Optoelectronic Packaging of Arrayed-Waveguide Grating

1/1 6.06.001 16:00 Optoelectronic Packaging of Arrayed-Waveguide Grating Modules and their Environmental Stability Tests H. Ehlers, M. Biletzke, B. Kuhlow, G. Przyrembel and U. H. P. Fischer (Member IEEE) Heinrich-Hertz-Institut für Nachrichtentechnik GmbH Einsteinufer 37, D-10587 Berlin Email: fischer@hhi.de ABSTRACT We designed and fabricated arrayed-waveguide grating (AWG) modules with thermoelectric coolers/heaters. At these modules we measured the optical fiber-chip coupling loss and the optical reflections. Further we investigated the temperature stability of the center wavelengths. The fabricated AWGs had 8 and 16 channels, respectively with a spacing of 0.8 nm (100 GHz) at 1540 nm center wavelength. The measurements show that the center wavelength could be kept constant within ±0.015 nm at ambient temperatures between 0 to 40 C. The center wavelength could be tuned over 0.3 nm by temperature adjustment. We performed environmental tests that revealed a good mechanical stability of the AWG modules. Index terms optoelectronic packaging, arrayed-waveguide grating, environmental stability, wavelength division multiplexing (WDM). Page 1 of 1

/ 6.06.001 16:00 1. INTRODUCTION Future optical communication systems will use more of the exceptional high bandwidth of optical fiber [1,,3]. Wavelength division multiplexing (WDM) systems are well suited to transport Terabits of information via the fiber. Multiplexers/demultiplexers (MUX/DEMUX) are essential components for dense WDM systems. Several different kinds of multiplexer types have been developed in the past: a) interference filters [4], b) fiber gratings [5,6] and c) planar lightwave circuit (PLC) MUX/DEMUXers. The planar fabrication process of the last mentioned PLCs allows the realization of high performance filters with a large number of wavelength channels. Further, an integration with other optical elements, e.g. Erbium doped amplifiers on one and the same chip seems to be possible. Arrayed-waveguide gratings (AWG) are a special kind of PLC- MUX/DEMUXers, which are very attractive components for WDM systems, because of their great flexibility in filter design [7,8]. Basically, an AWG is an optical spectrograph built in planar waveguide technique. Typically, AWGs works in a high grating order (50-50). For filter applications in communications systems a precise wavelength control and long term wavelength stability is demanded. Standard AWGs, however, show a change of center wavelength with temperature. A method to prevent this temperature drift is to package the AWG in a housing together with a temperature controller in order to tune and fix the desired filter-wavelength. In this paper, we describe the opto-mechanical set-up and the results of the wavelength controllability and environmental stability of AWG modules with 8x8 and 16x16 optical in/output ports. Additionally, we give the measured optical losses and the reflections at the interface between the chip end faces and the coupled fiber arrays. Page of

3/3 6.06.001 16:00. DEVICE CHARACTERISTICS The AWGs are designed with specially developed software tools. Fabrication on Silica wafers includes optical lithography with e-beam written masks and reactive ion etching (RIE). The processed wafers are finally cut and packaged, and the fiber arrays are aligned and attached to AWGs for system applications. A schematic drawing of the used AWG filters is shown in fig. 1. The AWG consists of a number of input/output waveguides, two focussing slab waveguides, and an arrayed waveguide grating in between with up to about 10 optical paths of graduate lengths. The constant path length difference L between neighboring arrayed-waveguides is chosen to be an integer multiple m of the center wavelength λ 0 of the AWG. m is the grating order and λ 0 =λ vacuum /n is the wavelength inside the grating material: L = m x λ 0. (1) Therefore, at the grating exit the center wavelength λ 0 has zero phase change and is focussed to the center output, whereas diverging wavelengths acquire an individual tilt of the optical phase resulting in a focussing into the different other outputs. The wavelength resolution of the filter is proportional to m. The temperature dependence of the center wavelength according to eq. (1) can be expressed by [9], dλ 0 dt = 1 d(n c L) m dt () where n c is the refractive index in the grating waveguides. The temperature dependence for our AWGs in fabricated Silica technique is calculated to be 0.011 nm/ C (1.38 GHz)/ C at 1.55 µm using for Silica: n c =1,447, dn c /dt= 1.0x10-5 / C and for the thermal expansion coefficient 1 dl T = = 10 6 α 0.7 / C (3) L dt Page 3 of 3

