IE20020187U1 - Method for optimising the calibration process of a tuneable laser - Google Patents
Method for optimising the calibration process of a tuneable laserInfo
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- IE20020187U1 IE20020187U1 IE2002/0187A IE20020187A IE20020187U1 IE 20020187 U1 IE20020187 U1 IE 20020187U1 IE 2002/0187 A IE2002/0187 A IE 2002/0187A IE 20020187 A IE20020187 A IE 20020187A IE 20020187 U1 IE20020187 U1 IE 20020187U1
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- matrix
- points
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- 238000000034 method Methods 0.000 title description 20
- 239000011159 matrix material Substances 0.000 claims abstract description 45
- 238000005259 measurement Methods 0.000 claims description 11
- 238000003708 edge detection Methods 0.000 claims description 7
- 230000003287 optical Effects 0.000 claims description 7
- 230000015572 biosynthetic process Effects 0.000 claims description 3
- 238000005755 formation reaction Methods 0.000 claims description 3
- 238000004519 manufacturing process Methods 0.000 description 7
- 230000000694 effects Effects 0.000 description 6
- 238000005070 sampling Methods 0.000 description 5
- 230000000875 corresponding Effects 0.000 description 3
- 238000004806 packaging method and process Methods 0.000 description 3
- 230000001276 controlling effect Effects 0.000 description 2
- 230000001419 dependent Effects 0.000 description 2
- 238000004886 process control Methods 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 230000000916 dilatatory Effects 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 230000001702 transmitter Effects 0.000 description 1
Abstract
ABSTRACT A method relating to the calibration of tuneable lasers is described. The method provides for the provision of correct control currents so as to achieve each of desired output frequencies from the laser. The correct currents are determined by forming a matrix of the output characteristics of the laser at specific tuning currents and processing that matrix to determine stable operating points within the matrix.
Description
Method for optimising the calibration process of a tuneable
laser
Field of the Invention
The invention relates to tuneable lasers, particularly to a
multi section laser diode that can be switched between
different wavelengths or frequencies and more particularly
to a method adapted to provide for the correct control
currents to achieve each of the desired output frequencies
from the laser.
Background to the Invention
Multi section laser diodes are well known in the art and
can be switched between different wavelengths. Typically
the diode is calibrated at manufacture to determine the
correct control currents that should be applied to the
laser so as to effect the desired output frequencies from
the laser.
One of the first known multi—section laser diodes is a
three—section tuneable distributed Bragg reflector (DBR)
Other types of multi—section diode lasers are the
(SG-DBR),
laser.
sampled grating DBR the superstructure sampled
DBR (SSG—DBR) and the grating assisted coupler with rear
sampled or superstructure grating reflector (GCSR). A
review of such lasers is given in Jens Buus, Markus
and “Widely Tuneable Semiconductor Lasers” ECOC’00. Beck
Christian Amann, “Tuneable Laser Diodes” Artect House,
Mason.
Figure l is a schematic drawing of a SG—DBR 10. The laser
comprises of back and front reflector sections 2 and 8 with
an intervening gain or active section 6 and phase section
. An anti—reflection coating 9 is usually provided on the
front and rear facets of the chip to avoid facet modes. The
back and the front reflectors take the form of sampled
Bragg gratings 3 and 5. The pitch of the gratings of the
back and front reflectors vary slightly to provide a
Vernier tuning effect through varying the current supplied
to these sections. The optical path length of the cavity
can also be tuned with the phase section, for example by
refractive index changes induced by varying the carrier
density in this section. A more detailed description of the
SG—DBR and other tuneable multi—section diode lasers can be
found elsewhere Jens Buus, Markus Christian Amann,
“Tuneable Laser Diodes” Artect House, 1998.
Multi—section diode lasers are useful in wavelength
division multiplexed (WDM) systems. Example applications
are as transmitter sources, as wavelength converters in
(OXC’s)
heterodyne receivers. Typically, WDM systems have channel
optical cross connects and for reference sources in
spacing conforming to the International Telecommunications
(ITU) standard G692, which has a fixed point at 193.1
THz and inter—channel spacing at an integer multiple of
(DWDM)
Union
5OGHz or 100 GHZ. An example dense WDM system could
have a 50 GHZ channel spacing and range from 191 THz to 196
THz (1525 — 1560 nm).
