We theoretically analyze the stability of the Brillouin fiber ring laser. This study allows us to draw the portrait of the unstable domains for a large range of the system parameters. The influence of the fiber length and of the cavity reinjection rate are highlighted. The effect of cavity detuning on the stability of the laser is investigated.
We report tuning range and efficiency measurements of a novel dual-gain medium Brillouin/erbium fiber laser which utilizes the Brillouin gain in single-mode optical fiber and gain in erbium-doped fiber. Analytical results on the efficiency of the laser are presented which fit well to the experimental results and allow for an intuitive interpretation of the observed properties of the laser efficiency, which varies according to the laser operating conditions. Tuning range of 4nm was observed upon tuning of the Brillouin pump wavelength.
A pulsed erbium-doped fiber source featuring a low-power laser diode and a fiber amplifier with internal spectral filtering is presented. This structure provides greater versatility than Q-switched fiber lasers and pulses of 180 W with 7 ns FWHM have been obtained. Applications include range finding and remote sensing.
We investigate hybrid erbium-doped fiber amplifiers (EDFAs) for use as broadband sources of amplified spontaneous emission (ASE) in spectrum-sliced wavelength-division multiplexed (WDM) networks. We report experimental results of two-stage, i.e., two-fiber sources that demonstrate the potential of hybrid EDFAs as high-power, broadband sources with high spectral uniformity. We have developed a numerical model in order to optimize the design of such sources. The model provides for use of multiple fibers and incorporation of optical filters. We present a preliminary theoretical design, derived using the model, of a high-power, broadband source that incorporates three fibers and two filters. The theoretical output spectrum of the source is a uniform comb of wavelength channels ideally suited for use in spectrum- sliced WDM networks. The output spectrum has 19 channels within a 3 dB bandwidth with 2 nm spacing and an average per-channel power of 1.0 mW.
We report the experimental demonstration of a polarized superfluorescent fiber source (SFS) made by inserting a low- loss polarizer at an optimum position along the erbium-doped fiber of a standards SFS. This source produces a strongly polarized output with nearly the same output power as a standard, unpolarized SFS. We show good agreement between the behavior of the source and numerical simulations.
Characteristics of the short wavelength upconversion fiber lasers are reviewed with emphasis on the Nd3+ and Ho3+ transitions. Improved performance of the green Ho3+:ZBLAN upconversion, including threshold incident pump powers < 15 mW for the pump wavelength range 641-645 nm, is discussed.
We present results of the manufacturing of ADD/DROP wavelength multiplexers, which combine an in-fiber Bragg grating written on one fiber in the middle of a polished coupler. When a fiber Bragg grating is written on one of the two fibers, and located in the middle of a 100 percent coupler, coupling is 'frustrated' at the Bragg wavelength while 100 percent coupling still occurs at other wavelengths. This performs an ADD/DROP wavelength multiplexer function, which is very useful for WDM telecommunications networks. Modeling of the device indicates that good performance needs a long interaction length with a strong grating, and predicts an attractive insertion loss with a selectivity of about 10 dB. Using the side-polishing technique, manufacturing of several devices indicates a good efficiency of the frustration effect but some transmission at wavelengths other than the Bragg wavelength. Results are in good agreement with the model. Insertion losses are within 1 dB. The selectivity spectrum shows an asymmetrical shape which is explained when considering the phaser matching and its wavelength dependence due to the grating.
We report on the fabrication f comb filters which operate on the principle of linear polarization rotation through the twisting of the birefringent axes of photosensitive elliptical-core fibers by UV radiation. The polarization coupling spectrum of these filters consist of a series of narrow resonances which are roughly equally spaced in frequency and which extend over the entire single-mode frequency range of the fiber. Some applications of these structures are also discussed.
