U.S. patent application number 14/312930 was filed with the patent office on 2015-12-24 for laser transceiver with improved bit error rate.
The applicant listed for this patent is Applied Optoelectronics, Inc.. Invention is credited to Yi Wang, Huanlin Zhang.
Application Number | 20150372763 14/312930 |
Document ID | / |
Family ID | 54870614 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150372763 |
Kind Code |
A1 |
Wang; Yi ; et al. |
December 24, 2015 |
LASER TRANSCEIVER WITH IMPROVED BIT ERROR RATE
Abstract
An optical transceiver generally includes an injection locked
(IL) laser configured to generate a transmit (Tx) optical signal
for transmission over an optical network and a laser driver circuit
configured to modulate the IL laser based on a Tx data signal. The
Tx data signal may be provided to the optical transceiver for
transmission over the optical network. The Tx data signal may
include a crossing point level associated with a transition between
a first signal level and a second signal level. The optical
transceiver may also include a crossing point control circuit
configured to apply distortion to the Tx data signal, the
distortion to increase the crossing point level.
Inventors: |
Wang; Yi; (Katy, TX)
; Zhang; Huanlin; (Sugar Land, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Applied Optoelectronics, Inc. |
Sugar Land |
TX |
US |
|
|
Family ID: |
54870614 |
Appl. No.: |
14/312930 |
Filed: |
June 24, 2014 |
Current U.S.
Class: |
398/81 ; 398/136;
398/193 |
Current CPC
Class: |
H04B 10/40 20130101;
H04B 10/504 20130101; H04B 10/58 20130101 |
International
Class: |
H04B 10/58 20060101
H04B010/58; H04B 10/40 20060101 H04B010/40; H04J 14/02 20060101
H04J014/02; H04B 10/50 20060101 H04B010/50 |
Claims
1. An optical transceiver comprising: an injection locked (IL)
laser configured to generate a transmit (Tx) optical signal for
transmission over an optical network; a laser driver circuit
configured to modulate said IL laser based on a Tx data signal,
said Tx data signal provided to said optical transceiver for
transmission over said optical network, said Tx data signal
comprising a crossing point level associated with a transition
between a first signal level and a second signal level; and a
crossing point control circuit to receive said Tx data signal, to
convert said Tx data signal into a first differential Tx data
signal and a second differential Tx data signal, and to apply a
pre-distortion by shifting levels of each of said first and said
second differential Tx data signals to increase said crossing point
level.
2. The transceiver of claim 1, further comprising a photodetector
configured to convert a received (Rx) optical signal from said
optical network to an electrical Rx data signal.
3. The transceiver of claim 2, further comprising a low pass filter
configured to reduce noise in said Rx data signal, said noise above
a cutoff frequency associated with said low pass filter.
4. The transceiver of claim 2, further comprising a decision
threshold circuit configured to adjust a threshold for determining
whether said Rx data signal corresponds to said first signal level
or said second signal level.
5. The transceiver of claim 1, further comprising a processor
configured to receive digital data and to control said distortion
applied by said crossing point control circuit, based on said
digital data.
6. The transceiver of claim 4, further comprising a processor
configured to receive digital data and to control said threshold
adjustment generated by said decision threshold circuit, based on
said digital data.
7. The transceiver of claim 1, wherein said IL laser generates said
Tx optical signal at a wavelength range associated with a filtered
broadband light source (BLS).
8. The transceiver laser of claim 1, wherein said transceiver is a
component of an Optical Networking Unit (ONU), said ONU conforming
to one of a Small Form Factor (SFF) or a Small Form Factor
Pluggable (SFP) transceiver size specification.
9. An optical networking unit comprising: an injection locked (IL)
laser configured to generate a transmit (Tx) optical signal for
transmission over an optical network at a transmission channel
wavelength, wherein the transmission channel wavelength is in one
of the L-band or the C-band; a laser driver circuit configured to
modulate said IL laser based on a Tx data signal provided for
transmission over said optical network, said Tx data signal
comprising a crossing point level associated with a transition
between a first signal level and a second signal level; a crossing
point control circuit to receive said Tx data signal, to convert
said Tx data signal into a first differential Tx data signal and a
second differential Tx data signal, and to apply a pre-distortion
by shifting levels of each of said first and said second
differential Tx data signals to increase said crossing point level;
and a photodetector configured to convert a received (Rx) optical
signal from said optical network to an electrical Rx data signal,
said Rx optical signal received at an Rx channel wavelength in one
of the L-band or the C-band.
