U.S. patent application number 12/022255 was filed with the patent office on 2008-08-14 for q-switched all-fibre laser.
Invention is credited to Fei Luo, Tung Feng Yeh.
Application Number | 20080192780 12/022255 |
Document ID | / |
Family ID | 39685769 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080192780 |
Kind Code |
A1 |
Luo; Fei ; et al. |
August 14, 2008 |
Q-SWITCHED ALL-FIBRE LASER
Abstract
A Q-switched all-fiber laser utilizes a long period fibre
grating (LPFG) modulator. The LPFG modulator is characterized by
optical spectral characteristics that are controlled by application
of stress via an actuator. In particular, the actuator applies
stress to selected sections of the LPFG in order to modulate a
light signal at a specified wavelength. Further, a controller is
utilized to control the application of stress in the time domain,
and thereby switch the Q-factor of the fiber laser cavity. In
addition to the LPFG, the laser cavity comprises a pair of fiber
Bragg gratings (FBGs) and a fiber gain medium.
Inventors: |
Luo; Fei; (Winchester,
MA) ; Yeh; Tung Feng; (Waltham, MA) |
Correspondence
Address: |
Anderson Gorecki & Manaras LLP
33 NAGOG PARK
ACTON
MA
01720
US
|
Family ID: |
39685769 |
Appl. No.: |
12/022255 |
Filed: |
January 30, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60901255 |
Feb 13, 2007 |
|
|
|
Current U.S.
Class: |
372/14 ;
372/10 |
Current CPC
Class: |
H01S 3/06791 20130101;
H01S 3/127 20130101; H01S 3/1067 20130101; H01S 3/106 20130101;
H01S 3/0675 20130101; H01S 3/11 20130101 |
Class at
Publication: |
372/14 ;
372/10 |
International
Class: |
H01S 3/121 20060101
H01S003/121 |
Claims
1. Apparatus comprising: at least one fiber Bragg grating
reflector; at least one pump light source; a gain fibre; and at
least one long period fiber grating modulator employed to switch Q
factor of the laser cavity, whereby a Q-switched fibre laser is
provided.
2. The apparatus of claim 1 wherein the long period fiber grating
modulator is operative in response to application of stress to at
least one area of the fibre to change refractive index of the area
while stress is applied.
3. The apparatus of claim 1 wherein the fiber Bragg grating is
characterized by a center wavelength matching a modulating
wavelength of the long period fiber grating modulator
4. The apparatus of claim 1 wherein the long period fiber grating
modulator includes at least one phase shift section.
5. The apparatus of claim 1 wherein the long period fiber grating
modulator includes first and second cascaded long period fibre
gratings or more cascaded long period fiber gratings in series.
6. The apparatus of claim 1 further including a pump source which
couples pump light into the laser cavity between the long period
fiber grating modulator and the gain fibre.
7. The apparatus of claim 1 wherein the laser is a ring laser
having a fibre loop, and wherein the long period fiber grating
modulator is part of the fibre loop.
8. The apparatus of claim 1 wherein the laser is a ring laser
having a fibre loop, and wherein the long period fiber grating
modulator is outside of the fibre loop.
9. The apparatus of claim 1 wherein the laser is a ring laser
having a fibre loop, and further including a pump source which
couples pump light directly into the fibre loop.
10. A method comprising: introducing pump light to laser cavity,
the laser cavity includes at least one fiber Bragg grating
reflector, a gain fibre, and at least one long period fiber grating
modulator; and switching the Q factor of laser cavity with the long
period fiber grating modulator, whereby a Q-switched fibre laser is
provided.
11. The method of claim 10 including the further step of
controlling the long period fiber grating modulator via application
of stress to at least one area of the fibre to change refractive
index of the area while stress is applied.
