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Register a new userDistributed delay-line interferometer based on a Bragg grating in transmission mode
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A novel approach for a delay line interferometer (DLI) based purely on forward Bragg scattering is proposed. We have numerically and experimentally demonstrated that a Bragg grating can deliver the functionality of a DLI in its transmission mode along a single common interfering optical path, instead of the conventional DLI implementation with two interfering optical paths. As a proof of concept, a fiber Bragg grating has been designed and fabricated, showing the desired functionality in the transmission mode of the Bragg grating. The proposed Bragg-DLI approach is applicable to any kind of Bragg grating technology, such as volume Bragg gratings, dielectric mirrors, silicon photonics, and other optical waveguide based Bragg structures.
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An analytic solution for Bragg grating with linear chirp in the form of confluent hypergeometric functions is analyzed in the asymptotic limit of long grating. Simple formulas for reflection coefficient and group delay are derived. The simplification makes it possible to analyze irregularities of the curves and suggest the ways of their suppression. It is shown that the increase in chirp at fixed other parameters decreases the oscillations in the group delay, but gains the oscillations in the reflection spectrum. The conclusions are in agreement with numerical calculations.
The combination of semiconductor quantum dots (QDs) with photonic cavities is a promising way to realize non-classical light sources with state-of-the-art performances in terms of brightness, indistinguishability and repetition rate. In the present work we demonstrate the coupling of an InGaAs/GaAs QDs emitting in the telecom O-band to a circular Bragg grating cavity. We demonstrate a broadband geometric extraction efficiency enhancement by investigating two emission lines under above-band excitation, inside and detuned from the cavity mode, respectively. In the first case, a Purcell enhancement of 4 is attained. For the latter case, an end-to-end brightness of 1.4% with a brightness at the first lens of 23% is achieved. Using p-shell pumping, a combination of high count rate with pure single-photon emission (g(2)(0) = 0.01 in saturation) is achieved. Finally a good single-photon purity (g(2)(0) = 0.13) together with a high detector count rate of 191kcps is demonstrated for a temperature of up to 77K.
Femtosecond laser writing is applied to form Bragg grating waveguides in the diamond bulk. Type II waveguides are integrated with a single pulse point-by-point periodic laser modification positioned towards the edge of the waveguide core. These photonic devices, operating in the telecommunications band, allow for simultaneous optical waveguiding and narrowband reflection from a 4th order grating. This fabrication technology opens the way towards advanced 3D photonic networks in diamond for a range of applications.
Optical microresonators are of paramount importance in photonic circuits requiring fine spectral filtering or resonant light recirculation. Key performance metrics improve with increasing resonance quality factor (Q) across all applications. The performance of silicon photonic circuits is often hampered by the low-quality factor of planar silicon microresonators, typically of Q~10^4-10^5. On the other hand, bulk whispering gallery mode resonators provide a wide range of materials with intriguing optical properties and exceptionally high resonant quality factors Q>10^7. However, the efficient coupling between bulk resonators and planar Si photonic waveguides is considered challenging, if not impossible, due to remarkably large mismatch in size and refractive index. Here, we show an efficient method to couple bulk resonators and Si waveguides based on subwavelength metamaterial engineering of silicon. Based on this approach, we experimentally demonstrate coupling between 220-nm-thick Si waveguides and bulk microresonators made of silica, lithium niobate and calcium fluoride with diameters in the 0.3-3.5 mm range, achieving high coupling efficiency of 75-99% and exceptional Q of 10^6-10^7. These results open a new route for the heterogeneous integration of bulk resonators and silicon photonic circuits, with great potential for applications in sensing, microwave-photonics, and quantum photonics, to name a few.
The unique spectral behavior exhibited by a class of non-uniform Bragg periodic structures, namely chirped and apodized fiber Bragg gratings (FBGs) influenced by parity and time reversal ($mathcal{PT}$) symmetry, is presented. The interplay between the $mathcal{PT}$-symmetry and nonuniformities brings exceptional functionalities in the broken $mathcal{PT}$-symmetric phase such as wavelength selective amplification and single-mode lasing for a wide range of variations in gain-loss. We observe that the device is no more passive and it undergoes a series of transitions from asymmetric reflection to unidirectional invisibility and multi-mode amplification as a consequence of variation in the imaginary part of the strength of modulation in different apodization profiles, namely Gaussian and raised cosine, at the given value of chirping. The chirping affords bandwidth control as well as control over the magnitude of the reflected (transmitted) light. Likewise, apodization offers additional functionality in the form of suppression of uncontrolled lasing behavior in the broken $mathcal{PT}$-symmetric regime besides moderating the reflected signals outside the band edges of the spectra.
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