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OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Femtosecond laser writing of Bragg grating waveguide bundles in bulk glass Markus Thiel,1,* Günter Flachenecker,1 and Wolfgang Schade1,2 1
2
Department of Fiber Optical Sensor Systems, Fraunhofer Heinrich-Hertz-Institute, Am Stollen 19B, 38640 Goslar, Germany Department of Applied Photonics, Institute of Energy Research and Physical Technologies, Am Stollen 19B, 38640 Goslar, Germany *Corresponding author: [email protected] Received December 30, 2014; revised February 11, 2015; accepted February 17, 2015; posted February 18, 2015 (Doc. ID 231399); published March 19, 2015 Waveguide bundles in bulk glass materials, consisting of several parallel scans of refractive index modifications, have been generated with a low-repetition femtosecond laser. Additionally, Bragg grating (BG) structures for 840 and 1550 nm have been introduced by segmentation of the central scan. A spectral loss in the transmission signal of >36 dB was achieved at 1550 nm with a second-order Bragg grating waveguide (BGW) in fused silica, which corresponds to an intrinsic grating efficiency of >16 dB∕cm. This is to our knowledge the strongest BG structure realized in glass with a femtosecond laser. The BGW were proven to be stable up to a temperature of 250°C in fused silica. The diameter of the waveguide bundles can be adapted very easily for a broad range of wavelengths and have been demonstrated for diameters between 1 and 50 μm. The transmission properties of the waveguide bundles are affected minorly by the insertion of BG structures, which opens the ability for adjusting the BGW for a broad range of wavelength in single-mode or multimode optical circuits. BGW have been realized successfully in fused silica, borosilicate glass (BOROFLOAT 33), and AF 32 eco Thin Glass from Schott. © 2015 Optical Society of America OCIS codes: (130.2755) Glass waveguides; (130.0130) Integrated optics; (130.3990) Micro-optical devices; (130.7408) Wavelength filtering devices; (230.1480) Bragg reflectors; (230.7370) Waveguides. http://dx.doi.org/10.1364/OL.40.001266
In the past, femtosecond (fs) laser-writing techniques have been successfully used for direct implementation of three-dimensional optical waveguide structures in transparent bulk glasses.[1] Of special interest are integrated photonic elements like Bragg grating waveguides (BGW). Fs laser direct-written BGW have been realized as laser mirrors, spectral filters for wavelength multiplexing, or sensing elements for strain and temperature [1–5]. For the realization of such BGW, a periodic modulation of the refractive index in the waveguide has to be generated with the fs laser. However this is not straight forward, due to the fact that a waveguide itself, generated with fs pulses, has already experienced a refractive index modification in comparison to the original bulk glass material. Different approaches have been reported for the production of BWG in glass in the last years. A weak BGW of ∼5 dB reflection has been demonstrated for the first time in bulk fused silica with a two-step laser process by Marshall et al. [6]. Firstly they generated a waveguide in fused silica by using a 1-kHz Ti:Sapphire fs laser and moving the glass substrate inside the focus of a microscope objective. For the subsequent step, they reduced the laser power and increased the scanning velocity for writing point by point a second-order Bragg grating (BG) with a weak response signal at 1550 nm, overlaying the previously written waveguide, originating in a smaller volume of the Bragg structure than the waveguide itself. A similar technique was applied by Chung et al. [7], who generated a first-order grating in soda-lime glass. Zhang et al. [2] were able to create BGW for 1540– 1570 nm in borosilicate glass with a 1-kHz fs laser in a single process step. As a result, their waveguides consisted of periodical separated segments. Their resulting BG reflection was 40% and the transmission signal less than 11 dB. However, a drawback of their technique is low reflection signals and the restriction to certain glass materials. To overcome this, a technique, which is called “burst writing”, was first introduced by the same group 0146-9592/15/071266-04$15.00/0
