Electrically tunable metasurfaces have great potential for flexible response and high precision in wavefront control, making them highly applicable. However, there is currently a scarcity of electrically tunable metasurfaces working at visible wavelengths. An electrically tunable transmission metasurface working at 660nm was proposed in this paper. The metasurface integrates a transparent conductive oxide material ITO as a tunable electro-optical material. The design scheme of the electrically tunable metasurface is based on the classical Drude model. In the electric field, the variation of carrier concentration in the accumulation layer induced by bias voltage can enhance the nonlinear optical response and improve light field modulation effect. The proposed metasurface is structured with four symmetrically distributed rectangular patches nested with a circular ring. In addition, the phase modulation capability of this model has been theoretically analyzed. With a bias voltage of -4.9V~20V, a continuous transmission phase delay between 0°~191.45° at a wavelength of 660nm can be achieved. The proposal of the electrically tunable metasurface structure establishes a new means for transmitted beam wavefront shaping and modulation, and in the future, the metasurfaces designed with a continuous phase modulation at visible wavelength will suggest more applications in naked-eye 3D display, holographic imaging, and other fields.
A single-cavity triple-comb all-fiber laser is proposed by wavelength/polarization multiplexing. A variable optical attenuator is introduced to equalize the 1530-nm and 1550-nm gain profile of erbium-doped fiber for dual-wavelength pulses. Their repetition rate difference reach kHz level. Meanwhile, by further adjusting the intracavity polarization state, polarization-multiplexed dual-comb pulses with tens-of-Hz repetition rate difference in the 1550-nm gain region are obtained. The more than one-order-of-magnitude difference between the maximum and minimum repetition frequency difference and qualified passive mutual coherence of triple-frequency pulses is highlighted. These results indicate a highly potential triple-comb source for multiple-comb metrology such as triple-comb ranging and frequency measurement and so on.
We demonstrate a Q-switched mode-locked Er-doped fiber laser using an all-fiber grade-index multimode fiber-based modulator which generates dark-bright pair between bright pulse sequences and alternate bright and dark pulses. A section of dispersion compensation fiber (Nufern UHNA4) considered as a candidate normal group victory dispersion fiber is used to adjust the net dispersion of cavity. At a pump power of 410 mW, evident Q-switched instability modulating mode-locked bright pulses are observed, and the duration of Q-switched envelope changes from 1.8 μs to 8 μs along with the variation of power. Changing the state of polarization controller, the mode-locked bright pulse train is tuned to dark pulse train with reducing the duration of Q-switched envelope to 1.2 μs. What’s more, dark-bright pair between bright pulses train and alternate bright and dark pulses are also observed under second harmonic operations with suitable PC states. Coupled complex Ginzburg-Landau equation, field coupling model for propagation in multimode fiber, and fiber nonlinear effects are provided to reveal the underlying principles of the transition of these pulse trains. Because of the principal modes and filtering effect in multimode fibers, the formation and stable propagation of the dark-bright pair are precisely achieved. At the same time, the physical mechanism behind the unusual pairing of dark and bright pulses is that under certain conditions, cross-phase modulation can counteract the time extension of optical pulses caused by the combination of self-phase modulation and normal dispersion. Thus, the cross-phase modulation induced chirping on dark solitons enables dark-bright pair between bright pulse sequences to coexist.
In this work, we demonstrate a single-walled carbon nanotubes-based wavelength multiplexed fiber laser, which generates dual-comb pulse in the train of soliton rain. The fiber laser cavity is manipulated in repetition frequency of 16.58 MHz, 3 dB spectral bandwidth of 8.4 nm. Two asynchronous pulses constitute the soliton rain pulse sequences, which the intensity difference is about 5.72 dB between the dual frequencies. A piece of graded-index multi-mode fiber as a filter based on the multi-mode interference effect is introduced into cavity to improving the signal to noise ratio to ~62 dB, and locate the central wavelength of the dual-comb at 1556.7 nm and 1561.5 nm. The repetition rate difference of the dual-frequency is about 169 Hz with the resolution bandwidth of 1 Hz. The time delay of the dual-frequency pulse detected by cross-correlation method is 5.78 ms, which is well matched with the results in radio frequency spectrum. Different from the stable period of the general cross-correlation signal, our experimental results show several different sub-periods due to the existence of the drifting solitons in the soliton rain sequences. Meanwhile, the number of different sub-periods in the correlation decreases from six to three as the pump power reduced from 100 mA to 97.3 mA. Our work provides a new sight into the quasi-steady multi-soliton dynamics process in fiber lasers, and will be promising solutions for interference ranging, and synchronization and timing.
The measurement of ultra-weak magnetic fields relies on the conversion of magnetic field information to atom spins using alkali metals. In this context, the detection of magnetic fields is accomplished through rotation angle measurement of linearly polarized light. This paper proposes a novel method to suppress mechanical errors between polarizers in the rotation angle measurement, taking advantage of the optical setup characteristics in atomic magnetometers. The method involves applying two separate frequency modulations to the pump beam and probe beam, effectively eliminating mechanical errors between the polarizers as a direct current component using a double-channel lock-in amplifier. Additionally, the double modulation method offers a solution to suppress shot noise caused by incident beam fluctuations or transverse spin relaxation, as well as mechanical errors among optical elements in the light path, enabling high-precision measurements.