The writing of gratings within heated hydrogen loaded, non H2 loaded germanosilicate or Ce3+ doped aluminosilicate fibers was carried out through UV exposure. The fibers were heated by means of a CO2 laser beam. The thermally induced change in the grating growth kinetics depends on the type of fiber used to write the grating. The thermal stability of gratings written in either heated or unheated germanosilicate fibers was investigated through isochronal annealing experiments. Difference between the thermal behaviors of gratings written in the H2 loaded and the non H2 loaded fibers was demonstrated. There exists strong evidence that structural changes are involved in the photosensitivity of the germanosilicate fibers.
In this paper, we describe the various behaviors of reflectivity and Bragg wavelength shift according to time and temperature when a grating is written in a germanosilicate core fiber. Four processes are pointed out which we have quoted type O, I, IIa, IIb. Then, they are analyzed. Type O is relevant of photochromism of color centers through Kramers-Kronig transformation. Type I is explained on the basis of densification induced by UV irradiation. Process giving rise to Type IIa is not yet known but the refractive index change is opposite to the others. Type IIb is an index change associated to ablation for open structure or fusion for closed ones. Finally, a general model is proposed interpreting the time-temperature dependence of the reflectivity and Bragg wavelength shift.
We apply a Kramers-Kronig model to several published experimental studies of photosensitivity in Ge-doped silica to calculate the predicted index change (Delta) nKK from the measured change in the absorption spectrum. In most cases (Delta) nKK is close to the measured index change (Delta) nexp. Most of (Delta) nKK arises from absorption changes between 165 and approximately 195 nm. In the few cases where (Delta) nKK is much smaller than (Delta) nexp, the absorption spectrum was not measured below 200 nm, which is probably the reason for the discrepancy. Based on several observations, we also make the hypothesis that in the majority of our study population, color centers are probably responsible for most of the observed absorption change, and thus of the index change.
Far-field properties of the optical power scattered from type IIA gratings photowritten within high NA germanosilicate fibers have been studied under various experimental conditions. First, the evolution of the out- coupling efficiency has been recorded in the course of the grating inscription. This revealed that the scattered power from fiber Bragg gratings sharply increased at the time of arrival of the type IIA photosensitivity. More, the out- coupling efficiency and the reflectivity of the type IIA grating seem to have a similar behavior under a thermal annealing process of the grating. It is then shown that the optical power detected in the far-field is scattered around a cone centered on the fiber axis, with a non-induced azimuthal distribution. The features of the observed pattern can be explained only if the UV induced index change, at the origin of he increase in the scattering efficiency, is non uniformly distributed in a plane perpendicular to the fiber axis.
We develop a simple method leading to the determination of important parameters for the Praseodymium doped fluoride fiber amplifier (PDFFA), such as the non-radiative energy transfer upconversion coefficient or pump excited state absorption cross section. We apply this method to five ZBLAN bulks of different concentration and to a ZBLAN: Pr3+ 500 ppmWt single-mode fiber. The resulting non-radiative energy transfer upconversion coefficients cT1 are 5 s-1 at 500 ppmWt and 9 s-1 at 1000 ppmWt. When the pump wavelength is set at 1017 nm, (sigma) ESAp is very weak: less than 5.10-26 cm2. We measure that the 3P0 population varies to the 2.7th pump power. We conclude that the upconversion mechanism leading to the excitation of 3P0-1-1I6 states in the PDFFA is different from all that has been previously suggested and we propose a three photon process. According to this study, a new model of the PDFFA is suggested.
Optical amplification at the 632.8 nm He-Ne wavelength has been investigated in praseodymium-doped fluorozirconate fiber. A slope efficiency of 0.15 dB/mW was realized with up to 14 dB gain which is a significant improvement on previously reported results. A rate equation approach is used to model and understand the gain dynamics. The lifetime of the 3P0 state was measured to be 46.5 +/- 0.9 microsecond(s) . Using Fuchtbauer-Ladenburg analysis, the absorption cross-section at the pump wavelength and the emission cross- section at the signal wavelength were calculated to be 0.39 +/- 0.03 pm2 and 1.65 +/- 0.15 pm2 respectively.
