Abstract:
An optical pulse generator, consisting of a semiconductor device and an optical output assembly. The semiconductor device includes an optically-active region having a gain section and a saturable absorber (SA) section, which are adapted to generate coherent optical pulses. The device also includes an output facet for coupling therethrough of the optical pulses generated in the optically-active region, and an SA electrode for application of a radio-frequency (RF) modulation of a desired frequency to the SA section.  
     The optical output assembly is optically coupled to the output facet of the semiconductor device so as to partially reflect the coherent optical pulses within a predetermined wavelength range. The assembly is positioned so as to form, together with the semiconductor device, a laser cavity having a resonant wavelength within the predetermined wavelength range and having an optical length such that a period of the RF modulation substantially equals a round-trip time for one of the pulses in the cavity, whereby the coherent optical pulses are output through the optical output assembly at a repetition rate substantially equal to the RF modulation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/177,405, filed Jan. 20, 2000, which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to lasers, and specifically to lasers generating pulses at high rates.  
         BACKGROUND OF THE INVENTION  
         [0003]    There is a continuing demand for increasing the rate of transference of data in data communication systems. Optical communication systems are able to satisfy the demand because of their inherently extremely high bandwidth, and one of the components of such a communication system is a source able to generate optical pulses at very high repetition rates. Semiconductor laser diodes operating at wavelengths of the order of 1 μm form the basis of many sources known in the art.  
           [0004]    An article titled “5.5-mm Long InGaAsP Monolithic Extended-Cavity Laser with an Integrated Bragg-Reflector for Active Mode-Locking,” by Hansen et al., in the March, 1992, issue of  IEEE Photonics Technology Letters,  which is incorporated herein by reference, describes a monolithic mode-locked semiconductor laser which generates transform-limited 20 ps wide pulses of 1.55 μm wavelength at a rate of 8.1 GHz.  
           [0005]    An article titled “Monolithic Colliding-Pulse Mode-Locked Quantum-Well Lasers,” by Chen et al., in the October, 1992, issue of  IEEE Journal of Quantum Electronics,  which is incorporated herein by reference, describes a monolithic mode-locked semiconductor laser generating pulses at ultra high rates, up to 160 GHz. The use of colliding pulses at a saturable absorber incorporated in the monolithic cavity further shortens the pulses, so that pulses having widths of the order of 1 ps are produced.  
           [0006]    A drawback common to all monolithic constructions, however, is that manufacturing process limitations cause inherently wide ranges in emitted wavelength and repetition rate. The drawback can be overcome by using an external cavity system, comprising a semiconductor laser chip and an external narrow band element, typically a fiber Bragg grating (FBG).  
           [0007]    European Patent Application 949,729/A2, to Meliga et al., whose disclosure is incorporated herein by reference, describes a module having a semiconductor laser chip coupled to an external grating written in a fiber optic. A portion of the fiber optic couples the chip and the grating. The grating acts as a partially reflecting mirror, emitting light having a wavelength defined by the grating spacing into the fiber optic.  
           [0008]    U.S. Pat. No. 5,305,336 to Adar et al., whose disclosure is incorporated herein by reference, describes a semiconductor laser chip coupled to an external grating. A DC bias and a radio-frequency (RF) current drive the chip via two electrodes, one of which functions as a ground electrode. The RF current and DC bias modulate the gain of the chip, and switches it between a net gain mode and a net absorption mode, so that the system provides pulses at the radio-frequency. The radio-frequency is set close to a fundamental cavity frequency, defined by a time for pulses within the system to travel a round-trip, which has the effect of mode-locking the system and emitting light pulses.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of some aspects of the present invention to provide apparatus and methods for generating short coherent optical pulses having a high repetition rate.  
           [0010]    It is a further object of some aspects of the present invention to provide apparatus and methods for setting the repetition rate of the optical pulses independent of a wavelength of the pulses.  
           [0011]    In preferred embodiments of the present invention, an optical system couples a semiconductor diode laser device to a fiber optic comprising a wavelength selective partial reflector, most preferably a fiber Bragg grating (FBG). The diode laser device comprises a relatively long gain section, and a short section operating as an electrically modulated saturable absorber, each section being controlled by a separate electrode. The device preferably has a third ground electrode. One facet of the device is coated to act as a first highly reflecting mirror. The partial reflector acts as a second mirror, so that an optical resonant cavity is formed between the two mirrors. When the partial reflector comprises an FBG, the wavelength at which the cavity resonates, and which is partially transmitted via the FBG into the fiber optic, is defined by the grating period of the FBG. An optical length of the cavity can be set by positioning the optical system and/or the fiber optic relative to the laser device, thus enabling the cavity to be tuned to the wavelength defined by the partial reflector.  
