High-power all-fiber linearly polarized Yb-doped chirped pulse amplifier based on active polarization control

High-power ultrafast laser amplification based on a non-polarization maintaining fiber chirped pulse amplifier is demonstrated.The active polarization control technology based on the root-mean-square propagation(RMS-prop)algorithm is employed to guarantee a linearly polarized output from the system.A maximum output power of 402.3 W at a repetition rate of 80 MHz is realized with a polarization extinction ratio(PER)of>11.4 dB.In addition,the reliable operation of the system is verified by examining the stability and noise properties of the amplified laser.The M2factor of the laser beam at the highest output power is measured to be less than 1.15,indicating a diffraction-limited beam quality.Finally,the amplified laser pulse is temporally compressed to 755 fs with a highest average power of 273.8 W.This is the first time,to the best of our knowledge,that the active polarization control technology was introduced into the high-power ultrafast fiber amplifier.

Toward a compact fiber comb with 1.66×10^(-12)instability based on acetylene-filled photonic microcells

We demonstrated an optical fiber frequency comb stabilized to an acetylene-filled photonic microcell.The short-term instability of the comb at 1 s gate time was 1.66×10^(-12)for a 4.2-h measurement in a laboratory environment with air conditioning.This is the best short-term stability reported for a compact fiber comb stabilized to an acetylene-filled photonic microcell at telecom wavelengths.It is particularly significant in the development of compact fiber combs with target instability of 10^(-13).Such a device has the potential to serve as an alternative to GPS in areas lacking signal coverage,including remote locations,regions with adverse weather conditions,and military intelligence areas.

Generation of multi-freedom controllable helical beam through high-order Bessel beams[Invited]

In this Letter,we propose a scheme to generate helical optical fields with multi-freedom controllable features.High-quality helical lobes with adjustable radii,chirality,and lobe numbers can be generated by tuning the phase term of two paired highorder Bessel beams.Furthermore,the pitch of the helical beam can be controlled by combining another rotational phase term.The validity of our scheme is demonstrated in both simulations and experiments.Our scheme is promising to facilitate the rapid fabrication of helical structures with diverse parameters,which are critical in various applications,such as optical metamaterials,biology,and particle transport.

Adaptive microwave photonic angle-of-arrival estimation based on BiGRU-CNN [Invited]

An adaptive microwave photonic angle-of-arrival(AOA) estimation approach based on a convolutional neural network with a bidirectional gated recurrent unit(BiGRU-CNN) is proposed and demonstrated.Compared with the previously reported AOA estimation methods based on phase-to-power mapping,the proposed method is unnecessary to know the frequency of the signal under test(SUT) in advance.The envelope voltage correlation matrix is obtained from dual-drive Mach–Zehnder modulator(N-DDMZM,N > 2) optical interferometer arrays first,and then AOA estimations are performed on different frequency signals with the aid of BiGRU-CNN.A three-DDMZM-based experiment is carried out to assess the estimation performance of microwave signals at three different frequencies,and the mean absolute error is only 0.1545°.

A cylindricalÖffner stretcher based on ternary reflector for femtosecond petawatt-level laser system

A cylindricalÖffner stretcher based on ternary reflector(COSTER)is proposed and analyzed.Compared with the traditionalÖffner stretcher,the COSTER has no off-axis aberration in the multipass configuration,and the output laser of COSTER has lower spectral phase noise and higher temporal contrast in the far field.The COSTER is quite suitable to be used in multipetawatt laser facilities,and it might be the preferred stretcher configuration for ultrafast and ultra-intense lasers.

Speckle-based interrogation system for quasi-distributed weak fiber Bragg gratings

A simple quasi-distributed fiber sensing interrogation system based on random speckles is proposed for weak fiber Bragg gratings(WFBGs) in this work. Without using tunable lasers or spectrometers, a piece of multimode fiber is applied to interrogate the WFBGs relying on the wavelength sensitivity of speckles. Instead of the CCD sensor, an InGaAs quadrant detector serves as the receiver to capture the fast-changing speckle patterns. A supervised deep learning algorithm of the multilayer perceptron architecture is implemented to process speckle data and to interrogate temperature changes or dynamic strains. The proposed demodulation system is experimentally demonstrated for WFBGs with 0.1% reflectivity.The experimental results demonstrate that the new system is capable of measuring temperature change with an accuracy of 1℃ and achieving dynamic frequency of 100 Hz. This speckle-based interrogation system paves a new way for distributed WFBGs sensing with a simple design.

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Chinese Optics Letters is a short-article journal for the rapid dissemination of new, important results in all branches of optical science and technology and related fields, published monthly by Chinese Laser Press and distributed abroad by Optica Publishing Group.

