AN OPTICAL WIRELESS COMMUNICATION RECEIVER WITH LARGE PHOTODETECTOR SUFACE AREA, LARGE FIELD OF VIEW AND HIGH BANDWIDTH

Abstract

An Optical Wireless Communication (OWC) receiver configured to receive an incoming optical beam modulated with data and output an output signal including the modulated data. A lens receives the incoming optical beam. Photodiodes positioned at a distance from the lens and proximal to the focal plane of the lens receive a fraction of the incoming optical beam and generate a photocurrent in correspondence with photons received. The photodiodes are provided in a two-dimensional array including rows and columns wherein outputs of the columns are combined and their photocurrents are summed. An amplifier connected to the combined output of the columns converts the summed photocurrents into an output signal. Interconnections of the photodiodes form at least two parallel branches wherein each branch includes a cascade of at least two photodiodes forming a combined photodetector surface.

Claims

1-13. (canceled)

14. An optical wireless communication (OWC) receiver configured to receive an incoming optical beam modulated with data and output an electrical output signal comprising the modulated data, the OWC receiver comprising: a lens configured to receive the incoming optical beam; a plurality of photodiodes positioned at a distance from the lens and positioned at or proximal to a focal plane of the lens, each of the plurality of photodiodes configured to receive a fraction of the incoming optical beam and generate a photocurrent in correspondence with photons received from the fraction of the incoming optical beam, wherein the plurality of photodiodes are arranged in a two-dimensional array comprising rows and columns, and wherein outputs of the columns are combined and their photocurrents are summed; and an amplifier connected to combined outputs of the columns of the two-dimensional array and configured to convert the summed photocurrents into an output signal of the amplifier; wherein interconnections of the photodiodes of the two-dimensional array are configured to form at least two parallel branches of photodiodes, and wherein each of the parallel branches comprises a cascade of at least two photodiodes forming a combined photodetector surface.

15. The OWC receiver according to claim 14, wherein the number of parallel branches of the two-dimensional array is greater than the number of cascaded photodiodes comprised in each of the branches.

16. The OWC receiver according to claim 14, wherein the number of parallel branches of the two-dimensional array is less than the number of cascaded photodiodes comprised in each of the branches.

17. The OWC receiver according to claim 14, wherein the number of parallel branches of the two-dimensional array equals the number of cascaded photodiodes comprised in each of the branches.

18. The OWC receiver according to claim 14, wherein the lens is configured to project the received optical beam onto a surface area that is larger than the combined surface area of the two-dimensional array of photodiodes.

19. The OWC receiver according to claim 14, wherein the two-dimensional array of the photodiodes comprising the rows and columns comprises interconnections of the photodiodes wherein respective interconnections defining each column are also interconnections defining the rows.

20. The OWC receiver according to claim 14, wherein each of the rows of the two-dimensional array further comprises a resistor positioned in parallel over the photodiodes of the respective row.

21. The OWC receiver according to claim 20, wherein each of the resistors is positioned outside of the two-dimensional array of the photodiodes.

22. The OWC receiver according to claim 20, wherein each of the resistors is integrated inside the two-dimensional array of the photodiodes.

23. An Optical Wireless Communication (OWC) system comprising at least one OWC receiver according to claim 14.

24. The OWC system according to claim 23 configured to provide optical communication via electromagnetic radiation with a wavelength in any one of the visible light spectrum, infrared light spectrum, near-infrared light spectrum or ultraviolet light spectrum.

