Transverse mode tailoring in a quasi-imaging high-finesse femtosecond enhancement cavity

doi:10.1364/OE.19.009551

« Show journal navigation

Transverse mode tailoring in a quasi-imaging high-finesse femtosecond enhancement cavity

Johannes Weitenberg, Peter Rußbüldt, Tino Eidam, and Ioachim Pupeza »View Author Affiliations

Optics Express, Vol. 19, Issue 10, pp. 9551-9561 (2011)
http://dx.doi.org/10.1364/OE.19.009551

View Full Text Article

Acrobat PDF (1324 KB)

Journal Search
Article Lookup

Article Tools

Citations

Alert me when this article is cited
Export Citation/Save

Article Navigation

▸ Article Outline
– Abstract
1. Introduction
2. Quasi-imaging
3. Experimental realization
4. Further discussion
5. Conclusion
– References and links

Article Tools
Full Text
Article Info
References
Cited By
Figures
Metrics
Download PDF

Viewing Equations in the
HTML Article

Optimal Viewing Recommendations

Full Text
Article Info
References (20)
Cited By
Figures (8)
Metrics

Abstract

We demonstrate a high-finesse femtosecond enhancement cavity with an on-axis obstacle. By inserting a wire with a width of 5% of the fundamental mode diameter, the finesse of F = 3400 is only slightly reduced to F = 3000. The low loss is due to the degeneracy of transverse modes, which allows for exciting a circulating field distribution avoiding the obstacle. We call this condition quasi-imaging. The concept could be used for output coupling of intracavity-generated higher-order harmonics through an on-axis opening in one of the cavity mirrors.

1. Introduction

The generation of high harmonics (HHG) of femtosecond radiation in a gas makes possible table-top coherent light sources with wavelengths down to a few nanometers. To reach the required peak intensities on the order of 10¹⁴ W/cm² for the nonlinear conversion at MHz repetition rates, as well as to increase the yield of the highly inefficient conversion process, resonant enhancement in a passive cavity can be employed [1

I. Pupeza, T. Eidam, J. Rauschenberger, B. Bernhardt, A. Ozawa, E. Fill, A. Apolonski, Th. Udem, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tünnermann, T. W. Hänsch, and F. Krausz, “Power scaling of a high-repetition-rate enhancement cavity,” Opt. Lett. 35(12), 2052–2054 (2010). [CrossRef] [PubMed]

–10

A. Ozawa, A. Vernaleken, W. Schneider, I. Gotlibovych, Th. Udem, and T. W. Hänsch, “Non-collinear high harmonic generation: a promising outcoupling method for cavity-assisted XUV generation,” Opt. Express 16(9), 6233–6239 (2008). [CrossRef] [PubMed]

]. In the infrared range, a power enhancement of three orders of magnitude has been demonstrated [1

]. High repetition rates (several tens of MHz and more) are desirable for several applications, including high precision spectroscopy in the XUV range [2

A. Ozawa, J. Rauschenberger, Ch. Gohle, M. Herrmann, D. R. Walker, V. Pervak, A. Fernandez, R. Graf, A. Apolonski, R. Holzwarth, F. Krausz, T. W. Hänsch, and Th. Udem, “High harmonic frequency combs for high resolution spectroscopy,” Phys. Rev. Lett. 100(25), 253901 (2008). [CrossRef] [PubMed]

] and coincidence spectroscopy of correlated electron dynamics [3

I. Pupeza, T. Eidam, J. Kaster, B. Bernhardt, J. Rauschenberger, A. Ozawa, E. Fill, Th. Udem, M. F. Kling, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tünnermann, Th. W. Hänsch, and F. Krausz, “Power scaling of femtosecond enhancement cavities and high-power applications,” Proc. SPIE 7914, 79141I , 79141I-13 (2011). [CrossRef]

]. The intracavity-generated high harmonics propagate along the resonator optical axis. Their output coupling from the cavity is a major challenge and has been subject to intensive research over the past few years [6

D. C. Yost, T. R. Schibli, and J. Ye, “Efficient output coupling of intracavity high-harmonic generation,” Opt. Lett. 33(10), 1099–1101 (2008). [CrossRef] [PubMed]

–10

]. An established method relies on a thin plate, placed behind the focus at Brewster’s angle for the fundamental radiation [2

R. J. Jones, K. D. Moll, M. J. Thorpe, and J. Ye, “Phase-coherent frequency combs in the vacuum ultraviolet via high-harmonic generation inside a femtosecond enhancement cavity,” Phys. Rev. Lett. 94(19), 193201 (2005). [CrossRef] [PubMed]

Ch. Gohle, Th. Udem, M. Herrmann, J. Rauschenberger, R. Holzwarth, H. A. Schuessler, F. Krausz, and T. W. Hänsch, “A frequency comb in the extreme ultraviolet,” Nature 436(7048), 234–237 (2005). [CrossRef] [PubMed]

]. The intracavity-generated XUV radiation experiences total reflexion at the plate and is thus separated from the fundamental beam. However, due to strong absorption of the evanescent field, the output coupling efficiency is typically 0.1-0.2 for wavelengths ~60 nm and significantly decreases for shorter wavelengths. For wavelengths around 10 nm this method is not suitable because the efficiency is typically <10⁻³. Moreover, nonlinearities introduced by the plate limit the peak power circulating in the cavity. A different output coupling method uses a grating, manufactured in the top coating layer of a cavity mirror behind the focus [6

D. C. Yost, T. R. Schibli, and J. Ye, “Efficient output coupling of intracavity high-harmonic generation,” Opt. Lett. 33(10), 1099–1101 (2008). [CrossRef] [PubMed]

Y.-Y. Yang, F. Süßmann, S. Zherebtsov, I. Pupeza, J. Kaster, D. Lehr, H.-J. Fuchs, E.-B. Kley, E. Fill, X.-M. Duan, Z.-S. Zhao, F. Krausz, S. L. Stebbings, and M. F. Kling, “Optimization and characterization of a highly-efficient diffraction nanograting for MHz XUV pulses,” Opt. Express 19(3), 1954–1962 (2011). [CrossRef] [PubMed]

]. While the XUV radiation is diffracted by the grating, the optical element acts as a highly reflecting mirror for the fundamental beam. The output coupling efficiency is typically <0.2 and also decreases for smaller wavelengths. Recently, it has been shown that the nanostructure leads to increased nonlinear effects on the mirror surface induced by the fundamental radiation [7

