Side Mode Research Articles

Experimental Demonstration of π Equivalent Phase-Shifted SBG Laser With Controlled Distributed Phase Shift

We designed and fabricated a π equivalent phase-shifted sampled Bragg grating (SBG) semiconductor laser with a controlled distributed phase shift. The phase shift is equivalently realized with reconstruction-equivalent-chirp (REC) technique. The laser is divided into three sections with the same length. By injecting different currents into the side and middle sections, a distributed phase shift can be introduced. The lasing wavelength can be continuously tuned by the injection currents. In our experiment, when the total current keeps 130 mA, the lasing wavelength can be continuously tuned by 1.2 nm. During the tuning, the side mode suppression ratio keeps above 40 dB and the output power varies only 1.3 dB. Therefore, the proposed method provides an alternative solution for the multiwavelength laser arrays in dense wavelength-division multiplexing (DWDM) systems.

High-Power Indium Phosphide Photonic Integrated Circuits

Indium phosphide (InP) is the most developed platform for photonic integrated circuits (PICs). Of interest is the advancement of this platform for applications that demand high performance, especially high output power, including free space communications and microwave photonics. In this paper, we summarize development of InP-based PIC transmitters. Two transmitter types were fabricated: one based on an offset quantum wells (OQW) platform and the other on a quantum well intermixing (QWI) platform. The OQW-based transmitter consists of a widely tunable laser, a high-speed semiconductor optical amplifier (SOA), a Mach–Zehnder modulator, and an output SOA. This transmitter demonstrates a 44-nm tuning range, >45 dB side mode suppression ratio, 14.5 dBm off -chip power, and a data rate of 7 Gbps. The second transmitter, based on QWI, utilizes an alternate epitaxial structure to achieve a lower confinement factor for higher SOA output saturation power. This QWI transmitter consists of a distributed Bragg reflector laser, a high-speed SOA, an electroabsorption modulator, and an output SOA. The measured off -chip power is 19.5 dBm, and a data rate of 20 Gbps is demonstrated. Based on the improved performance with the new epitaxial structure, a novel platform for high-power PIC transmitters integrated with low confinement and high-power SOAs is described.

Influence of Integrated Optical Feedback on Tunable Lasers

In this paper we explain how to use rate equations to describe a laser that includes integrated optical feedback. We find a relation between the threshold current, the voltage drop at the gain section, output power, linewidth, and side mode suppression ratio, and show experimental results.

An Ultra-High-SMSR External-Cavity Diode Laser with a Wide Tunable Range around 1550 nm

In this paper, a widely tunable external cavity diode laser (ECDL) with an ultra-high side mode suppression ratio (SMSR) was fabricated. Three configurations were constructed to investigate the relationship between the grating features and the SMSR. When a 1200 grooves/mm grating with a first order diffraction efficiency of 91% is utilized in the external-cavity laser system, a maximum SMSR of 65 dB can be achieved. In addition, the tunable range reaches 209.9 nm. The results show that the laser performance can be improved by proper high grating groove number and first-order diffraction efficiency.

850 nm GaAs/AlGaAs DFB lasers with shallow surface gratings and oxide aperture.

We present and experimentally demonstrate a novel oxide-confined ridge-waveguide distributed feedback (DFB) laser with the first-order surface grating using only a single growth step. The metal contacts are laterally offset from the ridge waveguide to inject current thus avoiding unwanted light absorption from the electrodes. The oxide aperture is defined by selective wet oxidation of aluminium-rich material, which confines the injection current from the electrodes to the active layer under the ridge waveguide. This allows that a thin ridge layer can be used with relatively higher refractive index compared to the active layer and thus the grating can be shallowly etched but provides a strong coupling effect. The fabricated 150 µm-long DFB laser exhibited a relatively low threshold current of 8 mA and a side mode suppression ratio (SMSR) up to 50 dB at the injected current of 32 mA around 4 times threshold at 20 °C. Stable single mode operation has been observed for the fabricated DFB laser over the temperature range from 10 to 50 °C. The variation of wavelength with temperature Δλ/ΔT was 0.06 nm/°C. The proposed laser may have advantages combined both DFB lasers and vertical-cavity surface-emitting lasers (VCSELs), such as single mode, stabilized polarization, potentially narrow linewidth and low power consumption. In addition, the laser is regrowth free, thus has advantages of low cost and high reliability.

