Combination of a spectrometer-on-chip and an array ...

Combination of a spectrometer-on-chip and an array of Young’s interferometers for laser spectrum monitoring A. Koshelev,1,2,* G. Calafiore, 3 C. Peroz,3 S. Dhuey, 4 S. Cabrini, 4 P. Sasorov, 1 A. Goltsov, 1,2 V. Yankov 1 2Moscow

1Nano-Optic Devices, 2953 Bunker Hill Lane, Santa Clara, CA 95054 USA. Institute of Physics and Technology (State University), Institutskii per. 9, Dolgoprudnyi, Moscow region, Russia 3aBeam Technologies, 5286 Dunnigan Ct., Castro Valley, CA 94546, USA 4The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA-94702, USA *Corresponding author: [email protected]

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX This paper presents the design and experimental results for an on-chip photonic device for laser spectrum monitoring that combines a nanospectrometer and an array of Young’s interferometers. The array of Young’s interferometers and the spectrometer measure the width and the wavelength of a spectrum in visible light, respectively. The accuracy of spectral width measurements is around 10% for FWHM higher than 2.5 pm. The spectrometer-on-chip is based on a digital planar hologram and provides a resolution around 145 pm within the spectral range of 719-861 nm (142 nm bandwidth). The performance of the device is demonstrated for distinguishing between the single and two longitudinal modes operation of a fiber Bragg grating laser diode with 23 pm mode separation. OCIS Codes: (130.3120) Integrated optics devices, (120.3180) Interferometry, (300.6190) Spectrometers. http://dx.doi.org/10.1364/OL.99.099999

One of the common applications for a spectrometer is the monitoring of a laser spectrum. It is important for numerous applications like interferometry and holography, optical component diagnostics, metrology, tunable laser calibration. The high brightness of the laser sources allows using spectrometers with a single mode input, which makes integrated optics ideal for these applications. Most on-chip spectrometers could be divided into two groups. The first group uses wavelength demultiplexers, such as arrayed-waveguide gratings [1,2], ring resonators [3, 4], microdonuts [5], echelle gratings [6], digital planar holograms (DPH) [7]. The second group is based on interferometry techniques and uses Fourier transform to obtain spectrum [8, Error! Reference source not found.]. These devices are able to obtain high resolution, however the spectral range is usually limited to several nanometers. To overcome the tradeoff between resolution and spectral range for the specific application of laser spectrum monitoring, we propose here to combine these two techniques into a single device. The main parameters describing a laser line are its center wavelength and spectral width. In the present approach, the linewidth is determined by measuring the visibility of the interference patterns created by the array of Young’s interferometers, while the value of the center wavelength is detected with a DPH spectrometer. The sketch of the device is shown in Error! Reference source not found.. Light coupled into a ridge waveguide (RWG) is separated into two parts with a Y-splitter. Then it is simultaneously guided to the array of interferometers and to the DPH. In each Young’s interferometer, the incoming light is split into two parts. Light that travels

through one of the interferometer arms is subject to a delay. Outputs of both arms are located near each other at some distance from the edge creating interference pattern on the output edge of the chip. The visibility of the interference pattern depends on the length of the delay line and the coherence length of the source. Unlike an array of Mach-Zehnder interferometers [8], where interference results in signal variation between different interferometers, even a single Young’s interferometer is sufficient to draw conclusions about the spectrum. Even if the visibility is too close to 0 or 1 to be accurately measured, it will still give lower or upper estimate of the spectral width. Due to its ease of fabrication and extreme sensitivity, Young’s interferometer has previously found an application in optical sensors [10-12]. Three interferometers are used to cover a wider range of spectral widths. Light that goes to the array of interferometers has to be polarized otherwise the interference patterns from TE and TM polarizations would overlap and make the data interpretation more difficult. Light is polarized by using an integrated polarizer based on shallow RWG [13].

channel the light with two different wavelengths and orthogonal polarization (TE and TM) is reflected. The difference between wavelengths is around 45 nm.

Fig. 1. Simplified sketch of the device. The actual device has three interferometers with different lengths of the delay line.

