Continuously tunable Yb:KYW femtosecond oscillator based on a tunable highly dispersive semiconductor mirror P. Wnuk,1,2,* P. Wasylczyk,2 Ł. Zinkiewicz,2 M. Dems,3 K. Hejduk,4 K. Regiński,4 A. Wójcik-Jedlińska,4 and A. Jasik4 2

1 Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/46, 01-224 Warsaw, Poland Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Hoza 69, 00-681 Warsaw, Poland 3 Institute of Physics, Lodz University of Technology, ul. Wólczańska 219, 90-924 Lódź, Poland 4 Institute of Electron Technology, Al. Lotnikow 32/46, 02-668, Warsaw, Poland * [email protected]

Abstract: The optimized nonuniform growth process was used to achieve spatially dependent reflectivity and dispersions characteristics in a highly dispersive semiconductor mirror. The mirror, together with a semiconductor saturable absorber mirror (SESAM), was used to demonstrate a tunable femtosecond Yb:KYW oscillator. In the passive modelocking regime the laser could be continuously tuned over 3.5 nm spectral band around 1032 nm with high resolution, maintaining the average output power above 140 mW. ©2014 Optical Society of America OCIS codes: (130.2035) Dispersion compensation devices; (130.7408) Wavelength filtering devices; (140.3600) Lasers, tunable; (140.7090) Ultrafast lasers; (160.6000) Semiconductor materials.

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

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Received 6 Jun 2014; revised 2 Jul 2014; accepted 3 Jul 2014; published 21 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018284 | OPTICS EXPRESS 18284

16. C. Chang-Hasnain, J. Harbison, C. Zah, M. Maeda, L. Florez, N. Stoffel, and T. P. Lee, “Multiple wavelength tunable surface emitting laser arrays,” IEEE J. Quantum Electron. 27(6), 1368–1376 (1991). 17. N. Matuschek, F. X. Kärtner, and U. Keller, “Analytical design of double-chirped mirrors with custom-tailored dispersion characteristics,” IEEE J. Quantum Electron. 35(2), 129–137 (1999). 18. A. Jasik, P. Wasylczyk, P. Wnuk, M. Dems, A. Wojcik-Jedlinska, K. Reginski, L. Zinkiewicz, and K. Hejduk, “Tunable semiconductor double-chirped mirror with high negative dispersion,” IEEE Photon. Technol. Lett. 26(1), 14–17 (2014). 19. A. P. Kovács, K. Osvay, Z. Bor, and R. Szipöcs, “Group-delay measurement on laser mirrors by spectrally resolved white-light interferometry,” Opt. Lett. 20(7), 788–790 (1995). 20. A. Wójcik, T. J. Ochalski, J. Muszalski, E. Kowalczyk, K. Goszczyński, and M. Bugajski, “Photoluminescence mapping and angle-resolved photoluminescence of MBE-grown InGaAs/GaAs RC LED and VCSEL structures,” Thin Solid Films 412(1–2), 114–121 (2002). 21. A. Jasik, P. Wasylczyk, M. Dems, P. Wnuk, A. Wójcik-Jedlińska, K. Regiński, Ł. Zinkiewicz, and K. Hejduk, “A passively mode-locked, self-starting femtosecond Yb:KYW laser with a single highly dispersive semiconductor double-chirped mirror for dispersion compensation,” Laser Phys. Lett. 10(8), 085302 (2013).

