Tilted Bragg grating (TBG) has been intensively investigated for photonic devices such as fiber sensors and on-chip filters. In this paper, we present that TBG can be equivalently realized by designing tilted sampled Bragg grating (TSBG), which is formed by superimposing the pre-designed tilted sampling structure on the basic grating. Since the 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 another step of micro-lithography. We designed and simulated the equivalent light responses of the TSBGs as that of the target TBG in the photonic devices. As examples, a mode converter based on TSBG has been proposed, which can be used for add-drop filter in the wavelength division multiplexing (WDM) technology combined with mode division multiplexing (MDM) technology. We also investigate the core-to-cladding mode coupling of TSBG in single-mode fiber for fiber sensing as well as the spatial filter in the α- DFB laser. The proposed technique may pave a new way for different 2D gratings with good design flexibility and easy fabrication in various photonic devices.
Recent advances in monolithically integrated multi-section semiconductor lasers (MI-MSSLs) have propelled microwave photonic technologies to new potentials with a compact, reliable, and green implementation. Much research has examined that MI-MSSLs can realize the same or even better microwave photonic functions compared to discrete lasers by taking advantages of enhanced light–matter interactions. They are beneficial towards the future of integrated microwave photonics (IMWP) once integrating the other optical components such as modulators, amplifiers, transmission waveguide and so on. Herein, these recent advances in this emerging field are reviewed and discussed. Three main kinds of MI-MSSL structures are demonstrated including passive feedback laser, active feedback laser, as well as monolithically integrated mutually injected semiconductor laser. Their pros and cons are distinguished and compared through analyzing the desired characteristic indicators in modern MWP subsystems. The focus of this paper is on the photonic microwave techniques based on the nonlinear dynamics of MI-MSSLs, consisting of electro-optic conversion characteristics enhancement, photonic microwave generation, microwave photonic filter, as well as multiwavelength laser array for wavelength division multiplexing radio-over-fiber (WDM-RoF) networks. We also take a look at the future prospective at the research directions and challenges in this area.
We propose and fabricate a linear frequency-swept DFB laser array based on the reconstructed-equivalent-chirp (REC) technique used in sensing system. During the fabrication process of the laser arrays, the reconstructed-equivalent-chirp technique is utilized to simplify the fabrication of the grating and precisely control the grating phase. A semiconductor optical amplifier (SOA) is monolithically integrated to enhance and balance the output optical power. The module achieves a wavelength range of more than 3 nm by covering 4 channels with an interval of 0.8 nm. The side mode suppression ratios (SMSRs) of all channels are above 50 dB and the output power are guaranteed above 10 dBm with the SOA providing 14 dBm saturation output power. To tune the wavelength on the microsecond scale, we adopt a combination of a MCU and a FPGA as the controlling core to turn on and off the driving current of all the 4 lasers on the DFB laser array, and the switching time between 2 channels is well controlled within 50 ns. At the same time, the module makes the wavelength output linearly with the current through the filter circuit, and achieves the sweep speed of 100 nm/s. This sweep speed, sweep range, output power, and good single-model performance meet the needs of sensing system for light sources.
Fast tunable lasers with switching time less than one microsecond are key components in high-speed optical switching networks. In this paper, we propose an effective method to achieve high wavelength switching speed by turning on/off individual lasers of a matrix-grating DFB laser array. The laser array consists of 16 DFB lasers, which are arranged as a 4-by-4 matrix. Besides, the REC technique is used to simplify the fabrication of the grating and precisely control the grating phase. 16 channels with 2.4-nm-spacing are obtained and the SMSRs of all the 16 channels are above 40 dB, indicating good single mode operation. A high-speed driving circuit is designed to supply stable direct current for the DFB laser array and to control the switching process. The experimental result shows that the switching time between 2 channels is less than 100 nanoseconds.
