Erbium doped materials are of great interest in thin film integrated optoelectronic technology, due to their Er3+ intra-4f emission at 1.54 μm, a standard telecommunication wavelength. Er-doped dielectric thin films can be used to fabricate planar optical amplifiers or lasers that can be integrated with other devices on the same chip. Semiconductors, such as silicon, can also be doped with erbium. In this case the Er may be excited through optically or electrically generated charge carriers. Er-doped Si light-emitting diodes may find applications in Si-based optoelectronic circuits. In this article, the synthesis, characterization, and application of several different Er-doped thin film photonic materials is described. It focuses on oxide glasses (pure SiO2, phosphosilicate, borosilicate, and soda-lime glasses), ceramic thin films (Al2O3, Y2O3, LiNbO3), and amorphous and crystalline silicon, all doped with Er by ion implantation. MeV ion implantation is a technique that is ideally suited to dope these materials with Er as the ion range corresponds to the typical micron dimensions of these optical materials. The role of implantation defects, the effect of annealing, concentration dependent effects, and optical activation are discussed and compared for the various materials.

Erbium implanted thin film photonic materials

Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

Graphene is an optical material of unusual characteristics because of its linearly dispersive conduction and valence bands and the strong interband transitions. It allows broadband light-matter interactions with ultrafast responses and can be readily pasted to surfaces of functional structures for photonic and optoelectronic applications. Recently, graphene-based optical modulators have been demonstrated with electrical tuning of the Fermi level of graphene. Their operation bandwidth, however, was limited to about 1 GHz by the response of the driving electrical circuit. Clearly, this can be improved by an all-optical approach. Here, we show that a graphene-clad microfiber all-optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps, limited only by the intrinsic carrier relaxation time of graphene. This modulator is compatible with current high-speed fiber-optic communication networks and may open the door to meet future demand of ultrafast optical signal processing.

/pdf/ultrafast-all-optical-graphene-modulator-29dpuypdgs.pdf

Ultrafast All-Optical Graphene Modulator

Under strong laser radiation, a Dirac material, the topological insulator (TI) Bi2Te3, exhibits an optical transmittance increase as a result of saturable absorption. Based on an open-aperture Z-scan measurement at 1550 nm, we clearly show that the TI, Bi2Te3 under our investigation, is indeed a very-high-modulation-depth (up to 95%) saturable absorber. Furthermore, a TI based saturable absorber device was fabricated and used as a passive mode locker for ultrafast pulse formation at the telecommunication band. This contribution unambiguously shows that apart from its fantastic electronic property, a TI (Bi2Te3) may also possess attractive optoelectronic property for ultrafast photonics.

Ultra-short pulse generation by a topological insulator based saturable absorber

Owing to their atomic layer thickness, strong light–material interaction, high nonlinearity, broadband optical response, fast relaxation, controllable optoelectronic properties, and high compatibility with other photonic structures, 2D materials, including graphene, transition metal dichalcogenides and black phosphorus, have been attracting increasing attention for photonic applications. By tuning the carrier density via electrical or optical means that modifies their physical properties (e.g., Fermi level or nonlinear absorption), optical response of the 2D materials can be instantly changed, making them versatile nanostructures for optical modulation. Here, up-to-date 2D material-based optical modulation in three categories is reviewed: free-space, fiber-based, and on-chip configurations. By analysing cons and pros of different modulation approaches from material and mechanism aspects, the challenges faced by using these materials for device applications are presented. In addition, thermal effects (e.g., laser induced damage) in 2D materials, which are critical to practical applications, are also discussed. Finally, the outlook for future opportunities of these 2D materials for optical modulation is given.

2D Materials for Optical Modulation: Challenges and Opportunities.

