IES83706Y1 - Method and apparatus for continuous sweeping of a tunable laser - Google Patents
Method and apparatus for continuous sweeping of a tunable laserInfo
- Publication number
- IES83706Y1 IES83706Y1 IE2003/0281A IE20030281A IES83706Y1 IE S83706 Y1 IES83706 Y1 IE S83706Y1 IE 2003/0281 A IE2003/0281 A IE 2003/0281A IE 20030281 A IE20030281 A IE 20030281A IE S83706 Y1 IES83706 Y1 IE S83706Y1
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- Prior art keywords
- laser
- wavelength
- regions
- continuous
- sweep
- Prior art date
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- 238000010408 sweeping Methods 0.000 title claims description 15
- 238000005259 measurement Methods 0.000 claims description 8
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Description
Method and Apparatus for continuous sweeping of a tunable
Title
laser
Field of the Invention
This invention relates to tunable lasers and the
requirement to sweep a range of wavelengths in a
continuous manner. This invention specifically relates to
a method to stitch two or more continuous tuning regions
of the laser together. Within the present specification
the term “continuous sweeping” means that there are no
discernable glitches or jumps in the output frequency of
the device outside of the smooth sweep across the
wavelength range_of interest.
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 controls that should be applied 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)
laser. Other types of multi—section diode lasers are the
sampled grating DBR (SG—DBR),
DER (SSG-DBR)
the superstructure sampled
and the grating assisted coupler with rear
sampled or superstructure grating reflector (GCSR). There
are also other laser types such as the External Cavity
(ECL)
Laser and gas lasers. A review of such lasers is
given in Jens Buus, Markus Christian Amann, “Tuneable
Laser Diodes” Artect House, 1998 and “Widely Tuneable
Semiconductor Lasers” ECOC’00. Beck Mason.
Figure 1 is a schematic drawing of a DBR 10. The laser
comprises of a Bragg reflector sections 2 with a gain or
active section 6 and phase section 4. An anti—reflection
coating 9 is sometimes provided on the front and/or rear
facets of the chip to avoid facet modes. The Bragg
reflector takes the form of Bragg gratings 5. The pitch
of the gratings of the Bragg reflector varies slightly to
provide a Bragg mode which moves in frequency through
varying the current supplied to this 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 DBR and other
tuneable multi—section diode lasers can be found
elsewhere Jens Buus, Markus Christian Amann, "Tuneable
Laser Diodes” Artect House, 1998.
As detailed above such tunable semiconductor lasers
contain sections where current is injected to control the
output frequency, mode purity and power characteristics
of the device. Various applications in
telecommunications/sensor fields require the laser to
sweep across a particular wavelength range in as
continuous a manner as possible. Moreover many
applications require that the range in wavelength that is
to be swept to be quite large, up to 80nm and higher.
However, certain types of tunable lasers have only a set
of narrow ranges of wavelengths over which they can be
continuously tuned. These individual continuously tunable
regions when put end to end will cover the full sweep
range of interest with some overlap.
It is desirable therefore to have a scheme by which each
of these continuous regions can be seamlessly stitched
together to give the appearance of continuous tuning.
Object of the invention
The object of the present invention is to provide a
method for stitching together continuous regions of a
multi—section tunable laser in an efficient and accurate
manner .
Summary of the Invention
Accordingly the present invention provides a method
adapted to identify for a predetermined wavelength the
end of a first continuously tunable region within the
range of operation of the laser and thereafter to once
again identify this same wavelength at the beginning of a
second continuously tunable region with high accuracy.
Desirably the first and second continuously tunable
regions are adjacent or overlap with one another. A
control signal can then be provided to highlight the
times when continuous tuning operation is being
performed. By limiting a suitable receiver to only make
measurements when the control signal is asserted, the
method of the present invention provides for a high level
of confidence in the continuous tuning behaviour in the
measurements. The methodology and technique is generic to
all types of laser devices and therefore can be applied
to several different types of tunable laser such as ECL,
DBR, SG-DBR, GCSR etc.
Desirably, once the first and second continuously tunable
regions are identified, a method of stitching them
together so as to form a usable tuning data set may be
provided.
