Abstract:
A mode-locked fiber MOPA delivers pulses of laser-radiation. A super-continuum generator including a bulk spectral-broadening element and a negative group-delay dispersion (NGDD) device is arranged to receive a pulse from the MOPA and cause the pulse to make a predetermined number of sequential interactions with the broadening element and the NGDD device. After making the predetermined interactions, the pulse is delivered from the super-continuum generator with a very broad spectral-bandwidth and a very short duration.

Description:
PRIORITY 
     This application is a continuation of application Ser. No. 14/080,366, filed Nov. 14, 2013, the disclosure of which is incorporated herein in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to super-continuum generation by spectral broadening of ultra-short laser-radiation pulses. The invention relates in particular to spectral broadening of the ultra-short radiation pulses in bulk nonlinear material. 
     DISCUSSION OF BACKGROUND ART 
     In a coherent super-continuum source, ultra short laser radiation pulses having relatively high peak power are typically spectrally broadened by passing the pulses through a short optical fiber which is either tapered to less than single mode dimensions or through a structured optical fiber such as a photonic crystal fiber (PCF), with the PCF being preferred. Typical sources of the laser-radiation pulses are mode-locked titanium sapphire (Ti:sapphire) lasers or Yb-doped mode-locked fiber lasers, both having pulse durations between tens of femtoseconds (fs) and hundreds of fs. 
     In order to maintain a phase correlation across the broadened spectrum, the typical length of a structured fiber is short, for example between about 5 millimeters (mm) and 10 mm. Depending on a particular application, the spectrally broadened pulses may be used with the entire broadened spectrum, or parts of the broadened spectrum may be tunable selected by a spectrometer or an interference filter. Applications of super-continuum sources include microscopy, spectroscopy and ultrafast amplifier seeding and phase stabilization. 
     A problem with using tapered or structured fibers for spectral broadening, while maintaining optical coherence across the spectrum, is that a relatively small core-diameter or about 5 micrometers (μm) or less is required. This leads to a poor coupling efficiency into the fiber, for example about 50% or less. In addition, the relatively small core-diameter limits the pulse-energy that can be broadened to about 5 to 10 nanojoules (nJ). At higher energy, damage to the fiber can occur. 
     Super-continua can also be generated by spectrally broadening ultra-short laser-radiation pulses by focusing the laser-radiation pulses in a bulk optical element of a highly nonlinear material such as Tellurite glass. Such a bulk element is significantly less expensive than a PCF and allows broadening the spectrum of high energy pulses with pulse energies of mJ and more. However, in order to overcome a problem of short interaction length of the focused pulses (resulting from a short Rayleigh range of the focused pulses) in the broadening element, the pulse-energy must be increased to a level where self-focusing effects create what is known as an elongated “filament” of different refractive index in the element. This filament behaves as a waveguide induced in the broadening element, which extends the interaction length of the pulse with the material of the broadening-element. 
     Filamentation, however, can cause permanent photo modification of the material of the broadening element, if not actual optical damage. This means that the broadening element would need to be periodically “shifted” with respect to a focused beam to expose an undamaged portion of the element to the beam. This would prolong the useful lifetime of the element. Eventually, however, the element would need to be replaced. There is a need for a method of spectral broadening in a bulk nonlinear element that is effective without a need for filamentation. 
     SUMMARY OF THE INVENTION 
     In one aspect, an apparatus in accordance with the present invention comprises a source of pulses of laser-radiation, each pulse having first pulse-duration and a first pulse-spectrum. The apparatus includes a spectral broadener. The spectral broadener includes a bulk spectral-broadening element and a negative group-delay dispersion device. The spectral broadener is arranged to receive a pulse from the source of laser pulses, cause the pulse to make a predetermined number of sequential interactions with the spectral-broadening element and the negative group-delay dispersion (NGDD) device, and deliver the pulse with a second pulse-duration and a second pulse-spectrum, the second pulse-duration being shorter than the first pulse-duration and the second pulse-spectrum being broader than the first pulse-spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
         FIG. 1  schematically illustrates a preferred embodiment of a tunable femtosecond laser-pulse source in accordance with the present invention including a pulsed laser-oscillator, a power amplifier for amplifying pulses from the laser oscillator, a pulse compressor for temporally compressing the amplified pulses, and a multi-pass super-continuum generator including a bulk spectral-broadening element. 
