Note: Descriptions are shown in the official language in which they were submitted.
CA 02341825 2001-03-21
Doc. No. OPR-5 CA(2) Patent
Multiple Wavelength Laser Source
Field of the Invention
This invention generally relates to laser sources and in particular to lasers
sources
suitable for wavelength-division multiplexed optical communications systems.
Background
Wavelength Division Multiplexing, WDM and Dense Wavelength Division
Multiplexing, DWDM optical transmission systems require light from multiple
laser
sources to match the International Telecommunications Union, ITU, channel
spacing. In
previous work this has been done with individual lasers tuned separately to
the channel
spacings.
Cochlain and Mears disclosed an example of a laser source producing a single
wavelength in an article entitled ''Broadband Tunable Single Frequency Diode
Pumped
Erbium Doped Fiber Laser", Elec tunics Letters, Vol. 28, No. 2, pp. 124 - 126,
1992.
Cochlain and Mears disclose a laser that comprises a loop mirror formed by a 3-
dB
coupler and an erbium-doped fiber having its ends connected to respective
ports of the
coupler via an isolator and a wavelength selective coupler (WSC), the latter
coupling
energy from a pump into the erbium-doped fiber. The WSC is used to pump 1480
nm
light into the loop mirror. A polarization controller between the WSC and the
3-dB
coupler controls the passage of amplified spontaneous emission (ASE) into the
coupler,
while the isolator blocks the ASE, from reaching the other port of the
coupler. A third
port of the 3-dB coupler is connei;te~d to a grating by way of a second
polarization
controller while the fourth port delivers the output signal. Rotation of the
grating selects
the individual wavelength at which the laser will lase. The ASE will be
reflected by the
grating back through the 3-dB coupler and will pass, via the isolator, around
the loop to
appear at the output of the laser. 'The grating reflects substantially all of
the light
reaching it, so the output signal can be extracted from only the one point in
the system,
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namely from the fourth port of the coupler. The device cannot readily be
adapted for
multiple wavelength use and so the common practice would be to use a number of
these
devices, each one tuned to a different wavelength, to provide the multiple
wavelengths
required for WDM or DWDM.. However, this approach is expensive since it
requires
duplication of components to provide multiple wavelengths.
The present invention seeks to overcome these disadvantages and to this end
provides a laser source capable of operating at multiple wavelengths.
Summary
In accordance with the presewt invention, there is provided a multiple
wavelength
laser source comprising
a loop mirror formedl by a loop of active fiber and a first coupler,
preferably a 3-dB (50:50) coupler, the fiber being connected between two ports
of
the coupler,
at least one pump means for injecting pump energy into the loop of active
fiber; and
a plurality of wavelength-selective reflection devices having different
selected wavelengths and coupled to at least a third port of the first
coupler;
wherein each reflection device is for reflecting into the fiber loop a first
portion, having a selected wavelength, of amplified spontaneous emission
produced by the active fiber., and directing a second portion of the amplified
spontaneous emission produced by the active fiber, to an output port.
Each reflection device will reflect back into the loop mirror ASE at its own
particular selected wavelength. C.'onsequently, the laser source will lase at
each of the
different wavelengths of the plurality of reflection devices, thereby
producing output light
at a plurality of different wavelengths which can be used for WDM or DWDM.
One or more of the reflection devices may transmit the second portion to the
output port. Additionally or alternatively, one or more of the reflection
devices may
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reflect the second portion to the output port, conveniently by way of an
additional
coupler.
In one embodiment of the invention, the plurality of reflection devices are in
series between the first coupler and the output port and each reflects the
first portion of
ASE back to the loop mirror and transmits the second portion to the output
port.
In another embodiment of tree invention, the plurality of the reflection
devices are
arranged in parallel. Specifically, a first plurality of wavelength-selective
reflection
devices may be coupled to a corresponding port of the first coupler by way of
a second
coupler, an output port of the first coupler being coupled to an output port
of the laser,
each reflection device reflecting both the first portion and the second
portion of ASE to
the second coupler and the second coupler directing a portion of the energy to
the loop
mirror and another portion to the associated output port. One or more of the
reflection
devices coupled to the second coupler may transmit a third portion of the ASE
to an
associated output port. Analogously, at least one other plurality of
wavelength-selective
reflection devices may be coupled to another port of the first coupler by way
of another
coupler in an arrangement similar to the one described above.
Various types of reflection devices meeting the requirements of the invention
may
be employed. Fiber Bragg gratings (FBGs) are one choice. Tunable filters with
partially
reflective and partially transmissive mirrors are another possibility.
