Hui Li

Alain Fourmigue

Sébastien Le Beux
email: sebastien.le-beux@ec-lyon.fr

Ian O'connor

Gabriela Nicolescu

Towards Maximum Energy Efficiency in Nanophotonic Interconnects with Thermal-Aware On-Chip Laser Tuning

Keywords: Nanophotonic interconnects, 3D integrated circuits, thermal simulation

Nanophotonic is an emerging technology considered as one of the key solutions for future generation on-chip interconnects. However, silicon photonic devices are highly sensitive to temperature variation. Under a given chip activity, this leads to a lower laser efficiency and a drift of wavelengths of optical devices (on-chip lasers and microring resonators (MRs)), which results in a higher Bit Error Ratio (BER). In this paper, we propose to jointly tune the on-chip lasers and and MRs in order to align the wavelengths of the emitted signals with the resonant wavelengths of the MRs. Our method allows significant improvements of the power consumption with regard to the related methods, while meeting the BER requirement. Compared to methods for which laser tuning is not possible, results show that a combined tuning of laser and MRs leads to 53% energy reduction when the uniform chip activity decreases from 20% to 5%. BER-energy tradeoffs have been explored and allow strategies to be defined to minimize either the energy, or the BER. As a key result, we have shown that, under specific chip activities, increasing the laser power consumption allows both energy and BER to be improved. This trend has been observed for a MWSR channel interconnecting 12 interfaces.

INTRODUCTION

I.

ECHNOLOGY scaling down to the ultra-deep submicron domain enables the integration of hundreds of cores. In order to enhance connecting many cores, chiplevel communication needs a new kind of interconnect to bring down the power budget. On-chip nanophotonic interconnects are an emerging technology considered as one of the key solutions for future generations of many cores [START_REF] Sun | Single-chip microprocessor that communicates directly using light[END_REF], providing high bandwidth and low latency. Interconnect architectures, e.g. MWSR (Multiple Writer Single Reader) [START_REF] Vantrease | Corona: System Implications of Emerging Nanophotonic Technology[END_REF], are composed of laser sources, microring resonators (MRs), waveguide and photodetector.

Laser sources can be fabricated in several ways and rely, for instance, on Fabry-Perot (FP), distributed feed-back (DFB) and distributed Bragg reflector (DBR) [START_REF] Roelkens | III-V/silicon photonics for on-chip and inter-chip optical interconnects[END_REF]. Among the available technologies, vertical-cavity surface-emitting lasers (VCSELs) provide an optical output power in the range of hundreds of microwatts, thus meeting the requirements for nanophotonic interconnects [START_REF] Roelkens | III-V/silicon photonics for on-chip and inter-chip optical interconnects[END_REF]. Furthermore, VCSELs based on a double set of Si/SiO2 photonic crystal mirrors (PCMs), so called PCM-VCSELs, are CMOS-compatible [START_REF] Sciancalepore | CMOS compatible ultra-compact 1.55-um emitting VCSEL using double photonic crystal mirrors[END_REF] [START_REF] Sciancalepore | CMOS-compatible integration of III-V VCSELs based on double photonic crystal reflectors[END_REF]. They can be used as on-chip laser sources since their footprint is of the same order of magnitude as the size of a MR used to modulate continuous waves emitted by off-chip lasers. VCSELs are thus sufficiently compact to be implemented in a large number. In this way, it saves the laser power since losses caused by power waveguides [START_REF] Vantrease | Corona: System Implications of Emerging Nanophotonic Technology[END_REF] are avoided, compared with off-chip lasers.

However, silicon photonic devices are highly sensitive to temperature variation induced by thermal effect over the chip, which leads to a drift of the laser wavelength and MRs resonant wavelength along a same photonic channel. Consequently, the Signal to Noise Ratio (SNR) of the signal received by the photodetector decreases, which leads to a higher Bit Error Ratio (BER). This is further accentuated by the significant reduction of on-chip lasers efficiency as the temperature increases.

This paper proposes a thermal-aware tuning method to overcome the wavelength variation induced by temperature variation, while achieving the targeted BER. The novelty of the method relies on the tuning of the laser driver current, which ideally complements traditional methods such as MRs tuning and channel remapping [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in WDM-Based Optical Networks-on-Chip[END_REF] [START_REF] Georgas | Addressing Link-Level Design Tradeoffs for Integrated Photonic Interconnects[END_REF]. While the method is evaluated for on-chip laser sources, it is also suitable for off-chip lasers since the impact of a temperature elevation would remain the same.

The paper is structured as follows. Section II presents the related work. Section III describes the problem induced by thermal variation and our proposed method. Section IV introduces an analytical model for tuning power method. Section V gives results and Section VI concludes the paper.

RELATED WORK II.

The issue related to thermal variation in on-chip optical communication has been addressed at both device and system levels. At device level, solutions relying on athermal devices [START_REF] Djordjevic | CMOS-compatible, athermal silicon ring modulators clad with titanium dioxide[END_REF] [START_REF] Mohamed | Reliability-Aware Design Flow for Silicon Photonics On-Chip Interconnect[END_REF], voltage tuning [START_REF] Manipatruni | Wide temperature range operation of micrometer-scale silicon electro-optic modulators[END_REF], local heating [START_REF] Biberman | Thermally Active 4×4 Non-Blocking Switch for Networks-on-Chip[END_REF][10], thermal-aware MR synthesis [START_REF] Condrat | Thermal-aware Synthesis of Integrated Photonic Ring Resonators[END_REF], and feed-back T control schemes [START_REF] Padmaraju | Thermal stabilization of a microring modulator using feedback control[END_REF] have been explored to limit the thermal impact on or control the resonant wavelength of MRs. At system level, the analyses allow the influence of temperature variation on the optical signal power received at receiver side [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in Optical Networks-on-Chip[END_REF] [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in WDM-Based Optical Networks-on-Chip[END_REF] to be evaluated. In our previous work [START_REF] Li | Thermal Aware Design Method for VCSEL-based On-Chip Optical Interconnect[END_REF], we propose a thermal-aware methodology to design an Optical Network-On-Chip (ONoC). Dynamic Voltage Frequency Scaling (DVFS) and workload migration techniques can be applied to reduce the temperature variation [START_REF] Li | Reliability Modeling and Management of Nanophotonic On-Chip Networks[END_REF]. Moreover, a thermally-aware job allocation policy [START_REF] Zhang | Thermal management of Manycore Systems with Silicon-Photonic Networks[END_REF] is used to minimize the temperature gradients among MRs. This work relies on device characteristics such as the Free Spectral Range (FSR), the number of waveguides and wavelengths. In our work, we propose a thermal-aware laser tuning method. We consider the influence of thermal variation on the BER and tuning power consumption, taking into account the actual efficiency of on-chip laser and chip activity. In [START_REF] Chen | Sharing and Placement of On-chip Laser Sources in Silicon-Photonic NoCs[END_REF], the authors explore the placement of shared on-chip lasers, which are located on a layer on top of the optical interconnect. In our contribution, we consider distributed CMOS-compatible on-chip lasers that are placed on the same layer with on-chip interconnect.

