Abstract:
A tunable fiber ring laser with a gain clamped semiconductor optical amplifier is a ring laser source working at room temperature. The laser has an inner cavity disposed inside an outer cavity. A pair of circulators disposed in the inner cavity is configured to assure counter-propagation of light between the inner cavity and the outer cavity. A gain-clamped semiconductor optical amplifier (GC-SOA) is formed by combining a semiconductor optical amplifier (SOA) and a fixed filter in conjunction with the pair of circulators. A Fiber Fabry-Perot Tunable Filter (FFP-TF) is disposed in the outer cavity and connects to the pair of circulators via a polarization controller and a fused coupler.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical laser technology, and particularly to a tunable fiber ring laser with a gain clamped semiconductor optical amplifier configured to provide stable tunable-lasing over a relatively wide lasing wavelength. 
     2. Description of the Related Art 
     Tunable fiber ring lasers have found a lot of attention recently for many applications. They are widely used in wavelength division multiplexing (WDM) communication systems, laser spectroscopy and fiber optic sensor systems. The advantage offered by these laser sources is that their emission lasing wavelength can be easily tuned in a certain spectral range using different techniques. This feature can be of importance, whereby a single source can replace several laser sources. Also, erbium doped fibers (EDF) have been used as the gain medium reported using tunable fiber Bragg gratings (TFBG) for wavelength tuning in the C-band. The gratings are embedded inside a 3-point bending device for achieving wavelength tuning. The laser is tunable from 1530 nanometers (nm) to 1565 nm. Other known designs include a mode restricting intra-cavity fiber Fabry-Perot (FP) filter in the EDF based ring laser. Such a laser operates from 1533.3 nm to 1574.6 nm. The laser is predominantly tunable in the C-band than in the L-band. Further, a widely tunable erbium doped fiber ring laser based on multimode interference effect is known and is tunable from 1549 nm to 1609 nm, where tuning is more pronounced in the L-band than in the C-band. Also, a semiconductor based linear optical amplifier used as the gain medium is known, and the laser is tunable from 1507 nm to 1600 nm. Further, it is known that a tuning wavelength of 80 nm can be achieved by varying the length of the EDF in the cavity, and that the EDF length is varied from 50 m to 200 m in order to achieve a tuning range of 80 nm. Additionally, a tunable fiber ring laser is known based on a two-taper Mach Zehnder interferometer, and the laser is tuned by mechanically bending one of the two taper waists. The tuning range in the L-band is from 1564 nm-1605 nm, which was achieved by employing an L band erbium doped fiber amplifier (EDFA) in the cavity. Also, a micro-electro-mechanical system (MEMS) based in-plane FP filter has been used to demonstrate a tuning range of 35 nm in the C-band. An advantage offered by semiconductor optical amplifier based lasers is their compact size, such as when compared to their EDF based counterpart. 
     Thus, a tunable fiber ring laser with a gain clamped semiconductor optical amplifier addressing the aforementioned problems is desired. 
     SUMMARY OF THE INVENTION 
     Embodiments of a tunable fiber ring laser with a gain clamped semiconductor optical amplifier can provide a widely tunable semiconductor fiber ring laser based on a gain clamped semiconductor optical amplifier working at room temperature. By incorporating embodiments of a gain clamped semiconductor optical amplifier (SOA) as a gain medium, the laser can be tuned from around 1522 nm to around 1599 nm, for example. Embodiments of a tunable fiber ring laser have an inner cavity disposed in an outer cavity. A pair of circulators is disposed in the inner cavity and is configured to provide a counter-propagation of light between the inner cavity and the outer cavity. Embodiments of a gain-clamped semiconductor optical amplifier (GC-SOA) are formed by combining a semiconductor optical amplifier (SOA) and a fixed filter where the circulator pair is inside the inner cavity. This configuration in the laser cavity can provide an improvement in terms of transient gain excursions by applying optical feedback in the tunable fiber ring laser. This attribute of the GC-SOA can enable realizing a stable or substantially stable tunable-wavelength laser source. The fiber Fabry-Perot tunable filter (FFP-TF) in embodiments of a tunable fiber ring laser is configured in the outer cavity to act as a wavelength selective element to selectively tune the laser wavelength of the tunable fiber ring laser. Embodiments of a tunable fiber ring laser are continuously or substantially continuously tunable over a 78 nanometer (nm) range of the C-band and the L-band, for example. Also, embodiments of a tunable fiber ring laser can produce a power equalized output from 1530 nm to up to 1570 nm with a side-mode-suppression ratio (SMSR) of greater than 60 dB, for example. 