According to first embodiment of the present invention a
method of providing a set of continuous tuning regions
from a discontinuously tuned laser is provided, the
method comprising the steps of:
providing a wavelength reference having at least
first and second resonance peaks,
sweeping the laser across a pre—determined
wavelength range of the wavelength reference, and
defining, within the laser sweep, one or more
regions of continuous tuning operation of the laser, each
of the regions corresponding to a response of the laser
between adjacent resonance peaks of the wavelength
reference.
Desirably, the one or more regions of continuous tuning
operation are adjacent to one another. Alternatively, the
one or more regions of continuous tuning operation are
displaced from one another across the pre-determined
wavelength range. In such an alternative the regions may
be chosen randomly within the pre—determined wavelength
range.
The method may further comprise the step of:
stitching two or more regions to one another so as
to form a usable tuning data set.
Such stitching the two or more regions to one another is
desirably effected by:
a. creating a lookup table of regions that have
continuous tuning over a first frequency region
with a frequency overlap on either side with the
previous and next continuous tuning regions,
b. asserting a control signal to denote a continuous
region when the first resonance peak is detected,
c. de—asserting the control signal and jumping to
the next continuous tuning region when the next
resonance peak is found within this continuous
tuning region, and
d. repeating the above steps (b—c) until a
sufficient range of wavelength has been swept.
The regions of continuing tuning operation of the laser
are desirably defined by:
calibrating the laser so as to provide a range of
currents with no mode jumps,
selecting continuous regions with a first frequency
overlap that have a resonance peak in the
wavelength response from their beginnings and
ends, and
setting the currents whilst sweeping through those
wavelengths so as to provide a smoothly
transitioning wavelength sweep.
The step of setting the currents is typically provided by
one or more of the following:
filtering,
shaping.
In preferred embodiments the method may include the
additional step of assigning a frequency (Egfis) or
wavelength ( kmws) value to discrete points within the
continuous region of operation of the laser device, the
value being assigned by:
measuring the time from the resonance peak at the
beginning of the sweep to the measurement instant
(Tmeas) ,
measuring the time required to sweep between
adjacent resonance peaks (Tsqmam), and
calculating the value by extracting a value for Tmws
from Tsegment
For application of a frequency value and using an etalon
as a wavelength reference the method typically includes
the step of calculating the frequency using the equation:
F p‘1xSR * 17neas
meas‘ ‘
lzlalrm Segmemstarz '
T segment
where FSRBmn is the free spectral range of the
reference etalon and F‘ is the absolute
SegmculSlarI
frequency of the first resonant etalon peak in the
segment.
The laser device may be used as a reference source for a
second device. Such an application may use the regions of
continuous tuning operation to define the spectral
characteristics of a second laser device, or to provide
an optical spectrum analyser.
The wavelength reference is desirably provided by one or
more of the following:
a fabry perot etalon,
a gas cell,
fabry bragg grating,
notch filter,
a reflective fabry perot etalon, and
optical filter.
Any portion of the resonance peak may used to determine
the location of the resonance peak.
The invention additionally provides an optical
arrangement adapted to provide a set of continuous tuning
regions from a discontinuously tuned laser, the
arrangement comprising:
a wavelength reference having at least first and
second resonance peaks associated therewith,
a tunable laser,
means for sweeping the laser across a pre—determined
wavelength range of the wavelength reference, and
means for defining, within the laser sweep, one or
more regions of continuous tuning operation of the laser,
each of the regions corresponding to a response of the
laser between adjacent resonance peaks of the wavelength
reference.
These and other features of the present invention will be
better understood with reference to the following
drawings.
grief Description of the drawings
Figure l shows a schematic of a Distributed Bragg
Reflector Laser diode.
Figure 2 shows a schematic of an embodiment of the system
in a typical application.
Figure 3 shows how a continuous sweep may be obtained
from a mode plane.
Figure 4 is an example of the output of the high finesse
etalon in one of the continuous sweep regions of the
laser.
Figure 5 is an example of the spectral characteristics of
a fabry—perot filter as used in the methodology of the
present invention.
Figure 6 is a plot of frequency vs. time for a system
utilising the method of the present invention.
