Note: Descriptions are shown in the official language in which they were submitted.
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AUTOMATIC BIAS CONTROL OF AN OPTICAL TRANSMITTER
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] None.
BACKGROUND
[0002] Optical communications systems use modulated light, e.g., optical
signals,
through light channels or fiber optic cables to transmit information between
devices. For
example, long distance transmission of broadband signal content, such as
analog
multichannel video, may include the use of narrow line width light sources in
conjunction in
a low loss wavelength window of single mode optical fibers (SMF). In optical
communication systems, a light beam is modulated in accordance with the
information to be
conveyed and transmitted along the optical fiber to a receiver.
[0003] The typical lowest loss of the SMF fiber window is in the convention
band (C-
band). In addition to the low loss in this window, the availability of the
optical amplifier in
this wavelength window is another advantage. There are two types of modulation
that can be
used for the light modulation, direct modulation and external modulation. In
the direct
modulation transmitter, light of a distributed feedback laser (DFB) laser is
directly modulated
through the modulation of the current going to the laser. In an external
modulation
transmitter, the light from the light source is modulated by an optical
external modulator.
[0004] The direct modulation transmitter is a cost-effective solution for
many
applications. Accompanying the intensity modulation of the light is the
frequency modulation
of the light, known as laser chirp. However, one factor that the optical
transmission system
needs to consider is the fiber dispersion. The interaction of the laser chirp
with the fiber
dispersion can cause some undesirable performance degradations, such as second
order
distortion in analog hybrid fiber coax (HFC) cable television (CATV)
transmission systems.
The distortion can be corrected through an electronic circuit. However, since
the fiber
dispersion is the function of fiber length, the distortion correction has to
be set for each
targeted fiber length. Therefore, this may add some additional tuning during
network
implementation. Also, this may cause some limitations in certain applications.
For example,
when the light is split in the transmission path and each portion of the split
light travels down
to different fiber lengths, performing a distortion correction becomes
difficult to satisfy both
transmission lengths. The other example is when a primary link and a secondary
link have
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different link lengths the distortion correction needs to be reset after a
switching happens
between the primary and secondary path. Furthermore, the electronic distortion
correction has
its own limit in terms of its correction capability, which limits the total
transmission link
length.
[0005] To overcome the aforementioned drawbacks of the directly modulated
transmitter,
an external modulation transmitter may be used, especially for long reach
transmission
applications since external transmitters are close to chirp free. There are
different types of
external modulator technologies, such as a lithium niobite (LN) based Mach-
Zehnder (MZ)
modulator and an electro-absorption based modulator.
[0006] For LN MZ transmitters, the light from the light source is split
equally and each
split is sent to phase modulator path in the MZ modulator. The phase of the
light from each
path is controlled by the voltage applied to the phase modulator through an
electro-optic
effect. The lights from the two paths of the phase modulator are then combined
and interfere.
If the phase difference between the two light beams are zero degrees, then the
max optical
output power is achieved. If the phase difference between the two light beams
is 180 degrees,
then the minimum optical output power is achieved. The LN MZ based external
modulator
thus provides very good analog performance over long transmission distance not
only
because its low modulator chirp, but also because of its intrinsic good second
order distortion
performance if biased at its quadrature point. However, LN MZ transmitters
also suffer some
drawbacks. First of all, the best second order distortion performance can only
be achieved at
a quadrature point of the modulator transfer function and a small bias
deviation from that
point makes the distortion degrade very quickly. Therefore, the modulator
voltage bias for the
best performance needs to be constantly monitored and controlled because of
its drift.
Secondly, the modulator is bulky. Thirdly, it is very costly as compared to
the directly
modulated transmitter.
[0007] An electro-absorption (EA) based external modulator is based on the
Franz-
Keldysh effect or quantum-confined Stark effect, where the effective band gap
of the
semiconductor of the absorption material of the modulator changes with its
bias voltage. The
absorbed light is converted to photocurrent and therefore the electro-
absorption modulator
(EAM) works in a similar way to that of a photodetector. When no bias voltage
is applied to
the electro-absorption modulator, the band gap is wide enough to allow the
light at the laser
wavelength to pass through transparently. As the bias voltage is increased,
the band gap is
narrowed enough to start absorbing the light. Therefore, changing the bias
voltage of the
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electro-absorption modulator modulates light. The electro-absorption modulator
based
external transmitters have several advantages. First, the electro-absorption
modulator has a
much lower chirp as compared to the directly modulated DFB laser. Second, the
electro-
absorption modulator requires a low bias voltage and driving power for
modulation. Third,
the electro-absorption modulator can be integrated with a DFB laser to form a
device called
an EML (electro-absorption modulated laser). Because of this integration, the
EML device is
very small with a package similar to a normal DFB laser, and therefore very
cost effective.
Like LN MZ modulator, the best second order distortion is achieved only when
the modulator
is biased at its inflection point of its extinction ratio (ER) curve. The
inflection point is where
the ER curve changes in its curvature from concave up to concave down.
However, EA
modulators also have some drawbacks. First, also like the LN MZ modulator, a
small EAM
bias deviation can make transmitter distortion degrade very fast or the bias
voltage needs to
stay in an extremely narrow window in order for distortions to be acceptable.
Second, its best
distortion bias voltage is a function of wavelength of the light and therefore
a change in light
wavelength induced for any reason can cause a system performance degradation.
[0008] Throughout the lifetime of a laser, its threshold (e.g., the laser
bias current at
which the laser turns on and starts emitting light) becomes larger, and its
slope efficiency
becomes smaller because of laser aging effects. Thus, the laser output power
becomes smaller
over time for a fixed laser bias current. An automatic power control circuit
(APC) is designed
to track the laser power by monitoring the laser power by a way, such as a
back-facet
photocurrent, and then increase the laser bias current to maintain the laser
output power.
However, the bias voltage increase of the laser causes wavelength of the laser
to change. In
electro-absorption modulator based transmitters, the modulator performance is
affected
unfavorably by the wavelength change due to the fact that EA modulator
performance is a
function of laser wavelength relative to the modulator's highest absorption
wavelength.
