Twist Sensitivity of Cladding-Mode Resonances and Its Cross-Sensitivity to Strain and Temperature in a Mechanically Induced Long-Period Fiber Grating

Abstract

Twist sensitivity of cladding-mode resonances in a mechanically induced long-period fiber grating formed over a single-mode fiber is experimentally demonstrated and theoretically analyzed. Of the two usual cladding-mode resonances corresponding to LP₁₁ and LP₁₂, higher-order mode LP₁₂ is more sensitive to twist in comparison with the lower-order mode LP₁₁. The extent of down-shifting of resonant wavelengths depends on twist-induced circular birefringence and the modal field distribution of the cladding-modes inside the fiber. When the fiber is severely twisted to 3.5 rad/cm, a shift sensitivity is observed of 1 nm/(rad/cm) for the LP₁₁ mode and 4.23 nm/(rad/cm) for the LP₁₂ mode. The fiber breaks when the twist rate exceeds 3.5 rad/cm. In comparison with LP₁₂, the LP₁₁ resonance is almost independent of the axial strain variation with an ultra-low sensitivity of 0.18 pm/με, and it is also almost insensitive to the temperature variation with a coefficient of 35 pm/°C. Forming the LP₁₁ resonance far away from its cut-off wavelength, a widely tunable band-pass filter is also demonstrated with a very high twist sensitivity of 8.75 nm/(rad/cm) and negligible cross-sensitivity to strain and temperature. The experimental and theoretical results are very useful in selecting sensitive and stable cladding-mode resonances in the design of new mechanically induced long-period fiber gratings based torsion sensors and tunable band-pass filters.     

Nonlinear beam clean-up using resonantly enhanced sum-frequency mixing

We investigate the possibility of improving the beam quality and obtaining high conversion efficiency in nonlinear sum-frequency generation. A 765 nm beam from an external cavity tapered diode laser is single-passed through a nonlinear crystal situated in the high intracavity field of a 1342 nm Nd:YVO₄ laser, generating a SFG beam at 488 nm. The ECDL have M _H ² =1.9 and M _V ² =2.4 and the solid-state laser has M ²<1.05. Varying the focusing of the 765 nm beam, the conversion efficiency and the beam quality of the generated 488 nm beam change correspondingly. We show that it is possible to improve the M ² of the 488 nm beam to less than 1.3 while preserving a high conversion efficiency of the SFG process.     

Theoretical and experimental investigations of nanosecond 177.3 nm deep-ultraviolet light by second harmonic generation in KBBF

We have presented theoretical and experimental investigations of nanosecond (ns) deep-ultraviolet (DUV) 177.3 nm radiation by means of second harmonic generation (SHG) from a frequency-tripled Nd:YAG laser (355 nm, 49 ns and 10 kHz) in KBe₂BO₃F₂ (KBBF) nonlinear crystal for the first time. A DUV KBBF-SHG numerical model, accounting for linear absorption, pump depletion, beam spatial birefringent walk-off and diffraction, is performed in the Gaussian approximation of spatial and temporal profiles. In the experiment, a maximum average output power of 14.1 mW at 177.3 nm was obtained. The dependence of 177.3 nm output power on the 355 nm pump power was simulated. The calculated results are in good agreement with the measured data. We used the model further to investigate the optical conversion efficiency, pulse width, beam spatial intensity profile and beam quality factor of the generated 177.3 nm light, in particular the effect of beam birefringent walk-off.     

Ultra-Compact and Efficient Integrated Multichannel Mode Multiplexer in Silicon for Few-Mode Fibers

Space-division multiplexing (SDM) is one of the key enabling technologies to increase the capacity of fiber communication systems. However, implementing SDM-based systems using multimode fiber has been challenging with the need for compact, low-cost, and scalable mode de/multiplexer (DE/MUX). Here a novel integrated mode MUX for few-mode fibers (FMFs) is presented which can launch up to eight spatial and polarization channels. The new design is composed of a 2D multimode grating coupler (MMGC), highly compact spot size converters (SSCs), and adiabatic directional couplers (ADCs). Eight data lanes in FMFs can be selectively launched with integrated optical phase shifters. Experimental results reveal efficient chip-to-fiber coupling with peak efficiencies of −3.8, −5.5, −3.6, and −4.1 dB for LP₀₁, LP_11a, LP_11b, and LP_21b modes, respectively. Thanks to the use of an integrated subwavelength Mikaelian lens for mode-independent field size conversion with loss ≤−0.25 dB, the total footprint of the MMGC and SSCs is only 35×35 µm². The proposed design shows great potential for densely integrated photonic circuits in future SDM applications.     

