We proposed the simultaneous wavelength extensions of single-cavity dual-wavelength pulses by using a single erbium-doped fiber amplifier and one section of high nonlinear fiber. By additionally introducing a polarization dependent isolator, polarization dependent loss based gain profile tuning is adopted to obtain dual-wavelength pulses centered at 1533.7 and 1554.8nm. When both dual-wavelength pulses are simultaneously launched into the same erbium-doped fiber amplifier with the bidirectional pump power, the spectral range of the output pulses could be expanded to be from 1500 to 1600nm. Subsequently, the amplified dual-wavelength pulses further pass through a section of high nonlinear fiber, extending the spectrum from 1.2μm to more than 1.75μm. Dual-wavelength pulses are amplified simultaneously by using only one amplifier, showing the feasibility of the simplification of single-cavity dual-comb pulse amplification. These results show the high potential in the applications such as multi-color laser generation and spectroscopy.
Synchronous nanosecond and femtosecond pulses delivered from a low-repetition-rate Er-doped fiber laser mode-locked by nonlinear polarization evolution is experimentally proposed. Here, the repetition rate is set as ~4.5 MHz by introducing sufficiently long fiber in a ring cavity. By fully exploiting long fiber and anti-saturation absorption characteristics, it is experimentally observed that dissipative-soliton-resonance pulse with the nanosecond-level pulsewidth and femtosecond soliton pulse synchronously propagate in the same cavity. Besides, the pulsewidth of dissipative-soliton-resonance pulse and laser output power could be tailored by finely configuring the bidirectional pump powers. These results provide deep understanding of low-repetition-rate pulse laser and an intriguing way to obtain tunable dual-scale synchronous pulses, indicating the high potential for multiple-pulse laser processing and so on.
We proposed the emission of wavelength-switchable dual-wavelength-comb pulses in a practical-filter-free cavity. Based on the polarization dependent loss based gain profile tuning, lasings in triple independent gain subregions, i.e. ~1530-, ~1543- and ~1555-nm gain subregions, of erbium-doped fiber, are experimentally observed. Mode-locked by hybrid mechanisms combining carbon nanotube and nonlinear polarization evolution, triple types of dual-wavelength pulses distributed in different dual gain subregions are experimentally obtained. They are distributed in above triple gain subregions and could be switched by adjusting the intracavity polarization controller. These results provide a simple yet effective route to obtain dual-wavelength-comb pulses without additional practical filter and show the high potential in the applications of single-cavity dual-comb metrology.
We investigated a stable single-cavity dual-wavelength-comb fiber laser with significant difference of pulse characteristics. Switchable single/dual-wavelength pulses across 1530- and 1550-nm gain regions are obtained by adjusting the intracavity linear loss. In the dual-wavelength operation, the repetition rates fluctuate and drift in more than 145 Hz, while the standard deviation of the repetition rate difference is measured as 64 mHz in 1000-second monitoring. The passive mutual coherence between pulses is comparable or somewhat better than the reported one under the similar disturbance and monitoring condition. Meanwhile, the significant difference of dual-wavelength pulse characteristics, including spectral bandwidth, pulse energy and dispersion is observed and discussed. The qualified stability is also attributed to the significant pulse difference, which could suppress the nonlinear pulse interaction induced instability. These results provide further physical understanding of the construction of dual-wavelength-comb pulse fiber laser, showing the high potential to promote the performance improvement of dual-comb metrology such as dual-comb spectroscopy, and ranging.
Metasurfaces are opening up promising flexibilities to reshape the wavefront of electromagnetic waves, and a notable optical phenomenon is observed with the tailored surface plasmon. However, meta-devices with metallic material face challenges of low transmission efficiency, especially in the visible spectrum. In this work, a multilayer metamaterial strategy is proposed to improve the transmission. By adding dielectric material as space layer is inserted along the propagation direction, vertical cavities are formed to modulate the transmission and reflection. The multilayer structure is numerically studied to discuss the transmission efficiency. A maximum transmission efficiency of 90% is achieved by adding a spin on glass material to the silver layer at the visible spectrum. The transmission efficiency with and without space layer is discussed as a control. Significantly, the surface plasma and the phase could still be modulated by adjusting the geometrical parameters of the multilayer metamaterial for diversified functionalities.
In this paper, the transmission properties of the double-walled carbon nanotube (DWCNT) films in terahertz spectrum are simulated and analyzed using the finite element method. Carbon nanotubes (CNTs) have excellent electromagnetic properties, oxidation resistance and flexibility, so it has become a material that has attracted much attention to replacing metal wire grids. The transmission of the transverse magnetic (TM) gives the power transmission of the polarizer for the component parallel to the grating vector and the transmission of the transverse electric (TE) for the perpendicular component. Our simulation results show that the transmission of the TM mode and the TE mode could be modulated by adjusting the diameter and spacing of the DWCNTs. In the range of 0.3-2.5THz, the transmission of the TM can be modulated from 90.7% to 99.3%, and TE can be modulated from 0.3% to 23%, which offers an alternative way of designing a polarizer with the desired degree of polarization. The simulation results provide numerical results in wide-band THz wave modulation with DWCNTs.
We proposed an absolute distance measurement method with a large non-ambiguity range based on a polarization-multiplexed dual-comb fiber laser. By fully exploiting the intracavity linear loss based gain profile tilting and residual birefringence, polarization-multiplexed dual-comb pulses with tunable repetition rate difference and overlapping spectra in the 1530-nm gain region are obtained. The repetition frequency difference could be continuously tuned from ~89 to ~194 Hz. The alternative sampling under different repetition rate difference is experimentally verified to be effective approach to extend the non-ambiguity range in the single-cavity dual-comb ranging. The non-ambiguity range could reach thousands of kilometer while the precision could reach at least on the order of hundreds of micrometers. These results indicate a simple and intriguing route with a free-running laser source to obtain ranging with large non-ambiguity range, showing high potential in the applications such as satellite formation flying, large-scale 3D surface morphology measurement and so on.