25. The OWC system according to claim 23 configured for application in at least one of a wireless local area network, a wireless personal area network, and a vehicular network.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1A: shows the Field-of-View half-angle α of an OWC receiver according to the present disclosure wherein the optical beam arrives at angle α which illuminates the photodiode (PD) with diameter D.sub.2 through a lens with a diameter D.sub.1 and a focal length f where the photodiode is defocused over a distance x with respect to the focal plane of the lens;

[0039] FIG. 1B: shows projecting the beam's spot onto the photodiode. The defocused spot with radius R.sub.c=D.sub.c/2 is illuminating the photodiode with radius R.sub.2=D.sub.2/2;

[0040] FIG. 2A shows the Field-of-View half angle α and FIG. 2B the photo-detected fraction T of incident optical beam versus the defocusing parameter p (where p is defined as p=x/f) for lens diameter D.sub.1=10 mm, and lens focal length f=5 mm, and for various diameters D.sub.2 of the photodiode;

[0041] FIG. 3: shows an electrical circuit equivalent circuit of a single photodiode;

[0042] FIG. 4: shows an optical transimpedance receiver;

[0043] FIG. 5: shows an example of the electrical circuit equivalent of putting N photodiodes in parallel;

[0044] FIG. 6: shows an example of the electrical circuit equivalent of putting N photodiodes in series;

[0045] FIG. 7: shows the DC bias voltage V.sub.n (n=1 . . . N) of each photodiode when putting N photodiodes in series;

[0046] FIG. 8: shows an example of the 2D matrix of photodiodes with a resistor R.sub.p positioned in parallel over the photodiodes of each row;

[0047] FIG. 9: Putting multiple photodiodes in a serial/parallel 2D matrix (M photodiodes in series, and K series of M photodiodes in parallel);

[0048] FIG. 10: shows frequency characteristics of OWC receiver (2D array of N=M×K photodiodes, followed by transimpedance amplifier).

DETAILED DESCRIPTION OF THE INVENTION

[0049] As illustrated by FIG. 1, the proposed OWC receiver receives the incoming optical beam that is modulated with the high-speed data, and converts this to an electrical output signal. To do this, the OWC receiver deploys a photodiode (PD in FIG. 1) which is followed by an electrical amplifier. The diameter of the optical beam is typically much larger than the active area of the photodiode, so to maximize the output signal the photodiode should capture as much as possible power of the incoming beam. As shown in FIG. 1, the optical beam is captured by a lens first, and then projected onto the photodiode. To maximize the captured optical power as well as avoiding the need for careful angular alignment with respect to the beam, the OWC receiver should have a large cross-sectional aperture, as well as a wide Field-of-View (FoV, i.e. large angular range). But to handle high data speeds, the OWC receiver should also have a large bandwidth, which typically requires a photodiode with a small active photodetection area.

[0050] If the photodiode is to be fully covered by the beam's defocused spot, then D.sub.c=p Δ D.sub.1>D.sub.2. For this full coverage, the allowable displacement Δ of the centre of the spot with respect to the centre of the photodiode (with defocusing parameter p=x/f, with 0≤p<1) is

[00004]$\Delta \le \frac{1}{2}\left(D_c-D_2\right)=\frac{1}{2}\left(p.Math.D_1-D_2\right)$

[0051] The optical power P.sub.det incident on the photodiode as fraction of the total optical power in the spot P.sub.spot is

[00005]$P_{\det }=P_{\mathrm{spot}}.Math.{\left(\frac{D_2}{D_c}\right)}^2=P_{\mathrm{spot}}.Math.\frac{1}{p^2}.Math.{\left(\frac{D_2}{D_1}\right)}^2$

[0052] For the Field-of-View half angle α holds

[00006]$\tan \alpha =\frac{\Delta }{f-x}=\frac{p.Math.D_1-D_2}{2f\left(1-p\right)}$

[0053] As shown in FIG. 2A, the FoV's half angle α is increased when defocusing is applied, with a diameter D.sub.2 of the photodiode of 0.2 mm as indicated by reference 211, a diameter of 0.4 mm as indicated by reference 212, a diameter of 0.8 mm as indicated by reference 213, and a diameter of 1.0 mm as indicated by reference 211. The FoV decreases slightly when the photodiode diameter is increased; on the other hand, the photodetected power increases considerably, as shown in FIG. 2B. In FIG. 2B the photo-detected fraction T of incident optical beam is shown versus the defocusing parameter p, where p is defined as p=x/f) for lens diameter D.sub.1=10 mm, and lens focal length f=5 mm, and for various diameters D.sub.2 of the photodiode, i.e. with a diameter D.sub.2 of the photodiode of 1.0 mm as indicated by reference 221, a diameter of 0.8 mm as indicated by reference 222, a diameter of 0.4 mm as indicated by reference 223 and a diameter of 0.2 mm as indicated by reference 212.

