Femtosecond passively modelocked diode laser with intracavity ...

Femtosecond passively modelocked diode laser with intracavity dispersion management Tobias Schlauch,1,* Jan C. Balzer,1 Andreas Klehr,2Götz Erbert,2 Günther Tränkle,2 and Martin R. Hofmann1 1

Chair for Photonics and Terahertz Technology, Building IC 6/133, D-44780 Bochum, Germany 2 Ferdidand Braun Institute, Gustav-Kirchhoff-Str. 4, D-12489 Berlin, Germany *[email protected]

Abstract: We report on the generation of ultrashort pulses by dispersion management of a passively modelocked external cavity diode laser. Pulse widths down to 200 fs are obtained at 830nm emission wavelength. We use intracavity dispersion management to increase the spectral bandwidth and compress the strongly chirped pulses externally with a grating compressor. ©2010 Optical Society of America OCIS codes: (140.2020) Diode lasers; (320.2250) Femtosecond phenomena; (320.7100) Ultrafast measurements; (320.7120) Ultrafast phenomena.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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Received 24 Aug 2010; revised 16 Sep 2010; accepted 13 Oct 2010; published 5 Nov 2010

8 November 2010 / Vol. 18, No. 23 / OPTICS EXPRESS 24316

18. K. Kim, S. Lee, and P. J. Delfyett, “1.4kW high peak power generation from an all semiconductor mode-locked master oscillator power amplifier system based on eXtreme Chirped Pulse Amplification(X-CPA),” Opt. Express 13(12), 4600–4606 (2005). 19. T. Schlauch, M. Li, M. R. Hofmann, A. Klehr, G. Erbert, and G. Tränkle, “High Peak Power Femtosecond Pulses From Modelocked Semiconductor Laser in External Cavity,” Electron. Lett. 44(11), 678–679 (2008). 20. C. Jördens, T. Schlauch, M. Li, M. R. Hofmann, M. Bieler, and M. Koch, “All-Semiconductor Laser Driven Terahertz Time-Domain Spectrometer,” Appl. Phys. B 93(2-3), 515–520 (2008). 21. A. Azouz, N. Stelmakh, and J. M. Lourtioz, “Passive Modelocking Of Semiconductor Lasers With Tunable Group Velocity Dispersion Cavity,” Electron. Lett. 29(16), 1437–1438 (1993). 22. M. Breede, S. Hoffmann, J. Zimmermann, J. Struckmeier, M. Hofmann, T. Kleine-Ostmann, P. Knobloch, M. Koch, and J. P. Meyn, “Fourier-transform external cavity lasers,” Opt. Commun. 207(1-6), 261–271 (2002). 23. S. Backus, C. G. Durfee, M. M. Murnane, and H. C. Kapteyn, “High Power Ultrafast Lasers,” Rev. Sci. Instrum. 69(3), 1207–1223 (1998). 24. O. E. Martinez, “3000 Times Grating Compressor With Positive Group Velocity Dispersion: Application To Fiber Compensation In 1.3-1.6 μm Region,” IEEE J. Quantum Electron. 23(1), 59–64 (1987). 25. J.-C. Diels, and W. Rudolph, Ultrashort Laser Pulse Phenomena, (Academic, Amsterdam, 2006). 26. S. Akturk, M. Kimmel, P. O’Shea, and R. Trebino, “Measuring spatial chirp in ultrashort pulses using single-shot Frequency-Resolved Optical Gating,” Opt. Express 11(1), 68–78 (2003). 27. A. Kwok, L. Jusinski, M. A. Krumbiigel, J. N. Sweetser, D. N. Fittinghoff, and R. Trebino, “Frequency-resolved optical gating using cascaded second-order nonlinearities,” IEEE J. Sel. Top. Quantum Electron. 4(2), 271–277 (1998). 28. H. A. Haus, “Theory Of Modelocking Of A Laser Diode In An External Resonator,” Appl. Phys. (Berl.) 51, 4042–4049 (1980). 29. H. A. Haus, and Y. Silberberg, “Theory of mode locking of a laser diode with a multiple-quantum-well structure,” J. Opt. Soc. Am. B 2(7), 1237–1243 (1985). 30. R. Trebino, Frequency-Resolved Optical Gating: The Measurement of Ultrashort Laser Pulses, (Kluwer Academic, Norwell, Massachusetts, 2000). 31. P. Klopp, U. Griebner, M. Zorn, A. Klehr, A. Liero, M. Weyers, and G. Erbert, “Mode-locked InGaAs-AlGaAs disk laser generating sub-200-fs pulses, pulse picking and amplification by a tapered diode amplifier,” Opt. Express 17(13), 10820–10834 (2009). 32. E. P. Ippen, “Principles Of Passive Mode Locking,” Appl. Phys. B 58(3), 159–170 (1994).

