Large Mode Size Research Articles

Spectral broadening in convex-concave multipass cells.

Since its first demonstration in 2016, the multi-pass spectral broadening technique has covered impressive ranges of pulse energy (3 µJ - 100 mJ) and peak power (4 MW - 100 GW). Energy scaling of this technique into the joule-level is currently limited by phenomena such as optical damage, gas ionization and spatio-spectral beam inhomogeneity. These limitations can be overcome by the novel multi-pass convex-concave arrangement, which exhibits crucial properties such as large mode size and compactness. In a proof-of-principle experiment, 260 fs, 15 µJ and 200 µJ pulses are broadened and subsequently compressed to approximately 50 fs with 90% efficiency and excellent spatio-spectral homogeneity across the beam profile. We simulate the proposed concept for spectral broadening of 40 mJ and 1.3 ps input pulses and discuss the possibility of further scaling.

Novel Low-Loss Fiber-Chip Edge Coupler for Coupling Standard Single Mode Fibers to Silicon Photonic Wire Waveguides

Fiber-to-chip optical interconnects is a big challenge in silicon photonics application scenarios such as data centers and optical transmission systems. An edge coupler, compared to optical grating, is appealing to in the application of silicon photonics due to the high coupling efficiency between standard optical fibers (SMF-28) and the sub-micron silicon wire waveguides. In this work, we proposed a novel fiber–chip edge coupler approach with a large mode size for silicon photonic wire waveguides. The edge coupler consists of a multiple structure which was fulfilled by multiple silicon nitride layers embedded in SiO2 upper cladding, curved waveguides and two adiabatic spot size converter (SSC) sections. The multiple structure can allow light directly coupling from large mode size fiber-to-chip coupler, and then the curved waveguides and SSCs transmit the evanescent field to a 220 nm-thick silicon wire waveguide based on the silicon-on-insulator (SOI) platform. The edge coupler, designed for a standard SMF-28 fiber with 8.2 μm mode field diameter (MFD) at a wavelength of 1550 nm, exhibits a mode overlap efficiency exceeding 95% at the chip facet and the overall coupling exceeding 90%. The proposed edge coupler is fully compatible with standard microfabrication processes.

Analysis of Optical Integration between Si3N4 Waveguide and a Ge-Based Optical Modulator Using a Lateral Amorphous GeSi Taper at the Telecommunication Wavelength of 1.55 µm

We report on the theoretical investigation of using an amorphous Ge0.83Si0.17 lateral taper to enable a low-loss small-footprint optical coupling between a Si3N4 waveguide and a low-voltage Ge-based Franz–Keldysh optical modulator on a bulk Si substrate using 3D Finite-Difference Time-Domain (3D-FDTD) simulation at the optical wavelength of 1550 nm. Despite a large refractive index and optical mode size mismatch between Si3N4 and the Ge-based modulator, the coupling structure rendered a good coupling performance within fabrication tolerance of advanced complementary metal-oxide semiconductor (CMOS) processes. For integrated optical modulator performance, the Si3N4-waveguide-integrated Ge-based on Si optical modulators could simultaneously provide workable values of extinction ratio (ER) and insertion loss (IL) for optical interconnect applications with a compact footprint.

Thickness optimization for an anti-ASE cap in a thin disk laser considering dioptric power and aberration-induced loss

A numerical model is developed to analyze the optical path difference (OPD) of a capped disk with different thickness anti-ASE caps. The spherical and aspherical parts of the OPD are described by dioptric power and diffraction loss, which are used to evaluate the stability range of the resonator and output power of a thin disk laser, respectively. The analytical results show that the anti-ASE cap increases thermal aberration and diffraction loss compared with the uncapped disk. However, with a particular thickness anti-ASE cap, the dioptric power of the capped disk almost keeps constant with the increase of pump power density, which will be beneficial to designing a stable resonator with large fundamental mode size. At the same time, the diffraction loss will reach the maximum value at this thickness, too. Thus, weighing the thermal lens effect and diffraction loss will be important for designing an anti-ASE cap with an optimal thickness to realize higher-power laser generation.

