Stable Single-mode Operation Research Articles

Design of a Second Harmonic Double-Beam Continuous Wave Gyrotron with Operating Frequency of 0.79 THz

This paper presents the most essential steps of a design study of a novel second harmonic gyrotron operating in CW (continuous wave) regime at a frequency of 0.79 THz and an output power of 1–100 W. It is based on a novel idea for suppression of the parasitic modes using a double-beam electron-optical system (EOS). It includes a triode magnetron injection gun (MIG), which forms two high-quality helical electron beams (HEB). Different schemes, namely one with two generating beams and another with one generating and one absorbing beam, have been investigated and compared. It has been shown that the scheme with two generating beams is more advantageous since it allows an effective suppression of the parasitic modes and a stable single-mode operation at the second harmonic resonance. A MIG which is appropriate for the realization of the latter scheme has been optimized using numerical codes for computer-aided design (CAD). It forms beams with practically equal pitch factors and moderate velocity spread. The construction of the gun is not sensitive to small misalignments and shifts of the electrodes and the magnetic field. Among the most promising characteristics of the presented design are an improved mode selection and a stable single-mode generation at currents that are two to three times higher than the currents in the single-beam (i.e., conventional) gyrotrons.

Improved performance of tunable single-mode laser array based on high-order slotted surface grating.

We present an improved design of a wavelength-tunable single-mode laser array based on a high order surface grating with non-uniformly spaced slots. The laser array consists of 12 slotted single-mode lasers. The fabricated device exhibits a quasi-continuous tuning range of more than 36 nm over the temperature range from 10°C - 45°C covering the full C-band. All lasers in the array have stable single-mode operation with side mode suppression ratio of 50 dB due to the modified slot design. A spectral linewidth of less than 500 kHz was obtained for all channels in the array.

Compact silicon photonic wavelength-tunable laser diode with ultra-wide wavelength tuning range

We present a wavelength-tunable laser diode with a 99-nm-wide wavelength tuning range. It has a compact wavelength-tunable filter with high wavelength selectivity fabricated using silicon photonics technology. The silicon photonic wavelength-tunable filter with wide wavelength tuning range was realized using two ring resonators and an asymmetric Mach-Zehnder interferometer. The wavelength-tunable laser diode fabricated by butt-joining a silicon photonic filter and semiconductor optical amplifier shows stable single-mode operation over a wide wavelength range.

An Equivalent-Asymmetric Coupling Coefficient DFB Laser With High Output Efficiency and Stable Single Longitudinal Mode Operation

An equivalent-asymmetric coupling coefficient (EACC) distributed feedback laser (DFB) semiconductor laser with equivalent-half apodization grating (EHAG) structure is proposed and experimentally demonstrated for the first time; the EHAG profile is equivalently realized by linearly changing the duty cycle of a sampled Bragg grating along the one half cavity, while that of the other half cavity is kept uniform. Compared with the equivalent-symmetric coupling coefficient DFB laser, the simulated intensity distribution of the EACC DFB laser shows that the light power is concentrated not only on near the phase-shift region and that the power at the front end is enlarged. Therefore, the longitudinal spatial hole burning may be reduced; the output efficiency and the single-mode stability may be improved. The experimental results show that the output power ratio between the front and rear facets is about 2.17 and the side-mode suppression ratios are over 50 dB when the injection current is in the range from 60 to 200 mA.

Stable single mode operation of quantum cascade lasers by complex-coupled second-order distributed feedback grating

Stable single mode operation of distributed feedback quantum cascade lasers operating in room-temperature continuous-wave mode is reported. Feedback is provided by the second-order grating in a both index- and loss-dominated coupling scheme. Reliable single mode emission at λ∼4.7μm with a side-mode suppression ratio ∼30dB is observed. A linear tuning of wavelength with heat-sink temperature or injection current indicates the stable single mode operation without mode hopping.

Single-mode laser by parity-time symmetry breaking.

Effective manipulation of cavity resonant modes is crucial for emission control in laser physics and applications. Using the concept of parity-time symmetry to exploit the interplay between gain and loss (i.e., light amplification and absorption), we demonstrate a parity-time symmetry-breaking laser with resonant modes that can be controlled at will. In contrast to conventional ring cavity lasers with multiple competing modes, our parity-time microring laser exhibits intrinsic single-mode lasing regardless of the gain spectral bandwidth. Thresholdless parity-time symmetry breaking due to the rotationally symmetric structure leads to stable single-mode operation with the selective whispering-gallery mode order. Exploration of parity-time symmetry in laser physics may open a door to next-generation optoelectronic devices for optical communications and computing.

Narrow linewidth 1560 nm InGaAsP split-contact corrugated ridge waveguide DFB lasers.

We demonstrate a split-contact corrugated ridge waveguide InGaAsP distributed feedback laser at 1560 nm. The laser cavity has been defined with uniform third-order gratings etched along the sidewalls of the ridge waveguide. The gratings were fabricated using a standard I-line stepper lithography technique along with an inductively coupled reactive ion-etching process. Stable single-mode operation has been achieved with side-mode suppression ratios ≥50 dB, output powers ≥7 mW, a wavelength tuning range ≥2.3 nm, and narrow linewidths (≤140 kHz) for different biasing conditions, with a minimum of 70 kHz. The effect of p-contact partition on device performance is also studied.