While the linewidth of the source is measured with interferometers, the wavelength is measured with the Digital Planar Hologram. DPH consists of millions of grooves fabricated in the core of a planar waveguide chip. In a DPH spectrometer, the light interacts with the DPH structure and is reflected back into several focal points (output channels) depending on its wavelength. Light that goes to the DPH is not polarized, which allows observing reflections of both polarizations. Due to difference in the effective refractive indices, the TE- and TM-polarized light will be reflected into different output channels. In a general purpose spectrometer this circumstance would complicate the interpretation of spectral measurements. However, for narrow spectral lines this property can be advantageously used to increase spectral range. The spectral range of the DPH for both polarizations combined is 719-861 nm (142 nm). The dispersion curve is linear in the wavenumber space with the average channel spacing around 145 pm. The detailed description of the DPH principles and design procedures are presented in our previous publications [14, 15]. The planar waveguide substrates consist of Si/SiO2(8 µm)/Si3N4(158 nm) purchased from Lionix BV. The photonic chips are fabricated by direct electron beam lithography (EBL) and plasma etching. More details are given in Ref [16]. The etching depth of our device is around 10 nm. A SiO2 upper cladding layer is finally deposited to protect the waveguide layer and to minimize the losses due to scattering [17]. Top-view of the device with a red laser input light (λ=660 nm) that is coupled into the chip is shown in Fig. 2. The TM-polarized light released from the polarizing RWG that leads to the interferometers is clearly visible. No reflection from the DPH is observed because the device is designed for Near Infrared (719-861 nm), while a red light laser was used in Fig. 2. Because of this mismatch, a small portion of TM polarized light falls out of the RWG that leads to the DPH (bottom-right corner of the picture). Example of spectral responses of the DPH spectrometer in its working bandwidth is depicted in Fig. 3. In this measurement, a super luminescent diode is coupled into the chip and then light from a single DPH channel is directed to a spectrum analyzer with 10 pm resolution. The average FWHM of the response curve is found to be around 145 pm and 130 pm for TE and TM polarization, respectively. As one can see from Fig. 3 in any given

Fig. 2. Top view of the 660 nm laser light propagating inside the chip. Some dust particles are present on top of the waveguide appearing as the bright spots. Their presence does not alter the general performance.

Fig. 3. Normalized spectral response of the 15 first channels (#1 to #15 from right to left) for the DPH spectrometer. a) TM

polarization (FWHM=130 pm, spectral spacing is 155 pm). b) TE polarization (FWHM=145 pm, spectral spacing is 165 pm).

where n – effective refractive index, L – length of the delay line, λ–wavelength.

To demonstrate the performance of the array of interferometers, the spectrum of a Fiber Bragg Grating (FBG) laser diode was characterized. The laser spectrum is quite narrow (~1MHz). However, due to changes in the ambient temperature, the two-longitudinal mode regime can occur from time to time (Fig. 7a). The distance between the modes (23 pm) is much smaller than the DPH resolution (145 pm), which makes it impossible to distinguish by DPH the difference between single- and two- mode regimes. However, this spectral difference can be clearly observed by the interferometers. Fig. 4 shows that for a single-mode regime all of the three visibilities are close to 1. In case of two-mode regime all of the visibilities decrease. The visibility of the interference pattern at the output of the medium interferometer is 0.39. The long interferometer provides a visibility value that is higher than that of the medium one. The reason for this is that the length of the delay line of the long interferometer is a multiple of the coherence length corresponding to 23 pm mode separation. To calculate the pattern visibility, the intensity profile was fitted with the curve:   x  xc 2   1  2 ( x  x0 )   y  x   A exp    cos     2* w2   V T    

(1)

A, xc , x0 , w, V , T – parameters, and the where parameter V represents the visibility of the interference pattern. Inevitable errors in dark current measurements may result in the measured visibility values to be slightly higher than 1. The measured visibilities agree with the theory (see Error! Reference source not found.). Table 1. Comparison between theoretical and measured visibilities, assuming narrow spectrum of the individual longitudinal mode. Interfero Single mode Two modes meter Theory Experiment Theory Experiment Long 1 0.99 0.94 0.92 Medium 1 1.02 0.38 0.39 Short 1 0.98 0.92 0.87

Fig. 4. a) Spectra taken with spectrum analyzer (resolution ~10 pm) of the single-and two-mode FBG laser diode regime. b), c), d) interference patterns and their profiles observed at the outputs of the long, medium and short interferometer, correspondingly. The lengths of interferometers is represented by a   2 / ( nL) ,

