U.S. patent application number 16/155566 was filed with the patent office on 2019-02-07 for dc-coupled laser driver with ac-coupled termination element.
The applicant listed for this patent is MACOM Technology Solutions Holdings, Inc.. Invention is credited to Cristiano Bazzani, Atul Gupta, Kevin McDonald, Matteo Troiani, Yanxin Will Wang.
Application Number | 20190045283 16/155566 |
Document ID | / |
Family ID | 56165564 |
Filed Date | 2019-02-07 |
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United States Patent
Application |
20190045283 |
Kind Code |
A1 |
Troiani; Matteo ; et
al. |
February 7, 2019 |
DC-COUPLED LASER DRIVER WITH AC-COUPLED TERMINATION ELEMENT
Abstract
An optical signal module including a driver and an optical
signal module. The driver includes a differential pair configured
to receive and process an input signal to create a drive signal. A
modulation current source provides a modulation current to the
differential pair. One or more termination resistors connected to
the differential pair for impedance matching. A first switch,
responsive to a first control signal, maintains charge on a charge
storage device. The optical signal module includes an optical
signal generator arranged between a supply voltage node and a bias
current node. The optical signal generator receives the drive
signal and generates an optical signal representing the input
signal. A second switch is between a supply voltage node the bias
current node. The second switch, responsive to second control
signal, selectively establishes a short between the supply voltage
node the bias current node.
Inventors: |
Troiani; Matteo; (Irvine,
CA) ; Bazzani; Cristiano; (Irvine, CA) ; Wang;
Yanxin Will; (San Marcos, CA) ; McDonald; Kevin;
(Portland, OR) ; Gupta; Atul; (Aliso Viejo,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MACOM Technology Solutions Holdings, Inc. |
Lowell |
MA |
US |
|
|
Family ID: |
56165564 |
Appl. No.: |
16/155566 |
Filed: |
October 9, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14588267 |
Dec 31, 2014 |
10097908 |
|
|
16155566 |
|
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|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04Q 11/0066 20130101;
H04Q 2011/0035 20130101; H04Q 2011/0033 20130101; H04B 10/504
20130101; H01S 5/0428 20130101; H04B 10/502 20130101; H04Q
2011/0086 20130101 |
International
Class: |
H04Q 11/00 20060101
H04Q011/00; H04B 10/50 20130101 H04B010/50; H01S 5/042 20060101
H01S005/042 |
Claims
1. An optical signal generator module including a driver, the
module comprising: a driver configured to process at least one
input signal to generate a drive signal; a first switch located
between a supply voltage node and the driver, the first switch
responsive to a first control signal; a charge storage device
located between the supply voltage node and the first switch; an
optical signal generator receiving the drive signal and responsive
to the drive signal generating an optical signal; a second switch
associated with the optical signal generator, the second switch
responsive to a second control signal to selectively short voltage
across the optical signal generator through the switch; one or more
termination elements selected to impedance match the driver with
the optical signal generator, the one or more termination resistors
located between the supply voltage node and the driver mirror.
2. The optical signal generator module of claim 9 wherein the at
least one input signal is a differential input signal.
3. The optical signal generator module of claim 9 wherein the
charge storage device comprise a capacitor.
4. The optical signal generator module of claim 9 further
comprising a controller configured to generate the first control
signal and the second control signal, such that when the optical
signal generator is active the first switch is closed and the
second switch is open, and when the optical signal generator is
inactive the first switch is open and the second switch is
closed.
5. The optical signal generator module of claim 12, wherein the
when the optical signal generator is transitioning to an active
state, the second switch is opened before the first switch is
closed and when the optical signal generator is transitioning to an
inactive state, the first switch is opened before the second switch
is closed.
Description
1. PRIORITY CLAIM
[0001] This application is a continuation of U.S. application Ser.
No. 14/588,267 filed on Dec. 31, 2014, which will issue as issue as
U.S. Pat. No. 10,097,908 on Oct. 9, 2018, the contents of which are
incorporated herein by reference in their entirety.
2. FIELD OF THE INVENTION
[0002] The invention relates to laser drivers and in particular to
a laser driver with improved response time and reduced power
consumption.
