Coherent quasi-cw 153nm light sourceat 33MHz repetition rate

Yutaka Nomura,1,2,3,* Yoshiaki Ito,1 Akira Ozawa,1,2 Xiaoyang Wang,4 Chuangtian Chen,4

Shik Shin,1,2 Shuntaro Watanabe,2,5 and Yohei Kobayashi1,21Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8581, Japan

2CREST, Japan Science and Technology Agency (JST), Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan3Present address: Institute for Molecular Science, National Institutes of Natural Sciences, 38 Nishigo-naka, Myodaiji, Okazaki 444-8585, Japan.

4Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, P.O. Box 2711, Beijing 100190, China5Research Institute for Science and Technology, Tokyo University of Science, Yamasaki 2641, Noda 278-8510, Japan

*Corresponding author: nomura@ims.ac.jp

Received February 9, 2011; revised April 5, 2011; accepted April 6, 2011;posted April 6, 2011 (Doc. ID 142336); published May 5, 2011

We present a quasi-cw laser in a vacuum ultraviolet region at megahertz repetition rate. The narrowband pulsesgenerated from an ytterbium-fiber laser system at 33MHz repetition rate at the central wavelength of 1074nm arefrequency-converted by successive stages of LiB3O5 crystals and KBe2BO3F2 crystals. The generated radiation at153nm has the shortest wavelength achieved through phase-matched frequency conversion processes in nonlinearoptical crystals to our knowledge. © 2011 Optical Society of AmericaOCIS codes: 190.2620, 140.3510, 160.4330.

Narrowband lasers in vacuum and extreme ultraviolet(VUV/XUV) regions have a diverse range of applicationsin various scientific fields, such as photoelectronspectroscopy and high precision spectroscopy. TheKBe2BO3F2 (KBBF) crystals [1,2] have been successfulin generating such radiation owing to their wide transpar-ency and large birefringence necessary for phase-matched frequency conversion processes in the VUV re-gion. In fact, 41mW of VUV radiation at 177 nm (7 eV) isgenerated through a second harmonic generation (SHG)process in a KBBF crystal at 80MHz repetition rate [3].Extending the photon energy to even higher energy hasgreat significance in many scientific fields, such as angle-resolved photoelectron spectroscopy (ARPES) [4,5],where the higher photon energy would enable the obser-vation of a broader region of the momentum space ofcuprate superconductors [4,6] to reveal the mechanismsof high-temperature superconductivity [7,8].Several VUV laser sources have been demonstrated at

the 8 eV region, such as the fifth harmonic of Ti:sapphirelaser using KBBF crystal [9], four-wave mixing in argongas [10], or high-harmonic generation in an enhancementcavity [11]. However, they do not satisfy the narrow line-width and high repetition rate, which are necessary toachieve high resolution and high signal-to-noise ratio.Although sum-frequency generation (SFG) in a KBBFcrystal is a very promising method to generate such aVUV radiation, it is a challenge to generate the radiationaround 150 nm. The seventh harmonic generation of acommon wavelength of 1064 nm would be hindered bystrong absorption by the KBBF crystal itself. This pro-blem can be alleviated by shifting to a longer wavelength.Since the wavelength is close to the absorption edge,even 1nm of shift in VUV wavelength can increase thetransmission significantly. However, achieving thismeans shifting the fundamental wavelength by 7 nm,which is not trivial with conventional neodymium-basedlasers.In this Letter, we report generation of quasi-cw VUV

radiation at 153:4nm at 33MHz repetition rate. This

is achieved by successive frequency conversion of theoutput of an ytterbium (Yb)-doped fiber laser centeredat 1074 nm using LiB3O5 (LBO) crystals and KBBF crys-tals. The obtained VUV radiation has the shortest wave-length generated through phase-matched processes innonlinear optical crystals to our knowledge.

