Direct frequency modulation characteristics in three different AlGaAs lasers-a channeled-substrate planar (CSP) laser, a buried-heterostructure (BH) laser, and a transverse-junction-stripe (TJS) laserare studied theoretically and experimentally. Experimental FM responses are measured by using the Fabry-Perot interferometer and birefringent optical filters in the 0-5.2 GHz modulation frequency region. Experimental FM response dependences on modulation frequency, dc bias level, and stripe structure are successfully explained by the theoretical analyses considering both the carrier density modulation effect and the temperature change effect. FM response in the low modulation frequency regin from 0 to 10 MHz, gradually decreasing with the modulation frequency, stems from the thermal effect. FM response in the high modulation frequency region from 10 MHz to 5.2 GHz is caused by the carrier effect. A flat FM response of several hundred MHz per 1 mA is observed in the CSP and TJS lasers, but a V-shaped FM response is obtained in the BH laser. Resonance peak due to relaxation oscillation and cutoff characteristics are observed in several gigahertz regions. Weak lateral mode confinement, strong vertical mode confinement, carrier injection outside the effective core region, and p-side down mounts are effective ways to achieve a flat and efficient FM response with a small spurious intensity modulation.

Direct frequency modulation in AlGaAs semiconductor lasers

Direct frequency modulation characteristics in three different AlGaAs lasers - a channeled-substrate planar (CSP) laser, a buried-heterostructure (BH) laser, and a transverse-junction-stripe (TJS) laser- are studied theoretically and experimentally. Experimental FM responses are measured by using the Fabry-Perot interferometer and birefringent optical filters in the 0-5.2 GHz modulation frequency region. Experimental FM response dependence on modulation frequency, dc bias level, and stripe structure are successfully explained by the theoretical analyses considering both the carrier density modulation effect and the temperature change effect. FM response in the low modulation frequency region from 0 to 10 MHz, gradually decreasing with the modulation frequency, stems from the thermal effect. FM response in the high modulation frequency region from 10 MHz to 5.2 GHz is caused by the carrier effect. A flat FM response of several hundred MHz per 1 mA is observed in the CSP and TJS lasers, but a V-shaped FM response is obtained in the BH laser. Resonance peak due to relaxation oscillation and cutoff characteristics are observed in several gigahertz regions. Weak lateral mode confinement, strong vertical mode confinement, carrier injection outside the effective core region, and p-side down mounts are effective ways to achieve a flat and efficient FM response with a small spurious intensity modulation.

Direct Frequency Modulation In AlGaAs Semiconductor Lasers

The theoretical bases and the experimental results of the dynamic behavior of 1.5-1.6 \mu m GaInAsP/InP dynamic-single-mode (DSM) lasers with distributed Bragg reflector (DBR) are given. A condition for the single-mode operation of a rapidly modulated DBR laser, called a "dynamic-single-mode laser," and the dynamic spectral width were theoretically presented. Experimentally, buried-heterostructure distributed-Bragg-reflector integrated-twin-guide (BH-DBR-ITG) and buried-heterostructure butt-jointed-built-in distributed-Bragg-reflector (BH-BJB-DBR) lasers emitting at the wavelength of 1.5-1.6 \mu m were directly modulated up to 3 GHz, and the stable single-mode operations were obtained in both types of lasers. The dynamic spectral width at the worst modulation condition was measured to be 0.27 nm, which was about \frac{1}{35} times smaller than that of conventional lasers.

1.5-1.6 &#181;m GaInAsP/InP dynamic-single-mode (DSM) lasers with distributed Bragg reflector

Carrier density dependence of the refractive index in the active layers of semiconductor lasers is evaluated from the wavelength shift with increases in current by taking into account effects of the active layer temperature rise and lateral carrier and optical field distributions on the wavelength shift. The derived refractive index change due to carrier density increase is -4 \times 10^{-27} m3, which is in good agreement with the theoretical value.

Carrier density dependence of refractive index in AlGaAs semiconductor lasers

Laser diode response to deep sinusoidal injection current modulation is investigated both theoretically and experimentally. By the numerical analysis of multilongitudinal rate equations with a modulation term, regular pulsation regions and the spectral broadening and wavelength shift of the optical pulse are given. These theoretical results are well supported by experimental observations with an AlGaAs laser diode, where a 40-70 ps optical pulse at a repetition rate of 200 MHz to 2.17 GHz was obtained. The possibility of generating a coherent optical pulse by harmonic frequency injection current modulation of laser diodes without external cavities is pointed out theoretically. Further, coherent pulsations by external light injection in sinusoidally modulated laser diodes are investigated both theoretically and experimentally.

Response of semiconductor laser to deep sinusoidal injection current modulation

The dependence of relaxation oscillation on carrier diffusion length in AlGaAs lasers is analyzed using multimode rate equations containing carrier diffusion and spontaneous emission terms. A parameter representing coupling between carrier and light field distribution is introduced. The carrier diffusion shortens the delay time, the relaxation oscillation period, and the decay time constant and also makes spectral narrowing faster. The damping factor is enlarged and relaxation oscillation is suppressed as the carrier diffusion length becomes large. The diffusion length, normalized by stripe width L D /W, is estimated to be about 0.6 by comparing the numerical results with the experimental tendency of transient response. Furthermore, relaxation oscillation is suppressed as the oscillation beam spot size becomes smaller.

Takeshi Ito

Papers

Dynamic properties of semiconductor lasers