As these are multi—section lasers they require some
calibration before use to determine the correct values of
current to achieve each of the desired output wavelengths
of the tuneable laser. For example an SG—DBR laser has 4
If each of these sections has a 300 possible
(0-90 mA in steps of O.3mA)
sections.
values of current and as each
of the sections of the laser are interdependent on the
output of the laser there are 300x300x300x3OO possible
combinations of current that can be applied to the laser.
|E020187
Added to this the laser must also meet the requirements for
line width, SMSR etc.. This means that the laser must be
calibrated and a lookup table of currents obtained where
each entry in the table consists of the currents required
to achieve each wavelength in the frequency plan. Each of
these entries are called operating points.
The manufacturing process of tuneable lasers is not fully
mature and each device will have its own unique wavelength
signature which means that each device is sufficiently
different to require a full calibration and data from
another laser will not work. This means that each device
must be fully calibrated to obtain the lookup table and
this table must be used with the device when in operation.
Several techniques to obtain this lookup table of
information have been published including “Fast Generation
of Optimum Operating Points for Tuneable SG-DBR Laser over
l535—l565nm Range” John Dunne et al. Conference on Lasers
and Electroptics (CLEO) Baltimore, May, 1999 pl47—148
proceedings, “Fast Accurate Characterisation of a GCSR
laser over the complete EDFA Band” Tom Farrell et a1.
LEOS’99 November, San Francisco, “Control of widely
tuneable SSG—DBR lasers for dense wavelength division
multiplexing” Gert Sarlet, G. Mbrthier, R. Baets J.
Lightwave Technol. vol. 18, no. 8, pp 1128-1138, August
2000, and also in patent WO OO/54380. The first publication
mentioned above also utilises a measurement set—up such as
that shown in Figure 2. The apparatus comprises a laser 600
which is controlled by current sources and a temperature
control element 610. The output of the laser is passed
through a first coupler 620, so as to provide a portion of
the output to a wavelength meter 630 and a second portion
to a second coupler where it is split further. A first
portion of the split light output is passed directly to a
IEMMB7
first photodiode (photodiode A), whereas the second portion
passes through a linear filter and the filtered signal is
then detected using photodiode B. By dividing the detected
voltage level at photodiode B (which is proportional to the
amount of light arriving at photodiode A) by the detected
voltage at photodiode A, a value is obtained that is
proportional to the wavelength of the light emitted by the
laser. Either the value measured by the wavelength meter or
the value of photodiode B divided by photodiode B can be
used as the wavelength of the light emitted by the laser.
While these methods offer solutions to the general concept
of calibration, they are over complicated as they involve
many operations and parameters (typical numbers for
conventional systems is anything between 10 and 20) to
guide the calibration process. Inevitably this leads to
several parameters that control the calibration and these
are sensitive to particular device structures and cannot
cope with device variation off a production line. Also
these parameters will often be interdependent leading to a
multidimensional space to set up the calibration where only
a small subset of the possible parameters will provide good
calibration results on the tuneable laser. Ideally the
calibration should have a small set of parameters that
greatly simplifies the calibration and its dependency on
particular device characteristics.
There is therefore a need to provide a method that enables
constant and accurate results to be obtained so as to
provide for a process control of the calibration process.
Object of the Invention
It is an object of the present invention to provide a
process control system for ensuring accurate calibration of
laser diodes.
Summary of the Invention
Accordingly the present invention provides a method of
calibration of a multi—section tuneable laser diode to a
specific frequency grid with a small set of parameters to
control this process. The methodology and technique of the
present invention is advantageous in that it is generic and
can be applied to several types of tuneable lasers such as
DBR, SG-DBR, SSG-DBR, GCSR etc..
This generic approach to the core calibration algorithm can
be used to reduce the number of calibration parameters to a
smaller set of values than heretofore possible.
Using the methodology of the present invention assumptions
are not made of the specific type of mode jumps that occur
in a laser and all are treated instead as regions of
instability where the laser should not be operated. The
present invention provides a technique whereby all possible
operating points of the laser are obtainable and it is
straightforward to obtain a quality parameter for each of
the operating points so as to ascertain its stability in
that operating region. This can then be used as a basis for
a screening method so as to effect a selection of operating
points that fall within a predefined criteria range.