Higher order mode propagation in an optical nanofiber

The propagation of higher modes, such as the LP₁₁ mode, in optical nanofibers using the exponentially tapered optical fiber as a basic model is investigated. In order to preserve the LP₁₁ mode in the downtaper as far as the nanofiber waist, the effect of varying the cladding-core radius ratio on the LP₁₁ adiabatic criterion is modeled. A Laguerre-Gaussian beam is created in free space using a spatial light modulator (SLM) and coupled to a few-mode fiber. This device allows convenient switching between the fundamental and LP₁₁ fiber modes. By selecting a few-mode fiber with a relatively low cladding-core ratio, the propagation of the LP₁₁ mode down to a submicron waist has been maintained. Furthermore, by observing the transmission profile during tapering, it is possible to decisively terminate the pulling process in order to eliminate the two degenerate HE₂₁ modes of the LP₁₁ mode. As a result, a nanofiber can be fabricated through which only the TE₀₁ and TM₀₁ components of the LP₁₁ mode propagate. Such a nanofiber has promising applications in the area of mode interference for controlled particle trapping sites.     

Optimal design of broadband long period grating-based LP01↔LP02 mode converters for dispersion compensation

Dispersion compensation with higher order mode is emerging as a promising technique that can provide full dispersion and dispersion slope compensation for a long distance high bit rate system. Long period grating-based LP₀₁LP₀₂ mode converters are designed in the wavelength region of 1550 nm for this purpose. The simulated mode converters have large bandwidth with conversion efficiencies of over 99% and the estimated figure of merit is 309 ps/(nm dB). The negative dispersion of –124 ps/(nm km) is obtained over a bandwidth (@20 dB) of 58 nm with relative dispersion slope of 0.008/nm.     

Non-adiabatic transition in C2H5OH+ on a light-dressed potential energy surface by ultrashort pump-and-probe laser pulses

We experimentally investigate how parameters of ultrashort laser pulses such as the pulse width and wavelength could induce changes in the dynamics of vibrational wave packets on the light-dressed-potential energy surface (LD-PES) of C₂H₅OH⁺ using a pump-and-probe pulse excitation scheme. The probability of non-adiabatic transition at 800 nm from the singly ionized ground state to the repulsive excited state leading to C–O bond breaking is enhanced when a probe laser pulse is delayed by ～180 fs. At this pulse delay, on the other hand, C–C bond breaking is significantly suppressed. Therefore, the deformation of LD-PES is considered to change the direction of the wave packet traveling originally along the C–C stretching into the direction along the C–O stretching. This non-adiabatic transition leading to the redirection of the dissociating wave packet is found to occur more efficiently at the probe laser wavelengths at 400 nm than at 800 nm. The critical pulse delay is still ～180 fs even at 400 nm.     

High-order mode based dispersion compensating modules using spatial mode conversion

High-Order Mode Dispersion Compensating Modules (HOM-DCM) using spatial optical transformations for mode conversion are reviewed. It is shown that mode transformers using this technology can be designed to transform the LP₀₁ mode of SMF fibers to the LP₀₂ mode of specially designed dispersion compensating High-Order Mode Fiber (HOMF), with typical insertion loss of ∼1 dB, and typical extinction ratio to other modes less than -20 dB. The HOMF itself can provide high negative dispersion [typically in the range of 400-600 ps/(nm km)], and high negative dispersion slope, allowing efficient compensation of all types of transmission fiber. Combining two mode transformers with HOMF and possibly trim fiber for fine-tuning, results, for example, in a HOM-DCM that compensates 100 km LEAF? fiber, with Insertion loss < 3.5 dB, and Multi-Path Interference (MPI) < -36 dB. MPI phenomena in HOM-DCMs is characterized, and shown to comprise both coherent and incoherent parts, and to result from both the mode transformers and fiber coupling within the HOMF. MPI values of < -36 dB have been shown to allow error free transmission of 10 Gb/s signals over up to 6000 km. Finally, a number of applications well suited to the properties of HOM-DCMs are reviewed.     