], which might limit the scaling of intracavity peak power. Moreover, the fact that the harmonics are spatially dispersed might constitute a drawback for some applications. In a third approach, the output coupling is achieved geometrically through an on-axis hole or slit in the mirror behind the focus. The smaller divergence angle of the high harmonics due to the shorter wavelength makes this approach possible. Because there is no interaction of the XUV radiation with any optical element, a geometrical access allows for efficient output coupling of short wavelengths (e.g. sub-10 nm). Moreover, no dispersion or nonlinearities are introduced by the output coupling mechanism. If the fundamental mode of the cavity is used, an on-axis hole has to be very small to allow for a high enhancement [8

K. D. Moll, R. J. Jones, and J. Ye, “Output coupling methods for cavity-based high-harmonic generation,” Opt. Express 14(18), 8189–8197 (2006). [CrossRef] [PubMed]

]. Approaches allowing for a broad hole or slit at small loss for the fundamental radiation have been proposed by employing either transverse modes with vanishing field on the optical axis [8

K. D. Moll, R. J. Jones, and J. Ye, “Output coupling methods for cavity-based high-harmonic generation,” Opt. Express 14(18), 8189–8197 (2006). [CrossRef] [PubMed]

], an imaging resonator [9

W. P. Putnam, G. Abram, E. L. Falcão-Filho, J. R. Birge, and F. X. Kärtner, “High-intensity Bessel-Gauss beam enhancement cavities,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMD1 (2010).

] or by using non-collinear high harmonic generation [8

K. D. Moll, R. J. Jones, and J. Ye, “Output coupling methods for cavity-based high-harmonic generation,” Opt. Express 14(18), 8189–8197 (2006). [CrossRef] [PubMed]

,10

In this paper we experimentally demonstrate the possibility of geometrical on-axis access to a high-finesse enhancement cavity by a technique which we call quasi-imaging. By particular choice of the resonator geometry, higher-order transverse modes can be simultaneously resonant with the fundamental mode. In our case, the Gauss-Hermite modes GH _0,0, GH _4,0 and GH _8,0 can be excited simultaneously. If an obstacle, e.g. a wire or a slit in a resonator mirror, is introduced into the beam path, these modes arrange to avoid the obstacle. We call this quasi-imaging, because only a subset of the transverse modes is involved, whereas in an imaging resonator all transverse modes are simultaneously resonant.

The manuscript is structured as follows: Section 2 contains an introduction to the concept of quasi-imaging, in Section 3 we present the experimental realization of a quasi-imaging resonator, Section 4 gives a brief discussion and Section 5 concludes the paper.

2. Quasi-imaging

2.1 Definition of a quasi-imaging resonator

For a stable spherical resonator in paraxial approximation, one set of eigen-modes, i.e. field distributions which are reproduced after one resonator round trip, are the Gauss-Hermite GH_n _, _m modes. When an obstacle is placed in the beam path, these modes are in general no longer the resonator eigen-modes. However, if a set of GH modes is simultaneously resonant, new eigen-modes can be constructed as a combination thereof, which avoid the obstacle and have small diffraction loss. We call this condition quasi-imaging. Quasi-imaging is related to imaging, because the hole in a field distribution is reproduced after a resonator round trip. However, quasi-imaging can be achieved in a stable resonator (i.e. with a distinguished eigen-q-parameter, or equivalently for |A + D| < 2 with the elements of the beam transfer matrix for one resonator round trip M = [[A, B],[C, D]]), which is not the case for imaging. The on-axis phase ϕ acquired at a resonator round trip by a GH_n _, _m mode with eigen-q-parameter (q_x and q_y ) and wave number k is given by

φ_{k, n, m} = k L + (n + \frac{1}{2}) ψ_{x} + (m + \frac{1}{2}) ψ_{y}

(1)

where L is the resonator length (length of a ring resonator or twice the length of a linear resonator) and ψ_x,y is a phase which we will refer to as the Gouy parameter. This phase is due to the Gouy effect and is defined by the resonator geometry [11

S. Gigan, L. Lopez, N. Treps, A. Maître, and C. Fabre, “Image transmission through a stable paraxial cavity,” Phys. Rev. A 72(2), 023804 (2005). [CrossRef]

]. The Gouy parameter and the eigen-q-parameter can be different for the two transverse directions in Cartesian geometry. The stability range of the resonator is determined by the condition 0 < ψ < π or π < ψ < 2π. Modes with mode number difference Δn are simultaneously resonant if the Gouy parameter is ψ_x = 2π h/Δn with an integer h, because their round-trip phases are equal mod(2π). The simplest case, in which a zero of the electric field at the position of the obstacle is achieved, is the combination of the fundamental mode with the next resonant transverse mode. In the middle of the stability range, where the Gouy parameter is ψ = π/2 or 3π/2, the mode number difference for simultaneously resonant modes is Δn = 4. The corresponding mode combination is shown in Fig. 1 . It constitutes a new eigen-mode of the resonator with a sufficiently small obstacle on the optical axis. The obstacle leads to a coupling between the modes [12

R. Paschotta, “Beam quality deterioration of lasers caused by intracavity beam distortions,” Opt. Express 14(13), 6069–6074 (2006). [CrossRef] [PubMed]

Fig. 1 Eigen-mode of a quasi-imaging resonator at the position of the obstacle and at the position of maximum intensity on the optical axis in Cartesian geometry with mode number difference Δn = 4. To achieve zero on the optical axis, the modes have to be added with the indicated coefficients and opposite sign. Because the modes acquire a different on-axis phase ϕ at propagation, at a different position they are added with equal sign, which yields an on-axis intensity maximum. The spatial overlap with a Gaussian beam with the same q-parameter can be calculated from the coefficients and is U = 3/11.

Further resonant GH modes can contribute to the field distribution. This can change the spatial overlap with the incident field, the width of the on-axis region of small intensity, and loss at apertures limiting the transverse extent of the field distribution. Such apertures can be used to suppress the contribution of higher-order GH modes.

If a resonator has cylindrical symmetry, the Gouy parameters in the transverse directions are equal, i.e. ψ_x = ψ_y , and the Gauss-Laguerre modes GL^l _p with radial and azimuth mode index p and l can be used instead of the GH modes as an orthonormal basis of eigen-modes. Various mode number differences and mode combinations can be chosen. A detailed theoretical description exceeds the scope of this paper and will be provided in [13

J. Weitenberg, P. Rußbüldt, I. Pupeza, Th. Udem, H.-D. Hoffmann, and R. Poprawe, “Geometrical on-axis access to high-finesse resonators by quasi-imaging,” (manuscript in preparation).