SUMMARY OF THE INVENTION
[0009] In one embodiment an electro-absorption modulator is configured to
receive an
optical light from an optical light source and output a modulated optical
signal. The electro-
absorption modulator includes a bias voltage used to set a predetermined
modulation
performance and an output power of the electro-absorption modulator. A
controller is
configured to measure a photocurrent generated by the electro-absorption
modulator and use
the photocurrent as a reference to automatically control the bias voltage of
the electro-
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absorption modulator to maintain the predetermined modulation performance and
the output
power of the electro-absorption modulator when a detuning change occurs
between the
electro-absorption modulator and the optical light source.
[0010] In another embodiment an electro-absorption modulator includes an
automatic
power control circuit that controls a bias current of the optical light source
to maintain a same
output power from the optical light source.
[0011] In another embodiment an electro-absorption modulator includes a
monitor
configured to monitor the photocurrent of the electro-absorption modulator, a
processor
configured to set a reference voltage, and a bias control logic configured to
control the
electro-absorption modulator bias voltage based on the reference voltage.
[0012] In another embodiment an electro-absorption modulator includes a
controller that
minimizes second order distortion changes that occur based on the detuning
change by
controlling a bias voltage.
[0013] In another embodiment, a method for electro-absorption modulator
includes
receiving an optical light from an optical light source and outputting a
modulated optical
signal. The electro-absorption modulator includes a bias voltage used to set a
predetermined
modulation performance and an output power of the electro-absorption
modulator. The
electro-absorption modulator measures a photocurrent generated by the electro-
absorption
modulator. The electro-absorption modulator uses the photocurrent as a
reference to
automatically control the bias voltage of the electro-absorption modulator to
maintain the
predetermined modulation performance and the output power of the electro-
absorption
modulator when a detuning change occurs between the electro-absorption
modulator and the
optical light source.
[0014] In another embodiment an optical light source is configured to
output an optical
light. An electro-absorption modulator is configured to receive the optical
light and output a
modulated optical signal. The electro-absorption modulator includes a bias
voltage used to
set a predetermined modulation performance and an output power of the electro-
absorption
modulator. A controller is configured to measure a photocurrent generated by
the electro-
absorption modulator and use the photocurrent as a reference to automatically
control the bias
voltage of the electro-absorption modulator to maintain the predetermined
modulation
performance and the output power of the electro-absorption modulator when a
detuning
change occurs between the electro-absorption modulator and the optical light
source.
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[0015] In another embodiment an electro-absorption modulator is configured
to receive
an optical light from an optical light source and output a modulated optical
signal. The
electro-absorption modulator includes a bias voltage that is used to set a
predetermined
modulation performance and an output power of the electro-absorption
modulator. A
controller is configured to measure a bias current of the optical light source
and use a change
of the bias current to determine a detuning change that occurs between the
electro-absorption
modulator and the optical light source. The controller uses the detuning
change to
automatically control the bias voltage of the electro-absorption modulator to
maintain the
predetermined modulation performance and maintain the output power of the
electro-
absorption modulator.
[0016] In another embodiment, a method includes receiving, by a computing
device, a
bias current of an optical light source that outputs an optical light to an
electro-absorption
modulator. The electro-absorption modulator is configured to output a
modulated optical
signal and includes a bias voltage that is used to set a predetermined
modulation performance
and an output power of the electro-absorption modulator. The method includes
comparing,
by the computing device, the received bias current with the initial bias
current to determine a
detuning change that occurs between the electro-absorption modulator and the
optical light
source. The method includes using, by the computing device, the detuning
change to
automatically control the bias voltage of the electro-absorption modulator to
maintain the
predetermined modulation performance and maintain the output power of the
electro-
absorption modulator.
[0017] In another embodiment an optical light source configured to generate
an optical
light. An electro-absorption modulator is configured to receive the optical
light from the
optical light source and output a modulated optical signal. The electro-
absorption modulator
includes a bias voltage that is used to set a predetermined modulation
performance and an
output power of the electro-absorption modulator. A controller is configured
to measure a
bias current of the optical light source and use a change of the bias current
to determine a
detuning change that occurs between the electro-absorption modulator and the
optical light
source. The controller uses the detuning change to automatically control the
bias voltage of
the electro-absorption modulator to maintain the predetermined modulation
performance and
maintain the output power of the electro-absorption modulator.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 illustrates an example optical communication system according
to some
embodiments.
[0019] FIG. 2A depicts a simplified system for a transmitter that includes
bias control
logic for an electro-absorption modulated laser (EML) according to some
embodiments.
[0020] FIG. 2B depicts a simplified system for a transmitter that includes
bias control
logic for an electro-absorption modulated laser (EML) according to some
embodiments.
[0021] FIG. 3 depicts a graph showing the wavelength detuning relationship
according to
some embodiments.
[0022] FIG. 4 depicts an example of extinction ratio curves versus
different temperatures
or detuning according to some embodiments.
[0023] FIG. 5 depicts a graph that shows the shift of a modulator
extinction curve due to
a small detuning change AA according to some embodiments.
[0024] FIG. 6 depicts an example of bias control logic according to some
embodiments.
[0025] FIG. 7 shows an example circuit that monitors the EAM photocurrent
and feeds
the current back to the EAM bias control circuit to generate a new bias
voltage according to
some embodiments.
[0026] FIG. 8 depicts a simplified flowchart for adjusting the EAM bias
voltage
according to some embodiments.
[0027] FIG. 9 shows an example of composite second order performance
according to
some embodiments.
[0028] FIG. 10 depicts a graph that shows the impact of the detuning change
on the
performance of the EML according to some embodiments.
[0029] FIG. 11A shows an example of the predetermined EAM bias curve over
detuning
change according to some embodiments.
[0030] FIG. 11B shows an example of the predetermined EAM bias curve over
laser
current change according to some embodiments.
[0031] FIG. 12 depicts a simplified flowchart of a method for adjusting the
bias voltage
of EAM 208 according to some embodiments.
[0032] FIG. 13 depicts a more detailed example of bias control logic
according to some
embodiments.
