High-efficiency and broadband tunable Raman fiber laser in a chalcogenide fiber based on the Fresnel reflection.

A high-efficiency and broadband tunable chalcogenide fiber Raman laser with the Fabry-Perot (F-P) cavity formed by the Fresnel reflection was established. A maximum average power slope efficiency of around 43% and a maximum output peak power of about 2.9 W at 2148 nm were demonstrated by using a 2 µm nanosecond pump source. The laser shows a broadened pulse width of 674 ns and a broadband tunability of the central wavelength from 2100 to 2186 nm. The Raman Fabry-Perot cavity constituted by the Fresnel reflection from chalcogenide fiber endfaces can operate at any wavelength without the aid of any additional optical feedback element. This will facilitate the realization of fiber lasers with excellent performance and compact system, especially in the mid-infrared region.

On-chip Er-doped Ta2O5 waveguide amplifiers with a high internal net gain.

We designed and fabricated a double-layered structure Er3+:Ta2O5 waveguide and investigated its optical amplification performance in C band. The pump laser threshold for zero gain at 1533 nm was 2.5 mW, and the internal net gain was ∼4.63 dB/cm for a lunched pump power of 36.1 mW at 980 nm and signal input power of -30.0 dBm (1 µW). The relationship between the internal gain and the signal input power was also investigated, and a large internal net gain of 10.58 dB/cm was achieved at a signal input power of ∼-47.1 dBm. The results confirm the potentials of the use of Ta2O5 as a host material for optical waveguide amplification.

Broadband SnS/Te photodetector to long-wavelength infrared: breaking the spectrum limit through alloy engineering.

Materials based on group IV chalcogenides, are considered to be one of the most promising materials for high-performance, broadband photodetectors due to their wide bandgap coverage, intriguing chemical bonding and excellent physical properties. However, the reported photodetectors based on SnS are still worked at relatively narrow near-infrared band (as far as 1550 nm) hampered by the nonnegligible bandgap of 1.1-1.5 eV. Here, a novel photodetector based on Te alloyed SnS thin film was demonstrated with an ultra-broadband response up to 10.6 µm. By controlling the Te alloyed concentration in SnS increasing to 37.64%, the bandgap narrows to 0.23 eV, exhibiting a photoresponse potential at long-wavelength infrared excitation. Our results show Te-alloying can remarkably enhance the detection properties of SnS/Te photodetectors. The photoresponsivity and detectivity of 1.59 mA/W and 2.3 × 108 Jones were realized at 10.6 µm at room temperature. Moreover, the nonzero photogain was observed generated by nonlinearly increased photocurrent density, resulting in a superlinear dependency between photoresponsivity and light intensity. Our studies successfully broaden photoresponse spectrum of SnS toward the mid-infrared range for the first time. It also suggests that alloying is an effective technique for tuning the band edges of group IV chalcogenides, contributing deep implications for developing future optoelectronic applications.

Simulation of the generation conditions and influence parameters of a self-mode-locked erbium-doped fiber laser.

The generation conditions and influence parameters of self-mode-locked pulses in fiber lasers are theoretically studied. By establishing the simulation model of a self-mode-locked erbium-doped fiber laser (EDFL) with a high-concentration erbium-doped fiber-based saturable absorber (SA), the effect of gain saturation energy, orientation angles of the polarizer and analyzer with respect to the fast axis of the fiber, laser coupling output ratio, dispersion value and condition on the self-mode-locked pulse generation and performances are quantitatively analyzed. The result shows that a low laser coupling output ratio can help the formation of a self-mode-locked pulse. The anomalous dispersion self-mode-locked EDFL has a relative high tolerance for dispersion value change but requires high gain energy for mode-locked pulse generation. The normal dispersion one possesses a low mode-locked pulse formation threshold but is relative polarization sensitive. This study is of important reference significance for the investigation of mode-locked fiber lasers.

Generation of watt-level supercontinuum covering 2-6.5 µm in an all-fiber structured infrared nonlinear transmission system.

