Using a volume Bragg grating instead of a Faraday isolator in lasers incorporating stimulated Brillouin scattering wavefront reversal or beam cleanup

A master-oscillator power-amplifier with stimulated Brillouin scattering (SBS) beam cleanup or wavefront reversal typically incorporates a Faraday isolator to outcouple the Stokes light, limiting the power scalability. Volume Bragg gratings (VBGs) have the potential for scaling to higher powers. We report here the results of tests on a VBG designed to resolve wavelengths 0.060 nm apart, corresponding to the 16 GHz frequency shift for SBS backscattering at 1064 nm in fused silica. Such an element may also find use in between stages of fiber amplifiers, for blocking the Stokes wave. ©2011 Optical Society of America OCIS codes: (050.7330) Volume gratings; (290.5900) Scattering, stimulated Brillouin; (290.5830) Scattering, Brillouin; (290.5855) Scattering, polarization; (290.1350) Backscattering; (230.1480) Bragg reflectors. References and links 1. R. Boyd, Nonlinear Optics Third Edition, (Elsevier Science & Technology Books, New York 2008). 2. L. Lombard, A. Brignon, J. P. Huignard, E. Lallier, and P. Georges, “Beam cleanup in a self-aligned gradientindex Brillouin cavity for high-power multimode fiber amplifiers,” Opt. Lett. 31(2), 158–160 (2006). 3. L. B. Glebov, V. I. Smirnov, C. M. Stickley, and I. V. Ciapurin, “New approach to robust optics for HEL systems,” in Laser Weapons Technology III, W.E. Thompson and P.H. Merritt, Editors. Proc. of SPIE 4724, 101–109 (2002). 4. L. B. Glebov, “High brightness laser design based on volume Bragg gratings,” Proc. SPIE 6216, 621601, 621601-10 (2006). 5. J. E. Hellström, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Quasi-two-level Yb:KYW laser with a volume Bragg grating,” Opt. Express 15(21), 13930–13935 (2007). 6. A. Gourevitch, G. Venus, V. Smirnov, D. A. Hostutler, and L. Glebov, “Continuous wave, 30 W laser-diode bar with 10 GHz linewidth for Rb laser pumping,” Opt. Lett. 33(7), 702–704 (2008). 7. N. Ter-Gabrielyan, L. D. Merkle, A. Ikesue, and M. Dubinskii, “Ultralow quantum-defect eye-safe Er:Sc2O3 laser,” Opt. Lett. 33(13), 1524–1526 (2008). 8. New Focus, 3635 Peterson Way, Santa Clara, CA 95054, model TLB-6300. 9. S. V. Mokhov, L. B. Glebov, V. I. Smirnov, and B. Ya. Zeldovich, “Propagation of Electromagnetic Waves in Non-uniform Volume Bragg Gratings,” presented at Frontiers in Optics, Rochester, NY, Oct. 19–23 2008. 10. H. Shu, S. Mokhov, B. Ya. Zeldovich, and M. Bass, “More on analyzing the reflection of a laser beam by a deformed highly reflective volume Bragg grating using iteration of the beam propagation method,” Appl. Opt. 48(1), 22–27 (2009). 11. H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909–2945 (1969). 12. Innovative Photonics, www.innovativephotonics.com, 4250 U.S. Rt. 1, Monmouth Junction, NJ 08852. 13. Nufern, www.nufern.com, 7 Airport Park Rd, E. Granby, CT 06026. 14. D. P. M. Photonics, www.dpmphotonics.com, P.O. Box 3002, Vernon, CT 06066. 15. O. F. S. Optics, www.osfoptics.com, 2000 Northeast Expressway, Norcross, GA 30071. #149284 $15.00 USD Received 17 Jun 2011; accepted 4 Jul 2011; published 15 Aug 2011 (C) 2011 OSA 29 August 2011 / Vol. 19, No. 18 / OPTICS EXPRESS 16885 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE JUN 2011 2. REPORT TYPE 3. DATES COVERED 00-00-2011 to 00-00-2011 4. TITLE AND SUBTITLE Using a volume Bragg grating instead of a Faraday isolator in lasers incorporating stimulated Brillouin scattering wavefront reversal or beam cleanup 5a. CONTRACT NUMBER


Form Approved OMB No. 0704-0188
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information.Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302.Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.A master-oscillator power-amplifier with stimulated Brillouin scattering (SBS) beam cleanup or wavefront reversal typically incorporates a Faraday isolator to outcouple the Stokes light, limiting the power scalability.Volume Bragg gratings (VBGs) have the potential for scaling to higher powers.We report here the results of tests on a VBG designed to resolve wavelengths 0.060 nm apart, corresponding to the 16 GHz frequency shift for SBS backscattering at 1064 nm in fused silica.Such an element may also find use in between stages of fiber amplifiers, for blocking the Stokes wave.

