Side Mode Research Articles

Performance enhancement of an optically-injected-semiconductor-laser-based optoelectronic oscillator by subharmonic microwave modulation.

An approach to enhancing the performance of an optically-injected-semiconductor-laser-based optoelectronic oscillator (OEO) is proposed by subharmonic microwave modulation. A free-running OEO is first established based on period-one dynamics of an optically injected semiconductor laser. The oscillation frequency can be tuned in the range of 8.87 to 18.41GHz by controlling the injection strength, but the output signal suffers from strong side modes and poor frequency stability. To address these problems, subharmonic microwave modulation is applied to the injected semiconductor laser. In the experiment, microwave modulation with 1/2, 1/4, and 1/6 subharmonics is demonstrated. The side-mode suppression ratio is improved by over 40dB, while the phase noise at a 1kHz offset is reduced by about 18dB. Furthermore, the frequency drift over a period of 20 min, which characterizes the long-term stability, is reduced from 8.7kHz to less than 1Hz, indicating a significant reduction of over three orders.

Experimental demonstration of DFB lasers with active distributed reflector

We experimentally demonstrated DFB lasers containing an active distributed reflector that has the same waveguide core as the active section. Although without current injection, the distributed reflector will be optically pumped to near transparency by the laser itself, and therefore can provide relatively high reflection to the laser. The laser, fabricated with processing steps similar to standard DFB lasers, has achieved 10-mA threshold current, 0.38-mW/mA slope efficiency, above 55-dB side mode suppression ratio, and 24-GHz modulation bandwidth at 60-mA current injection. 28-Gb/s transmission over 10-km single-mode fibers with a power penalty of less-than 0.5 dB has been demonstrated as well.

Tunable and switchable C-band and L-band multi-wavelength erbium-doped fiber laser employing a large-core fiber filter

In this paper, we propose and experimentally demonstrate a C- and L-band erbium-doped fiber laser (EDFL) with tunable single-, dual- and triple-wavelength lasing emissions. A tapered structure large-core fiber filter (LCFF) is inserted to serve as tuning component and wavelength selector for tunable and stable multi-wavelength oscillations. Based on the axial strain response of LCFF, two tunable triple-wavelength regimes within the range from 1558.71 to 1579.81 nm are obtained with side mode suppression ratio (SMSR) as high as 55 dB. Moreover, tunable single- and dual-wavelength regimes are also achieved with total tuning range of 11 nm and 6.23 nm, respectively. The tuning step for overall laser regimes is 0.02 nm, and the maximum wavelength sensitivity reaches to 1.87 pm/με. In addition, this proposed versatile EDFL is proved to be very stable since the lasing wavelength shifts and power fluctuations are less than 0.02 nm and 0.78 dB during an hour.

Tunable self-injection locked green laser diode.

We report, to the best of our knowledge, the first employment of a self-injection locking scheme for the demonstration of a tunable InGaN/GaN semiconductor laser diode. We have achieved a 7.11nm (521.10-528.21nm) tunability in a green color with different injection currents and temperatures. The system exhibited mode spectral linewidth as narrow as ∼69 pm and a side mode suppression ratio as high as ∼28 dB, with a maximum optical power of ∼16.7 mW. In the entire tuning window, extending beyond 520nm, a spectral linewidth of ≤100 pm, high power, and stable performance were consistently achieved, making this, to the best of our knowledge, the first-of-its-kind compact tunable laser system attractive for spectroscopy, imaging, sensing systems, and visible light communication.

Single-mode, high-power, mid-infrared, quantum cascade laser phased arrays

We demonstrate single-mode, 16-channel, optical phased arrays based on quantum cascade laser technology, with emission wavelengths around 4.8 µm. The integrated device consists of a distributed feedback seed section, a highly-efficient tree array multi-mode interferometer power splitter, and a 16-channel amplifier array with a 4° angled facet termination. With a single layer Y2O3 coating, the angled facet reflectivity is estimated to be less than 0.1% for suppressing amplifier self-lasing. A peak output power of 30 W is achieved with an emission spectrum narrower than 11 nm and a side mode suppression ratio over 25 dB. Far field distribution measurement result indicates a uniform phase distribution across the array output. Using the same phased array architecture, we also demonstrate single-mode 3.8 µm QCL amplifier arrays with up to 20 W output power.

Opto-electronic oscillator mediated by acoustic wave in a photonic crystal fiber stimulated in 1 μm band.