10. The optical networking unit of claim 9, further comprising a
low pass filter configured to reduce noise in said Rx data signal,
said noise above a cutoff frequency associated with said low pass
filter.
11. The optical networking unit of claim 9, further comprising a
decision threshold circuit configured to adjust a threshold for
determining whether said Rx data signal corresponds to said first
signal level or said second signal level.
12. The optical networking unit of claim 9, further comprising a
processor configured to receive digital data and to control said
distortion applied by said crossing point control circuit, based on
said digital data.
13. The optical networking unit of claim 11, further comprising a
processor configured to receive digital data and to control said
threshold adjustment generated by said decision threshold circuit,
based on said digital data.
14. The optical networking unit of claim 9, wherein said optical
networking unit conforms to one of a Small Form Factor (SFF) or a
Small Form Factor Pluggable (SFP) transceiver size
specification.
15. A wavelength division multiplexed (WDM) system comprising: a
plurality of terminals associated with different respective channel
wavelengths and configured to transmit optical signals on the
different respective channel wavelengths, at least one of the
plurality of terminals including at least an optical transceiver
comprising: an injection locked (IL) laser configured to generate a
transmit (Tx) optical signal for transmission over an optical
network; a laser driver circuit configured to modulate said IL
laser based on a Tx data signal, said Tx data signal provided to
said optical transceiver for transmission over said optical
network, said Tx data signal comprising a crossing point level
associated with a transition between a first signal level and a
second signal level; and a crossing point control circuit to
receive said Tx data signal, to convert said Tx data signal into a
first differential Tx data signal and a second differential Tx data
signal, and to apply a pre-distortion by shifting levels of each of
said first and said second differential Tx data signals to increase
said crossing point level.
16. The WDM system of claim 15, wherein the plurality of terminals
include optical networking units (ONUs) in a WDM passive optical
network (PON).
17. The WDM system of claim 16, further comprising: at least one
optical line terminal (OLT) configured to receive aggregate WDM
optical signals including the channel wavelengths; at least one
branching point coupled between the OLT and the plurality of ONUs,
the branching point being configured to combine the optical signals
at the channel wavelengths; and a trunk optical path coupling the
OLT and the branching point.
18. The WDM system of claim 15, wherein said optical transceiver
further comprises a photodetector configured to convert a received
(Rx) optical signal from said optical network to an electrical Rx
data signal.
19. The WDM system of claim 15, wherein said optical transceiver
further comprises a decision threshold circuit configured to adjust
a threshold for determining whether said Rx data signal corresponds
to said first signal level or said second signal level.
20. The WDM system of claim 15, wherein said optical transceiver
further comprises a processor configured to receive digital data
and to control said distortion applied by said crossing point
control circuit, based on said digital data.
21. The WDM system of claim 19, wherein said optical transceiver
further comprises a processor configured to receive digital data
and to control said threshold adjustment generated by said decision
threshold circuit, based on said digital data.
22. A method comprising: providing an injection locked (IL) laser
configured to generate a transmit (Tx) optical signal for
transmission over an optical network; modulating said IL laser
based on a Tx data signal provided for transmission over said
optical network, said Tx data signal comprising a crossing point
level associated with a transition between a first signal level and
a second signal level; converting said Tx data signal into a first
differential Tx data signal and a second differential Tx data
signal; applying pre-distortion by shifting levels of the first
differential Tx data signal and the second differential Tx data
signal to increase said crossing point level; converting a received
(Rx) optical signal from said optical network to an electrical Rx
data signal; and adjusting a threshold for determining whether said
Rx data signal corresponds to said first signal level or said
second signal level.
23. The method of claim 22, further comprising low pass filtering
said Rx data signal to reduce noise in said Rx data signal above a
cutoff frequency.