We demonstrate a compact Q-switched dual-wavelength erbium-doped fiber (EDF) laser based on graphene as a saturable absorber (SA). By optically driven deposition of graphene on a fiber core, the SA is constructed and inserted into a diode-pumped EDF laser cavity. Also benefiting from the strong third-order optical nonlinearity of graphene to suppress the mode competition of EDF, a stable dual-wavelength Q-switching operation has been achieved using a two-reflection peak fiber Bragg grating as the external cavity mirror. The Q-switched EDF laser has a low pump threshold of 6.5mW at 974nm and a wide range of pulse-repetition rate from 3.3 to 65.9kHz. The pulse duration and the pulse energy have been characterized. This is, to the best of our knowledge, the first demonstration of a graphene-based Q-switched laser.

Graphene-based passively Q-switched dual-wavelength erbium-doped fiber laser.

We experimentally confirm that graphene within fiber laser cavities can generate four-wave-mixing (FWM) by observing the laser spectral broadening and the transition from the single-longitudinal-mode oscillation to multiple-longitudinal-mode one. Then, by simultaneously exploiting the graphene-induced nonlinear FWM and its super-broadband saturable absorption, we further achieve for the first time to the best of our knowledge, multiwavelength Q-switched Yb3+- or Er3+-doped fiber lasers at 1 μm and 1.5 μm wavebands, respectively. Simultaneous 23-wavelength Q-switching oscillation with a wavelength spacing of 0.2 nm is stably generated at 1.5 μm waveband. The multiwavelength Q-switched pulses have the minimum pulse duration of 2.5 μs, the maximum pulse energy of 72.5 nJ and a wide range of pulse-repetition-rate (PRR) from 2.8 to 63.0 kHz. At 1 μm waveband, we also obtain five-wavelength simultaneous lasing in Q-switching regime with the pulse duration of ~ 3 μs, pulse energy of 10.3 nJ and PRR between 39.8 and 56.2 kHz.

Graphene-Induced Nonlinear Four-Wave-Mixing and Its Application to Multiwavelength Q-Switched Rare-Earth-Doped Fiber Lasers

We report continuous-wave laser operation at 698 nm in Pr3+-doped LiYF4 crystal using an InGaN laser diode emitting at 444 nm with a maximum output power of 760 mW By suppressing the oscillation at 640 and 721 nm, a maximum output power of 156 mW at 698 nm was obtained in a single transverse mode with a slope efficiency as high as 487% The beam quality factors M2 in the x and y directions were measured to be 14 and 12, respectively

Diode-pumped Pr^3+:LiYF_4 continuous-wave deep red laser at 698 nm

We propose and experimentally demonstrate a stable multiwavelength erbium-doped fiber (EDF) ring laser based on four-wave-mixing of atomic few-layer graphene. The compact laser cavity consists of 3-m EDF, an optically deposited ultrathin graphene device, and a polarization-maintaining fiber Sagnac loop filter. The comparison between the ring laser cavity without and with the graphene device shows that the highly nonlinear graphene can be helpful to mitigate the mode competition of EDF laser and stabilize the multiwavelength oscillation. Under the assistance of the graphene device, power-equalized eleven-channel simultaneous lasing with a wavelength-spacing of 0.54 nm is achieved at room temperature. This laser has a very high extinction ratio up to 57 dB and a narrow linewidth per channel of less than 0.01 nm.

Graphene-Assisted Multiwavelength Erbium-Doped Fiber Ring Laser

We have measured, for the first time, the complete 1.7µm excited state absorption spectrum in erbium doped glasses. The spectrum is obtained by measuring the gain of an erbium doped waveguide in the 1400-1800 nm region. Using the measured spectra, we estimate the 1.5µm uniform upconversion rate by calculating the spectral overlap between the 1.5µm emission and 1.7µm ESA cross sections. The technique is applied to erbium-doped, ion-exchangeable silicate glasses, yielding upconversion constants in the range of 1-10 x 10 -18 cm 3 /s.

/pdf/17-mu-m-excited-state-absorption-measurement-in-erbium-doped-5218te3vf8.pdf

17 mu-m excited-state absorption measurement in erbium-doped glasses

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