Patent Application
20120069860
The disclosed technique relates to fiber lasers, in general, and to methods and systems for constructing pulsed fiber lasers using gain switching, in particular.
Fiber lasers are lasers in which optical fibers are used as the gain media for the laser. The fibers can be made of glass or plastic. The optical fibers used in such lasers are usually doped using rare-earth metals such as neodymium, ytterbium, erbium or thulium and have applications in many fields, such as material processing, telecommunications, spectroscopy and medicine. Fiber lasers can be mode-locked and Q-switched for generating laser pulses on the order of nanoseconds, picoseconds and femtoseconds. Such lasers are known in the art.
U.S. Pat. No. 7,120,174 to MacCormack, et al. entitled, “Pulsed laser apparatus and method” is directed towards a laser apparatus for generating optical pulses. The laser apparatus has a reflecting gain element which includes a fiber gain medium. The reflecting gain element is coupled to a controllable reflecting/transmitting module having a reflecting state and a transmitting state. The controllable reflecting/transmitting modules are operable to switch from the transmitting state to the reflecting state to initiate a build-up of an optical pulse, and to switch back to the transmitting state for outputting the optical pulse before it reaches the reflecting/transmitting module after a cavity roundtrip. MacCormack also discloses a method for generating optical pulses by Q-switching. The method comprises a first step of providing a reflective gain element comprising a first reflective means, an input/output port and a gain medium therebetween. An optical pumping means is also provided for pumping radiation into the gain medium for enabling optical gain and for emitting optical radiation from the input/output port along a first optical path. In a second step, a controllable reflecting/transmitting means is provided and disposed in the first optical path. The controllable reflecting/transmitting means has a reflecting state for reflecting a controllable portion of the optical radiation back into the gain medium and a transmitting state for transmitting the optical radiation through the reflecting/transmitting means along the first optical path to form an output optical radiation. The controllable reflecting/transmitting means is also operable to switch between the reflecting state and the transmitting state. In a third step, the controllable reflecting/transmitting means is switched from the transmitting state to the reflecting state. This switching forms a temporal optical cavity between the first reflective means and the controllable reflective/transmitting means through the gain medium. The temporal optical cavity is formed for a duration of time less than the time required for the controllable portion of the optical radiation to make a roundtrip and to initiate an optical pulse. In a fourth step, the controllable reflecting/transmitting means is switched from the reflecting state to the transmitting state for transmitting the optical pulse propagating from the gain element through the controllable reflecting/transmitting means along the first optical path.
US Published Patent Application No. 2006/0045145 to Arahira, entitled, “Mode-locked laser diode device and wavelength control method for mode-locked laser diode device” is directed towards a laser for generating optical pulses in which the wavelength width in the wavelength's variable area is sufficiently wide and in which frequency chirping is suppressed enough to be used for optical communication systems. The laser is constructed from an optical pulse generation section which includes a mode-locked laser device, a continuous wave light source, a first optical coupling means and a second optical coupling means. An optical waveguide, which includes an optical gain area, an optical modulation area and a passive wave-guiding area, is created in the mode-locked laser device. Constant current is injected into the optical gain area from a first current source via a p-side electrode and an n-side common electrode. Reverse bias voltage is applied to the optical modulation area by a voltage source via a p-side electrode and an n-side common electrode. The modulation voltage, having a frequency obtained by multiplying the cyclic frequency of the resonator of the mode-locked laser device by a natural number, is applied to the optical modulation area by a modulation voltage source. The output light of the continuous wave light source is inputted to the optical wave guide of the mode-locked laser device via the first optical coupling means, and the output light of the mode-locked laser device is outputted to the outside via the second optical coupling means.
U.S. Pat. No. 6,400,495 to Zayhowski, entitled, “Laser system including passively Q-switched laser and gain-switched laser” is directed towards a two-stage laser system including a passively Q-switched microchip laser and a gain-switched microchip laser. A pulse train generated by the passively Q-switched laser is fed into the gain-switched laser, which in turn produces an optical output signal at a preferred wavelength. In particular, the passively Q-switched laser is pumped with an optical signal generated by a diode pump laser. Based on the absorption of the optical signal, energy in the passively Q-switched laser then accumulates in its optical cavity until a threshold is reached. At this point an output optical pulse is produced and then fed into the gain-switched laser. In turn, energy accumulates in the optical cavity of the gain-switched laser where the gain medium absorbs the optical pulse from the Q-switched laser. As a result, light from the optical pulse efficiently inverts the transition near a second wavelength. This results in a gain in the gain-switched cavity at the second wavelength. By choosing an appropriate output coupler on the gain-switched laser, the gain induced by the absorbed pulse leads to the development of an optical pulse at the second wavelength. Preferably, the output pulse at the second wavelength is at around 1.5 μm, which is an eye-safe wavelength.
