Liquid Crystal Technology for Telecom Applications
With increasing interest and required features in dynamically reconfigurable
network architectures, compact non-mechanical optical signal processors are in
great demand to develop reliable, fast-reconfiguring, energy/space-efficient
network elements such as dynamic add/drop multiplexers, dynamic optical routers
and all-optical cross connects. Liquid crystal technology has been drawing a lot
of attention to as an option for all-optical signal processors because of its
unique abilities to provide optical performance comparable to optomechanical
devices, but with the reliability and efficiency of solid-state devices.
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By applying a voltage on a liquid-crystal spatial modulator, liquid crystal
molecules realign and rotate the polarizations of lasers passing through them.
With a sufficient voltage, the laser polarization rotates to an orthogonal
state. We can utilize the discrete characteristics for optical switching by
routing the electrically-controlled polarizations to the desired ports. |
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With a variable voltage, a desired portion of the input power can be
adjusted to the output port by rotating the polarization. We can utilize the
analog characteristics for optical attenuation. |
WDM Gain/Power Equalization
This first OHE paper was presented in Optical Fiber
Conference, OFC, March 2001, as a Postdeadline Paper PD-29.
This work was
done at
Chorum Technologies
Publications
In wavelength-division-multiplexing (WDM) optical
links, it is important to keep all lasers in the same fiber at the same power
levels in order to avoid signal-to-noise-ratio degradation due to the power- and
wavelength-dependent gain characteristics in erbium-doped fiber amplifiers
(EDFAs) [1]. The non-flat gain profiles over the desired spectral ranges cause
variations in power levels for different lasers. With cascaded EDFAs in WDM
links, lower accumulated gain in some certain wavelengths reduces the
signal-to-noise ratios. Therefore, it limits the transmission distance. This
issue may be resolved by installing fixed-gain filters in EDFAs to achieve a
flatten gain. However, the gain profiles in EDFAs change with the numbers and
power levels of input lasers. In a dynamically reconfigurable WDM network, where
lasers are dynamically added or dropped from nodes or cross connects, the gain
profiles of EDFAs will vary with network reconfigurations. Even for simple
point-to-point fixed-add/drop WDM systems, there are design considerations
concerning future addition of lasers or reduction of WDM wavelength spacings.
The gain profiles in EDFAs will also change as channel numbers change.
Therefore, with fast growing interests in dynamic reconfigurable WDM networks
and upgradability considerations, dynamically-controlled all-optical gain
equalizers become essential elements for the next generation WDM networks.
Several dynamic gain equalization approaches have
been previously demonstrated [2-5]. In this work, we present a new gain
equalization approach using liquid-crystal modulators and the harmonic synthesis
approach. The functionality of the liquid-crystal optical harmonic equalizer
(OHE) has been demonstrated with dynamically gain equalization and tilting for
different EDFA gain profiles.
OPERATION PRINCIPLES
Figure 1: Configuration of
a liquid-crystal optical harmonic equalizer with EDFAs.
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Figure 1 shows the configuration of a liquid-crystal
harmonic gain equalizer. In this configuration, the equalizer has multiple
stages of harmonic elements. Each stage includes a liquid-crystal harmonic
filter to adjust the wavelength shift and a liquid-crystal attenuator to adjust
the amplitude of harmonic waveform. The optical spectrum analyzer obtains power
profiles at the input/output ports of OHE and/or the output of second EDFA. The
digital signal processor (DSP) compares the data with the desired gain profile,
which can be accessed through the user interface, and calculate the required
transfer function to flatten the power profile. The DSP then calculate the
amplitude and wavelength shift for each harmonic element by expanding the
required transfer function in a Fourier series. The driver circuit applies
accurate biasing voltages on the liquid-crystal modulators to achieve the
required harmonic shapes. |
The name ('OHE) is also
a Hawaiian
Bamboo
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PERFORMANCE
The response times of liquid-crystal
modulators to achieve required amplitude and wavelength shift adjustments in our
OHE are in the millisecond and submillisecond ranges. The required biasing
voltage magnitudes for liquid-crystal cells are less than 5V. Figures 2 show the
automatic equalization results for four different gain profiles. In these tests,
an amplified spontaneous emission (ASE) source is used as the input signal of
EDFA in the input of OHE, without a second EDFA at the output of OHE. The
flattened results were measured at the output of OHE. Fig.2 (a), (b), (c) and
(d) show flatness of ±0.15dB, ±0.3dB, ±0.3dB and ±0.15dB in
the wavelength ranges of 1528-1561nm, 1533-1568nm, 1532-1562nm and 1535-1576nm,
respectively. Generally speaking, the flattened output profiles can be reached
within less than ±0.3dB for the required flatness levels in less than four
equalization iterations. The insertion loss in the through status is around
4.5dB. The dynamic range for setting user-desired gain profiles is more than
10dB.