12. The method of claim 10 including the further step of reflecting
the light with at least one fiber Bragg grating characterized by a
center wavelength matching a modulating wavelength of the long
period fiber grating modulator
13. The method of claim 10 including the further step of switching
the Q factor of the laser cavity with a long period fiber grating
modulator having at least one phase shift section.
14. The method of claim 10 including the further step of switching
the Q factor of the laser cavity with a long period fiber grating
modulator having first and second cascaded long period fibre
gratings or more cascaded long period fiber gratings in series.
15. The method of claim 10 including the further step of coupling
pump light into the laser cavity between the long period fiber
grating modulator and the gain fibre.
16. The method of claim 10 wherein the laser is a ring laser having
a fibre loop, and including the further step of switching the Q
factor of the laser cavity with a long period fiber grating
modulator that is part of the fibre loop.
17. The method of claim 10 wherein the laser is a ring laser having
a fibre loop, and including the further step of switching Q factor
of the laser cavity with a long period fiber grating modulator that
is outside of the fibre loop.
18. The method of claim 10 wherein the laser is a ring laser having
a fibre loop, and further the pump light is coupled pump directly
into the fibre loop.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] A claim of priority is made to U.S. Provisional Patent
Application 60/901,255, filed Feb. 13, 2007, entitled Q SWITCHED
FIBER LASER WITH ALL FIBER CONFIGURATIONS, which is incorporated by
reference.
FIELD OF THE INVENTION
[0002] This invention is generally related to the field of lasers,
and more particularly to a Q-switched all-fibre laser.
BACKGROUND OF THE INVENTION
[0003] Characteristic features of fiber lasers include high output
beam quality, compact size, ease-of-use, and low running cost.
Fiber lasers can generate either continuous-wave (CW) radiation or
pulse radiation. Pulsed operation can be achieved via Q-switching
techniques. Q-switched fiber lasers are preferred for applications
such as micro-machining, marking, and scientific research due to
their high peak power and excellent beam quality. Q-switching is
achieved by inserting an optical modulator in the laser resonance
cavity to control optical loss in the cavity. In particular, the
modulator functions as an optical loss switch. Initially, cavity
loss is kept on a high level (low Q factor state). Laser
oscillation cannot occur at this initial period, but energy from a
pump source accumulates in the gain medium. Subsequently, cavity
loss is switched to a low loss level (high Q factor state), so that
laser oscillation builds up quickly in the cavity and generates a
high peak power laser pulse. When the laser cavity is switched
between low Q and high Q by the optical modulator, sequenced laser
pulses are produced.
[0004] Optical modulation for Q-switching can be achieved by either
active or passive means. Examples of active Q-switching modulation
means include acousto-optic modulators (AOMs) and electro-optic
modulators (EOMs). The AOM comprises optical crystals such as
tellurium dioxide, crystalline quartz, and fused silica. The EOM
comprises optical materials such as potassium di-deuterium
phosphate (KD*P), beta barium borate (BBO), lithium niobate
(LiNbO.sub.3), as well as NH.sub.4H.sub.2PO.sub.4 (ADP), and other
materials. One drawback of known AOM and EOM devices is that they
are relatively bulky. This is a drawback because the fibre core has
a relatively small diameter, the difference of which relative to
the size of the modulator complicates light coupling between the
device and an optical fiber. Further, AOM and EOM devices are
relatively expensive.
[0005] A typical configuration of a Q-switched fiber laser is
illustrated in FIG. 1. The laser cavity comprises a pair of fiber
Bragg grating (FBG) reflectors (15, 35) having the same center
wavelength, a gain fiber (18) which provides optical gain, and an
optical modulator (90) coupled to an optical fiber pigtail (20) for
coupling a light signal between the fiber and the modulator. The
optical modulator may be either an AOM or EOM type. A pump source
(1) provides pump light (5) which is coupled to the fiber laser
cavity to excite the gain fiber (18). The FBG reflectors provide
optical feedback for laser oscillation. The optical modulator (90)
is employed as a switch to control optical loss within the laser
cavity, and thereby provide Q-switching. Initially, the cavity loss
is kept on a high level with the modulator switch "off" (low Q
factor state of the laser cavity), at which time no light signal
passes through the modulator (90). As discussed above, laser
oscillation does not occur at this time, but energy from pump light
source (5) accumulates in the gain fiber (18). Subsequently, the
cavity loss is reduced over a relatively short time by "switching
on" the optical modulator to a low loss level (high Q factor state
of the laser cavity), at which time the light signal passes through
optical modulator (90). Consequently, laser oscillation builds up
quickly in the cavity and generates a high peak power laser pulse.