           [0012]    A radio-frequency (RF) signal with a DC bias is injected into the saturable absorber section. The period of the RF signal is set so that it corresponds to the time for a pulse to make a round-trip within the cavity, thus locking the modes of the laser in phase and causing the laser device to emit short mode-locked pulses at a repetition rate equal to the frequency of the RF signal. The saturable absorber section in the cavity is positioned to cause a colliding pulse effect in the saturable absorber, further shortening the pulses so that the temporal pulse widths are effectively at the transform limit set by the frequency bandwidth of the partial reflector. Thus, the combination of the dual-section laser device coupled to the wavelength selective partial reflector enables the laser cavity to be produced so as to generate short transform-limited pulses having a substantially invariant wavelength. Furthermore, the repetition rate of the pulses can be conveniently set independent of the wavelength by appropriately setting the length of the cavity.  
           [0013]    In some preferred embodiments of the present invention, the saturable absorber section is positioned adjacent to the highly reflecting facet, so that the pulses propagating in the cavity collide at the facet. In other preferred embodiments of the present invention, the saturable absorber section is positioned at an optical center of the cavity, so that pulses reflected from the opposing cavity mirrors collide in the absorber section.  
           [0014]    In some preferred embodiments of the present invention, the optical system coupling the output of the laser device to the fiber optic comprises a single converging lens separated from the device and the fiber optic. Positions of the lens and the fiber optic are independently set when adjusting the laser cavity. In an alternative preferred embodiment, the single lens is cemented to, or is integral with, the fiber optic, so that settings for the cavity are made by adjusting the position of the fiber optic. The single lens focuses the diverging output of the device onto the fiber optic.  
           [0015]    In other preferred embodiments of the present invention, the optical system comprises a plurality of lenses, one of which may be in contact or integral with the fiber optic. As for the single lens, the plurality of lenses focus the diverging output of the device onto the fiber optic.  
           [0016]    There is therefore provided, according to a preferred embodiment of the present invention, an optical pulse generator, including:  
           [0017]    a semiconductor device, which includes:  
           [0018]    an optically-active region including a gain section and a saturable absorber (SA) section, which are adapted to generate coherent optical pulses;  
           [0019]    an output facet for coupling therethrough of the optical pulses generated in the optically-active region; and  
           [0020]    an SA electrode for application of a radio-frequency (RF) modulation of a desired frequency to the SA section; and  
           [0021]    an optical output assembly, optically coupled to the output facet of the semiconductor device so as to partially reflect the coherent optical pulses within a predetermined wavelength range, and positioned so as to form, together with the semiconductor device, a laser cavity having a resonant wavelength within the predetermined wavelength range and having an optical length such that a period of the RF modulation substantially equals a round-trip time for one of the pulses in the cavity, whereby the coherent optical pulses are output through the optical output assembly at a repetition rate substantially equal to the RF modulation.  
           [0022]    Preferably, the semiconductor device includes a gain electrode for application of a current to the gain section.  
           [0023]    Further preferably, the current includes a substantially DC current.  
           [0024]    Preferably, the semiconductor device includes a common electrode which acts as a return for the gain electrode and the SA electrode.  
           [0025]    Preferably, the semiconductor device includes a highly reflecting facet which together with the output facet encloses the optically-active region.  
           [0026]    Further preferably, the output facet is coated by an antireflection coating.  
           [0027]    Preferably, the optical output assembly includes a fiber optic having a fiber Bragg grating (FBG) which partially reflects the optical pulses within the predetermined wavelength range responsive to a period of the FBG, and wherein the fiber optic transmits the optical pulses.  
           [0028]    Further preferably, the optical output assembly includes one or more lenses which focus the coherent optical pulses between the fiber optic and the output facet.  
           [0029]    Preferably, the one or more lenses include a lens fixedly coupled to the fiber optic.  
           [0030]    Further preferably, at least one of the one or more lenses and the fiber optic are positioned so as to form the laser cavity.  
           [0031]    Preferably, the generator includes a DC bias current which is applied to the SA electrode.  
           [0032]    Preferably, the gain section is positioned adjacent to the output facet.  
           [0033]    Preferably, a length of the SA section is substantially less than a length of the gain section.  
           [0034]    Preferably, the semiconductor device includes a passive waveguide section coupled to the optically-active region so as to form the laser cavity.  
