Differential Phase Shift Keying (DPSK) modulation format has been shown as a robust solution for next-generation optical transmission systems. One key device enabling such systems is the delay interferometer, converting the signal phase information into intensity modulation to be detected by the photodiodes. Usually, Mach-Zehnder interferometer (MZI) is used for demodulating DPSK signals. In this paper, we developed an MZI which is based on all-fiber Multimode Interference (MI) structure: a multimode fiber (MMF) located between two single-mode fibers (SMF) without any transition zones.  The standard MZI is not very stable since the two beams go through two different paths before they recombine. In our design the two arms of the MZI are in the same fiber, which will make it less temperature-sensitive than the standard MZI. Performance of such MZI will be analyzed from transmission spectrum. Finally such all-fiber MI-based MZI (MI-MZI) is used to demodulate 10 Gbps DPSK signals. The demodulated signals are analyzed from eye diagram and bit error rate (BER)

Chen, Xiaoyong

García Regodon, David

Martín Minguez, Alfredo

Rodriguez Horche, Paloma

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

 Mach-Zehnder Interferometer Based on 
All-fiber Multimode Interference Device 
for DPSK Signal Demodulation  
Xiaoyong Chen(1), David Garcia(1), Alfredo Martín Minguez(2), Paloma R. Horche(1) 
xiaoyong.chen@alumnos.upm.es, David.garcia.regodon@alumnos.upm.es, alfredo.martin@upm.es, 
phorche@tfo.upm.es 
(1) Departamento de Tecnología Fotónica y Bioingeniería 
(2) Departamento de Tecnología Electrónica 
Escuela Técnica Superior de Ingenieros de Telecomunicación, Universidad Politécnica de Madrid, Avda. 
Complutense 30, Madrid - 28040. 
 
 
 
Abstract- Differential Phase Shift Keying (DPSK) modulation 
format has been shown as a robust solution for next-generation 
optical transmission systems. One key device enabling such 
systems is the delay interferometer, converting the signal phase 
information into intensity modulation to be detected by the 
photodiodes. Usually, Mach-Zehnder interferometer (MZI) is 
used for demodulating DPSK signals. In this paper, we 
developed an MZI which is based on all-fiber Multimode 
Interference (MI) structure: a multimode fiber (MMF) located 
between two single-mode fibers (SMF) without any transition 
zones.  
The standard MZI is not very stable since the two beams go 
through two different paths before they recombine. In our 
design the two arms of the MZI are in the same fiber, which will 
make it less temperature-sensitive than the standard MZI. 
Performance of such MZI will be analyzed from transmission 
spectrum. Finally such all-fiber MI-based MZI (MI-MZI) is 
used to demodulate 10 Gbps DPSK signals. The demodulated 
signals are analyzed from eye diagram and bit error rate (BER). 
 
