Feedback Laser Research Articles

Distributed feedback laser with methylammonium lead bromide embedded in channel-type waveguides

Distributed feedback (DFB) lasing is achieved with CH3NH3PbBr3 perovskite embedded in channel-type waveguides. By using a focused ion beam, DFB gratings are engraved within the bottom surface of channel-type waveguides etched on a SiO2/Si wafer. The oriented CH3NH3PbBr3 crystallized inside the waveguide with the DFB gratings shows second-order DFB lasing mode in lateral and normal directions above a threshold excitation density of 47 μJ cm−2 which is one order of magnitude lower than that in the non-DFB laser. The splitting DFB peaks are well reproduced by finite-differential time-domain calculation. The peaks change to a power-broadened amplified spontaneous emission as the excitation fluence is increased far above the threshold.

Review of laser feedback interference and its applications in biological medicine

Review of laser feedback interference and its applications in biological medicine

All-fiber laser feedback interferometer using a DBR fiber laser for effective sub-picometer displacement measurement.

A fiber autodyne laser feedback displacement sensor based on the effect of the frequency shift is demonstrated. The all-fiber structure enables our system to be easily employed in diverse complex and narrow scenes. By virtue of adopting an ultra-high sensitivity distributed Bragg reflector (DBR) fiber laser as the laser source and the frequency-shift technology to avoid the phenomenon that the measured signal of the low frequency is submerged in the noise, the measurement of the sub-picometer displacement under weak feedback condition has been achieved, which shows a great potential in the field of micro-vibration measurement. Moreover, the proposed system has advantages such as simplicity in system structure, low cost of implementation, and immunity to electromagnetic interference.

Generation and transmission of 60-GHz mmWave emitted by VCSEL, DFB–EAM over RoF

This paper introduces a radio-over-fiber (RoF) architecture for transmission of 60-GHz multi-gigabit millimeter wave (mmWave), emitted by two different sources, a vertical cavity surface emitting laser (VCSEL) and distributed feedback (DFB) laser connected with electro-absorption modulator (EAM) using wavelength-division multiplexing (WDM) technique. These light generation techniques are used for delivering in-building wireless personal area network (PAN). At centralized office (CO), data are modulated simultaneously over two light waves under one infrastructure that will provide a 60 GHz mmWave, which is a cost-effective and efficient system. Due to the use of dynamic integrated architecture, millimeter-wave radio-frequency (RF) component will reduce the cost at the central office. With the proposed system, acceptable bit error rate (BER) less than 10–9 with a reasonably good carrier suppression of 40 dB is achieved after modulation, which facilitates lower power consumption architecture. Hence our proposed system using VCSEL and DFB can be best suited for indoor communication in a power-efficient and cost-effective manner. The susceptibility scale for investigation of these light-waves comes under 3 dB.

A Lower Frequency Shift Based on Mode Conversion for Optical Heterodyne Micro-Vibration Measurement

We experimentally obtain a lower frequency shift based on mode conversion by using a mode-selective coupler (MSC) and an acoustically induced fiber grating (AIFG) for optical heterodyne micro-vibration measurement. Through efficient conversion between LP11 and LP01 core modes in a few-mode fiber, a mode conversion frequency shifter (MCFS) can be achieved and the magnitude of frequency shift is several hundred kHz, which is generally challenging for traditional bulk acousto-optic frequency shifter. Moreover, the signal-to-signal beating interference (SSBI) can be eliminated by using the MCFS without requirement of guard band. The scheme of optical heterodyne micro-vibration measurement based on the MCFS has a high-quality signal-to-noise ratio (SNR) over 85 dB. An all-fiber Bragg grating (FBG) scheme is also designed to improve the measurement sensitivity of heterodyne detection, which has a measurable vibration frequency range of 1 Hz to 300 kHz and minimal detectable amplitude of 0.019 nm. The all-fiber MCFS has the advantages of compactness, easy integration and can be applied directly to satellite ground station and laser feedback tomography.

Laser feedback interferometry in multi-mode terahertz quantum cascade lasers.