Spinor Bose-Einstein condensate interferometer within the undepleted pump approximation: Role of the initial state

Most interferometers operate with photons or dilute, non-condensed cold atom clouds in which collisions are strongly suppressed. Spinor Bose-Einstein condensates (BECs) provide an alternative route toward realizing three-mode interferometers; in this realization, spin-changing collisions provide a resource that generates mode entanglement. Working in the regime where the pump mode, i.e., the m=0 hyperfine state, has a much larger population than the side or probe modes (m=+1 and m=-1 hyperfine states), f=1 spinor BECs approximate SU(1,1) interferometers. We derive analytical expressions within the undepleted pump approximation for the phase sensitivity of such an SU(1,1) interferometer for two classes of initial states: pure Fock states and coherent spin states. The interferometer performance is analyzed for initial states without seeding, with single-sided seeding, and with double-sided seeding. The validity regime of the undepleted pump approximation is assessed by performing quantum calculations for the full spin Hamiltonian. Our analytical results and the associated dynamics are expected to guide experiments as well as numerical studies that explore regimes where the undepleted pump approximation makes quantitatively or qualitatively incorrect predictions.

High-Resolution X -Band Frequency Synthesizer Using SILPLL Optoelectronic Oscillators.

Optoelectronic oscillators (OEOs) with their demonstrated low phase noise are popular due to the increasing demand of very clean local oscillator and clock signal sources. A forced technique of self-injection locked and phase-locked loop (SILPLL) reduces phase noise and suppresses side mode observed in long fiber delay of the standard OEO. A frequency synthesizer working at X -band is developed using an electrically coarse-tuned YIG filter cascaded with an optical wavelength fine-tuned dispersive optical transversal filter by tuning the oscillation frequency of the synthesized OEO frequency, as a replacement for fixed-frequency narrowband filter using high- Q aluminum cavities. A close-to-carrier phase noise of -115 dBc/Hz at 1 kHz is reported by the Vπ/2 operation of a Mach-Zehnder modulator (MZM) over 8-12 GHz with a frequency tuning step as small as 2.6 kHz. Phase noise introduced by a photodetector is also explored at a very close-to-carrier frequency. It shows 10-dB phase noise degradation at PD reverse bias voltage of 3 V compared with 6 V at close to the Vπ operation of an MZM. A 19-in rack mount frequency synthesizer system is also realized at X -band with a demonstrated long-term (up to 60 min) frequency drift stability of 3 kHz over whole X -band working frequencies.

Ni-NPs doped PVA: An efficient saturable absorber for generation multiwavelength Q-switched fiber laser system near 1.5 μm

We report on the use of Nickel nanoparticles (Ni-NPs) as saturable absorber (SA) to generate Q-switched fiber laser pulses in C-band. The Ni-NPs were incorporated in polyvinyl alcohol (PVA) to produce thin film-based saturable absorber (SA) and integrated into an erbium-doped fiber laser (EDFL) ring cavity to generate passively Q-switching. The Ni-doped PVA SA reveals modulation depth of 15% and a saturation intensity of 200 MW/cm2. Lasing in CW region begins at 77 mW pump power, whereas stable self-starting Q-switching with a central wavelength of 1563.1 nm begins at 85 mW. By fine adjustment of the laser diode pump power up to 236.4 mW dual-, and triple- lasing lines have been observed at 1563.1, 1563.3 and 1563.4 with side mode suppression ratio (SMSR) of more than 49 dB. The proposed laser is useful for generating pulse laser with a minimum pulse width of 1.5 μs and a maximum repetition rate of 56.79 kHz with a pulse energy of about 4.33 nJ at the maximum pump power of 275 mW. The stability of the pulse is verified from the radio-frequency (RF) spectrum with a measured signal-to-noise ratio (SNR) of 44 dB. The ability of Ni-PVA as an effective SA may lead to further development of pulsed fiber laser in the field of photonics.

