t is a well-established practice for communications systems designers to use variable optical
attenuators (VOAs) for controlling optical power levels in front of receivers, amplifiers and for
channel equalization. Over the last several years there has been a variety of electrically
actuated opto-mechanical VOAs commercialized for such uses. Many important new applications
for VOAs are emerging, especially for metro and access networks. Many of these applications
require high speed VOAs, eliminating the slow, electrically actuated opto-mechanical VOAs from
consideration. To address these new network requirements, Kotura has introduced a line of very
high-speed UltraVOA
TM
Arrays that have no such speed limitations. The UltraVOA
TM
Arrays are
in production and are qualified to Telcordia GR-1221, GR-1209, and the applicable portions of
GR-468.

Koturas UltraVOA
TM
Arrays are solid-state, electrically controlled variable optical attenuators
based on silicon opto-electronic integrated circuit (SOEIC) technology. SOEIC technology
derives from and is fully compatible with standard CMOS technology, therefore it is the platform
of choice for creating optical components that are compact, easy to manufacture, and can
combine multiple functions in the same package. Figure 1 shows a comparison of a CMOS wafer
and a SOEIC wafer: the key difference with SOEIC is the addition of waveguides for the
propagation of optical signals; other features are the same. It is expected that the SOEIC
technology platform will put optical components on similar performance and cost curves as those
of electronic ICs. Koturas SOEIC technology is described in detail in a paper available from
Kotura titled Silicon Opto-Electronic Integrated Circuits: Bringing the Excellence of Silicon into
Optical Communications.























Figure 1. Comparison of CMOS and SOEIC Wafers
VLSI CMOS
SOEIC
Electrons & Photons in Silicon
Electrons in Silicon
Silicon
SiO
2
Passivation
p+ / n+ doped
metal
Optical waveguide
VLSI CMOS
SOEIC
Electrons & Photons in Silicon
Electrons in Silicon
Silicon
SiO
2
Passivation
p+ / n+ doped
metal
Optical waveguide
Silicon
SiO
2
Passivation
p+ / n+ doped
metal
Optical waveguide 2

Kotura UltraVOA Application Note

The UltraVOA
TM
Arrays make use of single-mode optical waveguides along with p-i-n diodes
integrated on the same substrate to create up to eight independently variable VOAs in a 14-pin
butterfly package with a footprint smaller than half of that of a business card. The structure in
Figure 2 represents one of up to eight channels of a Kotura UltraVOA
TM
Array. The devices rely
on the fast free carrier absorption effect to control optical power through an in-line waveguide.
As shown in the diagram, free carriers across the p-i-n diode absorb photons propagating in the
waveguide; the higher the current across the diode, the greater the attenuation. Because the
attenuation mechanism is temperature dependent, the UltraVOA Arrays also feature a built-in
thermistor and thermo-electric cooler (TEC). Koturas UltraVOA
TM
Arrays exhibit best-in-class
characteristics of sub-microsecond response time, wide attenuation range, low PDL, and compact
footprint.












Figure 2. SOEIC VOA Detail


Koturas UltraVOA
TM
Arrays are very simple to control because of the monotonic response of
attenuation vs drive current as depicted in Figure 3. This characteristic response is inherent to
the physics of the devices and is very repeatable. The curves in Figure 4 are from
measurements of response time; the red curve shows a 10 dB change in attenuation as observed
at a photodetector, and the blue curve is the drive voltage.
