The typical modal characteristics arising during laser feedback interferometry (LFI) in multi-mode terahertz (THz) quantum cascade lasers (QCLs) are investigated in this work. To this end, a set of multi-mode reduced rate equations with gain saturation for a general Fabry-Pérot multi-mode THz QCL under optical feedback is developed. Depending on gain bandwidth of the laser and optical feedback level, three different operating regimes are identified, namely a single-mode regime, a multi-mode regime, and a tuneable-mode regime. When the laser operates in the single-mode and multi-mode regimes, the self-mixing signal amplitude (peak to peak value of the self-mixing fringes) is proportional to the feedback coupling rate at each mode frequency. However, this rule no longer holds when the laser enters into the tuneable-mode regime, in which the feedback level becomes sufficiently strong (the boundary value of the feedback level depends on the gain bandwidth). The mapping of the identified feedback regimes of the multi-mode THz QCL in the space of the gain bandwidth and feedback level is investigated. In addition, the dependence of the aforementioned mapping of these three regimes on the linewidth enhancement factor of the laser is also explored, which provides a systematic picture of the potential of LFI in multi-mode THz QCLs for spectroscopic sensing applications.

Dual-Functional Transmitter for Simultaneous RF/LFM Signal Using a Monolithic Integrated DFB Array

A photonics-assisted approach to dual-functional analog signal transmitter that can simultaneously generate radio-frequency (RF) signal and linear frequency modulation (LFM) signal is proposed and experimentally demonstrated. Two types of beat notes can be independently generated by heterodyning of two lasers in a photodiode using a 4-channel monolithic integrated distributed feedback (DFB) laser array. In this system, the frequency scanning for the analog signal is achieved by adding a saw-tooth signal modulating on DFB3. To the LFM signal, the center frequency and bandwidth can be tuned only by changing the direct current and the voltage of driving signal, respectively. In the experimental demonstration, the center frequency for the generated analog signal is ranging from 2 GHz to 35 GHz, limited only by the bandwidth of PD. The instantaneous bandwidth reaches to 10 GHz as well as the time-bandwidth product ~ 480,000 at the peak-to-peak voltage ~ 3.5 V. Dual-functional integration analog signal transmitter is possible to adapt to variable environments, with more flexible frequency tunability for multiple function and broadband bandwidth for high resolution.

Laser Scanning Feedback Imaging System Based on Digital Micromirror Device

Most of the traditional scanning feedback imaging systems cannot break through the limitations of scanning speed. In this letter, a fast scanning system with changeable scanning sequences by projecting patterns alternatively on the digital micromirror device is proposed. Combined with the laser feedback techniques, the system exhibits high sensitivity to weak signals from targets with low reflectivity. In experiments, the images of handwriting ink patterns on a piece of glass are obtained in 6 seconds, and the resolution of the system is evaluated to be 124.5 μm with a 3.24 mm by 3.24 mm field of view. This scanning feedback imaging system can be combined potentially with single pixel imaging techniques to reconstruct phase objects more efficiently.

Influence of Fluorescence to Photon Lifetime Ratio on Detection Sensitivity in Laser Self-Mixing Interferometry

Laser self-mixing interferometry (SMI) has been widely applied in the fields of precision measurement in scientific research, industry and biomedicine, and most researchers preferentially utilize laser diodes (LD) as light sources due to their compact structure and low cost. In most SMI cases, detection sensitivity rather than structure and cost is always the first concern. So in this article, we concentrate on an influencing factor on detection sensitivity named fluorescence to photon lifetime ratio (FPLR) which is an inherent parameter of the active material in a laser cavity. Derivation and simulation based on the time delayed rate equations are followed to validate the significant role played by the FPLR, then experiments according to different laser types including solid state lasers (SSL) and LDs are implemented to further prove the factor mentioned. Results demonstrate that SSLs always have higher detection sensitivity to feedback light than LDs, and is always more suitable to be applied in precision measurement. Additionally, the Yb:YAG laser is more sensitive than the Nd:YAG one. Increasing the pumping level from the threshold will decrease the SMI signal's sensitivity, and reducing laser feedback strength will also decrease the SMI signal's sensitivity. The findings from this article are beneficial to studying laser feedback sensitivity and selecting appropriate laser types in designing SMI sensors or instruments. Though with the disadvantages of large size and high cost, SSLs may be more suitable light sources in the field of high performance SMI sensing if miniaturization and cost are not the significant consideration.

基于激光回馈共聚焦的层析成像系统

基于激光回馈共聚焦的层析成像系统

基于激光回馈共聚焦的层析成像系统

基于激光回馈共聚焦的层析成像系统

Combined frequency-locking technology of a digital integrated resonator optic gyroscope with a phase-modulated feedback loop.

