# Demonstration of an All-Optical 2-to-4 Level Encoder Based on an Optical Parametric Amplifier

Authors: Yu Liang; P. C. Chui; Kenneth Kin-Yip Wong
Journal: Research Letters in Optics
 (2009)

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## Abstract

We demonstrated a novel technique for all-optical 2-to-4 level amplitude-shift keying (ASK) coding based on a fiber optical parametric amplifier. A 20-Gb/s signal is realized by multiplexing two 10-Gb/s data streams.


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## Body

## 1. Introduction

With growing
internet, multilevel modulation has received great interest as the
increasing demands of the transmission capacity of the optical fiber network
while increasing WDM channels is limited by the spectral  bandwidth. Multilevel
modulation formats are effective ways to overcome such problems because of
their higher information packing capacity in a single symbol, that is, coding
multiple bits per symbol. Some schemes have been proposed [1–3] to achieve this
goal. However, all of these schemes generated multilevel signal electronically,
then modulated an optical carrier. Recently, some all-optical methods have
been demonstrated to avoid the speed limitation of electronic components. For
the multilevel phase-shift keying format, differential quaternary phase-shift
keying (DQPSK) and 8-level PSK have been widely studied recently [4]. In order
to further increase signal level, simultaneous modulation of amplitude and
phase-shift keying (APSK) has been demonstrated [5]. On the other hand,
multilevel ASK modulation is also an efficient way to realize higher bit rate
in the communication system. Cross-polarization modulation (XPolM) has been
explored in semiconductor optical amplifier (SOA) for multilevel modulation
implementation [6]. However, this approach requires a dedicated setting polarization-state
angle of input waves in SOA.In this paper, we present
experimental demonstration of 4-level ASK at 20 Gb/s using an optical
parametric amplifier (OPA). The great triumph of using fiber-based OPA was its
inherent femtosecond respond time governed by theχ(3) nonlinear susceptibility of an optical fiber  [7]
and potential signal gain. The sole use of OPA effects in our setup makes it a
possibility for higher speed operation.


## 2. Principle

Amongst nonlinear effects induced byχ(3) nonlinear susceptibility
in silica, one of the important phenomena is four-wave mixing (FWM). FWM is
also a nonlinear effect that leads to OPA, as a special case of which the waves
satisfy the phase matching conditions. In the degenerate FWM case [7], two
frequencies of light at ωp   and  ωs  copropagate in a
nonlinear medium, then a new frequency component, ωi, can be generated, satisfying ωi=2ωp−ωs. During this
process, the interplay between FWM, self-phase modulation (SPM), and cross-phase
modulation (XPM) effects can lead to an exponential amplification if ωp is a high-power pump and ωs is a weak probe. This
resultant exponential amplification is known as OPA. However, ωp, ωs, and ωi are named pump, signal
and idler, respectively, as shown in Figure 1. The dashed line represents the
gain spectrum of OPA. This optical amplification is achieved by choosing the
frequencies of interplaying waves such that they satisfy a phase-matching
condition optimal for an exponential gain in power on the signal, and the idler.
In the specific case of single pump, OPA occurs at regions satisfying −4γPP<Δβ<0, where Δβ=2β(ωp)−β(ωs)−β(ωi), and β(ω) is the propagation
constant as a function of frequency, γ  is the nonlinear coefficient, PP is the pump power.Figure 1
Schematic of OPA.In order to achieve highly efficient
OPA, the phase-matching condition should be satisfied by setting appropriate wavelengths
of pump and signal. Assuming an undepleted pump, parametric gainGs≫1, and phase-matching condition
is satisfied, Gs can
approximately be obtained in decibel as [7] (1)Gs≈10log(14exp(2γPPL)), where L is
the fiber length. It shows that the parametric gain is exponentially dependent
on the pump power.The principle of the proposed
multilevel encoder is shown in Figure2. Three waves were launched into a spool
of highly nonlinear dispersion-shifted fiber (HNL-DSF) as the nonlinear medium.
Two of them were amplitude modulated with pseudo-random bit sequence (PRBS) as
a pump and probe number 1, respectively. The third one, probe number 2, was a continuous-wave
(CW). When the pump and probe number 1 are “0,”
only the probe number 2, low CW power comes out, this is the first level. When the
pump is “1” and the
probe number 1 is “0,” the probe number 2
can be amplified to the second level by OPA process. When the pump is “0” and probe number 1 is “1,” the power comes out from fiber is the sum of probe number 1
and probe number 2 which is higher than the second level and forms the third level.
Finally, when the pump and probe number 1 are both “1,” the sum of probe number 1 and probe number 2 can be amplified
together, resulting in the strongest power and the fourth level. Then, the two
data streams information can be transferred and 4-level ASK encoding was
realized through OPA effect.Figure 2
The operating principle of multilevel encoding.


