8.2 Gbps Optical Wireless Link Using SiPM at NIR Wavelength

Silicon Photomultipliers (SiPMs) can provide optical wireless communication (OWC) receivers with better sensitivities than PIN photodiodes, thanks to their photon counting capabilities. The commercial availability of SiPM with fast output pulse width allows high-speed data transmission at low irradiance levels. This article investigates the use of a commercially available SiPM with an active area of 1 mm2 and fast output pulse width of 0.6 ns in a Near-infrared (NIR) OWC link and compares the performance of a 3 mm x 3 mm SiPM with a fast output pulse width of 1.4 ns. Although both SiPMs have maximum detection efficiency at 450 nm, a wavelength of 850 nm is used owing to the availability of high-speed VCSELs, higher eye safety limit and ability to filter ambient light when using this wavelength. The OWC link is designed and tested at a 35 cm transmission distance using On-Off-Keying (OOK) and Decision Feedback Equalization (DFE), and a maximum data rate of 8.2 Gbps at a BER of 3.8 × 10−3 was obtained with 15.34 W/m2 irradiance at the SiPM. These results are achieved in the dark and under 500 lux of White Light Emitting Diode (WLED) ambient light. A long pass colour glass filter is used to reject light up to 700 nm wavelength, thus rejecting most of the interference from WLEDs, resulting in data rates similar to dark environments. This is the highest data rate achieved using SiPMs in an NIR link.


I. INTRODUCTION
O PTICAL Wireless Communications (OWC) has become a highly competitive solution for wireless communications due to their broad, unlicensed bandwidth [1] which allows very high data throughput.OWC systems are also immune to RF electromagnetic interference (EMI) and do not generate EMI, making them suitable for use in places where the electromagnetic interference level has to be minimised, such as in flight cabins or in dense ad-hoc networks, typical in vehicular communications with a dense traffic, or in settings with a high level of EMI such as factories [2].Moreover, OWC offers higher security in the physical layer over its RF counterpart as optical signals do not penetrate walls.OWC has the potential to achieve multi-gigabit data rates using low-cost commercial off-the-shelf sources and receivers.Due to these advantages, OWC has attracted an increasing amount of research attention in recent years.Many efforts have been put into improving the data rate [3], [4], designing novel high-bandwidth transmitters/receivers [3], [5], [6], extending the link distance [6], [7] or improving the link capacity/robustness through the adoption of advanced modulation schemes [8], [9].However, one aspect of OWC that is important but requires more research attention is the need in many applications to achieve high-speed data transmission under low light intensity levels.Many practical OWC applications, such as light fidelity (LiFi), require operation under eye-safe conditions limiting the maximum intensity at the transmitter.Reducing the required light intensity at the receiver helps reduce the requirement for accurate optical alignment and increase range.However, severe challenges are placed on the receiver's sensitivity to achieve high-speed data transmission with a low light intensity.
The development of silicon photomultipliers (SiPMs) significantly alleviates this problem.SiPMs are implemented by integrating multiple single-photon avalanche diodes (SPADs) together with metal or polysilicon quenching resistors [10].SPADs operate through DC biasing above their break-down voltage, which subsequently generates electron-hole pairs by absorbing an incoming photon.The SPAD undergoes an avalanche breakdown which is passively quenched by a series resistance which dynamically reduces the bias below the breakdown voltage in response to the photogenerated current.The combination of SPAD and quenching resistor is referred as a micro-cell.The micro-cell needs some time (known as dead time or recovery time) to revert to the original biased state to detect another photon due to the quenching process.During the recovery time, photons can still be detected by the microcell but with lower PDE [11].However, when multiple micro-cells are concurrently read, the resulting SiPM device retains the SPAD's high sensitivity while being able to generate an analogue signal with a dynamic range proportional to the number of micro-cells [10].The high sensitivity of SiPMs makes them good candidates for achieving high-speed communications under low light intensities.A 2.4 Gbps OWC link was built with a commercial blue laser diode (LD), OOK modulation and an off-the-shelf SiPM achieving a bit error rate of 10 −3 at a received power level of only 133 mW/m 2 [12].This datarate was further improved to 3.45 Gbps using OOK with decision feedback equalisation (DFE) at a light intensity of 500 mW/m 2 [13].A 5 Gbps data transmission was achieved at a power level of 1.06 W/m 2 with Orthogonal Frequency Division 0733-8724 © 2023 IEEE.Personal use is permitted, but republication/redistribution requires IEEE permission.
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TABLE I KEY PARAMETERS OBTAINED FROM THE MANUFACTURES DATA SHEET FOR A
C-SERIES 10010 AND J-SERIES 30020 SIPM [17], [18] Multiplexing (OFDM) modulation with adaptive bit and energy loading [14].In addition, a 1 Gbps eye-safe OWC transmission system using a wide field of view receiver was also demonstrated using a 6 mm 2 SiPM with a combination of colour glass filters under 500 lux ambient light [15].Despite the SiPM's high sensitivity, its potential to achieve high-speed data transmission is hindered by saturation and intersymbol interference (ISI) caused by finite output pulse width.The mitigation of these effects requires techniques such as the use of ambient light filters, peak to average power ratio (PAPR) reduction techniques, a careful selection of SiPM bias voltage and ISI mitigation schemes such as DFE [12], [14].
In this article, a 1 mm 2 SiPM is investigated at a Near Infra-Red (NIR) wavelength of 850 nm and the performance compared to a 3 mm x 3 mm SiPM.An OWC link, comprising a high-speed Vertical Cavity Surface Emitting Laser (VCSEL) transmitter and the 1 mm 2 SiPM receiver, is used to achieve an 8.2 Gbps data rate using On-Off Keying (OOK) and DFE.These results are achieved under dark conditions and under 500 lux of white LED ambient light with a signal irradiance of 15.32 W/m 2 .To the best of the authors knowledge, this is the highest data rate achieved using a SiPM.
The rest of the article is organised as follows.Section II describes SiPM characteristics.The experimental OWC setup is described in Section III.The results of the relationship between data rate and irradiance are highlighted in Section IV.The operating wavelength selection discussion is mentioned in Section V. Finally, conclusions are provided in Section VI.

