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A high-precision tunable single-frequency fiber laser (T-SFFL) at 1.5 μm based on self-injection locking is experimentally demonstrated. By virtue of an effective filtering and self-injection locking scheme, a stable single-longitudinal-mode operation of the fiber laser is obtained. The fine wavelength tunability is implemented by stretching a uniform fiber Bragg grating in the self-injection loop, and the laser wavelength is tuned from 1550.28 to 1560.40 nm with a tuning accuracy of less than 54 pm. In the whole tuning range of 10.12 nm, an output power of approximately 5 mW and an optical signal-to-noise ratio of 71 dB are synchronously obtained with this laser system. Additionally, the measured laser linewidth is less than 630 Hz and the relative intensity noise is lower than -149 dB/Hz at frequencies from 0.4 to 7.5 MHz. It is believed that this 1.5 μm high-precision T-SFFL can be a promising candidate for the optical communications system applications.

The receiver sensitivity is a very important metric in optical communication links operating at low received signal powers. Phase sensitive optical amplifiers (PSAs) can amplify optical signals without excess noise, thus providing a fundamental sensitivity improvement of 3 dB when employed as a pre-amplifier compared to conventional erbium doped fiber amplifiers (EDFA). In this paper, we investigate, both theoretically and experimentally, the sensitivities achieved using power efficient multi-dimensional modulation formats such as M-ary pulse position modulation format (M-PPM) and M-PPM combined with quadrature phase shift keying (QPSK) along with a near-noiseless PSA pre-amplified coherent intradyne receiver. We find that at high signal to noise ratios (SNRs) corresponding to low bit-error-rates (BER), M-PPM+QPSK results in the best sensitivity, which is improved with the order M, while at low SNRs corresponding to high BER (~14% where 100% overhead forward error correction codes (FEC) would be needed to recover the data), QPSK is the most sensitive format, while at the same time providing the best spectral efficiency. We report experimental sensitivities of 2.1 photons per information bit (PPB) at a pre-FEC BER=10-3 using 64-PPM+QPSK and assuming 7% FEC, and 0.8 PPB at a pre-FEC BER= 0.14 using QPSK and assuming 100% FEC.

Ensuring high speed physical layer confidential data exchange in classical optical fiber channels is a critical challenge. In this letter, we experimentally demonstrated a physical secure optical communication system based on temporal spreading-then-random phase encryption scheme for secure transmission of 25 Gb/s intensity-modulated signal. Commercial low cost continuous-wave laser is employed as the optical source, and a chirped fiber Bragg grating followed by a phase modulator is utilized as a two-dimensional encryption module. Successful encryption and decryption of 25 Gb/s confidential data have been achieved by optimizing optical hardware parameters. The demonstrated system can combine the advantages of hardware and digital encryption, exhibiting great potential for providing physical layer security in future high-speed secure optical communication systems.

High-speed and robust optical beam-steering device will be the key component for various applications, such as LiDAR (light detection and ranging) and free-space optical communication. Optical phased arrays (OPAs) integrated on semiconductor chips have recently received increasing attention due to the high-speed operation, compactness, and low cost. In particular, indium phosphide (InP)-based OPAs are advantageous for high-output-power applications at 1.55- $\mu \text{m}$ eye-safe wavelength, owing to their capability of monolithically integrating active components, such as high-power lasers and optical amplifiers. In this letter, we design and fabricate, to the best of our knowledge, the largest-scale InP-based OPA and experimentally demonstrate its beam-steering operations. The fabricated OPA consists of 100 waveguides with carrier-injection-based phase shifters, densely integrated on 7 mm $\times5$ mm footprint. A focused beam width of 0.11° is steered across the free spectral range of 8.88°, corresponding to more than 80 resolvable points. This is the largest number of resolvable points obtained by a monolithic InP OPA. The response time of the OPA is confirmed to be less than 16 ns, which is limited by the driver circuit. This work paves the way for realizing compact beam-steering modules for high-speed and high-output-power imaging applications.

Avalanche photodiodes (APDs) are used in optical receivers of high-speed optical communication systems to improve signal-to-noise ratio over conventional photodiodes. Low excess noise characteristics are crucial for APDs to preserve the benefits associated with high internal gains. In this work, we presented room temperature data of avalanche gain and excess noise factors of Al0.85Ga0.15As0.56Sb0.44 APDs using pure and mixed carrier injection profiles. Using pure electron injection, the best possible excess noise performance for a given avalanche width was measured with an excess noise factor https://doi.org/10.15131/shef.data.15082455 )

Photonic reservoir computing (RC) enables us to introduce artificial neural networks (ANNs) as a deployable hardware-based processing unit beyond the von Neumann architecture. Its unique approach to consider only the output layer weights as adaptive, reduces training complexity and enables the use of ANNs in time-sensitive areas like optical communications. In this paper, we numerically investigate and evaluate the requirements of a photonic RC using an active silicon micro-ring resonator as a photonic integrated circuit (PIC) based reservoir system to compensate for transmission impairments such as chromatic dispersion and nonlinear effects occurring in optical transmission systems. We compare the performance of this RC with results from digital signal processing using Kramers-Kronig reception of distorted PAM-4 symbols from a 56 GBaud WDM transmission experiment over 100 km uncompensated SSMF.

