Abstract:
A mode-locked laser has optical components integrated into a single apparatus and interrelated via optical free-space coupling. The laser optical cavity path is reduced to less than ten meters, primarily composed of optical gain fiber. A Fabry-Perot filter is matched to the laser pulse repetition frequency. Utilizing a Fabry-Perot filter within the laser optical cavity suppresses supermode spurs in the phase noise spectrum; thereby reducing total timing jitter.

Description:
BACKGROUND 
     Conventional mode-locked lasers utilize fiber pigtailed optical components spliced together to form the laser optical cavity. Optical fiber inter-connections between each optical component produce a laser optical cavity path length that can be 100 m or more. Such a long optical cavity produces an optical path length that is very sensitive to disturbances such as temperature change and vibration, adversely affecting the frequency stability of the laser output. 
     SUMMARY 
     In one aspect, embodiments of the inventive concepts disclosed herein are directed to a mode-locked laser having optical components integrated into a single apparatus. The laser optical cavity path is reduced to less than ten meters, primarily composed of optical gain fiber. 
     In a further aspect, a Fabry-Perot filter is matched to the laser pulse repetition frequency. Utilizing a Fabry-Perot filter within the laser optical cavity suppresses supermode spurs in the phase noise spectrum; thereby reducing total timing jitter. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and should not restrict the scope of the claims. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the inventive concepts disclosed herein and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the embodiments of the inventive concepts disclosed herein may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a conventional, prior art, mode-locked laser; 
         FIG. 2  shows an exemplary embodiment of a mode-locked laser according to the inventive concepts disclosed herein; 
         FIG. 3  shows a graph showing phase noise supermode spurs of conventional mode-locked lasers in a balanced optical correlator; 
         FIG. 4  shows a block diagram of a digital receiver system with an optical clock according to exemplary embodiments of the inventive concepts disclosed herein; and 
         FIG. 5  shows a block diagram of a photonic analog-to-digital converter according to exemplary embodiments of the inventive concepts disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     Before explaining at least one embodiment of the inventive concepts disclosed herein in detail, it is to be understood that the inventive concepts are not limited in their application to the details of construction and the arrangement of the components or steps or methodologies set forth in the following description or illustrated in the drawings. In the following detailed description of embodiments of the instant inventive concepts, numerous specific details are set forth in order to provide a more thorough understanding of the inventive concepts. However, it will be apparent to one of ordinary skill in the art having the benefit of the instant disclosure that the inventive concepts disclosed herein may be practiced without these specific details. In other instances, well-known features may not be described in detail to avoid unnecessarily complicating the instant disclosure. The inventive concepts disclosed herein are capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     As used herein a letter following a reference numeral is intended to reference an embodiment of the feature or element that may be similar, but not necessarily identical, to a previously described element or feature bearing the same reference numeral (e.g., 1, 1a, 1b). Such shorthand notations are used for purposes of convenience only, and should not be construed to limit the inventive concepts disclosed herein in any way unless expressly stated to the contrary. 
     Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by anyone of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     In addition, use of the “a” or “an” are employed to describe elements and components of embodiments of the instant inventive concepts. This is done merely for convenience and to give a general sense of the inventive concepts, and “a’ and “an” are intended to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. 
     Finally, as used herein any reference to “one embodiment,” or “some embodiments” means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the inventive concepts disclosed herein. The appearances of the phrase “in some embodiments” in various places in the specification are not necessarily all referring to the same embodiment, and embodiments of the inventive concepts disclosed may include one or more of the features expressly described or inherently present herein, or any combination of sub-combination of two or more such features, along with any other features which may not necessarily be expressly described or inherently present in the instant disclosure. 
     Broadly, embodiments of the inventive concepts disclosed herein are directed to a mode-locked laser having optical components integrated into a single apparatus. 
     Referring to  FIG. 1 , a conventional mode-locked laser  100  is shown. A conventional mode-locked laser  100  comprises a pump laser diode  102  that feeds light to a pump combiner  104 . The conventional mode-locked laser  100  comprises one or more gain fiber elements  106 , a Lyot filter  108 , an isolator  110 , one or more lead-zirconate-titanate (PZT) fiber elements  112 , an electro-optic modulator  114 , a laser tap  118 , and a plurality of optical component connections  116  connecting the fiber-pigtailed optical components  104 ,  106 ,  108 ,  110 ,  112 ,  114 , and  118  to produce a laser output  120  from the conventional mode-locked laser  100 . 
     The pump combiner  104  combines light from the electro-optic modulator  114  and other optical components  106 ,  108 ,  110 ,  112 , and  114  via the plurality of optical component connections  116 . Each element in the plurality of optical component connections  116  adds to the optical cavity length. The optical cavity length must be stabilized to much less than 1 micron for lasers with short duration light pulses. Complex controls in the plurality of optical component connections  116  and the PZT fiber elements  112  (configured to stretch in response to temperature fluctuations) are required to stabilize the laser output  120 . 
