Side-mode Suppression Ratio Research Articles

Sub-Kilohertz Narrow Linewidth Fiber Laser Based on Nanoparticles Doped Self-Injection Module

Fiber lasers with narrow linewidth are essential light sources for many prospects in advanced scientific fields. In this article, we proposed a scheme of self-injection locked single-frequency fiber laser based on a hybrid cavity configuration and a novel type of nanoparticles doped fiber. The newly proposed hybrid cavity was applied to achieve mode selection in the case of a long cavity which helped increase output power without sacrificing mode instability and single frequency performance. The nanoparticles doped self-injection module helped achieve a fiber laser with 3dB linewidth compressed from 16 kHz to 497 Hz and side mode suppression ratio of more than 60 dB. Furthermore, the sub-kilohertz fiber laser was amplified to more than 1 W with the polarization extinction ratio of ~30 dB. Throughout the work, experimental verification and analysis have been carried out to confirm the single frequency lasing performance, including center wavelength drift and slope efficiency variation. The demonstrated watt-level narrow linewidth fiber laser with low RIN and frequency noise has full potential for practical applications.

Sub-kHz-Linewidth External-Cavity Laser (ECL) With Si3N4 Resonator Used as a Tunable Pump for a Kerr Frequency Comb

Combining optical gain in direct-bandgap III-V materials with tunable optical feedback offered by advanced photonic integrated circuits is key to chip-scale external-cavity lasers (ECL), offering wideband tunability along with low optical linewidths. External feedback circuits can be efficiently implemented using low-loss silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) waveguides, which do not suffer from two-photon absorption and can thus handle much higher power levels than conventional silicon photonics. However, co-integrating III-V-based gain elements with tunable external feedback circuits in chip-scale modules still represents a challenge, requiring either technologically demanding heterogeneous integration techniques or costly high-precision multi-chip assembly, often based on active alignment. In this work, we demonstrate Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based hybrid integrated ECL that exploit 3D-printed structures such as intra-cavity photonic wire bonds and facet-attached microlenses for low-loss optical coupling with relaxed alignment tolerances, thereby overcoming the need for active alignment while maintaining the full flexibility of multi-chip integration techniques. In a proof-of-concept experiment, we demonstrate an ECL offering a 90 nm tuning range (1480 nm–1570 nm) with on-chip output powers above 12 dBm and side-mode suppression ratios of up to 59 dB in the center of the tuning range. We achieve an intrinsic linewidth of 979 Hz, which is among the lowest values reported for comparable feedback architectures. The optical loss of the intra-cavity photonic wire bond between the III-V gain element and the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based tunable feedback circuit amounts to approximately (1.6 ± 0.2) dB. We use the ECL as a tunable pump laser to generate a dissipative Kerr soliton frequency comb. To the best of our knowledge, our experiments represent the first demonstration of a single-soliton Kerr comb generated with a pump that is derived from a hybrid ECL.

Numerical simulation of linewidth compression dynamics of external-cavity semiconductor laser

We theoretically simulate the linewidth compression dynamics of the semiconductor laser with a single external-cavity feedback. The evolution time from the periodic oscillation state to the equilibrium state is calculated by tracking the change of the intrinsic linewidth. The influences of the external-cavity length and feedback level are also explored. The linewidth evolution time of the external-cavity semiconductor laser can be extended with longer external cavity length and higher feedback level. Simultaneously, the intrinsic linewidth can be compressed while the side mode suppression ratio will be decreased. The simulation work show that the dynamic linewidth compression can be obtained by single external-cavity feedback, and nevertheless it is difficult to achieve the single-frequency output, which can instruct the design of the dynamical parameter tuning of the external-cavity semiconductor laser.

Single-frequency violet and blue laser emission from AlGaInN photonic integrated circuit chips.

Chip-based, single-frequency and low phase-noise integrated photonic laser diodes emitting in the violet (412 nm) and blue (461 nm) regime are demonstrated. The GaN-based edge-emitting laser diodes were coupled to high-quality on-chip micro-resonators for optical feedback and mode selection resulting in laser self-injection locking with narrow emission linewidth. Multiple group III-nitride (III-N) based photonic integrated circuit chips with different waveguide designs including single-crystalline AlN, AlGaN, and GaN were developed and characterized. Single-frequency laser operation was demonstrated for all studied waveguide core materials. The best side-mode suppression ratio was determined to be ∼36 dB at 412 nm with a single-frequency laser emission linewidth of only 3.8 MHz at 461 nm. The performance metrics of this novel, to the best of our knowledge, type of laser suggest potential implementation in next-generation, portable quantum systems.

