使用10-GHz双并行马赫曾德尔调制器生成40-GHz CSRZ脉冲串的新方法

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"一种利用10-GHz双并行马赫-曾德尔调制器产生40-GHz CSRZ脉冲序列的新方案被提出并实验验证。该技术通过模拟计算了生成的CSRZ脉冲的光谱，并将其与基于传统MZM的RZ和CSRZ脉冲序列进行了比较。实验结果显示，成功得到了CSRZ脉冲序列，且载波和不期望的20-GHz低频成分被压制了25dB。这种方法还可以扩展到生成160-GHz的CSRZ脉冲。" 本文主要探讨的是在光学通信领域中，如何高效地生成高频率的脉冲序列，特别是40-GHz的载波抑制归零（CSRZ）脉冲。CSRZ是一种光学脉冲编码方式，它在脉冲返回零的同时消除了载波，从而减少了信号带宽需求，提高传输效率。传统的CSRZ脉冲生成通常需要更高的调制器工作频率，但该研究提出了一种创新方法，仅使用一个10-GHz的双并行马赫-曾德尔调制器（MZM）就能实现40-GHz CSRZ脉冲的产生。马赫-曾德尔调制器（MZM）是光纤通信中常用的调制设备，它基于光的干涉原理，通过改变输入光路的相位差来调制光信号。双并行结构的MZM可以实现更复杂的光信号处理，例如这里用于生成高频率脉冲。通过巧妙设计调制器的工作模式和驱动信号，研究人员能够有效地将10-GHz的输入信号转换为40-GHz的CSRZ脉冲序列。文章通过仿真计算了生成的CSRZ脉冲的光谱特性，这有助于理解脉冲的形成过程和性能。对比实验表明，生成的CSRZ脉冲不仅保持了其特有的归零特性，而且成功抑制了不需要的20-GHz低频成分，这对于减少信号间的干扰和提高系统信噪比至关重要。实验结果的25dB抑制表明，这种方法对于噪声抑制非常有效。此外，文中还指出，这种技术有潜力进一步扩展到生成160-GHz的CSRZ脉冲，这预示着在高速光通信系统中，使用低频率组件生成超高速脉冲成为可能，这将极大地推动高容量、低延迟的通信网络的发展。这项研究提供了一种高效、实用的技术，有望简化高速光学通信系统的复杂性，降低设备成本，同时提升系统的整体性能。

010602-1 CHINESE OPTICS LETTERS / Vol. 9, No. 1 / January 10, 2011

A novel scheme to generate 40-GHz CSRZ pulse trains

using a 10-GHz dual-parallel Mach-Zehnder modulator

Yanfei Xing (000ÿÿÿ)

∗

and Caiyun Lou (£££æææ)

Tsinghua National Lab oratory for Information Science and Technology, State Key Laboratory of Integrated Optoelectronics,

Department of Electronic Engineering, Tsinghua University, Beijing 100084, China

∗

Corresp onding author: yanfei.xing@gmail.com

Received May 12, 2010; accepted Septemb er 21, 2010; posted online January 1, 2011

A new technique to generate 40-GHz carrier-suppressed return-to-zero (CSRZ) optical pulse trains using

only a 10-GHz dual-parallel Mach-Zehnder mo dulator (MZM) is presented and experimentally demon-

strated. The spectrum of the generated CSRZ pulses is calculated by simulation and compared with

conventional MZM-based RZ and CSRZ pulse trains. The experimental results demonstrate that CSRZ

pulse trains are obtained, and that the carrier and the unwanted 20-GHz low-frequency component are

suppressed by 25 dB. The technique can also b e extended to 160-GHz CSRZ pulse generation when 40-GHz

devices are employed.

OCIS co des: 320.5550, 060.2360.

doi: 10.3788/COL201109.010602.