Channel hopping, along with additional MRs on the ends of the spectral range, is utilized for tuning in [START_REF] Mohamed | Power-Efficient Variation-Aware Photonic On-Chip Network Management[END_REF] [START_REF] Nitta | Addressing System-Level Trimming Issues in On-Chip Nanophotonic Networks[END_REF]. In [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in WDM-Based Optical Networks-on-Chip[END_REF], the thermal tuning is adopted to adjust the MR resonant wavelength to the neighboring channel wavelength. In order to counter-balance the temperature variation, communication channels can be remapped through ONoC reconfiguration [START_REF] Zhang | Power-Efficient Calibration and Reconfiguration for Optical Network-on-Chip[END_REF].

In [START_REF] Wu | Variation-Aware Adaptive Tuning for Nanophotonic Interconnects[END_REF], the laser output power is reduced until BER requirements can still be reached for optical interconnects using on-chip lasers. The MRs wavelength mismatch is first compensated with electrical tuning; then (i.e. once the wavelengths are aligned), the laser output power is reduced. However, the impact of the driver current onto the laser temperature is not taken into account, which will significantly impact i) the laser output power and ii) the emitted signal wavelength. Moreover, the optical crosstalk for the signal is not taken into consideration, which may significantly impact the BER.

In our work, we propose to jointly tune MRs and lasers wavelengths, which results in a global reduction of the optical channel power consumption (i.e. including laser and MRs), taking into account the temperature of the optical devices and the crosstalk, along with the laser output power change. Our method is also complementary to channel remapping, which allows the remapping of MRs wavelength to the closest (from an energy point of view) laser wavelength.

THERMAL SENSITIVITY OF NANOPHOTONIC

III. INTERCONNECTS

This section presents communication channels in Wavelength Division Multiplexing (WDM)-based optical interconnect and illustrates the problem we tackle in this paper. Finally, our proposed method is illustrated.

A. Optical Communications based on WDM

WDM-based silicon photonic interconnects allow multiple wavelengths to be utilized for communication channels. Figure 1 illustrates a MWSR-like architecture with on-chip lasers such as PCM-VCSELs, utilizing NWL wavelengths (λ0, λ1, … , λNWL-1). The light vertically emitted by the laser is coupled into a waveguide by using a taper (not shown in the figure for the sake of clarity). Multiple laser wavelengths are combined together into a single waveguide with a multiplexer ("MUX" for short in Figure 1), which can be implemented by a multimode interference (MMI) coupler [START_REF] Mandorlo | Controlled Multi-Wavelength Emission in Full CMOS Compatible Micro-Lasers for on Chip Interconnections[END_REF]. The wavelengths are equally distributed in order to reduce the crosstalk.

B. Network Interface and Thermal Variation

The Optical Network Interfaces (ONIs) are responsible for modulating and receiving the optical signal on the optical layer, as shown in Figure 1. The optical signal (supplied by on-chip lasers) is modulated by one of the writers and propagates to the reader for detection and conversion. An interface is composed of i) writers for modulating signal and ii) one reader for detecting and receiving signal. The writer is composed of NWL MRs. The reader uses passive MRs and photodetectors to filter and detect the expected signal respectively.

In the architecture example illustrated in Figure 1, we assume 4 interfaces, meaning that there are 4 MWSR channels. In this example, we also assume a single waveguide per channel. There are thus 3 writers and 1 reader per channel, each writer being composed of NWL modulators and the reader being composed of NWL photodetector/passive MR couples. For proper communications, it is mandatory that the wavelengths of the laser sources are well aligned with the resonant wavelengths of the MRs (in the writers) and those of the passive MR (in the reader). However, silicon based optical devices are sensitive to thermal variation (0.1nm/°C typically [START_REF] Li | Reliability Modeling and Management of Nanophotonic On-Chip Networks[END_REF]), which induces the mismatch of the wavelengths of silicon photonic devices along one communication channel. As a result, the optical signal power received at the reader side decreases, which results in a lower SNR and a higher BER. In addition, the efficiency of on-chip lasers is reduced when the temperature increases [START_REF] Li | Thermal Aware Design Method for VCSEL-based On-Chip Optical Interconnect[END_REF], i.e., the BER is further degraded. In order to deal with this issue, we propose a thermal-aware tuning method to ensure a targeted BER at the reader side while minimizing the power consumption.

bundle of TSVs

C. Signals and MRs Wavelengths Alignment

Figure 2 illustrates our method in the context of a MWSR channel [START_REF] Vantrease | Corona: System Implications of Emerging Nanophotonic Technology[END_REF] with a single laser source, 2 writers and one reader. In our work, we consider distributed lasers that are located in the same layer with MRs, waveguide and photodetectors. For this purpose, we assume the use of CMOS-compatible laser sources such as VCSELs, for which the size is similar to the one of MRs [START_REF] Li | Thermal Aware Design Method for VCSEL-based On-Chip Optical Interconnect[END_REF]. As illustrated in Figure 2-a, ONIm communicates with ONIr: the MR in the intermediate ONIi is turned OFF while the MR in ONIm is in the modulation state (state OFF and ON to modulate data '1' and '0' respectively).

Figure 2-b illustrates the ideal transmission of a data '1' that occurs when there is no temperature variation along the communication path. The optical signal injected by the laser (which is characterized by a power OPideal and a wavelength λlaser) crosses ONIm and ONIi, propagates along the waveguide until ONIr where it is dropped to the photodetector (as illustrated by the blue transmission lines in the Figure 2-b). The power of the optical signal decreases along the path due to the waveguide propagation losses and the MRs crossing losses (blue line in Figure 2-b). The BER is estimated based on the received optical power OPpd,ideal, the receiver sensitivity and the crosstalk (induced by other transmitting signals at different wavelengths, not illustrated here for the sake of clarity but taken into account in our models).

1) Existing MR Tuning Only Method

In case of temperature gradient over the communication path, the resonant wavelength of the MRs will drift (see the red transmission lines in Figure 2-c) while the wavelength of the emitted signal (λlaser) remains the same in case off-chip lasers are considered. Without compensating the effect of this drift, the misalignment between the signal wavelength and the MRs resonant wavelengths leads to significantly increased BER. To overcome this effect, the MRs along the waveguide are tuned back to their initial positions (the grey transmission lines in Figure 2-c) using thermal tuning or voltage tuning. The posttuning signal transmission is illustrated by the blue line in Figure 2-c: the received optical power is slightly lower than in the ideal scenario due to marginal wavelengths misalignment. The MRs tuning power depends on their temperature drift (∆TMR,m, ∆TMR,i and ∆TMR,r) and the thermal sensitivity coefficient ρMR. The total power consumption of the channel is given by the sum of the laser power consumption Plaser and the MRs tuning power ∑PMR.

2) Proposed Laser and MR Tuning Method

The key novelty of our method relies on the laser bias current tuning: since the laser temperature varies with its bias current, the wavelength of the optical signal can be tuned. Hence, in addition to tuning the MRs to align the resonant wavelengths with the optical signal, we also tune the wavelength of the optical signal itself, which contributes to reduce the power required to compensate the thermal variation.

As illustrated in Figure 2-d, tuning the driver current Ilaser has an impact on the laser power consumption and the emitted signal wavelength: the former varies from Plaser to P'laser and the latter shifts from λlaser to λ'laser. Therefore, under the same temperature gradient considered in Figure 2-c, the MRs wavelengths need to be tuned to λ'laser, instead of λlaser (see the green transmission lines in Figure 2-d). The MRs tuning power decreases since the wavelengths distance is reduced. As a drawback, the power of the emitted signal is reduced, meaning that a tradeoff needs to be defined in order to reach the target BER while decreasing the total power consumption of the channel. It is worth noticing that, although the method is illustrated with a 1-wavelength channel, it is generic and it can be applied to WDM channels, as illustrated in the results section. Moreover, it is complementary to related methods providing channel remapping [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in WDM-Based Optical Networks-on-Chip[END_REF][15] that we have adapted to minimize the tuning power instead of the

ANALYTICAL MODELS IV.