The high data rates enabled by free-space optical communication links and additional factors, such as the unregulated spectrum, make them an attractive solution in future geostationary feeder link applications with capacities beyond one Terabit per second. However, optical links through the atmosphere are heavily affected by atmospheric turbulence, leading to significant scintillation. Transmitter diversity is an effective way to improve the signal quality of optical links. Transmitter diversity with Phase-Division in Bit-Time is a novel scheme for mitgating interference and thus enabling the use of equal wavelengths for multiple transmitters. This paper will evalute the Phase-Division in Bit-Time transmitter diversity scheme and present measurement results from a lab testbed.

Bit-error rate (BER) performance of free-space optical communication (FSO) systems is compromised by pointing errors. Few-mode fiber (FMF) based receiver emerges as a promising solution for FSO, because FMF can obtain high coupling efficiency in the presence of pointing errors. The high coupling efficiency allows FMF based system to achieve improved BER performance. In this letter, based on a simplified mode demultiplexer model, we propose a method to use the coupling efficiencies of each mode of FMF to characterize the BER of the FMF based FSO system having nonzero boresight pointing errors. We find that the boresight displacement can significantly deteriorate the BER performance, and the jitter variance can reduce the impact of boresight within a suitable range.

We propose and experimentally demonstrate the mitigation of adverse effects of scintillation using a spectrum-sliced incoherent light source based on gain-saturated reflective semiconductor optical amplifiers (RSOAs). This incoherent light source, originally developed for the cost-effective realization of wavelength-division-multiplexed passive optical networks, employs RSOAs for both data modulation and the suppression of large excess intensity noise inherent in incoherent light sources. We show through experiment that this incoherent light source is effective in suppressing the power fluctuations induced by scintillation when compared to a coherent light source, and thus improves the receiver sensitivity of free-space optical communication systems under weak atmospheric turbulence.

We have demonstrated direct generation of high-order orbital angular momentum (OAM) modes by chiral long-period gratings (CLPGs) written in a few mode fiber (FMF) via a CO2 laser. The high-order core modes (LP11, LP21, and LP31 modes) were realized by the CLPGs through high order diffraction modes with efficiencies of more than 99.69%. The -1st, −2nd, and −3rd order OAM modes can be generated directly by the CLPGs with various grating periods. The proposed method opens a new path to implement all fiber high-order OAM mode generation. The OAM mode converters can be adopted as wavelength tunable mode converters by adjusting twist rate of the CLPGs, which could be widely used in optical communication and fiber sensing system based on FMFs.

We report on the experimental observation of wavelength-switchable bound solitons in a passively mode-locked erbium-doped fiber ring laser based on a tapered graded-index multimode fiber. Due to the multimode interference filtering effect, by adjusting the polarization controller while maintaining the pump power of 158 mW at 1480 nm, the center wavelength of the fiber laser output can be switched between 1565 nm and 1595 nm. At the same time, bound soliton pairs and bound soliton triplets can be both observed in these two wavelengths. Such a simple, compact all-fiber mode-locked laser with switchable wavelength has great potential for high capacity optical communication.

A multi-wavelength Brillouin-Raman random fiber laser (BRRFL) with wavelength spacing of double Brillouin frequency shift (BFS) has been proposed for the first time, to the best of our knowledge. This BRRFL has an extremely simple full-open cavity configuration made up of only one roll of fiber, where bidirectional stimulated Raman/Brillouin scattering (SRS/SBS) and Rayleigh scattering based random distributed feedback coexist. In order to further enhance the performance of the BRRFL, a combination of erbium-doped fiber amplifier and single-mode fiber is incorporated into one side for random lasing regeneration. Both the side-mode suppression ratio (SSR) and order number of generated Brillouin Stokes lines (BSLs) show some improvements. A total of 66 BSLs with double BFS spacing has been obtained. Furthermore, the demonstrated laser system offers a wide tunable range with excellent flatness of spectral envelope. The proposed laser featuring simple structure and double BFS spacing would enable promising potentials in optical communication and sensing systems.

