Optical signal modulation

ES 2 579 604 T3

DESCRIPTION

Optical signal modulation

Technical field

The invention relates to a quadrature amplitude modulation optical modulator 2ⁿ (QAM), a quadrature 2 amplitude modulation methodⁿ, and an optical signal transmission apparatus comprising the 2-phase optical modulatorⁿ-QAM.

Background

16-level quadrature amplitude modulation (16-QAM) is one of the candidate modulation formats for 100 gigabit / s (Gb / s) transmission over fiber optics. It encodes four bits of a sixteen-point constellation, with four different amplitude values ​​of the in-phase and quadrature components of the transmitted signal. When considering a diversity bias 16-QAM format for 100 Gb / s transmission, the symbol rate required for the transmitter is 12.5 Gbaud. Even at this low symbol rate, generating multi-level drive signals can be tricky. Another complication arises from the need to apply differential digital coding to the optical signal prior to optical modulation, to resolve the π2 phase ambiguity of the QAM constellation that would otherwise arise when estimating the carrier phase at the receiver. .

There are four known 16-QAM transmitter / modulator schemes, as follows. The first comprises a conventional in-phase / quadrature (IQ) modulator in which each of the in-phase and quadrature components is a four-level signal obtainable using a four-level driving voltage. Despite the simplicity of the optical scheme, the requirement to generate four-level drive voltages makes the transmitter less attractive for application in 100 Gb / s systems.

A second scheme comprises a single dual-drive Mach Zehnder modulator (MZM), as reported in K.-P. Ho and H.-W. Cuei, Generation of arbitrary quadrature signals using one dual-drive modulator, J. Lightwave Technology., Vol. 23, no. 2, Pages 764-770, February 2005. The output signal can assume any value in the complex plane through a correct choice of the driving voltages for the two MZMs. This scheme incorporates the simplest optical components, however, the generation of the 16-QAM constellation requires a very complex drive voltage scheme with up to 16-level signals.

A third 16-QAM transmitter structure, comprising an IQ amplitude and phase modulator, requiring only two-level driving voltages, has been reported in M. Seimetz's paper, Multi-format transmitters for coherent optical M-PSK and M-QAM transmission, in Proc. ICTON'05, 2005, pages 225-229, paper Th.B1.5. The basic structure is similar to a conventional IQ modulator but each branch also comprises a phase modulator. In each arm, the MZM generates the two amplitude levels {1/3, 1}, and the phase modulator (PM) sets the phase to zero or π, to obtain the necessary signals from four levels in each quadrant of the constellation diagram.

The fourth 16-QAM transmitter structure comprises two quaternary phase shift modulators (QPSK) nested within a Mach-Zehnde interferometer that has an 80:20 output combination ratio, as reported in the JME Kahn and Ip paper. , Carrier synchronization for 3- and 4-bit-per-symbol optical transmission, J. Lightwave technology, vol. 23, no. 12, pp. 4110-4114, December 2005. This scheme achieves 16-QAM modulation using binary driver signals for each QPSK modulator. However, you still need electrical processing of the input bit stream to apply quadrant differential encoding.

A document related to a Mach-Zehnder modulator is known, specifically Sakamoto et al: 50-Gb / s 16QAM by a quad-parallel Mach-Zehnder modulator, Proceedings of the European Conference on Optical Communication, January 1, 2007, page PD2 .8. However, devices and operations such as those now described in the invention are neither disclosed nor suggested in this document.

Lower and higher order QAM, such as quaternary amplitude modulation (4-QAM) and 64-QAM, are also known for encoding digital data, and are collectively referred to herein as 2ⁿ-QAM.

Any occurrence of the term "embodiment" in the description is to be considered as an aspect of the invention, the invention being defined in the accompanying independent claims.

Compendium

The goal is to avoid at least some of the disadvantages mentioned above and to provide a better 2ⁿ-QAM and an optical signal transmission apparatus.