1. Introduction Ultrashort laser pulses have a huge application potential including material processing, communication technology, biomedical imaging and ultrafast spectroscopy in biology, physics and chemistry. For the generation of femtosecond laser pulses modelocking is the method of choice [1]. That holds for the well established femtosecond Ti:sapphire lasers as well as for semiconductor lasers. Ti:sapphire lasers emit light pulses with durations of only some fs with an average optical power of some W but they are extremely expensive and rather complex. Consequently, there has been a lot of research towards more compact and cost effective alternative fs laser sources as, for example, fiber lasers [2], Cr:LiSAF lasers [3], or semiconductor based lasers. Optically pumped vertical external-cavity surface-emitting lasers (VECSELs) have also been shown to be attractive compact sources of picosecond pulses with average power above 2 W [4] or even of femtosecond pulses with pulse widths down to 60 fs [5] and peak powers up to 315 W [6]. However, among those cost effective alternatives, diode lasers might be the most promising because they can be pumped directly with an electrical current while all other concepts are optically pumped and thus intrinsically more complex. During the last decades a lot of activity has taken place on investigation and development of semiconductor diode laser short-pulse systems [7–10]. Recently, particular progress has been made with modelocked edge emitting diode lasers based on quantum dots providing subpicosecond pulse widths at repetition frequencies in the GHz range [11,12]. However, usually two problems complicate the realization of specifications competitive with those of Ti:sapphire lasers. First, the average power emitted out of a diode laser system is usually only in the low mW range. This introduces the need for further amplification by, for example, semiconductor tapered amplifiers [13]. Second, sub-picosecond pulse durations are difficult to generate in practice. This is due to the considerable chirp of the pulses as a consequence of the strong coupling of real and imaginary part of the susceptibility in the semiconductor [14–16]. This coupling is reduced in quantum dot lasers so that the direct generation of weakly chirped sub-picosecond pulses becomes possible [11]. With standard diode lasers, sub-picosecond pulse widths could in most cases only be realized by external pulse compression to reduce the

