Polarisation behaviour of diode lasers with frequency selective feedback

The paper reviews recent results obtained with diode lasers used in external hybrid cavities with frequency selective feedback. Very wide discreet tunable ranges are achieved. They can be made to oscillate in a tunable mode having the desired polarization state TE or TM. High-speed polarization switching in the GHz range is possible by inserting passive or active polarization selecting elements in the cavity. Hybrid external cavities are attractive for applications in optical metrology spectroscopy and optical communications.

The terahertz quantum-cascade (QC) VECSEL is a recently demonstrated approach to designing single-mode terahertz lasers based on the coupling of an amplifying reflect-array metasurface with an external optical cavity. The QC-VECSEL has demonstrated single-mode terahertz lasing with high output power and near-diffraction limited beam quality. The QC-VECSEL is also a natural candidate for demonstrating broadband, continuous, single-mode frequency tuning as the VECSEL's lasing frequency is determined by the length of the its external cavity, which can be mechanically tuned. In this work, we use a piezoelectric translational stage to actively adjust the length of the QC-VECSEL's external cavity and demonstrate >500 GHz of single-mode tuning around a center frequency of 3.5 THz (>20% fractional tuning). High-quality, circular beam patterns are observed with a divergence angle of ~15° throughout the tuning range, and tens of milliwatts of peak terahertz output power are observed. In order to maintain single mode behavior, the external cavity is made to be extremely short, increasing the spacing between the external cavity's neighboring longitudinal resonances. Cavity lengths as short as 250 µm have been studied, but the free-spectral range of the external cavity could not be made larger than the gain bandwidth of the metasurface, providing testament to the bandwidth of both the metasurface and the QC-gain material.

Chip-scale lasers with a wide tunable range and a narrow linewidth have rich applications in optical communications, sensing, and light detection and ranging systems. External cavity lasers (ECLs) have the advantage of a wide tuning range and a narrow linewidth compared with their counterparts such as distributed feedback lasers and distributed Bragg reflector lasers because the passive filter separated from the gain section can be widely tuned and the low-loss external cavity extends the photon lifetime. However, so far, the tuning range achieved by the chip-scale ECLs is in the range of 80–120 nm, smaller than that of the bulk free-space ECL (typically >160 nm). The bottleneck in reaching an ultrabroad tuning range for chip-scale ECLs lies in the unsuppressed side modes of the external cavity. Here, we demonstrate a hybrid-integrated ECL consisting of a broadband III–V reflective semiconductor optical amplifier and a low-loss Si3N4 wavelength-selective reflector. A record 172-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio is achieved beyond the free spectral range of the Vernier ring filter due to the enhanced lasing mode selection from a tunable Sagnac loop reflector. The Si3N4 platform enables a low-loss external cavity, facilitating laser linewidth reduction below 4 kHz over the full tuning range. The on-chip output power reaches 26.7 mW at a wavelength of 1550 nm.

Stretched and soliton femtosecond pulse generation with graphene saturable absorber by manipulating cavity dispersion

External optical feedback can significantly alter the operating characteristics and output radiation properties of semiconductor diode lasers. A semiconductor injection laser operating in an external cavity is attractive for numerous applications requiring greater spectral purity, frequency stability, and smaller beam divergence than can be obtained from a solitary laser diode. A linear array of diode lasers was fabricated from a bar of five, 6-µm wide, 250-µm center spacing, single-stripe emitters. The external cavity was formed by a 0.75 pitch graded-index rod lens positioned ~1 µm from the front facet of the bar array. The rod lens transfers an image of the radiating bar facet directly from the back to the front of the rod. The external cavity is therefore formed by optical feedback from the front surface of the rod with the rod lens also performing the function of the intracavity lens element. This dual function of the graded-index rod lens enables the array laser to assume a very compact form in comparison to other external cavity configurations. It was observed that the wavelength of each diode element could be controlled through proper alignment of the external cavity. Once the diode elements were operating at the same wavelength, the array could be tuned over a 15-nm range (798-813 nm). A mutual coherence interferometer was constructed to determine qualitatively the existence of a phase locked condition. A very clear interference fringe pattern was produced. Frequency locked, mutually coherent, operation of five diode elements was achieved, with external wavelength control demonstrated.

