TECHNICAL FIELD
The present invention relates generally to fiber laser systems and method for optical output characteristics measurements. More particularly, the present invention relates to method and apparatus for monitoring the spectral and temporal dynamics of fiber ring laser.
BACKGROUND
Fiber lasers and amplifiers have been widely used in communication, scientific research, industrial, medical and defence applications. They uniquely combine high average powers, unparalleled beam quality, small footprint, high efficiencies and easier thermal management. The fiber laser uses an optical fiber as the active medium, which usually has a rare-earth doped core. Fiber cavity exists in two basic configurations: the linear configuration (Fabry-Perot) and the ring configuration. A linear cavity is formed by inserting the gain fiber between two reflectors, most commonly fiber Bragg gratings. Ring cavities are constructed using the gain fiber and either a fused fiber coupler, or an optical circulator and reflector. Ring cavity configuration is more suitable for highly stable single frequency tunable fiber lasers. Conventionally, method of monitoring spectral and temporal measurement of fiber ring laser has additional components making the setup cumbersome and not user friendly. Further, the conventional method taps the output of the laser using a fused fiber coupler thus interrupting the output.
US Patent Application No.8744224-B2 discloses a system and a method for monitoring output of an optical amplifier utilizing one of the plurality of the tapered fiber bundle (TFB)/pump combiner as the monitor fiber. To monitor the laser signal, a single fiber of TFB is used as a monitor fiber. A monitor or coupler may be integrated to TFB during manufacturing. The systems and methods disclosed allow for monitoring the signal without increasing the length of the signal's path. Alternative embodiments disclosed include replacing the multimode

monitor fiber with single mode fiber and integrating an in-line monitor or coupler into the tapered fiber bundle.
[0004] Another US Patent Application No. 5504771 discloses a single
frequency fiber optic ring laser with improved frequency stability, reduced laser intensity noise, broad tunability and narrow linewidth. The spectrum and intensity noise measurements are done using the light emitted from the output port of the output coupler of the ring laser.
[0005] Hence, there is still a need of a better or an alternative invention which
will solve the above defined problems.
SUMMARY
[0006] This summary is provided to introduce concepts related to fiber laser
systems and method for optical output characteristics measurements. This summary is neither intended to identify essential features of the present invention nor is it intended for use in determining or limiting the scope of the present invention.
[0007] For example, various embodiments herein may include one or more
fiber laser systems and method thereof are provided. In one of the embodiments, a measurement setup for monitoring the spectral and temporal dynamics of a fiber ring laser comprising of fiber ring laser source is provided. The fiber ring laser includes pump diode, tapered fiber bundle (TFB), active double clad fiber, 2x2 output coupler, tunable filter and isolator. The tapered fiber bundle couples the light from the fiber coupled pump diode into the gain fiber, wherein the gain fiber amplifies the signal light along its length. The 2x2 output coupler has an unused input port (also referred as measurement port) which can be used for the spectral and temporal measurement of laser output. Thus the output port of laser is not intervened, hence enabling the setup for real time measurement while laser is in use. Further, the measurement port of the output coupler is connectorized for direct connection to the measurement instruments. Further, no additional components and splices required in proposed setup thus making measurement simpler. Further, for

simultaneous spectrum and temporal measurement an additional 1x2 splitter is further added to the measurement port.
[0008] In another embodiment, the present invention makes the real-time measurements possible using the unused input port of output coupler without adding any extra component in the ring cavity output. Further, simultaneous measurement of spectral and temporal dynamics can be done by adding one more 1x2 splitter to the same measurement port. The spectrum measurement in the present invention relates to the measurement of center wavelength, linewidth, signal-to-noise-ratio (SNR) and the on-set of nonlinearities. The high gain and small core of fiber laser leads to nonlinear effects generating the undesired wavelengths in the spectrum. These wavelengths use the laser wavelength as pump light and may induce instability and damages in the fiber laser. The temporal measurements in the present invention are related with the stability of power in continuous wave (CW) lasers. Because of nonlinear effects, short duration pulses can occur even at CW operation in fiber lasers. These pulses being of high peak power may induce damages of fiber laser components, splices, recoats and the fiber itself. Therefore spectral and temporal measurements are crucial for a good cavity design to measure generated wavelength and to ensure a stable laser output without any fluctuations.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0009] The detailed description is described with reference to the
accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and modules.
[0010] Figure 1 illustrates a fiber ring laser setup, according to an exemplary
implementation of the present invention.
[0011] Figure 2 illustrates a 2x2 fiber fused coupler of Fig.1, according to an
exemplary implementation of the present invention.