3. RELATED ART
[0003] The use of optical devices in electronic system is common
and widespread. The optical devices may include laser or LED for
use in communication systems, projection systems, or optical media
readers and writers. To improve performance and increase product
life while decreasing power consumption and heat generation, it is
preferred to reduce the response time of the driver and also reduce
power consumption.
[0004] One type of light source driver is a DC-coupled laser
driver. This type of laser driver may benefit from near end
termination elements to reduce reflection when driving 25-ohm
transmission lines such as for example, a TOSA (transmitter optical
sub-assembly) in high data rate applications. Different application
may be established with different termination resistances. Failure
to properly terminate will result in signal reflections due to
impedance mismatch, which limit system operation.
[0005] In a typical configuration, the light source, such as a
laser device includes a cathode terminal and an anode terminal. The
cathode termination resistor, which connects to the cathode
terminal and to a high voltage, creates a current path for laser
bias current which may be sourced by a bias current driver.
Therefore, some portion of the bias current is wasted in the
termination resistor. This is undesirable in all applications, but
in system which rely on battery power for operation, is highly
undesirable.
[0006] In addition, many applications require a rapid response time
for both laser on time and laser off time. One such application is
burst mode communication systems which assign timeslots to various
users of a shared optical cable and then each user transmits in
bursts during their assigned time slot. As can be appreciated,
rapid response time allows an assigned user to begin transmitting
sooner and terminate transmission faster, during their assigned
time slot, translates to higher effective data rates during the
time slot. Therefore, there is a need in the art to reduce burst-on
time and burst-off time.
SUMMARY
[0007] To overcome the drawbacks of the prior art and provide
additional benefits, an optical signal module including a driver is
disclosed. The driver includes a differential pair with a first
transistor configured to receive a first input signal and a second
transistor configured to receive a second input signal. The first
signal and the second signal being a differential input signal such
that the differential pair is configured to supply a first drive
signal and a second drive signal in response to the first input
signal and the second input signal. A modulation current source
provides a modulation current to the differential pair and one or
more termination elements, such as resistors, connected to the
differential pair. A first switch, responsive to a first control
signal, has a first terminal and a second terminal such that the
first terminal connects to at least one of the one or more
termination elements. A charge storage device is also part of this
embodiment and is connected to the second terminal of the first
switch and a supply voltage node. In other embodiments,
differential pair may be replaced with a singled ended modulation
circuit. The termination element may be one or more inductors,
capacitors, resistors, active devices (such as in the case of
active termination) or any other impedance matching element. The
termination element may be referred to herein as a termination
resistor.
[0008] The optical module includes an optical signal generator
arranged between the supply voltage node and a bias current node.
The optical signal generator receives the first drive signal and a
second drive signal to generate an optical signal representing the
first input signal and the second input signal. A second switch is
configured between a supply voltage node and the bias current node
such that the second switch is responsive to second control signal
and is configured to establish a short between the supply voltage
node the bias current node. The first switch and/or second switch
can by any device or element that creates an open circuit or blocks
current flow.
[0009] In one embodiment the module further comprises a first
inductor located between the optical signal generator and the
supply voltage node and a second inductor located between the
optical signal generator and the bias current node. In one
configuration, the optical signal generator is a laser diode.
[0010] The first control signal and second control signal may be
generated by a controller responsive to a burst-on signal or a
burst-off signal. Responsive to a burst-on signal, the second
control signal opens the second switch and thereafter the first
control signal closes the first switch. In one configuration
responsive to a burst-off signal, the first control signal opens
with first switch to thereby maintain charge on the charge storage
device, and thereafter the second control signal closes the second
switch. A cascade transistor may be located between one of the
termination elements and the differential pair.
[0011] Also disclosed is an optical signal generator module
including a driver. The module comprises a driver configured to
process at least one input signal to generate a drive signal. A
first switch is located between a supply voltage node and the
differential pair and the first switch is responsive to a first
control signal. A charge storage device is located between the
supply voltage node and the first switch. An optical signal
generator receives the drive signal, and responsive to the drive
signal, generates an optical signal. A second switch is associated
with the optical signal generator. The second switch is responsive
to a second control signal to selectively short voltage across the
optical signal generator through the switch. One or more
termination elements are also provided and selected to impedance
match the driver with the optical signal generator, the one or more
termination elements located between the supply voltage node and
the driver mirror.