         FIG. 2  schematically illustrates one preferred embodiment of the super-continuum generator of  FIG. 1 , including a multi-pass optical relay formed between a first plane mirror surmounted by a bulk spectral-broadening element, and a second plane mirror having a negative group-delay dispersion (NGDD) mirror coating thereon for providing pulse re-compression. 
         FIG. 3  schematically illustrates another preferred embodiment of the super-continuum generator of  FIG. 1 , similar to the super-continuum generator of  FIG. 2 , but wherein the second plane mirror has a conventional reflective coating and pulse-recompression is provided by a pair of prisms. 
         FIG. 4  is a graph schematically illustrating computed normalized intensity as a function of wavelength for an initial pulse, and for the initial pulse after 5, 6 and 7 passes in the super-continuum generator of  FIG. 2 . 
         FIG. 4A  is a graph schematically illustrating computed normalized intensity as a function of wavelength for the 7-pass curve of  FIG. 4 , with and without re-compression between passes. 
         FIG. 5  is a graph schematically illustrating computed normalized intensity as a function of time for an initial pulse, and the initial pulse after 2, 4, and 7 passes in the super-continuum generator of  FIG. 2 . 
         FIG. 6  is a plot schematically illustrating computed NGDD required after each pass in an example of the apparatus of  FIG. 2  for full re-compression of a spectrally broadened pulse compared with a selected fixed NGDD. 
         FIG. 7  is a graph schematically illustrating computed relative intensity as a function of wavelength the apparatus example of  FIG. 6  for a pulse after 4 passes with complete re-compression on each pass, and after 6 passes with the same fixed re-compression after each pass. 
         FIG. 8  is a plot schematically illustrating initial pulse-duration and pulse duration after each pass in the apparatus example of  FIG. 6  for complete re-compression after each pass and for fixed re-compression after each pass. 
         FIG. 9A  is a graph schematically illustrating computed relative intensity as a function of time for an initial pulse and for the pulse after one pass through a zinc sulfide spectral-broadening element in a simple experimental one-pass broadening apparatus. 
         FIG. 9B  is a graph schematically illustrating measured relative intensity as a function of time for an initial pulse and for the pulse after one pass through a zinc sulfide spectral-broadening element in the experimental one-pass broadening apparatus. 
         FIG. 10A  is a graph schematically illustrating computed intensity in dB as a function of wavelength for an initial pulse and for the pulse after one pass through a zinc sulfide spectral-broadening element in the experimental one-pass broadening apparatus. 
         FIG. 10B  is a graph schematically illustrating measured intensity in dB as a function of wavelength for an initial pulse and for the pulse after one pass through a zinc sulfide spectral-broadening element in the experimental one-pass broadening apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates a preferred embodiment  10  of a tunable femtosecond laser-pulse source in accordance with the present invention. Source  10  includes a pulsed laser-oscillator  12 , a power amplifier  14  for amplifying pulses from the laser oscillator, and a pulse compressor  16  for temporally compressing the amplified pulses. The pulse-compressor can include gratings or prisms as is known in the art. The temporally-compressed, amplified pulses are spectrally broadened in multi-pass super-continuum generator  20  in accordance with the present invention. A spectral selector  22 , such as a grating or prism spectrometer, selects one or more wavelength bands of the spectrally broadened pulses to provide output pulses. 
