Experimental investigation of continuous-wave supercontinuum ring laser composed of clad-pumped Er/Yb codoped fiber and highly-nonlinear optical fiber

We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470 nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process.     

Resonant optical nonlinearity measurement of Yb(3+) / Al(3+) codoped optical fibers by use of a long-period fiber grating pair

A new method of measuring optical nonlinearity for resonant nonlinear optical fibers is proposed. A long-period fiber grating (LPG) pair was used to measure the nonlinear refractive index n(2) of a Yb(3+)/Al (3+) codoped optical fiber, which was spliced between the two LPGs, as the fiber was pumped with a laser diode. The nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm depended on the pump power. As the pump power increased, the nonlinear refractive index decreased. At launched pump powers of 2-8 mW, the nonlinear refractive index of the Yb(3+)/Al (3+) codoped fiber near 1580 nm was ~7.5x10(-15)m (2)/W .     

Enhanced upconverted photoluminescence in Er and Yb codoped ZnO nanocrystals with and without Li ions

The upconversion luminescence spectral intensity of Er³⁺ in Er³⁺ and Yb³⁺ codoped ZnO nanocrystals with and without Li⁺ are investigated. Yb³⁺ ions as a tradition sensibilizer have efficient energy transfer processes from Yb³⁺ (²F_5/2) to Er³⁺ (⁴I_13/2, ⁴I_11/2, ⁴F_9/2), which lead to the increment of upconversion luminescence intensity. Following by adding Li⁺ to the Er³⁺ and Yb³⁺ codoped ZnO nanocrystals, the upconversion intensity emitted by Er³⁺ ions is found greatly enhanced. The enhancement is attributed to the distortion of the local field symmetry of Er³⁺ ions, so increases various intra-4f transitions of Er³⁺ ions. Both Yb³⁺ and Li⁺ can disperse Er³⁺ ions in specimen, so reduced the interaction between neighboring Er³⁺ ions.     

Spectroscopic properties of Er/Yb codoped fluorophosphate glasses

Er³⁺/Yb³⁺ codoped fluorophosphate glasses were prepared and their thermal stabilities, Raman spectra, absorption spectra, and fluorescence spectra were measured. It is found that proper content of NaF or PbF₂ is helpful for the increase of stability against crystallization. The variation of Al(PO₃)₃ or NaF content in the composition affects not the maximum phonon energy but the phonon density. The introduction of PbF₂ decreases the phonon energy slightly. Intense green and red upconversion luminescence was observed for the fluorophosphate glass with low phosphate content. A glass matrix for upconversion luminescence requiring neither expensive raw material nor special atmospheric conditioned preparation is provided. Infrared luminescence around 1530 nm was researched. Fluorophosphate glasses with bandwidth properties and stimulated-emission cross sections better than tellurite, germanate and silicate glasses are obtained. Through the introduction of NaF, the bandwidth properties are decreased. Through the introduction of PbF₂ the gain properties are increased. On the whole, it is difficult to obtain a material with the best gain properties and bandwidth properties simultaneously. There should be a compromise between them according to the demand.     

Growth and optical properties of Yb and Tb codoped BaB2O4 crystals

Optical absorption and luminescence spectra of ytterbium and terbium codoped BaB₂O₄ (β-BBO and α-BBO) crystals grown in different conditions have been studied. Low-temperature absorption peaks were observed in all samples. Features related to rare earth ions were observed in absorption and luminescence spectra. Absorption and emission in the range 860-1000 nm are caused by ²F_5/2↔²F_7/2 transitions in Yb³⁺ ions. Emission peaks at 500, 550, 590 and 630 nm correspond to ⁵D₄→⁷F₆, ⁷F₅, ⁷F₄, and ⁷F₃ transitions of Tb³⁺ ions, respectively. The probable reasons of variations in spectroscopic features related to Yb in BBO host are discussed. It has been shown that the replacement of Ва²⁺ by Yb³⁺ in the lattice of ВаВ₂О₄ results in the decrease in the symmetry of oxygen surrounding of Yb³⁺.     