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According to a first aspect of the invention there is provided a quadrature amplitude modulation optical modulator 2ⁿ comprising:

an optical input;

a first optical modulation apparatus configured to apply an amplitude modulation scheme 2^n-2 who has 2^n-2 constellation points arranged in a constellation in a first quadrant of its constellation diagram to a received optical signal; and a second optical modulation apparatus configured to selectively rotate the phase of a received optical signal, wherein the optical input is configured to deliver an optical signal to be modulated to one of the first and second modulation apparatus. optical, said apparatus generates an intermediate optical signal and has also been configured to deliver the intermediate optical signal to the other one between the first and the second optical modulation apparatus, to thereby generate an output modulated optical signal having a square constellation diagram comprising 2ⁿ constellation points distributed over the four quadrants of the constellation diagram.

The constellation diagram quadrant 2ⁿ-QAM is thus established by rotating the constellation points in the first quadrant, rather than translating them as in the nested QPSK 16QAM transmitter scheme. This rotation is equivalent to applying a four-quadrant differential encoding. The 2 optical modulatorⁿ-QAM thereby automatically performs quadrant differential encoding, without requiring additional optical components to implement the encoding.

Preferably n is an even number and is at least 4, more preferably 4 or 6, the constellation points of the first optical modulation apparatus are arranged in a substantially square constellation. Therefore a 16-QAM or 64-QAM optical modulator is provided which has inherent differential encoding per quadrant.

The first quadrant is preferably quadrant I.

The first optical modulation apparatus preferably comprises:

an in-phase / quadrature optical modulator comprising an optical input configured to receive an optical signal to be modulated, an in-phase branch comprising a first optical modulator, a quadrature branch comprising a second optical modulator and a phase shifter π / 2, and an optical output; and a driving apparatus configured to deliver driving electrical signals of ^ (2ⁿ'²) levels to the first and second optical modulators, such that the in-phase / quadrature optical modulator can operate to apply the amplitude modulation scheme 2^n-2.

A 16-QAM optical modulator therefore requires only two-level (binary) driving electrical signals, and performs differential encoding per quadrant without the need for additional optical components. Furthermore, the use of binary driving electrical signals substantially avoids the heavy distortion that multi-level driving signals can experience, due to modulator bandwidth limitations and non-linearity in high speed transmission applications.

A 64-QAM optical modulator would only need four-level driving electrical signals, and therefore requires simpler driving signals than are needed for known 64-QAM modulators.

The first and second optical modulators preferably comprise Mach-Zehnder modulators. Any noise in the driving electrical signals, therefore, will be substantially absorbed by the transfer function without² of the Mach-Zehnder modulators. The 2 optical modulatorⁿ-QAM also generally produces straight transitions between the output signal symbols which offers the advantage of creating stable decision points for all combinations of symbols.

Preferably, n is 4 and the two-level driving electrical signals comprise about 0.39 ν_π, where ν_π is the switching voltage of the respective Mach-Zehnder modulator, and zero volts. The Mach-Zehnder modulators thereby generate the 1/3 and 1 amplitude levels of the output signal, giving the in-phase / quadrature optical modulator a constellation of output signal symbols having points in the I quadrant at the locations I1 (in-phase axis), Q1 (quadrature axis): 1 / 3.1 / 3; 1, 1/3; 1/3, 1; and 1, 1.

Preferably, the secondary optical modulation apparatus is configured to apply a quaternary phase shift modulation scheme having four modulation levels of substantially equal amplitude and phases separated by 90 degrees, the modulation scheme giving a square constellation diagram having a constellation point in each quadrant, to the received optical signal to selectively rotate the phase of the received optical signal.

