Journal of the Korean Physical Society, Vol. 52, No. 4, April 2008, pp. 1043∼1047

A Sub-10-fs Ti:Sapphire Oscillator with a Simple Four-Mirror Cavity

Baatarchuluun Tsermaa,∗ Byeong Kwan Yang, Jae Myung Seo and Jin Seung Kim

Institute of Photonics and Information Technology, Chonbuk National University, Jeonju 561-756

(Received 21 December 2007)

Optical pulses of 9.4 fs are stably generated from a Kerr-lens mode-locked Ti:sapphire laseroscillator containing only four mirrors, including two double-chirped mirrors. To the best of ourknowledge, the oscillator has the simplest cavity design compared with other sub-10-fs oscillatorsreported yet. The balanced distribution of the negative group delay dispersion within the cavityyields improved stability against intensity fluctuations.

PACS numbers: 42.55.Rz, 42.60.By, 42.65.ReKeywords: Sub-10-fs, Ti:sapphire laser, Kerr-lens mode-locked, SHG-FROG

I. INTRODUCTION

Recent technological advances in the development ofhighly doped, broadband gain medium and dispersion-engineered chirped mirrors have led to better perfor-mances of Kerr-lens mode-locked (KLM) sub-10-fs laseroscillators [1, 2]. Both prism-controlled and mirror-controlled dispersion compensation methods are used forgenerating sub-10-fs pulses from KLM lasers [3–9]. Theearly designs mainly used prisms that provided fine tun-ing for the intracavity net dispersion. However, theyhave drawbacks including uncompensated higher-orderdispersion components of the prism substrate, a cavityalignment sensitive stability, and a low repetition rate.The development of dispersion-engineered chirped mul-tilayer dielectric mirrors [10, 11] has opened the pos-sibility to overcome the limitations inherent in prism-controlled dispersion compensation. The main advan-tages of mirror-dispersion-controlled (MDC) oscillatorsare simplicity, compactness, and stability of the cav-ity, which are essential for time-resolved experiments,carrier-envelope phase stabilization, and biomedical op-tical coherence tomography (OCT) applications [12,13].

One of the objectives of ultrashort pulsed laser re-search is the development of operationally effective, sim-ple, and cost-effective laser oscillators. In order tofurther improve the current performance of ultrashortlasers, we have constructed a Ti:sapphire oscillator witha reduced component count that combines greater sim-plicity with improved performance. This paper reportsgeneration of sub-10-fs optical pulses from a mirror-dispersion-controlled KLM oscillator which employs only4 mirrors: a pair of double-chirped mirrors, a totally re-flecting end mirror, and an output coupler. Even thoughfour-mirror oscillators are not new [14], to the best of

∗E-mail: bch@chonbuk.ac.kr

our knowledge, this is the simplest sub-10-fs oscillatorwhich uses only a pair of negative group delay disper-sion (GDD)-oscillation-compensated mirrors. In orderto tailor the required negative GDD in the sub-10-fsKLM laser cavity, more than two chirped mirrors areutilized, which increases the degrees of freedom for dis-persion control [15, 16]. However, the increased num-ber of chirped mirrors can either generate an asymmet-rical distribution of dispersion that eventually leads todegraded KLM efficiency [17–19] or, at least, compli-cate the cavity geometry. Our four-mirror oscillator hasfully solved the aforementioned weaknesses because itemploys only two double-chirped mirrors symmetricallypositioned in the two arms of the cavity. The arrange-ment of this paper is as follows: In Section II, the lay-out of the sub-10-fs Ti:sapphire oscillator is discussed.In Section III, pulse characteristics measured by usinga second-harmonic-generation frequency-resolved opticalgating (SHG-FROG) method are presented. In SectionIV, the results are summarized.

II. EXPERIMENT

The schematic of the constructed oscillator is shownin Figure 1. It is a Z-folded oscillator consisting ofa Ti:sapphire crystal (X), a pair of GDD-oscillation-compensated curved double-chirped mirrors (DCM’s)(M2, M3), a highly reflecting end mirror (M1) and anoutput coupler (OC). A highly doped (0.33 wt%), 2-mm-thick Ti:sapphire crystal is oriented at Brewster’s angle(θ ≈ 60.4◦). M1 is a mirror with protected silver coat-ing which has 98 % reflectivity in the spectral range of600 – 1200 nm and almost zero dispersion. The curvedmirrors M2 and M3 (LAYERTEC GmbH) have 50-mmradii of curvatures, and the mirror pair produces the re-quired negative dispersion inside the cavity. M2 and

-1043-

-1044- Journal of the Korean Physical Society, Vol. 52, No. 4, April 2008

Fig. 1. Schematic diagram of the mirror-dispersion-controlled sub-10-fs oscillator (X, Brewster-orientedTi:sapphire crystal; M1, highly reflecting end mirror;M2 and M3, a pair of GDD-oscillation-compensated double-chirped mirrors; OC, output coupler; L, focusing lens for thepump laser; S, vertical slit; θ, folding angle).