The method of the present invention enables the performance
of the calibration to be checked without requiring the
determination of a full set of calibrations at all possible
calibration parameter values.
In accordance with a first embodiment of the present
invention a method of calibrating a multi—section tuneable
lEn2uze7
laser to a specific frequency grid is provided, the method
comprising the steps of:
forming a first discrete matrix of output values from
the laser, the matrix being defined by an optical
characteristic of the output of the laser at specific
determining tuning currents, and
processing the matrix so as to determine stable
operating points within the matrix, the stable operating
points defining specific frequencies where the laser may be
operated.
The invention additionally provides a method of calibrating
a tuneable laser, the method comprising the steps of:
forming a first discrete matrix of output value from
the laser, the output values being defined by values of at
least one optical characteristic of the output of the laser
as determined at specific tuning currents, and
processing the matrix so as to determine stable
operating points within the matrix, the stable operating
points defining specific frequencies where the laser may be
operated.
The output values from the laser are desirably measurements
indicative of the characteristics of the laser, the
characteristics being selected from one or more of the
following:
the output power of the laser,
the wavelength of the laser, or
the SMMR of the laser
the linewidth, or
some other laser characteristics.
Typically the first matrix is determined by measuring the
output values from the laser as a function of coarse tuning
currents of the laser.
’E"20137
The matrix typically may be viewed graphically as a plane
of values relating to the output power of the laser at
specific controlling tuning parameters.
Desirably the step of processing the matrix includes the
steps of:
defining regions within the matrix where an edge or
discontinuity is present, and
determining points which are bounded by
discontinuities or edges, the points determined
representing stable operating regions for the specific
tuning parameters.
The step of defining regions within the matrix where an
edge of discontinuity is present is desirably performed by
effecting an edge detection on the matrix values, the edge
detection effecting the formation of a processed matrix set
of values, the processed matrix set of values having values
indicative of whether an edge is present.
The edge detection is desirably effected by:
processing the matrix using a filter algorithm in a
direction substantially equivalent to the direction of mode
jumps of the laser output,
determining a set of maximum points within the
filtered matrix,
determining a set of minimum points within the
filtered matrix,
establishing a set of maximum and minimum pairs,
determining the difference between the maximum and
minimum of each pair so as to provide a plurality of
difference values, and
thresholding the difference values determined such
that those point pair values greater than a certain
’E”20:sz
threshold value are areas which are contributing to an edge
within the matrix.
The thresholding is desirably performed using the value of
the mode jump parameter as a threshold value. The value of
this may be determined by arbitrarily selecting a sequence
of values and selecting the value which provides the best
result.
Other measurements may additionally be made such as the use
of a coarse wavelength filter to convert wavelength changes
in the laser to power changes. As the power through the
filter is proportional to the input power and the
wavelength by dividing the input power into the power
output of the filter a value proportional to wavelength is
determined. Similar techniques described above or a simple
Laplacian operation and threshold can extract more edges
from the laser and result in an edge matrix which can be
combined to the previous matrix using a logical OR operator
to provide more robust results and guarantee the detection
of all edges. It will be noted that just using the coarse
wavelength filter measurement alone will not provide
comprehensive edges for all mode jumps of the laser, but
may be considered sufficient or satisfactory for certain
applications.
The step of defining stable operating points within the
matrix set of values is typically effected by performing a
distance map operation on the processed matrix set so as to
determine distances between adjacent edges and selecting
those points which are in the centre of the region bounded
by the edges.
The method may additionally provide the process of
determining whether each stable operating point obtained
'Enzaya7
represents the optimum stable operating point for that mode
includes the following steps:
dilating the set of points that are defined as
contributing to an edge in the edge matrix thereby forcing
the edges to join where gaps exist,
determining whether more than one operating point
resides in each dilated bounded region, and
if more than one operating point is found in the
region as above measure the frequency of the laser at these
points,
determining whether the difference between the
measured frequency of the laser and the mode jump spacing
is within a predetermined value and if it is within the
value averaging the plurality of operating points to
provide a single operating point within that bounded region
or if it is not within that predetermined value allowing
for a plurality of points within that region.
It will be appreciated that the mode jump spacing is a
physical characteristic of all such tuneable devices and is
dependent on the main Fabry Perot cavity set up in the
laser diode.
