New scheme to generate DQPSK payload/ASK label signal

Laser DQPSK Pulse carver ASK label … Data 1

Fig 1-2 Structure of RZ-DQPSK/ASK label signal transmitter and router

The RZ-DQPSK/ASK label signal transmitter and label router setup is shown Fig 1-2 However, to implement such an ASK/RZ-DQPSK orthogonal modulation format, three cascaded optical modulators are required for phase encoding, pulse carving and label impressing, an arrangement that is extremely costly and difficult to manage due to the size and the electronic components required in each modulator. In addition, the heritage loss is usually so high that two EDFAs will be needed in the transmitting end.

This thesis proposes a simple and elegant method to generate ASK/RZ-DQPSK signal which used the two dual-drive Mach-Zehnder modulator. The first Mach-Zehnder modulator generates NRZ-DQPSK. The second Mach-Zehnder modulator is used to impress the label data and perform pulse carving. First, the sin wave mix the low bit rate label data used mixer into Mach-Zehnder modulator before.

Chapter 2

DQPSK payload/ASK label signal

2-1 Mach-Zehnder modulator

At bit rates of 10 Gb/s or higher, the frequency chirp imposed by direct modulation becomes large enough that direct modulation of semiconductor lasers is rarely used. For such high-speed transmitters, the laser is biased at a constant current to provide the CW output, and an optical modulator placed next to the laser converts the CW light into a data-coded pulse train with the right modulation format. The most commonly used Mach-Zehnder external modulators are based on LiNbO3 (lithium niobate) technology, therefore this section will deal only with these types of devices.

The linear electro-optic effect that is used to produce the phase changes in the branches of the Mach-Zehnder modulator is known as the “Pockels-effect”. For LiNbO3, the application of an electric field results in a change in the refractive index and therefore varies the phase of the propagating light. The strength of this electro-optic effect is dependent on the direction of the applied electric field and the orientation of the LiNbO3 crystal. Hence, the electrode placement and configuration is a critical design issue.

Intensity trimmer

Intensity trimmer

Fig 2-1 The Mach-Zehnder modulator having two intensity trimmers

Fig 2-1 shows a Mach-Zehnder modulator having two intensity trimmers in the arms.

By using the trimmers, we can compensate the amplitude imbalance due to fabrication

errors, where the intensity trimmers can be also constructed byMZ structures.

The most general case is a dual-drive modulator with two possibly independent drive signals. Where the dual-drive modulator formula is:

1 2

2-2 Differential quadrature phase shift keying (DQPSK)

In the case of PSK (phase shift keying) format, the optical bit stream by modulating the phase while the amplitude and the frequency of the optical carrier are kept constant. For binary PSK, the phase takes two values, commonly chosen to be 0 andπ. The PSK formats carry the information in the optical phase itself. Due to the lack of an absolute phase reference in direct-detection receivers, the phase of the preceding bit is used as a relative phase reference for demodulation. This results in differential phase shift keying (DPSK) formats, which carry the information in optical phase changes between bits.

The commonly used DPSK transmitter setups are shown in Fig 2-2. The transmitters consist of a continuously oscillating laser followed by one external modulator, typically based on LiNbO3 technology. Phase modulation can either be performed by a Mach–Zehnder modulator.

Laser MZM

Fig 2-2 the DPSK transmitter and DPSK modulation

There have been a number of applications that have been propose and demonstrated for PSK. One of these is to increase spectral efficiency through the use of multilevel signaling is DQPSK. The basic principle of optical DQPSK (differential quadrature phase shift keying) modulation is to represent each couple of two bits (so called “dibit”) of the information sequence to be transmitted by optical phase differences between consecutive

symbols taking values into ,0, ,

2 2

π π π

⎧− ⎫

⎨ ⎬

⎩ ⎭.Each transmitted symbol therefore corresponds to two bits of information, meaning that the symbol rate (in baud) is equal to half of the bit rate (B, in bit/s).

The Fig 2-3 illustrates the partitioning of a typical pulse stream for DQPSK modulation.

