Miniature Spectrographs: Characterization of Arrayed Waveguide Gratings for Astronomy

Transcription

1 Miniature Spectrographs: Characterization of Arrayed Waveguide Gratings for Astronomy Nick Cvetojevic *ab, Nemanja Jovanovic ab, Joss Bland-Hawthorn c, Roger Haynes d, Jon Lawrence ab a Department of Physics and Astronomy, Macquarie University, NSW, 2109, Australia b Anglo-Australian Observatory, NSW, 2122, Australia c Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW, 2006, Australia d Astrophysikalisches Institut Potsdam, Potsdam, Germany ABSTRACT We present results from a laboratory characterization of integrated photonic arrayed-waveguide grating chips, which are a modified version of commercial arrayed-waveguide grating multiplexors, for the purposes of creating an integrated photonic spectrograph. Using a robust probing setup we measure the peak total efficiency of the chips to be ~75%. We measure the spectral resolution full-width half maximum to be 0.22 ± 0.02 nm, (giving R = λ/δλ = 7000 ± 700 at 1500 nm). For our device we find the free-spectral range is ~60 nm and slightly larger than the full-width half-maximum of the efficiency profile (53 nm). Finally, we briefly discuss the importance of an integrated cross-dispersion component for the new integrated photonic spectrograph prototype. Keywords: IPS, Integrated Photonic Spectrograph, AWG, Arrayed Waveguide Grating, Miniature Spectrographs, 1. INTRODUCTION The next generation of major ground-based optical and near-infrared astronomical telescopes are planned to have aperture sizes from 20 to 40 meters in diameter, making them immensely larger than existing telescopes. This has a major impact on seeing limited spectroscopic instrumentation, as the size of the instrument grows in proportion to the telescope aperture for traditional designs and more importantly, the cost of the instrument increases with the telescope aperture squared, or faster [1]. Furthermore, the desire to obtain spectra from thousands of locations in the telescope focal plane simultaneously places another demand on the size and cost of next-generation astronomical spectrographs. This unsustainable trend has necessitated a miniaturization of devices for astronomy, with integrated photonics showing great promise. Particularly of interest is the integrated photonic spectrograph (IPS) [1,2]. One manufactured IPS format employs an arrayed-waveguide grating (AWG) structure, which inputs light from a standard optical fiber to a single photonic chip and outputs a spectrum that is dispersed over several millimeters, rather than the hundreds of centimeters with conventional spectrographs [3]. The IPS devices are small, light weight, have no moving parts, and are relatively inexpensive compared to conventional spectrographs used in astronomy. They potentially offer not only the miniaturization of spectrographs and required components, but also substantial economic and logistical advantages. In fact, the IPS can provide these benefits without sacrificing properties required for an astronomical spectrograph (i.e resolution of a few thousand and an operating wavelength range of a few hundred nanometers). The feasibility of the concept has already been demonstrated with a successful on-sky demonstration of a working, AWG-based, IPS prototype [4]. We build on that previous body of work with a new range of integrated AWG chips, which are a backbone for a new generation of the integrated photonic spectrograph. * nick.cvetojevic@mq.edu.au; phone ; fax

2 2. ARRAYED WAVEGUIDE GRATING The standard AWG comprises an input fiber port, which is typically an SMF-28 single-mode fiber for infrared devices, which feeds into a multiplexor. The multiplexor, or free propagation zone, allows the light to diverge into a parallel array of closely spaced single-mode waveguides, which in turn feed a demultiplexor (another free-propagation zone). The array waveguides behave analogously to a grating, i.e they create multiple point source which interfere, and as they have incrementally increasing lengths a specific central wavelength can be designed to be focused on the output surface. The demultiplexor allows the light to interfere, and focuses it onto the output ports that lie along its concave output facet [5]. AWGs have seen extensive use in the telecommunications industry as wavelength division multiplexers and demultiplexers, and have therefore been optimized for such applications [5,6]. Some of the requirements, for instance the minimization of cross-talk between neighboring channels, are less stringent for astronomical applications and hence some parameters of the device s design are over engineered in their current formats. The AWG chip used for the initial IPS prototype was modified by removing the output channels and polishing the output plane, hence creating a continuous spectral output rather than the discrete wavelength ports required for telecommunications. An example is shown in Fig. 1 below. As AWGs typically operate at high orders (20-30), the different grating orders are output superimposed on top of each other. Arrayed waveguides Free Fibre Parabolic Taper Direct-Fibre Launching Old IPS Prototype Launched light Free propagation zones Dispersed output Overlapping orders Figure 1. Schematic of the AWG chip. Inset left: The input method used on two different AWG chips. The original IPS prototype has a parabolic taper at the interface between a single-mode waveguide and the free propagation zone. The new AWG chips have had the input waveguide polished back such that it is possible to directly buttcouple an optical fiber. Inset right: overlapping of different spectral orders. This original IPS design still retains the input waveguide as used in telecommunications applications. A new set of AWG chips, which are this papers primary concern, have both the output and input waveguides removed. In effect this makes the chip consist only of the waveguide array and two free propagation zones. This is done to provide greater flexibility with regards to how the light is input into the device, which will be utilized for multi-fiber launch scenarios in the future. 3. CHIP CHARACTERIZATION As astronomical spectrographs have substantially different requirements and tolerances than that of wavelength multiplexers and de-multiplexers, a typical application of AWGs for telecommunications systems, a characterization of these new parameters is conducted before proceeding with any integration with a telescope system. The most important