     It is assumed for purposes of this description that laser-oscillator  12  is a mode-locked fiber-laser delivering pulses at a wavelength of 1040 nanometers (nm) having a duration of about 100 femtoseconds (fs) and a spectral bandwidth of about 11 nm, at a PRF of 80 Megahertz (MHz), with an average power of 2 milliwatts, (mW). The power amplifier includes plural stages of fiber (optical) amplification, with the pulses being stretched to 100 ps duration prior to at least the final stage of amplification. Pulse compressor  16  compresses the amplified, stretched pulses back to 100 fs duration. Compressed amplified pulses from pulse-compressor  16  are assumed to have an average power of about twenty Watts (20 W) and a spectral bandwidth (FWHM) of about 20 nm.  FIG. 2  schematically illustrates one preferred embodiment  20 A of super-continuum generator (spectral broadener)  20  of  FIG. 1 . Super-continuum generator  20 A includes a multi-pass optical relay formed between a first plane mirror  36  surmounted by a bulk spectral-broadening element  34 , and a second plane mirror comprising a negative group-delay dispersion (NGDD) mirror coating  40  on a substrate  42 . A detailed description of the design and properties of NGDD mirrors is presented in U.S. Pat. No. 6,081,379, and U.S. Pat. No. 6,154,318, assigned to the assignee of the present invention and the complete description of which is hereby incorporated by reference. NGDD mirrors are sometimes referred to by a questionable terminology “chirped mirrors”. Mirror  36  is backed by a substrate  38  which can be optionally cooled. 
     Mirrors  36  and  40  are at the foci of off-axis parabolic mirrors  32 A and  32 B respectively. The spectral-broadening element may be made from tellurium oxide (TeO 2 ), tellurite glass, sapphire (Al 2 O 3 ), zinc sulfide (ZnS), and silicon carbide (SiC), among other optically nonlinear materials. 
     A pulse from compressor  16  to be spectrally broadened is directed by a turning mirror  30  to parabolic mirror  32 A. Mirror  32 A focuses the pulse (assumed to be collimated) through spectral-broadening element  34  onto mirror  36 . Mirror  36  reflects the pulse back through the optically nonlinear element back onto mirror  32 A. Such a double pass through broadening element  34  is referred to hereinafter a “bounce”. The spectral-broadening element is preferably only about thick enough to accommodate one half of the Rayleigh range of the focused beam inside the material. For the pulse parameters exemplified above and ZnS as the nonlinear material, a thickness of less than 1 mm is preferred More preferably the thickness is between about 150 μm and 300 μm. After the bounce off the spectral-broadening element  34 , the pulse is spectrally broadened to a certain extent, and temporally stretched by positive dispersion introduced by the broadening element. 
     Mirror  32 B re-collimates the pulse and directs the re-collimated pulse to mirror  32 B which focuses the pulse on NGDD mirror  40 . Mirror  40  temporally re-compresses the pulse. Mirror  40  may only provide partial compression or even over-compression to have a pulse with negative frequency chirp incident on the nonlinear material on the next bounce. Here it should be noted that the term re-compression as used herein means canceling positive GDD introduced by the broadening element by NGDD introduced by a NGDD device. This re-compression may be complete re-compression, i.e., with the NGDD equal to the positive GDD; partial-recompression with less NGDD than positive GDD; or over re-compression with more NGDD than positive GDD. This is discussed in detail, with examples, further hereinbelow. 
     Continuing with reference to  FIG. 2 , NGDD mirror  40  is tilted at an angle with respect to mirror  36  such that a pulse returned to mirror  32 B returns to mirror  32 A along a path spaced apart from the previous incidence path. A result of this is that the pulse follows multiple parallel paths between mirrors  32 A and  32 B, and makes corresponding multiple bounces from the spectral-broadening element and multiple reflections from the NGDD mirror. Eventually one of the parallel paths is intercepted by a turning mirror  44  which directs the pulse out of the super-continuum generator to the spectral selector of  FIG. 1 . As depicted in  FIG. 2 , the pulse makes 5 bounces from the element  34  and 5 reflections from NGDD mirror  40 . Those skilled in the art will recognize that such a multi-pass relay arrangement can be made to provide more than 5, or less than 5 bounces by suitable placement of the turning mirrors. 