[0054] Single Photodiode

[0055] FIG. 3 (left) describes the electrical circuit equivalent of a single photodiode, with serial resistance R.sub.s, parallel resistance R.sub.d, capacitance C.sub.d, dark current i.sub.d(t), and photo-current i.sub.s(t). A p-i-n photodiode is typically biased by a reverse voltage across its intrinsic region, and when photons penetrate into this region electron-hole pairs are generated which due to the electric field travel to the anode and cathode electrodes of the photodiode, respectively, and thus generate the photocurrent i.sub.s(t). The (typically small) serial resistance R.sub.s includes the bonding wires to the photodiode chip, the (high) parallel resistance R.sub.d includes the leakage current, and the (small) dark current i.sub.d(t) is generated by thermal effects. The photo-current is i.sub.s(t)=R.Math.a.Math.P(t) where R is the responsivity of the photodiode (in A/W), and a is the fraction of the instantaneous optical beam power P(t) which is detected by the active area of the photodiode.

[0056] Applying Thevenin's theorem, as shown in FIG. 3 (middle) the photodiode can be represented by an equivalent circuit with a voltage source e(t) and serial impedance Z, and alternatively by applying Norton's theorem as shown in FIG. 3 (right) also by an equivalent circuit with a current source i.sub.tot(t) and the same impedance Z.sub.tot=Z in parallel, where

[00007]$e\left(t\right)=Z_d.Math.\left(i_s\left(t\right)+i_d\right)=\frac{R_d}{1+j\omega R_d{C}_d}.Math.\left(i_s\left(t\right)+i_d\right)i_s\left(t\right)=a.Math.R.Math.P\left(t\right)Z_{\mathrm{tot}}=Z=Z_d+R_s=\frac{R_d}{1+j\omega R_d{C}_d}+R_s{i}_{\mathrm{tot}}\left(t\right)=\frac{e\left(t\right)}{Z_{\mathrm{tot}}}=\frac{i_s\left(t\right)+i_d}{1+\frac{R_i}{R_d}\left(1+j\omega R_d{C}_d\right)}$

[0057] Optical Receiver

[0058] To process the signals after the photodetection, an electronic high-frequency low-noise amplifier is applied after the photodiode. A well-known typical example is a transimpedance amplifier (TIA), of which the circuit schematics are shown in FIG. 4 (where A is the open-loop gain of the amplifier, and Z.sub.t its feedback impedance). Its amplification is expressed as a transimpedance Z.sub.T, for which can be derived that

[00008]$Z_T=\frac{v_{\mathrm{out}}\left(t\right)}{i_{\mathrm{tot}}\left(t\right)}=\frac{A.Math.Z_t}{1+A+\left(Z_t/Z_{\mathrm{tot}}\right)}$

[0059] Substituting the single-photodiode model yields

[00009]$Z_T=\frac{A.Math.Z_t}{1+A+Z_t/\left(\frac{R_d}{1+j\omega R_d{C}_d}+R_s\right)}\approx \frac{A.Math.Z_t}{1+A+j\omega .Math.C_d.Math.Z_t}\mathrm{for}{R}_sR_d\mathrm{and}{R}_d1$

[0060] which gives for the low-frequency receiver transimpedance and the receiver's −3 dB bandwidth, respectively,

[00010]$Z_r\left(\omega =0\right)=\frac{A}{1+A}{Z}_t{\omega }_{-3dB}=\frac{1+A}{C_d.Math.Z_t}$

[0061] Multiple Photodiodes in Parallel

[0062] To increase the active area, N photodiodes can be put in parallel. The circuit model of such parallel arrangement is shown in FIG. 5. The current source model of FIG. 3 (right) for the single photodiode is extended to connecting N of these models in parallel, which implies that the currents can be simply added and the impedance is divided by N:

