We describe a Yb-fiber based laser comb, with a focus on the relationship
between net-cavity dispersion and the frequency noise on the comb. While tuning
the net cavity dispersion from anomalous to normal, we measure the amplitude
noise (RIN), offset frequency (f_CEO) linewidth, and the resulting frequency
noise spectrum on f_CEO. We find that the laser operating at zero net-cavity
dispersion has many advantages, including an approximately 100x reduction in
free-running f_CEO linewidth and frequency noise power spectral density between
laser operation at normal and zero dispersion. In this latter regime, we
demonstrate a phase-locked f_CEO beat with low residual noise

Adler

Dong

Fermann

Gordon

Grüner-Nielsen

Holman

Jones

Knox

Lora Nugent-Glandorf

Namiki

Newbury

Prochnow

Schibli

Scott A. Diddams

Sugg

Telle

Todd A. Johnson

Washburn

Wise

Yohei Kobayashi

Yost

Zhou

English

arXiv

We describe a Yb-fiber-based laser comb, with a focus on the relationship between the net-cavity dispersion and frequency noise on the comb. While tuning the net-cavity dispersion from anomalous to normal, we measure the relative intensity noise, offset frequency (fCEO) linewidth, and the resulting frequency noise spectrum on the fCEO. We find that the laser operating at zero net-cavity dispersion has many advantages, including an approximately 100× reduction in free-running fCEO linewidth and frequency noise power spectral density when compared to the normal-dispersion regime. At the zero-dispersion point, we demonstrate a phase-locked fCEO beat with low residual noise

Nugent-Glandorf, Lora

Johnson, Todd A.

Kobayashi, Yohei

Diddams, Scott A.

