High power fiber lasers are now mature products and have numerous applications as medical, military, industrial processing and telecommunication. Usually single mode fiber lasers are pumped with single mode pump diode featuring low output power. Double-clad fiber lasers (DCFL) overcome the issue related to low power single mode pump diodes to take fully advantage of large low cost multimode diodes. Improvement in high power multimode diode combined with double-clad fiber technology permit to DCFLs to be very efficient and promising. They are mainly operating in CW regime however Q-switch or mode lock regimes are also achievable. DCFLs in CW regime can feature several watt output power. Up to 30W CW Ytterbium DCFL has been demonstrated indicating that fiber laser is able to handle very high optical power without any optical damage. Also fiber lasers included tens of meters long fiber, improving heat dissipation and leading to a less critical thermal management compare to bulk lasers. For terrestrial telecommunication applications, DCFLs are mainly operating in CW regime and in particular, Ytterbium DCFL is largely used as a pump for Raman Fiber Laser (RFL). The RFLs are used for components testing, pumping EDFAs and Raman amplifiers.
We present a theory and experiment of the pulse train generated by a rational harmonic mode locked ring fiber laser. The pulse width is calculated as a function of the rational harmonic order and the optical transfer function of the modulator. The theoretical work is based on a time domain analysis, which predicts that the pulse width decreases when the rational harmonic order goes up. The pulse width as a function of the modulation amplitude and bias level of the modulator was measured, the experimental results agree with the theory.
Subsystems based on LiNbO3 are attractive because the modulators are now commercially available, can operate up to very high speeds, and can operate over a wide wavelength range. We describe the principle of operation and performance of high repetition rate pulsed sources based on LiNbO3 modulators. We have generated transform limited pulses at 20 GHz repetition rate with a pulsewidth of 8 ps and at 40 GHz repetition rate with a pulsewidth of 6.5 ps using two modulators in series. These modulators were driven by sinusoidal signals at 10 GHz. The analysis shows generation of shorter pulses at higher repetition rate is feasible with higher bandwidth modulators.
We present a new, to our knowledge, method of Stokes-anti-Stokes Raman amplification in fiber, based on our idea of anti-Stokes generation at stimulated Raman scattering in media with variations of third order nonlinearity along longitudinal coordinate. The quasi-phase matching conditions at different pump wave intensity are obtained by numerical simulation. The dependence of layers lengths on pump wave intensity is computed. We received the models of media in which the Stokes-anti-Stokes amplification coefficient is higher than 10 dB. We propose the analytical model of quasi-phase matching stimulated Raman scattering in silica fiber which shows significant calculations rate improvement (~120 times) comparison with differential equations model. The results of our research can be used for creation of new effective optical fiber amplifiers.
An analytic computer-oriented model for the Er / Yb co-doped silicate fiber amplifier is presented. The model is based on iterative solving the rate and propagation equations in their recursive forms at uniform discrete points along the fiber. It is capable of handling double-clad multimode-pumped fiber configurations, as well as conventional single-mode-pumped ones (with the signal featuring single-mode propagation in both cases). Arbitrary number of signals and pumps including wavelength-multiplexed ones from any direction can be treated. Amplification-deteriorating physical effects characteristic of the Er / Yb co-doped system, such as Yb amplified spontaneous emission, Er direct pumping, Er ions up-conversion and pairs formation, pump excited-state absorption and Er - Yb back-transfer, have been accounted for, in addition to the direct Yb - Er energy transfer. The relative importance of these effects in the amplification process has been modeled and discussed on the basis of experimentally determined fiber physical parameters.
We propose a novel cascaded amplifier system for long-haul PSK or FSK transmission that equalizes WDM channel powers completely passively. An unflattened EDFA is asymmetrically placed within an all-fiber Sagnac interferometer. The Kerr nonlinearity of the Sagnac-loop fiber induces a net phase difference between the counterpropagating signals. Individual channels have independent, nonlinear transfer functions upon exiting the loop. This system provides higher gain for weak channels, while strong channels receive reduced gain. Thus, all channels approach and maintain a steady-state power level through successive amplifications. This system requires no active feedback mechanisms to maintain channel power equality. Its performance is not affected by changes in the gain spectrum of the optical amplifier and, unlike all other power equalization or gain-flattening schemes, the degree of equalization improves with increasing number of amplifications. This presentation will discuss the operating principle of this device, theoretical predictions of its properties, and work in progress towards an experimental proof of principle.
