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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 20 — Oct. 4, 2004
  • pp: 4935–4940
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Generation of turquoise light by sum frequency mixing of a diode-pumped solid-state laser and a laser diode in periodically poled KTP

Sandra Johansson, Stefan Spiekermann, Shunhua Wang, Valdas Pasiskevicius, Fredrik Laurell, and Katrin Ekvall  »View Author Affiliations


Optics Express, Vol. 12, Issue 20, pp. 4935-4940 (2004)
http://dx.doi.org/10.1364/OPEX.12.004935


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Abstract

We report a simple and efficient method to achieve visible light by sum-frequency mixing radiation from a diode-pumped solid-state laser and a laser diode in a periodically poled KTiOPO4 crystal. Since high-power laser diodes are available at a wide range of wavelengths, it is thereby possible to obtain essentially any wavelength in the visible spectrum by appropriate choice of lasers. For demonstration we choose to construct a light source in the blue-green region. A turquoise output power of 4.0 mW was achieved.

© 2004 Optical Society of America

1. Introduction

Low power, CW Ar+-ion lasers are presently used in applications like bioanalysis, graphics and semiconductor inspection. A trend today is to replace these lasers with diode-pumped alternatives, since the Ar+-ion lasers are bulky and inefficient. So far most attempts to reach the blue-green wavelength range have been based on frequency doubling of semiconductor lasers. Efficient blue light generation was first obtained at 425 nm by direct doubling of the radiation from a single stripe laser diode in a quasi-phase matched LiNbO3 waveguide.1

1. J. Webjörn, F. Laurell, and G. Arvidsson: “Blue light generated by frequency doubling of laser diode light in a lithium niobate channel waveguide,” IEEE Photon. Technol. Lett. 1, 316–318 (1989). [CrossRef]

However, it was found that such units were unstable in output power as the waveguides had a very narrow phase-matching bandwidth (typically 0.2 nm) and the laser diodes drifted in wavelength with time due to mode hops, temperature and mechanical drift and aging.2

2. F. Laurell: “Periodically poled materials for miniature light sources,” Opt. Mat. 11, 235–244 (1999). [CrossRef]

In many biotech applications the 488 nm Ar+-line is used. For this wavelength, particular chemistry is developed and matched to fluorophores with relatively narrow absorption bands. With the development of high power pump diode lasers for Er:Yb-fiber amplifiers at 980 nm it became possible both to reach this wavelength band and to obtain higher frequency doubled power at the same time. These lasers can be wavelength stabilized with pig-tailed fibre Bragg gratings and with such configurations it has been possible to obtain a long term stable blue-green source emitting more than 5 mW in the 488 nm band.2 This early work with frequency-doubled semiconductor lasers involved edge-emitting diodes, but recently, optically3

3. T.D. Raymond, W.J. Alford, M.H. Crawford, and A.A. Allerman: ”Intracavity frequency doubling of a diode-pumped external-cavity surface-emitting semiconductor laser,” Opt. Lett. 24, 1127–1129 (1999). [CrossRef]

,4

4. A. Caprara, J.L. Chilla, and L.A. Spinelli: ”High power external-cavity optically-pumped semiconductor lasers,” US Patent 6,097,742 (2000).

,5

5. E. Schielen, M. Golling, and P. Unger: “Diode-pumped semiconductor disk laser with intracavity frequency doubling using lithium triborate (LBO),” IEEE Photonics Technol. Lett. 14, 777–779 (2002). [CrossRef]

and electrically pumped surface-emitting lasers (VCSELs)6

6. E.U. Rafailov, W. Sibbett, A. Mooradian, J. G. McInerney, H. Karlsson, S. Wang, and F. Laurell: ”Efficient frequency doubling of a vertical-extended-cavity-surface-emitting laser diode by use of a periodically poled KTP crystal,” Opt. Lett. 28, 2091–2093 (2003). [CrossRef] [PubMed]

have been used with intra-cavity frequency doubling. The bandgap of the InGaAs semiconductor has in both these cases been tailored to emit at around 976 nm to obtain the desired 488 nm wavelength after frequency doubling. Single-frequency blue output with a linewidth down to a few MHz, has been obtained from an intra-cavity configuration7

7. M. A. Holm, D. Burns, A.I. Ferguson, and M.D. Dawson: “Single-frequency second-harmonic generation in a vertical external-cavity semiconductor laser,” Conference on Lasers and Electro-Optics (CLEO 2000), TOPS Vol.39, 2000, pp. 440–441.