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[0033] FIG. 14 depicts results of the composite second order (CSO)
performance at
different EML detuning changes with and without EAM bias voltage control
according to
some embodiments.
DETAILED DESCRIPTION
[0034] Described herein are techniques for an optical transmission system.
In the
following description, for purposes of explanation, numerous examples and
specific details
are set forth in order to provide a thorough understanding of some
embodiments. Some
embodiments as defined by the claims may include some or all of the features
in these
examples alone or in combination with other features described below, and may
further
include modifications and equivalents of the features and concepts described
herein.
[0035] In an optical communication system, information is transmitted via
message
signals through a physical medium from a source to a destination. For example,
a cable-based
system can be used to deliver analog and/or high-definition digital
entertainment and
telecommunications, such as video, voice, and high-speed internet services,
from a headend
to subscribers over an existing cable television network using optical
signals. The cable
television network can take the form of an all-fiber network or hybrid
fiber/coax (HFC)
network. In either network, an optical communication system, such as an
optical transmitter,
in a headend/hub converts electrical signals (e.g., data, video, and voice
signals) to optical
signals. The optical signals are transmitted downstream via a fiber to a fiber
node that serves
a group of end users (e.g., a service group). The fiber node can include an
optical receiver
that converts the received optical signals to electrical signals that then are
transmitted to the
service group, for example, via receiving devices such as cable modems (CMs)
and/or set top
boxes (STBs).
[0036] FIG. 1 illustrates an example optical communication system 100
according to
some embodiments. System 100 delivers analog and/or high-definition digital
entertainment
and telecommunications, such as video, voice, and high-speed Internet
services, over a
fiber connection 112 between a headend/hub 110 and fiber node 130 for delivery
to a service
group 120 of receiving devices such as cable modems (CMs) and/or set top boxes
(STBs).
[0037] An optical transmitter (TX) 114 in the headend/hub 110 may convert
electrical
signals representing various services (e.g., video, voice, and Internet) to
optical signals for
transmission over the fiber 112 to the fiber node 130. The optical signal from
the
transmitter 114 may be amplified by an optical amplifier 115 (e.g., an erbium
doped fiber
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amplifier (EDFA)) before reaching the fiber node 130. An example EDFA 115 is
shown
between headend/hub 110 and the fiber node 130, but it is noted that the EDFA
115 may be
located in the headend/hub 110 and/or in the fiber node 130 or along the
fiber.
[0038] A single fiber node 130 is shown in FIG. 1, but it should be
understood that a
network of nodes may exist between the headend/hub 110 and the service group
120 for
delivery of cable services to consumers, and networks may be designed with
fiber, coax, or a
combination thereof for transmission of optical and/or electrical signals. In
the example
system shown in FIG. 1, the fiber node 130 includes an optical receiver (RX)
116 that
converts the received optical signals to electrical signals. The electrical
signals then are
transmitted to service group 120.
[0039] In fiber transmission systems, especially long transmission systems,
external
modulator based transmitters may be used. Among external modulator
technologies, electro-
absorption laser (EML) based transmitters provide the required system
performance. An
EML based transmitter includes some unique performance advantages because of
its
extremely low chirp, its small package size, and its low cost. A chirp is a
signal in which the
frequency increases (up-chirp) or decreases (down-chirp) with time. EML based
transmitters
produce the external transmitter performance at a cost close to DFB based
transmitters.
Although EML transmitters are described, transmitters other than EML based
transmitters
that generate a photocurrent at the modulator may also be used.
[0040] For network implementations, such as an HFC network, the EA
modulator can
also offer good second order distortion performance if biased at the
inflection point of its
extinction ratio curve. However, for the external modulator, whether a LN MZ
modulator or
an EA modulator, the optimum bias voltage for the good second order distortion
performance
or even order distortion performance needs to be tightly controlled because
the bias window
for an acceptable second order performance is very narrow. MZI optical
modulators
fabricated in lithium niobate (LiNb0) have been shown to be sensitive to
thermal and
mechanical stresses that cause dynamic shifts of the quadrature bias point.
For these reasons,
the bias point of a typical external modulator may vary due to temperature
variations, signal
fluctuations, manufacturing tolerances and other environmental factors. If the
proper bias
point is not maintained, the modulator will exhibit stronger nonlinearity,
especially even-
order harmonics and the reduction of the signal strength in one of the
outputs. The variations
induced by stresses therefore require an active control to maintain an optimum
distortion
performance.
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[0041] The electro-absorption modulator (EAM) may suffer from a similar
problem when
the modulator is packaged separately from a light source package, such as a
laser package,
and therefore may need similar tight control of the modulator bias as
described for MZI based
modulators. However, when integrated with the DFB laser, the modulator is
hermetically
packaged together with the laser in the EML laser module. For this reason, the
temperature of
the DFB laser and EAM are set the same and maintained because the temperature
of both is
controlled by a temperature source, such as a thermoelectric cooler (TEC).
Therefore, in
theory, the bias voltage of the EAM is not affected by the environmental
temperature and
some other conditions if the laser bias and EAM bias are stable.
[0042] Nevertheless, bias control for an EML is still needed. FIG. 2A
depicts a simplified
system 200 for transmitter 114 that includes bias control logic 202 for an EML
201 according
to some embodiments. A light source, referred to as laser 206, may output a
light that is
received by EAM 208. EAM 208 absorbs the light, which is converted to
photocurrent.
Photocurrent is the electric current from EAM that is the result of exposure
to radiant power.
As described above, the effective band gap of the semiconductor of the
absorption material
changes with its bias voltage. When no bias is applied to EAM 208, the band
gap is wide
enough to allow the light at the laser wavelength to pass through
transparently. As the bias
voltage is increased, the band gap is narrowed enough to start absorbing the
light. Therefore,
EAM 208 with its changing bias voltage offers its capability of modulating
light. The
modulated light from EAM 208 is output by EML 201 and transmitted to a
receiver 210,
which may be an optical receiver at a node.
