Page 1

Quantum model for electro-optical amplitude

modulation

José Capmany1,* and Carlos R. Fernández-Pousa2

1ITEAM Research Institute, Universidad Politécnica de Valencia, C/ Camino de Vera s/n, 46022 Valencia, Spain

2Signal Theory and Communications, Department of Physics and Computer Science, Universidad Miguel Hernández,

Av. Universidad s/n, E03202 Elche, Spain

*jcapmany@dcom.upv.es

Abstract: We present a quantum model for electro-optic amplitude

modulation, which is built upon quantum models of the main photonic

components that constitute the modulator, that is, the guided-wave

beamsplitter and the electro-optic phase modulator and accounts for all the

different available modulator structures. General models are developed both

for single and dual drive configurations and specific results are obtained for

the most common configurations currently employed. Finally, the operation

with two-photon input for the control of phase-modulated photons and the

important topic of multicarrier modulation are also addressed.

©2010 Optical Society of America

OCIS codes: (130.4110) Modulators; (060.5565) Quantum communications; (060.5625) Radio-

frequency photonics; (060.0060) Fiber optics and optical communication; (270.0270) Quantum

optics.

References and links

1. L. Thylen, U. Westergren, P. Holmström, R. Schatz, and P. Jänes, “Recent developments in high-speed optical

modulators,” in Optical Fiber Telecommunications V.A, I.P Kaminow, T. Li, and A.E. Willner, eds. (Academic

Press, 2008), Chap. 7.

2. G. P. Agrawal, Fiber-Optics Communications Systems (John Wiley & Sons, New York, 2002).

3. C. H. Cox III, Analog Optical Links: Theory and Practice (Cambridge Univ. Press, 2004).

4. M. Howerton, and W. K. Burns, “Broadband traveling wave modulators in LiNbO3,” in RF Photonic Technology

in Optical Fiber Links, W.S. Chang, ed. (Cambridge Univ. Press, 2002), Chap. 5.

5. G. E. Betts, “LiNbO3 external modulators and their use in high performance analog links,” in RF Photonic

Technology in Optical Fiber Links(Cambridge Univ. Press, 2002), Chap. 4.

6. A. Yariv, and P. Yeh, Photonics: Optical Electronics in Modern Communications (Oxford Univ. Press, 2006).

7. N. Kashima, Passive optical Components for optical Fiber Transmission (Artech House, Boston, 1995).

8. R. Syms, and J. Cozens, Optical Guided waves and Devices (McGraw-Hill, New York, 1992)

9. B. E. A. Saleh, and M. C. Teich, Fundamentals of Photonics (John Wiley & Sons, New York, 1991)

10. H. A. Bachor, and T. C. Ralph, A Guide to Experiments in Quantum Optics (Wiley-VCH, Weinheim, 2003).

11. J. M. Mérolla, Y. Mazurenko, J.-P. Goedgebuer, L. Duraffourg, H. Porte, and W. T. Rhodes, “Quantum

cryptographic device using single-photon phase modulation,” Phys. Rev. A 60(3), 1899–1905 (1999).

12. O. Guerreau, J.-M. Mérolla, A. Soujaeff, F. Patois, J. P. Goedgebuer, and F. J. Malassenet, “Long distance QKD

transmission using single sideband detection scheme with WDM synchronization,” IEEE J. Sel. Top. Quantum

Electron. 9(6), 1533–1540 (2003).

13. G. B. Xavier, and J.-P. von der Weid, “Modulation schemes for frequency coded quantum key distribution,”

Electron. Lett. 41(10), 607–608 (2005).

14. A. Ortigosa-Blanch, and J. Capmany, “Subcarrier multiplexing optical quantum key distribution,” Phys. Rev. A

73(2), 024305 (2006).

15. P. Kolchin, C. Belthangady, S. Du, G. Y. Yin, and S. E. Harris, “Electro-optic modulation of single photons,”

Phys. Rev. Lett. 101(10), 103601 (2008).

16. C. C. Gerry, and P. L. Knight, Introductory Quantum Optics (Cambridge Univ. Press, 2005).

17. M. Fox, Quantum optics: An introduction (Oxford Univ. Press, 2006).

18. J. C. Garrison, and R. Y. Chiao, Quantum Optics (Oxford Univ. Press, 2008).

19. U. Leonhardt, “Quantum physics of simple optical instruments,” Rep. Prog. Phys. 66(7), 1207–1249 (2003).

20. P. Kumar, and A. Prabhakar, “Evolution of quantum states in an electro-optic phase modulator,” IEEE J.

Quantum Electron. 45(2), 149–156 (2009).

21. J. Capmany, and C. R. Fernández-Pousa, “Quantum model for electro-optical phase modulation,” J. Opt. Soc.

Am. B 27(6), A119–A129 (2010).

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Page 2

22. J. É. Capmany, A. Ortigosa-Blanch, J. É. Mora, A. Ruiz-Alba, W. Amaya, and A. Martinez, “Analysis of

subcarrier multiplexed quantum key distribution systems: signal, intermodulation and quantum bit error rate,”

IEEE J. Sel. Top. Quantum Electron. 15(6), 1607–1621 (2009).

1. Introduction

The electro-optic amplitude modulator (EOM) is the most popular modulating device

employed in high speed optical communications systems featuring line rates in excess of

2.5 Gb/s [1,2]. It is also widely employed in analog photonic applications [3] and radio over

fiber systems [4,5] where the modulating subcarriers are located in the RF, microwave and

millimeter –wave regions of the electromagnetic spectrum.