It is an object of the disclosed technique to provide a novel system for a fiber laser setup for generating laser pulses based on the method of gain switching which overcomes the disadvantages of the prior art. In accordance with the disclosed technique, there is thus provided a pulsed fiber laser including an electronic driver, a laser diode, and a laser cavity, the laser cavity including a combiner, a doped optical fiber, and a coupler. The laser diode is coupled with the electronic driver, the combiner is coupled with the laser diode, the doped optical fiber is coupled with the combiner, and the coupler is coupled with the doped optical fiber and the combiner. The electronic driver is for providing a drive current, the laser diode is for generating a pump pulse, the doped optical fiber is for absorbing the pump pulse and for generating a circulating laser pulse and the coupler is for outputting a first portion of the circulating laser pulse and for returning a second portion of the circulating laser pulse to the combiner. The electronic driver operates the laser diode at a specific pump pulse repetition rate (PRR), a specific pump pulse shape and a specific pump pulse width and the combiner provides the pump pulse and the second portion of the circulating laser pulse to the doped optical fiber.
In accordance with another aspect of the disclosed technique, there is thus provided a pulsed fiber laser including an electronic driver, a laser diode, and a laser cavity, the laser cavity including a doped optical fiber and a coupler. The laser diode is coupled with the electronic driver, the doped optical fiber is coupled with the laser diode, and the coupler is coupled with a first side of the doped optical fiber and a second side of the doped optical fiber. The electronic driver is for providing a drive current, the laser diode is for generating a pump pulse, the doped optical fiber is for absorbing the pump pulse and for generating a circulating laser pulse, and the coupler is for outputting a first portion of the circulating laser pulse and for returning a second portion of the circulating laser pulse to the second side of the doped optical fiber. The electronic driver operates the laser diode at a specific pump pulse repetition rate (PRR), a specific pump pulse shape and a specific pump pulse width, and the pump pulse and the second portion of the circulating laser pulse are provided to the second side of the doped optical fiber.
In accordance with a further aspect of the disclosed technique, there is thus provided a pulsed fiber laser including a plurality of electronic drivers, a plurality of laser diodes, and a laser cavity, the laser cavity including a plurality of combiners, a doped optical fiber, and at least one coupler. Each one of the plurality of laser diodes is coupled with a respective one of the plurality of electronic drivers and each of one the plurality of combiners is coupled with a respective one of the plurality of laser diodes. The doped optical fiber is coupled with each of the plurality of combiners, and the coupler is coupled with a first one of the plurality of combiners and with a second one of the plurality of combiners. Each one of the plurality of electronic drivers is for providing a respective drive current and each one of the plurality of laser diodes is for generating a respective pump pulse. The doped optical fiber is for absorbing each of the respective pump pulses and for generating a circulating laser pulse. The coupler is for outputting a first portion of the circulating laser pulse and for returning a second portion of the circulating laser pulse to one of the plurality of combiners. The plurality of electronic drivers respectively operate the plurality of laser diodes at specific pump pulse repetition rates (PRRs), specific pump pulse widths and specific pulse shapes. The plurality of combiners provide the respective pump pulses and the second portion of the circulating laser pulse to the doped optical fiber.
In accordance with another aspect of the disclosed technique, there is thus provided a pulsed fiber laser including an electronic driver, a laser diode, and a laser cavity, the laser cavity including a combiner, a doped optical fiber, a circulator, and a fiber Bragg grating (FBG). The laser diode is coupled with the electronic driver, the combiner is coupled with the laser diode, the doped optical fiber is coupled with the combiner, the circulator is coupled with the doped optical fiber and the combiner, and the FBG is coupled with the circulator. The electronic driver is for providing a drive current, the laser diode is for generating a pump pulse and the doped optical fiber is for absorbing the pump pulse and for generating a circulating laser pulse. The circulator provides the circulating laser pulse to the FBG and the FBG outputs a first portion of the circulating laser pulse and returns a second portion of the circulating laser pulse to the circulator. The circulator provides the second portion of the circulating laser pulse to the combiner. The electronic driver operates the laser diode at a specific pump pulse repetition rate (PRR), a specific pump pulse width and a specific pump pulse shape, and the combiner provides the pump pulse and the second portion of the circulating laser pulse to the doped optical fiber.
