Chirped RZ-AMI optical transmitter

[0001] This application claims priority to an application entitled “Chirped RZ-AMI Optical Transmitter,” filed in the Korean Intellectual Property Office on Oct. 20, 2004 and assigned Serial No.2004-83926, the contents of which are incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical transmitter, and more particularly to an optical transmitter using a Mach-Zehnder Modulator (MZM).

[0004] 2. Description of the Related Art

[0005] FIG. 1 is a block diagram showing the basic construction of a typical Return-to-Zero Alternate-Mark-Inversion (RZ-AMI) optical transmitter using an MZM and a Delay Interferometer (DI), and FIG. 2 is a diagram showing processing signals of the RZ-AMI optical transmitter shown in FIG. 1. The RZ-AMI optical transmitter 100 includes a pre-coder 110, a modulator driver 120, a Continuous Wave (CW) laser 130, an MZM 140 and a DI 150.

[0006] The pre-coder 110 pre-codes and outputs binary data S₁₁ which are input Non-Return-to-Zero (NRZ) signals. The modulator driver 120 receives the input from the pre-coder 110, amplifies it and outputs the amplified signal as a pre-coded signal S₁₂. The pre-coder 110 may include a 1-bit delay element and an exclusive-OR element. The MZM 140 intensity & phase-modulates and outputs, according to the amplified signal, a light input from the CW laser 130. The bias position of the MZM 140 is located at a null point corresponding to a minimum value in a transfer characteristic function of the MZM 140. The DI 150 splits the modulated S₁₃ input from the MZM 140 into a first and a second optical signal, delays the first optical signal by 0.5 bit, i.e., one half of a bit period, and outputs an optical signal S₁₅ obtained by combining the first delayed optical signal and the second optical signal so that they destructively interfere. Then, an RZ-AMI optical signal is obtained by phase-modulating the destructively-interfered optical signal S₁₅ each bit by means of a phase modulator so that the optical signal S₁₅ has an inversed phase. The RZ-AMI modulation scheme known in the art has characteristics in which an optical signal includes intensity information and a phase of the optical signal is inverted alternately with each bit. In particular, in indicating the intensity of an RZ-AMI optical signal, as in the case of an RZ signal, a shift in energy of the RZ-AMI optical signal from a level 0 to a level 1, with a subsequent return to the level 0, indicates a single bit. Accordingly, since the RZ-AMI optical signal has the same change of intensity as that in the RZ signal, the RZ-AMI optical signal has an advantage in an RZ modulation scheme. For example, the RZ-AMI optical signal is tolerant to a non-linearity of an optical fiber in a transmission system having a data speed more than 20 Gb/s. Further, since the phase of the optical signal is inverted alternately each bit, a frequency component of a carrier is suppressed. Therefore, the RZ-AMI optical signal is tolerant to not only the Brillouin non-linearity effect but also the non-linearity effect such as the Intra-channel Four-Wave-Mixing (IFWM) and the Intra-channel Cross-Phase-Modulation (IXPM).

[0007] However, the RZ-AMI optical transmitter 100 as described above is expensive due to the expensive parts, particularly the MZM 140, the DI 150 and the phase modulator. Therefore, a system with the RZ-AMI optical transmitter 100 tends to require a non-competitive price.

[0008] Since a chirped RZ signal has been known to be tolerant to the non-linearity effect, it is observed by the present inventors that the RZ-AMI optical signal may also have the same advantages. Accordingly, a chirped RZ-AMI modulation scheme may be a very superior modulation scheme having advantages of a chirped RZ signal and an RZ-AMI optical signal. What is needed is a chirped RZ-AMI optical transmitter that is low-priced and tolerant to the non-linearity effect.

[0009] The present invention has been made to solve the above-mentioned problems occurring in the prior art, and, in one aspect, the present invention provides a chirped RZ-AMI (Return-to-Zero Altemate-Mark-Inversion) optical transmitter having two logic gates and a Mach-Zehnder Modulator (MZM). The first logic gate receives a first signal obtained by pre-coding binary data, and a second signal having a waveform obtained by inverting and delaying the first signal. An output signal is obtained by logically operating on the first and second signals. A second logic gate receives a fourth signal having a waveform obtained by inverting the first signal, and a fifth signal having a waveform obtained by delaying the first signal. A sixth signal is obtained by logically operating on the fourth and fifth signals. The MZM modulates, according to the third and sixth signals, inputted light and outputs an RZ-AMI optical signal.

