Low Loss 16-Channel Photodetector Array Receiving Module With Fine Tuning Ability

A 16-channel photodetector array receiving module with good performance as low loss, fine tuning and high reliability is manufactured in our laboratory. The whole module was consisted with a silica arrayed waveguide grating chip (AWG), a thermo-optical type variable optical attenuator (VOA), a photodetectors (PDs) array with direct-current (DC) bias circuit, all components were integrated via 3D package method. In the article, the structure parameters of Y branch and edge couplers were optimized for lower loss. In order to realize excellent tuning ability, Titanium was selected as heating material for precisely tuning and its electric attenuation range could reach at a level more than 20 dB. Periodically segmented waveguide coupler designed in passive chips could minimize coupling loss less than 0.4 dB/coupling. The high-performance receiving module could effectively improve detection quality and significantly enlarge capacity in communication system.


I. INTRODUCTION
R EVEIVING module was a powerful device in resource exploration, optical fiber hydrophone and telecommunication. During decades, the rapidly increasing demand for high-capacity in communication had resulted in a dramatic development for multi-channel device, which could effectively improve transmission efficiency. In order to expand amount value of channel, wavelength division multiplexing technology is one of options for improve capacity of module. In [1], a fourchannel receiver based on micro-ring filter was reported. The rate could achieve at 10 Gb/s per channel and total insertion loss was 7.5 dB. Heat-tuning micro-ring-based wavelength division multiplexing (WDM) receiver could obtain 4×25 Gb/s rate [2]. Single monitoring photodetector was applied to get feedback signal from through port of micro-ring filter. A compact fourchannel optical receiver with p-i-n type photodetector array was reported in [3]. A transimpedance-amplifier array was exploited in the receiver to improve gain flatness and reduce insert loss interference. The crosstalk could be less than 17dB at 25GHz operating frequency. A five-channel receiver with 300 GHz channel spacing was presented in [4]. The increase amount of channel was obtained via increasing number of rings in demultiplexing structure. Optical receivers were usually designed in four or five channels, module with more than ten channels has seldom been reported. A 16-channel receiver consisted with AWG and Si-Ge PD was fabricated through monolithic integration platform [5]. Compared with hybrid integration, there had been no mature process flow for monolithic integration because of complex structure design and demanding processing conditions.
For power modulation, VOA rarely applied in receiver in recent research. As the result, AWG, VOA and PDs array which hybrid integrated in one module could be a potential method to expand capacity and decrease whole volume of device. In order to implement functions of power modulation and pre-equalization in receiving system, variable optical attenuator was typically applied to achieve the goal. Generally, VOA had been designed in three types: micro-electro-mechanical system (MEMS) type [6], electro-optical type [7]- [9] and thermo-optical type [10]- [13]. MEMS as an early commercialized modulator product had not yet satisfied the demand of miniaturization in recent year. Even though electro-optical VOA had advantages like faster response time and lower power consumption, doping process in fabrication increased complexity compared to thermo-optical VOA. As the result, thermo-optical VOA was an ideal choice due to simple fabrication process and miniaturization. The thermo-optical VOA used in the device was optimized for better performance. To reduce coupling losses, the Y branch structure was designed and Titanium was also chosen as heater material to obtain larger attenuation range and more precise control step.
Different kinds of structure couplers had been proposed to minimize coupling loss. Multiple layers coupler with inversely tapered waveguide could achieve 0.8 dB coupling loss per conversion [14]. Suspended structure was also another option for improving coupling efficiency which prevent leakage loss from substrate [15]. Irregular pattern of coupler profile was topologically created via genetic algorithm which could achieve extremely low coupling loss at 0.11 dB in an ultra-compacted size [16]. In this paper, periodically segmented waveguide (PSW) converter which had a relatively simple structure might be better choice for edge mode coupling. Coupling loss between fiber and AWG could be minimized to −0.103 dB in simulation via calculating the optimal value of duty cycle of PSW. Grating waveguide was patterned with a few degrees of rotation to  prevent loss from reflection. The actually measured coupling loss was up to 0.4 dB per conversion.
Based on market requirements for high capacity, miniaturization, integration, high reliability and high-performance modules in communication and resource detection, in our research, we successfully hybrid integrated a 16-channel photodetector array receiving module via 3D integrating method. Structure parameters of passive chips were optimized in order to improve the performance of device. PSW structure coupler was proposed to improve coupling efficiency. The Y branch and heater material of VOA were also designed to achieve high manufacture tolerance and fine tuning. Reliability of whole module was effectively improved via rearranging integrating sequence and adding supporting spacer beneath VOA.

