When dual-passband of the WaveShaper is programmed to coincide with two neighboring longitudinal modes defined by the two FP-LDs, single-frequency operation at these two wavelengths can be obtained, according to the principle mentioned above. Wavelength spacing of the two neighboring longitudinal modes defined by the two FP-LDs can be changed by adjustment of the drive currents of the two FP-LDs, and the passband spacing of the WaveShaper can be programmed to vary with a certain step size. Consequently, dual-wavelength single-frequency output with switchable wave-length spacing can be achieved. Furthermore, when the passband spacing of the WaveShaper is fixed, wavelength tunable dual-wavelength output can be obtained by injection locking different
longitudinal modes of the FP-LDs through adjustment of the center wavelength of the WaveShaper.