M3 have a nominal GDD of 70 fs2 ± 30 fs2 at 720– 1020 nm and almost cancel out the GDD oscillationsof each other over the bandwidth. Their reflectivity ex-ceeds 99.8 % in the spectral range of 630 – 980 nm, andalmost transparent (R < 10 %) for a pump beam in thespectral range of 505 – 545 nm. We used a frequency-doubled, diode-pumped Nd:YVO4 laser (532 nm, 5 W)as a pump source. The pump beam is focused with a40-mm focal-length lens (L) into the Ti:sapphire crystal.A water cooling system is used to remove the heat fromthe crystal.

The output coupler is 1-mm thick, and it has a 5 %transmittance over the spectral range of 660 – 920 nm.The cavity has a round-trip length of about 2.1 m, whichmakes the repetition rate 142 MHz. The lengths ofthe cavity arms confined by M1 −M2 and M3 − OChave been set equal to d1 ≈ 300 mm and d2 ≈ 700mm, respectively. We have done prior calculations us-ing the ABCD matrix method [20, 21] in order to esti-mate the distance between the folding mirrors and theposition of the gain medium to ensure stability of theresonator. Even though the oscillator can support bothhard-aperture and soft-aperture KLM, we mainly usedthe hard-aperture approach by introducing a vertical slit(S) on the longer arm side of the cavity. The proper val-ues of the folding angles (θ = θ2 = θ3 ≈ 15.22◦ ) at M2

and M3 were determined by requiring compensation ofthe astigmatism introduced by the tilted crystal.

A more balanced distribution of the negative GDD inthe two arms of the cavity allows the pulse to be short atthe each end of the cavity and the center of gain medium[1]. A possible way for equal distribution of the negativeGDD is use of the same negative GDD mirrors in the twoarms of the cavity. When the spatial and the temporalfoci of the intracavity pulse coincide in the crystal, thepeak intensity may reach a high value in a narrow spa-tial range, which enhances the best self-focusing. Thus,a symmetrical dispersion distribution may lead to an ef-fective saturable absorption that is twice as strong asan asymmetrical dispersion distribution, which results insubstantially shorter pulses [17–19]. The optimal spatialand temporal focusing yields improved stability against

fluctuations of the intensity. As we used a highly doped,2-mm-thick Ti:sapphire crystal, we had a chance to re-duce the number of negative chirped mirrors of the cavitydown to only a pair. Using only a pair of negative GDDmirrors as a dispersion compensator is efficient for notonly simplicity but also better stability against pertur-bations. The latter can be attributed to the symmetri-cal distribution of the intra-cavity dispersion because ityields more efficient KLM.

In order to achieve appropriate dispersion compensa-tion, we estimated the dispersion characteristics of all theoptical components in the cavity. The refractive indicesof the Ti:sapphire gain medium and air were calculatedaccording to Sellmeier equation [22] and results in esti-mates of the positive second- and third-order dispersionsof the cavity. The nominal GDD of the contributing el-ements at the central wavelength of 800 nm are 260 fs2

for double passage through the gain medium, 40 fs2 fora 2.1-m optical path in air, –280 fs2 for 4 bounces on thenegative GDD mirrors and –20 fs2 for a bounce on theoutput coupler [23]. The net GDD of the cavity takesa value near zero over the bandwidth of 720 – 1020 nm,which makes it suitable to enhance the KLM conditionin the cavity. Instead of keeping the repetition rate con-stant, we changed the cavity length to fine tune the netintracavity dispersion and found the optimum length tosatisfy the most favorable Kerr-lens mode-locking condi-tion.

After fine adjustments of the cavity mirrors and thecrystal, pulses with sufficiently broad spectrum and anoutput power of 400 mW were easily generated by usinga pumping power of about 4.5 W. The output retainedgood stability against environmental perturbations forhours in both the spectrum and the power.

III. PULSE CHARACTERIZATION

For pulse characterization, we used the SHG-FROGmethod [24,25], together with the crystal angle-ditheringtechnique [26]. The angle dithering was used to achievephase matching over the whole spectrum of the pulseto be measured with a modest signal power. Figure 2depicts the experimental setup for the pulse character-ization. The pulse to be measured was split into twoidentical pulses by using a beam splitter, and one of themdirected to a variable delay stage and then recombinedwith the other pulse by using another beam splitter toform a pair of pulses with some relative delay but mov-ing parallel to each other. The pulse pair was focused byusing an off-axis parabolic mirror (f = 75 mm) into the100-µm-thick BBO crystal, where their second harmonicis generated. The second harmonic signal was spectrallyresolved by using a spectrometer (HR2000, Ocean Op-tics) and was recorded at every delay step of 1.05 fs. Themovement of the delay arm mirrors was performed by us-ing a piezo-actuator, which was attached to one of the

