We summarize our recent work on the development of multi-wavelength fiber lasers. The multi-wavelength fiber laser is constructed by all-fiber components and capable of continuous tuning in absolute wavelength and discrete tuning in wavelength spacing. One of the key components of the multi-wavelength fiber laser is a novel fiber comb filter. First, we will discuss the novel fiber comb filters based on fiber Lyot and Sagnac filters. Second, we will discuss multi-segment all-PM fiber ring lasers based on the fiber comb filters to demonstrate the concept of tunable multi-wavelength laser sources with continuous tuning in absolute wavelength and discrete tuning in wavelength spacing. The tuning method is based on birefringence filtering properties of a segmented all-PM fiber ring resonator. Based on this method, we show a discrete wavelength spacing of dual-wavelength output from 0.9 to 2.5 nm and continuous wavelength tuning over 10 nm. The demonstrated lasers can be easily modified to tune over a greater wavelength spacing and absolute wavelength range.
Bi2O3-based thulium (Tm3+) doped glass fiber (Bi-TDF) for S-band amplification was investigated. Tm3+ was doped in Bi2O3-SiO2 based glass and melted using a conventional method. Emission spectra of the 3H4 - 3F4 were measured with pumping at a wavelength of 792 nm using laser diode (LD). Full width of half maximum (FWHM) of the emission is 1.4 times and 1.1 times broader than that of fluoride glass and tellurite glass, respectively. Moreover, the emission peak shifted towards longer wavelength as compared with fluoride and tellurite glasses. Single mode Bi-TDFs with Tm3+ concentrations of 2000 ppm, 3900 ppm and 6000 ppm were fabricated and evaluated with fusion splicing to SiO2 fibers. Gain profiles were measured with bi-directional pumping using 1047 nm Yb fiber lasers. The gain-peaks observed around 1470 nm shifted towards longer wavelength with increasing Tm3+ concentration. Gain properties of Bi-TDF were improved by additional pumping at the wavelength of 1560 nm with Tm3+ concentrations of 2000 ppm and 3900 ppm. A maximum gain over 10 dB of Bi-TDF was obtained using a fusion spliceable Bi-TDF with a length of only 100 cm.
Low-temperature photoluminescence of epitaxial GaAs codoped with erbium and oxygen is investigated. Samples are prepared by low-pressure organometallic vapor phase epitaxy at optimized growth conditions providing formation of Er-2O luminescent centers. The low-temperature spectral bandwidth of the predominant emission line at 1538.1 nm is measured to be approximately 0.04 nm. The decay time of approximately 1.1 ms is measured for the Er-related emission at 77 K. Stimulated emission is identified at 77 K from gain measurements by variable-stripe-length method. The measured gain up to 45 cm- at pumping power density of 0.1 kW/cm2. The model is presented that explains sublinear power dependence of the Er-related spontaneous luminescence.
Phosphate glasses have become increasingly popular for planar waveguide devices owing in part to the development of a number of different commercial compositions with a wide range of optical, physical, chemical and laser properties. In addition, the recent development of low temperature bonding technology has made possible the fabrication of structures involving multiple glasses prepared as a single hybrid substrate. Combined, these new materials and technologies make possible the creation of devices with increasing integration and complexity. Here, we present passive characterization data collected on glass joints prepared with the low temperature bonding technology and active performance data of a hybrid DBR laser where the surface relief grating has been fabricated in the passive glass region of a hybrid substrate.
Optical devices made by using rare-earth doped fluoride fibers are now commercially available, and have been widely used, for example, as test sources to evaluate WDM components. We will review these fiber-based devices, which include ASE (Amplified Spontaneous Emission) light sources and optical fiber amplifiers in the S, C, and L bands and fiber lasers in the near infrared region.
A new glass in the fluoroarsenate family was fabricated and doped with Er3+ ions. The compositions made are (Na4As2O7)40 (BaF2)30 (YF3)x with xequals 0, 0.1, 0.5, 1, 3, and 5. The optical properties of Er3+ ions were established in terms of absorption and emission spectra, Judd-Ofelt calculations, and lifetime measurements in the visible and infrared domains. Because of the presence of arsenic, these glasses show lower phonon energies than their fluorophosphate counterparts. As a result, the green emission from the 4S3/2 level of Er3+ can be observed. At concentration of 0.1 mol $ ErF3, the experimental lifetime for 4S3/2 is well accounted for by thermalization processes and the occurrence of multiphonon relaxations. For the 4I11/2 and 4I13/2 levels, the lifetime is maximum for 1% erbium concentration, due to the occurrence of signal self-absorption. Excellent agreement is obtained between the radiative and experimental lifetimes of the 4I13/2 level, which validates the application of the Judd-Ofelt theory.
