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Lasers stabilized to optical fiber delay lines have been shown to deliver a comparable short-term (<1 s) frequency noise performance to that achieved by lasers stabilized to ultra-low expansion (ULE) cavities, once the linear frequency drift has been removed. However, for continuous stable laser operations, the drift can be removed only when it can be predicted, e.g., when it is linear over very long timescales. To date, such long-term behaviour of the frequency drift in fiber delay lines has not been, to the best of our knowledge, characterised. In this work we experimentally characterise the frequency drift of a laser stabilised to a 500â m-long optical fiber delay line over the course of several days. We show that the drift still follows the temperature variations even when the spool temperature is maintained constant with fluctuations below tens of mK. Consequently, the drift is not linear over long timescales, preventing a simple feed-forward compensation. However, here we show that the drift can be reduced by exploiting the high level of correlation between laser frequency and the fiber temperature. In our demonstration, by applying a frequency correction proportional to temperature readings, a calculated frequency drift of less than 16â Hz/s over the several days of our test was obtained, corresponding to a 23-fold improvement from uncorrected values.
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Fast (nanoseconds) optical wavelength switching is emerging as a viable solution to scaling the size and capacity of intra-data center interconnection. A key enabling technology for such systems is low-jitter optical clock synchronization, which enables sub-nanosecond clock and data recovery for optically switched frames using low-cost methods such as clock phase caching. We propose and demonstrate real-time low-latency wavelength-switched clock-synchronized intra-data center interconnection at 51.2 GBd using a fast tunable laser (with ns scale switching time) and ultra-stable-latency hollow core fiber (HCF) for optically-switched data center networks. For wavelength-switched systems, we achieve a physical layer latency below 46â ns, consisting of 28â ns transceiver latency and a 18â ns inter-packet gap. Finally, we show that by exploiting the low chromatic dispersion and thermally-stable latency features of HCF, active clock phase tracking can be entirely eliminated.
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By modifying the interconnection design between standard single-mode fiber (SSMF) and nested antiresonant nodeless type hollow-core fiber (NANF), we create an air gap between SSMF and NANF. This air gap enables the insertion of optical elements, thus providing additional functions. We show low-loss coupling using various graded-index multimode fibers acting as mode-field adapters resulting in different air-gap distances. Finally, we test the gap functionality by inserting a thin glass sheet in the air gap, which forms a Fabry-Perot interferometer and works as a filter with an overall insertion loss of only 0.31 dB.
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Optical fibers with a low thermal coefficient of delay (TCD) have been developed for frequency and timing transmission/distribution. However, their temperature sensitivity changes as a function of temperature and, to date, no study of such fibers has demonstrated improved performance over extended temperature ranges, especially at sub-zero temperatures. Here, we show that a hollow core fiber (HCF) with a thin acrylate coating can have a TCD within ±2.0 ps/km/°C over a broad temperature range from -150°C to +60°C. In addition, this thinly coated HCF can be fully insensitive to temperature around -134°C, making it of interest, e.g., for laser stabilization close to cryogenic temperatures.
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In the original publication of our research article "Hollow core fiber Fabry-Perot interferometers with reduced sensitivity to temperature" [Opt. Lett.47, 2510 (2022)10.1364/OL.456589OPLEDP0146-9592], we identified an error that requires correction. The authors sincerely apologize for any confusion that may have arisen from this error. The correction does not affect the overall conclusions of the paper.
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We report simultaneous low coupling loss (below 0.2 dB at 1550 nm) and low back-reflection (below -60 dB in the 1200-1600 nm range) between a hollow core fiber and standard single mode optical fiber obtained through the combination of an angled interface and an anti-reflective coating. We perform experimental optimization of the interface angle to achieve the best combination of performance in terms of the coupling loss and back-reflection suppression. Furthermore, we examine parasitic cross-coupling to the higher-order modes and show that it does not degrade compared to the case of a flat interface, keeping it below -30 dB and below -20 dB for LP11 and LP02 modes, respectively.
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Today's lowest-loss hollow core fibers are based on antiresonance guidance. They have been shown both theoretically and experimentally to have very low levels of backscattering arising from the fiber structure - 45 dB below that of traditional optical fibers with a solid silica glass core. This makes their longitudinal characterization using conventional reflectometric techniques very challenging. However, it was recently estimated that when filled with air, their backscattering coefficient increases to about 30 dB below that of standard solid core fibers. This level should be measurable with commercially available high performance optical time domain reflectometers (OTDR). Here we demonstrate - for the first time to the best of our knowledge - the measurement of backscattering from the air inside a hollow core fiber. We show that the characterization of multi-km long hollow core fibers with 15 m spatial resolution is possible using a commercial OTDR instrument. To benefit from its full dynamic range, we strongly suppress the 4% back-reflections that ordinarily occur at the OTDR's standard fiber output when directly-connected to a hollow core fiber. Furthermore, low coupling loss into the hollow core fiber (0.3 dB in our experiment) also helps to maximize the achievable OTDR signal-to-noise ratio. This approach enables distributed characterization and fault-finding in low-loss hollow core fibers, a topic of increasing importance as these fibers are now starting to be installed in commercial optical communication networks.