[0010] In accordance with another aspect of the present invention, there is provided a chirped RZ-AMI (Return-to-Zero Alternative-Mark-Inversion) optical transmitter having a first logic gate for receiving a first signal obtained by pre-coding binary data and a second signal having a waveform obtained by inverting and delaying the first signal. A third signal for output is obtained by logically operating on the first and the second signals. A second logic gate receives a fourth signal having a waveform identical to a waveform of the second signal and a fifth signal having a waveform identical to a waveform of the first signal. A sixth signal for output is obtained by logically operating on the fourth and the fifth signals. A Mach-Zehnder Modulator (MZM) modulates, according to the third signal and sixth signals, inputted light and outputs an RZ-AMI optical signal.

[0022] For the purposes of clarity and simplicity, detailed description of known functions and configuration incorporated herein is omitted for clarity of presentation.

[0023] FIG. 3 is a block diagram showing a chirped RZ-AMI optical transmitter according to a first preferred embodiment of the present invention.

[0024] The chirped RZ-AMI optical transmitter 200 includes a pre-coder 210, a first and a second branching means 212, 214, a first and a second delay 220, 230, a first and a second logic gate 250, 255, a first and a second modulator driver 260, 270, a CW laser 280, and an MZM 290.

[0025] The pre-coder 210 pre-codes input NRZ binary data S₂₁, divides the pre-coded signal (or 2 level signal) into two branch signals, inverts one of the branch signals, and outputs the remaining branch signal (non-inverted signal) S₂₂ and the inverted signal S₂₂. The pre-coder 210 may include a 1-bit delay element, an exclusive-OR element, a branching means (e.g., parallel connection of conductive wires) for dividing the outputs of the delay element and the exclusive-OR element into two branch signals, and an inverter for inverting one of the two branch signals.

[0026] The first branching means 212 divides the branch signal input from the pre-coder 210 into two branch signals. This may be accomplished through parallel connection of conductive wires.

[0027] The first delay 220 delays one of the branch signals having passed through the first branching means 212 by 0.5 bit, and outputs the delayed signal.

[0028] The second branching means 214 divides the inverted signal input from the pre-coder 210 into two branch signals. This may, as in the case of the first branching means 212, be accomplished through parallel connection of conductive wires.

[0029] The second delay 230 delays one of the branch signals having passed through the second branching means 214 by 0.5 bit, and outputs the delayed signal.

[0030] The first logic gate 250 receives the branch signal (first signal) S₂₂ having passed through the first branching means 212 and the delayed signal (second signal) S₂₄ input from the second delay 230. The first logic gate 250 logically operates on the received first and second signal to output the third signal S₂₅.

[0031] The second logic gate 255 receives the branch signal (fourth signal) S₂₂ having passed through the second branching means 214 and the delayed signal (fifth signal) S₂₃ input from the first delay 220. The second logic gate 255 logically operates on the received fourth and fifth signal to output the sixth signal S₂₆.

[0032] The first modulator driver 260 amplifies the third signal S₂₅ input from the first logic gate 250.

[0033] The second modulator driver 270 amplifies the sixth signal S₂₆ input from the second logic gate 255.

[0034] The CW laser 280 outputs a light having a predetermined wavelength and the MZM 290 outputs a chirped RZ-AMI optical signal S₂₇ obtained by modulating, according to the amplified third and the sixth signals, the light input from the CW laser 280. The MZM 290 includes a dual-arm. The third signal is applied to one arm of the dual-arm, and the sixth signal is applied to the other arm of the dual-arm. The MZM 290 may have a z-cut structure having a dual-arm. The bias position of the MZM 290 is located at a null point corresponding to a minimum value of a modulator transfer characteristic.

[0035] The electric field S₂₇(E) of the chirped RZ-AMI optical signal is determined by a difference between waveforms of the third signal and the sixth signal output from the first and the second logic gates 250, 255. The phase S₂₇(P) of the chirped RZ-AMI optical signal is determined by a sum of waveforms of the third signal and the sixth signal output from the first and the second logic gates 250, 255. A chirp characteristic of the chirped RZ-AMI optical signal changes according to combinations of the first and the second logic gates 250, 255, providing the chirped RZ-AMI optical signal with a positive chirp or a negative chirp. Hereinafter, the chirped RZ-AMI optical signal will be said to have positive chirp when it has a phase that increases as its intensity increases and decreases when the intensity decreases, i.e., a phase that varies directly with intensity. FIG. 6 shows a polar map representing positive chirp. By contrast, the chirped RZ-AMI optical signal has negative chirp when its phase varies inversely with intensity, so that phase increases when intensity decreases and such that phase decreases when intensity increases. FIG. 9 shows a polar map representing negative chirp.

