The invention relates to the field of telecommunications, and in particular to a device of and a
method for optical data transmission in an optical access network.
Background
These days, access networks, also called passive optical networks (PON), are used, for
connecting a plurality of customers to a core network of data transportation.
In such an access network, the interconnection between the core network and the access
network is given at a so-called central office, which contains an optical line terminal (OLT).
The OLT is connected via at least one optical fiber, preferably called optical feeder fiber, to a
so-called remote node. At this remote node, an optical downlink signal transmitted by the OLT
is split onto different optical branches, to which one or more customers are connected by
means of optical network units (ONU).
The customers send via the optical branches optical uplink signals towards the remote node,
which combines these optical uplink signals to a combined uplink signal and transmits this via
the optical feeder fiber to the optical line terminal.
In order to determine the transmission properties of the transmission channel into which a
device transmits an optical signal, a measurement technique of optical time domain
reflectometry (OTDR) can be applied. In such an OTDR measurement, a reflection profile of
the transmission channel is estimated. Preferably, the technique of OTDR is carried out at the
OLT.
For the purpose of OTDR, a measurement signal in the form of an optical pulse may be
transmitted into the optical channel. The received response signal, which is reflected by the
optical channel, can then be traced in the time domain, for determining the reflection profile.
In a more advanced technique of OTDR, a measurement signal may be generated as an
optical signal carrying a correlation sequence. The received response signal is then recorded
and used for determining the reflection profile. The reflection profile can be determined, by
correlati ng a sam pled version of the received response signal with the initial correlation
sequence itself. In the case, that the auto-correlation function of the correlation sequence is
equal to or approximated by the dirac delta function, the result of the correlation yields an
estimate of the impulse response of the optical channel in the time domain, which is an
approximation of the reflection profile.
When transmitting an optical transmission signal carrying transmission data into the optical
channel using a transmission device, it is one possibility to carry out the technique of OTDR by
using a separate device. The transmission device and the separate OTDR device are in this
case both coupled to the same optical channel, preferably via an optical coupler. The optical
channel is preferably an optical fiber connected to the devices.
A more advanced technique is that of embedded OTDR, in which the transmission device itself
contains the hardware for generating the optical transmission signal as well as the hardware
that is necessary for carrying out an OTDR measurement. Preferably, the optical transmission
signal is directly modulated in dependence on a correlation sequence, wherein the frequency
of this direct modulation is chosen such that it does not disturb data reception at a receiving
side. After transmitting the optical transmission signal, carrying the directly modulated
measurement signal, into the optical channel, the response signal of the optical channel can
be obtained, by filtering out from received optical signal that frequency, at which the optical
transmission signal was modulated.
This received response signal can then be used for determining a reflection profile via the
technique of signal correlation as it has been described previously above.
Summary
It is an aim of the invention, to improve optical data transmission devices, which carry out
embedded OTDR measurement.
Proposed is an optical data transmission device for an optical access network. The device
comprises a laser transmission unit, which is operable to generate an optical transmission
signal. Furthermore, the device comprises a driving unit, which is operable to control the laser
transmission unit, such that the optical transmission signal is modulated in dependence on a
data signal, which is preferably an electrical data signal.
Furthermore, the device comprises an optical reception unit, which is adapted to convert at
least a fraction of a received optical signal into an electrical measurement signal. For this
conversion, the optical reception unit contains and makes use of a photo-diode and an
electrical amplifier, which are both an integral part of the optical reception unit. The optical
reception unit is furthermore separate from the laser transmission unit.
The device comprises furthermore at least one control unit, which is operable to control the
laser transmission unit, such that the optical transmission signal is modulated in dependence
o n a measurement signal.
The control unit is furthermore operable to measure multiple electrical measurement signals
during a measurement interval and to determine an averaged received electrical
measurement signal based on the measured electrical signals.
Even furthermore, the control unit is operable to control the electrical amplifier, such that the
electrical amplifier is turned off during at least a fraction of a time interval, which is prior to
the measurement interval .