2.2 Geometrical on-axis access to a bow-tie ring resonator by quasi-imaging

The simplest setup of a ring resonator, as it is e.g. commonly used for HHG, is a resonator with two focusing mirrors with radius of curvature R_C and resonator length L (see Fig. 2 ). The distance d of the focusing mirrors determines the Gouy parameters ψ_x and ψ_y in the transverse directions. The angle of incidence α on the curved mirrors causes different Gouy parameters in the transverse directions, i.e. ψ_x ≠ ψ_y . Therefore, the condition for quasi-imaging can be achieved only in one transverse direction. In the middle of the stability range, for ψ_y = 3π/2, the modes GH _0,0 and GH _0,4 are simultaneously resonant and can be combined to a “slit mode” avoiding an on-axis slit in the output coupling mirror. The on-axis intensity oscillates along the propagation direction, with zero on the optical axis in the focal plane and an intensity maximum one Rayleigh length from the focus (see Fig. 2).

Fig. 2 Quasi-imaging ring resonator. In the middle of the stability range, the Gouy parameter ψ_y for one round trip in the sagittal transverse direction (orthogonal to the plane of the beam path) is ψ_y = 3π/2 and the modes GH _0,0 and GH _0,4 are simultaneously resonant. The Gouy parameter acquired in the short arm is ψ_y ~π and in the long arm ψ_y ~π/2. The intensity distribution in the transverse direction of the quasi-imaging is shown at selected positions. The slit allows for a geometrical access (blue arrow) to the optical axis which could be used for output coupling of high harmonics generated in a gas jet near the focus.

2.3 Adjustment of transverse mode degeneracy

Quasi-imaging relies on a degeneracy of transverse modes, which are simultaneously excited to form a field distribution avoiding an obstacle. A degeneracy can be easily adjusted by interpreting the scan pattern, i.e. the transverse mode resonances at a scan of the oscillator cavity length, for the enhancement cavity without an obstacle. Figure 3 shows a schematic diagram of such a scan pattern for a femtosecond bow-tie ring resonator. The abscissa shows the varied oscillator cavity length multiplied with the wave number and the ordinate shows the corresponding intracavity power. Resonances of the fundamental mode (GH _0,0) appear with a period of 2π, corresponding to a change in cavity length by one wavelength λ. Because higher-order transverse modes acquire an additional phase (Eq. (1)), they are resonant at smaller frequencies than the fundamental mode, i.e. at longer oscillator cavity lengths. As can be easily seen, the Gouy parameter in tangential direction ψ_x is larger than in sagittal direction ψ_y for a bow-tie ring resonator. This difference between the Gouy parameters becomes visible in a resonator with finesse F, if the resonance width 2π/F is smaller than the Gouy parameter difference ψ_x − ψ_y . The position in the stability range can be read from the position of the resonances, the Gouy parameter being the distance between the 0-resonance and 1-resonance. In practice, the transverse modes can be easily identified by the number of adjacent resonances and by tilting of the incident beam and thereby changing its spatial overlap with the cavity transverse modes. Mode GH _4,0 acquires an additional phase compared to the fundamental mode, which is four times the Gouy parameter ψ_x . Therefore, the two modes are simultaneously resonant if ψ_x = 3π/2 holds. In the situation shown in Fig. 3, ψ_x has to be increased (by increasing the distance d between the curved mirrors) in order to achieve the degeneracy.

Fig. 3 Schematic diagram of the resonances for a scan of the oscillator cavity length L_osc . k is the wave number corresponding to the central wavelength of the incident femtosecond pulse train. The relative height of the resonances depends on the spatial overlap with the incident beam and is arbitrarily chosen. Resonances of higher order than 4 are not shown. To a group of resonances with constant sum of the two transverse mode order numbers n + m = r we refer to as r-resonances. The dashed line is an envelope for the resonance height.

The main resonance is the resonance for which all comb lines of the incident frequency comb simultaneously coincide with a cavity resonance. The side resonances drop in height and increase in width (width not indicated in Fig. 3) depending on the cavity finesse and spectral bandwidth, because different spectral parts of the frequency comb are on resonance for slightly different cavity lengths.

3. Experimental realization

3.1 Experimental setup

In order to demonstrate quasi-imaging, we employed the experimental setup shown in Fig. 4 , and described in [1

]. The Yb-based fiber laser system is described in [14

T. Eidam, F. Röser, O. Schmidt, J. Limpert, and A. Tünnermann, “57 W, 27 fs pulses from a fiber laser system using nonlinear compression,” Appl. Phys. B 92(1), 9–12 (2008). [CrossRef]

]. The laser emits almost Fourier-limited τ = 200 fs pulses (τ∙Δν = 0.39) with a central wavelength λ = 1040 nm and bandwidth Δλ = 7 nm. The repetition rate is ν _rep = 78 MHz. Throughout the experiments presented in this paper we used an incident average power of P_in = 1.4 W. Two spherical lenses were used to match the laser output mode to the enhancement cavity fundamental mode. The passive cavity of length L = 3.84 m consists of eight mirrors. Mirrors M2-M8 are highly reflective (R_M = 0.99995) over the entire laser bandwidth and M1 is the input coupler (R_IC = 0.9986). Mirrors M5 and M6 are spherically curved (radius of curvature R_C = 150 mm), all other mirrors are plane. This yields a Gaussian waist radius of w ₀ = 22 µm in the middle of the stability range. All cavity mirrors exhibit low dispersion. In order to minimize round-trip dispersion, as well as to avoid nonlinearities in air, the cavity is placed in a vacuum chamber.

Fig. 4 Experimental setup. (a) Laser, enhancement cavity and locking scheme as in [1

], diagnostics: power meter, optical spectrum analyzer. (b) Image of the 100 µm thick on-axis wire with the cavity off resonance.

For resonant enhancement, the incident laser frequency comb modes have to spectrally overlap with the cavity resonances. As discussed in [3

], due to the relatively narrow optical bandwidth of the femtosecond radiation, an active locking of the frequency comb to the cavity resonances can be realized by controlling only one of the two comb parameters, once they are brought in vicinity of optimum spectral overlap. The latter is performed by manually adjusting the enhancement cavity length and the carrier-envelope offset (CEO) phase by using a pair of wedges in the oscillator cavity. The locking electronics drives a piezoelectric transducer (PZT) varying the laser oscillator cavity length. The error signal for the locking is generated with the Hänsch-Couillaud scheme [15

Th. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35(3), 441–444 (1980). [CrossRef]

] from the signal reflected by the cavity input coupler. The only difference towards the setup described in [1

] is a pinhole, enabling the spatial selection of the reflected beam profile for the generation of the error signal. This is particularly useful if the incident transverse mode does not match the excited cavity mode. The intracavity beam leaking through one of the highly reflecting cavity mirrors (M8) with a transmission of 1.65·10⁻⁶ is characterized using a power meter and an optical spectrum analyzer. The circulating power P_circ is calculated from the power P_leak leaking through this mirror according to P_circ = P_leak /1.65·10⁻⁶.