Preferably, but not necessarily, the first portion and the second portion each
comprise about 50 per cent of the emission energy.
The laser source may comprise a plurality of attenuators, each between the
fiber
loop and one of the reflection devices. The attenuators may be used to adjust
the amount
of ASE reflected and hence control the amplitude of the laser output signal at
the
corresponding selected wavelength. In addition, they are used to control
lasing modes
competition in the loop. 'The attenu;~tors may be adjusted to "flatten" the
output spectrum.
The number of reflection devices coupled to each port of the 3-dB coupler need
not be
the same.
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In accordance with another aspect of the present invention, there is provided
a
laser source system for producing multiple sets of lasing wavelengths, said
system
comprising:
laser source combining means for combining output from a plurality of multiple
wavelength laser sources, each multiple wavelength laser source comprising::
a loop mirror means, said loop mirror means comprising:
a loop of active fiber; and
a splitter/coupler means that is coupled, via a first and a second
port, to both ends of the loop of active fiber;
at least one pump mf:ans for injecting pump energy into the loop of active
fiber; and
a plurality of wavelength-selective reflecting devices, said devices having
different selected wavelengths and coupled to at least a third port of the
splitter/eoupler means;
wherein each reflecting device is for reflecting a first portion of amplified
spontaneous emission, supplied by the loop of active fiber, and directing a
second
portion of the amplified spontaneous emission, supplied by the loop of active
fiber, to an output port.
Brief Description of The Drawings
Figure 1 illustrates, as a first embodiment of the invention, a laser source
having a
plurality of gratings, with attenuators, in series;
Figure 2 illustrates, as a second embodiment of the invention, a laser source
having a plurality of gratings in parallel;
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Figure 3 illustrates the spectrum of the laser output from the first four
output ports
of the laser source of Figure 2; and
Figure 4 illustrates the spectrum of the laser output from fifth and sixth
output
ports of the laser source of Figure 2.
Detailed Description of the Invention
In the drawings, identical or corresponding items in the different Figures
have the
same reference number.
Referring first to Figure 1. a laser source comprises a loop mirror formed by
a 3-
dB fiber coupler 10 having four ports, A, B, C and D, and a loop of active
fiber 12 doped
with any rare earth, preferably erbium-doped f ber, EDF, with its ends
connected to ports
C and D, respectively, through wavelength-selective couplers 14 and 16 that
are
connected also to pump sources (lasers) 18 and 20, respectively.
WDM couplers are used to combine the pump energy at 980 nm and the amplified
signal at 1550 nm inside the EDF loop 12. Using 3-dB couplers to combine the
980 nm
pump energy and the 1550 nm laser output is problematic and therefore it is
more
appropriate to use a WDM: coupler, inside of the EDF loop, to combine these
light
signals.
A polarization controller i 1 is placed in the loop to divide amplified
spontaneous
emission (ASE) generated by the pump source to the output terminals of the
coupler 10.
Port A of the coupler 10 is coupled to a first output port Pl«UT by a
plurality of
fiber Bragg gratings, FBGs, 22A, 24A, ..., having characteristic wavelengths
~,~, ~,3, ...,
respectively, in series. Port B of the coupler 10 is coupled to a second
output port P2ou~
by a second plurality of FBGs, 22B, 248, ..., having characteristic
wavelengths ~,2, ;~,4, ...,
respectively, all in series. Each of the FBGs 22A, 24A, 22B, 24B, ..., is
selected to
reflect, preferably, about 50% of the light incident upon it at the selected
wavelength and
transmit the remainder.
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Attenuators 32A, 34A...are coupled in series with each FBG 22A, 24A..., and
attenuators 32B, 34B.. are coupled in series with each FBG 22B, 24B... to
balance the
energy available between the output wavelengths. Since ASE has a non-flat
output
function, the energy imbalance is inherent.
In operation, the pump ener~,ry from pump sources 18 and 20 produces ASE in
the
EDF 12. Assuming a symmetric configuration, 50 per cent of the ASE will appear
at
each of the ports C and D of the 3-dB coupler 10 and will be coupled to ports
A and B.
Further, setting the attenuators 32A., 34.A, ..., to zero attenuation, FBG 22A
will reflect
50 per cent of the light leaving port A so that it re-enters the loop mirror
and transmits the
remainder to FBG 2.4A which, in a similar manner, will reflect 50 per cent of
the ASE at
its own selected wavelength and transmit the remainder of the ASE and other
light
including any lasing frequency. All of the other gratings/attenuators in
series with the
first output port P 1 ~>uT will operate in a similar manner. Once lasing
conditions have
been established, the output signal appearing at output port Plou~~ will
comprise all the
lasing wavelengths ~,i, , 7~_;, , ... of the gratings 22A, 24A and so on.