Figure 7 is a schematic of a second application of the
methodology of the present invention.
Detailed Description of the Drawings
The invention will now be described with reference to
exemplary embodiments thereof and it will be appreciated
that it is not intended to limit the application or
methodology to any specific example. The techniques used
by the method of the present invention are specifically
provided to enable the formation of a continuous sweep
from a discontinuously tuned laser. A continuous sweep is
taken to be one which has no discernibly or substantial
glitches in frequency along its length, only a smooth
transition across the full wavelength range.
The methodology of the present invention will now be
described with reference to a three section DER device,
and it will be appreciated from a person skilled in the
art that this is only exemplary of the type of device
that may be used with the method of the present
invention. It will be further appreciated that the
technique and methodology herein described is a generic
technique being applicable to all narrow and widely
tunable lasers such as the SG—DBR, SSG-DBR, DBR and GCSR
types.
Figure 2 shows a typical embodiment of the system for a
typical application, the system shown enabling the
spectral characterisation of the device under test. A
tunable laser and control electronics 300 provide the
optical output where the output wavelength and power of
the laser are controlled by the electronics. The optical
power from the laser is passed through a 3—way splitter
310. Part of the light goes to a wavelength reference
340,
test
a direct power measurement 350 and to a device under
(DUT) 320. Three receivers are provided ( 370) to
monitor the wavelength reference power, direct power and
the DUT power. These are typically measured with photo-
detectors 331, a, b, c and a trans—impedance amplifier
a, b, c and the voltages are passed back to the laser
and control electronics 300. Connector blocks or
interface units 390 are also provided to enable the
removable coupling of the DUT, one unit coupling it to
the 3-way splitter and the second to the corresponding
receiver 360 for the DUT. In operation such an embodiment
>—'
(D
of the invention provides for the analysis of the
spectrum of the device under test by comparison with the
control laser/wavelength reference combination.
This application to an accurate wavelength spectral
characterisation is only one exemplary embodiment of the
invention. Other embodiments may include the use of the
methodology within an interrogators or optical spectrum
analyser.
To achieve the high accuracies required a wavelength
reference is required that can be used in conjunction
with the laser. In this exemplary technique, a Fabry
Perot etalon is used but it will be appreciated that
other references can be used which provide a similar
The Fabry Perot (FP)
(>30).
will be less dependent on output power variations of the
characteristic. filter that is used
desirably has a high Finesse A high finesse filter
tunable laser. The spectral characteristics of such a
filter is shown in Figure 4. It will be apparent, as
shown in Figure 5, that such a filter has at least two
resonance peaks, each being associated with a high level
of transmissivity at specific frequencies. Between these
peaks the filter exhibits low levels of transmissivity.
This means that there is a large extinction ratio between
the resonance peaks and away from the resonance peak.
This also means that the spectral width of the filter is
small (<5pm). As the laser sweeps across the wavelength
ranges, when the wavelength coincides with a resonance
peak a response is obtained on a receiving photodiode. As
the Free Spectral Range (FSR and the start frequency of
the FP filter is well know and an approximate frequency
of the laser is known (from, for example, an initial
calibration of the laser) when the laser wavelength is at
a resonance of the FP filter the wavelength of the laser
can be determined very accurately (to much better than
O.lpm resolutions). This enables high accuracy of the
laser source to be determined at the wavelength reference
peaks as the wavelength at each peak is known accurately.
and the laser is calibrated so that in—between each
resonance the wavelength tuning is linear in time,
therefore the wavelength at any point between two such
resonance's can be referenced from the time taken for the
laser from the first resonance to the second resonance,
i.e.
kl: Wavelength of the first resonance
%4+FSR= wavelength of the second resonance
T: time to sweep from X1 to the second resonance peak
K: time to reach a wavelength between the two resonance’s
X: wavelength at time K
Therefore
X=K/T*FSR+k1
Another example of a suitable wavelength reference would
be a gas cell. It will be understood that a gas cell will
provide a multitude of reference peaks to reference the
tunable laser while sweeping, and while these peaks are
not typically displaced regularly from one another, the
use of such a cell is advantageous in that it is less
susceptible to pressure or temperature effects and can
therefore serve as a more accurate reference than for
example the fabry—perot etalon. In a further example an
etalon could be referenced to a gas cell, so as to
provide an absolute reference and then the substantially
regular intervals between the resonance peaks of the
fabry—perot could be used in combination with the tunable
laser in the method of the invention.