A copper wire was placed vertically on a motor-driven translation stage to function as an obstacle on the optical axis of the resonator. The position of the wire along the optical axis determines the positions of the on-axis intensity minima and maxima in the cavity. In order to yield the situation shown in Fig. 2 offering a geometrical access to the focus, the wire would have to be placed in front of the curved mirror behind the focus (M6). However, this position is not relevant for the demonstration of quasi-imaging. This is because the Gouy parameter ψ = 3π/2 acquired at one round trip starting at the wire is independent of its position. Therefore, the convenient implementation of the wire (400 mm before mirror M5) as well as the vertical orientation is chosen for good accessibility. The plane of the wire is imaged 1:1 with a lens (f = 400 mm) on a CCD camera (see Fig. 4(b)). This allows for adjusting the wire to the optical axis and observing the beam profile at that position. To record the beam profile at different positions in the cavity the camera can be moved on a rail. The 100 µm width of the wire equals 5% of the Gaussian beam diameter of 2w = 2·0.99 mm at that position. An additional vertical aperture is placed in the cavity at the same longitudinal position and with a distance from the optical axis of roughly three times the Gaussian beam radius. This aperture serves as a spatial filter, i.e. it suppresses contributions of higher-order transverse modes other than GH _0,0 and GH _4,0 to the circulating field.

A variation of the position in the stability range is achieved by displacing mirror M5 and compensating the change in the resonator length by displacing mirror M7 (see Fig. 4(a)). The cavity alignment is not significantly affected.

Throughout the experiments presented here, the q-parameter of the incident beam was adjusted for maximum spatial overlap with the fundamental mode of the enhancement cavity without the wire. The spatial overlap with higher-order transverse cavity modes is non-zero. We determined the spatial overlap with the fundamental mode by analysis of a scan pattern. We scanned the oscillator cavity length over more than one free spectral range and recorded the intracavity power measured with a photodiode placed behind mirror M8 and connected to an oscilloscope. For this experiment, we replaced mirrors M1 (the input coupler) and M8 by mirrors with a reflectivity of R_M = 0.99. With the resulting finesse F = 100π and with a scan time t_fsr = 7 ms for one free spectral range, the scan time over one resonance width t_res = t_fsr /F = 22 µs is about 17 times larger than the cavity build-up time t_bu = (F/π)/ν _rep = 1.3 µs. This is long enough to allow for a full build-up of the intracavity radiation for each transverse mode, and short enough to avoid distortions by mechanical vibrations. We assume a linear behavior of the scanning PZT. Then the intracavity power integrated over the illumination time of a transverse mode or a group of transverse modes is proportional to the spatial overlap of the incident beam with the respective mode or group of modes. Transverse modes with mode order higher than 4 were not measurable and therefore not considered. In this manner, the spatial overlap of the incident beam with the fundamental mode was estimated to be U = 0.94.

3.2 Enhancement of the cavity modes GH_0,0 and GH_1,0

In two first experiments, we locked the cavity without the wire to the fundamental mode (GH _0,0) and, subsequently, to the GH _1,0 mode. The purpose of these experiments was to characterize the empty cavity, in particular the round-trip loss and impedance matching. Also, the elaborate evaluation of finesse and overlap from enhancement and coupling was tested for consistency.

With the fundamental mode we achieved stable locking over several minutes with a circulating average power of P_circ = 1950 W. This corresponds to a power enhancement of P_circ /P_in = 1400. The circulating power fluctuates in time and repeatedly drops to ~0.8 of the maximal value. We attribute this strong fluctuation to the relatively slow PZT used for locking (measured mechanical bandwidth <20 kHz) in conjunction with the high cavity finesse. This was confirmed by decreasing the cavity finesse, which led to a significant decrease of the fluctuations. Furthermore, this is not a fundamental limitation of cavity enhancement, since a larger locking bandwidth can be achieved by advanced design of PZT-actuated mirrors [16

T. C. Briles, D. C. Yost, A. Cingöz, J. Ye, and T. R. Schibli, “Simple piezoelectric-actuated mirror with 180 kHz servo bandwidth,” Opt. Express 18(10), 9739–9746 (2010). [CrossRef] [PubMed]

]. The time-averaged circulating power P_circ was measured with a power meter. We assume that the maximal power level P_circ ^max is determined by the loss and overlap, and the dropping is due to a deviation from resonance. This maximum value is P_circ ^max = 1.1·P_circ (compare Fig. 5(a) ). The corresponding maximum level of the enhancement, which was used to calculate the loss and the overlap, is E = P_circ ^max /P_in = 1540. The coupling ratio K of the power coupled into the cavity to the incident power was determined from the drop of the signal reflected from the input coupler compared to the unlocked case. This signal was measured with a photodiode. The maximum level of the coupling was determined to be K = 0.70. From these values the round trip loss factor is calculated to be R = 1 − K/E = 0.99955. It follows for the cavity finesse F = 2π/(1 − R_ICR) = 3400. The difference between the measured enhancement E = 1540 and the enhancement expected from the loss E_R = (1 − R_IC )/(1 − (R_ICR)^1/2)² = 1630 can be attributed to an incomplete spatial overlap of U = E/E_R = 0.94, since a complete spectral overlap was confirmed by measuring the incident and circulating spectra. This spatial overlap matches the value estimated from the scan. The pulse duration was measured to be τ = 200 fs.

Fig. 5 (a) Lock of the slit mode with the wire on the optical axis. Transmitted signal (green, 50 mV/div) and reflected signal (blue, 500 mV/div). The graph shows the fluctuation of the circulating power with time and the meaning of P_circ and P_circ ^max . (b) Measured resonances (transmitted signal, green, 1 mV/div) for a linear scan (PZT signal, magenta, 5 V/div) of the oscillator cavity length without wire (compare Fig. 3). The abscissa is in fact a time axis with 1 ms/div. Transverse mode resonances up to order 4 are visible. Modes with the same sum of the two transverse mode order numbers m + n = const. can be clearly distinguished. The Gouy parameter is ψ_x = 0.78·2π and ψ_y = 0.76·2π assuming a linear response of the PZT.

