Stable Single-mode Oscillation Research Articles

Hepta-cross-ring based erbium laser with stable single-mode oscillation in L-band range

In this research, a hepta-cross-ring based erbium-doped fiber (EDF) laser is designed and studied to achieve continuous-wave (CW) and single-longitudinal-mode (SLM) tunability in L-band region. The hepta-cross-ring design can introduce the mode-filter function caused by the Vernier effect to effectively suppress dense multi-longitudinal-mode (MLM). Due to the L-band erbium gain medium, the wavelength-tuning range of the fiber laser is achieved from 1567.6 to 1612.0 nm. Furthermore, the output implementations of optical signal to noise ratio (OSNR), corresponding power, stability and wavelength linewidth in the proposed fiber laser are also measured and discussed.

High-power, stable single-mode CW operation of 1550 nm wavelength InP-based photonic-crystal surface-emitting lasers

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

Parity–Time-Symmetric Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

Parity–time (PT) symmetry has been intensively exploited in optoelectronic oscillators (OEOs), enabling mode selection in mutually coupled feedback loops. However, two mutually coupled feedback loops are usually established to achieve a spatial PT-symmetric architecture, which increases system complexity and instability. Here, we propose and experimentally prove a PT-symmetric OEO based on stimulated Brillouin scattering (SBS). The PT symmetry is implemented in an overlapping spatial loop in which the gain and loss are realized based on SBS gain and SBS loss, respectively. By adjusting the pump power and the polarization states of the probe and pump lights, the gain and loss can be balanced, thus achieving stable single-mode oscillation. In experiment, a 9.66 GHz microwave signal with a side-mode suppression ratio of 41 dB is generated. At 10 kHz offset frequency, the single-sideband phase noise reaches as low as −120.1 dBc/Hz. This demonstration opens new avenues to enhance mode selectivity in an OEO and can potentially be integrated on a chip.

Optoelectronic oscillator with improved sidemode suppression by joint use of spectral Vernier effect and parity-time symmetry.

An optoelectronic oscillator (OEO) with improved sideband suppression by joint use of the spectral Vernier effect and parity-time (PT) symmetry is proposed and experimentally demonstrated. The spectral Vernier effect is implemented using two mutually coupled loops with different loop lengths, to increase the effective free spectral range (FSR). To further increase the mode selection capability to ensure stable single-frequency oscillation with an increased sidemode suppression ratio (SMSR), PT symmetry is implemented, in which the two mutually coupled loops are controlled with balanced gain and loss. Thanks to the combined effects, stable single-mode oscillation with a significantly increased SMSR is achieved. The proposed OEO is studied theoretically and evaluated experimentally. The results show that for a generated microwave signal at 10 GHz, the SMSR is 67.68 dB, which is increased by 11.20 dB or 26.05 dB, when using only the spectral Vernier effect or only the PT symmetry. Thanks to the long length of the longer loop, good phase noise performance is still maintained. The measurement shows that a phase noise as low as -124.5 dBc/Hz at an offset frequency of 10 kHz is achieved.

Parity-time symmetric tunable OEO based on dual-wavelength and cascaded PS-FBGs in a single-loop.

The ability to achieve low phase noise single-mode oscillation within an optoelectronic oscillator (OEO) is of fundamental importance. In the frequency-tunable OEO, the wide microwave photonic filter (MPF) bandwidth is detrimental to select single-mode among the large number of cavity modes, thus leading to low signal quality and spectral purity. Stable single-mode oscillation can be achieved in a large time delay OEO system by harnessing the mechanism from parity-time (PT) symmetry. Here, a PT-symmetric tunable OEO based on dual-wavelength and cascaded phase-shifted fiber gratings (PS-FBGs) in a single-loop is proposed and experimentally demonstrated. Combining the merits of wide frequency tuning of PS-FBG-based MPF and single mode selection completed by the PT-symmetric architecture of the OEO, where the gain and loss modes carried by dual-wavelengths to form two mutually coupled resonators in a single-loop, signals range from 1 GHz to 22 GHz with the low phase noise distributed in -122∼ -130 dBc/Hz at 10 kHz offset frequency are obtained in the experiment.