The EOM is built by embedding one or two electro-optic phase modulators into a Mach-

Zehnder interferometric setup closed by two guided-wave beamsplitters [5,6]. For instance, in

Fig. 1 we show the most common EOM designs that are encountered in practice [5].

EOMs with only one internal phase modulator such as (B), (D), (F) in Fig. 1 are known as

single drive or asymmetric modulators, while EOMs with two internal phase modulators, such

as (A), (C) and (E) in Fig. 1 are known as dual drive modulators. For each phase modulator

there are two ports, one for the modulator DC bias voltage and another to inject the time

varying modulating signal. Designs (A) and (B) correspond to Y-Branch modulators, where

both the input and output beamsplitters in the Mach-Zehnder interferometric structure are

Y-Branch power splitters. These are the most common commercial devices. Designs (C) and

(D) represent the so-called DC-Modulators where both the input and output beamsplitters in

the Mach-Zehnder interferometric structure are guided wave directional couplers. These

modulators bring the added value of an extra input and an extra output port but are more

expensive to produce since the fabrication of a symmetric directional coupler is more

challenging as compared to the Y-Branch power splitter which particularly simple to

implement in integrated fashion [7,8]. DC modulators were common in the early days of

integrated optics but today, although available upon request from different vendors they are

not a mainstream commercial product. Finally, designs (E) and (F) correspond to hybrid

Y-Branch DC modulators which feature one input and two outputs. These modulators, which

can be found upon request in the market, are common for Cable TV applications [5], where

the dual output permits a first signal splitting in the broadcasting header. The operation and

design principles of the EOM under classical conditions are quite well established and

understood and the interested reader can find useful information in innumerable references in

the literature [3,9].

Fig. 1. Different possible layouts for amplitude electro-optic modulators. After [5].

As far as its operation under quantum regime there is however little if any work reported

in the literature but nevertheless, the use of electro-optic modulation is finding increasing

application in several areas including the control of quantum measurements [10], sources of

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approximated single-photon states in quantum key distribution (QKD) systems [11],

frequency-coded [12,13] and subcarrier multiplexed [14] QKD systems or, more recently, for

tailoring the wave function of heralded photons [15]. It is thus, an objective of this paper, to

provide an accurate modeling of this device under such conditions. To do so, we have

organized the paper as follows: In Section 2 we briefly present the existing quantum models

for the building blocks of the EOM, that is, the beamsplitter and the phase modulator. In

Section 3 we first develop the general transformation equations for the cases of single photon

and coherent input states. The general results are then specialized in Section 4 as examples for

several popular designs and operation modes of EOMs. In section 5 we briefly analyze in

detail the operation of DC modulators under two-photon inputs, and show how they can be

used to interpolate between two-port entangled and separable states composed of phase-

modulated photons. In section 6 we develop the equation for the important case of multitone

radiofrequency modulation. Finally the main summary and conclusions of the paper are

presented in Section 6.

2. Quantum models for the beamsplitter, directional coupler, Y-branch power splitter

and electro-optic phase modulator

2.1 Bulk-optics beamsplitter

The operation of the bulk optics BS under quantum regime can be found in several texts and

references in the literature [10,16–19]. For instance, in [16] and [10,17,18] an excellent

description based on the Heisenberg picture is developed. Here we focus as well on a

description based on the Schrödinger picture that, for reasons to be apparent later in this

chapter, is more convenient to our purposes of describing the operation of more complex

photonic devices such as the amplitude electro-optic modulator. We then specialize the

description to the two more prominent guided-wave versions of the device [6–8]; the

Directional Coupler (DC) and the Y-Branch power splitter (YB) which are employed in the

EOM. To describe the quantum operation of the optical beamsplitter we will refer to Fig. 2.

The upper part of the figure describes the general framework of the Hilbert spaces and states

that characterize both the input and the output of the BS. We will assume that the input is

given by the product state |in = |Ψ11|Ψ22, where |Ψj j belongs to Hj, the multimode Hilbert

space corresponding to states of input port j = 1,2 (Although the Hilbert space is assumed

multimode, the BS is a singlemode device and does not mix frequencies. So it is enough to

analyze its operation over a single mode. Furthermore, we assume polarization independent

operation.)

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Fig. 2. Quantum state labeling (upper) and single photon behavior (lower) of an optical

beamsplitter.

In the same way, an output state from the BS is given by |out = |Φ1’1’ |Φ2’2’, with |Φj’ j’

Hj’ the Hilbert space corresponding to the states of output port j’ = 1’,2’. Upon these

definitions, the action of the BS in the Heisenberg picture is described by an unitary scattering

operator ˆSBS: |in |out which leaves invariant the vacuum state, ˆSBS |vac = |vac and such

that, with the usual notations for the creation operators

action can be expressed in matrix form:

††

1ˆ

a

ˆ

a

1 and

††

2ˆ

a

ˆ

a

1

, its

†

1

†

2

†

1

†

2

†

BS

''

ˆ

ˆ

a

ˆ

ˆ

a

ˆˆ

,

BS

t

r

r

t

aa

SS

(1)

where t’ and r’ define the field transmission and reflection coefficients for a classical signal

fed through port 1 and correspondingly t and r are those for inputs from port 2. The explicit

form for the bulk-optics beamsplitter scattering operator is [16]:

