Analog modulation of optical signals

[0001] This invention relates to optical transmission systems and, more particularly, to analog modulation of optical signals.

[0002] There is a clear trend towards using higher carrier frequencies and high date rates in optical communications systems. One such arrangement employs Mach Zehnder modulators supplied with individual light, i.e., optical carrier, signals to be modulated. Each of the Mach Zehnder modulators is modulated directly by a microwave sub-carrier that has been modulated with the desired digital data. See for example, an article authored by W. D. Jemison et al., entitled “Microwave Photonic Vector Modulator for High-Speed Wireless Digital Communications”, IEEE Microwave And Wireless Components Letters, pp 125–127, Vol. 12, No. 4, April 2002. This and other known optical signal modulation arrangements have limited bandwidth and, hence, there is a limit on the data rate that may be used in optical communications systems employing such modulation schemes.

[0003] These and other problems and limitations of prior known optical modulation arrangements are overcome in applicants' unique invention by generating a so-called sub-carrier modulated optical signal and, then, vector modulating the sub-carrier modulated optical signal to yield the desired modulated optical signal for transmission. The vector modulation includes a phase component and/or an amplitude component.

[0004] In one specific embodiment of the invention, the vector modulation is effected by splitting the sub-carrier modulated optical signal comprising a carrier and an at least one unmodulated sub-carrier into a plurality of similar optical signals, phase modulating the carrier of each of the similar optical signals with the data and delaying the resulting optical signals in a prescribed manner relative to one another. Then, the “delayed” signals are combined to yield the signal comprising the vector modulated sub-carrier modulated optical signal to be transmitted. Wherein a signal can be “delayed” by a zero (0) delay interval.

[0005] In another embodiment of the invention, the “sub-carrier modulated optical signal” is obtained by using two optical carriers that are slightly offset in wavelength relative to each other so as to generate a beating tone in a remote photodiode detector. The two optical carriers are coupled together and supplied to the vector modulator to be vector modulated with the data signal.

[0006] In yet another embodiment of the invention, the phase modulators in the vector modulator are grouped in sets. Then, the phase modulated optical signals in each set are each delayed by the same interval for that set. The delay intervals for the individual sets are in a prescribed relationship to each other. In one example, a first set has zero delay interval, a second set has a delay interval of τ, a third set has a delay interval of 2τ, a fourth set has a delay interval of 3τ and so on.

[0007] In still another embodiment of the invention, the data signal is precoded to generate signals to drive particular ones of the phase modulators of the vector modulator to desired vector modulation states for each state of the data signal.

[0017] FIG. 1 shows, in simplified block diagram form, one embodiment of the invention. Specifically, shown is optical light source 101 typically including a laser to generate an optical carrier signal at a desired wavelength. Exemplary optical carrier signals to be processed have optical frequencies of about 2.3×10¹⁴ Hertz to about 1.8×10¹⁴ Hertz, i.e., a wavelength of about 1.3 microns to about 1.7 microns. In one example, an optical carrier signal having a wavelength of approximately 1.55 microns, i.e., a frequency of 1.93×10¹⁴ Hertz is generated by light source 101 and supplied to amplitude modulator 102. Also supplied to amplitude modulator 102 via input 103 is a desired amplitude modulation signal that modulates the optical carrier from source 101 to obtain a sub-carrier modulated optical signal. As is known, the spectrum of this signal includes the optical carrier and two sidebands, i.e., sub-carriers. This modulation may be achieved in a number of ways. It can be realized by use of an electro-optical modulation scheme through either direct modulation of the laser in source 101 or by an external modulation using, for example, a Mach Zehnder modulator or an electro-absorption modulator. The frequency of the modulation signal is in the microwave/millimeter-wave range. Another approach using two optical carrier signals is described below in conjunction with the embodiment of FIG. 3. The sub-carrier-modulated optical signal from amplitude modulator 102 is supplied to vector modulator 104 where the subcarrier is phase and/or amplitude modulated, i.e., vector modulated with a data signal supplied via input 105 to yield the optical transport signal at output 106. In this embodiment, the data rate can be as high as the sub-carrier frequency. Embodiments of vector modulators 104 that may be employed in the embodiment of the invention of FIG. 1 are shown in FIGS. 4 and 5 and described below.

[0018] FIG. 2 also shows, in simplified block diagram form, a second embodiment of the invention. The embodiment of FIG. 2 is similar to that of FIG. 1 and includes similar elements that are essentially identical from both a physical and functional standpoint. These similar elements have been similarly numbers as those in FIG. 1 and will not be discussed again in detail. The only significant difference between the embodiments of FIG. 1 and FIG. 2 is the use of Precoder and Driver 201. As can be seen, one or more data signals are supplied to precoder and driver 201 via input connections 105-1 through 105-N. In precoder and driver 201, the data signals are employed to code and generate voltage signals for driving, phase modulators in vector modulator 104. These voltage signals are supplied via output connections 107-1 through 107-M to vector modulator 104. Again, details and operation of vector modulator 104 are described below regarding FIGS. 4 and 5 and a simple example of operation of precoder and driver 201 in conjunction with vector modulator 104 is described below regarding FIG. 7.