       FIG. 3  schematically illustrates another preferred embodiment  20 B of the super-continuum generator of  FIG. 1 , similar to the super-continuum generator of  FIG. 2  with an exception that pulse re-compression is provided by prisms  46  and  48  arranged as a complementary prism pair. The NGDD mirror coating  40  of super-continuum generator  20 A is replaced in generator  20 B by a conventional reflective coating  50 . After each bounce from spectral-broadening element  34 , the pulse is further spectrally broadened, and after pass through the prisms, the pulse is further temporally compressed. 
       FIG. 4  is a graph schematically illustrating computed normalized intensity as a function of wavelength for an initial pulse (solid curve), and the initial pulse after 5, 6 and 7 passes (dashed curves) in a super-continuum generator of  FIG. 2 . While some broadening will occur with just one bounce from (double pass through) broadening element  34 , in practice, at least 2 bounces from (2×2 passes through) the broadening element will be required for broadening to be truly regarded as a continuum generation. 
       FIG. 4A  is a graph schematically illustrating computed normalized intensity as a function of wavelength for the 7-pass curve of  FIG. 4  with (solid curve) and without (dashed curve) re-compression between bounces on the spectral-broadening element. It can be seen that the re-compression after each pass is particularly important in the spectral broadening process. 
     The amount of positive group delay dispersion GDD introduced by the broadening element on each bounce will be different, and, in fact, will decrease. Ideally, the negative GDD of mirror  40  should be selected to exactly compensate GDD of the spectral-broadening element. However, with a fixed negative GDD used for recompression as depicted in  FIG. 2 , the negative GDD can be selected such that the net GDD is reduced to zero after the final bounce of any selected number of bounces. This will mean that the net GDD is positive for all prior bounces. In the computation of  FIG. 4  it is assumed that the net GDD for each bounce is exactly zero. In a prism arrangement as depicted in  FIG. 3 , it may be possible to arrange the prisms such that the negative GDD sequentially decreases due to the different paths taken by a pulse through the prism on sequential passes. The spectral-broadening element in the computation of  FIG. 4  is assumed to be a tellurium oxide (TeO 2 ) element having a thickness of 150 μm. 
       FIG. 5  is a graph schematically illustrating computed normalized intensity as a function of time for an initial pulse, and the initial pulse after 2, 4, 7 passes in the super-continuum generator of  FIG. 2 . It can be seen that the initial pulse has a FWHM duration of about 100 femtoseconds, and is compressed to an FWHM duration of less than 10 femtoseconds after the seventh pass through the super-continuum generator. In this calculation, the net GDD is exactly nulled after each bounce. As a result the pulse-duration becomes shorter and shorter on successive passes because of the increasing spectral bandwidth and optical coherence across the bandwidth. 
     In effect, the multi-pass relay arrangement of  FIGS. 2 and 3  effectively increases the interaction length of a focused pulse in spectral-broadening element by twice the number of passes. So, effectively, in an element  34  having a thickness of 100 μm, and a pulse beam Rayleigh range of 100 μm, seven bounces in the broadening element are equivalent to an interaction length of 1.4 mm in a thicker element with the pulse-beam in focus over the entire 1.4 mm, without any requirement for filamentation to achieve this interaction length. In addition, the pulse duration stays short over the entire interaction length due to the pulse-compression between bounces. 
     It should be noted here that the multi-pass relay arrangement of  FIGS. 2 and 3  is just one arrangement that can be used to provide multiple, focused double-passes of a pulse beam in a thin, bulk, spectral-broadening element. One such arrangement is contemplated which similar to the arrangements of  FIG. 2 , hut wherein there is a single ellipsoidal mirror, with the mirror-backed spectral-broadening element at one conjugate focus of the ellipsoidal mirror and the NGDD mirror at the other conjugate focus of the ellipsoidal mirror. This and any other multi-mirror arrangement for creating a predetermined number of interactions of a laser beam with an optical element may be used in the apparatus of  FIG. 1  without departing from the spirit and scope of the present invention. 
