Abstract Wave-guiding was achieved in Tm3+ doped YAG

cladding waveguide fabricated by ultrafast laser inscription. With a graphene based saturable absorber mirror, Q-switched mode-locking operation in the 2 μm spectral region was realized from the waveguide.

I. INTRODUCTION Ultrafast Laser Inscription (ULI) has been proved to be

a powerful technique for fabricating buried waveguides in transparent optical materials. Waveguides with cladding morphology fabricated by ULI, since first reported in Nd:YAG in 2005, has drawn much attention due to low propagation loss, and geometric flexibility, and guiding both transverse mode [1-3].

Pulsed lasers, especially ultra-short lasers, have widespread applications in industry, military and medical. As one of the dominant pulse compression methods, Q-switched mode-locking (QML) generates mode-locked pulses underneath a Q-switched envelope. Thereby, QML can provide ultra-short mode-locked pulses as well as high energy Q-switched pulses at the same time, which is attractive for many fields such as nonlinear frequency conversion [4], micro machining [5], or surgery [6].

For passively pulse operation, graphene has attracted growing attention as promising saturable absorbers (SAs). Compared with traditional SAs (such as doped bulk crystals and semiconductor saturable absorber mirrors), graphene features broadband saturable absorption, low saturation intensity, controllable modulation depth, ultrafast carrier relaxation time, low-cost and easy fabrication, etc. So far, graphene saturable absorbers have been used to generate laser pulses at a wavelength ranging from 1 μm to 2 μm [7].

Here, we mainly demonstrate, for the first time to our knowledge, a compact Q-switched mode-locking laser generation from a Tm:YAG ceramic cladding waveguide with a graphene-based saturable absorber mirror.

II. WAVEGUIDE FABRICATION AND CHARACTERIZATION

The depressed cladding waveguide was fabricated by ULI in Tm:YAG (1%), as shown in the schematic in Fig. 1.A master oscillator power amplifier fiber laser (IMRA FCPA µ-Jewel D400), delivering 460 fs pulses and operating at 500 kHz was used as a laser source. The center wavelength of the laser was 1047 nm, which was

circularly polarized in this work. The beam was focused through a 0.4 NA aspheric lens at a depth of about 250 μm under one of the polished surfaces with pulse energy of 200 nJ. 16 tracks were inscribed inside the sample to form a clad with reduced refractive index and a diameter of 30 μm. The inset of Fig. 1 shows the cross sectional image of this waveguide.

Fig. 1. Schematic of the experiment for cladding waveguide writing.

The inset is the microscope image of the waveguide cross section.

A Ti:Sapphire laser was used as a pump source for both cw laser and Q-switched mode-locking. In the former case, as previously reported [8], the waveguide started lasing at low threshold, around 100 mW. The waveguide laser showed single transmission mode with a wavelength of 1985 nm. The maximum laser output power was measured to be 48 mW, when the incident pump power was 515 mW, resulting in an optical conversion efficiency of about 9.3%. The lasing slope efficiency was determined to be 11.5%.

III. Q-SWITCHED MODE-LOCKING OF THE WAVEGUIDE LASER

In this work, a graphene film was coated on a 5% output coupler to form a graphene saturable absorber mirror (SAM). The input mirror and output SAM were then adhered to two end facets of the sample to form the Fabry-Perot laser cavity, the length of which was 10.5 mm. The schematic of the laser cavity is shown in Fig.2, in which M1 and M2 are input mirror and graphene SAM, respectively. The dark film in the internal surface of M2 represents the graphene film.

By adjusting the laser cavity and the position of

Y. Y. Ren*, S. J. Beecher**, G. Brown**, A. Ródenas***, A. Lancaster**, F. Chen* and A. K. Kar**

* School of Physics, State Key Laboratory of Crystal Materials and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China.

** School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, Scotland *** Departament de Química Física I Inorgánica, Universitat Rovira i Virgili, Tarragona, Spain

Q-switched Mode-locking of a Mid-infrared Tm:YAG Waveguide Laser with Graphene Film

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graphene SAM, pulsed laser generation was realized from the waveguide. Fig. 3 depicts the recorded pulse trains. As expected, QML regime was obtained. Fig. 3 (a) shows the Q-switched pulses, while Graphene mode-locked pulses are shown in Fig. 3 (b).

Fig. 2. The schematic of pulsed laser generation cavity. M1: Pump

mirror; M2: graphene SAM

Fig. 3 Q-switched (a) and mode-locked (b) pulse trains generated from the waveguide

The maximum average output power is about 6.5 mW

at pump power of 665 mW. The repetition rate of the Q-switching is measured to be around 684 kHz, while the mode-locking repetition rate is about 7.8 GHz, which shows good agreement with the calculated results.

CONCLUSIONS Single-mode cladding waveguide was fabricated by

ultrafast laser inscription. CW waveguide laser at 2μm was realized. Furthermore, with graphene film being used as a saturable absorber, Q-switched mode-locking was achieved from the waveguide.

ACKNOWLEDGMENT This work was supported by the National Natural

Science Foundation of China (11111130200), and Royal Society international joint projects NSFC 2010 (JP 100985) and the UK EPSRC through grant

EP/GO30227/1. Authors gratefully acknowledge the Royal Academy of Engineering, Emmanuel College and King’s College, Cambridge.

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[8] Y. Y. Ren, G. Brown, A. Ródenas, S. Beecher, F. Chen, and A. K. Kar, “Mid-infrared waveguide lasers in rare-earth-doped YAG,” Opt. Lett., Vol. 37, pp. 3339-3341, August 2012.

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