Method and apparatus for controlling laser emmision wavelength using non-linear effects

The present invention is generally directed to a laser, such as a soliton fiber laser, having an emission wavelength controlled by non-linear effects. Although the emission wavelength of such lasers is typically limited to the center of the gain profile, exemplary embodiments of the present invention provide relatively broad bandwidth control by producing significant gain-pulling using non-linear effects. Any non-linear effects in a laser cavity can be used to provide significant gain pulling and a broadband wavelength tuning range including, for example, the soliton self-frequency shift (SSFS) and cross-phase modulation (CPM). As a result, non-linear tuning can be achieved. Exemplary embodiments provide gain-pulling which allows a significant separation to be induced between the peak emission wavelength of the modelocked fiber laser (i.e., the modelocked emission wavelength, or MLEW) and the emission wavelength of the non-modelocked laser (i.e., the continuous wave emission wavelength, or CWEW).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an apparatus and method for 
generating light energy, and more particularly, to passively modelocked 
lasers having the emission wavelength controlled by balancing non-linear 
effects. 
2. State of the Art 
Practically speaking, ultra-short pulse sources which can consistently emit 
pulses having pulse durations less than one picosecond should be compact, 
environmentally stable and require relatively low power. A document 
entitled "Additive-Pulse-Compression Modelocking of a Neodymium Fiber 
Laser", by Fermann M. E. et al, Optics Letters, Vol. 16, No. 4, Feb. 15, 
1991, pages 244-246, describes a passively modelocked fiber laser for 
generating ultra-short pulses using a rare-earth doped fiber. 
An environmentally stable, passively modelocked laser is described in 
co-pending U.S. application Ser. No. 08/169,707 entitled "Environmentally 
Stable Passively Modelocked Fiber Laser Pulse Source", filed in the U.S. 
Patent and Trademark Office on Dec. 20, 1993, by Dr. Martin E. Fermann and 
Dr. Donald J. Hatter, the disclosure of which is hereby incorporated by 
reference in its entirety. As referenced therein, the phrase 
"environmentally stable" refers to a pulse source which is substantially 
immune to a loss of pulse generation due to environmental influences such 
as temperature drifts and which is, at most, only slightly sensitive to 
pressure variations. 
Exemplary embodiments of an environmentally stable, ultra-short pulse 
source have been implemented by differentially exciting two linearly 
polarized, fundamental eigenmodes of a highly birefringent fiber (HBF) 
such that they accumulate a differential non-linear phase delay after a 
particular propagation distance. Due to interference of the eigenmodes at 
a polarizer, the non-linear phase delay translates into an amplitude 
modulation, which can provide sufficient pulse-shortening per round-trip 
to produce stable passive modelocking. The amount of amplitude modulation 
is sensitive to the linear phase delay between the two interfering 
eigenmodes. 
Linear phase drifts between two polarization eigenmodes of a cavity, such 
as the cavity described in accordance with exemplary embodiments of the 
previously mentioned co-pending application, can be eliminated using a 
pigtailed Faraday rotator mirror (FRM) as one of the cavity mirrors. A 
document entitled "Single-Polarisation Fibre Amplifier", by I. N. Duling 
III et al, Electronics Letters, Jun. 4, 1992, Vol. 28, No. 12, pages 
1126-1128 also generally describes using a Faraday rotator mirror a as an 
end mirror. The Faraday rotator mirror permits environmental stability to 
the amplifier. 
Passively modelocked lasers are typically subject to a variety of processes 
that affect the output pulses. For example, in a document entitled "Mode 
Locking In Solitary Lasers", by T. Brabec et al, Optics Letters, Vol. 16, 
No. 24, Dec. 15, 1991, pages 1961-1963, pulse formation in modelocked 
lasers is described wherein the presence of isolated (i.e., discrete) 
cavity elements results in the occurrence of instability. Noticeable 
affects on stability can also result from third-order dispersion, and such 
instabilities can result in the formation of spectral sidebands as 
described, for example, in a document entitled "Characteristic Sideband 
Instability of the Periodically Amplified Average Soliton", by S. M. J. 
