488nm Pump in LiNbO3 with temperature tuning
Blue-pumped whispering gallery optical
parametric oscillator
Christoph Sebastian Werner,1Tobias Beckmann,1Karsten Buse,1,2and Ingo Breunig1,* 1Laboratory for Optical Systems,Department of Microsystems Engineering—IMTEK,University of Freiburg,
Georges-K?hler-Allee102,Freiburg79110,Germany
2Fraunhofer Institute of Physical Measurement Techniques,Heidenhofstra?e8,Freiburg79110,Germany
*Corresponding author:ingo.breunig@imtek.de
Received August7,2012;accepted August21,2012;
posted September4,2012(Doc.ID173997);published October5,2012
We demonstrate a whispering gallery optical parametric oscillator pumped at488nm wavelength.This millimeter-sized device has a pump threshold of160μW.The signal field is tunable between707and865nm wavelength and the idler field between1120and1575nm through temperature variation.Although the conversion efficiency is fundamentally limited to several percent because of absorption loss for the pump wave,the results provide evi-dence that such oscillators will be able to cover finally the entire visible range.?2012Optical Society of America OCIS codes:190.4970,190.4360,230.5750.
Optical parametric oscillators(OPOs)can be employed to generate coherent light at almost arbitrary frequency
[1].The only fundamental limitation is that the optical
pump field has a shorter wavelengthλp compared to
the wavelengthsλs andλi of the two generated fields,
called signal and idler waves.Here,the relation1∕λp 1∕λs 1∕λi holds.Conventional OPOs comprise a non-linear-optical crystal inside of a mirror cavity.The mir-
rors have to be specially coated for the spectral range
in which the device is aimed to operate.Recently,a
new class of OPOs has been demonstrated—whispering gallery OPOs[2,3].They do not require any external mirrors.In these exceptional devices,light is guided by total internal reflection inside the nonlinear-optical crystal.Thus,whispering gallery OPOs provide a low-loss cavity over the whole transparency range of the crystal. This has enabled pump thresholds below10μW[2].The green-pumped devices demonstrated so far have a tuning range between1010and1120nm[2,4]while the near-infrared-pumped devices are tunable between1800and 2500nm[3].Pump lasers operating in the blue can extend the tunability into the visible spectral range.This is of interest,e.g.,for coupling of nonclassical light from a whispering gallery OPO[5]to rubidium atoms or diamond vacancies.Such experiments require signal wavelengths around795or639nm,respectively.
In this Letter,we present a whispering gallery OPO
pumped in the blue at488nm wavelength.The experi-
mental setup sketched in Fig.1comprises a spheroidally
shaped monolithic cavity(1.8mm major diameter,
0.7mm minor diameter,1mm thickness)made of a z-cut stoichiometric lithium niobate crystal doped with 1.2%magnesium oxide.The resonator is mounted in an oven whose temperature can be varied between30 and170°C.We use a rutile prism to couple the pump light with the power P p into the resonator.The gap between the resonator and the coupling prism can be adjusted using a piezo translator.Inside the cavity,the pump wave is converted to signal and idler waves,while all interact-ing waves oscillate in the cavity.The pump field is extra-ordinarily polarized,whereas the signal and the idler fields are ordinarily polarized.This ensures type I phase matching.Two detectors are used to measure the power P p of the transmitted portion of the pump light and the power P s of the signal wave.The silicon-based detector for the signal wave is not influenced by the idler light (wavelength larger than1200nm).Furthermore,we investigate the spectrum of the light generated by the parametric process in the wavelength range between 300and2500nm.
In order to determine the quality factor of the optical whispering gallery,we scan the pump laser frequencyνacross a cavity mode and measure the transmitted power P p versus the frequency shift.This is shown in Fig.2.The linewidthΔνin absence of parametric oscillation ranges from80(prism far from the resonator)to140MHz(prism touching the resonator).The first value yields the intrin-sic quality factor Q p≡ν∕Δν 7.7×106for the pump wave.From here,we can deduce the absorption coeffi-cientαp 2πn p∕λp Q p≈3.8m?1with the refractive index n p 2.25[6],assuming that the intrinsic cavity loss is dominated by absorption.This value agrees quite well with the previously measured coefficient 2…3 m?1 for congruent lithium niobate[7].The ratio
between
Fig.1.(Color online)Illustration of the experimental setup: P p,P p,and P s represent powers of the pump wave at488nm, of its transmitted portion,and of the signal light,respectively. The inset shows a microscope picture of the resonator,whereas the arrow indicates the direction of the crystallographic z-axis.
4224OPTICS LETTERS/Vol.37,No.20/October15,2012
0146-9592/12/204224-03$15.00/0?2012Optical Society of America
the highest and lowest linewidth is smaller than two.From this,we can conclude that we are not able to achieve critical coupling or overcoupling;i.e.,the inter-nal loss is always larger than the coupling loss.The coupling efficiency κwas limited to 40%for a vanishing gap between the prism and the resonator.
Later,we want to compare the pump threshold as well as the conversion efficiency of the OPO to the theoreti-cally expected values.For this purpose,we take here a closer look on the measured linewidths and coupling ef-ficiencies.With the coefficient r p being the ratio between the coupling loss and the internal loss for the pump wave,we get [8]
Δν ν
1Q p 1Q c
Δν0 r p 1 ;(1)
κ 4Q p Q c Q p Q c 2 4r p
r p 1 2
;
(2)
with Q c as the coupling quality factor and Δν0 ν∕Q p as the linewidth determined by the intrinsic loss.At r p <1,the resonator is undercoupled,and at r p >1the resona-tor is overcoupled.From Eq.(1)we deduce r p 0.75for the prism touching the resonator.For this value,Eq.(2)shows that 98%of the pump light could be coupled into the resonator for perfect mode matching.Since we achieve 40%coupling efficiency instead,only the power ~P
p 0.41P p is available for the parametric process.This reduction mainly originates from an imperfect spatial overlap between the pump beam and the whispering gallery mode.