As can be seen from Figure 3 a continuous tuning region
is obtained by finding a line which crosses no mode
boundaries 210. This is shown as a black line 200 in a
phase, bragg plane, where the degree of shading denotes
the output wavelength of the laser. By choosing a region
to sweep the currents that do not cross a mode jump of
the laser continuous tuning is achieved. This can be
accomplished in a number of ways as reported in “Complete
Wavelength Control of a GCSR Laser over EDFA Band” IEEE
LEOS’99, San Francisco, Nov’99, T. Farrell et al. This
results in a set of continuous tuning regions and a
subset of these is taken which cover the wavelength range
of interest.
Figure 4 shows the current on the phase section of the
laser vs. the output frequency of the laser. It will be
appreciated that similar plots may be obtained for the
other sections of the laser which must be controlled in a
similar manner to achieve the continuous tuning.
Specifically, a similar control current function is
required for the Bragg section in the case of a 3 section
DBR laser,
of the SG—DBR laser.
and of the front and back reflector sections
If there is a requirement to keep
the output power of the laser constant during the sweep
the gain section can also be controlled to compensate.
Use of a Digital to Analog Converter (DAC) which will
step the currents at the required resolution, can be used
to implement this, as will be appreciated by those
skilled in the art. Also an electronic filter can be used
to smooth the currents to achieve smooth continuous
tuning of the laser source.
Once a series of these continuous sweeps has been
identified, a set of continuous regions can be selected
each of which overlap by a predetermined frequency,
typically 10 GHZ, and each of which starts and end 10 GHZ
from one of the peaks of the high finesse etalon. Each
continuous tuning range of the laser will sweep the
wavelength of the laser across two different peaks, so
that all the continuous tuning ranges cover all the gaps
between reference peaks in the wavelength range of
interest. The laser may then be swept through each of
these continuous regions. In operation, the wavelength
reference of the filter is used to identify the regions
of the sweep that are used. This is performed by using a
control signal which is asserted when the sweep hits a
first wavelength reference and de—asserted when the sweep
hits a second reference. A control signal can be
generated by use of a threshold operation on the response
of the wavelength reference. When the light level is
above a threshold value, this signifies that the
wavelength of the laser is at the wavelength reference
wavelength of that peak. This can be implemented with the
use of a comparator and some simple logic elements. A
reference is identified when the optical power received
through the reference is above a threshold. This is
explained in more detail in the following sections.
When sweeping the laser across one continuous tuning
region the sweep will cross two wavelength reference
peaks, equivalent to the two peaks of the etalon. Upon
the wavelength of the laser hitting the first wavelength
reference peak the control signal is asserted. This means
that the laser is now sweeping linearly across a
wavelength range between two reference wavelengths. Upon
hitting the second etalon peak in this continuous sweep
it de—asserts the control signal associated with the
continuous regions of the sweep and starts sweeping along
the beginning of the overlapping region of the next
continuous region. Then the next continuous wavelength
tuning region is used. During this sweep it once again
detects the peak_of the wavelength reference it will know
it is at exactly the same frequency as the end of the
last sweep and can reassert the control signal thereby
providing seamless continuity in the tuning with no gaps
or overlaps in wavelength in the measurements.
Figure 6 shows a plot of Frequency vs. Time for the
system with measurements only being taken when the
control signal is asserted. The gaps in the response
allow for the sweeping of the laser in regions where the
control signal is not asserted and therefore not used. It
will be appreciated that although there are breaks in
between each of the sets of continuous regions that these
breaks represent time delays and that the end point of
one set and the start point of a second set are at the
( as could be seen from the intercept of
line 430). It will be
same wavelength
both points against the Y-axis,
also appreciated that time can be allowed for the laser
to reach thermal equilibrium between each continuously
tunable region.