Parity-time symmetry in parameter space of polarization

Establishing parity-time (PT) symmetry in non-spatial space is a promising way to simplify the PT-symmetry system structure. In this paper, an implementation of PT symmetry in optical polarization space is reported. By utilizing the polarization multiplexing technology, PT symmetry is formed in overlapping spatial parameter space. The degeneracy of eigenmodes with two distinct PT phases in polarization space is demonstrated. In the PT-symmetric phase, the eigenmodes have real eigenfrequencies that respect PT symmetry, exhibiting broken degeneracy (mode splitting); in the PT-broken phase, the eigenmodes are degenerate with a pair of complex conjugate eigenfrequencies. The sharp-pointed peak filter response in the PT-broken phase due to the strong field localization is characterized, which explains the mode-selection mechanism of PT-symmetry breaking. The polarization-space PT symmetry is applied in a 7-km single-loop optoelectronic oscillator, and a stable single-mode oscillation signal is generated with a phase noise of −138 dBc/Hz at 10 kHz and side-mode suppression ratio of 49 dB. The approach expands the parameter spaces to carry out PT symmetry and could promote the integration of the PT-symmetry photon system.

High-finesse frequency tunability of optoelectronic oscillator based on SiO[formula omitted] optical waveguide ring resonator with the frequency locking technology

Fine tunability of oscillation frequency of optoelectronic oscillator (OEO) is crucial to its practicality. An approach of utilizing frequency locking technology is presented to realize the high-finesse tunable output of OEO based on SiO2 optical waveguide ring resonator (OWRR). Then the OEO system based on SiO2 OWRR is constructed, consisting of OEO loop and frequency locking loop. When the laser frequency is locked at the resonant frequency of the SiO2 OWRR, the stable single-mode oscillation is realized based on the filtering effect of SiO2 OWRR. The OEO oscillation frequency can be precisely tuned by adjusting the locking position of laser frequency, and a high-finesse tunable output of microwave signal is obtained with a tuning step of 20 kHz. Moreover, the change of locking position would not deteriorate the phase noise; for the generated signal at 2 GHz at different locking point, the phase noise is about -89.18 dBc/Hz at 10 kHz. These results show that the OEO based on SiO2 OWRR using frequency locking technology is feasible to obtain fine tunability of oscillation frequency. It indicates that the SiO2 OWRR has great potential in monolithic integrated OEO.

Parity-Time Symmetric Optoelectronic Oscillator Based on an Integrated Mode-Locked Laser

Originating from the non-Hermitian quantum mechanics, the parity-time (PT) symmetry has been extensively explored to ensure a stable single-mode oscillation for optoelectronic oscillators (OEOs), while avoiding additional extremely challenging ultrahigh-Q filters. Enabled by the emerging photonics integrated circuit (PIC) technology, we report the tunable PT-symmetric OEO by using an integrated mode-locked laser, as a striking advance to reduce the footprint of OEOs. Stable single-mode oscillating signal at 24.5 GHz has been generated with a phase noise of -108 dBc/Hz @ 10 kHz frequency offset. In particular, a tunable frequency range from 24 to 25 GHz is achieved by manipulating the injection currents into the well-designed on-chip linear laser cavity. This work exhibits a breakthrough in conceptual integrated microwave photonic devices, and also accelerates the real-world applications of fundamental physical theory of PT symmetry to information industry.

A Precisely Frequency-Tunable Parity-Time-Symmetric Optoelectronic Oscillator

Mode selection is crucial for a stable single-mode oscillation of an optoelectronic oscillator (OEO). In this article a parity-time (PT)-symmetric frequency-tunable OEO is proposed and experimentally demonstrated, to realize the precise frequency tuning without using any narrowband filter. The proposed PT-symmetric OEO is implemented based on two identical optoelectronic hybrid loops mainly consisting of a shared dual-parallel Mach-Zehnder modulator (DPMZM) and a photodetector. The PT-symmetry condition can be achieved by enabling antisymmetric gain/loss in the two loops through manipulating the bias voltages of the two child-MZMs of DPMZM, and continuous frequency tunability is realized by adjusting the bias voltage of the parent-MZM. A theoretical model is developed for the proposed PT-symmetric OEO to analyze the dynamics of the PT-supermodes as the gain/loss coefficient changing. Furthermore, an experiment is performed. Without high-Q filter, stable single-mode oscillation of the PT-broken supermode at 4.2 GHz with side-mode suppression ratios of 60 and 50 dB is demonstrated in the OEO with 6 and 2000 m fiber loop lengths, respectively. The frequency tuning ranges are 1.4 MHz and 1.5 kHz for two OEOs with different fiber loop lengths.

