Volume Bragg grating (VBG) with refractive index modulation (RIM) apodized with sinusoidal semiperiod profile is studied theoretically and experimentally. An apodized VBG of this type was fabricated with a sequential recording of two VBGs with slightly different resonant Bragg wavelengths in the same glass wafer. As a result, a moiré pattern was produced with a constant average refractive index and a slow sinusoidal envelope of RIM. Modeling showed that an apodized VBG with a sinusoidal semiperiod has provided a suppression of the sidelobes in the reflection spectrum. The experimental measurements are in good agreement with the theoretical predictions. This type of VBG is suitable for high-resolution spectroscopy applications due to a significant reduction of sidelobes.
The design of Q-switched lasers capable of producing pulse widths of 100’s of picoseconds necessitates the cavity length be shorter than a few centimeters. Increasing the amount of energy extracted per pulse requires increasing the mode area of the resonator that for the same cavity length causes exciting higher order transverse modes and decreasing the brightness of the output radiation. To suppress the higher order modes of these multimode resonators while maintaining the compact cavity requires the use of intra-cavity angular filters. A novel Q-switched laser design is presented using transmitting Bragg gratings (TBGs) as angular filters to suppress the higher order transverse modes. The laser consists of a 5 mm thick slab of Nd:YAG, a 3 mm thick slab of Cr:YAG with a 20% transmission, one TBG aligned to suppress the higher order modes along the x-axis, and a 40% output coupler. The gratings are recorded in photo-thermo-refractive (PTR) glass, which has a high damage threshold that can withstand both the high peak powers and high average powers present within the resonator. Using a 4.1 mrad TBG in a 10.8 mm long resonator with an 800μm x 400 μm pump beam, a nearly diffraction limited beam quality of M2 = 1.3 is obtained in a 0.76 mJ pulse with a pulse width of 614 ps.
Power scaling using a higher order mode in a ribbon fiber has previously been proposed. However, methods of selecting the higher order mode and converting to a single lobe high brightness beam are needed. We propose using a multiplexed transmitting Bragg grating (MTBG) to convert a higher order mode into a single lobe beam. Using a ribbon fiber with core dimensions of 107.8 μm by 8.3 μm, we use the MTBG to select a higher order mode oscillating within the resonator with 51.4% efficiency, while simultaneously converting the higher order mode to a beam with diffraction limited divergence of 10.2 mrad containing 60% of the total power.
Increasing the dimensions of a waveguide provides the simplest means of reducing detrimental nonlinear effects, but such systems are inherently multi-mode, reducing the brightness of the system. Furthermore, using rectangular dimensions allows for improved heat extraction, as well as uniform temperature profile within the core. We propose a method of using the angular acceptance of a transmitting Bragg grating (TBG) to filter the fundamental mode of a fiber laser resonator, and as a means to increase the brightness of multi-mode fiber laser. Numerical modeling is used to calculate the diffraction losses needed to suppress the higher order modes in a laser system with saturable gain. The model is tested by constructing an external cavity resonator using an ytterbium doped ribbon fiber with core dimensions of 107.8μm by 8.3μm as the active medium. We show that the TBG increases the beam quality of the system from M2 = 11.3 to M2 = 1.45, while reducing the slope efficiency from 76% to 53%, overall increasing the brightness by 5.1 times.
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.
Power scaling of high power laser resonators is limited due to several nonlinear effects. Scaling to larger mode areas can offset these effects at the cost of decreased beam quality, limiting the brightness that can be achieved from the multi-mode system. In order to improve the brightness from such multi-mode systems, we present a method of transverse mode selection utilizing volume Bragg gratings (VBGs) as an angular filter, allowing for high beam quality from large mode area laser resonators. An overview of transverse mode selection using VBGs is given, with theoretical models showing the effect of the angular selectivity of transmitting VBGs on the resonator modes. Applications of this ideology to the design of laser resonators, with cavity designs and experimental results presented for three types of multimode solid state lasers: a Nd:YVO4 laser with 1 cm cavity length and 0.8 mm diameter beam with an M2 of 1.1, a multimode diode with diffraction limited far field divergence in the slow axis, and a ribbon fiber laser with 13 cores showing M2 improved from 11.3 to 1.5.