Error! Reference source not found. shows that the accuracy of visibility measurement is better than 0.05. This results in the average spectral measurement accuracy being around 10% for spectral width varying from 2.5 pm to 160 pm for Gaussian line. The same accuracy is obtained for the spectrum consisting of the two lines that are spaced 2.1 pm through 185 pm (spectral width) considering a width of individual lines is small compared to line spacing. Spectral widths larger than that are impossible to measure with these interferometers because the visibility of the interference pattern would be too low in all of the interferometers. However, those widths can be measured with the DPH since its resolution is ~145 pm. Conclusion

We designed, fabricated and tested an integrated photonic device for laser spectrum monitoring based on the combination of DPH spectrometer and an array of Young’s interferometers. This combination allows measuring the spectral width of laser lines as narrow as 2.5 pm over a spectral bandwidth around 142 nm (719861 nm). The resolution of the spectrometer is around 145 pm. Using this approach we were able to distinguish between single- and two-mode (23 pm spaced) operation of a FBG laser. The device is suitable for the use with a pulsed laser, since no scanning is necessary to obtain the spectrum. Acknowledgements. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the United States Department of Energy under contract DEAC02- 05CH11231. This study is partially supported by the Air Force Office of Scientific Research (AFOSR), Air Force Material Command, USAF, under grant/contract FA9550-12-C-0077.

References 1. P. Cheben, J. H. Schmid, A. Delâge, A. Densmore, S. Janz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, and D.X. Xu, Opt. Express, vol. 15, pp. 2299-2306, 2007. 2. Zirngibl, M., Dragone, C. & Joyner, C. H., Technol. Lett. 4, 1250–1253 (1992). 3. F. Xia, M. Rooks, L. Sekaric, and Y. Vlasov, Opt. Express, vol. 15, pp. 11934–11941, 2007. 4. Little, B. E. et al., IEEE Photon.Technol. Lett. 10, 549–551 (1998). 5. Xia, Z. et al., Opt. Express 19, 12356–12364 (2011). 6. S. Janz, A. Balakrishnan, S. Charbonneau, P. Cheben, M. Cloutier, A. Delâge, K. Dossou, L. Erickson, M. Gao, P.A. Krug, B. Lamontagne, M. Packirisamy, M. Pearson, and D.-X. Xu, IEEE Photon. Technol. Lett., vol. 16, pp. 503– 505, 2004. 7. C. Peroz, C. Calo, A. Goltsov, S. Dhuey, A. Koshelev, P. Sasorov, I. Ivonin, S. Babin, S. Cabrini, and V. Yankov, Opt. Lett. 37, 695-697 (2012). 8. Velasco, A.V., Cheben, P., Calvo, M.L., Florjanczyk, M., Bock, P.J., Delage, A., Schmid, J.H., Lapointe, J., Janz, S., Dan-Xia Xu, Vachon, M., Transparent Optical Networks (ICTON), 2013 15th International Conference on , vol., no., pp.1,4, 23-27 June 2013 9. le Coarer, E., Blaize, S., Benech, P., Stefanon, I., Morand, A., Le Rondel, G., Leblond, G., Kern, P., Fedeli, J.-M., Royer, P., Nature Photonics (2007), 1, 8, 473 – 478 10. WANG, M. et al., Journal of the European Optical Society - Rapid publications, Europe, v. 7, jun. 2012. 11. A. Brandenburg and R. Henninger, Appl. Opt. 33, 59415947 (1994). 12. A. Ymeti, J. Kanger, J. Greve, P. Lambeck, R. Wijn, and R. Heideman, Appl. Opt. 42, 5649-5660 (2003). 13. D. Dai, Z. Wang, N. Julian, and J. Bowers, Opt. Express 18, 27404-27415 (2010). 14. Babin, S. et al., Applied Physics Lett. 95, 041105-041105041103 (2009).

15. Yankov, V. et al., IEEE Photon. Technol. Lett.,15, 410-412 (2003). 16. C. Calò, V. Lacatena, A. Koshelev, S. Dhuey, I. Ivonin, S. Babin, A. Goltsov, S. Cabrini, V. Yankov, C. Peroz, J. Vac. Sci. Technol B. 30, 2166 (2012) 17. C. Peroz, A. Goltsov, S. Dhuey, P. Sasorov, I. Ivonin, B. Harteneck, S. Kopyatev, S. Cabrini, S. Babin, V. Yankov, IEEE Photonics Journal 3, 888 (2011).