We demonstrate a watt-level mid-infrared supercontinuum source, with the spectrum covering the infrared region from 2 to 6.5 µm, in an all-fiber structured laser transmission system. To further improve the SC spectral bandwidth, power and system compactness in the follow-up As2S3 fiber, we theoretically and experimentally explored some knotty problems that would potentially result in the As2S3 fiber end-facet failure and low SC output power during the high-power butt-coupling process and proposed an optimal coupling distance on the premise of the safety of As2S3 fiber end face. In addition, we also built a multi-pulse pumping model for the first time to more precisely estimate the SC spectral evolution in As2S3 fiber. This work will give an important reference to someone who is working on the all-fiber structured, high-power mid- and far-infrared supercontinuum source.

Metallic 2H-Tantalum Selenide Nanomaterials as Saturable Absorber for Dual-Wavelength Q-Switched Fiber Laser.

A novel 2H-phase transition metal dichalcogenide (TMD)-tantalum selenide (TaSe2) with metallic bandgap structure is a potential photoelectric material. A band structure simulation of TaSe2 via ab initio method indicated its metallic property. An effective multilayered TaSe2 saturable absorber (SA) was fabricated using liquid-phase exfoliation and optically driven deposition. The prepared 2H-TaSe2 SA was successfully used for a dual-wavelength Q-switched fiber laser with the minimum pulse width of 2.95 µs and the maximum peak power of 64 W. The repetition rate of the maximum pulse energy of 89.9 kHz was at the level of 188.9 nJ. The metallic 2H-TaSe2 with satisfactory saturable absorbing capability is a promising candidate for pulsed laser applications.

Size effect of WSe2 on red passively Q-switched fiber laser output performance.

Transition metal dichalcogenides (TMDs) possess a direct bandgap in the visible frequency range and can be applied as attractive visible saturable absorbers (SAs). In this paper, a new TMDs tungsten selenide (WSe2)-based red Q-switcher is fabricated and successfully used for Q-switched pulse generation in a red Pr3+-doped ZBLAN fiber laser. The passive Q-switching fiber laser at 635.2 nm generates a stable pulse with pulse duration of 504 ns and average output power of 4.91 mW as well as a tunable pulse repetition rate of 131.9-260.4 kHz. Furthermore, the size effect of WSe2 on the 635 nm passive Q-switching output performance is also investigated. Our work will provide certain significance for the optimization of passive Q-switching red fiber lasers based on TMDs.

Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers.

Passive Q-switching or mode-locking by placing a saturable absorber inside the laser cavity is one of the most effective and popular techniques for pulse generation. However, most of the current saturable absorbers cannot work well in the visible spectral region, which seriously impedes the progress of passively Q-switched/mode-locked visible pulsed fibre lasers. Here, we report a kind of visible saturable absorber-two-dimensional transition-metal dichalcogenides (TMDs, e.g. WS2, MoS2, MoSe2), and successfully demonstrate compact red-light Q-switched praseodymium (Pr(3+))-doped all-fibre lasers. The passive Q-switching operation at 635 nm generates stable laser pulses with ∼200 ns pulse duration, 28.7 nJ pulse energy and repetition rate from 232 to 512 kHz. This achievement is attributed to the ultrafast saturable absorption of these layered TMDs in the visible region, as well as the compact and all-fibre laser-cavity design by coating a dielectric mirror on the fibre end facet. This work may open a new route for next-generation high-performance pulsed laser sources in the visible (even ultraviolet) range.

Gold nanoparticles as a saturable absorber for visible 635 nm Q-switched pulse generation.

Gold nanoparticle (GNP) possesses saturable absorption bands in the visible region induced by surface plasmon resonance (SPR). We firstly applied the GNP as a visible saturable absorber (SA) for the red Q-switched pulse generation. The GNPs were embedded in polyvinyl alcohol (PVA) for film-forming and inserted into a praseodymium (Pr(3+))-doped fiber laser cavity to achieve 635 nm passive Q-switching. The visible 635 nm Q-switched fiber laser has a wide range of pulse-repetition-rate from 285.7 to 546.4 kHz, and a narrow pulse width of 235 ns as well as the maximum output power of 11.1 mW. The results indicate that the GNPs-based SA is available for pulsed operation in the visible spectral range.

Intermode beating mode-locking technique for O-band mixed-cascaded Raman fiber lasers.