## 3. Experimental Setup

Figure3  shows the experimental setup. The nonlinear
medium used for OPA was a spool of 1 km HNL-DSF with
nonlinear coefficient γ≈14 W−1km−1 and zero-dispersion wavelength λ0≈1560 nm. The two input 10-Gb/s return-to-zero
(RZ) signals were generated by coupling two tunable laser sources, TLS1, with
wavelength λpump  at 1561 nm
and TLS2 with wavelength λprobe♯1 at 1545.5 nm using a 3-dB coupler. They
were launched into two external Mach-Zehnder
intensity modulators (MZ-IM) for return-to-zero on-off keying (RZ-OOK)
modulation, as pump and probe number 1. The two MZ-IMs were driven by 10-Gb/s 231-1
PRBS and 10 GHz clock, respectively. Then the pump and probe waves were split by
a WDM coupler, WDMC1, after amplitude modulation. The pump wave was then
amplified to 20.4 dBm by two erbium doped amplifiers, EDFA1 and EDFA2 at EDFA2
output. A 0.8-nm bandwidth tunable band-pass filter, TBPF1, was inserted
between two EDFAs to reduce amplified spontaneous emission (ASE) noise at the
input of EDFA2. On the other branch, the probe number 1 wave was boosted by EDFA3 to
11 dBm after passing through a 0.8 nm TBPF2 to reduce the noise and optical
delay line (ODL) to align the pattern of pump and probe. Then, these two
branches were combined by WDMC2. The probe number 2 from the third tunable laser
source, TLS3, with input power of HNL-DSF at 0.5 dBm and wavelength λprobe♯2 at 1545.9 nm, was shifted 0.4 nm from λprobe♯1 to avoid interference. TLS3 was combined with the output of WDMC2 using a 3-dB
coupler and the polarization controller, PC4, was used to adjust state of
polarization (SOP) of probe number 2 to obtain maximum gain. The probe number 1 and probe number 2
propagated through the HNL-DSF together to experience OPA effects. The 0.8 nm
TBPF3 placed after HNL-DSF, set at 1545.5 nm, was used to filter out the multilevel
signal. The TBPF3 output port was monitored by a digital communication analyzer
(DCA). The variable optical attenuator (VOA) before DCA was used to prevent
damage.Figure 3
Experimental setup of multilevel encoder.


## 4. Results and Discussion

The waveforms of coding
process are showed in Figure4, which represent the 4-level data patterns in time
domain at 1545.5 nm. According to the encoding principle, the upper and middle
patterns are level 1 and level 2 versus level 0, respectively. All 4-level can
be identified in the bottom of Figure 4. As demonstrated in Figure 2, the upper
pattern was a duplicate of pump pattern. The middle pattern was probe number 1 signal.
Then the bottom pattern consisted of the above two. Figure 5 is the corresponding
eye diagram. It was found that the level spacing can be arbitrary but we chose
around 0:1:2:4 for clear identification [8]. The extinction ratio of the
three separated 2-level eye patterns corresponding to level 1, 2, and 3 were
7.56 dB, 11.07 dB, and 13.18 dB, respectively. While the respective signal-to-noise
ratios (SNR) were 18.01 dB, 26.31 dB, and 26.56 dB. As the lowest amplitude, the
performance of level 1 was relatively weaker than that of the other two due to
the existence of the CW. The narrower pulsewidth of levels 1 and 3 whose
operation principle was like an AND gate due to pulse compression in FWM
process [9]. The mark level of level 3 was noisier than the other levels due to
the ASE from EDFA amplified by OPA effect.Figure 4
The waveforms of 4-level signal. Time base: 200 ps/div.Figure 5
Eye diagram of 4-level signal. Time base: 50 ps/div.Furthermore,
note that the scheme  we proposed here can also be potentially applied to an
all-optical digital-to-analog conversion (DAC) system [10]. The incoming serial
pulses which represent a digital signal are converted to the parallel pulses by
an optical serial-to-parallel converter [11]. The parallel pulses are assigned
to the probe and pump as the most significant bit (MSB) and the least
significant bit (LSB), respectively. So as indicated in Figure 2, the 2-bit
digital signals “00,”  “01,” “10”, and “11” correspond to the analog signals “0,”
“1,” “2”, and “3”. Consequently, a 2-bit all-optical DAC scheme with
binary code can be realized. The advantage of this technique is its easy implementation
and flexibility of level spacing optimization.


## 5. Conclusions

We have experimental
demonstrated a novel technique for all-optical 2-to-4 level ASK coding based on
OPA. A conversion from two optical RZ-OOK 10-Gb/s signals to a 4-level ASK
signal at 20 Gb/s was achieved. The results in this paper have demonstrated
that this device has potential usage in improving the performance of the
available channel in WDM communication system. Thus, the proposed scheme
provides a new way to increase the speed of optical network and meets the
growing demand of the internet in future.The results
also exhibit that this device can be applied in all-optical DAC system
potentially. This propose is left for future work.



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*Source: 981620-2009-03-23.xml*