In-fiber microchannel device filled with a carbon nanotube dispersion for passive mode-lock lasing

Upconversion-Luminescent Fiber Microchannel Sensors for Temperature Monitoring at High Spatial Resolution in the Brains of Freely Moving Animals

Brain temperature is a critical factor affecting neural activity and function, whose fluctuations may result in acute life-threatening health complications and chronic neuropathology. To monitor brain temperature, luminescent nanothermometry (LN) based on upconversion nanoparticles (UCNPs) with low autofluorescence has received extensive attention for its advantages in high temperature sensitivity and high response speed. However, most of current the LNs are based on optical imaging, which fails in temperature monitoring in deep brain regions at high spatial resolution. Here, the fiber microchannel sensor (FMS) loaded with UCNPs (UCNP-FMS) is presented for temperature monitoring at high spatial resolution in the deep brains of freely moving animals. The UCNP-FMS is fabricated by incorporating UCNPs in microchannels of optical fibers, whose diameter is ∼50 µm processed by femtosecond laser micromachining for spatially resolved sensing. The UCNPs provide thermal-sensitive upconversion emissions at dual wavelengths for ratiometric temperature sensing, ensuring a detection accuracy of ± 0.3 °C at 37 °C. Superior performances of UCNP-FMS are demonstrated by real-time temperature monitoring in different brain regions of freely moving animals under various conditions such as taking food, undergoing anesthesia/wakefulness, and suffering external temperature changes. Moreover, this study shows the capability of UCNP-FMS in distributed temperature sensing in mammalian brains in vivo.

Novel nanomaterials based saturable absorbers for passive mode locked fiber laser at 1.5μm

Compared with continuous wave lasers, ultrafast lasers have the advantages of ultra-short pulse width and ultra-high peak power, and have significant applications in optical communications, medical diagnostics, and precision machining. Saturable absorber (SA) technology is the most effective technique for the generation of ultra-fast lasers, which are based on artificial SAs and natural SAs. Among them, the semiconductor saturable absorber mirror has become the most commonly used form at present. Recently, basic research and application of nanomaterials such as carbon nanotubes (CNTs) and graphene have been developed rapidly. Researchers have found that nanomaterials exhibit extraordinary characteristics in ultrafast photonics, such as the low saturation intensity of CNTs, zero-band gap of graphene, and extremely high modulation depth of the topological insulator nano-films. Since graphene was first reported as an SA in 2009, many other nanomaterials have been successively explored, resulting in the rapid development of novel nanomaterial-based SAs. In this paper, we classified the nanomaterials used in SA mode-locking technology at 1.5μm and reviewed their research progress with a particular focus on nonlinear optical properties, integration strategies, and applications in the field of ultrafast photonics.

Conformal Graphene Directly Synthesized on a Femtosecond Laser-Scribed In-Fiber Microstructure for High-Energy Ultrafast Optical Pulses

Despite extensive efforts to explore femtosecond lasers functionalized by nonlinear graphene (Gf) that relies on the traditional transfer process, maximizing the efficiency, customizing the nonlinear interaction, and minimizing the optical loss remain critical challenges, especially in high-energy pulse generation. We demonstrate an ultrafast nonlinear all-fiber device based on conformal Gf directly synthesized in three dimensions on the surface of an in-fiber microstructure. A femtosecond laser-induced selective etching process is used to fabricate a customized microstructure that ensures the minimum but efficient laser-Gf interaction as well as possesses excellent surface conditions to suppress absorption and scattering loss. Conformal Gf is prepared by a spatial diffusion-based atomic carbon spraying process that enables nanocrystals to be synthesized homogeneously even onto the complex surface of the microstructure. The demonstration of high-energy pulses from the Gf saturable absorber highlights its simple, process-efficient, adjustable, and robust performance. The resultant hyperbolic secant pulses display individual pulse energy and peak power of up to 13.2 nJ and 20.17 kW, respectively.

Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

Wide-spectral saturable absorption (SA) in low-dimensional (LD) nanomaterials such as zero-, one-, and two-dimensional materials has been proven experimentally with outstanding results, including low saturation intensity, deep modulation depth, and fast carrier recovery time. LD nanomaterials can therefore be used as SAs for mode-locking or Q-switching to generate ultrafast fiber laser pulses with a high repetition rate and short duration in the visible, near-infrared, and mid-infrared wavelength regions. Here, we review the recent development of emerging LD nanomaterials as SAs for ultrafast mode-locked fiber laser applications in different dispersion regimes such as anomalous and normal dispersion regimes of the laser cavity operating in the near-infrared region, especially at ~1550 nm. The preparation methods, nonlinear optical properties of LD SAs, and various integration schemes for incorporating LD SAs into fiber laser systems are introduced. In addition to these, externally (electrically or optically) controlled pulsed fiber laser behavior and other characteristics of various LD SAs are summarized. Finally, the perspectives and challenges facing LD SA-based mode-locked ultrafast fiber lasers are highlighted.