[0043] The EML's modulation characteristic is described by an extinction
ratio (ER)
curve, which is the normalized transmission coefficient versus EAM bias
voltage. The
characteristic of the ER curve, such as its shape and its relative position,
is determined by a
parameter, called wavelength detuning. FIG. 2B depicts a simplified system
1200 for an
transmitter 114, similar to FIG. 2A, that includes bias control logic 1202 for
an EML 1201
according to some embodiments. EML 1201 includes an optical light source, such
as a laser
1206, that outputs a light that is received by EAM 1208 included in EML 1201.
EAM 1208
absorbs the light, which is converted to photocurrent. Photocurrent is the
electric current
from EAM 1208 that is the result of absorption of radiant power. As described
above, the
effective band gap of the semiconductor of the absorption material changes
with its bias
voltage. When no bias is applied to EAM 1208, the band gap is wide enough to
allow the
light at the laser wavelength to pass through transparently. As the bias
voltage is increased,
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the band gap is narrowed enough to start absorbing the light. Therefore, EAM
1208 with its
changing bias voltage offers its capability of modulating light. The modulated
light from
EAM 1208 is output by EML 1201 and transmitted to a receiver 1210, which may
be an
optical receiver at a node.
[0044] As previously mentioned, a predetermined modulation performance,
such as a
predetermined optimal second order distortion or even order distortion, is
achieved only
when the modulator is biased near its inflection point of its extinction
ration (ER) curve. The
EAM modulation extinction ratio curve and thus the absorption are also a
function of
wavelength detuning. FIG. 3 depicts a graph showing the wavelength detuning
relationship
according to some embodiments. The graph shows that the detuning is defined as
wavelength difference between the DFB laser wavelength at 304 and the EAM peak
absorption wavelength at 302 according to some embodiments.
[0045] Wavelength detuning can also be changed by the EML temperature
because
temperature coefficients of the EAM and the DFB are different. FIG. 4 depicts
an example of
ER curves versus different temperatures or detuning according to some
embodiments. The
detuning may result in a different absorption by the modulator, which changes
a
predetermined modulation performance that was set for the EAM 208 by an
original bias
voltage setting. In addition to the movement along the ER curve due to the
bias voltage, the
shape of the ER curve is also changed when the temperature changes. For
example, the ER
curve changes as the temperature changes from 10 C to 85 C. Therefore, the EML
transmitter
performance in terms of output power and/or modulation performance will change
and/or
degrade and the transmitter output power will vary because of the detuning
change, such as
for a fixed EAM bias voltage. In some embodiments, it is desirable to limit
the EML detuning
difference to maintain the transmitter performance.
[0046] The wavelength detuning, however, may not be constant through the
lifetime of
EML 201 because of the aging process of the laser. Throughout the lifetime of
a laser, its
threshold (e.g., the laser bias current at which the laser turns on and starts
emitting light)
becomes larger, and its slope efficiency becomes smaller because of laser
aging effects. Thus,
the laser output power becomes smaller over time for a fixed laser bias
current. A transmitter
automatic power control (APC) circuit can be used to increase the laser bias
current to
maintain a constant laser output power based on the photocurrent of laser 206,
such as the
current from a back-facet photodetector in the package of EML 201. Increasing
the bias
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current of laser 206, however, causes the laser wavelength to change. The
wavelength change
with the laser bias current change may be due to a plasma effect and Joule
heating effect.
[0047] The wavelength change due to the laser aging and APC circuit
changing the bias
current of laser 206 may cause the detuning change between the DFB laser
wavelength and
EAM peak wavelength. The transmitter performance of EML 201 is then affected
due to the
ER curve's deviation from the original ER curve due to the detuning.
Accordingly, some
embodiments adjust the EAM voltage bias to optimize and to regain the
transmitter
performance and transmitter output power.
[0048] Conventionally, the EAM bias was controlled by an optical coupler at
the output
of the EML that taps off a small portion of light power and feeds it to an
optical receiver
(e.g., a photo detector (PD)) to convert the optical signal back to an
electrical signal. The
converted electrical signal is then sent to a feedback and bias control (F/C)
circuit. The
feedback and bias control circuit may contain an RF amplifier and a filter
network so that the
distortion beat power can be extracted. Using the extracted distortion beat
power as an
indicator, the bias control circuit can be automatically adjusted to minimize
the transmitter
distortion. While effective in optimizing the bias automatically, this
approach needs an
optical coupler, a photo detector, and electronic feedback and tracking
network that all add
significant cost and some real estate to the EML transmitter in addition to
some complexity in
the product design. The optical coupler also adds some insertion loss and thus
reduces the
transmitter output power.
[0049] Another conventional approach uses electrical signals converted by
the EAM,
which allows the optical coupler and photodetector to be omitted. Unlike the
system using the
optical coupler and a photodetector to convert the optical signal back to the
electrical signal
for the feedback and bias control (F/C) network, the optical to electrical
signal conversion is
accomplished using the intrinsic nature of the EAM because the EAM itself
functions as a
photo detector. That is, the electro-absorption of the EAM converts a portion
of optical
power to electrical power. An RF coupler is attached to the EAM to tap off a
portion of the
signal. The tapped signal is then sent to the feedback and bias control
circuit (F/C circuit) to
automatically track the distortion beat power and control the EA modulator
bias for an
optimal distortion performance.
[0050] Despite the advantages of the second system over the first one in
many aspects,
both of them work based on the same principle, controlling the EA modulator
bias based on
tracked distortion beat power after the optical signal is converted back to
the electrical signal.
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The approach may have some limitations. For example, the power of distortion
beat at some
targeted frequency may not be strong enough when the transmitter modulation
signal's
frequency band patterns or signal systems, such as National Television System
Committee
(NTSC) and Phase Alternating Line (PAL), are different for different
applications. Another
example is that for some other applications, the system setting may not be
based on solely on
analog distortion performance. In this case, the distortion beat power may not
be a proper
gauge for the bias control.
