Laser Phys. Lett. 1, No.1, 1-5 DOl Abstract: We have developed a linearly polarized Ytterbium doped fiber ring laser with a single longitudinal mode output at 50"", DET 1064 run. A fiber-coupled intracavity phase modulator ensured mode-hop free operation and allowed fast frequency tuning. The fiber laser was locked with high stability to an iodine-stabilized laser, showing a frequency nOIse suppressIOn of a factor,...., 105 at 1 ':IlHz Configuratron of single-mode mode-hop-free Yb-doped fiber ring laser with intIacavity modulator @ 2012 by ASTRO, Ltd. Published exclusively by WILEY·¥CH Verlag GmbH & Co. KGaA Experimental performance of a single-mode Ytterbium-doped fiber ring laser with intracavity modulator Kenji Numata, 1,2,. Jordan Camp 2 ~ Department of Astronomy, University of Maryland, College Park, Maryland, 20742, USA NASA Goddmd Space Flight Center, Code 663, 8800 Greenbelt Rd., Greenbelt, MlII)'land, 20771, USA Received: X XXX 2012, Revised: X XXX XXXX, Accepted: X XXX XXXX Published online: X XXX XXXX Key words: Fiber laser, Single frequency, Frequency noise, Intensity noise PACS: 42.55.wd,42.60.Mi 1. Introduction In the field of interferometry, 1064 run is the common wavelength choice, due to the availability of high-quality bulk optics and the traditional Nd:YAG laser source repre We report on the development of a stable Ytterbium sented by NPRO. Single-frequency fiber lasers at 1064 nm (Yb )-ioped fiber ring laser that emits a linearly polar have been commercialized in the form of distributed feed ized, single longitudinal-mode, and continuous-wave light back [3], distributed Bragg reflector [4], and ring [5] at 1064 nm. Our work is originally motivated by the configurations, and they have been replacing traditional LISA (Laser Interferometer Space Antenna) mission [I], Nd:YAG laser in some fields. Our ring laser was built in which picometer-Ievel interferometry is performed in solely with commercially available components, allowing space over 1000 seconds to detect the passage of gravita fiexibility of design. We achieved stable mode-hop free op tional waves. Single-frequency fiber lasers have attracted eration by active frequency control using an intra-cavity interest due to their interferometric applications; compared fiber-coupled phase modulator, which additionally enabled with. traditional laser based on bulk crystal, for example fast frequency tuning. We measured and suppressed the the non-planar ring oscillator (NPRO) [2], the fiber laser is laser's frequency and intensity noise, which are important robust against mechanical disturbances and has a cleaner for low-frequency interferometry. Our fiber ring laser ap output in terms of shape and polarization. For the same pears to have enough stability for sensing applications. reason, it can also be readily coupled to a fiber amplifier when optical amplification is required . • Corresponding author: e-mail: [email protected] @2012by_\STRO,LId.. Published cxclusive1~ by WILEY. VCH Verlag GmbH &. Co. KGaA K. Numata, 1. Camp: A Ytterbium-doped fiber ring laser with intracavity modulator The unidirectional operation of this fiber ring laser, E~rw DET which is important to suppress instability due to spatial hole burning, was achieved by the circulator. The main laser output was taken from the transmission of the FBG Yb-dopad fiber WDM PZT after a band-pass filter isolator, which filtered out the am S81. .'.ii""r D",T TEC 95/5 Coupler F,~q(lf.,!cy' .. plified spontaneous eroission (ASE). Polarization parallel GrJI;';oI to the fast axis was blocked in the circulator and isola tors. The temperatures of the FBG and the FBG-FP were actively controlled aod stabilized in a copper block by a thermo-electric cooler (TEe) for coarse wavelength tun ing and for a stable operation. Figure 1 Ring laser configuration. WDM: wavelength division 2.3. Control systems multiplexing coupler, FBG: fiber Bragg grating, FBG-FP: FBG Fabry-Perot, BPF: band-pass filter, ISO: isolator, DET: detector, An overlap of the lasing longitudinal mode aod FBG-FP TIA: transimpedance amplifier, TEC: thenno electric cooler, SG: resonance was actively maintained to ensure single lon signal generator. The light going into ports I, 2, and 3 of the circulator comes out from ports 2, 3, and 4, respectively. gitudinal mode