The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed

Photonic-Crystal Fibers

Femtosecond-laser micromachining (also known as inscription or writing) has been developed as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. This paper focuses on crystalline dielectric materials, and is a review of the state-of-the-art in the fabrication, characterization and applications of femtosecond-laser micromachined waveguiding structures in optical crystals and ceramics. A brief outlook is presented by focusing on a few potential spotlights.

Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining

Rare-earth ion doped TeO2 and GeO2 glasses as laser materials

In this paper we review the micromachining of photonic devices in several materials by means of ultrashort laser pulses. The physical mechanisms underlying the refractive index modification and the different laser systems used to induce such modification are discussed. A thorough review of the photonic devices demonstrated with the femtosecond laser microfabrication technique is presented. In particular, this paper is focused on photonic devices based on optical waveguides. The devices are organized into two categories: passive and active devices. In the former category power splitters, directional couplers, interferometers and Bragg gratings are reviewed, while in the latter amplifiers and lasers are discussed. Finally, conclusions and future perspectives of femtosecond laser micromachining in photonics are provided.

https://iopscience.iop.org/article/10.1088/1464-4258/11/1/013001/pdf

Micromachining of photonic devices by femtosecond laser pulses

The rapid development of the femtosecond laser has revolutionized materials processing due to its unique characteristics of ultrashort pulse width and extremely high peak intensity. The short pulse width suppresses the formation of a heat-affected zone, which is vital for ultrahigh precision fabrication, whereas the high peak intensity allows nonlinear interactions such as multiphoton absorption and tunneling ionization to be induced in transparent materials, which provides versatility in terms of the materials that can be processed. More interestingly, irradiation with tightly focused femtosecond laser pulses inside transparent materials makes three-dimensional (3D) micro- and nanofabrication available due to efficient confinement of the nonlinear interactions within the focal volume. Additive manufacturing (stereolithography) based on multiphoton absorption (two-photon polymerization) enables the fabrication of 3D polymer micro- and nanostructures for photonic devices, micro- and nanomachines, and microfluidic devices, and has applications for biomedical and tissue engineering. Subtractive manufacturing based on internal modification and fabrication can realize the direct fabrication of 3D microfluidics, micromechanics, microelectronics, and photonic microcomponents in glass. These microcomponents can be easily integrated in a single glass microchip by a simple procedure using a femtosecond laser to realize more functional microdevices, such as optofluidics and integrated photonic microdevices. The highly localized multiphoton absorption of a tightly focused femtosecond laser in glass can also induce strong absorption only at the interface of two closely stacked glass substrates. Consequently, glass bonding can be performed based on fusion welding with femtosecond laser irradiation, which provides the potential for applications in electronics, optics, microelectromechanical systems, medical devices, microfluidic devices, and small satellites. This review paper describes the concepts and principles of femtosecond laser 3D micro- and nanofabrication and presents a comprehensive review on the state-of-the-art, applications, and the future prospects of this technology.

Femtosecond laser three-dimensional micro- and nanofabrication

A fan-out device has been fabricated using ultrafast-laser waveguide-inscription that enables each core of a multicore optical fiber (MCF) to be addressed by a single mode fiber held in a fiber V-groove array (FVA). By utilizing the unique three-dimensional fabrication capability of this technique we demonstrate coupling between an FVA consisting of a one-dimensional array of fibers and an MCF consisting of a two-dimensional array of cores. When coupled to all cores of the MCF simultaneously, the average insertion loss per core was 5.0 dB in the 1.55 mum spectral region. Furthermore, the fan-out exhibited low cross-talk and low polarization dependent loss.

Ultrafast-laser inscription of a three dimensional fan-out device for multicore fiber coupling applications.

Waveguide structures are fabricated in z-cut lithium niobate (LiNbO3) using focussed femtosecond pulses Two different types of waveguide structure are fabricated depending on the pulse energy used In the first, guiding occurs in regions directly surrounding a visible laser-damage region In the second, guiding occurs in a material modification region created at the focus High confinement guiding at 1550nm is demonstrated in the first type of waveguide but found to be temporary, thus indicating that at least part of the refractive index change is due to phenomena such as stress that are subject to relaxation Finally, the polarization dependent guiding properties of the structures are investigated

Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime

We describe delivery of femtosecond solitons at 800nm wavelength over five meters of hollow-core photonic bandgap fiber. The output pulses had a length of less than 300fs and an output pulse energy of around 65nJ, and were almost bandwidth limited. Numerical modeling shows that the nonlinear phase shift is determined by both the nonlinearity of air and by the overlap of the guided mode with the glass.

Femtosecond soliton pulse delivery at 800nm wavelength in hollow-core photonic bandgap fibers.

A three dimensional fan-out device has been fabricated using ultrafast laser inscription. The device allows each core of a multicore fibre to be addressed individually by a single mode fiber held in an FVA.

Ultrafast laser inscription of a three dimensional fan-out device for multicore fiber coupling applications

A channel waveguide is fabricated inside an erbium-doped oxyfluoride silicate glass sample using femtosecond pulses in the low repetition rate regime. The waveguide cross section is controlled using the multiscan fabrication technique. The 1.85-cm-long waveguide exhibits a total background insertion loss of 4.3 dB when coupled to Corning SMF-28 fibers. Under the maximum available pump power, the device exhibits an internal gain of 1.7 dB at 1537 nm.

S. Campbell

Papers

Ultrafast-laser inscription of a three dimensional fan-out device for multicore fiber coupling applications.

Optical waveguide fabrication in z-cut lithium niobate (LiNbO3) using femtosecond pulses in the low repetition rate regime

Femtosecond soliton pulse delivery at 800nm wavelength in hollow-core photonic bandgap fibers.

Ultrafast laser inscription of a three dimensional fan-out device for multicore fiber coupling applications

Internal gain from an erbium-doped oxyfluoride-silicate glass waveguide fabricated using femtosecond waveguide inscription