A novel dual-channel thermo-optic locking method for the whispering gallery mode microresonator

Mode locking can be effectively achieved by using the thermo-optic effects in the whispering gallery mode(WGM)optical microcavity,without the help of external equipment.Therefore,it has the advantages of small size,low integration costs,and self-locking,which shows great potential for application.However,the conventional single-channel microcavity thermal-locking method that relies solely on internal thermal balance will inevitably be disturbed by the external environment.This limitation affects the locking time and stability.Therefore,in this paper,we propose a new method for closed-loop thermal locking of a dual-channel microcavity.The thermal locking of the signal laser and the thermal regulation of the control laser are carried out respectively by synchronously drawing a dual-path tapered fiber.The theoretical model of the thermal dynamics of the dual-channel microcavity system is established,and the influence of the control-laser power on the thermal locking of the signal laser is confirmed.The deviation between the locking voltage of the signal laser and the set point value is used as a closed-loop feedback parameter to achieve long-term and highly stable mode locking of the signal laser.The results show that in the 2.63 h thermal-locking test,the locking stability is an order of magnitude higher than that of the single tapered fiber.This solution addresses the issue of thermal locking being disrupted by the external environment,and offers new possibilities for important applications such as spectroscopy and micro-optical sensor devices.

Towards electronic-photonic-converged thermo-optic feedback tuning

As Moore’s law approaching its end,electronics is hitting its power,bandwidth,and capacity limits.Photonics is able to overcome the performance limits of electronics but lacks practical photonic register and flexible control.Combining electronics and photonics provides the best of both worlds and is widely regarded as an important post-Moore’s direction.For stability and dynamic operations considerations,feedback tuning of photonic devices is required.For silicon photonics,the thermooptic effect is the most frequently used tuning mechanism due to the advantages of high efficiency and low loss.However,it brings new design requirements,creating new design challenges.Emerging applications,such as optical phased array,optical switches,and optical neural networks,employ a large number of photonic devices,making PCB tuning solutions no longer suitable.Electronic-photonic-converged solutions with compact footprints will play an important role in system scalability.In this paper,we present a unified model for thermo-optic feedback tuning that can be specialized to different applications,review its recent advances,and discuss its future trends.

Design of Thermo-Optic Variable Optical Attenuator Based on Quartz Substrate

In this paper, we designed a thermo-optic variable optical attenuator (VOA) based on quartz substrate, which consists of a Mach-Zehnder interferometer (MZI) and a thin film heater above the phase-modulation arm. The transmission properties of the waveguide and attenuation characteristics of the device have been simulated by beam propagation method (BPM), and the simulated results illustrated that the designed VOA had good performance.

Study on the thermo-optic properties of DR1/PMMA composite

This paper reports that the thermo-optic coefficient （dn/dT） as well as thermal expansion coefficients （β） of DR1/PMMA polymer film are measured for both TE （transversal electric） and TM （transversal magnetic） polarizations by using an attenuated total reflection configuration at the wavelengths of 832nm. The thermo-optic coefficients of DR1/PMMA are negative and as high as the order of 10^-4/℃. The influences of dopant concentration, poling process and photobleaching process on the thermo-optic properties of DR1/PMMA are also investigated.

In-fiber photoelectric device based on graphene-coated tilted fiber grating

Graphene and related two-dimensional materials have attracted great research interests due to prominently optical and electrical properties and flexibility in integration with versatile photonic structures.Here,we report an in-fiber photoelec-tric device by wrapping a few-layer graphene and bonding a pair of electrodes onto a tilted fiber Bragg grating(TFBG)for photoelectric and electric-induced thermo-optic conversions.The transmitted spectrum from this device consists of a dense comb of narrowband resonances that provides an observable window to sense the photocurrent and the electrical injection in the graphene layer.The device has a wavelength-sensitive photoresponse with responsivity up to 11.4 A/W,allowing the spectrum analysis by real-time monitoring of photocurrent evolution.Based on the thermal-optic effect of electrical injection,the graphene layer is energized to produce a global red-shift of the transmission spectrum of the TF-BG,with a high sensitivity approaching 2.167×10^(4)nm/A^(2).The in-fiber photoelectric device,therefore as a powerful tool,could be widely available as off-the-shelf product for photodetection,spectrometer and current sensor.

Four-stage cascaded variable optical attenuator with large attenuation range for quantum key distribution

A four-stage cascaded variable optical attenuator(VOA)with a large attenuation range is presented.The VOA is based on a Mach–Zehnder interferometer(MZI)and fabricated in a silica-based planar lightwave circuit(PLC)platform.The thermo-optic effect is used to achieve intensity modulation.The measured maximum attenuation of the four-stage cascaded VOA is 88.38 d B.The chip is also tested in a quantum key distribution(QKD)system to generate signal and decoy states.The mean photon number after attenuation of the four-stage cascaded VOA is less than 0.1,which can meet the requirement of QKD.