Effect of group velocity dispersion on supercontinuum generation and filamentation in transparent solids

We experimentally investigate the spectral extent and spectral profile of the supercontinuum (SC) generated in transparent solids: barium fluoride, calcium fluoride, and fused silica upon irradiation by intense femtosecond-long pulses of 800, 1,380, and 2,200 nm light. These wavelengths correspond to the normal and anomalous group velocity dispersion (GVD) regimes in fused silica calcium fluoride and barium fluoride. We observe an isolated (anti-Stokes) wing on the blue side most prominently in fused silica but also in CaF₂. The SC conversion efficiency is measured for the long wavelengths used in our experiments. We also present results on filamentation in BaF₂ in the anomalous GVD regime, including visualization of focusing–refocusing events within the crystal; the size of a single filament is also determined. The 15-photon absorption cross section in BaF₂ is deduced to be 6.5 × 10^?190 cm³⁰ W^?15 s^?1.     

Multiple second-harmonic waves in a nonlinear photonic crystal with fractal structure

We report the first poling fabrication of high quality LiNbO₃ nonlinear photonic crystal with H-shaped fractal superlattice. Due to the self-similarity character of this structure, the collinear quasi-phase matched second-harmonics at 10 wavelengths, such as 674, 592, 577, 544, 517, and 499 nm, have been achieved in one crystal. Especially, for 499 nm second-harmonic laser spot, the normalized conversion efficiency can be as high as 0.53 %/mW.     

Sol–gel combustion synthesis of magnetic MnFe2O4 oxide and FeNi alloy: product dependence on the reduction ability

We report on fabrication of on-chip calcium fluoride (CaF₂) microdisk resonators using water-assisted femtosecond laser micromachining. Focused ion beam (FIB) milling is used to create ultra-smooth sidewalls. The quality (Q) factors of the fabricated microresonators are measured to be 4.2 × 10⁴ at wavelengths near 1,550 nm. The Q factor is mainly limited by the scattering from the bottom surface of the disk whose roughness remains high due to the femtosecond laser micromachining process. This technique facilitates the formation of on-chip microresonators on various kinds of bulk crystalline materials, which can benefit a wide range of applications such as nonlinear optics, quantum optics, and chip-level integration of photonic devices.     

Stable MHz-repetition-rate passively Q-switched microchip laser frequency doubled by MgO:PPLN

We present an Nd³⁺:YVO₄ microchip laser that is passively Q-switched by a semiconductor saturable absorber mirror. The system generates 520 ps pulses at 1064 nm with 340 mW average output power at up to 2.3 MHz repetition rate. Single longitudinal and transverse mode operation with a peak-to-peak timing jitter less than 1 % is achieved. We discuss the influence of different setup parameters by using numerical simulations of the coupled rate equations and FEM simulations of the thermal management. The infrared light was frequency doubled in an MgO:PPLN crystal with up to 75 % conversion efficiency, which to our knowledge is the highest conversion efficiency that was ever achieved with passively Q-switched microchip lasers.     

Efficient laser operation of diode-pumped Pr3+,Mg2+:SrAl12O19

We report on diode-pumped laser operation of Pr³⁺,Mg²⁺:SrAl₁₂O₁₉ at lasing wavelengths of λ _L = 724.4 nm, λ _L = 643.5 nm, and λ _L = 622.8 nm. Furthermore, the laser threshold could be reached in the green spectral range. By pumping the crystal longitudinally from each side with two polarization beam combined InGaN laser diodes, a total pump power of ≈4 W was available. In the deep red spectral range, a maximum output power of 564 mW was achieved with a maximum slope efficiency of 50 % with respect to the absorbed pump power. The maximum possible internal losses were estimated to ≈1 %. Beam quality factors M ² were in the range of 1.2–1.5.     

Solid-state laser system for laser cooling of sodium

We demonstrate a frequency-stabilized, all-solid laser source at 589 nm with up to 800 mW output power. The laser relies on sum-frequency generation from two laser sources at 1064 nm and 1319 nm through a PPKTP crystal in a doubly resonant cavity. We obtain conversion efficiencies as high as 2 W/W² after careful optimization of the cavity parameters. The output wavelength is tunable over 60 GHz, which is sufficient to lock on the sodium D₂ line. The robustness, beam quality, spectral narrowness and tunability of our source make it an alternative to dye lasers for atomic physics experiments with sodium atoms.     

