Patent Application: US-57231205-A

Abstract:
method and device for the generation of a comb of stabilized frequency lines and / or a train of ultrashort laser pulses for stabilization of the position of the carrier wave with respect to the amplitude envelope of few cycles laser pulses . interference between spectral components generated by means of difference frequency generation and self phase modulation in one and the same non - linear crystal allows detecting and stabilizing the temporal evolution of the carrier - envelope offset phase . the described technique improves dramatically the accuracy of the stabilization and has very small insertion losses .

Description:
as mentioned above , fig1 a and 1b show a schematic representation of laser light pulses in the time domain ( fig1 a ) and in the frequency domain ( fig1 b ). the spectrum of a train of laser light pulses shown in fig1 b consists of spectral lines separated by the repetition frequency f r such that f n + 1 − f n = f r . furthermore , the frequency f ceo which , may be denoted as offset frequency f 0 , too , is shown in fig1 b , and in fig1 a , also the ceo phase shift δφ and the period t and its inverse , the repetition frequency f r , are shown . fig2 shows a schematic block diagram of a preferred embodiment of the device according to the invention comprising a f ceo stabilization scheme . as to the components of this device , there is a pump laser 1 , e . g . a second harmonic of diode pumped nd : yvo4 laser ( for instance the commercially available laser coherent , verdi : 532 nm , 3 . 85 w ). the pump laser beam 1 ′ is applied to a ti : sapphire laser oscillator 2 where a laser light beam 3 is generated in accordance with the well - known mode - locking principle . the laser light beam 3 is then coupled into a non - linear optical medium 4 after passing a pair of fused silica wedged plates w ( which may be used to optimize the duration of the pulses carried by the beam 3 ) and chirped mirrors cm 1 and cm 2 , as well as further mirrors 5 , 6 and 7 . the chirped mirrors cm 1 , cm 2 provide for a negative group delay dispersion ( gdd ), as is known per se , whereas the wedged plates w introduce a positive gdd ; accordingly , gdd compensation may be achieved by cm 1 , cm 2 . the non - linear optical medium 4 may comprise a periodically poled magnesium oxide - doped lithium niobate ( pp — mgo : ln ) crystal , as is indicated in fig2 , but may alternatively comprise also other optically non - linear periodically poled crystal materials which are capable of quasi - phase matching ( qpm ), as disclosed e . g . in u . s . pat . no . 5 , 787 , 102 a , and of the difference frequency generation described , compare also ref . [ 23 ]; so , for instance , periodically poled lithium niobate crystals , periodically poled lithium tantalate crystals , or periodically poled potassium niobate crystals may be used , too . the output of the non - linear medium 4 , or crystal 4 , respectively , is coupled into a delay line 8 comprising chirped mirrors cm 3 , cm 4 ( with multiple reflections ) via a concave mirror 9 . at the output 8 ′ of the delay line , e . g . 6 - fs phase - stabilized pulses are obtained , i . e . a train of laser light pulses , the laser light having a spectrum spanning the wavelength range of 0 . 6 - 1 . 2 μm . furthermore , the output light of the non - linear crystal 4 is sent to a detector and stabilizing unit 10 comprising a detector 11 which includes a long pass filter lf having a cutoff wavelength at 1400 nm and a photo diode pd , for instance an in — gaas photo diode . for stabilizing the frequency , a feedback loop 12 is provided comprising a low - pass amplifier 13 , e . g . an electronic amplifier available from stanford research system ( model sr560 ); a phase - locking electronics 14 , as e . g . the “ lock box ” from menlosystems ; and a rf ( radio frequency ) reference oscillator 15 , for instance a signal generator , marconi , 2022d , which is operated at 1 mhz . from fig2 , it may further be seen that the — electronic — output put of the “ lock box ” 14 is applied to an electro - optic modulator eom , to control the amplitude of the pump laser beam 1 ′, to effect self phase modulation in the oscillator 2 , for maintaining the offset frequency f 0 = f ceo constant . ( instead of this type of control , it would also be possible , e . g ., to control the power of the pump laser 1 , as will be well - known to persons skilled in the art ). the device according to fig2 allows a dramatically better stabilization of the temporal evolution of ceo phase , when compared with the prior