In accordance with a further aspect of the disclosed technique, there is thus provided a pulsed fiber laser including a first electronic driver, a laser diode, and a laser cavity, the laser cavity including a combiner, a doped optical fiber, a high reflection fiber Bragg grating (HRFBG), and a low reflection fiber Bragg grating (LRFBG). The laser diode is coupled with the first electronic driver, the combiner is coupled with the laser diode, the doped optical fiber is coupled with the combiner, the HRFBG is coupled with the combiner, and the LRFBG is coupled with the doped optical fiber. The first electronic driver is for providing a drive current, the laser diode is for generating a pump pulse and the doped optical fiber is for absorbing the pump pulse and for generating a circulating laser pulse. The HRFBG is for reflecting the pump pulse and the LRFBG is for outputting a first portion of the circulating laser pulse and for returning a second portion of the circulating laser pulse to the combiner. The combiner provides the pump pulse and the second portion to the HRFBG and the first electronic driver operates the laser diode at a specific pump pulse repetition rate (PRR), specific pump pulse width and a specific pulse shape.
The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:
The disclosed technique overcomes the disadvantages of the prior art by providing a novel fiber laser setup for generating laser pulses based on the method of gain switching. According to the disclosed technique, the gain medium of the fiber laser is pumped by a semiconductor laser diode having a repetition rate and pulse duration which are electronically controlled. The laser cavity of the fiber laser is formed by a partial feedback of the stimulated radiation of the laser back into the gain medium. As a convention, the terms “radiation,” “laser radiation,” “laser light,” “laser beam,” “photons,” “laser pulse,” “pulse,” “stimulated emissions” and “stimulated radiation” are used interchangeably throughout the specification to denote the light produced by the fiber laser of the disclosed technique. Also, the terms “fiber” and “optical fiber” are used interchangeably throughout the specification to denote an optical fiber.
Lasers usually comprise an optical cavity, also known as an optical resonator, in which radiation can circulate, as well as a gain medium, positioned inside the optical cavity, for amplifying the radiation. The gain medium represents a substance, such as a compound, in a particular state of matter (i.e., solid, liquid, gas or plasma) which can amplify the radiation in the optical cavity. In fiber lasers, the optical cavity is usually an optical fiber. A part of the optical fiber is usually doped with an element or compound, such as a rare-earth metal or a compound of rare-earth metals, to form the gain medium of the laser.
In general, the sub-atomic particles of a substance, such as the gain medium of a laser, remain in a low energy state, known as the ground state. If energy is applied to a substance, these sub-atomic particles can absorb the energy and move to a higher energy state, known as an excited state. In a laser, the act of supplying energy to the gain medium is known as pumping the gain medium. The energy source can be referred to as a pump source, a laser pump or simply a pump. As the gain medium is pumped, a population inversion begins to occur. Population inversion refers to the amount of sub-atomic particles in the gain medium in an excited state versus the amount of sub-atomic particles in the gain medium in the ground state. It is noted that the particles in the gain medium which can be excited can also be generally referred to as active atoms or ions.
Some of the excited sub-atomic particles return to their ground state energies via a process known as spontaneous emission. As these sub-atomic particles return to their ground state, they release their stored energy as photons. If a photon passes another sub-atomic particle in a particular excited state, it can induce that sub-atomic particle to also release its stored energy in the form of a photon. This process is referred to as stimulated emission. As mentioned above, lasers usually have an optical cavity for circulation radiation, or laser light. As photons are initially released, they circulate, or reflect, inside the optical cavity of the laser, thereby inducing many sub-atomic particles in the gain medium to release their energy as photons. Usually the optical cavity is arranged such that a portion of the photons circulating inside the cavity is released via an output coupler, leading to the emission of laser light.
