光纤通信中的相干检测技术解析

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"这篇文档详细探讨了相干检测技术在光学纤维系统中的应用，涉及相干检测的基本原理、接收与发送端的数字信号处理流程，以及如何应对相关挑战。文章还比较了非相干检测、差分相干检测和相干检测等不同检测方法，并介绍了利用不同自由度（DOF）进行调制的方法，特别提到了极化复用正交幅度调制（PM-QAM）的优势。此外，双极化同相或异相下变频作为线性过程，能完全恢复接收到的信号场的四个DOF。通过在奈奎斯特定理采样率下对下变频信号进行采样，可以利用数字信号处理（DSP）来补偿传输损耗，包括色散和极化模色散等线性损伤。" 本文主要讨论了光纤通信领域的关键技术创新——相干检测技术，这种技术因其高效率和性能提升而在现代光纤系统中受到了广泛关注。首先，文章对比了不同类型的检测方法，包括非相干检测、差分相干检测和相干检测。非相干检测通常简单但效率较低，而相干检测则能提供更高的灵敏度和带宽效率，但实现起来更为复杂。接着，文章详细介绍了相干检测的原理，特别是在接收和发送端采用的数字信号处理技术。这些技术对于克服光纤传输中的各种问题至关重要，如色散、极化模色散和光纤损耗等。色散会导致光信号在光纤中传播时不同频率的成分速度不同，造成信号失真；极化模色散则会影响光的极化状态，进一步影响信号质量。此外，文章还强调了极化复用正交幅度调制（PM-QAM）的重要性，这是一种利用光信号的两个极化方向和两个正交幅度状态来编码信息的高效调制方式。通过这种方式，可以充分利用所有四个自由度，显著提高频谱效率和功率效率。在实际应用中，双极化同相或异相下变频被用来将接收到的光信号转换为电信号，这个过程是线性的，有助于完整恢复信号的四个自由度。随后，利用数字信号处理（DSP）技术，可以在奈奎斯特定理的采样率下对信号进行处理，实时补偿传输过程中产生的各种线性损伤，从而提高系统的整体性能和稳定性。相干检测在光纤通信中的应用不仅涉及到复杂的物理原理，还包括了先进的数字信号处理技术。这一领域的发展对于推动下一代高速、长距离的光纤通信网络至关重要，同时也为解决当前网络容量需求的挑战提供了可能的解决方案。

polarized signals are denoted as vectors and are written in bold face, such as

y . Unless

stated otherwise, we employ a linear (x, y) basis for decomposing dually polarized signals. In

Section 5 dealing with impairment compensation, matrix and vector quantities are denoted in

bold face.

3. Optical detection methods

3.1 Noncoherent detection

Fig. 1. Noncoherent receivers for (a) amplitude-shift modulation (ASK) and (b) binary

frequency-shift keying (FSK).

In noncoherent detection, a receiver computes decision variables based on a measurement of

signal energy. An example of noncoherent detection is direct detection of on-off-keying

(OOK) using a simple photodiode (Fig. 1(a)). To encode more than one bit per symbol, multi-

level amplitude-shift keying (ASK) – also known as pulse-amplitude modulation – can be

used. Another example of noncoherent detection is frequency-shift keying (FSK) with wide

frequency separation between the carriers. Fig. 1(b) shows a noncoherent receiver for binary

FSK.

The limitations of noncoherent detection are: (a) detection based on energy measurement

allows signals to encode only one degree of freedom (DOF) per polarization per carrier,

reducing spectral efficiency and power efficiency, (b) the loss of phase information during

detection is an irreversible transformation that prevents full equalization of linear channel

impairments like CD and PMD by linear filters. Although maximum-likelihood sequence

detection (MLSD) can be used to find the best estimate of the transmitted sequence given only

a sequence of received intensities, the achievable performance is suboptimal compared with

optical or electrical equalization making use of the full electric field [13].

3.2 Differentially coherent detection

Fig. 2. Differentially coherent phase detection of (a) 2-DPSK (b) M-DPSK, M > 2.