Summary form only given. Laser cavity solitons (LCS) are transverse, nonlinear, self-localized and dissipative states that possess both translational and phase invariance. LCS interaction and synchronisation properties in broad-area semiconductor lasers have the potential for massive parallelism and the formation of complex arrays. Phase-locked bound states of solitons have been predicted in mode-locked lasers for the temporal case [1] and in lasers with saturable absorbers for the spatial case [2]. Corresponding phase-quadrature states have been observed in fibre lasers [1].Here we present a different kind of soliton locking. We demonstrate experimentally and theoretically Adlertype locking and synchronization of spatial LCS in a vertical-cavity surface-emitting laser (VCSEL) with an external volume Bragg grating (VBG) that provides frequency-selective feedback (see left panel of Fig.1) [3]. In particular we explain the role played by defects resulting from fluctuations during the epitaxial growth process. The experiment has been performed with a temperature tuned 981 nm VCSEL of 200 μm circular aperture and a VBG with a single reflection peak at 981.1 nm, and a peak reflectivity of 99% [3]. The external cavity for the frequency-selective feedback is arranged in a self-imaging configuration that maintains the high Fresnel number of the VCSEL cavity (see the left panel of Fig. 1). This potentially enables large parallelism of solitons and hence the prospect of synchronisation in large disordered networks.A piezo-electric transducer was used to minutely tilt the external cavity's end reflector (VBG) with respect to the optical axis. The differential change of the external cavity length for the two LCS leads to a differential change in feedback phase. In this way the detuning between two LCS can be tuned. During the scan, a region of frequency and phase locking appears, identified in the right panel of Fig. 1 by the region of high fringe visibility in the far field. This illustrates that locking - once achieved - is a robust phenomenon. We have theoretically and numerically investigated the interaction of pinned LCS upon the change of the difference in the depth of the trapping defects for both class-B semiconductor [4] and Ginzburg-Landau [5] models. In the locking region we recover an Adler scenario, where the fringe phase varies smoothly and quasilinearly with the detuning of the external cavity (see the red curve in the right panel of Fig. 1). The transitions to and from frequency and phase-locking are rather abrupt. Finally we discuss theoretical and experimental spectra in the locked and unlocked regions and the effect of the external cavity modes. Independent switching and erasure of locked and unlocked LCS is demonstrated.

Analysis of parameter influence law of waveguide Bragg grating

Optical fibers play an important role in general and, in particular, in the field of sensors. As part of a sensor system, quite often fibers are coupled by a ball lens. For efficient usage, the fiber ball lens systems (FBLSs) have to be optimized. The present work presents analytic expressions for the design parameters of such systems. FBLSs comprise sections of a single-mode optical fiber, a coreless fiber (CLF) and a ball lens. Their geometric dimensions have to be optimized for their use in different applications in optical metrology. The derived expressions facilitate the optimum parameter choice, which is usually done by expensive numerical simulations. For comparison and validation of the results by experiments, FBLSs with different ball lens radii and CLF sections have been prepared by a fusion splicer technique. Their characteristics were investigated in the optical spectral ranges at 630 nm and 1550 nm. Experimental methods comprising far-field, near-field and reflection/transmission measurements with optical fibers validate the theoretical considerations. To our knowledge, this is the first time that simple analytic approaches have been applied to the fabrication of FBLSs. This facilitates and quickens their general design for different applications in optical metrology significantly.

A stable tunable external cavity diode laser (ECDL) which uses an intracavity electro- optic deflector incorporated into a Littrow-type external cavity as a wavelength tunable element is proposed. The deflector comprises an antireflection-coated rectangle Lead Lanthanum Zirconate Titanate (PLZT) electro-optic ceramic with a pair of parallel trapezoidal electrodes on the up and down surfaces, which can simultaneously change the cavity length and the beam-grating angle to achieve mode-hop-free tuning. The theory for mode-hop-free tuning will be presented in this paper, and the model calculations predict that the laser can be tuned continuously by as much as 110GHz. This new approach is simple in structure with no mechanical moving parts and has potential configuration of compact grating-stabilized ECDL. Keywords: Laser tuning, Mode-hop-free, Electro-optic defl ectors, External cavity diode laser, Ceramics 1. INTRODUCTION Due to the merits of single-mode, narrow-linewidth, fine tuna bility and large tunable range, external cavity diode lasers (ECDLs) have found more and more applications in high resolution spectroscopy, synthetic aperture laser radar, optical communication systems, and so on [1~3]. Usually, the wavelength of ECDLs is usually tuned by rotating a diffraction grating in the Littrow configuration [4] or a mirror in the Littman–Metcalf configuration [5] via a piezo-electrical actuator. Continuous single-mode scanning without mode hopping can be achieved by synchronous tuning of external cavity modes and grating selectivity [6]. However, the modulation frequency and reliability are restricted by the movement mechanical construction of the laser system. To avoid the drawbacks referred above, using a deflector as the tuning element without moving component in the extended cavity appears to be more attractive, among which the electro-optic crystal (EOC), acousto-optic modulator (AOM), and nematic liquid-crystal (NLC) deflectors have had a large development [7~10]. However, the mode-hop-free tuning range is limited to 10 GHz because of limitation of the high driving voltage, as reported in reference [7]. Although using two AOMs or NLC can achieve wider tuning rage (including mode hops), up to 225GHz for AOMs [9] and 12nm for NLC [10], these kinds of tuning have a drawback in application for high repetition rates.