References with titles 1. P. Cheben, J. H. Schmid, A. Delâge, A. Densmore, S. Janz, B. Lamontagne, J. Lapointe, E. Post, P. Waldron, and D.X. Xu, “A high-resolution silicon-on-insulator arrayed waveguide grating microspectrometer with submicrometer aperture waveguides”, Opt. Express, vol. 15, pp. 22992306, 2007. 2. Zirngibl, M., Dragone, C. & Joyner, C. H. “Demonstration of a 15×15 arrayed waveguide multiplexer on InP”, IEEE Photon. Technol. Lett. 4, 1250–1253 (1992). 3. F. Xia, M. Rooks, L. Sekaric, and Y. Vlasov, “Ultra-compact high order ring resonator filters using submicron silicon photonic wires for on-chip optical interconnects”, Opt. Express, vol. 15, pp. 11934–11941, 2007. 4. Little, B. E. et al. “Ultra-compact Si–SiO2 microring resonator”, IEEE Photon.Technol. Lett. 10, 549–551 (1998). 5. Xia, Z. et al. “High resolution on-chip spectroscopy based on miniaturized microdonut resonators”, Opt. Express 19, 12356–12364 (2011). 6. S. Janz, A. Balakrishnan, S. Charbonneau, P. Cheben, M. Cloutier, A. Delâge, K. Dossou, L. Erickson, M. Gao, P.A. Krug, B. Lamontagne, M. Packirisamy, M. Pearson, and D.-X. Xu, “Planar waveguide echelle gratings in silica-onsilicon”, IEEE Photon. Technol. Lett., vol. 16, pp. 503–505, 2004. 7. C. Peroz, C. Calo, A. Goltsov, S. Dhuey, A. Koshelev, P. Sasorov, I. Ivonin, S. Babin, S. Cabrini, and V. Yankov, "Multiband wavelength demultiplexer based on digital planar holography for on-chip spectroscopy applications", Opt. Lett. 37, 695-697 (2012). 8. Velasco, A.V.; Cheben, P.; Calvo, M.L.; Florjanczyk, M.; Bock, P.J.; Delage, A.; Schmid, J.H.; Lapointe, J.; Janz, S.; Dan-Xia Xu; Vachon, M., "High resolution fouriertransform microspectroscopy based on spiral silicon waveguides", Transparent Optical Networks (ICTON), 2013 15th International Conference on , vol., no., pp.1,4, 23-27 June 2013 9. le Coarer, E., Blaize, S., Benech, P., Stefanon, I., Morand, A., Le Rondel, G., Leblond, G., Kern, P., Fedeli, J.-M., Royer, P., “Wavelength-scale stationary-wave integrated Fourier transform Spectrometry”, Nature Photonics (2007), 1, 8, 473 – 478 10. WANG, M. et al. “An integrated Young interferometer based on UV-imprinted polymer waveguides for label-free biosensing applications”, Journal of the European Optical Society - Rapid publications, Europe, v. 7, jun. 2012. ISSN 1990-2573. 11. A. Brandenburg and R. Henninger, "Integrated optical Young interferometer", Appl. Opt. 33, 5941-5947 (1994). 12. A. Ymeti, J. Kanger, J. Greve, P. Lambeck, R. Wijn, and R. Heideman, "Realization of a multichannel integrated Young interferometer chemical sensor", Appl. Opt. 42, 5649-5660 (2003). 13. D. Dai, Z. Wang, N. Julian, and J. Bowers, "Compact broadband polarizer based on shallowly-etched silicon-oninsulator ridge optical waveguides", Opt. Express 18, 27404-27415 (2010). 14. Babin, S. et al. “Digital optical spectrometer-on-chip”, Applied Physics Lett. 95, 041105-041105-041103 (2009). 15. Yankov, V. et al. “Multiwavelength Bragg gratings and their application to optical MUX/DEMUX devices”, IEEE Photon. Technol. Lett.,15, 410-412 (2003). 16. C. Calò, V. Lacatena, A. Koshelev, S. Dhuey, I. Ivonin, S. Babin, A. Goltsov, S. Cabrini, V. Yankov, C. Peroz, “Fabrication of digital planar holograms into high

refractive index waveguide core for spectroscopy-on-chip applications”, J. Vac. Sci. Technol B. 30, 2166 (2012) 17. C. Peroz, A. Goltsov, S. Dhuey, P. Sasorov, I. Ivonin, B. Harteneck, S. Kopyatev, S. Cabrini, S. Babin, V. Yankov, “High-Resolution Spectrometer-on-Chip Based on Digital Planar Holography”, IEEE Photonics Journal 3, 888 (2011).