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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 18 — Sep. 9, 2013
  • pp: 21523–21529
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Design and fabrication of ultra-steep notch filters

Jinlong Zhang, Alexander V. Tikhonravov, Michael K. Trubetskov, Yongli Liu, Xinbin Cheng, and Zhanshan Wang  »View Author Affiliations


Optics Express, Vol. 21, Issue 18, pp. 21523-21529 (2013)
http://dx.doi.org/10.1364/OE.21.021523


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Abstract

We present the design and production approach of an ultra-steep notch filter. The notch filter that does not have thin layers is optimized utilizing the constrained optimization technique, and this is well suitable for accurate monitoring with the electron beam deposition technique. Single layer SiO2 and Ta2O5 films were deposited and carefully characterized in order to determine tooling factors and refractive indices wavelength dependencies accurately. We produced the ultra-steep notch filter with indirect monochromatic monitoring strategy and demonstrated the excellent correspondence to the theoretical spectral performance.

© 2013 OSA

1. Introduction

Notch filters are optical filters that selectively reject a wavelength band and transmit at both shorter and longer wavelengths [1

1. J. A. Dobrowolski, “Optical properties of films and coatings,” in Handbook of Optics, M.Bass ed. (McGraw-Hill, New York, 2010), IV, 7.15–7.53.

, 2

2. H. A. Macleod, Thin-film Optical Filters, 4th ed., (CRC Press/Taylor & Francis, 2010).

]. Such filters have various scientific and technological applications, including protection from laser radiation, laser-based fluorescence instrumentation, raman spectroscopy, and other applications.

Rugate filters with continuously modulated refractive indices are a promising method to design notch filter [3

3. B. G. Bovard, “Rugate filter design: the modified Fourier transform technique,” Appl. Opt. 29(1), 24–30 (1990). [CrossRef] [PubMed]

]. They offer various advantages in optical performances, including extremely broad passbands and perfectly suppressed ripples by apodization [4

4. W. H. Southwell and R. L. Hall, “Rugate filter sidelobe suppression using quintic and rugated quintic matching layers,” Appl. Opt. 28(14), 2949–2951 (1989). [CrossRef] [PubMed]

, 5

5. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, M. A. Kokarev, N. Kaiser, O. Stenzel, S. Wilbrandt, and D. Gäbler, “New optimization algorithm for the synthesis of rugate optical coatings,” Appl. Opt. 45(7), 1515–1524 (2006). [CrossRef] [PubMed]

]. Even though the theoretical aspects of this filter type and the achievable extraordinary spectral characteristics are clarified, the realization and sufficient reproduction of gradient-index filters still pose a challenging problem [6

6. M. Lappschies, B. Görtz, and D. Ristau, “Application of optical broadband monitoring to quasi-rugate filters by ion-beam sputtering,” Appl. Opt. 45(7), 1502–1506 (2006). [CrossRef] [PubMed]

, 7

7. C. C. Lee, C. J. Tang, and J. Y. Wu, “Rugate filter made with composite thin films by ion-beam sputtering,” Appl. Opt. 45(7), 1333–1337 (2006). [CrossRef] [PubMed]

].

Two-material multilayers are the simplest way to design notch filters. Researchers proposed many methods to improve the performance of two-material filters. Utilizing the approach with equivalent layers, high-performance two-material notch filters have been designed and produced [8

8. A. Thelen, “Design of Optical minus filter,” J. Opt. Soc. Am. 61(3), 365 (1971). [CrossRef]

10

10. U. Schallenberg, B. Ploss, M. Lappschies, and S. Jakobs, “Design and manufacturing of high performance notch filters,” Proc. SPIE 7739, 77391X, 77391X-9 (2010). [CrossRef]

]. However, two-material designs often have very thin layers that are difficult to control precisely, and such notch filters are typically produced by ion beam sputtering which results in a long deposition time and high stress.

