System description

Figure 2 shows a four wavelength multiplexed LD based VLC source, combined with a MEMS mirror based beam-steering module. First, beams from the red, green, blue, and violet LDs are collimated and combined into a single beam using three dichroic mirrors as shown in Fig. 2(a). The divergence of each beam can be independently adjusted depending on the desired beam characteristics, and each beam can be independently tuned to ensure the beams are co-aligned. Each beam path also incorporates an attenuator, based on two rotating polarisers, that allows the relative intensity of each colour component to be adjusted.

Figure 2 Four colour WDM VLC system incorporated with a MEMS mirror based beam-steering module. (a) Schematic and (b) set-up implementation of the transmitter and (c) the receiver structure. Full size image

The co-aligned beams then pass through an iris, which ensures the beams are the same diameter, then into a beam-steering system. A MEMS mirror with an aperture diameter of 5 mm and a mechanical and optical steering range of +/−5 degree and +/−10 degree, respectively, is used to steer the beam, and a well-collimated beam with a minimum diameter of 10 mm at 4 m (divergence half angle <0.072 deg) is obtained from this system. Detailed information about the sources, optics, and the MEMS mirror is given in the methods section.

Communication performance

An orthogonal frequency division multiplexing (OFDM) scheme is used to check the communication capacity of the proposed system11. OFDM is one of the most widely used modulation schemes, especially when pursuing high data-rate links since it allows adaptive bit and power loading to the sub-carriers according to the communication channel condition.

For OFDM, first, M-ary quadrature amplitude modulation (M-QAM) symbols are encoded from a distinct bit stream for each colour channel. An inverse fast Fourier transform (IFFT) is used to convert the M-QAM symbols allocated on different subcarriers to time-domain samples. The samples outside of the linear dynamic range set by each LD are clipped. Then, an arbitrary waveform generator (AWG) generates an analogue signal from the conditioned time samples, followed by a wide band high-power electrical amplifier. With the addition of DC bias, the analogue signal is applied to each LD. The LDs emit red, green, blue, and violet light with a wavelength distribution centred at 660 nm, 520 nm, 450 nm, and 400 nm, respectively. The driving condition can cause instantaneous fluctuation of an illumination level, but when DC-balanced modulation scheme (such as DCO-OFDM) is used, with a stable average current, the impact is negligible20.

The system shown in Fig. 2 then steers the resulting beam, which propagates to a receiver. To support the WDM configuration, optical band-pass filtering is performed using the filtering module similar to the colour combining unit in the transmitter, as shown in Fig. 2(c). A 2-inch diameter high numerical aperture lens is used to capture the filtered light onto a 1.4 GHz BW free-space photo-receiver. This is a valid approach as a negligible crosstalk is expected between the four implemented colour channels. Each channel is from a narrow linewidth LD with no spectral overlap, hence allowing for independent WDM characterisation as widely reported in literature8,9,10,11,12. (The specifications of LDs and the pass-bands configured by the dichroic mirrors are in the methods section.) Each separated and optical-to-electrical (O-E) converted signal is sampled by an 2.5 GHz BW oscilloscope for communication performance tests. Finally, MATLAB® is used to process the captured electrical signal for decoding and analysing the channel adaptive optical OFDM.

The time taken to send one OFDM frame is determined by the total number of samples in one frame and the sampling rate. The total number of samples includes the IFFT (N fft ) samples generated from the M-QAM symbols on the sub-carriers and the cyclic prefix samples (N c ) dealing with the interference between frames and sub-carriers. The sampling rate is determined by over sampling rate (f os ) and system sampling clock (f clk ). Then, the frame time (T f ) is calculated as

$${T}_{f}=({N}_{{\rm{fft}}}+{N}_{{\rm{c}}})\frac{{f}_{os}}{{f}_{clk}}[\sec \,.]$$ (1)