[8]. For this method, usually a laser repetition rate in the MHz range is used. In opposition to the former techniques, one “volume dot” is written by a burst of pulses, consisting of thousands of pulses, causing a higher refractive index change, and high reflecting first-order BG (89% reflectivity) were fabricated in fused silica. Even most promising and used for several applications [1,3,9], the burst writing technique of Zhang et al. [8] still lacks in flexibility on the desired waveguide cross-section, which depends on parameters such as pulse energy, repetition rate, and focal length of the microscope objective. An alternative technique to produce a defined cross-section of a waveguide is the multiscan technique, which was applied successfully by different groups [5,10,11]. Brown et al. [5] used a multiscan technique consisting of 20 parallel scans in borosilicate glass for production of BGW. With this technique, they achieved strong spectral filtering, but the reflectivity of their BG signal was only 40%. In this Letter, we present a new enhanced multiscan technique for Fs laser direct-written waveguides including BGW structures. In our approach, the waveguide is constructed around the BG structure via multiscan technique. The advantage is a maximally achievable refractive index contrast for the grating, as well as an increased structural flexibility by meaning the diameter of the waveguide and the grating period. This enables us to produce strong BGs in waveguides with tailored diameter for single-mode operation of a broad range of wavelengths from the visible to infrared. This is in contrast to the former works [2,5,7,8], where the BG and waveguide structures are formed simultaneously by the laser process, and therefore, their properties are linked. In consequence, their limitations are a more or less fixed diameter of the waveguide [2,7,8], restricted BG period [2,7,8] or certain glass materials [2,5]. For our experiments, we used a Ti:Sapphire laser providing 100-fs laser pulses at 800 nm with a repetition rate of 5 kHz. The glass samples were mounted on top of three © 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
precision linear stages (XMS series from Newport). With a λ∕4 zero order waveplate, the linear polarization of the laser was transformed into circular polarization. The laser was focused with a 20×, 0.45 NA microscope objective 150 μm below the surface of the glass sample, which was moved linearly horizontal with constant velocity of 1 mm∕s. By moving the glass sample under the laser focus, a single line (scan) with a positive refractive index contrast to the bulk material is generated. We have written waveguides from 5 to 500 μm below the surface. The limitation of writing depths clearly depends on the imaging quality of the microscope objective. Going close toward the surface unwanted, irregular destructions appear instead of structures with a smooth refractive index modification. The refractive index contrast itself depends sensitively on the power of the laser pulses, scanning speed, and depth, which has to be figured out for different glass materials. In fused silica for the waveguide scans, laser pulse energies of 224 nJ were used, which is 10% less than the damage threshold. According to the laser focus, the diameter of a single scan waveguide is about >1 μm in glass, which is well suited for guiding light in the visible spectrum. To enhance the transmission to the NIR spectrum and also to improve the transmission in the visible, the diameter of the waveguide has to be increased. To achieve this, we used a multiscan technique to form an effective waveguide with larger diameter consisting of a set of parallel single waveguide scans in the volume, which we call waveguide bundle. By this procedure, the waveguide diameter can be adapted in a flexible way for the mode field and the wavelength of the guided light. As an example, we have written single and multimode waveguide bundles for 800 and 1550 nm (Fig. 1). The diameters of these waveguides vary between 7 and 50 μm with a scan to scan distance of 2 μm. We have tested the transmission properties of the waveguide bundles in dependence of the density of the parallel single waveguide scans. For this, we prepared waveguide bundles with a homogeneous hexagonal packing of scans shown in the inset of Fig. 2 containing 7, 19, or 49 scans. For comparison of all configurations, the bundle diameter was kept constant at 9 μm. The insertion loss at 1550 nm for the waveguide bundles was
Fig. 1. Top: images of the intensity distribution of guided NIR light (1550 nm) at the end facet of the waveguide bundles with different diameters (scan numbers) of: (a) 7 μm (7), (b) 10 μm (19), (c) 50 μm (931). Bottom: optical microscopy top view images of the corresponding waveguide bundles.
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Fig. 2. Insertion loss at 1550 nm of waveguide bundles regarding the number of waveguide scans. Inset: cross-sections of waveguide bundles of (a) 7 scans, (b) 19 scans, (c) 49 scans. The length of the waveguide bundles was 21 mm. The diameters and distances of the scans are true to scale.
