Volume Bragg Lasers

Ivan Divliansky1, Apurva Jain1, Derrek Drachenberg1, Alexey Podvyaznyy2, Vadim Smirnov2, George Venus1, Leonid Glebov1

1 CREOL/ The College of Optics and Photonics, University of Central Florida, USA 2 OptiGrate Corp. Orlando, FL, USA

1. ABSTRACT

This paper is a survey of recent achievements at the College of Optics and Photonics/CREOL at the University of Central Florida in the use of newly developed diffractive optical elements which are volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass. Three levels of semiconductor laser design are proposed to achieve high-power low-divergence output. The first level is coherent coupling of emitters by means of PTR Bragg gratings which provide excitation of only one common mode in a multichannel resonator. This type of phase locking automatically leads to a narrow spectral width of emission usually not exceeding a few tens of picometers. The second level is a change of the mechanism of transverse mode selection from spatial selection by apertures to angular selection by PTR Bragg gratings. This approach allows increasing of the aperture size without increasing the length and selecting of arbitrary mode but not necessarily a fundamental one. The third level is spectral beam combining by PTR Bragg gratings which re-direct radiation from several high-power fiber lasers to co-propagate in the same direction with diffraction limited divergence. This approach allows simplification of the thermal management because only passive devices with low absorption (a PTR volume Bragg gratings) are placed in the path of high power laser beam. Keywords: Diode-pumped alkali laser (DPAL), volume Bragg grating, spectral narrowing, spectral beam combining, mode selection, coherent beam combining.

2. INTRODUCTION This paper is a survey of recent achievements at the College of Optics and Photonics/CREOL at the University of Central Florida and at OptiGrate Corporation in the development of diffractive optical elements which are volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass for dramatic improvement of the parameters of different types of lasers. This new technology enabled recording of high efficiency phase volume holograms in an optical quality silicate glass. These holograms are used for the creation of a number of unique diffractive optical elements that provide new opportunities for laser beam control, sensing and signal processing. The same type of glass can be used for the design of conventional optical elements and windows working in high power laser beams and under high level of ionizing radiation. Moreover, this material could be used for the creation of complex elements such as all-solid-state lasers and combined refractive and diffractive elements. Many conventional photosensitive materials for volume hologram recordings have their own merits, combined with considerable drawbacks1. None of those materials though, has a combination of high phase sensitivity, low losses and high tolerance to laser radiation. All these requirements are necessary if this photosensitive material is going to be used in laser design. The availability of diffractive optical elements fabricated from PTR glass has changed the situation allowing for such elements to be part of laser systems with power levels ranging from mW to kW.

3. PHOTO-THERMO REFRACTIVE GLASS One of the hot practical problems of modern optoelectronics (photonics) is fine spectral, angular and spatial filtering. Theoretically, the most efficient solution of the problem can be achieved by the use of thick volume holographic gratings (Bragg gratings). After several decades of intense work, such diffractive elements may be found in several laboratories. However, they are not widely present on the market. We believe that the main cause restricting the wide use of holographic optical elements for a long time was the absence of the appropriate photosensitive material for volume hologram recording. Known photosensitive materials for volume hologram recording include: silver halide photographic emulsions, dichromated gelatin, photoresists, photopolymers, photothermoplastics, polymers with spectral hole-burning, chalcogenide glasses, oxide glasses doped with variable valence rare-earth elements, porous glasses doped with photopolymers, germanium doped silica, and photorefractive crystals1. Each of these materials has its own merits, combined with considerable drawbacks. In particular, organic materials (photographic emulsions, dichromated gelatin, and photopolymers) have high sensitivity due to multistep process including chemical development. However, they shrink in the development process and are sensitive to humidity. Spectral hole burning does not provide high diffraction efficiency. Photo-thermoplastics have low spatial resolution and low thickness. Porous glasses have high level of scattering and low optical quality. Germanium doped silica has extremely low sensitivity; corresponding gratings are realized in fiber configuration only. Photo-refractive crystals have low tolerance to elevated temperatures and are photosensitive after recording. None of the mentioned materials has tolerance to laser radiation comparable with the main materials for optical design – dielectric glasses and crystals. Photo-thermo-refractive (PTR) glass is a relatively new photosensitive material for phase hologram recording. It combines high sensitivity achieved due to two-step process and high optical quality resulting from rich experience accumulated in optical glass technology. PTR glass is a Na2O-ZnO-Al2O3-SiO2 glass doped with silver (Ag), cerium (Ce), and fluorine (F). It is transparent from 350 nm to 2500 nm. The chain of processes, which occurs in these glasses and produces refractive index variation, is as follows2. The first step is the exposure of the glass sample to UV radiation, somewhere in the range from 280 nm to 350 nm. This exposure results in photo-reduction of silver ions Ag+ to atomic state Ag0. This stage is similar to formation of a latent image in a conventional photo film; while no significant changes in optical properties of PTR glass occur here. A number of commercially available lasers with a long length of coherence can be used for such exposure. The further processing is secured by a thermal development. A number of silver containing clusters arise in exposed regions of glass after aging at elevated temperatures, apparently due to increased mobility of Ag0