The recently emerged photonic integration technology based on thin-film lithium niobate (LN) have been regarded as a very promising candidate for advanced photonic integrated circuits (PICs) due to its attractive nonlinear properties, wide-spread use in electro-optic applications, and etc. Generally, the thin-film LN optical waveguide used in PICs is sub-micrometer scale. Mode mismatch between fiber and sub-micrometer LN waveguide in chip is the main factor of increasing the fiber-to-chip coupling loss and the total insertion loss of LN PICs. Therefore, for practical applications, low-loss mode size converter for coupling between fiber and sub-micrometer LN waveguide is essential. In this paper, an efficient and novel fiber-to-chip mode size converter for thin-film LN PICs was designed and fabricated. The converter consists of a LN nano-taper and a cantilevered SiO2 waveguide. The nano-taper is embedded in the center of SiO2 waveguide. Laterally connected SiO2 cantilever beams are fabricated to provide structural support for the cantilevered SiO2 waveguide. Our work provides an efficient way to realize low-loss fiber-to-chip interface for thin-film LN PICs.
This paper presents our recent studies on photonic devices based on antisymmetric Bragg gratings (ASBGs). ASBGs can realize backward mode conversion between the fundamental and first order transverse electric mode, i.e., TE0 and TE1. If a π phase shift is inserted in the middle of ASBG, light resonance along with mode conversion can be set up. We call this new resonance as the “Hybrid mode resonance”. Based on this structure, several photonic devices are realized. For example, we experimentally demonstrated an on-chip light filter with dropped reflected light. It can be equivalent to the light circulator in some cases and benefits the application to photonic integrated systems. Besides, other photonic devices such as single wavelength resonator and narrow band reflector are also realized. We think the proposed grating structure may give a new way to design high-performance photonic devices.
A weak double-peak fiber Bragg grating (FBG) temperature sensor is proposed and demonstrated. Wavelength-swept tunable laser is regarded as one of the most popular demodulation methods for fiber Bragg grating (FBG) sensors. However, due to the limitations of the existing tunable laser technologies, a fast, compact, stable and low-cost tunable laser for FBG sensors is still unavailable, which will become one of the major barriers for more widespread applications of FBG sensors. To further improve the efficiency and accuracy of the FBG interrogation system, a FBG temperature sensor is proposed and demonstrated by using tunable laser and a weak double-peak FBG. Since the reflection of the weak double-peak FBG has two main reflection peaks and relatively wide bandwidth, it is convenient to track the two characteristic peaks to accurately obtain the wavelength shift during the alteration of ambience temperature. A proof-ofconcept experiment is also conducted to verify the theory. By demodulating a weak double-peak FBG in the temperature experiment, a sensor sensitivity of 10.17 pm/ °C is measured for the proposed interrogation system.
Dual-wavelength semiconductor lasers have various potential applications in microwave photonics and laser radars fields. Monolithic integrated dual-wavelength DFB laser with equivalent-chirp sampled grating is proposed and investigated theoretically in this paper. The grating of the dual-wavelength DFB laser is designed by the reconstruction-equivalentchirp technique. The effects of the grating with different phase-shifts and different chirp ratios on the characteristics of the dual-wavelength DFB laser are theoretically demonstrated. The results proposed in this paper have great reference value for fabricating dual-wavelength semiconductor lasers.
Proc. SPIE. 11192, Real-time Photonic Measurements, Data Management, and Processing IV
KEYWORDS: Demodulation, Fiber lasers, Sensors, Temperature metrology, Frequency modulation, Sensing systems, Fiber Bragg gratings, Single mode fibers, Radio optics, Reflectors
A multi-longitudinal mode (MLM) laser sensor system based on software radio demodulation is proposed, which realizes the simultaneous measurement of vibration and temperature. The software demodulation is realized through the SDR (Software Defined Radio) technique due to the applied vibration can be analyzed as the superposition of a series of frequency modulation (FM) signals. Furthermore, the absorption of erbium-doped fiber (EDF) decreases with the increase of temperature, so the output power of the laser can be measured as another sensing signal. As a result, by demodulating these two sensing signals, the vibration from 10 Hz to 2 kHz can be successfully measured and the sensitivity of the temperature is around 0.011 dBm/°C.
