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Spectral encoding and decoding of ultrashort pulses using 63-bit Gold sequence-coded fiber Bragg gratings for high-dimensional quantum communication applications

A. F. G. F. Filho, J. L. S. Lima, K. Z. Nobrega, W. F. da Silva, M.M. Freire, A.S.B. Sombra

Published May 21, 2026• 0 citations
Publisher

Abstract

Abstract In this work, we investigate the spectral encoding and decoding of ultrashort optical pulses using fiber Bragg gratings encoded with a 63-bit Gold sequence. The encoding is implemented through discrete ± π phase shifts in the periodic modulation of the refractive index along the grating. Using numerical simulations based on coupled-mode theory, we demonstrate that an input pulse with a 2 ps duration can be spectrally encoded by the grating, producing a distributed temporal structure after a Fourier transform. The original pulse can be recovered by propagating the encoded signal through a second grating with an inverse sequence, characterizing a correlated filtering process. The results show that encoded Bragg gratings can act as compact optical devices for pulse encoding and decoding in the spectral domain. Beyond the classical description, we discuss the quantum interpretation of the process. In the single-photon regime, the spectral transformation implemented by the encoded grating maps a single optical temporal mode into a superposition of multiple time-bin modes, enabling the generation and manipulation of high-dimensional temporal quantum states. These results indicate that encoded Bragg gratings constitute a promising platform for temporal mode engineering and may find applications in high-dimensional quantum communication systems [9, 10].

