Compact Femtosecond Laser Direct Written Integrated Retarders Based on Embedded Nanogratings

Abstract

Femtosecond Laser Direct Writing (FLDW) is a well-known rapid prototyping method to fabricate integrated optical circuits in glass chips [1]. These circuits have been used to show various quantum information applications, using the states of photons as qubits [2]. Generally, when transmitting information via single photons, it is desirable to make use of all possible degrees of freedom that this photon has to offer, in order to increase the amount of information transferred per photon. One of these degrees of freedom is the photon's polarization. To make use of this degree of freedom, devices capable of manipulating the polarization are required. Various approaches for manipulating the polarization of photons in a FLDW circuit have been demonstrated before [3–6], of which some were used for quantum information applications [7–11]. In our work, we present a novel method of polarization control using embedded nanogratings as waveplates. These nanogratings are highly birefringent self-assembled structures. Due to their relatively high form birefringence on the order of Δη = 10−3, they can be used as a compact waveplate enabling further miniaturization of integrated optical circuits. The properties of these gratings can be altered by using different laser parameters during the inscription process. We have added various waveplates made of nanogratings into FLDW waveguides. We demonstrate their functionality as waveplates of different retardation and optical axis orientations using crossed polarizer birefringence measurements. Due to the chosen writing geometry, a full control over the direction of the optical axis can be achieved. The thickness of these structures is on the order of a few hundred micrometers. Former approaches for polarization control in FLDW circuits required structures in the range of millimetres to centimeters [9–11]. Some of these approaches were limited in the achievable optical axis orientations [9, 10]. Our waveplates can be both used for classical applications and as single qubit quantum gates, which will be demonstrated. Waveplate structures usable as Hadamard, Pauli-x, Pauli-z and Pi/8th gates have been fabricated. The transferability to fibers will be discussed.