Optical wireless communication using a fully integrated 400 µm diameter APD receiver

Transcription

1 Optical wireless communication using a fully integrated 400 µm diameter APD receiver Dinka Milovancěv, Tomislav Jukic, Bernhard Steindl, Paul Brandl, Horst Zimmermann Institute of Electrodynamics, Microwave and Circuit Engineering, Vienna University of Technology, Gusshausstrasse 25/354, Vienna 1040, Austria dinka.milovancev@tuwien.ac.at Published in The Journal of Engineering; Received on 12th June 2017; Accepted on 17th July 2017 Abstract: An optical wireless communication (OWC) system is demonstrated which uses a highly sensitive lens-less optoelectronic integrated circuit with a 400 µm diameter avalanche photodiode as a high-speed receiver, and a transmitter with highly collimated steerable beam at 680 nm wavelength. The benefits of integrated receiver with large-area photodiode in an OWC scenario are proven experimentally. At a data rate of 2 Gb/s OWC is established over a distance of 12 m without using any optics at the receiver side to increase the collection area. The maximum receiving angle of the lens-less receiver is 18. The receiver is tested under different lighting conditions and immunity to background light is validated up to 2000 lux without using any filter to reduce the photons outside the wavelength range of interest. 1 Introduction The increasing demand for wireless services requires new supporting technologies and architectures. Optical wireless communication (OWC) with its high bandwidth capabilities, inherent security and immunity to electromagnetic interference emerges as a viable supplement to existing wireless technology. In [1], photonic solutions are investigated that are suitable for indoor communication, i.e. photonic home area networks; among them particular attention is given to OWC. However, the OWC is still facing some genuine challenges especially in the Gb/s range. In [2], some of the challenges were thoroughly investigated. This paper suggests employing avalanche photodiodes (APDs) and beam steering in order to overcome the problems of limited receiver sensitivity whilst ensuring reasonable coverage. The progress and overview of some systems using beam steering was done in [3]. However, these systems did not concentrate on the receiver development. The main requirements of OWC receiver are: high sensitivity, wide bandwidth, wide dynamic range, large collection area, ability to reject ambient light, low cost and small size. The integrated approach offers lightweight solutions, and using the visible range of wavelengths gives the opportunity to integrate the photonic and electronic devices on the same silicon wafer leading to cheaper mass production. Additionally, by using an optoelectronic integrated circuit (OEIC) the problems of bond wires between photodetector and circuits are mitigated resulting in better reliability, smaller size and better sensitivity. One of the first integrated solutions for OWC application was presented in [4], where positive intrinsic negative (PIN) photodiodes were flip-chip bonded to an array of receivers. By using APDs the receiver sensitivity can be further improved. In our previous studies, the focus was on OWC systems using beam steering via a microelectromechanical (MEMS) mirror with the possibility of an adaptable beamwidth [5] and fixed beamwidth with a superior focusing [6]. Integration of a 200 µm diameter APD was successfully done in [7], where a 0.35 µm high-voltage (HV) CMOS process was used with a substrate voltage of 65 V, and in [8] where a 0.35 µm bipolar-complementary-metal-oxidesemiconductor (BiCMOS) process was used which needed a lower substrate voltage ( 35 V) for proper operation of the APD. The 200 µm diameter APD OEIC described in [8] was used in an OWC scenario [6], where without any optics at the receiver side a data rate of 2 Gb/s was achieved over a distance of 6.5 m. Besides the high-speed receiver used for communication, four additional surrounding (satellite) PIN photodiodes were implemented on the same chip together with low-speed circuits for monitoring the position of optical beam. The output of the monitoring circuits could be used for a back channel in order to align the beam. In this work, we test the 400 µm diameter APD OEIC described in [9] as a lens-less receiver for wireless optical communication under various conditions. In contrast to work in [6], the satellite photodiodes and accompanying circuits are omitted in order to save chip area, so there is no possibility for feedback information regarding the position of the beam, i.e. back channelling, such as a voltage signal at the output of the non-linear differential transimpedance amplifier (TIA) connected to satellite photodiodes. However, the large photodiode area can ease the alignment requirements, thus making the receiver suitable for tracking architectures where the transmitter beam is searching for the receiver with the aim to position itself, so that the best signalto-noise ratio is achieved. In one of our previous studies, tracking via a camera and via image recognition was demonstrated [10]. 