3 characteristics of the modified AWG chips are its throughput efficiency, its spectral resolution, and the free spectral range. The efficiency of the device is obviously of paramount importance for any astronomical instrument. However, as AWGs are typically characterized by manufacturers with regards to their output channels, rather than for a continuous focal plane, our modified AWGs require a more comprehensive efficiency assessment. We measure how the efficiency behaves as a function of wavelength and its position on the focal surface, providing not only a throughput profile, but also an indication of the wavelength dependant losses of the device. In addition, we monitor the shape of the output mode profile to determine the smallest resolvable spectral feature, and hence determine the devices spectral resolution. 3.1 Experimental Setup The experimental setup used to characterize the direct fiber launch AWG chip is shown in Fig. 2. To probe the AWG chips we coupled a narrow-band tunable laser diode (Santec TSL-210), which is capable of scanning at increments of 0.01nm over an 80nm range between 1500nm and 1580nm, to a 90/10 fibre optical splitter. 10% of the light was sent to a precision translation stage to probe the AWG chip with the rest going to a power meter. This is because the noise floor and saturation limit of the NIR camera are far lower than the power meter s. By accounting for the losses in the probing setup the power meter provides a real time measurement of input power and is used to account for fluctuations in laser power over time and over wavelength. Tuneable NIR Laser Optical Splitter 10% 90% Power meter X-Y-Z Translation Stage NIR CCD Camera AWG Microscope Objective Figure 2. The experimental setup for probing both the throughput and the point-spread function of the integrated photonic spectrograph. To monitor the output of the AWG chip we use an indium-gallium-arsenide (InGaAs) CCD camera which operates in the near infra-red. This camera was flux calibrated to allow for an accurate determination of total integrated power of any light at the AWG output. Further, a 40x, long focal-length objective lens was used to magnify the image so the point spread function is adequately sampled. The benefit of this type of characterization setup is its ability to accurately measure input power, output power and point spread function simultaneously in addition to precisely coupling light from an optical fiber to the chip. The setup is shown in Fig. 2. This setup is a more rigorous and flexible probing method when compared to the technique used in previous IPS characterizations [4]. 3.2 Results The throughput efficiency was determined by scanning the laser wavelength and measuring the input and output powers. Figure 3 shows the efficiency of an AWG chip between the wavelengths of 1510 nm and 1580 nm at 5 nm intervals. The shape of the efficiency curve is similar to the typical efficiency responses for these devices found in the literature and is comparable to the results from the first prototype IPS [4].

4 Throughput Efficiency 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% FWHM = 53nm. 0% Wavelength (nm) Figure 3. The efficiency as a function of wavelength for the arrayed waveguide grating chips with a single fiber input, showing the full-width half maximum to be 53 nm and a peak efficiency of ~75% at the central operating wavelengths. The peak efficiency of the device is ~75% over a 25nm window of wavelengths between 1530nm and 1555nm, reducing to ~5% 35nm from the center. Our measured peak efficiency is slightly higher than that measured for the original prototype (60-70%), which demonstrates a direct-fiber launch as a viable and practical method of coupling light into an AWG chip. The higher efficiency is attributed to the more robust probing system which allowed for much more careful measurements to be made. The free-spectral range of the device is defined as the largest wavelength range for a given order that does not overlap with, the same range in an adjacent order. For our device we find it is ~60 nm and slightly bigger than the full-width half-maximum of the efficiency profile. While the laser s tuning range prohibited us from comprehensively probing the device in higher or lower orders, we did observe secondary peaks, caused by light coupling to a higher order when the laser wavelength was offset from the central operating wavelength by 30 nm (i.e nm). The separation of the two peaks being 60 nm. The output intensity profile was measured using the CCD camera which provides a direct measurement of the intensity distribution along the spectrally dispersed axis. The images are shown in Fig. 4. From this we are able to directly ascertain the point-spread function of the device and hence calculate the IPS resolution. The typical full-width halfmaximum of the original IPS prototype s point-spread function (Fig. 4a) is 0.63 ± 0.02 nm, making the resolution of the spectrograph R = 2460 ± 75. The direct-fiber launched AWG chips (Figure 4b) had a typical FWHM of 0.22 nm ± 0.02 nm, and hence a resolution of R = 7000 ± 700. The output intensity profile in Fig. 4a) is of the original IPS prototype used in our previous work [4]. In contrast, Fig. 4b) shows the intensity profile for the latest AWG chips probed with our new setup. It can be seen that the profiles are vastly different in shape. The new probing method reveals more structure in the intensity distribution than originally observed, and interestingly is absent in the direct-fiber launched AWG. This is likely due to the fact that since the original IPS prototype did not have the input face polished; it retained a parabolic taper on the input interface to the free propagation zone.

