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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]

Fast optical switches have the potential of lowering the cost, power, and latency of today’s data-center networks. The on-off switching of DFB lasers in arrays is poised to provide extremely fast switching because the on-off time of a DFB laser is the level of sub-nanosecond. However, the key of the DFB laser array based tunable laser lies in the high precision control of the lasing wavelength. In this letter, we proposed and experimentally demonstrated a tunable laser by integrating 16 DFB lasers based on the reconstruction-equivalent-chirp (REC) technique. Without using the conventional e-beam lithography, an equivalent $\lambda $ /4 phase-shift is fabricated in each DFB laser to ensure the single-longitudinal-mode (SLM) yield. Moreover, high precise wavelength spacing is realized. Wavelength deviations of 89.3% DFB lasers are within ±0.2 nm. The output power of all the channels is above 7 mW, and the relative intensity noise is below −130 dB/Hz. By on-off switching the lasers, sixteen channels with 2.4-nm spacing can be fast switched. The switching time between any two channels is less than 50 ns. In addition, a wide-band continuous wavelength tuning range of 38.4 nm can be obtained with a small temperature variation of 18 °C. Owing to the REC technique, the tunable laser via high-density integration of DFB lasers is demonstrated with simple fabrication, high SLM yield, and high wavelength precision, which opens up a path for wide-band and especially fast tunable lasers. [ABSTRACT FROM AUTHOR]

We present a new configuration of narrow linewidth semiconductor lasers (NLSLs) based on a novel narrow-band reflector (NBR) with π phase-shifted anti-symmetric Bragg grating (π-ASBG). Based on this configuration, two specific laser structures are theoretically analyzed. The effective length of Bragg grating is highly increased due to the double hybrid-mode resonances between the fundamental transverse electric (TE0) mode and first-order transverse electric (TE1) mode in the π-ASBG with the aid of a uniform grating reflector. The narrowest linewidth of ∼2.4 kHz is obtained of the proposed NLSLs, which is much lower than the traditional NLSLs with external uniform Bragg grating in the case of the same grating length. [ABSTRACT FROM AUTHOR]

In this study, a tunable in-series multi-wavelength distributed feedback laser array (DFB-MLA) based on reconstruction-equivalent-chirp (REC) technique with simple wavelength tuning scheme is presented and experimentally demonstrated. Unlike traditional DFB laser tuned by thermal-electric cooler (TEC), the proposed laser tuned by injection current can realize faster wavelength-tuning and reduce TEC power consumption. By comparing the thermal influence of different structures of heatsink blocks, several optimized block structures are selected for appropriate thermal conductivity and controllable heat dissipation, resulting in a broad and continuous wavelength tuning range. The proposed in-series DFB-MLA on optimized heatsink achieves better tuning performance with a wavelength tuning range of ~2.4 nm per channel from 40 to 120 mA and stable output power of over 20 mW compared to the laser mounted on the standard heatsink block with the tuning range of 1.4 nm per channel. Furthermore, the tunable laser keeps good single-mode stability, with a side mode suppressing ratio (SMSR) of over 40 dB. Thus, this study provides a new perspective for designing a low-cost continuous tunable semiconductor laser with a simple tuning mechanism in the future. [ABSTRACT FROM AUTHOR]

Recently space communication technology develops rapidly and space chaotic laser communication serves as an excellent scheme to enhance the security of data transmission. To achieve long-distance transmission, Erbium-doped fiber amplifier (EDFA) is used in a space chaotic laser communication system, but this will inevitably bring amplifier spontaneous emission (ASE) noise. In order to investigate the effect of ASE noise, we give a modified bit error rate (BER) calculation model and numerical simulations are conducted based on it. Quantitative results show that most of ASE noise in the system is caused by the EDFA at the receiver rather than the emitter. Apart from this, deeper researches further indicate that ASE noise will induce mismatch. When ASE noise is small, inherent mismatch noise is the main factor causing mismatch. But as ASE noise power becomes larger, mismatch noise will be dominated by external mismatch noise and internal mismatch noise. Our results have a good reference value for the design of space chaotic laser communication systems. [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]

