ACCURATE AGILE WAVELENGTH OPTICAL SOURCE AND USE THEREOF

SOURCE OPTIQUE DE LONGUEUR D'ONDE PRECISE ET AGILE, ET UTILISATION CONNEXE

English Abstract

This disclosure applies to an improvement on conventional optical transmitter
architecture which allows the
production of bursts of data signals with accurate selectable optical
frequency within the target optical band.
The invention overcomes the limitation caused when the monitor and feedback
circuits do not provide sufficient
control within the required time. A key aspect of this invention is the
ability to ensure the output frequency is
well controlled, even for rapid changes in output frequency, as in packet on
wavelength systems. Two
approaches are discussed: one for cases when the laser cannot be tuned within
an allowed switching time, and
one which applies when the laser can be tuned sufficiently quickly but
requires on-going stabilization. The
second approach may be used to improve the performance of the first approach.
The first approach, in which a
single laser cannot tune accurately within the required time, makes use of
multiple lasers implemented with an
appropriately fast optical switch to select the desired laser, each laser
being tuned and locked on the frequency
required for the packet it will address while an other is (or others are)
transmitting. In the second approach, in
which a single laser can be tuned to the nominal frequency during the
transition time, the implementation
improves the initial spectral quality and frequency accuracy on switching to a
new frequency through: -setting
the initial frequency as determined by a set of parameters in a table, -
improving the output quality during the
data transmission, -updating the table parameters based on the changes
necessary to improve the signal. The
second approach is particularly appropriate for rapid cyclical requirements
for a particular wavelength, as the
conditions for any wavelength may be nominally unchanged from cycle to cycle.
If the tunable lasers cover only
a fraction of the desired optical band, multiple lasers may be operated in
paralleled to provide full coverage of
the wavelength band, and combined with a fast optical switch, while being
controlled with either of the two
control approaches. This disclosure also applies to the use of such rapidly
tuned optical sources to provide data
units, frames or packets where each data unit can be of arbitrary frequency
within a set of operational
frequencies.

Claims

We claim:
1) An optical transmitter consisting of
a) a wavelength agile optical source which can provide an optical signal at
any of the desired wavelengths
and which can generate the optical signal to the required wavelength accuracy,
within the required time,
through the use of feedback signals. The agile source consisting of sufficient
number and disposition of
tunable sources to cover the wavelengths of interest and provide sufficient
redundant wavelength
coverage for stabilization operations. The various tunable sources being
operated nominally
sequentially to provide more time for adjusting the output wavelength to the
desired accuracy; the
alternative source or sources being set up while one of the sources is
transmitting data.
b) A set of parameters and an algorithm, used for determining the open loop
settings for a tunable laser,
where the parameters may be variable over time; determination based on the
modifications required to
maintain accurate output during the operation of the source.
c) a fast optical switch to select between the optical sources, the speed of
the optical switch being
sufficient for the intended use.
d) modulation means, by which data is impressed on the optical signal.
e) wavelength locking means for ensuring each source output either being set
prior to sending data or
transmitting data is controlled to the desired accuracy.
f) control means which selects the desired output signal to set the output
wavelength of the transmitter,
and which sets the output wavelength of the additional sources to the desired
wavelength within the
required frequency accuracy and within the required time based on feedback
signals from wavelength
referencing units and in response to system level management.
And used to send data units of various wavelengths

Description

CA 02339911 2001-03-07 '
MOL7VIltlOn:
Conventional long haul optical transmitters use a CW laser source coupled to
an external modulator to impress
the data on the light, while shorter reach systems directly modulate the
optical output by modulating the
pumping current. In both cases the sources produce light of a fixed, or nearly
fixed, optical frequency, which
must be accurately controlled for wavelength division multiplexing (WD11~
applications. 08en small changes
in operating frequency can be made by adjusting the driving current. In cases
where one desires a larger tuning
range, it is necessary to use so-called tunable lasers. These lasers are able
to tune over their accessible frequency
band in times ranging from nanoseconds to seconds, depending on the technology
used. While tuning rate is
almost irrelevant for many optical systems, there are proposals for systems in
which data units are transmitted
with wavelength coding: these systems require rapid transitions from one
frequency to another. Tuning rate is
not the only requirement on the devices however. It is necessary to ensure the
optical frequency is precisely on
the target frequency; typically the TTL1 optical gid. In general, the lasers
can not be set to the grid points with
sufficient accuracy over their operating life and conditions using 'open loop'
control. Wavelength lockers,
constructed of detectors and wavelength sensitive optical elements, are used
to provide feedback signals to
ensure the laser output is precisely aligned to the grid. These lockers
require suffi~ent signal, and finite locking
times, to operate. While some tunable laser technologies can change from one
optical frequency to another in a
few ns, the lockers generally require substantially more than 1 microsecond to
pull the laser precisely 'on-grid'.
Fast optical switching systems may require that transitions from one well
defined optical frequency to another be
achieved in much less than one microsecond. Limited frequency tuning range
also presents a problem, as not all
lasers can tune over the full optical bands of interest. This disclosure
describes methods for improving the
accuracy for rapidly switched wavelength applications while enabling full
range tuning.
Aonroach:
The first approach: Redundant sources:
A key aspect of one form of this invention is the use of more than one laser
to cover a given region of the optical
spectrum, where the format of the wavelength switching allows them to be used
in sequence. In general, if there
is a requirement for first (sub ms) switching from one wavelength to another,
the application will involve packet-
like transmission, and the output will be modulated emission at a stable
optical frequency for a period of time,
followed by a rapid transition to a modulated output at a different optical
frequency (though it could be the same

