WO2018035954A1

WO2018035954A1 - System and method for photonic digital to analog conversion

Info

Publication number: WO2018035954A1
Application number: PCT/CN2016/103264
Authority: WO
Inventors: Hadi BAHRAMIABARGHOUEI; Irfan Muhammad FAZAL
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2016-08-25
Filing date: 2016-10-25
Publication date: 2018-03-01
Anticipated expiration: 2019-02-25

Abstract

An n-bit electro-optic modulator (EOM) (300) for converting an electrical signal into an optical signal is provided. The EOM (300) includes a plurality of n optical modulators (313 ₁ _……n), pairs of the plurality of n optical modulators (313 ₁ _……n),each receiving light at a common wavelength of n/2 different wavelengths ( ₁ _……n/2), and each of the n optical modulators (313 ₁ _……n) amplitude encoding one of n bits of an input digital n-bit source (215) using the corresponding received common wavelength of light according to a significance of that digital bit. A plurality of n/2 beam combiners (315 ₁ _……n/2) are provided. Each of the beam combiners (315 ₁ _……n/2) receives a pair of modulated outputs (317 ₁ _……n) from one of the pairs of optical modulators (313 ₁ _……n) that received the light at common wavelength and combines the modulated outputs (317 ₁ _……n) pair to produce a single dual-polarized output (318 ₁ _……n/2). A system and a method are provided for converting an electrical signal into an optical signal. Transmission and reception methods are provided for transmitting and receiving a radio-over-fiber signal.

Description

SYSTEM AND METHOD FOR PHOTONIC DIGITAL TO ANALOG CONVERSION

REFERENCE TO RELATED APPLICATIONS

The present application claims priority from US Patent Application No. 15/247,238, filed on 25 August 2016 and entitled “System and Method for Photonic Digital to Analog Conversion” , the entire disclose of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention pertains to the field of photonics and in particular to a system and method for photonic digital to analog conversion.

BACKGROUND

Communication networks require large amounts of data to be exchanged between control centres and the radio heads that transmit data to, and receive data from, recipients. The data needs to be converted between the digital domain for processing and the analog domain for transmission.

One solution is to transmit the data digitally to each radio head, and provide the necessary digital-to-analog (D/A) conversion equipment at the radio head. A problem with this solution is that each radio head, typically located in a remote location, must be supplied with complicated and sensitive equipment to perform the D/Aconversion. Furthermore, the solution requires fairly high transmission bandwidth, and accordingly does not scale well at higher rates of analog bandwidth.

Another solution to the problem is to transmit the analog signal to the radio head, allowing the radio head to amplify the analog signal for transmission. This solution eliminates the need for D/Aconversion at each radio head. However, analog RF systems have a rather limited bandwidth capacity, and may suffer from electrical interference. To improve the transmission bandwidth, an optical fiber link may be used to convey the analog optical signal to the radio head, where the signal is detected and amplified. Such radio over fiber (RoF) systems do not suffer from electrical interference problem.

The utilization of RoF systems has hitherto been somewhat hindered by the complexity of the analog optical modulators used to provide the analog optical signal. Depending on specific construction, the modulators may be subject to internal electrical interference between sections driven by individual bits of the digital signal being transmitted.

This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.

SUMMARY

In an implementation an n-bit electro-optic modulator (EOM) for converting an electrical signal into an optical signal is provided. The EOM may include a plurality of n optical modulators, pairs of the plurality of n optical modulators each one receiving light at a common wavelength of n/2 different wavelengths, and each one of the plurality of n optical modulators amplitude encoding one of n bits of an input digital n-bit source using the corresponding received common wavelength of light according to a significance of that digital bit； and a plurality of n/2 beam combiners, each receiving a pair of modulated outputs from one of the pairs of optical modulators and combining the pair of modulated outputs into a single dual-polarized output.

In an aspect each one of the plurality of n/2 beam combiners comprises a polarization beam combiner. Each of the n/2 polarization beam combiners may be configured to convert one of the corresponding pair of modulated outputs to an orthogonal polarization. In an aspect the EOM may further comprise a polarization-insensitive WDM multiplexer for receiving and combining the n/2 dual-polarized outputs for coupling to an optical fiber. The plurality of n optical modulators and the plurality of n/2 polarization beam combiners may, in an aspect, be monolithically integrated on a silicon photonic chip.