The gain of a laser refers to the amount of amplification, i.e., the amount of stored energy in the excited states of the sub-atomic particles of the gain medium. It is noted that without sufficient gain, the laser radiation would dissipate as it circulates inside the optical cavity. In this respect, the optical cavity can be said to have energy losses, or laser losses. When the gain is substantially equal to the laser losses, the gain medium is said to be at the lasing threshold. Any increase in the population inversion above the lasing threshold will result in sustainable amplification, which will result in laser light being produced. The lasing threshold can be maintained continuously thereby yielding a continuous wave (CW) laser. The lasing threshold can also be maintained for short durations of time using various known techniques in the art, thereby yielding a pulsed laser.
The wavelength of light emitted from a laser is usually determined by the excited states of the sub-atomic particles of the gain medium. Photons having different wavelengths can be released when the sub-atomic particles return to their ground state, depending on which excited state the sub-atomic particles were at. In the art, the wavelength dependence of the gain coefficient (i.e., the emission) of the gain medium is specified via the emission cross-section spectral line. Correspondingly, the wavelength dependence of the pump absorption coefficient is specified via the absorption cross-section.
Reference is now made to
Laser diode 104 can be a semiconductor laser diode. Combiner 106 can be substituted for any known pump coupler. It is noted that in one embodiment of the disclosed technique, fiber laser 100 can be constructed without a combiner. In such an embodiment, laser diode 104 can be coupled directly to doped optical fiber 108 by fusion or adhesion, with the optical fiber coupled with output port 112D also being fused or adhered to doped optical fiber 108 directly. It is noted that all the components in fiber laser 100 are coupled via optical fibers. It is also noted that the optical fibers in cavity 105, including doped optical fiber 108, can be polarization maintaining optical fibers, and that combiner 106 and coupler 110 can be polarization maintaining components. Doped optical fiber 108 is doped with an active, rare-earth element, which can include, but is not limited to, ytterbium (Yb), erbium (Er), erbium-ytterbium (Er-Yb), Thulium (Tm), Neodymium (Nd) and Germanium (Ge). In one embodiment of the disclosed technique, doped optical fiber 108 is a double-clad fiber having a single mode core. In this embodiment, laser diode 104 is coupled with combiner 106 using known pump coupling techniques. In this embodiment, fiber laser 100 produces a higher power output laser beam. In another embodiment of the disclosed technique, doped optical fiber 108 is a single-clad fiber. In this embodiment, combiner 106 is substituted for a wavelength division multiplexing (WDM) coupler and laser diode 104 is coupled to laser cavity 105 via the WDM coupler. It is noted that in this embodiment, the wavelength of the laser light produced by laser diode 104 and the wavelengths at which the WDM coupler operates must be substantially similar.
Electronic driver 102 operates laser diode 104 by providing laser diode 104 with a drive current. Electronic driver 102 can operate laser diode 104 at specific pulse repetition rates (PRR) and can operate laser diode 104 to produce specific pulse shapes, such as a square shape, sawtooth shape and the like, as is known in the art. In general, electronic driver 102 operates laser diode 104 to give off pulses in the microsecond (μs) range. The drive current of electronic driver 102 may be modified to produce different types of pulse shapes in laser diode 104. In general, the modification of the drive current depends on the specific response of laser diode 104, e.g. the permitted electronic rise time, as well as the desired effect on features of the pulse shape, such as symmetry, power residing in the tail of the pulse, and the like. Laser diode 104 acts as a pump laser for pumping doped optical fiber 108. In general, laser diode 104 operates at a wavelength corresponding to the absorption spectrum of doped optical fiber 108. Laser diode 104 can operate at a frequency, or PRR of kilohertz, tens of kilohertz or up to hundreds of kilohertz, having an output peak power of tens of watts, for example, 10 to 30 watts, or as high as hundreds of watts. It is noted that the output peak power of laser diode 104 in the disclosed technique, operating in a pulsed mode, may be higher than the output peak power of laser diode 104 operating in a continuous wave (CW) mode, since the operational duty cycle of laser diode 104 in the disclosed technique is less than 100%.