[0051] In some embodiments, an alternative way of automatic tracking and
bias control is
proposed in FIG. 2A. Different from the above two approaches, this method uses
a
characteristic of EML 201 to control the transmitter output power and to
maintain the
transmitter performance. For example, bias control logic 202 monitors the
output power of
EAM 208 and adjust the bias voltage of EAM 208. This approach has the
advantage that the
output power can be monitored as a direct current (DC) value. Thus, monitoring
the output
power does not require any dependence on the modulation signal (e.g., the
optical signal after
modulation). To maintain the performance of the transmitter using the output
power, the
relationship of the detuning change of the transmitter and the temperature
performance effect
on the modulator extinction curve will be described.
[0052] The modulator ER curve moves and varies when a detuning change
occurs.
However, it is also seen in FIG. 4 that if the detuning change is not
significant, the modulator
extinction curve move is almost a shift of the original ER curve to a new
position but the
shape stays the same. This small detuning change can happen during the EML
lifetime. For
one example of an EML laser 206, a DFB laser is biased at 250mA while the
maximum laser
current is 300mA. The laser wavelength change versus laser bias change is
0.008nm/mA. The
wavelength change caused by the current change from 250mA to 300mA is 0.4nm,
which is
the detuning change if the EML temperature stays constant. The 0.4nm detuning
change can
be simulated by the temperature change knowing the fact that the wavelength
change over
temperature for both the DFB and EAM 208 is a lot larger than the wavelength
change due to
the laser bias change. For the EML, the temperature coefficient of the DFB is
0.1nm/C and
that of EAM is 0.4nm/C. Therefore, 0.4nm detuning change can be simulated by
0.4nm/(0.4-
0.1)=1.3C temperature change. The lines in FIG. 4 can thus be used to
visualize the change in
the ER curve due to a detuning change by relating the change in the ER curve
due to
temperature change to the equivalent change due to a detuning change. The
small DFB
temperature can still cause some laser power change. The bias change due to
the APC circuit
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is less than 4mA, which introduces additional detuning change of
4*0.008=0.032nm and can
be ignored compared to 0.4nm detuning change.
[0053] For a very small detuning change, the modulator extinction curve may
only
experience a very small shift and retains its shape. FIG. 5 depicts a graph
500 that shows the
shift of a modulator extinction curve due to a small detuning change AA
according to some
embodiments. The Y axis of graph 500 shows the EAM output power and the X axis
shows
the bias voltage Vbias for EAM 208. When EAM 208 is connected directly to an
output of
the EML transmitter, then the output power of the EML transmitter is the same
as the output
power of EAM 208. In this case, the output power of either maybe monitored. If
a device is
connected between the output of EAM 208 and the output of the EML transmitter,
either
output power may be monitored, but the power level may be different, but have
the same
characteristics of change.
[0054] At 502, a negative detuning change of -AA causes the shown shift in
the modulator
extinction curve from 501 to 502. Also, a positive detuning change of +AA
causes the shift in
the modulator extinction curve from 501 to 504. However, due to the shift, the
EAM bias
voltage moves off of a predetermined or best second order distortion bias
point or
predetermined or best even order distortion bias point and the transmitter
output is no longer
the same no matter how small the shift. The original bias point on curve 501
is at 508 for the
bias voltage of v0. Where the detuning is decreased by -AA, if the EAM stays
at its original
bias point, vO, the EML output is changed from PO (point 508) to P1 (point
506) because of
the ER curve shift even though the transmitter APC circuit can still keep the
DFB laser output
the same. That is, the constant DFB back-facet photocurrent does not guarantee
a constant
transmitter output power. In this case, the transmitter output power becomes
smaller, but the
photocurrent from the EAM becomes larger. Likewise, the best distortion bias
point is also
missed because point 506 on curve 502 is not in the same position of point 508
on curve 501.
[0055] Since the modulator extinction curve after its shift due to a small
detuning change
can be deemed as the replica of the original modulator extinction curve, if
the EA bias moves
from voltage v0 to voltage vi, both the best distortion bias and the
transmitter power are set
back from a point 506 to a point 510 where the output power is similar
compared to before
the detuning change occurred (point 508 and point 510 have the same power PO).
Further,
the position at 510 on the curve 502 is similar to the position 508 on the
previous curve 501,
which maintains the linear modulation properties of the EML output due to
being around the
inflection point of both curves. In the meantime, the EAM photocurrent is also
set back to
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the same value due to the bias voltage change maintaining the same output
power. That is,
when the EAM photocurrent can be used as an indicator or monitoring parameter
for bias
EAM control circuit, maintaining the same EAM photocurrent maintains the best
distortion
bias and the transmitter output power simultaneously. Maintaining the best
distortion bias and
the transmitter output power is performed by attempting to keep the EAM output
power
substantially the same.
[0056] FIG. 6 depicts an example of bias control logic 202 according to
some
embodiments. A photocurrent monitor 602 monitors the EAM photocurrent. A
processor 604
then processes the changes in the photocurrent and sends a signal for the
changes to an EAM
bias control 606. EAM bias control 606 may then generate a new bias voltage
for EAM 208.
Then, photocurrent monitor 602 monitors the change in photocurrent based on
the adjustment
in the bias voltage. This adjustment continues until the original EAM
photocurrent is
achieved. The adjusting of the bias voltage maintains the photocurrent the
same, and keeps
the output power of EAM 208 constant, which maintains the output power of EML
201.
Although monitoring a change in photocurrent is discussed, the photocurrent
can be
monitored in different forms that allow the output power of EAM 208 to be
monitored, such
as monitoring the voltage that is proportional to the photocurrent or power of
EAM 208.
Changing the bias voltage of EAM 208 maintains the output power and a
predetermined
modulation performance. However, the signal output or distortion beats after
modulation by
EAM 208 is not used to adjust the bias voltage.
[0057] FIG. 7 shows an example circuit that monitors the EAM photocurrent
and feeds
the current back to the EAM bias control circuit to generate a new bias
voltage according to
some embodiments. In the circuit, EAM 208 is biased by a voltage source,
composed of an
op-amp Ul and a transistor Ql, that provides a constant bias voltage. When
there is EAM
photocurrent change, the photocurrent change at 702 can be sensed by the
resistor R8 at 704.