†

1

†

2 1

ˆ ˆ

a a

12

ˆ

ˆ

ˆ ˆ

a a

exp() exp[()/ 2],

BS

S j J

j

(2)

where θ determines the power division fraction between the output ports of the device (θ = π/2

for a 3-dB beamsplitter), and definition Eq. (10) is consistent with Eq. (1) provided that:

'cos/ 2

ttr

'sin/ 2 .

rj

(3)

The action over single-mode displacement operators is given by [16]

BS

S

†

BS

†

BS

ˆˆ

ˆˆˆ

( )

''

ˆˆ

ˆˆˆ

( )

,

BS

SDSDtDr

DSDrDt

1

1

(4)

from which we have the general transformation rule for arbitrary single-mode coherent-state

inputs in both ports:

12

BS

St

1'2'

ˆ

'',

rrt

(5)

where

ˆ( )

D

vac

.

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2.2 Directional coupler

The most widespread version of the optical beamsplitter in guided-wave format is the

directional or 2 × 2 coupler [6–9]. It is composed on two input and two output optical fibers or

integrated input waveguides. Signal coupling is achieved on the central part of the device by

creating the adequate conditions such that the evanescent fields of the fundamental guided

mode in one waveguide can excite the fundamental guided mode in the other and vice-versa.

Different techniques to achieve signal coupling and for analyzing the device performance and

carry out a proper design have been developed in the last 20 years and are quite well known

and understood [6]. The beamsplitting power ratio of the 2 × 2 coupler is characterized by its

coupling constant k [6] that defines the fraction of power coupling (i.e crossing) from one

waveguide to the other. Another characteristic is the π/2 phase shift that the optical field

experiences when coupling from one waveguide to the other, which means that the reflection

coefficients r, r’ in Eq. (3) are imaginary. The results for the general bulk-optics beamsplitter

can thus be employed to describe the 2 × 2 directional coupler by making:

'1'.

ttkrr j k

(6)

2.3 Y-branch power splitter

The Y-Branch or 1 × 2 Power Splitter is another popular guided-wave implementation of the

optical beamsplitter although it is more commonly employed in its integrated optics version

than in optical fiber format [8]. As the 2 × 2 directional coupler it is characterized by its

coupling constant k that again, defines the fraction of power coupling (i.e crossing) from one

waveguide to the other. However, in this case, there is no phase shift experienced by the

optical field when coupling from one waveguide to the other. The behavior of the Y-Branch

splitter under classical regime is shown in Fig. 3.

One distinctive feature of the Y-Branch is that it only has one physical input port. To

characterize the device as a beamsplitter [18], we need to assume that there is a second input

port although with no physical access from the classical point of view (on the reverse

operation this port corresponds to the radiated field due to the antisymmetric supermode [8]).

However, it should be borne in mind that under quantum regime this port is accessible by a

vacuum state [16].

Fig. 3. Classical behavior of Y-branch guided-wave power splitter.

The energy conservation and reciprocity relationships can be summarized as:

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2222**

| '|

t

| '|

r

| |

t

| |

r

1''0.

r tr t

(7)

Now, if t, t’, r and r’ are real and k characterizes the fraction of power coupling (i.e crossing)

from one waveguide to the other then the above relationships are verified provided that:

'1',

ttkrkr

(8)

so:

†

1

†

2

†

1

†

2

†

YB

1

ˆ

ˆ

a

ˆ

ˆ

a

ˆˆ

.

1

YB

kk

aa

SS

kk

(9)

Alternatively, one can employ an explicit form for the Y-Branch power splitter scattering

operator, which in this case is given by a different expression than that of Eq. (2):

†

1

†

2 1

ˆ ˆ

a a

22

ˆ

ˆ

ˆ ˆ

a a

exp()exp[()/2].

YB

Sj J

(10)

Again, it can be checked that Eq. (10) verifies the transformation given by Eq. (9) if:

'1cos/ 2'sin/ 2 .

ttkrrk

(11)

2.4 Electro-optic phase modulator

Quantum models for the electro-optic phase modulator have been derived in detail in [20,21].

Figure 4 shows a typical layout of the device. It consists of a dielectric waveguide and two

electrodes placed at both sides. The dielectric material is subject to the electro-optic effect and

the voltage applied to the electrodes changes linearly the refractive index undergone by one of

the two possible input linear polarizations while not affecting the refractive index in the other

linear polarization.

Using the overcomplete basis of coherent states and by analogy with the classical behavior

it can be shown [21] that the modulator action subject to sinusoidal radiofrequency

modulation is described by a unitary operatorˆSPM that couples modes with different

frequencies. Such modulating tone at frequency Ω is given by V(t) = VDC + Vmcos(Ωt +

θ).where φb = πVDC/Vπ is a static phase shift related to the modulator bias voltage normalized

by the modulator Vπ parameter, m = πVm/Vπ represents the radiofrequency modulation index.

The optical output shows frequencies ω0 + k Ω, with k the integer determining the order of the

output optical sideband. After quantizing the travelling-wave radiation in a length L, allowed

frequencies become multiples of the quantity 2π v/L, where v is the speed of light in the

medium. This implies that the incoming optical radiation can be described by index n0>0 such

that ω0 = 2π n0v/L, and correspondingly the radio-frequency Ω is determined by integer N>0

with Ω = 2π Nc/L. The scattering between waves with positive frequency is then given by:

0

0

†

PM

0

†

n

†

qN r

a

1

.