Nonlinear upconversion quenching in high Er-doped glasses has been studied with use of Monte Carlo modeling and a new experimental steady-state technique. The experimental and simulated results exhibit that the upconversion coefficient depends on the following factors: (a) erbium content, (b) migration of excitation, (c) pump power, and (d) duration of pulse pumping.
In this work we report the upconversion luminescence of Pr3+ doped Kpb2Cl5 crystal upon excitation in the near infrared region inside the 1G4 level. Direct excitation in the visible region has been also carried out. A strong orange luminescence from the 1D2 leve, and a less intense blue, green, and red emission from the 3P0,1 levels have been observed under infrared excitation and compared with those obtained under one photon excitation. The upconverted fluorescence from the 1D2 level shows a quadratic dependence on the pump power indicating a two photon process, whereas the pump power dependence of the blue fluorescence from level 3P0 is nearly cubic (2.62), which indicates a three photon mechanism. Emission from levels 3P0,1 is also observed under pulsed excitation in the 1D2 state. The possible excitation mechanisms responsible for these upconversion processes are discussed.
Laser induced internal cooling has been investigated in a new fluorochloride glass (CNBZn) and a fluoride glass (BIG) doped with 2.1x1020 Yb3+ ions/cm3 and in a Kpb2Cl5 crystal doped with 5x1019 Yb3+ by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency for CNBZn glass which is approximately 2% relative to the absorbed laser power at 1010 nm and 300 K falls about 20% at 77 K. The cooling efficiency for BIG glass was only approximately 6% at room temperature. For the Yb3+ doped Kpb2Cl5 crystal we have shown internal laser cooling with a cooling efficiency of about 0.2% at room temperature. This is the third ytterbium-doped crystal, after Kgd(WO4) (Ref.10) and YAG (Ref.11), in which anti-Stokes laser-induced internal cooling has been demonstrated. The observed temperature dependence of the cooling process can be explained by a simple model accounting for the photon-ion- photon interaction.
Microstructure resonators have attracted considerable attention because of their versatility for signal processing applications. We use our Er3+ doped telluride glass to fabricate Er3+ glass microsphere. The excitation of whispering-gallery modes is realized by coupling from an angle-polished single-mode optical fiber tip to an optical microsphere cavity. In the experimental setup, one fiber was used as an input coupler and the other fiber was used as an output coupler. 975nm fiber pigtailed laser diode was used to side pump the microspheres. Very nice whispering-gallery modes (WGM's) near 1.5mm were observed successfully. Signal enhancement was observed in this Er3+ doped telluride glass microsphere with a radius around 50mm. The best enhancement of this Er3+ doped telluride glass microsphere reaches 12dB.
The application of frequency up-conversion in rare earth (Ho3+ and Yb3+) co-doped glasses for optical storage has been investigated. Optical discs with the storage layer made from rare-earth doped tellurite glasses have been studied in terms of minimum bit-size and up-conversion efficiency. The possibility of multi-layered architecture has also been investigated. The choice of using tellurite glasses is based on previous experimental results indicating this type of materials gives extremely high up-conversion efficiency. Since the up-conversion effect is dependent on the square of the intensity of the pump beam, it is possible that the effective spot size of the signal region can be a significantly smaller than that limited by diffraction, hence leading to super-resolution in the storage layer and increased storage density.
Waveguide laser arrays operating at 1.5 micrometers have been fabricated on Er:Yb-doped glass substrates by a two-step silver-sodium ion-exchange process based on thermal diffusion followed by a field-assisted burial step. The fabrication parameters have been optimized to achieve low propagation losses and good mode matching between waveguide channels and standard single-mode output fibers. Each laser cavity is formed by two Bragg gratings butt-coupled to the two ends of the channel waveguide. Fiber-coupled output power in excess of 10 mW is readily available from a single channel, when pumped from both sides by two 980-nm laser diodes providing a maximum power of ~300 mW. Single-longitudinal mode operation with output power of the order of 1 mW has been achieved using narrow reflectivity band (<30 GHz) Bragg gratings with different laser configurations. The laser operating wavelength can be selected across the whole C-band of optical communications (1530-1565 nm) by changing the Bragg grating peak reflectivity wavelength. A relative intensity noise lower than -150 dB/Hz has been measured for frequencies larger than 2 MHz. Stability of single-frequency operation, low intensity noise and flexibility in the choice of the operating wavelength make this laser array particularly attractive for wavelength division multiplexing optical transmissions.
