Elastic optical networks (EON) have been proposed to meet the network capacity and dynamicity challenges. Hardware and software resource optimization and re-configurability are key enablers for EONs. Recently, innovative multi-carrier transmission techniques have been extensively investigated to realize high capacity (Tb/s) flexible transceivers. In addition to standard telecommunication lasers, optical carrier generators based on optical frequency combs (OFC) have also been considered with expectations of reduced cost and inventory, improved spectral efficiency, and flexibility. A wide range of OFC generation techniques have been proposed in the literature over the past few years. It is imperative to summarize the state of the art, compare and assess these diverse techniques from a practical perspective. In this survey, we identify salient features of optical multicarrier generators, review and compare these techniques both from a physical and network layer perspective. OFC demultiplexing/filtering techniques have also been reviewed. In addition to transmission performance, the impact of such sources on the network performance and real-world deployment strategies with reference to cost, power consumption, and level of flexibility have also been discussed. Field trials, integrated solutions, and flexibility demonstrations are also reported. Finally, open issues and possible future directions that can lead to real network deployment are highlighted.

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A Survey of Optical Carrier Generation Techniques for Terabit Capacity Elastic Optical Networks

A monolithically integrated low linewidth optical comb is demonstrated by gain switching of a three-section laser device. The device consists of a slave and master section separated by a shared slotted mirror section. Wavelength tunability has been demonstrated by varying the electrical bias of each section. The number of comb lines is shown to almost double with the addition of optical injection from the master section into the slave. The unmodulated device has a full width half max linewidth of ∼ 500 kHz, while the comb line set were measured to be ∼ 600 kHz, with little degradation as a result of gain switching. The FSR (free spectral range) of the demonstrated comb is 4 GHz, which is tunable within the bandwidth of the device, with a central wavelength of 1580.3 nm.

Monolithically integrated low linewidth comb source using gain switched slotted Fabry-Perot lasers.

We show, both experimentally and theoretically, that a slave laser injected with an optical frequency comb can undergo two distinct locking mechanisms, both of which decrease the output optical comb’s frequency spacing. We report that, for certain detuning and relative injection strengths, slave laser relaxation oscillations can become undamped and lock to rational frequencies of the optical comb spacing, creating extra comb tones by nonlinear dynamics of the injected laser. We also study the frequency locking of the slave laser at detunings in between the injected comb lines, which add the slave laser’s frequency to the comb. Our results demonstrate the effect of the α parameter and stability of the locked states and indicate how the frequency of the relaxation oscillations affect both of these locking mechanisms. These optical locking mechanisms can be applied to regenerate or multiply optical combs.

Harmonic frequency locking and tuning of comb frequency spacing through optical injection.

A photonic integrated circuit is demonstrated that couples three Fabry–Perot (FP) lasers together via an integrated star coupler. The FP lasers rely on a common cleaved facet and individual multimode interference reflectors (MIRs) for lasing operation. The MIRs are monolithically integrated with each FP laser using a single growth step, which allows for epitaxial regrowth-free processing. By suitable independent tuning of each Fabry–Perot, a single mode operation and a mode selection can be attained from the coupled cavity system. The mode selectivity and side-mode suppression ratios of $\sim 40$ dB are demonstrated.

Coupled Cavity Single-Mode Laser Based on Regrowth-Free Integrated MMI Reflectors

Design and fabrication tolerance analysis of multimode interference couplers

We investigate the selective amplification and filtering of injection-locked slotted Fabry-Perot semiconductor lasers. Current and temperature tuning are used to selectively filter and amplify subcarriers of coherent optical combs with a selectivity of at least 10 GHz with an optical gain of up to 18 dB for filtered lines. A side mode suppression ratio in excess of 20 dB is also achieved.

Investigation of active filter using injection-locked slotted Fabry-Perot semiconductor laser

Optical injection locking has been demonstrated as an effective filter for optical communications. These optical filters have advantages over conventional passive filters, as they can be used on active material, allowing them to be monolithically integrated onto an optical circuit. We present an experimental and theoretical study of the optical suppression in injection locked Fabry–Perot and slotted Fabry–Perot lasers. We consider both single frequency and optical comb injection. Our model is then used to demonstrate that improving the Q factor of devices increases the suppression obtained when injecting optical combs. We show that increasing the Q factor while fixing the device pump rate relative to threshold causes the locking range of these demultiplexers to asymptotically approach a constant value.

https://www.mdpi.com/2304-6732/6/1/27/pdf

Mode Suppression in Injection Locked Multi-Mode and Single-Mode Lasers for Optical Demultiplexing

We demonstrate a monolithically photonic integrated circuit (PIC) comprising a single facet slotted laser (SFSL), a semiconductor optical amplifier (SOA), and a 1 × 2 multimode interference (MMI) coupler. The integrated SFSL generates a tunable single longitudinal mode output, and utilizes a simplified fabrication process for the PIC when compared with distributed feedback or distributed Bragg reflector lasers by eliminating the epitaxial regrowth as well as the ebeam or holographically generated gratings. The integration technique is implemented by biasing the MMIs to transparency, which makes the fabrication comparable the standard ridge waveguide laser. The demonstrated PIC can be used as a 1 × 2 splitter or by integrating with other waveguide devices, such as laser, modulators, or SOAs, to realize different functionality.

Monolithic Integration of Single Facet Slotted Laser, SOA, and MMI Coupler

This work investigates the optical phase locking performance of Slotted Fabry Perot (SFP) lasers and develops an integrated variable phase locked system on chip for the first time to our knowledge using these lasers. Stable phase locking is demonstrated between two SFP lasers coupled on chip via a variable gain waveguide section. The two lasers are biased differently, one just above the threshold current of the device with the other at three times this value. The coupling between the lasers can be controlled using the variable gain section which can act as a variable optical attenuator or amplifier depending on bias. Using this, the width of the stable phase locking region on chip is shown to be variable.

On-chip optical phase locking of single growth monolithically integrated slotted fabry perot lasers

We present a photonic integrated circuit (PIC) designed to create multiple coherent optical signals for the generation of coherent modulation formats, such as DPSK. A Multimode Interference Coupler (MMI) is monolithically integrated with single facet Slotted Fabry Perot (SFP) lasers at its input and output arms. In this work we show that light from a master SFP laser can be used to injection lock both output slave SFPs for use in the generation of coherent optical channels.

William Cotter

Papers

Investigation of active filter using injection-locked slotted Fabry-Perot semiconductor laser

Mode Suppression in Injection Locked Multi-Mode and Single-Mode Lasers for Optical Demultiplexing

Monolithic Integration of Single Facet Slotted Laser, SOA, and MMI Coupler

On-chip optical phase locking of single growth monolithically integrated slotted fabry perot lasers

Multiple coherent outputs from single growth monolithically integrated injection locked tunable lasers