     Output stability is dependent, at least in part, on optical path length, which in turn is dependent on temperature changes based on thermos-optical index change and thermal expansion in the fiber material. A change in optical path length is correlated to a change in temperature multiplied by fiber length; therefore, a reduction in fiber length reduces temperature sensitivity. 
     Referring to  FIG. 2 , an exemplary embodiment of a mode-locked laser  200  according to the inventive concepts disclosed herein is shown. The laser  200  comprises a compact enclosure  202  containing a modulator  208 , output splitter  210 , Lyot filter  212 , isolator  214 , and pump combiner  206 . A pump laser diode  204  delivers light to the pump combiner  206  that combines the light with previous filtered laser pulses from the optical components  208 ,  210 ,  212 , and  214  in the enclosure  202 . The combined output from the pump combiner  206  is delivered as an input to the modulator  208  and the output splitter  210  produces a laser output  220 . In some embodiments, the optical components  208 ,  210 ,  212 , and  214  are affixed to the enclosure  202  along an optical pathway defined by an axis connecting the optical components  208 ,  210 ,  212 , and  214 , and optically coupled in free-space. Such embodiments obviate the need for fiber-pigtailed optical components and a plurality of optical component connections, resulting in an optical path length 1/100 as long (or less) as compared to a conventional mode-locked laser such as shown in  FIG. 1 . A shorter optical path length requires less optical fiber in the optical cavity. Reducing the length of optical fiber in the optical cavity greatly reduces the effects of environmental disturbances on the stability of the output  220  of the laser  200  and reduces the complexity of any required active system stability controls. 
     For example, in a mode-locked laser  200  according to some embodiments utilizing optical fiber having a thermo-optical index coefficient of 6.8×10 −6 /° C. and a thermal expansion coefficient of 5.5×10 −7 /° C., a 25 m length of fiber would experience a change in length of 184 μm/° C. By comparison, 2.5 m length of fiber would experience a change in length of 18.4 μm/° C. Because of the reduced sensitivity to temperature, the enclosure  202  may be smaller than a convention mode-locked laser due to insulation requirements. 
     In some embodiments, one or more gain fiber elements  218  are interposed in the optical path between the pump combiner  206  and the modulator  208 . Because of the shorter optical path length as compared to conventional mode-lock lasers, only gain fiber elements  218  are required in the laser cavity; no PZT fiber elements are necessary. 
     In some embodiments, the mode-locked laser  200  includes a Fabry-Perot filter  216  interposed between the isolator  214  and the pump combiner  206 . The Fabry-Perot Filter  216  is also affixed to the enclosure  202  and optically coupled in free-space to the isolator  214  and pump combiner  206 . The Fabry-Perot filter  216  reduces phase noise in the laser output  220 . 
     Referring to  FIG. 3 , a graph showing phase noise supermode spurs  302  of conventional mode-locked lasers in a balanced optical correlator is shown. Jitter spectral density  300  and reverse integrated timing jitter  306  are dependent on the offset frequency of mode-locked lasers in the balanced optical correlator. Within a particular offset frequency band, supermode spurs  302  develop. Within such range, it may be desirable to remove supermode spurs  302  such that the jitter spectral density approaches the single mode-locked laser phase noise floor  308  of approximately 168 dBc/Hz. A Fabry-Perot filter suppresses the supermode spurs  302  to produce an output within a target phase noise  304  approaching the single mode-locked laser phase noise floor  308 . Accordingly, some embodiments are directed to a compact mode-locked laser with very low timing jitter that is insensitive to environmental disturbances. 
     Referring to  FIG. 4 , a block diagram of a digital receiver system  400  with an optical clock according to exemplary embodiments of the inventive concepts disclosed herein is shown. A digital receiver system  400  includes a receiver  402  that receives a clock signal  404  comprising photo pulses produced by a mode-locked laser  406  according to one embodiment of the inventive concepts disclosed herein. The receiver  402  may receive the clock signal  404  via a photodiode or any other mechanism capable of distinguishing pulses in a frequency range corresponding to the operating frequency of the mode-locked laser  406 . The receiver  402  also receives an input signal  408  and produces a digitized output  410  based on the input signal  408  and the clock signal  404 . Such a digital receiver system  400  may be incorporated into a radar system or communication system such as a software defined radio. 
     Referring to  FIG. 5 , a block diagram of a photonic analog-to-digital converter  500  according to exemplary embodiments of the inventive concepts disclosed herein is shown. A photonic analog-to-digital converter  500  receives an analog signal  502 . The analog signal  502  may drive a pump laser diode in a mode-locked laser  504  according to embodiments of the inventive concepts disclosed herein, or otherwise indirectly drive pulses in the mode-locked laser  504 , which then produces a corresponding stream of photo pulses that may be converted to a digital signal output  508  via some mechanism such as a photodiode  510 . 
     It is believed that the inventive concepts disclosed herein and many of their attendant advantages will be understood by the foregoing description of embodiments of the inventive concepts disclosed, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the broad scope of the inventive concepts disclosed herein or without sacrificing all of their material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.