art . when the peak intensity of the laser pulse and the nonlinearity of the non - linear frequency mixing crystal , namely the optically non - linear medium 4 , are large enough , second - order non - linear frequency mixing ( second harmonic generation or difference frequency generation ; → f d ) as well as self - phase modulation (→ f spm ) occur at the same time with the aid of the non - linear medium 4 . if there is a spectral overlap between these two generated components f d and f spm , a beat signal ( beating frequency ), f 0 between them should emerge at f ceo , that is f 0 = f ceo . as mentioned above , a prior art scheme making use of a thin zno crystal for spectral broadening and second harmonic generation was demonstrated for observation of a beat signal at f ceo , s . ref . [ 21 ]; however , phase stabilization could not be accomplished . in the present case , 6 - fs 3 - nj pulses from the ti : sapphire oscillator 2 are tightly focussed on the non - linear optical medium 4 , e . g . in form of a periodically poled magnesium oxide - doped lithium niobate bulk crystal ( pp — mgo : ln ), which has a higher non - linear conversion efficiency than the zno crystal , and both self - phase modulation and difference - frequency generation occur in the crystal 4 , and their spectra overlap at about 1400 nm . as a result , a strong interference beat signal is observed at this wavelength of 1400 nm , and stabilization of f ceo of the laser is possible . a most remarkable feature of this phase stabilization technique is that the beat signal is generated outside of the original laser spectrum . this means that the pulses used for phase stabilization can be exploited for further applications . additionally , all beams are collinear and no delay lines are needed to adjust the two non - linear mixing components f d and f spm . thus , in contrast to the prior art f to 2f technique , the present system is insensitive to misalignment , and better phase locking quality can be expected . the underlying processes of this scheme are explained in fig3 a : the difference frequency f d between high frequency and low frequency components ( e . g . 600 nm and 1050 nm ) is generated ( by frequency mixing ) at 1400 nm . at the same time , self phase modulation inside the crystal 4 also generates light at this wavelength . the carrier - envelope offset frequency f ceo of the difference frequency is always 0 , s . ref . [ 19 , 20 ], whereas the supercontinuum carries f ceo of the original pulse train . consequently , one can observe the interference beat signal at 1400 nm . the horizontal arrows in fig3 a indicate pairs of frequency lines that are mixed in the process of difference frequency generation (“ dfg ”), giving rise to the spectrum labeled “ dfg ” signal ”. the label “ original spectrum ” is associated to the spectrum of the pulses focused into the non - linear crystal 4 . this spectrum is broadened in the non - linear crystal 4 due to self phase modulation ( spm ). in the spectral region in which the dfg signal and the broadened spectrum overlap , a beat signal having the frequency f ceo emerges . fig3 b shows the long wavelength edge spectrum of the pulses after passing through the crystal . the spectrum of fig3 b has been measured with an optical spectrum analyser ( ando , aq - 6315a ). in fig3 b , the solid line shows the spectrum when the beam 3 is focused into the crystal 4 , whereas the dotted line shows this spectrum when the beam is not focused into it . beat signals are observed in the shaded regions . newly generated spectral components in this region are clearly visible when the pulses are focused more tightly into the crystal 4 . this is attributed mainly to the self - phase modulation by the crystal 4 as well as to difference - frequency mixing where phase matching occurs . fig4 shows the out - of - loop phase noise power spectral density ( psd ) and integrated ceo phase error ( ceo pe ) versus frequency , as a function of observation time ( frequency − 1 ). in an experiment , the pulses passing through the non - linear crystal 4 were re - compressed by the delay line 8 down to 6 f s , which is few - cycle pulse , and the measured out of loop phase noise was 0 . 0427π rad ( from 10 μs to 35 minutes observation time ), which is approximately five times better than that of the prior art phase stabilization methods , s . ref . [ 18 , 22 ]. the large phase error step - like structure around the observation time corresponding to about 200 hz ( indicated by 16 in fig4 ) is much less pronounced than that of ref . 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