We report on a technique that exploits the control of the carrier -envelope offset to suppress the frequency-to-intensity noise conversion in the locking of a mode-locking laser against a high-finesse optical enhancement resonator. A proper combination of the laser carrier-envelope offset and the resonator finesse allows the improvement of the signal-to-noise ratio of the optical intensity trapped into the optical resonator. In this paper, we show the application of this technique in the laser system of the inverse Compton scattering source BriXSinO, currently under development in Milan, Italy, demonstrating the possibility of achieving an intracavity intensity noise reduction of a factor of 20

Canella, F

Cialdi, S

Galzerano, G

Giannotti, D

Suerra, E

English

We report on a technique that exploits the control of the carrier- envelope offset to suppress the frequency-to-intensity noise conversion in the lock- ing of a mode-locking laser against a high-finesse optical enhancement resonator. A proper combination of the laser carrier-envelope offset and the resonator finesse allows the improvement of the signal-to-noise ratio of the optical intensity trapped into the optical resonator. In this paper, we show the application of this technique in the laser system of the inverse Compton scattering source BriXSinO, currently under development in Milan, Italy, demonstrating the possibility of achieving an intracavity intensity noise reduction of a factor of 20

Suerra, E.

Canella, F.

Giannotti, D.

Galzerano, G.

Cialdi, S.

Scientific Open-access Literature Archive and Repository

Intracavity intensity noise suppression in the inverse Compton scattering source BriXSinO exploiting carrier-envelope offset manipulation