Description
TECHNICAL FIELD
[0001] The present invention relates to laser transceivers, and
more particularly, to an injection locked laser transceiver with
crossing point adjustment circuitry for improved bit error rate for
use in a wavelength division multiplexed passive optical
network.
BACKGROUND INFORMATION
[0002] Optical communications networks, at one time, were generally
"point to point" type networks including a transmitter and a
receiver connected by an optical fiber. Such networks are
relatively easy to construct but deploy many fibers to connect
multiple users. As the number of subscribers connected to the
network increases and the fiber count increases rapidly, deploying
and managing many fibers becomes complex and expensive.
[0003] A passive optical network (PON) addresses this problem by
using a single "trunk" fiber from a transmitting end of the
network, such as an optical line terminal (OLT), to a remote
branching point, which may be up to 20 km or more. One challenge in
developing such a PON is utilizing the capacity in the trunk fiber
efficiently in order to transmit the maximum possible amount of
information on the trunk fiber. Fiber optic communications networks
may increase the amount of information carried on a single optical
fiber by multiplexing different optical signals on different
wavelengths using wavelength division multiplexing (WDM). In a
WDM-PON, for example, the single trunk fiber carries optical
signals at multiple channel wavelengths to and from the optical
branching point and the branching point provides a simple routing
function by directing signals of different wavelengths to and from
individual subscribers. At each subscriber location, an optical
networking terminal (ONT) or optical networking unit (ONU) is
assigned one or more of the channel wavelengths for sending and/or
receiving optical signals.
[0004] A challenge in a WDM-PON, however, is designing a network
that will allow the same transmitter to be used in an ONT or ONU at
any subscriber location. For ease of deployment and maintenance in
a WDM-PON, it is desirable to have a "colorless" ONT/ONU whose
wavelength can be changed or tuned such that a single device could
be used in any ONT/ONU on the PON. With a "colorless" ONT/ONU, an
operator only needs to have a single, universal transmitter or
transceiver device that can be employed at any subscriber
location.
[0005] One or more tunable lasers may be used to select different
wavelengths for optical signals in a WDM system or network such as
a WDM-PON. Various different types of tunable lasers have been
developed over the years, but most were developed for high-capacity
backbone connections to achieve high performance and at a
relatively high cost. Less expensive tunable lasers have been
developed, such as, for example the injection locked (IL) laser
which is seeded by a filtered broadband light source (BLS). The IL
laser is effectively tuned to the wavelength associated with the
pass band of the BLS filter. The IL laser, however, is typically
noisier than other, more expensive, tunable lasers and lacks the
linearity properties of those more expensive lasers. This can cause
distortion of the pulse width of the modulating signal which
results in an increased communication bit error rate (BER).
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] These and other features and advantages will be better
understood by reading the following detailed description, taken
together with the drawings wherein:
[0007] FIG. 1 is a top level schematic diagram of a wavelength
division multiplexed (WDM) optical communication system including
at least one transceiver, consistent with embodiments of the
present disclosure.
[0008] FIG. 2 is a schematic diagram of a wavelength division
multiplexed (WDM) passive optical network (PON) including at least
one transceiver, consistent with embodiments of the present
disclosure.
[0009] FIG. 3 is a schematic diagram of an optical transceiver with
improved bit error rate, consistent with embodiments of the present
disclosure.
[0010] FIG. 4 is a signal diagram illustrating an eye pattern,
consistent with an embodiment of the present disclosure.
[0011] FIG. 5 is a schematic diagram of a crossing point control
circuit of an optical transceiver, consistent with another
embodiment of the present disclosure.
DETAILED DESCRIPTION
[0012] A laser transceiver with improved bit error rate, consistent
with embodiments described herein, generally includes an injection
locked (IL) laser transmitter module with driver circuitry
configured to adjust the crossing point of the modulating RF signal
to reduce distortion during transmission. The transceiver may also
include a receiver module with a low-pass filter to reduce high
frequency received noise, and a decision threshold circuit
configured to lower the received signal decision threshold to a
level where noise is reduced. The adjustments of the crossing point
for transmission and the decision threshold for reception may be
adaptively set and/or updated by a microcontroller or other
processor based on operating characteristics of the system, such
as, for example, the type of IL laser being used.
