Sampled Bragg Gratings Research Articles

Dual-wavelength distributed feedback laser array based on four-phase-shifted sampled Bragg grating for terahertz generation.

We propose and experimentally demonstrate a dual-wavelength distributed feedback (DFB) laser array utilizing a four-phase-shifted sampled Bragg grating. By using this grating, the coupling coefficient is enhanced by approximately 2.83 times compared to conventional sampled Bragg gratings. The devices exhibit a stable dual-mode lasing achieved by introducing further π-phase shifts at 1/3 and 2/3 positions along the cavity. These devices require only one stage of lithography to define both the ridge waveguide and the gratings, mitigating issues related to misalignment between them. A dual-wavelength laser array has been fabricated with frequency spacings of 320 GHz, 500 GHz, 640 GHz, 800 GHz, and 1 THz. When integrated with semiconductor optical amplifiers, the output power of the device can reach 23.6 mW. Furthermore, the dual-wavelength lasing is maintained across a wide range of injection currents, with a power difference of <3 dB between the two primary modes. A terahertz (THz) signal has been generated through photomixing in a photoconductive antenna, with the measured power reaching 12.8 µW.

DFB laser array based on four phase-shifted sampled Bragg gratings with precise wavelength control.

A four-laser array based on sampled Bragg grating distributed feedback (DFB) lasers in which each sampled period contains four phase-shift sections is proposed, fabricated, and experimentally demonstrated. The wavelength spacing between adjacent lasers is accurately controlled to 0.8 nm ± 0.026 nm and the lasers have single mode suppression ratios larger than 50 dB. Using an integrated semiconductor optical amplifier, the output power can reach 33 mW and the optical linewidth of the DFB lasers can be as narrow as 64 kHz. This laser array uses a ridge waveguide with sidewall gratings and needs only one metalorganic vapor-phase epitaxy (MOVPE) step and one III-V material etching process, simplifying the whole device fabrication process, and meeting the requirements of dense wavelength division multiplexing systems.

Low-κ, narrow linewidth III-V-on-SOI distributed feedback lasers with backside sampled Bragg gratings.

We demonstrate a heterogeneously integrated III-V-on-SOI distributed feedback laser with a low grating strength (κ < 40 cm-1) and a narrow linewidth of Δν = 118 kHz. The laser operates single mode with a side-mode suppression ratio over 45 dB, provides a single-sided waveguide-coupled output power of 22 mW (13.4 dBm) and has a wall-plug efficiency of 17%. The dynamic characteristics were also evaluated, obtaining an intrinsic 3 dB modulation bandwidth of 14 GHz and a photon lifetime of 8 ps. Large-signal intensity modulation using a 231-1 PRBS pattern length revealed open eye diagrams up to 25 Gb/s and a penalty on the dynamic extinction ratio lower than 1 dB after transmission over a 2 km standard single mode optical fiber.

A Stable Dual-Wavelength DFB Semiconductor Laser With Equivalent Chirped Sampled Grating

A monolithic integrated dual-wavelength DFB semiconductor laser is fabricated and experimentally demonstrated. Utilizing the reconstruction-equivalent chirp (REC) technique, dual-wavelength lasing is obtained by introducing two symmetric equivalent phase shifts in a chirped sampled Bragg grating. The laser operates at 1650 nm and the wavelength spacing is about 0.8 nm at room temperature. The frequency of the beat signal between the two wavelengths is 88.47 GHz, and the linewidth of the beat signal is 5.1 MHz. Based on the REC technique, the grating pattern can be fabricated by using holographic exposure combining with conventional lithography. This work provides a simple and cost-effective way to achieve dual-wavelength semiconductor laser.

A Monolithic Integrated Dual-Wavelength DFB Laser With Equivalent Inverse-Gaussian Apodized Grating

A monolithic integrated dual-wavelength DFB laser with inverse-Gaussian apodized sampled Bragg grating (IGASBG) is proposed and investigated. The IGASBG in the DFB laser is achieved by varying the duty cycle of sampled Bragg grating. The frequency spacing between the two lasing modes can be designed from 38 GHz to 660 GHz which covers almost the entire millimeter-wave range and reaches terahertz. The results proposed in this paper have great reference value for fabricating dual-wavelength semiconductor lasers.

Photonic integrated circuits for terahertz source generation

The authors introduce four kinds of terahertz photonics components based on photonic integrated circuits (PICs). A PIC-based integrated optoelectronic synthesiser for THz communications is described, which can be tuned continuously over the range 0.254-2.723 THz using photomixing. A laterally-coupled dual-grating distributed feedback laser (DFB) diode integrated with an electroabsorption modulator is used to generate an 820 GHz beat signal. THz signal production is reported using a dual-wavelength DFB diode laser with a two-section phase-shifted sampled Bragg grating. Finally, a THz source at 640 GHz, based on a sampled grating distributed Bragg reflector semiconductor mode-locked laser diode, is reported offering superior reproducibility, controllability, and a wider operation range than other reported mode-locked THz laser diodes. Each of these sources is a monolithic construction emitting light at 1.5 μm. The light can be amplified in an erbium-doped fibre amplifier, delivered over silica optical fibre and used to generate THz radiation via a photodiode antenna or photoconductive antenna in a remote location.