11 dB for 7 scans, 4.8 dB for 19 scans, and 1.1 dB for 49 scans as can be seen in Fig 2. The NA of the bundles was measured by imaging the output of the guided light on a CCD, which was for bundles with 7 scans 0.024, for 19 scans 0.034, and for 49 scans 0.041. Considering for simplification for the waveguide bundles a step index profile, the change of refractive index in the waveguide bundles could be determined approximately by their NA to be Δn 2.0 × 10−4 for 7 scans, Δn 3.9 × 10−4 for 19 scans, and Δn 5.8 × 10−4 for 49 scans. By using the cutback method, the propagation loss for waveguides with 7 scans was 16 dB∕cm. In comparison, the BG efficiency in transmission in Ref. 2 was 2.2 dB∕cm, 8.4 dB∕cm in Ref. 8, and in Ref. 5, it was 13.2 dB∕cm. In addition our reflectivity of the BGW is in comparison to Refs. 2 and 5 (both ∼40%) significantly higher. Only Zhang et al. have reported a higher reflectivity of 91% for a BGW of 50-mm length in fused silica with their “burst writing” technique [8]. The thermal stability of a BGW-bundle in fused silica was investigated by heating a sample up to a defined temperature and keeping it constant for 1 h. After cooling down the glass sample, we have measured the BG signal again. This procedure was repeated for different temperature steps, and the results are shown in Fig. 6. The BGW bundle was stable up to temperatures of 250°C. Above 300°, the signal decreases rapidly. Beside fused silica, we have successfully written BGW in borosilicate glass (BOROFLOAT 33, Schott) as well as in 100-μm thin, flexible glasses of AF 32 eco (Schott).
Fig. 6. Thermal stability of a BGW bundle. For each temperature step, the sample was heated up and kept on a constant temperature for 1 h. After every temperature step the sample was cooled down, and the reflected Bragg grating signal was measured.
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
In conclusion, we have demonstrated that optical waveguide bundles, consisting of several single waveguide scans, are very well suited for integration of BG structures. The advantage of our technique is the integration of short and strong BG structures, caused by a maximal achievable refractive index contrast for the grating, as well as the ability of an easy adaption of the diameter of the waveguides, tailored for the mode field and wavelength of the guided light for a broad wavelength range from the visible to NIR. This flexibility is due to the fact that the choice of grating period for the BGW does nearly not affect the transmission losses, which is a limiting parameter in former works. For the production of BGW bundles, we demonstrated that single pulse writing with a low repetition kHz fs laser system is sufficient. Nevertheless, it should be possible to use our technique with high repetitive fs lasers. This would reduce the processing time, and additional thermal effects during the laser process may support the quality of the waveguide bundles. However, in comparison to lithographic techniques for fabrication of waveguides, no time-consuming development for masking and etching is necessary. Finally, we could demonstrate that our approach of fs laser direct writing is a further promising step to establish fs laser processing of integrated optics in different applications such as integrated optical sensors or optical components in telecommunication.
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References 1. H. Zhang and P. R. Hermann, in Femtosecond Laser Micromachining, R. Osellame, G. Cerullo, and R. Ramponi, eds. (Springer, 2012), p. 227. 2. H. B. Zhang, S. M. Eaton, J. Z. Li, and P. R. Herman, Opt. Lett. 31, 3495 (2006). 3. J. R. Grenier, L. A. Fernandes, J. S. Aitchison, P. V. S. Marques, and P. R. Herman, Opt. Lett. 37, 2289 (2012). 4. H. B. Zhang, S. Ho, S. M. Eaton, J. Li, and P. Herman, Opt. Express 16, 14015 (2008). 5. G. Brown, R. R. Thomson, A. K. Kar, N. D. Psaila, and H. T. Bookey, Opt. Lett. 37, 491 (2012). 6. G. D. Marshall, M. Ams, and M. J. Withford, Opt. Lett. 31, 2690 (2006). 7. J. Chung, Y. Gu, and J. G. Fujimoto, Submicron-Period Waveguide Bragg Gratings Direct Written by an 800-nm Femtosecond Oscillator (Optical Society of America, 2007), paper CThS4. 8. H. B. Zhang, S. M. Eaton, and P. R. Herman, Opt. Lett. 32, 2559 (2007). 9. K. K. Lee, A. Mariampillai, M. Haque, B. A. Standish, V. X. Yang, and P. R. Herman, Opt. Express 21, 24076 (2013). 10. N. D. Psaila, R. R. Thomson, H. T. Bookey, A. K. Kar, N. Chiodo, R. Osellame, G. Cerullo, G. Brown, A. Jha, and S. Shen, Opt. Express 14, 10452 (2006). 11. Y. Nasu, M. Kohtoku, and Y. Hibino, Opt. Lett. 30, 723 (2005).
