Narrow-linewidth Tunable Laser Research Articles

High-precision fiber-optic salinity sensor by micro-cavity and intensity demodulation

In this paper, a hollow-core fiber based micro-cavity Mach-Zehnder interferometer is realized and demonstrated by arc-discharged large lateral-offset splicing. The transmission spectra are investigated and compared with respects to loss and extinction ratio, also the cavity length and offset are theoretically and experimentally optimized. The obtained results show that the concentration of salinity can affect the wavelength response, the average salinity and temperature sensitivities are −2.275 nm/‰ and 1.71 nm/°C, respectively. Moreover, more than 5-fold improvement of the real detection limit is gained by a narrow line-width tunable laser based intensity-demodulation system. And the detection limit of 0.031 ‰ is experimentally witnessed with a time-response of ∼280 ms.

High-Repetition-Rate 2.3–2.7 µm Acousto-Optically Tuned Narrow-Line Laser System Comprising Two Master Oscillators and Power Amplifiers Based on Polycrystalline Cr2+:ZnSe with the 2.1 µm Ho3+:YAG Pulsed Pumping

High-average-power narrow-linewidth tunable solid-state lasers in the wavelength region between 2 and 3 μm are attractive light sources for many applications. This paper reports a narrow-linewidth widely tunable laser system based on the polycrystalline Cr2+:ZnSe elements pumped by repetitively pulsed 2.1 µm Ho3+:YAG laser operating at a pulse rate of tens of kilohertz. An advanced procedure of ZnSe element doping and surface improvement was applied to increase the laser-induced damage threshold, which resulted in an increase in the output power of the Cr2+:ZnSe laser system. The high-average-power laser system comprised double master oscillators and power amplifiers: Ho3+:YAG and Cr2+:ZnSe laser oscillators, and Ho3+:YAG and Cr2+:ZnSe power amplifiers. The output wavelength was widely tuned within 2.3–2.7 µm by means of an acousto-optical tunable filter inside a Cr2+:ZnSe master oscillator cavity. The narrow-linewidth operation at the pulse repetition rate of 20–40 kHz in a high-quality beam with an average output power of up to 9.7 W was demonstrated.

Widely Tunable Narrow-Linewidth Laser Based on a Multi-Period-Delayed Feedback Photonic Circuit

Widely Tunable Narrow-Linewidth Laser Based on a Multi-Period-Delayed Feedback Photonic Circuit

Online Testing Method for the Fine Spectral Characteristics of Narrow-Band Interference Filters Based on a Narrow-Linewidth Tunable Laser.

The transmission spectrum of a narrow-band interference filter is crucial and highly influenced by factors such as the temperature and angle, thus requiring precise and online measurements. The traditional method of measuring the transmission spectrum of an interference filter involves the use of a spectrometer, but the accuracy of this method is limited. Moreover, placing a narrow-band interference filter inside a spectrometer hinders real-time online measurements. To address this issue, there is demand for high-precision online spectral testing methods. In response to this demand, we propose and experimentally validate a fine spectral characterization method for narrow-band interference filters. This method uses a narrow-linewidth tunable laser, achieving a spectral resolution in the MHz range for online testing. Two types of narrow-band interference filters were tested using the constructed laser spectroscopy experimental system, obtaining a transmission spectrum with a spectral resolution of 318 MHz. In comparison to spectrometer-based methods, our proposed method demonstrates higher spectral accuracy, enables online measurements, and provides more accurate measurements for special spectral interference filters. This approach has significant application value and promising development prospects.

Spectroscopic study of cooperative states of Nd3+ pair centers in fluorite-type crystals: A competition of dipole-dipole and quadrupole-quadrupole energy exchange

Pair centers of Nd3+ ions in the CdF2, CaF2, and SrF2 doped crystals at T=6.5K were studied under tunable narrow-linewidth laser excitation. It has been established that the exchange of electronic excitation leads to the splitting of the one-exciton cooperative 4G5/2(1) x 4I9/2(1) state by 2–3 cm−1 with a non-monotonic dependence on the distance between the ions in the pairs in these crystals. This dependence is explained by the competition of forced dipole-dipole and quadrupole-quadrupole interactions with spin flip. The high rate of such exchange mechanisms is due to strong spin–orbit interaction and small energy difference between the 4f levels of Nd3+ ions. It is shown that the pair centers in all one-exciton cooperative states, in contrast to the ground and two-exciton excited states, have nonzero static dipole moments. This makes it possible to use the latter states as ultrafast qubits, and one-exciton states as control qubit to implement a CNOT quantum gate.