[0013] The laser transceiver may be used in a wavelength division
multiplexed (WDM) passive optical network (PON). The transceiver
may be incorporated, for example, in an optical networking terminal
(ONT), optical line terminal (OLT) or optical networking unit (ONU)
of the WDM PON. The reduction of noise and distortion may lower
communication bit error rates and improve communication over the
optical network.
[0014] As used herein, "channel wavelengths" refer to the
wavelengths associated with optical channels and may include a
specified wavelength band around a center wavelength. In one
example, the channel wavelengths may be defined by an International
Telecommunication (ITU) standard such as the ITU-T dense wavelength
division multiplexing (DWDM) grid. As used herein, "tuning to a
channel wavelength" refers to adjusting a laser output such that
the emitted laser light includes the channel wavelength. The term
"coupled" as used herein refers to any connection, coupling, link
or the like and "optically coupled" refers to coupling such that
light from one element is imparted to another element. Such
"coupled" devices are not necessarily directly connected to one
another and may be separated by intermediate components or devices
that may manipulate or modify such signals.
[0015] Referring to FIG. 1, a WDM optical communication system 100
including one or more transceivers 102 with reduced bit error rate,
consistent with embodiments of the present disclosure, is shown and
described. The WDM system 100 includes one or more terminals 110,
111, 112 coupled at each end of a trunk optical fiber or path 114
for transmitting and receiving optical signals at different channel
wavelengths over the trunk optical path 114. Terminal 110 may be an
optical line terminal (OLT) while terminals 111 and 112 may be
optical networking units (ONUs). The terminals 110, 111, 112 at
each end of the WDM system 100 include one or more transceivers 102
which further include transmitters 120 (e.g., T.sub.X1 to T.sub.Xn)
and receivers 122 (e.g., R.sub.X1 to R.sub.Xn) associated with
different channels (e.g., Ch. 1 to Ch. n) for transmitting and
receiving optical signals at the different channel wavelengths
between the one or more terminals 110, 111, 112.
[0016] Each terminal 110, 111, 112 may include one or more
transmitters 120 and receivers 122, and the transmitters 120 and
receivers 122 may be separate or integrated as a transceiver within
a terminal. Optical multiplexers/demultiplexers 116, 118 at each
end of the WDM system 100 combine and separate the optical signals
at the different channel wavelengths. Aggregate WDM optical signals
including the combined channel wavelengths are carried on the trunk
optical path 114. One or more of the transmitters 120 may be a
tunable transmitter capable of being tuned to the appropriate
channel wavelength through injection locking based on seeding from
a broadband light source, as will be described in greater detail
below. Thus, the transmitters 120 may be constructed as universal
transmitters capable of being used in different locations in the
WDM system 100 and tuned to the appropriate channel wavelength
depending upon the location in the WDM system 100.
[0017] Referring to FIG. 2, an embodiment of the WDM optical
communication system of FIG. 1 is shown in greater detail. One or
more transceivers, consistent with embodiments of the present
disclosure, may be used to transmit and receive optical signals in
a WDM-PON 200. The WDM-PON 200 provides a point-to-multipoint
optical network architecture using a WDM system. According to one
embodiment of the WDM-PON 200, at least one optical line terminal
(OLT) 110 may be coupled to a plurality of optical networking
terminals (ONTs) or optical networking units (ONUs) 111, 112, . . .
, etc. via optical fibers, waveguides, and/or paths 114. The OLT
110 and the ONUs 111, 112 include one or more optical transceivers
102 configured to provide reduced bit error rates, as described in
greater detail below.
[0018] The OLT 110 may be located at a central office of the
WDM-PON 200, and the ONUs 111, 112 may be located in homes,
businesses or other types of subscriber location or premises. The
optical demultiplexer 118, or branching point, may be configured to
couple a trunk optical path 114 to separate optical paths to the
ONUs 111, 112, at the respective subscriber locations. The
branching point may include one or more passive coupling devices
such as a splitter or optical multiplexer/demultiplexer. An optical
multiplexer/demultiplexer, for example 116, 118, may be an arrayed
waveguide grating (AWG) configured to combine and/or split the
optical signals at the different respective channel wavelengths
(e.g., .lamda..sub.L1, .lamda..sub.L2, . . . .lamda..sub.Ln). In
one example, the ONUs 111, 112 may be located about 20 km or less
from the OLT 110.