[0012] In one embodiment, at least one input signal is a
differential input signal. The charge storage device may comprise a
capacitor. The module may include a controller configured to
generate the first control signal and the second control signal,
such that when the optical signal generator is active the first
switch is closed and the second switch is open, and when the
optical signal generator is inactive the first switch is open and
the second switch is closed. In one embodiment, when the optic
signal generator is transitioning to an active state, the second
switch is opened before the first switch is closed and when the
optical signal generator is transitioning to an inactive state, the
first switch is opened before the second switch is closed.
[0013] Also disclosed herein is a method for driving an optical
signal generator with a driver. The optical signal generator and
driver include a driver configured to amplify at least one input
signal, a first switch located between a supply voltage node and
the driver such that the first switch is responsive to an
activation control signal and a de-activation control signal. A
charge storage device is also present and is located between the
supply voltage node and the first switch. An optical signal
generator receives the drive signal and responsive to the drive
signal generates an optical signal while a second switch that is
associated with the optical signal generator, is responsive to the
activation control signal and the de-activation control signal to
selectively short voltage across the optical signal generator
through the switch. In operation, the system receives a one or more
input signals and also receives a supply voltage from a supply
voltage node. Then receiving an activation signal to initiate a
transmit period for the optical signal generator and responsive to
the activation signal, generating the activation control signal.
The method of operation then provides the activation control signal
to the second switch to open the second switch and then provides
the activation control signal to the first switch to close the
first switch. The system also amplifies the input signal with the
driver to generate a drive signal representative of the input
signal. Thereafter, the system receives a de-activation signal to
terminate a transmit period for the optical signal generator and
responsive to the de-activation signal, generate the de-activation
control signal. The system provides the de-activation control
signal to the first switch to open the first switch and provides
the de-activation control signal to the second switch to close the
second switch.
[0014] In one embodiment, the activation signal initiates a
transmit period or a burst-on period. Responsive to the activation
control signal, the second switch may be opened before the first
switch is closed. Responsive to the de-activation control signal
the first switch may be opened before the second switch is
closed.
[0015] In one configuration, the one or more input signals
comprises two input signals presented as a differential pair. The
method may also include impedance matching the driver to the
optical signal generator with one or more termination elements such
that the one or more termination elements located between the
supply voltage node and the driver or as part of the driver. In one
exemplary method of operation, closing the second switch shorts
voltage across the optical signal generator, thereby terminating
optical signal generation. Opening the first switch maintains
charge on the charge storage device during periods when the optical
signal generator is not generating an optical signal.
[0016] Other systems, methods, features and advantages of the
invention will be or will become apparent to one with skill in the
art upon examination of the following figures and detailed
description. It is intended that all such additional systems,
methods, features and advantages be included within this
description, be within the scope of the invention, and be protected
by the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The components in the figures are not necessarily to scale,
emphasis instead being placed upon illustrating the principles of
the invention. In the figures, like reference numerals designate
corresponding parts throughout the different views.
[0018] FIG. 1 is a block diagram illustrating an exemplary light
source driver with unified switching and a charge storage device to
reduce power consumption and improve response time.
[0019] FIG. 2A is a circuit diagram illustrating an exemplary laser
driver configured with switching elements and charge storage device
for increasing response time and decreasing power consumption.
[0020] FIG. 2B is a circuit diagram illustrating an exemplary
driver circuit with an alternative switch arraignment.
[0021] FIG. 2C is a circuit diagram illustrating an exemplary
driver circuit with an alternative switch arraignment.
[0022] FIG. 3 is an operational flow diagram of an exemplary method
of operation.
DETAILED DESCRIPTION
[0023] To overcome the drawbacks of the prior art and to provide
additional benefits, an optical signal driver is disclosed that
includes a charge storage device and a unified switching system to
maintain charge on the charge storage device during transmit and
non-transmit periods, such as in a time multiplexed
environment.
[0024] FIG. 1 is a block diagram illustrating an exemplary light
source driver with unified switching and a charge storage device to
reduce power consumption and improve response time. This is one
possible configuration and as such it is contemplated that other
embodiments which utilize a charge storage device in connection
with one or more switching elements maybe developed which do not
depart from the claims that follow.