atoms. These silver containing clusters serve as the nucleation centers for NaF crystal growth. Interaction of those nanocrystals with the surrounding glass matrix causes the decrease of refractive index. Refractive index change Δn about 10-3

(1000 ppm) can be achieved (Fig. 1). Dependence of refractive index decrement on exposure is a hyperbolic function but not an exponential one3. Such Δn enables high efficiency hologram recording in PTR glass wafers with thickness exceeding several hundreds of microns.

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Fig. 2. Absorption spectrum of PTR glass and spectral lines of main laser sources in the window of transparency. Gray arrow shows the region of photosensitivity.

Fig. 1. Dependence of refractive index decrement in PTR glass on exposure at 325 nm after thermal development at 520°C for 2 hours. Solid lines – modeling by hyperbolic functions.

The refractive index modulation that resulted from microcrystalline phase precipitation in glass matrix has two important consequences. The first one is negative. Two-phase system usually shows scattering of optical radiation resulted from difference between refractive indices of vitreous and crystalline phases. However, one can see in Fig. 2 that losses in the near IR region are in the range of 10-2

cm-1. It was found that induced absorption is in the range of 10-3 cm-1

while the main contribution to losses is produced by scattering. This level of scattering is low enough to enable the use of such optical elements in laser resonators. The level of induced scattering is not a basic property of PTR glass but strongly depends on conditions of glass technology and processing. The second consequence of crystalline phase precipitation in PTR glass is extremely positive. There is no way to destroy crystalline particles of NaF in glass matrix by any type of radiation. This is why PTR holograms are stable under exposure to IR, visible, UV, X-ray, and gamma-ray irradiation. Laser damage threshold is in the range of 40 J/cm2

for 8 ns laser pulses at 1064 nm4. Nonlinear refractive index in PTR glass is the same as that for fused silica8 which allows the use of PTR diffractive elements in all types of pulsed lasers. Testing of PTR diffractive grating under irradiation of 4 kW Yb-fiber laser focused to a 4-mm-diameter spot (IPG Photonics, Inc.) showed its stability while heating did not exceed 15 K. Thermal variations of refractive index in PTR glass are very low (dn/dt=5×10-8

1/K). This feature leads to thermal shift of Bragg wavelength in PTR diffractive gratings of around 7 pm/K. Melting temperature of NaF crystals is almost 1000°C. This is why PTR holograms are stable at elevated temperatures and could tolerate thermal cycling up to 400°C. This temperature is determined by plasticity of silicate glass matrix. Volume holographic optical element is an interference pattern recoded in a volume of a photosensitive medium by means of refractive index modulation. Such elements produce transformation of optical beams resulting from diffraction of the propagating wave at the pattern with a modified refractive index. The simplest volume holographic element is a volume Bragg grating which is a system of planar layers with modified refractive index. Depending on the diffraction angle and the orientation of the grating in the plate, one can distinguish several types of Bragg gratings (Fig. 3). A grating is called a transmitting Bragg grating if diffracted beam crosses the back surface, reflecting - if diffracted beam crosses the front surface, and prismatic - if diffracted beam crosses one of the side surfaces.