[0019] One application of the WDM-PON 200 is to provide
fiber-to-the-home (FTTH) or fiber-to-the-premises (FTTP) capable of
delivering voice, data, and/or video services across a common
platform. In this application, the central office may be coupled to
one or more sources or networks providing the voice, data and/or
video.
[0020] In the WDM-PON 200, different ONUs 111, 112 may be assigned
different channel wavelengths for transmitting and receiving
optical signals. In one embodiment, the WDM-PON 200 may use
different wavelength bands for transmission of downstream and
upstream optical signals relative to the OLT 110 to avoid
interference between the received signal and back reflected
transmission signal on the same fiber. For example, the L-band
(e.g., about 1565 to 1625 nm) may be used for downstream
transmissions from the OLT 110 and the C-band (e.g., about 1530 to
1565 nm) may be used for upstream transmissions to the OLT 110. The
upstream and/or downstream channel wavelengths may generally
correspond to the ITU grid. In one example, the upstream
wavelengths may be aligned with the 100 GHz ITU grid and the
downstream wavelengths may be slightly offset from the 100 GHz ITU
grid.
[0021] The ONUs 111, 112 may thus be assigned different channel
wavelengths within the L-band and within the C-band. Transceivers
or receivers located within the ONUs 111, 112 may be configured to
receive an optical signal on at least one channel wavelength in the
L-band (e.g., .lamda..sub.L1, .lamda..sub.L2, . . .
.lamda..sub.Ln). Transceivers or transmitters located within the
ONUs 111, 112 may be configured to transmit an optical signal on at
least one channel wavelength in the C-band (e.g., .lamda..sub.C1,
.lamda..sub.C2, . . . .lamda..sub.Cn) based on seeding of the laser
as will be explained in greater detail below. Other wavelengths and
wavelength bands are also within the scope of the system and method
described herein.
[0022] One embodiment of the ONUs 111, 112 includes a transceiver
102 comprising an (IL) laser for transmitting an optical signal at
the assigned channel wavelength (.lamda..sub.C1) and a
photodetector, such as a photodiode, for receiving an optical
signal at the assigned downstream channel wavelength
(.lamda..sub.L1).
[0023] The OLT 110 may be configured to generate multiple optical
signals at different channel wavelengths (e.g., .lamda..sub.L1,
.lamda..sub.L2, . . . .lamda..sub.Ln) and to combine the optical
signals into the downstream WDM optical signal carried on the trunk
optical fiber or path 114. The OLT 110 may also be configured to
separate optical signals at different channel wavelengths (e.g.,
.lamda..sub.C1, .lamda..sub.C2, . . . .lamda..sub.Cn) from an
upstream WDM optical signal carried on the trunk path 114 and to
receive the separated optical signals.
[0024] Transceivers or transmitters located within the OLT 110 may
be configured to transmit an optical signal on at least one channel
wavelength in the L-band (e.g., .lamda..sub.L1, .lamda..sub.L2, . .
. .lamda..sub.Ln) based on seeding of the laser as will be
explained in greater detail below. Other wavelengths and wavelength
bands are also within the scope of the system and method described
herein.
[0025] The IL lasers of transceivers 102 may be modulated by RF
data signals to generate the respective optical signals. The lasers
may be modulated using various modulation techniques including
external modulation and direct modulation.
[0026] In one embodiment, one or more broadband light sources
(BLSs), for example a C-band BLS 232 and an L-band BLS 234, may be
configured to generate broadband light over a desired wavelength
range such as the C-band or the L-band, respectively. The broadband
light generated by module 232 and 234 may be coupled, by optical
coupler 236, into the trunk path 114 such that L-band seeding is
provided to the OLT 110 and C-band seeding is provided to ONUs 111,
112. C/L filter modules 230 may be provided in the path to each
transceiver 102 and configured to separate incoming C-band (or
L-band) wavelength light from outgoing L-band (or C-band)
wavelength light respectively. Thus, for example, the receivers 122
of each transceiver 102 of OLT 110 will receive the appropriate
C-band signal wavelength assigned to that receiver. Likewise, the
IL lasers of each transmitter 120 of each transceiver 102 of the
OLT will receive the appropriate L-band wavelength seeding signal
so that the IL laser may transmit at the assigned wavelength within
the L-band.