The same applies to the second output port P2~l~T. 'fhe ASE light leaving port
B
of the 3-dB coupler 10 will be reflected and transmitted in a similar manner
by the fiber
gratings 22B, 24B... in series with second output port P2~ur, so that the
light leaving
output port P2ouT comprises the wavelengths 7~2, ~,4... of gratings 22B, 24B
and so on.
For most WDM or DWDM applications, it is desirable for the amplitude of the
output signal to be the same at each wavelength. Consequently, the attenuators
previously mentioned may be used to adjust the amount of light reflected by
the
respective FBG, and hence the amplitude of the output light at the
corresponding
wavelength. There is competition for the ASE in EDF 12 to induce lasing at the
chosen
wavelengths. The ASE energy is shared between the competing lasing
wavelengths.
The laser source illustrated iin Figure 2 is based on a loop mirror formed by
a 3-dB
(first) fiber coupler 10 having four, ports A, B, C and D, and a loop of
active fiber 12,
preferably erbium-doped, with its ends connected to ports C and D,
respectively, through
wavelength-selective couplers 14 and 16 that are also connected to pump
sources (lasers)
18 and 20, respectively. The laser source in Figure 2 differs from that shown
in Figure 1
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in that the FBGs are not connected :in series to the port A of fiber coupler
10 but are
instead connected in parallel. Thus, fBGs 22A and 22B are connected to ports A
and B
of a second 3-dB coupler 30 by way of attenuators 32A and 32B, respectively.
FBGs
24A and 24B are connected to ports A and B of a third 3-dB coupler 40 by way
of
attenuators 36A and 36B, respectively. The transmissive ports of FBGs 22A,
22B, 24A,
and 24B are coupled to four output ports PIoIJT~ P2ouT~ P3ouT and P4ouT,
respectively.
Ports D of couplers 30 and 40 are connected to ports A and B, respectively, of
the 3-dB
coupler 10, and ports C of couplers 30 and 40 are connected to fifth and sixth
output ports
PSouT and P6ouT, respectively.
In operation, the ASE leaving the coupler 10 will be split again by couplers
30
and 40 before reaching FBGs 22A, 228, 24A and 24B. Each of these gratings will
reflect
about 50 per cent of the ASE at its own selected wavelength and transmit the
remainder,
as before. Consequently, when lasing conditions have been established, the
light
appearing at ports PlouT, P2ouw P3oiJT, and P4ouT will have wavelengths ~,,,
~,2, ~,3, ~4, ...
and the ASE_ Figure 3 shows the measured output at terminal P 1 ouT when two
gratings
are used in the setup. Note that the signal to noise ratio is about 45 dB.
The light leaving the output pons PSouT and P6ouT will be reflected by the
gratings and will pass through the ataenuators again, as compared with the
light leaving
the first output ports Plow, P2ou~, f3ouT, and P4ou~t. Consequently, the light
at output
port s PSouT and P6ouT have a better signal-to-noise ratio (better than 55 dB)
compared to
the other output terminals, as illustrated in Figure 4. The reason that the
signal-to-noise
ratio is improved at the output ports PSou-I~ and P6ouT is that the signals
contains less ASE,
because the gratings reflects only the signal to couplers 30, and 40, while
they passes
ASE to the output ports P 1 ou~r~ P2 ouT, P3ouT, and P4ou~r.
It should be appreciated that, if only two wavelengths were needed, one of the
couplers 30 and 40, and its associated pair of gratings, could be omitted.
Conversely,
additional wavelengths could be obtained by adding more couplers and pairs of
gratings,
in a tree-like configuration. It is also envisaged that the embodiment of
Figures 1 and 2
could be combined, with some of the parallel branches of the laser source
having a series
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of gratings, or each embodiment of Figures l and 2 could be repeated more than
one time
to achieve multiple of DWDM source
The attenuators 32A, 32B, 36A and 36B allow the amplitude of the light at each
wavelength to be adjusted so that, if desired, they are equal.
Although, in the above-described laser sources, the gratings each reflect
about 50
per cent of the selected wavelength light, other proportions could be used.
It is an advantage of the present invention that a multiplicity of wavelengths
can
be provided using a single active~fiber loop mirror and a grating for each
wavelength.
Also, the number of wavelengths can be increased simply by adding more fiber
gratings,
and perhaps increasing pump energy, as appropriate.
Numerous other embodiments may be envisioned without departing from the
spirit and scope of the present invention.
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