The experimental setup is shown in Fig. 1. In our os-cillator, highly doped Yb fiber with the length of 20 cm ispumped by a fiber-coupled laser diode delivering 680mWat 976 nm. The oscillator operates in an all-normal disper-sion regime [12] within a unidirectional ring cavity anddelivers 150mW of output at the repetition rate of33MHz. Owing to strong self-phase modulation, the spec-trum exhibits two peaks located at 1056 nm and 1074 nm.The peak at the longer wavelength side is extracted bysending the beam through a 4-f system with a mechanical

Fig. 1. (Color online) Experimental setup. LD, laser diode;YDF, Yb-doped fiber; GR, grating; KBBF-PCD, KBBF prism-coupled device. (a) The spectrum measured after the amplifierstages. (b) An autocorrelation trace of the output beam after theamplifier stages. The red solid curve shows the measureddata. The blue dashed curve shows a fit assuming a Gaussiantemporal profile.

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slit in the Fourier plane. The spectral width of the ex-tracted beam is ∼0:1 nm [Fig. 1(a)].The output from the oscillator was amplified with

three Yb-fiber-based amplifier stages. The first amplifierstage consists of 20 cm-long single-mode Yb fiberpumped by a fiber-coupled laser diode delivering up to350mW at 976 nm. This stage is two-pass, i.e., the outputbeam is sent back to the same fiber again after reflectingoff a 600mm−1 grating to remove any amplified sponta-neous emission (ASE). The amplified beam after the sec-ond pass is coupled out by a polarizing beam splitter andwave plates. The power of the output beam after the firststage is ∼50mW. Reflection from another grating re-moves ASE contained in the output beam. The secondstage is basically the same as the first stage except thatit is single-pass. The beam is amplified to 100mW, whichis again reflected off a grating to suppress any ASE. Thefinal stage consists of a 1:2m-long, Yb-doped, single-mode double-clad photonic crystal fiber (NKT Photonics,DC-200/40-PZ-Yb-01) pumped with a high-power laserdiode (Jenoptik, JOLD-100-CPXF-2P) that can deliverup to 100W at 976 nm. Typical output after the finalamplifier stage is 7W with the pulse duration of 20 ps.The spectrum and an autocorrelation trace of the outputpulse are shown in the inset of Fig. 1.The amplified infrared beam is frequency-converted by

a 20mm-long type I LBO crystal, which typically pro-duces 3:6W of the second harmonic (SH) signal at537 nm. The SH process utilizes the noncritical phase-matching scheme with the temperature set to 138 °C.The SH beam and the remaining fundamental beamare focused into a 15mm-long type II LBO crystal to gen-erate the third harmonic (TH) beam at 358 nm throughSFG between the fundamental and SH beams. Typicaloutput power of the TH beam is 1W.The TH beam is focused onto a KBBF–prism-coupled

device (KBBF-PCD) [2], which is a KBBF crystal sand-wiched with a SiO2 prism and a CaF2 prism. The KBBFcrystal is 1:8mm thick and the apex angle of each prismis 60°. When the fundamental wavelength is 1074 nm, thetype I phase-matching angle of the SHG process 3ωðoÞ þ3ωðoÞ → 6ωðeÞ in the KBBF crystal is obtained to be 64:9°according to the Sellmeier equation [13]. The device isplaced in a chamber filled with nitrogen gas at 1 atmto avoid absorption of VUV radiation by oxygen gas.The concentration of oxygen in the chamber during

the experiments is measured to be ≲2 ppm. When 1Wof the TH beam is focused onto the KBBF crystal witha lens with the focal length f ¼ 200mm, ∼1mW of thesixth harmonic (6H) beam of the original IR beam at179 nm is generated through an SHG process.

The 6H beam is recollimated after the KBBF-PCD andcombined with the remaining fundamental IR beam to befocused onto another KBBF-PCD, which consists of a1:5mm-thick KBBF crystal with a SiO2 prism and aCaF2 prism with the apex angle of 52°. When the funda-mental wavelength is 1074 nm, the type I phase-matchingangle of the SFG process ωðoÞ þ 6ωðoÞ → 7ωðeÞ in theKBBF crystal is obtained to be 50:4°.