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We demonstrate a 3× thermal phase sensitivity reduction for a hollow-core fiber (HCF) Fabry-Perot interferometer by winding the already low temperature sensitivity HCF on to a spool made from an ultralow thermal expansion material. A record low room temperature fiber coil phase thermal sensitivity of 0.13â ppm/K is demonstrated. The result is of particular interest in reducing the thermal sensitivity of HCF-based Fabry-Perot interferometers (for which existing thermal sensitivity reduction methods are not applicable). Our theoretical analysis predicts that significantly lower (or even zero) thermal sensitivity should be achievable when a spool with a slightly negative coefficient of thermal expansion is used. We also suggest a method to fine-tune the thermal sensitivity and analyze it with simulations.
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The optical phase accumulated when light propagates through an optical fiber changes with temperature. It has been shown by various authors that this thermal phase sensitivity is significantly smaller in hollow core fibers (HCFs) than in standard single-mode fibers (SSMFs). However, there have been considerable differences in the level of sensitivity reduction claimed, with factors in the range ×3 to ×20 improvement for HCFs relative to SSMFs reported. Here we show experimentally that this large variation is likely attributable to the influence of fiber coating, which is exacerbated in HCFs with a relatively thin silica glass outer wall (e.g., the wall thickness is typically just 20 µm in a 125 µm diameter HCF). Further, we show that the coating also causes the optical phase stability to suffer from relaxation effects, which have not been previously discussed in the HCF literature, to the best of our knowledge. In addition to demonstrating these relaxation effects experimentally, we analyze them through numerical simulations. Our results strongly suggest that they originate from the viscoelastic properties of the coating. To minimize the adverse effects of the coating, we have fabricated a HCF with a relatively thick wall (â¼50µm) and a very thin coating (10 µm). This results in an almost 30-fold reduction in HCF thermal phase sensitivity relative to SSMFs - a significantly lower sensitivity than in previous reports. Moreover, our thinly coated HCF exhibits no discernable relaxation effects while maintaining good mechanical properties.
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In this Letter, we conceptually demonstrate the potential of a phase-sensitive amplifier to operate as an active detector of stochastic phase changes in fiber-based frequency dissemination systems with two orders of magnitude better sensitivity than state-of-the-art one-way systems relying on two-wavelength dissemination schemes. Theoretical and experimental analyses show that these stochastic phase changes (caused by environmental changes, e.g., due to temperature) can be detected with high sensitivity via optical phase comparison performed within the phase-sensitive amplifier. Experimental results are in close agreement with theoretical predictions showing that phase-sensitive amplifiers may find a niche application in metrology, with potential to significantly improve one-way fiber-based frequency dissemination systems.
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This Letter outlines radio-over-fiber combined with radio-over-free-space optics (RoFSO) and radio frequency free-space transmission, which is of particular relevance for fifth-generation networks. Here, the frequency band of 24-26 GHz is adopted to demonstrate a low-cost, compact, and high-energy-efficient solution based on the direct intensity modulation and direct detection scheme. For our proof-of-concept demonstration, we use 64 quadrature amplitude modulation with a 100 MHz bandwidth. We assess the link performance by exposing the RoFSO section to atmospheric turbulence conditions. Further, we show that the measured minimum error vector magnitude (EVM) is 4.7% and also verify that the proposed system with the free-space-optics link span of 100 m under strong turbulence can deliver an acceptable EVM of <9% with signal-to-noise ratio levels of 22 dB and 10 dB with and without turbulence, respectively.
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We investigate for the first time, to the best of our knowledge, both theoretically and experimentally, how the phase noise of the radio frequency (RF) drive signal affects the phase noise of the individual tones of a Fabry-Perot (F-P) modulator-based optical frequency comb. We observe that the expected deleterious effect of the RF drive signal phase noise on the comb output is partially suppressed due to the filtering characteristics of the F-P cavity. We found that the cavity-induced phase noise suppression is strongest for high-order comb tones, e.g., reaching up to 40 dB for the 100th comb tone at high offset frequencies. The phase noise suppression becomes even stronger for low RF-drive powers, or when the seed laser does not resonate in the F-P cavity. For both cases, we observe up to a 10 dB increase in phase noise suppression. We also evaluate the timing jitter improvement obtained, thanks to the cavity-induced phase noise reduction. The timing jitter (integrated from 2.5 MHz to 2.5 GHz) decreased by a factor of 7 for the beat signal obtained between two comb tones that are 100 tones apart (in comparison with the timing jitter obtained in a cavity-less comb generator).
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We demonstrate coherent wavelength conversion capable of covering the entire C-band by modulating the incoming optical carrier with a compact Fabry-Perot cavity embedded phase modulator and by optical injection locking a semiconductor laser to a tone of the generated optical frequency comb. The phase noise of the converted optical carrier over 1 THz frequency interval is measured to be -40 dBc/Hz at 10 Hz offset and the frequency stability is better than 2 × 10(-17) level for averaging times >1000 s, making this technique a promising solution for comparisons of state-of-the-art optical clocks over complex fiber networks.