Proc. SPIE. 11192, Real-time Photonic Measurements, Data Management, and Processing IV
KEYWORDS: Switching, Semiconductor lasers, Field programmable gate arrays, Switches, Fabrication, Analog electronics, Transform theory, Digital electronics, Feedback control, Time metrology
We propose and fabricate a rapidly wavelength switching DFB laser array based on Reconstructed-Equivalent-Chirp technique. A Semiconductor Optical Amplifier(SOA) is applied to enhance and balance the output optical power. The module covers 8 channels from 1554.5nm to 1566.1nm with an interval of 1.6nm. To tune the wavelength on the microsecond scale, we adopt a combination of a MCU and a FPGA as the controlling core to turn on and off the driving current of 8 lasers on the DFB laser array through a collector feedback circuit, and the switching time between 2 channels is well controlled within 300ns. The side mode suppression ratios(SMSRs) of all channels are above 50dB and the output power are guaranteed above 10dBm with the SOA providing 14dBm saturation output power.
A parallel hybrid-integrated optical injection locking (PHOIL) DFB laser with narrow linewidth is experimentally demonstrated by using REC technology. The linewidth of the PHOIL DFB laser is significantly narrowed under the injection locking state. The linewidth of the PHOIL DFB laser is measured to be under 100 kHz, compared with 1034.2 kHz under free running state. The narrowest linewidth of the PHOIL DFB laser can be as low as 31.9 kHz. The nonlinear distortions of the PHOIL DFB lasers, including third-order intermodulation distortion (IMD3) and spurious-free dynamic range (SFDR), are also suppressed significantly. The IMD3 is reduced by 4.33 dB and SFDR is also increased from 77.63 dB·Hz2/3 to 82.71 dB·Hz2/3.
In recent years, with the development of eavesdropping technology, how to improve the security of data transmission has become a hot research issue. In order to enhance the security of the secure communication system, a chaotic laser secure communication system with variable laser power is proposed in this paper. Based on the original optoelectronic feedback chaotic laser communication system, the change of laser power is regarded as a new key. The simulation results show that the error rate of the eavesdropper is above 10-2 over most of the laser power, and the average error bit rate of the eavesdropper is 7 orders of magnitude higher than the average error rate of the authorized receiver. In addition, the appropriate masking efficiency will reduce the bit error rate of the eavesdropper, while it has little effect on the eavesdropper. The system scheme can be used in communication with high confidentiality requirements in the future.
A compact cascaded tunable distributed Bragg reflection (DBR) semiconductor laser is proposed and simulated. Each laser section (LS) is formed by two passive adjacent grating sections (GSs) with slightly different Bragg wavelengths and an active section (AS) between them. A step-wise grating period profile is designed to realize wide range 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 four GSs and three ASs was designed and analyzed, resulting in a continuous tuning range of 13.2 nm. Furthermore, an improved structure with apodized grating in each GS is proposed for good single mode property. The single mode stability and fabrication tolerance are significantly improved. Particularly, this structure based lasers has a fast switching speed of about 5ns. The proposed structure would benefit the practical applications to the low cost tunable lasers in wavelength division multiplexing (WDM) systems.
A wavelength-tunable small form-factor pluggable (SFP) optical module is proposed and implemented, which is based on a self-designed 4-channel DFB laser array. The module adopts the widely used SFP packaging standard so that it is convenient to connect with other devices. It has an I2C interface for receiving wavelength tuning commands and downloading digital diagnostics monitoring information to the host processor. Three parts are included: the receiver, the transmitter and the microcontroller unit, to complete the conversion of optical-electro, electro-optical. A large range and high precision wavelength tuning is realized through innovative tuning methods. Two wavelength tuning methods are utilized: channel switching of 4-channel for coarse tuning and temperature tuning combined with current tuning for fine tuning to actualize the tunable output of the DFB laser array. This wavelength-tunable SFP optical module can replace several fixed wavelength optical modules in a traditional WDM system, thus greatly reducing costs and improving the utilization ratio of resources. Experimental results show the SFP optical module can achieve the continuity of wavelength tuning covering 1539.0 nm to 1551.0 nm. It can switch over 16 channels in a 100G-DWDM system or 31 channels in a 50G-DWDM system. The side mode suppression ratios (SMSRs) of most channels are above 40dB over the wavelength tuning range of 12 nm. The optical signal transmission rate is up to 1.25Gbps.