[0027] Similarly, for example, the receivers 122 of each
transceiver 102 of ONUs 111, 112 will receive the appropriate
L-band signal wavelength assigned to that receiver. Likewise, the
IL lasers of each transmitter 120 of each transceiver 102 of the
ONUs will receive the appropriate C-band wavelength seeding signal
so that the IL laser may transmit at the assigned wavelength within
the C-band.
[0028] Referring to FIG. 3, a transceiver with improved bit error
rate is described in greater detail. In some embodiments, the
transceiver 102 includes a transmitter component 330 (e.g., TX 120
of FIG. 1) and a receiver component 340 (e.g., RX 122 of FIG. 1),
either or both of which may be under the control of a processor or
micro-controller unit (MCU) 312, as will be explained below.
[0029] The transmitter component 330 may include an IL laser diode
306 configured to generate laser light in a desired wavelength
range for transmission over an optical network, for example the WDM
PON 200. The IL laser is considered to be a "colorless" laser
because it does not have a predefined lasing wavelength, but rather
it lases at the wavelength of an injected seeding light and may
lock onto the injected seeding light over a relatively wide range
of wavelengths. The laser diode 306 is seeded by a broadband light
source (BLS) 308 that is filtered by a WDM PON filter 310 which is
configured as a narrow band-pass optical filter. The BLS 308 may
emit light that covers a wide range of wavelengths. The filter 310
is configured to filter the light provided by the BLS 308 down to a
wavelength range that corresponds to the desired wavelength range
for the laser 306 and thus seeds the laser for transmission at that
wavelength. In some embodiments, the filter 310 may be a thin-film
filter or an array waveguide grating (AWG). The BLS 308 may
correspond, for example, to the C-band BLS 232 and/or the L-band
BLS 234 of FIG. 2. The WDM PON filter 310 may be incorporated, for
example, in the optical multiplexer/demultiplexer (e.g., AWG)
modules 116, 118 of FIG. 2.
[0030] Laser diode driver circuit 304 is electrically coupled to
laser 306 and may be configured to drive the laser by applying a
driving current to induce lasing. The laser driver circuit 304 may
modulate the laser 306 with an electrical signal that represents
the signal intended for transmission, Tx Data 332, which will
typically be provided as a radio frequency (RF) signal. The driver
304 thus causes the laser 306 to generate a modulated optical
signal for transmission at the desired channel wavelength. The
crossing point control circuit 302 may be configured to adjust the
waveform shape of the Tx Data signal 332 to improve the
transmission characteristics of the signal, as explained below.
[0031] The TX data signal 332 may be a binary signal (e.g., on-off
keying modulated signal) having an amplitude or voltage that
transitions between a first value associated with a logical `0`
signal level and a second value associated with a logical `1`
signal level, as illustrated in FIG. 4(a), which is commonly
referred to as an "eye" diagram. The signal shown in FIG. 4(a) is
relatively clean and symmetric (e.g., the crossing point being
approximately halfway between the two signal levels). In such a
case it may be straightforward to distinguish a `1` from a `0`
after transmission and reception of the modulated optical signal.
Unfortunately, due to the nature of the BLS 308, which is typically
an amplified spontaneous emission (ASE) light source, the light
from an IL laser 306 is generally noisier than the light produced
by a more expensive distributed feedback (DFB) or Fabry-Perot (FP)
laser. Additionally, the fabrication techniques for an IL laser may
result in a laser chip design having a longer dimensional length,
which may adversely affect the linearity of the IL laser. This
non-linearity may shift the crossing point of the eye diagram down
towards the `0` level causing pulse width distortion (e.g., the
pulse width of the `0` signal is different from the pulse width of
the `1` signal) resulting in communication errors (e.g., higher bit
error rates). FIG. 4(b) illustrates an example of such a noisier
and distorted signal.