2 Optical receiver with 400 µm diameter APD The APD OEIC has a large-area photodiode (400 µm diameter) and it was developed in the same 0.35 µm BiCMOS process as the 200 µm diameter APD OEIC [8]. The structure of the APD is shown in Fig. 1. It has a separate absorption zone (thick epitaxial layer) and multiplication zone (at the n++/p-well junction). The photodiode benefits from the low-doped thick epitaxial layer (12 µm) available in the 0.35 µm BiCMOS technology used. Unfortunately, an antireflection coating (ARC) on top of the APD was not available for the used process. The block diagram of the receiver s building blocks is shown in Fig. 1. The first stage of the receiver is a single-ended TIA with a common-emitter (CE) input stage buffered by two emitter followers, Fig. 2a [9]. To enable pseudo-differential operation, a replica of the TIA was made biased by a feedback loop consisting of a low-noise operational amplifier and resistor capacitor (RC) network. This feedback loop is a low-pass filter and the circuit as a whole cuts off low-frequency components of the incoming optical signal such as ambient light and has a break frequency of about 40 khz. A low break frequency is needed, so that long runs of 1 and 0 can be detected without experiencing baseline wander. However, this requires quite large capacitors in the RC network; therefore, the whole feedback loop was placed off-chip to save chip area and have more flexibility in adjusting a break frequency. For low-noise amplifier, we used LMP7732 from Texas Instruments, the R1/R2 and C1/C2 formed the low-pass filters

2 Fig. 1 Cross-section of APD and circuit block diagram Fig. 2 Circuit diagram of a TIA b Post-amplifier c Output buffer and R3 and C3 filters the feedback voltage signal from the noise of the preceding stages. The resistor R4 converts the feedback voltage into the input current for the dummy TIA. The post-amplifying stages are two cascoded limiting differential amplifiers suitable for on off keying modulation. A CE-common-base cascode circuit structure was employed at both limiting amplifiers due to its inherent high speed (eliminates Miller capacitance), see Fig. 2b. The emitter followers provide level shifting and buffering. The cascade of two limiting amplifiers was sized up from first to second stage regarding device dimensions and bias currents to maintain the wide bandwidth. The last stage uses a common current mode logic output stage; a simple differential amplifier with large input transistors capable of delivering high-output currents driving the 50 Ω loads, Fig. 2c. The output stage can provide a differential voltage swing of up to 1.1 V pp. The overall single-ended transimpedance of the receiver is 85 kω. The whole circuit has a measured power consumption of mw at a supply voltage of 3.3Vandoccupiesanareaof960µm 1540µm[9], see Fig OWC receiver design challenges The receiver design for large photodiode diameter has many challenges. The motivation for increasing the diameter from 200 to Fig. 3 Micro-photograph of the integrated APD optical receiver 400 µm is facilitating alignment and increasing collected optical power. Hence, the required irradiance (W/m 2 ) at the receiver is smaller. Since the irradiance decreases with the square of the distance, the increase of the diameter for the same divergence angle of the beam can lead to a larger transmitting distance. The extent

3 of the performance gain will depend also on the amount of ambient light that is collected and, more importantly, on receiver sensitivity. As the photodiode diameter is doubled, the photodiode capacitance is increased about four times (from 500 ff to 1.77 pf). The photodiode capacitance has a detrimental influence on the receiver sensitivity especially in the Gb/s range, since it gives rise to frequency ( f 2 ) dependent noise. Additionally, for TIA topologies with a CE input transistor, the capacitance of the input transistor should match the capacitance of the photodiode in order to optimise the noise performance of a TIA [11]. For this reason, the input transistor size and collector current were scaled up compared with the 200 µm receiver in order to match the initially estimated 2.3 pf photodiode capacitance perceived at the input. To keep the bandwidth needed for 2 Gb/s operation, the feedback resistance R fb was decreased three times (from 1500 to 500 Ω) due to the increased capacitances. Small feedback resistance results in smaller transimpedance gain and, more importantly, the noise current spectrum of the feedback resistor given by thermal noise 4 kt/r fb contributes directly to the input-referred current noise of the TIA. Therefore, the smaller feedback resistance increases the input-referred current noise. In summary, all circuitdependent main noise sources will scale proportionally with the increase of the photodiode diameter; therefore, increasing the equivalent input-referred noise current. The overall receiver sensitivity depends on the input-referred noise current coming from the electrical circuitry as well as on the noise coming from the APD itself. The noise of the APD is dependent on avalanche multiplication M and the technology process. High multiplication gain is desirable since this operating range of APD also enables high bandwidth (high substrate voltage). However, it also generates a large portion of signal-dependent noise. The optimum multiplication gain is the one at which APD noise and equivalent input-referred noise from the electrical circuitry are equal [12]. The receivers with high equivalent input-referred circuit noise will demand for high multiplication gains in order to achieve optimum noise performance (sensitivity). High multiplication gain for a given photodiode is achieved using a high electric field (high reverse biasing voltage) for which the APD can reach its maximum bandwidth due to carrier velocity saturation. Therefore, the high equivalent input-referred circuit noise of an OWC receiver with large-area photodiode makes it easier to optimise for this high multiplication and maximum bandwidth operating region of the APD itself. The optimum working point of the APD was found by sweeping the substrate potential in order to achieve optimum avalanche multiplication for which the receiver becomes most sensitive. The APD achieved a gain of 31.9 at a substrate voltage of 24.0 V and a responsivity of 14.2 A/W at 675 nm. The dependence of APD responsivity on operating wavelength ( nm) at different biasing conditions can be found in [13]. For the substrate potential of 24 V, the bandwidth of the APD receiver was 960 MHz which is at the lower limit for 2 Gb/s communications. At a data rate of 2 Gb/s, the receiver reached a sensitivity of 30.6 dbm for a bit error rate (BER) lower than 10 9 at a wavelength of 675 nm [9]. As stated in [9], the expected drop in receiver sensitivity by doubling the photodiode diameter is 1.25 db for APD OEIC. However, the collecting area is four times larger, so the needed irradiance would drop by 4.75 db resulting in a 1.73 times increase of transmission distance. Fig. 4 Measurement setup for OWC 4 OWC system structure and experiments The system setup for OWC is shown in Fig. 4. The measurements were performed within our laboratory facilities with normal lighting (500 lux) coming from the ambient light sources. Regarding the transmitter side of the system, we used a 680 nm single-mode (SM) vertical cavity surface emitting laser (VCSEL) together with an adjustable collimating lens. The collimating lens together Fig. 5 BER dependence on transmission distance a BER versus distance between transmitter and APD receiver b Eye diagram at a distance of 12 m at 2 Gb/s

4 with the SM VCSEL offered better focusing abilities compared with [5, 7]; therefore, a smaller laser beam spot. The operating wavelength of 680 nm was chosen mainly due to the high responsivity of the APD in this spectral range. The output power of the VCSEL was 0.85 mw which makes it eye safe for class 1 laser devices (maximum value is 1 mw) and the extinction ratio was set to 8. The transmitter was connected to a bit pattern generator and modulated up to a data rate of 2 Gb/s with a pseudorandom binary sequence. The transmitted beam was steered with an MEMS-based mirror described in [7]. No optics were used at the receiver side. The receiver was placed on a rotatable platform in order to determine the maximum receiving angle. One of the OEIC outputs was connected to a BER tester, while the other was connected to an oscilloscope to capture eye diagrams. The setup in Fig. 2 also includes adjustable light sources for testing the receiver under different ambient light conditions. 4.1 BER dependence on transmission distance The maximum achievable transmission distance depends on the transmitted optical power, beam divergence angle, receiver sensitivity and photodiode diameter. The BER dependence on the transmission distance is depicted in Fig. 5a. The maximum achievable error-free transmission (BER < 10 9 ) distance was 12 m at 2 Gb/s. For this transmission distance, the eye diagram at the single-ended output of the receiver is shown in Fig. 5b. The diameter of a focused spot at this distance was around 1.15 cm, which corresponds to an angle of divergence of roughly 0.96 mrad (full-width at half maximum). The power density at this distance is 7.5 µw/mm 2 which for a photodiode area of mm 2 results in a received optical power of dbm. This value corresponds well to the known receiver sensitivity of 30.6 dbm (BER < 10 9 ) [9] at 2 Gb/s. Compared to [6] where a 200 µm diameter APD (four Fig. 6 BER versus rotation of the receiver against the optical axis (11 m distance and 2 Gb/s) times smaller collecting area) was used, the working distance increased 1.84 times at a data rate of 2 Gb/s. For comparison, the maximum error-free transmission distance was 20 m and was measured for a data rate of 1 Gb/s; this corresponds to an increase of 1.81 times compared with [6]. For both data rates, the increase in transmission distance is well in accordance with the theoretically predicted value of 1.73 for APD receivers in this technology when the photodiode diameter is doubled [9]. Compared to the 200 µm diameter HV CMOS APD receiver from [7] which operated at a data rate of 1 Gb/s in an OWC system that had the same setup and the same transmitter [14], the distance is increased 2.8 times. The large difference is due to the poorer receiver sensitivity obtained in HV CMOS. 4.2 BER dependence on receiving angle In case of perfect alignment between transmitter and receiver, the signal beam is perpendicular to the receiver surface and the signal power density is at maximum for a given distance. In a real case scenario, a receiver should be able to operate together with additional transmitters at different locations and the need for (precise) adjustment of the receiver should be avoided. The angular shift will cause a power penalty which is directly proportional to the cosine of the angle between the receiver s surface normal and the incoming beam. To test the