Q-switched operation of a femtosecond-laser-inscribed Yb : YAG channel waveguide laser using carbon nanotubes

We demonstrate a diode-pumped femtosecond-laser-inscribed Yb:YAG channel waveguide laser, Q-switched by using single-walled carbon nanotubes (SWCNTs) near 1029 nm. We used saturable absorber mirrors (SAMs) fabricated by depositing SWCNTs on three different output couplers. Best performance of the 9.3-mm-long ultra-compact Q-switched waveguide laser is obtained with an output coupling transmission of 20%. In this case, a maximum average output power of 60 mW with a corresponding pulse energy of 37.7 nJ and a pulse duration of 88 ns at 1.59MHz repetition rate were achieved. The highest pulse energy of 39.2 nJ and the shortest pulse duration of 78 ns were obtained with 30% and 10% output couplers, respectively. ©2015 Optical Society of America OCIS codes: (140.3615) Lasers, ytterbium; (140.3540) Lasers, Q-switched; (230.7380) Waveguides, channeled; (190.4400) Nonlinear optics, materials. References and links 1. R. Aviles-Espinosa, G. Filippidis, C. Hamilton, G. Malcolm, K. J. 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Introduction
Compact pulsed laser sources in the 1-μm spectral region are of particular interest for a variety of applications in microscopy, spectroscopy and metrology [1][2][3].One of the important aspects for the development of compact laser systems is pumping with laser diodes.In recent years, Yb 3+ -and Nd 3+ -doped single crystals and ceramics exhibiting large emission cross sections, low quantum defects and low thermal loads turned out to be very promising candidates as gain media for the development of diode-pumped and highly efficient lasers both in continuous-wave (CW) and pulsed operation near 1 μm.Waveguiding structures allow for high intra-cavity pump intensities at reduced pump powers, an excellent overlap of the pump and laser modes and efficient heat removal.To date, various CW and pulsed Yb 3+ -and Nd 3+ -doped solid-state lasers based on planar and channel waveguides are actively investigated .In particular, the channel waveguide structures tend to deliver circular single-mode output in more efficient and stable operation due to an enhanced light confinement in small volumes.In addition e. g. to diffusion bonding, ion implantation and liquid-phase epitaxy [4,5,17], femtosecond-laser (fs-laser) inscription is a well-known technique for manufacturing channel waveguides inside dielectric gain materials.This relatively simple technique allows direct writing of straight and curved channels without masks [10,12,22,26].Recently, diode-pumped high-power operation of a femtosecond-laserinscribed Yb:YAG waveguide laser delivering CW output powers of up to 2.35 W was successfully demonstrated [12].
In this work, we report on single-mode diode-pumped fs-laser-inscribed Yb:YAG channel waveguide lasers, Q-switched by single-walled CNT SA mirrors (SWCNT-SAMs) for the first time.SWCNTs are deposited on different output couplers (OCs).The 9.3-mm-long laser cavity consists of an Yb:YAG waveguide and two mirrors (i.e., the end mirror and the output coupler (OC) acting simultaneously as a SAM) which are directly attached to both end-facets of the waveguide.In this configuration, operation characteristics using three SWCNTdeposited OCs of different transmissions (10%, 20% and 30%) are investigated.

Fabrication of channel waveguides and SWCNT-SAMs
The channel waveguides were fabricated by the fs-laser direct writing method [26] inside a 7% Yb 3+ -doped YAG crystal using 150 fs pulses at a center wavelength of 775 nm from a 1 kHz Ti:sapphire chirped pulse amplifier (CPA) system (Clark-MXR CPA-2010).The fabrication process is described in detail in Ref. 22.The laser beam is focused with an aspheric lens (f = 3.1 mm) 345 μm below the polished surface of the Yb:YAG crystal and the tracks were written at a pulse energy of ~1.5 μJ.Two parallel tracks are written by moving the sample mounted on a motorized translation stage with a velocity of 25 μm/s, while the fs-laser beam is incident on the sample perpendicularly.The tracks inside the crystal induce stress which changes the refractive index in the material surrounding the tracks.The refractive index decrease of the tracks lead to a total refractive index change in the order of 10 −3 -10 −4 and this enables waveguiding between two written tracks.A separation of 28 μm between the tracks results in a good confinement of the fundamental mode near 1 μm [21].The waveguide length after the polishing process is 9.3 mm.
To fabricate reflection-type SAs applicable for Q-switching of the Yb:YAG channel waveguide laser, arc-discharged SWCNTs (Meijo Carbon Co., Ltd) are used as starting material.The SWCNT powder is dissolved in 1,2-dichlorobenzene (o-DCB) at a concentration of 0.5 mg/ml and agitated for several hours in an ultrasonic bath.To remove metallic impurities and large bundles, the SWCNT dispersion is centrifuged for about 30 minutes.The well-dispersed SWCNT solution is then mixed with poly(methyl methacrylate) (PMMA) and the SWCNT/PMMA composite solution is spin-coated on different OCs to a layer thickness of about 300 nm.Finally, the coated OCs are used as SWCNT-SAMs in Qswitching experiments.It is noted that, for stable Q-switched operation of lasers, a larger modulation depth of the SA than for the case of mode-locking [29,30]  different OCs.The linear transmission of the SWCNT-SA is measured to be > 98% near the 1-μm region and a broad absorption band which arises from the E 22 transition of the semiconducting SWCNTs appears around 1050 nm.The nonlinear transmission of the SWCNT-SA is measured at 1080 nm by employing a femtosecond Ti:sapphire laser-pumped optical parametric oscillator (OPO) with a resolution of 0.1%.The SWCNT-SA has a saturation fluence of < 40 μJ/cm 2 , a modulation depth of 0.65% and nonsaturable losses of about 1%.