           [0035]    Preferably, the semiconductor device includes a highly reflecting facet which together with the output facet encloses the optically-active region and the passive waveguide section, and wherein the SA region is positioned adjacent to the output facet, so that a first optical length from the SA section to the highly reflecting facet is substantially equal to half a second optical length of the laser cavity.  
           [0036]    There is further provide, according to a preferred embodiment of the present invention, a method for generating an optical pulse, including:  
           [0037]    applying radio-frequency (RF) modulation of a predetermined frequency to a saturable absorber (SA) section of an optically-active region in a semiconductor device, the optically-active region comprising a gain section separate from the SA section, so as to generate coherent optical pulses at a repetition rate substantially equal to the predetermined frequency; and  
           [0038]    coupling an optical output assembly to the optically-active region, so as to form a laser cavity that includes the optically-active region and has a resonant wavelength range substantially defined by the optical output assembly, and such that a period of the repetition rate substantially equals a round-trip time for one of the pulses in the cavity.  
           [0039]    Preferably the method includes providing a gain electrode for application of a current to the gain section and an SA electrode for application of the RF modulation to the SA section and a common electrode which acts as a return for the gain electrode and the SA electrode.  
           [0040]    Further preferably, the method includes enclosing the semiconductor device by a highly reflecting facet and an antireflection (AR) coated output facet, and wherein coupling the optical assembly to the optically-active region includes coupling the assembly via the AR coated facet.  
           [0041]    Preferably, the optical output assembly includes a fiber optic having a fiber Bragg grating (FBG), and the method includes partially reflecting the optical pulses within the resonant wavelength range responsive to a period of the FBG.  
           [0042]    Further preferably, the optical output assembly includes one or more lenses, and coupling the optical output assembly includes positioning at least one of the one or more lenses and the fiber optic so as to form the laser cavity.  
           [0043]    Preferably, the method includes applying a DC bias current to the SA section.  
           [0044]    Preferably, the method includes coupling a passive waveguide section to the optically-active region so as to form the laser cavity.  
           [0045]    Further preferably, the method includes positioning the SA section substantially at an optical center of the laser cavity.  
           [0046]    The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]    [0047]FIG. 1 is a schematic diagram of a Mode-Locked Fiber-Grating Laser (ML-FGL) system, according to a preferred embodiment of the present invention;  
         [0048]    [0048]FIG. 2 is a flowchart showing steps for implementing the system of FIG. 1, according to a preferred embodiment of the present invention;  
         [0049]    [0049]FIG. 3 is a schematic diagram of an alternative ML-FGL system, according to a preferred embodiment of the present invention;  
         [0050]    [0050]FIG. 4 is a schematic diagram of another alternative ML-FGL system, according to a preferred embodiment of the present invention; and  
         [0051]    [0051]FIG. 5 is a schematic diagram of yet another alternative ML-FGL system, according to a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0052]    Reference is now made to FIG. 1, which is a schematic diagram of a Mode-Locked Fiber-Grating Laser (ML-FGL) system  10 , according to a preferred embodiment of the present invention. An optically-active semiconducting region  28 , comprised in a semiconductor laser device  25  having a generally box-like shape, is formed in a gain medium  26  of the device, between non-lasing semiconductor regions  30  of the medium, by methods known in the art. Region  28  has a length, represented by L chip , which is of the order of 300 μm-1000 μm, and has a refractive index n chip . Device  25  comprises an antireflection (AR) coated front facet  24 , and a high-reflection (HR) coated back facet  36  which acts as a first mirror of a laser cavity  11 .  
         [0053]    Device  25  comprises a relatively long gain section  31 , controlled by a gain electrode  32 , and a very short saturable absorber (SA) section  33 , controlled by an SA electrode  34 . Electrodes  32  and  34  are coupled to an upper surface of device  25 , and a common electrode  35  is coupled to a lower surface of the device. SA section  33  is adjacent to facet  36 , and gain section  33  is adjacent to facet  24 . Electrode  32  is preferably of the order of 30 μm in length, and electrode  34  is preferably of the order of the remaining length of chip  25 . As described in more detail hereinbelow, when region  28  lases it emits coherent diverging light from facet  24 , which acts as an output facet.  