High-quality microwave generation and frequency up-conversion are demonstrated utilizing a photonic integrated two-section DFB laser. Both the DFB lasers are fabricated by the reconstruction-equivalent-chirp (REC) technique. We acquire microwave signals by optical heterodyne. High-quality microwave signal can be generated by the optical injection locking technique with low phase noise of -96.3 dBc/Hz at 10-kHz and narrow linewidth of a few kHz level. Besides, Frequency-doubled and frequency-quadrupled signals are achieved respectively.
An innovative demodulation system for multilongitudinal mode fiber laser sensor has been proposed. By using a bandpass filter and a low-speed analog-to-digital converter (ADC), the high-frequency sensing signal can be downconverted and sampled simultaneously when the unaliasing condition is satisfied. Since the MLM fiber laser sensor could generate a wideband electrical signal after optical-to-electrical conversion, it is convenient to filter the signal to meet the unaliasing condition by a tunable bandpass filter while keeping the sample rate unchanged. Moreover, each tone of the beating frequency signal has the full information for demodulation of measurand. The demodulation system only needs a bandpass filter and a low-speed ADC which reduces the cost of the system and make the system more stable. A proof-of-concept experiment is conducted to verify the proposed scheme. Eventually by demodulating a beat frequency in 1.625GHz, a sensitivity of -5.87kHz/°C is achieved in a fiber laser sensing system with a sample rate of 500MHz.
We designed and experimentally studied a sampled Bragg grating semiconductor laser with π equivalent phase shift (EPS) and three equally separated electrodes. When the central electrode is injected different current from the other electrodes, a distributed phase shift (DPS) can be introduced into the studied laser. By changing the injection current ratio into three electrodes, the DPS can be controlled and then the lasing wavelength can be tuned while the laser keeps single longitudinal mode operation.
Reconstruction equivalent chirp (REC) technique is widely used in the design and fabrication of semiconductor laser arrays and tunable lasers with low cost and high wavelength accuracy. Bent waveguide is a promising method to suppress the zeroth order resonance, which is an intrinsic problem in REC technique. However, it may introduce basic grating chirp and deteriorate the single longitudinal mode (SLM) property of the laser. A nonlinear equivalent chirp pattern is proposed in this paper to compensate the grating chirp and improve the SLM property. It will benefit the realization of low-cost Distributed feedback (DFB) semiconductor laser arrays with accurate lasing wavelength.
We studied the in-line DFB tunable laser based on REC technique. We mapped the contour lines of the P-I contour diagram of this in-line tunable laser into the wavelength-current contour map. And the maximum output power is obtained.
Also the tuning currents are obtained. Firstly, we simulated the P-I contour as well as wavelength-current contour. Secondly, we experimentally demonstrated the mapped
contour.
A fiber Bragg grating (FBG) wavelength shift demodulation method based on tunable distributed feedback laser diode (DFB-LD) is proposed. A highly effective algorithm is proposed to demodulate the strain of the sensing FBG, which is scanned by the tunable DFB-LD. The system is unique in that accurate demodulation can be achieved by scanning only a 0.4-nm bandwidth FBG spectrum instead of relying on scanning the FBG’s principal power peak and the scanned spectrum can be any part of the FBG. Thus, the demodulation system is much more simplified in comparison to a traditional demodulation system, which enables using wavelength-division multiplexing for the FBG sensing network. With the strain experiment of the FBG, the results match the theory of the proposed demodulation method very well, and the errors of the strain experiment are within 2 pm. The tunable DFB-LD is based on reconstruction-equivalent chirp (REC) technology. The REC technology potentially offers a low-price, compact, and high-performance solution for the demodulation.
