WO2019117808A1

WO2019117808A1 - Laser system and method of forming the same

Info

Publication number: WO2019117808A1
Application number: PCT/SG2018/050603
Authority: WO
Inventors: Xia YU; Biao Sun
Original assignee: Agency For Science, Technology And Research
Priority date: 2017-12-12
Filing date: 2018-12-11
Publication date: 2019-06-20

Abstract

Various embodiments may relate to a laser system. The laser system may include an oscillator, a first amplifier stage optically coupled to the oscillator, a gain equalizer stage optically coupled to the first amplifier stage, a pulse picker stage optically coupled to the gain equalizer stage, a second amplifier stage optically coupled to the pulse picker stage, and a third amplifier stage optically coupled to the second amplifier stage.

Description

LASER SYSTEM AND METHOD OF FORMING THE SAME

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10201710314V filed December 12, 2017, the contents of it being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] Various aspects of this disclosure relate to a laser system. Various aspects of this disclosure relate to a method of forming a laser system.

BACKGROUND

[0003] Ultrafast high energy laser emissions at 2 um are usually generated by three approaches.

[0004] The first approach involves using a 800nm titanium (Ti) : Sapphire laser seeded amplifier to pump a parametric crystal. This approach is called optical parametric amplification (OP A). Light Conversion is manufacturing a system called TOPAS based on this approach. The output specification at 2 um is > 200m J, at 1 kHz, 20fs. The cost of the whole system is approximately Singapore $1.2 million.

[0005] The second approach relates to direct emission from gain crystals doped with Thulium (Tm) or Holium (Ho) ions. This approach is called stimulated emission-based amplification, which includes regenerative amplification, cryogenic amplification and chirped pulse amplification (CPA) etc. A seed laser together with one or more powerful pumps are used to generate amplified laser emission. Qpeak is manufacturing a CPA plus regenerative amplifier architecture. The output specification at 2 um is > lmJ, at 1 kHz, 2ps. The cost of the whole system is approximately Singapore $450,000.

[0006] The third approach relates to direct emission from gain fibers doped with Tm or Ho ions. This approach is similar to the second approach, but involve confining the light in a piece of flexible glass (fiber optics). The approach may also include chirped pulse amplification (CPA). The state-of-the art output specification at 2 um is > 200uJ, at 100 kHz, lps. The estimated cost of the whole system is about Singapore $200,000. [0007] The different technical approaches for amplification as described above have their advantages and limitations.

[0008] OPA may achieve wide spectral coverage. The ultrafast pulse property of OPA may allow it to be a good scientific tool for few-cycle and even attosecond research. Regenerative amplification and cryogenic amplification can boost up the energy to more than 10 mJ. There has been interest from many research groups to further scale up the energy to Joule level. However, the repetition rate of regenerative amplification and cryogenic amplification may be limited by the available average power of the pump and conversion efficiency.

SUMMARY

[0009] Various embodiments may relate to a laser system. The laser system may include an oscillator configured to generate a seed optical signal. The laser system may also include a first amplifier stage optically coupled to the oscillator. The first amplifier stage may be configured to generate an amplified optical signal based on the seed optical signal received from the oscillator. The laser system may further include a gain equalizer stage optically coupled to the first amplifier stage. The gain equalizer may be configured to modify a pulse shape of the amplified optical signal from the first amplifier stage to generate a modified optical signal. The laser system may additionally include a pulse picker stage optically coupled to the gain equalizer stage. The pulse picker stage may be configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal. The laser system may also include a second amplifier stage optically coupled to the pulse picker stage. The second amplifier stage may be configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage to generate a noise reduced optical signal. The laser system may further include a third amplifier stage optically coupled to the second amplifier stage. The third amplifier stage may be configured to amplify the noise reduced optical signal from the second amplifier stage to generate a laser.

[0010] The gain equalizer stage may include a polarization controller, and a polarization- dependent isolator optically coupled to the polarization controller. The pulse picker stage may include an acousto-optical modulator (AOM) optically coupled to the polarization-dependent isolator. The second stage amplifier may include a gain medium optically coupled to the pulse picker stage, and an assistive fiber laser (AFL) also optically coupled to the gain medium so that the assistive fiber laser (AFL) provides a source for simulated emission and reduces the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage. The third amplifier stage may include a photonic crystal fiber (PCF) configured to amplify the noise reduced optical signal from the second stage amplifier.

[0011] Various embodiments may relate to a method of forming a laser system. The method may include optically coupling a first amplifier stage to an oscillator configured to generate a seed optical signal. The first amplifier stage may be configured to generate an amplified optical signal based on the seed optical signal received from the oscillator. The method may also include optically coupling a gain equalizer stage to the first amplifier stage. The gain equalizer may be configured to modify a pulse shape of the amplified optical signal from the first amplifier stage to generate a modified optical signal. The method may additionally include optically coupling a pulse picker stage to the gain equalizer stage. The pulse picker stage may be configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal. The method may further include optically coupling a second amplifier stage to the pulse picker stage. The second amplifier stage may be configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage to generate a noise reduced optical signal. The method may also include optically coupling a third amplifier stage to the second amplifier stage. The third amplifier stage may be configured to amplify the noise reduced optical signal from the second amplifier stage to generate a laser. The gain equalizer stage may include a polarization controller, and a polarization- dependent isolator optically coupled to the polarization controller. The pulse picker stage may include an acousto-optical modulator (AOM) optically coupled to the polarization-dependent isolator. The second stage amplifier may include a gain medium optically coupled to the pulse picker stage, and an assistive fiber laser (AFL) also optically coupled to the gain medium so that the assistive fiber laser (AFL) provides a source for simulated emission and reduces the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage. The third amplifier stage may include a photonic crystal fiber (PCF) configured to amplify the noise reduced optical signal from the second stage amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

FIG. 1 A shows a schematic of a fiber laser.