         [0054]    The diverging light emitted from facet  24  is focussed by an optical system  20  into a fiber optic  12 , which comprises a fiber Bragg grating (FBG)  14 . FBG  14  acts as a partially reflecting narrow-band mirror, reflecting a portion of the light in a wavelength defined by the grating period back to optical system  20 . The non-reflected portion is transmitted into fiber optic  12  to form the output of cavity  11 . Optical system  20  focuses light reflected from FBG  14  back to facet  24 , so that the optical system couples the FBG to facet  24  of device  25 . An optical output assembly  17  comprises optical system  20  and fiber optic  12 . Thus device  25 , optical system  20 , and FBG  14  comprise laser cavity  11 , and laser system  10  comprises device  25  coupled to output assembly  17 .  
         [0055]    Optical system  20  most preferably comprises a single lens  21 . Lens  21  is preferably any industrial-standard lens, or alternatively any custom lens, which is able to focus light emitted from facet  24  to FBG  14 . Preferably, lens  21  is a bi-convex, aspheric, ball, cylindrical, or graded refractive index (GRIN) lens.  
         [0056]    The description hereinbelow explains how lengths of elements of cavity  11  relate to each other in order for the cavity to operate. FBG  14  is assumed to be a distance D g  from a tip  15  of fiber optic  12 . The portion of FBG  14  participating in cavity  11  is assumed to have an effective length L eff , so that a length L f  of fiber optic  12  participating in cavity  11  is given by:  
           L   f   =L   eff   +D   g   (1)  
         [0057]    A total optical length L cavity  of cavity  11  is given by:  
           L   cavity   =L   chip   ·n   chip   +u+L   os   +v+L   f   ·n   f   (2)  
         [0058]    where  
         [0059]    L chip  is the length of region  28 ,  
         [0060]    n chip  is the refractive index of region  28 ,  
         [0061]    L os  is an optical length of optical system  20 ,  
         [0062]    u is a distance from system  20  to facet  24 ,  
         [0063]    v is a distance from system  20  to tip  15 , and  
         [0064]    n f  is a refractive index of fiber optic  12 .  
         [0065]    For a cavity of optical length L cavity , a time taken for the round-trip of a photon within the cavity is  
             t   =     2   ·       L   cavity     c               (   3   )                               
 
         [0066]    where c is the speed of light.  
         [0067]    Injection of DC current to electrode  32  activates gain section  31  of device  25 , enabling cavity  11  to lase, and the cavity will lase continuously except for the effect provided in section  33 , wherein the gain is effectively neutralized at specific times. To activate section  33 , a DC reverse bias combined with a radio-frequency (RF) modulation is applied to electrode  34 . The frequency f of modulation is set to correspond with the round-trip time t, i.e.,  
             f   =     c     2   ·     L   cavity                 (   4   )                               
 
         [0068]    The application of an RF modulation at a frequency corresponding to the round-trip time results in mode-locking the laser, and generates pulses at the modulating frequency. The pulses are shortened by combining the DC bias with the RF modulation, which effectively results in shortening the time during the modulation period when the system experiences net gain. Further pulse shortening is achieved by the location of SA section  33  close to HR coated facet  36 , so that a pulse collides with its own reflection in section  33 .  
         [0069]    [0069]FIG. 2 is a flowchart showing steps for implementing system  10 , according to a preferred embodiment of the present invention. Initially nominal values for parameters comprised in the right side of equation (2), so that equation (4) is obeyed for a predetermined frequency, for example 40 GHz, are calculated. System  10  is then implemented, preferably by moving fiber optic  12  relative to device  25  so as to vary values of u and v. Most preferably, while fiber optic  12  is moved, optical system  20  is also moved so as to maintain light output from facet  24  focused onto end  15  of fiber optic  12 , and a spectrum of the laser output is monitored until correct mode spacing is achieved. Once positions for fiber optic  12  and optical system  20  have been determined, the fiber optic and optical system are fixed in place.  
         [0070]    Once cavity  11  has been constructed, frequency f is injected to SA section  33 , and the frequency is varied, most preferably around the predetermined value, until an optimal output from fiber optic  12  is achieved.  
         [0071]    [0071]FIG. 3 is a schematic diagram of an ML-FGL system  50 , according to a preferred embodiment of the present invention. Apart from the differences described below, the operation of system  50  is generally similar to that of system  10  (FIG. 1), so that elements indicated by the same reference numerals in both systems  50  and  10  are generally identical in construction and in operation. In system  50 , optical system  20  preferably comprises a lens  56  which is constructed as an integral part of fiber optic  12 . Alternatively, lens  56  is a distinct lens cemented onto tip  15  of fiber optic  12 . A radius of a surface  58  of lens  56  is preferably set so that light is substantially focussed from facet  24  into fiber optic  12 . Most preferably, distance u between surface  58  and facet  24  is set to be generally equal to the focal length of lens  56 . Device  25  is activated, using RF and DC bias injected at electrode  34 , and DC current injected at electrode  32 , as described above with reference to system  10 . System  50  is most preferably implemented substantially as described above for system  10 , with reference to FIG. 2. It will be appreciated that system  50  is relatively more compact than system  10 , at the cost of having a tuning range which is somewhat smaller, since only distance u can be varied.  