[00011]$i_{\mathrm{tot}}\left(t\right)={.Math.}_{n=1}^N{i}_n\left(t\right)={.Math.}_{n=1}^N\left(a_n.Math.R.Math.P\left(t\right)+i_d\right)=N.Math.i_d+R.Math.P\left(t\right){.Math.}_{n=1}^N{a}_n=N.Math.i_d+N.Math.\stackrel{\_}{a}.Math.R.Math.P\left(t\right)Z_{\mathrm{tot}}=\frac{1}{N}Z=\frac{1}{N}\left(\frac{R_d}{1+j\omega R_d{C}_d}+R_s\right)$

[0063] From this analysis, it can be observed that the equivalent capacitance C.sub.eq≈N.Math.C.sub.d and the generated photocurrent is i.sub.tot(t)≈N.Math.ā.Math.R.Math.P(t), where each photodiode is illuminated by a fraction a.sub.n of the beam's power and

[00012]$\stackrel{\_}{a}=\frac{1}{N}{.Math.}_{n=1}^N{a}_n$

denotes the average fraction of the beam power which is incident on a single photodiode. So, the generated photocurrent is N times the average current generated by an individual photodiode.

[0064] Applying the N parallel photodiodes in a transimpedance amplifier, its transimpedance Z.sub.T is found to be

[00013]$Z_T=\frac{A.Math.Z_t}{1+A+N.Math.Z_t/\left(\frac{R_d}{1+j\omega R_d{C}_d}+R_s\right)}\approx \frac{A.Math.Z_t}{1+A+j\omega .Math.N.Math.C_d{Z}_t}\mathrm{for}{R}_sR_d\mathrm{and}{R}_d1$

[0065] And thus, the low-frequency receiver gain and the receiver's −3 dB bandwidth are, respectively,

[00014]$Z_T\left(\omega =0\right)=\frac{A}{1+A}{Z}_t{\omega }_{-3dB}=\frac{1+A}{N.Math.C_d.Math.Z_t}$

[0066] In comparison with the single-photodiode receiver, the generated photocurrent is N times larger, hence a reference transimpedance Z.sub.T,,ref may be defined which expresses the output signal v.sub.out(t) as generated by the average photo-generated current per photodiode ā.Math.R.Math.P(t)

[00015]$Z_{T,\mathrm{ref}}\left(\omega =0\right)=\frac{v_{\mathrm{out}}\left(t\right)}{\text{?}}=N.Math.\frac{A}{1+A}{Z}_t$$\text{?}\text{indicates text missing or illegible when filed}$

[0067] Hence the output signal generated by the receiver with the N photodiodes in parallel is N times larger than that of the receiver with a single photodiode, but the receiver's bandwidth is N times smaller.

[0068] Multiple Photodiodes in Series

[0069] Alternatively, the active area can be extended by putting N identical photodiodes in series. The equivalent circuit of such cascade of photodiodes is shown in FIG. 6. Applying the equivalent voltage source circuit of the single photodiode in FIG. 3 (middle), this cascade is represented by FIG. 6 (left), where the n-th photodiode receives a fraction a.sub.n of the beam's power P(t), hence its photocurrent is i.sub.s,n(t)=a.sub.n.Math.R.Math.P(t), and according to Thevenin can be represented again (similar as in FIG. 3 (middle)) as a voltage source e.sub.n(t) with serial impedance Z. As FIG. 6 (middle) shows, this series of voltage sources can be summed, as well as the impedances, into a single voltage source e.sub.tot(t) and single serial impedance N×Z, which subsequently according to Norton can be represented by a single current source i.sub.tot(t) and same impedance Z.sub.tot=N×Z in parallel. The relations are given by