In an aspect the EOM may further comprise a plurality of n/2 beamsplitters each for splitting light at a particular wavelength of the n/2 different wavelengths for coupling to the corresponding pair of the plurality of n optical modulators. In an aspect, ach one of the plurality of n/2 beamsplitters may be configured for a 1: 2 splitting ratio.

In an aspect each one of the plurality of n optical modulators comprises a Mach-Zehnder optical modulator.

In an aspect the EOM may further comprise a multiplexer for receiving and combining the n/2 dual-polarized outputs for coupling to an optical fiber.

In an aspect the plurality of n optical modulators are electrically isolated from one another.

In an implementation a receiver is provided. The receiver may include a photodetector for receiving an optical signal composed of n/2 dual-polarized optical signals each encoded with one of n/2 wavelengths of light, and converting the received n/2 dual-polarized optical signals into an electrical signal composed of a sum of the n/2 dual-polarized optical signals.

In an implementation a system is provided for transmitting an input digital n-bit source to a broadcast location. The system may include a plurality of n optical modulators, pairs of the plurality of n optical modulators each receiving light at a common wavelength of n/2 different wavelengths, and each one of the plurality of n optical modulators amplitude encoding one of n bits of an input digital n-bit source using the corresponding received common wavelength of light according to a significance of that digital bit； a plurality of n/2 beam combiners, each one receiving a pair of modulated outputs from one of the pairs of optical modulators and combining the pair of modulated outputs into a single dual-polarized output； a multiplexer for receiving and combining the n/2 dual-polarized outputs for injection into an optical fiber to transmit the n/2 dual-polarized outputs to the broadcast location； and, a photo-detector at the broadcast location for receiving the n/2 dual-polarized outputs from the optical fiber and converting the received the n/2 dual-polarized outputs into an electrical signal composed of a sum of the n/2 dual-polarized outputs.

In an aspect each one of the n/2 beam combiners comprises a polarization beam combiner for converting one of the corresponding pair of modulated outputs to an orthogonal polarization. In an aspect the multiplexer comprises a polarization-insensitive WDM multiplexer for receiving and combining the n/2 dual-polarized outputs for coupling to the optical fiber. In an aspect the plurality of n optical modulators and the plurality of n/2 polarization beam combiners are monolithically integrated on a silicon photonic chip.

In an aspect the system further comprises a plurality of n/2 beamsplitters each for splitting light at a particular wavelength of the n/2 different wavelengths for coupling to the corresponding pair of the plurality of n optical modulators. In an aspect each one of the plurality of n/2 beamsplitters is configured for a 1: 2 splitting ratio.

In an aspect of the system each one of the plurality of n optical modulators comprises a Mach-Zehnder optical modulator.

In an aspect of the system the plurality of n optical modulators are electrically isolated from one another.

In an aspect the system further comprises an RF transmitter for amplifying and broadcasting the electrical signal.

In an implementation a method is provided for transmitting a radio-over-fiber signal to a broadcast location. The method may include optically amplitude encoding n bits of an input digital bit source according to a significance of each bit using n/2 wavelengths of light, pairs of bits being optically encoded with a same wavelength of light, and combining each of the pairs of bits optically encoded with the same wavelength of light to produce n/2 dual-polarized signal outputs； combining the n/2 dual-polarized signal outputs into an optical signal； and, coupling the optical signal into an optical fiber for transmission to the broadcast location.

In an aspect the n/2 dual-polarized signal outputs are produced by orthogonally polarizing the pairs of bits amplitude encoded with the same wavelength of light.

In an aspect the method further comprises receiving the optical signal at the broadcast location and converting the received optical signal into an electrical signal composed of a sum of the n/2 dual-polarized signal outputs； and, amplifying and transmitting the electrical signal.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Figure 1 illustrates a phase-shift modulated electro-optic modulator.

Figure 2 illustrates an amplitude modulated electro-optic modulator.

Figure 3 illustrates an embodiment of an amplitude modulated electro-optic modulator.

Figure 4 illustrates an embodiment of a receiver for receiving an amplitude modulated optical signal.

Figure 5 is a process flow diagram representing an embodiment for transmitting a radio-over-fiber signal to a broadcast location.

