Abstract:
A simple and flexible WDM laser source is disclosed using a loop erbium-doped fiber amplifier (LEDFA) configuration. The loop serves as a mirror and as an amplification medium. The laser cavity was made from the loop mirror and a set of fiber Bragg gratings (FBGs) which select the proper lasing wavelengths. The FBGs can be placed either in parallel or in series at the output of the loop configuration. Optical attenuators are placed in front of the FBG to control the flatness of the laser source output and determine the required lasing condition for each wavelength to avoid competition of the different lasing wavelengths. This configuration is flexible for adding any number of wavelengths as long as enough amplified spontaneous emission (ASE) is generated in the loop. Signal to noise ratio as high as 55-dB can be achieved.

Description:
[0001]    This application claims priority of U.S. provisional application No. 60/191,732 filed Mar. 24, 2000 and Canadian application No. 2,302,105 filed Mar. 24, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention generally relates to laser sources and in particular to lasers sources suitable for wavelength-division multiplexed optical communications systems.  
         BACKGROUND  
         [0003]    Wavelength Division Multiplexing, WDM and Dense Wavelength Division Multiplexing, DWDM optical transmission systems require light from multiple laser sources to match the International Telecommunications Union, ITU, channel spacing. In previous work this has been done with individual lasers tuned separately to the channel spacings.  
           [0004]    Cochlain and Mears disclosed an example of a laser source producing a single wavelength in an article entitled “Broadband Tunable Single Frequency Diode Pumped Erbium Doped Fiber Laser”, Electronics Letters, Vol. 28, No. 2, pp. 124-126, 1992. Cochlain and Mears disclose a laser that comprises a loop mirror formed by a 3-dB coupler and an erbium-doped fiber having its ends connected to respective ports of the coupler via an isolator and a wavelength selective coupler (WSC), the latter coupling energy from a pump into the erbium-doped fiber. The WSC is used to pump 1480 nm light into the loop mirror. A polarization controller between the WSC and the 3-dB coupler controls the passage of amplified spontaneous emission (ASE) into the coupler, while the isolator blocks the ASE from reaching the other port of the coupler. A third port of the 3-dB coupler is connected to a grating by way of a second polarization controller while the fourth port delivers the output signal. Rotation of the grating selects the individual wavelength at which the laser will lase. The ASE will be reflected by the grating back through the 3-dB coupler and will pass, via the isolator, around the loop to appear at the output of the laser. The grating reflects substantially all of the light reaching it, so the output signal can be extracted from only the one point in the system, namely from the fourth port of the coupler. The device cannot readily be adapted for multiple wavelength use and so the common practice would be to use a number of these devices, each one tuned to a different wavelength, to provide the multiple wavelengths required for WDM or DWDM. However, this approach is expensive since it requires duplication of components to provide multiple wavelengths.  
           [0005]    The present invention seeks to overcome these disadvantages and to this end provides a laser source capable of operating at multiple wavelengths.  
         SUMMARY  
         [0006]    In accordance with the present invention, there is provided a multiple wavelength laser source comprising  
           [0007]    a loop mirror formed by a loop of active fiber and a first coupler, preferably a 3-dB (50:50) coupler, the fiber being connected between two ports of the coupler,  
           [0008]    at least one pump means for injecting pump energy into the loop of active fiber; and  
           [0009]    a plurality of wavelength-selective reflection devices having different selected wavelengths and coupled to at least a third port of the first coupler;  
           [0010]    wherein each reflection device is for reflecting into the fiber loop a first portion, having a selected wavelength, of amplified spontaneous emission produced by the active fiber, and directing a second portion of the amplified spontaneous emission produced by the active fiber, to an output port.  
           [0011]    Each reflection device will reflect back into the loop mirror ASE at its own particular selected wavelength. Consequently, the laser source will lase at each of the different wavelengths of the plurality of reflection devices, thereby producing output light at a plurality of different wavelengths which can be used for WDM or DWDM.  
           [0012]    One or more of the reflection devices may transmit the second portion to the output port. Additionally or alternatively, one or more of the reflection devices may reflect the second portion to the output port, conveniently by way of an additional coupler.  
           [0013]    In one embodiment of the invention, the plurality of reflection devices are in series between the first coupler and the output port and each reflects the first portion of ASE back to the loop mirror and transmits the second portion to the output port.  
           [0014]    In another embodiment of the invention, the plurality of the reflection devices are arranged in parallel. Specifically, a first plurality of wavelength-selective reflection devices may be coupled to a corresponding port of the first coupler by way of a second coupler, an output port of the first coupler being coupled to an output port of the laser, each reflection device reflecting both the first portion and the second portion of ASE to the second coupler and the second coupler directing a portion of the energy to the loop mirror and another portion to the associated output port. One or more of the reflection devices coupled to the second coupler may transmit a third portion of the ASE to an associated output port. Analogously, at least one other plurality of wavelength-selective reflection devices may be coupled to another port of the first coupler by way of another coupler in an arrangement similar to the one described above.  