The feasibility of a planar channel waveguide amplifier and laser made of praseodymium (Pr3+) doped chalcogenide glass, based on the binary composition GeS2, is investigated by means of an appositely implemented computer code. The buried channel waveguide amplifier, having both core and substrate doped with a dopant concentration Nequals3000 ppm, shows gain close to Gequals7 dB for an input signal power of Psequals1 (mu) W at the wavelength (lambda) sequals1310 nm and a pump power Ppequals 200 mW at the wavelength (lambda) pequals1020 nm. The Fabry-Perot laser, based on the aforesaid buried channel waveguide terminated pump power Pthequals16mW and a slope efficiency Sequals8%.
The feasibility of high gain planar waveguide amplifiers, made by Er-doped, silica-titania SiO$02) - TiO2 thin films on silica SiO2 glass substrate, is proved by means of an on purpose developed computer code. The computer code implemented for simulation is based on a complete model, which takes into account the secondary transitions pertaining to the ion-ion interactions, the pump and signal propagation, and the ASE. In the small signal operation, a gain close to 4.7 dB is theoretically demonstrated for a waveguide of 5 cm length, by using a pump power of 200 mW. A first set of waveguiding samples is fabricated and characterized.
Recently the effects of glass-host nonlinearities, particularly XPM and FWM, in EDFAs have received much attention . The reason is that in WDM communication systems these effects in typical EDFAs can be almost as large as in the transmission fibers themselves, and may affect communication system performance. Here we show theoretically that by using highly-doped waveguide amplifiers (EDWAs), which are just a few cm long, we can reduce these effects by several orders of magnitude compared to typical EDFAs which are several meters long. These effects remain moderate even if highly-nonlinear hosts such as tellurite or bismuth oxide are used; this may also open the way for the utilization of such materials, which are problematic for EDFAs. We have used analytical approximations as well as very accurate numerical simulations to calculate XPM and FWM in EDWAs and EDFAs. Nonlinearities are treated as a perturbation: we first calculate the power evolution for each wave along the amplifier, neglecting the effect of nonlinearities; we then use the power maps of all the relevant signals to calculate the nonlinear effects, by numerically integrating the corresponding differential equations. We will present the significant features of our simulation program, particularly the incorporation of the nonlinear effects. We will present examples of calculations for representative lengths, gains, and glass host materials, illustrating the considerable nonlinearity reduction achievable by using short waveguide amplifiers.
Long period fiber grating (LPFG) can be used as active gain controlling device in EDFA. However, LPFGs fabricated in the standard telecom fiber only have a typical temperature sensitivity of 3-10nm/100 degree(s)C, which may not be sufficient for implementing tuneable filters capable of wide tuning range and high tuning efficiency. In this paper, we report a theoretical and experimental investigation of thermal properties of LPFGs fabricated in B/Ge co-doped optical fiber. We have found that the temperature sensitivity of the LPFGs in the B/Ge fiber is considerably increased compared with those produced in the standard fiber. The LPFGs written in the B/Ge fiber have achieved, on average, one order of magnitude higher sensitivity than that of the LPFGs produced in the standard telecom fiber. We have also identified that the thermal response of LPFG is strongly dependent on the order of the coupled resonant cladding mode. The maximum sensitivity of 1.75nm/ degree(s)C achieved by the 10th cladding mode of the 240micrometers LPFG is nearly 24 times that of the minimum value (0.075nm/C) exhibited by the 30th mode of the 34micrometers LPFG. Such devices may lead to high-efficiency and low-cost thermal/electrical tunable loss filters or sensors with extremely high temperature resolution.
Upconversion luminescence of Yb3+-doped and Yb3+ Ho3+-codoped oxyfluoride vitroceramics when excited by a diode laser operating at a wavelength of 960 nm is investigated. A strong 479-nm upconversion cooperative radiation luminescence, attributed to the coupled states of Yb3+Yb3+ clusters, is discovered. This result suggests an interesting and effective path to achieve compact blue upconversion luminescence. The presence of holmium in the Yb3+Ho3+-codoped oxyfluoride vitroceramics substantially reduces intensity of the cooperative upconversion luminescence.