Raman lasing and other nonlinear effects based on ultrahigh-Q CaF_(2)optical resonator

The calcium fluoride(CaF_(2))whispering gallery mode crystalline resonator is an excellent platform for nonlinear optical applications because of the decreasing in threshold caused by ultrahigh quality(Q)factor.In this paper,we achieved the observation of Raman lasing,first-order Raman comb,and second-order Raman lasing in a CaF_(2)disk resonator with a diameter of 4.96 mm and an ultrahigh-Q of 8.43×10^(8)at 1550-nm wavelength.We also observed thermal effects in CaF_(2)disk resonator,and the threshold of thermo-optical oscillation is approximately coincident with Raman lasing,since the intracavity power increases rapidly when the power reaches the threshold,and higher input pump power results in longer thermal drift and higher Raman emission power.With a further increase in pump power,the optical frequency combs range is from 1520 nm to 1650 nm,with a wavelength interval of 4×FSR.It is a promising candidate for optical communication,biological environment monitoring,spectral analysis,and microwave signal sources.

Novel wavelength-accurate InP-based arrayed waveguide grating

A 13-channel, InP-based arrayed waveguide grating （AWG） is designed and fabricated in which the on-chip loss of the central channel is about -5 dB and the crosstalk is less than -23 dB in the center of the spectrum response. However, the central wavelength and channel spacing are deviated from the design values. To improve their accuracy, an optimized design is adopted to compensate the process error. As a result, the central wavelength 1549.9 nm and channel spacing 1.59 nm are obtained in the experiment, while their design values are 1549.32 nm and 1.6 nm, respectively. The route capability and thermo-optic characteristic of the AWG are also discussed in detail.

Investigation of photodamage by femtosecond laser to cells via gold nanorods

Usually,only focused femtosecond(fs)lasers at near-infrared(NIR)range can induce photo-damage to transparent cells,making it difficult to treat large amount of cells by such optical methods for photostimulation.In this study,we clarify the mechanism of photodamage to cells that are co-cultured with gold nanorods(GNRs)by fs laser.The pulse duration and repetition rate of the fs laser play a key role in cell damage suggesting that the heat accumulation con-tributes to the major part for the cell damage rather than the high peak power which mainly determines the efficiency of multiphoton excitation.We further show that cellular Ca^(2+)can also be released in this scheme,but the process is more sensitive to peak power.Our results can provide a large-scale GNR-mediated photostimulation for cell signaling modulation.

Physical origin of observed nonlinearities in Poly(1-naphthyl methacrylate):Using a single transistor–transistor logic modulated laser beam

A thermal lens technique is adopted using a single modulated continuous wave （cw） 532-nm laser beam to evaluate the nonlinear refractive index n2, and the thermo-optic coefficient dn/dT, in polymer Poly （1-naphthyl methacrylate） （P-1-NM） dissolved in chloroform, tetrahydrofuran （THF）, and dimethyl sulfoxide （DMSO） solvents. The results are compared with Z-scan and diffraction ring techniques. The comparison reveals the effectiveness and the simplicity of the TTL modulation technique. The physical origin is discussed for the obtained results.

Coupled thermal-optic effects and electrical modulation mechanism of birefringence crystal with Gaussian laser incidence

We study the Gaussian laser transmission in lithium niobate crystal（LiNbO3） by using the finite element method to solve the electromagnetic field＇s frequency domain equation and energy equation. The heat generated is identified by calculating the transmission loss of the electromagnetic wave in the birefringence crystal, and the calculated value of the heat generated is substituted into the energy equation. The electromagnetic wave＇s energy losses induced by its multiple refractions and reflections along with the resulting physical property changes of the lithium niobate crystal are considered.Influences of ambient temperature and heat transfer coefficient on refraction and walk-off angles of O-ray and E-ray in the cases of different incident powers and crystal thicknesses are analyzed. The E-ray electrical modulation instances, in which the polarized light waveform is adjusted to the rated condition via an applied electrical field in the cases of different ambient temperatures and heat transfer coefficients, are provided to conclude that there is a correlation between ambient temperature and applied electrical field intensity and a correlation between surface heat transfer coefficient and applied electrical field intensity. The applicable electrical modulation ranges without crystal breakdown are proposed. The study shows that the electrical field-adjustable heat transfer coefficient range becomes narrow as the incident power decreases and wide as the crystal thickness increases. In addition, it is pointed out that controlling the ambient temperature is easier than controlling the heat transfer coefficient. The results of the present study can be used as a quantitative theoretical basis for removing the adverse effects induced by thermal deposition due to linear laser absorption in the crystal, such as depolarization or wave front distortion, and indicate the feasibility of adjusting the refractive index in the window area by changing the heat transfer boundary conditions in a wide-spectrum laser.

Ultralow cross talk arrayed waveguide grating integrated with tunable microring filter array

The silicon-based arrayed waveguide grating(AWG)is widely used due to its compact footprint and its compatibility with the mature CMOS process.However,except for AWGs with ridged waveguides of a few micrometers of cross section,any small process error will cause a large phase deviation in other AWGs,resulting in an increasing cross talk.In this paper,an ultralow cross talk AWG via a tunable microring resonator(MRR)filter is demonstrated on the SOI platform.The measured insertion loss and minimum adjacent cross talk of the designed AWG are approximately 3.2 and-45.1 d B,respectively.Compared with conventional AWG,its cross talk is greatly reduced.