A carbon nanotube (CNT) is a cylinder made of graphene. CNTs possess excellent properties of carbon materials, and the parallel-arranged nanotubes exhibit anisotropy additionally. The finite element method is adopted to simulate single-walled carbon nanotube (SWCNT) array in the terahertz spectrum. Under the standard size parameters we have set, Our simulation results show that between 1-3 THz, P-polarization transmittance, which is perpendicular to the SWCNTs, fluctuates around 0.5, while S-polarization transmittance, which is parallel to the SWCNTs, is around 4.8×10-3. The reflectance parallel to the SWCNTs gradually increases to 0.407, and the reflectance perpendicular to the SWCNTs gradually decreases to nearly 1×10-8 as the frequency increases. The simulation results provide theoretical support for further applications of SWCNTs in optical field.
We proposed the triple-wavelength pulses across the 1530- and 1550-nm gain regions are emitted from a carbon nanotube mode-locked ring fiber laser by simultaneously exploiting intracavity loss-based gain profile tuning, Lyot filter effect, and nonlinear polarization evolution. A polarization beam splitter with 2×1-m intracavity polarization-maintaining fiber pigtails is additionally introduced in a typical ring fiber cavity. Polarization-dependent loss is firstly adjusted to equalize the 1530- and 1550-nm gain regions. Except for the triple-wavelength pulses based on Lyot filter and loss-based gain profile tuning, another type of triple-wavelength pulses, i.e. single-wavelength pulse centered at 1530-nm gain region and spectral-overlapping dual-wavelength pulses centered at 1550-nm gain region, are observed by additionally introducing nonlinear polarization evolution. These intriguing results show the feasibility of multi-wavelength pulse generation based on multiple soliton formation mechanisms and the high potential to construct a single-cavity multiple-comb source with versatile pulse characteristics.
Recently, the nonlinear multimodal interference-based all fiber saturable absorber has been the focus of attention on ultrafast fiber lasers, owing to its intriguing properties of versatility, high damage threshold and instantaneous response time. Although, challenges present in the technology, such as complex perturbation induced by quasi-degenerate modes in multimode fiber, it is presented as an effective solution to control the output characterization and study the nonlinear dynamics in fiber lasers. In this work, we experimentally and numerically demonstrate the spectral sidebands in a passively Er-doped fiber laser based on multimodal interference technique. Kelly-type and triangular-type sidebands are achieved, and can be switchable by changing the polarization states of cavity, which are asymmetric distribution on both sides of the output spectrum. When the polarization states are varied, a wide sideband is obtained, which the width of sideband can be tuned from 0.13 nm to 2.3 nm. Coupled complex Ginzburg-Landau equation are provided to reveal the underlying principles of the tunable features in sidebands. The results of numerical simulation show the relevance between filtering induced by modal interference, high-order dispersion, polarization modal dispersion and experimental results. Our work lays a foundation for understanding of nonlinear dynamics in mode locking fiber lasers based on multimodal interference effect and provides a new way to generating versatile ultrafast source in engineering and scientific research.
We experimentally investigated the build-up dynamics of single-cavity dual-wavelength-comb pulses emitted from a ring fiber cavity with Lyot filter configuration. Dual-wavelength lasers are firstly observed by adjusting the polarization controller to control Lyot filter effect. When the pump powers of the bidirectional pumps are set as 57 mW and 49 mW respectively, dual-wavelength pulses with the center wavelengths of 1546.2 nm and 1563.6 nm and spectral bandwidths of 2 nm and 1.6 nm are obtained. Subsequently, time-stretched dispersive Fourier transform spectroscopy is adopted to monitor the build-up process of dual-wavelength pulses. When switching on the pump diode, the three-stage build-up process from background noise to stable dual-wavelength pulses is experimentally observed. The build-up time is at the level of hundreds of milliseconds. These results provide a deep understanding of single-cavity dual-wavelength-comb pulse generation and contribute to the design and control of the single-cavity dual-comb pulses.
Based on PbS quantum dots and single-walled carbon nanotube, we have successfully demonstrated a Er-doped fiber laser capable of switching between two different types of output pulses. By finely adjusting both the pump power and the states of polarization controller, flexible switchable Q-switched and mode-locked pulses can be achieved. At pump power of 29 mW, Q-switched pulses are obtained at a central wavelength of 1560.2 nm. When the pump power increases from 29 mW to 92 mW, the Q-switched rate varies from 25 kHz to 75.22 kHz. Accordingly, the output pulse energy rises from 3 nJ to 5.46 nJ, and the output power changes from 0.08 mW to 0.41 mW. When the pump power is set in the ranges of 92 mW to 107 mW, the fiber laser enters the transition region of Q-switching operation. In this region, evident Q-switched instability with large fluctuations is observed, which is independent of the polarization states. When the laser pump power exceeds 107 mW, the Q-switched pulse disappears, and mode-locked pulses are obtained by altering the state of the polarization controller. The central wavelength of the mode-locked pulses output spectrum is 1561.1 nm, and the corresponding 3 dB spectral bandwidth is 4.22 nm. Coupled Ginzburg-Landau equation are provided to reveal the underlying principles of the transition of these pulse trains. Our work provides a new prospect for achieving fiber lasers capable of flexibly switching output pulse types, further expanding their applications in fields such as laser microprocessing, optical communication and medical lasers.
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.
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.
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 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.
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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