1.3-µm and 10-Gbps tunable DBR-LD for low-cost application of WDM-based mobile front haul networks.

We report a 1.3-µm and 10-Gbps tunable distributed Bragg reflector laser diode (DBR-LD) for the low-cost application of a wavelength division multiplex based mobile front-haul network. The device consists of gain, phase control, and DBR sections, implemented using a butt-coupling method through a monolithic integration and through the introduction of an etched mesa planar buried hetero-structure in a waveguide structure. From the work, a 560-µm long DBR-LD with a 220-µm long micro-heater DBR section has a threshold current of 10 mA ± 1 mA and a tuning range of more than 15 nm within a heater injection current of ∼100 mA. Spectral and dynamic tests for this LD show 16 channels spaced a wavelength grid of 0.8 nm with a side mode suppression ratio of greater 40 dB and clear eye openings with a dynamic extinction ratio of over 5.4 dB at an operating current of 60 mA.

Novel Dual band SIW Filter Using Quad mode Cavity

A dual-band bandpass substrate integrated waveguide (SIW) filter is proposed using a quad-mode cavity in this paper. First two degenerative modes (TE102 and TE201) with via perturbation give the first passband. The second passband is realised by using higher modes (side and diagonal modes of TE202) which are obtained by putting square slot at the center of the cavity. The square slot increases the frequency ratio of the center frequencies of first and second passbands. Moreover, orthogonal feed-lines are used in the proposed design to increase transmission zeros (TZs) which helps to improve the selectivity and out-of-band rejection of the filter. Designed and fabricated a dualband filter prototype using a single layer printed circuit board (PCB) technology, size is only 19 mm × 19 mm. The insertion losses are 2.1 dB and 2.4 dB, and fractional bandwidths of 3.40 per cent and 2.00 per cent at 11.00 and 15.58 GHz, respectively. The measurement results show close agreement with the simulation results.

Large linewidth reduction in semiconductor lasers based on atom-like gain material

With a new generation of quantum dot (QD) optical gain material comprising atom-like features, the fundamental spectral characteristics of laser emission have been improved significantly. We describe the spectral and power characteristics of continuous wave (CW) single-mode InAs/AlGaInAs/InP QD distributed feedback lasers operating at 1.5 μm. Linewidths as narrow as 60 kHz (30 kHz±10 kHz intrinsic linewidth) at 20°C, which broadens to only 280 kHz (80 kHz±10 kHz intrinsic linewidth) at 80°C, have been achieved. The laser exhibits high output powers of 58 mW at 20°C and 26 mW at 80°C with side mode suppression ratios exceeding 50 dB. These record values stem from high uniformity of the QDs and a large dot density. The linewidth was measured by two techniques that confirm each other: delayed self-heterodyne interferometry and optical frequency comb interferometry. A model fits the experimental results well and enables extraction of the bias and temperature dependent α parameter. At 20°C, α is less than 0.5 at threshold and increases to only 0.9 at 150 mA above threshold. The corresponding values at 80°C are 2 and 2.5. These results imply a great potential of QD lasers for the most demanding applications in terms of spectral purity, such as coherent optical communication systems and optical metrology.

996 nm high-power single-longitudinal-mode tapered gain-coupled distributed feedback laser diodes.

A high-power single-longitudinal-mode regrowth-free tapered gain-coupled distributed feedback laser diode based on periodic current injection is achieved at 996 nm. It enhances the output power without beam quality degrading. A continuous-wave output power of over 1.12 W is achieved at 3 A. The maximum output power in single-longitudinal-mode operation is up to 0.56 W at 1.4 A. The power conversion efficiency is over 24%, and the slope efficiency is 0.58 W/A. The side mode suppression ratio is over 38 dB; the 3 dB spectral linewidth is less than 2.4 pm. The lateral far-field divergence angle is only 14.98°, and the beam quality factor M2 is 1.64, achieving a near-diffraction-limit emission. Our device has great potential in commercial applications and the experimental study of high-power near-diffraction-limit laser diodes for its low-cost fabrication technique and narrowband single-longitudinal-mode emission at high power.

