resolution of 30 m and a one-way dynamic range of 28 dB for Rayleigh backscattered light have been achieved.<<ETX>>

Effective differential gain and modulation bandwidth reduction in quantum well lasers due to confinement factor modulation

A mechanism that may reduce the effective differential gain due to the modulation of the confinement factor with carrier density in quantum-well lasers is described. This mechanism may limit modulation bandwidth for quantum-well lasers with high threshold carrier density and narrow confining layer.<<ETX>>