Junsu Lee

A femtosecond pulse fiber laser at 1935 nm using a bulk-structured Bi 2 Te 3 topological insulator

We experimentally demonstrate a femtosecond mode-locked, all-fiberized laser that operates in the 2 μm region and that incorporates a saturable absorber based on a bulk-structured bismuth telluride (Bi(2)Te(3)) topological insulator (TI). Our fiberized saturable absorber was prepared by depositing a mechanically exfoliated, ~30 μm-thick Bi(2)Te(3) TI layer on a side-polished optical fiber platform. The bulk crystalline structure of the prepared Bi(2)Te(3) layer was confirmed by Raman and X-ray photoelectron spectroscopy measurements. The modulation depth of the prepared saturable absorber was measured to be ~20.6%. Using the saturable absorber, it is shown that stable, ultrafast pulses with a temporal width of ~795 fs could readily be generated at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity. This experimental demonstration confirms that bulk structured, TI-based saturable absorbers can readily be used as an ultra-fast mode-locker for 2 μm lasers.

A femtosecond pulse erbium fiber laser incorporating a saturable absorber based on bulk-structured Bi 2 Te 3 topological insulator

We experimentally demonstrate the use of a bulk-structured Bi(2)Te(3) topological insulator (TI) as an ultrafast mode-locker to generate femtosecond pulses from an all-fiberized cavity. Using a saturable absorber based on a mechanically exfoliated layer about 15 μm thick deposited onto a side-polished fiber, we show that stable soliton pulses with a temporal width of ~600 fs can readily be produced at 1547 nm from an erbium fiber ring cavity. Unlike previous TI-based mode-locked laser demonstrations, in which high-quality nanosheet-based TIs were used for saturable absorption, we chose to use a bulk-structured Bi(2)Te(3) layer because it is easy to fabricate. We found that the bulk-structured Bi(2)Te(3) layer can readily provide sufficient nonlinear saturable absorption for femtosecond mode-locking even if its modulation depth of ~15.7% is much lower than previously demonstrated nanosheet-structured TI-based saturable absorbers. This experimental demonstration indicates that high-crystalline-quality atomic-layered films of TI, which demand complicated and expensive material processing facilities, are not essential for ultrafast laser mode-locking applications.

Mode-locked, 1.94-μm, all-fiberized laser using WS₂ based evanescent field interaction.

We demonstrate the use of an all-fiberized, mode-locked 1.94 μm laser with a saturable absorption device based on a tungsten disulfide (WS2)-deposited side-polished fiber. The WS2 particles were prepared via liquid phase exfoliation (LPE) without centrifugation. A series of measurements including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the prepared particles had thick nanostructures of more than 5 layers. The prepared saturable absorption device used the evanescent field interaction mechanism between the oscillating beam and WS2 particles and its modulation depth was measured to be ~10.9% at a wavelength of 1925 nm. Incorporating the WS2-based saturable absorption device into a thulium-holmium co-doped fiber ring cavity, stable mode-locked pulses with a temporal width of ~1.3 ps at a repetition rate of 34.8 MHz were readily obtained at a wavelength of 1941 nm. The results of this experiment confirm that WS2 can be used as an effective broadband saturable absorption material that is suitable to passively generate pulses at 2 μm wavelengths.

A Q-switched, mode-locked fiber laser using a graphene oxide-based polarization sensitive saturable absorber

We demonstrate the use of a graphene oxide (GO)-deposited D-shaped fiber as a polarization sensitive saturable absorber for the implementation of a stable Q-switched, mode-locked fiber laser. Using both features of nonlinear saturable absorption and large polarization dependence loss of GO-deposited D-shaped fiber, stable Q-switched mode-locked pulses are readily obtained from an erbium-doped fiber ring laser through simple intra-cavity polarization control under a fixed pump power. It is shown that bursts of sub-picosecond, mode-locked pulses with a Q-switching envelope of a ~2.63 μs temporal width and a ~71.3 kHz repetition rate can readily be generated from the laser. It is also demonstrated that the operating regime of the pulsed laser can be readily changed between mode-locking, Q-switched mode-locking, and Q-switching simply by changing the polarization state of the oscillated beam within the cavity.

All-fiberized, passively Q-switched 1.06 μm laser using a bulk-structured Bi2Te3 topological insulator

We experimentally demonstrate the use of a bulk-structured Bi2Te3 topological insulator (TI) deposited on a side-polished fiber as an effective saturable absorber for the implementation of an all-fiberized passively Q-switched 1.06 μm fiber laser. Unlike previous Q-switched laser demonstrations based on nanosheet-based TIs, which require complicated and sophisticated fabrication procedures, a ~9.1 μm-thick bulk-structured Bi2Te3 TI film that was prepared using a mechanical exfoliation method was chosen to be used in this experimental demonstration due to ease of fabrication. The modulation depth of our prepared bulk-structured Bi2Te3 TI-deposited side-polished fiber was measured to be ~2.5% at 1.06 μm. By incorporating such an absorber in an all-fiberized ytterbium-doped fiber ring cavity, stable Q-switched pulses were obtained through evanescent field interaction between the oscillated beam and the Bi2Te3 TI film. The temporal width and repetition rate of the output pulses were tunable from ~1 to ~1.3 μs and from ~77 to ~35 kHz, respectively, depending on the pump power. Through a performance comparison of our laser with recently demonstrated Q-switched 1 μm fiber lasers using carbon nanotubes, graphene, and nano-structured TIs, our bulk-structured Bi2Te3 TI-based saturable absorber is shown to have passive Q-switching performance comparable to saturable absorbers based on carbon nanotubes or nano-structured TIs.