The ion-assisted electron-beam deposition technology is the most widely used technology in coating production. It possesses the advantages of high deposition rate and good stress quality. In order to fabricate notch filters with this widely spread deposition technique, layer thickness constraints must be taken into account. An accurate control of index of refraction and thickness of each layer during deposition may be greatly improved if layer thicknesses are not too thin or too thick [11

11. A. V. Tikhonravov and M. K. Trubetskov, “Modern design tools and a new paradigm in optical coating design,” Appl. Opt. 51(30), 7319–7332 (2012). [CrossRef] [PubMed]

].

In this paper, we present the design and production approach of an ultra-steep notch filter. The notch filter with layer thicknesses that are close to half-wave of rejection wavelength is optimized utilizing the constrained optimization technique, and therefore are better suitable for accurate monitoring during the deposition. Single layer SiO2 and Ta2O5 films were deposited and characterized for accurate determination of tooling factors and refractive indices wavelength dependencies. We produced the ultra-steep notch filter with indirect monochromatic monitoring strategy by ion-assisted electron-beam deposition and demonstrated an excellent correspondence to the theoretical spectral performance. Moreover, we performed reverse engineering of the deposited filter to show that remaining deviations from the target curve are connected to the low-index layers.

2. Design of the ultra-steep notch filter

We consider the notch filter with the performance that should satisfy the following demands under normal incidence: high transmittance (no back side reflections include) in the wavelength ranges of 400-500 nm and 550-700 nm, and high reflectivity in the spectral region from 500 to 550 nm, as shown by the purple dots in Fig. 1
Fig. 1 Layer-thickness profile (a) and spectral transmittance (b) of an ultra-high steep notch filter obtained by constrained optimization.
.

In reference [12

12. J. L. Zhang, Y. J. Xie, X. B. Cheng, H. F. Jiao, and Z. S. Wang, “Thin-film thickness-modulated designs for optical minus filter,” Appl. Opt. 52(23), 5788–5793 (2013). [CrossRef] [PubMed]

], we proposed an approach to design notch filters with layer thicknesses around half-wave of the rejection wavelength. The reason for designing filters without thin layers is as follows. In order to achieve a practical coating design, layer thickness constraints can be an important additional demand that should be taken into account. There may be practically proved lower limits for physical layer thicknesses for conventional deposition processes [13

13. S. Wilbrandt, O. Stenzel, and N. Kaiser, “All-oxide broadband antireflection coatings by plasma ion assisted deposition: design, simulation, manufacturing and re-optimization,” Opt. Express 18(19), 19732–19742 (2010). [CrossRef] [PubMed]

]. Lower thickness limits can be also applied in connection with monitoring strategies used for coating production. For non-quarter wave multilayers, level monitoring is often used [14

14. C. J. van der Laan, “Optical monitoring of nonquarterwave stacks,” Appl. Opt. 25(5), 753–760 (1986). [CrossRef] [PubMed]

]. To minimize thickness error, active monochromatic monitoring strategies involving corrections of termination levels require that at least one monitoring signal extremum is registered during a layer deposition [15

15. C. C. Lee, K. Wu, C. C. Kuo, and S. H. Chen, “Improvement of the optical coating process by cutting layers with sensitive monitoring wavelengths,” Opt. Express 13(13), 4854–4861 (2005). [CrossRef] [PubMed]

, 16

16. B. Chun, C. K. Hwangbo, and J. S. Kim, “Optical monitoring of nonquarterwave layers of dielectric multilayer filters using optical admittance,” Opt. Express 14(6), 2473–2480 (2006). [CrossRef] [PubMed]

]. This is possible only if a monitored layer is thick enough.

It may be also desirable to apply upper limits for thicknesses of individual layers. With a large film thickness, essential changes of film structure can take place. Upper constraints for individual layer thicknesses may also help to avoid unreasonably high total optical thickness values.