It will be further appreciated that each operating point
lies in a region bounded by dilated edges, and that if a
point is determined to be the sole occupier of such a
region and no other operating point lies in the same
bounded region then this operating point is optimised.
The method may additionally provide for a repetition of the
one or more of the preceding steps at different tuning
parameters so as to provide a plurality of matrices, each
matrix being indicative of a set of operating points for a
particular set of tuning parameters facilitating the
lE02fl?
linking of operating points from different matrices so as
to form a continuous tuning region.
The linking of points between two different matrices is
typically effected by joining points that meet the criteria
that:
a point from a first matrix and a point from a second
matrix are joined if the point from the second matrix has a
larger front and back current but these currents are within
a predetermined distance value of the two operating points.
This predetermined distance value is easily obtained from
the characteristics of the devices as it is dependent on
the tuning efficiency of the coarse wavelength tuning
sections of the laser. It can be approximated by selecting
a value greater than current on the front and/or back that
causes the laser to step from one wavelength to another.
Alternatively the frequency of each operating point may be
measured and those operating points that are adjacent and
have a frequency difference within a predetermined range
are joined. This it will be appreciated forms a tuning rate
parameter which defines the largest amount by which
frequencies can differ and yet still be joined.
In accordance with another embodiment of the present
invention a method is provided comprising the steps of:
a) effecting a measurement of a first set of operating
points for single phase current,
b) plotting this first set of operating points as a
function of a mode jump parameter,
c) repeating steps a) and b) for a plurality of phase
currents, and
analysing the resultant graph and determining a median mode
jump parameter that may be used for all planes of the laser
diode.
lEu2nv8?
These and other features of the present invention will be
better understood with reference to the following drawings.
Brief Description of the Drawings
Figure 1 is a schematic drawing of a known laser diode,
Figure 2 shows a measurement set—up for measuring
frequencies and power of a tuneable laser,
Figure 3 shows an output power plane of a laser diode as
measured according to two tuning currents,
Figure 4 shows a graphical representation of a matrix as
formed according to the method of the present invention,
Figure 5 is a graphical representation of a processed
matrix identifying operating points according to the
present invention,
Figure 6 is a graphical representation of the output of the
filter used to detect the mode jumps of the laser according
to a method of the present invention,
Figure 7 shows a sub set of the continuous tuning regions
of a laser as a function of phase current, and
Figure 8 shows a subset of the continuous tuning regions of
a laser as a function of the two coarse tuning currents and
the fine tuning current, Front grating, Back Grating and
Phase section respectively for an SG—DBR laser.
Detailed Description of the Drawings
The present invention provides a method of calibrating a
multi—section tuneable laser diode to a specific frequency
grid with a small set of parameters controlling the
process. The technique is generic and can be applied to
several types of tuneable lasers such as DBR, SG—DBR, SSG—
DBR, GCSR etc..
|E20‘i
The techniques used by the method of the present invention
are not based on assumptions with regard to specific type
of mode jumps that occur in the laser. According to the
present invention mode jumps are treated as regions of
instability where the laser should not be operated.
Furthermore all the possible operating points of the laser
are obtained along with a corresponding quality parameter
for each of the operating points to quantify it's
stability. Then a screening method can be implemented to
select operating points that meet a certain criterion.
In preferred embodiments of the present invention several
measurements are performed on the laser. These are
measuring the output power vs. all of the coarse tuning
currents, i.e. for a 4 section SG—DBR laser this results in
a plane. Figure 3 shows an output power plane of a laser
according to two tuning currents.
This plane can be obtained using a coarse wavelength
selective element using suitable techniques such as those
described in “Fast Accurate Characterisation of a GCSR
laser over the complete EDFA Band” Tom Farrell et al.
LEOS’99 November, San Francisco. These measurements are
repeated as a function of any other tuning sections that
the device may have. For example with an SG—DBR laser
several planes are measured of Front grating vs. Back
grating against phase current.
Each of these planes are then processed to obtain the
stable operating points available at that particular phase
current. This is performed by the following steps:
1. Perform an edge detection algorithm which will
produce a similar sized plane where the pixels are
turned on if there is an edge and off if the is no
”?W2o1presence of an edge at the corresponding location in
the plane. An example of such a plane is shown In
Figure 4
2. Perform a distance map operation on the edge map
3. Select the peaks of the edgemap
4. Join peaks in the edgemap which are closer together
than the sum of their two distance map values
The edge detection can be performed usually in two steps.