           [0015]    Various types of reflection devices meeting the requirements of the invention may be employed. Fiber Bragg gratings (FBGs) are one choice. Tunable filters with partially reflective and partially transmissive mirrors are another possibility.  
           [0016]    Preferably, but not necessarily, the first portion and the second portion each comprise about 50 per cent of the emission energy.  
           [0017]    The laser source may comprise a plurality of attenuators, each between the fiber loop and one of the reflection devices. The attenuators may be used to adjust the amount of ASE reflected and hence control the amplitude of the laser output signal at the corresponding selected wavelength. In addition, they are used to control lasing modes competition in the loop. The attenuators may be adjusted to “flatten” the output spectrum. The number of reflection devices coupled to each port of the 3-dB coupler need not be the same.  
           [0018]    In accordance with another aspect of the present invention, there is provided a laser source system for producing multiple sets of lasing wavelengths, said system comprising:  
           [0019]    laser source combining means for combining output from a plurality of multiple wavelength laser sources, each multiple wavelength laser source comprising:  
           [0020]    a loop mirror means, said loop mirror means comprising:  
           [0021]    a loop of active fiber; and  
           [0022]    a splitter/coupler means that is coupled, via a first and a second port, to both ends of the loop of active fiber;  
           [0023]    at least one pump means for injecting pump energy into the loop of active fiber; and  
           [0024]    a plurality of wavelength-selective reflecting devices, said devices having different selected wavelengths and coupled to at least a third port of the splitter/coupler means;  
           [0025]    wherein each reflecting device is for reflecting a first portion of amplified spontaneous emission, supplied by the loop of active fiber, and directing a second portion of the amplified spontaneous emission, supplied by the loop of active fiber, to an output port. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 illustrates, as a first embodiment of the invention, a laser source having a plurality of gratings, with attenuators, in series;  
         [0027]    [0027]FIG. 2 illustrates, as a second embodiment of the invention, a laser source having a plurality of gratings in parallel;  
         [0028]    [0028]FIG. 3 illustrates the spectrum of the laser output from the first four output ports of the laser source of FIG. 2; and  
         [0029]    [0029]FIG. 4 illustrates the spectrum of the laser output from fifth and sixth output ports of the laser source of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    In the drawings, identical or corresponding items in the different Figures have the same reference number.  
         [0031]    Referring first to FIG. 1, a laser source comprises a loop mirror formed by a 3-dB fiber coupler  10  having four ports, A, B, C and D, and a loop of active fiber  12  doped with any rare earth, preferably erbium-doped fiber, EDF, with its ends connected to ports C and D, respectively, through wavelength-selective couplers  14  and  16  that are connected also to pump sources (lasers)  18  and  20 , respectively.  
         [0032]    WDM couplers are used to combine the pump energy at 980 nm and the amplified signal at 1550 nm inside the EDF loop  12 . Using 3-dB couplers to combine the 980 nm pump energy and the 1550 nm laser output is problematic and therefore it is more appropriate to use a WDM coupler, inside of the EDF loop, to combine these light signals.  
         [0033]    A polarization controller  11  is placed in the loop to divide amplified spontaneous emission (ASE) generated by the pump source to the output terminals of the coupler  10 .  
         [0034]    Port A of the coupler  10  is coupled to a first output port P 1   OUT  by a plurality of fiber Bragg gratings, FBGs,  22 A,  24 A, . . . , having characteristic wavelengths λ 1 , λ 3 , . . . , respectively, in series. Port B of the coupler  10  is coupled to a second output port P 2   OUT  by a second plurality of FBGs,  22 B,  24 B, . . . , having characteristic wavelengths λ 2 , λ 4 , . . . , respectively, all in series. Each of the FBGs  22 A,  24 A,  22 B,  24 B, . . . , is selected to reflect, preferably, about 50% of the light incident upon it at the selected wavelength and transmit the remainder.  
         [0035]    Attenuators  32 A,  34 A . . . are coupled in series with each FBG  22 A,  24 A . . . , and attenuators  32 B,  34 B . . . are coupled in series with each FBG  22 B,  24 B . . . to balance the energy available between the output wavelengths. Since ASE has a non-flat output function, the energy imbalance is inherent.  
         [0036]    In operation, the pump energy from pump sources  18  and  20  produces ASE in the EDF  12 . Assuming a symmetric configuration, 50 per cent of the ASE will appear at each of the ports C and D of the 3-dB coupler  10  and will be coupled to ports A and B. Further, setting the attenuators  32 A,  34 A, . . . , to zero attenuation, FBG  22 A will reflect 50 per cent of the light leaving port A so that it re-enters the loop mirror and transmits the remainder to FBG  24 A which, in a similar manner, will reflect 50 per cent of the ASE at its own selected wavelength and transmit the remainder of the ASE and other light including any lasing frequency. All of the other gratings/attenuators in series with the first output port P 1   OUT  will operate in a similar manner. Once lasing conditions have been established, the output signal appearing at output port P 1   OUT  will comprise all the lasing wavelengths λ 1 , , λ 3 , . . . of the gratings  22 A,  24 A and so on.  