Kelly, Electronics Letters, Vol. 28, page 806, 1992. Further, in a 
document entitled "Ultrabroad-Band Femtosecond Lasers", by Christian 
Spielmann et al, Journal of Quantum Electronics, instabilities due to 
third-order dispersion which give rise to asymmetric pulse spectra are 
described. 
Thus, while conventional passively modelocked lasers are typically subject 
to processes which can result in instabilities that affect the output 
pulses (e.g., emission wavelength), any gain-pulling due to these 
instabilities is minimal such that the spectrum of the modelocked pulses 
remains located close to the peak of the gain profile. In particular, the 
tuning range of standard modelocked lasers is governed dominantly by their 
finite gain bandwidth and cannot be extended or controlled by non-linear 
processes. 
Because the tuning range of standard modelocked lasers is primarily 
governed by their finite gain bandwidth, any ability to control selection 
of a particular emission wavelength of the modelocked laser is 
substantially limited. Accordingly, it would be desirable to provide a 
tunable laser having an emission wavelength which can be controlled over a 
relatively broad bandwidth. 
SUMMARY OF THE INVENTION 
The present invention is generally directed to a laser, such as a soliton 
fiber laser, having an emission wavelength controlled by non-linear 
effects. Although the emission wavelength of such lasers is typically 
limited to the center of the gain profile, exemplary embodiments of the 
present invention provide relatively broad bandwidth control by producing 
significant gain-pulling using non-linear effects. Any non-linear effects 
in a laser cavity can be used to provide significant gain pulling and a 
broadband wavelength tuning range including, for example, the soliton 
self-frequency shift (SSFS) and cross-phase modulation (CPM). As a result, 
non-linear tuning can be achieved. Exemplary embodiments provide 
gain-pulling which allows a significant separation to be induced between 
the peak emission wavelength of the modelocked fiber laser (i.e., the 
modelocked emission wavelength, or MLEW) and the emission wavelength of 
the non-modelocked laser (i.e., the continuous wave emission wavelength, 
or CWEW). 
Exemplary embodiments of the present invention include a cavity having an 
axis along which light energy travels, a medium having a non-linear 
refractive index for inducing a predetermined wavelength shift of a 
modelocked emission wavelength of the cavity relative to a continuous wave 
emission of the cavity, and means for outputting energy generated within 
the cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an apparatus for amplifying light energy, represented as 
a laser energy generating means, such as a passively modelocked soliton 
fiber laser 100. As illustrated in the FIG. 1 embodiment, the passively 
modelocked laser 100 includes a cavity 200 having an axis along which 
light energy travels. In accordance with exemplary embodiments, the cavity 
can be a Fabry-Perot cavity as illustrated in FIG. 1. 
The passively modelocked laser further includes a means for pumping the 
laser energy generating means, the pumping means being generally 
designated 300. The pumping means includes an energy source (e.g., 
electrical or optical energy source, depending on laser type) generally 
represented as a pump 302. A wavelength-division multiplexing coupler 
(WDM) 304 formed using low birefringent fiber is provided for coupling the 
pumping means to the cavity 200. In an exemplary embodiment, the pump 302 
can produce energy in the 980 nanometer range, and the wavelength division 
multiplexer coupler can be an Aster WDM 1550/980 to accommodate a 980 
nanometer pump and a 1550 nanometer signal. However, those skilled in the 
art will appreciate that the wavelength division multiplexing coupler can 
be any multiplexer which allows pumping of the laser cavity 200 without 
substantial loss of signal light; i.e., one which allows differential 
coupling between the pump 302 and the signal light. 
