Abstract:
In a laser such as a mode-locked or Q-switched laser, one of the resonator reflectors comprises a switchable Faraday rotator mirror coupled to a switchable magnetic field source. The operation of the laser is therefore controlled by the application of the magnetic field to the Faraday rotator device. When no magnetic field is applied, the device behaves as an isolator and thereby breaks the signal path between the resonator reflectors. When a saturation magnetic field is applied, the reflectors, disposed on opposite sides of the optical gain medium, thereby form a cavity such that lasing will occur. The device may be formed of discrete components or fabricated as an integrated optical device.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to switchable lasers and, more particularly, to Q-switched lasers and mode-locked lasers. 
     BACKGROUND OF THE INVENTION 
     Pulsed lasers are used in a wide variety of applications ranging from signal sources in telecommunications systems to optical sources in sensing and measuring equipment. Q-switched lasers, for example, provide high power, short duration pulses for optical sensing functions, optical time domain reflectometry, and the measurement of nonlinearities in optical fibers. Illustratively, Q-switched lasers are capable of generating peak pulse powers of the order of a few hundred watts or more at repetition rates in the tens of kilohertz range. Pulse durations of about 1-100 nanoseconds are typical. Mode-locked lasers, on the other hand, may serve as high speed (e.g., multi-gigabit) signal sources in telecommunications systems, particularly soliton transmission systems. As such, the mode-locked laser may generate peak pulse powers of a few hundred milliwatts at repetition rates in excess of 10 Ghz. Pulse durations of a few picoseconds are typical. 
     Q-switched and mode-locked lasers have been extensively reported in the scientific literature. Two basic structures have been successfully demonstrated: a fiber laser ring topology of the type described by F. Fontana et al. in U.S. Pat. No. 5,381,426 issued on Jan. 10, 1995 and a Fabry-Perot (FP) fiber laser configuration of the type shown in U.S. Pat. No. 5,450,427 granted to M. E. Fermann et al. on Sep. 12, 1995. Most conventional laser designs rely on the use of an electro-optic, acoustic-optic or absorption modulators. These modulators are all bulk optic components, rendering the overall laser design less integrable, or limited to certain configurations or materials. 
     Thus, a need remains in the art for a relatively simple switched laser design. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention, which relates to a switched laser configuration and, more particularly, to a switched laser that uses a switchable Faraday rotator to control the switching activity in the laser. 
     In a preferred embodiment the switchable laser comprises a switchable Faraday mirror coupled to a gain medium, with a second reflective surface disposed beyond the output of the medium. The laser&#39;s cavity length is defined by the combination of the switchable Faraday mirror, gain medium and second reflective surface. The switching function within the Faraday mirror is controlled by an applied magnetic field. When no magnetic field is applied, the Faraday rotator isolates the first reflective surface from the second reflective surface and no lasing occurs. When a magnetic field is then applied, the Faraday rotator becomes transmissive and a lasing cavity is formed by the pair of reflectors on either side of the gain medium, providing lasing and gain at the frequency determined by the total cavity length. Therefore, the switching activity of the laser is controlled by switching the magnetic field applied to the laser. A current or voltage applied to a magnetic source is used to provide the switchable magnetic filed in the first instance. 
     In general, a Faraday rotator yields a 90° polarization rotation to the signal passing therethrough. Thus, in order to provide an appropriate aligned amplified output signal, the signal must make two passes through the laser cavity so that it is fully rotated 180°. In an alternative embodiment of the present invention, the Faraday rotator device may be formed to comprise twice its conventional length so that the signal will rotate through the full 180° before entering the laser gain medium. Additionally, the second mirror forming the laser may be a bandwidth-limited mirror including a grating (such as a UV fabricated grating) structure to produce the necessary wavelength selectivity. 
     Other and various features of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, where like numerals represent like parts in several views: 
     FIG. 1 illustrates an exemplary switchable laser using a Faraday rotator in accordance with the present invention; 
     FIG.  2 . contains a graph illustrating the application of a series of magnetic field pulses to a switchable Faraday rotator and the associated laser output pulses; 
     FIG. 3 illustrates an exemplary switchable laser configuration of the present invention, incorporating a grating structure within the laser cavity; 
     FIG. 4 is an alternative embodiment of a switchable laser including an in-line reflection grating; 
     FIG. 5 is a monolithic realization of the laser arrangement of FIG. 3; and 
     FIG. 6 is a monolithic realization of the laser arrangement of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     The principles of the present invention may be readily understood by reviewing the arrangement of switchable laser  10  of FIG.  1 . Laser  10  comprises a gain medium  12  disposed on an optical axis between a switchable Faraday rotator mirror  14  and a second mirror or reflector  16 . Laser  10  may operate, for example, as either a Q-switched or mode-locked laser. A source  18  of pump energy is coupled to gain medium  12  and supplies electromagnetic energy at a wavelength and intensity sufficient for gain medium  12  to provide optical gain. The interconnections between components, depicted as black solid lines, are optical waveguides; for example, optical fibers or substrate-supported integrated waveguides. Indeed, as will be discussed below, the gain medium itself may be a suitable semiconductor-doped optical fiber or solid state planar waveguide. As depicted, the output of the laser is taken through reflector  16 , which, therefore, is made to be partially transmissive. The output signal is coupled to a utilization device (not shown) through an isolator  20 . The latter serves to prevent unwanted reflections from being coupled back into the laser and causing instability in the laser performance. As is well known in the art and discussed below, the output signal can be taken from other locations within the laser structure. 