oscillation. We utilized the Pouod-Drever Hall techoique [8] to lock FBG-FP to the laser ring cav ity. The reflected light from FBG-FP is passed though an intra-cavity lithium-niobate phase modulator that phase 2. Experimental setup modulates the light at 80 MHz, and is directed through the circulator to a fiber-coupled detector. The detected signal 2.1. Overview is demodulated using a mixer. Once lasing is achieved, the demodulated signal at the mixer represents the difference Figure I shows our ring laser configuration. The Yh-doped between the laser frequency and the FBG-FP resanaace. gain fiber in the ring cavity was core-pumped by a laser The signal was filtered after a proper offset was added, diode (LD) through a WDM coupler. Single longitudinal and it was fed back to a piezoelectric actuator (PZT) that mode selection was achieved by cascading an FBG and an stretches FBG-FP, forcing it to follow the laser frequency. FBG-FP filter. This configuration is similar to an earlier The control bandwidth of this frequency control loop was work done at the 1.5 I-'m range using Erbimn-doped, non about I kHz. polarization maintaining (PM) fibers [6] [7]. Using a PM The output intensity of the laser was actively stabilized Yb fiber and PM components, our ring laser produces a by monitoring a small portion of the main laser output and linearly polarized output at 1064 mn. by controlling the injection current of the pump LD with a bandwidth of ~ I 00 kHz. This control baodwidth is limited by the tuning speed of the pmnp LD. 2.2. Filters and mode selection The inset of Fig. I illustrates how a single longitudinal 2.4. Pump source and gain media mode is selected in this setup. We used FBG as a coarse filter to select the lasing wavelength out ofYb's wide gain The pmnp LD was a single-mode, PM, fiber-coupled laser bandwidth (~100 nm). FBG was written on the slow axis at 976 mn. The pmnp light was coupled into the cav of a PM980 fiber, and its peak reflectivity was about 70%. ity through a PM WDM coupler after passing through The center wavelength of its reflection was specified to be a narrow-band fllter. The filter prevented the ASE aod at 1064.5 nm at room temperature, and the 3 dB reflec 1064 om laser from reaching the pump LD, suppress tion bandwidth was 0.06 mn (16 GHz). FBG was spliced ing spurious lasing. The gain fiber was a double-cladding, to one of the four ports of the circulator so that it was used single-mode, PM, Yh-doped fiber. We used it as a single in reflection mode. clad fiber, pmnping its 6 I-'m core. The small signal absorp FBG-FP was used as the second filter to select one tion of the core was 1200 dB/m at 976 mn, and the length of the longitudinal modes within the FBG bandwidth. of the gain fiber was about 40 em. In FBG-FP, an FP cavity is formed between two FBGs written next to each other. The free spectral range (FSR) and 3 dB bandwidth of the FP cavity were 12 GHz and 3. Experimental results 60 MHz, respectively. FBG-FP's 12 GHz FSR restricted the lasing to the center of the FBG bandwidth of 16 GHz. 3.1. Output optical power and spectrum The 60 MHz FBG-FP's BW then selected one longitudi nal mode of the 4 m laser cavity, whose FSR was about Figure 2 shows the relationship between the pump power 51 MHz. and the main laser output power. The excessive insertion @2012"'·ASTRO,Ltd Publi$hed exclusively by WILEY·VCH Verl~g GmbH &: Co. KGlA Laser PhysIcs Laser Phys. Lett. 1, No.1, 1-5 (2012) I www.lphys.org --------------L-e-tt-er-s 3 3rr----r---r---.----r---~--~ • Slope efficiency - 1.6 % Threshold pump power =2 14 mW 4 2 oU-__~ __~ __~ ____~ __~ __~ -3 -2 -1 o 2 3 o 100 200 300 400 500 600 Voltage to PZT amp M Pump power [mW] Figure 4 Wavelength tuning by FBG temperature and FBG-FP Figure 2· Laser output power dependence on pump power. spacing. The spacing is controlled by applying a voltage onto the amplifier for the PZT that stretches the FBG-FP. 1.0 • 10' Fast frequency tUrjing (Phase modulator) 150 :c,e - 0.8 10' ........ -- - APmhapslietu (drieg h(lte aftx aisx)i s) 100 ~ - i! 0.6 e; -.-- 50 ~ u• 10' .... ' -"'''.--------- , o ~•c 0.4 .