In the FIG. 1 embodiment, the cavity includes a gain medium 202 for 
amplifying energy within the cavity 200. The gain medium can be any 
rare-earth-doped fiber capable of providing light amplification. For 
purposes of the following discussion, reference will be made to an 
optically pumped laser having an active fiber doped with erbium ions as 
the gain medium 202. However, those skilled in the art will appreciate 
that other rare-earth-doped fibers, such as fibers doped with neodymium 
ions can be used. Further, the present invention is not limited to fiber 
lasers, but can also be used with other types of lasers such as bulk 
solid-state lasers comprising a gain medium of bulk solid-state materials, 
and semiconductor lasers. Optical or electrical pumping can be used, 
although optical pumping is generally preferred for use with bulk 
solid-state lasers while electrical pumping is generally preferred for 
semiconductor lasers. 
In accordance with the present invention, the gain medium amplifies energy 
along the axis of the cavity and can be a medium having a non-linear 
refractive index. Alternately, a medium having a non-linear refractive 
index, separate from the gain medium, can be provided. The use of a medium 
having a non-linear refractive index induces a predetermined wavelength 
shift of a modelocked emission wavelength of the cavity relative to a 
continuous wave emission wavelength of the cavity. 
The continuous wave emission wavelength of the laser 100 is located at the 
peak of the spectral gain band. The location of the modelocked emission 
wavelength can be shifted by up to, for example, 14 nanometers (i.e., by 
nearly 30 percent of the gain bandwidth of erbium) away from the 
continuous wave emission wavelength of the laser 100. The location to 
which the modelocked emission wavelength is shifted is power dependent and 
can be suppressed by the selection of a suitable polarization state inside 
the fiber. 
The laser cavity 200 further includes means for reflecting energy along an 
axis within the cavity which passes through the gain medium, the axis 
being generally designated by the arrow 204. The energy reflecting means 
includes a first cavity mirror 206, located at a first end of the cavity 
200, for reflecting signal light within the cavity. The cavity mirror can 
be any standard laser mirror readily available and known to those skilled 
in the art. In the exemplary FIG. 1 embodiment, the cavity mirror 206 also 
functions as a laser energy outputting means for outputting energy 
generated within the cavity. Thus, the cavity mirror 206 in the FIG. 1 
embodiment serves two functions: it reflects a fraction of energy 
impinging onto it back into the laser cavity 200; and, it allows the 
remaining fraction of energy to leak through the cavity mirror 206 to 
provide output energy. Alternately, the first cavity mirror 206 can be 
separate from an output coupler means if desired. 
The FIG. 1 embodiment includes two interfering polarization directions of a 
Kerr-type modelocked fiber laser. These interfering polarization 
directions include two linearly polarized eigenmodes of a 
highly-birefringent fiber (HBF). In the exemplary FIG. 1 embodiment, the 
erbium-doped fiber used as the gain medium 202 can be a 
highly-birefringent fiber. 
In accordance with the present invention, the FIG. 1 cavity 200 can also 
include low-birefringent fiber (LBF). In exemplary embodiment, the length 
of low-birefringent fiber can be relatively short in comparison to the 
length of the highly-birefringent fiber (e.g., on the order of eight to 
ten times shorter). The highly-birefringent fiber thereby dominates 
non-linear pulse-shaping in these embodiments, with such pulse shaping 
being negligible in the low-birefringent fiber. 
In an exemplary implementation of a cavity, 2.6 meters of 
highly-birefringent fiber can be used with 0.6 meters of standard 
communications-type low-birefringent fiber. The highly-birefringent fiber 
can have a polarization beat length of 10 centimeters at a lasing 
wavelength of 1.567 microns, an effective core area of 28 microns and a 
numerical aperture of 0.19. Further, the highly-birefringent fiber can be 
doped with, for example, approximately 5.times.10.sup.18 erbium 
ions/centimeters.sup.3. 
By using both high-birefringent fiber and low-birefringent fiber in the 
cavity, non-polarization maintaining couplers can be used for coupling 
light into and out of the laser cavity 200 to simplify laser assembly and 
packaging and thus reduce overall costs. Alternately, the cavity can 
include only low birefringement fiber where short-pulse oscillation is 
induced by a saturable absorber. Equally, fibers sections with both 
positive and negative group velocity dispersion (GVD) can be included in 
the cavity. 