[00016]$\begin{array}{c}{i}_{s,n}\left(t\right)=a_n.Math.R.Math.P\left(t\right)\\ e_{\mathrm{tot}}\left(t\right)={.Math.}_{n=1}^N{e}_n\left(t\right)=\frac{N.Math.N_d}{1+j\omega R_d{C}_d}\left(i_d+\stackrel{\_}{a}.Math.R.Math.P\left(t\right)\right)\mathrm{with}\stackrel{\_}{a}=\frac{1}{N}{.Math.}_{n=1}^N{a}_n\\ Z_{\mathrm{tot}}=N.Math.Z=N\left(\frac{R_d}{1+j\omega R_d{C}_d}+R_s\right)\\ i_{\mathrm{tot}}\left(t\right)=\frac{e_{\mathrm{tot}}\left(t\right)}{N.Math.Z}=\frac{i_d+d.Math.R.Math.P\left(t\right)}{1+\frac{R_s}{R_d}\left(1+j\omega R_d{C}_d\right)}\end{array}$

[0070] where

[00017]$\stackrel{\_}{a}=\frac{1}{N}{.Math.}_{n=1}^N{a}_n$

denotes the average fraction of the beam power which is incident on a single photodiode. Assuming small R.sub.s and large R.sub.d, it is observed that the equivalent capacitance C.sub.eq≈C.sub.d/N, so the cascading of the N photodiodes yields a reduction of the equivalent capacitance with a factor N, whereas the generated photocurrent i.sub.tot(t) at low frequencies (so ω<<1) is i.sub.tot(t)=i.sub.s(t)≈ā.Math.R.Math.P(t), i.e. the same as the current generated by an individual photodiode when all photodiodes are illuminated by an equal fraction of the beam's power.

[0071] Applying the series of photodiodes in the transimpedance amplifier scheme, it is found that

[00018]$Z_T=\frac{A.Math.Z_t}{1+A+Z_t/\left[N.Math.\left(\frac{R_d}{1+j\omega R_d{C}_d}+R_s\right)\right]}\approx \frac{A.Math.Z_t}{1+A+j\omega .Math.Z_t.Math.C_d/N}\mathrm{for}{R}_s\ll R_d\mathrm{and}{R}_d\gg 1$

[0072] And thus, for the low-frequency receiver gain and the receiver's −3 dB bandwidth, respectively,

[00019]$\begin{array}{c}{Z}_T\left(\omega =0\right)=\frac{A}{1+A}{Z}_t\\ {\omega }_{-3\mathrm{dB}}=N.Math.\frac{1+A}{C_d.Math.Z_t}\end{array}$

[0073] In comparison with the single-photodiode receiver, the generated photocurrent is the same, hence a reference transimpedance Z.sub.T,ref may be defined which expresses the output signal as generated by the average signal

[00020]$Z_{T,\mathrm{ref}}\left(\omega =0\right)=\frac{v_{\mathrm{out}}\left(t\right)}{\text{?}}=\frac{A}{1+A}{Z}_t$$\text{?}\text{indicates text missing or illegible when filed}$

Hence the output signal generated by the receiver with the N photodiodes in series is the same as that of the receiver with a single photodiode, but the receiver's bandwidth is N times larger.

[0074] As the photodiodes each need to be biased with a reverse voltage which is adequate for achieving its detection and bandwidth performance, the reverse bias voltage V.sub.n (with n=1 . . . N) per photodiode n when putting them in series needs to be assessed, including its sensitivity for slight asymmetries in the characteristics of the photodiodes and in the photocurrents generated by them.

[0075] The DC current I.sub.n generated by incident light with power P.sub.n on the active area of a photodiode n which is reverse-biased with a voltage V.sub.n is known to be

[00021]$I_n=I_{\mathrm{On}}\left(1-e\text{?}\right)+a_n.Math.R_n.Math.P$$\text{?}\text{indicates text missing or illegible when filed}$

[0076] where I.sub.on is the photodiode's dark current, R.sub.n photodiode responsivity, P.sub.n incident optical power on photodiode n (note that R.sub.n.Math.P.sub.n<I.sub.n<R.sub.n.Math.P.sub.n+I.sub.on; at room temperature, kT/q»25 mV).