Tilting of Bragg Waveguide Gratings Using Two-Dimensional Sampling Structures

Tilted Bragg grating (TBG) has been intensively investigated for photonic devices such as fiber sensors and on-chip filters. In this article, we present that TBG can be equivalently realized by designing the tilted sampled Bragg grating (TSBG). As its basic grating is uniform and the sampling structure is in the order of micrometer, the fabrication of TSBG can be realized with one step of holographic exposure combined with micro-lithography. As examples, we designed and simulated the equivalent light responses of the TSBGs in the multimode planar waveguide and single-mode fiber respectively. Moreover, the fabrication tolerance of TSBG has also been analyzed. The proposed technique may pave a new way for different 2D gratings with good design flexibility and easy fabrication in various photonic devices.

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.

Multi-wavelength sampled Bragg grating quantum cascade laser arrays

A multi-wavelength sampled Bragg grating (SBG) quantum cascade laser array operating between 7.32 and 7.85 μm is reported. The sampling grating structure, which can be analyzed as a conventional grating multiplied by a sampling function, is fabricated by holographic exposure combined with optical photolithography. The sampling grating period was varied from 8 to 32 μm, and different sampling order (−1st, −2nd, and −3rd order) modes were achieved. We propose that higher-order modes with optimized duty cycles can be used to take full advantage of the gain curve and improve the wavelength coverage of the SBG array, which will be beneficial to many applications.

Sampled Bragg gratings formed in helically twisted fibers and their potential application for the simultaneous measurement of mechanical torsion and temperature.

We propose and demonstrate a novel type of sampled Bragg gratings by combining a helically twisted fiber and a Bragg grating. A comb-like spectrum with a series of harmonic narrow resonances is observed, and the influence of geometrical parameters on the resonances is studied. As a special application, the intrinsic nature of the device that contains the Bragg grating and helical fiber spectral responses permits the temperature to be detected from the former, whereas the mechanical torsion is extracted from the latter, suggesting a potential for the simultaneous measurement of these two parameters. The proposed configuration features simplification, easy fabrication, high flexibility, stability, and low cost, and therefore has good prospects for sensor applications, as well as other applications, such as multi-channel filters, distributed Bragg reflectors, etc.

Stable single-mode operation of a distributed feedback quantum cascade laser integrated with a distributed Bragg reflector

We report an index-coupled distributed feedback quantum cascade laser by employing an equivalent phase shift (EPS) of quarter-wave integrated with a distributed Bragg reflector (DBR) at λ∼5.03 μm. The EPS is fabricated through extending one sampling period by 50% in the center of a sampled Bragg grating. The key EPS and DBR pattern are fabricated by conventional holographic exposure combined with the optical photolithography technology, which leads to improved flexibility, repeatability, and cost-effectiveness. Stable single-mode emission can be obtained by changing the injection current or heat sink temperature even under the condition of large driving pulse width.

Multiple-wavelength distributed-feedback laser arrays with high coupling coefficients and precise channel spacing.

Special designs of sampled Bragg gratings have been used to fabricate distributed-feedback semiconductor laser arrays with very precise wavelength spacing and strong coupling coefficients. By dividing one sampling period into two equal sections, each with a grating but with a π-phase shift between the sections, the ±1st-order channels are enhanced while eliminating the zeroth-order reflection. By using multiple phase-shifted sections, only the -1st-order channel is enhanced. Using a single electron beam lithography step and two π-phase-shifted sections, we have fabricated an eight-channel laser array with a channel spacing of 100GHz.

Experimental Demonstration of the Anti-Symmetric Sampled Bragg Grating

In this letter, we provided a proof-of-concept study on the anti-symmetric sampled Bragg grating (ASBG) with π equivalent phase shift (π-EPS) in silicon-on-insulator (SOI). A four-channel narrow-band passive filter array was fabricated. Experimental results show that the 0th-order resonance is totally eliminated, while the +1st-order resonance is maintained. In addition, 3-D finite-difference-time-domain (3-D-FDTD) algorithm was also used to analyze the other characteristics of this structure, such as the radiation phenomenon caused by anti-symmetric feature. Since the anti-symmetric basic grating combined with sampling pattern could equivalently realize various grating structures, ASBG can be used for multi-wavelength photonic devices, such as DFB laser array, sensor array, electrooptic modulator array, and so on.

Reflection and Transmission Spectra of Multimode Interference Devices With Bragg Gratings

We numerically investigate the reflection and transmission spectra of multimode interference (MMI) devices incorporating uniform and sampled Bragg gratings with emphasis on their applications to an add-drop multiplexer (ADM). In numerical analysis, a rectangular index modulation is assumed as the grating and a 2×2 MMI device is treated to operate as a nearly two-mode interference (TMI) device, which is frequently used as an alternative to conventional directional couplers that have a disadvantage of large size. Although the first two guided modes are used in the TMI device, it is especially important to overlap their reflection bands. Therefore, a design method for attaining a sufficient overlap and some considerations are given. The spectral response of the TMI device with a uniform Bragg grating is analyzed by a combination of the mode-propagation method and the transfer-matrix method and discussed on the basis of the state of overlap. In the case of a sampled Bragg grating, it is found that although many modes are excited in the MMI section, multiple reflection bands (multi-channel) are created as expected. The treated device can be applied to single- and multi-channel ADMs.