[0025] In this exemplary embodiment, an optical device driver 104
connects to or is in communication with an optical device 108. The
driver 104 may be any arrangement of active and/or passive devices
configured to receive and process an input signal into a drive
signal capable of driving the optical device 108 to thereby
generate an optical signal as shown. The optical device driver 104
may be configured to include one or more impedance matching devices
or networks to match the impedance at the input 116, such as from a
pre-driver impedance, and at the output to the optical device 108.
Impedance matching devices may include one or more resistors,
inductors, capacitors, active devices, or a combination of one or
more of those devices. Proper impedance matching insures maximum
signal transfer and reduced reflection, which in turn result in
improved performance and higher effective bandwidth and data
transfer rates.
[0026] The optical device may comprise any device capable of
generating an optical signal including but not limited to one or
more lasers, LED, optical modulator or any other type of optical
signal generator or conditioner, with or without flex or any
embedded trace. The applications or operating environment for the
system of FIG. 1 could be an optical communication system,
projection systems, optical media read/write applications such as
CD, DVD, and Blu-ray, or any other application or environment which
utilizes an optical signal.
[0027] An input signal is provided to the optical device driver 104
through one or more input ports 116. The input to the driver 104
may be single ended or a differential pair, or it may assume any
other format or number of inputs.
[0028] A modulation current source 120 provides a modulation
current and a bias current to the optical device driver 104 as
shown while a bias current source 124 provides a bias current to
the optical device 108. In this environment, modulation current
sources 120 and bias current source 124 are known elements and as
such, these devices are not discussed in detail. In one embodiment,
the driver 104 is includes two elements, one that provides
modulating electrical signal to the optical device 108 and another
that provides biasing current to the optical device.
[0029] A supply voltage node Vdd 112 is shown at the top of FIG. 1
as establishing a supply voltage to the system. Connected to the
supply voltage node 112 is the optical device 108 and three
switches, referred to herein as switch S3 132, switch S1 134, and
switch S2 136. Switch S3 132 connects to the voltage supply node
112 and the optical device driver 104. Switch S2 136 connects to
the voltage supply node 112 and a node on the opposing terminal of
the optical device 108, such as the node between the bias current
source 124 and the optical device. Switch S1 connects between a
charge storage device 140 and the driver 104. Any of the switches
S1, S2, and S3 may be made to be in an open condition (open
circuit) or a closed condition (short circuit) and one of ordinary
skill in the art would understand that each switch may be
selectively opened or closed based on the control signal.
[0030] The switches may be any type switch, either passive or
active, and may be controlled by one or more control signals, shown
as inputs C to the switch modules 132, 134, 136. The switch control
signals (C) determine the switch positions as discussed below in
general and in particular in FIG. 3. The switch control signals are
responsive to or controlled by one or more other signals in the
optical transmit module and may be generated directly from these
signals are processed by a controller or processor. In one
embodiment, the switch control signals, which open and close the
switches, are responsive to burst-on mode and burst-off mode, or
transmit on and transmit off signals.
[0031] The switch S3 132 is optional, but is included in this
embodiment to prevent or cut off any leakage to the optical device
during burst off periods.
[0032] A charge storage device 140 also connects to the supply
voltage node 112 and the switch S1 134. The charge storage device
may be any device that stores or supplements charge including but
not limited to a capacitor, active device such as transistor, FET,
MOSFET, or any active circuit. In one embodiment the charge storage
device also blocks DC current from passing between the supply
voltage node 112 and the optical driver when the switch S1 134 is
closed.
[0033] In operation, the input signal(s) is presented to the driver
104 on input 116. The driver processes the signal to establish the
signal at a magnitude (voltage, current) suitable for driving the
optical device to generate an optical signal. In one embodiment,
the input is a modulated signal containing signal data. The supply
voltage is provided from node 112. During active transmit sessions,
the switch S1 is closed in response to the switch control signal
causing the charge storage device to become charged and store
charge, while also acting to block DC current from flowing from the
supply node 112 to the optical signal driver. During the transmit
period (or active period) for the optical device 108, the switch S2
is open, to allow the driver signal to drive the optical
device.
