Patent Publication Number: US-2012027033-A1

Title: Multi-segment all-fiber laser

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a multi-segment all-fiber laser device and method for generating optical pulses and/or pulse trains. 
     The compactness, ruggedness, high beam quality, and efficiency of fiber lasers make them attractive devices for applications in optical communications, signal processing and sensing as well as in medicine and industry. In recent years, much effort has been directed towards the development of pulsed fiber lasers based on Q-switching and mode-locking. Pulsed fiber lasers can be low-cost and low-maintenance alternative light sources for conventional pulsed solid-state lasers. 
     In traditional pulsed fiber lasers mode-locking and Q-switching are achieved through external, bulk optical elements such as saturable absorbers or acousto-optic and electro-optic modulators (B. C. Collins K. Bergman, S. T. Cundiff, S. Tsuda, J. N. Kurz, J. e. Cunningham, W. Y. Jan, M. Koch, and W. H. Knox, “Short cavity erbium/ytterbium fiber lasers mode-locked with a saturable Bragg reflector”, IEEE J. Sel. Top. Quantum Electron. 3, 1065 (1997); G. P. Lees, D. Taverner, D. J. Richardson, and L. Dong, “Q-switched erbium doped fibre laser utilising a novel large mode area fibre”, Electron. Lett. 33, 393 (1997)) 
     These bulk elements make the laser design rather complex. Alternatively, mode-locked fiber ring lasers with linear polarizers or figure-eight fiber lasers with nonlinear interferometry have been demonstrated. While the first two categories lose the many advantages of an all-fiber format, the second pair of configurations suffer from stability problems. Importantly, none of the all-fiber approaches allow for an externally controlled, adjustable repetition rate. 
     There also exists the effect of self-pulsing in fiber lasers in cavities free from active modulation or passive mode-locking devices that have been reported more than a decade ago (J. L. Zyskind, V. Mizrahi, D. J. DiGiovanni, and J. W. Sulhoff, “Short single frequency erbium-doped fiber laser”, Electron. Lett. 28, 1385 (1992); P. Le Boudec, M. Le Flohic, P. L. Francois, F. Sanchez, and G. Stephan, “Self-pulsing in Er3+-doped fiber laser”, Opt. Quantum Electron. 25, 359 (1993). 
     These self-pulsation phenomena are based on instabilities and can generally be classified as either sustained self-pulsing (SSP) or self-mode-locking (SLM) (F. Fontana, M. Begotti, E. M. Pessina, and L. A. Lugiato, “Maxwell-Bloch modelocking instabilities in erbium-doped fiber lasers”, Opt. Commun. 114, 89 (1995)). 
     SSP is the periodic emission of laser pulses at a repetition rate associated with relaxation oscillations. It is enhanced at particular pumping rates and by low cavity photon lifetimes. SSP is generally considered a detrimental effect in high-power fiber lasers because in combination with stimulated Brillouin scattering it leads to the emission of intense irregular pulses. 
     SML involves laser signal modulations at a period corresponding to the cavity round-trip time and can typically be observed close to the laser threshold. Therefore, any self-pulsation occurs either at the rate of the relaxation oscillations (typically a few hundred Hz to a few hundred kHz in fiber lasers) or the inverse cavity roundtrip time (typically a few MHz to 1 GHz depending on the fiber laser cavity length) and can neither be easily controlled nor manipulated. 
     OBJECTIVE OF THE PRESENT INVENTION 
     Accordingly, the objective of the present invention is to provide a method and system which is capable of emitting well-defined optical pulses and/or pulse trains of well-defined but adjustable wavelength. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the invention relates to a multi-segment all-fiber laser device including: a first active fiber laser segment; a first grating; a second grating; and a gain-phase coupling fiber segment arranged between the first and second gratings, said gain-phase coupling segment simultaneously providing coupling of gain and phase between said first and second gratings. 
     The first and second gratings may be distributed feed-back grating structures. 
     Preferably, the first grating is located in the first active fiber laser segment, and the second grating is preferably located in a second active fiber laser segment. Accordingly, the gain-phase coupling segment may be positioned between both active fiber laser segments. 
     The gain-phase coupling segment may comprise a passive optical fiber of specific length, and/or an active fiber having a variable optical gain depending on the optical power of a pump radiation, and/or a nonlinear optical fiber with an intensity dependent refractive index. 
     The gain-phase coupling segment is preferably connected to a control pump source for providing pump radiation in the gain-phase coupling segment. A gain-phase control unit may control the optical power of pump radiation provided by the control pump source. This allows adjusting the gain and/or phase in said gain-phase coupling segment in order to maintain or enable gain-phase coupling between the gratings. 
     Furthermore, the first active fiber laser segment and/or the second active fiber laser segment may be pumped by a single or a plurality of pump sources in order to provide population inversion in those active fiber laser segments. 
     The multi-segment all-fiber laser device may further comprise a temperature control unit which is connected to the gain-phase coupling segment. The temperature control unit may control the temperature and thus the refractive index of the gain-phase coupling segment. 
     An embodiment of the invention further relates to a method of emitting optical pulses and/or pulse trains, including the steps of:
         activating a first active fiber laser segment of a multi-segment all-fiber laser device to emit radiation;   at least partially reflecting the radiation between a first grating of said multi-segment all-fiber laser device and a second grating of said multi-segment all-fiber laser device; and   adjusting a gain-phase coupling fiber segment arranged between the first and second gratings in order to simultaneously couple gain and phase between said first and second gratings.       