Figure 6 is a process flow diagram representing an embodiment for receiving a radio-over-fiber signal at a broadcast location.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

Figure 1 illustrates an electro-optic modulator (EOM) 100 for converting an electrical digital signal into an analog optical signal. The EOM 100 is based on a Mach-Zehnder interferometer having an optical input 105 for receiving a continuous wave light beam 106 generated by a laser source 107, first 101 and second 102 branches coupled to the optical input 105, and an optical output 110 coupled to the

branches

101 and 102 for outputting an analog modulated light beam 108. The first branch 101 includes segments 121-128 having different lengths, preferably in a binary fashion, i.e. each subsequent segment length is twice longer or shorter than the previous. A digital bit source 115 provides the electrical information to be encoded in form of an 8-bit digital signal including bits Bit₁-Bit₈. In operation, the EOM 100 applies electrical voltages corresponding to individual Bit₁-Bit₈ to different segments 121-128, each length corresponding to a significance of different one of the bits. The application of the electric field induces a modulation in the refractive index of the segments 121-128, thereby causing a phase shift of a magnitude proportional to the length of the different segments 121-128. In the case of this exemplar 8-bit modulator, as illustrated below the induced phase shift differs for each bit section of refractive material is different, which allows for an additive modulation:

RF_1 -π/2

RF_2 -π/4

RF_3 -π/8

RF_4 -π/16

RF_5 -π/32

RF_6 -π/64

RF_7 -π/128

RF_8 -π/256

In operation, the phase shifts RF1. . RF8 are added together, providing an overall phase shift in the first branch 101 varying from 0 to 255π/256. The optical interference of the phase delayed light in the first branch 101 with light in the second branch 102 causes the output light beam 108 to be amplitude modulated. A limitation of the EOM 100 is that, as illustrated, the segments 121-128 are physically close to one another, since they are in a same branch, i.e. the first branch 101. The close physical proximity of the electrodes can lead to electrical crosstalk between the bits when encoding the signal.

Figure 2 illustrates an n-bit EOM 200 for converting an electrical digital signal into an analog optical signal. The EOM 200 of Figure 2 has an input digital bit source 215 that provides the information to be encoded. The digital bit source 215 provides n bits of digital information, each bit to be encoded by an optical intensity modulator 213_1…n . The optical intensity modulator 213_1…n may include a ring modulator, a Mach-Zehnder modulator, an electro absorption modulator, and the like. The modulators 213_1…n each receive an optical input consisting of a continuous wave laser light beam generated by a corresponding laser source 207_1…n. The laser sources 207_1…n each emit light at a different wavelength of light λ_1…n.

The modulators 213_1…n modulate the received light sources to encode one of the n bits supplied by the digital bit source 215. Modulated outputs 217_1…n have an optical power in the relation of P_o/2ⁱ (where i is the index from 1 to n corresponding to that modulated outputs 217_1…n) . The outputs 217_1…n from each modulator 213 are amplitude modulated signals corresponding to the significance of each encoded bit. The outputs 217_1…n may be additively combined using a wavelength division multiplexer (WDM) 230. The combined output may be injected into an optical fiber 240 for transmission (e.g. to a base station for transmission) . At the base station, the optical signal may be converted from the optical domain to the electrical domain, and the resultant electrical signal may be amplified for broadcast. In some aspects a plurality of m signals may be so encoded and transmitted to a plurality of m receiving stations using optical multiplexing techniques known in the art.

Figure 3 illustrates an embodiment of an n-bit amplitude modulated electro-optic modulator (EOM) 300 for converting an electrical signal into an optical signal. The EOM 300 of Figure 3 receives an input from the n-bit digital bit source 215 that provides the information to be encoded. The digital bit source 215 provides n bits at a time, each bit to be encoded by an optical modulator 313_1…n (such as a ring modulator, Mach-Zehnder modulator, electro-absorption modulator, and the like) . The modulators 313_1…n each receive one of n optical inputs 302_1…n consisting of a continuous wave laser light beam. Pairs of optical modulators 313_1…n receive optical inputs having the same wavelength of light λ_1…n/2. The n continuous wave laser light beams may be generated by a corresponding n laser sources or, as illustrated in Figure 3, the n continuous wave laser light beams may be generated by n/2 laser sources 307_1…n/2, each producing a different wavelength of light λ_1…n/2. In either case, each light beam 302_1…n has a different power such that the optical powers of the n light beams correspond to an n-bit binary relationship Pⁿ.