A Sub-10-fs Ti:Sapphire Oscillator with a Simple· · · – Baatarchuluun Tsermaa et al. -1045-

Fig. 2. Experimental layout of the SHG-FROG apparatusfor characterization of sub-10-fs pulses.

two arms of the Michelson interferometer. The interfer-ometer provided accurate positioning of the delay armmirrors. The phase-matching bandwidth of the 100-µm-thick BBO crystal was insufficient for the entire band-width of the pulses, so the BBO crystal was mounted ona galvanometer to dither the crystal, which makes the ef-fective phase-matching bandwidth could cover the wholebandwidth of the pulses.

Transmission of the pulses through the beam splittersubstrate induced substantial broadening of the pulses.To compress and recover their initial pulse shape, weestimated the GDD of the fused silica substrate of thebeam splitter using Sellmeier’s equation and achievedpre-compensation for this positive dispersion by usingpairs of negative GDD mirrors.

Figure 3 shows measured and retrieved SHG-FROGtraces of 128 by 128 grid points. The symmetry of themeasured FROG trace along the time delay axis indi-cates that the optical components of the FROG systemwere well aligned. The phase was retrieved by using theprincipal components generalized projection algorithm[27] for the iterative procedure. The FROG error ofthis reconstruction procedure amounts to 0.0081. Fig-ure 4 shows the reconstructed pulse characteristics in thetemporal and the spectral domains. As is seen in Fig-ure 4(a), the temporal width of the pulse was 9.4 fs atFWHM. The flatness of the temporal phase in the centralpeak suggests that the pulse is nearly transform limited.Figure 4(b) shows the spectrum and the spectral phaseof the retrieved pulse. The retrieved spectrum covers aspectral range of 710 – 980 nm with a spectral width of100.2 nm at FWHM. The small oscillation in the spectralphase may be attributed to the residual GDD oscillationinduced by the DCMs and the output coupler.

The pulse characteristics of sub-10-fs oscillators crit-ically depend on the nature of the constituent opticalcomponents. Selection of cavity mirrors well-matched

Fig. 3. (a) Measured and (b) retrieved FROG traces.

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

1

7 0 0 7 5 0 8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0W a v e le n g th , n m

Spec

tral

Inte

nsi

ty,a

rb.u

nits

-4

-3

-2

-1

0

1

2

3

4

Phase,rad

.

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

1

-4 0 -3 0 -2 0 -1 0 0 1 0 2 0 3 0 4 0T im e , fs

Tem

pora

lInte

nsi

ty,a

rb.u

nits

-2

-1

0

1

2

3

4

Phase,rad

(a)

(b)

Fig. 4. Reconstructed pulse profiles. (a) Temporal in-tensity (solid) and phase (dashed) profiles of a 9.4-fs pulse(FWHM). (b) Spectrum (solid) and spectral phase (dashed)profiles (spectral width is 100.2 nm at FWHM).

with the spectral bandwidth and the dispersion char-acteristics of the intracavity optical components is thekey issue for construction of an ideal oscillator with apulse duration limited only by the bandwidth of the gainmedium. However, commercially available componentsdo not allow realization of such an ideal oscillator.

In order to check the consistency of the retrieved pulseparameters, we compared the retrieved results with theindependently measured data. Figure 5 depicts a com-parison of the retrieved and the measured pulse spectra.The retrieved spectrum fairly well reproduces its mea-sured counterpart over the whole bandwidth. For thetime-domain consistency check, we measured the inten-sity autocorrelation of the pulse and compared it with theintensity autocorrelation calculated from the retrievedspectrum and the spectral phase of the pulse (Figure 6).

-1046- Journal of the Korean Physical Society, Vol. 52, No. 4, April 2008

700 750 800 850 900 950 1000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Inte

nsi

ty,a.u

.

Wavelength, nm

Fig. 5. Comparison of the retrieved (thick line) and theindependently measured (thin line) fundamental pulse spec-tra.

0

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

1

-8 0 -6 0 -4 0 -2 0 0 2 0 4 0 6 0 8 0

D e la y , fs

Inte

nsi

ty,a

rb.u

nits

Fig. 6. Comparison of the independently measured inten-sity autocorrelation (dashed) with the autocorrelation (solid)calculated from the reconstructed spectrum and phase.

The agreement was fairly good, except for slight discrep-ancies at the leading and the trailing parts, which aresensitive to residual error.

IV. SUMMARY

We have designed and constructed a very simple sub-10-fs Kerr-lens mode-locked, Ti:sapphire laser oscillatorby using only four mirrors: a pair of DCMs for intra-cavity dispersion compensation, one high reflector, andone output coupler. The average output power wasabout 400 mW for a pumping power of 4.5 W. The bal-anced distribution of the negative group delay dispersionwithin the cavity yielded good stability against environ-mental fluctuations, and the oscillator kept working forseveral hours without any problem.