Powerful oversized W-band free-electron maser with advanced Bragg resonator based on coupling of propagating and cutoff waves

We study, both theoretically and experimentally, a free-electron maser (FEM) with a two-mirror resonator that includes the advanced Bragg structure based on coupling of propagating and cutoff waves as an upstream reflector. Similar to gyrotrons, the presence of a cutoff wave in the feedback loop improves significantly the selectivity of the resonator at large values of the oversize factor and facilitates a stable single-mode oscillation regime in short wavelength bands. Proof-of-principle experiments conducted at the induction linac LIU-3000 (Dubna) demonstrate the operability of the FEM with the proposed modification of the Bragg resonator. At the transverse oversize factor of about 5, stable narrow-band generation was achieved with an output power of up to 7 MW and the oscillation frequency of about 80 GHz corresponding to the frequency of the cutoff mode excited in the advanced Bragg reflector. These investigations encourage the application of the advanced Bragg resonators for the development of high-power long-pulse FELs driven by induction linacs and operating in the subterahertz to terahertz frequency range.

Frequency-Tunable Parity-Time-Symmetric Optoelectronic Oscillator Using a Polarization-Dependent Sagnac Loop

A widely frequency-tunable parity-time (PT) symmetric optoelectronic oscillator (OEO) implemented based on a single polarization-dependent Sagnac loop is proposed and experimentally demonstrated. Two mutually coupled feedback loops having an identical geometry with balanced gain and loss are implemented to achieve PT-symmetry, which is essential for single-mode selection without using an ultra-narrow passband microwave filter. In the implementation, a Sagnac loop consisting of a polarization beam splitter (PBS) and two polarization controllers (PCs) is employed which functions equivalently as two mutually coupled feedback loops, with the gain, loss and coupling coefficient being independently controllable. Since only a single physical loop is used, the implementation is greatly simplified and the stability is highly improved. The operation of the proposed OEO is experimentally verified. Stable single-mode oscillation with a wide frequency tunable range from 2 to 12 GHz is achieved. The phase noise is −128 dBc/Hz at an offset frequency of 10 kHz.

Parity-time-symmetric optoelectronic oscillator.

An optoelectronic oscillator (OEO) is a hybrid microwave and photonic system incorporating an amplified positive feedback loop to enable microwave oscillation to generate a high-frequency and low-phase noise microwave signal. The low phase noise is ensured by the high Q factor of the feedback loop enabled by the use of a long and low-loss optical fiber. However, an OEO with a long fiber loop would have a small free spectral range, leading to a large number of closely spaced oscillation modes. To ensure single-mode oscillation, an ultranarrowband optical filter must be used, but such an optical filter is hard to implement and the stability is poor. Here, we use a novel concept to achieve single-mode oscillation without using an ultranarrowband optical filter. The single-mode operation is achieved based on parity-time (PT) symmetry by using two identical feedback loops, with one having a gain and the other having a loss of the same magnitude. The operation is analyzed theoretically and verified by an experiment. Stable single-mode oscillation at an ultralow phase noise is achieved without the use of an ultranarrowband optical filter. The use of PT symmetry in an OEO overcomes the long-existing mode-selection challenge that would greatly simplify the implementation of OEOs for ultralow-phase noise microwave generation.

Narrow-spectral-linewidth silicon photonic wavelength-tunable laser with highly asymmetric Mach–Zehnder interferometer

We propose a narrow-spectral-linewidth silicon photonic wavelength-tunable laser with a novel external wavelength-tunable filter, which consists of two silicon ring resonators with different circumferences and a highly asymmetric Mach-Zehnder interferometer (MZI), the two optical paths of which have significantly different lengths. Calculations and experimental results indicated that the gain difference between longitudinal modes was increased by the highly asymmetric MZI. Consequently, a narrow spectral linewidth of 12kHz and a stable single-mode oscillation were obtained.