The first step is to pass a high pass filter such as those
1
or of the 2-D type 0 0 O
-1 -2 -1
of the type [—l -2 O 2 1]
across each plane in the predominant direction of the mode
jumps. Then, by following in the direction of the filter
and looking for a max. to min set of points in the line a
group of max./min and difference points can be obtained.
This is performed by looking for local maximum where the
point being considered is higher or equal to the points on
either side. Similarly local minima are located by looking
for a point that is less than or equal to the points on
either side.
Then in the same direction as the filter was passed through
the data it is possible to locate max. to min. pairs. For
each of these pairs we record the value at the max. and the
min and subtract. This is called the difference value. By
thresholding this value using the mode jump parameter we
allow max. to min jumps of a certain size to be accepted as
a edge. The selected mode jump parameter can be selected on
the basis of analysis of performance of one or more mode
jump parameters and using the parameter that results in the
optimum performance. This can be seen in Figure 4, which
shows an edgemap of a SG—DBR laser.
'En2n:s7
It will be appreciated that the mechanism performs equally
well on all edges that are obtained from mode jumps in
tuneable lasers.
If we calculate the effects of this filter on data [xO, xh
xmi, xng, .] using a filter having
[r_2IOI2I
X21 -- -r Xn—2/ Xn—lr Xnr
a filter value such as it will be appreciated
that the function:
f(n) : _xn—-2 _2xn—] +2xn+1 + xn+2
where jKn) is the result at n after the filter operation
is performed effects a resultant relationship:
f(n) = + (xm — xi] )]
This corresponds to the slope of the points around the
centre point i.e. the slope of points xmpxml and the slope
of x,,_2,xM as:
m _ xn+1_xn—l =
1_.__j_——:
(n+1)—(n—1)
x -X _
m1 = n+1 n] I
2
xn+2 _xn—2
By combining the two slopes it can be shown that:
xn+2 —xn—2 xn+l _xn—l
mf+m2=
IEQZDH7
m1+m2=5 2
{xn+2 _xn—2
It will be appreciated therefore that jKn)=40nf+n5) i.e.
the filter response is equal to four times the sum of the
two sets of slopes about the centre point.
The next step is to find the local max. and min i.e.
fmax (nm) = f (rzmx ), where f (nmax ) 2 f (nm j: 1)
and
fmm(’7min)=f(”mm)= Where f("mm)3f(”mm i1)
and the difference value d is
d=f@Am)—fimOmQ ,wmm I1 >n zmd n.—n sxmmmmw
D1111 max mm max
Therefore an edge is present when d:>Rw where Rm is the
mode jump parameter.
Figure 3 shows a power plane of an SG—DBR laser as a
function of the two coarse tuning currents (Front grating
and back grating). A line is taken from the plane in the
direction we are looking for mode jumps i.e. a fixed front
grating current. Figure 6 shows one of these lines after
passing the filter [—1,—2,0,2,l] across it. This will then
be thresholded using the techniques described above to
obtain all points contributing to edges.
The result of this when repeated for all front grating
current values is shown in Figure 4.
A distance map operation is performed next where D(x,y) is
the distance from (x,y) to the nearest edge. There are some
examples of how to perform this in “Control of widely
tuneable SSG—DBR lasers for dense wavelength division
|E020F
multiplexing” Gert Sarlet, G. Mbrthier, R. Baets J.
Lightwave Technol. vol. 18, no. 8, pp ll28—ll38, August
. By locating the distance map peaks i.e. where D(x,y)
Z D(xil,yil) it is possible to obtain the centre of the
regions bounded by edges. In many cases multiple local max.
peaks will be found and these can be merged using the
following rule.
Merge D(Xmax]’ymaxl)+D(xmaJa2=ymmc2) S \/(Xmaxl ~Xmax2 )2 +(ymaxl _ymax2 )2
Following this we obtain a set of points which are in the
centre of regions bounded by edges. These points are
operating points of the laser for that phase and gain
current, and are shown in Figure 5 which is the
corresponding edgemap to Figure 4 with the operating points
selected.
