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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 12 — Jun. 13, 2005
  • pp: 4370–4378
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Ultra low-loss low-efficiency diffraction gratings

T. Clausnitzer, E.-B. Kley, A. Tünnermann, A. Bunkowski, O. Burmeister, K. Danzmann, R. Schnabel, S. Gliech, and A. Duparré  »View Author Affiliations


Optics Express, Vol. 13, Issue 12, pp. 4370-4378 (2005)
http://dx.doi.org/10.1364/OPEX.13.004370


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Abstract

The realization of ultra low-loss dielectric reflection gratings with diffraction efficiencies between 7% and 0.02% is presented. By placing the grating beneath the highly reflective layerstack scattering was significantly reduced. This concept allows the all-reflective coupling of high laser radiation to high finesse cavities, thereby circumventing thermal effects caused by absorption in the substrate.

© 2005 Optical Society of America

1. Introduction

Fig. 1. Gratings as cavity coupler (a) high efficiency grating: coupling by the specular 0th order (b) low efficiency grating: coupling by the weak -1st diffraction order
Fig. 2. Two approaches to combine grating and layerstack: (a) grating on top, (b) grating beneath the stack

2. General considerations on the sample design

R0°=1(2η1+T+S).
(1)

Our general goal is a high power build-up in the cavity, which requires reduction of the transmission T as well as the scattering losses S. The finesse of the cavity is determined by the value of R . Hence, for fixed values of S and T, the finesse can be controlled by changing η1. For the devices discussed above, the aim was to realize efficiencies in the range below 5%.

Fig. 3. (a) Incidence from 2nd order Littrow angle and (b) retroreflection for normal incidence. The transmission T (blue) is defined as the sum of all transmitted diffraction orders. Due to the same optical path h1 is the same in (a) and (b) (red arrows).

The propagation directions of the diffraction orders of a grating in air are given by the grating equation

sinφm=sinφin+mλd,
(2)

where λ is wavelength, d the grating period, φin the incident angle, and φm the propagation angle of the mth diffraction order. The setup demands that the -1st order propagates normally to the grating. That is, for the incident beam

sinφin=λd.
(3)

Since φin has to be smaller than 90° in air, the period has to be larger than the wavelength. A further issue is that every propagating order carries an amount of energy. In order to reduce scattering losses by additional diffraction orders, only those orders that are going to be used in the setup should be allowed to propagate. The period is thus restricted to less than 2λ. The wavelength the devices are designed for is 1064 nm (Nd:YAG- laser). Therefore, periods between 1064 nm and 2128 nm are eligible.

Fig. 4. Measured reflection spectrum of the dielectric layerstack composed of Ta2O5 and SiO2 (λ=1.064µm)

3. Grating on top of the HR stack

Fig. 5. Theoretical calculation of the diffraction efficiency as a function of groove depth hg and the thickness of the residual layer beneath the grating tr

For fabrication of the HR-stack, a flat fused silica substrate has been coated first by ion beam sputtering. In accordance with Fig. 5 we chose fused silica with a thickness of 550 nm as the topmost layer. The grating with a 1.45µm period was fabricated by electron-beam lithography and reactive ion beam etching. A groove depth of 290 nm and a fill factor of 0.5 were measured by an atomic force microscope (AFM). If the grating grooves were ideal, the grating would theoretically posses a diffraction efficiency of η1=1.5% and thus a reflectivity of R =97% [Eq. (1)]. To analyze the losses of the real gratings we measured the angle-resolved scattering (ARS) using the high-sensitivity ARS-instrumentation described in [13

13. A. Duparré, J. Ferre-Borrull, S. Gliech, G. Notni, J. Steinert, and J. M. Bennett, “Surface characterization techniques for determing the root-mean-square roughness and power spectral densities of optical components,” Appl. Opt. 41, 154–171 (2002) [CrossRef] [PubMed]

]. These measurements, of course, do not provide the value of the total scattering S of the device. Estimation of total scattering by either direct measurement with an integrating or a Coblentz sphere or by calculation from measured ARS curves still rises several problems, which are a topic of further detailed studies. These investigations address the question how to estimate and even how to define the total scattering for a grating - and in particular the value of the angle separating efficiency from scatter - in harmony with the instructions given for plane surfaces in the international standard ISO 13696. However, the ARS-measurements give insight into the scattering processes and are a good measure for S. The samples were illuminated by a Nd:YAG laser at 1064 nm wavelength, TE- polarization, and normal incidence. Figure 6 shows the results of these measurements. A first order diffraction efficiency of 1.5% was also measured by a calibrated integrating sphere. Beside the expected peaks there are some additional diffraction peaks in the range of 10-6 of the incident intensity between the 0th and the two first orders. These orders are a result of periodic fill factor variations that are typical for e-beam lithographic pattern generation [14