[0012] Figure 3 illustrates the use of unused single mode input port of 2x2fiber
fused coupler of Fig. 2 with an in-line optical spectral analyser for spectral measurements, according to an exemplary implementation of the present invention.
[0013] Figure 4 illustrates the use of unused single mode input port of 2x2 fiber
fused coupler of Fig. 2 along with a photo detector and oscilloscope for temporal measurements, according to an exemplary implementation of the present invention.
[0014] Figure 5 illustrates unused single mode input port of 2x2 fiber fused
coupler connected to a 1x2 splitter, according to an exemplary implementation of the present invention.
[0015] It should be appreciated by those skilled in the art that any block
diagrams herein represent conceptual views of illustrative systems embodying the principles of the present invention. Similarly, it will be appreciated that any flowcharts, flow diagrams, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.
DETAILED DESCRIPTION
[0016] In the following description, for the purpose of explanation, specific
details are set forth in order to provide an understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, some of which are described below, may be incorporated into a number of systems.
[0017] The various embodiments of the present invention provide a fiber laser
system and method for optical output characteristics measurements.
[0018] Furthermore, connections between components and/or modules within
the figures are not intended to be limited to direct connections. Rather, these

components and modules may be modified, re-formatted or otherwise changed by intermediary components and modules.
[0019] References in the present invention to “one embodiment” or “an
embodiment” mean that a particular feature, structure, characteristic, or function described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
[0020] The present invention relates to fiber laser systems and method for
optical output characteristics measurements. In one of the embodiments, a measurement setup for monitoring the spectral and temporal dynamics of a fiber ring laser comprising of fiber ring laser source is provided. The fiber ring laser includes pump diode, tapered fiber bundle (TFB), active double clad fiber, 2x2 output coupler, tunable filter and isolator. The tapered fiber bundle couples the light from the fiber coupled pump diode into the gain fiber, wherein the gain fiber amplifies the signal light along its length. The 2x2 fiber fused coupler includes a first and a second input ports and a first and a second output ports. The 2x2 fiber fused coupler couples out a portion of signal light from the ring laser cavity and feeds the remaining signal light back to the ring laser cavity. Further, the second output port provide signal light as a feedback to the laser cavity. The first output port collects the output light from the fiber ring laser. The first input port collects an amplified signal light from the ring laser cavity. Further, the input signal light in the first input port leaks out to the second input port, which is an unused port, due to back reflections inside the tapered fused coupled region in the 2x2 fiber fused coupler. The leaked signal light to the second input port carries the signal light and monitors the spectral and temporal dynamics of the fiber ring laser. The 2x2 output coupler has an unused input port (also referred as measurement port) which can be used for the spectral and temporal measurement of laser output. Thus the output port of laser is not intervened, hence enabling the setup for real time measurement while laser is in use. Further, the measurement port of the output coupler is

connectorized for direct connection to the measurement instruments. Further, no additional components and splices required in proposed setup thus making measurement simpler. Further, for simultaneous spectrum and temporal measurement an additional 1x2 splitter is further added to the measurement port.
[0021] In another implementation, the present invention makes the real-time
measurements possible using the unused input port of output coupler without adding any extra component in the ring cavity output. Further, simultaneous measurement of spectral and temporal dynamics can be done by adding one more 1x2 splitter to the same measurement port. The spectrum measurement in the present invention relates to the measurement of center wavelength, linewidth, signal-to-noise-ratio (SNR) and the on-set of nonlinearities. The high gain and small core of fiber laser leads to nonlinear effects generating the undesired wavelengths in the spectrum. These wavelengths use the laser wavelength as pump light and may induce instability and damages in the fiber laser. The temporal measurements in the present invention are related with the stability of power in continuous wave (CW) lasers. Because of nonlinear effects, short duration pulses can occur even at CW operation in fiber lasers. These pulses being of high peak power may induce damages of fiber laser components, splices, recoats and the fiber itself. Therefore spectral and temporal measurements are crucial for a good cavity design to measure generated wavelength and to ensure a stable laser output without any fluctuations.
[0022] In another implementation, a method for measuring the spectral and
temporal dynamics of a fiber ring laser is provided. The fiber ring laser includes a 2x2 fiber fused coupler which couples out the optical power in a defined coupling ratio. Further, to monitor the spectral and temporal stability of the laser system, an unused input port of the fiber coupler is used. The method is to use the leaked laser light to the free input port of the fused coupler for measurements. The single mode capability of coupler gives the true and accurate measurements. This method is simple to implement, no additional components, no additional splices, no intervening of output and real time measurements while laser is in use.