~Ii ~I. , '\'\~ " , , -50 ~ 0.2 10' \-\, -100 .... -150 10' 0.0 10' 10' 10' 10' 10' 10' 0 100 200 300 Frequency {Hz] Frequency [MHz] Figure 5 Response function of frequency tuning by the phase Figur~ 3 Output optical spectra measured by a 300 MHz FSR modulator. scanning FP. in a center wavelength shifting of 0.25 om (66 GHz). The loss in the FBG-FP (4.2 dB), the phase modulator (2.6 dB), wavelength jump corresponds to the FSR of the FBG-FP the circulator (4.9 dB), and the filler-isolator (1.9 dB) re (12 GHz). sulted in a high pump threshold of ~200 mW aod 00 out When the frequency control servo is active, the laser put power of 1"V6 mW under a maximum available pump frequency was continuously tuned by another PZT on the power of ......,600 mW . The output polarization extinction ra cavity over 4 GHz (minimum). The cavity PZT functions tio was better thoo 20 dB. as the slow but wide frequency actuator below the 1 kHz Figure 3 shows the detailed optical spectrum mea frequency loop boodwidth. sured by a scOOhing FP cavity with a 300 MHz FSR. The The optical length of the cavity can also be fast-tuned fiber laser oscillates in single longitudinal mode, and the by varying the voltage applied to the phase modulator. (A spectrum linewidth is below I MHz (resolution limited)_ similar frequency tuning method in fiber ring laser coo be Single-mode lasing was maintained over several days by found in [9J.) We merged the modulation signal and the the frequency control loop. fast tuning signal with a wide-bandwidth operational am_ plifier. As shown in Fig. 5, the troosfer function amplitude of such frequency tuning remains flat up to ~ I 0 MHz. 3.2. Frequency tuning Coarse wavelength (frequency) tuning was achieved by 3.3. Frequency and intensity noise changing the FBG-FP spacing and FBG temperature as shown in Fig_ 4_ FBG-FP spacing was chooged by varying Figure 6 shows the frequency noise spectrum of our fiber the voltage applied to the PZT on the FBG-FP. The tem laser compared with that of a commercial NPRO laser. Be perature was tuned between 18°C and 34 °C, resulting low I kHz, our fiber laser had a comparable frequency © 2012 by ASTRO, Ltd. Published exc[us.;vely by WH.EY·VCH '!erlag GrnbH& Co. KGaA K. Numata, 1. Camp: A Ytterbiwn-doped fiber ring laser with intracavity modulator .. ----R;,;';:;'; _ ••• 111 _.. ...... - 1-- .. Flberlaserl ~ 10' ~ 1" i Irf ~ .!. ~ 10' Figure 8 Setup for frequency stabilization of the fiber laser by phase-locking to a more stable laser. Only essential components 10-1 are shown for the fiber laser. 10~ 10-2 10" Iii 10' 10' Frequency [Hz] 10' Figure 6 Frequency noise spectrum of the fiber laser. The fre .:F. quency noise of commercial NPRQ is also plotted for compari " 10' =. son. .. • • 10' c Iii ! 111 c~,.•. 1([' 10' ~ a 10-2 U. .~ 10· 10-1 •~• 1O~ 10.2 10' Iii 10' 10' 10~ i Frequent;y [HzI 10" ~ 10· Figure 9 Frequency noise spectra of the fiber laser with and ~ 10.7 without frequency stabilization. The noise level oCthe reference '" 10· laser, to which the fiber laser was phase-locked, is also shown. 10~ 10-2 Ie!' "I 10' 10' 10' Frequency [Hz] 3.4. Frequency stabilization Figure 7 Relative intensity noise spectra of the fiber laser with and without the active intensity controL The intensity noise of The fiber laser was phase-locked to an iodine-stabilized conunercial NPRO is also plotted for comparison. The shot noise NPRO using slow- and mst-frequency actuators in order level is indicated for the fiber laser at ..... 3 mW detection power level. to demonstrate the stability and controllability of the fiber laser. Figure 8 shows the' setup for the stabilization ex periment. The optical offset frequency, set by the signal generator, was 100 MHz. The beatoote between the fiber noise to the NPRO. The noise peaks around 100 Hz are laser and the iodine-stabilized NPRO was demodulated due to acoustic noise, which could be suppressed by proper by the mixer at this frequency an.d was fed back to the enclosure. The excess noise (larger drift) below 10 mHz is fiber laser through the two control loops. Figure 9 ahows due to the change in overall cavity length, whose temper the frequency noise of the frequency-stabilized fiber laser. ature is not stabilized. Above 1 kHz, our fiber laser had a The noise was suppressed within the control bandwidth of larger frequency noise than the NPRO, which was in part ......,2 MHz, and it reached' the noise of the master NPRO due to relaxation oscillation. below ",80 kHz. The noise