Broadband generation of mid ir, coherent continua with optical fibers

Abstract

Coherent and compact supercontinuum light sources for the mid IR spectral regime are disclosed and exemplary applications thereof. The supercontinuum generation is based on the use of highly nonlinear fibers or waveguides. In at least one embodiment the coherence of the supercontinuum sources is increased using low noise mode locked short pulse sources. Compact supercontinuum light sources can be constructed with the use of passively mode locked fiber or diode lasers. Wavelength tunable sources can be constructed using appropriate optical filters or frequency conversion sections. Highly coherent supercontinuum sources further facilitate coherent detection schemes and can improve the signal/noise ratio in lock in detection schemes.

What is claimed is: 
     
         1 . A method for coherent detection comprising:
 providing a short pulse source with a central emission wavelength >1700 nm;   generating a coherent supercontinuum in a waveguiding material;   inducing coherent signal emission by irradiating a sample with an output of said short pulse source, said signal emission covering a signal emission spectral region;   selecting a spectral region within said coherent supercontinuum that corresponds with said signal emission spectral region;   interfering two spectrally overlapping signals from said signal emission spectral region and from the corresponding region from said supercontinuum on a detector; and   amplifying the detected signal.   
     
     
         2 . A method for determining a characteristic of a sample, the method comprising:
 providing a short pulse source with a central emission wavelength >1700 nm;   generating a low noise supercontinuum in a waveguiding material;   irradiating a sample with at least part of said supercontinuum;   detecting a response to said irradiating step produced by said sample; and   determining a characteristic of said sample based upon said response.   
     
     
         3 . The method according to method 2, wherein said supercontinuum is coherent and comprises a plurality of monochromatic spectral lines. 
     
     
         4 . The method according to method 3, wherein at least one of said monochromatic spectral lines is locked to an external frequency reference signal. 
     
     
         5 . The method according to method 3, wherein a carrier envelope offset frequency of said spectral lines is locked to an external reference signal. 
     
     
         6 . A coherent supercontinuum source comprising;
 a fiber-based laser source generating short optical pulses, said fiber-based source generating an output at a central wavelength >1700 nm, said short optical pulses comprising a pulse width <10 ps;   a pump source to pump said fiber-based source;   a nonlinear waveguide for self-frequency shifting the output of said fiber-based source to frequency shifted output wavelengths >2200 nm; and   a highly nonlinear material receiving pulses having said frequency shifted output wavelengths and generating said coherent supercontinuum therewith.   
     
     
         7 . The coherent supercontinuum source according to  claim 6 , wherein said pump source comprises at least one single-frequency seed source. 
     
     
         8 . The coherent supercontinuum source according to  claim 6 , wherein said pump source comprises a seed source nearly free of any longitudinal mode structure. 
     
     
         9 . The coherent supercontinuum source according to  claim 6 , wherein said pump source comprises a seed source that generates amplified spontaneous emission. 
     
     
         10 . The coherent supercontinuum source according to  claim 6 , wherein said fiber-based source comprises a passively mode locked fiber oscillator, and wherein an output of said pump source is injected into the core of a gain fiber within said mode locked fiber oscillator. 
     
     
         11 . The coherent supercontinuum source according to  claim 6 , wherein said fiber-based source comprises a passively mode locked fiber oscillator based on a Tm, Tm:Ho, or a Ho doped fiber. 
     
     
         12 . The coherent supercontinuum source according to  claim 6 , wherein said fiber based source produces pulses with a pulse width <300 fs. 
     
     
         13 . The coherent supercontinuum source according to  claim 6 , wherein said fiber-based source produces pulses with a pulse width <100 fs. 
     
     
         14 . The coherent supercontinuum source according to  claim 6 , wherein said highly nonlinear material comprises a high numerical aperture photonic crystal fiber (PCF) having a core and a single layer of air holes at least partially surrounding said core. 
     
     
         15 . The coherent supercontinuum source according to  claim 6 , wherein coherence of said coherent supercontinuum source is measurable by RF beat signal with a S/N ratio of at least 10 dB when measured with an RF frequency analyzer at 100 kHz resolution, measurable with an f−2f interferometer.