We then locked the cavity without the wire to the GH _1,0 mode. Because the GH modes form an orthogonal set of modes, the spatial overlap of an incident GH _0,0 mode with the circulating GH _1,0 mode vanishes, unless the incident beam has a tilt or transverse offset with respect to the cavity optical axis. In the experiment, the q-parameter was left the same as before and the beam was tilted horizontally to optimize the spatial overlap. The maximum spatial overlap for an incident Gaussian beam with eigen-q-parameter is easily calculated to be U = 1/e = 0.37 for a tilt angle of γ = 2/(k·w) or an offset of Δx = w with the wave number k and Gaussian beam radius w. The pinhole in the locking scheme was used to select one of the two lobes in the reflected signal. The CEO phase of the incident pulse train had to be adjusted for maximum spectral overlap of the frequency comb modes with the cavity resonances corresponding to the GH _1,0 mode. This is due to the fact that the cavity resonances for mode order n = 1 are shifted by the Gouy parameter ψ_x compared to the fundamental mode (see Fig. 3). A power enhancement of P_circ /P_in = 550, P_circ ^max = 1.1·P_circ and a coupling of K = 0.28 were measured. The finesse is determined to be F = 3400, which is the same as for the fundamental mode. The spatial overlap is U = 0.37, which matches the value calculated for an incident Gaussian beam. The smaller power enhancement compared to the fundamental mode is due to the reduced spatial overlap alone, the resonator loss and therefore the finesse is identical in both cases. As in the case of the fundamental mode, the entire spectrum was coupled to the GH _1,0 mode and evenly enhanced.

3.3 Quasi-imaging: enhancement of slit modes with an on-axis obstacle

In the final experiment the incident beam was aligned to the cavity optical axis and the CEO phase was adjusted for maximum spectral overlap with the cavity resonances of the fundamental mode. The wire was placed as an obstacle on the optical axis, which can be done with high precision by monitoring the image of the wire while the cavity is not locked (see Fig. 4(b)). In order to achieve the transverse mode degeneracy, we adjusted the position in the stability range by changing the distance d between the curved mirrors by displacing mirror M5 and compensating the change in the resonator length by displacing mirror M7. The position in the stability range can be read from the scan pattern of the cavity with the wire removed from the beam path, as is discussed in Section 2.3. Figure 5(b) shows a recorded scan pattern.

In the locked case, different field distributions with different numbers of lobes were observed, depending on the position in the stability range (see Fig. 6 ). Field distributions with more than the four lobes for the simple slit mode depicted in Fig. 1(a) involve the contribution of higher-order transverse modes (GH _8,0 and GH _12,0). In order to suppress these contributions, a vertical aperture was placed in the cavity and moved towards the optical axis until additional lobes besides the four were suppressed. This was the case for a distance from the optical axis of about three times the Gaussian beam radius (see Fig. 7(a) ).

Fig. 6 Different intensity distributions at the position of the wire on the optical axis in the locked cavity recorded at different positions in the stability range. Besides the wire, no additional obstacle is used here.

Fig. 7 Measured intensity distribution of the excited slit mode. (a) Image of the plane with the wire on the optical axis and spatial filtering aperture, Gaussian beam radius w = 0.99 mm. Wire and aperture are indicated by dashed lines. (b) Image of a position with maximal intensity on the optical axis, which is 2.4 m before the wire, Gaussian beam radius w = 0.77 mm. (c) Fit of the mode combination c ₀·GH _0,0 + c ₄·GH _4,0 + c ₈·GH _8,0 with complex coefficients to a cut through the intensity distribution from (a). The contribution of the three GH modes is indicated.

As in the previous experiment, we used the pinhole in the locking scheme to select one of the lobes for the generation of the error signal. We went through a region in the stability range around the degeneracy by varying the distance d by some tens of µm. Figure 8(a) shows the enhancement as a function of the detuning from the quasi-imaging condition in terms of the Gouy parameter ψ_x and of the distance d between the curved mirrors. We have derived a theoretical model for the detuning curve. However, the model exceeds the scope of this paper and will be described in [13

J. Weitenberg, P. Rußbüldt, I. Pupeza, Th. Udem, H.-D. Hoffmann, and R. Poprawe, “Geometrical on-axis access to high-finesse resonators by quasi-imaging,” (manuscript in preparation).

]. The experimentally measured width (FWHM) of the detuning curve is Δδ = 17 µm. On the one hand, this is a small fraction of the entire stability range of 3.2 mm. On the other hand, it is so broad that the quasi-imaging can be easily adjusted manually and does not demand for an active control of the distance d.

Fig. 8 (a) Power enhancement P_circ /P_in versus detuning from the resonator degeneracy. The model used here will be described in [13

J. Weitenberg, P. Rußbüldt, I. Pupeza, Th. Udem, H.-D. Hoffmann, and R. Poprawe, “Geometrical on-axis access to high-finesse resonators by quasi-imaging,” (manuscript in preparation).

]. (b) Normalized intracavity spectra of the GH _0,0, GH _1,0 and slit mode (logarithmic scale). The curves show that the spectral coupling to the excited transverse modes is identical in all cases.

For maximum power enhancement P_circ /P_in = 330 at circulating power P_circ = 460 W, a maximum level of the circulating power P_circ ^max = 1.08·P_circ (see Fig. 5(a)) and a coupling of K = 0.24 were measured. The finesse is determined to be F = 3000, which represents only a small decrease compared to the cavity without the wire F = 3400. The additional round-trip loss at the wire amounts to 2.3·10⁻⁴. With the wire the enhancement expected from the loss is E_R = 1290. This represents a reduction by 0.79 compared to the cavity without the wire. However, the smaller power enhancement compared to the fundamental mode is mainly due to the reduced spatial overlap. Figure 7 shows the measured intensity distribution of the enhanced field at the position of the wire and at a position of maximum intensity on the optical axis. A fit of a mode combination of GH _0,0, GH _4,0 and GH _8,0 with complex coefficients to the intensity distribution yields a power fraction of |c ₀|² = 0.25 in the fundamental mode. The power fraction in the GH _8,0 mode is only |c ₈|² = 0.03, because it is suppressed by the aperture. The power fraction of |c ₀|² = 0.25 defines the expected spatial overlap with an incident Gaussian beam with the eigen-q-parameter. The spatial overlap calculated from enhancement and coupling is U = 0.27, which is in good agreement with the value of |c ₀|². The entire spectrum was coupled to the slit mode in the cavity and evenly enhanced (see Fig. 8(b)).