As a pump laser, laser diode 104 provides a pump pulse to laser cavity 105 via combiner 106. It is noted that in this embodiment, laser diode 104 pumps doped optical fiber 108 from the left hand side. In another embodiment, the combiner may be situated on the right hand side of doped optical fiber 108, such that laser diode 104 pumps the gain fiber from the right hand side. The pump pulse is provided by combiner 106 to doped optical fiber 108, which is used to pump doped optical fiber 108. The pump pulse generated by laser diode 104 is absorbed by doped optical fiber 108. Recall that doped optical fiber 108 represents the gain medium of fiber laser 100. Laser diode 104 pumps doped optical fiber 108 thereby causing a population inversion, which leads to stimulated radiation in doped optical fiber 108 to be produced. The stimulated radiation is provided to coupler 110 via input port 112A. A portion of the stimulated radiation is outputted from coupler 110 via output port 112C whereas the remaining portion of the stimulated radiation is provided as feedback, via output port 112D, to combiner 106. Coupler 110 is provided with a coupling ratio which determines the amount of stimulated radiation provided to output port 112C and to output port 112D. Combiner 106 then combines the stimulated radiation provided from output port 112D and the pump pulse provided from laser diode 104 to doped optical fiber 108.
In general, laser diode 104 provides a pump pulse to doped optical fiber 108 in order to induce a fast build-up of the population inversion of the active atoms or ions in doped optical fiber 108. This build-up continues until the lasing threshold is reached, at which point the produced stimulated radiation begins to circulate in laser cavity 105, via coupler 110 and combiner 106. The stimulated radiation is amplified in doped optical fiber 108 over the course of one or more round trips in laser cavity 105, until the available gain of doped optical fiber 108 is depleted and the cavity radiation intensity falls off. The cavity radiation intensity refers to the intensity of the light circulating inside laser cavity 105. The pumping of doped optical fiber 108 is then terminated, by temporarily switching off laser diode 104, to cease further gain increase and to prevent the generation of subsequent pulses. Switching off laser diode 104 after the available gain of doped optical fiber 108 has been depleted results in the cavity radiation intensity falling off to zero. The build up of the population inversion and the subsequent depletion of the available gain substantially cause the laser light outputted from coupler 110 via output port 112C to be a laser pulse. In the art, this is referred to as gain switching. Once the laser pulse is outputted, laser diode 104 is turned on again to generate the next laser pulse, according to the desired pulse repetition rate. The outputted laser pulse has a pulse width that is shorter than the pulse width of the pump pulse provided by laser diode 104, with its pulse repetition rate being determined by the pulse repetition rate of the pump pulse. The power level and duration of the pump pulse are determined based on the properties of the gain medium as well as the components and the design of the laser cavity. The power level and the duration of the pump pulse are controlled to obtain output pulses of a desired power, pulse width and pulse repetition rate. The output peak power of the outputted laser pulse is on the order of hundreds of milliwatts (mW). The pulse width of the outputted laser pulse is on the order of nanoseconds (ns). In one embodiment of the disclosed technique, when the laser pulse is outputted from coupler 110 via output port 112C, laser diode 104, which is the pump laser, operates at an output power level of zero, i.e., it is turned off, so that the gain fiber is not pumped when the laser pulse is outputted from fiber laser 100. In another embodiment of the disclosed technique, when the laser pulse is outputted from coupler 110 via output port 112C, laser diode 104 operates at an output power level which is sufficiently low to maintain the gain of doped optical fiber 108 below its threshold value, i.e. the gain fiber does not produce sustained stimulated emissions. In general, there is no need to detect when the laser pulse is outputted from output port 112C, as the time required for the laser pulse to be produced in fiber laser 100 can be calculated based on various parameters of fiber laser 100, such as the pump current of laser diode 104 and the pump pulse repetition rate, as is known in the art.
In general, the coupling ratio of coupler 110 is such that a larger portion of the stimulated radiation is outputted from coupler 110 than the portion returned to combiner 106. For example, the coupling ratio may be such that 90% of the stimulated radiation is outputted via output port 112C and 10% is returned via output port 112D. In is noted that other ratio breakdowns are possible. For example, if laser diode 104 is a weak laser diode, i.e., its output peak power is low, then returning a larger portion of the stimulated radiation to the gain fiber can expedite the amplification process and the pulse generation process. The spectral properties of coupler 110, such as the wavelength dependence of its coupling ratio, as well as the emission cross-section spectral line shape of the gain fiber determine the spectral properties of the output laser pulse, such as its wavelength and linewidth. Coupler 110 may be custom designed to provide the laser pulse returned to laser cavity 105 via output port 112D with specific spectral properties, such as a particular central wavelength, a particular spectral width and a particular extinction ratio. In general, these spectral properties determine the spectral properties of the outputted laser pulse. It is noted that a plurality of different wavelengths can be generated for the outputted laser pulse of a given doped optical fiber according to the emission cross-section spectral line of its gain medium. For example, the outputted pulse of a Yb-doped laser may have a wavelength ranging from 1030 nanometers (nm) to 1080 nm. It is noted that in this embodiment, no fiber Bragg gratings (FBG) are used, which results in fiber laser 100 being quieter during operation and which increases the operational stability of fiber laser 100 between outputted pulses.