The creation and manipulation of photonic skyrmions provide a novel degree of freedom for light-matter interactions, optical communication and nanometrology. Since the localized vortex within skyrmions arises from the twist and curl of the phase structure, the orbital angular momentum of light is essential for their construction. While numerous skyrmionic textures have been proposed, they are formed within the spatial domain and induced by the longitudinal orbital angular momentum. Here we theoretically propose and experimentally observe spatiotemporal skyrmions within a picosecond pulse wavepacket, generated through vectorial sculpturing of spatiotemporal wavepackets. The skyrmionic textures emerge within the spatiotemporal distribution of a vector field encompass all possible polarization states. Constructed upon the transverse orbital angular momentum, spatiotemporal skyrmions, in contrast to spatial skyrmions, exhibit no helical twisting perpendicular to the skyrmion plane, demonstrating potential stability against deformations or perturbations. These results expand the skyrmion family and offer new insights into optical quasiparticles, potentially leading to advanced applications in optical metrology, sensing, and data storage.

Experimental demonstration of lasing in a broad area twin-contact semiconductor laser which operates as a phase-conjugation (PC) mirror in an external cavity configuration is reported. This allows “self-aligned” and self-pumped spatially nondegenerate four-wave mixing to be achieved without the need for external optical signals. The external cavity laser system is very insensitive to tilt misalignments of the external mirror in the PC regime and exhibits very good mechanical stability. The resonant frequency of the external cavity lies in the GHz range which corresponds to a subnanosecond time response of phase conjugation processes in the semiconductor laser.

A theoretical analysis has been undertaken of steady-state transverse-mode selection in vertical-cavity surface-emitting lasers (VCSEL’s) that are subject to optical feedback in an external-cavity configuration. Multiple reflections in the external cavity are taken into account. The influence of thermal effects on mode selection is also treated, and noise properties of the configuration are discussed. In the analysis specific attention is paid to the competition between the two lowest-order modes (LP01 and LP11 degenerate modes) that are appropriate to the chosen circularly symmetric vertical-cavity surface-emitting laser structure. It is shown that preferential excitation of either mode can be effected by the appropriate choice of optical feedback delay. It is also shown that multiple reflections in the external cavity can affect the mode selection, depending on the optical feedback delay. Thermal effects are shown to be detrimental for transverse-mode selection. Strategies for the minimization of thermal effects on mode selection are also discussed. Finally, the relative intensity noise spectrum for each transverse mode is shown to maintain its double-peaked structure and low mode-partition noise in the presence of optical feedback. A change from a twin-peaked to a single-peaked structure of this spectrum is also observed when the optical feedback delay is changed.

We report on an experiment to study the properties of nonlasing subthreshold sidemodes in a semiconductor laser operating in an external grating cavity configuration. We measure optical spectra consisting of the lasing mode and subthreshold nonlasing sidemodes, as well as radio frequency spectra at low frequency (0-100MHz) and at high frequency near the external grating cavity mode frequency. As the laser frequency is varied, the features of the rf spectra and the optical sidemode spectra all vary systematically. We present results on how these variations depend upon the external grating cavity mode frequency. Our results are compared with previous experiments and with theoretical predictions based upon four-wave mixing between the lasing mode and the adjacent sidemodes.

The laser emission of transverse electric (TE) and transverse magnetic (TM) polarization, in alternating successive spectral ranges, that can occur when external grating diode lasers are wavelength tuned, is theoretically and experimentally investigated. The modulated threshold gain curves of the external cavity for the two polarizations can be made to intersect with each other if a grating is used in such a way as to practically eliminate the difference between the laser diode losses and confinement factors for TE and TM polarization. The result is a sequence of wavelength ranges where the threshold gain is alternately lower for the TE resp. TM polarization. Within each range, a wavelength tunable single mode laser emission is obtained. The phenomenon is described, taking into account the characteristics of the key components of the external cavity, i.e., laser diode structure, antireflective (AR) coating, and grating efficiency. Design tools are proposed for the characteristics of the components necessary for the development of such an external cavity. The theoretical and design concepts are experimentally confirmed in a number of external cavities, using diode lasers emitting around 1.5 /spl mu/m, with different types of antireflective coatings, as well as different grating configurations. >

Broadly tunable hetero-cascading quantum cascade lasers: Design, growth, and external cavity operation