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chirp [8,17]. A combination of amplification and compression provides the required increase of the optical power and a reduction of the chirp. With this concept, all semiconductor laser systems could be realized that provide peak powers of 1.4 kW with a colliding pulse modelocked semiconductor laser [18] or of 2.5 kW with a passively mode-locked semiconductor laser [19]. Though such systems are already attractive compact alternatives to the standard Ti:sapphire lasers for applications like THz time domain sampling [20], the full potential of semiconductor lasers for ultrashort pulse generation has not yet been exploited. In particular, the gain bandwidth of semiconductor lasers would allow for the generation of sub-100 fs pulse widths but pulse widths in that range have not yet been reported by an electrically pumped semiconductor laser. Previous studies show that intracavity dispersion management enables control of the pulse characteristics of a diode laser system [21]. In this paper we investigate in detail the interplay between intracavity dispersion management with external pulse compression for a passively mode-locked semiconductor laser system and its influence on the pulse characteristics like the chirp and the resulting spectral bandwidth and pulse duration. By an extensive analysis including second harmonic generation (SHG) frequency resolved optical gating (FROG) measurements, we show how a considerable improvement of spectral width and temporal duration can be achieved. We use a Fourier transform external cavity laser (FTECAL) geometry [22] which, in contrast to standard external cavity geometries, enables easy straightforward geometrical influence on the intracavity dispersion [21]. 2. Laser setup The complete system is shown in Fig. 1. It consists of the FTECAL resonator on the left side and an external pulse compressor on the right side. The FTECAL resonator contains the edge emitting laser diode separated into two parts, i.e. the gain section and the saturable absorber section. The laser structure used is based on an InGaAsP double quantum well (DQW) active region embedded in a super large AlGaAs optical waveguide layer of 3.4 µm to realize a narrow vertical divergence of 20 degree to reach a good collimation of the output beam. An antireflection coating is placed on the facet facing the external cavity. The total length of the laser diode is 1200 μm with an absorber length of 80 μm. The latter is operated with a reverse bias in order to realize passive mode-locking. Furthermore the FTECAL-setup consists of a collimator (not shown in Fig. 1), a diffraction grating, a Fourier lens and an end mirror (the second end mirror is given by the second side of the laser diode with a reflectivity of 95% achieved by application of a high reflectivity coating). The collimated light of the laser diode is split into its spectral components via first-order diffraction by the grating. The Fourier lens (f = 150mm) focuses the individual spectral components onto the focal plane in which the end mirror is placed to reflect the components back in themselves. The end mirror and the Fourier lens are mounted on translation stages in order to enable changes in the distances of the components inside the cavity. The 0th order reflection of the beam from the laser diode at the grating is coupled out and directed to the pulse compressor. The laser output can be analyzed before and after the compressor with an autocorrelator with additional SHG-FROG-option (APE Pulse Check) and an optical spectrum analyzer (OSA).

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Fig. 1. Experimental setup with oscillator and external pulse compressor.

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The external pulse compressor has a compact design. It also consists of a diffraction grating, a lens (f = 150mm) and an end mirror. An optical isolator enables coupling out of the light without backreflections into the laser. A variation of the distances between grating, lens, and end mirror introduces a group delay dispersion (GDD) that permits the compensation of linear chirp [19]. We keep the distance between lens and end mirror fixed to the focal length of the lens and move both with respect to the grating. By increasing the distance, up-chirp can be compensated. In contrast, down-chirp is reduced by shifting lens and end mirror nearer to the grating. The setup is similar to a so often called pulse stretcher [23] or grating compressor [24,25]. It should be mentioned that this compact compressor might induce spatial chirp, i.e. the wavelengths might vary spatially across the beam, such that the measurements might be adulterated [26]. Spatial chirp could be avoided with a much more complex double-pass grating compressor [17]. However, in the forefront of the measurements individual comparison measurements provided the same results using the described one gratingcompressor and a double-pass grating compressor (f = 400mm) and thus confirmed that spatial chirp is negligible in our system. Hence for compactness and easy alignment purposes the one-grating-compressor was applied in the following. It is obvious from Fig. 1 that the external pulse compressor and the external cavity have very similar geometries. With other words, movement of lens and end mirror relative to the grating within the FTECAL resonator changes the intracavity dispersion [21]. In the following we study the influences of this variable dispersion on the modelocking. 3. Experimental results In all experiments described in this paper, we operated the laser in passive modelocking operation only. Figure 2 shows a typical autocorrelation trace and the corresponding spectrum of the pulses emitted directly from the laser. The pulse width is 7.4 ps and the spectral bandwidth is 0.77 THz, corresponding to a time-bandwidth product of 5.7. This is 18 times the Fourier limit for a sech-shaped pulse and indicates that the pulses are strongly chirped. The average optical power is 6.3 mW at a repetition rate of 266 MHz. The pulse peak power is 3.2 W. The FROG trace in Fig. 3 confirms that the pulses are chirped and shows a strong linear chirp component associated with smaller chirp components of higher order. The source and retrieved FROG trace are in a good agreement. The FWHM of the spectrum is 1.78 nm and thus nearly identical to the spectral width obtained with the OSA in Fig. 2 of 1.81 nm. The spectral phase is dominated by the quadratic term that leads to a mainly linearly chirped pulse. The pulse duration obtained from the temporal intensity (d) does with 8.4 ps not perfectly agree with the value of 7.4 ps obtained from the autocorrelation in Fig. 2. In the following, we use the FROG measurements not for determination of absolute values for temporal and spectral widths but restrict ourselves to the analysis of the chirp. Due to the inherent symmetric shape of the SHG-FROG-trace [27] we cannot conclude on the sign of the chirp from this trace alone.

