Complex-coupled Distributed Feedback Lasers Research Articles

Strong Laser Emission Modulation by Coherent Perfect Absorption Inside Complex-Coupled Distributed Feedback Laser Diodes.

The proof-of-concept of the exploitation of Coherent Perfect Absorption (CPA) in electrically-injected distributed-feedback laser sources is reported. Capitalizing on the essence of CPA as "light extinction by light", an integrated laser-modulator scheme emerges. The key ingredient compared to conventional single-frequency laser diodes is a careful periodic in-phase modulation of both real and imaginary parts of the complex grating index profile that enables both single-frequency operation and 40dB line purity at the Bragg frequency. It is shown that this combination is most apt for the operation of CPA as a modulation mechanism that respects the laser spectral purity. The specific proof-of-concept is based on an ultra-short external cavity formed by a metallic micro-mirror, whose role is to generate the second beam of more conventional CPA interferometric approaches. The implemented complex-coupled grating is compatible with existing industrial technologies and promising for real-life laser source applications. Furthermore, the concept can be directly transferred to other material platforms and other wavelengths ranging from terahertz to ultraviolet.

Parity-time symmetric complex-coupled distributed feedback laser with excellent immunity to external optical feedback

In this paper, we propose an external optical feedback resistant distributed feedback (DFB) laser diode (LD) by exploiting parity-time symmetric complex coupling. With its complex refractive index followed a parity-time symmetry, the grating shows a strongly asymmetric reflection to the contra-propagating light inside the DFB cavity, which effectively rejects the returning light from one end. Consequently, the DFB LD is much less sensitive to external optical feedback. On the contrary, the transmissivity of such grating is still symmetric so that the output light of the DFB LD is not affected. Numerical simulation result shows that the lasing wavelength drift can be less than 0.2 nm with a SMSR exceeding 45 dB under a coherent external optical feedback as high as -10 dB.

Widely Tunable Amplified Feedback Laser With Beating-Frequency Covering 60-GHz Band

A monolithic integrated amplified feedback laser is experimentally demonstrated as a new solution to generate dual-mode lasing with a wide frequency tuning range covering the entire 60-GHz band. This laser consists of a complex-coupled distributed feedback laser and an integrated feedback cavity formed by a phase section and an amplifier section. Different laser sections are integrated by a simple quantum well intermixing technique. By tuning the injection currents of the laser sections, the beating-frequency can be continuously tuned from 46.8 to 72 GHz with a 3-dB linewidth below 5 MHz. Benefit from the simple fabrication process and wide tuning range, the device will be a potential candidate for hybrid fiber-wireless access network and high frequency optical microwave generation.

The Fabrication of Eight-Channel DFB Laser Array Using Sampled Gratings

An eight-channel monolithically integrated complex-coupled distributed-feedback laser array based on sampled gratings has been designed and fabricated. Selective lasing at different wavelengths is obtained. The frequency separation between each adjacent channel is about 200 GHz. The typical threshold current is between 30 and 40 mA. The optical output power of each channel is about 10 mW at an injection current of 100 mA. The continuous tuning of emission wavelength with injected currents is also demonstrated.

Application of sampled grating to control the lasing wavelength in complex-coupled DFB laser

This paper reports that the complex-coupled distributed feedback laser with the sampled grating has been designed and fabricated. The +1st order reflection of the sampled grating is utilized for laser single mode operation, which is 1.5387 μm in the experiment. The typical threshold current of the device is 30 mA, and the optical output power is about 10 mW at the injection current of 100 mA.

Complex-Coupled DFB Laser Using a Buried SiO$_{2}$ Grating

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this letter, we report a distributed feedback (DFB) laser having a dielectric grating formed by SiO<formula formulatype="inline"><tex>$_{2}$</tex> </formula> and InP to explore both the large refractive index difference and the partial gain coupling. Epitaxy lateral overgrowth by metal–organic chemical vapor deposition is conducted to grow the top p-type InP waveguide layer in the dielectric grating template. An index coupling coefficient <formula formulatype="inline"><tex>$\kappa$</tex></formula> of about 250 cm<formula formulatype="inline"><tex>$^{-1}$</tex></formula> is estimated on the laser vertical waveguide structure. The insulating nature of the dielectric grating also partially blocks the injection current flow and modulates the optical gain of the active quantum-well region underneath it. A prototype DFB laser was fabricated and showed a stable single-mode operation with a sidemode suppression ratio larger than 47 dB measured at room temperature and continuous-wave operation. The technology developed can also be used for other applications that require high efficiency grating structures. </para>

Temperature insensitive 1.3 [micro sign]m InGaAs∕GaAs quantum dot distributed feedback lasers for 10 Gbit∕s transmission over 21 km

Long wavelength monomode InGaAs/GaAs quantum dot (QD) distributed feedback (DFB) lasers with emission wavelength around 1325 nm are presented. Threshold currents below 19 mA for operating temperatures up to 70°C and output powers of 10 mW at 25°C (6 mW at 70°C) are observed. Error-free 10 Gbit/s transmission over 21 km fibre with an extinction ratio of 8.5 dB at room temperature (5.1 dB at 70°C) is demonstrated. The low threshold, low temperature sensitivity and high modulation speed were realised using complex coupled DFB lasers with ten stacks of self-organised MBE-grown QD layers and p-type modulation doping.