In order to grasp the advantages of the proposed device, the following aspects have to be
taken into consideration.
When generating the optical transmission signal using the laser transmission unit, this optical
signal may have a fixed wavelength with a narrow line width, preferably called bandwidth.
The wavelength is fixed, in the sense that for a given constant temperature of the laser
transmission unit also the wavelength is constant.
When transm itting an optical signal with a narrow line and a fixed center wavelength into the
optical channel, this may result in coherent Raleigh noise present within the received response
signal. Such coherent Raleigh noise will then inevitably degrade the accuracy of the
measurement. One possibility to reduce the coherent Raleigh noise would be, to use an optical
transmission signal with a broader line width, but this will in turn cause limitations for the data
transmission due to the dispersion effect present within the optical channel.
Another counter measure for reducing the coherent Raleigh noise is achieved by the proposed
device in the following way:
By turning the electrical amplifier off during a time interval prior to the measurement interval,
and then turning the electrical amplifier on for the time of the measurement interval, the
electrical amplifier will generate thermal energy within the optical reception unit. This thermal
energy may then dissipate from the optical reception unit to the laser transmission unit, where
it then may cause a temperature change and thus also a change of the optical center
wavelength at which the laser transmission unit generates the optical transmission signal . The
change of the optical signal's center wavelength used for data transmission as well as OTDR
measurement will in turn reduce the impact of coherent Raleigh noise onto the measurement
during the measurement interval.
The proposed device has the advantage, that no extra thermoelectric cooler or thermoelectric
heater has to be foreseen within or directly at the laser transmission unit for changing the
temperature in order to change the center wavelength of the generated optical signal . Instead,
the thermal energy of the electrical amplifier can be used for this. This allows a higher degree
of integration within the optical transmission device. Since the electrical amplifier is already
present anyway within the optical reception unit for converting the received optical signal into
an electrical measurement signal, no extra component has to be foreseen within the optical
transmission device for generating thermal energy in order to cause a temperature change of
the laser transmission unit for achieving a change of the center wavelength. All that has to be
foreseen is, that the control unit is operable to turn the amplifier off during a time interval
prior to the measurement interval. Thus, no additional costs and no extra electronic board nor
optical sub module dimension increase are required. Even furthermore, no extra power supply
for any extra thermo electrical cooler or heater has to be foreseen. This even allows a higher
integration of the embedded OTDR system, preferably into a small factor pluggable optical
transceiver module.
Brief description of the Figures
Figure 1 shows a proposed optical transmission device according to an embodiment.
Figure 2 shows alternating time intervals of measurement and non-measurement.
Description of embodiments
Figure 1 shows an optical transmission device OTD, which is preferably used within an optical
line terminal of an optical access network.
The device OTD contains a laser transmission unit LD, which is preferably a laser diode.
Preferably, the laser transmission unit LD is a semiconductor laser, which may be in a
preferred embodiment a distributed feedback (DFB) laser. The transmission unit LD generates
an optical sig nal OS, which has a center wavelength and a line width. The optical signal OS is
transmitted into an optical sub assembly OSA. The optical signal OS passes an optical isolator
I as well as an optically partially transmitting device M and an optical filter F towards an
optical interface O IF, at which preferably a lens L is located. Instead of a lens L, shown in this
preferred embodiment of Figure 1, the optical interface O IF may contain a plug for
connecting an optical fiber, or as an even further alternative a so call fiber pigtail .
The optical isolator I transmits optical signals propagating from the transmission unit LD
towards the lens L, while it suppresses a transmission of optical signals propagating from the
lens L towards the transmission unit LD.