Phase modulation polarization encoding module applied to one-to-many QKD network based on wavelength division multiplexing

The quantum key distribution(QKD)network is a promising solution for secure communications.In this paper,we proposed a polarization-independent phase-modulated polarization encoding module,and it can be combined with a dense wavelength division multiplexer(DWDM)to achieve multi-user QKD.We experimentally test the encoding module with a repetition rate of 62.5 MHz,and its average quantum bit error rate(QBER)is as low as 0.4%.Finally,we implement a principle verification test for simultaneous QKD for 1 to 2 users in 100 min,and the average QBER of two users under the transmission distance of 1 km and 5 km is kept below 0.8%.Due to the use of polarization encoding,the module can also realize scalable network architecture in free-space QKD systems in the future.

Effect of concentration on the formation time of diffraction rings in spatial self-phase modulation

A new unsaturated wind-chime model is proposed for calculating the formation time of the diffraction rings induced by spatial self-phase modulation(SSPM) in molybdenum disulfide suspension.To optimize the traditional wind-chime model,the concentration variable of 2 D materials was introduced.The results of the unsaturated wind-chime model match quite well with the SSPM experimental results of molybdenum disulfide.Based on this model,the shortest formation time of diffraction rings and their corresponding concentration and light intensity can be predicted using limited data.Theoretically,by increasing the viscosity coefficient of the solution,the response time of the diffraction ring,to reach the maximum value,can be significantly reduced.It has advanced significance in shortening the response time of photonic diodes.

High laser damage threshold LiNa_(5)Mo_(9)O_(30) prism:for visible to mid-infrared range

In this study,an excellent polarization optical crystal LiNa_(5)Mo_(9)O_(30) with wide transmission range and high laser damage threshold was researched in detail.The laser damage threshold of the LiNa_(5)Mo_(9)O_(30) crystal was measured to be 2.64GW/cm^(2),which was the highest among polarized optical crystals.The birefringence in the range of 0.435-5μm was larger than 0.14,while the wedge angle between 31.94°and 32.12°would satisfy the application in this waveband.The extinction ratio of the fabricated prism with the wedge angel of 31.09°was larger than 15,000:1.The results show that the LiNa_(5)Mo_(9)O_(30) prism is an excellent polarization device,especially in the mid-infrared range and high-power applications.

Ultrafast all-optical switching of dual-band plasmon-induced transparency in terahertz metamaterials

An active ultrafast formation and modulation of dual-band plasmon-induced transparency(PIT) effect is theoretically and experimentally studied in a novel metaphotonic device operating in the terahertz regime,for the first time,to the best of our knowledge.Specifically,we designed and fabricated a triatomic metamaterial hybridized with silicon islands following a newly proposed modulating mechanism.In this mechanism,a localized surface plasmon resonance is induced by the broken symmetry of a C_(2)structure,acting as the quasi-dark mode.Excited by exterior laser pumps,the photo-induced carriers in silicon promote the quasi-dark mode,which shields the near-field coupling between the dark mode and bright mode supported by the triatomic metamaterial,leading to the dynamical modulation of terahertz waves from individual-band into dual-band PIT effects,with a decay constant of 493 ps.Moreover,a remarkable slow light effect occurs in the modulating process,accompanied by the dual-transparent windows.The dynamical switching technique of the dual-band PIT effect introduced in this work highlights the potential usefulness of this metaphotonic device in optical information processing and communication,including multi-frequency filtering,tunable sensors,and optical storage.

Inverse design of 1D color splitter for high-efficiency color imaging

We introduce a simple one-dimensional(1D)structure in the design of 1D color splitters(1D-CSs)with RGB unit cells for color imaging and propose a single-to-double-layer design in 1D-CSs.Based on inverse design metasurfaces,we demonstrate numerically a single-layer 1D-CS with a full-color efficiency of 46.2%and a double-layer 1D-CS with a full-color efficiency of48.2%;both of them are significantly higher than that of traditional color filters.Moreover,we demonstrate a 1D-CS that has application value by evaluating the double-layer 1D-CS’s performances in terms of incident angle sensitivity,polarization angle sensitivity,and assembly tolerance.

