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The tunable laser based on distributed feedback (DFB) laser array has a good single-longitudinal-mode property, high wavelength stability, narrow spectrum linewidth, and feasibility for monolithic integration. The tunable in-series DFB laser array (TIDLA) was proposed to avoid the large extra power loss by the optical combiner. But in the TIDLA, a large laser count will cause a large chip length, which brings about low utilization ratio of material, poor mechanical strength, and large power consumption. In this paper, we demonstrated a 4-channel TIDLA with duplex DFB sections. The total chip length was greatly shortened owing the duplexing of DFB sections. A long linearly chirped Bragg grating is implemented in the laser cavity to reduce the grating interference among different lasers. Multiple π phase shifts and two reflector sections are introduced to improve the mode stability. The designed channel spacing is 100 GHz, and the reconstruction-equivalent-chirp technique is used to provide high precision control of the grating phase. Noted that a variation of 0.1 nm in grating period will cause that of 0.65 nm in wavelength. Output power of above 4 mW, SMSR of above 42 dB, and small wavelength deviation of within 0.15 nm was obtained. Besides, fast channel switches of less than 1 μs are obtained by laser on/off switching. The proposed TIDLA might be applied in the NG-PON2 network where the dense wavelength division multiplexing technique and fast channel switching are required. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a high-power eight-wavelength distributed feedback (DFB) laser array with 100 GHz spacing using the grating reflector (GR). The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of a random grating phase at the HR facet. Therefore, more output power, better single-longitudinal-mode operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with HR/AR facet coatings. The maximum output power and side mode suppression ratio of the fabricated eight-channel GR-DFB laser array as high as 70 mW and 55 dB can be achieved, which may benefit wavelength division multiplexing transmission systems, gas sensors, optical computing systems, and microwave photonic signal processing. [ABSTRACT FROM AUTHOR]

A method of engineering spectrum of mode converter based on cascaded Bragg gratings is investigated in detail. Due to the cascaded structures, the proposed mode converter owns higher design degrees of freedom comparing with mode converter based on traditional Bragg grating. Meanwhile, thanks to various light responses of gratings, conversion spectrum can be easily tailored by designing grating structures. As examples, mode converters with specific conversion spectra such as single-band pass, double-band pass, multi-band pass, narrow-band backward spectra have been designed. Besides, the same triangle-shaped band pass spectrum is obtained by two converters based on different grating structures with the aid of distribute-layer-peeling algorithm, which further shows the high design flexibility of the proposed method. We think the proposed method gives a new way for advanced mode converter with required conversion spectrum. [ABSTRACT FROM AUTHOR]

We proposed a new kind of add-drop multiplexer based on $\pi $ phase-shifted antisymmetric Bragg grating ($\pi $ -ASBG). Light resonance is formed in the $\pi $ -ASBG with mode conversion between backward fundamental transverse electric (TE0) mode and the forward first order transverse electric (T1) mode. With the aid of the TE1 mode generated in the $\pi $ -ASBG, add-drop multiplexer can be designed. As examples, two multiplexer structures based on single- and dual-bus resonator, on silicon-on-insulator (SOI) platform have been studied in detail. Besides, no light is reflected to the input port, if the structure is carefully designed with critical coupling condition satisfied. For the fabricated add-drop multiplexer based on single-bus resonator, the extinction ratio is 23.90 dB and full width at half maximum (FWHM) is 0.33 nm at the through port. For the fabricated add-drop multiplexer based on dual-bus resonator, the extinction ratio is 17.22 dB and FWHM is 0.29 nm at through port. The results show that proposed structures provide a new method to realize add-drop multiplexer and other integrated optical filters for optical communications and sensors. [ABSTRACT FROM AUTHOR]

Multi-wavelength distributed feedback laser array based tunable semiconductor lasers have been widely used owing to their high mode stability and simple tuning scheme. Comparing with the in-parallel laser array, the in-series one can avoid a large power loss caused by the optical combiner. However, the single-mode stability of an in-series laser array is vulnerable to the reflections of other lasers. Here we experimentally demonstrated a tunable in-series laser array with high single-mode stability and high uniformity of wavelength spacing. To reduce the impact of such reflections, the Bragg gratings in different lasers were designed in phase, and the grating phase error was reduced by utilizing the reconstruction-equivalent-chirp technique. Besides, a three-section laser structure was applied in each laser to increase the priority of the dominant lasing mode. Four lasers with small wavelength-spacings of 2.5 nm were integrated in the in-series laser array. According to the measurement results of 40 in-series laser arrays (160 lasers), wavelength deviations of 90.6% lasers were within ±0.2 nm, average wavelength spacings of 97.5% of the measured laser arrays were deviated from the design within ±0.2 nm, and SMSRs of 96.3% lasers were above 45 dB. The output power was above 25 mW and the relative intensity noise was below −130 dB/Hz. Like the in-parallel laser array, the proposed laser array can be applied in either continuous wavelength tuning or fast channel switching. For the continuous wavelength tuning applications, the 10-nm tuning range was obtained with a temperature tuning range of less than 20 °C. For the fast channel switching applications, four channels with 2.5-nm spacing are available, and the switching time between two channels was less than 300 ns. [ABSTRACT FROM AUTHOR]