The optically partially transmitting device M is preferably an optically partially reflecting mirror,
which allows a greater fraction of the signal OS to propagate in the direction from the
transmission unit LD towards the lens L, while it reflects a smaller fraction of the signal OS
propagating in this direction. Furthermore, this mirror allows a greater fraction of an optical
signal to propagate from the lens L towards the transmission unit LD, while it reflects a smaller
fraction of such a signal propagating in this direction. As an example, 90 % of the energy of
an optical signal are transmitted by the optically partially reflecting mirror, while 10 %of the
energy of an optical signal are reflected by the optically partially reflecting mirror.
The optically partially transmitting device M is alternatively an optical wavelength division
multiplex (WDM) tap coupler. Such a tap coupler transmits optical signals in the direction of
the optical signal OSfor first predefined wavelength or wavelength range, while it suppresses
transmission optical signals in this direction for a second predefined wavelength or wavelength
range. Preferably, the f irst predefined wavelength is coincident with the wavelength of the
optical signal OS. Furthermore, the tap coupler transmits optical signals in the direction from
the lens Ltowards the transmission unit LD for the second wavelength or wavelength range,
while it directs optical signals coming from the lens L towards the reception unit ORX for the
first predefined wavelength or wavelength range.
The optical filter F allows optical signals to propagate from the transmission unit LD towards
the lens L. The filter F filters optical signals propagating from the lens L towards the filter F in
the way, that optical signals having the wavelength of the transmitted optical signal OS pass
the filter Ftowards the optically partially transmitting device M, while optical signals of a
different wavelength are reflected by the filter Ftowards an optical data reception unit ODRX.
Preferably, the optical transmission signal O S is a downlink signal with a wavelength of
1490 nm, while an optical uplink signal OUS, which is preferably a combined uplink signal,
received at the device OTD, preferably from other units of a network, has a wavelength of
3 10 nm.
The optical transmission device OTD is connectable at the lens L to an optical fiber OF, into
which the device OTD transmits the transmission signal OS and receives the optical uplink
signal OUS. The lens L can be seen as an optical interface of the device OTD.
The laser transmission unit LD is controlled by a driving unit LDU, such that the optical
transmission signal OS is modulated in dependence on an electrical data signal. The electrical
data signal is preferably received at the device OTD via a data interface DTX.
The control unit OC controls the laser transmission unit, LD, such that the optical transmission
signal O S is modulated in dependence on a measurement signal . The measurement signal is
preferably a digital electrical signal representing a correlation sequence.
The laser driving unit LDU provides an electrical bias signal to the transmission unit LD for
power adjustment as well as an electrical modulation signal for modulating the optical signal
OS in dependence on digital data received at the driving unit LDU. This data is preferably
received at the device OTD via a data interface DTX.
The laser driver LDU itself is controlled by a control unit MC, which is preferably a micro
controller, wherein the unit MC controls a working point of the driving unit LDU . Furthermore,
the unit MC controls the driving unit LDU, such that temperature compensation is carried out.
The modulation of the transmission signal OS in dependence on the measurement signal is
preferably carried out, by using as a correlation sequence a pseudo-random bit sequence.
This bit sequence has a pre-defined bit rate, which is preferably a bit rate of 10 MHz. The bit
sequence has preferably a length of 2 - l .
The device OTD receives from the fiber O F at the lens L a response signal ROS of the optical
channel. This signal ROS then passes the optical filter F. A fraction FROS 1 of this signal ROS is
reflected or suppressed by the optically partially transmitting device M towards an optical
reception unit ORX. A remaining fraction FROS2 of the signal ROS is transmitted by the
optical ly partially transmitting device M towards the isolator I, which suppresses this remaining
fraction FROS2, such that it does not reach the laser unit LD. Preferably, in the case that the
optically partially transmitting device M is a tap coupler, the signal fraction FROS1 has a
wavelength that is coincident with the first predefined wavelength or wavelength range, while
the signal fraction FROS 2 has wavelength that is coincident with the second predefined
wavelength or wavelength range.