A distributed feedback (DFB) laser for high-speed modulation with asymmetric corrugation-pitch-modulated (ACPM) grating structure and high-reflection and anti-reflection coating (HR-AR) coating is demonstrated theoretically and experimentally in this paper. The ACPM gating structure is fabricated by the reconstruction equivalent chirp (REC) technology, which is with low cost. The impact of the longitudinal spatial hole burning (LSHB) effect and the facet phase on the characteristics of the ACPM DFB laser is discussed in detail. The numerical simulation results show that the proposed ACPM DFB laser can suppress the LSHB effect significantly. Comparing with the conventional HR-AR DFB laser, the ACPM DFB laser can achieve higher output efficiency, better single-mode yield, and wider modulation bandwidth. The modulation eye diagrams of HR-AR DFB lasers with different grating structures are also presented. The experimental results show that the LSHB effect in the HR-AR DFB laser with ACPM structure is suppressed effectively. The laser with ACPM structure fabricated by the REC technique has high slope efficiency, stable single longitudinal mode operation, and wide modulation bandwidth. [ABSTRACT FROM AUTHOR]

Multi-wavelength distributed feedback laser array (DFB-MLA) based tunable semiconductor lasers have been widely used owing to the high mode stability and simple wavelength tuning scheme. In this paper, we proposed and experimentally demonstrated a matrix-grating tunable DFB laser array. And the reconstruction-equivalent-chirp (REC) technique is utilized to simplify grating fabrication and realize precise control of the grating phase. Traditionally, the laser is tuned by thermal-electric cooler (TEC), with large power consumption, slow wavelength tuning capability, and added device. To improve the wavelength tuning speed of laser, we varied laser’s injection current instead of adjusting TEC’s temperature. In addition, we analyzed the thermal influence of submount on the laser and then chose the SiO2 as the submount material for its low thermal conductivity and poor heat dissipation, resulting in wide wavelength tuning range via injection current. The DFB-MLA chip achieves a better tunable effect as the lasers can be tuned at 3.5–3.7 nm per channel from 50 to 120 mA with stable output power of over 3 mw as contrasted with the laser on the AlN submount being tuned at 1.4–1.5 nm per channel. And the chip has a side-mode suppressing ratio (SMSR) of over 35 dB and strong stability of wavelength in the repeated experiments. The proposed method may give a new way to realize the wide tuning range tunable DFB laser with a simple wavelength tuning mechanism in the future, especially for the applications of uncooled low-cost tunable lasers. [ABSTRACT FROM AUTHOR]

An integrated single sideband (SSB) modulation method benefitting from the sideband amplification injection locking (SAIL) effect for radio-over-fiber (ROF) link is proposed and experimentally demonstrated, which is based on an integrated multi-section mutual injection (IMSMI) DFB laser. The IMSMI-DFB laser, which overcomes the drawbacks of using discrete devices, simplifies the whole structure. The SAIL effect, which is regulated by the input RF power and bias current of IMSMI-DFB laser, promises a good pathway for adjusting and controlling the modulation frequency, optical carrier to sideband ratio (OCSR), and sideband suppression ratio (SSR), meanwhile, provides high gain in the link. The characteristics of SAIL effect are investigated. The OCSR can be optimized to 0 dB, the SSR is over 26 dB, and the transmission link gain improves over 20 dB from 16 GHz to 26 GHz. The ROF transmission experiments demonstrate that the power fading introduced by fiber dispersion is dramatically eliminated, and the error vector magnitudes (EVMs) of the received 20 MSymbol/s 32 QAM signals at frequencies from17 GHz to 22.3 GHz after 50 km fiber transmission are below 4.8%. This work provides an effective way to realize integrated direct SSB modulation with outstanding features for ROF link. [ABSTRACT FROM AUTHOR]