DOI
10.21203/rs.3.rs-9383871/v1

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Abstract In this work, we investigate the spectral encoding and decoding of ultrashort optical pulses using fiber Bragg gratings encoded with a 63-bit Gold sequence. The encoding is implemented through discrete ± π phase shifts in the periodic modulation of the refractive index along the grating. Using numerical simulations based on coupled-mode theory, we demonstrate that an input pulse with a 2 ps duration can be spectrally encoded by the grating, producing a distributed temporal structure after a Fourier transform. The original pulse can be recovered by propagating the encoded signal through a second grating with an inverse sequence, characterizing a correlated filtering process. The results show that encoded Bragg gratings can act as compact optical devices for pulse encoding and decoding in the spectral domain. Beyond the classical description, we discuss the quantum interpretation of the process. In the single-photon regime, the spectral transformation implemented by the encoded grating maps a single optical temporal mode into a superposition of multiple time-bin modes, enabling the generation and manipulation of high-dimensional temporal quantum states. These results indicate that encoded Bragg gratings constitute a promising platform for temporal mode engineering and may find applications in high-dimensional quantum communication systems [9, 10].
Spectral-temporal manipulation of optical pulses has enabled numerous developments within a broad range of research topics, ranging from fundamental science to practical applications. Within quantum optics spectral-temporal degree of freedom of light offers a promising platform for integrated photonic quantum information processing. An important challenge in experimentally realizing spectral-temporal manipulation of quantum states of light is the need for highly efficient manipulation tools. In this context the intrinsically deterministic electro-optic methods show great promise for quantum applications. We experimentally demonstrate application of electro-optic platform for spectral-temporal manipulation of ultrashort pulsed quantum light. Using techniques analogous to serrodyne frequency shifting we show active spectral translation of few-picosecond single photon pulses by up to 0.5 THz. By employing an approach based on an electro-optic time lens we demonstrate up to 6-fold spectral compression of heralded single photon pulses with efficiency that enables us to significantly increase single photon flux through a narrow bandpass filter. We realize the required temporal phase manipulation by driving a lithium niobate waveguided electrooptic modulator with 33 dBm sinusoidal RF field at the frequency of either 10 GHz or 40 GHz. We use a phase lock loop to temporally lock the RF field to the 80 MHz repetition rate of approximately 1 ps long optical pulses. Heralded single photon wavepackets are generated by means of spontaneous parametric down-conversion in potassium dihydrogen phosphate (KDP) crystal, which enables preparation of spectrally pure single photon wavepackets without the need for spectral filtering. Spectral shifting is achieved by locking single-photon pulses to the linear slope of sinusoidal 40 GHz RF phase modulation. We verify the spectral shift by performing spectrally resolved heralded single photon counting, using frequency-to-time conversion by means of a highly dispersive chirped fiber Bragg grating. We verify the non-classicality of spectrally shifted single photons by measuring high-visibility Hong-Ou-Mandel interference using a reference single photon pulse. Spectral compression is based on the time lens principle, which requires locking optical pulses to approximately quadratic region of sinusoidal phase modulation. We utilize both 10 GHz and 40 GHz RF driving frequencies. Bandwidth compression is achieved by chirping the single photon pulse using an appropriate length of single-mode fibre and subsequently subjecting it to the action of the time lens. We verify spectral compression directly using the aforementioned spectrally-resolved heralded single photon counting method. We achieve 3-fold spectral compression of 2 nm bandwidth single photon pulses using 40 GHZ modulation frequency, and 6-fold spectral compression of 0.9 nm bandwidth single photon pulses using 10 GHz modulation frequency. Overall transmission of our set-up exceeding 30% enables practical usability of our spectral compression method which we demonstrate experimentally by showing an increased photon flux through a narrowband filter. Our results present an important contribution towards implementing quantum information processing in the spectral-temporal degree of freedom of a photon. In the context of quantum networks they present an enabling tool towards efficient photonic interfacing of different quantum information processing platforms.
Creating and controlling well-defined single-photon states is important for many quantum enhanced technologies. Information can be encoded in any degree of freedom associated with a single-photon field excitation, for example in the polarization or transverse spatial mode structure. The ability to encode multiple qubits in these states is desirable for high data transmission rates and increased information processing capacity. The spectral-temporal domain offers a large Hilbert space for encoding information, well suited to integrated optical architectures owing to the low cross-talk between channels. Indeed, time-frequency encoding is an integral component of existing information technologies infrastructure. Thus, complete, coherent control of the spectral-temporal mode structure of light is essential to advancing optical quantum technologies. Arbitrary control of single-photon states has been demonstrated in the polarization and spatial-momentum degrees of freedom however, is yet to be established in the spectral-temporal domain. Recently it was shown that spectral-temporal pulse shaping is experimentally feasible using nonlinear optical methods. However, such techniques can introduce noise photons, are challenging to implement deterministically and require specially prepared auxiliary pump pulses. Furthermore, they are typically only possible at relatively low repetition rates to avoid damage of nonlinear optical materials. Here, we demonstrate deterministic pulse shaping of single-photon wave packets by introducing a time-varying phase. Applying linear or quadratic phase to the wave packet introduces a spectral shift or spectral broadening, respectively. Achieving significant spectral manipulation requires temporal-phase modulation on the order of π. The phase must also vary on the same time scale as the optical pulses, approximately 1 ps in duration here. This relatively rapid phase variation is achieved using a fast electro-optic phase modulator. The modulator is driven by a time-varying voltage that is synchronized with a pulse train of single-photon wave packets for accurate temporal phase control. In this thesis, we experimentally demonstrate control of the spectral-temporal state of single-photon wave packets using this method. Heralded single-photon wave packets are generated by spontaneous parametric down conversion pumped by a frequency-doubled Titanium-Sapphire laser oscillator with 80 MHz repetition rate. To achieve significant modulation of single-photon pulses a 10 V peak-to-peak radio-frequency signal of 40 GHz drives the phase modulator, which requires approximately 3.6 V to achieve a π phase shift. The signal is synchronized with the oscillator pulse train using custom designed electronics. Details of these electronics are presented, resulting in a robust phase and amplitude controllable 40 GHz voltage source, phase locked to the single-photon pulse train. To demonstrate the utility of this pulse-shaping method we experimentally show spectral shearing of almost 1 nm for single-photon wave packets with 830 nm central wavelength and 1 nm bandwidth. Measurement of the second-order intensity correlation before and after the wave-packet manipulation remains constant within uncertainty of the measurement, showing that the quantum nature of the single-photon source is not deteriorated by the modulator. Preservation of the wave-packet coherence is verified by two-photon interference between spectrally sheared and non-modulated single-photons.
Secure optical communication systems based on general bent sequences have been described which encode/decode information with arrays of fiber Bragg gratings (FBG). A broadband pulse coming from an optical source will be split into a series of separated pulses when it reaches the encoding FBG array. These pulses have different spectrums and different time delays. Because it is difficult for an attacker to recover the original pulse if he doesn't know the exact structure of the encoding FBG array, one can say that the information from the optical source has been encrypted. The coded signal will be recovered by the decoding FBG array having the inverse structure of the encoding FBG array. Because various structures of encoding FBG arrays could generate different reflected pulses to ensure the security of communication in the unsafe channels, grating tuning techniques are used broadly to increase the modes of the systems. Some models of the systems will be introduced and their security will be discussed. Bent sequence is generated by a general class of bent functions to charge the modes of FBG arrays in order to increase the security of our systems because of its excellent characteristics in cryptology. That makes the optical communication system safe against the interception. Simulation and discussion will be included and the applicability of the system in the future has been prospected.