To detect the seventh harmonic (7H) signal with highsignal-to-noise ratio, a lock-in amplifier (Stanford Re-search Systems, SR850) is used. A chopper wheel is in-serted in the fundamental beam to modulate it. As adetector, a solar-blind phototube (Hamamatsu, R1187)is used to suppress the background from the fundamentalbeam. Thus, the lock-in amplifier sees only modulatedsignals with wavelength below 200 nm. To resolve the sig-nal spectrally, a simple spectrometer is built with a2400mm−1 grating and twomechanical slits. The detectorand the exit slit are scanned with a linear stage. A clearpeak is observed with the lock-in amplifier as shown inFig. 2 as red circles, which is expected to be the 7H of thefundamental laser light of 1074 nm. The angle of theKBBF crystal and focusing of each incident beam areoptimized by maximizing the output signal. We did notobserve any significant deviation of the phase-matchingangle from the theoretically expected value. To confirmthe wavelength of the signal, scattered light from the 6Hbeam is detected by measuring the unmodulated DCcomponents of the signal by digital multimeter (Keithley,Model 2000) without using lock-in detection. The mea-sured spectrum is shown as blue triangles in Fig. 2.The 6H peak is observed at a theoretically expected posi-tion. The peaks are broadened because the resolution ofthe system is ∼3 nm.

The strength of the 7H signal is measured by changingvarious parameters. For the following measurements, thesignal is measured directly after the KBBF crystal with-out the reflection from the grating. Although we used alock-in detection system, some signal at the wavelengthof the 6H signal is still observed. This signal seems tocome from the modulation of the 6H signal by the funda-mental beam that occurred within the SiO2 or CaF2prism, but its origin is not identified. To suppress thisscattered light, specially manufactured filters (SigmaKoki) were used. The filters transmit more than 50% ofthe 7H beam while blocking more than 99% of the 6Hbeam. After transmitting through two filters, the scat-tered light was suppressed below the detection limit.With this setup, the 7H signal is measured by changingthe power of the incident IR beam. The result is shownin Fig. 3(a). The 7H signal scales linearly to the IR poweras expected. The maximum signal strength is estimatedto be ∼1nW by using the provided values of the photo-tube sensitivity and filter transmission.

The conversion efficiency seems rather low comparedto the previous work that used KBBF crystals to generate6H radiation [3]. This is most probably due to the absorp-tion of the KBBF crystal itself, which strongly depends on

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the quality of the crystal. Although the power of 1 nWmay seem low, the photon number per longitudinal modeper second is as high as ∼105, which is enough for certainprecision spectroscopy experiments. For ARPES mea-surements, however, higher output power is desired.The output power could be increased by shifting to evenlonger wavelength to avoid absorption of the KBBFcrystal. Another possibility is to decrease the absorptionof the crystal by cooling the crystal itself [14].Figure 3(b) shows the KBBF angle dependence of the

7H signal. The red circles show the experimental values,while the blue curve shows theoretical curve obtained bycalculating the wave vector mismatch for the interactionlength of 200 μm, which is much shorter than the opticalpath length of 2mm within the KBBF crystal. Althoughthe calculation assumes the interaction between planewaves and, therefore, does not precisely reproduce theexperimental conditions, this result indicates that the

interaction length might be shortened by effects suchas walk-off or absorption in the crystal.

In summary, we have demonstrated the generation of153:4nm radiation at 33MHz repetition rate throughsuccessive frequency conversion with two LBO crystalsand two KBBF crystals. The narrowband quasi-cw VUVradiation at 8 eV can be used in many applications, suchas the study of high-temperature supercondutivity withARPES measurements.

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Fig. 3. (Color online) The 7H signal measured by changingparameters. (a) The 7H signal plotted against the power ofthe incident IR beam. Red circles show the experimental data,while the blue curve is the linear fit. (b) Crystal-angle depen-dence of the 7H signal. Red circles show the 7H signal plottedagainst the angle of the KBBF crystal. Blue curve is calculatedby assuming the phase-matching length of 200 μm.

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