In the resonator fiber optic gyroscope (RFOG), the conventional laser feedback loop is realized by adjusting the laser central frequency to tracking the resonance point of the resonator. This method generally relies on the laser tuning coefficient and digital-to-analog converter, which inevitably produces quantization error and limits frequency-locking accuracy and gyro resolution. In addition, the output drift caused by low-frequency noise also is a problem with long-term lock-in tests. In this paper, a novel and simple combined frequency-locking technology based on a phase-modulated feedback loop and a laser feedback loop is proposed by using the high-precision tuning of a phase modulator to improve the resolution and suppress the low-frequency drift. Furthermore, it has been proved by experiments that the resolution is increased by 10 times, while the frequency-locking accuracy is improved from 10°/s to 0.5°/s by using the combined frequency-locking mode. In addition, the low-frequency drift is eliminated with the long-term lock-in of RFOG, and the system resolution from 1°/s to 0.1°/s is accurately displayed.

Slot-Waveguide Silicon Nitride Organic Hybrid Distributed Feedback Laser

One of the major barriers for a widespread commercial uptake of silicon nitride photonic integrated circuits for cost-sensitive applications is the lack of low-cost monolithically integrated laser light sources directly emitting into single-mode waveguides. In this work, we demonstrate an optically pumped organic solid-state slot-waveguide distributed feedback laser designed for a silicon nitride organic hybrid photonic platform. Pulsed optical excitation of the gain medium is achieved by a 450 nm laser diode. The optical feedback for lasing is based on a second-order laterally coupled Bragg grating with a slot-waveguide core. Optimized material gain properties of the organic dye together with the increased modal gain of the laser mode arising from the improved overlap of the slot-waveguide geometry with the gain material enable single-mode lasing at a wavelength of 600 nm. The straightforward integration and operation with a blue laser diode leads to a cost-effective coherent light source for photonic integrated devices.

Spectral Processing of Self-Mixing Interferometric Signal Phase for Improved Vibration Sensing Under Weak- and Moderate-Feedback Regime

In this paper, spectral processing of laser Self-Mixing (SM) interferometric signal phase has been carried out allowing better measurement accuracy for harmonic and arbitrarily shaped vibrations for an optical feedback based SM interferometric Laser Diode (LD) sensor. The resulting algorithm not only improves the measurement accuracy but also reduces the processing time (by a factor 3.45) as compared with a previous time-domain based displacement retrieval technique called the Phase Unwrapping Method (PUM). Fourier series based analysis of laser feedback phase is carried out to determine processing limits. The proposed algorithm has also been found to be robust against variation in optical feedback coupling C as well as additive noise. This use of spectral analysis not only increases the measurement accuracy but also retrieves information about target movement harmonics which can also be used in modal analysis applications of remote mechanical targets. Using an SM vibration sensor based on a LD emitting at 785 nm, this technique has provided an average RMS error of 12.5 nm (while that of PUM is 39 nm RMS) for harmonic target vibrations of $5~\mu \text{m}$ amplitude. For reduced range of $1 , an average RMS error of ~8 nm ( $\sim \lambda $ /100) is obtained.

Core/Shell CdSe/CdS Bone‐Shaped Nanocrystals with a Thick and Anisotropic Shell as Optical Emitters

AbstractColloidal core/shell nanocrystals are key materials for optoelectronics, enabling control over essential properties via precise engineering of the shape, thickness, and crystal structure of their shell. Here, the growth protocol for CdS branched nanocrystals is applied on CdSe nanoplatelet seeds and bone‐shaped heterostructures are obtained with a highly anisotropic shell. Surprisingly, the nanoplatelets withstand the high growth temperature of 350 °C and structures with a CdSe nanoplatelet core that is overcoated by a shell of cubic CdS are obtained, on top of which tetrahedral CdS structures with hexagonal lattice are formed. These complex core/shell nanocrystals show a band‐edge emission around 657 nm with a photoluminescence quantum yield of ≈42% in solution, which is also retained in thin films. Interestingly, the nanocrystals manifest simultaneous red and green emission and the relatively long wavelength of the green emission indicates charge recombination at the cubic/hexagonal interface of the CdS shell. The nanocrystal films show amplified spontaneous emission, random lasing, and distributed feedback lasing when the material is deposited on suitable gratings. This work stimulates the design and fabrication of more exotic core/shell heterostructures where charge carrier delocalization, dipole moment, and other optical and electrical properties can be engineered.