In accordance with exemplary embodiments, the laser cavity 200 can further 
include means for compensating linear phase drifts of the gain medium 202. 
For example, a phase drift compensating means can be provided to control 
polarized light generated within the cavity using at least one Faraday 
rotator, such as a pigtailed Faraday rotator mirror 210 formed with low 
birefringent fiber. Faraday rotator mirrors are known devices which can be 
chosen to reflect, in an orthogonal state, any polarization state which 
impinges upon them. The at least one Faraday rotator mirror can therefore 
compensate for linear phase drifts between the polarization eigenmodes of 
the gain medium. The reflective qualities of the pigtailed Faraday rotator 
mirror 210 permit this device to serve as a second cavity mirror of the 
reflecting means located at a second end of the cavity 200, opposite the 
first end, thereby defining the boundaries of the cavity 200. For example, 
the Faraday rotator mirror 210 can be a 45 degree rotator which rotates 
the polarization of reflected light by 90 degrees relative to incoming 
light such that reflected light propagates back down the gain medium 202 
in exactly an orthogonal polarization state. 
Due to the use of the Faraday rotator mirror 210, the total linear phase 
delay between the polarization eigenmodes of the fiber is exactly zero 
after one round-trip. Non-linear phase changes remain uncompensated and 
accumulate along the polarization eigenmodes of the highly-birefringent 
fiber after reflection by the Faraday rotator mirror 210. Because the 
highly-birefringent fiber eliminates random mode-coupling, and because the 
low-birefringent fiber is relatively short in length, the non-linear phase 
changes are governed by the relative power in the polarization eigenmodes 
and are not susceptible to environmental influence. When low-birefringent 
fiber is used in conjunction with a saturable absorber in the cavity, 
short pulse oscillation is dominated by the saturable absorber, which 
reduces the differential phase delay between the polarization eigenmodes 
to a negligible level. Thus, the Faraday rotator still stabilizes the 
linear polarization state of the cavity. 
The Faraday rotator mirror 210 suppresses spurious back-reflections from 
intra-cavity fiber ends (e.g., fiber, including the gain medium 202, 
included within cavity 200) and eliminates a possible continuous 
wavelasing background. For example, scattered light which is reflected 
back to the Faraday rotator mirror 210 will again be rotated therein and 
absorbed by an optical polarizing element, such as polarizer 216. 
The Faraday rotator mirror 210 can be incorporated at a point in the laser 
cavity 200 at which group-velocity walk-off between the polarization 
eigenmodes is maximum. Thus, non-linearity of fiber components in front of 
the Faraday rotator mirror 210 along the cavity axis can be reduced to 
minimize unwanted non-linearity of the low-birefringent fiber. Thus, 
environmentally stable operation can be achieved using relatively long 
lengths of low-birefringent fiber, if desired. 
The phase drift compensating means can further include a second Faraday 
rotator 212. The second Faraday rotator 212 can be a 45.degree. Faraday 
rotator which, in an exemplary embodiment, can be centrally poised in the 
cavity to compensate for polarization rotation of the Faraday rotator 
mirror 210. 
The exemplary FIG. 1 apparatus also includes means for focusing energy 
generated along the axis 204, with the energy focusing means including at 
least a first lens 228 for focusing energy received from the gain medium 
202 onto the first cavity mirror 206, and for directing energy from the 
cavity mirror 206 onto the gain medium 202. A highly-birefringent fiber 
section can therefore extend from a location adjacent the lens 228, or as 
close to it as possible, to ensure that an amount of power in the 
polarization eigenmodes of the highly-birefringent fiber stays absolutely 
constant. In such an arrangement, the first Faraday rotator mirror 210, 
the wavelength division multiplexer coupler 304 and the highly 
birefringent fiber 218 can be interconnected using fusion splices. 
The lens 228 can be any optical element available for focusing light from 
the gain medium. In exemplary embodiments, the focal point of the lens can 
be selected to coincide with the first cavity mirror 206 so that the power 
density on the cavity mirror 206 is maximized. Similarly, the focal point 
of the lens can be selected to coincide with maximizing power density on 
the gain medium 202. 