This section presents the analytical models used for BER and tuning power estimations.

A. MWSR Channel

Figure 3 illustrates a generic MWSR channel with lasers and NONI interfaces (NONI-1 writers and 1 reader). NWL lasers inject the optical signals, i.e., NWL wavelengths can be used for communicating within the MWSR channel. In the figure, the signal modulation occurs in ONI0 (i.e., the first writer). In the following, we define analytical models to evaluate i) the MWSR channel communication quality through a BER estimation and ii) the tuning power consumption (Ptuning) required to align the wavelengths of the optical signals with the MRs.

B. Bit Error Ratio (BER) Models

In Figure 3, the optical signals modulated with data dj propagate through the intermediate writers (i.e., ONIi, i=1, …, NONI-2) until reaching the reader, i.e., the targeted (NONI-1) th ONI. The optical power received by the photodetector at λj (

j d j pd OP , , ) is composed of i) the expected optical signal at λj ( j d j signal OP , ,
) and ii) the crosstalk (

j d j crosstalk OP , ,
) from the other signals at λk (where k=0, 1, … , NWL-1, and k≠j). Following [START_REF] Li | Complementary Communication Path for Energy Efficient On-Chip Optical Interconnects[END_REF], the signal transmission is thus composed by i) the transmission through the writers (Tw) and ii) the transmission through the reader part (Tr). Hence, the received optical power is defined as:

) ] [ ] [ ] [ ( 1 0 , , , ,       WL k j N k laser j r d w d j pd k OP k T k T OP (1)
The expected received signal power is:

] [ ] [ ] [ , , , , j OP j T j T OP laser j r d w d j signal j j    (2)
And the received crosstalk is:

) ] [ ] [ ] [ ( 1 0 , , , ,       WL k j N k laser j r d w d j crosstalk k OP k T k T OP (k≠j) (3)
We consider the signal at wavelength λk received by the j th photodetector as a general case. The transmission in the writers

part ( ] [ , k T k d w ) and the reader part ( ] [ , k T j r
) are detailed as following equations.

)) , ( ( )) , ( ( ) ( ) ( ] [ 1 0 1 1 0 ] [ ] [ , , ,                      n n k t N n N n k t N n k l p k N b d w d L L k T WL W WL total w b w k (4) ) , ( ) , ( ) ( ) ( ] [ 1 0 ] [ ] [ , , , j k d n k t j n k l p k N b j r to ta l r b r L L k T           (5)
Here, φt and φd are the signal transmission on MR through port and drop port [START_REF] Li | Complementary Communication Path for Energy Efficient On-Chip Optical Interconnects[END_REF]. Lb and Lp are the bending loss and the propagation loss, which are assumed to be 0dB and 0.5dB/cm in this work. lw,total[k] and lr,total[k] (resp. Nw,b[k] and Nr,b[k]) are the total waveguide length (resp. number of waveguide bends) experienced by signal at λk in writers and reader parts. ∆λ is the MR wavelength shift between ON and OFF states.

On the reader side, the receiver sensitivity gives the minimum optical signal power that can be detected by a photodetector. For a given receiver sensitivity, the SNR can be calculated as follow.

n j pd j pd i OP OP SNR ) ( 0 , , 1 , ,     (6)
where in is the photodetector internal noise (4uA [START_REF] Vivien | Zero-bias 40Gbit/s germanium waveguide photodetector on silicon[END_REF] in this work), R. is the responsivity of the photodetector (1A/W in this work). OPpd,j,1 and OPpd,j,0 is the optical power received for the modulation of data "1" and "0" respectively. The worst-case SNR is estimated by considering

OP pd,j,1 =OP signal,j,1 (7)
OP pd,j,0 =OP crosstalk,j,0 (8)
The BER is obtained as following:

) 2 2 ( 2 1 SNR erfc BER  (9)

C. Power Models

The total tuning power consumption of a MWSR channel is divided into two parts: i) the lasers power consumption (Plaser) and ii) the MRs tuning power (PMR) as in equations ( 10), [START_REF] Condrat | Thermal-aware Synthesis of Integrated Photonic Ring Resonators[END_REF] and [START_REF] Padmaraju | Thermal stabilization of a microring modulator using feedback control[END_REF]. Table 1 summarizes the parameters involved in the model and the following details the lasers and MR models we assume.

1) PCM-VCSEL for On-Chip Laser Sources

In this work, we consider the PCM-VCSELs (illustrated in Figure 4-a) as on-chip laser sources. They rely on a double set of Si/SiO2 photonic crystal mirrors (PCMs). PCM-VCSELs are considered due to their micrometerscale layer thickness (thinner than VCSELs using DBR), OP pd,0,d0 OP pd,j,dj OP pd,NWL-1,dNWL-1

OP laser [N WL -1] writers (Transmission: T w ) reader (Transmission: T r )

OP laser [j] OP laser [0] their broadband reflectivity, full control over the cavity modal and polarization emission features [START_REF] Sciancalepore | Thermal, Modal, and Polarization Features of Double Photonic Crystal Vertical-Cavity Surface-Emitting Lasers[END_REF]. Moreover, PCM-VCSELs are CMOS compatible. The fabrication employs standard CMOS pilot line processing tools and high-yield full-wafer bonding of group III-V alloys on silicon [START_REF] Sciancalepore | Thermal, Modal, and Polarization Features of Double Photonic Crystal Vertical-Cavity Surface-Emitting Lasers[END_REF]. Coupling the vertical light from VCSEL into a horizontal waveguide can be achieved by using a taper located on the layer of the top PCM and the waveguide. We assume an 80% coupling efficiency, which is slightly pessimistic compared to the 85% simulated in [START_REF] Ohira | On-chip optical interconnection by using integrated III-V laser diode and photodetector with silicon waveguide[END_REF]. The signal wavelength (λlaser) can be tuned by changing the laser temperature (Tlaser) since the laser is sensitive to the thermal variation (assumed as 0.1nm/°C [START_REF] Sciancalepore | Thermal, Modal, and Polarization Features of Double Photonic Crystal Vertical-Cavity Surface-Emitting Lasers[END_REF]), as shown in Figure 4-b. Under a given driver current (Ilaser), the laser efficiency will decrease with an increase of the temperature (Tlaser), which leads to a reduction of the emitted optical signal (OPlaser), as shown in Figure 4-c. The relationship between the wavelength and the temperature is given as: λ laser,j =λ laser,j,room +∆λ laser,j [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in Optical Networks-on-Chip[END_REF] ∆λ laser,j =ρ laser •∆T laser,j [START_REF] Ye | System-Level Modeling and Analysis of Thermal Effects in WDM-Based Optical Networks-on-Chip[END_REF] ∆T laser,j =T laser,j -T room [START_REF] Georgas | Addressing Link-Level Design Tradeoffs for Integrated Photonic Interconnects[END_REF] Since the lasers are located above the processing layer, their temperature is influenced by i) the driver current (Ilaser) and ii) the chip activity. The laser power consumption (Plaser,j for a laser at λj) can be estimated as:

P laser,j =[slop ohm ×(I laser,j -I th )+1]×I laser,j -slop W/A ×(I laser,j -I th )(16)
where slopohm, slopW/A and Ith are the laser voltage slope, the output power slope and the threshold current, respec-

2) Microring Resonators (MRs)

The modulation is realized by electro-optic effect on the MRs. Forward biased is applied to perform voltage tuning, which leads to a blue shift of the resonant wavelength. MRs can be classified according to their junction, PN or PIN. With a PN junction, the reverse biasing changes the refractive index through carrier depletion while, with a PIN junction, the refractive index is changed by carrier injection. Although PN junctions allow a faster switching time compared to PIN junctions, the higher extinction ratio provided by PIN junction leads to better communications [START_REF] Li | Complementary Communication Path for Energy Efficient On-Chip Optical Interconnects[END_REF]. In this work, we thus consider the use of PIN junctions only. Since the MRs are sensitive to temperature, their resonant wavelength needs to be tuned to ensure a proper alignment of the signal wavelength λlaser,j with the MR wavelength λMR,i,j. The MR tuning power (PMR,i,j) depends on the distance between the wavelengths. MR tuning can be achieved by using electro-optic and thermo-optic effects [START_REF] Li | Active resonance wavelength stabilization for silicon microring resonators with an inresonator defect-state-absorption-based photodetector[END_REF], as illustrated in Figure 5 (monitor and feedback-control parts are not shown). The following details the tuning methods:

j i MR j laser j i j i MR TE P , , , , , ,      (17) j i MR MR room j MR,i j i MR T , , , , , ,        (18) room j i MR j i MR T T T    , , , , (19)
Voltage Tuning (VT) for blue shift: Voltage tuning is fast but its range (VTr) is limited to 1nm [START_REF] Li | Reliability Modeling and Management of Nanophotonic On-Chip Networks[END_REF]. We denote VTe as the voltage tuning efficiency (typically 0.13mW/nm [START_REF] Nitta | Addressing System-Level Trimming Issues in On-Chip Nanophotonic Networks[END_REF] [START_REF] Xu | Tolerating Process Variations in Nanophotonic On-chip Networks[END_REF]).

Thermal Tuning (TT) for red shift: the MR resonant wavelength can be red shifted by using local micro-heater [START_REF] Shen | Eight-channel reconfigurable microring filters with tunable frequency, extinction ratio and bandwidth[END_REF]. Thermal tuning is slower than voltage tuning but its operating range (TTr) can reach 20nm [START_REF] Gan | Maximizing the Thermo-Optic Tuning Range of Silicon Photonic Structures[END_REF]. The thermal tuning efficiency TTe is lower (typically 0.24mW/nm [START_REF] Nitta | Addressing System-Level Trimming Issues in On-Chip Nanophotonic Networks[END_REF] [START_REF] Xu | Tolerating Process Variations in Nanophotonic On-chip Networks[END_REF]).

D. Simulation Model

From the estimation of the tuning power and the BER, we define exploration strategies to i) reduce the power consumption by maintaining a targeted BER and ii) decrease the BER as much as possible and estimate the power cost. Both methods require tuning the laser bias current and will be further detailed in Section V-B.

To evaluate these methods, the temperature of the optical devices (i.e., on-chip lasers and MRs) along the communication channel and under a given chip activity is needed. For this purpose, IcTherm 1 [START_REF] Fourmigue | Efficient Transient Thermal Simulation of 3D ICs with Liquid-Cooling and Through Silicon Vias[END_REF] is used to provide thermal maps of the silicon photonic interconnect. Ic-Therm is a thermal simulator for electronic devices which accurately models their complex structure and provides 3D full-chip temperature maps. IcTherm solves the physical equations that govern the temperature in the chip, using the Finite Volume Method [START_REF] Chapra | Numerical Methods for Engineers[END_REF], a numerical method for solving partial differential equations. It was validated against the commercial simulator COMSOL [START_REF]COMSOL[END_REF]: its maximal error was found to be less than 1% [START_REF] Fourmigue | Efficient Transient Thermal Simulation of 3D ICs with Liquid-Cooling and Through Silicon Vias[END_REF]. The structure of the system is discretized into small cubic cells that match the distribution of the materials and the heat sources. 1 IcTherm website : http://www.ictherm.com/

RESULTS

V.

This section describes the system we consider and then evaluates the efficiency of the proposed method.

A. Case Study

Figure 6 shows the global view of the targeted system, which contains the following components: steel backplate, motherboard, socket, Intel's Single-Chip Cloud Computer (SCC chip) with silicon-photonic interconnects and on-chip laser sources, copper lid and heat sink. The considered 3D architecture is based on SCC (Figure 7-a) with a stacked optical layer [START_REF] Li | Thermal Aware Design Method for VCSEL-based On-Chip Optical Interconnect[END_REF]. Figure 7-b shows the abstract layout of SCC we use for the considered architecture layout. Following the method described in [START_REF] Li | Thermal Aware Design Method for VCSEL-based On-Chip Optical Interconnect[END_REF], the silicon photonic interconnect is placed on top of the SCC chip and thermal simulations are performed using IcTherm [START_REF] Fourmigue | Efficient Transient Thermal Simulation of 3D ICs with Liquid-Cooling and Through Silicon Vias[END_REF]. From the resulting thermal map, the proposed laser tuning method is applied in order to i) evaluate the BER using the transmission models from [START_REF] Li | Complementary Communication Path for Energy Efficient On-Chip Optical Interconnects[END_REF] and ii) evaluate the total channel power consumption Ptuning (including the laser power consumption Plaser and the MR tuning power PMR). We assume 4 architectures including 4, 6, 8 and 12 interfaces, as illustrated in Figure 8. The number of interfaces gives the number of MWSR channels. In the figure, a single waveguide per channel is represented and only parts of the channels are illustrated for the 12 interfaces architecture for the sake of clarity. For comparison purpose, we assume architectures with on-chip lasers (Figure 8 a-d) and off-chip lasers (Figure 8 e-h). On-chip lasers are placed out of the interface in order to limit the temperature variation of the MRs as the laser driver current is tuned. The off-chip lasers are placed all around the chip in order to reduce the additional-insertion losses (i.e., from waveguide propagation and waveguide crossing.

a) SCC b) Abstract SCC layout a) b) c) d) e) f) g) h)
Figure 9 illustrates photonic layer temperature maps for the layout given in Figure 8-a, assuming 4 waveguides per MWSR channel and 16 wavelengths per waveguide (i.e., there is a total of 64 laser sources per channel). DDR3 memory controllers and PLL have been shut down to highlight the impact of chip activity and laser power consumption on the temperature. In order to illustrate the variety of application to be executed, we consider three values for the laser power consumption (4mW, 8mW, and 12mW) and three chip activities (uniform 10%, diagonal 25%-50% and corner 50%-5%). This figure highlights the important thermal gradient in the lasers region. It is worth noticing that, by considering the VCSELs described in Figure 4, only very limited lasing effect is obtained for temperature above 80°C. Such scenario occurs for a 50% local activity and 12mW laser power consumption (see the red hotspot region). The impact of the chip activity and the laser power consumption on the BER will be investigated in the following.