A spatial light modulator (SLM) unit by using polycrystalline silicon (poly-Si) as resonant thin film is demonstrated here in order to improve the extinction ratios in C band (1530 to 1565 nm). The curves of the reflectivity of SLM unit versus wavelength with voltage OFF and ON are simulated and verified by experiments. The SLM unit presents high extinction ratios in C band with maximum of 42.2 dB at 1547.5 nm. The high extinction ratio is due not only to the excellent thermo-optic effect but also to the thermo-elastic deformation effect of the poly-Si thin film. The resonance wavelength has a red shift of 2 nm. The high-extinction-ratio C-band spatial light modulator unit has a giant application potential in the space optical communication field.

Achieving high speed physical layer security is a constant pursuit but critical challenge for the information society. In this paper, a novel symbol-by-symbol optical phase encryption technique relying on ultra-long, reconfigurable optical phase patterns and commercial off-the-shelf dispersive components is proposed for high speed physical layer security. A record 40Gb/s secure optical communication system with symbol overlapped, optical phase encrypted differential-phase-shift-keying modulated signal is experimentally demonstrated based on the proposed technique. Security robustness against various eavesdropper’s attacks has been validated for three optical codes with a chip rate of 40Gchip/s and variable code-length of 128-chip, 512-chip and 1024-chip, respectively. The demonstrated technique exhibits the advantages of supporting high bit rate operation and advanced optical modulation formats, improving code flexibility and cardinality, which makes it very promising for future ultra-fast secure optical communication.

A frequency-modulated continuous-wave (FMCW) coherent lidar with downlink communication capability is proposed based on phase-diversity coherent detection, by which laser ranging, laser velocimetry and free-space optical downlink communications can be simultaneously achieved. In the local transceiver, a linear-frequency modulated (LFM) optical signal is generated and used as the lidar signal. The LFM optical signal is received by the remote transceiver and coded with data via amplitude modulation. Then, the light is transmitted back to the local transceiver, in which a phase-diversity coherent optical receiver is used to simultaneously extract the range, velocity, and communication data. The in-phase and quadrature outputs from the phase-diversity coherent optical receiver are simultaneously recorded to reform a complex signal. The data is extracted from the intensity of the complex signal, while the range and velocity are obtained from the argument of it. A demonstration experiment is carried out. Different digital baseband signals are encoded on the reflected signal via an intensity modulator, while the 80-MHz Doppler frequency shift is simulated by an acousto-optic modulator. The impact of the digital signal on lidar detection is removed. Meanwhile, the communication quality is little affected by the varied frequency of the de-chirped signal. Therefore, the lidar detection and the free-space optical communications are simultaneously implemented.

In this study, Fabry-Perot quantum well lasers with shortened cavity designs (25 and 50 μm) were fabricated. The devices had dry-etched and cleaved facets and exhibited an output power of up to 2.36 mW for a cavity length of 25 μm; good spectral performance was achieved. Single-optical-mode operation with better than 25dB of side mode suppression ratio can be achieved for the fabricated devices with 25- and 50-μm cavities. The short-cavity devices also exhibited highspeed modulation at elevated temperatures. Moreover, 25 Gb/s eye diagrams could be obtained at 65°C for the 25μm device and 55°C for the 50μm device. Due to their small area, ultra-short-cavity Fabry-Perot laser can be suitably used in next-generation light sources for optical communication.

We report the characterization of continuous-wave four-wave mixing (FWM) in SM1950 and SMF-28 silica fibers in the 2 μm wavelength band. Maximum conversion efficiencies of -21 dB and -25 dB are achived for SM1950 and SMF-28 fibers, respectively. In order to explore multichannel wavelength conversion at 2 μm, we characterize the wavelength conversion of a three-tone signal, which is generated by intensity modulation of a laser using a 10 GHz radio frequency (RF) signal. For all the three channels, we achieve a conversion efficiency of -25 dB using a 21.8 dBm pump. Recent demonstrations of efficient splicing of hollow-core fibers, which have been proposed for optical communications at 2 μm, with silica-based fibers paves the way for all-optical processing of multi-channel signals such as orthogonal frequency-division multiplexing (OFDM).

Tunable coherent light source emitting at the visible region has long been recognized as a key asset, especially with a monolithic design involved, for the increasing number of emerging applications in the scientific research and industrial manufacturing. Here, an efficient 640 nm solid-state laser is used as a pump source, and the fiber end-facet mirrors construct a fiber laser resonator. We experimentally developed green wavelength-tunable Ho3+-doped all-fiber upconversion lasers, which corresponds to the transitions originating from the Stark levels within thermally-coupled 5 S2, 5 F4 states and terminating on the 5 I8 ground state. Moreover, a high power 236 mW and a high efficiency 38.9% of green upconversion fiber laser operating at 544 nm was demonstrated, and the output power achieved is six-times higher than before. Such compact tunable and high power green fiber lasers are very attractive for the practical applications in optical communications, spectroscopy, sensing, and optical switching devices.