     According to a preferred embodiment the temperature of the gain-phase coupling fiber segment is controlled in order to maintain or enable gain-phase coupling between both gratings. 
     Moreover, if the gain-phase coupling fiber segment includes an active fiber having a variable optical gain depending on the optical power inside, the active fiber will preferably be pumped in order to adjust the optical gain of the active fiber and to maintain or enable gain-phase coupling between both gratings. 
     The method may also include the step of regulating the output power of the first active fiber laser segment in order to control the refractive index of a nonlinear optical fiber included in said gain-phase coupling fiber segment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the manner in which the above-recited and other advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail by the use of the accompanying drawings in which 
         FIG. 1  shows an exemplary embodiment of a multi-segment all-fiber laser device having two active fiber laser segments; 
         FIG. 2  depicts the radiation intensity generated by the device shown in  FIG. 1 , over wavelength; 
         FIG. 3  depicts the radiation intensity generated by the device shown in  FIG. 1 , over frequency; 
         FIG. 4  depicts the intensity of radiation generated by the device shown in  FIG. 1 , in time domain; 
         FIG. 5  shows a second exemplary embodiment of a multi-segment all-fiber laser device having two temperature control units for controlling two active laser segments; and 
         FIG. 6  shows a third exemplary embodiment of a multi-segment all-fiber laser device having a single active fiber laser segment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention will be best understood by reference to the drawings, wherein identical or comparable parts are designated by the same reference signs throughout. 
     It will be readily understood that the device features of the present invention, as generally described and illustrated in the figures herein, could vary in a wide range of different device features. Thus, the following more detailed description of the exemplary embodiments of the present invention, as represented in  FIGS. 1-6  is not intended to limit the scope of the invention, as claimed, but is merely representative of presently preferred embodiments of the invention. 
       FIG. 1  shows an exemplary embodiment of a multi-segment all-fiber laser device  10  that can emit well-defined optical pulses and/or pulse trains of well-defined but adjustable wavelength. The optical output radiation is designated by reference signs Pout  1  and Pout  2 . 
     Device  10  comprises several segments arranged in direction along the fiber comprising a first active laser segment  20  having a first distributed feed-back grating  25 , a second active laser segment  30  having a second distributed feed-back grating  35 , and a gain-phase coupling fiber segment  40  arranged between the first distributed feed-back grating  25  and the second distributed feed-back grating  35 . The gain-phase coupling segment provides coupling of gain and phase between gratings  25  and  35 . 
     The embodiment shown in  FIG. 1  comprises three segments; however, the device may include even more segments, e.g. more active fiber laser segments, propagation segments, grating segments, and/or nonlinear refraction segments, where these segments assume a cooperative mode of operation created by self-organization based on the gain-phase coupling of the segments. Pulse shape, duration, repetition rate, and/or pulse power may be adjusted or tuned by either the frequency detuning of the laser segments, the propagation time delays between the segments, the nonlinear phase changes induced by the segments, or by a combination of these parameters. 
     For generating optical output radiation preferably both fiber laser segments  20  and  30  are optically pumped to achieve optical gain. Pump signals P 1  and P 2  are generated by activation pump sources  50  and  60  which are connected to active fiber laser segments  20  and  30  via wavelength sensitive couplers WDM 1  and WDM 2 . 
     In order to enable coupling of gain and phase between the first distributed feed-back grating  25  and the second distributed feed-back grating  35 , the gain-phase coupling fiber segment  40  is preferably tunable. 
     E.g., the gain-phase coupling fiber segment  40  may include an active fiber having a variable optical gain depending on the optical power of a pump radiation. Alternatively or additionally, the gain-phase coupling segment  40  may comprise a nonlinear optical fiber with an intensity dependent refractive index. 
     For external tuning, a control pump source  70  is connected to gain-phase coupling segment  40  via an additional coupler  80 . The control pump source  70  provides a pump radiation Pcontrol which is coupled into the gain-phase coupling segment  40  and which varies the optical characteristics inside the gain-phase coupling segment  40 . The control pump source is controlled by gain-phase control unit  75  which is adapted to adjust the gain and/or phase in said gain-phase coupling segment  40  and to enable gain-phase coupling between the distributed feed-back gratings  25  and  35 . 