B. Impact of the Laser Bias Current on the BER and the Power Consumption

We evaluate our method for a 12 interfaces architecture (i.e., 12 MWSR channels with 11 writers and 1 reader per channel), 1 waveguide and 4 wavelengths (i.e., 4 lasers per waveguide and 4 MRs per writer). The study focuses on a single MWSR channel. We run thermal simulations under uniform 15% and 20% chip activity (i.e., 18.75W and 25W, respectively), with the tuning current Ilaser ranging from 0mA to 16mA. Figure 10 represents the laser temperature drift ∆Tlaser and the optical output power OPlaser deduced from the resulting thermal maps. It is worth noticing that, compared to Figure 4-c, a much higher sensitivity to the driver current is obtained due to the locally dissipated energy, which contributes to the temperature elevation. Indeed, Figure 4-c corresponds to characterization results obtained under a temperature stabilized using a Peltier cooling system. In Figure 10, the laser response is simulated in a 3D integrated circuit without cooling system.

For Ilaser=0.05mA under uniform 15% chip activity, ∆Tlaser = 26°C and there is no lasing effect (i.e. OPla-ser=0mW) since the lasing current threshold for the corresponding temperature is not reached. Lasing effect starts at Ilaser= 1mA and a maximum of 0.65mW optical power is obtained for 8mA. For this current value, the laser drift temperature is 36.8°C. Above 8mA, the laser efficiency drastically decreases due to the temperature elevation. From the laser only point of view, the most relevant Ilaser current values ranges from 1mA to 8mA. However, from the whole MWSR channel point of view, current values above 8mA can also be relevant since a drift of the signal wavelengths accompanies the laser temperature elevation. Depending on the context, this may allow the reduction of the power needed to tune the MRs on the MWSR channel. For 20% chip activity, a similar trend is observed but ∆Tlaser is slightly higher, which in turns leads to a reduction of the outputted light.

1) Results for Uniform Chip Activities

From the thermal maps over the optical layer, with a laser power consumption Plaser ranging from 1mW to 22mW (obtained by tuning the laser injection current Ilaser), we evaluate the total power required to align the MRs resonant wavelengths with the emitted signals. We also evaluate the worst-case BER among all the channels in the network, as illustrated in Figure 11. For 20% chip activity (in red), for small laser injection current (i.e., Plaser ranging from 1mW to 6mW), the optical power of the emitted signal is too low to compensate the channel losses, which results in a high BER. Then, the communication quality improves as the laser injection current increases: the BER reaches its optimal value at Plaser=9mW. By considering a static design method, the corresponding injection current will be selected to ensure the best communication quality. Above 9mW, the laser efficiency decreases due to the increase of the local temperature, leading to a higher BER. The channel power consumption is also given in the figure. It is composed of the laser power consumption (simply reported from the x-axis) and the MRs tuning power.

Figure 11 gives the tuning power consumption and the BER under 20% chip activity (in red). When the chip activity drops to 15% (in blue), two methods can be followed:

 To achieve a given BER (e.g., 10 -12 ), the laser current can be reduced to minimize the tuning power, as represented by ab arrow in Figure 11. In this example, the tuning power drops from 10.1mW to 7.5mW, i.e., 26% reduction.  To minimize the BER, the laser current is slightly increased, as illustrated by cd arrow in Figure 11. In this example, the tuning power increases from 12.9mW to 14.4mW (i.e., 12% power cost). However, the BER remains optimal and decreases from 10 -16 to 10 -28 (for 20% and 15% chip activity respectively). As reflected in the thermal maps, the gradient temperature within each interface remains below 1°C. The total tuning power of each MWSR channel can thus be obtained.

Simulations results show that the targeted 10 -12 BER couldn't be reached for chip activity higher than 25%, independently from the selected laser power consumption. This limitation is directly related to the thermal sensitivity of the laser, for which the efficiency drops for temperature above 40°C. The laser efficiency is further explored in Section V-C.

2) Results for Diagonal Chip Activities

We also run thermal simulations for 5%-25% and 25%-5% diagonal chip activities. Figure 12 gives the results for the MWSR channel with laser sources located on the topleft hand side of the chip (i.e., blue channel in Figure 8-d). 5mW and 6mW laser power consumption allow the total power consumption (while still achieving 10 -12 BER) to be minimized for 5%-25% and 25%-5% activities, respectively. As expected, the maximum reachable BER is better for 5%-25% activity due to the lower heat locally dissipated by the chip (i.e., the optical signals outputted by the lasers is higher). An interesting trend is the slightly lower MRs tuning for 5%-25% activity, which is due to a reduced distance between the MRs resonant wavelengths and the laser wavelengths. While this power saving only slightly influences the total channel energy figures, significant gains are reachable for larger chip activity gradient, which is evaluated in the following.

3) Results for Corner Chip Activities

Figure 13 illustrates the total tuning power and the BER under a) 15%-65%, b) 15%-75% and c) 15%-85% corner activities. In this scenario, we only consider the MWSR channel with laser sources located in the 15% chip activity region since an insufficient lasing effect is obtained for lasers located in the region with higher activity.

In Figure 13-a, for Plaser=1mW, 30mW are required to align the MRs resonant wavelengths with the, significantly distant, optical signals. Increasing Plaser to 2mW leads to a reduction of the wavelengths distance, which in turn helps reducing PMR to 20mW. PMR continues shrinking until wavelengths are aligned, which is obtained for Pla-ser=5mW (mark a in the figure). This value also corresponds to the minimum total tuning power allowing the targeted 10 -12 BER to be reached. The optimal BER is obtained for Plaser=11mW (mark b). In Figure 13-b, the targeted BER is obtained starting from Plaser=6mW. However, for this value, the wavelengths distance to compensate is still significant and leads to PMR=4mW. By increasing Plaser to 7mW (mark c), 3mW can be saved on the MRs tuning, which leads to a more power efficient solution. This corresponds to the scenario sketched in Figure 2. Due to a slightly higher temperature, the optimal BER is obtained for Plaser=10mW (mark d). The same trend can be observed in Figure 13-c: Plaser=8mW allows the BER requirements to be reached but Plaser=9mW is a globally more power efficient solution. Furthermore, Plaser=9mW also leads to the optimal BER (mark e), thus demonstrating the potential for adaptive laser tuning methods to maximize the energy efficiency of nanophotonic interconnects. Finally, a slight 2% increase of the corner gradient activity (i.e., 15%-87%, not shown in the figure) will avoid reaching the targeted BER for Plaser=11mW (which leads to the optimal BER in Figure 13-a, while Plaser=9mW still achieves it. This demonstrates that for the considered system, an adaptive method relying on laser tuning not only reduces the energy, but also helps covering a wider range of chip activities.

C. Laser Efficiency Comparison

In this section, we consider two laser efficiencies and we evaluate their impact on the tuning power. Figure 14-a represents a conservative and an aggressive scenario. The conservative scenario corresponds to the laser already detailed in Figure 4 and the aggressive one is obtained by considering a x2 laser efficiency. The tuning power is illustrated in Figure 14-b for uniform chip activities ranging from 5% to 20%, where we assume 12 interfaces. The targeted BER is set to 10 -12 , which requires less energy with aggressive scenario since the receiver optical power is higher for a same bias current.

Hence, for a 5% chip activity, Ptuning is reduced from 6.4mW (conservative) to 3.6mW (aggressive). The improvement of the laser efficiency leads to an increase of the ratio of the MR tuning in the channel total power consumption. For instance, for 5% chip activity, the MR tuning power ratio increases from 37% to 45%. However, due to the higher efficiency of the laser, tuning the laser becomes more efficient than tuning the MRs, which leads to a reduction of the MRs tuning power (i.e., 2.36mW and 1.62mW for conservative and aggressive scenario respectively). The combined reduction of laser tuning and MRs tuning leads to significant global energy saving, e.g., 43% for 5% chip activity. Further power saving are obtained for higher chip activities (e.g., 54% under 20% chip activity). These results would help investigating laser requirements for ultra-low power silicon photonics interconnects.