FIG. !B is a plot of emission as a function of wavelength (in micrometers or mih) illustrating the emission wavelengths of different fiber lasers doped with different elements.

FIG. 1C illustrates the operation of a chirped pulse amplifier (CPA).

FIG. 2A is a schematic illustrating a laser system according to various embodiments.

FIG. 2B is a plot of power (in decibel with reference to 1 milli-Watt or dBm) as a function of wavelength (in nanometer or nm) showing a measured gain spectrum of a thulium (Tm) doped fiber.

FIG. 2C is a schematic illustrating the higher gain at short wavelengths of a conventional fiber amplifier.

FIG. 2D illustrates the operation of the polarization controller, the polarization-dependent isolator (PDISO) according to various embodiments.

FIG. 2E shows (top) polarization shaping of the pulse in the temporal domain according to various embodiments, and (bottom) polarization shaping of the pulse in the spectral domain according to various embodiments.

FIG. 2F shows a schematic illustrating the difference between spontaneous emission and stimulated emission.

FIG. 2G shows a plot of normalized power as a function of wavelength showing the effect of using a continuous wave (CW) laser of a wavelength to force stimulated emission according to various embodiments.

FIG. 2H is a plot of power (in decibel with reference to 1 milli-Watt or dBm) as a function of wavelength (in nanometer or nm) illustrating the measured spectra with (W/) or (W/O) the assistive fiber laser (AFL) at different pump currents according to various embodiments.

FIG. 21 illustrates the amplified spontaneous emission (ASE) accumulation in an amplifier when the pulse to pulse separation is 10 /ts. FIG. 2J shows (top) the amplified spontaneous emission (ASE) in a conventional gain medium, and (bottom) the reduction in ASE when an assistive fiber laser (AFL) is used for ASE suppression in a low repetition rate laser amplifier according to various embodiments.

FIG. 2K shows a photonic crystal fiber (PCF) according to various embodiments.

FIG. 2L illustrates the photonic crystal fiber (PCF) connected to a step index fiber such as FUD- 3440 via an intermediate fiber according to various embodiments.

FIG. 2M shows the photonic crystal fiber (PCF) being placed in a water tube for thermal management according to various embodiments.

FIG. 2N shows an image of the laser system according to various embodiments.

FIG. 3A shows a plot of normalized intensity as a function of wavelength (in nanometer or nm) illustrating the output spectrum at various pulse energies of the laser generated by the laser system according to various embodiments.

FIG. 3B shows a plot of normalized intensity as a function of time (in seconds or s x l0^~9) illustrating two pulse profiles of the laser generated by the laser system according to various embodiments.

FIG. 3C shows a plot of output power (in Watts or W) / pulse width (in nanoseconds or ns) as a function of pump power (in Watts or W) illustrating the output power and pulse width at various pump power values according to various embodiments.

FIG. 4 is a schematic illustrating design of the laser system according to various embodiments. FIG. 5 is a schematic illustrating a laser system according to various embodiments.

FIG. 6 is a schematic illustrating a method of forming a laser system according to various embodiments.

FIG. 7 shows (left) a schematic diagram for welding two highly transparent plastics with a butt joint; and (right) images of welding examples prepared with a 2 mia Thulium- Yttrium- Aluminum- Garnet (Tm : YAG) laser according to various embodiments.

FIG. 8 shows a table comparing some parameters of conventional laser system and a laser system according to various embodiments.

DETAILED DESCRIPTION [0013] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0014] Embodiments described in the context of one of the methods or laser systems are analogously valid for the other methods or laser systems. Similarly, embodiments described in the context of a method are analogously valid for a laser system, and vice versa.

[0015] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0016] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0017] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[0018] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items.

[0019] Fiber-based laser technology may be suitable for applications which requires high repetition rate of photons, for example, imaging and spectroscopy applications.

[0020] A fiber laser is an optical fiber doped with rare-earth element such that it acts as a gain medium. FIG. 1A shows a schematic of a fiber laser. The fiber laser may include a core, an inner cladding around the core, and an outer cladding around the inner cladding. Pump light may be introduced at a first end of the fiber laser, and the fiber laser may emit laser at a second end of the fiber laser. [0021] FIG. 1B is a plot of emission as a function of wavelength (in micrometers or mpi) illustrating the emission wavelengths of different fiber lasers doped with different elements.

[0022] FIG. 1C illustrates the operation of a chirped pulse amplifier (CPA). A short pulse is initially emitted by a short-pulse oscillator. A pair of gratings disperse the spectrum and stretches the pulse by a factor of a thousand. The dispersed pulse is long, has low power, and is safe for amplification. Power amplifiers then amplifies the dispersed pulse into a high energy pulse. A second pair of gratings then reverses the dispersion and recompresses the high energy pulse into a resultant high-energy, ultra-short pulse.

[0023] Conventional laser systems may have disadvantages. Rod-type fiber laser system may have limited flexibility in terms of operation, while free-space optics may suffer from limited stability and large footprint.

[0024] Various embodiments may seek to address one or more issues or disadvantages facing conventional laser systems.

[0025] FIG. 2A is a schematic illustrating a laser system 200 according to various embodiments. The laser system 200 may be an all-fiber laser system. The setup illustrated in FG. 2A may be in stackable modules laid over an optical table. The laser system 200 may include an oscillator 202 configured to generate a seed optical signal. The seed optical signal may be a pulse train including a plurality of pulses. The oscillator 202 may be a non-linear polarization rotation- based mode-lock oscillator.