II. SIPM CHARACTERISTICS
SiPMs are an important alternative to positive-intrinsicnegative diodes (PINs) and APDs in optical wireless communication links, thanks to their single photon counting capability that improves the overall sensitivity of the receiver.Previously, an Onsemi J series 30020 3 mm 2 SiPM was explored in detail, both in the dark and under ambient light [13], [16] for VLC applications.As shown in Table I, this SiPM offers Full-Width Half Maximum (FWHM) fast output pulse width of 1.4 ns.The fast output is formed from the parallel connection of all micro-cells where each micro-cell in the SiPM has capacitively coupled output [17].Based on the 30020 SiPM, a maximum data rate of 5 Gbps was reported using OFDM [14].However, to achieve data rates greater than 5 Gbps, a SiPM with shorter FWHM pulse width is required.
Therefore, an Onsemi C-series 10010 SiPM with an active area of 1 mm 2 is selected for this study.The 30020 SiPM is also evaluated and data transmission comparison between 30020 and 10010 SiPMs are performed under 850 nm wavelength.Fig. 1 shows measured fast output pulses of the 10010 and 30020 SiPMs where the 10010 SiPM provides a narrower pulse width of 0.6 ns and can result in better BW and expected to achieve higher data rates compared to the 30020 SiPM.Nevertheless, the 10010 SiPM has a very low peak Photon Detection Efficiency (PDE) <18% compared to the 30020 SiPM, which has a peak PDE of 48% at 420 nm.These SiPMs are designed to provide peak PDE around 420 nm, and the PDE drops considerably in the NIR region.At 850 nm, the PDE of 10010 SiPM is < 1%.The parameter details of both the 10010 and 30020 SiPMs are presented in Table I.
As explained in Section I, during the recovery time the PDE of a microcell is reduced [11], so incident photons may not be detected.At high irradiances, photons arrive at each microcell with an average inter-photon arrival time lower than the recovery time, resulting in saturation.Nevertheless, due to the large number of microcells which make up the SiPM, some photons continue to be detected.However, the irradiance required to achieve a particular data rate at a target BER increases rapidly due to decreasing average PDE.The fast output of the Onsemi SiPM is capacitively coupled between the SPAD and the quench resistor providing the response to a 0.6 ns pulse in Fig. 1.The fast output pulse also governs the bandwidth of the SiPM (see Section III).Fig. 2 shows the photocurrent of the 10010 SiPM as a function of irradiance under a fixed bias voltage.It is observed that current is approximately linear up to an irradiance of 1 W/m 2 , beyond which it begins to saturate.Complete saturation will occur at an intensity greater than 10 W/m 2 and could not be reached due to setup limitations.
The bias current (I bias ) of the SiPM can be estimated [13] using Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Where C rate is the count rate,Q cell is a charge required to recharge each microcell, N cells are number of microcells in the SiPM, Irrad is the irradiance, τ r is the recovery time and: where η λ is the PDE, A is the area of SiPM and E photon is the photon energy.
The estimated 10010 SiPM bias current as a function of irradiance is also plotted in Fig. 2 using (1).C rate is evaluated by using parameters of Table I and τ r = 2.2τ (where τ is 19 ns for the 10010 SiPM).Q cell of 0.15 pC is estimated by taking the ratio of bias current to the fast pulse generation rate.This results in a PDE (η λ ) of 0.85% providing the close match with the measured values.