A Model of Dual Fabry–Perot Etalon-Based External-Cavity Tunable Laser Using Finite-Difference Traveling-Wave Method

A physical model of an external-cavity tunable laser (ECTL) utilizing the vernier effect of a dual Fabry–Perot (FP) etalon is presented and simulated using the finite-difference traveling wave (FDTW) method. In this paper, we provide a detailed explanation of the physical principle and construction process of the model, as well as the simulation results for the laser. The model is precisely established by studying the time-dependent changes in the carrier concentration and optical field of different wavelengths inside the laser before reaching a steady state. By determining multiple parameters in the tuning region and gain region, the proposed model can calculate and predict various laser parameters, such as output power and side-mode suppression ratio (SMSR). Moreover, the FDTW method displays the change process of various parameters, such as carrier concentration and spectrum, in the convergence of various positions in the laser with femtosecond time resolution. This capability is promising for in-depth research on the inner mechanism of lasers.

Characterization of Thermo-Optically Tuned Multi-Channel Interference Widely Tunable Semiconductor Laser for Quasi-Continuous Tuning

Characterization of the thermo-optically tuned multi -channel interference widely tunable semiconductor laser for quasi-continuous tuning is demonstrated for the first time. The characterization strategy is carried out by simultaneously controlling seven arm phase sections to shift the reflection peak and adjusting the common phase section to fine-tune the longitudinal mode. Output wavelengths at 50 GHz frequency spacing are characterized, and a quasi-continuous tuning range beyond 60 nm is achieved at two temperatures. In the entire tuning range: side-mode suppression ratios are higher than 40 dB; wavelength deviations are less than ± 10 pm from international telecommunication union grid; total thermal tuning powers are lower than 35 mW, and Lorentzian linewidths are less than 100 kHz. The tuning information near any output wavelength is obtained and the characterization speed is greatly improved. This strategy provides a new route for characterizing tunable lasers with multiple tuning currents.

High side-mode suppression ratio with a Vernier effect single-mode laser using triple coupled microrings

The development of single-mode lasers with a high side-mode suppression ratio (SMSR) is challenging but highly desirable for integrated photonics devices and long-distance communications due to their high spectral purity and stability. Here, we demonstrate a single-mode laser with a high side-mode suppression ratio based on size-mismatched triple-coupled microrings. With the exact engineering of several key parameters of the structure like air gap and radii of microrings for controlling the free spectral range (FSR), a predominant mode is selected to lase in amplified spontaneous emission (ASE) of the gain material and all side and high order modes are suppressed by Vernier effect. In this work, we show that a single-mode operation is efficiently generated with an improved side-mode suppression ratio of over 20 dB in a three-ring-coupled microcavity laser. The single-frequency output persists for a wide power range. The theoretical calculations and numerical simulations’ results confirm the validity of the experimental results. Our structural engineering creates new opportunities in a variety of frontier applications in single-mode lasers and high-quality sensors.

Self-injection locking of a low-noise erbium-doped random fiber laser by a random fiber grating ring.

We demonstrate a self-injection locking (SIL) in an Er-doped random fiber laser by a high quality factor (high-Q) random fiber grating ring (RFGR) resonator, which enables a single-mode narrow-linewidth lasing with ultra-low intensity and frequency noise. The RFGR resonator includes a fiber ring with a random fiber grating to provide random feedback modes and noise suppression filters with self-adjusted peak frequency adaptable to small perturbations allowing single longitudinal mode over 7000 s with frequency jitter below 3.0 kHz. Single-mode operation is accomplished by carefully controlling phase delays and mode coupling of resonant modes between main ring and RFGR with a side-mode suppression ratio of 70 dB and narrow linewidth of 1.23 kHz. The relative intensity noise is -140 dB/Hz above 100 kHz and the frequency noise is 1 Hz/Hz1/2 above 10 kHz.

Single-mode GaSb-based laterally coupled distributed-feedback laser for CO2 gas detection

We report on a GaSb-based laterally coupled distributed feedback (LC-DFB) laser with Cr gratings operating at 2004 nm for CO2 detection application. Butterfly packaged with single-mode fiber pigtailed, the laser diode operates in the continuous-wave mode in a temperature range from −10 °C to 60 °C, with a maximum output power of 2 mW and a maximum side-mode suppression ratio over 30 dB. Wavelength-modulated absorption spectroscopy of CO2 demonstrates the applicability of the LC-DFB laser to tunable diode laser absorption spectroscopy. Furthermore, the diode junction temperature, which is measured by using the wavelength shift method, exhibits a maximum value of 17 °C in the single-mode operation range.