, and using master-oscillator-power-amplifier (MOPA) systems8

8. D. Woll, B. Beier, K.J. Boller, R. Wallenstein, M. Hagberg, and S. O’Brien: ”1 W of blue 465-nm radiation generated by frequency doubling of the output of a high-power diode laser in critically phase-matched LiB3O5,” Opt. Lett. 24, 691–693 (1999). [CrossRef]

,9

9. D. Woll, J. Schumacher, A. Robertson, M. A. Tremont, R. Wallenstein, M. Katz, D. Eger, and A. Englander: “250 mW of coherent blue 460-nm light generated by single-pass frequency doubling of the output of a mode-locked high-power diode laser in periodically poled KTP,” Opt. Lett. 27, 1055–1057 (2002). [CrossRef]

. Also methods based on sum-frequency mixing (SFM) have been exploited, so far mostly to attain yellow radiation10

10. J.C. Bienfang, C.A. Denman, B.W. Grime, P.D. Hillman, G.T. Moore, and J.M. Telle:”20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers,” Opt. Lett. 28, 2219–2221 (2003). [CrossRef] [PubMed]

,11

11. N. Saito, K. Akagawa, Y. Hayano, Y. Saito, H. Takami, and S. Wada: “An efficient method for quasi-continuous-wave generation of 589 nm by sum-frequency mixing in periodically poled KTP,” in Advanced Solid-State Photonics, J. J. Zayhowski and G.J. Quarles, eds., Nineteenth Topical Meeting and Tabletop Exhibit (Optical Society of America, Santa Fe, New Mexico, 2004).

. Although output powers as high as 20 W in the visible wavelength range has been achieved10

10. J.C. Bienfang, C.A. Denman, B.W. Grime, P.D. Hillman, G.T. Moore, and J.M. Telle:”20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers,” Opt. Lett. 28, 2219–2221 (2003). [CrossRef] [PubMed]

, remaining problems are power stability and high-frequency noise. In this paper we present a simple scheme to obtain visible light by sum-frequency mixing radiation from a diode-pumped solid-state laser (DPSSL) and a laser diode (LD) in a periodically poled KTiOPO4 (PPKTP) crystal. The two mixing lasers are combined for single-pass SFM, which is advantageous for stability and low noise. Since high-power laser diodes are available at a wide range of wavelengths, whereas diode-pumped solid-state lasers are fixed at certain wavelengths, it is thereby possible to obtain essentially any wavelength in the visible spectrum by appropriate choice of lasers. Furthermore, scaling in power is possible by using a high power DPSSL. As a proof of concept we choose to construct the light source in the above-mentioned technologically interesting wavelength band of the Ar+-laser.

2. Experiment

The DPSSL was a CW, diode-pumped Nd:YVO4 laser operating at 1064.3 nm. It was constructed of a Nd:YVO4 crystal (0.7 atm%, a-cut) AR coated for 808 nm and HR coated for 1064 nm on the incoupling side and AR coated for 1064 nm on the opposite side. A mirror having a radius of curvature of 100 mm and a reflectivity of 95 % was used for outcoupling of the laser light. It was pumped at 808 nm with a 1.4 W fiber-coupled laser diode (Osram GmbH). This gave a maximum output power of 730 mW, corresponding to an efficiency of 52%. The output beam was polarized in the σ-direction and the beam quality was close to Gaussian (M2 ~1.5). For collimation of the beam, a lens with a focal length of 150 mm was used, giving a circular beam with a diameter of approximately 1 mm.