Referring to the example of Figure 3 in which n/2 laser sources 307_1…n/2 are used, each laser source 307_1…n/2 produces a different wavelength of light λ _1…n/2. Furthermore, the n/2 laser sources 307_1…n/2 may produce different power levels, each source supplying two of the n optical powers. The outputs from each of the n/2 laser sources 307_1…n/2 may be input to a corresponding one of n/2 1: 2 optical beam power splitters (BS) 308_1…n/2. The BS 308_1…n/2 each split the received laser beam into two optical beams, one having twice higher optical power than the other, such that the resulting optical beams have a corresponding Pⁿ power relationship 1P, 2P, 4P, 8P, …, 128P, as indicated in Figure 3. In an aspect, the laser sources 307_1…n/2 may each produce a same power level and attenuators (not shown in Figure 3) are used to provide the appropriate power level to input into the modulators 313_1…n.

In the example of Figure 3, the laser sources 307_1…n/2 each power two optical modulators 313_1…n using a same wavelength of light λ_1…n/2. In the embodiment of Figure 3, the laser sources 307_1…n/2 power optical modulators 313_1…n encoding successive bits, but they could encode any other of the n bits in practice and this selection is not intended to be limiting.

The optical modulators 313_1…n each modulate the received light to encode a corresponding one of the n bits supplied by the digital bit source 215. The input digital bit corresponding to each of the optical modulators 313_1…n is used to selectively pass or block the light provided by the corresponding optical input 302_1…n. As a result, the modulated outputs 317_1…n from each modulator 313 are amplitude modulated optical signals corresponding to the significance of the encoded bits, based upon the power of the associated optical input 302_1…n. In an embodiment the pairs of modulated outputs 317_1…n of common wavelength are combined using polarization beam combiners (PBC) 315_1…n/2 to convert the pairs of modulated outputs 317_1…n into two orthogonal linear polarizations (e.g. TE &TM polarization modes) and combined to produce n/2 dual-polarized outputs 318_1…n/2.

The embodiment above described the use of PBC 315_1…n/2 to produce the dual-polarized outputs 318_1…n/2 as an illustrative example only. There are a number of other arrangements that may be used to produce n/2 dual-polarized outputs 318_1…n/2, and all are considered aspects of the present invention. For instance, an embodiment may employ polarization beam splitters in place of the BS 308_1…n/2. The polarization beam splitters splitting the n/2 laser beams produced by the n/2 laser sources 307_1…n/2 into n/2 pairs of orthogonally polarized beams. In this embodiment the n optical inputs 302_1…n comprise orthogonally polarized optical beams that are then modulated by the optical modulators 313_1…n. The polarized modulated outputs may then be combined by the n/2 PBC 315₁, n/2 beam combiners, an n-input multiplexer, or a combination of a multiplexer and beam combiners. As will be appreciated, use of the PBC is preferable as regular BCs are lossier.

In an implementation using polarization beam splitters (PBS) , two photonic chips may be used to provide optical modulators 313_1…n One photonic chip would support n/2 of the optical modulators 313_1…n to modulate n/2 optical beams of a first polarization, while the other photonic chip would support the remaining n/2 of the optical modulators 313_1…n to modulate the n/2 optical beams of the orthogonal polarization. Separating adjacent input bit bits for modulation between the two photonic chips may provide for additional isolation between input digital bits.

The n/2 dual-polarized outputs 318_1…n/2 may be coupled using an optical multiplexer 340, e.g. a polarization-insensitive WDM multiplexer, for injection into the optical fiber 240. In some aspects, the optical multiplexer 340 may be a part of the EOM 300. In other aspects, as illustrated in Figure 3, the optical multiplexer 340 may be separate from the EOM 300. As with the EOM 200 of Figure 2, in some aspects a plurality of m signals may be so encoded and transmitted to a plurality of m receiving stations (e.g. base station) using optical multiplexing techniques known in the art.

Referring to Figure 4, an embodiment of a transmission system 400 may include a transmission end 405 including the EOM 300 of Figure 3 which produces an amplitude modulated optical signal encoding n channels with only n/2 wavelengths of light λ_1…n/2, and a receiving end 410 for receiving the amplitude modulated optical signal is presented. The receiving end 410 may be located, for instance, at the receiving station to extract the amplitude modulated optical signal from the optical fiber 240. The receiving end 410 includes a photo-detector 420 which receives the amplitude modulated optical signal from the optical fiber 240 and converts the received amplitude modulated optical signal into an electrical signal. The amplitude of the electrical signal corresponds to a sum of the amplitudes of the modulated outputs 317_1…n. The converted electrical signal may then be amplified for broadcast by an RF transmitter 450.

