We demonstrate a new and extremely compact design for a directly diode-pumped Yb:glass laser oscillator that is used as femtosecond light source for supercontinuum generation. The laser is capable of generating femtosecond pulses of 150 fs and pulse energies up to 39 nJ at a repetition rate of 20 MHz. By using a Herriott-type multi-pass cell, 70 % of the total resonator length are folded to only 30 cm. With off-the-shelf components, our setup has a footprint of 62 × 23 cm2. Using smaller mechanical components, the size can easily be further decreased. In combination with a tapered fiber, the laser forms a cheap, stable, and compact femtosecond supercontinuum source with up to 400 mW of average whitelight power.
©2006 Optical Society of America
Broadband laser sources have become an important tool for applications in metrology and spectroscopy in various scientific areas, for example as light source for multi-photon microscopy  or pump-probe-experiments . They can cover the whole spectral range from the near-ultraviolet to the near-infrared region  and can be pumped with nano- to femtosecond pulses to produce directed beams that can be focused to a diffraction limited spot or be as well collimated like conventional laser beams. Such sources can replace single-wavelength lasers, provide access to wavelength regions that are hard to reach with other laser sources, or they might even enable possibilities that conventional whitelight sources cannot, such as coherent two-color CARS-spectroscopy .
Current supercontinuum laser sources usually consist of a combination of a suitable pump laser and a microstructured fiber that could be either a photonic crystal fiber (PCF)  or a tapered fiber [6, 7]. Depending on the chosen fiber parameters such as length or fiber cross section, one can use a wide range of different pulsed lasers as pump sources. Our goal was to design a compact and cheap directly-diode pumped femtosecond laser oscillator with peak intensities that are high enough for supercontinuum generation with a wide variety of different tapered fibers. The whole system should have the potential to work as a portable whitelight source.
Supercontinuum generation by pumping tapered fibers with femtosecond lasers was already successfully shown with 80 fs pulses from a Ti:sapphire laser  as well as with 200 fs pulses from an Yb:glass laser [8, 9]. Since the supercontinua generated from tapered fibers depend on the energy as well as on the width of the input pulses, our aim was to be build a versatile pump laser that was able to provide pulse widths shorter than 200 fs and pulse energies of some tens of nanojoules.
The laser material Yb:glass is well suited to build cheap and compact directly diode-pumped femtosecond lasers. It is isotropic and thus all parameters are polarization independent. This allows to use unpolarized fiber coupled laser diodes as pump sources that have a better beam quality than laser diode bars. By this way it is possible to achieve high output powers with simple pump optics and moderate pump powers. Furthermore, Ytterbium doped laser materials are very efficient and show a low quantum defect because of their quasi-3-level nature. This allows to cool the laser medium with a small Peltier cooler instead of water cooling. Nonetheless, Yb:glass shows a relatively broad emission spectrum that allows the generation of ultrashort laser pulses shorter than 60 fs .
As mode-locking regime, we chose solitary mode-locking [11, 12] which is stabilized by a semiconductor saturable absorber mirror (SESAM) [13, 14, 15]. To use as few components as possible, the intra-cavity dispersion and self-phase-modulation (SPM) are compensated by dispersive mirrors, and the laser resonator is folded by using a Herriott-type multi-pass cell  to fit onto a very small footprint.