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Preferably, the second optical modulation apparatus comprises:

a second in-phase / quadrature optical modulator comprising an optical input, an in-phase branch comprising a third optical modulator, a quadrature branch comprising a fourth optical modulator and a π / 2 phase shifter, and an optical output; and a second driving apparatus configured to deliver bi-level driving electrical signals to the third and fourth modulators, the driving electrical signals are of different voltages to the driving electrical signals to the first and second optical modulators, such that the second optical modulation apparatus applies a quaternary phase shift modulation scheme having four modulation levels of substantially equal amplitude and phases separated by 90 degrees, giving the modulation scheme a square constellation diagram having a point constellation in each quadrant, to the received modulated optical signal.

The output signal symbols at the constellation points in quadrant I are therefore respectively left in quadrant I or rotated to quadrant II, III or IV of the constellation diagram.

The third and fourth optical modulators preferably comprise Mach-Zehnder modulators.

The bi-level driving electrical signals to the third and fourth modulators preferably comprise V ^ where V_π is the switching voltage of the respective Mach-Zehnder modulator, and zero volts. The third and fourth modulators thereby generate output signal amplitude levels of -1 and 1, giving the second in-phase / quadrature optical modulator a constellation diagram comprising four constellation points at I2 locations (axis in -phase), Q2 (quadrature axis): 1, 1; eleven; -1, -1, -1, 1.

This rotation is equivalent to applying a four-quadrant differential encoding. The second in-phase / quadrature optical modulator thereby inherently applies differential quadrant coding to the modulated optical signal.

Preferably, the optical output of the in-phase / quadrature optical modulator is coupled to the optical input of the second in-phase / quadrature optical modulator, thereby placing the optical modulators in cascade.

Each of the first and second drive apparatus preferably comprises a first and a second drive respectively, each driving a Mach-Zehnder modulator. Alternatively, the first and second drive apparatus can be provided as a single device.

The second optical modulation apparatus may alternatively comprise a phase modulator configured to selectively rotate the phase of the received signal zero degrees, 90 degrees, 180 degrees, or 270 degrees. This rotation is equivalent to applying a four-quadrant differential encoding. The phase modulator thereby inherently applies quadrant differential coding to the modulated optical signal.

The Quadrature Amplitude Modulation Optical Modulator 2ⁿ it may further comprise a differential encoder that may function to apply differential encoding to the first two bits of the n bits of a data symbol generated by the quadrature amplitude modulation optical modulator 2ⁿ.

According to a second aspect of the invention there is provided an optical signal transmission apparatus comprising:

an optical source that has an optical output and that can function to generate an optical data signal; and a quadrature amplitude modulation optical modulator 2ⁿ According to the first aspect of the invention, the optical output of the optical source is coupled to the first optical modulation apparatus.

The optical signal transmission apparatus thus inherently applies quadrant differential coding to a transmitted optical signal, with no additional operations necessary to achieve the coding.

According to a third aspect of the invention there is provided a method for quadrature amplitude modulation 2ⁿcomprising the stages:

a) receiving an optical signal to be modulated;

b) generate an intermediate optical signal by applying one of the following to the received optical signal:

to. an amplitude modulation scheme 2^n-2 who has 2ⁿ constellation points arranged in a constellation in a first quadrant of its constellation diagram to the optical signal; Y

b. a phase selective rotation of the optical signal to selectively rotate the phase of the optical signal; Y

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c) apply the other of points a to the intermediate optical signal. and b., to thereby generate an output optical signal having a constellation diagram comprising 2ⁿ constellation points distributed over the four quadrants of the constellation diagram.

The method inherently applies differential quadrant coding during modulation of an optical signal, with no additional processing steps required to achieve the necessary coding.

Preferably n is an even number and is at least 4, more preferably 4 or 6, the constellation points are arranged in a substantially square constellation.

Preferably apply to. understands:

I. delivering the optical signal to an in-phase / quadrature optical modulator comprising an optical input configured to receive an optical signal to be modulated, an in-phase branch comprising a first optical modulator, a quadrature branch comprising a second optical modulator and a π / 2 phase shifter, and an optical output; Y

II. deliver electrical signals driving ^ (2ⁿ'²) levels to the first and second optical modulators, such that the in-phase / quadrature optical modulator applies an amplitude modulation scheme 2^n-2 who has 2^n-2 constellation points arranged in a square constellation in the first quadrant of its constellation diagram of the received optical signal.