[0034] In this exemplary embodiment, the supply voltage Vdd voltage
shown in FIG. 2 can be a supply voltage or can be controlled by a
regulator. In the event the voltage is sourced or controlled by a
regulator, there are two solutions. First, a regulator may be
configured to establish voltage Vdd close to a target value
voltage. The target voltage can be selected by the user. Second, a
feedback loop may be provided that is configured to sense the DC
voltage (the frequency components much lower than data frequency)
on OUTP terminal (connect point between driver module and optical
module) and set the Vdd voltage in order to keep the OUTP DC
voltage close to a target value. The target can be selected by the
user.
[0035] Immediately after or close in time after the end of the
transmit period (end of the burst-on period), the switch S1 134 is
first opened, and thereafter switch S2 136 is closed. The opening
of switch S1 should not interfere with the burst on transmit
period. The time between switch S1 134 opening and switch S2 136
may be of a short duration. Opening switch S1 134 prior to the
closing of switch S2 136 results in the charge storage device 140
maintaining storage of the charge established during the transmit
period. Charge is maintained on the storage device 140 because the
switch creates an open circuit in the discharge path for the charge
storage device to ground. Closing of the switch S2 136 rapidly
thereafter eliminates any voltage differential across the optical
device thereby rapidly terminating emission of light energy.
[0036] Maintaining the charge on the charge storage device 140
improves performance by establishing a fast response time when
re-establishing optical signal generation during a subsequent
transmit period. Absent the charge maintained on the charge storage
device 140, at the start of a subsequent transmit session there
would be a delay to charge the capacitor or other circuit elements
before transmission begins. Establishing the charge storage device
as a blocking element or open circuit to DC current reduces power
consumption by not providing current into the termination. This
configuration may be referred to as AC coupled termination.
[0037] At the beginning of a subsequent transmit period, when an
optical signal is to be generated, the switch S2 136 opens and
thereafter the switch S1 134 closes and optical signal generation
commences.
[0038] Turning now to FIG. 2, a circuit diagram is shown for an
exemplary laser driver configured with switching elements and
charge storage device for increasing response time and decreasing
power consumption. This is but one possible circuit configuration
and other arrangements are contemplated that also include the novel
and beneficial aspects of this circuit as discussed below. In
addition, not all elements and aspects of the circuit are shown as
would be understood by one of ordinary skill in the art.
[0039] In this example embodiment, the circuit may be divided for
purposes of discussion into several separate sub-systems, each of
which are discussed below. This embodiment includes an optical
module 256 (which may include additional elements not shown), a
driver module 212, termination elements, shown in this embodiment
as termination resistors 216A, 216B, a switching system defined by
a controller 220, and switches 222, 224, 225, and a capacitor 240.
Each sub-system is discussed below.
[0040] The optical module includes the optical signal generator
256. A first and second inductor 250, 252 connect to the optical
signal generator 256 as shown. The optical signal generator 256
generates the optical signal and may comprise one or more lasers,
LEDs, or any other element that generates an optical signal.
[0041] In this embodiment, the optical signal generator 256 include
a cathode terminal 260 and an anode terminal 258. The anode
terminal of the optical signal generator connects to a supply
voltage node 242 through the inductor 250. At the cathode terminal
260, the optical signal generator 256 connects to the inductor 252,
which in turn connects to a switch 225 and a bias current source
268 as shown.
[0042] In this embodiment, the inductors 250, 252 are included to
account for the time constant of the circuit created by the closing
of switch S2 225. The inductors affect the time constant of the
burst on/burst off transition. As discussed above, switch S1 is
closed after switch S2, and after the system is settled. Thus,
there are two aspects to consider. First, during burst off, switch
S2 shunts impedances which causes the time constant to be small, or
of short duration, since the series resistance of the circuit is
also small due to the switch S2. Thus, the time constant is
minimized since switch S2 resistance is small. Second, for the
transition to burst on mode, it is important to time the operation
of switch S2 and switch S1 so that switch S1 is closed only after
the current and voltage of the optical branch are settled after the
switch S2 is opened. Thus, the controller is configured to
incorporate a timing or delay function to close switch S1 after
settling of the circuit according to the time constant which
prevents switch S1 from closing immediately after switch S2. The
amount of time may be fixed or programmable, such as through a user
interface accessible through the timer of controller. The amount of
time depends on the value of the inductors L1 & L2.
[0043] In an alternative embodiment, the controller or other
element may be made to monitor the voltage on node 260 and then
detect when the circuit is settled, at which time switch S1 can be
closed. Such an embodiment may include a feedback component or a
detector. The monitoring may occur at the node 260 which is defined
by the letter M. Inductor L1 252 also shields the optical signal
generator 256 from the capacitance of the current bias source
268.