I. INTRODUCTION 
Differential Phase Shift Keying (DPSK) is an attractive 
modulation format for next-generation optical transmission 
systems since it exhibits high tolerance to chromatic 
dispersion (CD) and robustness to nonlinear effects. Besides 
these advantages, it also offers an improved receiver 
sensitivity by balanced detection. In DPSK communications, 
one key device which is able to convert the differential phase 
information into intensity information to be detected by the 
photodiodes is a delay interferometer, such as Mach-Zehnder 
interferometer (MZI). Usually, MZI is used for demodulating 
DPSK signals because of its simple structure.  In recent years, 
all-fiber MZI is proposed to take place of the standard MZI 
for improving the performance. It is not only used for 
measurement of temperature, pressure, flatness of plane 
optical plates, thickness of thin film, coherence length of a 
laser, but also used in optical communication, such as DPSK 
demodulator [1]. However, in the DPSK demodulation 
scheme proposed in ref. [1], there is one problem of stability 
due to the two beams go through two different paths and the 
effect, which is generated from temperature and pressure, on 
both arms is different. For further improving the performance 
of all-fiber MZI, several schemes are proposed recently. In [2] 
a photonic crystal fiber (PCF) based MZI for DPSK signal 
demodulation is presented. In this scheme the device is 
fabricated by mismatch splicing of a PCF with standard 
single mode fibers (SMFs). In [3] a multimode fiber (MMF) 
based interferometer for differential phase demodulation is 
proposed. 
All-fiber-based MZI is attractive since it is low-cost and 
easy implementation. In this paper we will present an all-
fiber Multimode Interference-based MZI (MI-MZI) for 
DPSK signals demodulation. Performance of the MI-MZI is 
study deeply from transmission spectrum based on a 
theoretical study and experiments. Finally, a DPSK 
demodulator based MI-MZ will also be presented and 
discussed from theory and simulations.  
The paper is organized as the following. The theory of 
MI-MZI will be presented in section 2. Experimental setup 
and results will be shown and discussed in section 3. In 
section 4 we will present the simulation structure for 
demodulating DPSK signals and discuss the simulation 
results. Finally, the conclusions will be presented in section 5. 
II. THEORY 
The modal interference process is a well known 
phenomenon and it had been studied by several authors [3-9]. 
Based on this principle we can make an interferometer as a  
Fig 1 Comparison between MI-MZI structure (a) and a standard MZI (b) 
 MZI. The structure of the MI-MZI is shown in Fig. 1a where 
only core fibers are drawn. The structure is composed of a 
multimode region located between two single-mode regions 
without any transition zones. This passive device has been 
made with just a multimode fiber (50/125m) spliced 
between two single-mode fibers (9/125m). Moreover, a 
simple connector (with a matched liquid index to avoid 
reflection interfaces) can be employed to couple single-mode 
and multimode fibers without any fusion. Both 
configurations have been employed by us and similar results 
have been obtained [5, 7].  
Since the central region of a MI-MZI is multimode, 
coupling between the LP01 single-mode fiber and all the other 
modes of the multimode fiber is possible. However, coupling 
occurs preferentially between the LP01 mode and the LP02 
mode since both modes have similar azimuthal symmetry 
and the phase mismatch is minimized. Since excited 
fundamental (LP01) and first order symmetry (LP02) modes 
are propagating in the multimode fiber with different phase 
constant, these modes interfere with each other along the 
direction of propagation. The light power coupled into the 
SMF-output fiber depends on the relative phase difference at 
the end of the multimode fiber (L).  
Compared with the standard MZI (as shown in Fig. 1b) 
which obtains a time delay through a physical path length 
difference, the MI-MZI obtains a time delay from time 
difference for both modes passing through the MMF 
transmission path.  
The number of modes that a fiber can support is related to 
the V-parameter of the fiber. The V-parameter is defined as 
[10]: 
ܸ ൌ ଶగ௔ఒ ඥ݊௖௢௥௘ଶ െ݊௖௟௔ௗଶ                           (1) 
where a is the core radius, λ is the wavelength in vacuum, 
݊௖௢௥௘ is the maximum refractive index of the core and ݊௖௟௔ௗ 
is the refractive index of the cladding. Ideally, in our scheme 
we want to have only two circular-symmetric modes.  
The MMF we will used has a w-type index profile shown 
in Fig. 2. This refractive index dip decreases power coupling 
from input mode to zero-order mode and increases coupling 
to next-order symmetric modes. This effect causes w-type 
index profile to have an extinction ratio higher than step 
index profile. Also, with this kind of fibers we can achieve a 
better LP01 - LP02 modes beating because the non 
symmetrical modes, such as modes LP11 and LP21, are less 
coupling.  
 