III. DATA TRANSMISSION EXPERIMENT
The 10010 SiPM optical wireless data transmission experiment was performed using the experimental setup shown in Fig. 3.A directly modulated 7 GHz 850 nm multimode VCSEL was used as the transmitter.OOK modulation signals were generated (one sample per bit) by a 10 GHz Tektronix Arbitrary Waveform Generator (AWG) and amplified (SHF 20 GHz amplifier) to 630 mVpp.An optimal VCSEL bias current of 5 mA was selected for all data transmission experiments.A biconvex lens was placed in front of the VCSEL to form a collimated beam.The SiPM receiver was positioned 35 cm away from the transmitter for experimental convenience and the irradiance varied using Thorlabs ND absorptive filters and a wire grid polariser.A diffuser was placed in front of the SiPM to uniformly illuminate its 1 mm 2 active area.
The SiPM was enclosed in an opaque box with an aperture opening to restrict any stray light and allow a colour glass filter to be placed in the aperture for filtering.The output of the SiPM was connected to a 20 GHz amplifier and then captured by the Tektronix MSO64 oscilloscope.The bandwidth of the  VCSEL, AWG, amplifiers and oscilloscope were over specified for the application to ensure that they did not limit the system performance.The irradiance at the SiPM was measured by replacing the SiPM with the power meter sensor positioned at the same location as the SiPM active area.
The system frequency response was measured by replacing the AWG and oscilloscope with a vector network analyser.Fig. 4 shows the measured frequency response with a 3 dB bandwidth of 310 MHz.Although a relatively small 3 dB BW is observed, its slow roll-off allowed higher data rates by adopting DFE which was implemented in software on the digitised received signal.Fig. 4 shows that the measured frequency response matches well with the Fast Fourier Transform (FFT) response of the 10010 SiPM fast output pulse shown in Fig. 1.A pseudorandom binary sequence (PRBS) OOK signal was generated using the AWG.Simple OOK modulation is chosen in preference to more spectrally efficient schemes which are often preferred in band limited channels due to the severe non-linearity which occurs in the SiPM [19], [20].8b10b coding was used as this was found to improve the OWC link Bit Error Ratio (BER).The signal from the oscilloscope was then post-processed offline.The signal was low-pass filtered to reduce noise, and DFE applied with 80 feedforward and 20 feedback taps to reduce ISI.The DFE feedforward and feedback tap values were iteratively found to obtain the optimum BER.A BER limit of 3.8 × 10 −3 was used to define the maximum achievable data rate.The BER is calculated by taking the ratio between total bit errors and total transmitted bits for a 2 15 bit frame that is transmitted 10 times.
The OWC link was also demonstrated under 500 lux of ambient light generated using Philips 18 W warm white LEDs, whose spectrum is shown in Fig. 5 along with the long pass filter (RG-780) response.As an 850 nm wavelength VCSEL was employed, the ambient light could be effectively excluded while allowing the wanted signal to pass.The colour glass filters also widen the FOV of the receiver compared to bandpass filters.The colour glass filters are absorptive filter which absorbs unwanted light and are not angle dependent.However, bandpass filters typical operate by reflecting the undesired light and only work at limited angles.Colour glass filters FOV performance are evaluated in detail by W. Ali et al. [15].The RG-780 filter has less than 10% transmission up to 760 nm and reduced the SiPM DC current from 8.06 mA to 0.05 mA under WLED illumination, thus preventing saturation of the SiPM.The dark current at 29 V bias voltage was ∼1 nA showing that although the filter works well, some residual WLED light remains.The transmittance at 850 nm is ∼ 99% resulting in 0.3 dB of loss to the wanted signal.

