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

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.

Optical Fibers Embedded with As-Grown Carbon Nanotubes for Ultrahigh Nonlinear Optical Responses

Photonic crystal fiber (PCF) embedded with functional materials has demonstrated diverse applications ranging from ultrafast lasers, optical communication to chemical sensors. Many efforts have been made to fabricating carbon nanotube (CNT) based optical fibers by ex situ transfer method; however, often suffer poor uniformity and coverage. Here, the direct growth of CNTs on the inner walls of PCFs by the chemical vapor deposition (CVD) method is reported. A two-step growth method is developed to control the narrow diameter distribution of CNTs to ensure desirable nanotube optical transitions. In the as-fabricated CNT- embedded fiber, third-harmonic generation (THG) has been enhanced by ≈15 times compared with flat CNT film on fused silica. A dual-wavelength all-fiber mode-locked ultrafast laser (≈1561 and ≈1064 nm) is further demonstrated by integrating the 1.36±0.15 nm-diameter CNTs into two kinds of photonic bandgap hollow core PCF (named HC-1550 and HC-1060) as saturable absorbers, using their S11 (≈0.7 eV) and S22 (≈1.2 eV) interband transition respectively. The fiber laser shows stable output of ≈10 mW, ≈800 fs pulse width, and ≈71 MHz repetition rate at 1561 nm wavelength. These results can enable the large-scale applications of CNTs in PCF-based optical devices.

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.

Optical properties and applications of molybdenum disulfide/SiO2 saturable absorber fabricated by sol-gel technique

We investigate a new type of molybdenum disulfide (MoS₂)-doped sol-gel glass saturable absorber (SA) fabricated by sol-gel technique. The reagents used for the sol-gel glass contain Tetraethyl orthosilicate (TEOS), ethanol, water, and hydrochloric acid. Different from the traditional ways of fabricating SAs, the MoS₂ in our method is encapsulated by inorganic sol-gel glass instead of polymer compound with low laser damage resistance, which greatly increases the optical damage threshold of MoS₂ SA. The MoS₂-doped sol-gel glass as an SA is experimentally demonstrated in a passively mode-locked ytterbium-doped fiber laser (YDFL). Stable mode-locked pulse trains are successfully generated in the normal dispersion regime with a pulse width of 13.8 ps and the average output power of 34.6 mW. The fluctuation of the central wavelength and spectral bandwidth is as low as 0.9% in one week, which indicates that the mode-locking state has good environmental stability. To the best of our knowledge, it is the first example of sol-gel glass SA for ultrafast pulses generated in YDFL, which potentially gives a new approach to improve optical damage threshold and long-term working stability for broadband absorbers.

Revealing the nature of morphological changes in carbon nanotube-polymer saturable absorber under high-power laser irradiation

Composites of single-walled carbon nanotubes (SWNTs) and water-soluble polymers (WSP) are the focus of significant worldwide research due to a number of applications in biotechnology and photonics, particularly for ultrashort pulse generation. Despite the unique possibility of constructing non-linear optical SWNT-WSP composites with controlled optical properties, their thermal degradation threshold and limit of operational power remain unexplored. In this study, we discover the nature of the SWNT-polyvinyl alcohol (PVA) film thermal degradation and evaluate the modification of the composite properties under continuous high-power ultrashort pulse laser operation. Using high-precision optical microscopy and micro-Raman spectroscopy, we have examined SWNT-PVA films before and after continuous laser radiation exposure (up to 40 hours) with a maximum optical fluence of 2.3 mJ·cm⁻². We demonstrate that high-intensity laser radiation results in measurable changes in the composition and morphology of the SWNT-PVA film due to efficient heat transfer from SWNTs to the polymer matrix. The saturable absorber modification does not affect the laser operational performance. We anticipate our work to be a starting point for more sophisticated research aimed at the enhancement of SWNT-PVA films fabrication for their operation as reliable saturable absorbers in high-power ultrafast lasers.

Double-Wall Carbon Nanotube Hybrid Mode-Locker in Tm-doped Fibre Laser: A Novel Mechanism for Robust Bound-State Solitons Generation

The complex nonlinear dynamics of mode-locked fibre lasers, including a broad variety of dissipative structures and self-organization effects, have drawn significant research interest. Around the 2 μm band, conventional saturable absorbers (SAs) possess small modulation depth and slow relaxation time and, therefore, are incapable of ensuring complex inter-pulse dynamics and bound-state soliton generation. We present observation of multi-soliton complex generation in mode-locked thulium (Tm)-doped fibre laser, using double-wall carbon nanotubes (DWNT-SA) and nonlinear polarisation evolution (NPE). The rigid structure of DWNTs ensures high modulation depth (64%), fast relaxation (1.25 ps) and high thermal damage threshold. This enables formation of 560-fs soliton pulses; two-soliton bound-state with 560 fs pulse duration and 1.37 ps separation; and singlet+doublet soliton structures with 1.8 ps duration and 6 ps separation. Numerical simulations based on the vectorial nonlinear Schr¨odinger equation demonstrate a transition from single-pulse to two-soliton bound-states generation. The results imply that DWNTs are an excellent SA for the formation of steady single- and multi-soliton structures around 2 μm region, which could not be supported by single-wall carbon nanotubes (SWNTs). The combination of the potential bandwidth resource around 2 μm with the soliton molecule concept for encoding two bits of data per clock period opens exciting opportunities for data-carrying capacity enhancement.