In general, the following parameters are specified according to the desired effect on the power, pulse shape and pulse width of the output laser pulse: the amount of doping and the core size of doped optical fiber 108, the coupling ratio of coupler 110 and the length of laser cavity 105. The length of laser cavity 105 includes the length of doped optical fiber 108 as well as the passive optical fibers which couple doped optical fiber 108 with coupler 110 and combiner 106, and which couple coupler 110 with combiner 106. Passive optical fibers refer to optical fibers which not are doped. In general, except for doped optical fiber 108, all optical fibers in fiber laser 100 are passive optical fibers. According to the disclosed technique, the output peak power of laser diode 104 and the duration of the pump pulses are adjusted and fine-tuned in order to specify a particular output peak power and particular pulse duration of the output laser pulse. The output peak power and the duration of the pump pulses are also determined such that subsequent pulses, except for the desired output laser pulse, are not generated. Other parameters that affect the output peak power and the pulse duration of the output laser pulse include the particular shape of the laser pulse of laser diode 104 as well as the repetition rate at which the laser pulse of laser diode 104 is provided.
Reference is now made to
In fiber laser 130, doped optical fiber 138 is pumped by two laser diodes, first laser diode 134A and second laser diode 134B. It is noted that doped optical fiber 138 can also be pumped by a plurality of laser diodes (not shown), which can be combined by a standard pump combiner or, alternatively, can each be coupled with the gain fiber individually. In one embodiment, first laser diode 134A and second laser diode 134B pump doped optical fiber 138 simultaneously. In another embodiment, a delay in time is placed on one of the laser diodes such that first laser diode 134A and second laser diode 134B do not pump doped optical fiber 138 at the same time. It is noted that each of the electronic drivers provide respective diode drive signals to their respective laser diodes. In general, the various parameters specifying first electronic driver 132A and second electronic driver 132B as well as the various parameters specifying first laser diode 134A and second laser diode 134B can be substantially similar or different. For example, the laser diodes may pump doped optical fiber 138 with the same output peak power or with different output peak powers. Also, the duration of time each electronic driver operates its respective laser diode may be the same or may differ. In general, different diode drive signals may be combined to achieve a desired pump pulse shape. Furthermore, one of the laser diodes may be operated in a CW mode at a low output power level as a laser bias, while the other laser diode operates as a laser pump. Operating one of the laser diodes in a CW mode may expedite the population inversion of the active atoms or ions in doped optical fiber 138. For example, electronic driver 132A may operate laser diode 134A in a CW mode at an output power level which is sufficiently low to maintain the gain of doped optical fiber 138 below its threshold value, whereas electronic driver 132B may operate laser diode 134B in a pulsed mode for pumping doped optical fiber 138. In this example, laser diodes 134A and 134B can be of lower output peak power. It is noted that in an embodiment where a plurality of laser diodes are provided to pump doped optical fiber 138, at least one of the laser diodes may be operated in a CW mode as a laser bias, whereas at least another one of the laser diodes may be operated in a pulsed mode for pumping the gain fiber.