IV. RESULTS
The SiPM bias was optimised to achieve the highest data rate of 8.2 Gbps by plotting the BER as a function of SiPM bias voltage as shown in Fig. 6.The maximum SiPM bias was limited to 29 V to prevent damage.As the bias voltage is increased, the PDE increases [18] which results in improved BER.However, higher SiPM bias may also lead to a high probability of cross-talk, after-pulse effects and Dark Count Rate (DCR) [21].Optical cross-talk occurs when a micro-cell, undergoes avalanche and accelerated carriers in the high field region emit photons which excite surrounding cells.After-pulsing is due to charge carriers  trapped in defects within the silicon, which are then released after some delay (several nanoseconds), triggering an avalanche and producing an after pulse in the same micro-cell.However, cross-talk and after-pulse effects are dependent on SiPM gain, and due to the very low PDE at 850 nm (<1%), we do not see their impact up to the maximum rated bias voltage.In addition, DCR is due to the thermal electrons generated in the active volume and it also increases with the SiPM bias voltage.Nevertheless, the result in Fig. 6 shows that DCR also has a minimum impact because the 10010 SiPM receiver only generates 1 nA dark current at 29 V bias.Under 500 lux ambient light, the current increases to 0.05 mA but has negligible effect on the BER.Consequently, all further measurements are obtained at a bias voltage of 29 V.
The 10010 SiPM performance with the RG-780 filter has been analysed in more detail.The required irradiance to achieve a target BER of 3.8 × 10 −3 at various data rates is shown in Fig. 7 under dark conditions and under 500 lux of ambient WLED Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.It is evident that ambient light, once filtered at the receiver, has a very small effect on the higher data rates from Fig. 7, however at lower datarates where a lower irradiance is required, the effect of ambient light is more significant.For example, under dark conditions, at 1 Gbps a signal irradiance of 0.14 W/m 2 is required, which corresponds to an average 0.67 mA SiPM current (Fig. 2).A slightly higher irradiance (0.16 W/m 2 ) was needed under ambient light to compensate for the 0.05 mA SiPM current due to ambient leakage through the long wavelength filter.This ambient light interference impact was reduced for 2 Gbps and 3 Gbps datarate and completely diminished beyond 3 Gbps because of the higher signal-to-interference ratio (or effective modulation depth) as a result of the higher signal irradiance required.At the highest data rates (>7 Gbps) high irradiances (> 3.3 W/m 2 ) are required and, from Fig. 2, it can be observed that the SiPM is operating in saturation at these irradiances.The maximum achievable data rate is therefore limited due to both saturation and recovery time of the 10010 SiPM.
The 10010 SiPM was also tested using OOK without DFE, and a maximum of 2.25 Gbps was obtained at 3.8 × 10 −3 BER, as depicted in Fig. 7. Fig. 8 (bottom) shows eye diagram of 2.25 Gbps data transmission.The 10010 SiPM results are also compared with the 3 mm SiPM (30020) using the same setup.Due to its larger size, a higher number of cells and larger PDE value, the 30020 SiPM required a lower irradiance up to 4 Gbps (0.6 W/m 2 compared to 10010, which required 0.86 W/m 2 ).However, due to 30020 SiPM's lower bandwidth, a maximum data rate of 5 Gbps was obtained at an irradiance of 12.9 W/m 2 .The results are also consistent with the simulation results reported by W. Matthews et al. [22], which showed that 10010 SiPM will perform better at data rates higher than 3 Gbps using 405 nm transmitter than 30020 SiPM.The slight deviation from the results reported by W. Matthews et al. [22] might be due to considering different bandwidth transmitters.