College of Saint Benedict and Saint John’s University: DigitalCommons@CSB/SJU

College of Saint Benedict and Saint John's University DigitalCommons@CSB/SJU Physics Faculty Publications Physics 5-1-2011 Impact of dispersion on amplitude and frequency noise in a Yb-fiber laser comb Lora Nugent-Glandorf Todd A. Johnson College of Saint Benedict/Saint John's University, toddjohnson@csbsju.edu Yohei Kobayashi Scott A. Diddams Follow this and additional works at: https://digitalcommons.csbsju.edu/physics_pubs  Part of the Optics Commons Recommended Citation Nugent-Gladorf L, Johnson TA, Kobayashi Y, Diddams SA. 2011. Impact of dispersion on amplitude and frequency noise in a Yb-fiber laser comb. Optics Letters 36(9): 1579-1580. This Article is brought to you for free and open access by DigitalCommons@CSB/SJU. It has been accepted for inclusion in Physics Faculty Publications by an authorized administrator of DigitalCommons@CSB/SJU. For more information, please contact digitalcommons@csbsju.edu. Impact of dispersion on amplitude and frequencynoise in a Yb-fiber laser combLora Nugent-Glandorf,1,* Todd A. Johnson,1 Yohei Kobayashi,2 and Scott A. Diddams1,31National Institute of Standards and Technology, Time and Frequency Division, Mail Stop 847,325 Broadway, Boulder, Colorado 80305, USA2The University of Tokyo Institute for Solid State Physics, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan3e-mail: scott.diddams@nist.gov*Corresponding author: LNG@boulder.nist.govReceived February 14, 2011; revised March 22, 2011; accepted March 24, 2011;posted March 30, 2011 (Doc. ID 142510); published April 22, 2011We describe a Yb-fiber-based laser comb, with a focus on the relationship between the net-cavity dispersion andfrequency noise on the comb. While tuning the net-cavity dispersion from anomalous to normal, we measure therelative intensity noise, offset frequency (f CEO) linewidth, and the resulting frequency noise spectrum on the f CEO.We find that the laser operating at zero net-cavity dispersion has many advantages, including an approximately100× reduction in free-running f CEO linewidth and frequency noise power spectral density when compared tothe normal-dispersion regime. At the zero-dispersion point, we demonstrate a phase-locked f CEO beat with lowresidual noise. © 2011 Optical Society of AmericaOCIS codes: 140.3510, 140.7090, 260.2030.Yb-doped fiber femtosecond laser combs have been thesubject of recent attention due to their high efficiency,high repetition rate capabilities, low noise, and ease ofdirect diode pumping and amplification [1–4]. Yb-fibercombs have proven particularly useful in spectroscopicapplications, where amplification and nonlinear fre-quency generation can push the initial 1:03 μm wave-length into the mid-IR [5,6] or ultraviolet [7]. There areseveral fiber laser designs [8] that can operate in variousgroup delay dispersion (GDD) regimes, from soliton-like(net GDD is negative) to similariton pulse propagation(net GDD is normal). In all of these designs, net-cavitydispersion plays a significant role in the laser dynamicsand must be carefully managed.In order to explore the relationship between disper-sion and amplitude and frequency noise on the frequencycomb, we employ a laser design that can be tuned readilyacross a range of net-cavity dispersions. Because a fre-quency comb with low amplitude and frequency noiseis necessary for precision spectroscopy and other appli-cations, it is important to understand the properties ofthese Yb-fiber laser combs in all dispersion regimes.Noise on the comb teeth can originate from many differ-ent sources inside the cavity, including amplitude noisefrom the pump diode, amplified stimulated emission(ASE) jitter, cavity loss, and cavity length changes [9].With a pair of volume holographic gratings, we are ableto adjust the dispersion inside the cavity in order to studythe dependence of the frequency comb noise on the net-cavity dispersion and to identify the optimum dispersionfor obtaining a low-noise comb with narrow linewidthcarrier-envelope offset frequency (f CEO). In the time do-main picture of frequency combs, f CEO is the product ofthe repetition rate and the fractional carrier-envelopephase change (Δϕce=2π) per cavity round trip. Noiseon f CEO can originate from noise on the repetition rate(timing jitter) or the carrier-envelope phase (phase jitter)[9–12], both of which are affected by the various noisesources in the system. The linewidth of f CEO, whilenot the complete picture of noise in the laser system,is a sensitive indicator of frequency noise when tuningthe dispersion in the laser.We have designed and built the Yb-fiber laser shown inFig. 1 [1], where 70 cm of Yb gain fiber is pumped by a976 nm laser diode (350 to 450mW). The free-space sec-tion contains a polarizing beam splitter for output cou-pling, an isolator that promotes lasing in the correctdirection, and a double pass through two volume holo-graphic transmission gratings. The second grating canbe translated a total of 1 cm, corresponding to a disper-sion change of 16; 000 fs2 at a center wavelength of1035 nm. Mode locking is initiated by careful movementsof the three free-space wave plates. Depending on themode-locking conditions, the laser output power rangesfrom 50 to 100mW at a 100MHz repetition rate. As is seenin the inset of Fig. 1, the optical spectrum varies con-siderably when changing the grating separation to tunethe net-cavity dispersion of the laser from normal toanomalous.To understand the effects of cavity dispersion on laseroperation, it is first necessary to understand specificallywhat dispersion regimes are