II. VOA DESIGN
In the receiving model, thermo-optical VOA with Mach-Zehnder (MZ) structure was applied for power modulation. The whole VOA chip was presented in Fig. 1. Y branch with structural details was also partially enlarged at up-right insert in Fig. 1. Rectangular waveguide L 1 was posited after input waveguide and its width was narrowed down to 90% of original width to minimize radiative loss, L 2 was a tapered waveguide and had the same function as L 1 . Taper width and length of L 2 were scanned for optimal structure. According to the results in Fig. 2, it was clearly shown that L 2 tapered structure had significant ability to reduced radiative loss. Taper with 50 μm length and 6 μm width had best performance in transmission efficiency.
All structure parameters of Y branch were optimized for better transmission efficiency. According to simulated resulted of whole MZ structure, insert loss was successfully lower to 0.04dB and transmission result was exhibited in Fig. 3(a). It was confirmed that evenly distributed power and extremely low loss could be achieved basic on this Y branch structure.
According to VOA working principle, it was the most important process for symmetrical Y branches to evenly distribute light into separated waveguides. The maximum attenuation value in dB was calculated as: where I 1 and I 2 were power distributed in different arm waveguides. It was obvious that attenuation threshold was affected by equally power division. The more equal the power distribution, the greater attenuation range it could be obtained. Based on optimal structure of Y branch, Fig. 3(b). presented the trend of maximum attenuation with increasing error of waveguide width. As the results, power difference between two paths was increased with growing width error that shown in orange solid line. Maximum attenuation presented by blue solid line lowered down at the same time. According to the result, the optimized Y branch had a desirable tolerance and maximum attenuation still could be 20 dB while width difference in two arm waveguides was 0.4 μm. Choice of heating metal material was significant consideration for achieving high precision tuning. The cross section of thermo-optical VOA structure was exhibited in down-left insert in Fig. 1. Metal arm deposited on the top of waveguide was used to heat core to modulate refractive index in order to obtain phase difference. Required voltages were different for various metal to generate the same heat flow. The larger drive voltage it required, the finer tuning step we could achieve. Required power had the following relationship: where k w was thermal conductivity of heating material, w h , t c , t w were structure parameters for waveguide geometry and Δ∅ h was phase difference between two paths. According to Fig. 4. Attenuation curve as a function of applied voltage for five heating materials. equation above, it could obtain attenuation threshold for five metal heaters with their own modulation voltage which shown in Fig. 4. The dimension of heating arm was determined, it was evident that modulation voltage threshold was proportional to material resistivity and maximum attenuation value also varied directly with resistivity. As the result, Titanium was the superior choice to achieve precise control and obtain maximum attenuation among all five materials. Power consumption is an important consideration in practical application, in addition, thickness of upper cladding and dimensions of heating arm were also optimized to reduce power consumption. Thinner upper cladding thickness could reduce heat loss during heat transfer process. Moreover, thicker electrodes partially mitigated undesired resistance generated by nonuniformity deposition. 250 μm-wide output ports and same-width tuning arm spacing were designed to prevent heat cross-talking. However, multiple channels and larger output space would obviously increase difficulty in coupling alignment process.

III. PSW COUPLER DESIGN
In our research, spot size converter in periodically segmented waveguide structure was introduced to reduce insert loss caused by mode field mismatch. PSW structure presented in Fig. 5(a) was constituted with a section of rectangular waveguide, a widely tapered waveguide with duty cycle linearly gradated from 0 to ultimate value and a constant width waveguide with fixed ultimate duty cycle. Linearly gradated width was used as amplifier for mode field dimension that would expand to the around size of fiber. Gradient duty cycle was designed for modulating equivalent refractive index of channel waveguide. Refractive index of PSW structure could be modified by changing value of duty cycle. Detail formula as follow: where Δn f iber is refractive index difference of fiber and Δn waveguide is refractive index difference of channel waveguide. Assumed that Δn f iber is 0.45% and Δn waveguide is 2.0%, cladding index of each component was the same. Used the following calculation to determine optimal value of duty cycle: where n cladding is refractive index of cladding for both FA and AWG chip. According to the equation, optimal theoretical value of duty cycle was 0.22 for maximum coupling efficiency. It is an equivalent modal of PSW structure that a continuous waveguide with similar refractive index gradient [17]. In order to verify the accuracy of above theory, we established two coupler models, a PSW coupler and a continuous waveguide coupler with exactly the same structure parameters and refractive index, to simulate coupling efficiency separately. In PSW structure, the value of duty cycle was scanned for obtaining optimized structure parameter. In continuous waveguide model, final index value was set to multiply by a factor that was equal to duty cycle, and scanned the factor for optimizing coupling efficiency. The results presented in Fig. 5(b) showed that both coupler structures got maximum output power at optimized duty cycle value of 0.225 which was consistent with theoretical value calculated before. Output power values for two couplers had the same trend with scanning duty cycle while PSW coupler had higher coupling loss. One reason for larger loss was reflection existed at multiple surfaces of segmented waveguide. Fig. 6(a) presented transmission result of PSW coupler, all of structure parameters were set to optimal values. PSW structure coupler exhibited high coupling efficiency, the coupling loss could be reduced to merely 0.103dB. Fig. 6(b) also illustrated mode expanding process in PSW coupler, obvious enlargement of mode field dimension could be found in cross-section view.  Therefore, PSW structure could be applied in ports of chips to minimize coupling loss.
In order to minimize loss from tiny gap between fiber and chip, coupling glue was used to stick two edges together at maximum output position. After 15-minute UV exposing, mechanical glue was covered around connection to make sure a high strength bonding for following integration process. Second UV exposing lasting for 15 minutes were required for further strengthening. After whole coupling and strengthening process, we measured the actual coupling loss between FA and AWG. The total loss contained three types of loss: system loss, transmission loss of AWG and coupling loss. 13 dB input power was applied and obtained 8.7 dB output power measured at the end port of AWG. We calculated the coupling loss to be 0.8 dB according 1.63 dB system loss and 1.87 dB transmission loss. Because of twice coupling process at both sides of AWG, single coupling loss between FA and AWG should be around 0.4 dB. Compared with other type couplers, PSW could satisfy demand of actual application and easily adjust its structure to adapt different requirement. Fig. 7. exhibited the inner configuration of receiving module. General integrating process followed by the steps: First, passive components were coupled and glued together in a sequence of fiber array (FA), AWG and VOA to constitute VAWG part; Second, PDs array were vertically adhered next to RF transmission circuit. PDs chip applied in the module had average performances of 0.8 A/W responsivity, 8.0 GHz −3 dB bandwidth and 15 nA dark current. Then, the VAWG well coupled with PDs array was sticked on the sink beneath AWG. Final, VOA was wire bonded with pin on package though DC circuit. In the procedure operated above, thermostatic drying oven were used for drying glue to fix PDs array and control circuit. Because of high temperature and air current cycling lasting as least for two hours, some undesirable micro-offset in different chips might be introduced into VAWG. Plastic cladding of FA fiber had a relatively high percentage of heating shrinking, which would lead to direct exposure of bare fiber and severe reduction of device reliability. Adjustment of integration process sequence is an obviously effective method to deal with those drawbacks. Glued stabilization for PDs array chip, spacer and power control circuit was rearranged at first order. The process which required drying oven would be finished first. Coupling process of passive components would begin after all fixation procedure completed. Because passive components entirely circumvented high temperature drying, no bare fiber exposure evidence that updated integrating operation significantly increased reliability of whole receiving module.

