Although, in principle, single mode fibers can transport information at bit rates larger than 10Tb/s (10¹³ bits per second), the low speed of electronic components and the dispersive and nonlinear effects inside optical fiber significantly limit the bit rate in reality. One of ways to overcome the limit of the bit rate is to simultaneously transmit multiple optical channels at different wavelengths. This method is called wavelength-division multiplexing (WDM) that has been used in the past few decades. Recently, with the rapid growth of the demand for bandwidth intensive services, more advanced WDM techniques have been required to achieve a higher information spectral density. This leads to the advent of coherent WDM (CoWDM) in 2005, highly increasing the information spectral density with the use of a coherent comb and the demultiplexing and multiplexing of subcarriers without losing coherence. A coherent comb can be generated using mode-locked lasers or cascaded Mach-Zehnder modulators; however, in photonic integrated circuits (PICs), as the authors note, there can be some difficulties on integrating demultiplexers incorporating a wave selective mechanism.

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.

---

---

 Add Comment

You must log in to add comments.