The FBG pair (15, 35) have the same center wavelength and function
as narrow band reflective mirrors which provide optical feedback to
the laser cavity and confine the laser oscillation wavelength to
the FBG wavelength. Since the FBG has a relatively narrow
reflective bandwidth, the laser oscillates only at this wavelength
and the output has a narrow wavelength spectrum. When the laser
cavity is switched sequentially between the low Q factor state and
the high Q factor state by means of the optical modulator (90),
sequenced laser pulses are produced. Modulator switch control is
achieved by means of a signal (95) from an external controller
(96). One device of the FBG pair (15, 35) is partially transparent
and has relatively lower reflectivity, resulting in a percentage of
the generated laser light being permitted to leave laser cavity and
deliver the laser output (38 or 42).
[0006] Referring to FIG. 2a, the FBG is formed by introducing a
periodic changes of refractive index in the fiber core. The
modified area (151) within the fiber core has a smaller refractive
index difference of period .LAMBDA..sub.B relative to the adjacent
unmodified area (152). Several techniques are known for changing
the refractive index of discreet areas of the fibre core. One
technique is to expose the area to a UV laser beam, e.g., area
(151) is altered by exposure to UV light, but area (152) is neither
exposed nor altered.
[0007] The principle characteristic parameters of a FBG are center
wavelength .lamda..sub.B, bandwidth .DELTA..lamda..sub.B, and
reflectivity. The condition for high reflection, known as the Bragg
condition, relates the reflected wavelength, or Bragg wavelength,
.lamda..sub.B to the grating period .LAMBDA..sub.B and the
effective refractive index of the fiber core n via:
.lamda..sub.B=2n.LAMBDA..sub.B.
FIGS. 2b, 2c, and 2d illustrate the spectral characteristics of a
FBG. When broad band light (110, FIG. 2a) having spectrum (120,
FIG. 2b) is input into the FBG as shown, the reflected light (112,
FIG. 2a) has a corresponding spectrum (122, FIG. 2c), and the
transmitted light (111, FIG. 2a) has a corresponding spectrum (121,
FIG. 2d).
[0008] Somewhat similar to the FBG in terms of physical
configuration, a Long Period Fiber Grating (LPFG) has a grating
period .LAMBDA..sub.L which is considerably longer than the period
.LAMBDA..sub.B of the FBG, i.e., typically .LAMBDA..sub.L is
200.about.2000 times longer than .LAMBDA..sub.B. The LPFG couples
the fundamental mode in the fiber core with the cladding modes of
the fiber and propagates them in the same direction. The excited
cladding modes are attenuated, resulting in the appearance of
resonance loss in the transmission spectrum. However, in contrast
with the FBG, the LPFG does not produce reflected light. FIGS. 3a,
3b and 3c illustrate the physical configuration and the spectral
transmission characteristics of a LPFG. The periodic grating
structure (22, FIG. 3a) can be made by using a UV laser beam to
"burn" discreet, periodically spaced areas in the fiber core in a
manner which is similar to that described above with reference to
the FBG, where the modified area (251) exhibits a refractive index
change in comparison with unmodified area (252). Recent research
suggests that the modified areas can be also formed by using a high
voltage electric arc discharge or CO.sub.2 laser to "burn" the
fiber, i.e., introducing structural changes and slight geometrical
deformation in the irradiated area of the fibre. Alternatively,
mechanical stress can be used, i.e., by applying static stress to
the areas of the fibre to be modified through a corrugated plate.