The system clock is normally fixed for a specific system. The oversampling rate is an integer value with which the modulation BW is adjusted. To maximize the use of all available resources within the modulation BW, a channel adaptive bit and power loading algorithm based on21 is used. For a given target bit error rate (BER) performance, the algorithm determines the number of bits on each sub-carrier with optimized power level, leading to the maximum total bits (b sum ) maintaining the same total power \(({P}_{RF}^{total})\).

$${b}_{sum}=\,\max (\sum _{k=1}^{\frac{{N}_{{fft}}}{2}-1}{lo}{{g}}_{2}{M}_{k})[{\rm{bits}}]\,{\rm{and}}\,{P}_{RF}^{total}=\sum _{k=1}^{\frac{{N}_{{\rm{fft}}}}{2}-1}{P}_{k}[{\rm{W}}]$$ (2)

where M k is the optimized level of M-QAM symbols on kth sub-carrier, which is set to 1 when the sub-carrier is determined to be empty by the algorithm. Then, the maximum data-rate (DR max ) from a WDM link with a total optical power constraint \(({P}_{optical}^{total})\) is given as

$$D{R}_{{\rm{\max }}}=\,\max (\sum _{\lambda =1}^{{N}_{wdm}}\frac{{b}_{sum,\lambda }}{{T}_{f,\lambda }})[b/s],\,{\rm{subject}}\,{\rm{to}}\,{P}_{optical}^{total}=\sum _{\lambda =1}^{{N}_{wdm}}{P}_{\lambda }[{{\rm{W}}}_{{\rm{optical}}}]$$ (3)

where N wdm is the number of WDM channels, and the subscript λ is the index of each WDM channel.

Figure 3(a) shows the maximum data-rate achieved below the target BER of 3.8 × 10−3 from the four colour WDM system presented in Fig. 2. The target BER guarantees a reliable communication link by applying forward-error-correction coding with 7% overhead22. In the demonstration, due to the receiver aperture wider than input beam size from the line of sight pencil beam, the received power is equated with the transmitted power. The maximum achievable data rate depends both on the total power transmitted and the proportion of each colour used. The four colours are optimally mixed to achieve the highest data-rate given the same total power. The maximum data-rates logarithmically increase with increased the total received optical power, achieving 10.9 Gb/s at 0.2 mW, 21.2 Gb/s at 0.7 mW, and 35.6 Gb/s at 4 mW. The figure suggests that the rates could be further increased with greater numbers of WDM channels, as channel rates are saturated beyond 20 Gb/s. For instance, from 4 mW (35.6 Gb/s) to 8 mW (37.8 Gb/s), there is only a 2.2 Gb/s improvement. If the additional 4 mW is used for accommodating more WDM channels, the rate could become double depending on the source BWs and colour dependent property of the receiver. Figure 3(b–d) show the optical spectrum, CIE colour coordinate, and assigned bits for each colour, at the data-rate point of 35.6 Gb/s, respectively.

Figure 3 WDM results for maximum data-rate. (a) Received optical power in total vs. measured aggregate data-rate using channel adaptive OFDM, where the power proportion is adjusted to maximise the data-rate for a given power level. (b) Optical spectrum, (c) CIE colour coordinate, and (d) Assigned bits for each colour leading to 35.6 Gb/s (BER < 3.8 × 10−3). Full size image

