Longitudinal Mode Operation Research Articles

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.

Sub-nanosecond pulse, single longitudinal mode Q-switched Nd:YVO4 laser controlled by reflecting Bragg gratings

We demonstrated sub-nanosecond pulse width at single longitudinal mode operation in a diode-pumped electro-optically Q-switched Nd:YVO4 laser. The laser used one reflecting Bragg gratings (RBG) as output coupler for mode selection. Stable, single-mode longitudinal operation with laser pulse of 0.43mJ energy, peak power of 0.66MW and duration of 645ps at 5 to 150Hz was realized from a 38mm long optical resonator.

Narrow linewidth laterally coupled 155 μm AlGaInAs/InP distributed feedback lasers integrated with a curved tapered semiconductor optical amplifier

We present a laterally coupled 1.55 μm AlGaInAs/InP distributed feedback laser monolithically integrated with a curved tapered optical amplifier, providing an output power of 210 mW with single transverse and longitudinal mode operation exhibiting a record low linewidth of 64 kHz.

Multicore fiber distributed feedback lasers

We demonstrate parallel fabrication of seven fiber distributed feedback (DFB) lasers in a hexagonally arrayed multicore core Er doped fiber with 40 μm core spacing. DFB grating cavities 8 cm long and operating near 1545 nm were fabricated with a single UV inscription exposure. We observed dual polarization, single longitudinal mode operation with a linewidth below 300 kHz for each laser.

Distributed feedback polymer laser with an external feedback structure fabricated by holographic polymerization technique

Distributed feedback polymer laser with a grating in the waveguide cladding layer was prepared by holographic polymer dispersed liquid crystals (HPDLCs). The holographic polymerization technique enables one-step formation of an organic grating on top of a homogenous semiconducting polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) layer. Under optical pumping, a lasing threshold of 21μJ/cm2 and a bandwidth of 0.7nm were obtained in a single longitudinal mode operation. Further studies on the effect of the semiconducting polymer thickness show the amount of evanescent wave spread into the grating layer determines the laser performance of the proposed device.

Experimental demonstration of the three phase shifted DFB semiconductor laser based on Reconstruction-Equivalent-Chirp technique

A three phase shifted (3PS) distributed feedback (DFB) semiconductor laser based on Reconstruction-Equivalent-Chirp (REC) technique is experimentally demonstrated for the first time. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as that of the true 3PS laser. However, it only changes the μm-level sampling structures but the seed grating is uniform. So, its cost of fabrication is low. The measurement results exhibit its good single longitudinal mode (SLM) operation even at high bias current and surrounding temperature.

Experimental demonstration of the three phase shifted DFB semiconductor laser based on Reconstruction-Equivalent-Chirp technique

A three phase shifted (3PS) distributed feedback (DFB) semiconductor laser based on Reconstruction-Equivalent-Chirp (REC) technique is experimentally demonstrated for the first time. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as that of the true 3PS laser. However, it only changes the μm-level sampling structures but the seed grating is uniform. So, its cost of fabrication is low. The measurement results exhibit its good single longitudinal mode (SLM) operation even at high bias current and surrounding temperature.

96 mW longitudinal single mode red-emitting distributed Bragg reflector ridge waveguide laser with tenth order surface gratings

Red-emitting ridge waveguide lasers with integrated tenth order surface distributed Bragg reflector gratings were developed. The grating was implemented by the use of a BCl3-Ar-plasma, while the shape of the grating trench was controlled by additional He-backside cooling of the wafer. The devices exhibit longitudinal single mode operation up to 96 mW at 635.3 nm with a side mode suppression ratio of 18 dB and a good beam quality of M2<3. The spectrum is free of mode hops for a span of more than 55 pm.

Resonantly pumped 1.645 μm single longitudinal mode Er:YAG laser with intracavity etalons.

We report on a 1.645 μm single longitudinal mode Er:YAG laser that was resonantly pumped by a 1.532 μm fiber laser, using intracavity etalons to generate single longitudinal mode operation. We obtained 0.749 W single longitudinal mode output power at 1.645 μm from an Er:YAG laser with two intracavity etalons. The M2-factors of the Er:YAG laser were 1.041 and 1.068 in the x and y directions, respectively.

Tunable Er-doped fiber ring laser with single longitudinal mode operation based on Rayleigh backscattering in single mode fiber

A tunable and single longitudinal mode Er-doped fiber ring laser (SLM-EDFRL) is proposed and demonstrated based on Rayleigh backscattering (RBS) in single mode fiber-28e (SMF-28e). Theory and experimental study on formation of SLM from normal multi-mode ring laser is demonstrated. The RBS feedback in 660 m SMF-28e is the key to ensure SLM laser oscillation. This tunable SLM laser can be tuned over 1549.7-1550.18 nm with a linewidth of 2.5-3.0 kHz and a side mode suppression ratio (SMSR) of ~72 dB for electrical signal power. The tuning range is determined by the bandpass filter and gain medium used in the experiment. The laser is able to operate at S+C+L band.