The refractive index at the areas subjected to stress is changed in
accordance with the photo-elastic effect, but the adjacent areas
which are not subjected to stress are unmodified.
[0009] When a broad band light (210, FIG. 3a) having spectrum (220,
FIG. 3b) is input into the LPFG, the transmitted light (211, FIG.
3a) has a corresponding spectral characteristic (221, FIG. 3c),
several resonance loss peaks (222, 223), including the fundamental
mode coupling with different cladding modes of the fiber. However,
there is no light reflection. Considering resonance loss peak (222,
FIG. 3c), having a center wavelength .lamda..sub.L, and bandwidth
.DELTA..lamda..sub.L, the resonance loss of the LPFG is due to the
coupling of the fundamental mode in the fiber core with the
cladding modes of the fiber. The phase matching between the
fundamental mode and cladding modes at wavelength .lamda..sub.mL
can be expressed as:
.lamda..sub.mL=(n.sub.core-n.sub.cl.sup.m).LAMBDA..sub.L,
where n.sub.core is the effective refractive index of the
fundamental mode, n.sub.cl.sup.m is the effective refractive index
of the m.sup.th cladding mode, and .LAMBDA..sub.L is the period of
the LPFG. Since several cladding modes can satisfy this condition,
each one is at a different center wavelength .lamda..sub.mL, and
thus the transmission spectrum of the LPFG exhibits a series of
transmission loss notch peaks (222, 223, FIG. 3C).
[0010] FIGS. 4a-4c illustrate the physical configuration and the
spectral transmission characteristics of a phase shifted LPFG. In
the phase shifted LPFG, a part of the grating period is shifted at
the grating center by .LAMBDA.p. As a result, a phase shift is
introduced into the LPFG. For example, by introducing a .pi.-phase
shift at the center of the LPFG, the notch peak (See FIG. 3c) is
changed to a reverse peak (232, FIG. 4c). For a broad band input
(220, FIG. 4b), a corresponding transmission spectrum (231, FIG.
4c) of the phase shifted LPFG is produced, enabling transmission at
wavelength .lamda..sub.L.
[0011] FIGS. 5a-5c illustrate the physical configuration and the
spectral transmission characteristics of cascaded LPFGs. Cascaded
LPFGs are formed by connecting a pair of LPFGs (25, 26) in series.
Each of the LPFGs has a grating length d.sub.1 and d.sub.2, and
together define a separation distance of L. When broad band light
(210) having spectrum (220, FIG. 5b) is input into the cascaded
LPFGs, the corresponding transmitted light (211) has a
corresponding spectral transmission response (241, FIG. 5c). It can
be seen from FIGS. 5b and 5c that the spectrum of the transmitted
light has several spectral transparent peaks (242, 244 and 246) and
several spectral loss peaks (245, 243). This is due to interference
between the fundamental mode and cladding modes. The first LPFG
couples part of the fundamental mode to the cladding modes, and
then the coupled cladding modes and fundamental mode travel along
the fiber simultaneously to the second LPFG. At the second LPFG,
the two modes interact with each other and generate spectral
interference fringe patterns. The fringe spacing
.DELTA..lamda..sub.PL is related to the grating length d.sub.1,
d.sub.2, d and the separation distance L between the two LPFGs. An
increase in L corresponds with a decrease in the fringe spacing
.DELTA..lamda..sub.PL. For multi-channel filter applications the
distance L is typically less than 600 mm.
SUMMARY OF THE INVENTION
[0012] In accordance with one embodiment of the present invention,
a Q-switched fibre laser apparatus comprises at least one pump
source, one reflector, a gain fibre, and a long period fiber
grating modulator employed to switch Q factor of the laser
cavity.