1. Introduction Contemporary femtosecond oscillators have made a long journey to became compact, turnkey maintenance free devices. Besides stability and reliability, a possibility to vary the central wavelength of the generated pulses is often a desired feature. In particular multiphoton microscopy [1] and florescence lifetime imaging [2] call for matching the laser band with the fluorescing molecules spectral characteristics. Femtosecond pump-probe spectroscopy [3], wavelength multiplexing [4] or nano-printing [5] are among other areas benefiting from pulse tunability. In the early designs the intracavity negative dispersion – fundamental for the stable passive modelocked operation – was introduced with a pair of prisms [6]. As the laser spectrum is spatially dispersed inside the prism line, a movable slit could be used for the laser tuning, its position defining the central wavelength, while its width controls the bandwidth [7]. However simple and universal, this scheme has several drawbacks: any additional element, such as prism, acts as a parasitic output couplers, thus compromising the usable output power. Due to the nonlinear dependence of the refractive index on frequency, spectral phase introduced by the prism line is not purely quadratic and different slit configurations require different settings of the external compressor to achieve the shortest pulses. Further, the prism-equipped cavity becomes sensitive to misalignment, often preventing or destabilizing modelocking. More importantly, due to intrinsic spatial constrains, prism dispersion lines with their tuning capability cannot be used with short cavities for generation of pulses at high repetition rates [8]. A milestone in the femtosecond sources stability and reliability was the invention of the chirped mirrors [9], where negative dispersion is achieved by the wavelength dependent light penetration depth into a multilayer mirror structure. In contrast to the classical quarter wavelength mirrors where alternating layers of high and low index layers are deposited with constant thickness, the thickness of the chirped mirror layers varies with the layer number. If the layer thickness increases with depth, longer wavelengths penetrate deeper into the mirror stack and thus the normal (positive) dispersion introduced by dispersive cavity elements may be compensated. Different algorithms are used to design mirrors with the desired Group Delay Dispersion – GDD [10]. It was soon realized there exists a reciprocal dependence between the maximum GDD value and the compensated bandwidth – the higher the GDD absolute value, the narrower the bandwidth it can span. Recently, mirrors with negative GDD of a few thousands of fs2 became available, with the bandwidth of few nanometers [11]. To increase the absolute value of the GDD even further, the mirror stack made of materials with low refractive indices difference may be used. Recently we have demonstrated such structure with a semiconductor materials in a highly dispersive laser mirror [12]. While the chirped mirror technology has paved the way towards stable, turn-key operation femtosecond oscillators, at the same time the laser tunability has been sacrificed. Additional spectral filtering devices – birefringent Lyot filters [13] or angle or position tunable dielectric band pass filters [14] are used, but they inevitably introduce extra intracavity dispersion, and often reduce the available output power. #213578 - $15.00 USD (C) 2014 OSA

Received 6 Jun 2014; revised 2 Jul 2014; accepted 3 Jul 2014; published 21 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018284 | OPTICS EXPRESS 18285

In this paper we demonstrate a novel approach where dispersion compensation and tunability are combined into a single element – the tunable semiconductor double-chirped mirror (TSDCM). The precise control of the mirror growth conditions and the substrate position relative to the effusion cells in the molecular beam epitaxy (MBE) chamber result in the layer thickness varying with position on the mirror. Thus the spectral mirror properties – reflection and dispersion – continuously shift across the mirror area. The idea of variable coatings has been used before, e.g. in Gaussian mirrors (also known as Variable Reflective Mirrors) [15] for the unstable resonator lasers to increase the energy extraction efficiency. Another demonstration of mirrors with varying thickness is a monolithic array of VCSEL lasers where each of the emitter lases at different wavelength [16]. 2. Mirror design and production The mirror design is based on 65 AlAs/GaAs layer pairs. The 10 deepest layers form the fixed wavelength quarter wavelength Bragg reflector. This is followed by 25 layer pairs with a single chirp and topped with 30 double chirped pairs. The chirped layer thicknesses were calculated using an analytical approach [17] and the whole structure was capped by an SiNx antireective layer. The semiconductor mirror stack was grown with MBE in a 32P Riber machine. Both the effusion cells and the substrate wafer were positioned in the configuration favoring a radial gradient growth – see Fig. 1. Subsequently, the structure was protected with a 3 nm GaAs antioxidation layer and finally the SiNx antireflective (AR) coating was deposited using plasma enhanced chemical deposition. The mirror radius of curvature, measured by High Resolution X-Ray Diffraction, is 6 m. Detailed description of the mirror design and the fabrication procedure can be found in [18].

Fig. 1. The wavelength tunable semiconductor chirped mirror nonuniform growth process concept. The effusion cells with angularly dependent deposition density are used in the configuration resulting in a spatially dependent layer thickness.

The main idea behind the TSDCM is to harness the growth spatial non-uniformities in a well controlled process to obtain a multilayer structure where the layers thickness varies with position. The highest thickness of the layers in the center of the substrate results in the high reflectivity and the negative chirp bands shifted to longer wavelength. With the decreasing thickness towards the mirror edge, the spectral features shift towards blue. 3. Results and discussion The mirror reflectivity and GDD were characterized versus the position on the mirror surface. The reflectivity curves are presented in Fig. 2, where the central wavelength of the highreflectivity plateau variation with position is visible, following the structure design.