In this paper we apply the constrained optimization in [11

11. A. V. Tikhonravov and M. K. Trubetskov, “Modern design tools and a new paradigm in optical coating design,” Appl. Opt. 51(30), 7319–7332 (2012). [CrossRef] [PubMed]

] for designing of the notch filter. The lower and upper constraints for layer optical thicknesses are 100 and 350nm. The layer thicknesses profile and spectral response of the final design obtained using OptiLayer software [17

17. A. V. Tikhonravov and M. K. Trubetskov, Optilayer Thin Film Software, http://www.optilayer.com.

] are shown in Fig. 1. The average transmittance is higher than 99%, and the bandwidth of the transmission zone at the −30 dB and −0.5dB attenuation level are 30nm and 52nm respectively, and the steepness is defined by the ratio of two bandwidth values [11

11. A. V. Tikhonravov and M. K. Trubetskov, “Modern design tools and a new paradigm in optical coating design,” Appl. Opt. 51(30), 7319–7332 (2012). [CrossRef] [PubMed]

]. As compared to the notch filter designs obtained using apodization technique in [12

12. J. L. Zhang, Y. J. Xie, X. B. Cheng, H. F. Jiao, and Z. S. Wang, “Thin-film thickness-modulated designs for optical minus filter,” Appl. Opt. 52(23), 5788–5793 (2013). [CrossRef] [PubMed]

] some layers near the substrate have been additionally optimized to suppress transmittance ripples in the passbands. However the main structure is still an apodized multilayer stack. Additionally, as we can expect, all layers of the obtained design have optical thicknesses close to half-wave of the central rejection wavelength and therefore are suitable for accurate monitoring during the deposition. Such coating can be produced by ion-assisted electron-beam deposition technology with high deposition rate and good stress quality.

3. Deposition and characterization of single layers

For the coating production we used the Optorun Electron-beam deposition plant. During deposition layers were densified with an RF-type ion source. Thereby, the selected deposition parameters resulted in coatings without any relevant shift after cooling and venting. The deposition temperature was 473K, and the deposition rates were 0.2nm and 0.6nm for Ta2O5 and SiO2 respectively. The indirect monochromatic back-reflection optical monitor is utilized to control thicknesses of deposited layers. A preliminary calibration is done by pre-production depositions of single Ta2O5 and SiO2 layers aimed at accurate determination of tooling factors and refractive indices wavelength dependencies.

We deposited SiO2 film with the thickness of about 440 nm and Ta2O5 film with the thickness of about 300 nm. In each case there are two films: one is on the monitor glass and another one is on the calotte. Both film pairs were monitored at the wavelength of 750 nm (this wavelength was chosen because it is most often present in our monitoring runsheet for the notch filter). Depositions of both film pairs were terminated at a swing level of 50% around the above mentioned thickness values, and the monitoring signals passed two minima and two maxima before termination instants.

The fitting of measurement data by the model transmittance curve is shown in Fig. 3(a)
Fig. 3 (a) Fitting of measured transmittance data (red crosses) by model transmittance (solid curve) at the end of discrepancy function minimization: non absorbing model of Ta2O5 film. (b) Wavelength dependence of the refractive index of the Ta2O5 films found in the frame of the homogeneous thin film model.
. The obtained wavelength dependencies of the refractive index are depicted in Fig. 3(b). There is an excellent correspondence between Ta2O5 indices on monitor glass and on the calotte substrate, and this allows us to neglect possible small offsets in layer refractive indices and determine only errors in layer thicknesses. Moreover there is also an excellent correspondence between found thicknesses of the Ta2O5 film on monitor glass and the planned value. Found thickness value is 3.55 quarter wave optical thickness (QWOT) at 750nm and the planned value corresponding to the 50% swing is 3.54 QWOT. Thickness of the film on the calotte is 3.64 QWOT. This means that monitor film is thinner with the tooling factor 0.975.

Figure 4(a)
Fig. 4 (a) Fitting of measured reflectance data (red crosses) by model reflectance (solid curve) at the end of discrepancy function minimization: non absorbing model of SiO2 film. (b) Wavelength dependence of the refractive index of SiO2 film.
presents the fitting of measured reflectance data (gray crosses) by model reflectance (solid curve) at the end of discrepancy function minimization for SiO2 film. And wavelength dependence of the refractive index of the SiO2 film is shown in Fig. 4(b). Here we don not have reflectance data for the SiO2 film on monitor glass, since the deposition area in the monitor glass is too small for the reflectance measurement. Nevertheless, the refractive index of the film on calotte is derived from R data with high enough accuracy. Then we used this fixed index to determine thicknesses of the film on calotte and on monitor glass according the transmittance data shown in the Fig. 5
Fig. 5 Transmittance of the uncoated substrate (solid curve) and the SiO2 coatings on calotte (blue dashed line) and monitor glass (red solid line).
. These thicknesses are 3.88 QWOT and 3.56 QWOT respectively, and this gives tooling factor of 0.917.