Stretched pulse Yb(3+)silica fiber laser

We report what we believe to be the first results on short-pulse generation in Yb(3+) :silica fiber. By applying the stretched pulse technique in a unidirectional, polarization-switch Yb(3+) fiber laser incorporating a prism-based dispersive delay line, we obtain self-start mode locking and 100-pJ pulses that can be compressed to give clean chirp-free <100-fs pulses. We believe such sources to have great potential for use not only in all-solid-state, high-power femtosecond pulse systems based on Yb(3+) :silica glass but also as seeds for conventional Nd(3+) :glass amplifier chains.     

Characteristics of photoluminescence and Raman spectra of InP doped silica fiber

We fabricated a silica optical fiber doped with InP sandwiched in the core and the inner cladding layers by using the conventional modified chemical vapor deposition process. We presented the experimental X-ray analysis on the optical properties and found that compound InP was contained in the fiber core after annealing process. Broadband photoluminescence observed in InP doped fiber was well coincided with those coming from the recombination of deep levels in InP. The occurrence of temperature-dependent photoluminescence both at the lower and room temperature would be related with the localized defects and their confinement in the micro-network structure of Si-O-Si. The Raman spectra reveal that Si-O-In vibration would disorder the silica ring structures and enervate their vibrations.     

Efficient single-frequency fiber lasers with novel photosensitive Er/Yb optical fibers

Boron- and germanium-doped highly photosensitive cladding is used in a novel design to achieve photosensitive Er/Yb-doped fibers, permitting short, strong gratings (length ~1 cm, reflectivity >99%) to be written without hydrogenation. The high absorption at 980 nm in Er/Yb fibers permits efficient pump absorption over a short device length, which is ideal for achieving highly efficient single-frequency fiber lasers. Both single-frequency Bragg-grating reflector and distributed-feedback lasers with slope efficiencies of 25% with respect to launched pump power have been realized in such fibers.     

基于铒镱共掺光纤光栅光学上转换的实验研究

通过载氢和紫外光曝光在铒镱共掺光纤上制作了光纤光栅,当波长为976.4nm的半导体连续激光泵浦该光栅时,在只有4mW的抽运功率水平下,便可以观察到显著的上转换蓝绿光辐射,随泵浦功率的增加,辐射强度也在增加。通过单色仪和光电倍增管记录到了绿光、蓝光、紫光和紫外等多波长的上转换辐射光。利用显微镜数码相片证实在同种铒镱共掺光纤中没有上转换现象。     

Q-switched mode-locking femtosecond all-fiber laser based on Er3+/Yb3+ codoped phosphate fiber

A Q-switched mode-locking femtosecond all-fiber laser based on a 2 cm long homemade Er³⁺/Yb³⁺ codoped phosphate glass fiber has been reported. By using the nonlinear polarization evolution technique, a nearly 100% modulation depth of mode-locking pulse train is achieved. At a pump power of 410 mW, the energy of each Q-switched envelope, whose width is about 220 μs, is 10 μJ, while the duration of mode-locking pulse within the Q-switched envelope is 318-fs.     

Gain evaluation of compact designing on Er/Yb codoped germanate glass channel waveguide

Bent waveguide structures (U- and F-bend) based on UV-sensitive Er³⁺/Yb³⁺ codoped germanate glass substrates have been designed to achieve high-gain C-band amplification. Using simulated-bend method, the optimal radius for curved structure is offered to be 1.90 cm with loss coefficient of 0.0015 dB/cm, as the substrate size is minimally schemed. In the wavelength range of 1528–1559 nm, obvious gain enhancement for the bent structure waveguides is anticipated, while, for the F-bend waveguide, the internal gain at 1533.8 nm wavelength is derived to be 22.55 dB, which is much higher than the value of 14.06 dB in the U-bend waveguide, and over three times higher than that of the straight one, after compensating both the bend loss and the transition loss. The simulation results indicate that the bent structure designing is beneficial in attaining high optical gain in Er³⁺/Yb³⁺ codoped germanate glass substrates, which assures that long-period grating can be applied to implement practical C-band gain-flattened amplification.     