In operation, the EOM 300 takes as input from the n-bit digital bit source 215, and amplitude encodes each bit onto a separate channel. The channels are differentiated by wavelength λ_1…n/2 and by polarization to produce n channels with only n/2 wavelengths of light λ_1…n/2. The n channels are multiplexed together and transmitted over the optic fiber 240. At the receiving end 410, the output from the optical fiber 240 is directed onto the photo-detector 420 to obtain a signal that has a magnitude corresponding to the sum of the n channels.

In step 500 n bits of an input digital bit source are optically amplitude encoded according to a significance of each bit using n/2 wavelengths of light. Pairs of the bits being optically encoded with a same wavelength of light. The input digital bit source being a digital representation of the radio waveform to be transmitted from the broadcast location.

In step 510 each of the pairs of bits optically encoded with the same wavelength of light are combined to produce n/2 dual-polarized optical signal outputs. In an aspect, the n bits may be optically encoded onto polarized optical beams that are then combined. In an aspect, the n bits may be optically encoded onto polarized optical beams produced by PBSs that split input non-polarized laser sources into two orthogonally polarized optical beams. The optically encoded polarized optical beams are then combined. In an aspect, the n bits may be optically encoded onto optical beams that are not polarized or have a same polarization state, and the encoded optical beams may then be orthogonally polarized. In an aspect the n bits may be optically encoded onto optical beams that are then polarized and combined by PBCs. In either case, n/2 dual-polarized optical signal outputs are produced.

In step 520 the n/2 dual-polarized optical signal outputs are combined into an optical signal.

In step 530 the optical signal may be coupled to an optical fiber for transmission to the broadcast location.

In step 600 an optical signal is received, at a broadcast location. The optical signal being an amplitude encoded optical signal that represents the radio waveform to be transmitted at the broadcast location. The optical signal is composed of n/2 dual-polarized optical signals, each encoded with one of n/2 wavelengths of light. In this fashion the optical signal is can transport n channels of information using n/2 wavelengths of light since each wavelength carries two orthogonally polarized channels of information.

In step 610 the received optical signal, i.e. the n/2 dual-polarized optical signals, are directed onto a photo-detector.

In step 620, using the photo-detector, the directed n/2 dual-polarized optical signals are converted into an electrical signal. The resulting electrical signal being a sum of the n/2 dual-polarized optical signals. Accordingly, the electrical signal may be produced without performing any signal processing at the broadcast location. Since the input digital bits are amplitude encoded, and differentiated by wavelength and polarization, the output from the photo-detector represents a sum of the input amplitude encoded digital bits.

In step 630 the electrical signal may be amplified and transmitted from the broadcast location. As indicated above, the present method allows for a radio-over-fiber transmission where the receiving end converts the received optical signal directly into an electrical signal.

Although the present invention has been described with reference to specific features and embodiments thereof, it is evident that various modifications and combinations can be made thereto without departing from the invention. The specification and drawings are, accordingly, to be regarded simply as an illustration of the invention as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present invention.

Claims

An n-bit electro-optic modulator (EOM) for converting an electrical signal into an optical signal, comprising:

a plurality of n optical modulators, pairs of the plurality of n optical modulators each one receiving light at a common wavelength of n/2 different wavelengths, and each one of the plurality of n optical modulators amplitude encoding one of n bits of an input digital n-bit source using the corresponding received common wavelength of light according to a significance of that digital bit； and,