The pulse characteristics were measured by using theSHG-FROG method combined with crystal ditheringtechnique. The intensity and the phase of the pulses inthe temporal and the spectral domains retrieved from themeasured FROG traces showed that the pulse width was9.4 fs and the spectral bandwidth was 100.2 nm. Consis-tency checks of the phase retrieval were also performedin the temporal and the spectral domain with other inde-

pendent measurements. The agreements between themwere fairly good.

ACKNOWLEDGMENTS

This work is supported in part by a Korea Re-search Foundation grant (KRF-2006-005-J00302) and bythe Nano-Bioelectronics and Systems Research Center(Seoul National University), an Engineering ResearchCenter supported by the Korea Science and Engineer-ing Foundation.

REFERENCES

[1] F. X. Kartner, Few-cycle Laser Pulse Generation and ItsApplications (Springer, Berlin, 2004).

[2] J. Ye and S. T. Cundiff, Femtosecond Optical FrequencyComb Technology (Springer, New-York, 2005), Chap. 2-3.

[3] L. Xu, G. Tempia, A. Poppe, M. Lenzner, Ch. Spielman,F. Krausz, A. Stingl and K. Ferencz, Appl. Phys. B 65,151 (1997).

[4] G. Steinmeyer, D. H. Sutter, L. Gallmann, N. Matuschekand U. Keller, Science 286, 1507 (1999)

[5] E. Slobodchikov, S. Sakabe, T. Kuge and S. Kawato,Opt. Rev. 6, 149 (1999).

[6] A. Baltuska, Z. Wei, M. S. Pshenichnikov, D. A. Wiersmaand R. Szipocs, Appl. Phys. B 65, 175 (1997).

[7] U. Morgner, F. X. Kartner, S. H. Cho, Y. Chen, H. A.Haus, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. An-gelow and T. Tschudi, Opt. Lett. 24, 411 (1999).

[8] Y. S. Lim, H. S. Jeon, Y. C. Noh, K. J. Yee, D. S. Kim,J. H. Lee, J. S. Chang and J. D. Park, J. Korean Phys.Soc. 40, 837 (2002).

[9] Y. W. Lee and J. H. Yi, J. Korean Phys. Soc. 49, 412(2006).

[10] R. Szipocs, K. Ferencz, C. Spielmann and F. Krausz,Opt. Lett. 19, 201 (1994).

[11] N. Matuschek, F. X. Kartner and U. Keller, IEEE J.Quantum Elect. 35, 129 (1999).

[12] S. T. Cundiff, J. Korean Phys. Soc. 46, 1181 (2006).[13] T. Brabec and F. Krausz, Rev. Mod. Phys. 72, 545,

(2000).[14] A. Penzkofer, M. Wittmann, M. Lorenz, E. Siegert and

S. Macnamara, Opt. Quantum Elect. 28, 423 (1996).[15] A. Stingl, M. Lenzner, Ch. Spielmann and F. Krausz,

Opt. Lett. 20, 602 (1995).[16] T. Fuji, A. Unterhuber, V. S. Yakovlev, G. Tempia, A.

Stingl, F. Krausz and W. Drexler, Appl. Phys. B 77, 125(2003).

[17] P. Christov and V. D. Stoev, Opt. Lett. 20, 2111 (1995).[18] I. P. Christov, V. Stoev, M. Murnane and H. Kapteyn,

Opt. Lett. 21, 1493 (1996).[19] Y. H. Cha, K. Lee, J. M. Han and Y. J. Rhee, J. Opt.

Soc. Am. B 20, 1369 (2003).[20] Y. W. Lee, J. H. Yi, Y. H. Cha, Y. J. Rhee, B. C. Lee

and B. D. Yoo, J. Korean Phys. Soc. 46, 1131 (2005).[21] J. Xia and M. H. Lee, Appl. Opt. 41, 453 (2002).

A Sub-10-fs Ti:Sapphire Oscillator with a Simple· · · – Baatarchuluun Tsermaa et al. -1047-

[22] M. Bass, Handbook of Optics Vol. II, 2nd ed. (McGrawHill, New-York, 1995).

[23] B. Tsermaa, Ph.D dissertation, Chonbuk National Uni-versity, 2007.

[24] R. Trebino, Frequency-resolved Optical Gating (Kluwer,Boston, 2002).

[25] B. K. Yang and J. S. Kim, Hankook Kwanghak Hoeji

15, 278 (2004).[26] P. O’Shea, M. Kimmel and R. Trebino, Opt. Expr. 7,

342 (2000).[27] D. J. Kane, IEEE J. Sel. Top. Quantum Elect. 4, 278

(1998).