In this paper, we propose and experimentally demonstrate a new method for optical frequency transfer over fibre.Instead of dual acousto-optic modulators (AOMs) as adopted in the traditional fibre phase noise compensation setup, here an active fibre phase noise compensation scheme with a single acousto-optic modulator (AOM) is used.The configuration simplifies the equipment of the user end while maintaining a high-performance optical frequency transfer stability.We demonstrate an actively stabilized coherent transfer at an optical frequency of 193.55THz over 10-km spooled fibre, obtaining a relative frequency stability (Allan deviation) of 3.84 × 10 -16 /1 s and 4.08 × 10 -18 /10 4 s, which is improved by about 2∼3 orders of magnitude in comparison with the one without any phase noise compensation that achieves a relative frequency stability of 1.81 × 10 -14 /1 s and 2.48 × 10 -15 /10 4 s.
In this paper, we propose and experimentally demonstrate a new method for optical frequency transfer over fibre.Instead of dual acousto-optic modulators (AOMs) as adopted in the traditional fibre phase noise compensation setup, here an active fibre phase noise compensation scheme with a single acousto-optic modulator (AOM) is used.The configuration simplifies the equipment of the user end while maintaining a high-performance optical frequency transfer stability.We demonstrate an actively stabilized coherent transfer at an optical frequency of 193.55THz over 10-km spooled fibre, obtaining a relative frequency stability (Allan deviation) of 3.84 × 10 -16 /1 s and 4.08 × 10 -18 /10 4 s, which is improved by about 2∼3 orders of magnitude in comparison with the one without any phase noise compensation that achieves a relative frequency stability of 1.81 × 10 -14 /1 s and 2.48 × 10 -15 /10 4 s.
We present a technique for coherence transfer of laser light through a branching fiber link, where the optical phase noise induced by environmental perturbations via the fiber link is passively compensated by remote users without the requirements of any active servo components. At each remote site, an acousto-optic modulator (AOM) is simultaneously taken as a frequency distinguisher for distinguishing its unique frequency from other sites' and as an optical actuator for compensating the phase noise coming from the optical fiber. With this configuration, we incorporate a long outside loop path consisting of a fiber-pigtailed AOM into the loop, enabling the significant reduction of the outside loop phase noise in the passive way. To further address the residual out-of-loop phase noise coming from the interferometer and the two-way optical frequency comparison setup, we design a low-noise active temperature stabilization system. Measurements with a back-to-back system show that the stability in our stabilization system is 2×10-16at 1 s, reaching 2×10-20after 10,000s. Adopting these techniques, we demonstrate transfer of a laser light through a branching fiber network with 50 km and 145 km two fiber links. After being compensated for the 145 km fiber link, the relative frequency instability is 3.4×10-15at the 1 s averaging time and scales down to 3.7×10-19at the 10,000 s averaging time. This proposed technique is suitable for the simultaneous transfer of an optical signal to a number of independent users within a local area.
We describe and experimentally evaluate a stabilized radio-frequency transfer technique that employs optical phase sensing and optical phase actuation. This technique is achieved by modifying existing optical frequency transfer equipment and also exhibits advantages over previous stabilized radiofrequency transfer techniques in terms of size and complexity. Acousto-optic modulators (AOMs) are used to modulate an optical carrier. Stabilization of frequency fluctuations in the link is achieved by steering the frequency of one of the AOMs. We demonstrate the stabilized transfer of a 160-MHz signal over a 166-km fiber optical link, achieving an Allan deviation of 9.7 × 10-12at 1 s of integration, and 6.4 × 10-15at 104s. This technique was considered for application to the Square Kilometre Array SKA1-low radio telescope.