We propose a new kind of distributed feedback (DFB) semiconductor laser based on the sampled Moiré grating (SMG) integrated with a grating reflector (GR). Coupling coefficient along the laser cavity can be controlled, and $\pi $ phase shift ($\pi $ -PS) can be introduced by SMG. The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of random grating phase at the HR facet. Therefore, more output power, better single longitudinal mode (SLM) operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with AR/HR facet coatings. As an example, an 8-channel DFB laser array based on the proposed structure has been simulated. The proposed laser may benefit the applications requiring both high output power and wavelength precision, such as wavelength division multiplexing (WDM) transmission systems and gas sensors. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

To evaluate the effect of crosstalk caused by wavelength deviation of reconstruction-equivalent-chirp (REC) integrated multi-wavelength laser arrays (MWLAs) on an optical free-space communication system in which wavelength division multiplexing (WDM) is applied, we derive the model of power penalty caused by the wavelength deviation of REC-MWLAs and the bit error rate (BER) of the system under atmospheric turbulence and conduct a numerical simulation. The two adjacent channels in arranged REC laser arrays are defined as the analysis scope. We find that the power penalty is more sensitive to the deviation of the channel itself than the adjacent channel as the frequency shift increases from 0 to 50 GHz. The numerical relationship between the BER and power penalty is drawn to further investigate the tolerance for wavelength deviation under the requirements of BER < 10 − 6 and BER < 10 − 9. In addition, for a smaller deviation of 5 GHz of the analyzed channel, the linewidth compression of the REC laser can decrease the power penalty from 1.3 to 0.7 dB, improving the system performance and moderating the demands for deviation of REC-MWLAs, and the gemination of channel spacing does not evidently reduce the requirement for deviation. The results of this work demonstrate the feasibility of REC-MWLAs as the light source of WDM free-space communication systems under atmospheric turbulence and provide an effective reference for the allowable deviation in the design and testing of MWLAs. [ABSTRACT FROM AUTHOR]

A compact cascaded tunable distributed feedback semiconductor laser is proposed and theoretically analyzed. Each laser section (LS) is formed by two adjacent grating sections (GSs) with slightly different Bragg wavelengths and a $\pi $ phase shift in the joint between them. A step-wise grating period profile is designed to realize multi-channel lasing. Since two LSs share a common GS, the total cavity length of the tunable laser is significantly reduced. As an example, a tunable laser with six GSs and five $\pi $ phase shifts was designed and analyzed, resulting in a continuous tuning range of 12 nm. The total length of the laser is reduced by 40% compared with the conventional structure. Furthermore, an improved structure with apodized grating in each GS is also proposed. The single mode stability and fabrication tolerance are significantly improved. Due to the compact structure, the cost of the proposed laser can be highly reduced. Therefore, we think the proposed structure would benefit the practical applications to the low cost tunable lasers in wavelength-division multiplexing systems. [ABSTRACT FROM AUTHOR]

In this letter, we present a novel and simple method to suppress the signal fading phenomena in polarization optical time-domain reflectometry (POTDR). A combination of polarization multiplexing and multi-directional polarization analyzing is utilized to achieve the goal. A short polarization maintaining fiber is connected to a multi-wavelength laser source to generate polarization multiplexing probe lightwaves. The signals for different probe lightwaves are demultiplexed by a dense wavelength division multiplexer. Then, the three polarizers with different orientations are used to analyze the signals, respectively. The principle of the method is analyzed in detail. Experiments demonstrate the validity of the system to suppress the signal fading. The proposed method can eliminate the location error of event caused by the signal fading and the signal-to-noise ratio is also improved compared with a traditional POTDR. [ABSTRACT FROM PUBLISHER]

A real-time interrogation technology for a large-scale fiber laser sensor array is proposed and experimentally investigated by employing a hybrid topological structure based on the wavelength- and frequency-division multiplexing techniques. In this system, a 2-D n × m sensor array is established by using a 1 × n coupler and n 1 × m couplers. All sensors are divided into n units by the 1 × n coupler. In each unit, there are m fiber-ring laser sensors with approximately the same wavelength and different beat frequencies due to different laser cavity lengths. Meanwhile, different units are characteristic of different operating wavelengths by splicing different fiber Bragg grating (FBG) as a reflector. Therefore, each sensor has a unique wavelength- and frequency-encoded information. All sensors can be interrogated by the combination of wavelength and frequency multiplexing technology. In the paper, a 4 × 4 sensor array is set up experimentally to verify the interrogation ability, and the sensing information of the strain and temperature has been successfully extracted. The experimental results show that the proposed interrogation system has the ability of real-time monitoring for a large-scale fiber laser sensor array. [ABSTRACT FROM PUBLISHER]