4/4 6.06.001 16:00 which is of minor influence here. It shows that the temperature change of the device must be controlled to ±1 C to fix the center wavelength to ±0.0l nm. On the other hand the center wavelength can be tuned up to 5 nm (65 GHz) at 1.55 µm wavelength by changing the temperature of the AWG by 50 C. The parameters of the used AWGs are shown in table 1. AWG 8x8 DWDM AWG 16x16 DWDM Channel spacing nm/ 50 GHz 0.8 nm /100 GHz Insertion loss 3.8 db 6.5 db 3dB passband width 8 GHz 34 GHz Crosstalk <-7 db <-5 db Table 1: AWG parameters [10] 3. OPTICAL COUPLING SET-UP AWGs were realized with a PIRI [11] Si/SiO -wafer as a basic element, fabricated by flame hydrolysis deposition. The waveguides were structured by reactive ion etching (RIE). The dimensions of our AWGs are 50x30 mm and 78x3 mm for the 8x8 and the 16x16 AWG, respectively. The core dimensions of the monomode waveguides are 6x6 µm. The difference of refractive index between core (n co ) and cladding (n cl ) was 0.7 %, with a core index of n=1.454. The calculation of the numerical aperture (NA AWG ) of the light spot at the end of the AWG waveguide is the following [1]: NA α AWG AWG = n co n cl = sin α = 0.17 = 9.8 (angle of aperture) (4) The angle of aperture of the standard monomode fiber (α SMF ) could be calculated with the international standardized parameters of the ITU [13] as 6.5. For efficient transfer of optical Page 4 of 4

5/5 6.06.001 16:00 energy from the SMF and the AWG waveguide, the mode profiles should overlap as much as possible which is described by Saruwatari et al. [14,15]. The coupling efficiency η between two Gaussian beams can be expressed by: x 0 η = κ exp -κ (1/ w where κ = 4w w AWG AWG w ( z) = w SMF AWG AWG + 1/ w SMF /[( wawg+ wsmf) + λ z / π ngap], [ 1+ ( λz/ πw ) ] AWG ) + π θ [ w ( z) + w ] AWG SMF /λ x θz / w 0 AWG (5) with? n x? z gap 0 = wavelengt h = refractive = lateral misalignme = angular misalignme = longitudin index of medium nt nt al misalignme nt between the waveguide and fiber The db loss can be expressed in terms of : L ( η ) = 10log( η) [ db] (6) To calculate the mode field mismatch between the single mode components and the corresponding mismatch loss, all lateral and angular misalignments of the fiber axis relative to the incident beam of the AWG waveguide are set to zero. Than the coupling efficiency of eq. (5) simplifies to: η 0 = 4w AWG w SMF /( w where z = 0,? = 0, x = 0 AWG + w SMF ) (7) The mode field diameter is a function of wavelength (here 1.540 µm) and aperture angle of: λ w = π n tanα (8) Page 5 of 5

6/6 6.06.001 16:00 With eq. (8) the mode field radii can be calculated: w AWG = 3.6 µm and w SMF, = 4.9 µm. Than the calculation of the mismatch loss L(η 0 ) between the two waveguides using eq. (7) gives a value of 0.41 db. Comparative calculations of the mode field mismatch loss by beam propagation method (BPM) program [16] give the same result. Measured average values for the coupling losses of 0.94 db for the 8x8 AWG and 0.91 db for the 16x16 AWG are depicted in fig (a+b). The additional loss of 0.53 db and 0.5 db, respectively are caused by imperfections in the alignment and fixing process of the fiber ribbon at the chip interface. It is not possible to adjust all fibers of the array optimal to the SiO waveguides. A misfit in angle of 1 results in 0.1 db additional loss and an offset of 1µm - 1.5 µm in the lateral position gives an additional loss of 0.4 db 0.53 db and loss of 0.04 db for longitudinal gap of 10µm calculated by eq. (6) and by BPM. The fiber array is fixed by UV-hardening glue with an optimal adaptation of the reflective index of n gap =1,48 (Panacol, Vitralit 7104) to the fiber and the SiO waveguide. Concluding, the loss by the fixing process is due to a misfit in angle of 1 combines with a misfit in lateral position of 1µm - 1.5 µm and a gap for the adhesive of 10µm, which are realistic alignment parameters for this coupling technique. We used no anti reflection coating of the end surfaces of the waveguides, neither of the SMF nor of the SiO waveguide, which are straight cut. Therefore, we expected values for the back reflection at these fiber-chip-interfaces of 30 db. The obtained results for reflections are shown in fig 3a for the 8x8 AWG and in fig. 3b for the 16x16 AWG, respectively. For the 16x16 device we measured a mean value of 3.5 db and for the 8x8 AWG we reached an even lower average of 38 db. The probable reason for this value lower than expected is a tilted fixing of the fiber array to the SiO waveguide. An additional tilting would further reduce the reflection values but would increase the coupling loss which indeed is slightly higher than expected for pure modemismatch loss. Page 6 of 6