Fig 2-3(a) shows the original data stream d t_k

( )

=d d d0, , ,...1 2 consisting of bipolar pulses;

Note that d t and I

( )

d t each have half the bit rate ofQ

( )

d t . k

( )

Fig 2-3 (a) QPSK modulation (b) in-phase stream and quadrature stream d0 d₁ d2 d₃ d4 d5 d6 d₇

2-2.1 Convention optical DQPSK transmitter

π 0

Fig 2-4 The structure of an optical DQPSK system π/4

The most widely used implementation of a DQPSK transmitter is shown in Fig 2-4.

The transmitter consists of two parallel DPSK modulators that are integrated together in order to achieve phase stability (a serial arrangement is also possible, and has been used in experimental demonstrations).

The electric field of DPSK signal at the modulator output is

( ) ( )

( ) 1 exp exp

2 2 2

j j

e t = ^⎧⎨⎩ ^⎡⎢⎣ πφ t ^⎤⎥⎦− ^⎡⎢⎣− πφ t ^⎤⎥⎦^⎫⎬⎭ (6) Now consider, t he electric field of QPSK signal at the modulator output is

( ) ( )

φ is the normalized binary drive signal

( )

( )

( )

the drive signal, n=1 and 2, and T is the bit interval of the data. The receiver essentially consist of two DPSK receivers, although the phase difference in the arms of the delay interferometers is now set to +π/ 4 and−π/ 4. Whose differential delay is equal to the bit period. The benefit of DQPSK is that, for the same data rate, the symbol rate is reduced by a factor of two.

2-2.2 Variety of dual-drive DQPSK signals

A DQPSK signal can also be generated using a conventional dual-drive Mach-Zehnder modulator. The dual-drive Mach-Zehnder modulator consists of two

phase modulators that can be operated independently. The Fig 2-5 shows the three kinds of method to generate DQPSK signals and the conventional method to generate DQPSK signals, which are the eye diagram of the drive signal and output intensity of the DPQSK transmitter. The fig2-5(a) is used with two two-level drive signals having a peak to peak drive voltage of 2V_π. The fig 2-5 (b), (c) and (d) separately represent to drives the dual-drive Mach-Zehnder modulator with a four-level drive signal, two-level drive signal and three-level drive signal. The peak to peak drive voltage is reducing from 1.5V_π for four-level signal to V_π for two- and three- level drive signals. The output intensity of conventional transmitter has optical intensity ripples between consecutive symbols. With two or three levels of drive signal, the output intensity of the dual-drive Mach-Zehnder modulator also has ripples between consecutive symbols.

(a) (b) (c) (d)

Fig 2-5 Eye diagram of the drive signal and output intensity, where (a) the conventional transmitter and dual-drive transmitter (b) four-, (c) two-, and (d) three-level drive signals

2-2.3 Two level drive DQPSK transmitter

We focused how to generate DQPSK signals which used two-level drive signals at this thesis. The Fig 2-6 shows the input data streams and bias point of the dual-drive Mach-Zehnder modulator to generate DQPSK signals. First, a continuous wave is externally modulated by a dual-drive Mach-Zehnder modulator. Biased at the quadrature, the two arms of the MZM are fed by two independent data streams, V₁ and V₂, and each

Fig 2-6 Two-level drive signals used one dual drive Mach-Zehnder modulator

The output electrical field can be written as:

1 2 data1 is zero and data2 is one, the input data1 is one and data2 is zero and the input data1 is one and data2 is one ) in two independent data streams.

The case 1 (0, 0)

Fig 2-7 the symbol constellation of DQPSK signal

According to those examples, the (0, 0) case is at the first coordinates, the (1, 0) case is at

the second coordinates, the (0, 1) case is at the third coordinates and the (1, 1) case is at the forth coordinates. The Fig 2-7 illustrates the symbol constellation of DQPSK signal, which has four phases depending on the input signal V1 and V2.

2-2.4 Transition of two level drive DQPSK

The two-level scheme has overshoot ripples doubling the output intensity. Those variations of electric field are equivalent to frequency chirp. The output intensity of two-level scheme has five kind of power fluctuates that show in Fig 2-5(c). This thesis provides two kind of methods to discuss how to happen overshoot ripples when symbol change to another symbol. First, the Fig 2-8 shows the symbol constellation with electric field locus of a dual-drive transmitter.