[0013] In accordance with another embodiment of the invention, a
method for producing laser light comprises introducing pump light
to laser cavity which includes at least one reflector, a gain
fibre, and a long period fiber grating modulator, and switching Q
factor of the laser cavity with the long period fiber grating
modulator, whereby a Q-switched fibre laser is provided.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 illustrates the physical configuration of a
Q-switched fiber laser.
[0015] FIGS. 2a, 2b, 2c and 2d illustrate the physical
configuration and spectral transmission characteristics of a
FBG.
[0016] FIGS. 3a, 3b, and 3c illustrate the physical configuration
and spectral transmission characteristics of a LPFG.
[0017] FIGS. 4a, 4b, and 4c illustrate the physical configuration
and spectral transmission characteristics of a phase shifted
LPFG.
[0018] FIGS. 5a, 5b, and 5c illustrate the physical configuration
and spectral transmission characteristics of a cascaded LPFG
pair.
[0019] FIGS. 6a, 6b, and 6c illustrate a LFPG optical
modulator.
[0020] FIG. 6d and FIG. 6e illustrate the spectral transmission
behavior of the innovative LPFG modulator.
[0021] FIG. 7 illustrates use of the LPFG modulator as a component
of an all-fiber in-line device such as an all-fiber Q-switched
laser.
[0022] FIGS. 8a, 8b, and 8c illustrate an alternative LPFG
modulator formed by employing a phase shifted LPFG.
[0023] FIG. 9a illustrates an embodiment of the modulator based on
cascaded LPFGs.
[0024] FIGS. 9b, 9c, 9d and 9e illustrate the spectral transmission
characteristics of the embodiment of FIG. 9a.
[0025] FIG. 10 illustrates a Q-switched fiber laser employing two
LPFG modulators in the fiber laser cavity.
[0026] FIGS. 11a, 11b, and 11c illustrate an LPFG modulator
assembly using two LPFGs.
[0027] FIG. 12 illustrates a Q-switched fiber laser employing a
LPFG modulator in which the pump light is coupled into the laser
cavity from the middle of the laser cavity.
[0028] FIG. 13 illustrates a Q-switched fiber laser employing a
LPFG modulator in which a ring laser cavity is used and the LPFG
modulator is placed outside of the fiber loop.
[0029] FIG. 14 illustrates a Q-switched fiber laser employing a
LPFG modulator in which a ring laser cavity is used and the LPFG
modulator is placed inside of the fiber loop.
[0030] FIG. 15 illustrates a Q-switched fiber laser in which a LPFG
modulator is employed in a ring laser cavity and an optical
isolator is employed to achieve unidirectional laser
oscillation.
[0031] FIG. 16 illustrates a Q-switched fiber laser having a ring
laser cavity in which the pump light is coupled into the laser
cavity from the middle of the cavity.
DETAILED DESCRIPTION
[0032] Referring to FIGS. 6a-6c, a LFPG optical modulator is
provided via the controlled (time, area and force) application of
stress to an optical material to introduce refractive index changes
in the material in accordance with the photo-elastic effect. As
illustrated, a small section (253) of the LPFG (22) is subjected to
stress (203) through force applied by an actuator (202). The stress
may be applied by mechanical, acoustic or other means. The actuator
(202) may include a piezo actuator that operates in response to a
modulating voltage (205) from a controller (206). The applied
stress (203) causes a temporary deformation of the material at
section (253) and a corresponding refractive index change at
section (253). The periodic structure and spectral transmission
behavior of the LPFG are changed in a corresponding manner. In
particular, the magnitude of the refractive index change is related
to the magnitude of applied force, the periodic structure and
spectral transmission behavior is related to (a) which areas are
subjected to stress and (b) the period and frequency at which
stress is applied.