ˆˆ

ˆ

a S

ˆ

PMq

q

Cq

S

(12)

Here the coefficients

q

0

00

000

0

()( )

m

1( )

m

()( ),

m

bb

q

jq q

jq q

jj

qq q

q q

q q

CejeJJejeJ

(13)

represent the scattering amplitudes from the input frequency represented by a mode number n0

= q0N r0 (q0 and r0 being integers with 0 r0<N) to an output mode of frequency qNr0. In

the last part of Eq. (13) we have introduced the so-called optical limit which recover the usual

expressions of the classical theory of electro-optic phase modulation: when the incoming

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radiation is optical (n0>>0) the second Bessel function in Eq. (13) can be neglected and

Eq. (12) can be written as:

0

0

†

PM

†

n

1

†

n

ˆ

()( ),

ˆˆ

ˆ

a S

b

o

j

js

PMssN

sq

ejeJ m a

S

(14)

and then Cq(q0) represents the amplitude of the scattering from the incoming frequency to its

sideband of order qq0.

Fig. 4. Typical configuration of a waveguide electro-optic phase modulator. (Lower) Black-box

representation of the phase modulator under quantum regime.

The phase modulator can be described by [21]:

*†

N

) ,

ˆ

ˆˆˆ

exp(

PM

N

b ph

SjTTN

(15)

where χ = ejθm/2 and

†

m N

a

†

m

0

11

ˆˆˆ

ˆˆ

a

ˆ ˆ ,

a a

Nmphm

mm

TNT

(16)

3. General quantum model for the electro-optic amplitude modulator

We consider a general structure of the amplitude modulator as shown in Fig. 5. The layout is

obviously an abstract representation as the beamsplitters are of the guided wave type [5,6,8].

However, the representation of Fig. 5 clearly shows the three building blocks of the

modulator; an input beamsplitter opening the two paths of the interferometer, two different

paths (labeled 1 and 2 in the figure) which include, each one, a phase modulator and an output

beamsplitter which closes the interferometer and provides two possible output ports. Now, the

operator describing the action of the EOM, ˆSEOM, is given by:

12

ˆˆˆˆˆˆˆˆ

(),

oioi

EOM BSPM BSBSPMPMBS

SSSSSSSS

(17)

where ˆSPM1 and ˆSPM2 represent the scattering operators corresponding, respectively, to the

phase modulators located in the lower and upper paths of the layout of Fig. 5. ˆSEOM is unitary,

since it is the product of three unitary operators, so that ˆSEOM ˆS†

We also mention that, although we use this complete layout to develop the model for the

amplitude modulator, the equations developed here can and will be particularized to the case

of asymmetric modulators as well where, say, ˆSPM1 is substituted by the identity operator.

EOM = ˆS†

EOM ˆSEOM = 11.

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Fig. 5. Generic layout of an electro-optic amplitude modulator.

3.1 Single Photon input states

We shall start with the characterization of the EOM response to single photon inputs, that is,

we wish to evaluate:

ˆ

1

EOMEOMn

n

S vacSa vac

1

†

n

†

n

†

EOM

††

n

†

EOM

ˆˆˆˆ

ˆ

a

ˆ

a

1

ˆˆˆˆ

ˆ

a

,

EOMEOM EOM

n

EOM

S vacS vac vacSS vacvac

vacSS vacvac

11

1

(18)

where we have used that, since all the operators in (17) leave invariant the vacuum state,

ˆSEOM also leaves it invariant. Using the results derived in section 2 we obtain, for instance:

11

ˆ

i o PMnPMi o

t rSa SrtS

1122

22

†

n

†

EOM

' '

i o

t t

†

n

†

PM

'†

n

†

PM

' '††'†

n

†

PM

ˆˆˆˆˆˆ

ˆ

a

ˆ

a S

ˆ

a S

ˆˆˆˆ

ˆ

a S

,

EOMPMi o

r r S

PM

PM

SSS

111

11

(19)

a transformation that possesses a straightforward physical interpretation. The first term in the

right member represents the transformation corresponding to a state entering the EOM

through port 1, transmitted by the input beamsplitter BSi to the lower path, being modulated

by PM1 and finally transmitted by the output beamsplitter BSo to output port 1. The second

term represents the transformation corresponding to a state entering the EOM through port 1,

transmitted by the input beamsplitter BSi to the lower path, being modulated by PM1 and

finally reflected by the output beamsplitter BSo to output port 2. The third term represents the

transformation corresponding to a state entering the EOM through port 1, reflected by the

input beamsplitter BSi to the upper path, being modulated by PM2 and finally reflected by

beamsplitter BSo to output port 1. The fourth and last term represents the transformation

corresponding to a state entering the EOM through port 1, reflected by the input beamsplitter

BSi to the upper paths, being modulated by PM2 and finally transmitted by the output

beamsplitter BSo to output port 2. Similarly we obtain:

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1122

1122

†

n

†

EOM

'†

n

†

PM

†

n

†

PM

'†

n

†

PM

†

n

†

PM

ˆˆˆˆˆˆ

ˆ

a

ˆ

a S

ˆ

a S

ˆˆˆˆ

ˆ

a S

ˆ

a S

,

EOMi o

rt

PMi o

t r S

PM

i o

rr

PMi o

t t

PM

SSS

SS

111

11

(20)

with a similar physical interpretation for its four terms but taking into account that now the

input is in port 2. Although Eqs. (19) and (20) have been derived for a perfectly balanced

interferometer they hold for unbalanced structures since any phase imbalance between the

upper and the lower arms can be incorporated into the DC bias term φb.