Due to the simple configuration, qualified passive coherence between pulses, and cost-effective characteristics, single-cavity dual-comb sources attract increasing research interest. Actually, such lasers have been experimentally verified in dual-comb metrology such as dual-comb frequency measurement and spectroscopy. Unlike the single-cavity dual-comb fiber laser multiplexed in other dimensions such as wavelength, direction and mode-locked mechanism, polarization-multiplexed pulses own the unique characteristics of overlapping spectra, intrinsic spectral coherence, and tunable repetition rate difference. They are beneficial for the simplification of additional optical amplification and the satisfaction of versatile requirements of dual-comb metrology. Here, we demonstrated a single-wall carbon nanotube saturable absorber mode-locked Er-doped fiber laser to emit wavelength-switchable polarization-multiplexed dual-comb pulses. The intracavity loss is carefully tuned by an additional optical variable attenuator to define the oscillation windows. In both the 1530- and 1550-nm gain regions, spectral-overlapping, polarization-multiplexed pulses are experimentally obtained with the fine configuration of the intracavity state of polarization. The polarization dynamics and tunable repetition rate difference are experimentally revealed. The repetition rate difference is at the tens-of-hertz level, which is somewhat lower than that of the reported polarization-multiplexed fiber laser with additionally introduced polarization-maintaining fiber. Since there are no additional birefringent media, the polarization mode dispersion for polarization-multiplexed pulses is attributed to the residual birefringence. Moreover, the passive mutual coherence is also highlighted. There results provide a simple yet effective way to design switchable and versatile single-cavity dual-comb pulses.
In our work, we experimentally demonstrate wavelength multiplexed dual-comb pulses based on multi-modal interference effect in a passively single-walled carbon nanotube mode-locking all fiber ring laser. The laser cavity achieves a variety of dual-wavelength mode-locked states by switching the polarization controller in the laser cavity. A piece of 25 cm long graded-index multi-mode fiber as a filter based on the multi-mode interference effect is introduced into cavity to fixing wavelength and to improving the signal to noise ratio. With optimized length of multi-mode fiber, we observed the two different filter state which located at 1559 nm and 1562 nm, 1561 nm and 1563 nm respectively in the different polarization dual-comb states. With suitable filtering state by stretching the multi-mode fiber, the two asynchronous pulse sequences coexist with diverse operation, which propagate with singlet and double pulses, respectively. The repetition rate of the laser is 16.59 MHz and the time period corresponding to the asynchronous pulse is ~60 ns. The repetition rate difference of dual-wavelength states reaches 100 Hz. In addition, we recorded the output modulation state of the laser cavity. Our research provides experimental basis for optical fiber sensing, wavelength division multiplexing communication system and high resolution spectroscopy.
The beam shaping system to convert the Gaussian beam to top-hat beam is widely used in modem optics such as laser technologies. A general beam shaper is normally composed of a convex or concave lens in nonspherical or freeform, which is bulky in the optical path and turns to a barrier in size minorizing for a compact system. A novel beam shaper with a plane structure with flat surfaces on both bottom and top sides are provided in this paper. Taking advantages of the phase changes by the subwavelength structures and the general Fresnel principle for discrete structures, a metalens with beam shaping function is designed. The phase variation between a Gaussian beam and a top-hat beam is studied with Fourier optics and then is adopted to the layout of the beam shaping metalens. Afterwards, the finite domain time difference method is adopted to simulate the energy distribution of the modulated beam to study the effectiveness of the novel ultra-thin beam shaping metalens. Examples to convert the Gaussian beam to top-hat beam calculated with convex surface and nanopillar array with flat surfaces are illustrated in the paper to demonstrate and discuss the beam shaping results with the novel design in plane form and ultra-thin thickness. According to our study, a beam shaping lens with flat surfaces and thickness smaller than 1 um with the uniformity better than 98% can be achieved at wavelength of 790 nm. Variable beam shaping results could be obtained by the design method to figure out the phase distribution with ray optics and then design the metalens according to the desired phase modulation by arranging the subwavelength structures accordingly. Tue numerical results may pave the way for further design of metalens and offers a solution for compact systems with optical paths.
We have demonstrated a partial mode-locked Er-doped fiber laser with nonlinear multimodal interference technique, which generates a tunable condensed phase with numerous unresolved aggregated solitons. The condensed phase has a h-shaped envelop with sharp leading edge and low amplitude trailing edge. The maximum span of a single envelop of condensed phase reaches 115 ns, and is beyond half of cavity roundtrip time (200 ns). Changing polarization states in cavity, the span varies from 115 ns to 24 ns, with the disappearance of condensed state of solitons, and the h-shape pulse grows. Benefiting from the quasi-degenerate modes existing in multimode fiber, the perturbation by mode noise in multimode fiber causes partial mode-locked operation in our experiments and results into the generation of condensed phase. The feature of saturated absorption and reverse saturated absorption of nonlinear modulator shapes the h-shaped envelop. The presented fiber laser is a promising tool to deep insight into complex nonlinear dynamics and laser physics in fiber lasers.
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