         [0072]    [0072]FIG. 4 is a schematic diagram of an ML-FGL system  70 , according to a preferred embodiment of the present invention. Apart from the differences described below, the operation of system  70  is generally similar to that of system  10  (FIG. 1), so that elements indicated by the same reference numerals in both systems  70  and  10  are generally identical in construction and in operation. In system  70 , optical system  20  comprises a first converging lens  76  and a second converging lens  72 , separated by a distance d. Preferably, lens  72  is constructed to be integral with, or is cemented to, fiber optic  12 , generally as described above with reference to lens  56  (FIG. 2). Alternatively, lens  72  is generally similar to lens  76 , and is separated from tip  15  of fiber  12 .  
         [0073]    Most preferably, lens  76  has a focal length substantially equal to its distance from facet  24 , so that diverging light from the facet is collimated by the lens. Lens  72  focuses the collimated light onto FBG  14 . Tuning of system  70  is accomplished by varying the separation of lens  72  and  76 , so that system  70  has a relatively large tuning range with substantially constant coupling between facet  24  and FBG  14 . System  70  is most preferably implemented as described above with reference to FIG. 2.  
         [0074]    [0074]FIG. 5 is a schematic diagram of an ML-FGL system  90 , according to a preferred embodiment of the present invention. Apart from the differences described below, the operation of system  90  is generally similar to that of system  50  (FIG. 3), so that elements indicated by the same reference numerals in both systems  90  and  50  are generally identical in construction and in operation. System  90  comprises a semiconductor laser device  105  having an SA section  103  of length L SA , adjacent to AR coated facet  24 , which is implemented and controlled by an SA electrode  98 . Device  105  also comprises a central gain section  107  which is implemented and controlled by a central gain electrode  106 . Apart from their positioning, SA electrode  98  and gain electrode  106  are respectively substantially similar in construction and operation to SA electrode  34  and gain electrode  32 .  
         [0075]    Device  105  further comprises a passive waveguide section  109 . Section  109  comprises a waveguide  108 , which is substantially similar in dimensions to region  28  (FIG. 1) and which is terminated by an HR coated facet  110 . Preferably, section  109  comprises a semiconductor having a band-gap causing the semiconductor to be substantially passive and non-absorbing. However, unlike region  28 , there is substantially no current injected into section  109 , so that waveguide  108  acts as a passive light guide, and a cavity  101  is formed between facet  110  and FBG  14 . In cavity  101 , lengths of elements of the cavity are set so that an optical length from the center of SA section  103  to facet  110  is substantially equal to an optical length from the center of section  103  to FBG  14 . It will be appreciated that each of these optical lengths is substantially equal to half the optical length of cavity  101 , so that SA section  103  is substantially at an optical center of the cavity. In this configuration, pulses from section  103 , propagating in opposite directions within the cavity, will collide (after reflecting at ends of cavity  101 ) in the section, and so be shortened.  
         [0076]    The description hereinbelow explains how lengths of elements of cavity  101  relate to each other in order for the cavity to operate. Assume that a distance between facet  110  and face  24  is L chip , and that a distance from facet  24  to surface  58  is L fc . L fc  is adjusted so that the optical lengths to the ends of cavity  101 , as measured from the center of section  103 , are substantially equal.  
         [0077]    The optical length from the center of section  103  to facet  110  is given by:  
               (       L   chip     -       L   SA     2       )     ·       n   chip     .             (   5   )                               
 
         [0078]    The optical length from the center of section  103  to FBG  14  is given by:  
                   L   SA     2     ·     n   chip       +     L   fc     +       L   f     ·     n   f               (   6   )                               
 
         [0079]    Equating equations (5) and (6), and rearranging, gives:  
         ( L   chip   −L   SA )· n   chip   =L   fc   +L   f   ·n   f   (7)  
         [0080]    In implementing cavity  101 , L fc  is calculated so that equation (7) is satisfied. System  90  is then most preferably implemented substantially as described above for system  10 , with reference to FIG. 2.  
         [0081]    It will be appreciated that other methods of coupling device  105  with FBG  14 , such as described hereinabove with reference to systems  10  and  70 , may be utilized to form cavity  101 .  
         [0082]    It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.