Hybrid free space optical (FSO)/radio frequency (RF) systems have been proved to be reliable for high-data-rate wireless backhauls. In this article, we present a unified performance analysis of the hybrid FSO/RF transmission system which transmits identical data in both links and implements two popular diversity combining schemes, namely, selection combining (SC) and maximal ratio combining (MRC), in the receiver. Specially, for the FSO link, the Gamma-Gamma turbulence with pointing errors under heterodyne detection (HD) and intensity modulation/direction detection (IM/DD) is considered in our analysis while the general κ-μ shadowed fading which unifies popular RF fading models is employed for the analysis of the RF link. As a result, unified closed-form expressions of outage probabilities and average bit error rates for different modulation schemes are derived. A preliminary extension of the RF link to multi-input–multi-output (MIMO) transmission is also analyzed. Analytical and Monte Carlo simulation results are provided to characterize the performance of the hybrid FSO/RF link which is compared to the single FSO link and the single RF link. The agreement between the analytical and simulation results confirms the unification of various FSO channels and RF fading scenarios into a single closed-form expression. [ABSTRACT FROM AUTHOR]

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. [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]

In this letter, we designed and experimentally demonstrated a compact mode converter in silicon-on-insulator (SOI) waveguide based on conformal dielectric metasurface, which uses polygonal slot structure to manipulate the wavefront of guided modes. Mode conversion from TE0 to TE2 mode can be realized within an ultracompact size ($\sim 7.66~\mu \text{m}$) by designing the arrow-like slot structure. With the help of asymmetric directional coupler (ADC), the TE0-to-TE2 mode converter was measured in the experiment with low insertion loss (IL) (< 0.5 dB) and low crosstalk (CT) (<−8.0 dB) from 1525 to 1565 nm. Moreover, a TE0-to-TE1 mode converter using another polygonal slot structure was also experimentally demonstrated. Our proposed mode converter shows great potential in high-density integration of photonic devices for on-chip mode control. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A four-channel wavelength-selectable in-series DFB laser array with good single-longitudinal-mode (SLM) property is proposed and theoretically analyzed. To realize good SLM property, a linearly chirped Bragg grating with multiple π phase shifts are implemented in the laser cavity. In addition, two reflector sections with chirped Bragg grating are specially designed for reflection compensation at both ends. In our simulation, the split-step time-domain model with coupled-wave equations and carrier rate equations are used. Based on the simulation results, all the channels work with low threshold currents and the SLM properties are considerably enhanced compared with the conventional in-series laser array. Notably a high precision control of grating phase is required in the practical fabrication due to the ultra-small variation of grating period. Therefore, the reconstruction-equivalent-chirp technique is discussed to provide the fabrication of high-precision grating. In addition, the complex heterogeneous integration or butt-joint technique is not required during the whole fabrication process, which simplifies the fabrication and reduces the cost. The proposed wavelength-selectable in-series laser array is deemed to be applied in the dense wavelength division multiplexing technology, especially when fast wavelength switching is needed, such as in the application of NG-PON2. [ABSTRACT FROM AUTHOR]

Single sideband (SSB) modulation based on a monolithic integrated optically injection-locked (MOIL) DFB laser is demonstrated experimentally. To our knowledge, this is the first time to realize SSB modulation utilizing a MOIL DFB laser. Tunable SSB signals with large lower-to-upper sideband power ratios are achieved by changing the bias currents of the MOIL DFB laser. The largest lower-to-upper sideband power ratio is measured to be 24.4 dB, which is very useful for improving the transmission performance of a radio-over-fiber link. Due to the generated SSB modulation, the fiber transmission response is flattened significantly and a signal of 25 GHz is transmitted through 85-km fiber without being affected by the fiber chromatic dispersion. 20 MSymbol/s 32-QAM signals with subcarriers of 21 GHz, 23 GHz, and 25 GHz are also transmitted by the proposed SSB modulation link, and both the error vector magnitude (EVM) and the bit error rate (BER) of the link are improved distinctly. The proposed method can effectively suppress the fiber chromatic dispersion. [ABSTRACT FROM AUTHOR]