Tunable Fiber Laser Using an Er/Yb Codoped Fiber Based Modal Interferometer Filter

A tunable erbium-doped fiber laser (EDFL) is proposed and demonstrated experimentally. It is based on a tunable fiber filter using the pump induced refractive index change characteristics of the Er/Yb codoped phosphosilicate fiber (EYDF) based modal interferometer (MI). The laser achieves a tunability of 2.39 nm and 1.60 nm, with the wavelength linearly tuned from 1534.05 nm to 1536.44 nm and from 1554.36 nm to 1555.96 nm by changing the pump power injected into the MI. Its side mode suppression ratios are 30 dB and 33 dB, respectively. The stability of the tunable EDFL is getting better with the continuous improvement of the power of pump 1, and the dual-wavelength power fluctuations are less than 0.22 dB and 0.12 dB within 20 min at 250 mW of pump 1, respectively.

All-Optical VCSEL-to-VCSEL Injection Based on Cross Gain Modulation for Routing in Multinode Flexible Spectrum Optimization in Optical Fibre Transmission Links

In this paper, we experimentally present a novel, all-optical spectral efficient vertical-cavity surface-emitting laser- (VCSEL-) based technique for routing and spectrum assignment in optical networks. Exploiting all optical VCSEL-to-VCSEL injection to attain cross gain modulation, the optical transmitter is optimized for optical transmission paths to assure quality of service by overcoming blockage for differentiated bandwidth demands during network congestion incidences. A 10 Gbps directly modulated 1549 nm master VCSEL is optically injected into the 1549 nm side modes of a 1550 nm slave VCSEL. The Shannon limit is considered for higher transmission rates with the problem decomposed into degraded routing and spectrum assignment and chromatic dispersion in the optical transmission link penalties. In this work, the proposed technique achieved a 1.3 dB penalty for transmission over a 25 km G.655 nonzero dispersion-shifted single-mode optical fibre, a value within the transmission media and optical system characteristics of 3 dB as recommended by the International Telecommunication Union-Telecommunication (ITU-T). The number of transceivers, switches, and optical transmission links in the network was reduced, increasing the number of satisfied bandwidth requests, thus optimizing the spectral resource utilization.

RETRACTED: Widely tunable optoelectronic oscillator based on selective parity-time-symmetry breaking

Optoelectronic oscillators (OEOs) are promising solutions for generating microwave signals with low phase noise and wideband tunability, and they can be applied to converging systems such as communications, radars, and electronic warfare systems. However, a significant challenge remains in ensuring a low phase noise, wideband tunability, and ultra-high side mode suppression ratio (SMSR) simultaneously. Parity-time (PT) symmetry breaking provides an excellent tool for single-mode oscillation by exploiting the interplay between the gain and loss. The oscillation mode was previously fixed because the breaking of the PT symmetry cannot be accurately manipulated. Herein, we propose an OEO with selective PT-symmetry breaking showing a wideband tunability and ultra-high SMSR. The tunability of the proposed OEO is attributed to the selection of different modes to break the PT symmetry using a widely tunable microwave photonic filter (MPF). The large roll-off of the MPF significantly enhances the gain difference between the selected and competing modes. Consequently, both the output power and SMSR of the OEO increase. During the experiment, the measured oscillation frequency is tuned from 2.6 to 40 GHz. The output power of the selected mode is enhanced by 12.9 dB, and the maximal SMSR reaches up to 71.4 dB. Further, the measured phase noise of the OEO at 17.74 GHz reaches −129 dBc/Hz at a 10-kHz offset frequency. Exploration of the selective PT-symmetry breaking provides the possibility of developing classes of widely tunable OEOs with an ultra-high SMSR and excellent low phase noise simultaneously.

Hybrid dual-injection locked 1610 nm quantum-dash laser for MMW and THz applications

Dual-mode hybrid injection-locking scheme is reported over an L-band InAs/InP quantum-dash laser to generate tunable millimeter (MMW) and Terahertz (THz) beat frequencies. In addition to the inherent enhancements that are provided by injection locking, the system provides comparable high dual-mode peak power, side mode suppression ratio (SMSR) up to 30 dB, and large beat frequency tunability of ∼1.7 THz. As such, four different beat frequencies are demonstrated, namely 62.6- and 263-GHz MMW and 1.21- and 1.75-THz signals. The stability of the generated waves is then investigated showing a minimal fluctuation of 0.35 dB and 0.1 dB in the SMSR and peak power, respectively.