Incident

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Bragg gratings in PTR glass were recorded by an exposure to the interference pattern of radiation from a He-Cd laser operating at 325 nm with average power of 35 mW. The spatial frequency of the gratings was varied from 50 mm-1

up to about 10,000 mm-1. Volume gratings in both transmitting and reflecting mode were recorded with thickness ranging from 0.5 mm to 25 mm. Maximum aperture of the gratings was up to 35 mm x 35 mm. Maximum diffraction efficiency of the PTR Bragg gratings exceeded 99 %. Detailed mathematical modeling of transmitting PTR Bragg gratings is presented in Ref. [5]. Angular selectivity becomes narrower with increasing of spatial frequency and thickness of the grating. It is important to note that angular selectivity below 1 mrad can be achieved for Bragg gratings with thickness of several millimeters and spatial frequency exceeding several hundreds of lines per millimeter. Spectral selectivity of transmitting grating can be narrowed down to sub-nanometer region.

Fig. 3. Transmitting (left) and reflective (right) volume Bragg gratings in PTR glass. The hologram occupies the whole volume of the glass plate.

An example of angular selectivity of a transmitting PTR grating is shown in Fig. 4. It was found that the experimental profiles of diffraction efficiency coincided with theoretical functions for the uniform sinusoidal Bragg grating within fluctuations which did not exceed 5 %. One can see that angular selectivity of about 2 mrad is achieved in the PTR Bragg grating having thickness of 1 mm. Good coincidence of theoretical and experimental angular distributions shows that the value of refractive index modulation is uniform across the aperture of the hologram. It is important to note that some discrepancy between the model and experimental data are caused not by distortions of a PTR grating but by diffraction limited divergence of a laser beam with an aperture of 1.5 mm. Thus, PTR Bragg gratings can have angular selectivity comparable and even narrower than angular divergence of single-mode laser radiation.

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An example of spectral selectivity of a reflecting PTR grating is shown in Fig. 5. Deflection angles could range from 120 to 180°, angular selectivity from 10 to 100 mrad, spectral selectivity from 0.01 to 2 nm. Absolute diffraction efficiency up to 98.5% and relative diffraction efficiency for plane wave of 99.99% were demonstrated. The example in Fig. 5 shows laser mirror for a semiconductor diode with spectral width of 150 pm and efficiency of about 98%. One can see that spectral width (FWHM) of this filter coincides with the theoretical value within accuracy of few percents. Thus, both transmitting and reflecting gratings with high diffraction efficiency are recorded in PTR glass. These gratings can have different orientation inside a glass plate. Moreover, a number of different gratings can be recorded in the same volume. Low losses in these gratings enable their use as components of laser resonators.

4. VOLUME BRAGG LASERS

It is important to note, that contrary to conventional optical elements, volume diffractive gratings are slits or apertures in space of wave vectors (or in spectral and angular spaces). This feature of thick Bragg gratings allows some unusual approaches for laser design resulting in dramatic decreasing of spectral width, conversion of wide stripe diodes to single mode emitters, and to transformation of high power semiconductor lasers to tunable sources. Such lasers that have volume Bragg gratings as the components of their resonators are called volume Bragg lasers (VOBLAs).

Fig. 5. Spectral selectivity and diffraction efficiency of a reflecting Bragg grating.

Fig. 4. Angular selectivity of transmitting PTR Bragg grating at 1550 nm. Spatial frequency 703 mm-1, refractive index modulation 640 ppm, thickness 1.16 mm.