For a thicker wire with 220 µm width (which equals 11% of the Gaussian beam diameter) placed at the same position we measured a power enhancement P_circ /P_in > 300. Table 1 summarizes the results for the different resonator modes.

Table 1 Finesse, Enhancement, Spatial Overlap and Pulse Duration for the Different Resonator Modes

	GH _0,0	GH _1,0	slit mode
	without wire		with wire
finesse F	3400	3400	3000
enhancement P _circ/P _in	1400	550	330
spatial overlap U	0.94	0.37	0.27

The power enhancement in the experiment is limited by the imperfect spatial overlap of U = 0.27 for the simple slit mode. The spatial overlap is expected to reach 0.44 for an incident Gaussian beam with an adapted q-parameter [13

J. Weitenberg, P. Rußbüldt, I. Pupeza, Th. Udem, H.-D. Hoffmann, and R. Poprawe, “Geometrical on-axis access to high-finesse resonators by quasi-imaging,” (manuscript in preparation).

]. A cylindrical telescope has to be used then for mode matching. Theoretically, the spatial overlap can reach unity if the incident field is adequately shaped. Beam shaping can e.g. be achieved with free form optics. The increase of the spatial overlap is a problem which can be solved outside the enhancement cavity.

4. Further discussion

The experiment presented here shows that it is possible to combine simultaneously resonant transverse modes in a stable degenerate cavity to form a field distribution reproducing an on-axis hole or slit at every round trip. A hole or slit is also reproduced in an imaging resonator. However, unlike imaging, quasi-imaging can be achieved in a stable resonator. In the simple case of a ring resonator with two focusing mirrors and a constant length, only one parameter, namely the distance of the focusing mirrors, has to be adjusted to achieve quasi-imaging. An imaging resonator is understood to be telecentrically imaging with magnification ± 1, because only then a field distribution can be reproduced after one resonator round trip. This can be achieved in a symmetrical confocal resonator, for which the length has to equal the radius of curvature [9

]. If one wants to choose the resonator length and the radius of curvature independently, further curved mirrors have to be added, leading to additional distances which have to be adjusted [17

J. A. Arnaud, “Degenerate optical cavities,” Appl. Opt. 8(1), 189–195 (1969). [CrossRef] [PubMed]

]. An imaging resonator is not stable in the sense that it does not possess a distinguished eigen-q-parameter. Instead, any q-parameter is reproduced after a resonator round trip. Both concepts, imaging and quasi-imaging, are promising as geometrical output coupling methods for intracavity-generated XUV radiation, offering the prospect of average and peak power scalability in enhancement cavities for HHG.

In order to achieve a geometrical on-axis access to the cavity, the obstacle can be an opening in a cavity mirror. This is preferably a curved mirror, if an access to the focus is desired. The fabrication of a standard quarter-inch-thick mirror with a hole or slit with dimensions on the order of 100 µm is challenging, nevertheless first experiments are promising. Fabrication of such mirrors is currently being pursued by direct laser drilling with a frequency doubled ns-laser and a sub-ns-laser [18

D. Esser, W. Bröring, J. Weitenberg, and H.-D. Hoffmann, “Laser-manufactured mirrors for geometrical output coupling of intracavity-generated high harmonics,” (manuscript in preparation).

] and by in-volume selective laser etching (ISLE).

The on-axis access to the cavity could be used for output coupling of high harmonics generated in a gas jet near the focus. The suitability of the field distribution for efficient high harmonic generation still has to be shown. As can be seen from Fig. 2, the field distribution changes its shape rapidly in the region around the focus. Therefore, the coherence length is somewhat reduced compared to the fundamental mode with the same Rayleigh length. The field one Rayleigh length from the focus has a central lobe on the optical axis containing 0.45 of the power. This central lobe has an elliptical shape, as can be seen in Fig. 7(b). In the focal plane the two central lobes contain 0.59 of the power. These values hold for the simple slit mode of GH _0,0 and GH _4,0 alone. Contributions of higher-order transverse modes can change the shape of the field. This can be manipulated by apertures, by the shape of the incident field and by the position in the stability range. The interaction region for HHG could be limited to one of these two regions. The lobes in the focal plane are in phase, which means that the harmonic signal will exhibit a maximum on the optical axis in the far field. In contrast, the GH _1,0 mode, as well as other GH or GL modes with vanishing intensity on the optical axis, are unfavourable for HHG, because the lobes oscillate with opposite phase and the harmonic signal vanishes on the optical axis [8

K. D. Moll, R. J. Jones, and J. Ye, “Output coupling methods for cavity-based high-harmonic generation,” Opt. Express 14(18), 8189–8197 (2006). [CrossRef] [PubMed]

The geometrical on-axis access to a high-finesse cavity could also be used for other applications besides output coupling of high harmonics, e.g. for Thomson back-scattering at electrons for the generation of coherent X-radiation [19

F. V. Hartemann, W. J. Brown, D. J. Gibson, S. G. Anderson, A. M. Tremaine, P. T. Springer, A. J. Wootton, E. P. Hartouni, and C. P. J. Barty, “High-energy scaling of Compton scattering light sources,” Phys. Rev. ST Accel. Beams 8(10), 100702 (2005). [CrossRef]

]. Here, a cavity can be used to enhance the fs-radiation which is back-scattered at a high-energy electron beam. A geometrical access could be used for output coupling of the X-radiation and/or the input of the electron beam.

The rapidly changing shape of the intensity distribution could be advantageous for other applications. E.g. by the combination of two GL modes, an intensity minimum in the focus can be formed, which is strongly localized in radial as well as in axial direction. Such a field distribution, possibly enhanced in a cavity, can serve as a dipole trap for neutral atoms [20

R. Grimm, M. Weidemüller, and Y. B. Ovchinnikov, “Optical dipole traps for neutral atoms,” Adv. At. Mol. Opt. Phys. 42, 95–170 (2000). [CrossRef]

]. Moreover, such field distributions can also be excited in an active resonator by the use of a degeneracy of transverse modes and adapted apertures.

5. Conclusion

We demonstrated quasi-imaging in a high-finesse femtosecond enhancement cavity. This is a stable cavity with a degeneracy of transverse modes and an obstacle in the beam path. The obstacle exacts a hole in the field distribution. This hole is reproduced after a resonator round trip due to the mode degeneracy. Therefore, only small loss is introduced by the obstacle. The concept allows for a geometrical on-axis access to the cavity, which could be used as a dispersion-free and peak as well as average power scalable method for output coupling of intracavity-generated high harmonics with increasing efficiency for decreasing wavelengths. In order to achieve quasi-imaging, the distance of the curved mirrors in the bow-tie ring cavity has to be adjusted with a precision of a few µm, which is only a small fraction of the stability range but still can be attained manually.