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Femtosecond laser writing of Bragg grating waveguide bundles in bulk glass Markus Thiel,1,* Günter Flachenecker,1 and Wolfgang Schade1,2 1
2
Department of Fiber Optical Sensor Systems, Fraunhofer Heinrich-Hertz-Institute, Am Stollen 19B, 38640 Goslar, Germany Department of Applied Photonics, Institute of Energy Research and Physical Technologies, Am Stollen 19B, 38640 Goslar, Germany *Corresponding author: [email protected] Received December 30, 2014; revised February 11, 2015; accepted February 17, 2015; posted February 18, 2015 (Doc. ID 231399); published March 19, 2015 Waveguide bundles in bulk glass materials, consisting of several parallel scans of refractive index modifications, have been generated with a low-repetition femtosecond laser. Additionally, Bragg grating (BG) structures for 840 and 1550 nm have been introduced by segmentation of the central scan. A spectral loss in the transmission signal of >36 dB was achieved at 1550 nm with a second-order Bragg grating waveguide (BGW) in fused silica, which corresponds to an intrinsic grating efficiency of >16 dB∕cm. This is to our knowledge the strongest BG structure realized in glass with a femtosecond laser. The BGW were proven to be stable up to a temperature of 250°C in fused silica. The diameter of the waveguide bundles can be adapted very easily for a broad range of wavelengths and have been demonstrated for diameters between 1 and 50 μm. The transmission properties of the waveguide bundles are affected minorly by the insertion of BG structures, which opens the ability for adjusting the BGW for a broad range of wavelength in single-mode or multimode optical circuits. BGW have been realized successfully in fused silica, borosilicate glass (BOROFLOAT 33), and AF 32 eco Thin Glass from Schott. © 2015 Optical Society of America OCIS codes: (130.2755) Glass waveguides; (130.0130) Integrated optics; (130.3990) Micro-optical devices; (130.7408) Wavelength filtering devices; (230.1480) Bragg reflectors; (230.7370) Waveguides. http://dx.doi.org/10.1364/OL.40.001266
In the past, femtosecond (fs) laser-writing techniques have been successfully used for direct implementation of three-dimensional optical waveguide structures in transparent bulk glasses.[1] Of special interest are integrated photonic elements like Bragg grating waveguides (BGW). Fs laser direct-written BGW have been realized as laser mirrors, spectral filters for wavelength multiplexing, or sensing elements for strain and temperature [1–5]. For the realization of such BGW, a periodic modulation of the refractive index in the waveguide has to be generated with the fs laser. However this is not straight forward, due to the fact that a waveguide itself, generated with fs pulses, has already experienced a refractive index modification in comparison to the original bulk glass material. Different approaches have been reported for the production of BWG in glass in the last years. A weak BGW of ∼5 dB reflection has been demonstrated for the first time in bulk fused silica with a two-step laser process by Marshall et al. [6]. Firstly they generated a waveguide in fused silica by using a 1-kHz Ti:Sapphire fs laser and moving the glass substrate inside the focus of a microscope objective. For the subsequent step, they reduced the laser power and increased the scanning velocity for writing point by point a second-order Bragg grating (BG) with a weak response signal at 1550 nm, overlaying the previously written waveguide, originating in a smaller volume of the Bragg structure than the waveguide itself. A similar technique was applied by Chung et al. [7], who generated a first-order grating in soda-lime glass. Zhang et al. [2] were able to create BGW for 1540– 1570 nm in borosilicate glass with a 1-kHz fs laser in a single process step. As a result, their waveguides consisted of periodical separated segments. Their resulting BG reflection was 40% and the transmission signal less than 11 dB. However, a drawback of their technique is low reflection signals and the restriction to certain glass materials. To overcome this, a technique, which is called “burst writing”, was first introduced by the same group 0146-9592/15/071266-04$15.00/0