Narrow Linewidth 1550 nm Corrugated Ridge Waveguide DFB Lasers

We report on the design and characterization of InP-based multiple quantum well corrugated ridge waveguide distributed feedback diode lasers operating at 1550 nm. Third-order gratings have been etched along the sidewalls of the ridge waveguide using the standard I-line stepper lithography technique with an inductively coupled reactive ion etching process. An as-cleaved 1500-μm-long laser diode shows stable continuous wave single-mode operation at 1550 nm with high side-mode suppression ratios (>50 dB), a temperature-dependent wavelength shift dλ/dT ~0.095 nm/°C, and output powers ≥7 mW at 25 °C. Linewidth determination has been carried using the delayed self-heterodyne interferometric technique. Narrow linewidths (≤250 kHz) have been observed for a wide range of current injection, with a minimum of 184 kHz at 300 mA.

Index-coupled multi-wavelength distributed feedback quantum cascade lasers based on sampled gratings

Single-mode, wide wavelength range, high output power and low threshold current density DFB QCLs emitting around 4.7 \(\upmu \)m were presented. The multi-wavelength lasers were fabricated simultaneously on a single wafer by different sampled gratings. Instead of electron beam lithography, conventional holographic exposure combined with the optical photolithography was used to fabricate the sampled grating. Single-mode lasing at different wavelengths with a mean side mode suppression ratio above 20 dB was obtained. A wide wavelength range of 238 nm (\(\sim \)106 cm\(^{-1}\)) was achieved in pulsed mode at room temperature with different sampling periods. All the DFB-QCLs showed a stable single-mode operation with threshold current density in the range from 1.067 to 1.3 kA/cm\(^{2}\), and output power from a single facet in the range from 666.7 to 944.4 mW at 293 K.

Honeycomb photonic crystal vertical-cavity surface-emitting lasers: coupled cavities enhancing the single-mode range

A new design is proposed to increase the single-mode current range of low threshold current, photonic crystal vertical-cavity surface-emitting lasers. Our method is based on the optical coupling between the usual central defect and six others of nearly identical sizes. The resulting honeycomb configuration aims to spread higher supermodes in transverse directions, and to reduce their overlap with a central current-injected region. Simultaneous confinement of the fundamental mode within the central cavity and the shift of higher modes toward outer trapping defects can lead to stable single-mode operation. Simulations confirm that either in-phase or out-of-phase coupled supermodes could be promoted by slightly modifying the areas of outer cavities, while thermal lensing always stabilizes the in-phase coupled supermodes. The single-mode current regime should be extended noticeably compared to the conventional single-defect lasers.

A laterally-coupled distributed feedback laser with equivalent quarter-wave phase shift

We report the first laterally-coupled distributed feedback (LC-DFB) laser with a quarter-wave equivalent phase shift (EPS) realized by interference lithography (IL) and conventional photolithography. A specially designed sampled grating is fabricated on both sidewalls of a ridge waveguide to provide a quarter-wave EPS at the center of the cavity. The resulting laser exhibits stable single-mode lasing operation over a wide range of injection currents, with a side mode suppression ratio (SMSR) of 41.1 dB. This provides a practical, low-cost method to fabricate quarter-wave phase shifted DFB lasers with high performance without any epitaxial regrowth or the use of electron-beam lithography, thereby simplifying the fabrication of DFB lasers with stable and precise wavelengths, as single devices or as arrays in photonic integrated circuits.

Temperature Characteristics of Monolithically Integrated Wavelength-Selectable Light Sources

The temperature characteristics of monolithically integrated wavelength-selectable light sources are experimentally investigated. The wavelength-selectable light sources consist of four distributed feedback (DFB) lasers, a multimode interferometer coupler, and a semiconductor optical amplifier. The oscillating wavelength of the DFB laser could be modulated by adjusting the device operating temperature. A wavelength range covering over 8.0nm is obtained with stable single-mode operation by selecting the appropriate laser and chip temperature. The thermal crosstalk caused by the lateral heat spreading between lasers operating simultaneously is evaluated by oscillating-wavelength shift. The thermal crosstalk approximately decreases exponentially as the increasing distance between lasers.

Stable single axial mode operation of injection-seeded Q-switched Nd:YAG laser by real-time resonance tracking method

A simple real-time resonance tracking method is described and experimentally demonstrated for stable single axial mode operation of an injection-seeded Q-switched Nd:YAG laser. We have achieved stable single axial mode operation with mW of seeding power in noisy environment over hours. The mode selectivity dependence on finite Q-switch opening time is demonstrated and quantitatively modeled for Q-switch opening time optimization.