The stable propagation of ultrashort optical solitons in an ultralong active fiber is investigated. We show that the soliton self-frequency shift is very sensitive to the nonresonance of the carrier frequency and that the linearly frequency dependent gain with positive slope is the effective component of gain spectrum for the compensation of this effect. Furthermore, we demonstrate that the inclusion of nonlinear gain that contains terms proportional tot he second and fourth power of the amplitude can provide stable propagation of ultrashort optical solitons.
The use of rare-earth dopants for all-optical switching is of interest because of the very low pump powers required to achieve full switching. In addition, the ease with which rare-earth ions can be incorporated into silica based fibers and waveguides makes them ideally suited for the fabrication of fiber compatible components. The principal disadvantages of this type of nonlinearity are that the relaxation times are characteristically slow, and relatively long interaction lengths are required to achieve phase changes of the order of (pi) . In this presentation, an overview of the work being carried out in Australia will be given, concentrating on the techniques developed to minimize the relaxation times and power length products of these nonlinearities.
To explain the high third-order nonlinearity observed in rare earth doped fibers, we examine the role of the various transitions of the dopants. We present experimental evidence that in fibers doped with Er3+, Nd3+, or Yb3+, most of the nonlinearity is caused by the dopant's UV transitions, i.e., it is non-resonant in nature. The magnitude of the nonlinearity is shown to be in excellent quantitatively agreement with a model. Fiber modulators and switches based on this widespread mechanisms are predicted to operate over a large range of signal wavelengths and with negligible signal loss.
The resonant nonlinearity observed in rare-earth doped fiber is of interest for all-optical switching due to the very low pump powers needed to achieve complete switching. Large nonlinear effects have been observed previously in both erbium and neodymium doped fibers, however, these large effects cannot be explained by taking into account the change in absorption of local transitions alone. Two mechanisms have been suggested to account for these anomalously large index changes, namely thermally induced effects and far from resonance population dependent effects. In this work, an ytterbium doped twin-core fiber is used to investigate the tow possible mechanisms, and by allowing the doped fiber to lase and observing the clamping of the induced phase change, it is demonstrated that the effect is population dependent in nature. The observed wavelength response of the effect shows an increase in induced phase towards shorter wavelengths, which suggests that the effect is due principally to changes in strong absorptions in the UV. This far-from-resonance effect has been used to demonstrate large phase changes at the preferred telecommunications windows of 1300nm and 1550nm.
When an electric field is applied to a heated piece of bulk silica glass, which is subsequently cooled with the field applied, a permanent second order nonlinearity is formed.Here we examine the formation of the nonlinearity in situ via real-time observation of the second harmonic signal. The dynamics of the growth and decay of the signal are dependent on filed polarity, sample history and ambient environment during heating. The in situ dynamics indicate that under field reversal, there can be a long delay in creating or destroying the nonlinearity.
We describe interferometric measurements of the DC Kerr susceptibilities in an SiO2-GeO2 channel waveguide. The measured magnitude of (chi) 1111(3)(-(omega) ; (omega) , 0,0) equals 2.44 +/- 0.2 X 10-22 m2/V2 is comparable to the previously measured optical Kerr susceptibility. However, the ratio of nonlinear tensor elements (chi) 1111(3)/(chi) 1122(3) equals 1.1 for the DC Kerr effect differs from that observed in the optical Kerr effect where (chi) 1111(3)/(chi) 1122(3) equals 3. We provide theoretical interpretation for this difference in the polarization dependence.
A simple fabrication technique, readily extensible to volume manufacturing, is presented to produce an electro-optically active fiber segment hat can be simply integrated into optoelectronic devices. The fabrication technique offers a dielectric isolation structure surrounding the fiber to a low high field poling, a pair of electrodes used both for poling and for inducing an electro-optic effect, and needs of the fiber unaffected by the fabrication and available for splicing to additional fiber sections. An electro-optic coefficient, including overlap between the nonlinearity and the fiber mode, as high as 0.4 pm.V was obtained by combined temperature/electric field poling. The related device applications are discussed.