D. Tuning Methods Comparison

We compare our method with related solutions in which only MR tuning is used. We consider related solutions relying on off-chip and on-chip lasers. For off-chip lasers, the laser wavelength is fixed and the resonant wavelengths of the MRs along a given channel may need to be tuned back to be aligned with the laser, considering channel remapping. We also take into account waveguide crossing losses (considered as 0.05dB per crossing), assuming a single waveguide is used per channel. We assume the same characteristics for off-chip and on-chip lasers and a 80% coupling efficiency has been considered, which is slightly optimistic compared to the 74% demonstrated in [START_REF] Sacher | Wide bandwidth and high coupling efficiency Si3N4-on-SOI dual-level grating coupler[END_REF]. For on-chip lasers with MR tuning only method, we take into account the wavelength drift of the emitted optical signal due to the temperature variation to evaluate the energy needed to tune the MRs. With our method, the laser bias current Ilaser is tuned.

We consider a 12 interfaces architecture with 4 to 16 wavelengths per MWSR channel and a 10Gb/s data rate. The initial chip activity is assumed to be 10% and the laser power consumption is set to achieve a given BER (e.g., 10 -12 ). Figure 15 illustrates the contributions of the laser and the MR tuning power (without considering the modulation) for 4 to 16 wavelengths. Our method is the most power efficient for all the studied cases. For instance, for 4 wavelengths MWSR channels, we reach 2.5pJ/bit wrt. 3.1pJ/bit and 10.7pJ/bit for related on-chip and off-chip methods respectively. When the number of wavelength increases, additional energy is needed since the number of MRs considerably increases. The results show that, for 16 wavelengths, the optical channels using off-chip lasers give comparable power results to our method. It is also worth noticing that the results obtained for the off-chip lasers method are optimistic for 3 reasons. First, we assume off-chip lasers working at a constant 25°C room temperature, which requires an energy consuming cooling system not taken into account here. Second, we assume a single waveguide per MWSR channel, which leads to at least 11 waveguide crossing for a 12 interfaces architecture illustrated in Figure 8-h (e.g., it leads to 0.55dB losses on each waveguide). MWSR channels with more than 4 waveguides are regularly considered in the literature, which would significantly increase the losses. Third, the propagation loss due to the longer waveguides has not been considered. However, additional scenarios, including application dependant chip activities, should be considered to further consolidate this comparison. To further compare our method with the MR tuning only method, we also evaluate for which number of wavelengths the BER is the best. For this purpose, we lower as much as possible the BER when the chip activity decreases from 20% to 15% and we evaluate the required energy. We assume 12 interfaces and a number of wavelengths ranging from 4 to 16. As shown in Figure 16, the energy/bit remains slightly constant with the number of wavelengths (approx. 1.2 pJ/bit). However, the BER significantly increases from 10 -28 (4 wavelengths) to 10 -10 (16 wavelengths), i.e., the communication quality is better for small number of wavelengths. Such method allows the maintaining of a given BER level to satisfy applications constrained by communication quality. Overall, these results show that the MWSR channels with a small number of wavelengths provide a better BER. This leads us to

BER

conclude that our method tends to be the most power efficient for this range of wavelengths.

E. Energy Saving under a Given BER

We evaluate the potential gain of the proposed laser tuning method with the traditional on-chip laser (for which only MRs tuning is possible). For this purpose, we first set the laser power consumption to 9mW in order to reach a targeted 10 -12 BER for the highest considered chip activity (25% in this example). We then reduce the chip activity to 20%, 15% and 10%, and we evaluate the achievable power reduction while still reaching 10 -12 BER [START_REF] Petracca | Design Exploration of Optical Interconnection Networks for Chip Multiprocessors[END_REF] [START_REF] Li | Spectrum: A Hybrid Nanophotonic-Electric On-Chip Network[END_REF]. As illustrated in Table 2, up to 42% reduction is obtained for a 10% chip activity. In case a 10 -9 BER [START_REF] Xie | Crosstalk Noise and Bit Error Rate Analysis for Optical Network-on-Chip[END_REF][40] turns to be acceptable (i.e., in order to match the requirements of an application to be executed), the energy consumption can be further decreased. These results demonstrate the significant energy reductions achievable by using chip activity-aware methods. We evaluate the energy saving of our method for various numbers of interfaces and wavelengths and by assuming the same technique as previously described. As shown in Figure 17, the energy saving is the smallest for 12 interfaces and 16 wavelengths but it still reaches 31% wrt. the MRs tuning only method. On average, 53% energy saving is obtained.

CONCLUSION VI.

In this paper, we propose to jointly tune lasers and MRs to improve the energy efficiency of fully integrated nanophotonic interconnects. For this purpose, we have defined a method relying on thermal simulations and crosstalk analyses, taking into account the thermal sensitivity of the laser sources. Evaluations have been carried out for a 3D stacked architecture interconnecting processors with MWSR-like optical channels. We have assumed CMOS compatible PCM-VCSELs for the light sources and the layout of the optical layer has been designed to avoid waveguide crossing. Design space exploration covered the number of interfaces, the number of wavelengths and the laser efficiency, taking into account uniform, diagonal and corner chip activities.

Compared to methods for which laser tuning is not possible, results show that a combined tuning of laser and MRs leads to 53% energy reduction when the uniform chip activity decreases from 20% to 5%. BER-energy tradeoffs have been explored and allow strategies to be defined to minimize either the energy, or the BER. As a key result, we have shown that, under specific chip activities, increasing the laser power consumption allows both energy and BER to be improved. This trend has been observed for a MWSR channel interconnecting 12 interfaces. We also showed that, by being able to tune the laser power within the 7-13mW ranges, the targeted 10 -12 BER is reachable in all the scenarios we covered, assuming a maximum 20% chip activity in the region where the lasers are located. This strong limitation directly depends on the laser efficiency, which drastically decreases above 40°C. Major technological improvements are thus needed to make silicon photonics becoming a realistic and viable solution for on-chip interconnects in fully integrated 3D architectures. While such improvement would lead to more energy efficient optical channels by reducing the laser power consumption, it is worth noticing that our joint laser and MRs tuning method would further contribute to this objective by also reducing the MRs tuning power.

A major challenge for the practical use of our method at run-time is the calibration process, which has to rapidly explore lasers and MRs tuning options. For this purpose, transient thermal simulations are mandatory to optimize the controller, which will be investigated in the near future. In parallel, we are currently investigating whether layout optimization contributes to improve the energy efficiency of nanophotonic interconnects. Together with the run-time calibration process, one can define novel group tuning methods for lasers that could ideally complement the already existing group tuning for MRs. Finally, our method is generic and can be applied to other types of light sources, included on-chip and off-chip lasers. This leads to a need for comparisons we will achieve in the future.

Figure 1 :

 1 Figure 1: MWSR based optical interconnect with on-chip lasers.

Figure 2 :

 2 Figure 2: a) MWSR channel with one wavelength and two writers, b) transmission without thermal variation (ideal case), c) transmission considering MR tuning only method with off-chip laser sources, and d) transmission with our proposed on-chip laser tuning method.