[0019] FIG. 3 illustrates, in simplified block diagram form, a third embodiment of the invention. The embodiment of FIG. 3 is similar to that of FIG. 2 and includes similar elements that are essentially identical from both a physical and functional standpoint. These similar elements have been similarly numbers as those in FIG. 2 and will not be discussed again in detail. The only significant difference between the embodiments of FIG. 2 and FIG. 3 is how the sub-carrier modulated optical signal is generated in the embodiment of FIG. 3. As indicated above, there are several ways of generating the desired sub-carrier modulated optical signal upon which the data signal will eventually be modulated. In the embodiment of FIG. 3, a coherent technique is advantageously employed to realize the desired sub-carrier modulated optical signal. Specifically, in this example, two carrier optical signals are generated by employing tunable laser light sources 301 and 302. Signals for controlling the frequency (wavelength) of the laser light are supplied via connections 303 and 304 to tunable light sources 301 and 302, respectively. The resulting two optical carrier signals are tuned to have a prescribed wavelength offset and are overlaid coherently. At a receiving detector, typically a photodiode, the difference in wavelength generates a beating signal. The lasers, in this example, are single mode tunable lasers. This allows very wide tunability, since tuning the wavelength of one laser directly changes the frequency of the beating signal. Another advantage of using the two tunable lasers is that the resulting optical synthesizer generates a virtually harmonic free optical source. This results in an improved dynamic range and spurious free dynamic range of the optical link. Then, the two optical carriers that are offset relative to each other are supplied to optical coupler 305 where they are combined, i.e., mixed, to form the desired o sub-carrier modulated optical signal, which is supplied to vector modulator 104.

[0020] FIG. 4 shows, in simplified block diagram form, details of one vector modulator 104 that may be employed in the embodiments of the invention. The optical vector modulator 104 is based on the summing of multiple optical tapped delay lines. The principle of operation is as follows: The to be phase-shifted and/or to be amplitude-modulated signal could either be an amplitude-modulated single optical carrier generated by amplitude modulator 102 (FIG. 1 or FIG. 2) or include two optical carriers that are slightly offset in wavelength to generate a beating signal generated by tunable light sources 301 and 302 and coupler 305 of FIG. 3. This sub-carrier modulated optical signal is, when fed via 401 into the optical vector modulator 104 of FIG. 4, split into a plurality of similar branches by a power splitter 402, e.g., a multimode interference (MMI) coupler. Each branch is equipped with a phase modulator 403-1 through 403-M to adjust the phase of the optical carrier followed by an optical delay line. The delay in series with phase modulator 403-1, in this example, is zero (0). The delays in the remaining branches including phase modulators 403-2 through 403-M are generated by delay units 404-1 through 404-(M−1), respectively. Each of these delay lines in delay units 404-1 through 404-(M−1) changes the phase of the sub-carrier of the optical signal from phase modulators 403-2 through 403-M, respectively, by a fixed amount. For example, the delay line in unit 404-1 provides a delay of τ, delay unit 404-2 provides a delay of 2τ and delay unit 404-(M−1) provides a delay of (M−1)τ. Typically, a delay τ of 1/(M*sub-carrier frequency) is required. Thus, if the sub-carrier frequency is 40 GHz, the total delay range should be 0, . . . 25 picoseconds (ps). Another MMI coupler, i.e., power combiner 405, combines all of the phase-shifted and delayed optical signals from all branches to produce a modulated output optical signal at output 106, which will interfere constructively or destructively depending on the summing optical phases from all tributary branches. Therefore, by interfering signals with different carrier phase, the phase and the amplitude of the carrier of the summing signal can be set to an arbitrary state. These interfered optical carriers will produce microwave phasors with prescribed amplitude and phase at the remote optical detector.

[0021] The phase modulator 403 of each branch can be fabricated e.g. in a material system with linear electro-optic effect, as InP, GaAs or LiNbO₃. The effective refractive index of an optical waveguide changes in proportion to the applied electrical field perpendicular to this waveguide. A high frequency distributed electrical waveguide is engineered to co-propagate with the optical wave with matched propagating velocity to deliver the local electrical field with high modulation bandwidth. The different branches will delay the optical signal by a different length of time. This results in different sub-carrier phases at the outputs of these delay lines in units 404. In the combiner 405, these different output signals that interfere constructively have a different sub-carrier phase due to the different time delays these signals experienced. The sub-carrier of the signal after the MMI coupler, i.e., power combiner 405, is the sum of all sub-carriers of the signals that interfere constructively.