V. OPERATING WAVELENGTH SELECTION
The selection of 850 nm helps to transmit higher power compared to blue wavelengths under which SiPMs are widely evaluated [11], [12], [13], [14], [15], [16].The laser standard [23] allows 19.5 times more power at 850 nm compared to the blue light (400-450 nm) range.Wavelength of 850 nm also allows selection of low cost off-the-shelf colour glass filters to reject WLED ambient light and preserving FOV.However, choosing 850 nm reduces PDE of the 30020 SiPM by 18 times and the 10010 SiPM by 21 times (0.85% PDE at 850 nm) compared to peak PDE at 420 nm.The increase in transmitter power at 850 nm mitigates the impact of lower PDE of the SiPMs besides allowing the use of a higher bandwidth transmitter.However, due to lower PDE and smaller irradiance from an eye safe laser transmitter at 850 nm, optics in front of the 10010 SiPM will be used to achieve desired irradiance levels.In future work, off-the-shelf commercial optics will be selected carefully to achieve higher speeds along with wider coverage for OWC links based on eye safe sources.

VI. CONCLUSION
Commercially available SiPMs can achieve faster data transmission at low irradiance levels than PIN and APD photodiodes due to their unique photon counting ability.The use of the fast output in combination with DFE allows data transmission at rates well beyond the 3 dB bandwidth.Combined with a large area and high gain this also allows higher data rate transmission with low irradiance.
In this article, a commercially available SiPM with an area of 1 mm 2 was investigated.Although the selected SiPM is not optimised for NIR wavelengths, resulting in a low PDE, an 850 nm VCSEL was chosen as the transmitter due to the off-the-shelf availability of high bandwidth devices optimised for datacomms and increased eye-safe light levels.An additional benefit of NIR is that optical filters can be used to exclude common sources of interference, such as WLED lighting, while allowing the NIR signal to pass.
An OWC link was designed and demonstrated at a 35 cm distance.Using OOK modulation and DFE, the maximum data rate of 8.2 Gbps was demonstrated at a BER of 3.8 × 10 −3 with an irradiance of 15.34 W/m 2 in dark conditions.The system was also investigated under 500 lux of ambient light from WLEDs with little impact on the required irradiance for 8.2 Gbps transmission.This irradiance is three orders of magnitude lower than that needed by a PIN photodiode with an active area of 0.4 mm [4] and [24] to achieve a similar datarate.Longer transmission distances could readily be achieved by using a simple lens arrangement before the SiPM.In addition, the 10010 SiPM and 30020 SiPM data transmission results are compared and it is shown that 10010 SiPM needs lower irradiance beyond 4 Gbps which is also consistent with the results reported in [22].
To the best of the authors' knowledge this is the highest data rate reported using SiPM with an NIR transmitter.These results highlight that SiPMs can play an important role in designing very high-speed optical wireless links.

Fig. 2 .
Fig. 2. Measured and estimated SiPM bias current as a function of irraiance at 850 nm wavelength for 29 V bias.

Fig. 4 .
Fig. 4. Measured and estimated frequency response from fast pulse width of 10010 SiPM.

Fig. 6 .
Fig. 6.BER as a function of SiPM bias voltage at the maximum data rate of 8.2 Gbps.