Figure 3 :

 3 Figure 3: Generic MWSR channel.

P

  MR,NONI-1,0 P MR,NONI-1,j P MR,NONI-1,NWL-1

Figure 4 :

 4 Figure 4: PCM-VCSEL: a) 3D view extracted from [26] and the cross-section view including the taper, b) wavelength drift of VCSEL (∆λ laser ) according to temperature drift (∆T laser ) considering temperature sensitivity (ρ laser ) as 0.1nm/°C, and c) output power of laser (OP laser ) wrt. T laser and I laser . Curves b) and c) are obtained by extrapolation of data from [26].

Figure 5 :

 5 Figure 5: Top-view schematic of a PIN MR-based modulator with integrated voltage tuner and thermal tuner, inspired from [31].

Figure 6 :

 6 Figure 6: Packaging of the SCC chip and the optical interconnect.

Figure 7 :

 7 Figure 7: Considered electrical layer: a) SCC and b) its abstract layout.

Figure 8 :

 8 Figure 8: Considered interconnect layouts including 4, 6, 8 and 12 interfaces with a-d) on-chip lasers and e-h) off-chip lasers.

Figure 9 :

 9 Figure 9: 2D temperature maps of the photonic layer for various chip activities (i.e., uniform 10%, diagonal 25%-50% and corner 50%-5%) and laser power consumption (i.e., 4mW, 8mW, and 12mW).

Figure 10 :

 10 Figure 10: Impact of I laser on the optical output power (OP laser ) and the laser temperature drift (∆T laser ) under uniform 15% and 20% chip activities.

Figure 11 :

 11 Figure 11: Tuning power (P tuning ) and BER for a MWSR channel with 12 interfaces, 4 wavelengths per waveguide under 15% and 20% uniform chip activities. FSR=59nm, m=17, voltage tuning efficiency VTe=0.13mW/nm, thermal tuning efficiency TTe=0.24mW/nm [22][35].

Figure 12 :

 12 Figure 12: P tuning and BER under 5%-25% and 25%-5% diagonal activities. Interconnect and technologies assumptions are those used in Figure 11.

Figure 13 :

 13 Figure 13: P tuning and BER under a) 15%-65%, b) 15%-75% and c) 15%-85% corner chip activities. Interconnect and technologies assumptions are those used in Figure 11.

Figure 14 :

 14 Figure 14: a) Laser efficiency for conservative and aggressive scenarios and b) P tuning for 5%, 10%, 15% and 20% chip activities. Interconnect and technologies assumptions are those used in Figure 11.

Figure 15 :

 15 Figure 15: Channel energy efficiency w and w/o laser tuning for 12 interfaces. FSR=59nm, m=17, VTe=0.13mW/nm, TTe=0.24mW/nm [22][35].

Figure 16 :

 16 Figure 16: Design tradeoff between BER and energy efficiency for 12 interfaces.

Figure 17 :

 17 Figure 17: Energy saving for our joint laser and MR tuning method wrt. MR tuning only method for different channel configurations.

Electrical layer IP core ONI 1 ONI 2 ONI 3 ONI 4 On-chip lasers reader Optical layer writer Waveguide MRs ON Photodetector On-chip laser OFF MUX

TABLE 1 : TECHNOLOGICAL PARAMETERS

 1

	Parameters Description	Unit
	NONI	Number of ONIs in the network
	NWL	Number of wavelengths per interface
	PMR,i,j	Tuning power of the MRj in ONIi	mW
	Plaser,j	Power consumption of laserj	mW
	TEi,j	Tuning efficiency (voltage tuning or thermal	mW/n
		tuning) of the MRi in ONIj	m
	λlaser,j	Wavelength of laser corresponding to the	nm
		wavelengthj after tuning
	λMR,i,j	Wavelength of MRj in ONIi considering the	nm
		wavelength drift
	∆λlaser,j	Wavelength drift of laserj	nm
	λlaser,j,room	Wavelength of laserj at room temperature	nm
	λMR,i.j,room	Wavelength of MRj in ONIi at room tempera-	nm
		ture
	ρMR, ρlaser	Temperature sensitivity factor of MR and laser	nm/°C
		wavelengths
	∆TMR,i,j,	Temperature drift of MRj in ONIi and laserj	°C
	∆Tlaser,j
	TMR,i,j,	Temperature of the MRj in ONIi and laserj	°C
	Tlaser,j
	Troom	Room temperature as reference	°C
	FSR	Free Spectral Range	nm
	m	Resonant mode number of MR

MR,i,j ) Waveguide Tuning power: P MR,i,j ∝│λ laser,j -λ MR,i,j │

		p +	p +	Voltage tuner
	Microring	p -	p +	n +
	Resonator			Thermal tuner
	Optical SoC Substrate (λ Motherboard	Heat Sink Fins Fan Heat Sink Base	Back Plate Socket Copper LID 1 cm
					Copper LID (2 mm)
					TIM (75 µm)
					Optical Layer (~4 µm)
					Silicon (50 µm)
					Silicon (50 µm)	TSV (ø5 µm)
					Metal Layers (15 µm)
				Optical SoC
				(355 µm)	Bonding Layer (20 µm)
					Silicon Interposer
					(200 µm)
				C4	Epoxy (80 µm)
					Substrate (1 mm)

TABLE 2 : ENERGY CONSUMPTION REDUCTION: W/ LASER TUN- ING WRT. W/O LASER TUNING

 2

	Chip activity Target BER	20%	15%	10%
	10 -12	21%	42%	42%
	10 -9	32%	42%	52%

Gabriela Nicolescu obtained her B. Sc. A and her MSc degree from Politechnica Bucharest. She obtained her PhD degree, in 2002, from INPG (Institut National Polytechnique de Grenoble) in France. She has been working at Ecole Polytechnique de Montréal (Canada) since 2003, where she is a professor in the Computer and Software Engineering Department. Her research interests are in the field of design methodologies, programming and security for systems with advanced technologies, such as 3D multiprocessor systems-on-chip integrating liquid cooling and optical networks. She published five books and she is the author of more than hundred articles in journals, international conferences and book chapters.

ACKNOWLEDGMENTS

The authors would like to thank the editor and anonymous reviewers for their valuable feedback. The authors also gratefully acknowledge Xavier Letartre from INL lab for the fruitful discussions. Hui LI is supported by China Scholarship Council (CSC).


Mauro Avila

Katrin Wolf
email: katrin.wolf@acm.org

Anke Brock
email: anke.brock@inria.fr

Niels Henze

Remote Assistance for Blind Users in Daily Life: A Survey about Be My Eyes

Keywords: Human-centered computing → Accessibility systems and tools, Visual impairment, remote assistance, mobile interaction

The mobile application Be My Eyes connects blind users with sighted volunteers through a video and audio connection. In contrast to previous attempts, Be My Eyes has a large number of blind and sighted users. In this paper we report from a survey among 30 blind Be My Eyes users to guide future work in this domain. We describe for which situations blind users find Be My Eyes useful and which are challenging. Furthermore, we identify users' concerns and areas for improvements. Finally, we derive directions for future work.