To obtain turquoise radiation, a LD (Axcel Photonics) was employed. For 500 mA of drive current a power of 400 mW was emitted in a single transverse mode. The free-running wavelength at this power and at a temperature of 36 °C was 915.7 nm. A laser diode lens (Melles Griot 06GLC001) was used for collimation of the LD beam. Its N.A. was 0.615 and its focal length was 6.5 mm. After collimation, the LD beam was astigmatic with a diameter of 4.8 mm in the x-direction and 1.9 mm in the y-direction. To achieve a better mode-match in the PPKTP crystal, the diode beam was reduced to 1.7 mm in the x-direction with the help of two uncoated cylindrical lenses acting as a telescope. Due to Fresnel reflections from the cylindrical lenses and the beam combiner, the effective power from the LD reaching the PPKTP crystal was 280 mW.

Fig. 1. The first version of the experimental set-up.

The two lasers were aligned perpendicular to each other and the beams were then combined with a dichroic mirror and focused to a spot size of 14 μm and 16 μm for the LD and 28 μm for the DPSSL into the PPKTP crystal. For comparison, a numerical simulation of the mixing of two focused circular Gaussian beams was done, based on an analysis made by S. Guha and J. Falk12

12. S. Guha and J. Falk: “The effects of focusing in three-frequency parametric upconverter,” J. Appl. Phys. 51, (1), 50–60 (1980). [CrossRef]

. The numerical simulation gave that the optimum focused spot sizes in the PPKTP crystal are 23 μm and 25 μm, respectively. Unfortunately, the available optics limited the choice of focused beam widths to the above stated values. The 9 mm long PPKTP crystal had a grating period of 6.99 μm designed for first order quasi-phase matching using the d33 coefficient. Its non-linear coefficient was deff = 10 pm/V, measured by SHG of a Ti:Sapphire laser. This value is close to that of an ideal crystal. The crystal had antireflection coating for 914 nm and 1064 nm on one side, and antireflection coating for 914 nm, 1064 nm and 492 nm on the other side. The spectral width of both the LD and the DPSSL was measured with an optical spectrum analyser (OSA), and it showed to be less than the resolution of the OSA, which was 0.06 nm. However, neither the LD nor the DPSSL were single longitudinal mode. As for the DPSSL, three longitudinal modes with an interspacing of 0.1 nm were present.

3. Results

Two versions of the experimental set-up were tested. In the first set-up (see Fig.1), no attempt was made to long-term stabilize the LD:s spectral output. This gave a maximum SFM output power at approximately 492 nm of 4.0 mW. The experimentally achieved output power should be compared to 6.8 mW, calculated from the numerical simulation for two circular Gaussian beams12

12. S. Guha and J. Falk: “The effects of focusing in three-frequency parametric upconverter,” J. Appl. Phys. 51, (1), 50–60 (1980). [CrossRef]

. This discrepancy is attributed to the non-ideal overlap with one circular beam and one elliptical beam. The maximum power was reached at a KTP temperature of 36.2 °C and the effective temperature bandwidth (FWHM) was approximately 5.8 °C, see Fig. 2. Regarding the temperature bandwidth, basic QPM theory13

13. M.M. Fejer, G.A. Magel, D.H. Jundt, and R.L. Byer: “Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances,” IEEE J. Quantum Electron. 28, 2631–2654 (1992). [CrossRef]

gave a numerical value of 3.4 °C. The larger experimental value of 5.8 °C is due to the presence of several longitudinal modes in the two lasers, giving a larger thermal acceptance bandwidth compared to the ideal single mode case.

Fig. 2. The turquoise output power as a function of the temperature of the PPKTP crystal. The measured data is represented by the squares and the curve is a sinc-fit of the data. The temperature bandwidth, ΔT = 5.8 °C.

The normalized conversion efficiency

ηSFM=P3P1·P2·L,
(1)

was measured to 2.2 %W-1cm-1, where P3 is the SFM power, P2 the DPSSL power, P1 the LD power and L the length of the PPKTP crystal. The SFM power is given by P 3 = γSFM P 1 P 2, where the parameter γSFM is dependent on the non-linear coefficient of the material, the optical frequencies taking part in the non-linear process and the beam focusing conditions14

14. W.P. Risk, T.R. Gosnell, and A.V. Nurmikko, Compact Blue-Green Lasers (Cambridge University Press, 2003). [CrossRef]

. Hence if P1 is fixed and P2 is varied, the SFM power should vary linearly as a function of P2. This is also the case as seen from Fig.3, where the SFM power is plotted vs. the DPSSL power. In this way, power scaling is simple, for example by increasing the DPSSL power.