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.
Volume Bragg gratings (VBG) recorded in photo-thermo refractive glass (PTR) have high stability, and high damage threshold, allowing for many applications to the design of high power lasers. Gratings recorded in the transmitting geometry (TBG) have narrow angular selectivity, and can be used as a spatial filter in a resonator. Such gratings have previously been useful for improving the brightness of high power diodes, and increasing the beam quality in rod geometry solid state lasers. As the gratings have narrow angular selectivity, losses for higher order modes in the resonator no longer depend on the cavity length, allowing for the construction of short cavities with large mode areas. In this paper, we explore the design of short 1cm cavities using two TBGs as a spatial filter and no aperture in the cavity. The M2 parameter as a function of pump size and angular selectivity of the TBG are explored, using pump diameters ranging from 800um to 2mm and angular selectivity ranging from 11mrad to 1.8mrad. An M2 parameter of 1.05 is reported for an 800μm pump diameter, a 6.2mrad TBG, and a 1cm long cavity.
We demonstrate the recording of volume phase masks in the bulk of photo-thermo-refractive glass. Recording was
produced by exposing the glass to UV radiation through binary amplitude masks. Depending on the profile of the
amplitude mask either a binary volume phase mask or a grayscale phase mask may be produced. Volume phase masks
have been used to generate Fresnel lenses, convert a Gaussian beam into higher order Hermite-Gauss and Laguerre-Gauss modes, to produce optical vortices, and to create aberration-correcting optical components.
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.
We present a new method for phase stabilization of a holographic recording system for volume Bragg gratings. The primary feature of this method is that it is extremely flexible and simple to integrate into an existing holographic recording setup. The setup allows for Bragg gratings with arbitrary tilt and resonant wavelength to be recorded. An analysis of the effects of phase stabilization and a method for analyzing the effectiveness of this phase stabilization approach are also introduced and successfully demonstrate its benefits.
In the Spring Semester of 2011, Univ. of Central Florida's CREOL introduced an elective course in Optomechanical Design. In addition to homework assignments and exams, one component of the course grade was a design project. Rather than the traditional "assigned" project, the instructor experimented with a novel research-centric approach. Specifically,
students were asked to select a project directly applicable to their graduate research. While challenging for the instructor to grade, student motivation and performance remained exceptionally high throughout the semester. This paper summarizes the background, projects, and pedagogical benefits of such a research-centric approach to project-based learning.
The recent development of kW fiber laser sources makes the concept of laser systems operating at power levels from
tens of kilowatts up to 100-kilowatt levels a reality. The use of volume Bragg gratings for spectral beam combining is
one approach to achieve that goal. To make such systems compact, lower the complexity and minimize the induced
thermal distortions we propose and demonstrate the use of special volume Bragg elements which have several Bragg
gratings written inside as combining optical components. The multiplexed volume Bragg gratings (MVBGs) were
recorded in photo-thermo refractive glass and three beams with total power of 420 W were successfully combined using
one MVBG. The combining efficiency was 97% and there was no significant beam quality degradation. The results
demonstrated that the approach of using multiplexed volume Bragg gratings for spectral beam combining is an excellent
extension to the current state of the art combining techniques. Especially valuable is the capability to reduce the number
of optical elements in the system and while being able to manage the expected thermal load when kilowatt level sources
are used for beam combining.
A Fabry-Perot etalon, consisting of two π phase shifted reflecting volume Bragg gratings, is presented. These gratings
are obtained as a moiré pattern resulting from sequential recording of interference patterns with different periods in
photo-thermo-refractive glass and called moiré volume Bragg gratings (MVBGs). A detailed investigation of the
fundamental operating principles and measurement techniques for phase shifted gratings is shown. Experimental results
demonstrating a MVBG with a 15 pm bandwidth and 90% transmission at resonance are presented. The use of the
MVBG for longitudinal mode selection in a laser resonator is shown.