光子模数转换新方案：相同半波电压的多比较器Mach-Zehnder调制器

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"使用多个比较器和Mach-Zehnder调制器以相同的半波电压进行光子模数转换" 这篇研究论文探讨了一种新颖的光子模拟到数字转换（ Photonics Analog-to-Digital Converter，PADC）方案，该方案利用了具有相同半波电压的Mach-Zehnder调制器（MZM）以及多级比较器，以提高位分辨率。Mach-Zehnder调制器是光纤通信和光子学中常用的一种设备，它通过改变输入光信号的相位来实现光信号的调制。在传统的PADC系统中，MZM的半波电压通常需要通过几何比例来调整，以便在不同阶段实现精确的相位变化，这增加了系统的复杂性。然而，该论文提出的新型方案则利用了具有相同半波电压的MZM，简化了系统的实现，降低了设计和操作的难度。这不仅减少了硬件的需求，而且可能提高了转换效率和精度。论文中提到的关键概念包括： 1. 光子模拟到数字转换器 (PADC)：这是一种将连续变化的光学信号转化为离散数字值的设备。在光子学和光纤通信领域，PADC是至关重要的，因为它允许光学信号与电子信号之间的高效接口。 2. Mach-Zehnder调制器 (MZM)：MZM是一种基于干涉原理的光调制器，由两个互相独立的偏振控制器和一个分束器组成。当施加电压时，MZM能够改变通过其的光波的相位，从而实现对光信号的调制。 3. 相同的半波电压：半波电压是指在MZM中产生相位差180度所需的电压。如果所有MZM都使用相同的半波电压，那么它们在转换过程中将更一致，简化了系统设计，并可能减少误差。 4. 多级比较器：在PADC中，比较器用于将光信号的相位信息转化为数字信息。使用多个比较器可以提高分辨率，因为每个比较器可以识别相位的一个更小的间隔。 5. 对称数系统：虽然文中没有深入讨论，但这个关键词可能指的是在转换过程中使用的数据表示方法，可能有助于优化转换过程中的位分配和计算效率。这篇论文提出了一种创新的方法，通过优化MZM和比较器的使用，以实现更高效、更简单的光子模数转换系统。这种方法对于提升光通信系统中信号处理的性能和可靠性具有重要意义。通过减少系统复杂性和提高转换精度，该技术可能为未来光子学和光纤通信领域的应用打开新的可能性。

Photonic analog-to-digital conversion using multiple comparators

and Mach-Zehnder modulators with identical half-wave voltages

Shuna Yang

, Zhiguo Shi

, Hao Chi

, Xianmin Zhang

, Shilie Zheng

, Xiaofeng Jin

, Jianping Yao

a,b

Department of Information and Electronic Engineering, Zhejiang University, Hangzhou 310027, China

Microwave Photonics Research Laboratory, School of Information Technology and Engineering, University of Ottawa, Ottawa, Canada K1N 6N5

article info

Article history:

Received 10 August 2008

Received in revised form 6 October 2008

Accepted 17 October 2008

Keywords:

Photonic analog-to-digital converter

Mach-Zehnder modulator

Symmetrical number system

abstract

A novel photonic analog-to-digital conversion scheme using Mach-Zehnder modulators (MZMs) with

identical half-wave voltages and multiple comparators with improved bit resolution is proposed and

demonstrated. Compared with the scheme using MZMs with geometrically scaled half-wave voltages,

the proposed scheme has the advantage of using MZMs with identical half-wave voltages which simplify

signiﬁcantly the implementation. The bit resolution is improved based on the use of multiple compara-

tors in each MZM channel, with the digital coding realized based on the symmetrical number system the-

ory. A proof-of-concept experiment is implemented. An analog-to-digital converter with a 4-bit

resolution using two MZMs and nine comparators is demonstrated.

1. Introduction

Analog-to-digital converter (ADC) realized in the optical do-

main has been an attractive research topic for many years thanks

to the unique ability of ultra high speed sampling offered by the

state-of-the-art optical mode-locked lasers. By now, numerous ap-

proaches to realizing optical sampling and optical quantization

have been proposed and demonstrated [1]. A well-known architec-

ture for photonic ADC is the scheme proposed by Taylor, in which

an array of parallel Mach-Zehnder modulators (MZMs) are used

[2], with geometrically scaled half-wave voltages (V

) realized by

controlling the electrode lengths of the MZMs. The major limita-

tion related to Taylor’s scheme is that the V

of the MZM at the

least signiﬁcant bit (LSB) should be very low, which is difﬁcult to

realize using current waveguide technology. To avoid using MZMs

with very low V

, an approach employing cascaded MZMs with

identical V

was proposed [3,4].

Instead of using an array of MZMs, a photonic ADC can also be

realized using a single modulator. Recently, Stigwall et al. proposed

a photonic ADC with a free-space interferometric structure in

which a phase modulator is incorporated in one arm of the inter-

ferometer [5,6]. By placing an array of photodetectors (PDs) at

the speciﬁc locations of the diffraction pattern generated at the

output of the interferometer, digital data in the form of a linear

binary code are generated. A similar scheme was recently demon-

strated by Li et al., where the interferometric structure was real-

ized based on a ﬁber-optic platform, which makes the system

more compact [7]. More recently, we proposed a new scheme of

photonic ADC using MZMs with identical half-wave voltages [8].

The quantization and digital coding are achieved by properly bias-

ing the MZMs, with each MZM being connected to a comparator

with the threshold setting at half of the maximum voltage. A bin-

ary code representing the input analog signal is generated. Com-

pared with the Taylor’s scheme, the bit resolution achieved in [8]

is smaller. For an ADC using n MZMs, the bit resolution is log

(2n); while in the Taylor’s scheme, the number of bit resolution

equals to the number of MZMs employed.

To increase the bit resolution without increasing the number of

MZMs, Pace et al. proposed to use multiple comparators with each

having a different threshold level [9]. The digital coding is achieved

based on the theory of symmetrical number system (SNS). It is

shown that by using multiple comparators with the SNS encoding,

a bit resolution greater than one for each channel (corresponding

to a MZM) can be obtained. To avoid the ambiguity problem, the

MZMs should have a small difference in V

[9]. Therefore, the dif-

ﬁculty in implementing the MZMs with different V

still exists.

In this paper, a novel scheme for photonic ADC using an array of

MZMs with identical V

and multiple comparators for each channel

is presented. By setting the threshold values of the comparators

based on the SNS technique, an increased bit resolution is obtained.

To avoid the ambiguity problem, the MZMs are biased at different

bias points. This is the key advantage compared with the approach

by Pace et al. [9], where the MZMs are with different V

, which

complicated signiﬁcantly the system implementation. A photonic

ADC scheme with a 4-bit resolution using two MZMs and nine

doi:10.1016/j.optcom.2008.10.035

* Corresponding author. Tel.: +86 571 87953855.

E-mail addresses: shizg@zju.edu.cn (Z. Shi), chihao@zju.edu.cn (H. Chi),

zhangxm@zju.edu.cn (X. Zhang), zhengsl@zju.edu.cn (S. Zheng), jpyao@site.

uottawa.ca (J. Yao).

Optics Communications 282 (2009) 504–507

Contents lists available at ScienceDirect

Optics Communications

journal homepage: www.elsevier.com/locate/optcom

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