Experimental results
Figure 1 shows the experimental setup of the Q-switched Yb:YAG channel waveguide laser including the SWCNT-coated OC acting simultaneously as SAM.A single-mode pump beam from a can-type CW 940-nm laser diode (9-mm TO Can LD, Axcel photonics Inc.) is collimated using an aspheric lens of f = 6.24 mm.A Faraday isolator is inserted into the beam path to prevent back-reflection from the waveguide facets and the mirrors.A half-wave plate and a Glen-Taylor polarizer which is transparent for p-polarization are installed for pump beam attenuation by rotating the wave plate.The linearly p-polarized pump beam is focused onto the waveguide using a second aspheric lens (L1, f = 18.4 mm) through an incoupling mirror which is highly transparent at the pump wavelength and highly reflective at the laser wavelength.This mirror serves as an end mirror in the resonant cavity.The maximal CW pump power measured directly in front of the waveguide amounts to 264 mW.The 9.3-mmlong Yb:YAG channel waveguide as gain medium is mounted on a multiple-axis stage without active cooling.The SWCNT-deposited OC is then attached to the other end of the waveguide.Three different output coupling transmissions of 10%, 20% and 30% are investigated.For detection, a long-pass filter for filtering out the residual pump beam and a convex lens with f = 11 mm (L2) as a beam collimator are used outside the laser cavity.Figures 1(a) and 1(b) show the microscopic image of two 28-μm-separated tracks with the waveguide in between them and the top-view of this channel waveguide, respectively.The depth of tracks from the crystal surface is about 345 μm.As shown in Fig. 1(c), the end mirror and SWCNT-coated OC are directly attached to each end-facet, and hence, the total cavity length of the Q-switched Yb:YAG channel waveguide laser is determined by the length of the gain medium, i.e., 9.3 mm.To prevent Fresnel losses and undesired feedback, index matching oil is used between both waveguide facets and mirrors.quality of the SWCNT-coating.The Q-switched channel waveguide laser shows a pumppower-dependent behavior which is similar to that typically observed in lasers passively Qswitched with saturable absorbers [31].By increasing the pump power, the repetition rate and pulse energy increase, while the pulse duration decreases (Figs.3(b) and 3(c)).The repetition rate with the SWCNT-coated 10% and 20% OCs can be tuned between about 1 and 1.6 MHz depending on the pump power, whereas it varies between about 0.8 and 1.4 MHz with the 30% OC, as shown in Fig. 3(b).In Fig. 4(a), an exemplary laser spectrum and pulse shape (upper inset) for 20% output coupling are depicted.The lower inset shows the beam profile of the output beam with a nearly circular fundamental mode recorded by a beam profiler (LaserCam-HR, Coherent Inc.).The output beam diameter measured after collimation in a distance of about 30 cm from OC is 0.4 mm.The Q-switched laser operates also near 1029 nm for the other output coupler transmissions and the beam modes are very similar in all cases such as in the case of CW operation.The shortest pulse duration and the maximum pulse energy are measured with the 10% and 30% OCs to be 78 ns and 39.2 nJ, respectively, at the maximum-available pump power of 264 mW.It should be noted that in contrast to many other Q-switched waveguides the pulses are fully modulated.Moreover, the stability of Q-switching was excellent, which was confirmed by measurements of the pulse train in different time-spans (Fig. 4(b)).Although the waveguide is not externally cooled, the Q-switching is stable for hours with negligible changes.Furthermore, the application of a laser diode as a pump source paves the way towards a further miniaturization of crystalline pulsed active optical devices.

Conclusion
We report the first diode-pumped femtosecond-laser-inscribed Yb:YAG channel waveguide laser, Q-switched by SWCNT-coated output couplers.The channel waveguide created between two precisely fs-laser-inscribed parallel tracks delivers a spatially circular fundamental-mode both in CW and Q-switched regimes.Using SWCNT-coated different output couplers acting simultaneously as saturable absorber mirrors in the present laser setup, Q-switched operation characteristics of a 9.3-mm-long ultra-compact laser is systematically investigated.High transmission, low saturation fluence and suitable modulation depth of SWCNT-SAMs enable us to generate stable Q-switched pulses.Improvements of the laser performance including higher output power, shorter pulse duration and even mode-locked operation at GHz repetition rates should be possible by applying higher pump powers, dispersion compensation and fine control of the saturable absorber parameters.This work is currently in progress.

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234521 -$15.00USD Received 12 Feb 2015; revised 13 Mar 2015; accepted 15 Mar 2015; published 19 Mar 2015 (C) 2015 OSA is generally required.The fabricated SWCNT-SAMs used in the present work exhibit appropriate parameters for Qswitching.This was verified by measurements of the linear and nonlinear characteristics of SWCNTs deposited on fused silica substrate under identical conditions to those fabricated on #234521 -$15.00USD Received 12 Feb 2015; revised 13 Mar 2015; accepted 15 Mar 2015; published 19 Mar 2015 (C) 2015 OSA

Fig. 3 .
Fig. 3. Yb:YAG channel waveguide laser Q-switched by SWCNT-coated OCs: (a) average output power characteristics and (b) repetition rates for three different output couplers.(c) Pulse width and (d) pulse energy as function of incident pump power for 20% OC