[0036] When the delay times of the first and the second delay 220, 230 are set to correspond to about one half of a transmission speed of the chirped RZ-AMI optical signal, for example, about 12.5 ps in a case of a transmission speed of 40 Gb/s, an RZ of about 50% occurs. The duty cycle of the chirped RZ-AMI optical signal (RZ signal) may be adjusted by adjusting the delay times of the first and the second delay 220, 230. As is typical of the AMI signal, the phase of the chirped RZ-AMI optical signal inverts with each bit.

[0037] The optical transmitter 200 may be realized by means of various combinations of the first and the second logic gates 250, 255.

[0038] FIG. 4 is a diagram showing processing signals of the optical transmitter 200 according to a first combination of the first and the second logic gates 250, 255. The first combination is the case in which NAND logic gates as employed as the first and the second logic gates 250, 255.

[0039] In particular, the first logic gate 250 receives the first signal S₂₂ having passed through the first branching means 212 and the second signal S₂₄ input from the second delay 230. The first logic gate 250, implemented as a NAND gate, outputs the third signal S₂₅ as a 0 bit when the first signal is a 1 bit and the second signal is a 1 bit. The first logic gate 250 outputs the third signal S₂₅ as a 1 bit in the other three case, i.e., when the first signal is a 1 bit and the second signal is a 0 bit, the first signal is a 0 bit and the second signal is a 1 bit, and the first signal is a 0 bit and the second signal is a 0 bit.

[0040] The second logic gate 255 receives the fourth signal S₂₂ having passed through the second branching means 214 and the fifth signal S₂₃ input from the second delay 220. The second logic gate 255 outputs the sixth signal S₂₆ as a 0 bit when the fourth signal is 1 bit and the fifth signal is 1 bit. Likewise, in the other three cases, the second logic gate 255 outputs the sixth signal as 1 bit, i.e., when the fourth signal is a 1 bit and the fifth signal is a 0 bit, the fourth signal is a 0 bit and the fifth signal is a 1 bit, and the fourth signal is a 0 bit and the fifth signal is a 0 bit.

[0041] The NAND operation performed by each of the first and the second logic gates 250, 255 can be realized through a serial connection of a NOT logic gate and an AND logic gate. Since the intensity and the phase S₂₇(P) of the chirped RZ-AMI optical signal S₂₇ output from the MZM 290 vary together, as evident from the bottom two waveforms in FIG. 6, it follows that the chirped RZ-AMI optical signal of the first combination of logic gates 250, 255 has positive chirp.

[0042] FIG. 5 is a diagram showing processing signals of the optical transmitter 200 according to a second combination of the first and the second logic gate 250, 255, in which the second logic gates are implemented as OR logic gates.

[0043] As in the first combination, the first logic gate 250 receives the first signal S₂₂ having passed through the first branching means 212 and the second signal S₂₄ input from the second delay 230. The first logic gate 250, however, outputs the third signal S₂₅ as a 0 bit when at least one of the first and second signals is a 1 bit, and outputs the third signal as a 1 bit when the first signal is a 0 bit and the second signal is a 0 bit.

[0044] Again, as in the first combination, the second logic gate 255 receives the fourth signal S₂₂ having passed through the second branching means 214 and the fifth signal S₂₃ input from the second delay 220. The second logic gate 255, however, outputs the sixth signal S₂₆ as a 0 bit when at least one of the fourth and fifth signals is a 1 bit, and outputs the sixth signal of 1 bit when the fourth signal is a 0 bit and the fifth signal is a 0 bit. Since the intensity and the phase S₂₇(P) of the chirped RZ-AMI optical signal S₂₇ output from the MZM 290 vary directly, as shown in FIG. 6, one can see that the chirped RZ-AMI optical signal of the second combination has, like that of the first combination, positive chirp.

[0045] FIG. 7 illustrates signal processing of the optical transmitter 200 according to a third combination of the first and the second logic gates 250, 255, which uses AND logic gates as the first and the second logic gates. The first logic gate 250 accordingly outputs the third signal S₂₅ as a 1 bit when the first signal is 1 bit and the second signal is 1 bit. The third signal is outputted as a 0 bit when one of the first and second signals is a 1 bit and the other is a 0 bit, , or when the first and second signals are both 0 bits. The second logic gate 255 operates analogously to output the sixth signal S₂₆ as a 1 bit when both the fourth and fifth signals are 1 bits, and to otherwise output the sixth signal as a 0 bit. Since the intensity and the phase S₂₇(P) of the chirped RZ-AMI optical signal S₂₇ output from the MZM 290 of the third combination vary inversely, the chirped RZ-AMI optical signal has negative chirp as shown in FIG. 9.

