Benoit Sévigny

High power cladding light strippers

The ability to strip cladding light from double clad fiber (DCF) fibers is required for many different reasons, one example is to strip unwanted cladding light in fiber lasers and amplifiers. When removing residual pump light for example, this light is characterized by a large numerical aperture distribution and can reach power levels into the hundreds of watts. By locally changing the numerical aperture (N.A.) of the light to be stripped, it is possible to achieve significant attenuation even for the low N.A. rays such as escaped core modes in the same device. In order to test the power-handling capability of this device, one hundred watts of pump and signal light is launched from a tapered fusedbundle (TFB) 6+1x1 combiner into a high power-cladding stripper. In this case, the fiber used in the cladding stripper and the output fiber of the TFB was a 20/400 0.06/0.46 N.A. double clad fiber. Attenuation of over 20dB in the cladding was measured without signal loss. By spreading out the heat load generated by the unwanted light that is stripped, the package remained safely below the maximum operating temperature internally and externally. This is achieved by uniformly stripping the energy along the length of the fiber within the stripper. Different adhesive and heat sinking techniques are used to achieve this uniform removal of the light. This suggests that these cladding strippers can be used to strip hundreds of watts of light in high power fiber lasers and amplifiers.

Pump combiner loss as a function of input numerical aperture power distribution

High-power combiner designs (such as kilowatt-class combiners and beyond) are increasingly aggressive on brightness conservation in order to reduce the brightness loss of the pumps as much as possible in both direct diode combining and pump and signal coupling, especially with the advent of next-generation high-power pumps. Since most of the pump loss is due to brightness loss across the combiner, tighter designs (close to the brightness limit) are considerably more sensitive to variations in the input power distribution as a function of numerical aperture; for instance, next-generation, high-power multi-emitter pumps are likely to have larger numerical apertures than conventional single-emitter diodes. As a consequence, pump insertion loss for a given combiner design sitting close to the brightness limit should be dependant on the input power distribution. Aside from presenting a manufacturing challenge, high brightness combiners also imply more sophisticated testing to allow a deeper understanding of the loss with respect to the far-field distribution of the pump inputs and thus enable the extrapolation of loss for an arbitrary, cylindrically symmetric radiant intensity distribution. In this paper, we present a novel test method to measure loss as a function of numerical aperture (NA) fill factor using a variable NA source with square-shaped far field distributions. Results are presented for a range of combiners, such as 7x1 and 19x1 pump combiners, with different brightness ratio and fiber inputs. Combiners violating the brightness conservation equation are also characterized in order to estimate the loss as a function of input power vs. NA distribution and fill factor.

Modal sensitivity analysis for single mode operation in large mode area fiber

Most of the current large mode area (LMA) fibers are few-moded designs using a large, low numerical aperture (N.A.) core, which promotes mode coupling between core modes and increases bending losses (coupling with claddingmodes), which is undesirable both in terms ofmode area and beamquality. Furthermore, short LMA fiber lengths and small cladding diameters are needed to minimize nonlinear effects and maximize pump absorption respectively in high-power pulsed laser systems. Although gain fiber coiling is a widely used technique to filter-out unwanted modes in LMA fibers, coupling between modes can still occur in component leads and relay fibers. In relay fiber, light coupled into higher-order modes can subsequently be lost in the coiling or continue as higher-order modes, which has the overall effect of reducing the effective transmission of the LP01 mode and degrading the beam quality. However, maximum transmission of the LP01 mode is often required in order to have the best possible beam quality (minimal M2). Launching in an LMA fiber with a mode field adapter (MFA)1 provides an excellent way of ensuring maximum LP01 coupling, but preservation of this mode requires highmodal stability in the output fiber. Small cladding, low N.A. LMA fibers have the disadvantage of being extremely sensitive to external forces in real-life applications, which is unwanted for systems where highly sensitive mode coupling can occur. In this paper, we present a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter. Furthermore, we present the impact of higher N.A. as a solution to increase mode stability in terms of its effect on peak power, effective mode area and coupling efficiency.

