Sergiy Mokhov

Reflection of light by composite volume holograms: Fresnel corrections and Fabry-Perot spectral filtering

Effects in composite volume Bragg gratings (VBGs) are studied theoretically and experimentally. The mathematics of reflection is formulated with a unified account of Fresnel reflections by the boundaries and of VBG reflection. We introduce the strength S of reflection by an arbitrary lossless element such that the intensity of reflection is R=tanh(2) S. We show that the ultimate maximum/minimum of reflection by a composite lossless system corresponds to addition/subtraction of relevant strengths of the sequential elements. We present a new physical interpretation of standard Fresnel reflection: strength for TE or for TM reflection is given by addition or by subtraction of two contributions. One of them is an angle-independent contribution of the impedance step, while the other is an angle-dependent contribution of the step of propagation speed. We study an assembly of two VBG mirrors with a thin immersion layer between them that constitutes a Fabry-Perot spectral filter. The transmission wavelength of the assembly depends on the phase shift between the two VBGs. Spectral resolution Deltalambda(FWHM)=25 pm at lambda=1063.4 nm is achieved with the device of small total physical thickness 2L=5.52 mm.

More on analyzing the reflection of a laser beam by a deformed highly reflective volume Bragg grating using iteration of the beam propagation method

A further extension of the iteration method for beam propagation calculation is presented that can be applied for volume Bragg gratings (VBGs) with extremely large grating strength. A reformulation of the beam propagation formulation is presented for analyzing the reflection of a laser beam by a deformed VBG. These methods will be shown to be very accurate and efficient. A VBG with generic z-dependent distortion has been analyzed using these methods.

Ultranarrow bandwidth moiré reflecting Bragg gratings recorded in photo-thermo-refractive glass

Over the last 15 years, a new photosensitive glass called photo-thermo-refractive (PTR) glass has been developed for the recording of volume holographic elements such as volume Bragg gratings. Main advantage of such elements is that they allow obtaining narrowband filters in both spectral and angular spaces [1]. However, the further decrease of the spectral bandwidth of volume Bragg gratings represents a technological issue and is thus limited to values between 50 and 100 pm depending on their central wavelength. For some applications such as LIDARS or the longitudinal mode selection of Nd:YAG lasers [2], bandwidth in the range of a few picometers or tens of picometers would be required. Hence, new technology must be developed in order to achieve ultra-narrowband selectivity. In this paper we present the fabrication of so-called Moiré reflecting Bragg gratings (Moiré RBG), resulting from the recording of two reflecting Bragg gratings with shifted periods (figure 1). Such structure was widely investigated in the past in germanium doped silica fibers [3]. However, due to the unavailability of bulk photosensitive materials, no experimental demonstration of Moiré RBG was performed. PTR glass allows recording of high efficiency reflecting Bragg gratings in a few millimeters glass samples and losses can be kept low below 1%. Hence, PTR glass makes an ideal candidate for the recording of Moiré RBG with very high transmission at resonance.

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.

Stretching and compressing of short laser pulses by chirped volume Bragg gratings: analytic and numerical modeling

Abstract. The theory of stretching and compression of short light pulses by chirped volume Bragg gratings (CBGs) is reviewed based on spectral decomposition of short pulses and on wavelength-dependent coupled wave equations. Analytic theory of diffraction efficiency of CBG with constant chirp and approximate theory time-delay dispersion is presented. Comparison of approximate analytic results with exact numeric coupled-wave modeling shows excellent agreement for smooth heterogeneities of CBGs.

200 fs, 50 W fiber-CPA system based on chirped-volume-Bragg-gratings

Record-short pulses of 200fs have been obtained from a power-scalable-Yb-fiber-CPA system that uses chirped-volume-Bragg-gratings for the stretcher and compressor. The power was scaled up to 50W with a corresponding 33W of compressed power.

Strength of electromagnetic, acoustic and Schrödinger reflections

The notion of reflection strength S of a plane wave by an arbitrary non-absorbing layer is introduced, so that the intensity of reflection is R=tanh2S. We have shown that the total strength of reflection by a sequence of elements is expressed through particular element strengths and mutual phases between them by a simple addition rule; in particular, its possible maximum is the sum of the absolute strengths of constituents. We show that the standard Fresnel reflection may be understood in terms of variable S as a sum or difference of two separate contributions, due to an impedance step and a speed step. Strength of reflection for propagating acoustic and quantum mechanical waves is also discussed.

Longitudinal mode selection in laser cavity by moiré volume Bragg grating

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.

Moiré volume Bragg grating filter with tunable bandwidth

We propose a monolithic large-aperture narrowband optical filter based on a moiré volume Bragg grating formed by two sequentially recorded gratings with slightly different resonant wavelengths. Such recording creates a spatial modulation of refractive index with a slowly varying sinusoidal envelope. By cutting a specimen at a small angle, to a thickness of one-period of this envelope, the longitudinal envelope profile will shift from a sine profile to a cosine profile across the face of the device. The transmission peak of the filter has a tunable bandwidth while remaining at a fixed resonant wavelength by a transversal shift of incidence position. Analytical expressions for the tunable bandwidth of such a filter are calculated and experimental data from a filter operating at 1064 nm with bandwidth range 30-90 pm is demonstrated.

Multiplexed Reflective Volume Bragg Grating for Passive Coherent Beam Combining

Multiplexed Bragg grating recorded in photo-thermo-refractive glass can be efficient combiner for coherent locking of high-power lasers. Spectral properties of two-channel combiner are studied theoretically and experimentally. Multi-channel scaling of this approach is discussed.