16-QAM optical modulation therefore requires the use of only two-level (binary) driving electrical signals, thereby realizing per quadrant differential encoding without the need for additional processing steps. In addition, the pulse of the first and second optical modulations with driving binary electrical signals substantially avoids the heavy distortion that multi-level driving signals can experience, due to modulator bandwidth limitations and non-linearity in high power applications. Transmission speed. The 64-QAM optical modulation utilizing the present method requires only four-level driving electrical signals, and similarly inherently performs quadrant differential encoding.

Preferably, the first and second optical modulators comprise Mach-Zehnder modulators, n is 4 and stage II. comprises delivering two-level driving electrical signals of about 0.39 ν_π, where ν_π is the switching voltage of the respective Mach-Zehnder modulator, and zero volts to each of the Mach-Zehnder modulators.

Any noise in the driving electrical signals, therefore, will be substantially absorbed by the transfer function without² of the Mach-Zehnder modulators. The method of 2ⁿ-QAM also generally produces straight transitions between the output signal symbols which offers the advantage of creating stable decision points for all combinations of symbols.

Preferably b. it comprises applying a quaternary phase shift modulation scheme having four substantially equal amplitude modulation levels and phases 90 degrees apart, giving the modulation scheme a square constellation diagram having a constellation point in each quadrant.

Applying the QPSK modulation scheme selectively leaves the output signal symbols at constellation points in quadrant I or rotates them a desired amount to quadrant II, III or IV of the constellation diagram.

Preferably apply b. understands:

I. delivering the optical signal to a second in-phase / quadrature optical modulator comprising an optical input configured to receive an optical signal to be modulated, an in-phase branch comprising a third optical modulator, a quadrature branch that it comprises a fourth optical modulator and a π / 2 phase shifter, and an optical output; Y

II. deliver bi-level driving electrical signals to the third and fourth optical modulators, such that the in-phase / quadrature optical modulator applies a phase shift modulation scheme that has four substantially equal amplitude modulation levels and phases separated 90 degrees, giving the modulation scheme a square constellation diagram having a constellation point in each quadrant.

Selectively rotating the phase of the optical signal is equivalent to applying four-quadrant differential coding. Applying the QPSK modulation scheme thereby inherently applies quadrant differential coding during modulation of the optical signal.

Preferably, the third and fourth optical modulators comprise Mach-Zehnder modulators and stage ii. it comprises delivering two-level driving electrical signals of approximately νπ, where νπ is the switching voltage of the respective Mach-Zehnder modulator, and zero volts to each of the Mach-Zehnder modulators.

Apply b. It may alternatively comprise delivering the optical signal to a phase modulator and selectively applying a zero degree, 90 degree, 180 degree or 270 degree phase rotation to the optical signal.

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Applying phase rotation is equivalent to applying four-quadrant differential coding. The modulation method 2ⁿ-QAM thus inherently applies quadrant differential coding during modulation of the optical signal.

The Quadrature Amplitude Modulation Method 2ⁿ It may also comprise applying differential encoding to the first two bits of the n bits of a data symbol generated using said method.

An embodiment of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings.

Brief description of the drawings

Figure 1 is a schematic representation of a 16-QAM optical modulator according to a first embodiment of the invention;

Figure 2 is the constellation diagram of the first in-phase / quadrature optical modulator (first modulation apparatus) of the 16-QAM optical modulator of Figure 1;

Figure 3 is the constellation diagram of the second in-phase / quadrature optical modulator (second modulation apparatus) of the 16-QAM optical modulator of Figure 1;

Figure 4 illustrates that the product of the constellation diagrams of Figures 2 and 3 produces a constellation diagram for a square 16-QAM optical modulation scheme;

Figure 5 illustrates the general schematic for a prior art QPSK or 16-QAM transmitter;

Figure 6 is a flow chart illustrating the steps of a 16-QAM method according to a second embodiment of the invention, implemented using the 16-QAM optical modulator of Figure 1;

Figure 7 is a schematic representation of a 16-QAM optical modulator according to a third embodiment of the invention;

Figure 8 is a flow chart illustrating the steps of a 16-QAM method according to a fourth embodiment of the invention, implemented using the 16-QAM optical modulator of Figure 7; Y

Figure 9 is a schematic representation of an optical signal transmission apparatus according to a fifth embodiment of the invention.