Analysis of a Special Period Sampled Bragg Grating (PSBG) Structure Combined with the Large Chirp Parameter

Chirped fiber Bragg grating (CFBG) provides an attractive solution for low cost dispersion compensation in a fiber optic transmission system. This kind of grating can be produced with a large chirp parameter and period sampled distribution along the grating length. The present work carries out the design optimization of a chirped FBG in respect of chirp bandwidth and apodization in achieving optimum dispersion compensation in a 20Gbps optical transmission link for different modulation formats.

Study on sampled waveguide grating with anti-symmetric periodic structure.

An anti-symmetrically sampled Bragg grating (ASBG) with single mode waveguide is proposed and investigated for the first time. Based on anti-symmetric periodic structure, the coupling coefficient between the forward and backward guided modes becomes zero, thus nearly no light is reflected. Besides, the equivalent tilted grating effect with radiation mode coupling is found. If another anti-symmetrically sampling structure is imposed to form a sampled grating, the 0th sub-grating can be avoided, while the ± 1st sub-gratings are adjusted as uniform gratings with normal performances. This will be very benefit for some special applications such as distributed feedback (DFB) lasers based on Reconstruction-equivalent-chirp (REC) technique where 0th order resonance can be avoided. In addition, error analysis for the proposed structure is also performed for practical applications.

Monolithically Integrated Four-Channel DFB Semiconductor Laser Array With an Equivalent-Distributed Coupling Coefficient

An equivalent-distributed coupling coefficient (EDCC) distributed feedback (DFB) semiconductor laser is proposed, and a four-channel EDCC DFB laser array is experimentally demonstrated. In this paper, the EDCC grating profile is realized by varying the duty cycle of a certain subsection of the whole sampled Bragg grating (SBG) along the laser cavity, instead of the true DCC structure obtained by changing the height of the uniform grating pitch. Moreover, the EDCC grating array can be designed by stepping the sampling period of the SBG and fabricated by combining the conventional holographic exposure with a micrometer-level photolithography on the same wafer. The experimental results show that the uniform channel spacing in the array has been obtained and that the residual of the lasing wavelengths after linear fitting changes in the scope of -0.078 and 0.048 nm. Each laser has good single longitudinal mode operation with the minimum side-mode suppression ratio (SMSR) of about 47.65 dB and the average SMSR of 49.83 dB under the same injection current of 80 mA.

An equivalent-stepped-index-coupled DFB semiconductor laser and laser array realized by stepping the duty cycle of the Sampled Bragg grating

An equivalent-stepped-index-coupled (ESIC) distributed feedback semiconductor laser with an equivalent-stepped duty cycle (ESDC) is proposed and investigated numerically. The ESDC grating profile is realized by stepping the duty cycle of a Sampled Bragg grating (SBG) with the uniform seeding grating. Numerical results show that the light will be distributed in larger region along the laser cavity , that is to say, light field will be flattened and spatial hole burning can be suppressed in such flexible structure. Based on the ESDC structure, different sampling structures in a DFB laser array can also be obtained by stepping the sampling periods; therefore, multi-wavelength laser array can be designed and fabricated, which would simplify the fabrication process , and may be more suitable for the high precision and massive fabrication of DFB laser and laser array with reduced spatial hole burning. • We proposed an equivalent-stepped-duty cycle (ESDC) grating structure. • The ESDC profile is realized by stepping the duty cycle of a Sampled Bragg grating. • An equivalent-stepped-index-coupled (ESIC) DFB laser is based on the ESDC grating. • The SHB is suppressed effectively in the ESIC DFB laser. • The ESIC DFB laser array can be designed and fabricated easily based on the REC.

Experimental demonstration of DFB semiconductor lasers with varying longitudinal parameters

The distributed-coupling-coefficient and distributed-coupling-coefficient corrugation-pitch-modulated DFB lasers are experimentally demonstrated. The proposed lasers maintain good side mode suppression ratio over 50dBfrom 2.5 times to 12.5 times threshold current. The grating profiles of varying longitudinal parameters are equivalently obtained by specially designed sampled Bragg gratings and fabricated by conventional holographic exposure and μm-level photolithography.

Multi-wavelength surface emitting quantum cascade laser based on equivalent phase shift

A novel surface emitting distributed feedback quantum cascade laser emitting around λ ∼ 4.6 μm is demonstrated by employing an equivalent phase shift (EPS) of quarter-wave (λ/4). The EPS is fabricated through extending one sampling period by 50% in the center of a sampled Bragg grating. Single-lobed far-field radiation pattern with a low divergence angle of about 0.6° × 16.8° is obtained. Selective single-mode lasing with a mean side mode suppression ratio above 20 dB and wavelength coverage range of 72 nm is achieved simultaneously on a single wafer only by changing the sampling period.