An opto-electronic oscillator based on guided acoustic wave Brillouin scattering in a photonic crystal fiber (PCF) stimulated by a light wave in 1μm band is proposed and demonstrated. A short length of a homemade PCF stimulated by relatively low pump power leads to strong coupling between the pump and probe waves. The oscillation is realized in a feedback loop, in which the acoustic wave bridges the pump and probe. Oscillation is achieved at 1.237GHz, which matches the resonance of the acoustic mode, in a single-longitudinal-mode operation of the hybrid cavity. It has a high side mode suppression ratio of over 60dB.

Tunable single-wavelength erbium-doped fiber ring laser using a large-core fiber

In this paper, a tunable single-wavelength erbium-doped fiber ring laser is proposed. A novel fiber filter based on a large-core fiber is developed for suppressing the mode competition in erbium-doped fiber lasers (EDFLs). Under the pump power of 250 mW, single-wavelength lasing output is implemented with a side mode suppression ratio (SMSR) higher than 55 dB. By adjusting the curvature applied on the large-core fiber, stably tunable single-wavelength lasing output within the wavelength range from 1552.4 nm to 1563.2 nm is obtained with a 3 dB linewidth of 0.2 nm. Furthermore, the wavelength shift and peak power fluctuation are controlled within 0.02 nm and 0.5 dB by monitoring the lasing output with an interval of 6 min in one hour, respectively. A contrast experiment of a laser without tapering LCF is carried out to examine the importance of the taper in LCF-based filter. Experimental results demonstrate that the side modes of the lasing output can be effectively suppressed and thus SMSR can be improved from 41 dB to 58 dB by using a taper.

Inverse Scattering Method Design of Regrowth-Free Single-Mode Semiconductor Lasers Using Pit Perturbations for Monolithic Integration

An inverse scattering method is used to design regrowth-free single-mode lasers, using etch depth insensitive pits as perturbations in the laser cavity. These pit perturbations are circular holes etched into the waveguide of each laser to a depth 0.8 $\mu \text{m}$ below the quantum wells in the laser material, and are used in place of the etch depth sensitive slots more commonly implemented for optical feedback in photonic integrable lasers. We compare the number of these pit perturbations required to achieve the target wavelength of the devices, and report strongly single mode lasers (greater than 40 dB). Devices were then designed to lase at wavelengths between 1545 nm and 1565 nm, to demonstrate the wavelength accuracy of the designs. Thermal tuning was used to tune device wavelength over 2 nm, while maintaining side mode suppression ratios greater than 30 dB.

Interaction of two drops in the bag breakup regime by a continuous air jet

Fuel atomization characteristics are important in the performance of gasification and combustion. This work concerns the interaction of two drops in a continuous air jet stream. Morphological classifications of interaction modes have been analyzed using high speed camera. Behaviors of drops group and the isolated drop in the airflow are significantly different. Experimental photos show that there are four main interaction modes, which are coalescence mode, puncture mode, side by side mode and no direct contact mode. Influence of dimensionless distance on drops interaction mode is researched. Then the drops interaction regime map is obtained. The model prediction is in good agreement with the experimental results. The total dimensionless breakup time of puncture mode is the longest, then the coalescence mode, and the third the side by side mode. Results present an evaluation of the role of proximity to other drops in affecting the outcome, which are useful in understanding atomization mechanism and improving simulation model.

Spontaneous Exact Spin-Wave Fractals in Magnonic Crystals.

Exact fractals of nonlinear waves that rely on strong dispersion and nonlinearity and arise spontaneously out of magnetic media were observed for the first time. The experiments make use of a microwave to excite a spin wave in a quasi-one-dimensional magnonic crystal. When the power of the input microwave (P_{in}) is low, the output signal has a power-frequency spectrum that consists of a single peak. When P_{in} is increased to a certain level, new side modes are generated through modulational instability, resulting in a comblike frequency spectrum. With a further increase in P_{in}, each peak in the frequency comb can evolve into its own finer comb through the modulational instability. As P_{in} is increased further, one can observe yet another set of finer frequency combs. Such a frequency-domain fractal manifests itself as multiple layers of amplitude modulation in the time-domain signal.

The diverse magneto-optical selection rules in bilayer black phosphorus

The magneto-optical properties of bilayer phosphorene is investigated by the generalized tight-binding model and the gradient approximation. The vertical inter-Landau-level transitions, being sensitive to the polarization directions, are mainly determined by the spatial symmetries of sub-envelope functions on the distinct sublattices. The anisotropic excitations strongly depend on the electric and magnetic fields. A uniform perpendicular electric field could greatly diversify the selection rule, frequency, intensity, number and form of symmetric absorption peaks. Specifically, the unusual magneto-optical properties appear beyond the critical field as a result of two subgroups of Landau levels with the main and side modes. The rich and unique magnetoabsorption spectra arise from the very close relations among the geometric structures, multiple intralayer and interlayer hopping integrals and composite external fields.