To proceed further we do compact the notation by naming [see Eqs. (12) and (13)]:

q a

q a

010111020222

†

n

†

n

†

PM

†

qN

†

n

†

n

†

PM

†

qN

00

11

ˆ

ˆˆˆˆ

ˆ

a S

ˆˆ

c

ˆ

a S

ˆ

,

PMqr PMqr

qq

bSCSC

(21)

where

Eq. (20) we will assume in general that the frequencies of the RF signals modulating each of

the two phase modulators are different (given by mode indexes N1 and N2, respectively). Thus,

following the same discussion as after Eqs. (12) and (13) we have n0 = q0N1 r1 for phase

modulator 1 and n0 =

show different characteristics (for example, they might be biased at different voltages), so we

employ Cq(x) and Cq(x) to express the fact that the coefficients given by Eq. (13) might be

different for both modulators even if they are modulated by the same RF frequency.

Use of Eq. (21) in Eqs. (19) and (20) yields the expression of the transition amplitudes for

a single-photon input to each of the multimode output in each output port as a function of BS

modulator couplings. Although general, those expressions are rather involved and of limited

use in practice. Simpler expressions result in the especially representative case of the same

microwave frequency of the modulating signal in both RF ports so N1 = N2 = N, r1 = r2 = r,

q0 =

† ˆb and

†ˆ c have the interpretation of creators of phase-modulated photons. Referring to

0q N2 r2 for phase modulator 2. Furthermore, both modulators might

0q . In this case Eqs. (19) and (20) reduce to:

†

n

†

EOM

' '

i o

t t C

'†

qN r

00

1

'''†

qN r

00

1

ˆˆ

ˆ

a

ˆ

a

ˆ

a

,

EOMqi o

r r C

q

q

i o

t r C

q i o

r t C

q

q

SSqq

qq

1

11

(22)

†

n

†

EOM

'†

qN r

a

00

1

'†

qN r

00

1

ˆˆ

ˆ

a

ˆ

ˆ

a

.

EOMi o

rt C

qi o

t r C

q

q

i o

rr C

qi o

t t C

q

q

SSqq

qq

1

11

(23)

The interpretation of these formulas is now straightforward: due to the action of the phase

modulators, an input photon couples to the same ladder of sideband modes

output port. The particular value of the transition amplitude depends on an interference term,

i.e., on the coherent sum of the transition amplitudes Cq and Cq set by each phase modulator,

which can be tuned independently by different configuration of bias (φ b1, φ b2), radio-

frequency phase (θ1, θ2) and modulation indices (m1, m2). This interference term determining

the coupling to the output optical sidebands is finally weighted (in amplitude and phase) by

the corresponding beamsplitter’s reflection and transmission coefficients.

†ˆqN r

a

in each

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3.2 Coherent input states

In this case we need to evaluate:

000

0

0

0

†

EOM

†

EOM

ˆˆˆ

ˆ

()(())()

ˆˆˆ

ˆ

()(())(),

oo

EOMn EOMnn

n

EOMnEOMnn

n

SvacSDSvacvac

SvacSDS vacvac

1

1

(24)

where |

computing the transformations. Following a similar procedure to that of subsection 3.1 we

arrive after a straightforward but lengthy process to:

oo

nn

represents the coherent state for mode number n0. We are thus interested in

q

q

q

00

110221122

†

EOM

' '

i o

t t C

' ' '

i o

t r C

'

000

11

ˆˆ

ˆ

(())

ˆˆˆˆ

,

ooo

EOM

nn

qNrnqqNrni o

r r C

qqNrnqoqNrni o

rt C

q

q

q

SDS

DDDqD

1

(25)

where the brackets separate the multimode outputs in each of the two ports, and, similarly:

00

11221122

†

EOM

''

0000

11

ˆˆ

ˆ

(( ))

ˆˆˆˆ

()()()( ) .

oooo

EOM

nn

qNrni o

rt C q

q qNrni o

t r C q

q qNrni o

rr C q

q qNrni o

t t C q

q

q

q

SDS

DDDD

1

(26)

Again, when the microwave frequency of the modulating signal is the same in both RF ports

(N1 = N2 = N, r1 = r2 = r, q0 =

0q ), the former equations reduce to:

00

0

†

EOM

' '

i o

t t C q

'' '

i o

t r C q

'

0000

11

ˆˆ

ˆ

(())

ˆˆ

()()()(),

o

EOM

nn

qN rnq i o

r r C q

q qN rnqi o

rt C q

q

q

q

SDS

DD

1

(27)

00

00

†

EOM

''

0000

11

ˆˆ

ˆ

(())

ˆˆ

()()()().