     These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an embodiment of a GC-SOA equipped tunable fiber ring laser according to the present invention. 
         FIG. 2  is a wavelength comparison plot of an embodiment of a GC-SOA versus a conventional SOA. 
         FIG. 3  is a spectral density plot showing tuning peaks of an embodiment of a GC-SOA equipped tunable fiber ring laser according to the present invention. 
         FIG. 4  is a spectral bandwidth plot of an embodiment of a GC-SOA equipped tunable fiber ring laser according to the present invention. 
         FIG. 5  is a SMSR versus Wavelength versus Output Power plot of an embodiment of a GC-SOA equipped tunable fiber ring laser according to the present invention. 
     
    
    
     Unless otherwise indicated, similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The schematic diagram of  FIG. 1  shows an embodiment of a tunable fiber ring laser  12  in an experimental setup as can be used to demonstrate the operation of the widely tunable semiconductor fiber ring laser  12  at room temperature, for example. Embodiments of a tunable fiber ring laser, such as the tunable semiconductor fiber ring laser  12  have a gain-clamped semiconductor optical amplifier (GC-SOA) including a SOA  14  formed inside an inner cavity  10   a  which acts as a gain medium. The tunable semiconductor fiber ring laser  12  laser source has two cavities, for example. The inner cavity  10   a , as an inner short cavity, is incorporated to provide an optical feedback to realize the GC-SOA, whereas the outer cavity  10   b  serves as the main cavity of the laser source. The SOA  14  is incorporated in the tunable semiconductor fiber ring laser  12  setup to realize the GC-SOA within the inner cavity  10   a  of the tunable semiconductor fiber ring laser  12 . 
     A polarization controller (PC)  20  in a main outer cavity  10   b , as an outer long cavity, of the tunable semiconductor fiber ring laser  12  adjusts the state of polarization in the laser cavity to achieve a relatively high signal-to-noise ratio (SNR) and to achieve a relatively stable output power. 
     A fiber Fabry-Perot Tunable Filter (FFP-TF)  16 , desirably a thin filmed filter, for example, is included in the main outer cavity  10   b  in-line between the PC  20  and a fused 90/10 coupler having a 90% pass through  27   a  and a 10% diverter  27   b . The FFP-TF  16  provides a feedback light beam and acts as a wavelength selective element in the tunable fiber ring laser  12  to selectively tune the laser wavelength of the tunable fiber ring laser  12 . The tuning of the laser wavelength is achieved by tuning the pass-band of the FFP-TF  16  employed in the main outer cavity  10   b.    
     The direction of the feedback light beam in the inner cavity  10   a  is established by the two circulators  11   a  and  11   b . The light in the inner cavity  10  as cavity circulates in a counterclockwise direction, or in a counter-propagating direction, as indicated by the curved arrow underneath the SOA  14  in  FIG. 1 . The light beam in the main outer cavity  10   b  circulates in the clockwise direction, as indicated by the linear arrows inside main outer cavity  10   b  in  FIG. 1 . The two circulators  11   a  and  11   b  in the GC-SOA determine or establish the direction of feedback light, such as a feedback light beam, as well as enhance avoiding unwanted reflections from the tunable filter FFP-TF  16  to the SOA  14 . 
     A fixed filter  18  in the feedback loop can have a 3 decibel (dB) bandwidth of 0.025 nm and can be fixed at around 1532 nm, for example, which can enhance providing a relatively fine gain control. The fixed filter  18  and the SOA  14  is operably connected to the pair of circulators  11   a  and  11   b  within the inner cavity  10   a , the fixed filter  18  providing a feedback path and combined with the SOA  14  forming a gain-clamped semiconductor optical amplifier (GC-SOA) in the tunable fiber ring laser  12  having an optical output directing first optical signals to the main outer cavity  10   b  and an optical input accepting second optical signals from the main outer cavity  10   b.    