ABSTRACT A distributed-feedback semiconductor laser array with buried heterostructure waveguide based on reconstruction equivalent chirp technique is experimentally demonstrated. The fabricated six-element laser array has channel spacing of 800 GHz and well meets the International Telecommunication Union standard, with relatively low threshold and high efficiency. Good single-longitudinal mode operation has been achieved at high injection current (120 mA) and high temperature (80°C), with side mode suppression ratios not lower than 40 dB, which is promising for the future wavelength division multiplexing systems. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1278-1281, 2015 [ABSTRACT FROM AUTHOR]

A dual-wavelength distributed feedback fiber laser with two equivalent phase shifts is proposed and demonstrated experimentally. By studying varied parameters of the fiber laser, which show different impacts on the lasers' characteristic, a dual-wavelength laser of 31 pm spacing is designed and fabricated. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1453-1456, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25969 [ABSTRACT FROM AUTHOR]

With the widespread deployment of wavelength division multiplexing (WDM), optical transceivers increasingly use many glass micro-optical components (GMOC). Visual inspection of these GMOCs is a critical manufacturing step to ensure quality and reliability. However, manual inspection is often labor-intensive and time-consuming due to the transparent nature of glass components and the small, randomly located defects in three dimensions. Although automated optical inspection (AOI) exists, it has not yet been able to provide the desired level of accuracy and efficiency. This paper reports the development of an AOI platform for 3D defect detection on GMOCs. The platform incorporates 3D video acquisition and a novel two-stage neural network machine-learning algorithm. It includes a robotic arm for moving parts in 3D, a camera with an illumination module for video acquisition, and a video streaming processing unit with a machine vision algorithm for real-time defect detection on a production line. The robotic arm enables multi-perspective video capture of a test sample without refocusing. The two-stage machine learning network uses a modified YOLOv4 architecture with color channel separation (CCS) convolution, an image quality evaluation (IQE) module, and a frame fusion module to integrate the single frame detection results. This network can process multi-perspective video streams in real-time for defects detection in a coarse-to-fine manner. The AOI platform was trained with only 30 samples and achieved promising performances with a recall rate of 1, a detection accuracy of 97%, and an inspection time of 48 s per part. • In order to solve the challenging problem of defect detection on transparent optical components with 3D shape and random defect locations, a novel optical inspection platform based on video stream is developed to capture multi-perspective images of the object, and machine learning based algorithms is used to identify the defects. [ABSTRACT FROM AUTHOR]

In this paper, a strain sensing system based on Distributed-Feedback (DFB) laser array and a high effective demodulate algorithm is proposed. The uniqueness and innovation of the algorithm is in the following two aspects: one is that accurate demodulation can be achieved by scanning only 0.4 nm bandwidth of fiber Bragg grating (FBG) spectrum instead of relying on scanning FBG’s principal power peak; and the other is that the scanned spectrum can be any part of reflected spectrum of FBG sensor, which make it possible of using wavelength-division multiplexing (WDM) for FBG sensing network. In the experiment, DFB laser array with single-channel and four-channel are used to demonstrate the proposed algorithm. Experiments results show that the single-channel’s error is below 5 pm, four-channel’s is below 2 pm, strain and algorithm results show good linearity. [ABSTRACT FROM AUTHOR]

The tunable distributed feedback (DFB) semiconductor lasers with cascaded configuration were designed and fabricated using equivalent phase shift (EPS) technique. 32-wavelength division multiplexing (WDM) channels with 50-GHz and 100-GHz wavelength spacing were experimentally demonstrated. In such a tunable laser, single mode operation can be ensured by introducing EPS and the wavelength tuning can be realized by cascaded sections with specific sampling periods combining with chip heating. The proposed approach has some advantages such as good single mode property, low fabrication cost and feasibility of monolithic integration. The experimental results show that the tuning range can be nearly up to 25 nm and the side mode suppression ratios (SMSRs) of most channels are above 40 dB. The proposed laser should be promising for cost-effective applications in WDM systems. [ABSTRACT FROM AUTHOR]

Abstract: A novel stable multi-wavelength fiber Bragg grating laser is achieved and its sensing characteristic with the strain is studied. The beat signals generated by the multi-wavelength fiber laser are measured under different strains. Four-wave mixing (FWM) is firstly observed in such a short-cavity fiber laser. [Copyright &y& Elsevier]