[0023] In another implementation, the second input port is configured to carry
the leaked signal light in order to measure the spectrum of the laser signal light using an optical spectrum analyser.
[0024] In another implementation, the second input port is configured to be
connectorized to directly interface with the optical spectrum analyser.
[0025] In another implementation, the second input port is placed in front of a
photodetector which is connected to an oscilloscope, wherein the oscilloscope is configured to monitor the temporal variations continuously.
[0026] In another implementation, the second input port is configured to
measure the laser signal characteristics without affecting the first output port, wherein the first output port is configured to provide the laser signal.
[0027] In another implementation, an additional 1x2 splitter, connected to the
second input port, is configured to measure the spectral and temporal dynamics simultaneously.
[0028] Figure 1 illustrates a fiber ring laser setup, according to an exemplary
implementation of the present invention. The fiber ring laser 100 includes fiber coupled pump diode 101, tapered fiber bundle (TFB) 102, gain fiber 103, a 2x2 fiber fused output coupler 104, an isolator 105, and a tunable filter 106. The TFB 102 couples the pump light from the fiber coupled laser diode 101 into the double clad gain fiber 103 which amplifies the signal along its length. The 2x2 fiber fused coupler 104 includes a first and a second input ports (201, 202) and a first and a second output ports (203, 204). The 2x2 fiber fused coupler 104 couples out a portion of signal from the ring cavity and feeds the rest back to the cavity. Further, the second output port 204 provide signal light as a feedback to the laser cavity 100. The first output port 203 collects the output light from the fiber ring laser. The first input port 201 collects an amplified signal light from the ring laser cavity 100. Further, the input signal light in the first input port 201 leaks out to the second input port 202, which is an unused port, due to back reflections inside the tapered fused

coupled region in the 2x2 fiber fused coupler 104. The leaked signal light to the second input port 202 carries the signal light and monitors the spectral and temporal dynamics of the fiber ring laser. Further, the feedback arm of the ring cavity consists of the isolator 105 and the tunable filter 106. The fed back signal is coupled back to the gain media 103 through the signal port of TFB 102.
[0029] In the embodiment shown in Fig. 1, the coupling of pump light is not
limited to TFB 102 and may be accomplished through a wavelength division multiplexer in case of a low power setup. The gain fiber 103 used in the present embodiment is double clad ytterbium doped fiber having 5 µm core diameter and 130 µm cladding diameter. However, a variety of other rare earth doped fibers maybe used with different core and cladding configurations. An optical isolator 105 is used to maintain a unidirectional propagation of signal in the fiber ring laser 100. Tunable optical filter 106 acts a wavelength selective element in the fiber ring laser 100.
[0030] Figure 2 illustrates a 2x2 fiber fused coupler of Fig.1, according to an
exemplary implementation of the present invention. A fused coupler basically consists of two parallel optical fibers that have been twisted, stretched and fused together so that their cores are very close to each other. This forms a coupling region and the length of this coupling region, determines the coupling ratio from one fiber to the other. The 2x2 fiber fused coupler has two input ports 201 and 202 and two output ports 203 and 204. The input port 201 is connected to the gain fiber which collects the amplified signal light from the ring laser cavity 100 and couples out in a specified ratio. In the present embodiment, the signal port 203 and tap port 204 is split in a 90:10 ratio. However, this ratio is not limited and a wide variety of tap ratios are available. The tap port 204 is used to provide feedback to the laser cavity and the signal port 203 is used to collect the laser output light. However, the signal and tap port are not limited to this method and maybe be interchanged in the given fiber laser design. Some of the input light in the port 201, also leaks out to the unused single mode input port 202 due to back reflections inside the tapered fused coupled region in the 2x2 fiber fused coupler 104. The leaked power to the unused