If the input power exceeds the pump threshold P th for the optical parametric oscillation,we can measure the signal power P s while scanning the pump fre-quency across a cavity mode.Figure 2shows that at P p 200μW,we generate P s ≈0.9μW of signal light at 40°C resonator https://www.360docs.net/doc/72752434.html,ing a spectrometer,we have confirmed the parametric oscillation at λs 800nm and λi 1250nm.The signal power grows with increasing pump power as shown in Fig.3.Theoretically,this input –output curve should have a shape according to [8]
P s 4
~P p ∕P th q ?1 P th ×λp
λs r ?1p 1 r ?1
s
1 ;(3)
with the pump threshold
P th πε0c 0n 2p n 2s n 2i 2p
V eff
1Q p Q s Q i × r p 1 2 r s 1 r i 1
r p
:
(4)
Here,the coefficients r s;i are the ratios between the cou-pling losses and the intrinsic losses of the signal and idler waves,respectively.The second factor of Eq.(4),V eff ,is the effective mode volume,and Q j are the intrinsic qual-ity factors of the three interacting waves.The latter fac-tor of Eq.(3)gives the maximum achievable conversion
efficiency ηmax at ~P
p 4P th .Only in the case of strong overcoupling for both waves it reaches λp ∕λs correspond-ing to the Manley –Rowe limit.We determine ηmax 2.6%and P th 66μW (160μW total pump threshold)by a fit of Eq.(3)to our experimental data.Reducing the gap be-tween the resonator and the prism,hence increasing r p;s ,enhances the conversion efficiency to 7%.However,as indicated before,even for a vanishing gap we stay in the undercoupled regime for the pump wave.This limits the conversion efficiency.
The gap reduction also increases the pump threshold of the parametric oscillation up to 330μW.According to Eq.(4),this is expected.In order to compare the measured pump thresholds with the prediction of this equation,we chose the following realistic parameters:n j 2.2[6],d 5pm ∕V [9],V eff 10?12m 3,Q p 7.7×106,Q s 5×107[10],and Q i 108[10].From the linewidth measurements,we know that the pump field is undercoupled;we set r p 0.5.The signal and idler fields have larger wavelengths and might be criti-cally coupled or overcoupled r s 1,r i 2.This yields P th 10μW,which is a factor of 6smaller than the measured value.One reason might be a
significantly
Fig.2.(Color online)Transmitted pump power P p and signal power P s as a function of the frequency shift of the pump laser at 40°C resonator
temperature.
Fig. 3.(Color online)Signal power P s versus total pump
power P p and versus the fraction ~P
p available for the para-metric process at 40°C resonator temperature.The dots are the experimental data,and the solid line corresponds to a fit according to Eq.(3).
October 15,2012/Vol.37,No.20/OPTICS LETTERS 4225
larger effective mode volume due to different transversal mode structures of the three interacting waves.
By changing the resonator temperature from 30to 170°C in steps of 10°C,we can tune the signal and idler wavelengths.Figure 4shows this behavior for 4mW total pump power and a vanishing gap between the coupling prism and the resonator.At every investigated tempera-ture value,we scan the pump frequency over 6GHz,en-abling us to couple to different transversal pump modes.The signal wavelength λs ranges from 707to 865nm,and the idler wavelength λi from 1120to 1575nm.In order to compare the measured tuning behavior to the theore-tically expected one,we calculate the wavelengths λs;i fulfilling the phase matching condition n p ∕λp n s ∕λs n i ∕λi .Here,the effective refractive index de-pends on the bulk refractive index [6],on the tempera-ture,on the resonator shape,and on the transversal mode structure of the interacting waves [11].Figure 4shows a good agreement between experiment and simulation.If the whispering gallery OPO is aimed to generate tun-able signal light in the green or yellow spectral range,the system should be pumped at wavelengths around 400nm.Based on our experimental results,we can predict that in this case the pump threshold will increase to hundreds of microwatts.This is mainly due to decreasing values for the intrinsic quality factor Q p and for the ratio r p .Furthermore,the decreasing r p will limit the maximum achievable conversion efficiency to several percent,which can be seen from Eq.(3).Thus,for a practical de-vice with some 10%conversion efficiency,a material with significantly lower absorption in the ultraviolet region
compared to that of lithium niobate is required.Possible candidates are lithium tantalate and borates.Lithium tantalate is transparent down to 300nm wavelengths [12]and can be periodically poled to achieve quasi phase matching [13].Borates are transparent even below 200nm wavelengths [14].However,here birefringent phase matching is the only current option since no meth-od to grow orientation-patterned borates is known so far.In conclusion,we demonstrated and characterized a blue-pumped whispering gallery OPO.This monolithic device with about 2mm diameter has a submilliwatt pump threshold and is tunable over hundreds of nan-ometers between the red and near infrared.Furthermore,our experiments show that the conversion efficiency is limited to several percent because of absorption loss for the pump wave.We consider this special OPO as a step to a compact system that covers the whole visible spectral range with tunable nonclassical light.
We gratefully acknowledge the financial support from the Deutsche Forschungsgemeinschaft.
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Fig.4.(Color online)Signal and idler wavelengths versus the resonator temperature.The solid and dashed curves corre-spond to theoretically expected values under the assumption of different transversal mode combinations sketched in the in-set for pump (p ),signal (s ),and idler (i )waves.The curved line in the inset indicates the resonator rim.
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