We excited a field distribution avoiding a wire on the optical axis and at a different position exhibiting an on-axis intensity maximum. Compared to the fundamental mode with F = 3400, the cavity finesse was only slightly reduced to F = 3000 by the insertion of the wire with a width equal to 5% of the Gaussian beam diameter. This corresponds to an additional round-trip loss of 2.3·10⁻⁴. The decrease of the power enhancement from P_circ /P_in = 1400 to P_circ /P_in = 330 is mainly due to the imperfect spatial overlap, which could be increased by shaping the incident beam.

Acknowledgments

This work was supported within the cooperation project KORONA between the Max-Planck-Institut für Quantenoptik (MPQ) and Fraunhofer-Institut für Lasertechnik (ILT). We thank the fiber laser group at the Institute of Applied Physics at Friedrich-Schiller-University Jena for providing the laser source for the experiments.

References and links

1.	I. Pupeza, T. Eidam, J. Rauschenberger, B. Bernhardt, A. Ozawa, E. Fill, A. Apolonski, Th. Udem, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tünnermann, T. W. Hänsch, and F. Krausz, “Power scaling of a high-repetition-rate enhancement cavity,” Opt. Lett. 35(12), 2052–2054 (2010). [CrossRef] [PubMed]
2.	A. Ozawa, J. Rauschenberger, Ch. Gohle, M. Herrmann, D. R. Walker, V. Pervak, A. Fernandez, R. Graf, A. Apolonski, R. Holzwarth, F. Krausz, T. W. Hänsch, and Th. Udem, “High harmonic frequency combs for high resolution spectroscopy,” Phys. Rev. Lett. 100(25), 253901 (2008). [CrossRef] [PubMed]
3.	I. Pupeza, T. Eidam, J. Kaster, B. Bernhardt, J. Rauschenberger, A. Ozawa, E. Fill, Th. Udem, M. F. Kling, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tünnermann, Th. W. Hänsch, and F. Krausz, “Power scaling of femtosecond enhancement cavities and high-power applications,” Proc. SPIE 7914, 79141I , 79141I-13 (2011). [CrossRef]
4.	R. J. Jones, K. D. Moll, M. J. Thorpe, and J. Ye, “Phase-coherent frequency combs in the vacuum ultraviolet via high-harmonic generation inside a femtosecond enhancement cavity,” Phys. Rev. Lett. 94(19), 193201 (2005). [CrossRef] [PubMed]
5.	Ch. Gohle, Th. Udem, M. Herrmann, J. Rauschenberger, R. Holzwarth, H. A. Schuessler, F. Krausz, and T. W. Hänsch, “A frequency comb in the extreme ultraviolet,” Nature 436(7048), 234–237 (2005). [CrossRef] [PubMed]
6.	D. C. Yost, T. R. Schibli, and J. Ye, “Efficient output coupling of intracavity high-harmonic generation,” Opt. Lett. 33(10), 1099–1101 (2008). [CrossRef] [PubMed]
7.	Y.-Y. Yang, F. Süßmann, S. Zherebtsov, I. Pupeza, J. Kaster, D. Lehr, H.-J. Fuchs, E.-B. Kley, E. Fill, X.-M. Duan, Z.-S. Zhao, F. Krausz, S. L. Stebbings, and M. F. Kling, “Optimization and characterization of a highly-efficient diffraction nanograting for MHz XUV pulses,” Opt. Express 19(3), 1954–1962 (2011). [CrossRef] [PubMed]
8.	K. D. Moll, R. J. Jones, and J. Ye, “Output coupling methods for cavity-based high-harmonic generation,” Opt. Express 14(18), 8189–8197 (2006). [CrossRef] [PubMed]
9.	W. P. Putnam, G. Abram, E. L. Falcão-Filho, J. R. Birge, and F. X. Kärtner, “High-intensity Bessel-Gauss beam enhancement cavities,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMD1 (2010).
10.	A. Ozawa, A. Vernaleken, W. Schneider, I. Gotlibovych, Th. Udem, and T. W. Hänsch, “Non-collinear high harmonic generation: a promising outcoupling method for cavity-assisted XUV generation,” Opt. Express 16(9), 6233–6239 (2008). [CrossRef] [PubMed]
11.	S. Gigan, L. Lopez, N. Treps, A. Maître, and C. Fabre, “Image transmission through a stable paraxial cavity,” Phys. Rev. A 72(2), 023804 (2005). [CrossRef]
12.	R. Paschotta, “Beam quality deterioration of lasers caused by intracavity beam distortions,” Opt. Express 14(13), 6069–6074 (2006). [CrossRef] [PubMed]
13.	J. Weitenberg, P. Rußbüldt, I. Pupeza, Th. Udem, H.-D. Hoffmann, and R. Poprawe, “Geometrical on-axis access to high-finesse resonators by quasi-imaging,” (manuscript in preparation).
14.	T. Eidam, F. Röser, O. Schmidt, J. Limpert, and A. Tünnermann, “57 W, 27 fs pulses from a fiber laser system using nonlinear compression,” Appl. Phys. B 92(1), 9–12 (2008). [CrossRef]
15.	Th. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35(3), 441–444 (1980). [CrossRef]
16.	T. C. Briles, D. C. Yost, A. Cingöz, J. Ye, and T. R. Schibli, “Simple piezoelectric-actuated mirror with 180 kHz servo bandwidth,” Opt. Express 18(10), 9739–9746 (2010). [CrossRef] [PubMed]
17.	J. A. Arnaud, “Degenerate optical cavities,” Appl. Opt. 8(1), 189–195 (1969). [CrossRef] [PubMed]
18.	D. Esser, W. Bröring, J. Weitenberg, and H.-D. Hoffmann, “Laser-manufactured mirrors for geometrical output coupling of intracavity-generated high harmonics,” (manuscript in preparation).
19.	F. V. Hartemann, W. J. Brown, D. J. Gibson, S. G. Anderson, A. M. Tremaine, P. T. Springer, A. J. Wootton, E. P. Hartouni, and C. P. J. Barty, “High-energy scaling of Compton scattering light sources,” Phys. Rev. ST Accel. Beams 8(10), 100702 (2005). [CrossRef]
20.	R. Grimm, M. Weidemüller, and Y. B. Ovchinnikov, “Optical dipole traps for neutral atoms,” Adv. At. Mol. Opt. Phys. 42, 95–170 (2000). [CrossRef]