2. Laser design
The laser setup (Fig. 1) consisted of a z-folded cavity with a total resonator length of 7.5 m which led to a pulse repetition rate of 20 MHz. The laser medium was placed between the two curved mirrors M7 and M9. Both mirrors had a focal length of 75 mm and focused the laser beam into the crystal with a theoretical mode radius of approximately 53 μm. The pump beam was collimated by an achromatic lens L1 with 30 mm focal length and re-focused into the laser medium with the achromatic lens L2 with a focal length of 75 mm. The pump beam was injected into the laser resonator through a dichroic mirror M10 and formed a pump spot with an estimated mode radius of 51 μm. As laser medium, we used a piece of 9.5 % Ytterbium-doped phosphate glass (QX-glass, Kigre Inc.) with dimensions 5 × 2 × 4 mm3. The optical path length in the laser medium was 4 mm at normally incident laser beam. The front surfaces were antireflection coated for the pump wavelength 980 nm and the laser wavelength 1040 nm. We used this combination because it yielded the lowest reflection losses for the unpolarized pump-light. The laser medium was pumped by a fiber coupled laser diode (Jenoptik Unique Mode GmbH) that provided 5.2 W power at 976 nm from a multimode fiber with 50 μm core diameter and an NA of 0.15 with an excellent beam quality. The laser resonator was extended by a Herriott-type multi-pass cell (M4, M5) and two curved mirrors M1 and M6 with a focal length of 250 mm and 1000 mm, respectively. Mirror M1 focused the laser beam onto the SESAM with a spot diameter of approximately 440 μm. Mirror M3 was a flat dichroic mirror to filter out remaining pump light and prevent it from saturating the SESAM. Mirrors M2 and M8 were flat dispersive mirrors to compensate the intra-cavity dispersion and SPM. As output coupler (OC), we used a flat mirror with a reflectivity of 95 %.
A Herriott-type multi-pass cell consists of two mirrors M4 and M5 and two small flat mirrors inbetween M4 and M5 by which the beam can enter and exit the multi-pass cell. The laser beam bounces back and forth several times between M4 and M5 to achieve a long beam path.
Therefore the radius of curvature of M4 and M5 and the distance between them must be chosen in such a way that they forme a stable resonator. That requires that the complex q-parameter of a Gaussian beam is preserved after one roundtrip in the multi-pass cell and results in beam paths that form elliptical traces on M4 and M5 . The criteria to design such Herriott-type multi-pass cells are well described in . As multi-pass cell, we chose a confocal cavity with mirrors M4 and M5, both curved with the same focal length of 2500 mm and thus the same radius of curvature R = 5000 mm. The beam path was intended to form a circle with nine bounces on each mirror. For such a confocal cavity the necessary distance L between M4 and M5 was calculated by a formula that can be derived from the description in :
R denotes the radius of curvature of the two mirrors, n is the total number of bounces on one mirror, and the ratio . π describes the angle between two consecutive bounces. The combination m=2 and n=9 leads to traces on the mirrors that form a whole circle with nine bounces and one obtains a necessary distance of L = 301.5 mm between the mirrors M4 and M5.
To achieve mode-locking, a SESAM [13, 14, 15] introduced saturable absorption into the laser resonator. We used a SESAM from Batop GmbH with a modulation depth of ΔR = 1.6 % and a saturation fluence of F = 70 μJ/cm 2. The laser mode diameter on the SESAM was approximately 440 μm. To operate the laser in the solitary mode-locking regime [11, 12], anomalous dispersion was introduced into the cavity by two dispersive mirrors M2 and M8. Each reflection on one of these mirrors provided -900 fs2 ± 100 fs2 dispersion. The group-delay dispersion could be fine-tuned by inserting uncoated glass plates of N-SF57 into the beam path at Brewster angle. Each plate was 1 mm thick and provided approximately +80 fs2 of normal dispersion. Mode-locking with pulse widths of 180 fs or 150 fs was achieved with one or three plates in the cavity, resulting in a net group-delay dispersion per round-trip of D 2 = -3440 fs2 or D 2 = -3120 fs2, respectively.
The laser wavelength could be tuned by inserting a 1 mm thick crystalline quartz plate at Brewster angle which acted as birefringent filter . For pulses with 180 fs pulse width, the center wavelength was slightly tunable between 1038 nm and 1047 nm. For shorter pulses with 150 fs pulse width, the center wavelength at stable mode-locked operation was fixed, either around 1040 nm or 1045 nm, depending on the position of the birefringent filter. In that case the birefringent filter helped to support stable mode-locking.
3. Experimental study of the laser parameters
The shortest pulses and highest output power were achieved with a 5 % output coupler and a SESAM modulation depth of ΔR = 1.6 %. Shorter pulses might be possible with a SESAM with higher modulation depth, but this would decrease the output power. The SESAM was strongly saturated with a fluence of 440–520 μJ/cm2, but no double-pulsing or cw-breakthroughs could be observed.