[8]. For this method, usually a laser repetition rate in the MHz range is used. In opposition to the former techniques, one “volume dot” is written by a burst of pulses, consisting of thousands of pulses, causing a higher refractive index change, and high reflecting first-order BG (89% reflectivity) were fabricated in fused silica. Even most promising and used for several applications [1,3,9], the burst writing technique of Zhang et al. [8] still lacks in flexibility on the desired waveguide cross-section, which depends on parameters such as pulse energy, repetition rate, and focal length of the microscope objective. An alternative technique to produce a defined cross-section of a waveguide is the multiscan technique, which was applied successfully by different groups [5,10,11]. Brown et al. [5] used a multiscan technique consisting of 20 parallel scans in borosilicate glass for production of BGW. With this technique, they achieved strong spectral filtering, but the reflectivity of their BG signal was only 40%. In this Letter, we present a new enhanced multiscan technique for Fs laser direct-written waveguides including BGW structures. In our approach, the waveguide is constructed around the BG structure via multiscan technique. The advantage is a maximally achievable refractive index contrast for the grating, as well as an increased structural flexibility by meaning the diameter of the waveguide and the grating period. This enables us to produce strong BGs in waveguides with tailored diameter for single-mode operation of a broad range of wavelengths from the visible to infrared. This is in contrast to the former works [2,5,7,8], where the BG and waveguide structures are formed simultaneously by the laser process, and therefore, their properties are linked. In consequence, their limitations are a more or less fixed diameter of the waveguide [2,7,8], restricted BG period [2,7,8] or certain glass materials [2,5]. For our experiments, we used a Ti:Sapphire laser providing 100-fs laser pulses at 800 nm with a repetition rate of 5 kHz. The glass samples were mounted on top of three © 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
precision linear stages (XMS series from Newport). With a λ∕4 zero order waveplate, the linear polarization of the laser was transformed into circular polarization. The laser was focused with a 20×, 0.45 NA microscope objective 150 μm below the surface of the glass sample, which was moved linearly horizontal with constant velocity of 1 mm∕s. By moving the glass sample under the laser focus, a single line (scan) with a positive refractive index contrast to the bulk material is generated. We have written waveguides from 5 to 500 μm below the surface. The limitation of writing depths clearly depends on the imaging quality of the microscope objective. Going close toward the surface unwanted, irregular destructions appear instead of structures with a smooth refractive index modification. The refractive index contrast itself depends sensitively on the power of the laser pulses, scanning speed, and depth, which has to be figured out for different glass materials. In fused silica for the waveguide scans, laser pulse energies of 224 nJ were used, which is 10% less than the damage threshold. According to the laser focus, the diameter of a single scan waveguide is about >1 μm in glass, which is well suited for guiding light in the visible spectrum. To enhance the transmission to the NIR spectrum and also to improve the transmission in the visible, the diameter of the waveguide has to be increased. To achieve this, we used a multiscan technique to form an effective waveguide with larger diameter consisting of a set of parallel single waveguide scans in the volume, which we call waveguide bundle. By this procedure, the waveguide diameter can be adapted in a flexible way for the mode field and the wavelength of the guided light. As an example, we have written single and multimode waveguide bundles for 800 and 1550 nm (Fig. 1). The diameters of these waveguides vary between 7 and 50 μm with a scan to scan distance of 2 μm. We have tested the transmission properties of the waveguide bundles in dependence of the density of the parallel single waveguide scans. For this, we prepared waveguide bundles with a homogeneous hexagonal packing of scans shown in the inset of Fig. 2 containing 7, 19, or 49 scans. For comparison of all configurations, the bundle diameter was kept constant at 9 μm. The insertion loss at 1550 nm for the waveguide bundles was
Fig. 1. Top: images of the intensity distribution of guided NIR light (1550 nm) at the end facet of the waveguide bundles with different diameters (scan numbers) of: (a) 7 μm (7), (b) 10 μm (19), (c) 50 μm (931). Bottom: optical microscopy top view images of the corresponding waveguide bundles.
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Fig. 2. Insertion loss at 1550 nm of waveguide bundles regarding the number of waveguide scans. Inset: cross-sections of waveguide bundles of (a) 7 scans, (b) 19 scans, (c) 49 scans. The length of the waveguide bundles was 21 mm. The diameters and distances of the scans are true to scale.