1

Fig 2-8 the symbol constellation with electric field locus of a dual-drive transmitter

When symbol (0, 0) change to symbol (1, 1), the electric field locus follow route 1 to arrive symbol (1, 1) simultaneously, vice versa. The output intensity of DQPSK signal is constant power at transition state. When the symbol (0, 0) change to (0, 1), the electric field locus follow route 2 to arrive symbol (0, 1) simultaneously, vice versa. When the symbol (1, 1) change to (0, 1), the electric field locus follow route 3 to arrive symbol (0, 1) simultaneously, vice versa. The output intensities of route 2 and route 3 will increase

to maximum value at transition state. When the symbol (0, 0) change to (1, 0), the electric field locus follow route 4 to arrive symbol (1, 0) simultaneously, vice versa. When the symbol (1, 1) change to (1, 0), the electric field locus follow route 5 to arrive symbol (1, 0) simultaneously, vice versa. The output intensities of route 4 and route 5 will reduce to minimum value at transition state. When the symbol (0, 1) change to (1, 0), the electric field locus follow route 6 to arrive symbol (1, 0) simultaneously. The output intensities will reduce to minimum value then increase to maximum value at transition state. When the symbol (1, 0) change to (0, 1), the electric field locus follow route 6 to arrive symbol (0, 1) simultaneously. The output intensities will increase to maximum value then reduce to minimum value at transition state. When symbol does not change its state, the output intensity is constant power at transition state.

We also can use symbol position of Mach-Zehnder modulator transfer curve that shows in Fig 2-9. Since the symbol transition passes through the minimum and the maximum of the transfer curve, the power fluctuates around the symbol edge owing to the rise-fall time of the drive signal.

Fig 2-9 The symbol position of Mach-Zehnder modulator transfer function

The symbol (0, 0) and symbol (1, 1) position were at V_π/ 2 of Mach-Zehnder modulator transfer curve, so that the output intensities are constant power at transition

state of symbol (0, 0) to symbol (1, 1) or symbol (1, 1) to symbol (0, 0). The symbol (0, 1) position is at −V_π / 2 of Mach-Zehnder modulator transfer curve, so that the output intensities will increase to maximum value at transition state of symbol (0, 0), symbol (1, 1) to symbol (0, 1) or symbol (0, 1) to symbol (0, 0), symbol (1, 1). The symbol (1, 0) position is at 3 / 2V_π of Mach-Zehnder modulator transfer curve, so that the output intensities will reduce to minimum value at transition state of symbol (0, 0), symbol (1, 1) to symbol (1, 0) or symbol (1, 0) to symbol (0, 0), symbol (1, 1). The symbol (0, 1) to symbol (1, 0) will follow transfer curve which increase to maximum value then reduce to minimum value at transition state. Adversely, the symbol (1, 0) to symbol (0, 1) also will follow transfer curve which reduce to minimum value then increase to maximum value at transition state.

Fig 2-10 the five kind power fluctuates of symbol transition

There are sixteen kind of symbol transition which have five kind of power fluctuates

that shows in Fig 2-10. The overshoot ripples between symbols will influence NRZ-DQPSK signal quality so that the dual-drive Mach-Zehnder modulator pulse carver will change NRZ-DQPSK to RZ-DQPSK signal.

2-2.5 Eye spreading of two level drive DQPSK

The ripple of the drive signal transfer to the optical signal. With the conventional transmitter, no amplitude ripple of the drive signal transfers to the phase ripple. Even when the drive signal has a large ripple, the intensity ripple of the transmitted signal is compressed by the nonlinear transfer function of Mach-Zehnder modulator. For the NRZ-DQPSK signal, the ripples from the drive signal may be increase by the Mach-Zehnder modulator. This section will discuss and calculate optical eye spreading of NRZ-DQPS. The eye spreading is defined asΔ =_e (δ δ₁+ ₂) /d, where δ₁ and δ₂ are the spreading in the upper and lower level, and d is the high of the eye diagram, the Fig 2-11 shows eye spreading of the two-level eye spreading.

Fig 2-11 eye spreading of the two-level eye spreading

Assume δ₁=δ₂→ = Δδ _ed/ 2

(14)

2

Because 1/2 is influence of bias point, we don’t consider this item.