     Device  10  may also include a temperature control unit  90  which controls the temperature of the gain-phase coupling segment  40 . By controlling the temperature of the gain-phase coupling segment  40 , the gain and the refractive index inside the gain-phase coupling segment  40  may also be tuned in order to enable gain-phase coupling between the distributed feed-back gratings  25  and  35 . 
     Numerical simulations of the embodiment in a wider parameter range demonstrate that the device  10  is capable of pulsed operation regimes as illustrated by the graphs shown in  FIG. 2-4 . The numerical simulations are based on computer programs that have been previously applied to simulate coupled semi-conductor lasers and their dynamics and are modified according to the materials parameters of phosphate glass fiber lasers (H. J. Wünsche, S. Bauer, J. Kreissl, O. Ushakov, N. Korneyev, F. Henneberger, E. Wille, H. Erzgräber, M. Peil, W. Elsässer, I. Fischer, “Synchronization of delay-coupled oscillators: A study of semiconductor lasers”, Phys. Rev. Lett. 94, 163901 (2005); S. Schikora, P. Hovel, H. J. Wünsche, E. Schöll, F. Henneberger, “All-optical noninvasive control of unstable states in a semiconductor laser”, Phys. Rev. Lett. 97, 213902 (2008)). The segment lengths l for simulation were as follows: active laser segments  20  and  30 : l=3.5 cm; gain-phase coupling fiber segment  40 : l=3.0 cm. The simulation assumes that the structure is homogeneously pumped along the fiber axis. 
       FIG. 2  depicts the intensity I of the optical radiation over the relative wavelength in nanometers. On top of the optical spectrum reflection spectra of the distributed feed-back gratings  25  and  35  are plotted. 
       FIG. 3  depicts the intensity I of the optical radiation over the frequency in GHz. 
     Preferably, a gap is placed in both distributed feed-back gratings  25  and  35  in order to produce a round-trip phase shift of π/3. 
     The 7-GHz peak in  FIG. 3  is associated with prominent and highly regular intensity pulsations in the device output with pulse duration in the sub-ns range. This is possible despite a response time of the inversion that is as long as 13 ms. The origin of this form of self-pulsing is gain coupling between the segments leading to a cooperative mode of operation of the entire three-segment device. 
       FIG. 4  shows a time-resolved laser emission from the device as shown in  FIG. 1 . 
       FIG. 5  depicts another embodiment of a multi-segment all-fiber laser device  10  which is capable of emitting radiation. In addition to the embodiment of  FIG. 1 , device  10  of  FIG. 5  further comprises temperature control units  100  and  110 . Temperature control unit  100  allows to control the temperature of the first active laser segment  20 , whereas temperature control unit  110  allows to control the temperature of the second active laser segment  30 . 
     With both temperature control units  100  and  110 , the temperatures of the active fiber laser segments  20  and  30  can be individually regulated. Thus, these segments can also be detuned relative to each other. 
       FIG. 6  depicts a third embodiment of a multi-segment all-fiber laser device  10  which is capable of emitting radiation. In contrast to the embodiments discussed above with reference to  FIGS. 1-5 , the embodiment of  FIG. 6  comprises a single active fiber laser segment  20  and a single activation pump source  50  for generating a pump signal P 1 . The second distributed feed-back grating  35 ′ is not pumped. 
     In summary, the operation modes of the devices  10  as described above may include:
         Pulse repetition rates can be tuned by changing the frequency detuning as well as the coupling strength between both active fiber laser segments  20  and  30 .   Pulse repetition rates can be tuned by changing the optical length of the coupling fiber segment between the two DFB (DFB: distributed feed back) grating structures.   In one mode of operation, device  10  emits a stable train of optical pulses.   In another mode of operation, two pulse trains with stable phase relations can be emitted.   The frequency difference between the two pulse trains can be tuned.   The operation wavelengths of both active fiber laser segments  20  and  30  can be tuned relative to each other, e.g., by temperature tuning.   The device  10  can provide repetition rates between 100 Hz and 200 GHz, even up to 10 THz when one segment exhibits sufficiently strong Kerr-type non-linear refraction.       

     REFERENCE NUMERALS 
     
         
           10  multi-segment all-fiber laser device 
           20  first active laser segment 
           25  first distributed feed-back grating 
           30  second active laser segment 
           35  second distributed feed-back grating 
           35 ′ second distributed feed-back grating 
           40  gain-phase coupling fiber segment 
         P 1  pump radiation 
         P 2  pump radiation 
           50  pump source 
           60  pump source 
           70  control pump source 
           75  gain-phase control unit 
           80  coupler 
           90  temperature control unit 
           100  temperature control unit 
           110  temperature control unit 
         Pout 1  optical output radiation 
         Pout 2  optical output radiation 
         WDM 1  wavelength sensitive coupler 
         WDM 2  wavelength sensitive coupler 
         Pcontrol pump radiation