Theoretical and experimental study of liquid refractive index measurement method based on the double external-cavity-laser feedback effect

A liquid refractive index (RI) measurement method based on the double external-cavity laser feedback effect is proposed. The method uses a laser beam to simultaneously enter the top and bottom surfaces of the liquid to be tested, with the light reflected from the two surfaces returning to the laser cavity along the original path, forming a double external-cavity system. When the liquid level rises, the self-mixing interference signal, caused by the optical path from the top and bottom liquid surfaces, superimposes on the tuning intensity of the laser, and the RI of the liquid can be obtained by demodulating the amplitude-frequency feedback tuning curve. This method offers the following innovative performances. Firstly, the structure of the system is simple and the measuring range is large. Secondly, the measurement results can be traced back to one physical quantity, i.e. the RI of air. Thirdly, it has been proved that the laser feedback effect is ubiquitous in all types of lasers, and the change of wavelength does not affect the feedback phenomenon, so that the method is not limited by the type and wavelength of the laser.

Ultrasound-modulated laser feedback tomography in the reflective mode.

A novel method of ultrasound-modulated optical tomography (UOT) detection based on the laser feedback technology is proposed in this Letter. The system has advantages such as a simple structure, high sensitivity, and reflective configuration. Effective penetration depths of up to 9cm and 5cm in phantom and biological tissues, respectively, have been demonstrated experimentally. The detection capability is comparable with the state of the art in the transmission mode but with a much lower photon consumption. Although a lot remains to be improved, the proposed method is promising for further development toward practical applications.

Distributed feedback lasing based on a negative-index metamaterial waveguide.

This Letter lays the foundation of a new type of distributed feedback (DFB) laser whose optical feedback is due to the evanescent coupling between an active positive-index material (PIM) waveguide and a lossy negative-index metamaterial (NIM) waveguide. Active PIM-NIM coupled-mode equations are presented and solved to characterize the dispersion relation, resonant optical gain, and lasing. The photonic bandgap of this grating-less DFB laser does not depend on a Bragg wavenumber, but depends on the difference between the wavenumbers of the PIM and NIM waveguides; controlling this wavenumber difference allows for single-mode lasing and, ultimately, single-mode broadband lasing.

Dual-Modality Confocal Laser Feedback Tomography for Highly Scattering Medium

The combination of optical modalities is a common practice to improve the efficiency of biomedical optics systems. We propose confocal laser feedback tomography for volumetric dual-modality imaging. Two major modalities of laser Doppler flowmetry and reflectance confocal microscopy were integrated, and the signals were concurrently acquired for cross-sectional and volumetric imaging, by means of a compact laser scanning system. The technique was applied to a two-layered skin tissue phantom containing a 200- $\mu \text{m}$ -diameter intralipid flow channel that features functional and morphological changes at the same time. Results show the potential for concurrent 3-D mapping of dynamic and static inhomogeneities in a highly scattering medium which can improve the contrast in non-invasive biological tissue imaging, providing a confocal image in addition to the Doppler flowmetry image.

A Novel Laser Lithotripsy System with Automatic Real-Time Urinary Stone Recognition: Computer Controlled Ex Vivo Lithotripsy is Feasible and Reproducible in Endoscopic Stone Fragmentation.

Urinary stone treatment has been strongly influenced by advances in technology. Nevertheless, the photonic characteristics of stones as the treatment target have been neglected. Monitoring fluorescence spectra is sufficient for automatic target differentiation and laser feedback control as previously described. We investigated the characteristics of fluorescence signals and the clinical practicability of real-time laser feedback control during lithotripsy. Fluorescence excitation light was superimposed on a holmium laser beam into the treatment fiber. Spectra were recorded and signal amplitude changes were analyzed during increases in distance between the fiber tip and the stone to identify the optimal threshold level for stone recognition. Ho:YAG lithotripsy was performed under in vitro surgical conditions in porcine tissue while our feedback system autonomously controlled the laser impulse release during lithotripsy. The tissue was then endoscopically and macroscopically examined for laser induced lesions. Mean ± SD autofluorescence signal amplitudes from urinary stone samples varied between 142 ± 29 and 1,521 ± 152 ADU while tissue and endoscope coating emission was negligible. Signal amplitude decreased rapidly at distances larger than 1 to 2 mm. Clinically reliable threshold values for target recognition could be set to prevent laser pulse emission if the stone was out of range or urothelial tissue might be harmed by laser irradiation. We observed no incorrectly released laser pulse or injury to tissue during autonomously controlled holmium laser lithotripsy. Our laboratory study strengthens the evidence that tracking real-time autofluorescence spectra during endoscopic stone surgery via automatic feedback control of the laser impulse release may become a potentially useful clinical tool for surgeons who navigate in the upper urinary tract.