10 mJ femtosecond Ti: Sapphire regenerative amplifier with large mode size

With advent of chirped-pulse amplification, the peak power of femtosecond laser pulse was reached to petawatt (PW) or hundreds of terawatt (TW). Many progresses of high-field physics and ultrafast dynamics in matter are achieved using TW or PW laser. Pre-amplifier is an exponential growth amplifier which is also a bridge between oscillator and power amplifier. The best choice of pre-amplifier is amplification in regenerative cavity, due to its high stability and beam quality. The quality of pre-amplified laser pulse is significant to efficiency and beam quality of the successive power amplifier. High energy pre-amplifier with high beam quality will reduce the requirement of pump laser in final power amplifier. But typical regenerative amplifier only support low output energy of few millijoule. Higher energy from only one regenerative amplifier is crucial to whole laser system. High energy regenerative amplifier can be achieved by increasing the size of TEM00 in cavity. A new femtosecond Ti:sapphire regenerative amplifier with large mode size was demonstrated in this letter. The regenerative cavity is designed as stable linear resonator in which end mirrors are planar, the diameter of beam waist in Ti:sapphire crystal is larger than 2 mm, which can support high energy pulse amplified in cavity. By matching the focal spot of pump laser with the size of mode and optimization of cavity, the output laser energy up to 17.4 mJ was achieved under the pump energy of 60 mJ at repetition rate of 10 Hz, which corresponds to the efficiency of 29%. The amplified laser pulse from regenerative amplifier was compressed in a grating-pair compressor. By carefully alignment of incident angle and distance between the two gratings of compressor, the shortest pulses duration of 40.6 fs and energy of 13.9 mJ are obtained, which is a little bit longer than Fourier-transform limit based on spectrum of laser. The dispersion in the CPA laser system was also analyzed, after optimization of compressor, there are still high order dispersions uncompensated, which results in the duration of compressed pulses longer than Fourier-transform limit. Based on this large mode size regenerative amplifier, peak power of 1.9 TW laser pulses which compressed pulse energy of 81.4 mJ in 43 fs were also further realized by following only one stage of multipass amplifier. The beam quality (M2) was measured to be 1.6 and 1.5 in X and Y directions respectively, and the energy stability is 2.15% (rms). The results show that this large mode size regenerative amplifier is an ideal choice of pre-amplifier in TW laser system.

Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides.

Fiber-chip edge couplers are extensively used in integrated optics for coupling of light between planar waveguide circuits and optical fibers. In this work, we report on a new fiber-chip edge coupler concept with large mode size for silicon photonic wire waveguides. The coupler allows direct coupling with conventional cleaved optical fibers with large mode size while circumventing the need for lensed fibers. The coupler is designed for 220 nm silicon-on-insulator (SOI) platform. It exhibits an overall coupling efficiency exceeding 90%, as independently confirmed by 3D Finite-Difference Time-Domain (FDTD) and fully vectorial 3D Eigenmode Expansion (EME) calculations. We present two specific coupler designs, namely for a high numerical aperture single mode optical fiber with 6 µm mode field diameter (MFD) and a standard SMF-28 fiber with 10.4 µm MFD. An important advantage of our coupler concept is the ability to expand the mode at the chip edge without leading to high substrate leakage losses through buried oxide (BOX), which in our design is set to 3 µm. This remarkable feature is achieved by implementing in the SiO2 upper cladding thin high-index Si3N4 layers. The Si3N4 layers increase the effective refractive index of the upper cladding near the facet. The index is controlled along the taper by subwavelength refractive index engineering to facilitate adiabatic mode transformation to the silicon wire waveguide while the Si-wire waveguide is inversely tapered along the coupler. The mode overlap optimization at the chip facet is carried out with a full vectorial mode solver. The mode transformation along the coupler is studied using 3D-FDTD simulations and with fully-vectorial 3D-EME calculations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 µm.