influence of the angular shift, the receiver was mounted on a rotatable platform, while the transmitter was kept fixed at a distance of 11 m. This distance was chosen such that the signal-to-noise ratio was comparable with that in our previous work where a 200 µm diameter APD OEIC was used [6]. By rotating the receiver against the optical axis and measuring the BER, an allowable incidence angle of ±9 was determined for a secure 2 Gb/s data transmission (BER < 10 9 ), as shown in Fig. 6. This result is similar to the one in [6] where the receiving angle of ±11 was obtained for a transmission distance of 6 m at 2 Gb/s. The BER curve dependence on the receiving angle is not only shaped by the cosine power penalty, though also by an angledependent oscillation effect caused by optical interference in oxide and passivation layers on top of the APD. The obtained receiving angle is cut back compared with a lens-less receiver in [7] which used ARC to prevent this optical interference. Nevertheless, compared to [5], where a PIN photodiode was used together with a lens to increase the transmitting distance, the receiving angle is doubled. 4.3 Background light immunity A wireless optical receiver should operate in various environment scenarios; therefore, it must be able to provide for error-free communication even in the presence of background light. Common indoor light levels are in the range of lux [15]. Owing to the statistical nature of ambient light, additional shot noise is Fig. 7 Influence of background illuminance on BER (starting BER < at distance of 11 m at 2 Gb/s)

5 Fig. 8 Background photocurrent for 400 µm APD OEIC (dashed red) and 200 µm APD OEIC (black) produced which is proportional to the DC current of the photodiode produced by the average power of this background radiation. Adjustable cold-light sources (Euromex LE.5210) were used to emulate high background light conditions. These cold-light sources use a HLX halogen lamp from OSRAM which is aluminium coated. Halogen lamps typically have a broad light spectrum from near ultraviolet into the infrared range. These cold-light sources were placed in front of the receiver, whereas the transmitter was at the distance of 11 m from the receiver. At the transmitting distance of 11 m, a BER of <10 10 could be obtained under normal light conditions (500 lux). At this distance, the signal power density at the receiver side is approximately the same as in the experiment where the receiver with 200 µm diameter APD was used [6]. During the measurement, the signal power was constant whereas the background light level was increased. Fig. 7 shows the BER dependence on the background illuminance; we can see that as the background illuminance increases the BER deteriorates. Up to an illuminance of 2000 lux error-free transmission (BER < 10 9 ) can be guaranteed. This result is very promising for APD receivers, especially since there was no filter to reduce the light outside the wavelength range of interest. The photocurrent generated by the photodiode was measured with a Keithley 2612 source meter which supplied the APD OEIC substrate voltage of 24 V. An additional curve (red line) showing the photocurrent resulting only from the background illuminance is displayed in Fig. 7. This current gives rise to shot noise which increases the BER. The DC current generated by background illumination is proportional to photodiode area. Fig. 8 compares the generated photocurrent of the receiver with 400 µm diameter APD reported in this work, with that of a receiver with 200 µm APD [6], and as can be seen the latter has around four times smaller background current for the same illuminance. This is the reason why the receiver in [6] showed better immunity to background illuminance (up to 6000 lux). 5 Conclusion This paper presents the application of a fully integrated optical receiver with large-area APD in an OWC scenario. The receiver shows promising results in terms of working distance, where error-free transmission was possible over 12 m at a maximum data rate of 2 Gb/s, and over 20 m at 1 Gb/s. The working range increased as predicted compared with our previous work [6] as the diameter of the photodiode is doubled. The large diameter of the APD together with the high sensitivity of the receiver enables these high working distances without using any optics at the receiver side. The full receiving angle of the receiver is 18 which represents a two-fold improvement compared with a lensbased PIN receiver [5]. However, the receiving angle is not as wide as that of a lens-less APD receiver using an ARC on the APD and described in [7]. Error-free wireless transmission can be ensured even at high background light levels up to 2000 lux, which is suitable for indoor applications where expected light levels are around 500 lux. For future work, a fully automated version should be considered where tracking is done via image recognition such as in [10]. Additionally, the benefits of the increased photodiode diameter could be furthermore investigated with different photodiode diameters to exploit the limits in OWC performance gains. 6 Acknowledgments This work has been supported in part by the TU Wien research funds and by the TU Wien Library through its Open Access Funding Program. 7 References [1] Koonen A.M.J., Tangdiongga E.: Photonic home area networks, J. 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