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DOI 10.1393/ncc/i2023-23150-6Communications: SIF Congress 2022IL NUOVO CIMENTO 46 C (2023) 150Intracavity intensity noise suppression in the inverse Comptonscattering source BriXSinO exploiting carrier-envelopeoffset manipulationE. Suerra(1)(2), F. Canella(3)(2), D. Giannotti(4), G. Galzerano(3)(2)and S. Cialdi(1)(2)(1) Dipartimento di Fisica, Università degli Studi di Milano - Milano, Italy(2) INFN, Sezione di Milano - Milano, Italy(3) IFN CNR - Milano, Italy(4) Dipartimento di Fisica, Politecnico di Milano - Milano, Italyreceived 31 January 2023Summary. — We report on a technique that exploits the control of the carrier-envelope offset to suppress the frequency-to-intensity noise conversion in the lock-ing of a mode-locking laser against a high-finesse optical enhancement resonator.A proper combination of the laser carrier-envelope offset and the resonator finesseallows the improvement of the signal-to-noise ratio of the optical intensity trappedinto the optical resonator. In this paper, we show the application of this techniquein the laser system of the inverse Compton scattering source BriXSinO, currentlyunder development in Milan, Italy, demonstrating the possibility of achieving anintracavity intensity noise reduction of a factor of 20.1. – IntroductionInverse Compton Scattering (ICS) sources are promising tools for the generation ofmonochromatic X-rays, which play an increasingly important role in many scientificfields, in particular physics, medicine, materials science, and cultural heritage, thanksto their reduced dimensions and costs [1-4]. In the area of Milan, the realization of anew ICS source is supported by Università degli Studi di Milano and Istituto Nazionaledi Fisica Nucleare (INFN) within the framework of the BriXSinO project [5]. Thissource will be a test bench for a new kind of Energy Recovery Linac (ERL) based on apush-pull scheme [6], implementing at the same time unique features such as fast dual-energy X-ray generation [7, 8]. BriXSinO will provide X-ray beams with an averageenergy tunable in the range 9–37 keV, a relative bandwidth of ΔE/E = 1–10%, andintensities of 109–11 ph/s, with a maximum repetition rate of nearly 100MHz. Oneof the main requirements for these ICS facilities is the high stability of the laser, inparticular its average intensity in the interaction point with the electron bunches. InBriXSinO, the limit for the intracavity laser intensity noise has been set to the 2% levelby the users [5]. One of the main sources of amplitude noise is given by the frequency-to-intensity conversion that happens intracavity. In this work, we apply a novel solutionCreative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0) 12 E. SUERRA et al.to reduce the residual frequency-to-intensity noise conversion, and we demonstrate itseffectiveness in the BriXSinO test laser system.2. – Theory and setupThe core of our technique resides in the dependence of the frequency-to-intensitynoise conversion on the carrier-envelope offset of the laser. A detailed description ofthe related theory can be found in [9], and it has been presented at SIF Congress 2022.Both the mode-locking laser and the enhancement cavity have a comb spectrum, andtheir modal frequencies can be written, respectively, as [10] ν(laser)m = mfrep + fceo andν(cav)n = nFSR+ fcav. Here m and n are integers, frep is the repetition rate of the laser,FSR is the Free Spectral Range of the cavity, and fceo is the carrier-envelope offset ofthe laser, while fcav is the offset of the enhancement cavity containing the contributionsof the Guoy phase, the air, and the mirrors’ dispersion. In stacking the laser pulses intothe cavity, the highest coupled power is achieved when the two combs are matched [11].This condition is satisfied and maintained via the well-known Pound-Drever-Hall (PDH)technique [12], which, in our setup, stabilizes FSR so that νlaserm0 = νcavn0 , with n0 = m0,and where νm0 corresponds to the barycenter of the laser spectrum. This means thatFSR = frep + f0/m0, where we introduced f0 = fceo − fcav as the relative offset betweenthe laser and the cavity combs. We have that detuning between the teeth of order m oflaser and cavity is Δνm = f0(m−m0)/m0, and thus it directly depends on f0. Since weare interested in intracavity power fluctuations, we write the expression of intracavity gainas Γ = 1N∑m SmΓm(Δνm). Here Γm(Δνm) = (1−R1)[1+R−2√R cos (2πΔνm/FSR)]−1is the gain of the single tooth m, Sm is the amplitude of the laser spectrum as a functionof the tooth m, and N =∑m Sm. This means that the total gain is the sum of the singletooth gains, weighted on the laser spectrum. If we introduce a frequency fluctuation δfequal for all the teeth, then detuning becomes Δνm + δf . Supposing that δf followsa Gaussian distribution with variance σ2δf , then the corresponding variance of the gainfluctuations can be found to be σ2Γ = 〈δΓ2〉 − 〈δΓ〉2 = α2σ2δf + 2β2σ4δf , where α =1N∑m SmdΓmdν |Δνm and β =12N∑m Smd2Γmdν2 |Δνm . If the PDH signal is 0, then α = 0,so that the frequency-to-intensity conversion factor of interest is β. In the expression ofβ, however, the contribution of the teeth m in the sum depends on f0 through Γm. Iff0 = 0 one can demonstrate that the contribution of the teeth far from m0 and the one ofthe teeth near m0 cancel out, so that β decreases and σΓ as a consequence, too. On theother hand, also the gain Γ decreases when f0 differs from 0, but less than σΓ, so that thetrend is advantageous in terms of the relative noise of intracavity power σΓ,rel = σΓ/Γ.Indeed, once a desired gain Γ is fixed, it is convenient to set the cavity with a highergain value Γ̃ (i.e., a higher finesse), and then increment f0 until Γ is reached: in thiscondition, the relative intracavity power noise is lower than the one