A novel intermode beating mode-locking (IBML) technique combined with a cascaded Raman process is proposed to mode-lock an O-band two-cascaded Raman fiber laser. Using a 980-m-long phosphosilicate fiber pumped by a 1064 nm laser, the second-order Raman oscillation at 1319 nm is generated by the mixed-cascaded Raman shifts of P2O5 and SiO2. By precisely matching the intermode beating frequencies of the 1064 nm pump laser and the second-order Raman cavity frequency, harmonic mode-locking at 1319 nm is initiated. The dynamic process of the IBML operation in the cascaded Raman laser is experimentally investigated. The 131st-order harmonic mode-locking with a repetition rate of 27.247 MHz is very stable with the radio-frequency (RF) signal-to-noise ratio of >56 dB and the RF supermode-suppression ratio of >43 dB. The mode-locked pulses with the square profile are confirmed as the noise-like pulses by an autocorrelator. The IBML technique, in combination with the cascaded Raman process, could offer an exciting new prospect for obtaining simple, compact, and arbitrary-wavelength mode-locked laser sources.

Large-energy, wavelength-tunable, all-fiber passively Q-switched Er:Yb-codoped double-clad fiber laser with mono-layer chemical vapor deposition graphene.

We demonstrate a large-energy, wavelength-tunable, all-fiber passively Q-switched Er:Yb-codoped laser using a mono-layer chemical vapor deposition (CVD) graphene saturable absorber (SA). By exploiting the large laser gain of Er:Yb double-clad fiber and optimizing the coupling ratio of the output coupler, not only can the mono-layer CVD graphene SA be protected from oversaturation and thermal damage, but also a large pulse energy up to 1.05 µJ (corresponding to the average output power of 25.6 mW) is thus achieved. Using a tunable fiber Fabry-Perot filter, stable Q-switched pulses can operate with a tunable range from 1530.97 to 1546.92 nm, covering a wavelength range of ∼16 nm. The Q-switching states at the different lasing wavelengths have been observed and recorded. The Q-switched repetition rate and the pulse duration (with the minimum one of 2.6 µs) have been characterized as well. This is, to the best of our knowledge, the largest pulse energy from an all-fiber graphene Q-switched laser.

Self-mode-locked 2 µm Tm(3+)-doped double-clad fiber laser with a simple linear cavity.

We demonstrate the self-mode-locking operation of a thulium (Tm)-doped fiber laser (TDFL) with a simple linear cavity. Since the laser cavity does not include any specific mode-locker, we experimentally investigate and analyze the self-mode-locking mechanism. The mode-locking operation is attributed to the combination of the self-phase modulation effect and the weak saturable absorption of the high-concentration Tm-doped fiber. The mode-locked TDFL operates at a central wavelength of 1985.5 nm with the 3 dB spectral linewidth of 0.18 nm. The self-mode-locking generates a large pulse energy of 32.7 nJ with a pulsed repetition rate of 2.05 MHz and is stable with a radio-frequency signal-to-noise ratio of more than 54 dB. To the best of our knowledge, it is the first demonstration of a 2 µm Tm-doped fiber laser mode-locked by such technique.

High-energy passively Q-switched 2 µm Tm(3+)-doped double-clad fiber laser using graphene-oxide-deposited fiber taper.

We have demonstrated a high-energy Q-switched double-clad thulium-doped fiber laser (TDFL) using a graphene-oxide-deposited tapered fiber (GODTF) device as a saturable absorber operating at a wavelength of 2 µm for the first time. Because of the side-interaction of the graphene-oxide with the evanescent field on the taper waist, the GODTF devices have potential for offering high laser damage threshold. Using a 788 nm laser diode as the pump source, the TDFL generated stable single transverse mode Q-switched pulses with a single pulse energy of 6.71 µJ (corresponding to an average power of 302 mW) at a wavelength of 2032 nm. This is significantly higher than the highest pulse energy/average power from any rare-earth-doped fiber lasers employing a graphene or graphene-oxide based Q-switch so far. The demonstrated TDFL in this paper represents an encouraging step towards the practical applications of graphene or graphene-oxide based Q-switched 2 µm TDFLs.