Single Step Generation of Micro and Radio Wave Signals in a Short Cavity Fiber Laser

Generation of stable micro or radio waves by frequency down-conversion has always been a challenging but rewarding task in radio over fiber systems. In this letter, a novel method for such a generation using a short-cavity fiber laser is demonstrated. For this purpose, an all-fiber laser is designed to have two stable longitudinal laser modes, each of them having two orthogonal polarization modes. Hence, four microwave signals, two in 1.2 GHz range and the other two in 14 GHz range, and one radio wave signal were generated by beating these lasing modes on a square law detector. Various measurements show high level of stability in generated micro/radio waves because they originate from the single laser cavity. According to the authors' knowledge, this is the first demonstration of the dual longitudinal mode operation of a short-cavity fiber laser for down-conversion from optical frequency directly to radio frequency.

Effects of Asymmetric Grating Structures on Output Efficiency and Single Longitudinal Mode Operation in $\lambda/4$-Shifted DFB Laser

We report on the design of the λ/4 -shifted DFB laser that can provide the high output efficiency as well as the stable single-mode operation. In particular, we investigate the effects of the asymmetry in the grating structure on the lasing characteristics of the λ/4-shifted DFB laser theoretically and experimentally. The steady-state and mode-stability analyses are performed for the λ/4 -shifted DFB lasers having the asymmetric phase shift (APS) and asymmetric coupling coefficient (ACC) grating structures by numerical simulations. Although the λ/4-shifted DFB laser fabricated with APS structure is found to be capable of providing high output efficiency, its lasing mode is relatively unstable due to the increased longitudinal spatial hole-burning. On the other hand, by using the ACC grating structure, we can significantly improve both the output efficiency and single-mode stability. To verify these simulation results, we fabricate both types of λ/4-shifted DFB lasers having APS and ACC grating structures. The measured data agree well with the simulated results. For example, when we fabricate the DFB laser with the ACC structure and coupling coefficient ratio of 0.55, the output power ratio between the front and rear facets of ~ 2.5 and the side-mode suppression ratio (SMSR) of >;50 can be achieved as long as the operating current is in the range of 60 mA ~200 mA.

Volume Bragg Grating-Based 70 nm Tunable and Narrow Output Spectra from a Ti:sapphire Laser

In this study, a simple V-shaped cavity Ti:sapphire laser setup is developed using a volume Bragg grating (VBG) as the folding mirror. The laser can reach single longitudinal mode operation with an output power of up to 30 mW and a tunability exceeding 70 nm. The unexpected asymmetric reduction observed in the VBG reflectivity is discussed.

Optical generation of microwave signal using fiber Bragg gating‐based double‐ring fiber laser assisted by saturable absorber

AbstractWe propose and demonstrate a simple approach to optical generation of microwave signal by exploiting a uniform fiber Bragg gating (FBG)‐based double‐ring laser and a saturable absorber.Parallel arranged uniform FBGs have a wavelength spacing corresponding to the microwave signal. A piece of unpumped polarization‐maintaining erbium‐doped fiber (EBF) serves as a saturable absorber, ensuring the dual‐wavelength single longitudinal mode operation. Stable dual‐wavelength single longitudinal mode lasers are obtained in the experiment. A microwave signal at 28.38 GHz is successfully generated by beating the dual‐wavelength single longitudinal mode lasers. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2478‐2481, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26303

Tunable Fabry-Perot filter using hollow-core photonic bandgap fiber and micro-fiber for a narrow-linewidth laser

A novel tunable fiber Fabry-Perot (FP) filter is proposed and demonstrated by using a hollow-core photonic bandgap fiber (HC-PBF) and a micro-fiber. The interference cavity is a hollow core of HC-PBF. One of the reflection mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other reflection mirror is a gold-coated end of micro-fiber that uses chemical etching process to obtain the similar diameter as the core of HC-PBF. Hence the movable mirror can be adjusted with long distance inside the hollow core of HC-PBF. Tunable FP filter is used as a mode selecting component in the reflection mode to implement stable single longitudinal mode (SLM) operation in a ring laser. With FP cavity length of 0.25 ± 0.14 mm, the wavelength of SLM laser can be tuned over 1554-1562 nm with a tuning step of 0.2-0.3 nm, a side-mode suppression ratio (SMSR) of 32-36 dB and a linewidth of 3.0-5.1 kHz. With FP cavity length of 2.37 ± 0.37 mm, the SLM laser can be tuned over 1557.3-1560.2 nm with a tuning step of 0.06-0.1 nm, a SMSR of 44-51 dB and a linewidth of 1.8-3.0 kHz.