in a cavity with again Γ and f0 = 0Hz. Figure 1 shows a simulation of this technique. Here we takeΓ = 1130 and Γ̃ = 2680, corresponding to finesse values of 1800 and 4300, respectively,with a frequency noise σδf = 5kHz. By incrementing f0 to 30MHz, the gain of F = 4300drops to 1130, the same of F = 1800 (panel (a)), but the relative noise drops by a factorof 6 (panel (b)).3. – Experimental setup and resultsThis research has been developed in the framework of the BriXSinO ICS source,schematized in fig. 2(a). The laser system used for the Compton interaction is basicallyformed by a mode-locking oscillator, amplified firstly by a fiber amplification module,INTRACAVITY INTENSITY NOISE SUPPRESSION IN BRIXSINO 3Fig. 1. – Simulations of the gain (a) and the relative noise (b) as functions of f0, for finessevalues of 1800 and 4300. f0 can be exploited to obtain a fixed gain (1130), but a lower relativeintensity noise.then by a four-mirror crossed enhancement cavity. At the moment, a low power versionof the final laser system is implemented, and its simplified scheme is shown in fig. 2(b).Exploiting this setup and the technique described in [13] to measure the finesse and to cal-culate the cavity gain, we acquired the Relative Intensity Noise (RIN) of the intracavitypulses through the transmitted beam for two different configurations. The first corre-sponds to a low finesse value of 1800 and zero offset, while the second to a high finessevalue of 4300 and an offset different from zero. Notice that in this last configuration, wechanged the offset to f0 = 45MHz, until the same gain of 1140 of the first configurationwas reached. For a significant comparison, we kept the PDH feedback parameters con-stant in the two configurations (they were optimized for the first configuration). However,a third measurement has been performed in the high-finesse and high-offset configurationand with the feedback parameters optimized for that case. Results are shown in fig. 3(a).Starting from the upper curve, in orange we have the configuration with F = 1800 andf0 = 0MHz, then in green the configuration with F = 4300 and f0 = 45MHz, then inblue the configuration with F = 4300, f0 = 45MHz and the feedback parameters opti-mized, and finally in gray the external laser noise, which is our background. The directcomparison between the orange and green curves shows a noise improvement of 10 dB (onaverage) between 100Hz and 10 kHz, and a difference of 20 dB in the range 10–50 kHz.The resulting integrated noises are σΓ,rel = 8% and σΓ,rel = 2.5%, respectively. Since theintracavity power is the same in the two cases, we demonstrated that our method can beFig. 2. – (a) Scheme of the BriXSinO machine [5]. The enhancement cavity will be centeredin the Compton Interaction Point (IP) to generate X-rays. (b) Simplified scheme of the laserline. A Yb mode-locking laser is fiber-amplified up to 1.1W, and stacked in a cavity. A PDHfeedback loop locks the cavity to the laser.4 E. SUERRA et al.Fig. 3. – (a) RIN of the intracavity power in three different configurations with the same gainΓ = 1140 ± 30. From top to bottom: (orange) F = 1800 and f0 = 0MHz, (green) F = 4300and f0 = 45MHz, (blue) F = 4300, f0 = 45MHz and feedback optimized, (gray) external laserbackground. (b) Temporal traces of the intracavity power in the same three configurations ofpanel (a). From left to right: orange, green, blue. The relative noise is reduced by a factor 20,while the intracavity power remains the same.effectively used to reduce the relative noise. Moreover, optimized feedback parametersfor the high-finesse and high-offset configuration (in blue) leads to an even lower noiseof 0.4%. Note that the noise reduction of −20 dB occurs within the feedback bandwidthbelow 10 kHz. To better highlight the noise reduction obtained with our technique, infig. 3(b), we show the temporal trace of the intracavity power, measured by exploitingthe transmitted beam (configurations and colors correspond to the ones of panel (a)).4. – ConclusionsIn conclusion, we have demonstrated the suppression effect of f0 on the frequency-to-intensity noise conversion of a pulsed laser-cavity locked system. In particular, we showedthat an increase in f0 causes a considerable reduction in intensity noise, which we couldexperimentally reduce by a factor of 20, while maintaining a constant average intracavitygain Γ = 1140. Thus, once the necessary gain Γ is fixed, one could set a higher nominalgain Γ̃, and then increase f0 until Γ is reached, resulting in lower intracavity powerfluctuations than operating with a nominal gain Γ and maximizing spectral coupling(f0 = 0MHz). Our technique can be effectively implemented in every optical system,where a high average intracavity power and a low intensity noise are necessary, such asin ICS sources like BriXSinO.REFERENCES[1] Sun C. et al., Phys. Rev. ST Accel. Beams, 14 (2011) 044701.[2] Günther B. et al., J. Synchrotron Rad., 27 (2020) 1395.[3] Petrillo V. et al., Appl. Sci., 13 (2023) 752.[4] Cardarelli P. et al., Phys. Med., 77 (2020) 127.[5] BriXSinO TDR, https://bit.ly/BriXSinOTDR (Accessed on 2nd May 2023).[6] Drebot I. et al., Instruments, 3 (2019) 49.[7] Suerra E. et al., Nucl. Instrum. Methods Phys. Res. A, 1019 (2021) 165852.[8] Drebot I. et al., EPL, 120 (2017) 14002.[9] Canella F. et al., Appl. Phys. B, 128 (2022) 205.[10] Telle H. et al., Appl. Phys. B, 69 (1999) 327.[11] Holzberger S. et al., Opt. Lett., 40 (2015) 2165.[12] Drever R. W. P. et al., Appl. Phys. B, 31 (1983) 97.[13] Galzerano G. et al., IEEE Trans. Instrum. Meas., 69 (2020) 9119.

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Intracavity intensity noise suppression in the inverse Compton scattering source BriXSinO exploiting carrier-envelope offset manipulation

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