634 CHINESE OPTICS LETTERS / Vol. 6, No. 9 / S eptember 10, 2008
Millimeter-wave modulated optical pulse generated by pulse
repetition rate multiplication and temporal Talbot effect
Zhengqing Pan (
), Qing Ye (
), Haiwen Cai (
ééé
°°°
©©©
),
Ronghui Qu (
YYY
JJJ
), and Zujie Fang (
yyy
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)
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800
Received February 18, 2008
A novel scheme was proposed to generate a millimeter-wave (MMW) optical pulse by combining pulse
rep etition rate multiplication (PRRM) technology and t emporal Talbot effect (TTE). A cascaded Mach-
Zehnder interferometer (MZI) lattice was used for PRRM, and a linearly chirped fiber grating (LCFG) was
used as TTE. The basic principle was analyzed by using a Gaussian input short pulse and its characteristics
were discussed by numerical simulation. It is shown that the p roposed scheme is feasible for MMW signal
generation and has potential merits for practical application of radio over fiber (ROF) technology.
OCIS codes: 060.2330, 060.2630, 060.4080, 060.1810.
doi: 10.3788/COL20080609.0634.
In recent years, radio over fiber (ROF), which is expected
to be used in future mobile communication and advanced
microwave technologies, has attracted great attention.
Generation of millimeter-wave (MMW) signals is one of
the key problems to be solved in tra ns mitter of ROF sys-
tems. To overcome the electronic bottleneck, photonic
assisted methods have been paid more attention, such
as heterodyne method, which uses beating between two
light waves with frequency difference corresponding to
the required microwave frequency
[1]
. Another attractive
method is schemes by using passive devices, such as di-
rect space-time pulse s hape r
[2]
, which separates different
frequency composition of the input optical pulse, adjusts
their amplitudes and phases, and then recombines them
to generate a microwave modulated optical pulse. A spe-
cially designed superstr uctur e fiber Bragg grating can
also generate self beating within an optical pulse
[3]
. How-
ever, there are still some problems to be solved in pas-
sive device-based methods, one of which is the low en-
ergy efficiency. Temporal Talbot effect (TTE), which
is ca used by interferences be tween the identical pulse
trains traveling in the dispersive medium, is use d to mul-
tiply the repetition rate of a periodic pulse sequence
[4,5]
,
and is proposed to generate microwave modulated optical
signals
[6,7]
. New schemes by using different mechanisms
are still to be investigated.
In this letter, a novel scheme combining TTE and pas-
sive pulse repetition rate multiplication (PRRM) was
proposed to realize MMW modulated optical pulse signal
generation. The passive PRRM consists of a fiber Ma ch-
Zehnder interferometer (MZI) lattice
[8]
, which trans-
forms a single optical pulse to a N -pulse train with a
relatively wide interval. A linearly chirpe d fib e r grating
(LCFG) is used as a dispersive component in which TTF
occurs. T he pulse interval is further reduced to transform
the pulse train into a hig h fre quency modulated signal.
The basic characteristics of microwave modulated o ptical
signal generated by PRRM and TTF are analyze d theo-
retically, a nd its advantages and technological issues are
discussed. It is shown that the new scheme is feasible
and suitable for some applications.
A schematic diagram of our proposal is shown in Fig. 1.
Firstly, a single optical short pulse is transformed into
a N -pulse train by a cascaded MZI. The path difference
∆L of each MZI is designed to change with its stage num-
ber i as ∆L
i
= ∆L
1
/2
i−1
(i = 1 ···k), if k stages of MZI
are used. A simple calculation gives that N = 2
k
, and
the interval between the pulses in the ge nerated train
is T
1
= (n∆L
1
/c)/2
k−1
, where n is the index and c is
the light velocity in vacuum. Then, ideally, the impulse
response function of the PRRM can be written as
[8]
h
1
(t) =
N−1
X
p=0
a(t)δ(t − p T
1
), (1)
where a(t) is the envelope o f pulse sequence which de-
pends mainly on splitting ratios of the couplers, and will
be a constant if ideal 3-dB couplers used; δ(t) is the delta
function, p is the sequence number in the pulse train.
Consequently, the repetition rate of the pulse tr ain is
f
1
= 1/T
1
= 2
k−1
c/(n∆L
1
).
The second part of the proposed scheme plays a role of
TTE, which consists of a circulator and a LCFG designed
with a constant first-order dispers ion coefficient defined
as Φ
′′
= ∂
2
Φ/∂ω
2
, where Φ is the phase shift of light
wave passing through the LCFG, and the c orresponding
impulse response function is
[9−11]
h
2
(t) =
i
√
i2πΦ
′′
exp
−it
2
2Φ
′′
. (2)
Each pulse in the tra in will suffer changes in its phase
when passing thro ugh the LCFG, and the pulse waveform
will be broadened. They will interfere with each other,
Fig. 1. Schematic for MMW modulated optical signal gener-
ation. OEC: optical to electrical connection.
1671-7694/2008/090634-04
c
2008 Chinese Optics Letters