Narrow-Linewidth Tunable Fiber Laser Based on Laser-Induced Graphene Heated Fiber Bragg Grating with Low Voltage

In this paper, we demonstrate a narrow-linewidth tunable fiber laser based on laser-induced graphene (LIG) paper-heated fiber Bragg grating (FBG) with low voltage. A linewidth of less than 600 Hz is achieved by the combination of a piece of unpumped Er-doped fiber and an FBG. Changing the temperature of the FBG will result in the central transmission spectrum shifting, and hence the laser wavelength tuning. LIG-heated (LIG-H) fabrication on polyimide (PI) paper by CO2 laser is used to offer temperature control of the FBG. By adjusting the voltage of the LIG-H from 0 to 5 V, the temperature of the LIG-H can be changed from room temperature up to 220 °C, while the central wavelength of the output laser can be continuously adjusted from 1549.5 nm to 1552 nm with a full range of 2.5 nm. The proposed technique by electric control of LIG-H can provide a low-cost and compact wavelength tunable laser design.

Study on external cavity diode laser with a wide mode-hopping free tuning range

A wide mode-hopping free and narrow linewidth tunable laser diode source with a Littman-Metcalf arrangement based on a diffraction grating is presented. Several experiments are carried out to demonstrate the tuning characteristics and spectral linewidth of the proposed external-cavity diode laser source. A wide no mode-hopping continuous wavelength tuning range about 59.13 nm in ultra-C-band with a narrow spectral linewidth of less than 100 kHz is obtained. An optical signal to noise ratio of more than 65 dB and an output power more than 14.8 dBm over the whole tuning range can also be realized in a long-term free running condition. The proposed tunable laser allows simultaneously mode-hopping free and narrow linewidth tunable radiation, thus opening a door for practical application such as coherent detection.

Widely tunable narrow linewidth laser source based on photonic molecule microcombs and optical injection locking.

We demonstrate a method to generate a widely and arbitrarily tunable laser source with very narrow linewidth. By seeding a coupled-cavity microcomb with a highly coherent single-frequency laser and using injection locking of a Fabry-Perot laser to select a single output comb tone, a high power, high side mode suppression ratio output wave is obtained. The system is demonstrated across 1530 -1585 nm with a linewidth below 8 kHz, having 5 dBm output power and sidemode suppression of at least 60 dB. Prospects of extending the performance are also discussed.

High energy widely tunable narrow-linewidth Ti:sapphire laser using combined-cavity configuration.

A high-energy narrow-linewidth Ti:sapphire laser with widely tunable wavelength was investigated. The Littman cavity was seeded by an extended prism cavity, and they were coupled together by sharing a partial reflection mirror. The widely wavelength tunability of the prism cavity and the linewidth compression of Littman cavity were incorporated together, which resulted in a significantly increased tunable wavelength range from 720 nm to 884 nm with linewidth of less than 100 MHz. The coupling effect and the synchronization between the two cavities in temporal and spectral domain were discussed. The narrow-linewidth laser centered at 786 nm was further amplified to 36 mJ and frequency-doubled to 393 nm with pulse energy of 18.8 mJ while maintaining the narrow linewidth at a repetition rate of 10 Hz. This widely tunable narrow-linewidth laser is a promising light source for high-resolution fluorescence spectroscopy.

Widely tunable Er:Yb fiber laser using a fiber Bragg grating embedded in a 3D printed beam

A narrow linewidth (Δλ < 0.07 nm), low noise, widely tunable Er:Yb ring fiber laser is demonstrated using a fiber Bragg grating mirror embedded in a 3D printed polymer beam. By bending the polymer beam, continuous tuning of the laser was achieved over 30 nm, from 1543 nm to 1574 nm, with power variation below 1 dB, showing high temporal and spectral stability and a signal-to-background value exceeding 50 dB. These results present a versatile and simple method for tailoring tunable narrow-linewidth lasers.

Fabrication and characterization of deformed microdisk cavities in silicon dioxide with high Q-factor.

We demonstrate the excitation and characterization of whispering gallery modes in a deformed optical microcavity. To fabricate deformed microdisk microresonators we established a fabrication process relying on dry plasma etching tools for many degrees of freedom and a shape-accurate morphology. This approach allowed us to fabricate resonators of different sizes with a controlled sidewall angle and underetching in large quantities with reproducible properties such as a surface roughness RQ≤2nm. The excitation and characterization of these modes were achieved by using a state-of-the-art tapered fiber coupling setup with a narrow linewidth tunable laser source. The conducted measurements in shortegg resonators showed at least two modes within a spectral range of about 237 pm. The highest Q-factors measured were in the range of 105. Wave optical eigenmode and frequency domain simulations were conducted that could partially reproduce the observed behavior and therefore allow us to compare the experimental results.

Tunable narrow-linewidth laser at 2 μm wavelength for gravitational wave detector research.