Fig. 3. Turquoise CW output power measured for fixed LD power (at 280 mW) and changing the DPSSL power.

The short-term power fluctuations were less than 2.5 %. As can be expected, the beam shape was slightly elliptical, but it was nevertheless Gaussian in both directions.

Occasionally it happened that the LD mode hopped resulting in a large instability of the SFM signal. In this case the wavelength shift was large enough to exceed the phase-matching bandwidth of the PPKTP crystal, 0.2 nm14

14. W.P. Risk, T.R. Gosnell, and A.V. Nurmikko, Compact Blue-Green Lasers (Cambridge University Press, 2003). [CrossRef]

. The mode hops, typically 0.3 nm, were primarily attributed to uncontrolled back-reflections from the optics and the crystal. In order to tune and long-term stabilize the LD in wavelength, a transmission grating (Spectrogon AB) with 2000 lines/mm was used in a Littrow configuration, i.e., the -1:st order reflection was coupled back into the LD. The transmission grating was placed after the collimating lens and it was oriented with an angle of 66.3° towards the LD to lock the diode at the phase-matching wavelength 915.7 nm. The LD could be tuned for wavelengths between 906 nm and 916 nm where it operated perfectly stable with a linewidth below the resolution bandwidth of the OSA.

The diode pump power reaching the PPKTP crystal was in this case 110 mW, whereas the DPSSL power was 620 mW. A maximum turquoise power of 0.74 mW was then obtained.

The noise of the turquoise light was studied with an electrical spectrum analyser (HP 8560A). It showed a very low noise, but a small peak at 2.3 MHz could be observed (see Fig. 4) which is assumed to originate from relaxation oscillations of the Nd:YVO4 laser. The signal-to-noise ratio at 2.3 MHz was then 104 dB.

Fig. 4. Noise spectra of the turquoise light (upper) and detection system noise (lower). Relaxation oscillations originating from the DPSSL are present at 2.3 MHz.

4. Conclusions

In conclusion, a novel scheme to generate visible radiation at a desired wavelength by mixing a diode-pumped solid-state laser with a single-mode laser diode has been investigated. The main advantage of this scheme is the possibility to reach essentially any wavelength in the visible spectrum by appropriate choice of lasers. PPKTP was used for the frequency mixing and the lasers were chosen to give turquoise light at approximately 492 nm. An output power of 4 mW was obtained. It was long-term stabilized by inserting a transmission grating in order to lock the diode laser at quasi-phase matching. In this case, the output power was reduced to 0.74 mW. A signal-to-noise ratio at 2.3 MHz of 104dB was then obtained.

By using a more powerful pump source for the solid-state laser, the turquoise power could be scaled considerably in power. As an example, a 3 W pump diode would increase the turquoise output power to about 14 mW according to our numerical simulation. Thus, if assumed that we reach 60% of this as in our experiments, about 8 mW could be expected experimentally. A double-pass SFM configuration15

15. S. Spiekermann, F. Laurell, V. Pasiskevicius, H. Karlsson, and I. Freitag: ”Optimizing non-resonant frequency conversion in periodically poled media,” Appl. Phys. B 79, 211–219 (2004). [CrossRef]

and a longer PPKTP crystal are established methods of increasing the non-linear conversion efficiency. Furthermore, the results would improve if better collimation and focusing optics for the two beams would be used.

It also deserves to be mentioned that this scheme gives a very inexpensive and easy way of modulating the visible radiation, i.e. by modulating the single mode laser diode via the driving current, which is not possible with DPSSL:s.

Acknowledgments

The Göran Gustafsson Foundation, the Carl Tryger Foundation and the Lars Hierta Foundation are acknowledged for financial support to this project.

References

1.

J. Webjörn, F. Laurell, and G. Arvidsson: “Blue light generated by frequency doubling of laser diode light in a lithium niobate channel waveguide,” IEEE Photon. Technol. Lett. 1, 316–318 (1989). [CrossRef]

2.

F. Laurell: “Periodically poled materials for miniature light sources,” Opt. Mat. 11, 235–244 (1999). [CrossRef]

3.