Numerical study on the minimum modulation depth of a saturable absorber for stable fiber laser mode locking

We conducted a numerical study to investigate the minimum modulation depth of a saturable absorber that is essentially required for stable optical pulses to be generated from a passively mode-locked fiber laser cavity. More specifically, an extended nonlinear Schrodinger equation was numerically solved in order to analyze the impact of the cavity group velocity dispersion (GVD), cavity χ(3) nonlinearity, and saturation gain of an active fiber on the minimum modulation depth that is required. The net cavity GVD is shown to significantly influence the required minimum modulation depth level of a saturable absorber. A cavity with little net anomalous dispersion was found to be readily mode locked through the use of a saturable absorber with a very small amount of modulation depth. Furthermore, the reason that a fiber laser cavity with a zero cavity GVD becomes unstable and needs a much higher modulation depth was investigated, and such a condition was found to be associated with the fiber χ(3) nonlinearity. The minimum modulation depth was also shown to vary according to the saturation gain level of an active fiber.

Large energy, all-fiberized Q-switched pulse laser using a GNRs/PVA saturable absorber

We experimentally demonstrate the use of gold nanorods (GNRs)/PVA composite-deposited side-polished fiber as an efficient saturable absorber for use in an all-fiberized, high-energy Q-switched fiber laser. The modulation depth of the prepared saturable absorber was of ~7.5% at wavelength of 1549.8 nm. Stable Q-switched pulses with a single pulse energy of ~2.56 μJ were readily generated from a fiber Bragg grating-based Fabry-Perot cavity at a pump power of ~229 mW. The pump efficiency was estimated at ~19.2%, and the impact of the variation in the cavity length with respect to the output pulse characteristics was investigated with fixed pump power. The output performance of the proposed all-fiberized laser was compared to that of Q-switched, 1.5-μm fiber lasers incorporating other types of nonlinear optical nanomaterials that had been previously proposed.

Passively Q-Switched 1.89-μm Fiber Laser Using a Bulk-Structured Bi 2 Te 3 Topological Insulator

We experimentally demonstrate that a bulk-structured Bi2Te3 topological insulator (TI) film deposited on a side-polished fiber can act as an effective Q-switch for a 1.89-μm laser. Our bulk-structured Bi2Te3 TI film with a thickness of ~31 μm, was prepared using a mechanical exfoliation method, and the fabricated film was transferred onto a side-polished SM2000 fiber to form a fiberized saturable absorber based on evanescent field interaction. By incorporating the saturable absorber into a thulium (Tm)-holmium (Ho) co-doped fiber-based ring cavity, it is shown that Q-switched pulses with a minimum temporal width of ~1.71 μs can readily be produced at a wavelength of 1.89 μm. The output pulse repetition rate was tunable from ~35 to ~60 kHz depending on the pump power. The maximum output pulse energy was ~11.54 nJ at a pump power of 250 mW. The output performance of our laser is compared to that of the 1.98-μm Q-switched fiber laser based on a nanosheet-based Bi2Se3 TI demonstrated previously by Luo et al.

Ultrawideband doublet pulse generation based on nonlinear polarization rotation of an elliptically polarized beam and its distribution over a fiber/wireless link.

Proposed herein is an alternative photonic scheme for the generation of a doublet UWB pulse, which is based on the nonlinear polarization rotation of an elliptically polarized probe beam. The proposed scheme is a modified optical-fiber Kerr shutter that uses an elliptically polarized probe beam together with a linearly polarized control beam. Through theoretical analysis, it was shown that the optical-fiber-based Kerr shutter is capable of producing an ideal transfer function for the successful conversion of input Gaussian pulses into doublet pulses under special elliptical polarization states of the probe beam. An experimental verification was subsequently carried out to verify the working principle. Finally, the system performance of the generated UWB doublet pulses was assessed by propagating them over a 25-km-long standard single-mode fiber link, followed by wireless transmission. Error-free transmission was successfully achieved.

Harmonically mode-locked femtosecond fiber laser using non-uniform, WS2-particle deposited side-polished fiber

We investigated the feasibility of using a WS2-deposited side-polished fiber as a harmonic mode-locker to produce a femtosecond fiber laser with a frequency of 1.51 GHz. Our work focuses on using a side-polished fiber platform with non-uniform WS2 particles prepared through liquid phase exfoliation method without centrifugation. Femtosecond optical pulses were generated from an all-fiberized erbium-doped fiber-based ring cavity by increasing the pump power to achieve a tunable pulse repetition rate from 14.57 MHz to 1.51 GHz (104th harmonic). The characteristics of the output pulse were systematically investigated to analyze the pulse repetition rate, harmonic order, average output power, pulse energy, and pulse width as a function of the pump power. The output performance of the laser was compared to that of a laser based on a microfiber-based WS2 film SA described in (Yan et al 2015 Opt. Mater. Express 5 479–89). This experimental demonstration reaffirms that a side-polished fiber is an effective platform to implement an ultrafast harmonic mode-locker, and non-uniform WS2 particles prepared via simple liquid phase exfoliation method without centrifugation provide a suitable saturable absorption response at 1.55 μm.