EOM

nn

qN rni o

rt C q

q i o

t r C q

q qN rni o

rr C q

qi o

t t C q

q

q

q

SDS

DD

1

(28)

Comparison of Eqs. (27) and (28) with Eqs. (22) and (23) shows that the multimode coherent-

state outputs are governed by the same interference term or coherent sum of phase-modulator

settings (through Cq and Cq) weighted by the beamsplitter reflection and transmission

coefficients. The situation is thus similar to that of a single beamsplitter or a passive

interferometer [16], where the classical output amplitudes described by the product states

Eqs. (27) and (28) are in direct correspondence of the transition amplitudes of the genuine

quantum output state Eqs. (22) and (23) when the modulator is operated at single-photon

level. In classical, coherent-state operation, the interference between modes is designed to

produce outputs with specific temporal features, such as pulse trains with different duty cycle

for digital communications or amplitude waveforms with distortion-free, linear response for

analog communications. At single-photon level, the action of the modulator is best viewed in

the spectral domain, where the device operates as an active, multimode beamsplitter, with

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different output transition amplitudes that can be tuned by each of the phase-modulator

settings.

4. Particular results for selected EOM configurations

Despite the generality of the expressions developed in the previous section, in practical terms

however the range of amplitude modulators which are available either on the shelf or upon

request from manufacturers is restricted to a few number of designs, most of which have been

shown in Fig. 1. We now proceed to specialize the results obtained in 3.1 and 3.2 to those

configurations that are more commonly encountered in practice [5], restricting our analysis to

single-photon operation.

4.1 Y-Branch modulator with two modulating inputs

The layout of this modulator is shown in Fig. 1(A). In practical devices the design is such that:

''''

1/ 21/ 2.

ooiiioio

ttttrrrr

(29)

Restricting the analysis to the case when the microwave frequency of the modulating signal is

the same in both RF ports, and a single-photon input at the only accessible port, we get

†

n

†

EOM

†

qN r

a

†

qN r

a

0000

11

1

2

1

2

ˆˆ

ˆ

a

ˆˆ

,

o

EOMqqqq

qq

SSCqCqCqCq

111

(30)

with, according to Eq. (13)

1

2

0

10

00

0

20

00

011

022

.

()()1()

()()1()

b

b

q

q

q

j

q q

j

q q

q q

q

j

q q

j

q q

q q

Cqç

Cq

e jeJmJm

ejeJmJm

(31)

Then, the response to a single photon input is given by:

0

0000

11

1

2

1

2

ˆ

1()( ) 1()( ) 1.

EOMqqqq

n qN r qN r

qq

SvacC qC qvacvacC q

C q

(32)

Now, we specialize this output for two standard settings of this modulator through the

definition of bias, modulation indices and radio-frequency phase. The fist case is the so called

Double Sideband (DSB) operation in quadrature [4,5] which, due to the low harmonic

distortion represents one of the most standard setting in analog communications. It is

characterized by m1 = m2 = m, θ1 = 0, θ2 = π, φ b1 = φ b2 = π /2. Substituted into Eq. (31)

yields:

0

0

00

0

0

00

1

0

1

*

q

00

()

()(),

( )

j

( )

m

1 ( )

m

() ( )

m

1( )

m

q

q

q

q q

q q

q q

q

q q

q q

q q

C q

C q C q

JJ

jJJ

(33)

so Eq. (30) is now:

0

†

n

†

EOM

†

qN r

a

†

qN r

a

00

11

ˆˆ

ˆ

a

ˆˆ

Re[()]Im[( )].

EOMqq

qq

SSC qjC q

111

(34)

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The first term in the right member of Eq. (34) corresponds to the output field from the

modulator, while the second term identifies the radiated field. Turning our attention to the

output field term, we can see that the real part of Cq(q0) is zero if qqo is even. This means

that no modes corresponding to even harmonics are expected at the modulator output. This is

a well-known characteristic of this particular modulator design under classical operation.

The second interesting operation regime is the Single Sideband (SSB) operation [5] in

which by properly dephasing by 90° one of the input RF modulating tones one of the RF

sidebands (upper or lower) is eliminated at the output of the modulator. This operation mode

is also of widespread use in analog communication due to its resilience to link’s dispersion. In

this case the values of the parameters are m1 = m2 = m, θ1 = 0, θ2 = π/2, φ b1 = π/2 and φ b2 = 0,

substituted into Eq. (31) yields:

q

00

00

00

00

1

0

0

22

().

( )

j

( ) ( 1)

m

( )

m

( )

j

( ) ( 1)

m

( )

m

q

q

q q

q

q q

q q

qqq

q q

q q

C

C q

JJ

JJ

(35)

Note that for the special case when q = q01, that is, for the lower RF sideband one gets:

0

10

q

Cq

0

10,

q

q

C

(36)

and thus, as expected, the contribution to the lower RF sideband is cancelled in the first term

of the right hand-side member of Eq. (32). It can be readily checked that the upper sideband is

cancelled if θ1 = 0, θ2 = π/2 while keeping unaltered the rest of the parameters.

4.2 Y-Branch modulator with one modulating input

The layout of this modulator is shown in Fig. 1(B), and its action can be derived from the

results of the previous subsection after substituting the action of the second phase modulator

PM2, which is not present, by the identity. Specifically:

010101010

110110

†

n

†

EOM

†

n

†

PM

†

n

†

n

†

PM

†

n

†

qN

†

n

†

qN

†

n

00

11

1

2

1

2

ˆˆˆˆˆˆ

ˆ

a

ˆ

a S

ˆ

a

ˆ

a S

ˆ

a

()()()

1

2

1

2

ˆˆ

a

ˆˆ

a

()(),

EOMPMPM

qrqr

qq

SSSS

Cq aC q a

111

11

(37)

and the response to a single photon input is:

00

00

0

0

00

1,

0

1,

1

2

ˆ

1(() 1) 1

() 1

1

2

(( ) 1) 1

1.

o

o

EOMqq

nn qN r

qq q

qqo

nqN r

q q q

S vacCqCq vac

vacCqCq

(38)

Here, except for the input mode index n0, the output is determined by the phase modulator

PM1 whose multimode output is split in the output beamsplitter. For n0, the output is

governed by the interference between the unmodulated photon and the phase-modulated

photon at mode n0, whose transition amplitude is

0

0

()

q

Cq

.

