Marc SeGall

Strong Bragg Gratings in Highly Photosensitive Photo-Thermo-Refractive-Glass Optical Fiber

A new type of photosensitive fiber is demonstrated. Long lengths (>;100 m) of coreless optical fiber are fabricated from highly photosensitive photo-thermo-refractive glass. A minimum loss of <;2 dB/m is measured. A holographic technique using low power near-UV two-beam interference patterns is applied to record strong and robust Bragg gratings inside the fiber. The gratings show no degradation when heated up to 425 °C for several hours.

Binary volume phase masks in photo-thermo-refractive glass

Permanent binary phase masks with planar surfaces and high tolerance to laser radiation are recorded in the volume of photo-thermo-refractive glass using the contact copying technique and binary amplitude master masks. Conversion of a Gaussian beam to higher order modes is shown.

Stabilization system for holographic recording of volume Bragg gratings using a corner cube retroreflector

Volume Bragg gratings serve an important role in laser development as devices that are able to manipulate both the wavelength and angular spectrum of light. A common method for producing gratings is holographic recording of a two collimated beam interference pattern in a photosensitive material. This process requires stability of the recording system at a level of a fraction of the recording wavelength. A new method for measuring and stabilizing the phase of the recording beams is presented that is extremely flexible and simple to integrate into an existing holographic recording setup and independent of the type of recording media. It is shown that the presented method increases visibility of an interference pattern and for photo-thermo-refractive glass enables enhancement of the spatial refractive index modulation. The use of this technique allows for longer recording times that can lead to the use of expanded recording beams for large aperture gratings.

Holographically encoded volume phase masks

Abstract. We present here a method to create spectrally addressable phase masks by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Moreover, this approach enables the capability to encode and multiplex several phase masks into a single holographic element without cross-talk while maintaining a high diffraction efficiency. As examples, we demonstrate fiber mode conversion with near-theoretical conversion efficiency as well as simultaneous mode conversion and beam combining at wavelengths far from the original hologram recording wavelength.

Effect of aberrations in a holographic system on reflecting volume Bragg gratings

The effect of aberrations in the recording beams of a holographic setup is discussed regarding the deterioration of properties of a reflecting volume Bragg grating. Imperfect recording beams result in a spatially varying grating vector, which causes broadening, asymmetry, and washed out side lobes in the reflection spectrum as well as a corresponding reduction in peak diffraction efficiency. These effects are more significant for gratings with narrower spectral widths.

High-contrast filtering by multipass diffraction between paired volume Bragg gratings.

High-contrast filtering via multiple reflections between matched volume Bragg gratings (VBGs) is demonstrated. The use of multiple reflections serves to increase the suppression ratio of the out-of-band spectral content such that contributions of grating sidelobes can be mitigated. The result is a device that retains spectral and angular selectivity and diffracts light into a single order with high efficiency but reshapes the spectral/angular response to achieve higher signal-to-noise ratios. We demonstrate that multipass spectral filters can be recorded with extremely high suppression ratios using reflecting Bragg gratings (RBGs) in three different configurations. These filters demonstrate roll-offs of over 150 dB/nm. Similarly, we demonstrate angular filtering by multipass transmitting gratings.

Simultaneous laser beam combining and mode conversion using multiplexed volume phase elements

To scale to power levels of up to tens of kW with a few fiber lasers, the best candidates are large core fibers guiding a few large-area higher order modes with the outputs of these fibers combined into a single beam. However, in many applications it is desirable to convert these higher order modes into a Gaussian profile. Here, we propose a method to accomplish this task via single volume phase element. This element accepts multiple higher order mode beams and simultaneously converts and combines them to a single Gaussian profile in the far field.

The effect of aberrated recording beams on reflecting Bragg gratings

The effect of aberrations present in the recording beams of a holographic setup is discussed regarding the period and spectral response of a reflecting volume Bragg grating. Imperfect recording beams result in spatially varying resonant wavelengths and the side lobes of the spectrum are washed out. Asymmetrical spectra, spectral broadening, and a reduction in peak diffraction efficiency may also be present, though these effects are less significant for gratings with wider spectral widths. Reflecting Bragg gratings (RBGs) are used as elements in a variety of applications including spectral beam combining1,2, mode locking3,4, longitudinal and transverse mode selection in lasers5,6, and sensing7,8. For applications requiring narrow spectral selectivity9, or large apertures10, these gratings must have a uniform period throughout the length of the recording medium, which may be on the order of millimeters. However, when using typical recording techniques such as two-beam interference for large aperture gratings and phase-mask recording of fiber gratings, aberrations from the optical elements in the system result in an imperfect grating structure11-13. In this paper we consider the effects of aberrations on large aperture gratings recorded in thick media using the two-beam interference technique. Previous works in analyzing the effects of aberrations have considered the effects of aberrations in a single recording plane where the beams perfectly overlap. Such an approach is valid for thin media (on the order of tens of microns), but for thick recording media (on the order of several millimeters) there will be a significant shift in the positions of the beams relative to each other as they traverse the recording medium. Therefore, the fringe pattern produced will not be constant throughout the grating if one or both beams have a non-uniform wavefront. Such non-uniform gratings may have a wider spectral width, a shifted resonant wavelength, or other problems. It is imperative therefore to know what the effects of aberrations will have on the properties of the RBGs. Thus, in this paper we consider the imperfect fringe pattern caused by the recording beams and its effect on the diffraction efficiency and spectral profile of the recorded reflecting volume Bragg gratings.

Achromatic phase elements based on a combination of surface and volume diffractive gratings

Phase masks are important optical elements that have been utilized for several decades in a large variety of applications. Recently, we demonstrated a new type of phase masks fabricated by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Here, we present a new method of true achromatization of this type of phase masks that removes the need for angle tuning and is implemented by combining this holographic phase masks approach with a pair of surface diffraction gratings.

High spectral contrast filtering produced by multiple pass reflections from paired Bragg gratings in PTR glass

The properties of multiple reflections from narrow bandwidth reflection Bragg gratings are presented. The use of multiple reflections serves to increase the suppression ratio of the out-of-band spectral content such that contributions of grating sidelobes can be mitigated. The result is a device which retains spectral and angular selectivity in a single high efficiency diffraction order but reshapes spectral/angular response to achieve higher signal to noise ratios (SNR). The material for recording these high suppression devices is photo-thermo-refractive (PTR) glass. PTR is a highly homogeneous photosensitive glass with features such as low losses and high laser damage threshold. It has recently been used with good success to record permanent volume Bragg gratings with high efficiency and narrow band selectivity for use in laser cavities. Multiple reflections from the grating surface are achieved using several different arrangements. The multiple pass grating reflections are demonstrated and compared to the performance of a single reflection from a volume Bragg grating.