[0033] FIGS. 6b and 6c are cross-sectional views of the LFPG of
FIG. 6a that illustrate different configurations for applying
stress to the fibre (20). In FIG. 6b the LPFG fiber (20) is
disposed between actuator (202) and a plate (215). The fiber can be
fixed in place with glue (207). FIG. 6c shows an alternative
embodiment in which a V-groove plate (216) is employed in lieu of
the flat plate (215, FIG. 6b) for enhanced fiber fixing and
enhanced stress distribution.
[0034] FIG. 6d and FIG. 6e illustrate the spectral transmission
behavior of the innovative LPFG modulator. The transmission
spectrum of the LPFG when no stress is applied is shown by a first
section (221, FIG. 6d), i.e., a narrow band input light (122) with
center wavelength .lamda..sub.L is blocked since the resonance loss
peak (222) of the LPFG is just at this wavelength. This corresponds
to the "switch off" state of the LPFG modulator. The bandwidth of
the signal light is narrower than the bandwidth
.DELTA..lamda..sub.L of the LPFG. When stress is applied to section
(253, FIG. 6a), the transmission spectrum is changed as shown in
FIG. 6e, with the resonance loss peak (222, FIG. 6d) becoming peak
(222a, FIG. 6e). The narrow band input light (122) can now pass
through the LPFG. This corresponds to the "switch on" state of the
LPFG modulator. Thus, the input light (122) with center wavelength
.lamda..sub.L can be modulated in response to the control signal
applied to the actuator.
[0035] FIG. 7 illustrates use of the LPFG modulator as a component
of an all-fiber in-line device such as an all-fiber Q-switched
laser. The illustrated laser cavity has a Fabry-Perot configuration
and includes a pair of FBG reflectors (15, 35) having the same
center wavelength .lamda..sub.B, a gain fiber (18), and an LPFG
modulator (201). The resonance loss peak .lamda..sub.L of the LPFG
is matched with center wavelength .lamda..sub.B of the FBGs. The
bandwidth .DELTA..lamda..sub.B of the FBGs is narrower than
bandwidth .DELTA..lamda..sub.L of the LPFG, i.e.,
.DELTA..lamda..sub.B<<.DELTA..lamda..sub.L. The laser
oscillation wavelength is confined by the FBGs at wavelength
.lamda..sub.B. The LPFG modulator is employed to switch the Q
factor of the laser cavity, i.e., control optical loss in the time
domain. Switching is provided in response to a modulating voltage
(205) applied to the actuator by a controller (206). Pump source
(1) couples pump light (5) into the laser cavity to pump gain fiber
(18). One or both of the FBG reflectors (15, 35) are partially
transparent at its wavelength. Consequently, the laser output (38
or 42) can be provided from either fiber end (37) or fibre end (9),
or both fiber ends.
[0036] An alternative LPFG modulator can be formed by employing a
phase shifted LPFG as shown in FIG. 8a. In this embodiment the
stress (203) is applied to the phase shift section on the LPFG
through actuator (202). The transmission spectrum of the phase
shifted LPFG with and without applied stress is shown in FIG. 8b
and FIG. 8c. When no stress is applied to the LPFG, a narrow band
signal light (122) can pass through the area (232) of the phase
shifted LPFG, i.e., in the "switch on" state. When stress is
applied to area (253) the LPFG has resonance loss (232a) at
wavelength .lamda..sub.L, i.e., in the "switch off" state. As with
the previous embodiment, the bandwidth of the signal light is
narrower than bandwidth .DELTA..lamda..sub.L of the LPFG.
[0037] FIG. 9a illustrates an embodiment of the modulator based on
cascaded LPFGs. A pair of LPFGs (25, 26) are disposed in series.