14. E.-B. Kley, T. Clausnitzer, M. Cumme, K. Zöllner, B. Schnabel, and A. Stich, “Investigation of Large-Area Gratings Fabricated by Ultrafast E-Beam Writing,” in Advanced Optical Manufacturing and Testing Technology , L. Yang, H. M. Pollicove, Q. Xin, and J. C. Wyant, eds., Proc. SPIE 4231, 116–125 (2000)

]. The scattering reveals a largely uniform decay over the whole space surrounding the grating, which is an indication for statistical roughness-induced scattering [15

15. J. Stover, Optical Scattering — Measurement and Analysis, (SPIE, Bellingham, WA, 1995) [CrossRef]

]. These losses might be reduced by low-pass filtering the grating structure. It is well-known that dielectric coatings can smooth a profile [16

16. A. Duparré, “Light scattering of thin dielectric films,” in Handbook of Optical Properties - Thin Films for Optical Coatings, R.E. Hummel and K.H. Guenther, eds. (CRC Press, Boca Raton, 1995)

]. Consequently, covering a grating structure with a dielectric layer is a possible way to remove high frequency perturbations like roughness or sharp edges. If the smoothing is very strong only the period information is retained, therefore statistical or systematical fill factor variations can be removed. This is the idea of the following concept.

Fig. 6. Angle resolved scattering measurement of the grating on top of the multilayer (λ=1.064µm)

4. Grating beneath the HR stack

To investigate this issue we placed gratings with several fill factors on one substrate and covered them with standard HR-layerstacks as used in the grating-on-top-concept. In Fig. 7 the cross-section of several gratings with a groove depth of 40 nm and 150 nm (measured before the coating process by AFM) is illustrated. The corrugation depth of the volume grating is obviously decreased by increasing the number of layers. Small fill factors and therefore narrow grating ridges disturb the layerstack more than larger fill factors. However, if Fig. 7(a) and Fig. 7(b) are compared, the surface reliefs become nearly equal after a certain number of layers, independent from the depth of the original grating. It is therefore likely that the coating also smooths roughness and sharp edges.

Fig. 7. Scanning electron microscope images of coated gratings

We measured the value of η1 as well as the 0th transmitted order with an integrating sphere. Since the diffraction of the grating is set to be weak, measuring the 0th transmitted order is a good estimation for the whole transmission of the sample. The measurement results are shown in Fig. 8(a) and (b) (black and red lines). The dashed line in Fig. 8(b) indicates the transmission of the substrate without perturbation by the grating. As discussed above, a smaller fill factor causes a larger corrugation of the volume grating, resulting in higher diffraction efficiency and transmission. For larger fill factors the corrugation is smoother, and the diffraction efficiency approaches zero, and the transmission becomes comparable to an undisturbed mirror. Furthermore, the graphs for the 150 nm-deep gratings approach those of the 40 nm-deep gratings if the fill factor increases. This fact confirms the observations already made in the SEM-images. To quantify the scattering losses ARS-measurements were again performed for selected gratings.

Fig. 8. (a) Diffraction efficiency and (b) 0° transmission measured by an integrating sphere

Figure 9 shows the measurement results of two gratings with a depth of 40 nm and fill factors of 0.49 and 0.83 (illustrated by the hollow circles in Fig. 8(a)). In Fig. 9(a) the intensity of the two first orders is nearly the same as in Fig. 6. Thus, this grating fills nearly the same optical function. The scattered light, however, is significantly reduced. There is only one parasitic diffraction order with an intensity of 10-6 between the 0th and the two first orders; the other peaks are in the range of 10-7 or less, while in Fig. 6 all parasitic orders are higher than 5·10-7. Also, the background signal caused by statistical roughness is reduced from 3·10-8 to 1·10-8. In Fig. 9(b) the scattering is further decreased, and only one parasitic order could be resolved by the measurement setup. Therefore the coating smooths not only the statistical errors but also periodical variations of the fill factor.