An asymmetric sampled Bragg grating (SBG) semiconductor laser, which consists of two sections with same length but different sampling duty cycle, can be introduced an arbitrary equivalent-phase-shift (EPS) into its center. At the same time, to adjust the sampling duty cycles in the two sections as different magnitude, the studied laser can output more lasing power from its one facet than that from the other one. That is to say, this method can be used to design and fabricate the EPS SBG semiconductor laser with higher output efficiency.
A dual-wavelength laser array is obtained by two asymmetric phase shifts. Different wavelength spacings are obtained by varying the magnitude of the phase shifts. The phase shifts are distributed along two phase-arranging regions, which are obtained equivalently by specially-designed sampled structures with uniform seed gratings.
A laser array, which is consisted of 56 π equivalent phase shift (EPS) sampled Bragg grating (SBG) semiconductor laser, is experimentally investigated. The experimental results show the influence of the sampling duty cycle fabrication error on the lasing wavelength of an SBG semiconductor laser can be ignored.
A simple wireless-fiber laser sensor is proposed base on directly photonic generation
of microwave beat signal. In this scheme, a multi-longitudinal modes fiber laser is formed by two fiber Bragg gratings and a section of erbium-doped fiber. Two same 2G-GSM mobile antennas are used as wireless transmitter and receiver. By this method, the real-time monitoring of fiber laser sensors can be achieved through over ultra-long distance. This technique offers a simple, all-electrical and cheap way for fiber sensor information accessing wireless net. The experiment result shows the root mean square deviations of the sensor are about 4.7 με and 6.7 με at 2.38 GHz before and after wireless transmission, respectively.
KEYWORDS: Sensors, Fiber lasers, Fiber Bragg gratings, Fiber optics sensors, Signal to noise ratio, Laser resonators, Modes of laser operation, Reflectors, Signal detection, Signal generators
A multilongitudinal mode fiber ring laser sensor is proposed and experimentally demonstrated by measuring the strain applied on the laser sensor head. The ring cavity of the laser is formed by a 3-dB coupler, a section of erbium-doped fiber, and one fiber Bragg grating. Photonic generation of beat signals and strain measurement theory are discussed in detail. The strain applied on the fiber ring cavity is obtained by measuring the beat frequency shift. The selection way of the optimal beat signal for strain measurement is obtained by experimental research and discussion. The root-mean-square deviation of the strain and the response of beat frequency to the strain are 2.7 μɛ and 1.5 kHz/μɛ at 1993 MHz, respectively. The proposed sensor scheme offers a cost-effective and high-stability device for strain measurement.
A special sampling structure based on the double exposure technology is proposed to achieve dual-wavelength lasing in
the distributed feedback (DFB) fiber laser. This structure is composed of two grating pitches in one sampling period,
which could be realized by changing the fiber's length in the fabrication. And through employing an equivalent phase
shift, only a submicrometer-level precision is required for precise phase control. Then a stable dual-wavelength laser
with the spacing of 400pm is obtained in the experiment successfully. The output power is 30.46uW and the SMSR is
46dB under a pumped power of 146mw.
A corrugation-pitch-modulated (CPM) distributed feedback (DFB) semiconductor laser based on
reconstruction-equivalent-chirp (REC) technology is first experimentally demonstrated. The laser can suppress
spatial hole burning (SHB) effectively and operates in good stable single longitudinal mode even the injection
current very high.
A multi-longitudinal-mode fiber laser sensor is proposed and experimentally
demonstrated base on beat frequency demodulation method. A novel laser cavity is formed by a
FBG, a section of erbium-doped fiber and a broadband reflector. The proposed laser sensor has
ultra-stable frequency information due to self-phase matching of FBG, and high signal to noise
ratio.