[0026] The laser system 200 may also include a first amplifier (amp) stage 204 (alternatively referred to as a pre-amp stage) optically coupled to the oscillator 202. The first amplifier stage 204 may be configured to generate an amplified optical signal based on the seed optical signal received from the oscillator 202. The amplified optical signal may also be a pulse train including a plurality of pulses, each pulse having an amplitude greater than a pulse of seed optical signal.

[0027] As shown in FIG. 2 A, the first amplifier stage 204 may include an optical circulator 214 optically coupled to the oscillator 202. The optical circulator may have a first port (port a) coupled to the oscillator 202, a second port (port b) coupled to an optical coupler 218, and a third port (port c) coupled to a gain equalizer stage 206. The optical circulator 214 may be configured so that an optical signal entering one port may only exit from the next port. As such, the seed optical signal from the oscillator 202 entering port a may exit from port b, and may travel to the optical coupler 218. The first amplifier stage 204 may include a gain fiber 216, and a further optical coupler 220 in addition to the optical coupler 218. The gain fiber 216 may be connected to the optical coupler 218 at a first end and to the further optical coupler 220 at a second end. Ml and MG coupled to the optical coupler 220 may be monitoring ports for laser diagnosis. The gain fiber 216 may be configured to amplify the seed optical signal received from the oscillator 202. The first amplifier stage 204 may further include a chirped fiber Bragg grating (CFBG) 222 connected to the further optical coupler 220. The chirped fiber Bragg grating 222 may be configured to reflect a predetermined range of wavelengths of the amplified seed optical signal from the gain fiber 216. The amplified seed optical signal reflected from grating 222 may then be further amplified as it passes through the gain fiber 216 to generate the amplified optical signal.

[0028] The laser system 200 may also include the gain equalizer stage 206 optically coupled to the first amplifier stage 204. The amplified optical signal may then travel to the gain equalizer stage 206 as it enters port b of the optical circulator 214, and exits port c of the optical circulator 214. The gain equalizer 206 may be configured to modify a pulse shape of the amplified optical signal from the first amplifier stage 204 to generate a modified optical signal. In other words, the gain equalizer 206 may be for spectral shaping. The gain equalizer stage 206 may include a polarization controller (PC) 224, and a polarization-dependent isolator (PDISO) 226 optically coupled to the polarization controller 224.

[0029] A challenge facing a conventional multi-stage fiber amplifier is the higher gain at short wavelengths. It causes a significant temporal pulse shortening under high power conditions. The increased peak power due to the shortening of the temporal pulse duration may cause detrimental nonlinear effects. The reduction by amplitude shaping of the seed pulses in the stretcher has been previously demonstrated and known. FIG. 2B is a plot of power (in decibel with reference to 1 milli-Watt or dBm) as a function of wavelength (in nanometer or nm) showing a measured gain spectrum of a thulium (Tm) doped fiber. The absorption of the fiber us 27 dB at about 793 nm while the ion concentration is 8.2e+24 m ³. FIG. 2C is a schematic illustrating the higher gain at short wavelengths of a conventional fiber amplifier. As shown in FIG. 2C, this may result in a significant reduction of the 3dB bandwidth for the laser spectrum.

[0030] The polarization controller 224 as shown in FIG. 2A may be an optical device configured to modify the polarization of light, i.e. the amplified optical signal from the first amplifier stage 204. The polarization controller 224 may be configured such that different wavelengths of the amplified optical light passing through may be polarized in different directions. The polarization-dependent isolator 226 may allow the amplified optical signal to pass through with loss based on the polarization to generate the modified optical signal. FIG. 2D illustrates the operation of the polarization controller 224, the polarization-dependent isolator (PDISO) 226 according to various embodiments. The inset shows the phase and path difference of differently polarized light, which lead to different losses.

[0031] The polarization controller 224 together with the polarization-dependent isolator (PD ISO) 226 may be able to pre-compensate the lower gain of the main amplifier at predetermined range of wavelengths, e.g. wavelengths > l950nm. By controlling the birefringence, the polarization dependent loss may shape the pulse in both temporal and spectral domains as shown in FIG. 2E. FIG.2E shows (top) polarization shaping of the pulse in the temporal domain according to various embodiments, and (bottom) polarization shaping of the pulse in the spectral domain according to various embodiments.

[0032] The laser system 200 as shown in FIG. 2A may further include a pulse picker stage 208 optically coupled to the gain equalizer stage 206. The pulse picker stage 208 maybe for repetition down frequency, i.e. may be configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal. The pulse picker stage 208 may include an acousto-optical modulator (AOM) 228 optically coupled to the polarization-dependent isolator 226.

[0033] The acousto-optical modulator 228 may be configured to use acousto-optic effect to diffract and shift the frequency of light using sound waves (e.g. at radio-frequency). The acousto- optical modulator 228 may include a medium, e.g. a crystal such as glass or quartz, configured to allow the modified optical signal from the gain equalizer stage 206 to pass through. The acousto- optical modulator 228 may also include a piezoelectric transducer attached to the medium. The piezoelectric transducer may be configured to generate sound waves in the medium based on a radio frequency (RF) signal so that a pulse repetition rate of the down frequency optical signal generated by the pulse picker stage 208 is based on a frequency of the radio frequency signal. The pulse picker stage 208 may further include a radio frequency (RF) driver 230 coupled to the acousto-optical modulator 228, the radio frequency driver 230 configured to generate the radio frequency signal. The pulse picker stage 208 may additionally include a syn-unit 232 coupled to the radio frequency (RF) driver 230. The syn-unit 232 may be configured to reduce a repetition rate of the laser system 200, e.g. from 65 MHz to 100 kHz.