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Spectrally pure microwave sources are highly desired for several applications, ranging from wireless communication to next generation radar technology and metrology. Additionally, to generate very pure signals at even higher frequencies, these advanced microwave sources have to be compact, low in weight, and low energy consumption to comply with in-field applications. A hybrid optical and electronic cavity, known as an optoelectronic oscillator (OEO), has the potential to leverage the high bandwidth of optics to generate ultrapure high-frequency microwave signals. Here we present a widely tunable, low phase noise microwave source based on a photonic chip. Using on-chip stimulated Brillouin scattering as a narrowband active filter allows single-mode OEO operation and ultrawide frequency tunability with no signal degeneration. Furthermore, we show very low close-to-carrier phase noise. This Letter paves the way to a compact, fully integrated pure microwave source.
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We demonstrate the use of an optical injection phase locked loop (OIPLL) as a regenerative amplifier for optical frequency transfer applications. The optical injection locking provides high gain within a narrow bandwidth (<100 MHz) and is capable of preserving the fractional frequency stability of the incoming carrier to better than 10(-18) at 1000 s. The OIPLL was tested in the field as a mid-span amplifier for the transfer of an ultrastable optical carrier, stabilized to an optical frequency standard, over a 292 km long installed dark fiber link. The transferred frequency at the remote end reached a fractional frequency instability of less than 1×10(-19) at averaging time of 3200 s.
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A main, yet-unsolved challenge in splicing hollow-core fiber (HCF) into standard single-mode fiber (SMF) systems lies in managing the strong Fresnel back-reflection that occurs when the light travels from the empty core of the HCF into the glass core of the SMF or vice versa. This impacts the performance of fiber systems that combine SMFs and HCFs due to effects such as multipath interference. Here, we demonstrate a new technique that combines angle-cleaving the HCF, which reduces the back-reflection, with offset-splicing the mode-field adapter to the SMF, which compensates for the refraction at the glass-air interface, enabling us to achieve low coupling loss. We first analyze this novel configuration via simulations and show that it is possible to achieve a coupling loss that is comparable to a conventional flat-cleaved splice. Subsequently, we fabricate an SMF-HCF connection with a loss of 0.6 dB prior to arcing (1.2 dB after splicing) and ultralow back-reflection (-64 dB) by applying an optimized 4.5° angle and 5 µm offset. To the best of our knowledge, this is the first low-insertion-loss spliced SMF-HCF connection where a widely acceptable level of back-reflection of <-60 dB is achieved.
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We explore a new class of Distributed Feedback (DFB) and Distributed Bragg Reflector (DBR) structures that employ the recently-developed concept of Parity-Time (PT) symmetry in optics. The approach is based on using so-called unidirectional Bragg gratings that are non diffractive (transparent) when illuminated from one side and diffracting (Bragg reflection) when illuminated from the other side, thus providing a uni-directional Bragg functionality. Such unusual property is achieved through diffraction through a grating having periodic variations in both, phase and amplitude. DFB and DBR structures traditionally consist of a gain medium and reflector(s) made via periodic variation of the (gain media) refractive index in the direction of propagation. As such structures are produced in a gain material. It becomes just possible to add periodic amplitude modulation in order to produce the unidirectional Bragg functionality. We propose here new and unique DFB and DBR structures by concatenating two such unidirectional Bragg gratings with their nonreflective ends oriented outwards the cavity. We analyze the transmission and reflection properties of these new structures through a transfer matrix approach. One of the unique characteristics of the structure is that it inherently supports only one lasing mode.
Asunto(s)
Diseño Asistido por Computadora , Modelos Teóricos , Refractometría/instrumentación , Resonancia por Plasmón de Superficie/instrumentación , Simulación por Computador , Diseño de Equipo , Análisis de Falla de Equipo , Luz , Dispersión de RadiaciónRESUMEN
Recently, significant efforts have been devoted to enable light resonating inside various resonators for long time, leading to high Q factors. Achieving tunability of the free spectral range while maintaining high Q has been, however, challenging.
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We experimentally demonstrate phase regeneration of a 40-Gb/s differential phase shift keying (DPSK) signal in a 1.7-m long highly nonlinear lead silicate W-type fiber using a degenerate two-pump phase-sensitive amplifier (PSA). Results show an improvement in the Error Vector Magnitude (EVM) and a reduction of almost a factor of 2 in the phase noise of the signal after regeneration for various noise levels at the input.
Asunto(s)
Amplificadores Electrónicos , Tecnología de Fibra Óptica/instrumentación , Procesamiento de Señales Asistido por Computador/instrumentación , Telecomunicaciones/instrumentación , Diseño de EquipoRESUMEN
We experimentally demonstrate phase-sensitive amplification in a highly nonlinear and low-dispersion lead-silicate W-type fiber. A phase-sensitive gain variation of 6 dB was observed in a 1.56-m sample of the fiber for a total input pump power of 27.7 dBm.