accessible. Determinationof the actual cavity dispersion is not straightforward,Fig. 1. (Color online) Schematic of Yb-fiber oscillator: WDM,wavelength division multiplexer; PBS, polarizing beam splitter;M1–M3, silver mirrors (M2 is a half mirror). Inset, optical spec-tra at four distinct dispersion regimes in the cavity (þn ¼ morenormal, n ¼ normal, z ¼ zero, a ¼ anomalous).1578 OPTICS LETTERS / Vol. 36, No. 9 / May 1, 20110146-9592/11/091578-03$15.00/0 © 2011 Optical Society of Americabecause fiber dispersion curves (both Yb-doped and sin-gle mode) can vary significantly from manufacturing spe-cifications and from fiber to fiber. Therefore, we firstmeasured the cavity dispersion in situ using the tech-nique of Knox [13]. By putting a slit that is narrower thanthe beam profile in front of the retroreflection mirror (M1in Fig. 1), the optical spectrum, and thus the centralwavelength of the mode-locked laser, can be scanned.The width of the mode-locked spectrum remains no lessthan 75% of the unfiltered spectrum. As the slit isscanned, the free-running repetition rate (f rep) is moni-tored with a photodiode and recorded with a frequencycounter. Differentiating the group delay, Tgð¼ 1=f repÞversus the center frequency of the optical spectra (ω0)yields the net-cavity GDD. The experimental results forfour grating separations are shown in Fig. 2 (circles).The solid lines in Fig. 2 are the calculated dispersionbased on the sum of the elements inside the cavity, in-cluding reasonable estimates of fiber dispersion (Yb fiberestimate from [14] and SMF from FLEX 1060 fiber data),dispersion from high-refractive-index materials (glass op-tics and terbium gallium garnet [15], a material found inoptical isolators), and grating dispersion [16]. The resultsshow that the four mode-locked spectra in the inset ofFig. 1 correspond to one anomalous, one near-zero,and two normal-dispersion regimes. While third-orderdispersion is evident in the slope of the lines in Fig. 2,we define the net-cavity GDD (from normal to anoma-lous) using the second-order dispersion value at the nom-inal center wavelength (1035 nm for all spectra in Fig. 1).With a clear understanding of the cavity dispersion, wemeasured the free-running offset frequency, f CEO, of thelaser operating in several dispersion modes. To this end,we first amplified the chirped oscillator output in a core-pumped Yb-doped fiber amplifier then recompressed thepulses with an external double-pass grating pair (averagepower ∼400mW, 80 fs pulse duration). Continuum gen-eration was achieved with a 15 cm piece of nonlinearsuspended-core fiber [17]. An f CEO beat was then mea-sured with a standard f -2f interferometer [18]. The mi-crowave spectra of two sample f CEO beat signals areshown in Fig. 3(a) for the normal- and zero-dispersionregimes, illustrating the significant difference in line-width. Similar effects have been observed in other Erand Yb laser systems [19]; however, to our knowledge,this dependence has not been mapped over a range ofcavity dispersions.A complete set of measurements of the free-runningf CEO linewidth versus dispersion is given in Fig. 3(b).A clear trend of decreasing f CEO linewidth toward zerodispersion (from both the normal and anomalous sides)is seen. For each dispersion regime (i.e., a fixed gratingseparation), the f CEO linewidth can also change withpump power (which directly affects the internal cavitycirculating power and the spectral width of the pulse).At each grating separation, the pump power was tunedwithin the full range of stable mode locking (i.e., nocw breakthrough). Many factors, including the pumpingrate, cavity loss, and the laser’s nonlinear response canimpact the f CEO linewidth, including the “turning points”[20] of the f CEO beat and the polarization settings.Nevertheless, in the normal-dispersion mode, we neverobtained an f CEO linewidth as narrow as in the zero-dispersion regime.As the microwave power spectrum [Fig. 3(a)] can am-biguously mix amplitude and frequency noise, we alsodirectly measured the frequency noise spectrum of f CEO.This was accomplished by sending the f CEO beat signalinto a frequency divider followed by a delay line discri-minator and a vector signal analyzer. These results aregiven in Fig. 4(a) for laser operation at normal and zerodispersion, corresponding to measured free-running f CEOwidths of 1:45MHz and 130 kHz, respectively. The broadf CEO beat clearly has considerably higher FM noise.Calculation of rf spectra from the frequency noise spec-tra in Fig. 4(a) (using numerical methods) yields a FWHMof 1:50MHz (normal) and 104 kHz (zero), a correspond-ingly 3% and 20% deviation from the experimental values.This might suggest a small contribution from AM noise inthe zero-dispersion regime, while the much higher FMnoise in the normal regime masks any AM contribution.Nonetheless, it is clear that the frequency noise domi-nates the f CEO linewidths. It is also possible to tightlylock the f CEO beat in the zero-dispersion regime, asshown in the inset of Fig. 3(a), where the coherent sig-nature on a undivided copy of f CEO (at 226MHz) is seen.Fig. 2. (Color online) Experimental measurement of the lasercavity GDD (circles) and calculation of dispersion as a sum ofall the cavity elements (solid lines).Fig. 3. (Color online) (a) Spectrum of free-running f CEO beatat normal- and zero-dispersion points with Lorentzian fits. Inset,locked f CEO beat. (b) Collection of f CEO linewidths versus net-cavity dispersion (measured by grating separation): dottedcurve, minimum linewidth obtained and dashed curve, averagevalue.May 1, 2011 / Vol. 36, No. 9 / OPTICS LETTERS 1579In this spectrum, 90% of the power is located within thecoherent carrier.In most frequency combs, there is a strong couplingbetween amplitude and frequency noise. The amplitudenoise of the laser oscillator was measured [Fig. 4(b)] atthe same four net-cavity-dispersion regimes shown inFigs. 