The sensed photocurrent is sent to the processor 604 through an analog to
digital convertor
(ADC) and then a new bias voltage, Vset, is then sent to the control voltage
of the voltage
source through a digital to analog convertor (DAC). EAM bias control 606 sets
a new bias
voltage Vset to maintain the voltage across resistor R8. This adjustment
continues until the
original EAM photocurrent is achieved.
[0058] FIG. 8 depicts a simplified flowchart 800 for adjusting the EAM bias
voltage
according to some embodiments. At 802, bias control logic 202 monitors output
power of
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EAM 208. At 804, bias control logic 202 determines if the output power
changes. If not, the
monitoring continues.
[0059] If the output power changes, then bias control logic 202 calculates
a bias voltage
change to maintain the output power. Then, at 808, bias control logic 202
applies the bias
voltage change to EAM 208.
[0060] FIG. 9 shows an example of composite second order performance
according to
some embodiments. The EML temperature was originally set to 28.5C and then was
tuned to
28.5C+/-1.5C. The transmitter was loaded with 79 analog subcarriers and 75 QAM
channels
at -6dB relative to the subcarrier power to make a total bandwidth of 1GHz.
The composite
second order (CSO) beats are measured and monitored at around 55.25MHz at 900
and
547.25MHz at 902 where the most distortion beats happen. With EAM bias
automatic
control, the EAM photocurrent composite second orders and transmitter output
power as
shown at 904are maintained for the temperature range and without automatic
control, the
composite second orders are degraded. The process proposed here is therefore
effective or
else the distortion performance and output power of the transmitter will be
degraded without
the automatic EAM bias control circuit in the case that the temperature change
causes ER
curve shift and the transmitter performance in terms of output power and
distortion
performance are changed even though the APC circuit can still maintain the DFB
laser output
power.
[0061] It is noted that the circuit depicted in FIG. 7 only serves as an
example. Therefore,
the photocurrent sensing can be performed in other ways. Although the laser
wavelength
coefficient due to bias current and the wavelength temperature coefficient for
both DFB laser
206 and EAM 208 are used in the previous calculation, the proposed process is
valid without
knowledge of these coefficients because the magnitudes of the wavelength
change caused by
DFB laser bias current change and temperature change are similar and the
method only relies
on monitoring photocurrent of EAM 208 and therefore the principle still holds
without
knowing the actual coefficients.
[0062] With the method proposed here, it is possible to not only
automatically adjust the
EML's bias for maintaining the targeted performance and output power but the
process can
also be used for maintaining the wavelength of EML 201. As mention earlier,
the APC circuit
can maintain the DFB laser output power by increasing the laser bias current
but it also
changes the laser wavelength. In DWDM applications, nevertheless, the EML
transmitters'
wavelength should also be maintained due to the narrow filter bandwidth in the
multiplexer
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(MUX) and the demultiplexer (DEMUX). The wavelengths in a DWDM system should
be
maintained also to avoid some system performance degradation due to some
undesired fiber
nonlinearities, such as four wave mixing. Wavelength maintenance by the laser
temperature
tuning and APC can also cause detuning change. The method given here can also
be used to
maintain the transmitter performance while the wavelength is tuned to be
constant.
[0063] In another embodiment, referring to FIG. 2B, the ER curve changes as
the
temperature changes from 10 C to 85 C. Therefore, the EML transmitter
performance in
terms of transmitter output power and modulation performance will change
because of the
detuning change for a fixed EAM bias voltage. For example, if an EML 1201
based
transmitter is used for HFC applications or other applications, EML 1201 may
need to
produce a very good linearity for a good analog performance and a good carrier
to noise ratio.
A good second order or even order distortion performance is achieved when the
EAM 1208 is
biased near the inflection point of its ER curve. The inflection point is
where the ER curve
changes in its curvature from concave up to concave down. Like in the LN MZ
modulator,
the EAM bias window for a good composite second order distortion performance
is very
narrow around the inflection point of the ER curve. It is therefore strongly
desirable to track
the EML's ER curve through predicting the EML's detuning and to set the bias
voltage of
EAM 1208 to the optimum performance bias voltage point to maintain the
transmitter
performance with constant characteristics.
[0064] EAM 1208 may be integrated together with laser 1206 by being
hermetically
sealed in an EML package. For this reason, the temperature of laser 1206 and
EAM 1208 are
set the same and maintained because the temperature of both is controlled by a
thermoelectric
cooler (TEC). Therefore, in theory, the optimal bias point on the ER curve is
not affected by
the environmental temperature and some other conditions.
[0065] However, in transmitter 114, the EAM bias still needs to be
automatically adjusted
to maintain good transmitter performance. Referring again to FIG. 2B, bias
control logic
1202 is necessary due to two reasons. First, if the laser bias voltage is not
constant over
temperature the transmitter distortion performance is then affected. Second,
the wavelength
detuning, however, may not be constant through the lifetime of EML 1201 even
though the
temperature of both the laser 1206 and EAM 1208 is tightly controlled and
maintained by the
thermoelectric cooler because of laser aging and due to the laser bias current
change by an
automatic power control (APC) circuit. Throughout the lifetime of a laser, its
threshold
becomes larger, and its slope efficiency becomes smaller because of laser
aging effects.
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Thus, the laser output power becomes smaller over time for a fixed laser bias
current. The
APC circuit in this case increases the laser bias current to maintain a
constant laser output
power based on the current of a back-facet photodetector in the package of EML
1201.
Increasing the bias current, however, causes the laser wavelength change. The
wavelength
change caused by the laser bias current change may be due to a plasma effect
and Joule
heating effect.
[0066] The wavelength change due to the laser aging and APC circuit
changing the bias
current of laser 1206 causes the detuning between the DFB laser wavelength and
EAM peak
wavelength. The transmitter performance of EML 1201 is then affected due to
the ER curve's
deviation from the original ER curve. Accordingly, some embodiments adjust the
EAM
voltage bias to optimize and to regain the transmitter performance and
transmitter output
power.