INTRODUCTION AND BACKGROUND

People with visual impairments face a large number of challenges that today cannot be addressed by assistive technology alone [START_REF] Thinus-Blanc | Representation of space in blind persons: vision as a spatial sense?[END_REF][START_REF] Golledge | Cognitive mapping and wayfinding by adults without vision[END_REF]. Bigham et al. [START_REF] Bigham | Human-backed access technology[END_REF] argue that intelligent technology can be made more useful in everyday life by involving human assistance. A number of attempts have been made to connect blind people with remote assistance from sighted persons. VizWiz [START_REF] Bigham | Vizwiz: nearly real-time answers to visual questions[END_REF], for example, is an audiobased mobile app that provides blind people with descriptions of photographs they took with their smartphone. Multiple workers are recruited for a small amounts of money to answer questions in a short amount of time. Similarly, the mobile apps Camfind and Taptapsee [START_REF] Holton | A review of the taptapsee, camfind, and talking goggles object identification apps for the iphone[END_REF] provide help using both, human operators and matching images to an existing database. Crowdviz [START_REF] Holton | Crowdviz: Remote video assistance on your iphone[END_REF] directly connects visually impaired users with paid and trained sighted persons through a smartphone-based video and audio connection. Recently, ACM ISBN 000-0000-00-000/00/00. DOI: 00.000/000 0 the conceptually similar iPhone application Be My Eyes became available 1 .

Be My Eyes connects blind persons with untrained volunteers through a free of charge service. According to the developers, Be My Eyes had almost 25,000 blind and over 300,000 sighted users in December 2015. We believe that blind users' benefits, the challenges they experience, and the services' current limitations can provide important insights for the development of future accessibility tools. Therefore, we conducted a survey among visually impaired users of Be my Eyes. In this paper, we provide insights about how the service is used and perceived by visually impaired users aiming to gain a deeper understanding about remote assistance systems. In the following, we first describe the design of the survey and the collected results. Finally, we discuss the findings and conclude with design recommendations for using members of social networks as human assistants to help visually impaired people in solving daily tasks.

DESIGN OF THE SURVEY

We conducted an online survey to learn how Be My Eyes is used and to identify its qualities and limitations.

In the first part of the survey, we asked participants for what situations they find Be My Eyes useful. We provided the following six common situations where visual assistance typically is needed: (1) Reading text, (2) Finding things, (3) Wayfinding and navigation, (4) Shopping assistance, (5) Using ticket machines, (6) Using ATM machines.

Participants rated each of the situations through the statement "I think Be My Eyes is useful when..." on a 5-item Likert scale from 1=totally agree to 5=totally disagree. Furthermore, we asked participants to rate whether Be My Eyes was useful when [START_REF] Thinus-Blanc | Representation of space in blind persons: vision as a spatial sense?[END_REF] The hands are busy or carrying things using the same Likert scale. We were interested in this question, because blind people are often carrying a cane in one hand and thus, have busy hands. The second part of the survey contained the following five open questions about the application: [START_REF] Bigham | Human-backed access technology[END_REF] In which other situations is the application useful? (2) What do you like most about Be My Eyes? (3) What do you most dislike about Be My Eyes? (4) How do you consider Be My Eyes could be improved? (5) Feel free to provide further comments about Be My Eyes.

Furthermore, participants were asked how often they use the app and how long each session lasts. Finally, we collected demographic data, including gender, age range, country of residence, and level of visual impairment.

The survey was published using our LimeSurvey server in English, Spanish and German. The survey was spread in social media channels for blind people, including email lists and Facebook. 15 females and 15 males completed the questionnaires. 80% were totally blind, and the others considered themselves as legally blind. 5 participants (17%) were aged between 18 and 25 years, 6 (20%) between 26 and 35, 10 (33%) between 36 and 50 years, and 9 (30%) between 51 and 65 years. 50% of the participants were from the US, the rest were from many countries over the world including Australia, Iran, Germany, and countries in South America.

RESULTS

Our results showed that 13% of the participants have used the app only once, 60% of the participants have used the app between 2 and 5 times, and 27% have used the app more often. 50% of the participants used the app in sessions between 1 and 3 minutes, 40% used the app between 3 and 5 minutes, and 10% used the app longer than 5 minutes.

On the scale from 1=useful to 5=not useful, the app was found most useful for reading text (M=2.17, SD=1.28). Be My Eyes also received useful for finding things (M=2.60, SD=1.28) and shopping assistance (M=2.79, SD=1.45). For wayfinding the app was rated slightly negatively with a score of M=3.10 (SD=1.24). For helping at ticket machines, the app got a slightly positive rating of M=2.61 (SD=1.37); but helping at ATMs was rated more negatively with a rating of M=3.61 (SD=1.62). Participants agreed with a rating of M=4.03 (SD=1.17) that Be My Eyes is not especially useful when the hands are busy or while carrying things.

Qualitative replies were translated to English if necessary. 8 participants found the app useful for working with nonaccessible displays like unknown microwave oven panels, diverse electronic devices, and public vending machines. 12 participants appreciated the app to get help for matching clothing and makeup: "I like it for when I have interviews and need to make sure my clothes match and that I haven't horribly misapplied my makeup." Furthermore, participants used the app for verifying the expiration date of food. Although, 10 users found communicating with a human instead of a machine more reliable, 16 participants complained about operators delay and connection issues: "It only works well with fast wifi ". Two had privacy concerns about showing personal information to strangers. One user suggested that Be My Eyes should have a guideline for these kind of things.

For improvement, one user proposed the app should work at night, two users proposed assistance category settings to inform the operators about the kind of help needed, and three participants suggested to improve the communications skills of volunteers through training.

DISCUSSION AND CONCLUSION

We conducted a survey with 30 visually impaired users of Be My Eyes to gain a deeper understanding about remote assistance systems. The results show that most participants use the app mainly between 2 to 5 times in short sessions between 1 to 3 minutes. Therefore, our participants might be still in a test period of the app. Alternatively, the low use rate (73% used it 5 times or less) could indicate a high abandoning rate as often observed for assistive technology and especially in the first year of adoption [START_REF] Phillips | Predictors of assistive technology abandonment[END_REF]. Our results cannot clearly answer this question, but the comments in the open questions section indicate that the users expect improvements in service reliability.

Users find Be My Eyes useful for reading text, finding things, shopping assistance and helping at ticket machines. They found the app useful for dealing with non-accessible displays and for selecting clothing and makeup. The app is found less useful when hands are busy, which is not surprising as the application requires the user holding the device for taking pictures. Wayfinding was judged neutral. Due to the session's duration, we infer that the participants have not used Be My Eyes for navigation tasks because it might require more time than the sessions took. Furthermore, users perceived wayfinding tasks to violate privacy, e.g., because they may not want a stranger to know where they go and when. Indeed, prior work on navigation habits of visually impaired people has shown that asking strangers for wayfinding advice has been perceived as risky [START_REF] Williams | Pray before you step out": describing personal and situational blind navigation behaviors[END_REF]. The application was also found less useful for using ATMs compared to using ticket machines. This can also be explained by the mentioned privacy issues and concerns about showing personal information to strangers. This is in line with previous findings by Bigham et al. [START_REF] Bigham | Human-backed access technology[END_REF].

The qualitative answers showed that participants value talking with real people. Potential areas of improvement are being able to specify the type of help needed and providing training for assistants. The latter is for instance the case in the Crowdviz [START_REF] Holton | Crowdviz: Remote video assistance on your iphone[END_REF] application, which is however not free.

Limited network connectivity was a serious problem. This issue is not in the hands of mobile application developers and not specific for Be My Eyes. Such problems should, however, be kept in mind when trying to transfer results from highly controlled lab studies to the real world. Although participants pointed at a number of challenges, they mostly provided very positive comments.

http://www.bemyeyes.org/