T.D. Raymond, W.J. Alford, M.H. Crawford, and A.A. Allerman: ”Intracavity frequency doubling of a diode-pumped external-cavity surface-emitting semiconductor laser,” Opt. Lett. 24, 1127–1129 (1999). [CrossRef]

4.

A. Caprara, J.L. Chilla, and L.A. Spinelli: ”High power external-cavity optically-pumped semiconductor lasers,” US Patent 6,097,742 (2000).

5.

E. Schielen, M. Golling, and P. Unger: “Diode-pumped semiconductor disk laser with intracavity frequency doubling using lithium triborate (LBO),” IEEE Photonics Technol. Lett. 14, 777–779 (2002). [CrossRef]

6.

E.U. Rafailov, W. Sibbett, A. Mooradian, J. G. McInerney, H. Karlsson, S. Wang, and F. Laurell: ”Efficient frequency doubling of a vertical-extended-cavity-surface-emitting laser diode by use of a periodically poled KTP crystal,” Opt. Lett. 28, 2091–2093 (2003). [CrossRef] [PubMed]

7.

M. A. Holm, D. Burns, A.I. Ferguson, and M.D. Dawson: “Single-frequency second-harmonic generation in a vertical external-cavity semiconductor laser,” Conference on Lasers and Electro-Optics (CLEO 2000), TOPS Vol.39, 2000, pp. 440–441.

8.

D. Woll, B. Beier, K.J. Boller, R. Wallenstein, M. Hagberg, and S. O’Brien: ”1 W of blue 465-nm radiation generated by frequency doubling of the output of a high-power diode laser in critically phase-matched LiB3O5,” Opt. Lett. 24, 691–693 (1999). [CrossRef]

9.

D. Woll, J. Schumacher, A. Robertson, M. A. Tremont, R. Wallenstein, M. Katz, D. Eger, and A. Englander: “250 mW of coherent blue 460-nm light generated by single-pass frequency doubling of the output of a mode-locked high-power diode laser in periodically poled KTP,” Opt. Lett. 27, 1055–1057 (2002). [CrossRef]

10.

J.C. Bienfang, C.A. Denman, B.W. Grime, P.D. Hillman, G.T. Moore, and J.M. Telle:”20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers,” Opt. Lett. 28, 2219–2221 (2003). [CrossRef] [PubMed]

11.

N. Saito, K. Akagawa, Y. Hayano, Y. Saito, H. Takami, and S. Wada: “An efficient method for quasi-continuous-wave generation of 589 nm by sum-frequency mixing in periodically poled KTP,” in Advanced Solid-State Photonics, J. J. Zayhowski and G.J. Quarles, eds., Nineteenth Topical Meeting and Tabletop Exhibit (Optical Society of America, Santa Fe, New Mexico, 2004).

12.

S. Guha and J. Falk: “The effects of focusing in three-frequency parametric upconverter,” J. Appl. Phys. 51, (1), 50–60 (1980). [CrossRef]

13.

M.M. Fejer, G.A. Magel, D.H. Jundt, and R.L. Byer: “Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances,” IEEE J. Quantum Electron. 28, 2631–2654 (1992). [CrossRef]

14.

W.P. Risk, T.R. Gosnell, and A.V. Nurmikko, Compact Blue-Green Lasers (Cambridge University Press, 2003). [CrossRef]

15.

S. Spiekermann, F. Laurell, V. Pasiskevicius, H. Karlsson, and I. Freitag: ”Optimizing non-resonant frequency conversion in periodically poled media,” Appl. Phys. B 79, 211–219 (2004). [CrossRef]

OCIS Codes
(140.3480) Lasers and laser optics : Lasers, diode-pumped
(140.3580) Lasers and laser optics : Lasers, solid-state
(140.7300) Lasers and laser optics : Visible lasers
(190.2620) Nonlinear optics : Harmonic generation and mixing

ToC Category:
Research Papers

History
Original Manuscript: May 19, 2004
Revised Manuscript: September 24, 2004
Published: October 4, 2004

Citation
Sandra Johansson, Stefan Spiekermann, Shunhua Wang, Valdas Pasiskevicius, Fredrik Laurell, and Katrin Ekvall, "Generation of turquoise light by sum frequency mixing of a diode-pumped solid-state laser and a laser diode in periodically poled KTP," Opt. Express 12, 4935-4940 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-20-4935