[0034] The laser system 200 as shown in FIG. 2A may further include a second amplifier (amp) stage 210 (alternatively referred to as an amp 1 stage) optically coupled to the pulse picker stage 208. The second amplifier stage 210 may be configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage 208 to generate a noise reduced optical signal.

[0035] The second amplifier stage 210 may include an optical coupler 230 optically coupled to the acousto-optical modulator 228, an optical circulator 232 optically coupled to the optical coupler 230, and a gain medium, e.g. a gain fiber 234, optically coupled to the optical circulator 232. The second amplifier stage 210 may also include an assistive laser fiber (AFL) (alternatively referred to as an auxiliary fiber laser) coupled to the optical coupler 230. A monitoring port M2 may also be coupled to the optical coupler 230. The optical circulator 232 may be configured so that light from the optical coupler 230 entering port a may exit from port b to the gain medium 234. As such, the gain medium 234 may be optically coupled to the pulse picker stage 208 and the assistive fiber laser (AFL) via the optical coupler 230 and the optical circulator 232.

[0036] Amplified spontaneous emission (ASE) may be produced when a laser gain medium in a conventional laser system is pumped to produce a population inversion. FIG. 2F shows a schematic illustrating the difference between spontaneous emission and stimulated emission. In conventional high energy laser amplifiers, ASE may limit the temporal intensity contrast, namely signal to noise ratio (SNR). Various embodiments may provide a mechanism to absorb or extract the incoherent ASE.

[0037] FIG. 2G shows a plot of normalized power as a function of wavelength showing the effect of using a continuous wave (CW) laser of a wavelength to force stimulated emission according to various embodiments. Using the laser may result in“squeezing the noise” from broadband to narrowband.

[0038] FIG. 2H is a plot of power (in decibel with reference to 1 milli-Watt or dBm) as a function of wavelength (in nanometer or nm) illustrating the measured spectra with (W/) or (W/O) the assistive fiber laser (AFL) at different pump currents according to various embodiments. [0039] As shown in FIG. 2H, when the pump current is increased from 12.7A to 13.5A and then to 14A, the spectrum gets broader and the signal-to-noise ratio (SNR) degrades. This phenomenon may be especially pronounced in a next stage amplifier when the repetition rate is reduced, as the pulse to pulse separation gets longer, leaving more time window for ASE accumulation. The ASE may propagate and may be further amplified in a subsequent amplifier stage. This phenomenon may contribute to the noise. Otherwise, the excitation of the gain medium may be depleted by the incoherent ASE rather than by the desired coherent laser radiation, limiting the amplification in the subsequent amplifier stage.

[0040] FIG. 21 illustrates the amplified spontaneous emission (ASE) accumulation in an amplifier when the pulse to pulse separation is 10 / s.

[0041] Various embodiments may include an assistive fiber laser AFL, e.g. a continuous wave mode thulium-doped fiber laser (CW-TDFL) with emission wavelength at 1950nm. The AFL may serve as a source of signal for stimulated emission and hence suppresses the ASE. In other words, the assistive fiber laser (AFL) may provide a source for simulated emission and may reduce the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage 208, thus improving the signal-to-noise ratio (SNR).

[0042] FIG. 2J shows (top) the amplified spontaneous emission (ASE) in a conventional gain medium, and (bottom) the reduction in ASE when an assistive fiber laser (AFL) is used for ASE suppression in a low repetition rate laser amplifier according to various embodiments.

[0043] As shown in FIG. 2H, the SNR has been improved from 11.4 dB to 35.5 dB at pump current of 14A when the AFL is used.

[0044] The laser system 200 as shown in FIG. 2A may also include an optical coupler 236 optically coupled to the gain medium 234, e.g. the gain fiber. The optical coupler 236 may be connected to a clipped chirped fiber Bragg grating (CFBG) 238. The light exiting from the gain medium 234 may pass through the optical coupler 236 to the CFBG 238. A range of wavelengths of the light may be reflected by the CFBG 236 may travel back to the gain medium 234 via the coupler 236. As such, the second amplifier stage 210 may be configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage 208 to generate the noise reduced optical signal. The noise reduced optical signal may then enter port b of the optical circulator 232, and may then exit from port c of the optical circulator 232.

[0045] The laser system 200 may also include a third amplifier (amp) stage 212 (alternatively referred to as a photonic crystal fiber (PCF)-amp stage) optically coupled to the second amplifier stage 2lO.The third amplifier stage 212 may be optically coupled to port c of the optical circulator 232. The third amplifier stage 212 may be configured to amplify the noise reduced optical signal from the second amplifier stage 210 to generate a laser. The third amplifier stage 212 may include an optical coupler 240 connected to port c of the optical circulator 232. The third amplifier stage 212 may further include a photonic crystal fiber (PCF) 242 optically coupled to the optical coupler 240.The photonic crystal fiber 242 may be configured to amplify the noise reduced optical signal from the second stage amplifier 210 to generate the laser. The third amplifier stage 212 may additionally include a monitoring port M3 coupled to the optical coupler 240.

[0046] The microstructured photonic crystal fiber (PCF)-amplifier 242 may boost the pulse energy to hundreds-of micro-Joules. The configuration is illustrated in FIG. 2K. A 2.4 m thulium doped PCF with 37 um mode field diameter (MFD) may be used in the last stage fiber based amplifier.