Unified picture of modal loss rates from microwave to optical frequencies in deep-subwavelength metallic structures: A case study with slot waveguides

The behavior of the modal loss rate in deep-subwavelength metallic structures depends strongly on frequency: as the mode size decreases, at optical frequencies, the modal loss rate always increases to the theoretical upper bound Γ/2, whereas at microwave frequencies, it remains far lower than Γ/2, where Γ is the electron collision frequency of the metal. By analyzing the metallic slot waveguide as a model system, we show that these significantly different behaviors of the modal loss rate at optical and microwave frequencies are actually two extreme cases of a single universal behavior. Specifically, we show that as the mode size decreases, the loss rate always plateaus first and then increases to Γ/2, regardless of frequency. The only difference between frequencies is the properties of the plateau: at optical frequencies, the plateau is narrow, allowing the loss rate to reach Γ/2 at a relatively large mode size, whereas at microwave frequencies, the plateau is wide and formed at 13ω, defining a practically attainable maximum loss rate that is far lower than Γ/2.

CMOS-compatible 75 mW erbium-doped distributed feedback laser.

On-chip, high-power, erbium-doped distributed feedback lasers are demonstrated in a CMOS-compatible fabrication flow. The laser cavities consist of silicon nitride waveguide and grating features, defined by wafer-scale immersion lithography and an erbium-doped aluminum oxide layer deposited as the final step in the fabrication process. The large mode size lasers demonstrate single-mode continuous wave operation with a maximum output power of 75mW without any thermal damage. The laser output power does not saturate at high pump intensities and is, therefore, capable of delivering even higher on-chip signals if a stronger pump is utilized. The amplitude noise of the laser is investigated and the laser is shown to be stable and free from self-pulsing when the pump power is sufficiently above threshold.

Low-loss ultra-subwavelength hybrid plasmonic waveguide based on metallic bump structures

Sharp metal edges (like wedge or triangular geometries) have been widely studied as low-loss surface plasmon polariton (SPP) waveguiding structures. However, fabrication difficulties lead to round-shaped tips which have larger mode size and shorter propagation length. As a solution, here we present an easy to realize structure for turning an edge-less bump metallic geometry (whose fabrication is less troublesome) into a hybrid one. Through investigating the geometrical dispersion, propagation lengths of up to half a millimetre and effective mode areas as small as λ2/330 are obtained for the 1.55 µm telecommunication wavelength. Gain-assisted SPP propagation is also considered by introducing gain in the InGaAsP material, for which critical gain as low as 11 cm−1 is calculated. Finally, obtained results suggest that the designed waveguide can find various applications in nanoscale waveguiding.

Computation of extreme eigenvalues in higher dimensions using block tensor train format

We consider approximate computation of several minimal eigenpairs of large Hermitian matrices which come from high-dimensional problems. We use the tensor train (TT) format for vectors and matrices to overcome the curse of dimensionality and make storage and computational cost feasible. We approximate several low-lying eigenvectors simultaneously in the block version of the TT format. The computation is done by the alternating minimization of the block Rayleigh quotient sequentially for all TT cores. The proposed method combines the advances of the density matrix renormalization group (DMRG) and the variational numerical renormalization group (vNRG) methods. We compare the performance of the proposed method with several versions of the DMRG codes, and show that it may be preferable for systems with large dimension and/or mode size, or when a large number of eigenstates is sought.