[0046] FIG. 8 represents optical transmitter 200 signal processing according to a fourth combination of the first and the second logic gate 250, 255, the latter being implemented as NOR logic gates. The first logic gate 250 therefore outputs the third signal S₂₅ as a 1 bit when the first and second signals are 0 bits, and otherwise as a 1 bit. The second logic gate 255 analogously realizes NOR logic by the same input signals S₂₃, S₂₂ described above for the first three combinations. Since the intensity and the phase S₂₇(P) of the chirped RZ-AMI optical signal S₂₇ of the fourth combination output from the MZM 290 vary inversely, it follows that the chirped RZ-AMI optical signal has negative chirp as shown in FIG. 9.

[0047] The following table 1 represents formats and chirp signs for the first to the fourth combination.

[0048]

	TABLE 1
	First logic gate	Second logic gate	format	Sign of chirp
First	NAND	NAND	AMI	Positive
combination
Second	OR	OR	AMI	Positive
combination
Third	AND	AND	AMI	Negative
combination
Fourth	NOR	NOR	AMI	Negative
combination

[0049] FIG. 10 is a block diagram showing a chirped RZ-AMI optical transmitter according to a second preferred embodiment of the present invention. The optical transmitter 300 has nearly the same construction as that of the optical transmitter 200 shown in FIG. 3. However, they differ in that the optical transmitter 300 uses only one delay, that delay being disposed at a different position. Accordingly, the following discussion focuses, for brevity, on these differences.

[0050] The optical transmitter 300 includes a pre-coder 310, first and a second branching means 312, 314, a delay 320, first and a second logic gate 330, 340, a first and a second modulator driver 350, 360, a CW laser 370, and an MZM 380.

[0051] The pre-coder 310 pre-codes input NRZ binary data, divides the pre-coded signal (or 2 level signal) into two signals, inverts one of the branch signals, and outputs the remaining branch signal (non-inverted signal) and the inverted signal.

[0052] The first branching means 312 divides the branch signal input from the pre-coder 310 into two branch signals.

[0053] The delay 320 delays the branch signal input from the pre-coder 310 by 0.5 bit, and outputs the delayed signal.

[0054] The second branching means 314 divides the delayed signal input from the delay 320 into two branch signals.

[0055] The first logic gate 330 receives one (first signal) of the branch signals having passed through the first branching means 312 and one (second signal) of the branch signals having passed through the second branching means 314, and logically operates on the received first and second signals to output a third signal.

[0056] The second logic gate 340 receives the remaining branch signal (fourth signal) having passed through the first branching means 314 and the remaining branch signal (fifth signal) having passed through the second branching means 312, and logically operates on the received fourth and fifth signals to output a sixth signal.

[0057] The first modulator driver 350 amplifies the third signal input from the first logic gate 330.

[0058] The second modulator driver 360 amplifies the sixth signal input from the second logic gate 340.

[0059] The CW laser 370 outputs a light having a predetermined wavelength and the MZM 380 outputs a chirped RZ-AMI optical signal obtained by modulating the light input from the CW laser 370 according to the amplified third and sixth signals.

[0060] The optical transmitter 300 may be realized by means of various combinations of the first and the second logic gates 330, 360.

[0061] The following table 2 represents formats and chirp signs for a fifth to an eighth combination. Description for each combination is omitted.

[0062]

	TABLE 2
	First logic gate	Second logic gate	format	Sign of chirp
Fifth	NAND	OR	AMI	Positive
combination
Sixth	OR	NAND	AMI	Positive
combination
Seventh	AND	NOR	AMI	Negative
combination
Eighth	NOR	AND	AMI	Negative
combination

[0063] According to the present invention as described above, a chirped RZ-AMI optical transmitter with a low price can be realized by using only one MZM without an expensive DI. Moreover, since the chirped RZ-AMI optical transmitter outputs a chirped RZ-AMI optical signal, the chirped RZ-AMI optical transmitter is tolerant to the non-linearity effect.

[0064] Although preferred embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims, including the full scope of equivalents thereof.