A Compact Self-Injection-Locked Narrow-Linewidth Diode Laser with Narrowband Dielectric Filter

The self-injection-locked diode laser can significantly reduce the linewidth down to a sub-100 kHz level but requires optical filtering that is much narrower than the laser cavity linewidth. Such optical filtering is usually sophisticated and beyond the capability of the normal dielectric coating approach. Here we develop a 0.16 nm narrowband dielectric coating using a Fabry–Perot dielectric filter (HFFPF) for the narrow linewidth self-injection-locked laser demonstration using a cost-effective Fabry–Perot laser diode. Single-longitude-mode output is achieved, with beat linewidth down to ~66 kHz and a high side mode suppression ratio over 60 dB. This setup of self-injection-locked laser is cost-effective for mass production and thus is suitable for various applications such as laser ranging, LIDAR, optical communication, and holographic.

Heterogeneously integrated III–V-on-Si laser with embedded photonic molecule mirrors

We have designed, fabricated, and characterized a III–V-on-Si laser employing photonic molecule mirrors with high potential for laser tunability and control. Resonant mirrors are realized by tailoring supermodes of coupled microrings. A laser threshold of 40 mA (L-band), series resistance of 10 Ω, and side mode suppression ratio of 40 dB are demonstrated.

Monolithically Integrated Tunable Three- Section All-Active DBR Laser Diodes With 30 nm Tuning Range

We present a monolithically integrated three-section all-active AlGaInAs/InP tunable distributed Bragg reflector (DBR) ridge waveguide laser diode that uses thermal tuning mechanism and mode-hopping tuning principle to achieve wide-range wavelength tuning. It is similar to the traditional three-section DBR laser structure, including a gain section, a phase section, and a DBR grating section. The difference is that the phase section and the DBR section incorporate the same active quantum well structure as the gain section. In this case, weak effective refractive index changes of the three active sections affected by the current injection will occur since the carrier density is clamped by the threshold gain condition of the DBR laser diode. Despite this limitation, a noncontinuous tuning up to ∼30 nm (1564–1594 nm) can be observed using only current injection at a constant temperature of 25 °C. Significant thermal effects occurred during the tuning process. In addition, a discontinuity is introduced in the tuning because it does not contain a passive region for phase adjustment. However, the device still provides a stable single longitudinal mode output with nine channels, a wavelength spacing of about 4 nm, and a side mode suppression ratio (SMSR) of more than 30 dB. Furthermore, the DBR laser diode can be easily fabricated utilizing the conventional distributed feedback laser diode fabrication process without secondary epitaxy and heater metal. The all-active DBR laser diode will be expected to be applied in the coarse wavelength division multiplexing (CWDM) systems with the advantage of low cost.

Inverse-Designed Non-Uniform Silicon Nitride Gratings With High Beam Steering Performance

Non-uniform silicon nitride grating antennas are designed by inverse-design method. In the wavelength range of 1500-1590 nm, the non-uniform gratings exhibit a large scanning range of 13.2° and high wavelength tuning efficiency of 0.147 °/nm, nearly 2.4 times that of the uniform gratings, along with superior transmission and directionality performance. At 1550 nm, a high side mode suppression ratio of 4.32 dB is obtained for the non-uniform gratings, much higher than that of the uniform gratings (1.00 dB). This work may pave the way for the development of silicon nitride-based miniaturized optical phased arrays with excellent beam steering performance.

A 40-Gbps fiber-FSO convergent transmission system employing OFCL-based WDM and external modulation technique

We have proposed and demonstrated a 40Gbps fiber-free space optics (FSO) convergent system based on external modulation and wavelength division multiplexing (WDM) technique. In this experiment, 4010×4 Gbps signal is successfully transmitted employing WDM scheme over 50 km single mode fiber plus 2.5 km FSO distance. Optical frequency comb lines (OFCL) are generated using a continuous wave laser diode, a radio frequency signal generator, a dual drive Mach Zehnder modulator, and an electrical attenuator. OFC lines, having a side mode suppression ratio (SMSR) value of ∼40 dB and flatness of ∼0.4 dB, are employed as broadband laser source in the presented convergent system. The performance of the fiber-FSO system is investigated theoretically for different weather conditions. Very low power penalty with low bit error rate (BER), clear eye diagrams, and standard Q-factor are achieved in the proposed system. The power penalty is restricted within 2.5 dB and 5.8 dB at a BER of 10−9 for 50 km SMF plus 2.5 km FSO distance in clear weather and 50-km SMF plus 0.4-km FSO distance in heavy fog condition respectively. Thus, the proposed WDM-convergent system is efficient enough for high data rate transmission along with low BER value of ∼10−9 and high Q-factor for different environmental scenarios.