In this paper, we present a remote time-base-free technique for a coherent optical frequency transfer system via fiber.At the remote site, the thermal noise of the optical components is corrected along with the link phase noise caused by environmental effects.In this system, a 1 × 2 acousto-optic modulator (AOM) is applied at the remote site, with the first light being used to eliminate the noise of the remote time base and interface with remote users while the zeroth light is used to establish an active noise canceling loop.With this technique, a 10 MHz commercial oscillator, used as a time base at the remote site, does not contribute to the noise of the transferred signal.An experimental system is constructed using a 150 km fiber spool to validate the proposed technique.After compensation, the overlapping Allan deviation of the transfer link is 7.42 × 10 -15 at 1 s integration time and scales down to 1.07 × 10 -18 at 10,000 s integration time.The uncertainty of the transmitted optical frequency is on the order of a few 10 -19 .This significantly reduces the time-base requirements and costs for multi-user applications without compromising transfer accuracy.Meanwhile, these results show great potential for transferring ultra-stable optical frequency signals to remote sites, especially for point-to-multi-users.
SummaryOptical fibers have been regarded as one of the most promising materials for coherent optical frequency dissemination over thousands of kilometers. For suppressing the fiber-induced phase noise along the fiber network, we demonstrate the passive phase noise cancellation (PNC) technique without the need of phase detector could be preferable for noisy fiber links. A fiber-pigtailed acousto-optic modulator (AOM) with two diffraction order outputs (0 and -1 order) has been installed at the local site and been employed as the phase compensated device in the optical frequency transfer system. With a 550 km noisy fiber link, we demonstrate a fractional transfer frequency instability of 1.3 × 10-14at the integration time of 1 s and scales down to 5.5 × 10-19at 10,000 s in terms of modified Allan deviation.
High-precision time–frequency is widely used in time measurement, satellite navigation, scientific research, and other fields. With the rapid development of optical clock technology, the fractional frequency instability and uncertainty of optical clock have reached 10−18 orders of magnitude, which is expected to contribute to generating the International Atomic Time and may even be used to redefine the “second” in the future. Therefore, the long-distance transfer of time–frequency signals between optical atomic clocks is of great significance. Free-space optical frequency transfer technology is one of the important technologies for solving the space-based optical clock comparison because of its high transfer precision and easy networking characteristics. In order to solve the long-distance space-based optical clock comparison, this paper investigates a free-space active phase noise compensation method using an Acousto-Optic Modulator (AOM), based on the traditional optical fiber phase noise compensation scheme. This new method is more flexible and scalable than the optical fiber time–frequency transfer technology. The optical frequency transfer over a 64 m free-space link is demonstrated. The fractional frequency transfer instability during free running is 9.50 × 10−16 at 1 s, and 4.44 × 10−16 at 2000 s, and the fractional frequency instability after compensation is 7.10 × 10−17 at 1 s, 3.07 × 10−19 at 2000 s, which is about 1–3 orders of magnitude better than that in free running, and provides a feasible scheme for space-based optical clock comparison.
Optical fibers have been recognized as one of the most promising host material for high phase coherence optical frequency transfer over thousands of kilometers. In the pioneering work, the active phase noise cancellation (ANC) technique has been widely used for suppressing the fiber phase noise introduced by the environmental perturbations, in which an ideal phase detector with high resolution and unlimited detection range is needed to extract the fiber phase noise, in particular for noisy fiber links. We demonstrate the passive phase noise cancellation (PNC) technique without the need of phase detector could be preferable for noisy fiber links. To avoid the effect of the radio frequency (RF) from the time base at the local site in the conventional active or passive phase noise cancellation techniques, here we introduce a fiber-pigtailed acousto-optic modulator (AOM) with two diffraction order outputs (0 and +1 order) with properly allocating the AOM-driving frequencies allowing to cancel the time base effect. Using this technique, we demonstrate transfer of coherent light through a 260 km noisy urban fiber link. The results show the effect of the RF reference can be successfully removed. After being passively compensated, {we demonstrate a fractional frequency instability of 4.9×10−14 at the integration time of 1 s and scales down to 10−20 level at 10,000 s in terms of modified Allan deviation over the 260 km noisy urban fiber link}. The frequency uncertainty of the retrieved light after transferring through this noise-compensated fiber link relative to that of the input light achieves (0.41±4.7)×10−18. The proposed technique opens a way to a broad distribution of an ultrastable frequency reference with high coherence without any effects coming from the RF reference and enables a wide range of applications beyond metrology over fiber networks.