As a final check on the operating points gathered the edge
map can be dilated by one pixel and the broken into regions
bounded by mode jumps. If more than one operating point
found in the distance map is in any of the regions found
above then the frequency of the laser at these points
should be measured. Any points which have a frequency
difference of less than 10% of the mode jumps spacing is
the same point and they can be averaged to obtain only one
point. Points which have a large frequency difference are
allowed as they are different operating points.
The next stage after the above has been performed on all
measured planes is to join operating points that have
continuous tuning between them. Examples of the continuous
tuning lines are shown in Figures 7 and 8. This is
performed by taking the first plane (at the lowest phase
current) and joining operating points that meet the
following criterion.
REn2nw
A point from the lower phase current and a point from the
higher phase current where the higher phase current point
has larger front and back currents but these currents are
within the average distance map peak values of the two
operating points.
Alternatively the frequency of each operating point can be
measured using a wavelength meter in an known arrangement
such as that described in Figure 2.
As detailed previously the apparatus comprises a laser 600
which is controlled by current sources and a temperature
control element 610. The output of the laser is passed
through a first coupler 620, so as to provide a portion of
the output to a wavelength meter 630 and a second portion
to a second coupler where it is split further. A first
portion of the split light output is passed directly to a
first photodiode (photodiode A), whereas the second portion
passes through a linear filter and the filtered signal is
then detected using photodiode B.
By dividing the detected voltage level at photodiode B
(which is proportional to the amount of light arriving at
photodiode A) by the detected voltage at photodiode A, a
value is obtained that is proportional to the wavelength of
the light emitted by the laser. Either the value measured
by the wavelength meter or the value of photodiode B
divided by photodiode B can be used as the wavelength of
the light emitted by the laser.
The main advantage of using the photodiode value for
wavelength is that it is much quicker to measure than using
a wavelength meter by several orders of magnitude. Then
operating points that are adjacent and have a small
frequency (or wavelength as the speed of light divided by
the frequency equals wavelength) difference can be joined.
This provides a parameter “tuning rate parameter” which is
the largest amount the frequencies can differentiate by and
still be joined.
When all these points are joined for all phases, they
represent the continuous tuning regions of the laser. These
lines are then sampled in output frequency of the laser.
This is performed by setting a few points on each line on
the laser and measuring the output wavelength of the laser.
Then by interpolation the desired frequencies can be
calculated as the tuning of the laser is continuous between
the sample points.
This allows the generation of a lookup table of currents
where each entry in the table has the required currents on
each section that when set to the laser it will output
light at a desired wavelength of light. Typically this is
performed for the ITU G692 frequency specification but can
be used to generate any frequency plan required. The
advantages of this approach are that it is capable of
generating the lookup table using an automated calibration
system. This is extremely significant in the manufacture of
these devices as the test time required adds a significant
cost to the production of tuneable lasers. The ability to
generate the lookup table for a device in an automated and
fast method allows easy configuration of the diode to the
customers requirements and proof of conformance of the
laser to a specification.
Also this system can be used at different stages in
production of tuneable lasers to improve the yield at each
stage of production. For example packaging corresponds to
about 70% of component cost so when this system is used to
'Efl20as7
test the devices before packaging, a known good device will
only be allowed to be packaged where a known good device is
one that can achieve the desired frequency output range
required. This ensures that the packaging process is as
efficient as possible and improves the yield of the device.
It will be appreciated that as the method of the present
invention uses only the output power of the laser to
identify good stable operating points, that this method
provides a fast method of calibration. A frequency
reference is required to calibrate the laser to a desired
frequency plan and methods are described that allow fast
measurement of this. Also the use of components such as a
wavelength locker can be used at this stage to optimise the
speed of calibration.
It will be appreciated that the above description is
exemplary of the techniques as provided by the present
invention and it is not intended to limit the invention to
any specific filter technique or other parameter. The above
is provided for exemplary purposes only and it will be
appreciated that the method of the present invention can be
adapted and will perform equally well for any multi-section
tuneable laser.
l. Measure the output power of the device as a function
(e.g. DBR laser — Bragg
Section, SG—DBR laser Front grating and back grating
of the coarse tuning sections
sections, GCSR laser — Coupler and reflector sections)
. Find the edgemap or discontinuities in the measured
data
. Pick points which are in—between the edges found.