Excessively tilted fiber grating based Fe3O4 saturable absorber for passively mode-locked fiber laser

A novel approach to saturable absorber (SA) formation is presented by taking advantage of the mode coupling property of excessively tilted fiber grating (Ex-TFG). Stable mode-locked operation can be conveniently achieved based on the interaction between Ex-TFG coupled light and deposited ferroferric-oxide (Fe₃O₄) nanoparticles. The central wavelength, bandwidth and single pulse duration of the output are 1595 nm, 4.05 nm, and 912 fs, respectively. The fiber laser exhibits good long-term stability with signal-to-noise ratio (SNR) of 67 dB. For the first time, to the best of our knowledge, Ex-TFG based Fe₃O₄ SA for mode-locked fiber laser is demonstrated.

Prospects for strongly coupled atom-photon quantum nodes

We discuss the trapping of cold atoms within microscopic voids drilled perpendicularly through the axis of an optical waveguide. The dimensions of the voids considered are between 1 and 40 optical wavelengths. By simulating light transmission across the voids, we find that appropriate shaping of the voids can substantially reduce the associated loss of optical power. Our results demonstrate that the formation of an optical cavity around such a void could produce strong coupling between the atoms and the guided light. By bringing multiple atoms into a single void and exploiting collective enhancement, cooperativities ~400 or more should be achievable. The simulations are carried out using a finite difference time domain method. Methods for the production of such a void and the trapping of cold atoms within it are also discussed.

WS(2)/fluorine mica (FM) saturable absorbers for all-normal-dispersion mode-locked fiber laser

The report firstly propose a new WS(2) absorber based on fluorine mica (FM) substrate. The WS(2) material was fabricated by thermal decomposition method. The FM was stripped into one single layer as thin as 20 μm and deposited WS(2) on it, which can be attached to the fiber flank without causing the laser deviation. Similar to quartz, the transmission rate of FM is as high as 90% at near infrared wavelength from one to two micrometers. Furthermore, FM is a highly elastic material so that it is not easy to break off even its thickness was only 20 μm. On the contrary, quartz is hard to be processed and easy to break off when its thickness is less than 100 μm. Compared to organic matrix such as polyvinyl alcohol (PVA), FM has higher softening temperature, heat dissipation and laser damage threshold than those of organic composites. In our work, the modulation depth (MD) and non-saturable losses (NLs) of this kind of saturable absorber were measured to be 5.8% and 14.8%, respectively. The WS(2)/FM absorber has a high damage threshold of 406 MW/cm(2), two times higher than that of WS(2)/PVA. By incorporating the saturable absorber into Yb-doped fiber laser cavity, a mode-locked fiber laser was achieved with central wavelength of 1052.45 nm. The repetition rate was 23.26 MHz and the maximum average output power was 30 mW. The long term stability of working was proved to be good too. The results indicate that WS(2)/FM film is a practical nonlinear optical material for photonic applications.

Tunable optical limiting optofluidic device filled with graphene oxide dispersion in ethanol

An optofluidic device with tunable optical limiting property is proposed and demonstrated. The optofluidic device is designed for adjusting the concentration of graphene oxide (GO) in the ethanol solution and fabricated by photolithography technique. By controlling the flow rate ratio of the injection, the concentration of GO can be precisely adjusted so that the optical nonlinearity can be changed. The nonlinear optical properties and dynamic excitation relaxation of the GO/ethanol solution are investigated by using Z-scan and pump-probe measurements in the femtosecond regime within the 1.5 μm telecom band. The GO/ethanol solution presents ultrafast recovery time. Besides, the optical limiting property is in proportion to the concentration of the solution. Thus, the threshold power and the saturated power of the optical limiting property can be simply and efficiently manipulated by controlling the flow rate ratio of the injection. Furthermore, the amplitude regeneration is demonstrated by employing the proposed optofluidic device. The signal quality of intensity-impaired femtosecond pulse is significantly improved. The optofluidic device is compact and has long interaction length of optical field and nonlinear material. Heat can be dissipated in the solution and nonlinear material is isolated from other optical components, efficiently avoiding thermal damage and mechanical damage.

Enhanced stability of nitrogen-sealed carbon nanotube saturable absorbers under high-intensity irradiation

Due to their broadband saturable absorption and fast response, carbon nanotubes have proven to be an excellent material for the modelocking of fiber lasers and have become a promising device for the implementation of novel laser configurations. However, it is imperative to address the issue of their long-term reliability under intense optical pulses before they can be exploited in widespread commercial applications. In this work, we study how carbon nanotubes degrade due to oxidation when exposed to high-intensity continuous-wave light and we demonstrate that by sealing the carbon nanotubes in a nitrogen gas, the damage threshold can be increased by over one order of magnitude. We then monitor over 24 hours the performance of the carbon nanotube saturable absorbers as the passive modelocking device of an erbium-doped fiber laser with intracavity powers ranging from 5 mW to 316 mW. We observe that when the carbon nanotubes are sealed in nitrogen environment, oxidation can be efficiently prevented and the laser can operate without any deterioration at intracavity powers higher than 300 mW. However, in the case where carbon nanotubes are unprotected (i.e. those directly exposed to the air in the environment), the nanotubes start to deteriorate at intracavity powers lower than 50 mW.