The laser transmission unit LD is controlled by a control unit OC, such that the transmission
signal OS is modulated in dependence on a measurement signal . Within the optical reception
unit ORX, the fraction FROS1 of the received optical signal is converted into an electrical
measurement signal EMS. For this purpose, the reception unit ORX contains a photo diode PD,
which converts the signal fraction FROS1 into an electrical signal, as well an ampl ifier TIA,
which amplifies the converted electrical signal for obtaining the electrical measurement signal
EMS. The amplifier TIA is preferably a transimpedance amplifier. The amplifier TIA uses
preferably electrical energy provided by a power supply PS.
The modulation of the transmission signal OS in dependence on the measurement signal is
preferably an amplitude modulation . Furthermore, the modulation of the optical signal OS in
dependence o n the received data is also an amplitude modulation at a predefined bit rate
using on and off keying. This on and off keying is preferably carried out at a bit rate of 2.5
Gigabits per second.
The amplitude, by which the optical signal OS is modulated in dependence on the
measurement signal, is preferably within the range of 5 to 0 % of the optical signal's OS
maximal ampl itude.
Within a measurement interval, the measurement signal is modulated onto the optical signal
OS successively multiple times. Within this measurement interval, the optical reception unit
ORX thus measures multiple electrical measurement signals, by converting a fraction of the
received optical signal FROS1 to these respective electrical measurement signals. The multiple
electrical measurement signals are then provided to an OTDR control unit OC, which stores
these multiple electrical measurement signals and determines an average received electrical
measurement signal based on the measured electrical signals. This averaging may be an
unweighted or a weighted averaging. By this way of measurement including averaging of
multiple received measured signals, an increased accuracy of the measurement is achieved in
comparison to a way measurement consisting of only one single measurement.
The averaged received electrical measurement signal determined within the control unit OC
may then be provided via a data bus I2C to a device, not explicitly shown in Figure 1. Such a
device may the correlate the averaged measurement signal with the correlation sequence itself
for final ly determining a reflection profile of the optical channel. Alternatively, this correlation
may be carried out directly within the control unit OC.
The control unit MC controls the amplifier TIA, such that the amplifier TIA is turned off during
at least a fraction of a time interval prior to the measurement interval.
For this purpose, the control unit MC is connected to a power supply PS of the amplifier TIA via
an electrical connection. For turning the power supply PS on and off, the control unit MC
sends an electrical control signal CS, also shown as "TIA enable", to the power supply PS. By
turning on and off the power supply PS of the amplifier TIA, also the amplifier TIA is turned on
and off in dependence on the control signal CS sent by the control unit MC.
Alternatively, instead of turning on and off the power supply PS as a separate power supply of
the amplifier TIA, the control unit MC is connected electrically directly with the amplifier TIA,
wherein the amplifier TIA provides an interface, preferably a PIN-connection not explicitly
shown in Figure , at which an electrical control signal can turn the amplifier TIA directly on
and off.
As it has been described previously above, the amplifier TIA will generate thermal energy
when it is turned on. Thus, by turning the amplifier TIA on and off during different intervals of
measurement and non-measurement, the amount of thermal energy present within the
reception unit ORX can be varied. Furthermore, the thermal energy present within the
reception unit ORX may dissipate from the reception unit ORX towards the separate laser
transmission unit LD. Thus, by turning on and off the amplifier TIA during different intervals,
the amount of thermal energy present within the transmission unit LD due to thermal energy
dissipation can be varied. Thus, a change or variation of the center wavelength of the optical
transmission signal generated by the transmission unit LD can be achieved.
The reception unit ORX and the laser transmission unit LD are separate in the sense, that they
have respective casings that are not directly connected to each other. These casing may
preferably be indirectly connected to each other via a casing of an optical sub assembly
(OSA), which will be described in detail further below.
Figure 2 shows different time intervals of measurement and non-measurement.
Due to the fact, that a measurement of a reflection profile does not have to be carried out at
all times when operating the transmission device within the network, the amplifier for
measuring the electrical measurement signals within the reception unit has to be turned on
only during such time intervals, during which a measurement of the reflection profile shall be
carried out.