7/7 6.06.001 16:00 4. MODULE PACKAGING Fig. 4 and 5 show photographs and the structure of AWG modules. The modules consists of an AWG chip, a pair of optical fiber arrays, and a chip carrier in a case with an electrical plug for the cooling of the device. The AWG was packaged as follows: first, the chip with the fiber arrays was fixed to a heat spreader by using a stress-free mount technique which employs an efficient adjustment of the extension coefficients of the AWG-Chip substrate (α Si =7*10-6 / C) to that of the heat sink (Covar αco=4.8*10-6 / C). Assuming a chip length of Lchip= 78 mm and a temperature difference of T= C between heat sink and chip we have a difference in length of only l Kovar - l Si = 0.3 µm which is a near perfect adaptation. Additionally the mounting of chip and heat sink employs a low Young's modulus adhesive. A thermal sensor (AD590) for monitoring the chip temperature was embedded in the chip carrier. Finally, the heat sink was mounted on the Peltier element which was attached to the case. The case made of aluminum was designed to work as a heat sink for the device. After finishing the electromechanical packaging, the chip, and the fiber arrays were adjusted and bonded together with an UV-curable adhesive (Panacol, Vitralit 7104). For chip-fiber coupling the chip module and submount were fixed (fig. 6) onto a three axes stage (x, y, z). The fiber array was also fixed on a stage with three linear and two angular adjustments. Light of a tunable laser source (HP 8168A) was fed via a 1xN (N=8,16) optical splitter to all fibers of the array. The prealignment was made by manual adjusting the fiber array in a line with the waveguides of the AWG using a microscope. The output at the opposite side of the AWG was monitored via microscope lens, a split-field optic, and an infrared camera system (Hamamatsu). First, the optical outputs near the center of the AWG were aligned. Later, the exterior outputs of the array Page 7 of 7

8/8 6.06.001 16:00 were aligned more precisely. Therefore, the laser had to be tuned to the center wavelength of the filter curve of the observed fiber port. For optimal quantitative survey of the alignment process, two laterally adjustable IR-detectors (Anritsu ML910B & MA9611) were used. Finally, the adhesive was inserted into the gap between the array and the AWG-chip and hardened by UV light. To stabilize the connection a glass beam was positioned onto the chip directly in face with the array and fixed there as shown in fig. 7 (a, b). After hardening, the opposite chip interface was aligned and connected in the same way as described before, now using the already fixed fibers for launching the input wavelengths. 5. ENVIRONMENTAL STABILITY For application in optical nets, AWG modules must be stable with respect to temperature changes and mechanical stresses. At present, there are no definite environmental and mechanical criteria for dense WDM devices such as AWG modules. Therefore, we describe the stability results of the AWG modules, which we investigated with reference to the IEC-68- requirements [17]. We carried out several kinds of environmental tests, including a vibration test as described in the following. In the tests, insertion losses, center wavelength and return losses were measured online for each sample at a constant chip temperature of 3 C. To confirm the temperature controllability of the AWG module, we measured the changes in center wavelength both, driving with and without the TEC-cooler device at ambient temperatures. The set-up for the temperature testing is depicted in fig. 8. Further, the device under test (DUT) is placed into a humidity controlled environmental test chamber (Weiss SB/160/40). To stabilize the chip temperature a controller-unit (Profile LDC 1000) was used. For testing the wavelength shift, a tunable laser source HP 8504C in combination with a polarization controller was used. Signal detection was done with an Anritsu power meter (ML 9001A). In fig. 9, the temperature Page 8 of 8

9/9 6.06.001 16:00 dependence of the center wavelength as a function of ambient temperature is shown. The chip temperature was held constant (T=3 C) and the ambient temperature was varied between 10 C and 40 C, which is depicted in fig. 9a. No significant deviation between the three measured curves can be observed which reveals a stability of the temperature control circuit of better than 0.1 C. Without temperature control (fig. 9b)the center wavelengths vary with the expected drift of 0.01 nm (1.5 GHz)/ C as shown in fig. 10. Here the results of the center wavelength drift are summarized. To confirm the temperature stability of the set-up we stressed the module by several temperature cycles between 15 C and 40 C (fig. 11) which reflects the assumption that the AWGs will be used mostly in in-house applications. The variation of the optical coupling loss is less than ±0.1 db which is an acceptable good value for the use of this AWG set-up in WDM system applications. After thermal cycling, we completed the test with mechanical shock and vibrational stress. Furthermore, vibrational tests were performed with an acceleration of higher than 16 g within a broad spectral bandwidth of 50-5000 Hz, measured with an acceleration sensor (Entran, range 0-1000 g) and a digital oscilloscope. After these tests, no significant degradation (<0. db) of the coupling efficiency was detected. 6. SUMMARY We designed and fabricated arrayed-waveguide grating (AWG) modules with thermoelectric coolers, and additionally we measured the optical coupling loss, the optical reflection values, and the temperature stability of the center wavelengths. The fabricated AWGs had 8 and 16 channels with a spacing of nm and 0.8 nm at 1540 nm, respectively. We performed environmental tests that showed also good mechanical stability of the AWG modules. In summary the packaged AWGs are well suited for use in dense WDM transmission systems. Page 9 of 9