[0047] FIG. 2K shows a photonic crystal fiber (PCF) 242 according to various embodiments. The insets show (top left) connection of the PCF 242 with an intermediate 30/250 fiber using fusion splicing, (top right) an uncollapsed cross-sectional view of the PCF 242, (bottom left) an angle cleaved end of the PCF 242, and (bottom right) a collapsed cross-sectional view of the PCF 242.

[0048] The connections between 3 amplifier stages 204, 210, 212 and the pulse picker stage 208 may be connected by fusion splicing. The third amplifier stage 212 may include a step-index fiber. However, the step-index fiber and the thulium doped PCF 242 may not be directly connected for optical coupling, and may require connection via an intermediate fiber.

[0049] The first reason for the above is the large difference of MFD (21 mih for step index fiber, 37 mpi for PCF 242). The second reason is the microstructure inside the PCF 242. In various embodiments, the third amplifier stage 212 may include an intermediate fiber to connect the PCF 242 and the step index fiber. FIG. 2L illustrates the photonic crystal fiber (PCF) 242 connected to a step index fiber 244 such as FUD-3440 via an intermediate fiber 246 according to various embodiments.

[0050] The laser system 200 may include a water tube configured to hold the photonic crystal fiber (PCF) 242 and the intermediate fiber 246 so that a joint between the photonic crystal fiber (PCF) 242 and the intermediate fiber 246 is contained within the water tube. The water tube may be further configured to allow water to flow from a first portion of the tube to a second portion of the tube for thermal management.

[0051] FIG. 2M shows the photonic crystal fiber (PCF) 242 being placed in a water tube 248 for thermal management according to various embodiments. The spliced connection between fibers 242, 246 may be held by the water tube 248, and may be sealed by metal caps 250a, 250b. The angle cleaved end may be very easily damaged. Therefore, it may be better to start the packaging process of the water tube from the other end. T-shape tubes 252a, 252b may be used to connect the water tube 248 and guide water through the tube 242 as shown in FIG. 2M.

[0052] In various embodiments, the laser system 200 may be configured to generate a laser having a central wavelength of about 1975 nm, and a 10 dB bandwidth of about 30 nm. The laser may have a tunable repetition rate tunable from about 100 kHz to about 65 MHz. The laser may have a pulse energy of about 480 joJ at a repetition rate of about 100 kHz. The pulse width may be about 2 ns. The laser system 200 may occupy a footprint of about 50 cm by 50 cm by 50 cm. FIG. 2N shows an image of the laser system 200 according to various embodiments.

[0053] FIG. 3A-C show the laser output of a laser system according to various embodiments measured in the time domain, the spectral domain and the spatial domain. The pulse energy is indirectly measured by a power meter.

[0054] FIG. 3 A shows a plot of normalized intensity as a function of wavelength (in nanometer or nm) illustrating the output spectrum at various pulse energies of the laser generated by the laser system according to various embodiments.

[0055] FIG. 3B shows a plot of normalized intensity as a function of time (in seconds or s x 10 ⁹) illustrating two pulse profiles of the laser generated by the laser system according to various embodiments.

[0056] FIG. 3C shows a plot of output power (in Watts or W) / pulse width (in nanoseconds or ns) as a function of pump power (in Watts or W) illustrating the output power and pulse width at various pump power values according to various embodiments. The circle data points in FIG. 3C relate to the output power while the diamond data points relate to pulse width.

[0057] FIG. 4 is a schematic illustrating design of the laser system according to various embodiments. In step 1, the all- fiber laser system may be designed in simulation software. In step 2, the oscillator may be sourced or designed. In step 3, the multi-stage amplifier for pulse shaping may be designed. In step 4, the pre-amplifier stage including the auxiliary fiber laser (AFL) for amplified spontaneous emission (ASE) suppression may be designed. In step 5, splicing and thermal management in the photonic crystal fiber based amplifier may be designed. In step 6, measurement and characterization may be carried out.

[0058] FIG. 5 is a schematic illustrating a laser system 500 according to various embodiments. The laser system 500 may include an oscillator 502 configured to generate a seed optical signal. The laser system 500 may also include a first amplifier stage 504 optically coupled to the oscillator 502. The first amplifier stage 504 may be configured to generate an amplified optical signal based on the seed optical signal received from the oscillator 502. The laser system 500 may further include a gain equalizer stage 506 optically coupled to the first amplifier stage 504. The gain equalizer 506 may be configured to modify a pulse shape of the amplified optical signal from the first amplifier stage 504 to generate a modified optical signal. The laser system 500 may additionally include a pulse picker stage 508 optically coupled to the gain equalizer stage 506. The pulse picker stage 508 may be configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal. The laser system 500 may also include a second amplifier stage 510 optically coupled to the pulse picker stage 508. The second amplifier stage 510 may be configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage 508 to generate a noise reduced optical signal. The laser system 500 may further include a third amplifier stage 512 optically coupled to the second amplifier stage 510. The third amplifier stage 512 may be configured to amplify the noise reduced optical signal from the second amplifier stage 510 to generate a laser.

[0059] The gain equalizer stage 506 may include a polarization controller, and a polarization- dependent isolator optically coupled to the polarization controller. The pulse picker stage 508 may include an acousto-optical modulator (AOM) optically coupled to the polarization-dependent isolator. The second stage amplifier 510 may include a gain medium optically coupled to the pulse picker stage, and an assistive fiber laser (AFL) also optically coupled to the gain medium so that the assistive fiber laser (AFL) provides a source for simulated emission and reduces the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage. The third amplifier stage 512 may include a photonic crystal fiber (PCF) configured to amplify the noise reduced optical signal from the second stage amplifier 510.