Enhanced 10-Gbit/s link performance for directly Modulated complex-coupled DFB lasers via resonance frequency, damping rate, and chirp

This paper demonstrates the respective roles that combined index- and gain-coupling play in the overall link performance of distributed feedback (DFB) lasers. Their impacts on both static and dynamic properties such as slope efficiency, resonance frequency, damping rate, and chirp are investigated. Simulation results are compared with experimental data with good agreement. Transmission-oriented optimization is then demonstrated based on a targeted specification. The design tradeoffs are revealed, and it is shown that a modest combination of index- and gain-coupling enables optimum transmission at 10 Gbit/s.

Analysis of single-mode yields above threshold for complex-coupled distributed feedback lasers with asymmetric facet reflectivities

Single-mode yields above threshold of complex-coupled (CC) distributed feedback (DFB) lasers with antireflection and high-reflection-coated facets for various values of ∣kL∣ and coupling coefficient ratio (CR) are presented and compared with the spatial hole burning- (SHB-) corrected yield at threshold. If the criterion necessary for the SHB-corrected yield is selected properly, the two yields give good agreement. We present two single-mode yields above threshold, showing the difference between in-phase (IP) and antiphase (AP) CC DFB lasers, even though the two yields are the same at threshold. Single-mode yields above threshold of IP CC DFB lasers are higher than those of AP CC DFB lasers, since the threshold gain of the next strongest mode of AP CC DFB lasers, increases rapidly compared with that of IP CC DFB lasers because of the SHB effect. As the CR increases, the single-mode yield of IP CC DFB lasers initially increases and has a nearly constant value of more than 90% for a CR greater than or equal to 0.3. As the CR and ∣kL∣ increase, the effect of the reflectivity of the antireflection facet on the single-mode yield decreases.

Widely Tunable Complex-Coupled Distributed Feedback Laser With Photonic Crystal Mirrors and Integrated Optical Amplifier

We report the development of a current-controlled widely tunable laser diode based on InP with photonic crystal mirrors and an integrated amplifier section for stabilized power output. The laser consists of three longitudinally coupled ridge waveguide cavities that are coupled through photonic crystal mirror segments. A laterally defined binary superimposed grating provides complex-coupled distributed feedback into two of the laser cavities. The wavelength can be switched over a range of 26 nm. Through adjustment of the current into a third amplifier section, the output power can be stabilized over the tuning range of the laser.

Complex-coupled DFB lasers: advantages and drawbacks of gain and radiation loss grating

Two types of InGaAs-GaAs-AlGaAs complex-coupled distributed feedback laser structures grown by molecular beam epitaxy are compared. The first structure uses interrupted quantum wells (QW's) grown over a grating. Its properties strongly depend on the interface cleaning resulting in nonreproducible threshold current densities. The second structure includes three uniform QW's, the gain coupling being induced by the vertical radiation of the second order grating. This last structure has reproducible, low continuous wave threshold current densities (jth=300 A/cm/sup 2/). The coupling strength and its origin are described for the two structures. Both structures have high mono-mode yield (>75%) and side-mode suppression ratios larger than 40 dB.

Analysis of a dephased complex-coupled distributed-feedback laser by the power-series method

A novel power-series method to solve the coupled-wave equations is introduced. The method is used to calculate the threshold gain margins of a complex-coupled distributed-feedback laser as functions of the ratio of gain coupling to index coupling (|kappa(g)|/|kappa(n)|) and of the phase difference between the index and the gain gratings. For coupling coefficient |kappa|l < ., the laser shows a mode degeneracy at specific values of the ratio |kappa(g)|/|kappa(n)| for cleaved facets. At phase differences pi/2 and 3pi/2 between the gain and the index gratings, an antireflection-coated complex-coupled laser becomes multimode, and a different mode starts to lase. The effect of facet reflectivity (both magnitude and phase) on the gain margin of a complex-coupled DFB laser is also investigated. Although the gain margin varies slowly with the magnitude of the facet's reflectivity, it shows large variations as a function of the phase. Spatial hole burning was found to be minimum at phase difference npi, n =, ..., and maximum at phase differences pi/2 and 3pi/2.

A Novel 1.3-µm High T0 AlGaInAs/InP Strained-Compensated Multi-Quantum Well Complex-Coupled Distributed Feedback Laser Diode

A 1.3-µm AlGaInAs/InP buried heterostructure (BH) stripe distributed feedback laser with a novel AlInAs/InP complex-coupled grating grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) is proposed and demonstrated. A high characteristic temperature (T0=90 K between 20–80°C) and temperature-insensitive slope efficiency (0.25 dB drop from 20 to 80°C) in 1.3 µm AlGaInAs/InP DFB lasers was obtained by introducing Al(Ga)InAs graded-index separate-confinement heterostructure (GRINSCH) layers and a strained-compensated (SC) multi-quantum well (MQW).

