Passively modelocked fiber lasers operating in the soliton regime can generate pulses at multi-gigahertz harmonic repetition rates. The lasers are modelocked with an ultrafast saturable absorber and the low loss cavities support the formation of multiple equally spaced soliton pulses. These sources are potentially attractive for applications in high speed fiber optic communications systems. The design and construction of these laser sources as well as their application to spectrally sliced wavelength division multiplexed transmission is described.
Low threshold operation of the 550 nm holmium laser is reported in a Ho3+:ZBLAN fiber pumped near 650 nm. The 550 nm transition has been pumped by an InGaAlP diode laser producing approximately 30 mW at 643 nm. Over 1.2 mW of green laser output and an optical conversion efficiency of 12% has been obtained. The threshold diode laser pump power was 3.5 mW launched.
Green upconversion signals were observed from the 5S2 levels of holmium ion under 2 mW HeNe laser excitation. Under a dye laser or Ar+ laser excitation, we also observed blue upconversion signals from 3H6, 5G4 and 5G5 levels in addition to the green and red signals.
Changes in the pump polarization state are shown to have a large effect on the mean wavelength of erbium-doped superfluorescent fiber sources due to polarization-dependent gain. This effect causes long-term drift as the fiber birefringence in the source changes over time. By depolarizing the pump, this effect is reduced to under 3 ppm, the noise level of our instrument. A second depolarizer is used to compensate for polarization-dependent transmission of the other source components, namely the WDM coupler and the isolator. The resulting source is insensitive to drifts in the fiber birefringence due to environmental effects.
A coupled supermode approach is proposed to determine Bragg grating reflection effects in waveguides, in particular for gratings written in directional fiber couplers. The coupled supermode theory applies to a wide variety of devices and when compared to coupled individual mode theory, offers high accuracy and added precision that enable more efficient component designs.
In this paper, we propose a comparison between experimental and simulated results of the transmission change of fused tapered fiber couplers exposed to uniform UV irradiation. Numerical simulations use the supermode propagation theory and reveals the decrease of the supermode propagation constant difference. This decrease induces a spectral shift of the coupler response to higher wavelengths and is found to be more significant for taper ratios corresponding to core to cladding mode transition. The model shows a very good agreement with experimental results observed for low power expositions.
We propose a new method for analyzing fiber Bragg gratings having an arbitrary index profile and an arbitrary individual grating length. The proposed method is based on the lattice filter model which are widely used in the signal processing community ranging from digital filtering to speech synthesis and explosive seismic signal processing applications. Lattice filter interpretation provides us with an accurate and simple tool for analyzing arbitrary aperiodic fiber grating structures, and gives us further insight to the understanding of the fiber Bragg gratings. To verify the validity of the proposed model, we have fabricated two grating structures; the short period (periodic) fiber Bragg grating structure and the chirped fiber Bragg grating structure. We have observed that the predicted transmission spectrum and the predicted reflectivity using the lattice filter model match very closely to the corresponding measured spectrum curves in the band of the wavelength of our interest.
Index gratings written in fused-fiber couplers have been shown to make promising WDM devices featuring good wavelength selectivity. Reflection gratings especially show a narrowband wavelength response. Codirectional gratings, however, can exhibit broadband behavior under certain circumstances, such as when coupled modes of a waveguide have similar effective indices, the coupling length is short or the grating is strongly chirped. We propose a device that effects 2 X 2 power division over a large wavelength band through the fused coupler supermode coupling effected using a long period grating inscribed in the coupler waist. Coupling from one branch of the coupler to the other is null without the grating due to a strong coupler asymmetry, thus preventing the overall wavelength response from being too oscillatory. The variation of the supermode beat lengths in the coupler inducing an intrinsic chirp and the convergence at a certain coupler waist diameter of the effective indices of the two lower-order supermodes representing power transfer from one branch to the other facilitate the process of obtaining a broadband response. Though the device shown is a 2 X 2 coupler, the concept can be easily transposed to couplers having more branches than in this case.
The dynamics of grating growth in both H2-loaded and non-H2-loaded GeB-doped fiber is investigated with cw 244 nm light and pulsed 266 nm light. The growth function is found to be the same in both cases, and both the growth rate and the saturation levels are proportional to the writing power density. Such a dynamics can be explained by a two- photon defect creation mechanism counterbalanced by a one- photon destruction mechanism, the saturation level being determined not by depletion of available defects, but by an equilibrium between the two mechanisms. We discuss the implications of such a model.