IV. 3D HYBRID INTEGRATION
A piece of metal sink was used as spacer which height was a little bit lower than the distance between VOA chip and inner bottom of package. The spacer was design for supporting VOA chip, which suspending in original design and merely supporting by the connection with AWG chip. Furthermore, all stress only focused on connection was more likely to cause micro-offset or eventually broken when wedge pressed on VOA chip during wire bonding process. As the result, metal sink as a support would effectively stabilize and fix VOA chip in optimal position. There was a gap between VOA spacer and heat sink of AWG. The air-filled gap still had a heat insulated effect which would successfully avoid heating crosstalk from temperature control system.
Due to application of 3D-package method, volume could minimize to 8.8 cm×4.9 cm×0.8 cm for the whole 16-channel photodetector module. The well-sealed module was posited on external control circuit. All electrically controlled system were compactly integrated on the circuit board with function of temperature control, power attenuation, electrical signal amplification and signal differential output.
V. TEST AND RESULT Fig. 8(a) presented sealed model which was fixed on peripheral circuit board and all functions were controlled through the electronic component. Incident optical signal was demultiplexed and tunned via V-AWG and then transferred to photocurrent by photodetector array. The electric signal was differential output after amplification. Fig. 8(b) presented modulation ability of VOA. Maximum attenuation values were calculated for all 16 channels and the result was listed in Table I. According to the results, maximum attenuation value had a decrease trend along wavelength shortening. One of possible reason for photocurrent declining was series connection of detector chips.  Real-time eye diagram was shown in Fig. 9. Input sinusoidal signals could be observed in oscilloscope. However, the quality of transmission signals still needs further improvement. There was an unignorable noise during transmission which seriously deteriorated the restoration of signals. Some noise might come from amplifying process. Other possible source of noise might come from electronic components mismatch which caused unknown filtering. Furthermore, differences in coupling process and PDs performance leaded to discrepancy of signal quality in different channels. Next step, it is the main direction to improve performance of overall circuit, such as wider bandwidth, higher sensitivity and high uniformity.

VI. CONCLUSION
In the research, a 16-channel photodetector receiving model was fabricated through 3D hybrid integration. PSW coupler structure was designed for more efficient coupling process. Parameters of Y-branch of VOA were optimized to evenly distribute optical power and minimize transmission loss. Simulated maximum attenuation could be at over −60 dB with Titanium heating arm to achieve high precision modulation. Temperature control components were set to calibrate output wavelength to satisfy application requirement. It was effective to improve reliability of module via rearranging integration procedure sequence. The actual coupling loss between FA and AWG could be reduced to 0.4 dB/conversion base on PSW structure. The test results of attenuation evidenced that all 16 outputs could successfully achieve large modulation range over 20 dB. Transmission real-time diagram presented a sinusoidal signal while noise deteriorated quality of signal.