4.1. Spectral narrowing of laser diodes and bars The increasing interest in diode-pumped alkali laser (DPAL) systems direction is connected with energy-directed civil and military applications. That requires developing laser solutions with high power (~1 kW, ~10 kW orders of magnitude), high beam quality, high efficiency, compactness. Firstly chemical gas (oxygen-iodine) lasers were perused to solve this problem. But they appeared quite inconvenient in terms of their construction. Significant progress of diode-pumping devices which has been made in last decade increased efficiency, power and compactness of solid state lasers. These devices successfully provide kilowatts regime with excellent beam quality but they appear limited in power because of thermo-optical distortions in static gain medium. Finally more than 6 years ago the idea of diode-pumped alkali laser (DPAL) had received a second life due to W. Krupke6 to substitute diode-pumped solid state gain medium with a cell with alkali vapor. That system consolidates efficiency and power scaling of high power diode pumped solid state lasers and high beam quality of gaseous gain medium.

The most attractive feature of alkali media is their extremely high quantum efficiency - 95.3% for Cs, 98.1% for Rb and 99.6% for K as compared to 76% for 1.06 µm Nd:YAG laser. In addition, the use of efficient high power diode laser pumping can significantly increase not only the output power but the wall plug efficiency as well.

The main problem in using pumping commercially available diode laser bars (LDB) with a typical spectral line width of several nm (~1 THz) at tens of watts power is matching with absorption line of pure alkali vapors which is about 500 MHz. There are two approaches to solve this difficult task. First one is to broad the absorption line up to 1 nm by increasing a pressure of buffer gas (required component of alkali gain media cell to provide excited electrons population of alkali laser working energy level) up to 25 atm. However that causes a number of problems in alkali medium such as absorption cross-section reduce, higher operation temperature, chemical reaction between alkali atoms and buffer gas and as a result an alkali laser total power efficiency drop occurs. Another way is to narrow LDB spectrum to match to low buffer gas pressure (~1 atm) alkali gain medium mixture absorption line-width of 10 GHz. Such narrowing can be provided by volume Bragg gratings (VBGs) as an output coupler in external resonator with LDB. In this work we present 250 W external resonator coupled with VBGs based laser with the spectrum line-width of 20 pm (~10 GHz @ 780 nm) for pumping rubidium D2 line.

The laser system developed by us incorporates 7 laser modules and a control electronics rack (Fig. 6). All of the modules are tuned to the same wavelength. The output emission has a line-width of 10 GHz and central wavelength stability of 5 pm which is sufficient to provide more than 90% absorption in low pressure Rb-gain medium. Also the system has electronics rack containing LDBs power source, thermal management system and water chiller for heat dissipation. The output emission of all the modules is coupled to a fiber bundle and is tunable within 140 pm range.

Fig. 6. Configuration of a Rb-pump laser system and a single laser module.

The stability and position of the central wavelength is determined by the resonant wavelength of the VBG which can be controlled by temperature variations. The wavelength shift due to temperature change is about 7 – 14 pm/°C for 780 – 1500 nm spectral region. This is low enough to guarantee stability of the pumping wavelength within 1 pm for temperature controlled with precision of 0.1°C. At the same time, it is high enough to provide thermal tuning of the wavelength for maximum overlapping with the absorption spectrum of the Rb-cell. Ultimately, one module can be considered as an independent laser unit (Fig. 6) requiring power supply and heat dissipation. The maximum emission power such module could provide is 35 W. At the same time the output spectrum line-width is less than 20 pm with side lobes spectral suppression better than 30 dB.

A similar approach was used as a foundation for the development of a resonant pumping source for cryogenic Er-doped lasers (Fig. 7). The achieved parameters for this new system were as follows:

240 W output power

20 GHz spectral width emission

500 pm wavelength tunability

± 5 pm stability over 8 hours

fiber delivery system

Laser diode bars from Princeton Lightwave were chosen working in the range 1525 ÷ 1538 nm, with output power of 35 W each (at 100 A pumping current). Each LB was mounted on a microchannel block and was water cooled to 15ºC at all times. To condition the output emission of each bar we used a commercially available optical system from LIMO. It consisted of several optical elements responsible for collimating and rearranging the emission of the individual laser diodes inside the bar. After the beam was collimated a VBG was aligned to lock the laser in an external resonator configuration and provided narrowing of the laser emission spectrum.