Observation of 550 MHz passively harmonic mode-locked pulses at L-band in an Er-doped fiber laser using carbon nanotubes film**Project supported by the National Natural Science Foundation of China (Grant No. 61605107), Young Eastern Scholar Program at Shanghai Institutions of Higher Learning, China (Grant No. QD2015027), the “Young 1000 Talent Plan” Program of China, the Open Program of the State Key

We demonstrate a passively harmonic mode-locked (PHML) fiber laser operating at the L-band using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film. Under suitable pump power and an appropriate setting of the polarization controller (PC), the 54th harmonic pulses at the L-band are generated with the side mode suppression ratio (SMSR) better than 44 dB and a repetition frequency of 503.37 MHz. Further increasing the pump power leads to a higher frequency of 550 MHz with compromised stability of 38.5 dB SMSR. To the best of our knowledge, this is the first demonstration on the generation of L-band PHML pulses from an Er-doped fiber laser based on CNTs.

Experimental study on linewidth compression based on Rayleigh backscattering in 1064 nm fiber laser

Abstract Attributed to the unique energy level structure of the Yb 3 + , it is arduous to effectively compress the linewidth of Ytterbium-doped fiber (YDF) laser by conventional methods. Herein, we investigated Rayleigh backscattering (RBS) characteristics in single mode fiber-1060-XP (SMF-1060-XP) and the linewidth compression efficacy of RBS in 1064 nm laser system. A single longitudinal mode (SLM) fiber laser operating at 1064 nm has been obtained, and the 3 dB linewidth can be compressed from 8.01 kHz to 1.35 kHz thereby yielding a side mode suppression ratio as high as ∼ 50 dB provided the pump power is set to 390 mW. The theoretical prediction suggests the feasibility to actualize a linewidth of hundreds of Hertz provided the experimental setup is further optimized. The investigation of our work has a potential application in the study of linewidth compression of short cavity lasers or DFB lasers operating at a 1064 nm wavelength.

Indium Phosphide Photonic Integrated Circuits for Free Space Optical Links.

An indium phosphide (InP)-based photonic integrated circuit (PIC) transmitter for free space optical communications was demonstrated. The transmitter consists of a sampled grating distributed Bragg reflector (SGDBR) laser, a high-speed semiconductor optical amplifier (SOA), a Mach-Zehnder modulator, and a high-power output booster SOA. The SGDBR laser tunes from 1521 nm to 1565 nm with >45 dB side mode suppression ratio. The InP PIC was also incorporated into a free space optical link to demonstrate the potential for low cost, size, weight and power. Error-free operation was achieved at 3 Gbps for an equivalent link length of 180 m (up to 300 m with forward error correction).

SWNT Saturable Absorption Application in Telecom Wavelength Range

Abstract The current study investigated saturable absorption of a single-wall carbon nanotube (SWNT) layer in the optical spectral range. Initially, a glass substrate was covered with an SWNT layer several μm in width to determine its attenuation properties in the optical telecom spectrum. The spectral response of the broadband source (BBS) was 1260 to 1640 nm, linear and non-gradient. Attenuation behavior in the narrow spectrum sources depended on optical power variation. Saturation absorption occurred at the peak of optical power. This characteristic was observed for laser diodes with wavelengths of 1310 and 1550 nm. The EDFA optical amplifier showed saturable absorption. This property can be applied to increase side mode suppression ratio 7 dB in laser diodes and improve optical signal-to-noise ratio (OSNR) 8 dB in optical amplifiers.

Erbium Ring Fiber Laser Cavity Based on Tip Modal Interferometer and Its Tunable Multi-Wavelength Response for Refractive Index and Temperature

A tunable multi-wavelength fiber laser is proposed and demonstrated based on two main elements: an erbium-doped fiber ring cavity and compact intermodal fiber structure. The modal fiber interferometer is fabricated using the cost-effective arc splice technique between conventional single-mode fiber and microfiber. This optical fiber structure acts as a wavelength filter, operated in reflection mode. When the refractive index and temperature variations are applied over the fiber filter, the ring laser cavity provides several quad-wavelength laser spectra. The multi-wavelength spectra are tuned into the C-band with a resolution of 0.05 nm. In addition, the spectra are symmetric with minimal power difference between the lasing modes involved, and the average of the side mode suppression ratio is close to 37 dB. This laser offers low-cost implementation, low wavelength drift, and high power stability, as well as an effect of easy controllability regarding tuned multi-wavelength.