All-fiber mode-locked laser oscillator with pulse energy of 34 nJ using a single-walled carbon nanotube saturable absorber

We demonstrate a dissipative soliton fiber laser with high pulse energy (>30 nJ) based on a single-walled carbon nanotube saturable absorber (SWCNT-SA). In-line SA that evanescently interacts with the high quality SWCNT/polymer composite film was fabricated under optimized conditions, increasing the damage threshold of the saturation fluence of the SA to 97 mJ/cm(2). An Er-doped mode-locked all-fiber laser operating at net normal intra-cavity dispersion was built including the fabricated in-line SA. The laser stably delivers linearly chirped pulses with a pulse duration of 12.7 ps, and exhibits a spectral bandwidth of 12.1 nm at the central wavelength of 1563 nm. Average power of the laser output is measured as 335 mW at an applied pump power of 1.27 W. The corresponding pulse energy is estimated to be 34 nJ at the fundamental repetition rate of 9.80 MHz; this is the highest value, to our knowledge, reported in all-fiber Er-doped mode-locked laser using an SWCNT-SA.

Mode-locking of Er-doped fiber laser using a multilayer MoS2 thin film as a saturable absorber in both anomalous and normal dispersion regimes

Application of a multilayer Molybdenum Disulfide (MoS2) thin film as a saturable absorber was experimentally demonstrated by realizing a stable and robust passive mode-locked fiber laser via the evanescent field interaction between the light and the film. The MoS2 film was grown by chemical vapor deposition, and was then transferred to a side polished fiber by a lift-off method. Intensity-dependent optical transmission through the MoS2 thin film on side polished fiber was experimentally observed showing efficient saturable absorption characteristics. Using erbium doped fiber as an optical gain medium, we built an all-fiber ring cavity, where the MoS2 film on the side polished fiber was inserted as a saturable absorber. Stable dissipative soliton pulse trains were successfully generated in the normal dispersion regime with a spectral bandwidth of 23.2 nm and the pulse width of 4.98 ps. By adjusting the total dispersion in the cavity, we also obtained soliton pulses with a width of 637 fs in the anomalous dispersion regime near the lasing wavelength λ = 1.55 μm. Detailed and systematic experimental comparisons were made for stable mode locking of an all-fiber laser cavity in both the normal and anomalous regimes.

Pulse width shaping of passively mode-locked soliton fiber laser via polarization control in carbon nanotube saturable absorber

We report the continuous control of the pulse width of a passively mode-locked fiber laser via polarization state adjustment in a single-walled carbon nanotube saturable absorber (SWCNT-SA). The SWCNT, coated on the side-polished fiber, was fabricated with optimized conditions and used for stable mode-locking of the fiber laser without Q-switching instabilities for any polarization state of the laser intra-cavity. The 3-dB spectral bandwidth of the mode-locked pulses can be continuously tuned from 1.8 nm to 8.5 nm with the polarization control for a given laser cavity length and applied pump power. A pulse duration varying from 470 fs to 1.6 ps was also observed with a change in the spectral bandwidth. The linear and the nonlinear transmission properties of the SA were analyzed, and found to exhibit different modulation depths depending on the input polarization state in the SA. The largest modulation depth of the SA was observed at the polarization state of the transverse electric mode that delivers shortest pulses at the laser output.

Graphene-filled hollow optical fiber saturable absorber for efficient soliton fiber laser mode-locking

We demonstrate a novel in-line saturable absorber based on hollow optical fiber (HOF) filled with graphene composite for high power operation of mode-locked fiber laser. Evanescent field of guided mode propagating in few centimeter-long HOF interacts with the graphene/polyvinyl acetate (PVAc) composite, which enables robust and efficient nonlinear absorption leading to stable passive mode-locking. The mode-locked fiber laser generates soliton pulses with 5.9-nm spectral bandwidth and its maximum output power is measured up to 80 mW. We also observe passive harmonic mode-locking of soliton laser delivering stable pulses with a repetition rate of 506.9 MHz at 33rd harmonics.

Low noise GHz passive harmonic mode-locking of soliton fiber laser using evanescent wave interaction with carbon nanotubes

Passive harmonic mode-locking in soliton fiber laser is presented with excellent noise characteristics by employing a single-walled carbon nanotubes saturable absorber designed to interact with evanescent wave of the laser field. The 34(th) harmonic mode-locking pulses at 943.16 MHz repetition rate were stably generated with 18 mW output power, >50 dB side-mode suppression and -140 dB/Hz relative intensity noise. Soliton energy control with polarization controller further increased the harmonic order to 61st, 1.692 GHz, but with compromised performance. Scaling to higher-order harmonic mode-locking is discussed for practical application in optical communication system.

Linear and nonlinear optical properties of carbon nanotube-coated single-mode optical fiber gratings

Single-wall carbon nanotube deposition on the cladding of optical fibers has been carried out to fabricate an all-fiber nonlinear device. Two different nanotube deposition techniques were studied. The first consisted of repeatedly immersing the optical fiber into a nanotube supension, increasing the thickness of the coating in each step. The second deposition involved wrapping a thin film of nanotubes around the optical fiber. For both cases, interaction of transmitted light through the fiber core with the external coating was assisted by the cladding mode resonances of a tilted fiber Bragg grating. Ultrafast nonlinear effects of the nanotube-coated fiber were measured by means of a pump-probe pulses experiment.

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.

A 113 fs fiber laser operating at 1.56 mum using a cascadable film-type saturable absorber with P3HT-incorporated single-wall carbon nanotubes coated on polyamide

We successfully fabricated a cascadable film-type single-wall carbon nanotube (SWNT) saturable absorber coated on aromatic polyamide film, in which the saturable absorption effect can be controlled with the number of films. A conductive polymer P3HT (poly-3-hexylthiophene) was adopted to obtain a uniform SWNT solution. We applied saturable absorber films to a passively mode-locked fiber laser and successfully generated a 113 fs, 42 MHz pulse by inserting two film layers between fiber connectors in the cavity.