In this work we have tested characteristics of EDFA based on a single-mode Er-doped fiber, pumped at 1480 nm by two-stage Raman fiber converter. As an active media of Raman fiber converter a single-mode fiber with phosphorus doped core was used. The conversion efficiency of Raman converter was measured to be 36%. Output power of EDFA as high as 26 dBm was achieved at wavelengths 1554 and 1582 nm corresponding to C and L-band of WDM systems.
While fiber optic Bragg grating sensors have emerged as a viable commercial product and principle component within numerous fiber optic components, they possess the inherent inflexibility that once written, the nonstressed grating spacing is fixed. An alternative method of fabricating Bragg gratings where a photosensitive fiber core material (similar to that used in photogrey sunglasses) exists at the grating site along the fiber has been examined. As opposed to a conventional grating writing method using lateral illumination of the fiber core, in this instance the diffraction rating is written via a deliberate intensity variation in the light which is injected into the fiber. The slight difference in refractive index between the photogrey section of the core and the regular fiber causes an internal Fabry-Perot resonator cavity to be established. By using an intensity-modulated high power laser, the illuminating modulation pattern reflects back and forth within this cavity establishing a standing wave pattern. Different patterns may be launched into the fiber resulting in a grating spacing which is variable. This standing wave pattern effectively illuminates the photogrey section nonuniformly with the high power portions of the standing wave pattern causing more darkening - thereby in essence creating the Bragg diffraction grating. Removal of this illumination source results in a grating that fades away yielding a re-writable component. An examination of this type or re-writable component will be reported along with its suitability for 100- and 5-GHz DWDM applications.
We have developed long-period fiber gratings (LPFGs) utilizing the photoelastic effect and have demonstrated polarization-independent operation. The LPFG is made by pressing a standard, jacketed single-mode fiber between a flat plate and a plate with grooves mechanically machined with a suitable period. The grating's transmission spectrum is easily tuned by adjusting pressure, grating tilt, and length. Furthermore, the grating can be completely erased by removing the pressure from the fiber. Grating attenuation greater than 25 dB has been demonstrated with a notch-location polarization dependence of +-4 nm. In this paper we report drastic reduction in this polarization dependence by two different approaches. Passing through the grating a second time after reflecting off a Faraday rotator mirror was successful; this method may be used with other types of LPFGs. The second approach utilizes our mechanical grating's ability to be double-passed with two fibers side-by-side. Between passes, a fiber-loop half-wave plate aligned at 45 degrees to the plane of the grooved plate swaps power between x- and y-polarization states. The resulting output's measured polarization dependence was smaller than +/- 0.2 nm. Further improvement is expected through careful tuning of the wave plate. We also report a computer model of the filter spectrum and its polarization dependence, which takes into account non-uniform index perturbation, lossy cladding modes, cladding index perturbation, as well as the polarization dependence of the photoelastic effect, characteristics not usually present in UV-induced LPFGs. The model generates transmission spectra that agree quite well with experimental results.
This paper contrasts the photosensitivity responses and processing windows between two extreme approaches in laser structuring of photonic devices: ultrafast and deep-ultraviolet F2 lasers. Low-loss single mode waveguides were formed by scanning in fused silica the focused light from a 50-fs Ti:Sapphire laser and a 157-nm 15-ns F2 laser. The latter source represents the first known demonstration of writing buried waveguide structures in bulk glass without driving ultrafast-laser interaction physics. For the ultrafast laser, a refractive index change of 1.0 x 10-3 was noted after an accumulated fluence of ~10 kJ/cm2, a high scanning speed of ~100 micrometers /s, and 100-kHz repetition rate. Longitudinal and side-writing techniques were employed and waveguides were characterized at 0.633-micrometers and 1.5-micrometers wavelengths. For the F2 laser, photosensitivity responses were similar in germanosilicate planar waveguides, and ~10-fold smaller in fused silica. Waveguide writing speeds were ~100-fold slower than for the ultrafast laser because of the smaller 100-Hz repetition rate. Overall, ultrafast lasers and ultraviolet lasers offer strong photosensitivity responses in silica-based glasses that address niche applications in fabricating complex three-dimensional photonic structures and trimming optical circuits for telecommunication applications.