Although the phase drift compensating means provides environmental 
stability, a means for transforming linear polarization of energy can be 
used to optimize non-linear polarization evolution for modelocking. The 
linear polarization transformation means can include one or more 
waveplates 214 and 215 within the laser cavity 200 for introducing a 
linear phase delay, with interference of polarization eigenmodes occurring 
at the polarizer 216. 
In the exemplary FIG. 1 embodiment, the waveplates 214 and 215 are 
illustrated as a .lambda./4 waveplate and a .lambda./2 waveplate, 
respectively. The linear polarization transformation means provides a 
unique polarization transformation from the intra-cavity polarizer to the 
fiber. This transformation is defined on the well-known Poincare sphere as 
(0,0).fwdarw.(2.PSI., 2.alpha.); that is, linearly polarized light 
emerging from the polarizer 216 is transformed into elliptically polarized 
light with an ellipticity .PSI. (representing the round-trip linear 
polarization state and the cavity loss), where the ellipse is rotated by 
an angle e with respect to x-axis of the fiber, and wherein the tangent of 
.PSI. is b/a where b and a are minor and major axes of the polarization 
ellipse, respectively. 
In calculating the polarization transformation, the action of the 
intra-cavity Faraday rotator is ignored. The polarization state at the 
intra-cavity polarizer is (0,0) and the polarization at the fiber end 
closest to the polarizer 216 is (2.PSI., 2.alpha.), where .alpha. is the 
angle of the continuous wave polarization ellipse with respect to the slow 
axis of the fiber. Note that the continuous wave loss of the cavity is 
given by sin.sup.2 2.PSI.. 
Generally speaking, a physical meaning can be attached to the polarization 
transformation by assuming that the .lambda./4 waveplate is aligned with 
its axes at 45.degree. with respect to the polarizer 216. By tilting the 
.lambda./4 waveplate, a phase delay of .delta.=2.PSI. is introduced along 
its axes. 
Thus, the polarization transformation induced by the waveplates can be 
adjusted by holding .PSI. constant and by changing .alpha.. This is 
performed by leaving the .lambda./4 waveplate untouched and rotating the 
.lambda./2 waveplate. The resulting action does not change the continuous 
wave loss in the cavity; i.e., the action leaves the linear (round-trip) 
polarization state of the cavity essentially unaffected. 
On the other hand, changing .alpha. changes the power distribution in the 
fiber axes and will thus lead to a change in the non-linear polarization 
state of the cavity. Hence, using this particular control, the non-linear 
polarization state of the cavity can be separately changed without 
affecting the (round-trip) linear polarization in the cavity. 
In accordance with exemplary embodiments, means for initiating a 
modelocking process in a cavity 200 can also be included using, for 
example, a saturable absorber, a vibrating cavity mirror, a fiber 
stretcher or an optical modulator. The modelocking process can be 
sustained in the cavity using the saturable absorber in conjunction with 
Kerr-type non-linearity of the cavity. The saturable absorber can be a 
semiconductor material with its band edge close to the lasing wavelength 
of the fiber laser. 
Those skilled in the art will appreciate that the laser system 
configuration of the FIG. 1 embodiment is by way of example only and that 
alternate embodiments can be used in accordance with the present 
invention. For example, the FIG. 1 system can be implemented using any 
type of fiber in the cavity. Further, the entire Fabry-Perot configuration 
of FIG. 1 can be reversed so that the Faraday rotator mirror 216 is to the 
lefthand side of the cavity and the cavity mirror 206 is to the righthand 
side of the cavity. 
In accordance with the present invention, the exact locations of the 
Faraday rotator mirror 210 and the Faraday rotator 212 can be readily 
determined by those skilled in the art. However, in accordance with 
exemplary embodiments, the Faraday rotator mirror 210 and the Faraday 
rotator 212 define an intra-cavity portion of the cavity 200 wherein the 
gain medium is located. 