4.3 Hybrid Y-Branch modulator with two modulating inputs

The layout of this modulator is shown in Fig. 1(E), where:

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''''

1/ 2/ 2.

ooiiiioo

ttttrrrrj

(39)

For this configuration, again, there is only one physically accessible input port (port 1), and

the former equation transforms to:

q

´0

†

n

†

EOM

†

qN r

a

†

qN r

a

0000

11

ˆˆ

ˆ ()

1

2

1

2

ˆˆ

()()().

EOM

qqqq

qq

SaS

C qjC qjC C q

1

11

(40)

Here, the interference terms produced in each sideband by the modulator represent the sum of

the transition amplitudes with an additional relative phase j.

5. Two-photon input

In the previous section, the focus has been the multimode transition capabilities of one-photon

states allowed by conventional designs of EOM. However, as interferometric devices with

control of the relative phase, modulation index and modulating driving tone in each arm,

EOMs permit compact implementations of standard, single-mode, quantum effects. In fact, if

the modulation indices of the phase modulators are set to zero, they behave as phase shifters

after the control of the bias voltage. Then, tasks based on bulk optics interferometers can be

directly translated to the DC modulators of type 1(C) and 1(D), since the four ports of the

interferometer are physically accessible and the mathematical descriptions of beamsplitter and

directional coupler are the same. However, proper selection and operation of the EOM permit

the integration in the same device of phase modulator and interferometer, thus allowing a

direct extension of certain operations to multimode, phase-modulated fields.

In this section we show how DC-modulators can be used to switch and interpolate

between two-port entangled and separable states composed of phase-modulated, multimode

photons. The transformation corresponding to an operator representing two input photons (one

at each input port) is given by:

0000

ˆˆˆ ˆ

2

i o i onni o i onn

r r t tccr t t tc c

1

000

i ii io onno on

t t rrr tb ct tbc

ˆ

´0000

00000000

†

n

†

n

†

EOM

†

n

†

EOM

†

n

†

EOM

' '

i o i o

t t rt

'†

n

†

n

' '

i o i o

t t rr b

'†

n

†

n

'''†

n

†

n

'†

n

†

n

'††'††

ˆˆˆˆˆˆ

ˆ

a

ˆ

a

ˆ

a

ˆ

a

ˆ ˆ

b b

ˆˆ ˆ ˆ

b b

ˆ ˆ

2

EOMEOMEOM

i o i o

t r rr

i o i o

r r t r c c

SSSSSS

b

11

111

00000

'''††'††

n

'†

n

†

n

'†

n

†

n

ˆˆˆ

ˆˆˆˆ

.

o o

r r c

o o

t rbb c

11

(41)

In practice the input beamsplitter to all modulators is always balanced, so that |ti| = |ti’| = |ri| =

|ri’| = 1/2 which, combined with the reciprocity relations, yields titi’ + riri’ = 0. This means

that both photons circulate in the same modulator’s arm due to the well-know effect [16].

Introducing in Eq. (41) the coefficients of the DC-modulators,

''''

1/ 2 / 2,

ooiiiioo

ttttrrrrj

(42)

we get:

´00000000

000000

†

n

†

n

†

EOM

†

n

†

n

†

n

†

n

†

n

†

n

†

n

†

n

†

n

†

n

†

n

†

n

1

2

ˆ ˆ

b b

ˆˆ ˆ ˆ

b b

ˆˆ

ˆ

a

ˆ

a

44

1

2

ˆ ˆ

c c

ˆ

c

ˆ

c

ˆ ˆ

c c

,

44

EOM

ii

SSbb

ii

11

11

(43)

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so that both output photons are modulated (scattered) by the same phase modulator.

Furthermore, if we consider set equal modulation indices and radio-frequency driving tones in

both phase modulators, we get

ˆ

exp(

o

n

cj

difference between the upper and lower phase modulators, we obtain

´00

ˆˆ

EOMnn EOM

SaaS

e

b b b b

1 1

†† ˆ

)

b

o

bn

, where Δφb represents the bias voltage

000000

†††

†

n

†

n

†

n

†

n

†

n

†

n

ˆˆ

ˆ ˆ ˆ ˆˆˆ

sin[] cos().

2

b

b

j

j

bb

ebb

(44)

Now, the structure of the output state can be controlled by Δφb. For instance, if Δφb = 0 then

the output is a product state, whereas for Δφb = π/2 the output state is entangled, and at this

point the overall modulator behaves in a similar fashion to a balanced beamsplitter acting on

phase-modulated states.

6. Multitone amplitude modulation

We turn now our attention to the modeling of the EOM subject to multitone RF signal

modulation as we previously considered in [21] for the electro-optic phase modulator. The

interest in this analysis is justified by the fact that this is the regime under which several

quantum experimental setups [10] and subcarrier multiplexed quantum key distribution

systems [4,22], operate. We will assume, as in [21] linear and small signal modulation

conditions, that is, m <<1.