Wide-area and multi-point remote sensing technology plays an important role in the fields of disaster monitoring and locating. We demonstrate a demultiplexing method for quasi-distributed identical strong fiber Bragg grating (FBG) sensors based on optoelectronic oscillator (OEO). When the FBG sensor is affected by environmental changes, the wavelength of the FBG will shift. An oscillation will be stimulated in the OEO since the laser power is reflected by the FBG. The demultiplexing system utilizes the oscillating frequency of the OEO to encode the location information of grating series. Compared with other schemes, the proposed system has three superior advantages. First, as a digital discrete position encoding system, it has a strong anti-interference ability and does not require continuous sweeping wavelength and rapid digital signal processing, which can achieve high speed and high stability. Second, it can demodulate identical FBGs with strong reflectivity, thus ensuring long-distance and large capacity. Third, it can work in a low-frequency range without using high-frequency devices, which has the potential to be a low-cost interrogator in practical applications. The theoretical capacity of the system is 62 multiplexed points in one line and the experimental results show that the sensing area can range from 1 m∼1 km with the response time of ∼0.61 s and the oscillating frequency stability of <28 kHz. The linear relationship between the wavelength of the laser and the alarm strain/temperature, corresponding to 1.3 μϵ/pm or 0.1 °C/pm, can be used to set alarm threshold values. [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]

A dual-frequency Doppler Lidar (DFDL) with high precision utilizing a monolithic integrated two-section DFB laser as the dual-frequency light source is proposed and experimentally demonstrated. The DFDL can be realized with smaller size using the monolithic integrated two-section DFB laser which is fabricated by the reconstruction-equivalent-chirp (REC) technique with high precision and low fabrication cost. The range of the measured speed is from 13.62 μm /s to 1.56 m/s, which covers 6 orders of magnitude. The largest relative error of the DFDL system is 3.16%. The DFDL system has an excellent resolution of 1.95 μm/s, which is suitable to detect micro speed changes. [ABSTRACT FROM AUTHOR]

We propose and theoretically analyze a hybrid transverse mode resonance DFB semiconductor laser which consists of one active section with π-phase shifted anti-symmetric Bragg grating (π-ASBG) and two passive sections with uniform Bragg grating. The hybrid light resonance along with mode conversion between the fundamental transverse electric (TE0) mode and first order transverse electric (TE1) mode is formed by the π-ASBG. The emitted light from one facet with the TE0 or TE1 mode can be controlled by tuning the injection currents of the passive sections. As an example, a laser with ridge waveguide is designed and simulated. The transverse mode with the same wavelength is tuned by changing the injection currents of the passive sections. The suppression ratio between the two transverse modes reaches 18.4 dB and 18.7 dB when the TE0 and TE1 modes are emitted, respectively. A simple method for equivalently realizing the grating in the proposed laser was also analyzed. Therefore, as the transverse mode tunable light source, the proposed laser may benefit the future mode division multiplexing (MDM) systems for reconfigurability and more flexibility. [ABSTRACT FROM AUTHOR]

A unified data communication and radar sensing system towards millimeter-wave (mm-wave) applications with extended coverage is proposed and experimentally demonstrated. In the unified system, the transmission and reception of mm-wave signals are implemented by microwave photonics technologies, and the orthogonal frequency division multiplexing (OFDM) signal is employed for communications and radar sensing simultaneously. The superheterodyne optical carrier suppression (HeteroOCS) is used to up-convert the intermediate frequency (IF) OFDM signal to the mm-wave band in the central office (CO). The optical mm-wave signal is transmitted to the remote antenna units (RAUs) via optical fibers. The echo signal is received by the RAU which is remotely connected to the CO by an analog photonic link based on a polarization modulator (PolM). In the transmitter and receiver links, the chromatic dispersion of optical fibers is overcome by the HeteroOCS and the PolM, respectively. The baseband units (BBU) centralized in the CO is used to process the radar sensing signal. A pre-delayed trigger signal is proposed to overcome the fiber length limitation. A proof-of-concept experiment is performed. Two targets with distances of 1.64 m and 3.40 m are successfully detected separately. The communication function of the system is also verified. 1 m wireless transmission of 1.56 Gb/s data rate is achieved below the forward error correction limit. [ABSTRACT FROM AUTHOR]