As such it will be appreciated that Figure 6 represents a
reference spectrum for the tunable laser 300, which can
then be used for examination of the characteristics of
the DUT 325. A linear continuous curve may be formed from
extrapolation of the data provided in Figure 6.
The wavelength reference used allows exact wavelength
calibration of the sweep. This is significant as the
output wavelength of the laser can change due to
degradation, ambient temperature etc. An important
property of this system is that as the laser degrades
etc. the absolute wavelength of the laser can vary but
the linearity of the sweep is constant. The rate of
change of wavelength of the sweep can also vary but by
the novel implementation defined in this patent the
calculation of the wavelength at any time is not
affected.
Lu)
Lfl
A second embodiment can be described once again with
reference to a three section DBR device.
Again as can be seen from Figure 3 a continuous tuning
region is obtained by finding a line that crosses no mode
boundaries. This can be accomplished in a number of ways
such as in “Complete Wavelength Control of a GCSR Laser
over EDFA Band” IEEE LEOS’99, San Francisco, Nov’99, T.
Farrell et al. The simplest is to locate each of the mode
boundaries using some appropriate type of filter and
compute the equation of the line between them.
Once a series of these continuous sweeps has been
identified, a set of continuous regions can be selected
each of which overlap by a predetermined frequency such
as l0 GHZ and each of which starts and ends 10 GHZ from
one of the peaks of the high finesse etalon. The laser
then sweeps through each of these continuous regions.
Upon hitting the second etalon peak in a particular
continuous sweep it de—asserts the control signal
associated with the continuous regions of the sweep and
starts sweeping along the beginning of the overlapping
region of the next continuous region. When during this
sweep it once again detects the peak of the etalon it
will know it is at exactly the same frequency and can
reassert the control signal thereby providing seamless
continuity in the tuning with no gaps or overlaps in
wavelength in the measurements.
The accuracy and performance of the tunable laser is
dependent on the accuracy of the calibration. By
calibrating the laser to high accuracy and achieving a
linear wavelength sweep with respect to time this novel
technique can deliver high speed and high accuracy not
available with existing techniques technology.
If a sweeper with sub 0.1 pm accuracy is required over a
typical 40nm sweep range this results in 40,000 points
that have to be set on the laser. For each of these
points multiple currents have to be controlled to ensure
linear tuning, good SMSR, line width and constant output
power. This can result in a large amount of data, i.e.
for a SG—DBR laser where 4 laser sections are controlled
(Front reflector, Back reflector, Phase and Gain or SOA)
the number of bytes of data in a lookup table is 40,000
different set points multiplied by 4 controlled sections
by 2 bytes to store the set value = 40,000 x 4 x 2 = 320
Kbytes. While this is straightforward for a PC in an
actual instrument this will rapidly become the bottleneck
and the sweep speed will suffer. For example for a sweep
speed of l00ms, the data needs to be retrieved at
3.2Mbytes/sec processed and set to the laser. The
methodology of the present invention may be extended to
provide a method of approximating the tuning of each
section that allows for data minimisation and speed of
sweep.
Figure 4 shows the characteristic tuning of a segment of
the laser. It will be noted that the other sections of
the laser must be controlled to achieve this type of
tuning. To get a good approximation of this curve a
complex equation is required to fit, an example of which
will be given as below.
An approximation that works well over most of the tuning
rate is
Y:a+b/(c—x) where y is the output frequency of the laser
and x is the phase current.
As this function requires a divide to calculate this
takes significant time and reduces the overall speed of
sweep possible. Use of a polynomial approximation can
provide sufficient accuracy with enough coefficients to a
given or required wavelength accuracy. Also reducing the
segment size can greatly reduce the error of a low order
polynomial fit. The advantage of using a polynomial fit
is that this can be implemented by using a set of
difference equations and result in a set of additions to
be performed rather than time consuming multiplies or
divides.
By using a polynomial which can be calculated as a set of
additions this means that the speed of the calculations
can be performed quickly and not effect the speed of the
sweep. This means that the sweep speed can be limited by
the physical properties of the laser instead of the
supporting electronics.