Fig. 2 Refractive index profile of a w-type fiber 
The transmission is periodic with respect to the frequency 
and is usually represented by a comb-filtering spectrum as 
depicted, i.e., in Figs. 4-6. The spacing ∆λ  between the 
adjacent constructive peaks (destructive valleys) can be 
written as [11]: 
              ∆ߣ ൌ ఒమ	|௡బభି௡బమ|௅ ൌ
ଶగఒ
|ఉబభିఉబమ|௅ ൌ
ఒ	௭್
௅                    (2) 
Where n01 and n02 are the effective index of refraction for 
modes LP01 and LP02 respectively; ݖܾ ൌ 2ߨ|ߚ01െ	ߚ02|  [12] is the 
beat length of the MMF. 
     From (2) we can see that it is possible to adjust the 
separation between peaks by adjusting the length of the 
MMF. Using these properties we can do a transmission 
spectrum very similar to a standard MZI spectrum. 
The propagation constant of the two beating modes can 
be written as: 
	ߚ଴ଵ െ	ߚ଴ଶ ൌ ஛ସగ௔మ௡ౙ౥౨౛ ሺܷ଴ଵ
ଶ െ ܷ଴ଶଶ ሻ                     (3) 
where 
U01	ൌ	2.405	e	‐1/V		 	 			for	the	mode	LP01	ሺHE11ሻ	
U02	ൌ	5.520	e	‐1/V																	for	the	mode	LP02	ሺHE12ሻ	
The transmission spectrum of this MI-MZI can be 
described as follows: 
ܶ ൌ ଵܶ ൅ ଶܶ ൅ 2ඥ ଵܶ ଶܶܿ݋ݏ	ሺ߮ሻ                     (4) 
߮ ൌ ׬ ∆ߚ	ሺݖሻ݀ݖ ൌ ܮ∆ߚଵଶ௅଴                            (5) 
where T1 and T2 are the intensity of the two modes LP01 and 
LP02, respectively; L is the length of the MMF; ∆ߚଵଶ	is the 
difference between the propagation constant of the modes 
LP01 and LP02. The operation principle is based on standard 
MZI with the LP01 and LP02 modes taken as the two 
interference arms.  
Also, combining with the Eq. (2), the time delay between 
two modes can be defined by the following function: 
∆ݐ ൌ ௅௖/௡బభ െ
௅
௖/௡బమ ൌ
∆௡௅
௖ ൌ
ఒమ
௖∆ఒ                    (6) 
     Here we define another parameter which is called delay 
coefficient ∆݀. It is corresponding to the time delay in a one-
meter MMF. It can be described as: 
∆݀ ൌ ∆௧௅ ൌ
∆௡
௖ ൌ
ఒమ
௖௅∆ఒ                             (7) 
     We can see that the delay time of the MI-MZI is only 
determined by the effective index difference ∆݊ between two 
modes, which is governed by the MMF structure. 
III. EXPERIMENTAL RESULTS OF THE MI-MZI 
     The experimental setup using for the measurements is 
shown in Fig 2. For testing the transmission spectrum of the 
MI-MZI, a white light is used as light source because it has a 
very wide spectrum from 0.4µm to 1620µm. The MMF used 
in the experiment has the core and clad diameter of 50µm and 
125µm, respectively. The numerical aperture (NA) is 0.22. 
The refraction index of core and clad is 1.4730 and 1.4567, 
 respectively. Finally a spectrum analyzer is used to observe 
and record the transmission spectrum. 
 
Fig. 3 Experimental setup to measure wavelength transmission 
     In order to observe the spacing ∆ߣ  between adjacent 
constructive peaks (destructive valleys) changing with the 
length of MMF, we use different length of MMF (12cm, 
18.9cm and 24cm). The experimental results are shown in 
Figs. 4-6. Fig. 4 (a) shows the transmission spectrum of the 
MI-MZI with MMF length of 12cm and the line (b) shows the 
spectral respond of the white source used in the experimental 
setup. As can be seen, it decreases for  > 1400nm, this 
causes lower amplitude oscillation for wavelengths greater 
than that.  
     The experimental results clearly show that when the length 
of MMF increases, the spacing ∆ߣ decreases. These results 
are in agreement with the theory which requires that the 
spectral period should be inversely proportional to the length 
of the MMF. As it can be seen in Figs. 4-6, the power 
oscillation has a certain modulation indicating the existence 
of more than two modes. Numerical calculations have 
indicated that four of the modes carrying most of the energy. 
Because every pair of modes has a different propagation 
constant difference, different oscillation amplitudes are 
obtained. 
 
 
 
 
 
 
 
 
 
 
 
Fig.4 (a) Transmission spectrum of MI-MZI with MMF length of 12 cm;            
(b) Transmission spectrum of white source 
 
Fig. 5 Transmission spectrum of  MI-MZI with MMF length of 18.9 cm 
 
Fig. 6 Transmission spectrum of  MI-MZI with MMF length of 24 cm 
IV. MI-MZI FOR DPSK SIGNALS DEMODULATION  
     Considering the MI-MZI as a delay line interferometer 
(DLI), it can be directly used for demodulating DPSK signals. 
A basic diagram of MI-MZI for DPSK signals demodulation 
is shown in Fig. 7a. And we follow it to build a simulation 
system as shown in Fig. 7b. 
 