[0022] FIG. 5 shows, in simplified block diagram form, details of a specific vector modulator that may be employed in the embodiments of the invention. Such an optical vector phase modulator 104 can be realized in a number of different configurations. One of the simplest designs is to use eight branches, which are grouped into pairs of two with the same delay line length. Basically, the branch pairs form Mach-Zehnder modulators 403. The delay lengths of these different pairs are designed such that they differ by a quarter period of the center frequency of the envelope signal. If now only one of the Mach-Zehnder structures is biased such that the optical signal of each arm interferes constructively, one can set four different phase states each separated by 90°. If two neighboring Mach Zehnder phase modulators 403 are switched on together, this results in a phase offset of 45° in comparison to the signal from a single Mach Zehnder phase modulator 403.

[0023] Specifically, FIG. 5 shows input 401 to which an appropriate sub-carrier modulated optical signal is supplied, for example, from amplitude modulator 102 of FIG. 2 or the two combined offset optical carriers generated by tunable light sources 301 and 302 and coupler 305 of FIG. 3. The supplied signal is split into eight branches and a branch is supplied to each one of phase modulators 403-1 through 403-8. Again, the phase modulators 403-1 through 403-8 are grouped into sets of two and each set has a different delay path. For example, a first set including phase modulators 403-1 and 403-2, has a zero delay; a second set including phase modulators 403-3 and 403-4, has a delay of τ generated by delay lines in delay units 501-1 and 501-2; a third set including phase modulators 403-5 and 403-6, has a delay of 2τ generated by delay lines in delay units 501-3 and 501-4; and a fourth set including phase modulators 403-7 and 403-8, has a delay of 3τ generated by delay lines in delay units 501-5 and 501-6. The optical signals from the eight branches are combined in power combiner 405 to yield the desired output optical signal at output 106.

[0024] FIG. 6 shows all possible phase/amplitude states of the optical envelope produced by vector modulator 104 of FIG. 5, if each phase modulator is driven either with 0V or with a Voltage that results in a π/2 phase shift of the optical carrier or with a Voltage that results in a π phase shift of the optical carrier. It is possible to use digital, multi-level or analog control signals, which allows obtaining all possible phase/amplitude states in-between the discrete ones shown in FIG. 6.

[0025] FIG. 7 illustrates, in simplified block diagram form, a simplified version of the invention to illustrate operation of the precoder and driver 201 shown in FIGS. 2 and 3. Specifically, optical light source 101 supplies an sub-carrier modulated optical signal to the vector modulator 104 and, therein, to power splitter 402. In this example, power splitter 402 splits the supplied optical carrier into three equal versions, i.e., branches, each of which is supplied to one of phase modulators 403-1, 403-2 and 403-3. An output of phase modulator 403-2 is supplied to delay unit 404-1 wherein a delay line delays the phase modulated signal by τ. An output of phase modulator 403-3 is supplied to delay unit 404-2 wherein a delay line delays the phase modulated signal by 2τ. In turn, the output from phase modulator 403-1 and the outputs from delay units 404-1 and 404-2 are supplied to power combiner 405 where they are combined, as described above, to generate the desired transport output optical signal at output 106. The transport optical signal is transmitted to a remote receiver where it is detected via, for example photodiode 701, which generates a desired signal at terminals 702. Returning, phase modulators 403-1, 403-2 and 403-2 are supplied control signals via paths 107-1, 107-2 and 107-3, respectively, from precoder and driver 201. The operation of precoder 201 can be explained by referring to the table in FIG. 8. In this example, it is desired to generate symbols A, B, C and D. This is realized by supplying a binary input to terminal 105-1 and 105-2 of precoder and driver 201. As seen in FIG. 9, the desired vectors corresponding to the location of the symbols A, B, C and D in the constellation shown in FIG. 9 are (0,0) 1<60°, (0,1) 1<−60°, (1,0) 1<180° and (1,1) 0, respectively. These vectors are realized by controlling phase modulators 403-1, 403-2 and 403-3 by the modulation voltages V₁, V₂ and V₃, respectively. Thus, for symbol A (binary input 0,0) V₁=V₂=V₃=0, for symbol B (binary 0,1) V₁=V_π and V₁=V₂=0, for symbol C (binary 1,0) V₁=V₂0 and V₃=V_π and for symbol D (binary 1,1) V₁=V₃=0 and V₂=V_π.

[0026] The above-described embodiments are, of course, merely illustrative of the principles of the invention. Indeed, numerous other methods or apparatus may be devised by those skilled in the art without departing from the spirit and scope of the invention.