It will be appreciated that the present invention
provides a method for establishing a continuous tuning
set from a discontinuous laser. Such a method of
operating a tunable laser, as provided by the present
invention provides a high accuracy sweeping tunable
source by means of a wavelength reference. The method
desirably provides for a calibration of a tunable laser
in a manner that provides for continuously tunable
segments between each known wavelength from the
wavelength reference. As will be understood from the
above mentioned exemplary embodiment, any gaps between
known wavelengths of the wavelength reference are
traversed by a single segment, and desirably each of the
continuously tunable segments overlaps with the adjacent
segments.
By calibrating the tunable laser in a manner wherein the
wavelength tuning is constant with time while in
operation, the wavelength of the laser at any time can be
calculated from the wavelength references at the time for
the laser to tune between them
In a further embodiment, as shown in Figure 7, the DUT
can be an optical source and the system can be configured
to measure the wavelength of the optical source as a
wavelength meter, or perform as an optical spectrum
analyser. This involves a heterodyne detection method 371
where the optical light from the tunable laser and the
optical device under test are mixed (in coupler 311) and
a beat signal generated. This beat signal is at a
frequency of the difference between the two optical
signals. By low pass filtering the photodiode detector a
beat will only be present on the receiver when the
frequency difference between the two optical signals in
less than the bandwidth of the low pass filter. The
heterodyne receiver performs the following actions
“ AC couple the photodiode response
. Low Pass the photodiode response
. Rectify the low passed output
. Low pass the rectified output or integrate and dump.
With use of the correct bandwidths when the optical
wavelength of the DUT and the tunable laser are within
the required wavelength of each other a signal is
received on the heterodyne receiver. As the wavelength of
the tunable laser is known the wavelength of the DUT can
be measured.
It will be appreciated that the present invention
provides a method that enables the use of resonance peaks
in a wavelength reference to define regions of continuous
operation of a discontinuous laser device. By subdividing
the operating region of the laser device into segments or
regions of continuous operation the present invention
provides for continuous operating regions in an extended
sweep of the laser device. This has application in a
number of different fields for example as an optical
spectrum analyser or the like.
Claims (5)
- l. A method of providing a set of continuous tuning regions from a discontinuously tuned laser, the method comprising the steps of: providing a wavelength reference having at least first and second resonance peaks, sweeping the laser across a pre—determined wavelength range of the wavelength reference, and defining, within the laser sweep, one or more regions of continuous tuning operation of the laser, each of the regions corresponding to a response of the laser between adjacent resonance peaks of the wavelength reference.
- 2. The method as claimed in claim 1 further comprising the step of stitching two or more regions to one another so as to form a usable tuning data set, effected by one or more of the following substeps: a. creating a lookup table of regions that have continuous tuning over a first frequency region with a frequency overlap on either side with the previous and next continuous tuning regions, b. asserting a control signal to denote a continuous region when the first resonance peak is detected, c. de—asserting the control signal and jumping to the next continuous tuning region when the next resonance peak is found within this continuous tuning region, and d. repeating the above steps (b—c) until a sufficient range of wavelength has been swept.
- 3. The method as claimed in claim 1 or claim 2 wherein the regions of continuing tuning operation of the laser are defined by: 19 calibrating the Laser so as to provide a range or currents with no mode jumps, selecting continuous regions with a first treguency overlap that have a resonance peak in the 3 M wavelength response from their beginnings and ends, and setting the currents whilst sweeping through those wavelengths so as to provide a smoothly transitioning wavelength sweep. 10
- 4. The method as claimed in any preceding claim further comprising the step of assigning a frequency (Fmws) or wavelength ( Xmws) value to discrete points within the 15 continuous region of operation of the laser device, the value being assigned by: measuring the time from the resonance peak at the beginning of the sweep to the measurement instant 20 (Tmeas)/ measuring the time required to sweep between adjacent resonance peaks (Tsmmam), and calculating the value by extracting a value for Tmas from Twmmnt 25
- 5. A method substantially as hereinbefore described with reference to and/or as illustrated in
Publications (2)
Publication Number | Publication Date |
---|---|
IE20030281U1 IE20030281U1 (en) | 2004-10-20 |
IES83706Y1 true IES83706Y1 (en) | 2004-12-15 |
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