[0067] Conventionally, the EAM bias was controlled by an optical coupler at
the output
of the EML that taps off a small portion of light power and feeds it to an
optical receiver
(e.g., a photo detector (PD)) to convert the optical signal back to an
electrical signal. The
converted electrical signal is then sent to a feedback and bias control (F/C)
circuit. The
feedback and bias control circuit may contain an RF amplifier and a filter
network so that the
distortion beat power can be extracted. Using the extracted distortion beat
power as an
indicator, the bias control circuit can be automatically adjusted to minimize
the transmitter
distortion.
[0068] While effective in optimizing the bias automatically, this approach
needs an
optical coupler, a photo detector, and electronic feedback and tracking
network that all add
significant cost and some real estate to the EML transmitter in addition to
some complexity in
the product design. The optical coupler also adds some insertion loss and thus
reduces the
transmitter output power.
[0069] Another conventional approach uses electrical signals converted by
the EAM,
which allows the optical coupler and photodetector to be omitted. Unlike the
system using the
optical coupler and a photodetector to convert the optical signal back to the
electrical signal
for the feedback and bias control (F/C) network, the optical to electrical
signal conversion is
accomplished using the intrinsic nature of the EAM because the EAM itself
functions as a
photo detector. That is, the electro-absorption of the EAM converts a portion
of optical
power to electrical power. An RF coupler is attached to the EAM to tap off a
portion of the
signal. The tapped signal is then sent to the feedback and bias control
circuit (F/C circuit) to
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automatically track the distortion beat power and control the EA modulator
bias for an
optimal distortion performance.
[0070] Despite the advantages of the second system over the first one in
many aspects,
both of them work based on the same principle, controlling the EA modulator
bias based on
tracked distortion beat power after the optical signal is converted back to
the electrical signal.
The approach may have some limitations. For example, the power of distortion
beat at some
targeted frequency may not be strong enough when the transmitter modulation
signal's
frequency band patterns or signal systems, such as National Television System
Committee
(NTSC) and Phase Alternating Line (PAL), are different for different
applications. Another
example is that for some other applications, the system setting may not be
based on solely on
analog distortion performance. In this case, the distortion beat power may not
be a proper
gauge for the bias control.
[0071] In some embodiments, an alternative way of automatic tracking and
bias control is
proposed in FIG. 2B. Different from the above previous approaches, bias
control logic 1202
predicts EML detuning changes as it evolves over its lifetime. Bias control
logic 1202 uses
the wavelength temperature coefficient of both laser 1206 and EAM 1208 to
generate the
EML transmitter performance at different detuning changes. Then, bias control
logic 1202
uses a predetermined EAM bias curve to optimize EML transmitter performance
under
different detuning levels. Bias control logic 1202 derives the EAM bias
voltage from the
predetermined EAM bias curve based on the detuning change of EML 1201, which
is
converted from the bias current change of laser 1206. The derived bias voltage
(or current)
from laser 1206 is then used to derive the bias voltage source or the bias
current source of
EAM 1208. This also maintains the output power of EML 1201 while optimizing
its
performance throughout its lifetime.
[0072] The modulator ER curve moves and varies with a detuning change due
to a
change in the EML temperature because of the difference of the wavelength
temperature
coefficient of laser 1206 and EAM 1208. As described earlier, the EML detuning
change can
happen throughout its lifetime because of the laser aging and the APC circuit
adjustment of
the laser's bias voltage even though the EML's temperature is strictly
maintained. It is
possible that this detuning change is solely determined by the aging of laser
1206 because the
laser current may be significantly higher than the EAM current, such as for
the EML current
in the transmitters for HFC applications. For example, the laser current may
be as high as
250mA while the EAM photocurrent might be only about 13mA. If the laser is
biased at
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250mA while the maximum laser current is 300mA and the coefficient laser
wavelength
change versus laser bias current change is 0.008nm/mA, then the wavelength
change caused
by the current change from 250mA to 300mA is 0.4nm. This 0.4nm can represent
the EML
detuning change over its lifetime if the EML temperature stays constant.
100731 FIG. 10 depicts a graph 1500 that shows the impact of the detuning
change on the
performance of EML 1201 according to some embodiments. An original ER curve is
shown
at 1502 and is biased at v0 at 1504. The inflection point for predetermined
modulation
performance, such as the optimal composite second order performance, is
achieved when the
output power of EML 1201 is then set to PO at 1506. When a detuning, AA, of
EML 1201
occurs, then the ER curve shifts, such as to the right to a curve at 1508 or
to the left to a curve
at 1510. However, when the ER curve shifts, the original EAM bias is off from
the new ER
curve's inflection point, and the composite second order distortion of EAM
1208 is degraded.
That is, a point 1512 on ER curve 1508 is off from the inflection point on
that curve if the
EAM bias voltage stays unchanged at voltage v0.
[0074] To maintain a similar composite second order performance, bias
control logic
1202 causes the EAM bias point to be moved to another inflection point 1514 on
the new ER
curve 1508. This inflection point 1514 corresponds to an EAM bias voltage vi.
To perform
this change, bias control logic 1202 tracks the detuning change by monitoring
the laser bias
current change and sets the EAM bias to its inflection point automatically.
Bias control logic
1202 predicts the ER curve change during the lifetime of EML 1201 based on the
known
laser and EAM temperature coefficients and optimum EAM bias setting curve, and
can set
the predetermined optimal EAM bias voltage for EAM 1208 near each detuning
point.
[0075] The prediction of the EML detuning curve is made possible if the
wavelength
temperature coefficient of both laser 1206 and EAM 1208 is known. In some
examples, the
wavelength temperature coefficient of laser 1206 is 0.1nm/C and that of EAM
1208 is
0.4nm/C. Therefore, a 0.4nm detuning change can be simulated by 0.4nm/(0.4-
0.1)=1.3C
temperature change of EML 1201. The lines in FIG. 4 can thus be used to
visualize the
change in the ER curve due to a detuning change by relating the change in the
ER curve due
to temperature change to the equivalent change due to a detuning change caused
by the laser
bias change. That means the performance of EAM 1208 over the laser bias
current change
over the lifetime of EAM 1208 can be produced by setting the temperature of
EML 1201.