[0077] FIG. 7 shows putting N photodiodes in series and applying the bias voltage V.sub.b across the whole series. A resistance R.sub.d in parallel and resistance R.sub.s in series with the photodiode has been included (in accordance with FIG. 3).

[0078] For the current/generated by the series of photodiodes holds

[00022]$I=I_n+\frac{1}{R_d}{V}_n=I_{\mathrm{On}}\left(1-e\text{?}\right)+a_n{R}_{n}P+\frac{V_n}{R_d}$$\text{?}\text{indicates text missing or illegible when filed}$

which under typical reverse bias conditions where V.sub.n>>kT/q is well approximated by

[00023]$I\approx I_{\mathrm{On}}+a_n{R}_{n}P+\frac{V_n}{R_d}$

[0079] Hence

[00024]$\begin{array}{c}{V}_b=N.Math.I.Math.R_s+{.Math.}_{n=1}^N{V}_n=N.Math.I.Math.\left(R_s+R_d\right)-N.Math.R_d\left(\stackrel{\_}{I_{\mathrm{On}}}+\stackrel{\_}{a_n{R}_n}.Math.P\right)\\ I=\frac{V_b+N.Math.R_d\left(\stackrel{\_}{I_{\mathrm{On}}}+\stackrel{\_}{a_n{R}_n}.Math.P\right)}{N.Math.\left(R_s+R_d\right)}\\ V_n=R_d\left[\frac{V_b+N.Math.R_d\left(\stackrel{\_}{I_{\mathrm{On}}}+\stackrel{\_}{a_n{R}_n}.Math.P\right)}{N.Math.\left(R_s+R_d\right)}-I_{\mathrm{On}}-a_n{R}_n.Math.P\right]\\ V_n=\frac{R_d}{R_d+R_s}.Math.\frac{V_b}{N}+\frac{R_d^2}{R_d+R_s}\left(\stackrel{\_}{I_{\mathrm{On}}}+\stackrel{\_}{a_n{R}_n}.Math.P\right)-R_d\left(I_{\mathrm{On}}+a_n{R}_n.Math.P\right)\end{array}$

[0080] In good approximation, as typically R.sub.d>>R.sub.s

[00025]$V_n\approx \frac{V_b}{N}+R_d\left(\stackrel{\_}{I_{\mathrm{On}}}-I_{\mathrm{On}}+\stackrel{\_}{a_n{R}_n}.Math.P-a_n{R}_n.Math.P\right)\mathrm{for}{R}_d\gg R_s$

[0081] Hence, the total bias voltage V.sub.b is equally divided among the N photodiodes, yielding a bias voltage V.sub.b/N for each photodiode, where per photodiode n a bias voltage deviation can occur which is the product of its parallel resistance R.sub.d and the difference of the individual dark current I.sub.on and the individual generated photocurrent R.sub.n.Math.P.sub.n with respect to the average values of these currents. By having all photodiodes integrated on the same chip, these differences will be small. By lowering R.sub.d, e.g. by putting an external resistor in parallel to each photodiode (as shown in FIG. 8 in which each row of the matrix has such a resistor R.sub.p which is located outside of the matrix), the individual bias voltage deviation resulting from these differences can be reduced (if that is needed for adequate performance).

[0082] Multiple Photodiodes in Series/Parallel in a 2D Matrix

[0083] It is proposed to put the photodiodes in a two-dimensional M×K matrix, which has M photodiodes in series, and K of such series in parallel; see FIG. 9.

[0084] The relations determining the current source i.sub.tot(t) and impedance Z.sub.tot in the Thevenin equivalent circuit are (using the before-mentioned results of the analyses for the series- and parallel-connected 1D photodiode structures)

