In this Applied Optics article, the authors propose the use of slotted Fabry-Perot (SFP) semiconductor lasers with injection locking for amplifying and filtering a selected subcarrier from a coherent optical comb, since the SFP lasers can be easily integrated in PICs because of low energy consumption and a single epitaxial growth step required. To create a coherent optical comb, the authors use a tunable laser source (TLS) and a LiNbO3 Mach-Zehnder modulator, biased to operate in quadrature with a modulation frequency of 10 GHz (1010 Hz). This modulated signal is automatically coupled to a SFP laser through optical feedback, provided by using an optical circulator with a piezo controller. The output signals from the SFP laser are directly analyzed using an optical spectrum analyzer.
First, the authors show plots of the spectrum on the optical spectrum analyzer versus TLS wavelength, both with single carrier and coherent comb injection, to describe the performance of the SFP laser as a selective amplifier and filter. In both cases, a side mode suppression ratio (SMSR) of greater than 20 dB is achieved. Next, to demonstrate the use of SFP lasers as tunable filters, the temperature and injection current on the SFP laser are controlled to tune its resonant condition. On both single carrier and coherent injection, controls of temperature and injection current on the SFP laser each successfully filter and amplify a selected subcarrier also with an SMSR of higher than 20 dB. Therefore, as the authors claim, a wavelength-selective and tunable optical amplifier with the use of injection-locked SFP lasers can possibly be very useful for CoWDM in PICs, since each subcarrier can be filtered and amplified with great suppression of unwanted wavelengths while retaining coherence.
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