The enormous transmission capacity of the order of thousand of Gb/s offered by single mode fibers cannot be fully utilized with the conventional step index fiber. To increase the bit rate and the repeater spacing, some more advanced fiber designs have to be considered. However for these designs and the new ones to come to a numerical technique, which is fast and accurate, is required for calculating the dispersion. Most of the existing numerical approaches are good enough for the calculation of propagation constant, though some of them are not suited for arbitrary refractive index profile. However for the numerical calculation of dispersion, first and second derivatives of the propagation constant are required. So the propagation constant must be highly accurate so as to ensure accuracy of dispersion calculations. A powerful numerical technique, the quadratic finite element method (FEM) is used for analyzing the modal characteristic of single mode optical fiber with arbitrary refractive index profile. The simulated results when compared with earlier reported ones for step index profile, confirms the accuracy of the proposed numerical technique. It is shown that multiple cladded fiber is better suited for Wideband Systems. Dispersion flattened fibers have been proposed in the past with W-profile, but all such fiber designs have been observed to be bend sensitive in the long wavelength window, as one has to operate very close to the cutoff of the fundamental mode. The cutoff wavelength of the fundamental mode can be increased by introducing a second region of raised index into W-structure and thus, the light which penetrated (leaked) into the outer cladding earlier can now be retrapped. Computations show that by properly optimizing the profile parameters, a fiber can be designed where the dispersion can be kept confined with in +/- 1.0 ps/km-nm over a wide entire wavelength span from 1290 to 1550 nm. The wavelength span includes both the low loss transmission window near 1310 and 1550 nm. The results suggest an excellent wideband optical waveguide for future WDM systems.
Based on our line shape analysis of temperature dependent absorption spectra on InGaAs/AlAsSb single quantum wells, we expect a fast carrier redistribution with in the broad inhomogeneous intersubband absorption spectrum from a wavelength as short as 1.72 micrometers . In addition, due to large resonant 3rd order susceptibility but weak absorption, we expect small saturation intensity (Is) at this short wavelength. We present wavelength dependent saturation measurements to show that the Is is, indeed, lower by more than an order of magnitude compared to that at the main peak (1.88 micrometers ). We also show from the figure of merit estimates that the carrier relaxation time at 1.72 micrometers is expected to be faster at 1.72 micron, consistent with the line shape analysis predictions.
A novel scheme of an optical fiber connector is proposed, which enables coupling between a passive device and two optical fibers without using any bulk optics. By using long-period fiber gratings (LPGs), appreciable coupling was obtained without the help of expensive and alignment-sensitive optics such as lenses and collimators. The incident beam that has been propagated along the core of a fiber is coupled to a cladding mode with a help of a LPG at first. The beam coupling between two fiber-ends occurs in the free-space through the claddings of both fibers. The coupled beam in the cladding of the second fiber is re-coupled to the core mode by the second LPG. The effective beam coupling is possible in this scheme due to the large diameter of the fiber cladding compared with the core. The large beam size reduces the divergence angle in the free space, which reduces the insertion loss. A passive device such as a DWDM filter can be inserted in the free space between the fiber ends. The working distance giving a 3 dB relative loss of the proposed connector was measured to be about 500 micrometers , which was 4-times longer than the case of the core-to-core coupling.
We investigate an extremely complex mechanism of spectral supercontinuum generation at the propagation of intensive femtosecond laser pulses in microstructure (photonic crystal) fibers beyond the slowly-varying envelope approximation. We discuss and compare theoretical approaches grounded on the considerations of field and spectral evolution of the pulses. We present new nonlinear spectral equation, first-order in propagation coordinate, applicable for the study of femtosecond supercontinuum generation in microstructure fiber with arbitrary linear dispersion. We demonstrate a very good agreement to independently published experimental data found in numerical simulations. We study formation and evolution of few-cycle solitary waves under the effects of Kerr and Raman nonlinearities.
A novel method, which utilizes amplified spontaneous emission (ASE) as a secondary pump source, is presented for implanting a linear cavity erbium-doped fiber laser operating in L-Band. The output wavelength tuned from 1583nm to 1600nm, about 17nm tuning range, was obtained in the experiment and the stability of the laser is very good.
Time domain ABCD matrix formalism is a useful model for analyzing the characteristics of actively modelocked fiber laser. Basing on this model and giving more consideration on the influences of optical fiber dispersion and optical fiber nonlinearity, we analyzed the laserí»s characteristics of actively modelocked fiber laser, and compared the theoretical analysis results with experimental ones.