11 dB for 7 scans, 4.8 dB for 19 scans, and 1.1 dB for 49 scans as can be seen in Fig 2. The NA of the bundles was measured by imaging the output of the guided light on a CCD, which was for bundles with 7 scans 0.024, for 19 scans 0.034, and for 49 scans 0.041. Considering for simplification for the waveguide bundles a step index profile, the change of refractive index in the waveguide bundles could be determined approximately by their NA to be Δn 2.0 × 10−4 for 7 scans, Δn 3.9 × 10−4 for 19 scans, and Δn 5.8 × 10−4 for 49 scans. By using the cutback method, the propagation loss for waveguides with 7 scans was 16 dB∕cm. In comparison, the BG efficiency in transmission in Ref. 2 was 2.2 dB∕cm, 8.4 dB∕cm in Ref. 8, and in Ref. 5, it was 13.2 dB∕cm. In addition our reflectivity of the BGW is in comparison to Refs. 2 and 5 (both ∼40%) significantly higher. Only Zhang et al. have reported a higher reflectivity of 91% for a BGW of 50-mm length in fused silica with their “burst writing” technique [8]. The thermal stability of a BGW-bundle in fused silica was investigated by heating a sample up to a defined temperature and keeping it constant for 1 h. After cooling down the glass sample, we have measured the BG signal again. This procedure was repeated for different temperature steps, and the results are shown in Fig. 6. The BGW bundle was stable up to temperatures of 250°C. Above 300°, the signal decreases rapidly. Beside fused silica, we have successfully written BGW in borosilicate glass (BOROFLOAT 33, Schott) as well as in 100-μm thin, flexible glasses of AF 32 eco (Schott).
Fig. 6. Thermal stability of a BGW bundle. For each temperature step, the sample was heated up and kept on a constant temperature for 1 h. After every temperature step the sample was cooled down, and the reflected Bragg grating signal was measured.
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
In conclusion, we have demonstrated that optical waveguide bundles, consisting of several single waveguide scans, are very well suited for integration of BG structures. The advantage of our technique is the integration of short and strong BG structures, caused by a maximal achievable refractive index contrast for the grating, as well as the ability of an easy adaption of the diameter of the waveguides, tailored for the mode field and wavelength of the guided light for a broad wavelength range from the visible to NIR. This flexibility is due to the fact that the choice of grating period for the BGW does nearly not affect the transmission losses, which is a limiting parameter in former works. For the production of BGW bundles, we demonstrated that single pulse writing with a low repetition kHz fs laser system is sufficient. Nevertheless, it should be possible to use our technique with high repetitive fs lasers. This would reduce the processing time, and additional thermal effects during the laser process may support the quality of the waveguide bundles. However, in comparison to lithographic techniques for fabrication of waveguides, no time-consuming development for masking and etching is necessary. Finally, we could demonstrate that our approach of fs laser direct writing is a further promising step to establish fs laser processing of integrated optics in different applications such as integrated optical sensors or optical components in telecommunication.
1269
References 1. H. Zhang and P. R. Hermann, in Femtosecond Laser Micromachining, R. Osellame, G. Cerullo, and R. Ramponi, eds. (Springer, 2012), p. 227. 2. H. B. Zhang, S. M. Eaton, J. Z. Li, and P. R. Herman, Opt. Lett. 31, 3495 (2006). 3. J. R. Grenier, L. A. Fernandes, J. S. Aitchison, P. V. S. Marques, and P. R. Herman, Opt. Lett. 37, 2289 (2012). 4. H. B. Zhang, S. Ho, S. M. Eaton, J. Li, and P. Herman, Opt. Express 16, 14015 (2008). 5. G. Brown, R. R. Thomson, A. K. Kar, N. D. Psaila, and H. T. Bookey, Opt. Lett. 37, 491 (2012). 6. G. D. Marshall, M. Ams, and M. J. Withford, Opt. Lett. 31, 2690 (2006). 7. J. Chung, Y. Gu, and J. G. Fujimoto, Submicron-Period Waveguide Bragg Gratings Direct Written by an 800-nm Femtosecond Oscillator (Optical Society of America, 2007), paper CThS4. 8. H. B. Zhang, S. M. Eaton, and P. R. Herman, Opt. Lett. 32, 2559 (2007). 9. K. K. Lee, A. Mariampillai, M. Haque, B. A. Standish, V. X. Yang, and P. R. Herman, Opt. Express 21, 24076 (2013). 10. N. D. Psaila, R. R. Thomson, H. T. Bookey, A. K. Kar, N. Chiodo, R. Osellame, G. Cerullo, G. Brown, A. Jha, and S. Shen, Opt. Express 14, 10452 (2006). 11. Y. Nasu, M. Kohtoku, and Y. Hibino, Opt. Lett. 30, 723 (2005).