     In a GC-SOA, such as in the tunable semiconductor fiber ring laser  12 , the induced lasing oscillations can clamp the gain and can suppress the gain saturation. This effect can reduce the gain-competition among the lasing modes, for example. The main outer laser cavity  10   b  includes the GC-SOA, the polarization controller PC  20 , the FFP-TF  16 , such as from Micron Optics Co., and the 10% fused coupler  27   a ,  27   b . The PC  20  is useful in achieving an optimized polarization state inside the main outer cavity  10   b  to achieve a relatively stable output power. The 10% fused coupler  27   a ,  27   b  is used to tap the output from the laser, the 10% light being directed to an optical spectrum analyzer (OSA), such as illustrated in  FIG. 1 , for example. 
     The resonator design in embodiments of the tunable semiconductor fiber ring laser  12  can provide a positive feedback, for example. A first portion of the split output from the optical coupler, such as the fused coupler  27   a ,  27   b , provides a tunable wavelength coherent beam emitted from the tunable fiber ring laser  12 , and a second portion of the split output includes optical signals retained in the inner cavity  10   a  and the main outer cavity  10   b  via a second circulator of the pair of circulators  11   a ,  11   b.    
     In the tunable semiconductor fiber ring laser  12 , the gain-clamped semiconductor optical amplifier (GC-SOA) includes the SOA  14  driven by a laser diode driver at a biasing current of approximately 200 milli-amperes (mA), for example. The SOA  14  can offer a relatively small signal gain of 25 dB with a saturation output power of 11.2 decibels-milli-watt (dBm), for example. The gain ripple of the SOA  14  is less than 0.2 dB and the gain difference between the transverse-electric (TE) and the transverse-magnetic (TM) polarization is less than 1 dB, for example. The average noise figure (NF) of the SOA  14  is around 6.64 dB. Gain clamping is achieved in the tunable semiconductor fiber ring laser  12  by the introduction of a feedback light beam realized by employing a narrow line-width tunable filter, such as the FFP-TF  16 , and the two 3-port fiber circulators  11   a  and  11   b  in the loop. 
     The direction of feedback light beam in the GC-SOA is established by the two circulators  11   a  and  11   b . Ports 2 of the circulators  11   a  and  11   b  are connected to the OSA  14 . Port 3 of the circulator  11   a  is connected to input of the fixed filter  18 . Output of the fixed filter  18  is connected to port 1 of the circulator  11   b . Port 3 of the circulator  11   b  is connected in the main outer cavity  10   b  to the polarization controller, such as the PC  20 , which is connected to the tunable FFP filter, such as the FFP-TF  16 , the output of which feeds the fused coupler  27   a ,  27   b  having split feedback path  27   a  and diverter path  27   b . The 90% path output from the fused coupler  27   a ,  27   b  is connected to port 1 of the circulator  11   a  to complete the circuit in the tunable semiconductor fiber ring laser  12 . The tunable filter FFP-TF  16  facilitates a continuous or substantially continuous tuning of the tunable fiber ring laser  12  in the wavelength range from 1522 nm to around 1599 nm to 1600 nm, for example. 
     The feedback light in the tunable fiber ring laser  12  operates in a counter-propagating direction to the main outer cavity  10 . The two circulators  11   a ,  11   b  in the GC-SOA determine the direction of feedback light, as well as enhance avoiding unwanted reflections from the tunable filter, such as the FFP-TF  16 , to the SOA  14 . The fixed filter  18  in the feedback loop can have a 3 dB bandwidth of 0.025 nm and can be fixed at around 1532 nm which can enhance providing a relatively fine gain control, for example. Also, it is known that in a GC-SOA the induced lasing oscillations can clamp the gain and can suppress the gain saturation, as can reduce gain-competition among lasing modes, for example. 