Stand-off detection of ethanol gas in turbulent state using a 1.7 μm/1.5 μm near-infrared spectroscopy system

Considering that the stand-off detection of ethanol gas is significant for monitoring ethanol-free occasions, we propose and experimentally demonstrate a 1.7 μm/1.5 μm near-infrared spectroscopy system. Then based on the generated laser signals and the system, the ethanol gas and ethanol solution was stand-off detected and researched. Firstly, we measured the absorption characteristics of ethanol solution in the near-infrared band for wavelength selection. Secondly, the Tm-doped fiber laser in the system was achieved based on a ring cavity and a customized FBG; its output laser wavelength was 1728.45 nm. The linewidth of the 1.7 μm band signal was about 0.18 nm and the side mode suppression ratio (SMSR) was about 57 dB. Thirdly, ethanol gas and ethanol solution were stand-offs detected using the established 1.7 μm/1.5 μm near-infrared spectroscopy system. The concentration of ethanol solution was measured by 1.7 μm band laser, and the R-Square of the linear fit was 0.98. However, the concentration of ethanol gas in a turbulence state cannot be easily measured due to scintillation. Therefore, we detected the ethanol gas a turbulence state using a 1.7 μm/1.5 μm dual-band system, R-Square of the linear fit was 0.83 when the power jitter variance was 2.35. the experimental results offer a technical reference for stand-off detection of ethanol gas.

High-power tapered two-section distributed feedback laser based on a chirped sampled grating.

We propose and theoretically study a two-section high-power distributed feedback (DFB) laser with three equivalent phase shifts (3EPSs). A tapered waveguide with a chirped sampled grating is introduced to amplify the output power and keep a stable single-mode operation. The simulation exhibits the maximum output power and side mode suppression ratio of a 1200µm length two-section DFB laser as high as 306.5mW and 40dB, respectively. Compared with traditional DFB lasers, the proposed laser has a higher output power, which may benefit wavelength division multiplexing transmission systems, gas sensors, and large-scale silicon photonics.

Narrow linewidth optical frequency comb based on a directly modulated microcavity laser with optical feedback.

A narrow linewidth optical frequency comb (OFC) based on a directly modulated microcavity laser with external optical feedback is investigated numerically and demonstrated experimentally. Based on the numerical simulations with rate equations, the evolution of the optical and electrical spectra is presented for the direct-modulated microcavity laser with increased feedback strength, and the linewidth property is improved at suitable feedback conditions. The simulation results also show good robustness for the generated OFC in terms of feedback strength and phase. Moreover, the OFC generation experiment is performed by combining with the dual-loop feedback structure to suppress the side mode, and an OFC with a side-mode suppression ratio of 31 dB is realized. Thanks to the high electro-optical response of the microcavity laser, a 15-tone OFC with a frequency interval of 10 GHz is obtained. Finally, the linewidth of each comb tooth is measured to be around 7 kHz under the feedback power of 47 µW, which indicates an enormous compression of approximately 2000 times compared with the free-running continuous-wave microcavity laser.

Intra-cavity coherent combining of DBR lasers on an InP generic foundry platform.

We report an intra-cavity coherent combining of two distributed Bragg reflector (DBR) lasers with a combining efficiency of ∼84% on an InP generic foundry platform. The on-chip power of the intra-cavity combined DBR lasers is ∼9.5 mW at the injection current of 42 mA in both gain sections simultaneously. The combined DBR laser operates in a single-mode regime with a side-mode suppression ratio of 38 dB. This monolithic approach paves the way toward high-power and compact lasers, which is useful in scaling integrated photonic technologies.

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.

Single-Mode Tunable Interband Cascade Laser Emitting at 3.4 μm With a Wide Tuning Range Over 100 nm

We report a widely tunable monolithically integrated V-coupled cavity interband cascade laser (ICL) emitting near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.4~\mu \text{m}$ </tex-math></inline-formula> . The mode selection is achieved using a half-wave V-coupler designed for the ICL in the mid-infrared range without any grating. The side-mode suppression ratio is demonstrated up to 35.7 dB. In the heat-sink temperature range from 130 K to 140 K, single-mode wavelength tuning range of over 100 nm from 3329.9 nm to 3431.3 nm has been attained by adjusting both current and temperature. These results show that the V-coupled cavity ICLs are very promising for multi-species trace gas absorption spectroscopy with a high sensitivity.