Tunable terahertz-wave generation from DAST crystal pumped by a monolithic dual-wavelength fiber laser

For developing a continuous-wave (CW) tunable Terahertz-wave (THz-wave) source using difference-frequency generation (DFG) in highly nonlinear optical crystals, we proposed and demonstrated a dual-wavelength fiber ring laser system operating around 1060 nm based on wideband chirped fiber Bragg gratings (CFBGs) and semiconductor optical amplifier (SOA). Thermo-induced phase shift along the CFBG produces a very sharp transmission spike therefore two lasing wavelengths with single longitudinal mode operation are oscillating simultaneously within the fiber ring cavity. Due to the inhomogeneous gain broadening property of SOA, the wavelength spacing of our dual-wavelength fiber laser can be continuously adjusted from 0.3 to 9.5 nm. By using this single emitter dual-wavelength fiber laser to pump an organic nonlinear DAST crystal, type-0 collinear phase matching of DFG process can be fulfilled and monochromatic THz wave ranging from 0.5 to 2 THz has been successfully generated.

Distributed feedback transistor laser

Single longitudinal mode operation of an InGaP/GaAs/InGaAs quantum well heterojunction bipolar transistor laser is reported. A third order distributed feedback surface grating is fabricated in the top emitter AlGaAs confining layers using soft photocurable nanoimprint lithography. The device produces continuous wave laser operation with a peak wavelength λ=959.75 nm and threshold current IB=13 mA operating at −70 °C. For devices with cleaved ends a sidemode suppression ratio >25 dB has been achieved.

Special Laser Wavelength Generation Using a Volume Bragg Grating as Nd:GdVO4 Laser Mirror

A 1071 nm laser using Nd:GdVO4 as the gain medium is demonstrated. A volume Bragg grating (VBG) with a reflection central wavelength at 1069.8 nm was used. The laser was operated in CW mode. The output wavelength range could exceed 1.3 nm by tuning the VBG temperature. Single longitudinal mode operation was achieved with an output power larger than 100 mW using a simple linear cavity setup. A brief discussion about the VBG thermal effect is also presented.

Novel DFB laser structure suitable for stable single longitudinal mode operation

This paper shows that a suitable design of a single phase-shift (1PS) distributed coupling coefficient (DCC) distributed feedback (DFB) laser structure can strongly improve both the normalized mode selectivity S and the flatness of the electrical field profile of DFB lasers—usually quantified by a flatness parameter F —in order to ensure the required criteria for single longitudinal mode operation ( S ≥ 0.25 and F ≤ 0.05 ) through an extended range of biasing current injection ( I). It is shown that a symmetric longitudinal laser cavity should be used in order to accomplish the requirements imposed by the modern optical communication systems. Photon and carrier rate equations have been used to evaluate the performance of the proposed laser structure in the above-threshold regime. The variations of the selectivity, the flatness, the lasing-wavelength, the emitted power ( P) and the side-mode suppression ratio (SMSR) versus current have been assessed. For I=5 I th , where I th is the laser threshold current, substantial improvements in S (60% higher), in F (40% better), in P (45% higher) and in the SMSR (about 4 dB higher) are achieved in the proposed 1PS-DCC-DFB laser when compared to similar single phase-shifted DFB structures referred elsewhere. The improvements are even better when compared to the standard QWS-DFB laser.

Optimisation of a corrugation-pitch-modulated DFB laser structure with inhomogeneous coupling coefficient for stable single longitudinal mode operation

This paper shows that a suitable design of a corrugation-pitch-modulated (CPM) distributed-coupling-coefficient (DCC) distributed feedback (DFB) laser structure can strongly improve the mode selectivity $(\mathfrak{G})$ and the flatness $(\mathfrak{F})$ of DFB laser structures in order to ensure the required criteria for single longitudinal mode operation ( $\mathfrak{G}$ ≥ 0.25 and $\mathfrak{F}$ ≤ 0.05), through an extended range of current injection. It is shown that a symmetric structure should be used in order to accomplish the requirements imposed by the modern optical communication systems. Photon and carrier rate equations have been used in order to evaluate the performance of the proposed laser structure in the above-threshold regime. The variations of $\mathfrak{G}$ , $\mathfrak{F}$ , the lasing wavelength, the emitted power ( P ) and the side-mode-suppression-ratio (SMSR) with the current injection ( I ) have been evaluated. For I = 5 I th , where I th is the laser threshold current, substantial improvements in $\mathfrak{G}$ ( 3.8 times better), in $\mathfrak{F}$ ( 2 times better), in P (15% higher) and in the SMSR (about 3.4 dB higher) are achieved in the proposed CPM-DCC-DFB laser when compared to an optimised CPM-DCC-DFB laser referred elsewhere. The improvements are even better when compared to the standard QWS-DFB laser.