 
Fig. 7a Principle of MI-MZI for DPSK signals demodulation. MZM: Mach-
Zehnder modulator; BER: bit error rate 
 
Fig. 7b Simulation system of MI-MZI for DPSK signals demodulation 
     In the simulation a tunable laser is used to align the source 
with the transmission peak of the interferometer. A Mach-
Zehnder modulator (MZM), which is driven by a 10Gbps 
DPSK data stream, is used to modulate the light to generate a 
10Gbps optical NRZ-DPSK signal. The simulated MMF has 
the same parameters as the MMF used in the experiment, with 
length of 28.7m to generate 100ps delay between the two 
modes. Finally, a photodiode is used to receive the 
demodulated signal and a bit error rate (BER) analyzer is 
used to analyze the performance of the received signal. 
Laser MZM 
MMF 
SMF SMF 
Photodetector 
BER 
Data 
BER Analyzer 
  
Fig. 8a Eye diagram 
 
Fig. 8b BER vs Received power  
     The simulation results are shown in Fig. 8. The eye 
diagram shows a relatively open intensity eye which can 
provide error-free operation. Fig. 8b shows the BER of the 
received signal as a function of the received power. We can 
see that the sensitivity of the MMF receiver is about -22dBm 
at a BER of 10-15 and about -23.5dBm at a BER of 10-9. 
Compared with the typical DLI DPSK receiver which is built 
with two fiber couplers and has the sensitivity of -23.6dBm at 
a BER of 10-15 and -24.8dBm at a BER of 10-9, there is about 
1.4 ± 0.2dB power penalty by using the MMF receiver. This 
penalty is caused by several aspects, such as the loss in the 
MMF, and the low extinction ratio (ER) of the interference 
between the two modes. The ER depends on two factors: the 
first one is the coupling efficiency between the mode LP01 in 
the SMF and the modes LP01 and LP02 in the MMF. From (4) 
we know that only when the powers of the LP01 and LP02 
modes are the same, the transmission spectrum of the MI-
MZI would reach the highest ER. The second factor is the 
non symmetrical modes excited in the MMF. Although the 
MMF with w-type index profile decreases power coupling 
from input mode to LP01 mode and increases coupling to LP02 
and reduce the non symmetrical modes coupling, there are 
still non symmetrical modes, such as LP11 and LP21, 
propagating in the MMF, which would decrease ER and lead 
to power penalty. 
 
V. Conclusion 
     We have demonstrated a MI-MZI, which is based on a 
MMF between two SMFs. In this structure the excited 
fundamental (LP01) and first order symmetry (LP02) modes 
are propagating in the multimode fiber with different phase 
constants. These modes interfere with each other along the 
direction of propagation and the light power coupled into the 
SMF-output fiber depends on the relative phase difference at 
the end of the multimode fiber (L). So we can obtain an 
interferometer pattern at the exit of the second section of 
SMF. The results of an experimental verification are in 
agreement with the theory. Also, a simulation for using the 
MI-MZI to demodulate a DPSK signal is done; the simulation 
results show that MMF receiver has similar performance as 
the typical DLI DPSK receiver [13] with a 1.4 ± 0.2dB power 
penalty.  
     Since the design of MI-MZI is low cost and easily 
manufactured, it will be used widely in many optical 
applications in the future, such as using as filters or 
temperature sensors. Moreover, with the proper design it also 
can be used as WDM multiplex and demultiplex. 
ACKNOWLEDGMENTS 
The authors would like to acknowledge support from the 
China Scholarship Council (CSC). 
REFERENCES 
[1] F. Séguin, and F. Gonthier, ‘‘Tuneable All-Fiber Delay-Line 
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[2]  Jiangbing Du, Yongheng Dai, Gordon K. P. Lei, Weijun Tong, and 
Chester Shu, “Photonic crystal fiber based Mach-Zehnder interferometer 
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7917-7922 (2010) 
[3] Yannick Keith Lize, Robert Gomma and Raman Kashyap, “Low-cost 
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[9] Marco N. Petrovich, Francesco Poletti, David J. Richardson, “Analysis of 
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[10] Govind P. Agrawal, “Fiber-Optic communication systems”, ed. John 
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[11] Paloma R. Horche, Manuel López-Amo, Miguel A. Muriel and José A. 
Martin-Pereda, “Spectral Behavior of a Low-Cost All-Fiber Component 
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[12] Paloma R. Horche, “Transmissions Propoerties of All-Fibre Modal 
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[13] A. H. Gnauck, “40-Gb/s RZ-differential phase shift keyed transmission”, 
in Proc. OFC’03, paper ThE1 (2003) 
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Mach-Zehnder interferometer based on all-fiber multimode interference device for DPSK signal demodulation

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Mach-Zehnder interferometer based on all-fiber multimode interference device for DPSK signal demodulation

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