Abstract
In this paper, we propose an external optical feedback resistant distributed feedback (DFB) laser diode (LD) by exploiting parity-time symmetric complex coupling. With its complex refractive index followed a parity-time symmetry, the grating shows a strongly asymmetric reflection to the contra-propagating light inside the DFB cavity, which effectively rejects the returning light from one end. Consequently, the DFB LD is much less sensitive to external optical feedback. On the contrary, the transmissivity of such grating is still symmetric so that the output light of the DFB LD is not affected. Numerical simulation result shows that the lasing wavelength drift can be less than 0.2 nm with a SMSR exceeding 45 dB under a coherent external optical feedback as high as -10 dB.
Highlights
Distributed feedback (DFB) lasers are widely exploited in telecommunication and data communication systems and networks due to their superiorly pure spectrum,[1] which is extremely beneficial for maintaining the signal fidelity during its transmission over optical fibers
It is well known that external optical feedback affects the performance of the DFB laser diodes (LDs) on the lasing spectrum,[2] the side mode suppression ratio (SMSR),[3] the eye diagram,[4] and the bit-error-rate (BER).[5]
A common solution is to cascade an optical isolator after the DFB LD to prevent the returning of the output light back to the laser cavity due to the external reflection.[6]
Summary
Distributed feedback (DFB) lasers are widely exploited in telecommunication and data communication systems and networks due to their superiorly pure spectrum,[1] which is extremely beneficial for maintaining the signal fidelity during its transmission over optical fibers. Equation (1) clearly obeys the PT symmetric condition, yet it creates an asymmetric coupling between the contra-propagating waves, as the coupling coefficients for the forward-to-backward and backward-to-forward propagating waves will be in proportion to (∆nr - ∆ni)/2 and (∆nr + ∆ni)/2, respectively, if the phase shift between the imaginary and real part of the grating refractive index is π/2, and vice versa. The single-mode yield,[14] threshold gain, and lasing frequency[15] have been investigated theoretically for complex-coupled DFB LDs with arbitrary phase shift between the index and gain/loss grating, which has the PT symmetric complex-coupled grating included as a special case, no study can be found in the literature on its immunity to external optical feedback, as the aforementioned unique feature of the asymmetric coupling was not quite identified . The DFB laser structures based on PT symmetric gratings[17] was proposed for better single-mode operation performance
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