a plurality of n/2 beam combiners, each receiving a pair of modulated outputs from one of the pairs of optical modulators and combining the pair of modulated outputs into a single dual-polarized output.
The n-bit EOM of claim 1, wherein each one of the plurality of n/2 beam combiners comprises a polarization beam combiner.
The n-bit EOM of claim 2, wherein each one of the n/2 polarization beam combiners is configured for converting one of the corresponding pair of modulated outputs to an orthogonal polarization.
The n-bit EOM of claim 3 further comprising a polarization-insensitive WDM multiplexer for receiving and combining the n/2 dual-polarized outputs for coupling to an optical fiber.
The n-bit EOM of claim 2, wherein the plurality of n optical modulators and the plurality of n/2 polarization beam combiners are monolithically integrated on a silicon photonic chip.
The n-bit EOM of claim 1 further comprising a plurality of n/2 beamsplitters each for splitting light at a particular wavelength of the n/2 different wavelengths for coupling to the corresponding pair of the plurality of n optical modulators.
The n-bit EOM of claim 6, wherein each one of the plurality of n/2 beamsplitters is configured for 1: 2 splitting ratio.
The n-bit EOM of any one of claim 1 to claim 7, wherein each one of the plurality of n optical modulators comprises a Mach-Zehnder optical modulator.
The n-bit EOM of any one of claim 1 to claim 7 further comprising:

a multiplexer for receiving and combining the n/2 dual-polarized outputs for coupling to an optical fiber.
The n-bit EOM of any one of claim 1 to claim 7, wherein the plurality of n optical modulators are electrically isolated from one another.
A receiver comprising:

a photodetector for receiving an optical signal composed of n/2 dual-polarized optical signals each encoded with one of n/2 wavelengths of light, and converting the received n/2 dual-polarized optical signals into an electrical signal composed of a sum of the n/2 dual-polarized optical signals.
A system for transmitting an input digital n-bit source to a broadcast location comprising:

a plurality of n optical modulators, pairs of the plurality of n optical modulators each receiving light at a common wavelength of n/2 different wavelengths, and each one of the plurality of n optical modulators amplitude encoding one of n bits of an input digital n-bit source using the corresponding received common wavelength of light according to a significance of that digital bit；

a plurality of n/2 beam combiners, each one receiving a pair of modulated outputs from one of the pairs of optical modulators and combining the pair of modulated outputs into a single dual-polarized output；

a multiplexer for receiving and combining the n/2 dual-polarized outputs for injection into an optical fiber to transmit the n/2 dual-polarized outputs to the broadcast location； and,

a photo-detector at the broadcast location for receiving the n/2 dual-polarized outputs from the optical fiber and converting the received the n/2 dual-polarized outputs into an electrical signal composed of a sum of the n/2 dual-polarized outputs.
The system of claim 12, wherein each one of the n/2 beam combiners comprises a polarization beam combiner for converting one of the corresponding pair of modulated outputs to an orthogonal polarization.
The system of claim 13, wherein the multiplexer comprises a polarization-insensitive WDM multiplexer for receiving and combining the n/2 dual-polarized outputs for coupling to the optical fiber.
The system of claim 13, wherein the plurality of n optical modulators and the plurality of n/2 polarization beam combiners are monolithically integrated on a silicon photonic chip.
The system of claim 12 further comprising a plurality of n/2 beamsplitters each for splitting light at a particular wavelength of the n/2 different wavelengths for coupling to the corresponding pair of the plurality of n optical modulators.
The system of claim 16, wherein each one of the plurality of n/2 beamsplitters is configured for 1: 2 splitting ratio.
The system of any one of claim 12 to claim 17, wherein each one of the plurality of n optical modulators comprises a Mach-Zehnder optical modulator.
The system of any one of claim 12 to claim 17, wherein the plurality of n optical modulators are electrically isolated from one another.
The system of any one of claim 12 to claim 17 further comprising an RF transmitter for amplifying and broadcasting the electrical signal.
A method for transmitting a radio-over-fiber signal to a broadcast location comprising:

optically amplitude encoding n bits of an input digital bit source according to a significance of each bit using n/2 wavelengths of light, pairs of bits being optically encoded with a same wavelength of light, and combining each of the pairs of bits optically encoded with the same wavelength of light to produce n/2 dual-polarized signal outputs；

combining the n/2 dual-polarized signal outputs into an optical signal；

coupling the optical signal into an optical fiber for transmission to the broadcast location.
The method of claim 21 wherein the n/2 dual-polarized signal outputs are produced by orthogonally polarizing the pairs of bits amplitude encoded with the same wavelength of light.
The method of claim 21 or claim 22 further comprising:

receiving the optical signal at the broadcast location and converting the received optical signal into an electrical signal composed of a sum of the n/2 dual-polarized signal outputs； and,

amplifying and transmitting the electrical signal.