Subpicosecond Colliding Pulse Mode Locking at 126 GHz in Monolithic GaAs/AlGaAs Quantum Well Lasers: Experiments and Theory

In this paper, we present results from monolithic passively mode-locked GaAs/AlGaAs quantum well lasers operating at 830 nm. Colliding pulse mode locking is achieved at repetition rates of 126 GHz with pulsewidths as short as 0.43 ps, an unprecedented value in monolithic semiconductor lasers operating at such high pulse repetition rates. We use a double quantum well laser epistructure with larger mode size d/Γ (d is the quantum well thickness and Γ is the optical confinement) and investigate the effect of the saturable absorber length on the mode-locking operation. The experimental results are theoretically explained and reproduced using a traveling wave model with an improved time-domain response for both the gain and the absorber sections of the device. The model confirms that the thermally induced spectral detuning of the absorber relative to the gain section determines both the optimal absorber length and the optimal biasing conditions to achieve the shortest pulse duration and highest peak power.

A long ring regenerative cavity for high energy chirped pulse amplification

We demonstrate a novel regenerative amplifier with a long ring cavity. Calculation on cavity parameters supports a large mode size. Based on the simulation with the cavity design software, we successfully established a 12 m ring regenerative amplifier. The amplified laser energy up to 20 mJ was obtained under 150 mJ green pump laser at a repetition rate of 10 Hz, corresponding to 13.3% conversion efficiency. The spectral bandwidth of the output pulse was 30 nm, which could support a sub 30 fs pulse duration. After seed injection, the ASE of the amplifier was effectively suppressed.

Output Power Limitations and Improvements in Passively Mode Locked GaAs/AlGaAs Quantum Well Lasers

We report a novel approach for increasing the output power in passively mode locked semiconductor lasers. Our approach uses epitaxial structures with an optical trap in the bottom cladding that enlarges the vertical mode size to scale the pulse saturation energy. With this approach we demonstrate a very high peak power of 9.8 W per facet, at a repetition rate of 6.8 GHz and with pulse duration of 0.71 ps. In particular, we compare two GaAs/AlGaAs epilayer designs, a double quantum well design operating at 830 nm and a single quantum well design operating at 795 nm, with vertical mode sizes of 0.5 and 0.75 μm, respectively. We show that a larger mode size not only shifts the mode locking regime of operation towards higher powers, but also produces other improvements in respect of two main failure mechanisms that limit the output power: the catastrophic optical mirror damage and the catastrophic optical saturable absorber damage. For the 830 nm material structure, we also investigate the effect of non-absorbing mirrors on output power and mode locked operation of colliding pulse mode locked lasers.

High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser

We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6.

Stable High Power and High Beam Quality Diode-Side-Pumped Continuous-Wave Intracavity Frequency-Doubled Nd:YAG Laser

We report a stable high power and high beam quality diode-side-pumped cw green laser from intracavity frequency-doubled Nd:YAG laser with KTP. By using a L-shaped concave–convex resonator, designed with two Nd:YAG rods birefringence compensation, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously. The green laser delivers a maximum 532 nm output power of 23.2 W. Under 532 nm output power of 20.9 W, the beam quality factor is measured to be 4.1, and the fluctuation of the output power is less than 1.4% in an hour.

A high power quasi-continuous-wave yellow laser based on intracavity sum-frequency generation

This paper reports a high-power all-solid-state yellow laser based on intracavity KTP crystal sum-frequency generation. In order to achieve high output power，three key techniques are used：Firstly，the system is designed to operate in a pulsed mode to increase the beam quality and optical-to-optical conversion efficiency under the same pump level. Secondly，thermal near-unstable resonator design and two-rod birefringence compensation is used to acquire large fundamental mode size in Nd：YAG rod. Thirdly，by optimizing an L-shaped co-folding-arm plane-concave symmetrical cavity，the two fundamentals have a high spatial overlap in the sum-frequency crystal and satisfy the power ratio requirement. As a result of the applications of these techniques，a quasi-continuous-wave yellow laser with output power of 7.6W and repitition frequency of 1.1kHz is obtained. To the best of our knowledge，this is the highest yellow laser output power produced by an intracavity sum-frequency Nd：YAG laser.

