In this report, we experimentally analyze the effect of optical injection of a signal with a single-sideband (SSB) modulation on a multi-wavelength laser (MWL) integrated on a photonic integrated circuit. The optical injection of an SSB modulation into the MWL leads to spectral multiplication of the signal around the un-injected modes of the MWL. This multiplication arises from the modulation of the carrier density inside the cavity and the strong nonlinear coupling between different modes of the MWL. We report an asymmetric power evolution of the generated sidebands around the injected and un-injected modes of the MWL while the modulation frequency is swept. The power and modulation bandwidth of the signal emerging around the injected and un-injected modes strongly depend on the position of the cavity resonance frequency of the injected mode, which can be tailored by adjusting the injection strength and the detuning of the injected signal.
Exposing semiconductor lasers to external optical feedback can significantly affect their emission properties by, e.g., generating complex dynamics or enabling coherence control. When considering semiconductor multi-wavelength lasers, i.e., lasers designed to emit at two or more distinct wavelengths, feedback has been used to demonstrate emission switching between the different wavelengths. In particular, by controlling the phase shift in the feedback cavity, fast (nanosecond timescale) wavelength switching between two, three and even four of the lasing modes, separated by more than a THz, has been observed experimentally.
The wavelength control is not straightforward since the response of the laser depends on various parameters such as the differential gain between the modes or the cross-saturation parameters (or carrier population gratings). Based on a model relying on a multi-mode extension of Lang-Kobayashi rate equations, we explore the parameter space in terms of feedback strength, delay and modal phases, to identify regions suitable for wavelength switching. In this direction, we determine a feedback strength threshold and detect the appearance of dynamics with longer delays.
Optical frequency combs (OFCs) have been identified as a key building block for many applications ranging from spectroscopy to optical communications. In these applications, the fixed phase relation between the individual spectral components of the comb is a crucial aspect of frequency combs. Recently multi-wavelength lasers that are in essence multimode lasers have shown a promising capability to enable frequency comb multiplication over a broad range of frequency offsets surpassing 1 THz. Despite the robust phase coherence within each comb's lines, there remains a notable lack of phase coherence among distinct sub-combs. In this work, we show that the injecting of an adapted frequency comb, i.e., a narrowband comb comprising five lines and an extra tone separated by a certain frequency offset from the central line of the comb, facilitates cascaded phase locking between three adjacent sub-combs. The interaction between the regenerated comb and the multiplied comb induced by the extra tone initiates a modulation at their beating frequency. By fine-tuning the frequency of the extra tone, we can adjust the position of the resulting beating, and eventually achieve a cascaded phase locking for the third mode. We envision that cascaded phase-locking can advantageously be extended to additional modes leading to cover higher frequency offsets up to a few THz.
Lasers capable of emitting two or more distinct wavelengths with a control over their power balance appear as promising and versatile key devices in the context of THz signal generation, telecommunication or wavelength conversion using optical injection. For such applications, fast wavelength switching is required but devices implying tunability through thermal or mechanical actions can be slow or bulky. Photonic integration has the potential to overcome these obstacles. While merging the beam of distinct lasers on the same chip seems to be a straightforward solution, the absence of modal competition and phase noise correlation limits their use in some applications.
Here, we consider DBR-based multi-cavity lasers with multiple wavelengths sharing a common broadband gain medium. The wavelength switching principle relies on controlling the phase in a monolithically integrated optical feedback cavity. In this contribution, by monitoring the emitted time-resolved power signal, we characterize the dynamics of the wavelength switching mechanism and report a measured response time below 4 ns. Additionally, from numerical investigations using a multi-mode extension of the Lang-Kobayashi rate equations, we identify key parameters influencing the switching time.
All-optical wavelength conversion is a key functionality for large WDM networks with dynamic traffic, e.g., to enable flexible wavelength allocation within the subnetworks. All-optical techniques mostly rely on exploiting the nonlinearities in semiconductor optical amplifiers, such as gain-saturation, FWM, and difference-frequency generation. These methods, however, require additional probe laser, or phase matching. Here we describe an alternative approach based on a feedback-controlled integrated DBR-based multi-wavelength laser (MWL). Our MWL is designed to emit at multiple and controllable modes, thus removing the need for an additional external probe light. Injecting an optical signal around one of the modes of the MWL leads to the spectral multiplication of the signal to the other modes of the MWL. By varying the phase and amplitude of the feedback we show frequency conversion of a 1 GBd ASK signal at offsets ranging from tens of GHz to 1.2 THz. The emission of the MWL can be controlled at nanosecond time scales by changing the feedback phase from a monolithically-integrated feedback cavity. Our approach is, in principle, only limited by the gain bandwidth of the active medium which can reach up to 10 THz in InP.
Laser designed to emit at multiple and controllable modes, or multi-wavelength lasers, have the potential to become key building blocks in future compact THz or mm-wave transceivers. Combined with optical injection, these lasers can function as low-noise THz sources or even enable all-optical THz signal processing. Among the various multi-wavelength laser concepts, DBR-based lasers stand out because of their simplicity principle to control and switch the output wavelength of the laser. The extra wavelengths also add new degrees of freedom and interesting new features for laser dynamics. Yet, the coupling between the different laser modes has not been carefully considered so far. Here, we experimentally and numerically analyze the effect of nonlinear mode coupling and interactions in a dual-wavelength laser under optical injection. We focus particularly on the evolution of the locking bandwidth for different gain coefficients between the injected and non-injected modes. In addition, we report a wavelength shift of the non-injected mode which follows the evolution of the detuning in the other mode. Our work brings a new important insight into the mode competition taking place in multi-wavelength lasers, pushing them forward towards novel applications.
C. Koos, T. Kippenberg, L. Barry, A. Ramdane, F. Lelarge, W. Freude, P. Marin, J. Kemal, C. Weimann, S. Wolf, P. Trocha, J. Pfeifle, M. Karpov, A. Kordts, V. Brasch, R. Watts, V. Vujicic, A. Martinez, V. Panapakkam, N. Chimot
Chip-scale frequency comb sources are key elements for a variety of applications, comprising massively parallel optical communications and high-precision optical metrology. In this talk, we give an overview on our recent progress in the area of integrated optical comb generators and of the associated applications. Our experiments cover modulator-based comb sources, injection locking of gain-switched laser diodes, quantum-dash mode-locked lasers, as well as Kerr comb sources based on cavity solitons. We evaluate and compare the performance of these devices as optical sources for massively parallel wavelength division multiplexing at multi-terabit/s data rates, and we report on comb-based approaches for high-precision distance metrology.
Correction of atmospheric effects on the propagation of laser light can be achieved with adaptive optics (AO) by relying on adequate wavefront sensors. For free-space laser communications and for tracking of high-speed airborne objects, conventional wavefront sensing methods e.g. those based on the Shack-Hartmann sensor (SHS), are not always effective. Partial obscuration and saturation of the detector due to strong turbulence lead to errors in wavefront reconstruction. Another drawback of Shack-Hartmann wavefront-sensing is the timeconsuming readout of the whole detector and subsequent matrix-vector multiplication necessary to reconstruct the wavefront. We characterize a promising modal alternative: digital holographic wavefront sensor (DHWS).We examine the performance of the sensor for single-, and multimode operation and its dependence on the detector size, scintillation, residual tip/tilt and misalignments.
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