. Repeat steps 1-3 for different values of fine tuning
current (normally wither the phase section or
temperature of the diode)
. Measure the optical frequency of all the points found
and join points as a function of the fine tuning
current where the tuning is continuous
. Interpolate/ Curvefit the lines found in part 5 to
obtain the actual currents to achieve the desired
output frequencies of the device
. If output power control is desired steps 1 to 6 can be
repeated for different gain currents.
Then the final points obtained by step 7 can be
interpolated between to find the desired output power
for each output frequency.
From the above description it will be appreciated that
hereintobefore there has been no description of the
extension of the present invention to cover situations of
hysteresis. Some devices such as DBR’s and GCSR’s exhibit
hysteresis in the Bragg/Reflector section which means that
if the Bragg is ramped up and then down the mode jumps are
not in the same region. These regions cannot be used as
they are sensitive to the direction of approach and when
switching the laser from one operating point to another the
jump in current could be from either a higher or lower
Bragg current. This involves one additional step in that
the line in both directions must be measured but the region
in—between is easily identified as being stable when the
output power is the same in both directions. Then the mode
jumps can be identified and then algorithm can proceed as
defined above.
It will be appreciated that the present invention provides
for a method that reduces the number of parameters that are
required to effect a measurement of stable operating points
for a laser diode. The formation of a matrix representing
the output of the laser for specific tuning parameters and
'Efl2oza7
the selection of specific operating points within the
matrix which represent stable operating points for a laser
diode at specific operating currents enables a more
efficient calibration of the laser.
The words “comprises/comprising” and the words
“having/including” when used herein with reference to the
present invention are used to specify the presence of
stated features, integers, steps or components but does not
preclude the presence or addition of one or more other
integers, components or groups thereof.
features, steps,
Claims (1)
- Claims A method of calibrating a multi—section tuneable laser to a specific frequency, the method comprising the steps of: a) forming a first discrete matrix of output values from the laser, the matrix being defined by an optical characteristic of the output of the laser at specific determining tuning currents, and b) processing the matrix so as to determine stable operating points within the matrix, the stable operating points defining specific frequencies where the laser may be operated. The method as claimed in claim 1 wherein the output Values from the laser are measurements indicative of the characteristics of the laser, the characteristics being selected from one or more of the following: a) the output power of the laser, b) the wavelength of the laser, c) the SMMR of the laser d) the linewidth, or e) some other laser characteristics. The method as claimed in any preceding claim wherein the step of processing the matrix includes the steps of: a) defining regions within the matrix where an edge or discontinuity is present, and b) determining points which are bounded by discontinuities or edges, the points determined representing stable operating regions for the specific tuning parameters. and wherein the step of defining regions within the matrix where an edge of discontinuity is present is performed by effecting an edge detection on the matrix values, the edge detection effecting the formation of a processed matrix set of values, the processed matrix set of values having values indicative of whether an edge is present. A method of calibrating a tuneable laser comprising one or more of the following the steps of: a) measuring the output power of the device as a function of the coarse tuning sections b) determining an edgemap or discontinuities in the measured data, c) defining points which are in—between the edges found, d) repeating steps a—c for different values of fine tuning current, e) measuring the optical frequency of all the points found and joining points so as to form continuous lines, the lines being determined as a function of the fine tuning current where the tuning is continuous, f) interpolating the lines found in part e to obtain the actual currents to achieve the desired output frequencies’of the device, and g) repeating steps a—f for different gain currents if output power control is required, and the final points are interpolated to find the desired output power for each output frequency, ”Efi2@a scanning the laser across each of the lines found in step (e) while monitoring the output of a photodiode where part of the light of the laser is passed through a high finesse filter onto the respective photodiode, the high finesse filter is configured such that the peaks in its transmitivity are located at the desired frequencies to calibrate the laser whereby the output of the photodiode indicates light is present at the output of the filter such that the laser is at a required frequency for calibration. recording currents set on the laser in step (h) for each of the points to generate a lookup table. A method substantially as described herein with reference to
Publications (2)
Publication Number | Publication Date |
---|---|
IE20020187U1 true IE20020187U1 (en) | 2003-09-17 |
IES83362Y1 IES83362Y1 (en) | 2004-03-24 |
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