[0032] The crossing point control circuit 302 may be configured to
adjust the waveform shape of the Tx Data signal 332, used to
modulate/drive the laser, by pre-distorting the signal to shift the
crossing point to a higher value or level. This pre-distortion may,
at least partially, compensate for the subsequent signal distortion
introduced by the non-linear characteristics of the IL laser. The
resulting transmitted optical signal may therefore have a crossing
point closer to the desired halfway point between the level `1` and
level `0` signals. The amount of pre-distortion may be controlled
by the MCU 312 and may depend on the characteristics of the
particular IL laser being used, for example measured or otherwise
known distortion, and/or any other relevant factors.
[0033] The receiver component 340 may include a photodetector 320
configured to receive an optical signal from an optical network,
for example the WDM PON 200. The received signal may also be a
binary signal (e.g., on-off keying modulated signal). In some
embodiments, the photodetector may include a trans-impedance
amplifier to provide an initial amplification of the received
signal before subsequent processing operations are performed. The
photodetector converts the received optical signal into an
electrical signal, which may, for example, be in the RF frequency
range. A low-pass filter 318 may process the output of the
photodetector 320 to limit the bandwidth of the received signal and
remove the higher frequency noise that may have been introduced by
the IL laser and/or the transmission through the optical network.
The low pass filter may have a cut-off frequency, above which noise
is filtered. In some embodiments, the cut-off frequency may be
fixed or adjustable.
[0034] A decision threshold circuit 316 may be configured to set a
threshold for determining whether the received signal represents a
logical `0` signal level or a logical `1` signal level. FIG. 4(b)
illustrates an example decision threshold 402. In the absence of
noise, distortion or other undesirable interference, the decision
threshold might be set to approximately 50 percent of the full
scale signal amplitude or approximately halfway between the
expected signal amplitude associated with a level `1` and a level
`0.` However, in practice, a lower decision threshold may improve
receiver performance since more noise is typically associated with
the `1` level due to the operating characteristics of the IL laser.
In some preferred embodiments, a decision threshold in the range of
approximately 20 to 30 percent of the full scale signal amplitude
(e.g., the expected signal amplitude associated with a level
`1`).
[0035] In some embodiments, the decision threshold may be
adaptively set in response to changing characteristics or
conditions of the transceiver system and/or the optical network.
The threshold may be set, for example, by the MCU 312.
[0036] In some embodiments, the decision threshold adjustment may
be performed as part of the post-amplifier circuit or module 314
which is configured to provide the received data signal RX Data 342
to the ONU or OLT as, for example, an RF signal in a desired
voltage range.
[0037] The crossing point control circuit 302 and decision
threshold circuit 316 may be under the control of a processor or
MCU 312 which may receive data/commands, for example over a digital
bus 350, from an external entity that is employing the transceiver
102. In some embodiments, the digital bus may conform to the
inter-integrated circuit (I.sup.2C) standard or the small form
factor (SFF) multi-source agreement (MSA) standard. For example,
the MCU may be configured to receive a request or instruction to
adjust the crossing point of the modulating transmit signal or
adjust the decision threshold of the received signal. In response
to that request, the MCU may generate the control signals necessary
to achieve these conditions and provide these control signals to
the crossing point control circuit 302 and/or the decision
threshold circuit 316. The MCU may operate based on software
execution/programming, firmware, hardware or any combination
thereof.
[0038] In some embodiments, the transceiver circuit 102 may conform
to the dimensions of the Small Form Factor (SFF) or a Small Form
Factor Pluggable (SFP) transceiver size specification. These
dimensions are set forth, for example, in the "Small Form Factor
Transceiver Multisource Agreement," dated Jan. 6, 1998, and the
"Small Form Factor Pluggable Transceiver Multisource Agreement,"
dated Sep. 14, 2000. It will be appreciated that the bit error rate
reduction techniques described herein, which enable the use of the
relatively less complex IL laser, allows for a decrease in size
(and cost) of the transceiver. This may contribute, at least in
part, to the ability to conform to the SFF/SFP specification.