Detailed description

Referring to Figures 1 to 4, a first embodiment of the invention provides a quadrature amplitude modulation optical modulator 2ⁿ (QAM) in the form of a 16-QAM optical modulator 10 comprising an optical input 11, a first optical modulation apparatus 12, and a second optical modulation apparatus 20. In this embodiment, an optical signal to be modulated it is received by the optical input 11 and is coupled to the first optical modulation apparatus 12, which applies a quaternary amplitude modulation scheme to the optical signal, generating an intermediate optical signal. The intermediate optical signal is then delivered to the second optical modulation apparatus 20, which is configured to selectively rotate the phase of the intermediate optical signal. A 16-QAM modulated optical signal is therefore output from the optical modulator 10 which has a square constellation diagram 19, as shown in Figure 4, comprising 16 constellation points 16 distributed over the 4 quadrants of the diagram constellation.

The first optical modulation apparatus 12 comprises an in-phase / quadrature (IQ) optical modulator 22, comprising an optical input 24 configured to receive an optical signal to be modulated, an in-phase branch 26 and a quadrature branch. 30. In-phase branch 26 comprises a first optical modulator in the form of a Mach-Zehnder modulator (MZM) 28 driven by a first drive unit (not shown). The quadrature branch comprises a second optical modulator in the form of a second MachZehnder modulator 32 driven by a second drive unit (not shown), and a phase shift ^ 34. The phase branch 26 and quadrature branch 30 are end-coupled output to form the optical output 36 of the IQ modulator 22.

The first Mach-Zehnder 28 modulator is driven with binary drive voltages (Vx-ι) of approximately 0.39 Vj, where V is the switching voltage of the Mach-Zehnder 28 modulator, and zero volts. The second Mach-Zehnder 32 modulator is similarly driven with the same binary voltage levels (Vyi). Mach-Zehnder modulators 28, 32, thereby generate output signal amplitude levels of% and 1 giving the IQ optical modulator 22 a constellation 14 of output signal symbols having four constellation points 16 in quadrant I at the in-phase axis (x1) and quadrature axis (y1) locations:%,%; 1, %; %, 1; and 1, 1, as shown in Figure 2.

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In this embodiment, the second optical modulation apparatus 20, comprises a second IQ optical modulator 38, comprising an optical input 40, in an in-phase branch 42 and a quadrature branch 46. The in-phase branch comprises a third modulator Mach-Zehnder 44. Quadrature branch 46 comprises a fourth Mach-Zehnder modulator 48 and a second π / 2 phase shifter 50. The optical output ends of the in-phase and quadrature branches 42, 46 are coupled together to form the output 52 of the second IQ modulator 38, which forms the output of the 16-QAM optical modulator 10.

The third Mach-Zehnder 44 modulator is driven with the binary electric drive voltages (V_X2) of V „and zero volts, and the fourth Mach-Zehnder 48 modulator is similarly driven with binary electrical driving signals (V_Y2) of V „and zero volts. The third and fourth Mach-Zehnder modulators are therefore driven with different voltages than the first and second Mach-Zehnder modulators 28, 32. The applied binary driving electrical signals give the second IQ modulator 38 a displacement modulation scheme of quaternary phase (QPSK) having 4 levels of substantially equal amplitude modulation and 90 ° phase separation. The second IQ modulator 38 therefore applies a modulation scheme having a square constellation diagram 39 comprising 16 constellation points located in each quadrant, as shown in Figure 3.