A $\lambda/4$-Shifted Distributed-Feedback Laser Diode With a Fiber Ring Cavity Configuration Having an OSNR of 85 dB and a Linewidth of 7 kHz

We newly propose a lambda/4-shifted distributed-feedback laser diode (DFB-LD) that employs a fiber ring cavity configuration and which incorporates an ultranarrow band fiber Bragg grating (FBG) in the cavity. By controlling the FBG wavelength to match the oscillation wavelength of the DFB-LD, we achieved stable single-mode oscillation where the sidemodes of the DFB-LD and the Fabry-Perot modes induced by the ring configuration were sufficiently suppressed. Large performance improvements with an optical signal-to-noise ratio of 85 dB and a linewidth of 7 kHz were obtained.

Lasers producing tailored beams

Compact lasers that can produce a range of beam patterns are important for progress in several areas, including the improvement of optical tweezers, ultra-high-density optical memory and microfluidics. Here we engineer photonic crystals to generate semiconductor lasers that produce a range of beam patterns while maintaining stable single-mode oscillation. Our results could contribute to the realization of compact lasers that are capable of producing diverse beam patterns on demand.

Hybrid-integrated external-cavity laser without temperature-dependent mode hopping

We propose and demonstrate an integrated external-cavity laser (ECL) with a novel configuration designed to suppress temperature-dependent mode hopping. The laser cavity is composed of a spot-size converter integrated laser diode (SS-LD), a UV-written grating, and silicone-filled grooves on a planar lightwave circuit (PLC). The silicone is used to compensate for the difference between the temperature coefficients of the SS-LD and the silica waveguides. We calculate theoretically the temperature interval of the mode hopping in the ECL to optimize the length of the silicone-filled grooves. We fabricated test and practical lasers with the proposed configuration using PLC hybrid-integration technologies. With the test laser, we confirmed stable single-mode oscillation and a large increase in the mode-hopping temperature interval for both static and dynamic operation. We obtained a low threshold of 15 mA, an optical power of 1.0 mW at 25/spl deg/C, and an injection current of 60 mA. In the practical laser, which we additionally integrated with a front monitor photodiode, we realized auto power control with mode-hopping-free operation from 18/spl deg/C to 56/spl deg/C. These results indicate the good performance levels of our proposed laser.

Frequency spectra and waveguiding of a family of daisy modes in vertical-cavity surface-emitting lasers

We have observed stable single-mode oscillation in a family of high-order daisy modes for a vertical-cavity surface-emitting laser that is optically-pumped with a doughnut-shaped beam. Measurements of the frequency separations and the radial profiles of the various modes show that the waveguiding has a near-quadratic nature.

Pinning of daisy modes in optically pumped vertical-cavity surface-emitting lasers

We have observed stable single-mode oscillation of optically pumped vertical-cavity surface-emitting lasers (i.e., without fabricated aperture) in several high-order daisy modes. The surprising stability of the nodal lines of a daisy mode is explained as being due to the combined action of many microscopic defects; when a daisy mode is moved across the wafer, the effective correlation length of the orientation of the pinning is of the order of 1 μm.

Wide-wavelength range detuning-adjusted DFB-LDs of different wavelengths fabricated on a wafer

We report for the first time different-wavelength distributed-feedback laser diodes (DFB-LDs) fabricated on a single wafer, whose lasing wavelengths cover the expanded erbium-doped fiber amplifier (EDFA) gain band from 1.52 to 1.59 /spl mu/m with adjusted detuning between lasing-wavelengths and gain-peak-wavelengths. Uniform lasing characteristics such as a low-threshold current of less than 12 mA with stable single-mode oscillation of more than 35-dB side-mode suppression ratio (SMSR) are achieved for the 75-nm wavelength range.

1.55 μm High Speed Partly Gain-Coupled DFB Lasers with Periodically Etched Strained-Layer Quantum Wells

The high speed performance of partly gain-coupled (GC) DFB lasers consisting of periodically etched strained-layer quantum wells (QW's) is reviewed with comparisons to the equivalent index-coupled (IC) DFB lasers with the same active layers. It is shown that the GC DFB laser has a –3 dB modulation bandwidth of 22 GHz at 10 mW with a stable single mode oscillation at the longer side of the Bragg Stop-band due to in-phase gain coupling. A theoretical analysis is also presented based on the local-normal-mode transfer-matrix laser model which takes into account both the longitudinal distribution of laser parameters and carrier transport effects. The mechanism for high modulation bandwidth of the GC DFB laser is attributed to a higher differential gain due to a reduced carrier transport time which is provided by an effecient carrier injection from the longitudinal etched interface of the QW's.