Fig. 2 shows the laser output power in dependence on the pump power for the laser configuration that supported pulses with 150 fs pulse duration. Mode-locking set in at a pump power of approximately 4.5 W as indicated by the step of the linear curve and the different slope efficiencies. The laser was self-starting and showed pure single-pulse mode-lockig for pump powers between 4.5 W to 5.2 W. Below 4.5 W pump power, the laser operated in the cw regime. In the mode-locked regime, a stable 20 MHz pulse train could be observed.
The width of the generated pulses could be adjusted by changing the net group-delay dispersion per round-trip by inserting or removing one or more of the N-SF57 glass plates. With a net group-delay dispersion of D2=-3440 fs2, pulses with 180 fs pulse width at an average power of 790 mW were generated. By reducing the net group-delay dispersion to D2=-3120 fs2, the pulse width decreased to 150 fs at an average power of 665 mW. Fig. 3(a) shows an autocorrelation measurement of pulses with approximately 150 fs length. The pulses were nearly transform limited and unchirped as one can conclude from an interferometric autocorrelation measurement as depicted in the inset of Fig. 3(a). The corresponding optical spectrum of the laser output showed a bandwidth of approximately 8 nm (Fig. 3(b)).
4. Use as pump source for supercontinuum generation
To demonstrate the capabilities of the system, the laser oscillator was used for supercontinuum generation with three different kinds of tapered fibers.
When choosing appropriate laser parameters for supercontinuum generation, one has to consider that with equally high input power into a tapered fiber, shorter laser pulses produce broader spectra, because non-linear effects such as self-phase-modulation are stronger with higher peak intensities. However, the laser average power is higher with longer pulses, and thus longer laser pulses result in a higher average whitelight power.
For the following measurements, we chose a laser operation mode with two intra-cavity glass plates, resulting in 160 fs pulses at approximately 700 mW laser power. We chose this combination since our pump diode showed a temperature problem which resulted in a slow decay of laser power over several hours. This configuration allowed to assure a minimum laser power of 600 mW over many hours of operation.
To protect the laser against backreflections from the fiber surface, a Faraday isolator (EOT Inc.) was placed between the laser output coupler and the fiber coupling optics.
Supercontinua were generated with three different tapered fibers. All fibers had a waist length of 90 mm but varied in waist diameter. Fig. 4(a) shows the power dependent spectra of the supercontinuum generated with a tapered fiber with 2.7 μm waist diameter. With approximately 600 mW average laser power coupled into the fiber, a supercontinuum average power of 290 mW was achieved. The spectrum stretched from 400 nm to 1650 nm. A more red-shifted spectrum was obtained by using a thicker taper waist of 4.3 μm (Fig. 4(b)). The average output power was 400 mW in that case. The obtained spectrum is relatively smooth and could be reshaped into a completely flat spectrum by discarding less than half of the average power. Shorter wavelengths reaching into the near ultraviolet region were obtained by using fibers with very small diameters. With a fiber-waist diameter of 2.0 μm we achieved a spectrum with 320 mW average power and a significant fraction of light in the blue to ultraviolet region (Fig. 4(c)).
The stability of the generated supercontinua was characterized by measuring the intensity noise of the total whitelight power and the noise of a small spectral section. Over a duration of 20 minutes, the noise of the pump laser accounted to 0.43 % rms which led to a noise of the the total average whitelight power of 0.79 % rms. To examine the noise of a small spectral section, a region at 633 nm center wavelength was cut out of the supercontinuum generated by a fiber with 4.0 μm waist diameter. The average power of this 15 nm broad part of the supercontinuum showed an intensity noise of 2.34 % rms.