[00026]$\begin{array}{c}{i}_{\mathrm{tot}}\left(t\right)={.Math.}_{k=1}^K{i}_{\mathrm{tot},k}\left(t\right)={.Math.}_{k=1}^K\frac{i_d\text{?}}{1+\text{?}\left(1+j\omega R_d{C}_d\right)}=K.Math.\frac{\text{?}}{1+\text{?}\left(1+j\omega R_d{C}_d\right)}\\ Z\text{?}={\left(M.Math.Z\right)}_1//{\left(M.Math.Z\right)}_2//.Math.//{\left(M.Math.Z\right)}_K=\frac{M}{K}Z=\frac{M}{K}\left(\frac{R_d}{1+j\omega R_d{C}_d}+R\text{?}\right)\end{array}$$\text{?}\text{indicates text missing or illegible when filed}$

[0085] where the operator “∥” means “connecting in parallel”, e.g., Z.sub.a∥Z.sub.b=Z.sub.a.Math.Z.sub.b/(Z.sub.a+Z.sub.b). Applying the 2D photodiode matrix in the transimpedance amplifier scheme, it follows that

[00027]$Z_T=\frac{A.Math.Z_t}{1+A+Z_t/\left[\frac{M}{K}\left(\frac{R_d}{1+j\omega R_d{C}_d}+R\text{?}\right)\right]}\approx \frac{A.Math.Z_t}{1+A+j\omega \text{?}{Z}_t.Math.C_d}\mathrm{for}{R}_s\ll R_d\mathrm{and}{R}_d\gg 1$$\text{?}\text{indicates text missing or illegible when filed}$

[0086] Hence the low-frequency receiver gain and the receiver's −3 dB bandwidth are, respectively,

[00028]$\begin{array}{c}{Z}_T\left(\omega =0\right)=\frac{A}{1+A}.Math.Z_t\\ {\omega }_{-3\mathrm{dB}}=\frac{M}{K}\frac{1+A}{C_d.Math.Z_t}\end{array}$

Comparing with the single-photodiode receiver, the generated photocurrent is K times the current generated in each series of connected photodiodes; of each series, the current is equal to the average current generated in a single photodiode. The reference transimpedance Z.sub.T,ref therefore is

[00029]$Z_{T,\mathrm{ref}}\left(\omega =0\right)=K.Math.\frac{A}{1+A}.Math.Z_t$

[0087] Therefore the output signal generated by the receiver with the 2D matrix of N=M×K photodiodes (thus with a total photodetection area which is N times larger than that of a single photodiode) is for low frequencies K times larger than that of the receiver with a single photodiode, and the receiver's bandwidth is M/K times larger.

[0088] In FIG. 10 it is shown how the frequency characteristics 901, 902, 903, 904 of the OWC receiver (consisting of the 2D array of N=M×K photodiodes) behave (where Z.sub.T,ref=V.sub.out(t)/i.sub.PD(t) expresses the ratio between the transimpedance amplifier's output voltage v.sub.out(t) and the average photocurrent generated per photodiode i.sub.PD(t)=ā.Math.R.Math.P(t)). The characteristics are shown in FIG. 10 on a log-log scale, i.e. both the horizontal axis and the vertical axis have a logarithmic scale. As can be observed, when K=M, the bandwidth is the same as for a single photodiode (N=1).

[0089] With respect to a single photodiode 905: if the number of photodiodes N is constant, but K>M as shown by curve 902, then the bandwidth is smaller by a factor M/K, and the gain is a factor K larger. Reversely, when N is constant, but K<M, as shown by curve 904, the bandwidth increases but the gain decreases.

[0090] With respect to a single photodiode 905, by means of applying a 2D array of photodiodes with K=M as shown by curve 903 the active area for photodetection can be increased while the bandwidth stays the same and the output signal of the OWC receiver is increased. For K>M, i.e. curve 902, the output voltage is further increased and the bandwidth decreased with respect to the case K=M, i.e. curve 903; reversely, for K<M, i.e. curve 904, the output voltage is decreased and the bandwidth increased.

[0091] The present invention has now been described in accordance with several exemplary embodiments, which are intended to be illustrative in all aspects, rather than restrictive. Thus, the present invention is capable of many variations in detailed implementation, which may be derived from the description contained herein by a person ordinary skilled in the art. All such variations are considered to be within the scope and spirit of the present invention as defined by the following claims and their legal equivalents.