4. Fabrication of notch filter

In this experiment we used the indirect monochromatic monitoring with level monitoring method. Specifically the Optorun monochromatic back-reflection monitor was used for the notch filter production. The monitoring strategy is using one monitoring chip to control thicknesses of two subsequent high and low index layers (except for the first layer). The main goal of using several monitoring chips instead of one monitoring chip is to prevent the accumulation of errors in the determination of an instant when a layer deposition should be terminated [14

14. C. J. van der Laan, “Optical monitoring of nonquarterwave stacks,” Appl. Opt. 25(5), 753–760 (1986). [CrossRef] [PubMed]

].

We produced the notch filter with the tooling factors and refractive indices derived above. After the deposition, transmittance of the sample was measured at normal incidence in the spectral range from 400 nm to 800 nm using a Cary5000 spectrophotometer. In Fig. 6(a)
Fig. 6 (a) Comparison of measurement transmittance data (red crosses) and theoretical transmittance (solid black curve) of the notch filter. (b) Fitting of measured notch filter transmittance (red crosses) by the model transmittance (solid curve) when the model with random errors in thicknesses of low index layers was applied.
we compare the measured transmittance data and theoretical transmittance data of the sample. In this figure an excellent agreement between theoretical and experimental data is observed.

Although we produced the notch filter with excellent performance, there are still some deviations between experimental and theoretical transmittance data. These deviations are caused by errors in layer parameters. In this experiment we used stable deposition process that produces high density films. Refractive indices of Ta2O5 and SiO2 thin-film materials deposited are stable and known with a high accuracy. This allows us to neglect possible small offsets in layer refractive indices and to attribute the observed deviations only to errors in layer thicknesses.

We performed reverse engineering of the deposited notch filter sample with the help of OptiRE software [17

17. A. V. Tikhonravov and M. K. Trubetskov, Optilayer Thin Film Software, http://www.optilayer.com.

] using the above assumption. Figure 6(b) shows the fitting of measured notch filter transmittance by the model transmittance when the model with random errors in thicknesses of low index layers was applied. The obtained values of these random errors were all in the range of 3.5% of planned theoretical layer thicknesses. The achieved excellent fitting of measurement data confirms that deviations observed in Fig. 6(a) could be indeed attributed to errors in low index layers.

5. Conclusion

In summary, we designed ultra-steep notch filters using constrained optimization with lower and upper constraints for layer thicknesses. The filter was produced using indirect monochromatic monitoring strategy with conventional deposition technology. The experimental result shows an excellent correspondence with the theoretical spectral performance. Moreover, reverse engineering was performed to demonstrate that the monitoring procedure provides an accurate control of the high-index layers, and that the remaining deviations from the target curve are due to errors in the low-index layers. In the following study, we are going to work on further improvement of our design-monitoring-production chain by improving an accuracy of monitoring of low-index layers.

Acknowledgments

This work was partly supported by the National Natural Science Foundation of China (Grant Nos. 61235011, 61008030, 61108014, 61205124), and the National 863 Program.

References and links

1.

J. A. Dobrowolski, “Optical properties of films and coatings,” in Handbook of Optics, M.Bass ed. (McGraw-Hill, New York, 2010), IV, 7.15–7.53.

2.

H. A. Macleod, Thin-film Optical Filters, 4th ed., (CRC Press/Taylor & Francis, 2010).

3.

B. G. Bovard, “Rugate filter design: the modified Fourier transform technique,” Appl. Opt. 29(1), 24–30 (1990). [CrossRef] [PubMed]

4.