         [0037]    The same applies to the second output port P 2   OUT . The ASE light leaving port B of the 3-dB coupler  10  will be reflected and transmitted in a similar manner by the fiber gratings  22 B,  24 B... in series with second output port P 2   OUT , so that the light leaving output port P 2   OUT  comprises the wavelengths λ 2 , λ 4 . . . of gratings  22 B,  24 B and so on.  
         [0038]    For most WDM or DWDM applications, it is desirable for the amplitude of the output signal to be the same at each wavelength. Consequently, the attenuators previously mentioned may be used to adjust the amount of light reflected by the respective FBG, and hence the amplitude of the output light at the corresponding wavelength. There is competition for the ASE in EDF  12  to induce lasing at the chosen wavelengths. The ASE energy is shared between the competing lasing wavelengths.  
         [0039]    The laser source illustrated in FIG. 2 is based on a loop mirror formed by a 3-dB (first) fiber coupler  10  having four, ports A, B, C and D, and a loop of active fiber  12 , preferably erbium-doped, with its ends connected to ports C and D, respectively, through wavelength-selective couplers  14  and  16  that are also connected to pump sources (lasers)  18  and  20 , respectively. The laser source in FIG. 2 differs from that shown in FIG. 1 in that the FBGs are not connected in series to the port A of fiber coupler  10  but are instead connected in parallel. Thus, FBGs  22 A and  22 B are connected to ports A and B of a second 3-dB coupler  30  by way of attenuators  32 A and  32 B, respectively. FBGs  24 A and  24 B are connected to ports A and B of a third 3-dB coupler  40  by way of attenuators  36 A and  36 B, respectively. The transmissive ports of FBGs  22 A,  22 B,  24 A, and  24 B are coupled to four output ports P 1   OUT , P 2   OUT , P 3   OUT  and P 4   OUT , respectively. Ports D of couplers  30  and  40  are connected to ports A and B, respectively, of the 3-dB coupler  10 , and ports C of couplers  30  and  40  are connected to fifth and sixth output ports P 5   OUT  and P 6   OUT , respectively.  
         [0040]    In operation, the ASE leaving the coupler  10  will be split again by couplers  30  and  40  before reaching FBGs  22 A,  22 B,  24 A and  24 B. Each of these gratings will reflect about 50 per cent of the ASE at its own selected wavelength and transmit the remainder, as before. Consequently, when lasing conditions have been established, the light appearing at ports P 1   OUT , P 2   OUT , P 3   OUT , and P 4   OUT  will have wavelengths λ 1  , λ 2 , λ 3 , λ 4 , . . . and the ASE. FIG. 3 shows the measured output at terminal P 1  OUT when two gratings are used in the setup. Note that the signal to noise ratio is about 45 dB.  
         [0041]    The light leaving the output ports P 5   OUT  and P 6   OUT  will be reflected by the gratings and will pass through the attenuators again, as compared with the light leaving the first output ports P 1   OUT , P 2   OUT , P 3   OUT , and P 4   OUT . Consequently, the light at output port s P 5   OUT  and P 6   OUT  have a better signal-to-noise ratio (better than 55 dB) compared to the other output terminals, as illustrated in FIG. 4. The reason that the signal-to-noise ratio is improved at the output ports P 5   OUT  and P 6   OUT  is that the signals contains less ASE, because the gratings reflects only the signal to couplers  30 , and  40 , while they passes ASE to the output ports P 1   OUT , P 2   OUT , P 3   OUT , and P 4   OUT .  
         [0042]    It should be appreciated that, if only two wavelengths were needed, one of the couplers  30  and  40 , and its associated pair of gratings, could be omitted. Conversely, additional wavelengths could be obtained by adding more couplers and pairs of gratings, in a tree-like configuration. It is also envisaged that the embodiment of FIGS. 1 and 2 could be combined, with some of the parallel branches of the laser source having a series of gratings, or each embodiment of FIGS. 1 and 2 could be repeated more than one time to achieve multiple of DWDM source  
         [0043]    The attenuators  32 A,  32 B,  36 A and  36 B allow the amplitude of the light at each wavelength to be adjusted so that, if desired, they are equal.  
         [0044]    Although, in the above-described laser sources, the gratings each reflect about  50  per cent of the selected wavelength light, other proportions could be used.  
         [0045]    It is an advantage of the present invention that a multiplicity of wavelengths can be provided using a single active-fiber loop mirror and a grating for each wavelength. Also, the number of wavelengths can be increased simply by adding more fiber gratings, and perhaps increasing pump energy, as appropriate.  
         [0046]    Numerous other embodiments may be envisioned without departing from the spirit and scope of the present invention.