[0060] In other words, the laser system 500 may include an oscillator 502, a first amplifier stage 504 connected to the oscillator 502, a gain equalizer stage 506 connected to the first amplifier stage 504, a pulse picker stage 508 connected to the gain equalizer stage 506, a second amplifier stage 510 connected to the pulse picker stage 508, and a third amplifier stage 512 connected to the second amplifier stage 510.

[0061 ] In various embodiments, the laser may have a range of values selected from 1800 nm to 2200 nm.

[0062] In various embodiments, each of the first amplifier stage 504 and the second amplifier stage 510 may include a chirped fiber Bragg grating.

[0063] In various embodiments, the oscillator 502 may be a fiber based seed oscillator 502 configured to generate femtosecond pulses. Each pulse may have a duration of 100 femtoseconds or less.

[0064] In various embodiments, the polarization controller may be configured to modify a polarization of the amplified optical signal from the first amplifier stage 504 to generate a polarized optical light.

[0065] In various embodiments, the polarization-dependent isolator may be configured to control loss based on the polarization of the polarized light from the polarization controller, thereby modifying the pulse shape of the amplified optical signal to generate the modified optical signal.

[0066] In various embodiments, the acousto-optical modulator (AOM) may include a medium configured to allow the modified optical signal from the gain equalizer stage to pass through. The acousto-optical modulator may further include a piezoelectric transducer attached to the medium. The piezoelectric transducer may be configured to generate sound waves in the medium based on a radio frequency signal so that a pulse repetition rate of the down frequency optical signal generated by the pulse picker stage is based on a frequency of the radio frequency signal. The medium may include glass or quartz.

[0067] The pulse picker stage may further include a radio frequency (RF) driver coupled to the acousto-optical modulator (AOM), the radio frequency driver configured to generate the radio frequency signal. The pulse picker stage may further include a syn-unit coupled to the radio frequency (RF) driver.

[0068] In various embodiments, the third amplifier stage may include a photonic crystal fiber (PCF). The photonic crystal fiber (PCF) may be doped with thulium, or any other suitable dopant(s).

[0069] In various embodiments, the laser system 500, e.g. the second amplifier stage 510 or the third amplifier stage 512, may include a step-index fiber. The laser system 500, e.g. the second amplifier stage 510 or the third amplifier stage 512 , may further include an intermediate fiber to optically couple the step-index fiber to the photonic crystal fiber (PCF).

[0070] In various embodiments, the laser system 500 may include a water tube configured to hold the photonic crystal fiber (PCF) and the intermediate fiber so that a joint between the photonic crystal fiber (PCF) and the intermediate fiber is contained within the water tube. The water tube may be further configured to allow water to flow from a first portion of the tube to a second portion of the tube.

[0071] The laser generated may have a central wavelength of 1975 nm, and a 10 dB bandwidth of 30 nm.

[0072] The laser generated may include a plurality of pulses. Each pulse of the plurality of pulses may have a pulse width of 2 ns or less.

[0073] In various embodiments, the laser system 500 may be an all-fiber laser system.

[0074] FIG. 6 is a schematic illustrating a method of forming a laser system according to various embodiments. The method may include, in 602, optically coupling a first amplifier stage to an oscillator configured to generate a seed optical signal. The first amplifier stage may be configured to generate an amplified optical signal based on the seed optical signal received from the oscillator. The method may also include, in 604, optically coupling a gain equalizer stage to the first amplifier stage. The gain equalizer may be configured to modify a pulse shape of the amplified optical signal from the first amplifier stage to generate a modified optical signal. The method may additionally include, in 606, optically coupling a pulse picker stage to the gain equalizer stage. The pulse picker stage may be configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal. The method may further include, in 608, optically coupling a second amplifier stage to the pulse picker stage. The second amplifier stage may be configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage to generate a noise reduced optical signal. The method may also include, in 610, optically coupling a third amplifier stage to the second amplifier stage. The third amplifier stage may be configured to amplify the noise reduced optical signal from the second amplifier stage to generate a laser. The gain equalizer stage may include a polarization controller, and a polarization-dependent isolator optically coupled to the polarization controller. The pulse picker stage may include an acousto- optical modulator (AOM) optically coupled to the polarization-dependent isolator. The second stage amplifier may include a gain medium optically coupled to the pulse picker stage, and an assistive fiber laser (AFL) also optically coupled to the gain medium so that the assistive fiber laser (AFL) provides a source for simulated emission and reduces the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage. The third amplifier stage may include a photonic crystal fiber (PCF) configured to amplify the noise reduced optical signal from the second stage amplifier.

[0075] In other words, the method may include coupling various stages and components to form a laser system.

[0076] In various embodiments, the laser may have a range of values selected from 1800 nm to 2200 nm.

[0077] In various embodiments, each of the first amplifier stage and the second amplifier stage may include a chirped fiber Bragg grating.

[0078] In various embodiments, the oscillator may be a fiber based seed oscillator configured to generate femtosecond pulses.

[0079] In various embodiments, the polarization controller may be configured to modify a polarization of the amplified optical signal from the first amplifier stage to generate a polarized optical light. [0080] In various embodiments, the polarization-dependent isolator may be configured to control loss based on the polarization of the polarized light from the polarization controller, thereby modifying the pulse shape of the amplified optical signal to generate the modified optical signal.