Nonradiative decay mechanisms in doped fibers introduce heat into the fiber core and often decrease the upper state lifetime of a dopant. This can be detrimental to doped-fiber lasers, amplifiers, and in some cases all-optical switches. In this paper we report theoretical and experimental studies of thermal effects in doped fibers, with particular emphasis on their impact on all-optical nonlinear switches using resonant nonlinearities. We observe significant thermal effects in transition metal-doped silica fibers. We determine that sub-microsecond(s) switching using a resonant nonlinear effect in a conventional switch architecture requires dopant oscillator strengths of 3.6 10-3 or greater when nonradiative processes are the predominant decay mechanism.
A resonance-enhanced nonlinearity is used to demonstrate a digital switching response in a two-mode fiber interferometer constructed from ytterbium(III)-doped optical fiber. The fiber used was single-mode at the pump wavelength (980 nm) and two-moded at the signal wavelength (514 nm). Interferometer-based optical switches are generally sensitive to fluctuations in pump power, because of the sinusoidal dependence of the output state on pump power. However by allowing the doped fiber to lase, thus clamping the phase shift, this sensitivity can be eliminated to yield a bi-stable, digital switching response. In our interferometer configuration, a laser cavity was defined in the ytterbium-doped fiber by a matched pair of Bragg gratings, and the magnitude of the clamped phase shift was controlled by stretching one of the gratings to adjust the laser threshold. Digital switching of a 514 nm signal with 11 dB isolation and a power-length product of 1.3 mW.m was demonstrated. The relaxation time of the switch was determined by the ytterbium(III) excited state lifetime, while the rise time could be reduced by increasing the pump power without compromising the isolation. The clamping technique demonstrated in this paper should be applicable to any optical switch incorporating a gain medium.
We demonstrate a fast, stable, all-optical switch using a thermal effect in cobalt-doped fiber. 46 ns switching is accomplished in a 10-m Sagnac loop containing a 2.55-cm length of doped fiber. Compared to other demonstrated all- optical switches using doped fibers, this switch offers a significant increase in speed and stability, as well as a significant reduction in fiber length.
A large second-order nonlinearity in Ge-doped silica glass induced by simultaneous applications of a high DC electric field and ultraviolet irradiation (UV-poling) is reported. The nonlinearity thus induced has been found to be quite relevant to the GeE' center. The d coefficient so far obtained exceeds the second biggest d-tensor component of LiNbO3.
We report the first measurement of the frequency and polarization dependence of the low-frequency third-order nonlinear susceptibility (chi) (3)(-(omega) ;(omega) ,0,0) of silica. In the frequency range tested (0.5 - 19 MHz), we observed sharp resonances with a complex dependence on polarization. Observations are quantitatively well explained by a theoretical model that assumes the presence of two contributions to (chi) (3), namely the Kerr effect and electrostriction. The model shows that our measurements are consistent with (1) a DC Kerr constant of 1.9 X 10-22 m2/V2 with a 3:1 polarization dependence, comparable to the Kerr constant at optical frequencies, and (2) an electrostriction modulation that is greatly enhanced by mechanical resonances of the sample and exhibits a polarization dependence of 2.3:1, in agreement with an elasto-optic model. This work suggests new means of producing a low-voltage, high-frequency phase modulator by operating at a fundamental resonance of the structure. It also lends credence to the general belief that DC rectification does not fully account for the large second- order nonlinearity that occurs in poled silica.
The space charge region associated with the second-order nonlinearity in thermally poled fused silica was examined using secondary ion mass spectrometry (SIMS). Results for four samples are presented here: two flame-fused quartz samples (type-II fused silica), a higher purity flame-fused synthetic (type-III) fused silica, and a type-II fused silica treated with deuterated water. SIMS results show: (1) ionic depletion regions formed by migration of alkali ion impurities with different mobilities, (2) injection of surface alkali contaminants arising from the high surface fields, and (3) evidence of field injection as well as thermal diffusion of adsorbed surface water.
We show that the Maker fringe analysis previously used to characterize the nonlinearity of thermally poled silica does not yield a unique nonlinear profile d33(z). This problem can be solved by using this method in conjunction with another independent measurement technique in order to first determine the general shape of the nonlinear profile (e.g. whether it exhibits one or multiple peaks). We demonstrate the validity of this approach by characterizing the same poled silica sample first by the Maker fringe analysis, then by imaging the second-harmonic intensity distribution generated by its nonlinear region. The latter indicates that the second-harmonic intensity exhibits a single-peaked distribution, and the former that the d33(z) profile is closely modeled by a buried depth of 5 micrometers , a 1/e width of 10 micrometers , and a peak d33 of 0.44 pm/V.
Recent work on the thermal poling of silicate optical fiber is presented. This paper includes the background on thermal poling, optimization of poling conditions, lifetime of the electro-optic effect as well as discussions on the change in (chi) (3) after poling and the validity of the frozen-in field model for thermal poling.