Introduction
A master-oscillator power-amplifier (MOPA) with stimulated Brillouin scattering (SBS) wavefront reversal [1] requires an optical element to couple the seed into the amplifier and outcouple the Stokes wave after the second, backward pass through the amplifier.SBS beam cleanup has a similar requirement [2].In both cases, a Faraday isolator is typically used because the laser and Stokes waves are counterpropagating.Alternatively, a volume Bragg grating (VBG) can separate the two based on the wavelength shift, just as in a conventional diffraction grating.Photo-Thermo-Refractive (PTR) glass can be made with a loss below 10 3 cm 1 , and a damage threshold above 10 4 W/cm 2 , therefore VBGs have the potential for scaling to higher powers, provided the area is large enough [3,4].A 6.3 mm-thick VBG has previously been used as the input coupler for a low quantum defect Yb:KYW laser [5].An 18 mm-thick VBG was used to narrow the linewidth of a diode bar to 20 pm (10 GHz) at 780 nm [6].A 3 mm-thick VBG was used as an input coupler for a low quantum defect Er:Sc 2 O 3 laser [7].We have designed and fabricated a 12 mm-thick VBG to resolve the 0.06 nm (16 GHz) Stokes shift in fused silica at 1064 nm.Initial testing has been carried out with up to 27 W incident upon the VBG.
In the wavefront reversal MOPA geometry, the Stokes beam is coupled out after the second pass amplification.A VBG could be used to reflect λ L and transmit λ S (Fig. 1).This geometry may have a more graceful failure mode in the event of a misalignment of the VBG, or an accidental shift in its resonance due to a change in temperature.In the beam cleanup MOPA geometry, diffracting λ L and transmitting λ S would again have the more graceful failure mode, but for ease of alignment, transmitting λ L is possible as well (Fig. 2).

Experiment & calculations
Our simulation with coupled wave theory shows that a 12 mm-thick grating should have sufficient resolution and an excellent contrast ratio.We then fabricated a sample and antireflection coated the 8 × 10 mm 2 entrance and exit faces.Low-power reflection measurements made with a tunable diode laser [8] agree well with the simulation (Fig. 3).The full width at half maximum (FWHM) of the simulation is 0.063 nm; the experimental data has a FWHM of 0.057 nm.The spectral selectivity of reflecting Bragg gratings widens if the efficiency is increased too much, so the VBG was designed to have a peak reflectivity below 0.95.Scattering losses are less than 1% and comparable to the residual reflectivity of the antireflection coated entrance and exit faces.The asymmetry in the side lobes of the measured curve could be due to a z-dependent background index change, or grating period distortion [9,10].The polarization dependence of a volume holographic grating should be negligible when the angle between the incident and diffracted beams is <10° [11], and we have confirmed this experimentally.For high-power testing, the VBG is mounted in a temperature-controlled holder with two rotation axes approximately normal to the grating vector.The source is a VBG-stabilized diode laser [12] amplified to 30 W with a single-mode, polarization-maintaining Yb-doped fiber amplifier [13].The delivery fiber has a numerical aperture of 0.06; the output is collimated to a 3.0 mm diameter with a 25 mm focal length doublet [14].A half-wave plate and Faraday isolator serve as a variable optical attenuator.To obtain the backward Stokes beam, light at λ L is focused with a 30 mm focal length doublet into a 2.7 km graded-index fiber with a 50 µm core and numerical aperture of 0.2 [15].Beam samplers at a small angle of incidence monitor incident, reflected, and transmitted powers.The VBG is aligned to maximize the Bragg reflection at an angle of 10°.

Results
High power measurements were taken in the geometry of Fig. 1 and of Fig. 2. Based on the low power measurements in Fig. 3, the figure of merit appropriate for Fig. 1, R L T S , could be as high as 0.96.The figure of merit appropriate for Fig. 2, T L R S , would be slightly less, 0.94, but easier to obtain experimentally because the short wavelength sidelobes in Fig. 3 are much smaller.
Measured in the geometry of Fig. 2, the VBG transmittance at λ L is 0.95 and the VBG reflectance at λ S is 0.94 at an input power of 27 W (Fig. 4).The figure of merit of the VBG is T L R S = 0.89.The light reflected from the fiber shows the characteristic threshold behavior of SBS.The highest SBS reflectance we observe is 0.81.The threshold is 0.2 W incident upon the fiber.We also tested the VBG in the geometry of Fig. 1 for input powers up to 7.5W.In this case, R L is 0.95, and T S is 0.88; both are nearly independent of input power.The figure of merit in this geometry is R L T S = 0.84.

Discussion
One issue is whether absorption will heat the VBG enough to shift the resonance.The index change and thermal expansion of photo-thermal refractive (PTR) glass are such that the resonant wavelength red shifts ~0.009 nm/°C at this spectral range.To investigate this we solved the 3D heat diffusion equation inside an 8×10×12 mm 3 piece of glass with specific heat 0.84 J/gm°C and thermal conductivity 1 W/m°C.An absorption coefficient α = 10 3 cm 1 corresponds to an absorption of 0.12%.Taking the case of a 100 W beam, we assume that 0.12 W is deposited uniformly in a cylinder 6 mm in diameter, centered in the sample, the two surfaces are in contact with a heat sink at 300 K, and there is no heat flow through the other four surfaces.The steady state results show a 1°C temperature difference between the beam axis and the 3 mm radius of the beam (Fig. 5, Fig. 6).This implies a 0.009 nm shift in resonance, which will produce only a small change in the transmission at λ L and λ S .

Fig. 1 .
Fig. 1.The preferred configuration using a VBG to outcouple in the wavefront reversal geometry.

Fig. 2 .
Fig. 2. A possible configuration to outcouple in the beam cleanup geometry.

Fig. 5 .
Fig. 5. Cross section of the steady state temperature profile inside an 8 × 10 × 12 mm 3 piece of PTR glass, with 0.12 W of heat deposited in a cylinder of the same length and 6 mm in diameter.