Coherent light generators – Particular component circuitry – For driving or controlling laser
Reexamination Certificate
2002-07-09
2004-12-21
Harvey, Minsun Oh (Department: 2828)
Coherent light generators
Particular component circuitry
For driving or controlling laser
C372S038100
Reexamination Certificate
active
06834065
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical network and, more particularly, a hardware implementation of the APON protocols, a direct digital drive of a laser diode, and measurement and control system for the direct digital drive of the laser diode.
BACKGROUND OF THE INVENTION
A passive optical network (PON) is an optical network that distributes signals to multiple terminal devices using passive splitters without active electronics, such as, for example, repeaters. Conventionally, signal delivery over passive networks uses a variety of transfer protocols, such as, for example, a synchronous optical network (SONET) or an asynchronous transfer mode (ATM) protocols. From time to time, the International Telecommunication Union (ITU) issues recommendations and standards for PONs under standard G.983.1, which standard is incorporated herein by reference. Generally, the present invention is described with relation to APON, asynchronous transfer mode passive optical networks and the associated protocols, but one of ordinary skill in the art would understand that the use of APON is illustrative of the present invention and the present invention could be used for other types of passive optical networks, for example, EPON.
FIG. 1
illustrates a conventional APON
100
. APON
100
could be either a fiber to the building (FTTB) or a fiber to the home (FTTH) network configuration. Generally, the PON system comprises an optical line terminal (OLT)
102
, at least one optical network unit (ONU)
104
, and at least one network termination (NT)
106
where an end user can access the system using, for example, a conventional computer, processor, or the like (not specifically shown). Connections
108
o
from the OLT
102
to the ONU
104
are fiber or optical connections and connections
108
e
from the ONU
104
to the NTs
106
are electrical connections. Depending on the number of ONUs
104
and NTs
106
, one or more optical distribution nodes (ODN, a.k.a optical splitters and combiners)
110
may be situated between OLT
102
and ONU
104
. Generally, ONUs
104
and NTs
106
reside at the end user or subscriber location (not specifically shown).
FIG. 9
illustrates a laser diode
902
using a conventional power source control system
904
. As shown, laser diode
902
emits useful light
906
and rear facet light
908
. Useful light
906
refers to light transmitted to connection
108
o
. A light monitor
910
, which could be a photodiode, a light meter, or the like, senses the intensity of rear facet light
908
. The intensity of rear facet light
908
corresponds to the intensity of useful light
906
. Substantially simultaneously with sensing the intensity of rear facet light, light monitor
910
supplies a light level feedback signal to a laser power controller
912
. Laser power controller
912
supplies a zero level current data signal
914
to a first programmable current source
916
. First programmable current source
916
supplies the current necessary to drive laser diode
902
at the light intensity that corresponds to a logic level zero. Laser power controller
912
also supplies a modulation current data signal
918
to a second programmable current source
920
. The modulation current data signal
918
determines the light intensity of the useful light output
906
. A modulation signal
922
is supplied to the gate of a bi-stable switch
924
to turn the switch on and off based on whether the useful light intensity
906
should be at the logic 1 or the logic 0 intensity. The bi-stable switch passes current from the programmable current source
920
to be summed with the current from programmable source
916
. The sum of the two currents drives laser diode
902
. The feedback signal to laser power controller
912
allows fine-tuning of the drive currents so the average intensity of the light signal remains within the protocol requirements for logic levels 1 and 0. These current vary widely from laser diode to laser diode ranging from as low as 2 or 3 milliamps to as high as 50 to 60 milliamps.
FIG. 10
is a diagrammatic representation of useful light intensity to drive current. In particular,
FIG. 10
shows transmission of an information cell
1002
. As is known in the art, cell
1002
is an ATM protocol for transmitting information.
FIG. 10
(and
FIG. 11
) does not actually represent transmission of a complete cell of information, but rather a short burst of information for convenience. Cell
1002
a
represents drive current for exemplary cell
1002
and cell
1002
b
represents light intensity for exemplary cell
1002
. As shown, cell
1002
a
can be considered in discrete parts
1004
a
and
1004
b
. Part
1004
a
is the drive current necessary for the transmission of light bearing information having an intensity of logic 1s and 0s. Part
1004
b
is the drive current for the transmission of light having an intensity of logic 0 to allow for a zero level measurement only; in other words, no information is being transmitted during the zero level measurement. The duration and timing of part
1004
b
is generally controlled by the associated transmission protocols. Similarly, light intensity shown by cell
1002
b
over the course of cell
1002
transitions between the high and low drive currents for the laser diode. As the diagram shows, because of difficulties in controlling the drive current for the laser diode, first logic pulse
1006
is typically wasted adjusting the drive current for the existing operating conditions and temperatures. Part of the difficulty of controlling current occurs because the lasing cavity needs to charge the photons sufficiently to begin emitting light. Also, when the photons in the lasing cavity are sufficiently charged to the threshold or knee level, the laser emits a burst of light and oscillates until the photons are properly charged and the laser is correctly operating above the threshold level.
As can be seen from
FIG. 10
, during non-transmission period
1008
, laser diode
902
is driven at a 0 current. Laser diode
902
is driven with a 0 current to inhibit the accidental transmission of light from laser diode
902
when laser diode
902
does not have a transmission grant. The drive current for a logic level 0 light intensity is some current greater than 0 current. Thus, one reason the first logic pulse
1006
is wasted is that time during the transmission of cell
1002
is required to charge the photons in the laser. While maintaining the laser drive current at the zero logic drive current (which is greater than 0 amps) would maintain the laser charged, it might allow for inadvertent light emission from the laser, which would cause interference with other transmitting lasers.
Transmission of a cell of information will be further explained with reference to
FIGS. 1-3
. Using ATM protocols, OLT
102
receives incoming cells
202
of information from a transport network or service node
112
destined for NTs
106
. For simplicity, this example has three cells of information ABC destined for three separate NTs
106
. The transport network could be any style network, such as the Internet, a Plain Old Telephone Service (POTS), digital video and/or audio streams. OLT
102
routes the incoming cell
202
over optical connection
108
o
through ODN
110
to three ONUs
104
1-3
. Using conventional protocols associated with APON, ONU
104
1-3
selects the data for its associated NT
106
and converts the optical signal to an electrical signal for distribution to the NT
106
over connection
108
e
. For example, ONU
a
selects data cell A from incoming cell
202
and converts that data into an electrical signal for NT
106
.
FIG. 3
shows the transmission of outgoing information from two NTs
106
4
and
106
5
, for example. NT
106
4
transmits an outgoing data cell D and NT
106
5
transmits an outgoing data cell E over connection
108
e
to ONUs
104
4
and
104
5
. The ONUs
104
4
and
104
5
converts the electrical signal to an optical signal for transmission to ODN
110
over connection
108
o
. ODN
110
combine
Carrier Access Corporation
Harvey Minsun Oh
Holland & Hart
LandOfFree
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