Fig. 9. Angle resolved scattering measurement (λ=1064 nm) of gratings coated by the multilayer

It can be concluded from theses measurements that scattering in diffractive cavity couplers can be efficiently reduced by placing a thick dielectric layerstack on top of the grating. Because of the resultant smoothing the resulting surface profile looks very similar for all the profiles considered here; roughness, sharp edges, and even small periodic fill factor variations are suppressed. The transmission is slightly enhanced compared to the undisturbed mirror. A smaller difference could be achieved by increasing the thickness of the layerstack. The smoothing of the grating profile is strongly influenced by the applied deposition technology and its parameters. A detailed analysis of the SEM-images showed that a layer consisting of SiO2 changes the corrugation much more than a layer consisting of Ta2O5, which is a result of different angular distributions of the sputter particles during the ion beam coating process used here. Instead of increasing the number of layers we therefore prepared a sample by inserting a 1.5µm thick layer of fused silica between the grating and the HR-stack. The measured diffraction efficiencies are illustrated by the grey line in Fig. 8(a), in comparison to the other samples. While for smaller fill factors these are significantly different from the samples without this additional layer, the two graphs approach each other for larger values. Obviously the large fill factor profiles have reached a limiting surface corrugation, which is hardly changed by additional coatings. Figure 10 shows the ARS-measurements for the largest fill factor grating. The first order efficiency is comparable to Fig. 9(b), but the small peaks near 35° are not observable anymore. For this grating a transmission of 10-4 was measured, which is the same as for the undisturbed mirror.

5. Conclusions

We have investigated the realization of low-efficiency dielectric reflection gratings by two concepts: gratings made on top of a highly reflective layerstack and the converse assembly. By coating the multilayer on top of the grating the losses of a grating can be effectively reduced. High-frequency profile features such as roughness, sharp edges or periodical fill factor variations are decreased, as are the scattering losses. The smoothing of the surface corrugation also corresponds to a decrease in diffraction efficiency. This effect can be used to tune the diffraction efficiency. Diffraction efficiencies between 7% and 0.02% have been demonstrated, with ultra low scattering losses. For a grating with 1.5% diffraction efficiency, a reduction of the angle-resolved scattering losses from 3·10-8 to 1·10-8 due to statistical scattering has been demonstrated, while parasitic diffraction orders have been drastically reduced.

Fig. 10. Angle resolved scattering measurement (λ=1064 nm) of a grating coated by a 1.5µm thick SiO2 layer and the HR-stack

Acknowledgments

This work was supported by the German Research Association (DFG) within the Sonderforschungsbereich TR 7 “Gravitational Wave Astronomy”. The authors would like to thank Layertec Optical Coatings GmbH for the coating of the dielectric stacks and Dr. U. Hübner (IPHT Jena) for the high-resolution SEM-images.

References and links

1.

T. Clausnitzer, J. Limpert, K. Zöllner, H. Zellmer, H.-J. Fuchs, E.-B. Kley, A. Tünnermann, M. Jupé, and D. Ristau, “Highly efficient transmission gratings in fused silica for CPA systems,” App. Opt. 42, 6934–6938 (2003) [CrossRef]

2.

Z. S. Liu, S. Tibuleac, D. Shin, P.P. Young, and R. Magnusson, “High-efficiency guided mode resonance filter,” Opt. Lett. 23, 1556–1558 (1998) [CrossRef]

3.

T. Clausnitzer, A.V. Tishchenko, E.B. Kley, J. Fuchs, D. Schelle, O. Parriaux, and U. Kroll, “Narrow band, polarization independent free space wave notch filter,” accepted for publication in JOSA A

4.

T. Clausnitzer, H.-J. Fuchs, E.-B. Kley, A. Tuennermann, and U. D. Zeitner, “Polarizing metal stripe gratings for a micro-optical polarimeter,” in Lithographic and Micromachining Techniques for Optical Component Fabrication II , E.B. Kley and H. P. Herzig, eds., Proc. SPIE 5183, 8–15 (2003)

5.

B Willke et.al., “The GEO 600 gravitational wave detector,” Class. Quantum Grav. 19, 1377–1387 (2002) [CrossRef]

6.

R. W. P. Drever, “Concepts for Extending the Ultimate Sensitivity of Interferometric Gravitational Wave Detectors Using Non-Transmissive Optics with Diffractive or Holographic Coupling,” in Proceedings of Seventh Marcel Grossman Meeting on General Relativity, M. Kaiser and R. T. Jantzen, eds., 1401–1406 (1995)

7.