Multi-point free-space optical communication (FSOC) has recently received extensive interest as a valuable solution for providing increased capacity in the upcoming communication networks. Diverse optical multiple access control (O-MAC) techniques have proved to successfully support multi-user FSOC. However, to adaptively and automatically select the most fitting O-MAC technique or combination thereof for signal demodulation, the number of transmitting users should be known a priori at the receiver side. In this letter, a technique based on histogram peak detection and unsupervised machine learning (e.g., k-mean, k-medoid, hierarchical, and fuzzy clustering), is proposed and experimentally demonstrated for number of users detection. The proposed methodology relies on amplitude information of received signals. Results show that when multiple users’ signals are received with equal amplitude, under-estimation of user number was observed; hence, a modified approach—a weighted clustering—was employed and experimentally validated. The work reported herein demonstrated the ability of the proposed methodology to simultaneously and accurately detect multiple transmitting users in real time and under severe atmospheric turbulence.

A simple all-fiber Solc filter (AFSF), which consists of two 45°-tilted fiber gratings (45°-TFGs) UV-inscribed in the polarization-maintaining fiber (PMF) and a series of PMF cavities, is proposed and demonstrated. The performance of the proposed filter has been theoretically simulated and experimentally verified. Both the simulated and experimental results show that the bandwidth of the filter could be tuned by the PMF sub-cavity length and the number of PMF cavities. And the free spectral range (FSR) only depends on the sub-cavity length. As a proof of that, the bandwidths of AFSF with different number of PMF sub-cavity (N=2, N=3, N=4) and the same PMF sub-cavity length of 30cm are 4 nm, 2.6nm, and 2nm, respectively. The FSRs of 3-stage AFSF with different PMF sub-cavity length (L=20 cm and L=40 cm) are 15.3 nm and 7.97 nm, respectively. Furthermore, we have also investigated the tunability of the AFSF by controlling the temperature of PMF cavity with a tuning sensitivity around 1.205 nm°/c. Compared with existing fiber-optic Solc filters, the AFSF with prominent advantages such as extremely simple and robust structure, thermal tunability in wavelength, and low cost will bring a bright future for applications in optical communication and sensing systems.

We experimentally demonstrate a real-time polarization tracking and equalization method based on the state of polarization detection from the adaptive finite impulse response (FIR) filter coefficients in a polarization division multiplexed (PDM) coherent optical communication system. By calculating the variation of the polarization within a time interval, the polarization rotation speed can be evaluated. In a field-programmable gate array (FPGA), a feedback structure for polarization rotation speed detection and FIR coefficients update can be built up. In this feedback structure, the update coefficient of the filter will be adjusted to an optimal value with the change of detection result. The experimental results show that the establishment of this feedback structure improves the polarization tracking ability in the real-time coherent optical receiver.

Photodetectors made of amorphous materials enable low-cost optical imaging and communications over non-semiconductor platforms. The key challenges are to improve efficiency, sensitivity, and frequency response. Using the localized surface plasmon resonance (LSPR) effect and an efficient carrier multiplication process, cycling excitation process (CEP), the plasmonically enhanced amorphous silicon photodetector (PEASP) with a thin (60 nm) absorption layer achieves a high external quantum efficiency with a record fast impulse response of 170 ps (FWHM). This approach offers the possibility of making detectors out of amorphous material for high frame rate imaging and optical communications in spite of the material’s low carrier mobility.

Here, we report embedded spiral zone plates (SZPs) processed by tightly focused femtosecond pulses, inducing a refractive index change in a sapphire crystal. The change in the index is investigated both experimentally and theoretically and found to be 0.93% lower than that before the laser irradiation. Benefiting from the investigation of the photorefraction, a high diffractive efficiency of up to 78% is achieved for the SZPs. In addition, the SZPs are embedded under the surface of the sapphire, which ensures that the components possess stable optical properties in the case of external refractive index changes. The high-performance elements having a high diffractive efficiency and stable optical performance will have tremendous applications in optical communication and optical manipulation.

When the near-infrared femtosecond Bessel laser beam (fs-BLB) is obliquely incident on the silica glass, annular beams (ABs) with bandwidths of several tens of nanometers and different peak wavelengths discretely distributed in a visible and near-infrared range are generated simultaneously. It is considered that four-wave mixing (FWM) and the corresponding cascaded parametric processes are responsible for the generation of multi-wavelength ABs. It is also found that the broken symmetry of the optical setup due to the obliquely oriented silica glass and the absorption band of the sample significantly enhance the generation efficiency of ABs. Since the diffraction angles of these colorful ABs mainly depend on their wavelengths and are insensitive to the input laser energy, this technique may find important applications where multi-wavelength ultrashort pulses are needed. For example, these multi-wavelength ABs can be used as light sources in nonlinear dark-field microscopy and multiphoton imaging to further suppress the background noise and improve resolution. They can also be used in multidimensional multiplexing optical communications.