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References

  1. J. Webjörn, F. Laurell and G. Arvidsson: �??Blue light generated by frequency doubling of laser diode light in a lithium niobate channel waveguide,�?? IEEE Photon. Technol. Lett. 1, 316-318 (1989). [CrossRef]
  2. F. Laurell: �??Periodically poled materials for miniature light sources,�?? Opt. Mat. 11, 235-244 (1999). [CrossRef]
  3. T.D. Raymond, W.J. Alford, M.H. Crawford and A.A. Allerman: �??Intracavity frequency doubling of a diode-pumped external-cavity surface-emitting semiconductor laser,�?? Opt. Lett. 24, 1127-1129 (1999). [CrossRef]
  4. A. Caprara, J.L. Chilla and L.A. Spinelli: �??High power external-cavity optically-pumped semiconductor lasers,�?? US Patent 6,097,742 (2000).
  5. E. Schielen, M. Golling and P. Unger: �??Diode-pumped semiconductor disk laser with intracavity frequency doubling using lithium triborate (LBO),�?? IEEE Photonics Technol. Lett. 14, 777-779 (2002). [CrossRef]
  6. E.U. Rafailov, W. Sibbett, A. Mooradian, J. G. McInerney, H. Karlsson, S. Wang and F.Laurell: �??Efficient frequency doubling of a vertical-extended-cavity-surface-emitting laser diode by use of a periodically poled KTP crystal,�?? Opt. Lett. 28, 2091-2093 (2003). [CrossRef] [PubMed]
  7. M. A. Holm, D. Burns, A.I. Ferguson and M.D. Dawson: �??Single-frequency second-harmonic generation in a vertical external�??cavity semiconductor laser,�?? Conference on Lasers and Electro-Optics (CLEO 2000), TOPS Vol.39, 2000, pp. 440-441.
  8. D. Woll, B. Beier, K.J. Boller, R. Wallenstein, M. Hagberg and S. O�??Brien: �??1 W of blue 465-nm radiation generated by frequency doubling of the output of a high-power diode laser in critically phase-matched LiB3O5,�?? Opt. Lett. 24, 691-693 (1999). [CrossRef]
  9. D. Woll, J. Schumacher, A. Robertson, M. A. Tremont, R. Wallenstein, M. Katz, D. Eger and A. Englander: �??250 mW of coherent blue 460-nm light generated by single-pass frequency doubling of the output of a mode-locked high-power diode laser in periodically poled KTP,�?? Opt. Lett. 27, 1055-1057 (2002). [CrossRef]
  10. J.C. Bienfang, C.A. Denman, B.W. Grime, P.D. Hillman, G.T. Moore and J.M. Telle: �??20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers,�?? Opt. Lett. 28, 2219-2221 (2003). [CrossRef] [PubMed]
  11. N. Saito, K. Akagawa, Y. Hayano, Y. Saito, H. Takami and S. Wada: �??An efficient method for quasi-continuous-wave generation of 589 nm by sum-frequency mixing in periodically poled KTP,�?? in Advanced Solid-State Photonics, J. J. Zayhowski and G.J. Quarles, eds., Nineteenth Topical Meeting and Tabletop Exhibit (Optical Society of America, Santa Fe, New Mexico, 2004).
  12. S. Guha and J. Falk: �??The effects of focusing in three-frequency parametric upconverter,�?? J. Appl. Phys. 51, (1), 50-60 (1980). [CrossRef]
  13. M. M. Fejer, G. A. Magel, D. H. Jundt and R. L. Byer: �??Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances,�?? IEEE J. Quantum Electron. 28, 2631-2654 (1992). [CrossRef]
  14. W.P. Risk, T.R. Gosnell and A.V. Nurmikko, Compact Blue-Green Lasers (Cambridge University Press, 2003). [CrossRef]
  15. S. Spiekermann, F.Laurell, V. Pasiskevicius, H. Karlsson and I. Freitag: �??Optimizing non-resonant frequency conversion in periodically poled media,�?? Appl. Phys. B 79, 211-219 (2004). [CrossRef]

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