Theory for optimum design and analysis of distributed-feedback lasers

Theory for optimum design of a distributed-feedback (DFB) laser is presented, based on a new set of coupled-power equations. A novel /spl lambda//4 phase shift index-coupled AR-coated DFB laser structure, which theoretically exhibits a longitudinally uniform power density, is proposed. It is shown that the theory is very simple and provides an easy way to analytically understand the physical mechanism crucial for the optimum design. Our theory can also be easily applied to the optimization of the design of gain-coupled, complex-coupled DFB laser structures as well as high power DFB lasers.

Enhanced wavelength tuning range in two-section complex-coupled DFB lasers by alternating gain and loss coupling

A novel tunable two-section complex-coupled distributed feedback (DFB) lasers were demonstrated which exhibited an enhanced wavelength tuning range beyond 7 nm by current injection only. By a simple master-and-slave type of injection current control, either in-phase gain-coupling or antiphase loss-coupling mechanism can be activated, in such a way that one of the two Bragg modes around DFB stopband will become the lasing mode, and its wavelength can be simply and independently tuned by corresponding injection current. More then 45 dB side-mode suppression ratio (SMSR) over entire tuning range were obtained. Good system performance for 375 km transmission with butt-coupled III-V Mach-Zehnder (MZ) modulator at 2.5 Gb/s over nondispersion-shifted fiber was also demonstrated.

The symmetry of the amplified spontaneous emission spectrum in complex-coupled DFB lasers

We present relations between facet reflections and coupling coefficient for the amplified spontaneous emission (ASE) spectrum of a complex-coupled, an index-coupled, and a gain-coupled distributed feedback (DFB) laser to have a mirror image with respect to the Bragg wavelength. We show that the ASE spectrum of a complex-coupled DFB laser cannot be symmetric in general with respect to the Bragg wavelength. The index-coupled and gain-coupled DFB lasers can have symmetric ASE spectrum for some values of the phase of the facet reflection coefficient that are determined.

Modeling and design of a DFB laser integrated with a Mach-Zehnder modulator

A comprehensive model for a complex-coupled distributed feedback laser diode monolithically integrated with a Mach-Zehnder modulator is developed and validated by comparison with experimental data. Methods and their tradeoffs for reduction of adiabatic frequency chirp due to the parasitic reflections from the modulator are discussed.

Non-reciprocal effects of complex-coupled distributed-feedback structures resulting from the phase difference between the coupling constants

We investigate theoretically the non-reciprocal properties of a complex-coupled periodical structure resulting from the phase difference between the index- and gain-coupling constants. We show that this effect can give rise to a spectacular non-reciprocity of the reflectivity spectrum. If the two contributions are out of phase by ± π 2 , they act together in one direction, but they cancel out each other by destructive interference in the other direction. We also investigate the non-reciprocal emission of a complex-coupled distributed-feedback laser at threshold. We believe that such properties could be useful in designing one-way-only filters in communication or data processing systems.

Fluctuations of the laser characteristics and the effect of the index-coupling component in the gain-coupled DFB laser

The normalized oscillation frequency /spl delta/L and threshold gain /spl alpha/L of gain-coupled distributed feedback (DFB) lasers with an AR/AR (symmetric) structure vary significantly with a variation in the corrugation phase at the facets when the facet reflectivity |r| is large, even though the spatial-hole-burning-corrected yield is 100%. For example, the standard deviation for the /spl delta/L, /spl sigma/(/spl delta/L), increases from 0.018 to 0.40 when |r| is increased from 0.03 to 0.30. The magnitude of /spl sigma/(/spl delta/L) and /spl sigma/(/spl alpha/L) are small, however, for the gain-coupled, index-coupled, and complex-coupled DFB lasers when the |r| is kept small. For the HR/AR (asymmetric) structure, these two values are more than an order of magnitude larger than those for the AR/AR structure, except for the /spl sigma/(/spl alpha/L) for the index-coupled laser, even though yields of each of these lasers are relatively large. In the AR/AR structure, the contamination by the index-coupling and gain-coupling components always degrades the device characteristics because of the difference in the phase shifter and when the amount of contamination is small this degradation is particularly severe for the index-coupled laser. Several properties of the index-coupled laser with the HR/AR structure can be improved, however, by introducing the gain-coupling component.

New Structure of 1.3 µm Strained-Layer Multi-Quantum Well Complex-Coupled Distributed Feedback Lasers

A new structure of 1.3 µm strained-layer multi-quantum well (MQW) complex-coupled distributed feedback lasers having an InAsP absorptive grating is demonstrated. In this structure, the InAsP absorptive grating and the strained-layer MQW active layer can be fabricated by a metalorganic vapor phase epitaxy process involving only one step. Furthermore, the fabricated laser has a low threshold current (16.8 mA) and a high efficiency (0.45 mW/mA).