Frequency-tripling OEO based on frequency multiplication in saturated electronic amplifier

A Frequency-Tripling optoelectronic oscillator (FT-OEO) based on frequency multiplication in a saturated in-loop electronic amplifier has been proposed and demonstrated. The significant 3rd harmonic signal generated at the saturated amplifier is coupled out and filtered as the desired output. A prototype of the FT-OEO is constructed, where the fundamental oscillating signal (fosc) locates at 2 GHz. The generated 6 GHz frequency-tripling signal (3fosc) is of 0 dB m and the phase noise is −108dBc/Hz @ 1 kHz and −130dBc/Hz @ 10 kHz. Under external injection, the side modes are suppressed to below −110dBc/Hz. This FT-OEO provides a compact and cost-efficient scheme for frequency multiplication of OEO so as to alleviate the bandwidth limit of electro-optic components.

Influence of Average Cavity Dispersion and Spectral Bandwidth on Passively Harmonic Mode Locked L-Band Er-Doped Fiber Laser

We systematically study the impact of average cavity dispersion and pulse spectral bandwidth on harmonic mode locking in the long-wavelength band from an Er-doped fiber laser using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film mode locker. By carrying out pulse energy management through optimizing the average cavity dispersion and optical spectral bandwidth, 2.08 GHz repetition rate corresponding to 201st harmonic with 36.5 dB side mode suppression ratio (SMSR) can be realized under 205 mW pump power at 1597.53 nm. Furthermore, we systematically investigate the characteristics of the harmonically mode locked ultrashort optical pulses with different spectral bandwidths versus pump power under fixed cavity dispersion first. Key laser parameters have been studied including pumping efficiency, duration, energy, SMSR, and time bandwidth product. Enlightened by the experimental results, a simple law on preliminary and fast judgment of pulses frequency under fixed pump power is proposed, namely, by monitoring the pulse spectral bandwidth. The laser source featuring high repetition rate and L-band operation is greatly sought-after in some specific applications such as modern optical communication system etc.

Compact Diode Laser-Based Dual-Wavelength Master Oscillator Power Amplifier at 785 nm

A compact, micro integrated, diode laser-based dual-wavelength master oscillator power amplifier at 785 nm is presented. Laser emission from a Y-branch distributed Bragg reflector laser is coupled into a tilted ridge waveguide amplifier on a $5\times 25$ mm2 micro optical bench. At 25°C and 20 mW pump power, an optical output power of 0.5 W is obtained. The corresponding emission wavelengths are 784.62 and 785.24 nm, resulting in a spectral distance of 10 cm $^{-1}$ . The measured spectral bandwidths and side mode suppression ratios are 0.02 nm (0.3 cm $^{-1}$ ) and 30 dB, respectively. At both wavelengths beam propagation parameters of 1.3 ( $\text{M}{^{2}}_{4\boldsymbol \sigma }$ ) are obtained. This allows for efficient dual-wavelength single-mode fiber coupling. In addition, the device does not show any effects from optical feedback. It neither shows a lateral spatial tilt of the far field intensity distributions for the two wavelengths, previously reported for single Y-branch diode lasers. Based on these parameters, the device is suitable for demanding applications such as shifted excitation Raman difference spectroscopy or confocal Raman microscopy.

Research on Multi-wavelength Fiber Laser Based on Nonlinear Effect

A multi-wavelength erbium-doped fiber laser (MWEDFL) is introduced, which uses highly erbium-doped fiber (EDF) as the gain medium and uses the nonlinear effect of the single mode fiber to suppress the mode competition. In order to obtain higher side mode suppression ratio and flatness, a nonlinear fiber loop mirror is added to the structure. The results show that a stable multi-wavelength laser with 16 lasing wavelengths and a side mode suppression ratio (SMSR) higher than 30dB is obtained.