Sten Helmfrid

Influence of randomly varying domain lengths and nonuniform effective index on second-harmonic generation in quasi-phase-matching waveguides

The degradation of second-harmonic generation in quasi-phase-matching waveguides, when random errors occur in the domain boundary position and when the effective index varies along the waveguide, is studied theoretically. Two models for random errors are used, one assuming independent shifts of the domain boundaries and one assuming independent domain lengths. Only the influence of random errors following the statistics in the second model might be of any significance in practical implementations. It is shown that in this case the normalized output power is a decreasing function of the product of the number of domains and a relative variance of the stochastic disturbances. If the difference in effective indices between the interacting modes varies along the waveguide, the same kind of ripple that has been observed in birefringence phase matching occurs.

Fabrication of advanced fiber Bragg gratings by use of sequential writing with a continuous-wave ultraviolet laser source

We present a novel scheme based on sequential writing for fabrication of advanced fiber Bragg gratings. As opposed to earlier sequential methods this technique uses a cw UV laser source and allows for very precise control and repetitivity of the formation of the gratings. Furthermore it is possible to use high average irradiances without destroying the fiber, resulting in considerable reduction in fabrication time for complex gratings. The method has been applied to several test gratings, which proved its versatility and quality.

Observation of strongly nonquadratic homogeneous upconversion in Er/sup 3+/-doped silica fibers and reevaluation of the degree of clustering

From careful studies of the 1530-nm fluorescence decay, we obtain the rate of energy transfer upconversion as a function of the inverted population for a series of nine highly Er-doped silica fibers (with concentrations from 0.3 to 8.6/spl middot/10/sup 25/ Er/sup 3+/-ions per m/sup 3/). The results demonstrate that the slow component (microsecond to millisecond scale) of the upconversion, usually referred to as the homogeneous upconversion, is clearly nonquadratic in its dependence on the inverted population, contrary to previous assumptions in the literature. In a second part, we present a new detailed model for energy transfer upconversion, permitting-to our knowledge for the first time-calculation of the rate of migration accelerated upconversion for any given spatial distribution of Er/sup 3+/-ions. We demonstrate that the results from the decay measurements may be explained with this model. Next, we review the results from a CW green fluorescence detection experiment for the determination of the degree of clustering, which was previously performed on five of the nine fibers. We find accordance between these results and our model, with parameters consistent with those needed to fit the results of the decay experiment, and we arrive at a new conclusion about the nature of clustering.

Second-harmonic generation in quasi-phase-matching waveguides with a multimode pump

Second-harmonic generation of blue light in quasi-phase-matching waveguides is analyzed with respect to the linewidth of the pump radiation. From the coupled-wave equation, when the pump linewidth is small, the conversion efficiency is shown to be enhanced by a factor of 2, compared with a single-mode case. A degradation of 3 dB occurs for a linewidth of three times the acceptance bandwidth. We also analyze the influence of the pump spectrum on measurements when the frequency response of a waveguide is evaluated by tuning the fundamental wavelength or by measuring the spectrum of the generated second-harmonic radiation.

Influence of various imperfections on the conversion efficiency of second-harmonic generation in quasi-phase-matching lithium niobate waveguides

The generation of coherent blue light by frequency doubling of radiation from semiconductor lasers in quasi-phase-matching lithium niobate waveguides has recently been demonstrated, but with a conversion efficiency significantly lower than the theoretically predicted value. We have experimentally investigated the performance of some quasi-phase-matching waveguides by measurements of the second-harmonic power and the integral of the second-harmonic power with respect to the fundamental wavelength at a wavelength scan. These experimental results are combined with a theoretical analysis. It is concluded that the inhomogeneities in the effective index, the absence of domain inversion in some regions along the waveguide, and the suppression of the nonlinearity that is due to the proton exchange process are the most important imperfections to deal with in the future optimization of the domain inversion process and the waveguide formation.

Optical parametric amplification of a 1.54- mu m single-mode DFB laser in a Ti:LiNbO/sub 3/ waveguide

Optical parametric amplification of radiation from a 1.54- mu m single-mode DFB laser in a Ti:LiNbO/sub 3/ waveguide has been investigated by measuring the idler created in the nonlinear process. The pump was a frequency-doubled Nd:YAG laser at 532-nm wavelength, operated either Q-switched or both Q-switched and mode-locked to avoid permanent surface damage and to minimize photorefractive index changes. On comparison with theoretical predictions, the output was found to correspond to a pump power one order of magnitude below the true power in the waveguide. The highest gain obtained was 4.7 dB with 49 W of pump power in a 27-mm-long waveguide. Although the experimental results can be improved, it is obvious that the major difficulty to overcome is the large pump power required. The prospect of decreasing the power requirement by using materials with higher effective nonlinear coefficients or by using quasi-phase-matching techniques is discussed. A more fundamental drawback is that the amplification is polarization dependent, and it is unclear how this can be handled. >

Gain saturation and pump depletion in high-efficiency distributed-feedback rare-earth-doped lasers

We propose a model for designing and analyzing rare-earth-doped distributed-feedback (DFB) lasers. The effects of pump depletion and cross-gain saturation are included and shown to be significant for high-efficiency lasers. Our simulations show good agreement with experimental data from an ytterbium-doped fiber laser. We also give some design guidelines for DFB fiber lasers.

Tunable and programmable optical bandpass filter

We demonstrate a novel tunable and programmable optical bandpass filter with a transmission peak less than 5 pm wide that is tunable over 18.5 nm. The number of transmission windows within the stop band, as well as each windows position and strength, is set by applying a heat profile to a chirped fibre Bragg grating with the aid of a thermal printer head comprising a 640 pixel array. It is shown that the width of a transmission peak formed by local heating is mainly determined by the refractive index modulation depth and chirp rate and not by the width of the heated region.

Limitations of the interferometric side diffraction technique for fibre Bragg grating characterization

We make a quantitative investigation of the interferometric side diffraction technique for characterization of fibre Bragg gratings. As opposed to more commonly used non-interferometric alternati ...

Fibre Bragg grating characterization with ultraviolet-based interferometric side diffraction

The interferometric side diffraction method for fibre Bragg grating characterization is evaluated using a set-up with an ultraviolet interrogation beam. In its present configuration, the reproducibility errors are 2–5% for the index and typically 10°–20° for the phase profile. The reflection spectra of chirped test gratings have been successfully reproduced with the aid of the acquired phase and refractive index envelope data. It is verified that the power of the interrogating beam, within some limits, is unimportant as regards the phase reproducibility, and the importance of relating the analysis to the fabrication parameters of the grating under investigation is stressed.