In principle, the general expressions [Eqs. (17)–(20)] are valid but Eq. (20) has to be

modified according to Eq. (49) of [21] to:

1111

010100101

2222

010100202

†

n

†

n

†

PM

†

n

†

n

†

n

1

1

†

n

†

n

†

PM

†

n

†

n

†

n

2

1

ˆ

ˆˆ

ˆ

a S

ˆ

a

ˆ

a

ˆ

a

ˆˆ

ˆ

c

ˆ

a S

ˆ

a

ˆ

a

ˆ

a

.

bbkk

kk

bbkk

kk

M

jjjj

PMkNN

k

M

jjjj

PMkNN

k

bSeemjeje

Seem jeje

(45)

Here M stands for the number of RF tones in the modulating signal. For SCM-QKD systems

in particular we are principally interested in the behavior under a coherent state input, since

this is the state characterizing the output of a faint pulsed laser source. For this case we can

follow the same process as in section 5 of [21] and sections 3 and 4 but taking into account the

prescriptions given by Eq. (45) to find:

0001

1

k

00

121122

00010020

12

0

†

EOM

' '

i o

t t e

' ' '

i o

t t m j

'

12

1

'''''

1

ˆˆˆ

A

ˆˆ

ˆ

A

ˆˆˆ

ˆˆˆ

bbbkbk

kk

bb

k

EOMnn

M

jjjjjj

nni o

r r e

nNknnNi o

r r m j

kn

k

N

jj

nni o

t r e

i o

r t e

nN i o

t r m j

kn

SDSB

DDeeDee

BDD

1

1122

020

'

2

ˆ

,

bkbk

k

jjjj

nNi o

r t m j

kn

eeDee

(46)

and:

00

121122

00010020

121

00010

†

EOM

''

12

1

''

1

1

ˆˆ

ˆˆˆ

ˆˆˆˆ

ˆˆˆ

bbbkbk

kk

bbb

k

EOMnn

M

jjjjjj

nni o

rt e

i o

t r e

nNi o

rt m j

knnNi o

t r m j

kn

k

N

jjj

nni o

rr e

i o

t t e

nN i o

rr m j

kn

k

SDSEF

EDDeeDee

FDDee

1

122

020

2

ˆ

.

kbk

k

jjj

nNi o

t t m j

kn

Dee

(47)

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The former expressions can be simplified if the same microwave modulation frequencies are

assumed for both RF ports. In this case:

121122

000

121122

000

0

' '

i o

t t e

'' '

i o

t t m e

'

12

1

''''''

12

1

ˆ

A

ˆˆ

ˆˆˆ

ˆˆ

bbbkbk

ko

bbbkbk

ko

M

jjjjjj

nni o

r r e

nNnki o

r r m e

k

k

M

jjjjjj

nni o

t r e

i o

r t e

nNn i o

t r m e

k i o

r t m e

k

k

n

DDjee

BDDjee

ED

121122

00

121122

000

''

12

1

''

12

1

ˆ

ˆˆˆ

.

bbbkbk

ko

bbbkbk

ko

M

jjjjjj

ni o

rt e

i o

t r e

nNn i o

rt m e

k i o

t r m e

k

k

M

jjjjjj

nni o

rr e

i o

t t e

nNni o

rr m e

ki o

t t m e

k

k

Djee

FDDjee

(48)

These expressions are obviously general and can be particularized to the different modulator

designs as we did in Section 4.

It is useful to compute the value of the mean fields corresponding to Eqs. (46)–(48). To do

so we will assume that the input coherent state is fed to only one of the device ports (port 1)

and assume equal modulation frequencies at both ports. In such a case, the output state is

given by:

00

0

121122

00

0

12

00

0

†

EOM

12

12

' '

i o

t t e

' ' '

i o

t t m e

'

112

1

1

'''''

21

2

1

ˆˆˆ

ˆ

(())

()()

()(

o

bbbkbk

ok

bb

EOMnEOMnn

n

M

jjjjjj

ni o

r r e

nk i o

r r m e

k

nnN

k

M

jjj

ni o

t r e

i o

r t e

ni o

t r m e

k

n

k

S vacSDSvac vac

jee

j

1

1122

'

2

).

bkbk

ok

jjj

i o

r t m e

k

nN

ee

(49)

To evaluate the mean field at the output port 1 of the modulator we consider the electric field

operator given by [16]:

1ˆ

E

ˆ

a e

,

2

j t

o

j

V

x1

(50)

where x is the unit vector relevant polarization. Then, applied to Eq. (51), yields an average

field:

0

01

1

k

120

0

0

1122

' '

i o

t t e

'

10

' '

i o

t t m e

'

2

ˆ

.

bb

k

bkbk

jjjt

ni o

r r e

M

jt

jjjj

knki o

r r m e

k

Ehcje

jjeee cc

x

x

(51)

Equation (51) shows the presence of photons in the mode corresponding to the input coherent

state as well as in the sidebands corresponding to the M modulating RF subcarriers.

6. Summary and conclusions

We have presented a quantum model for electro-optic amplitude modulation, which is built

upon quantum models of the main photonic components that constitute the EOM, that is, the

guided-wave beamsplitter and the electro-optic phase modulator and accounts for all the

different modulator structures which are currently available. General models have been

developed both for single and dual drive configurations and specific results were obtained for

the most common configurations currently employed. Finally, the operation with two-photon

input for the control of phase-modulated photons and the important topic of multicarrier

modulation are also addressed.

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