Stable single-mode operation of surface-emitting terahertz lasers with graded photonic heterostructure resonators

Graded photonic heterostructures (GPH) can be regarded as energy wells for photons. We show that judicious engineering of such photonic wells, obtained by tailoring the grading and the slit width of the GPH resonator, allows one to ensure spectrally single-mode emission on the fundamental symmetric mode in the whole lasing dynamical range of terahertz quantum cascade lasers. Furthermore, the radiative character of the symmetric mode leads to single-mode emission with mW output power in continuous-wave operation, as well as to single-lobed far-field beam patterns. A careful combination of theoretical analysis and experimental observations reveals that the results stem from interplay between mode competition and spatial hole burning effects.

Low-noise, single-frequency, single-polarization Brillouin/erbium fiber laser

We demonstrate a low-noise Brillouin/erbium fiber laser (BEFL), which uses only 1.5 m polarization-maintaining erbium-doped fiber as both the Brillouin and erbium gain media. This BEFL presents a phase noise of -125 dB/Hz(1/2) at 1 kHz frequency, at 2 mW Brillouin pump (~3 MHz linewidth) power and 200 mW 980 nm pump power. The polarization extinction ratio of the laser output light is 31 dB. Stable (~2 h) single-mode operation is observed. This BEFL presents potential applications in distributed Brillouin fiber sensors, inteferometric fiber sensors, and optical communications.

EML Based on Side-Wall Grating and Identical Epitaxial Layer Scheme

We, for the first time, present electroabsorption modulator lasers based on side-wall gratings and the identical epitaxial layer scheme using very simple fabrication processes. The devices show stable single-mode operation with little mode-hopping and a side mode suppression ratio of more than 45 dB, which is maintained until the applied current exceeds 6 times that of the threshold value. The drive voltage required to obtain a 12-dB extinction ratio is ${-}{\rm 2}~{\rm V}$ . The devices can be operated with a 3-dB bandwidth of 7 GHz allowing for error-free large signal modulation at 5 Gb/s with a dynamic extinction ratio of 11 dB.

Directional single-mode emission from coupled whispering gallery resonators realized by using ZnS microbelts

Ring microcavities were formed by wrapping ZnS microbelts, which act as the waveguide and gain region of the microcavities on the surface of optical fibers. The ring microcavities with the formation of whispering gallery modes have lasing threshold lower (Q-factor higher) than that of the ZnS microbelts. The excitation of TM modes could also be suppressed by the ring geometries of ZnS microbelts. Furthermore, directional single-mode lasing was realized from a coupled asymmetric ring microcavity. The Vernier coupling effect and deformed geometry of the asymmetric ring microcavity were contributed to the stable single-mode operation and directional emission, respectively.

Mode Selection and Coaxial Cavity Design for a 4-MW 170-GHz Gyrotron, Including Thermal Aspects

Gyrotron oscillators are millimeter wave sources, capable of reaching megawatt power levels. Such high RF power is required for electron cyclotron resonance heating and current drive systems for current and future nuclear fusion facilities. With total heating system powers in the range up to about 100 MW, these installations call for unit powers above 1 M W, to reduce cost and complexity of the complete heating system. In this paper, a mode selection process for a 4-MW 170-GHz coaxial-cavity gyrotron is presented and stable operating parameters are elaborated. The employed formalism, based on normalized variables, suggests one mode, namely the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-52,31</sub> , which is sufficiently separated from its competitors and supports an advanced two-beam quasi-optical mode converter . Through the utilization of time-dependent and self-consistent approaches, a coaxial cavity is optimized and stable single-mode operation at 4.35 MW of generated output power with an interaction efficiency of 33% is predicted. This paper discusses the mode selection process under consideration of realistic technical limitations, optimization of the cavity for stable operation with a pure output mode, and finally thermo-mechanical calculations on cavity cooling and surface temperature.

High Performance Asymmetric Three Corrugation-Pitch-Modulated DFB Lasers Suitable for Stable Single Longitudinal Mode Operation

This paper presents an optimized asymmetric three corrugation-pitch-modulated DFB laser (3CPM-DFB) with extremely high mode selectivity(△αL= 0.97) and low flatness(F = 0.009), which are two key parameters to indicate the laser’s single longitudinal mode(SLM) performance. In threshold analysis, the optimization process based on transfer matrix method is demonstrated to maximize △αL and minimize F simultaneously. In the above-threshold regime, the evolutions of △αL and longitudinal distribution of photon density with injection current are evaluated. More importantly, nanoimprint lithography which was proved an efficient way to fabricate DFB gratings can provide completely same simple fabrication procedure for both 3CPM grating and conventional uniform grating. So the big practical value of 3CPM-DFB can be expected because of its advanced performance and easy manufacturability.

Single-mode lasing of GaN nanowire-pairs

Stable single-mode lasing operation from a pair of coupled GaN nanowires is demonstrated through optical pumping. GaN nanowires with different lengths were placed side-by-side in contact to form a coupled cavity through nanoprobe manipulation. Unlike individual nanowire lasers, which operate in a combined multiple transverse and multiple longitude mode oscillation, a coupled nanowire-pair provides a mode selection mechanism through the Vernier effect, which can strongly enhance the free spectrum range between adjacent resonant modes and generate a stable single-mode operation with a high side-mode suppression ratio.