Actuator (202 or 202b or 202a) applies stress to the section of
LPFG (25) or LPFG (26) or on the fiber section (227) between LPFG
(25) and LPFG (26). Initially, when no stress is applied, the
transmission spectrum is as shown at section (241) in FIG. 9b. The
wavelength of the signal light (122) is matched at the wavelength
.lamda..sub.L1, which is at loss peak (243) on the spectrum of the
cascaded LPFGs. Consequently, the signal light (122) cannot pass
through and the modulator is in the "switch off" state. When the
stress is applied at any of points (202, 202b or 202a), the
transmission spectrum is changed as shown in FIG. 9c, where the
signal light (122) can pass through since .lamda..sub.L1 at peak
(243a) is transparent.
[0038] FIGS. 9d and 9e illustrate an alternative embodiment in
which, when no stress is applied, the signal light (122) can pass
through the cascaded LPFGs since the wavelength of the signal light
is set to match .lamda..sub.L2 at (244, FIG. 9d). When stress is
applied, the signal light (122) is blocked since the spectrum of
the cascaded LPFGs is changed as shown in FIG. 9e where the signal
light (122) is at the loss peak (244a) in the spectrum of the
cascaded LPFGs.
[0039] Generally, any of the LPFG modulators described above can be
utilized to provide an all-fibre Q-switched laser. FIG. 10, for
example, illustrates an alternative embodiment of the Q-switched
laser in which two LPFG modulators (201a, 201b) are employed in the
fiber laser cavity to enhance switch extinction. Two or more LPFGs
can also be packaged together as shown in FIGS. 11a, 11b and 11c.
The fibers (262, 271) with LPFGs (265, 275) are sandwiched between
actuator (202) and plate (215) or V-groove (216). Again, glue (207)
may be used to protect and fix the fiber. Modulating voltage (209)
is applied to actuator (202). FIG. 12 illustrates an embodiment of
the Q-switched fiber laser system in which the pump light is
coupled into the fiber laser cavity from the middle of the laser
cavity. In particular, the pump light (5) is coupled into laser
cavity through pump coupler (4). The Q-switched fiber laser can
also be implemented with ring laser cavity configurations as shown
in FIG. 13. The laser cavity comprises FBG reflector (15), LPFG
modulator (201), fiber coupler (60) and gain fiber (18). Two arms
(62, 64) of the fiber coupler (60) are spliced with gain fiber (18)
to form a fiber loop. The LPFG modulator (201) is placed outside of
the fiber loop between the FBG (15) and the fiber coupler (60). The
LPFG modulator is transparent at the pump wavelength, and the
resulting laser output comes from the arm (63) of the fiber
coupler. FIG. 14 illustrates another possible embodiment of the
ring fiber laser cavity where the LPFG modulator (201) is placed
inside the fiber loop. Furthermore, the gain fiber can be placed
outside of the fiber loop. In this case the fiber loop forms a
fiber loop mirror. FIG. 15 illustrates another alternative
embodiment of the ring fiber laser cavity in which an isolator (70)
is placed in the fiber loop in order to achieve unidirectional
laser oscillation in the laser cavity. The LPFG modulator (201) can
be placed either in the fiber loop or outside of the fiber loop
between the FBG (15) and the fiber coupler (60). FIG. 16
illustrates an embodiment of the LPFG modulator based Q-switched
fiber laser having ring laser cavity in which the pump light is
coupled into the laser cavity from the middle of the cavity through
the pump coupler (4). In any of the embodiments of the Q-switched
fiber laser employing an LPFG modulator, the LPFG modulator may be
a simple LPFG based modulator, a phase shifted LPFG based modulator
or a cascaded LPFGs based modulator. Further, one or more LPFG
modulators may be used in a fiber laser cavity in order to improve
switch extinction.
[0040] While the invention is described through the above exemplary
embodiments, it will be understood by those of ordinary skill in
the art that modification to and variation of the illustrated
embodiments may be made without departing from the inventive
concepts herein disclosed. Moreover, while the preferred
embodiments are described in connection with various illustrative
structures, one skilled in the art will recognize that the system
may be embodied using a variety of specific structures.
Accordingly, the invention should not be viewed as limited except
by the scope and spirit of the appended claims.
* * * * *