Further, those skilled in the art will appreciate that the selection and 
location of non-polarization maintaining, low-birefringent fiber can vary 
widely to satisfy the design constraints of a particular implementation. 
Further, in accordance with exemplary embodiments, highly-birefringent 
fiber can be used for the entire cavity. 
In accordance with another exemplary embodiment, a 2.3 meter length of such 
fiber can be doped with approximately 5.times.10.sup.18 erbium 
ions/centimeter.sup.3. In this case, the core diameter can be 5 microns, 
the polarization beat length 10 centimeters and the group-velocity 
dispersion a negative 13,000 femtoseconds.sup.2 /meter. For such an 
embodiment, an exemplary stability range can be defined as follows: 
55 degrees&lt;.PSI.&lt;77.degree. 
9 degrees&lt;.alpha.&lt;32 degrees 
Thus, modelocking can be sustained for a continuous wave intra-cavity loss 
between 20 and 90 percent. Modelocking can be initiated in about 30 
percent of the stability range or greater where modelocking was initiated 
by either vibrating one of the cavity end mirrors or by using a saturable 
absorber inside the cavity. In exemplary embodiments, pulse widths inside 
the stability range can be varied over a range of between 200 and 400 
femtoseconds or greater, while sustaining a range of intra-cavity pulse 
energies of between 50 and 150 picoJoules or greater. When keeping the 
polarization transforming means fixed, stable modelocking without a 
continuous wave background can be obtained for pump power variations of up 
to .+-.20 percent or greater. 
The pulse spectra as a function of increasing intra-cavity pulse energy in 
the presence of uncompensated gain-pulling and in the presence of 
compensated gain-pulling are illustrated in FIGS. 2A and 2B. FIG. 3 
illustrates pulse widths as a function of intra-cavity pulse energy. The 
FIG. 3 pulse spectra can be obtained with the polarization transforming 
means set at .alpha.=10 degrees, whereas .PSI.=75 degrees and 57 degrees 
for FIG. 2A and FIG. 2B, respectively.degree. 
In FIG. 3, the solid line 302 represents a parallel measurement relative to 
the intra-cavity polarizer 216 of FIG. 1. The dotted line 304 of FIG. 3 
represents an orthogonal measurement of the pulse widths representing an 
orthogonal measurement relative to the intra-cavity polarizer 216 of FIG. 
1. 
The pulse spectrum in FIG. 2A exhibits a large power-dependent wavelength 
shift, which can extend up to 14 nanometers to the red side of the peak of 
the gain profile (as represented by the marked position of the continuous 
wave emission near 1570 nanometers). Note that for red-shifts larger than 
10 nanometers, a continuous wave instability arises at the spectral gain 
peak. The continuous wave instability occurs near the continuous wave 
emission wavelength and thus the large red-shift becomes even more 
apparent. The red-shift allows the tuning range of the fiber laser to be 
extended. Further the red-shift allows the separation of the continuous 
wave emission wavelength from the modelocked emission wavelength described 
previously. 
On the other hand, in FIG. 2B the position of the pulse spectrum is nearly 
power-independent and located close to the spectral gain peak. A 
significant difference between the FIG. 2A spectrum and the FIG. 2B 
spectrum is the intra-cavity loss, which is 25 percent and 83 percent in 
FIGS. 2A and 2B, respectively. As the intra-cavity loss is increased, the 
pulling force from the gain profile also increases and tends to center the 
pulse near the peak of the gain profile. 
By keeping .PSI., the round-trip linear polarization state and the cavity 
loss constant, and by varying .alpha., large spectral red-shifts can be 
obtained for large values of .alpha., which maximize the intra-cavity 
pulse energy and minimize the pulse width. In particular, by simply 
rotating the .lambda./2 waveplate of the polarization transforming means, 
wavelength tuning of the fiber laser can be achieved with a single 
control. 