The spectral widths of the VBGs used were 110-120 pm (~14 GHz) with spectral uniformity across the working aperture of approximately 30 to 50 pm. The locked LBs demonstrated minimum spectral widths of 160 pm (20 GHz) which is away from the level of 10GHz observed for the LD bars in the Rb-vapor pumping system. Such high line-widths were attributed to two reasons: first to the significantly wider emission spectrum of the LBs emitting in the range of 1.5 μm if compared to the ones used for generating 780 nm, and second to the anti-reflection coating on the output facet of the LBs which was not optimized for working in external cavity regime. The VBGs provided suppression of the side lobes on the order of 25 dB or better.

Fig. 7. Configuration of pump laser system for cryogenic Er-doped laser.

To achieve tuning of the system each volume grating was heated from two sides and its temperature was monitored simultaneously with a thermistor. Heating the glass plate in which a grating was recorded leads to its expansion and thus to a change in the grating's period. Using a feedback loop and active control made possible the tuning and locking of all the lasers with very high precision. Figure 8 demonstrates the stability of the spectral widths of four LD bars upon heating of their corresponding VBGs to temperatures up to 90ºC. As it was mentioned before if laser bars with narrower spectra are used a smaller than the measured 20-25 GHz line-widths could be achieved.

After individual calibration of all twelve gratings’ temperatures and their corresponding emission wavelengths the Lab View code written to manage the system was able to lock all lasers at any wavelength within the tuning range with precision better than 10 pm which is lower than the spectrum analyzer resolution used for the measurements. In conclusion, we presented the development of two high-power laser systems operating at 780 and 1552 nm, with line-widths of 10 and 20 GHz correspondingly. The narrow line-widths were achieved by using volume Bragg gratings in external resonator configuration. Both systems used thermal tuning of the VBG to tune the wavelength with precision of approximately 10 pm and stability of few pm. Lasers with such narrow line-widths are ideal for pumping laser media with high efficiency.

4.2. Angular selection in laser resonators We propose an approach for reducing the divergence of high-power wide-stripe semiconductor diodes to a diffraction limit based upon the use of new transmitting and reflecting volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass. Transverse modes with different numbers in laser diodes have different angular distributions of their emission. A typical distribution of the intensity in far field for a multimode laser diode has two distinguished side lobes with intensities significantly higher if compared to the axial intensity. Also, the great number of overlapping transverse modes results in a very wide distribution, usually in the range of 10˚. The absence of optical elements which could provide efficient selection of radiation in angular space led to the use of existing selective elements working in geometrical space (slits or diaphragms) in the far field of radiation. This approach works by increasing the length (aspect ratio) of the resonator or by conversion from angular distribution to space distribution by focusing elements. However, providing preferential gain for off-axis modes which are most intensive in wide-stripe laser diodes is a problem if the just described approaches are used.

Due to their strong angular selectivity, volume Bragg gratings provide mode selection directly in angular space or in space of wave vectors. In other words, the transmitting volume Bragg grating functions as a slit and the reflecting volume Bragg grating functions as a diaphragm in angular space. This feature allows a single mode volume Bragg

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Fig. 8. Stability of the spectral widths of four LD bars tuned across 500 pm.

resonator design that matches the direction of the propagation and angular divergence of any particular mode which would be selected with the angle of incidence and angular selectivity of a volume Bragg grating placed in a resonator.

A design of a semiconductor VOBLA is shown in Fig. 9. A laser diode with a stripe width of 150 µm and reflection from the front facet of 5% was used for these experiments. A fast axis collimator provided close to diffraction limit divergence in direction perpendicular to the plane of drawing while multimode radiation along the slow axis has shown typical divergence of about 8˚. A transmitting PTR grating with angular selectivity adjusted to divergence of a single transverse mode was placed in vicinity of the fast axis collimator and aligned in a manner to provide selection of one of the transverse modes emitting in the range of maximum of one of the main side lobes. The second grating which was a Bragg mirror was placed next to the transmitting grating. This mirror had a grating vector tilted to have retroreflection corresponding to a diffraction angle for the first grating. This feature enabled positioning of the Bragg mirror’s surface normal along the axis of the device. The Bragg mirror provided feedback to the external resonator. The total length of this device including a laser diode and all optics was about 4 mm.