suppression factor was "-' 105 Figure 7 shows the relative intensity noise (RIN) spec around I mHz. tra of the fiber laser, both in free-running and stabilized conditions, and the NPRO, whose intensity is controlled by the internal servo (so-called "noise eater"). The fiber laser 4. Discussion had a smaller RlN below I Hz. However, above 10kHz, our fiber laser had a larger intensity noise due to the relax ation oscillation at ~ 130 kHz, which is typical for Yb fiber 4.1. Performance features lasers (see for example [10]). The narrow intensity tuning bandwidth of the pump LD prevented the intensity servo Th..."ugh the experiments described in the previous section, from suppressing the RIN peak. the following performance features are demonstrated: @)20l2byASTRO,Ltd. PubliS:ulCl ClI.clWli~I:· bo, WILEY-VCH Verla& GmbH &; Co. KO ..... Laser Phys. Lett.t, No.1, 1-5 -The frequency and intensity noise of the fiber laser are [2] T.J. Kane and R.L. Byer. Opt. Lett. 10, 65·67 (1985). equivalent to those of NPRO (best-known low-noise [3] NKT Photonics, http://www.nktphotonics.com. laser to date) below ~ I kHz. [4] NP Photonics, http://www.npphotonics.com. -The frequency noise at a higher frequency can be mit [5] Orbits Lightwave, http://www.orbitslightwave.com. Igated by the fast-frequency actuator and by locking [6] H. lnaba, A. Onoo, Y. Akimoto, T. Komukai, and to a frequency reference, a common technique used in M. Nakazawa, J. Quantum. Electron. 38, 1325·1330 (2002). precision measurement. A slow-frequency actuator al [7] X.P. Cheng, P. Shum, C.H. Tse, J.L. Zhou, M. Tang, lows stable frequency locking over long timescales. w.e. Tan, R.F. Wu, and 1. Zhang, Photon; Tech. Lett. 20, 976·978 (2008). -The unsupressed large relaxation oscillation peak pro [8] R.W.P. Drover, J.L. Hall, F.V. Kowalski, J. Hough, hibits the RIN from reaching the shot noise limit above G.M. Ford, AJ. Munley and H. Ward, Appl. Phys. B 31, '" 1 MHz, a commonly used frequency for modulation 97-105 (1983). in heterodyne interferometric measurement. A better [9] P. Ou, Y. Jia, B. eao, C. Zhang, S. Hu, andD. Feng, Chinese active intensity control would be required to perform Opt Lett. 6, 845-847 (2008). shot-noise limited operation in precision measurement. [l0] S. Huang, Y. Feng, J. Dong, A. Shirakawa, M. Mnsha. and K. Deda, Laser Phys. Lett. 2, 498-501 (2005). [II] K. Numata, J. Camp, M.A. Krainak. and L. Stolpner, Opt. 4.2. Space applications Exp.18, 22781-22788 (2010). For possible space use, we have performed gamma radi ation tests of the optical components used in fiber laser. The result showed that fiber laser would operate in space environment without significant performance degradation. Together with the high stability and tuning capability, it makes this design a potential candidate for precision space metrology applications. Potential disadvantages of this de sign for space use are: 1) Many components are required. 2) The optical efficiency is low due to lossy components. 3) Noise performance at high frequency is not good as NPRO. However, a practical advantage of this design is that it can easily be rebuilt as higher performance compo nents become available, especially a lower loss FBG-FP. phase modulator, and circulator. We will keep investigat ing space applications of optical-fiber technology, includ ing lasers with active gain fiber and a new type of semi conductor laser with waveguide [II]. 5. Summary We developed a stable single-mode fiber ring laser at 1064 run. Our fiber laser offers comparable frequency and intensity noise to an NPRO, and can be frequency stabilized to the standard iodine absolute reference. It also offers fast frequency tuning capability, high polarization extinction ratio, and open architecture in which aU optical components are commercially available. We think that this work is an important initial step for space fiber laser, and that it will also be useful for general laboratory applica tions in which stability is important. References [1] P. Bender, K. Danzmann, and the LISA Study Team, "Laser interferometer space antenna for the detection of gravita tional waves, pre-Phase A report," Tech. Rep. MPQ233, Max-Planck-Institut flir Quantenoptik, Giirching (1998). 2nd ed. @2012byASTRO,Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGlIA