Passive integrated circuits utilizing slow light in photonic crystal waveguides

The phenomenon of slow light in photonic crystal waveguides is discussed. Rather than maximising the slowdown factor, we believe that slow light is only useful for all-optical data processing if there is sufficient bandwidth, hence a slowdown factor of order 10-100 is more favourable, given that it enables bandwidths of order 1 THz or more to be realised. As a specific example, we demonstrate a slowdown factor of 12 (group index of 25) over a bandwidth of 2.5 THz in a W2 photonic crystal waveguide. Furthermore, slow light can only be useful if it is not compromised by losses. Due to recent improvements in our technology, we can now achieve losses of order 4 dB/cm, which is amongst the best reported for W1 photonic crystal waveguides.

Broadly tunable and ultra-wideband slow light was theoretically demonstrated in a line defected photonic crystal waveguide (PCW) by infiltrating fluid with suitable refractive index into the first row of air holes adjacent to the waveguide. Simulation results showed that the working wavelength of the flat slow light in the optimized optofluidic PCW could be tuned from 1542.5 nm to 1569 nm with nearly constant group index of 80. Namely, broadly tunable slow light could be realized with the ultra-wide bandwidth over 26.5 nm and the group index up to 80. These achievements could not only open the possibility for achieving many excellent optical devices based on slow light in PCW, but also highlight the versatility, flexibility, and tunability offered by optofluidic infiltration schemes.

A photonic crystal waveguide that features slow light without noticeable dispersion is demonstrated using a higher order even mode in a W2 waveguide on a SOI platform.

We experimentally demonstrate topological slow light waveguides in valley photonic crystals (VPhCs). We employed a bearded interface formed between two topologically-distinct VPhCs patterned in an air-bridged silicon slab. The interface supports both topological and non-topological slow light modes below the light line. By means of optical microscopy, we observed light propagation in the topological mode in the slow light regime with a group index ng over 30. Furthermore, we confirmed light transmission via the slow light mode even under the presence of sharp waveguide bends. In comparison between the topological and non-topological modes, we found that the topological mode exhibits much more efficient waveguiding than the trivial one, demonstrating topological protection in the slow light regime. This work paves the way for exploring topological slow-light devices compatible with existing photonics technologies.

We present a compact integrated refractive index sensor using slow light in photonic crystal waveguides (PCWs). The PCWs are used in a Mach-Zahnder interferometer configuration and operated close to the edge of the mode-gap to achieve a compact design and high sensitivity.

The realization of mode-locked lasers capable of generating short optical pulses in the near-infrared is a key enabler for various applications including high data rate optical interconnects. The related technology mostly consists of bulky and stand-alone devices, which tends to hinder their widespread use. In addition, the integration of such devices onto chip-based platforms could bring advantages in terms of robustness and stability, while offering some prospects for the realization of advanced hybrid photonic-electronic architectures. There has been recently some progress regarding the miniaturization of fast pulsed lasers [1], but the underlying cavity typically remains longer than a few millimetres and their integration on a chip is still a challenge [2]. Here, we aim at realizing a compact integrated pulsed micro-laser using an innovative photonic crystal (PhC) cavity based on low dispersion slow light modes. Most of the characteristics of mode-locked lasers are tightly linked to the product of the cavity length L and the group index ng of the modes traveling in the cavity. While miniaturization thus tends to degrade the quality of the generated optical pulses, we counterbalance this effect through exploiting slow light (high ng) modes in PhC structures. Dispersion engineering techniques have been successfully developed in passive Si/air suspended PhC cavities for decreasing the otherwise typically high dispersion of these modes [3]. We adapted these techniques to our asymmetric InP/silica structures so as to provide a frequency-equidistant comb of modes - a pre-requisite to laser mode-locking. While our geometry well improves laser heat dissipation with respect to PhC cavities suspended in air, it poses additional constraints and could increase the cavity mode optical losses. Using 3D FDTD simulations, we demonstrated the validity of this approach and designed, as an example, a 30 μm long, linear dispersion PhC cavity, with a group index of 29, providing at least 9 equally-spaced modes over an 11.5 nm bandwidth, which can afford the generation of sub-picosecond optical pulses. Despite the structure asymmetry, quality factors were all above 10,000, i.e. sufficient for reaching laser operation. Compared with a standard strip-waveguide-based laser cavity, this design provides a length reduction factor of almost one order of magnitude, as given by the ratio of the group index of each structure. Various designs enabled us to achieve a range of low dispersion bands with different group index (15-40), offering a relevant trade-off between device miniaturization and short pulse duration. Based on these designs, PhC active structures including InAsP/ InP quantum wells or InAs/InP quantum dashes were fabricated using molecular beam epitaxy, III-V/silica bonding, e-beam lithography and reactive ion etching. Near-field optical microscopy was used to identify both the spectral and spatial signatures of the cavity modes, which were in good agreement with the simulations. We also probed our cavities by microphotoluminescence. These cavities provide a first step towards the realization of miniaturized integrated mode-locked lasers. Their robust hybrid InP/silica geometry offers the potential for integration with silicon photonic devices, while enabling the integration of a saturable absorber, such as graphene, for achieving mode-locked laser operation.

Mach–Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p–n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p–n diode. This length is shorter than commercial LiNbO3 modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index ng. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.

Slow light in photonic crystal waveguides allows the size reduction in MZ modulators. Even with 50 - 200 micron phase shifters, modulators operate at 10 Gbps and beyond in the wide spectral and temperature ranges.

Improving slow light effect in photonic crystal line defect waveguide by using eye-shaped scatterers

We substitute different types of photonic crystal waveguide components by approximate transmission line circuits. The proposed distributed circuits exploit the analogy of wave propagation in photonic crystal waveguides and transmission lines. They are either cascaded to each other or inserted like stubs to imitate wave propagation within the photonic structure. Notable examples, e.g. coupled waveguide-cavity systems, sharp 90° bends, and T-junctions, are studied in detail. It is shown that analysis of the proposed circuits here can yield accurate enough results and thus substitute the brute-force numerical methods. The privilege of having analytical models is exploited to improve the performance of sharp 90° bends and T-junctions. The presented results are verified by using the standard finite-difference time domain (FDTD) method.

The Raman-induced nonreciprocity of light propagation in photonic-crystal silicon waveguides is analysed. Although the Raman gain increases in the slow-light regime, the nonreciprocity is smaller. Thus, it is interesting that Raman-pumped optical isolators benefit from slow-light operation. Significant reductions in pump power and/or device length are found to be possible.

Recently, planar photonic crystals (PhCs) have emerged as one possible and flexible platform for integrating various optical components onto the same chip. Of the extraordinary dispersion properties that are exhibited by planar PhC waveguides lies the possibility of significantly reducing the group velocity at which light propagates [1]. Most significantly, combining slow light PhCs with a nonlinear medium enhances all kinds of optical nonlinear processes [2], with the potential for realising compact all-optical nonlinear devices that operate at low power. In addition to providing slow light propagation, PhC waveguides can be further engineered, so that slow light modes can present very low dispersion. A number of approaches have been used to achieve this, for example by slightly modifying the geometry of the PhC waveguide [3] or by using selective liquid infiltration [4]. The possibility of managing the dispersion in PhC waveguides is highly advantageous, as it provides a way to control higher order dispersion parameters in addition to the group velocity for optimizing processes that strongly depend on dispersive characteristics and nonlinearities. In our previous works, we have investigated both numerically [5] and experimentally [6] four-wave mixing (FWM) in short (80 µm) dispersion engineered slow light silicon photonic crystal waveguides. Compared with silicon PhC waveguides, chalcogenide waveguides do not suffer from two photon absorption (TPA) and free carrier absorption (FCA). Therefore, the high nonlinearity of chalcogenide glass and merging it with slow light in photonic crystals can potentially be used to make a very compact GVD and SPM compensation device. Mid-span phase conjugation is a technique that can compensate for both GVD and SPM, thereby allowing higher power transmission with fewer amplifiers.

We report FWM (−9dB conversion) in a short (80μm) silicon dispersion engineered slow (c/30) light photonic crystal waveguide through launching a pulsed pump and a CW probe signal.

We present a simulation approach to estimate the effect of disorder induced backscattering in slow light photonic crystal line-defect waveguides. The backscattering leads to localization and thus limits the maximal length of such waveguides. Loss in passive waveguides in the localization regime reduces ripples in spectral transmission and group delay whereas gain increases these resonant multiple scattering effects.

We report controllable manipulation of slow and fast light in a whispering-gallery-mode microtoroid resonator fabricated from Erbium (Er3+) doped silica. We observe continuous transition of the coupling between the fiber-taper waveguide and the microresonator from undercoupling to critical coupling and then to overcoupling regimes by increasing the pump power even though the spatial distance between the resonator and the waveguide was kept fixed. This, in turn, enables switching from fast to slow light and vice versa just by increasing the optical gain. An enhancement of delay of two-fold over the passive silica resonator (no optical gain) was observed in the slow light regime. Moreover, we show dynamic pulse splitting and its control in slow/fast light systems using optical gain.