[0044] A drive signal is provided to the optical signal generator
256 from the driver 212. In this embodiment, the driver 212
includes three transistors Q1 276, Q2 272, Q3 274. In other
embodiments, active devices other than transistor may be used.
Transistors Q1 270 and Q2 272 are arranged as a differential stage
or as a differential pair with the emitter terminal connected to a
modulation current source 278. In other embodiments, the
differential pair may be configured as or replaced with an emitter
follower generator, current mirror, a modulation circuit, or any
circuit providing modulated signal to the optical signal generator.
In addition, although shown as a differential pair, the driver
could be configured as singled ended.
[0045] The base terminals of the transistors Q1 270 and Q2 272
receive input modulation signals, which may be considered as the
inputs to the driver, and which control the output of the optical
signal generator 256. The driver inputs provided to the base
terminals of transistors Q1, Q2 270, 272 may arrive from a
pre-driver circuit or other element depending on the
application.
[0046] The collector terminal of transistor Q1 270 connects to
transistor Q3 274 as shown. The base of transistor Q3 is presented
with a cascade voltage Vcasc to avoid breakdown of transistor Q1
270 which may result due to the signal on node outN being too large
and also to balance bias voltage of the transistor Q1 270 and
transistor Q2 272 which may occur due to voltage mismatch between
outP and outN. The collector terminal of transistor Q3 274 connects
to a termination resistor 216A. The collector terminal of
transistor Q2 272 connects to a termination resistor 216B as shown.
Although FIG. 2A is shown with bipolar transistors it is possible
that any other semiconductor technology, such as CMOS/GaAs/InP can
be used.
[0047] The termination resistors 216A, 216B are provided and
selected to impedance or resistance match the optical signal
generators 256 (or to match any trace between the optical signal
generator 256 and the driver) to prevent reflections and maximize
energy transfer to the optical signal generator 256. In one
embodiment, the resistance R2 216B is set to 25 ohms to match a 25
ohm optical signal generator 256. The purpose and selection of the
termination resistors 216A, 216B are discussed below in greater
detail.
[0048] The opposing terminal of resistor R1 216A connects to a
switch S3 222 and the opposing terminal of the switch S3 222
connects to the supply voltage node 242. The switch S3 222 also
receives a control signal C3 from a controller 220. The control
signal determines the position of the switch S3 222, such as open
or closed.
[0049] The second terminal of the resistor R2 216B connects to
switch S1 224 as shown and the opposing terminal of the switch S1
connects to a capacitor 240. The switch S1 224 also receives a
control signal C1 from the controller 220 such that the control
signal determine the open/close status of the switch. The opposing
terminal of the switch S1 224 connects to capacitor 240, which in
turn connects to the voltage supply node 242 as shown.
[0050] The switch S2 225 is connected between a node formed by the
inductor 252 and the bias current source 258, and at an opposing
terminal to the supply voltage node 242. The switch S2 225 receives
a control input C2 which controls the switch position between an
open and closed position. The switch S2 225 shorts the voltage
across the light source 256 to rapidly prevent light output during
burst-off periods. In other embodiments, the switch S2 may be
replaced with any device configured to steer the bias current and
the modulation current away from the laser during burst-off
periods. For example, in an alternative embodiment a differential
stage may be used, but such an embodiment would be more costly and
complex.
[0051] The controller 220 may comprise any type controller
including but not limited to control logic, ASIC, processor, delay,
state machine, or any combination thereof. The controller 220
receives and processes a status input to generate the one or more
control signals C1, C2, C3 provided to the switches. In one
embodiment, the status input comprises information regarding
whether the driver and light source is in burst-on mode or
burst-off mode. It is contemplated that one or more control
signal(s) may be delayed relative to other control signals.
[0052] In operation, the circuit of FIG. 2 operates as follows.
Modulation signals, which define the signal to be generated as an
optical signal, presented to the base nodes of transistor 270, 272
as differential signals. These signals are presented at the
collector node of the transistor 270, 272 and presented to the
light source to drive the light source. Termination resistors R1,
R2 216A, 216B are provided for impedance matching between the
driver 212 and the light source 256. The termination resistors may
be referred to as termination elements. The capacitor 240 is
provided to allow AC current flow from the supply voltage node 242
to the outP node of the driver, while blocking DC coupling.