Eu3+/Yb3+共掺杂ZrO2粉末的制备和发光性质研究

利用共沉淀方法制备了Eu3+/Yb3+单掺和共掺的ZrO2粉体材料,研究了煅烧温度和掺杂浓度对结构和发光性质的影响。XRD结果表明:所制备单掺样品含有单斜相和四方相2种不同结构,随着热处理温度的升高,四方相向单斜相转变,经1 150℃处理后,四方相消失,呈现单一的单斜相;Yb3+离子的掺入有稳定ZrO2四方相的作用,随着掺杂浓度的增加,单斜相转变为四方相。由于晶相的不同,Eu3+处在四方相和单斜相2种发光中心,二者发光性质不同。Eu3+/Yb3+共掺后,在270 nm激发Eu3+时,观测到了Yb3+在近红外波段(980 nm)的发光,同时证实Eu3+的激发光谱和Yb3+的激发光谱相一致,表明存在Eu3+到Yb3+的能量传递,交叉弛豫和共合作能量传递过程是其可能的能量传递机理。     

Carbon dioxide gas sensor derived from a 547-hole microstructured polymer optical fiber preform

In this Letter, we report a carbon dioxide gas sensor having 547 pieces of thin-film modified capillaries, which are derived from a microstructured polymer optical fiber preform. Compared with the conventional absorption-based sensor, the monolithic polymer capillary waveguide arrays have better sensitivity, because the huge sensing surfaces, composed of 547 pieces of dye-indicator-doped porous ethyl cellulose layers, interact directly with the gas molecules. As far as we know, a gas sensor based on multichannel capillary waveguide arrays has not been reported before.     

Dispersion compensation in optical transmission systems using high negative dispersion chalcogenide/silica hybrid microstructured optical fiber

In this paper, a new hybrid microstructured optical fiber (H-MOF) based upon photonic bandgap (PBG) light guiding mechanism which can be used for dispersion compensation in optical transmission systems is designed and simulated. The H-MOF core is made up of silica glass and the holes in the cladding network are filled with As₂Se₃ chalcogenide glass. By selecting an appropriate geometrical parameters for the structure, the dispersion and confinement losses of the proposed H-MOF at 1.55 µm are calculated to be ?6700 ps/nm/km and 6?×?10^?4 dB/m, respectively. Relative dispersion slope (RDS) of the H-MOF at 1.55 µm is about 0.00347 nm^?1. The proposed H-MOF is suitable for use in wavelength division multiplexing and dispersion compensating systems in optical fiber transmission networks.     

Magneto-oscillation of mid-gap photoluminescence in AlAs:Yb/GaAs superlattices

On AlAs:Yb/GaAs superlattice samples, we measured photoluminescence (PL) spectra including their temperature dependence, magnetic field dependence and resistance up to 25 T. In case of selective excitation of well layers, two broad band PLs were observed in additional to the exception of intra-4f PL from Yb. These peaks show oscillatory behavior similar to that of two-dimensional electron system. From the periods of the oscillation, the electron densities are estimated of the order of which are cannot be archived by usual photoexcitation. It was found that the electron density shows a linear dependence on the excitation energy. To explain such distinctive phenomena, we proposed a new model where Yb ions form hole traps in AlAs.     

A numerical method for solving gain characteristics of Er/Yb codoped waveguide amplifiers

By means of the interpolation and iteration methods to deal with the rate equations and the light propagation equations for the Er³⁺/Yb³⁺ codoped waveguide amplifier (EYCDWA), the gain characteristics in the forward, backward, and bidirectional pumped styles are analyzed. In order to obtain high gain, some parameters such as waveguide length, dopant concentration, pump power, and signal power are optimized. The results show that the pump power and signal power are about 50 mW and 1 mW, respectively, the gain is about 6.5 dB for the 2.0 cm-long EYCDWA with an Er³⁺ ion concentration of 1.5 × 10²⁶ m^–3 and Yb³⁺ ion concentration of 1.9 × 10²⁷ m^–3. Moreover, the upconversion is investigated for the designed device.     