A. Bunkowski, O. Burmeister, P. Beyersdorf, K. Danzmann, R. Schnabel, T. Clausnitzer, E.-B. Kley, and A. Tünnermann, “Low-loss grating for coupling to a high-finesse cavity,” Opt. Lett. 29, 2342–2344 (2004) [CrossRef] [PubMed]

8.

A. Bunkowski, O. Burmeister, K. Danzmann, and R. Schnabel, “Input-output relations for a three-port grating coupled Fabry-Perot cavity,” Opt. Lett. 30, 1183–1185 (2005) [CrossRef] [PubMed]

9.

B. W. Shore, M. D. Perry, J. A. Britten, R. D. Boyd, M. D. Feit, H. T. Nguyen, R. Chow, G. E. Loomis, and Lifeng Li, “Design of high-efficiency dielectric reflection gratings,” J.Opt. Soc. Am. A 14, 1124–1136 (1997) [CrossRef]

10.

M. D. Perry, R. D. Boyd, J. A. Britten, D. Decker, B. W. Shore, C. Shannon, and E. Shults, “High-efficiency multilayer dielectric reflection gratings,” Opt. Lett. 20, 940–942 (1997) [CrossRef]

11.

K. Hehl, J. Bischoff, U. Mohaupt, M. Palme, B. Schnabel, L. Wenke, R. Bödefeld, W. Theobald, E. Welsch, R. Sauerbrey, and H. Heyer, “High-Efficiency Dielectric Reflection Gratings: Design, Fabrication, and Analysis,” Appl. Opt. , 38, 6257–6271 (1999) [CrossRef]

12.

J. Turunen, “Diffraction theory of microrelief gratings,” in Micro-optics, H.P. Herzig, ed. (Taylor & Francis, Inc., 1997)

13.

A. Duparré, J. Ferre-Borrull, S. Gliech, G. Notni, J. Steinert, and J. M. Bennett, “Surface characterization techniques for determing the root-mean-square roughness and power spectral densities of optical components,” Appl. Opt. 41, 154–171 (2002) [CrossRef] [PubMed]

14.

E.-B. Kley, T. Clausnitzer, M. Cumme, K. Zöllner, B. Schnabel, and A. Stich, “Investigation of Large-Area Gratings Fabricated by Ultrafast E-Beam Writing,” in Advanced Optical Manufacturing and Testing Technology , L. Yang, H. M. Pollicove, Q. Xin, and J. C. Wyant, eds., Proc. SPIE 4231, 116–125 (2000)

15.

J. Stover, Optical Scattering — Measurement and Analysis, (SPIE, Bellingham, WA, 1995) [CrossRef]

16.

A. Duparré, “Light scattering of thin dielectric films,” in Handbook of Optical Properties - Thin Films for Optical Coatings, R.E. Hummel and K.H. Guenther, eds. (CRC Press, Boca Raton, 1995)

17.

L. Li and J. Hirsh, “All-dielectric high-efficiency reflection gratings made with multilayer thin-film coatings,” Opt. Lett. 20, 1349–1351 (1995) [CrossRef] [PubMed]

18.

J. K. Guha and J. A. Plascyk, “Low-Absorption Grating Beam Samplers,” in Optical Components: Manufacture and Evaluation , D. Nicholson ed., Proc. SPIE 171, 117–124 (1979)

OCIS Codes
(050.1950) Diffraction and gratings : Diffraction gratings
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(230.1360) Optical devices : Beam splitters

ToC Category:
Research Papers

History
Original Manuscript: April 19, 2005
Revised Manuscript: May 23, 2005
Published: June 13, 2005

Citation
Tina Clausnitzer, E.-B. Kley, A. Tünnermann, A. Bunkowski, O. Burmeister, K. Danzmann, R. Schnabel, S. Gliech, and A. Duparré, "Ultra low-loss low-efficiency diffraction gratings," Opt. Express 13, 4370-4378 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-12-4370