2

4

e

P_δ Ee ^Δ π

→ = −

Now NRZ eye spreading is defined as:

2

2-3 Pulse carver for RZ-DQPSK

Since DQPSK carries information in the phase of the optical signal, optical phase distortions (such as chirp) will have a severe impact on DQPSK receiver performance. At the transmitter, phase distortions may be caused by imperfect pulse carvers.

In order to operate chirp-free, a dual-drive MZM pulse carver has to have infinite DC extinction, and has to work in perfect push-pull operation, i.e., the sinusoidal drive amplitudes have to be of the same amplitude and of opposite phase. The Fig. 2-12 shows three commonly used ways of pulse carving by applying a sinusoidal drive signal to a

MZM-based pulse carver. The resulting RZ duty cycles are 33%, 50%, and 67%.

Fig 2-12 Three commonly used ways of pulse carving by applying MZM-based pulse carver

Three important facts are evident from the optical intensity and phase waveforms shown in Fig 2-13: First, when sinusoidally carving at the data rate (50% RZ), the residual optical phase variations are identical for each bit, while they are different for adjacent bits when carving at half the data rate (33% and 67% RZ). Since it is the difference between the optical phases of two adjacent bits that is used to decode DPSK signals at the receiver, higher degradations due to pulse carver chirp are found for 33% and 67% duty cycle RZ-DQPSK than for 50% RZ-DQPSK. Second, we see from the opposite phase curvatures (50% and 67%) or slope (33%) that chirp due to finite DC extinction ratios of the MZM can partially be compensated by imbalancing the drive amplitudes. Third, we notice that for 33% RZ a drive-signal amplitude imbalance leads to linear phase transitions (i.e., to optical frequency shifts) at pulse center, while a drive-signal phase error produces a phase offset at pulse center. Since pure bit-alternating frequency offsets do not disturb the phase difference between adjacent bits at pulse center (where the intensity is highest, and thus the contribution to the demodulated signal is largest), a higher tolerance is found for drive amplitude imbalance than for drive phase errors in the case of 33% RZ. For 67% RZ, the situation is opposite, and we find a higher tolerance to

drive phase errors than to drive amplitude imbalance.

Fig 2-13 the optical intensity and phase waveforms (50% RZ, 33% RZ and 67%RZ)

2-4 DQPSK demodulation

A typical balanced DQPSK receiver is shown in Fig. 2-14. The receiver essentially consist of two DPSK receivers, although the phase difference in the arms of the Mach-Zehnder delay-interferometer (DI) is now set to +π/ 4 and−π/ 4. The optical signal is first passed through a Mach-Zehnder delay-interferometer (DI), whose differential delay is equal to the bit period. This optical preprocessing is necessary in direct-detection receivers to accomplish demodulation, since photodetection is inherently insensitive to the optical phase; a detector only converts the optical signal power into an electrical signal. In a direct-detection DQPSK receiver, the DI lets two adjacent bits interfere with each other its output ports. This interference leads to the presence (absence) of power at a DI output port if two adjacent bits interfere constructively (destructively) with each other.

π/4

Fig 2-14 typical balanced DQPSK receiver

Thus, the preceding bit in a DQPSK-encoded bit stream acts as the phase reference for demodulating the current bit. (Note that in the case of coherent detection, this phase reference can be provided by a local laser within the receiver, which beats with the received signal to produce constructive and destructive interference.)

Ideally, one of the DI output ports is adjusted for destructive interference in the absence of phase modulation (“destructive port”), while the other output port then automatically exhibits constructive interference due to energy conservation (“constructive port”). For the same reason, the two DI output ports will carry identical, but logically inverted data streams under DQPSK modulation.

Now we consider DI how to determine 0 and 1, that the RZ-DQPSK signal through Mach-Zehnder delay-interferometer. The optical signal is first passed through a Mach-Zehnder delay-interferometer whose differential delay is equal to the bit period and does not delay bit will rotate +π/ 4 and−π/ 4. The intensity of two bit through Mach-Zehnder delay-interferometer compose vector will decide the zero and one The Fig 2-15 shows the constellation of zero and one determined method. Assume into Mach-Zehnder delay-interferometer signal sequence is the symbol (0, 1) and the symbol

(0, 0). That rotated +π/ 4 and−π/ 4 represents the transmitter input two kind of data,

Every symbol change to another symbol has sixteen case of symbol changing totally.