We present and characterize a narrow-linewidth external-cavity diode laser at 2 μm, and show that it represents a low-cost, high-performance alternative to fiber lasers for research into 2 μm photonic technologies for next-generation gravitational-wave detectors. A linewidth of 20 kHz for a 10 ms integration time was measured without any active stabilization, with frequency noise of ∼ 15 Hz/Hz between 3 kHz and 100 kHz. This performance is suitable for the generation of quantum squeezed light, and we measure intensity noise comparable to that of master oscillators used in current gravitational wave interferometers. The laser wavelength is tunable over a 120 nm range, and both the frequency and intensity can be modulated at up to 10 MHz by modulating the diode current. These features also make it suitable for other emerging applications in the 2 μm wavelength region including gas sensing, optical communications and LIDAR.

Tandem-Pumped High-Power Narrow-Linewidth Fiber Laser Tunable From 1060–1090 nm

Tandem pumping is a promising technique for gaining higher power from ytterbium-doped fiber (YDF) lasers. However, the characteristics of tandem pumped laser light, especially their variation with the lasing wavelengths, were not thoroughly investigated. We investigated the characteristics with the first 1018-nm tandem-pumped high-power tunable YDF laser. The 1018-nm tandem pumping was not applicable in tunable fiber lasers before because its much-lower absorption requires compensation of long YDFs, which enhances destructive nonlinear effects as a barrel effect for tuning wavelengths. Here, the setup used a fiber ring-cavity as seed, which was much more cost-effective than conventional setups, and only three stages of amplification, of which the last stage was unidirectional tandem-pumped. By deliberate system design, it gained over 1-kW output powers at wavelengths tuning from 1060–1090 nm with ∼100-pm 3 dB linewidths and 79.4%∼85.6% slope efficiencies. Amplified spontaneous emissions (ASE) and stimulated Brillouin scattering (SBS) caused no limits for further scaling powers. Variations of spectra and beam qualities of the outputs with the tuning wavelengths were revealed in detail in the experiment. Drastically varying linewidths at ∼1062 and ∼1080 nm suggested complex evolution of the nonlinear effects with the pump light. This study offers not only an economic seed setup to high-power narrow-linewidth tunable fiber lasers, but detailed characteristics of laser amplified with 1018-nm pump in YDFs. The knowledge of the characteristics can usefully guide future design of high-performance tandem-pumped fiber lasers.

Broadened-Laser-Driven Polarization-Maintaining Hollow-Core Fiber Optic Gyroscope

A state-of-the-art fiber optic gyroscope (FOG) using a 250-m quadrupolar-wound coil of polarization-maintaining hollow-core fiber (HCF) interrogated by a broadened laser is demonstrated. The fiber coil is directly connected to the multi-function integrated optics chip (MIOC) to reduce instabilities and reflections. The HCF FOG is first driven with a broadband source, yielding an angular random walk (ARW) of 2.12 μrad/√Hz and a drift of 0.375 μrad. This drift is 3.4 times lower than reported in any previous HCF FOG. Measuring the loss and ARW versus wavelength of the HCF FOG driven by a tunable narrow-linewidth laser reveals that the HCF supports a dense spectrum of high-scattering surface modes (∼70 modes/nm). The HCF FOG was also interrogated with a 22.5-GHz broadened-laser source, yielding an ARW of 6 μrad/√Hz. This ARW is most likely limited by backscattering mediated by coupling to the high-scattering surface modes. The drift of the HCF FOG driven by the broadened laser was 0.88 μrad. This drift is limited by polarization coupling, and confirms the holding parameter given by the fiber provider for this fiber (10−3 m−1).

High-resolution fiber Bragg grating sensor and its applications of geophysical exploration, seismic observation and marine engineering

With the development of fiber Bragg grating (FBG) and FBG based resonant cavity writing and signal demodulation technique, the measurement precision and frequency bandwidth of FBG sensor continue to be improved. It can highly promote its application in several fields of high precision detection requirements, such as geophysical exploration, seismic observation and marine observation. At present, the development of high-precision and wide bandwidth FBG sensors still face some challenges of key devices and techniques, including high-fineness FBG based resonant and low-noise narrow-linewidth tunable laser, high-precision broadband FBG wavelength demodulation technique, large-scale networking technique and high-sensitivity signal pickup probe design. Firstly, this paper introduces the development of high-precision FBG sensing technique. Secondly, it focuses on the cord devices and key techniques required for high-precision FBG sensing system and their applications in geophysical exploration, seismic observations and ocean observations. Finally, the high-precision FBG sensing technique and its application are prospected. In order to provide references for the development and application of high-precision fiber Bragg grating sensing technology, this paper aims to analyze and summarize some of the core techniques involved in high-precision FBG sensing technique and its application and the key issues that need to be solved.