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Figure 2: Equalization results and the
input power profiles of EDFA. |
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The OHE does not only dynamically equalize power
profiles, but also provides functionality for variable transfer functions, which
allows different power levels at both ends of passband. The DSP can generate a
required transfer function to achieve the desired output levels in a linear
fashion. This functionality can provide a gain-tilting profile that may add more
operation flexibility in networks with different types of optical amplifiers.
Two tilting profiles, along with the flattened profile, are shown in Fig. 3. The
flatness of gain is within ±0.3dB and the tilting slopes are ±4dB
across 1530-1560nm. The maximum dynamic ranges for tilting slopes in this design
are ±10dB over a 30-nm range.
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| Figure 3: Flattened and tilting results with the input power profile of
EDFA. |
Figure 4: Equalization result and the power profile of two-stage EDFAs with
the OHE in the through state. |
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| (a) |
(b) | |
Figure 4 shows the flattened result for a two-stage
configuration of EDFAs with a mid-stage OHE as the one shown in Fig. 1. The
flatness is within ±0.5dB in the wavelength range of 1528-1563nm with four
equalization iterations. The polarization dependent losses (PDLs) were measured
at two different states. Fig. 5(a) and (b) show PDLs of less than 0.15dB and
0.1dB for flattened and non-flattened profiles, respectively. The required
transfer functions for the flattened result and the through state are also
shown. The measured polarization mode dispersions are less than 0.15ps, as shown
in Fig. 6, under the through, flattened and 10-dB attenuation states. The
measured chromatic dispersion is less than ±7 ps/nm in the wavelength range
of 1525-1565nm. |
| Figure 5: Polarization dependent losses and the transfer functions of OHE. |
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An optical harmonic amplifier-gain equalizer using
liquid-crystal modulators is demonstrated. Optical performances, including gain
equalization and tilting, polarization dependent losses, polarization mode
dispersion and chromatic dispersion, are presented. The results show great
promises for dynamic, quick manipulation of gain profiles in WDM systems. The
liquid-crystal optical harmonic equalizers can also be used in other wavelength
ranges, such as L-band, since liquid-crystal modulators are broadband devices
and the harmonic approach is wavelength independent.
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Figure 6: Polarization mode dispersion of
OHE in different states. |
REFERENCES
1. G. Keiser, "A review of WDM technology and applications,"
Optical Fiber Tech., Vol.5, pp.3, 1999.
2. Y. Li and C. Henry, "Silica-based Optical Integrated Circuits,"
IEE Proc.-Opto., Vol.143, No.5, Oct. 1996.
3. S. Parry, J. King, K.
Roberts, N. Jolley, R. Keys and J. Mun, "Dynamic gain equalization of EDFAs
with Fourier filters," The Optical Amp. & Their App., June 9, 1999.
4. S. Yun, B. Lee, H. Kim and B. Kim, "Dynamic erbium-doped fiber
amplifier based on active gain flattening with fiber acoustooptic tunable
filters," IEEE Photonics Technology Letters, Vol. 11, No. 10, Oct. 1999.
5. B. Offrein, F. Horst, G. Bona, R. Germann, H. M. Salemink and R.
Beyeler, "Adaptive gain equalizer in high-index-contrast SiON technology,"
IEEE Photonics Technology Letters, Vol.12, No.5, 2000.
Created by J.C. Chiao