The constellation do not consider phase, the large compose vectors represent one and the large compose vectors represent zero.

2-5 Structure of DQPSK/ASK Label

Future Internet routers will need optical label switching to route and forward a massive number of packets per second independently of IP packet length and payload bit rate. Orthogonal amplitude shift keying /differential quadrature phase shift keying (ASK/DQPSK) optical label has been proposed as a competing scheme to sub-carrier multiplexed optical label due to its compact spectrum, simple label swapping and remarkable scalability to high bit rates.

At the ingress edge router, the incoming IP packets are assigned an amplitude shift keying (ASK) label, orthogonally modulated to the RZ-DQPSK payload. The packet switched network architecture requires the optical label to be swapped during the routing process to establish an appropriate optical path through the transmission fiber

network, as shown in the Fig. 2-16.

RZ-DQPSK payload ASK label B

RZ-DQPSK payload

Fig 2-16 system architecture for DQPSK/ASK label signal

The high speed packet data is in DQPSK format, while the low speed label is written with a low extinction ratio amplitude shift keying format. At network nodes, the label read by detecting and low pass filter some fraction of the signal. The label on the routed signal can then be erased and rewritten using an intensity modulator. At the packet destination, the data is read using a DQPSK receiver. The RZ-DQPSK/ASK label signal transmitter and label router setup is shown Fig 2-17.

Laser DQPSK Pulse carver ASK label … Data 1

Fig 2.17 The RZ-DQPSK/ASK label signal transmitter and router setup

2-5.1 Simple RZ-DQPSK/ASK Label transmitter

However, to implement such an ASK/RZ-DQPSK orthogonal modulation format, three cascaded optical modulators are required for phase encoding, pulse carving and label impressing, an arrangement that is extremely costly and difficult to manage due to the size and the electronic components required in each modulator. In addition, the heritage loss is usually so high that two EDFAs will be needed in the transmitting end. This thesis proposes a simple and elegant method to generate ASK/RZ-DQPSK signal which used the two dual-drive Mach-Zehnder modulator. The first Mach-Zehnder modulator generates NRZ-DQPSK. The second Mach-Zehnder modulator is used to impress the label data and perform pulse carving. First, the sin wave mix the low bit rate label data used mixer into Mach-Zehnder modulator before. Thus the pulse will include ASK label signal is shown the Fig 2-18.

Laser DQPSK Pulse carver … Data 1

Data 2

label 1 0 1 1 Transmitter

Fig 2-18 The simple RZ-DQPSK/ASK label signal transmitter ASK label Sinwave (bate/2HZ)

-1

Chapter 3

Experiment setup and result (DQPSK)

3-1 Dual-Drive DQPSK experiment setup

DQPSK Payload

The Fig 3-1 Dual-Drive DQPSK signal experiment setup

The experimental setup is shown in the Fig 3-1, which mainly contains three part:

payload transmitter, transmission fiber, and payload receiver. The RZ-DQPSK transmitter consists of continuously oscillating laser at 1546.96nm, two external dual-drive Mach-Zehnder modulators. The first modulator generates a 20Gbit/s NRZ-DQPSK signal. Biased atV_π / 2, two independent electrical data streams, which is driven by 10Gbit/s (PRBS 2¹⁵− ) NRZ data stream individually. The second 1 modulator generates a 10GHz RZ pulse train with 33% (The modulator is biased at the

payload transmitter, transmission fiber, and payload receiver. The RZ-DQPSK transmitter consists of continuously oscillating laser at 1546.96nm, two external dual-drive Mach-Zehnder modulators. The first modulator generates a 20Gbit/s NRZ-DQPSK signal. Biased atV_π / 2, two independent electrical data streams, which is driven by 10Gbit/s (PRBS 2¹⁵− ) NRZ data stream individually. The second 1 modulator generates a 10GHz RZ pulse train with 33% (The modulator is biased at the

在文檔中 比較傳統與簡化的傳送器用來產生歸零-差分四元相位移鍵/振幅移鍵 (頁 14-0)