Brillouin comb generation in a ring cavity with tellurite single mode fiber

We experimentally demonstrate tunable Brillouin comb generation in a ring cavity composed of a piece of tellurite single mode fiber. The tunable Brillouin comb lines are separated with frequency space of 7.9 GHz, which is equal to the Brillouin frequency shift in the tellurite fiber. Due to the high Brillouin gain coefficient of the tellurite fiber, a tellurite fiber as short as 200 m was inserted into the ring cavity. Cascaded stimulated Brillouin Stokes and anti-Stokes lines are generated in the ring cavity leading to multi-wavelength Brillouin comb generation bi-directionally. By tuning the narrow-linewidth tunable laser, we can tune the Brillouin comb in the wide range of 1533 nm to 1565 nm.

Upconversion detection of long-wave infrared radiation from a quantum cascade laser

Broadly tunable upconversion is demonstrated for long-wave infrared (LWIR) detection. The upconversion system is evaluated by the detection of 50 ns pulses from a narrow linewidth tunable quantum cascade laser (QCL) in the 9.4 to 12 µm range. The LWIR signal is mixed with a 1064 nm laser beam in a silver gallium sulfide (AgGaS2) crystal, resulting in an upconverted signal in the 956 to 977 nm range, using angle tuning for optimal phase-matching. This allows for efficient, high speed detection using a standard silicon detector. A theoretical model including absorption and diffraction shows qualitative agreement with experimental data.

A Coated Spherical Microresonator for Measurement of Water Vapor Concentration at PPM Levels in Very Low Humidity Environments

This paper presents a novel approach to measurements of low relative humidity (RH) values based on the whispering gallery modes (WGMs) effect in a small silica microsphere coated with a thin layer of Agarose hydrogel. The light from a narrow-linewidth tunable laser with a few GHz tuning range is launched into an adiabatic tapered fiber, which excites WGMs in the Agarose layer via evanescent coupling. Adsorption and desorption of water vapor by the Agarose layer in response to changes in ambient humidity lead to changes in the layer's refractive index, which in turn leads to a spectral shift of the WGM resonances. We experimentally demonstrate the WGMs spectral shift over a range of RH values from 1%RH to 25%RH. The proposed sensor displays linear response and is capable of measurement of low concentrations of water vapor (corresponding to about 11.7 ± 0.32 ppm water molecules in air). In addition, the proposed sensor offers the advantages of a very small form factor and good repeatability. Detailed studies of the sensor stability, cross sensitivity to temperature, response times, and hysteresis are also presented.

Widely-tunable, narrow-linewidth III-V/silicon hybrid external-cavity laser for coherent communication.

We demonstrate a III-V/silicon hybrid external cavity laser with a tuning range larger than 60 nm at the C-band on a silicon-on-insulator platform. A III-V semiconductor gain chip is hybridized into the silicon chip by edge-coupling the silicon chip through a Si3N4 spot size converter. The demonstrated packaging method requires only passive alignment and is thus suitable for high-volume production. The laser has a largest output power of 11 mW with a maximum wall-plug efficiency of 4.2%, tunability of 60 nm (more than covering the C-band), and a side-mode suppression ratio of 55 dB (>46 dB across the C-band). The lowest measured linewidth is 37 kHz (<80 kHz across the C-band), which is the narrowest linewidth using a silicon-based external cavity. In addition, we successfully demonstrate all silicon-photonics-based transmission of 34 Gbaud (272 Gb/s) dual-polarization 16-QAM using our integrated laser and silicon photonic coherent transceiver. The results show no additional penalty compared to commercially available narrow linewidth tunable lasers. To the best of our knowledge, this is the first experimental demonstration of a complete silicon photonic based coherent link. This is also the first experimental demonstration of >250 Gb/s coherent optical transmission using a silicon micro-ring-based tunable laser.

Phase tuning by length contraction.

Typical integrated optical phase tuners alter the effective index. In this paper, we explore tuning by geometric deformation. We show that tuning efficiency, Vπ L, improves as the device size shrinks down to the optimal bend radius, contrary to conventional index-shift based approaches where Vπ L remains constant. We demonstrate that this approach is capable of ultra-low power tuning across a full FSR in a low-confinement silicon nitride based ring resonator of 580 μm radius. We demonstrate record performance with VFSR = 16 V, Vπ L = 3.6 V dB, Vπ Lα = 1.1 V dB, tuning current below 10 nA, and unattenuated tuning response up to 1 MHz. We also present optimized designs for high confinement silicon nitride and silicon based platforms with radius down to 80 μm and 45 μm, respectively, with performance well beyond current state-of-the-art. Applications include narrow-linewidth tunable diode lasers for spectroscopy and non-linear optics, optical phased array beamforming networks for RF antennas and LIDAR, and optical filters for WDM telecommunication links.