To further investigate the possibilities to shape the supercontinua by changing the pump laser parameters, the influence of the chirp of the input pulses on the spectra was investigated. For efficient supercontinuum generation with femtosecond pulses in tapered fibers, it is necessary that the fiber shows anomalous dispersion. That allows the laser pulses to propagate through the fiber as solitons, and high peak powers remain along the whole waist length. When entering the taper waist, normally chirped input pulses are initially compressed until they finally form a soliton. Therefore a pulse that shows a flat phase when entering the taper waist might experience a “longer” effective waist length. The chirp of the input laser pulses was changed by bouncing the laser beam off dispersive mirrors before coupling it into the fiber. One bounce on a dispersive mirror provided -1300±150 fs2 of anomalous dispersion. Before the laser beam was then coupled into the fiber, it passed the Faraday isolator, the lenses of the focusing optics and a fiber pigtail of approximately 14 cm in front of the taper waist. All these elements accounted for a normal pulse chirp. The number of bounces on the dispersive mirrors was varied between none and ten, resulting in 0 fs2 to -13000 fs2 dispersion. Fig. 5(a) shows the results for a fiber with 2.0 μm waist diameter, and Fig. 5(b) shows the same for 4.3 μm waist diameter. The output powers were held constant at 280 mW and 325 mW, respectively. One recognizes, that the broadest spectra were achieved by providing input pulses with a slight anomalous chirp of -2600 fs2 in case (a) and -1300 fs2 in case (b). However, pulse pre-chirp showed little influence on the smoothness of the spectra. This concurs with similar results that could be found theoretically as well as experimentally for supercontinuum generation with pulses from a Ti:sapphire laser in . In our case the influence of a normal pre-chirp due to a long fiber pigtail seems to be less significant than in  since in contrast to Ti:sapphire wavelengths around 800 nm, tapered fibers show stronger anomalous dispersion at the taper waist and less normal dispersion at the fiber pigtail for 1040 nm.
We presented a new and very compact design for a directly-diode pumped Yb:glass laser oscillator. With standard components the laser setup fits onto an area of 62 × 23 cm2. With smaller mechanical components, the footprint of the laser can easily be decreased. The laser is capable of producing femtosecond pulses with pulse energies up to 39 nJ at a repetition-rate of 20 MHz. With 5.2 W pump power we achieved femtosecond pulses with 150 fs at an average power of 665 mW, or pulse widths of 180 fs at an average power of 790 mW. The laser is self-starting and very stable due to the few elements and the properties of the used Herriott-multi-pass cell. To show the applicability of the laser as a suitable pump source for microstructured fibers, successful supercontinuum generation was shown with three different kinds of tapered fibers. The generated supercontinua were very stable and showed an intensity noise of less than 1% rms for the total average whitelight power and less than 2.5% rms for a 15 nm broad spectral section. Furthermore it could be shown that only a slight pulse pre-chirp is necessary to exploit the broadest supercontinua from a tapered fiber. Since this setup can easily be realized with dispersive mirrors, the system has the potential to form a very compact and portable femtosecond supercontinuum laser source.
The authors would like to thank Dr. Jürgen Kuhl for his support and Dr. Ursula Keller for invaluable advice and support with SESAMs. We also would like to thank Dr. Alexander Killi and Dr. Uwe Morgner for many helpful discussions. Furthermore we thank the DFG (FOR557, SPP1113, FOR730) and BMBF (13N8340/1).