Optical deposition of graphene and carbon nanotubes in a fiber ferrule for passive mode-locked lasing

Mode-locked fiber lasers are currently undergoing a significant evolution towards higher pulse energies and shorter pulse durations. A key enabler in this progress has been the discovery of novel saturable absorbers (SA) such as carbon nanotubes (CNT) and graphene. The exceptional properties of CNTs as SA have been extensively studied in recent years. Graphene, a one atom thick planar sheet of carbon atoms arranged into a hexagonal lattice, has been recently proposed as an alternative to CNTs in several photonics applications. Here, we propose a method for the integration of graphene into a fiber ferrule using an optical deposition technique, which has been also employed for the deposition of CNT directly on the core of a fiber edge and in tapered fibers. We investigate and compare the optical properties of CNT-SA and graphene-SA fabricated by this optical deposition technique. Soliton-like, mode-locked lasing is confirmed using an erbium doped optical fiber in an all-fiber ring cavity laser configuration.

All-fiber Er-doped dissipative soliton laser based on evanescent field interaction with carbon nanotube saturable absorber

We report on an Er-doped fiber pulse laser at large net normal dispersion cavity by employing a dispersion compensating fiber in combination with a single-walled carbon nanotube (SWCNT) saturable absorber. A SWCNT/polymer composite film uniformly spin-coated on the side-polished fiber is prepared for robust and efficient nonlinear interaction with evanescent fields in the waveguide expecting increase of optical and thermal damage threshold compared to previously reported direct coating of SWCNTs on fiber ferrules. The fabricated dissipative soliton fiber laser exhibits high average output power of 55.6 mW, corresponding to pulse energy about 2.35 nJ. Highly chirped 5.8 ps pulses are generated with a spectral bandwidth of 13.9 nm and compressed down to 226 fs using additional length of conventional optical fiber at extra-cavity.

Passive mode-locked lasing by injecting a carbon nanotube-solution in the core of an optical fiber

In this paper, we propose a saturable absorber (SA) device consisting on an in-fiber micro-slot inscribed by femtosecond laser micro fabrication, filled by a dispersion of Carbon Nanotubes (CNT). Due to the flexibility of the fabrication method, efficient and simple integration of the mode-locking device directly into the optical fiber is achieved. Furthermore, the fabrication process offers a high level of control over the dimensions and location of the micro-slots. We apply this fabrication flexibility to extend the interaction length between the CNT and the propagating optical field along the optical fiber, hence enhancing the nonlinearity of the device. Furthermore, the method allows the fabrication of devices that operate by either a direct field interaction (when the central peak of the propagating optical mode passes through the nonlinear media) or an evanescent field interaction (only a fraction of the optical mode interacts with the CNT). In this paper, several devices with different interaction lengths and interaction regimes are investigated. Self-starting passively modelocked laser operation with an enhanced nonlinear interaction is observed using CNT-based SAs in both interaction regimes. This method constitutes a simple and suitable approach to integrate the CNT into the optical system as well as enhancing the optical nonlinearity of CNT-based photonic devices.

Femtosecond mode-locked fiber laser employing a hollow optical fiber filled with carbon nanotube dispersion as saturable absorber

We propose a novel in-line saturable absorber incorporating a hollow optical fiber (HOF) filled with single-walled carbon nanotube (SWCNT) dispersion. The evanescent field of the propagating light in the ring core interacts with the SWCNT/polymer composite distributed over the whole length of the HOF. The proposed saturable absorber with all-fiber format offers the robust and long nonlinear interaction along the waveguide direction expecting the increase of the threshold for optical and thermal damages with simple fabrication process. Low concentration SWCNT/polymer composite exhibiting very broadband resonant absorption around 1.5 microm with low scattering loss is prepared and based on this, we successfully demonstrate the passively mode-locked fiber laser including the SWCNT-filled HOF where the spectral bandwidth and the pulse duration of the laser output are 5.5 nm and 490 fs, respectively, with a repetition rate of 18.5 MHz.

Polarization-maintaining, high-energy, wavelength-tunable, Er-doped ultrashort pulse fiber laser using carbon-nanotube polyimide film

A high-energy, wavelength-tunable, all-polarization-maintaining Er-doped ultrashort fiber laser was demonstrated using a polyimide film dispersed with single-wall carbon nanotubes. A variable output coupler and wavelength filter were used in the cavity configuration, and high-power operation was demonstrated. The maximum average power was 12.6 mW and pulse energy was 585 pJ for stable single-pulse operation with an output coupling ratio as high as 98.3%. Wide wavelength-tunable operation at 1532-1562 nm was also demonstrated by controlling the wavelength filter. The RF amplitude noise characteristics were examined in terms of their dependence on output coupling ratio and oscillation wavelength.