Femtosecond Laser Direct-write Technique Research Articles

Short-length nanosecond pulsed 1.653-μm DBR fiber Raman laser oscillator fabricated by femtosecond laser direct-writing

We demonstrate a nanosecond pulsed 1.653-μm fiber Raman laser (FRL) oscillator in a short-length distributed Bragg reflector (DBR) cavity made by femtosecond laser direct-writing technique. The DBR FRL cavity is composed of commercial ultrahigh NA germanium-doped silica fiber with a short length of 10 m and a pair of fiber Bragg gratings (FBGs). The FBGs were directly inscribed on the ends of the Raman gain fiber, using femtosecond-laser point-by-point (PbP) writing method. A home-made 1.55-μm nanosecond fiber laser source was used as the pump of the FRL. 1.653-μm nanosecond pulsed laser oscillation with maximum average output power of 125 mW (corresponding to peak power of 6.6 W), 3 dB linewidth of ≤ 0.15 nm, and slope efficiency of 48 % was obtained under 1.55-μm pump with optimized repetition rate of 10 MHz. The optical signal-to-noise-ratio (OSNR) of the 1.653-μm narrow-linewidth Raman lasing is above 70 dB. The short-length DBR FRL output exhibits decently high degree of polarization (DOP) and high degree of linear polarization (DOLP) above 80 % in 3-hour period, benefitted from the birefringent FBGs made by femtosecond laser direct-writing method. It is expected that by further reducing the cavity length to < 1 m, the 1.653-μm DBR FRL oscillator made by femtosecond laser direct-writing method is promising as single-frequency narrow-linewidth, highly linearly polarized, high-OSNR laser sources for high-sensitivity CH4 gas detection.

Toward a10 dB net-gain waveguide amplifier in an Er3+-Yb3+ co-doped phosphate glass.

High-gain materials and high-quality structures are the two main conditions that determine the amplification performance of optical waveguides. However, it has been hard to balance each other, to date. In this work, we demonstrate breakthroughs in both glass optical gain and optical waveguide structures. We propose a secondary melting dehydration technique that prepares high-quality Er3+-Yb3+ co-doped phosphate glass with low absorption loss. Additionally, we propose a femtosecond laser direct-writing technique that allows controlling the cross section, size, and mode field of waveguides written in glass with high accuracy, leveraging submicron-resolution multi-scan direct-writing optical waveguide technology, which is beneficial for reducing insertion loss. As a proof of concept demonstration, we designed and fabricated two kinds of waveguides, namely, LP01- and LP11-mode waveguides in the Er3+-Yb3+ co-doped phosphate glass, enabling insertion loss as low as 0.9 dB for a waveguide length of 2 mm. Remarkably, we successfully achieved an optical amplification for both the waveguides with a net gain of >7 dB and a net-gain coefficient of >3.5 dB/mm, which is approximately one order of magnitude larger than that in the Er3+-Yb3+ co-doped phosphate glass fabricated by the traditional melt-quenching method. This will open new avenues toward the development of integrated photonic chips.

Direct femtosecond laser inscription of Bragg gratings in Ho3+/Pr3+ co-doped AlF3-based glass fibers for a 2.86 µm laser.

In this Letter, we report the fabrication of fiber Bragg gratings (FBGs) in home-made Ho3+/Pr3+ co-doped single-cladding fluoroaluminate (AlF3) glass fibers and its application in watt-level lasing at the mid-infrared (MIR) wavelength of 2.86 µm. The FBGs were inscribed using an 800 nm femtosecond (fs) laser direct-writing technique. The FBG properties were investigated for different pulse energies, inscription speeds, grating orders, and transversal lengths. A second-order FBG with a high reflectivity of 99% was obtained at one end of a 16.5-cm-long gain fiber. Under 1150 nm laser pumping, this fiber yielded a power exceeding 1 W at 2863.9 nm with an overall laser efficiency of 17.7%. The fiber laser showed a FWHM bandwidth of 0.46 nm and long-term spectral stability.

MXene Ti3C2Tx saturable absorber for passively Q-switched mid-infrared laser operation of femtosecond-laser–inscribed Er:Y2O3 ceramic channel waveguide

Abstract We report on an Er:Y2O3 ceramic channel waveguide laser operating at continuous-wave and passively Q-switched regimes, for the first time to the best of our knowledge. The Er:Y2O3 ceramic channel waveguide has been fabricated by a femtosecond laser direct-writing technique. The maximum output power reached 123.5 mW in continuous-wave mode with a slope efficiency of about 21%, and the lasing behavior has been found simultaneously at 2710.28, 2716.76, and 2723.18 nm. We have further fabricated an MXene Ti3C2Tx saturable absorber mirror to operate the Er:Y2O3 waveguide in passively Q-switched regime. At the same time, property of the MXene Ti3C2Tx saturable absorber has been characterized, and nonlinear saturable absorption of the saturable absorber has also been measured. The average output power is about 67.6 mW, and the shortest pulse width is about 58.1 ns. This work indicates that femtosecond laser direct writing is a reliable technique for processing mid-infrared ceramic waveguides, and MXene Ti3C2Tx is promising for operating pulsed mid-infrared lasers.

Femtosecond laser written arrayed waveguide gratings with integrated photonic lanterns.

We demonstrate for the first time functional arrayed waveguide gratings (AWGs) fabricated using the femtosecond laser direct-write technique. This fabrication technique is a mask-less alternative to lithography enabling design flexibility and rapid prototyping. It is ideal for customized small scale production for new applications. The devices were demonstrated in the visible region at 632.8 nm with a measured free spectral range (FSR) of 22.2 nm, and 1.35 nm resolution. To highlight the advantages of using a 3-dimensional fabrication technique, a 3-port photonic lantern was integrated with an AWG in a single monolithic chip. Integration of this type is not feasible with lithography-based AWG fabrication and can increase the functionality of AWGs for sensing applications.

Design and fabrication of reconfigurable laser-written waveguide circuits

Reconfigurability is an important requirement for implementing quantum photonic processing using waveguide circuits in which both high fidelity and the ability to change the optical transformation dynamically are necessary. This work aims to address the issue of scalability in reconfigurable waveguide circuits fabricated using the femtosecond laser direct-write (FLDW) technique. A set of reconfigurable waveguide Mach-Zehnder interferometers were designed and fabricated using a combination of femtosecond laser waveguide inscription and picosecond laser ablation. Thermal cross-talk between adjacent phase-shifters was managed by machining microchannels into the chip surface to isolate individual waveguides from the rest of the substrate. The tuning efficiency as defined by the dissipated power per unit of induced phase shift was improved by a factor of two in this way while maintaining a smaller device footprint than previous demonstrations of phase tuning in laser-written waveguides. The phase response of the waveguide interferometers to the heaters was thoroughly characterised and was well predicted by simulation. A characterization of the time-dependent response of the reconfigurable interferometers was also performed. A rise time measurement revealed that the circuit can be reconfigured within seconds. No long-term phase drifts away from a set point were observed over a period of more than 12 hours, and the short term phase stability was better than 6 mrad.

Towards femtosecond laser written arrayed waveguide gratings.

The fabrication of arrayed waveguide gratings (AWGs) using the femtosecond laser direct-write technique is investigated. We successfully demonstrate the fabrication of large planar waveguides that act as 2D free propagation zones. These slabs were found to have a highly uniform refractive index with a standard deviation of 1.97% relative to the total index contrast. The incorporation of low loss linear adiabatic tapers resulted in an increase of transmission by 90%. Strategies for manufacturing integrated laser written AWGs using continuous contouring to avoid lossy defects are discussed and demonstrated.

Low loss mid-infrared ZBLAN waveguides for future astronomical applications.

Photonic technologies will be at the heart of future terrestrial planet hunting interferometers. In particular the mid-infrared spectral region between 3.5 - 4.2 μm is the ideal window for hunting for young extra-solar planets, since the planet is still hot from its formation and thus offers a favorable contrast with respect to the parent star compared to other spectral regions. This paper demonstrates two basic photonic building blocks of such an instrument, namely single-mode waveguides with propagation losses as low as 0.29±0.03 dB/cm at a wavelength of 4 μm as well as directional couplers with a constant splitting ratio across a broad wavelength band of 500 nm. The devices are based on depressed cladding waveguides inscribed into ZBLAN glass using the femtosecond laser direct-write technique. This demonstration is the first stepping stone towards the realization of a high transmission mid-infrared nulling interferometer.

Waveguide inscription in Bismuth Germanate crystals using high repetition rate femtosecond lasers pulses

We report on the inscription of optical waveguides into crystalline bismuth germanate (Bi4Ge3O12, BGO) via the femtosecond laser direct-write (FLDW) technique. We found that by utilizing femtosecond laser pulses at Megahertz (MHz) repetition rates, a uniquely different fabrication regime can be exploited. In this paper, we show that cumulative heating effects can initiate a local transformation of the crystalline structure into an amorphous (glass-like) state that is characterized by an increased refractive index. We compare and contrast this novel, type-I modification based waveguide inscription regime with the previously reported fabrication of type-II damage/stress field structures in BGO and present measurements that indicate that the femtosecond laser writing process unavoidably causes a reduction in the electro-optic coefficient in the waveguides as compared to the bulk material. We discuss the potential of this technique for the fabrication of advanced sensor arrays for high-energy radiation detection and voltage sensing applications.

Narrow Linewidth DFB Waveguide Laser Fabricated via Ultrafast Laser Inscription

Linewidth measurement of a femtosecond laser direct-written distributed feedback (DFB) waveguide laser (WGL) is reported. The WGL was fabricated in Yb-doped phosphate glass using the femtosecond laser direct-write technique. The linewidth was measured using a loss-compensated recirculating delayed self-heterodyne interferometer. By recirculating the output signal in a 10.2-km fiber delay loop, the linewidth was measured to be 35.4±1.4 kHz at a delay time of 306 $\mu \text{s}$ , which is comparable with that of narrow-linewidth fiber DFB lasers.

Femtosecond direct-written integrated mode couplers.

We report the design and fabrication of three-dimensional integrated mode couplers operating in the C-band. These mode-selective couplers were inscribed into a boro-aluminosilicate photonic chip using the femtosecond laser direct-write technique. Horizontally and vertically written two-core couplers are shown to allow for the multiplexing of the LP11a and LP11b spatial modes of an optical fiber, respectively, with excellent mode extinction ratios (25-37 + dB) and low loss (~1 dB) between 1500 and 1580 nm. Furthermore, optimized fabrication parameters enable coupling ratios close to 100%. When written in sequence, the couplers allow for the multiplexing of all LP01, LP11a and LP11b modes. This is also shown to be possible using a single 3-dimensional three-core coupler. These integrated mode couplers have considerable potential to be used in mode-division multiplexing for increasing optical fiber capacity. The three-dimensional capability of the femtosecond direct-write technique provides the versatility to write linear cascades of such two- and three-core couplers into a single compact glass chip, with arbitrary routing of waveguides to ensure a small footprint. This technology could be used for high-performance, compact and cost-effective multiplexing of large numbers of modes of an optical fiber.

Three-dimensional ultra-broadband integrated tapered mode multiplexers

The demonstration of a three-dimensional tapered mode-selective coupler in a photonic chip is reported. This waveguide-based, ultra-broadband mode multiplexer was fabricated using the femtosecond laser direct-write technique in a boro-aluminosilicate glass chip. A three-core coupler has been shown to enable the multiplexing of the LP01, LP and LP spatial modes of a multimode waveguide, across an extremely wide bandwidth exceeding 400 nm, with low loss, high mode extinction ratios and negligible mode crosstalk. Linear cascades of such devices on a single photonic chip have the potential to become a definitive technology in the realization of broadband mode-division multiplexing for increasing optical fiber capacity.

Creating large second-order optical nonlinearity in optical waveguides written by femtosecond laser pulses in boro-aluminosilicate glass

The thermal poling technique was applied to optical waveguides embedded in a commercial boro-aluminosilicate glass, resulting in high levels of induced second-order optical nonlinearity. The waveguides were fabricated using the femtosecond laser direct-write technique, and thermally poled samples were characterized with second harmonic optical microscopy to reveal the distribution profile of the induced nonlinearity. It was found that, in contrast to fused silica, the presence of waveguides in boro-aluminosilicate glass led to an enhancement of the creation of the second-order nonlinearity, which is larger in the laser written waveguiding regions when compared to the un-modified substrate. The magnitude of the nonlinear coefficient d33 achieved in the core of the laser-written waveguides, up to 0.2 pm/V, was comparable to that in thermally poled fused silica, enabling the realization of compact integrated electro-optic devices in boro-aluminosilicate glasses.

Towards low-loss lightwave circuits for non-classical optics at 800 and 1,550 nm

We review the recent advances in integrated quantum optical technologies, with specific emphasis on the femtosecond laser direct-write technique. We present a comparative study of the processing windows which produce low-loss waveguides in two glasses, Schott AF-45 and Corning Eagle-2000. We report the losses at wavelengths 800 and 1,550 nm, the two most critical wavelengths for quantum information science. We find the iron absorption in Eagle-2000 to be the limiting factor for propagation losses, suggesting that low Fe\(^{2+}\) glasses are better suited for quantum optical science.

Integrated photonic building blocks for next-generation astronomical instrumentation II: the multimode to single mode transition

There are numerous advantages to exploiting diffraction-limited instrumentation at astronomical observatories, which include smaller footprints, less mechanical and thermal instabilities and high levels of performance. To realize such instrumentation it is imperative to convert the atmospheric seeing-limited signal that is captured by the telescope into a diffraction-limited signal. This process can be achieved photonically by using a mode reformatting device known as a photonic lantern that performs a multimode to single-mode transition. With the aim of developing an optimized integrated photonic lantern, we undertook a systematic parameter scan of devices fabricated by the femtosecond laser direct-write technique. The devices were designed for operation around 1.55 μm. The devices showed (coupling and transition) losses of less than 5% for F/# ≥ 12 injection and the total device throughput (including substrate absorption) as high as 75-80%. Such devices show great promise for future use in astronomy.

Molding topologically-complex 3D polymer microstructures from femtosecond laser machined glass

The fabrication of complex, three-dimensional microscale shapes that can be replicated over large surfaces is an ongoing challenge, albeit one with a wide range of possible applications such as engineered surfaces with tuned wetting properties, scaffolds for cell studies, or surfaces with tailored optical properties. In this work, we use a two-step femtosecond laser direct-write technique and wet-etching process to fabricate monolithic glass micromolds with complex three-dimensional surface topologies, and demonstrate the replication of these structures in a soft polymer (polydimethylsiloxane, PDMS). To estimate the forces experienced during the demolding for one representative structure, we use a combination of two models – a simple linear elastic model and a numerical hyperelastic model. These models are used to support the high experimental success rates of the demolding process observed, despite the high strain induced in the material during demolding. Since the process used is scalable, this work opens new avenues for low-cost fabrication of surfaces having complex microscale patterns with three-dimensional geometries.

Theoretical modeling and experiments on a DBR waveguide laser fabricated by the femtosecond laser direct-write technique

We present a model for a Yb-doped distributed Bragg reflector (DBR) waveguide laser fabricated in phosphate glass using the femtosecond laser direct-write technique. The model gives emphasis to transverse integrals to investigate the energy distribution in a homogenously doped glass, which is an important feature of femtosecond laser inscribed waveguide lasers (WGLs). The model was validated with experiments comparing a DBR WGL and a fiber laser, and then used to study the influence of distributed rare earth dopants on the performance of such lasers. Approximately 15% of the pump power was absorbed by the doped "cladding" in the femtosecond laser inscribed Yb doped WGL case with the length of 9.8 mm. Finally, we used the model to determine the parameters that optimize the laser output such as the waveguide length, output coupler reflectivity and refractive index contrast.

Fabrication of fully integrated antiresonant reflecting optical waveguides using the femtosecond laser direct-write technique

Utilizing a recently developed two-step fabrication process, we have experimentally demonstrated what is to the best of our knowledge the first ever integrated antiresonant reflecting optical waveguide (ARROW), directly inscribed into bulk optical glass. The waveguide consists of an array of high-index contrast rods that surround an unmodified core. The transmission spectrum reveals the formation of discrete bandgaps that can be tuned by varying the diameter of the individual rods.

Non-classical interference in integrated 3D multiports

We demonstrate three and four input multiports in a three dimensional glass platform, fabricated using the femtosecond laser direct-write technique. Hong-Ou-Mandel (HOM) interference is observed and a full quantum characterization is performed, obtaining two photon correlation matrices for all combinations of input and output ports. For the 3-port case, the quantum visibilities are accurately predicted solely from measurement of the classical coupling ratios.

Femtosecond direct-write überstructure waveguide Bragg gratings in ZBLAN

Strong waveguide Bragg gratings (10.5dB transmission dip) were fabricated using the femtosecond (fs) laser direct-write technique in ZBLAN glass. The Bragg gratings are based on depressed cladding waveguides and consist of planes, periodic according to the Bragg condition, which are constructed from a transverse hexagonal lattice of smaller point features. Such gratings are a key step toward the realization of mid-infrared monolithic waveguide lasers using the fs laser direct-write technique.