B-scan-sectioned dynamic micro-optical coherence tomography for bulk-motion suppression

In this Letter, we present B-scan-sectioned dynamic micro-optical coherence tomography(BD-MOCT) for high-quality subcellular dynamic contrast imaging. Dynamic micro-optical coherence tomography(D-MOCT) is a functional optical coherence tomography(OCT) technique performed on high-resolution(micron level) OCT systems;hundreds of consecutive B-scans need to be acquired for dynamic signal extraction, which requires relatively long data acquisition time. Bulk motions occurring during data acquisition(even at the micron level) may degrade the quality of the obtained dynamic contrast images. In BD-MOCT, each full B-scan is divided into several sub-B-scans, and each sub-B-scan repeats multiple times before the sample beam moves to the next sub-B-scan. After all of the sub-B-scans for a full B-scan are completely acquired, we stitch all of the sub-B-scans into the same number of full B-scans. In this way, the time interval between two consecutive stitched B-scans could be reduced multiple times for bulk-motion suppression. The performed scanning protocol modulates the scanning sequences of fast scanning and repeat scanning for improving the dynamic contrast image quality, while the total data acquisition time remains almost the same.

On-chip short-wave infrared multispectral detector based on integrated Fabry–Perot microcavities array

We demonstrate an ultra-compact short-wave infrared[SWIR]multispectral detector chip by monolithically integrating the narrowband Fabry–Perot microcavities array with the In Ga As detector focal plane array.A 16-channel SWIR multispectral detector has been fabricated for demonstration.Sixteen different narrowband response spectra are acquired on a 64×64 pixels detector chip by four times combinatorial etching processes.The peak of the response spectra varies from1450 to 1666 nm with full width at half-maximum of 24 nm on average.The size of the SWIR multispectral detection system is remarkably reduced to a 2 mm^(2) detector chip.

Design of low frequency fiber optic Fabry–Perot seismometer based on transfer function analysis

We develop a low frequency fiber Fabry–Perot(F-P)seismometer based on transfer function analysis.The seismometer structure and demodulation system accuracy are limitations of low frequency seismic monitoring.The transfer function of the F-P seismometer is analyzed,and the mass displacement spectrum(MDS)is introduced.MDS provides guidance for mechanical structure design and optical interferometer analysis to achieve low noise.The F-P seismometer prototype is built.The experiment shows that the prototype has an average noise of 6.74 ng=p Hz below 50 Hz,and its noise is less than that of the global new high noise model within 0.16–50 Hz,whose potential is considerable.

Information for Authors

Chinese Optics Letters is a short-article journal for the rapid dissemination of new, important results in all branches of optical science and technology and related fields, published monthly by Chinese Laser Press and distributed abroad by the Optical Society(OSA).

Information for Authors

Chinese Optics Letters is a short-article journal for the rapid dissemination of new, important results in all branches of optical science and technology and related fields, published monthly by Chinese Laser Press and distributed abroad by the Optical Society (OSA).

Ultra-long-period grating-based multi-wavelength ultrafast fiber laser [Invited]

We propose and demonstrate the cascaded multi-wavelength mode-locked erbium-doped fiber laser(EDFL) based on ultralong-period gratings(ULPGs) for the first time, to the best of our knowledge.Study found that the ULPG can be used as both a mode-locker for pulse shaping and a comb filter for multi-wavelength generation simultaneously.Using the dual-function of ULPG, three-, four-, five-, six-, and seven-wavelength mode-locked pulses are obtained in EDFL, seven of which are the largest number of wavelengths up to now.For the four-wavelength soliton pulses, their pulse width is about 7.8 ps.The maximum average output power and slope efficiency of these pulses are 8.4 m W and 2.03%, respectively.Besides the conventional pulses, hybrid soliton pulses composed of a four-wavelength pulse and single soliton are also observed.Finally, the effect of cavity dispersion on the multi-wavelength mode-locked pulses is also discussed.Our findings indicate that apart from common sensing and filtering, the ULPG may also possess attractive nonlinear pulse-shaping property for ultrafast photonics application.

Reduced threshold current density of GaN-based green laser diode by applying polarization doping p-cladding layer

Absorption induced by activated magnesium(Mg)in a p-type layer contributes considerable optical internal loss in Ga Nbased laser diodes(LDs).An LD structure with a distributed polarization doping(DPD)p-cladding layer(CL)without intentional Mg doping was designed and fabricated.The influence of the anti-waveguide structure on optical confinement was studied by optical simulation.The threshold current density,slope efficiency of LDs with DPD p-CL,and Mg-doped CL,respectively,were compared.It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency,which were caused by decreasing internal loss and hole injection,respectively.

Highly flexible Nyquist pulse generation based on multi-wavelength control

By controlling the wavelength and power of multiple light sources, we have realized a highly flexible Nyquist pulse generation scheme, in which the pulse repetition frequency, pulse multiplication factor, waveform envelope shape, and duty cycle are all tunable. By modulating the 3.2 GHz RF signal, we experimentally generated Nyquist pulses with repetition rates of 6.4 GHz and 9.6 GHz, a rectangular wave with a duty cycle of 0.26,and a sawtooth wave with a duty cycle of 0.52.