Modulation current source 278 provides current to the driver while
bias current source 268 provides the current through the light
source 256. During operation, the switches S1, S2, S3 224, 225, 222
are selectively controlled as discussed in FIG. 3.
[0053] During burst off, any configuration or element can be
employed that prevents device current from going into the optical
module, such as the laser 256. The switches S1, S2 are shown for
enablement and understanding. Thus, any device that, when the bias
is active, shunts bias current to the supply to keep it away from
the laser can be employed for the reasons stated herein. In one or
more embodiments, this could be a switch, or a diff. pair, any
active device, such as a FET or transistor, or any other device or
means to turn off the bias modulation. The purpose of such as
device is to establish the bias, which is a constant current to the
laser, when in burst off, to rapidly be effectively zero current to
laser. In certain example environments of use, such a TDM system,
any current leakage causes the laser to pollute the fiber, which in
turn reduces the SNR for other users. Using this innovation, the
system is able to maintain the bias on, but use the switch (or
other element) to rapidly shunt the current away from the laser to
keep the laser off. It is also contemplated that the Ibias current
258 could be turned off as a way to prevent current from being
presented to the optical module. This applies to both the bias
current and modulation current. Thus, any circuit may be used to
block the current into the laser, for instance any circuit that
steers the current (both modulation and bias) away from the driver
or any circuit that turns off both modulation and bias source.
[0054] FIG. 2B is a circuit diagram illustrating an exemplary
driver circuit with an alternative switch arraignment. As compared
to FIG. 2A identical elements are identified with identical
reference numbers. In this embodiment, a second switch S2 280 is
located across the outN node and the outP node a shown. By placing
the switch S2 in this location, the current and voltage across the
optical signal generator 256 may be rapidly shunted upon closing of
the switch S2. Operation of this embodiment is generally similar to
the operation of the embodiment of FIG. 2A.
[0055] FIG. 2C is a circuit diagram illustrating an exemplary
driver circuit with an alternative switch arraignment. As compared
to FIG. 2A identical elements are identified with identical
reference numbers. In this embodiment, a second switch S2 284 is
located between the Ibias current source 258 and inductor L1 252.
In other related embodiments, the switch S2 284 could be series
connected at any location in the path between node 242 and the
current source 258 to interrupt operation of the optical signal
generator 256. By placing the switch S2 284 in this location, the
current through the optical signal generator 256 may be rapidly
ceased upon opening of the switch S2. Operation of this embodiment
is generally similar to the operation of the embodiment of FIG. 2A,
except that the opening of switch S2 stops transmission from the
optical signal generator.
[0056] FIG. 3 is an operational flow diagram of an exemplary
protocol for switch control and driver operation. Discussion of the
operation of the system of FIG. 2 begins when the circuit is
initially in an off state, or a burst off mode. It is also assumed
that the capacitor 240 is charged as the method shown in FIG. 3 is
initiated, due to charging during prior burst mode periods.
[0057] At a step 304, the switches are set to a default position
during burst-off mode. In this embodiment, switch S1 is open,
switch S2 is closed, and switch S3 is closed. Establishing switch
S1 as open, prevents the charge on the capacitor 240 from passing
to ground and thus, the capacitor 240 remains charged. Setting
switch S2 to a closed position, shorts any voltage charge across
the light source thereby terminating any output from the light
source which may otherwise occur after the burst-on transmit
period. Optical signal transmission during a burst-off period is
unwanted because it interferes with other transmitters.
[0058] At a step 308 the system of FIG. 2 waits for, or monitors
for, a burst-on period. When to enter burst mode, or active
transmit mode, may be defined by a circuit or controller not shown
in FIG. 2. Enabling burst-on mode enables or causes the light
source 256 to generate an optical signal, such as during a transmit
period. A timing circuit or other control element may determine
when to enter burst mode. In addition, a burst-off signal, other
events, or periods may occur to enable or disable transmission by
the light source 256.