Reference is now made to
In fiber laser 170, isolator 178 enables the stimulated radiation produced in doped optical fiber 184 to propagate in only one direction, thereby causing uni-directional lasing in laser cavity 175 and increasing the output power of the outputted laser pulse. The direction of propagation enabled by isolator 178 corresponds to the direction of propagation of laser light through coupler 180, depicted in
Reference is now made to
It is noted that fiber laser 210 can include an isolator (not shown) substantially similar to isolator 178 (
Reference is now made to
It is noted that fiber laser 250 can include an isolator (not shown) substantially similar to isolator 178 (
Reference is now made to
In fiber laser 290, uni-directional lasing is achieved via circulator 300 and FBG 302. Laser radiation provided to combiner 296 is provided to circulator 300, via doped optical fiber 298. Circulator 300 transfers the laser radiation to FBG 302, which reflects a portion of it back to circulator 300 while the rest is outputted as a laser pulse. Circulator 300 then provides the reflected laser radiation back to combiner 296. In this respect, uni-directional lasing is achieved in fiber laser 290. In general, FBG 302 can be used to determine the spectral properties of the outputted laser pulse, as a portion of the laser radiation circulating inside laser cavity 295 is provided to FBG 302. FBG 302 may have a specified central wavelength, which is either tunable or constant, as well as a particular spectral response. Laser radiation, which is provided to FBG 302, is reflected back to circulator 300 at specific wavelengths according to the specified central wavelength, the spectral response, or both of FBG 302. This increases the portion of laser radiation in laser cavity 295 having a particular wavelength, thereby determining the wavelength of the laser pulse circulating inside laser cavity 295. It is noted that the optimal amount of reflected laser radiation provided to circulator 300 via FBG 302, may vary according to various parameters of fiber laser 290. In general, the portion of laser light reflected from FBG 302 back to circulator 300 is small to enable a greater portion of the laser radiation circulating inside the cavity to be outputted as the laser pulse. It is noted that in another embodiment, FBG 302 may be replaced by an optical fiber mirror (not shown). The optical fiber mirror may include a selective wavelength optical coating. The optical coating may be anti-reflective. In such an embodiment, fiber laser 290 may also include a band pass filter (not shown), coupled between circulator 300 and the optical fiber mirror.
Reference is now made to
Laser cavity 335 is formed via HRFBG 336, combiner 337 and LRFBG 340. Both HRFBG 336 and LRFBG 340 are used to determine the spectral properties of the outputted laser pulse. In general, both HRFBG 336 and LRFBG 340 have substantially similar specified central wavelengths, which are either tunable or constant, as well as substantially similar spectral widths and linewidths. Laser pulses are provided to combiner 337, which provides the laser pulses to HRFBG 336. The laser pulses are then reflected in HRFBG 336 and provided to LRFBG 340 via combiner 337 and doped optical fiber 338. Laser pulses which are provided to HRFBG 336 are provided to LRFBG 340. LRFBG 340 reflects back a portion of the laser pulses to HRFBG 336, via combiner 337, at specific wavelengths according to at least one of the specified central wavelength, the spectral response, or the linewidth of the fiber Bragg gratings in fiber laser 330. This increases the portion of laser radiation in laser cavity 335 having a particular wavelength, thereby determining the wavelength of the outputted laser pulse.
Reference is now made to
Saturable absorber 370 may be positioned anywhere inside laser cavity 365, for example, between coupler 372 and combiner 366. It is noted that saturable absorber 370 may be positioned and used in any of the embodiments described above in
The properties of saturable absorber 370 that affect the formation of the laser pulse include: initial transmittance value, saturation fluence and modulation depth. Initial transmittance value is a measure of how much of the laser radiation in laser cavity 365 can initially pass through saturable absorber 370. Saturation fluence refers to the fluence (i.e., energy per unit area) it takes to reduce the initial value of the fluence to 1/e of its initial value, where e is the base of the natural logarithm. Modulation depth refers to the maximum amount of change in optical losses. The selected values of the initial transmission value, saturation fluence and modulation depth of saturable absorber 370 are adjusted and fine-tuned depending on the desired effect on the outputted laser pulse, such as an increase in its power and a decrease in its width. In addition, the absorption spectrum of saturable absorber 370 should substantially correspond to the wavelength of the stimulated radiation circulating in laser cavity 365. Furthermore, the absorption cross-section of saturable absorber 370 should be higher than the emission cross-section of doped optical fiber 368 at the wavelength of the stimulated radiation circulating in laser cavity 365, so that saturable absorber 370, as described below, can increase the lasing threshold of fiber laser 360. Also, the saturation recovery time of saturable absorber 370 should be on the order of magnitude of the desired pulse width of the outputted laser beam. The saturation recovery time can also be longer than the desired pulse width of the outputted laser beam, but shorter than the time between consecutive pump pulses.