The general schematic of a known differential optical transmitter is shown in Figure 5. Due to the 4-fold rotational symmetry, QAM formats suffer from 4-fold phase ambiguity (multiples of π / 2) in estimating the carrier phase. The optimal encoding to solve the phase ambiguity problem in QAM with minimal encoding penalty is quadrant differential encoding. Given a format of 2ⁿ-QAM, with n even, each symbol is represented by 2ⁿ bits. The first 2 bits are differentially encoded, as in the QPSK differential format, and represent the change in the quadrant, while the remaining bits are Gray encoded in each quadrant.

In the known nested IQ and QPSK Phase and Amplitude modulators, two driving voltages determine the quadrant, while the other two determine the point within the quadrant. Therefore, the encoder can be performed by differential encoding of the first two bits through the same logical operations given for a DQPSK:

& U = '(' 'Ί, λ-Φ) + (αΐ₅ϋ © «2Λ) É-Λ © * 2, S-1) & 2jt - (^ Ι, έ © ^) + (^α1, ί®^β2^) · (^ñí)

The four points in each of the quadrants of these modulators are obtained respectively by a reflection or translation of the points in quadrant I. Differential coding per quadrant requires that the last two bits be encoded by Gray in each quadrant, with a pattern which is rotated according to the quadrant itself. As regards the Phase and Amplitude modulator IQ, the reflection can be converted into a rotation by observing that the double reflection from quadrant I to quadrant III is equivalent to one rotation, while the reflection to quadrant II or IV can be convert to a rotation simply by swapping the two bits. The corresponding logical operations are:

^ 3 Λ - (& ι ^,£33λ + (¿ιλΦ ^ λ) ¿M, * = (& Ι¿ · 4Ι4Λ +

As for the nested QPSK modulator, the translation can be converted into a rotation by rotating the two bits according to the chosen quadrant. A similar operation is performed for a DQPSK, in which the preceding coded bits, bi, ki and b2, ki, are rotated according to the actual information bits, ai.k and a2, k. Therefore, the logical operations required for this modulator can be obtained by:

In the 16-QAM optical modulator 10 of this embodiment, differential encoding is applied to the first two bits (bi, b2) across the driving voltages (such as V_xi, V_Yi) applied to the first and second Mach7 modulators

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Zehnder28, 32, according to the logic operations described above. However, the effective rotation of the constellation diagram points 16 from the I quadrant 18 of the constellation diagram 14 of the first IQ modulator 22 by the QPSK modulation scheme of the second IQ modulator 38 is equivalent to four quadrant differential coding. The 16-QAM 10 optical modulator therefore applies four-quadrant differential coding without the need to perform any additional operations on the optical signal, i.e .:

Λ = ^to3 Λ

In use, a 16-QAM modulation method according to a second embodiment of the invention is implemented by the 16-QAM optical modulator 10, as illustrated in the flow diagram of Figure 6. An optical signal to be modulated is received by the optical input 11 of the 16-QAM optical modulator 10, and is coupled to the optical input 24 of the first IQ modulator 22, which applies a quaternary amplitude modulation scheme (4- QAM) having the constellation diagram 14 shown in Figure 2. Differential coding is applied to the first two bits via drive voltages (V_xi, V_Yi) applied to the first and second Mach-Zehn modulators 28, 32. The resulting intermediate optical signal output from the first IQ modulator 22 is coupled to the input 40 of the second IQ modulator 38, which applies a QPSK modulation having the constellation diagram 39 as shown in figure 3. The phase of the intermediate optical signal is thereby selectively rotated, causing the constellation points of the intermediate optical signal, as shown in Figure 2, to be selectively rotated in each of the remaining 4 quadrants, to therefore produce a 16-QAM modulated optical output signal having a 19 constellation diagram, as shown in Figure 4.