OCIS Codes
(140.7240) Lasers and laser optics : UV, EUV, and X-ray lasers
(190.4160) Nonlinear optics : Multiharmonic generation
(320.7110) Ultrafast optics : Ultrafast nonlinear optics
(070.5753) Fourier optics and signal processing : Resonators

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 4, 2011
Revised Manuscript: April 27, 2011
Manuscript Accepted: April 27, 2011
Published: May 2, 2011

Citation
Johannes Weitenberg, Peter Rußbüldt, Tino Eidam, and Ioachim Pupeza, "Transverse mode tailoring in a quasi-imaging high-finesse femtosecond enhancement cavity," Opt. Express 19, 9551-9561 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-9551

Sort: Author | Year | Journal | Reset

References

I. Pupeza, T. Eidam, J. Rauschenberger, B. Bernhardt, A. Ozawa, E. Fill, A. Apolonski, Th. Udem, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tünnermann, T. W. Hänsch, and F. Krausz, “Power scaling of a high-repetition-rate enhancement cavity,” Opt. Lett. 35(12), 2052–2054 (2010). [CrossRef] [PubMed]
A. Ozawa, J. Rauschenberger, Ch. Gohle, M. Herrmann, D. R. Walker, V. Pervak, A. Fernandez, R. Graf, A. Apolonski, R. Holzwarth, F. Krausz, T. W. Hänsch, and Th. Udem, “High harmonic frequency combs for high resolution spectroscopy,” Phys. Rev. Lett. 100(25), 253901 (2008). [CrossRef] [PubMed]
I. Pupeza, T. Eidam, J. Kaster, B. Bernhardt, J. Rauschenberger, A. Ozawa, E. Fill, Th. Udem, M. F. Kling, J. Limpert, Z. A. Alahmed, A. M. Azzeer, A. Tünnermann, Th. W. Hänsch, and F. Krausz, “Power scaling of femtosecond enhancement cavities and high-power applications,” Proc. SPIE 7914, 79141I, 79141I-13 (2011). [CrossRef]
R. J. Jones, K. D. Moll, M. J. Thorpe, and J. Ye, “Phase-coherent frequency combs in the vacuum ultraviolet via high-harmonic generation inside a femtosecond enhancement cavity,” Phys. Rev. Lett. 94(19), 193201 (2005). [CrossRef] [PubMed]
Ch. Gohle, Th. Udem, M. Herrmann, J. Rauschenberger, R. Holzwarth, H. A. Schuessler, F. Krausz, and T. W. Hänsch, “A frequency comb in the extreme ultraviolet,” Nature 436(7048), 234–237 (2005). [CrossRef] [PubMed]
D. C. Yost, T. R. Schibli, and J. Ye, “Efficient output coupling of intracavity high-harmonic generation,” Opt. Lett. 33(10), 1099–1101 (2008). [CrossRef] [PubMed]
Y.-Y. Yang, F. Süßmann, S. Zherebtsov, I. Pupeza, J. Kaster, D. Lehr, H.-J. Fuchs, E.-B. Kley, E. Fill, X.-M. Duan, Z.-S. Zhao, F. Krausz, S. L. Stebbings, and M. F. Kling, “Optimization and characterization of a highly-efficient diffraction nanograting for MHz XUV pulses,” Opt. Express 19(3), 1954–1962 (2011). [CrossRef] [PubMed]
K. D. Moll, R. J. Jones, and J. Ye, “Output coupling methods for cavity-based high-harmonic generation,” Opt. Express 14(18), 8189–8197 (2006). [CrossRef] [PubMed]
W. P. Putnam, G. Abram, E. L. Falcão-Filho, J. R. Birge, and F. X. Kärtner, “High-intensity Bessel-Gauss beam enhancement cavities,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper CMD1 (2010).
A. Ozawa, A. Vernaleken, W. Schneider, I. Gotlibovych, Th. Udem, and T. W. Hänsch, “Non-collinear high harmonic generation: a promising outcoupling method for cavity-assisted XUV generation,” Opt. Express 16(9), 6233–6239 (2008). [CrossRef] [PubMed]
S. Gigan, L. Lopez, N. Treps, A. Maître, and C. Fabre, “Image transmission through a stable paraxial cavity,” Phys. Rev. A 72(2), 023804 (2005). [CrossRef]
R. Paschotta, “Beam quality deterioration of lasers caused by intracavity beam distortions,” Opt. Express 14(13), 6069–6074 (2006). [CrossRef] [PubMed]
J. Weitenberg, P. Rußbüldt, I. Pupeza, Th. Udem, H.-D. Hoffmann, and R. Poprawe, “Geometrical on-axis access to high-finesse resonators by quasi-imaging,” (manuscript in preparation).
T. Eidam, F. Röser, O. Schmidt, J. Limpert, and A. Tünnermann, “57 W, 27 fs pulses from a fiber laser system using nonlinear compression,” Appl. Phys. B 92(1), 9–12 (2008). [CrossRef]
Th. W. Hänsch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35(3), 441–444 (1980). [CrossRef]
T. C. Briles, D. C. Yost, A. Cingöz, J. Ye, and T. R. Schibli, “Simple piezoelectric-actuated mirror with 180 kHz servo bandwidth,” Opt. Express 18(10), 9739–9746 (2010). [CrossRef] [PubMed]
J. A. Arnaud, “Degenerate optical cavities,” Appl. Opt. 8(1), 189–195 (1969). [CrossRef] [PubMed]
D. Esser, W. Bröring, J. Weitenberg, and H.-D. Hoffmann, “Laser-manufactured mirrors for geometrical output coupling of intracavity-generated high harmonics,” (manuscript in preparation).
F. V. Hartemann, W. J. Brown, D. J. Gibson, S. G. Anderson, A. M. Tremaine, P. T. Springer, A. J. Wootton, E. P. Hartouni, and C. P. J. Barty, “High-energy scaling of Compton scattering light sources,” Phys. Rev. ST Accel. Beams 8(10), 100702 (2005). [CrossRef]
R. Grimm, M. Weidemüller, and Y. B. Ovchinnikov, “Optical dipole traps for neutral atoms,” Adv. At. Mol. Opt. Phys. 42, 95–170 (2000). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Click here to see a list of articles that cite this paper

Figures


Fig. 1	Fig. 2	Fig. 3


Fig. 4	Fig. 5	Fig. 6


Fig. 7	Fig. 8

« Previous Article | Next Article »

OSA is a member of CrossRef.

OpticsInfoBase

Optics Express

Transverse mode tailoring in a quasi-imaging high-finesse femtosecond enhancement cavity

Article Tools