Within a measurement interval I , measurement is carried out and therefore, the amplifier is
turned on. During the time interval 0 of non-measurement, no OTDR measurement is to be
performed, and thus the amplifier is turned off during this time interval of non-measurement.
The amplifier does not have to be turned off during the whole time interval 10, but may
preferably be turned off only during a fraction the time interval 10 of non-measurement.
During the time interval II of OTDR measurement, preferably a number of 218 correlation
sequences are transmitted, such that an averaged electrical measurement signal is based on
2 individual electrical measurement sequences.
Preferably, the time interval II of measurement is followed by another time interval 12 of nonmeasurement,
which then is followed by a further time interval 3 of measurement.
During at least a fraction of the time interval 2 of non-measurement, the control unit controls
the amplifier, such that it is turned off during at least a fraction of the time interval 12.
During the time interval of measurement 13 the amplifier is turned on under control of the
control unit.
Coming back to Figure , the turning on and off of the amplifier TIA during the respective time
intervals, as previously described, may be carried out by a software task running on the
control unit MC.
While the control unit MC is preferably a micro controller, the control unit OC is preferably
implemented as an application specific integrated circuit (ASIC).
The control units MC and OC may be provided as separate control units or alternatively as
one control unit. The control units O C und MC may be implemented as a common control
unit o r separate control units relying on different combinations of hardware and/or software.
The optical subassembly OSA is optically connected to the laser transmission unit LD and the
optical reception unit ORX. Thus, the optical subassembly OSA is able to provide the optical
transmission signal O F to an optical interface OIF, preferably given by the lens L.
Furthermore, the optical subassembly OSA is able to receive the received optical signal ROF
from the optical interface OIF.
The optical subassembly OSA is preferably a hermetically sealed device with a casing CA. To
this casing, the transmission unit LD and the reception unit ORX are mechanically connected.
Furthermore, also an optical data reception unit ODRX is mechanically connected to the
subassembly OSA.
The optical sub-assembly may have a casing CA made of plastic. According to an alternative,
the casing CA of the subassembly OSA is a metallic casing, which provides the advantage of
allowing a faster thermal energy dissipation from the reception unit ORX towards the
transmission unit LD. This is especially of advantage in the case, that the reception unit ORX
and the transmission unit LD are both sealed in respective metallic cases CA1 , CA2, which are
welded to the casing CA of the subassembly OSA.
The transmission unit OTD contains preferably the optical data reception unit ODRX, which
receives data via the uplink signals OUS. The optical data reception unit ODRX may also
make use of a photo diode as well as an amplifier for deriving from the optical uplink signal
OUS received data, which then may be provided to a data interface DRX.
The proposed device discloses also a method of optical data transmission, as claimed in an
independent claim, which comprises different steps corresponding to the independent claim
claiming the proposed device.
The functions of the various elements shown in the Figure , including any functional blocks
labeled as' control unit', may be provided through the use of dedicated hardware as well as
hardware capable of executing software in association with appropriate software. When
provided by a processor, the functions may be provided by a single dedicated processor, by a
single shared processor, or by a plurality of individual processors, some of which may be
shared. Moreover, explicit use of the term 'control unit' should not be construed to refer
exclusively to hardware capable of executing software, and may implicitly include, without
limitation, digital signal processor (DSP) hardware, application specific integrated circuit
(ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software,
random access memory (RAM), and non volatile storage. Other hardware, conventional
and/or custom, may also be included.