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References

  1. T. Clausnitzer, J. Limpert, K. Zöllner, H. Zellmer, H.-J. Fuchs, E.-B. Kley, A. Tünnermann, M. Jupé, and D. Ristau, "Highly efficient transmission gratings in fused silica for CPA systems,�?? App. Opt. 42, 6934-6938 (2003). [CrossRef]
  2. Z. S. Liu, S. Tibuleac, D. Shin, P.P. Young, R. Magnusson, "High-efficiency guided mode resonance filter,�?? Opt. Lett. 23, 1556-1558 (1998). [CrossRef]
  3. T. Clausnitzer, A.V. Tishchenko, E.B. Kley, J. Fuchs, D. Schelle, O. Parriaux, U. Kroll, "Narrow band, polarization independent free space wave notch filter," accepted for publication in JOSA A.
  4. T. Clausnitzer, H.-J. Fuchs, E.-B. Kley, A. Tuennermann, U. D. Zeitner, "Polarizing metal stripe gratings for a micro-optical polarimeter," in Lithographic and Micromachining Techniques for Optical Component Fabrication II, E.B. Kley, H. P. Herzig, eds., Proc. SPIE 5183, 8-15 (2003).
  5. B Willke et.al., "The GEO 600 gravitational wave detector,�?? Class. Quantum Grav. 19, 1377-1387 (2002). [CrossRef]
  6. R. W. P. Drever, �??Concepts for Extending the Ultimate Sensitivity of Interferometric Gravitational Wave Detectors Using Non-Transmissive Optics with Diffractive or Holographic Coupling,�?? in Proceedings of Seventh Marcel Grossman Meeting on General Relativity, M. Kaiser and R. T. Jantzen, eds., 1401-1406 (1995).
  7. A. Bunkowski, O. Burmeister, P. Beyersdorf, K. Danzmann, R. Schnabel, T. Clausnitzer, E.-B. Kley, A. Tünnermann, �??Low-loss grating for coupling to a high-finesse cavity,�?? Opt. Lett. 29, 2342-2344 (2004). [CrossRef] [PubMed]
  8. A. Bunkowski, O. Burmeister, K. Danzmann, and R. Schnabel, �??Input-output relations for a three-port grating coupled Fabry-Perot cavity,�?? Opt. Lett. 30, 1183-1185 (2005). [CrossRef] [PubMed]
  9. B. W. Shore, M. D. Perry, J. A. Britten, R. D. Boyd, M. D. Feit, H. T. Nguyen, R. Chow, G. E. Loomis, and Lifeng Li, �??Design of high-efficiency dielectric reflection gratings,�?? J. Opt. Soc. Am. A 14, 1124-1136 (1997). [CrossRef]
  10. M. D. Perry, R. D. Boyd, J. A. Britten, D. Decker, B. W. Shore, C. Shannon, E. Shults, �??High-efficiency multilayer dielectric reflection gratings,�?? Opt. Lett. 20, 940-942 (1997). [CrossRef]
  11. K. Hehl, J. Bischoff, U. Mohaupt, M. Palme, B. Schnabel, L. Wenke, R. Bödefeld, W. Theobald, E. Welsch, R. Sauerbrey, and H. Heyer, �??High-Efficiency Dielectric Reflection Gratings: Design, Fabrication, and Analysis,�?? Appl. Opt. 38, 6257-6271 (1999). [CrossRef]
  12. J. Turunen, "Diffraction theory of microrelief gratings", in Micro-optics, H.P. Herzig, ed. (Taylor & Francis, Inc., 1997).
  13. A. Duparré, J. Ferre-Borrull, S. Gliech, G. Notni, J. Steinert, J. M. Bennett, "Surface characterization techniques for determing the root-mean-square roughness and power spectral densities of optical components,�?? Appl. Opt. 41, 154-171 (2002). [CrossRef] [PubMed]
  14. E.-B. Kley, T. Clausnitzer, M. Cumme, K. Zöllner, B. Schnabel, A. Stich, "Investigation of Large-Area Gratings Fabricated by Ultrafast E-Beam Writing,�?? in Advanced Optical Manufacturing and Testing Technology, L. Yang, H. M. Pollicove, Q. Xin, J. C. Wyant, eds., Proc. SPIE 4231, 116-125 (2000).
  15. J. Stover, Optical Scattering �?? Measurement and Analysis, (SPIE, Bellingham, WA, 1995). [CrossRef]
  16. A. Duparré, �??Light scattering of thin dielectric films,�?? in Handbook of Optical Properties - Thin Films for Optical Coatings, R.E. Hummel, K.H. Guenther, eds. (CRC Press, Boca Raton, 1995).
  17. L. Li, J. Hirsh, �??All-dielectric high-efficiency reflection gratings made with multilayer thin-film coatings,�?? Opt. Lett. 20, 1349-1351 (1995). [CrossRef] [PubMed]
  18. J. K. Guha, J. A. Plascyk, �??Low-Absorption Grating Beam Samplers,�?? in Optical Components: Manufacture and Evaluation, D. Nicholson ed., Proc. SPIE 171, 117-124 (1979).

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