This device emitted up to 2 W within narrow angle corresponding to the diffraction limit. The strong selectivity of the PTR volume Bragg gratings prevented stratification of the emitted radiation caused by non-uniformity of the current density in the laser diode at high levels of pumping and enabled low divergence emission from a source having very strong optical distortions. The spectral width of the emission was narrowed from 1.5 to 0.26 nm and together with the emission position was independent from the temperature and the pumping current of the laser.

4.3. Spectral and coherent beam combining of fiber lasers using volume Bragg gratings Compact high-power lasers with good beam quality and narrow line-widths are desired for a great number of applications. Today, 1 μm ytterbium-doped LMA fiber laser sources with several kilowatts of CW power in both single and multi-mode regimes are commercially available. However, further scaling to higher power levels is fundamentally limited due to the onset of thermal and nonlinear effects in the fiber. Spectral beam combining (SBC) and coherent beam combining (CBC) are the two major complimentary methods of beam combining in the effort to reach multi-kilowatt diffraction limited beams. In spectral beam combining, outputs from an array of sources operating at different wavelengths are superimposed spatially by means of filters or dispersive elements to form a beam with combined power. Spectral beam combining is commonly implemented using either surface diffraction gratings or volume Bragg gratings (VBGs)7,8. When high powers are desired, VBGs have the advantage of spreading the deposited heat within the entire volume of the glass. VBGs have the advantage of being modular and can be easily scaled when high channel numbers are desired. Figure 10 shows the general architecture for beam combining by a series of volume Bragg gratings while a compact module for multi-channel spectral beam combining has been proposed in [7].

Fig. 9. Design of a volume Bragg laser incorporating transmitting grating for angular selection and reflecting grating for spectral selection.

A five channel SBC system with channel separation of 0.25 nm between adjacent wavelengths was developed with a total combined power of 0.75 kW within a 1 nm spectral range. To effectively combine high power beams with such a narrow spectral separation, very precise control of the VBG resonant wavelength must be implemented. The most common method of tuning the resonant wavelength is by angular tuning of the VBG. However, due to the narrow angular selectivity of the VBG, mechanical tuning becomes a challenge and is impractical at high output powers. We have developed a thermal tuning technique for maintaining high efficiency of beam combining throughout the power range of the system. By changing the temperature of the VBG, the glass expands or contracts, changing the period of the VBG and hence the resonant Bragg wavelength. This thermal method of tuning the resonant wavelength has much greater resolution than angle tuning and, once implemented, can be controlled electronically. With this tuning method, we were able to maintain peak combining efficiency of the system from low to peak power operation without mechanical realignment. The achieved beam quality parameter M2 was 1.8 for 4 channels and power of approximately 600 W. At maximum power the system’s M2 was slightly higher due to imperfection of one of the VBGs. A subsequent substitution of the grating will improve this parameter.

Coherent locking of two fiber lasers was demonstrated using this doubled VBG splitter in the configuration shown in Fig. 11. In this experiment, 3.3 m of Yb-doped panda-type PM LMA fiber with 25 μm core and 250 μm cladding and birefringence of about 1.2x10-4 was used to construct each channel. Two 915 nm laser diodes with maximum output power of 6 W are used to pump each channel through a (6+1)x1 PM high-power pump-signal combiner. The fibers in both channels were coiled with a diameter of >12 cm and all fiber ends were angle cleaved to 6o.

Fig. 10. Architecture for spectral beam combining by volume Bragg gratings.

Fig. 11. Experimental setup for coherently locking two fiber lasers with a doubled VBG splitter. OC: output coupler; L1, L2, L3, L4: f = 8 mm 0.5 NA aspheric lens.