     In accordance with the teachings of the present invention, Faraday rotator  14  is controlled by a magnetic field source  22 . When source  22  applies no saturating magnetic field H to Faraday rotator  14 , Faraday rotator  14  functions as an isolator between reflective surface  24  and reflector/polarizer  16 . In particular, Faraday rotator  14  leads to the introduction of high optical loss into the cavity (greater than 20 dB). Therefore, no lasing occurs. With the application of a saturation magnetic field H to Faraday rotator  14 , the device becomes transmissive and reflective surface  24  of Faraday rotator defines a reflection endpoint for the laser cavity. Therefore, a laser of cavity length L defined by Faraday rotator  14 , gain medium  12  and reflector  16  is formed, wherein the lasing waveguide is determined by cavity length L. FIG. 2 is a simplified graph depicting the relationship between the applied saturation magnetic field H and the output laser pulses. In accordance with the present invention, therefore, pulse switching can easily be controlled by the action of magnetic field source  22  and the frequency of the saturation magnetic field pulses applied to switchable Faraday rotator  14 . Continuous lasing may be provided simply by maintaining the application of the saturation magnetic field, as shown in FIG.  2 . 
     In general, switchable Faraday rotator yields a polarization that is rotated 90° with respect to the polarization of the input signal (that is, the polarization of the signal will be rotated by an angle of 45° on each pass through the device). Therefore, the signal must traverse the entire laser cavity twice to achieve the full 180° required to exit through reflector  16 . Alternatively, switchable Faraday rotator  14  may be formed of a length l twice the nominal length of a conventional switchable Faraday rotator. In this case, the optical signal will experience the full 180° rotation on a single pass through the laser cavity. 
     In accordance with the present invention, the bandwidth of reflector  16  can be limited to a specific wavelength range around a particular wavelength λ by incorporating a grating into the laser structure. FIG. 3 illustrates one such arrangement. In particular, switchable laser  30  comprises, like arrangement  10  of FIG. 1, a gain medium  12  and a switchable Faraday mirror  14  controlled by a magnetic field source  22 . For the particular embodiment of FIG. 3, an optical fiber  32  is disposed between Faraday mirror  14  and gain medium  12 . Gain medium  12  is an active gain medium and may comprise either a doped-fiber amplifier or an integrated waveguide amplifier structure. A second optical fiber  34  is coupled to the output of gain medium  12 . Pump signal P from pump source  18  is coupled into gain medium  12  using a multiplexer including an optical fiber  36  that is coupled, as shown, to second fiber  34 . As is well-known in the art, the propagation direction of the pump signal is irrelevant to achieving gain within the doped media. Alternatively, therefore, pump signal P could be multiplexed through first optical fiber  32  into gain medium  12 . A reflective grating  38 , such as a partially reflective linear grating or chirped grating, is coupled to second fiber  34 . Grating  38  may be formed using well-known UV fabricating techniques and may comprise either an optical fiber or optical substrate device. As with the arrangement of FIG. 1, isolator  20  is disposed at the output of laser  30  to prevent reflections from being re-introduced into the laser cavity. 
     An alternative switched laser arrangement  40  is illustrated in FIG.  4 . In this case, the propagation direction of the system has been reversed. Accordingly, fully reflective surface  24  of Faraday mirror  14  (as shown in the embodiments of FIGS. 1 and 3) has been replaced by a partially reflecting surface  42 , where reflecting surface  42  is chosen to be able to pass the output wavelength of the laser structure. The pump signal P from source  18  passes through reflective grating  44  (chosen to be fully reflective at the lasing wavelength) and thereafter enters gain medium  12 . Switchable Faraday mirror  14  is controlled in the manner described above to provide the second reflective surface for the optical cavity when a saturating magnetic field is present. The in-line arrangement as shown is expected to experience less loss than the arrangement of FIG. 3, which requires a multiplexer to introduce the pump signal into the system. 
     The switched laser embodiments of FIGS. 3 and 4 are illustrated as comprising discrete components. However, switchable lasers of the present invention may also be formed as integrated device structures. FIG. 5 illustrates an arrangement of switched laser  30  as shown in FIG. 3, using a set of three optical substrates to form the laser. Switchable Faraday rotator  14  is formed on a first optical substrate, with source  22  applying the saturating magnetic field. A second optical substrate  60 , for example, lithium niobate, is formed to comprise a rare earth-doped waveguide section  62  that is used as the gain medium for the laser. The formation of such integrated optical waveguides is well-known in the art. A second waveguide  64  is used to couple the pump signal P into the doped waveguide structure. A filter grating  38  is etched into the surface of substrate  60  using well-known UV lithography techniques. The output isolator is formed on a third substrate and coupled to receive the optical signal passing through grating  38 . Isolators and Faraday rotators would be integrated as reported in the prior art. In accordance with the present invention, the arrangement could be provided without the use of an external magnet (by using a thin film Faraday material) resulting in a more compact integrated arrangement. 
     An integrated embodiment of laser  40  of FIG. 4 is shown in FIG.  6 . In this arrangement, the fiber grating  72  is formed on the same substrate  70  as the doped amplifying waveguide  74 . In particular, grating  72  may be localized, as shown, or distributed over the amplifying waveguide section, thus reducing the overall cavity length of the laser. The reduction in cavity length would lead to higher laser performance; that is, shorter pulses and higher output power. 
     It is to be understood that the above-described embodiments are merely illustrative of the many possible specific embodiments that can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.