Patent Application: US-201314391260-A

Abstract:
stabilization of an injection locked optical frequency comb is achieved through polarization spectroscopy of an active laser cavity , eliminating optical pm sidebands inherent in previous stabilization methods . optical snr of 35 db is achieved . a monolithic alingaas quantum well fabry - prot laser injection locked to a passively mode - locked monolithic laser is presented here . the fp laser cavity can be used as a true linear interferometric intensity modulator for pulsed light .

Description:
a schematic of the laser system is shown in fig1 ( a ). the laser consists of a commercially available semiconductor optical amplifier ( soa ) 100 in an external fiber ring cavity . a mach - zehnder style intensity modulator ( im ) 110 is driven at the desired pulse repetition rate and frequency comb spacing , 10 . 24 ghz with ˜ 20 dbm of rf power . two ( 2 ) couplers are included for a laser output and injection input , in addition to multiple polarization controllers ( pc ) 120 , 130 , 140 , 150 and an optical isolator ( iso ) 160 to ensure unidirectional operation . a cw narrow linewidth (˜ 1 khz ) laser 170 at 1550 nm is used as the injection seed source while the injected power is controlled using a variable optical attenuator ( voa ) 180 . the injection polarization is adjusted for optimal injection locking . the soa and im both act as polarization discriminating devices and the injected tone experiences a phase shift while interacting with the cavity . the detection apparatus is placed at the injection coupler reflection port , where the output signal consists of the preferred injection polarization as well as the orthogonal polarization directly reflected from the cavity . a pc is used to rotate both signals such that they interfere in both outputs of a fiberized polarization beam splitter ( pbs ) 190 . a balanced photodetector ( bpd ) 200 takes the difference between the two signals to produce an error signal . fig1 ( b ) shows the optical spectrum without injection , as well as the full optical bandwidth achieved by the injection locked system , which has been observed to be constant for times greater than twenty ( 20 ) minutes . fig2 ( a ) shows the high resolution optical spectra of the harmonically mode - locked laser system centered 50 ghz below the injection frequency . when not injection locked ( black ), multiple axial mode groups are allowed to lapse , resulting in a seemingly continuous spectrum due to the resolution of the optical spectrum analyzer (& lt ; 100 mhz ). when injection locked , a high quality comb is generated at the repetition frequency . when the cavity is stabilized using the modified pdh scheme ( red ), sidebands from phase modulation ( 550 mhz ) of the injection seed laser also appear on other combs of the optical spectrum due to modulation by the intracavity im , visible in fig2 ( a ). accordingly , rf spurs appear 20 db above the noise floor at offsets from the carrier frequency in the photodetected rf spectrum , shown in fig2 ( b ). when stabilized using polarization spectroscopy ( blue in fig1 and 2 ) , the high resolution optical spectrum shows a sideband - free optical frequency comb with greater than 35 db osnr . accordingly , the elimination of phase modulation in the detection scheme produces a sideband - free rf spectrum with snr of 72 db . stabilization via polarization spectroscopy of an injection locked , harmonically mode - locked laser is demonstrated for the generation of 10 . 24 ghz - spaced optical frequency comb . injection locking is maintained for greater than twenty ( 20 ) minutes with an osnr of greater than 35 db and sideband - free photodetected rf snr of 72 db . widely spaced optical frequency combs with easily tunable spacing from this injection locked laser system are ideal for use in the field of multi - heterodyne spectroscopy . a new and promising alingaas - inp strained quantum well material at 1 . 55 - pm has been used for fabricating fpl and mlls . this material system is going to replace the conventional ingaasp - inp material because of a larger conduction band discontinuity ( ae c = 0 . 