Return-to-zero (RZ) and carrier-suppressed RZ (CSRZ)

optical pulses have been widely discussed and demon-

strated in high-speed optical transmission systems due

to their robustness to various fiber-based degradations,

not only for traditional on-off keying (OOK), but also for

newly presented differential phase shift keying (DPSK)

and differential quadrature phase shift keying (DQPSK)

transmission systems

[1−5]

. Therefore, an optical pulse

train generator or a pulse caver for RZ or CSRZ might

be a key component of future RZ-based optical transmis-

sion systems. Several techniques to generate RZ pulses

have been proposed and demonstrated. One of the most

commonly used methods is to employ a LiNbO

Mach-

Zehnder modulator (MZM) that can generate both chirp-

free RZ and CSRZ pulses by adjusting the bias point of

the MZM

[6]

However, the repetition rate of the pulses acquired by

modulating a MZM either equals the electrical driving

clock rate (50% RZ), or doubles the clock rate (33% RZ

and CSRZ). With the increase of the optical transmission

bit rate, the electrical devices reach severe bottlenecks

to speed up. Therefore, generating much higher repe-

tition rate optical pulses by an electrical clo ck with an

even lower frequency is necessary. Recently, techniques

for generating CSRZ optical pulse trains with repetition

rates of four times the clock rate have been reported, in-

cluding 1) using a phase modulator plus two polarization-

maintaining (PM) fibers and two polarizers

[7−9]

, 2) using

a MZM followed by a delay-line interferometer (DLI)

[10]

and 3) using two cascaded dual-drive MZMs

[11]

. Among

the above techniques, more than two devices have been

employed, adding to the complexity of the whole system,

which might lead to more instability.

In this letter, a new technique to generate 40-GHz

CSRZ optical pulse trains using a 10-GHz electrical clock

is presented and experimentally demonstrated, employ-

ing only a single dual-parallel MZM (DP-MZM). DP-

MZM is a commercially available device typically used

as a DQPSK modulator. It also acts as a multi-format

transmitter, generating signals with three other for-

mats, including duobinary, RZ alternate-mark-inversion

(AMI), and Manchester code format

[12]

. DP-MZM is

also used in pulse train generating schemes

[13]

. However,

when DP-MZM is used as a high-extinction-ratio opti-

cal intensity modulator, a DLI is still required, similar

to the scheme proposed in Ref. [8]. In our schematic

configuration, no extra optical devices are needed, aside

from a single integrated DP-MZM, which is driven by

two relatively delayed electrical clock signals. Thus, a

CSRZ optical pulse train with a repetition rate that is

four times the clock rate is immediately obtained at the

output port of the modulator. Experimentally, we utilize

a 10-GHz clock to drive the modulator in order to gener-

ate pulses with a repetition rate of 40 GHz. Furthermore,

this method might be potentially extended to a 160-GHz

CSRZ pulse generation technique, in which the frequency

of the driving clock should be changed to 40 GHz.

Figure 1(a) shows the configuration of the 40-GHz

CSRZ optical pulse train generator. The DP-MZM is

an integrated device consisting of three MZMs, among

which, two x-cut MZMs (MZM-a, MZM-b) are embed-

ded in the two arms of the primary Mach-Zehnder (MZ)

structure (MZM-c). The structure is originally intended

for DQPSK modulation; however, in this letter, we

demonstrate that it can also be used to generate CSRZ

pulses.

A single MZM can generate 33% and 50% RZ and 67%

CSRZ with its direct current (DC) bias set at different

points. When the bias is set at the maximum transmis-

sion point of the MZM, and a 10-GHz electrical clock is

used to drive it between adjacent minima, a 33% RZ pulse

train with a repetition rate of 20 GHz can be obtained at

the output port of the MZM. If MZM-a and MZM-b of

the DP-MZM are biased similarly as the MZM mentioned

above, two 33% RZ pulse trains would be formed respec-

tively in the two arms. However, the two 33% RZ pulse

trains cannot be observed simultaneously at the output

port, where they combine to form a new pulse train. By

extinguishing the light in the other arm, the pulse train

generated by each MZM can be seen. An electrical delay

line is adopted to make sure that a constant phase delay

1671-7694/2011/010602(4)

° 2011 Chinese Optics Letters

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