[0039] Referring to FIG. 5, a schematic diagram of one example
embodiment of the crossing point control circuit 302 is shown in
greater detail. Differential driver circuit 502 may be configured
to convert the Tx Data signal 332 into a differential version of
that signal (e.g, Tx+ and Tx-) each of which is coupled to a bias
circuit 504 and 506 respectively. The MCU 312 provides bias balance
signals 508 to each of the bias circuits 504, 506 which may shift
the levels of each differential component such that the crossing
point of the resulting differential signal 510 may be adjusted to a
higher value.
[0040] Accordingly, an optical transceiver, with signal crossing
point control and decision threshold circuitry, consistent with
embodiments described herein, may provide communications with
reduced bit error rate over a WDM PON. The optical transceiver may
use a relatively inexpensive IL laser and may conform to a
relatively small form factor.
[0041] Consistent with one embodiment, an optical transceiver
generally includes an injection locked (IL) laser configured to
generate a transmit (Tx) optical signal for transmission over an
optical network and a laser driver circuit configured to modulate
the IL laser based on a Tx data signal. The Tx data signal may be
provided to the optical transceiver for transmission over the
optical network. The Tx data signal may include a crossing point
level associated with a transition between a first signal level and
a second signal level. The optical transceiver may also include a
crossing point control circuit configured to apply distortion to
the Tx data signal, the distortion to increase the crossing point
level.
[0042] Consistent with another embodiment, an optical networking
unit includes an injection locked (IL) laser configured to generate
a transmit (Tx) optical signal for transmission over an optical
network at a transmission channel wavelength, wherein the
transmission channel wavelength is in one of the L-band or the
C-band. The ONU also includes a laser driver circuit configured to
modulate the IL laser based on a Tx data signal, the Tx data signal
provided to the optical transceiver for transmission over the
optical network. The Tx data signal includes a crossing point level
associated with a transition between a first signal level and a
second signal level. The ONU further includes a crossing point
control circuit configured to apply distortion to the Tx data
signal, the distortion to increase the crossing point level. The
ONU further includes a photodetector configured to convert a
received (Rx) optical signal from the optical network to an
electrical Rx data signal, the Rx optical signal received at an Rx
channel wavelength in one of the L-band or the C-band.
[0043] Consistent with a further embodiment, a wavelength division
multiplexed (WDM) system includes a plurality of terminals
associated with different respective channel wavelengths and
configured to transmit optical signals on the different respective
channel wavelengths. At least one of the plurality of terminals
includes at least an optical transceiver. The optical transceiver
includes an injection locked (IL) laser configured to generate a
transmit (Tx) optical signal for transmission over an optical
network. The optical transceiver also includes a laser driver
circuit configured to modulate the IL laser based on a Tx data
signal, the Tx data signal provided to the optical transceiver for
transmission over the optical network. The Tx data signal includes
a crossing point level associated with a transition between a first
signal level and a second signal level. The optical transceiver
further includes a crossing point control circuit configured to
apply distortion to the Tx data signal, the distortion to increase
the crossing point level.
[0044] Consistent with yet another embodiment, a method includes
providing an injection locked (IL) laser configured to generate a
transmit (Tx) optical signal for transmission over an optical
network. The method also includes modulating the IL laser based on
a Tx data signal, the Tx data signal provided to the optical
transceiver for transmission over the optical network. The Tx data
signal includes a crossing point level associated with a transition
between a first signal level and a second signal level. The method
further includes applying distortion to the Tx data signal to
increase the crossing point level. The method further includes
converting a received (Rx) optical signal from the optical network
to an electrical Rx data signal; and adjusting a threshold for
determining whether the Rx data signal corresponds to the first
signal level or the second signal level.
[0045] While the principles of the invention have been described
herein, it is to be understood by those skilled in the art that
this description is made only by way of example and not as a
limitation as to the scope of the invention. Other embodiments are
contemplated within the scope of the present invention in addition
to the exemplary embodiments shown and described herein.
Modifications and substitutions by one of ordinary skill in the art
are considered to be within the scope of the present invention,
which is not to be limited except by the following claims.
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