Harmonic characteristics of semiconductor lasers and generation of ultrashort optical pulse train by gain-switching and fiber-optic pulse compression scheme are studied, including the following major areas:
i) Laser parameter measurement by harmonic characteristics
ii) Gain-switched picosecond pulse generation
iii) Fiber-optic compression and separation of gain-switched pulses.
Novel measurement methods of semiconductor laser parameters such as spontaneous emission carrier lifetime, photon lifetime, spontaneous emission factor, and carrier diffusion parameter are proposed, which utilize the harmonic distortion characteristics. Spontaneous emission carrier lifetime is measured by the rate of amplitude change of 3rd harmonies at threshold to the modulation frequency below a few MHz. Photon lifetime is measured by the 3rd harmonic characteristics with modulation frequency of several hundreds of MHz. Spontaneous emission factor is measured by 5th harmonic characteristics with bias current below threshold. Carrier diffusion parameter is measured by the 2nd harmonic characteristics with bias current above thresholds. It is also found that these measurement methods using harmonic characteristics are very precise ones, because they measure the relative amplitude of harmonic distortions.
Picosecond optical pulse(bout 20 ps) train with single longitudinal mode and 1 Gbit/s repetition rate is obtained using a gain switching for an 1.3 μm GaInAsP/InP distributed feedback laser. A systematic investigation of pulse width and pulse waveform of gain-switched pulse including bias current dependence and current pulse amplitude dependence is carried out. It is found that the optimum condition for minimizing pulse width can be obtained at the DC bias level around the half of the threshold, which is confirmed in a numerical analysis based on the rate equations considering the gain saturation effect due to both spatial and spectral hole burning. Numerical analysis also indicate that the gain saturation effect is the major limiting factor of pulse shortening by gain-witching scheme. From the curve fitting of experimental data and the theoretical calculation for bias current dependence of pulse width, gain compression parameter $\varepsilon$ in the form of g(S)=$g_0$/(1+$\epsilon$S) is estimated about $1~2\times 10^{-17}cm^3$, which corresponds to the intraband relaxation time of 0.05~0.1 ps. A comparison of picosecond pulse measurement methods, by SHG auto-correlation and by streak camera, is discussed also.
Separation and compression of pulses from a gain-switched Fairy-Pert semiconductor laser is achieved by using a dispersion-shifted optical fiber. Expermentally, 1.26μm GaInAsP/InP laser diode with 2nm mode separation in wavelength is gain-switched to generate around 26 ps pulse, which is launched into a dispersion-shifted(1.55μm) fiber with chromatic dispersion of -26ps/nm-Km at 1.26μm wavelength to produce pulse train separtated by 120 ps interval in time after propagation of 2Km. Evolution of pulse waveform is analyzed theoretically by an improved expression of semiconductor laser chirp and rate equations with consideration of gain saturation due to both spatial and spectral hole burning which is suitable for large signal modulation.
A discussion is made on the possible application of this technique to a novel hybrid time/wavelength division multiplexing scheme for very high capacity signal transmission through optical fiber.