     Referring now to  FIG. 2 , plot  200  of  FIG. 2  shows the amplified spontaneous emission (ASE) spectrum of a conventional SOA and a GC-SOA in an embodiment of a tunable fiber ring laser, such as the tunable fiber ring laser  12 , at a fixed biasing current of 200 mA. The ASE peak wavelength of the SOA is at around 1526 nm which is shifted to 1560 nm when a GC-SOA is used. The feedback light beam at 1532 nm is also observed in  FIG. 2 . The 3 dB spectral width of the conventional SOA is around 55 nm which is extended to around 65 nm in the case of a GC-SOA. This extended spectral width can enable achieving a broadband tunable laser covering the whole or substantially the whole C-band and the L-band, for example. 
     Continuing with reference to  FIG. 2 , a narrow band, wide tunable range FFP-TF from the Micron Optics Co, such as the tunable FFP-TF  16 , was employed in the main outer cavity  10   b  to tune the lasing wavelength. By varying the voltage applied to the tunable filter, such as the tunable FFP-TF  16 , the tunable fiber ring laser, such as the tunable fiber ring laser  12 , was tuned over the C- and the L-band. The operable temperature range of the FFP-TF  16  is from −20 to +80 degrees centigrade (C) and its tuning Voltage/Free-Spectral-Range (FSR) is in a range of from approximately 0 volts to 16 volts, for example. The optical 3-bandwidth of the tunable filter FFP-TF  16  is 30 petameters (pm) (3.75 gigahertz (GHz)) and its FSR is around 102 nm, hence the Finesse of the FFP-TF  16  is 3400, for example. Also, the insertion loss at the peak of its pass-band is about 2.2 dB. 
     Plot  300  of  FIG. 3  shows the superimposed optical spectra of an embodiment of a tunable fiber ring laser, such as the tunable fiber ring laser  12 , while various external voltages were applied on the lead-zirconate titanate (PZT) film of the FFP-TF  16  in the tuning range of 1522 nm to almost 1600 nm. The output peak power is almost constant or substantially constant in the range from 1530 nm to 1570 nm. The measurements were performed with an optical spectrum analyzer (OSA), such as the OSA in  FIG. 1 , with a resolution of 0.01 nm, for example. 
     Referring to  FIG. 4 , a typical lasing spectrum of an embodiment of a tunable fiber ring laser, such as the tunable fiber ring laser  12 , tuned at 1551.2 nm and measured at around 1551 nm is shown in plot  400  of  FIG. 4 . The asymmetric shape of the laser is mainly due to the response of the OSA. The laser has a 3-dB bandwidth of around 0.015 nm limited by the resolution of the OSA. The total output power of the laser is around −1 dBm, for example. 
     Plot  500  of  FIG. 5  shows the output power and the SMSR versus the tuning wavelength in the C+L band of the laser, such as the tunable fiber ring laser  12 . The maximum and minimum output powers of −5 dBm and −15 dBm are observed at 1540 nm and 1522 nm, respectively. The maximum peak power variation of the laser is within 1 dB in the entire or substantially the entire C-band starting at 1530 nm and up to 1570 nm, for example. 
     The SMSR in the C-band is observed to be over 60 dB. However, outside this range, both the output power and the SMSR were reduced due to the smaller gain provided by the GC-SOA in an embodiment of a tunable fiber ring laser, such as the tunable fiber ring laser  12 . The maximum and minimum SMSRs are 62.5 dB and 50 dB over the entire tuning range in the C+L band. This demonstrates that embodiments of a tunable fiber ring laser, such as the tunable fiber ring laser  12 , can have a potential to be employed as a power equalized source in the C-band, for example. 
     In conclusion, embodiments of a tunable fiber ring laser with a gain clamped semiconductor optical amplifier can provide a widely tunable power equalized fiber ring laser using a GC-SOA. The embodiments of a tunable fiber ring laser are tunable from 1522 nm to around 1599 nm, for example, due to the relatively broad ASE generated by the GC-SOA in the embodiments of a tunable fiber ring laser. Also, embodiments of a tunable fiber ring laser have an advantage of a compact design along with an advantage of a relatively stable operation at room temperature. Further, embodiments of a tunable fiber ring laser can have potential applications in the WDM communication systems and fiber sensors, for example. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.