Large red-shifts can be attributed primarily to the soliton self-frequency 
shift and cross-phase modulation. The soliton self-frequency shift is 
well-known from soliton communications and can lead to red-shifts in 
soliton transmission. Frequency shifts from cross-phase modulation are 
known from studies of so-called vector solitons; i.e., the propagation of 
soliton in birefringent fibers. In accordance with the present invention, 
gain-pulling can be used to broaden the tuning range of fiber lasers 
whenever any linear or non-linear effects are present in the fiber cavity 
that can lead to a large separation between the continuous wave emission 
wavelength and the modelocked emission wavelength. 
The correct magnitude of the red-shift expected from both the soliton 
self-frequency shift and cross-phase modulation can be simulated 
numerically, as can the onset of continuous wave instability for large 
red-shifts. For the numerical simulations, the well-known non-linear 
Schrodinger equation can be solved by a split-step Fourier transform 
method. To model the laser correctly, two coupled non-linear Schrodinger 
equations, with a cross-phase modulation and a group-velocity walk-off 
term to account for the two axes of the fiber, can be used with a 
time-delayed non-linear response (accounting for the soliton 
self-frequency shift). Further, gain and periodic loss in the cavity, 
along with a term accounting for any assumed parabolic gain profile of the 
erbium fiber can be incorporated into the numerical simulation. 
In contrast to the soliton self-frequency shift, the cross phase modulation 
can also lead to spectral blue-shifts depending on the exact setting of 
.PSI. and .alpha. in the polarization transforming means. FIG. 2B reveals 
that the pulse spectrum exhibits a small blue shift with an increase in 
pulse power, which indicates that in this case the pulling force from 
cross-phase modulation opposes Raman self-scattering. In fact the 
polarization transforming means can be adjusted to give blue-shifts as 
large as 2 nanometers. In particular, gain-pulling from one or more 
effects in the cavity can be compensated by one or more other effects in 
that cavity. Those skilled in the art will appreciate that by compensating 
sufficiently strong gain-pulling from one or more effects in the cavity 
with gain-pulling from other equally strong effects, a continuous wave 
emission wavelength will move closer to the modelocked emission 
wavelength. A balance of non-linear effects in the cavity can therefore be 
used to control gain-pulling, and thereby provide the broadband emission 
wavelength control which can be achieved in accordance with exemplary 
embodiments. 
Thus, gain-pulling can be used to broaden the tuning range of a passively 
modelocked fiber laser. Further, the control of the (round-trip) linear 
polarization state of a fiber laser cavity can be separated from the 
control of the non-linear polarization state of the cavity, such that 
gain-pulling can be used to separate the continuous wave emission 
wavelength from the modelocked emission wavelength. This separation can be 
used to improve the quality of modelocked pulses, and can be achieved by 
setting the polarization transforming means to give a large separation and 
by allowing a small amount of continuous wave emission to be present 
simultaneously with the modelocked pulses. By filtering out the continuous 
wave component, amplified spontaneous emission in the cavity can be 
reduced, and/or the noise of the modelocked pulses can be reduced. The 
continuous wave emission wavelength can be removed by optical filtering 
techniques. 
Those skilled in the art will appreciate that the gain-pulling described in 
accordance with exemplary embodiments of the present invention can be used 
with lasers other than those described above including, but not limited 
to, any modelocked fiber laser, semiconductor lasers, or any other 
waveguide or bulk laser. Further, those skilled in the art will recognize 
that whenever gain-pulling manifests itself in a separation of the 
continuous wave emission wavelength from the modelocked emission 
wavelength, the continuous wave emission wavelength can be filtered out 
using an optical filter, thus permitting such features as laser and 
amplified spontaneous emission noise suppression. 
It will be appreciated by those skilled in the art that the present 
invention can be embodied in other specific forms without departing from 
the spirit or essential characteristics thereof. The presently disclosed 
embodiments are therefore considered in all respects to be illustrative 
and not restricted. The scope of the invention is indicated by the 
appended claims rather than the foregoing description and all changes that 
come within the meaning and range and equivalence thereof are intended to 
be embraced therein.