Octave-spanning super-continuum from a silica photonic crystal fiber pumped by a 386 MHz Yb:fiber laser

We report octave-spanning super-continuum generation in a silica photonic crystal fiber (PCF) pumped by a compact, efficient, mode-locked all-normal dispersion Yb:fiber laser. The laser achieved 45% optical-to-optical efficiency by using an optimized resonator design, producing chirped 750 fs pulses with a repetition rate of 386 MHz and an average power of 605 mW. The chirped pulses were compressed to 110 fs with a loss of only 4% by using multiple reflections on a pair of Gires-Tournois interferometer mirrors, yielding an average power of up to 580 mW. The corresponding peak power was 13.7 kW and produced a super-continuum spectrum spanning from 696-1392 nm.     

Experiments and analysis of infrared quantum cutting luminescence phenomena of Ho/Yb codoped nanophase oxyfluoride vitroceramics

The infrared quantum cutting phenomenon, which is an international hot research field, of Ho³⁺Yb³⁺: oxyfluoride vitroceramics (FOV) was studied in the present paper. It was found from the fluorescence spectroscopy experiments that the excitation spectrum of 973.0 nm fluorescence of Yb³⁺ ion is very similar to the absorption and excitation spectra of Ho³⁺ ion. It suggests that the energy transfer from Ho³⁺ ion to Yb³⁺ ion is very efficient. Then, all the possible important energy transfer passages were analyzed. It was found that the energy transfers {⁵G₄(Ho³⁺)→⁵F₅(Ho³⁺), ²F_7/2(Yb³⁺)→²F_5/2(Yb³⁺)} and {⁵F₅(Ho³⁺)→⁵I₇(Ho³⁺), ²F_7/2(Yb³⁺)→²F_5/2(Yb³⁺)} result in the effective two-photon quantum cutting 973.0 nm fluorescence of Yb³⁺ ion when ⁵G₄ or ³K₇ or the above energy level of Ho³⁺ ion are excited. Finally, the quantum efficiency η_QE,1%Yb=43.0% and η_QE,5%Yb=171.7% of two-photon quantum cutting was calculated for Ho(0.5)Yb(1):FOV and Ho(0.5)Yb(5):FOV respectively. This research would be beneficial for the enhancement of solar cell efficiency.     

Fabrication and characteristics of silica optical fiber doped with InP nano-semiconductor material

We fabricated a novel special silica optical fiber doped with nano-semiconductor material InP in double cladding layers by using conventional modified chemical vapor deposition (MCVD) process. During the deposition process, InP powder was placed at the entrance of silica substrate tube and vaporized by high temperature. With the help of nitrogen gas flow, the vaporized InP was carried into silica substrate tube and deposited on the inner surface in the form of nano-particle. The physical structure and amplification characteristics of the special fiber were investigated. From the refractive index distribution of the preform, we can observe obvious increase in inner cladding index, which attributes to InP dopant. By using 532 nm LD as pump, a broadband amplified spontaneous emission (ASE) was demonstrated in the wavelength range from 1,080 to 1,350 nm. The proposed nano- semiconductor doped fiber with optical amplification is expected to a wide application in optical fiber communications.     

Stable high-power continuous-wave Yb(3+)-doped silica fiber laser utilizing a point-by-point inscribed fiber Bragg grating

We report on a narrowband 5 W cw fiber laser incorporating a point-by-point fiber Bragg grating inscribed into the core of a Yb(3+)-doped double-clad fiber. The laser featured excellent long-term wavelength and power stability (0.3%), as well as a very narrow (15 pm) linewidth, when passive temperature stabilization of the grating was implemented.