CA 02339911 2001-03-07
frequency) for a period of time, etc. If the tuning plus locking time for a
source is longer than the transition
time, but comparable to, while ideally less than, the packet time, then
installing multiple sources is useful.
Consider the case where a single laser can be tuned over the frequency range
of interest. If there is only one
laser, then it must be able to accurately make the transition from the first
frequency to the second frequency
during the transition time. If the laser cannot be locked onto the desired
frequency to sufficient accuracy within
that time the deviant output will ai~ect other optical signal channels or the
signal will undergo distortion in
traversing the optical network. To mitigate this one can add a second laser
and a fast optical switch. As the first
laser is transmitting a data unit at a particular optical frequency through
one channel of the optical switch, the
second laser may be set to the frequency required for the subsequent data unit
and stabilized. At the appropriate
time the optical switch inserts the second source, freeing up the first source
to be tuned for the next data unit
frequency. This process can go on as long as the locking time is less than one
data unit length. If the data units
are too short for the locker to stabilize the frequency during transmission of
one data unit more lasers can be
added, with each tuned to the appropriate packet in the stream. In cases where
the tunable laser cannot cover the
entire range of frequencies required, multiple lasers, each covering part of
the optical band, can be combined to
achieve fizll coverage. Multiplexing the lasers with an optical switch, rather
than a passive star coupler, allows
the laser to be 'set up' before the signal is required. Combining multiple
lasers for increased spectral coverage,
multiple instances of each of the lasers (sufficient to allow stabilized
tuning prior to the signal being required)
for redundancy in wavelength coverage and a fast optical switch, to select the
appropriate stabilized signal,
results in a wavelength agile signal source suitable for transmitting optical
packets of arbitrary colour in each
packet. Figure I shows an example for a system using an external modulator,
rather than directly modulated
tunable lasers, and shows 4 lasers within the transmitter. In the example
presented in figure l, the tunable lasers
are assumed to only reliably access ~ half of the desired band, and so two
lasers are required to access the entire
band. Since the lasers in figure I are assumed to lock within one packet
length, only two devices per wavelength
range are needed to maintain the packet by packet wavelength agility. Though
only two sources of the same
wavelength range are shown, since the locker is assumed to lock within one
frame or packet length, more
sources of each band could be used for longer locking times. The number of
wavelength locking systems
required depends on the locking times and the numbers of sources required, but
can range from two {one for the
transmitting channel and one for the channel being set up) to the number of
sources in the system (one locking
system for each tunable source). Multiple transmitters may be multiplexed
using star couplers and amplifiers to
increase the signal level if desired.
The second approach: Dynamic lookup system:
In this form of this invention, applicable to reducing the time required to
reach the required accuracy, the lookup
table used to set the nominal wavelength of a tuneable laser is modified
during operation to ensure the targeting
algorithm has the most up-to-date values. Accurate operation can be achieved
by first finding the appropriate
conditions during a 'set up' phase, prior to sending data on the transmission
fibre, and then operating the laser
open loop each time it starts to send a particular wavelength, based on the
previous 'set up' parameter values.
Refinement of the wavelength is done during the data transmission itself, with
new parameters for the optimum
open loop settings replacing the previous ones. This method is particularly
applicable to formats in which the
wavelengths used cycle repeatedly over a short period of time: each wavelength
state will follow the same
previous state, reducing any deviation due to patterning, and the rapid
repetitive nature ensures little change
required due to device drift. Cyclic operation allows additional processing to
predict the best start-up operating
point given the known operating pattern.
An example of the 'set up and modify' flow is presented (the example assumes
there is a period of time for set
up, when the output is not injected into the transmission fibre, and that the
requirement for the particular optical
frequency is periodic):
1 ) system request for a particular wavelength within a cycle of wavelength
requests
2) at the appropriate point in the first cycle requiring the wavelength (the
set up time), the transmission to the
fibre is blocked and the laser is operated close to the desired frequency
3) The laser wavelength accuracy is then improved during the packet time
4) The parameters for the highest accuracy are recorded
5) The output block is removed, the laser is switched to the next wavelength,
and the next, etc, until returning
to the wavelength of interest
6) The output is blocked if necessary, and the laser is set to the best
position using the previous parameters
7) Follow steps 3) to 6) until sufficient accuracy achieved and the data is
ready for transmission
8) Start transmitting data, making slight modifications to the drive
parameters during transmission to ensure
the best accuracy.

CA 02339911 2001-03-07
9) Record the parameters for best quality for use the next time (for example,
one cycle later) that wavelength,
or one sufficiently close, is required.
A combination approach:
As any tuneable source, such as the ones in the first approach, needs an
initial set-point from which optimisation
occurs, it makes sense to use the method of the second approach to increase
the accuracy of the initial value even
in the first approach, as it will speed up the process of achieving sufficient
accuracy.
Figure 2 shows an efficient way to combine the signals from multiple sources
onto a single fibre.
Such laser sources are useful in optical transmission systems which use
dynamic wavelength allocation.
Examples are optically packet switched networks, optical MPLS networks, or
high speed re-routing in circuit
switched systems.