Based on a special sampling technique, we present a special structure for distributed feedback (DFB) lasers. As a
conventional equivalent π phase-shift sampling Bragg grating (SBG), this proposed structure also introduces an
equivalent π phase-shift into both its ±1st order channel. Combined with the conventional equivalent π phase-shift, the
sampling technique can be used to design and fabricate multiwavelength semiconductor or fiber lasers conveniently.
A distributed feedback (DFB) semiconductor laser with multiple phase shifts based on reconstruction equivalent chirp
(REC) technology is numerical studied and fabricated. The simulation results show that the performances of the multiple
phase shifts DFB semiconductor laser based on REC technology are nearly the same as the actual multiple phase shifts
DFB laser. They have the same P-I curves, the internal power distributions and the output ASE spectra. However, it only
changes the sampling structures of the REC based laser with the uniform seeding waveguide grating. So the fabrication
of such laser is very easy. In this paper, the fabrication of this structure was realized for the first time to the best of our
knowledge. The experimental results show that it has good single longitudinal mode operation even under high injection
current with side mode suppression ratio (SMSR) above 55dB even at high injection current.
A fiber distributed feedback laser accelerometer base on the double flexure strips structure is designed and
experimentally studied. In this paper, the vibration response of this double flexure strips structure is theoretically
analyzed and simulated. The ripple response of the proposed sensor is less than 1dB from 50 Hz to 350 Hz. The
experimental results show that the sensor has a higher sensitivity of 1591rad/g at 400Hz by introducing a 3x3 coupler
combining with an unbalanced Mach-Zehnder fiber interferometer demodulator. Signal-to-Noise and cross-sensitivity
coefficient of the sensor signal are about 43 dB and 10 dB respectively.
We propose and experimentally demonstrate a novel FBG dual-wavelength fiber laser sensor based on the beat
frequency demodulation technology. The dual-wavelength beat frequency sensing signal of about 5.224 GHz has been
obtained in a photodetector and observed by a radio-frequency spectrum analyzer (RFSA). Furthermore, by employing a
LiNbO3 modulator, the high-frequency beating signal can be tuned arbitrarily to tens or hundreds of MHz without
distortions. Thus a very cheap and low-frequency RF spectrum analyzer can be used in frequency signal detection. When
a strain is applied on the sensor, the beating signal will shift with a stain sensitivity of about (-3.92) kHz/με.
To our knowledge, this is the first report of a monolithically integrated distributed feedback (DFB) semiconductor laser
array based on reconstruction-equivalent-chirp (REC) technology. A laser bar with 30 different lasers is obtained, lasing
at 30 different wavelengths under single longitudinal mode. The typical threshold is about 40mA to 60mA. The typical
slope efficiency is about 0.07 mW/mA to .13 mW/mA. Tested under the injection current of 100mA, the side mode
suppression ratios (SMSR) range from 24.9dB to 46.8 dB, with an average of 37.2dB. The proposed method is presented
in hoping to make a positive contribution to large-scale photonics integrated circuits (PIC) research for the nextgeneration
fiber-optic networks.
A complex-coupled DFB 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 1.5387μm in the experiment. The typical
threshold current of the device is 30mA, and the optical output power is about 10mW at the injected current of 100mA.
Instead of real phase shifts, equivalent phase shifts (EPS) are adopted to construct ultra narrow phase-shifted band-pass
filer in sampled Bragg gratings (SBG). Two optimized distributions of multiple equivalent phase shifts, using 2 and 5
EPSs respectively, are given in this paper to realize flat-top and ripple-free transmission characteristics simultaneously.
Also two demonstrations with 5 EPSs both on hydrogen-loaded and photosensitive fibers are presented and their
spectrums are examined by an optical vector analyzer (OVA). Given only ordinary phase mask and sub-micrometer
precision control, ultra-narrowband flat-top filters with expected performance can be achieved flexibly and cost-effectively.
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