Crystalline waveguides with carbon nanomaterials for miniaturized pulsed lasers - INVITED

. This presentation discusses the recent results on miniaturized pulsed solid-state lasers by utilizing femtosecond-laser inscribed crystalline channel waveguides and carbon-nanomaterial-based saturable absorbers. Based on optical characterization and optimization of the optical materials, integrated compact waveguide lasers present diverse pulsed operation regimes from Q-switching to continuous-wave mode-locking. Pulsing mechanism and various parameters in waveguide lasers are investigated to provide a basis for achieving higher performance of novel on-chip ultrafast lasers.


Introduction
Compact solid-state lasers have been actively studied for practical applicability, integrated photonic circuits and their potential high-repetition-rate mode-locking.GHzlevel repetition frequencies particulaly provide fast sampling rate and high precision for broad application areas including spectroscopy, optical communication and material processing.One of the most reliable and successful strategies to downsize solid-state lasers is embedding waveguide structures into dielectric laser gain materials.
Channel waveguides fabricated by femtosecond-laser inscription [1] or diamond-saw dicing [2] enable high intracavity intensities with well-defined mode distributions for highly efficient laser operation in a simplified cavity design.
As a nonlinear saturable absorber device for pulsed operation of lasers by passive Q-switching and modelocking, low-dimensional carbon nanomaterials such as graphene and carbon nanotubes exhibit superior optical properties.They benefit from intrinsic ultrabroadband absorption with large ultrafast nonlinearities, various application types and relatively simple fabrication processes.Especially for compact cavities with short round-trip time and low critical mode-locking pulse energy, carbon nanomaterials are highly advantageous thanks to ultrashort relaxation times, finely controllable nonlinearity with low non-saturable loss and flexible integration types [3,4].
Here, we report on the recent results of diverse pulsed laser operation of crystalline channel waveguides with carbon nanomaterials.By utilizing those unique optical materials, pulsing mechanism and key parameters in waveguide lasers are extensively investigated in nearinfrared spectral range.These studies are expected to guide the development of novel compact laser sources with improved performance.

Fabrication & Optical characterization 2.1 Channel waveguides inscribed by fs laser
In the 1-µm spectral range, ytterbium ions (Yb 3+ ) have been recognized as a prominent dopants for excellent laser gain crystals with small quantum defects, high absorption/emission cross-sections and diode-pumping.In particular, Yb 3+ -doped monoclinic potassium double tungstates such as Yb:KLu(WO4)2 (KLuW) exhibit relatively high and broad emission cross-sections [5] which is beneficial for ultrashort pulse generation in highrepetition-rate mode-locking.
In uncoated Yb-crystals (length of < ~1 cm), fs-laser inscribed channel waveguides are fabricated by using an amplified Ti:sapphire laser system.The track is inscribed by a translation system with focusing the laser below the crystal surface to achieve waveguiding based on Type-2 or Type-3 index modification [1].Straight or curved beam-splitter structures (Fig. 1) are measured to exhibit propagation losses of < 1 dB/cm (~3 dB for splitting).

Nanocarbon-based saturable absorbers
Monolayer graphene grown by chemical vapor deposition on copper foil is used with polymethyl methacrylate (PMMA) buffer layers, and arc-discharged single-walled carbon nanotubes (SWCNTs) whose broad E22 interband transition is located near the 1-µm-wavelength region as shown in Fig. 2

Pulsed operation of lasers
To effectively combine nanomaterials into waveguides, interaction types of SWCNTs are comparatively studied in waveguides placed near the surface [6,7].Evanescentfield interaction provides monolithic integration of materials with more efficient Q-switched operation than direct type, and prevents thermal damage issues (Fig. 3(a)).In addition, pumping cavity schemes are investigated in surface-SWCNT Q-switched waveguide lasers for power scaling strategies [8].In the beam-splitter waveguide, selective excitation of monolayer graphene enables to generate dual-channel Q-switched pulses with tunability (Fig. 3 (b, c)) [9].The dual-channel operation shows potential platform for dual-comb sources.
From MHz nanosecond Q-switched pulses to sub-GHz mode-locking, diverse pulsed regimes are controllable by gap distance between waveguide and SWCNTs (Fig. 3(d)).Finely adjusted nonlinearity of SWCNTs satisfies certain cavity criteria for transition of pulsed operation [3].Based on these studies, multi-GHz femtosecond mode-locking up to 3.55 GHz is achieved by using slightly wedged waveguide structure and fluencecontrolled SWCNTs (Fig. 3 (e)) [4].The result reveals superior optical characteristics of the Yb:KLuW waveguide for high-repetition-rate mode-locking.The noise spectra of the GHz lasers are also analyzed depending on nonlinearity of absorber materials while showing low timing jitter.

Conclusion
Effective integration of crystalline channel waveguides with carbon nanomaterials provides compact chip-scale pulsed operation of solid-state lasers and diverse novel pulsing regimes owing to the properly designed optical materials.The results will be extended beyond the near-infrared spectrum, and can be employed in practical applications.

Fig. 1 .
Fig. 1.Optical microscope images of (a) input facet and (b) top view of fs-laser inscribed crystalline channel waveguides.
(a) are utilized in this work.The prepared nanocarbon layers are transferred/spin-coated on the desired substrate such as waveguide surfaces or laser cavity mirrors.The fabricated saturable absorbers are characterized for linear/nonlinear absorption, response times, Raman spectra, etc. (Fig. 2 (b)).Their optical properties are optimized by precise control of fabrication conditions including concentrations, thickness and number of layers.

Fig. 2 .
Fig. 2. (a) Linear absorption spectra of SWCNTs with several chiralities and (b) nonlinear transmission of SWCNTs (red) and monolayer graphene (blue) at the wavelength of 1030 nm.