72 ae g ) and a smaller valence band discontinuity [ 8 ]. the former enables uncooled operation over a large dc bias range and the latter enables a large range of bias voltage , which in fact makes the pulses shorter . the multiple quantum well layers consist of five ( 5 ) compressively strained wells ( 6 nm ) and six ( 6 ) slightly tensile strained barriers ( 10 nm ). these wells and barriers are followed by a 60 nm graded index separate - confinement hetrostructure ( grinsch ) and a 60 nm alingaas waveguide layer . these active regions are protected by a 20 nm of ingaasp as a wet etch stop layer . the upper cladding is 1 . 6 pm of inp followed by a 50 nm of ingaasp and 200 nm of ingaas both being heavily doped . the fabricated monolithic integrated fpl and 2 - section mll are shown in fig3 . the 2 . 5 pm wide vertical mesa waveguides are fabricated by standard lithography and plasma etching techniques followed by two step wet etching terminating at 1 . 85 pm to form a single mode waveguide . the electrical isolation between the saturable absorber ( sa ) and gain section is achieved by removing the first two heavily doped contact layers . bcb polymer has been used for planarization . because of slightly different group refractive indices of the two devices which is due to different injection currents , they have to be cleaved with appropriate lengths to match the mode spacing . the fpl has a total length of 1950 μm and the 2 - section mll was cleaved with the total length of 1912 μm with a sa of 56 μm ( 3 % of the total length ) separated by 10 μm gap from the gain section . the threshold currents of the fp and 2 - section fp laser were 53 ma and 55 ma respectively , with slope efficiencies of 0 . 12 and 0 . 11 w / a , respectively . passive mode - locking was achieved by forward biasing the gain section and reverse biasing the sa . with an optimized 3 % sa , stable mode - locking was observed for a wide range of bias currents ( from threshold to as large as three ( 3 ) times the threshold ) and reverse bias voltages of − 0 . 5v to − 3 . 8v . the optical spectrum is centered at 1555 nm with a mode spacing of 22 . 15 ghz and 10 db bandwidth of 14 . 1 nm . fig4 ( a ) and ( b ) show the optical and the rf spectra of the mll with the bias current of 147 ma and reverse bias of − 2 . 8 at 20 ° c . fig2 ( c ) and ( d ) show the optical spectra of the fpl at 23 ° c . with the bias current of 56 ma before and after injection locking respectively . these parameters were carefully tuned to match the rf tones . fig4 ( e ) shows the rf spectra of the fpl before and after injection locking with 0 . 1 mw injection power . frequency pulling of the rf beat tone of the fpl to the rf tone of the mll is observed and the rf tone of 2 mhz linewidth has been measured which matches the 2 - section fpl , as expected . furthermore the optical spectrum of the fpl after injection locking is much wider , by 11 nm ( 10 db width ). fig4 ( f ) shows the autocorrelation trace from the fp injection locked laser . the pulses are 4 . 6 ps if a guassian pulse shape is assumed . these results clearly demonstrate that the each axial modes of the fpl is coherent and locked to the injected comb from the passively mll . by modulating the current of the injection locked fpl , one imparts arcsine phase modulation on each injected comb component . by interfering this modulated signal with its unmodulated counterpart , the desired true linear intensity modulator for pulsed light is generated . an injection locked alingaas quantum well fpl as a resonant cavity is demonstrated . the fp monolithic device is locked to a monolithic passively mll with the same free spectral range . this injection locked fpl can be used as the resonant cavity with gain in a linear interferometric intensity modulator for pulsed light . 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[ 8 ] j . r . karin , r . j . helkey , d . j . derickson , r . nagarajan , d . s . allin , j . e . bowers , and r . l . thornton , “ ultrafast dynamics in field - enhanced saturable absorbers ,” appl . phys . lett ., vol . 64 , pp . 676 - 678 , 1994 . while the present invention has been disclosed both generically , and with reference to specific alternatives , those alternatives are not intended to be limiting unless reflected in the claims set forth below . the invention is limited only by the provisions of the claims , and their equivalents , as would be recognized by one of skill in the art to which this application is directed .