Coverage

A MEMS steering mirror provides the coverage. The mirror’s optical beam-steering range of +/−10 degree can cover 0.35 × 0.35 m2 at 1 m and 1.4 × 1.4 m2 at a 4 m link distance. However, these areas can be insufficient for a full coverage of some indoor environments. Figure 4(a) shows an angle magnifier module located at the transmitter output. The module magnifies the pointing angle of the combined light. The angle magnification factor is defined by focal lengths of the lenses and the distances between them. Here, the parameters are optimized to achieve the magnification factor of 3.8, allowing a wide coverage of 6.25 × 6.25 m2 at 4 m link distance, but enlarging the spot size by a similar factor. Figure 4(b) shows the received optical power and the data-rate for different steering angles with and without the angle magnifier module. Without the angle magnifier, 8 mW (37.8 Gb/s) is received within +−10 degrees of beam-steering. With the use of angle magnifier, +−38 degrees with data-rates greater than 36 Gb/s are achieved, despite the relative loss of 1 to 1.5 dB due to the enlarged beam size while magnifying the angle. In this report, WDM capable hot-spot lighting system and its coverage are mainly proposed and examined. For a receiver alignment, a beam-steering unit similar to the one in the transmitter can be used. Or, fluorescent concentrators which selectively capture the incoming light by its absorption property in a non-imaging way23 can be applied.

Figure 4 Coverage. (a) Angle magnifier module (b) received optical power and data-rate by steering the beam with and without the angle magnifier. Full size image

Considerations for lighting

LDs are considered efficient lighting sources with high luminous efficacy24,25. Also, LD based lighting systems can bring new lighting architectures with high controllability from low divergence26. For the use of LDs for lighting, however, eye-safety should be taken into account. Laser radiation hazards in the visible spectrum (400–780 nm) are dominated by retinal hazards, as described in the standard27. These are divided into photochemical and thermal hazards, which determine the accessible emission limits (AEL) for a given laser source. The AEL is the maximum allowed power for a laser to be part of a class, Class 1 being a safe source. The AEL not only depends on the wavelength of operation but also on the apparent source size and beam divergence. These two factors, together with the eye lens accommodation at the back of the retina determine the retinal image size. When the retinal image size becomes smaller, the hazard increases since the power density of the LD’s radiation on the retina becomes bigger. In this paper, the ‘small source’ condition has been applied to estimate the AEL for the emitted power, which is the most conservative safety calculation and worst-case scenario according to the standard27. In addition, the photochemical hazard limits the power level of blue wavelengths, and this often provides the most restrictive eye-safety condition for a safe white light generation.

Figure 5 shows the maximum data-rates by received power levels for an eye-safe link meeting the criteria for Class 127, with which all conditions of normal use are considered safe. The highest data-rate from the eye-safe case is 10.4 Gb/s at 370 µW, and the generated colour point at the rate is shown in Fig. 5(a). Achieving both higher data-rates and eye-safe operation is an area of current investigation, examining the geometrical modification of the sources (which controls the retinal image size) and the optimal wavelength distribution of the sources.

Figure 5 Examples. Maximum data-rate vs. received power for an eye-safe link, and white-light link with colour points in CIE coordinates at their highest data-rate of (a) 10.4 Gb/s and (b) 7.8 Gb/s, at BER < 3.8 × 10−3. Full size image

Figure 5a shows that the maximum eye-safe data rate is achieved using green light for the particular devices in this demonstration. However, white light can be preferred for general lighting. In this case, the white light requirements fix the proportion of RGBV colours. Then, the photochemical blue hazards become the most restrictive factor. Figure 5(b) shows the optimum point resulting in the highest data-rate satisfying both eye-safety and white-light constraint. Here, 20% deviation from the absolute white point (x = 0.33, y = 0.33) is used to approximate white-light boundary shown in28. The maximum data-rate for the eye-safe white-light cases is 7.8 Gb/s at 170 µW, and the generated colour point at the rate is shown in Fig. 5(b).

For general LD based illumination, there are still active and challenging topics of research. Illumination quality parameters such as colour rendering index (CRI) have to be further explored20. Discrete laser line sources are known to have a poor CRI, thus filling the uncovered spectrum with more WDM sources or with colour converted light from phosphor plates can help to improve this metric. Engineered extended sources defined by LD sources incident on a diffuser or a phosphor plate can release the limitation set by eye-safety regulation. Also, in depth study on laser products designed to function as conventional lamps is needed as its power emission limits are dictated by the lamp standard29.