Geometrically-controlled polarisation processing in femtosecond-laser-written photonic circuits

Abstract

Polarisation of light is a powerful and widely used degree of freedom to encode information, both in classical and quantum applications. In particular, quantum information technologies based on photons are being revolutionised by the use of integrated photonic circuits. It is therefore very important to be able to manipulate the polarisation of photons in such circuits. We experimentally demonstrate the fabrication by femtosecond laser micromachining of components such as polarisation insensitive and polarising directional couplers, operating at 1550 nm wavelength, where the two opposite behaviours are achieved just by controlling the geometric layout of the photonic circuits, being the waveguides fabricated with the same irradiation recipe. We expect to employ this approach in complex integrated photonic devices, capable of a full control of the photons polarisation for quantum cryptography, quantum computation and quantum teleportation experiments.Integrated optics is a very powerful platform to produce miniaturised complex photonic devices with improved scalability, robustness and suitability for field application 1 . This approach greatly benefited classical optics communications, which is at the basis of today's information society. A similar trend is being adopted also for quantum optical devices, fostering an exponential increase in the layout complexity of integrated photonic circuits for various quantum applications, from computation to simulation [2][3][4][5][6] . Integrated quantum photonics is currently based on consolidated technologies, such as silicon on insulator 3 and silica on silicon 2 , as well as on more innovative approaches as waveguide writing by femtosecond laser micromachining (FLM) [4][5][6] . The two main advantages that FLM introduced in integrated quantum photonics are: the possibility to manipulate polarisation-encoded photons on-chip 5 and unique 3D fabrication capabilities 7 . The former enables the direct on-chip transfer of many protocols already developed in quantum optics and based on polarisation encoding. The latter opens the way to innovative 3D layouts that can produce novel functionalities in more compact geometries.The polarisation sensitivity of the FLM fabricated devices comes from the low (~10 −4 -10 −5 ) optical birefringence that is typically present in this type of waveguides. Depending on the material, birefringence in FLM can originate from one or more of the following sources. It can be due to the creation of periodic nano-structures aligned orthogonally to the writing beam polarisation 8 , it can arise due to asymmetric material stresses induced in the focal volume 9 , or it may be due to ellipticity of the written waveguide cross section 10,11 . While high birefringence is desirable to achieve effective polarisation manipulation in compact devices, on the other hand this same property induces unwanted strong delays between the two main polarisation components also during plain propagation in the chip, thus rapidly destroying the original polarisation state. T...

Symmetric polarization-insensitive directional couplers fabricated by femtosecond laser writing

“…The first method is based, and further elaborates, on the PIC concept that was demonstrated in Ref. 11 . There, the polarization insensitivity in the splitting ratio was produced by writing the two waveguides composing the couplers very close to each other.…”

“…The inscription of the second waveguides alters the optical properties of the first one ( Fig. 2a), affecting on the one hand the coupling coefficients k H,V for the two polarizations, and on the other hand the birefringence of the first waveguide 11,19 . As a consequence, for a certain optimal waveguide separation, the beating frequencies for the two polarizations are experimentally observed to become equal and the power splitting ratio of the coupler can be made polarizationinsensitive.…”

“…On the one hand, the low birefringence of femtosecond laser written waveguides enables the propagation of polarization encoded qubits without coherence degradation 7 . On the other hand, the versatility of this technology has allowed to exploit such minimal birefringence for the fabrication of devices for polarization manipulation, such as polarization splitters [8][9][10][11] and integrated retarders and waveplates [12][13][14] .…”

“…However, an accurate characterization showed that power splitting is rigorously equal only for horizontally and vertically polarized input states, and small unwanted polarization rotations may anyway occur to light propagating in those devices 16 . More recently, Pitsios et al reported PICs operating at telecom wavelengths, fabricated by FLM in borosilicate glass with a fully planar layout 11 , which provide equal power splitting for every input polarization state. Actually, as we will study in more detail in the following, despite the equal power splitting, all couplers made of birefringent waveguides may produce polarization rotations that depend on the specific input-output path taken by the light into the coupler.…”

Integrated-optics heralded controlled-NOT gate for polarization-encoded qubits

“…The two PBSs used for our CNOT gate are implemented using a scheme that exploits the waveguides' birefringence. Evanescent coupling between two guided modes can occur for separation distances ranging from 20 − 30 µm, and different types of directional couplers can be constructed depending on the length of the coupling region and the relative position of the modes with respect to their ellipse axes [43][44][45]. Since the strength of the evanescent coupling is different for H and V , it is possible to tune the length of the coupling region in such a way that H returns entirely to the input mode while V transfers entirely to the other mode.…”