[0081] In various embodiments, the acousto-optical modulator (AOM) may include a medium configured to allow the modified optical signal from the gain equalizer stage to pass through, and a piezoelectric transducer attached to the medium. The piezoelectric transducer may be configured to generate sound waves in the medium based on a radio frequency signal so that a pulse repetition rate of the down frequency optical signal generated by the pulse picker stage is based on a frequency of the radio frequency signal.

[0082] In various embodiments, the medium may include glass or quartz.

[0083] In various embodiments, the pulse picker stage may further include a radio frequency (RF) driver coupled to the acousto-optical modulator (AOM), the radio frequency driver configured to generate the radio frequency signal.

[0084] In various embodiments, the pulse picker stage may further include a syn-unit coupled to the radio frequency (RF) driver.

[0085] In various embodiments, the third amplifier stage may include a photonic crystal fiber (PCF).

[0086] In various embodiments, the photonic crystal fiber (PCF) may be doped with thulium, or any other suitable dopant(s).

[0087] In various embodiments, the laser system, e.g. the second amplifier stage or the third amplifier stage, may include a step-index fiber. The laser system may further include an intermediate fiber to optically couple the step-index fiber to the photonic crystal fiber (PCF). The method may include optically coupling the step-index fiber and the PCF via the intermediate fiber.

[0088] In various embodiments, the method may also include providing a water tube configured to hold the photonic crystal fiber (PCF) and the intermediate fiber so that a joint between the photonic crystal fiber (PCF) and the intermediate fiber is contained within the water tube. The water tube may be further configured to allow water to flow from a first portion of the tube to a second portion of the tube.

[0089] The laser generated may have a central wavelength of 1975 nm, and a 10 dB bandwidth of 30 nm. [0090] The laser generated may include a plurality of pulses. Each pulse of the plurality of pulses may have a pulse width of 2 ns or less. The laser may have a pulse repetition rate selected from 100 kHz to 65 MHz.

[0091] The laser system may be an all-fiber laser system.

[0092] Various embodiments may include a fiber based oscillator which is a femtosecond seed.

Various embodiments may include multi-stage chirped pulse amplification involving chirped Bragg gratings. Various embodiments may include a gain equalizer arranged before the pulse picker stage, and which include a polarization controller and a polarization dependent isolator to compensate the uneven gain at longer wavelengths. Various embodiments may include an assistive fiber laser arranged before the photonic crystal fiber (PCF) amplifier for suppressing the ASE, hence improving the signal to noise ratio. Various embodiments may include a microstructured photonic crystal fiber based main amplifier with low-loss splicing and efficient thermal management. Various embodiments may be used for laser material processing, laser surgery, laser imaging, laser spectroscopy, and/or as a scientific laser source for various nonlinear processes.

[0093] Various embodiments may be used as a scientific laser tool for mid- infrared (MIR) supercontinuum generation, MIR frequency combs, thulium-or holmium-doped amplifier seeding, ultrafast spectroscopy, material characterization, and/or nonlinear optics, parametric amplification, high harmonic generation (HHG) for extreme ultraviolet (EUV)/X-ray generation.

[0094] Various embodiments may be used as an industrial laser for mid- infrared laser welding, cutting and making of plastics without the need for absorption-enhancing additives. FIG. 7 shows (left) a schematic diagram for welding two highly transparent plastics with a butt joint; and (right) images of welding examples prepared with a 2 mih Thulium-Yttrium- Aluminum-Garnet (Tm : YAG) laser according to various embodiments. The first image shows welding of butt joint on 4 mm poly(methyl methacrylate) (PMMA), while the second image shows T-bonding of 3 mm PMMA.

[0095] Various embodiments may be used for surgical applications, e.g. bloodless laser surgery. At 2 mih wavelength, the laser may be easily absorbed by water molecules, which are the main constituents of human tissue. In the realm of high precision surgery, they can be used to target water molecules during an operation and make incisions in very small areas of tissue without penetrating deeply. The energy from the laser may cause the blood to coagulate on the wound, which prevents bleeding. For instance, thulium laser may benefit patents with a type of cancer found in the inner ear called acoustic neuroma. As an acoustic neuroma enlarges, it puts pressure on the facial and cochlear nerves, resulting in severe pain and facial deformity. Due to limited imaging capabilities with rough cutting and nonspecific cautery tools, a conventional surgical approach to acoustic neuroma may result in a damaged nerve and a distorted facial appearance.

[0096] One conventional system is a 2 mia femtosecond fiber laser having an all polarization- maintaining fiber design from Thorlabs which costs about Singapore $63,000. The fiber laser has mode-locking using a saturable absorber. It has ultrashort pulses (< 80 fs) at 1950 nm ± 30 nm center wavelength. The average output power is > 500 mW, the pulse energy is > 10 nJ , and the repetition rate is 50 MHz.

[0097] Another conventional system is a 2 mth femtosecond fiber laser having an all fiber design and mode-locking using nonlinear polarization rotation from SIMTech. It has ultrashort pulses of < 65 fs at 1980 nm ± 30 nm center wavelength, > 580 mW average output power, pulse energy > 3 nJ, and 170 MHz repetition rate.

[0098] Yet another conventional system is from US PolarOnyx Inc, which is an all fiber based laser (156 mΐ, 100 kHz, 2 mt ).

[0099] Helmholtz-Institute Jena also report a laser based on free-space stretcher and rod fiber (570 mί, 61 kHz, 2 mhi).

[00100] FIG. 8 shows a table comparing some parameters of conventional laser system and a laser system according to various embodiments. LPF represents large-pitch fiber, while LMA represents large mode area.