This paper studies upconversion luminescence of oxyfluoride glass codoped with holmium and ytterbium. When the sample was excited in the infrared at a wavelength of 960 nm, several visible luminescence peaks were observed. The two strongest upconversion luminescence peaks are located at 544 and 658 nm, which are due to the 5S2 to 5I8 and 5F5 to 5I8 transitions of Ho3+, respectively. Additional upconversion luminescence peaks are present at 485, 751, and 799 nm. The upconversion luminescence process is initiated by excitation of the codoped Yb3+ ions, followed by a transfer of energy from the excited Yb3+ ions to the Ho3+ ions. The dependence of the upconversion luminescence intensity with the excitation laser power obeys a power law with fractional exponent. This unusual saturation phenomenon is likely a result of energy diffusion.
Praseodymium doped arsenic sulfide single mode fibers were prepared and investigated with regard to their suitability for 1.3 micrometers amplification. Although these fibers have relatively low basic losses of about 2 dB m-1 at pump (1.02micrometers ) and signal wavelength, till now no signal amplification could be measured because of a strong attenuation induced by the continuous wave (cw) pump light. In order to investigate the more detailed optical behavior, we have carried out time resolved measurements with short pump pulses of 1-5 ms at a repetition rate of 1 Hz on a 2 m fiber length. The evaluation of the time dependent signal intensity at the fiber end in comparison to the fluorescence intensity without signal has shown the fast development of a transient absorption within 0.5-1 s. Moreover, an inner gain could be clearly detected which increased with the pump power (0.75-75 mW). The gain coefficient related to the input pump intensity has been determined to be 0.5 dB- mW-1.
There is considerable interest in Er-doped silica-based materials with photosensitive properties for lasing and photorefractive applications. Tin doping has been recently found to induce strong and stable photosensitivity in silica glass. In this work, Er co-doped Sn-doped silica samples are obtained by sol-gel method by hydrolysis and condensation of TEOS and tin dibutyl diacetate, Er(NO3)3, or Er(CH3COO)3 as dopant precursors. Samples with Er content ranging between 0.1 and 1 mol% are investigated. Sn doping (0.4 mol%) is employed to induce photorefractivity properties in the glassy host. Time resolved photoluminescence, optical absorption and refractive index measurements are carried out and analyzed as a function of the Er content. The specific effects of Er content and Sn doping on the rare earth emission efficiency and photosensitivity of the glass are discussed. In particular it is shown that the Er emission shows the typical features of Er doped sol gel silica and the strong photosensitivity induced by tin doping is not affected by rare earth ions.
A novel Yb3+-doped double-clad silica fiber with rectangular inner cladding was designed and developed using MCVD process, solution-doping and optical machining altogether. The dimension of inner classing is 100(Mu) mx70micrometers , and Yb3+-doped concentration in the core is about 0.24wt%. The operation of the fiber laser pumped by inner cladding is reported. The threshold of laser is 34mW. When the pump power launched is 141mW, the laser output is 84mW at the wavelength and 1075.6nm, and slope efficiency is 77%.
The Nd3+ Cr3+ single and codoped lead silicate glasses (SiO2-Na2O-Sb2O3- PbO2) were synthesized by means of fusion method. Their optical properties were studied by means of absorption and photoluminescence (PL), recorded at room temperature. The wear 4F3/2 yields 4I9/2 emission band centered at 890 nm observed in the Nd3+ single doped glasses was strongly enhanced in the Nd3+ and Cr3+ codoped glass. The effective mechanisms based on both direct donor-donor energy transfer processes assisted by localized phonons and energy cross-relaxation between donor-acceptor were proposed.
The synthesis and optical properties of Nd3+-doped lead silicate glasses (SiO2-Na2O-Sb2O3- PbO2) were reported. The photoluminescence (PL) spectra in the range of 600 to 1100 nm reveal four well resolved emission bands, centered at (lambda) equals 810, 890, 920 and 1060 nm, respectively. In the dependence on the temperature, the PL line at the higher energy side exhibited a peculiar behavior, compared to that in the lower-energy side - a strong enhancement of PL intensity with an increase of temperature up to 300 K. It was well explained by the localized phonon-assisted energy transfer process in combination with thermal activation of Nd3+ by absorption of localized phonons. The build up in PL intensity of 810nm PL emission band with increasing temperature would be expected to have many practical applications, such as thermometer.
Tm3+ are new candidates for optical amplifiers in the 1.5micrometers region, the S band. For the moment, Tm3+-doped Fibers Amplifiers (TDFA) h ave been studied in fluoride matrices because of the lowest phonon energy of this glasses compared to the silica ones. We proposed to evaluate the spectroscopic potential of this ion in sulfide GeGaSSb glasses where the phonon energy is around 350cm-1. Absorption and emission cross sections were measured. Lifetime measurements were carried out. Results were compared with the Judd-Ofelt calculations.