References and links
1. T. Betz, J. Teipel, D. Koch, W. Härtig, J. Guck, J. Käs, and H. Giessen, “Excitation beyond the monochromatic laser limit: simultaneous 3-D confocal and multiphoton microscopy with a tapered fiber as white-light laser source,” J. Biomed. Optics 10, 054009 (2005). [CrossRef]
2. M. Punke, F. Hoos, C. Karnutsch, U. Lemmer, N. Linder, and K. Streubel, “High-repetition-rate white-light pump-probe spectroscopy with a tapered fiber,” Opt. Lett. 31, 1157–1159 (2006). [CrossRef] [PubMed]
3. F.G. Omenetto, N.A. Wolchover, M.R. Wehner, M. Ross, A. Efimov, A.J. Taylor, V.V.R.K. Kumar, A.K. George, J.C. Knight, N.Y. Joly, and P.St. Russell, “Spectrally smooth supercontinuum from 350 nm to 3 μm in sub-centimeter lengths of soft-glass photonic crystal fibers,” Opt. Express 14, 4928–4934 (2006). [CrossRef] [PubMed]
4. H. Kano and H. Hamaguchi, “Vibrationally resonant imaging of a single living cell by supercontinuum-based multiplex coherent anit-Stokes Raman scattering microscopy,” Opt. Express 13, 1322–1327 (2005). [CrossRef] [PubMed]
5. W.J. Wadsworth, N. Joly, J.C. Knight, T.A. Birks, F. Biancalana, and P.St.J. Russell, “Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibres,” Opt. Express 12, 299–309 (2006). [CrossRef]
6. D.A. Akimov, A.A. Ivanov, M.V. Alfimov, S.N. Bagayev, T.A. Birks, W.J. Wadsworth, P.St.J. Russell, A.B. Fedotov, V.S. Pivtsov, A.A. Podhivalov, and A.M. Zheltikov, “Two-octave spectral broadening of subnanojoule Cr:forsterite femtosecond laser pulses in tapered fibers,” Appl. Phys. B 74, 307–311 (2002). [CrossRef]
7. J. Teipel, K. Franke, D. Türke, F. Warken, D. Meiser, M. Leuschner, and H. Giessen, “Characteristics of super-continuum generation in tapered fibers using femtosecond laser pulses,” Appl. Phys. B 77, 245–251 (2003). [CrossRef]
8. J. Teipel, D. Türke, H. Giessen, A. Killi, U. Morgner, M. Lederer, D. Kopf, and M. Kolesik, “Diode-pumped, ultrafast, multi-octave supercontinuum source at repetition rates between 500 kHz and 20 MHz using Yb:glass lasers and tapered fibers,” Opt. Express 13, 1477–1485 (2005). [CrossRef] [PubMed]
9. A. Killi, U. Morgner, M.J. Lederer, and D. Kopf,“Diode-pumped femtosecond laser oscillator with cavity dumping,” Opt. Lett. 29, 1288–1290 (2005). [CrossRef]
10. C. Hönninger, F. Morier-Genoud, M. Moser, and U. Keller, “Efficient and tunable diode-pumped femtosecond Yb:glass lasers,” Opt. Lett. 23, 126–128 (1998). [CrossRef]
11. F.X. Kärtner, I.D. Jung, and U. Keller, “Soliton Mode-Locking with Saturable Absorbers,” IEEE J. Sel. Top. Quantum Electron. 2, 540–556 (1996). [CrossRef]
12. F.X. Kärtner, J. aus der Au, and U. Keller, “Mode-Locking with Slow and Fast Saturable Absorbers - What’s the Difference?,” IEEE J. Sel. Top. Quantum Electron. 4, 159–168 (1998). [CrossRef]
14. U. Keller, D.A.B. Miller, G.D. Boyd, T.H. Chiu, J.F. Ferguson, and M.T. Asom, “Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor Fabry-Perot saturable absorber,” Opt. Lett. 17, 505–507 (1991). [CrossRef]
15. U. Keller, K.J. Weingarten, F.X. Kärtner, D. Kopf, B. Braun, I.D. Jung, R. Fluck, C. Hönninger, N. Matuschek, and J. aus der Au, “Semiconductor Saturable Absorber Mirrors (SESAM’s) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers,” IEEE J. Sel. Top. Quantum Electron. 2, 435–451 (1996). [CrossRef]
16. D.R. Herriott and H.J. Schulte, “Folded Optical Delay Lines,” Appl. Opt. 4, 883–889 (1965). [CrossRef]
18. K. Naganuma, G. Lenz, and E.P. Ippen, “Variable Bandwidth Birefringent Filter for Tunable Femtosecond Lasers,” IEEE J. Quantum Electron. 28, 2141–2150 (1992). [CrossRef]
19. D. Türke, W. Wohlleben, J. Teipel, M. Motzkus, B. Kibler, J. Dudley, and H. Giessen, “Chirp-controlled soliton fission in tapered optical fibers,” Appl. Phys. B 83, 37–42 (2006). [CrossRef]