Intensity noise in high-frequency range of an external cavity diode laser and its reduction by second harmonic generation.

Characteristics of intensity noise of optically pumped vertical-cavity surface-emitting lasers and solid-state lasers, induced by the beating of the main lasing mode and non-lasing side modes and its reduction, have been reported in recent years. The mode beating noise of an external cavity diode laser composed of an electrically pumped edge-emitting laser diode chip is studied experimentally in this Letter. The noises due to the beating of the main mode with the first- to third-order side modes are observed, and multiple sub-peaks in the beating noise are measured. It is assumed that the new phenomena are coming from the enhanced four-wave mixing in the longer-active medium. Intensity noise reduction is also demonstrated by using the second harmonic generation of a beta barium borate crystal as a nonlinear absorber.

Application of optical frequency comb generation with controlled delay circuit for managing the high capacity network system

The generation of optical frequency comb (OFC) with novel controlled delay circuit is presented in this study. The proposed OFC is based on a single laser source which is cascadingly connected with three modulators; all the modulators are tailored by RF signal while incorporating no phase shifter or electrical/optical amplifier. The proposed OFC is used as a source at optical line terminal (OLT) of WDM-PON setup, which serves multiple users with a single laser source making OLT side very cost effective. 41 comb lines with over 40 dB tone to noise ratio and high side mode suppression ratio, least amplitude difference of under 0.3 dB, with cost effective setup is an attractive source for WDM-PON system. The frequency spacing is 32 GHz between OFCG lines which provides enough bandwidth for data transmission, the overall bandwidth provided by this scheme is 1.2 THz. Using DPSK modulation technique across each frequency of OFCG, the controlled delay circuit increases the capacity by factor two, whereas by deploying DQPSK modulation technique, it quadrupled the overall capacity in downlink transmission. Thus, the system offers four times increased capacity ∼1.6 Tbps by deploying state of the art technique for modulation and generating OFC with controlled delay. The average power penalty in the downlink and uplink transmission is 2.5 dBm and 3.13 dBm.

1 × 8 MMI based multi-channel interference laser integrated with SOA through a 2-port multimode interference reflector.

A 1 × 8 multimode interferometer (MMI) based multi-channel interference (MCI) widely tunable laser integrated with semiconductor optical amplifier (SOA) through a 2-port multimode interference reflector (MIR) is reported in this paper. The 2-port MIR can be fabricated with deeply etched waveguides of the MCI laser with no extra fabrication process, which decreases the fabrication complexity greatly. To reduce the loss arising from mode mismatch between surface and deep ridge waveguides, a tapered shallow-deep transition structure is designed and fabricated with a loss of 0.09 dB ± 0.03 dB. The MCI laser with integrated SOA achieves a tuning range of 52.5 nm with side mode suppression ratios higher than 40 dB across the tuning range and more than 13 dBm output power coupled into a lensed fiber at room temperature.

High-power tunable low-noise coherent source at 106 μm based on a surface-emitting semiconductor laser

Exploiting III-V semiconductor technologies, vertical external-cavity surface-emitting laser (VECSEL) technology has been identified for years as a good candidate to develop lasers with high power, large coherence, and broad tunability. Combined with fiber amplification technology, tunable single-frequency lasers can be flexibly boosted to a power level of several tens of watts. Here, we demonstrate a high-power, single-frequency, and broadly tunable laser based on VECSEL technology. This device emits in the near-infrared around 1.06µm and exhibits high output power (>100 mW) with a low-divergence diffraction-limited TEM00 beam. It also features a narrow free-running linewidth of <400 kHz with high spectral purity (side mode suppression ratio >55 dB) and continuous broadband tunability greater than 250GHz (<15 V piezo voltage, 6kHz cutoff frequency) with a total tunable range up to 3THz. In addition, a compact design without any movable intracavity elements offers a robust single-frequency regime. Through fiber amplification, a tunable single-frequency laser is achieved at an output power of 50W covering the wavelength range from 1057 to 1066nm. Excess intensity noise brought on by the amplification stage is in good agreement with a theoretical model. A low relative intensity noise value of -145 dBc/Hz is obtained at 1MHz, and we reach the shot-noise limit above 200MHz.