W. H. Southwell and R. L. Hall, “Rugate filter sidelobe suppression using quintic and rugated quintic matching layers,” Appl. Opt. 28(14), 2949–2951 (1989). [CrossRef] [PubMed]

5.

A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, M. A. Kokarev, N. Kaiser, O. Stenzel, S. Wilbrandt, and D. Gäbler, “New optimization algorithm for the synthesis of rugate optical coatings,” Appl. Opt. 45(7), 1515–1524 (2006). [CrossRef] [PubMed]

6.

M. Lappschies, B. Görtz, and D. Ristau, “Application of optical broadband monitoring to quasi-rugate filters by ion-beam sputtering,” Appl. Opt. 45(7), 1502–1506 (2006). [CrossRef] [PubMed]

7.

C. C. Lee, C. J. Tang, and J. Y. Wu, “Rugate filter made with composite thin films by ion-beam sputtering,” Appl. Opt. 45(7), 1333–1337 (2006). [CrossRef] [PubMed]

8.

A. Thelen, “Design of Optical minus filter,” J. Opt. Soc. Am. 61(3), 365 (1971). [CrossRef]

9.

V. Pervak, A. V. Tikhonravov, M. K. Trubetskov, J. Pistner, F. Krausz, and A. Apolonski, “Band filters: two-material technology versus rugate,” Appl. Opt. 46(8), 1190–1193 (2007). [CrossRef] [PubMed]

10.

U. Schallenberg, B. Ploss, M. Lappschies, and S. Jakobs, “Design and manufacturing of high performance notch filters,” Proc. SPIE 7739, 77391X, 77391X-9 (2010). [CrossRef]

11.

A. V. Tikhonravov and M. K. Trubetskov, “Modern design tools and a new paradigm in optical coating design,” Appl. Opt. 51(30), 7319–7332 (2012). [CrossRef] [PubMed]

12.

J. L. Zhang, Y. J. Xie, X. B. Cheng, H. F. Jiao, and Z. S. Wang, “Thin-film thickness-modulated designs for optical minus filter,” Appl. Opt. 52(23), 5788–5793 (2013). [CrossRef] [PubMed]

13.

S. Wilbrandt, O. Stenzel, and N. Kaiser, “All-oxide broadband antireflection coatings by plasma ion assisted deposition: design, simulation, manufacturing and re-optimization,” Opt. Express 18(19), 19732–19742 (2010). [CrossRef] [PubMed]

14.

C. J. van der Laan, “Optical monitoring of nonquarterwave stacks,” Appl. Opt. 25(5), 753–760 (1986). [CrossRef] [PubMed]

15.

C. C. Lee, K. Wu, C. C. Kuo, and S. H. Chen, “Improvement of the optical coating process by cutting layers with sensitive monitoring wavelengths,” Opt. Express 13(13), 4854–4861 (2005). [CrossRef] [PubMed]

16.

B. Chun, C. K. Hwangbo, and J. S. Kim, “Optical monitoring of nonquarterwave layers of dielectric multilayer filters using optical admittance,” Opt. Express 14(6), 2473–2480 (2006). [CrossRef] [PubMed]

17.

A. V. Tikhonravov and M. K. Trubetskov, Optilayer Thin Film Software, http://www.optilayer.com.

18.

A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, G. DeBell, V. Pervak, A. K. Sytchkova, M. L. Grilli, and D. Ristau, “Optical parameters of oxide films typically used in optical coating production,” Appl. Opt. 50(9), C75–C85 (2011). [CrossRef] [PubMed]

OCIS Codes
(310.1860) Thin films : Deposition and fabrication
(310.6860) Thin films : Thin films, optical properties
(310.4165) Thin films : Multilayer design

ToC Category:
Thin Films

History
Original Manuscript: July 22, 2013
Revised Manuscript: August 27, 2013
Manuscript Accepted: August 27, 2013
Published: September 5, 2013

Citation
Jinlong Zhang, Alexander V. Tikhonravov, Michael K. Trubetskov, Yongli Liu, Xinbin Cheng, and Zhanshan Wang, "Design and fabrication of ultra-steep notch filters," Opt. Express 21, 21523-21529 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-18-21523