#213578 - $15.00 USD (C) 2014 OSA

Received 6 Jun 2014; revised 2 Jul 2014; accepted 3 Jul 2014; published 21 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018284 | OPTICS EXPRESS 18286

Reflectance (arb. units)

2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 900

0 mm

0 mm 3 mm 6 mm 9 mm 12 mm 15 mm 18 mm 21 mm

950

1000 1050 1100 1150 1200 Wavelength (nm)

Fig. 2. Position dependent reflectivity curves of the TSDCM. The inset shows the measurement locations across the substrate, 0 mm corresponding to the 2” GaAs wafer center. The subsequent curves are vertically offset for clarity.

The mirror was designed for a femtosecond Yb:KYW oscillators, typically having the lasing bandwidth of about 15 nm which is narrow compared to the designed high reflectivity bandwidth. Since the central wavelength of the laser falls within the high reflectivity range for each location on the mirror, the latter does not affect the laser spectral characteristics. Thus, tunable but broadband reflectivity is insufficient for tuning a narrowband lasers. Still, we believe that the lasers with broader bandwidth (such as Ti:Sapphire) could benefit from the spatial tunability of such mirrors. In our design the laser tuning is based on the mirror dispersive properties, namely the GDD spatial dependence. Position dependent dispersive characteristics were measured with a modified white-light Michelson interferometer, similar to the one described in [19] and the results are plotted in Fig. 3. b)

2

GDD (fs )

0 -2000 -4000

3mm 6mm 9mm 12mm

-6000 -8000 980

990

1080 GDD minimum position (nm)

a) 2000

1000 1010 1020 1030 1040 1050 Wavelength (nm)

Experimental data Quadratic polynomial fit

1060 1040 1020 1000 980 960 940 920 0

5 10 15 20 Distance from the center (mm)

25

Fig. 3. Dispersive characteristics of the tunable semiconductor double chirped mirror; (a) spectral dependence of the GDD measured for four different distances from the mirror center, (b) spectral position of the second GDD minimum plotted vs. position across the mirror radius, together with the quadratic polynomial fit.

For each position on the mirror, in the vicinity of the lasing wavelength, two GDD minima are present. The first one, at the shorter wavelengths (bluer), has a smaller negative value but covers a broader band. The longer wavelength one (redder) has three times higher absolute GDD of around −8000 fs2 spanning a significantly narrower bandwidth. In Fig. 3(b) the position of the redder dip is plotted versus position on the mirror across the entire mirror substrate. Spectral shift of more than 100 nm with translation of the mirror by 24 mm was achieved. Similar dependence was observed for the redder dip. As expected, the central frequency of the GDD minima scales quadratically with the radial coordinate [20].

#213578 - $15.00 USD (C) 2014 OSA

Received 6 Jun 2014; revised 2 Jul 2014; accepted 3 Jul 2014; published 21 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018284 | OPTICS EXPRESS 18287

The TSDCM was used as an end mirror in an Yb:KYW femtosecond oscillator having the cavity design similar to the one described in [21]. The laser was pumped at 980 nm by a 580 mW single mode fiber coupled laser diode and the mode locked operation was initiated and supported with a 1030 nm optimized SESAM. The TSDCM was the only element responsible for the cavity dispersion management – compare Fig. 4(a). To vary the laser spot location on the tunable mirror, the latter was mounted on the translation stage so that it could be shifted perpendicularly to the beam axis. The estimated beam size on the TSDCM is 800–1000 μm. Once the mirror was translated to the position where the redder GDD minimum falls in the vicinity of the gain peak, a stable femtosecond pulse train was generated. It should be noticed, that in this region, a self-starting modelocking operation was achieved, thanks to the properties of the SESAM.

a)

b)

TSDCM OC M2 M1

L

SESAM

Normalized spectrum

LD

1,0

Beam position (mm):

0,8 0,6 0,4 0,2 0,0 1026

1028

1030

1032

1034

1036

Wavelength (nm)

Fig. 4. (a) Schematic of the Yb:KYW tunable femtosecond oscillator based on a 1 mm crystal (X) and the tunable semiconductor double-chirped mirror (TSDCM) on a translation stage, assisted by a SESAM mirror. The laser is pumped by a single mode 980 nm fiber-coupled laser diode (LD), focused on a crystal by a pair of lenses (L) (collimating f = 15 mm aspheric and focusing f = 63 mm). The cavity ends by an 8% transmission output coupler (OC). M1, M2 are concave R = 50 mm pump mirrors. (b) The oscillator spectra for different positions of the cavity mode on the TSDCM mirror surface – the tuning range spans more than 3.5 nm around 1032 nm (b).