Claims
1) Optical data transmission device (OTD) for an optical access network,
comprising
-a laser transmission unit (LD), operable to generate an optical transmission signal
(OS),
-at least one driving unit (LDU), which is operable to control said laser transmission unit
(LD), such that said optical transmission signal (OS) is modulated in dependence on a
data signal,
-an optical reception unit (ORX), adapted to convert at least a fraction of a received
optical signal (ROS) into an electrical measurement signal (EMS), using a photo diode
(PD) and an electrical amplifier (TIA), wherein said optical reception unit (ORX) is
separate from said laser transmission unit (LD),
- at least one control unit (MC, OC), operable to control said laser transmission unit,
such that said optical transmission signal (OS) is modulated in dependence on a
measurement signal,
wherein said control unit (MC, OC) is furthermore operable
to measure multiple electrical measurement signals during a measurement interval (I I )
and to determine an averaged received electrical measurement signal based on the
measured signals,
and to control said electrical amplifier (TIA), such that said electrical amplifier (TIA) is
turned off during at least a fraction of a time interval (10) prior to said measurement
interval ( 1 ).
2) Optical data transmission device (OTD) according to claim 1,
wherein said control unit (MC, OC) is connected to a power supply of said electrical amplifier
(TIA) by an electrical connection,
and wherein said control unit turns said electrical amplifier (TIA) off, by turning off said power
supply (PS) of said electrical amplifier (TIA) via said electrical connection.
3) Optical data transmission device (OTD) according to claim 1,
wherein said control unit (MC, OC) is connected to said electrical amplifier (TIA) by an
electrical connection,
and wherein said control unit turns off said electrical amplifier (TIA) via said electrical
connection.
4) Optical data transmission device (OTD) according to claim 1,
wherein said laser transmission unit (LD) and said optical reception unit (ORX) are optically
connected to an optical sub assembly (OSA), which is operable
to provide said optical transmission signal (OS) to an optical interface
- and to receive said received optical signal (ROS) from said optical interface.
5) Optical data transmission device (OTD) according to claim 4,
wherein said laser transmission unit (LD) and said optical reception unit (ORX) are
mechanically connected to said optical sub assembly (OSA).
6) Optical data transmission device (OTD) according to claim 5,
wherein said optical sub assembly (OSA) is encapsulated in a metallic casing.
7) Optical data transmission device (OTD) according to claim 1,
wherein said control unit (MC, OC) is operable to control said electrical amplifier (TIA) during
alternating time intervals of non-measurement and measurement, such that said electrical
amplifier (TIA) is turned off during at least respective fractions of time intervals (10, 12) of nonmeasurement.
8) Optical data transmission device (OTD) according to claim 1,
wherein said measurement signal represents a bit sequence with a predefined bitrate.
9) Optical data transmission device (OTD) according to claim 1,
wherein said driving unit (LDU) is operable to control said laser transmission unit (LD), such
that said optical signal (OS) is modulated in dependence on said data signal using on-off
keying.
10) Optical data transmission device (OTD) according to claim ,
wherein said control unit (MC, OC) is furthermore operable to provide a measurement data
signal representing said averaged received electrical measurement signal to a data bus (I2C
Bus).
11) Optical data transmission device (OTD) according to claim 1,
wherein said control unit (MC, OC) is operable to control a working point of said driving unit
(LDU).
12) Optical data transmission device (OTD) according to claim 1,
comprising furthermore an optical data reception unit (ODRX),
wherein said laser transmission unit (LD) is operable to transmit said optical signal (OS) at a
first wavelength,
and wherein said optical data reception unit (ODRX) is operable to receive data via the
received optical signal at a second wavelength, different from said first wavelength.
13) Optical data transmission device (OTD) according to claim 1,
wherein said control unit (MC, OC) turns said electrical amplifier (TIA) indirectly off, by turning
off a power supply (PS) of said electrical amplifier (TIA),
or wherein said control unit (MC, OC) turns said electrical amplifier (TIA) directly off, by
providing said control signal to said electrical amplifier (TIA).
14) Method of optical data transmissionn,
com rising
comprising furthermore,
measuring multiple electrical measurement signals during a measurement interval (N )
and determine an averaged received electrical measurement signal based on the
measured signals,
and controlling said electrical amplifier (TIA), such that said electrical amplifier (TIA) is
turned off during at least a fraction of a time interval (10) prior to said measurement
interval ( ),
using said control unit (MC, OC).