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References

  1. J. A. Dobrowolski, “Optical properties of films and coatings,” in Handbook of Optics, M.Bass ed. (McGraw-Hill, New York, 2010), IV, 7.15–7.53.
  2. H. A. Macleod, Thin-film Optical Filters, 4th ed., (CRC Press/Taylor & Francis, 2010).
  3. B. G. Bovard, “Rugate filter design: the modified Fourier transform technique,” Appl. Opt.29(1), 24–30 (1990). [CrossRef] [PubMed]
  4. W. H. Southwell and R. L. Hall, “Rugate filter sidelobe suppression using quintic and rugated quintic matching layers,” Appl. Opt.28(14), 2949–2951 (1989). [CrossRef] [PubMed]
  5. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, M. A. Kokarev, N. Kaiser, O. Stenzel, S. Wilbrandt, and D. Gäbler, “New optimization algorithm for the synthesis of rugate optical coatings,” Appl. Opt.45(7), 1515–1524 (2006). [CrossRef] [PubMed]
  6. M. Lappschies, B. Görtz, and D. Ristau, “Application of optical broadband monitoring to quasi-rugate filters by ion-beam sputtering,” Appl. Opt.45(7), 1502–1506 (2006). [CrossRef] [PubMed]
  7. C. C. Lee, C. J. Tang, and J. Y. Wu, “Rugate filter made with composite thin films by ion-beam sputtering,” Appl. Opt.45(7), 1333–1337 (2006). [CrossRef] [PubMed]
  8. A. Thelen, “Design of Optical minus filter,” J. Opt. Soc. Am.61(3), 365 (1971). [CrossRef]
  9. V. Pervak, A. V. Tikhonravov, M. K. Trubetskov, J. Pistner, F. Krausz, and A. Apolonski, “Band filters: two-material technology versus rugate,” Appl. Opt.46(8), 1190–1193 (2007). [CrossRef] [PubMed]
  10. U. Schallenberg, B. Ploss, M. Lappschies, and S. Jakobs, “Design and manufacturing of high performance notch filters,” Proc. SPIE7739, 77391X, 77391X-9 (2010). [CrossRef]
  11. A. V. Tikhonravov and M. K. Trubetskov, “Modern design tools and a new paradigm in optical coating design,” Appl. Opt.51(30), 7319–7332 (2012). [CrossRef] [PubMed]
  12. J. L. Zhang, Y. J. Xie, X. B. Cheng, H. F. Jiao, and Z. S. Wang, “Thin-film thickness-modulated designs for optical minus filter,” Appl. Opt.52(23), 5788–5793 (2013). [CrossRef] [PubMed]
  13. S. Wilbrandt, O. Stenzel, and N. Kaiser, “All-oxide broadband antireflection coatings by plasma ion assisted deposition: design, simulation, manufacturing and re-optimization,” Opt. Express18(19), 19732–19742 (2010). [CrossRef] [PubMed]
  14. C. J. van der Laan, “Optical monitoring of nonquarterwave stacks,” Appl. Opt.25(5), 753–760 (1986). [CrossRef] [PubMed]
  15. C. C. Lee, K. Wu, C. C. Kuo, and S. H. Chen, “Improvement of the optical coating process by cutting layers with sensitive monitoring wavelengths,” Opt. Express13(13), 4854–4861 (2005). [CrossRef] [PubMed]
  16. B. Chun, C. K. Hwangbo, and J. S. Kim, “Optical monitoring of nonquarterwave layers of dielectric multilayer filters using optical admittance,” Opt. Express14(6), 2473–2480 (2006). [CrossRef] [PubMed]
  17. A. V. Tikhonravov and M. K. Trubetskov, Optilayer Thin Film Software, http://www.optilayer.com .
  18. A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, G. DeBell, V. Pervak, A. K. Sytchkova, M. L. Grilli, and D. Ristau, “Optical parameters of oxide films typically used in optical coating production,” Appl. Opt.50(9), C75–C85 (2011). [CrossRef] [PubMed]

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