High power monolithically integrated all-fiber laser design using single-chip multimode pumps for high reliability operation

We present an all-fiber monolithically integrated fiber laser based on a custom tapered fused bundle pump combiner with 32 inputs ports connected to a double clad gain fiber. The pump combiner is designed to provide high isolation between signal and pumps fibers providing intrinsic pump protection. This configuration can generate more than 100W of continuous wave (CW) laser light using single-chip multimode pumps enabling long term reliability.

High-resolution refractive index anisotropy measurement in optical fibers through phase retardation modulation

We present an improved, high-resolution method for the measurement of phase retardation induced by the material birefringence of optical fibers. Such a method can be used to retrieve information about the spatial distribution of refractive index anisotropy in the fiber by comparing the accumulated phase of a polarization component oriented along the fiber transmission axis and another located in the transverse plane. The method is based on the nonlinear regression of a phase modulated signal of known modulation amplitude altered by the sample. Experimental results obtained with our method for a standard telecommunications fiber (the Corning SMF-28) as well as photosensitive fibers (Fibercore PS1250 and PS1500) are presented with a noise-limited phase resolution below 10(-4) radians and a spatial resolution below 1 microm. An analysis of the limitation of such measurement methods is also presented including diffraction by the fibers.

LMA fibers modal decomposition using image factor analysis

A new method is presented for the analysis of the modal content of a beam travelling in a waveguide. This method uses a simple optical set up to record beam images. Depending on the application, the source can be broad band (BBS) or a tunable laser. The method uses the eigenmode profiles of the waveguide under test, either theoretical or experimental ones. In this case, the technique is applied to characterize the modal content of few moded large mode area (LMA) fibers. Such LMA fibers are typically used in high power fiber lasers and amplifiers to reduce sensitivity to non-linear effects. By calculating the scalar products of the unfolded experimental and theoretical 2D profiles, the modal content is obtained. Access to such cost effective and easy to implement diagnosis tool will greatly help improving modal quality preservation in components and systems based on the fundamental mode operation of few moded LMA fibers. The high precision and performance of the method is evaluated using both computer generated and experimental data sets.

High core and cladding isolation termination for high-power lasers and amplifiers

As overall power increases in fiber lasers and amplifiers, the amount of optical power which must be dealt with in order to obtain high core to core and core to cladding isolation also increases. This unwanted light can represent hundreds of watts and must be managed adequately. By combining a proper termination (end cap) design and cladding stripping techniques it is possible to obtain a robust output beam delivery component. The cladding stripping techniques are inspired by previous work done on high power cladding strippers. All measurement presented here are done with a flat end cap. Both core to core and core to cladding isolation will be better with an angled end cap. A core-to-core isolation of over 25dB was measured, while core to cladding was over 30dB. Power handling was characterized by the capability of the device to handle optical power loss, rather than transmitted power. The component dissipated over 50 watts of optical power due to isolation. The above results show that understanding the mechanisms of optical loss for forward and backward propagating light in a end cap and the heat load that these losses generate is the key to deliver kilowatts of optical power and protect the integrity of the system.

All-fiber 32x1 pump combiner with high isolation for high power fiber laser

We present an all-fiberreg pump combiner with 32 ports. This device is designed to provide high isolation providing intrinsic pump protection. We demonstrated that over 200 W of pump power can be deliver in 20/400 mum fiber.

Drawing-Induced Index Anisotropy in Single-Material Endlessly Single-Mode Microstructured Optical Fibers

We report evidence of frozen-in viscoelastic strain and viscosity gradient arising from fiber drawing of pure silica endlessly single-mode holey optical fiber through phase retardation measurements. Tomographic reconstruction indicates non-uniform stress distribution during cooling.

Improved Method for Two-dimensional Determination of the Magnitude and Orientation of Weak Birefringence

A method for measuring refractive index anisotropy and orientation is described. Birefringence characterization of laser-written integrated optics devices is presented as an example application of the method.