Figure 3. Attenuation vs. Drive Current
Figure 4. Response Time


The sub-microsecond response time of Koturas UltraVOA
TM
Arrays gives systems designers the
unprecedented ability to use VOAs for high-speed transient suppression as well as for subcarrier
modulation which can be used to add wavelength and channel identification markers thereby
enabling additional functionality in the optical control plane. These novel applications, which can
only be implemented with very fast VOAs are described in the remainder of this note.
Silicon
SiO
2
Passivation
p+ / n+ doped
metal
Optical waveguide
Silicon
SiO
2
Passivation
p+ / n+ doped
metal
Optical waveguide
Silicon
SiO
2
Passivation
p+ / n+ doped
metal
Optical waveguide 3

Kotura UltraVOA Application Note


2. Transient Suppression
Faults, including fiber cuts and network element failures that require traffic re-routes can cause
optical power transients that must be controlled in order to avoid component damage and ensure
error-free transmission. However, these fault events are only part of the story in a transport
network where even normal operations could lead to transients
1
. Optical transport networks are
becoming increasingly complex topologically in both long haul and metro segments of the
network. Wavelengths can be terminated, or switched at any point in the network, giving rise to
unpredictable optical power transients that can propagate unchecked, even if the optical
amplifiers have some level of transient control. The likelihood of generating optical power
transients will only increase as service providers increase the deployment of optical transport
systems closer to the customer. For example, the action of dropping some preemptible services
in order to accommodate higher service level agreements (SLAs) can require switching or
rerouting some wavelengths, which in turn can lead to optical transients. A closed loop control
circuit that takes advantage of the sub-microsecond response time of Koturas UltraVOA
TM
arrays
is a straightforward and elegant solution to suppressing these transients that would otherwise
affect network performance
2
.

It has been shown that a control loop using VOAs with microsecond response time can effectively
suppress transients as large as 12 dB for a 120 microsecond pulse as, for example, could be
caused by a signal reroute due to a fiber cut
2
. Figure 5 shows the block-diagram level of a typical
transient suppression circuit. Similar analog control techniques have been in use for decades
and the constituent components, including logarithmic amplifiers and voltage comparators are
well understood. However, it is only with the availability of high-speed VOAs that these methods
can be applied for optical power transient suppression since the response time of the control
circuit and of the VOA must be fast compared to the offending transient
2,3
.
















Figure 5. Block Diagram of Transient Suppression Circuit

It is even possible to further simplify the transient suppression circuit by integrating the 1x2 tap
and the photodetector for the transient detection circuit into a single component.



UltraVOA
1x2 Tap
Transient
Detection Circuit
Loop Control Circuit
VOA Drive Circuit
Optical Signal Input
Optical Signal Output
UltraVOA
1x2 Tap
Transient
Detection Circuit
Loop Control Circuit
VOA Drive Circuit
Optical Signal Input
Optical Signal Output 4

Kotura UltraVOA Application Note
3. Wavelength Identification and Control
Another function that is important in making optical transport more manageable in metro core and
edge networks is transparent optical channel identification. The ability to identify optical channels
regardless of underlying protocols and payloads, paves the way towards implementation of the
optical control plane. With legacy transport systems, the only way to identify optical channels is
to either read the overhead bytes of the SONET frames, or in the case of native IP, to read the
actual packets being transmitted. In both instances, this functionality requires the additional cost
and complexity of performing optical-to-electrical-to-optical (OEO) conversions. Transparent
optical channel identification enables a wide range of very useful functions including automated
flow-through provisioning of wavelengths, enhanced performance monitoring, and automated
topology discovery. Once all network elements can take advantage of transparent optical
channel identification, full fault management, configuration management, accounting
management, provisioning, and security (FCAPS) capability can be implemented at the
wavelength level in a practical, cost-effective manner.

Because of the sub-microsecond response time of Koturas UltraVOA
TM
Arrays, the devices can
be modulated with analog signals with digitally encoded subcarriers that will uniquely identify a
particular wavelength. As shown in Figure 6, for example, an UltraVOA
TM
placed at the output of
a transmitter to control the optical power level for that specific wavelength can simultaneously be
modulated with an analog signal that will uniquely mark that wavelength throughout the network.
In the simplified diagram of Figure 6, a VOA driver and closed loop power control circuit
communicate with the Element Management System (EMS) and Network Management System
(NMS) in order to control optical power level and superimpose wavelength identification and other
supervisory information for each wavele