The cascade of the two IQ modulators 22, 38, therefore results in an optical modulation scheme that has a constellation diagram 19, which is equivalent to the product of the constellation diagram 14 of the first IQ modulator 22 and the constellation diagram 39 of the second modulator IQ 38. The first lQ modulator 22 generates the small square points 14 found in the I quadrant 18, while the second lQ modulator 38 generates the large square points 39, which, apart from an irrelevant general phase rotation of π / 4 , correspond to the phase rotations necessary to obtain the remaining 16QAM constellation points in quadrants II, III or IV. Including the phase rotation of π / 4, the low-pass equivalent of the output signal is:

= y + jy = (_X1 + Jyi) (x₂ + where the temporal dependence of the signal is omitted for the simplicity of notation.

Referring to Figure 7, a third embodiment of the invention provides a 16-QAM optical modulator 60, which is substantially the same as the 16-QAM optical modulator 10 of Figure 1, with the following modifications. The same reference numbers are retained for corresponding functions.

In this embodiment, the second optical modulation apparatus 20 comprises a phase modulator 62 that can function to selectively apply a phase rotation of 0 °, 90 °, 180 °, and 270 ° to the intermediate optical signal output of the first IQ optical modulator 22. The phase rotation provided by phase modulator 62 similarly inherently applies four quadrant differential coding to the modulated optical signal.

A fourth embodiment of the invention provides a 16-QAM method implemented using the 16-QAM optical modulator of Figure 7, and illustrated in the flow diagram of Figure 8. The method of this embodiment is substantially the same as the method shown in Figure 6, with the modification that the phase rotation is applied by the phase modulator 62 instead of by the QPSK implemented by the second optical modulator IQ 38.

Referring to Figure 9, a fifth embodiment of the invention provides an optical signal transmission apparatus 70 comprising the 16-QAM optical modulator 10 of the first embodiment and a data signal source in the form of a laser diode 72 which can function to generate a data stream 74 of optical signals, which is coupled to the optical input 24 of the first optical modulator IQ 22. The 16-QAM optical modulator 10 operates as described above to implement the method shown in the flow diagram of Figure 6 to the optical data signal.

Various modifications can be made to the described embodiments without departing from the scope of the present invention, as follows. The IQ optical modulators of the first embodiment can be inverted, so that the QPSK optical modulation is applied first to the optical signal followed by the 4-QAM optical modulation, similarly for the optical signal transmission apparatus of the fifth embodiment. Modulator

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IQ 22 and phase modulator 62 of the third embodiment of the invention can be similarly inverted, so that the phase modulator first applies the phase modulation to the optical signal followed by the IQ modulator that applies the 4-QAM optical modulation . It is evident that the IQ optical modulators can be substituted for a different type of optical modulator that can work to apply the 4-QAM optical modulation scheme having the constellation diagram shown in Figure 2.

Although the described embodiments are related to 16-QAM optical modulators and 16-QAM optical modulation methods, it will be appreciated that the first optical modulation apparatus can be substituted for an optical modulation apparatus that can function to apply a scheme different optical modulation 2ⁿ-QAM, such as an optical modulator and a 64-QAM modulation method. The driving electrical signals to the Mach-Zehnder modulators would be changed correspondingly, for example to 4-level driving signals in the 64-QAM optical modulation example.

16-QAM is one of the candidate modulation formats for 100 Gb / s transmission over fiber optics. It is a multi-level signal, and is not trivially generated using conventional optical modulators. On the other hand, quadrant differential coding is useful for resolving π / 2 phase ambiguity at the receiver but requires high-speed digital processing of the transmitted bit sequence. The 2 optical modulatorⁿ-QAM of the present invention allows a 16-QAM modulator to be provided that only requires two-level electrical signals, which provide an advantage over multi-level electrical signals that can be heavily distorted, due to bandwidth limitations and the non-linearity of the modulator. In addition, the 2ⁿ-QAM does not require additional bit sequence processing in order to apply quadrant differential encoding, which is implemented automatically by the modulator and method of the present invention.