5 110 a) b) 1555nmPSF200us 170 b Figure 4. The output intensity profiles of a) the original IPS prototype, and b) the new direct-fiber input AWG chips at 1555 nm. At this orientation the camera is imaging the chip face on to the output such that the horizontal axis for both images is the spectrally dispersed axis. Line profiles are taken through the mode s centre showing the intensity distribution in both axes. The parabolic-type taper creates a double-peaked electric field distribution by the interference of the fundamental mode and second-order mode [7]. The parabolic taper is typically used in telecommunication systems where a flattened and broadened spectral response is required for AWG multiplexers due to the tight restrictions imposed by various parameters (temperature, transmission losses, wavelength tolerances). Using R-Soft [8], we were able to simulate the electric field distribution for an AWG system with and without the parabolic taper shown in Fig. 5. The shape of the theoretical point spread functions closely match experimentally the observed shape for both of the AWG chips Intensity (arb.) Output Position (um) No Taper Parabolic Taper Figure 5. Theoretical simulations of electric field distribution for a parabolic taper and for a butt-coupled optical waveguide. The width and length of the parabolic horn are similar to the measured dimensions from the original IPS prototype.

6 4. CROSS-DISPERSION AND ORDER DECOUPLING As the AWG chips output multiple orders in the same active area, and as typical astronomical sources are relatively broadband, this information will be layered on top of itself producing a forest of undistinguishable spectral lines and features. A simple series of narrow-band filters can be used to limit the incoming light, but in this case, all spectral information outside the 60 nm free-spectral range is lost. This narrow spectral range of the device is not ideal for most astronomical applications. Fortunately, the orders can be uncoupled from each other using a cross-disperser and hence the spectral information contained therein can be maintained. A cross-disperser is a device that will disperse light spectrally in a direction perpendicular to the dispersion of the IPS and effectively appear to pull the orders apart when viewed on an imaging device. Which orders are shown, and how many there are, depend on the cross-disperser design and the imaging instruments used. It is entirely feasible to cover the entire H-band using this technique [4]. Figure 6. Example of a sky spectrum taken during early twilight (i.e., very strong atmospheric emission) with the prototype IPS. The figure shows ~1000 x 300 pixels. The IPS spectrally disperses in the vertical direction, with six spectral orders of the AWG device being dispersed horizontally by using the existing infrared spectrometer IRIS2 at the Anglo-Australian Telescope as a cross disperser. The AWG chips characterized above will use the technique of cross-dispersion to expand their operating range from the single free-spectral range of 60nm to a few hundred nanometers, see for example Fig. 6. However, unlike our previous work, we hope to integrate a compact and cost-effective cross-disperser to the overall IPS design rather than retrofitting spectral dispersers on the telescope, which are more specialized and expensive. 5. CONCLUSION With the successful characterization of the direct-fiber launched AWG chips we have shown that they are suitable for the new IPS prototype. Furthermore, we have established a test bed system for comprehensive characterization of AWG chips for astronomy. The measured resolution for the new design chips was R~7000, which is much higher than the R~2500 resolution provided by the original IPS design. This improvement comes about by the removal of the parabolic taper at the end of the input waveguide and using the direct-fiber launching technique. The R~7000 resolution of the device is sufficient for a broad range of astronomical applications. The peak efficiency of the device is ~75% over a 25 nm window of wavelengths between 1530 nm and 1555 nm, about 10% higher than the original prototype. This is most likely due to an improved characterization procedure and equipment allowing us a far more accurate measurement of throughput power. We outline the concept of an integrated cross-dispersing device which will decouple the information located in the higher orders, allowing far broader wavelength coverage. The next phase of research is to demonstrate the feasibility of a new IPS device employing the new AWG chips on the Anglo-Australian Telescope. This device will explore stacking multiple AWG chips to provide multiple independent spectral outputs, off-axis launch of multiple input fibers to improve collection efficiency, and an integrated crossdisperser.

7 REFERENCES [1] J. Bland-Hawthorn, and A. Horton, Instruments without optics: an integrated photonic spectrograph, Proc. SPIE 6269, 21 (2006). [2] J. Bland-Hawthorn, and P. Kern, Astrophotonics: a new era for astronomical instruments, Opt. Express 17(3), (2009). [3] R. R. Thomson, A. K. Kar, and J. Allington-Smith, Ultrafast laser inscription: an enabling technology for astrophotonics, Opt. Express 17(3), (2009). [4] N. Cvetojevic, J.S. Lawrence, S.C. Ellis, J. Bland-Hawthorn, R.Haynes. A. Horton Characterisation and on-sky demonstration of an integrated photonic spectrograph for astronomy, Optics Express 17, , (2009). [5] M. K. Smit, New focusing and dispersive planar component based on an optical phased-array, Electron. Lett. 24(7), (1988). [6] H. Takahashi, S. Suzuki, K. Kato, and I. Nishi, Arrayed-waveguide grating for wavelength division multi/demultiplexer with nanometer resolution, Electron. Lett. 26(2), (1990). [7] Okimoto, K., [Fundamentals of Optical Waveguides], Elsevier, Boston, (2006). [8] Rsoft, AWG Utility- BeamPROP,