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Fig. 2. Typical autocorrelation traces of compressed and uncompressed pulse (a). The black traces show the measured traces, the red ones the corresponding fits. The autocorrelation trace for the compressed pulse has been vertically shifted for better visibility. Trace (b) shows the spectrum measured with the OSA.

However, a theoretical analysis of modelocked diode lasers predicts blue chirp for passively modelocked semiconductor lasers which arises due to different values of the α-parameter in the gain and the absorber section [15]. 844

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This prediction is confirmed by our further experimental investigations. We send the output of the laser through our compressor and obtain a considerable reduction of the linear chirp by increasing the distance between the fixed unit of lens and mirror relative to the grating. As mentioned above, increasing the distance reduces up-chirp and we can conclude that the pulses emitted from our passively modelocked laser are strongly up-chirped. Figure 4 #133940 - $15.00 USD

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shows the retrieved FROG trace (a) and the temporal intensity with its temporal phase of the compressed pulse (b). While the FROG trace of the uncompressed pulse has a triangular shape due to the combination of strong linear and quadratic chirp, the FROG trace of the compressed pulse shows a rectangular shape. This is due to the compensated quadratic phase and the residual cubic phase, which cannot be compensated by the external grating compressor. This can be seen on the right hand side of Fig. 4. Compared to the temporal phase of the uncompressed pulse, the temporal phase of the compressed pulse is nearly constant. This leads to pulse duration of 904 fs, which is in good agreement with the result obtained from the autocorrelation function.

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Fig. 4. Retrieved FROG trace (a) and temporal intensity and phase (b) of the compressed pulse.

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While in standard femtosecond lasers intracavity dispersion management is state of the art, this tool has not often been used in modelocked diode lasers so far. In our experimental arrangement, we can simply introduce intracavity dispersion by the above mentioned variation of the relative positions of the cavity components. In Fig. 5 the second derivative of the spectral phase, i.e. the linear up-chirp, as determined from the SHG–FROG - measurements is plotted as a function of distance of grating and lensmirror unit.

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Note that these data are taken without external compressor. The change in repetition rate per displacement is about 150 kHz per mm, the average optical power remains nearly constant around 6.3 mW. The data confirm that the second derivative of the spectral phase depends on the distance almost linearly with a slope of 1.58 x 105 fs2/mm, which correspondents to a group velocity dispersion (GVD). In Fig. 6 we plot the corresponding spectra for different positive distances. Obviously, the spectra shift slightly to a shorter central wavelength and broaden considerably for increasing positive distance.

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Fig. 6. Measured spectra for different positions out of the focus.