10/10 6.06.001 16:00 7. ACKNOWLEDGEMENT The Department of Research and Development of the Federal Republic of Germany and the Senate of Berlin supported this work. We want to thank especially Th. Rosin for the developing of the long-term temperature test programs. Page 10 of 10

11/11 6.06.001 16:00 8. FIGURES AND FIGURE CAPTIONS Fig. 1: Schematic sketch of AWG-Chip outline Page 11 of 11

1/1 6.06.001 16:00 1. Loss average 0.94 db 1 Pigtail loss (db) 0.8 0.6 0.4 0. 0 1 3 4 5 6 7 8 Input/Output Fig. a) Pigtailing loss of 8x8 AWG Page 1 of 1

13/13 6.06.001 16:00 1,4 Loss average =0.91 db 1, 1 Pigtail loss (db) 0,8 0,6 0,4 0, 0 1 3 4 5 6 7 8 9 10 11 1 13 14 15 16 Input/Output Fig. b) Pigtailing loss of 16x16 AWG Page 13 of 13

14/14 6.06.001 16:00 0-5 -10-15 Reflection (db) -0-5 -30-35 -40-45 1 3 4 5 6 7 8 Input/Output Input, average: -39,09 db Output, average: -37,00 db Input: Reflection (db) Output: Reflection (db) Fig. 3 a) Reflections of an 8x8 AWG module for all inputs and outputs Page 14 of 14

15/15 6.06.001 16:00 0-5 -10 Reflection (db) -15-0 -5-30 -35-40 1 3 4 5 6 7 8 9 10 11 1 13 14 15 16 Input/Output Input, average: -3,1 db Output, average: -3,89 db Input: Reflection (db) Output: Reflection (db) Fig. 3 b) Reflections of an 16x16 AWG module for all inputs and outputs Page 15 of 15

16/16 6.06.001 16:00 Fig. 4: Photograph of an AWG module with 16x16 fiber ports Page 16 of 16

17/17 6.06.001 16:00 Fig. 5: Sectional view of AWG housing with ribbon fiber pigtails Page 17 of 17

18/18 6.06.001 16:00 Fig. 6: Experimental set-up for fiber array-chip coupling Page 18 of 18

19/19 6.06.001 16:00 Fig. 7 a) Photograph of fiber-chip-connection Page 19 of 19

0/0 6.06.001 16:00 Fig. 7 b) Sectional view of the fiber array-chip connection Page 0 of 0

1/1 6.06.001 16:00 Fig. 8: Environmental test set-up Page 1 of 1

/ 6.06.001 16:00-30 Peltier on Input 5 - Output 4-5 -0 Loss (db) -15-10 -5 0 1538.5 1539.0 1539.5 1540.0 1540.5 1541.0 Wavelength (nm) Loss (db), 10 C Loss (db), 0 C Loss (db), 40 C Fig. 9 a) Controlled spectral response of T chip = 3 C at T ambient = 10, 0, and 40 C Page of

3/3 6.06.001 16:00-30 -5-0 -15 Loss (db) -10-5 0 1538.5 1539.0 1539.5 1540.0 1540.5 1541.0 Wavelength (nm) Loss (db), 10 C Loss (db), 0 C Loss (db), 40 C Fig. 9 b) Spectral response of center wavelength at T ambient = 10, 0, and 40 C Page 3 of 3

4/4 6.06.001 16:00 1540.3 and ambient temperature 1540. Center Wavelength (nm) 1540.1 1540.0 1539.9 1539.8 1539.7 Tp uncontrolled (0.01 nm/k) Tp = 40 C Tp = 5 C Tp = 15 C 1539.6 1539.5 0 10 0 30 40 50 Ambient Temperature ( C) Fig. 10: Relationship between ambient temperature and center wavelength Page 4 of 4

5/5 6.06.001 16:00 AWG 8x8, Input 5 - Output 4 Center Wavelength 1539,54nm -5,8 45 Loss [db] -6-6, -6,4 40 35 30 5 0 Ambient Temperature [ C] -6,6 15-6,8 10 0:00:00 0:30:00 1:00:00 1:30:00 :00:00 :30:00 3:00:00 3:30:00 Time [h:m:s] Loss (db) Ambient Temperature ( C) Fig. 11: Temperature cycling of (T chip = 3 C) 8x8 module Page 5 of 5

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