In the embodiment of
Reference is now made to
In the embodiment of
In general, fiber Bragg gratings enable radiation to be reflected to varying degrees in particular wavelength regions. For example, HRFBG 406 reflects substantially all radiation impinging on it having a wavelength similar to its specified central wavelength, whereas LRFBG 412 reflects only a portion of the radiation impinging on it having a wavelength similar to its specified central wavelength. Tuner 410 enables the specified central wavelength of a fiber Bragg grating to be slightly shifted. In fiber laser 400, second electronic driver 411 causes tuner 410 to slightly shift the specified central wavelength of HRFBG 406 synchronously with the pump pulses provided by laser diode 404 to laser cavity 405. It is noted that the operation of first electronic driver 402 and second electronic driver 411 is synchronized. The specified central wavelength of HRFBG 406 is shifted sufficiently such that the wavelengths at which HRFBG 406 and LRFBG 412 reflect at do not fully overlap, thereby causing losses in laser cavity 405. In other words, laser radiation is not reflected back and forth between the two fiber Bragg gratings as they now reflect at different wavelengths. As losses in the cavity occur, laser diode 404 can provide more energy to laser cavity 405, thereby increasing the population inversion of doped optical fiber 408 before the lasing threshold of fiber laser 400 is reached, i.e., increasing the lasing threshold of fiber laser 400. Once a desired increased population inversion is achieved, tuner 410 can be used to shift the specified central wavelength of HRFBG 406 back to its initial value such that laser radiation reflects back and forth in laser cavity 405, thereby causing optical feedback in laser cavity 405. Due to the optical feedback, the stimulated radiation is rapidly amplified and the gain of doped optical fiber 408 is depleted thereby causing the generation of a laser pulse which is outputted via LRFBG 412. The outputted pulse has a higher power and shorter pulse width as compared to an outputted pulse generated in the setup of
Tuner 410 can be embodied as a piezoelectric or magneto-mechanic actuator. In such an embodiment, HRFBG 406 includes a strain which can be induced by the actuator, resulting in a physical change in the length of HRFBG 406 due to pressure. The length change alters the reflection spectrum of HRFBG 406, shifting its central wavelength. Tuner 410 can also be embodied as a thermo-electric cooler, which can result in a physical change in the length of HRFBG 406 due to variations in temperature. Tuner 410 is controlled by second electronic driver 411 and can be controlled by any pulse shape from second electronic driver 411 to repetitively prevent overlap of the wavelengths at which the fiber Bragg gratings reflect.
Reference is now made to
Laser cavity 505 is formed via optical fiber mirror 508, combiner 506 and coupler 512. The spectral properties of the outputted laser pulse are determined by either the spectral properties of optical fiber mirror 508, the spectral properties of coupler 512 or both. Optical fiber mirror 508 can include, for example, a fiber pigtailed collimator and a mirror. The collimator may have an anti-reflective optical coating to reduce transmission losses and the mirror may be an optically coated glass surface or metal surface, for example. In such a case, the spectral properties of the optical fiber mirror will be defined by the combined spectral properties of the collimator, the collimator coating, the mirror and the mirror coating. The spectral properties of coupler 512 are similar to the spectral properties of coupler 110 (
In fiber laser 500, electronic driver 502 operates laser diode 504 by providing laser diode 504 with a drive current. Laser diode 504 then provides pump pulses to combiner 506, which provides the pump pulses to doped optical fiber 510 which generates laser pulses. The laser pulses are reflected in coupler 512 and are provided back to doped optical fiber 510 and then to optical fiber mirror 508. Optical fiber mirror 508 reflects the received laser pulses and provides the reflected laser pulses to coupler 512 via doped optical fiber 510. The output ports of coupler 512, as shown in section 516, reflect a portion of the laser pulses back to optical fiber mirror 508, via doped optical fiber 510 and combiner 506, whereas another portion of the laser pulses are outputted as laser light via the second input port of coupler 512. Laser pulses are substantially reflected between optical fiber mirror 508 and coupler 512 until the lasing threshold is reached, at which point laser light is outputted by one of the input ports of coupler 512.
Isolator 434 provides the laser pulse to amplifier 436 which amplifies the laser pulse, thereby increasing its power. Amplifier 436 may include a plurality of amplification stages. Amplifier 436 can be constructed to amplify laser pulses only at the wavelength of the outputted laser pulses of fiber laser 432.
It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL2009/001136 | 12/2/2009 | WO | 00 | 11/28/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61119466 | Dec 2008 | US | |
| 61262178 | Nov 2009 | US |