In differentially coherent detection, a receiver computes decision variables based on a

measurement of differential phase between the symbol of interest and one or more reference

symbol(s). In differential phase-shift keying (DPSK), the phase reference is provided by the

previous symbol; in polarization-shift keying (PolSK), the phase reference is provided by the

signal in the adjacent polarization. A binary DPSK receiver is shown in Fig. 2(a). Its output

photocurrent is:

() () ( )

{

}

sssDPSK

TtEtERtI −=

Re ,

where

()

is the received signal, R is the responsivity of each photodiode, and T

is the

symbol period. This receiver can also be used to detect continuous-phase frequency-shift

()

iDPSK,

()

qDPSK,

-90°

()

iDPSK,

()

qDPSK,

-90°-90°

(

)

(a)

(

)

()

DPSK

()

DPSK

()

Decision

()

Decision

(a)

(

)

()

Decision

()

Decision

#86543 - $15.00 USD Received 20 Aug 2007; revised 9 Nov 2007; accepted 12 Nov 2007; published 9 Jan 2008

keying (CPFSK), as the delay interferometer is equivalent to a delay-and-multiply

demodulator. A receiver for M-ary DPSK, M > 2, can similarly be constructed as shown in

Fig. 2(b). Its output photocurrents are:

() () ( )

{

}

sssiDPSK

TtEtERtI −=

Re , and

() () ( )

{

}

sssqDPSK

TtEtERtI −=

Im .

A key motivation for employing differentially coherent detection is that binary DPSK has

2.8 dB higher sensitivity than noncoherent OOK at a BER of 10

−9

[5]. However, the constraint

that signal points can only differ in phase allows only one DOF per polarization per carrier,

the same as noncoherent detection. As the photocurrents in Eq. (1) to (3) are not linear

functions of the E-field, linear impairments, such as CD and PMD, also cannot be

compensated fully in the electrical domain after photodetection.

A more advanced detector for M-ary DPSK is the multichip DPSK receiver, which has

multiple DPSK receivers arranged in parallel, each with a different interferometer delay that is

an integer multiple of T

[14,15]. Since a multichip receiver compares the phase of the current

symbol to a multiplicity of previous symbols, the extra information available to the detector

enables higher sensitivity. In the limit that the number of parallel DPSK receivers is infinite,

the performance of multi-chip DPSK approaches coherent PSK [15]. In practice, the number

of parallel DPSK receivers required for good performance needs to be equal to the impulse

duration of the channel divided by T

. Although multi-chip DPSK does not require a local

oscillator (LO) laser, carrier synchronization and polarization control, the hardware

complexity can be a significant disadvantage.

3.3 Hybrid of noncoherent and differentially coherent detection

A hybrid of noncoherent and differentially coherent detection can be used to recover

information from both amplitude and differential phase. One such format is polarization-shift

keying (PolSK), which encodes information in the Stokes parameter. If we let

() ()

()

etatE

= and

() ()

()

etatE

= be the E-fields in the two polarizations, the

Stokes parameters are

1 yx

aaS −= ,

()

cos2

2 yx

aaS = and

()

sin2

3 yx

aaS = , where

() () ()

ttt

φφδ

−= [16]. A PolSK receiver is shown in Fig. 3. The phase noise tolerance of

PolSK is evident by examining S

to S

. Firstly, S

is independent of phase. Secondly, S

and

depend on the phase difference

() ()

φφ

− . As

()

and

()

are both corrupted by the

same phase noise of the transmitter (TX) laser, their arithmetic difference

()

is free of

phase noise. In practice, the phase noise immunity of PolSK is limited by the bandwidth of the

photodetectors [16]. It has been shown that 8-PolSK can tolerate laser linewidths as large as

01.0≈Δ

[17], which is about 100 times greater than the phase noise tolerance of coherent

8-QAM (Section 5.3.3). This was a significant advantage in the early 1990s, when symbol

rates were only in the low GHz range. In modern systems, symbol rates of tens of GHz, in

conjunction with tunable laser having linewidths <100 kHz, has diminished the advantages of

PolSK. Recent results have shown that feedforward carrier synchronization enables coherent

detection of 16-QAM at

Δ ~10

−5

[18], which is within the limits of current technology.

As systems are increasingly driven by the need for high spectral efficiency, polarization-

multiplexed QAM is likely to be more attractive because of its higher sensitivity (Section 4).

(

)

(

)

#86543 - $15.00 USD Received 20 Aug 2007; revised 9 Nov 2007; accepted 12 Nov 2007; published 9 Jan 2008

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