1 and 2. Amplitude noise was measured by monitor-ing a portion of the laser output with a fast photodiode(1GHz bandwidth) and using a vector signal analyzer toobtain a power spectral density of the relative intensitynoise (RIN ¼ ½ΔPrms=Po2 per Hz bandwidth, where P isthe optical power). We observed significant differencesin the RIN as a function of net-cavity dispersion, withzero net-cavity dispersion having the lowest RIN. It isnot possible to state that “all other parameters are equal”except the dispersion when comparing the different laseroperating modes. Each dispersion point requires adjust-ment of the pump diode power and waveplate settings inorder to achieve stable mode locking, leading to differ-ences in output coupling and cavity Q. However, in prac-tice, the zero-dispersion mode consistently shows thelowest RIN. We also present the pump laser RIN inFig. 4(b). The white-noise level of the pump RIN is nearlyconstant with the changing pump current (data notshown), and it is on average 10 dB higher than theoscillator RIN.Experimentally, the Yb-fiber oscillator presented hereoperates in its quietest mode near zero dispersion, in-cluding reduced amplitude noise on the oscillator, nar-rowest f CEO linewidth, and less frequency noise onf CEO itself. This fact alone should be important for thefurther development of such lasers for frequency combapplications. Additionally, it is interesting to note thatour measurements of f CEO noise are consistent withthe prediction of minimum timing jitter for stretched-pulse fiber lasers near zero net-cavity dispersion [9,21].This could imply a direct correlation between noise onf CEO and f rep, where ASE-induced center wavelengthchanges are coupled to timing jitter through the groupvelocity dispersion [9,10,21]. However, the connectionbetween various noise sources and the frequency noiseon the comb itself can be complicated [9,12,21,22], anddirect measurements of the timing jitter would help pro-vide a more complete picture of the noise properties ofYb-fiber laser combs [23].The authors thank IMRA America, Inc., for the use ofthe suspended-core nonlinear fiber [17]. This researchwas funded in part by the United States Department ofHomeland Security’s Science and Technology Directo-rate through the National Institute of Standards and Tech-nology (NIST). We further thank Ingmar Hartl, NateNewbury, Frank Quinlan, and Alex Zolot for helpful com-ments. Any mention of commercial products does notconstitute an endorsement by NIST.References1. X. Zhou, F. Yoshitomi, Y. Kobayashi, and K. Torizuka, Opt.Express 16, 7055 (2008).2. T. Schibli, I. Hartl, D. Yost, M. Martin, A. Marcinkevičius,M. Fermann, and J. Ye, Nat. Photon. 2, 355 (2008).3. M. Fermann and I. Hartl, IEEE J. Sel. Top. Quantum Elec-tron. 15, 191 (2009).4. I. Hartl, L. Fu, B. Thomas, L. Dong, M. Fermann, J. Kim,F. Kartner, and C. Menyuk, in Conference on Lasers andElectro-Optics/Quantum Electronics and Laser ScienceConference and Photonic Applications Systems Technolo-gies, OSA Technical Digest (CD) (Optical Society of Amer-ica, 2008), paper CTuC4.5. F. Adler, K. Cossel, M. Thorpe, I. Hartl, M. Fermann, andJ. Ye, Opt. Lett. 34, 1330 (2009).6. T. Johnson and S. Diddams, in Conference on Lasers andElectro-Optics, OSA Technical Digest (CD) (Optical Societyof America, 2010), paper CPDB11.7. D. Yost, T. Schibli, J. Ye, J. Tate, J. Hostetter, M. Gaarde,and K. Schafer, Nat. Phys. 5, 815 (2009).8. F. Wise, A. Chong, and W. Renninger, Laser Photon. Rev. 2,58 (2008).9. N. Newbury and B. Swann, J. Opt. Soc. Am. B 24, 1756(2007).10. J. Gordon and H. Haus, Opt. Lett. 11, 665 (1986).11. O. Prochnow, R. Paschotta, E. Benkler, U. Morgner,J. Neumann, D. Wandt, and D. Kracht, Opt. Express 17,15525 (2009).12. B. Washburn and N. Newbury, IEEE J. Quantum Electron.41, 1388 (2005).13. W. H. Knox, Opt. Lett. 17, 514 (1992).14. L. Grüner-Nielsen, S. Ramachandran, K. Jesperen,S. Ghalmi, M. Garmund, and B. Palsdottir, Proc. SPIE6873, 68730Q (2008).15. U. Sugg and B. Schlarb, Phys. Status Solidi (b) 182,K91 (1994).16. A. M. Weiner, Ultrafast Optics (Wiley, 2009).17. L. Dong, B. K. Thomas, and L. Fu, Opt. Express 16,16423 (2008).18. D. Jones, S. Diddams, J. Ranka, A. Stentz, R. Windeler,J. Hall, and S. Cundiff, Science 288, 635 (2000).19. N. Kuse and Y. Kobayashi, The University of Tokyo Insti-tute for Solid State Physics, Kashiwanoha 5-1-5, Kashiwa,Chiba 277-8581, Japan; I. Hartl, IMRA America, 1044 Woo-dridge Avenue, Ann Arbor, Michigan 48105, USA (personalcommunication, 2010).20. K. Holman, R. Jones, A. Marian, S. Cundiff, and J. Ye, Opt.Lett. 28, 851 (2003).21. S. Namiki and H. Haus, IEEE J. Quantum Electron. 33,649 (1997).22. H. Telle, B. Liphardt, and J. Stenger, Appl. Phys. B 74,1 (2002).23. Y. Song, K. Jung, and J. Kim, arXiv:1102.1046v1 (2011).Fig. 4. (Color online) (a) Frequency noise on two sample f CEObeat signals (division factor included to give true f CEO noise).(b) Amplitude noise (RIN) for four dispersion regimes.1580 OPTICS LETTERS / Vol. 36, No. 9 / May 1, 2011

Impact of dispersion on amplitude and frequency noise in a Yb-fiber laser comb

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The Impact of Dispersion on Amplitude and Frequency Noise in a Yb-fiber
  Laser Comb

The Impact of Dispersion on Amplitude and Frequency Noise in a Yb-fiber Laser Comb

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