Note that for negative distances the bandwidth remains almost unchanged while for distances above + 4 mm the modelocking collapses. Figure 7 shows the spectral bandwidth, the pulse width and the time-bandwidth product of the emitted pulses as a function of the grating to lens distance. It might be surprising that the pulse width remains almost constant upon distance variation while the spectral bandwidth and correspondingly the time-bandwidth show a strong increase for increasing positive distances (i.e. for negative GDD). We explain this as follows: the theoretical model for passive modelocking by Haus et al. shows that the dynamics of the absorber saturation and the gain saturation open a short net gain time window in which the blue chirped passively modelocked pulse is positioned [28,29]. Our modification of the GDD in the cavity does not primarily change the interplay of the gain and absorber dynamics but it enables more spectral components to enter the temporal net gain window for positive distances. Accordingly, the spectrum broadens but the pulse width is not considerably altered. 6

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Consequently, the additional use of an external pulse compressor is required. Figure 8 shows the results behind the external pulse compressor. In the left graph we plot the minimal achievable pulse width after compression as a function of the distance between grating and lens within the cavity. The shortest pulses could be generated at 4 mm behind the focal length were the spectrum was the broadest.

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Fig. 8. Pulse durations (autocorrelation with Lorentzian fit) behind pulse compressor for different distances of the FTECAL geometry (a). Resulting distances of diffraction grating and lens within the pulse compressor against the distances of the FTECAL geometry (b).

The right part of Fig. 8 shows the corresponding distance of grating and lens in the compressor as a function of the distance of grating and lens in the FTECAL cavity. As expected from Fig. 5 there is a nearly linear relation between the distances due to the linear relation of the GDD and the distance of the FTECAL shift. The shortest pulse in this series was measured when the FTECAL was shifted 4 mm out of the focal length. The pulse width after compression is 252 fs (assuming a Lorentzian pulse shape) at 266.6 MHz repetition frequency. With an average optical power of 3.5 mW the pulse peak power is 52 W. Note that much higher average power and peak powers in the kW range can be obtained by expanding the system with an additional tapered amplifier [19]. The time-bandwidth product of 0.58 is still 2.6-times higher than the theoretical minimum because of higher order phase distortions. The latter still influence the pulse shape as linear chirp is completely compensated. Third order dispersion leads to temporal sidelobes and thus broader pedestal in the autocorrelation trace [30] which are responsible for the Lorentzian pulse shape after compression. It should be noted that by fine tuning of absorber voltage and injection current, we got a further increase of the spectral width up to 6.48 nm and were able to compress the pulse width down to 200 fs assuming a Lorentzian pulse shape (Fig. 9). However; under these conditions the laser operated at the triple cavity roundtrip frequency, i.e. 800 MHz. This is a consequence of the modified absorber and gain saturation dynamics and has already been observed for other passively modelocked semiconductor laser systems [31]. spectral intensity / arb. units

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Fig. 9. Autocorrelation trace with Lorentzian fit function (a) and measured spectrum (b) for the optimum combination of intracavity dispersion management and external compression.

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4. Summary and conclusion We have investigated the interplay between intracavity dispersion management and external pulse compression for a passively modelocked diode laser. By changing the intracavity dispersion, we increased the spectral bandwidth from 1.1 nm to 5.3 nm and simultaneously the time-bandwidth product by a factor of 4.6. In detail, the linear chirp, i.e. the second derivative of the spectral phase, is changed from 1,160,000 fs 2 to 230,000 fs 2 by geometrical shifting of intracavity components. In contrast to earlier studies [21] we find that adding GDD in the cavity increases the bandwidth at almost constant pulse width and therefore enables considerable pulse shortening by external pulse compression. Combining intracavity dispersion management and external pulse compression we achieved a shortest pulse duration of 252 fs at a repetition rate of 266.6 MHz with a peak power of 52 W. A further increase of the intracavity GDD above the optimum position leads to an abrupt breakdown of modelocked operation. The reason for this breakdown is still under investigation. A possible explanation has been suggested by Ippen [32] who showed that in a modelocked laser stable oscillation is often not obtainable with zero GDD or with small values of GDD of the wrong sign in the presence of self-phase modulation. However, we expect that much shorter pulse widths even below 100 fs could be obtained by alternative intracavity dispersion management including also higher order dispersion components and avoiding spatial chirp. Acknowledgments This project was supported by the German Ministry for Education and Research (BMBF) within the project “Indilas” contract FKZ 13N9816.

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