Water vapor differential absorption lidar measurements using a diode-pumped all-solid-state laser at 935 nm

A diode-pumped, single-frequency laser system emitting at 935 nm has recently been developed to serve as the transmitter for water vapor differential absorption lidar (DIAL) measurements. This laser uses Nd:YGG (Y₃Ga₅O₁₂) as the active medium and emits radiation directly at 935 nm without the need of additional frequency conversion processes. The system was diode-pumped at 806 nm and was built up in a master-oscillator-power-amplifier configuration. It generates more than 30 mJ of pulse energy at 100 Hz repetition rate with a beam quality (M ²) of better than 1.4. Since water vapor DIAL demands for stringent requirements of the spectral properties those were carefully investigated in the scope of this paper. Single-frequency operation is achieved by injection seeding and active length control of the oscillator cavity. The range of continuously tunable single-frequency radiation extends to ～0.4 nm centered around 935.31 nm. Values of the spectral purity of >99.996% were determined using long-pass absorption measurements in the atmosphere exceeding the requirements by a large margin. Finally, for the first time water vapor DIAL measurements were performed using a Nd:YGG laser. The reported results show much promise of these directly pumped lasers at 935 nm for future spaceborne but also airborne water vapor lidar systems.     

Mid-infrared supercontinuum generation in a four-hole As2S5 chalcogenide microstructured optical fiber

We demonstrate the supercontinuum (SC) generation in a four-hole As₂S₅ chalcogenide microstructured optical fiber (MOF) experimentally. The As₂S₅ glass has better property of transmission than As₂S₃ glass in the visible range. The four-hole As₂S₅ MOF is fabricated by a rod-in-tube method. The SCs generated by different pump wavelengths at 2,000, 2,300 and 2,500 nm in the MOF whose length is from 2.3 to 20 cm are demonstrated. Those pump wavelengths correspond to the chromatic dispersion wavelength in the normal chromatic dispersion region, the anomalous chromatic dispersion region close to zero-dispersion wavelength (ZDW) and the anomalous chromatic dispersion region far from ZDW, respectively. Wider SCs can be obtained when pumped at a wavelength in the anomalous dispersion region close to ZDW. The widest SC range of 4,280 nm (from 1,370 to 5,650 nm) covering two octaves was obtained in a 4.8-cm-long fiber pumped at 2,300 nm.     

Three‐dimensional ultra‐broadband integrated tapered mode multiplexers

The demonstration of a three‐dimensional tapered mode‐selective coupler in a photonic chip is reported. This waveguide‐based, ultra‐broadband mode multiplexer was fabricated using the femtosecond laser direct‐write technique in a boro‐aluminosilicate glass chip. A three‐core coupler has been shown to enable the multiplexing of the LP₀₁, LP and LP spatial modes of a multimode waveguide, across an extremely wide bandwidth exceeding 400 nm, with low loss, high mode extinction ratios and negligible mode crosstalk. Linear cascades of such devices on a single photonic chip have the potential to become a definitive technology in the realization of broadband mode‐division multiplexing for increasing optical fiber capacity.     

Analysis of low power spatial light modulation characteristics of bacteriorhodopsin

We present a detailed and accurate analysis of low power spatial light modulation characteristics of bacteriorhodopsin (bR) based on nonlinear intensity induced absorption. Amplitude modulation of probe laser read beam transmissions at 410 nm and 640 nm, corresponding to the peak absorption of M^II and O states of D96N bR and WT bR respectively, by the modulation laser write beam intensity-induced population changes at 570 nm has been analyzed, considering all intermediate states with both forward and backward transitions in the respective bR photocycles, using the rate equation approach. The SLM characteristics are shown to be sensitive to the normalized small signal absorption coefficient β, rate constants of M^II and O intermediate states and the absorption cross-section of the initial B state at the probe wavelength (σ_Bp). There exists an optimum value of β for which maximum percentage modulation can be achieved. It is shown that for extended M^II state lifetime of 250 s in D96N bR and O state lifetime of 2.2 s in WT bR, with σ_Bp= 0, 100% modulation of read beam transmissions can be achieved, leading to high dynamic range and sensitivity for low laser write beam intensities of 50 μW/cm² and 4 mW/cm² at 570 nm, respectively.     

Radially polarized LG01-mode Nd:YAG laser with annular pumping

We demonstrated a radially polarized, LG₀₁-mode Nd:YAG laser by applying annular-shaped pump light. The annular profile of the pump light was originated from the mode conversion inside a conventional multimode fiber under off-focus coupling condition. This laser gave a maximum output power of 1.2 W at 1,064 nm with a slope efficiency of 28.3 %.