A 4.8-W M2 = 4.6 Continuous-Wave IntracavitySum-Frequency Diode-Pumped Solid-State Yellow Laser

A yellow continuous wave with beam quality M2 = 4.6 andoutput power of 4.8 W at 589 nm is generated by intracavitysum-frequency mixing of 1064 nm and 1319 nm radiations of a Nd:YAG laser.To achieve high beam quality at high power, thermally near-unstableflat–flat resonators with two-rod birefringence compensation aredesigned to obtain the large fundamental mode size inside the Nd:YAGrods and the same beam width inside the KTP crystal. The optimalintracavity power ratio of both 1064 nm and 1319 nm beams is alsoconsidered. The output power fluctuation of the yellow laser remainsless than 5% in four hours.

One hundred and twenty one W green laser generation from a diode-side-pumped Nd:YAG laser by use of a dual-V-shaped configuration

To ensure high-average-power and long-lifetime second harmonic generation, a dual-V-shaped configuration was designed. In this thermally near-unstable resonator, large fundamental mode size in Nd:YAG rod and small one in LBO crystal could be generated, which improved both the beam quality and the power density. A green output power of 121 W with pulse width of 68 ns and repetition rate of 10 kHz was obtained. To the best of our knowledge, the dual-V-shaped configuration is the first time used for green laser with average output power of more than 100 W.

광섬유 수동정렬을 위한 단일 모드 대형 코어 폴리머 광도파로

정력 오차 허용 범위를 증가 시켜서 효과적인 수동정렬이 가능토록 하기 위한 단일 모드 대형 코어 폴리머 광도파로를 구현하였다. 대형 코어 광도파로는 TEC(thermally expanded core) 광섬유의 모드와 일치하는 큰 도파모드를 가진다. 이로 인해 광섬유와 도파로의 정렬오차로 인한 모드 결합 손실을 줄일 수 있게 된다. 코어와 클래딩의 굴절률 차이가 5 <TEX>${\times}$</TEX> <TEX>$10^{-4}$</TEX> 인 폴리머 재료를 이용하여 25 <TEX>${\times}$</TEX> 25 <TEX>$\mu\textrm{m}$</TEX><TEX>$^2$</TEX> 크기의 정사각형 광도파로를 제작하였으며 도파모드 관측결과 단일모드로 동작함을 확인하였다. 이와 같이 두꺼운 형태의 광도파로 구조 제작을 위하여 자외선 경화를 이용한 인젝션 몰딩(injection molding) 공정을 사용하였다. 제작된 광도파로 소자를 TEC 광섬유와 정렬연결을 할 때 정렬오차가 4.5 <TEX>$\mu\textrm{m}$</TEX> 까지 증가하더라도 삽입손실 증가는 0.5 dB 이하고 유지됨을 확인하였다. To increase the tolerance for passive fiber alignment, a single mode polymer waveguide with a large core structure is demonstrated. The large core waveguide is designed to have a mode profile comparable to that of a thermally expanded core (TEC) fiber, and it can be connected to a high-contrast waveguide through an adiabatic transition taper structure. From a waveguide with a rectangular core of 25 <TEX>${\times}$</TEX> 25 <TEX>${\mu}{\textrm}{m}$</TEX><TEX>$^2$</TEX>, a single mode propagation is observed when the index contrast is as low as 0.0005. A UV-cured injection molding method is used to fabricate the thick core structure. Due to the large mode size, the insertion loss of the device is below 0.5 dB until the lateral displacement of the TEC fiber is 4.5 <TEX>${\mu}{\textrm}{m}$</TEX>. The low insertion loss is important for reproducible passive alignment.

Robust slab-coupled buried-rib semiconductor laser with high fibre coupling efficiency

A robust, single transverse mode slab-coupled buried-rib waveguide laser by combining the concept of slab-coupled waveguide and the technique of diluted waveguide is demonstrated. The large mode size and near-circular mode profile result in high fibre coupling efficiency and improved alignment tolerance.