A cascade fiber laser operating at 3.9 micrometers and at 1.2 micrometers in the attenuation minimum of the 3-5 micrometers atmospheric window has been realized with a HO3+- doped fluoride fiber. Two different pump wavelength ranges around 640 nm and 890 nm were used to characterize the laser cascade. A maximum CW laser output of more than 11 mW at 3.9 micrometers and 70 mW at 1.2 micrometers was obtained.
A systematic spectroscopic study of 22 rate-earth-ion doped ZBLAN glass samples was conducted to investigate the feasibility of sensitizing Tm:ZBLAN with Yb to facilitate the development of an efficient and conveniently pumped blue upconversion fiber laser. It was determined that, under conditions of single-color pumping, 480 nm emission from Tm3+ is strongest when Yb, Tm:ZBLAN is excited at a wavelength of approximately 975 nm. In this case, the strongest blue emission was obtained from a ZBLAN glass sample with a nominal dopant concentration of approximately 2.0 wt percent Yb + 0.3 wt percent Tm. Additionally, it was demonstrated that for weak 975 nm pump intensities, the strength of the blue upconversion emission can be greatly enhanced by simultaneously pumping at approximately 785 nm. This increase in upconversion efficiency is due to a reduction in the number of energy transfer steps needed to populate the Tm3+1G4 energy level. Measurements of fluorescence lifetimes as a function of dopant concentration wee also made for Yb3+, and Pr3+ transitions in ZBLAN in order to better characterize concentration quenching effects. Energy transfer between Tm3+ and Pr3+ in ZBLAN is also described.
The successful fabrication of channel waveguide in rare earth doped fluoride glass substrates is reported. A new process using anionic exchange between F- ions of the substrate and OD- ions of D2O vapor was developed. The guiding configuration has been qualified by m-liens method and near filed measurement, and the active properties of the channel waveguides are presented.
Bragg gratings were written in the cores of 7 percent mole germania doped preform plates through exposure to cw UV light. The diffraction efficiencies of the holograms were measured at a probe wavelength of 633 nm. The photoinduced refractive index modulations were calculated from these data. Microscopic inspections of the plate surfaces have shown that UV exposure induced corrugations at the grating places. A phase shift interferometric microscope was used to measure the depths of the corrugations. These measurements have shown that the corrugation depths do not depend on the nature of the gas surrounding the plate at the time of the UV exposure. Afterwards, both the refractive index modulations and the depths of the corrugations were measured in the course of 30 min isochronal step annealing experiments. The heating induced refractive index modulation changes were calculated from the changes in the corrugation depths by means of as train densification model and then compared to the experimental ones. Heating the gratings up to 600 degrees C has shown that the densification process is thermally reversible.
The mechanical resistance of UV irradiated optical fibers has been investigated by dynamic fatigue tests. The intrinsic Weibull distribution of the pristine and chemically stripped fiber were almost identical. The high germanium doped fiber was homogeneously irradiate using a KrF excimer with different pulse energy densities and different total doses, both corresponding to typical Bragg grating fabrication conditions. The Weibull distribution of the irradiated fibers depends on the pulse energy density and the total irradiation dose. Both a strong broadening and a reduction of the fiber strength were observed. With increasing total irradiation dose, the median breaking stress of the fiber decreases from 4.8 GPa for the pristine fiber to 2.5, 2.2, and 1.8 GPa for pulse energy densities of 50, 100 and 200 mJ/cm2, respectively, for a total dose of 1 kJ/cm2. At higher total irradiation levels the median breaking stress and hence the strain were found to be independent of laser fluence with a value of 2.7 GPa for 200 mJ/cm2.
Use of rare-earth doped fiber as the sensing element for temperature sensing is reviewed. The advantages of employing a fluorescence intensity ratio technique is discussed and examples of practical applications of the technique are given.