Nonreciprocal waveguide Bragg gratings

Abstract

The use of a complex short-period (Bragg) grating which combines matched periodic modulations of refractive index and loss/gain allows asymmetrical mode coupling within a contra-directional waveguide coupler. Such a complex Bragg grating exhibits a different behavior (e.g. in terms of the reflection and transmission spectra) when probed from opposite ends. More specifically, the grating has a single reflection peak when used from one end, but it is transparent (zero reflection) when used from the opposite end. In this paper, we conduct a systematic analytical and numerical analysis of this new class of Bragg gratings. The spectral performance of these, so-called nonreciprocal gratings, is first investigated in detail and the influence of device parameters on the transmission spectra of these devices is also analyzed. Our studies reveal that in addition to the nonreciprocal behavior, a nonreciprocal Bragg grating exhibits a strong amplification at the resonance wavelength (even with zero net-gain level in the waveguide) while simultaneously providing higher wavelength selectivity than the equivalent index Bragg grating. However, it is also shown that in order to achieve non-reciprocity in the device, a very careful adjustment of the parameters corresponding to the index and gain/loss gratings is required.

Optical mesh lattices withPTsymmetry

“…As recently predicted [35,36] PT -symmetric periodic structures like gratings can exhibit surprising behavior like unidirectional invisibility and intriguing reflection characteristics. More specifically, light propagating in such a system can experience reduced or enhanced reflections depending on the direction of propagation.…”

“…In other words the index distribution must be an even function of position whereas the gain/loss must be anti-symmetric. Thus far, several works have pointed out that PTsymmetry can lead to altogether new optical dynamics which are otherwise impossible in standard passive optical arrangements [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. These may include for example the occurrence of abrupt phase transitions along with the appearance of the so-called exceptional points [24][25][26], power oscillations [18], breaking left-right symmetry and the occurrence of secondary emissions [18].…”

“…These may include for example the occurrence of abrupt phase transitions along with the appearance of the so-called exceptional points [24][25][26], power oscillations [18], breaking left-right symmetry and the occurrence of secondary emissions [18]. In addition new classes of optical solitons [19,38] and nonlinear PT optical isolators [29], have been suggested along with unidirectional invisibility [35,36], broad area PT single mode lasers [30], and coherent perfect absorbers [32][33][34]. Note that phase transitions similar to those occurring due to PT -symmetry breaking have also been reported in other systems involving gain/loss modulation [42].…”

Nonlinear switching and solitons in PT‐symmetric photonic systems

“…In particular, many scientific works address the questions of existence and stability of nonlinear states in PT-symmetric oligomers, which may drastically differ from eigenmodes of underlying linear systems. The nonlinear effects in PT-symmetric systems can be utilized for an efficient control of light including all-optical low-threshold switching and unidirectional invisibility [24,56,57]. The possibility to engineer PT-symmetric oligomers, which may include nonlinearity, triggers a broad variety of studies on both the few-site systems and entire PT-symmetric lattices, including onedimensional PT-symmetric dimer [35,58], trimer [58,59], quadrimer [58,60], 2D PT-symmetric plaquettes [60,61], PT-symmetric finite/infinite chains [62][63][64][65], necklaces [66] and multicore fibers [67].…”

Wide-range-tunable Dirac-cone band structure in a chiral-time-symmetric non-Hermitian system

“…Extra imaginary potentials induce many unusual features even in certain simple or trivial systems, which include quantum phase transition occurred in a finite system [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], unidirectional propagation and anomalous transport [4,[21][22][23][24][25][26][27][28], invisible defects [29][30][31], coherent absorption [32] and self sustained emission [33][34][35][36][37], lossinduced revival of lasing [38], as well as laser-mode selection [39,40]. Most of these phenomena are related to the critical behaviours near exceptional or spectral singularity points.…”