input port carries the signal light and can be used to monitor spectral and temporal dynamics of the signal.
[0031] Figure 3 illustrates the use of unused single mode input port of 2x2fiber
fused coupler of Fig. 2 with an in-line optical spectral analyser for spectrum measurements, according to an exemplary implementation of the present invention. The measurement port 202 of the 2x2 fiber fused coupler 104 carries the leaked signal light in order to measure the wavelength of the laser signal using optical spectrum analyser. The measurement port 202 being a single mode fiber does not excite higher order modes making it convenient for accurate linewidth measurements of the fiber ring laser 100. Commercially available optical spectrum analysers have high sensitivity (typically of the order of nW) but low damage threshold (typically of the order of mW). The leaked power in the measurement port 202 is typically six orders of magnitude lesser than the signal power in the input port 201, thereby precluding the introduction of additional attenuators while interfacing with optical spectrum analyser 301. In accordance with embodiment of the present invention, the spectrum can be monitored continuously in real time without disturbing the signal power measurement through signal port 203. In accordance with an alternate exemplary embodiment of the present invention, the single mode measurement port 202, when connectorized can be directly interfaced with the optical spectral analyser 301.
[0032] Figure 4 illustrates the use of unused single mode input port of 2x2 fiber
fused coupler of Fig. 2 along with a photodetector and oscilloscope for temporal measurements, according to an exemplary implementation of the present invention. In general, fiber lasers maybe subjected to many temporal instabilities in power such as self-pulsing which could be due to various effects such as spatial hole burning, saturable absorption effect, re-absorption of laser photons in the un-pumped part of the doped fiber, relaxation oscillations and onset of nonlinearities such as stimulated Raman scattering and stimulated Brillouin scattering. Further, monitoring the temporal instabilities in lasers is of utmost importance. In accordance with embodiment of the present invention, the measurement port 202

of the 2x2 fiber fused coupler 104 may be used to measure the laser signal variations in time domain without disturbing the signal port 203 which may be used in conjunction to measure the fiber laser power. The measurement port 202 is placed in front of a fast photodetector 401 which is connected to oscilloscope 402, to continuously monitor the temporal variations.
[0033] Figure 5 illustrates unused single mode input port of 2x2 fiber fused
coupler connected to a 1x2 splitter, according to an exemplary implementation of the present invention. A setup and apparatus to simultaneously monitor the temporal and spectral measurements without disturbing in-use laser is illustrated in this figure. The measurement port 202 is further connected to a 1x2 splitter 501 which splits power into two ports. The two output ports of the 1x2 splitter 501 can be further used to monitor the spectral and temporal measurements.
[0034] It should be noted that the description merely illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present invention. Furthermore, all examples recited herein are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

We Claim:
1. A measurement setup for monitoring the spectral and temporal dynamics of
a fiber ring laser, said fiber ring laser comprising:
a fiber coupled pump diode (101), a tapered fiber bundle (102), a gain fiber (103), a 2x2 fiber fused coupler (104), an isolator (105), and a tunable filter (106);
the tapered fiber bundle (102) is configured to couple the light from the fiber coupled pump diode (101) into the gain fiber (103), wherein the gain fiber (103) is configured to amplify the signal light along its length;
the2x2 fiber fused coupler (104) includes a first and a second input ports (201, 202) and a first and a second output ports (203, 204), wherein the 2x2 fiber fused coupler (104) is configured to couple out a portion of signal light from the ring laser cavity (100) and to feed the remaining signal light back to the ring laser cavity (100);
the second output port (204) is configured to provide as a feedback to the laser cavity (100);
the first output port (203) is configured to collect the output light from the fiber ring laser;
the first input port (201) is configured to collect an amplified signal light from a ring laser cavity (100); wherein the input signal light in the first input port
(201) leaks out to the second input port (202), which is an unused port, due to back
reflections inside the tapered fused coupled region in the 2x2 fiber fused coupler
(104); and
the leaked signal light to the second input port (202) is configured to carry the signal light and to monitor the spectral and temporal dynamics of the fiber ring laser.
2. The measurement setup as claimed in claim 1, wherein the second input port
(202) is configured to carry the leaked signal light in order to measure the spectrum
of the laser signal light using an optical spectrum analyser (301).

3. The measurement setup as claimed in claim 1, wherein the second input port (202) is configured to be connectorized to directly interface with the optical spectrum analyser (301).
4. The measurement setup as claimed in claim 1, wherein the second input port (202) is placed in front of a photodetector (401) which is connected to an oscilloscope (402), wherein the oscilloscope (402) is configured to monitor the temporal variations continuously.
5. The measurement setup as claimed in claim 2 or claim 4, wherein the second input port (202) is configured to measure the laser signal characteristics without affecting the first output port (203), wherein the first output port (203) is configured to provide the laser signal.
6. The measurement setup as claimed in claim 1, wherein an additional 1x2 splitter (501), connected to the second input port (202), is configured to measure the spectral and temporal dynamics simultaneously.