     It should also be noted that while the present invention is described above with reference to broadening laser-radiation pulses from a fiber MOPA, ultra short pulses from any other type of laser or MOPA may be used without departing from the spirit and scope of the present invention. Such lasers and MOPAs include solid-state lasers and MOPAs, or hybrid MOPAs with a fiber-laser oscillator and solid-state amplification. 
     As noted above after each pass through the inventive super-continuum generator, positive GDD deceases. This is illustrated in  FIG. 6  which is a plot schematically illustrating computed NGDD (circles) required after each pass in an example of the apparatus of  FIG. 2  for complete re-compression (exact nulling of net GDD in the apparatus). It can be seen that the NGDD required becomes almost asymptotic to some relatively low NGDD after 4 passes in the apparatus. This suggests that a fixed NGDD value (triangles) comparable to the asymptotic value may be selected. Here, the fixed NGDD value is selected to be equal to the value required for complete re-compression (net GDD nulling) after three passes. This is slightly greater than the value required for 4 or more passes. 
     In the calculation of  FIG. 6  it is assumed that spectral-broadening element  34  of apparatus  20  is a ZnS element having as thickness of 300 μm. The initial pulse is assumed to be a pulse of 100 fs duration with an energy per pulse of 250 nJ. This is equivalent to an average power of 20 W at a PRF of 80 MHz. The beam diameter in the spectral-broadening element is assumed to be 40 μm. 
       FIG. 7  is a graph schematically illustrating computed relative intensity as a function of wavelength for the example of  FIG. 6  for a pulse after 4 passes with complete re-compression on each pass, and after 6 passes with the same fixed re-compression (the fixed NGDD of  FIG. 6 ) after each pass. It can be seen that the spectral width is about the same in each case with somewhat more asymmetry in the fixed NGDD case. 
       FIG. 8  is a plot schematically illustrating initial pulse-duration and pulse duration after each pass in the apparatus example of  FIG. 6  for complete re-compression after each pass and for fixed re-compression after each pass. It can be seen that the pulse-duration becomes progressively lower toward some asymptotic value less than 10 fs. In the case of complete re-compression on each pass, that value is reached after 4 passes. In the case of fixed re-compression on each pass, that value is reached after 6 passes. 
     In order to test the mathematical model used for all above-discussed calculations, some simple experiments were performed with pulses passed once (and focused in) a ZnS spectral-broadening element with net GDD nulled after the one pass. The pulses were provided by a fiber MOPA pre-compressed to about 370 fs before the pass through the ZnS element. The ZnS element had a thickness of 6 mm. 
       FIG. 9A  and  FIG. 9B  are graphs schematically illustrating respectively computed and measured relative intensity as a function of time for an initial pulse, and for the pulse after one pass through a the zinc sulfide spectral-broadening element in the experimental one-pass broadening apparatus. It can be seen that there is very good correlation between the initial and reduced pulse durations. In each case the initial pulse duration was 370 fs with the duration being about 200 fs after the single pass. 
       FIG. 10A  and  FIG. 10B  are graphs schematically illustrating respectively computed and measured intensity in dB as a function of wavelength for an initial pulse, and for the pulse after one pass through a zinc sulfide spectral-broadening element in the experimental one-pass broadening apparatus. Here the correlation between computed and experimental data is not as good as in the case of the pulse duration examples with the spectral broadening being less in the experimental case. It is believed that this is due to the excessive thickness of the ZnS element which introduces a correspondingly excess positive dispersion and self-focusing effects. Nevertheless, the form of the measured, broadened spectrum, with side lobes beginning to develop around a central peak is similar to that of the computed spectrum. 
     In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.