The output radiation from the two fiber lasers is incident on the beam splitter at an angle of +/- 9.6o in air. The mutually coherent part of the radiation that also satisfies the Bragg condition is diffracted towards the output coupler (80% reflective mirror at 1064 nm). The combined reflected radiation from the output coupler is split by the VBG for feedback to the two fiber lasers. A single coherent output is obtained through the output coupler. Spectral measurement with a Fabry-Perot scanning interferometer revealed a number of longitudinal modes oscillating under an envelope of ~2.5 pm FWHM. A small portion of coherent emission leaking through the VBG is used to make fringe measurements. The maximum contrast was about 96% indicating a significant degree of coherence between the two outputs of the two channels. A combining efficiency of >80% was measured with respect to individual laser outputs and the maximum combined power was pump-limited to ~4 W. Good contrast fringes and stable average output power show that two laser channels can be coherently locked by using VBGs recorded in PTR glass.

As the power requirements for many applications is reaching hundreds of kilowatts, various beam combining techniques are being proposed for efficient power scaling of fiber laser sources. Volume Bragg gratings in PTR glass have been used to demonstrate both efficient spectral and passive coherent combining of fiber lasers. In particular, 5 channel SBC has been demonstrated with a total combined output power of 0.75 kW within 1 nm spectral width. Thermal tuning of large aperture VBGs, designed to maintain high efficiency of beam combining without mechanical adjustment over a wide range of laser power, is disucssed and demonstrated. A novel method of passive coherent beam combining of fiber lasers using volume Bragg gratings has been presented. Coherent locking of 2 Yb-doped large mode area fiber lasers has been demonstrated using a multiplexed VBG (M-VBG) recorded in PTR glass. The combined laser system demonstrates an interference pattern fringe visibility of >95%, FWHM linewidth of ~2.5 pm, and maximum combined powers of ~4 W (limited by pump).

5. CONCLUSION

New volume Bragg laser systems are proposed base upon the use of volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass together with laser diodes or fiber lasers. These grating are used for conversion of wide-stripe laser diodes to emitters of single transverse mode radiation, for spectral narrowing of laser diode bars that are used as pumping sources for alkali and cryogenic lasers, for spectral combining of a number of beams with shifted wavelengths, and for coherent beam combining through multiplexed VBGs. Acknowledgments: The work was supported by SHEDS and ADHELS programs at DARPA, High Energy Laser – Joint Technology Office, Army Research Laboratory, and Air Force Research Laboratory. [1] Hariharan, P., [Optical Holography. Principles, techniques, and applications], Cambridge University Press, (1996).

[2] Glebov, L.B., “Photosensitive glass for phase hologram recording,” Glass Sci. Technol., 71C, 85-90 (1998).

[3] Glebov, L.B., “Kinetics modeling in photosensitive glass,” Optical Materials 25, 413–418 (2004).

[4] Glebov, L.B., Glebova, L.N., Smirnov, V.I., Dubinskii, M., Merkle, L.D., Papernov, S. and Schmid, A.W., “Laser damage resistance of photo-thermo-refractive glass Bragg gratings,” Proceedings of Solid State and Diode Lasers Technical Review, Albuquerque (2004).

[5] Ciapurin, I.V., Glebov, L.B. and Smirnov, V.I., “Modeling of phase volume diffractive gratings, part 1: transmitting sinusoidal uniform gratings,” Optical Engineering 45, 015802, 1-9 (2006).

[6] Krupke, W., "Diode-pumped alkali lasers (DPALs): an overview," Proc. SPIE 7005, 700521 (2008).

[7] Andrusyak, O., Smirnov, V., Venus, G., Vorobiev, N., and Glebov, L. B., “Applications of volume Bragg gratings for spectral control and beam combining of high power fiber lasers,” Proc. SPIE 7195, 71951Q (2009).

[8] Daneu, V., Sanchez, A., Fan, T. Y., Choi, H. K., Turner, G. W., and Cook, C.C., "Spectral beam combining of a broad-stripe diode laser array in an external cavity," Opt. Lett., 25, 405-407, (2000).