Novel techniques to modulate the refraction index used to ‘write’ Bragg gratings directly on the core of optical fibers have recently opened the way to high sensitivity optical-based strain measurements. The method, known as Fiber Bragg grating (FBG) dilatometry has a resolution L/L < 10-6, and can be used at cryogenic temperatures, in high magnetic fields, to measure samples as small as one millimeter in length. Most importantly, FBG dilatometry delivers data that is unaffected by electromagnetic noise ‘at the speed of light’!! In other words, the speed at which reflection spectra from the FBG can be read and recorded is only limited by the speed at which In-GaAs line array detectors can presently be read. Current state-of-the-art commercially available cameras operate acquiring one full 1024-pixel reflection spectrum with 14 bits resolution every 22 microseconds. These characteristics set optical FBG dilatometry apart as the only nanometer-resolution strain measurement method available and suitable for the very high magnetic fields produced at the NHMFL with resistive, resistive-superconductive hybrid and pulsed magnetic fields.

 The schematic diagram in Fig. 1 on the                             page describes the operation principles of optical FBG strain measurements. A ‘white’ light source, in our case a solid state light emiting diode (SLED), is used to send broad spectrum infrared light into an optical fiber furbished with a 1mm-long Bragg grating that reflects light at  λB = 1550 nm at room temperature. By means of an optical circulator (not shown) the reflected light is diverted into an spectrometer where the beam is spread, and then used to iluminate an In-GaAs line array camera acquiring the data at 46kHz. The sample is attached to the FBG section of the 9μm silica core/125 μm cladding single mode fiber with a cyano-acrilate bond as shown in Fig. 2 , and cooled to cryogenic temperatures using standar 3He/4He cryostats. Magnetic fields generated with continuous or pulsed resistive magnets are applied parallel or perpendicular to the strain measurement, to a recent world record of 100.75 T. The upper left pannel in Fig. 1 show data recently obtained in the spin transition system LaCoO₃, where the single crystal sample lattice and magnetization response to a change of spin state induced by an applied magnetic field was observed. The results were interpreted as a transition of some of the Co³⁺ ions in the perovskite material from a spin S = 0 state to a spin S = 1 state. It is believed that a similar lattice effect takes places in related ferric perovskites in Earth’s lower mantle under pressure. A full understanding of spin state transitions is yet ellusive, but these studies in high magnetic fields allowed for useful comparisons with theoretical predictions.

 The optical FBG dilatometry technique is now available to all qualified NHMFL users.

• Magnetostriction and magnetic texture to 100.75 Tesla in frustrated

   SrCu2(BO3)2. M. Jaime et al., Proc. Natl. Acad. Sci. 109, 12404 (2012).

• Cascade of Magnetic Field Induced Spin Transitions in LaCoO3.  M.

   Altarawneh et al., Phys. Rev. Lett. 109, 037201 (2012).

• Anisotropic cascade of field-induced phase transitions in the

   frustrated spin-ladder system BiCu2PO6. Y. Kohama et al., Phys., Rev.

   Lett. (2012) in the press.