Patent Application: US-201113117271-A

Abstract:
an optoelectronic oscillator is used to provide a continuous , high q , modulated signal for a variety of purposes , including a carrier wave for communications , and radar emissions . the oeo of this invention replaces an rf filter in the conventional oeo with an interferometer , preferably a high finesse fabry - perot etalon as the mode selector , providing lower phase noise and higher rf frequency stability .

Description:
the conventional oeo design shown in fig1 ( a ), has an electro - optic modulator ( eom ), optical delay , photodiode , rf filter and rf amplifier . the microwave signal is imposed on the optical beam and the optical delay acts as a microwave energy storage device . long delay lines are required in order to achieve a high microwave q , which results in many closely separated microwave modes . a narrow - band rf filter is necessary to filter out the closely separated rf modes in order to obtain stable oscillation . in fig1 ( b ), av is the supported rf tone separation and w is the oscillation frequency which is determined by the rf filter bandpass frequency . our inventive oeo design , using a fabry - perot etalon , is shown in fig1 ( c ). the oeo has an eom , an etalon as the resonant mode selector , optical delay line , photodiode and rf amplifier . the supported optical modes are shown in fig1 ( d ). the periodic transmission function of the etalon allows only the optical frequencies which are separated by the free spectral range ( fsr ) of the etalon to oscillate and eliminates the frequencies outside the resonance width . in this technique , the microwave oscillation frequency is determined by the fsr of the etalon . in fig1 ( d ), av represents the optical mode separation and co is the fsr of the etalon . since the rf domain signal results from the beating of the optical modes which are separated by w , only one microwave oscillation mode is allowed in the rf domain and the rf filter is no longer required , as shown in fig1 ( e ). the schematic of the oeo with the fabry - perot etalon is shown in fig2 . a 1550 nm cw laser is used as the light source . the optical frequency of the cw laser is tunable by applying an external voltage . the output of the cw laser is amplified and sent to the electro - optic modulator ( eom ) which is biased at quadrature . a 1000 finesse fabry - perot etalon with 10 . 287 ghz free spectral range ( fsr ) is used as the mode selector , as described . the etalon &# 39 ; s stability is advantageous as it becomes the primary frequency reference in the oeo . the etalon is made of ultralow expansion quartz and is sealed , so it is less susceptible to environmental changes , such as temperature or air pressure . the full width half maximum ( fwhm ) of the resonance of the etalon is 10 mhz . the fiber to fiber insertion loss of the etalon is only 1 db . a total optical delay of ten ( 10 ) meters is employed in order to get sufficient electrical supermode suppression . the photodiode has 16 ghz bandwidth with 0 . 8 a / w responsitivity , and is followed by a 40 db gain rf amplifier . a 10 db output coupler is used to extract the rf power and the remaining power fed back to the eom . the rf spectrum of the oeo is shown in fig3 . the rf power of the tone centered at 10 . 287 ghz is 12 dbm . no other rf modes or mode hopping is observed during the experiment . the phase noise and rf frequency stability performance of the oeo is also measured and compared with the standard oeo . in the standard oeo , the fabry - perot etalon is replaced by an rf filter with other components and parameters kept constant . the rf filter used in the experiment has a 3 db bandwidth of 13 mhz , centered at 10 . 287 ghz and has an insertion loss of 5 db . the total optical delay is kept constant by adding an appropriate amount of fiber to the cavity . the comparison of the phase noise of the oeo with an rf filter and the oeo with an etalon is shown in fig4 . the oeo with the rf filter has 5 - 10 db less signal - to - noise ratio ( snr ) than the oeo with fabry - perot etalon at close offset frequencies . the snr at 10 khz offset from the carrier for the oeo with fabry - perot etalon is 100 dbc / hz . the rf stability of the oscillation frequency is also measured using a real time rf spectrum analyzer in the spectrogram mode and compared for the two ( 2 ) oeo designs ( fig5 ). the rf frequency stability over one ( 1 ) minute for the oeo with the fabry - perot etalon is 2 . 6 khz whereas the stability for the standard oeo is 5 . 75 khz . fig5 clearly shows the improved stability due to the ultra stable fabry - perot etalon used as the filter . this disclosure demonstrates an optoelectric oscillator ( oeo ) which uses an ultra stable 1000 finesse fabry - perot etalon as the oscillator mode selector . the selection of the modes is performed in the optical domain by the use of the periodic etalon transfer function . the oeo oscillation frequency is defined by the free spectral range of the etalon . the performance of the new oeo is compared with the standard oeo design with an rf filter . when all the other parameters are kept constant , except the mode selector , the oeo with the fabry - perot etalon results in a better phase noise performance and higher rf frequency stability . the resonance bandwidth of the etalon can be narrowed easily by using a higher finesse etalon which allows inserting very long optical fiber delays to the oeo . when higher finesse etalons are used , a cw laser frequency locking system such as pound - drever - hall ( pdh ) method may be required to lock the laser frequency to the etalon resonance peak . a . neyer and e . voges , “ high - 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78 , january 2000 . x . s . yao and l . maleki , “ multiloop optoelectronic oscillator ,” ieee i . quantum electron ., vol . 36 , no . 1 , pp . 79 - 84 , january 2000 . e . shumakher and g . eisenstein , “ a novel multiloop optoelectronic oscillator ,” ieee photon . technol . lett ., vol . 20 , no . 22 , pp . 1881 - 1883 , november 2008 . j . yang , y . jin - long , w . yao - tian , z . li - tai , and y . en - ze , “ an optical domain combined dual - loop optoelectronic oscillator ,” eee photon . technol . lett ., vol . 19 , no . 11 , pp . 807809 , june 2007 . d . strekalov , d . aveline , y . nan , r . thompson , a . b . matsko , and l . maleki , “ stabilizing an optoelectronic microwave oscillator with photonic filters ,” j . lightw . technol ., vol . 21 , no . 12 , pp . 3052 - 3061 , december 2003 . 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