To characterize the TSDCM structure performance, several parameters of the laser were measured for each position of the cavity mode on the tunable semiconductor mirror, where stable mode locking was achieved. Figure 5(a) presents the average optical power of the laser along with the central wavelength of the emitted near infrared pulses for different positions of the cavity mode on the tuning mirror. Continuous and gapless tuning across 3.5 nm under constant output power, exceeding 140 mW, was achieved. This indicates a constant high reflectivity of the TSDCM, confirming the high optical quality of the nonuniform mirror structure. Interferometric autocorrelation of the laser pulses was measured for different positions across the mirror and the retrieved pulse durations (FWHM - sech2 pulse shape was assumed) and spectral widths are plotted in Fig. 5(b).

#213578 - $15.00 USD (C) 2014 OSA

Received 6 Jun 2014; revised 2 Jul 2014; accepted 3 Jul 2014; published 21 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018284 | OPTICS EXPRESS 18288

1033

100

1032 50 1031 0

5,0

5,5

6,0

6,5

Distance from the center (mm)

1030 7,0

550 3 500 2 450

1

0 5,0

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1034

Central wavelength (nm)

150

Optical power (mW)

b)

1035

Spectrum FWHM (nm)

a)

400 7,0

Distance from the center (mm)

Fig. 5. Femtosecond tunable Yb:KYW laser performance for different position of the beam on the TSDCM. The output power (black dots, left axis) and the central wavelength of the generated pulses (red squares, right axis) (a). FWHM of the laser spectra (black triangles, left axis) and the FWHM of the pulse duration (red squares, right axis) (b).

The tuning range of the laser, smaller than the one expected from the spectral width of the lasing medium, may indicate the presence of an element limiting the laser tunability. This could be either the reflectivity or dispersion characteristics of the SESAM or dichroic pump mirror which has quite steep spectral features. The pulse duration is governed by the spectral dependence of the TSDCM GDD minima. Further optimized TSDCM design should support much shorter pulses if the first (bluer) minima were used (with upscaled central wavelength) or the GDD value was reduced which would increase its bandwidth. Yet, as the pulses get shorter, the laser tuning becomes problematic, as the pulse bandwidth approaches the spectral width of the gain medium. 4. Conclusion Development of a tunable and highly dispersive semiconductor mirror allowed us to demonstrate, for the first time to our knowledge, a tunable femtosecond oscillator in the allreflective cavity configuration. The laser offers high stability, constant power and Fourier transform-limited femtosecond pulses across the 3.5 nm tuning range around 1032 nm. It is also possible to fabricate a set of two (or more) similar mirrors, with slightly shifted GDD characteristic, to extend the laser tuning range. The concept of the spatially varying multilayer structure deposition can be implemented in other coating techniques and the mirrors for different spectral regions may be fabricated in a similar fashion. One exemplary technique can be the metalorganic chemical vapor deposition with stationary susceptor (without rotation). A mirror with the linear dependence of the GDD spectral characteristic vs. position could be thus grown. Among the application fields of the tunable chirped mirrors we note the high repetition rate (tens to hundreds of GHz) mode locked oscillators where the millimeter long cavities leave no room for additional tuning and/or dispersion compensating elements. We believe that the idea and the proof-of-principle experimental demonstration presented in our manuscript can be further extended, both in terms of the achievable laser pulse duration as well as in the laser tuning range. Even with the present tuning span, some application can be envisaged, e.g. in experimental quantum optics where fine tuning of the phase matching process in spontaneous parametric downconversion can be beneficial for correlated/entangled single photon sources. Acknowledgments This work was partially supported by the National Centre for Research and Development (NCBiR) under project 02-0009-10/2011.

#213578 - $15.00 USD (C) 2014 OSA

Received 6 Jun 2014; revised 2 Jul 2014; accepted 3 Jul 2014; published 21 Jul 2014 28 July 2014 | Vol. 22, No. 15 | DOI:10.1364/OE.22.018284 | OPTICS EXPRESS 18289

Continuously tunable Yb:KYW femtosecond oscillator based on a tunable highly dispersive semiconductor mirror.

The optimized nonuniform growth process was used to achieve spatially dependent reflectivity and dispersions characteristics in a highly dispersive se...
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