[0059] At a step 312, the controller receives instructions, such as
a trigger signal to initiate a burst-on period. In response, the
controller generates switch control signals. At a step 316, the
control signal is provided to switch S2 to enable the laser (light
source) and a short time thereafter, the controller provides a
control signal to switch S1 to close switch S1. Closing switch S1
couples the capacitor and Vdd to the driver. By opening switch S2,
a voltage may be established across the light source since thereby
enabling operation of the light source subject to the signal from
the driver. Forcing switch S1 closed after opening switch S2,
prevents the charge stored on the capacitor or charged storage
device (element 240 in FIG. 1) from discharging.
[0060] Next, at a step 320 the driver receives the modulation input
signals from the laser driver, which are the signals to be
converted to optical signals by the light source. Accordingly, at
step 324 the driver generates the drive signals for the light
source according the established principles for the driver and at
step 328 the drive signal is presented to the light source which in
turn generates the optical signal. The drive signal activation is
synchronized with action of switch S2, such that the drive signal
follows the same timing rule as switch S2, namely, the drive signal
is activated before closing switch S1 during transition from burst
off to burst on, and the drive signal is disabled after opening
switch S1 during transition from burst on to burst off. The
activity of steps 324 and 328 continue for the duration of the
burst-on period.
[0061] At a step 332 the end of the burst-on period or burst mode
is detected or signaled, such that a burst-off signal is provided
to the controller to terminate the transmit period. In turn, the
controller generates one or more switch control signals. At a step
336, prior to ending or at the time of ending the burst-on period,
a control signal is sent to open switch S1. By opening switch S1,
any charge accumulated on the capacitor (element 240 in FIG. 2),
remains accumulated on the capacitor and does not dissipate.
Burst-on mode may be rapidly re-initiated because the capacitor
need not charge prior to initiating transmission.
[0062] At step 340, to end the burst-on mode and enter burst off
mode, and directly after opening of switch S1, switch S2 is closed
thereby disabling the light source transmission. By shorting the
voltage across the light source with the closing of switch S2,
transmission may be immediately stopped, without the delay that
might otherwise occur if the voltage driving light source were
allowed to dissipate through the light source to ground.
[0063] Then, at a step 344, the system monitors for the burst-on
period or other instruction to transmit. The process repeats with
operation of the driver and light source during burst-on and
burst-off modes. Although defined in terms of burst-on and burst-of
modes, any type on time frame and off time frame may be used.
[0064] The innovation disclosed herein has many benefits over the
prior art. One such prior art reference is U.S. Patent Application
U.S. 20120201260. The system of this prior art reference does not
include the switches (S1, S2, S3) of the present application. In
particular, switch S1 is not part of this prior art system and
hence the prior art system lacks the benefit provided by the switch
in connection with the charge storage device along with the novel
methods associated with the switch system. As described above,
switch S1 make the transition period to fully active burst-on and
burst-off mode shorter because the system does not need to
charge/discharge the capacitance C or other charge storage device.
The system of U.S. Patent Application U.S. 20120201260 does not
address this technical problem (recovering fast in burst mode
operation).
[0065] In addition, switch S2 allows to completely shunt the laser
during (or other optical signal generator) burst off because the
laser is shorted, guaranteeing that the laser will not transmit
during this period (strict requirement in TDMA systems). As
compared to the system of U.S. Patent Application U.S. 20120201260
the solution disclosed herein is more reliable because the prior
art does not have a switch that is steering current away from the
laser through a differential stage and as a result, the
differential stage will leak current during burst-off periods.
[0066] In addition, in the system of U.S. Patent Application U.S.
20120201260 the termination resistance on cathode side (R2) is
parallel to the ferrite L2 in the prior art solution whereas the
termination resistance as disclosed herein is in series with the
capacitance and the switch S1. The solution disclosed herein better
isolates the capacitance of the larger bias source from the
laser.
[0067] Also, the system disclosed in U.S. Patent Application U.S.
20120201260 does not control VCC voltage whereas the present
innovation does control the supply voltage Vcc. Vcc control make
the output transistors (in particular Q2) operate in a safe region
resulting in no breakdown. Other benefits over the prior art are
also provided by the present innovation.
[0068] While various embodiments of the invention have been
described, it will be apparent to those of ordinary skill in the
art that many more embodiments and implementations are possible
that are within the scope of this invention. In addition, the
various features, elements, and embodiments described herein may be
claimed or combined in any combination or arrangement.
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