The small DFB temperature for the simulation can still cause some laser power
change and
laser bias change due to APC circuit. The bias change due to the APC circuit
is less than
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4mA when the laser temperature changed by 1.3C, which introduces additional
detuning
change of 4*0.008=0.032nm and can be ignored compared to a 0.3nm detuning
change.
[0076] By predicting the detuning or the optimum EAM bias point at
different detuning
levels due to the EML aging and the APC circuit control of the laser bias
voltage, bias control
logic 1202 uses an optimum detuning-EAM bias curve, which may be established
by
experimental simulation of some detuning points due to aging and optimum
setting of the
EAM at these detuning points, to control the transmitter performance. The EAM
bias curve
may include several preset temperature points for the targeted detuning points
to cover the
range of detuning changes and a desired bias voltage for EAM 1208 at each
detuning point.
Since the EML detuning change and laser bias current change can be inter-
converted, the
detuning change can then be tracked by the bias current change, the
predetermined EAM bias
curve can be in the form of bias voltage versus detuning change or bias
voltage versus laser
bias current change. The predetermined EAM bias curve is then used to
automatically
control EAM bias and to maintain the optimal targeted transmitter performance
throughout its
lifetime. In some embodiments, the predetermined EAM bias curve may be
implemented in
different ways, such as via a lookup table or other type of data structure.
[0077] FIG. 11A shows an example of the predetermined EAM bias curve 1600
over
detuning change according to some embodiments. The detuning may change because
of
temperature changes and instead of using the detuning change on the X axis,
the X axis may
be the temperature change of EML 1201. In this example, the initial
temperature of the EML
is 23.3C and temperature range is +/-1.5 degrees, which causes a detuning
change from
around -0.45 to +0.45nm. Other changes may also be appreciated.
[0078] A curve 1602 shows the relationship between EAM bias voltage and the
detuning
change. As the detuning changes, bias control logic 1202 can determine the new
EAM bias
voltage. For example, if the detuning starts at zero at 1604 and changes to
0.2 at 1606, bias
control logic 1202 can change the EAM bias voltage from approximately -1.425
to -1.432
(from point 1608 to point 1610).
[0079] FIG. 11B shows an example of the predetermined EAM bias curve 1620
over
laser bias current change according to some embodiments. The initial
temperature of the
EML is 23.3C and temperature range is +/-1.5 degrees. A curve 1622 shows the
relationship
between EAM bias voltage and the current change. As the laser current changes,
bias control
logic 1202 can determine the new EAM bias voltage. For example, if the laser
current starts
at OmA at 1624 and changes to 40mA at 1626, bias control logic 1202 can change
the EAM
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bias voltage from approximately -1.425 to -1.437 (from point 1628 to point
1630). Although
the above curves were shown in FIGS. 11A and 11B, other curves may be used.
[0080] FIG. 12 depicts a simplified flowchart 1700 of a method for
adjusting the bias
voltage of EAM 1208 according to some embodiments. At 1702, bias control logic
1202
monitors for a laser bias current change. At 1704, bias control logic 1202
determines if the
laser bias voltage has changed. If not, the process goes back to laser bias
current change
monitoring.
[0081] At 1706, bias control logic 1202 calculates the detuning change that
results from
the laser bias voltage change. Then, at 1708, bias control logic 1202 analyzes
the detuning-
bias curve and calculates the new EMA bias voltage. Alternatively, the laser
current change-
bias curve may be used. Bias control logic 1202 can then determine a new EAM
bias voltage
based on the curve. Then, at 1710, bias control logic 1202 changes the EAM
bias voltage
based on the detuning-bias curve.
[0082] FIG. 13 depicts a more detailed example of bias control logic 1202
according to
some embodiments. Bias control logic 1202 includes a laser bias monitor 1802
that in turn
monitors the detuning change due to the bias current change of laser 1206.
When a change is
detected, EAM bias control 1808 uses the change to look up a change in the EAM
bias
voltage using lookup table 1804. In some embodiments, the change in the
detuning or the
current corresponds to a value interpolated or extrapolated from the lookup
table 1804. EAM
bias control 1808 then changes the bias voltage of EAM 1208 based on the value
derived
from lookup table 1804.
[0083] FIG. 14 depicts results of the composite second order (CSO)
performance at
different EML detuning changes with and without EAM bias voltage control
according to
some embodiments. The detuning represented is caused by the laser bias current
change and
is simulated by setting the EML's temperature. The EAM control voltage at each
detuning
point is derived from the predetermined EAM bias curve/table for the targeted
optimal CSO.
Some EAM biases are derived from interpolation of the predetermined
curve/table while
some others are derived from the extrapolation. The transmitter was loaded
with 79 analog
subcarriers and 75 QAM channels at -6dB relative to the subcarrier power to
make a total
bandwidth of 1GHz. The composite second order (CSO) beats are measured and
monitored at
near 55.25MHz at 1900 and 547.25MHz at 1902 where the most distortion beats
happen. It is
seen that with EAM bias automatic control based on the predetermined EAM bias
curve/table, the transmitter's CSO performance is well maintained.
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[0084] Some embodiments may be implemented in a non-transitory computer-
readable
storage medium for use by or in connection with the instruction execution
system, apparatus,
system, or machine. The computer-readable storage medium contains instructions
for
controlling a computer system to perform a method described by some
embodiments. The
computer system may include one or more computing devices. The instructions,
when
executed by one or more computer processors, may be configured to perform that
which is
described in some embodiments.
[0085] As used in the description herein and throughout the claims that
follow, "a", "an",
and "the" includes plural references unless the context clearly dictates
otherwise. Also, as
used in the description herein and throughout the claims that follow, the
meaning of "in"
includes "in" and "on" unless the context clearly dictates otherwise.
[0086] The above description illustrates various embodiments along with
examples of
how aspects of some embodiments may be implemented. The above examples and
embodiments should not be deemed to be the only embodiments, and are presented
to
illustrate the flexibility and advantages of some embodiments as defined by
the following
claims. Based on the above disclosure and the following claims, other
arrangements,
embodiments, implementations and equivalents may be employed without departing
from the
scope hereof as defined by the claims.
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