Compact, submilliwatt, 2 × 2 silicon thermo-optic switch based on photonic crystal nanobeam cavities

We propose and experimentally demonstrate a 2×2 thermo-optic(TO) crossbar switch implemented by dual photonic crystal nanobeam(PCN)cavities within a silicon-on-insulator(SOI)platform.By thermally tuning the refractive index of silicon,the resonance wavelength of the PCN cavities can be red-shifted.With the help of the ultrasmall mode volumes of the PCN cavities,only～0.16 mW power is needed to change the switching state.With a spectral passband of 0.09 nm at the 1583.75 nm operation wavelength,the insertion loss(IL)and crosstalk(CT)performances were measured as IL(bar)=-0.2 dB,CT(bar)=-15 dB,IL(cross)=-1.5 dB,and CT(cross)=-15 dB.Furthermore,the thermal tuning efficiency of the fabricated device is as high as1.23 nm/mW.

Vertically Integrated Thermo-Optic Waveguide Switch Using Optical Polymers

We propose and fabricate a vertically integrated thermo-optic waveguide switch. It controls the optical path between two vertically stacked waveguides using the thermo-optic effect of optical polymer. The measured crosstalk is less than -10 dB.

Fast response 2×2 thermo-optic switch with polymer/silica hybrid waveguide

A polymer/silica hybrid 2×2 multimode-interference switch is designed and fabricated. Instead of polymer Mach-Zehnder interferometer thermo-optic （TO） silica is used as under-cladding to accelerate heat release because of its large thermal conductivity. The developed switch exhibits low power consumption of 6.2 mW, low crosstalk of about 28 dB, and short response time. The rise and fall times of 103 and 91 its for this hybrid switch are shortened by 40.8% and 52.4%, respectively, compared with those of the fabricated TO switch （174 and 191 μs） using polymer as both upper- and under-claddings.

9.4 nm Tunable Vertically Coupled Microring Resonator Filter by Thermo-Optic Effect

A wide range (9.4nm) tuning of vertically coupled microring resonator filter was demonstrated utilizing a large TO coefficient of polymer. The power consumption was about 60m W and no degradation of filter response was observed.

A Fiber Optic Temperature Sensor Based on the Combination of Epoxy and Glass Particles With Different Thermo-Optic Coefficients

This paper describes the development and function of an optical fiber temperature sensor made out of a compound of epoxy and optical glass particles. Because of the different thermo-optic coefficients of these materials, this compound exhibits a strong wavelength and temperature dependent optical transmission, and it therefore can be employed for fiber optic temperature measurements. The temperature at the sensor, which is integrated into a polymer optical fiber （POF）, is evaluated by the ratio of the transmitted intensity of two different light-emitting diodes （LED） with a wavelength of 460nm and 650rim. The material characterization and influences of different sensor lengths and two particle sizes on the measurement result are discussed. The temperature dependency of the transmission increases with smaller particles and with increasing sensor length. With glass particles with a diameter of 43 Bm and a sensor length of 9.8 mm, the intensity ratio of the two LEDs decreases by 60% within a temperature change from 10℃ to 40℃.

Design and fabrication of wavelength tunable AWGs based on the thermo-optic effect

In this Letter, a 16 channel 200 GHz wavelength tunable arrayed waveguide grating（AWG） is designed and fabricated based on the silicon on insulator platform. Considering that the performance of the AWG, such as central wavelength and crosstalk, is sensitive to the dimension variation of waveguides, the error analysis of the AWG with width fluctuations is worked out using the transfer function method. A heater is designed to realize the wavelength tunability of the AWG based on the thermo-optic effect of silicon. The measured results show that the insertion loss of the AWG is about 6 d B, and the crosstalk is 7.5 d B. The wavelength tunability of 1.1 nm is achieved at 276 m W power consumption, and more wavelength shifts will gain at larger power consumption.

Thermo-optic characterization of KDP single crystals by a modified Snarmont setup for electro-optic modulation system

For our KDP crystal orientation, various tbermo-optic （TO） and relevant temperature-dependence param- eters are defined, presented, and studied in the framework of a transverse and a longitudinal electro-optic （EO） modulation systems. This study is based on the concept of the so-called opto-electrical bias （~） ap- plied to the system. For both of the above EO-modulation systems, a set of original equations is extracted and investigated with regard to each of the more important TO or temperature coefficients. Using these equations, for these parameters the role of the transverse configuration is examined in comparison with its corresponding longitudinal configuration. A comparison is done with other orientation of the same KDP crystal.

Fabrication of 2×2 Thermo-Optic Switches with Organic-Inorganic Hybrid Materials Prepared by Sol-Gel Technique

2×2 Mach-Zehnder interferometric thermo-optic switch was fabricated with organic/inorganic hybrid materials by sol-gel technique and direct UV patterning. The switching time of device was measured to be 4.2 ms and switching power 9.3 mW.