[00101] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A laser system comprising:

an oscillator configured to generate a seed optical signal;

a first amplifier stage optically coupled to the oscillator, the first amplifier stage configured to generate an amplified optical signal based on the seed optical signal received from the oscillator;

a gain equalizer stage optically coupled to the first amplifier stage, the gain equalizer configured to modify a pulse shape of the amplified optical signal from the first amplifier stage to generate a modified optical signal;

a pulse picker stage optically coupled to the gain equalizer stage, the pulse picker stage configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal;

a second amplifier stage optically coupled to the pulse picker stage, the second amplifier stage configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage to generate a noise reduced optical signal; and

a third amplifier stage optically coupled to the second amplifier stage, the third amplifier stage configured to amplify the noise reduced optical signal from the second amplifier stage to generate a laser;

wherein the gain equalizer stage comprises a polarization controller, and a polarization-dependent isolator optically coupled to the polarization controller; wherein the pulse picker stage comprises an acousto-optical modulator (AOM) optically coupled to the polarization-dependent isolator;

wherein the second stage amplifier comprises a gain medium optically coupled to the pulse picker stage, and an assistive fiber laser (AFL) also optically coupled to the gain medium so that the assistive fiber laser (AFL) provides a source for simulated emission and reduces the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage; and wherein the third amplifier stage comprises a photonic crystal fiber (PCF) configured to amplify the noise reduced optical signal from the second stage amplifier.

2. The laser system according to claim 1,

wherein the laser has a range of values selected from 1800 nm to 2200 nm.

3. The laser system according to claim 1,

wherein each of the first amplifier stage and the second amplifier stage includes a chirped fiber Bragg grating.

4. The laser system according to claim 1 ,

wherein the oscillator is a fiber based seed oscillator configured to generate femtosecond pulses.

5. The laser system according to claim 1

wherein the polarization controller is configured to modify a polarization of the amplified optical signal from the first amplifier stage to generate a polarized optical light.

6. The laser system according to claim 5,

wherein the polarization-dependent isolator is configured to control loss based on the polarization of the polarized light from the polarization controller, thereby modifying the pulse shape of the amplified optical signal to generate the modified optical signal.

7. The laser system according to claim 1,

wherein the acousto-optical modulator (AOM) comprises:

a medium configured to allow the modified optical signal from the gain equalizer stage to pass through; and a piezoelectric transducer attached to the medium;

wherein the piezoelectric transducer is configured to generate sound waves in the medium based on a radio frequency signal so that a pulse repetition rate of the down frequency optical signal generated by the pulse picker stage is based on a frequency of the radio frequency signal.

8. The laser system according to claim 7,

wherein the medium comprises glass or quartz.

9. The laser system according to claim 7,

wherein the pulse picker stage further comprises a radio frequency (RF) driver coupled to the acousto-optical modulator (AOM), the radio frequency driver configured to generate the radio frequency signal

10. The laser system according to claim 9,

wherein the pulse picker stage further comprises a syn-unit coupled to the radio frequency (RF) driver.

11. The laser system according to claim 1,

wherein the third amplifier stage comprises a photonic crystal fiber (PCF).

12. The laser system according to claim 11,

wherein the photonic crystal fiber (PCF) is doped with thulium.

13. The laser system according to claim 11,

wherein the second amplifier stage comprises a step-index fiber; and wherein the laser system further comprises an intermediate fiber to optically couple the step-index fiber to the photonic crystal fiber (PCF).

14. The laser system according to claim 13, further comprising: a water tube configured to hold the photonic crystal fiber (PCF) and the intermediate fiber so that a joint between the photonic crystal fiber (PCF) and the intermediate fiber is contained within the water tube; and

wherein the water tube is further configured to allow water to flow from a first portion of the tube to a second portion of the tube.

15. The laser system according to claim 1,

wherein the laser generated has a central wavelength of 1975 nm, and a 10 dB bandwidth of 30 nm.

16. The laser system according to claim 1,

wherein the laser generated comprises a plurality of pulses.

17. The laser system according to claim 16,

wherein each pulse of the plurality of pulses has a pulse width of 2 ns.

18. The laser system according to claim 16,

wherein the laser has a pulse repetition rate selected from 100 kHz to 65 MHz.

19. The laser system according to claim 1 ,

wherein the laser system is an all-fiber laser system.

20. A method of forming a laser system, the method comprising:

optically coupling a first amplifier stage to an oscillator configured to generate a seed optical signal, the first amplifier stage configured to generate an amplified optical signal based on the seed optical signal received from the oscillator;

optically coupling a gain equalizer stage to the first amplifier stage, the gain equalizer configured to modify a pulse shape of the amplified optical signal from the first amplifier stage to generate a modified optical signal; optically coupling a pulse picker stage to the gain equalizer stage, the pulse picker stage configured to reduce a pulse repetition rate of the modified optical signal to generate a down frequency optical signal;

optically coupling a second amplifier stage to the pulse picker stage, the second amplifier stage configured to amplify and further configured to reduce amplified spontaneous emission of the down frequency optical signal received from the pulse picker stage to generate a noise reduced optical signal; and

optically coupling a third amplifier stage to the second amplifier stage, the third amplifier stage configured to amplify the noise reduced optical signal from the second amplifier stage to generate a laser;

wherein the second stage amplifier comprises a gain medium optically coupled to the pulse picker stage, and an assistive fiber laser (AFL) also optically coupled to the gain medium so that the assistive fiber laser (AFL) provides a source for simulated emission and reduces the amplified spontaneous emission (ASE) of the down frequency optical signal from the pulse picker stage; and

wherein the third amplifier stage comprises a photonic crystal fiber (PCF) configured to amplify the noise reduced optical signal from the second stage amplifier.